During this project we have achieved several goals:
- First, we negotiated, purchased, and installed a unique instrumentation, namely a low temperature scanning tunneling microscope (STM) specifically designed for optical measurements. This instrument, whose main part was imported from Japan, is a delicate tool whose handling is not easy. Formation of the team to its use constituted an important part of the initial period. Eventually, whereas the core of the STM is commercial, the external optical setup allowing to record the emitted light is home made, and required an important time investment over the whole project duration. Altogether, this was a challenging technical work, but the instrument was rapidly running nicely, allowing to perform all the announced types of optical spectroscopies with ultimate spatial resolution.
- As announced, we investigated the fundamental atomic-scale properties of single-photon sources announced in the proposal. Here, several axes were run in parallel:
(i) On one side, we started to explore the STM-induced fluorescence properties of individual chromophores which constitute natural candidates for single-photon sources. We established the concept of hyper-resolved fluorescence microscopy and provided “optical images” with sub-nanometric resolution. We also investigated the influence of the close environment of the source on its fluorescence characteristic. Hence, the role of the plasmon-chromophore coupling and the influence of different adsorption sites of the chromophore are demonstrated. We realized early the first time-correlated measurements of the project. These measurements, that constitutes one of the main technical challenge of APOGEE, were used to reveal first the millisecond diffusion of protons at the heart of the molecule, a process known as tautomerization. Overall, this work as led to a first publication in Nature Nanotechnology (2020). Results obtained in the last months of the project, showed that we can extend the millisecond time resolution of these early measurement to ~100 ps, which eventually enabled us to identify the quantum nature of those sources (arXiv:2402.17536). At the end of the project Also, the impact of other sources in the close environment of the sources has revealed some unexpected results that we have explored in many details. We discovered that, at close distances, those sources are coupled, and that energy can be transferred from one to the next. This a led to a publication in Nature Chemistry (2021). Eventually we published two fundamental articles where we explain in many details how we can reach sub-nanometer scale resolution in fluorescence maps and what it means (PRX 2022, PRL2023)
(ii) On the second side, we explored the properties of single-photon sources embedded in 1D organic structures (graphene nanoribbons) and 2D transition metal dichalcogéneides (2D-TMD). Fabricating those samples in a way that allows their study in a low-temperature STM environment is challenging in its own, and was the focus of an important part of project. This eventually led to two seminal publications (Science 2023, and Nature Materials 2023) where we demonstrated for the first time that STML can be used to image and probe fluorescence sources in those two materials with ultimate spatial resolution.
