Dichroic Electro-Optical Behavior of Rhenium Sulfide Layered Crystal

Dichroic behaviors of layered ReS2 have been characterized using angular dependent polarized-absorption and resistivity measurements in the van der Waal plane. The angular dependent optical and electrical measurements are carried out with angles ranging from  = 0 ̊ (E || b) to  = 90 ̊ (E  b) with respect to the layer crystal’s b-axis. The angular dependence 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 a potential usage for fabrication of a polarized optical switch suitable for polarized optical communication in near-infra-red (NIR) region. Angular dependence of resistivities of ReS2 in the van der Waal plane has also been evaluated. The relationship of in-plane resistivities shows a sinusoidal-like variation from  = 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 optical-dichroic layer but also an electrical-dichroism material presented in the layer plane.


Introduction
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 NaFe 3 B 3 Al 6 Si 6 O 27 (OH) 4 or the birefringent crystal such as Calcite (CaCO 3 ) are usually the crucial substances for making linear polarizers [1].Such kinds of polarizers are usually suitable for making visible-to-ultraviolet lasers or for fabrication of linearly polarized devices.
Dichroic semiconductorrhenium disulfide (ReS 2 ) has been synthesized and proposed to possess the potential capability for fabrication of polarization sensitive photodetector applied in multi-channel optical communications [2].The polarization sensitive behavior of ReS 2 is arisen from its specific in-plane optical anisotropy in the layer [2].The crystallography of ReS 2 is usually crystallized in a distorted CdCl 2 layer-type structure with triclinic symmetry (space group P 1 ) [3][4][5].ReS 2 is an indirect semiconductor with an unpolarized gap of about 1.37 eV [5].From the observation of polarization-dependent spectral quantum efficiency, the layered ReS 2 presents optically biaxial behavior with the linearly polarized lights incident normal to the basal plane [i.e.E || (001) and k  (001)] [6].Polarized photocurrent measurements of an ITO/ReS 2 device with different azimuthal angles revealed that angular dependence of photoresponses in ReS 2 follows the Malus law [1,6].The polarization-dependent spectral quantum efficiency also verified ReS 2 an effective polarization sensitive photodetector applied in visible range [6].
In this paper, the optical and electrical dichroic properties of ReS 2 are shown to simultaneously present in the van der Waal plane.The dichroic behaviors of ReS 2 are characterized by angular dependent polarized-absorption and electrical-resistivity measurements in the layer plane.The measurements were done in the angular range from  = 0˚ (E || b) to  = 90˚ (E  b) with respect to the b-axis of the layer crystal.The polarized energy gaps of ReS 2 were obtained from the analysis of polarization-dependent absorption spectra.The relation of the polarized gaps of ReS 2 was determined to be E g () = 1.366 − 0.025•cos(2) eV.Angular dependence of resistivities of ReS 2 in the van der Waal plane showed a sinusoidal-like variation from  = 0˚ (E || b) to  = 90˚ (E  b).The angular dependency of in-plane resistivities of ReS 2 is () = 21.054− 14.535cos(2) -cm.The optical and electrical evidences showed that ReS 2 is not only an optical-dichroic layer but also an electrical-dichroism crystal in the van der Waal plane.

Growth and Structural Property
Single crystals of ReS 2 were grown by chemical vapor transport (CVT) method [7] using ICl 3 as a transport agent.The growing process of ReS 2 using CVT had been described elsewhere [8].The as-grown crystals of ReS 2 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  10 16 cm −3 .

Optical Measurement
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 m 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.chains, which corresponds to the longest edge of the plate [6,9].The angular dependence of electrical-resistivity and polarized-absorption measurements of ReS 2 was evaluated by the rotation indication as shown in Figure 1(b).All the measurements were carried out with the angular range starting from  = 0˚ to  = 90˚.The angular increment of the polarized-absorption measurement is 5˚ while the increment is 10˚ for the electricalresistivity measurement.

Dichroic Optical Property
Polarized optical-absorption measurements were implemented with the polarization angles from  = 0˚ to  = 90˚.Absorption coefficient  of ReS 2 can be determined from the transmittance T r by taking into account the spectral dependence of the reflectance R using the relation   equation assumes that there are multiple reflections within [10].This the sample, but that they add incoherently due to sample inhomogeneity or a sufficiently large spread of the incident angles.Because d is large for the sample crystals, the second term in the denominator of the T r relation can be neglected.is proportional to (h-E g ) n with n = 2.0  gests 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 where E is related to the unpolarized band gap, and

The
The d ical property of the p-type layered g0 Δ E is the energy amplitude of the sinusoidal variation.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 g0 E and E are determined to be 1.366 eV and 0.025 respectively.As shown in for the layered ReS 2 .This result concludes that the energy variation of polarized energy gaps for a dichroic semiconductor also follows the generalized Malus rule.

Axial Dependence of Electrical Carrier Transport
ichroic electr ReS 2 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

Conclusion
In conclusion, the iors of layered Re 2 dependent polarized-absorption as well as electrical-resistivity measurements in the van der Waal plane.The polarized energy gaps of ReS 2 show a sinusoidal angular dependence of E g () = 1.366 − 0.025•cos(2) eV.The in-plane resistivities of ReS 2 were measured on ten different rectangular-shape samples with dissimilar orientations.The angular dependent in-plane resistivities of ReS 2 also show a sinusoidal-like variation from  = 0˚ (E || b) to  = 90˚ (E  b).The relationship of angular dependent resistivities of ReS 2 was determined to be () = 21.054− 14.535•cos (2) -cm.The angular dependence of resistivity is due to the in-plane axial anisotropy of mobilities in the triclinic-layered ReS 2 .The experimental observations verified that ReS 2 is not only an optical-dichroic layer but also an electrical dichroism in the van der Waal plane.

Figure 1 .
Figure 1.(a) SEM image of layered ReS 2 on the van der Waal plane, where the Re 4 clustering chains that corresponding to the crystal orientation of b-axis are illustrated.(b) Schematic illustrations for the angular dependent optical-absorption and electrical-resistivity measurements.The angle  is defined as the angular difference between the orientation of electric field and that of the crystal's b-axis.

Figure 2
(b)  by the solid squares (in polar-coordination plot) are the values of polarized energy gaps of ReS 2 ranging from  = 0˚ to  = 90˚ with an increment of 5˚.The average phonon energies for the indirect ReS 2 from the polarized absorption spectra are determined to be about 25  5 meV.The angular dependence of polarized energy gaps of ReS 2 shows a sinusoidal-like variation of energies from 1.341 eV ( = 0˚) to 1.391 eV ( = 90˚).The angular dependence of the polarized gaps of ReS 2 can be analyzed by fitting the experimental data to a sinusoidal functional form of

Figure 2 (
b), a polar-coor- Δ eV, dination plot of a double circle line just depicts the curve trace of the polarized gaps moving from  = 0˚ to 360˚