Electrically driven molecular light emitters are considered to be one of the promising candidates as single-photon sources. However, it is yet to be demonstrated that electrically driven single-photon emission can indeed be generated from an isolated single molecule notwithstanding fluorescence quenching and technical challenges. Here, we report such electrically driven single-photon emission from a well-defined single molecule located inside a precisely controlled nanocavity in a scanning tunneling microscope. The effective quenching suppression and nanocavity plasmonic enhancement allow us to achieve intense and stable single-molecule electroluminescence. Second-order photon correlation measurements reveal an evident photon antibunching dip with the single-photon purity down to g (2)(0) = 0.09, unambiguously confirming the single-photon emission nature of the single-molecule electroluminescence. Furthermore, we demonstrate an ultrahigh-density array of identical single-photon emitters.

The energy and charge transfer dynamics of directly coupled Au-CdSe hybrid nanocrystals have been studied using time-resolved photoluminescence (PL) techniques. The PL of such nanohybrids was found to be quenched dramatically compared to that of both CdSe quantum dots and mixtures of CdSe quantum dots with Au nanoparticles. Fluorescence decay curves of the Au-CdSe nanohybrids show three distinct decay channels with the fastest one associated with the transfer of electrons from the CdSe portion to the Au portion. The holes on the CdSe portion created by such charge transfer were then quickly taken away by the solution, while the electrons on the Au portion slowly leaked into the solution as well, thus serving as a reductant for redox reactions. Using a model reaction based on the reduction of methylene blue by the leaking electrons, our photocatalytic experiments indicate that the electrons can be temporarily retained in the Au portion (most likely at the Au-capping agent interface) for a dramatically long timescale, up to 100 min. Finally, by merging all of the observations in the time-resolved PL measurements, we were able to figure out a relatively complete picture of charge transfer and retention in the Au-CdSe nanohybrids. This picture is expected to guide researchers in designing modern photocatalysts and solar cells constructed from nanoscale metal-semiconductor hybrids.

We provide an overview of the development of a merged system of low-temperature ultrahigh-vacuum scanning tunneling microscope (STM) with photon collection and detection units for optical imaging at the nanoscale. Focusing on our own work over the past ten years, the paper starts from a brief introduction of the STM induced luminescence (STML) technique and the challenge for nanoscale optical imaging, and then describes the design and instrumentation on the photon collection and detection system. The powerful potentials of the technique are illustrated using several selected examples from STML to tip enhanced Raman scattering that are mainly related to photon mapping. Such photon maps could reveal not only the local electromagnetic properties and the nature of optical transitions in the junction, but also exhibit spatial imaging resolution down to sub-molecular and sub-nanometer scale. The paper is concluded with a brief overlook on the future development of the STML technique.

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