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Cite This: Chem. Rev. XXXX, XXX, XXX−XXX

Pressure Induced Nanoparticle Phase Behavior, Property, and Applications Feng Bai,*,† Kaifu Bian,‡ Xin Huang,§ Zhongwu Wang,*,§ and Hongyou Fan*,‡,∥,⊥ †

Key Laboratory for Special Functional Materials of the Ministry of Education, Henan University, Kaifeng 475004, P. R. China Sandia National Laboratories, Albuquerque, New Mexico 87185, United States § Cornell High Energy Synchrotron Source, Cornell University, Ithaca, New York 14853, United States ∥ Department of Chemical and Biological Engineering, Albuquerque, University of New Mexico, Albuquerque, New Mexico 87106, United States ⊥ Center for Integrated Nanotechnologies, Sandia National Laboratories, Albuquerque, New Mexico 87185, United States Chem. Rev. Downloaded from pubs.acs.org by BOSTON COLG on 05/06/19. For personal use only.

‡

ABSTRACT: Nanoparticle (NP) high pressure behavior has been extensively studied over the years. In this review, we summarize recent progress on the studies of pressure induced NP phase behavior, property, and applications. This review starts with a brief overview of high pressure characterization techniques, coupled with synchrotron X-ray scattering, Raman, fluorescence, and absorption. Then, we survey the pressure induced phase transition of NP atomic crystal structure including size dependent phase transition, amorphization, and threshold pressures using several typical NP material systems as examples. Next, we discuss the pressure induced phase transition of NP mesoscale structures including topics on pressure induced interparticle separation distance, NP coupling, and NP coalescence. Pressure induced new properties and applications in different NP systems are highlighted. Finally, outlooks with future directions are discussed.

CONTENTS 1. Introduction 2. High Pressure Characterization Techniques 3. Pressure Induced NP Atomic Structure Phase Transition 3.1. CdSe NPs 3.2. PbS NPs 3.3. TiO2 NPs 3.4. CeO2 NPs 4. Pressure Induced NP Mesoscale Structure Phase Transformation 4.1. Pressure Induced Balanced NP Interactions 4.1.1. Pressure Induced NP Crystallization 4.1.2. Pressure Induced NP Coupling 4.2. Pressure Induced NP Coalescence 4.2.1. Formation of 1−3D Nanostructures 4.2.2. Correlation of Orientations and Final Nanostructures 4.2.3. Microstructures within Coalesced NP Architectures 5. Properties and Applications of NPs under Pressure 5.1. Mechanical Properties of NPs under Pressure 5.1.1. Mechanical Stiffness of NPs 5.1.2. NP Structural Stability during Phase Transitions 5.1.3. Compressibility at NP Mesostructures © XXXX American Chemical Society

5.2. Optical and Electronic Properties of NPs under Pressure 5.2.1. Surface Plasmon Resonance of NPs under Pressure 5.2.2. Band Gap of Semiconductor NPs under Pressure 5.2.3. Electronic Properties of NPs under Pressure 6. Summary and Outlook 6.1. Heterogeneous Coalescence and Doping 6.2. New NP Materials 6.3. Simulation and Modeling 6.4. New in Situ Characterization Methods 6.5. Pressure Induced NP Coupling Author Information Corresponding Authors ORCID Notes Biographies Acknowledgments References

B C E F I J O O Q Q S U U U V

AD AD AF AI AI AI AJ AJ AJ AJ AK AK AK AK AK AK AK

X Y Y AA AC

Received: January 11, 2019

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DOI: 10.1021/acs.chemrev.9b00023 Chem. Rev. XXXX, XXX, XXX−XXX

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Figure 1. Diamond anvil cell (DAC) high-pressure characterization techniques. (a) Schematic illustration of DAC. (b) Four-axis control system for a Mar345 detector used for tuning the sample-to-detector distance at an optimized X-ray wavelength. The components include (1) Mar345 detector, (2) horizontal x, (3) backward−forward y, (4) vertical z, (5) rotational x−y, and (6) moving tracks.44 Adapted with permission from ref 44. Copyright 2010 AIP Publishing

1. INTRODUCTION Pressure serves as a fundamental thermodynamic variable that has been used to create novel structure, accelerate chemical reaction, and control the properties of nanoparticles (NP).1−11 As one efficient tuning knob, it provides a driving force not only to reduce interatomic distances but also to modify electronic orbitals and bonding structures within a NP. Accordingly, pressure is emerging as a convenient and versatile tool to make quick discovery and fabrication of new NP materials, which cannot be accessible at ambient conditions. Through this review, we want to overview of how the study of NP materials under high pressure offers completely different and new ways not only in scientific investigation of fundamental phenomena but also in technological elaboration of new materials with enhanced or completely new properties. NPs have been widely used as functional building blocks to fabricate multidimensional (D) ordered assemblies for the development of “artificial solids” (e.g., metamaterials).12−24 NPs exhibit size- and shape-dependent chemical and physical properties that enable their integration into a wide range of applications, including surface-enhanced Raman sensors, optical and electronic coatings, biolabeling, and catalysts.14−20,25,26 According to the classical crystallization theory, crystallization proceeds though addition of molecular monomers or ions into a crystal lattice, which is governed by the minimization of system energy or the Gibb’s free energy G = E + PV − TS due to crystallization.27 While current NP material syntheses are largely developed through the solution chemistry at ambient conditions, the externally applied pressure through mechanical compression stresses, instead of solution chemistry, provides an additional processing parameter to change material structures via altering the term of PV in the system energy. This term enables the formation of new nanomaterial phases and configurations that impart innovative chemical and physical properties that have not been possible using current solutionbased chemical methods.28−40 Different from bulk materials, nanoparticles exhibit size and shape dependent high-pressure

phase transition behavior. Prior studies have discovered a series of interesting phenomena such as pressure induced shift of band gap energy of semiconductor NPs, elevated first-order phase transition pressure in NPs, pressure induced amorphization, and enhanced hardness and toughness, etc. Discovery of these new properties and metastable phases is crucial for design and engineering of new metastable materials which are structurally stable at ambient pressure. The rapid recent advances in precise control in size, shape, and composition of synthesized nanocrystals as well as formation of self-assembled superlattice with controlled orientations finally enable studies to address critical fundamental questions. It is now possible to discover how the key chemical/energetic factors such as NP size, shape, organic ligand, or interparticle interactions, as well as the nature of pressure fields affect NP high pressure behavior. It will be possible to know whether the orientations of the NP assembly influence nanostructures after pressure induced coalescence. Additionally, under certain threshold pressures, pressure induced mesoscale assembly can precisely and reversibly tune interparticle separation distance to achieve NP coupling and to reveal efficient charge/energy transfer, which provides great potentials for high performance nanomaterial integrations such as stress sensors, chemical sensors, high density memory storage, etc.30−33,35−37,39,41 We highlight previous researches on how an externally applied pressure changes chemical and crystal structures and thus drives phase transformation of NPs and how these changes accordingly influence their electronic and optical properties as well as their mechanical stability. This review will focus on discussion of pressure induced NP phase behavior, property, and applications with a major pressure range 0−40 GPa. For NP atomic structure, we discuss the phase transformation, size-dependent pressure behavior, and associated influence on physical property. For NP mesoscale structures, we discuss the pressure-induced coupling of NPs and subsequent coalescence to form the new category of advanced materials that have not been achieved by currently available solution-based chemical methods. Specific NP B


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Figure 2. (a−c) Optical absorption of CdSe NPs (2.8 nm in diameter) showing (a) the discrete features of wurtzite phase before a pressure-induced phase transition, and their blue-shift during compression; (b) the disappearance of discrete features around the phase transition; (c) the recovered features after a complete release of pressure; (d) Hysteresis curves of CdSe constructed from integration of the discrete feature of wurtzite in the optical absorption.52 Adapted with permission from ref 52. Copyright 1995 AIP Publishing.

configuration of the NP materials. DACs have been used as a major tool for high-pressure characterizations of structural changes, phase transitions, and properties of NP materials under static environments. DACs consist of two diamond anvils that sandwich NP samples (Figure 1). Through DACs, an external pressure overcomes atomic bonding within individual NPs or balances interparticle interactions within NP arrays, enables engineering of NP structure or assembly, allowing fine-tuning of lattice structure and interparticle separation distance and to fabricate new NP-based architectures. Inside DACs, pressure mediums are needed to transmit pressure and maintain a hydrostatic pressure field to achieve a uniform compression of target materials. Without adding pressure transmitting mediums, the pressure field becomes uniaxial inside the DACs. In general, there are three types of pressure transmitting mediums including liquid soft solids, solvents, and noble gases. The pressure transmitting mediums are required to be inert to avoid reaction with the compressed samples. Soft solids such as NaCl and BN are mostly used as a pressure-transmitting medium to create a quasi-hydrostatic conditions for some typical high pressure studies such as the measurement of electric resistance and high temperature heating of samples under pressure. Liquid solvents such as silicone oil, alcohols, fluorinert, etc., have been used as a pressure-transmitting medium to create a hydrostatic pressure field. Noble gases such Ne, He, and Xe have also been used to maintain a hydrostatic environment of the sample to a higher pressure.42 In general, the type of transmitting mediums will not affect the measurements. However, because gases penetrate into samples much easier, gases can isolate individual NPs from those in near vicinity within the NP assemblies. This prevents NPs from contacting with other NPs in near vicinity to be coalesced under pressure. Pressure is monitored and calibrated by a standard pressure-dependent ruby fluorescence technique using fluorescence of the R1 peak of small ruby pieces loaded together with NP samples inside the DAC chamber.43 Wang et al. developed combined small- and wide-angle synchrotron X-ray scattering techniques for in situ high-pressure

materials such as metal NPs (i.e., gold and silver) and semiconductor NPs (i.e., CdSe, PbS, TiO2, CeO2, etc.) will be used as examples for detailed discussion of pressure induced assembly science. This review will start with a brief overview of high pressure characterizations in section 2, which include discussion of compression devices such as diamond anvil cell (DACs), pressure media, and processes such as hydrostatic and dynamic compression. We will also discuss various characterization tools that can be coupled with DACs to in situ monitor the high pressure structural and spectral variations of NP materials. These characterization tools include synchrotron X-ray scattering, Raman, fluorescence, absorption, etc. Detailed instrumental discussions will be extended and expanded into several typical NP systems in subsequent sections. Then in section 3, we survey the pressure induced atomic structure phase transition of NPs in several important systems, such as CdSe, PbS, and TiO2, etc. Next, we address the pressure induced mesoscale structural transitions in section 4. Through different NP material systems, this section will focus on the discussion on the pressure induced interparticle separation distance, NP coupling, NP coalescence, and formation of new NP-based architectures and associated threshold pressures. In addition, simulation and modeling of NP under pressure will also be surveyed with focus on NP-based mesoscale structure changes. Section 5 provides survey on the pressure induced manifestation of new properties and applications in different NP systems. Finally, we wrap up summary and outlook with future directions in section 6.

2. HIGH PRESSURE CHARACTERIZATION TECHNIQUES Chemical synthesis of NPs relies mainly on kinetics and/or thermodynamics through solution-based synthetic processes. Alternatively, under high-pressure conditions, the free-energy change of the materials system (due to a PV term in the Gibbs free energy of G) allows a new opportunity to tune the phase or C


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Figure 3. (a,b) diffraction patterns of CdSe NP (4.2 nm in diameter) during (a) compression and (b) decompression. (c) Compression of the unit cell volume of rock-salt CdSe NP (4.2 nm in diameter), with the best fitted curve from Murnaghan equation.52 Adapted with permission from ref 52. Copyright 1995 AIP Publishing.

Figure 4. (a) Se EXAFS, (b) fractional shift in the Cd−Se bonding length, and (c) unit cell volume compression of CdSe NP (5.4 nm in diameter). Adapted with permission from ref 57. Copyright 1994 Springer Nature.

studies of NP samples.44 These techniques enable exploration of both atomic structure and mesoscale high-pressure phase behavior of nanomaterials. In this case, it is critical for DAC to have a large downstream angular opening that allows collection of both the small- and wide-angle synchrotron X-ray scattering of the samples. In addition, an adjustable detector is made to tune the sample-to-detector distance as desired. Wang et al. used a four-axial Mar345 detector to adjust vertically or horizontally to cover the scattering angle as wide as possible.44 Most recently, two detectors were synchronized with X-rays and samples, allowing for in situ collection of small-angle X-ray scattering with higher spatial resolution and wide-angle X-ray scattering with much wider angular range.

The transparent nature of diamond anvils allows in situ characterizations of NP materials through either the property measurement, such as the Raman spectrum, optical absorption, and photoluminescence spectra, or the structure measurement, such as X-ray/neutron diffraction and X-ray absorption spectrum.45−47 Taking CdSe NPs as an example, a series of discrete features can be observed by a reasonable combination of the two effects of direct band gap (wurtzite/zinc blende phase) and finite size of NP, using optical absorption spectroscopy at ambient condition, as shown in Figure 2a−c.48 Upon increase of pressure, the absorption features shift monotonically toward a higher energy (blue-shift). Eventually, the discrete features are replaced by a featureless profile that monotonically increases with photon energy. This variation is caused by a pressureD


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conditions is known as the principal Hugoniot. Because shock compression is an adiabatic process and is highly irreversible, the principal Hugoniot consists of high-temperature thermodynamic end states that are significantly offset from the principal isotherm. Consequently, shock compression is generally unsuitable for the high pressure NP experiments because the associated temperature rise through a shock (which can be hundreds or even thousands of degrees Celsius) can induce a breakdown or a melting of NPs. Other regions of the EOS can be accessed by using compression isentropes. The principal isentrope originating at ambient conditions lies between the principal isotherm and the principal Hugoniot at sufficiently high pressures and represents the response to continuous, adiabatic, reversible compression. Temperatures associated with pressure states on the principal isentrope (∼100 °C) are substantially lower than those for the principal Hugoniot, which prevents the NPs from thermally induced sintering. Achievement of truly isentropic loading is not an easy task; however, loading by a smoothly rising highpressure pulse or ramp wave, also referred to as the shockless compression, serves as one approach for realizing isentropic loading. Initial efforts to create quasi-isentropic loading, which involves the impact of gas-gun projectiles with ramp-wavegenerator buffer materials such as fused silica or pyroceram, were limited to stresses of ∼20 GPa.59 Further work on developments for ramp loading made use of graded density impactors, called “pillows,” that produce a small shock upon direct impact on a sample, followed by a gradual increase of loading stress. However, the inescapable formation of the initial shock wave from the impact is undesirable for NP assembly. Alternatively, an intense laser energy deposition technique has been developed that produces a stagnating plasma originating from a planar specimen that shocklessly loads it within a relatively rapid time of ∼10 ns to a pressure or stress of several hundred GPa.60 Unfortunately, specimens used with the laser ramp loading technique are limited to several hundred micrometers in diameter. The goal of inducing lower strain rates on large samples has led to the development of magnetically driven ramp loading techniques utilizing pulsed-power generators.61 This scheme has proven to be quite successful and has matured significantly over the past decade, making it suitable for the fabrication of functionally designed nanomaterials and structures. Using a pulse power generator, Li et al. ramp compress spherical gold NP arrays to pressures of tens of GPa, demonstrating the pressure-driven NP assembly beyond the DAC-achieved quasi-static regime (Figure 5).36 The demonstrated dynamic magnetic ramp compression produces smooth, shockless (isentropic) uniaxial loading with low temperature states suitable for the NP-based nanostructure synthesis.

induced phase transition to the rock-salt phase, which has an indirect band gap. Obviously, the disappearance of absorption features upon phase transition makes the optical absorption spectroscopy as an ideal tool to quantify the ratio of phase transition and to construct a hysteresis curve (Figure 2d). However, it is worthy of mentioning that the optical property of NPs does not always represent a direct evidence to identify the phase transition because the optical properties are also related to other properties besides the structure of NP itself, such as surface-coating ligands.49,50 For example, it is reported that, due to the surface tailoring of organic ligands, even though the structure of CdSe nanoplatelets returned to the original phase at ambient conditions after a cycle of pressurization and associated phase transition, the optical absorption and photoluminescence spectra do not come back to the discrete features.51 On the other hand, the structural characterization, such as Xray diffraction (XRD), could provide a direct evidence of phase transition.53−55 Figure 3 presents typical XRD patterns of CdSe NPs collected at different pressures. Although the diffraction peaks are broadened because of the finite size of NP (Debye− Scherrer effect), the positions of diffraction peaks of CdSe NPs at ambient conditions perfectly match a wurtzite structure and then evolve to that of a rock-salt phase at 6.3 GPa.52 Besides the phase transition, the volumetric compressibility could be obtained through the positional change of diffraction peaks with the pressure. From the compression of unit cell volume, the bulk modulus B0 (the inverse volume compressibility) can be calculated using a Murnaghan equation of state (EOS) below: V0

V=

(

1+

B0 ′ P B0

1/ B0

)

′ (1)

Figure 3C shows the volumetric compression in unit cell for the rock-salt phase of CdSe NPs (4.2 nm in diameter) and best fit from Murnaghan equation, with B0 = 74 ± 2 GPa. Another X-ray technique sensitive to the structure transformation is the extended X-ray absorption fine-structure (EXAFS) spectroscopy.56 For CdSe, Se K-edge (12.7 keV) was commonly selected, as shown in Figure 4.2,57 In the EXAFS of Se K-edge, a clear oscillation feature was observed, where the frequency and amplitude are related to the bond length between Cd and Se and the coordination number. Similar to the XRDbased results, the bond length between Cd and Se slightly decreases upon increase of pressure. At the phase transition pressure, the bond length and coordination number increase abruptly, agreeing with the fact that the bond length in wurtzite (2.62 Å) is shorter than that in rock salt (2.70 Å) (i.e., the packing density is higher). EXAFS can be used to obtain the bulk modulus of the NP as well. In XRD, the peaks are significantly broadened because of smaller sized particles and, as a result, the peaks can be overlapped, making it difficult to track the positional change of individual peaks during pressurization. Alternatively, EXAFS serves as an additional and sensitive tool to track the bond length, thus allowing for deriving the bulk modulus of nanocrystals. Static compression techniques are used to examine a material’s principal isotherm, which represents the pressure− volume response obtained by compressing a material at constant temperature originating at ambient conditions. Shock-wave techniques have traditionally been used to determine the EOS of materials under dynamic compression.58 The locus of end states produced by shock compression of a material from ambient

3. PRESSURE INDUCED NP ATOMIC STRUCTURE PHASE TRANSITION In this section, we focus on the pressure-induced changes of atomic structure, phase transformation, transition pressure, critical size, and threshold pressure. All these changes are related to the variation of atomic structures of individual NPs. In the subsection, we detail the discussion through specific examples of NPs based on their different properties. Thanks to the significant advance of synthetic methods, highly monodisperse and defectfree nanomaterials with a variety of well-controlled nanoscale morphologies, such as spherical/faceted NP, nanosheet, nanotube, and nanowire, have been achieved.12,62−65 As a result, the critical morphological characteristics of nanostructure, such as E


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transitions usually start at defects which are capable of accumulate the mechanical deformation forces and eventually the forces become large enough to cause the domain fracture.68,70 The nanoscale CdSe samples exhibit the similar pressureinduced phase transition pathway as that in bulk.67 However, starting from the pioneering work by Tolbert et al. on CdSe NPs,67,73 it has been discovered that the phase transition pressure of nanoscale CdSe is significantly elevated as compared with bulk counterpart (6.3 GPa for 4.4 nm CdSe NP67). During decompression, the rock-salt phase can be sustained at even a lower pressure (0.7 GPa67). Moreover, the domain size could be preserved upon phase transition, indicating that a single nucleation event undergoes across the domain of individual NPs68,70 (however, multiple nucleation events in the phase transition of NPs were also observed under use of other compression methods, such as laser-induced shock waves79). The size of the NP can significantly influence the phase transition behavior.73,80 Figure 6a shows the hysteresis curves for the phase transition of spherical CdSe NPs with different sizes.67 The hysteresis curves are shifted negatively with the particle size but merely change the width. It is thus suggested that the thermodynamic transition pressure decreases with particle size, but a kinetic effect (or the activation energy per atom) remains. Figure 6b presents the relation between transition pressure (represented by the midpoint of the hysteresis curve) and particle radius.2 A similar dependence of phase transition on the size of NPs was found in other geometric shapes of nanoscale CdSe. For example, a higher phase transition pressure was observed in thinner CdSe nanoplatelets (four monolayers vs five monolayers).51 For both spherical NPs and nanoplatelets, such a developing tendency of transition pressure as a function of particle size was mostly related to the fact that the contribution of surface energy becomes stronger when the size of NPs becomes smaller.2 The crystallographic facets were altered over the course of phase transition, resulting in the formation of higher energy facets in rock-salt phase, such as (111) facet (Figure 6c).2,71 These high energy facets are not naturally annealed crystals. Additionally, it is theoretically predicted that the CdSe nanorods should have the same dependence of transition pressure on the length of nanorod81 (note that the change of nanorod length does not significantly change the ratio of the surface area to volume.). However, this prediction has not yet been proved experimentally because, for nanorod, there exists a critical length scale (aspect ratio), beyond which the nanorod fractures into small NPs upon phase transition.82 Another critical morphological parameter influencing the phase transition of nanoscale CdSe is the surface structure.83 For example, it was theoretically predicted that the phase transition pathways were significantly different between spherical and faceted nanocrystals.84 Although the proposed transition pathways have not been fully proved experimentally, the influence of crystallographic facet on the phase transition was clearly observed experimentally. For example, considering that the spherical CdSe NPs only exhibit a clear elevation in the phase transition pressure when the diameter is small enough (10 years), which could be considered as the thermodynamic transition pressure. Additionally, the activation volumes derived from either the forward or reverse transition (∼0.2%) are apparently much smaller than the decreased ratio of the volume from wurtzite to rock-salt phase (∼18%) and even smaller than the volumetric change caused by a thermal vibration.70 These observations suggest that the

activation nucleation of phase transition is not fulfilled by a coherent movement of all the atoms, which should have led to a much higher activation volume. Furthermore, Wickham et al. reported that the locations of stacking faults, which cause the mixed phases of wurtzite and zinc blende, were generated during the reverse-phase transition from rock-salt, were “refreshed” over the cycles of phase transitions (Figure 7b,c).69 The formation of such stacking faults upon structural transition indicates that the structure transition most likely undergoes a plane sliding mechanism rather than a H


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coherent movement of all the atoms. In addition, Wang et al. performed a systematical study on highly oriented CdSe nanosheets, which captured much detailed information on pressure-induced phase transition pathway through a singlecrystal-like diffraction study of CdSe.72 Upon evolution of diffraction images from wurtzite to rock-salt phase (Figure 7d), it is suggested that the two sliding steps occur across the process of phase transition, as shown in Figure 7e: (i) first, the sliding of basal planes (001)⟨110⟩ in wurtzite phase is activated (the wurtzite/zinc blende stacking faults are usually generated in this sliding), (ii) then the sliding of (102) plane along 1̅01 with an angle of 40°. Additional computations have been performed to understand the kinetics of the phase transition.83,84,90−95 The dependences of both transition pressure and activation enthalpy on morphological shape from theoretical model match the experimental results qualitatively.91,92,95 However, the predicted transition pathways from such computations have not been fully supported by experimental observations. For example, many theoretical works proposed that an intermediate structure with five-coordination, often termed as h-MgO structure,83,94 should be formed in the phase transition but has not yet been observed experimentally.

Figure 8. Reported phase transition pressures from the literature. The red circle marks the results obtained from the mechanical milled samples.100,101 The blue region highlights the experiments conducted on the samples without use of a pressure transmitting medium102 (ref A,100 ref B,101 ref C,102 ref D,103 ref E,106 ref F110).

milling typically exhibit lower transition pressures than those made by other methods.100−103 Nevertheless, by comparing the phase transition pressure with the samples with similar synthesis methods and PTM, it tends to agree that the transition pressure increases with decrease of particle size. Podsiadlo et al. proposed that this structural transformation is achieved through a single nucleation event, similar to that in CdSe NPs.103,104 The elevation of transition pressure is also considered to be related to both the difference of surface energy between structural phases and the exposure of high energy surface facets during phase transition. Besides the difference of structure transformations, the properties of PbS NPs were significantly modified as well. Jiang et al. found that the electrical resistivity of PbS NPs shows a dramatic change upon phase transition (Figure 9a).101 Through the whole transformation, the electrical resistivity was significantly increased from a few kΩ cm at the starting of the phase transformation to several MΩ cm at the later stage. Upon accomplishment of phase transition, the resistivity continues to drop exponentially with pressure. This change is most likely related to a pressure-induced decrease of energy band gap in B16-structured PbS phase.101,105 A similar variation of optical properties was also observed using an optical absorption measurement on PbS quantum dots (QD), as shown in Figure 9b.102 A size-tuned excitonic absorption peak is clearly observed in the rock-salt PbS phase. During the phase transition, such an absorption feature vanishes, indicative of a dramatic change of electronic structure of PbS over the course of phase transition. This work also reveals a pressure-induced decrease of energy band gap in the rock-salt phase, which is less sensitive in the smaller NPs. Additionally, previous studies indicate that high pressure B16 phase in PbS nanomaterials can be quenched to ambient conditions by controlling the morphology of NPs. For example, Wang et al. observed that in PbS nanocubes with an average edge length of 13 nm (Figure 9c), the high pressure orthorhombic phase (B16) starts to form at a pressure of 5.7 GPa and remains stable to the peak pressure of 15.4 GPa.106 After a complete release of pressure, the B16 phase is largely preserved at ambient conditions. However, the studies on bulk modulus display much controversy results. In Podsiadlo et al.’s report, the bulk

3.2. PbS NPs

Bulk PbS has a cubic rock-salt structure (B1) with a lattice constant of 5.936 Å at ambient conditions. At a pressure of 2.2− 2.5 GPa, PbS undergoes the first-order structural transformation and transforms to an orthorhombic structure (B16/B33, black phosphorus-type).96−99 Upon increase of pressure, the secondphase transition to a CsCl type structure (B2) occurs above 22 GPa. Upon release of pressure, the starting rock-salt phase is completely recovered. Qadri et al. reported that PbS NPs (sizes ranging from 2.6 to 8.8 nm) undergo the same phase transition pathway as that in bulk and display a significant elevation of transition pressure.100 Table 1 and Figure 8 summarize all the reported transition pressures from the literature. There are apparent discrepancies in the size-dependent behaviors of transition pressure in PbS NPs, which are likely caused by different synthetic methods and various pressure transmitting mediums. For example, PbS NPs synthesized by mechanical Table 1. Phase Transition Pressure and Bulk Modulus in Nanoscale PbS (ME = Methanol:Ethanol)

size/shape 8.8 nm NP 5.4 nm NP 2.6 nm NP 7−9 nm NP 3 ± 0.3 nm NP 3.7 ± 0.3 6.7 ± 0.6 3 ± 0.3 nm NP 3.7 ± 0.3 nm NP 6.7 ± 0.6 nm NP 8.5 ± 0.8 nm NP 11.3 ± 0.9 nm NP 16.1 ± 1.9 nm NP 7 nm NP 15−20 nm NP PbS nanocube length 13 nm

phase transition pressure GPa 2.4 3.0 3.3 5.0 8.5 7.4 7.6

8.1−9.2 5.7 6.8

bulk modulus GPa

54.7 58.5 66.4 54.8 est 58.6 est 66.6 est 61.5 est 57.9 est 54.3 est 51

PTM

ref

NaCl NaCl NaCl NaCl no no no no no no no no no neon 4:1 ME No

100 100 100 101 102 102 102 109 109 109 109 109 109 103 110 106

I


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Figure 9. (a) Electrical resistance of PbS NP (8 nm) and bulk (10 μm). Reproduced with permission from ref 101. Copyright 2000 AIP Publishing. (b) Optical absorbance of PbS QDs with particle sizes of 3 and 3.7 nm, respectively. Adapted with permission from ref 102. Copyright 2014 Royal Society of Chemistry. Blue and red curves represent the rock-salt phase and orthorhombic phase, respectively. (c) TEM and XRD patterns of PbS nanocubes (self-assembled supercrystal) with the edge length of 13 nm. Adapted with permission from ref 106. Copyright 2015 John Wiley and Sons. Patterns marked in blue, red, and purple represent the rock-salt phase, orthorhombic phase, and the coexistence of two phases.

modulus of 7 nm PbS NP has a similar value of ∼51 GPa to the bulk.103,107,108 However, in Bian et al.’s report, the bulk modulus of PbS NPs does not follow a monotonic relation upon decrease of particle size, and instead, the results show a bimodal dependence of particle size, which is observed at ∼7 nm (least pronounced compressibility) as shown in Figure 10.109 A core− shell model well explains this observation. It turns out that the core behaves like the bulk, whereas the shell manifests the observed distinct behaviors of compressibility. This not only well explains the surface-dominant effect, but also helps understand the early observed size-dependent compressibility by Qadri et al.100

3.3. TiO2 NPs

Nanosized TiO2 is one of the model systems in which the size/ morphology not only affects the pressure where the phase transition takes place but also substantially changes the pathway of phase transition. In nature, TiO2 crystallizes mostly in three structural polymorphs: anatase (tetragonal, I41/amd), rutile (tetragonal, P42/mnm) and brookite (orthorhombic, Pbca). Note: previous studies also show that the TiO2-B phase sometimes exists in natural samples.111,112 Among these structural phases,113−116 anatase has been widely explored to understand the nanoscale/morphological effects on phase transition and compressibility. Bulk anatase TiO2 undergoes a phase transition from the anatase structure to a columbite αPbO2-type structure (orthorhombic, Pbcn, referred as TiO2-II in references) at 2−5 GPa, and then to baddeleyite structure (monoclinic, P21/c) at 12−15 GPa.117−123 During decompression, it transforms into a columbite structure at ∼7 GPa, which remains stable to ambient conditions.120 When particle reduces in size down to nanoscale (below 50 nm), anatase TiO2 displays a dramatically different behavior of phase transformation. Tables 2 and 3 list the observed phase transformations and compressibilities of anatase TiO2 with various particle sizes at room temperature. Dating back to 2001, Wang et al. employed Raman spectroscopy and studied anatase TiO2 NPs with diameter of 9 nm (see Figure 11a,b).124 They found that the Raman peaks (from anatase phase) shift to higher wavenumber and remain to a much higher pressure of 24 GPa than that in bulk. Upon continuous compression, the Raman peaks disappear, showing a featureless profile of Raman spectrum (Figure 11a,b). The results suggest that anatase TiO2 NPs become amorphous at 24 GPa, rather than transformation to either one of crystallized structural poly-

Figure 10. Size dependence of bulk modulus of PbS NPs. Blue line indicates the bulk modulus of bulk PbS, and red line represents the fit based on a core−shell model. Adapted with permission from ref 109. Copyright 2014 American Chemical Society. J


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Table 2. Pressure-Induced Amorphization Pressure and Bulk Modulus of Nanoscale TiO2 (ME = methanol:ethanol; MEW = methanol:ethanol:water size/shape 9 nm NP 4 nm NP 8 nm NP 6 nm NP 4 nm NP 6.5 nm NP 7.2 nm NP 9 nm NP 6 nm NP 6 nm NP 4/8 nm NP 6/11.4 nm NP

compression anatase → HDA GPa 24 21 26 20/15

decompression HDA → LDA GPa

bulk modulus

11 185 199 211 230

19.4−22.1 18 20−24 27.1

nanotube, 8−10 nm (diameter) along [100] nanoribbon TiO2-B 50−200 nm (width) 20 nm (thickness) along [010]

5.5 237 15−16

25.5 16.3−19.4

185

PTM

ref

4:1 ME no no silicon oil 4:1 ME 4:1 ME 4:1 ME 4:1 ME no Fluorinert FC70-FC77 1:1 with or without NaCl or ME 16:3:1 MEW (XRD) NaCl (Raman) no 4:1 ME

124 127 127 131 136 136 136 136 132 133 128 130 142 113

Table 3. Direct Transition from Anatase to Baddeleyite and Bulk Modulus of Nanoscale TiO2 (ME = methanol:ethanol; MEW = methanol:ethanol:water) size/shape

compression anatase → baddeleyite GPa

bulk modulus

30 nm NP 30−40 nm NP 12 nm NP 30 nm NP 30 nm NP 15 nm NP 6 nm NP 20 nm NP 40 nm NP 12 nm NP 20 nm NP 34 nm NP 13.5 nm NP 15 nm NP 15 nm NP 21.3 nm NP 24 nm NP 30.1 nm NP 45 nm NP nanosheet {001} facet 20−40 nm in length 5−8 nm in thickness curved surface NP 20 nm, est nanowire along [200] diameter εΔl ϕdoϕo/aΔϕ, the driving force due to pressure PΔV is greater than internal energy itself ΔU. Using ε = 0.5 eV,194 Δl = 1, ϕo = 0.744, ϕdo = 0.64, and a = 500 nm3, the calculation approximately gives P > 1 MPa. This threshold is 3 orders of magnitude lower than the reported experimental pressure scale, GPa.35 It is hence clear that the PΔV created by high pressure is the dominant driving force in the mechanical annealing.

It is believed that the plastic deformation by the DAC tips is also critical, which makes pressure annealing kinetically possible. Hydrostatic pressure tends to jam NP assembly by decreasing the free volume in the system.195,196 Jamming makes structural rearrangement impossible. This is also supported by Podsiadlo et al.’s study,42 in which they found the size of crystalline domains remained unchanged for NP supercrystals under hydrostatic pressure alone. Through the DAC tip, a plastic deformation occurs and the system can be unjammed.195 In particular, the local force balance of NPs is broken by the nonequilibrium deformation process, which prompts the particle relocation and grain reorientation. This helps the system overcome the free energy barrier before evolving to a new energy minimum state. Therefore, the mechanical annealing is only possible when combining both a high pressure and the plastic deformation. Overall, it is the combined effects of external pressure and plastic deformation that eliminate the structural defects from the NP assembly, resulting in a quasi-single-crystalline structure with a long-range order. The pressure annealing study provides a simple and efficient way to improve the structural quality of NP assemblies. 4.1.2. Pressure Induced NP Coupling. Precise control of NP exchange coupling is one important goal in the fabrication of NP-based devices. Structural characteristics of self-assembled NP materials such as interparticle separation distance are the most essential factor for achieving NP coupling in order to enhance the efficiency of charge coupling or transfer for applications in sensing, solid-state lighting, solar energy conversion, and electrical energy storage.197−201 As discussed S


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pressure, can be manipulated reversibly at the angstrom level until the particles make contact and the coalescence occurs. The method allows detection of new surface plasmonic behavior below the interparticle spacing limitation of 2 nm. The pressure induced NP assembly opens exciting and new avenues for investigation of NP coupling. Wu et al. reported the high pressure DAC compression of ordered 3D arrays of spherical gold NP arrays with an fcc mesophase, which has a lattice constant of 10.4 nm.30,31 The structural evolution of the fcc mesophase of the NP arrays was investigated by using high-pressure small-angle X-ray scattering (HP-SAXS) (Figure 22). They found that under hydrostatic

above, within the NP assemblies, the balanced interparticle interactions result in an intrinsic interparticle separation distance. This separation distance is ultimately determined by the organic ligands bound to the NP surfaces. For example, when dodecanethiol is used as the surface ligand of gold NPs, the interparticle separation distance is ∼2 nm within the selfassembled NP films. Bottom-up methods have been developed to change the length of the alkane chains to control interparticle separation in order to achieve NP coupling. For example, nearfield optical coupling has been demonstrated in the gold NP superlattices by varying the length of the organic ligands.201,202 Chen et al. reported the synthesis of 2D gold superlattices of alkanethiolate-stabilized gold NPs through a layer-by-layer assembly, where the interparticle separation distance was controlled by the alkyl chain lengths, ranging from C12 (1dodecanethiolate), C14 (1-tetradecanethiolate), C16 (1-hexadecanethiolate), to C18 (1-octadecanethiolate) (Figure 21).202 Because of the different lengths of the ligands, the interparticle separation was tuned from 2.2 to 3.4 nm. Through the varied interparticle separations, they were able to tune plasmonic response to shift from visible to near-infrared wavelengths. There are some limitations for this method. First, the availability of alkane ligands is limited, which prevents the method for a wide range control of the interparticle separation. Second, depending on the length of the alkane chains, the organic ligands with a minimum of 8−10 carbons are desirable to stabilize the ordered NP assemblies.18 When the alkane chain length is less than eight carbons, the overall NP assembly becomes disordered or collapsed. This makes the interparticle separation distance become ill defined. Thus, it is a challenge to achieve the separation distance less than 2 nm. Cha et al. reported a method to form gold NP dimers to address the issues of disordered or collapsed NP assemblies due to the short ligands.203 Through formation of dimers instead of NP assemblies, the interparticle distance and the surface plasmon coupling are readily tuned at the molecular level using self-assembled monolayers of alkanedithiols. The NP dimers allow investigating the classical electromagnetic model in which as the interparticle distance is reduced, the resonance surface plasmon coupling progressively red-shifts. Uniquely, they found that when the interparticle distance entered the subnanometer regime, the resonance band began to blue-shift and significantly broadened, revealing the quantum tunneling effect in the plasmonic response of gold NP dimers in the subnanometer separation region. Overall, both theoretical and experimental studies have been reported to describe the surface plasmonic scaling laws. However, because of the limitation of length by organic layer (∼2 nm), the scaling behavior has been studied only in the regime of interparticle spacing larger than 2 nm. For example, in the El-Sayed group’s work establishing scaling of SPR coupling behavior,204,205 it has been theoretically predicted that new physical phenomena emerge at extremely small interparticle distances below 2 nm, resulting in shifts that did not fit their generalized plasmon ruler equation; so far, these phenomena could not be experimentally observed. Recently, a pressure induced assembly method has been developed for effective tuning of assembly symmetry and separation distance of NPs, ideal for controlled investigation of distance-dependent exchange couplings and collective chemical and physical properties such as surface plasmonic resonance.30,31 Essentially, the pressure induced assembly and coupling method allows precise and systematic tuning of interparticle separation that, with control of the applied

Figure 22. Structural evolution of Au-NP assemblies during compression and decompression. (a) HP-SAXS patterns of Au-NP assemblies at ambient pressure, 7.7, 9.6, and 13 GPa. (b) Integrated spectra from HP-SAXS patterns collected at varied pressures during compression (black spectra) and decompression (green spectra). (c) Graph showing the d-spacing of the first Bragg reflection in each HPSAXS spectrum in (b). The data points marked in green correspond to release of the pressure. (d) Graph showing the d-spacing ratio (R) at different pressures. Reprinted with permission from ref 30. Copyright 2010 John Wiley & Sons, Inc.

pressures up to ∼9 GPa, the unit cell dimension of 3D ordered NP arrays reversibly shrinks and springs back upon application and release of external pressure, respectively. There are several typical features over the course of the compression process. First, a significant d-spacing shrinkage occurs at the beginning of compression, consistent with what has been observed in the pressure induced recrystallization.35 With increasing pressure, the interparticle separation is systemically and gradually reduced by the applied pressure. For example, the lattice constant shrinks from 10.4 nm at ambient pressure to 9.1 nm at ∼9 GPa. This essentially increases the NP coupling within the NP arrays. The volumetric shrinkage stops at 9 GPa. Second, the compression process is reversible. The lattice constant returns to 10.4 nm when the pressure is completely released. As a result, the centerto-center particle distance varies from 74 to 64 Å further until the NPs contact at ∼10 GPa. This is important as one can investigate reversibly the hysteresis of the compression process. In addition, it allows investigation of the surface plasmonic coupling beyond the ligand-enabled limitation of 2 nm. Finally, the hydrostatic pressure field is essential for the uniform volume shrinkage of NP lattice, which results in a uniform reduction of interparticle separation distance. Although uniaxial pressure or deviatoric stress can also cause volume shrinkage of NP arrays, the shrinkage occurs only in one direction so that the T


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compressed at mesoscale and the NPs get closer along the pressure field until the proximity NPs contact at the threshold pressure (Figure 22). Before the threshold pressure, the NP lattice box elastically bounces back when the pressure is released. The entire compression process is reversible. However, above the threshold pressure, when the applied pressure is released, the compression process is irreversible. The NPs start to coalesce together when the pressure is over the threshold pressure, forming dimers, trimmers, nanorods, and nanowires depending on the applied pressure. For example, Wu et al. demonstrated the threshold pressure of 9 GPa was needed for fabrication of Au NP nanowires.30 There are several key features for the pressure induced NP coalescence process. First, the coalesced nanostructures are polycrystalline in comparison with singlecrystalline structures that are synthesized by using chemical solution methods. They consist of individual NP nanodomains with the nanostructures. Second, the key dimensional parameter of the nanostructures is close to the size of the initial NPs. For example, the diameters of the resultant 1-D nanorods and nanowires are similar to those of the starting spherical NPs.30,33,36,37 Third, pressure medium is crucial for whether the pressure induced NP coalescence occurs to form 1−3D nanostructures. Liquid silicone oil has been used as the pressuretransmitting medium in the most of previous high-pressure studies.211 In general, it is capable of maintaining a hydrostatic environment of loaded samples to the pressures as high as ∼9− 10 GPa.212,213 Above this threshold, a generation of deviatoric stress results in a pressure gradient vertically across the samples. This nonhydrostatic stress has been confirmed by careful evaluation and analysis of the quantitative textures by unraveling 2D HP-SAXS pattern (Debye−Scherrer rings) into a Cartesian (cake) plot of azimuthal angle as a function of 2θ.214,215 An evident waviness at pressures at or above 8.9 GPa can be observed as an indication of the generation of a nonhydrostatic stress field during compression of NP films. Podsiadlo et al. performed a quasi-hydrostatic high-pressure compression study on the faceted 3D supercrystals self-assembled from 7 nm spherical PbS nanocrystals.42 They did not obtain any coalesced nanostructures in their experiments in which neon gas was used as the pressure-transmitting medium. Instead, they showed nearly perfect structural stability of the supercrystals. This is most likely because the neon gas penetrates into the supercrystals and isolated the individual PbS nanocrystals from being contacted, which prevents the formation of coalesced nanostructures. Finally, it is believed that the framework of the initial NP assembly is crucial for the phase transformation and subsequent formation of coalesced nanostructures. The mechanical flexibility of organic matrices such as the organic ligands on NP surfaces or polystyrene provide sufficient compressibility to maintain the ordered NP mesophase and favorable orientation while the unit cell dimension shrinks. In an ordered Au NP/mesophase in a silica framework,18 nanowires could not be obtained. This might be because the rigid, condensed silica layer between Au NPs prevents adjacent NPs from being contacted and sintered.30 The fabrication method of pressure induced NP coalescence has been successfully demonstrated in several material systems including Au,30,31,33,36 Ag,33 CdSe,5,37 Au/CdSe composites,29 and PbS.5,32,39

deformation is irreversible. For example, Yin et al. reported the use of a uniaxial stress to disassemble Au NP arrays in a polymer matrix.206 The stress process takes advantage of the intensity and time-dependent viscous flow of polymeric materials in response to the mechanical stress. They demonstrated that the uniaxial stress induced 1D deformation of Au NP composite film. Thus, by design of the plasticity of the polymer, the pressure induced plastic deformation leads to the disassembly of embedded Au NP chains and causes a noticeable shift of plasmonic band, which has been correlated to the applied mechanical stress. The pressure-regulated fine-tuning of interparticle separation distance overcomes current limitations (∼2 nm) resulting from the intrinsic organic monolayers and is ideal for controlled investigation of distance-dependent collective optical and electronic phenomena. Experimental studies have clearly demonstrated strong and collective 3D surface plasmon NP assembly (Figure 22). The observed red-shifts in the peak positions of surface plasmon resonance bands are much larger than any previously reported wavelength shifts by varying particle size (from 3 to 25 nm) or dielectric environments (10 nm shift for the change of refractive index n from 1.33 to 1.46).207−210 Another distinctive phenomenon is that the surface plasmonic behavior is reversible when the pressure is applied and released. This provides a unique opportunity to investigate the surface plasmonic resonance in a reversible manner over the course of compression and decompression, which is impossible for the current methods. The fact that the sizes of the NPs and the organic chains, as well as the dielectric environments, all remain unchanged throughout the course of these compression and release experiments, together with the apparent reversibility of plasmonic shift, indicates that the large observed shifts of surface plasmon bands are a consequence of the strong collective behavior. Finally, the pressure induced spectroscopic shifts of the surface plasmon bands can be correlated to the corresponding structures of finally harvested NP materials. This provides essential insights into how NP interactions influence both final NP packing at mesoscale and chemical and physical properties.33 4.2. Pressure Induced NP Coalescence

When two NPs are close together, they tend to sinter or coalesce to form a new particle with larger size. This is the so-called Oswald ripening process, in which larger particles grow at the expenses of smaller particles. The coalescence or sintering process often occurs in a liquid or solution phase at ambient conditions. Recently, pressure induced NP assembly and coalescence have been demonstrated in solid phase. Wu et al. reported that over the certain threshold pressures, NPs get close and contact by applied pressure to fuse into new types of nanostructures.30,32 Depending on the magnitude and environment of applied pressure, various nanostructures including NP dimers, trimmers, nanorods, and nanowires have been formed.30−37 In addition, depending on the orientation of the beginning NP arrays, 1−3D nanostructures have been demonstrated in metallic and semiconductor NP materials30−37 as well as metal−semiconductor composites.29 In this section, we discuss the recent progresses on the pressure induced NP coalescence and formation of 1−3D NP-based architectures. 4.2.1. Formation of 1−3D Nanostructures. Unlike the Oswald ripening process in which particles grow bigger, pressure induced NP coalescence results in a fusion of NPs into a series of specific 1−3D nanostructures. It has been discussed above in section 4.1 that, upon compression, the NP lattice box is

4.2.2. Correlation of Orientations and Final Nanostructures

Experimental results of pressure induced NP coalescence provide essential insights into the correlation of initial U


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Figure 23. 1−3D nanostructures formed by pressure-induced NP coalescence. (A−C) TEM images of the sintered Au NP dimers, trimers, nanorods, and nanowires. (D) Au nanosheets. (E) High resolution TEM image of Au nanowires that consist of Au NP domains. (F) 3D interconnected structures. (G) High resolution TEM image of 3D Au nanostructure showing the branched 3D connection by Au NPs. (A−C) Reprinted with permission from ref 30. Copyright 2010 John Wiley & Sons, Inc. (D,E) Reprinted with permission from ref 36. Copyright 2014 Macmillan Publishers Limited. (F,G) Reprinted with permission from ref 31. Copyright 2010 American Chemical Society.

orientation of the NP arrays to the final nanostructures. Depending on the initial orientation of the NP arrays, nanostructures including 1D nanowires, 2D nanosheet, and 3D interconnected framework have been demonstrated (Figure 23). For example, the [110] orientated NP arrays tend to form 1D nanowires under compression.30,33,36,37 At ambient pressure, the Au NP assembly with an fcc mesophase with a preferred orientation along the [110] direction was compressed in a DAC. Because silicone oil was used as the pressure medium, the sample was compressed under a hydrostatic pressure field up to 7.7 GPa. The pressure spreads isotropically over all directions within the Au NP assembly. Therefore, the shrinkage of the fcc unit cell lattice box was uniform. The fcc mesophase of the NP arrays and its [110] orientation have been successfully retained. For pressures over the course of 8.9−13 GPa, a nonhydrostatic stress was generated and applied perpendicularly to the NP film. Because of the [110]-orientation of NP mesophase, Au NPs along the [110] direction received greater stress than other crystallographic directions, which drove the spherical NPs to contact and eventually coalesce into Au nanowires preferentially along the [110] direction. Ultimately, these nanowires remain well-oriented in 2D hexagonal bundle that has the c-axis in parallel to the former fcc [110] direction. In another example, the 3D Au interconnected nanostructures have been fabricated via a pressure induced NP coalescence using a [111] orientated fcc NP mesophase.31 Atomistic simulation studies have shown that the dimensionality of the final structure depends on the orientation of superlattice and uniaxial loading (Figure 24). At ambient conditions, in an fcc NP superlattice, each NP has 12 closest neighbors. The various crystalline planes contain different numbers of NPs that correspond to different orientations. Given the nearest NP numbers are different at different orientations, compression along [110], [111], and [100], results in 2, 6, and 8 nearest neighbors, respectively. In an fcc NP mesophase, the closest interparticle spacings are those within {110} planes. Consequently, these particles touch and sinter first, forming the 1D nanowires in the [110] orientation. The coalescence along [111] orientation is consistent with

either 3D or possibly planar structures when the six closest neighbors are consistent with an in-plane hexagonal structure. Finally, the compression along [100] orientation gives threedimensional structures, but the closest-neighbor number of 8 indicates a three-dimensional structure with a reduced symmetry from a perfect fcc lattice. Simulations provide deeper insight into the ligand−surface interactions and bonding on NP sintering under pressure. Lane et al. suggested that the binding energy of S−Au is also critical. They found that the degree of sintering was significantly affected by the change of the S−Au binding energy.216 Lowering the binding energy seems to indicate that NP−NP sintering threshold pressures could be reduced. This seems consistent with experimental results. In the Ag NP lattice, the threshold pressure was found to be ∼8 GPa to induce the NP coalescence and formation of 1D nanowire.33 This pressure is lower than the observed threshold pressure of ∼9 GPa in Au NP lattice array, consistent with the fact that S−Ag is weaker than S−Au.30 Li et al. also emphasized the importance of the stress conditions that are required to enable such a fusion including both hydrostatic background pressure and uniaxial pressure.34 The hydrostatic pressure compress uniformly through all directions to the NP arrays, causing the fcc unit cell dimension to shrink uniformly. However, the overall pressure is not high enough to break the balanced forces between adjacent NPs. A uniaxial force is finally required to sinter the NPs along different orientations. On the basis of this understanding, a phase diagram was established in the Ag−Au NP superlattice systems, which provide structural and compositional tunability.34 4.2.3. Microstructures within Coalesced NP Architectures. Microstructures within coalesced NP structures by pressure compression are quite different from those by chemical methods such as orientated attachment (OA) that occurs in solutions. Aabdin et al. discovered that the angle at which the two NPs meet is critical for achieving oriented attachment. When the two Au NPs meet at a small angle (15°), dislocation occurs. V


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Figure 24. Dependent formation of different nanostructures by pressure induced coalescence. Illustrations of in each row show nearest-neighbor cluster within the fcc lattice from along [100], [110], and [111] orientations after compression and initial uncompressed state. The clusters are cut along three lattice planes (columns) to show the three-dimensional nature of the resulting structures. Reprinted with permission from ref 36. Copyright 2014 Macmillan Publishers Limited.

formation of smooth and single-crystalline nanostructures.219−223 In a solid state, the microstructure is polycrystalline and consists of NP domains from the original NPs. TEM results demonstrated that the coalescences are random. For example, Li et al. observed random coalescences of CdSe NPs into polycrystalline nanowires through pressure compression.37 High pressure deviatoric stress induces neither nanocrystal rotation nor preferential NP alignment during compression and pressure release. The final CdSe nanowire microstructures display a twisted morphology but lack a necking structure between adjacent nanocrystals. These results have clearly testified that no structure relaxation occurred in solid state, which is mainly due to the lack of NP rotation through the contacting process of NPs. Although the coalescences of CdSe NPs are achieved majorly through the ⟨111⟩/⟨111⟩ config-

Their molecular dynamics simulations validated that the contact angle favors achievement of a surface energy minimization for Au NP sintering.217 The pressure induced coalescence occurs in a solid state in contrary to OA that occurs in solutions. In solutions, NPs tend to align due to the dipole−dipole interactions. They are free to align to their crystallographic orientation through rotational or translational motion. When contacting along the aligned direction, they merge together to form larger nanocrystals. Different types of defects, such as twinning and dislocation, can be favorably formed at the grain boundary of coalesced nanocrystals as a consequence of the orientated attachment.218 After the NPs sinter together, structural relaxation often occurs through a solution-mediated mass redistribution or atomic rearrangement to eliminate defects, such as the necking structures between nucleated nanocrystals, leading to the W


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Figure 25. Random coalescences of NPs through a pressure induced coalescence. (A) Representative TEM image of CdSe nanowires synthesized by a pressure-induced assembly. The nanowire samples were collected after releasing the pressure from 15 GPa. Inset: CdSe nanowire bundles. (B) HRSTEM image of the CdSe nanowires. Inset: Statistical analyses of crystallographic interfaces between nanocrystals. (C) Schematic model showing the coalescence of CdSe nanocrystals in random crystallographic orientations, as marked in arrow. (D) Nanowires synthesized by compression of 6 and 8 nm NP arrays. Reprinted with permission from ref 37. Copyright 2017 AAAS.

variety of nanomaterials including metals, semiconductors, and organics are reviewed. The related existing and potential applications of these materials are also discussed. As discussed above in section 3, the inorganic cores of NPs undergo a series of atomically structural transformation upon pressurization. Under a moderate pressure, a reversible and isotropic shrinkage of lattice constant is commonly seen. As pressure further rises to a threshold value, a phase transition often occurs. Most of these behaviors are inherited from the corresponding bulk counterparts. However, very often quantitative difference was discovered, and they will be discussed in the following sections. The size-induced deviation from bulk behavior is heavily originated from the surfaces of NPs that display a reconstructed atomic structure and therefore different physical and chemical properties from bulk materials.4,44 At the mesoscale, an elevated pressure reduces interparticle separation and even transforms NPs into higher dimensional nanomaterials via the aforementioned pressure induced coalescence. As the structure evolves under pressure, the properties of NPs change accordingly. By in situ structure and property character-

uration because of their low energy surfaces, there are other random interfaces (Figure 25) within final microstructures. In another example, Li et al. demonstrated the defects in Ag NPs have not been removed during the pressure-induced process of NP coalescence. When Ag NPs merged together, the final microstructures displayed a NP-based multiply twinned polycrystalline feature with a zigzag morphology, which is different from those of single-crystalline particles in solution.183,224−227

5. PROPERTIES AND APPLICATIONS OF NPS UNDER PRESSURE As one of the two most fundamental thermodynamic variables, pressure plays an equally important role as temperature in determining a variety of material properties with no exception for NPs. Understanding the properties of NPs under elevated pressure serves both fundamental scientific interests and realworld applications. Pressure is a highly efficient tool to tune the structures in NP systems.228 In this section, the pressuredependent mechanical, optical, and electronic properties of a X


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B0′ is a positive dimensionless parameter that describes the pressure-induced hardening of the material as it cannot be compressed infinitely. Generally speaking, large B0 and small B0′ are desirable if minimum pressure sensitivity of structure is needed. Not surprisingly, the stiffness of NPs typically differs from the corresponding bulk counterpart, which is sizedependent and heavily affected by the particle surface structure and chemistry. However, whether the bulk modulus of a given nanomaterial is higher or lower than the bulk value has been reported to be arbitrary, and it will be discussed separately as follows. A majority of NP species studied so far were found to be stiffer than bulk including but not limited to Au, Ag, Pt, TiO2, TiC, Fe2O3, CoFe2O4, CeO2, PbS, and BaF2.41,103,126,130,149,230−238 Nobel metal NPs are the common subjects in material research due to their ease of synthesis and ambient stability. As reported by Gu et al., high-pressure XRD measurements revealed a significantly higher B0 of 290 GPa of 30 nm gold NPs over the bulk value of 170 GPa as well as a B0 of silver increased noticeably from 116 to 140 GPa.237 The increased firmness is shown by Figure 26a that presents a slower compression of these NPs than the bulk materials. This phenomenon was explained by the two factors: decreased interatomic distance of dA−A by surface and boundary effects and increased Einstein temperature with decreasing particle size. According to the Einstein’s expression of bulk modulus (eq 8), where M is the molar mass and θE is the Einstein temperature, these two factors together

ization of NPs, the important structure−property relationships can be systematically mapped.44 Pressure is also a very clean tool for structure tuning compared with other methods such as modification of the chemistry. For example, the interparticle separation can be modified by choosing or exchanging with one type of ligands with desired molecular size.229 However, the ligand manipulation inevitably alters chemical environment, especially at NP surfaces that are crucial to electronic and optical properties. Such an uncertainty can be naturally eliminated by using pressure as a structure-tuning method. On the other hand, understanding the pressure-property correlations is important to the applications of NPs in a wide range of fields such as chemical, oil/energy, machinery, and defense industries, where a high pressure environment is commonly involved. 5.1. Mechanical Properties of NPs under Pressure

NP structure under pressure is of essential importance although often overlooked when the research interest is the pressure− property correlation. Because of the relatively poor reproducibility of NP synthesis and processing, the structure information is critical for the true understanding of the pressure−property relationship. For example, the band gap of a semiconductor NP is directly altered by its atomic structure including lattice shrinkage and possible phase transition rather than the pressure itself. As efforts to serve this purpose, the structures in NP systems under pressure and related mechanical properties have been studied usually by high-pressure X-ray scattering using DACs. In this subsection, mechanical property studies associated with three subjects are reviewed: elastic compression and stiffness of NP cores, atomic lattice phase transition, and compression of superlattices. 5.1.1. Mechanical Stiffness of NPs. At relatively low pressures up to a few gigapascals, typical pressure media in a DAC remain in a fluid state and maintain a hydrostatic pressure. NPs under such circumstances are considered to shrink isotropically and reversibly. The compression pathway of a solid material is usually described by the third-order Birch− Murnaghan EOS (B-M-EOS, eq 4) or its modified version, Vinet EOS (V-EOS eq 5). When the pressure is released, the NPs return to the initial structure via the same pathway as long as the deformation remained in the elastic regime. P = 1.5B0 (v−7/3 − v−5/3)[1 + 0.75(B0 ′ − 4)(v−2/3 − 1)] (4)

P = 3B0 v−2/3(1 − v1/3) exp[1.5(B0 ′ − 1)(1 − v1/3)]

(5)

In these equations, v = V/V0 is the normalized volume of a material. At near ambient conditions, i.e., P = 0, the first derivative of either EOS yields the following. jij ∂P zyz = −B0 j z k ∂v { P = 0

(6)

The EOSs reduce to a linear relationship as the first-order approximation and can be easily translated to the Hooke’s law. B0, which shares the units of pressure, is defined as the bulk modulus of one given material to measure its stiffness against external pressure. The higher the bulk modulus the stiffer the material is. The first derivative of the bulk modulus leads to the other parameter B0’. i ∂B y B0 ′ = jjj zzz k ∂P { P = 0

Figure 26. NPs with higher stiffness than bulk material. (a) Pressure dependence of the unit cell volume of less compressible Ag and Au NPs compared to the bulk materials. Reprinted with permission from ref 236. Copyright 2008 American Physics Society. (b) Room temperature equation-of-state data for nanocrystalline CeO2. Reprinted with permission from ref 149. Copyright 2004 American Institute of Physics.

(7) Y


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Figure 27. NPs display size-dependent softening effect: (a) Al2O3. Reprinted with permission from ref 242. Copyright 2002 American Physics Society. (b) UO2. Reprinted with permission from ref 243. Copyright 2013 Elsevier. (c) ZnS, Reprinted with permission from ref 244. Copyright 2006 American Physics Society. (d) CdSe. Reprinted with permission from ref 245. Copyright 2014 Institute of Physics.

property and a shell of surface layer whose mechanical property was affected by surface curvature. The surface layer would be more rigid on bulk material when no curvature presents due to a surface reconstruction. As the NP diameter decreased, the increasing curvature weakened the surface layer by introducing exposed atoms and imperfections. A similar core−shell model was referred to explain the increased bulk modulus in Ho2O3 NPs.239 As another example to demonstrate the non-negligible influence of the surface structure of NPs on their properties, Ni NPs have been found to show either higher or lower deviation from bulk B0 (180 GPa) depending on the synthetic method. Ni NPs synthesized by a gas-phase condensation displayed a slightly enhanced stiffness of B0 = 185 GPa.240 In contrast, Ni NPs of similar size produced by mechanical ball milling were found to be weakened, with B0 = 161 GPa.241 The difference is attributed to the difference in NP surface layer morphology. The gas-phase condensation allows the NP surfaces to reach a low free energy state that is structurally stable and therefore enhance the stiffness. On the other hand, the surfaces of mechanically separated NPs are expected to be weakened due to the lack of an annealing process. In contrast, a few species of NPs were also found to be relatively softened from their bulk parents. In addition, a consistent size-dependency has been noticed that the smaller the NPs the lower the B0. It can be explained by the core−shell model as in the aforementioned PbS NP case. However, the materials in this category have a surface layer similar to or weaker than bulk solid to begin with. Some examples including Al2O3, UO2, ZnS, and CdSe NPs are presented in Figure 27.242−245 In these works, the softening effect was attributed to multiple mechanisms such as a size-induced reduction of vibration entropy, change in valence electron density and additional modes of structural response to external pressure in NPs.

resulted in a higher bulk modulus. In addition, high-energy X-ray pair distribution function measurements on Au and Pt NPs showed faster hardening of NPs, i.e., higher B0′, which was attributed to a pressure-induced reduction of grain size.236 B0 ∝

MθE2 dA − A

(8)

A similar size-induced stiffening effect has been also observed in ceramic and magnetic materials, including TiO2, TiC, and CoFe2O4 which exhibited 35%, 10%, and 19% increment in B0, respectively.126,234,238 By tuning the surface structure via a ligand decoration, an enormous enhancement of B0 of 84% has been achieved in γ-Fe2O3 NPs. It is worth pointing out that the sizeinduced stiffening could break down when the external pressure exceed the certain threshold. As discovered by Wang et al.,149 CeO2 NP initially displayed a significantly enhanced bulk modulus of 328 GPa which, however reduced back to the bulk value of 230 GPa when the exerting pressure exceeded 20 GPa (Figure 26b). The weakening of elastic stiffness of CeO2 NPs above 20 GPa was contributed to the pressure-induced enlargement of NP size. As a promising semiconductor for applications such as photovoltaic and infrared LED, the compressibility of PbS NPs was studied by high-pressure experiments, which showed less pronounced stiffening. Podsiadlo et al. reported a similar B0 to bulk value.103 A more systematic study by Bian et al. revealed a nonmonotonic size-dependence of B0 in PbS NPs.102,109 As presented in Figure 10, while the particle size increased from 3 nm, the stiffness of NPs increased to a peak value of B0 = 66 GPa, 25% higher than the bulk material, at approximately 7 nm. As the particle size further grew, B0 dropped asymptotically to the bulk value. Such a Hall−Petch-like relationship and the maximum were explained by a NP model containing a core with bulk Z


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Figure 28. Wurtzite-to-rock-salt transition in CdSe NPs. (a) Wurtzite to rock-salt transformation pressure as a function of CdSe NP radius. (b) Energy volume curves for bulk CdSe and three sizes of CdSe NPs in both the wurtzite and the rock-salt phases. Transition pressures and NP sizes are indicated. (c) Transformation of a wurtzite phase to a rock-salt phase following a two-dimensional analogy. Reprinted with permission from ref 52. Copyright 1995 American Institute of Physics. (d) Illustration of the various size regimes of the kinetics of solid−solid phase transitions. Defects, which act as nucleation sites, are indicated by asterisks in the cartoon of the bulk solid. Reprinted with permission from ref 246. Copyright 1997 American Association for the Advancement of Science.

5.1.2. NP Structural Stability during Phase Transitions. Within the elastic compression regime, the atomic lattice in NP cores remains and returns to their original symmetry upon pressure release. As pressure further increases, the lattice could become unstable and undergo phase transitions to different symmetries. The new phase is normally detected directly by changes in XRD patterns. Comparing with the elastic compression case in which the existing XRD peaks shift simultaneously to higher 2θ, the symmetrical changes result in the erection of new peaks as well as the disappearance of some original peaks. As a natural consequence of the structure− property relationship, phase transitions often induce abrupt changes of properties such as electronic structure and atomic vibration modes. Therefore, the corresponding phase transition can also be measured indirectly by optical absorption and Raman spectroscopy, respectively. The phase-dependent properties facilitate the development of materials with desirable characteristics not achievable with ambient phases. It also promotes applications such as sensitive pressure sensors. Phase transition is perhaps the most studied aspect in NPs. Not surprisingly, the extremely small size of NPs results in unique behaviors that had not been observed in bulk materials. As a common observation in almost every case, NPs display phase transition pressure (Ptran) diverted from bulk value, sometimes higher and sometimes lower and usually sizedependent. More interestingly, the strong surface effect of NPs can also introduce new structures under pressure which are not achievable in bulk materials. In some scenarios, the new phases can be even preserved after pressure release. These aspects of phase transition in NPs will be discussed as following. Enhanced structural stability of NPs, i.e., Ptran higher than bulk value has been reported for the majority of oxides and similar materials including, CeO2, Ga2O3, Ho2O3, In2O3, SnO2, ZnCo2O4, PbMoO4, CdSe, PbS, HgS, and BaF2.1,52,103,159,232,233,239,247−254 The two most popular explanations are the surface energy effect and unique nucleation mechanism both induced by the tiny particle size. As an exemplary system, the high-pressure structure of CdSe NPs has been intensely studied over the past two decades and will be reviewed with details in this section. Most synthetic CdSe NPs exist as a hexagonal wurtzite structure. By pressurization, the lattice transforms to a cubic rock-salt symmetry. As early as

1994, Tolbert et al. reported their pioneer work which explored the phase transition behavior of CdSe NPs of radius ranged from 1 to 2.1 nm.1 A followup paper was published in the following year to provide further details and insights.52 As the primary discovery of this work, the NPs showed a size-dependent Ptran from 4.9 to 3.6 GPa (Figure 28a), all higher than the bulk number of 2.0 GPa. A thermodynamic model was proposed to explain such an observation in terms of surface energy. This model can be used as a universal guideline to explain the size dependency of Ptran. As illustrated by Figure 28b, the internal energy of U as a function of volume of a unit cell were calculated based on the compressibility and surface energy for both wurtzite and rocksalt phases. For each particle size, the energy curve associated to the rock-salt structure shows an offset to high energy and smaller volume when compared with the wurtzite phase. The lower volume accommodates the PV term in free energy to compensate the cost of elevated energy stored and a higher surface energy in the high energy phase. On the other hand, the curve of the same phase also shifts toward high energy as the particle size decreases. This is a result of the increased contribution of surface energy that is significantly higher than the bulk energy. It also shifts slightly to the low volume side because of the surface tension-induced particle contraction that preexists under ambient pressure. At Ptran, the two phases coexist and share the same energy. Therefore, the following relation must be satisfied. Uwurtzite + PtranΔV = Urocksalt

(9)

This equation can be translated onto the energy−volume plane by drawing a straight line tangent to both energy curves. The slope of this line is then exactly Ptran. An origin of the higher energy associated with the rock-salt structure was attributed to the distortion of NP surfaces which convert the low index terrain into morphology containing a large portion of high index surface atoms (Figure 28c). The phase transition mechanism was also explored by studying the hysteresis phenomenon. NPs of different size displayed a similar width of hysteresis that is, however, broader than the bulk case. It indicates a singlenucleation transformation mechanism rather than the multinucleation events and grain size reduction in bulk material. A later study by the same group proposed the three regimes of AA


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ligands and therefore a higher pressure is needed to provide enough energy to the NP cores to promote the phase transition. Additionally, it has been also found even with the same NP and ligand, the structural stability can be tuned by how the NPs are arranged or organized at mesoscale, i.e., the mesostructure. An example was reported by Bian et al., in which by assembling 7 nm PbS NPs into face-centered cubic (fcc) or body-centered cubic (bcc) superlattice, different Ptran of 7.0 and 8.5 GPa were achieved respectively for the rock-salt-to-orthorhombic transition (Figure 29c−e).253 This was attributed to the higher compressibility and thus higher capacity to absorb mechanical energy of the bcc superlattice that will be discussed in details later in this section. It is worth pointing out that there also exist NPs with diminished structural stability than bulk material. For example, γ-Fe2O3 NPs were found by XRD measurements to have Ptran between 25 and 30 GPa, lower than the bulk value of 35 GPa.230,256 It was also confirmed by a high-pressure Raman spectroscopy study by Wang et al.257 The early transition into α phase was simply attributed to a surface or boundary effect. It can also be explained by the aforementioned energy−volume diagram as in the CdSe case. A lower Ptran shall be expected if the surface energy is higher for the low pressure phase. Other NPs display reduced Ptran include Bi2Te3 and CoFe2O4, both by approximately 10 GPa. The phase transitions in NPs discussed so far follow the same polymorphic path as in bulk materials but at a usually higher or occasionally lower pressure. As an exciting opportunity offered by NPs, new phases not attainable in macroscopic samples have been achieved. These new phases have the potential to possess attractive properties. As mentioned earlier, the phase transition pressure increases as particle diameter shrinks in most materials due to a strengthened surface effect. However, it is intuitive to not expect the NPs to be hardened infinitely. A commonly observed phenomenon is amorphization of atomic structure when the particle size drops below a threshold value. As early as the late 1990s, Tolbert et al. reported amorphization in Si NPs.3 As an element in the same group, Ge NPs were found to be prone to amorphization instead of a transition into the β-Sn metallic phase as bulk Ge does.258 More recently, a systematic study by Quan et al. on PbTe NPs have provided a more comprehensive view of this topic.163 By experiments with NP of different sizes, a phase diagram of PbTe NPs was produced as in Figure 30a. Typical rock-salt to orthorhombic transition is observed in PbTe NPs larger than 9 nm. As the NP size further decreased, the polymorphic transition path broke down and was replaced by a low density amorphous (LDA) phase. Further investigation at higher pressure revealed a metastable high density phase (HPA) which can be preserved under ambient conditions.38 Similarly, a new high-pressure cubic phase have been achieved in γ-Al2O3 NPs and maintained under ambient pressure.242 It offers a new approach to create material structures not achievable in bulk counterparts. The size-dependent amorphization in TiO2, Y2O3, and PbTe was further explored.259−261 Machon et al. explained it by three different phase transition mechanisms. As shown by Figure 30c, in the first scenario for large particles or bulk materials, the defect concentration is too low to induce an amorphization. Therefore the growth of the high-pressure phase within the low-pressure phase is homogeneous. In scenario 2, the amount of defects is enough to induce a percolating system of amorphous domains. In the case of small NPs, abundant defects present at particle surfaces. In scenario 3, the polymorphic and amorphization

phase transition types depending on particle size (Figure 28d). In addition, molecular simulations revealed that the nucleation mainly occurred at the surface of CdSe NPs due to the high likelihood of imperfections there.255 The size effect on structural stability is not limited to spherical NP only. Wang et al. discovered that the thinner CdSe nanosheets have higher Ptran than thicker sheets and bulk material (Figure 29a).5 This is again caused by an increasing role

Figure 29. Mesoscale effect on phase transition pressure in NPs. (a) Calculated thermodynamic transition pressure as a function of thickness of CdSe nanosheets and resultant saw-like rock-salt layer. (b) Observed transformation pressure in 1.4 nm CdSe nanosheets as a function of intersheet ligand length. Reprinted with permission from ref 5. Copyright 2010 National Academy of Science. An in situ highpressure XRD patterns of PbS NP superlattice (SL) under compression: (c) fcc and (d) bcc SLs. The phase transition occurs near 7.0 GPa in the fcc SL, whereas at 8.5 GPa. (e) Schematic of the two PbS crystal structures: left, low-pressure rock-salt phase, and right, highpressure orthorhombic phase. Reprinted with permission from ref 253. Copyright 2012 National Academy of Science.

played by surface atoms as dimension decreased. In addition, as shown by Figure 29b they also reported a dependence on the length of the coating ligand. When the ligand length increased from 2.3 to 2.6 nm, Ptran increased significantly by more than 2 GPa, equivalent to over 20000 atmospheric pressure. The extra structural stability was caused by that of a considerable amount of the input mechanical energy that was distributed into the AB


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However, when particle size drops to a few nanometers, new phases appear under pressure. Several groups have independently reported the phase transitions in Ag NPs that had been never observed in bulk silver.183,262,263 The Ag fcc lattice transforms to trigonal, rhombohedral (a special case of trigonal), or tetragonal symmetry. Koski et al. explained the development of new high-pressure phase by surface effect-assisted migration of twin penta-junctions. Similarly, Pd NPs were found to undergo an fcc to face-centered tetragonal phase transition.264 5.1.3. Compressibility at NP Mesostructures. Unlike the atomic lattice in NP cores, the mechanical properties of NP mesostructures have not been studied as intensely so far for mainly three reasons: (1) self-assembled NP samples with sufficient size and NP ordering were not available until about a decade ago; (2) synchrotron-based small-angle X-ray scattering (SAXS) is the most commonly used and efficient tool, which is unfortunately not widely accessible, to characterize mesoscale structure under pressure;44 (3) the characteristics of NP mesostructures highly depend on the particle size and shape, ligand type, and grafting density and even the sample preparation method. These parameters add up countless uncertainties which compromise reproducibility of the NP mesostructures. Therefore, only a very limited amount of works which focus on the mechanical property of NP mesostructures have been reported. Studies of PbS NP mesostructures are discussed to shed some lights on this topic. As one of the earliest works addressing the mechanical aspects of NP mesostructures, Tam et al. probed the mechanical properties of supercrystals self-assembled from 7 nm PbS NPs using traditional indentation measurements (Figure 31).42 The modulus and hardness of the sample were measured to be 1.7 and 70 MPa, respectively, similar to the hard polymer materials. The fracture toughness was only 40 KPa/m1/2, significantly lower than inorganic crystals with a typical value of 1 MPa/m1/2. The brittle nature revealed that the mechanical characteristics are determined mainly by the relatively weak interactions between soft ligands. As an analogy, the strength of a chain is determined by the weakest link. The ligands in NP system are often liquid in their free state and remain soft when grafted on NP surfaces. During the early pressurizing stage, these soft materials rather than the hard inorganic cores of the NPs are responsible for the major part of superlattice (SL) contraction. In a later work, Podsiadlo et al. reported quantitative details of the elasticity of the PbS NP SLs.103 As shown by Figure 31b, in situ high-pressure SAXS measurements found bulk modulus B0 of ca 5 GPa and the pressure derivative B0′ on the order of 10. In contrast, the values for macroscopic PbS crystals are 57 and 4.0 GPa, respectively. This result again confirmed that the softness of the SLs was directly contributed by the soft ligands occupying the interstitial spaces between NPs. Not surprisingly, the SL stiffness was found to be influenced by the length of the ligand chains. On the basis of this concept, a method was invented to compress and study the stiffness and related molecular forces in organic molecule chains by uniaxial compression of these chains between NPs.41 Furthermore, as mentioned in the discussion of phase transition, Bian et al. reported that the compressibility is also a function of the NP packing symmetry.253 Both fcc and bcc SLs were prepared from the same synthetic batch of 7 nm PbS NPs via different sample preparation approaches. As shown by Figure 31c, the bcc SL is shrunk considerably faster than the fcc SL and therefore absorbs more mechanical energy. This is attributed to the lower packing

Figure 30. Pressure-induced amorphorization in NPs. (a) The relationship between the first-order transition pressure and particle size of PbTe NPs. Reprinted with permission from ref 163. Copyright 2011 American Chemical Society. (b) Schematic phase transitions associated with Gibbs free energy observed in PbTe NPs, as well as the HRTEM images of the samples recovered from 15.6 GPa (left inset) and from 19.7 GPa at the initial stage of electron beam irradiation (right inset). The scale bars in both TEM images represent 5 nm. Reprinted with permission from ref 38. Copyright 2013 American Chemical Society. (c) Summary of the different scenarios during the pressurization of NPs. Reprinted with permission from ref 261. Copyright 2015 Royal Society of Chemistry.

transitions compete and the amorphous and high-pressure crystalline phases coexist. Another example of NPs showing exclusive high-pressure phase is noble metals. The typical close packing and ionic bond nature make the atomic lattice very stable in bulk materials. AC


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importantly, it is worth emphasizing that material properties are directly determined by microscopic structures rather than pressure. As mentioned earlier in this section, pressure serves as an efficient and clean tool to tune the structures of NP systems. Therefore simultaneously monitoring structural changes is extremely helpful to the understanding of the structure−property correlations that can further guide the development of nanomaterials with optimized structures for even ambient pressure applications. As the two widely studied optical phenomena in NPs, the variations of surface plasmon resonance and band gap as a function of pressure are discussed as follows. 5.2.1. Surface Plasmon Resonance of NPs under Pressure. Surface plasmon resonance (SPR) is defined as the oscillation of conduction electrons at the surface of a negative permittivity material, typically noble metal, which interferes with a media with positive permittivity such as air, solvent, and ligand.209,210 It enables the metal to convert incident light into electromagnetic field. The SPR is responsible for the unique colors of gold and copper and has been utilized intensively in areas such as sensors even before the bloom in nanomaterial research. When a metal enters the nano regime by reducing its size below the wavelength of incident light, the SPR is localized in individual NPs and coupled with neighboring NPs remotely via an electromagnetic field (Figure 32a). Therefore NPs offer the exclusive capability to manipulate light at nanoscale in a desired manner which can be achieved by optimizing the size, shape, and surrounding environments of NPs.265,266 Among all the metals that carry SPR effects, a significant portion of research efforts have been committed to silver NPs due to their unique capability to accommodate a strong SPR across a broad spectrum from ultraviolet (300 nm) all the way to near-infrared (1200 nm) at a much lower cost than other noble metals like Au, Pd, and Pt. A comprehensive summary of various aspects of SPR in Ag NPs can be found in a well-organized review article by Rycenga et al.207 Here we only focus on their SPR behaviors under pressure. As early as 1997, Collier et al. conducted a SPR study of Ag NPs in 2D Langmuir monolayer under varied surface pressures.162 This work demonstrated that the overall SPR behavior not only depends on particle size and shape but also the distance between NPs at mesoscale. Figure 32b shows that, when the interparticle separation distance decreased by applying surface pressure, the SPR peak red-shifted due to enhanced coupling between neighboring NPs. In addition, the reflectivity also increased at the early stage of compression. Both effects were attributed to the changing local electromagnetic field around each NP according to the classic model proposed by Farbman and Efrima.269 As the interparticle distance further decreased, the tunneling effect began to break down the coupling model and resulted in the decreased reflectance and the suppression of the SPR peak. The tunneling effect on SPR was directly probed by a series of delicate experiments performed by Scholl and co-workers.208 The process of SPR variation in the Ag NP monolayer was reversible when the surface pressure was removed, indicative of no permanent structural transformation. Similar investigations have also been performed by other groups. For example, Tao et al. reported that under increasing surface pressure, the Langmuir monolayer of cuboctahedra Ag NPs displayed reducing interparticle spacing (Figure 32c) and a simultaneous red shift of the SPR peak (Figure 32d) and eventually transformed into a metallic film.268 In addition, new

Figure 31. Compression and mechanical property of PbS NP supercrystals (SC). (a) SEM image of the residual imprint of an indentation made at a maximum load of 600 μN. Reprinted with permission from ref 42. Copyright 2010 American Chemical Society. (b) Calculation of bulk modulus from fitting of Birch−Murnaghan and Vinet’s equations of state to the SAXS data. Reprinted with permission from ref 103. Copyright 2011 American Chemical Society. (c) Effect of applied pressure on unit cell volume for fcc and bcc NC SCs. Unit cell volume was normalized with respect to ambient pressure. Red curves are by fitted Vinet EOS for visual aid. Reprinted with permission from ref 253. Copyright 2012 American Chemical Society.

density of NPs and smaller portion of interstitial spaces filled with ligands in the bcc SL. 5.2. Optical and Electronic Properties of NPs under Pressure

A large portion of existing and potential applications of NPs rely on their optical and electronic properties. These applications include but not limited to solar cells, light emitting devices, sensors, transistors, biological imaging, and therapeutic tools. Research on optical and electronic characteristics of NP under pressure directly benefits the development of applications involving elevated pressure such as pressure sensors. More AD


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Figure 32. Surface plasmon resonance (SPR) in 2D monolayer of Ag NPs. (a) Schematics of localized SPR in NPs and the related electromagnetic field. Reprinted with permission from ref 267. Copyright 2016 Royal Society of Chemistry. (b) Ultraviolet−visible reflectance spectra from a Langmuir monolayer of 40 Å Ag NP passivated with n-propanethiol, collected in situ as the film was compressed. The dashed line corresponds to the reflectance of a clean water surface. Correlations with the nonlinear optical response are indicated. Below 5 Å separation between particles, the reflectance spectra [(4) and (5)] begin to resemble that of a thin metallic film. Reprinted with permission from ref 162. Copyright 1997 American Association for the Advancement of Science. SEM images (c) and scattering spectrum (d) showing tunable plasmon response of a 2D SL of Ag NP as a function of surface pressure. Reprinted with permission from ref 268. Copyright 2007 Nature Publishing Group.

Figure 33. SPR in 3D Ag NP arrays. In situ ultraviolet−visible spectra of thin films of ordered Ag NP arrays. (b) Schematic of NP assembly and sintering under high pressure: (1) below 8 GPa, Ag NP superlattices shrink and spring back reversibly; (2) at the threshold pressure of 8 GPa, Ag NPs contact, driven by high pressure and start to sinter; (3) above 8 GPa, Ag NPs sinter and form nanorods and nanowires depending on applied pressure. (c) TEM images of corresponding Ag nanostructures at different stages of Ag NP aggregation in (b) and optical signature in (a). Scale bars, 100, 10, and 10 nm. Reprinted with permission from ref 33. Copyright 2014 Macmillan Publishers Limited. (d) The UV−vis extinction spectra of a typical composite film after experiencing different pressures. (e) Digital images of films doped with 11 wt % PEG after experiencing different pressures. Reprinted with permission from ref 206. Copyright 2014 American Chemical Society.

peaks arose due to collective charge polarizations of the ordered monolayer. Recently, the mesostructures and the SPR of 3D metal NP assemblies under pressure have been studied as well. Li et al. reported the compression of ordered spherical Ag NP arrays with an fcc mesostructure in a DAC,33 which revealed the two stages of deformation (Figure 33a−c). Under relatively low pressures, the contraction of the NP arrays was reversible and so was the coupling-induced red-shift of the SPR peak. As the

pressure was elevated to above 7 GPa, the NPs began to connect with neighbors via a stress-induced coalescence. Such a permanent morphology change from spheres to nanowires caused a nonreversible SPR red-shift as well as the appearance of new modes. The advances in understanding the pressuredependent SPR of NPs have been transformed into applications such as pressure/stress sensors. As demonstrated by Han and coworkers, a sensor which measures and memorizes the tensile stress was readily fabricated by integrating Au NPs into a AE


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Figure 34. Size-dependent pressure coefficient of band gap of QDs. (a) Peak position of the 1S absorption feature as a function of pressure for 3, 5, and 7 nm PbSe QDs. Reprinted with permission from ref 272. Copyright 2007 American Institute of Physics. (b) Pressure-dependent PL spectra of 3.4 and 5.1 nm CdTe QDs. (c) Pressure-dependence of observed PL peak energies of CdTe QDs of different sizes. Solid lines are quadratic polynomial fits to experimental data. Reprinted with permission from ref 273. Copyright 2013 The Royal Society of Chemistry. (d) Normalized atomic unit cell volume and bulk modulus of PbS QDs measured by high-pressure XRD. (e) Summary of experimental values of diameter (D0), ambient pressure lattice constant (a0), bulk moduli (B0), and pressure variation of energy gap of PbS NQDs of different sizes compared with calculated values based on the basic model (calc 1) and the detailed model (calc 2). Reprinted with permission from ref 102. Copyright 2014 The Royal Society of Chemistry.

polymer matrix.206 Upon calibration, the stress can be calculated from the SPR peak position (Figure 33d). It can be even conveniently read out by eyes when comparing to a standard, similar to the pH test strips (Figure 33e). On the basis of a similar concept and a stamping method, Minati et al. deposited Au NPs into a striped pattern on polydimethylsiloxane substrate which can sense strain up to 18% via the variation of the SPR peak.270 5.2.2. Band Gap of Semiconductor NPs under Pressure. Besides SPR, the band gap (Eg) of semiconductor NPs or QDs represents another significant subject that had attracted significant investigation efforts. The band gap is the most important characteristic of a semiconductor material as it is directly related to the design and performance of application devices. The band gaps of QDs directly translate to their optical properties and therefore are commonly measured by optical absorption or photoluminescence (PL) spectroscopy. The position of absorption peak corresponds to the energy of band gap in a direct gap material. As being pressurized, the atomic structure of a semiconductor undergoes a series of transformations including elastic contraction and phase transitions as discussed in section 3. The structural variation inevitably alters the electronic structures and therefore the band gaps. Depending on the electron density distribution and the emergent interactions upon structure transformation, the pressure response of band gap is material-specific. Some materials such as PbS display a red-shift of Eg while blue-shift occurs in other materials, e.g., CdSe. In a previous paper by Tolbert et al., the optical behavior of CdSe QDs under pressure was reported.52 CdSe is an n-type semiconductor with a bulk band gap of 1.74 eV that has been

utilized in optoelectronic devices such as LED and photoresistors. The three main phenomena were discovered in this work (Figure 2), including: (a) when pressure was lower than 4.8 GPa, the QDs remained in the initial wurtzite phase during compression. The absorption peak, i.e., Eg shifted to higher energy reversibly due to increased overlap of the wave function; (b) at a higher pressure of approximately 7 GPa, the wurtzite-torock-salt phase transition took place and caused the transformation from a direct to an indirect band gap, which eliminated the absorption peak and left only a slope in the spectrum. Such a spectroscopic discontinuity can be used to detect phase transition in similar materials; (c) when the pressure was released, the direct band gap was restored with a slight red-shift that was attributed to the coexistence of wurtzite and zinc blende structures. These findings revealed that QDs inherit the same qualitative behavior from their parent materials while showing quantitative deviations that are determined by factors including particle size, shape, ligand, and pressure profile. These factors are discussed as following. 5.2.2.1. Particle Size Effect on Optical Property. It has long been known that the ambient pressure band gap of QDs, Eg,0 depends on particle radius when it is smaller than the Bohr radius. The smaller the particle, the greater Eg,0, mainly due to quantum confinement effect. In essence, the electrons and holes in a QD are confined spatially and therefore must possess an additional energy similar to the solutions in the famous “particle in a box” problem of quantum mechanics. By solving the wave function in a spherical coordinate, the analytical expression for the band gap of QDs is described by eq 10 as published by Brus back in 1984.271 On the right-hand side of this equation, the second term represents the quantum confinement energy that AF


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Figure 35. Shape and surface chemistry effects of pressure response of QD band gap. (a−c) Transmission electron microscopy (TEM) images and (d− f) fluorescence peak positions of CdSe/CdS core/shell QDs with different shapes: spheres, rod, and tetrapods. Reprinted with permission from ref 275. Copyright 2009 American Chemical Society. (g) Schematic illustrating aggregation of tetrapods in L-lactic fiber, as well as stress transfer to the aggregates during stretching. Reprinted with permission from ref 276. Copyright 2013 American Chemical Society. (h) Pressure dependence of HOMO−LUMO gaps for CdS QDs passivated by phenyl (ph) or hydrogen (H) and bulk crystals (DFT and experiments). Reprinted with permission from ref 277. Copyright 2017 American Chemical Society.

XRD and optical absorption measurements to correlate the changes in band gap to the transformation in atomic lattice structure.102 In addition to a size-tunable pressure coefficient, the compressibility or bulk modulus B0 was also unveiled to be size-dependent (Figure 34d). The measured bulk modulus was then used to calculate ∂Egap/∂P by plugging individual B0 into eq 11 that was derived from eq 10. The results (calc 2) agree with experimental data as compared in Figure 34e. In contrast, simply using the bulk value of B0 returned the numbers in column calc 1, which failed to match the experiments. This work reiterates the importance of incorporate structural information to obtain insights into the properties of NPs

has a major contribution. A less important but non-negligible Coulombic interaction between electron and hole corresponds to the third term. The fourth term counts for the weak dielectric polarization energy which is usually neglected. This equation is the key to understand many size-tunable optical properties of QDs Eg,QD = Eg,bulk +

h2π 2 1.8e 2 e2 − + 2 ε2R R 2μR

i S yz zz kR{

∑ αnjjj ∞

n=1

2n

(10)

In the past decade, researchers found that not only the band gap, but its pressure coefficient ∂Egap/∂P is also size-dependent in a variety of QDs including CdSe, PbSe, PbS, CdTe, and Si.245,272−274 For example, Zhuravlev et al. reported different ∂Egap/∂P = −47, −54, and −56 meV/GPa for 3, 5, and 7 nm PbSe QDs, respectively (Figure 34a), different from bulk value of −59.9 meV/GPa.272 Although the authors noticed the tendency that the smaller particles are the less sensitive than their band gaps to pressure, neither deeper investigation nor discussion were reported. In another publication, Lin and coworkers investigated the pressure response of band gap of CdTe QDs by measuring their PL spectra.273 As shown in Figure 34b, pressure caused a blue-shift, weakening and eventually disappearance of the PL peak. The red-shift is the same as the macroscopic CdTe and similar to CdSe. The peak weakening is a result of the gradual phase transition from zinc blende to rock salt. As presented in Figure 34c, the size-dependence of ∂Egap/∂P was discovered and attributed to the difference in the compressibility of CdTe QDs of different sizes. Unfortunately, because of the lack of structural characterization, no quantitative analysis could be performed and discussed in this work. To overcome such a limitation and obtain quantitative understanding of this subject, Bian et al. combined high pressure

2y ij ∂Eg,QD yz i ∂E y i 2 2 jj zz = jjj g,bulk zzz + 1 jjj h π − 1.8e zzz jj zz zz jj 3B0 jjk μR2 ε2R zz{ k ∂P {T k ∂P {T

(11)

5.2.2.2. Shape Effect. As another major morphological factor of individual NPs, the particle shape also plays a role in the pressure-tuning of optical properties. An excellent example was reported by Choi and co-workers.275 In this work, the PL spectra of CdSe/CdS core/shell NPs of sphere, rod, and tetrapod shapes (Figure 35a−c) were obtained under pressurization. It was found that under deviatoric stress or nonhydrostatic pressure, the PL peak split due to varying local pressure felt by individual QDs. As demonstrated by the red symbols in Figure 35d−f, the further shift of the lower energy peak was found to be distinct with different QD shapes. The highest sensitivity was found in tetrapods and explained by a lever effect of the braches that magnified the nonuniformity in stress. In a follow up work performed by the same group, this concept was transformed into application by integrating them into a polymer matrix of poly LAG


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lactic (Figure 35e). The resultant product successfully passed the tests for use as a fluorescent stress sensor.276 5.2.2.3. Surface Chemistry Effect. Considering the important role played by surface effects in NP systems, it is not surprising to find that the surface chemistry also influences the band gap behavior under pressure. To overcome the difficulty in direct observation of this effect, Corsini et al. performed the timeresolved density functional theory simulations to probe this effect.277 It was discovered that the bulk-like linear shift of band gap was superposed by the surface chemistry-induced distortion, disorder, and orbital hybridization of the frontier orbital. Therefore by substituting the surface coating ligands, ∂Egap/∂P could be tuned (Figure 35h). 5.2.2.4. Interparticle Interactions. Similar to other electronic and optical phenomenon, the interparticle separation influences the explicit band gap by shifting it from the intrinsic value of an isolated QD. Unlike the SPR of metal NPs which heavily relies on the mesostructures due to the long-range electromagnetic field generated by individual NPs, the band gap of QDs is affected by interparticle coupling in a much less extent. A systematic study on this topic was presented by Kim et al.278 In this work, the interparticle separation between CdSe QDs was tuned by both ligand substitution and external pressure while the band gap was characterized by optical spectroscopes. When typical ligands such as trioctylphosphine oxide (TOPO) or tributylphosphine oxide (TBPO) were used, no coupling effect was detected. Only when a very small ligand molecule such as pyridine was used, a slight blue-shift of Eg was observed under pressure and was attributed to either rapid coupling between neighboring QDs which reduces the confinement energy or direct contact between QDs. As pressure further increases, an attachment could be achieved between QDs and permanently alter the band gap. Li and co-workers reported a pressure-based synthesis of uniform CdSe nanowires (NWs) from CdSe QDs.37 As shown in Figure 25, similar to the aforementioned cases of Ag and Au, CdSe QD cores achieved physical contact (dominantly ⟨111⟩/⟨111⟩) and sintered with neighbors in one dimension to form nanowires. Upon release of pressure, a permanent red-shift of Eg was observed (Figure 36a). This is a result of the delocalization of electron and hole in one dimension that weakened the confinement effect. Similarly, a permanent shift of Eg due to attachment in perovskite QDs was also recently reported.28 5.2.2.5. Pressure Profile Effect. In addition to the configuration of QDs themselves, the pressure response of band gap also depends on the profile of exerting pressure, i.e., hydrostatic pressure vs deviatoric stress. Under hydrostatic pressure, the core of a QD shrinks isotropically while deviatoric stress induces directional compression, which breaks the lattice symmetry and thus alters the way of the electronic structure change. Grant et al. studied the band gap of CdSe QDs.279 Different pressure profiles were achieved by using a variety of pressure media including gas (Ar), liquid (4:1 methanol/ ethanol, silicone fluid), and solid (sodium chloride). Also shown are the measured energies in the absence of any pressure medium. As shown in Figure 36a, in all cases an initial blue-shift was observed up to about 3−4 GPa. As pressure entered the nonhydrostatic or deviatoric regime, the Eg−P curve of QDs in a poor pressure medium environment leveled off and even dropped slightly. In contrast, in a good pressure medium environment, the band gap continued to increase. This result could inspire the invention of pressure probing applications with widely tunable responding behavior by embedding QDs in a

Figure 36. Band gap of QDs under pressure. (a) STEM image of CdSe NWs formed by pressure-induced attachment. Inset: Statistical analyses of crystallography of interfaces between QDs. (b) Schematic model showing the coalescence of CdSe QDs in different crystal orientations, as marked by the arrows. Reprinted with permission from ref 37. Copyright 2017 American Chemical Society. (c) Summary plot of PL energies from CdSe QDs in several different pressure media The solid and dotted lines serve to guide the eye. They illustrate the different PL peak energy behavior depending on uniform (solid line) or nonuniform (dotted line) stress environments. Reprinted with permission from ref 279. Copyright 2008 Wiley-VCH Verlag GmbH & Co. KGaA. (d) Shift of band gap energy of CsPbBr3 QDs with increasing pressure. The shadow region indicates the phase transition interval. Reprinted with permission from ref 280. Copyright 2017 American Chemical Society.

properly chosen polymer matrix.281 On the other hand, the kinetics of the pressure also influences the pressure response of QD band gap. The band gap of CdSe QDs was found to be less sensitive to shock impact than continuous pressure.282 5.2.2.6. Other Aspects. Some additional aspects are discussed here. The pressure coefficient ∂Egap/∂P can be altered by the composition of alloy QDs. For example, ∂Egap/∂P was doubled from 34 to 70 meV/GPa by changing the composition of 2.3 nm QD from CdS0.59Se0.41 to CdS0.22Se0.78.283 Similar alloying effect was also reported by Zhao et al.284 As another aspect, the phase transitions usually cause a dramatic change to the Eg−P profile. For materials like CdSe and PbS, the band gap becomes indirect and thus loses the characteristic optical peaks upon phase transition. It makes it difficult to use these materials to monitor pressure above the phase transition point. To overcome such a limitation, alternative semiconductors such as perovskite (Figure 36b) which remain a direct band gap shall be chosen.280,285−287 AH


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Figure 37. Electronic properties of NPs under pressure. (a) Schematics of microcircuit on diamond anvil (left) and the profile of DAC (right). Numbers marked on profile represent (1) Mo, (2) alumina layer, (3) insulating layer, (4) sample, (5) ruby, and (6) diamond. (A,B) Contact ends of the microcircuit. Reprinted with permission from ref 288. Copyright 2017 The Royal Society of Chemistry. (b) Pressure dependence of electrical resistance of CrSi2 QDs. Reprinted with permission from ref 289. Copyright 1999 Elsevier Science BV. (c) Pressure-dependent Hall coefficient of CuInS2 NPs. Reprinted with permission from ref 290. Copyright 2015 American Chemical Society. (d) Pressure−temperature phase diagram of bulk AgI with data points of variable-temperature synchrotron PXRD measurements. Circles: open, heating; closed, cooling. Red, green, and blue colors correspond to the different phases of AgI NPs. Yellow squares denote transition temperatures upon heating, determined using Rietveld refinement. Reprinted with permission from ref 291. Copyright 2017 American Chemical Society.

5.2.3. Electronic Properties of NPs under Pressure. Last but not least, the electronic properties of NPs under pressure are briefly reviewed. The electronic properties are equally important as the optical properties in terms of device applications. For example, a sufficient charge mobility is required in solar cells to extract the exited electron and holes into external circuits. Unfortunately, high-pressure studies are relatively scarce due to the difficulty of measurement. While most high-pressure optical measurements can be performed by transmitting light through the transparent diamond windows, electronic characterizations require to be specially designed with delicate integration of microprobes inside the DAC (Figure 37a). Among the limited available reports, a common phenomenon observed for NPs is the increase of electric conductivity.288 Semiconductor QDs may even transform into a metal phase under pressure. As presented in Figure 37b, Lu et al. showed that the electric resistance of 15 nm CrSi2 decreased by the 2 orders of magnitude at ∼3 GPa.289 This is a combined effect of the semiconductor-to-metal transition and decreased interparticle separation. Similar behavior has also been reported in Ge QDs.258 As additional exemplary discoveries, a pressure-induced n- to p-type carrier inversion was revealed in CuInS2 QDs by Hall effect measurements (Figure 37c).290 In AgI NPs, the superionic conducting α-phase, which is not accessible in bulk material under 147 °C, was preserved at room temperature under a moderate pressure of 0.18 GPa (Figure 37d).291

6. SUMMARY AND OUTLOOK Pressure serves as an important thermodynamic variable that has been used to change chemical and crystal structures, drive phase transformation of NPs and accordingly influence the electronic and optical properties of NPs and their mechanical stability. Pressure induces the atomic phase transformation of individual NPs, which exhibits a series of size-dependent pressure behaviors. This leads to the innovation of chemical and physical properties that have not been possible for the current solutionbased chemical methods. For ordered NP assemblies, the pressure can precisely and reversibly tune interparticle separation distance toward achievement of strong NP exchange coupling. After the pressure reaches to a certain threshold pressure, NP coalescence occurs and imparts new categories of advanced nanomaterials. The emergences of new characterization techniques and new synthetic approaches of nanomaterials bring new opportunities for future understanding of high pressure nanomaterials and creation of ever more intricate and controlled nanostructures. Some important directions are highlighted here. 6.1. Heterogeneous Coalescence and Doping

Herterostructural NPs are an important class of materials that contain two components of different natures and exhibit both collective properties and synergistically enhanced functionalities. Ability to precisely control structure of heterostructural NPs to tune physical properties, such as exciton recombination lifetime and blinking effect, are critical for their applications in light harvesting, optoelectronics, etc. While the solution-based chemical methods have achieved certain success, they have also AI


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platelets.28 They discovered that the structural transformations were correlated at both atomic and mesoscale levels with the band gap evolution through a pressure cycle of 0 ↔ 17.5 GPa. The pressure-synthesized CsPbBr3 nanoplatelets exhibited a 1.6-fold enhanced photoluminescence quantum yield and a longer emission lifetime than the starting CsPbBr3 NPs. It is reasonable to believe that pressure processing will serve as an effective tool to tune the structure and property of these new NP materials for metastable phases296 and enhanced applications.

been heavily involved in a series of side reactions, which require post purification. High-pressure compression represents a physical process and is therefore environmentally benign, using no organic solvents. No further purification is needed to remove byproducts as that in the chemical synthesis process. This will certainly greatly reduce cost and complexity of synthetic procedures. In addition, it overcomes the issue of variability in product property and structure associated with chemical synthesis methods that are very sensitive to subtle differences in kinetic conditions (e.g., temperature, concentration, etc.). For example, Zhu et al. reported pressure-enabled synthesis of CdSe−Au heterostructural dimers and nanorods by using the two-component superlattice of CdSe and Au NPs as the starting materials.29 During compression, a threshold pressure of 15.8 GPa is required to induce a superlattice transformation into lamellar structures through effective breaking of interfacial Au−S bonds. The detachment of surface coating ligands coincides with the phase transition of CdSe NPs from wurtzite to rock salt, which induces the formation of herterostructural dimers and nanorods. We believe that using a combination of semiconductor QDs and metallic NPs with controlled stoichiometry allows for the synthesis of new herterostructures. Inorganic NPs undergo a series of chemical transformations including ionic exchange, addition, and branching reactions.292 Similarly, the pressure induced doping appears as another direction which is highly worth pursuing. We envision that by replacing one NP component with metallic ions, the atomic doping of NPs can be accomplished through a high pressure compression. NPs exhibit specific surface defects that offer a series of active sites or instable surface planes for ease of further reaction and additional functionalization. For example, the formation of 1−2D PbSe nanostructures was reported by using spherical PbSe nanocrystals.222,293,294 The formation process relies on the reactive surfaces exposed on PbSe NPs that are prone to the detachment and oxidation of surface ligands. Li et al. demonstrated that one can take advantage of NP surface defects to tune nanostructures through a pressure induced coalescence.33 We expect that much complex architecture can be formed by pressure tuning surface defects.

6.3. Simulation and Modeling

Although great success has been achieved through experiments, there are still fundamental challenges that need theoretical simulation and modeling for understanding the pressure induced NP assembly process. For example, currently, it is not clear how organic ligands are detached from NP surfaces under pressure and how NPs are fused together at the contact point. Because of the small amount of samples, it is a challenge to use molecular spectroscopy to get enough signals to interpret the coalescence or fusion mechanism. In addition, how the organic ligands and NP compositions change the threshold pressure is another area in which theoretical research can help address. Overall, theoretical studies are needed to investigate novel NP interaction regimes that affect NP arrangement, size, and shape and defect motions under pressure. 6.4. New in Situ Characterization Methods

In situ TEM has been recently developed to observe the formation and transformation of NPs at atomic scale and NP self-assembly at mesoscale in real time.220,297−299 Future development of in situ techniques by design of new TEM sample holder to add mechanical forces to the NP arrays may provide new capability to make real-time visualization of pressure induced aggregation, attachment, and coalescence. This will provide invaluable insights for theoretical studies and synthesis of more complex materials.300 6.5. Pressure Induced NP Coupling

The ability to achieve precise control for macroscopic NP assemblies of structural characteristics such as interparticle separation distance is required to enhance the efficiency of electronic or plasmonic coupling for applications in sensing, solid-state lighting, solar energy conversion, and biosensing. Both the top-down processes such as e-beam and ion-beam lithography and bottom-up methods such as self-assembly of colloidal NPs have been vigorously pursued. While these fabrication methods have provided certain successes, they have essential fundamental limitations. The top-down processes are limited by their spatial resolutions of ∼10 nm by the e-beam or ion-beam size and by their inability to fabricate complicated and tunable 3D nanostructures for new and strong SPR coupling. Among the bottom-up synthesis methods, NP self-assembly has been extensively explored to tune interparticle separation distance by controlling the length of organic ligands. Despite these extensive efforts having been made, the NP structures formed using these methods have been limited only to the interparticle distance of ∼2 nm by the intrinsic organic monolayer on the NP surfaces. Recently, the pressure induced assembly method has demonstrated effective tuning of interparticle symmetry and separation distance, ideal for controlled investigation of distance-dependent electronic or plasmonic couplings to achieve collective chemical and physical properties.30,31 Importantly, the pressure induced NP assembly allows precise and systematic tuning of interparticle separation

6.2. New NP Materials

Nanoscience has opened up the ability to synthesize the whole new classes of materials such as perovskite NPs, carbon quantum dots, synthetic graphene, and so on. A variety of well-controlled nanoscale morphologies, such as spherical/faceted NP, nanosheet, nanotube, and nanowire, have been achieved. As a result, the newly emergent research directions are expected to explore the influences of critical morphological characters of nanostructures, such as the geometric shape, the size of nanocrystals, and the facet at the interfaces between nanomaterials and surrounding pressure-transmitting medium, to the phase transition behavior as well as how the high pressure behavior influences materials properties. Taking perovskite NPs as an example, Fang et al. recently studied the high-pressure phase behavior of formamidinium-based perovskite FAPbI3 NPs. They discovered an irreversible phase of δ-FAPbI3 under high pressure that promotes an efficient harvest of photons.295 The photoluminescence behaviors of δ-FAPbI3 were controlled by a radiative recombination at the defect levels rather than a bandedge emission during the whole cycling of pressurizaiton. These studies provide invaluable insights for design new perovskite materials for photovoltaic applications. Chen et al. reported the pressure-enabled synthesis of new CsPbBr3 perovskite nanoAJ


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that is under control of applied pressure and can be manipulated reversibly at the angstrom level until the particles make contact and coalescence occurs. The method allows detection of new surface plasmonic behavior below the interparticle spacing limitation of 2 nm. The pressure induced NP assembly opens exciting and new avenues for investigation of NP coupling toward development of new nanodevices.

focused on designer solids with controllable materials processing towards achievement of programmable design and controllable fabrication of advanced materials with desired and enhanced property and increased functionality. Hongyou Fan received a B.Sc. degree in chemistry major from Jilin University, an M.Sc. degree from the Chinese Academy of Sciences in polymer chemistry and physics, and a Ph.D. degree in Chemical Engineering from University of New Mexico in 2000. He was a postdoctoral fellow at Sandia National Laboratories, Albuquerque, New Mexico, before working there full time. He currently is a Distinguished Member of the Technical Staff at Sandia National Laboratories and is a National Laboratory Professor in the Department of Chemical and Biological Engineering at the University of New Mexico. His current research focuses on fundamental understanding of nanoparticle assembly for active nanomaterials and for nanoelectronic and nanophotonic applications. Fan is a Fellow of the American Physical Society and the Materials Research Society (MRS). He received the MRS Mid-Career Researcher Award in 2019, the MRS Fred Kavli Distinguished Lectureship Award in Nanoscience in 2015, four R&D 100 Awards for the development of technically significant products in 2007, 2010, 2016, and 2018, two Federal Laboratory Consortium for Technology Transfer−Outstanding Technology Development Awards in 2008 and 2013, The University of New Mexico Outstanding Faculty Mentor Award in 2005, and the Asian American Engineer of the Year Award in 2012.

AUTHOR INFORMATION Corresponding Authors

*H.F.: phone, (505)272-7128; E-mail, [email protected]. *F.B.: [email protected]. *Z.W.: [email protected]. ORCID

Zhongwu Wang: 0000-0001-9742-5213 Hongyou Fan: 0000-0001-6174-4263 Notes

The authors declare no competing financial interest. Biographies Feng Bai received a B.Sc. degree in 2000 with chemistry major and a Ph.D. degree in 2005 with polymer chemistry and physics majors from Nankai University. He worked as a postdoctoral fellow in the Fan group at Sandia National Laboratories from 2005 to 2010. Currently, he is a professor in Key Laboratory for Special Functional Materials of the Ministry of Education, Henan University, China. His current research focuses on the self-assembly and applications of nanomaterials, such as inorganic nanocrystals, block polymer, and supermolecules.

ACKNOWLEDGMENTS H.F. acknowledges the support from Center for Integrated Nanotechnology (CINT), a U.S. Department of Energy, Office of Basic Energy Sciences user facility. F.B. acknowledges the support from the National Natural Science Foundation of China (21771055, U1604139, 21422102, 21403054), Plan for Scientific Innovation Talent of Henan Province (no. 174200510019), and Program for Changjiang Scholars and Innovative Research Team in University (no. PCS IRT_15R18). We acknowledge Dr. Wenbin Li for discussions and communications on pressure annealing and Dr. Jack Wise on dynamic compression. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. CHESS is supported by the NSF award DMR1332208.

Kaifu Bian received his B.E. degree in chemical engineering from Tsinghua University in 2004 and his Ph.D. degree in chemical engineering and material science from Cornell University in 2015. He was a postdoctoral fellow in the Fan group at Sandia National Laboratories from 2015 to 2018. Currently, he works at Intel. His research interests include self-assembly of nanocrystals, synchrotronbased X-ray scattering, and structure−property relationship of nanomaterials under high pressure. Xin Huang received his B.Sc. degree in physics from University of Science and Technology of China in 2010 and his Ph.D. degree in applied physics from Cornell University in 2017. Currently, he is a postdoctoral associate in Cornell Laboratory for accelerator-based sciences and education, under the supervision of Dr. Zhongwu Wang. He is working on synchrotron based X-ray studies on self-assembly of nanoparticles. Zhongwu Wang is currently a staff scientist at the Cornell High Energy Synchrotron Source (CHESS) of the Cornell University. He obtained his Bachelor and Master degrees in earth science at the Central South University and Chinese Academy of Science, respectively. In 1998, he started his Ph.D. program on high pressure mineral/materials physics at Uppsala University in Sweden and finished his Ph.D. degree in 2001 at Florida International University. After a three-year’s term of training as a director-funded postdoctoral fellow at Los Alamos National Laboratory, he joined the CHESS as a staff scientist in 2006 and started his new and independent adventure of synchrotron-based technical development and scientific research. He led a team effort and built a high pressure beamline, capable of simultaneous collection of small-angle and wide-angle X-ray scatterings as well as multiple spectroscopes from the samples under in situ/real time environments. He developed a synchrotron-based “supercrystallography” approach, enabling precisely structural reconstruction of nanocrystal assembly from atomic through nano to mesoscale. His current research interest is

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