Yamin Leprince-Wang

Electrical and Photoresponse Properties of an Intramolecular p-n Homojunction in Single Phosphorus-Doped ZnO Nanowires

The single-crystal n-type and p-type ZnO nanowires (NWs) were synthesized via a chemical vapor deposition method, where phosphorus pentoxide was used as the dopant source. The electrical and photoluminescence studies reveal that phosphorus-doped ZnO NWs (ZnO:P NWs) can be changed from n-type to p-type with increasing P concentration. Furthermore, we report for the first time the formation of an intramolecular p-n homojunction in a single ZnO:P NW. The p-n junction diode has a high on/off current ratio of 2.5 x 10(3) and a low forward turn-on voltage of approximately 1.37 V. Finally, the photoresponse properties of the diode were investigated under UV (325 nm) excitation in air at room temperature. The high photocurrent/dark current ratio (3.2 x 10(4)) reveals that the diode has a potential as extreme sensitive UV photodetectors.

Single ZnO Nanowire/p‐type GaN Heterojunctions for Photovoltaic Devices and UV Light‐Emitting Diodes

We fabricate heterojunctions consisting of a single n-type ZnO nanowire and a p-type GaN film. The photovoltaic effect of heterojunctions exhibits open-circuit voltages ranging from 2 to 2.7 V, and a maximum output power reaching 80 nW. Light-emitting diodes with UV electroluminescence based on the heterojunctions are demonstrated.

Strain induced exciton fine-structure splitting and shift in bent ZnO microwires

Lattice strain is a useful and economic way to tune the device performance and is commonly present in nanostructures. Here, we investigated for the first time the exciton spectra evolution in bent ZnO microwires along the radial direction via high spatial/energy resolution cathodeluminescence spectroscopy at 5.5 K. Our experiments show that the exciton peak splits into multi fine peaks towards the compressive part while retains one peak in the tensile part and the emission peak displays a continuous blue-shift from tensile to compressive edges. In combination with first-principles calculations, we show that the observed NBE emission splitting is due to the valence band splitting and the absence of peak splitting in the tensile part maybe due to the highly localized holes in the A band and the carrier density distribution across the microwire. Our studies may pave the way to design nanophotonic and electronic devices using bent ZnO nanowires.

On the optical and morphological properties of microstructured Black Silicon obtained by cryogenic-enhanced plasma reactive ion etching

Motivated by the need for obtaining low reflectivity silicon surfaces, we report on (sub-) micro-texturing of silicon using a high throughput fabrication process involving SF6/O2 reactive ion etching at cryogenic temperatures, leading to Black Silicon (BS). The corresponding high aspect ratio conical spikes of the microstructured surface give rise to multiple reflections and hence, enhanced absorption under electromagnetic radiation. Aiming a better understanding of this mechanism, we performed a systematic study by varying several plasma process parameters: O2/SF6 gas flow rate ratio, silicon temperature, bias voltage, and etching time. We determined the process window which leads to BS formation and we studied the influence of the process parameters on the surface morphology of the obtained BS samples, through analysis of scanning electron microscopy images. The measured optical reflectance of BS is in the order of 1% in the visible and near infrared ranges (400–950 nm). We noticed that the lowest refle...

Thermal annealing effect on optical properties of electrodeposited ZnO thin films

ZnO thin films were electrodeposited in aqueous solution on gilded p-type Si wafer substrates. Thermal treatments were carried out on different films in Ar atmosphere at different temperatures, between 200 and 600 °C. Surface morphology studies using scanning electron microscopy and atomic force microscopy show a smooth surface for an annealing temperature of 400 °C with a roughness mean square value of about 15 nm and a precipitation of ZnO microcrystals on the deposit surface at 600 °C. X-ray diffraction experiments reveal a decrease in the c-parameter value from 5.223 to 5.206 A after treatment at 600 °C, due to the removal of hydrogen from the film. Raman spectroscopy analyses show an improvement in the crystal quality of the film and a decrease in the compressive stress inside the deposit. Photoluminescence observations reveal an important change in the UV emission band after annealing at 200 °C. A visible region emission band at 580 nm, ascribed to interstitial oxygen, is observed for the as-grown deposit and decreases as the annealing temperature increases. An emission band appears near 525 nm for samples annealed at 400 and 600 °C; this band is ascribed to oxygen vacancies created during annealing treatment. This result is in agreement with the energy dispersive x-ray spectroscopy experiments which revealed loss of oxygen.

Study on the structural and physical properties of ZnO nanowire arrays grown via electrochemical and hydrothermal depositions

We report here the systematic study of well controlled ZnO nanowire arrays grown via two different chemical ways: electrodeposition and hydrothermal method, which are frequently used for low cost synthesis of ZnO nanowire arrays. Both methods consist of two elaboration steps: a seed layer ZnO was first deposited on the substrate and then the growth of the ZnO nanowire arrays on the seed layer was performed. Scanning electron microscopy observations show a similar morphology, while x-ray diffraction (XRD) and Raman spectra revealed a preferred orientation of ZnO nanowires towards the c-axis. High resolution transmission electron microscopy analysis showed excellent monocrystallinity of the nanowire. A p-n junction structure based on above two kinds of nanowire arrays with a p-CuSCN layer was fabricated and their photoluminescence (PL) and conductance were measured in comparison. PL measurements demonstrated a higher defects concentration in ZnO nanowires obtained by hydrothermal method which leads to a ver...

Black silicon with sub-percent reflectivity: Influence of the 3D texturization geometry

In this paper we study the impact of the three-dimensional geometry of a micro/nanostructured silicon surface on its reflectivity under incident electromagnetic (EM) illumination. We simulate the optical reflectance of 3D micro/nano silicon cones of different dimensions. Based on the favorable simulation results, maskless textured silicon, called “black silicon” is processed by deep reactive ion etching (DRIE) under cryogenic temperatures. By varying the process parameters, we fabricate conical black silicon substrates with excellent anti-reflective behavior. Notable among the results, one of the samples exhibits the lowest reflectivity in the optical wavelength published to date for plasma-etched black-silicon.

Liquid-metal-based metasurface for terahertz absorption material: Frequency-agile and wide-angle

Terahertz metasurface absorption materials, which absorb terahertz wave through subwavelength artificial structures, play a key role in terahertz wave shielding and stealth technology, etc. However, most of the metasurface absorption materials in terahertz suffer from limited tuning range and narrow incident angle characteristics. Here, we demonstrate a liquid-metal-based metasurface through microfluidic technology, which functions as a terahertz absorption material with broadband tunability and wide-angle features. The proposed terahertz metasurface absorption material exhibits an experimental tuning range from 0.246 THz to 0.415 THz (the tuning range of central frequency reaches 51.1%), and the tuning range maintains at high level with wide-angle response up to 60°.

Formation mechanism of homo-epitaxial morphology on ZnO (000 ± 1) polar surfaces

The formation mechanism of experimentally observed epitaxial morphologies on (000 ± 1) polar surfaces of ZnO microwire was studied by density functional theory simulation of atomic step models on both polar surfaces. In situ observations by environmental scanning electron microscopy were employed to control and detect the morphology evolutions during the epitaxial growth process. Edge formation energies for representative step models were calculated as a function of Zn chemical potential. The energetically favorable step structures were determined under different conditions, which could be related to certain surface morphologies regarding the aspects of crystallography and growth kinetics. Our experiments can be well explained by theoretical simulations.

A tunable metamaterial for wide-angle and broadband absorption through meta-water-capsule coatings

We report a tunable metamaterial for wide-angle and broadband absorption by tuning the height of meta-water-capsule through microfluidics. An 80% absorption is achieved within 67% bandwidth when the incident angle varies from 0° to 45°.