J. Phys. Chem. C 2007, 111, 12625-12631
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Emergence of New Collective Properties of Cobalt Nanocrystals Ordered in fcc Supracrystals: I, Structural Investigation Isabelle Lisiecki,† Caroline Salzemann,† Dinah Parker,† Pierre-Antoine Albouy,‡ and Marie-Paule Pileni*,† Laboratoire des Mate´ riaux Me´ soscopiques et Nanome´ triques, UMR CNRS 7070, UniVersite´ Pierre et Marie Curie Baˆ t F, 4 Place Jussieu, 75005 Paris, France, and Laboratoire de Physique des Solides, UMR 8502, UniVersite´ Paris-Sud, Baˆ t. 510, 91405 Orsay, France ReceiVed: March 6, 2007; In Final Form: June 18, 2007
As is now well-established, metallic nanocrystals with very low size distribution self-organize into longrange ordered three dimensional (3D) superlattices. Here, we report the annealing induced atomic phase transition of Co nanocrystals, arranged in long-range ordered fcc supracrystal 3D assemblies, from a poorly crystallized fcc structure to hcp monocrystals. We stress that this transition takes place at the atomic scale and that the fcc suprastructure of the superlattice remains unchanged. In addition, neither coalescence between nanocrystals nor oxidation of the Co material is observed. In fact, improvement in the long-range fcc order of the supracrystals is found. To our knowledge, this is the first report of the fabrication of long-range fcc supracrystals made with hcp Co nanocrystal building blocks.
I. Introduction A new class of materials has been developed in which nanocrystals are self-organized into highly ordered fcc supracrystals.1,2 In these materials, the nanocrystals are arranged in three dimensional (3D) superlattices, analogous to atoms in a crystalline solid. The first reported 3D organizations, consisting of a few layers of nanocrystals, were built of Ag2S3 and CdSe nanocrystals.4 More recently, 3D superlattices consisting of other nanomaterials have been made including CoPt3,5 FePt,6 and over a much longer range, Ag,7-9 Au,10 and Co.11 In some cases, these supracrystals show interesting collective properties, which arise from the long-range ordered periodic arrangement of nanocrystals. Supracrystals of Ag nanocrystals have shown many collective optical properties,8,12 the most significant being the observation of coherence in atomic lattice vibrations by Raman spectroscopy.13 These collective properties are not observed for disordered 3D assemblies of the same nanocrystals. Another system in which different properties are observed for ordered and disordered 3D assemblies consists of CdSe nanocrystals where the photoluminescence spectra are different in each case.14 A final example concerns collective transport properties of Ag supracrystals, which show a well-defined ohmic behavior compared with the isolated nanocrystals where a Coulomb blockade contribution behavior is observed.15 In our group, we have developed synthesis16 and deposition11 methods which allow us to build fcc supracrystals of cobalt nanocrystals. These superlattices are formed on a large scale (some tens of micrometers square with a thickness of up to 5 µm (about 500 Co layers)). By controlling the deposition conditions, we are also able to form disordered 3D assemblies with the same nanocrystals.17 It has already been shown that there is a difference in the magnetic behavior when Co nanocrystals are organized in a two dimensional (2D) hexagonal * Corresponding author. † Universite ´ Pierre et Marie Curie. ‡ Universite ´ Paris-Sud.
network compared with the isolated nanocrystals.18,19 More recently, we have reported differences in the magnetic behavior of fcc supracrystals of Co nanocrystals compared with disordered 3D assemblies of the same nanocrystals which arise from the narrow distribution of energy barriers in the supracrystal.20 From an application point of view, these systems are already interesting for potential use in magnetic data storage. For both the study of collective properties and future applications, it is desirable to use nanocrystals with a high magnetic anisotropy. This can be achieved by annealing the nanocrystals, which induces the crystallographic transition to an hcp structure. This has already been demonstrated for cobalt nanocrystals either isolated or in 2D assemblies.21 Improving the crystalline structure of Co nanocrystals in 3D self-assemblies by in situ annealing is not trivial as in many systems (e.g., FePt22 and CoPt23) this leads to an undesirable nanocrystal coalescence. Here we report, for the first time to our knowledge, the annealing of long-range ordered fcc supracrystals, with neither coalescence nor oxidation. Hence, we show that we are able to control the atomic crystalline order while retaining the fcc suprastructure. In this paper, we report a comprehensive structural investigation of these ordered 3D assemblies, and in the second part of this two part publication, the corresponding magnetic investigation will be discussed. II. Experimental Section II.1. Products. All materials were used without further purification: cobalt acetate, dodecanoic acid, and sodium borohydride are from Aldrich; iso-octane and hexane are from Fluka, and sodium di(ethylhexyl) sulfosuccinate (NaAOT) is from Sigma. The synthesis of cobalt(II) bis(2-ethylhexyl)sulfosuccinate, (Co(AOT)2) was described previously.24 II.2. Apparatus. Transmission electron microscopy (TEM) was performed using a JEOL 1011 microscope. Scanning electron microscopy (SEM) was performed with a JMS-5510LV microscope. Grazing incidence small-angle X-ray diffraction experiments (GISAXS) were performed with a rotating anode
10.1021/jp0718193 CCC: $37.00 © 2007 American Chemical Society Published on Web 08/03/2007
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Figure 1. TEM images of 7.5 nm cobalt nanocrystals ordered in a compact hexagonal monolayer: native (A), in situ annealed at 350 °C (B). Insets: contrast profiles made over the line marked 1 (top left) and electron diffraction patterns (top right). C and D, thin 3D superlattices in the native state and annealed at 350 °C respectively.
TABLE 1: Various Parameters Extracted from the Histograms (†) Made on Approximately 500 Nanocrystals and from the Contrast Profiles (*) Averaged over Three Directionsa annealing T
native
250 °C
300 °C
350 °C
D† (nm) σ† (%) D* (nm) Dc-c* (nm) Di-p* (nm)
7.5 ( 0.4 9.4 7.8 ( 0.4 10.3 ( 0.4 2.5 ( 0.4
6.8 ( 0.4 11 6.8 ( 0.4 10.2 ( 0.4 3.4 ( 0.4
7.0 ( 0.4 10 6.7 ( 0.4 10.3 ( 0.4 3.6 ( 0.4
7.0 ( 0.4 10.3 ( 0.4 3.3 ( 0.4
a D, σ, Dc-c, and Di-p are the average diameter, the size distribution, the center-to-center nanocrystal distance, and the interparticle distance, respectively.
generator operated with a small-size focus (copper anode; focus size 0.2 mm × 0.2 mm; 50 kV, 30 mA). The optics consist of two parabolic multilayer graded mirror in KB geometry followed by a bent nickel-coated mirror at right angles providing a parallel monochromatic beam. The sample is mounted on a rotation stage, and the diffraction patterns are recorded on photostimulable imaging plates. Vacuum pipes are inserted between the sample and the imaging plate so as to reduce air scattering. Magnetometry was performed using a Cryogenics S600 SQUID magnetometer. II.3. Synthesis of Cobalt Nanocrystals. The synthesis and characterization of cobalt nanocrystals coated with dodecanoic acid have been described in a previous paper.16 Reverse micelles of 5 × 10-2 M Co(AOT)2 form an isotropic phase. The amount of water added in solution is fixed to reach a water concentration defined as w ) [H2O]/[AOT] ) 32. Sodium borohydride, NaBH4, added to the micellar solution reduces the cobalt ions. Sodium borohydride content is defined as R ) [NaBH4]/[Co(AOT)2] ) 6. Immediately after NaBH4 addition, the micellar
solution color changes from pink to black, indicating the formation of colloidal cobalt nanocrystals. The nanocrystals are coated and then extracted from the surfactant. The coating process is obtained as follows: In the solution containing nanocrystals, surfactants, water, and isooctane, 0.2 M lauric acid, C11H23COOH, is added inducing a chemical bond between oxygen of dodecanoic acid and Co atoms located at the interface. The coated cobalt nanocrystals are then washed and centrifuged several times with ethanol to remove all of the AOT surfactant, and the obtained black powder is dispersed in hexane. In order to eliminate the largest nanocrystal size formed, the solution is centrifuged, and only the upper phase containing the smallest sized nanocrystals is collected. At the end of the synthesis, 7.5 nm cobalt nanocrystals coated with dodecanoic acid and characterized by a 9.4% size distribution are produced. The entire synthesis is carried out in a N2 glovebox using deoxygenated solvents to prevent particle oxidation. II.4. fcc Supracrystal Preparation. The 3D supracrystals are prepared by horizontally immersing a highly oriented pyrolitic graphite (HOPG) substrate (10 mm × 5 mm) in 200 µL of a 5.5 × 10-7 M colloidal solution of cobalt nanocrystals dispersed in hexane. The solvent evaporation takes place at room temperature under nitrogen. This is carried out in an almost completely isolated system saturated with hexane such that evaporation takes in total approximately 72 h. In order to avoid oxidation of the cobalt nanoparticles, all of the samples are stored under nitrogen. We estimate that the total exposure time to air for each sample (e.g., during transport, GISAXS measurements, etc.) is approximately 2 h. II.5. Annealing Process. After deposition, the 3D assemblies are placed in a closed quartz ampule with a nitrogen atmosphere
Colbalt Nanocrystals Ordered in fcc Supracrystals
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Figure 2. SEM image of a native supra-crystal of 7.5 nm Co nanocrystals: (A) A thicker region. Inset top right: high magnification images; (B) a thinner region. Inset left: low magnification image showing both thin and thick regions.
and are annealed in a furnace at various temperatures (250, 300, and 350 °C) for 15 min. II.6. Profiles and Histograms. The histograms of the nanocrystals are obtained by measuring their diameter Di from at least 500 nanocrystals. The standard deviation, σ, is calculated from the experimentally determined distribution using the formula
σ ) {[
∑ ni(Di - D)2]/[n - 1]}1/2
where n corresponds to the number of measured particles and D corresponds to the average diameter. III. Results and Discussion The average diameter and size distribution of the cobalt nanocrystals in the native state and after annealing are determined by measuring around 500 particles imaged by TEM (see Figure 1A,B and Table 1). The nanocrystals are deposited on TEM grids covered with a thin layer of HOPG which deteriorates slightly after annealing at temperatures above 250 °C. This deterioration of the film increases the error in the particle size determination. The slight decrease in nanocrystal diameter measured after annealing is coherent with values found from the contrast profile, taken along a line of some tens of nanocrystals (see lines marked in Figure 1A,B and Table 1). The profile technique can be used because, at all annealing temperatures investigated, the nanocrystals are still selforganized in a compact hexagonal network without any coalescence (Figure 1B). There is an increase in interparticle gap (Di-p) after annealing while the center-to-center distance (Dc-c) remains unchanged. This decrease in the nanocrystal size could
be explained by the structural transition of the nanocrystals from a poorly crystallized fcc structure to hcp monocrystals. In coexistence with the 2D self-organizations on the HOPG TEM grid, regions of thin 3D organizations characterized by an area of a few micrometers square and a few layers of cobalt nanocrystals (5 maximum) are produced (Figure 1C). Figure 1D shows that the 3D nanocrystal order is well maintained after annealing at 350 °C without any observable coalescence. The high stability of these native and annealed organizations against coalescence and oxidation can be related to the organic coating (dodecanoic acid) surrounding the nanocrystals. This coating, attached to the metallic nanocrystal by a covalent bond between the Co surface and the O atom of the acid group, plays a key role in maintaining the integrity of the mesoscopic organization and consequently prevents coalescence. The electron diffraction pattern of native nanoparticles shows three diffuse diffraction rings indicating a very low crystallinity due to a few small fcc domains (see right inset, Figure 1A). After annealing at 350 °C, we observe all of the reflections corresponding to a pure hcp structure with both the first and the second-order reflections clearly visible (see right inset, Figure 1B). Eight rings characterized by 0.217, 0.203, 0.192, 0.145, 0.125, 0.116, 0.106, and 0.104 nm distances correspond respectively to the (100), (002), (101), (102), (110), (103), (112), and (201) planes of the Co hcp phase. Hence, from the electronic diffraction patterns a crystallographic transition from poorly crystallized fcc (nearly amorphous) nanoparticles to hcp nanocrystals is observed. No Co phase25,26 can be detected contrary to many other reports where the nanocrystals are synthesized via chemical methods. A few other groups have obtained hcp nanorods27 as well as hcp/fcc mixed phase nanocrystals.28 This
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Figure 3. SEM image of a supra-crystal of 7.5 nm Co nanocrystals, in situ annealed at 350 °C: (A) a thicker region. Inset top right: high magnification images; (B) a thinner region. Note that the annealed images correspond to the same regions presented in the native images (Figure 2).
shows that the nanocrystal structure is strongly related to the synthetic route, that is, the additive in the reactional medium, and cannot be easily predicted. Figure 2 shows the supracrystals obtained by deposition on an HOPG substrate. As shown in the low magnification SEM image in the left inset, the coverage is not homogeneous as most of the material is concentrated in aggregates (up to 1 mm2), which are located largely in the center of the substrate. Thicker regions, found in the center of the larger aggregates, show a deep cracked topology. The maximum thickness of the supracrystal blocks is found to be approximately 3-4 µm, which corresponds to 300-400 cobalt nanocrystal layers. The higher magnification SEM image (Figure 2A) shows that the block surface appears smooth and that the distance between blocks varies from 1 to 5 µm. Thinner regions (shown in Figure 2B) less than approximately 0.5 µm thick are also observed either at the periphery of the larger aggregates or in small aggregates. We see that, unlike the thicker film, the surface is largely free of cracks and has a terraced structure. On annealing the sample to 350 °C, formation of cracks (Figure 3B) is observed. In the region of thicker film, the SEM images (Figure 2A) do not show drastic changes. However, the higher magnification images (insets, Figures 2A and 3A) reveal qualitatively that the existing cracks are wider after annealing (see arrows). This is accompanied by a decrease in the block surface area. The fact that no cobalt oxide nanoparticles are present is confirmed by magnetization versus temperature measurements. Figure 4A shows the zero field cooled (ZFC) magnetization versus temperature curves for the native (dashed lines) and annealed (full lines) supracrystal assemblies. The absence of a lowtemperature peak in the ZFC curves indicates that the cobalt
nanocrystals are not severely oxidized. It has been shown that more than 1 nm of CoO on the surface of Co particles of similar size to those presented here gives rise to a peak at around 8 K.29 We cannot rule out a thin (
