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Monolayer tellurene is a novel two-dimensional semiconductor with excellent intrinsic properties. It is helpful in understanding doping and scattering mechanism to study the electronic structure of defective tellurene, thus it is important for the application of tellurene in electronic and photo-electronic devices. Using first-principles calculation based on the density functional theory, we investigate the effects of commonly seen point defects on the electronic structure and optical properties of monolayer β-Te. Seven kinds of point defects that may be present in β-Te are designed according to the lattice symmetry, including two single vacancies (SV-1, SV-2), two double vacancies (DV-1, DV-2) and three Stone-Wales (SW) defects (SW-1, SW-2, SW-3). It is found that the defect formation energies of these defects are 0.83–2.06 eV, which are lower than that in graphene, silicene, phosphorene and arsenene, suggesting that they are easy to introduce into monolayer β-Te. The two most stable defects are SV-2 and SW-1 where no dangling bond emerges after optimization. The calculated band structures show that all seven defects have little effect on the band gap width of monolayer β-Te, but they can introduce different numbers of impurity energy levels into the forbidden band. Among them, the SV-1, SV-2, DV-1 and SW-2 each act as deep level impurities which can be recombination centers and scattering centers of carriers, SW-1 acts as a shallow level impurity, DV-2 and SW-3 act as both deep level impurity and shallow level impurity. Besides, SW-1, SW-2 and DV-1 can change the band gap of monolayer β-Te from direct band gap to indirect band gap, which may result in the increase of the lifetime of carriers and decrease of photoluminescence of monolayer β-Te. The optical properties of monolayer β-Te, which are sensitive to the change in band structure, are also affected by the presence of defects. New peaks are found in the complex dielectric function and the absorption coefficient of defective monolayer β-Te in an energy range of 0–3 eV, of which the number and the position are dependent on the type of defect. The SV-1, DV-1, DV-2 and SW-2 can enhance the light response, polarization ability and light absorption in the low energy region of monolayer β-Te. This research can provide useful guidance for the applications of β-Te in the electronic and optoelectronic devices.

The geometries and electronic properties of armchair MoS2 nanoribbons were investigated by the first-principles method based on density functional theory. It was found that the stability and electronic properties of armchair MoS2 nanoribbons sensitively depend on edge modification. Increasing the number of hydrogen atoms on the edge caused the nanoribbons to become more stable and transition between indirect-gap semiconductor, semi-metal and direct-gap semiconductor. The band structure and densities of states of the nanoribbons indicated that low energy bands contributed to edge states. Different hydrogen adsorption patterns on each edge induce two kinds of edge state on the nanoribbons and these two kinds of edge state have little effect on each other. The relationships between the bandgap and width of three types of nanoribbons were studied. Nanoribbons terminated with zero or eight hydrogen atoms in each unit cell have a bandgap that oscillates with width in a period of three, while the bandgap changes nonperiodically in those terminated with four hydrogen atoms.

Based on Stone-Wales (SW) defect evolution theory and molecular dynamics, we simulate the docking process of two caped carbon nanotubes (CNTs) of different types to form a heterojunction using Monte Carlo methods. First, an algorithm for a fast simulation of the cap change in CNTs is put forward and the cap formation of single CNTs with open ends is simulated, by applying this method. SW defect evolution is designed as a leap change simulation of these caps, represents C-C bond formation and breakage, while molecular dynamics is used to simulate the gradient change of the relative bond distance between the C atoms. The coalescence process of forming heterojunction is also studied here. These simulations show that the process of docking is first to generate a large number of defects, which will precipitate the coalescence, then many defects disappear through the compound, finally the remaining defects transfer to the ends of this heterojunction in the form of pentagon/heptagon rings, thus leading to the reduced overall energy.

By using first-principles electronic structure and transport calculations based upon the density functional theory, we have studied the divacancy-defect effect of armchair graphene nanoribbons with zigzag edges. It is shown that the existence of 585 divacancy defects do not change the metallic characteristics of graphene nanoribbon, while changing the energy band structures near the Fermi energy level. Moreover, the spatial orientations of the divacancy defects have obvious effect on the transport properties of armchair graphene nanoribbons: for armchair graphene nanoribbons with odd width, slanting divacancies defects weaken conducting performace of graphene nanoribbons, while graphene nanoribbons with vertical divacancies defects basically remain linear I-V characteristics and little decrease in conducting capacity；For armchair graphene nanoribbons with even width, inclined divacancy defects increase the conducting property of graphene nanoribbons, while graphene nanoribbons with vertical divacancy defects have I-V characteristics of the perfect graphene nanoribbons.

Based on first-principles calculation of the electronic structure and transport properties, we study the electronic structure and transport properties of graphene nanoribbons （GNRs） with 585 divacancies defects. It is shown that because of the existence of 585 divacancies defects, there are defected band belts in the energy gap located at the defects for the zigzag graphene nanoribbon （ZGNRs） and their energy gaps will increase. The spatial orientations of the divacancies defects have effect on the electronic structure of the system. In addition, the influences of 585 divacancies defects on the transport properties of the ZGNRs with small energy gaps are great, while that on the transport properties of the ZGNRs with large energy gaps are small. And the spatial orientations of the defects have no obvious influence on the transport properties.

By performing first-principles electronic structure and transport calculations, we have demonstrated the electronic structure and transport properties of single layer zigzag graphene nanoribbons with armchair edges and the effect of edge-vacancy defects. It is shown that perfect zigzag graphene nanoribbons are semiconductor with certain energy gaps which will become smaller due to the edge-vacancy defects combining with semiconductor-metal transition. This result may contribute to the electronic structure sewing of the graphene nanoribbons in the energy-band engineering.

Based on first-principles electronic structure and transport calculations, we have studied electronic structure and transport properties of graphene nanoribbons with single vacancy defects. It is shown that introduction of the single vacancy defects leads to a flat band belt at the Fermi energy level for graphene nanoribbons and the semiconductor-metal transition in zigzag semiconducting graphene nanoribbons, which is useful in the energy-band engineering. Armchair graphene nanoribbons with odd width are metallic with good electric conduction while armchair graphene nanoribbons with even width have metallic band structures with character of the group IV semiconductor. Single vacancy defects weakens the conduction of armchair graphene nanoribbons with odd width while obviously strengthens the conduction of armchair graphene nanoribbons with even width.

The temperature dependence of VBL group formation in stripe domain walls of epitaxial garnet bubble films was investigated by means of the method of single-pulse bias field. A critical temperature T02 was found, which relats to the parameters of bubble films. When T >T02, hard bubbles no longer form. It was also found that, when T02, there are two different stages for VBL group formation. Their demarcation temperature is T01. The experimental curves in the first stage have been qualitatively interpreted. And the difference of soft domain segment movements, which lead ro their hardening, in the two stages has been revealed by means of the method of double exposure photography.