3d Assembly Of Fe Co Ni Nano Particles

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Monodisperse 3d

Transition-Metal (Co, Ni, Fe) Nanoparticles and Their Assembly into Nanoparticle Superlattices

spins aligned in a single direction.3 Further reducing the size of a single-domain particle decreases the number of spins exchange-coupled to resist spontaneous reorientation of its magnetization at a given temperature. A double-well potential (Figure 1f) is used to conceptualize the rotation of the magnetization direction for a uniaxial magnetic particle.8 The energy barrier E between the orientations is proportional to the particle volume V and the material’s anisotropy constant K, which describes the preference for spins to align in a particular direction within the particle due to the influence of crystal symmetry, shape, and surface effects. As the particle size decreases, E becomes comparable to thermal energy (kBT), and the energy barrier no longer pins the magnetization on the time scale of observation. The particle is said to be superparamagnetic. Mapping the scaling limits of magnetic storage technology8,9 and understanding

C.B. Murray, Shouheng Sun, Hugh Doyle, and T. Betley Introduction Magnetic colloids, or ferrofluids, have been studied to probe the fundamental size-dependent properties of magnetic particles and have been harnessed in a variety of applications.1–4 The magnetorheological properties of magnetic colloids have been exploited in high-performance bearings and seals.5 The deposition of magnetic dispersions on platters and tapes marked the earliest embodiments of magnet information storage.6 Magnetic particles enhance contrast in magnetic resonance imaging and promise future diagnostic and drug delivery applications.7 The need to explore the scaling limits of magnetic storage technology has motivated the preparation of size-tunable monodisperse magnetic nanoparticles8–10 with controlled internal structures. The study of these nanoparticles is critical to efforts to separate the role of defects from intrinsic, finite size effects. We report the preparation of colloidal magnetic nanoparticle samples with controlled size, surface coordination, and crystallinity that are monodisperse to 1 atomic shell. Figures 1a–d depict the stages of nanoparticle synthesis, size-selective pre-

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cipitation, self-assembly, and nanoparticle superlattice formation, respectively. Figure 1e shows a schematic diagram of a model nanoparticle with its crystalline metallic core, oxidized surface, and a monolayer coat of organic stabilizers (surfactants). In this article, crystalline particles with low concentrations of defects are referred to as nanocrystals (NCs), while the more general term nanoparticle (NP) will refer to particles containing gross internal grain boundaries, fractures, or internal disorder. Comparing the size evolution of the magnetic properties in these systems reveals the importance of internal crystal structure. The magnetic properties of nanometersized particles arise from the competition between strong, short-range exchange interactions and long-range, dipolar couplings of electron spins in the atoms that make up magnetic solids. These competing interactions favor parallel alignment of nearby spins and antiparallel alignment of distant spins, forming magnetic domains in “bulk” magnets.1–3 In a small particle, truncation of the long-range, dipolar forces results in a “single-domain” magnet with all of the

Figure 1. Schematic representation of (a) nanoparticle (NP) synthesis by high-temperature solution-phase routes; (b) size-selective precipitation, used to narrow NP sample-size distributions; (c) self-assembly of NP dispersions; and (d) formation of ordered NP assemblies (superlattices). (e) Model nanoparticle with its close-packed metallic core, oxidized surface, and a monolayer coat of organic stabilizers (surfactants). (f) Graph of the energydependence of NP magnetization. E represents the energy barrier to the rotation of the magnetization. The left and right arrows represent spin states.

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spin-dependent transport phenomena in nanoscale devices11 motivate studies of magnetic NPs.

General Synthesis of Monodisperse 3d Transition-Metal Nanoparticles One general scheme for preparing monodisperse (standard deviation s , 5%) 3d transition-metal NPs relies on producing a single short nucleation event followed by slower growth on the nuclei formed.12–15 This is achieved by either injecting a reducing agent into a hot solution of metal precursors and surfactants, or injecting thermally unstable, zero-valent metal precursors into a hot solution containing colloidal stabilizers (Figure1a). Adjusting the temperature and the metal-precursor-tosurfactant ratio controls NP size. Higher temperatures and larger metal-precursorto-surfactant ratios produce larger NPs. Surfactants adsorb reversibly to NP surfaces, providing a dynamic organic shell that mediates growth, stabilizes the NPs in solution, and limits oxidation after synthesis. An effective strategy employs a pair of surfactants, where one binds tightly to the metal NP surface (e.g., oleic acid), favoring slow growth, and the other binds weakly, permitting rapid growth (e.g., trioctylphosphine, TOP, and tributylphosphine, TBP).12– 15 The ratio of the strongly and weakly binding surfactants is adjusted to control NP size. Surfactants with similar binding chemistry, but with greater steric bulk (bulky TOP instead of TBP) slow NP growth, reducing the average NPsize. Although weakly binding surfactants are important in mediating growth, elemental analysis indicates that only the strongly binding surfactant remains coordinated to the surface of 3d transitionmetal NPs when isolated from the growth solution. The surfactant can be exchanged for surfactants of different length or chemical functionality. Adding a large excess of a substitute surfactant to a NP dispersion and heating and stirring the solution are generally sufficient to exchange the majority of the stabilizers. NPs can be reextracted, and repeating the procedure increases the fraction exchanged.14 Colloidal synthesis can yield monodisperse NPs directly from the growth solution,15–17 but generally an initial size distribution of s , 7–10% is considered a success. Size-selective precipitation is used to narrow the NPsize distribution. A nonsolvent miscible solution with the dispersing solvent is added dropwise, gradually destabilizing the dispersion until a slight cloudiness is apparent. The dispersion is centrifuged, yielding a small amount

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of precipitate enriched in the largest NPs, which is discarded. The supernatant solution, containing the majority of the NPs, is decanted, and more nonsolvent is added dropwise while stirring until a dense, cloudy suspension is formed. Centrifuging isolates most of the NPs in the precipitate, leaving the smallest NPs and synthetic by-products in the supernatant solution, which is discarded. At this stage, the precipitate generally exhibits a nearGaussian size distribution (s , 7%). Sizeselective precipitation of magnetic NPs is enhanced by magnetic fields (e.g., holding a strong magnet next to the centrifuge tube or using a magnetic stirrer), as the larger magnetic NPs are more effectively separated and retained in the precipitate. The size distribution is further narrowed (s , 5%) by redispersing the NPs in fresh solvent with ,0.1% stabilizer added to maintain a stable colloid and repeating the procedure.

Synthesis of Monodisperse hcp Cobalt Nanocrystals Reduction of cobalt salts by polyalcohols (polyols) at temperatures between 1008C and 3008C in the presence of stabilizers produces Co NPs with diameters of 2–20 nm.18 We find that using long-chain 1,2-diols instead of commonly used ethylene glycol or glycerol reducing agents has an advantage; the precursors and NPs remain well dispersed during growth, avoiding the formation of insoluble powders that preclude size-selective precipitation.19 We report the synthesis of Co NCs with a predominantly hcp internal crystal structure. In a typical synthesis of 6–8-nm hcp Co NCs, 1.0 g (4mmol) of Co(CH3COO)2 ? 4H2O is combined with 1.28 ml (4mmol) of oleic acid in a flask with 40 ml of diphenylether (DPE) and heated to 2008C under N2. One can substitute dioctylether for DPE in any of the following procedures, but it is significantly more costly. When the reaction reaches 2008C, ,2.0 mmol TOP is added, and the mixture is heated to 2508C. In a separate flask, 2.1 g of 1,2-dodecanediol is dissolved in 10 ml of dry DPE. The reducing solution is heated to 808C under vacuum for 15 min, back-flushed with N2, and injected into the hot (2508C) reaction vessel. The solution changes color from blue to black over a period of 2 min as the Co NCs nucleate and begin to grow. The reaction is held at 2508C for ,15–20 min, consuming the reagents. The Co NC dispersion is cooled, and the NCs are isolated by size-selective precipitation. NC size is tuned by tailoring the concentration or composition of stabilizers: increasing the concentration of oleic acid and TOP by

a factor of 2 yields smaller, 3–6-nm NCs, while substituting TBP for TOP forms larger, 10–13-nm Co NCs. A transmission electron microscope (TEM) image (Figure2a) of a large ensemble of 6-nm hcp Co NCs (s , 5%) shows uniform NCs assembling into a superlattice. High-resolution TEM (HRTEM) (Figure 2b) reveals a crystalline core with a lattice spacing of ,2 Å. A series of TEM images are analyzed to develop a statistical model of the NC sample size, size distribution, shape, and frequency and mode of defects (stacking faults along the c axis in hcp NCs are the predominant source of disorder). An atomistic model of the NC ensemble is built using collections of atomic coordinates to represent the atoms in the NCs. The atomic coordinates for a series of NC sizes are input into a discrete form of the Debye equation, and a weighted sum is taken to simulate the wide-angle x-ray scattering (WAXS) of NC ensembles with a specific size and size distribution.14,20 Stacking faults are introduced in model NCs by assigning a faulting probability as the atomic coordinates are generated. The parameters of size, size distribution, shape, and defect density are refined to provide the best simultaneous fit to the experimental diffraction and TEM results. Figure 2c shows the WAXS pattern for a sample of 8-nm hcp Co NCs (open circles) and the corresponding computer simulation (solid line). Patterns and simulations for 4-, 6-, 8-, and 10-nm samples are shown in Figure 2d. Finite size effects broaden each x-ray reflection, and in hcp Co NC samples, the (100), (102), and (103) reflections are further broadened and attenuated by stacking faults along the [002] direction that introduce the fcc character. Fits are consistent with 70–80% hcp and 20–30% fcc characters in each NC. Substituting 4.0mmol of Ni(CH3COO)2 ? 4H20 for the Co precursor and using a mixture of 2 mmol of oleic acid, 1 mmol of TBP, and 8 mmol of tributylamine (TBA) stabilizers in the procedure yields 12–13-nm Ni NPs. Replacing TBP and TBA with TOP and trioctylamine (TOA), respectively, produces 8–10-nm Ni NPs with even smaller NPs synthesized by increasing the amount of oleic acid. Co/Ni alloys are prepared using a mixture of Co(CH3COO)2 ? 4H2O and Ni(CH3COO)2 ? 4H2O. Figure3a shows a low-magnification TEM image of an ensemble of 8-nm Co/Ni alloy NPs (s ­ 5%). Although equal amounts of Co and Ni precursors were employed, energy-dispersive x-ray spectroscopy reveals that the NPs are 40% Co and 60% Ni, indicating Ni is more readily incorporated into the growing NPs. The

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Monodisperse 3d Transition-Metal (Co, Ni, Fe) Nanoparticles

Figure 2. (a) Low-magnification transmission electron microscope (TEM) image of a large ensemble of 6-nm hcp Co nanocrystals (NCs) packing into an ordered superlattice. (b) High-resolution TEM (HRTEM) image of a 7-nm hcp Co NC. (c) Wide-angle x-ray scattering (WAXS) pattern of 8-nm hcp Co NCs (open circles) and computer simulation (solid line) generated from TEM measurements of sample size, internal structure, and shape; (d) inset shows the WAXS patterns for 4-, 6-, 8-, and 10-nm hcp Co NCs with their respective fits.

electron-diffraction pattern (Figure 3b) shows that this alloy has a predominantly fcc crystal structure.

Multi-Twinned Co Nanoparticles One established preparation of Co NPs employs thermal decomposition of Co2(CO)8 in the presence of surfactants under an inert atmosphere.5,21,22 When air is not excluded from the reaction, metal-oxide particles are isolated.16,23 These carbonyl decomposition procedures are scalable to large quantities for the commercial production of magnetic fluids.5 Carbonyl-based syntheses typically do not yield single-crystal particles (NCs); rather, the NCs are comprised of multiple fragments of the bulk fcc lattice with several radial twin planes in each particle.24,25 These multi-twinned (mt) particles are members of a family of polyhedral structures and have been studied in several transition-metal systems.26,27 We adopt the label mt-fcc Co NPs for these samples. While we describe the synthesis of mt-fcc Co NPs, a recent work reports the synthesis of crystalline samples using carbonyl precursors.28

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Figure 3. (a) Low-magnification TEM image of 8-nm Co/Ni alloy NPs; (b) inset shows an electron-diffraction pattern from an 8-nm Co/Ni NP sample, confirming the predominantly fcc structure. (c) TEM image of an ensemble of 8-nm multi-twinned fcc (mt-fcc) Co NPs; (d) inset shows HRTEM image highlighting the mt internal structure of the 8-nm mt-fcc NPs. (e) TEM image of an ensemble of 6-nm Fe NPs; (f) at higher magnification (inset), the surface oxide layer is clearly visible. (g) Image of an ensemble of 6-nm FePt NPs; (h) inset shows HRTEM image of a FePt NC after annealing and forming of the face-centered-tetragonal (fct) phase.

To synthesize 8–10-nm mt-fcc Co NPs, a reaction vessel with 40 ml of DPE is heated under N2 to 200C, and 2.0 mmol (0.64 ml) of oleic acid and 2.0 mmol of TBP are added. In a separate flask, 684 mg of Co2(CO)8 is combined with 10 ml of dry DPE, warmed briefly to 40–60C under N2 until fully dissolved, and then rapidly injected into the reaction vessel. The solution turns black and foams as Co2(CO)8 decomposes, nucleating Co NPs and releasing CO gas. The vessel is held at 200C for 15–20 min, allowing the NPs to grow. The solution is cooled, and the NPs are isolated by size-selective precipitation. Figure 3c shows 8-nm mt-fcc Co NPs, and a higher-magnification image of the same sample (Figure 3d) highlights the presence of radial twin planes. Fe NPs are prepared by replacing Co2(CO)8 with Fe(CO)5 in the described procedure and carrying out the growth at 250C.5 After injection of a yellow-brown Fe(CO)5 solution, the reaction rapidly turns from brown to black and foams as CO is released and Fe NPs nucleate. The Fe NPs are sensitive to oxidation, and thus size-selective precipitation is carried out

under N2. The Fe NPs (Figure 3e) have a structure resembling the bulk bcc lattice, but show evidence of disorder and have a 2-nm-thick oxide shell (Figure 3f). Alloys of Fe and Co are accessible using a mixture of Co2(CO)8 and Fe(CO)5 at 250C, although the isolated NPs are poorly crystallized. Alloys and intermetallic phases are produced when 3d transition metals such as Fe are combined with Pt. Sun et al. have reported the preparation of 3–10-nm FePt NPs (  5%) by reduction of Pt[CH3COCHC(O)CH3]2 and decomposition of Fe(CO)5 in the presence of oleic acid and oleylamine stabilizers.10,17 Each NP consists of a disordered Fe/Pt fcc core and a surface coordinated by both amine and carboxylate stabilizers, which are retained when the NPs are isolated. These alloy NPs are converted to ordered facecentered-tetragonal (fct) NCs upon annealing to temperatures 550C. The high magnetocrystalline anisotropy of this intermetallic phase allows NCs as small as 3.5 nm to display strong, roomtemperature ferromagnetism,10,17 making these phases interesting for future mag-

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netic media.9 An ordered assembly of unannealed 6-nm FePt NPs (Figure 3g) with a 1-nm NP spacing has been produced by exchanging the original oleic acid and oleylamine stabilizers with hexanoic acid and hexylamine. An HRTEM image (Figure 3h) of an annealed 4-nm FePt NC shows lattice fringes due to the alternating planes of Fe and Pt.17

-Cobalt Nanocrystals

Several synthetic routes yield Co NCs having a third distinct crystal structure, called -cobalt.12,13,19,24 The -cobalt crystal structure consists of a complex, 20-atom unit cell (isomorphic with the  phase of Mn) that does not favor the formation of low-energy stacking faults as does the hcp/fcc system. The -Co NCs are converted to hcp and fcc Co by annealing at temperatures of 300C and 500C, respectively.12,13 Magnetic measurements [ferromagnetic resonance and superconducting quantum interference device (SQUID) magnetometry], TEM, and WAXS studies have been combined with computer simulations for mt-fcc Co NPs and -Co NCs.29 Comparison of the sizedependent magnetic properties of hcp Co NCs, mt-fcc Co NPs, and -Co NC samples provides an opportunity to observe the effects of internal crystal structure in a single elemental system.

Magnetic Properties of Nanoparticles Each NP (Co, Ni, Fe, or alloy) is a singledomain magnet, as described earlier. The magnetization M of an ensemble of noninteracting, single-domain NPs decays after removing a saturating field as kv/kBT M(t) e t/, where   f 1 and f0 (the 0 e attempt frequency) is 109–1011 Hz.3 To investigate the magnetic properties inherent in independent NPs, the NP samples must be monodisperse and the particle interactions must be minimized.29,30 In our experiments, the oleic-acid-stabilized particles are dispersed at   0.5% by weight in paraffin or polyvinyl pyrrolidone. The coercivity Hc, the field required to reverse the magnetization, decreases with decreasing NC size, as seen in M versus H loops measured at 5 K (Figure 4a) for a series of hcp Co NC sizes.25 The sharp drop in saturation magnetization (relative to that for bulk Co) with decreasing NP size results from the increase in the NP surface-to-volume ratio and the presence of 1–2 monolayers of cobalt oxide.31 In Figure 4b, a temperature series of M versus H loops for a dispersion of 9.5-nm -Co NCs is displayed. Increasing thermal energy helps the NCs reorient with the applied field H, reducing hysteresis and

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Figure 4. (a) Magnetization versus applied-field hysteresis (M versus H) loops at 5 K for 3-, 6-, 8-, and 11-nm hcp Co NC samples. (b) Comparison of M versus H loops for 9.5-nm -Co NCs at 5, 40, 100, and 300 K. (c) A series of zero-field-cooled (ZFC) and field-cooled (FC) magnetization scans for hcp Co NC samples (3, 6, 8, and 11 nm). (d) Plots of Hc versus particle diameter for a collection of hcp Co, -Co, and mt-fcc Co samples.

leading to superparamagnetic behavior above the blocking temperature TB, where hysteresis drops to zero.4 This 9.5-nm -Co NC sample is superparamagnetic above 125 K. A series of zero-field-cooled (ZFC) and field-cooled (FC) scans are shown in Figure 4c, in which magnetization is measured as a function of temperature for several sizes of hcp Co NCs. In ZFC scans, a sample is cooled in zero applied field, and the magnetization is recorded as the temperature is increased in the presence of a small field (10 Oe). As the thermal energy increases, the NCs

become unpinned and align with the applied field, increasing the sample’s net magnetization. At TB, the thermal energy causes the NC moments to begin to fluctuate in the small field, reducing the net magnetization. In FC scans, samples are cooled in a small field (10 Oe), freezing-in a net alignment of the NC moments. The field is removed, and the magnetization is measured as the sample is slowly warmed. Thermal energy unpins and randomizes the NC moments, lowering the sample’s net magnetization. At TB, the ZFC and FC curves converge.32

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In magnetic recording, the remanent magnetization Mr of the grains (in this case, NCs) encodes the information. Thus, the stability of magnetic information has a double exponential dependence on K and V, leading to an extraordinarily steep drop in thermal stability at nanometer sizes.8 A plot of coercivity Hc versus NC diameter (Figure 4d) summarizes 5 K hysteresis loops measured for several sizes of hcp Co, mt-fcc Co, and -Co. The Hc of the smallest NPs is dominated by coupling to the surface CoO layer. For each system, Hc is lower than that predicted for idealized NPs, as internal structural defects (stacking faults, twinned planes) reduce Hc. While 3d transition-metal NPs provide model systems for studying the sizedependent stability of magnetic media, magnetic NP superlattices (described in the next section) are also being investigated as model granular magnetoresistive materials.11 Future ultrahigh-density recording media will require uniform particles ( 10%) with an average diameter of 6 nm and a room-temperature Hc of 3000–5000 Oe. Although the Hc of elemental Co is too low for advanced media, we have extended this synthetic approach to prepare ordered arrays of highercoercivity intermetallic NCs such as FePt (Figures 3g and 3h).17

Magnetic NP Superlattices Colloidal NPs self-assemble from solution, forming close-packed NP arrays on a variety of substrates as the dispersing solvent evaporates. Tailoring the composition and combination of dispersing solvents and the temperature during deposition induces assembly of close-packed glassy films or ordered NP superlattices. Preparing samples with narrow size distributions is critical to achieving long-range order in NP assemblies: for size distributions of   7%, short- to medium-range order is observed; reducing  to 5% enables the formation of assemblies with long-range order, known as colloidal crystals or superlattices.14,33–35 However, even a sample with   5% can yield close-packed glassy NP films having only short-range order if the deposition conditions are not appropriately tailored and the evaporation rate is too high.14 Figure 5a shows an ensemble of 9-nm Ni NPs that has been deposited onto an amorphous carbon TEM grid from pentane. Rapid drying ( 10 s) has frustrated the formation of long-range order. The higher-magnification image (inset in Figure 5a) shows a degree of local order. Depositing the same sample of Ni NPs from 80% hexane and 20% octane slows the evaporation rate (5 min) and gives the NPs time to find energetically favor-

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Figure 5. (a) TEM image of a close-packed glassy film of 9-nm Ni NPs deposited onto an amorphous carbon TEM grid from pentane; the higher-magnification inset shows a degree of local order. (b) TEM image of a 9-nm Ni NP superlattice grown by slow drying of a hexane/octane dispersion; the inset shows that ordering has improved. (c) WAXS pattern for a 9-nm Ni NP sample. (d) Small-angle x-ray scattering (SAXS) patterns for 9-nm NPs: (curve A) dispersed at 1% by weight in poly(vinyl butyral), (curve B) deposited as a glassy film from pentane, (curve C) deposited over 5 min from hexane/octane, and (curve D) covered and dried slowly from hexane/octane over a period of 40 min. (e) ZFC-FC scans from 9-nm Ni NPs: (curve A) dispersed, (curve B) in a superlattice, and (curve C) in a superlattice annealed under vacuum.

able lattice sites in a growing assembly.14 Ordering is improved, as seen in Figure 5b for a Ni NP superlattice, by covering the substrate and further lowering the evaporation rate. The inset in Figure 5b shows a higher-magnification image. Figure 5c shows a WAXS pattern for the 9-nm Ni NP sample, confirming the predominantly fcc crystal structure of the individual NPs. A series of small-angle x-ray scattering (SAXS) patterns following the development of ordering is seen in Figure 5d. The SAXS pattern (curve A in Figure 5d) of 9-nm NPs dispersed at 1% by weight in a poly(vinyl butyral) matrix shows the oscillations characteristic of the NP size and size distribution.14,34 Curve B in Figure 5d shows scattering from a glassy film deposited from pentane; curve C shows scattering from a film deposited

over 5 min from hexane/octane; and in curve D, the film has been covered, extending the drying time to 40 min. With increasing drying time, the Bragg reflections from the hexagonally packed sheets of NPs become sharper and more intense.14,34 The reflections correspond to 9-nm NPs separated by a 4-nm surfactant layer. In Figure 5e, the ZFC-FC scans for (curve A) dispersed Ni NPs and (curve B) closepacked assemblies show that TB shifts and broadens as dipolar coupling between NPs increases. As the sample is annealed under vacuum (curve C), TB continues to shift as the surfactant is partially desorbed, and the interparticle distance decreases to 2 nm.13,33 The wetting properties of the substrate influence the NP superlattice morphology. If the NP dispersion wets the substrate,

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the superlattice growth is more twodimensional, first forming a monolayer and then multilayers as coverage increases. NPs adsorb to ledges and kinks of the growing structures, forming terraces that extend laterally across the substrate. A step-like structure is seen in the scanning electron micrograph (Figure 6a) at the edge of a 10-nm mt-fcc Co NP superlattice grown on a hydrofluoric-acid-dipped silicon wafer. Figure 6 shows the large central terrace of this same superlattice domain. If the NP dispersion does not wet the substrate, superlattice growth is more three-dimensional, forming facets that reflect the packing symmetry of the NPs. Figure 7a shows a TEM image of a hexagonal superlattice with rows of 8-nm mt-fcc Co NPs aligning to form facets. An image

Figure 6. (a) High-resolution scanning electron microscope (HRSEM) image showing the 10-nm mt-fcc Co NPs forming ledges at the edge of a superlattice; inset shows the ledges at higher magnification. (b) HRSEM image of the top surface of the NP superlattice imaged in 6a; inset shows higher-magnification image of the same sample.

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of a larger hexagonal superlattice (colloidal crystal) is seen in Figure 7b.14 When a magnetic field of 1000 Oe is applied in the plane of the substrate during deposition of this same NP dispersion, the superlattice morphology changes markedly.25,33 Spindle-shaped deposits grow with their long axes aligned with the field. Figures 7c–7e show this sample at increasing magnifications. Deposition in a magnetic field introduces anisotropy in the assembly and thus in magnetic properties of the NP films.

Conclusions High-temperature, solution-phase syntheses yield Co, Ni, Fe, and alloy nanoparticle samples that are uniform in size (to 1 atomic layer), composition, shape, internal structure, and surface chemistry, allowing the properties of nanoscale magnetic materials to be mapped. These nanoparticles constitute a versatile new set of nanoscale building blocks with which to assemble magnetic materials and devices.

Under controlled deposition conditions, these nanoparticles self-assemble from solutions to form glassy close-packed solids, extended two- and three-dimensional superlattice films, and faceted colloidal crystals. The resulting magnetic nanoparticle assemblies provide model systems with which to explore collective interparticle interactions (e.g., dipolar, exchange, spin-dependent tunneling) that are critical to the optimization of future magnetic technologies.

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Figure 7. (a) TEM image of a hexagonal superlattice grown from 8-nm mt-fcc Co NPs with no applied magnetic field. (b) Lower-magnification image of large hexagons formed in the sample. (c)–(e) Images at increasing magnification of spindle-shaped superlattices grown with a 1000-Oe field applied in the plane of the substrate.

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MANAGEMENT

NUCLEAR WASTE

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