Hui Chen's research while affiliated with Chinese Academy of Sciences and other places

Two-dimensional (2D) materials can take a large amount of mechanical deformation before reaching the fracture limit due to their high Young’s modulus, and this in return, provides a way to tune the properties of 2D materials by strain engineering. Previous works have shown that the optical band gap of transition metal chalcogenides (TMDs) can be modulatd by strain, resulting in a drift of photoluminescence (PL) peak position and a decrease (or little change) in PL intensity. Here, we report on a member of post-transition metal chalcogenides (PTMCs), 2D-GaSe sheets, displaying vastly different phenomena under strain. Strained 2D-GaSe emits photons at almost the same wavelength as unstrained parts but appears an order of magnitude brighter. In contrast to TMDs, optical spectroscopy measurements reveal changes in the optical properties are mostly related to the colossal optical absorption anisotropy of GaSe, instead of commonly accepted strain-induced band renormalization. Results suggest that the light-matter interaction and the optical properties of 2D-GaSe can be controlled at will by manipulating the optical absorption.

Heterostructure engineering of atomically thin two-dimensional materials offers an exciting opportunity to fabricate atomically sharp interfaces for highly tunable electronic and optoelectronic devices. Here, we demonstrate abrupt interface between two completely dissimilar material systems, i.e, GaTe-MoS2 p-n heterojunction transistors, where the resulting device possesses unique electronic properties and self-driven photoelectric characteristics. Fabricated heterostructure transistors exhibit forward biased rectifying behavior where the transport is ambipolar with both electron and hole carriers contributing to the overall transport. Under illumination, photo-excited electron-hole pairs are readily separated by large built-in potential formed at the GaTe-MoS2 interface efficiently generating self-driven photocurrent within < 10 ms. Overall results suggest that abrupt interfaces between vastly different material systems with different crystal symmetries still allow efficient charge transfer mechanisms at the interface and are attractive for photo-switch, photo-detector, and photovoltaic applications owing to large built-in potential at the interface.

The structural, electronic, and magnetic properties of the GaS monolayer doped by 12 different kinds of atoms were investigated systemically using first-principles calculations. N is found to be the most promising candidate for p-type doping among dopants at the S site, including nonmetal atoms H, B, C, N, O, and F and transition metal atoms V, Cr, Mn, Fe, Co, and Ni. Transition metal atoms appear to be hardly incorporated in the GaS monolayer under either S- or Ga-rich conditions. While the net magnetic moments of doped GaS by nonmetal atoms are either 0 or 1 μB, the value of transition metal dopants decreases from 5 to 0 μB by adding the number of valence electrons from V to Ni. In the case of transition metal dopants at the Ga site, the majority spin states of Cr and Co are located closest to the conduction band minimum and valence band maximum, respectively. Magnetic ground states exist in all of the monolayers doped by these impurities. Indirect band gap of the pristine GaS monolayer is regulated to be direct from one type of spin channel by introducing B and Mn in the S site and V, Fe, Co, and Ni in the Ga site.

The structural and electronic properties of black phosphorus/MoS2 (BP/MoS2) van der Waals (vdW) heterostructure are investigated by first-principles calculations. It is demonstrated that the BP/MoS2 bilayer is a type-II p-n vdW heterostructure, and thus the lowest energy electron–hole pairs are spatially separated. The band gap of BP/MoS2 can be significantly modulated by external electric field, and a transition from semiconductor to metal is observed. It gets further support from the band edges of BP and MoS2 in BP/MoS2 bilayer, which show linear variations with E⊥. BP/MoS2 bilayer also exhibits modulation of its band offsets and band alignment by E⊥, resulting in different spatial distribution of the lowest energy electron–hole pairs. Our theoretical results may inspire much interest in experimental research of BP/MoS2 vdW heterostructures and would open a new avenue for application of the heterostructures in future nano- and optoelectronics.Keywords: work function; electron−hole pairs separation; semiconductor-to-metal transition; band alignment

This demo presents an extremely efficient concept detection system based on a novel bag of words extraction method and sparse ensemble learning. We will show that the presented system can efficiently build the concept detectors upon millions of images, and achieve real-time concept detection on unseen images with the state-of-the-arts accuracy. To do so, we first develop an efficient bag of visual words (BoW) construction method based on sparse non-negative matrix factorization (NMF) and GPU enabled SIFT feature extraction. We then develop a sparse ensemble learning method to build the detection model, which drastically reduces learning time in order of magnitude over traditional methods like Support Vector Machine. The demo video of the system is available at YouTube: http://youtu.be/57obnlCxqAs

The electronic and magnetic properties of native point defects including vacancies VGa and VS, antisites GaS and SGa, and interstitials Gai and Si in monolayer and bulk GaS have been systemically studied using density functional theory (DFT) method. For the monolayer, it is found that the impurity states appear in the band gaps of all defect structures except the interstitial Si. Furthermore, half-metallic behaviors can be obtained in the presence of VGa and Gai. Monolayers with VGa, GaS, SGa, and Gai obtain total magnetic moment of 1.0 μB, so does the bulks with VGa, GaS, and SGa, while monolayers with VS and Si and bulk with Gai are proved to be spin-unpolarized. In addition, n- and p-type GaS monolayers can be obtained in Ga-rich and S-rich conditions, respectively. Moreover, GaS and SGa are identified as suitable n- and p-type defects, respectively.

The dielectric properties of multilayer GaS films have been investigated using a Berry phase method and a density functional perturbation theory approach. A linear relationship has been observed between the number of GaS layers and slab polarizability, which can be easily converged at a small supercell size and has a weak correlation with different stacking orders. Moreover, the intercoupling effect of the stacking pattern and applied vertical field on the electronic properties of GaS bilayers has been discussed. The band gaps of different stacking orders show various downward trends with the increasing field, which is interpreted as giant Stark effect. Our study demonstrates that the slab polarizability as the substitution of conventional dielectric constant can act as an independent and reliable parameter to elucidate the dielectric properties of low-dimensional systems and that the applied electric field is an effective method to modulate the electric properties of nanostructures.Keywords: dielectric properties; GaS layers; Berry phase; DFPT; slab polarizability; stacking orders; GSE

Creating materials with ultimate control over their physical properties is vital for a wide range of applications. From a traditional materials design perspective, this task often requires precise control over the atomic composition and structure. However, owing to their mechanical properties, low-dimensional layered materials can actually withstand a significant amount of strain and thus sustain elastic deformations before fracture. This, in return, presents a unique technique for tuning their physical properties by "strain engineering". Here, we find that local strain induced on ReSe2, a new member of the transition metal dichalcogenides family, greatly changes its magnetic, optical, and electrical properties. Local strain induced by generation of wrinkle (1) modulates the optical gap as evidenced by red-shifted photoluminescence peak, (2) enhances light emission, (3) induces magnetism, and (4) modulates the electrical properties. The results not only allow us to create materials with vastly different properties at the nanoscale, but also enable a wide range of applications based on 2D materials, including strain sensors, stretchable electrodes, flexible field-effect transistors, artificial-muscle actuators, solar cells, and other spintronic, electromechanical, piezoelectric, photonic devices.