Dielectric Silica Research Articles

Molecular Monolayers for Electrical Passivation and Functionalization of Silicon-Based Solar Energy Devices.

Silicon-based solar fuel devices require passivation for optimal performance yet at the same time need functionalization with (photo)catalysts for efficient solar fuel production. Here, we use molecular monolayers to enable electrical passivation and simultaneous functionalization of silicon-based solar cells. Organic monolayers were coupled to silicon surfaces by hydrosilylation in order to avoid an insulating silicon oxide layer at the surface. Monolayers of 1-tetradecyne were shown to passivate silicon micropillar-based solar cells with radial junctions, by which the efficiency increased from 8.7% to 9.9% for n+/p junctions and from 7.8% to 8.8% for p+/n junctions. This electrical passivation of the surface, most likely by removal of dangling bonds, is reflected in a higher shunt resistance in the J-V measurements. Monolayers of 1,8-nonadiyne were still reactive for click chemistry with a model catalyst, thus enabling simultaneous passivation and future catalyst coupling.

Polymer particles filled with multiple colloidal silica via in situ sol-gel process and their thermal property

The in situ formation of dielectric silica (SiO2) particles was carried out in the presence of temperature-responsive poly(N-isopropylacrylamide) particles. Unlike the typical sol-gel method used to prepare various SiO2 particles, the highly uniform growth of SiO2 particles was achieved within the cross-linked polymer particles (i.e., the polymer particles were filled with the SiO2 particles) simply by utilizing interfacial interactions, including the van der Waals attractive force and hydrogen bonding in nanoscale environments. The structural and morphological features as well as the thermal behaviors of these composites were thoroughly examined by electron microscopes, dynamic light scattering, and thermal analyzers. In particular, the thermal properties of these composites were completely different from the bare polymer, SiO2 particles, and their mixtures, which clearly suggested the successful incorporation of multiple SiO2 particles within the cross-linked polymer particles. Similarly, titanium oxide (TiO2) particles were easily embedded within the polymer particle template which exhibited improved overall properties. As a whole, understanding in situ formation of nanoscale inorganic particles within polymer particle templates can allow for designing novel composite materials possessing enhanced chemical and physical properties.

Silica-coated silver nanowire-loaded hybrid plasmonic waveguide for low-loss waveguiding on the nanoscale

A hybrid plasmonic waveguide with an alternative structure is presented in this paper, which consists of a silver cylindrical nanowire coated with a low-index dielectric silica layer attached on a silicon-on-insulator (SOI) substrate with a rectangular ridge. The effects of the silver nanowire, silica-coated layer, silicon substrate layer, and the ridge’s width to thickness ratio on the modal properties have been numerically investigated based on the finite-difference time-domain method. The simulation results demonstrate that the introduction of the low-loss dielectric-coated layer and the ridge of SOI substrate can greatly increase the propagation distance while maintaining strong confinement of light. The propagation distance as long as 660 μm and mode field with subwavelength confinement can be simultaneously realized at telecommunication wavelength. The proposed waveguide consisting of the silica-coated silver nanowire and the SOI ridge substrate would be a beneficial complement to the expanding family of hybrid plasmonic structures.

Exploring the selectivity of WO3 with iridium catalyst in an ethanol/naphthalene mixture using multivariate statistics

Temperature cycled operation and multivariate statistics have been used to compare the selectivity of two gate (i.e. sensitive) materials for gas-sensitive, silicon carbide based field effect transistors towards naphthalene and ethanol in different mixtures of the two substances. Both gates have a silicon dioxide (SiO2) insulation layer and a porous iridium (Ir) electrode. One of it has also a dense tungsten trioxide (WO3) interlayer between Ir and SiO2. Both static and transient characteristics play an important role and can contribute to improve the sensitivity and selectivity of the gas sensor.The Ir/SiO2 is strongly influenced by changes in ethanol concentration, and is, thus, able to quantify ethanol in a range between 0 and 5ppm with a precision of 500ppb, independently of the naphthalene concentrations applied in this investigation. On the other hand, this sensitivity to ethanol reduces its selectivity towards naphthalene, whereas Ir/WO3/SiO2 shows an almost binary response to ethanol. Hence, the latter has a better selectivity towards naphthalene and can quantify legally relevant concentrations down to 5ppb with a precision of 2.5ppb, independently of a changing ethanol background between 0 and 5ppm.

Mesoporous silica layer on plasmonic array: light trapping in a layer with a variable index of refraction

Mesoporous silica layers with open and accessible mesopores were fabricated on aluminum nanoparticle arrays. The system can support plasmonic-photonic hybrid modes that are radiatively coupled surface plasmon polaritons in the nanoparticles. The coupling is mediated either by diffraction in the plane of the array or by waveguiding in the dielectric silica layer covering the array. Upon irradiation of an array with visible light, these hybrid modes are excited and appear as sharp spectral dips in optical transmission, a manifestation of light trapping in the system. The porous nature of the layers can be taken advantage of, via pore infiltration, to facilitate control over the layer’s refractive index. We demonstrated tuning in the wavelength and spatial distribution of trapped light in the system.

Perfectly absorbing ultra thin interference coatings for hydrogen sensing

Here we numerically demonstrate a straightforward method for optical detection of hydrogen gas by means of absorption reduction and colorimetric indication. A perfectly absorbing metal-insulator-metal (MIM) thin film interference structure is constructed using a silver metal back reflector, silicon dioxide insulator, and palladium as the upper metal layer and hydrogen catalyst. The thickness of silicon dioxide allows the maximizing of the electric field intensity at the Air/SiO2 interface at the quarter wavelengths and enabling perfect absorption with the help of highly absorptive palladium thin film (∼7 nm). While the exposure of the MIM structure to H2 moderately increases reflection, the relative intensity contrast due to formation of metal hydride is extensive. By modifying the insulator film thickness and hence the spectral absorption, the color is tuned and eye-visible results are obtained.

New sample preparation method using room-temperature direct bonding in backside SIMS analysis

Secondary ion mass spectrometry (SIMS) is one of the tools to investigate the dopant or impurity profiles of the semiconductor materials. In particular, the backside SIMS is a useful technique to examine the diffusion of dopant at the surface while minimizing knock-on effect. However, it is very difficult to prevent insulating samples from charging when O2+ source is used for positive secondary ion detection in magnetic sector SIMS. In order to overcome the problem, the authors have employed a new sample preparation method using a room-temperature direct bonding (RTDB). The authors investigated the depth profiling of insulating samples, which were stacked on a dielectric silicon dioxide films with silicon wafer as a magnetic sector SIMS instrument. The samples treated with and without RTDB were compared, and the results show that the depth profiles of RTDB-treated sample could be obtained no sign of charge-up in the backside SIMS analysis.

Plasmonically Enhanced Photocatalytic Hydrogen Production from Water: The Critical Role of Tunable Surface Plasmon Resonance from Gold-Silver Nanoshells.

Gold-silver nanoshells (GS-NSs) having a tunable surface plasmon resonance (SPR) were employed to facilitate charge separation of photoexcited carriers in the photocalytic production of hydrogen from water. Zinc indium sulfide (ZnIn2S4; ZIS), a visible-light-active photocatalyst, where the band gap varies with the [Zn]/[In] ratio, was used as a model ZIS system (E(g) = 2.25 eV) to investigate the mechanisms of plasmonic enhancement associated with the nanoshells. Three types of GS-NS cores with intense absorptions centered roughly at 500, 700, and 900 nm were used as seeds for preparing GS-NS@ZIS core-shell structures via a microwave-assisted hydrothermal reaction, yielding core-shell particles with composite diameters of ∼200 nm. Notably, an interlayer of dielectric silica (SiO2) between the GS-NSs and the ZIS photocatalyst provided another parameter to enhance the production of hydrogen and to distinguish the charge-transfer mechanisms. In particular, the direct transfer of hot electrons from the GS-NSs to the ZIS photocatalyst was blocked by this layer. Of the 10 particle samples examined in this study, the greatest hydrogen gas evolution rate was observed for GS-NSs having a SiO2 interlayer thickness of ∼17 nm and an SPR absorption centered at ∼700 nm, yielding a rate 2.6 times higher than that of the ZIS without GS-NSs. The apparent quantum efficiencies for these core-shell particles were recorded and compared to the absorption spectra. Analyses of the charge-transfer mechanisms were evaluated and are discussed based on the experimental findings.

Influence of dielectric nanoparticles addition on electroconductivity and other properties of carbon nanotubes-based acrylic coatings

Abstract The coating compositions based on commercial acrylic aqueous dispersion and carbon nanotubes (CNT) were modified with dielectric inorganic oxide nanoparticles (DNP), i.e. alumina, silica or titania. The influence of DNP type and content (0, 20 or 50 wt.%) on electrical, optical, mechanical and thermal features of the coatings has been investigated. The samples containing silica reached significantly lower electrical surface resistivity and higher transparency (e.g. 3.6 × 10 7 Ω and 70%, respectively for the material containing 0.25 wt.% of CNT and 50 wt.% of silica) than the reference sample (with CNT only) as well as the samples with similar CNT and other DNP (alumina or titania) content. Moreover, the samples containing 1 wt.% of CNT and 50 wt.% of silica or titania nanoparticles exhibited higher hardness (ca. 300% increment) and lower gloss (reduction from 80 G.U. to 48 G.U. and 0.3 G.U., respectively) in relation to the sample with CNT only. Additionally, the presence of CNT and DNP resulted in thermal stability increase of acrylic coatings up to 12 °C (for the samples filled with alumina or silica).

Surface Plasmon Resonance of an Individual Nano‐Object on an Absorbing Substrate: Quantitative Effects of Distance and 3D Orientation

Modification of the plasmonic response of a metal nano‐object due to interaction with a substrate is experimentally investigated measuring the quantitative optical extinction spectra of individual nano‐objects with various elongated shapes (bipyramids and rods) deposited on a dielectric (silica) or absorbing (carbon) membrane. Apart from the expected dependence of the nanoparticle surface plasmon resonance (SPR) frequency on the nature of the substrate, large substrate and particle shape dependent modifications of its SPR width are demonstrated. These dependencies are ascribed to strong localization of the electromagnetic field associated with the longitudinal SPR of an elongated nano‐object around its tips, leading to different interaction with the substrate depending on the particle shape and 3D orientation relative to the substrate. Both parameters have been precisely determined by electron tomography, permitting excellent reproduction of the experimental data. Experiments performed on silver‐encapsulated bipyramids, whose shape evolves from a pyramidal one towards a cylindrical one, further confirm this effect.

Analysis of Localized Surface Plasmon Resonance in Ag/ITO/CdS/SiO2 Multilayered Nanostructured Composite

Multilayered nanoshells have attracted much attention due to their unique optical, electronic and magnetic properties. In this work, numerical calculation using discrete dipole approximation (DDA) is conducted to investigate the quad-layered metal nanoshell consisting of a particle with a dielectric silica (SiO2) core, inner cadium sulfide (CdS) shell, middle indium tin oxide (ITO) shell and outer metal silver (Ag) shell. The phenomenon is interpreted by plasmon hybridization theory and the Ag–ITO–CdS–SiO2 multilayered nanoshells are studied by extinction spectra of localized surface plasmon resonance. The variation in the spectrum peak with nanoparticle thickness and refractive index of the surrounding medium is derived. The electric field enhancement contour around the nanoparticles under illumination is analyzed at the plasmon resonance wavelength. The [Formula: see text], [Formula: see text], and [Formula: see text] modes red-shift with the refractive index of the surrounding medium and increase in the layer thickness causes either blue-shift or red-shift as shown by the extinction spectra. The mechanism of the red-shift or blue-shift is discussed. The [Formula: see text] mode blue-shifts and furthermore, the [Formula: see text] and [Formula: see text] modes of the Ag coated multilayered nanostructure are noticeable by comparing the extinction efficiency spectra of the Au–ITO–CdS–SiO2 and Ag–ITO–CdS–SiO2 multilayered nanoshells.

Catalyst-free approach for growth of graphene sheets on high-density silica nanowires by CVD

A novel “two-step annealing” method is proposed for the direct synthesis of graphene sheets on high-density dielectric silica nanowires without using metal catalysts. During the first annealing at 1000°C, the extremely thin SiO2 layer on Si substrate shrinks and forms dense nanoparticles. Using these silica nanoparticles as templates, graphene sheets and silica nanowires are synthesized simultaneously after the second annealing process at 800–850°C. The experiment results suggest that the graphene sheets grow along the nanowires and its crystalline quality and domain size are determined by the temperature and duration of the second annealing process.

Capacitive Micromachined Ultrasonic Transducers (CMUTs) for Underwater Imaging Applications

A capacitive micromachined ultrasonic transducer structure for use in underwater imaging is designed, fabricated and tested in this paper. In this structure, a silicon dioxide insulation layer is inserted between the top electrodes and the vibration membrane to prevent ohmic contact. The capacitance-voltage (C-V) characteristic curve shows that the transducer offers suitable levels of hysteresis and repeatability performance. The −6 dB center frequency is 540 kHz and the transducer has a bandwidth of 840 kHz for a relative bandwidth of 155%. Underwater pressure of 143.43 Pa is achieved 1 m away from the capacitive micromachined ultrasonic transducer under 20 excitation. Two-dimensional underwater ultrasonic imaging, which is able to prove that a rectangular object is present underwater, is achieved. The results presented here indicate that our work will be highly beneficial for the establishment of an underwater ultrasonic imaging system.

Permittivities of Ultra-Low Dielectric Silica Beads and Aerogels

Materials with relative permittivity or dielectric constant near to that of air (εr~1) are known as ‘ultra-low k’ materials. They find a number of applications in inter-connects of micro-electronic circuits, antennae, high-speed communication substrates etc. Among the inorganic materials, porous silica is the widely studied candidate. Porous silica can be of many types depending upon the extent of porosity and size and connectivity of pores. This paper presents the details of measurement of permittivities and the results of silica beads and silica aerogels. Silica beads, prepared by microwave heating of silica gels, are spherical beads of average 1mm size. Hydrophobic silica aerogels, prepared by ambient pressure drying of silica gels, are irregular chunks of 5-10 mm size. Both are potential bulk fill insulation materials and hence the permittivity can be measured as an aggregate filling a definite volume. The permittivities of these have been measured upto 1 MHz by 3-terminal method using a precision LCR meter and a powder-paste cell as per ASTM-D150-11. The εr values of silica aerogels and silica beads in 20 Hz-1 MHz range could be measured and are less than 1.6 at 1 MHz.

Atomic origin of ultrafast resistance switching in nanoscale electrometallization cells.

Nanoscale resistance-switching cells that operate via the electrochemical formation and disruption of metallic filaments that bridge two electrodes are among the most promising devices for post-CMOS electronics. Despite their importance, the mechanisms that govern their remarkable properties are not fully understood, especially for nanoscale devices operating at ultrafast rates, limiting our ability to assess the ultimate performance and scalability of this technology. We present the first atomistic simulations of the operation of conductive bridging cells using reactive molecular dynamics with a charge equilibration method extended to describe electrochemical reactions. The simulations predict the ultrafast switching observed in these devices, with timescales ranging from hundreds of picoseconds to a few nanoseconds for devices consisting of Cu active electrodes and amorphous silica dielectrics and with dimensions corresponding to their scaling limit (cross-sections below 10 nm). We find that single-atom-chain bridges often form during device operation but that they are metastable, with lifetimes below a nanosecond. The formation of stable filaments involves the aggregation of ions into small metallic clusters, followed by a progressive chemical reduction as they become connected to the cathode. Contrary to observations in larger cells, the nanoscale conductive bridges often lack crystalline order. An atomic-level mechanistic understanding of the switching process provides guidelines for materials optimization for such applications and the quantitative predictions over an ensemble of devices provide insight into their ultimate scaling andperformance.

Non-metal catalytic synthesis of graphene from a polythiophene monolayer on silicon dioxide

Direct synthesis of graphene without metal catalysts on a dielectric substrate is a major goal in graphene-based electronics and is an increasingly popular nanotechnology alternative to metal oxide semiconductor technology. However, current methods for the synthesis of these graphenes have many limitations, including the use of metal catalyst. Herein, we report a facile approach to the direct synthesis of graphene sheets based on the self-assembled monolayers (SAMs) technique. The new method for metal catalyst-free direct synthesis of a graphene sheet is through a solution-processable, inexpensive, easy, and reproducible cross-linked polythiophene self-assembled monolayer (SAM) that is formed via the [4+2] π cycloaddition reaction of π-electron conjugated thiophene layer self-assembled on the dielectric silicon dioxide substrate. The bifunctional molecules were carefully designed to create an SAM via silanization of alkoxy silane groups on the SiO2 substrate, and at the other end, a thin cross-linked polythiophene layer via a [4+2] π-electron cycloaddition reaction of π-electron conjugated thiophene SAM. By heating the cross-linked polythiophene SAM up to 1000°C under a high vacuum, single-layered or few-layered graphene sheets were successfully prepared on the dielectric silicon oxide substrate.

Measurement of 3D-forces on a Micro Particle in Acoustofluidic Devices Using an Optical Trap

Here, we use a calibrated high gradient laser trap to directly measure the total time-averaged 3D force on a dielectric silica parti- cle in the regime of an ultrasonic standing wave. Acoustic radiation and acoustic streaming apply forces on an optically trapped particle within an acoustofluidic device. From measuring the induced displacements from the laser trap center in three dimen- sions the acoustic forces (0.2-50pN) can be calculated in dependence of the particle position and excitation frequency. Thus, the real pressure distributions within acoustofluidic devices can be mapped out. The three dimensional direct measurement, as pre- sented here, opens up the possibility to quantify so far inaccessible small scale phenomena such as the effects of: a.) local and global acoustic streaming, and b.) boundaries or close-by objects.

Room temperature curable silica ink

A facile formulation of room temperature curable silica ink screen printed on flexible and rigid substrate for printed electronic applications is presented. The colloidal stability and room temperature curability of the developed ink is controlled by optimizing the solvent, filler, dispersant and binder compositions. The printed ink has a porous microstructure with average surface roughness of 370 nm, and dielectric characterization of printed silica ink on BoPET film reveals relative permittivity (er) of 3.2 and dielectric loss (tan δ) of 0.02 at 1 MHz, and values of the above parameters at microwave frequencies of 8.2–18 GHz vary from 2.4 to 2.3 and from 0.003 to 0.006, respectively, which is suitable for printed electronics. These findings suggest that the newly developed dielectric silica ink is cost effective and is curable at room temperature, which enables printing and curing of flexible electronic circuits in ambient conditions.

Monolithic integrable capacitive humidity sensing method for material characterization of dielectric thin films

A monolithic integrable capacitive humidity sensing method to determine water vapour transmission rates (WVTRs) of dielectric thin films is presented. The capacitive sensor, being used to detect transmission of water vapour, as well as the dielectric thin film to be tested can be processed subsequently with standard semiconductor technology. First measurements yield a reliable value of the well investigated dielectric silicon dioxide (SiO2). A 330nm thick plasma enhanced chemical vapour deposited film of SiO2 showed a WVTR of ∼1.6∗10-2±0.7∗10-2gm2∗d at 124°C and a step in surrounding relative humidity from 65% to 85%. The working principle of the sensor, its drawbacks and improvements are discussed and compared with other methods.

Optimization of magnetoresistive sensor current for on-chip magnetic bead detection using the sensor self-field

We investigate the self-heating of magnetoresistive sensors used for measurements on magnetic beads in magnetic biosensors. The signal from magnetic beads magnetized by the field due to the sensor bias current is proportional to the bias current squared. Therefore, we aim to maximize the bias current while limiting the sensor self-heating. We systematically characterize and model the Joule heating of magnetoresistive sensors with different sensor geometries and stack compositions. The sensor heating is determined using the increase of the sensor resistance as function of the bias current. The measured temperature increase is in good agreement with a finite element model and a simple analytical thermal model. The heat conductance of our system is limited by the μ 1 m thick electrically insulating silicon dioxide layer between the sensor stack and the underlying silicon wafer, thus the heat conductance is proportional to the sensor area and inversely proportional to the oxide thickness. This simple heat conductance determines the relationship between bias current and sensor temperature, and we show that μ 2 5m wide sensor on a μ 1 m oxide can sustain a bias current of 30 mA for an allowed temperature increase of 5 °C. The method and models used are generally applicable for thin film sensor systems. Further, the consequences for biosensor applications of the present sensor designs and the impact on future sensor designs are discussed.