Guowei Xu

Suspending single-wall carbon nanotube thin film infrared bolometers on microchannels

Suspended single-wall carbon nanotube (SWCNT) thin film bolometers have been fabricated on microchannels patterned on Si substrates using electron-beam lithography. The much improved bolometric photoresponse is attributed to the reduced thermal link between SWCNT bolometer and substrate, which can be controlled by tuning the width and spacing of the microchannels. The detectivity D∗ up to 4.5×105 cm Hz1/2/W has been obtained at room temperature, which is at least five times better than that of the unsuspended counterpart and may be further improved via elimination of metallic SWCNTs and improvement of the charge and heat transport across the intertube junctions.

Development of Nanopatterned Fluorine-Doped Tin Oxide Electrodes for Dye-Sensitized Solar Cells with Improved Light Trapping

Transparent conductors (TCs) are an important component of optoelectronic devices and nanoscale engineering of TCs is important for optimization of the device performance through improved light trapping. In this work, patterned periodic arrays of nanopillars and nanolines of pitch size of ~700 nm were created on fluorine-doped tin oxide (FTO) using nanoimprint lithography and reactive ion etching using environmentally friendly gases. The patterned FTO exhibits enhanced light trapping as compared to the unpatterned FTO, which agrees well with simulations based on Finite-Difference Time-Domain method for up to a distance of 4 μm. Dye sensitized solar cells (DSSCs) fabricated on the patterned FTO exhibited improved performance (fill factor and power conversion efficiency), which can be attributed to enhanced light absorption in the range 400-650 nm. Further, electrochemical impedance measurements revealed lower recombination resistance for the patterned FTO/TiO(2) electrode compared to the unpatterned FTO electrode/TiO(2) electrode as a result of better light capturing properties of patterned FTO. The direct fabrication of nanopatterns on TCs developed in the present study is expected to be a viable scheme for achieving improved performance in many other optoelectronic devices.

Development of pulsed laser deposition for CdS/CdTe thin film solar cells

This work explores in situ fabrication of thin film CdS (100 nm)/CdTe (1.5 μm) solar cells using pulsed laser deposition (PLD). Optimization of the PLD processing conditions, including laser energy density, substrate temperature, and the PLD chamber pressure, was achieved with respect to pinhole-free CdS and CdTe layers and solar power conversion efficiency. High efficiency up to 6.68% has been demonstrated and better performance is anticipated with optimization of the PLD process.

Doped graphene nanohole arrays for flexible transparent conductors

Graphene nanohole arrays (GNAs) were fabricated using nanoimprint lithography. The improved optical transmittance of GNAs is primarily due to the reduced surface coverage of graphene from the nanohole fabrication. Importantly, the exposed edges of the nanoholes provided effective sites for chemical doping using thionyl chloride was shown to enhance the conductance by a factor of 15–18 in contrast to only 2-4 for unpatterned graphene. GNAs can provide a unique scheme for improving both optical transmittance and electrical conductivity of graphene-based transparent conductors.

Effects of thermal annealing on noise property and temperature coefficient of resistance of single-walled carbon nanotube films

The effect of thermal annealing on the electrical transport properties of purified and COOH-functionalized single-walled carbon nanotube (SWCNT) films has been investigated and the correlation between the noise property and temperature coefficient of resistance (TCR) has been derived. Thermal annealing has been found highly efficient to improve both noise and TCR properties of the SWCNT films, which is important to applications of SWCNT bolometers. While the improvement may be attributed mainly to the enhanced intertube coupling in the purified SWCNT films, a combined change in both intratube and intertube charge transport is responsible in the case of COOH-functionalized SWCNT films.

Second-harmonic generation induced by electric currents in GaAs.

We demonstrate a new, nonlinear optical effect of electric currents. First, a steady current is generated by applying a voltage on a doped GaAs crystal. We demonstrate that this current induces second-harmonic generation of a probe laser pulse. Second, we optically inject a transient current in an undoped GaAs crystal by using a pair of ultrafast laser pulses and demonstrate that it induces the same second-harmonic generation. In both cases, the induced second-order nonlinear susceptibility is proportional to the current density. This effect can be used for nondestructive, noninvasive, and ultrafast imaging of currents. These advantages are illustrated by the real-time observations of a coherent plasma oscillation and spatial resolution of current distribution in a device. This new effect also provides a mechanism for electrical control of the optical response of materials.

Plasmonic Graphene Transparent Conductors

Plasmonic graphene is fabricated using thermally assisted self-assembly of silver nanoparticles on graphene. The localized surface-plasmonic effect is demonstrated with the resonance frequency shifting from 446 to 495 nm when the lateral dimension of the Ag nanoparticles increases from about 50 to 150 nm. Finite-difference time-domain simulations are employed to confirm the experimentally observed light-scattering enhancement in the solar spectrum in plasmonic graphene and the decrease of both the plasmonic resonance frequency and amplitude with increasing graphene thickness. In addition, plasmonic graphene shows much-improved electrical conductance by a factor of 2-4 as compared to the original graphene, making the plasmonic graphene a promising advanced transparent conductor with enhanced light scattering for thin-film optoelectronic devices.

Probing Nucleation Mechanism of Self-Catalyzed InN Nanostructures

The nucleation and evolution of InN nanowires in a self-catalyzed growth process have been investigated to probe the microscopic growth mechanism of the self-catalysis and a model is proposed for high pressure growth window at ~760 Torr. In the initial stage of the growth, amorphous InNx microparticles of cone shape in liquid phase form with assistance of an InNx wetting layer on the substrate. InN crystallites form inside the cone and serve as the seeds for one-dimensional growth along the favorable [0001] orientation, resulting in single-crystalline InN nanowire bundles protruding out from the cones. An amorphous InNx sheath around the faucet tip serves as the interface between growing InN nanowires and the incoming vapors of indium and nitrogen and supports continuous growth of InN nanowires in a similar way to the oxide sheath in the oxide-assisted growth of other semiconductor nanowires. Other InN 1D nanostructures, such as belts and tubes, can be obtained by varying the InN crystallites nucleation and initiation process.

Strong nanopore pinning enhances Jc in YBa2Cu3O7−δ films

Transport critical current density (Jc) has been studied in YBa2Cu3O7−δ (YBCO) thin films doped with nanopores as pins on magnetic vortices. The density of the nanopores in the range of 5±3 pores/μm2 corresponds to an accommodation field Hm∼4.1–16.6 mT. High Jc up to 8.3 MA/cm2 has been observed on these porous YBCO films at 77 K and self-field. A close correlation between Jc and the magnetic pinning potential of the nanopores has been demonstrated below Hm, suggesting that nanopores are strong pins on the magnetic vortices.