Optical and AFM studies of ZnO: excitonic properties, surface morphology and etching effects

The oxygen- and zinc-terminated polar surfaces of ZnO bulk crystals are examined using both optical spectroscopy and surface probe microscopy techniques. An initial study of as-received ZnO wafers purchased from three different companies reveals that there are large optical differences between each grower’s material in terms of bound-exciton and green band luminescence, and free-exciton reflectance resonances. Topographic data obtained using atomic force microscopy (AFM) and x-ray diffraction results suggest that these optical variations are partly due to the different polishing methods applied to the surfaces. The effects of etchant solutions on each polar face, both in terms of surface morphology and optical characteristics are examined. The O-terminated surface reacts strongly to hydrogen ions in acidic solutions and one observes high vertical-to-lateral etch ratios on such surfaces and dramatic increases in surface roughness, with corresponding decreases in bound-exciton luminescence and free-exciton reflectance resonances. The morphology of Zn-terminated surfaces is dominated by lateral etching which has a tendency to reduce roughness and improve luminescence and reflectance characteristics. Differences in the efficacy of cleaning for each polar face is explored through the analysis of AFM force-displacement plots and lateral force microscopy. The data show that the O-terminated face is strongly hydrophilic (as also seen in the etching behaviour) and that fluid layers of significant thickness can form on this face in ambient conditions. In comparison, the fluid layers present on the hydrophobic Zn-terminated surfaces are approximately half as thick or less. Finally the reflectance spectra of polycrystalline thin film and bulk ZnO wafers are modelled using a semi-classical model of the exciton-polariton interaction. A twoband dielectric response function is found to adequately describe the excitonic resonances, and the effects of Fabry-Perot oscillations in the thin film material are examined

Photoluminescence studies of radioactive mercury and gold impurity centres in silicon

The implantation of radioactive isotopes into semiconductor materials is a powerful technique that enables researchers to correctly assign chemical identities to defects observed by spectroscopic methods. Since each radioactive isotope has a characteristic decay rate, the intensity of spectroscopic features related to those atoms should also change with a corresponding rate, allowing an accurate identification of defect centres. This report shall discuss a number of Hg- and Au-related defects in silicon studied by Photoluminescence Spectroscopy. Radioactive 197Hg decays to stable Au, enabling defects involving both elements to be studied. Data presented here, based on implantations of 197Hg and stable 200Hg, confirms a previously observed Hg-related feature. It contains three Zero Phonon Lines and is due, we believe, to a three-level ground state and double excited states for the Hg atoms. The ‘FeB’ defect centre, previously shown to involve Au, is also studied. Whilst confirmation of the involvement of Au could not be established, an analysis has shown that thermal dissociation of this defect may negatively influence the results of spectroscopic measurements. A defect probably involving Au and Li is also studied, illustrating the techniques necessary to produce it. Again, this defect appears to dissociate upon room temperature storage at a rate similar to the ‘FeB’ defect, suggesting that Au is involved in this process for each. Finally, the results of an examination of Si:Ge Multi-Quantum Well (MQW) structures are presented. These indicate that much of the luminescence observed is related to either bulk- or surface-related dislocations in the Si rather than the quantum structures themselves

Plenary Session: \u3cem\u3eWater Research Needs in Kentucky\u3c/em\u3e

Four panelists were invited to present their visions/predictions of current and future water research needs in Kentucky in an opening plenary session at the 2012 Kentucky Water Resources Annual Symposium. The requested overall general focus was ground water research needs

Exciton-polariton behaviour in bulk and polycrystalline ZnO

We report detailed reflectance studies of the exciton–polariton structure of thin film polycrystalline ZnO and comparison with bulk crystal behaviour. Near-normal incidence reflectance spectra of these samples are fitted using a two-band dielectric response function. Our data show that the reflectance data in polycrystalline ZnO differ substantially from the bulk material, with Fabry–Perot oscillations at energies below the transverse A exciton and above the longitudinal B exciton in the films. In the strong interaction regime between these energies no evidence is seen of the normally rapid oscillations associated with the anomalous waves. We demonstrate that the strong interaction of the damped exciton with the photon leads to polaritons in this region with substantial damping such that the Fabry–Perot modes are eliminated. Good qualitative agreement is achieved between the model and data. The importance of the polariton model in understanding the reflectance data of polycrystalline material is clearly see

Optical properties of nanocrystalline ZnO thin films grown using pulsed laser deposition

Raman spectroscopy, x-ray diffractometry, atomic force microscopy, photoluminescence spectroscopy and reflectance spectroscopy have been used to characterize ZnO thin films grown by pulsed laser deposition as a function of the post-growth annealing temperature. Raman results show enhancement and broadening of certain Raman features which correlate with changes in the widths of the x-ray diffraction peaks for samples with varying grain size in the 50-100 nm range. These data suggest that electric fields, arising from charge trapping at grain boundaries, in conjunction with localised and surface phonon modes, are the cause of the intensity enhancement and asymmetry of the Raman features. Band-edge photoluminescence and reflectance spectra also altered considerably with increases in grain size, showing clearly observable excitonic structure in the reflectance spectra. An analysis using a deformation potential Hamiltonian demonstrates that the experimental exciton energies are not explicable solely in terms of sample strain and give additional evidence for electric fields in the samples due to charge trapping at grain boundaries. This is supported by theoretical estimates of the exciton energy perturbation due to electric fields and also by the behaviour of the green band in the samples. Detailed studies show that reflectance spectra in nanocrystalline ZnO differ substantially from bulk material. Interaction of excitons, damped by strong electric field effects, with photons leads to exciton-polaritons with substantial damping, eliminating the normal Fabry-Perot structure seen in thin films. Good qualitative agreement is achieved between the model and data and the conclusions are also in good agreement with the photoluminescence and Raman data. Finally, high intensity optical pumping data of these samples again shows a dependence on grain size. All samples show evidence of high excitation effects and the sample with the largest grain size displays random lasing at room temperature. All our results indicate the very strong influence of electric fields due to charge trapping at grain boundaries on the optical properties of nanocrystalline ZnO

Are women with history of pre-eclampsia starting a new pregnancy in good nutritional status in South Africa and Zimbabwe?

Background Maternal nutritional status before and during pregnancy is an important contributor to pregnancy outcomes and early child health. The aim of this study was to describe the preconceptional nutritional status and dietary intake during pregnancy in high-risk women from South Africa and Zimbabwe. Methods This is a prospective observational study, nested to the CAP trial. Anthropometric measurements before and during pregnancy and dietary intake using 24-h recall during pregnancy were assessed. The Intake Distribution Estimation software (PC-SIDE) was used to evaluate nutrient intake adequacy taking the Estimated Average Requirement (EAR) as a cut-off point. Results Three hundred twelve women who had pre-eclampsia in their last pregnancy and delivered in hospitals from South Africa and Zimbabwe were assessed. 73.7 and 60.2% women in South Africa and Zimbabwe, respectively started their pregnancy with BMI above normal (BMI ≥ 25) whereas the prevalence of underweight was virtually non-existent. The majority of women had inadequate intakes of micronutrients. Considering food and beverage intake only, none of the micronutrients measured achieved the estimated average requirement. Around 60% of pregnant women reported taking folic acid or iron supplements in South Africa, but almost none did so in Zimbabwe. Conclusion We found a high prevalence of overweight and obesity and high micronutrient intake inadequacy in pregnant women who had the previous pregnancy complicated with pre-eclampsia. The obesity figures and micronutrient inadequacy are issues of concern that need to be addressed. Pregnant women have regular contacts with the health system; these opportunities could be used to improve diet and nutrition. Trial registration PACTR201105000267371 . Registered 06 December 2010

PRICES PAID INDEXES FOR MAJOR U.S. CROPS

Commodity-specific Prices Paid Indexes and Prices Received/Prices Paid ratios are calculated for corn, cotton, soybeans, and wheat. These are compared to the current U.S.D.A. indexes. Target prices are recalculated using the commodity-specific Prices Paid Indexes and then compared to the actual tartet prices for the 1976-73 period

Optical properties of nanocrystalline ZnO thin films grown using pulsed laser deposition

Raman spectroscopy, x-ray diffractometry, atomic force microscopy, photoluminescence spectroscopy and reflectance spectroscopy have been used to characterize ZnO thin films grown by pulsed laser deposition as a function of the post-growth annealing temperature. Raman results show enhancement and broadening of certain Raman features which correlate with changes in the widths of the x-ray diffraction peaks for samples with varying grain size in the 50-100 nm range. These data suggest that electric fields, arising from charge trapping at grain boundaries, in conjunction with localised and surface phonon modes, are the cause of the intensity enhancement and asymmetry of the Raman features. Band-edge photoluminescence and reflectance spectra also altered considerably with increases in grain size, showing clearly observable excitonic structure in the reflectance spectra. An analysis using a deformation potential Hamiltonian demonstrates that the experimental exciton energies are not explicable solely in terms of sample strain and give additional evidence for electric fields in the samples due to charge trapping at grain boundaries. This is supported by theoretical estimates of the exciton energy perturbation due to electric fields and also by the behaviour of the green band in the samples. Detailed studies show that reflectance spectra in nanocrystalline ZnO differ substantially from bulk material. Interaction of excitons, damped by strong electric field effects, with photons leads to exciton-polaritons with substantial damping, eliminating the normal Fabry-Perot structure seen in thin films. Good qualitative agreement is achieved between the model and data and the conclusions are also in good agreement with the photoluminescence and Raman data. Finally, high intensity optical pumping data of these samples again shows a dependence on grain size. All samples show evidence of high excitation effects and the sample with the largest grain size displays random lasing at room temperature. All our results indicate the very strong influence of electric fields due to charge trapping at grain boundaries on the optical properties of nanocrystalline ZnO

Utilization of Tryptophan-like Fluorescence as a Proxy for <i>E. coli</i> Contamination in a Mixed-Land-Use Karst Basin

Karst aquifers are susceptible to contamination by pathogenic microorganisms, such as those found in human and animal waste, because the surface and subsurface drainage are well integrated through dissolution features. Fecal contamination of water is commonly assessed by the concentration of thermotolerant coliform bacteria, especially E. coli. This method is time-consuming, taking ≥18 h between the start of incubation and subsequent enumeration, as well as the time required to collect and transport samples. We examined the utility of continuous monitoring of tryptophan-like fluorescence (TLF) as a real-time proxy for E. coli in a mixed-land-use karst basin in the Inner Bluegrass region of central Kentucky (USA). Two logging fluorometers were sequentially deployed at the outlet spring. During storm flow, TLF typically peaked after discharge, which suggests that TLF transport in the phreatic conduit is likely related to sediment transport. The ability of TLF and other parameters (48 h antecedent precipitation, turbidity, and air temperature) to predict E. coli concentrations was assessed using the Akaike information criterion (AIC) applied to linear regression models. Because both the models and baseline concentrations of TLF differed between fluorometers, TLF and instrument interaction were accounted for in the AIC. TLF was positively correlated with E. coli and, in conjunction with antecedent precipitation, was the best predictor of E. coli. However, a model that included air temperature and antecedent precipitation but not TLF predicted E. coli concentrations similarly well. Given the expense of the fluorometers and the performance of the alternate model, TLF may not be a cost-effective proxy for E. coli in this karst basin