Article pubs.acs.org/Langmuir
Hierarchy in Au Nanocrystal Ordering in Supracrystals: A Potential Approach to Detect New Physical Properties Y. F. Wan,† N. Goubet,† P. A. Albouy,‡ and M. P. 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, UMR CNRS 8502, Université Paris-Sud, 91405 Orsay, France S Supporting Information *
ABSTRACT: Here we describe the morphologies of Au nanocrystals self-assembled in fcc 3D superlattices called supracrystals. The average size of the nanocrystals is either 5 or 7 nm with a very small size distribution (98%, Riedel de Haën), ethanol (99.8%, Prolabo), chlorotriphenylphosphine gold(I) (98%, STREM), and 1-dodecanethiol, tetradecanethiol, hexadecanethiol, and tert-butylamine borane complex (97%, Aldrich). The water used was purified with a Millipore system (18.2 MΩ). II.2. Equipment. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), and high-resolution scanning electron microscopy (HRSEM) images were obtained with a JEOL JEM 1011(100 kV) or JEOL JSM-5510LV and Hitachi Su-70 instruments, respectively. Small-angle X-ray diffraction (SAXRD) measurements were performed with a homemade system with a rotating copper anode generator operated with a small-size focus (0.1 × 0.1 mm2 in cross-section) at 40 kV and 20 mA.34 II.3. Syntheses of 5 and 7 nm Au Nanocrystals. Syntheses of 5 and 7 nm Au nanocrystals (Au5 and Au7), described in a previous paper,34 are performed by using two solutions. For 5 nm nanocrystals, the first solution consists of 0.25 mmol of chlorotriphenylphosphine gold(I) dissolved in 25 mL of toluene to which 2 mmol of a thiol derivative is added. The second solution is obtained by dissolving 5 mmol of tert-butylamine borane complex in 2 mL of toluene. Both solutions are heated to 100 °C and stirred until complete dissolution of all products. After mixing, a rapid change in color is observed from colorless to brown and finally dark red. The colloidal solution is then dried in a flow of nitrogen, and ethanol is subsequently added to the dark powder. After being stirred, the solution is centrifuged and a black precipitate is recovered after the removal of the supernatant. The precipitate is then dried in a flow of nitrogen in order to eliminate the remaining ethanol. A nanocrystal dispersion in toluene and centrifugation allow for the removal of impurities. Intermediate steps were all performed in a glovebox under nitrogen. For 7 nm crystals, the procedure described above is similar except that (i) the first solution contains only 1 mmol of thiol derivative and (ii) the second solution is obtained by dissolving 2.5 mmol of tert-butylamine borane complex in 15 mL of toluene. The different thiol derivatives used to coat Au nanocrystals are dodecanethiol (C12H25−SH), tetradecanethiol (C14H29−SH), and hexadecanethiol (C16H33−SH). According to Bain et al.,36 the lengths of alkyl chains in the trans configuration are 1.77 (LC12), 2.03 (LC14), and 2.28 nm (LC16), respectively. At the end of the synthesis, Au nanocrystals are dispersed in toluene. The final nanocrystal average diameter is determined from TEM measurements. For simplicity, 5 and 7 nm Au nanocrystals coated with alkyl chains with different numbers of carbon atoms (C12, C14, and C16) are called Au5C12, Au7C12, Au5C14, Au7C14, Au5C16, and Au7C16, respectively. The corresponding TEM pictures and size distributions are shown in the Supporting Information (Figure S1). The Au nanocrystal concentration ([Au]) is measured by UV−vis spectroscopy at λ = 520 nm (extinction coefficient ε = 3 × 106 mol−1·L·cm−1) with a conventional Varian Cary 1 spectrophotometer (Figure S2). II.4. Preparation of the Three-Dimensional Superlattices Called Supracrystals. A colloidal Au nanocrystals dispersion (100 μL) is poured into a beaker (1.52 cm3) with a silicon wafer (5 × 5 mm2) at the bottom, and the solvent progressively evaporates under a flow of nitrogen. At the end of the evaporation process, one observes the formation of films or aggregates on the silicon wafer. Four procedures are used (Table 1). Procedure I: The filled beaker is placed in a glovebox having a volume estimated at 0.97 m3. The solvent evaporation is accomplished in 7 to 8 h at room temperature (∼22 °C). Procedure II: The beaker is placed in a glass vessel having a volume of 8.2 × 10−4 m3 (Scheme 1). A heated bath is connected to a copper holder where the beaker is placed (Scheme 1b), and no N2 passes through the flask (Scheme 1a) during the evaporation process. This allows us to control the substrate temperature to 25 °C. The evaporation time is 8 to 9 h. Procedure III: It is similar to procedure II with an increase in the evaporation time from 8 to 9 h to about 25 h. Here the nitrogen flow
procedure
chamber
V (m3)
gas
T (°C)
I II III IV
GB GV GV GV
0.97 0.82 × 10−3 0.82 × 10−3 0.82 × 10−3
N2 flow N2 CTV CTV
RT 25 25 50
t (h)
σ (%)
± ± ± ±
0 39 75 25
8 9 25 4.5
0.5 0.5 1 0.5
a
Glove box (GB), glass vessel (GV), chamber volume (V), temperature (T), evaporation time (t), toluene vapor pressure (σ) indicating the ratio of existent solvent molecules inside the evaporation chamber to pure solvent, room temperature (RT), concentrated toluene vapor (CTV).
Scheme 1. Setup of a Glass Vessela
(a) A flask with two thin exits to allow N2 to flow through the toluene solution. (b) A glass vessel containing an evaporation beaker on a copper holder. (c) A heater controlling the temperature in the range of 15 to 60 °C. a
passes through a flask containing toluene solution before reaching the colloidal solution (Scheme 1a). The temperature remains at 25 °C. Procedure IV: As in procedure III, the nitrogen gas has a certain concentration of toluene molecules and the substrate temperature is fixed at 50 °C. The corresponding evaporation time is around 4 to 5 h. In a first approximation, we assume that the difference in temperature between procedures I and II (around 3 °C) is negligible and does not impact the supracrystal growth process. The major difference is related to the change in the volume chamber and consequently to the toluene vapor pressure. The toluene vapor pressure and consequently the evaporation rate result from the change between procedures II and III. In procedures III and IV, the two parameters (temperature and toluene vapor pressure) are related. II.5. Structural Study of the Three-Dimensional Superlattices. The structural information for such assemblies is deduced from SAXRD. The relationship between the position of the various spots observed on diffraction patterns and the mean center-to-center distance D between the coated particles for an fcc structure is detailed elsewhere.34 The full width at half-maximum (fwhm) of the main peak of the diffraction pattern is determined by Gaussian fitting (Origin 6.1 software).
III. RESULTS AND DISCUSSION Au nanocrystals with average diameters of 5 and 7 nm are coated with surfactants differing by their chain lengths (C12, C14, and C16) and dispersed in toluene. Using various procedures (Table 1), we evaporated the solvent of the colloidal dispersion. For each procedure, the structural information and morphology of the 3D assembly of nanocrystals are probed. Let us first consider procedures I and II. In the glovebox (procedure I), there are Au5C12 nanocrystals, self-ordered by the heterogeneous supracrystal growth process with the formation of a well-defined layer-by-layer film (Figure 1A). The SAXRD pattern is indexed by using an fcc structure (Figure 1C). Terraces of nanocrystal layers with linear steps having various heights and dislocations are observed by HRSEM (Figure 1B). Dislocations and slip planes govern the deformation of crystals that are strongly bonded in 3D, as already reported.26 On increasing the alkyl chain length of the coating agent by two B
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Figure 1. SEM images, HRSEM images, and SAXRD patterns for Au5C12 (A−C), Au5C14 (D−F), and Au5C16 (G−I), respectively, obtained at the end of toluene evaporation on a silicon substrate at room temperature in procedure I (glovebox) with an evaporation time of about 8 h.
confirms the data obtained previously by our group38 and Prasad et al.39 By increasing the NC size from 5 to 7 nm, rather small changes are observed. With Au 7 C12 NCs, as already observed,34,35 aggregates with well-defined shapes are produced (Figure 2A). The HRSEM pattern of the aggregates (Figure 2B) is characteristic of nearly perfect order between nanocrystals with an fcc structure (Figure 2C). By increasing the alkyl chain length to C14 and C16, the SEM patterns (Figure 2D,G) show well-defined flat aggregates with an underneath film of Au7C14/Au7C16 nanocrystals, respectively. From the HRSEM patterns (Figure 2E,H), the corresponding FFT patterns recorded in various areas (patterns a and b in Figure 2E,H), and the SAXRD patterns (Figure 2F,I), the aggregates are fcc supracrystals sitting on disordered (amorphous) films, as already observed for Au7C14/Au7C16 nanocrystals. Hence, through procedure I, control of the nanocrystal ordering is achieved for a rather short coating agent alkyl chain length (C12). A progressive increase in the chain length, from C12 to C16, induces the appearance of the amorphous phase in the presence of well-defined supracrystals to reach a pure disordered assembly (Au5C16). The increase in van der Waals interactions produced on increasing the nanocrystal size from 5 to 7 nm induces a change in supracrystal morphology with the formation of shaped instead of layer-by-layer supracrystals. With procedure II, very few changes in the morphologies and crystalline structures are observed: for Au5C12 and Au5C14 nanocrystals, the formation of fcc layer-by-layer supracrystals is observed with the appearance of 5% shaped supracrystals (Figure S3A). For Au7C12 nanocrystals, faceted shape aggregate
carbon atoms (C14), the SEM pattern (Figure 1D) shows aggregates sitting on films at the end of the evaporation of a Au5C14 nanocrystal solution. The HRSEM image corresponding to the zone limited by a circle (Figure 1D) shows that the aggregate is highly ordered in a hexagonal pattern as confirmed by fast Fourier transform, FFT (Figure 1E, FFT pattern of c), whereas the underlying film is disordered (Figure 1E, FFT pattern of a). Note that at the edge of the aggregate the nanocrystals are highly ordered near some disordered nanocrystals (Figure 1E, FFT pattern b). The wide fwhm of the main peak of the SAXRD pattern (Figure 1F) confirms the formation of fcc supracrystals simultaneously with some amorphous films. A further increase, by two carbon atoms, of the coating agent chain length (C16) shows a homogeneous film (Figure 1G). The HRSEM image (Figure 1H) of the zone denoted by a circle in Figure 1G and the SAXRD pattern (Figure 1I) reveals that the film is amorphous with a random distribution of Au5C16 nanocrystals. From these, it is concluded that by increasing the chain length of the coating agent the system evolves progressively from fcc supracrystals to disordered films. The ligand shells, especially for small particles, dominate the order−disorder transition when the van der Waals attraction between the particles is weak. From a theoretical study,37 the crystallization temperatures for different chain lengths are −7 °C for C12, 28 °C for C14, and above 42 °C for C16. Therefore, it is easy to understand why nanoparticles could crystallize layer-by-layer film supracrystals at room temperature for C12, an ordered film plus a disordered film for C14, and only an amorphous film for C16. Such behavior C
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Figure 2. SEM images, HRSEM images, and SAXRD patterns of Au7C12 (A−C), Au7C14 (D−F), and Au7C16 (G−I), respectively, obtained at the end of toluene evaporation on a silicon substrate at room temperature in procedure I (glovebox) with an evaporation time of about 8 h.
Figure 3. SEM images, HRSEM images, and SAXRD patterns of Au5C14 (A−C) and Au7C14 (D−F), respectively, obtained at the end of toluene evaporation on a silicon substrate at 25 °C under procedure II with an evaporation time of 9 h and a temperature of 25 °C.
formation is obtained (Figure S3G). With Au5C14 nanocrystals, the underlying disordered film observed for procedure I disappears and is replaced by a thin, well-ordered layer-bylayer film (Figure 3A,B) with an fcc crystalline structure (Figure 3C), whereas with Au7C14 nanocrystals the underlying amorphous film is still present (Figure 3D,E). With Au5C16 and Au7C16 nanocrystals, the observed films are amorphous
(Figure S3D−F,J−L). Note that the supracrystal surfaces produced by procedure II are smoother (Figures 3 and S3) than those observed for procedure I (Figure 1). At this point, we need to know the influence of the evaporation time on the morphology of nanocrystal assemblies. This is achieved through procedure III. When the N2 flow is passed through a toluene solution, the evaporation time D
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(FFT pattern a in Figure 4F). In coexistence with the triangular supracrystals, a layer-by-layer supracrystal film covers 10% of the substrate area (Figure 4G) with well-defined terraces (Figure 4H). The increase in nanocrystal size to 7 nm induces the formation of shaped supracrystals with the disappearance of the layer-by-layer film supracrystals (Figure S4P). With AuC16 nanocrystals, the films remain disordered (Figures S4O,R). From these data, it is concluded that by increasing the evaporation time with Au5C12 and Au5C14 nanocrystals the morphologies of the supracrystals evolve from films to shaped supracrystals. In both cases (Au5C12 and Au5C14 nanocrystals), around 10% of the film remains and disappears simultaneously on increasing the nanocrystal size. Hence, the morphologies of the fcc supracrystals markedly change with the procedure used. With Au5C12 nanocrystals, the morphology evolves from film to shaped supracrystals. Major changes are observed with Au5C14 nanocrystals. The morphology evolves from an fcc supracrystal film sitting on a disordered film (Figure 1D,E) to highly ordered films (Figure 3A,B) and finally to equilateral fcc supracrystals (Figure 4E,F) with fcc supracrystal films covering 10% of the substrate area. This feature remains similar on increasing the nanocrystal size. With Au5C16 and Au7C16 nanocrystals, amorphous films are produced for any procedure except procedure I, where fcc supracrystals of Au7C16 with an underlying amorphous film are obtained. At this stage, we have to take into account the substrate temperature effect. In procedure IV, the experimental conditions remain similar to those in procedure III except that the substrate temperature increases from 25 to 50 °C. Consequently, the evaporation time drops from 25 to 5 h. For any coating agent, the nanocrystal size, fcc supracrystals, and compacted structure (cps) are produced as shown from the SAXRD patterns in Figure 5. The morphologies of Au5 supracrystals are layer-by-layer films (Figure 6A−F). A few percent of shaped supracrystals are present on the substrate for Au5C12 and Au5C14 nanocrystals (inset of Figure 6A,C). Furthermore, contrary to what was obtained at 25 °C, fcc (Figure 5C) layer-by-layer supracrystals are produced for Au5C16 at 50 °C. On increasing the nanocrystal size from 5 to 7 nm, for any coating agent, more shaped supracrystals are obtained (Figure 6G,I,K) with well-ordered surfaces (Figure 6H,J,L). Hence, for Au5C16 nanocrystals, on increasing the substrate temperature from 25 to 50 °C a transition from amorphous (disordered) to supracrystal (highly ordered) films is observed. This can be explained as follows. First, the crystallization temperature increased with the chain length of the coating ligands, especially above 42 °C for C16,37 so only on increasing the substrate temperature to 50 °C can the Au5C16 particles be self-organized into a superlattice. Second, the increase in temperature induces a dynamic process with the transmission of the gauche defect from the end of the long-chain cone to the head direction.40 Hence these gauche defects preventing supracrystal formation at 25 °C disappear and dynamics offers the needed driving force for the chains to interdigitate and consequently to form an ordered film. The increase in the dynamic process indicates that the configuration of capping ligand chains is sensitive to temperature, as already observed.41 Crystalline agglomerates of supracrystals occur at high temperature.37 Note that with other nanocrystals and coating agents the first step in ordering the Au5C16 nanocrystals in 3D superlattices is heterogeneous growth with the formation of layer-by-layer supracrystals. With Au5C12 and Au5C14 nanocryst-
increases from 9 to 25 h. The SAXRD patterns (Figures S2M− R) are characteristic of fcc supracrystals except for AuC16 nanocrystals where an amorphous film is produced for any nanocrystal size (Figures S2O,R). Most of the Au 5 C12 nanocrystals form shaped fcc supracrystal aggregates (Figure 4A,B). Note that 10% of the substrate area is covered by a
Figure 4. SEM (A, C), the corresponding HRSEM (B, D) and SAXRD (I) of Au5C12, and SEM (E, G), HRSEM (F, H), and SAXRD (J) of Au5C14 dispersed in toluene and deposited at 25 °C on a silicon substrate with a 25 h evaporation time in procedure III.
layer-by-layer supracrystal film (Figure 4C). The nanocrystals ordered in hexagonal networks exhibit some dislocations (Figure 4D). By increasing the alkyl chain length to C14, sharp, well-defined triangular supracrystals of Au5C14 nanocrystals are produced (Figure 4E). The HRSEM image and the corresponding FFT pattern (Figure 4F) clearly show that these equilateral triangular supracrystals are composed of highly ordered Au5C14 nanocrystals deposited on a bare substrate. The nanocrystals are highly ordered at the edges of the supracrystal E
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Figure 5. SAXRD patterns obtained for Au5C12 (A), Au5C14 (B), Au5C16 (C), Au7C12 (D), Au7C14 (E), and Au7C16 (F) of supracrystals produced by controlling the substrate temperature at 50 °C with an evaporation time of 5 h (procedure IV). cps stands for compacted structure.
Figure 6. SEM images obtained for Au5C12 (A, B), Au5C14 (C, D), Au5C16 (E, F), Au7C12 (G, H), Au7C14 (I, J), and Au7C16 (K, L) of supracrystals produced by controlling the substrate temperature at 50 °C with an evaporation time of 5 h (procedure IV). F
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Under these experimental conditions, it is demonstrated that the supracrystal morphologies differ markedly with the various experimental conditions used, such as the evaporation time, the volume of the chamber used to evaporate the solvent, and the substrate temperature. For a given nanocrystal size, evaporation time, and volume of the chamber used, the ability of nanocrystals to self-order in supracrystals decreases with increasing chain length of the coating agent from C12 to C16. With a rather short evaporation time (8 and 9 h), such behavior remains similar on increasing the nanocrystal size (i.e., the van der Waals interactions). However, under well-known and controlled conditions, it is possible to produce highly ordered supracrystals with coating ligands of C14 and C16. A hierarchy in the nanocrystal ordering in supracrystals takes place with a transition from disordered assemblies to a supracrystal film grown layer-by-layer and finally the supracrystals grown in solution with various well-defined shapes. We already know that mechanical properties change with the supracrystal growth mechanism.33,34 Right now we are studying the optical and mechanical properties of such assemblies differing by their level of ordering in 3D superlattices. That opens a new research area related to the hierarchy in the nanocrystal ordering in supracrystals.
als, both layer-by-layer and shaped supracrystals are produced, indicating that two-crystal growth processes take place simultaneously. Hence by increasing the substrate temperature, as in procedure IV under a nitrogen flow of the concentrated solvent molecules, the system evolves from layer-by-layer to shaped fcc supracrystals. An overview of the data presented here shows a hierarchy in the mesoscopic ordering of nanocrystals. Sequential transitions from a disordered−ordered film to shaped assemblies are observed (Table S1). This is well demonstrated with AuC16 nanocrystals with a transition from disordered (Figures 1G, 2G, and S3D,J) to ordered films (Figure 6E) to reach faceted film supracrystals (Figure 6K). With AuC14 nanocrystals, the transition is disordered with ordered films on the top (Figures 1D and 2D), ordered films (Figure 3A), well-defined equilateral (Figure 4E), and various shaped assemblies plus film (Figure 6I). With AuC12 nanocrystals, the growth of fcc supracrystals evolves from a film (Figures 1A, S1A, and 6A) to shaped (Figures 2A, S1E, 4A, and 6G) assemblies. This sequential hierarchy in supracrystal growth (disordered/ordered films and shaped supracrystals) is supported by the fwhm of the Bragg peak deduced from the diffraction patterns shown in Figure S5. Figure 7 shows that the fwhm of the Bragg peak is larger when nanocrystals form fcc films rather than shaped fcc assemblies.
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ASSOCIATED CONTENT
S Supporting Information *
TEM images and size distributions of Au nanocrystals. Absorption spectra of Au nanocrystal solutions. SEM and HRSEM images and SAXRD patterns of Au nanocrystals. Morphologies corresponding to the various experimental conditions. 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]. Notes
Figure 7. Full width at half-maximum (fwhm) of all SAXRD diffraction patterns in various procedures. Red represents Au5, and black represents Au7. Various shapes of points denote C12 (■), C14(▲), and C16 (★).
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS M.P.P.’s contribution to this research has been funded by the European Community’s Seventh Framework Program (FP7/ 2008-2011) under grant 213382. The research leading to this article has been partially supported by an advanced grant of the European Research Council under grant 267129.
From these data, it is claimed that a hierarchy in the supracrystal growth process takes place from a disordered film to layer-by-layer to shaped fcc supracrystals. The kinetic process governs such hierarchy in the nanocrystal ordering with a transition from heterogeneous (layer-by-layer) to homogeneous (shaped supracrystals) growth. Furthermore, it is demonstrated that the increase in the chain length of coating agents induces a decrease in the ability of nanocrystals to selfassemble in fcc supracrystals.
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REFERENCES
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IV. CONCLUSIONS A systematic study of fcc supracrystal formation is presented in this article. The Au nanocrystals are characterized by 5 and 7 nm average sizes with a very low size distribution (
