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Nat Commun ; 10(1): 3283, 2019 Jul 23.
Artigo em Inglês | MEDLINE | ID: mdl-31337765


Control of atomic-scale interfaces between materials with distinct electronic structures is crucial for the design and fabrication of most electronic devices. In the case of two-dimensional materials, disparate electronic structures can be realized even within a single uniform sheet, merely by locally applying different vertical gate voltages. Here, we utilize the inherently nano-structured single layer and bilayer graphene on silicon carbide to investigate lateral electronic structure variations in an adjacent single layer of tungsten disulfide (WS2). The electronic band alignments are mapped in energy and momentum space using angle-resolved photoemission with a spatial resolution on the order of 500 nm (nanoARPES). We find that the WS2 band offsets track the work function of the underlying single layer and bilayer graphene, and we relate such changes to observed lateral patterns of exciton and trion luminescence from WS2.

Langmuir ; 31(35): 9700-6, 2015 Sep 08.
Artigo em Inglês | MEDLINE | ID: mdl-26308879


We present a method for synthesizing large area epitaxial single-layer MoS2 on the Au(111) surface in ultrahigh vacuum. Using scanning tunneling microscopy and low energy electron diffraction, the evolution of the growth is followed from nanoscale single-layer MoS2 islands to a continuous MoS2 layer. An exceptionally good control over the MoS2 coverage is maintained using an approach based on cycles of Mo evaporation and sulfurization to first nucleate the MoS2 nanoislands and then gradually increase their size. During this growth process the native herringbone reconstruction of Au(111) is lifted as shown by low energy electron diffraction measurements. Within the MoS2 islands, we identify domains rotated by 60° that lead to atomically sharp line defects at domain boundaries. As the MoS2 coverage approaches the limit of a complete single layer, the formation of bilayer MoS2 islands is initiated. Angle-resolved photoemission spectroscopy measurements of both single and bilayer MoS2 samples show a dramatic change in their band structure around the center of the Brillouin zone. Brief exposure to air after removing the MoS2 layer from vacuum is not found to affect its quality.

Nano Lett ; 12(12): 6187-91, 2012 Dec 12.
Artigo em Inglês | MEDLINE | ID: mdl-23131144


In this Letter we report a comparative study, in the infrared regime, of surface plasmon polariton (SPP) propagation in epitaxially grown Ag films and in polycrystalline Ag films, all grown on Si substrates. Plasmonic resonance features are analyzed using extraordinary optical transmission (EOT) measurements, and SPP band structures for the two dielectric/metal interfaces are investigated for both types of film. At the Si/Ag interface, EOT spectra show almost identical features for epitaxial and polycrystalline Ag films and are characterized by sharp Fano resonances. On the contrary, at the air/Ag interface, dramatic differences are observed: while the epitaxial film continues to exhibit sharp Fano resonances, the polycrystalline film shows only broad spectral features and much lower transmission intensities. In corroboration with theoretical simulations, we find that surface roughness plays a critical role in SPP propagation for this wavelength range.

Science ; 337(6093): 450-3, 2012 Jul 27.
Artigo em Inglês | MEDLINE | ID: mdl-22837524


A nanolaser is a key component for on-chip optical communications and computing systems. Here, we report on the low-threshold, continuous-wave operation of a subdiffraction nanolaser based on surface plasmon amplification by stimulated emission of radiation. The plasmonic nanocavity is formed between an atomically smooth epitaxial silver film and a single optically pumped nanorod consisting of an epitaxial gallium nitride shell and an indium gallium nitride core acting as gain medium. The atomic smoothness of the metallic film is crucial for reducing the modal volume and plasmonic losses. Bimodal lasing with similar pumping thresholds was experimentally observed, and polarization properties of the two modes were used to unambiguously identify them with theoretically predicted modes. The all-epitaxial approach opens a scalable platform for low-loss, active nanoplasmonics.