For two-dimensional (2D) layered semiconductors, control over atomic defects and understanding of their electronic and optical functionality represent major challenges towards developing a mature semiconductor technology using such materials. Here, we correlate generation, optical spectroscopy, atomic resolution imaging, and ab initio theory of chalcogen vacancies in monolayer MoS2. Chalcogen vacancies are selectively generated by in-vacuo annealing, but also focused ion beam exposure. The defect generation rate, atomic imaging and the optical signatures support this claim. We discriminate the narrow linewidth photoluminescence signatures of vacancies, resulting predominantly from localized defect orbitals, from broad luminescence features in the same spectral range, resulting from adsorbates. Vacancies can be patterned with a precision below 10 nm by ion beams, show single photon emission, and open the possibility for advanced defect engineering of 2D semiconductors at the ultimate scale.

We investigate the conditions yielding plasmon-exciton strong coupling at the single emitter level in the gap between two metal nanoparticles. A quasi-analytical transformation optics approach is developed that makes possible a thorough exploration of this hybrid system incorporating the full richness of its plasmonic spectrum. This allows us to reveal that by placing the emitter away from the cavity center, its coupling to multipolar dark modes of both even and odd parity increases remarkably. This way, reversible dynamics in the population of the quantum emitter takes place in feasible implementations of this archetypal nanocavity.PACS numbers: 73.20. Mf, 42.50.Nn, 71.36.+c Plasmon-exciton-polaritons (PEPs) are hybrid lightmatter states that emerge from the electromagnetic (EM) interaction between surface plasmons (SPs) and nearby quantum emitters (QEs) [1,2]. Crucially, PEPs only exist when these two subsystems are strongly coupled, i.e., they exchange EM energy coherently in a time scale much shorter than their characteristic lifetimes. Recently, much attention has focused on PEPs, since they combine the exceptional light concentration ability of SPs with the extreme optical nonlinearity of QEs. These two attributes makes them promising platforms for the next generation of quantum nanophotonic components [3].A quantum electrodynamics description of plasmonic strong coupling of a single QE has been developed for a flat metal surface [4], and isolated [5,6] and distant nanoparticles [7][8][9], where SP hybridization is not fully exploited. From the experimental side, in recent years, PEPs have been reported in emitter ensembles [10][11][12][13], in which excitonic nonlinearities are negligible [14][15][16]. Only very recently, thanks to advances in the fabrication and characterization of large Purcell enhancement nanocavities [17][18][19], far-field signatures of plasmonexciton strong coupling for single molecules have been reported experimentally [20].In this Letter, we theoretically investigate the plasmonic coupling of a single emitter in a paradigmatic cavity: the nanometric gap between two metallic particles [13,19,20]. We consider spherical-shaped nanoparticles, and develop a transformation optics (TO) [21,22] approach that fully accounts for the rich EM spectrum that originates from SP hybridization across the gap. Our method, which is the first application of TO concepts to treat quantum optical phenomena, yields quasianalytical insight into the Wigner-Weisskopf problem [23] for these systems, and enables us to reveal the prescriptions that nanocavities must fulfil to support single QE PEPs. (1) level system (with transition frequency ω E and z-oriented arXiv:1605.09443v2 [cond-mat.mes-hall]

Polymethine dyes are linear π-conjugated compounds with an odd number of carbons that display a much greater delocalization in comparison to polyenes that have an even number of carbon atoms in their main chain. Herein, we perform scanning tunneling microscope based break-junction measurements on a series of three cyanine dyes of increasing length. We demonstrate, at the single molecule level, that these short chain polymethine systems exhibit a substantially smaller decay in conductance with length (attenuation factor β = 0.04 Å) compared to traditional polyenes (β ≈ 0.2 Å). Furthermore, we show that by changing solvent we are able to shift the β value, demonstrating a remarkable negative β value, with conductance increasing with molecular length. First principle calculations provide support for the experimentally observed near-uniform length dependent conductance and further suggest that the variations in β with solvent are due to solvent-induced changes in the alignment of the frontier molecular orbitals relative to the Fermi energy of the leads. A simplified Hückel model suggests that the smaller decay in conductance correlates with the smaller degree of bond order alternation present in polymethine compounds compared to polyenes. These findings may enable the design of molecular wires without a length-dependent decay for efficient electron transport at the nanoscale.

Ferrocenes are ubiquitous organometallic building blocks that comprise a Fe atom sandwiched between two cyclopentadienyl (Cp) rings that rotate freely at room temperature. Of widespread interest in fundamental studies and real-world applications, they have also attracted some interest as functional elements of molecular-scale devices. Here we investigate the impact of the configurational degrees of freedom of a ferrocene derivative on its single-molecule junction conductance. Measurements indicate that the conductance of the ferrocene derivative, which is suppressed by two orders of magnitude as compared to a fully conjugated analog, can be modulated by altering the junction configuration. Ab initio transport calculations show that the low conductance is a consequence of destructive quantum interference effects that arise from the hybridization of metal-based d-orbitals and the ligand-based π-system. By rotating the Cp rings, the hybridization, and thus the quantum interference, can be mechanically controlled, resulting in a conductance modulation that is seen experimentally. File list (2) download file view on ChemRxiv ferrocene_main.pdf (0.97 MiB) download file view on ChemRxiv ferrocene_SI.pdf (8.17 MiB) Mechanically-tunable Quantum Interference in Ferrocene-based Single-Molecule Junctions