Reliable prediction of the band gap properties of graphene buffer layer on SiC using meta-GGA approximation

On the electronic properties of defective graphene buffer layer on 6H–SiC(0001)

Understanding the role of defects in the magnetic properties of the graphene buffer layer (BL) grown on substrates should be important to provide hints for manufacturing future graphene-based spintronic devices in a controlled fashion. Herein, density functional theory was applied to assess the structure and magnetic properties of defective BL on 6H-SiC(0001). Particularly, we conducted a thorough study of one and two vacancies and Stone-Wales defects in the BL. Our results reveal that the removal of a carbon atom in the BL framework that was originally bonded to a Si atom in the substrate is preferred over that of a sp2-bonded atom. As a result, a hexacoordinated silicon atom is formed with a slightly deviated octahedral geometry. A stable antiferromagnetic (AF) state was verified for the single vacancy system, with a quite different spin-density distribution to the one obtained for the perfect BL. Also, this AF state is nearly degenerate with the non-magnetic and low magnetic states. As for the Stone-Wales defect, the AF sate is almost degenerate with the most stable M = 2 μB magnetic configuration. However, the introduction of two vacancies in the carbon network of BL causes the loss of magnetism of the BL-SiC system. Our theoretical calculations support experimental predictions favoring the BL as the site for single vacancy formation rather than the epitaxial monolayer graphene, by 4.3 eV.

We investigate the interfacial electronic structure of monolayer iron tetraphenylporphyrin chloride (FeTPP-Cl) adsorbed on graphene (Gr) buffer layers supported by Ni(111) and Pt(111). This study unveils the role of a...

The buffer layer is an important structure of cable, and its performances have a notable impact on the reliability of cable. In recent years, some problems especially faults in buffer layer have occurred frequently. However, the failure mechanism of buffer layer is unclear, which makes it difficult to prevent this accident. In this paper, the electrical properties of buffer layer were investigated under different environment. Moreover, in order to explore the cause of fault, the radial model of cable was established to compute the electrical field distribution in cable buffer layer. The results show that the electrical properties of the buffer layer will change under the variety of temperature, humidity and pressure in buffer layer. In the simulation analysis, it notes that the maximum electrical field intensity in buffer layer increases with the decline of its conductivity. Furthermore, when the maximum electrical field intensity exceeds air breakdown field, the discharge will occur and consequently lead to failure.

Buffer layer free large area bi-layer graphene on SiC(0 0 0 1)

Magnetism in graphene is of fundamental as well as technological interest, with potentialapplications in molecular magnets and spintronic devices. While defects and/oradsorbates in freestanding graphene nanoribbons and graphene sheets have beenshown to cause itinerant magnetism, controlling the density and distributionof defects and adsorbates is in general difficult. We show from first principlescalculations that graphene buffer layers on SiC(0001) can also show intrinsicmagnetism. The formation of graphene–substrate chemical bonds disrupts the grapheneπ-bonds and causes localization of graphene states near the Fermi level. Exchangeinteractions between these states lead to itinerant magnetism in the graphene buffer layer.We demonstrate the occurrence of magnetism in graphene buffer layers on bothbulk-terminated as well as more realistic adatom-terminated SiC(0001) surfaces. Ourcalculations show that adatom density has a profound effect on the spin distribution in thegraphene buffer layer, thereby providing a means of engineering magnetism in epitaxialgraphene.

The present work shows the construction of CuO grains on the electrodeposited Cu2O layer through annealing method and the photovoltaic effect of Cu2O:CuO/ZnO heterojunctions with graphene buffer layer is also discussed. The annealing temperature of Cu2O layer is varied from 100 to 300 °C. The graphene monolayer was deposited by chemical vapor deposition method. The morphology, structural and electrical properties of Cu2O layer were characterized by using Scanning Electron Microscopy (SEM), High Resolution Transmission Electron Microscopy (HR-TEM), X-ray Diffractometry (XRD) and I-V measurement, respectively. The Cu2O grains size increase as the annealing temperature increased and the CuO grains could be observed at 300°C. The graphene monolayer was successfully inserted in between Cu2O:CuO/ZnO heterojunction. The Cu2O:CuO/Gr/ZnO heterojunction shows high electrical rectification with threshold voltage of 0.5 V.

The paper presents a comparative approach concerning the properties of TiO2 thin layers obtained via spray pyrolysis deposition (SPD) using hydrophilic and hydrophobic polymers as additives. The influences of composition (X-ray diffraction), morphology (Atomic Force Mycroscopy, Contact angle) and optoelectric (electrical conductivity, absorption, band gap and photocurrent) properties on the photocatalytic activity of the layers were studied.

Graphene has attracted much attention because it has quite high electron mobility due to the characteristic electronic properties such as Dirac cone and then has been expected as a future electronic device material to contribute to the low energy consumption, which could help combat the global warming. Among graphene fabrication methods suggested so far, thermal decomposition of SiC substrate, in which graphene sheets are formed after Si atom sublimation from SiC surface at high temperature, has been intensively studied. The C-atom layer directly grown on the SiC substrate is not graphene but so-called buffer layer (BL), which has similar honeycomb structure to graphene but does not have the graphene's characteristic electronic properties such as the Dirac cone, because BL is covalently bonded to the SiC surface and lacks the hexagonal symmetry. To utilize BL grown on SiC substrate as graphene, it is necessary to anneal it under hydrogen (H) ambient to intercalate H atoms between BL and SiC substrate, so that the graphene’s characteristic properties is recovered. However, the lack of knowledge on the intercalation mechanism make it difficult to control this process to obtain high quality graphene. In this paper, we report our recent studies on the interaction between H atoms and BL grown on SiC substrate with the periodicity of (6√3x6√3)R30˚ by using first-principles density functional calculations [1]. We first investigated the adsorption process of H2 molecule on BL. The total adsorption energy for two H atoms is 1.6 eV, then H2 molecules prefer to dissociatively adsorb on BL. The activation energy of this process is 0.9 eV and that of the reverse process is 2.5 eV. Considering the fact that H atoms on Si(001) surface desorb from the surface at around 780K with the activation energy of about 2.5eV, we conjecture that H atoms on BL would also desorb from it at the temperature at which the experiments are conducted (above 870K). The H atom adsorption energy ranges between 0.94 eV and -0.35 eV depending on the adsorption sites. Adsorption energies are evaluated based on a H2 molecule in vacuum, and the positive values mean stable states, while negative ones mean unstable states. This variety comes from the C atom bonding states. On the C atom with π bonds, H atom is quite stable with a large adsorption energy, while on the C atom without π bonds due to the bonding to a substrate Si atom, H atom is not stable (sometimes it is endothermic). For the H atom diffusion process, it is found that the activation energy ranges from 1.7 eV to 2.3 eV depending on the diffusion path. This variety comes from the initial and the final states of H atom. If the initial and the final states are stable (on C atom with π bonds), the activation barrier is small like 1.7 eV, while if one of the two is unstable (on C atom without π bonds), the activation energy is large like 2.3 eV. We also investigate the penetration process of H atoms through BL. In the final state, H atom stays on a Si atom of the SiC substrate below BL. The final state is 0.8 eV higher in energy than the initial state where H atom stays on BL. So, H atoms prefers to stay on BL rather than penetrate through BL. The activation barrier of this penetration process is about 4.9 eV, which is quite larger than that of H2 desorption process described above, meaning that H atoms prefer to desorb from BL rather than penetrate through BL. We also study the behaviour of H atoms or molecules below BL. It is found that H molecules put between BL and SiC surfaces easily dissociate into two H atoms, which then make covalent bonds to Si dangling bonds. Surprisingly, there is no energy barrier for the adsorption of H molecule onto Si dangling bonds on a SiC(0001) surface. This is quite contrastive to the adsorption of H molecule onto BL. To investigate the diffusion of H atoms below BL, we performed first-principles molecular dynamics simulation in NVT condition (T=1500K). Even in a short simulation time of three picoseconds, we observed several H atom hoppings from one Si dangling bond to another, meaning that H atom easily diffuse below BL. Acknowledgments: This research was partially supported by MEXT within the priority issue 6 of the FLAGSHIP2020. Earth Simulator of JAMSTEC and NIMS Numerical Materials Simulator were used for this study. [1] PHASE code: https://azuma.nims.go.jp/.

A CdS thin film buffer layer has been widely used as conventional n-type heterojunction partner both in established and emerging thin film photovoltaic devices. In this study, we perform numerical simulation to elucidate the influence of electrical properties of the CdS buffer layer, essentially in terms of carrier mobility and carrier concentration on the performance of SLG/Mo/p-Absorber/n-CdS/n-ZnO/Ag configured thin film photovoltaic devices, by using the Solar Cell Capacitance Simulator (SCAPS-1D). A wide range of p-type absorber layers with a band gap from 0.9 to 1.7 eV and electron affinity from 3.7 to 4.7 eV have been considered in this simulation study. For an ideal absorber layer (no defect), the carrier mobility and carrier concentration of CdS buffer layer do not significantly alter the maximum attainable efficiency. Generally, it was revealed that for an absorber layer with a conduction band offset (CBO) that is more than 0.3 eV, Jsc is strongly dependent on the carrier mobility and carrier concentration of the CdS buffer layer, whereas Voc is predominantly dependent on the back contact barrier height. However, as the bulk defect density of the absorber layer is increased from 1014 to 1018 cm−3, a CdS buffer layer with higher carrier mobility and carrier concentration is an imperative requirement to a yield device with higher conversion efficiency and a larger band gap-CBO window for realization of a functional device. Most tellingly, simulation outcomes from this study reveal that electrical properties of the CdS buffer layer play a decisive role in determining the progress of emerging p-type photo-absorber layer materials, particularly during the embryonic device development stage.

Tin dioxide and metallic impurity (Cu, Fe) doped stannic oxide Nano layers were produced by chemical bath deposition method on glass substrates. The effects of doping on optoelectronic properties of stannic oxide Nano layers were studied. Optical Reflectance measured in the wavelength range of 220–2500 nm by spectrophotometer. Other optical properties and optical band gaps were calculated using Kramers-Kronig relations on reflectivity curves. Electronic properties were calculated by full potential linearized augmented plane wave (FP-LAPW) method, within density functional theory (DFT). In this approach, the generalized gradient approximation (GGA) in the form of the LSDA functional was used for the exchange-correlation potential calculations. Band gap structures and density of states were calculated. Doping impurity changes optical properties of Tin dioxide layers. Metallic impurities, especially copper, decreases the band gap energy and increases conductivity of layers. Value of band gap calculated by DFT method for SnO2 compound obtained 1.2 eV. All results are in good agreement with each other.

The prevention of environmental vibration pollution induced by train operation is one of the inevitable problems in the construction of urban rail transit. With the advantage of flexible adjustment, phononic crystals (PCs) have a broad application prospect in suppressing elastic wave propagation of rail transit. In this paper, a damped rail with two‐dimensional honeycomb PCs was proposed, and its band structure was analysed with FEM. Then, a parametric study was used to investigate the influences of design parameters of the honeycomb PCs on its band gap property. Furthermore, with a 3D half‐track model, the vibration reduction property of the damped rail with honeycomb PCs was discussed. The results show that the damped rail with honeycomb PCs has an absolute band gap in the frequency range of 877.3–1501.7 Hz, which includes the pinned‐pinned resonance frequency of the rail internally. Reducing the filling fraction and elastic modulus of the matrix can obtain an absolute band gap in a lower frequency range but also bring a narrower bandwidth. The decrease of scatterer density leads to higher boundary frequencies of the absolute band gap and descends the bandwidth. In order to obtain an absolute band gap which can suppress the pinned‐pinned resonance of the rail and keep a wider bandwidth, the filling fraction is suitable to be about 0.5, and the elastic modulus of the matrix is proposed to be not more than 0.6 MPa. Metals with heavy density can be used as the scatterer to obtain a better vibration reduction effect. It is hoped that the research results can provide a reference for the application of PCs in track vibration reduction.

Graphene coating-modified LiCoO2 films as high-performance cathode material in quasi-solid-state thin-film lithium batteries

The possibility of using chemical vapor deposition (CVD) graphene as a 2D buffer layer for epitaxial growth of III-nitrides by plasma assisted-MBE on amorphous substrates (SiO2 prepared by thermal oxidation of Si wafer) was investigated. The comparative study of graphene-coated parts of the wafers and the parts without graphene was carried out by scanning electron microscopy and X-ray diffractometry. It was shown that epitaxial GaN and AlN films with close to 2D surface morphology can be obtained by plasma assisted-MBE on amorphous SiO2 substrates with a multilayer graphene buffer using the HT AlN nucleation layer.

Growth of ZnO thin film on graphene transferred Si (100) substrate