Mapping of functionalized regions on carbon nanotubes by scanning tunneling microscopy

Scanning tunneling microscopy (STM) gives us the opportunity to map the surface of functionalized carbon nanotubes in an energy resolved manner and with atomic precision. But this potential is largely untapped, mainly due to sample stability issues which inhibit reliable measurements. Here we present a simple and straightforward solution that makes away with this difficulty, by incorporating the functionalized multiwalled carbon nanotubes (MWCNT) into a few layer graphene - nanotube composite. This enabled us to measure energy resolved tunneling conductance maps on the nanotubes, which shed light on the level of doping, charge transfer between tube and functional groups and the dependence of defect creation or functionalization on crystallographic orientation.Comment: Keywords: functionalization, carbon nanotubes, few layer graphene, STM, CITS, ST

Synthesis, structure, and opto-electronic properties of organic-based nanoscale heterojunctions

Enormous research effort has been put into optimizing organic-based opto-electronic systems for efficient generation of free charge carriers. This optimization is mainly due to typically high dissociation energy (0.1-1 eV) and short diffusion length (10 nm) of excitons in organic materials. Inherently, interplay of microscopic structural, chemical, and opto-electronic properties plays crucial role. We show that employing and combining advanced scanning probe techniques can provide us significant insight into the correlation of these properties. By adjusting parameters of contact- and tapping-mode atomic force microscopy (AFM), we perform morphologic and mechanical characterizations (nanoshaving) of organic layers, measure their electrical conductivity by current-sensing AFM, and deduce work functions and surface photovoltage (SPV) effects by Kelvin force microscopy using high spatial resolution. These data are further correlated with local material composition detected using micro-Raman spectroscopy and with other electronic transport data. We demonstrate benefits of this multi-dimensional characterizations on (i) bulk heterojunction of fully organic composite films, indicating differences in blend quality and component segregation leading to local shunts of photovoltaic cell, and (ii) thin-film heterojunction of polypyrrole (PPy) electropolymerized on hydrogen-terminated diamond, indicating covalent bonding and transfer of charge carriers from PPy to diamond

Investigating DNA Double Strand Breaks (DSB) in mammalian cells by novel fluorescent reporters

An efficient DNA damage response is critical for maintaining the integrity of the mammalian genome, and ensuring the accurate transfer of genetic information between generations. Of particular biological relevance are DNA double strand breaks (DSB), which if repaired incorrectly may contribute to carcinogenesis. Review of contemporary literature has led to the identification of protein interactions and transcriptional events, tightly associated with the mammalian DSB response. Characteristics of selected events have been manipulated, with the notion of developing a reporter system that offers a sensitive and rapid method of detecting DSB in living mammalian cell models. Work presented here provides a quantitative evaluation of DSB generation in various mammalian cell lines, following chemical and irradiation treatment, and highlights the limitations of currently used markers. A series of recombinant proteins comprising peptide interacting domains, which exhibit altered spatio-temporal dynamics in relation with each other following DSB induction, are proposed as potential reporters of damage in mammalian cells. Novel gene constructs have been engineered that encode these peptide interacting domains, sandwiched between fluorescence-resonance-energy transfer (FRET) capable proteins. DSB specific events are predicted to induce peptide interactions that may be tracked in real time, by monitoring alterations in the fluorescent properties of such a recombinant protein. In an alternative approach, the transcriptional up-regulation of RAD52 mRNA following DSB induction was extended to whole cells. Optimisation of a fluorescent molecular beacon probe complementary to mammalian RAD52 mRNA is described, and data obtained in mammalian cells following DSB induction supports the notion that RAD52 is actively transcribed as part of the DSB response.EThOS - Electronic Theses Online ServiceBiotechnology and Biological Sciences Research Council (BBSRC) and GlaxoSmithKline (GSK).GBUnited Kingdo

Chaos:The speed limiting phenomenon in dynamic atomic force microscopy

\u3cp\u3eThis paper investigates the closed-loop dynamics of the Tapping Mode Atomic Force Microscopy using a new mathematical model based on the averaging method in Cartesian coordinates. Experimental and numerical observations show that the emergence of chaos in conventional tapping mode AFM strictly limits the imaging speed. We show that, if the controller of AFM is tuned to be faster than a certain threshold, the closed-loop system exhibits a chaotic behavior. The presence of chaos in the closed-loop dynamics is confirmed via bifurcation diagrams, Poincaré sections, and Lyapunov exponents. Unlike the previously detected chaos due to attractive forces in the AFM, which can be circumvented via simple changes in operation parameters, this newly identified chaos is seemingly inevitable and imposes an upper limit for the closed-loop bandwidth of the AFM.\u3c/p\u3

Open-Circuit Voltage Limitation in Low-Bandgap Diketopyrrolopyrrole-Based Polymer Solar Cells Processed from Different Solvents

Low-bandgap diketopyrrolopyrrole-based polymer bulk-heterojunction solar cells prepared from different solvents are studied by means of capacitance measurements. Large variations of both photovoltage and photocurrent are induced during device processing by using different solvents that allow internal operating energetics to be addressed. Addition of o-dichlorobenzene to chloroform produces an upward offset of the polymer highest occupied molecular orbital level in accordance with the red shift of the absorption spectra, which correlates with the flat-band potential offset extracted from Mott–Schottky analysis of the reverse and low-forward capacitance. However, the open-circuit voltage does not reach the expected value in the case of chloroform-processed devices because of the reduced occupancy of the acceptor fullerene lower unoccupied molecular orbital states extracted from the chemical capacitance analysis under illumination. This photovoltage loss is linked with the limitation to the increase in the electron Fermi level caused by the reduced charge density involved in the photovoltaic process, as derived from the morphology-influenced short-circuit current density