Amorphous Al2O3 Matrix Research Articles

Growth characteristics and properties of RuAlO hybrid films fabricated by atomic layer deposition

In this work, RuAlO hybridfilmswere synthesized by alternating ALD sequences of Al2O3and Ru for electrical conductivity regulation.O2exposure has significant effectson the nucleation and etching processes of Ru, therefore the O2exposure was first optimized for efficient growth of Ru. A set of RuAlOhybrid films with different Ru subcycle ratioswere fabricated.In-situ quartz crystal microbalance (QCM) measurementrevealed acertain nucleation delaywhenRu was deposited on the surface ofAl2O3, whileno nucleation delay was observed when Al2O3was deposited on Ru. High resolution transmission electron microscopy(HRTEM) characterization confirmed formation ofRu grains embedded in amorphous Al2O3matrix. Elemental mapping results demonstrated that RuandAlelementsareuniformly distributedon the cross section ofthe film.In the grazing incidence angle X-ray diffraction (GIAXRD) patterns ofRuAlOhybrid films,characteristic diffraction peaks of Ru appear with aRusubcycle ratio ≥ 70 %. By properly tuning the Ru to Al2O3ALD subcycle ratio, electrical resistivity of theRuAlOhybrid film could beadjusted over a wide range of 10-3 ∼ 1011Ω·cm.

High temperature electrical insulation and adhesion of nanocrystalline YSZ/Al2O3 composite film for thin-film thermocouples on Ni-based superalloy substrates

Electrical insulation and adhesion are two vital aspects for the integrated thin film sensors with Ni-based superalloy substrates. In this paper, nanocrystalline yttria-stabilized zirconia/alumina (YSZ/Al2O3) composite film was fabricated on Ni-based superalloy substrate by reactive magnetron sputtering. The X-ray diffraction and scanning electron microscope results show that tetragonal zirconia (t-ZrO2) nanocrystalline is homogeneously dispersed in the amorphous Al2O3 matrix. The crystal size of the t-ZrO2 nanocrystalline is well maintained with the temperature up to 1000 °C. The resistivity of the film is about 108 Ω·cm at 1000 °C, which is three orders of magnitude higher than that of the pure Al2O3 film. Furthermore, this YSZ/Al2O3 film demonstrates superior toughness and adhesion with the Ni-based superalloy substrate even after undergoing high-temperature cycles. Finally, a S-type thin-film thermocouple was fabricated on the YSZ/Al2O3 film shows good stability, repeatability, and linearity in multiple thermal cycling tests up to 900 °C exceeded 48 h.

Novel Ni–Al nanosheet catalyst with homogeneously embedded nickel nanoparticles for hydrogen-rich syngas production from biomass pyrolysis

In this work, an innovative porous Ni–Al nanosheet catalyst synthesized by a homogeneous precipitation method via urea hydrolysis is proposed for enhanced hydrogen-rich syngas production from catalytic pyrolysis of rice husk in a two-stage reactor system. The role of synthesis temperature in modulating the crystalline composition, particle size, metal dispersion as well as porous structure of resulting Ni–Al nanocomposite has been delineated. The results indicate that fine spherical NiO and metallic Ni0 nanoparticles are homogeneously embedded in amorphous Al2O3 matrix for all Ni–Al catalysts, which also have developed bimodal micro/mesoporous structure with high surface areas (513–948 m2/g). Catalytic tests show that these highly active catalysts exhibit almost ten times higher hydrogen production rate (7.74–17.39 mmol/g biomass) and H2/CO molar ratio (1.96–2.74) than that in the absence of catalyst (0.56–1.64 mmol/g, 0.11–0.24) by tuning catalytic temperatures. The Ni–Al catalysts that with the presence of metallic Ni0 and developed porous structure exhibit higher catalytic activity and suppression of coke deposition through providing more active sites for catalytic cracking and reforming reactions, and rapid diffusion of intermediate products.

Real-time tracking of the self-assembled growth of a 3D Ge quantum dot lattice in an alumina matrix

A laboratory in situ grazing-incidence small-angle X-ray scattering (GISAXS) tracking of the self-assembled growth of a regular 3D Ge quantum dot (QD) structure in an amorphous Al2O3 matrix during the ion beam sputter deposition of a periodic Ge/Al2O3 multilayer on silicon is reported. A 573 K substrate temperature proved to be necessary to achieve the self-assembly effect. Relying on a fast repeated acquisition of GISAXS patterns, the temporal evolution of the growing 3D Ge QD structure was analyzed bilayer by bilayer to determine its type, lateral and vertical correlation lengths, and inter-QD distance. The QD structure was found to have body-centered tetragonal lattice type with ABA stacking, with the lattice parameters refined by fitting the final GISAXS pattern relying on a paracrystal model. A single set of paracrystal parameters enables one to simulate the temporal evolution of the in situ GISAXS patterns throughout the deposition process, suggesting that the Ge QD self-assembly is driven from the very beginning solely by the growing surface morphology. Ex situ GISAXS and X-ray reflectivity measurements along with a cross-section high-resolution transmission electron microscopy analysis complete the study.

Significantly enhanced dielectric constant and energy density in Au/Al2O3 nanocomposite thin films

Nano-Au self-producing process was investigated to improve the dielectric properties of amorphous Al2O3 thin films via controlling the heat-treatment temperature. The results of high-resolution transmission electron microscopy (HTEM) showed that the nano-Au particles were uniformly dispersed in the amorphous Al2O3 matrix. More importantly, nano-Au particles could supply more space polarization charges and the dielectric constant of the nanocomposite thin film was enhanced from 8.2 to 32.2. As a result, the ultimate energy density was up to 13.3 J/cm3 with 2 mol% doping content, which is increased by about 137.5% compared to the pure counterpart. Furthermore, the thermally stimulated depolarization current (TSDC) analysis provided a powerful evidence to testify that the enhancement of the dielectric constant was originated from the trap charge polarization after the nano-Au doping. The finite element simulation results showed that the more space charges were generated around the nano-Au particles, which is more intuitive to understand the reason for the increase of the dielectric constant.

Tunable optical properties of plasmonic Au/Al2O3nanocomposite thin films analyzed by spectroscopic ellipsometry accounting surface characteristics

In the present work, we have fabricated plasmonic gold/alumina nanocomposite (Au/Al2O3 NC) thin films on a glass substrate at room temperature by RF magnetron co-sputtering. The influence of the film thickness (∼10-40 nm) on the optical and other physical properties of the samples was investigated and correlated with the structural and compositional properties. The X-ray diffractometer measurement revealed the formation of Au nanoparticles with average crystallite size (5-9.2nm) embedded in an amorphous Al2O3 matrix. The energy-dispersive X ray and X-ray photoelectron spectroscopy results confirmed the formation of Au/Al2O3 NC quantitatively and qualitatively and it was observed that atomic% of Au increased by increasing thickness. The optical constants of the plasmonic Au/Al2O3 NC thin films were examined by variable angle spectroscopic ellipsometry in the wide spectral range of 246-1688nm, accounting the surface characteristics in the optical stack model, and the obtained results are expected to be unique. Additionally, a thickness-dependent blueshift (631-590nm) of surface plasmon resonance peak was observed in the absorption spectra. These findings of the plasmonic Au/Al2O3 NC films may allow the design and fabrication of small, compact, and efficient devices for optoelectronic and photonic applications.

Mechanical behavior of ultrathin sputter deposited porous amorphous Al2O3 films

The determination of the mechanical properties of porous amorphous Al2O3 thin films is essential to address reliability issues in wear-resistant, optical and electronic coating applications. Testing the mechanical properties of Al2O3 films thinner than 200 nm is challenging, and the link between the mechanical behavior and the microstructure of such films is largely unknown. Herein, we report on the elastic and viscoplastic mechanical properties of amorphous Al2O3 thin films synthesized by reactive magnetron sputtering using a combination of internal stress, nanoindentation, and on-chip uniaxial tensile testing, together with mechanical homogenization models to separate the effect of porosity from intrinsic variations of the response of the sound material. The porosity is made of voids with 2–30 nm diameter. The Young's modulus and hardness of the films decrease by a factor of two when the deposition pressure increases from 1.2 to 8 mTorr. The contribution of porosity was found to be small, and a change in the atomic structure of the amorphous Al2O3 matrix is hypothesized to be the main contributing factor. The activation volume associated to the viscoplastic deformation mechanism is around 100 Å3. Differences in the atomic structure of the films could not be revealed by electron diffraction, pointing to a minute effect of atomic arrangement on the elastic properties.

Molecular level mixing: An approach for synthesis of homogenous hybrid ceramic nanocomposite powders

An approach for the production of homogenous Al2O3–5SiC-1CNTs hybrid nanocomposite via molecular level mixing is reported. Aluminum nitrate nonahydrate, dimethylformamide, SiCβ nanoparticles, and functionalized CNTs were magnetically stirred and sonicated to prepare the desired slurry. Heating of the dried mixture in normal atmosphere at 400°C for 4h resulted in the formation of amorphous Al2O3 matrix, imbedded with CNTs and SiC nanoparticles, which crystallized during subsequent sintering. FT-IR was used to characterize bonds formed during powder synthesis. The uniform distribution of CNTs and SiC in the synthesized powder and consolidated sample was ascertained using x-ray mapping, FE-SEM, and TEM. Dense alumina and homogeneous Al2O3–5SiC-1CNT nanocomposite were consolidated by spark plasma sintering at 1500°C for 10min. The composite possessed improved fracture toughness (by ∼33%) and slightly reduced hardness (by ∼4%) with respect to monolithic Al2O3.

Morphology-engineered strain transformation in Ge/GeO2 core/shell nanoparticles

Growth of nanoparticles embedded in a host matrix can lead to substantial strain. Ge/GeO2 core/shell nanoparticles embedded in amorphous Al2O3 matrix is fabricated by the pulsed laser deposition method and rapid thermal annealing technique, which is confirmed by the experimental HRTEM result and consistent with Zhdanov׳s theoretical prediction. A finite-element calculation is performed to investigate the tuning effect on the strain by the morphology evolution of the Ge/GeO2 core/shell nanoparticle embedded in Al2O3 matrix. The simulated result indicates that the strain at the interface between the core and the shell strongly depends on the morphology of the nanoparticles. Moreover, it can be found that there is a dramatic transformation of the strain on Ge core from tensile to compressive strain during the shrinkage of Ge core and the expansion of GeO2 shell. The simulated results indicate that the strain can be designed by tuning the morphology of the nanoparticles, which provides an opportunity to engineer the properties of the nano-sized core/shell structures.

Strain Distribution of Au and Ag Nanoparticles Embedded in Al2O3 Thin Film

Au and Ag nanoparticles embedded in amorphous Al2O3 matrix are fabricated by the pulsed laser deposition (PLD) method and rapid thermal annealing (RTA) technique, which are confirmed by the experimental high‐resolution transmission electron microscope (HRTEM) results, respectively. The strain distribution of Au and Ag nanoparticles embedded in the Al2O3 matrix is investigated by the finite‐element (FE) calculations. The simulation results clearly indicate that both the Au and Ag nanoparticles incur compressive strain by the Al2O3 matrix. However, the compressive strain existing on the Au nanoparticle is much weaker than that on the Ag nanoparticle. This phenomenon can be attributed to the reason that Young’s modulus of Au is larger than that of Ag. This different strain distribution of Au and Ag nanoparticles in the same host matrix may have a significant influence on the technological potential applications of the Au‐Ag alloy nanoparticles.

Influence of sputtering atmosphere on hopping conductance in granular nanocomposite (FeCoZr)x(Al2O3)1−x films

The present paper investigates conductivity σ for a wide temperature range of 2K<T<300K in the nanocomposite (FeCoZr)x(Al2O3)1−x films (1–6μm thicknesses, 0.30<x<1.00) which were sputtered on glass–ceramic substrates from the compound target in vacuum chamber filled with Ar (set 1 films) or Ar+O2 gas mixture (set 2 films). TEM and HRTEM images revealed that granular films of set 1 containing nanosized crystalline metallic bcc α-FeCo-based nanoparticles embedded in the amorphous Al2O3 matrix. In case of Ar+O atmosphere of deposited α-FeCo-based crystalline “cores” with stabilized sizes were covered with an amorphous FeCo-based oxide “shells”. The σ(T) dependences were found to be in agreement with this phase structure. They show transition of the set 1 films σ(T) curves with x from tunneling (hopping) behavior below the percolation threshold x<xC≈0.46 to metallic one for x>xC. Moreover, as opposed to such behavior, for the set 2 films hopping mechanism of σ(T) was revealed far beyond xC due to the formation of semiconducting FeCo-based oxide “shells” separating electrically contacting FeCoZr cores.

Influence of annealing conditions on the structural and photoluminescence properties of Ge quantum dot lattices in a continuous Ge + Al2 O3 film

We investigate the structural and photoluminescence (PL) properties of regularly ordered lattices of Ge quantum dots (QDs) formed by self-assembly in a thick film of Ge + Al2O3 mixture. The effects of annealing at different temperatures and different annealing environments (vacuum and air) are studied. We show that the regular ordering of QDs remains unchanged for annealing up to 800 °C in vacuum, while it is preserved up to 700 °C when annealing is performed in air. The inner/crystalline structure of Ge QDs and their shape depend on the annealing conditions. The crystalline grains of γ-Al2O3 form in the initially amorphous Al2O3 matrix after annealing at 800 °C. The PL spectra are found to consist of two main contributions, one from the matrix and the other from Ge QDs. Both contributions depend strongly on the annealing conditions. The annealing treatment of Ge quantum dot lattices formed by self-assembly in alumina matrix influence significantly the internal structure and photoluminescence properties of the films.

Growth of a three-dimensional anisotropic lattice of Ge quantum dots in an amorphous alumina matrix

Simple processes for the preparation of semiconductor quantum dot lattices embedded in dielectric amorphous matrices play an important role in various nanotechnology applications. Of particular interest are quantum dot lattices with properties that differ significantly in different directions parallel to the material surface. Here, a simple method is demonstrated for the fabrication of an anisotropic lattice of Ge quantum dots in an amorphous Al2O3matrix by a self-assembly process. A specific deposition geometry with an oblique incidence of the Ge and Al2O3adparticles was used during magnetron sputtering deposition to achieve the desired anisotropy. The observed Ge quantum dot ordering is explained by a combination of directional diffusion of adparticles from the Ge and Al2O3targets and a shadowing process which occurs during deposition as a result of the specific surface morphology. The prepared material shows a strong anisotropy of the electrical conductivity in different directions parallel to the sample surface.

Fabrication and magnetic properties of FePt/Al2O3 composite film by atomic-layer-deposition

Self-assembled face-centered cubic FePt nanoparticles were capped by a amorphous Al2O3 capping layer with the thickness of 10nm using the atomic layer deposition (ALD) technology, and transmission electron microscopy indicates that the FePt nanocrystals are well-separated. After annealing the composite film at 700°C, well-monodispersed face-centered tetragonal FePt particles can be obtained, yielding a coercivity value Hc=5.9kOe with the magnetocrystalline anisotropy of 3.86MJ/m3. The thermal factor for the composite film is 68.5, meeting the industry requirement (KuV/kBT ≥ 50). The protection of amorphous Al2O3 matrix can effectively inhibit grain growth and particle aggregation, and preserve the ordered domains of FePt nanoparticles during the L10 ordering transition. The combination of ALD-capping layer and self-assembled FePt nanoparticles provides a new potential approach to fabricate patterned magnetic recording media with ultrahigh areal density.

Interfacial microstructure between thick aluminium wires and aluminium alloy pads formed by ultrasonic bonding

Interfacial microstructures between thick Al wires and pads formed by ultrasonic bonding were analysed to investigate the behaviour of bond preventing phases. The wire and the substrate form direct bond interface in the central region of the bond area, whereas the surfaces of them in the peripheral region are covered by amorphous Al2O3 bond preventing layer. The amorphous Al2O3 layer is chipped off the surfaces by application of ultrasonic vibration and forms a number of spherical particles intervening between the wire and the substrate. The particles consist of amorphous Al2O3 matrix and lamellae of Al. The particles grow by accumulating the amorphous Al2O3 on the surfaces of the wire and the substrate in contact, moving outward the bond area. The bond area expands through the area where the bond preventing layer has been removed.

Influence of annealing conditions on the formation of regular lattices of voids and Ge quantum dots in an amorphous alumina matrix

In this work, the influence of air pressure during the annealing of Ge quantum dot (QD) lattices embedded in an amorphous Al2O3 matrix on the structural, morphological and compositional properties of the film is studied. The formation of a regularly ordered void lattice after performing a thermal annealing process is explored. Our results show that both the Ge desorption from the film and the regular ordering of the QDs are very sensitive to the annealing parameters. The conditions for the formation of a void lattice, a crystalline Ge QD lattice and a disordered QD lattice are presented. The observed effects are explained in terms of oxygen interaction with the Ge present in the film.

Cu2Sb–Al2O3–C nanocomposite alloy anodes with exceptional cycle life for lithium ion batteries

Cu2Sb–Al2O3–C nanocomposite alloy material has been synthesized via a mechanochemical reduction of Sb2O3 with Al and Cu metals in the presence of carbon (acetylene black). X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) reveal that these nanocomposites are composed of nanostructured Cu2Sb particles in an amorphous Al2O3 and conductive carbon matrix. The presence of Al2O3 in the nanocomposite drastically improves the cycle life of the Cu2Sb–C material and enables a remarkable capacity retention of 80% over 500 cycles (330 mAh/g) after the first cycle. With a tap density of ∼1.4 g cm−3, Cu2Sb–Al2O3–C offers significantly higher volumetric capacity than the commonly used graphite anode. A conductive Cu framework supports the electrochemically-active Sb and results in low impedance and good rate capability, while the matrix of ceramic oxide and conductive carbon reduce the agglomeration of the nanocrystalline Cu2Sb particles and buffer the volume expansion/contraction that occurs during cycling. TEM of the electrodes that were cycled between 1 and 500 cycles shows the evolution of the morphology of Cu2Sb particles in the presence and absence of Al2O3. Because the Cu2Sb–Al2O3–C nanocomposite electrodes can operate well above the potential where lithium plating occurs, it is viewed as a safer alternative to graphite anodes.

Magnetoresistance in FeCoZr–Al2O3 nanocomposite films containing ‘metal core–oxide shell’ nanogranules

Temperature and magnetic field dependences of electrical conductivity are systematically studied in granular films (Fe45Co45Zr10)x(Al2O3)100−x (28 ⩽ x ⩽ 64) containing crystalline metallic α-FeCo-based nanoalloy cores encapsulated in an amorphous oxide shell embedded in an amorphous Al2O3 matrix. Formation of ‘metallic core–oxide shell’ nanogranules is confirmed by transmission electron microscopy (TEM) and HRTEM. The structure of core and shell is governed with the difference in the oxidation states of Fe and Co ions investigated with EXAFS, XANES and Mössbauer spectroscopy. A considerable negative magnetoresistance (MR) effect of spin-dependent nature is observed in the whole range of x values. Its increase with decreasing temperature is correlated with the magnetic saturation of superparamagnetic metallic nanogranules. The enhanced MR effect in ‘core–shell’ granular films is related to the percolation of oxide shells and their influence through spin filtering processes. A considerable high field MR at low temperatures and the resulting deviation of MR and squared magnetization are attributed to a magnetic randomness and/or strong magnetic anisotropy of the magnetic oxide shell.

Strain-Induced Structural Phase Transition of Si Nanoparticles

The growth of nanoparticles embedded in a host matrix can lead to substantial strain. Strain can have a substantial influence on the properties of nanoparticles. In this work, Si nanoparticles embedded in an amorphous Al2O3 matrix were fabricated using pulsed laser deposition and rapid thermal annealing. The strain-induced structural transition from the cubic phase to the hexagonal phase was observed during the rapid thermal annealing process and led to the decrease of the band gap caused by the hexagonal crystalline phase. It was found that the microstructures and photoluminescence of Si nanoparticles are closely related to strain. Thus, strain engineering is an effective tool for tailoring the optical properties of Si nanoparticles.

Self-formation of nanocomposite Cu/Al2O3 thin films on CuAl dilute alloys by annealing in inert atmospheres

Self-formation of the surface protective layer on dilute CuAl alloys by annealing in inert atmospheres is investigated with thermodynamic consideration, first principles density functional simulation, and transmission electron microscopy (TEM). The calculations predict that when Cu atoms are involved in the surface Al2O3 layer, the Cu-Al2O3 solution does not exist. Instead, a phase structure of Cu and Al2O3 should be formed. Consistent with this prediction, TEM observations showed that the surface thin film consists of the nanocomposite Cu/Al2O3 structure, in which spherical Cu nanoparticles 3–5 nm in diameter are formed and disperse in the amorphous Al2O3 matrix. The results are discussed with respect to surface segregation and the stability of Cu in Al2O3.