Search

Semiconductor nanowires are believed to play a crucial role for future applications in electronics, spintronics and quantum technologies. A potential candidate is HgTe but its sensitivity to nanofabrication processes restrain its development. A way to circumvent this obstacle is the selective area growth technique. Here, in-plane HgTe nanostructures are grown thanks to selective area molecular beam epitaxy on a semi-insulating CdTe substrate covered with a patterned SiO$_{\mathrm{2}}$ mask. The shape of these nanostructures is defined by the in-plane orientation of the mask aperture along the <$110$>, <$1\bar{\mathrm{1}}0$>, or <$100$> direction, the deposited thickness, and the growth temperature. Several micron long in-plane nanowires can be achieved as well as more complex nanostructures such as networks, diamond structures or rings. A good selectivity is achieved with very little parasitic growth on the mask even for a growth temperature as low as $140${\deg}C and growth rate up to $0.5$ ML/s. For <$110$> oriented nanowires, the center of the nanostructure exhibits a trapezoidal shape with {$111$}B facets and two grains on the sides, while <$1\bar{\mathrm{1}}0$> oriented nanowires show {$111$}A facets with adatoms accumulation on the sides of the top surface. Transmission electron microscopy observations reveal a continuous epitaxial relation between the CdTe substrate and the HgTe nanowire. Measurements of the resistance with fourpoint scanning tunneling microscopy indicates a good electrical homogeneity along the main NW axis and a thermally activated transport. This growth method paves the way toward the fabrication of complex HgTe-based nanostructures for electronic transport measurements., Comment: 18 pages and 8 figures. Submitted to Nanotechnology

Standoff chemical sensing of chemicals and toxic compounds is very desirable for actively monitoring residential, commercial, and industrial spaces. Analytes such as toxic industrial compounds (TICs), chemical warfare agents (CWAs), and environmental pollutants (EPs) can pose serious threats to civilians, warfighters, and the environment. We report on highly sensitive, robust, and inexpensive distributed standoff fiber optic chemical sensors (FOCSs). FOCSs are activated by analytes reaching a sensing region and inducing a change in its optical properties. The information of a spectral response is transmitted by a shift in transmission spectra probed by evanescent waves, corresponding to the detection of an analyte. However, there are many factors that play a role in the implementation and optimization of such fiber sensors. We find it beneficial to construct the theory of general optochemical waveguide sensors to guide the analysis. We apply it to a specific fiber platfom that is robust, broadband, multi-target, scalable, and compatible to texitle fibers and fabrics - a focus of this work. The good agreement between the predicted and measured spectra of FOCSs exposed to Ammonia (NH$_3$) lends support to the analysis. The experiment yields a 1ppm sensitivity, representing the current best in class. The use of NH$_3$ in the test serves as a representative for a larger set of amine-based TICs, CWAs, EPs, and other chemical species, targeted for wide-area distributed coverage and textile-compatible sensors.

We study single-electron tunneling (SET) characteristics in crystalline PbS/InP junctions, that exhibit single-electron Coulomb-blockade staircases along with memory and memory-fading behaviors. This gives rise to both short-term and long-term plasticities as well as a convenient non-linear response, making this structure attractive for neuromorphic computing applications. For further insights into this prospect, we predict typical behaviors relevant to the field, obtained by an extrapolation of experimental data in the SET framework. The estimated minimum energy required for a synaptic operation is in the order of 1 fJ, while the maximum frequency of operation can reach the MHz range., Comment: 9 pages, 5 figures

After decades of explorations, suffering from low critical temperature and subtle nature, whether a metallic ground state exists in a two-dimensional system beyond Anderson localization is still a mystery. Supremely, phase coherence could be the key that unlocks its intriguing nature. This work reveals how quantum phase coherence evolves across bosonic superconductor-metal-insulator transitions via magneto-conductance quantum oscillations in high-Tc superconducting films. A robust intervening anomalous metallic state characterized by both resistance and oscillation amplitude saturations in the low temperature regime is detected. By contrast, with decreasing temperature the oscillation amplitude monotonically grows on the superconducting side, but decreases at low temperatures on the insulating side. It suggests that the saturation of phase coherence plays a prominent role in the formation of this anomalous metallic state.

We report on experimental observation of laser-like emission from a periodic all-silicon nanostructure formed on an electronic grade silicon-on-insulator wafer, using a highly-uniform hexagonal self-organized array of nanopores as an etch mask. Classical lasing characteristics such as threshold behavior and stimulated emission, optical gain, emission linewidth narrowing and far-field modal pattern were observed at temperatures below 70 K under optical pumping., Comment: Submitted to Physical Review Letters (Nov.8th, 2004)