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Dr Cuong Ton-That

Biography

Cuong Ton-That obtained a BSc in Physics (Honours, University Medal) from UTS and a PhD degree in the field of surface science from the Robert Gordon University, UK in 2000. He joined UTS in 2003 as a lecturer. Prior to working at UTS, he was a postdoctoral research associate in the Nanoscience Centre, University of Cambridge from 2000-2003.

His research is focused upon the development of oxide materials and nanostructures for solid state lighting applications. He has expertise in the growth and characterisation of zinc oxide nanostructures together with experience in cathodoluminescence, synchrotron-based X-ray spectroscopy, deep-level transient spectroscopy and scanning probe microscopy techniques.

Image of Cuong Ton-That
Senior Lecturer, School of Mathematical and Physical Sciences
Core Member, Research Strength Materials and Technology for Energy Efficiency Member
BSc (Hons) (UTS), PhD (Aberdeen)
 
Phone
+61 2 9514 2201
Room
CB04.05.310
Can supervise: Yes

Chapters

Ton-That, C. & Phillips, M.R. 2015, 'Cathodoluminescence microanalysis of ZnO nanowires' in Arbiol, J. & Xiong, Q. (eds), Semiconductor Nanowires, Woodhead Publising, Cambridge, England, pp. 393-408.

Conferences

Phillips, M., Manning, T.J., Nenstiel, C., Lockrey, M.N., Ton-That, C. & Hoffmann, A.V. 2011, 'High Temperature In-Situ Cathodoluminescence Studies of the Thermal Stability of Hydrogen in p-type Magnesium Doped Gallium Nitride', Microscopy and Microanalysis, Vol 17, Supplement 2, High Temperature In-Situ Cathodoluminescence Studies of the Thermal Stability of Hydrogen in p-type, Nashville, Tennessee, pp. 1-2.
Foley, M., Ton-That, C. & Phillips, M.R. 2010, 'Cathodoluminescence characterisation of vapour transport grown ZnO structures', ICONN 2010 - Proceedings of the 2010 International Conference on Nanoscience and Nanotechnology, pp. 207-209.
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ZnO structures grown under controlled vapour-phase transport growth conditions were characterised by electron microscopy and high-resolution cathodoluminescence techniques. Variations in the defect related emission and morphology were observed to be dependent on the distance from the source material. Annealing of grown structures under oxygen eliminated the defect emission. These experimental observations suggest that oxygen deficiency in ZnO is linked to the defect related emission, and that defect emission is strongly influenced by the oxygen gas content during vapour transport growth. © 2010 IEEE.
Dowd, A.R., Armstrong, N.G., Ton-That, C., Johansson, B. & Phillips, M. 2008, 'Cathodoluminescence for High Resolution Non-Destructive Luminescence Depth Profiling.', IUMRS-ICEM 2008 | COMMAD 2008, A-MRS, Parkville, VIC.
Foley, M., Ton-That, C. & Kirkup, L. 2007, 'Electrical Properties of Pure and Oxygen-Intercalated Fullerene Films', Proceedings of the 31st Annual Condensed Matter and Materials Meeting, Australian National University, ACT, Australia, pp. 1-3.
The conductivity of polycrystalline fullerene films as a function of oxygen concentration was investigated and found to be affected significantly by changes in oxygen partial pressure. The conductivity of the film was fitted to an Arrhenius curve. Analysis of the data indicated that a change of 0.11eV in activation energy occurred as the oxygen pressure was varied over two orders of magnitude.
Dowd, A., Johansson, B., Armstrong, N., Ton-That, C. & Phillips, M. 2006, 'Cathodoluminescence as a method of extracting detailed information from nanophotonics systems: A study of silicon nanocrystals', Proceedings of SPIE - The International Society for Optical Engineering.
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We investigated Si nanocrystal samples produced by high dose 600 keV Si+ implantation of fused silica and annealing using cathodoluminescence (CL). CL spectra collected under 5-25 keV electron irradiation show similar features to reported photoluminescence spectra, including the strong near IR peak. The CL intensity distribution is formulated as a linear inverse problem and two methods namely the régularisation method and maximum entropy method can be applied to determine the depth profile without making any assumptions concerning the profile function, i.e. a free form solution. We show using simulated CL data that the maximum entropy method is the most appropriate as it preserves the positivity and additivity of the depth profile. This method is applied to experimental CL data and we have localised the spatial origin of the near IR emission to the near-surface region of the implant, 400 nm from the surface, containing the smallest Si nanocrystals.

Journal articles

Tran, V.S., Ngo, H.H., Guo, W., Zhang, J., Liang, S., Ton-That, C. & Zhang, X. 2015, 'Typical low cost biosorbents for adsorptive removal of specific organic pollutants from water', Bioresource Technology, vol. 182, pp. 353-363.
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Specific organic pollutants (SOPs) such as phenolic compounds, PAHs, organic pesticides, and organic herbicides cause health and environmental problems due to their excessive toxic properties and poor biodegradability. Low-cost biosorbents are considered as a promising alternative for conventional adsorbents to remove SOPs from water. These materials have several advantages such as high sorption capacities, good modifiability and recoverability, insensitivity to toxic substances, simple operation in the treatment processes. However, previous reports on various types of biosorbents for removing SOPs are still moderately fragmented. Hence, this paper provides a comprehensive review on using typical low-cost biosorbents obtained from lignocellulose and chitin/chitosan for SOPs adsorption. Especially, their characteristics, biosorption mechanism together with utilization for eliminating SOPs are presented and discussed. The paper also gives a critical view regarding future applications of low-cost biosorbents in SOPs-contaminated water treatment.
Choi, S., Phillips, M.R., Aharonovich, I., Pornsuwan, S., Cowie, B.C.C. & Ton-That, C. 2015, 'Photophysics of Point Defects in ZnO Nanoparticles', Advanced Optical Materials.
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Zinc oxide (ZnO) nanoparticles have recently been identified as a promising candidate for advanced nanophotonics applications and quantum technologies. This work reports the formation of luminescent point defects and describes their photophysical properties. In particular, it is shown using correlative photoluminescence, cathodoluminescence, electron paramagnetic resonance (EPR), and X-ray absorption near-edge spectroscopy that green luminescence at 2.48 eV and an EPR line at g = 2.00 belong to a surface oxygen vacancy (Vo,s+) center, while a second green emission at 2.28 eV is associated with zinc vacancy (VZn) centers. It is established that radiative point defects can be excited in the visible that exhibits nanosecond lifetimes using both above bandgap and sub-bandgap 405 and 532 nm excitation. This work provides important knowledge towards employment of point defects in ZnO in nanophotonics technologies.
Choi, S., Berhane, A.M., Gentle, A., Ton-That, C., Phillips, M.R. & Aharonovich, I. 2015, 'Electroluminescence from Localized Defects in Zinc Oxide: Toward Electrically Driven Single Photon Sources at Room Temperature', ACS Applied Materials and Interfaces, vol. 7, no. 10, pp. 5619-5623.
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Single photon sources are required for a wide range of applications in quantum information science, quantum cryptography, and quantum communications. However, the majority of room temperature emitters to date are only excited optically, which limits their proper integration into scalable devices. In this work, we overcome this limitation and present room temperature electrically driven light emission from localized defects in zinc oxide (ZnO) nanoparticles and thin films. The devices emit in the red spectral range and show excellent rectifying behavior. The emission is stable over an extensive period of time, providing an important prerequisite for practical devices. Our results open possibilities for building new ZnO-based quantum integrated devices that incorporate solid-state single photon sources for quantum information technologies. (Graph Presented).
Choi, S., Johnson, B.C., Castelletto, S., Ton-That, C., Phillips, M.R. & Aharonovich, I. 2014, 'Single photon emission from ZnO nanoparticles', Applied Physics Letters, vol. 104, no. 26.
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Room temperature single photon emitters are very important resources for photonics and emerging quantum technologies. In this work, we study single photon emission from defect centers in 20 nm zinc oxide (ZnO) nanoparticles. The emitters exhibit bright broadband fluorescence in the red spectral range centered at 640 nm with polarized excitation and emission. The studied emitters showed continuous blinking; however, bleaching can be suppressed using a polymethyl methacrylate coating. Furthermore, hydrogen termination increased the density of single photon emitters. Our results will contribute to the identification of quantum systems in ZnO. © 2014 AIP Publishing LLC.
Lem, L.L.C., Phillips, M.R. & Ton-That, C. 2014, 'Controlling the visible luminescence in hydrothermal ZnO', Journal of Luminescence, vol. 154, pp. 387-391.
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Cathodoluminescence spectra have been measured in hydrothermal and hydrogen-doped ZnO at different excitation densities and temperatures to investigate the emission efficiencies of near-band-edge (NBE), green and yellow luminescence bands. The NBE intensity depends linearly on the electron beam excitation as expected for excitonic recombination character. The intensities of the green and yellow bands are highly dependent not only on the excitation density but also on temperature. At high excitation densities ZnO exhibits dominant green emission at room temperature; the intensity of the green band can be further controlled by doping ZnO with hydrogen, which passivates green luminescence centers. Conversely at small excitation densities (< 0.1 nA) and low temperatures the visible luminescence from ZnO is predominantly yellow due to the abundance of Li in hydrothermal ZnO. The results are explained by differences in the recombination kinetics and the relative concentrations of the green and yellow centers, and illustrate that single-color emission can be achieved in ZnO by adjusting the excitation power and temperature. &copy; 2014 Published by Elsevier B.V. All rights reserved.
Chan, K.S., Ton-That, C., Vines, L., Choi, S., Phillips, M.R., Svensson, B.G., Jagadish, C. & Wong-Leung, J. 2014, 'Effects of high temperature annealing on defects and luminescence properties in H implanted ZnO', Journal of Physics D: Applied Physics, vol. 47, no. 34.
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The evolution of luminescence properties and voids formation with respect to annealing temperature in H implanted ZnO was investigated by depth-resolved cathodoluminescence spectroscopy (DRCLS), transmission electron microscopy and secondary ion mass spectrometry (SIMS). The annealing temperature is found to induce noticeable changes to the shape, size and empty volume density of the cavities. DRCLS results also reveal that the green emission is influenced by different annealing temperatures. In particular, the 600 &deg;C anneal produces a strong quenching of the green emission in the implanted region, while after the 800 &deg;C anneal a significant enhancement near the surface is observed. The annealing at 600 &deg;C also results in an uncommon violet emission at ?3.1 eV that is not observed after a higher annealing temperature. A clear correlation between the violet emission, vacancies and Li is revealed from comparison between the DRCLS intensities and SIMS data. &copy; 2014 IOP Publishing Ltd.
Ton-That, C., Lem, L.L.C., Phillips, M.R., Reisdorffer, F., Mevellec, J., Nguyen, T.-.P., Nenstiel, C. & Hoffmann, A. 2014, 'Shallow carrier traps in hydrothermal ZnO crystals', New Journal of Physics, vol. 16.
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Native and hydrogen-plasma induced shallow traps in hydrothermally grown ZnO crystals have been investigated by charge-based deep level transient spectroscopy, photoluminescence and cathodoluminescence microanalysis. The as-grown ZnO exhibits a trap state at 23 meV, while H-doped ZnO produced by plasma doping shows two levels at 22 meV and 11 meV below the conduction band. As-grown ZnO displays the expected thermal decay of bound excitons with increasing temperature from 7 K, while we observed an anomalous behaviour of the excitonic emission in H-doped ZnO, in which its intensity increases with increasing temperature in the range 140-300 K. Based on a multitude of optical results, a qualitative model is developed which explains the Y line structural defects, which act as an electron trap with an activation energy of 11 meV, being responsible for the anomalous temperature-dependent cathodoluminescence of H-doped ZnO. &copy; 2014 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
Anantachaisilp, S., Anantachaisilp, S., Smith, S.M., Ton-That, C., Osotchan, T., Moon, A.R. & Phillips, M.R. 2014, 'Tailoring deep level surface defects in ZnO nanorods for high sensitivity ammonia gas sensing', Journal of Physical Chemistry C, vol. 118, no. 46, pp. 27150-27156.
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&copy; 2014 American Chemical Society. The influence of deep level surface defects on electrical and gas sensing properties of ZnO nanorods NH3(g) sensors was studied. ZnO nanorods 50-60 nm in diameter were synthesized via low-temperature hydrothermal growth at 90&deg;C on sapphire substrates. The as-grown nanorods exhibited a cathodoluminescence (CL) peak centered at 1.90 eV (YL), attributed to LiZn deep acceptors or O interstitials. Subsequent annealing in O2 at 1 atm and Zn vapor at 650&deg;C produced broad CL peaks centered at 1.70 eV (RL) and 2.44 eV (GL), respectively. The RL and GL have been ascribed to acceptor-like VZn and donor-like VO related centers, respectively. Electrical and gas sensing measurements established that the NH3 gas response sensitivity was 22.6 for O2 anneal (RL), 1.4 for Zn vapor anneal (GL), and 4.1 for the as-grown (YL) samples. Additionally, treatment in H-plasma quenched the RL and inverted the NH3 electrical response due to the incorporation of H donors. Changes in the gas sensing response are explained by a shift in the position of the ZnO Fermi level relative to the chemical potential of NH3 gas due to the creation of near surface donor or acceptors. These data confirm that ZnO nanorods arrays can be tailored to detect specific gas species. (Chemical Equation Presented).
Chan, K.S., Ton-That, C., Vines, L., Choi, S., Phillips, M.R., Svensson, B.G., Jagadish, C. & Wong-Leung, J. 2014, 'Effects of high temperature annealing on defects and luminescence properties in H implanted ZnO (vol 47, 342001, 2014)', JOURNAL OF PHYSICS D-APPLIED PHYSICS, vol. 47, no. 41.
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Zhu, L., Ton-That, C. & Phillips, M.R. 2013, 'Nitrogen incorporation in ZnO nanowires using N2O dopant gas', Materials Letters, vol. 99, pp. 42-45.
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Aligned nitrogen-doped ZnO nanowires were grown by chemical vapour deposition using Au catalyst. N incorporation was achieved through the introduction of N2O gas as a dopant source and confirmed by Raman spectroscopy, which reveals additional N-related modes at 275, 580 and 642 cm-1. The nanowires have a hexagonal faceted shape and are predominantly grown along the [001] direction. The nanowire morphology is unaffected by N incorporation. The luminescence peak at 3.24 eV was monitored as a function of N2O content. Intensity analysis of this band reveals that it can be partly attributed to donor-acceptor pair (DAP) emission originating from the N doping. &copy; 2013 Elsevier B.V. All rights reserved.
Ton-That, C., Nguyen, T.-.P. & Dan, Y. 2013, 'Enhanced photoluminescence of polyfluorene by incorporation of Al-doped ZnO nanoparticles', Thin Solid Films, vol. 538, pp. 85-88.
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Photoluminescent and structural properties of nanocomposite films consisting of green-emitting polyfluorene (PF) and Al-doped ZnO (AZO) nanoparticles were investigated. Topographical analysis shows that AZO nanoparticles can be homogeneously dispersed in PF films with less than 10 wt.% AZO. Photoluminescence of the composite films was investigated under wavelength selective excitation of AZO. Photoexcitation with laser energy below the AZO bandgap produces a slight increase in the PF emission intensity with increasing AZO concentration, accompanied by a spectral blueshift. These effects are attributed to separation of polymer chains and twisting of the PF backbone as revealed by Raman spectroscopy. On the other hand, photoexcitation above the AZO bandgap shows a remarkable enhancement, with the intensity at 5-10 wt.% AZO being three times greater than that of pure PF. The luminescence enhancement is attributed to the efficient transfer of energy from AZO nanoparticles to PF. &copy; 2013 Elsevier B.V. All rights reserved.
Cortie, M.B., Coutts, M.J., Ton-That, C., Dowd, A., Keast, V.J. & McDonagh, A.M. 2013, 'On the coalescence of nanoparticulate gold sinter ink', Journal of Physical Chemistry C, vol. 117, no. 21, pp. 11377-11384.
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We examine the mechanism by which thiol-protected gold nanoparticle inks can sinter at surprisingly low temperatures. At room temperature the sample is comprised of randomly close-packed gold nanoparticles of about 2.3 nm diameter with a ligand shell of about 0.2 nm effective thickness. As the particles are heated through 80 C they begin to coarsen, reaching about 10 nm diameter at 180 C. Upon further heating, rapid sintering and grain growth occurs at a temperature that depends on environment and heating rate. Sintering in vacuum requires a higher temperature than in oxidizing environments. Mass spectrometry in the former case is consistent with volatile species such as C 4H9, C2SH, and C2H4 being displaced, whereas XPS shows that the exposed surface of the Au is rich in C and S. However, when sintering is performed in the presence of even trace O 2, it is the Au-S bond that is cleaved, and the sintering temperature is lowered by up to 50 C. In this case mass spectrometry shows the generation of alkane and thiol fragments, some S2 and H2S, and oxidized sulfur-containing species, whereas XPS shows that C and S on the Au surface is much reduced. &copy; 2013 American Chemical Society.
Choi, S., Ton-That, C., Phillips, M.R. & Aharonovich, I. 2013, 'Observation of whispering gallery modes from hexagonal ZnO microdisks using cathodoluminescence spectroscopy', Applied Physics Letters, vol. 103, no. 17.
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Zinc oxide hexagonal microdisks with diameters ranging from 3 ?m up to 15 ?m were fabricated by thermal chemical vapour deposition. Optical characterisation of ZnO microdisks was performed using low temperature (80 K) cathodoluminescence (CL) imaging and spectroscopy. The microdisks exhibited green luminescence locally distributed near the hexagonal boundary of the ZnO microdisks. High resolution CL spectra of the ZnO microdisks revealed whispering gallery modes (WGMs) emission. The experimentally observed WGMs were in excellent agreement with the predicted theoretical positions calculated using a plane wave model. This work could provide the means for ZnO microdisk devices operating in the green spectral range. &copy; 2013 AIP Publishing LLC.
He, R., Tang, B., Ton-That, C., Phillips, M. & Tsuzuki, T. 2013, 'Physical structure and optical properties of Co-doped ZnO nanoparticles prepared by co-precipitation', Journal of Nanoparticle Research, vol. 15, no. 11.
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The structural and optical properties of cobalt-doped zinc oxide (Co-doped ZnO) nanoparticles have been investigated. The nanopowder with Co concentrations up to 5 at% was synthesized by a coprecipitation method. The physical structure and the chemical states of the Co-doped ZnO were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, UV-Visible reflectance and cathodoluminescence (CL) spectroscopy. The results show that cobalt ions predominantly occupy Zn2+ sites in the wurtzite crystal lattice and possess a valence state of 2+. CL analysis revealed that the incorporation of Co2+ creates a new emission band at 1.85 eV, but quenched the near-band-edge luminescence. &copy; Springer Science+Business Media 2013.
Ton-That, C., Foley, M., Phillips, M.R., Tsuzuki, T. & Smith, Z. 2012, 'Correlation between the structural and optical properties of Mn-doped ZnO nanoparticles', Journal of Alloys and Compounds, vol. 522, pp. 114-117.
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The crystallographic and optical properties of Mn-doped ZnO nanoparticles prepared by a sol-gel process have been investigated by X-ray diffraction, UV-visible absorption spectroscopy and cathodoluminescence microanalysis. X-ray diffraction reveals that the nanoparticles have hexagonal wurtzite crystal structure, with the lattice constants along the a- and c-axes increasing with increasing Mn concentration from 0 to 2.4 at%. For all Mn concentrations in this range, the nanoparticles are essentially free of native point defects so that they exhibit only band-edge luminescence. The optical bandgap and band-edge emission energies for Mn-doped ZnO were found to increase in proportion to the lattice constants. The direct correlation between the bandgap and crystal structure suggests that the band-edge optical properties of Mn-doped ZnO is predominantly influenced by the amount of Mn atoms substituting Zn on the lattice sites. &copy; 2012 Elsevier B.V. All rights reserved.
Weston, L., Ton-That, C. & Phillips, M.R. 2012, 'Doping properties of hydrogen in ZnO', Journal of Materials Research, vol. 27, no. 17, pp. 2220-2224.
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The doping properties and stability of hydrogen in zinc oxide (ZnO) crystals have been investigated by cathodoluminescence (CL) spectroscopy. Hydrogen incorporation was achieved by hydrogen plasma at 200 &deg;C. The ZnO near-band-edge (NBE) peak is dramatically enhanced, while the green emission at 2.4 eV is quenched with increasing hydrogen incorporation. These effects are attributed to hydrogen passivating green luminescence centers, which are most likely negatively charged zinc vacancy defects. E-beam irradiation of H-doped ZnO crystals by an intense electron beam with ?W power reverses the hydrogen doping process. This effect is ascribed to the dissociation of H-related defects, formation of Chidden H2, and electromigration of H + under the influence of the local trapped charge-induced electric field. These results highlight the potential to modify the local luminescent properties of ZnO by e-beam irradiation. &copy; 2012 Materials Research Society.
Ton-That, C., Weston, L. & Phillips, M.R. 2012, 'Characteristics of point defects in the green luminescence from Zn- and O-rich ZnO', Physical Review B - Condensed Matter and Materials Physics, vol. 86, no. 11.
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Cathodoluminescence spectra have been measured to determine the characteristics of ubiquitous green luminescence (GL) in nonstoichiometric zinc oxide (ZnO). Zn- and O-rich ZnO were found to exhibit characteristic emissions at 2.53 eV [full width at half-maximum (FWHM) 340 meV] and 2.30 eV (FWHM 450 meV), respectively. Hydrogen was used to probe the physical nature of GL centers. The Zn-rich GL is enhanced upon H incorporation, whereas the O-rich GL is completely quenched as its underlying acceptor-like V Zn centers are passivated by H. The GL emission bands each exhibit remarkably different excitation-power dependencies. The Zn-rich GL follows a close to linear relationship with excitation power, while the O-rich GL exhibits a square-root dependence. Calculations based on bimolecular recombination equations show the defect concentration in Zn-rich ZnO is three orders of magnitude greater than that in O-rich ZnO, indicating V O is more readily formed than V Zn in thermochemical treatments of ZnO. &copy; 2012 American Physical Society.
Lem, L.L.C., Ton-That, C. & Phillips, M.R. 2011, 'Distribution of visible luminescence centers in hydrogen-doped ZnO', Journal of Materials Research, vol. 26, no. 23, pp. 2912-2915.
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ZnO crystals have been investigated by scanning cathodoluminescence microscopy and spectroscopy at 80 K following hydrogen incorporation by plasma exposure. The intensity of the ZnO near-band-edge (NBE) emission is greatly enhanced while the defect-related green emission is quenched following plasma treatment. These effects are attributed to the passivation of zinc vacancies by hydrogen. The green and yellow intensities and their intensity ratios to the NBE vary with excitation depth for both undoped and H-doped ZnO crystals. The intensities of the green and yellow emissions exhibit sublinear dependencies on electron beam excitation density while the NBE intensity increases linearly with the excitation density. These saturation effects with increasing excitation density must be taken into account when assessing defects in ZnO by luminescence characterization. &copy; Copyright Materials Research Society 2011.
Foley, M., Ton-That, C. & Phillips, M.R. 2010, 'Luminescent properties of ZnO structures grown with a vapour transport method', Thin Solid Films, vol. 518, no. 15, pp. 4231-4233.
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ZnO structures were synthesised on the sapphire (112?0) substrate by a vapour transport method in a gas flowing furnace. The influence of the oxygen content in the gas mixture on the morphology and luminescent properties of ZnO structures grown on a strip-like substrate was investigated, with all other growth parameters being kept nominally identical. Integrated electron microscopy and cathodoluminescence analysis shows gradual variations of structural and optical emission properties for ZnO structures grown on the long substrate. Defect-related green luminescence of ZnO is found to be highly dependent on the oxygen vapour in the growth region of the furnace. Our findings demonstrate that the green luminescence is associated with oxygen deficiency in ZnO. &copy; 2009 Elsevier B.V. All rights reserved.
Ton-That, C., Foley, M., Lem, L.L.C., McCredie, G., Phillips, M.R. & Cowie, B.C.C. 2010, 'Diffusion synthesis and electronic properties of Fe-doped ZnO', Materials Letters, vol. 64, no. 3, pp. 386-388.
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Fe-doped ZnO was successfully fabricated by thermal in-diffusion of Fe into ZnO crystals. X-ray absorption near edge structure (XANES), photoemission and cathodoluminescence (CL) spectroscopy have been combined to examine the Fe diffusion and its effects on the electronic and optical properties of the crystal. Depth-resolved CL demonstrates that Fe in-diffusion occurs to at least 4 ?m depth and results in intense green luminescence, whereas the undoped crystal exhibits only the ZnO near-band-edge emission. XANES and valence-band photoemission show that Fe is incorporated as Fe2+/3+ ions on substitutional Zn sites. The results suggest that the variation in the CL properties is due to a change in the oxygen vacancy charge state as a result of electron transfer from Fe. Crown Copyright &copy; 2009.
Phillips, M.R., Drouin, D., Moody, S.J. & Ton-That, C. 2009, 'Imaging fundamental electronic excitations at high spatial resolution using scanning cathodoluminescence microscopy', Microscopy and Microanalysis, vol. 15, no. SUPPL. 2, pp. 670-671.
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Ton-That, C., Phillips, M.R., Foley, M., Moody, S.J. & Stampfl, A.P.J. 2008, 'Surface electronic properties of ZnO nanoparticles', Applied Physics Letters, vol. 92, no. 26.
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The surface electronic structure of ZnO nanoparticles has been studied with photoemission and x-ray absorption spectroscopies. Contrary to expectation, ZnO:Zn phosphor nanoparticles were found to contain a lower oxygen vacancy density on the surface than undoped ZnO counterparts, but oxygen vacancies are in different chemical environments. Cathodoluminescence shows intense green luminescence from the ZnO:Zn surface, while the undoped nanoparticles exhibit only the near-band-edge emission. The results indicate the roles of surface oxygen vacancies and their environment in the previously unexplained green luminescence from the ZnO:Zn material. &copy; 2008 American Institute of Physics.
Foley, M., Ton-That, C. & Phillips, M.R. 2008, 'Cathodoluminescence inhomogeneity in ZnO nanorods', Applied Physics Letters, vol. 93, no. 24.
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Luminescence properties of vertically aligned, crystalline ZnO nanorods are studied by cathodoluminescence (CL) spectroscopy and microscopy. Results show that luminescence characteristics vary dramatically with location on the nanorod as well as CL excitation depth. CL inhomogeneity is observed between the nanorod tip and sidewalls, accompanied by a variation in the chemical environment of surface oxygen ions as probed by photoemission spectroscopy. Our findings demonstrate that CL can provide useful information on the local optical properties of nanostructured materials, which is simply beyond the capability of other methods. &copy; 2008 American Institute of Physics.
Ton-That, C., Stockton, G., Phillips, M.R., Nguyen, T.-.P., Huang, C.H. & Cojocaru, A. 2008, 'Luminescence properties of poly- (phenylene vinylene) derivatives', Polymer International, vol. 57, no. 3, pp. 496-501.
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Background: Conjugated polymers, especially those of the poly(phenylene vinylene) (PPV) family, are promising candidates as emission material in light-emitting devices. The aim of this work was to investigate the dependence of the luminescence properties of PPV-based derivatives on their polymer structure, especially side groups. Results: Three PPV derivatives, BEHPPV, MEHPPV and MEHSPPV, were synthesised and characterised by photoluminescence (PL) and cathodoluminescence (CL) spectroscopies in the temperature range 10-300K. PL and CL spectra of the polymers exhibit similar luminescence peaks, which undergo a blue shift with increasing temperature. The shift in wavelength is accompanied by variations in the relative intensities of emission peaks. Both BEHPPV and MEHPPV display emission characteristics of the PPV backbone, but the peak of MEHPPV shifts to a longer wavelength in comparison with the corresponding peak of BEHPPV at the same temperature. The luminescence spectra of MEHSPPV, which has a sulfanyl incorporated in the side chain, are considerably different from those of the two other derivatives. Conclusions: The results demonstrate that the luminescence properties depend strongly on the chain conformations of the conjugated backbone, which are affected by polymer side chains. &copy; 2007 Society of Chemical Industry.
Ton-That, C., Foley, M. & Phillips, M.R. 2008, 'Luminescent properties of ZnO nanowires and as-grown ensembles', Nanotechnology, vol. 19, no. 41.
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Vertically aligned ZnO nanowires were synthesized on a sapphire (1120) substrate by vapour deposition and their light-emitting properties were characterized using photoluminescence and cathodoluminescence spectroscopies. Besides the nanowires, there exists a mosaic ZnO film on the substrate surface. Comparison of the luminescent properties of the as-grown ensemble and the nanowires extracted from it shows that the mosaic film is the major source of the defect-related green emission while the nanowires possess highly crystalline quality with virtually no defects. Photoemission spectroscopy shows that the valence band states associated with O 2p from the as-grown sample are diminished compared with those from the extracted nanowires. These findings suggest that the green emission partly arises from oxygen vacancies located on the surface of the mosaic film. &copy; IOP Publishing Ltd.
Ton-That, C., Phillips, M.R. & Nguyen, T.-.P. 2008, 'Blue shift in the luminescence spectra of MEH-PPV films containing ZnO nanoparticles', Journal of Luminescence, vol. 128, no. 12, pp. 2031-2034.
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Luminescence properties of nanocomposites consisting of ZnO nanoparticles in a conjugated polymer, poly [2-methoxy-5-(2?-ethyl hexyloxy)-phenylene vinylene] (MEH-PPV), were investigated. Photoluminescence measurements reveal a blue shift in the emission spectrum of MEH-PPV upon incorporation of ZnO nanoparticles into the polymer film while the emission is increasingly quenched with increasing ZnO concentration. In contrast, the structure of the polymer and its conjugation length are not affected by the presence of ZnO nanoparticles (up to 16 wt% ZnO) as revealed by Raman spectroscopy. The blue shift and photoluminescence quenching are explained by the separation of photogenerated electron-hole pairs at the MEH-PPV/ZnO interface and the charging of the nanoparticles. Crown Copyright &copy; 2008.
Ton-That, C., Foley, M. & Phillips, M.R. 2008, 'Luminescent properties of ZnO nanowires and as-grown ensembles.', Nanotechnology, vol. 19, no. 41, p. 415606.
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Vertically aligned ZnO nanowires were synthesized on a sapphire ([Formula: see text]) substrate by vapour deposition and their light-emitting properties were characterized using photoluminescence and cathodoluminescence spectroscopies. Besides the nanowires, there exists a mosaic ZnO film on the substrate surface. Comparison of the luminescent properties of the as-grown ensemble and the nanowires extracted from it shows that the mosaic film is the major source of the defect-related green emission while the nanowires possess highly crystalline quality with virtually no defects. Photoemission spectroscopy shows that the valence band states associated with O 2p from the as-grown sample are diminished compared with those from the extracted nanowires. These findings suggest that the green emission partly arises from oxygen vacancies located on the surface of the mosaic film.
Ton-That, C., Dowd, A., Shard, A.G., Dhanak, V.R., Taninaka, A., Shinohara, H. & Welland, M.E. 2007, 'Electronic structure of potassium-doped La@ C82 metallofullerene studied with photoelectron spectroscopy', Physical Review B - Condensed Matter and Materials Physics, vol. 76, no. 16.
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The electronic structure of potassium-doped La@ C82 has been studied with synchrotron-radiation photoelectron spectroscopy. Ultraviolet photoemission measurements indicate evolution of the valence-band states of La@ C82 with increasing potassium content, but Kx La@ C82 remains semiconducting for all x between 0 and 6, with a band gap of at least 0.4 eV, in contrast to K-doped C60. The valence-band features, including those which arise from the interactions of C82 cages with La and K, show oscillatory behavior of photoemission intensity with incident photon energy. The strength of the oscillations varies between the states, and we associate diminished oscillations with increased localization of electron density. Finally, although core and valence states of La@ C82 change considerably with potassium dosage, the encapsulated La atom is well shielded from the outside chemical environment and its valency remains close to 3+. &copy; 2007 The American Physical Society.
Ton-That, C., Shard, A.G., Dhanak, V.R., Shinohara, H., Bendall, J.S. & Welland, M.E. 2006, 'Electronic structure of pristine and potassium-doped Y@ C82 metallofullerene', Physical Review B - Condensed Matter and Materials Physics, vol. 73, no. 20.
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Ultraviolet photoelectron spectra (UPS) of Y@ C82 have been measured with a synchrotron light source. UPS spectra reveal the photoelectron onset at 0.3 eV below the Fermi level, indicating the semiconducting nature of the metallofullerene, and contain low-binding-energy features associated with charge transfer from the encapsulated atom to molecular orbitals. As for empty fullerenes, when the incident photon energy is varied, the relative intensities of the valence-band features of Y@ C82 oscillate, showing that the endohedral atom does not significantly disturb the photoelectron interference effects of the fullerene cage. For K2.3 Y@ C82, which forms a fcc-like structure, core and valence states of Y@ C82 broaden and shift to higher binding energies but there is no evidence of density of states at the Fermi level. Finally, although core and valence states of Y@ C82 change considerably upon an intercalation of potassium, the encapsulated atom is well shielded from the outside chemical environment and its valency remains close to 3+. &copy; 2006 The American Physical Society.
Ton-That, C., Welland, M.E., Larsson, J.A., Greer, J.C., Shard, A.G., Dhanak, V.R., Taninaka, A. & Shinohara, H. 2005, 'Electrostatic ordering of the lanthanum endoatom in La@C 82 adsorbed on metal surfaces', Physical Review B - Condensed Matter and Materials Physics, vol. 71, no. 4.
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An investigation of the dynamic behavior of the endohedral La atom in La@C 82 adsorbed on Ag(111) or Cu(111) using the normal-incidence x-ray standing wavefield (NIXSW) technique is presented. For the surface-normal (111) reflection, NIXSW demonstrates different La absorption profiles between the La@C 82 monolayer and multilayer. Analysis of the profiles reveals significant La ordering in the monolayer; the degree of ordering increases as the film is cooled. First-principles calculations provide a description of the endohedral metal atom-cage bonding and the fullerene-surface interaction. Combined, these effects give rise to preferential binding sites for the endoatom resulting from electrostatic interactions within the adsorbed fullerene. &copy;2005 The American Physical Society.
Shard, A.G., Ton-That, C., Campbell, P.A. & Dhanak, V.R. 2004, 'Site occupancy of chlorine on Cu(111) using normal-incidence x-ray standing waves: The energy difference between fcc and hcp hollow sites', Physical Review B - Condensed Matter and Materials Physics, vol. 70, no. 15.
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It has previously been established that the lowest energy site for chlorine atoms on Cu(111) is the "fcc" hollow. However, substantial population of the "hcp" hollow at room temperature indicates that there is a relatively small difference in energy between the two sites. We show that this energy difference must be less than 10 meV by measuring the relative populations using normal-incidence x-ray standing waves and comparing the results to Monte Carlo simulations. This result is consistent with recent density functional theory calculations which indicate an energy difference of approximately 5 meV.
Ton-That, C., Shard, A.G., Egger, S., Dhanak, V.R. & Welland, M.E. 2003, 'Modulations of valence-band photoemission spectrum from C-60 monolayers on Ag(111)', PHYSICAL REVIEW B, vol. 67, no. 15.
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Ton-That, C., Shard, A.G., Egger, S., Dhanak, V.R., Taninaka, A., Shinohara, H. & Welland, M.E. 2003, 'Orientation and constraints of endohedral lanthanum in La@C-82 molecules adsorbed on Cu(111)', PHYSICAL REVIEW B, vol. 68, no. 4.
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Ton-That, C., Shard, A.G., Egger, S., Taninaka, A., Shinohara, H. & Welland, M.E. 2003, 'Structural and electronic properties of ordered La@C82 films on Si(1 1 1)', Surface Science, vol. 522, no. 1-3.
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The growth of endohedral fullerene La@C82 films on Si(1 1 1) has been explored by scanning tunneling microscopy (STM). The STM images reveal the molecules are hexagonally close-packed in multilayer films with the inter-molecular separation similar to that of the La@C82 crystal. Electronic properties of the films have been investigated by ultraviolet photoelectron spectroscopy (UPS) and tunneling spectroscopy, which provide comparable results for the electronic structure of the occupied electronic states. Tunneling spectroscopy also gives an insight into the unoccupied states and shows that the edges of the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) both approach the Fermi level, indicating that the film is metallic or semi-metallic. An electronic structure of the La@C82 film is proposed from the spectroscopic results. &copy; 2002 Elsevier Science B.V. All rights reserved.
Ton-That, C., Shard, A.G., Egger, S., Dhanak, V.R. & Welland, M.E. 2003, 'Modulations of valence-band photoemission spectrum from C60 monolayers on Ag(111)', Physical Review B - Condensed Matter and Materials Physics, vol. 67, no. 15, pp. 1554151-1554156.
The photon-energy dependences of valence-band photoemission spectra have been investigated for monolayer, submonolayer, and multilayer C60 films on Ag(111). The intensity ratios of the highest and next-highest occupied molecular orbitals (the HOMO and NHOMO) of the films are found to oscillate with the same period as the incident photon energy is varied, but the amplitude of the oscillation is smaller with lower film coverage. Since the characteristics of the molecular orbitals of C60 in the monolayers are considerably different from those of the thick film due to the charge-transfer interaction between the metallic surface and the molecule the observation opposes the proposal in which the oscillation is entirely dependent on the nature of the empty and occupied states involved in the photoemission process. The results instead support the photoelectron interference model by Hasegawa et al. [Phys. Rev. B 58, 4927 (1998)]. Variation of the oscillation amplitude with C60 coverage is explained in terms of hybridization of the HOMO and NHOMO with the surface, Depositing C60 onto Ag(111) held at a high temperature produces a single-phase (2?3 2?3) R30&deg; C60 monolayer. Angle-resolved photoemission experiments reveal changes in the relative intensities of the two bands with the azimuth-emission angle, measured with respect to the symmetry axes of the ordered monolayer. The intensity modulation is due to anisotropic scattering of photoelectrons along various molecular axes.
Ton-That, C., Shard, A.G., Egger, S., Dhanak, V.R., Taninaka, A., Shinohara, H. & Welland, M.E. 2003, 'Orientation and constraints of endohedral lanthanum in La@C82 molecules adsorbed on Cu(111)', Physical Review B - Condensed Matter and Materials Physics, vol. 68, no. 4, pp. 454241-454246.
The orientation of the endohedral La atom in La@C82 adsorbed on Cu(111) has been determined using the normal-incidence x-ray standing wave (NIXSW) technique. The NIXSW measurements were carried out using both the normal ?111? and 70.5&deg; off-normal ?111? Bragg reflections. The La ?111? profiles of the La@C82 monolayer and submonolayers are found to be similar to the ?111? profile of the multilayer, while the ?111? profiles of the monolayer and submonolayers are distinctly different with much lower peak height. This indicates significant ordering of the La atoms in the monolayers with respect to height above the Cu surface but little lateral ordering. The results of curve fits to the monolayer profiles show that the La atoms are regionally located at 1.62 (&plusmn;0.77) &Aring; above an extended scattering plane from the crystal. Cooling the monolayers to a temperature of 170 K increases ordering of the La atoms with respect to height above the surface. The NIXSW results demonstrate that the endohedral molecules adopt a restricted orientation upon adsorbing on the surface and the La atom resides on either the upper or lower half of the cage. Theoretical calculations show that the most stable La positions are located in the top half of the fullerene cage, which supports the La upper configuration. The results are explained in terms of electrostatic interactions between the endohedral atom, the fullerene cage, and the substrate.
Ton-That, C., Shard, A.G. & Bradley, R.H. 2002, 'Surface feature size of spin cast PS/PMMA blends', Polymer, vol. 43, no. 18, pp. 4973-4977.
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Thin films of polystyrene (PS)/polymethylmethacrylate (PMMA) blends have been spin cast on mica from chloroform solutions. When the concentration of PMMA in the casting solution is less than that of PS a pitted morphology is formed. The average sizes of the pits are shown to increase with both the total concentration of the casting solution and the relative concentration of PMMA. The change in pit size is explained in terms of incomplete dewetting of a PMMA solution from an underlying PS solution. For a given ratio of PMMA/PS the average pit diameters appear to increase linearly with the square of the film thickness, the gradient of which is dependent on the film composition. &copy; 2002 Elsevier Science Ltd. All rights reserved.
Teare, D.O.H., Emmison, N., Ton-That, C. & Bradley, R.H. 2001, 'Effects of serum on the kinetics of CHO attachment to ultraviolet-ozone modified polystyrene surfaces', Journal of Colloid and Interface Science, vol. 234, no. 1, pp. 84-89.
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Attachment kinetics of Chinese hamster ovarian (CHO) cells were investigated on ultraviolet-ozone oxidized polystyrene (UVO-PS) dishes in the presence and absence of serum. The surface chemistry of UVO-PS has been extensively characterized. Although cells attached rapidly to the oxidized dishes with serum present it was found that serum actually inhibits the rate of attachment. Spreading of attached cells was favored by the presence of serum. It is suggested that the increased quantity of hydrophilic carboxyl groups on longer exposed UVO-PS leads to a change in the protein layer adsorbed from serum and also a higher affinity of the surface for extracellular proteins secreted by the attached cells. The UVO-PS surfaces present a new way of producing tissue culture grade polystyrene (TCPS) in a highly controllable method, which would ensure greater consistency in TCPS surfaces. &copy; 2001 Academic Press.
Ton-That, C., Shard, A.G., Teare, D.O.H. & Bradley, R.H. 2001, 'XPS and AFM surface studies of solvent-cast PS/PMMA blends', Polymer, vol. 42, no. 3, pp. 1121-1129.
Films of polystyrene (PS) and poly(methyl methacrylate) (PMMA) blends of two different thicknesses have been examined by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Blends with different compositions were spin-cast onto a mica substrate with chloroform as the mutual solvent. XPS measurements revealed surface enrichment of PMMA in all compositions. The thicker (66 nm) films exhibit a higher degree of PMMA surface enrichment than the thinner (17 nm) films. AFM imaging allows distinctions to be drawn between blends with differing compositions. The blend films with less than 50% PMMA bulk concentration generally exhibit pitted surfaces; the pit size varies with film thickness and bulk composition. When the PMMA bulk concentration is greater than 50%, the film surface changes to show island-like phase-separated structure. The surface segregation and morphology are explained in terms of solubilities of the two polymers in the solvent and dewetting of PMMA relative to PS. The phase domains on the film surface have also been resolved by frictional force microscopy (FFM) using hydrophilic tips bearing hydroxyl groups. &copy; 2000 Elsevier Science Ltd. All rights reserved.
Ton-That, C., Teare, D.O.H. & Bradley, R.H. 2000, 'Friction, surface oxidation, and polar free energy for polymer surfaces by chemical force microscopy', Chemistry of Materials, vol. 12, no. 8, pp. 2106-2111.
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The surfaces of polystyrene (PS) films and polystyrene/poly(methyl methacrylate) (PS/PMMA) blend films have been studied using atomic force microscopy with probe tips coated with specific chemical functional groups (chemical force microscopy). PS films have been surface oxidized using an ultraviolet/ozone technique to give substrates with total surface oxygen levels between 0 and 20 atom % measured using XPS. XPS has also been used to study the chemistry of these substrates. AFM tips carrying hydroxyl-terminated (polar) and methyl-terminated (apolar) groups have been produced and used to characterize the statistical effects of oxidation surface chemistry. Scans have been acquired in frictional force mode with the tip moving orthogonal to the long axis of the cantilever. The torsional constant of the AFM cantilever has been used to derive absolute friction values for the systems studied. Increased frictional force is observed when surfaces of increasing polarity (defined by the oxygen level measured by XPS or by the water contact angle) are probed using the hydroxyl-terminated polar tips. Absolute coefficients of friction are shown to be directly proportional to the surface oxygen level and to the water contact angle, and these can therefore be used to approximate changes in polar free energy caused by oxidation. The phase-separated domains of PS and PMMA in blend thin films can be resolved at a submicrometer level using the CFM approach with a polar tip.
Ricketts, B.W. & Ton-That, C. 2000, 'Self-discharge of carbon-based supercapacitors with organic electrolytes', Journal of Power Sources, vol. 89, no. 1, pp. 64-69.
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The self-discharge behaviour of supercapacitors is an important factor when considering their suitability for some applications. In this paper, measurements of the self-discharge rates of carbon-based supercapacitors with organic electrolytes are presented and interpreted in terms of two mechanisms. The first is the diffusion of ions from regions of excess ionic concentration formed during the charging of the capacitor and the second is leakage of charge across the double-layer at the electrolyte-carbon interfaces in the capacitor. The dependence of the self-discharge rate on temperature and on the initial voltage across the capacitor is described.
Ton-That, C., Shard, A.G., Daley, R. & Bradley, R.H. 2000, 'Effects of annealing on the5 surface composition and morphology of PS/PMMA blend', Macromolecules, vol. 33, no. 22, pp. 8453-8459.
Films of polystyrene (PS) and poly(methyl methacrylate) (PMMA) blend have been annealed at a temperature above their glass transition temperatures for up to 48 h. Surface chemical compositions of the cast and annealed films were measured by X-ray photoelectron spectroscopy (XPS) while surface topographical changes were followed by atomic force microscopy (AFM). The blend films spin-cast from chloroform produce nonequilibrium surfaces with a significant excess of PMMA. The polymer component with a lower surface free energy, PS, is shown to segregate to the surface upon annealing. The PS surface concentration of the films, containing 50% PS:50% PMMA in the bulk, was evaluated using the ester peak in XPS C Is spectra (sampling depth ~ 9 nm) and found to increase from ~5% (freshly spin-cast film) to a saturated level of ~47% after 17 h of annealing. AFM imaging reveals evolution of blend morphology with annealing time. The spin-cast films prior to annealing exhibit pitted topography with typical pit size of ~1.2 /im and depth of 30-40 nm. As the annealing process proceeds, these pits get continually shallower. Frictional force microscopy with hydroxylated tips recorded surface phase separations for the films of 2-4 h annealing. As the annealing continues to above 14 h, the pitted structure becomes distorted. The surface enrichment and morphology changes upon annealing are explained by dewetting of PMMA relative to PS. &copy; 2000 American Chemical Society.
Teare, D.O.H., Emmison, N., Ton-That, C. & Bradley, R.H. 2000, 'Cellular attachment to ultraviolet ozone modified polystyrene surfaces', Langmuir, vol. 16, no. 6, pp. 2818-2824.
The surfaces of standard untreated polystyrene cell culture dishes have been oxidatively modified for up to 8 min exposure time using an ultraviolet ozone treater in order to promote cell adhesion. Surface oxygen chemisorption and topographical modification has been characterized using monochromatic X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), respectively. The oxidation process is shown to proceed at low exposure times (<60 s) via the formation of C-OR groups, although some R2C=0 and RO-C=0 groups are also formed. At longer treatments, RO-C=O groups become the dominant species, although the other groups are also present. The maximum level of oxygen reached is 36 atomic %. Some of the oxygen present at surfaces treated at times of >60 s is in the form of loosely bound low molecular weight oxidized material (LMWOM) which is produced by oxidative scission of the PS backbone. Water washing leads to a reduction in surface oxygen content mainly by the removal of RO-C=0 and R2C=0 functional groups. The residual stable oxygen levels, which can be introduced, are approximately 20-25 atomic %. Surface chemistry changes are accompanied by the formation of surface spikes which are about 30 nm high and 300-400 nm wide. A correlation between treatment time/oxygen level and overall roughness is observed. The effect of washing upon the topography is to slightly increase the surface roughness, although not to a significant degree. The attachment kinetics of adhesion for Chinese hamster ovary cells show that adhesion occurs much more rapidly for oxidized surfaces than for untreated control materials. A direct correlation between the levels of oxidation and the rate of cell adhesion is demonstrated. &copy; 2000 American Chemical Society.
Teare, D.O.H., Ton-That, C. & Bradley, R.H. 2000, 'Surface characterization and ageing of ultraviolet-ozone-treated polymers using atomic force microscopy and x-ray photoelectron spectroscopy', Surface and Interface Analysis, vol. 29, no. 4, pp. 276-283.
Ultraviolet-ozone (UVO) treatment of poly(ethyleneterephthalate) (PET) films and polystyrene (PS) dishes of up to 10 min exposure has been studied. Surface polarity, oxygen chemisorption and topographical change were analysed by contact angle measurement, x-ray photoelectron spectroscopy and atomic force microscopy. Studies of the treated surface reveal the chemistry behind the increasing oxygen content. The oxidation process is shown to proceed via different mechanisms for the two polymers. Polyethyleneterephthalate appears to undergo a Norrish-type chain depolymerization reaction, whereas PS undergoes a much more random chain scission attack. Atomic force microscopy analysis shows an increase in the surface roughness with increasing exposure to UVO for both polymers, with grains of low-molecular-weight oxidized material (LMWOM) forming at the surface. This material can be removed by washing. Surfaces that remain after washing have a higher concentration of oxygen species than the native surfaces. Analysis of aged surfaces shows that for oxidized PET a relaxation process occurs, lowering the levels of surface oxygen. This appears to occur due to the diffusion of LMWOM into the PET bulk. Relaxation of the oxidized PS surface is less thermodynamically favourable owing to the apolar nature of the PS. Copyright &copy; 2000 John Wiley & Sons, Ltd.
Ton-That, C., Shard, A.G. & Bradley, R.H. 2000, 'Thickness of spin-cast polymer thin films determined by angle-resolved XPS and AFM tip-scratch methods', Langmuir, vol. 16, no. 5, pp. 2281-2284.
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Polystyrene (PS) and poly(methyl methacrylate) (PMMA) thin films (<100 nm thickness) have been spin-cast from chloroform solution onto cleaved mica surfaces (roughness within 0.2 nm). An algorithm for calculating the film thicknesses based on the relative intensities of the C 1s peak of the films and the Si 2s peak of the mica from angle-resolved X-ray photoelectron spectroscopy (XPS) is presented. The film thickness changes as a function of casting conditions. Data from this approach are comparable with thickness measured by an atomic force microscopy (AFM) tip-scratch method in the range 1.5-5.5 nm. Thicknesses of the films are shown to increase linearly with concentration of cast solutions.
Ton-That, C., Campbell, P.A. & Bradley, R.H. 2000, 'Frictional force microscopy of oxidized polystyrene surfaces measured using chemically modified probe tips', Langmuir, vol. 16, no. 11, pp. 5054-5058.
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Chemical force microscopy (CFM) using probe tips that have been chemically functionalized to give hydroxylated (polar) or methylated (apolar) surfaces has been used to investigate frictional properties of polystyrene (PS) films that have been oxidatively modified to varying degrees using an ultraviolet-ozone (UVO) treatment. Oxygen chemisorption levels and functional group chemistry of the PS films have been followed using X-ray photoelectron spectroscopy (XPS), and the resulting data have been correlated with lateral (frictional) force measurements for each surface type. CFM was performed on the native and treated films using tips that were either hydroxyl- or methyl-terminated. The treated films exhibit substantially higher friction than untreated films when imaging with the hydroxyl-terminated tips. Friction is reduced when nonpolar methyl-functionalized tips are employed. The higher frictional forces and coefficients measured for the oxidized surfaces using the hydroxyl-terminated tip are due to hydrogen bonding between the polar hydroxyl groups of the tip and the oxygen functional groups of the PS surfaces.
Ton-That, C., Teare, D.O.H., Campbell, P.A. & Bradley, R.H. 1999, 'Surface characterisation of ultraviolet-ozone treated PET using atomic force microscopy and X-ray photoelectron spectroscopy', Surface Science, vol. 433, pp. 278-282.
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The effects of ultraviolet-ozone (UVO) oxidation of polyethyleneterephthalate (PET) surfaces have been studied using atomic force microscopy and X-ray photoelectron spectroscopy. Surface oxygen increases from 26 at.% (untreated) to 37 at.% for the most oxidised surfaces produced and an increase in mean surface roughness and grain size is also observed. The larger grains appear to result from the formation of low molecular weight oxidised species by PET chain scission at the ester group and the material formed is therefore COOH/COOR rich. These species are mobile on the polymer surface and coalesce to form 200 nm grains which can be partially removed by water washing. The surface beneath is still rougher and more highly oxidised than untreated PET but the increase in stable oxygen is due to the formation of COH/COR groups.
Kirkup, L. & Ton-That, C. 1998, 'A direct reading thermometer based on a silicon diode', Physics Education, vol. 33, pp. 302-305.