SiO2 Multilayers Research Articles

Direct observation of conductive filaments from 3D views in memristive devices based on multilayered SiO2: Formation, Dissolution, and vaporization

Memristive devices, also known as memristors or ReRAMs, are promising candidates for accessing next-generation memory. In classic electrochemicalmetallization (ECM) theory, there are only two states of conductive filaments: formation and dissolution. In our experiment, we found that the metallic filaments also vaporized, leaving observable defects in a series of memristive devices based on a Cu-doped multilayered SiO2 electrolyte layer. Furthermore, the vapour from conductive filaments exfoliated adjacent single layers of multilayered SiO2. The morphologies of the conductive filaments in a memristive device (W/Cu-doped SiO2/Ag) were studied using an SEM (scanning electron microscope) instrument equipped with an FIB (focused ion beam) module. With the gradual removal of the electrolyte layer, cross-sectional images of the conductive filaments were captured from perspective, top and side views. Based on these images, a three-dimensional model of the conductive filaments was proposed. All the findings suggested that the SET and RESET processes were complex and involved the simultaneous formation, dissolution and vaporization of conductive filaments. The vaporization of the conductive filaments permanently changed the surface morphology of the devices. This model, presented at the end of our paper, explains the irregular phenomena that occurred in I-V measurements.

Excitation and modulation of long-range surface plasmons based on InSb multilayer structure

We proposed an Ag-based multilayer InSb and SiO2 system in the terahertz region. Based on the surface plasmon polariton (SPP) interaction among each interface, the spontaneous emission of a dipole in multilayer systems supporting SPP and waveguide modes is theoretically studied. It found that the coupling efficiency of long-range SPP (LRSPP) is about 78% with a propagation length of up to 28.25 mm. The thermally tunable characteristics of InSb enable LRSPP devices to be realized with adjustable coupling efficiency. Our work could be applied as a future guide for integrated plasmonic device design.

Waterproof and ultrasensitive paper-based wearable strain/pressure sensor from carbon black/multilayer graphene/carboxymethyl cellulose composite

Huge electronic wastes motivated the flourishing of biodegradable electrically conductive cellulosic paper-based functional materials as flexible wearable devices. However, the relatively low sensitivity and unstable output in combination with poor wet strength under high moisture circumstances impeded the practical application. Herein, a superhydrophobic cellulosic paper with ultrahigh sensitivity was proposed by innovatively employing ionic sodium carboxymethyl cellulose (CMC) as bridge to reinforce the interfacial interaction between carbon black (CB) and multilayer graphene (MG) and SiO2 nanoparticles as superhydrophobic layer. The resultant paper-based (PB) sensor displayed excellent strain sensing behaviors, wide working range (−1.0 %–1.0 %), ultrahigh sensitivity (gauge factor, GF = 70.2), and satisfied durability (>10,000 cycles). Moreover, the superhydrophobic surface offered well waterproof and self-cleaning properties, even stable running data without encapsulation under extremely high moisture conditions. Impressively, when the fabricated PB sensor was applied for electronic-skin (E-skin), the signal capture of spatial strain of E-skin upon bodily motion was breezily achieved. Thus, our work not only provides a new pathway for reinforcing the interfacial interaction of electrically conductive carbonaceous materials, but also promises a category of unprecedentedly superhydrophobic cellulosic paper-based strain sensors with ultra-sensitivity in human-machine interfaces field.

Quantum size-dependent luminescence and nonlinear optical properties of silicon quantum dots/SiO2 multilayer

The luminescence and nonlinear optical properties of silicon quantum dots (Si QDs)/SiO2 multilayers with different QDs diameters (2.5, 3 and 4 nm) were investigated by utilizing continue wave (325 nm) and picosecond laser (tp = 25 ps, λ = 1064 nm), respectively. It was found that both the linear and nonlinear optical properties of Si QDs/SiO2 multilayers strongly depended on the QDs size. The photoluminescence intensity could be enhanced by ∼32-fold, and about 17 times increase in the saturable absorption (SA) coefficient was achieved, which could be ascribed to the controllable density of interface-states as well as their coupling effect with the Si QDs core states. Moreover, a transformation from reverse saturable absorption (RSA) characteristic through two-step absorption process into the SA behavior due to the interface-state filling effect was observed when increasing the incident laser intensity. We proposed that the ultrafast free carries trapping process by the interface states would greatly influence the self-defocusing effect. In addition, the free carries absorption and SA effect should be considered in accounting for the variation of nonlinear parameters as changing the pump intensity. Our results suggest that the widely tunable SA feature through controlling the Si QDs size provides great potential to develop high-efficient Si QDs-based nonlinear optical devices such as passive mode-locking or Q-switching laser.

Ultralow Index SiO2 Antireflection Coatings Produced via Magnetron Sputtering.

Antireflection (AR) coatings with graded refractive index profiles approaching air offer unparalleled AR performance but lack a scalable fabrication process that would enable them to be used more widely in applications such as architecture and solar energy conversion. This work introduces a sputtering-based sacrificial porogen process to fabricate multilayer nanoporous SiO2 coatings with tunable refractive index down to neff = 1.11. Using this approach, we demonstrate a step-graded bilayer AR coating with outstanding wide-angle AR performance (single side average reflectivity in the visible spectrum ranges from 0.2% at normal incidence to 0.7% at 40°), good adhesion, and promising environmental durability. These results open up a path to produce ultrahigh performance AR coatings over large area by using industrial-scale magnetron sputtering systems.

XPS Studies of the SiO2 Substrates and Thermoelectric Thin Films of Sn/Sn+SnO2 under the Effects of the Different Thermal Treatments

Multilayered thermoelectric Sn/Sn+SnO2 thin films were prepared using KJL DC/RF magnetron sputtering system under Ar gas plasma on the SiO2 substrates. The thicknesses of the fabricated thin films were found using Filmetrics UV thickness measurement system. The fabricated thin films were annealed at different temperatures for one hour to tailor the thermoelectric properties. In this study, unannealed, annealed at 150 and 300°C samples were characterized using Thermo Fisher XPS system brought to the Alabama A&M University by the NSF-MRI support. X-ray Photoelectron Spectroscopy (XPS), also known as Electron Spectroscopy for Chemical Analysis (ESCA) is a type of analysis used for characterization of various surface materials. XPS is mostly known for the characterization of thin films-which are coatings that have been deposited onto a substrate and may be comprised of many different materials to alter or enhance the substrate’s performance. XPS analysis provides information for composition, chemical states, depth profile, imaging and thickness of thin film. This paper focuses on the application of XPS techniques in thin film research for Sn/Sn+SnO2 multilayered thermoelectric system and SiO2 substrates annealed at different temperatures. Since SiO2 substrates were used during the deposition of the multilayer thin films, we would like to perform detailed XPS studies on the SiO2 substrates. SiO2 substrates is being used with many researchers, this manuscript will be good reference for the researchers using SiO2 substrates. Thermal treatment of the substrates and the multilayered thin films has caused some changes of the XPS characterization including binding energy, depth profile, peak value and FWHM. The treatment effects were discussed and compared to each other.

Fabrication of oxide-based near infrared-shielding coatings for a smart window to prevent infrared-induced photoaging in human skin

Oxide-based near infrared (IR)-shielding coatings consisting of quarter‐wave stacks of oxygen-deficient tantalum oxide (Ta2O5−x) and silicon oxide (SiO2) multilayers and tin-doped indium oxide (In2O3) (ITO) films with the thicknesses of 200–600 nm can block the passage of IR-A (wavelength: 760–1400 nm) and IR-B (wavelength: 1400–3000 nm) radiation, respectively. In this study, the optical properties and microstructure of these oxide-based IR-shielding coatings were investigated. Transmission electron microscopy images indicated that amorphous Ta2O5−x/amorphous SiO2 multilayers were uniform and dense. ITO films were found to be highly crystalline and show carrier concentrations of up to 7.1 × 1020 cm−3, resulting in the strong IR-B optical absorption due to the plasma excitation of the free carriers. Oxide-based IR-shielding coatings with an ITO thickness of 420 nm were found to have near-IR shielding rates of >90% and an average visible light transmittance of >70%. The effects of IR on human keratinocytes were studied to evaluate the IR-induced photoaging in human skin. It was found that the downregulation of cellular proliferation and the enhancement of senescence-associated β-galactosidase activity induced by IR irradiation were significantly inhibited by oxide-based IR-shielding coatings. Thus, this study provides a facile method for the development of coatings for smart windows with high IR-shielding ability and high visible light transmittance.

Enhanced microwave absorption and electromagnetic shielding property of (1-x)K0.5Na0.5NbO3 ~ xAl2O3 nano-ceramics

(1-x) K0.5Na0.5NbO3 ~ xAl2O3 (x = 0, 0.2, 0.4, 0.6) ceramics were prepared via a traditional solid-state reaction method. The phase structure, micro-morphology, dielectric properties and electromagnetic properties of ceramic samples were studied and analyzed. Results indicate that all the samples are similar to K0.5Na0.5NbO3 (KNN) in perovskite structure. With the increase of Al2O3 content, the X-ray diffraction peaks move to a large angle region, suggesting the substitution of niobium ions by aluminium ions and the distortion of the KNN lattice with a new phase arising. With the increase of Al2O3 content the grain size reduces and the dielectric constant decrease, yielding to the decrease of the electromagnetic shielding performance of ceramic. When the x is 0.4, the minimum value of reflectivity of sample is −28 dB at the frequency of 11.6 GHz. It can be concluded that both the grain size and Al2O3 content can obviously affect the electromagnetic properties of ceramics, which can be easily turned through a multi-layer SiO2 heterojunction structure.

The preparation and study of multilayer structured SiO2–TiO2 film: the effects of photonic crystals on enhanced photocatalytic properties

Herein, SiO2-photonic crystal/TiO2 composite film (STF) with multilayer structure was designed and fabricated via self-assembly method and spin coating method. The micro-morphology and optical properties of the STF were characterized by scanning electron microscope (SEM) and UV/Vis/NIR spectrophotometer respectively. The photocatalytic activity of the STF was tested via the degradation rate of rhodamine B. The characterization results indicate that the STF has a regular ordered structure when the amount of ammonia is 3 mL and the concentration of SiO2 suspension is 1%, and SiO2 can be effectively combined with TiO2, so the STF has the best photocatalytic performance. The double-layer SiO2-photonic crystal/TiO2 composite film (DSTF) was further prepared and compared with the single-layer SiO2-photonic crystal/TiO2 composite film (SSTF). The photocatalytic properties of the DSTF are better than that of the SSTF. Finally, the mechanism of STF with enhanced photocatalytic performance is discussed, indicating that photonic crystal can effectively enhance the utilization of light which are near the wavelength of the center band gap, and then improve the photocatalytic performance of the photocatalyst.

Positron annihilation analysis of nanopores and growth mechanism of oblique angle evaporated TiO2 and SiO2 thin films and multilayers

The nano-porosity embedded into the tilted and separated nanocolumns characteristic of the microstructure of evaporated thin films at oblique angles has been critically assessed by various variants of the positron annihilation spectroscopy. This technique represents a powerful tool for the analysis of porosity, defects and internal interfaces of materials, and has been applied to different as-deposited SiO2 and TiO2 thin films as well as SiO2/TiO2 multilayers prepared by electron beam evaporation at 70° and 85° zenithal angles. It is shown that, under same deposition conditions, the concentration of internal nano-pores in SiO2 is higher than in TiO2 nanocolumns, while the situation is closer to this latter in TiO2/SiO2 multilayers. These features have been compared with the predictions of a Monte Carlo simulation of the film growth and explained by considering the influence of the chemical composition on the growth mechanism and, ultimately, on the structure of the films.

Ultra-high transparent sandwich structure with a silicon dioxide passivation layer prepared on a colorless polyimide substrate for a flexible capacitive touch screen panel

Indium tin oxide (ITO)-based multilayer structures with ultrathin silver (Ag) deposition on colorless polyimide (CPI) were investigated as touch sensor electrode materials in this study. The electrical, optical, structural, and morphological properties of the ITO-based multilayer structures were compared. The figure of merit of the ITO/Ag/ITO(160)/CPI multilayer structure (bottom ITO film deposited at 160 °C), in which the bottom ITO layer was deposited at 160 °C, was calculated using Haacke's formula, and the value was found to be 63.6 × 10−3 Ω−1. Moreover, the sheet resistance and optical transmittance at 550 nm were 6.4 Ω/□ and 91.4%, respectively. Transmission electron microscopy measurements demonstrated that the continuous Ag layer had a uniform film of 10.9 nm. When an ITO/Ag/ITO(160) multilayer structure was used as a substitute for an X–Y ITO layer electrode for fabricating a flexible capacitive-type touch screen panel (FCTSP) and a low-refractive-index silicon dioxide (SiO2) layer was used as the passivation layer, the ultra-high optical transmittance of the SiO2/ITO/Ag/ITO(160) multilayer structure was measured to be 94.6% (at 550 nm), and the average optical transmittance over the visual region (400–700 nm) was measured to be 92.2%. An FCTSP with multiple touch points was successfully fabricated using an X–Y ITO/Ag/ITO(160) multilayer electrode and SiO2 passivation layer. Finally, we also derive the related theoretical formulas to fix our experimental data. Moreover, based on the theory, we can predict the optimized thickness to support the desirable ultra-high transparency of multilayer structures. This auxiliary simulation also paves an alternative route to explore the possibilities of optimized multilayer flexible devices.

High-speed heterojunction photodiodes made of single- or multiple-layer MoS2 directly-grown on Si quantum dots

Recently, chemical vapor deposition-grown molybdenum disulfide (MoS2) has been actively employed for MoS2/Si-wafer heterojunction (HJT) photodetectors due to the attractive optoelectronic properties. However, the MoS2/Si HJT is not so advantageous in that it exhibits low photoresponse due to the low light absorption despite the simple device structure. In addition, there is a limitation in achieving high-quality MoS2 films due to the defects at the MoS2/Si interface developed during the transfer of the MoS2 films to the target substrate, resulting from the restriction of the direct growth on the Si wafer. Here, we first report successful direct growth of single- and multi-layer MoS2 films on Si quantum dots (SQDs) multilayers (MLs) embedded SiO2 (SQDs:SiO2 MLs) substrates. The multilayer MoS2/SQDs HTJ photodiodes show response speed of rise time: ∼60 ns/fall time: ∼756 ns, highest than ever achieved, and detectivity of 6.1 × 1013 cm Hz1/2 W−1. This excellent performance can be attributed to well formation of the HJT at the MoS2/SQDs:SiO2 interface by the direct growth, resulting in the reduction of the defects, thereby facilitating the carrier transport, and high light absorptivity of the SQDs:SiO2 MLs.

Mechanical properties and in situ fracture behavior of SiO2f/phosphate geopolymer composites

Geopolymers, such as phosphate-based geopolymer, are promising materials that have drawn extensive attentions as wave-transparent candidates with low dielectric loss factor. In this paper, multilayer SiO2 fibers are introduced into phosphate-based geopolymer matrix to prepare composites. Crack generation and fracture behaviors during the three-point bending tests in as-prepared SiO2f/phosphate geopolymer composites are clarified by in situ observation. Before high-temperature treatment, composite with 17 vol% fiber content exhibits the best mechanical properties. A decreasing mechanical strength is observed after being treated at higher temperature, which is attributed to the stronger interface binging between fibers and phosphate-based geopolymer matrix. Most importantly, the presence of SiO2 fibers can strengthen mechanical properties of phosphate-based geopolymer matrix but shows negligible impact on its dielectric properties, achieving dielectric loss factor as low as 3.53 × 10−3 after being treated at 700 °C.

Structural analysis of silicon nanostructures obtained from thermal annealing of PVD deposited SRO/SiO2 multilayers

We report the synthesis and characterization of silicon nanostructures obtained by thermal annealing of silicon-rich oxide/silicon dioxide (SRO/SiO2) multilayers deposited by magnetron sputtering in an argon and oxygen atmosphere. The main motivation to study silicon comes from its success and dominance in microelectronics. Light sources, modulators, waveguides, and logical gates are a few examples of microelectronic materials applications in the various photonic devices which have been developed based on silicon nanocrystals.SRO/SiO2 multilayers were deposited by 13.56 MHz radio-frequency magnetron sputtering. The as-deposited multilayers and the crystallized films were investigated using in-situ X-ray diffractometry (XRD), High-resolution transmission electron microscopy (HRTEM) and Secondary ion mass spectrometry (SIMS).Multilayers composed of alternating stacks of amorphous SRO and SiO2 layers were crystallized by thermal annealing in vacuum. The different stages from nucleation until full crystallization were investigated by in situ X-ray diffractometry. XRD in agreement with HRTEM confirmed the presence of silicon crystalline fraction in annealed films. It is calculated that there is 17–25% of crystalline volume fraction in the multilayers after the annealing procedure with density of particles of 2–2.8 × 1018cm−3. The crystalline fraction obtained consists of nanoparticles, whose average size is 3.5–5 nm.

Broadband LWIR and MWIR absorber by trapezoid multilayered grating and SiO2 hybrid structures

A broadband metamaterial absorber with high absorption simultaneously in mid-wave infrared (MWIR) and long-wave infrared (LWIR) was proposed. In the MWIR, the absorption higher than 0.8 is from 4 to 6.3 µm, while the absorption in the LWIR is from 8.7 and 9.6 µm. The absorber is insensitive to the incident angle. The broadband absorption in the MWIR is due to the slow-light effect of the trapezoid multilayered grating structure. And the broadband absorption in the LWIR is due to the phonon polariton resonant of trapezoid SiO2 layer. In the broadband high absorption region, the atmosphere is transparent, which may greatly promote the practical application of the absorber in double-color IR imaging, detecting, infrared stealth and thermal emitting.

Silica and carbon decorated silica nanosheet impact on primary human immune cells.

Silica nanosheets (SiO2 NS) are considered to be a promising material in clinical practice for diagnosis and therapy applications. However, an appropriate surface functionalization is essential to guarantee high biocompatibility and molecule loading ability. Although SiO2 NS are chemically stable, its effects on immune systems are still being explored. In this work, we successfully synthesized a novel 2D multilayer SiO2 NS and SiO2 NS coated with carbon (C/SiO2 NS), and evaluated their impact on human Peripheral Blood Mononuclear Cells (PBMCs) and some immune cell subpopulations. We demonstrated that the immune response is strongly dependent on the surface functionalities of the SiO2 NS. Ex vivo experiments showed an increase in biocompatibility of C/SiO2 NS compared to SiO2 NS, resulting in a lowering of hemoglobin release together with a reduction in cellular toxicity and cellular activation. However, none of them are directly involved in the activation of the acute inflammation process with a consequent release of pro-inflammatory cytokines. The obtained results provide an important direction towards the biomedical applications of silica nanosheets, rendering them an attractive material for the development of future immunological therapies.

Process study about silica anti-reflection coatings prepared by sol–gel method for cadmium telluride solar cells

Silica (SiO2) thin films with different refractive indexes as the anti-reflection coating for cadmium telluride (CdTe) solar cells were prepared by sol–gel method in this paper. In order to obtain the preparation process of SiO2 films with suitable refractive index, the different hydrolysis processes relied on the acid-base environment as well as the ratio of reactants was studied, respectively. It mainly includes that (I) the gel growth model in hydrogen or hydroxide ion environment provided by hydrochloric and ammonia. (II) The ratio of solvent, precursor and catalyst under base catalysis. (III) The weight and amount of polyethylene glycol molecular which is an additive agent to increase the porosity of sol film under acid catalysis. Based on the results above, a monolayer SiO2 coating with optimized refractive index was applied onto the glass substrate of CdTe solar cell, and the quantum efficiency has been enhanced at the band of 400–800 nm. After that, multilayer SiO2 coatings with high–low refractive indexes were designed according to the Fresnel theory to achieve wider spectrum (390–870 nm) enhancement. The broadband anti-reflection coatings (BARCs) were deposited on CdTe solar cell. The results were not consistent with expectations, which required further study about process matching between BARCs and device manufacture.

P‐18: Improvement of Reliability in Coplanar a‐IGZO TFTs by Multilayer SiO2 Gate Insulator

We investigated the PBTS instability of top‐gate self‐aligned coplanar a‐IGZO TFTs with multilayer SiO2 GI. It is confirmed that the PBTS instability due to the deep charge trapping in GI bulk can be improved by increasing activation energy through the interfacial formation with hydrogen‐low SiO2 deposition, and it is demonstrated experimentally that the PBTS instability due to the excess oxygen in the active region can be improved by hydrogen diffusion process through sequential hydrogen‐rich SiO2 deposition after ACT/GI interface formation.

Modulated spin orbit torque in a Pt/Co/Pt/YIG multilayer by nonequilibrium proximity effect

We have compared the spin orbit torque (SOT) induced magnetization switching in Pt/Co/Pt/Y3Fe5O12 (YIG) and Pt/Co/Pt/SiO2 multilayers. The critical switching current in Pt/Co/Pt/YIG is almost half of that in Pt/Co/Pt/SiO2. Through harmonic measurements, we demonstrated the enhancement of the effective spin Hall angle in Pt/Co/Pt/YIG. The increased efficiency of SOT is ascribed to the nonequilibrium proximity effect at the Pt/YIG interface, which suppresses the spin current reflection and enhances the effective spin accumulation at the Co/Pt interface. Our method can effectively reduce the switching current density and provide another way to modulate SOT.

Modulation of surface states by phosphorus to improve the optical properties of ultra-small Si nanocrystals

Here, we report the enhanced luminescence and optical gain by appropriate P-doping in Si nanocrystals (NCs)/SiO2 multilayers with ultra-small size of ∼1.9 nm. The luminescence intensity is enhanced by 19.4% compared to that of an un-doped NC and the optical gain is as high as 171.8 cm−1, which can be attributed to the reduction of surface defect states by the passivation of P impurities as revealed by electron spin resonance spectra. Further increasing the P-doping ratios results in the increase of conduction electrons due to the substitutional doping of phosphorus in the Si NCs, which favors the Auger recombination process. Consequently, both the luminescence intensity and the optical gain decrease rapidly. It is demonstrated that introduction of the suitable impurities can effectively modulate the surface chemical environment of Si NCs, which provides a new way to control the physical properties of Si NCs.