Polarized Raman Reveals Alignment of Few-Layer MoS₂ Films

Abstract

Ongoing interest in two-dimensional (2D) layered materials has motivated extensive studies of transitional metal dichalcogenides, especially its most pronounced representative, MoS 2 . The few-layer MoS 2 exhibits distinct properties from those in bulk, which predetermine its potential usage in optoelectronics and flexible devices. Recently, it was found that the layer orientation in MoS 2 thin films is a key parameter for their utilization in specific devices. Thus far, the alignment of MoS 2 layers has been detected mostly by transmission electron microscopy (TEM). The drawback of this method is that it requires elaborate sample preparation and probes only a nanometer-scale area of the sample surface. Here we present polarized Raman spectroscopy which provides information about the MoS 2 layer orientation on the area a few orders larger than in TEM. We show that the depolarization ratio of the significant Raman peaks A 1g and E 2g show specific values for the vertical and horizontal alignment of the MoS 2 layers. We also analytically calculated the depolarization ratio for a thin MoS 2 layer, which is in good agreement with the measured values. Polarized Raman spectroscopy thus provides a simple, reliable, and general way for specifying the layer alignment in various 2D layered materials.

High carrier mobility epitaxially aligned PtSe2 films grown by one-zone selenization

“…Pre-deposited platinum on the substrate and the selenium powder are placed next to each other in the center of a furnace. Recently, we used this method in preparing MoS2 thin films on different substrates with the possibility to control the layer alignment [19][20][21][22]. In this paper, we study how the selenization temperature, growth time and the thickness of an initial Pt layer affect the structural and electrical properties of as-prepared PtSe2 films.…”

Collapse Mechanism in Few-Layer MoS2Langmuir Films

“…Thanks to the layered structure of the MoS 2 crystal, the 002 diffraction vector is aligned along the surface normal of the MoS 2 nanosheets. 47,48 The angular distribution of the 002 diffracted intensity, termed as the pole figure, directly quantifies the changes in the distribution of the nanosheet tilts as the angles in reciprocal and real space are the same. Consequently, the angular width evaluated as fwhm of the 002 diffraction along the χ 002 represents a direct measure of the tilts' distribution.…”

Revealing the nanogeometry of WS2 nanoflowers by polarization-resolved Raman spectroscopy

“…The response of Raman modes to polarized light is described by Raman polarizability tensors, based on the crystal symmetries in the material [20,[33][34][35]. It is interesting to point out that the measured E 2g and A 1g Raman features exhibit the same polarization response (see Figure 9 in the Supplementary Materials).…”

“…As mentioned in the main text, the Raman polarizablity tensors are defined with respect to the crystal axes of flat TMDs layers, e.g., a frame of reference with the excitation light perpendicular to it. As the petals of the WS 2 flowers exhibit a variety of orientations with respect to the incident light, the Raman tensor needs to be defined in a different frame of reference (see [33][34][35]). Figure 12a presents schematically a WS 2 flake in the horizontal x-y plane, excited by circularly polarized light propagating along z.…”

“…Thus, the polarization-resolved Raman response of for instance a vertical TMDs wall will be completely different than that of a flat layer, e.g. modes that are usually allowed/forbidden in cross-polarization will now be absent/observed [20,[33][34][35]. Therefore, polarizationresolved Raman studies will provide insight in the flowers' nanogeometry and orientation.…”