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 structural stability and diversity of elemental boron layers are evaluated by treating them as pseudoalloy B(1-x)[hexagon](x), where [hexagon] is a vacancy in the close-packed triangular B lattice. This approach allows for an elegant use of the cluster expansion method in combination with first-principles density-functional theory calculations, leading to a thorough exploration of the configurational space. A finite range of compositions x is found where the ground-state energy is essentially independent of x, uncovering a variety of stable B-layer phases (all metallic) and suggesting polymorphism, in stark contrast to graphene or hexagonal BN.

Interfaces play a key role in low dimensional materials like graphene or its boron nitrogen analog, white graphene. The edge energy of hexagonal boron nitride (h-BN) has not been determined as its lower symmetry makes it difficult to separate the opposite B-rich and N-rich zigzag sides. We report unambiguous energy values for arbitrary edges of BN, including the dependence on the elemental chemical potentials of B and N species. A useful manifestation of the additional Gibbs degree of freedom in the binary system, this dependence offers a way to control the morphology of pure BN or its carbon inclusions and to engineer their electronic and magnetic properties.

We investigate the possibility of band structure engineering in the recently predicted 2D layered form of blue phosphorus via an electric field (Ez) applied perpendicular to the layer(s). Using density functional theory, we study the effect of a transverse electric field in monolayer, as well as three differently stacked bilayer structures of blue phosphorus. We find that, for Ez > 0.2 V/Å the direct energy gap at the Γ point, which is much larger than the default indirect band gap of mono-and bilayer blue phosphorus, decreases linearly with the increasing electric field; becomes comparable to the default indirect band gap at Ez ≈ 0.45 (0.35) V/Å for monolayer (bilayers) and decreases further until the semiconductor to metal transition of 2D blue phosphorus takes place at Ez ≈ 0.7 (0.5) V/Å for monolayer (bilayers). Calculated values of the electron and hole effective masses along various high symmetry directions in the reciprocal lattice suggests that the mobility of charge carriers is also influenced by the applied electric field.

T wo-dimensional materials have drawn tremendous attention in the recent past in terms of both interesting fundamental physics and possible applications in future generation devices. Graphene and hexagonal boron nitride (h-BN) are the two most promising candidates for this purpose. 1À3 Single layers of graphene and h-BN have been fabricated and found to be stable at room temperature. 4À8 The most significant difference between the two isostructured materials lies in their electrical conductivity. Whereas graphene is a semimetal and a very good conductor, 1,9 BN is an insulator (band gap ∼ 6 eV), 10 which limits their applications in electronic devices. This void can be bridged by combining graphene and BN to make semiconducting material with a stoichiometry of B x C y N z . 11 Other than the solid solution of B, C, and N, it is also necessary to explore the possibility of fabricating a grapheneÀBN composite material, where the two phases coexist separately, but in the same plane. Such a novel composite is the focus of the present work.Free standing nanostructures, such as nanoribbons 12,13 and quantum dots 14 (QDs) of graphene, have been discussed extensively in the literature. The effect of electron confinement leads to size-dependent electronic properties in graphene nanostructures. Interestingly, properties of graphene nanostructures are also dependent on the edge shapes, namely, zigzag (ZZ) and armchair (AC). 1 For example, the tight binding model predicts ZZ and AC nanoribbons to be metallic and seimconducting, respectively. 1,15 Density functional theory-based calculations further show that ZZ edges are spin-polarized and corresponding nanoribbons are also semiconducting in nature. 12À14 Similar properties have been reported for graphene nanoroads 16 and QDs 17 embedded in graphane. Fundamentally free-standing and graphane-embedded nanostructures of graphene are similar. Whereas electrons are