Amorphous Al2O3 Matrix Research Articles

Formation and strain distribution of Ni/NiO core/shell magnetic nanoparticles fabricated by pulsed laser deposition

Nucleation and growth lead to substantial strain in nanoparticles embedded in a host matrix. The distribution of strain field plays an important role in the physical properties of nanoparticles. Magnetic Ni/NiO core/shell nanoparticles embedded in the amorphous Al2O3 matrix were fabricated by pulsed laser deposition. The results from a high-resolution transmission electron microscope also revealed that the core/shell nanoparticles consist of a single crystal Ni core with a faced-centered cubic structure (Space Group FM-3M) and polycrystalline NiO shell with a trigonal/rhombohedral structure (Space Group R-3mH). The growth strain of Ni/NiO core/shell nanoparticles embedded in the Al2O3 matrix was investigated. Finite element calculations clearly indicate that the NiO shell incurs large compressive strain. The compressive strain existing at the NiO shell area enables the shell material at the interface to adapt to the lattice parameters of Ni core. This process results in a relatively good crystallinity near the interface, which may be associated with the higher exchange coupling between the ferromagnetic Ni core and antiferromagnetic NiO shell.

Effect of B content on structure and magnetic properties of FeCoB-Al2O3 nanogranular films

The effect of B content on the structure, soft magnetic properties, and high frequency characteristics of as-deposited FeCoB-Al2O3 nanogranular films fabricated by radio frequency magnetron co-sputtering was studied in this work. The introduction of B into the FeCo-Al2O3 films leads to a refinement of granular microstructure. The FeCoB-Al2O3 nanogranular films consist of the FeCoB nanoparticles uniformly embedded in the amorphous Al2O3 matrix. An addition of a small amount of B into the FeCo-Al2O3 films can markedly decrease the coercivity of the films. The excellent magnetic softness with a low coercivity of about 0.08 kA/m was achieved in the FeCoB-Al2O3 films. The Henkel plots confirm the existence of intergranular exchange coupling in the FeCoB-Al2O3 films. The FeCoB-Al2O3 films with low B content exhibit a high permeability over 200 at low frequency and a high-resonance frequency of 3.2 GHz, implying a high cut-off frequency for high frequency applications.

Room-Temperature Coercivity of Ni/NiO Core/Shell Nanoparticles Fabricated by Pulsed Laser Deposition

Ni/NiO core/shell nanoparticles embedded in an amorphous Al2O3 matrix were fabricated by pulsed laser deposition. The core/shell nanoparticles consist of a single-crystal Ni core and polycrystalline NiO shell. High coercivity at room temperature was achieved in the sample with Ni/NiO nanoparticles, which can be attributed to the interfacial interaction between the ferromagnetic Ni core and the antiferromagnetic NiO shell. These results indicate that the formed Ni/NiO core/shell structure is favorable for the large coercivity application.

Sb-MOx-C (M = Al, Ti, or Mo) Nanocomposite Anodes for Lithium-Ion Batteries

Sb-MOx-C (M = Al, Ti, and Mo) nanocomposites have been synthesized by a mechanochemical reduction of Sb2O3 with, respectively, Al, Ti, and Mo, in the presence of carbon (acetylene black). X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and scanning transmission electron microscopy (STEM) data reveal that these nanocomposites are composed of uniformly dispersed nanostructured antimony in the amorphous Al2O3, TiO2, or MoO3 matrix, along with conductive carbon. These composite electrodes exhibit excellent electrochemical cycling performance and rate capability in lithium cells, compared to pure antimony. Among the three Sb-MOx-C systems studied, the M = Al system with Al2O3 as the amorphous phase exhibits the best electrochemical performance, offering a capacity of >430 mAh/g after 100 cycles. The improvement in the cycling performance, compared to that of pure antimony, is attributed to a homogeneous distribution of the electrochemically active Sb nan...

Magnetron Sputtered nc-Al/<I>α</I>-Al<SUB>2</SUB>O<SUB>3</SUB> Nanocomposite Thin Films for Nonvolatile Memory Application

In this paper, we developed nc-Al/a-Al2O3 nanocomposite thin films using magnetron sputtering. The nc-Al/a-Al2O3 films were sputtered on p-type Si substrates from pure Al target in gas mixture of Ar and O2. X-ray photoelectron spectroscopy and high resolution transmission electron microscope studies confirm that the nanocrystalline Al are embedded in amorphous Al2O3 matrix thus nc-Al/ a-Al2O3 nanocomposite forms. This nanocomposite thin film exhibits memory effect as a result of charge trapping.

Thermal Stability and Phase Transformations of γ‐/Amorphous‐Al2O3 Thin Films

AbstractMagnetron‐sputtered Al2O3 thin films were annealed in ambient air. The phase compositions of the as‐deposited Al2O3 films were (i) fully amorphous, (ii) nanocrystalline γ‐Al2O3 in an amorphous Al2O3 matrix, and (iii) fully crystalline γ. For all samples, annealing to 1 100–1 150 °C resulted in a transformation to α‐alumina. The transformation paths depend on the phase fraction of γ in the as‐deposited films. For amorphous films and films with low initial γ fraction, the intermediate phase θ‐Al2O3 appeared in the range of 1 000–1 100 °C. For predominantly crystalline γ‐Al2O3 as‐deposited films no intermediate Al2O3 phases were observed, indicating a direct γ‐to‐α phase transformation at ∼1 100 °C.

Acoustic Vibration Modes and Electron–Lattice Coupling in Self-Assembled Silver Nanocolumns

Using ultrafast spectroscopy, we investigated electron-lattice coupling and acoustic vibrations in self-assembled silver nanocolumns embedded in an amorphous Al2O3 matrix. The measured electron-lattice energy exchange time is smaller in the nanocolumns than in bulk silver, with a value very close to that of isolated nanospheres with comparable surface to volume ratio. Two vibration modes were detected and ascribed to the breathing and extensional mode of the nanocolumns, in agreement with numerical simulations.

Evidence of bayerite clusters within the AAO amorphous bulk alumina. Consequence for AAO SANS matching properties.

ABSTRACTSmall Angle Neutron Scattering (SANS) and neutron diffraction are used to probe the structure of Anodized Aluminium Oxide (AAO) in an extended Q range, from 7.10−3 to 16 Å−1. In the small angle region [7. 10−3 Å−1 − 7. 10−2 Å−1], impregnation of a D2O/H2O mixture within the AAO porous structure, leads to a dramatic decrease of the coherent SANS signal by two orders of magnitude, but perfect matching of the membrane cannot be reached. We explain such an imperfect matching by the presence of 1 nm in size Bayerite domains within the bulk of the amorphous Al2O3 matrix, as detected by Neutron diffraction in the [0.3 Å−1 − 4 Å−1] Q range. Traces of sulfur, probably incorporated during the anodic synthesis process in a H2SO4 solution, are also detected by Scanning Electron Microscopy with Energy Dispersive X-ray analysis (SEM-EDX). We estimate the AAO neutron scattering length to be 4.22 1010 cm−2. This value is an important input, when taking advantage of neutron contrast matching to probe the large scale filling homogeneity (adsorption of a liquid or deposition of a solid) within the porous structure of AAO membranes.

Observation of Co/CoO nanoparticles below the critical size for exchange bias

We compare the magnetic properties of pure and oxidized Co nanoparticles embedded in an amorphous Al2O3 matrix. Nanoparticles with diameters of 2 or 3 nm were prepared by alternate pulsed laser deposition in high vacuum conditions, and some of them were exposed to O2 after production and before being embedded. The nanoparticles are organized in layers, the effective edge-to-edge in-depth separation being 5 or 10 nm. The lower saturation magnetizations per Co atom for the samples containing oxidized nanoparticles provide evidence for the formation of antiferromagnetic CoO shells in the nanoparticles. None of the samples with Co/CoO nanoparticles show exchange bias, while vertical hysteresis loop shifts and enhanced coercivities (as compared to samples with pure Co nanoparticles) are observed. This constitutes evidence for the nanoparticles size being in all cases smaller than the critical size for exchange bias. The difference in coercivity versus temperature dependences for the samples with pure and oxidized Co nanoparticles shows that the exchange anisotropy in Co/CoO nanoparticles appears at temperatures lower than 50 K.

Structural Characterization of Sputtered Composite Stabilized ZrO<sub>2</sub> Thin Films

Zirconia (ZrO2) exhibits three different polymorphic phases as a function of the thermal and pressure conditions (cubic, tetragonal and monoclinic). The use of zirconia coatings at high temperatures requires it to be stabilized at room temperature in order to maintain the high temperature phases when subjected to thermal cycles. For this purpose, this work reports different ways to stabilize ZrO2 coatings produced by DC reactive magnetron sputtering. We have produced stabilized ZrO2 coatings by doping with other metallic and rare earth oxides (Y2O3 and Gd2O3), depositing nanostructured ZrO2 crystallites in an amorphous Al2O3 matrix and using a ZrO2/Al2O3 nanolaminated structure. A comparative study of the coatings produced is presented along with their structural stabilization using different approaches. For the doped coatings the tetragonal or cubic phases were obtained as a function of the dopant percentage and for the nanostructured and nanolayered structures the stabilization mechanism is related to the constraining of the zirconia nanocrystallites and the capacity to maintain its size under certain value.

Characterization of (Co0.45Fe0.45Zr0.10)x(Al2O3)1–x nanocomposite films applicable as spintronic materials

AbstractThe combined use of scanning electron microscopy, Mössbauer spectrometry, X‐ray diffraction and also DC/AC electrical and magnetic characterization techniques allowed this work to elucidate the influence of metal‐to‐dielectric component ration, and effect of oxygen in gas mixture on phase structure and properties of the Co0.45Fe0.45Zr0.10 ferromagnetic nanoparticles in amorphous dielectric Al2O3 matrix. The films of 3–5 µm thickness with compositions of 30 at.% < X < 60 at.% were sputtered on a single substrate from the compound target in the chamber containing either pure Ar or Ar‐O gas mixture. The films sputtered in Ar‐O mixture resulted in partial oxidation of metallic nanoparticles causing the shift of percolation threshold X c to 55 at.% as compared to 45–47 at.% for the samples deposited in pure Ar. Mössbauer studies of the films sputtered in Ar gas have shown that at X < X c metallic nanoparticles were at superparamagnetic state but for X > X c metallic phase showed a ferromagnetic behavior. DC carrier transport at the temperatures of 77–350 K were explained by hopping mechanisms and described by Mott and Shklowski‐Efros laws. That was also confirmed by the relationship power law of Z ′(f ) ∼ f –s (s ≤ 2) for real part of impedance at low temperatures (<120 K) for the samples on dielectric side of MIT. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Nanoparticle morphology in FeCo-Al2O3granular films with tunneling giant magnetoresistance

A series of FeCo–Al2O3 granular films were prepared by a magnetron-controlled sputtering system. The tunneling giant magnetoresistance and nanoparticle morphology of FeCo particles in FeCo–Al2O3 granular films were directly determined utilizing a conventional four-probe method and TEM (HRTEM) observation, respectively. The results indicated that the tunneling giant magnetoresistance can reach a maximum of 6.9% at about 32.8 vol% FeCo particles, so far the highest value reported at room temperature and under an applied field of 12.5 kOe. Meanwhile, the sensitivity of TMR also reaches a maximum at about 32.8 vol% FeCo particles. In addition, TEM and HRTEM observation disclosed that FeCo–Al2O3 films consist of FeCo nanoparticles with bcc structure or amorphous FeCo phase dispersed in amorphous or crystalline Al2O3 matrix. For films with lower volume fraction of FeCo particles, the size distribution of FeCo particles satisfied a log-normal function. With increasing volume fraction of FeCo particles, the size distribution of FeCo particles deviated gradually from a log-normal function. Meanwhile, the average size of FeCo particles increased monotonically with increasing volume fraction of FeCo particles, leading to the fact that TMR can reach a peak value at a certain middle particle size. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Production of self-aligned metal nanocolumns embedded in an oxide matrix film

Self-aligned silver nanocolumns embedded in amorphous Al2O3 matrix are produced by alternate pulsed laser deposition at room temperature in a single step process. The morphology and the spatial distribution have been investigated by standard, high resolution, and scanning transmission electron microscopy. Polycrystalline nanocolumns having diameters close to 3nm and heights up to 7nm are all oriented perpendicular to the substrate surface. The self-alignment of the nanocolumns is the consequence of the deposition of consecutive layers of Ag nanoparticles using the first layer as template for the ordered growth of the nanocolumns. Control over the density, diameter and height of the nanocolumns is achievable by tailoring the deposition sequence.

Critical size for exchange bias in ferromagnetic-antiferromagnetic particles

We present a study of the magnetic properties of oxidized Co nanoparticles with an average grain size of 3nm, embedded in an amorphous Al2O3 matrix. These nanoparticles can be considered as imperfect Co-core CoO-shell systems. Magnetization measurements after magnetic field cooling show a vertical shift of the hysteresis loop, while no exchange bias is observed. With a simple model, we show that there is a critical grain size for hybrid ferromagnetic-antiferromagnetic particles, below which exchange bias is absent for any ratio of ferromagnetic and antiferromagnetic constituents. The reason is that the interfacial exchange energy dominates over other energies in the system due to a large surface-to-volume ratio in the nanoparticles.

X-ray diffraction and X-ray photoelectron spectroscopy studies of stabilised cobalt nanoparticles with a thin Al2O3 surface layer

Cobalt nanoparticles supported in a self confined crystallite dimension of diameter D =34–41 nm, with a thin Al2O3 surface layer were synthesised in a hydrogen gas environment at 975–1125 K temperature by co-reduction of Co2+ ions dispersed in an amorphous AlO(OH).αH2O gel. During heating, the gel decomposes at low temperatures, between 425 and 675 K, resulting in a dispersed high energy structure of Co2+ cations amorphous Al2O3 matrix. An effective reconstructive Co2++H2→Co+2H+ co-reduction reaction occurs at a temperature as low as 975 K, producing Co nanoparticles encapsulated in thin amorphous Al2O3 layers. The formation and existence of an Al2O3 layer (with a limited thickness up to terc, where 2rc≈4.28 nm is the critical dimension in a stable crystallite) over a growing Co particle controls the dynamics of its growth in a self confined dimension in a high energy state of fcc or bcc allotrope. The results pertaining to the Co nanoparticles stabilised by Al2O3 surface film are analysed and discussed in terms of thermal analysis, X-ray diffraction, microstructure and X-ray photoelectron spectroscopy.

Optical response of mixed Ag-Cu nanocrystals produced by pulsed laser deposition

Ag - Cu mixed nanocrystals (NCs) have been embedded in an amorphous Al2O3 matrix by alternate ablation of pure metal and Al2O3 targets. The composition of the NCs has been varied from pure Cu to pure Ag. Structural analysis by high-resolution electron microscopy (HREM) has shown that in the whole range of compositions, the NCs have average in-plane diameters in the range 3–4nm and are wider in the in-plane direction than their height, the aspect ratio being 1.4–1.6. The optical properties of the resulting nanocomposite films have instead a strong dependence on the NCs composition. A single surface plasmon resonance (SPR) has been observed whose wavelength blueshifts from that of Cu to that of Ag as the Ag content increases, thus allowing the SPR to be tuned over a broad interval (424–572nm). Although the nucleation and growth mechanisms have been found to depend on the metal that was deposited first, the optical response has no significant sensitivity to the deposition sequence and thus, it is concluded that more controllable results are achieved by depositing the lighter element first. Finally, HREM together with selective area electron diffraction show that the Ag-CuNCs are most likely formed by metastable solid solutions of Ag and Cu.

Third order nonlinear optical susceptibility of Cu:Al2O3 nanocomposites: From spherical nanoparticles to the percolation threshold

The third order optical susceptibility of metal-dielectric nanocomposite films (Cu:Al2O3) has been determined by degenerate four wave mixing. The films have been synthesized by alternate pulsed laser deposition and consisted of Cu nanoparticles in an amorphous Al2O3 matrix. They have metal volume fractions, p, ranging from 0.07 to 0.45, and morphologies that range from spherical particles (diameter, φ∼2 nm) to a random network when close to the percolation threshold. In nanocomposites containing isolated oblate spheroids (p⩽0.17), the optical response at wavelengths close to that of the surface plasmon resonance (SPR) can be described in the frame of the Maxwell-Garnett effective medium theory. Above the particle coalescence threshold, in nanocomposites with higher Cu content (p⩾0.2), both the linear absorption in the near-infrared and the third order nonlinear optical susceptibility at the SPR are greatly enhanced, the latter achieving values as high as 1.8×10−7 esu. These results are discussed in terms of multipolar interactions among particles and giant local resonance effects that appear as a consequence of the particle coalescence and the increase in size of the nanocrystals.

Low-temperature metalorganic chemical vapor deposition of Al2O3 for advanced complementary metal-oxide semiconductor gate dielectric applications

A low-temperature metalorganic chemical vapor deposition process was developed and optimized, using a design of experiments approach, for the growth of ultrathin aluminum oxide (Al2O3) as a potential gate dielectric in emerging semiconductor device applications. The process used the aluminum β-diketonate metalorganic precursor [aluminum(III) 2,4-pentanedionate] and water as, respectively, the metal and oxygen source reactants to grow Al2O3 films over a temperature range from 250 to 450 °C. The resulting films were analyzed by x-ray photoelectron spectroscopy, x-ray diffraction measurements, Rutherford backscattering spectrometry, nuclear-reaction analysis for hydrogen profiling, scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. The as-deposited Al2O3 phase was amorphous and dense and exhibited carbon and hydrogen incorporation of, respectively, 1 and 10 at.%. Postannealing at 600 °C led to a reduction in hydrogen concentration to 1 at.%, while maintaining an amorphous Al2O3 matrix.

Structure and thermal stability of Fe : Al2O3 nanocomposite films

Nanocomposite films consisting of Fe nanocrystals (NCs) embedded in an amorphous Al2O3 matrix have been fabricated by alternate pulsed laser deposition in vacuum. The size, shape and distribution of the Fe NCs have been analysed by high resolution electron microscopy. The thermal stability of the films has been studied by in situ annealing in the electron microscope, and the chemical state of the Fe has been investigated by electron energy loss spectroscopy. The nucleation and growth of the Fe NCs has been studied as a function of the number of pulses. Nucleation is homogenous over the surface of the Al2O3 at the beginning of deposition. The distribution of the NCs is uniform and their in-plane shape can be approximated by ellipses for a small number of pulses on the Fe target. As the Fe content increases some adjacent NCs coalesce resulting in a sudden increase in dimensions and in-plane aspect ratio. Isothermal annealing treatments for up to 160 min show that the Fe NCs are stable up to 400\\r{}C, with morphological changes occurring at higher temperatures. The as-prepared Fe NCs were found to be α-Fe and no changes of oxidation state of Fe were observed after annealing.

Structural and Optical Properties of NdAlO3 Nanocrystals Embedded in an Al2O3 Matrix

Fabrication of an oxide−oxide nanocomposite, containing a homogeneous dispersion of NdAlO3 nanocrystals in an Al2O3 matrix, is achieved by employing a single molecular compound, [NdAl3(OPri)12(PriOH)], in the sol−gel process. X-ray diffraction patterns of the composite material show NdAlO3 to be the only crystalline phase, up to 1200 °C, with an equimolar part of amorphous Al2O3. The crystallization of alumina (δ-, γ- and κ-phases) occurs at higher temperatures (>1300 °C). TEM investigations reveal a bimodal distribution of particles where NdAlO3 crystallites are uniformly dispersed in an amorphous Al2O3 matrix. HR-TEM analysis shows a composite structure formed by NdAlO3 nanocrystals (50−60 nm) homogeneously incorporated in a matrix of smaller alumina particles (10−12 nm). The existence of two chemically different Al(III) species (Al2O3 and NdAlO3), in the composite, is proven by 27Al solid-state NMR and XPS studies. The line deconvolution of the Al 2p XPS spectrum reveals two components in a 1:2 ratio w...