Silicon Nanocrystal Superlattice Nucleation and Growth - Langmuir

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Silicon Nanocrystal Superlattice Nucleation and Growth Adrien Guillaussier, Yixuan Yu, Vikas Reddy Voggu, Willi Aigner, Camila Saez Cabezas, Daniel Houck, Tushti Shah, Detlef-M. Smilgies, Rui Nuno Pereira, Martin Stutzmann, and Brian A. Korgel Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02710 • Publication Date (Web): 23 Oct 2017 Downloaded from http://pubs.acs.org on October 25, 2017

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Silicon Nanocrystal Superlattice Nucleation and Growth Adrien Guillaussier,† Yixuan Yu,† Vikas Reddy Voggu,† Willi Aigner,‡ Camila Saez Cabezas,† Daniel W. Houck,† Tushti Shah,† Detlef-M. Smilgies,§ Rui N. Pereira,‡ǁ Martin Stutzmann,‡ Brian A. Korgel†* †

McKetta Department of Chemical Engineering and Texas Materials Institute, The University of

Texas at Austin, Austin, Texas 78712-1062, USA. ‡

Technische Universität München, Walter Schottky Institut, Am Coulombwall 4, 85748

Garching beiMünchen, Germany. §

Cornell High Energy Synchrotron Source (CHESS), Cornell University, Ithaca, NY 14853,

United States. ǁ

Institute for Nanostructures, Nanomodelling and Nanofabrication, Department of Physics,

University of Aveiro, 3810-193 Aveiro, Portugal *Corresponding Author: (T) +1-512-471-5633; (F) +1-512-471-7060; [email protected]

ABSTRACT Colloidal dodecene-passivated silicon (Si) nanocrystals were dispersed in hexane or chloroform and deposited onto substrates as face-centered cubic (FCC) superlattices by slowly evaporating the solvent. The uniformity of the nanocrystals enables extended order; however, the solvent and the evaporation protocol significantly influence the self-assembly process, determining the 1

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morphology of the films, the extent of order and the superlattice orientation on the substrate. Chloroform yielded superlattices with step-flow growth morphologies and (111)SL, (100)SL and (110)SL orientations.

Hexane led to mostly island morphologies when evaporated at room

temperature with exclusively (111)SL orientations. Higher evaporation temperatures led to more extensive step-flow deposition. A model for the surface diffusion of nanocrystals adsorbed to the superlattice surface is developed.

INTRODUCTION Ordered arrays of ligand-stabilized nanocrystals, or superlattices, have been made from many different kinds of materials, with a diverse range of nanocrystal shapes, including spheres, rods, disks and even tetrapods, by self-assembly at solid or liquid interfaces, or as free-standing films, by evaporating the solvent from concentrated dispersions.1-23

Superlattice formation

requires only that the nanocrystals be uniform in size and shape and well-dispersed in the solvent.

Because of the size-dependent properties of the nanocrystals and the emergent

collective behavior of their assemblies, superlattices have been explored for numerous applications.24-31

One approach for making significant quantities of well-characterized Si

nanocrystals involves an initial high temperature reactant (hydrogen silsequioxane, HSQ) decomposition and nanocrystal formation step followed by alkene surface passivation by hydrosilylation.32-34

Si nanocrystals are generated with sufficient uniformity to enable a

subsequent size-selective precipitation to provide the monodispersity needed for superlattice assembly.35-37 Like other semiconductor quantum dots, Si nanocrystals exhibit size-tunable visible luminescence and optoelectronic properties,34,38-44 but are much more stable at elevated temperature because of the strong covalent Si-C bonding of their hydrocarbon capping ligands.45

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Si nanocrystals are also biocompatible and non-toxic, making them appealing for commercial applications.51-55 However, even with uniform size, Si nanocrystals do not self-assemble into superlattices as readily as other kinds of nanocrystals, typically requiring long drying times (>30 min), perhaps as a result of weaker van der Waals attractions between Si cores than other types of more polarizable materials, such as ionic solids and especially metals.35

In contrast,

alkanethiol-capped gold nanocrystals for example form superlattices by drop-casting from nearly any good solvent in a matter of several seconds.56,57 It is well-known that the solvent and drying conditions influence superlattice assembly, but the effects of these parameters on Si nanocrystal superlattice assembly have not yet been reported in any significant detail. Here, we examine in detail how the solvent and drying conditions influence Si nanocrystal superlattice assembly. Although nanocrystal uniformity is the primary requirement for superlattice formation, subtle changes in the solvent and drying protocol significantly influence the extent of order, superlattice orientation and film morphology. Chloroform and hexane are both good solvents for the Si nanocrystals, but chloroform was found to be a significantly better solvent than hexane for superlattice deposition. When evaporated at room temperature, hexane dispersions gave superlattices with rough surfaces, characteristic of island growth morphologies; whereas, chloroform dispersions led to superlattices with smooth surfaces and step-flow growth morphologies. This is qualitatively similar to what we observed previously for alkanethiol-capped gold and silver nanocrystal superlattices;56 however, the drying times for the Si nanocrystals were significantly longer (30 min-2 hr) in order to achieve superlattice formation. There was also a significant difference in the preferred orientation of the facecentered cubic (FCC) superlattices of Si nanocrystals compared to gold and silver nanocrystals. FCC superlattices of gold and silver nanocrystals tend to orient nearly exclusively with (111)SL

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planes on the substrate;58 whereas, Si nanocrystal superlattices exhibited both (111)SL and (100)SL orientations on the substrate, with a few rare instances of (110)SL orientations as well. By slowing the evaporation time and/or increasing the evaporation temperature (to 55°C), superlattices with significantly longer range order were formed. Both hexane and chloroform gave smooth films with step-flow growth morphologies.

A model for nanocrystal surface

diffusion reveals that the differences in superlattice morphology and texture relate primarily to differences in interparticle attractions in the solvent. The addition of a polar solvent, ethanol, into the dispersion significantly reduced the range of order in the superlattices.

EXPERIMENTAL DETAILS Materials. FOx 16 (Dow Corning Corporation, 16% hydrogen silsequioxane (HSQ) by weight in methyl isobutyl ketone), ethanol (Sigma, 99%), hydrofluoric acid (Sigma, 48%), 1dodecene (Acros Organics, 93%), hydrochloric acid (Fischer, 25%), hexanes (Fisher Chemical, 99.9%) were purchased and used as received. Si nanocrystal superlattices were deposited on ptype (0.01-0.02 Ω.cm) Si wafers obtained from GlodiTech and on p type (1-20 Ω.cm) borondoped wafers obtained from Crystec for SEM and GISAXS measurements. Si nanocrystal synthesis. Si nanocrystals were synthesized using published procedures.33,36,44 HSQ (3 g) is heated in a tube furnace under forming gas (95% N2, 5% H2) at 1400°C for 1 hr. An agate mortar and pestle is used to grind the resulting solid product into a brown powder, which is then shaken in a wrist action shaker for 9 hr with 30 g of 3 mm diameter borosilicate glass beads. One gram of the resulting fine powder is etched using a mixture of 3 mL of 25% HCl and 30 mL of 40% HF in the dark for 7 hr. This results in hydride-terminated Si nanocrystals, which are isolated by centrifugation and rinsed three times—twice with EtOH and 4


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once with chloroform. These nanocrystals are then dispersed in 12 mL of 1-dodecene and sonicated to from a turbid brown dispersion, which is transferred to a three-neck flask, degassed with four freeze-pump-thaw cycles, and finally heated at 190°C under nitrogen for 16 hr. After cooling to room temperature, the resulting dispersion is transferred to a glass centrifuge tube and centrifuged at 8000 rpm for 5 min. The precipitate (unpassivated particles) is discarded. The supernatant is then dispersed in hexane and reprecipitated four more times to remove residual reaction byproducts, once using acetone and three additional times using ethanol as the antisolvent. For the studies carried out here, the uniformity of the nanocrystals was further improved by size selective precipitation. Ethanol was added dropwise to the dispersion until a slight turbidity is observed. The dispersion is then heated to 30°C until it regains its optical transparency. This resulting dispersion is then centrifuged at 10°C at 8500 rpm for 8 min. During this centrifugation step, the larger nanocrystals in the sample precipitate while the smaller nanocrystals remain dispersed. The supernatant is collected and redispersed in hexane at a concentration of 5 mg/mL for further use. Transmission Electron Microscopy (TEM). TEM images were obtained with a Tecnai Biotwin TEM operated at 80 kV. Samples were prepared for imaging by dipping a carbon-coated copper TEM grid (Electron Microscopy Science, CF200-Cu) into a vial with a diluted dispersion of nanocrystals in hexane (1 mg/mL). (See video in Supporting Information) Superlattice preparation. Superlattices were prepared on two different substrates: ptype (1-20 Ω.cm) boron-doped Si wafers obtained from Crystec with 275 μm thickness and a 50 nm of silicon nitride grown on the surface or p-type (0.01-0.02 Ω.cm) Si wafer obtained from

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GlobiTech with 620 μm thickness and a 300 nm silicon oxide layer on the surface. Substrates were cut into 2 mm by 2 mm squares. To deposit the nanocrystals onto the substrate, 20 μL of Si nanocrystal dispersion is first dried in a 3 mL glass vial. Then, 300 μL of either chloroform or hexane are added to redisperse the nanocrystals. The substrate is then placed with a vertical orientation into the vial. (See the video in Supporting Information). The drying time was manipulated by changing the size of the vial and either 3 mL or 20 mL vials were used. It took either 30 min or 2 hr to fully dry depending on the size of the vial. Since the vapor pressures of hexane and chloroform are similar (17.6 kPa and 21.3 kPa at room temperature, respectively), both solvents evaporate at similar rates. The evaporation temperature was increased when desired by placing the vial on a hot plate. The dried substrates were rinsed with a few drops of ethanol, dried with air and then placed on an SEM holder for imaging. Scanning Electron Microscopy (SEM). High resolution SEM images were acquired with a Zeiss Supra 40 VP SEM at 3 keV accelerating voltage. Images were collected through the in-lens detector. To prevent sample charging, substrates were electrically grounded to the SEM base with a strip of copper tape and silver paste. Small Angle X-ray Scattering (SAXS). GISAXS and solution SAXS measurements were performed using the D1 beam line of the Cornell High Energy Synchrotron Source (CHESS). Monochromatic X-ray radiation with a wavelength of 1.166 Å was used. GISAXS and solution SAXS images were collected using a fiber-coupled CCD camera (MedOptics) of 1024 × 1024 pixels with pixel size of 46.9 μm × 46.9 μm and 14-bit dynamic range per pixel. The sample-to-detector distance was 588 mm and the incident beam angle was 0.25o. Patterns

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were background corrected and integrated using Fit2D software (version: 12_077_i686_WXP). Diffraction data were analyzed using indexGIXS-2L software.59,60

RESULTS AND DISCUSSION Island and Step-Flow Growth Morphologies of Si Nanocrystal Superlattices. Dodecene-capped Si nanocrystals with a relatively narrow size distribution, and an average diameter of 12.2 ± 0.8 nm as determined by SAXS, were dispersed in chloroform or hexane and deposited onto silicon substrates by slowly evaporating the solvent at room temperature. (See Supporting Information for SAXS data and a video of the solvent evaporation process.) Figures 1 and 2 show SEM images of the assemblies that were formed, as well as GISAXS data. The SEM images have sufficient resolution to make out individual nanocrystals. It is clear in both the SEM images and the GISAXS data that the nanocrystals assemble into face-centered cubic (FCC) superlattices. Indexing the Bragg diffraction peaks in the GISAXS data in Figures 1e and 2e show that deposition from either chloroform or hexane leads to FCC superlattices with the same lattice constant of aSL=21.5 nm. This corresponds to an interparticle spacing of 3.0 nm,61 which is slightly larger than what is typically observed for FCC superlattices of Au nanocrystals with similar C12 alkyl capping,62,63 but is similar to what we have previously observed for slightly smaller ~8 nm diameter Si nanocrystals.35,36 The SEM and GISAXS data also show however that the solvent used to deposit the nanocrystals leads to qualitative differences in thin film morphology, the range of order, and the orientation of the superlattices on the substrate.

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Figure 1. Si nanocrystal superlattices formed by evaporating chloroform dispersions: (a-c) SEM images and (d,e) GISAXS data. The nanocrystals in (a) and (b) were deposited by evaporating the dispersion for 2 hr at room temperature and the superlattices with FCC structure exhibited step morphology as shown here with superlattices oriented predominantly on (a) (111)SL and (b) (100)SL planes. The nanocrystals in (c) were deposited by evaporating the dispersion for 2 hr at 55oC and the resulting FCC superlattices exhibited step morphology with preferential (111)SL orientation on the substrate as shown here. The GISAXS patterns in (d) and (e) were obtained from nanocrystals dried at room temperature for 30 min and 2 hr, respectively. The FCC superlattices in the SEM images in (a)-(c) are oriented with (111)SL planes on the substrate.

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Figure 2. Si nanocrystal superlattices formed by evaporating hexane dispersions: (a-c) SEM images and (d,e) GISAXS data. The nanocrystals in (a) and (b) were deposited by evaporating the dispersion for 2 hr at room temperature and the superlattices exhibited a mixture of the two different superlattice morphologies shown here: (a) islands and (b) steps. The nanocrystals in (c) were deposited by evaporating the dispersion for 2 hr at 55oC and the resulting superlattices exhibited the step morphology shown here. The GISAXS patterns in (d) and (e) were obtained from nanocrystals dried at room temperature for 30 min and 2 hr, respectively. The pattern in (d) corresponds to a superlattice with small superlattice domains and no preferential superlattice orientation. The pattern in (e) corresponds to an FCC superlattice. The FCC superlattices in the SEM images in (a)-(c) are oriented with (111)SL planes on the substrate. 9


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The superlattice films deposited at room temperature from hexane are rough and exhibit both islands (Figure 2a) and steps (Figure 2b).56 The films deposited from chloroform are smooth with almost exclusively terrace and step morphology (Figures 1a and 1b). The range of order is much more extended when chloroform is used as the solvent and required less drying time to still obtain long range order. Nanocrystals dried for only 30 min in hexane gave amorphous GISAXS patterns, although SEM images showed that there are still observable domains of ordered assembly (Figure 2d, See also Supporting Information) and a radial integration of the diffraction pattern (Supporting Information) shows that the nanocrystals have FCC order, but not much correlation perpendicular to the surface. The prevalence of weak powder rings shows a random orientation of superlattice crystal domains. Longer drying time led to improved order, and 2 hr of solvent evaporation yielded superlattices with relatively longrange order as the GISAXS data show in Figure 2e. Longer drying times have also been observed to significantly enhance the order of PbSe nanocrystal superlattices.64 Superlattices deposited from hexane orient exclusively with (111)SL planes on the substrate; whereas, superlattices deposited from chloroform exhibit both (111)SL and (100)SL orientations. (In rare instances, (110)SL orientations were also observed, see Supporting Information). Figure 1b shows a Si nanocrystal superlattice with (100)SL orientation. The range of order in the superlattices was found to increase when the drying temperature was increased. The temperature also influenced the morphology of the films. Films dried from hexane at 55°C no longer exhibit islands and show only step-flow growth morphologies, as in Figure 2c. The nanocrystals no longer deposit into circular islands and form more continuous layers on the substrate.

(See Supporting Information for SEM images).

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The increased

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deposition temperature also improves the order of the superlattices deposited from chloroform. Figure 1c shows an SEM image of a film dried from chloroform at 55°C. The increased temperature produces films with terraces and steps like those produced at room temperature, but these are much more extended. Additionally, (100)SL orientations on the substrate are no longer observed.

The Terrace-Step-Kink (TSK) Model.

The influence of the solvent and deposition

conditions on the superlattice morphology can be explained by the TSK (Terrace Step Kink) model illustrated in Figure 3.56,65,66 During the drying process, a nanocrystal adsorbs to the superlattice and then can diffuse along the surface to settle on a terrace, step edge or kink. Some nanocrystals may desorb and then readsorb to the growing superlattice as the solvent slowly evaporates, but the final location of the nanocrystal (i.e., terrace, step edge or kink) depends on how far the nanocrystal can diffuse. The number of nearest neighbors for a nanocrystal is 6, 5 and 3 at a kink, a step edge and on a terrace, respectively; therefore, the most stable position for the nanocrystal is at a kink. Step-flow growth or deposition results when the majority of nanocrystals add to the growing superlattice at kink sites (Figures 3b and 3d). If the surface diffusion length is too short, nanocrystals become trapped on a terrace (or a step edge) and produce island growth morphologies (Figure 3c). Step-flow leads to smoother films, whereas island growth creates rough surfaces.67

A characteristic of island growth is the formation of

circular mounds like those in Figure 2a. The steps produced by step-flow growth tend to have a a triangular shape, as in Figures 1a, 2b and 2c. This is because the fast lateral growth direction of the terraces is along the SL direction.56 Based on the observed superlattice morphologies—i.e., island growth versus step-flow growth—the nanocrystals must be able to

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diffuse longer distances on the superlattice surface when dispersed in chloroform than in hexane during the evaporation process.

Figure 3. Nanocrystal superlattice assembly described by the Terrace-Step-Kink (TSK) model. (a) Nanocrystals can settle on a terrace, at a step edge or a kink. (b) An exposed (111)SL plane of an FCC superlattice with step-growth morphology; nanocrystals are adding to the superlattice step in the SL direction. (c) An exposed (111)SL plane of an FCC superlattice with island morphology. (d) An exposed (100)SL plane of an FCC superlattice with step-growth morphology; nanocrystals are adding to the superlattice step in the SL direction.

Given that step-flow growth morphologies are observed in many of the Si nanocrystal superlattice films, nanocrystals must be diffusing a significant distance along the superlattice surface after adsorption. Here, we consider a hopping mechanism for the surface diffusion of adsorbed nanocrystals on the superlattice to determine the relative surface diffusion rates of nanocrystals in hexane or chloroform. Figure 4 illustrates the proposed hopping mechanism of an adsorbed nanocrystal from an initial hollow site  with threefold coordination, to a

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neighboring hollow site  via an intermediate bridge site , with twofold coordination. The hopping rate  ,    exp   kT 

(1)

is related to the energy barrier for a nanocrystal to move from to  sites,  .68

k is

Boltzmann’s constant, T is temperature, and  is the vibrational frequency, estimated to be 10 cm-1 based on the values reported for silver and CdSe nanocrystals in a superlattice.69 The surface diffusion coefficient D s ,68

Ds 

 2 3

is related to the distance between  sites,  : for the (111)SL surface,   is the nearest neighbor distance in the superlattice.

(2)





3 3 d near , where d near

When a nanocrystal moves from an site to a

neighboring  site, the nanocrystal goes from threefold to twofold coordination and one nanocrystal “bond” must be broken.

This bond energy (and correspondingly  ) can be

estimated from the pair interaction potential between two neighboring nanocrystals at the interparticle separation in the superlattice determined experimentally (by GISAXS).

The

relevant pair interaction potentials are those of the nanocrystals dispersed in the solvent, since the late stages of the assembly process proceed while the assembly is still wet. Based on the pair interaction potentials for Si nanocrystals dispersed in hexane and chloroform (provided as Supporting Information),  =7.1 meV (hexane) and  =4.9 meV (chloroform), which are both less than kT (26 meV at room temperature) and are consistent with the formation of superlattices in both solvents.

Table 1 summarizes the values of Ds for Si nanocrystals dispersed in

chloroform or hexane at room temperature and 55oC. Surface diffusion is faster in chloroform than in hexane, and is also faster at higher temperature in both solvents. 13


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Figure 4. (a) The potential energy barrier  , used in Eqn (1) for nanocrystal diffusion along the surface of the growing superlattice was estimated by considering that a nanocrystal in stable position α interacts with three nearest neighbors (in blue) and a nanocrystal in position β (saddle position) interacts with two nearest (blue) and two second-nearest (orange) neighbors. The nearest neighbor and second nearest neighbor separations, d near and d 2nd near , were determined from the superlattice parameter a, that was measured using GISAXS: d near  2a 2 and

d 2nd near 





3 2 d near . The interparticle interaction energies,  A and  B , were calculated from

pair interaction potentials for solvent-dispersed nanocrystals as shown in Supporting Information. (b) The activation energy for a Si nanocrystal to hop between two hollow sites α, across a bridge site . The separation between two α positions is  





3 3 d near 





6 6 a . The

potential  , calculated for surface diffusion in hexane is about 40% higher than in chloroform.

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Table 1. Calculated values of the surface diffusion coefficient (Ds), of Si nanocrystals dispersed in hexane and chloroform at 25oC and 55oC. Solvent Hexane Chloroform

Ds 25o C, cm 2 sec

Ds 55o C, cm 2 sec

5.9  10 6 6.4  10 6

6.1  106 6.6  10 6

Superlattices with (100)SL Orientations on the Substrate. The appearance of (100)SL orientations for FCC superlattice films deposited on substrates is unusual. Nanocrystals tend to settle on the closest-packed planes in a superlattice to maximize the van der Waals attraction between the nanocrystals and the substrate and to maximize the entropy in the basal plane.70,71 For FCC superlattices, this is the (111)SL plane and for BCC superlattices, it is the (110)SL plane. The planar density of the FCC(100)SL surface is quite low (it is a square lattice) and neither van der Waals attraction with the substrate, or nearest neighbor nanocrystals, nor configurational entropy provide an explanation for why this orientation would occur.72 Monolayers of square lattice assemblies of nanocrystals have not been observed in drop-cast nanocrystal films. We are aware of only one other case of nanocrystal superlattices being observed with (100)SL orientations, in which spin-coated PbSe nanocrystal superlattices exhibited a “ravioli”-shape with (100)SL planes oriented on the substrate.64 When these same PbSe nanocrystals were dropcast onto substrates, the superlattices oriented as expected on (111)SL planes (with similar interparticle spacing). The kinetics of the superlattice assembly process in that case was also clearly influencing the superlattice orientation.

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The (100)SL superlattice orientation was observed extensively when Si nanocrystals were deposited from chloroform at room temperature. Figure 5 shows additional SEM images of FCC Si nanocrystal superlattices oriented with (100)SL planes on the substrate. Even the drying cracks take on the square geometry of the (100)SL plane (Figures 5a and 5b). The (100)SL orientations always coexisted with (111)SL orientations, so these orientations were not preferred over (111)SL, but there was a sufficient driving force for them to occur regularly and compete with the more typical (111)SL orientation. GISAXS also shows the evidence of the mixture of (111)SL and (100)SL orientations in the Si nanocrystal superlattices dried from chloroform at room temperature. (See Supporting Information). Figure 5c shows a boundary between two domains with (100)SL and (111)SL orientations.

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Figure 5. SEM images of Si nanocrystal superlattices obtained by drying a chloroform dispersion at room temperature for 2 hr with {100}SL orientation on the substrate. (a-b) SEM images showing several (100)SL orientated superlattice domains separated by drying cracks,

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giving rise to a gross rectangular morphology. (c) A grain boundary between superlattice domains with (100)SL and (111)SL planes oriented on the substrate.

Here, we speculate about why the (100)SL orientations occur so frequently. Superlattices dried from hexane exhibited only (111)SL orientations, as shown in Figures 2a-2b. And when superlattices were formed from chloroform at higher temperature (55oC), only (111)SL orientations appeared. These facts indicate that the (100)SL orientation must relate to the kinetics of the superlattice formation process. The (100)SL planes always exhibit step-flow growth morphologies like those in Figure 1b, indicating that surface diffusion is always efficient and relatively long range. The rectangular shape of the terrace edges—instead of the characteristic triangular shape on (111)SL surfaces—results from the relative orientation of the SL direction on the (100)SL surface, as illustrated in Figures 3b and 3d. The observation of step-flow growth rules out the possibility that superlattices are nucleating homogeneously and then settling on the substrate with (100)SL orientations.

The most likely explanation for the (100)SL

orientations has been provided by computer simulations of crystallization of Lennard-Jones (LJ) particles at a solid-fluid interface. This work has shown that the crystallization of an FCC assembly of LJ particles in the direction is not thermally activated—unlike crystallization in the direction, which exhibits an activation barrier—and at low enough temperature, can outpace crystallization in the direction.72,73 The reason for the activation barrier for crystallization in the direction is that there are two different possible lattice positions available for the particles adding to the (111) surface, either at B or C positions on top of a close-packed A layer. (The FCC lattice is made up of ABC stacking of close-packed planes). If a particle happens to land in a C position, it must shift over

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to the B position to avoid creating a defect. On the (100) plane, there is no such degeneracy— only fourfold coordinated hollow sites. With the attraction forces between nanocrystals being relatively low in chloroform, the nanocrystals are perhaps approaching an ideal system of LJ particles with relatively weak interparticle interactions, and occasionally crystallize in the SL direction. Crystallization of a superlattice film thicker than a monolayer off a crystal plane that is a square lattice is also relatively common when the crystal structure dictates it. For example, BCC superlattices of alkanethiol-capped Au nanocrystals commonly orient on the (110)SL planes,71 which is the densest plane in the structure, but happens to also have a cubic geometry. (Incidentally, those same nanocrystals will form hexagonal close-packed planes when deposited as monolayers).71

Crystallization of the Si nanocrystal superlattices in the SL

direction was never observed in hexane perhaps because of the stronger interparticle interactions, which would tend to favor the (111)SL planes on the substrate with more nearest neighbors and larger bond energy in the 2D plane and stronger adhesion to the substrate. When dried from chloroform at higher temperature, it is likely that deposition the (111)SL planes simply outpaces any possible deposition on the (100)SL plane.

Influence of Increased Solvent Polarity on the Nanocrystal Assemblies. Nanocrystals were deposited from dispersions with increasing volumes of ethanol as a polar antisolvent to determine how far the system can be pushed and still obtain ordered superlattices of the Si nanocrystals. Increasing the polarity of the solvent increases the interparticle attractions and reduces the surface mobility of the nanocrystals.74 The Si nanocrystals were dispersed in hexane with various amounts of added ethanol (0%, 10%, 20% and 30% in volume) and evaporated onto substrates. For a small volume ratio of EtOH (10%), the nanocrystals deposit with irregular

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morphology as large “branch”-like mounds can be observed (Figure 6a). A zoomed in view of these spots, however, show that these regions are still made up of ordered superlattices (Figure 6b). When the ethanol volume ratio was further increased to 30%, the nanocrystals deposited into films as large mounds as shown in Figure 6c. The nanocrystals were not ordered in these films (Figure 6d).

Figure 6. (a-b) SEM images of a Si nanocrystals branch-like mound obtained from a hexane dispersion dried two hours with a 10 % volume ratio of ethanol. A zoomed in view confirms these are ordered mounds (b). (c-d) SEM image of a Si nanocrystals amorphous film obtained from a hexane dispersion dried two hours with a 30% volume ratio of ethanol. A magnified image shows that the nanocrystals are in a disordered arrangement. CONCLUSIONS Here, we show that uniform, well-dispersed Si nanocrystals can be assembled into superlattices by evaporating the solvent under a relatively wide range of conditions, although the solvent and drying protocols do significantly influence the extent of order, the film morphology 20


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and the superlattice orientation. The high quality of the Si nanocrystal samples enable SEM imaging of the assemblies with single particle resolution, and the superlattice order is clearly observable in GISAXS measurements. Compared to other systems like alkanethiol-capped Au nanocrystals for example that have been more extensively studied, drying times need to be more carefully controlled and extended for significantly longer time periods to achieve long-range order. For example, drop casting from hexane with less than 30 min drying time at room temperature does not lead to significant long-range order. Chloroform works better as a solvent, but the drying conditions still matter. Increasing the drying temperature from room temperature to 55oC, while maintaining a long drying time, significantly increases the extent of order in the superlattice. We developed a model for nanocrystal surface diffusion. Both hexane and chloroform are good solvents for the ligands and Si nanocrystals; however, subtle differences in the solvent dielectric constant lead to sufficient differences in interparticle attraction to give rise to significant differences in the extent of order, superlattice orientation and thin film morphology. At room temperature, hexane tends to deposit superlattices in mounds as a result of poor surface diffusion lengths and island growth. Higher temperatures improve the range of order and leads to smoother films with step-flow growth morphologies. Chloroform deposits superlattices with step-flow growth morphologies at room temperature, and interestingly generates superlattices with (100)SL orientation to a significant extent.

Acknowledgements Financial support of this work was provided by the Robert A. Welch Foundation (Grant no. F-1464), the National Science Foundation (Grant no. CHE-1308813), and the Portuguese 21


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Foundation for Science and Technology (FCT) via the UT Austin-Portugal Program and the project I3N (Project No. UID/CTM/50025/2013). The Cornell High Energy Synchrotron Source (CHESS) is a national user facility supported by the National Science Foundation under award DMR-1332208.

Supporting Information SAXS data used to determine the Si nanocrystal size; indexing of the GISAXS patterns; SEM images of Si nanocrystals thin film morphology at 25oC and 55oC; calculations of the Hamaker constant, pair interaction potentials, images of superlattices with (110)SL orientation; a video showing the superlattice deposition procedure. This material is available free of charge via the internet at http://pubs.acs.org.

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