Browsing by Subject "Nanotubes"

Now showing 1 - 10 of 10
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    Electrical transport measurements of individual bismuth nanowires and carbon nanotubes
    (2005) Jang, Wan Young; Yao, Zhen, Ph. D.
    Nanostructures are defined by reducing dimensions. When the reduced size of materials is comparable to the Fermi wavelength, quantum size effect occurs. Dimensionality plays a critical role in determining the electronic properties of materials, because the density of states of materials is quite different. Nanowires have attracted much attention recently due to their fundamental interest and potential application s. A number of materials have been tried. Among them, bismuth has unique properties. Bismuth has the smallest effective mass as small as 0.001me. This small effective mass of Bi nanowires allows one to observe the quantum confinement effect easily. Also Bi nanowires are good candidates for a low-dimensional transport study due to long mean free path. Because of these remarkable properties of Bi nanowires, many efforts have been made to study Bi nanowires. However, because bismuth is extremely sensitive to the oxide, it is very difficult to make a reliable device. So far, array measurements of Bi nanowires have been reported. The study is focused on the synthesis and electric transport measurements of individual Bi nanowires. Bi nanowires are synthesized by electrodeposition using either anodic aluminum oxide (AAO) templates or commercially available track etched polycarbonate membranes (PCTE). The desired nanowire has a heterostructure of Au – Bi – Au. Au wires on both sides serve as contact electrodes with Bi. To extract nanowires from PCTE or AAO, several attempts have been made. Devices consisting of single Bi nanowires grown by hydrothermal method are fabricated and electrical measurements have been carried out after in-situ deposition of Pt electrodes. The temperature dependence of resistance of majority of nanowires increases with decreasing temperature, showing polycrystalline nature of nanowires. However, some nanowires show resistance peaks at low temperature, suggesting quantum size effect (QSE). Magnetoresistance (MR) has also been measured. We have also studied electric transport measurements of carbon nanotubes grown in AAO templates. These vertically grown carbon nanotubes (CNTs) are useful for field emission device. In addition, ultra-density vertical CNT transistor arrays have also been proposed based on these nanotube structures. To realize these interesting electronic applications, a detailed understanding of the electronic transport properties of the nanotubes is needed. In particular, nanotubes grown in the AAO templates are known to possess significant amount of structural disorder. It is thus important to elucidate the effect of disorder on the electronic properties of these nanotubes. Electrical transport measurements of individual carbon nanotubes are studied, The four-terminal resistance at room temperature scales linearly with the nanotube length indicating diffusive nature of transport. The conductance shows an exp[(-1/T)1/3] dependence on temperature T, suggesting that two-dimensional variable-range hopping is the dominant conduction mechanism. The maximum current density carried by these nanotubes is on the order of 106 A/cm2.
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    Electrochemical evaluation of nanocarbons for biogenic analyte detection
    (2007-12) Lyon, Jennifer Lee, 1980-; Stevenson, Keith J.
    This dissertation explores the use of nanocarbons both as conductive supports for redox enzyme electrochemistry and as electrocatalytic components for the nonmediated detection of biogenic analytes. More specifically, the influence of nitrogen doping of these nanocarbons (referred to herein as nitrogen-doped carbon nanotubes, or N-CNTs) on their bioelectrocatalytic performance is studied through direct enzyme adsorption and exploitation of the N-CNTs' inherent reactivity toward H₂O₂ to create H₂O₂-based sensing strategies. Both nondoped CNTs and N-CNTs may be effectively incorporated into biogenic sensing assemblies, as demonstrated herein using a variety of electrochemical techniques. Chapter 1 gives a general overview of the scope of this research and describes previous studies conducted within our laboratories that demonstrate our CNTs' promise as biogenic electrode materials. Chapter 2 describes the chemical vapor deposition (CVD) method used to prepare both CNTs and N-CNTs and establishes their suitability for use in the detection schemes outlined in later chapters through long-term stability studies. Additionally, the redox activity of Fe nanoparticles entrapped in the CNTs as a result of this CVD growth process is examined using a host of electrochemical experiments. Importantly, the data presented in this chapter show that these Fe particles do not explain the observed electrocatalytic response of the CNTs. Chapter 3 explores the direct adsorption of horseradish peroxidase (HRP) at both nondoped and N-CNTs. Spectroscopic and electrochemical assays are used to compare the extent of HRP enzymatic activity upon immobilization at both types of CNTs. Both types of HRP/CNT composites are then utilized in a quantitative H₂O₂ sensing strategy. Chapter 4 discusses the intrinsic reactivity of N-CNTs toward H₂O₂. Koutecky-Levich plots are used to demonstrate differences in H₂O₂ consumption mechanisms between NCNTs and traditional peroxidases. By replacing HRP with N-CNTs in an amperometric glucose detection scheme, the versatility of N-CNTs as a peroxidase substitute for biogenic analyte detection is demonstrated. Chapter 5 outlines future directions for this research, including possible strategies for improving electron transfer between HRP and both types of CNTs. This chapter also presents a newly developed, mediated oxidase-substrate electrochemical detection method that can easily be modified to incorporate CNTs.
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    Interaction of intense laser fields with carbon nanotubes
    (2006) Hsu, Han; Reichl, L. E.
    The main topic addressed in this dissertation is the interaction of intense laser fields with the π electrons of single-walled carbon nanotubes. Since it is very difficult to carry out ab initio calculations for nanotubes in the presence of intense laser fields, we first propose a simple but realistic model to simulate the π electrons in a graphene layer and single-walled carbon nanotubes. We replace the atomic potential of each carbon atom of graphene or nanotubes by a two-dimensional attractive regularized δ function. By adjusting the parameters in this simple δ-potential model, we successfully reproduce the band structures and wave functions for graphene and nanotubes calculated by ab initio methods. With its simplicity and accuracy, we can use δ-potential model to study the interaction of intense laser fields with nanotubes. Combining the δ-potential model and Floquet-Bloch theory, we calculate the electronic states and the electron motion in an armchair nanotube driven by monochromatic intense laser fields with polarization parallel to the nanotube axis. The intensity and frequency (photon energy) of the applied laser fields are varied so their effect on the electrons can be understood. In each case, Floquet-Bloch theory is used to calculate the Floquet-Bloch states, quasienergy band, mean energy band, and electron current density. By summing up the current density of all occupied Floquet-Bloch states, the harmonic generation spectrum can be determined. We demonstrate that the deformation of quasienergy band and mean energy band is related to high-order harmonic generation. Only the states deviating from field-free eigenstates may contribute to high-order harmonic generation.
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    Preparation and characterization of nitrogen doped carbon nanotube electrode materials
    (2006) Maldonado, Stephen; Stevenson, Keith J.
    This dissertation describes the preparation and characterization of nitrogen doped carbon nanotube films by a chemical vapor deposition (CVD) process. Thorough description of the preparation method is given. A variety of techniques are used to demonstrate the relationship between the physicochemical properties and electrochemical properties of nitrogen doped carbon nanotubes, shedding insight to the influence of nitrogen doping on observed electrocatalysis phenomena. Chapter 1 comprises a general overview of the presented material and scope of the work. Chapter 2 details the CVD preparation of nitrogen doped carbon nanotubes. Scanning electron microscopy, thermal gravimetric analysis, and x-ray photoelectron spectroscopy are used to demonstrate the degree of control afforded by the CVD process over the resultant properties of the prepared carbon nanotubes. The direct preparation of carbon nanotube films on a current collector is shown. Chapter 3 consists of a thorough characterization and comparison of nitrogen doped carbon nanotubes and non-doped carbon nanotubes. Transmission electron microscopy, thermal gravimetric analysis, and Raman spectroscopy demonstrate the increased disorder caused by nitrogen doping into the graphitic lattice structure of carbon nanotubes. X-ray photoelectron spectroscopy highlights the existence of multiple carbon-nitrogen surface functionalities that change in relative abundance as the nitrogen content is varied. Titration analyses indicate that nitrogen doped carbon nanotubes are basic and acquire a cationic surface charge in solutions of neutral pH. Chapter 4 presents a collection of voltammetric responses of several outer-sphere and innersphere redox probes. The similarities and differences between the responses at non-doped carbon nanotube electrodes and nitrogen doped carbon nanotube electrodes are noted and discussed in the context of the conclusions of the physical characterizations. A detailed mechanistic analysis of the O2 reduction process at nitrogen doped carbon nanotube electrodes is presented. Chapter 5 details continuing work with carbon nanotube materials for fundamental and applied studies.
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    Spin and charge transport through carbon based systems
    (2007) Jung, Suyong, 1976-; Yao, Zhen, Ph. D.
    In this thesis, we investigate spin-dependent transport through ferromagnet-contacted single-walled carbon nanotubes (SWCNTs), in which charge transport shows the Fabry-Perot (FP) interference effect, the Kondo effect and the Coulomb blockade effect at low temperatures. Hysteric magnetoresistance (MR) is observed in all three transport regimes, which can be controlled by both the external magnetic field and the gate voltage. The MR in the FP interference regime can be well understood by a model considering the intrinsic electronic structure of SWCNTs and the quantum interference effect. In the strongly interacting Kondo regime, the Kondo effect is not suppressed by the presence of nearby ferromagnetism. Several observed MR features including the non-splitted zero-bias Kondo peak and positive MR switching can be explained by the strong Kondo effect and weak ferromagnetism in the leads. In the Coulomb blockade regime, several effects that can be associated with the magneto-Coulomb effect have been observed, and isolated spin accumulation and transport through the SWCNT quantum dot have been realized by a four-probe non-local measurements. We also studied charge transport behavior through organic semiconductor pentacene thin film transistors (OTFTs) in the limit of single- or a few molecular layers of pentacene films. The charge transport in these devices can be well explained by the multiple trapping and release model. The structural disorders induced by the physical and chemical causes, such as grain boundaries, interactions with gate insulator, metal contacts and ambient conditions can be responsible for the localized trap states in the ultrathin layer OTFTs, which are further confirmed by the electric force microscopy (EFM) measurements.
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    Synthesis and characterization of carbon nanotube supported nanoparticles for catalysis
    (2007-12) Vijayaraghavan, Ganesh, 1978-; Stevenson, Keith J.
    This dissertation describes the synthesis and characterization of nitrogen doped carbon nanotube (NCNT) supported nanoparticles for catalysis, specifically, the cathodic oxygen reduction reaction (ORR) in fuel cells. Strategies for synthesis of mono- and bimetallic nanoparticle catalysts through dendrimer based templating techniques and with the aid of metal organic chemical vapor deposition (MOCVD) precursors and efficient assembly protocols of the catalysts with the NCNTs are discussed in detail. Physicochemical properties of the NCNTs and NCNT supported catalysts were characterized using a host of tools including scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, x-ray photoelectron spectroscopy (XPS), thermo gravimetric analysis, BET surface area and pore size analysis and electrochemical techniques including cyclic voltammetry, chronocoulometry, chronoamperometry and rotating disk electrode voltammetry. Chapter 1 serves as a general introduction and provides a brief overview of challenges associated with the synthesis, characterization and utilization of graphitic carbons and graphitic carbon supported catalysts in heterogeneous catalysis. Chapter 2 provides an overview of the synthesis and characterization of systematically doped iron and nickel catalyzed NCNTs in an effort to understand the effect of nitrogen doping on ORR. Chapter 3 describes the use of NCNTs as supports for dendrimer templated nanoparticle catalysts for ORR. A facile synthetic strategy for the immersion based loading of catalysts onto NCNTs by spontaneous adsorption to achieve specific catalyst loadings is explored. Chapter 4 details the loading of monodisperse Pt, Pd and PtPd catalysts on the as synthesized NCNTs using MOCVD precursors. The MOCVD route offers promise for direct dispersion and activation of ORR catalysts on NCNT supports and eliminates a host of problems associated with traditional solvent based catalyst preparation schemes. Chapter 5 details future directions on a few topics of interest including efficient electrodeposition strategies for preparing NCNT supported catalysts, studies on PtCu catalysts for ORR and finally prospects of using NCNT supported catalysts in fuel cell applications.
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    Thermal and thermoelectric transport measurements of one-dimensional nanostructures
    (2005) Zhou, Jianhua; Shi, Li, Ph. D.
    This dissertation presents thermal and thermoelectric transport measurements of onedimensional nanostructures including bismuth telluride (BixTe1-x) nanowires and singlewalled carbon nanotubes (SWCNT). Theoretical calculations have predicted that BixTe1-x nanowires may have enhanced thermoelectric figure of merit defined as ZT = (S2 σ/κ)T, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity, and T is the absolute temperature. Our measurements showed that the σ of BixTe1-x nanowires was very close to, and the κ was largely reduced compared to the bulk values at 300 K. For a BixTe1-x nanowire with x ≈ 0.46, the room temperature S of 260 µV/K was 60% higher than that of its bulk counterpart, while small negative S was measured for four nanowires with x ≈ 0.54. High ZT can be expected for BixTe1-x nanowires with optimized x. The unique electron transport and heat dissipation mechanisms in current – carrying metallic and semiconducting SWCNTs were studied with the use of Scanning Probe Microscopy (SPM) methods including Electrostatic Force Microscopy (EFM), Scanning Gate Microscopy (SGM) and Scanning Thermal Microscopy (SThM). For several metallic SWCNTs with low-bias resistance above 40 x 103 Ω, the electrical potential profile along the SWCNTs was linear at a voltage bias of 0.1 V, suggesting that the electron mean free path was shorter than the length of the nanotube at the low bias. Heat dissipation along these metallic SWCNTs was uniform at voltage biases above 0.22 V. For several semiconducting SWCNTs with low-bias resistance as low as 20 x 103 Ω, large conduction barriers were induced by SGM probes at locations where defects existed, and the heat dissipation was uniform at voltage biases above 0.12 V. This observation suggests diffusive and dissipative heat dissipation in semiconducting SWCNTs. A large tip-sample thermal contact resistance has made it challenging to obtain the actual temperature rise of the sample surface using the SThM method. We have developed a nanocontact thermometry technique that can potentially be employed for quantitatively mapping surface temperature profiles of nanoelectronics with spatial resolution below 20 nm. This method was tested with metal interconnect structures.
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    A UHV variable temperature STM and its application to the study of high-T(C) superconductors and carbon nanotubes
    (2002) Lee, Jinho, 1969-; de Lozanne, Alejandro L.
    The first part of this dissertation describes brief theoretical background for scanning tunneling microscopy(STM), single electron tunneling, and Coulomb blockade phenomena. The second part addresses issues on the design and construction of Ultra High Vacuum Low Temperature Scanning Tunneling Microscope (UHV LTSTM)and also it’s operation, the third part describes topographic, spectroscopic data obtained on high-TC superconductors and HOPG with carbon nanotube(CNT) tips with hitherto made UHV LTSTM system. The fourth part discusses on the possibility of new microscopy using CNT tip’s remarkable I-V characteristic. Our UHV LTSTM can reach from room temperature down to ∼8K. To control the STM head, we used W.A.Technology’s TOPS °r system and it’s software. STM experiments were carried out using double etched W wire tips, and also carbon nanotubes grown on sharpened W wires. SEM images of these nanotube tips show bundles of SWNT or MWNT structures. Atomic-scale STM images on HOPG and 3 nm structural modulations on Bi2Sr2CaCu2O8+x crystals were observed with these nanotube tips. Rectifying I-V characteristics with a threshold voltage of ∼0.7 V were observed with a nanotube tip and a Bi2Sr2CaCu2O8+x sample at 84K. This unexpected characteristics were explained in terms of double barrier tunneling. Quantized differential conductance peaks were also observed reproducibly with different carbon nanotube tips and different sample at different temperature from 13K to 84K, with peak to peak ∆V as large as few 100 mV, which suggests coulomb blockade phenomena. Performing constant imaging tunneling spectroscopy(CITS) with these CNT tips with single electron box behavior, a new possibility of microscopy was discussed in virtue of Fourier transform.