Amorphous SiO2 Surface Research Articles

Hybrid Organic–Inorganic Biomaterials as Drug Delivery Systems: A Molecular Dynamics Study of Quercetin Adsorption on Amorphous Silica Surfaces

Many important drugs in pharmaceutical applications are poorly soluble. Solubilization, which is diffusion through biological barriers, and the control of local administration are crucial steps for bioavailability and to avoid cytotoxic effects. Hybrid organic/inorganic biomaterials can incorporate drugs for in situ release after implantation. Molecular Mechanics (MM) and Molecular Dynamics (MD) simulations are useful tools for investigating intermolecular interactions between drug and biomaterial surfaces at the atomistic level for these applications. This work studies quercetin, a flavonoid drug important for its anti-inflammatory, antioxidant, and anticancer properties, and the amorphous SiO2 surface using a simulation protocol proposed in previous work related to ketoprofen drugs. After adsorption on the amorphous silica surface, the adsorption process of quercetin drug molecules at two different drug concentrations near a hydrated and then dried silica surface is investigated. Interestingly, these theoretical results are compared with experimental data obtained via Fourier Transform Infrared Spectroscopy (FT–IR) spectra related to quercetin molecules homogenously entrapped in a silica matrix obtained via the Sol–Gel method. Favorable H– bonds and some π–π interactions among drug molecules are crucial surface interactions for the new generation of biocompatible materials capable of incorporating anti-inflammatory agents for release into the human body.

Amorphous SiO2 Surface Irregularities and their Influence on Liquid Molecule Adsorption by Molecular Dynamics Analysis

As the semiconductor industry relentlessly reduces device sizes, efficient and precise cleaning processes have become increasingly critical to address challenges such as nanostructure stiction. Gaining insight into the molecular behavior of water and isopropyl alcohol (IPA) on silicon dioxide (SiO2) surfaces is essential for controlling semiconductor wet cleaning processes. This study investigated the interactions between these liquids and SiO2 surfaces. Using molecular dynamics (MD) simulations, we examined the adsorption behavior of water and IPA molecules on both amorphous and crystalline SiO2 (a-SiO2 and c-SiO2) surfaces. Our findings reveal a preferential adsorption of water molecules on a-SiO2 surfaces compared to c-SiO2. This preference can be ascribed to the irregularity of the a-SiO2 surface, which results in the presence of silanol groups that remain inaccessible to the liquid molecules. In contrast, the c-SiO2 surface exhibits a more uniform and accessible structure. This study not only imparts crucial insights into the molecular behavior of water and IPA on SiO2 surfaces but also provides valuable information for future enhancements and optimization of semiconductor wet surface preparation, cleaning, etching and drying.

Mechanistic Origin of Selective Methane to Methanol Oxidation on Vanadium-Doped Mesoporous Amorphous Silica Photocatalyst

Selective direct oxidation of methane to methanol is an important reaction for efficient utilization of natural gas. Photocatalytic routes for methane oxidation processes have advantages over traditional thermal catalysis as they can be more easily deployed at small-scale and remote off-grid sites, potentially reducing the rate of methane release and flaring. Hu and co-workers identified single-site vanadium-doped mesoporous amorphous silica photocatalysis which demonstrated methane conversion with high methanol selectivity ( J. Photochem. Photobiol., A 2013, 264, 48−55, DOI: 10.1016/j.jphotochem.2013.05.005). Photocatalysts using amorphous SiO2 frameworks are significantly less studied than other heterogeneous photocatalysts. As a result, computational studies are important for elucidating mechanisms in order to gain fundamental understanding as to how amorphous SiO2-based photocatalysts can be used for partial oxidation of methane. This work uses density functional theory to establish the reaction barriers associated with the photocatalytic methane to methanol reaction combined with microkinetic modeling. The work is able to elucidate the role of terminal versus bridging oxygens at the photocatalytic center in enhancing photocatalytic efficiency. In addition, avoiding non-Mars–van Krevelen reaction pathways, in which the oxygen vacancy is filled before methanol is formed, is found to be important, owing to overstabilization of reaction intermediates leading to subsequent high energy reaction barriers. Finally, the amorphous SiO2 surface is found to modify reaction barriers by stabilizing intermediates through dispersion and preventing the photocatalytic center from forming the optimum coordination geometry.

Quantum dots TiO2 loaded amorphous SiO2 composite photocatalysts: Significant performance enhancement and the effect of SiO2 surface hydroxyl groups

Using amorphous SiO2 (A-SiO2) and tetrabutyl titanate (TBOT) as raw material, a composite photocatalyst with surface loaded quantum dot size TiO2 (A-SiO2/TiO2, AST) was prepared. The degradation of tetracycline hydrochloride as well as mechanism of performance improvement by A-SiO2 were investigated. The TiO2 on the composite was anatase with a content of 9.96 wt% and a size of 7–8 nm (quantum dot level). TiO2 was uniformly distributed and was closely bonded at the interface by chemical bonds. The degradation rate of AST was 95% after 60 min UV exposure, which was comparable to pure nano-TiO2. The particle size of TiO2 in AST was reduced from 20–50 nm to 7–8 nm comparing with pure nano-TiO2 and changed from a highly agglomerated to a well-dispersed state due to abundant hydroxyl groups on A-SiO2 surface. TiO2 bandgap width increased from 3.25 eV to 3.39 eV. These led to the exposing of active sites on TiO2 and significantly increased photogenerated carrier separation efficiency. This study provides a feasible and simple method to improve TiO2 nanoparticles’ photocatalytic activity, reduce the usage and achieve recycling to cut the application cost by surface coated.

Effect of Na+ on the Adsorption Behavior of Polystyrene Nanoparticles onto Coal and Quartz Surfaces

The recovery of difficult-to-float coal using traditional nonpolar hydrocarbon oil collectors can be challenging, particularly for low-rank or oxidized coal. Thus, there is a need for more efficient flotation agents. Nanoparticle flotation collector technology has become increasingly popular in the field of mineral processing, and the presence of various ions in the slurry can significantly affect the interaction between collectors and mineral surfaces. In this study, cationic polystyrene (PS) nanoparticles were prepared using an emulsion polymerization method, and the effects of Na+ ion concentration on the in situ adsorption and desorption processes, adsorption layer configuration, and adsorption kinetics of PS particles on amorphous carbon (coal model) and SiO2 sensors (quartz mineral model) were analyzed using the quartz crystal microbalance with dissipation (QCM-D) technique. Our results showed that the hydrophobic PS nanoparticles irreversibly adsorbed onto both amorphous carbon and SiO2 sensors under different environmental conditions, and their adsorption capacity decreased gradually with increasing Na+ ion concentration. Increasing Na+ ion concentration from 0 M to 1.0 M resulted in a 24.4% and 30.9% decrease in equilibrium adsorption capacities of PS nanoparticles onto amorphous carbon and SiO2 surfaces, respectively. The adsorption rate of PS nanoparticles onto the SiO2 surface was much greater than that on the amorphous carbon surface. The adsorption rate constant of PS nanoparticles onto SiO2 surfaces was 0.782 at 0.1 M Na+ ion concentration, while its adsorption rate constant onto amorphous carbon surfaces was only 0.060. Moreover, the adsorption process was found to be more in line with the quasi-primary kinetic model. These findings suggest that PS nanoparticles may serve as promising flotation collectors for the recovery of difficult-to-float coal, and highlight the importance of considering the effect of dissolved ions on the adsorption properties of flotation collectors.

Sputtered L10-FePd and its Synthetic Antiferromagnet on Si/SiO2 Wafers for Scalable Spintronics.

As a promising alternative to the mainstream CoFeB/MgO system with interfacial perpendicular magnetic anisotropy (PMA), L10-FePd and its synthetic antiferromagnet (SAF) structure with large crystalline PMA can support spintronic devices with sufficient thermal stability at sub-5 nm sizes. However, the compatibility requirement of preparing L10-FePd thin films on Si/SiO2 wafers is still unmet. In this paper, we prepare high-quality L10-FePd and its SAF on Si/SiO2 wafers by coating the amorphous SiO2 surface with an MgO(001) seed layer. The prepared L10-FePd single layer and SAF stack are highly (001)-textured, showing strong PMA, low damping, and sizeable interlayer exchange coupling, respectively. Systematic characterizations, including advanced X-ray diffraction measurement and atomic resolution-scanning transmission electron microscopy, are conducted to explain the outstanding performance of L10-FePd layers. A fully-epitaxial growth that starts from MgO seed layer, induces the (001) texture of L10-FePd, and extends through the SAF spacer is observed. This study makes the vision of scalable spintronics more practical.

Cold sintering of SiC ceramics with the assistance of amorphous SiO2 surface layer

The cold sintering process (CSP) densifies ceramics powder at lower temperatures (<300 °C) than conventional sintering by applying a small amount of solvent such as water and a uniaxial pressure of several hundred megapascals. However, this method can only be applied to materials with high solubility in solvents, and direct hardening of low solubility materials such as SiC is difficult. In this study, hardened SiC/SiO 2 bodies were fabricated by CSP from SiC particles whose surface was oxidized by preheating, using NaOH solution as a solvent. The compressive strength and relative density of the hardened SiC/SiO 2 bodies fabricated from a mixture of SiC particles with 15.3 mass% SiO 2 and 5 mol/L NaOH solution were 520 MPa and 80%, respectively. These values were considerably higher than those of hardened SiC/SiO 2 bodies fabricated by CSP from a mixed powder consisting of untreated SiC particles and amorphous SiO 2 particles. When the surface-oxidized SiC particles were used as a starting material, the mismatch in chemical bonding between SiC and SiO 2 and uneven distribution of the binder were reduced, resulting in high mechanical strength for the hardened SiC/SiO 2 bodies.

Surface Interactions between Ketoprofen and Silica-Based Biomaterials as Drug Delivery System Synthesized via Sol-Gel: A Molecular Dynamics Study.

Biomaterial-based drug delivery systems for a controlled drug release are drawing increasing attention thanks to their possible pharmaceutical and biomedical applications. It is important to control the local administration of drugs, especially when the drug exhibits problems diffusing across biological barriers. Thus, in an appropriate concentration, it would be released in situ, reducing side effects due to interactions with the biological environment after implantation. A theoretical study based on Molecular Mechanics and Molecular Dynamics methods is performed to investigate possible surface interactions between the amorphous SiO2 surface and the ketoprofen molecules, an anti-inflammatory drug, considering the role of drug concentration. These theoretical results are compared with experimental data obtained by analyzing, through Fourier transform infrared spectroscopy (FT-IR), the interaction between the SiO2 amorphous surface and two percentages of the ketoprofen drug entrapped in a silica matrix obtained via the sol–gel method and dried materials. The loaded drug in these amorphous bioactive material forms hydrogen bonds with the silica surface, as found in this theoretical study. The surface interactions are essential to have a new generation of biomaterials not only important for biocompatibility, with specific structural and functional properties, but also able to incorporate anti-inflammatory agents for release into the human body.

Effect of the External Velocity on the Exfoliation Properties of Graphene from Amorphous SiO2 Surface

External action has a significant influence on the formation of high-quality graphene and the adhesion of graphene on the surface of the MEMS/NEMS device. The atomic-scale simulation and calculation can further study the exfoliation process of graphene by external actions. In multilayer graphene systems where graphene layers were simulated weakly contacted with SiO2 substrate, a constant vertical upward velocity (Vup) was applied to the topmost layer. Then two critical velocities were found, and three kinds of distinct exfoliation processes determined by critical upward velocities were observed in multilayer graphene systems. The first critical velocities are in the range of 0.5 Å/ps–3.18 Å/ps, and the second critical velocities are in the range of 9.5 Å/ps–12.1 Å/ps. When the Vup is less than the first critical velocity, all graphene layers will not be exfoliated. When Vup is between the first and second critical Vup, all layers can be exfoliated almost synchronously at last. When Vup is larger than the second critical Vup, the topmost layer can be exfoliated alone, transferring energy to the underlying layers, and the underlying layers are slowly exfoliated. The maximum exfoliation force to exfoliate the topmost layer of graphene is 3200 times larger than that of all graphene layers. Moreover, it is required 149.26 mJ/m2 to get monolayer graphene from multilayers, while peeling off all layers without effort. This study explains the difficulty to get monolayer graphene and why graphene falls off easily during the transfer process.

Impact of Silane Monolayers on the Adsorption of Streptavidin on Silica and Its Subsequent Interactions with Biotin: Molecular Dynamics and Steered Molecular Dynamics Simulations.

Protein adsorption on surfaces is used in analytical tools as an immobilization mean to trap the analyte to be detected. However, protein adsorption can lead to a conformational change in the protein structure, resulting in a loss of bioactivity. Here, we study adsorption of a streptavidin-biotin complex on amorphous SiO2 surfaces functionalized with five different silane self-assembled monolayers by all-atom molecular dynamics simulations. We find that the streptavidin global conformational change, as well as the nature of residues with high mobility, depends on the alkyl chain length and head-group charge of silane molecules. Effects on interactions with biotin are further investigated by steered molecular dynamics (SMD) simulations, which mimics atomic force microscopy (AFM) with the biotin attached on the tip. We show the combined effects of adsorption-induced global conformational changes and of the position of residues with high mobility on the streptavidin-biotin rupture force. By comparing our results to experimental and SMD rupture forces obtained in water, without any surface, we conclude that silane with uncharged and short alkyl chains allows streptavidin immobilization, while keeping biotin interactions better than silanes with long alkyl chains or charged head groups.

Molecular Dynamics Simulations of Hydrophobic Nanoparticle Effects on Gas Hydrate Formation

To investigate the effect of particle-based hydrate inhibitors on hydrate crystallization, molecular dynamics (MD) simulations were performed at the interface of hydrate-forming liquids where 2 nm sized silica nanoparticles are stabilized. The hydrophobic nanoparticles were prepared by functionalization of CH3 groups on the amorphous SiO2 surface, and the three-phase contact angle of the oil (CH4 + decane), the aqueous phase (water + tetrahydrofuran (THF)), and the solid was 108.7°. Although the hydrates were not crystallized on the solid substrate, there was a trend that hydrate nucleation occurred on the side of the aqueous phase in the presence of nanoparticles. Through the four-body structural order parameter profiles, the formation of a low order parameter layer around the nanoparticles was observed, which originates from the reduced diffusivity by water binding. Adjacent to this layer, an elevation of the order parameter appeared in the same region where nucleation occurs because the less ordered layer acts as nucleation seeds. Indeed, a well of low free energy is formed in the presence of nanoparticles, in contrast to the non-nanoparticle system where no significant deviations were observed. Also, the driving force of hydrate nucleation is reduced with the increased nanoparticle injection. The nanoparticles prevented the diffusion of methane into the aqueous phase, resulting in a V-shape graph of induction time and delayed hydrate growth rate. These simulation results on the nanoparticle effect will provide a fundamental basis for improving the performance of particle-based inhibitors.

Apparatus and method for the growth of epitaxial complex oxides on native amorphous SiO2 surface of (001) oriented single crystal silicon

The design, fabrication, and performance of an apparatus for the deposition of complex oxides with highly uniform thicknesses at controllable deposition rates over large area, even on the native amorphous SiO2 layer of (001) oriented single crystal Si, are described. The apparatus makes use of the lateral port of a spherical chamber. The port is maintained at uniform temperature, and it houses a substrate heater. The deposition process is controlled by varying different parameters such as target-to-substrate distance, sputtering power, sputtering gas atmosphere, substrate temperature, and pulsed plasma growth. The system has been tested by growing a series of La0.7Sr0.3MnO3 thin films on Si. The systematic strain relaxation and thus the tunable magnetic properties along with the presence of high-quality surface morphology of the films indicate that the designed system could be used to fabricate different components of oxide electronics-based devices over larger area.

High-dose MeV electron irradiation of Si-SiO2 structures implanted with high doses Si+

The influence was studied of 22-MeV electron irradiation on Si-SiO2 structures implanted with high-fluence Si+ ions. Our earlier works demonstrated that Si redistribution is observed in Si+-ion-implanted Si-SiO2 structures (after MeV electron irradiation) only in the case when ion implantation is carried out with a higher fluence (1016 cm-2). We focused our attention on the interaction of high-dose MeV electron irradiation (6.0×1016 cm-2) with n-Si-SiO2 structures implanted with Si+ ions (fluence 5.4×1016 cm-2 of the same order magnitude). The redistribution of both oxygen and silicon atoms in the implanted Si-SiO2 samples after MeV electron irradiation was studied by Rutherford back-scattering (RBS) spectroscopy in combination with a channeling technique (RBS/C). Our results demonstrated that the redistribution of oxygen and silicon atoms in the implanted samples reaches saturation after these high doses of MeV electron irradiation. The transformation of amorphous SiO2 surface into crystalline Si nanostructures (after MeV electron irradiation) was evidenced by atomic force microscopy (AFM). Silicon nanocrystals are formed on the SiO2 surface after MeV electron irradiation. The shape and number of the Si nanocrystals on the SiO2 surface depend on the MeV electron irradiation, while their size increases with the dose. The mean Si nanocrystals height is 16-20 nm after irradiation with MeV electrons at the dose of 6.0×1016 cm-2.

Hybrid Nanocomposite Films Comprising Dispersed VO2 Nanocrystals: A Scalable Aqueous-Phase Route to Thermochromic Fenestration.

Buildings consume an inordinate amount of energy, accounting for 30-40% of worldwide energy consumption. A major portion of solar radiation is transmitted directly to building interiors through windows, skylights, and glazed doors where the resulting solar heat gain necessitates increased use of air conditioning. Current technologies aimed at addressing this problem suffer from major drawbacks, including a reduction in the transmission of visible light, thereby resulting in increased use of artificial lighting. Since currently used coatings are temperature-invariant in terms of their solar heat gain modulation, they are unable to offset cold-weather heating costs that would otherwise have resulted from solar heat gain. There is considerable interest in the development of plastic fenestration elements that can dynamically modulate solar heat gain based on the external climate and are retrofittable onto existing structures. The metal-insulator transition of VO2 is accompanied by a pronounced modulation of near-infrared transmittance as a function of temperature and can potentially be harnessed for this purpose. Here, we demonstrate that a nanocomposite thin film embedded with well dispersed sub-100-nm diameter VO2 nanocrystals exhibits a combination of high visible light transmittance, effective near-infrared suppression, and onset of NIR modulation at wavelengths <800 nm. In our approach, hydrothermally grown VO2 nanocrystals with <100 nm diameters are dispersed within a methacrylic acid/ethyl acrylate copolymer after either (i) grafting of silanes to constitute an amorphous SiO2 shell or (ii) surface functionalization with perfluorinated silanes and the use of a perfluorooctanesulfonate surfactant. Homogeneous and high optical quality thin films are cast from aqueous dispersions of the pH-sensitive nanocomposites onto glass. An entirely aqueous-phase process for preparation of nanocrystals and their effective dispersion within polymeric nanocomposites allows for realization of scalable and viable plastic fenestration elements.

Influence of temperature on morphological and optical properties of MoS2 layers as grown based on solution processed precursor

A novel liquid phase synthesis of MoS2 thin films has been achieved. Solubility of Molybdenum(V) chloride in appropriate solvent was optimized while having good coating properties essential for thin film formation. Chemical conversion of the deposited Mo-precursor films on silicon/silicon dioxide substrate to MoS2 were obtained by annealing in presence of sulfur. This novel method allows facile upscaling process to large substrates with uniform film thickness down to 4nm, which are of high interest for future low-cost fabrication of electronic devices. UV–Vis, Raman spectroscopy, and X-ray photoelectron spectroscopy measurements confirm the formation of MoS2 films with a stoichiometric chemical composition of Mo/S ~0.47. Furthermore, X-ray diffraction and Transmission electron microscopy measurements revealed formation of polycrystalline films with random grain orientation attributed to the amorphous SiO2 surface of the substrate. Improved crystallinity of deposited films was achieved by increasing the process temperature. Annealing at temperatures above 750°C increased the uniformity of multilayer films, together with the increase of MoS2 grain size to 100nm. This simple wet-chemical synthesis approach allows upscaling, controllable film thickness and is suitable for preparation of other transition metal dichalcogenides thin films for applications in the future electronics.

(Invited) Functionalization of Graphene By CVD Using Transition Metal Carbonyls and Their Characterization

Functionalization of Graphene By CVD Using Transition Metal Carbonyls and Their Characterization K. Vinodgopal (North Carolina Central University), X. You (University of North Carolina, Chapel Hill), E. Laws, S. Sendlinger, N. Sciortino, G. Dai (North Carolina Central University), and J. Atkin (University of North Carolina, Chapel Hill) The goal of carbon based electronics can be realized by creating reproducible and controllable band gaps in the normally “metallic” graphene.1 Functionalization of the graphene surface via the formation of hexahapto(η 6-metal) bonds yield structures that should not show any significant structural distortion or rehybridization as is the case with organic functionalization of graphene. Several approaches are possible to achieve such a “subtle” covalent functionalization. Haddon et. al. have developed a wet-chemistry synthetic approach by refluxing exfoliated graphene with either Cr(CO)6 or (η 6-benzene)Cr(CO)3 in a high boiling solvent under an inert atmosphere.2-4 Our approach to achieve this same objective of functionalizing the graphene surface with zero-valent transition metal carbonyl precursors is to use chemical vapor deposition (CVD) methods. CVD enables us to grow high quality single-layered graphene on copper as well as carry out the subsequent functionalization of the graphene surface. The essential requirement for such a process is that the carbonyl compound must have high vapor pressure which Cr(CO)6 satisfies. In fact, Anpo et. al. have used a simple CVD process to functionalize the organosilica framework of phenylene (-C6H4-)-bridged hybrid mesoporous materials (HMM-ph) with Cr(CO)6 and Mo(CO)6.5 The CO vibration provides a convenient handle to confirm the functionalization. The IR spectral peak for Cr(CO)6 was observed at 1991 and 2020 cm-1 which were assigned to the T1u and Eg vibrational modes, respectively. After functionalization, the IR spectral peaks were slightly red shifted to 1981 cm-1 and a broad peak at 1940-1840 cm-1 corresponding to a1 (1981 cm-1) and e (1911, 1884 cm-1) vibrational mode of physically absorbed C6H6Cr(CO)3 on the amorphous SiO2 surface. Our CVD procedure involves a) preparation of mostly single layered graphene on a copper substrate by CVD and b) subsequent functionalization by vapor deposition of the Cr(CO)6 at 80°C at atmospheric pressure under a flow of Ar/H2 to yield graphene. However, given the difficulties of obtaining IR spectra on the copper substrate, we have focused on Raman imaging using a 473 nm laser and correlated Raman AFM mapping. The correlated Raman-AFM mapping shown below indicates fairly uniform deposition of the chromium carbonyl on the surface and red shifts in the CO vibration consistent with formation of a zero- valent complex on the graphene surface. Additional confirmation of the formation of the hexa-hapto complexes of is provided by Scanning Electron Microscopy with EDX. Figure Caption: A) AFM image of the CVD functionalized graphene-Cr(CO)3 surface on copper; B) correlated Raman map of the same region obtained with 473 nm laser excitation and C) representative Raman spectrum obtained from one of the bright green spots displayed on the correlated Raman map. These bright green spots in Panel B correspond to the region defined by the green spectral limits shown in the spectrum in panel C.

Influence of the Lattice Mismatch on the Atomic Ordering of ZnO Grown by Atomic Layer Deposition onto Single Crystal Surfaces with Variable Mismatch (InP, GaAs, GaN, SiC)

It has previously been reported that epitaxial growth of ZnO can be obtained at low temperatures by atomic layer deposition (ALD) onto a GaN (0001-Ga) surface, corresponding to a ~2.3% compressive lattice mismatch of the deposited ZnO. The question addressed here is the atomic ordering of deposited ZnO as a function of the lattice mismatch between ZnO and several single-crystal seeding surfaces. We have deposited ZnO using ALD onto either the (111) cubic or (0001) hexagonal surfaces of a set of available single-crystal substrates (GaAs, InP, GaN, SiC), for which the lattice mismatch varies over a wide range of values, positive and negative. It is found that deposition onto surfaces with very high extensive lattice mismatch (GaAs, InP) leads to polycrystalline ZnO, similar to the configuration obtained on an amorphous SiO2 surface. In contrast, ZnO ALD deposition onto both 2H-GaN (0001-Ga) and 4H-SiC (0001-Si) surfaces with lower and compressive mismatch leads to epitaxial ordering over the whole substrate temperature range of 180–250 °C.

H2O incorporation in the phosphorene/a-SiO2 interface: a first-principles study

Based on first-principles calculations, we investigate (i) the energetic stability and electronic properties of single-layer phosphorene (SLP) adsorbed on an amorphous SiO2 surface (SLP/a-SiO2), and (ii) the further incorporation of water molecules at the phosphorene/a-SiO2 interface. In (i), we find that the phosphorene sheet binds to a-SiO2 through van der Waals interactions, even in the presence of oxygen vacancies on the surface. The SLP/a-SiO2 system presents a type-I band alignment, with the valence (conduction) band maximum (minimum) of the phosphorene lying within the energy gap of the a-SiO2 substrate. The structure and the surface-potential corrugations promote the formation of electron-rich and electron-poor regions on the phosphorene sheet and at the SLP/a-SiO2 interface. Such charge density puddles are strengthened by the presence of oxygen vacancies in a-SiO2. In (ii), because of the amorphous structure of the surface, we consider a number of plausible geometries for H2O embedded in the SLP/a-SiO2 interface. There is an energetic preference for the formation of hydroxyl (OH) groups on the a-SiO2 surface. Meanwhile, in the presence of oxygenated water or interstitial oxygen in the phosphorene sheet, we observe the formation of metastable OH bonded to the phosphorene, and the formation of energetically stable P–O–Si chemical bonds at the SLP/a-SiO2 interface. Further x-ray absorption spectra simulations are performed, which aim to provide additional structural/electronic information on the oxygen atoms forming hydroxyl groups or P–O–Si chemical bonds at the interface region.

Study on external electric field-induced structural changes in the initial growth of pentacene on amorphous SiO2

External electric field-induced structural changes in the initial growth processes of pentacene on a sylanol-saturated amorphous SiO2 surface have been investigated using molecular dynamics simulations. A series of electric fields with different directions and strengths are applied to the initial lateral-orientated pentacene cluster. After that different molecular orientations are observed, which obviously present the regulatory effects of an electric field on the molecular orientation. When the electric field perpendicular to the molecular plane is applied to the pentacene cluster, the pentacene molecules keep the lying-down conformation regardless of changes in the electric field strength. Contrary to that, under the electric field parallel to the pentacene long axes or short axes with proper strength, the pentacene cluster undergoes a conformational transition from a lying-down conformation to a standing-up one. The results of this work may enrich the information on the electric field-induced orientation controllable fabrication approaches of organic semiconductor thin films.

Temperature-dependent orientation study of the initial growth of pentacene on amorphous SiO2 by molecular dynamics simulations

Temperature-dependent molecular orientations in the initial growth processes of pentacene on amorphous SiO2 surface with different substrate temperatures have been investigated using molecular dynamics simulations. As the substrate temperature ranges from 270K to 600K, there exists a transition behavior for pentacene cluster from the normal-oriented, ordered configuration to the lateral-oriented, disordered one as measured by the decreased average orientation angle and order parameter, showing the significant effect of the substrate temperature on the molecular orientation. The transition behavior is related to the strength relationship between molecule–molecule interactions and molecule–substrate interactions. During the optimal temperature range between 300K and 350K, the pentacene molecules tend to form the normal-oriented, well-ordered cluster driven by the dominant molecule–molecule interactions, which is affected by the substrate temperature in a greater degree than the molecule–substrate interactions. When the temperature is lower than 300K, the ordering of pentacene cluster becomes a little worse. A higher substrate temperature results in the lateral orientation with the weakening of the molecule–molecule interactions. Then the further intensification of molecular thermal motion gradually makes the molecules separate from the cluster or the substrate surface, resulting in the appearance of the undesirable separated configuration.