Dang-Qi Fang

Silicane nanoribbons: electronic structure and electric field modulation

We present electronic band structure, Gibbs free energy of formation, and electric field modulation calculations for silicane nanoribbons (NRs), i.e., completely hydrogenated or fluorinated silicene NRs, using density functional theory. We find that although the completely hydrogenated silicene (H-silicane) sheet in the chair-like configuration is an indirect-band-gap semiconductor, a direct band gap can be achieved in the zigzag H-silicane NRs by using Brillouin-zone folding. Compared to H-silicane NRs, the band gaps of completely fluorinated silicene (F-silicane) NRs reduce at least by half. For all silicane NRs considered here, the Gibbs free energy of formation is negative but shows different trends by changing the ribbon width for H-silicane NRs and F-silicane NRs. Furthermore, by analyzing the effect of transverse electric fields on the electronic properties of silicane NRs, we show that an external electric field can make the electrons and holes states spatially separated and even render silicane NRs self-doped. The tunable electronic properties of silicane NRs make them suitable for nanotechnology application.

Electronic and magnetic properties of zigzag silicene nanoribbons with Stone–Wales defects

The structural, electronic, and magnetic properties of zigzag silicene nanoribbons (ZSiNRs) with Stone–Wales (SW) defects were investigated using first-principles calculations. We found that two types of SW defects (named SW-Ι and SW-ΙΙ) exist in ZSiNRs. The SW defect was found to be the most stable at the edge of the ZSiNR, independently of the defect orientation, even more stable than it is in an infinite silicene sheet. In addition, the ZSiNRs can transition from semiconductor to metal or half-metal by modifying the SW defect location and concentration. For the same defect concentration, the band structures influenced by the SW-Ι defect are more distinct than those influenced by the SW-ΙΙ when the SW defect is at the edge. The present study suggests the possibility of tuning the electronic properties of ZSiNRs using the SW defects and might motivate their potential application in nanoelectronics and spintronics.

Band gap engineering of ZnSnN2/ZnO (001) short-period superlattices via built-in electric field

Using density-functional-theory calculations combined with hybrid functional, we investigate the band gaps and built-in electric fields of ZnSnN2/ZnO (001) short-period superlattices. The band gap of ZnSnN2/ZnO (001) superlattice can be tuned from 1.9 eV to 0 eV by varying the thickness of both the ZnSnN2 and ZnO regions. Compared to the III-nitride superlattices, stronger built-in electric fields, induced by the polarizations, form inside the ZnSnN2/ZnO superlattices. The lowest electron and uppermost hole states are mainly localized at the two opposite interfaces of the superlattice, but the tails of the lowest electron states extend over several atomic layers. Based on the electrostatic argument, we demonstrate that variations of the band gap are approximately described by a geometric factor. The influence of the in-plane strain is also discussed. The results will be valuable in the design of ZnSnN2/ZnO heterostructures for electronics and optoelectronics applications.

Novel electronic transport of zigzag graphdiyne nanoribbons induced by edge states

The electronic structure and transport of graphdiyne nanoribbons are investigated theoretically by ab initio calculations. We find that some edge states of zigzag graphdiyne nanoribbons are confined in a narrow energy range. For non-magnetic zigzag graphdiyne nanoribbons, the edge states whose energy is near the valence band top form a special electronic transport channel and lead to current peaks (about several μA) at small bias below the conduction voltage. However, ferromagnetic graphdiyne nanoribbons do not have such current peaks because the edge states energy is much higher than the valence band top and the transport channel cannot be formed. Such special effect, which is not found in graphene nanoribbons, does not depend on the width of zigzag graphdiyne nanoribbons. According to the result, it is feasible to apply this novel property to design a magnetically controllable nanoscale switch.

Enhanced visible light absorption in ZnO/GaN heterostructured nanofilms

Abstract First-principles calculations have been employed to investigate structural stability and electronic properties of non-polar ( 1 1 ¯ 00 ) and ( 11 2 ¯ 0 ) ZnO/GaN heterostructured nanofilms. The effects of nanofilm thickness and GaN ratio are considered. It has been found that all studied heterostructured nanofilms are less stable than the corresponding pure ZnO film but more stable than pure GaN one, exhibiting a much thicker film with better stability. Electronic band structures display that both two types of heterostructured nanofilms are semiconductors with their band gaps strongly depending on the GaN ratios as well as the thicknesses. Of particular interest is that the band gaps decrease firstly, and then increase with the increasing GaN ratio, showing flexibly tunable band gaps that cover a wide range of the solar spectrum. Furthermore, spatial charge distribution to the valence band maximum and the conduction band minimum has been studied. By calculating the complex dielectric function, the properties of optical absorption has been explored to exploit their potential application in the solar energy harvesting.

Magnetism from 2p states in K-doped ZnO monolayer: A density functional study

Using density-functional–based methods, we have studied 2p-based magnetic moments and magnetic coupling in potassium (K)-doped ZnO monolayer. We find that the substitution of a K atom at a Zn site in a ZnO monolayer induces a magnetic moment of per cell mainly originating from the O-2p states and has much lower formation energy than a magnetic Zn vacancy. A half-metallic electronic property and long-range ferromagnetic coupling between the magnetic moments are obtained based on the generalized gradient approximation (GGA) calculations, which is explained by a double-exchange–like mechanism. Moreover, with stronger correlation correction on 2p states, the structure of the substitutional K impurity undergoes a Jahn-Teller–like distortion. Incorporating magnetism into a two-dimensional ZnO monolayer will promote its application in nanodevices.

First-principles study of the atomic structure and edge state of zigzag carbon nanoscrolls

The atomic structure and edge state of zigzag carbon nanoscrolls (ZCNSs) are investigated using first-principles calculations based on density functional theory. The results show a non-monotonic dependence of the total energy of ZCNS on the surface curvature due to a competition between the elasticity and the van der Waals interactions in a scroll. The edge states can be tuned by using different forms of edge hydrogenation and inner radius. It is found that the edge state range of monohydrogenated ZCNSs is smaller than that of monohydrogenated zigzag-edged graphene nanoribbons (ZGNRs), which is also verified using the tight-binding approximation. With the different edge hydrogenations, ZCNSs prefer the sp2 hybridization more than the sp3 one. Our present study could suggest the possibility of adjusting the electronic properties of ZCNSs and may provide potential applications in the electronic devices.