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This work reports molecular beam epitaxy (MBE) of two-dimensional (2D) GaSe on a three-dimensional (3D) GaN/sapphire platform, which is widely recognized as a potential candidate for electronics and optoelectronic applications. Herein, we have demonstrated that regulating the adatoms' mobility via growth temperature can enable a growth mode transition from screw dislocation-driven (SDD) to layer-by-layer (LBL) in the epitaxy of 2D-GaSe. Typically, the high-density and uniform spiral structure is observed in the SDD-GaSe at low temperatures (≤500 °C), while μm-scale triangular LBL-GaSe morphology was dominant at high-temperature regime. The diverse optical properties of 2D-GaSe layers under different growth modes were comprehensively investigated, where the unique behaviors of the in-plane propagation (E1g) Raman mode in the SDD-GaSe as well as the resonant effect in the LBL-GaSe have been reported for the first time. Moreover, a significant blueshift of ∼0.21 eV in PL spectra of the LBL-GaSe layer with respect to the SDD-GaSe layer is indicated. This opens up the probability for band structure engineering of the 2D-GaSe epitaxial layers by switching the growth mode. Attractively, the LBL-GaSe multilayers exhibited a current density ∼120 nA/cm2 at zero bias; thus, it could be an auspicious candidate for self-powered photodetecting applications. [ABSTRACT FROM AUTHOR]

Herein, a tight‐binding description is utilized to investigate electronic structures and density of states of single‐layer (SL) and bilayer (BL) graphene ribbons with and without the influence of external electric fields. Analyses are implemented to reveal the similarity and difference among electronic properties of three types of structures (specifically SL and BL ribbons with AA and AB stackings). Moreover, both armchair and zigzag edge orientations are considered. It is indicated in the results that variation in electronic properties of these structures in the presence of external electric fields depends on both structural form and edge orientation. The following two points are demonstrated: 1) a transverse field has a significant effect on adjusting bandgap of the zigzag configurations, whereas a vertical electric field has a distinct impact on energy bands of armchair edge structures, and 2) among considered structures, AB‐stacking BL ribbons are invariably the structures that are most strongly influenced by external fields. Interestingly, AB‐stacking BL ribbons are also capable of enlarging bandgap with both edge terminations. Herein, an insight into how the electronic structure and charge distribution of both SL and BL graphene nanoribbons can be controlled not only in armchair but also in zigzag edge terminations using electric stimulants is provided by the results. [ABSTRACT FROM AUTHOR]

We present an efficient parameterized tight-binding (TB) model which can induce the energy bands that are consistent with results from ab-initio calculations. Besides the comparison of our simple TB model to the previous ones, we also supplement further neighbor interactions into the TB Hamiltonian to analyze the band structure and consider the impact of each type of TB parameters such as onsite and hopping energies, overlap terms of wave functions on the shape and the position of the energy bands. Then a set of appropriate parameters is selected to reproduce reasonably important characteristics of the band structure captured by ab-initio calculations, particularly at the low energy region near the Fermi level, which is relevant for many applications. Moreover, the TB model is applied to predict the impact of a vertical electric field on the electronic properties and the thermoelectric Seebeck effect of penta-graphene. It unveils that the bandgap is strongly modulated under the external field and the intrinsic Seebeck coefficient of penta-graphene is found to be 67 times as high as that of graphene. [ABSTRACT FROM AUTHOR]