Article pubs.acs.org/Langmuir
Hierarchy in Au Nanocrystal Ordering in a Supracrystal: II. Control of Interparticle Distances Yanfen Wan,† Nicolas Goubet,† Pierre-Antoine Albouy,‡ Nicolas Schaeffer,† and Marie-Paule Pileni*,† †
Laboratoire des Matériaux Mésoscopiques et Nanométriques (LM2N), UMR CNRS 7070, Université Pierre et Marie Curie, bât F, BP 52, 4 place Jussieu, 75252 Paris Cedex 05, France ‡ Laboratoire de Physique des Solides, Université Paris-Sud, 91405 Orsay, France S Supporting Information *
ABSTRACT: Au nanocrystals coated with thiol derivatives differing by the length of their alkyl chains are used to build 3D superlattices called supracrystals. In this study, we used two sets of Au nanocrystals differing by their sizes and size distributions. The average sizes are 5 nm (Au5) and 7 nm (Au7). From one experiment to the other, the size distribution slightly changes. For Au5 nanocrystals, it evolves from 6 to 8%, and for Au7 nanocrystals, it varies from 5 to 6%. The Au nanocrystals (Au5 and Au7) are first dispersed in toluene and produce fcc supracrystals by solvent evaporation. Here, by small-angle grazing X-ray diffraction, we observe a control in the average interparticle distance within the supracrystals. When the supracrystals are grown at zero toluene vapor pressure, the interparticle distances increase linearly with the alkyl chain length of the nanocrystals’ coating agent regardless of their diameters. Furthermore, the dry supracrystals can swell and the interparticle distance within the superstructure be increased by subjecting the material to toluene vapor pressure after initial growth. This swelling process is reversible, and retraction occurs when the toluene vapor pressure drops. This indicates a strong ability of the dried supracrystals to trap toluene molecules. On increasing the toluene vapor pressure during the solvent evaporation process, the slope of the linear dependency of the interparticle distances to the alkyl chain length is markedly decreased and the interparticle distance reaches a quasi-plateau. This is explained by the influence of depletion forces created by the presence of thiol-containing molecules physisorbed on the coating molecules on the internal structure of these supracrystals. Recently, we demonstrated that, by using the same nanocrystals (Au5 and Au7), a hierarchy in the supracrystal growth process takes place from heterogeneous nucleation with the formation of a layer-by-layer film to homogeneous nucleation in solution with the formation of shaped supracrystals. Here it is shown that the interparticle distance is independent of the supracrystal growth mechanisms.
I. INTRODUCTION Colloidal crystals of nanocrystals, called supracrystals, are usually produced by slow and controlled solvent evaporation of a nanocrystal suspension characterized by a low size distribution and acting as artificial atoms. The first 3D superlattices were produced using semiconductor particles.1,2 In the last few decades, supracrystals of spherical metallic nanocrystals with mesoscopic dimensions were extensively studied.3−17 The supracrystal growth mechanisms, remaining an open question, are known to be governed by complex softsphere interactions. Because of the collective behavior of the nanocrystals within the supracrystals, these 3D superlattices have shown promising intrinsic properties. This led to the study and the emergence of a new class of materials.6−8,18−24 Control of the structure and interactions of these nanocrystal assemblies on different scales is an exciting route to designing new materials with tunable optical, magnetic, and electronic properties.25 An in-depth understanding of the roles of nanocrystal size and interparticle © 2013 American Chemical Society
distance (the latter being defined as the average edge-to-edge distance between two nanocrystals) is crucial for the manufacture of well-defined supracrystals and their potential integration on technological devices. To reach a better understanding of the supracrystal growth processes and to predict their final internal structures, a large number of simulations were carried out on Au nanocrystals selfordered in 3D superlattices.26−31 Most of these simulations were carried out in vacuum without taking into account the solvent evaporation process of the colloidal solutions, and only a few studies were carried out while considering the solvent molecules to be a parameter. Vlugt’s group described the effective interactions between nanocrystals in the presence of solvent molecules and demonstrated that they are markedly different from those occurring in vacuum as a result of solventReceived: September 16, 2013 Revised: September 30, 2013 Published: October 1, 2013 13576
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capping-layer interactions.28,30,32 Thus, crystalline metallic cores capped with organic ligands should be considered to be building units that can be modified by tailoring the metallic core size, ligand length, ligand−ligand interactions, and ligand− solvent interactions in order to obtain different supracrystal structures.26−33 Recently, various experimental studies on the influence of supracrystal growth conditions on the superlattices’ crystalline structures were carried out. They include the investigation of the solvent evaporation rate and the interactions between nanocrystals, substrates and nanocrystals, and coating alkyl chains and solvent molecules.16,17,34,35 We recently demonstrated35 a hierarchy in the nanocrystals’ ordering ranging from disordered assemblies to a supracrystalline film sitting on a disordered nanocrystal film, layer-by-layer grown supracrystalline films (heterogeneous growth process), and finally micrometric supracrystals grown in solution with polyhedral shapes (homogeneous growth process). Here it is shown that the average interparticle distances within the supracrystals described previously35 are independent of the supracrystal growth mechanisms (heterogeneous or homogeneous) and change markedly with the solvent vapor pressure during the evaporation process. Furthermore, the supracrystals formed at zero vapor pressure can be swelled reversibly.
proportional to the evaporation rate of toluene at the level of the wet junction, the voltage difference is thus zero in a saturated atmosphere and maximized under high nitrogen flow (close to zero toluene vapor pressure). A linear dependence is assumed between temperature and voltage. The voltage is measured by using a microvoltameter (Agilent 34401A digital multimeter). II.3. Supracrystal Growth. The supracrystal growth procedure is described in great detail in ref 35. Briefly, 100 μL of a colloidal Au nanocrystals dispersion is placed in a beaker (1.52 cm3) containing a silicon wafer (5 × 5 mm2) at the bottom. The solvent is allowed to evaporate under a flow of nitrogen, inducing the formation of films or aggregates on the silicon wafer. Three distinct sets of experimental parameters were used while keeping the temperature constant (around 25 °C). They mainly differ by the evaporation time and the toluene vapor pressure. The various evaporations were carried out as follows: (i) in a glovebox (estimated volume: 0.97 m3), in which the evaporation time is approximately 7 to 8 h and the toluene vapor pressure is zero (Pt = 0); (ii, iii) in a glass vessel (volume = 8.2 × 10−4 m3). The evaporation time is approximately 8 to 9 h under a dry nitrogen atmosphere and 25 h when the nitrogen flow passes through a flask containing toluene. The partial toluene vapor pressures are estimated to be 39 and 75%, respectively (Scheme S1). The supracrystals are imaged using a JEOL JSM-5510LV scanning electron microscope (SEM) operating at a 20 kV acceleration voltage. II.4. Structural Study of the Three-Dimensional Superlattices. The structural information for such assemblies is deduced from small-angle X-ray diffraction (SAXRD). A rotating copper anode operating at 40 kV and 20 mA provided the X-ray beam that was focused with Kirkpatrick−Baez optics. From the experimental reflection coordinates and the sample−detector distance obtained from SAXRD patterns, the center-to-center interparticle distance D = (3/2)1/2λB/(2sinθB) determined through Bragg angle 2θB is calculated with λB = 0.154 nm. The edge-to-edge interparticle spacing δpp is given by δpp= D − 2R, with R being the average radius of the Au nanocrystals determined from TEM images. The width of the reflections is usually resolution-limited (excepted for AuC16, see below), indicating good ordering of nanocrystals. The average interparticle distance, δpp, is calculated from the d111 spacing. II.5. In Situ Small-Angle X-ray Diffraction (SAXRD) Measurements. The in situ SAXRD system was carried out in a small volatilization chamber placed on the specimen stage, with the substrate remaining inside this chamber during the measurements (Figure S2 in Supporting Information). Toluene-swelled cotton swabs were placed in the chamber prior to the measurements and then taken out. Pt went from 0 prior to cotton swab insertion to 100% after insertion and then back to 0, more than 15 h after the removal of the swabs. The incident X-ray beam was aligned along the grazing direction of the substrate’s surface plane. The samples were initially aligned using the substrate as a reference; thus the top surface of the substrate was centered in the Xray beam along the grazing direction. The substrate was then tilted relative to the incident beam in a fixed angular position.
II. EXPERIMENTAL SECTION II.1. Syntheses of Au Nanocrystals. Here the syntheses of 5 nm (Au5) and 7 nm (Au7) Au nanocrystals differing by their coating agents were carried out according to the procedure reported in ref 35. Au5 nanocrystals are produced by mixing 25 mL of a toluene (Riedel de Haën) solution containing 0.25 mmol of chlorotriphenylphosphine AuI (purchased from STREM) and 2 mmol of an alkanethiol (C12H25−SH, C14H29−SH, or C16H33−SH) as coating agents into a second 2 mL toluene solution containing 5 mmol of a tertbutylamineborane complex (Aldrich) at 100 °C under vigorous stirring. The colorless, clear solution mixture rapidly turned brown and then dark red. The colloidal solution was dried under a flow of nitrogen and then redispersed in ethanol. After being shaken, the solution was centrifuged, the supernatant was removed, and the black precipitate appeared to be dry under a flow of nitrogen in order to eliminate the remaining ethanol. An MA-E2-11 inductively coupled plasma atomic emission spectrometer (ICP-AES) was used to quantify the amounts of gold and sulfur in the dried powder; these values were found to be approximately 40 and 2 wt %, respectively (Table S1 in Supporting Information). Au7 nanocrystals were prepared following the same general procedure but using 1 mmol of a thiol derivative and 2.5 mmol of the tert-butylamineborane complex dissolved in 15 mL of toluene. All syntheses were carried out in a glovebox under nitrogen. After the synthesis and purification procedures, the Au nanocrystals were dispersed in toluene. Transmission electron microscopy (TEM) specimens were prepared by depositing 1 drop of the colloidal solution onto an amorphous carbon-coated copper grid placed on a piece of absorbent paper. The nanocrystals were imaged on a JEOL JEM 1011 TEM operated at a 100 kV acceleration voltage. Their average diameter and size distribution were estimated from these pictures using ImageJ software (Figure S1 in Supporting Information). Note that the size distribution slightly changes from one experiment to another. For Au5 nanocrystals, it varies from 6 to 8%, whereas with Au7 nanocrystals it varies from 5 to 6%. Hence, Au5 and Au7 nanocrystals differing by their coating agent (C12, C14, and C16) are used. They will be referred to as Au5C12, Au7C12, Au5C14, Au7C14, Au5C16, and Au7C16 throughout this report. II.2. Toluene Vapor Pressure Measurement during the Evaporation Process. The toluene vapor pressure, Pt, is evaluated using the principle of a wet−dry hygrometer. One solder of a copperconstantan thermocouple is wetted by toluene whereas the other is set close to the sample. With the temperature difference being
III. RESULTS AND DISCUSSION The supracrystals prepared from Au5 and Au7 nanocrystals coated with dodecanethiol (AuC12), tetradecanthiol (AuC14), and hexadecanethiol (AuC16) are produced as described in the Experimental Section. As already described in our previous paper,35 the morphologies of the supracrystals change markedly under the experimental conditions (Figure S3). Briefly, when the evaporation is carried out under Pt = 0, a highly ordered film of Au5C12 nanocrystals is obtained whereas well-defined shaped assemblies of Au7C12 nanocrystals are observed under the same conditions. Increasing the alkyl chain length of the nanocrystals’ coating agents from C12 to C14 resulted in the formation of well-ordered films characterized by a face-centered cubic (fcc) structure sitting on an amorphous film of disordered nanocrystals for the two sizes of nanocrystals studied. A further increase in the alkyl chain length of the coating agent to C16 13577
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induces the formation of amorphous films with Au5C16, whereas in the case of Au7C16 nanocrystals additional fcc supracrystals are produced. For any supracrystals formed, the SAXRD patterns show that the nanocrystals are self-assembled in an fcc crystalline structure (inset of Figure S3).35 From the SAXRD reflections (Experimental Section), the average edge-to-edge interparticle distances, δpp, are deduced (Table S2). Figure 1
from Pt = 0 to 39 and 75%, respectively (conditions ii and iii, see Experimental Section). For Pt = 39%, very few changes in the morphologies and crystalline structures of the supracrystals are observed compared to the number produced at Pt = 0 (Figure S3).35 For Au5C12 and Au5C14 nanocrystals, the fcc layer-by-layer supracrystal structure remains, and only 5% of the shaped supracrystals are observed in the corresponding SEM images. Furthermore, the underlying disordered nanocrystal film observed at Pt = 0 for Au5C14 nanocrystals disappears and is replaced by a thin, well-ordered layer-by-layer film with an fcc crystalline structure, whereas with Au7C14 nanocrystals some disordered assembly remains. When Pt = 75%, the morphologies of the fcc supracrystals markedly change. For Au5C12 nanocrystals, shaped supracrystals with a few films are produced whereas well-defined truncated tetrahedral fcc supracrystals with 10% of fcc films are obtained in the case of Au5C14 nanocrystals. For any nanocrystal size, AuC16 nanocrystals form amorphous films. Furthermore, for any coating agent, the interparticle distances remain quasi-unchanged when the supracrystals grow under Pt = 39 and 75% respectively (Figure 1, Tables S3 and S4). The values of the standard deviations corresponding to the plot of the linear regression are around 5 to 6%, which is higher than the expected experimental limit obtained from SAXRD (2%). This indicates that, under such experimental conditions, large fluctuations take place and consequently the alkyl chain length of thiol-containing molecules used to passivate the nanocrystals plays a negligible role in the control of the interparticle distance. To the best of our knowledge, no theoretical model has been accurately designed to describe such a process. Nevertheless, if we consider the geometrical model developed by Schapotschnikow et al.,32 the overlap between neighboring cones still occurs for experimental values within the limitation of the supracrystal stability (Tables S3 and S4). Hence, the solvent vapor pressure used in the colloidal nanocrystal solution markedly changes the interparticle distance. At Pt = 0, linear behavior between the interparticle distance and the alkanethiol chain length is observed; this agrees with theoretical simulations carried out under vacuum (i.e., at Pt = 0). In contrast, a nearly constant value of the interparticle distance is obtained when the system is subjected to toluene vapor pressure (Pt = 39 and 75%) in confined media. When the geometrical arrangement of the cones described in OCM32 (Supporting Information), is considered, the size of the cones is no longer dictated by the length of the alkyl chains or the nanocrystal sizes when Pt ≠ 0. Hence, the major changes observed in the interparticle distances can be attributed to the toluene vapor pressure effect. As mentioned above, according to the experimental setup, the morphology of the supracrystals changes from the formation of a film grown layer by layer (heterogeneous growth) to the appearance of shaped supracrystals (homogeneous growth). A careful comparison between the overall morphologies obtained under various experimental conditions (Figure S3) and δpp values (Tables S2, S3 and S4) clearly shows that no correlation exists between δpp and the supracrystal growth mechanism. This is somewhat unexpected if the important changes in the stiffness of the supracrystals (varying by a factor of 5 to 10 with the growth process) are being considered.37 Nevertheless, the change in the interparticle distance of nanocrystals within the supracrystals from a procedure used to produce them needs to be addressed. Note that the stiffness also changes with Pt: at Pt = 0, the
Figure 1. Variations of the interparticle edge-to-edge distance δpp (nm) with the number of carbon atoms (n) of the alkyl chain of the coating agent obtained through various procedures characterized at Pt = 0 (■), Pt = 39% (•), and Pt = 75% (▲). Black indicates supracrystals grown from 5 nm nanocrystals. Red indicates supracrystals grown from 7 nm nanocrystals.
(■) shows a linear dependence of the interparticle distance on the number of carbon atoms of the alkyl chain of the coating agents with a standard deviation of 1.7%. These data confirm that there is no significant effect of the nanocrystal size on the final interparticle distance after the formation of the supracrystals. It is also noteworthy that such a linear dependence, when extrapolated to a coating agent’s chain of zero carbon atoms, reaches δpp = 0.51 nm. This value is in full agreement with the data obtained previously.36 Schapotschnikow et al.32 developed the overlapped cone model (OCM) supported by Monte Carlo simulations and based on the assumption that the motion of the ligands adsorbed onto the particle’s surface is limited in space and assumes a conical shape, with the cones overlapping when two or more nanoparticles are in close proximity to one another. The alkyl chains used to coat the nanocrystals are assumed to be in a semicrystalline state in their trans configuration and to control the cone’s volume and length and consequently the average distance between the nanocrystals. Considering the two variables that are the scaled equilibrium distance τ = D/d and the scaled ligand length λ = L/R= 2L/d, with d (nm) being the average diameter and L (nm) being the length of alkyl chain developed in this model (Supporting Information), the best agreement with the experimental data hereby presented corresponds to a fourbody interaction process (Table S2). This indicates that, under such experimental conditions (Pt = 0), the length of the cone portions that overlap one another is directly related to the length of the alkyl chains used to passivate the nanocrystals. This clearly shows that the interparticle distances are well predicted by Monte Carlo simulations when Pt is 0 during supracrystal growth. From our previous paper,35 it is known that by changing the solvent vapor pressure during the toluene evaporation the supracrystal morphology may evolve. Hence, here we investigate the role of the toluene vapor pressure during the solvent evaporation process. The vapor pressure was increased 13578
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is repeated twice, and the variations in the interparticle distances are reversible; thee results are depicted in Figure 2d, e. The supracrystal lattices can be expanded or retracted reversibly through elastic deformations by tuning the solvent vapor pressure. This observation is consistent with a model developed previously39 describing the elastic contribution of the ligand chains that can interpenetrate and be compressed when the value of the scaled distance τexp = D/d is below 1.36. Removing the toluene molecules from the nanocrystal arrays takes longer than including them within the superlattices, and this behavior shows the strong ability of the supracrystal to trap the solvent molecules even though it is initially dried. This leads to a rapid increase in the δpp values. These variations in the internal structure of the supracrystals are in good agreement with those obtained with PbSe and PbS nanocrystals34 for which the δpp values increase with Pt. This clearly indicates that dried supracrystals do not entirely retain their internal integrity even though they remain ordered. At such point one may wonder if a disordered dried film of nanocrystals with a small size distribution could form supracrystals under solvent exposure. To answer this question, we consider Au5C16 nanocrystals forming an amorphous film (Pt = 0). The Au5C16 nanocrystal film was subjected to Pt = 100%, as described previously. The SAXRD pattern of the dry film before exposure to toluene shows, as expected, a diffuse ring characteristic of an amorphous structure that is in agreement with our previous paper (Figure 3a).35 By subjecting the sample
supracrystal’s Young modulus of nanocrystals coated with C14 is larger than with a coating of C12, whereas at Pt = 39 and 75% the moduli remain similar. This is explained as the change in the conformation of the alkyl chains.38 To explain such discrepancies in the interparticle distances with the solvent evaporation mode, in situ SAXRD measurements are carried out on a dried fcc supracrystal of Au5C12 nanocrystals and characterized by the smallest δpp value obtained within the framework of this work (1.8 nm). The well-defined Bragg reflection width (Figure 2a) confirms the
Figure 2. In situ SAXRD profiles. (a) Original profile of Au5C12 supracrystal grown at Pt = 0, δpp = 1.8 nm, (b) first increase of solvent vapor pressure, δpp = 2.5 nm, (c) first decrease of solvent vapor pressure, δpp = 1.9 nm, (d) second increase of solvent vapor pressure, δpp = 2.3 nm, and (e) second decrease of solvent vapor pressure, δpp = 1.9 nm.
long-range ordering of the Au5C12 nanocrystals in the plan parallel to the surface of the substrate. The Pt value increases from 0 to 100% when including toluene-saturated cotton swabs. In a 2 to 3 s time frame, the {111} Bragg reflection shifted toward smaller angles and no longer varied with time (Figure 2b), indicating a rapid increase in the interparticle distances between Au5C12 nanocrystals from 1.8 to 2.5 nm (Table 1). Table 1. Interparticle Distance (δpp) Varying Reversibly with Increasing/Decreasing Toluene Vapor Pressure within the Chamber by Adsorption/Desorption of a Cotton Swab Impregnated with Toluenea
a
step
origin
first adsorption
first desorption
second adsorption
second desorption
time δpp (nm)
0s 1.8
2 to 3 s 2.5
15 h 1.9
2 to 3 s 2.3
15 h 1.9
Figure 3. In situ SAXRD profiles. (a) Original profile of Au5C16 supracrystal grown at Pt = 0, (b) first increase of solvent vapor pressure for 15 h, (c) first decrease of solvent vapor pressure for 24 h, (d) second increase of solvent vapor pressure for 15 h, and (e) second decrease of solvent vapor pressure for 24 h.
to Pt = 100% for 15 h, the SAXRD pattern shows a retraction of the ring and the appearance of shadow peaks, indicating an increase in the average distance between nanocrystals and the emergence of local nanocrystal ordering (Figure 3b). The sample is then stored in the absence of toluene for 24 h and allowed to dry. The corresponding SAXRD pattern shows similar features to those depicted in Figure 3a, characteristic of a disordered film (Figure 3c). Such a switch between a disordered and a locally ordered film is successfully repeated several times, and similar structural variations are observed (Figure 3d,e), indicating a reversible process. However, the
s, seconds; h, hours; δpp, edge-to-edge distance between particles.
If the cotton swabs are taken off of the chamber and the sample is allowed to dry in air for 15 h, then the Bragg peak is shifted back toward large angles (Figure 2c). The interparticle distance reaches 1.9 nm, which is close to the value obtained before saturating the atmosphere with toluene (1.8 nm). This clearly shows that the average distance between nanocrystals decreases when Pt is reduced from 100 to 0%. This experiment 13579
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experiments, several conclusions can be drawn: (1) The solvent vapor pressure is a major parameter in the internal structure of the supracrystals. However, other contributions such as the presence of foreign molecules during the growth process have to be taken into account in order to control this final overall structure fully. (2) Elastic, reversible swelling of dry supracrystals takes place, and this effect can be tuned through variations of the solvent vapor pressure. (3) The interparticle distance does not depend on the supracrystal growth mechanism (i.e., homogeneous or heterogeneous growth). (4) The length of the alkyl chain of the coating agent controls the interparticle distance when the supracrystals are grown under zero vapor pressure. However, this process cannot be generalized. Hence, it is hereby shown that predicting the structure of supracrystals (shape, interparticle distance, etc.) is far from being trivial and can be achieved only through a careful consideration of many different parameters. These results also emphasize the limitations of the various models and theoretical simulations that have been developed to reach theoretical predictions for these types of structures.
SAXRD profiles presented in Figure 3 show only the presence of local order when the film is submitted to toluene-saturated vapor pressure, and the emergence of long-range order within the supracrystals is not observed. Nevertheless, the overall ordering within the structure can be greatly improved by swelling. On the basis of the results depicted above, it is obvious that the solvent vapor pressure during the supracrystal growth process and/or after its growth has a major impact on its internal structure and on the interparticle distances within it in particular. However, another effect can be important in the structure of the superlattices. In spite of the purification process after the nanocrystal synthesis, the presence of free thiols within the colloidal solutions and hence during the evaporation process has to be considered. This has been confirmed by chemical analysis (Supporting Information, Table S1). Hence, during the evaporation process and under Pt = 0, a part of the residual “unreacted” thiol-containing molecule physisorbs on the alkyl chain used as a coating agent and could likely be expelled with the solvent during evaporation. The depletion forces created by these thiol-containing molecules favor the nanocrystal assemblies whereas the length of the alkyl chains can control the interparticle distances. This process could markedly decrease upon increasing the Pt value, with the total decrease at Pt = 75%. Then, the excess thiol derivatives cannot be excluded with the solvent molecules by depletion forces. Under these conditions, the solvent and thiol-containing molecules remain in the superlattices and render the contribution of the coating agent’s alkyl chain length negligible. Hence, both residual thiols and solvent molecules control the length of the cone and consequently the interparticle distances. A tentative explanation of such a confinement effect could be as follows: during the evaporation process, a liquid−gas phase transition takes place and the supracrystals are formed before the end of the evaporation process.40,41 The increase in the Pt values (39 and 75%) could induce, as previously observed with various microemulsions,42 a change in the chemical potential of the system and consequently in the composition of the microphases. Hence, the chemical potential at Pt = 0 markedly decreases compared to higher values. Here, one can assume that the change in the chemical potential induces a variation in the phase compositions and consequently an evolution of the system from a liquid−gas phase transition to a triple-point (liquid−gas−solid) transition. By analogy to what was observed with 7.8 nm silica nanoparticles, at high volume fractions,43 the size of the geometrical cones could be regulated to induce a vitrification process, hence keeping the cone’s length quasiconstant for the various alkyl chain lengths used. The large standard deviation of the interparticle distance can be explained by the fact that the number of thiol-containing molecules could evolve slightly from one sample to another.
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ASSOCIATED CONTENT
S Supporting Information *
Scheme of the supracrystal synthesis experimental setup, color picture of the SAXRD setup, TEM pictures of the nanocrystals, SEM images of the various supracrystals, elemental analysis data, and SAXRD data. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Special thanks are due to Dr. I. Arfaoui for fruitful discussions and criticism of the manuscript. An Advanced Grant of the European Research Council under grant agreement no. 267129 supported this work.
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REFERENCES
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IV. CONCLUSIONS Here, the interparticle distance of 5 and 7 nm Au nanocrystals coated with alkanethiols differing by their length (from 12 to 16 carbon atoms) was measured by small-angle X-ray diffraction. The interparticle distance within the supracrystals is determined either after evaporating a colloidal solution under various solvent vapor pressures (0, 39, and 75%) or after submitting dry supracrystals to 100% solvent vapor pressure. It was demonstrated that the interparticle distance in the supracrystals markedly changed with the experimental environments used to grow these assemblies. Built on these 13580
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