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Energy Transfer Properties of Rhodobacter Sphaeroides Chromatophores During Adaptation to Low Light Intensity

Description

Time-resolved fluorescence spectroscopy was used to explore the pathway and kinetics of energy transfer in photosynthetic membrane vesicles (chromatophores) isolated from Rhodobacter (Rba.) sphaeroides cells harvested 2, 4, 6 or 24 hours after a transition from growth in high to low level illumination. As previously observed, this light intensity transition initiates the remodeling of the photosynthetic apparatus and an increase in the number of light harvesting 2 (LH2) complexes relative to light harvesting 1 (LH1) and reaction center (RC) complexes. It has generally been thought that the increase in LH2 complexes served the purpose of increasing the overall energy transmission to the RC. However, fluorescence lifetime measurements and analysis in terms of energy transfer within LH2 and between LH2 and LH1 indicate that, during the remodeling time period measured, only a portion of the additional LH2 generated are well connected to LH1 and the reaction center. The majority of the additional LH2 fluorescence decays with a lifetime comparable to that of free, unconnected LH2 complexes. The presence of large LH2-only domains has been observed by atomic force microscopy in Rba. sphaeroides chromatophores (Bahatyrova et al., Nature, 2004, 430, 1058), providing structural support for the existence of pools of partially connected LH2 complexes. These LH2-only domains represent the light-responsive antenna complement formed after a switch in growth conditions from high to low illumination, while the remaining LH2 complexes occupy membrane regions containing mixtures of LH2 and LH1–RC core complexes. The current study utilized a multi-parameter approach to explore the fluorescence spectroscopic properties related to the remodeling process, shedding light on the structure-function relationship of the photosynthetic assembles. Possible reasons for the accumulation of these largely disconnected LH2-only pools are discussed.

ContributorsDriscoll, Brent (Author) / Lunceford, Chad (Author) / Lin, Su (Author) / Woronowicz, K. (Author) / Niederman, R. A. (Author) / Woodbury, Neal (Author) / College of Liberal Arts and Sciences (Contributor)

Created2014-08-28

Exploration of the cold-wall CVD synthesis of monolayer MoS2 and WS2

Description

A highly uniform and repeatable method for synthesizing the single-layer transition metal dichalcogenides (TMDs) molybdenum disulfide, MoS2, and tungsten disulfide, WS2, was developed. This method employed chemical vapor deposition (CVD) of precursors in a custom built cold-wall reaction chamber designed to allow independent control over the growth parameters. Iterations of this reaction chamber were employed to overcome limitations to the growth method. First, molybdenum trioxide, MoO3, and S were co-evaporated from alumina coated W baskets to grow MoS2 on SiO2/Si substrates. Using this method, films were found to have repeatable coverage, but unrepeatable morphology. Second, the reaction chamber was modified to include a pair of custom bubbler delivery systems to transport diethyl sulfide (DES) and molybdenum hexacarbonyl (MHC) to the substrate as a S and Mo precursors. Third, tungsten hexacarbonyl (WHC) replaced MHC as a transition metal precursor for the synthesis of WS2 on Al2O3, substrates. This method proved repeatable in both coverage and morphology allowing the investigation of the effect of varying the flow of Ar, varying the substrate temperature and varying the flux of DES to the sample. Increasing each of these parameters was found to decrease the nucleation density on the sample and, with the exception of the Ar flow, induce multi-layer feature growth. This combination of precursors was also used to investigate the reported improvement in feature morphology when NaCl is placed upstream of the substrate. This was found to have no effect on experiments in the configurations used. A final effort was made to adequately increase the feature size by switching from DES to hydrogen sulfide, H2S, as a source of S. Using H2S and WHC to grow WS2 films on Al2O3, it was found that increasing the substrate temperature and increasing the H2S flow both decrease nucleation density. Increasing the H2S flow induced bi-layer growth. Ripening of synthesized WS2 crystals was demonstrated to occur when the sample was annealed, post-growth, in an Ar, H2, and H2S flow. Finally, it was verified that the final H2S and WHC growth method yielded repeatability and uniformity matching, or improving upon, the other methods and precursors investigated.

ContributorsLunceford, Chad (Author) / Drucker, Jeff (Thesis advisor) / Menéndez, Jose (Committee member) / Smith, David J. (Committee member) / Rez, Peter (Committee member) / Arizona State University (Publisher)

Created2019