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Directed Self-Assembly of Ultrasmall Metal Nanoclusters Zhennan Wu,† Qiaofeng Yao,† Shuangquan Zang,‡ and Jianping Xie*,† †
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Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117585 ‡ College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, China ABSTRACT: Directed self-assembly (DSA) of nanoparticles plays a key role in customizing advanced functional materials via collective and synergetic properties between neighbored building blocks. In comparison to the blossom of DSA in large (>3 nm) nanoparticle system witnessed in the past decades, the development of DSA in the sub-two-nanometer regime (e.g., underlying chemistry, fundamental properties of assemblies and potential applications) markedly lags behind. Thanks to continuous progress in synthetic chemistry and total structure determination of atomically precise metal nanoclusters (NCs, 3 nm),15−20 DSA is rarely explored in the sub-two-nanometer (nm) size regime that links discrete molecules and larger NPs (>3 nm).21−25 On one hand, NPs exhibit highly size-dependent electronic, optical, magnetic, and catalytic properties.26−28 Establishing DSA chemistry in the largely unexplored sub-2-nm regime holds a proven opportunity to enrich the functional diversity of nanomaterials. On the other hand, for each type of nanoscale driving force, its magnitude and length scale are critically dependent on the size and interdistance of nano-objects.29 Therefore, it is highly desired to map out individual nanoscale forces in a quantitative or semiquantitative manner in DSA setups accommodated in this transition size regime.30 Metal nanoclusters (NCs), the representative ultrasmall NPs, typically possess sub-2-nm metal core consisting of a few to hundreds of metal atoms.21 Metal NCs provide an ideal platform for exploring DSA chemistry in the sub-2-nm size regime. First, continuous progress in synthetic chemistry has produced a variety of metal NCs with customizable size, core, and surface composition, sustaining a diverse library of building blocks for subsequent DSA development.31 Second, ultrasmall size renders metal NCs with intriguing molecularlike properties (e.g., strong luminescence and molecular chirality). Upon regular packing by DSA, enhanced or even novel materials performance could be aroused from the aforementioned molecular-like properties.32 In addition, DSA exploration could greatly push forward crystallization science for metal NCs, which is crucial for perceiving the precise structure of metal NCs at angstrom or sub-angstrom levels.28 The accurate knowledge on cluster structure, in return, is indispensable for establishing a reliable structure−property relationship for metal NCs, thus adding to their acceptance in diverse sectors of practical applications. Third, the rich surface chemistry (including diverse surface functionalities and their programmable arrangement patterns) of metal NCs constitutes a versatile toolbox for DSA exploration, holding appealing potential for producing hierarchical superstructures with complex and even new packing fashion not yet observed in bulk metal materials.33−36 Last but not least, the size-sensitive properties of metal NCs, in tandem with their well-developed characterization techniques, facilitate facile monitoring of DSA process at an unprecedented atomic and molecular precision. As to ultrasmall metal NCs, the composition, size and atomic packing pattern, which are decisive to their physicochemical properties and self-assembly behavior, exhibit considerable susceptivity toward the surroundings. Moreover, their large specific surface area could preferentially induce random aggregation even fusion during the self-assembly process.37 Thus, extending the use of common directing agents, external fields or templates into DSA could be a nontrivial task, and relevant self-assembly chemistry should be carefully re-examined at sub-2-nm size regime. Therefore, manipulating driving forces and understanding their quantitative details are of core importance in engineering DSA and potential applications of ultrasmall metal NCs.37−40
In this Perspective, we systematically summarized recent progress in DSA of ultrasmall metal NCs, with an unvaried emphasis on the molecular/nanoscale driving forces governing these DSA processes. We aim to provide a concise summary and handy toolbox of nanoscale interactions that act as additional constraints and directing agents for DSA of functional building blocks in the sub-2-nm regime, of which ultrasmall metal NCs are a model, with the goal of moving toward the construction of anisotropic self-assemblies. Specifically, in the vein of various directing agents, the fundamentals of dipolar, van der Waals, electrostatic interactions, hydrogen bonds, template effects, and amphiphilicity in the DSA of ultrasmall metal NCs are discussed in a concise fashion. Subsequently, the DSA enhanced or even evoked properties and thus applications of metal NCs are explored. This paper ends with our perspectives on the development of DSA chemistry in the near future. We hope the fundamentals and methodologies highlighted here (with a number of demonstrative studies) may increase the acceptance of ultrasmall metal NCs in various sectors of practical applications. Of particular note, the past two decades have also witnessed great advances of crystallography of metal NCs, which have produced a diversity of liquid crystalline41,42 substances and the supracrystals with various morphology, symmetry, and crystallinity.43−51 Because the crystallization processes are mostly driven by entropy factors, only selected crystallization examples that involve directional molecular/ nanoscale forces are discussed in this perspective. We apologize for not being able to include all excellent contributions in self-assembly/crystallization of metal NCs because of the limited length.
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APPROACHES TO DSA OF METAL NCS Dipolar Interactions. Reminiscent of the colloidal synthesis (wet chemistry) of NPs by the agglomeration mechanism, where small crystal seeds are agglomerated via an oriented attachment-mediated self-assembly pathway. This process in general involves spontaneous self-organization of adjacent building blocks by sharing a common crystallographic orientation in near-perfect registry across their interface. The oriented attachment mechanism is of great importance in generating anisotropy in assembled nanomaterials with various morphologies (e.g., rods, wires, sheets, and ribbons).52−56 The oriented alignment in the aforementioned directed growth/ self-assembly scenarios should be mostly attributed to dipolar interactions between close-neighbored building blocks (e.g., small nanocrystals), whose small size concomitant noncentrosymmetric distribution of polar facets or asymmetric lattice truncations are main contributors to such dipolar interactions.57 In the case of metal NCs, their ultrasmall size featuring ill-defined and ambiguous crystal facets, dynamics of surface motifs, as well as the susceptivity of entire cluster structure to the surroundings,23 form the structure basis for development of considerable dipolar interactions between neighbored NCs. The specific triggers for intercluster dipolar interactions include heteroatom/impurity doping,58 distortions of core structure,59 exchange of surface motifs,60 and intercluster reactions.61 For example, Zhang et al. employed the Au15DT15 (DT = 1dodecanethiol) as a model NC and successfully demonstrated its tendency toward one-dimensional (1D) attachment prompted by dipole−dipole attraction (Figure 1).37 With consideration of the specific arrangement of intrinsic Au atoms 238
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between metal cores of neighbored NCs becomes negligible in comparison to that of their surface ligands. This is in sharp contrast to the scenario in conventional large-sized metal NPs, where the size of surface ligands is much smaller than that of metal core. Therefore, establishing effective van der Waals interactions of ligand−ligand or ligand−solvent provides an alternative means for DSA of metal NCs. For example, Zhang et al. exemplified usefulness of van der Waals forces in DSA of metal NCs by stimulating the hydrophobic−hydrophobic interactions between long-carbon chains at high temperature (140 °C).39 The model NCs used in their DSA exploration are Au NCs capped by hydrophobic alkylthiols. To enhance alkyl chain mobility and, thereby, facilitate Au NCs assembly by interligand interactions, the authors allowed the DSA occurred at an elevated temperature (i.e., 140 °C). Of particular note, the authors managed to quantify the energy contribution from DT ligands via the equation ÅÄ a1a 2 a1a 2 A ÅÅÅÅ UvdW(r ) = ÅÅ 2 + 2 3 ÅÅÅÇ r − (a1 + a 2)2 r − (a1 − a 2)2 É 2 2 Ñ 1 ijj r − (a1 + a 2) yzzÑÑÑÑ zzÑ + lnjj 2 2 jk r − (a1 − a 2)2 z{ÑÑÑÑÖ where ai represents the diameter of atom/molecule species i and r is the distance between two atomic or molecular centers, respectively. A is the Hamaker coefficient and can be estimated according to the Hamaker integral approximation of A = Cvdwπ2/v1v2, where Cvdw is a constant characterizing the interacting species and the surrounding medium (in the simplest approximation model of two spheres, it is related to the size and distance of two bodies), and vi is the molar volume of species i. For −CH2− group, A ≈ 5 × 10−20 J (Cvdw ≈ 50 × 10−79 J m6, v ≈ 30 Å3 mol−1). As a result, the van der Waals attraction between ligands around the neighboring Au NCs is calculated to be 7 kBT, where kB is the Boltzmann constant and T is the absolute temperature, in good accordance with the observed temperature dependence of DSA process. It should be mentioned that the van der Waals interaction can also align or orient involved bodies in their aggregate, although this orienting effect is typically weak in large NP systems.66 Thanks to the dominant role of ligands in dictating
Figure 1. TEM images of the Au15 NCs before (a) and after (b) dipolar interactions governed DSA. (c) Brownian dynamics (BD) simulation result of the dipole-induced linear arrangement and the subsequent aggregation of Au15 NCs, adopted from a model composed of 18 Au NCs, at the BD time unit t = 1 000 000Δt. Reproduced with permission from ref 37. Copyright 2015 American Chemical Society.
and the ligands distribution, theoretical calculation indicates that the permanent dipole moment (μ) of Au15 DT15 is up to 13.27 D. The typical energy of dipolar attractions between two neighbored Au15DT15 NCs is calculated according to the classical formula for aligned dipoles, E = −μ2/2πε0r(r2 − dNC2), where the absolute dielectric constant ε0= 8.85 × 10−12 C2 J−1 m−1, μ is the permanent dipole moment, and r and dNC are the center-to-center interdipolar separation and the diameter of NCs, respectively. The as-calculated energy of NC dipolar attractions could be up to 3.0 kJ/mol, which is substantially higher than the energy of regular molecular dipole−dipole attractions (∼1.5 kJ/mol). Such combined experimental and theoretical efforts unambiguously manifest that the dipolar attraction is a strong and effective force driving 1 D alignment of NCs in DSA processes. In addition to inducing dipolar attractions between building blocks, 1D attachment fashion could also be achieved via dipolar interactions of shape-directing agents in DSA of NCs or NPs.52 The past decade has witnessed remarkable diversity expansion of metal NCs or NPs assembled by dipolar interactions.53−57 However, there still exist much room for improving the structure and symmetry controllability in the dipolar-interaction-induced DSA. For this purpose, a molecular-level understanding on the compositional and structural evolution of NCs during their DSA process is highly desired. Recently, to decode compositional information on NCs in size growth and functionalization reactions, Xie et al. developed delicate in situ mass spectrometry methodology to monitor the size and composition evolution of NCs in the aforementioned cluster reactions.62−64 Similar mass spectrometry-based analysis kit could also be used to unravel the DSA fundamentals of NCs. It should also be pointed out that revealing more precise (at angstrom or sub-angstrom level) structural information on NCs during the DSA process should intensely rely on X-ray single crystallography analyses, which remain as one of the biggest challenges in current development of cluster chemistry. van der Waals Interactions. The van der Waals forces originate from the electromagnetic fluctuations because of the incessant movements of positive and negative charges within all types of atoms, molecules, particles, and bulk materials.65 They are, therefore, present between any two material bodies, usually acting in an attractive fashion to bring the bodies together. One marked structural feature of metal NCs is the comparable size of their protecting ligands and metal core. Given the fact that van der Waals attractions decay exponentially with increasing distance between the two involved bodies,29 the magnitude of van der Waals attraction
Thanks to the dominant role of ligands in dictating the recognition and selfassembly chemistry of metal NCs, the packing of NCs in their assemblies could become programmable via patterning the arrangement of their surface ligands and, thus, their van der Waals forces (particularly in an anisotropic fashion). the recognition and self-assembly chemistry of metal NCs, the packing of NCs in their assemblies could become programmable via patterning the arrangement of their surface ligands and, thus, their van der Waals forces (particularly in an anisotropic fashion). 239
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density-dictated DSA strategy is in accordance with the assembly behavior observed in larger sized NPs and is also deployed in the DSA of metal NCs into well-defined 2D sheets.37 Electrostatic and Hydrogen-Bonding Interactions. In addition to the intrinsic van der Waals interaction, the landscape of surface ligands is capable of being encoded information for introducing and modifying the strength of diverse supramolecular interactions as the directing agents. For example, aromatic ligands can endow efficient π···π stacking, π···ions, and π···C−H forces in most DSA systems in organic media.67 Considering the supramolecular interactions in aqueous or semiaqueous media, the electrostatic and hydrogen-bonding interactions between surface ligands should never be neglected.15 Electrostatic interactions could offer strong attraction or repulsion between interacting bodies. They are also easily tunable through the choice of solvent (e.g., dielectric constant), pH value, electrolyte concentration, as well as the chemical nature (e.g., size and valence) of the surrounding counterions.68 Hydrogen bonds are largely electrostatic in nature so that inevitably accompany with electrostatic effects in self-assembly. It can be multistimuli responsive (pH, temperature, and solvent), allowing reversible and responsive assemblies.69 Recent advances in self-assembly chemistry manifest that a deliberate balance of hydrogen-bond and electrostatic interactions is capable of directing self-assembly of metal NCs into superstructures with diverse morphology and symmetry. As a demonstrative example, Bigioni and Landman et al. reported a rhombus-shaped supracrystals of Ag44(pMBA)304− (p-MBA = para-mercaptobenzoic acid) NCs, where all p-MBA ligands were fully protonated. The authors demonstrated that the as-formed supracrystals are reinforced by an extensive network of hydrogen-bonds made possible by the fully protonated p-MBA ligands.70 Interestingly, their lateral orientation is featured by two hydrogen bonding dimerizations, while the interlayer terminations are triple bundles of hydrogen bonding. Such anisotropic distribution of hydrogen bonding contributes to the feasible chiral rotation of NCs, giving rise to a pressure softening behavior of superlattices under hydrostatic compression (Figure 3c). In a recent study, Xie et al. also suggested that by comprehensive deprotonation of p-MBA ligands at a highly alkaline condition (thus eliminating any directional hydrogen bonds), the supracrystals of Ag44(p-MBA)304− could be shaped into an octahedron. More interestingly, by altering the crystallization kinetics via electrostatic interactions (made possible by tuning the solvent polarity and ionic strength), the authors are able to customize the shape of supracrystals as octahedron or concaveoctahedron (Figure 3b).71 Another good example comes from balanced hydrogen-bond and electrostatic interaction promoted DSA of Au102(p-MBA)44, which is a recent discovery by Ikkala and coworkers.72 They delicately manipulated the population of protonated and deprotonated p-MBA under basic conditions, thus maintaining a good water solubility and a balance between the hydrogen-bonding and ionic interactions among Au102(p-MBA)44 NCs. As a result of such fine balance, well-defined two-dimensional (2D) superlattices could be formed by Au102(p-MBA)44 NCs, and they could be further bent into spherical capsids at suitable solvent conditions (Figure 3d−f). Of particular note, the electrostatic forces are predominately a surface effect that is activated through adsorption or
In a recent contribution, Jin et al. demonstrated that an inhomogeneous but symmetric distribution of surface ligands on cluster surface can serve as directional “sticky bonds” for DSA of Au246(p-MBT)80 NCs (p-MBT = p-methylbenzenethiolate) into lattices with orientational, rotational, and translational orderliness.35 To maximize the van der Waals interactions between surface ligands and, thus, minimize the total energy of assembled NCs, the p-MBT ligands are selforganized into two highly ordered patterns on the surface of Au246(p-MBT)80 NCs. That is, 25 of p-MBTs are arranged into four pentagonal circles at the pole site (α-rotation; two of such α-rotation patches cover on the top and bottom, respectively) and 6 of p-MBT ligands align into three alternating parallel pairs at the waist site (β-parallel; five of these β-parallel patches align along the waist), respectively. Such highly ordered and directional patterns of ligands are believed to be the root cause of the anisotropic packing of metal NCs in their supracrystals. As shown in Figure 2d and 2e, those vertex NCs can interact
Figure 2. (a) Overall structure of ligands on the surface of Au246(pMBT)80 NCs. (b) Rotational packing of ligands at the pole site of Au246(p-MBT)80. (c) Parallel packing of ligands at the waist of Au246(p-MBT)80. (d, e) Coordination geometry of Au246(p-MBT)80 in the ensembles: side view (d) and top view (e). Reproduced with permission from ref 35. Copyright 2016 American Association for the Advancement of Science.
with the central one via the α-rotation ligands at the pole sites, whereas few interactions could be witnessed among the βparallel ligands at the waist sites. The preferred alignment by αrotation ligands is reasonably correlated to the higher packing density of ligands in α-rotation (∼14 ligands nm−2) compared to that in β-parallel (∼6 ligands nm−2). A similar surface240
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reasonable model. For example, it has been documented that negatively charged Au NCs possess an ions-like step-like assembly behavior, mimicking the formation of ionic crystalline compounds at their solubility product (Ksp).73 Namely, the
Namely, the negatively charged Au NCs can be considered to be nanoions and behave similarly to multivalent ions. Their self-assembly would be triggered when a threshold concentration of counterions (defined by the Ksp) is reached. negatively charged Au NCs can be considered to be nanoions and behave similarly to multivalent ions. Their self-assembly would be triggered when a threshold concentration of counterions (defined by the Ksp) is reached. Template Effects. Templates could offer one direct constraint for the DSA of metal NCs.15 In general, there are soft and hard templates. Diversity in the templates could easily result in diversity in the self-assembly approaches and products, especially for the generation of anisotropic architectures. However, there are still distinct obstacles in using ultrasmall metal NCs as building blocks because they are extremely sensitive to the surroundings and easy to aggregate in response to even tiny disturbances.39 Despite the abovementioned technical challenges, fascinating successes have been made in DSA of metal NCs by using either soft or hard templates. These successes should be largely attributed to a deliberate balance between cluster−template interaction and cluster stability. For example, He et al. designed an oil/water interface as a soft template to direct self-assembly of Au NCs into large-area 2D nanosheets.74 This process involves an in situ formation and simultaneous self-assembly of DT-protected Au NCs (DT = 1-dodecanethiol) at the liquid/liquid interface. Au(I)−DT complexes were predissolved in an organic phase of octadecene, followed by the addition of an aqueous solution of reducing agent, formaldehyde. As an attempt to minimize the total surface energy, the Au(I)−DT complexes tend to behave like natural surfactants in terms of self-assembly at the liquid−liquid interface. Namely, the polar Au(I) terminals sit toward the aqueous phase, while the nonpolar hydrophobic tails of DT align in the organic phase. As a result, the heterogeneous reaction (reduction of Au ions to Au atoms) ensures the formation of Au NCs dominantly at the oil/water interface, followed by in situ DSA into well-defined nanosheets at the oil/water interface. Importantly, micro-soft-template of liquid−liquid interface can also be generated in the system of two miscible solvents under proper conditions. Figure 4 depicts 2D nanosheets by DSA of DT-capped Au NCs in two miscible solvents with high boiling points but slightly different polarities (e.g., liquid paraffin (LP) and benzyl ether (BE)).39 Upon reaching a threshold temperature, microphase separation occurs in these two miscible solvents, which creates a lamellar interface. Such a lamellar interface is capable of acting as a soft template to direct the self-assembly of Au NCs into well-defined twodimensional nanosheets. The formation of the lamellar nanosheets is favored at high LP/BE ratio and high concentration of Au NCs (Au NCs are more compatible
Figure 3. (a) Chemical and single crystal structure of Ag44(pMBA)304− NCs. (b) Formation diagram of Ag44(p-MBA)304− supracrystals at varied ionic strength (indexed by [Cs+]) and solvent polarity (indexed by the volume ratio of dimethyl sulfoxide (DMSO) in its mixture with water). The scale bar is 10 μm. (c) Rotational structural transition of two layers in Ag44(p-MBA)304− single crystals, showing ligand fixture and rotation of NCs. Ag44(pMBA)304− in two different planes rotate in opposite directions under hydrostatic compression (V = instantaneous volume under compression, V0 = initial volume). (d) Chemical and single-crystal structure of Au102(p-MBA)44NCs. (e) TEM image of assembled 2D architectures. The scale bar is 50 nm. (f) A close-look of 2D monolayer presenting hexagonal closed packed Au 102 (pMBA)44NCs. Panels a and c are reproduced with permission from ref 70. Copyright 2014 Springer Nature. Panel b is reproduced with permission from ref 71. Copyright 2015 WileyVCH. Panels d−f are reproduced with permission from ref 72. Copyright 2016 Wiley-VCH.
desorption of ionic species in large NP systems. As to metal core or “whole body” carried the charge, if any, it is typically weak and subject to screening by electrolytes in solution in large NPs. However, ultrasmall metal NCs feature a
However, ultrasmall metal NCs feature a comparable scale of ligands to the “whole body” and the unique structure (intrinsic charge could be developed within the metal core as an attempt for electronic shell closure). comparable scale of ligands to the “whole body” and the unique structure (intrinsic charge could be developed within the metal core as an attempt for electronic shell closure). Thus, treating the charged NCs as the integral nanoions is a 241
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Figure 4. Schematic illustration of the DSA of Au15 NCs into single-cluster-thick nanosheets at the LP−BE interface (LP = liquid paraffin and BE = benzyl ether). Reproduced with permission from ref 39. Copyright 2013 Wiley-VCH. Figure 5. Schematic illustration of the DSA process of Au25(MHA)18@xCTA (x = 6−9) NCs through amphiphilicity. Reproduced with permission from ref 77. Copyright 2015 American Chemical Society.
with BE solvent); otherwise, the self-assembled architectures undergo a structural transformation from nanosheets to bipyramids, nanospheres, and irregular aggregates (dependent on combined effects of LP/BE ratio and Au NC concentration). The surface energy (E = πr2γ(1 + cos θ)2) of Au NCs at the LP-BE interface can be estimated according to the contact angle of Au NCs with liquid paraffin (Θ), radius of Au clusters (r), and the interfacial tension (γ). Of note, the contact angle Θ is universally larger than 90°, which means Au NCs energetically favor benzyl ether, inhibiting their migration into liquid paraffin and thus aggregating into nanosheets at the LP/BE interface. Different from soft templates, hard templates often provide less control over the periodicity of nano-objects. The primary selection rules for hard templates largely depend on their morphology and functionality. For instance, the reduced graphene oxides (rGO) with strong absorption capacity toward metal ions and intrinsic efficient charge transfer character have been employed by Tang et al. as the substrates to scaffold the DSA of Au NCs, which are also explored for further electrocatalytic applications.75 In another study, Zeng et al. presented in situ formation and DSA of ultrasmall metal NCs (Au, Pd, and Pt) based on preformed molecular surfactant micelles. In particular, the giant micelles of CTA+-metal halide anions complexes (CTA+ = cetyltrimethylammonium) are prepared in water with various well-defined geometric shapes. Subsequent in situ reductions of metal ions gives rise to 3D assemblies of metal NCs, which inherit the geometric feature of template micelles.76 Amphiphilicity. The “molecular-like” structural and property features, which have largely prompted the total synthesis and application exploration of discrete metal NCs, could also render NCs with deliberately tunable “molecularlike” behavior in terms of DSA. For example, the amphiphilicity could be introduced to metal NCs by a molecularly precise manner, fueling “surfactant-mimicking” self-assembly of metal NCs. As presented in the work by Xie et al.,77 hydrophilic Au25(MHA)18 (MHA = 6-mercaptohexanoic acid) can be partially (near half of monolayer) patched with hydrophobic cations (CTA+ = cetyltrimethylammonium ion) via a phase-transfer driven ion-pairing reaction (Figure 5). Because of the coexistence of flexible hydrophilic MHA and hydrophobic MHA···CTA ligands in comparable amounts on the NC surface, the Au25(MHA)18@CTAx NCs (x = 6−9) exhibit good amphiphilicity, which drives their packing into regularly stacked bilayers, mimicking formation of lyotropic liquid crystalline phases by molecular surfactants.
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PROPERTY ENHANCEMENT BY DSA OF METAL NCS Apart from establishing the DSA chemistry in the sub-2-nm regime, property (and its diversity) enhancement is another center of pursuit in the DSA research of metal NCs. This should be on one hand attributed to unique molecular-like properties deliverable by discrete metal NCs and, on the other hand, ascribed to the performance synergism among regularly assembled NCs. It should be noted that the enhanced or, in some cases, newly arisen properties in NC assemblies are largely dependent on their packing orderliness and symmetry. In the following context, two most intriguingly molecular properties (i.e., luminescence and catalytic performance) are employed to exemplify the usefulness of DSA in property tuning or enhancement of metal NCs. Luminescence. Luminescent metal NCs are promising optical probes for a variety of applications (e.g., bioimaging and biosensing) because of their ultrasmall size, low toxicity, renal clearance, and good photostability.78−80 However, their low photoluminescence quantum yields (QYs) and unclear origin of emission greatly limit their acceptance in various sectors of practical applications. It has now become known that one possible emission mechanism of metal NCs lies on “aggregation-induced emission” (AIE), which is originally discovered by Tang’s group in 2001.81 For example, Xie et al. found that controlled aggregation of Au(I)−SR complexes on the in situ formed small Au(0) core could produce bright emission, leading to the formation of ultrabright Au(0)@ Au(I)−SR core−shell NCs.71,72 Thereafter, AIE mechanism has been widely accepted by the cluster research community for evoking and enhancing the emission of many other NCs, including Au, Ag, Cu, and AuAg alloy NCs.81−83 The aggregation of weakly/nonemissive NCs could be facilely induced by non-solvents, cations and many other slight disturbances. However, both these two approaches are challenging inhomogeneity control over the NC aggregates/ assemblies, which leads to the coexistence of inhomogeneous and multiple emitters in the system (i.e., poor color purity).32 In this vein, the DSA is promising for arranging component metal NCs into ordered, customizable and versatile patterns for enhancing NCs’ luminescence performance in terms of luminescence intensity and color purity. It could also provide a 242
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assemblies featuring ultrathin nanosheets emit bright yellow emission with QYs up to 15.4% (Figure 6c). It is suggested that the Cu(I) defects generated in the assembled nanosheets could facilitate the relaxation of intrinsic LMMCT-determined triplet state to the defect-related intermediate state. The increased content of emissive Cu(I) is also attributed to the anisotropy of assembled ultrathin nanosheets, which may polarize the charge distribution of Cu NCs. Consequently, the resultant larger surface dipole between Cu and S atoms can promote charge separation and arouse high emission intensity. Because of the DSA-induced enhancement, the Cu NC assemblies are further applied as phosphors in fabricating light-emitting diodes (LEDs). Different-colored LEDs are produced by employing different-structured assemblies (with controlled feed ratio) as the color conversion layer on a 365 nm GaN LED chip.32 In addition to emission intensity and color purity, the stability of luminescent metal NCs is also vital for their applications. For instance, the inherent poor stability of Ag NCs has greatly hampered the development of Ag NCs as a novel family of optical probes in many practical applications.78 Recently, Zang and co-workers grafted nonemissive and less stable Ag12 NCs (decomposed within 30 mins in discrete states) with adaptable bridging ligands, the interconnection of which promoted DSA of grafted NCs into rigid metal−organic frameworks (MOFs).85 The resultant cluster-based MOFs exhibit enhanced stability (stable over one year) and intensive blue emission (QY of 12.1%). Upon such improvement, the asobtained cluster-based MOFs hold potential applications as sensitive gas sensors via ultrafast dual-function fluorescence switching mechanism, where turn-off is triggered by O2 and multi-colored turn-on can be initiated by a variety of volatile organic compounds. To sum up, both the above-mentioned AIE and DSA enhanced emission of metal NCs rely heavily on property engineering of protecting ligands on individual cluster. Namely, they are a “single-cluster event” in nature. Beyond the single cluster level, synergism between neighbored NCs could also give rise to a couple of novel collective properties. For example, as reported by Konishi’s group,86 assemblies of [core + exo]-type [Au8]4+ clusters would facilely switch dominant radiative mode from fluorescence to phosphorescence. The monomeric Au8 NC exhibits fluorescent emission centered at ∼590 nm, while assembled NCs with patterned orientations would sustain a phosphorescent emission at ∼700 nm, which should be attributed to unique electronic structure coupling made possible by intercluster interactions among linearly-assembled NCs (Figure 7). Another good example of DSA-enabled collective properties is generation of permanent excimer in supramolecular networking of metal NCs. The permanent excimer-like colloidal suprastructures constitute noninteracting metal cores (at the ground state) embedded in a network of hydrogen-bond-linked surface ligands.87 As a result of repulsive forces between the metal cores, the embedded metal NCs behave as independent chromophores at the ground state, whereas, they form bimolecular excimers with largely Stokesshifted emission (∼1 eV) upon photoexcitation. By contrast, encapsulation of metal NCs in bulky vesicles hinders the excimeric interaction, resulting in an unvaried photophysics as isolated NCs. Such finding has already been verified and explored in Cu NC system,88 which proposes directly-
good platform to establish a reliable structure−property relationship for cluster emission. Of particular note, the versatility of the DSA permits fine-tuning over many structural attributes (e.g., spatial distribution and inter-NCs interactions) in the assembled metal NCs, which could be easily transmitted into deliberately customizable emission properties. For example, as demonstrated by Zhang et al., the emission of DT-capped Cu NCs could be significantly enhanced upon their morphology-controlled DSA (Figure 6a).32 The isolated
Figure 6. (a) Schematic diagram and corresponding TEM images reveal the DSA enhanced emission of Cu NCs. (b) A series of emission lifetimes of the Cu assemblies, in which a clear time lag presented as the counts of excited electrons up to the maximum when the emission wavelength varies from 570 to 590 nm. This is attributed to the defect state behavior, as shown in the schematic diagram (c) of the excited state relaxation dynamics of the Cu assemblies with the high QYs up to 15.4%. Panel a is reproduced with permission from ref 32. Copyright 2015 American Chemical Society. Panels b and c are reproduced with permission from ref 40. Copyright 2017 American Chemical Society.
Cu NCs are nonemissive, while bright emission emerges in assembled architectures (nanoribbons in this particular case), in which more ordered and compact architecture is capable of generating stronger emission. In this context, the improved interactions of DT ligands from neighbored Cu NCs can remarkably suppress nonradiative relaxation of excited states in DT ligands; subsequently, optimize the radiative energy transfer via ligand-to-metal charge transfer (LMCT) or ligand-to-metal−metal charge transfer (LMMCT) process.84 Furthermore, the mechanochromic luminescent properties of as-assembled ribbons of Cu NCs have been investigated. By grinding the assembled NC ribbons, the QY decreases from 6.5% to 3.8%, as the thickness and regularity of the Cu NC ribbons decrease. It should also be noted that accompanying the photoluminescence QYs decrease is varied emission color from blue to green, which could be attributed to decreased average distance of cuprophilic Cu(I)···Cu(I) induced by the grinding treatment. To further enhance the emission intensity of Cu NCs, the authors had managed to introduce metal defects in the self-assemblies in follow-up work.40 After elaborate introduction of ethanol in their colloidal system, a rapid assembly dynamics developed accompanying the formation of Cu(I) defect-rich surface. The defect-rich 243
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(in particular crystallization) of metal NCs may offer an excellent platform for unraveling the fundamentals of nanometal catalysis, such as insights into molecular activation, active centers, and reaction pathways. No matter from the view of colloidal self-assembly techniques (exploring the ultrasmall size law and acquiring ensembles with high level of hierarchy, complexity, and accuracy) or the perspective of engineering functional ultrasmall metal NCs (customizing and enhancing materials properties), the topic of DSA of metal NCs is of core interest but less developed in comparison to that of their synthetic chemistry. It has been widely accepted that various directing agents, templates, and external fields need to be well modeled and manipulated in this transition size regime (3 nm). Encoding functionalities in the surface landscape of metal NPs, such as polymer-grafting, host−guest pairs, complementary DNA pairs, and functional surfactants, is a prevalent approach to introduce supramolecular interactions in the DSA process. However, it is not trivial to linearly extend those well-developed NP assembly methodologies into DSA of metal NCs. They are typically challenged by NCs’ weak structural stability, high surface energy induced aggregation and even fusion, and inconstant properties after surface modification. Notably, different from large-sized metal NPs, the structure of metal NCs can be described as well-defined M(0)@M(I)− SR (M denotes metal atom) core−shell structure, in which oligomeric M(I)−SR motifs cover over the M(0) core.21 Engineering surface motifs by systematically considering their composition, length, spatial arrangements should be effective in tailoring the recognition chemistry and self-assembly of metal NCs. In addition to the above-mentioned molecular or
Figure 7. Schematic illustration of a possible aggregated state of the [Au8]4+ clusters in solution. Some clusters form assemblies that give phosphorescence-type emissions, whereas the rest of the cluster molecules dispersed in the shell (bulk solution), together with counteranion and solvent molecules, only exhibit weak fluorescence. Reproduced with permission from ref 86. Copyright 2017 American Chemical Society.
assembled metal NCs as a rising family of efficient and stable nonresonant single-particle emitters. Catalysis. In nanometal catalysis, the catalytic properties are highly related to the number of undercoordinated metal atoms (even a single atom matters).27 Ultrasmall metal NCs typically consist of tens to hundreds of metal atoms, featuring a high fraction of under-coordinated surface metal atoms.39 Such intrinsic abundance in undercoordinated surface atoms, together with recent advances in ligand-engineering and total synthesis of metal NCs, has greatly added to the acceptance of ultrasmall metal NCs as promising candidates for various catalytic applications (e.g., electrochemical reduction of CO2, catalytic oxidation of CO, and semihydrogenation of alkynes).27 For example, both the sphere and rod-shaped ligand-on Au25 (namely, without removing ligands) composed TiO2 catalysts perform high catalytic conversion for semihydrogenation of terminal alkynes with nearly 100% selectivity to the corresponding alkenes. The catalytic fundamentals of metal NCs have been explored in terms of metal cores (size and heteroatom substitution) and surface effects.89−91 However, the development of NC-based catalysts is also hampered by their intrinsic poor stability and relatively low efficiency of metal NCs. The catalytic performance can also be enhanced by DSA of catalytically active NCs into an ordered pattern. For example, Zhang et al. demonstrated that DSA of Cu NCs into 2D ribbons could significantly enhance their durability in electrocatalytic oxidation of oxygen.39 The freestanding NC ribbons show enhanced catalytic activity. The inner strong nanoscale interactions also render the asassembled ribbons with robust stability, which is conducive for good recycling in catalytic applications. Most importantly, DSA
In addition to the above-mentioned molecular or nanoscale interactions, the rich content of M(I) in the M(I)−SR protecting motifs suggest that the metallophilic interaction can be a good candidate driving the DSA of metal NCs. nanoscale interactions, the rich content of M(I) in the M(I)− SR protecting motifs suggest that the metallophilic interaction can be a good candidate driving the DSA of metal NCs. The attractive metallophilic interactions have been extensively reported as a governing force to direct supramolecular selfassembly of many M(I) compounds.92−94 However, its usefulness in DSA of ultrasmall metal NCs is less explored. In recent work, we demonstrated that aurophilic interaction, which is one of the most important metallophilic interactions, is effective in inducing DSA of Au(I) rich Au NCs into welldefined nanoribbons.95 More interestingly, the as-formed Au NC nanoribbons exhibit bright luminescence centered at ∼620 nm with an absolute QY of 6.2% at room temperature, which should be attributed to aurophilic interactions featured ligand-
Most importantly, DSA (in particular crystallization) of metal NCs may offer an excellent platform for unraveling the fundamentals of nanometal catalysis, such as insights into molecular activation, active centers, and reaction pathways. 244
DOI: 10.1021/acsmaterialslett.9b00136 ACS Materials Lett. 2019, 1, 237−248
ACS Materials Letters
Perspective
to-metal−metal charge transfer (LMMCT). The similar 1D architecture of Au NCs sustained by aurophilic interactions has also been observed by Maran et al. in a single crystal of Au25(SBu)18 NCs.96,97 As mentioned above, the atomically precise size and structure monodispersity of metal NCs allow us to acquire atomic-level insights into their DSA fundamentals (e.g., the relationship between packing patterns and driving forces).36 Such molecular-like structural features may also intrinsically form the structural basis for a library of NC assemblies with rich diversity and hierarchy comparable to that of biomolecules.35 More intriguingly, the unique structure of metal NCs
More intriguingly, the unique structure of metal NCs has generated a couple of novel packing modes (e.g., 6H lefthanded helical arrangement) that have not yet been observed in bulk metal materials, paving a new way for constructing metamaterials.
Figure 8. Schematic representation of a hydrated Ag4 cluster inside a sodalite zeolite cage. Reproduced with permission from ref 102. Copyright 2018 American Association for the Advancement of Science.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Zhennan Wu: 0000-0001-5887-7129 Qiaofeng Yao: 0000-0002-5129-9343 Shuangquan Zang: 0000-0002-6728-0559 Jianping Xie: 0000-0002-3254-5799
has generated a couple of novel packing modes (e.g., 6H lefthanded helical arrangement) that have not yet been observed in bulk metal materials, paving a new way for constructing metamaterials.34 The development of DSA chemistry for metal NCs with varied size and composition is still in its infancy, although the cocrystallization of two atomically precise, different-sized AuAg NCs has been recently reported by the Zheng group.98 DSA of a more complicated multicomponent system with a number of distinct pairwise interactions will be more fascinating both in fundamentals and applications, although its realization should be more technically challenging. Moreover, some smart assemblies (with light or thermal response) of NCs are also highly desirable and expected in the near future.99,100 Another interesting topic for further exploration is construction of chiral architectures by chiral metal NCs, which could be useful in enantioselective synthesis and separation of value-added chemicals.101 The susceptivity of metal NCs toward surroundings, on one hand, can cause a structural change of metal NCs during their DSA processes, but on the other hand, offers a good opportunity to capture some novel cluster species in their assemblies. This is helpful for enriching NC library as both functional materials and fundamental models to uncover some long-standing puzzles in nanoscience. Very recently, Grandjean et al. reported their discovery of bright green photoluminescence from Ag4 NCs confined in zeolites.102,103 Such bright emission is clearly assigned to the electronic states of Ag4 clusters bound with water molecules coconfined in the cavity of zeolites (Figure 8). To sum up, DSA chemistry is less developed in comparison to synthetic chemistry of metal NCs. More research efforts are thus required in the near future to establish reliable governing principles of DSA in this ultrasmall size regime (
