Wenhui Duan

High-Rate, Ultralong Cycle-Life Lithium/Sulfur Batteries Enabled by Nitrogen-Doped Graphene

Nitrogen-doped graphene (NG) is a promising conductive matrix material for fabricating high-performance Li/S batteries. Here we report a simple, low-cost, and scalable method to prepare an additive-free nanocomposite cathode in which sulfur nanoparticles are wrapped inside the NG sheets (S@NG). We show that the Li/S@NG can deliver high specific discharge capacities at high rates, that is, ∼ 1167 mAh g(-1) at 0.2 C, ∼ 1058 mAh g(-1) at 0.5 C, ∼ 971 mAh g(-1) at 1 C, ∼ 802 mAh g(-1) at 2 C, and ∼ 606 mAh g(-1) at 5 C. The cells also demonstrate an ultralong cycle life exceeding 2000 cycles and an extremely low capacity-decay rate (0.028% per cycle), which is among the best performance demonstrated so far for Li/S cells. Furthermore, the S@NG cathode can be cycled with an excellent Coulombic efficiency of above 97% after 2000 cycles. With a high active S content (60%) in the total electrode weight, the S@NG cathode could provide a specific energy that is competitive to the state-of-the-art Li-ion cells even after 2000 cycles. The X-ray spectroscopic analysis and ab initio calculation results indicate that the excellent performance can be attributed to the well-restored C-C lattice and the unique lithium polysulfide binding capability of the N functional groups in the NG sheets. The results indicate that the S@NG nanocomposite based Li/S cells have a great potential to replace the current Li-ion batteries.

Topological crystalline insulators in the SnTe material class

Topological crystalline insulators are new states of matter in which the topological nature of electronic structures arises from crystal symmetries. Here we predict the first material realization of topological crystalline insulator in the semiconductor SnTe by identifying its non-zero topological index. We predict that as a manifestation of this non-trivial topology, SnTe has metallic surface states with an even number of Dirac cones on high-symmetry crystal surfaces such as {001}, {110} and {111}. These surface states form a new type of high-mobility chiral electron gas, which is robust against disorder and topologically protected by reflection symmetry of the crystal with respect to {110} mirror plane. Breaking this mirror symmetry via elastic strain engineering or applying an in-plane magnetic field can open up a continuously tunable band gap on the surface, which may lead to wide-ranging applications in thermoelectrics, infra-red detection and tunable electronics. Closely related semiconductors PbTe and PbSe also become topological crystalline insulators after band inversion by pressure, strain and alloying.

Role of symmetry in the transport properties of graphene nanoribbons under bias.

The intrinsic transport properties of zigzag graphene nanoribbons (ZGNRs) are investigated using first-principles calculations. It is found that although all ZGNRs have similar metallic band structure, they show distinctly different transport behaviors under bias voltages, depending on whether they are mirror symmetric with respect to the midplane between two edges. Asymmetric ZGNRs behave as conventional conductors with linear current-voltage dependence, while symmetric ZGNRs exhibit unexpected very small currents with the presence of a conductance gap around the Fermi level. This difference is revealed to arise from different coupling between the conducting subbands around the Fermi level, which is dependent on the symmetry of the systems.

First-principles study on morphology and mechanical properties of single-walled carbon nanotube

Abstract In this Letter, the morphology, mechanical properties and electronic structure of single-walled carbon nanotube are investigated by use of a first-principles cluster method within the framework of local density approximation. The calculated C–C bond length, elastic modulus and tensile strength are in agreement with available experimental values. The C–C bond length is larger, and the elastic modulus and tensile strength are less in carbon nanotube than these in graphite. From the bonding analysis, we find that the bonding between carbon atoms in carbon nanotube is predominantly σ bond, and π bond takes an additional effect. Compared with graphite, the curvature effect of tube weakens the binding of σ bond, but strengthens the binding of π bond.

Quantum Manifestations of Graphene Edge Stress and Edge Instability: A First-Principles Study

We have performed first-principles calculations of graphene edge stresses, which display two interesting quantum manifestations absent from the classical interpretation: the armchair edge stress oscillates with a nanoribbon width, and the zigzag edge stress is noticeably reduced by spin polarization. Such quantum stress effects in turn manifest in mechanical edge twisting and warping instability, showing features not captured by empirical potentials or continuum theory. Edge adsorption of H and Stone-Wales reconstruction are shown to provide alternative mechanisms in relieving the edge compression and hence to stabilize the planar edge structure.

Experimental observation of topological Fermi arcs in type-II Weyl semimetal MoTe2

Observations of topological surface states provide strong evidence that MoTe2 is a type-II Weyl semimetal, hosting Weyl fermions that have no counterpart in high-energy physics.

Stable Nontrivial Z2 Topology in Ultrathin Bi (111) Films: A First-Principles Study

Recently, there have been intense efforts in searching for new topological insulator materials. Based on first-principles calculations, we find that all the ultrathin Bi (111) films are characterized by a nontrivial Z(2) number independent of the film thickness, without the odd-even oscillation of topological triviality as commonly perceived. The stable nontrivial Z(2) topology is retained by the concurrent band gap inversions at multiple time-reversal-invariant k points with the increasing film thickness and associated with the intermediate interbilayer coupling of the Bi film. Our calculations further indicate that the presence of metallic surface states in thick Bi (111) films can be effectively removed by surface adsorption.

Toward single-layer uniform hexagonal boron nitride-graphene patchworks with zigzag linking edges.

The atomic layer of hybridized hexagonal boron nitride (h-BN) and graphene has attracted a great deal of attention after the pioneering work of P. M. Ajayan et al. on Cu foils because of their unusual electronic properties (Ci, L. J.; et al. Nat. Mater. 2010, 9, 430-435). However, many fundamental issues are still not clear, including the in-plane atomic continuity as well as the edge type at the boundary of hybridized h-BN and graphene domains. To clarify these issues, we have successfully grown a perfect single-layer h-BN-graphene (BNC) patchwork on a selected Rh(111) substrate, via a two-step patching growth approach. With the ideal sample, we convinced that at the in-plane linking interface, graphene and h-BN can be linked perfectly at an atomic scale. More importantly, we found that zigzag linking edges were preferably formed, as demonstrated by atomic-scale scanning tunneling microscopy images, which was also theoretically verified using density functional theory calculations. We believe the experimental and theoretical works are of particular importance to obtain a fundamental understanding of the BNC hybrid and to establish a deliberate structural control targeting high-performance electronic and spintronic devices.

Making a field effect transistor on a single graphene nanoribbon by selective doping

Using first-principles electronic structure calculations, we show a metal-semiconductor transition of a metallic graphene nanoribbon with zigzag edges induced by substitutional doping of nitrogen or boron atoms at the edges. A field effect transistor consisting of a metal-semiconductor-metal junction can then be constructed by selective doping of the ribbon edges. The current-voltage characteristics of such a prototype device is determined by the first-principles quantum transport calculations.

Suppression of spin polarization in graphene nanoribbons by edge defects and impurities

We investigate the effect of edge defects (vacancies) and impurities (substitutional dopants) on the robustness of spin polarization in graphene nanoribbons (GNRs) with zigzag edges by using density-functional-theory calculations. The stability of the spin state and its magnetic moments is found to continuously decrease with increasing the concentration of the defects or impurities. The system generally becomes nonmagnetic at the concentration of one edge defect (impurity) per