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We report the first direct measurements of ultrafast electronic relaxation dynamics in PbI2 colloidal nanoparticles using femtosecond transient absorption spectroscopy. The PbI2 nanoparticles were prepared using colloidal chemistry methods in different solvents, including ethanol, 2-propanol, 1-butanol, water, and acetonitrile, as well as in poly(vinyl alcohol) (PVA) matrix. The particle sizes were determined using low- and high-resolution transmission electron microscopy and atomic force microscopy, which provided direct evidence of photodegradation of the nanoparticles. The ground state electronic absorption spectra of aged PbI2 nanoparticles in acetonitrile and alcohol solvents showed two major peaks near 360 and 292 nm, which slightly blue shift with decreasing size. In aqueous solution containing PVA a new sharp excitonic peak appeared at 414 nm, indicative of nanoparticle formation. With excitation at 390 nm and probing in the visible to near-infrared region, the electronic relaxation dynamics in PbI2 nanoparticles were directly monitored. The electronic relaxation is found to be sensitive to solvent and insensitive to particle size. In acetonitrile the relaxation was dominated by a 75 ps decay. In alcohol solvents, in addition to a 75 ps decay, a fast 6 ps decay was observed. The relaxation in aqueous PVA solution featured a double exponential decay with time constants of 1 and 40 ps. There appeared to be oscillations at early times with a period changing with solvent but not with particle size. The dynamics observed were somewhat dependent on the probe wavelength and independent of the excitation intensity. The results suggest that the surface plays a major role in the electronic relaxation process of PbI2 nanoparticles. The influence of particle size is relatively minor in the size range studied (3−100 nm), probably because the relaxation is dominated by surface characteristics that do not vary significantly with size and/or the size is much larger than the exciton Bohr radius (1.9 nm) and thereby spatial confinement is not significant in affecting the relaxation process.

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A new chemical deposition method has been developed to prepare photoconducting n-Sb2S3 thin polycrystalline films. The solution composition of the deposition bath was 0.025M potassium antimonyl tartarate, 0.4M triethanolamine, 0.025M thioacetamide, and 5 • 10-7M silicotungstic acid (STA), respectively. The as-deposited and the annealed films were characterized through x-ray diffraction, neutron activation analysis, and the optical absorption investigations. The photoelectrochemical performances of these films were examined. The best photoresponse was observed on the film prepared from a chemical bath containing 5 • 10-7M STA.

The first fabrication of low cost n-Sb2S3/p-Si heterojunction solar cells by chemical deposition method is reported. It is observed that in the case of n-Sb2S3 films chemically deposited with silicotungstic acid on p-Si and annealed, the photovoltaic properties of the n-Sb2S3/p-Si junctions are considerably improved. Under AM1 illumination, the improved junction exhibited an efficiency (η) of ∼5.19% on an active area of 0.05 cm2 without any antireflection coating whereas the n-Sb2S3 films deposited without STA on p-Si showed η=1.03%.

This work was supported by the Ookura Foundation.

We report optical second-harmonic generation (SHG) in reflection from GaSe crystals of 1 to more than 100 layers using a fundamental picosecond pulsed pump at 1.58 eV and a supercontinuum white light pulsed laser with energies ranging from 0.85 to 1.4 eV. The measured reflected SHG signal is maximal in samples of ∼20 layers, decreasing in thicker samples as a result of interference. The thickness-and frequency-dependence of the SHG response of samples thicker than ∼7 layers can be reproduced by a second-order optical susceptibility that is the same as in bulk samples. For samples 7 layers, the second-order optical susceptibility is reduced compared to that in thicker samples, which is attributed to the expected bandgap increase in mono-and few-layer GaSe.

Advances in the growth processes of 4H-SiC epitaxial layers have led to the continued expansion of epilayer thickness, allowing for the detection of more penetrative radioactive particles. We report the fabrication and characterization of high-resolution Schottky barrier radiation detectors on 250 μm thick n-type 4H-SiC epitaxial layers, the highest reported thickness to date. Several 8 × 8 mm2 detectors were fabricated from a diced 100 mm diameter 4H-SiC epitaxial wafer grown on a conductive 4H-SiC substrate with a mean micropipe density of 0.11 cm−2. From the Mott–Schottky plots, the effective doping concentration was found to be in the range (0.95–1.85) × 1014 cm−3, implying that full depletion could be achieved at ∼5.7 kV (0.5 MV/cm at the interface). The current-voltage characteristics demonstrated consistently low leakage current densities of 1–3 nA/cm2 at a reverse bias of −800 V. This resulted in the pulse-height spectra generated using a 241Am alpha source (5486 keV) manifesting an energy resolution of less than 0.5% full width at half maximum (FWHM) for all the detectors at −200 V. The charge collection efficiencies (CCEs) were measured to be 98–99% with no discernable correlation to the energy resolution. A drift-diffusion model fit to the variation of CCE as a function of bias voltage, revealed a minority carrier diffusion length of ∼10 μm. Deep level transient spectroscopy measurements on the best resolution detector revealed that the excellent performance was the result of having ultralow concentrations of the order of 1011 cm−3 lifetime limiting defects—Z1/2 and EH6/7.