Chemistry & Biochemistry Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2752
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Item Electrical Properties of a Tube-in-a-Tube Semiconductor(2016) Ng, Allen Lee; Wang, YuHuang; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Tube-in-a-tube (Tube^2) nanostructures were synthesized through the outer-wall selective covalent functionalization of double-walled carbon nanotubes (DWCNTs) at high functional densities. Upon functionalization, the properties of individual walls within the structure decouple resulting in an electrically insulating functional outer tube while the inner tube retains exceptional CNT properties. The exceptional electrical properties of Tube^2 semiconductor structures were demonstrated for applications that include molecular and biological sensors and patterning of CNTbased structures with electronic type specificity. Tube^2 thin film transistor (TFT) sensors exhibited simultaneous ultrahigh sensitivity and selectivity towards chemical and biological targets. Carboxylic acid terminated Tube^2 sensors displayed an NH3 sensitivity of 60 nM, which is comparable with small molecule aqueous solution detection using state-of-the-art TFT sensors while simultaneously attaining 6,000 times higher chemical selectivity towards a variety of amine containing analyte molecules over carboxylic acids. Similarly, 23-base ii oligonucleotide terminated Tube^2 sensors demonstrated concomitant sensitivity down to 5 nM towards their complementary sequence without amplification techniques and single mismatch selectivity without the use of a gate electrode. Unique sensor architectures can be designed with the requirement of a gate electrode, such as the creation of millimeter-scale point sensors. The optical features and unique structural features of Tube^2 thin films were also exploited to address the challenge of patterning CNT nanostructures with electronic type specificity. Patterned dot arrays and conductive pathways were created on an initially insulating Tube^2 thin film by tuning the resonance of the direct-writing laser with the electronic type of the inner tube (i.e., metallic or semiconducting). The successful patterning of Tube^2 thin films was unambiguously confirmed with in situ Raman spectral imaging and electrical characterization. Furthermore, a hybrid 2-D carbon nanostructure comprised of a functionalized graphene that covers a semiconducting (6,5) SWCNT network (fG/sSWCNT) was developed. The hybrid fG/sSWCNT nanostructure exhibits similar structural and electrical properties as a semiconducting Tube^2 thin film, but possesses a transconductance that is an order of magnitude larger than Tube^2 and ON/OFF ratios as high as 5400 without the useful of further processing steps such as electrical breakdown.Item CHEMICAL FUNCTIONALIZATION OF CARBON NANOTUBES FOR CONTROLLED OPTICAL, ELECTRICAL AND DISPERSION PROPERTIES(2013) Brozena, Alexandra; Wang, YuHuang; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A carbon nanotube is a graphitic sheet, rolled into a one-dimensional, hollow tube. This structure provides certain individual nanotubes with high conductivity and near-infrared optical activity. These properties are not necessarily translated at the macroscale, however, due to strong van der Waals attractive forces that determine the behavior at the bulk level - exemplified by aggregation of nanotubes into bundles with significantly attenuated functionality. Different methods of carbon nanotube covalent functionalization are studied to improve dispersion while simultaneously maintaining intrinsic electrical and optical properties. In addition to retention of known behavior, new carbon nanotube photoluminescence pathways are also revealed as a result of this same covalent functionalization strategy. With various wet chemistries, including super-acid oxidation, the Billups-Birch reaction, and various diazonium based reactions, that utilize strong reducing or oxidizing conditions to spontaneously exfoliate aggregated carbon nanotubes, we are able to covalently functionalize individually dispersed nanotubes in a highly scalable manner. Covalent addition to the nanotube sidewalls converts the native sp2 hybridized carbon atoms to sp3 hybridization, which helps disrupt inter-tube van der Waals forces. However, this change in hybridization also perturbs the carbon nanotube electronic structure, resulting in an undesired loss of electrical conductivity and optical activity. We observe that controlling the location of functionalization, such as to the outer-walls of double-walled carbon nanotubes or as discrete functional "bands," we avert the loss of desirable properties by leaving significant tracts of sp2 carbon atoms unperturbed. We also demonstrate that such functional groups can act as electron and hole traps through the creation of a potential well deviation in the carbon nanotube electronic structure. This defect-activated carrier trapping primes the formation of charged excitons (trions) which are observed as redshifted photoluminescence in the near-infrared region. Implications and impacts of these covalent functionalization strategies will be discussed.Item EXPLORATION OF NOVEL METHODS FOR THE FABRICATION AND CHARACTERIZATION OF ORGANIC FIELD-EFFECT TRANSISTORS AND EXAMINATION OF FACTORS INFLUENCING OFET PERFORMANCE(2009) Southard, Adrian Edward; Fuhrer, Michael S.; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis explores novel methods for fabricating organic field effect transistors (OFETs) and characterizing OFET devices. Transfer printing is a promising process for fabricating organic thin-film devices. In this work, a transfer-printing process is developed for the polymer organic semiconductor P3HT. Pre-patterned P3HT is printed onto different dielectrics such as PMMA, polystyrene and polycarbonate. The P3HT layer is spun on a smooth silicon interface made hydrophobic by treatment with octyltrichlorosilane, which functions as a release layer. This method has distinct advantages over standard OFET fabrication methods in that 1) the active layer can be pre-patterned, 2) the solvent for the P3HT need not be compatible with the target substrate, and 3) the electrical contact formed mimics the properties of top contacts but with the spatial resolution of bottom contacts. Transparent, conducting films of carbon nanotubes (CNTs) are prepared by airbrushing, and characterized optically and electronically. OFETs with CNT films as source and drain electrodes are fabricated using various patterning techniques, and the organic/CNT contact resistance is characterized. CNT films make transparent, flexible electrodes with contact resistance comparable to that found for Au bottom-contacted P3HT transistors and comparable to CNT-film bottom-contacted pentacene transistors with CNTs deposited by other less flexible methods. A transparent OFET is demonstrated using transfer printing for the assembly of an organic semiconductor (pentacene), CNT film source, drain, and gate electrodes, and polymer gate dielectric and substrate. The dependence of the conductance and mobility in pentacene OFETs on temperature, gate voltage, and source-drain electric field is studied. The data are analyzed by extending a multiple trapping and release model to account for lowering of the energy required to excite carriers into the valence band (Poole-Frenkel effect). The temperature-dependent conductivity shows activated behavior, and the activation energy is lowered roughly linearly with the square-root of electric field, as expected for the Poole-Frenkel effect. The gate voltage dependence of the activation energy is used to extract the trap density of states, in good agreement with other measurements in the literature.