Oxide-assisted Growth Research Articles

One-step chemical vapor deposition synthesis of Si NWs@C core/shell anodes without additional catalysts by the oxide-assisted growth mechanism for lithium-ion batteries.

Si NWs@C core/shell anodes for lithium-ion batteries were synthesized via a one-step environmental-pressure chemical vapor deposition (CVD) process utilizing nano-silicon and methane as raw materials. In this structure, the silicon nanowire core is obtained by controlling the temperature above 900 °C to catalyze the growth of nano-silicon particles coated with a natural oxide layer according to the oxide-assisted growth (OAG) mechanism, while the carbon as a protective coating shell is derived from methane cracking. In contrast to the conventional nanowire catalytic approach, this method obviates the addition of metal catalysts while ensuring a straightforward and scalable process. This Si NWs@C electrode displayed excellent electrochemical performance, exhibiting high reversible capacity (745.8 mA h g-1) and excellent cycling stability (91.3% after 100 cycles at 0.5 A g-1).

A numerical study on the influence of grain boundary oxides on dwell fatigue crack growth of a nickel-based superalloy

• A computational multi-physical modeling approach to oxide-assisted intergranular dwell crack growth of γ′ strengthened nickel-based superalloys is presented. The modeling framework explicitly couples mass transport of chemical species involved in oxide formation to the evolution of mechanical fields ahead of a crack tip. The viscoplastic response accounts for the influence of the γ′ dispersion. • Detailed Quantitative study of the interaction between the dynamic stress-assisted oxide formation ahead of the crack tip and the resulting near crack-tip stress fields was implemented. • A method for extracting crack growth rates based on failure of the oxide wedge ahead of a crack tip is presented • The predicted rates are shown to be in good agreement with available experimental data without any direct information or calibration of model parameters to crack growth rate data in advance. • Established model shows capability of reveal the effect of particle dispersion of nickel-based superalloy on the dwell fatigue crack growth rate. A theoretical treatment on the oxide-controlled dwell fatigue crack growth of a γ’ strengthened nickel-based superalloys is presented. In particular, this study investigates the influence of an externally applied load and variations in the γ’ dispersion on the grain boundary oxide growth kinetics. A dislocation-based viscoplastic constitutive description for high temperature deformation is used to simulate the stress state evolution in the vicinity of a crack at elevated temperature. The viscoplastic model explicitly accounts for multimodal γ’ particle size distributions. A multicomponent mass transport formulation is used to simulate the formation/evolution of an oxide wedge ahead of the crack tip, where stress-assisted vacancy diffusion is assumed to operate. The resulting set of constitutive and mass transport equations have been implemented within a finite element scheme. Comparison of predicted compositional fields across the matrix/oxide interface are compared with experiments and shown to be in good agreement. Simulations indicate that the presence of a fine γ’ size distribution has a strong influence on the predicted ow stress of the material and consequently on the relaxation in the vicinity of the crack-tip/oxide wedge. It is shown that a unimodal dispersion leads to reduced oxide growth rates (parabolic behavior) when compared to a bimodal one. Stability conditions for oxide formation are investigated and is associated with the prediction of compressive stresses within the oxide layer just ahead of the crack tip, which become progressively negative as the oxide wedge develops. However, mechanical equilibrium requirements induce tensile stresses at the tip of the oxide wedge, where failure of the oxide is predicted. The time taken to reach this critical stress for oxide failure has been calculated, from which dwell crack growth rates are computationally derived. The predicted rates are shown to be in good agreement with available experimental data.

Laser ablation behavior of SiO2-Nd2O3/Si-SiC-MoSi2 coated C/C composites repaired by laser cladding

A SiO2-Nd2O3/Si-SiC-MoSi2 coating was proposed to repair the damaged SiC coated C/C composites by laser cladding. Laser ablation was performed to evaluate the effect of coating repair. Results showed that the preparation of the SiO2-Nd2O3 outer layer could improve the laser ablation resistance of the specimens by weakening heat accumulation and enhancing heat consumption, which reduced the maximum temperature by 261.99 K and 694.75 K during ablation under 300 W and 500 W for 7 s. The growth of the ablative products, including the Si nanowires and SiOx spherical particles, was based on vapor deposition and oxide-assisted growth mechanism.

Graphene oxide-assisted electrochemical growth of Ni(OH)2 nanoflowers on nickel foam as electrode material for high-performance supercapacitors

Flower-like Ni(OH)2 nanostructures were electrochemically grown on nickel foam (NF) in an alkaline GO solution without adding nickel salts. The electrochemical deposition process was carried out using two modes of potentiostatic and galvanostatic to investigate the influences of the applied potential regime on the physicochemical and electrochemical properties of the resulting Ni(OH)2 nanostructures. Furthermore, to validate the promoting effect of GO on the electrochemical growth of Ni(OH)2, the same synthesis processes were performed in the absence of GO, using both the electrochemical modes. The chemical composition and morphological features of the electrodes, and the role of GO in the electrolyte greatly changed by altering the applied electrochemical mode. When a constant negative potential was applied, GO nanosheets were not reduced or deposited on the NF, and only flower-like Ni(OH)2 nanostructures were formed on the NF. Due to its hierarchical porous structure, this sample exhibited a significantly higher electrochemical performance compared to the samples prepared in the absence of GO and the sample prepared in the presence of GO by applying a constant cathodic current. Co-electrodeposition of reduced GO (rGO) nanosheets along with the Ni(OH)2 nanostructures occurred as a constant cathodic current was applied, leading to the formation of highly-packed rGO/Ni(OH)2 composite layer providing a considerably lower electrochemical performance. The electrode prepared by applying a constant potential to the NF in the alkaline solution containing GO exhibited specific capacitances of 2667.2 and 1586.8 mF cm-2 at the current densities of 4 and 40 mA cm-2, respectively. This electrode also showed a good rate capability with capacitance retention of 78% after 1000 charge/discharge cycles at the high current density of 40 mA cm-2. The assembled asymmetric supercapacitor device of Ni(OH)2/NF//AC/NF showed a high specific capacitance of 543.6 mF cm-2 at 2 mA cm-2, and the capacitance retention of 73.1% after 2000 cycles of charge/discharge at 16 mA cm-2. It also provided the highest energy density of 143 μWh cm-2 at power density of 1400 μW cm-2.

Graphene Oxide-Assisted Growth of Ultralong and Soft Single-Crystalline NaCl Ionic Nanowires for Potential Optical Nanodevices

Graphene Oxide-Assisted Growth of Ultralong and Soft Single-Crystalline NaCl Ionic Nanowires for Potential Optical Nanodevices

Visible light photocatalytic properties of one-step SnO2-templated grown SnO2/SnS2 heterostructure and SnS2 nanoflakes

The nanoflakes of SnS2/SnO2 heterostructure and SnS2 were synthesized by a one-step SnO2-templated chemical vapor deposition method. The metal oxide-assisted growth mechanism of SnS2/SnO2 heterostructure and SnS2 nanoflakes were realized through investigating serial microstructures of products with varied growth time. Furthermore, the photocatalytic activity for MB dyes degradation of varied growth time products was used to explore the effect of product microstructure under the visible light irradiation. The SnO2/SnS2 heterostructure and the oxide vacancies of nanoflakes demonstrated an improved visible light photocatalytic performance for MB degradation, which was around twice of the pure SnS2 nanoflakes and better than P25. The results of different scavengers on the degradation efficiency for MB indicate the·O2 −, and ·OH are the main active species in the photodegradation reaction. The one-step growth mechanism of SnS2/SnO2 could prove a facile process to grow metal oxide-metal sulfide heterostructure.

Adjusting the Morphology and Properties of SiC Nanowires by Catalyst Control.

We report on the growth of SiC nanowires on a single crystal Si substrate by pyrolysis of polycarbosilane and using two catalyst (Al2O3 and Ni) films with different thickness (2, 4, and 6 nm). The catalyst films were deposited on the Si substrate, and the SiC nanowires were grown according to two mechanisms, i.e., the oxide-assisted growth mechanism and vapor- liquid-solid mechanism. As a result, pearl-chain-like SiC nanowires and straight SiC nanowires were obtained. The prepared nanowires exhibited excellent photoluminescence properties, emission spectra displaying two emission peaks at 395 and 465 nm, and have good thermal stability below 1000 °C. The experimental results revealed the importance of the catalyst in controlling the morphology and properties of SiC nanowires.

Oxide-assisted growth of scalable single-crystalline graphene with seamlessly stitched millimeter-sized domains on commercial copper foils.

Chemical vapor deposition (CVD) is considered as an effective route to obtain large-area and high-quality polycrystalline graphene; however, there are still technological challenges associated with its application to achieve single crystals of graphene. Herein, we present the CVD growth of scalable single-crystalline graphene by seamless stitching millimeter-sized unidirectional aligned hexagonal domains using different types of commercial Cu foils without repeated substrate polishing and H2-annealing processes. Compared with that reported in previous studies, herein, the average size for the hexagonal graphene domains is enlarged by 1–2 orders of magnitude (from tens of micrometers to millimeter). The key factor for growth is the Cu surface monocrystallization achieved by a pre-introduced oxide layer and the sequential Ar annealing. The graphene domains exhibit an average growth rate of >20 μm min−1 and a misorientation possibility of <2%, and seamless stitching at the domain coalescence interfaces is confirmed by atomic force microscopy measurements.

Growth Mechanisms of Inductively-Coupled Plasma Torch Synthesized Silicon Nanowires and their associated photoluminescence properties.

Ultra-thin Silicon Nanowires (SiNWs) were produced by means of an industrial inductively-coupled plasma (ICP) based process. Two families of SiNWs have been identified, namely long SiNWs (up to 2–3 micron in length) and shorter ones (~100 nm). SiNWs were found to consist of a Si core (with diameter as thin as 2 nm) and a silica shell, of which the thickness varies from 5 to 20 nm. By combining advanced transmission electron microscopy (TEM) techniques, we demonstrate that the growth of the long SiNWs occurred via the Oxide Assisted Growth (OAG) mechanism, while the Vapor Liquid Solid (VLS) mechanism is responsible for the growth of shorter ones. Energy filtered TEM analyses revealed, in some cases, the existence of chapelet-like Si nanocrystals embedded in an otherwise silica nanowire. Such nanostructures are believed to result from the exposure of some OAG SiNWs to high temperatures prevailing inside the reactor. Finally, the intense photoluminescence (PL) of these ICP-grown SiNWs in the 620–950 nm spectral range is a clear indication of the occurrence of quantum confinement. Such a PL emission is in accordance with the TEM results which revealed that the size of nanostructures are indeed below the exciton Bohr radius of silicon.

Electron Transport through Thin SiO&lt;sub&gt;2&lt;/sub&gt; Films Containing Si Nanoclusters

The electron transport mechanisms through nanocomposite SiO2(Si) films containing Si nanoclusters into dielectric SiO2 matrix have been investigated. SiO2(Si) films were obtained by oxide assisted growth. At the first stage the SiOx films with different content of excess Si were deposited by LP CVD method. Second stage includes high temperature (T=1100 C) annealing of SiOx films that promotes formation of Si nanocrystals. Current transport through SiO2(Si) films were studied in temperature range 100-350 K. As it was observed the dominant mechanism of electron transport depends as on voltage and temperature. The Mott’s conductivity caused by traps near Fermi level was revealed in low-voltage range for all temperatures. At increasing the voltage the SCLC conductivity is observed for films with higher content of excess silicon while in case low content of Si the Pool-Frenkel mechanism dominates. The further increase in voltage results in a double carrier injection.

Large-Area, Transfer-Free, Oxide-Assisted Synthesis of Hexagonal Boron Nitride Films and Their Heterostructures with MoS2 and WS2.

Ultrasmooth hexagonal boron nitride (h-BN) can dramatically enhance the carrier/phonon transport in interfaced transition metal dichalcogenides (TMDs), and amplify the effect of quantum capacitance in field-effect gating. All of the current processes to realize h-BN-based heterostructures involve transfer or exfoliation. Rational chemistries and process techniques are still required to produce large-area, transfer-free, directly grown TMDs/BN heterostructures. Here, we demonstrate a novel boron-oxygen chemistry route for oxide-assisted nucleation and growth of large-area, uniform, and ultrathin h-BN directly on oxidized substrates (B/N atomic ratio = 1:1.16 ± 0.03 and optical band gap = 5.51 eV). These intimately interfaced, van der Waals heterostructures of MoS2/h-BN and WS2/h-BN benefit from 6.27-fold reduced roughness of h-BN in comparison to SiO2. This leads to reduction in scattering from roughness and charged impurities, and enhanced carrier mobility verified by an increase in electrical conductivity (5 times for MoS2/h-BN and 2 times for WS2/h-BN). Further, the heterostructures are devoid of wrinkles and adsorbates, which is critical for 2D nanoelectronics. The versatile process can potentially be extrapolated to realize a variety of heterostructures with complex sandwiched 2D electronic circuitry.

Understanding the role of silicon oxide shell in oxide-assisted SiNWs growth

The role of silicon oxide shell in oxide-assisted SiNWs growth is studied by performing ab initio molecular dynamics simulations on the structural and dynamical properties of the interface between crystalline Si(111) surface and disorder SiO thin film. Si atoms in the SiO film tends to aggregate into the vicinity of the Si(111)/SiO interface. In addition, the diffusion of Si atoms at the interface is anisotropic - the diffusion along the interface is several times faster than that perpendicular to the interface. The segregation and anisotropic diffusion of Si atoms at the Si(111)/SiO interface shed interesting light into the mechanism of oxide-assisted silicon nanowire growth.

Non-catalytic vapor synthesis of millimeter-scale α-Si3N4 nanowires from oxidized silicon powders

Millimeter-scale single-crystalline α-Si3N4 nanowires were synthesized via heating oxidized silicon powders, without adding carbon and metal catalysts, at 1390°C in N2 atmosphere based on an oxide-assisted growth process. The crystal structure, morphology and photoluminescence property of synthesized nanowires were investigated. With changing oxygen content of silicon powders from 3.28wt% to 24.97wt% by pre-oxidation, the average diameter of as-grown Si3N4 nanowires increased from 210nm to 365nm, however, the crystallized α-phase and [101] growth direction have not altered. The α-Si3N4 nanowires grown from the pre-oxided silicon powders have higher room temperature photoluminescence property than ones from the as-received silicon powders.

Microstructure and growth mechanism of ZrO2 nanorod network via oxyacetylene torch ablation

In this work, the network structure of ZrO2 nanorods was firstly synthesized without a metal catalyst. A developed vapor–liquid–solid (VLS) and oxide-assisted growth (OAG) model is discussed as the growth mechanism of ZrO2 nanorods. ZrO2 nanoparticles, as a growth catalyst, provide zirconium and oxygen atoms for the formation of the nanorods and serve as a growth site as well. In the center and transition regions, ZrO2 nanorods with a diameter of 30–54nm finally grow into a network of ZrO2 layer during the cooling process. In the border region, due to the appropriate growth temperature and adequate ZrO2 vapors, ZrO2 nanorods with a significant increase in length are synthesized during the ablation process.

Oxide-assisted crack growth in hold-time low-cycle-fatigue of single-crystal superalloys

Compressive hold-time low-cycle fatigue is one of the important damage modes in Ni-based superalloy hot-gas path components. In strain controlled LCF, the compressive hold typically degrades fatigue life significantly due to creep relaxation and the resultant generation of tensile stress upon returning to zero strain. Crack initiation typically occurs on the surface, and therefore, the cracks are covered with layers of oxides. Recent finite element modeling based on experimental observations has indicated that the in-plane compressive stress in the alumina layer formed on the surface of the bond coat assists rumpling and, eventually, leads to initiation of cracks. The stress in the oxide layer continues to assist crack extension by pushing the alumina layer along the crack front during the compressive hold. In-situ measurements of the growth strains of alumina were performed using high energy synchrotron X-rays at Argonne National Lab. Specimens of single-crystal superalloys with and without aluminide coatings were statically pre-oxidized to form a layer of alumina at 1093 and 982 ∘ C. For the in-situ synchrotron measurements, the specimens were heated up to the pre-oxidation temperatures with a heater. The alumina layers on both bare and coated specimens show compressive in-plane strains at both temperatures. The oxide strains on the superalloys showed dependency on temperature; on the other hand, the oxide strains in the aluminide coatings were insensitive to temperature. The magnitude of the compressive strains was larger on the superalloys than the ones on the aluminide coatings.

Electrical Properties of Composite Films with Silicon Nanocrystals in the Insulating Matrix

The electrical properties of nanocomposite SiO2(Si) films containing Si nanoclusters have been investigated. The films were formed by oxide assisted growth that included ion plasma sputtering (IPS) of Si target and following high temperature annealing. It was determined that electrical conductivity of the films correspond to the mechanism of hopping conductivity with variable hopping length through the traps near the Fermi level (Mott mechanism) due to the large number of silicon dangling bonds in the dielectric matrix. The peculiarities of charge capture in nanocomposite SiO2(Si) films for their application as the medium for charge storage in memory cells have been investigated by C-V method. The good charge storage possibility of SiO2(Si) films formed by IPS deposition with followed temperature annealing has been observed. The negative differential capacitance has been revealed in conditions of semiconductor surface accumulation. The physical model for explanation of the negative differential capacitance of MIS structures with nanocomposite SiO2(Si) films as the dielectric has been proposed. The model is based on the parallel conjunction of the oxide capacitance and nanocrystals capacitance.

ZrB2–SiC coating to protect carbon/carbon composites against ablation

To improve the ablation resistance of carbon/carbon (C/C) composites, a ZrB2–SiC coating was prepared by pack-cementation. The phase composition, microstructure and element distribution of the coating were analyzed using X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy. The coated sample exhibits a dense structure and has an outstanding ablation property. During exposure to the oxyacetylene flame at about 2000K for 40s, the coated sample had a linear ablation rate of 4.4×10−3mm/s and a mass ablation rate of 6.2×10−5g/s. Compared with the uncoated C/C composites, the linear and mass ablation rate decreased by 49% and 97%, respectively. During the oxyacetylene torch test, a silica-scale glass was generated on the surface of the coating, and the embed ZrO2 could provide pinning effect to avoid cracking and spalling of the silica-scale glass. After the ablation test, some silica nanowires (SiONWs) were found in the ablation edge region of the sample. The growth mechanism of SiONWs consists of oxide-assisted growth (OAG) and vapor-liquid-solid (VLS) formation.

Silicon nanowires prepared by electron beam evaporation in ultrahigh vacuum

One-dimensional silicon nanowires (SiNWs) were prepared by electron beam evaporation in ultrahigh vacuum (UHV). The SiNWs can be grown through either vapor–liquid-solid (VLS) or oxide-assisted growth (OAG) mechanism. In VLS growth, SiNWs can be formed on Si surface, not on SiO2 surfaces. Moreover, low deposition rate is helpful for producing lateral SiNWs by VLS. But in OAG process, SiNWs can be grown on SiO2 surfaces, not on Si surfaces. This work reveals the methods of producing large-scale SiNWs in UHV.

One-Dimensional Nanostructures by Pulsed Laser Ablation

One-dimensional (1D) nanostructures represent a unique system for investigating phenomena at the nanoscale and are also expected to play a critical role as both interconnects and functional components in the fabrication of nanoscale electronic devices. Pulsed laser can provide extremely intense energy in a small spot and it can ablate virtually any materials. Various nanostructures have been investigated and fabricated with pulsed laser ablation, and this technique is not limited by the type of materials or crystal structures. This article focuses on 1D nanostructures and presents an overview of the common growth mechanism of nanowires by pulsed laser ablation. We first introduce a general scheme of the pulsed laser ablation in a tube furnace for the nanowire growth. Subsequently, we discuss various growth mechanisms involved to generate 1D nanostructures from pulsed laser ablation, including vapor–liquid–solid growth, oxide-assisted growth, and vapor–solid growth. Defects can also play an important role and they have been observed in the nanowires from different growth process.

Growth mechanism of silica nanowires without a metal catalyst via oxyacetylene torch ablation

In this paper, for the first time, silica nanowires (SiONWs) were synthesized without a metal catalyst via ablation of SiC-containing coating under oxyacetylene torch. The growth mechanism of silica nanowires was discussed by using a developed vapor–liquid–solid (VLS) and oxide-assisted growth (OAG) model. Silica nanoparticles as a growth catalyst provided silicon and oxygen atoms for the formation of the nanowires and served as a growth site as well. In the center region, silica nanowires with an even diameter of 150nm and a length up to more than 100μm formed during the cooling process. In the outer region, silica nanowires were synthesized during the ablation process and finally grew into a cauliflower-like silica layer.