Dong Han

Phonon-Enabled Carrier Transport of Localized States at Non-Polar Semiconductor Surfaces: A First-Principles-Based Prediction

Electron-phonon coupling can hamper carrier transport either by scattering or by the formation of mass-enhanced polarons. Here, we use time-dependent density functional theory-molecular dynamics simulations to show that phonons can also promote the transport of excited carriers. Using nonpolar InAs (110) surface as an example, we identify phonon-mediated coupling between electronic states close in energy as the origin for the enhanced transport. In particular, the coupling causes localized excitons in the resonant surface states to propagate into bulk with velocities as high as 10(6) cm/s. The theory also predicts temperature enhanced carrier transport, which may be observable in ultrathin nanostructures.

Possible n/p-type conductivity of two-dimensional graphene oxide by boron and nitrogen doping: Evaluated via constrained excitation

As the first-principles calculations using the supercell approximation give widely scattered results in a two-dimensional charged system, making the evaluation of defect ionization energy difficult, here an alternative constrained excitation is applied to overcome this problem for defect analysis. As an example in graphene oxide with 50% oxygen coverage (according to the popular epoxy-chain-plus-hydroxyl-chain model), the structures, stabilities, and electronic properties of nitrogen and boron dopants are investigated. Generally, boron prefers to replace carbon in the sp3 region as an acceptor while nitrogen has a tendency to substitute the sp2 carbon close to the boundary between the sp2 region and the sp3 region as a donor. Their ionization energies are 0.24–0.42u2009eV for boron and 0.32–0.67u2009eV for nitrogen. However, a special case of nitrogen doped in the boundary-sp3 carbon can change to be an acceptor with the assistance of its neighboring (epoxy) oxygen “Lift-off,” leading to the shallowest ionization...

Slide fastener reduction of graphene-oxide edges by calcium: insight from ab initio molecular dynamics.

The reduction of graphene oxide can be used as a simple way to produce graphene on a large scale. However, the numerous edges produced by the oxidation of graphite seriously degrade the quality of the graphene and its carrier transport property. In this work, the reduction of oxygen-passivated graphene edges and the subsequent linking of separated graphene sheets by calcium are investigated by using first-principles calculations. The calculations show that calcium can effectively remove the oxygen groups from two adjacent edges. The joining point of the edges serves as the starting point of the reduction and facilitates the reaction. Once the oxygen groups are removed, the crack is sutured. If the joining point is lacking, it becomes difficult to zip the separated fragments. A general electron-reduction model and a random atom-reduction model are suggested for these two situations. The present study sheds light on the reduction of graphene-oxide edges by using reactive metals to give large-sized graphene through a simple chemical reaction.