Prof Michael Johnston

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.