Thin SiO2 Layer Research Articles

Advanced protective coatings for reflectivity enhancement by low temperature atomic layer deposition of HfO2 on Al surfaces for micromirror applications

We report on the implementation of thin atomic layer deposited coatings for environmental protection and reflectivity enhancement of Al-based micromirror devices. The reflectance characteristics of HfO2 and SiO2 thin layers on Al surfaces has been simulated by Rigorous Coupled-Wave Analysis (RCWA). Reflectance enhancement, especially in the 420–675 nm wavelength region, has been predicted by fine-tuning the layer thickness in HfO2/SiO2 dual layer stacks. This engineered solution has been developed and then implemented on real micro-electro-mechanical system (MEMS) devices demonstrating an enhancement of the reflectivity. Our approach suggests that atomic layer deposition is a valid method for both protecting and enhancing the performances of reflective surfaces of micro-opto-electro mechanical systems.

Nonreciprocal absorber of subwavelength metallic gratings

A dual-band non-reciprocal absorber, which is composed of the silver grating with a thin SiO2 layer, is proposed in near-infrared region. When the transverse magnetic wave illuminates on the thin metallic gratings, two SPPs modes (anti-symmetrical and symmetrical modes) can be excited, making the two absorption bands come into being. When a thin SiO2 layer is added on one side of the metallic gratings, one of the two SPPs modes can be enhanced and the other is inhibited. By carefully choosing the thickness of the SiO2 layer, the dual-band nonreciprocal absorber can be achieved.

Multi-band terahertz absorber exploiting graphene metamaterial

A dual-frequency broadband terahertz metamaterial absorber, composed of a periodically ellipse-shaped graphene array placed over a thin SiO2 layer and a lossy polyimide layer backed with a gold ground, is proposed in this article. The numerical simulation results reveal that the effective absorption band (absorption > 0.9) ranges from 0.3 THz to 0.75 THz and 1.57 THz to 1.87 THz, with a favorable independence of incident angle. Furthermore, replacing the gold ground with a frequency selective surface inserted with graphene, another absorbing peak appears between two broad absorbing bands and can be continuously tuned from 1 THz to 1.3 THz by controlling the chemical potential of graphene from 0 eV to 0.5 eV in the FSS. To improve the absorbance of the tunable absorbing peak, a backed cavity is added to the structure, which promotes the absorbance from 0.6 to more than 0.9 with unaffected tunability. These designs overcome limitations of traditional absorbers and have promising applications for filtering, detecting, and sensing.

Novel Leaking-Free, Green, Double Core/Shell, Palladium-Loaded Magnetic Heterogeneous Nanocatalyst for Selective Aerobic Oxidation

A new nanocatalyst containing Fe3O4 as a core, coated by a thin layer of SiO2, a mesoporous TiO2 shell, functionalized with diethylenetriamine (DETA) substituents and palladium nanoparticles dispersed on the surface and coordinated to organic groups was prepared. It exhibited spherical morphology with an average size of 178 nm and possess 195 m2 g−1 surface area and 2.4 nm pore diameter with a saturation magnetization of 20.7 emu g−1. Inductively coupled plasma spectroscopy proved Pd loading via porosity and functionality by DETA groups up to 10 wt% and the leaking-free property of the nanocatalyst. Furthermore, the nanocatalyst exhibited high efficiency for the selective aerobic oxidation of alcohols to aldehydes or ketones. Graphical Abstract

Hierarchical CdS Nanorod@SnO2 Nanobowl Arrays for Efficient and Stable Photoelectrochemical Hydrogen Generation.

An efficient photoanode based on CdS nanorod@SnO2 nanobowl (CdS NR@SnO2 NB) arrays is designed and fabricated by the preparation of SnO2 nanobowl arrays via nanosphere lithography followed by hydrothermal growth of CdS nanorods on the inner surface of the SnO2 nanobowls. A photoelectrochemical (PEC) device constructed by using this hierarchical CdS NR@SnO2 NB photoanode presents significantly enhanced performance with a photocurrent density of 3.8 mA cm-2 at 1.23 V versus a reversible hydrogen electrode (RHE) under AM1.5G solar light irradiation, which is about 2.5 times higher than that of CdS nanorod arrays. After coating with a thin layer of SiO2 , the photostability of the CdS NR@SnO2 NB arrays is greatly enhanced, resulting in a stable photoanode with a photocurrent density of 3.0 mA cm-2 retained at 1.23 V versus the RHE. The much improved performance of the CdS NR@SnO2 NB arrays toward PEC hydrogen generation can be ascribed to enlarged surface area arising from the hierarchical nanostructures, improved light harvesting owing to the NR@NB architecture containing multiple scattering centers, and enhanced charge separation/collection efficiency due to the favorable CdS-SnO2 heterojunction.

Uniformity Improvement of SiN x -Based Resistive Switching Memory by Suppressed Internal Overshoot Current

In this paper, we have investigated the effect of additional thin SiO2 layer on switching variability of SiN x -based resistive memory (RRAM). We found that excessive LRS state generated in set operation results in large reset current and abrupt reset operation. The abrupt reset operation leads to large distribution of HRS. To investigate the transient characteristics of switching procedure in detail, measurement environment was implemented with equivalent circuit, and measured current from equipment was separated into capacitive and resistive current of resistive memory cell. Consequently, we point the internal overshoot current occurring in set operation as the cause of the excessive leaky state leading to large resistance distributions. Finally, we confirm the effect of low resistance state value ( $R_{{\text{LRS}}}$ ) and cell capacitance ( $C_{{\text{DUT}}}$ ) on the internal overshoot current of RRAM.

Effects of oil-in-water based nanolubricant containing TiO2 nanoparticles in hot rolling of 304 stainless steel

In this paper, oil-in-water (O/W) based nanolubricants containing TiO2 nanoparticles were developed to reduce the rolling force and improve the surface quality of rolled 304 stainless steel product. Practical hot rolling tests with and without application of lubricant were conducted to systematically investigate the effects of the developed O/W based nanolubricants on the rolling force, surface roughness, oxide scale thickness and tribological behaviour. The obtained results indicate that the nanoparticles can enter the deform zone with oil droplets to take a lubrication effect. The optimal lubrication effect can be achieved when the O/W (1% oil mass fraction) based nanolubricant with a TiO2 mass fraction of 1.5% was applied. The microstructure of oxide scales formed on the hot rolled sample was not significantly affected by TiO2 nanoparticles in the O/W based lubricant, but the oxide scale growth could be induced by TiO2 nanoparticles. A discontinuous thin SiO2 layer at the substrate-scale interface was observed. It can effectively reduce the formation of iron-rich oxides. The novel nanolubricant has a great potential to be applied in the hot steel rolling, to realise the cost-effective and environmental-friendly manufacturing process.

Charge trapping properties of Ge nanocrystals grown via solid-state dewetting

In the present work, we report on the charge trapping properties of Germanium Nanocrystals (Ge NCs) self assembled on SiO2 thin layer for promising applications in next-generation non volatile memory by the means of Deep Level Transient Spectroscopy (DLTS) and high frequency C-V method. The Ge NCs were grown via dewetting phenomenon at solid state by Ultra-High Vacuum (UHV) annealing and passivated with silicon before SiO2 capping. The role of the surface passivation is to reduce the electrical defect density at the Ge NCs-SiO2 interface. The presence of the Ge NCs in the oxide of the MOS capacitors strongly affects the C-V characteristics and increases the accumulation capacitance, causes a negative flat band voltage (VFB) shift. The DLTS has been used to study the individual Ge NCs as a single point deep level defect in the oxide. DLTS reveals two main features: the first electron traps around 255 K could correspond to dangling bonds at the Si/SiO2 interface and the second, at high-temperature (>300 K) response, could be originated from minority carrier generation in Ge NCs.

Remarkably Improved Dispersion Stability and Thermal Conductivity of WO₃-H₂O Suspension by SiO₂ Coating.

The long term dispersion stability for an improved thermal conductivity is a challenging issue that needs to be solved for heat transfer applications. Hence, this research investigated that a thin layer of SiO2 coating (2-5 nm) over WO3 nanostructures (SiO2@WO3) of different shapes exhibit superior dispersion (0.01%) stability for longer duration (∼3 days) as evident by steady zeta potential (-30 ↔ -60.70 mV), no significant change in particle-size (139 ↔ 147 nm) distribution, density (1.001 ↔ 0.988 g/cm3) and refractive index (1.335 ↔ 1.332) etc., are indicator for colloidal stability relative to bare WO3 nanoparticles and bulk SiO2 aqueous suspension which quickly settles down within 1-2 hours after 30 min sonication at 23 °C. Thin Si-OH layer over WO3 surface imparts superior hydrophilicity, larger surface area for effective solute-solvent (SiO2@WO3-H2O) interaction for improved colloidal stability showing no sedimentation and color change of SiO2@WO3 dispersion (0.01%) even after 3 days due to repulsive interaction between negatively charged Si-O- particles. Thereby, thermal conductivity is found to be quite stable (0.631 ↔ 0.618 W/m K) up to 3 days, whereas aqueous suspension of bare WO3 and SiO2 particles quickly settle down and thermal conductivity rapidly decreased to a value of 0.584 W/m K for de-ionized water further indicates the significance of SiO2 coating. Depending on the thickness of SiO2 layer and volume fraction of SiO2@WO3, a maximum of 8-10% increment of thermal conductivity was achieved where anisotropic WO3 displayed always more (∼5%) thermal conductivity than typical spherical nanoparticles.

Improved interfacial strength of SiO2 coated carbon fiber in cement matrix

To improve the interfacial properties of carbon fiber in cement matrix, a thin SiO2 layer was coated onto carbon fibers through the condensation and polymerization of the tetraethyl orthosilicate under alkaline conditions. The morphology and chemical composition of the surficially grown SiO2 were characterized and analyzed. The surficially grown SiO2 layer reacted with Ca(OH)2 to form calcium silicate hydrate, which condensed the fiber-matrix interface. A single carbon fiber pullout from cement matrix test was conducted to evaluate the merit of surficial grown SiO2 on interfacial properties. Chemical debonding energy and frictional bond strength were obtained based on the pullout curves. The experimental results showed that the chemical debonding energy and frictional bond strength of the modified carbon fibers were significantly enhanced with respect to those of the plain carbon fiber, and this effect was primarily attributed to the improvement in interfacial hydration products that benefited from the reaction between surficial-coated SiO2 and Ca(OH)2.

Surface Enhanced DNP Assisted Solid-State NMR of Functionalized SiO2 Coated Polycarbonate Membranes

Abstract Surface enhanced solid-state NMR by dynamic nuclear polarization (DNP SENS) enables the characterization of the inner-pore surface functionalization of porous etched ion-track membranes exhibiting low specific surface areas compared to typical SBA- or MCM-type mesoporous silica materials. The membranes were conformally coated with a 5 nm thin SiO2 layer by atomic layer deposition. This layer was subsequently modified by aminopropyl silane linkers that allow further functionalization via the terminal amine group. The results evidence that in principle DNP SENS is a capable tool to analyze more complex porous systems, e.g. bioinspired functional etched ion-track membranes down to the molecular level. These results are relevant also for single nanopore systems, for which a direct analysis of the channel surface functionalization is not feasible by classical characterization methods. The applicability of DNP SENS to complex porous systems requires the optimization of the sample preparation and measurement parameters.

Adsorption properties of BSA and DsRed proteins deposited on thin SiO2 layers: optically non-absorbing versus absorbing proteins

Protein adsorption on solid surfaces is of interest for many industrial and biomedical applications, where it represents the conditioning step for micro-organism adhesion and biofilm formation. To understand the driving forces of such an interaction we focus in this paper on the investigation of the adsorption of bovine serum albumin (BSA) (optically non-absorbing, model protein) and DsRed (optically absorbing, naturally fluorescent protein) on silica surfaces. Specifically, we propose synthesis of thin protein layers by means of dip coating of the dielectric surface in protein solutions with different concentrations (0.01–5.0 g l−1). We employed spectroscopic ellipsometry as the most suitable and non-destructive technique for evaluation of the protein layers’ thickness and optical properties (refractive index and extinction coefficient) after dehydration, using two different optical models, Cauchy for BSA and Lorentz for DsRed. We demonstrate that the thickness, the optical properties and the wettability of the thin protein layers can be finely controlled by proper tuning of the protein concentration in the solution. These results are correlated with the thin layer morphology, investigated by AFM, FTIR and PL analyses. It is shown that the proteins do not undergo denaturation after dehydration on the silica surface. The proteins arrange themselves in a lace-like network for BSA and in a rod-like structure for DsRed to form mono- and multi-layers, due to different mechanisms driving the organization stage.

Enhancement of dielectric constant of polyimide by doping with modified silicon dioxide@titanium carbide nanoparticles.

Electrode materials used in supercapacitors must have a high dielectric constant and a low dielectric loss along with good mechanical properties. In this study, the dielectric constant of polyimide (PI) was improved by preparing a PI/SiO2@TiC composite. A homogeneous dispersion of SiO2@TiC nanoparticles was obtained by the hydrolyzation of tetraethyl orthosilicate, which resulted in the formation of a thin layer of SiO2 on TiC particles. Polyamic acid was doped with the modified SiO2@TiC nanoparticles by mechanical blending to synthesize the PI/SiO2@TiC composite, and its dielectric properties were investigated. Scanning electron microscopy studies confirmed that the SiO2@TiC particles were homogeneously dispersed in the PI matrix and did not exhibit agglomeration. An increase in the SiO2@TiC filler content increased the dielectric constant and the dielectric loss of the composite, but decreased its breakdown strength and deteriorated its mechanical properties. Thus, the addition of SiO2@TiC particles to PI is suitable for improving its dielectric properties while maintaining its flexibility.

The surface plasmon-induced hot carrier effect on the catalytic activity of CO oxidation on a Cu2O/hexoctahedral Au inverse catalyst.

The intrinsic correlation between an enhancement of catalytic activity and the flow of hot electrons generated at metal-oxide interfaces suggests an intriguing way to control catalytic reactions and is a significant subject in heterogeneous catalysis. Here, we show surface plasmon-induced catalytic enhancement by the peculiar nanocatalyst design of hexoctahedral (HOH) Au nanocrystals (NCs) with Cu2O clusters. We found that this inverse catalyst comprising a reactive oxide for the catalytic portion and a metal as the source of electrons by localized surface plasmon resonance (localized SPR) exhibits a change in catalytic activity by direct hot electron transfer or plasmon-induced resonance energy transfer (PIRET) when exposed to light. We prepared two types of inverse catalysts, Cu2O at the vertex sites of HOH Au NCs (Cu2O/Au vertex site) and a HOH Au NC-Cu2O core-shell structure (HOH Au@Cu2O), to test the structural effect on surface plasmons. Under broadband light illumination, the Cu2O/Au vertex site catalyst showed 30-90% higher catalytic activity and the HOH Au@Cu2O catalyst showed 10-30% higher catalytic activity than when in the dark. Embedding thin SiO2 layers between the HOH Au NCs and the Cu2O verified that the dominant mechanism for the catalytic enhancement is direct hot electron transfer from the HOH Au to the Cu2O. Finite-difference time domain calculations show that a much stronger electric field was formed on the vertex sites after growing the Cu2O on the HOH Au NCs. These results imply that the catalytic activity is enhanced when hot electrons, created from photon absorption on the HOH Au metal and amplified by the presence of surface plasmons, are transferred to the reactive Cu2O.

An ultrathin SiO2 blocking layer to suppress interfacial recombination for efficient Sb2S3-sensitized solar cells

An SiO2 thin layer efficiently suppresses the recombination at the TiO2/Sb2S3 interface and enhances the photovoltaic performance of Sb2S3 sensitized solar cells.

A novel and green fabrication of 3C-SiC nanowires from coked rice husk-silicon mixture and their photoluminescence property

This work provides a novel and green approach to synthesize 3C-SiC nanowires (NWs) by carbothermic reduction of coked rice husk and silicon powders without catalyst or extra protective atmosphere. Results show that the as-synthesized 3C-SiC NWs are well-crystallized and grow along the [1 1 1] direction, the diameters of them are in the range of 50–120 nm, and their lengths are up to dozens of micrometers. They own a face-centered cubic structure and are wrapped by a thin amorphous SiO2 layer (∼6 nm). Meanwhile, a few stacking faults perpendicular to the growth direction exist in their crystals. These 3C-SiC NWs exhibit three ultraviolet light emission peaks at wavelengths of 435 nm, 467 nm and 572 nm, respectively, showing they possess a wide range of emission wavelengths. This study is of great significance which extends the research of 3C-SiC NWs and will bring great benefits to their practical applications in optoelectronic devices.

Green synthesis and photoluminescence property of β-SiC nanowires from rice husk silica and phenolic resin

This paper reports a new and green method to synthesize β-SiC nanowires (NWs) via carbothermic reduction of rice husk silica and phenolic resin without any catalyst or extra protective atmosphere, and their microstructure and room temperature photoluminescence (PL) property have been investigated. Results show that the as-synthesized β-SiC NWs have different morphologies (columnar, acicular and flaky shapes) with lengths up to dozens of micrometers and diameters in the range of 47–380nm. They are surrounded by a thin amorphous SiO2 layer and grow along the [111] direction. Meanwhile, a few stacking faults and twin crystal phenomena can be observed in them. The PL spectrum of β-SiC NWs reveals a sharp and two broad ultraviolet light emission peaks at 563nm, 539nm and 662nm, respectively, showing an excellent candidate for optoelectronic applications.

Enhanced transmittance of sapphire by silicon oxynitride thin films annealed at high temperatures

Silicon oxynitride (SiON) and SiO2 thin films attracted considerable attention for various applications such as anti-reflection coatings and surface passivation layers. In this report, we present the enhancement of sapphire optical transmittance by over-layer deposition of the amorphous SiON by RF magnetron sputtering, and investigation on changes in microstructure and transmittance upon high-temperature annealing ∼1373 K. A thin layer of nanocrystalline SiO2 was formed at the sapphire/film interface induced by the annealing. Remarkably, the XPS spectra revealed that the silicon-nitrogen bonds disappear in the annealed films to form a non-stoichiometric silicon oxide (SiOx) phase. The films showed high visible transmittance ∼92%, which is comparable to that of soda-lime glass due to a significant reduction in the refractive index 1.41 of the films after annealing at 1373 K.

Robust SiO2 gate dielectric thin films prepared through plasma-enhanced atomic layer deposition involving di-sopropylamino silane (DIPAS) and oxygen plasma: Application to amorphous oxide thin film transistors

DIPAS (di-isopropylamino silane, H3Si[N(C3H7)2]) and O2 plasma were employed, using plasma-enhanced atomic layer deposition (PEALD), to deposit silicon oxide to function as the gate dielectric at low temperature, i.e., below 200°C. The superior amorphous SiO2 thin films were deposited through the self-limiting reactions of atomic layer deposition with a deposition rate of 0.135nm/cycle between 125 and 200°C. PEALD-based SiO2 thin layer films were applied to amorphous oxide thin film transistors constructed from amorphous In-Ga-Zn-O (IGZO) oxide layers, which functioned as channel layers in the bottom-gated thin film transistor (TFT) structure, with the aim of fabricating transparent electronics. The SiO2 gate dielectric exhibited the highest TFT performance through the fabrication of heavily doped n-type Si substrates, with a saturation mobility of 16.42cm2/V·s, threshold voltage of 2.95V and large on/off current ratio of 3.69 × 108. Ultimately, the highly doped Si was combined with the ALD-based SiO2 gate dielectric layers, leading to a saturation mobility of 16.42cm2/V·s, threshold voltage of 2.95V, S-slope of 0.1944, and on/off current ratio of 3.69 × 108. Semi-transparent and transparent TFTs were fabricated and provided saturation mobilities of 22.18 and 24.29cm2/V·s, threshold voltages of 4.18 and 2.17V, S-slopes of 0.1944 and 0.1945, and on/off current ratios of 9.63 × 108 and 1.03 × 107, respectively.

Silicon-Integrated Hybrid-Cavity 850-nm VCSELs by Adhesive Bonding: Impact of Bonding Interface Thickness on Laser Performance

The impact of bonding interface thickness on the performance of 850-nm silicon-integrated hybrid-cavity vertical-cavity surface-emitting lasers (HC-VCSELs) is investigated. The HC-VCSEL is constructed by attaching a III–V “half-VCSEL” to a dielectric distributed Bragg reflector on a Si substrate using ultrathin divinylsiloxane-bis-benzocyclobutene (DVS-BCB) adhesive bonding. The thickness of the bonding interface, defined by the DVS-BCB layer together with a thin SiO2 layer on the “half-VCSEL,” can be used to tailor the performance, for e.g., maximum output power or modulation speed at a certain temperature, or temperature-stable performance. Here, we demonstrate an optical output power of 2.3 and 0.9 mW, a modulation bandwidth of 10.0 and 6.4 GHz, and error-free data transmission up to 25 and 10 Gb/s at an ambient temperature of 25 and 85 °C, respectively. The thermal impedance is found to be unaffected by the bonding interface thickness.