Self-assembled Nanoparticle Arrays Research Articles

Nonvolatile memory cell effect in multilayered Ni1−xFex self-assembled nanoparticle arrays in polyimide

Transmission electron microscopy images showed that self-assembled Ni1−xFex nanoparticle arrays were periodically inserted in the polyimide (PI) layers. Capacitance-voltage (C-V) measurements on Al/PI/multiple-stacked Ni1−xFex nanoparticle arrays/PI/p-Si (100) structures at 300K showed a metal-insulator-semiconductor capacitor behavior with different flatband voltage shifts, which depended on the value of the sweep voltage, due to the variations of the charged electron density in the multiple-stacked Ni1−xFex nanoparticle arrays. Conductance-voltage (G-V) measurements showed that the conductance peak related to the interface trap disappeared, and that the positions of the C-V and the G-V hystereses at the sweep voltage were different.

A Simple and Rapid Method of Making 2D and 3D Arrays of Gold Nanoparticles

Monodispersive gold nanoparticles can be synthesized by a dropwise addition of a reducing agent microemulsion to a gold ion microemulsion followed by immediate stabilization with 1-decanethiol. No size-selective precipitations or digestive ripening procedures are necessary. There is no need for metal functionalization of the surfactant AOT. Gold nanoparticles with an average size of 3.8 nm and a relative size dispersion of 5.4% were observed using n-heptane as a solvent. It seems possible to adjust the nanoparticle size by small changes in the carbon chain length of the solvent. Self-assembled 2D and 3D arrays of gold nanoparticles with adjustable sizes have been obtained on carbon-coated copper grids and on a silicon wafer. The arrays have good crystallinity as evidenced by the external morphology and transmission electron diffraction results. The size of the gold nanoparticle 3D arrays depends on the immersion time and can be greater than 15 microm. This approach could be used to synthesize other noble metal nanoparticle arrays that may lead to new materials for electronic and photonic applications.

Agglomeration and sintering in annealed FePt nanoparticle assemblies studied by small angle neutron scattering and x-ray diffraction

In this work we give a detailed account of complementary small angle neutron scattering and x-ray diffraction studies of polymer mediated, self-assembled FePt nanoparticle arrays as a function of annealing temperature. The combination of these two techniques provides significantly greater physical insight than is available using either individually. Since both methods integrate over a large number of particles statistically meaningful data can be obtained in contrast to imaging techniques where typically only small areas are analyzed. The data show that the median particle size increases with annealing at temperatures of $580\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ and above. The data also demonstrate that the distribution of particle diameters is significant and increases with annealing temperature. These results allow a comprehensive structural model of the annealed assemblies to be developed in terms of particle sintering and agglomeration. This enhanced understanding will allow new strategies to be pursued in realizing the potential of nanoparticle assemblies as a monodispersed data storage medium.

Correlation between tunneling magnetoresistance and magnetization in dipolar-coupled nanoparticle arrays

The tunneling magnetoresistance (TMR) of a hexagonal array of dipolar coupled anisotropic magnetic nanoparticles is studied using a resistor network model and a realistic micromagnetic configuration obtained by Monte Carlo simulations. Analysis of the field-dependent TMR and the corresponding magnetization curve shows that dipolar interactions suppress the maximum TMR effect, increase or decrease the field-sensitivity depending on the direction of applied field and introduce strong dependence of the TMR on the direction of the applied magnetic field. For off-plane magnetic fields, maximum values in the TMR signal are associated with the critical field for irreversible rotation of the magnetization. This behavior is more pronounced in strongly interacting systems (magnetically soft), while for weakly interacting systems (magnetically hard) the maximum of TMR (Hmax) occurs below the coercive field (Hc), in contrast to the situation for non-interacting nanoparticles or in-plane fields (Hmax=Hc). The relation of our simulations to recent TMR measurements in self-assembled Co nanoparticle arrays is discussed.

Cluster size analysis of two-dimensional order in colloidal gold nanoparticle arrays.

A protocol for cluster size distribution analysis was developed in order to parametrize local two-dimensional (2D) order in a quantitative manner, using mean cluster sizes and fractional hcp cluster formation (fhcp). Cluster size analysis was performed on 2D arrays of Au nanoparticles encapsulated in resorcinarene tetrathiol, which were organized into close-packed films at aqueous interfaces. The degree of monolayer formation and 2D order within the self-assembled nanoparticle arrays was observed to be strongly dependent on the amount and type of electrolyte (chloride and/or citrate) adsorbed on the nanoparticle surface, prior to encapsulation and extraction to the solvent interface. Increasing the concentration of adsorbed electrolyte could promote monoparticulate film formation but had a variable effect on local 2D order.

Structural and magnetic model of self-assembled FePt nanoparticle arrays

Chemically ordered, self-assembled FePt nanoparticle arrays with high magnetic anisotropy are considered as a candidate medium for data storage beyond 1 Tbit/in2. We report comprehensive structural and magnetic studies on thin (three-layer) assemblies of polyethylenimine (PEI) and 4 nm Fe58Pt42 nanoparticles using x-ray diffraction, small angle neutron scattering, and magnetometry. We show that prior to annealing FePt nanoparticles in the PEI-FePt assembly consist of a metallic magnetic core surrounded by a weakly magnetic or nonmagnetic shell. High temperature annealing creates the desired L10 chemical ordering and results in high coercivity FePt nanoparticles. However, we find that the high temperatures necessary to establish full chemical ordering leads to particle sintering and agglomeration. Understanding the magnetic and physical properties of these assemblies allows future research directions to be clarified for nanoparticle arrays as data storage media.

Dipolar interactions and structural coherence in iron nanoparticle arrays

Self-assembled arrays of iron nanoparticles are characterized structurally by transmission electron microscopy and small-angle X-ray scattering, and magnetically by low-temperature hysteresis loops and magnetic relaxation. The differences in the magnetic properties are related to the strength of magnetic dipolar interactions and to the degree of structural ordering in the arrays.

Self-assembled arrays of zinc oxide nanoparticles from monolayer films of diblock copolymer micelles

A hexagonal array of optically active ZnO nanoparticles was synthesized in situ on the solid substrate by utilizing a single-layered film of diblock copolymer micelles as a nanostructured template.

Self-assembled arrays of ZnO nanoparticles and their application as varistor materialsElectronic supplementary information (ESI) available: XRD plots and FESEM images. See http://www.rsc.org/suppdata/jm/b4/b400927d/

Linear arrays of ZnO nanoparticles have been successfully prepared by a simple sol–gel condensation reaction involving chemical modifiers, followed by drying (80 °C) and calcination (500 °C). The calcined material (nano-array ZnO) is composed of approximately spherical nanoparticles of average diameter 21 ± 3 nm, self-assembled to form arrays extending in length to 2–4 µm. The morphology of the ZnO is found to depend sensitively on the amounts of chemical modifiers present. In their absence the ZnO produced (nano-ZnO) is an unstructured agglomerate of nanoparticles. The mechanism for formation of these linear arrays has been investigated by examining the intermediates formed at 80 °C and 250 °C using XRD and TEM and by following the decomposition reactions using TGA and DSC. Varistors prepared from the nano-array ZnO by sintering (1050 °C) with appropriate mixtures of metal oxides showed a breakdown voltage of 786 ± 30 V mm−1, which is substantially higher than that of samples prepared under similar conditions from either micron-sized commercial ZnO (507 ± 30 V mm−1) or from nano-ZnO (683 ± 30 V mm−1).

In-plane and out-of-plane transverse susceptibility in close-packed arrays of monodisperse Fe nanoparticles

The transverse susceptibility (TS) of arrays of self-assembled Fe nanoparticles has been studied using a sensitive radio-frequency resonant technique. Symmetrically located broad peaks in the TS data are observed below the blocking temperature as the applied field is swept from positive to negative saturation. These peaks occur at the effective anisotropy fields $(\ifmmode\pm\else\textpm\fi{}{H}_{K})$ with the peak width determined by the distribution in ${H}_{K}$ in the nanoparticle array system. These features are observed to be strongly affected by dipolar interactions as well as thermal relaxation. Systematically tracking the evolution of the TS curves across the superparamagnetic transition reveals distinct temperature ranges over which thermal activation and dipolar energy overcome the effective magnetic anisotropy energy. Hysteresis loops measured using a superconducting quantum interference device magnetometer indicate a smaller coercive field for in-plane field orientation compared to that for out-of-plane orientation. This is also reflected in the TS measurements. A comparison of the TS over a wide range in temperature and magnetic fields, applied in plane and out of plane, reveals the distinct influence of variation in dipolar interaction strengths for the two geometries.

Coercivity and remanence in self-assembled FePt nanoparticle arrays

FePt-based nanostructured materials are excellent candidates for high density recording beyond 1 Tbit/in2. We calculate remanence, coercivity, and loop shape of annealed monodisperse FePt nanocrystals, using a modified Stoner–Wohlfarth model. To justify the simplifications of a Stoner–Wohlfarth model detailed finite element micromagnetic simulations were performed. Magnetic measurements on arrays of chemically synthesized FePt nanoparticles show remanence ratios of about 0.6 which is greater than that predicted for a series of noninteracting Stoner–Wohlfarth particles. A small fraction of the particles (5%) is assumed to remain in the disordered fcc phase with low magnetocrystalline anisotropy. Both remanence and coercivity are highly sensitive to the strength of the exchange interactions within a multiple twined nanocrystal. The calculated values are in the range from Jr/Js=0.52, Hc=0.77 MA/m to Jr/Js=0.61, Hc=1.2 MA/m. The results of the modified Stoner–Wohlfarth model are confirmed by finite element micromagnetic simulations taking into account magnetostatic interactions and allowing nonuniform magnetic structures within a particle.

Magnetic properties of self-assembled interacting nanoparticles

The temperature-dependent magnetization and the hysteresis properties (remanence and coercivity) of magnetic nanoparticle arrays are studied by Monte Carlo simulations. An oscillatory variation of the remanence with layer coverage and accompanying peaks in the coercive field are predicted at low temperatures, due to dipolar interparticle interactions. The blocking temperature of the arrays decreases with the inverse cube of the interparticle spacing (Tb∼d−3) and it remains almost unchanged with film thickness above one monolayer. Our results are compared with recent experiments on self-assembled Co nanoparticle arrays.

Direct imaging of self-assembled magnetic nanoparticle arrays: Phase stability and magnetic effects on morphology

Chemically stabilized 7.0 nm Fe and 3.9 nm FePt nanoparticles have been self-assembled into thin arrays, and imaged by transmission electron microscopy. The local packing structures and stacking sequences of the arrays were uniquely identified by a combination of bright-field imaging and projected potential image simulations. Close-packed structures, hexagonal close-packed (hcp) and face-centered cubic (fcc), were observed for 7.0 nm Fe nanoparticles, while a loosely packed body-centered cubic (bcc) structure was detected for 3.9 nm FePt nanoparticles as well as fcc and hcp. The bcc structure formation is driven by the large vibrational entropy of particles and the soft interparticle repulsions originating from competing enthalpic and entropic effects on surfactant molecule conformation. The sole anomaly noted is a preference for forming hcp arrays with odd numbers of layers observed for Fe nanoparticles. This is attributed to the thickness- and layer-diameter-dependent magnetic coupling between superferromagnetically ordered array layers.

Resorcinarene Promotes Self-Assembled Arrays of 2D Gold Nanoparticles

Resorcinarene Promotes Self-Assembled Arrays of 2D Gold Nanoparticles

Magnetic Properties of Self-Assembled Fe Nanoparticle Arrays

ABSTRACTThe magnetic properties of multilayer arrays of Fe nanoparticles were compared with those of frozen, dilute suspensions of the same particles. The array sample displayed larger coercivity in both hysteretic and remanent magnetization measurements. However, the derivative of the remanent magnetization curve shows a much broader switching field distribution for the arrays than for the dilute sample. Magnetic relaxation measurements show the convergence of the time dependent properties of the samples at large times, and much more rapid relaxation in the arrays at short times.

Structure of Self-Assembled Fe and FePt Nanoparticle Arrays

AbstractArrays were self-assembled by evaporating suspensions of 4 nm FePt or 8 nm Fe nanoparticles. The monolayers had a hexagonal close packed (hcp) structure, but the multilayer structure varied. To identify the multilayer structures, transmission electron microscopy (TEM) images were compared with phase contrast image simulations. The results showed that Fe could be grown as both hcp and face-centered cubic (fcc), or fcc-like, structures. The results of image analysis of the FePt arrays were consistent with fcc structures.

Electron Microscopy of Mesoscale Arrays of Quantum-Confined CdS Nanoparticles Formed on DNA Templates

AbstractThis paper describes the fabrication method and initial characterization of self-assembled mesoscale arrays of quantum-confined CdS nanoparticles using DNA as a template for the overall shape. Three DNAs were used: the circular and linear forms of the plasmid pUCLeu4, and circular φ×174 RF II. In all three cases, the mesoscale lengths are consistent with the A-form of DNA. The structural signatures and crystallography were confirmed using conventional and high resolution transmission electron microscopy, and electron diffraction. Optical spectroscopy demonstrated that the particles display quantum-confinement effects. This research is a fundamental demonstration of the power of combining biochemical and solid-state processing techniques.