Nanodicing Single Crystalline Silicon Nanowire Arrays

Here, we demonstrate a novel method for the production of single-crystal Si nanowire arrays based on the top-down carving of Si-nanowall structures from a donor substrate, and their subsequent controlled and selective harvesting into a sacrificial solid material block. Nanosectioning of the nanostructures-embedding block by ultramicrotome leads to the formation of size, shape, and orientation-controlled high quality nanowire arrays. Additionally, we introduce a novel approach that enables transferring the nanowire arrays to any acceptor substrate, while preserving their orientation, and placing them on defined locations. Furthermore, crystallographic analysis and electrical measurements were performed, proving that the quality of the sectioned nanowires, which derive from their original crystalline donor substrate, are remarkably preserved

Controlled Formation of Radial Core–Shell Si/Metal Silicide Crystalline Heterostructures

The highly controlled formation of “radial” silicon/NiSi  core−shell nanowire heterostructures has been demonstrated for the first time. Here, we investigated the “radial” diffusion of nickel atoms into crystalline nanoscale silicon pillar 11 cores, followed by nickel silicide phase formation and the creation of a well-defined shell structure. The described approach is based on a two-step thermal process, which involves metal diffusion at low temperatures in the range of 200–400 °C, followed by a thermal curing step at a higher temperature of 400 °C. In-depth crystallographic analysis was performed by nanosectioning the resulting silicide–shelled silicon nanopillar heterostructures, giving us the ability to study in detail the newly formed silicide shells. Remarkably, it was observed that the resulting silicide shell thickness has a self-limiting behavior, and can be tightly controlled by the modulation of the initial diffusion-step temperature. In addition, electrical measurements of the core–shell structures revealed that the resulting shells can serve as an embedded conductive layer in future optoelectronic applications. This research provides a broad insight into the Ni silicide “radial” diffusion process at the nanoscale regime, and offers a simple approach to form thickness-controlled metal silicide shells in the range of 5–100 nm around semiconductor nanowire core structures, regardless the diameter of the nanowire cores. These high quality Si/NiSi core–shell nanowire structures will be applied in the near future as building blocks for the creation of utrathin highly conductive optically transparent top electrodes, over vertical nanopillars-based solar cell devices, which may subsequently lead to significant performance improvements of these devices in terms of charge collection and reduced recombination

Controlled Formation of Radial Core–Shell Si/Metal Silicide Crystalline Heterostructures

The highly controlled formation of “radial” silicon/NiSi  core−shell nanowire heterostructures has been demonstrated for the first time. Here, we investigated the “radial” diffusion of nickel atoms into crystalline nanoscale silicon pillar 11 cores, followed by nickel silicide phase formation and the creation of a well-defined shell structure. The described approach is based on a two-step thermal process, which involves metal diffusion at low temperatures in the range of 200–400 °C, followed by a thermal curing step at a higher temperature of 400 °C. In-depth crystallographic analysis was performed by nanosectioning the resulting silicide–shelled silicon nanopillar heterostructures, giving us the ability to study in detail the newly formed silicide shells. Remarkably, it was observed that the resulting silicide shell thickness has a self-limiting behavior, and can be tightly controlled by the modulation of the initial diffusion-step temperature. In addition, electrical measurements of the core–shell structures revealed that the resulting shells can serve as an embedded conductive layer in future optoelectronic applications. This research provides a broad insight into the Ni silicide “radial” diffusion process at the nanoscale regime, and offers a simple approach to form thickness-controlled metal silicide shells in the range of 5–100 nm around semiconductor nanowire core structures, regardless the diameter of the nanowire cores. These high quality Si/NiSi core–shell nanowire structures will be applied in the near future as building blocks for the creation of utrathin highly conductive optically transparent top electrodes, over vertical nanopillars-based solar cell devices, which may subsequently lead to significant performance improvements of these devices in terms of charge collection and reduced recombination

Supplement 1: Full rotational control of levitated silicon nanorods

Supplement 1 Originally published in Optica on 20 March 2017 (optica-4-3-356

Excited-State Proton Transfer and Proton Diffusion near Hydrophilic Surfaces

Time-resolved emission techniques were employed to study the reversible proton photoprotolytic properties of surface-attached 8-hydroxypyrene-1,3,6-trisulfonate (HPTS) molecules to hydrophilic alumina and silica surfaces. We found that the excited-state proton transfer rate of the surface-linked HPTS molecules, in H₂O and D₂O, is nearly the same as of HPTS in the bulk, while the corresponding recombination rate is significantly greater. Using the diffusion-assisted proton geminate-recombination model, we found that the best fit of the time-resolved fluorescence (TRF) signal is obtained by invoking a two-dimensional diffusion space for the proton to recombine with the conjugated basic form, RO^–*, of the surface-linked HPTS. However, we obtain an excellent fit by a three-dimensional diffusion space for diffusional HPTS in bulk water. These results indicate that the photoejected solvated protons are confined to the surface for long periods of time. We suggest two plausible mechanisms responsible for two-dimensional proton diffusion next to hydrophilic surfaces