Native Oxide Layer Research Articles

Self-sacrificial-reaction guided formation of hierarchical electronic/ionic conductive shell enabling high-performance nano-silicon anode

The inevitable surface oxidation of nano-Si inhibits its practical application as anode for lithium ion battery. Here, a hierarchical electronic/ionic conducting shell is smartly constructed by self-sacrificing reaction of native oxide layer on Si nanoparticles with lithium species. The morphological observation shows that Si nanoparticles are wrapped by lithium ionic conductor Li2SiO3 and amorphous carbon as the interlayer and outer layer, respectively. Such hierarchical structure not only provides a tight bond between Si and coating layer, but also improves the interfacial stability and conductivity of Si nanoparticles. Theoretical calculations demonstrate that the adsorption of Li2SiO3 on Si (111) and the adsorption of C on Si/Li2SiO3 are exothermic and spontaneous, and that the electron transfer and ionic conductivity at the surface of Si/Li2SiO3/C composite is enhanced. As a result, the as-prepared Si/Li2SiO3/C sample exhibits outstanding cycle stability and rate capability. By optimizing the lithium sources and thickness of native oxide layer, the resulted material exhibits 2106 mAh g−1 at 200 mA g−1, remains 1583 mA g−1 at 3000 mA g−1, and maintains 70.2% of capacity retention after 200 cycles. This work provides a robust strategy to promote the practical applications of Si nanoparticles as anode for lithium ion batteries.

Room temperature plasma-etching and surface passivation of far-ultraviolet Al mirrors using electron beam generated plasmas

The development of optical systems operating in the far ultraviolet range (FUV, λ=100-200 nm) is limited by the efficiency of passivated aluminum (Al) mirrors. Although it is presently possible to obtain high-reflectivity FUV mirrors through physical vapor deposition, the process involves deposition with substrates at high temperatures, which is technically challenging for large optical elements. A novel passivation procedure for bare Al mirrors is reported. The treatment consisted of using a low-temperature electron-beam generated plasma produced in a gas mixture of Ar and SF6 to etch away the native oxide layer from the Al film, while simultaneously promoting the generation of a thin aluminum tri-fluoride (AlF3) layer on the Al surface. In the first section we analyze the effect of varying both ion energy and SF6 concentration on the FUV reflectance, thickness, composition, and surface morphology of the resulting AlF3 protective layers. In the second section, the reflectivity of samples is optimized at selected important FUV wavelengths for astronomical observations. Notably, samples attained state-of-the-art reflectances of 75% at 108.5 nm (He Lyman γ), 91% at 121.6 nm (H Lyman α), 90% at 130.4 nm (OI), and of 95% at 155.0 nm (C IV). The stability over time of these passivated mirrors is also investigated.

Influence of storage conditions and reconditioning of AlSi10Mg powder on the quality of parts produced by laser powder bed fusion (LPBF)

Abstract The subject of this study is an investigation of the influence of storage conditions of AlSi10Mg powder on the LPBF parts. For this purpose, powders that were stored and aged under humid conditions in a climatic chamber for a certain period of time and subsequently dried in some instances were manufactured by LPBF. Afterwards, a comparison of the part density, mechanical properties and the surface quality of the LPBF parts was carried out. To gain a better understanding of the influences of the storage conditions considered on the powder and its behavior as feedstock, all aged powders were thoroughly characterized. A variation of the process parameters, laser power and scanning speed, were tested in order to identify the optimal parameter setting for the use of fresh, aged, and dried powder. It was found that the powders adsorb the water molecules in a mostly physical way. The natural oxide layer on the powder particles remains unaffected by the storage conditions. XPS investigations do not provide any clear evidence for the formation of hydroxides on the surface of the particles. Overall, the results of the printed parts show that the use of powder, which was stored under humid conditions, results in a lower part density and lower mechanical strength. This deterioration in processability can be reversed by drying the powder in a vacuum drying oven.

Graphene coating on a nickel-copper alloy (Monel 400) for microbial corrosion resistance: Electrochemical and surface characterizations

Graphene is an attractive 2D coating material because its remarkable physico-chemical properties enable numerous applications. Graphene coating was deposited on Monel 400 alloy by chemical vapour deposition (CVD) for corrosion resistance in a marine environment containing sulphate reducing bacteria (SRB). Surface morphology and coating characterization were performed using scanning electron microscopy (SEM) and Raman spectroscopy. Electrochemical results revealed that the multilayer graphene coating that replaced the native protective oxide layer on the Monel 400 surface provided superior corrosion resistance in the abiotic and biotic solutions. Post corrosion characterisation, using SEM and FTIR, supported the electrochemical findings.

Modulation of optoelectronic properties of the Bi2Te3 nanowire by controlling the formation of selective surface oxidation

In this study, a native oxide layer was transformed from TeO2 into Bi2O3 on Bi2Te3 by changing the annealing temperature, and the change in the effects of the interface between the oxide layers and Bi2Te3 nanowire (NW) was studied. Caused by the change in the surface oxide layer, the change in the interface between the Bi2Te3 NW and oxide layer depending on the annealing temperature alters the band-alignment and charge states of defects, which significantly modulates the carrier dynamics (optical excitation and recombination processes) at the interface. Because of the trap/de-trap processes at a defect of the Bi2Te3/TeO2 interface, the photoresponse of the Bi2Te3/TeO2 system shows the defect-induced photogating effect, whereas the photoresponse of the Bi2Te3/Bi2O3 system shows a capacitive response without trap/de-trap processes. The systematic analysis of the effects of the surface oxidation and related defects on the photoresponse of TIs gives insight into TI-related photodetector devices and their properties under various conditions. Finally, a simple method for controlling the oxidation of the surface and accompanied interface properties such as the valance band offset (VBO), conduction band offset (CBO), and trap/de-trap processes, which can directly affect the operation of TI-based photodetector devices, is proposed.

Surface-Functionalized Boron Nanoparticles with Reduced Oxide Content by Nonthermal Plasma Processing for Nanoenergetic Applications.

The development of an in situ nonthermal plasma technology improved the oxidation and energy release of boron nanoparticles. We reduced the native oxide layer on the surface of boron nanoparticles (70 nm) by treatment in a nonthermal hydrogen plasma, followed by the formation of a passivation barrier by argon plasma-enhanced chemical vapor deposition (PECVD) using perfluorodecalin (C10F18). Both processes occur near room temperature, thus avoiding aggregation and sintering of the nanoparticles. High-resolution transmission electron microscopy (HRTEM), high-angular annular dark-field imaging (HAADF)-scanning TEM (STEM)-energy dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) demonstrated a significant reduction in surface oxide concentration due to hydrogen plasma treatment and the formation of a 2.5 nm thick passivation coating on the surface due to PECVD treatment. These results correlated with the thermal analysis results, which demonstrated a 19% increase in energy release and an increase in metallic boron content after 120 min of hydrogen plasma treatment and 15 min of PECVD of perfluorodecalin. The PECVD coating provided excellent passivation against air and humidity for 60 days. We conclude in situ nonthermal plasma reduction and passivation lead to the amelioration of energy release characteristics and the storage life of boron nanoparticles, benefits conducive for nanoenergetic applications.

High speed picoliter droplet top-view analysis for advancing and receding contact angles, boiling regimes and droplet-droplet interaction

Free falling picoliter and sub-picoliter droplets generated by an ultrasonic atomizer are studied from a top-view perspective with a high-speed camera while falling on a heated surface. Due to the small droplet size processes are dominated by surface tension (Re = 0.5, We = 10−4, Oh = 2 × 10−2) resulting in almost perfect spherical shapes. Our presented model analyzes the droplets top-view appearance and allows the extraction of static and dynamic advancing and receding contact angles. In addition, the extracted evaporation rate allows to determine the critical heat flux (CHF) and Leidenfrost point (LFP). The evaporation regimes: nucleate boiling, transition boiling and film-boiling can be clearly defined by CHF and LFP. The presented model assumes the droplet evaporation starts with a shape corresponding to the static advancing contact angle Θa and continues to the static receding contact angle Θr with pinned contact line, i.e. with a constant visible droplet area. Afterward, it evaporates with a dynamic receding contact angle Θdr until the end of its lifetime. The initial advancing contact angle is calculated from the volume of the arriving droplet in air and the measured contact area directly after landing on the surface. The analysis yields Θa = 37.2 ± 3.3° and Θdr = 9.1 ± 1.6° for DI-water on silicon with natural oxide layer at T = 110°C. Furthermore, different complex scenarios of interactions between droplets are observed for sessile droplets formation and during the evaporation process.

Direct Growth of Si, Ge, and Si-Ge Heterostructure Nanowires Using Electroplated Zn: An Inexpensive Seeding Technique for Li-Ion Alloying Anodes.

A scalable and cost-effective process is used to electroplate metallic Zn seeds on stainless steel substrates. Si and Ge nanowires (NWs) are subsequently grown by placing the electroplated substrates in the solution phase of a refluxing organic solvent at temperatures >430°C and injecting the respective liquid precursors. The native oxide layer formed on reactive metals such as Zn can obstruct NW growth and is removed in situ by injecting the reducing agent LiBH4 . The findings show that the use of Zn as a catalyst produces defect-rich Si NWs that can be extended to the synthesis of Si-Ge axial heterostructure NWs with an atomically abrupt Si-Ge interface. As an anode material, the as grown Zn seeded Si NWs yield an initial discharge capacity of 1772 mAh g-1 and a high capacity retention of 85% after 100 cycles with the active participation of both Si and Zn during cycling. Notably, the Zn seeds actively participate in the Li-cycling activities by incorporating into the Si NWs body via a Li-assisted welding process, resulting in restructuring the NWs into a highly porous network structure that maintains a stable cycling performance.

Electron Transfer through a Natural Oxide Layer on Real Metal Surfaces Occurring during Sliding with Polytetrafluoroethylene: Dependence on Heat of Formation of Metal Oxides

Electron emission (EE) from real metal surfaces occurring during sliding contact with a polytetrafluoroethylene (PTFE) rider has been investigated using the thermodynamic data of metal oxides and the X-ray photoelectron spectroscopy (XPS) intensity ratio of oxygen/metal on the surfaces. EE was termed triboelectron emission (TriboEE). Rolled metal sheets of 18 types were used. The metal‒oxygen bond energy calculated from the heat of the formation of metal oxide, (D(M–O)), was shown to be a key factor in dividing the EE into two routes, the so-called Schottky effect and the tunnel effect, due to the surface oxide layer. The metals in periodic groups 4 (Ti and Zr), 5 (V, Nb, and Ta), and 6 (Mo and W) maintained higher values of D(M–O), while, moving down the groups, the TriboEE intensity increased, being ascribed to the former route. In groups 10 (Ni, Pd, and Pt) and 11 (Cu, Ag, and Au), the D(M–O) values decreased moving down the groups, but the TriboEE intensity increased significantly, which can be attributed to the latter route. Furthermore, with the increase in the electrical conductivity of metals, the TriboEE intensity became remarkably high, while the D(M–O) value fell rapidly and became almost constant. The XPS results showed that the dependence of the D(M–O) and XPS metal core intensity on the O1s intensity and the XPS intensity ratio of the O1s/metal core was different between groups 10 and 11 and groups 4, 5, and 6. It was concluded that, under the electric field caused on the real metal surface by the friction with PTFE, the electron from metals with small D(M–O) values predominantly tunnels the surface oxide layer as a surface barrier, while with large D(M–O) values, the electron passes over the top of the barrier.

The Effect of Hydrofluoric Acid Concentration in Sulfuric Acid-Based Desmutting Solution for Aluminium Alloys

The natural oxide layer, which spontaneously covers the surface of aluminium and its alloys, is well adherent to the surface, but it does not show adequate corrosion resistance in many conditions. In order to improve the material anti-corrosion performance, it is necessary to replace the surface oxide by a conversion coating. The first step to do this is to prepare the alloy surface to subsequent treatments. The pre-treatment implemented before the conversion treatment is one of the main factors responsible for the performance of the conversion coating. It involves several phases, including desmutting that encompasses the part immersion in an acid or alkaline solution bath. In this work, the influence of hydrofluoric acid concentration in the desmutting bath's formulation was investigated. The samples surface were characterised by using electrochemical techniques, glow discharge optical emission spectrometry and contact angle measurements.

Reactive Bilayers by Self-activated Electroless Nickel-Phosphorous Deposition on Pure Aluminum

Nickel-aluminum bilayers and multilayers are commonly used in reactive systems because of their high heat of mixing. Traditionally, the layers are sputter deposited, an expensive and time-consuming process. In this article, a chemical reduction technique, electroless nickel-phosphorous (EN) deposition, is used for formation of the Ni-Al bilayer using a pure Al substrate. Traditionally, EN deposition on Al requires surface activation due to the native oxide layer. Here, we tune the bath pH to dissolve this oxide layer, such that deposition can be obtained without surface activation. Deposition kinetics is measured by tracking the mass change with time and temperature, and we extracted the activation energies at two different pH values. At pH 7, the energy is 98.7 kJ/mol and reduces to 55 kJ/mol at a pH of 8. The process can also be extended to aluminum powders and flakes in order to form reactive core–shell structures.

Photoluminescence properties of cuprous phosphide prepared through phosphating copper with a native oxide layer.

Although cuprous phosphide (Cu3P) has been widely studied and applied in other fields, its photoluminescence (PL) properties are rarely investigated. Herein, we report that Cu3P can emit near-infrared light at 750 nm. We show that the annealing and the presence of cuprous oxide can enhance the PL emission. The mechanism of the PL enhancement is the improvement of crystal quality and the formation of a space charge region. Our results provide a reference for improving the PL properties of p-type semiconductors.

Surface roughness and substrate induced symmetry-breaking: influence on the plasmonic properties of aluminum nanostructure arrays.

The surface topography is known to play an important role on the near- and far-field optical properties of metallic nanoparticles. In particular, aluminum (Al) nanoparticles are commonly fabricated through evaporation techniques, therefore exhibiting elevated surface roughness additionally to their native oxide layer. In this study, the mode-dependent influence of surface roughness on the plasmonic properties sustained by Al nanodisks (NDs) is first numerically investigated using a realistic model taking into account the thin native oxide layer. Due to the symmetry-breaking induced by the supporting dielectric substrate to Al ND, it appears that the roughness affects differently the substrate-induced out-of-plane quadrupolar mode (below 300 nm) and the in-plane dipolar mode sustained by the Al ND. By increasing the top surface roughness of the Al ND, the substrate-induced quadrupolar mode is significantly damped especially in the ultraviolet regime, while the dipolar resonance is broadened and redshifted. The explanation of these effects relies in the decoherence and dissipation of the collective electronic oscillations as a result of the top surface roughness to the different near-field distribution of the out-of-plane quadrupolar mode and in-plane dipolar mode. Moreover, the influences of the diameter of Al ND, dielectric substrate with different refractive index, and the oxidation of Al ND on these two modes are also investigated. Particularly, the quadrupolar mode disappears with surface roughness and oxidation, explaining why this mode is very weak and sometimes barely visible on evaporated Al nanostructures reported in the literature. Finally, these results are experimentally confirmed by characterizing the optical properties of periodic Al ND arrays.

Real-time imaging of nanoscale electrochemical Ni etching under thermal conditions.

The ability to vary the temperature of an electrochemical cell provides opportunities to control reaction rates and pathways and to drive processes that are inaccessible at ambient temperature. Here, we explore the effect of temperature on electrochemical etching of Ni–Pt bimetallic nanoparticles. To observe the process at nanoscale resolution we use liquid cell transmission electron microscopy with a modified liquid cell that enables simultaneous heating and biasing. By controlling the cell temperature, we demonstrate that the reaction rate and dissolution potential of the electrochemical Ni etching process can be changed. The in situ measurements suggest that the destabilization of the native nickel oxide layer is the slow step prior to subsequent fast Ni removal in the electrochemical Ni dissolution process. These experiments highlight the importance of in situ structural characterization under electrochemical and thermal conditions as a strategy to provide deeper insights into nanomaterial transformations as a function of temperature and potential.

Образование акцепторных центров в CdHgTe под воздействием воды и термообработок

Influence storage and boiling in deionized water and heat treatments of epitaxial films CdxHg1-xTe on the Hall and ellipsometric parametres is investigated. Water treatment reduces refractive index of native CdxHg1-xTe oxide from 2.1 to 1.2-1.4. It means that matter with a lower refractive index, such as water, is introduced in the oxide. Boiling in water leads to formation of acceptors in CdxHg1-xTe with concentrations up to 1019 cm-3. Change of medium’s pH from alkaline to the acidic decreases the speed of acceptors formation. Heat treatments after storage in water also leads to formation of acceptors. The conclusion is made, that water medium or water absorbed by native oxide layer leads to formation of acceptors in CdxHg1-xTe. Concentration of acceptors grows with temperature of treatments and quantity of accessible water.

Determining the potential barrier presented by the interfacial layer from the temperature induced I–V characteristics in Al/p-Si Structure with native oxide layer

The temperature induced current-voltage (I–V-T) characteristics of the Al/SiO2/p-Si diodes with the interfacial native oxide layer were experimentally investigated in present our study. The values of 24.35 and 1.02 eV for the tunneling factor (aδχ) and the potential barrier presented by the interfacial layer (χ) were determined for the Al/SiO2/p-Si metal-insulator-semiconductor (MIS) diode. The χ value obtained from the forward bias I–V-T characteristics of the SiO2/p-Si diode is a crucial result according to those from only room temperature I–V curves in literature studies. Furthermore, the temperature dependence of the apparent barrier heights of the forward bias I–V-T characteristics displayed a Gaussian distribution (GD) of inhomogeneous barrier heights (BHs). Thus, the standard deviation values of σ1s0=147.65 mV for high temperature range and σ2s0=83.66 mV for low temperature range were obtained from the GD plot of the BHs (the apparent BH versus (2kT)−1 plot). Again, for the mean barrier height Φ‾b0 from the GD plot of the BHs, the values of 1.16 eV and 0.70 eV were obtained from the high temperature region and low temperature region, respectively. Furthermore, the Richardson constant values of 54.66 and 58.42 A/K2cm2 from these temperature regions are approximately 1.70 times higher than the theoretical value of 32 A/K2cm2 of the p-type Si semiconductor.

Formation of Graphene–Silicon Junction by Room Temperature Reduction With Simultaneous Defects Removal

A novel method of preparing reduced graphene oxide (rGO)–silicon (Si) junctions free from native silicon oxide layer at room temperature is reported. The method is based on a simultaneous reduction–dissolution reaction between graphene oxide (GO) and fresh Si atoms with the assistance of dilute hydrofluoric acid (HF). The rGO–Si junction is characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, and atomic force microscopy. A reaction mechanism is proposed that Si is oxidized and dissolved by HF, which transfers the electronic holes to GO and facilitates the removal of its oxygen-containing groups. The use of HF provides a unique benefit to the contact formation that native oxide on Si is removed by HF instantaneously and only fresh Si surface is in contact with rGO. Therefore, the method provides a new strategy of preparing rGO-native oxide-free Si interface, which has been a fundamental challenge in the fabrication of graphene–Si junction. Electrical tests show that rGO–Si interface has a slightly higher barrier than Al–Si.

Study on hydrogen absorption and surface properties of TiZrVNbCr high entropy alloy

In this study, we examined the microstructure, hydrogen absorption behavior, and native oxide layer of 20Ti–20Zr–20V–20Nb–20Cr high entropy alloy. It was revealed that the alloy consisted of two phases with the chemical composition of 31.1Ti-19.6Zr-16.6V-26.0Nb-6.7Cr (bright phase) and 9.1Ti-20.3Zr-23.4V-14.1Nb-33.1Cr (dark phase). Even though an oxide layer was formed on the surface of the alloy due to the exposure to an air atmosphere for 10 days, it could absorb hydrogen at a low hydrogen pressure without any thermal activation process. After the hydrogen absorption experiment, not only the hydrogen absorption as high as 1.77 wt% was achieved, but the phase transformation occurred; the bright phase transformed to dihydride and the dark one did monohydride. To clarify which phase mainly contributed to the hydrogen absorption behavior, ingots with chemical composition of the dark and bright phases were prepared separately. The hydrogen absorption occurred only for the dark phase ingot, not for the bright phase ingot, suggesting that only the oxide layer on the dark phase seems to have a reactivity with hydrogen. XPS results revealed that while the Cr element was not detected in the oxide film on the bright phase, the oxide film on the dark phase possessed the Cr element. Based on the results, the presence of the Cr element in the oxide would be ascribed to the high reactivity with hydrogen.

Natural surface oxidation consideration in first principles modeling of the X-ray absorption near edge fine structure of silicon

First-principles calculations of the electronic structure of crystalline silicon and its dioxide were performed on the basis of the density functional theory. The full-potential linearized augmented plane waves method was used with the generalized gradient approximation for the exchange-correlation potential with Perdew, Burke and Ernzerhof functional. X-ray absorption near edge structure of silicon L3 spectra for these materials were calculated with the use of core-hole approach and supercells. These calculated spectra were used as reference for modeling the X-ray absorption L3 spectrum of a real silicon sample covered with a layer of natural SiO2 oxide. Comparison of the spectra obtained by the first principles calculations with the results of a high-resolution synchrotron experiments and the suggested approach to their modeling demonstrated the possibility of a complex silicon-oxygen system prospective materials and structures detailed study.

Micro-spectroscopy of Buried Short-Range Surface Plasmon Polaritons Supported by Thin Polycrystalline Gold Films

The dispersive properties of short-range surface plasmon polaritons are investigated at the buried interfaces in vacuum/Au/fused silica and vacuum/Au/SiO2/Si multilayer systems for different gold film thicknesses of up to 50 nm using two-photon photoemission electron microscopy. The experimental data agrees excellently with results of transfer matrix method simulations, emphasizing the sensitivity of the plasmonic wave vector to the thickness of the gold film and an ultrathin native substrate oxide layer. The results furthermore illustrate the exceptional qualification of low-energy electron photoemission techniques in studying electronic excitations at buried interfaces.