Publications by Michael Johnston


III-V semiconductor nanowires for optoelectronic device applications

Progress in Quantum Electronics 35 (2011) 23-75

HJ Joyce, Q Gao, H Hoe Tan, C Jagadish, Y Kim, J Zou, LM Smith, HE Jackson, JM Yarrison-Rice, P Parkinson, MB Johnston

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.


Ultrafast charge separation at a polymer-single-walled carbon nanotube molecular junction.

Nano Lett 11 (2011) 66-72

SD Stranks, C Weisspfennig, P Parkinson, MB Johnston, LM Herz, RJ Nicholas

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.


Improved performance of GaAs-based terahertz emitters via surface passivation and silicon nitride encapsulation

IEEE Journal on Selected Topics in Quantum Electronics 17 (2011) 17-21

C Headley, L Fu, P Parkinson, X Xu, J Lloyd-Hughes, C Jagadish, MB Johnston

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.


Electron mobility and injection dynamics in mesoporous ZnO, SnO₂, and TiO₂ films used in dye-sensitized solar cells.

ACS Nano 5 (2011) 5158-5166

P Tiwana, P Docampo, MB Johnston, HJ Snaith, LM Herz

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.


All-optical full-color displays using polymer nanofibers

ACS Nano 5 (2011) 2020-2025

H Yu, D Liao, MB Johnston, B Li

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.


III-V semiconductor nanowires for optoelectronic device applications

PROGRESS IN QUANTUM ELECTRONICS 35 (2011) 23-75

HJ Joyce, Q Gao, HH Tan, C Jagadish, Y Kim, J Zou, LM Smith, HE Jackson, JM Yarrison-Rice, P Parkinson, MB Johnston


Dynamic terahertz polarization in single-walled carbon nanotubes

Physical Review B - Condensed Matter and Materials Physics 82 (2010)

XL Xu, P Parkinson, KC Chuang, MB Johnston, RJ Nicholas, LM Herz

We have investigated the anisotropic dynamic dielectric response of aligned and well-isolated single-walled carbon nanotubes using optical-pump terahertz (THz)-probe techniques. The polarization anisotropy measurements demonstrate that the THz radiation interacts only with radiation polarized parallel to the nanotubes which have been selectively excited by a polarized pump pulse thus allowing controlled THz polarization to be achieved from unaligned nanotubes. © 2010 The American Physical Society.


Terahertz emission from lateral photo-Dember currents

Optics Express 18 (2010) 4939-4947

G Klatt, F Hilser, W Qiao, M Beck, R Gebs, A Bartels, K Huska, U Lemmer, G Bastian, MB Johnston, M Fischer, J Faist, T Dekorsy

The photo-Dember effect is a source of impulsive THz emission following femtosecond pulsed optical excitation. This emission results from the ultrafast spatial separation of electron-hole pairs in strong carrier gradients due to their different diffusion coefficients. The associated time dependent polarization is oriented perpendicular to the excited surface which is inaptly for efficient out coupling of THz radiation. We propose a scheme for generating strong carrier gradients parallel to the excited surface. The resulting photo-Dember currents are oriented in the same direction and emit THz radiation into the favorable direction perpendicular to the surface. This effect is demonstrated for GaAs and In0.53Ga0.47As. Surprisingly the photo-Dember THz emitters provide higher bandwidth than photoconductive emitters. Multiplexing of phase coherent photo-Dember currents by periodically tailoring the photoexcited spatial carrier distribution gives rise to a strongly enhanced THz emission, which reaches electric field amplitudes comparable to a high-efficiency externally biased photoconductive emitter. © 2010 Optical Society of America.


Role of ultrafast torsional relaxation in the emission from polythiophene aggregates

Journal of Physical Chemistry Letters 1 (2010) 2788-2792

P Parkinson, C Müler, N Stingelin, MB Johnston, LM Herz

An understanding of aggregation effects in semiconducting polymers is essential for their use in optoelectronic devices; however, the dynamic evolution of such interchain states is not well understood. Here, we have investigated a blend of semiconducting poly(3-hexylthiophene) (P3HT) with an electronically inert ultrahigh-molecular-weight polyethylene (UHMW-PE) matrix that is shown to allow precise control over the extent to which the P3HT chains aggregate. We determined the singlet exciton population within isolated and aggregated P3HT regions using femtosecond time-resolved photoluminescence measurements and found a strong ultrafast decay pathway in the aggregated case only. Comparison of the emission from the two lowest vibronic bands demonstrates a changeover from an initial vibrationally "hot" photoexcited state to a geometrically relaxed aggregate state within ∼13 ps, corresponding to time scales for torsional relaxation in these materials. We conclude that formation of an aggregate excited state in conjugated polymers is mediated by vibrational relaxation from a low-symmetry to a high-symmetry ordered state for the ensemble. © 2010 American Chemical Society.


Improved performance of GaAs-based terahertz emitters

IRMMW-THz 2010 - 35th International Conference on Infrared, Millimeter, and Terahertz Waves, Conference Guide (2010)

C Headley, L Fu, P Parkinson, X Xu, J Lloyd-Hughes, C Jagadish, MB Johnston

We have improved the stability and performance of terahertz photoconductive (Auston) switches using a combination of (NH4)2S surface passivation and silicon nitride (Si3N4) encapsulation. The passivation and encapsulation processes increased the average terahertz power generated four-fold.


The role of ultrafast torsional relaxation in the emission from polythiophene aggregates

Conference on Optoelectronic and Microelectronic Materials and Devices, Proceedings, COMMAD (2010) 117-118

P Parkinson, C Müller, N Stingelin, MB Johnston, LM Herz

An understanding of aggregation effects in organic semiconductors is essential for their effective use in optoelectronic devices. Typically, the electronic dynamics in such systems are heavily dependant upon the aggregation state, and dynamics often occur on sub-nanosecond timescales. © 2010 IEEE.


Intense terahertz generation based on the photo-Dember effect

Lasers and Electro-Optics/Quantum Electronics and Laser Science Conference: 2010 Laser Science to Photonic Applications, CLEO/QELS 2010 (2010)

G Klatt, F Hilser, W Chao, R Gebs, A Bartels, K Huska, U Lemmer, G Bastian, MB Johnston, M Fischer, J Faist, T Dekorsy

We demonstrate a new scheme for generating THz radiation based on the photo-Dember effect in lateral geometry. By micro-structuring a semiconductor surface we achieve strongly enhanced THz emission comparable to high-efficiency externally biased photoconductive emitters. © 2010 Optical Society of America.


Terahertz radiation from multiplexed photo-dember currents

Optics InfoBase Conference Papers (2010)

G Klatt, F Hilser, W Qiao, R Gebs, A Bartels, K Huska, U Lemmer, G Bastian, MB Johnston, M Fischer, J Faist, T Dekorsy

We investigate a novel method to generate intense THz radiation by multiplexing coherent photo-Dember currents in a lateral geometry. These THz emitters are passive devices with a peak frequency at about 1.5 THz. © OSA / UP 2010.


Ultrafast terahertz conductivity dynamics in mesoporous TiO<inf>2</inf>: Influence of dye sensitization and surface treatment in solid-state dye-sensitized solar cells

Journal of Physical Chemistry C 114 (2010) 1365-1371

P Tiwana, P Parkinson, MB Johnston, HJ Snaith, LM Herz

We have used optical-pump terahertz-probe spectroscopy to explore the photoinduced conductivity dynamics in mesoporous anatase TiO2 films, commonly employed as the electron-transporting electrode in dye-sensitized solar cells. We find an intrinsic mobility value of 0.1 cm2/(V s) and diffusion length of ∼20 nm for electron motion through the TiO2 matrix. The photoconductivity dynamics in TiO2 films, both before and after sensitization with a ruthenium bypyridyl complex termed Z907, were examined in order to study the charge injection, trapping, and recombination time scales. We observe a biphasic charge injection from Z907, with a fast sub-500 fs component, followed by a slower 70-200 ps component. This is followed by photoconductivity decay over the first few nanoseconds, predominantly reflecting charge carrier trapping. In addition, we have utilized terahertz spectroscopy to investigate the influence of treating the titania surface with TiCl4 on early-time charge dynamics. In the solar cells, surface treatment of the mesoporous TiO2 with TiCl4 is critical to enable efficient operation. Here, we find that neither early-time charge mobility nor charge injection rate or decay times are significantly affected by the treatment, which suggests that it may, instead, have an impact on phenomena occurring on longer time scales. © 2010 American Chemical Society.


Characterisation of nanostructures via terahertz spectroscopy

Conference on Optoelectronic and Microelectronic Materials and Devices, Proceedings, COMMAD (2010) 23-24

P Parkinson, HJ Joyce, X Xu, Q Gao, HH Tan, C Jagadish, LM Herz, MB Johnston

We have used terahertz spectroscopy to measure the conductivity and time-resolved photoconductivity of a range of semiconducting nanostructures. This article focuses on our recent terahertz conductivity studies on semiconductor nanowires and single walled carbon nanotubes. © 2010 IEEE.


Intense terahertz generation based on the photo-dember effect

Optics InfoBase Conference Papers (2010)

G Klatt, F Hilser, W Chao, R Gebs, A Bartels, K Huska, U Lemmer, G Bastian, MB Johnston, M Fischer, J Faist, T Dekorsy

We demonstrate a new scheme for generating THz radiation based on the photo-Dember effect in lateral geometry. By micro-structuring a semiconductor surface we achieve strongly enhanced THz emission comparable to high-efficiency externally biased photoconductive emitters. © 2010 Optical Society of America.


Terahertz excitonic response of isolated single-walled carbon nanotubes

Journal of Physical Chemistry C 113 (2009) 18106-18109

X Xu, K Chuang, RJ Nicholas, MB Johnston, LM Herz

We have investigated the ultrafast far-infrared transmission of isolated single-walled carbon nanotubes using optical-pump THz-probe spectroscopy. The THz dielectric response is dominated by excitons with an initial, rapid decay due to Auger recombination followed by a slow decay of isolated single excitons. Frequencydependent analysis of the photomduced dielectric function suggest an internal excitonic excitation at ∼11 meV with further low-frequency (∼0.6 and 1.4 THz) absorption features at high densities ascribed to exciton complexes. A featureless conductivity bleaching is attributed to an exciton-induced reduction in the mobility of free carriers caused by phase-space filling. © 2009 American Chemical Society.


Extraction of the anisotropic dielectric properties of materials from polarization-resolved terahertz time-domain spectra

Journal of Optics A: Pure and Applied Optics 11 (2009)

E Castro-Camus, MB Johnston

The anisotropic complex dielectric properties of materials in the terahertz band is a topic that has attracted considerable attention recently in the fields of physics, chemistry and biochemistry. The mathematical formalism for analysing polarization-resolved terahertz time-domain data is presented, and particular cases including birefringence, optical activity and circular dichroism are discussed. © 2009 IOP Publishing Ltd.


Carrier lifetime and mobility enhancement in nearly defect-free core-shell nanowires measured using time-resolved terahertz spectroscopy.

Nano Lett 9 (2009) 3349-3353

P Parkinson, HJ Joyce, Q Gao, HH Tan, X Zhang, J Zou, C Jagadish, LM Herz, MB Johnston

We have used transient terahertz photoconductivity measurements to assess the efficacy of two-temperature growth and core-shell encapsulation techniques on the electronic properties of GaAs nanowires. We demonstrate that two-temperature growth of the GaAs core leads to an almost doubling in charge-carrier mobility and a tripling of carrier lifetime. In addition, overcoating the GaAs core with a larger-bandgap material is shown to reduce the density of surface traps by 82%, thereby enhancing the charge conductivity.


Conductivity of nanoporous InP membranes investigated using terahertz spectroscopy

Conference Proceedings - Lasers and Electro-Optics Society Annual Meeting-LEOS (2008) 707-708

SKE Merchant, J Lloyd-Hughes, P Parkinson, LM Herz, MB Johnston, L Sirbu, IM Tiginyanu

We report on extrinsic and photoexcited carrier behaviour in porous InP of various porosities and two orientations, studied using terahertz spectroscopy. We observed behaviour indicative of a surface electron depletion layer resulting from bandbending. ©2008 IEEE.

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