Single crystals of ReS₂ were grown by chemical vapor transport (CVT) method [7] using ICl₃ as a transport agent. The growing process of ReS₂ using CVT had been described elsewhere [8]. The as-grown crystals of ReS₂ were essentially layer type. X-ray diffraction measurement confirmed triclinic structure of the crystal [8]. Weak van der Waals bonding between the layers means that they display good cleavage property parallel to the layers. The strong bonding force along the Re cluster chains also implies that we can easily tear the layer along b-axis via the auxiliary of weak bonding force between the individual chains. Hall effect measurement revealed p-type semiconducting behavior of the layer. The carrier density is ~1×10¹⁶ cm⁻³.

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 Figure 1(a) is a scanning-electron microscope (SEM) photograph of ReS₂ in the layered plane. The crystal image is 50 × amplification and the crystal orientation of b-axis is shown. The b-axis is parallel to the metal Re cluster

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 Figure 1(b). All the measurements were carried out with the angular range starting from q = 0˚ to q = 90˚. The angular increment of the polarized-absorption measurement is 5˚ while the increment is 10˚ for the electricalresistivity measurement.

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 T_r by taking into account the spectral dependence of the reflectance R using the relation [10]. This equation assumes that there are multiple reflections within the sample, but that they add incoherently due to sample inhomogeneity or a sufficiently large spread of the incident angles. Because ad is large for the sample crystals, the second term in the denominator of the T_r relation can be neglected. Figure 2(a) displays the polarized absorption coefficients of ReS₂ as a function of photon energy from q = 0˚ to q = 90˚ at 300 K. The absorption edges of ReS₂ clearly indicate an energy blue-shift behavior with the increase of polarization angles from q = 0˚ to 90˚. The absorption-edge anisotropy in rhenium disulfide provides relevant evidence that the energy gap of ReS₂ is polarization dependent. This property also lends a potential ability for ReS₂ to fabricate a polarized optical switch suitable for polarized optical communication in the NIR region. The analysis of the experimental results in Figure 2(a) also shows that the absorption coefficient a for ReS₂

is proportional to (hn-E_g)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 [5] could be utilized to determine the polarized band gaps and average phonon energies from the polarized absorption spectra. Shown in Figure 2(b) by the solid squares (in polar-coordination plot) are the values of polarized energy gaps of ReS₂ ranging from q = 0˚ to q = 90˚ with an increment of 5˚. The average phonon energies for the indirect ReS₂ from the polarized absorption spectra are determined to be about 25 ± 5 meV. The angular dependence of polarized energy gaps of ReS₂ shows a sinusoidal-like variation of energies from 1.341 eV (q = 0˚) to 1.391 eV (q = 90˚). The angular dependence of the polarized gaps of ReS₂ can be analyzed by fitting the experimental data to a sinusoidal functional form of

where E_g0 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 [11] that describing the dependence of the linearly polarized lights. The solid curve displayed in Figure 2(b) is the results by fitting the polarized energy gaps to Equation (1). From the leastsquare fits using Equation (1), the energy values of and are determined to be 1.366 eV and 0.025 eVrespectively. As shown in Figure 2(b), a polar-coordination plot of a double circle line just depicts the curve trace of the polarized gaps moving from q = 0˚ to 360˚ for the layered ReS₂. This result concludes that the energy variation of polarized energy gaps for a dichroic semiconductor also follows the generalized Malus rule.

The dichroic electrical property of the p-type layered ReS₂ in van der Waal plane was studied using a regular four-point method [12]. The measurement configuration is shown in Figure 3(a). A dc current source supplied the current and a dc voltmeter used for detection unit. The in-plane resistivities were measured on ten different rectangular shape samples with different orientations. The direction of cutting-edge for each specimen was varied from q = 0˚ (|| b) to q = 90˚ (^ b) with an angular increment of 10˚. Displayed in Figure 3(b) by the solid circles are the experimental data points for the angular dependent in-plane resistivities of layered ReS₂. The in-plane electrical anisotropy of ReS₂ in Figure 3(b) clearly indicates the lowest resistivity is occurred with the electric field parallel to the b-axis while the maximum resistivity is arisen along the direction perpendicular to b-axis. The

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 Figure 3(b) also reveal a sinusoidal-like variation from q = 0˚ (E || b) to q = 90˚ (E ^ b). The relationship of angular dependent resistivities of ReS₂ can also be analyzed by using the generalized Malus formula of. The solid curve in Figure 3(b) is the fitting result of angular-dependent in-plane resistivities of ReS₂ extended from 0˚ to 360˚. The angular dependence of in-plane resistivities of ReS₂ is determined to be r(q) = 21.054 − 14.535·cos(2q) W-cm. The electric anisotropy in Figure 3(b) confirms that ReS₂ is an electrical dichroism which presents a special axial selectivity of electrical conduction along the layer plane. This property is attributed to the in-plane axial anisotropy of mobilities that existed in the triclinic-layered ReS₂.