Ching-Hwa Ho

Single-Layer ReS2: Two-Dimensional Semiconductor with Tunable In-Plane Anisotropy

Rhenium disulfide (ReS2) and diselenide (ReSe2), the group 7 transition metal dichalcogenides (TMDs), are known to have a layered atomic structure showing an in-plane motif of diamond-shaped-chains (DS-chains) arranged in parallel. Using a combination of transmission electron microscopy and transport measurements, we demonstrate here the direct correlation of electron transport anisotropy in single-layered ReS2 with the atomic orientation of the DS-chains, as also supported by our density functional theory calculations. We further show that the direction of conducting channels in ReS2 and ReSe2 can be controlled by electron beam irradiation at elevated temperatures and follows the strain induced to the sample. Furthermore, high chalcogen deficiency can induce a structural transformation to a nonstoichiometric phase, which is again strongly direction-dependent. This tunable in-plane transport behavior opens up great avenues for creating nanoelectronic circuits in 2D materials.

Optical properties of the interband transitions of layered gallium sulfide

Optical properties of GaS layered semiconductor have been characterized using temperature-dependent absorption and piezoreflectance (PzR) measurements in the temperature range between 15 and 300K. From the comparison of optical-absorption and PzR spectra at low temperature, the gallium sulfide layer was confirmed to be an indirect semiconductor. The band gap of GaS was determined to be 2.53±0.03eV at room temperature. PzR measurements of GaS were carried out in the energy range between 2 and 5eV. The low-temperature PzR spectrum obviously shows three doublet-excitonic structures (denoted as series A, B, and C) presented at the energies around 3, 4, and 4.5eV, respectively. The Rydberg constant and threshold energy of the excitonic series A, B, and C were determined. Transition origins of the A, B, and C series were examined. Temperature dependences of the interband transitions of the gallium sulfide are analyzed. The parameters that describe temperature variations of the transition energies of GaS are eva...

Crystal structure and band-edge transitions of ReS2-xSex layered compounds

Abstract Single crystals of the ReS 2− x Se x solid solution series have been grown by the chemical vapor transport method using Br 2 as a transport agent. From analyzing the X-ray patterns, the series crystals are determined to be single-phase and crystallized in the triclinic layered structure. The lattice parameters of the compound series are evaluated and discussed. The optical absorption edge was measured on basal-plane at room temperature. The results reveal that ReS 2− x Se x are indirect semiconductors and their energy gaps are determined. The band gap energy varies smoothly with the Se composition x , indicating that the nature of band edges are similar for the end members ReS 2 and ReSe 2 , and the compounds of intermediate compositions.

Practical thermoreflectance design for optical characterization of layer semiconductors

Modulation spectroscopy is a powerful characterization tool of semiconductors. In this article, we present a practical design for implementing the thermoreflectance (TR) measurements of sheet-type materials more effectively. Detailed design diagrams of the electronic circuits and heater structure of the TR measurements are described. Duty-cycle and frequency responses of the heated pulses used in the TR measurements of layered GaSe are tested. The heated pulses of low frequency and long duty cycle seem to be more efficient in the periodic thermal perturbation of the layered crystals. The thermoreflectance of layer-type GaSe0.9S0.1 and GaSe0.8S0.2 as well as the polarized thermoreflectance (PTR) of layered ReS2 and ReSe2 are, respectively, carried out. The experimental spectra are detailed analyzed and discussed. Experimental analyses show the well-behaved performance of this thermoreflectance design.

In-plane anisotropy of the optical and electrical properties of ReS2 and ReSe2 layered crystals

ReS 2 and ReSe 2 belong to the family of transition-metal dichalcogenides crystallized in the distorted octahedral layer structure of triclinic symmetry. Crystals with triclinic symmetry are optically biaxial. The interband transitions of ReS 2 and ReSe 2 are expected to show anisotropic character for linearly polarized light, incident normal to the basal plane. The optical anisotropic effects for E∥b and E⊥b polarizations were studied through polarization-dependent piezoreflectance (PzR) and optical absorption measurements. The polarization dependence of the PzR spectra provides conclusive evidence that the features associated with the interband excitonic transitions are originated from different origins. The results of absorption measurements indicate that ReS 2 and ReSe 2 are indirect semiconductors, in which E parallel to b-axis polarization exhibits a smaller band gap and a single phonon makes an important contribution in assisting the indirect transitions. The electrical conductivity along the b-axis was shown to be several times higher than that perpendicular to b-axis for both crystals.

Temperature dependence of energies and broadening parameters of the band-edge excitons of single crystals

We have measured the temperature dependence of the spectral features in the vicinity of direct band-edge excitonic transitions of single crystals from 25 to 300 K using piezoreflectance (PzR). From a detailed lineshape fit of the PzR spectra, the energies and broadening parameters of the A and B excitons have been determined accurately. The origin of these excitonic transitions is discussed. The transition energies and their splittings vary smoothly with the tungsten composition x, indicating that the natures of the direct band edges are similar for the compounds. In addition, the parameters that describe the temperature variation of the energies and broadening function of the excitonic transitions are evaluated and discussed.

The study of optical band edge property of bismuth oxide nanowires α-Bi 2 O 3

The α-phase Bi(2)O(3) (α-Bi(2)O(3)) is a crucial and potential visiblelight photocatalyst material needless of intentional doping on accommodating band gap. The understanding on fundamental optical property of α-Bi(2)O(3) is important for its extended applications. In this study, bismuth oxide nanowires with diameters from tens to hundreds nm have been grown by vapor transport method driven with vapor-liquid-solid mechanism on Si substrate. High-resolution transmission electron microscopy and Raman measurement confirm α phase of monoclinic structure for the as-grown nanowires. The axial direction for the as-grown nanowires was along < 122 >. The band-edge structure of α-Bi(2)O(3) has been probed experimentally by thermoreflectance (TR) spectroscopy. The direct band gap was determined accurately to be 2.91 eV at 300 K. Temperaturedependent TR measurements of 30-300 K were carried out to evaluate temperature-energy shift and line-width broadening effect for the band edge of α-Bi(2)O(3) thin-film nanowires. Photoluminescence (PL) experiments at 30 and 300 K were carried out to identify band-edge emission as well as defect luminescence for the α-Bi(2)O(3) nanowires. On the basis of experimental analyses of TR and PL, optical characteristics of direct band edge of α-Bi(2)O(3) nanowires have thus been realized.

Enhanced photoelectric-conversion yield in niobium-incorporated In2S3 with intermediate band

β-In2S3 is a nontoxic window layer material usually used in a thin-film solar cell. Transition metal (TM)-incorporated In2S3 has been proposed to promote conversion efficiency in In2S3 because multi-photon absorption by an intermediate band (IB) would happen in the sulfide. In this paper, band-edge and photoelectric-conversion properties of Nb-substituted In2S3 have been probed by thermoreflectance (TR), photoconductivity (PC), and photo–voltage–current (Photo V–I) measurements. The crystals of niobium-incorporated In2S3 with different Nb contents of In1.99S3:Nb0.01, In1.995S3:Nb0.005, and undoped In2S3 were grown by chemical vapor transport (CVT) method using ICl3 as a transport agent. X-ray diffraction measurements showed that the as-grown In2S3:Nb compounds are β-phase crystals with tetragonal structure. Lattice constants of the β-In2S3:Nb show shrinkage with the increase of the Nb content in the In2S3. Experimental TR spectra reveal four transition features including direct gap (E0), valence band to donor (EVS), valence band to IB (EIB), and acceptor to IB [Ed(IB)] transitions detected in the Nb-substituted In2S3 compounds. The band gap (E0) shows a reduction and the crystal color changes from fresh red, to red, to dark red with increasing Nb content in the undoped β-In2S3, β-In1.995S3:Nb0.005, and β-In1.99S3:Nb0.01. The PC and Photo V–I measurements verified that high photoelectric-conversion efficiency occurred in the β-In2S3 with higher niobium content. The intermediate band (IB) was formed by Nb substitution with indium in the β-In2S3 [i.e. detected by EIB≈1.52 eV and Ed(IB) ≈ 1.42 eV]. The IB state should mainly dominate the multi-photon absorption capacity and enhance the photoelectric-conversion yield in the indium sulfide.

Optical absorption of ReS2 and ReSe2 single crystals

The optical absorption of synthetic ReS2 and ReSe2 single crystals is reported over a temperature range from 25 to 300 K. Analysis reveals that the absorption edges of ReS2 and ReSe2 are indirect allowed transitions. The indirect band gaps at various temperatures are determined and their temperature dependence is analyzed by the Varshni equation [Physica 34, 149 (1967)] and an empirical expression proposed by O’Donnel and Chen [Appl. Phys. Lett. 58, 2924 (1991)]. The parameters that describe the temperature dependence of energy gaps of these two materials are evaluated and discussed.

In-plane anisotropy of the optical and electrical properties of layered ReS2 crystals

In-plane anisotropic optical and electrical properties of layered ReS2 crystals are reported. The optical anisotropic effects for E || b polarization and E b polarization were studied through polarization-dependent piezoreflectance and optical absorption measurements. The resistivity perpendicular to the b-axis was shown to be several times larger than that along the b-axis.