Physics
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Item 3D Imprint Lithography Using Sixny Molds(2002) Jayatissa, N.W.K.; Alkaisi, M.M.; Blaikie, R.J.Three-dimensional patterning reduces more complicated alignment steps in the fabrication of micro and nano-scale structures. Multiple lithography processes with interlevel alignment or single lithography with multi layer resist is essential for three dimensional patterning. A number of 3D structures have found immediate applications in a range of microelectronic systems such. as micro-optics, micro-electromechanical systems, and in monolithic microwave integrated circuits. We have previously demonstrated that imprint at low temperatures (well below the glass transition of the resist) is possible for a number of structures using silicon nitride (SixNy) molds. A low temperature process is important for pattering substrates or polymer-based materials that are intolerant of high temperatures. The advantages of using SixNy for mold making are the capability of forming reliable nanoscale structures and its surface properties that allows imprint without sticking. In this work we will present pattering results for three-dimensional structures using nanoimprint lithography with SixNy molds. The mold material consists of a 1?m thick low stress SixNy layer deposited by low-pressure chemical vapour deposition (LPCVD) on to a silicon substrate. Patterning of the SixNy was performed using electron beam lithography at 50keV into ma-N 24033 negative tone resist.Item A Correlation between activation energy and light absorption of WO3 incorporated TiO2(2002) Dharmaratna, W.G.D.; Roshan, P.W.C.; Siripala, W.; Wijesundera, R.P.The variation of electrical conductivity properties and light absorption properties are studied in W 6+ incorporated Ti02, Both conductivity and light absorption depend on the percentage of W 6+ incorporated into the crystal matrix of Ti02. The activation energy decreased by a maximum of 30% as a result of doping and the lowest activation energy was measured when the dopant concentration was 0.2%. The light reflectance decrease with the dopant percentage, but not in a monotonically decreasing passion. The variation of light reflectance as a function of dopant concentration showed a minimum when the dopant concentration was 0.2%. Both features are quite important in improving the photocatalytic properties of Ti02.Item A Cu2O/TiO2 heterojunction thin film cathode for photoelectrocatalysis(Solar Energy Materials and Solar Cells, 2003) Siripala, W.; Ivanovskaya, A.; Jaramillo, T.F.; Baeck Sung-Hyeon; McFarland, E.W.A thin film heterojunction photocathode was developed consisting of 100nm of n-type titanium dioxide (TiO2) cathode surface deposited on p-type cuprous oxide (Cu2O). The cuprous oxide was deposited electrochemically on Ti foil. A photocurrent of 0.7mA/cm2(at -1V bias) and an open circuit photovoltage of 460mV were obtained under an illumination of 700W/m2. The photoresponse as a function of pH demonstrated that the TiO2 film protected the Cu2O underlayer against corrosion. These results suggest that using a simple and inexpensive heterostucture configuration, the corrosion limitations of Cu2O alone may be overcome while maintaining a relatively high efficiency for photoelectrolysis.Item A Fundamental Study of the Transport Properties of Aqueous Superacid Solutions(Journal of Physical Chemistry B, 2010) Suarez, S.N.; Jayakody, J.R.P.; Greenbaum, S.G.; Zawodzinski, T.; Fontanella, J.J.An extensive investigation of the transport properties of aqueous acid solutions was undertaken. The acids studied were trifluoromethanesulfonic (CF3SO3H), bis(trifluoromethanesulfonyl)imide [(CF3SO2)2NH], and para-toluenesulfonic (CH3C6H4SO3H), of which the first two are considered superacids. NMR measurements of self-diffusion coefficients (D), spin?lattice relaxation times (T1), and chemical shifts, in addition to ionic conductivity (?), viscosity (?), and density measurements, were performed at 30 �C over the concentration range of 2?112 water to acid molecules. Results showed broad maxima in ? for all three acids in the concentration range of 12?20 water to acid molecules. This coincided with minima in anion Ds and is attributed to a local molecular ordering, reduced solution dielectric permittivity, and increased ionic interactions. The location of the maxima in ? correlates with what is observed for hydrated sulfonated perfluoropolymers such as Nafion, which gives a maximum in ionic transport when the ratio of water to acid molecules is about 15?20. Of the three acids, bis(trifluoromethanesulfonyl)imide was found to be the least dependent on hydration level. The occurrence of the anticorrelation between the ionic conductivity maximum and the anion self-diffusion minimum supports excess proton mobility in this region and may offer additional information on the strength of hydrogen bonding in aqueous media as well as on the role of high acid concentration in the Grotthuss proton transport mechanism.Item A Home Made Double Slab Pyranometer for Irradiance Measurements(Proceedings of the 44th Technical Session of the Sri Lanka Association for the Advancement of Science (SLAAS), 1988) Punyasena, M.A.; Jayasuriya, K.D.Item A Mössbauer effect study of barium ferrite ball-milled in air(Hyperfine Interactions, 1994) Jayasuriya, K.D.; Kaczmarek, W.A.; Wu, E.; Campbell, S.J.The effects of dry-milling BaFe12O19 in air for periods of 190, 360, 590, 690 and 1000 h have been investigated by X-ray diffraction and M�ssbauer effect measurements. The sizes of the BaFe12O19 particles decrease on milling, as expected, although a partial decomposition of BaFe12O19 to ?-Fe2O3 is found to take place on extended milling (1000 h). The room temperature M�ssbauer spectra are consistent with superparamagnetic relaxation associated with the fine BaFe12O19 and ?-Fe2O3 particles. The X-ray diffraction patterns of all the milled samples exhibit features indicative of a disordered structural state, consistent with the nanoscale particles and a nanostructured state.Item A Mössbauer study of ball-milled Co?Fe?Si?B: I: dry milling(Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 1993) Jayasuriya, K.D.; Campbell, S.J.; Calka, A.; Jing, J.X-ray diffraction and M�ssbauer effect measurements of ball-milled Co?Fe?Si?B material which had first been re-crystallized from amorphous melt-spun ribbon are presented. Fully amorphous powders have been obtained after only 50 h milling with a ball-to-powder ratio of BPR ? 84. The amorphous powders exhibit chemical and structural inhomogeneities compared with the starting melt-spun amorphous ribbons.Item A Mössbauer study of ball-milled Co?Fe?Si?B: II: with surfactants(Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 1993) Jayasuriya, K.D.; Campbell, S.J.; Calka, A.; Jing, J.The use of a range of organic surface active substances in the ball milling of Co?Fe?Si?B has been studied by X-ray diffraction and M�ssbauer effect measurements. Effects characteristic of superparamagnetic relaxation of the magnetic moments are observed in the M�ssbauer spectra of samples milled with either an anionic or a cationic surfactant. This behaviour is linked with the decreased particle size and high shape homogeneity of surfactant-aided milled samples compared with the amorphous powders resulting from standard dry milling. Impurity iron-oxide phases are also found to occur in samples milled in the presence of surfactants.Item A Mössbauer study of the oxidation state of Fe in silicate melts(American Mineralogist, 2004) Jayasuriya, K.D.; O-Neill, H.S.C.; Berry, A.J.; Campbell, S.J.Fe3+/?Fe ratios were determined from M�ssbauer spectra recorded for a series of 17 anorthite-diopside eutectic glasses containing 1 wt% 57Fe2O3 quenched from melts equilibrated over a range of oxygen fugacities from fO2~ 105 bars (Fe3+/?Fe = 1) to 10?13 bars (Fe3+/?Fe = 0) at 1682 K. Fe3+/Fe2+ was found to be proportional to fO2 to the power of 0.245 � 0.004, in excellent agreement with the theoretical value of 0.25 expected from the stoichiometry of the reaction Fe2+O + 0.25 O2 = Fe3+O1.5. The uncertainty in the Fe3+/?Fe ratios determined by M�ssbauer spectroscopy was estimated as � 0.01 (1?) from the fit of the data to the theoretical expression, which is significantly less than that quoted for previous measurements on silicate glasses; this results from fitting the spectra of a large number of systematically varying samples, which allows many of the ambiguities associated with the fitting procedure to be minimized. Fe3+/?Fe ratios were then determined for samples of the anorthite-diopside eutectic composition equilibrated at selected values of fO2, to which up to 30 wt% Fe2O3 had been added. Fe3+/?Fe was found to vary with ?Fe (or FeOT), but both the 1 wt% and high FeOT data could be satisfactorily fit assuming the ideal stoichiometry (i.e., Fe3+/Fe2+ ?fO2 1/4) by the inclusion of a Margules term describing Fe2+-Fe3+ interactions. The large negative value of this term indicates a tendency toward the formation of Fe2+-Fe3+ complexes in the melt. The resulting expression, using the ideal exponent of 0.25, gave a fit to 289 Fe3+/?Fe values, compiled from various literature sources, of similar quality as previous empirical models which found an exponent of ~0.20. Although the empirical models reproduce Fe3+/?Fe values of glasses with high FeOT reasonably well, they describe the data for 1 wt% FeOT poorly. The non-ideal values of the exponent describing the dependence of Fe3+/?Fe on fO2 at high FeOT are an artifact of models that did not include a term explicitly to describe the Fe2+-Fe3+ interactions. An alternative model in which Fe in the silicate melt is described in terms of three species, Fe2+O, Fe3+O1.5, and the non-integral valence species Fe2.6+O1.3, was also tested with promising results. However, at present there is no model that fits the data within the assessed accuracy of the experimental measurements.Item A multinuclear NMR study of ion transport in P(EO)nLiBETI complexes(Solid State Ionics, 2005) Suarez, S.N.; Abbrent, S.; Jayakody, J.R.P.; Greenbaum, S.G.; Shin, J.H.; Passerini, S.A study of ion transport in P(EO)nLiBETI complexes was undertaken, using both AC impedance and nuclear magnetic resonance (NMR) spectroscopy. 1H, 7Li and 19F NMR techniques were used to investigate structure and dynamics as a function of temperature for n=3, 6, 8, 12 and 20. Spin?lattice relaxation times (T1) and spectral information were obtained from ?50 to 100 �C. Variable temperature self-diffusion coefficients (D) and ionic conductivity (?) measurements were also performed. Anion diffusion (DF) results displayed a dependence on available free volume, increasing with decreasing salt concentration. On the other hand, cation diffusion (DLi) results did not follow this trend. DLi for n=3 and 6 suggest the presence of ionic mobility in the crystalline phase, with a significant rise above the melting point. A transition from a crystalline to amorphous phase dominated ion transport occurs at n=8. This is supported by ? results, which exhibited a VTF type of behavior for n?8 that is associated with ion transport in the amorphous phase.Item A Photoelectrochemical Investigation of n- and p-type semi�conducting behaviour of Copper Oxide Films(Semiconductor Science and Technology, 1989) Siripala W; Kumara K PCopper(I) oxide films were prepared on copper substrates by exposing them to solutions containing Cu2+ ions, and it was observed that the photoresponse of these films electrodes in a photoelectrochemical cell is both n- and p-type. However, it was observed that the n-type behaviour of these film electrodes could be enhanced and the p-type behaviour could be reduced by adjusting the pH of the solution in which the oxide films had been prepared. The simultaneous existence of spatially separated n- and p-type regions in the Cu2O film is suggested as the possible reason for these observations. The anodic oxidation of copper was considered to be the origin of the p-type regions while the n-type regions were considered to be the result of the cathodic deposition of Cu2O.Item A Photoluminescence Study on CuInS2 Thin Films Prepared by the Sequential Deposition Technique(Annual Research Symposium -Faculty of Graduate Studies, University of Kelaniya, 2001) Wijesundera, R.P.; Siripala, W.Solar energy conversion to electrical energy using low cost solar cells contributes substantially for a solution to the present global energy crisis. In this respect, various semiconductor materials are being studied for possible applications in low cost solar cell devices. Copper Indium Di Sulphide (CuInS2) is very promising semiconductor material because of its electronic and optical properties, which are suitable for these applications. Low-cost semiconductor material growth techniques normally produce unwanted electronic states in the material producing undesired effects on the solar cell application. In this investigation, CuInS2 thin films prepared by the electrodeposition of Cu films followed by the In deposition were used to prepare Cu-In alloy. Cu-In alloy was suplherized in an H2S chamber to grow CuInS2 films. X-ray diffraction and optical characterizations suggest that the films are of good quality. The photoluminescence study at low temperature produced two peaks at 815nm and 880 nm. This result suggests the band to band transition and the sulfur vacancy transition. Our study reveals that there are no other defect electronic states in the band gap except the S vacancies, confirming the good quality of the materialItem A specific heat study of natural haematite around the Morin transition and the effects of entrapped water(Journal of Physics and Chemistry of Solids, 1985) Jayasuriya, K.D.; Stewart, A.M.; Campbell, S.J.The Morintransition in samples of natural haemitite has been examined by specificheat and M�ssbauer measurements. Specificheat measurements on as-received samples reveal an anomaly at ? 273.2 K and irregular behaviour at higher temperatures. However, heat treatment of samples demonstrated that these effects were due to water trapped inside the natural specimens. No anomaly due to the Morintransition was observed in the specificheat although the expected changes in the M�ssbauer hyperfine parameters were observed for the different magnetic phases below and above the Morintransition. The results are consistent with a coexistence of the two phases and a spread of temperatures ? 40 K over which the transitions take place throughout the material of purity 99.6 wt%.Item A Study of CuInS2 Thin Films for Photovoltaic Applications(Proceedings of the Technical Session of Institute of Physics, 1999) Wijesundera, R.P.; Nadesalingam, M.P.; Siripala, W.; Jayasuriya, K.D.; Kalingamudali, S.R.D.; Jayanetti, J.K.D.S.; de Silva, K.T.L.Thin films of copper indium disulphide (CuInS2) on Ti Substrate were prepared by annealing potentiastatically electrodeposited Cu-In alloy in H2S gas at 5500 C. Films were characteristised by X-ray diffraction (XRD), scanning electron microscopy (SEM), and spectral response in a polysulphide electrolyte. XRD measurements revealed the formation of the polysrystaline CuInS2 thin films and the abscence of any other phases. SEM showed the formation of crystallites having the size about 0.2 ?m. Variations of spectral response, open-circuit voltage (Voc) and short circuit current (Isc) with annealing in air have been studied. As deposited CuInS2 films exhibit a direct band gap of 1.5 eV, and shows n-type behaviour when used in a Photoelectrochemical (PEC) cell. Heat treatment shows a considerable enhancement of the photoresponse.Item A study of Peltier effect in a thermoelectric couple of n-type cuprous oxide and p-type cuprous sulfide semiconductors(Proceedings of the Annual Reaserch symposium, 2003) Abeywarna, U.K.; Gunawardana, E.P.P.C.; Bopege, D.N.; Rajapakse, R.R.M.; Wijesundera, R.P.; Siripala, W.Item A Study of Temperature and Salinity Variations with Depth in Salt Pans at Palavi in North-West Region of Sri Lanka(Annual Research Symposium, Faculty of Graduate Studies, University of Kelaniya, 2000) Hewageegana, P.S.; Amarasekara, C.D.; Jayakody, J.R.P.; Punyasena, M.A.Item A Study of Wind Effects on Thermal Stability of Salt Pan Solar Ponds(Annual Research Symposium, Faculty of Graduate Studies, University of Kelaniya, 2001) Punyasena, M.A.; Amarasekara, C.D.; Jayakody, J.R.P.; Perera, P.A.A.Item Absolute phase effect in ultrafast optical responses of metal nanostructures(APPLIED PHYSICS A: MATERIALS SCIENCE & PROCESSING, 2007) Stockman, M.I.; Hewageegana, P.We predict that nonlinear ultrafast electron photoemission by strong optical fields and, potentially, other nonlinear optical responses of metal nanostructures significantly depend on the absolute (carrier?envelope) phase of excitation pulses. Strong enhancement of the local optical fields produces these responses at excitation intensities lower by order(s) of magnitude than for known systems. Prospective applications include control of ultrafast electron emission and electron injection into nanosystems. A wider class of prospective applications is the determination of the absolute phase of pulses emitted by lasers and atoms, molecules, and condensed matter at relatively low intensities.Item Advantages of the use of solar cells and light emitting diodes (LED) for traffic signal lights for the current power crises in Sri Lanka(2002) Kalingamudali, S.R.D.; Kalingamudali, M.L.During power failures and power cuts road users suffer immense difficulties due to non-availability of traffic signal lights. Use of solar cells with lead acid accumulators to provide required power and the use of light emitting diode (LED) displays to function as signal lamps to reduce the stored power consumption will be a feasible solution for the same. In addition to the low power consumption of LED displays, it is possible to use the same display to indicate different directions by having different switching circuits. Since the LED display consists of a number of LEDs, visibility from side ways is better than the conventional signal lamp having a single light source. Failure of the lamp in the conventional signals halts the entire operation of the signal lamp but in the case of failures of few LEOs in a matrix will not cease the entire operation. The other major advantage is during the installation of new signal posts; it is possible to erect the same as a wireless installation. Solar panel with an accumulator (lead acid battery) can provide power while infrared sensors and diodes could be used to transfer signals between each signal post; hence it is not necessary to dig across the road for control and power cables.Considering the signal light requirement for the whole country, the above solution will have a considerable impact on the national power grid.Item Ammonium sulfide surface treatment of electrodeposited p-type cuprous oxide thin films(Electronic Materials Letters, 2014) Jayathileke, K.M.D.C.; Kapaklis, V.; Siripala, W.; Jayanetti, J.K.D.S.The effects of ammonium sulfide surface treatment on electrodeposited p-type polycrystalline cuprous oxide (Cu2O) thin films deposited on Ti substrates were studied. The structural and morphological properties of the films were investigated using scanning electron microscopy, x-ray diffraction, and energy-dispersive x-ray spectroscopy. The changes in the conductivities and photocurrents of the films after the ammonium sulfide treatment were determined. Films that had undergone the ammonium sulfide treatment showed reduced resistivities, enhanced spectral photoresponses, and enhanced current-voltage characteristics. The results showed that ammonium sulfide treatment improved the peak output current of the p-type Cu2O films by about 400% compared with those of bare Cu2O films. This improvement is attributed to the passivation of defects in the films by sulfur, showing that sulfur passivation provides a good method for improving of Cu2O-based devices.