Dichroic behaviors of layered ReS2 have been characterized using angular dependent polarizedabsorption and resistivity measurements in the van der Waal plane. The angular dependent optical and electrical measurements are carried out with angles ranging from θ= 0°(E || b) to θ= 90°(E ^b) with respect to the layer crystal’s b-axis. The angular de pendence of polarized energy gaps of ReS2 shows a sinusoidal variation of energies from ~1.341 eV (E ||b to ~1.391 eV (E ^ b). The experimental evidence of polarized energy gap leaves ReS2 apotential usage for fabrication of a polarized optical switch suitable for polarized optical communication in nearinfrared (NIR) region. Angular dependence of resistivities of ReS2 in the vander Waal plane has also been evaluated. The relationship of inplane resistivities shows a sinusoidallike variation from θ= 0°(E ||b) to 90°(E ^ b) and repeated periodically to 360°. The experimental results of optical and electrical measurements indicated that ReS2 is not only an opticaldichroic layer but also an electricaldichroism material presented in the layer plane.
Dichroism and optical-anisotropic materials played an important role for the evolution of light-wave optics in past years. For generation of polarized light, dichroic crystal or birefringent solid is the most commonly used material because the specific axial anisotropy exists in the crystal structure. Among the dichroic or birefringent solids, the mineral tourmaline such as NaFe3B3Al6Si6O27(OH)4 or the birefringent crystal such as Calcite (CaCO3) are usually the crucial substances for making linear polarizers [
Dichroic semiconductor¾rhenium disulfide (ReS2) has been synthesized and proposed to possess the potential capability for fabrication of polarization sensitive photodetector applied in multi-channel optical communications [
In this paper, the optical and electrical dichroic properties of ReS2 are shown to simultaneously present in the van der Waal plane. The dichroic behaviors of ReS2 are characterized by angular dependent polarized-absorption and electrical-resistivity measurements in the layer plane. The measurements were done in the angular range from q = 0˚ (E || b) to q = 90˚ (E ^ b) with respect to the b-axis of the layer crystal. The polarized energy gaps of ReS2 were obtained from the analysis of polarization-dependent absorption spectra. The relation of the polarized gaps of ReS2 was determined to be Eg(q) = 1.366 − 0.025·cos(2q) eV. Angular dependence of resistivities of ReS2 in the van der Waal plane showed a sinusoidal-like variation from q = 0˚ (E || b) to q = 90˚ (E ^ b). The angular dependency of in-plane resistivities of ReS2 is r(q) = 21.054 − 14.535×cos(2q) W-cm. The optical and electrical evidences showed that ReS2 is not only an optical-dichroic layer but also an electrical-dichroism crystal in the van der Waal plane.
Single crystals of ReS2 were grown by chemical vapor transport (CVT) method [
Measurements of reflectance and transmittance at nearnormal incidence were made on a scanning monochromatic measurement system. An 150 W tungsten-halogen lamp filtered by a PTI 0.2 m monochromator provided the monochromatic light. Transmission intensity was closely monitored to obtain an incidence as close to 90˚ as possible. Single crystals with a thickness of about 100 mm were used in the transmission measurements. The reflected light of the sample was detected by an EG & G type HUV-2000B silicon photodiode and the signal was recorded from an EG & G model 7265 dual phase lock-in amplifier. A pair of OPTOSIGMA near-infrared-dichroic-sheet polarizers with the measurement range of 760 - 2000 nm was employed in the polarization dependent optical measurements.
Angular dependent resistivity measurements of layered ReS2 were made on ten different samples cut in bar type. The cutting edge of each sample was varied from q = 0˚ (||b) to q = 90˚ (^b) with an angle increment of 10˚ starting from the layer crystal’s b-axis. Displayed in
chains, which corresponds to the longest edge of the plate [6,9]. The angular dependence of electrical-resistivity and polarized-absorption measurements of ReS2 was evaluated by the rotation indication as shown in
Polarized optical-absorption measurements were implemented with the polarization angles from q = 0˚ to q = 90˚. Absorption coefficient a of ReS2 can be determined from the transmittance Tr by taking into account the spectral dependence of the reflectance R using the relation [
is proportional to (hn-Eg)n with n = 2.0 ± 0.1. This suggests an indirect allowed transition. A more complete analysis by taking into account both absorption and emission phonons [
where Eg0 is related to the unpolarized band gap, and
is the energy amplitude of the sinusoidal variation.
The energy-variation relation of Equation (1) is similar to the generalized Malus law [
The dichroic electrical property of the p-type layered ReS2 in van der Waal plane was studied using a regular four-point method [
lowest value of resistivity along b-axis is related to the strongest bonding force existed along the crystal orientation of the Re cluster chains. The angular-dependent in-plane resistivities of ReS2 in
In conclusion, the dichroic optical and electrical behaveiors of layered ReS2 were characterized using angular dependent polarized-absorption as well as electrical-resistivity measurements in the van der Waal plane. The polarized energy gaps of ReS2 show a sinusoidal angular dependence of Eg(q) = 1.366 − 0.025·cos(2q) eV. The in-plane resistivities of ReS2 were measured on ten different rectangular-shape samples with dissimilar orientations. The angular dependent in-plane resistivities of ReS2 also show a sinusoidal-like variation from q = 0˚ (E || b) to q = 90˚ (E ^ b). The relationship of angular dependent resistivities of ReS2 was determined to be r(q) = 21.054 − 14.535·cos (2q) W-cm. The angular dependence of resistivity is due to the in-plane axial anisotropy of mobilities in the triclinic-layered ReS2. The experimental observations verified that ReS2 is not only an optical-dichroic layer but also an electrical dichroism in the van der Waal plane.
The author would like to acknowledge the research funding supported by the National Science Council of Taiwan under the Project No. NSC 101-2221-E-011-052- MY3.