Browsing by Subject "Nanostructured materials--Electric properties"

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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.
Fabrication of silicon-based nano-structures and their scaling effects on mechanical and electrical properties
(2007-12) Li, Bin, 1974 May 21-; Ho, P. S.
Silicon-based nanostructures are essential building blocks for nanoelectronic devices and nano-electromechanical systems (NEMS), and their mechanical and electrical properties play an important role in controlling the functionality and reliability of the nano-devices. The objective of this dissertation is twofold: The first is to investigate the mechanical properties of silicon nanolines (SiNLs) with feature size scaled into the tens of nanometer level. And the second is to study the electron transport in nickel silicide formed on the SiNLs. For the first study, a fabrication process was developed to form nanoscale Si lines using an anisotropic wet etching technique. The SiNLs possessed straight and nearly atomically flat sidewalls, almost perfectly rectangular cross sections and highly uniform linewidth at the nanometer scale. To characterize mechanical properties, an atomic force microscope (AFM) based nanoindentation system was employed to investigate three sets of silicon nanolines. The SiNLs had the linewidth ranging from 24 nm to 90 nm, and the aspect ratio (Height/linewidth) from 7 to 18. During indentation, a buckling instability was observed at a critical load, followed by a displacement burst without a load increase, then a fully recoverable deformation upon unloading. For experiments with larger indentation displacements, irrecoverable indentation displacements were observed due to fracture of Si nanolines, with the strain to failure estimated to be from 3.8% to 9.7%. These observations indicated that the buckling behavior of SiNLs depended on the combined effects of load, line geometry, and the friction at contact. This study demonstrated a valuable approach to fabrication of well-defined Si nanoline structures and the application of the nanoindentation method for investigation of their mechanical properties at the nanoscale. For the study of electron transport, a set of nickel monosilicde (NiSi) nanolines with feature size down to 15 nm was fabricated. The linewidth effect on nickel silicide formation has been studied using high-resolution transmission electron microscopy (HRTEM) for microstructural analysis. Four point probe electrical measurements showed that the residual resistivity of the NiSi lines at cryogenic temperature increased with decreasing line width, indicating effect of increased electron sidewall scattering with decreased line width. A mean free path for electron transport at room temperature of 5 nm was deduced, which suggests that nickel silicide can be used without degradation of device performance in nanoscale electronics.
Germanium nanowires : synthesis, characterization, and utilization
(2004) Hanrath, Tobias, 1977-; Korgel, Brian Allan, 1969-
A supercritical fluid synthesis method was developed for the preparation of single crystal germanium (Ge) nanowires with diameters as small as 4 nanometer and several tens of micrometer in length. Alkanethiol protected gold nanocrystals were used to seed and direct nanowire growth. Nanowire processing and their implementation as building blocks in nanowire based devices requires rigorous control of nanowire surface chemistry, which differs from well-studied monolithic atomically-smooth single crystal substrate surface chemistry due to the nanowire’s high surface area to volume ratio and atomically rough surface. Ge nanowire surface oxidation was studied by Ge 3d x-ray photoelectron spectroscopy. A broad range of solution-phase routes to the Ge nanowire surface passivation were explored including sulfidation, hydride and chloride termination, and organic monolayer passivation. Nanowires with covalently bonded monolayer surface terminations formed via thermally-initiated hydrogermylation reactions with alkenes, alkynes or dienes exhibited excellent chemical stability compared to untreated or etched nanowire surfaces and enabled low contact resistance ohmic electrical contacts to be made to the nanowires. Device characteristics of single Ge nanowire devices fabricated with gold electrical contacts patterned by e-beam lithography were compared with devices prepared using focused e-beam or Ga-beam assisted Pt chemical vapor deposition. These device structures permitted direct investigation of the influence of nanowire surface chemistry, doping, and gate electrode architecture, on device operation. The impact of the surface chemistry on surface state dominated electron transport in single nanowire devices was investigated by room temperature field-effect measurements. The density and relaxation time distribution of electrically active surface states was found to be highly sensitive to the nanowire surface chemistry. Complimentary to the device measurements, fundamental electrical and optical properties were probed via electron energy loss spectroscopy on individual nanowires inside the transmission electron microscope. The volume plasmon energy increased with decreasing diameter for nanowires narrower than 24 nm. Below 24 nm, organic monolayer-coated nanowires also exhibited size-dependent Ge 3d core ionization spectra that shifted to higher energy with reduced diameter that are independent of probe position relative to the surface. In contrast, the Ge 3d edge for surface-oxidized nanowires exhibited a chemically-induced shift when positioned near the surface.
Silicon nanowires, carbon nanotubes, and magnetic nanocrystals: synthesis, properties, and applications
(2007) Lee, Doh Chang, 1978-; Korgel, Brian A., 1969-
Central to the practical use of nanoscale materials is the controlled growth in technologically meaningful quantities. Many of the proposed applications of the nanomaterials potentially require inexpensive production of the building blocks. Solution-based synthetic approach offers controllability, high throughput, and scalability, which make the process attractive for the potential scale-up. Growth kinetics could be readily influenced by chemical interactions between the precursor and the solvent. In order to fully utilize its benefits, it is therefore pivotal to understand the decomposition chemistry of the precursors used in the reactions. Supercritical fluids were used as solvent in which high temperature reactions could take place. Silicon nanowires with diameters of 20~30 nm was synthesized in supercritical fluids with metal nanocrystals as seeds for the nanowire growth. To unravel the effect of silicon precursors, several silicon precursors were reacted and the resulting products were investigated. The scalability of the system is discussed based on the experimental data. The nanowires were characterized with various characterization tools, including high-resolution transmission electron microscopy and electron energy loss spectroscopy. The crystallographic signatures were analyzed through the transmission electron microscopic study, and fundamental electrical and optical properties were probed by electron energy loss spectroscopy. Carbon nanotubes were prepared by reacting carbon-containing chemicals in supercritical fluids with organometallic compounds that form metal seed particles in-situ. A batch reaction, in which the temperature control was relatively poor, yielded a mixture of multiwall nanotubes and amorphous carbon nanofilaments with a low selectivity of nanotubes in the product. When reaction parameters were translated into a continuous flow-through reaction, nanotube selectivity as well as the throughput of the total product significantly improved. Magnetic properties of various metal nanocrystals were also studied. Colloidal synthesis enables the growth of FePt and MnPt3 nanocrystals with size uniformity. The as-synthesized nanocrystals, however, had compositionally disordered soft-magnetic phases. To obtain hard magnetic layered phase, the nanocrystals must be annealed at high temperatures, which led to sintering of the inorganic cores. To prevent sintering, the nanocrystals were encapsulated with silica layer prior to annealing. Interparticle magnetic interactions were also explored using particles with varying silica thickness.