We investigate atomic and electronic structures of boron nanotubes (BNTs) by using the density functional theory (DFT). The transport properties of BNTs with different diameters and chiralities are studied by the Keldysh nonequilibrium Green function (NEGF) method. It is found that the cohesive energies and conductances of BNTs decrease as their diameters decrease. It is more diffcult to form (N,0) tubes than (M,M) tubes when the diameters of the two kinds of tubes are comparable. However, the (N,0) tubes have a higher conductance than the (M,M) tubes. When the BNTs are connected to gold electrodes, the coupling between the BNTs and the electrodes will affect the transport properties of tubes significantly.
In this paper the growth mechanism of a Te/Bi2Te3 novel structure is studied by ab-initio calculations. The results show that the growth of Te nanorods is determined by the adsorption energy of Te atoms on different crystalline Te surfaces. The adsorption energy of Te on the Te (001) surface is 3.29 eV, which is about 0.25 eV higher than that of Te on the Te (110). This energy difference makes the preferential growth direction along the < 001 > direction. In addition, the higher surface energy of Bi2Te3 (110) and the lattice misfit between crystalline Bi2Te3 and Te along < 001 > direction are considered to explain the growth of the Bi 2 Te 3 nanoplatelets, in which Volmer-Weber model is used. The theoretical results are in agreement with experimental observation.
The atomic and electronic structures of a graphene monolayer on a Ru(0001) surface under compressive strain are investigated by using first-principles calculations.Three models of graphene monolayers with different carbon periodicities due to the lattice mismatch are proposed in the presence and the absence of the Ru(0001) substrate separately.Considering the strain induced by the lattice mismatch,we optimize the atomic structures and investigate the electronic properties of the graphene.Our calculation results show that the graphene layers turn into periodic corrugations and there exist strong chemical bonds in the interface between the graphene N × N superlattice and the substrate.The strain does not induce significant changes in electronic structure.Furthermore,the results calculated in the local density approximation(LDA) are compared with those obtained in the generalized gradient approximation(GGA),showing that the LDA results are more reasonable than the GGA results when only two substrate layers are used in calculation.
The molecular thin films of Rose Bengal (RB) embedded in polymethyl methacrylate matrix are fabricated by using the spin-coating technique. The macroscopic current-voltage (I-V) characterization of the film shows that the RB molecule has two conductance switching states with a high ON/OFF ratio in ambient conditions. The infrared spectra indicate that intermolecular hydrogen bonds can form in the RB thin films after their hydrolysis in air. With the first-principles calculations, we demonstrate that the hydrogen bonds will be destroyed in concomitance with the conformational change when the RB molecule switches to its high-conductance state after applying a voltage.
Deposition patterns of tetracyanoquinodimethane(TCNQ) molecules on different surfaces are investigated by atomic force microscopy.A homemade physical vapour deposition system allows the better control of molecule deposition. Taking advantage of this system,we investigate TCNQ thin film growth on both SiO_2 and mica surfaces.It is found that dense island patterns form at a high deposition rate,and a unique seahorse-like pattern forms at a low deposition rate.Growth patterns on different substrates suggest that the fractal pattern formation is dominated by molecule-molecule interaction.Finally,a phenomenal 'two-branch' model is proposed to simulate the growth process of the seahorse pattern.
We investigate the electronic structures of one and two monolayer iron phthalocyanine (FePc) molecules on Au(111) surfaces. The first monolayer FePc is lying flat on the Au(111) substrate, and the second monolayer FePc is tilted at～15° relative to the substrate plane along the nearest neighbour [101ˉ] direction with a lobe downward to the central hole of the unit cell in the first layer. The structural information obtained by first-principles calculations is in agreement with the experiment results. Furthermore, it is demonstrated that the electronic structures of FePc molecules in one-monolayer FePc/Au(111) system are perturbed significantly, while the electronic structures of FePc molecules in the second monolayer in two-monolayer FePc/Au(111) system remain almost unchanged due to the screening of the buffer layer on Au(111).
Three different methods are used to manipulate and control phthalocyanine based single molecular rotors on Au (111) surface: (1) changing the molecular structure to alter the rotation potential; (2) using the tunnelling current of the scanning tunnelling microscope (STM) to change the thermal equilibrium of the molecular rotor; (3) artificial manipulation of the molecular rotor to switch the rotation on or off by an STM tip. Furthermore, a molecular 'gear wheel' is successfully achieved with two neighbouring molecules.
We report on a forest-like-to-desert-like pattern evolution in the growth of an organic thin film observed by using an atomic force microscope.We use a modified diffusion limited aggregation model to simulate the growth process and are able to reproduce the experimental patterns.The energy of electric dipole interaction is calculated and determined to be the driving force for the pattern formation and evolution.Based on these results,single crystalline films are obtained by enhancing the electric dipole interaction while limiting effects of other growth parameters.
