Recent experiments have shown that the photocatalytic activity of g-C3N4 can be greatly enhanced by C60 modification, however, a fundamental understanding of its mechanistic operation is still lacking. Using first-principles calculations, the interfacial effects of C60/g-C3N4 nanocomposites on the electronic properties, charge transfer and optical response have been explored in detail. For different stacking patterns, the two constituents are always linked by van der Waals (vdW) forces without any exception, and form type-II heterojunctions in most cases. The valence band maximum and conduction band minimum of these heterostructures are dominated by the unsaturated nitrogen (N2) atoms and C60 molecule, respectively, which strongly interact with each other, resulting in strong charge transfer between the two involved constituents and an obvious bending of the g-C3N4 sheets. The unsaturated N2 atoms included in the interfaces have a significant influence on promoting the photocatalytic performance, while the existence of saturated nitrogen (N1 and N3) atoms lying in the interfaces will weaken the interfacial interactions between C60 molecules and the g-C3N4 monolayers. Moreover, the sensitive optical response and satisfactory type-II band alignment clearly show that the C60/g-C3N4 heterostructure is an outstanding photocatalyst for hydrogen production. We proposed a deep insight (the role of nitrogen) into understanding the improved photocatalytic ability of the C60/g-C3N4 nanocomposites, which may contribute to the rational design of both C60/g-C3N4 and g-C3N4-based nanocomposite photocatalysts.
By using the nonequilibrium Green's function formalism combined with the density-functional theory, we present a theoretical study of the spin-dependent electron transport of a chromium porphyrin-based molecule device.
Carbon-nanotube/graphene-nanoribbon junctions were recently fabricated by the controllable etching of single-walled carbon-nanotubes [Wei et al., Nat. Commun. 4, 1374 (2013)] and their electronic transport properties were studied here. First principles results reveal that the transmission function of the junctions show a heavy dependence on the shape of contacts, but rectifying is an inherent property which is insensitive to the details of contacts. Interestingly, the rectifying ratio is largely enhanced in the junction with a realistic contact and the enhancement is insensitive to the details of contact structures. The stability of rectifying suggests a significant feasibility to manufacture realistic all-carbon rectifiers in nanoelectronics.
Using the non-equilibrium Green's function formalism in combination with density functional theory, we calculated the spin-dependent electronic properties of molecular devices consisting of pristine and hydrogen-terminated zigzag gallium nitride nanoribbons (ZGaNNRs). Computational results show that the proposed ZGaNNR models display multiple functions with perfect spin filtering, rectification, and a spin negative differential resistance (sNDR) effect. Spin-dependent transport properties, spin density and transmission pathways with applied bias values were calculated to understand the spin filter and the sNDR effect. The spin filtering efficiency can be up to -100% or 100% within a large range of biases, and a dual spin filtering effect can also be found in these model devices. The highest rectification ratio reaches 4.9 × 109 in spin-down current of ZGaNNRs with only the passivated nitrogen edge, and only ZGaNNRs with the passivated gallium edge exhibit an obvious sNDR behavior with the largest peak to valley current ratio of 1.25 × 107. The proposed hydrogenated ZGaNNRs can be preferred materials for realizing oscillators, memory circuits and fast switching applications.
The spin-dependent electron transport properties through a single-carbon atomic chain (SCAC) sandwiched between two-zigzag-graphene-nanoribbon (zGNR) electrodes are investigated by performing first-principles calculations based on the nonequilibrium Green's function (NEGF) approach in combination with spin density functional theory (DFT). Our calculations show that SCAC connecting two zGNRs with asymmetry-contacting points is a perfect spin filter in the transmission function within a large energy range. Moreover, the spin-dependent electron transmission spectra exhibit robust transport polarization characteristics and a strong current polarization behavior (almost 100%) can be found. The microscopic mechanisms are proposed for the spin-related phenomena.
Ferroelectric (FE) materials have been extensively applied to the multifunctional electronic devices, particularly the FE memories due to their excellent physical properties. The FE memory is a kind of nonvolatile memory device, and it could overcome the shortcomings of the traditional memory. But the development of the FE memory is very slow due to the FE failure problem. However, with the continuous decrease of the thickness of FE thin film, when it reaches microns or nanometers in magnitude, the leakage current is the main cause of the FE failure of FE thin film. The leakage current of FE thin film is directly related to whether the FE memory is applicable, and it has been the hot spot of scientific researches. There are still a lot of factors influencing the FE memory leakage current except for the thickness of the film, such as interface, processing temperature, defect, domain wall, etc. Of these factors, the defect and domain wall are the most common and the most probable. In this paper, the first-principle calculation method through combining the density function theory with the nonequilibrium Green's function is used to systematically study the influence of oxygen vacancy defect on the leakage current of the FE thin film. The doping with four kinds of Cu, Al, V, and Fe cations is used to regulate and control the leakage current of the FE thin PbTiO3 film caused by the oxygen vacancy defects. We investigate the leakage current induced by oxygen vacancies in PbTiO3 films, and the doped PbTiO3 thin FE films having oxygen vacancies. It is found that Fe and Al doping will increase the leakage current of oxygen vacancy defects of FE thin films, while the Cu and V doping significantly reduce the leakage current of oxygen vacancy defects of FE thin films. This is because the Cu and V doping have obvious pinning effect on oxygen vacancy defect. In addition, we find that the oxygen vacancies are pinned by Cu and V atoms due to the fact that the formation energy of oxygen vacancies can be remarkably reduced. So Cu and V doping in PbTiO3 not only induce the leakage current but also improve the fatigue resistance of the FE thin film induced by oxygen vacancies. Moreover, since the ionic radius of V is closer to the ionic radius of Ti than the ionic radius of Cu, V is easier to implement doping to suppress the leakage current caused by the oxygen vacancy defects. These conclusions are of important theoretical significance and application value for improving the performance of FE thin films and their FE memories.
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