Going Green with Nanophotonics Plasmons are optically induced collective electronic excitations tightly confined to the surface of a metal, with silver being the metal of choice. The subwavelength confinement offers the opportunity to shrink optoelectronic circuits to the nanometer scale. However, scattering processes within the metal lead to losses. Lu et al. (p. 450 ) developed a process to produce atomically smooth layers of silver, epitaxially grown on silicon substrates. A cavity in the silver layer is capped with a SiO insulating layer and an AlGaN nanorod was used to produce a low-threshold emission at green wavelengths.

The emergence of two-dimensional electronic materials has stimulated proposals of novel electronic and photonic devices based on the heterostructures of transition metal dichalcogenides. Here we report the determination of band offsets in the heterostructures of transition metal dichalcogenides by using microbeam X-ray photoelectron spectroscopy and scanning tunnelling microscopy/spectroscopy. We determine a type-II alignment between MoS2 and WSe2 with a valence band offset value of 0.83 eV and a conduction band offset of 0.76 eV. First-principles calculations show that in this heterostructure with dissimilar chalcogen atoms, the electronic structures of WSe2 and MoS2 are well retained in their respective layers due to a weak interlayer coupling. Moreover, a valence band offset of 0.94 eV is obtained from density functional theory, consistent with the experimental determination.

Experimental determinations of the electronic band structures in TMDs are quite non-trivial.Optical spectroscopies [1][2][3][4][5] are unsuitable to measure the quasi-particle band structures due to the existence of large exciton binding energies. Using angle resolved photoemission (ARPES), it is difficult to probe the conduction band structures 6,7 . In principle, scanning tunneling spectroscopy (STS) would be an ideal probe to determine both the valence and conduction band structures.However, the reported results have been controversial thus far, even for the determination of the quasi-particle band gaps [8][9][10] . As we will show, this is due primarily to the intriguing influence of the lateral momentum in the tunneling process, making certain critical points difficult to access in the conventional scanning tunneling spectroscopy acquired at a constant tip-to-sampledistance (Z). By using a comprehensive approach combining the constant Z and variable Z spectroscopies, as well as state-resolved tunneling decay constant measurements, we have shown 3 that detailed electronic structures, including quasi-particle gaps, critical point energy locations and their origins in the BZs in TMDs can be revealed.The TMD samples are grown using chemical vapor deposition (CVD) for WSe 2 11 or molecular beam epitaxy (MBE) for MoSe 2 on highly-oriented-pyrolytic-graphite (HOPG)substrates. In addition, MoSe 2 has also been grown on epitaxial bi-layer graphene to investigate the environmental influences on the quasi-particle band structures. Figures 1a and b show scanning tunneling microscopy (STM) images of MoSe 2 and WSe 2 , respectively. Due to a nearly 4:3 lattice match with the graphite, the TMD samples also show Moiré patterns with a periodicity of ~ 1nm. An example is shown as an inset in Fig. 1b, similar to those reported earlier 9 . In Fig. 1c we show a generic electronic structure for SL-TMD materials. We first discuss the result of MoSe 2 due to the availability of experimentally determined E vs. k dispersion in the valence band which can be used to cross-check with our results. 4Such a large difference in the decay constant is responsible for the difficulty in detecting the states near the VBM. The lack of the sensitivity in the constant-Z STS can be overcome by acquiring spectra at variable Z as described before 14 . Here we adopt a form of variable Z spectroscopy by performing STS at constant current. In this mode, as the sample bias is scanned across different thresholds, Z (the dependent variable) will respond automatically in order to keep the current constant. The differential conductivity (I/V) I is measured by using a lock-in amplifier (Fig. 2b). In the meantime, the acquired Z-value is used to deduce (Z/V) I which can be used to identify individual thresholds (Fig. 2c).For the valence band (left panel), the state at the  point also appears as a prominent peak in the (I/V) I spectrum. Moreover, spectroscopic features above  are observed due to the significant enhancement of the sensitivity. A shoul...

We report on the first demonstration of broadband tunable, single-mode plasmonic nanolasers (spasers) emitting in the full visible spectrum. These nanolasers are based on a single metal-oxide-semiconductor nanostructure platform comprising of InGaN/GaN semiconductor nanorods supported on an Al2O3-capped epitaxial Ag film. In particular, all-color lasing in subdiffraction plasmonic resonators is achieved via a novel mechanism based on a property of weak size dependence inherent in spasers. Moreover, we have successfully reduced the continuous-wave (CW) lasing thresholds to ultrasmall values for all three primary colors and have clearly demonstrated the possibility of "thresholdless" lasing for the blue plasmonic nanolaser.

Semiconductor heterostructures are fundamental building blocks for many important device applications. The emergence of two-dimensional semiconductors opens up a new realm for creating heterostructures. As the bandgaps of transition metal dichalcogenides thin films have sensitive layer dependence, it is natural to create lateral heterojunctions (HJs) using the same materials with different thicknesses. Here we show the real space image of electronic structures across the bilayer–monolayer interface in MoSe2 and WSe2, using scanning tunnelling microscopy and spectroscopy. Most bilayer–monolayer HJs are found to have a zig-zag-orientated interface, and the band alignment of such atomically sharp HJs is of type-I with a well-defined interface mode that acts as a narrower-gap quantum wire. The ability to utilize such commonly existing thickness terraces as lateral HJs is a crucial addition to the tool set for device applications based on atomically thin transition metal dichalcogenides, with the advantage of easy and flexible implementation.