Terahertz photonics

Abstract:

High-performance dye-sensitized solar cells are usually fabricated using nanostructured TiO(2) as a thin-film electron-collecting material. However, alternative metal-oxides are currently being explored that may offer advantages through ease of processing, higher electron mobility, or interface band energetics. We present here a comparative study of electron mobility and injection dynamics in thin films of TiO(2), ZnO, and SnO(2) nanoparticles sensitized with Z907 ruthenium dye. Using time-resolved terahertz photoconductivity measurements, we show that, for ZnO and SnO(2) nanoporous films, electron injection from the sensitizer has substantial slow components lasting over tens to hundreds of picoseconds, while for TiO(2), the process is predominantly concluded within a few picoseconds. These results correlate well with the overall electron injection efficiencies we determine from photovoltaic cells fabricated from identical nanoporous films, suggesting that such slow components limit the overall photocurrent generated by the solar cell. We conclude that these injection dynamics are not substantially influenced by bulk energy level offsets but rather by the local environment of the dye-nanoparticle interface that is governed by dye binding modes and densities of states available for injection, both of which may vary from site to site. In addition, we have extracted the electron mobility in the three nanoporous metal-oxide films at early time after excitation from terahertz conductivity measurements and compared these with the time-averaged, long-range mobility determined for devices based on identical films. Comparison with established values for single-crystal Hall mobilities of the three materials shows that, while electron mobility values for nanoporous TiO(2) films are approaching theoretical maximum values, both early time, short distance and interparticle electron mobility in nanoporous ZnO or SnO(2) films offer considerable scope for improvement.

Abstract:

We report a number of crossed nanofiber structures for full-color micro/nanodisplays, which were formed by assembling flexible poly(trimethylene terephthalate) (PTT) nanofibers under an optical microscope with the assistance of micromanipulators. The color pixels of the displays consist of micro/nanometer sized color spots in a radius of 300-1500 nm, which were realized through crossed junctions of the PTT nanofibers. The colors of the spots were tuned by changing the power ratios of the launched red, green, and blue lights. We further present a new way to develop white light illumination by combination of red, green, and blue lights with assembly techniques and low production costs. © 2011 American Chemical Society.

Abstract:

Semiconductor nanowires have recently emerged as a new class of materials with significant potential to reveal new fundamental physics and to propel new applications in quantum electronic and optoelectronic devices. Semiconductor nanowires show exceptional promise as nanostructured materials for exploring physics in reduced dimensions and in complex geometries, as well as in one-dimensional nanowire devices. They are compatible with existing semiconductor technologies and can be tailored into unique axial and radial heterostructures. In this contribution we review the recent efforts of our international collaboration which have resulted in significant advances in the growth of exceptionally high quality IIIV nanowires and nanowire heterostructures, and major developments in understanding the electronic energy landscapes of these nanowires and the dynamics of carriers in these nanowires using photoluminescence, time-resolved photoluminescence and terahertz conductivity spectroscopy. © 2011 Elsevier Ltd. All rights reserved.

Abstract:

We have investigated the charge photogeneration dynamics at the interface formed between single-walled carbon nanotubes (SWNTs) and poly(3-hexylthiophene) (P3HT) using a combination of femtosecond spectroscopic techniques. We demonstrate that photoexcitation of P3HT forming a single molecular layer around a SWNT leads to an ultrafast (∼430 fs) charge transfer between the materials. The addition of excess P3HT leads to long-term charge separation in which free polarons remain separated at room temperature. Our results suggest that SWNT-P3HT blends incorporating only small fractions (1%) of SWNTs allow photon-to-charge conversion with efficiencies comparable to those for conventional (60:40) P3HT-fullerene blends, provided that small-diameter tubes are individually embedded in the P3HT matrix.

Abstract:

We have improved the stability and performance of terahertz (THz) photoconductive (Auston) switches using a combination of (NH4) 2S surface passivation (SP) and silicon nitride (Si3 N4) encapsulation. The influences of SP and encapsulation on the ultrafast electron dynamics in GaAs were examined using THz emission spectroscopy and optical pumpTHz probe spectroscopy. The power of THz radiation from the surface of photoexcited GaAs increased by a factor of 5 after passivation and encapsulation, while the process lengthened the trapping time for photoexcited charge carriers. By fabricating and assessing the performance of photoconductive switches, we found that passivation and encapsulation increased the average THz power generated fourfold. © 2010 IEEE.