Ab initio study of the (2 x 2) phase of barium on graphene

We present a first-principles density functional theory study on the structural, electronic and dynamical properties of a novel barium doped graphene phase. Low energy electron diffraction of barium doped graphene presents clear evidence of (2 x 2) spots induced by barium adatoms with BaC8 stoichiometry. First principles calculations reveals that the phase is thermodynamically stable but unstable to segregation towards the competitive BaC6 monolayer phase. The calculation of phonon spectrum confirms the dynamical stability of the BaC8 phase indicating its metastability, probably stabilized by doping and strain conditions due to the substrate. Barium induces a relevant doping of the graphene pi states and new barium-derived hole Fermi surface at the M-point of the (2 x 2) Brillouin zone. In view of possible superconducting phase induced by foreign dopants in graphene, we studied the electron-phonon coupling of this novel (2 x 2) obtaining lambda = 0.26, which excludes the stabilization of a superconducting phase

Environmental Control of Charge Density Wave Order in Monolayer 2H-TaS2

Contains fulltext : 208621.pdf (publisher's version ) (Open Access

Charge density wave phase of VSe2 revisited

Scanning tunneling microscopy and spectroscopy are used to image the charge density wave at the surface of cleaved VSe2 and to probe its local density of states at 5 K. The main features in the spectrum are linked to the contributions of the p-like and d-like bands of VSe2 found in angle-resolved photoemission spectroscopy and tight-binding calculations. Different from previous tunneling spectroscopy work, we find a narrow partial gap at the Fermi level that we associate with the charge density wave phase. The energy scale of the gap found in the experiment is in good agreement with the charge density wave transition temperature of VSe2, under the assumption of weak electron-phonon coupling, consistent with the Peierls model of Fermi surface nesting. The role of defects is investigated, which reveals that the partial gap in the density of states and hence the charge density wave itself is extremely stable, though the order, phase, and amplitude of the charge density waves on the surface are strongly perturbed by defects

Field-Effect Transistors Based on Networks of Highly Aligned, Chemically Synthesized N=7 Armchair Graphene Nanoribbons

We report on the experimental demonstration and electrical characterization of N = 7 armchair graphene nanoribbon (7-AGNR) field effect transistors. The back-gated transistors are fabricated from atomically precise and highly aligned 7-AGNRs, synthesized with a bottom-up approach. The large area transfer process holds the promise of scalable device fabrication with atomically precise nanoribbons. The channels of the FETs are approximately 30 times longer than the average nanoribbon length of 30 nm to 40 nm. The density of the GNRs is high, so that transport can be assumed well-above the percolation threshold. The long channel transistors exhibit a maximum I-ON/I-OFF current ratio of 87.5

Probing the origin of photoluminescence brightening in graphene nanoribbons

We measure the absolute absorbance of a single layer of seven atom wide armchair graphene nanoribbons and study the influence of laser-induced defects on the absorption spectrum of the ribbons. We find that the absorption spectrum shows a broad peak at approximately 2.4 eV that is attributed to excitonic transitions and a smaller peak at 1.77 eV. The low-energy peak is diminished when we induce defects in the material. Simultaneously the photoluminescence is significantly enhanced. We thus attribute the 1.77 eV spectral feature in the absorption spectrum to a quenching state, which energetically coincides with the emission. Our results clearly demonstrate the significance of this state in photoluminescence processes in the ribbons. We additionally measure the dependence of the generation of defects on the energy of the incident photons and the photoluminescence excitation spectrum. The photoluminescence excitation efficiency peaks at a higher photon energy than the maximum absorption, hinting at an efficient decay from higher energetic states to the emissive state

Photothermal Bottom-up Graphene Nanoribbon Growth Kinetics

We present laser-induced photothermal synthesis of atomically precise graphene nanoribbons (GNRs). The kinetics of photothermal bottom-up GNR growth are unravelled by in situ Raman spectroscopy carried out in ultrahigh vacuum. We photothermally drive the reaction steps by short periods of laser irradiation and subsequently analyze the Raman spectra of the reactants in the irradiated area. Growth kinetics of chevron GNRs (CGNRs) and seven atoms wide armchair GNRs (7-AGNRs) is investigated. The reaction rate constants for polymerization, cyclodehydrogenation, and interribbon fusion are experimentally determined. We find that the limiting rate constants for CGNR growth are several hundred times smaller than for 7-AGNR growth and that interribbon fusion is an important elementary reaction occurring during 7AGNR growth. Our work highlights that photothermal synthesis and in situ Raman spectroscopy are a powerful tandem for the investigation of on-surface reactions

Narrow photoluminescence and Raman peaks of epitaxial MoS2 on graphene/Ir(111)

We report on the observation of photoluminescence (PL) with a narrow 18 meV peak width from molecular beam epitaxy grown MoS2 on graphene/lr(1 1 1). This observation is explained in terms of a weak graphene-MoS2 interaction that prevents PL quenching expected for a metallic substrate. The weak interaction of MoS2 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 MoS2 as well as a different thermal expansion of both materials. Molecular beam epitaxy grown MoS2 on graphene is therefore an important platform for optoelectronics which allows for large area growth with controlled properties

Photodetection Using Atomically Precise Graphene Nanoribbons

In the search for high-sensitivity, low-noise, and high-bandwidth photodetectors, materials are a key ingredient. One- and two-dimensional materials are of particular interest in this area due to their extraordinary properties such as ballistic transport. Here, we demonstrate nanoscale photoconductive photodetectors using aligned atomically precise seven-atom wide armchair-edge graphene nanoribbons. The detector responsivity is 0.035 mAW(-1) at a bias voltage of 2 V. The dark current is below 30 pA for a bias voltage of 1.5 V, which is orders of magnitude lower than that of typical graphene photodetectors. The possibility to align the nanoribbons and to tune their optical and electronic properties by choice of ribbon width and edge structure enables nanoscale polarization-resolving photodetectors optimized for specific spectral ranges. Graphene nanoribbons with identical electronic and optical properties can be prepared on a large scale using bottom-up synthesis, making them a highly interesting material for electronics and optoelectronics

Comprehensive tunneling spectroscopy of quasifreestanding MoS2 on graphene on Ir(111)

We apply scanning tunneling spectroscopy to determine the band gaps of mono-, bi-, and trilayer MoS2 grown on a graphene single crystal on Ir(111). Besides the typical scanning tunneling spectroscopy at constant height, we employ two additional spectroscopic methods giving extra sensitivity and qualitative insight into the k vector of the tunneling electrons. Employing this comprehensive set of spectroscopic methods in tandem, we deduce a band gap of 2.53 +/- 0.08 eV for the monolayer. This is close to the predicted values for freestanding MoS2 and larger than is measured for MoS2 on other substrates. Through precise analysis of the comprehensive tunneling spectroscopy we also identify critical point energies in the mono- and bilayer MoS2 band structures. These compare well with their calculated freestanding equivalents, evidencing the graphene/Ir(111) substrate as an excellent environment upon which to study the many celebrated electronic phenomena of monolayer MoS2 and similar materials. Additionally, this investigation serves to expand the fledgling field of the comprehensive tunneling spectroscopy technique itself

Observation of Room-Temperature Photoluminescence Blinking in Armchair-Edge Graphene Nanoribbons

By enhancing the photoluminescence from aligned seven-atom wide armchair-edge graphene nanoribbons using plasmonic nanoantennas, we are able to observe blinking of the emission. The on-and off-times of the blinking follow power law statistics. In time-resolved spectra, we observe spectral diffusion. These findings together are a strong indication of the emission originating from a single quantum emitter. The room temperature photoluminescence displays a narrow spectral width of less than 50 meV, which is significantly smaller than the previously observed ensemble line width of 0.8 three optical transitions, which are energetically situated below the lowest bulk attribute the emission to transitions involving Tamm states localized at the end eV. From spectral excitonic state E(11)of the nanoribbon. time traces, we identify of the nanoribbons. We The photoluminescence from a single ribbon is strongly enhanced when its end is in the antenna hot spot resulting in the observed single molecule characteristics of the emission. Our findings illustrate the essential role of the end termination of graphene nanoribbons in light emission and allow us to construct a model for photoluminescence from nanoribbons