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In my research, I have performed many characterization and fabrication experiments that are based on tools of analytical chemistry, materials chemistry, and surface science. My research projects are as follows. (1) Fabrication of transparent polymer templates for nanostructured amorphous silicon photovoltaics was done using low-cost nanoimprint lithography of polydimethylsiloxane. This approach provides a test bed for absorption studies in nanostructured film geometries and should result in improved light capturing designs in thin-film solar cells. Nanopatterned polymer films were characterized by scanning electron microscopy and optical measurements. (2) A straightforward method for fabricating freely suspended, thin, carbon nanotube (CNT) membranes infiltrated with polymers was developed. This process is a new approach for making thin, reinforced, smooth films or membranes with high concentrations of CNTs, which may lead to higher performance materials. Characterization of the film and membrane was performed via scanning electron microscopy and atomic force microscopy. (3) Laser activation-modification of semiconductor surfaces (LAMSS) was carried out on silicon with a series of 1-alkenes. A key finding from this study is that the degree of surface functionalization in a LAMSS spot appears to decrease radially from the center of the spot. These laser spots were studied by time of flight secondary ion mass spectrometry (ToF-SIMS), and the resulting spectra were analyzed using a series of chemometrics methods. (4) A large ToF-SIMS data set from multiple coal samples spanning a wide range of coal properties was subjected to a chemometrics analysis. This analysis separates the spectra into clusters that correspond to measurements from classical combustion analyses. Thus ToF-SIMS appears to be a promising technique for analysis of this important fuel. (5) Several experiments on carbon nanotube processing were performed in my research, including carbon nanotube sheet formation, carbon nanotube purification, carbon nanotube dispersion, and carbon nanotube functionalization. X-ray photoelectron spectroscopy was a key characterization tool for many of these experiments.

A process, called carbon nanotube templated microfabrication (CNT-M) makes high aspect ratio microstructures out of a wide variety of materials by growing patterned vertically aligned carbon nanotubes (VACNTs) as a framework and then infiltrating various materials into the frameworks by chemical vapor deposition (CVD). By using the CNT-M procedure, a partial Si infiltration of carbon nanotube frameworks results in porous three dimensional microscale shapes consisting of silicon-carbon nanotube composites. The addition of thin silicon shells to the vertically aligned CNTs (VACNTs) enables the fabrication of robust silicon nanostructures with edibility to design a wide range of geometries. Nanoscale dimensions are determined by the diameter and spacing of the resulting silicon/carbon nanotubes while microscale dimensions are controlled by the lithographic patterning of CNT growth catalyst. The characterization and application of the new silicon nanomaterial, silicon-carbon core-shell nanotube (Si/CNT) composite, is investigated thoroughly in the dissertation.The Si/CNT composite is used as thin layer chromatography (TLC) separations media with precise microscale channels for fluid flow control and nanoscale porosity for high analyte capacity. Chemical separations done on the CNT-M structured media outperform commercial high performance TLC media resulting from separation efficiency and retention factor. The Si/CNT composite is also used as an anode material for lithium ion batteries. The composite is assembled into cells and tested by cycling against a lithium counter electrode. This CNT-M structured composite provides an effective test bed for studying the effects of geometry (e.g. electrode thickness, porosity, and surface area) on capacity and cycling performance. A combination of high gravimetric, volumetric, and areal capacity makes the composite an enabling materials system for high performance Li-ion batteries.Last, a thermal annealing to the Si/CNT composite results in the formation of silicon carbide nanowires (SiCNWs). This combination of annealing and Si/CNTs yields a unique fabrication approach resulting in porous three dimensional silicon carbide structures with precise control over shape and porosity.

Since the discovery of carbon nanotubes (CNT) in 1999, they have attracted much attention due to their unique mechanical and electrical properties and potential applications. Yet their nanosize makes the study of individual CNTs easier said than done. In our laboratory, carbon fibers with nanotube cores have been synthesized with conventional chemical vapor deposition (CVD) method. The single multiwall carbon nanotube (MWNT) sticks out as a tip of the carbon fiber. In order to pick up the individual CNT tips, focused ion beam (FIB) technique is applied to cut and adhere the samples. The carbon fiber with nanotube tip was first adhered on a micro-manipulator with the FIB welding function. Afterwards, by applying the FIB milling function, the fiber was cut from the base. This enables us to handle the individual CNT tips conveniently. By the same method, we can attach the nanotube tip on any geometry of solid samples such as conventional atomic force microscopy (AFM) silicon tips. The procedures developed for the FIB assisted individual CNT tip fabrication will be described in detail. Because of their excellent electrical and stable chemical properties, individual CNTs are potential candidates as electron guns for electron based microscopes to produce highly coherent electron beams. Due to the flexibility of the FIB fabrication, the individual CNT tips can be easily fabricated on a sharpened clean tungsten wire for field emission (FE) experimentation. Another promising application for individual CNT tips is as AFM probes. The high aspect ratio and mechanical resilience make individual CNTs ideal for scanning probe microscopy (SPM) tips. Atomic force microscopy with nanotube tips allows us to image relatively deep features of the sample surface at near nanometer resolution. Characterization of AFM with individual CNT tips and field emission properties of single CNT emitters will be studied and presented.

Single electron transistors (SET) have attracted significant attention as a potential building block for post CMOS nanoelectronic devices. However, lack of reproducible and parallel fabrication approach and room temperature operation are the two major bottlenecks for practical realization of SET based devices. In this thesis, I demonstrate large scale single electron transistors fabrication techniques using solution processed single wall carbon nanotubes (SWNTs) and studied their electron transport properties. The approach is based on the assembly of individual SWNTs via dielectrophoresis (DEP) at the selected position of the circuit and formation of tunnel barriers on SWNT. Two different techniques: i) metal-SWNT Schottky contact, and ii) mechanical templating of SWNTs were used for tunnel barrier creation. Low temperature (4.2K) transport measurement of 100 nm long metal-SWNT Schottky contact devices show that 93% of the devices with contact resistance (RT) > 100 K? show SET behavior. Majority (90%) of the devices with 100 K? < RT < 1 M?, show periodic, well-de?ned Coulomb diamonds with a charging energy ~ 15 meV, represents single electron tunnelling through a single quantum dot (QD), defined by the top contact. For high RT (> 1M?), devices show multiple QDs behaviors, while QD was not formed for low RT ( < 100 K?) devices. From the transport study of 50 SWNT devices, a total of 38 devices show SET behavior giving an yield of 76%. I also demonstrate room temperature operating SET by using mechanical template technique. In mechanical template method individual SWNT is placed on top of a Al/Al2O3 local gate which bends the SWNT at the edge and tunnel barriers are created. SET devices fabricated with a template width of ~20 nm shows room temperature operation with a charging energy of ~150 meV. I also discussed the detailed transport spectroscopy of the devices.

Fabrication of high-performance electronic devices using the novel semiconductors is essential for developing future electronics which can be applicable in large-area, flexible and transparent displays, sensors and solar cells. One of the major bottlenecks in the fabrication of high-performance devices is a large interfacial barrier formation at metal/semiconductor interface originated from Schottky barrier and interfacial dipole barrier which causes inefficient charge injection at the interface. Therefore, having a favorable contact at electrode/semiconductor is highly desirable for high-performance devices fabrication. In this dissertation, the fabrication of nanoelectronic devices and investigation of their transport properties using carbon nanotubes (CNTs) and graphene as electrode materials will be shown. I investigated two types of devices using (i) semiconducting CNTs, and (ii) organic semiconductors (OSC). In the first part of this thesis, I will demonstrate the fabrication of high-performance solution-processed highly enriched (99%) semiconducting CNT thin film transistors (s-CNT TFTs) using densely aligned arrays of metallic CNTs (m-CNTs) for source/drain electrodes. From the electronic transport measurements at room temperature, significant improvements of field-effect mobility, on-conductance, transconductance and current on/off ratio for m-CNT/s-CNT devices were found compared to control palladium (Pd contacted s-CNT devices. From the temperature dependent transport investigation, a lower Schottky barrier height for the m-CNT/s-CNT devices was found compared to the devices with control metal electrodes. The enhanced device performance can be attributed to the unique device geometry as well as strong ?- ? interaction at m-CNT/s-CNT interfaces. In addition, I also investigated s-CNT TFTs using reduced graphene oxide (RGO) electrodes. In the second part of my thesis, I will demonstrate high-performance organic field-effect transistors (OFETs) using different types of graphene electrodes. I show that the performance of OFETs with pentacene as OSC and RGO as electrode can be continuously improved by increasing the carbon sp2 fraction of RGO. The carbon sp2 fractions of RGO were varied by controlling the reduction time. When compared to control Pd electrodes, the mobility of the OFETs shows an improvement of ?200% for 61% sp2 fraction RGO, which further improves to ?500% for 80% RGO electrode. Similarly, I show that when the chemical vapor deposition (CVD) graphene film is used as electrodes in fabricating OFET, the better performance is observed in comparison to RGO electrodes. Our study suggests that, in addition to ?-? interaction at graphene/pentacene interface, the tunable electronic properties of graphene as electrode have a significant role in OFETs performance. For a fundamental understanding of the interface, we fabricated short-channel OFETs with sub-100nm channel length using graphene electrode. From the low temperature electronic transport measurements, a lower charge injection barrier was found compared to control metal electrode. The detailed investigations reported in this thesis clearly indicated that the use of CNT and graphene as electrodes can improve the performance of future nanoelectronic devices.

Organic field effect transistors (OFETs) are of significant research interest due to their promising applications in large area, low-cost electronic devices such as flexible displays, sensor arrays, and radio-frequency identification tags. A major bottleneck in fabricating highperformance OFET is the large interfacial barrier between the metal electrodes and organic semiconductors (OSC) which results in an inefficient charge injection. Carbon nanotubes (CNTs) are considered to be a promising electrode material which can address this challenge. In this dissertation, we demonstrate fabrication of high-performance OFETs using aligned array CNT electrodes and investigate the detailed electronic transport properties of the fabricated devices. The OFETs with CNT electrodes show a remarkable enhancement in the device performance such as high mobility, high current on-off ratio, higher cutoff frequency, absence of short channel effect and better charge carrier injection than those OFETs with metal electrodes. From the low temperature transport measurements, we show that the charge injection barrier at CNT/OSC interface is smaller than that of the metal/OSC interface. A transition from direct tunneling to Fowler-Nordheim tunneling observed in CNT/OSC system shows further evidence of low injection barrier. A lower activation energy measured for the OFETs with CNT electrodes gives evidence of lower interfacial trap states. Finally, OFETs are demonstrated by directly growing crystalline organic nanowires on aligned array CNT electrodes. In addition to investigating the interfacial barrier at CNT/OSC interface, we also studied photoconduction mechanism of the CNT and CNT/OSC nanocomposite thin film devices. We found that the photoconduction is due to the exciton dissociations and charge carrier separation caused by a Schottky barrier at the metallic electrode/CNT interface and diffusion of the charge iv carrier through percolating CNT networks. In addition, it is found that photoresponse of the CNT/organic semiconductor can be tuned by changing the weight percentage of CNT into the organic semiconductors.

Carbon nanotubes have been regarded as ideal building blocks for nanoelectronics and multifunctional nanocomposites due to their exceptional strength, stiffness, flexibility, as well as their excellent electrical properties. However, carbon nanotube itself has limitations to fulfill the practical application needs: 1) an individual carbon nanotube has a low density of states at the Fermi level, and thus its conductivity is only comparable to moderate metals but lower than that of copper. 2) Metallic and semiconducting nanotubes are inherently mixed together from the synthesis, and the selection/separation is very difficult with very low efficiency. 3) Carbon nanotubes alone cannot be used in practical application and a bonding material is normally needed as the join material for actual devices. In this work, we fundamentally explored the possibility that metals (Cu, Al) could tailor carbon nanotube’s electronic structure and even transit it from semiconducting to metallic, thus skipping the selection between the metallic and the semiconducting CNTs. We also found out a novel way to enhance a semiconducting CNT system’s conductance even better than that of a metallic CNT system. All these researches are done under density functional theory (DFT) frame in conjunction with non-equilibrium Green functions (NEGF). At first we studied the adsorbed copper’s influence on the electronic properties of CNT (10, 0) and CNT (5, 5). Results indicate that both the Density of States (DOS) and the transmission coefficients of CNT (5,5) /Cu have been increased. For CNT (10,0)/Cu, the band gap has been shrank, which means the improved conducting properties by the incorporation of copper . iv As a further case, semiconductor SWCNT (10, 0) with more adsorbed copper chains outside has been studied. 1, 4, 5 and 6 Cu chains have been added onto the carbon nanotube (10,0), and the adsorption of 6 Cu chains finally lead to the transform of the system from semiconducting to metallic. Considering the confining effect, the case that Cu filled into CNT (10, 0) is also studied. It is found that the filled copper chains could modify the system to be metallic more efficiently than the adsorbed Cu chain. Similarly, Al adsorbed on CNT (10, 0) is also studied, and it is found that Al has a better efficiency than copper in tuning the semiconducting CNT to metallic. The existing chemical bonds between the CNT and Al atoms may account for this higher efficiency. In addition, the resultant conductivity of the Al/CNT system is better than that of Cu/CNT system. The Cu/CNT (5,5)+Cu/Cu junction, as another realistic device setup, has been studied in terms of the conductance. The results show that the incorporation of Cu would enhance the conductance of the Cu/CNT/Cu system due to the interaction between Cu and the CNT.

This dissertation demonstrated that the manipulation of substances at the molecular or nanometer level can lead to the discovery and development of new materials with interesting properties and important applications. Chapter 1 describes the development of a nanoscale molecular thin film material for corrosion protection. By using a self-assembled monolayer film with a thickness of only about 1 nanometer as a linkage, a covalent bonding was achieved between a polyurethane top coating and an aluminum alloy substrate. This covalent bonding between polymer top coating and the aluminum alloy substrate significantly improved the corrosion resistance of the substrate. Chapter 2 and Chapter 3 describe the development of a gold nanoparticle-polymer composite material in different forms with a number of applications. Gold nanoparticles are among one of the most extensively studied nanomaterials. When the size of gold is shrunk to the nanometer scale, many interesting and new physical properties start to appear from gold nanoparticles. The optical properties of gold nanoparticles, particularly the surface plasmon resonance absorption, have been investigated in this dissertation for the development of multifunctional nanocomposite materials. Chapter 2 presents the preparation of a gold nanoparticle/poly(methyl methacrylate) (PMMA) nanocomposite film and the application of such films for microstructure fabrication using a direct laser writing technique. Gold nanoparticles are excellent photon-thermal energy converters due to their large absorption cross section at the surface plasmon resonance region. Upon laser irradiation of the nanocomposite film, the thermal energy converted from the absorbed photon energy by gold nanopaticles induced a complete decomposition of PMMA, leading to the formation of various microstructures on the nanocomposite films. Chapter 3 reports the further development of a nanoparticle/polymer composite nanofiber material fabricated through an electrospinning process. The matrix of the nanofiber is made of two polyelectrolytes, poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH). Three methods were developed to incorporate gold nanoparticles into the polymer matrix. The composite nanofiber materials developed in this study demonstrate multifunctional properties, including good electrical conductivity, photothermal response, and surface-enhanced IR absorption. This material may be used for many important applications including catalysis, chemical and biological sensors, and scaffold materials for tissue engineering. In Chapter 4, another most important nanomaterial, carbon naotubes (CNTs), were introduced as fillers to prepare polymer nanocomposites. A dispersion method for multi-walled carbon nanotubes (MWCNTs) using a conjugated conducting polymer, poly(3-hexylthiophene) (P3HT) as the third component and trifluoroacetic acid (TFA) as a co-solvent was developed. Due to the excellent dispersion of carbon nanotubes in PMMA and enhanced conductivity of the nanocomposites by the conjugated conducting polymers, the prepared composite materials has an extremely low percolation threshold of less than 0.006 wt% of MWCNT content. The potential use of MWCNT/conducting polymer composites for energy storage applications such as suppercapacitors was further investigated by Cyclic Voltammetry (CV), Electrochemical Impedance Spectroscopy (EIS) and charging-discharging cycles. Compared to pure carbon nanotubes, the nanocomposite materials have significantly improved properties and are promising for supercapacitor applications.