Characterization of 2D As 2 S 3 crystal by Raman spectroscopy
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MRS Advances © 2018 Materials Research Society DOI: 10.1557/adv.2018.201
Characterization of 2D As2S3 crystal by Raman spectroscopy S. Mamedov 1, N. Drichko 2 1
2
Horiba Scientific, Edison, NJ 08859 USA;
John Hopkins University, Department of Physics and Astronomy, Baltimore, MD, 21218, USA
Abstract
As2S3 is a semiconductor, which is known as a layer crystal with structure that is very similar to the metal chalcogenides, such as MoS2 and graphite. In such crystalline structure, the molecular unit is extended in two dimensions indefinitely. The unit cell of As2S3 contains two layers with bond length of 2.24A within the layer and 3.56A between the layers. Large difference between interlayer and intralayer bond length corresponds to a significant difference in bond strengths. The weak bonding between layers primarily occurs via van der Waals interactions. Optical phonons in 2D layer crystal As2S3 have been investigated by Raman scattering in temperature range of 4K-270K in two polarizations in the layer plane (ac plane). Our experimental data shows strong polarization dependence of Raman bands in ac plane for internal mode (intra-layer interactions). Additionally, it presents low frequency band, due to the weak inter-layer interaction. The important evidence for the dominance of layer symmetry with very weak interaction between the layers provides understanding of structural motives of As2S3 and may predict optical / electronic properties of similar 2D materials.
INTRODUCTION Layer crystals may be considered as a sub-class of molecular crystals, in which the individual layer (the molecular unit) is extended in two dimensions. Such twodimensional molecular symmetry is diperiodic factor-group symmetry and it differs from triperiodic crystal symmetry [1]. The low frequency optical phonons are predicted in the molecular crystals with two or more layers in the crystal unit cell. Furthermore, the layers move as rigid units for these phonons. The crystalline structure of As2S3 is monoclinic with 20 atoms in the unit cell [2, 3]. The crystalline symmetry is P21/n and it isomorphs to [4]. In monoclinic unit cell, the a and c axes are perpendicular to the b axis, where according to crystallographic data, the ac angle is . The b axis is the only unique axis of the symmetry. This angle is important for determination of the structural features of individual layers, as mentioned in [5,6]. The layer structure, within experimental uncertainty, can be considered the orthorhombic with all three axis are axis
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of symmetry, as seen in [2, 3]. The space group is Pmn21 and the isomorph group is . The factor-group representation determines the symmetries of the phonons (at q ≈ 0), which is shown in Table 1 and 2 for crystal and layer, respectively. Table 1. Crystal symmetry P21/n (
Representa
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