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Research interests

Electronic processes in organic and nanoscale semiconductors

• Ultra-fast carrier/quasi-particle generation
• Carrier transport
• Exciton lifetime
• Interface physics (particularly bulk heterojunction interfaces)

Ultrafast photophysical experiments

• Optical-pump terahertz-probe spectroscopy
• Photoluminescence up-conversion spectroscopy

Organic optoelectronic devices

• Organic photovoltaics (heterojunction and dye-sensitsed)
• Light emitting diodes

Publications

1. The influence of surfaces on the transient terahertz conductivity and electron mobility of {GaAs} nanowires,
   HJ Joyce, SA Baig, P Parkinson, CL Davies, JL Boland, HH Tan, C Jagadish, LM Herz, MB Johnston
   J. Phys. D-Appl. Phys., 50:224001 (2017) [ abstract | pdf | doi:10.1088/1361-6463/aa6a8f ]
   Bare unpassivated GaAs nanowires feature relatively high electron mobilities (400-2100 cm(2) V-1 s(-1)) and ultrashort charge carrier lifetimes (1-5 ps) at room temperature. These two properties are highly desirable for high speed optoelectronic devices, including photoreceivers, modulators and switches operating at microwave and terahertz frequencies. When engineering these GaAs nanowire-based devices, it is important to have a quantitative understanding of how the charge carrier mobility and lifetime can be tuned. Here we use optical-pump-terahertzprobe spectroscopy to quantify how mobility and lifetime depend on the nanowire surfaces and on carrier density in unpassivated GaAs nanowires. We also present two alternative frameworks for the analysis of nanowire photoconductivity: one based on plasmon resonance and the other based on Maxwell-Garnett effective medium theory with the nanowires modelled as prolate ellipsoids. We find the electron mobility decreases significantly with decreasing nanowire diameter, as charge carriers experience increased scattering at nanowire surfaces. Reducing the diameter from 50 nm to 30 nm degrades the electron mobility by up to 47%. Photoconductivity dynamics were dominated by trapping at saturable states existing at the nanowire surface, and the trapping rate was highest for the nanowires of narrowest diameter. The maximum surface recombination velocity, which occurs in the limit of all traps being empty, was calculated as 1.3 x 10(6) cm s(-1). We note that when selecting the optimum nanowire diameter for an ultrafast device, there is a trade-off between achieving a short lifetime and a high carrier mobility. To achieve high speed GaAs nanowire devices featuring the highest charge carrier mobilities and shortest lifetimes, we recommend operating the devices at low charge carrier densities.
2. Single $n^+$-i-$n^+$ {InP} nanowires for highly sensitive terahertz detection,
   K Peng, P Parkinson, Q Gao, JL Boland, ZY Li, F Wang, S Mokkapati, L Fu, MB Johnston, HH Tan, C Jagadish
   Nanotechnology, 28:125202 (2017) [ abstract | pdf | doi:10.1088/1361-6528/aa5d80 ]
   Developing single-nanowire terahertz (THz) electronics and employing them as sub-wavelength components for highly-integrated THz time-domain spectroscopy (THz-TDS) applications is a promising approach to achieve future low-cost, highly integrable and high-resolution THz tools, which are desirable in many areas spanning from security, industry, environmental monitoring and medical diagnostics to fundamental science. In this work, we present the design and growth of n(+)-i-n(+) InP nanowires. The axial doping profile of the n+-i-n+ InP nanowires has been calibrated and characterized using combined optical and electrical approaches to achieve nanowire devices with low contact resistances, on which the highly-sensitive InP singlenanowire photoconductive THz detectors have been demonstrated. While the n+-i-n+ InP nanowire detector has a only pA-level response current, it has a 2.5 times improved signal-tonoise ratio compared with the undoped InP nanowire detector and is comparable to traditional bulk THz detectors. This performance indicates a promising path to nanowire-based THz electronics for future commercial applications.
3. Broadband phase-sensitive single {InP} nanowire photoconductive terahertz detectors,
   K Peng, P Parkinson, JL Boland, Q Gao, YC Wenas, CL Davies, ZY Li, L Fu, MB Johnston, HH Tan, C Jagadish
   Nano Lett., 16:4925-4931 (2016) [ abstract | pdf | doi:10.1021/acs.nanolett.6b01528 ]
   Terahertz time-domain spectroscopy (THz-TDS) has emerged as a powerful tool for materials characterization and imaging. A trend toward size reduction, higher component integration, and performance improvement for advanced THz-TDS systems is of increasing interest. The use of single semiconducting nanowires for terahertz (THz) detection is a nascent field that has great potential to realize future highly integrated THz systems. In order to develop such components, optimized material optoelectronic properties and careful device design are necessary. Here, we present antenna-optimized photoconductive detectors based on single InP nanowires with superior properties of high carrier mobility (similar to 1260 cm(2) V-1 s(-1)) and low dark current (similar to 10 pA), which exhibit excellent sensitivity and broadband performance. We demonstrate that these nanowire THz detectors can provide high quality time-domain spectra for materials characterization in a THz-TDS system, a critical step toward future application in advanced THz-TDS system with high spectral and spatial resolution.
4. Low Ensemble Disorder in Quantum Well Tube Nanowires,
   CL Davies, P Parkinson, N Jiang, JL Boland, S Conesa-Boj, HH Tan, C Jagadish, LM Herz, MB Johnston
   Nanoscale, 7:20531--20538 (2015) [ abstract | pdf | doi:10.1039/C5NR06996C ]
   We have observed very low disorder in high quality quantum well tubes (QWT) in GaAs-Al$_{0.44}$Ga$_{0.56}$As core-multishell nanowires. Room-temperature photoluminescence spectra were measured from 150 single nanowires enabling a full statistical analysis of both intra- and inter-nanowire disorder. By modelling individual nanowire spectra{,} we assigned a quantum well tube thickness{,} a core disorder parameter and a QWT disorder parameter to each nanowire. A strong correlation was observed between disorder in the GaAs cores and disorder in the GaAs QWTs{,} which indicates that variations in core morphology effectively propagate to the shell layers. This highlights the importance of high quality core growth prior to shell deposition. Furthermore{,} variations in QWT thicknesses for different facet directions was found to be a likely cause of intra-wire disorder{,} highlighting the need for accurate shell growth
5. Single Nanowire Terahertz Detectors ,
   K Peng, P Parkinson, L Fu, Q Gao, N Jiang, Y Guo, F Wang, H Joyce, JL Boland, M Johnston, H Tan, C Jagadish
   , 2015:STu4H.8 (2015) [ abstract | pdf | doi:10.1364/CLEO_SI.2015.STu4H.8 ]
   Photoconductive terahertz detectors based on single GaAs/AlGaAs nanowire were designed, fabricated and incorporated into the pulsed time-domain technique, showing a promise for nanowires in terahertz applications such as near-field terahertz sensors or on-chip terahertz micro-spectrometers.
6. Single Nanowire Photoconductive Terahertz Detectors,
   K Peng, P Parkinson, L Fu, Q Gao, N Jiang, Y Guo, F Wang, HJ Joyce, JL Boland, HH Tan, C Jagadish, MB Johnston
   Nano Lett., 15:206-210 (2015) [ abstract | pdf | doi:10.1021/nl5033843 ]
   Spectroscopy and imaging in the terahertz (THz) region of the electromagnetic spectrum has proven to provide important insights in fields as diverse as chemical analysis, materials characterization, security screening, and nondestructive testing. However, compact optoelectronics suited to the most powerful terahertz technique, time-domain spectroscopy, are lacking. Here, we implement single GaAs nanowires as microscopic coherent THz sensors and for the first time incorporated them into the pulsed time-domain technique. We also demonstrate the functionality of the single nanowire THz detector as a spectrometer by using it to measure the transmission spectrum of a 290 GHz low pass filter. Thus, nanowires are shown to be well suited for THz device applications and hold particular promise as near-field THz sensors.
7. Modulation Doping of GaAs/AlGaAs Core--Shell Nanowires With Effective Defect Passivation and High Electron Mobility,
   JL Boland, S Conesa-Boj, P Parkinson, G Tutuncuoglu, F Matteini, D Ruffer, A Casadei, F Amaduzzi, F Jabeen, CL Davies, HJ Joyce, LM Herz, A Fontcuberta i Morral, MB Johnston
   Nano Lett., 15:1336-1342 (2015) [ abstract | pdf | doi:10.1021/nl504566t ]
   Reliable doping is required to realize many devices based on semiconductor nanowires. Group III–V nanowires show great promise as elements of high-speed optoelectronic devices, but for such applications it is important that the electron mobility is not compromised by the inclusion of dopants. Here we show that GaAs nanowires can be n-type doped with negligible loss of electron mobility. Molecular beam epitaxy was used to fabricate modulation-doped GaAs nanowires with Al0.33Ga0.67As shells that contained a layer of Si dopants. We identify the presence of the doped layer from a high-angle annular dark field scanning electron microscopy cross-section image. The doping density, carrier mobility, and charge carrier lifetimes of these n-type nanowires and nominally undoped reference samples were determined using the noncontact method of optical pump terahertz probe spectroscopy. An n-type extrinsic carrier concentration of 1.10 +- 0.06 × 1016 cm–3 was extracted, demonstrating the effectiveness of modulation doping in GaAs nanowires. The room-temperature electron mobility was also found to be high at 2200 +- 300 cm2 V–1 s–1 and importantly minimal degradation was observed compared with undoped reference nanowires at similar electron densities. In addition, modulation doping significantly enhanced the room-temperature photoconductivity and photoluminescence lifetimes to 3.9 +- 0.3 and 2.4 +- 0.1 ns respectively, revealing that modulation doping can passivate interfacial trap states.
8. Ultrafast Transient Terahertz Conductivity of Monolayer {MoS$_2$} and {WSe$_2$} Grown by Chemical Vapor Deposition,
   CJ Docherty, P Parkinson, HJ Joyce, M Chiu, C Chen, M Lee, L Li, LM Herz, MB Johnston
   ACS Nano, 8:11147-11153 (2014) [ abstract | pdf | doi:10.1021/nn5034746 ]
   We have measured ultrafast charge carrier dynamics in monolayers and trilayers of the transition metal dichalcogenides MoS2 and WSe2 using a combination of time-resolved photoluminescence and terahertz spectroscopy. We recorded a photoconductivity and photoluminescence response time of just 350 fs from CVD-grown monolayer MoS2, and 1 ps from trilayer MoS2 and monolayer WSe2. Our results indicate the potential of these materials as high-speed optoelectronic materials.
9. Electron Mobilities Approaching Bulk Limits in ``Surface-Free'' {GaAs} Nanowires,
   HJ Joyce, P Parkinson, N Jiang, CJ Docherty, Q Gao, HH Tan, C Jagadish, LM Herz, MB Johnston
   Nano Lett., 14:5989-5994 (2014) [ abstract | pdf | doi:10.1021/nl503043p ]
   Achieving bulk-like charge carrier mobilities in semiconductor nanowires is a major challenge facing the development of nanowire-based electronic devices. Here we demonstrate that engineering the GaAs nanowire surface by overcoating with optimized AlGaAs shells is an effective means of obtaining exceptionally high carrier mobilities and lifetimes. We performed measurements of GaAs/AlGaAs core–shell nanowires using optical pump-terahertz probe spectroscopy: a noncontact and accurate probe of carrier transport on ultrafast time scales. The carrier lifetimes and mobilities both improved significantly with increasing AlGaAs shell thickness. Remarkably, optimized GaAs/AlGaAs core–shell nanowires exhibited electron mobilities up to 3000 cm2 V–1 s–1, reaching over 65% of the electron mobility typical of high quality undoped bulk GaAs at equivalent photoexcited carrier densities. This points to the high interface quality and the very low levels of ionized impurities and lattice defects in these nanowires. The improvements in mobility were concomitant with drastic improvements in photoconductivity lifetime, reaching 1.6 ns. Comparison of photoconductivity and photoluminescence dynamics indicates that midgap GaAs surface states, and consequently surface band-bending and depletion, are effectively eliminated in these high quality heterostructures.
10. Ultrafast Energy Transfer in Biomimetic Multistrand Nanorings,
    P Parkinson, CEI Knappke, N Kamonsutthipaijit, K Sirithip, JD Matichak, HL Anderson, LM Herz
    J. Am. Chem. Soc., 136:8217-8220 (2014) [ abstract | pdf | doi:10.1021/ja504730j ]
    We report the synthesis of LH2-like supramolecular double- and triple-stranded complexes based upon porphyrin nanorings. Energy transfer from the antenna dimers to the pi-conjugated nanoring occurs on a subpicosecond time scale, rivaling transfer rates in natural light-harvesting systems. The presence of a second nanoring acceptor doubles the transfer rate, providing strong evidence for multidirectional energy funneling. The behavior of these systems is particularly intriguing because the local nature of the interaction may allow energy transfer into states that are, for cyclic nanorings, symmetry-forbidden in the far field. These complexes are versatile synthetic models for natural light-harvesting systems.
11. Single GaAs/AlGaAs Nanowire Photoconductive Terahertz Detectors ,
    K Peng, P Parkinson, L Fu, Q Gao, N Jiang, Y Guo, F Wang, HJ Joyce, JL Boland, MB Johnston, HH Tan, C Jagadish
    , 2014:221-222 (2014) [ abstract | pdf | doi:10.1109/COMMAD.2014.7038695 ]
    Photoconductive terahertz detectors based on single GaAs/AlGaAs core-shell nanowire have been designed and fabricated. The devices were characterised in a terahertz time-domain spectroscopy system, showing excellent sensitivity comparable to the standard bulk ion-implanted InP receiver, with a detection bandwidth of 0.1 ~ 0.6 THz. Finite-difference time-domain simulations were performed to understand the origin of the narrow bandwidth of current detectors as well as optimize antenna designs to improve detector performance.
12. Three-dimensional in situ photocurrent mapping for nanowire photovoltaics,
    P Parkinson, YH Lee, L Fu, S Breuer, HH Tan, C Jagadish
    Nano Lett., 13:1405-1409 (2013) [ abstract | pdf | doi:10.1021/nl304170q ]
    Devices based upon semiconductor nanowires provide many well-known advantages for next-generation photovoltaics, however, limited experimental techniques exist to determine essential electrical parameters within these devices. We present a novel application of a technique based upon two-photon induced photocurrent that provides a submicrometer resolution, three-dimensional reconstruction of photovoltaic parameters. This tool is used to characterize two GaAs nanowire-based devices, revealing the detail of current generation and collection, providing a path toward achieving the promise of nanowire-based photovoltaic devices.
13. Enhanced Minority Carrier Lifetimes in {GaAs}/{AlGaAs} Core–-Shell Nanowires through Shell Growth Optimization,
    N Jiang, Q Gao, P Parkinson, J Wong-Leung, S Mokkapati, S Breuer, HH Tan, CL Zheng, J Etheridge, C Jagadish
    Nano Lett., 13:5135-5140 (2013) [ abstract | pdf | doi:10.1021/nl4023385 ]
    The effects of AlGaAs shell thickness and growth time on the minority carrier lifetime in the GaAs core of GaAs/AlGaAs core–shell nanowires grown by metal–organic chemical vapor deposition are investigated. The carrier lifetime increases with increasing AlGaAs shell thickness up to a certain value as a result of reducing tunneling probability of carriers through the AlGaAs shell, beyond which the carrier lifetime reduces due to the diffusion of Ga–Al and/or impurities across the GaAs/AlGaAs heterointerface. Interdiffusion at the heterointerface is observed directly using high-angle annular dark field scanning transmission electron microscopy. We achieve room temperature minority carrier lifetimes of 1.9 ns by optimizing the shell growth with the intention of reducing the effect of interdiffusion.
14. Optically pumped room temperature {GaAs} nanowire lasers,
    D Saxena, S Mokkapati, P Parkinson, N Jiang, Q Gao, HH Tan, C Jagadish
    Nat. Photon., 7:963-968 (2013) [ abstract | pdf | doi:10.1038/nphoton.2013.303 ]
    Nearinfrared lasers are important for optical data communication, spectroscopy and medical diagnosis. Semiconductor nanowires offer the possibility of reducing the footprint of devices for three dimensional device integration and hence are being extensively studied in the context of optoelectronic devices. Although visible and ultraviolet nanowire lasers have been demonstrated widely, progress towards roomtemperature infrared nanowire lasers has been limited because of material quality issues and Auger recombination. (Al)GaAs is an important material system for infrared lasers that is extensively used for conventional lasers. GaAs has a very large surface recombination velocity, which is a serious issue for nanowire devices because of their large surfacetovolume ratio. Here, we demonstrate roomtemperature lasing in coreshellcap GaAs/AlGaAs/GaAs nanowires by properly designing the FabryPerot cavity, optimizing the material quality and minimizing surface recombination. Our demonstration is a major step towards incorporating (Al)GaAs nanowire lasers into the design of nanoscale optoelectronic devices operating at nearinfrared wavelengths.
15. Precursor flow rate manipulation for the controlled fabrication of twin-free {GaAs} nanowires on silicon substrates.,
    J Kang, Q Gao, P Parkinson, HJ Joyce, H Tan, Y Kim, Y Guo, H Xu, J Zou, C Jagadish
    Nanotechnology, 23:415702 (2012) [ abstract | pdf | doi:10.1088/0957-4484/23/41/415702 ]
    Vertically oriented GaAs nanowires (NWs) are grown on Si(111) substrates using metal-organic chemical vapor deposition. Controlled epitaxial growth along the 111 direction is demonstrated following the deposition of thin GaAs buffer layers and the elimination of structural defects, such as twin defects and stacking faults, is found for high growth rates. By systematically manipulating the AsH(3) (group-V) and TMGa (group-III) precursor flow rates, it is found that the TMGa flow rate has the most significant effect on the nanowire quality. After capping the minimal tapering and twin-free GaAs NWs with an AlGaAs shell, long exciton lifetimes (over 700ps) are obtained for high TMGa flow rate samples. It is observed that the Ga adatom concentration significantly affects the growth of GaAs NWs,
16. Noncontact Measurement of Charge Carrier Lifetime and Mobility in {GaN} Nanowires,
    P Parkinson, C Dodson, HJ Joyce, KA Bertness, NA Sanford, LM Herz, MB Johnston
    Nano Lett., 12:4600--4604 (2012) [ abstract | pdf | doi:10.1021/nl301898m ]
    The first noncontact photoconductivity measurements of gallium nitride nanowires (NWs) are presented, revealing a high crystallographic and optoelectronic quality achieved by use of catalyst-free molecular beam epitaxy. In comparison with bulk material, the NWs exhibit a long conductivity lifetime (>2 ns) and a high mobility (820 ± 120 cm2/(V s)). This is due to the weak influence of surface traps with respect to other III–V semiconducting NWs and to the favorable crystalline structure of the NWs achieved via strain-relieved growth.
17. Direct-write non-linear photolithography for semiconductor nanowire characterization,
    P Parkinson, N Jiang, Q Gao, HH Tan, C Jagadish
    Nanotechnology, 23:335704 (2012) [ abstract | pdf | doi:10.1088/0957-4484/23/33/335704 ]
    A practical bottleneck prohibiting the rapid, confident and damage-free electrical contacting of vapour-liquid-solid grown nanowires arises from the random spatial distribution and variation in quality of the nanowires, and the contact dimensions required. Established techniques such as electron-beam lithography or focused ion-beam deposition have challenges in scaling, damage or complexity that can make a large statistical sample difficult. We present a direct laser-writing technique to allow rapid electrical contacting of nanowires on a large variety of substrates.
18. Distinct Photocurrent Response of Individual {GaAs} Nanowires Induced by n-Type Doping,
    H Xia, Z Lu, T Li, P Parkinson, Z Liao, F Liu, W Lu, W Hu, P Chen, H Xu, J Zou, C Jagadish
    ACS Nano, 6:6005-6013 (2012) [ abstract | pdf | doi:10.1021/nn300962z ]
    The doping-dependent photoconductive properties of individual GaAs nanowires have been studied by conductive atomic force microscopy. Linear responsivity against the bias voltage is observed for moderate n-doped GaAs wires with a Schottky contact under illumination, while that of the undoped ones exhibits a saturated response. The carrier lifetime of a single nanowire can be obtained by simulating the characteristic photoelectric behavior. Consistent with the photoluminescence results, the significant drop of minority hole lifetime, from several hundred to subpicoseconds induced by n-type doping, leads to the distinct photoconductive features. Moreover, by comparing with the photoelectric behavior of AlGaAs shelled nanowires, the equivalent recombination rate of carriers at the surface is assessed to be >1 × 1012 s–1 for 2 × 1017cm–3 n-doped bare nanowires, nearly 30 times higher than that of the doping-related bulk effects. This work suggests that intentional doping in nanowires could change the charge status of the surface states and impose significant impact on the electrical and photoelectrical performances of semiconductor nanostructures.
19. Long minority carrier lifetime in Au-catalyzed GaAs/Al[sub x]Ga[sub 1 - x]As core-shell nanowires,
    N Jiang, P Parkinson, Q Gao, S Breuer, HH Tan, J Wong-Leung, C Jagadish
    Appl. Phys. Lett., 101:023111 (2012) [ abstract | pdf | doi:10.1063/1.4735002 ]
    GaAs/AlxGa1−xAs core-shell nanowires were grown by metal organic chemical vapor deposition with optimized AlxGa1−xAs shell and twin-free Au-catalyzed GaAs cores. Time-resolved photoluminescence measurements were carried out on single nanowires at room temperature, revealing minority carrier lifetimes of 1.02 ± 0.43 ns, comparable to self-assisted nanowires grown by molecular beam epitaxy. The long minority carrier lifetimes are mainly attributed to improvement of the GaAs/AlxGa1−xAs interface quality. The upper limit of surface recombination velocity of the structure is calculated to be 1300 cm/s with the AlxGa1−xAs shell grown at 750 °C, which is comparable with planar double heterostructures.
20. Surface energy relay between cosensitized molecules in solid-state dye-sensitized solar cells,
    MD Brown, P Parkinson, T Torres, H Miura, LM Herz, HJ Snaith
    J. Phys. Chem. C, 115:23204-23208 (2011) [ abstract | pdf | doi:10.1021/jp207075z ]
    We employ cosensitization of a visible absorbing organic sensitizer and a near IR absorbing Zn-phthalocyanine complex to significantly enhance the optical bandwidth in spiroOMeTAD based solid-state DSCs. The cosensitized cells exhibit greatly enhanced performance, with full sun AMES power conversion efficiencies of 4.7%, as compared to 3.9% for the best monosensitized device. Unexpectedly, further to broadening the spectral response, the addition of the near IR sensitizer greatly enhances the spectral response in the visible region. Through both electronic and spectroscopic investigations, we demonstrate that resonant energy transfer occurs from the visible to the near IR sensitizer. This unforeseen charge generation route works in conjunction with direct electron transfer from the visible sensitizer, improving the overall charge generation efficiency and explaining the panchromatic enhancements with the cosensitized system. This previously unobserved mechanism for charge generation relaxes the design criteria for visible absorbing sensitizers, providing a second, and possibly primary, channel for efficient charge generation.
21. {III}--{V} semiconductor nanowires for optoelectronic device applications ,
    HJ Joyce, Q Gao, HH Tan, C Jagadish, Y Kim, J Zou, LM Smith, HE Jackson, JM Yarrison-Rice, P Parkinson, MB Johnston
    Prog. Quantum Electron., 35:23-75 (2011) [ abstract | pdf | doi:10.1016/j.pquantelec.2011.03.002 ]
    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 III–V 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.
22. Improved performance of {GaAs}-based terahertz emitters via surface passivation and silicon nitride encapsulation,
    C Headley, L Fu, P Parkinson, XL Xu, J Lloyd-Hughes, C Jagadish, MB Johnston
    IEEE J. Sel. Top. Quantum Electron., 17:17-21 (2011) [ abstract | pdf | doi:10.1109/JSTQE.2010.2047006 ]
    We have improved the stability and performance of terahertz (THz) photoconductive (Auston) switches using a combination of (NH4)(2)S surface passivation (SP) and silicon nitride (Si3N4) encapsulation. The influences of SP and encapsulation on the ultrafast electron dynamics in GaAs were examined using THz emission spectroscopy and optical pump-THz 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.
23. Ultrafast Charge Separation at a Polymer-Single-Walled Carbon Nanotube Molecular Junction,
    SD Stranks, C Weisspfennig, P Parkinson, MB Johnston, LM Herz, RJ Nicholas
    Nano Lett., 11:66-72 (2011) [ abstract | pdf | doi:10.1021/nl1036484 ]
    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.
24. Role of Ultrafast Torsional Relaxation in the Emission from Polythiophene Aggregates,
    P Parkinson, C Muller, N Stingelin, MB Johnston, LM Herz
    J. Phys. Chem. Lett., 1:2788-2792 (2010) [ abstract | pdf | doi:10.1021/jz101026g ]
    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.
25. Dynamic terahertz polarization in single-walled carbon nanotubes,
    XL Xu, P Parkinson, K-C Chuang, MB Johnston, RJ Nicholas, LM Herz
    Phys. Rev. B, 82:085441 (2010) [ abstract | pdf | doi:10.1103/PhysRevB.82.085441 ]
    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.
26. Ultrafast Terahertz Conductivity Dynamics in Mesoporous {TiO}$_2$: {Influence} of Dye Sensitization and Surface Treatment in Solid-State Dye-Sensitized Solar Cells,
    P Tiwana, P Parkinson, MB Johnston, HJ Snaith, LM Herz
    J. Phys. Chem. C, 114:1365-1371 (2010) [ abstract | pdf | doi:10.1021/jp908760r ]
    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 cm(2)/(V s) and diffusion length of similar to 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.
27. Carrier lifetime and mobility enhancement in nearly defect-free core--shell nanowires measured using time-resolved terahertz spectroscopy,
    P Parkinson, HJ Joyce, Q Gao, HH Tan, X Zhang, J Zou, C Jagadish, LM Herz, MB Johnston
    Nano Lett., 9:3349-3353 (2009) [ abstract | pdf | doi:10.1021/nl9016336 ]
    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.
28. Efficient generation of charges via below-gap photoexcitation of polymer-fullerene blend films investigated by terahertz spectroscopy,
    P Parkinson, J Lloyd-Hughes, MB Johnston, LM Herz
    Phys. Rev. B, 78:115321 (2008) [ abstract | pdf | doi:10.1103/PhysRevB.78.115321 ]
    Using optical-pump terahertz-probe spectroscopy, we have investigated the time-resolved conductivity dynamics of photoexcited polymer-fullerene bulk heterojunction blends for two model polymers: poly[3-hexylthiophene] (P3HT) and poly[2-methoxy-5-(-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) blended with [6,6]-phenyl-C61 butyric acid methyl ester (PCBM). The observed terahertz-frequency conductivity is characteristic of dispersive charge transport for photoexcitation both at the −* absorption peak (560 nm for P3HT) and significantly below it (800 nm). The photoconductivity at 800 nm is unexpectedly high, which we attribute to the presence of a charge-transfer complex. We report the excitation-fluence dependence of the photoconductivity over more than four orders of magnitude, obtained by utilizing a terahertz spectrometer based upon on either a laser oscillator or an amplifier source. The time-averaged photoconductivity of the P3HT:PCBM blend is over 20 times larger than that of P3HT, indicating that long-lived hole polarons are responsible for the high photovoltaic efficiency of polymer:fullerene blends. At early times (~ps) the linear dependence of photoconductivity upon fluence indicates that interfacial charge transfer dominates as an exciton decay pathway, generating charges with mobility of at least ~0.1 cm2 V−1 s−1. At later times, a sublinear relationship shows that carrier-carrier recombination effects influence the conductivity on a longer time scale (>1 µs) with a bimolecular charge annihilation constant for the blends that is approximately two to three orders of magnitude smaller than that typical for neat polymer films.
29. Conductivity of nanoporous {InP} membranes investigated using terahertz spectroscopy,
    SKE Merchant, J Lloyd-Hughes, L Sirbu, IM Tiginyanu, P Parkinson, LM Herz, MB Johnston
    Nanotechnology, 19:395704 (2008) [ abstract | pdf | doi:10.1088/0957-4484/19/39/395704 ]
    We have investigated the terahertz conductivity of extrinsic and photoexcited electrons in nanoporous indium phosphide (InP) at different pore densities and orientations. The form of electronic transport in the film was found to differ significantly from that for bulk InP. While photo-generated electrons showed Drude-like transport, the behaviour for extrinsic electrons deviated significantly from the Drude model. Time-resolved photoconductivity measurements found that carrier recombination was slow, with lifetimes exceeding 1~ns for all porosities and orientations. When considered together, these findings suggest that the surfaces created by the nanopores strongly alter the dynamics of both extrinsic and photoexcited electrons.
30. Transient terahertz conductivity of {GaAs} nanowires,
    P Parkinson, J Lloyd-Hughes, Q Gao, HH Tan, C Jagadish, MB Johnston, LM Herz
    Nano Lett., 7:2162-2165 (2007) [ abstract | pdf | doi:10.1021/nl071162x ]
    The time-resolved conductivity of isolated GaAs nanowires is investigated by optical-pump terahertz-probe time-domain spectroscopy. The electronic response exhibits a pronounced surface plasmon mode that forms within 300 fs before decaying within 10 ps as a result of charge trapping at the nanowire surface. The mobility is extracted using the Drude model for a plasmon and found to be remarkably high, being roughly one-third of that typical for bulk GaAs at room temperature.
31. Dimensionality-dependent energy transfer in polymer-intercalated {SnS$_2$} nanocomposites,
    P Parkinson, E Aharon, MH Chang, C Dosche, GL Frey, A Kohler, LM Herz
    Phys. Rev. B, 75:165206 (2007) [ abstract | pdf | doi:10.1103/PhysRevB.75.165206 ]
    We have investigated the influence of dimensionality on the excitation-transfer dynamics in a conjugated polymer blend. Using time-resolved photoluminescence spectroscopy, we have measured the transfer transients for both a three-dimensional blend film and for quasi-two-dimensional monolayers formed through intercalation of the polymer blend between the crystal planes of an inorganic SnS2 matrix. We compare the experimental data with a simple, dimensionality-dependent model based on electronic coupling between electronic transition moments taken to be point dipoles. Within this approximation, the energy-transfer dynamics is found to adopt a three-dimensional character in the solid film and a two-dimensional nature in the monolayers present in the SnS2-polymer nanocomposite.