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...

The non-trivial electronic topology of a topological insulator is so far known to display signatures in a robust metallic state at the surface. Here, we establish vibrational anomalies in Raman spectra of the bulk that signify changes in electronic topology: an E 2 g phonon softens unusually and its linewidth exhibits an asymmetric peak at the pressure induced electronic topological transition (ETT)in Sb 2 Se 3 crystal. Our first-principles calculations confirm the electronic transition from band to topological insulating state with reversal of parity of electronic bands passing through a metallic state at the ETT, but do not capture the phonon anomalies which involve breakdown of adiabatic approximation due to strongly coupled dynamics of phonons and electrons. Treating this within a four-band model of topological insulators, we elucidate how non-adiabatic renormalization of phonons constitutes readily measurable bulk signatures of an ETT, which will facilitate efforts to develop topological insulators by modifying a band insulator.

Synchrotron-based high-pressure x-ray diffraction measurements indicate that compressibility, a fundamental materials property, can have a size-specific minimum value. The bulk modulus of nanocrystalline titania has a maximum at particle size of 15 nm. This can be explained by dislocation behavior because very high dislocation contents can be achieved when shear stress induced within nanoparticles counters the repulsion between dislocations. As particle size decreases, compression increasingly generates dislocation networks ͑hardened by overlap of strain fields͒ that shield intervening regions from external pressure. However, when particles become too small to sustain high dislocation concentrations, elastic stiffening declines. The compressibility has a minimum at intermediate sizes.

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