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Strong electron-phonon interaction which limits electronic mobility of semiconductors can also have significant effects on phonon frequencies. The latter is the key to the use of Raman spectroscopy for nondestructive characterization of doping in graphene-based devices. Using in-situ Raman scattering from single layer MoS2 electrochemically top-gated field effect transistor (FET), we show softening and broadening of A1g phonon with electron doping whereas the other Raman active E 1 2g mode remains essentially inert. Confirming these results with first-principles density functional theory based calculations, we use group theoretical arguments to explain why A1g mode specifically exhibits a strong sensitivity to electron doping. Our work opens up the use of Raman spectroscopy in probing the level of doping in single layer MoS2-based FETs, which have a high on-off ratio and are of enormous technological significance.PACS numbers: 78.30.-j Discovery of graphene 1 stimulated an intense research activity due to interesting fundamental phenomena it exhibits as well as the techonological promise it holds in a broad range of applications ranging from sensors to nanoelectronics. Vanishing bandgap of a single layer graphene is a sort of a limitation in developing a graphene-based field effect transistor with a high on/off ratio. This has spurred efforts to modify graphene to open up a gap and towards development of other two dimensional materials like MoS 2 , WS 2 and boron nitride (BN), both experimentally and theoretically. Avenues to open up gap through modification of graphene include quantum confinement in nanoribbons 2 , surface functionalization 3 , applying electric field in the bilayer 4,5 , deposition of graphene on other substrates like BN 6,7 , and B or N substitutional doping 8 , which require fine control over the procedure of synthesis.In contast to graphene, single layer MoS 2 consisting of a hexagonal planar lattice of Mo atoms sandwiched between two similar lattices of S atoms (S-Mo-S structure) with intralayer covalent bonding is a semiconductor with a direct band gap of ∼ 1.8 eV, and is quite promising for FET devices with a high on-off ratio. It has been shown that the luminescence quantum yield of monolayer MoS 2 is higher than its bulk counterpart 9,10 .Recently a monolayer MoS 2 transistor 11 has been shown to exhibit an on-off ratio of ∼10 8 and electron mobility of ∼200 cm 2 /V-sec. These values are comparable to silicon based devices and make MoS 2 based devices worth exploring further. It is known that in a field effect transistor, carrier mobility is limited by scattering from phonons and the maximum current is controlled by hot phonons. Both these issues in a FET depend on the electron-phonon coupling (EPC). Raman spectroscopy has been very effective to probe EPC for single 12-14 and bilayer graphene 15-17 transistors by investigating the renormalization of the G and 2D modes as a function of carrier density.Recent layer-dependent Raman studies of single and few layers of MoS 2 18 have shown th...

We report resonant Raman scattering of MoS 2 layers comprising of single, bi, four and seven layers, showing a strong dependence on the layer thickness. Indirect band gap MoS 2 in bulk becomes a direct band gap semiconductor in the monolayer form. New Raman modes are seen in the spectra of single-and few-layer MoS 2 samples which are absent in the bulk. The Raman mode at 230 cm À1 appears for two, four and seven layers. This mode has been attributed to the longitudinal acoustic phonon branch at the M point (LA(M)) of the Brillouin zone. The mode at~179 cm À1 shows asymmetric character for a few-layer sample. The asymmetry is explained by the dispersion of the LA(M) branch along the Γ-M direction. The most intense spectral region near 455 cm À1 shows a layer-dependent variation of peak positions and relative intensities. The high energy region between 510 and 645 cm À1 is marked by the appearance of prominent new Raman bands, varying in intensity with layer numbers. Resonant Raman spectroscopy thus serves as a promising non invasive technique to accurately estimate the thickness of MoS 2 layers down to a few atoms thick.

We show the evolution of Raman spectra with a number of graphene layers on different substrates, SiO 2 /Si and conducting indium tin oxide (ITO) plate. The G mode peak position and the intensity ratio of G and 2D bands depend on the preparation of sample for the same number of graphene layers. The 2D Raman band has characteristic line shapes in single and bilayer graphene, capturing the differences in their electronic structure. The defects have a significant influence on the G band peak position for the single layer graphene: the frequency shows a blue shift up to 12 cm -1 depending on the intensity of the D Raman band, which is a marker of the defect density. Most surprisingly, Raman spectra of graphene on the conducting ITO plates show a lowering of the G mode frequency by ~ 6 cm -1 and the 2D band frequency by ~ 20 cm -1 . This red-shift of the G and 2D bands is observed for the first time in single layer graphene.

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