C. H. Ho

Integrated digital inverters based on two-dimensional anisotropic ReS2 field-effect transistors

Semiconducting two-dimensional transition metal dichalcogenides are emerging as top candidates for post-silicon electronics. While most of them exhibit isotropic behaviour, lowering the lattice symmetry could induce anisotropic properties, which are both scientifically interesting and potentially useful. Here we present atomically thin rhenium disulfide (ReS2) flakes with unique distorted 1T structure, which exhibit in-plane anisotropic properties. We fabricated monolayer and few-layer ReS2 field-effect transistors, which exhibit competitive performance with large current on/off ratios (∼107) and low subthreshold swings (100u2009mV per decade). The observed anisotropic ratio along two principle axes reaches 3.1, which is the highest among all known two-dimensional semiconducting materials. Furthermore, we successfully demonstrated an integrated digital inverter with good performance by utilizing two ReS2 anisotropic field-effect transistors, suggesting the promising implementation of large-scale two-dimensional logic circuits. Our results underscore the unique properties of two-dimensional semiconducting materials with low crystal symmetry for future electronic applications.

Piezoreflectance study of near band edge excitonic-transitions of mixed-layered crystal Mo(SxSe1-x)2 solid solutions

The temperature dependence of the spectral features in the vicinity of the direct band edge of mixed-crystals Mo(SxSe1-x)2 solid solutions is measured in the temperature range of 25–295u2009K by using piezoreflectance (PzR). The near band-edge excitonic transition energies of Mo(SxSe1-x)2 solid solutions were determined accurately from a detailed line-shape fit of the PzR spectra. The near band-edge excitonic transition energies were found to vary smoothly with the increase of S content x, indicating that the natures of the direct band edges of Mo(SxSe1-x)2 solid solutions are similar. The temperature dependences of near band edge transition energies were analyzed using Bose-Einstein expressions in the temperature range from 25 to 295u2009K. The parameters that described the temperature variation of the energies and broadening function of the excitonic transitions were evaluated and discussed.

Preparation and characterization of molybdenum-doped ReS2 single crystals

Single crystals of Mo-doped rhenium disulphide (ReS2) have been grown by the chemical vapour transport method using bromine as a transporting agent. Single-crystalline platelets of up to 5×5xa0mm2 surface area and 100xa0µm in thickness were obtained. From the x-ray diffraction patterns, the doped crystals are found to crystallize in the triclinic layered structure. The Hall coefficient measurement indicates that the samples are n-type in nature. The doping effects of the material are characterized by temperature-dependent conductivity, optical absorption and piezoreflectance measurements. The activation energies for the impurity carriers increase with doping. The indirect energy gap of the doped sample shows a slight red-shift. The direct band-edge excitonic transition energies remain unchanged, while the broadening parameter of the excitonic transition features increases due to impurity scattering.

Polarized electrolyte-electroreflectance study of ReS2 and ReSe2 layered semiconductors

Polarization dependent electrolyte-electroreflectance (EER) measurements of ReS2 and ReSe2 layered crystals have been carried out in the energy range of 1.3 to 5.0 eV. The EER spectra of E∥b polarization exhibit distinct band-edge excitonic and interband transition features from those of E⊥b polarization. Analysing the polarization dependent EER spectra, the structures of the excitonic and interband transitions of ReS2 and ReSe2 with optical polarizations along the b-axis and perpendicular to the b-axis are examined clearly and the energy positions are determined accurately. The mechanism of field-lattice interaction is proposed to account for the in-plane anisotropy of the crystals. Based on the analysis of experimental results, a probable band-structure scheme of ReX2 (X = S, Se) is constructed.

Pressure-induced metallization and superconducting phase in ReS 2

Among the family of transition metal dichalcogenides, ReS2 occupies a special position, which crystalizes in a unique distorted low-symmetry structure at ambient conditions. The interlayer interaction in ReS2 is rather weak, thus its bulk properties are similar to those of monolayer. However, how compression changes its structure and electronic properties is unknown so far. Here using ab initio crystal structure searching techniques, we explore the high-pressure phase transitions of ReS2 extensively and predict two new high-pressure phases. The ambient pressure phase transforms to a “distorted-1T” structure at very low pressure and then to a tetragonal I41/amd structure at around 90u2009GPa. The “distorted-1T” structure undergoes a semiconductor–metal transition at around 70u2009GPa with a band overlap mechanism. Electron–phonon calculations suggest that the I41/amd structure is superconducting and has a critical superconducting temperature of about 2u2009K at 100u2009GPa. We further perform high-pressure electrical resistance measurements up to 102u2009GPa. Our experiments confirm the semiconductor–metal transition and the superconducting phase transition of ReS2 under high pressure. These experimental results are in good agreement with our theoretical predictions.High-pressure physics: transitions and superconductivity of compressed ReS 2ReS2 is a unique transition metal dichalcogenide (TMD) in terms of its distorted low-symmetry structure at ambient conditions. A subject that remains elusive so far is how its structure and electronic properties respond to pressure. Now a collaborative team led by Prof. Jian Sun from Nanjing University looks at the phase transitions in ReS2 under pressure utilizing ab initio crystal structure searching combining with high-pressure electrical resistance measurements. Upon small compression, the ambient phase transforms to a triclinic distorted 1T structure before changing to a tetragonal polymorph at higher pressure. The former transition is due to the layer sliding with a Peierls mechanism governing the energy stabilization and this semiconducting phase would be metallized with increasing pressure. The latter predicted structure is superconducting at a critical temperature of around 2u2009K at 100u2009GPa. This work suggests the role of pressure in tailoring the electronic structures of TMDs.

Cleavage tendency of anisotropic two-dimensional materials: ReX2 ( X=S,Se ) and WTe2

With unique distorted 1T structure and the associated in-plane anisotropic properties, mono- and few-layer ReX2 (X=S, Se) have recently attracted particular interest. Based on experiment and first-principles calculations, we investigate the fracture behavior of ReX2. We find that the cleaved edges of ReX2 flakes usually form an angle of ~120{deg} or ~60{deg}. In order to understand such phenomenon, we perform comprehensive investigations on the uniaxial tensile stress-strain relation of monolayer and multi-layer ReX2 sheets. Our numerical calculation shows that the particular cleaved edges of ReX2 flakes are caused by unique anisotropic ultimate tensile strengths and critical strains. We also calculate the stress-strain relation of WTe2, which explains why their cleaved edges are not corresponding to the principle axes. Our proposed mechanism about the fracture angle has also been supported by the calculated cleavage energies and surface energies for different edge surfaces.