Polarized Raman Reveals Alignment of Few-Layer MoS₂ Films

Ongoing interest in two-dimensional (2D) layered materials has motivated extensive studies of transitional metal dichalcogenides, especially its most pronounced representative, MoS2. The few-layer MoS2 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 MoS2 thin films is a key parameter for their utilization in specific devices. Thus far, the alignment of MoS2 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 MoS2 layer orientation on the area a few orders larger than in TEM. We show that the depolarization ratio of the significant Raman peaks A1g and E2g show specific values for the vertical and horizontal alignment of the MoS2 layers. We also analytically calculated the depolarization ratio for a thin MoS2 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.