Study of surface microstructure and optical properties of as-grown Mo0.6W0.4Se2 single crystals Sunil H. Chaki, M. P. Deshpande, Jiten P. Tailor, M. D. Chaudhary, and K. S. Mahato Citation: AIP Conf. Proc. 1512, 882 (2013); doi: 10.1063/1.4791321 View online: http://dx.doi.org/10.1063/1.4791321 View Table of Contents: http://proceedings.aip.org/dbt/dbt.jsp?KEY=APCPCS&Volume=1512&Issue=1 Published by the American Institute of Physics.
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Study of Surface Microstructure and Optical properties of as-grown Mo0.6W0.4Se2 Single Crystals Sunil H. Chaki#, M.P.Deshpande, Jiten P.Tailor, M.D.Chaudhary and K.S.Mahato Department of Physics, Sardar Patel University, Vallabh Vidyanagar, Gujarat-388120, India # Corresponding author email:
[email protected] Abstract. The surface microstructures and optical properties of as-grown Mo0.6W0.4Se2 single crystals were studied. The microstructures on the as-grown crystal surface reveals that spirals and Frank-Read dislocations are prominent. The study of the variation of refractive index (K), extinction coefficient (k), real and imaginary dielectric constants (εr and εi), and the optical conductivity (σ0) with incident photon energy from the analysis of optical absorption spectrum of Mo0.6W0.4Se2 single crystals, showed that near the bandgap energy value all sharply increases. Keywords: Mo0.6W0.4Se2, single crystals, surface microstructure, optical constants. PACS: 78.20. Ci, 78.40.Fy
vapor transport (DVT) technique. The as–grown single crystals surfaces were examined under ‘Axiotech 100 HD’ optical microscope (Carl Ziess India Private Limited, Bangalore, India) for their microstructures. The optical properties of as-grown Mo0.6W0.4Se2 single crystals were studied using UV-Vis-NIR spectrophotometer (Perkin Elmer Lambda 19) in the wavelength range of 200 nm to 1500 nm.
INTRODUCTION The Mo0.6W0.4Se2 single crystal is one of the members of the semiconducting family of the mixed transition metal dichalcogenides (TMDCs) involving IVB, VB, VIB and VA group of the periodic table. The basic structure of TMDCs is characterized by a stacking of two-dimensional sandwich units X-M-X, (where M is transition metal such as W, Mo, etc. and X is chalcogens namely S, Se and Te) along the c-axis separated from each other by the so called van der Waals gap. Within a layer (a-b basal plane) the bonds are strong while between the layers (along c-axis) they are remarkably weak. The TMDCs bandgap matches with solar spectrum making it significant for optical to electrical conversion through solid state photovoltaic and photoelectrochemical solar cell. Literature shows no study of optical and dielectric properties like refractive index (K), extinction coefficient (k), real (εr) and imaginary (εi) dielectric constants and optical conductivity (σ0) on Mo0.6W0.4Se2 single crystals. The composition, structural and electrical property of Mo0.6W0.4Se2 single crystals has already been reported [1]. Here the authors reports, surface microstructure and how the optical constants like dielectric constant and optical conductivity of Mo0.6W0.4Se2 single crystals varies with incident photon energy.
RESULTS AND DISCUSSION Optical Microscopy
(a)
(b)
FIGURE 1. Surface microstructure at (a) 10 μm and (b) 20 μm.
The common features visible on the as-grown Mo0.6W0.4Se2 single crystals are shown in Figure 1. The Figure 1(a) is a dark field image of the surface, which shows hexagonal spiral originating from screw dislocation and advancing in clockwise direction. The Figure 1(b) is bright field image, clearly showing presence of Frank-Read dislocation. This dislocation emerges due to multiplication in slip plane under the shear stress during the growth.
EXPERIMENTAL The Mo0.6W0.4Se2 single crystals of mixed transition metal dichalcogenides were grown by direct
SOLID STATE PHYSICS: Proceedings of the 57th DAE Solid State Physics Symposium 2012 AIP Conf. Proc. 1512, 882-883 (2013); doi: 10.1063/1.4791321 © 2013 American Institute of Physics 978-0-7354-1133-3/$30.00
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behavior of abrupt increase at band edge is due to anisotropy in spatial charge conductivity. Under the application of external electric field due to incident photon, the charge carriers easily migrate in the conducting layers (a-b plane) but are accumulated at the layer boundaries. The optical conductivity is one of the powerful tools for studying the electronic state in the semiconductor materials. The variation of electrical conductivity with the alternating electric field of the incident photon was determined by the expression [4],
Optical Properties of Mo0.6W0.4Se2 Single Crystal The absorption spectrum of Mo0.6W0.4Se2 single crystals is shown in Figure 2. The direct band gap determined from the spectrum using near band edge absorption relation [2] comes out to be 1.41 eV.
Optical conductivity (σ0) = αKc/4π
(1)
Where, α is absorption coefficient, K is refractive index and c is speed of light. The plot between the optical conductivity (σ0) verses incident photon energy is depicted in Figure 4. It indicates that optical conductivity (σ0) remains near constant in the range 0.8 to 1.3 eV followed by an increase near the crystal bandgap energy value of 1.46 eV (850 nm). The optical conductivity increase is due to the high absorbance of incident photon at the bandgap value by Mo0.6W0.4Se2 single crystals.
FIGURE 2. Absorption spectra of Mo0.6W0.4Se2 single crystals.
Optical constants and solid state properties of Mo0.6W0.4Se2 single crystal were studied using optical absorption theory [3]. The variation of optical constants like refractive index (K) and extinction coefficient (k) with incident photon energy were studied using relation [4]. Figure 3(a) shows that the refractive index (K) remains constant at low energy values (0.2 to 1.2 eV) and then sharply increases near bandgap value of 1.46 eV (850 nm). As seen in Figure 3(b), the extinction coefficient (k) is high at low photon energy (0.8 eV) followed by decrease as energy increases finally culminating to a sharp rise near the bandgap value of 850 nm (1.46 eV).
FIGURE 4. Plot of Optical conductivity verses photon energy.
Conclusions Surface microstructure study of the Mo0.6W0.4Se2 single crystals surface showed that hexagonal spirals and Frank-Read dislocations are prominent in this DVT grown single crystals. The study of optical constants like refractive index, dielectric constants and optical conductivity variation with incident photon energy shows that at the bandgap value of photon energy of the Mo0.6W0.4Se2 single crystals, the constants abruptly increases.
(a) (b) FIGURE 3. Plot of (a) Refractive index and extinction coefficient verses photon energy (eV) and (b) real (εr) and imaginary (εi) part of dielectric constant verses photon energy (eV).
REFERENCES 1. Sunil Chaki, M. P. Deshpande, J. P. Tailor, M. D. Chaudhary and P. N. Sakaria, AIP Conf. Proc. 1447, 987-988 (2012). 2. S. K. Srivastava, D. Palit, Solid State Ionics 176,513-521 (2005). 3. J. I. Pankove, “Relationship between optical constants” in Optical Processes in Semiconductors, New York, Dover Publications, 1971, pp. 87-95. 4. H. M. Pathan, J. D. Desai, C. D. Lokhande, Applied Surface Science, 202, 47-56 (2002).
The complex dielectric constant is a fundamental intrinsic property of the materials. The real and imaginary dielectric constants (ε) were determined using relation [4]. Here Figure 3(b) shows that value of real dielectric slowly increases while imaginary dielectric slowly decreases with incident photon energy in the range 0.8 to 1.32 eV respectively. Near the crystal bandgap energy value, 850 nm (1.46 eV), both dielectric constants suddenly increases. This
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