Silica Films Research Articles

Interface Effects in the Stability of 2D Silica, Silicide, and Silicene on Pt(111) and Rh(111).

Ultrathin two-dimensional silica films have been suggested as highly defined conductive models for fundamental studies on silica-supported catalyst particles. Key requirements in this context are closed silica films that isolate the gas phase from the underlying metal substrate and stability under reaction conditions. Here, we present silica bilayer films grown on Pt(111) and Rh(111) and characterize them by scanning tunneling microscopy and X-ray photoelectron spectroscopy. We provide the first report of silica bilayer films on Rh(111) and have further successfully prepared fully closed films on Pt(111). Interestingly, surface and interface silicide phases play a decisive role in both cases: On platinum, closed films can be stabilized only when silicon is deposited in excess, which results in an interfacial silicide or silicate layer. We show that these silica films can also be grown directly from a surface silicide phase. In the case of rhodium, the silica phase is less stable and can be reduced to a silicide in reductive environments. Though similar in appearance to the "silicene" phases that have been controversially discussed on Ag(111), we conclude that an interpretation of the phase as a surface silicide is more consistent with our data. Finally, we show that the silica film on platinum is stable in 0.8 mbar CO but unstable at elevated temperatures. We thus conclude that these systems are only suitable as model catalyst supports to a limited extent.

Zinc Oxide Nanostructure Deposition into Sub-5 nm Vertical Mesopores in Silica Hard Templates by Atomic Layer Deposition.

Nanostructures synthesised by hard-templating assisted methods are advantageous as they retain the size and morphology of the host templates which are vital characteristics for their intended applications. A number of techniques have been employed to deposit materials inside porous templates, such as electrodeposition, vapour deposition, lithography, melt and solution filling, but most of these efforts have been applied with pore sizes higher in the mesoporous regime or even larger. Here, we explore atomic layer deposition (ALD) as a method for nanostructure deposition into mesoporous hard templates consisting of mesoporous silica films with sub-5 nm pore diameters. The zinc oxide deposited into the films was characterised by small-angle X-ray scattering, X-ray diffraction and energy-dispersive X-ray analysis.

Spatially selective narrow band and broadband absorption in Ag/SiO2/Ag based trilayer thin films by oblique angle deposition of SiO2 layer

We have experimentally demonstrated spatially selective absorption in Ag-SiO2-Ag based trilayer thin films by tuning the deposition angle of SiO2 layer. These structures generate cavity resonance which can be tuned across the substrate locations due to spatially selective thickness and refractive index of silicon oxide (SiO2) film sandwiched between metallic silver (Ag) mirrors. Spatially selective property of SiO2 film is obtained by oblique angle deposition technique using an electron beam evaporation system. The resonance wavelength of absorption in this trilayer structure shifts across the substrate locations along the direction of oblique deposition. The extent of shift in resonance increases with increase in angle of deposition of SiO2 layer. 4.14 nm mm−1 average shift of resonance wavelength is observed when SiO2 is deposited at 40° whereas 4.76 nm mm−1 average shift is observed when SiO2 is deposited at 60°. We observed that the width of resonance increases with angle of deposition of the cavity layer and ultimately the resonant absorption disappears and becomes broadband when SiO2 is deposited at glancing angle deposition (GLAD) configuration. Our study reveals that there is a suitable range of oblique angle of deposition from 40° to 60° for higher spatial tunability and resonant absorption whereas the absorption becomes broadband for glancing angle deposition.

Assessing the Influence of Confinement on the Stability of Polyoxometalate-Functionalized Surfaces: A Soft Sequential Immobilization Approach for Electrochromic Devices.

A functionalization process has been developed and the experimental conditions optimized allowing the immobilization of first-row transition metal (Mn+) containing polyoxometalates (POMs) with the formula [M(H2O)P2W17O61](10-n)- on transparent indium-tin oxide (ITO) electrodes for electrochromic applications. Both flat ITO grafted with 4-sulfophenyl moieties and sulfonate-functionalized vertically oriented silica films on ITO have been used as electrode supports to evaluate possible confinement effects provided by the mesoporous matrix on the stability of the modified surfaces and their electrochromic properties. Functionalization involved a two-step sequential process: (i) the immobilization of hexaaqua metallic ions, such as Fe(H2O)63+, onto the sulfonate-functionalized materials achieved through hydrogen bonding interactions between the sulfonate functions and aqua ligands (water molecules) coordinated to the metallic ions facilitating and stabilizing the attachment of the metallic ions to the sulfonated surfaces; (ii) their coordination to [P2W17O61]10- species to generate "in situ" the target [Fe(H2O)P2W17O61]7- moieties. Comparison of the characterized surfaces clearly evidenced a significant improvement in the long-term stability of the nanostructured [Fe(H2O)P2W17O61]7--functionalized silica films compared to the less constrained flat [Fe(H2O)P2W17O61]7--modified ITO electrodes for which a rapid loss of [P2W17O61]10- species was observed. Concordantly, the [Fe(H2O)P2W17O61]7- POM confined in the mesoporous films coated on ITO gave rise to much better and stable electrochromic properties, with a transmittance modulation of 40% at 515 nm.

Engineering the SiOx interfacial layer of Si-based metal-insulator-semiconductor junction for photoelectrochemical hydrogen production

Photoelectrochemical (PEC) water splitting provides a potential method to produce renewable hydrogen energy, but there is still plenty of room for improving the efficiency and stability of photoelectrodes. In this paper, we present a metal–insulator-semiconductor (MIS) structure based on p-Si that enables stable and efficient water splitting by engineering the interfacial insulating layer. The silicon oxide (SiOx) film with appropriate thickness and low defects is regrown by a chemical oxidation process, which provides a high-quality insulating layer to passivate the p-Si. The carrier flux, barrier height and interfacial resistance of p-Si based MIS junction can be systematically tuned by controlling the thickness and quality of SiOx layer. Under AM 1.5G illumination, the optimized p-Si/SiOx/Ti/Pt photoelectrode shows an onset potential of 0.5 V vs. RHE, a maximum photocurrent of 28 mA/cm2 and a high applied bias photon-to-current efficiency (ABPE) of 6 %. These results have significant implications for constructing MIS photoelectrodes towards effective water splitting.

Optical response and temperature shielding coatings using tri-layer structure composed of titania/silica/titania

The fabrication of metamaterials with inspiration from nature paved the door for the creation of revolutionary passive and active devices. Butterfly wings are one of them, and their multilayered structure motivated nanotechnologists, physicists, and other optoelectronic and photonic engineers to develop photonic crystals, dielectric reflectors, and Bragg reflectors for use in visible light communication, solar cells, and other photonic and optoelectronic applications. We report the sol–gel spin coating of a dielectric reflector on the glass substrate, an optical passive component consisting of titania and silica thin films for infrared radiation reflection. Individual thin films of titania and silica on glass substrates were studied using an x-ray diffractometer, which indicated anatase of titania and the amorphous nature of silica at a Bragg angle of 25° with a sharp and wide peak, respectively. The multilayer structure of titania/silica/titania was further investigated using Fourier transform infrared spectroscopy, which revealed the presence of Ti–O–Ti and Si–O–Si vibrational bonds at wavenumbers 546 and 973 cm−1, respectively, as well as the presence of Ti–O–Si vibrational bond at 1100 cm−1. The thickness of the multilayer titania/silica/titania was measured using a cross-sectional field emission scanning electron microscope (FESEM) and found to be 160/240/160 nm, respectively. Finally, reflection investigation on the multilayer structure using ultraviolet–visible–near-infrared spectroscopy validated the reflection of the infrared spectrum area by around 70% and showed to be beneficial for temperature shielding applications on glass furnishings.

Highly-selective optical filter for NADH fluorescence detection in multiphoton microscopy

Colorectal cancer (CRC) is a pressing global health concern, emphasizing the need for early detection tools. In this study an optical filter for precise detection of nicotinamide adenine dinucleotide (NADH) fluorescence via two-photon excitation fluorescence (TPEF) was developed. Fabricated with silicon dioxide and titanium dioxide thin films in a Fabry-Perot structure, the filter achieved a peak transmittance of about 95% at 483 nm, with a 12 nm full-width at half maximum. TPEF measurements using a tailored setup and NADH liquid phantoms underscored the filter's significance in selectively capturing NADH fluorescence while mitigating interference from other fluorophores. This work marks a substantial stride towards integrating multiphoton microscopy into conventional colonoscopy, enabling non-invasive, objective optical biopsy for colorectal tissue analysis. Further refinements of the experimental setup are imperative to advance tissue differentiation and enhance CRC diagnosis.

Highly efficient anti-reflection coating on transparent material surface by a deposition-oxidation two-step process using very high-frequency plasma under atmospheric pressure

Using atmospheric-pressure (AP) helium (He)-based plasma excited by a 150 MHz very high-frequency (VHF) power, the formation of low-refractive-index silicon oxide (SiOx) layers for single-layer quarter-wave anti-reflection (AR) coatings of transparent substrate materials is studied. To effectively lower the refractive index of SiOx layers, a deposition-oxidation two-step process is adopted. Specifically, carbon and hydrogen-containing silicon oxide (SiOCH) films are coated in hexamethyldisiloxane and hydrogen-fed plasma on the substrates with fine polystyrene spheres arranged, and subsequently oxidized in oxygen-fed plasma. During the post-oxidation process, the SiOCH film is transformed into silica (SiO2)-like one and the polystyrene spheres underlying the film is simultaneously oxidized to be removed. As a result, a SiO2-like film with a very low refractive index of 1.23 and with excellent adhesion to the substrate is obtained. It is also demonstrated that the low-refractive-index coating functions well as a quarter-wave AR coating of a glass substrate.

Catalytic atomic layer deposition of amorphous alumina–silica thin films on carbon microfibers

Deposition of silica-based thin films on carbon microfibers has long been considered a challenge. Indeed, the oxidation-sensitive nature of carbon microfibers over 550 K and their submicron-textured surface does not bode well with the required conformity of deposition best obtained by atomic layer deposition (ALD) and the thermal oxidative conditions associated with common protocols of silica ALD. Nonetheless, the use of a catalytic ALD process allowed for the deposition of amorphous alumina–silica bilayers from 445 K using trimethylaluminium and tris(tert-pentoxy)silanol (TPS). In this study, first undertaken on flat silicon wafers to make use of optical spectroscopies, the interplay between kinetics leading to a dense silica film growth was investigated in relation to the applied operation parameters. A threshold between the film catalyzed growth and the complete outgassing of pentoxy-derived compounds from TPS was found, resulting in a deposition of equivalent growth per cycle of 1.1 nm c−1, at a common ALD rate of 0.3 nm min−1, with a flat thickness gradient. The deposition on carbon microfiber fabrics was found conformal, albeit with a thickness growth capped below 20 nm, imparted by the microfiber surface texture. STEM-EDX showed a sharp interface of the bilayer with limited carbon diffusion. The conformal and dense deposition of alumina–silica thin films on carbon microfibers holds great potential for further use as refractory oxygen barrier layers.

Measuring the refractive index and thickness of a thin film: an optical polarization experiment

We propose a teaching experiment for simultaneous measurement of the refractive index and thickness of a thin film. The experimental apparatus includes: a diode laser, a polarizer, a spectrometer, a photodiode and a portable digital multimeter. With the apparatus, we measure the refractive index and thickness of two thin films (a silicon dioxide film on silicon, a silicon nitride film on silicon). The maximum discrepancy between the results of our apparatus and an ellipsometer is 1.4%. The experiment described here can be used in optical polarization teaching for undergraduate sophomores and has several valuable pedagogical outcomes: (1) understanding the multi-beam interference in a film; (2) applying chi-square minimization; (3) carrying out the nonlinear curve fitting with multiple parameters.

Study on TSV insulating medium based on SACVD process

TSV silicon switching board technology has the advantages of short global interconnection, small delay, low power consumption and high integration. The dielectric insulation performance in the TSV hole will directly affect the signal integrity and power integrity of the TSV silicon switching board. The insulating dielectric layer of TSV requires good forming ability and step covering, and lower leakage current and good heat resistance reliability. In this paper, the filling capacity of SACVD process is analyzed, the coverage rate of SACVD process is studied under different process parameters, and the leakage current of silicon dioxide film is measured by planar capacitance structure. Finally, the leakage current test of the TSV pairs is carried out online. Before the substrate thinning, the leakage current of the two blind hole TSV pairs is measured by the probe station. The voltage ranges form 0 to 50V, and the leakage current ranges from a few pA to a few hundred pA.

Formation mechanism of two-dimensional hexagonal silica on SiO2/Si substrate

Owing to their remarkable electronic properties, silica ultrathin films have been utilized as an insulating layer in nanoelectronics systems. Silica films have been epitaxially grown on different substrates using various synthesis methods. Among all fabrication approaches, chemical vapor deposition has long been an advanced method for synthesizing two-dimensional (2D) materials due to its ability to ensure precise stacking control and minimize contamination between layers. This study harnessed the potential of CVD to atomically fabricate thin layered 2D silica on a SiO2/Si substrate. Significantly, a unique combination of multiple transition metals and salt as the catalysts aided the formation of 2D silica for the first time. Salt is a crucial catalyst in promoting the evaporation of high-melting-point metal catalysts, resulting in hexagonal nucleation sites on the SiO2/Si wafer. By meticulously controlling growth parameters, a distinctive hexagonal structure was obtained. Correspondingly, this work delves into the growth mechanism of 2D silica, as evidenced by experiments involving salt alone and individual transition metals. Group VB transition metals played a prominent role in achieving the hexagonal structure compared to their group IVB counterparts. This research offers insight into the formation and growth mechanism of 2D silica, expanding the understanding of silica nanostructures.

Silica-embedded Gold Nanoparticles Analyzed by Atom Probe Tomography.

Nanoparticles are utilized in a multitude of applications due to their unique properties. Consequently, characterization of nanoparticles is crucial, and various methods have been employed in these pursuits. One such method is Atom Probe Tomography (APT). However, existing sample preparation techniques for APT generally involve embedding of the nanoparticles in a matrix different from their environment in solutions or at solid-liquid interfaces. In this work, we demonstrate a methodology based on silica embedding and explore how it can be utilized to form a matrix for nanoparticles suitable for APT analysis. Through chemisorption to a surface, gold nanoparticles were densely packed, ensuring a high probability of encountering at least one particle in the APT analyses. The nanoparticle-covered surface was embedded in a silica film, replacing the water and thus making this method suitable for studying nanoparticles in their hydrated state. The nanoparticle's silver content and its distribution, originating from the nanoparticle synthesis, could be identified in the APT analysis. Sodium clusters, possibly originating from the sodium citrate used to stabilize the particles in solution, were observed on the nanoparticle surfaces. This indicates the potential for silica embedding to be used for studying ligands on nanoparticles in their hydrated state.

Highly Sensitive Electrochemical Determination of Butylated Hydroxyanisole in Food Samples Using Electrochemical-Pretreated Three-Dimensional Graphene Electrode Modified with Silica Nanochannel Film.

The sensitive detection of antioxidants in food is essential for the rational control of their usage and reducing potential health risks. A simple three-dimensional (3D) electrode integrated with an anti-fouling/anti-interference layer possesses great potential for the direct and sensitive electrochemical detection of antioxidants in food samples. In this work, a 3D electrochemical sensor was developed by integrating a 3D graphene electrode (3DG) with vertically ordered mesoporous silica film (VMSF), enabling highly sensitive detection of the common antioxidant, butylated hydroxyanisole (BHA), in food samples. A simple electrochemical polarization was employed to pre-activate the 3DG electrode (p3DG), enhancing its hydrophilicity. Using the p3DG as the supporting electrode, stable modification of VMSF was achieved using the electrochemical assisted self-assembly (EASA) method, without the need for any adhesive agents (VMSF/p3DG). Taking BHA in food as a model analyte, the VMSF/p3DG sensor demonstrated high sensitivity, due to the enrichment by nanochannels, towards BHA. Electrochemical detection of BHA was achieved with a linear range of 0.1 μM to 5 μM and from 5 μM to 150 μM with a low limit of detection (12 nM). Owing to the fouling resistance and anti-interference capabilities of VMSF, the constructed 3D electrochemical sensor can be directly applied for the electrochemical detection of BHA in complex food samples.

Tunable Neuromorphic Switching Dynamics via Porosity Control in Mesoporous Silica Diffusive Memristors.

In response to the growing need for efficient processing of temporal information, neuromorphic computing systems are placing increased emphasis on the switching dynamics of memristors. While the switching dynamics can be regulated by the properties of input signals, the ability of controlling it via electrolyte properties of a memristor is essential to further enrich the switching states and improve data processing capability. This study presents the synthesis of mesoporous silica (mSiO2) films using a sol-gel process, which enables the creation of films with controllable porosities. These films can serve as electrolyte layers in the diffusive memristors and lead to tunable neuromorphic switching dynamics. The mSiO2 memristors demonstrate short-term plasticity, which is essential for temporal signal processing. As porosity increases, discernible changes in operating currents, facilitation ratios, and relaxation times are observed. The underlying mechanism of such systematic control was investigated and attributed to the modulation of hydrogen-bonded networks within the porous structure of the silica layer, which significantly influences both anodic oxidation and ion migration processes during switching events. The result of this work presents mesoporous silica as a unique platform for precise control of neuromorphic switching dynamics in diffusive memristors.

Simple fabrication of electrochemical sensor based on integration of dual signal amplification by the supporting electrode and modified nanochannel array for direct and sensitive detection of vitamin B2.

Development of simple and reliable sensor for detecting vitamin content is of great significance for guiding human nutrition metabolism, overseeing the quality of food or drugs, and advancing the treatment of related diseases. In this work, a simple electrochemical sensor was conveniently fabricated by modification a carbon electrode with vertically-ordered mesoporous silica film (VMSF), enabling highly sensitive electrochemical detection of vitamin B2 (VB2) based on the dual enrichment of the analyte by the supporting electrode and nanochannels. The widely used glassy carbon electrode (GCE), was preactivated using simple electrochemical polarization, The resulting preactivated GCE (p-GCE) exhibited improved potential resolution ability and enhanced peak current of VB2. Stable modification of VMSF on p-GCE (VMSF/p-GCE) was achieved without introducing another binding layer. The dual enrichment effect of the supporting p-GCE and nanochannels facilitated sensitive detection of VB2, with a linear concentration ranged from 20 nM to 7 μM and from 7 μM to 20 μM. The limit of detection (LOD), determined based on a signal-to-noise ratio of three (S/N = 3), was found to be 11 nM. The modification of ultra-small nanochannels of VMSF endowed VMSF/p-GCE with excellent anti-interference and anti-fouling performance, along with high stability. The constructed sensor demonstrated the capability to achieve direct electrochemical detection of VB2 in turbid samples including milk and leachate of compound vitamin B tablet without the need for complex sample pretreatment. The fabricated electrochemical can be easily regenerated and has high reusability. The advantages of simple preparation, high detection performance, and good regeneration endow the constructed electrochemical sensor with great potential for direct detection of small molecule in complex samples.

Magnetic graphene oxide and vertically-ordered mesoporous silica film for universal and sensitive homogeneous electrochemiluminescence aptasensor platform

Herein, we demonstrated a convenient and highly sensitive homogeneous electrochemiluminescence (ECL) aptasensor platform by combination of magnetic composite probe and vertically-ordered mesoporous silica films (VMSF). Magnetic composite probe composed of biorecognition element (target-specific aptamer), ECL signal reporter (Ru(bpy)32+) and large-scale magnetic graphene oxide (M−GO), can be prepared by simple incubation, magnetic separation and redispersion prior to use. High-affinity and specific binding between target and aptamer can detach aptamer from M−GO and simultaneously release the slight amount of Ru(bpy)32+ into a solution, which can be sensitively detected by VMSF modified indium tin oxide (VMSF/ITO) due to the excellent electrostatic preconcentration capacity of VMSF. In this sensing system, detectable signal of Ru(bpy)32+ can be remarkably amplified by VMSF/ITO and meanwhile background signal can be reduced by the adsorption capacity of M−GO and size exclusion property of VMSF/ITO, effectively promoting the analytical performance of the present homogeneous ECL aptasensor. The applicability and universality of this sensing platform was confirmed by employing alpha fetoprotein (AFP) and prostate specific antigen (PSA) as the target models and their detection limits were 0.08 pg/mL and 9.6 pg/mL, respectively. Furthermore, the designed homogeneous ECL aptasensor platform is label-free, immobilization-free, low-cost, and free of additional electrode modification and sample pretreatment procedures, rendering a simple sensing strategy for quantitative determination of various biomarkers in clinical diagnosis by adapting the specific aptamer.

Development of n-Type, Passivating Nanocrystalline Silicon Oxide Films via Plasma-Enhanced Chemical Vapor Deposition

Nanocrystalline silicon oxide (nc-SiOx:H) is a multipurpose material with varied applications in solar cells as a transparent front contact, intermediate reflector, back reflector layer, and even tunnel layer for passivating contacts, owing to the easy tailoring of its optical properties. In this work, we systematically investigate the influence of the gas mixture (SiH4, CO2, PH3, and H2), RF power, and process pressure on the optical, structural, and passivation properties of thin n-type nc-SiOx:H films prepared in an industrial, high-throughput, plasma-enhanced chemical vapor deposition (PECVD) reactor. We provide a detailed description of the n-type nc-SiOx:H material development using various structural and optical characterization techniques (scanning electron microscopy (SEM), energy dispersive X-ray (EDX), Raman spectroscopy, and spectroscopic ellipsometry) with a focus on the relationship between the material properties and the passivation they provide to n-type c-Si wafers characterized by their effective carrier lifetime (τeff). Furthermore, we also outline the parameters to be kept in mind while developing different n-type nc-SiOx:H layers for different solar cell applications. We report a tunable optical gap (1.8–2.3 eV) for our n-type nc-SiOx:H films as well as excellent passivation properties with a τeff of up to 4.1 ms (implied open-circuit voltage (iVoc)~715 mV) before annealing. Oxygen content plays an important role in determining the crystallinity and hence passivation quality of the deposited nanocrystalline silicon oxide films.

Stresses in silicon dioxide films deposited from dielectric targets: results of atomistic modeling

The previously proposed method of molecular dynamics simulation of the deposition of thin films from metal targets is adapted to the case of dielectric targets and applied to silicon dioxide films. The possibility of not only silicon atoms leaving the target, but also clusters with oxygen atoms was taken into account by adding O=Si=O molecules to the flow of deposited atoms. Atomistic film clusters were obtained by high-energy and low-energy deposition at different percentages of molecules in the flow of deposited atoms. The values of the stress tensor components are calculated. With high-energy deposition, compressive stresses are observed, with low-energy deposition, tensile stresses are observed. The absolute values of the diagonal components of the stress tensor increase with increasing proportion of molecules in the flow of deposited atoms.

Revealing the mechanism of interfacial adhesion enhancement between the SiO2 film and the GaAs substrate via plasma pre-treatments

The formation mechanism of a highly adherent silicon dioxide (SiO2) film on gallium arsenide (GaAs) substrate by plasma enhanced chemical vapor deposition (PECVD) is proposed. Ar, N2, and NH3 were used as pre-treatment gas to improve the interfacial adhesion. The interfacial adhesion was measured by the cross-cut tape test. By the measurement of spectroscopic ellipsometry and x-ray photoelectron spectroscopy (XPS), it is revealed that nitrogen plasma pre-treatment had formed a very thin GaN transition layer on the surface, which was responsible for the improvement of interfacial adhesion. XPS depth-profiling further confirmed various pre-treatment gases generate plasma mixtures and form thin film layers with different compositions on the GaAs surface. These layers have a significant impact on the adhesion of the subsequently prepared SiO2 film. The primary mechanism for improving interfacial adhesion is the renovation of the substrate composition via plasma pre-treatment by PECVD, which forms a transition layer of nitrides that eliminates the negative effects of oxides on adhesion. This study reveals the mechanism of interfacial adhesion enhancement between SiO2 film and GaAs substrate, which is of significant importance in fabricating high-performance and reliable semiconductor devices.