Beniamino Sciacca

Solution-phase epitaxial growth of quasi-monocrystalline cuprous oxide on metal nanowires

The epitaxial growth of monocrystalline semiconductors on metal nanostructures is interesting from both fundamental and applied perspectives. The realization of nanostructures with excellent interfaces and material properties that also have controlled optical resonances can be very challenging. Here we report the synthesis and characterization of metal–semiconductor core–shell nanowires. We demonstrate a solution-phase route to obtain stable core–shell metal–Cu2O nanowires with outstanding control over the resulting structure, in which the noble metal nanowire is used as the nucleation site for epitaxial growth of quasi-monocrystalline Cu2O shells at room temperature in aqueous solution. We use X-ray and electron diffraction, high-resolution transmission electron microscopy, energy dispersive X-ray spectroscopy, photoluminescence spectroscopy, and absorption spectroscopy, as well as density functional theory calculations, to characterize the core–shell nanowires and verify their structure. Metal–semiconductor core–shell nanowires offer several potential advantages over thin film and traditional nanowire architectures as building blocks for photovoltaics, including efficient carrier collection in radial nanowire junctions and strong optical resonances that can be tuned to maximize absorption.

Au-Cu2O core-shell nanowire photovoltaics

Semiconductor nanowires are among the most promising candidates for next generation photovoltaics. This is due to their outstanding optical and electrical properties which provide large optical cross sections while simultaneously decoupling the photon absorption and charge carrier extraction length scales. These effects relax the requirements for both the minority carrier diffusion length and the amount of semiconductor needed. Metal-semiconductor core-shell nanowires have previously been predicted to show even better optical absorption than solid semiconductor nanowires and offer the additional advantage of a local metal core contact. Here, we fabricate and analyze such a geometry using a single Au-Cu2O core-shell nanowire photovoltaic cell as a model system. Spatially resolved photocurrent maps reveal that although the minority carrier diffusion length in the Cu2O shell is less than 1 μm, the radial contact geometry with the incorporated metal electrode still allows for photogenerated carrier collection...

Transformation of Ag Nanowires into Semiconducting AgFeS2 Nanowires

We report on the synthesis of semiconducting AgFeS2 nanowires, obtained from the conversion of Ag nanowires. The study of the conversion process shows that the formation of Ag2S nanowires, as an intermediate step, precedes the conversion into AgFeS2 nanowires. The chemical properties of AgFeS2 nanowires were characterized by X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy at intermediate steps of the conversion process and show that the temperature at which the reaction takes place is critical to obtaining nanowires as opposed to nanotubes. Optical measurements on nanowire ensembles confirm the semiconducting nature of AgFeS2, with a direct band gap of 0.88 eV.

Nanoscale chiral valley-photon interface through optical spin-orbit coupling

Nanoscale chiral valley-photon interface Occupation of different valleys within the band structure of some materials can be used to encode information. That information is typically encoded in terms of the chirality or polarization of emitted photons. Gong et al. combined a plasmonic silver nanowire with a flake of the transition metal dichalcogenide WS2 to form a nanophotonic platform for the transfer of solid-state spin into optical information over mesoscopic distances. The direction of light emission from the nanowire was strongly dependent on the spin-orbit coupling of light and the WS2 layer. Such a highly efficient interface should prove useful for developing valleytronics into a practical on-chip technology. Science, this issue p. 443 Valley-dependent directional emission is demonstrated via spin-orbit coupling with a plasmonic nanowire. The emergence of two-dimensional transition metal dichalcogenide materials has sparked intense activity in valleytronics, as their valley information can be encoded and detected with the spin angular momentum of light. We demonstrate the valley-dependent directional coupling of light using a plasmonic nanowire–tungsten disulfide (WS2) layers system. We show that the valley pseudospin in WS2 couples to transverse optical spin of the same handedness with a directional coupling efficiency of 90 ± 1%. Our results provide a platform for controlling, detecting, and processing valley and spin information with precise optical control at the nanoscale.

Integrating Sphere Microscopy for Direct Absorption Measurements of Single Nanostructures

Nanoscale materials are promising for optoelectronic devices because their physical dimensions are on the order of the wavelength of light. This leads to a variety of complex optical phenomena that, for instance, enhance absorption and emission. However, quantifying the performance of these nanoscale devices frequently requires measuring absolute absorption at the nanoscale, and remarkably, there is no general method capable of doing so directly. Here, we present such a method based on an integrating sphere but modified to achieve submicron spatial resolution. We explore the limits of this technique by using it to measure spatial and spectral absorptance profiles on a wide variety of nanoscale systems, including different combinations of weakly and strongly absorbing and scattering nanomaterials (Si and GaAs nanowires, Au nanoparticles). This measurement technique provides quantitative information about local optical properties that are crucial for improving any optoelectronic device with nanoscale dimensions or nanoscale surface texturing.

Monocrystalline Nanopatterns Made by Nanocube Assembly and Epitaxy

Monocrystalline materials are essential for optoelectronic devices such as solar cells, LEDs, lasers, and transistors to reach the highest performance. Advances in synthetic chemistry now allow for high quality monocrystalline nanomaterials to be grown at low temperature in solution for many materials; however, the realization of extended structures with control over the final 3D geometry still remains elusive. Here, a new paradigm is presented, which relies on epitaxy between monocrystalline nanocube building blocks. The nanocubes are assembled in a predefined pattern and then epitaxially connected at the atomic level by chemical growth in solution, to form monocrystalline nanopatterns on arbitrary substrates. As a first demonstration, it is shown that monocrystalline silver structures obtained with such a process have optical properties and conductivity comparable to single-crystalline silver. This flexible multiscale process may ultimately enable the implementation of monocrystalline materials in optoelectronic devices, raising performance to the ultimate limit.

Nanowires: Solution‐Grown Silver Nanowire Ordered Arrays as Transparent Electrodes (Adv. Mater. 5/2016)

Ordered arrays of metal nanowires are employed as transparent electrodes by E. C. Garnett and co-workers, as described on page 905. The subwavelength dimensions enable high transmittance, while the metal allows for low sheet resistance. The growth of crystalline silver in solution leads to a larger grain size than in evaporated films, which reduces electron scattering and increases the conductivity by a factor of 2-3, approaching that of bulk silver.