Thick SiO2 Research Articles

Obviously Enhanced Fluorescent Signal of Core‐Shell Nanostructures through Simultaneous Regulation of Spectral Overlap and Shell Thickness for Imaging and Photothermal Therapy of Ovarian Cancer Cells

AbstractIn this research, by simultaneously regulating the two major factors affecting the plasmonic enhanced fluorescence (PEF), spectral overlap and the distance between the fluororophores and the noble metal nanoparticles, a significantly enhanced fluorescent signal is achieved. Core‐shell nanostructures composed of aspect ratio (AR) adjustable gold nanorods (GNRs) and various thickness of SiO2 are prepared and the decorated fluorophores are realized optimized PEF. A typical stimuli‐responsive conjugated polymer, polydiacetylene (PDA), and a near‐infrared (NIR) dye Cy5.5 are selected as fluorophores and their fluorescent signal are enhanced 7.26 and 4.41 times, respectively. Based on the optimized optical properties, a multifunctional antibody modified Mab‐Cy5.5‐GNRs@SiO2 is successfully demonstrated the targeting, imaging, and photothermal therapy (PTT) effects on SKOV‐3 ovarian cancer cells.

A High Efficiency Grating Coupler Based on Hybrid Si-Lithium Niobate on Insulator Platform

In this paper, a high efficiency grating coupler based on the Si thin film on Lithium Niobate (LiNbO3, LN) on insulator (LNOI) platform (Si-LNOI) was developed and analyzed. Two-dimensional finite-different time-domain (2D-FDTD) simulations were performed to optimize the grating structure. Those parameters (e.g., the layer thickness of SiO2, LNOI, and Si) and grating configuration were numerically investigated for the coupling efficiency. A coupling efficiency exceeding 98% at λ=1550 nm was obtained for TE polarization. Furthermore, the deviation tolerances of the grating period, the filling factor, the etching depth, and the layer thickness of the Si waveguide layer were studied systematically. The results indicated a high deviation tolerance. The grating coupler presented in this study could be achieved without adding fabrication steps other than etching, thereby showing high coupling efficiency and low fabrication complexity.

Long-range polaritonic heat conduction in asymmetric surrounding media

Surface Phonon Polaritons (SPhPs) as an evanescent electromagnetic surface wave supports long propagation which readily surpasses the mean free path of classical heat carriers, e.g., phonons and electrons in solids. SPhPs have emerged as a promising candidate to dominate heat transfer in thin films. Polaritonic heat transfer has two distinct advantages: superior thermal conduction and a wide range of manipulation. Here, we study the upper limit of the thermal conductivity mediated by long-range polaritons in asymmetric surrounding media, where its surface effect overwhelms the volumetric one. The thin film structure strengthens the interactions of two surface waves at the top and bottom surfaces, and the asymmetric surrounding media makes an evanescent surface wave to further penetrate the free space with a higher refractive index, but it requires a fine tuning of asymmetric permittivity of the surrounding media to reach the upper limit of energy transmission efficiency near to the modal cut-off, where the transverse wavevector becomes zero. Both analytical and numerical simulations were introduced to investigate dispersion in asymmetric surrounding media and to model the thermal conductivity of glass thin films. Anomalously high thermal conductivity of 248 W/m-K was achieved with a 50 nm thick SiO2 film in asymmetric surrounding media, yet subtly dissimilar.

Low polarization-sensitive ultra-broadband anti-reflection coatings with improved reliability.

Broader spectra, lower reflectivity and higher reliability are the performance requirements for broadband antireflective (BBAR) films. In this work, a BBAR film structure was proposed, which maintains extremely low reflectivity, ultra-wide spectra, low polarization sensitivity and practical reliability. The BBAR film consists of a dense multilayer interference stack on the bottom and a nano-grass-like alumina (NGLA) layer with a gradient low refractive index distribution on the top. The film was deposited by atomic layer deposition, while the NGLA layer was formed by means of a hot water bath on Al2O3 layer. The top NGLA layer has extremely high porosity and ultra-low refractive index, along with extremely fragile structure. To surmount the fragility of NGLA layer, a sub-nano layer of SiO2 was grown by atomic layer deposition to solidify its structure and also to adjust the refractive index with different thicknesses of SiO2. Finally, in the wide wavelength range of 400-1100 nm, the average transmittance of the double-sided coated fused quartz reaches 99.2%. The absorption, light scattering, reliability and polarization characteristics of BBAR films were investigated. An optimized BBAR film with low polarization-sensitivity and improved reliability was realized, which should be potentially promising for application in optical systems.

Shortwave infrared surface plasmons in multilayered two-dimensional Ti3C2T x MXenes

MXene, an ultra-thin two-dimensional conductive material, has attracted considerable interest in various fields due to its exceptional material properties. In particular, Ti3C2Tx MXene exhibits distinct optical properties, enabling it to support surface plasmons in the shortwave infrared (SWIR) region. However, it is challenging to enhance the field confinement of MXene surface plasmons in a single-interface structure due to the substantial intrinsic absorption of Ti3C2Tx MXene. Herein, we explore various multilayer structures capable of supporting high field confinement of Ti3C2Tx MXene plasmons, including insulator–MXene–insulator (IMI), MXene–insulator–MXene (MIM), and insulator–MXene–insulator–MXene (IMIM) configurations. We observe that the field confinement of MXene plasmons improves as the thickness of either the MXene or insulator layers decreases, which is attributed to the strong coupling between plasmons at the multilayer interfaces. Furthermore, the IMIM structure demonstrates the most substantial enhancement in field confinement. In an IMIM structure with a 1.3 nm-thick MXene monolayer and a 1.0 nm thick SiO2 layer, the wavelength and effective field size of the plasmon at a frequency of 150 THz (λ 0 = 2.0 μm) are calculated to be 24.61 nm and 1.50 nm, respectively. These values demonstrate a reduction by factors of 55 and 596, respectively, compared to those obtained in a single SiO2–MXene interface structure. Multilayer-based MXene plasmons provide a solution for enhancing the field confinement of MXene plasmons in the SWIR region, and we expect them to play a crucial role in a variety of 2D material-based SWIR plasmonic applications.

Transparent quantum dot light-emitting diodes with a current focusing structure

We report transparent quantum dot light-emitting diodes with a current focusing structure. By depositing a SiO2 thin film to form the current focusing structure, the DC density and luminance significantly increased to over 8700 mA/cm2 and 360 000 cd/m2, respectively. The emission spectra and current densities as functions of SiO2 thickness and aperture width have been investigated and discussed. This current focusing design is proved effective and can be further applied to other planar light-emitting diode devices.

Tunable Au@SiO2/Au Film Metasurface as Surface Plasmon Resonance Enhancer for Direct and Ultrasensitive Detection of Exosomes.

Surface plasmon resonance (SPR) spectroscopy with non-labelling, sensitive, and real-time properties is critical for clinical diagnosis applications. However, conventional SPR sensors face the challenge of lower sensitivity and selectivity for trace exosomes assay in complex serum. We proposed a core-shell Au@SiO2-Au film (Au@SiO2-Au film) metasurface to enhance SPR signal based on systematic study on the relationship between gap modes and SPR enhancement. The self-assembled multifunctional peptide was designed as recognition layer with antifouling properties for ultrasensitive and selective detection of PD-L1+ exosomes in serum. The tuning electromagnetic (EM) field model by manipulating the gap was established to guide the preparation of Au@SiO2-Au film metasurface. The in-plane and out-of-plane coupling of Au@SiO2 nanoparticles (NPs) could greatly enlarge and enhance three-dimensional EM field to meet the size of exosomes located in the evanescent field. At the structural level, we achieved high sensitivity (0.16 particles/mL) and a broad response range (10-5 × 103 particles/mL) through optimizing the thickness of SiO2 and surface coverage of Au@SiO2. Furthermore, clinical sample assay achieved the optimal diagnostic accuracy (AUC = 0.97) for differentiating cancer patients from healthy controls. This work provides an opportunity for the construction of a tunable gap mode as SPR enhancer in a total internal reflection architecture. The systematic study on the relationship between gap modes and SPR sensitivity provides a broad scope for promoting direct, efficient, highly selective, and sensitive detection of SPR sensors for clinical application.

Influence of Silicon and Chromium on the Na2SO4-Induced Hot Corrosion Behavior of Titanium Alloys

Titanium alloys are widely used as construction materials in the aerospace and automotive industries. They have many advantages but also have limitations related to their susceptibility to high-temperature oxidation and hot corrosion. Many efforts to increase the lifetime of components made of titanium alloys have been reported in the literature; the most promising ones involve the deposition of coatings. The present paper is focused on the development of coatings containing chromium and silicon, and their further evaluation in hot corrosion tests. It was proved that the Cr-Si coatings were more effective than Si coatings alone in protecting the titanium alloys against Na2SO4-induced hot corrosion at 800 °C. The enhanced corrosion resistance was attributed to the preferential formation of a thick and continuous SiO2 layer on the surface and—in the case of titanium aluminide alloy—the growth of an Al2O3-rich inner layer of the scale, promoted by chromium.

Transfer of Heteroepitaxial Grown 3C-SiC Layers for Application in Optical Frequency Combs

We developed a process for the fabrication of SiCOI stacks which are a suitable platform for optical devices. Starting from 3C‑on‑Si samples the silicon substrate was removed by wet chemical etching and the remaining 3C‑SiC layers were bonded to two different low refractive substrates (Al2O3 and polycrystalline SiC with a 3 µm thick SiO2 layer on top deposited by PECVD). We found that also bonding onto Al2O3 was possible, the stability of the resulting stack wasn´t strong enough for further processing. In contrast mechanical stable SiCOI stacks could be realized using the oxide coated polycrystalline SiC as substrate. Besides the substrate materials three different bonding approaches (hydrophilic, hydrophobic and adhesive bonding using an HSQ resist) as well as multiple process parameters were analyzed with regard to the bonding performance. The best results could be achieved using the adhesive bonding approach with a bonding temperature ≥ 400°C, a process time ≥ 4 h and a bonding pressure of 96 N/cm2.

Spherical SiO2 growth on LaFeO3 perovskite to create core–shell structures for Cd(II) adsorption on its surface

In this study, LaFeO3 perovskite is synthesized by the flash method, and then a simple method is developed for growing SiO2 on its surface to construct a core/shell nanocomposite. The thickness of the SiO2 shell is controlled by varying the amount of tetra-ethyl orthosilicate (TEOS). Two LaFeO3@SiO2 samples with varying SiO2 thicknesses were synthesized. Herein, the lattice parameters, morphology, magnetic, and adsorption properties of the investigated core/shell nanocomposites are tuned by changing the content of SiO2. Results confirm that when the content of SiO2 increased, the prepared LaFeO3@SiO2 core/shell nanocomposite showed good adsorption performance, and the removal efficiency enhanced from 50 to 60% for LaFeO3 and LaFeO3@SiO2, respectively. It is thought that the enhancement of the adsorption performance is related to the high porosity and amorphous nature of SiO2.

Saturation Thickness of Stacked SiO2 in Atomic-layer-deposited Al2O3 Gate on 4H-SiC

Abstract High-k materials as an alternative dielectric layer for SiC power devices have the potential to reduce interfacial state defects and improve MOS channel conduction capability. Besides, under identical conditions of gate oxide thickness and gate voltage, the high-k dielectric enables a greater charge accumulation in the channel region, resulting in a larger number of free electrons available for conduction. However, the lower energy band gap of high-k materials leads to significant leakage currents at the interface with SiC, which greatly affects device reliability. By inserting a layer of SiO2 between the high-k material and SiC, the interfacial barrier can be effectively widened and hence the leakage current will be reduced. In this study, the optimal thickness of the intercalated SiO2 was determined by investigating and analyzing the gate dielectric breakdown voltage and interfacial defects of a dielectric stack composed by atomic-layer-deposited Al2O3 layer and thermally nitride SiO2. Current-voltage and high-frequency capacitance-voltage measurements were performed on metal-oxide-semiconductor test structures with 35 nm thick Al2O3 stacked on 1-, 2-, 3-, 6-, or 9 nm thick nitride SiO2. Measurement results indicate that the current conducted through the oxides was affected by the thickness of the nitride oxide and the applied electric field. Finally, a saturation thickness of stacked SiO2 that contributed to dielectric breakdown and interfacial band offsets was identified. The findings in this paper provides guideline for the SiC gate dielectric stack design with the breakdown strength and the interfacial state defects considered.

Numerical Analysis of the Influence of Fabrication Process Uncertainty on Terahertz Metasurface Quality

The purpose of this paper was to investigate the influence of fabrication process uncertainty on terahertz metasurface quality. The focus was on the effect of metasurface fabrication inaccuracy on resonances. To the best of our knowledge, this is the first paper to study the effect of the metasurface fabrication process on its resonant frequency. The terahertz split ring resonator-based metasurface is under consideration. Using a numerical model, the influence of the uncertainty of various geometrical parameters obtained during the fabrication process (mainly layer deposition, photolithography, and etching processes) is analyzed according to the resonance of the designed metasurface. The influence of the following parameters causes a shift of resonant frequencies of the considered metasurface: etching deviation e, metallization thickness tAl and SiO2 layer thickness tSiO2. The quality of the metasurface affected by the variations of obtained geometrical parameters was determined by the deviation of resonant frequency Δfr. The developed numerical model was verified by THz-TDS (terahertz time-domain spectroscopy) measurements of the fabricated structure.

Identification of the monolayer thickness difference in a mechanically exfoliated thick flake of hexagonal boron nitride and graphite for van der Waals heterostructures

Exfoliated flakes of layered materials, such as hexagonal boron nitride (hBN) and graphite with a thickness of several tens of nanometers, are used to construct van der Waals heterostructures. A flake with a desirable thickness, size, and shape is often selected from many exfoliated flakes placed randomly on a substrate using an optical microscope. This study examined the visualization of thick hBN and graphite flakes on SiO2/Si substrates through calculations and experiments. In particular, the study analyzed areas with different atomic layer thicknesses in a flake. For visualization, the SiO2 thickness was optimized based on the calculation. As an experimental result, the area with different thicknesses in a hBN flake showed different brightness in the image obtained using an optical microscope with a narrow band-pass filter. The maximum contrast was 12% with respect to the difference of monolayer thickness. In addition, hBN and graphite flakes were observed by differential interference contrast (DIC) microscopy. In the observation, the area with different thicknesses exhibited different brightnesses and colors. Adjusting the DIC bias had a similar effect to selecting a wavelength using a narrow band-pass filter.

Highly Rectifying Conical Nanopores in Amorphous SiO2 Membranes for Nanofluidic Osmotic Power Generation and Electroosmotic Pumps

Nanopore membranes are a versatile platform for a wide range of applications ranging from medical sensing to filtration and clean energy generation. To attain high-flux rectifying ionic flow, it is required to produce short channels exhibiting asymmetric surface charge distributions. This work reports on a system of track etched conical nanopores in amorphous SiO$_2$ membranes, fabricated using the scalable track etch technique. Pores are fabricated by irradiation of 1 $\mu$m thick SiO$_2$ windows with 2.2 GeV $^{197}$Au ions and subsequent chemical etching. Structural characterisation is performed using atomic force microscopy (AFM), scanning electron microscopy (SEM), small angle X-ray scattering (SAXS), ellipsometry, and surface profiling. Conductometric characterisation of the pore surface is performed using a membrane containing 16 pores, including an in-depth analysis of ionic transport characteristics. The pores have a tip radius of (5.7 $\pm$ 0.1) nm, a half-cone angle of (12.6 $\pm$ 0.1)$^{\circ}$, and a length of (710 $\pm$ 5) nm. The $pK_a$, $pK_b$, and $pI$ are determined to 7.6 $\pm$ 0.1, 1.5 $\pm$ 0.2, and 4.5 $\pm$ 0.1, respectively, enabling the fine-tuning of the surface charge density between +100 and -300 mC $m^{-2}$ and allowing to achieve an ionic current rectification ratio of up to 10. This highly versatile technology addresses challenges that contemporary nanopore systems face, and offers a platform to improve the performance of existing applications.

Plasma-enhanced atomic layer deposited SiO2 enables positive thin film charge and surface recombination velocity of 1.3 cm/s on germanium

The excellent field-effect passivation provided by aluminum oxide (Al2O3) on germanium surfaces relies on the high negative fixed charge present in the film. However, in many applications, a neutral or a positive charge would be preferred. Here, we investigate the surface passivation performance and the charge polarity of plasma-enhanced atomic layer deposited (PEALD) silicon oxide (SiO2) on Ge. The results show that even a 3 nm thick PEALD SiO2 provides a positive charge density (Qtot, ∼2.6 × 1011 cm−2) and a relatively good surface passivation (maximum surface recombination velocity SRVmax ∼16 cm/s). When the SiO2 thin film is capped with an ALD Al2O3 layer, the surface passivation improves further and a low midgap interface defect density (Dit) of ∼1 × 1011 eV−1 cm−2 is achieved. By varying the SiO2 thickness under the Al2O3 capping, it is possible to control the Qtot from virtually neutral (∼2.8 × 1010 cm−2) to moderately positive (∼8.5 × 1011 cm−2) values. Consequently, an excellent SRVmax as low as 1.3 cm/s is obtained using optimized SiO2/Al2O3 layer thicknesses. Finally, the origin of the positive charge as well as the interface defects related to PEALD SiO2 are discussed.

Low temperature electrical transport in thin carbon films deposited on SiO2/Si substrates by pulsed laser deposition

In this paper electrical transport studies are performed on thin carbon films deposited on SiO2/Si substrates by pulsed laser deposition (PLD) applying laser ablation of micro-crystalline graphite target. Experiments were carried out on 320 - 420 nm thick SiO2 on Si substrates as well as on hydrogenated diamond-like carbon (DLC) films deposited on SiO2/Si. Structural studies by means of XPS, SEM and Raman spectroscopy revealed that the films can be characterized as nano-sized carbon phases possessing different phase composition (i.e. the ratio sp3/sp2 hybridized carbon, etc.). The electrical conductivity/resistivity of the films was measured in the temperature range 10 K < T < 300 K. Four-contact Van der Pauw method as well as two contact schemes have been applied. Some films have low room temperature resistivity in the range ρ = (0.1–1.5)×10-3 Ω.·m and consist predominantly of sp2 hybridized carbon with Raman spectra, which resemble that of nano-sized graphene depending on the deposition conditions and substrates used. The thinnest only 0.5 nm layer deposited directly on SiO2 exhibits relatively low specific resistance (~10-3 Ω. m), which can be taken as an indication of good deposition conditions of graphene-like layers. The current flow mechanism was explored at temperatures from 300 K down to 10K. The temperature dependence reveals non-metallic behavior - the conductivity decreases at decreasing temperature as opposed to typical metal behaviour. A model of variable range hopping (VRH) mechanism is applied to explain the low temperature conductivity drawn from transport in nanocrystalline disordered systems.

Electrical characterization of thin silicon-on-insulator films doped by means of phosphorus end-terminated polymers

Ex-situ doping of 30 nm thick silicon-on-insulator (SOI) substrates is performed by using polymers terminated with a doping containing moiety. Poly(methylmetachrylate) polymers with a P containing moiety are used to create a phosphorus δ-layer at the interface between the Si device layer and a 10 nm thick SiO2 capping layer deposited on the SOI sample by conventional e-beam evaporation. Drive-in of the P atoms is performed by annealing the samples in a rapid thermal processing (RTP) system at temperatures (TA) ranging from 900 to 1200 °C in N2 atmosphere. Annealing time is properly selected to inject a constant P dose of ∼ 1 × 1013 cm−2 into the SOI substrate, achieving a uniform dopant concentration throughout the entire Si device layer as verified by Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) measurements. Sample resistivity (ρ), carrier concentration (ne) and mobility (μ) are determined combining sheet resistance and Hall measurements in the van der Pauw configuration. An average activation rate above 95% is observed at room temperature in the samples annealed at 1000 °C for 100 s, suggesting complete activation of the injected dopants. The detection of phosphorus signal by EPR analysis confirms the extremely high effective concentration of dopants. Low temperature (5–300 K) electrical characterization of a 30 nm thick SOI sample (ne ∼ 2.7 × 1018 cm−3 at 300 K) indicates that the evolution of ρ, ne and μ values as a function of temperature are perfectly consistent, within the experimental error, with those reported for a P doped bulk Si.

Optimization of sol-gel prepared SiO2 coating and FeSiCr@SiO2 soft magnetic composites based on critical ammonia concentration

The SiO2 insulating layer was prepared on the FeSiCr powders successfully by one-step dumping sol-gel method and ammonia was used as the catalyst. It is found that the growth of SiO2 layer can be tuned by the ammonia concentration. Within the reaction time of 0.5 h, SiO2 grows with heterogeneous nucleation when the ammonia concentration is 0.08–0.16 ml/g, and SiO2 coating layer was relatively uniform. With a high ammonia concentration of 0.20–0.24 ml/g, SiO2 grows with both heterogeneous and homogeneous nucleation, and SiO2 particles gathered and grew. The saturation magnetization and effective permeability of FeSiCr@SiO2 SMCs gradually decrease with the increase of ammonia concentration, while the specific resistance, quality factor and DC-bias property increase. With the ammonia concentration of 0.12 ml/g, the thickness of SiO2 insulating layer was about 30–60 nm. The annealed FeSiCr@SiO2 soft magnetic composite shows good soft magnetic performance with effective permeability and core loss of 45.52 and 418.32 kW/m3 at 0.05 T and 100 kHz, respectively, and the quality factor of 76.5 at 1 MHz.

Investigation on the thermal stability of reflective and anti-reflective coatings on freestanding GaN

The reflective (RC) and anti-reflective coatings (ARC) on freestanding GaN were fabricated using a stack of TiO2/SiO2 multi-layers, which changes the reflectance at 532 nm from 17.0% to 2.5% and 99.6%, respectively, and proves the effectiveness of RC and ARC. The reflection spectra of annealed RC and ARC both show a blueshift of about 52 nm after high-temperature annealing, because of the phase transition of TiO2 from amorphism to anatase at high temperatures. In order to improve the thermal stability of RC and ARC, we could increase the thickness of TiO2 and SiO2 proportionally during the coating process.

Development of in situ characterization of two-dimensional materials grown on insulator substrates with spectroscopic photoemission and low energy electron microscopy

Ultrathin two-dimensional (2D) materials offer great potential for next-generation integrated circuit and optoelectronic devices. Chemical vapor deposition (CVD)-grown 2D materials provide a way to mass production in industry. However, how to in situ characterize their intrinsic electric/photoelectric properties and carrier dynamics with electron/photoelectron probes is still a problem due to the interference from the conducting substrate. Here, we present a grounding Au grids method to realize in situ characterization of the CVD-grown MoS2 on the insulating thick SiO2 layer covered Si substrate with spectroscopic photoemission and low energy electron microscopy (SPELEEM). Through depositing Au grids afterwards, we have achieved good grounding of MoS2 flakes in the photoemission electron microscopy (PEEM), mirror electron microscopy (MEM), and micro-area low energy electron diffraction (µ-LEED) measurements. We have clarified the false signal caused by stray photoelectrons originated from the Au stripes, and as well as the space charge effects induced by intense photoemission. We have also confirmed that time-resolved PEEM results are not affected by the stray signal, and by adopting a small light spot, both static and time-resolved micro-area photoelectron spectroscopy (µ-PES) can be unaffected by space charge effects. Our results provide a reliable way to in situ investigate 2D materials grown on insulating substrates by probing photoelectrons or backscattered electrons.