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Research interests

Graphene and low dimensional systems

• Time-domain terahertz spectroscopy
• Optical-pump terahertz-probe spectroscopy
• Optical characterisation of graphene

Publications

1. 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.
2. 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.
3. An ultrafast carbon nanotube terahertz polarisation modulator,
   CJ Docherty, SD Stranks, SN Habisreutinger, HJ Joyce, LM Herz, RJ Nicholas, MB Johnston
   J. Appl. Phys., 115:203108 (2014) [ abstract | pdf | doi:10.1063/1.4879895 ]
   We demonstrate ultrafast modulation of terahertz radiation by unaligned optically pumped single-walled carbon nanotubes. Photoexcitation by an ultrafast optical pump pulse induces transient terahertz absorption in nanowires aligned parallel to the optical pump. By controlling the polarisation of the optical pump, we show that terahertz polarisation and modulation can be tuned, allowing sub-picosecond modulation of terahertz radiation. Such speeds suggest potential for semiconductor nanowire devices in terahertz communication technologies.
4. Electronic properties of {GaAs}, {InAs} and {InP} nanowires studied by terahertz spectroscopy,
   HJ Joyce, CJ Docherty, Q Gao, HH Tan, C Jagadish, J Lloyd-Hughes, LM Herz, MB Johnston
   Nanotechnology, 24:214006 (2013) [ abstract | pdf | doi:10.1088/0957-4484/24/21/214006 ]
   We have performed a comparative study of ultrafast charge carrier dynamics in a range of III–V nanowires using optical pump–terahertz probe spectroscopy. This versatile technique allows measurement of important parameters for device applications, including carrier lifetimes, surface recombination velocities, carrier mobilities and donor doping levels. GaAs, InAs and InP nanowires of varying diameters were measured. For all samples, the electronic response was dominated by a pronounced surface plasmon mode. Of the three nanowire materials, InAs nanowires exhibited the highest electron mobilities of 6000 cm^2/V/s , which highlights their potential for high mobility applications, such as field effect transistors. InP nanowires exhibited the longest carrier lifetimes and the lowest surface recombination velocity of 170 cm/s. This very low surface recombination velocity makes InP nanowires suitable for applications where carrier lifetime is crucial, such as in photovoltaics. In contrast, the carrier lifetimes in GaAs nanowires were extremely short, of the order of picoseconds, due to the high surface recombination velocity, which was measured as 5.4 × 10^5 cm/s. These findings will assist in the choice of nanowires for different applications, and identify the challenges in producing nanowires suitable for future electronic and optoelectronic devices.
5. Extreme sensitivity of graphene photoconductivity to environmental gases,
   CJ Docherty, C Lin, HJ Joyce, RJ Nicholas, LM Herz, L Li, MB Johnston
   Nat. Commun., 3:1228 (2012) [ abstract | pdf | doi:10.1038/ncomms2235 ]
   Graphene is a single layer of covalently bonded carbon atoms, which was discovered only 8 years ago and yet has already attracted intense research and commercial interest. Initial research focused on its remarkable electronic properties, such as the observation of massless Dirac fermions and the half-integer quantum Hall effect. Now graphene is finding application in touch-screen displays, as channels in high-frequency transistors and in graphene-based integrated circuits. The potential for using the unique properties of graphene in terahertz-frequency electronics is particularly exciting; however, initial experiments probing the terahertz-frequency response of graphene are only just emerging. Here we show that the photoconductivity of graphene at terahertz frequencies is dramatically altered by the adsorption of atmospheric gases, such as nitrogen and oxygen. Furthermore, we observe the signature of terahertz stimulated emission from gas-adsorbed graphene. Our findings highlight the importance of environmental conditions on the design and fabrication of high-speed, graphene-based devices.
6. Ultra-low Surface Recombination Velocity in InP Nanowires Probed by Terahertz Spectroscopy,
   HJ Joyce, J Wong-Leung, C Yong, CJ Docherty, S Paiman, Q Gao, HH Tan, C Jagadish, J Lloyd-Hughes, LM Herz, MB Johnston
   Nano Lett., 12:5325-5330 (2012) [ abstract | pdf | doi:10.1021/nl3026828 ]
   Using transient terahertz photoconductivity measurements, we have made non-contact, room temperature measurements of the ultrafast charge carrier dynamics in InP nanowires. InP nanowires exhibited a very long photoconductivity lifetime of over 1ns, and carrier lifetimes were remarkably insensitive to surface states despite the large nanowire surface area-to-volume ratio. An exceptionally low surface recombination velocity (170cm/s) was recorded at room temperature. These results suggest that InP nanowires are prime candidates for optoelectronic devices, particularly photovoltaic devices, without the need for surface passivation. We found that the carrier mobility is not limited by nanowire diameter, but is strongly limited by the presence of planar crystallographic defects such as stacking faults in these predominantly wurtzite nanowires. These findings show the great potential of very narrow InP nanowires for electronic devices, but indicate that improvements in the crystallographic uniformity of InP nanowires will be critical for future nanowire device engineering.
7. Terahertz properties of graphene,
   CJ Docherty, MB Johnston
   J Infrared Milli Terahz Waves, 33:797 (2012) [ abstract | pdf | doi:10.1007/s10762-012-9913-y ]
   Graphene has proved itself as being unique in terms of fundamental physics, and of particular importance for post–silicon electronics. Research into graphene has divided into two branches, one probing the remarkable electronic and optical properties of graphene, and the other pursuing technologically viable forms of the material. Terahertz time domain spectroscopy (THz TDS) is a powerful tool for both, able to characterise the free carrier response of graphene and probe the inter and intraband response of excited carriers with sub-ps time resolution. We review THz TDS and related THz measurements of graphene.