Roberto Guerra

Size, oxidation, and strain in small Si/SiO2 nanocrystals

The structural, electronic and optical properties of Si nanocrystals of different size and shape, passivated with hydrogens, OH groups, or embedded in a SiO2 matrix are studied. The comparison between the embedded and free, suspended nanocrystals shows that the silica matrix produces a strain on the embedded NCs, that contributes to determine the band gap value. By including the strain on the hydroxided nanocrystals we are able to reproduce the electronic and optical properties of the full Si/SiO2 systems. Moreover we found that, while the quantum confinement dominates in the hydrogenated nanocrystals of all sizes, the behaviour of hydroxided and embedded nanocrystals strongly depends on the interface oxidation degree, in particular for diameters below 2 nm. Here, the proportion of NC atoms at the Si/SiO2 interface becomes relevant, producing surface-related states that may affect the quantum confinement appearing as inner band gap states and then drastically changing the optical response of the system.

Silicon nanocrystallites in a SiO2 matrix: Role of disorder and size

We compare, through first-principles pseudopotential calculations, the structural, electronic, and optical properties of different size silicon nanoclusters embedded in a

High luminescence in small Si / SiO 2 nanocrystals: A theoretical study

{\text{SiO}}_{2}

Preferential positioning of dopants and co-dopants in embedded and freestanding Si nanocrystals

crystalline or amorphous matrix with that of freestanding, hydrogenated, and hydroxided silicon nanoclusters of corresponding size and shape. We find that the largest effect on the optoelectronic behavior is due to the amorphization of the embedded nanocluster. In that, the amorphization reduces the fundamental gap while increasing the absorption strength in the visible range. Increasing the nanocluster size does not change substantially this picture but only leads to the reduction in the absorption threshold, following the quantum confinement rule. Finally, through the calculation of the optical absorption spectra both in an independent-particle and a many-body approach, we show that the effect of local fields is crucial for describing properly the optical behavior of the crystalline case while it is of minor importance for amorphous systems.

Silicon and Germanium Nanostructures for Photovoltaic Applications: Ab-Initio Results

In recent years many experiments have demonstrated the possibility to achieve efficient photoluminescence from Si/SiO2 nanocrystals. While it is widely known that only a minor portions of the nanocrystals in the samples contribute to the observed photoluminescence, the high complexity of the Si/SiO2 interface and the dramatic sensitivity to the fabrication conditions make the identification of the most active structures at the experimental level not a trivial task. Focusing on this aspect we have addressed the problem theoretically, by calculating the radiative recombination rates for different classes of Si-nanocrystals in the diameter range of 0.2-1.5 nm, in order to identify the best conditions for optical emission. We show that the recombination rates of hydrogenated nanocrystals follow the quantum confinement feature in which the nanocrystal diameter is the principal quantity in determining the system response. Interestingly, a completely different behavior emerges from the OH-terminated or SiO2-embedded nanocrystals, where the number of oxygens at the interface seems intimately connected to the recombination rates, resulting the most important quantity for the characterization of the optical yield in such systems. Besides, additional conditions for the achievement of high rates are constituted by a high crystallinity of the nanocrystals and by high confinement energies (small diameters).

Static friction scaling of physisorbed islands: the key is in the edge

In this work we aim at understanding the effect of n- and p-type substitutional doping in the case of matrix-embedded and freestanding Si nanocrystals. By means of ab initio calculations we identify the preferential positioning of the dopants and its effect on the structural properties with respect to the undoped case. Subsequently, we consider the case of phosphorus and boron co-doped nanocrystals showing that, against the single-doping situation, the energetics strongly favors the binding of the impurities at the nanocrystal surface. Indeed we demonstrate that the polar B-P bond forms a stable permanent electric dipole that radially points inward in the nanocrystal. Such a noteworthy characteristic and its physical consequences are discussed alongside new potential applications.

Optical absorption and emission of silicon nanocrystals: From single to collective response

Actually, most of the electric energy is being produced by fossil fuels and great is the search for viable alternatives. The most appealing and promising technology is photovoltaics. It will become truly mainstream when its cost will be comparable to other energy sources. One way is to significantly enhance device efficiencies, for example by increasing the number of band gaps in multijunction solar cells or by favoring charge separation in the devices. This can be done by using cells based on nanostructured semiconductors. In this paper, we will present ab-initio results of the structural, electronic and optical properties of (1) silicon and germanium nanoparticles embedded in wide band gap materials and (2) mixed silicon-germanium nanowires. We show that theory can help in understanding the microscopic processes important for devices performances. In particular, we calculated for embedded Si and Ge nanoparticles the dependence of the absorption threshold on size and oxidation, the role of crystallinity and, in some cases, the recombination rates, and we demonstrated that in the case of mixed nanowires, those with a clear interface between Si and Ge show not only a reduced quantum confinement effect but display also a natural geometrical separation between electron and hole.

Frictional transition from superlubric islands to pinned monolayers

The static friction preventing the free sliding of nanosized rare gas solid islands physisorbed on incommensurate crystalline surfaces is not completely understood. Simulations modeled on Kr/Pb(111) highlight the importance and the scaling behavior of the islands edge contribution to static friction.

The influence of silicon nanoclusters on the optical properties of a-SiNx samples: A theoretical study

We report on the possibility of describing the absorption and emission characteristics of an ensemble of silicon nanocrystals (NCs) with realistic distributions in the NC size, by the sum of the responses of the single NCs. The individual NC responses are evaluated by means of ab initio theoretical calculations and the summation is performed by taking into account the trend of the optical properties as a function of NC size and oxidation degree. The comparison with experimental results shows a nice matching of the spectra, also without any tuning of the parameters. Finally, the possibility of adapting the model in order to reproduce the experimental data is explored and discussed.

Local-field effects in silicon nanoclusters

The inertial sliding of physisorbed submonolayer islands on crystal surfaces contains unexpected information on the exceptionally smooth sliding state associated with incommensurate superlubricity and on the mechanisms of its disappearance. Here, in a joint quartz crystal microbalance and molecular dynamics simulation case study of Xe on Cu(111), we show how superlubricity emerges in the large size limit of naturally incommensurate Xe islands. As coverage approaches a full monolayer, theory also predicts an abrupt adhesion-driven two-dimensional density compression on the order of several per cent, implying a hysteretic jump from superlubric free islands to a pressurized commensurate immobile monolayer. This scenario is fully supported by the quartz crystal microbalance data, which show remarkably large slip times with increasing submonolayer coverage, signalling superlubricity, followed by a dramatic drop to zero for the dense commensurate monolayer. Careful analysis of this variety of island sliding phenomena will be essential in future applications of friction at crystal/adsorbate interfaces.