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 notion that halide perovskite crystals (ABX , where X is a halide) exhibit unique structural and optoelectronic behavior deserves serious scrutiny. After decades of steady and half a decade of intense research, the question which attributes of these materials are unusual, is discussed, with an emphasis on the identification of the most important remaining issues. The goal is to stimulate discussion rather than to merely present a community consensus.

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.

Time-resolved, pulsed excitation methods are widely used to deduce optoelectronic properties of semiconductors, including now also Halide Perovskites (HaPs), especially transport properties. However, as yet no evaluation of their amenability and justification for the use of the results for the above-noted purposes has been reported. To check if we can learn from pulsed measurement results about steadystate phototransport properties, we show here that, although pulsed measurements can be useful to extract information on the recombination kinetics of HaPs, great care should be taken. One issue is that no changes in the material are induced during or as a result of the excitation, and another one concerns in how far pulsed excitation-derived data can be used to find relevant steady-state parameters. To answer the latter question, we revisited pulsed excitation, and propose a novel way to compare between pulsed and steady state measurements at different excitation intensities. We performed steady-state photoconductivity and ambipolar diffusion length measurements, as well as pulsed TR-MC and TR-PL measurements as function of excitation intensity on the same samples of different MAPbI 3 thin films, and find good quasi-quantitative agreement between the results, explaining them with a generalized single level recombination model that describes the basic physics of phototransport of HaP absorbers.Moreover, we find the first experimental manifestation of the boundaries between several effective recombination regimes that exist in HaPs, by analyzing their phototransport behavior as a function of excitation intensity.

Bound quasiparticles, negatively charged trions and neutral excitons are associated with the direct optical transitions at the K-points of the Brillouin zone for monolayer MoS 2 . The change in the carrier concentration, surrounding dielectric constant, and defect concentration can modulate the photoluminescence and Raman spectra. Here, we show that exposing the monolayer MoS 2 in air to a modest laser intensity for a brief period of time enhances simultaneously the photoluminescence intensity associated with both trions and excitons, together with ∼3 to 5 times increase of the Raman intensity of first-order and second-order modes. The simultaneous increase of photoluminescence from trions and excitons cannot be understood based only on known scenario of depletion of electron concentration in MoS 2 by adsorption of O 2 and H 2 O molecules. This is explained by laser-induced healing of defect states resulting in reduction of nonradiative Auger processes. This laser healing is corroborated by an observed increase of intensity of both the first-order and second-order longitudinal acoustic Raman modes at the M-point of Brillouin zone by a factor of ∼3 to 5. The A 1g mode hardens by ∼1.4 cm À1, whereas the E 1 2g mode softens by ∼1 cm À1. The second-order longitudinal acoustic Raman mode at the M-point of Brillouin zone at ∼440 cm À1 shows an increase in wavenumber by ∼8 cm À1 with laser exposure. These changes are a combined effect of change in electron concentrations and oxygen-induced lattice displacements.