We show that small π-conjugated molecules adopt a lying-down orientation when deposited on few-layer MoS2 with horizontally oriented layers. In contrast, for vertically aligned MoS2 layers, DIP molecules are arranged in a standing-up manner.
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.
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