Spatial defects nanoengineering for bipolar conductivity in MoS2.

Understanding the atomistic origin of defects in two-dimensional transition metal dichalcogenides, their impact on the electronic properties, and how to control them is critical for future electronics and optoelectronics. Here, we demonstrate the integration of thermochemical scanning probe lithography (tc-SPL) with a flow-through reactive gas cell to achieve nanoscale control of defects in monolayer MoS2. The tc-SPL produced defects can present either p- or n-type doping on demand, depending on the used gasses, allowing the realization of field effect transistors, and p-n junctions with precise sub-µm spatial control, and a rectification ratio of over 104. Doping and defects formation are elucidated by means of X-Ray photoelectron spectroscopy, scanning transmission electron microscopy, and density functional theory. We find that p-type doping in HCl/H2O atmosphere is related to the rearrangement of sulfur atoms, and the formation of protruding covalent S-S bonds on the surface. Alternatively, local heating MoS2 in N2 produces n-character.

Functional interfacing of Rhodospirillum rubrum chromatophores to a conducting support for capture and conversion of solar energy.

Owing to the considerable current interest in replacing fossil fuels with solar radiation as a clean, renewable, and secure energy source, light-driven electron transport in natural photosynthetic systems offers a valuable blueprint for conversion of sunlight to useful energy forms. In particular, intracytoplasmic membrane vesicles (chromatophores) from the purple bacterium Rhodospirillum rubrum provide a fully functional and robust photosynthetic apparatus, ideal for biophysical investigations of energy transduction and incorporation into biohybrid photoelectrochemical devices. These vesicular organelles, which arise by invagination of the cytoplasmic membrane, are the sites of the photochemical reaction centers and the light harvesting 1 (LH1) complex. The LH1 protein is responsible for collecting visible and near-IR radiant energy and funneling these excitations to the reaction center for conversion into a transmembrane charge separation. Here, we have investigated the morphology, fluorescence kinetics and photocurrent generation of chromatophores from Rsp. rubrum deposited directly onto gold surfaces in the absence of chemical surface modifications. Atomic force microscopy showed a significant coverage of the gold electrode surface by Rsp. rubrum chromatophores. By in situ fluorescence induction/relaxation measurements, a high retention of the quantum yield of photochemistry was demonstrated in the photoactive films. Chronoamperometric measurements showed that the assembled bioelectrodes were capable of generating sustained photocurrent under white light illumination at 220 mW/cm(2) with a maximum current of 1.5 µA/cm(2), which slowly declines in about 1 week. This study demonstrates the possibility of photoelectrochemical control of robust chromatophore preparations from Rsp. rubrum that paves the way for future incorporation into functional solar cells.

Structural and functional proteomics of intracytoplasmic membrane assembly in Rhodobacter sphaeroides.

The results of a detailed structural and functional proteomic analysis of intracytoplasmic membrane (ICM) assembly in the model purple phototrophic bacterium Rhodobacter sphaeroides are reviewed in this report. Proteomics approaches have focused upon identification of membrane proteins temporally expressed during ICM development and spatially localized within the internal cell membranes, together with their structural and functional correlates. For the examination of temporal protein expression, procedures were established for the induction of ICM formation at low oxygen tension and for ICM remodeling in cells adapting to low intensity illumination, which permitted isolation by rate-zone sedimentation of ICM growth initiation sites (CM invaginations) in an upper-pigmented band (UPB), together with more mature ICM vesicles (chromatophores) as the main band. Nondenaturing clear native gel electrophoresis of the chromatophore fraction gave rise to four pigmented bands: the top and bottom bands contained the reaction center-light-harvesting 1 (RC-LH1) core complex and the LH2 peripheral antenna, respectively, while two bands of intermediate migration exhibited distinct associations of LH2 and core complexes. Proteomic analysis of the gel bands revealed developmental changes including increasing levels of LH2 polypeptides relative to those of core complexes as ICM development proceeded, as well as a large array of other associated proteins including high spectral counts for the F1FO-ATP synthase subunits, and the cytochrome bc1 complex. High counts were also observed for RSP6124, a protein of unknown function, that were correlated with increasing LH2 levels. RC-LH1-containing clear native electrophoresis gel bands from the UPB were enriched in cytoplasmic membrane (CM) markers, including electron transfer and transport proteins, as well as general membrane assembly factors (viz., preprotein translocases YidC, YajC and SecY, bacterial type 1 signal peptidase and twin arg translocation subunit TatA), thereby confirming the origin of the UPB from both peripheral respiratory membrane and sites of active CM invagination in which preferential assembly of the RC-LH1 complex occurs. Functional aspects of the photosynthetic unit assembly process were monitored by fluorescence induction/relaxation measurements of the variable fluorescence arising from LH-bacteriochlorophyll a. Slowing of the rate of RC electron transfer turnover (τQA), as assessed from the relaxation phase, was correlated with the growth of the functional absorption cross section (σ) and LH2/LH1 molar ratios. This is thought to arise from the imposition of constraints upon free diffusion of ubiquinone (UQ) redox species between the RC and cytochrome bc1 complex as the ICM bilayer becomes densely packed with LH2 rings. Such LH2 packing was confirmed in a comparison by high-resolution atomic force microscopy of ICM patches from cells grown at high and low light intensity [Adams and Hunter: Biochim Biophys Acta 2012;1817:1616-1627], in which the increasing LH2 levels form densely packed LH2-only domains, representing the light-responsive antenna complement arising under low illumination. In contrast, LH2 is initially dispersed in rows and small cluster-separating linear arrays of largely dimeric RC-LH1 core complexes, which become filled with LH2 during acclimation to reduced light intensity. In phototrophically grown cells that were transferred to oxic conditions in the dark, fluorescence induction/relaxation measurements showed that despite a growth burst independent of photosynthetic pathways, functional photosynthetic units were maintained for up to 24 h after the transition. The τQA was accelerated from ∼1 to 0.5 ms by 8 h, reflecting the decrease in LH2 levels, facilitating more rapid UQ redox species diffusion in the membrane bilayer as crowding by LH2 is overcome. Under these circumstances, UPB levels were elevated with significant increases in LH1/LH2 molar ratio. These changes indicate that vesiculation of CM growth initiation sites to form vesicular ICM was arrested under oxic conditions.