Bare SiO2 Research Articles

Controlled Growth of Large-Area Uniform Multilayer Hexagonal Boron Nitride as an Effective 2D Substrate.

Multilayer hexagonal boron nitride (h-BN) is an ideal insulator for two-dimensional (2D) materials, such as graphene and transition metal dichalcogenides, because h-BN screens out influences from surroundings, allowing one to observe intrinsic physical properties of the 2D materials. However, the synthesis of large and uniform multilayer h-BN is still very challenging because it is difficult to control the segregation process of B and N atoms from metal catalysts during chemical vapor deposition (CVD) growth. Here, we demonstrate CVD growth of multilayer h-BN with high uniformity by using the Ni-Fe alloy film and borazine (B3H6N3) as catalyst and precursor, respectively. Combining Ni and Fe metals tunes the solubilities of B and N atoms and, at the same time, allows one to engineer the metal crystallinity, which stimulates the uniform segregation of multilayer h-BN. Furthermore, we demonstrate that triangular WS2 grains grown on the h-BN show photoluminescence stronger than that grown on a bare SiO2 substrate. The PL line width of WS2/h-BN (the minimum and mean widths are 24 and 43 meV, respectively) is much narrower than those of WS2/SiO2 (44 and 67 meV), indicating the effectiveness of our CVD-grown multilayer h-BN as an insulating layer. Large-area, multilayer h-BN realized in this work will provide an excellent platform for developing practical applications of 2D materials.

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

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

Hydrophobic modification of SiO2 surface by aminosilane derivatives

New aminosilanes with hydrolysable Si-N bond(s) were prepared by the amination of chlorosilanes and used as hydrophobic modifiers of SiO2 surface. The aminosilanes are rather stable towards hydrolysis in air compared to their chlorosilane analogs. The modified SiO2 was applied as a gate insulator of an organic thin-film transistor with a vapor-deposited film of pentacene as the active material. The transistor with the aminosilane-modified SiO2 showed two- to threefold higher hole mobility than the device with bare SiO2.

Surface enhanced Raman scattering due to interstitial gold nanoparticles into SiO2 spheres array

Films of SiO2 spheres in a periodic array infiltrated with Au nanoparticles (NPs), were obtained by the co-assembly method. Samples with low (M1), medium (M2) and high (M3) concentrations of Au NPs were characterized by SEM, TEM and UV-Vis and Raman Spectroscopies. The images from the microscope show that the average size is 275 nm and 22 nm for SiO2 sphere and Au NP, respectively. Also, with the infiltration procedure the NPs form clusters and reside in the interstitial sites of the FCC array without modifying significantly the distance between the centers of the SiO2 spheres. However, the photonic band gap is shifted to larger wavelengths compared to the bare SiO2 Film. These samples are proposed as SERS substrates, and to evidence their sensitivity, Methylene Blue was used as the molecular probe at different concentrations. An Enhancement Factor, EF, of ∼105 is reached when the M3 film is used as SERS substrate. To have an insight about the Au NPs concentration effect in the SERS EF, discrete dipole approximation was employed to calculate the near electric field intensity of a system of SiO2 spheres and Au NPs, observing that the near field is more intense in the region between the SiO2 sphere surface and the Au NPs close to it.

Polydimethylsiloxane thin-film coating on silica nanoparticles and its influence on the properties of SiO2–polyethylene composite materials

The thin films of polydimethylsiloxane (PDMS) were deposited on SiO2 nanoparticles using a thermal vapor deposition (TVD) method. Highly conformal thin films with thicknesses less than 5 nm were formed on the surface of SiO2, and the PDMS-coated particles exhibited strong hydrophobicity. We used bare and PDMS-coated SiO2 as additives to a polyethylene (PE) matrix to fabricate inorganic–organic composite materials. The mixtures were blended and injection molded. Our findings suggest that the PDMS coating on SiO2 reduces the formation of large hydroclusters of SiO2 in the PE matrix under high shear stress. Considering that the formation of the hydroclusters of additive particles causes problems during the fabrication processes of injection and extrusion, we suggest that coating these particles with PDMS via TVD can facilitate injection and extrusion processing of inorganic–organic composite materials.

Ambient condition bias stress stability of vanadium (IV) oxide phthalocyanine based p-channel organic field-effect transistors

High bias-stress stability and low threshold voltage (Vth) shift under ambient conditions are highly desirable for practical applications of organic field-effect transistors (OFETs). We demonstrate here a 20-fold enhancement in the bias-stress stability for hexamethyledisilazane (HMDS) treated vanadium (IV) oxide phthalocyanine (VOPc) based OFETs as compared to the bare VOPc case under ambient conditions. VOPc based OFETs were fabricated on bare (non treated) SiO2 and a HMDS monolayer passivated SiO2 layer, with an operating voltage of 40 V. The devices with top contact gold (Au) electrodes exhibit excellent p-channel behavior with a moderate hole mobility for the HMDS-treated device. It is demonstrated that the time dependent ON-current decay and Vth shift can be effectively controlled by using self-assembled monolayers of HMDS on the VOPc layer. For the HMDS-treated case, the bias stress stability study shows the stretched exponential decay of drain current by only ~15% during the long-term operation with constant bias voltage under ambient conditions, while it shows a large decay of >70% for the nontreated devices operated for 1000 s. The corresponding characteric decay time constant (τ) is 104 s for the HMDS treated case, while that of the the non-treated SiO2 case is only ~480 s under ambient conditions. The inferior performance of the device with bare SiO2 is traced to the charge trapping at the voids in the inter-grain region of the films, while it is almost negligible for the HMDS-treated case, as confirmed from the AFM and XRD analyses. It is believed that HMDS treatment provides an excellent interface with a low density of traps and passivates the dangling bonds, which improve the charge transport characteristics. Also, the surface morphology of the VOPc film clearly influences the device performance. Thus, the HMDS treatment provides a very attractive approach for attaining long-term air stability and a low Vth shift for the VOPc based OFET devices.

Hydrothermally generated and highly efficient sunlight responsive SiO2 and TiO2 capped Ag2S nanocomposites for photocatalytic degradation of organic dyes

Here, we report the synthesis of bare Ag2S nanoparticles (NPs), SiO2 and TiO2 capped Ag2S nanocomposites (NCs) by a simple one-pot hydrothermal method. The Ag2S NCs, prepared by hydrothermal treatment, could be maintained and uniformly distributed on the surface of SiO2 and TiO2 precursors. The physicochemical properties of the prepared photocatalysts were characterized by XRD, SEM, TEM, TGA, FTIR, zeta potential, PL, and UV–vis DRS. The synthesized Ag2S NPs, SiO2 and TiO2 capped Ag2S NCs were showed strong spectral absorption capabilities in the visible regions. The diameter of prepared Ag2S NPs, SiO2 and TiO2 capped Ag2S NCs are observed to be in the range of 10–30nm. Consequently, they exhibited the prominent photocatalytic behavior of the prepared Ag2S samples for the degradation of Eosin Y (EY) and Safranin O (SO) dyes under sunlight irradiation. It shows that TiO2 capped Ag2S NCs has exhibited an excellent photocatalytic activity in the degradation of EY (96.41%) and SO (91.38%) dyes than the bare Ag2S NPs and SiO2 capped Ag2S NCs. The extent of degradation was confirmed performing TOC analysis, and over 85% TOC removal was achieved. The photocatalytic degradation of EY and SO dyes was followed pseudo-first order kinetics. The prepared composite materials present properties which are highly promising for environmental clean-up applications.

Study of pool boiling of distilled water on SiO2 nanoparticle-coated wire

Experimental results of pool boiling of distilled water on bare nichrome wire and SiO2 nanoparticle-coated wire are presented. Nano-coated wires were obtained by first boiling them in SiO2-water nanofluid. The nanofluid was prepared based on distilled water and SiO2 nanoparticles. The average size of the nanoparticles was 100 nm. The volume concentration of particles was equal 2%. Boiling curves of distilled water were obtained on bare nichrome wire and SiO2 nanoparticles coated wire. The coating increased the critical heat flux during boiling of water.

Highly Sensitive Graphene–Semiconducting Polymer Hybrid Photodetectors with Millisecond Response Time

Graphene–semiconducting light absorber hybrid photodetectors have attracted increasing attention because of their ultrahigh photoconductive gain and superior sensitivity. However, most graphene-based hybrid photodetectors reported previously have shown a relatively long response time (on the order of seconds) caused by numerous long-lived traps in these hybrid systems, which greatly restricts device speed. In this work, graphene–thieno[3,4-b]thiophene/benzodithiophene polymer hybrid photodetectors fabricated on self-assembled-monolayer (SAM)-functionalized SiO2 substrates are demonstrated with a maximum responsivity of ∼1.8 × 105 A W–1 and a relatively short photocurrent response time of ∼7.8 ms. The fast and highly sensitive device characteristics provide great potential in low-light imaging applications. The hybrid photodetector on the SAM-coated SiO2 substrate shows better performance in responsivities and response times as compared with those of the device on the bare SiO2 substrate. The improved resp...

Cactus-like double-shell structured SiO 2 @TiO 2 microspheres: Fabrication, electrorheological performances and microwave absorption

The cactus-like double-shell structured SiO2@TiO2 microspheres (CDS SiO2@TiO2 microspheres) with high dielectric properties were successfully fabricated and utilized as high-performance electrorheological (ER) fluids and microwave absorbing materials. The SiO2 nanospheres with average diameters of 190 nm were synthesized by the hydrolysis of tetraethyl orthosilicate (TEOS), and used as the core for the first coating layer of TiO2 achieved by sol-gel method. The final cactus-like TiO2 layer was prepared via a hydrothermal method. The morphology of the resultant samples was characterized via transmission electron microscopy (TEM), scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM). This novel CDS SiO2@TiO2 microspheres based suspension exhibited improved ER properties compared to conventional bare SiO2 spheres.

Chemical effects of organo-silanized SiO2 nanofillers on epoxy adhesives

Although thermal and mechanical effects of organo-silanized SiO2 nanofillers (NFs) incorporated into epoxy resins have been actively studied by many researchers, their chemical properties have rarely been reported. In this work, we investigated the chemical effects of organo-silanized SiO2 NFs on epoxy adhesives with (3-glycidoxypropyl) methyldiethoxysilane (GPTMS) and (3-aminopropyl) trimethoxysilane (APTMS)-functionalized SiO2 NFs. After curing epoxy-functionalized NF (EPOXY-NF) with triethylenetetramine (TETA) hardener, we were able to verify chemical crosslinking of the TETA hardener on EPOXY-NF with X-ray photoelectron spectroscopy, vacuum Fourier-transform infrared spectroscopy, and zeta potential analysis. Similarly, we found that after curing amine-functionalized NF (NH2-NF) with bisphenol A diglycidyl ether (DGEBA) epoxy resin, the resin was chemically crosslinked on NH2-NF as well. Finally, when we evaluated adhesion properties of adhesives composed of DGEBA resin/NH2-NF and TETA hardener/EPOXY-NF on the basis of ASTM D3163, we observed improvements in the shear strength of 79 and 49%, respectively, in comparison with bare SiO2 NF. This suggests that the chemical effects of organo-silanized SiO2 NFs indeed contribute to the adhesion properties of adhesives.

Ultralow friction of ink-jet printed graphene flakes.

We report the frictional response of few-layer graphene (FLG) flakes obtained by the liquid phase exfoliation (LPE) of pristine graphite. To this end, we inkjet print FLG on bare and hexamethyldisilazane-terminated SiO2 substrates, producing micrometric patterns with nanoscopic roughness that are investigated by atomic force microscopy. Normal force spectroscopy and atomically-resolved morphologies indicate reduced surface contamination by solvents after a vacuum annealing process. Notably, the printed FLG flakes show ultralow friction comparable to that of micromechanically exfoliated graphene flakes. Lubricity is retained on flakes with a lateral size of a few tens of nanometres, and with a thickness as small as ∼2 nm, confirming the high crystalline quality and low defects density in the FLG basal plane. Surface exposed step edges exhibit the highest friction values, representing the preferential sites for the origin of the secondary dissipative processes related to edge straining, wear or lateral displacement of the flakes. Our work demonstrates that LPE enables fundamental studies on graphene friction to the single-flake level. The capability to deliver ultralow-friction-graphene over technologically relevant substrates, using a scalable production route and a high-throughput, large-area printing technique, may also open up new opportunities in the lubrication of micro- and nano-electromechanical systems.

Polysaccharide coating on environmental collectors affects the affinity and deposition of nanoparticles

The mobility and fate of engineered nanoparticles in aquatic environments drive the exposure of biota. This is affected by the naturally suspended particulate matter, porous media, or mineral substrate that constitutes the different environmental collectors to which nanoparticles may attach. Moreover, the bacterial biofilm colonization on these surfaces may act as a natural substrate to retain or repel the nanoparticles. Little is known of the effect of such organic coatings of the collector on the fate of nanoparticles.The objective of this work was to study the deposition of TiO2 nanoparticles on a model collector by exposing a bare SiO2 surface or a coating with bacterial polysaccharide to understand and compare the respective deposition mechanisms. The nanoparticle deposition was studied under favourable or unfavourable conditions, which were obtained by changing the electrostatic interactions between the nanoparticles and the collector from attractive to repulsive forces. The electrostatic interactions were tuned by modifying the nanoparticle coating (bare or PAA coatings), examining the pH vs. pHIEP, and varying the salt concentration. Atomic Force Microscopy (AFM) was used to measure the deposition after dipping the substrate in a nanoparticle suspension, and Quartz Crystal Microbalance with Dissipation monitoring (QCM-D) was conducted in flow mode to observe the deposition kinetics.Our results show that the physicochemical conditions strongly influence the mode of nanoparticle deposition. Under attractive interactions, the polysaccharide coating tends to decrease the deposition compared to the bare mineral substrate, due to lower surface charge density. In both cases, electrostatic repulsions between neighbouring nanoparticles were partially screened by the substrate with opposite charge during deposition. Along with a moderate salt concentration, this effect tended to favour high deposition density, although the nanoparticles experience dominant interparticle repulsion when they are suspended in the same electrolyte. The deposition rate was also dependent on the substrate capacity, and the rate displayed regimes that were either limited by the sticking reaction or by only transport, which are related to the ionic strength and the substrate type, respectively. Under repulsive electrostatic interactions between the nanoparticles and the substrate, while no deposition occurred on the silica substrate, a limited and partially reversible deposition occurred on the polysaccharide one, evidencing the existence of weak bonds with the nanoparticles.

Mechanism of rubrene thin film growth using α-quaterthiophene inducing layer at low temperature

Growth of rubrene thin films at room temperature on bare SiO2 substrates and on top of a α-quaterthiophene (α-4T) inducing layer on SiO2 substrates was investigated. The rubrene thin films and α-4T inducing layers were prepared by vacuum deposition method and were characterized by atomic force microscopy and X-ray diffraction. Rubrene thin films directly deposited on SiO2 substrates had a number of holes on their surface, while the ones grown on top of a α-4T inducing layer on SiO2 substrates exhibited highly ordered morphology and structure. It was found that large-area and ordered rubrene thin films could properly match with the α-4T inducing layers in the prepared heterostructures. Two different rubrene film growth mechanisms were observed from results obtained for films with different thicknesses. We conclude that rubrene thin films prepared at low temperature using α-4T inducing layer on SiO2 substrates hold a great promise of application in large-area and flexible displays.

Modification of gate dielectric on the performance of copper (II) phthalocyanine based on organic field effect transistors

Top contact Copper (II) Phthalocyanine (CuPc) based on Organic Field Effect Transistor (OFET) using various dielectrics were fabricated. The bare SiO2 and bilayer Polyvinyl alcohol (PVA)/SiO2 gate insulators with different thickness (50 and 100nm) onto thin SiO2 layers were used as gate dielectrics. It is found that using bilayer gate dielectrics can improve device performance. The PVA/SiO2 bilayer gate insulator architecture increases the field-effect mobility by 20 times and improves the on/off ratio from 103–104. Also the photosensitivity of device was examined under different light intensities. The photosensitivity CuPc-OFET was found as 10 at a light intensity of 100mW/cm2 at the off state. This proposes that the CuPc-OFET behaves as a phototransistor.

Detection of pH and Enzyme-Free H2O2 Sensing Mechanism by Using GdO x Membrane in Electrolyte-Insulator-Semiconductor Structure.

A 15-nm-thick GdOx membrane in an electrolyte-insulator-semiconductor (EIS) structure shows a higher pH sensitivity of 54.2 mV/pH and enzyme-free hydrogen peroxide (H2O2) detection than those of the bare SiO2 and 3-nm-thick GdOx membranes for the first time. Polycrystalline grain and higher Gd content of the thicker GdOx films are confirmed by transmission electron microscopy (TEM) and X-ray photo-electron spectroscopy (XPS), respectively. In a thicker GdOx membrane, polycrystalline grain has lower energy gap and Gd2+ oxidation states lead to change Gd3+ states in the presence of H2O2, which are confirmed by electron energy loss spectroscopy (EELS). The oxidation/reduction (redox) properties of thicker GdOx membrane with higher Gd content are responsible for detecting H2O2 whereas both bare SiO2 and thinner GdOx membranes do not show sensing. A low detection limit of 1 μM is obtained due to strong catalytic activity of Gd. The reference voltage shift increases with increase of the H2O2 concentration from 1 to 200 μM owing to more generation of Gd3+ ions, and the H2O2 sensing mechanism has been explained as well.

Enhanced mobility in organic field-effect transistors due to semiconductor/dielectric interface control and very thin single crystal

A perfect organic crystal while keeping high quality semiconductor/dielectric interface with minimal defects and disorder is crucial for the realization of high performance organic single crystal field-effect transistors (OSCFETs). However, in most reported OSCFET devices, the crystal transfer processes is extensively used. Therefore, the semiconductor/dielectric interface is inevitably damaged. Carrier traps and scattering centers are brought into the conduction channel, so that the intrinsic high mobility of OSCFET devices is entirely disguised. Here, very thin pentacene single crystal is grown directly on bare SiO2 by developing a ‘seed-controlled’ pentacene single crystal method. The interface quality is controlled by an in situ fabrication of OSCFETs. The interface is kept intact without any transfer process. Furthermore, we quantitatively analyze the influence of crystal thickness on device performance. With a pristine interface and very thin crystal, we have achieved the highest mobility: 5.7 cm2 V−1 s−1—more than twice the highest ever reported pentacene OSCFET mobility on bare SiO2. This study may provide a universal route for the use of small organic molecules to achieve high performance in lamellar single crystal field-effect devices.

Pickering emulsion polymerization kinetics of styrene: Comparison of bare and surface modified SiO2 nanoparticles

ABSTRACTA step towards the understanding of some mechanistic events occurring in the styrene Pickering emulsions, using a SiO2 dispersion, is presented. Polymerizations at 80°C with different levels of a water soluble initiator were performed. The emulsion polymer content was ca. 15% with conversions close to 90%. With conversion and particle size measurements, the particle density was estimated for bare and surface modified SiO2 particles. Then, the average number of radicals per particle was inferred, yielding a pseudo-bulk type polymerization. It was found that bare SiO2 nanoparticles do not participate in the nucleation mechanism; however, they, along with the initiator, promote an enhanced oligomer coagulation. On the other hand, the hexadecyltrimethylammonium bromide modified SiO2 nanoparticles do participate in the nucleation and coagulation mechanisms, yielding more stable and smaller poorly-covered polymer particles. This approach allowed untangling some events such as: particle nucleation, radical entry to particles, particle density, coagulation and vitreous and Trommsdorff effects.

A facile method for fabrication of highly integrated organic field-effect transistors on photoresist-unwettable insulators with remarkable stability

Environmental stability is one of the most important parameters for high-performance organic field-effect transistors (OFETs). Hydrophobic insulators usually possess much better air stability than some conventional inorganic and hydrophilic organic insulators. However, fabrication of devices with high integration by photolithography method cannot be directly performed on hydrophobic insulators due to their photoresist-unwettable properties. In this work, a simple yet efficient metal-assisted photolithography method is developed to achieve large-scale fabrication of highly integrated organic electronic devices on photoresist-unwettable insulators. By using copper (Cu) as sacrificial layer, photolithography can be performed on these insulators with nearly the same resolution, uniformity, and reproducibility as the conventional photolithography. This method shows excellent flexibility and is capable of fabricating high-integrated devices on a variety of hydrophobic insulators including hydrophobic amorphous fluoropolymer (CYTOP), poly(dimethylsiloxane) (PDMS), and octadecyltrichlorosilane (OTS)-modified SiO2. OFETs based on 6,13-dichloropentacene (DCP) microwires (MWs) with CYTOP as the insulator layer were fabricated, which exhibited excellent device performance. Much improved device stability with very low mobility degradation (less than 9%) was observed after 24 days. While devices fabricated on hydrophilic insulators of poly-(vinyl phenol) (PVP) and bare SiO2 experienced dramatic performances decay within 24 days. Furthermore, by using metal-assisted photolithography method, flexible OFETs arrays could be further fabricated on polyethylene naphthalate (PEN) substrate, which showed excellent mechanical bending flexibility and stability. This work unveils the great potential of metal-assisted photolithography method for long-term stable high-integration organic electronic devices.

Protecting the properties of monolayer MoS₂ on silicon based substrates with an atomically thin buffer.

Semiconducting 2D materials, like transition metal dichalcogenides (TMDs), have gained much attention for their potential in opto-electronic devices, valleytronic schemes, and semi-conducting to metallic phase engineering. However, like graphene and other atomically thin materials, they lose key properties when placed on a substrate like silicon, including quenching of photoluminescence, distorted crystalline structure, and rough surface morphology. The ability to protect these properties of monolayer TMDs, such as molybdenum disulfide (MoS2), on standard Si-based substrates, will enable their use in opto-electronic devices and scientific investigations. Here we show that an atomically thin buffer layer of hexagonal-boron nitride (hBN) protects the range of key opto-electronic, structural, and morphological properties of monolayer MoS2 on Si-based substrates. The hBN buffer restores sharp diffraction patterns, improves monolayer flatness by nearly two-orders of magnitude, and causes over an order of magnitude enhancement in photoluminescence, compared to bare Si and SiO2 substrates. Our demonstration provides a way of integrating MoS2 and other 2D monolayers onto standard Si-substrates, thus furthering their technological applications and scientific investigations.