Raman micro-spectroscopy is well suited for studying a variety of properties, including chemical, magnetic, electronic, symmetry, and layer orientation in 2D materials. Many interesting phenomena emerge at cryogenic temperatures, and often, additional or new information about a sample can be obtained with temperature dependent measurements. We have demonstrated efficient characterization of 2D materials with a closed-cycle micro-Raman spectroscopy and photoluminescence platform capable of maintaining high spatial resolution and collection efficiency across the entire temperature range (4K – 600K). The system overcomes challenges associated with variable temperature sample drift and optical alignment by incorporating a low thermal mass sample stage, in-vacuum high NA objective, and bridging optics to a low astigmatism, broadband spectrograph. The Raman spectra for single and double layer graphene were collected from 500K to 4K as a platform benchmark standard. 1- and 2-layer graphene can easily be differentiated by the G band to 2D band ratio. The expected monotonic dependence of the line width of the G band with respect to decreasing temperature, from 20cm-1 at 300K to 10cm-1 at 5K was observed for the 1-layer graphene sample. The transition metal dichalcogenides studied were MoS2 and WSe2 and temperature dependent Raman spectra were collected from 300K to 4K. Concurrently, the PL of WSe2 was collected from 300K down to 4K. The MoS2 on sapphire sample with decreasing temperature exhibits line sharpening FWHM (5cm-1 at 300K to 2.8cm-1 at 5K) as well as the predicted E12g band shifting from ~382 to ~386cm-1. Raman spectra of WSe2 thin films are very interesting because of the overlap of the E12g and A1g modes across the temperature range and the observed modes at 251cm-1 and 260cm-1 at room temperature. WSe2 also has a substrate mediated temperature dependent steady-state PL. On sapphire the ratios of the 3 emitting states for WSe2, localized exciton, free exciton and trion, change as the temperature is varied from 4K to 300K.

Variable temperature Raman micro-spectroscopy as a nanoanalytical tool for studying 2D materials

2019 Spring National Meeting

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