ABSTRACT
Contaminants in water were studied using ultraviolet absorption with light emitting diode and deuterium lamp sources, and a thresholding detector. The absorption spectra of potassium hydrogen pthalate, clothianidin, tryptophan, thiamethoxam, uric acid and metaldehyde were obtained in the range 200-360 nm. Only metaldehyde was not suitable for detection in this range. For the other contaminants, and mixtures of pairs of compounds, the transmitted signal could be approximately described with a simple spectral model of the source-absorption-detector system. Combined measurements at two wavelengths could allow relative concentrations in certain mixtures to be determined, and real-time absorption measurements were demonstrated in a flume.
ABSTRACT
Spin-coating of poly(ethylenimine) (PEI) has been used to reduce the work function of GaAs (001), (110), (111)A and (111)B. The magnitude of the reduction immediately after coating varies significantly from 0.51 eV to 0.69 eV and depends on the surface crystal face, on the GaAs bulk doping and on the atomic termination of the GaAs. For all samples, the work function reduction shrinks in ambient air over the first 20 hours after spin coating, but reductions around 0.2-0.3 eV persist after 1 year of storage in air. Core-level photoemission of thin film PEI degradation in air is consistent with a two-stage reaction with CO2 and H2O previously proposed in carbon capture studies. The total surface dipole from PEI coating is consistent with a combination of internal neutral amine dipole and an interface dipole whose magnitude depends on the surface termination. The contact potential difference measured by Kelvin probe force microscopy on a cleaved GaAs heterostructure is smaller on p-doped regions. This can be explained by surface doping due to the PEI, which increases the band bending on p-doped GaAs where Fermi level pinning is weak. Both surface doping and surface dipole should be accounted for when considering the effect of PEI coated on a semiconductor surface.