Browsing by Author "Giridharagopal, Rajiv"
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Item Atomic-scale investigation of polydiacetylene nanowires by scanning tunneling microscopy and spectroscopy(2006) Giridharagopal, Rajiv; Kelly, Kevin F.Nanowires comprised of polydiacetylene, a conjugated polymer, have been analyzed at the nanoscale using scanning tunneling microscopy (STM) and spectroscopy. STM analysis shows that these nanowires exhibit unique electronic behavior due to the different substrate electrode materials used, particularly graphite and molybdenum disulfide. The change in charge transfer behavior is evidence of the importance of polymer-electrode interactions. Nanowires are also shown to randomly desorb due to an interaction with the STM tip. A single disruption often results in the entire nanowire desorbing, and the underlying monolayer is reordered within milliseconds. Additionally, spectroscopic data has been acquired using a novel technique called alternating current STM (ACSTM). ACSTM allows for the acquisition of differential capacitance information. Analysis of the nanowires yields a peak in differential capacitance, as is typical of metal/insulator/semiconductor structures. The ACSTM is sensitive to both carrier concentration and dopant type, making it ideal for future metrological applications.Item Scanning probe analysis of polydiacetylene nanowires and poly(3-hexylthiophene) thin films(2010) Giridharagopal, Rajiv; Kelly, Kevin F.In molecular and organic electronic systems, the electrode material has considerable influence on the performance of the resulting device. The advent of scanning tunneling microscopy (STM) and its various spectroscopic extensions has allowed for exploration of the polymer-electrode interface with atomic-scale resolution. Here I present the use of STM to analyze such systems. Specifically, STM and microwave-frequency alternating current STM (ACSTM) of conducting polymers on different substrates can shed new light on how the electrode material electronically affects the adhered polymer structure. The polymers used in this work are polydiacetylene nanowires and poly(3-hexylthiophene) (P3HT) monolayer, bilayer, and thin films. For both polymers, we can use the convolution of electronic and topographic information inherent in STM topography images to extrapolate information about the electronic structure. It is also possible to acquire information about the work function, the density of states (DOS), relative energy level positions, and the differential capacitance via spectroscopic measurements. In particular, capacitance imaging requires a novel technique known as ACSTM that can be used to probe relative carrier concentration. This thesis presents analyses of PDA and P3HT on graphite and molybdenum disulfide. For P3HT thin films, gold and platinum substrates are also studied. The results indicate a strong substrate-dependent charge transfer that is further illuminated through ACSTM and other spectroscopic investigations. In this work, preliminary investigations of photovoltaic P3HT:fullerene films are also discussed.