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Kou, Liangzhi

Publications and source records attributed to Kou, Liangzhi.

9 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Two-dimensional ferroelectric topological insulators in functionalized atomically thin bismuth layers

We introduce a class of two-dimensional (2D) materials that possess coexisting ferroelectric and topologically insulating orders. Such ferroelectric topological insulators (FETIs) occur in noncentrosymmetric atomic layer structures with strong spin-orbit coupling (SOC). Here, we showcase a prototype 2D FETI in an atomically thin bismuth layer functionalized by CH 2 OH, which exhibits a large ferroelectric polarization that is switchable by a ligand molecule rotation mechanism and a strong SOC that drives a band inversion leading to the topologically insulating state. An external electric field that switches the ferroelectric polarization also tunes the spin texture in the underlying atomic lattice. Moreover, the functionalized bismuth layer exhibits an additional quantum order driven by the valley splitting at the K and K' points in the Brillouin zone stemming from the symmetry breaking and strong SOC in the system, resulting in a remarkable state of matter with the simultaneous presence of the quantum spin Hall and quantum valley Hall effect. These phenomena are predicted to exist in other similarly constructed 2D FETIs, thereby offering a unique quantum material platform for discovering novel physics and exploring innovative applications.

2-dimensional systems↗
DOE OSTI2017

Computational Dissection of Two-Dimensional Rectangular Titanium Mononitride TiN: Auxetics and Promises for Photocatalysis

Recently, two-dimensional (2D) transition-metal nitrides have triggered an enormous interest for their tunable mechanical, optoelectronic, and magnetic properties, significantly enriching the family of 2D materials. Here, by using a broad range of first-principles calculations, we report a systematic study of 2D rectangular materials of titanium mononitride (TiN), exhibiting high energetic and thermal stability due to in-plane d–p orbital hybridization and synergetic out-of-plane electronic delocalization. The rectangular TiN monolayer also possesses enhanced auxeticity and ferroelasticity with an alternating order of Possion’s Ratios, stemming from the competitive interactions of intra- and inter- Ti—N chains. Such TiN nanosystem is a n-type metallic conductor with specific tunable pseudogaps. Halogenation of TiN monolayer downshifts the Fermi level, achieving the optical energy gap up to 1.85 eV for TiNCl(Br) sheet. Overall, observed electronic features suggest that the two materials are potential photocatalysts for water splitting application. Furthermore, these results extend emerging phenomena in a rich family 2D transition-metal-based materials and hint for a new platform for the next-generation functional nanomaterials.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Electrostatic properties of two-dimensional WSe{sub 2} nanostructures

Recently, two-dimensional transition metal dichalcogenides have intrigued much attention due to their promising applications in optoelectronics. The electrostatic property investigation of WSe{sub 2} nanostructures is essential for device application. Here, the interlayer screening effects of WSe{sub 2} nanoplates with different thicknesses were investigated by measuring surface potential employing Kelvin probe force microscopy. Simultaneously, charges can be injected into WSe{sub 2} nanoplate by means of conducting atomic force microscopy to tune the electrostatic properties of WSe{sub 2} nanostructures. Our experimental results have some important implications for improving performance of WSe{sub 2}-based optoelectronic devices through interface or surface engineering.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Tetragonal bismuth bilayer: A stable and robust quantum spin hall insulator

In this study, topological insulators (TIs) exhibit novel physics with great promise for new devices, but considerable challenges remain to identify TIs with high structural stability and large nontrivial band gap suitable for practical applications. Here we predict by first-principles calculations a two-dimensional (2D) TI, also known as a quantum spin Hall (QSH) insulator, in a tetragonal bismuth bilayer (TB-Bi) structure that is dynamically and thermally stable based on phonon calculations and finite-temperature molecular dynamics simulations. Density functional theory and tight-binding calculations reveal a band inversion among the Bi-p orbits driven by the strong intrinsic spin–orbit coupling, producing a large nontrivial band gap, which can be effectively tuned by moderate strains. The helical gapless edge states exhibit a linear dispersion with a high Fermi velocity comparable to that of graphene, and the QSH phase remains robust on a NaCl substrate. These remarkable properties place TB-Bi among the most promising 2D TIs for high-speed spintronic devices, and the present results provide insights into the intriguing QSH phenomenon in this new Bi structure and offer guidance for its implementation in potential applications.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Toward Rational Design of Catalysts Supported on a Topological Insulator Substrate

Exotic and robust metallic surface states of topological insulators (TIs) have been expected to provide a promising platform for novel surface chemistry and catalysis. However, it is still not fully known how TIs affect the activity of catalysts. In this work, we study the effects of topological surface states (TSSs) on the activity of transition metal clusters (Au, Ag, Cu, Pt, and Pd), which are supported on a TI Bi2Se3 substrate. It was found the adsorption energy of oxygen on the supported catalysts can be always enhanced due to the TSSs. However, it does not necessarily mean an increase of the activity in catalytic oxidation reaction. Rather, the enhanced adsorption behavior in the presence of TSSs exhibits dual effects, determined by the intrinsic reactivity of these catalysts with oxygen. For the Au case, the activity of catalytic oxidation can be improved because the TSSs can enhance the dissociation rate of dioxygen. In contrast, a negative effect is found for the Pt and Pd clusters since the TSSs will suppress the desorption process of reaction products. We also found that the effect of TSSs on the activity of hydrogen evolution reaction (HER) is quite similar (i.e., the metals with original weak reactivity can gain a positive effect from TSSs). The present work can pave a way for more rational design and selection of catalysts when using TIs as substrates.

Xiao, Jianping↗