Observation of topologically protected states at crystalline phase boundaries in single-layer WSe2.

Transition metal dichalcogenide materials are unique in the wide variety of structural and electronic phases they exhibit in the two-dimensional limit. Here we show how such polymorphic flexibility can be used to achieve topological states at highly ordered phase boundaries in a new quantum spin Hall insulator (QSHI), 1T'-WSe2. We observe edge states at the crystallographically aligned interface between a quantum spin Hall insulating domain of 1T'-WSe2 and a semiconducting domain of 1H-WSe2 in contiguous single layers. The QSHI nature of single-layer 1T'-WSe2 is verified using angle-resolved photoemission spectroscopy to determine band inversion around a 120 meV energy gap, as well as scanning tunneling spectroscopy to directly image edge-state formation. Using this edge-state geometry we confirm the predicted penetration depth of one-dimensional interface states into the two-dimensional bulk of a QSHI for a well-specified crystallographic direction. These interfaces create opportunities for testing predictions of the microscopic behavior of topologically protected boundary states.

Substrate-induced enhancement of the chemical reactivity in metal-supported graphene.

Graphene is commonly regarded as an inert material. However, it is well known that the presence of defects or substitutional hetero-atoms confers graphene promising catalytic properties. In this work, we use first-principles calculations to show that it is also possible to enhance the chemical reactivity of a graphene layer by simply growing it on an appropriate substrate. Our comprehensive study demonstrates that, in strongly interacting substrates like Rh(111), graphene adopts highly rippled structures that exhibit areas with distinctive chemical behaviors. According to the local coupling with the substrate, we find areas with markedly different adsorption, dissociation and diffusion pathways for both molecular and atomic oxygen, including a significant change in the nature of the adsorbed molecular and dissociated states, and a dramatic reduction (∼60%) of the O2 dissociation energy barrier with respect to free-standing graphene. Our results show that the graphene-metal interaction represents an additional and powerful handle to tailor the graphene chemical properties with potential applications to nano patterning, graphene functionalization and sensing devices.

Unveiling the atomistic mechanisms for oxygen intercalation in a strongly interacting graphene-metal interface.

The atomistic mechanisms involved in the oxygen (O) intercalation in the strongly interacting graphene (G) on Rh(111) system are characterized in a comprehensive experimental and theoretical study, combining scanning tunneling microscopy and density functional theory (DFT) calculations. Experimental evidence points out that the G areas located just above the metallic steps of the substrate are the active sites for initializing the intercalation process when some micro-etching points appear after molecular oxygen gas exposure. These regions are responsible for both the dissociation of the oxygen molecules and the subsequent penetration to the G-metal interface. Unlike in other species, the DFT calculations exclude single-point defects as additional entry paths to the interface. After penetration, the intercalation proceeds inwards due to the high mobility of atomic oxygen at the interface following mid-height paths connecting the higher areas of the rippled graphene structure. At larger coverages, the accumulation of O atoms under the high areas increases the G-metal distance in the neighboring low areas, paving the way for the O incorporation and the G detachment that leads to the final O-(2 × 1) structure. Furthermore, our results show that these mechanisms are possible only at temperatures slightly lower than those in which graphene etching takes place.

Purely substitutional nitrogen on graphene/Pt(111) unveiled by STM and first principles calculations.

Nitrogen doping of graphene can be an efficient way of tuning its pristine electronic properties. Several techniques have been used to introduce nitrogen atoms on graphene layers. The main problem in most of them is the formation of a variety of C-N species that produce different electronic and structural changes on the 2D layer. Here we report on a method to obtain purely substitutional nitrogen on graphene on Pt(111) surfaces. A detailed experimental study performed in situ, under ultra-high vacuum conditions with scanning tunneling microscopy (STM), low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) of the different steps on the preparation of the sample, has allowed us to gain insight into the optimal parameters for this growth method, that combines ion bombardment and annealing. This experimental work is complemented by first-principles calculations and STM simulations that provide the variation of the projected density of states due to both the metallic substrate and the nitrogen atoms. These calculations enlighten the experimental findings and prove that the species found are graphitic nitrogen. This easy and effective technique leads to the possibility of playing with the amount of dopants and the metallic substrate to obtain the desired doping of the graphene layer.

Electronic Structure, Surface Doping, and Optical Response in Epitaxial WSe2 Thin Films.

High quality WSe2 films have been grown on bilayer graphene (BLG) with layer-by-layer control of thickness using molecular beam epitaxy. The combination of angle-resolved photoemission, scanning tunneling microscopy/spectroscopy, and optical absorption measurements reveal the atomic and electronic structures evolution and optical response of WSe2/BLG. We observe that a bilayer of WSe2 is a direct bandgap semiconductor, when integrated in a BLG-based heterostructure, thus shifting the direct-indirect band gap crossover to trilayer WSe2. In the monolayer limit, WSe2 shows a spin-splitting of 475 meV in the valence band at the K point, the largest value observed among all the MX2 (M = Mo, W; X = S, Se) materials. The exciton binding energy of monolayer-WSe2/BLG is found to be 0.21 eV, a value that is orders of magnitude larger than that of conventional three-dimensional semiconductors, yet small as compared to other two-dimensional transition metal dichalcogennides (TMDCs) semiconductors. Finally, our finding regarding the overall modification of the electronic structure by an alkali metal surface electron doping opens a route to further control the electronic properties of TMDCs.