Hydrogen is the smallest and one of the most reactive atoms. H diffuses easily in solids and interacts strongly with the electronic charge distribution. Furthermore, H is present in most growth processes and device mass-production steps. For these reasons, a great interest has been focused on the effects of H in insulators and semiconductors. In particular, the deliberate and controllable incorporation of H in crystals is extremely important since, on the one side, it allows understanding the best approaches to eventually cage H in solids and, on the other hand, it represents a means to modify on demand the electronic and structural properties of materials.
In this proposal, we apply for the purchase of a low-energy ion Kaufman source. This kind of system is particularly suited for studying the effects of H in solids thanks to the low energy (10-100 eV) of the ion beams employed. Indeed, such low energies increase considerably the cross-section of interaction between the ions and unpaired chemical bond distribution in the crystal and facilitate H incorporation. The Kaufman source will be used on a broad range of materials addressing the interests of many research groups. The aim is to investigate the electronic and optoelectronic properties of quantum materials (such as graphene, transition metal dichalcogenides, hexagonal boron nitride) and their heterostructures, of excitonic and topological insulators, and to augment the H storage capability in solid matrices (like metal hydrides and metal-organic frameworks) for energy applications. Moreover, the study of H and deuterium loading on single layer graphene is also relevant to predict the capability to load tritium on graphene - a key element in the design of next generation experiments to measure the mass of the electron neutrino or for a future observatory of the cosmic neutrino background, a messenger from the very early Universe.
The Kaufman source is an incredibly versatile scientific instrument that allows to incorporate relatively large amounts of H while minimizing sample damage. Thanks to its small footprint and to its compatibility with standard vacuum systems, the Kaufman source can be installed in and moved to different laboratories. The Kaufman source we intend to purchase allows varying the ion beam energy from 10 to 1000 eV with a high degree of reproducibility and control, resulting in a dose of incorporated H varying over 5 orders of magnitude. These parameters are typically hard to control by other systems, such as plasma sources or electrochemical methods [Pan91]. Furthermore, Kaufman sources are configured to have the region of gas ionization physically separated from the target, avoiding exposure of the samples to intense and potentially damaging electric fields. This turns out to be especially critical in ultra-thin samples, such as 2D crystals, or in advanced insulators, whose surface properties are pivotal for the observation of fundamental physical phenomena, such as dissipation-less currents and bosonic condensation. We'll first test the source in the coordinator's lab (where optimal vacuum levels of 10^-7 mbar can be reached) on materials of interest to the participants of this proposal and extended to collaborations with other groups. Afterwards, the Kaufman source will be installed in the forming Materials Laboratory associated to the Centro Amaldi research center. The activity of the Materials Laboratory runs in parallel with the development of the next generation mirrors to be installed in the interferometer of the Virgo gravitational-wave detector. This accounts for the interest of Ettore Maiorana and Ernesto Placidi in the use of the Kaufman source as a possible subsidiary means to investigate the reflectivity of the oxides employed for the mirrors used in Virgo. The core of the laboratory is a multi-purpose ultra-high-vacuum chamber (UHV, 10^-11 mbar) equipped with an x-ray photon-emission spectroscopy (XPS) setup featuring tens of micron spatial resolution. The facile integration of the Kaufman source in this system will permit to characterize in-situ the electronic states of 2D materials and of the surface of insulators before, during and after hydrogen irradiation. This is quite an important innovation, since H-treated materials are characterized typically ex-situ under conditions obviously much different from those established during exposure to H. We plan indeed also to use electron spectroscopy measurements during exposure to the ion beams. This will be extremely interesting. For instance, in the case of hydrogenated TMDs, this will provide us with valuable information on the ongoing processes (in first place the HER) leading to the dome blistering. Most importantly, in addition to XPS characterization tools, this system will also integrate a UHV micro-Raman and micro-PL setups, enabling us to have an in-situ picture of the evolution of the optical, vibrational and electronic properties of the samples with sub-micron spatial resolution at temperatures varying from ambient down to 10 K. As mentioned above, the effects of H on the material's properties will be studied avoiding the unavoidable contaminants that plague conventional, ex-situ experiments. This is an unprecedented experimental advancement. It will permit indeed to understand in detail and without external perturbations the physical and chemical mechanisms underlying many important phenomena like catalysis, atomic layer exfoliation, and semiconductor and topological insulator surface reconstruction, to cite a few examples of high scientific topicality.