Characterization of MoS2 flakes

Using Tip-Enhanced Optical Spectroscopic (TEOS)

A flake of MoS2: the TEPL shift image derived from fitting the PL peak through regression analysis.
(a) Curve Fitting of PL signal and (b) TEPL-shift image.

Molybdenum disulfide (MoS2) is a promising semiconducting Transition Metal Dichalcogenide (TMD) 2D nano-material for next generation photovoltaic solar cells, optoelectronic circuits and sensors due to its great excitonic recombination property, high carrier mobility and low leakage current.

One of the advantages of two dimensional (2D) TMDs, when compared to graphene, comes from quantum confinement, enabling the indirect-to-direct bandgap transition as a function of number of individual layers. Nano-scale characterization is needed for the understanding necessary to engineer nanodevices integrating monolayer MoS2.

Tip-enhanced optical spectroscopies (TEOS) based on the amplification of signal from the nano-region under the tip enables such nano-characterization. In the case of 2D TMD, Tip-Enhanced Photoluminescence (TEPL) is capable of revealing variation in emission within a submicron size flake. Complementary morphological, chemical, and electronic structure information may be acquired simultaneously and with nanometer spatial resolution through AFM imaging, Tip Enhanced Raman spectroscopy (TERS) and Kelvin probe measurements, respectively. 

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Characterization of MoS2 Flakes
DescriptionBoth TEPL and TERS images are well correlated with AFM morphological images obtained simultaneously, and all are consistent in revealing the nature (number of layers) of MoS2 flakes. Upon deconvolution, the TEPL signal is even capable of revealing local inhomogeneities within a MoS2 flake of 100 nm size. Kelvin probe measurement supports TEPL and TERS measurements and adds to the power of such tip-enhanced combinative tools. TEOS characterization of 2D materials is likely to contribute to further deployment of these materials into commercial products through a better understanding of their electrical and chemical properties at the nanoscale.
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