Amorphous SiO2 Surface Research Articles

Molecular orientation transformation of pentacene on amorphous SiO2: A computational study on the initial growth stage of physical vapor deposition

This study describes the transformation of molecular orientation which has a significant effect on the charge carrier mobility of organic thin film transistors. Theoretical analyses and molecular dynamics simulations are performed to uncover the mechanism of the orientation transformation of pentacene molecules on the amorphous SiO2 surface during the early stage of the physical vapor deposition process, where the initially grown lateral-oriented cluster will transfer to the normal-oriented one at a critical size nc. The reorientation behavior is related to the competition between the molecule-molecule interactions and the molecule-substrate interactions. For small size clusters, the pentacene molecules are found to prefer to form a lateral-oriented cluster as driven by the molecule-substrate interactions, which can be increased by the existence of the sylanol groups on the amorphous SiO2 surface. Furthermore, when the molecular number n is greater than nc, the normal orientation in the herringbone structure is gradually formed and becomes the stable configuration because the molecule-molecule interactions is dominant.

Early Stages of Sn-Doped In2O3 Film Growth on Amorphous SiO2 Surfaces Observed by Atomic Force Microscopy and Transmission Electron Microscopy

The early stages of Sn-doped In2O3 (ITO) film growth on amorphous SiO2 (a-SiO2) surfaces were observed by atomic force microscopy (AFM) and transmission electron microscopy (TEM). AFM measurements clearly showed that the surface morphologies of the ITO films (3–6 nm in thickness) possessed an extended hill-and-valley structure. TEM analysis revealed that small ITO islands on the a-SiO2 surface could aggregate, resulting in the formation of larger ITO islands approximately 15–30 nm in diameter. The combined results indicate that the ITO film growth on a-SiO2 surfaces is similar to that on glass surfaces.

Hydrophobicity of Hydroxylated Amorphous Fused Silica Surfaces

Understanding the mechanism of water adsorption on silica is important in many fields of science and technology, such as geo- and atmospheric chemistry. Vibrational IR-visible sum-frequency generation (SFG) spectroscopy of hydroxyls (~3100-3800 cm(-1)) at the amorphous SiO2 surface in contact with air of varying relative humidity provides information about the adsorption sites and orientation of water molecules. The similar magnitudes of the resonant and nonresonant contributions to the interfacial second-order susceptibility, χ((2)), allow the phases of the various hydroxyls (SiOH; HOH), and thus their orientations with respect to the surface, to be determined. The surface silanols (SiOH) appear to interact weakly with adsorbed water as indicated by the persistence of the narrow surface silanol (SiOH) peak at ~3750 cm(-1) as the relative humidity of ambient air increases from <5% to >95%. Adsorbed water molecules are represented by two oppositely oriented hydroxyl modes, at ~3350-3400 and ~3650 cm(-1), respectively. The weakly hydrogen-bonded water hydroxyls (~3650 cm(-1)) are oriented toward the silica substrate and are assigned to water molecules that aggregate over the hydrophobic silica areas with exposed siloxane bridges. We believe that this is the first experimental identification of water molecules in contact with siloxane network whose hydrophobic nature has been predicted by molecular dynamics simulations for tetrahedral (SiO4) surface of kaolinite. The SFG data suggest that, at the molecular level, hydroxylated amorphous fused silica has hydrophobic character.

Silicon Carbide Oxidation in High‐Pressure Steam

Silicon carbide is a candidate cladding for fission power reactors that can potentially provide better accident tolerance than zirconium alloys. SiC has also been discussed as a host matrix for nuclear fuel. Chemical vapor–deposited silicon carbide specimens were exposed in 0.34–2.07 MPa steam at low gas velocity (~50 cm/min) and temperatures from 1000°C to 1300°C for 2–48 h. As previously observed at lower steam pressure of 0.15 MPa, a two‐layer SiO2 scale was formed during exposure to these conditions, composed of a porous cristobalite layer above a thin, dense amorphous SiO2 surface layer. Growth of both layers depends on temperature, time, and steam pressure. A quantitative kinetics model is presented to describe the SiO2 scale growth, whereby the amorphous layer is formed through a diffusion process and linearly consumed by an amorphous to crystalline phase transition process. Paralinear kinetics of SiC recession were observed after exposure in 0.34 MPa steam at 1200°C within 48 h. High‐pressure steam environments are seen to form very thick (10–100 μm) cristobalite SiO2 layers on CVD SiC even after relatively short‐term exposures (several hours). The crystalline SiO2 layer and SiC recession rate significantly depend on steam pressure. Another model is presented to describe the SiC recession rate in terms of steam pressure when a linear phase transition kl governing the recession kinetics, whereby the reciprocal of recession rate is found to follow a negative unity steam pressure power law.

Ab initio derived force‐field parameters for molecular dynamics simulations of deprotonated amorphous‐SiO2/water interfaces

AbstractWe present a set of Coulomb point charges and van der Waals parameters for molecular dynamics simulations of interfaces between natively deprotonated amorphous SiO2 surfaces and liquid water, to be used in combination with standard biomolecular force fields. We pay particular attention to the extent of negative charge delocalisation in the solid that follows the deprotonation of terminal silanol groups, as revealed by extensive Bader analysis of electronic densities computed by density functional theory (DFT). The absolute charge values in our force field are determined from best‐fitting to the electrostatic potential computed ab initio (ESP charges). Our proposed parameter set is found to reproduce the energy landscape of single water molecules over neutral and deprotonated amorphous SiO2 surfaces and, after a minor adjustment, over thin oxide films on Si. Our analysis reveals a certain degree of arbitrariness in the choice of the DFT scheme used as the reference for the force‐field optimisation procedure, highlighting its intrinsic limits.magnified imageInteraction between a water molecule and an oxidised Si surface calculated with several DFT and force‐field schemes, and delocalisation of the negative charge upon deprotonation of an amorphous SiO2 surface.

Doping of graphene adsorbed on the a-SiO2 surface

We have performed an ab initio theoretical investigation of a graphene sheet adsorbed on amorphous SiO2 surface (G/a-SiO2). We find that graphene adsorbs on the a-SiO2 surface through van der Waals interactions. The inhomogeneous topology of the a-SiO2 clean surface promotes a total charge density displacement on the adsorbed graphene sheet, giving rise to electron-rich as well as hole-rich regions on the graphene. Furthermore, the adsorbed graphene sheet exhibits a net total charge density gain. In this case, the graphene sheet becomes n-type doped, however, no chemical bonds form at the graphene–SiO2 interface. The electronic charge transfer from a-SiO2 to the graphene sheet occurs upon the formation of a partially occupied level lying above the Dirac point. We find that this partially occupied level comes from the three-fold coordinated oxygen atoms in the a-SiO2 substrate.

Critical examination of growth rate for magnesium oxide (MgO) thin films deposited by molecular beam epitaxy with a molecular oxygen flux

The authors report a study of molecular beam deposition of MgO films on amorphous SiO2 and (0001) GaN surfaces over a large range of temperatures (25–400 °C) and molecular oxygen growth pressures (10−7–10−4 Torr). This study provides insight into the growth behavior of an oxide with volatile metal constituents. Unlike other materials containing volatile constituents (e.g., GaAs, PbTiO3), all components of MgO become volatile at normal epitaxial growth temperatures (≥250 °C). Consequently, defining which species is the adsorption controller becomes ambiguous. Different growth regimes are delineated by the critical substrate temperature for Mg re-evaporation and the Mg:O flux ratio. These regimes have impact on phase purity, quartz crystal microbalance calibration, and film microstructure. The universal decay in deposition rate above growth 10−5 Torr O2 is also considered. By introducing a third flux of inert argon gas, rate reduction is attributed to increased molecular scattering and not oxidation of the metal source.

Modelling of electron and hole trapping in oxides

We critically review several examples of successful modelling of electron and hole trapping in metal oxides, which demonstrate a breadth of polaronic behaviour. The examples range from self-trapping in the perfect lattice to trapping by structural defects and impurities and illustrate the important phenomenon of charge localization. We present recent results in four different systems: nanoporous mayenite, amorphous SiO2, crystalline hafnia and MgO surfaces and interfaces. The complex nature of charge trapping and polaronic behaviour in these systems can go beyond traditional cases and illustrate the different challenges involved.

The morphology of amorphous SiO2 surfaces during low energy ion sputtering

The morphology of different amorphous or amorphizedSiO2 surfaces, including thermally grown films, fused silica, and single crystalline quartz, duringlow energy ion sputtering has been investigated by means of atomic force microscopy. Forall three materials, the formation of periodic ripple patterns oriented normal to the direction ofthe ion beam is observed at intermediate incident angles. At near-normal incidence, theSiO2 surfaces remain flat, whereas a rotation of the ripple patterns is observed at grazing incidence.At intermediate angles, the patterns on the different surfaces exhibit wavelength coarseningof different strengths, which can be attributed to different amounts of near-surface masstransport by the surface-confined ion-enhanced viscous flow. In the framework of the recenthydrodynamic model of ion erosion, the observed differences in ripple coarsening are consistentwith this interpretation and indicate that the surface energies of thermally grownSiO2 and amorphized quartz are lower and higher than that of fused silica, respectively.

A 14-ps full width at half maximum high-speed photoconductor fabricated with intersecting InP nanowires on an amorphous surface

We demonstrate a high-speed polarization-insensitive photoconductor based on intersecting InP nanowires synthesized between a pair of hydrogenated silicon electrodes deposited on amorphous SiO2 surfaces prepared on silicon substrates. A 14-ps full width at half maximum de-embedded impulse response is measured, which is the fastest reported response for a photodetector fabricated using nanowires. The high-speed electrical signal measurements from the photoconductor are performed by an integrated coplanar waveguide transmission line. The demonstrated ability to grow intersecting InP nanowires on hydrogenated microcrystalline Si surfaces will facilitate the construction of ultra-fast photodetectors on a wide range of substrates.

Computational Study of the Adsorption of Trichlorophosphate, Dimethyl Methylphosphonate, and Sarin on Amorphous SiO2

Ab initio quantum chemical calculations have been performed to study the adsorption of trichlorophosphate, dimethyl methylphosphonate, and Sarin via hydrogen bonding to Si−OH groups on the amorphous SiO2 (a-SiO2) surface. Two SiO2 models are used: a small Si5O7H8 “cagelike” cluster and a larger Si21O56H28 structure designed to approximate the local environment in a-SiO2. In the latter case, regions of different local OH density are considered as adsorption sites. Adsorption energies, bonding geometries, and adsorbate vibrational modes are obtained, and anharmonicity is explicitly included in the treatment of the SiO−H stretching mode. The computed results for the adsorption-induced shift in frequency of the SiO−H stretch and of the molecular PO stretch are compared with the available experimental data. For all three species, the most stable adsorption geometry involves hydrogen bonding between two Si−OH groups and the O atom of the PO group.

Ethylenediamine at Air/Liquid and Air/Silica Interfaces: Protonation Versus Hydrogen Bonding Investigated by Sum Frequency Generation Spectroscopy

The adsorption process on mineral oxide surfaces is one of the most important factors influencing the migration and distribution of contaminants in the environment. Although there have been numerous studies carried out at the macroscopic scale, there is a lack of molecular-scale interfacial information. The molecular-scale information is often crucial for the determination of sorption mechanisms. In this study, sum frequency generation spectroscopy (SFG), a surface specific vibrational spectroscopy, has been employed to investigate the adsorption of ethylenediamine (H2N(CH2)2NH2, EDA) onto the amorphous SiO2 surface and EDA protonated products at air/liquid interfaces. The SFG spectra of EDA adsorbed on the silica surface and the singly protonated EDA solution are similar in both CH and NH stretching regions. These spectral similarities indicate that EDA molecules are strongly chemisorbed to the silica surface through the protonation of one EDA amine group by surface silanol OH groups, thereby forming a (H2N(CH2)2NH3)+(O-Si[triple bond])- surface complex. The SFG results also indicate that the surface acidity of the silanol OH groups (pKa (HOSi[triple bond])) is between the two pKa values of EDA (in the range of 7.56-10.71) at the air/silica interface.

Nanosized CeO2–SiO2, CeO2–TiO2, and CeO2–ZrO2 Mixed Oxides: Influence of Supporting Oxide on Thermal Stability and Oxygen Storage Properties of Ceria

The influence of SiO2, TiO2, and ZrO2 on the structural and redox properties of CeO2 were systematically investigated by various techniques namely, X-ray diffraction (XRD), Raman spectroscopy (RS), UV–Vis diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HREM), BET surface area, and thermogravimetry methods. The effect of supporting oxides on the crystal modification of ceria was also mainly focused. The investigated oxides were obtained by soft chemical routes with ultrahigh dilute solutions and were subjected to thermal treatments from 773 to 1073 K. The XRD results suggest that the CeO2–SiO2 sample primarily consists of nanocrystalline CeO2 on the amorphous SiO2 surface. Both crystalline CeO2 and TiO2-anatase phases were noted in the case of CeO2–TiO2 sample. Formation of cubic Ce0.75Zr0.25O2 and Ce0.6Zr0.4O2 (at 1073 K) were observed in the case of CeO2–ZrO2 sample. The cell ‘a’ parameter estimations revealed an expansion of the ceria lattice in the case of CeO2–TiO2, while a contraction is noted in the case of CeO2–ZrO2. The DRS studies suggest that the supporting oxides significantly influence the band gap energy of CeO2. Raman measurements disclose the presence of oxygen vacancies, lattice defects, and displacement of oxide ions from their normal lattice positions in the case of CeO2–TiO2 and CeO2–ZrO2 samples. The XPS studies revealed the presence of silica, titania, and zirconia in their highest oxidation states, Si(IV), Ti(IV), and Zr(IV) at the surface of the materials. Cerium is present in both Ce4+ and Ce3+ oxidation states. The HREM results reveal well-dispersed CeO2 nanocrystals over the amorphous SiO2 matrix in the case of CeO2–SiO2, isolated CeO2 and TiO2 (A) nanocrystals and some overlapping regions in the case of CeO2–TiO2, and nanosized CeO2 and Ce–Zr oxides in the case of CeO2–ZrO2 sample. The exact structural features of these crystals as determined by digital diffraction analysis of HREM experimental images reveal that the CeO2 is mainly in cubic fluorite geometry. The oxygen storage capacity (OSC) as determined by thermogravimetry reveals that the OSC of mixed oxides is more than that of pure CeO2 and the CeO2–ZrO2 exhibits highest OSC.

Structural Characterization of Nanosized CeO2—SiO2, CeO2—TiO2, and CeO2—ZrO2 Catalysts by XRD, Raman, and HREM Techniques.

Structural characteristics of nanosized ceria−silica, ceria−titania, and ceria−zirconia mixed oxide catalysts have been investigated using X-ray diffraction (XRD), Raman spectroscopy, BET surface area, thermogravimetry, and high-resolution transmission electron microscopy (HREM). The effect of support oxides on the crystal modification of ceria cubic lattice was mainly focused. The investigated oxides were obtained by soft chemical routes with ultrahighly dilute solutions and were subjected to thermal treatments from 773 to 1073 K. The XRD results suggest that the CeO2−SiO2 sample primarily consists of nanocrystalline CeO2 on the amorphous SiO2 surface. Both crystalline CeO2 and TiO2 anatase phases were noted in the case of CeO2−TiO2 sample. Formation of cubic Ce0.75Zr0.25O2 and Ce0.6Zr0.4O2 (at 1073 K) were observed in the case of the CeO2−ZrO2 sample. Raman measurements disclose the fluorite structure of ceria and the presence of oxygen vacancies/Ce3+. The HREM results reveal well-dispersed CeO2 nanocry...

Adsorption of 4-Picoline and Piperidine to the Hydrated SiO2 Surface: Probing the Surface Acidity with Vibrational Sum Frequency Generation Spectroscopy

Vapor adsorption is an important process influencing the migration and the fate of many organic pollutants in the environment. In this study, vibrational sum frequency generation (SFG) spectroscopy was used to study the adsorption of two surface acidity probe molecules, 4-picoline (pKa = 5.94) and piperidine (pKa = 11.24), onto the amorphous SiO2 surface. The adsorption of 4-picoline onto the silica surface occurs by forming weak hydrogen bonds between the nitrogen atoms of 4-picoline molecules and the hydrogen atoms of surface silanol OH groups. Piperidine molecules are strongly chemisorbed onto the SiO2 surface through the protonation of piperidine molecules by surface silanol OH groups. The SFG results indicate that the surface acidity constant of silanol OH groups (pKa-(HOSi triple bond)) is in the range of 5.94-11.24 at the air/solid interface. Although this range of surface acidity constants is quite wide, it is possible to narrow it by choosing probe molecules with a smaller pKa range. Together with theoretical prediction methods, adsorption studies using vibrational SFG spectroscopy are capable of quantifying the surface acidity of mineral oxides by carefully choosing the acidity probe molecules.

First-principles molecular-dynamics investigation of the hydration mechanisms of the (0001) α-quartz surface

Using first-principles molecular dynamics, we investigate the interactions of water with the (0001) α-quartz surface. Both the cleaved (unreconstructed) surface, which presents non-bridging oxygen atoms, and the dense surface, which is characterized by 3-membered and 6 membered-rings with siloxane bonds at the top, are considered. The cleaved surface is found to be very hydrophilic. When a water molecule arrives at the surface, its oxygen atom bonds to an under-coordinated Si atom. One of its hydrogen atoms is then transferred to a neighboring non-bridging oxygen through the formation of a hydrogen bond. When the surface is already partially hydrated, the formation of hydrogen-bond chains is observed between the water molecule and adjacent silanol groups. By contrast, the dense surface is shown to be hydrophobic. Our calculations provide qualitative information on the possible reaction paths for the dehydration mechanisms, and more generally on the interactions of water with amorphous SiO2 surfaces.

Effects of charge transfer on a-SiO2 surface structure: A molecular dynamics study

The effects of charge transfer between atoms on an amorphous SiO2 surface were studied by comparing a model with fixed charge (FQ) and a model that takes into consideration charge transfer by the charge equilibration (QEq) method. The QEq surface has more oxygen atoms and denser structures compared to the FQ sample. We found, on the surface of the QEq sample, three-coordinated Si with an O–Si–O bond angle in the range of 109–115°. This structure is similar to that of the experimentally observed E′ center (≡Si⋅). We also found a structure similar to the nonbridging oxygen hole center (≡Si–O⋅), which has been observed experimentally on a silica glass surface. These structures were not observed in the FQ model. The results suggest that consideration of the charge transfer is essential to reproduce the defect structure of the silica glass surface.

Highly (111)-oriented and conformal iridium films by liquid source metalorganic chemical vapor deposition

Highly (111)-oriented and conformal iridium (Ir) films were deposited by a liquid source metalorganic-chemical-vapor-deposition process on various substrates. An oxygen-assisted pyrolysis of (methylcyclopentadienyl) (1,5-cyclooctadiene) Ir precursor at a wide range of substrate temperatures (Tsub) between 300 and 700 °C was used. At a low Tsub of 350 °C, the randomly oriented polycrystalline films exhibited an I111/I200 x-ray intensity ratio of 6. However, the films deposited at Tsub = 700 °C on native SiO2 and amorphous SiO2 surfaces were highly oriented with the I111/I200 ratios of 277 and 186, respectively. The transmission electron microscopy study revealed continuous, dense, and faceted microstructures of Ir films. Also, the step coverage of Ir on TiN (64%) was higher than that on amorphous SiO2 (50%) surfaces.

Effect of Silica Additive on the Anatase‐to‐Rutile Phase Transition

The effect of SiO2 addition on the anatase‐to‐rutile phase transition was investigated by DTA, XRD, FTIR, and XPS. TiO2 xerogels containing SiO2 up to 20 mol% were prepared by mixing and hydrolyzing titanium tetraisopropoxide (TTIP) and tetraethylorthosilicate (TEOS) with HNO3 as a catalyst. With increased amounts of SiO2 in the xerogels, the following results were obtained: (1) the crystallization temperature of anatase increased from 415°C in pure TiO2 to 609°C in 20‐mol%‐SiO2‐containing xerogel in the DTA curves; (2) the formation temperature of rutile, according to quantitative XRD analysis, increased with increased SiO2 content up to 5 mol% SiO2 but became constant at higher SiO2 contents; (3) the crystallinity of anatase became lower; and (4) the lattice parameter a of the anatase decreased slightly, but the parameter c decreased greatly up to 20 mol% SiO2. Although the added silicon atoms were considered from these results to be incorporated into the amorphous TiO2 and anatase structures, the 29Si MAS NMR spectra of the xerogels containing 10 mol% SiO2 showed only tetrahedral silicon, with no indication of silicon in octahedral coordination. When calcined at higher temperatures, the xerogel showed polymerization of the SiO4 tetrahedra in the NMR spectra and the Si–O–Si vibration in the FTIR spectra. The chemical composition of the xerogel surfaces, measured using XPS, showed increased SiO2 content with increased calcining temperature, indicating the expulsion of silicon from inside the particles to form an amorphous SiO2 surface layer. The formation of this amorphous SiO2 surface layer was considered to be important in retarding the anatase‐to‐rutile phase transition by suppressing diffusion between anatase particles in direct contact and limiting their ability to act as surface nucleation sites for rutile. These effects of silicon additions were similar to those observed in the γ‐Al2O3‐ to‐α‐Al2O3 transition.

Hydrolysis of the amorphous silica surface. II. Calculation of activation barriers and mechanisms

Using a previously derived model of the dry, amorphous, hydrophilic SiO2 surface, the reactivity of generic defect sites on the surface with respect to water, and the local network rearrangement that accompanies hydrolysis at these sites, is investigated using cluster models. Ab initio methods are used to calculate reaction barriers and reaction pathways. Consequences of the various types of hydrolysis product found are discussed with reference to potential sites for polymer chemisorption on the hydrolyzed, amorphous SiO2 surface.