Browsing by Author "Robert Nemanich, Committee Chair"

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Domain Patterned Ferroelectric Surfaces for Selective Deposition Via Photochemical Reaction
(2007-12-07) Hanson, Jacqueline Nicole; Laura Clarke, Committee Member; Alexei Gruverman, Committee Member; Thomas Pearl, Committee Member; Robert Nemanich, Committee Chair
In this work, the use of domain patterned ferroelectric materials as a template to direct the assembly of nanostructures via photochemical reactions as well as to direct the assembly of polar molecules is presented. A distinct characteristic of ferroelectric materials is existence of a reversible spontaneous polarization. The direction of the spontaneous polarization can be reversed with the application of an electric field; thus, conductive tip atomic force microscopy (AFM) was employed to "write" nanoscale domain patterns in the ferroelectric and subsequently to visualize the configuration with piezoresponse force microscopy (PFM). Domain patterned lithium niobate and lithium tantalate, which are known as oxygen octahedra ferroelectrics, were used as templates. These materials exhibit unique surface electronic properties resulting from screening of the bound polarization charge, which dictate local reactivity on the surface. The effect of composition (stoichiometry and doping) on photochemical reactions at the surface is explored. Depending on the composition of the material, deposition can occur on the surface of domains or domain boundaries leading to a "bottom-up" method of nanowire formation. Nanowires of various shapes and sizes can be fabricated, as these parameters are dependent only upon the underlying domain configuration. Domain specific adsorption of polar molecules is achieved by utilizing the pyroelectric nature of ferroelectric materials. Surfaces may also be passivated with these polar molecules; subsequent UV irradiation induces photodecomposition of the molecules, which results in the formation of trenches at domain boundaries. Additionally, a scanning Kelvin probe microscopy (SKPM) study of the charge distribution on the surface of lithium niobate emphasizes the external screening mechanism and demonstrates the instability and screening of surface charges.
Electronic Transition Imaging of Carbon Based Materials: The Photothreshold of Melanin and Thermionic Field Emission from Diamond
(2006-12-06) Garguilo, Jacob Marshall; Glenn Edwards, Committee Member; Harald Ade, Committee Member; Robert Nemanich, Committee Chair; Keith Weninger, Committee Member; Ron Scattergood, Committee Member
This study explores electronic transitions in carbon based materials through the use of a custom built, non rastering electron emission microscope. The specifics and history of electron emission are described as well as the equipment used in this study. The materials examined fall into two groups, melanosome films isolated from the human body and polycrystalline diamond tip arrays. A novel technique for determining the photothreshold of a heterogeneous material on a microscopic or smaller scale is developed and applied to melanosome films isolated from the hair, eyes, and brain of human donors. The conversion of the measured photothreshold on the vacuum scale to an electrochemical oxidation potential is discussed and the obtained data is considered based on this conversion. Pheomelanosomes isolated from human hair are shown to have significantly lower photoionization energy than eumelanosomes, indicating their likelihood as sources of oxidative stress. The ionization energies of the hair melanosomes are checked with complimentary procedures. Ocular melanosomes from the retinal pigment epithelium are measured as a function of patient age and melanosome shape. Lipofuscin, also found in the eye, is examined with the same microscopy technique and shown to have a significantly lower ionization threshold than RPE melanosomes. Neuromelanin from the substantia nigra is also examined and shown to have an ionization threshold close to that of eumelanin. A neuromelanin formation model is proposed based on these results. Polycrystalline diamond tip arrays are examined for their use as thermionic energy converter emitters. Thermionic energy conversion is accomplished through the combination of a hot electron emitter in conjunction with a somewhat cooler electron collector. The generated electron current can be used to do work in an external load. It is shown that the tipped structures of these samples result in enhanced emission over the surrounding flat areas, which may prove valuable in limiting the negative space charge effect in vacuum energy converting devices. Additionally, the effects of exceeding a threshold temperature for the films are shown, establishing a maximum operating regime for any device which incorporates hydrogen terminated diamond.
Nanoscale Investigation of the Piezoelectric Properties of Perovskite Ferroelectrics and III-Nitrides
(2004-02-09) Rodriguez, Brian Joseph; Angus Kingon, Committee Co-Chair; Robert Nemanich, Committee Chair; Salah Bedair, Committee Member; Jacqueline Krim, Committee Member; Alexei Gruverman, Committee Member
Nanoscale characterization of the piezoelectric and polarization related properties of III-Nitrides by piezoresponse force microscopy (PFM), electrostatic force microscopy (EFM) and scanning Kelvin probe microscopy (SKPM) resulted in the measurement of piezoelectric constants, surface charge and surface potential. Photo-electron emission microscopy (PEEM) was used to determine the local electronic band structure of a GaN-based lateral polarity heterostructure (GaN-LPH). Nanoscale characterization of the imprint and switching behavior of ferroelectric thin films by PFM resulted in the observation of domain pinning, while nanoscale characterization of the spatial variations in the imprint and switching behavior of integrated (111)-oriented PZT-based ferroelectric random access memory (FRAM) capacitors by PFM have revealed a significant difference in imprint and switching behavior between the inner and outer parts of capacitors. The inner regions of the capacitors are typically negatively imprinted and consequently tend to switch back after being poled by a positive bias, while regions at the edge of the capacitors tend to exhibit more symmetric hysteresis behavior. Evidence was obtained indicating that mechanical stress conditions in the central regions of the capacitors can lead to incomplete switching. A combination of vertical and lateral piezoresponse force microscopy (VPFM and LPFM, respectively) has been used to map the out-of-plane and in-plane polarization distribution, respectively, of integrated (111)-oriented PZT-based capacitors, which revealed poled capacitors are in a polydomain state.
Synthesis and Field Emission Properties of Carbon Nanotube Films
(2005-02-17) Wang, Yunyu; Sanju Gupta, Committee Member; Griff Bilbro, Committee Member; Hans Hallen, Committee Member; Robert Nemanich, Committee Chair; Thomas Pearl, Committee Member
In this study, vertically aligned multiwalled carbon nanotube films were grown by microwave plasma chemical vapor deposition (MWCVD). Through controlling the thickness of the iron thin film, carbon nanotubes with different diameters were obtained. When the thickness of the iron layer was reduced to 0.3-0.5 nm, single-wall and double-wall nanotubes were obtained with a high areal density (~ 1012/cm2) and vertical alignment. Scanning electron microscopy, Raman spectroscopy and high resolution transmission electron microscopy were employed to characterize the as-deposited nanotubes. In addition, a systematic study of the internal structure transition of the carbon nanotubes has been conducted and a growth model was proposed in terms of carbon surface and bulk diffusion. The field emission of the carbon nanotube films has also been explored in this study. Different measurement systems including a variable distance field emission system, a field emission imaging system, and a field electron emission system (FEEM) were employed. The effects of the diameter (multi-wall vs single- and double- wall), the adsorbates, and the temperature on the field emission properties of carbon nanotubes have been exhibited. Finally, two processes including hydrogen plasma etching and re-growth were used to treat the as-deposited film, and an increased emission site density was observed for the re-grown carbon nanotube film.