Fangbo Xu

Two-Dimensional Mono-Elemental Semiconductor with Electronically Inactive Defects: The Case of Phosphorus

The deep gap states created by defects in semiconductors typically deteriorate the performance of (opto)electronic devices. This has limited the applications of two-dimensional (2D) metal dichalcogenides (MX2) and underscored the need for a new 2D semiconductor without defect-induced deep gap states. In this work, we demonstrate that a 2D mono-elemental semiconductor is a promising candidate. This is exemplified by first-principles study of 2D phosphorus (P), a recently fabricated high-mobility semiconductor. Most of the defects, including intrinsic point defects and grain boundaries, are electronically inactive, thanks to the homoelemental bonding, which is not preferred in heteroelemental system such as MX2. Unlike MX2, the edges of which create deep gap states and cannot be eliminated by passivation, the edge states of 2D P can be removed from the band gap by hydrogen termination. We further find that both the type and the concentration of charge carriers in 2D P can be tuned by doping with foreign atoms. Our work sheds light on the role of defects in the electronic structure of materials.

Riemann Surfaces of Carbon as Graphene Nanosolenoids.

Traditional inductors in modern electronics consume excessive areas in the integrated circuits. Carbon nanostructures can offer efficient alternatives if the recognized high electrical conductivity of graphene can be properly organized in space to yield a current-generated magnetic field that is both strong and confined. Here we report on an extraordinary inductor nanostructure naturally occurring as a screw dislocation in graphitic carbons. Its elegant helicoid topology, resembling a Riemann surface, ensures full covalent connectivity of all graphene layers, joined in a single layer wound around the dislocation line. If voltage is applied, electrical currents flow helically and thus give rise to a very large (∼1 T at normal operational voltage) magnetic field and bring about superior (per mass or volume) inductance, both owing to unique winding density. Such a solenoid of small diameter behaves as a quantum conductor whose current distribution between the core and exterior varies with applied voltage, resulting in nonlinear inductance.

Site-percolation threshold of carbon nanotube fibers—Fast inspection of percolation with Markov stochastic theory

We present a site-percolation model based on a modified FCC lattice, as well as an efficient algorithm of inspecting percolation which takes advantage of the Markov stochastic theory, in order to study the percolation threshold of carbon nanotube (CNT) fibers. Our Markov-chain based algorithm carries out the inspection of percolation by performing repeated sparse matrix–vector multiplications, which allows parallelized computation to accelerate the inspection for a given configuration. With this approach, we determine that the site-percolation transition of CNT fibers occurs at pc=0.1533±0.0013, and analyze the dependence of the effective percolation threshold (corresponding to 0.5 percolation probability) on the length and the aspect ratio of a CNT fiber on a finite-size-scaling basis. We also discuss the aspect ratio dependence of percolation probability with various values of p (not restricted to pc).