For quasi-freestanding 2H-TaS 2 in monolayer thickness grown by in situ molecular beam epitaxy on graphene on Ir(111), we find unambiguous evidence for a charge density wave close to a 3 × 3 periodicity. Using scanning tunneling spectroscopy, we determine the magnitude of the partial charge density wave gap. Angle-resolved photoemission spectroscopy, complemented by scanning tunneling spectroscopy for the unoccupied states, makes a tight-binding fit for the band structure of the TaS 2 monolayer possible. As hybridization with substrate bands is absent, the fit yields a precise value for the doping of the TaS 2 layer. Additional Li doping shifts the charge density wave to a 2 × 2 periodicity. Unexpectedly, the bilayer of TaS 2 also displays a disordered 2 × 2 charge density wave. Calculations of the phonon dispersions based on a combination of density-functional theory, density-functional perturbation theory, and many-body perturbation theory enable us to provide phase diagrams for the TaS 2 charge density wave as functions of doping, hybridization and interlayer potentials, and offer insight into how they affect lattice dynamics and stability. Our theoretical considerations are consistent with the experimental work presented and shed light on previous experimental and theoretical investigations of related systems.

We employ ultra-high vacuum (UHV) Raman spectroscopy in tandem with angle-

Graphene nanoribbons (GNRs) are promising materials for the production of nanoscale devices. [1][2][3][4][5] In contrast to graphene, GNRs can be metallic or semiconducting with a tunable band gap that depends on the ribbon width and the edge configuration. [6][7][8][9][10] Carbon nanotubes are a similar 1D material but despite a 20 year history, the large-area synthesis of monochiral carbon nanotubes remains challenging. [11] On the other hand, nanoribbons can be fabricated with atomically controlled precision thanks to the bottom-up nanofabrication techniques. [12][13][14][15][16][17][18][19][20] Thus, GNRs combine the best attributes of the nanotube and graphene worlds.Engineering of GNR-based optoelectronic devices requires an understanding of the charge transfer effect on the A semiconductor-to-metal transition in N = 7 armchair graphene nanoribbons causes drastic changes in its electron and phonon system. By using angle-resolved photoemission spectroscopy of lithium-doped graphene nanoribbons, a quasiparticle band gap renormalization from 2.4 to 2.1 eV is observed. Reaching high doping levels (0.05 electrons per atom), it is found that the effective mass of the conduction band carriers increases to a value equal to the free electron mass. This giant increase in the effective mass by doping is a means to enhance the density of states at the Fermi level which can have palpable impact on the transport and optical properties. Electron doping also reduces the Raman intensity by one order of magnitude, and results in relatively small (4 cm −1 ) hardening of the G phonon and softening of the D phonon. This suggests the importance of both lattice expansion and dynamic effects. The present work highlights that doping of a semiconducting 1D system is strikingly different from its 2D or 3D counterparts and introduces doped graphene nanoribbons as a new tunable quantum material with high potential for basic research and applications.

We report on the observation of photoluminescence (PL) with a narrow 18 meV peak width from molecular beam epitaxy grown MoS 2 on graphene/Ir(111). This observation is explained in terms of a weak graphene-MoS 2 interaction that prevents PL quenching expected for a metallic substrate. The weak interaction of MoS 2 with the graphene is highlighted by angle-resolved photoemission spectroscopy and temperature dependent Raman spectroscopy. These methods reveal that there is no hybridization between electronic states of graphene and MoS 2 and a different thermal expansion of graphene and MoS 2 . Molecular beam epitaxy grown MoS 2 on graphene is therefore an important platform for optoelectronics which allows for large area growth with controlled properties. arXiv:1809.01886v1 [cond-mat.mes-hall] 6 Sep 2018 Narrow photoluminescence peak of epitaxial MoS 2 on graphene/Ir (111)

Lateral heterojunctions of atomically precise graphene nanoribbons (GNRs) hold promise for applications in nanotechnology, yet their charge transport and most of the spectroscopic properties have not been investigated. Here, we synthesize a monolayer of multiple aligned heterojunctions consisting of quasi-metallic and wide-bandgap GNRs, and report characterization by scanning tunneling microscopy, angle-resolved photoemission, Raman spectroscopy, and charge transport. Comprehensive transport measurements as a function of bias and gate voltages, channel length, and temperature reveal that charge transport is dictated by tunneling through the potential barriers formed by wide-bandgap GNR segments. The current-voltage characteristics are in agreement with calculations of tunneling conductance through asymmetric barriers. We fabricate a GNR heterojunctions based sensor and demonstrate greatly improved sensitivity to adsorbates compared to graphene based sensors. This is achieved via modulation of the GNR heterojunction tunneling barriers by adsorbates.