High-Throughput Computational Screening of Two-Dimensional Semiconductors.

Two-dimensional (2D) materials have attracted great attention mainly due to their unique physical properties and ability to fulfill the demands of future nanoscale devices. By performing high-throughput first-principles calculations combined with a semiempirical van der Waals dispersion correction, we have screened 73 direct- and 183 indirect-gap 2D nonmagnetic semiconductors from nearly 1000 monolayers according to the criteria for thermodynamic, mechanical, dynamic, and thermal stabilities and conductivity type. We present the calculated lattice constants, formation energy, Young's modulus, Poisson's ratio, shear modulus, anisotropic effective mass, band structure, band gap, ionization energy, electron affinity, and simulated scanning tunnel microscopy for each candidate meeting our criteria. The resulting 2D semiconductor database (2DSdb) can be accessed via the Web site https://materialsdb.cn/2dsdb/index.html. The 2DSdb provides an ideal platform for computational modeling and design of new 2D semiconductors and heterostructures in photocatalysis, nanoscale devices, and other applications. Further, a linear fitting model was proposed to evaluate band gap, ionization energy, and electron affinity of 2D semiconductors from the density functional theory (DFT) calculated data as initial input. This model can be as precise as hybrid DFT but with much lower computational cost.

Importance of bulk states for the electronic structure of semiconductor surfaces: implications for finite slabs.

We investigate the influence of slab thickness on the electronic structure of the Si(1 0 0)- p([Formula: see text]) surface in density functional theory (DFT) calculations, considering both density of states and band structure. Our calculations, with slab thicknesses of up to 78 atomic layers, reveal that the slab thickness profoundly affects the surface band structure, particularly the dangling bond states of the silicon dimers near the Fermi level. We find that, to precisely reproduce the surface bands, the slab thickness needs to be large enough to completely converge the bulk bands in the slab. In the case of the Si(1 0 0) surface, the dispersion features of the surface bands, such as the band shape and width, converge when the slab thickness is larger than 30 layers. Complete convergence of both the surface and bulk bands in the slab is only achieved when the slab thickness is greater than 60 layers.

Transport properties of a biphenyl-based molecular junction system-the electrode metal dependence.

We investigate the transport properties of a biphenyl-dithiol molecule sandwiched between electrodes made of metal Y (Y = Cu, Ag and Au) using the non-equilibrium Green's function method based on a density functional theory. The electrode metal Y has an influence on the coupling between the molecule and electrodes, and thus on the transmission peak height. For the transmission T(Y) at the Fermi energy, we obtain T(Cu)∼T(Ag)

Formation mechanism of one-dimensional Si islands on a H/Si(001) surface.

The formation mechanism of one-dimensional Si islands on a H/Si(001)-(2x1) surface is studied using scanning tunneling microscopy/spectroscopy and first-principles calculations. We observed that one-dimensional islands that are made from dimer chains are formed at the initial growth stages similar to the bare Si(001) surface. It is found that the number of odd-numbered dimer chains is larger than that of even-numbered dimer chains. We propose the growth processes of the two types of growth edges to explain the observation.

Dependence of the conduction of a single biphenyl dithiol molecule on the dihedral angle between the phenyl rings and its application to a nanorectifier.

The transport properties of a biphenyl dithiol (BPD) molecule sandwiched between two gold electrodes are studied using the nonequilibrium Green's function method based on the density functional theory. In particular, their dependence on the dihedral angle (phi=90 degrees -180 degrees ) between two phenyl rings is investigated. While the dihedral-angle dependence of the density of states projected on the BPD molecular orbitals is small, the transport properties change dramatically with phi. The transmission at the Fermi energy exhibits a minimum at phi=90.0 degrees and greatly increases with phi. The ratio of the maximum obtained at phi=180 degrees to the minimum exceeds 100. As an application of this characteristic transport behavior, a BPD molecule functionalized with NH(2) and NO(2) groups is considered. It is found that this molecule works as a nanorectifier.

Electron transport in a pi-stacking molecular chain.

We have investigated the electronic structure and transport properties of a pi-stacking molecular chain which is covalently bonded to a H/Si(100) surface, using the first-principles density functional theory approach combined with Green's function method. The highest occupied molecular orbital (HOMO) dispersion is remarkably reduced, but remains noticeable ( approximately 0.1 eV), when a short pi-stacking styrene wire is cut from an infinitely long wire and sandwiched between metal electrodes. We find that the styrene chain's HOMO and lowest unoccupied molecular orbital (LUMO) states are not separated from Si, indicating that it does not work as a wire. By substituting -NO2 or -NH2 for the top -H of styrene, we are able to shift the position of the HOMO and LUMO with respect to the Fermi level. More importantly, we find that the HOMO of styrene-NH2 falls into the band gap of the substrate and is localized in the pi-stacking chain, which is what we need for a wire to be electrically separated from the substrate. The conductance of such an assembly is comparable to that of Au/benzene dithiolate/Au wire based on chemical bonding, and its tunability makes it a promising system for a molecular device.

Theoretical investigation on electron transport through an organic molecule: effect of the contact structure.

Knowing how the contact geometry influences the conductance of a molecular wire junction requires both a precise determination of the molecule/metallic-electrode interface structure and an evaluation of the conductance for different contact geometries with a fair accuracy. With a greatly improved method to solve the Lippmann-Schwinger equation, we are able to include at least one atomic layer of each electrode into the extended molecule. The artificial effect of the jellium model used for the electrodes is therefore significantly reduced. Our first-principles calculations on the transport properties of a single benzene dithiolate molecule sandwiched between Au(111) surfaces show that the transmission of the bridge site contact, which is the most stable adsorption configuration in equilibrium, displays different features from those of other configurations, and that the inclusion of the surface layers of Au electrodes into the extended molecule shifts and broadens the transmission peaks due to a stronger and more realistic S-Au bonding. We discuss the geometry dependence of the transport properties by analyzing the density of states of the molecular orbitals.

Density functional theory investigation of benzenethiol adsorption on Au(111).

We have studied the adsorption of benzenethiol molecules on the Au(111) surface by using first principles total energy calculations. A single thiolate molecule is adsorbed at the bridge site slightly shifted toward the fcc-hollow site, and is tilted by 61 degrees from the surface normal. As for the self-assembled monolayer (SAM) structures, the (2 square root of 3 x square root of 3)R30 degrees herringbone structure is stabilized against the (square root 3 x square root 3)R30 degrees structure by large steric relaxation. In the most stable (2 square root 3 x square root 3)R30 degrees SAM structure, the molecule is adsorbed at the bridge site with the tilting angle of 21 degrees, which is much smaller compared with the single molecule adsorption. The van der Waals interaction plays an important role in forming the SAM structure. The adsorption of benzenethiolates induces the repulsive interaction between surface Au atoms, which facilitates the formation of surface Au vacancy.

Tail molecule dependence of thiolate adsorption on Au(111) surface: Theoretical study.

The adsorption of thiolates with various tail molecules on the Au(111) surface has been investigated by first-principles calculations. We have considered six typical thiolate molecules, that is, methylthiolate, ethylthiolate, ethylenethiolate, acetylenethiolate, benzenethiolate, and thiophenethiolate. It is found that these thiolates exhibit little difference in their stable adsorption geometries. They are adsorbed at the bridge site with being significantly tilted from the surface normal. The adsorption energy of thiolate on Au, on the other hand, largely varies depending on the type of tail molecule, and is linearly proportional to the binding energy of thiolate with H. We discuss the tail molecule dependence in terms of the bonding environment around the C atom connected to the head S atom.