Scanning photocurrent microscopy study of photovoltaic and thermoelectric effects in individual single-walled carbon nanotubes Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/0r9676185

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  • Carbon Nanotubes are a unique family of nanostructures that have shown remarkable promise for mechanical, electrical, and optical applications. Fundamentally similar to the earlier discovered Buckminsterfullerene (C₆₀), carbon nanotubes are hollow cylinders formed from a single sheet of carbon atoms. The research presented in this dissertation investigates several carbon nanotube properties, with an emphasis on the interactions between light and electrons within the nanotube. Better understanding of these optoelectronic properties is of great interest for future advancement in solar energy conversion. Carbon nanotubes are a quintessential material for researchers studying nanoscience. Small changes in nanotube structure can lead to striking differences in electrical and optical properties. In addition to this rich variety of properties, nanotubes can be concentrically inset within one another resulting in single-walled, double-walled, and multi-walled versions. Single-walled carbon nanotubes are desirable for optoelectronic study as complicated interactions between shells are avoided; which, while an interesting topic, is beyond the scope of this work. In the past, determination of carbon nanotube wall number was achieved via transmission electron microscopy, a technique requiring difficult sample preparation, making it impractical for most working devices. This thesis presents an alternative technique based on the compressibility of nanotube sidewalls as measured by atomic force microscopy. This technique is readily applicable to common device designs and can distinguish single and double-walled carbon nanotubes. This technique has been applied to characterizing the output of our growth process, resulting in recipes that highly favor single-walled growth. The optical properties of single-walled carbon nanotubes have been studied intensely. Each single-walled nanotube has an optical fingerprint based on its unique atomic structure. Specific single-walled nanotube structures can be selected for a wide range of optical applications. Again, however, most techniques previously used to study these optical properties are not easily transferable to common device designs. In this research, the measurement of nanotube fingerprints is achieved by optoelectronic means through the use of a newly constructed scanning photocurrent microscope. This instrument measures changes in electrical transport as the device is locally illuminated by a spectrally-tunable light source, making it a versatile tool for studying nanostructures implemented in a variety device designs. Early work on nanotube optoelectronic properties attributed photocurrent generation to a photovoltaic mechanism. This thesis, however, shows that strong photothermoelectric effects can also be present in nanotube optoelectronic devices. Light absorbed by a nanostructure can locally raise the temperature, resulting in thermoelectric currents. If a carbon nanotube is being employed as the absorbing element of a solar cell, it is possible for thermoelectric currents to reduce or enhance the efficiency of energy conversion. The last portion of this work investigates the nature of optically generated thermoelectric effects in carbon nanotubes. With a better understanding of these thermoelectric effects, future nanostructure solar cell design can efficiently utilize thermoelectric currents.
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