Nanocrystal Superlattice Embedded within an Inorganic

Communication pubs.acs.org/cm

Nanocrystal Superlattice Embedded within an Inorganic Semiconducting Matrix by in Situ Ligand Exchange: Fabrication and Morphology Richa Sharma,†,‡ April M. Sawvel,† Bastian Barton,† Angang Dong,† Raffaella Buonsanti,† Anna Llordes,† Eric Schaible,§ Stephanus Axnanda,§ Zhi Liu,§ Jeffrey J. Urban,† Dennis Nordlund,⊥ Christian Kisielowski,† and Delia J. Milliron*,∥ †

The Molecular Foundry and §Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ⊥ Stanford Synchrotron Radiation Laboratory, Stanford Linear Accelerator Center, Stanford, California 94309, United States ∥ McKetta Department of Chemical Engineering, The University of Texas at Austin, Austin, Texas 78712, United States S Supporting Information *

ABSTRACT: Preparation of ordered nanocrystal superlattices that are also conductive remains an outstanding challenge. Typically, ligand exchange for small organic or inorganic molecules brings nanocrystals into close proximity at the cost of disrupting regular ordering. Here, we carry out ligand exchange, replacing bulky insulating organic ligands with inorganic chalcogenidometallate clusters, in nanocrystal assemblies floating at the liquid−air interface. The mobility of the nanocrystals allows lattice contraction without significantly diminishing the ordering observed by grazing incidence X-ray scattering and transmission electron microscopy (TEM). This approach produces inorganic nanocomposite films one to a few nanocrystals in thickness with highly regular arrangements of nanocrystals. Analysis of free-standing nanocrystal membranes by low-dose, aberration-corrected TEM allows direct visualization of the nanocrystal−matrix interface.

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Inorganic nanocomposites15,16 show novel and unique functionalities due to a combination of intrinsic material properties and their engineered interface. We and others have recently developed modular methods for making inorganic nanocomposite films, synthesized by exchanging long-chain organic ligands for molecular inorganic clusters that are annealed to form a continuous inorganic matrix surrounding the embedded nanocrystals.1,5,17−19 Using such techniques, inorganic nanocomposites can be fashioned from modular combinations of nanocrystals and inorganic clusters, including chalcogenidometallates (ChaMs, also called MCCs) and polyoxometalates (POMs). The resulting nanocomposites have shown promise for applications as diverse as thermoelectrics,20 electrochromic windows,18 transistors,1,5 and photovoltaic cells1 based on the flexible design opportunities offered by selecting the components while chemically controlling the structure of their interface. Inorganic clusters can be exchanged for organic ligands either in solution1,5 or through so-called solid-state ligand exchange,17,19 carried out by immersing a prefabricated nanocrystal thin film in a solution of inorganic clusters. In the latter case,

INTRODUCTION Densely packed assemblies of semiconductor nanocrystals have emerged as promising solution-processable semiconductors suitable for electronic applications such as thin film transistors1−5 and solar cells.6,7 Although regularly ordered arrays8−10 of nanocrystals, or superlattices, are expected to have the most favorable electronic properties, nearly all electronic devices reported to date have employed nanocrystal assemblies with little ordering.1,11,12 This is because although long-range supercrystalline order13 is routinely achieved by self-assembly of nanocrystals capped by long-chain aliphatic ligands, disordered films5,12 typically result from any of the diverse processes used to replace these insulating ligands, ultimately rendering assemblies conductive. Thus, constructing large-scale composites with order and no cracking remains a challenge. One notable exception has been the generation of conductive assemblies using long conjugated ligands.14 In this case, ligand exchange can be carried out after nanocrystal assembly with minimal superlattice contraction or the associated introduction of disorder. However, nanocrystal−nanocrystal coupling is fairly weak in such cases because of the large interparticle spacing and the relatively large HOMO−LUMO gap of the ligands compared to the nanocrystals. Still lacking has been a strategy capable of generating closely spaced, ordered nanocrystal assemblies within an inorganic semiconducting matrix. © XXXX American Chemical Society

Received: December 22, 2014 Revised: March 22, 2015

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DOI: 10.1021/cm504716s Chem. Mater. XXXX, XXX, XXX−XXX

Communication

Chemistry of Materials

Subsequent diffusion of the ChaMs into the floating assembly and diffusion of the organic ligands out of the assembly completes the ligand exchange process, which is therefore facilitated by the subphase solvent. Although the superlattice contracts, the mobility of the nanocrystals in their floating assembly is such that this contraction takes place without introducing microcracking and local disorder in the ultrathin film; instead the overall area of the film is reduced and it visibly pulls away from the sides of the assembly apparatus. The resulting inorganic cluster-capped nanocrystals can be readily transferred to any substrate for thermal annealing (to form ultrathin inorganic nanocomposites) and structural analysis. In this work, we chose PbSe nanocrystals as a model system, taking advantage of their well-defined and studied synthesis23 and surface chemistry.24,25 Starting with assemblies of oleic-acid capped PbSe nanocrystals, we found that both neutral and charged inorganic clusters could be employed to fabricate metal chalcogenide nanocomposites by this new approach. Specifically, the hydrazinium salts of [Sn2S6]4−, [Ge2S6]4−, and the hydrazinecomplex (N2H4)2ZnTe26,27 were each found to be suitable inorganic molecular precursors for the matrix phase. Thus, we conclude that the primary driving force for ligand replacement is mass action, and not electrostatic interactions between the nanocrystals and ligands. The subphase solvent had to be selected for each case to solvate a sufficient concentration of each precursor to drive mass-action exchange, while also avoiding side reactions between the solvent and the inorganic clusters. For ligand exchange, the ChaM concentration in the subphase is ∼10 mg/mL. We found distilled dimethylformamide (DMF) and acetonitrile (AN) to be suitable supporting solvents for the ligand exchange with [Sn2S6]4− and [Ge2S6]4− ChaMs and with neutral (N2H4)2ZnTe, respectively. Within seconds of adding the clusters to the subphase, the still-floating film contracts and turns darker due to the reduced interparticle spacing (Figure 1d, inset), indicating that ligand exchange is immediately successful. Volume contraction during the exchange of bulky organic ligands to small inorganic clusters results in shrinkage of the mobile, macroscopic, free-floating film mainly along the edges and induces some cracks at the macroscale. We wait for ∼15 min to ensure complete ligand exchange. After ligand exchange, the liquid in the well is removed using a micropipettor and the floating film is transferred to a substrate that was previously submerged in the well. The microscale continuity of films prepared by this method was found to be excellent, with very little micron-scale cracking (SEM, Figure 1g, h and AFM in the Supporting Information, Figure S9). By contrast, nanocomposite films synthesized by solid-state ligand exchange, with the superlattice supported on a silicon substrate, show typical massive cracking on the micrometer scale (Figure 1i).17 These cracks are evidence that the rigid substrate restricts the ability of the assembly to reorganize and accommodate the strain resulting from ligand exchange and the associated lattice contraction. Grazing incidence small-angle X-ray scattering (GISAXS) and transmission electron microscopy (TEM) were used to study the long-range and local morphology in the inorganic nanocomposites (Figure 2). Because the films are extremely thin, the GISAXS patterns are composed of largely vertical bars whose position along the qy axis is indicative of the in-plane periodicity, whereas the width of these reflections in the same direction indicates the degree of ordering. Figure 2a shows the GISAXS pattern for a PbSe assembly made by deposition on the surface of AN, then transferred to a silicon substrate without ligand

some regularity of nanocrystal packing is maintained in the nanocomposite based on the ordering in the original ligandcapped nanocrystal superlattice.13 However, ligand exchange is typically accompanied by a significant reduction of interparticle space, as bulky organic ligands are displaced. The inevitable result is the formation of abundant microcracks that present barriers to charge transport. Recently, Dong et al. reported that cracking could be largely avoided when exchanging long ligands with small organic molecules like formic acid or ethylene dithiol if the preassembled film was supported not on a rigid substrate, but rather was held suspended at a liquid−air interface.21 Here, we extend this floating ligand exchange process to inorganic clusters (ChaMs) and show how thin, even monolayer thick, nanocomposites incorporating ordered assemblies of nanocrystals can be fabricated without introducing cracks.

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RESULTS AND DISCUSSION We fabricate continuous, ultrathin films of semiconductor nanocrystal-in-matrix composites by carrying out ligand exchange on nanocrystal assemblies floating at a liquid surface (Figure 1). First, organic ligand-capped semiconductor nano-

Figure 1. Ligand exchange process. (a−c) Schematic of NC superlattice membrane growth at the liquid−air interface in a Teflon well. Teflon well is filled with DMF (liquid subphase) and substrate is placed at the bottom of the well. (a) Organic ligand capped NC dipsersion in hexane is introduced at the DMF surface. (b, c) Hexane spreads and evaporates resulting in self-assembled NC film at the DMF surface. (d) ChaMs introduced in the subphase via pipet. The inorganic clusters displace the organic ligands resulting in NC superlattice in ChaM matrix. Insets in c and d are the digital camera images of the ligand exchange process in Teflon well. (e) Post ligand exchange, DMF solution removed by pipet transferring the nanocomposite film to the substrate. (f) Dried nanocomposite film is thermally annealed to cross-link the matrix. (g, h) SEM images of nanocomposite films of PbSe-in-(N2H5)4Sn2S6, PbSein-(N2H4)2 ZnTe, respectively. (i) SEM of nanocomposite film synthesized by ligand exchange of nanoparticle film while supported on solid substrate.

crystals are self-assembled into ultrathin superlattices by depositing a controlled amount of nanocrystal dispersion in hexane atop a polar subphase. In earlier reports, such assembly processes were carried out using various subphase liquids including ethylene glycol, acetonitrile, or even water.21,22 In the second stage of our fabrication process, we trigger ligand exchange by adding dissolved ChaMs to the subphase. B

DOI: 10.1021/cm504716s Chem. Mater. XXXX, XXX, XXX−XXX

Communication

Chemistry of Materials

imaging. We can form free-standing, mechanically stable nanocomposite membranes with size of several micrometers (Supporting Information, Figure S1). The resulting, atomically resolved images offer supporting evidence for direct bonding between the nanocrystals and ChaMs. An atomic resolution phase image of PbSe nanocrystals ligand exchanged with [Ge2S6]−4 clusters is shown in Figure 3. In these initial images,

Figure 2. Structure of inorganic nanocomposites characterized (a−c) at long-range morphology by GISAXS and (d−f) at atomic length scale by TEM. (a) PbSe nanocrystal superlattice. (b) PbSe-in-ZnTe nanocomposite film after annealing. (c) I(q) versus q plots obtained by taking linecuts of the GISAXS data along the horizontal (qy) axis. Peaks correspond to center-to-center spacing. Traces refer to the original PbSe superlattice (black), PbSe-ZnTe nanocomposite (blue). (d) PbSe with organic ligands, (e) PbSe-in- (N2H5)4Ge2S6, (f) PbSe-in-(N2H4)2ZnTe.

Figure 3. Phase of a reconstructed exit wave function showing molecular structures at the surface and between nanocrystals (arrows). Their presence is compatible with the attachment of [Ge2S6]−4 clusters. Low dose rates and a voltage of 80 kV ensure that an alteration of the structure is minimal. (a) Arrows indicate the nanointerface. (b) Colors correspond to the phase of the exit wave. Color bar is the phase of exit wave in radians.

species dimensionally consistent with germanium(IV) sulfides can be found bound to the nanocrystal surface and filling the interparticle space (arrows, Figure 3). A low accelerating voltage of 80 kV was used in combination with a low dose rate (80 e−/ Å2s) to try to maintain the pristine structure of the ChaMs. Although further quantitative investigations are needed to reveal their detailed atomic arrangement of ChaMs at nanocrystal surfaces, these images provide initial direct microscopic evidence for their surface binding. Hence, our free-floating method for fabricating nanocrystal-in-matrix composites uniquely affords opportunities both for materials analysis and the production of densely spaced inorganic nanocomposites suitable for electronic applications.

exchange. The peak position along qy indicates an in-plane periodicity of 9.2 nm, or 2.2 nm spacing between the 6.8 ± 0.3 nm diameter nanocrystals. The scattering intensity is reduced in the film where ligand exchange with (N2H4)2ZnTe was carried out before transfer to the silicon substrate (Figure 2b), consistent with the reduced scattering contrast of an all-inorganic composition. Conversion to an inorganic nanocomposite also shifts the reflections to higher scattering angles in this case indicating a decrease of 2 nm in the average interparticle spacing, with only 7.2 nm remaining on average between the nanocrystals. The width (fwhm) of the first order GISAXS reflection is only slightly broader for the annealed PbSe-in-ZnTe nanocomposite (0.02 nm−1) than for the unexchanged nanocrystal assembly (0.014 nm−1), indicating excellent retention of regularity in the structure. To directly visualize the structure of the nanocomposites, floating films were transferred to TEM holey carbon films with holes of various sizes up to 100 μm for analysis. The highly ordered local morphology of PbSe-in-matrix films is apparent, with a very thin layer of inorganic matrix apparent surrounding the nanocrystals and filling the narrow spaces in between adjacent nanocrystals (Figure 2e, f). Figure 2e, f are nanocrystals capped with ChaMs and no thermal treatment. Figure 2d is a TEM image of PbSe nanocrystals with organic ligands. Overall, GISAXS and TEM analysis confirm that continuous ultrathin composites, free of gross defects and containing excellent local superlattice ordering are produced by our floating ligand exchange method. Additional SEM, STEM and TEM images of nanocomposites are included in the Supporting Information, Figures S1−S7, confirming that regular ordering is preserved through the ligand exchange and annealing processes for all the inorganic clusters studied. The continuity of the inorganic nanocomposites lends them significant mechanical integrity, permitting the preparation and analysis of free-standing membranes by aberration-corrected TEM. The absence of any background from a supporting film allowed us to visualize atomic arrangements at the interface of the inorganic matrix, derived from ChaMs, and the nanocrystals. Ultrathin composites were directly transferred from the liquid surface to holey carbon TEM supports for background-free

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CONCLUSION Our new approach to fabricating ultrathin composites resolves the challenge of creating densely spaced nanocrystal assemblies in inorganic matrices while avoiding microscale cracking and disorder. The ligand exchange and assembly method outlined herein was successfully applied to both charged and neutral inorganic clusters. These methods can be used to construct largearea nanocomposites with highly regular nanocrystal ordering, which have great potential for nanocrystal electronics and furthermore are amenable to high fidelity characterization by background-free aberration-corrected TEM.

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EXPERIMENTAL METHODS

Nanoparticle Synthesis. PbSe nanocrystals with rock-salt structure and average size of 7 nm and first absorption feature at 2153 nm were synthesized using a modified literature procedure23 from PbO and TOPSe in a noncoordinating solvent (1-octadecene). Precursor Synthesis. ChaMs were prepared at room temperature in a nitrogen atmosphere glovebox by dissolution of metal chalcogenides in anhydrous hydrazine in the presence of excess chalcogen.26,28 The Sn−S ChaM is synthesized by mixing 2 mmol of SnS and 4 mmol of S in 4 mL of anhydrous hydrazine and stirring for 3 days. The light yellow solution formed is filtered through a 0.2 μm filter to remove any undissolved solids and dried under nitrogen flow to yield a yellow powder with the chemical formula (N2H5)4Sn2S6. The Ge−S ChaM is similarly synthesized by stirring 2 mmol of GeS with 4 mmol of S in 2 mL of anhydrous hydrazine for 1 week. The resulting colorless solution is C

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filtered, and dried under nitrogen to yield a white crystalline powder with the expected chemical formula (N2H4)x(N2H5)4Ge2S6. For ligand exchange, the ChaMs of Sn−S/Ge-S are dissolved in formamide at concentrations ∼50 mg/mL. The (N2H4)2ZnTe is synthesized by mixing 3 mmol of ZnTe with 3 mmol Te in 6 mL of anhydrous hydrazine and stirring for 3 days.27 Addition of Te increases the solubility of ZnTe in hydrazine. The red colored solution is filtered and dried under nitrogen. For ligand exchange, (N2H4)2ZnTe is dissolved in ethanolamine and filtered to remove undissolved solids. Caution! Hydrazine is highly toxic and should be handled with extreme caution to prevent exposure by inhalation or absorption through the skin. Nanocomposite Characterization. GISAXS measurements were performed at beamline 7.3.3 at the Advanced Light Source, Lawrence Berkeley National Laboratory, using 10 keV X-ray beam. SEM imaging is carried out on a Zeiss Gemini Ultra-55 analytical scanning electron microscope, using beam energies of 2−10 kV and an in-lens detector. An inbuilt EDAX detector is used for elemental analysis. EDAX data is presented Figure S10 in the Supporting Information for PbSe-in-ZnTe. Atomic force microscopy (AFM) is used to characterize the thickness and surface morphology of nanocomposite membranes transferred to SiO2−Si wafers (Figure S8 in the Supporting Information). The membrane thickness is in the range between 12 and 13 nm. An Asylum MFP-3D atomic force microscope was used in tapping mode for AFM analysis. Aberration-Corrected TEM. Sample preparation: To enable this study, we fish-out a monolayer of floating nanocomposite film on a holey carbon TEM grid (i.e., holes in the grid have no supporting membrane). Interestingly, our method enables the fabrication of free-standing, mechanically stable nanocomposite membranes with size of several micrometers (Figure S1 in the Supporting Information). Our experiments are performed with the TEAM I microscope operated at 80 kV. Details of its capabilities are published elsewhere.29,30 In brief, we achieve deep sub-Ångstrom resolution at 80 kV acceleration voltage by limiting the focal spread by means of a beam monochromator together with a corrector for the chromatic aberration (Cc). Residual lens aberrations are removed post image acquisition during numerical reconstruction of the electron exit wave functions that creates the in-line or on-axis holograms.31 Low magnification bright field images were digitally denoised by nonlinear anisotropic diffusion (n = 20, δt = 1/7, κ = 250).32

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

STEM and AFM images and EDAX. This material is available free of charge via the Internet at http://pubs.acs.org.

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Communication

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address ‡

R.S. is currently at Schlumberger-Doll Research, 1 Hampshire Street, Cambridge, MA 02139, USA Funding

Work was performed in part at the Molecular Foundry, Lawrence Berkeley National Laboratory, supported by the Office of Science, Office of Basic Energy Sciences, U.S. DOE, under DEAC02-05CH11231. R.S., R.B., and A.D. were supported by a DOE Early Career Research Program grant, and A.L. was supported by a DOE ARPA-E grant, both to D.J.M. Support also provided by the Welch Foundation (F-1848). Notes

The authors declare no competing financial interest. D

DOI: 10.1021/cm504716s Chem. Mater. XXXX, XXX, XXX−XXX