FEATURE ARTICLE pubs.acs.org/JPCC
Ordering at Various Scales: Magnetic Nanocrystals Isabelle Lisiecki and Marie-Paule Pileni* Laboratoire LM2N, Universite Pierre et Marie Curie (Paris VI), BP 52, 4 Place Jussieu, F - 75231 Paris Cedex 05, France ABSTRACT: Here, it is shown that the internal crystallinity called nanocrystallinity of rather uniform Co nanoparticles can be improved by annealing. This induces marked changes in the magnetic properties such as an increase in the blocking temperature that can reach a value close to room temperature. It is shown that the acoustic breathing modes remain quite unchanged by changing the nanocrystallinity. Co nanocrystals with low size distribution are able to self-assemble either in fcc colloidal crystals called supracrystals or in films with voids. Collective intrinsic properties (chemical and physical) due to magnetic nanocrystal ordering in 2D and 3D superlattices are presented. Furthermore, when the nanocrystals are aligned, the magnetic properties of the assemblies are improved. By using magnetostatic bacteria, it is demonstrated that the magnetic anisotropy is mainly due to induced dipolar interactions with a low contribution of the influence of the orientation of the nanocrystal easy axes.
I. INTRODUCTION The control of the crystalline structure at the nanometer and micrometer scale is one of the most fundamental challenges in condensed matter science.14 One of the fundamental concepts regarding the internal crystallinity of nanomaterials, called nanocrystallinity, refers to the atom ordering in nanocrystals. The nanocrystallinity influence on the chemical and physical properties of metal nanocrystals suffers from an extensive lack of knowledge,5 whereas that of supracrystallinity still remains to be investigated. Recently, controversies have emerged concerning the influence of nanocrystallinity on both acoustic vibrations68 and mechanical6,9 properties of metal nanocrystals. Whereas the low sensitivity of the breathing mode frequency of the nanocrystallinity of silver nanoparticles is predicted from calculations,7,8 a significant dependence has been observed by time-resolved pumpprobe spectroscopy.6 Coated nanocrystals are used as building blocks to grow 2D and 3D close-packed arrays called supracrystals. Another fundamental concept regards the crystallinity at the micrometer scale called supracrystallinity. In the past, many groups have succeeded in controlling the supracrystallinity by using various coating agents1119 but this was never obtained from the same nanocrystal size and coating agent. For the first time,20 from the same batch of nanocrystals (i.e., same size and coating), formation of hcp and fcc supracrystals at a fixed pressure and various temperatures for Ag nanocrystals coated with decanethiol was observed. By replacing decanethiol by dodecanethiol, disordered aggregates, hcp and bcc supracrystals are produced by changing the substrate temperature. These supracrystals are in thermodynamic equilibrium states. There is increasing interest in this new class of “artificial solids” with tunable electronic, magnetic, and optical properties2126 in which the nanocrystals take the place of atoms in traditional solids.10,22 During the past few years, the influence of the ordering has been pointed out by comparing r 2011 American Chemical Society
the physical properties of single batches of nanocrystals packed either in disordered 3D assemblies or ordered in fcc supracrystals. Vibrational coherence in supracrystals27 was first established by low-frequency Raman spectroscopy (LFRS) measurements for fcc supracrystals of 5-nm Ag nanocrystals in good correlation with theoretical arguments.28 This vibrational coherence is evidenced by a decrease in the quadrupolar mode bandwidth. The existence of propagative vibrations in supracrystals has also been deduced from time-resolved pump probe (TRPP) measurements. Long-time scale differential reflectivity dynamics show large oscillations for the supracrystal, whereas in the disordered sample, a monotonous decay is observed.29 This was interpreted as the existence of propagative vibrational modes. The photoluminescence properties of CdSe nanocrystals differ according to whether they are ordered or disordered in a 3D assembly.30 In an fcc supracrystal composed of 7.5-nm Co polycrystals, a lower distribution of interaction energies, an inhibition of the flipping of the superspins, and a slower approach to magnetic saturation are observed compared to the disordered aggregates.31 The electron transport properties seem to be controlled by the degree of ordering in the nanocrystal assemblies.32,33 Here, we concentrate on the synthesis, ordering, and specific properties of magnetic nanocrystals either isolated or self-assembled in fcc supracrystals with formation of mesosocopic films.
II. CRYSTALLINITY OF NANOCRYSTALS CALLED NANOCRYSTALLINITY To self-organize nanocrystals, one of the major parameters is the size uniformity of nanocrystals. It has been observed that during the chemical synthesis, the use of a large amount of reducing agent favors a low size distribution of the nanocrystals Received: September 5, 2011 Revised: November 8, 2011 Published: November 17, 2011 3
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(Figure 3D), all of the hcp-Co phase reflections up to the second order are observed, i.e., the (100), (002), (101), (102), (110), (103), (112), and (201) reflections, indicating a drastic structural transition compared to the native state (Figure 1B). The HRTEM study mostly shows very large domain polycrystals (composed of ordered domains larger than 1 nm). When these domains are large enough, the planes can be characterized by a 2.00 Å spacing consistent with the hcp phase. This population coexists with some hcp single-crystals with and without stacking faults and other defects. At 286 (Figure 1F) and 316 C (Figure 1H), the electron diffraction signatures appear very similar to that obtained at 220 C, but an accurate image treatment allows us to conclude that the Co hcp rings become slightly sharper indicating an increase in the coherence length of the hcp nanocrystals. This behavior is well confirmed by the HRTEM images showing, at 286 C, the disappearance of the polycrystals in favor of hcp-Co single-crystals with (Figure 1E insets 1 and 3) and without defects (Figure 1G inset 1). As is well illustrated by the complicated power spectrum, defects induce splitting of the reflections (Figure 1E insets 2 and 4) obtained in their absence (Figure 1G inset 2). Annealing at 316 C results in the annihilation of the defects in the hcp nanocrystals (Figure 1G insets 1 and 2). This structural investigation shows that a solution-phase annealing occurring at various temperatures up to 316 C yields a progressive structural change of Co nanocrystals from, probably, fcc polycrystals to hcp single-crystals that remain highly stable with respect to coalescence and oxidation.35 By annealing a 2D array of Co nanocrystals at 350 C, the nanocrystallinity is drastically improved. Figure 2A and inset show that the average diameter remains the same (within the accuracy limit of the measurements) as those of the native nanocrystals obtained at R = 6, i.e., 7.5 ( 0.4 nm. The nanocrystal ordering also remains unchanged and no coalescence is detected. The HRTEM image of a single particle (Figure 2B) shows regular lattice planes with 2.00 Å spacing (the characteristic distance corresponding to the 002 plane of hcp Co). The electron diffraction pattern (Figure 2C) taken from a population of several hundred nanocrystals shows rings corresponding to pure hcp Co without the signature of cobalt oxide. These results clearly indicate that a dry annealing process at 350 C induces a drastic structural transition of Co nanomaterial from fcc polycrystals to hcp single-crystals.36 A general feature characterizing single magnetic nanocrystals is their superparamagnetic behavior. In the superparamagnetic regime, the magnetic anisotropy energy barrier is higher than the thermal energy. Then, the magnetization vector fluctuates among the easy directions of magnetization. In an interacting particle assembly, the energy barriers of these nanocrystals (Eb) take into account both the anisotropy energy, Ea, and the dipole interaction energy, Edd. When the sample is cooled in zero field, to 3 K, there is no net alignment of the spins, and the resulting magnetization is close to zero. As a magnetic field is applied (20 Oe) and the temperature is increased, the spins become progressively “unblocked”, aligning toward the field direction, and the magnetization increases until reaching a maximum which is defined as the blocking temperature, TB. Above TB, the thermal energy of the spins prevents alignment in the field direction and the magnetization decreases with increasing temperature. When an external field is applied to an assembly of nanocrystals, the spins align themselves with it. Its magnetization increases and reaches a maximum value called the saturation magnetization, Ms. As the magnitude of the magnetic field decreases, spins cease to be
Figure 1. TEM images (A, C, E, and G) and electron diffraction patterns (B, D, F, and H) of cobalt nanocrystals deposited on an amorphous carbon grid in the native state (A and B), solution-phase annealed at 220 (C and D), 286 (E and F), and 316 C (G and H). Reflections corresponding to the hcp (*) and fcc (+) structures. Insets: HRTEM images of some typical crystallographic structures of Co nanoparticles and their power spectra.
that consequently order on a rather large scale.34 Here, we demonstrate that it is possible to control the nanocrystallinity by two different ways. The first is a phase-solution process: the nanocrystals are dispersed in a suitable solvent having a boiling point high enough to be able to reach the desired annealing temperature. The second is a dry process: The freshly prepared nanocrystals are deposited on a substrate like amorphous carbon or highly ordered pyrolitic graphite (HOPG) and annealed at various temperatures. The annealing of Co nanocrystals is performed in various solvents with different boiling points (bp). To anneal the nanocrystals at 220 C, paraffin (bp = 220 C) is used, whereas octylether (bp = 286.17 C) and trioctylamine (bp = 316.15 C) are used to anneal the nanocrystals at 286 and 316 C. Once the desired temperature is reached, the solution is held for 30 min at this temperature before allowing it to cool progressively inside the flask. Figure 1, panels C, E, and G, shows that, whatever the annealing temperature is, the nanocrystals keep their integrity without aggregation and/or coalescence. The average diameter and size distribution of the annealed nanocrystals have the same order of magnitude as the native nanocrystals, i.e., 7 ( 0.2 nm (Figure 1A). When annealing the colloidal solution at 220 C 4
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Figure 2. (A) TEM image, (B) HRTEM image, and (C) electron diffraction pattern of Co hcp single-crystals obtained after dry annealing hexagonally ordered Co polycrystals. (D) Magnetization versus field curves at 5 K of a supracrystalline sample, native (dashed line) and annealed at 350 C (solid line). (E) ZFC/FC magnetization versus T/TB curves for the supracrystalline samples, native and, in the insert, annealed at 350 C. (F) ZFC magnetization versus T/TB curves for the supracrystalline sample, native (dashed line) and annealed at 350 C (solid line).
aligned with the field, and the total magnetization decreases. The magnetization versus field curves of 3D assemblies of Co nanocrystals in their native state and annealed at 350 C ,and deposited on HOPG substrate, are shown in Figure 2D. The increase in saturation magnetization observed on the hysteresis curve is attributed to the crystallographic transition of the nanocrystals from a polycrystalline phase to a single-crystalline phase. The ZFC magnetizations versus temperature curves show that annealing the sample at 350 C induces a drastic increase in TB, from 100 to 280 K, (Figure 2E). This confirms the crystallographic transition from Co polycrystals to single (hcp) nanocrystals. This leads to an increase in the anisotropy of the nanocrystals, which in turn gives an increase in the energy barriers and hence in TB. As shown in Figure 2F, the ZFC curves, normalized to TB of the samples in the native and annealed states, superimpose. This result shows that, after annealing at 350 C, the conversion to a single-domain hcp structure is complete and no coalescence between nanocrystals occurs. The absence of a peak at around 8 K in the ZFC curves confirms the absence of oxidation of the metallic nanomaterial before and after the annealing treatment. Additionally, these results show that the ZFC measurements could be used as a powerful tool to probe the particle crystallinity.37 Controversies have emerged concerning the effect of nanocrystallinity on acoustic vibrational properties. Acoustic breathing modes are impulsively excited by the rapid heating of the particle lattice that occurs after ultrafast laser excitation. This excitation mechanism is a two step process; the pump laser deposits energy into the electron distribution, and this energy is subsequently transferred to the lattice via electronphonon coupling. From calculations,7,8 it is predicted that the breathing mode frequency of nanocrystals does not depend on the arrangement of atoms in the nanocrystals, i.e., on the nanocrystallinity, whereas a significant dependence has been observed by time-resolved pump probe spectroscopy.6 To solve such controversy, we performed a
Figure 3. Short-time scale differential reflectivity dynamics of Co polycrystals (A) and hcp-Co single-crystals (B).
comparative femtosecond pumpprobe spectroscopy study of Co polycrystals and Co single (hcp) nanocrystals whose diameter and size distribution (7.3 nm and 10%, respectively) are the same (within the accuracy limit of the measurements). For any nanocrystallinity and the probe wavelength, the 2D maps related to the variation of differential reflectivity (ΔR/R) signal amplitude as a function of both probe delay and wavelength reveal similar and clear oscillations, which are visible for about five periods.39 Their damping is consistent with the low nanocrystal size distribution. The time-resolved fractions of the oscillating component of ΔR/R signals extracted from the 2D maps at 5
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Figure 4. SEM images of void structures obtained on Si substrate by evaporation of 200 μL of a 5 103 M concentrated solution (A), 400 μL of a 5 103 M concentrated solution (B), and 200 μL of a 102 M concentrated solution (C). SEM images of void structures obtained on HOPG substrate by evaporation of 200 μL of a 102 M concentrated solution (D). Snapshots of particle configurations of the confined Stockmayer fluid at T = 1.25, μ = 2.0, and L = 10: (E) F = 0.4, H = 30; (F) F = 0.42, H = 30; (G) F = 0.45, H = 30; (H) F = 0.5, H = 30; N = 3000 except for panels FH N = 2592.
three different selected probe wavelengths, i.e., 470, 500, and 540 nm, are shown in Figure 3, panels A and B. By averaging the results obtained at several probe wavelengths and for a number different positions on the sample, we obtain ν[poly] = 600 ( 8 GHz and ν[single] = 586 ( 10 GHz for the polycrystal- and single (hcp) nanocrystals, respectively. These frequencies are of the same order of magnitude as the ratio of the sound velocity to the dimension of the studied Co nanocrystals. So, it makes sense to assign the observed oscillations to the modulation of the signal by acoustic vibrations confined in the nanocrystals. To ascertain such assignment and allow for the proper identification of the vibrational modes observed through our pump probe measurements, it is interesting to compare the measured frequencies with those calculated using the RUS method, as reported in ref 7. The frequency of the fundamental radial (breathing) mode of 7.3 nm Co nanocrystals with either polyor single nanocrystal as being those calculated for Co nanospheres having isotropic and anisotropic elasticity (hexagonal crystallinity), i.e., ν[poly] = 692 GHz and ν[single] = 682 GHz, respectively. These two frequencies are found to be very close from one to the other, thus confirming the experimental observation of the low sensitivity of the breathing mode frequency in Co nanocrystals upon change in their crystallinity.38 Nevertheless, the theoretical frequencies overestimate by nearly 15% the value obtained from our measurements. The mismatch between the measured and calculated frequencies cannot be attributed to oxidation of Co nanocrystals. This is supported by the magnetic properties37 and by the frequency measurement reproducibility from one sample to the other. One possible contribution to the discrepancy between theory and calculations is related to the use of bulk elastic constants in this model, as these constants are probably different from that of the material confined at the nanoscale. The change in the physical parameters of metal (Young’s modulus, dielectric constant, stiffness, etc.) resulting from such confinement compared to the bulk ones remains an open question.6,8,9,39 It should however be noted that the slight decrease by ∼1.5% of the breathing mode frequency
predicted using RUS calculation from poly- to single crystalline Co NCs well agrees with the experimentally observed decrease, which is found to be ∼2.3%. This result provides therefore additional support to interpret the slight change in breathing mode frequency between poly- and single crystalline Co NCs as being due to the influence of crystallinity on their vibrational dynamics.
II. MESOSCOPIC FILMS OF CO NANOCRYSTALS: SUPRACRYSTALS OR VOID FILMS Magnetic nanocrystals, characterized by low size distribution, are coated with dodecanoic acid and dispersed in nonpolar solvent. By slow solvent evaporation of nanocrystals characterized by a very low size distribution, the nanocrystals are able to longrange self-organize in a compact hexagonal network. Periodic arrangement of nanocrystals in 3D superlattices with several hundreds of layers of organized nanocrystals takes place and forms colloidal crystals called supracrystals40 with the X-ray diffraction spots of a fcc structure. It is possible, with the same batch of nanocrystals to tune the nanocrystal ordering from colloidal crystals called supracrystals to disordered aggregates. This is obtained by controlling the substrate temperature during the deposition process of nanocrystals on the substrate. At low temperature, the deposition gives rise to the formation of a nonhomogeneous thin film coexisting with aggregates. The X-ray diffraction pattern has a broad diffuse ring attributed to a disordered material. By applying a magnetic field perpendicular to the substrate during the solvent evaporation process dots, labyrinths and voids are produced. Hence, a substrate as highly oriented pyrolitic graphite, HOPG, or a silicon wafer is immersed in a solution of Co nanocrystals dispersed in hexane and a magnetic field (0.4T) perpendicular to the substrate is applied during a very slow solvent evaporation. Different morphologies of 3D mesostructures, determined by thermodynamics (slow evaporation), are produced. Void structures are observed only in a very narrow range of concentrations (5 103 and 102 M), while under other conditions close films or columns were observed. Here we 6
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expected for Φ > 0.5 to minimize the surface energy, whereas labyrinths and columns should form for Φ = 0.5 and Φ < 0.5, respectively. If, in addition, we consider the repulsive dipolar interactions, a labyrinthine phase appears even at Φ > 0.5 in this approximate treatment. Therefore, to quantitatively predict the range of the labyrinthine and void phases as a function of volume fraction and field, simulations have to be carried out taking the entropy directly into account. Monte Carlo (MC) simulations44 is performed in the density range F = 0.40.6 for magnetic fields H = 530 in reduced units: Simulations are started by either inserting the particles randomly in the simulation box or by starting from an initial lattice configuration with random orientations of the dipole moments. The magnetic field is applied instantaneously. Typical simulation runs involved 1.8 106 cycles after equilibration of the system, a cycle corresponding to translation and rotation of the N particles. For notational convenience, stars in the reduced quantities will be omitted in the remainder of the paper. The MC results obtained for F = 0.40.6 can be combined with our previous MC results19 covering the density range F = 0.10.4. Thus, a full sequence of microstructures is found: they range from cylindrical columns arranged in a near hexagonal array to vertical sheet-like labyrinths and finally to hole (void) structures. In absence of the attractive van der Waals term, any structures as columns, labyrinths or voids are obtained by Monte Carlo simulations. Instead, at low density (F e 0.2) and high dipole moment (μ ≈ 3) isolated, rigid chains form in the direction of the field. However for denser systems (F g 0.5) and weaker dipole moments (μ ≈ 2) the system appears spatially disordered. Fixing the value of H at 30 (corresponding to 0.545 T) simulations are performed at various densities. At low density cylindrical columns and vertical sheetlike labyrinths are observed. At 0.4 < F < 0.55 void structures are obtained. Figure 4, panels E and G, shows how the cylindrical structure progressively evolves from an elongated to a more circular shape on increasing the density from 0.4 to 0.5: Elongated holes start to appear at F ≈ 0.4 for H g 5 (i.e., H e 0.09 T; Figure 4E). With increasing density these elongated holes become smaller and for densities F>0.45 (circular) cylindrical void structures arranged in a somewhat distorted hexagonal array are obtained (Figure 4, panels G and H). The diameter of the void cylinders decreases as a function of the density from F = 0.45 (Figure 4G) to F = 0.50 (Figure 4H). This type of organization corresponds to the inverted hexagonal structure predicted by free energy approaches.44 Quite remarkably, these hole structures are by no means static but migrate through the system on a rather rapid ’time’ scale (typically 5 105 cycles) during the MC process. To make a more quantitative comparison of the conditions for onset of the different morphologies with experiment, we calculated the volume fraction Φ, defined as the ratio of the volumes occupied by the aggregates and the simulation box. The analysis is restricted to fields 20 < H < 30 (corresponding to 0.36 < H < 0.545 T) as in the experiments. Within the statistical error, results for Φ turn out not to depend appreciably on H for H = 1030. At F = 0.45 one finds Φ = 0.83 ( 0.03. Simulations thus predict the formation of void cylinders at a volume fraction larger than 0.83 in fair agreement with experiment where these structures are observed at Φ = 0.91 ( 0.01. Elliptical holes are obtained in the simulations at densities between 0.4 and 0.45, i.e., for volume fractions between Φ = 0.70 ( 0.03 and 0.83 ( 0.03, which also well fits the experimental value of 0.73 ( 0.03. To conclude, void structures in Co nanocrystal films are here observed for the first
concentrate on void formation and consequently two different amounts of Co nanocrystals in solution are considered (4.66 10 11 and 9.32 1011 mol). During the evaporation, the volume fraction increases gradually. At the end of the evaporation the system exhibits a colloidal gasliquid transition.42 This leads to the coexistence of a concentrated and diluted phase of Co nanocrystals. The concentrated phase forms structures while the diluted one corresponds to the voids within this structure. The volume fraction, Φ, corresponds to the overall fraction of the volume occupied by the concentrated phase to the total volume occupied by both phases. Please note that this is not the volume fraction of the nanocrystals in the total volume. We assume that (i) the structure obtained after evaporation using SEM can be used experimentally to determine Φ and (ii) the void structures have the same thickness as the cobalt film, Φ, is given by (S s)/S where S is the surface area of the substrate selected on the SEM image and s the total area of the void regions. Figure 4A shows the SEM images of the mesostructures obtained on a Si substrate immersed in 200 μL of a 5 103 M colloidal solution. When the total amount of Co nanocrystals is n = 4.66 1011 mol, circular cylindrical void mesostructures forming a distorted hexagonal network are observed. The average diameter of such circular bases is 0.33 ( 0.01 μm and Φ = 0.91 ( 0.01. With n = 9.32 1011 mol, a cylindrical structure with an elliptic-like basis having an aspect ratio around 3.4 and Φ= 0.73 ( 0.02 is observed (Figure 4B). The void structures appear to locally orient in the same direction. Keeping the same amount of Co nanocrystals (n = 9.32 1011 mol) and using a higher concentration of colloidal solution (102 M), i.e., a smaller immersion volume (200 μL), panel C shows that cylindrical voids with a circular base are again observed. These results can be rationalized by observing that Φ is the relevant parameter for mesostructure morphologies rather than the total amount, n, of cobalt nanocrystals. In fact, during evaporation, nanocrystals can deposit along the beaker walls that depends on the volume of solution. The volume fraction Φ evaluated in the different experiments shows that cylindrical void (circular base) mesostructures correspond to Φ = 0.91 ( 0.01 to form these mesostructures. When the void morphology evolves from cylindrical void to elliptic cylindrical void mesostructures, Φ decreases from 0.91 to 0.73 ( 0.02. At a still lower volume fraction (Φ ≈ 0.7), the elliptic cylindrical mesostructures become more elongated leading to the inversed labyrinthine structures observed on HOPG. Coexistence of both cylindrical voids and inverted labyrinthine structures on HOPG is understood by the different amounts of Co nanocrystals deposited at different regions of the HOPG substrate. This is explained by irregularities of the HOPG substrate at a scale of 10 μm, which are not observed in the case of the silicon substrate. The physical origin of the structure lies in the competition between short-range attractive forces and long-range dipolar repulsion between the nanocrystals. The short-range attraction leads to a colloidal gas liquid transition42 during the solvent evaporation, which induces an assembly of the particles. The formation of modulated pattern can be qualitatively understood by the reduction of the demagnetization energy due to the void in the magnetized film. The appearance of spherical voids corresponds to a minimization of the interfacial energy. A free energy approach taking into account the attractive surface and the repulsive dipolar energies of the structures shows that the void structures are the most stable pattern at large volume fractions.43 If we only take into account the attractive surface term, the appearance of void structure is 7
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Figure 5. Sketch of the phase behavior of the confined Stockmayer system predicted by Monte Carlo simulation as a function of density and field strength. The symbols represent the different simulated state points considered: filled circles: columnar phase; diamonds: coexistence of columns and sheets; squares: sheets (labyrinths); open circles: void structures; crosses: homogeneous film. The state points for F < 0.4 are from the reference Lacoste, D.; Lubensky, T. C. Phys. Rev. E 2001, 64, 041506.
Figure 6. (A) TEM image of Cohcp/CoO core/shell nanocrystals produced after air exposure of an ordered array of hcp-Co single-crystals. (B) Electron diffraction pattern. (C) HRTEM image. (D) TEM image of almost fully oxidized CoO nanocrystals produced after air exposure of isolated and/or disordered hcp-Co single-crystals. Inset: HRTEM image.
time due to the use of stable concentrated colloidal solutions of cobalt nanocrystals. These structures are characterized by a higher volume fraction than 0.7 of Co nanoparticles deposited on the substrate in excellent agreement with the values predicted by our MC simulations. Both experimental and theoretical results show that the volume fraction (and not the initial conditions of the experiment) is the key factor to explain the appearance of void structures. In addition, a change in Φ changes the morphology of the voids. Finally, one can note that in recent computer simulations inverse phases (reverse micelles, voids) have been shown to arise even in very simple systems (spherically symmetrical potentials) from competition in the length scales of attractive and repulsive interactions between the particles45 and even more surprising, in purely repulsive systems from competition between hard-core and soft-shoulder length scales.46 This shows that the kind of void structures observed here experimentally for the first time can be expected in a large variety of other systems. Extended simulations at various field and density values allows sketching the different phases obtained from the MC simulations of the confined Stockmayer fluid as a function of density and field strength in Figure 5.
In some cases, the orientation of the nanocrystal is such that lattice planes can also be observed in part of the shell, where the characteristic distance of the lattice planes is 2.46 Å and corresponds to the (111) planes of CoO. These data clearly demonstrate the formation of core/shell nanocrystals composed of CoO as the shell and hcp Co as the core. The increase in the average diameter of the core/shell nanocrystals (∼1.5 nm) is due to the change in the density of the CoO material compared to the hcp metallic Co. A remarkable enhancement of hcp Co nanocrystals resistance to oxidation due to their ordering is evidenced through a comparative study of the oxidation process between nanocrystals close packed in a 2D array and their neighbors either disordered or isolated on the substrate. In the former case, hcp Co nanocrystals transform into Cohcp/CoO coreshell structures, keeping their integrity within the 2D arrays (Figure 6, panels A and C), whereas in the latter case, the same nanocrystals tend to fully oxidize finally coalesce (Figure 8D and inset). Hence, hcp Co nanocrystals, self-ordered in a compact hexagonal network, are more resistant to oxidation than the same nanocrystals disordered and/or isolated on the substrate. This enhanced stability observed in the 2D ordered arrays could be related to a decrease in the permeability to O2 molecules of the dodecanoic acid chains, which limits the oxygen diffusion toward the metallic core and consequently to the confinement of the coating chains at the nanocrystal surface and in the space between the nanocrystals. Mechanical Properties in 3D Superlattices48. Above it is shown that at high volume fraction corresponding to the overall fraction of the volume occupied by the concentrated phase to the total volume occupied in the total volume voids in mesoscopic films of Co nanocrystals are produced. At low volume fraction cylinders and labyrinths are predicted as shown in Figure 5. Here we concentrate on the intrinsic mechanical properties due to the nanocrystal ordering in supracrystals. By applying a magnetic field perpendicular to the substrate during the evaporation (process) of 5.7-nm Co nanocrystals dispersed in hexane and characterized by a very low size distribution (13%), mainly dots (columns) fallen (Figure 7A inset 1) or upright (Figure 7A inset 2) are produced at the end of the evaporation (Figure 7A). Conversely, when the nanocrystal size distribution is rather large
III. COLLECTIVE AND INTRINSIC PROPERTIES DUE TO COBALT NANOCRYSTAL ORDERING Oxidation of Co Nanocrystals47. After exposure to air for a few hours, of the annealed (350 C) Co sample described above, the TEM image of a 2D array (Figure 6A and inset) shows that Co nanocrystals are characterized by a fairly well contrasted core/shell structure. The average shell thickness and core diameter are ∼5 nm and ∼2 nm respectively, which in turn give a total average particle diameter equal to ∼9 nm, larger than that observed for nonoxidized Co hcp nanocrystals (7.5 nm). The corresponding electron diffraction pattern (Figure 6B) shows both the signature of hcp Co and cubic CoO through the ring at 2.46 Å typical of the (111) planes of CoO. The HRTEM image (Figure 6C) shows a single core/shell particle in a hexagonal network. The core shows regular lattice planes with a typical distance of 2.00 Å, corresponding to the (002) planes of hcp Co. 8
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Figure 8. (A) ZFC magnetization versus T/TB curves for a supracrystal (red) and a disordered 3D assembly of Co nanocrystals (black). (B) Magnetization versus field curves at 5 K, normalized to MS of a supracrystal (red) and disordered 3D assembly (black). Inset: Magnification of the low field region.
keeping the same average nanocrystal size. The mechanical property of the column formation controls the final patterns. Magnetic Properties31. By controlling the substrate temperature during the solvent evaporation, it is possible to control the degree of ordering of the 3D assemblies of Co nanocrystals. Hence, at high temperatures, (18 < T < 45 C), we form longrange fcc supracrystals (ordered sample), whereas at lower temperatures, (5 < T < 12 C), the 3D assemblies, made with the same batch of nanocrystals, do not present any long-range ordering (disordered sample). For both samples, TB is found to be around 100 K, indicative of strong interparticle interactions. Figure 8A shows the ZFC and FC curves normalized to the blocking temperature, TB, of both the ordered (red) and disordered (black) 3D assemblies composed of 7-nm cobalt polycrystals.31 The ZFC peak is significantly narrower for the ordered sample. The width of the ZFC peak is related to the distribution of energy barriers, Eb, in the system: a broader distribution gives a broader peak. As explained above, the barrier energy is the sum of the anisotropy energy (Ea = kaV) and the dipoledipole interaction energy, Edd, that varies with nanocrystal distance. Because the ordered and disordered samples are made with the same batch of nanocrystals, this change in the ZFC peak width cannot be attributed to a change in the nanocrystal volume distribution or in the anisotropy. We therefore explain the difference in the distribution of Eb by the change in the structural environment of the Co nanocrystals. As pointed out, dipolar forces have a strong directional dependence and, consequently, dipolar interactions in the assembly should be sensitive to the detailed geometrical arrangement of the nanocrystals. The supracrystals are characterized by an fcc ordering with a long coherence length and therefore their geometric environment is fairly uniform. In the disordered 3D assembly, we have a mixture of many fcc domains characterized by a short coherence length coexisting with amorphous domains. Therefore, the distribution
Figure 7. SEM patterns of mesostructures of 5.7-nm cobalt nanocrystals having 13% (A) and 18% (D) size distributions. Insets 1 and 2 are TEM images obtained at a higher magnification.
(18%), keeping a similar average diameter (5.9 nm), the formation of a large number of flower-like entities due to coalescence of either upright or fallen columns forming worm-like and labyrinthine structures are produced (Figure 7B and insets 1 and 2). The major difference between these structures is due to the size distribution of nanocrystals. The proposed mechanism of pattern formation is the following: during the evaporation, a liquidgas phase transition42 occurs with formation of a concentrated solution of nanocrystals in equilibrium with a diluted one. In the concentrated phase, columns (dots) are progressively formed and tend to migrate to self-organize in hexagonal patterns. When the size distribution is low enough, the nanocrystals dispersed in solution tend to self-assemble in fcc supracrystals with formation of well-defined and compact columns,whereas for higher size distribution values, the interactions between particles markedly decrease and the columns are formed with disordered entities. This creates defects and the cohesive forces between columns are not large enough to keep them ordered and the columns tend to fuse to form labyrinths. This result is in good agreement with that obtained by Ashby et al.,49 who found an increased hardness when 7-nm lead sulfide nanocrystals coated with oleic acid ligands are close packed. Hence, the mesoscopic structure of Co nanocrystals is tuned from well-defined dots to labyrinths with increasing the nanocrystal size distribution from 13% to 18% 9
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of Edd (and hence Eb) in the supracrystal is expected to be lower than in the disordered sample, leading to the observed narrowing of the ZFC peak. We acknowledge that this effect of order is fairly subtle, however, we have found that it is highly reproducible. Figure 8B shows the magnetization versus field curves for the ordered and disordered samples at 5 K. In both cases, saturation is reached at around 1 T and hysteresis is observed. For the ordered sample, we find that the coercivity, Hc is larger than for the disordered sample (Figure 8B, inset) and that the latter saturates at a slightly lower field. This is consistent with a more collective behavior in the supracrystal compared to the disordered sample. It is reasonable to imagine that the flipping of the spins could require a higher field when the nanocrystals are ordered in a long-range superlattice compared to the disordered system where we have only very short-range order. The first intrinsic magnetic property due to the ordering is observed.31 Figure 8B shows that, for the supracrystals, the approach to saturation is more gradual than for the disordered aggregates. This difference is explained by considering the model of amorphous ferromagnets proposed by Chudnovsky et al.51 In this model, there is only short-range structural order that leads to a local random anisotropy with a short-range correlation length. The randomness of this local anisotropy leads to a very small uniform anisotropy in the sample, referred to as coherent anisotropy. The model predicts an approach to saturation in these materials given by ΔM ≈ 1/H1/2, which has been confirmed experimentally.51 This was extended to disordered nanocrystallized films52 where a 1/H1/2 approach to saturation is also observed. In addition, the authors show that increasing the anisotropy in these films leads to a deviation from 1/H1/2 behavior toward a more linear approach to saturation, which would apply in the case of a perfect, uniaxial system. It is concluded that in the disordered aggregates, the direction of anisotropy randomly fluctuates from one magnetic nanocrystal to another and leads to an extremely small uniform local anisotropy with a square root magnetization law in approaching saturation. For supracrystals, due to long-range mesoscopic order, the total anisotropy is rather large with interparticle coupling energies leading to a smoother magnetization curve on approaching saturation. This clearly shows that the magnetic properties of ordered and disordered aggregates of nanocrystals31 behave similarly to those of highly crystallized and amorphous materials. Similarly, as predicted, the coercive field increases for supracrystals (900 Oe) compared to disordered aggregates (600 Oe). Again, as above, this agrees with a change in the local anisotropy with the nanocrystal ordering.51 From this comparison between the approach to saturation magnetization and the hysteresis loop of either amorphous material or highly crystallized supracrystals, it is reasonable to conclude that there is a similarity in the magnetic behavior between disordered aggregates or highly ordered supracrystals as in amorphous and highly crystalline materials of atoms. To go further, to a first approximation, we could assume that the magnetic behavior of atoms and nanocrystals is similar in a given order. Vibration Properties53. We know from data already published and those described in paragraph II (illustrated by Figure 3) that excitation of nanocrystals induces collective acoustic breathing modes of atoms in nanocrystals. Here we ask ourselves whether collective acoustic vibration of nanocrystals in supracrystals exist. This could support a hypothesis we offer from which nanocrystals self-assembled in supracrystals behave as atoms in nanocrystals. This study involves the same ordered and disordered 3D assemblies of Co polycrystals with intrinsic magnetic properties. Here, we
Figure 9. Long-time scale differential reflectivity dynamics of 3D assemblies of Co polycrystals. Solid line: disordered assemblies; open circles: fcc supracrystals.
compare their vibration dynamics properties using the femtosecond pumpprobe spectroscopy. On the long time scale (up to 400 ps), the energy deposited by a pump pulse dissipates to the environment of the nanocrystals. This can be described by a thermal diffusion mechanism. Figure 9 shows the time-dependent differential reflectivity signal ΔR/R(t) of 3D disordered and fcc supracrystals made of 6.3-nm Co nanocrystals. In the first case, we observe a monotonous decay whereas in the second, a periodic modulation of the decay is observed with a characteristic period of 125 ( 5 ps, which corresponds to a low frequency mode of 1.7 cm1. Such modulation is the signature of a coherent vibration, that is to say a collective breathing mode of perfectly ordered Co nanocrystals in fcc supracrystals via dodecanoic acid chains. This result is consistent with static measurements of Ag nanocrystals coated with aliphatic chains on 3D fcc supracrystals where an internanocrystal coherent vibration was observed using Raman spectroscopy.27,28 As a first approximation, the supracrystal can be described by a harmonic oscillator model involving spheres (Co nanocrystals) interconnected by springs (dodecanoic chains) with a rigidity constant k.54 From the model, we can deduce the rigidity constant of the chains and the value obtained, 1.4 ( 0.2 N m1, is in good agreement with the value obtained by force measurements on the rigidity of this type of aliphatic chains.55 To further confirm the existence of a collective motion of Co nanocrystals in supracrystals, a similar study was performed with a larger nanocrystal size, 7.3 nm. As in the previous case, only the dynamics of the supracrystal reflectivity show a modulation at long times with a greater period of 131 ps. This result could confirm the existence of an intrinsic property of coherent vibration related to the organization of cobalt nanocrystals in the 3D supracrystals. Superspin Glass Behavior56. In a conventional atomic spin glass, the lack of long-range magnetic order arising from magnetic frustration or highly dilute magnetic ions, leads to interesting behavior such as aging, rejuvenation and memory effects.57 In a magnetic nanoparticle system, if the nanoparticles are small enough to have a single magnetic domain, that is the case with 7 nm-Co nanoparticles, each nanoparticle will act like a giant or “super” spin. In sufficiently concentrated systems, these “superspins” can interact via long-range dipolar interactions, the random nature of which leads to a highly disordered and frustrated magnetic state analogous to that in atomic spin glasses. Until now, the interacting systems investigated was disordered aggregates without any regular array of nanocrystals.5860 Therefore, the question that arises is what is the influence of the long-range ordering of Co nanocrystals in a 3D fcc array on the superspin glass behavior. The blocking temperature of the Co fcc 10
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)
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supracrystals is 130 K, which is significantly larger than that observed for a dilute system of Co nanocrystals of a similar size due to the strong magnetic dipolar interactions between the nanocrystals in the supracrystal. The FC magnetization curve is nearly temperature independent below TB, characteristic of spin glass behavior.56 In less interacting systems, where the behavior is superparamagnetic, the FC magnetization curve is not temperature-independent below TB but increases with decreasing temperature. Figure 10, panels A and B, shows the in-phase (χ0 ) and out-of-phase (χ00 ) components of the ac susceptibility versus temperature, measured in a range of ac frequencies varying by 2 decades (in magnitude). The χ0 susceptibility shows a clear frequency dependence, where the temperature at which the maximum susceptibility is observed (Tpeak) increases with increasing frequency. The χ00 susceptibility also shows frequency dependence, where the maximum gradient of the slope for a given frequency corresponds to Tpeak in the χ0 curve. Frequency dependence in an ac measurement is observed both in superparamagnetic and superspin glass materials. In order to differentiate between these two types of behavior, one can analyze quantitatively the change in Tpeak with frequency. For a superparamagnet, where dipolar interactions between the magnetic moments are negligible, the frequency dependence should follow an Arrhenius law τ = τ0 expEa/kBT where Ea is the anisotropy energy, τ is the inverse of the measurement frequency, and τ0 is an attempt time. By plotting log10 τ versus 1/Tpeak and fitting the data to a straight line, the value of τ0 can be extracted and in this case we find τ0 = 1031 s. This nonrealistically- small value indicates that this system is not best described by simple energy barrier blocking and thermal activation and a different approach is needed. Hence, we have tried fitting the data to a critical power law τ = τ*(Tpeak/Tg 1)zν, where Tg was taken as the maximum in the dc ZFC magnetization curve. Fitting the data yielded τ* = 109(3 s and zν = 12 ( 2. This value of τ* is in good agreement with values found for spin glasses, and zν, although slightly high, is also compatible within the error to that expected for spin glasses.57,61 Spin glass and superspin glass materials are known to show aging and memory effects, which can be demonstrated by a simple dc magnetization experiment. The sample is zero field
)
Figure 10. (A) In phase and (B) out of phase part of the ac susceptibility versus temperature, measured at frequencies between 0.08 and 8 Hz in an ac driving field of 1.7 Oe for fcc supracrystals made of Co polycrystals.
)
cooled from above Tg to a temperature Ts typically equal to 0.7 Tg where a waiting time of tw = 104 s is imposed before continuing cooling (down) to low temperatures. A small field is then applied and the magnetization is measured on heating. A deviation from the reference ZFC curve (with no stop during cooling) is observed at Ts, which is known as a “memory dip”, so-called as the system has “remembered” the relaxation toward a zero magnetization value (aging) that occurred during the cooling process.57 The results of these ac and dc susceptibility investigations provide strong evidence for superspin glass behavior in these fcc Co supracrystals. Comparison between the Effects of the Orientation of the Easy Axes and Dipolar Interaction when Nanocrystals Are Ordered in 1D62. In our previous studies, we demonstrated that alignment of magnetic nanocrystals induces a squarer hysteresis loop compared to the same nanocrystals deposited on a substrate in 2D superlattices. This process is due to the orientation of the easy axis and to the dipolar interactions.63 To enable the distinction between the easy axis orientation and dipolar interaction effect, aligned iron oxide nanoparticles called magnetosomes are produced from AMB-1 magnetotactic bacteria. The magnetosomes are arranged in chains inside the bacteria, creating a strong magnetic dipole, which is being used by the bacteria to align and swim along the earth’s magnetic field. They are ferrite nanocrystals either in the reduced (Fe3O4) or oxidized (γFe2O3) forms. The size distribution is bimodal with the small magnetosomes having sizes ranging from 4 nm up to 30 nm and larger ones having sizes lying between 30 and 55 nm. Furthermore, the biological filaments, which surround the magnetosomes, maintain the alignment of the easy axes of the individual magnetosomes.64,65 These filaments might remain functional after the extraction of the magnetosomes from the bacterial cells.62 The magnetosomes are extracted from AMB-1 magnetotactic bacteria. During the deposition and evaporation, a magnetic field is applied. Figure 11A shows that the magnetosome chains organize within wide bands and orientate along the direction of the deposition field. Part of the biogenic material, which binds the chains of magnetosomes to the bacteria, is removed and the chains attract each other, forming a long string of nanoparticles. However, because of the remaining biogenic material, the chains are still bent in several regions. Due to the magnetic properties, as shown in Figure 11C, the hysteresis loop is squarer when the applied field is parallel (green) to the aligned particles than that recorded when it is perpendicular (red). The values of ΔMr/Ms (ΔMr/Ms = Mr/Ms Mr/Ms^/Mr/Ms ) and ΔHc (ΔHc = Hc Hc^/Hc ) are 80% and 50% respectively. Given a mean distance between the magnetosomes of ∼6 nm, the ratio Ems/Ek is estimated as ∼9 where Ems and Ek are the magnetostatic and anisotropy energies, respectively. This value is larger than 1 indicating a strong dipolar coupling between the magnetosomes. To estimate the orientation of the easy axes, the crystallographic planes of the magnetosomes with a truncated octahedral structure are identified by HRTEM. Figure 11B shows that the [111] crystallographic directions (white arrows) follow the orientation of the chain, confirming the results obtained by electron holography whereas some are randomly orientated (dark arrows). To estimate the influence of the orientation of the magnetic particle easy axes on the magnetic response, the extracted magnetosomes are treated to remove the remaining biological material, which binds the magnetosomes together, and to disorientate the magnetosome easy axes. To do so, the extracted magnetosomes are heated for one hour at 90 C in presence of 1% sodium dodecyl sulfate (SDS). The treated, extracted and untreated, 11
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Figure 11. TEM images of extracted, untreated (A) and SDS-treated (D) magnetosomes. HRTEM images of extracted, untreated (B), and SDS-treated (E) magnetosomes. The arrows indicate the Æ111æ crystallographic orientations. White arrows are used when the Æ111æ crystallographic orientations follow the direction of the chains. Black arrows are used when these orientations do not follow this direction. Hysteresis loops of the extracted, untreated (C), and SDS treated (F) magnetosomes. The magnetic field applied during the measurements of the hysteresis loops is either parallel (green line) or perpendicular (red line) to the direction of the alignment. The measurements are carried out at 300 K.
IV. CONCLUSIONS Here, the nanocrystallinity, i.e., the atomic ordering in nanocrystals, is improved by annealing nanocrystals either self-ordered on a substrate (dry process) or dispersed in solution (solution-phase process). This induces changes in the magnetic properties of nanocrystals with a significant increase in the saturation magnetization and in the blocking temperature. The measurement of the blocking temperature can be considered as a new approach to identify the nanocrystallinity transition from fcc polycrystals to hcp single-crystals. The shape of the mesoscopic structure of magnetic nanocrystals markedly influences their magnetic properties. When the nanocrystals are aligned, the dipolar interactions play a more important role in the magnetic response than the orientation of the easy axes. When Co nanocrystals are ordered in 2D superlattices, their resistance to oxidation is markedly improved. The magnetic properties of Co supracrystals markedly differ from those obtained when the same nanocrystals form a disordered assembly. These data show, for the first time, intrinsic chemical, mechanical and magnetic properties related to the ordering of Co nanocrystals.
magnetosomes are then deposited on the TEM grid. Figure 11D shows that the treated, extracted magnetosomes are also aligned forming a slightly more compact assembly than that observed with the extracted, untreated magnetosomes (Figure 11A). Using the same approach as described above, the Æ111æ crystallographic directions (black arrows) are randomly orientated and the magnetosomes behave like individual nanoparticles with a cubic symmetry (Figure 11E). Figure 11F shows a behavior similar to that observed with the extracted, untreated magnetosomes: a squarer hysteresis loop when the magnetic field is applied parallel to the oriented magnetosomes than when it is applied perpendicular to this direction. However, compared with the untreated, extracted magnetosomes, the hysteresis loop is less square in the parallel field configuration (green), while it remains identical in the perpendicular field configuration (red). The values of ΔMr/Ms and ΔHc decrease from 80% and 50% for the extracted, untreated magnetosomes down to 65% and 30% for the extracted, treated ones. This decrease cannot be attributed to a loss of dipolar interactions. Given a mean distance between the magnetosomes of ∼3 nm, the ratio Ems/Ek is estimated as ∼12, of the same order of magnitude as that obtained with the extracted, untreated magnetosomes. Therefore, the loss of magnetic anisotropy has to arise from the disorientation of the magnetosome easy axes. Then, it is concluded that the magnetic anisotropy of an assembly of aligned magnetosomes is mainly governed by the dipolar interactions between them and includes a smaller contribution due to the alignment of their easy axes.66 From the percentages given above, the dipolar interactions represent 70 ( 10% of the total magnetic anisotropy whereas the contribution of the easy axes is 30 ( 10%. To our knowledge, this is the first time that it is possible to establish the origin of the magnetic anisotropy in a system of aligned magnetic nanoparticles and to differentiate between the two contributions, i.e., the nanoparticle dipolar interactions and the orientation of the nanoparticle easy axes. The dominant contribution of the dipolar interactions might arise from the large sizes of the magnetosomes that, for the majority of these, are 20 to 50 nm.
’ BIOGRAPHIES
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Isabelle Lisiecki obtained her Ph.D. in physical chemistry from Pierre et Marie Curie University in 1993. She is currently research director in CNRS in the Laboratory “Laboratoire des Materiaux Mesoscopiques et Nanometriques”. After her Ph.D., she focused on the size, shape, and crystallinity control of copper nanocrystals. She has shown the fundamental role played by the degree of hydration of the reactants, the dynamic character of the micelles, the capping with the surfactant and/or salts, and the reducing agent concentration on the nucleation and growth processes. Since 2003, she has worked on magnetic systems and favored the formation of 2D and 3D long-range self-organizations of cobalt nanocrystals. She controlled the nanocrystallinity of Co nanoparticles from fcc polycrystals to hcp single-crystals, and the mesoscopic organization from 3D disordered assemblies to fcc superlattices. Hence, the magnetic properties of these artificial solids can be tuned from superpara- to almost ferromagnetism. Magnetic, vibrational, and chemical stability intrinsic properties are shown to arise from the mesoscopic organization. Her current research interests include the synthesis of hollow inorganic nanocrystals and the emergence of their unique and new physicochemical properties.
Tendeloo. The MPP research leading to these results has received funding from the Advanced Grant of the European Research Council under Grant Agreement No. 267129.
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Marie-Paule Pileni is a Distinguished Professor, member (1999 to present) and chair (20052010) of Institut Universitaire de France. She studied the development of structure reactivity relationships in organized molecular systems and, about the physical properties of these new materials, developed the self-organization of the nanocrystals in (2D) and (3D) at the mesoscopic scale. She studied the specific collective optical, magnetic, and chemical properties of these assemblies. She received the Langmuir award of the American Chemical Society, the lecture award of the Japanese Chemical Society, the Research Award of the Alexander von Humboldt Foundation in Germany and Descartes-Huygens Prize of the Royal Netherlands Academy of Arts and Science, the Emilia Valeri award from the French Academy of Science and the Catalan-Sabatier Lectureship award from the Royal Society of Chemistry of Spain. In addition, she was the French citation laureate, the Institute of Scientific Information Award for most-quoted French scientist between 1981 and 1998. She is a member of the Royal Swedish Academy of Engineering Sciences, of the European Academy of Science and has a doctorate honoris causa from Chalmers University, Goteborg, Sweden. She is “Officier dans l’Ordre National de la Legion d’Honneur”.
’ ACKNOWLEDGMENT Special thanks are due to Drs. E. Alphandery, A. T. Ngo, D. Parker, S. Turner, J. Richardi, C. Salzemann, and Pr. G. Van 13
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