Theoretical characterization of 2D and 1D unconventional systems for future quantum technologies: from on-surface carbon networks to position-controlled point defects in semiconductors

Network formed by sp- and sp2-hybridized carbon atoms with high degrees of π conjugation, namely graphynes and graphdiynes, are stimulating intense research effort for their potential use in nanoelectronics, catalysis and photo-conversion, due to their high-charge carrier mobility and tunable band gaps and band structures [1, 2,3]. The significant advancements in the synthesis of these materials through on-surface assembly of molecular precursors opens the way to the design of novel carbon networks, displaying different geometries and physical properties. Nevertheless, the fabrication of extended ordered 2D monolayers, their characterization and the definition of protocols for their manipulations represent challenging steps in view of their applications. We present a study of sp-, sp2- and hybrid sp+sp2 2D systems, obtained by on-surface synthesis of brominated molecular precursors on Au(111) surface. Through ab-initio calculations within Density Functional Theory, in connection with recent Scanning Tunneling Spectroscopy and Microscopy and Raman spectroscopy experiments, we focus on the interaction with the underlying metal substrate [4-8]. In particular we analyze the evolution of the structural, electronic and vibrational properties during the different stages of the formation, passing from the as deposited metallorganic network to pure sp-/ sp2- nanostructures during annealing, and we show the effect of the substrate coupling on their semiconducting properties. By contrast to 2D nanostructures, 1D point-like defect systems have the advantage of being more easily integrable within traditional electronics. Along this path, we also present a study of Germanium+vacancy (GeV) complex defects that can be experimentally realized in silicon by single-ion implantation followed by annealing [9]. GeV hybrid complexes combine the electronic properties of the silicon vacancy, which carries deep states in the bandgap, with an accurate spatial controllability and stability. For these reasons, GeV complexes have a double advantage with respect to single-atom conventional doping elements in silicon (such as phosphorous, arsenic and other shallow-level dopants): from one side, they have a small diffusivity, remaining at the desired positions, once implanted; on the other side, they induce deep and strongly localized ground state impurity levels, which may hence allow to operate without the need of cryogenic temperatures [10]. The development of on-demand individual deep impurities in silicon is indeed motivated by their employment as a physical substrate for qubits [11], nanoscale transistors [12], single photon emitters [13] and Hubbard-like quantum simulators [14], with the advantage of an easy integration in the silicon industry. We exploit a multiscale approach that combines ab initio calculation and Hubbard model to characterize electronic properties and quantum transport in an array of GeV complexes in silicon, supporting the experimental measurements. On the ground of Density Functional Theory calculations performed with hybrid screened-exchange functionals we demonstrate that the GeV defect is characterized by deep states in the gap, preventing thermal ionization, and high electronic correlation [15]. By including the effect of positional disorder we are able to reproduce the temperature-activation of the quantum transport observed in the experiments. We show the establishment of different regimes corresponding to different filling of the many body states in the array, as consequence of the balancing between localization due to disorder, Coulomb repulsion and delocalization of excited states [16]. Part of this study has been funded by the Horizon European Funding Programme (H2020-INFRAIA) - NFFA User Project, ID 188 and 517 [17].