Glass is transparent. Metals are shiny. A piece of wood is opaque. Each material has a different relationship with light. Some materials let light go through; others reflect back; others absorb it. This is true also at the nanoscale. In addition, optical properties at the nanoscale are even more interesting. Imagine if your gold ring would change its color if you break it in half, or if you change its size anyhow. In fact, this is what happens for nano-sized metals and semiconductors. In particular, metals can act as an antenna for light. Gold nanopads have the ability to redirect light in specific directions and locations. Semiconductors change their response to light depending on its size and shape. What about coupling the two systems?
Within this research field, I investigated the optical properties of new semiconductor architectures (V-shaped membranes). Fabrication of these structures is very challenging and we did it in collaboration with Prof. Fontcuberta group at EPFL. I studied the their optical properties, analyzing the change in the scattering spectra as the size is changed. I used second harmonic generation to probe the non-linear properties of such nanostructures, finding very interesting correlation between structural properties and optical behavior.
Second harmonic excitation spectroscopy is a very powerful too to investigate material properties. I used it to characterize substoichiometric silicon nitride thin films, to elucidate size-dependent effects on gold nanoparticles, and to obtain polarization-controlled multispectral nanofocusing of metal nanoantennas.
Moreover, I investigated the coupling between photonic and plasmonic properties. In collaboration with EPFL, we designed gold nanoantenna arrays coupled with gallium arsenide nanowires. By using second harmonic excitation spectroscopy, we elucidated all the coupling effects in these systems and we showed that new modes emerge at expenses of the expected structural resonances.
These projects have been funded by the Air Force Office of Scientific Research, and performed during my experience at Boston University.
Integration of metallic nanostructures on nanowires for modification of their optical properties A. Casadei, E. Alarcon-Llado, E. F. Pecora, J. Trevino, C. Forestiere, D. Ruffer, E. Russo-Averchi, F. Matteini, G. Tutuncuoglu, M. Heiss, L. Dal Negro, A. Fontcuberta i Morral Frontiers in Nanophotonics, CSF Conference 2015
Second harmonic excitation spectroscopy in studies of Fano-type coupling in plasmonic arrays G. F. Walsh, J. Tervino, E. F. Pecora, L. Dal Negro SPIE Optics + Photonics 2015
Engineering light coupling in single nanowire with metal nano-antennas A. Casadei, J. Trevino, E. F. Pecora, E. Alarcò- Lladò, D. Ruffer, E. Russo-Averchi, G. Tutuncuoglu, F. Matteini, C. Forestiere, L. Dal Negro, A. Fontcuberta i Morral International Conference on One dimensional Nanomaterials ICON 2013
Second-harmonic generation from plasmonic nanoantennas and arrays A. Capretti, C. Forestiere, E. F. Pecora, G. Walsh, J. Trevino, S. Minissale, L. Dal Negro, G. Miano The International Conference on Surface Plasmon Photonics SPP6
Second-harmonic generation in substoichiometric silicon nitride layers E. F. Pecora, A. Capretti, G. Miano, L. Dal Negro Bulletin of the American Physical Society, vol. 58, V1.00119
We present a novel approach for the direct synthesis of ultrathin Si nanowires (NWs) exhibiting room temperature light emission. The synthesis is based on a wet etching process assisted by a metal thin film. The thickness-dependent morphology of the metal layer produces uncovered nanometer-size regions which act as precursor sites for NW formation. The process is cheap, fast, maskless and compatible with Si technology. Very dense arrays of long (several micrometers) and small (diameter of 5–9 nm) NWs have been synthesized. An efficient room temperature luminescence, visible with the naked eye, is observed when NWs are optically excited, exhibiting a blue-shift with decreasing NW size in agreement with quantum confinement effects. A prototype device based on Si NWs has been fabricated showing a strong and stable electroluminescence at low voltages. The relevance and the perspectives of the reported results are discussed, opening the route toward novel applications of Si NWs.
Si and Ge have the same crystalline structure, and although Si-Au and Ge-Au binary alloys are thermodynamically similar (same phase diagram, with the eutectic temperature of about 360°C), in this study, it is proved that Si and Ge nanowires (NWs) growth by electron beam evaporation occurs in very different temperature ranges and fluence regimes. In particular, it is demonstrated that Ge growth occurs just above the eutectic temperature, while Si NWs growth occurs at temperature higher than the eutectic temperature, at about 450°C. Moreover, Si NWs growth requires a higher evaporated fluence before the NWs become to be visible. These differences arise in the different kinetics behaviors of these systems. The authors investigate the microscopic growth mechanisms elucidating the contribution of the adatoms diffusion as a function of the evaporated atoms direct impingement, demonstrating that adatoms play a key role in physical vapor deposition (PVD) NWs growth. The concept of incubation fluence, which is necessary for an interpretation of NWs growth in PVD growth conditions, is highlighted.
Controllable and uniform doping of nanowires (NWs) is the ultimate challenge prior to their effective application. Si NWs amorphize and bend toward the impinging ions under ion irradiation as a result of viscous flow. We demonstrate that thermal annealing induces a full recovery of the crystalline phase corresponding to the unbending of the NWs. The competition between Solid Phase Epitaxy and Random Nucleation and Growth at the nanoscale is the key parameter controlling the recovery.
We demonstrated the heteroepitaxial growth of single-crystal faceted Ge nanowires (NWs) by electron-beam evaporation on top of Si(111) substrates. Despite the non-ultrahigh vacuum growth conditions, scanning electron microscope and transmission elec- tron microscope images show that NWs have specific crystallographic growth directions (111), (110), and (112) and that specific surface crystallographic planes (111) or (110) correspond to the (110) and (112) growth directions. Moreover, we studied in detail the Ge NWs structural properties. The temperature dependence of the NW length and of the frequency of each crystallographic orientation has been elucidated. Finally, the microscopic growth mechanisms have been investigated.
The growth mechanisms of epitaxial Si nanowires (NWs) grown by electron beam evaporation (EBE) and catalyzed through gold droplets are identified. NWs are seen to grow both from adsorbed Si atoms diffusing from the substrate and forming a dip around them, and from directly impinging atoms. The growth of a 2D planar layer competing with the axial growth of the NWs is also observed and the experimental parameters determining which of the two processes prevails are identified. NWs with (111), (100) and (110) orientation have been found and the growth rate is observed to have a strong orientation dependence, suggesting a microscopic growth mechanism based on the atomic ordering along (110) ledges onto (111)-oriented terraces. By properly changing the range of experimental conditions we demonstrate how it is possible to favor the axial growth of the NWs, define their length and control their crystallographic orientation.
We have elucidated the mechanism for B migration in the amorphous (a-) Si network. B diffusivity in a-Si is much higher than in crystalline Si; it is transient and increases with B concentration up to 2 x 1020 B/cm3. At higher density, B atoms in a-Si quickly precipitate. B diffusion is indirect, mediated by dangling bonds (DB) present in a-Si. The density of DB is enhanced by B accommodation in the a-Si network and decreases because of a-Si relaxation. Accurate data simulations allow one to extract the DB diffusivity, whose activation energy is 2.6 eV. Implications of these results are discussed.
B clustering in amorphous Si D. De Salvador, G. Bisognin, M. Di Marino, E. Napolitani, A. Carnera, S. Mirabella, E. Pecora, E. Bruno, F. Priolo, H. Graoui, M. A. Foad, F. Boscherini The Journal of Vacuum Science and Technology B 26, 382 (2008)