David Lee Phillips

Defect emissions in ZnO nanostructures

Defects in three different types of ZnO nanostructures before and after annealing under different conditions were studied. The annealing atmosphere and temperature were found to strongly affect the yellow and orange-red defect emissions, while green emission was not significantly affected by annealing. The defect emissions exhibited a strong dependence on the temperature and excitation wavelength, with some defect emissions observable only at low temperatures and for certain excitation wavelengths. The yellow emission in samples prepared by a hydrothermal method is likely due to the presence of OH groups, instead of the commonly assumed interstitial oxygen defect. The green and orange-red emissions are likely due to donor acceptor transitions involving defect complexes, which likely include zinc vacancy complexes in the case of orange-red emissions.

A doorway state leads to photostability or triplet photodamage in thymine DNA

Ultraviolet irradiation of DNA produces electronic excited states that predominantly eliminate the excitation energy by returning to the ground state (photostability) or following minor pathways into mutagenic photoproducts (photodamage). The cyclobutane pyrimidine dimer (CPD) formed from photodimerization of thymines in DNA is the most common form of photodamage. The underlying molecular processes governing photostability and photodamage of thymine-constituted DNA remain unclear. Here, a combined femtosecond broadband time-resolved fluorescence and transient absorption spectroscopies were employed to study a monomer thymidine and a single-stranded thymine oligonucleotide. We show that the protecting deactivation of a thymine multimer is due to an ultrafast single-base localized stepwise mechanism where the initial excited state decays via a doorway state to the ground state or proceeds via the doorway state to a triplet state identified as a major precursor for CPD photodamage. These results provide new mechanistic characterization of and a dynamic link between the photoexcitation of DNA and DNA photostability and photodamage.

Mechanisms of Antibacterial Activity of MgO: Non‐ROS Mediated Toxicity of MgO Nanoparticles Towards Escherichia coli

The toxicity of metal oxide nanomaterials and their antimicrobial activity is attracting increasing attention. Among these materials, MgO is particularly interesting as a low cost, environmentally-friendly material. The toxicity of MgO, similar to other metal oxide nanomaterials, is commonly attributed to the production of reactive oxygen species (ROS). We investigated the toxicity of three different MgO nanoparticle samples, and clearly demonstrated robust toxicity towards Escherichia coli bacterial cells in the absence of ROS production for two MgO nanoparticle samples. Proteomics data also clearly demonstrate the absence of oxidative stress and indicate that the primary mechanism of cell death is related to the cell membrane damage, which does not appear to be due to lipid peroxidation.

Influence of annealing on stimulated emission in ZnO nanorods

Vertically aligned ZnO nanorod arrays with rod lengths in the range of 200–1500nm were fabricated by a hydrothermal method. No stimulated emission was observed in as grown nanorods. Annealing of the rods in forming gas and oxygen significantly affected their optical properties and enabled the achievement of stimulated emission. The lowest lasing threshold and defect emission as well as the longest spontaneous emission decay times were obtained for nanorods annealed in oxygen flow. This indicates that interstitial oxygen, which is commonly assumed to be the cause of yellow-green defect emission, is not the dominant defect in hydrothermally grown nanorods.

Time-resolved photoluminescence from ZnO nanostructures

Different ZnO nanostructures (tetrapods, shells, rods, and highly faceted rods) were characterized by photoluminescence (PL) and time-resolved PL measurements. It was found that different nanostructures exhibit very different optical properties in terms of defect emission and decay times of the spontaneous emission. No correlation was found between the PL decay times and defect emission intensities and defect emission positions. The short decay times of the UV emission are most likely due to nonradiative defects that are correlated with the crystalline quality and do not contribute to the visible emission. Neither short PL decay times nor intense defect emissions rule out achievement of stimulated emission.

Time-resolved photoluminescence study of the stimulated emission in ZnO nanoneedles

ZnO nanoneedles were fabricated by thermal evaporation of Zn nanoparticles at 800 °C and atmospheric pressure. The samples showed strong ultraviolet photoluminescence and weak orange defect luminescence. Time-resolved photoluminescence (TRPL) was measured using the Kerr-gated fluorescence technique in order to probe the ultrafast carrier dynamics in exciton-exciton scattering and electron hole plasma (EHP) regimes. In both regimes, the decay time of the photoluminescence is very fast (∼1ps). Even though no structure is detected in the time-integrated spectra of the EHP emission, the TRPL reveals the coexistence of the excitons and free carriers. Possible reasons for the observed phenomena are discussed.

Solvation and solvent effects on the short‐time photodissociation dynamics of CH2I2 from resonance Raman spectroscopy

Resonance Raman spectra of CH2I2 have been obtained at excitation wavelengths of 369, 355, and 342 nm in cyclohexane solution and in methanol solution at excitation wavelengths of 355 and 342 nm. Resonance Raman spectra were also measured for CH2I2 in the vapor phase with an excitation wavelength of 355 nm. The resonance Raman spectra of CH2I2 exhibit most of their intensity in fundamentals, overtones, and combination bands of modes nominally assigned as the I–C–I symmetric stretch, the I–C–I bend, and the I–C–I antisymmetric stretch vibrations. The absorption spectra and resonance Raman intensities of the gas phase and methanol solution phase diiodomethane spectra were simulated using a simple model and time‐dependent wave packet calculations. Normal mode coefficients from normal coordinate calculations were used to convert the motion of the wave packet on the excited electronic state surface from dimensionless normal coordinates into internal coordinates of the molecule. The short‐time photodissociation...

The degradation mechanism of methyl orange under photo-catalysis of TiO2

The properties of photo-generated reactive species, holes and electrons in bulk TiO(2) (anatase) film and nano-sized TiO(2) were studied and their effects towards decomposing pollutant dye methyl orange (MO) were compared by transient absorption spectroscopies. The recombination of holes and electrons in nano-sized TiO(2) was found to be on the microsecond time scale consistent with previous reports in the literature. However, in bulk TiO(2) film, the holes and electrons were found to be on the order of picoseconds due to ultra fast free electrons. The time-correlated single-photon counting (TCSPC) technique combined with confocal fluorescence microscopy revealed that the fluorescence intensity of MO is at first enhanced noticeably by TiO(2) under UV excitation and soon afterwards weakened dramatically, with the lifetime prolonged. Photo-generated holes in nano-sized TiO(2) can directly oxidize MO on the time scale of nanoseconds, while free electrons photo-generated in bulk TiO(2) film can directly inject into MO on the order of picoseconds. Through cyclic voltammetry measurements, it was found that MO can be reduced at -0.28 V and oxidized at 1.4 V (vs. SCE) and this provides thermodynamic evidence for MO to be degraded by electrons and holes in TiO(2). Through comparison of the hole-scavenging effect of MO and water, it was found that in polluted water when MO is above 1.6 × 10(-4) M, the degradation is mainly due to a direct hole oxidation process, while below 1.6 × 10(-4) M, hydroxyl oxidation competes strongly and might exceed the hole oxidation.

Solvation effects and short-time photodissociation dynamics of CH2I2 in solution from resonance Raman spectroscopy

Abstract Resonance Raman spectra of CH2I2 in cyclohexane solution have been obtained within the directly dissociative A-state absorption band of CH2I2 at excitation wavelengths of 355 and 341.5 nm. We find several intense vibrational bands in the solution phase spectra that are not observed in a previously reported 355 nm gas phase spectrum indicating that solvation effects are important in the Franck-Condon region of the first electronic excited state of CH2I2. The solution phase short-time photodissociation dynamics appears to have more motion in the ICI bending coordinate than the gas phase photodissociation.

Short-time photodissociation dynamics of A-band and B-band bromoiodomethane in solution: An examination of bond selective electronic excitation

We have obtained resonance Raman spectra and absolute Raman cross section measurements at eight excitation wavelengths in the A‐band and B‐band absorptions of bromoiodomethane in cyclohexane solution. The resonance Raman intensities and absorption spectra were simulated using a simple model and time‐dependent wave packet calculations. Normal mode vibrational descriptions were used with the results of the calculations to find the short‐time photodissociation dynamics in terms of internal coordinates. The A‐band short‐time photodissociation dynamics indicate that the C–I bond becomes much longer, the C–Br bond becomes smaller, the I–C–Br angle becomes smaller, the H–C–Br angles become larger, the H–C–I angles become smaller, and the H–C–H angle becomes a bit smaller. The B‐band short‐time photodissociation dynamics indicate the C–Br bond becomes much longer, the C–I bond becomes slightly longer, the I–C–Br angle becomes smaller, the H–C–I angles become larger, the H–C–Br angles become smaller, and the H–C–H...