RESUMO
Two-dimensional (2D) all-inorganic double perovskite materials have attracted great interest owing to their unique photoelectric characteristics, such as high quantum efficiency and relative stability. However, few studies have been conducted on the 2D all-inorganic double perovskite Cs3AgBiBr7, and its photoelectric properties are unclear. In this study, we present a detailed investigation of the band structure, optical absorption spectrum, carrier mobility and exciton binding energy of the double perovskite Cs3AgBiBr7 based on the first-principles. The results show that this system has an indirect band gap and low carrier mobility, high exciton binding energy (2041.38 meV) and significant light absorption in the UV region. We also find that the material may be a potential exciton insulation candidate owing to the exciton binding energy beyond the band gap. Our calculated results also show that low dimensional perovskite Cs3AgBiBr7 is more suitable for luminescence than a photovoltaic device. We hope our theoretical results will inspire and promote the experimental exploration of 2D all-inorganic double perovskite materials for photoelectric applications.
RESUMO
The rapid rise of two-dimensional (2D) materials has aroused increasing interest in the fields of microelectronics and optoelectronics; various types of 2D van der Waals heterostructures (vdWHs), especially those based on MoS2, have been widely investigated in theory and experiment. However, the interfacial properties of MoS2 and the uncommon crystal surface of traditional three-dimensional (3D) metals are yet to be explored. In this paper, we studied heterostructures composed of MoS2 and metal(001) slabs, based on the first-principles calculations, and we uncovered that MoS2/Au(001) and MoS2/Ag(001) vdWHs reveal Schottky contacts, and MoS2/Cu(001) belongs to Ohmic contact and possesses ultrahigh electron tunneling probability at the equilibrium distance. Thus, the MoS2/Cu(001) heterostructure exhibits the best contact performance. Further investigations demonstrate that external longitudinal strain can modulate interfacial contact to engineer the Schottky-Ohmic contact transition and regulate interfacial charge transport. We believe that it is a general strategy to exploit longitudinal strain to improve interfacial contact performance to design and fabricate a multifunctional MoS2-based electronic device.
RESUMO
An appropriate band structure and effective carrier separation are very important for the performance of a solar photocatalyst. In this paper, based on first-principles calculations, it was predicted that blue phosphorene (BlueP) and a C2N monolayer can form a promising metal-free type-II heterojunction. The electronic structure of the BlueP/C2N heterojunction facilitated the overall water splitting reactions well. The projected band structure showed that the conduction band edge was contributed by C2N and the valence band edge was dominated by BlueP. Under the combination of the driving force of the band offset and the built-in electric field between the two layers, the photo-generated electrons and holes were transferred spontaneously to the conduction band of C2N and the valence band of BlueP, respectively. An effective carrier separation in the heterostructure was thus achieved. More notably, the obtained light absorption of the BlueP/C2N junction showed an obvious red-shift, which greatly extended the area of light adsorption to the visible light region. We further proposed that strain could also be used to modulate the band gap and the band edge positions of the heterojunction. Our results not only provide a theoretical design, but also reveal the fundamental separation mechanism of the photo-generated carriers in the BlueP/C2N heterojunction.
RESUMO
Nitrogen doped amorphous carbon (a-C : N) thin films were prepared by DC magnetron sputtering. The films were investigated by AES, UV-Vis and ellipsometer. A parameter 'D' defined as the distance between the maximum of positive going excursion and the minimum of negative going excursion was calculated in the derivative AES spectra. The values of 'D' were used to calculate the percentage of sp2 hybrid bonds. The optical transmission and the optical band gap of the films were characterized by an UV-Vis spectrophotometer. The results showed that the optical band gap decreased and then increased with the increase of N2 gas source. The transmission and refractive index changed in reverse order. It was demonstrated that the thin film with low percentage of nitrogen was beneficial to the formation of sp3 hybrid bonds and caused the optical band gap of the thin film to increase. As a result, the thin film should be prepared under low percentage of nitrogen pressure to ensure that it possesses fine optical properties.