The combined spectral image is rendered through a color coded plot of Raman (green) and photoluminescence (red) signal strength over the corresponding color bracketed Raman shift positions shown in the upper traces. The perimeter of the crystal appears green because of the absence of photoluminescence at that location. The separate Raman and photoluminescence images show the spatially varying differences in solid state structure as revealed through vibrational and electronic spectroscopies.
Graphene is probably the most well-known of the emerging class of materials known as two-dimensional crystals. These materials are constituted by monolayer to few-layered structures.
In recent years, new inorganic two dimensional materials have emerged including MoS2, MoSe2, WS2, WSe2 among others. These materials have attracted significant interest because of special electronic, optical and optoelectronic properties in the monolayer to few-layer forms that are different from those manifest by the bulk form.
You may have observed the spatially varying colors in reflected white light images of 2D crystals, and so there have been developments to use optical microscopy to rapidly identify the number of molecular layers that make up the two-dimensional crystal.
Previously, we reported on the use of resonance Raman and photoluminescence spectroscopy and imaging of few-layer MoS2 to identify spatial variation in the number of layers and strain. In this Application Note, we focus on Raman and photoluminescence (PL) imaging for the characterization of 2D WS2 crystals.
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