Browsing by Subject "Nanowires"
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Item CuInSe₂ nanowires and earth-abundant nanocrystals for low-cost photovoltaics(2013-05) Steinhagen, Chet Reuben; Korgel, Brian Allan, 1969-Widespread commercialization of photovoltaics (PVs) requires both higher power conversion efficiencies and low-cost, high throughput manufacturing. High efficiencies have been achieved in devices made from materials such as CuIn[subscript x]Ga₁₋[subscript x]Se₂ (CIGS). However, processing of these solar cells still requires high temperature and vacuum, driving up cost. A reduction in manufacturing costs can be achieved by utilizing colloidal nanocrystals. Semiconductor nanocrystals can be dispersed in solvents and deposited via simple and scalable methods under ambient conditions to form the absorber layer in low-cost solar cells. Efficiencies of ~3% have been achieved with CIGS nanocrystal PVs, but this must be improved substantially for commercialization. These devices suffer from poor charge transport in the nanocrystal layer. Here, the synthesis of nanowires and their utilization in solar cells was explored as a way to improve charge transport. CuInSe₂ (CIS) nanowires were synthesized via the solution-liquid-solid method. PV devices were fabricated using the nanowires as the light absorbing layer, and were found to exhibit a measureable power output. Earth-abundant materials were also explored, motivated by the material availability concerns associated with CIGS. Pyrite FeS₂ nanocrystals were synthesized via an arrested precipitation reaction to produce phase-pure particles 15 nm in size. These nanocrystals were spray coated to form the active layer in several different common device architectures. These devices failed to produce any power output. The material was determined to be slightly sulfur deficient, leading to a high carrier concentration and metallic behavior in the thin films, with conductivities measured to be ~5 S/nm. A nanocrystal synthesis of Cu₂ZnSnS₄ (CZTS) was also developed to produce highly dispersible crystalline particles ~11 nm in size. These nanocrystals were spray coated onto glass substrates to form the absorber layer in test PV devices, and an efficiency of 0.23% was achieved without high-temperature or chemical post-processing. Additional studies included the synthesis of CZTS nanorods and their incorporation into functioning solar cells. The selenization of CZTS nanocrystal films was also studied as a way to improve solar cell performance. High temperature annealing in a Se atmosphere was found to produce CZTS(Se) layers, which could be used in working PV devices.Item Electrical properties of single GaAs, Bi₂S₃ and Ge nanowires(2005) Schricker, April Dawn; Korgel, Brian A.Item Electrical transport measurements of individual bismuth nanowires and carbon nanotubes(2005) Jang, Wan Young; Yao, Zhen, Ph. D.Item Electronic and spintronic transport in germanium nanostructures(2014-05) Liu, En-Shao; Tutuc, Emanuel, 1974-The digital information processing system has benefited tremendously from the invention and development of complementary metal-oxide-semiconductor (CMOS) integrated circuits. The relentless scaling of the physical dimensions of transistors has been consistently delivering improved overall circuit density and performance every technology generation. However, the continuation of this trend is in question for silicon-based transistors when quantum mechanical tunneling becomes more relevant; further scaling in feature sizes can lead to increased leakage current and power dissipation. Numerous research efforts have been implemented to address these scaling challenges, either by aiming to increase the performance at the transistor level or to introduce new functionalities at the circuit level. In the first approach, novel materials and device structures are explored to improve the performance of CMOS transistors, including the use of high-mobility materials (e.g. III-V compounds and germanium) as the channel, and multi-gate structures. On the other hand, the overall circuit capability could be increased if other state variables are exploited in the electronic devices, such as the electron spin degree of freedom (e.g. spintronics). Here we explore the potential of germanium nanowires in both CMOS and beyond-CMOS applications, studying the electronic and spintronic transport in this material system. Germanium is an attractive replacement to silicon as the channel material in CMOS technology, thanks to its lighter effective electron and hole mass. The nanowire structures, directly synthesized using chemical vapor deposition, provide a natural platform for multi-gate structures in which the electrostatic control of the gate is enhanced. We present the realization and scaling properties of germanium-silicon-germanium core-shell nanowire n-type, [omega]-gate field-effect transistors (FETs). By studying the channel length dependence of NW FET characteristics, we conclude that the intrinsic channel resistance is the main limiting factor of the drive current of Ge NW n-FETs. Utilizing the electron spins in semiconductor devices can in principle enhance overall circuit performance and functionalities. Electrical injection of spin-polarized electrons into a semiconductor, large spin diffusion length, and an integration friendly platform are desirable ingredients for spin based-devices. Here we demonstrate lateral spin injection and detection in Ge NWs, by using ferromagnetic metal contacts and tunnel barriers for contact resistance engineering. We map out the contact resistance window for which spin transport is observed, manifestly showing the conductivity matching required for spin injection.Item Fabrication and Characterization of Nanowires(2011-10-21) Phillips, Francis RandallThe use of nanostructures has become very common throughout high-tech industries. In order to enhance the applicability of Shape Memory Alloys (SMAs) in systems such as Nano-Electromechanical Systems, the phase transformation behavior of SMA nanostructures should be explored. The primary focus of this work is on the fabrication of metallic nanowires and the characterization of the phase transformation of SMA nanowires. Various metallic nanowires are fabricated through the use of the mechanical pressure injection method. The mechanical pressure injection method is a template assisted nanowire fabrication method in which an anodized aluminum oxide (AAO) template is impregnated with liquid metal. The fabrication procedure of the AAO templates is analyzed in order to determine the effect of the various fabrication steps. Furthermore, metallic nanowires are embedded into polymeric nano bers as a means to incorporate nanowires within other nanostructures. The knowledge obtained through the analysis of the AAO template fabrication guides the fabrication of SMA nanowires of various diameters. The fabrication of SMA nanowires with di fferent diameters is accomplished through the fabrication of AAO templates of varying diameters. The phase transformation behavior of the fabricated SMA nanowires is characterized through transmission electron microscopy. By analyzing the fabricated SMA nanowires, it is found that none of the fabricated SMA nanowires exhibit a size eff ect on the phase transformation. The lack of a size e ffect on the phase transition of SMA nanowires is contrary to the results for SMA nanograins, nanocrystals, and thin films, which all exhibit a size eff ect on the phase transformation. The lack of a size eff ect is further studied through molecular dynamic simulations. These simulations show that free-standing metallic nanowires will exhibit a phase transformation when their diameters are sufficiently small. Furthermore, the application of a constraint on metallic nanowires will inhibit the phase transformation shown for unconstrained metallic nanowires. Therefore, it is concluded that free-standing SMA nanowires will exhibit a phase transformation throughout the nanoscale, but constrained SMA nanowires will reach a critical size below which the phase transformation is inhibited.Item First principles-based atomistic modeling of the structure and nature of amorphous Au-Si alloys and their application to Si nanowire synthesis(2008-12) Lee, Soohwan; Hwang, Gyeong S.A great deal of attention has been paid to semiconductor nanowires due to their compatibility of conventional silicon-based technology. Metal-catalytic vapor-liquidsolid (VLS) and various solution-based techniques have widely been used to synthesize silicon/germanium (Si/Ge) nanowires. It is well characterized that the crystallographic orientations, diameter sizes, and surface morphologies of semiconductor nanowires can be controlled by varying process conditions and metal catalysts. Earlier experimental and theoretical studies have identified mechanism underlying metal catalyzed Si/Ge nanowire growth, involving Si/Ge diffusion into a metal catalyst, eutectic Si/Ge-catalyst alloy formation, and Si/Ge precipitation at the catalyst-nanowire interface. However, little is known about the atomic-level details of the structure, energetics and dynamics of amorphous metal alloys such as gold-silicon (Au-Si) and gold-germanium (Au-Ge) despite their importance for well controlled synthesis of Si/Ge nanowires, which is essential for the success of Si/Ge nanowires-based applications. Experiments provide many clues to the fundamental aspects of the behavior and properties of metal alloys, but their interpretations often remain controversial due largely to difficulties in direct characterization. While current experimental techniques are still limited to providing complementary atomic-level, real space information, first principles based atomistic modeling has emerged as a powerful means to address the structure, function and properties of amorphous metallic alloys. This thesis work has focused on developing a detailed understanding of the atomic structure, energetics, and oxidation of Au-Si alloys, as well as molecular mechanisms underlying Au-catalyzed Si nanowire growth. In addition, the surface reconstruction and chemistry of Si nanowires has been examined, with comparisons to planar Si surfaces. In this dissertation, based on first principles atomistic simulations, we present: 1) the atomic structure, energetics, and chemical ordering of amorphous Au-Si alloys with varying Au:Si composition ratios; 2) the behavior of boron (B) in the Au-Si alloy, such as diffusion and agglomeration, and the effect of B addition on the atomic distribution of Si and Au, with implications for in-situ doping of Si nanowires; 3) the origin and structural ordering of Si surface segregation in the Au-Si alloy, providing important insights into the nucleation and early-stage growth of Si nanowires; 4) the interfacial interaction between the Au-Si alloy and various facets of crystalline Si, such as (111), (211), (110), (110), which explains well the underlying reasons for the growth direction of Si nanowires; 5) the oxidation of the Au-Si alloy; and 6) the surface reconstruction and chemistry of Si nanowires with comparisons to planar Si surfaces. Outcomes from the thesis work contribute to: clarifying the atomic structure, energetics and chemical ordering of amorphous bulk Au-Si alloys, as well as their surfaces and interfaces; better understanding molecular mechanisms underlying the Aucatalyzed synthesis of Si nanowires; and identifying the surface reconstruction and chemistry of Si nanowires. The improved understanding can provide invaluable guidance on the rational design and fabrication of Si nanowire-based future electronic, chemical, and biological devices. This thesis work also offers a theoretical platform for studying metal alloy systems with various applications.Item 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.Item High performance germanium nanowire field-effect transistors and tunneling field-effect transistors(2010-12) Nah, Junghyo, 1978-; Tutuc, Emanuel, 1974-; Banerjee, Sanjay K.; Lee, Jack C.; Dodabalapur, Ananth; Register, Leonard F.; Shi, LiThe scaling of metal-oxide-semiconductor (MOS) field-effect transistors (FETs) has continued for over four decades, providing device performance gains and considerable economic benefits. However, continuing this scaling trend is being impeded by the increase in dissipated power. Considering the exponential increase of the number of transistors per unit area in high speed processors, the power dissipation has now become the major challenge for device scaling, and has led to tremendous research activity to mitigate this issue, and thereby extend device scaling limits. In such efforts, non-planar device structures, high mobility channel materials, and devices operating under different physics have been extensively investigated. Non-planar device geometries reduce short-channel effects by enhancing the electrostatic control over the channel. The devices using high mobility channel materials such as germanium (Ge), SiGe, and III-V can outperform Si MOSFETs in terms of switching speed. Tunneling field-effect transistors use interband tunneling of carriers rather than thermal emission, and can potentially realize low power devices by achieving subthreshold swings below the thermal limit of 60 mV/dec at room temperature. In this work, we examine two device options which can potentially provide high switching speed combined with reduced power, namely germanium nanowire (NW) field-effect transistors (FETs) and tunneling field-effect transistors (TFETs). The devices use germanium (Ge) – silicon-germanium (Si[subscript x]Ge[subscript 1-x]) core-shell nanowires (NWs) as channel material for the realization of the devices, synthesized using a 'bottom-up' growth process. The device design and material choice are motivated by enhanced electrostatic control in the cylindrical geometry, high hole mobility, and lower bandgap by comparison to Si. We employ low energy ion implantation of boron and phosphorous to realize highly doped contact regions, which in turn provide efficient carrier injection. Our Ge-Si[subscript x]Ge[subscript 1-x] core-shell NW FETs and NW TFETs were fabricated using a conventional CMOS process and their electrical properties were systematically characterized. In addition, TCAD (Technology computer-aided design) simulation is also employed for the analysis of the devices.Item Ignition studies of Al-Fe203 energetic nanocomposites(Texas Tech University, 2004-12) Patibandla, SridharEnergetic nanocomposites are materials consisting of both fuel and oxidizer elements with at least one constituent in the nanometer size range. When ignited they react exothermically and release large amounts of energy, much larger than that compared with bulk materials. In this work, a new kind of energetic nanocomposite, a nanowire array-based thin film is reported. The nanocomposite consists of oxidizer (or fuel) nanowires embedded in a thin fuel (or oxidizer) film. Such nanocomposites are reported for the first time. In this thesis, energetic nanocomposites of the type Al-Fe2O3 are prepared. Fe203 is in the form of nanowire arrays prepared using nanoporous alumina membranes. AI film is in the form of a thin film prepared by vacuum deposition. Nanowires with two different dimensions are studied, namely l0nm and 50nm. Thickness of the Al film is 50nm. The wires are either embedded perpendicularly (stmctured nanocomposite) in the Al film or randomly distributed (random nanocomposite) on the Al film. All of these samples are found to demonstrate ignition. This thesis describes the complete fabrication process for the nanocomposites. Demonstration of ignition in these nanocomposites is described. Experimental procedure to measure the ignition properties is discussed and an estimate of the energy released is presented.Item Magnetic domain wall dynamics in nanoscale thin film structures(2008-05) Knutson, Carl Oliver, 1980-; Tsoi, MaximThe dynamics of individual magnetic domain walls in permalloy nanowires fabricated through focused ion beam patterning is presented in this dissertation. The motion of an individual domain wall is detected directly via magneto-optical Kerr polarimetry. In the basic measurement, the velocity of the domain wall is measured over a range of external driving magnetic fields. The velocity measurement is found to have two distinct linear regions separated by a region with a negative differential. This measurement is then expanded upon by varying the width of the nanowire and applying a DC current though the nanowire. The means by which the domain wall enters the nanowire from a continuous film is investigated. A pinning potential traps the domain wall at the interface between the nanowire and continuous film at low magnetic fields. Fields below the critical “injection” field allow the domain wall to overcome the pinning potential through a thermal activation mechanism. The critical injection field is found to depend on the width of the nanowire. A moving domain wall has been found to create a voltage across the nanowire through which it traverses. The voltage produced is proportional to the velocity of the domain wall and is on the order of hundreds of nV.Item Nanofabrication via directed assembly: a computational study of dynamics, design & limits(2016-08) Arshad, Talha Ali; Bonnecaze, R. T. (Roger T.); Ellison, Christopher J.; Ganesan, Venkat; Sreenivasan, S. V.; Willson, Carlton G.Three early-stage techniques, for the fabrication of metallic nanostructures, creation of controlled topography in polymer films and precise deposition of nanowires are studied. Mathematical models and computational simulations clarify how interplay of multiple physical processes drives dynamics, provide a rational approach to selecting process parameters targeting specific structures efficiently and identify limits of throughput and resolution for each technique. A topographically patterned membrane resting on a film of nanoparticles suspended in a solvent promotes non-uniform evaporation, driving convection which accumulates particles in regions where the template is thin. Left behind is a deposit of particles the dimensions of which can be controlled through template thickness and topography as well as film thickness and concentration. Particle distribution is shown to be a competition between convection and diffusion represented by the Peclet number. Analytical models yield predictive expressions for bounds within which deposit dimensions and drying time lie. Ambient evaporation is shown to drive convection strong enough to accumulate particles 10 nm in diameter. Features up to 1 µm high with 10 nm residual layers can be deposited in < 3 minutes, making this a promising approach for continuous, single-step deposition of metallic nanostructures on flexible substrates. Selective exposure of a polystyrene film to UV radiation has been shown to result in non-uniform surface energy which drives convection on thermal annealing, forming topography. Film dynamics are shown to be a product of interplay between Marangoni convection, capillary dissipation and diffusion. At short times, secondary peaks form at double the pattern density of the mask, while at long times pattern periodicity follows the mask. Increased temperature, larger surface tension differentials and thick films result in faster dynamics and larger features. Electric fields in conjunction with fluid flow can be used to position semi-conducting nanowires or nanotubes at precise locations on a substrate. Nanowires are captured successfully if they arrive within a region next to the substrate where dielectrophoresis dominates hydrodynamics. Successful assembly is predicated upon a favorable balance of hydrodynamics, dielectrophoresis and diffusion, represented by two dimensionless groups. Nanowires down to 20 nm in length can be assembled successfully.Item Nanoscale Engineering for the Design of Efficient Inorganic-Organic Hybrid Thermoelectrics(2014-04-14) Brockway, Lance RobertResearch aimed at enhancing the thermoelectric performance of semiconductors comprised of only earth-abundant elements has recently come under renewed focus as these materials systems offer a cost-effective path for scavenging waste heat. In light of the prediction that nanostructuring could increase the thermoelectric performance of materials; semiconductor nanowires comprised of non-toxic, low cost, and earthabundant elements were synthesized and studied for their thermoelectric performance. For accomplishing this task, zinc phosphide (Zn_(3)P_(2)), zinc antimonide (Zn_(4)Sb_(3)), and zinc oxide (ZnO) nanowires were synthesized on the gram-quantity scale. Post-synthesis decomposition techniques were developed to controllably reduce the nanowire diameter and to create nanotubes. This decrease in nanowire diameter comes at an additional cost of an exponential decrease in surface stability. To combat this, a vapor phase surface passivation technique was developed to protect the nanowire surfaces from degradation and agglomeration. Finally, gram quantities of both functionalized and unfunctionalized nanowires were compressed into dense nanobulk pellets and characterized for their thermoelectric performance. The reactive vapor transport synthesis technique resulted in gram-quantities of single-crystalline nanowires with consistent 20 nm diameters. The nanowire diameters were further reduced to create sub-5 nm quantum wires and nanotubes using controlled decomposition. A self-consistent mechanism to describe this phenomenon was proposed. The nanowires were further surface-functionalized with various organic molecules to prevent surface degradation and to control the interfacial transport properties within the consolidated nanowire-pellets. The stability enhancement of the nanowires using this vapor-phase self-assembled monolayer technique was shown using traditional organic characterization techniques and suspension stability. Zn_(3)P_(2) and ZnO nanowires were then hot-pressed and spark plasma sintered, respectively, into nanobulk pellets. It was observed that the nanowires in the Zn_(3)P_(2) pellet did not break upon compaction, but bent elastically to achieve their sintered density; this was confirmed using a single nanowire inside a TEM. The thermoelectric performance of the functionalized nanowires was shown to be 3-fold higher than that of unfunctionalized nanowires due to less nanowire surface oxidation. Finally, the ZnO nanowire-bulk pellets were optimally-doped resulting in a 30% decrease in thermal conductivity compared to the bulk and the highest reported n-type oxide zT to date of 0.60.Item Nanostructured materials for solar energy conversion(2013-05) Hoang, Son Thanh; Mullins, C. B.The energy requirements of our planet will continue to grow with increasing world population and the modernization of currently underdeveloped countries. This will force us to search for environmental friendly alternative energy resources. Solar energy by far provides the largest of all renewable energy resources with an average power of 120 000 TW irradiated from the sun which can be exploited through solar electricity, solar fuel, and biomass. Nanostructured materials have been the subject of extensive research as the building block for construction of solar energy conversion devices for the past decades. The nanostructured materials sometimes have peculiar electrical and optical properties that are often shape and size dependent and are not expected in the bulk phase. Recent research has focused on new strategies to control nanostructured morphologies and compositions of semiconductor materials to optimize their solar conversion efficiency. In this dissertation, we discuss the synthesis and characterizations of one dimensional nanostructured TiO₂ based materials and their solar energy conversion applications. We have developed a solvothermal synthesis method for growing densely packed, vertical, single crystalline TiO₂ rutile nanowire arrays with unprecedented small feature sizes of 5 nm and lengths up to 4.4 [mu]m. Because of TiO₂'s large band gap, the working spectrum of TiO₂ is limited to the ultra violet region with photons shorter than 420 nm. We demonstrate that the active spectrum of TiO₂ can be shifted to ~ 520 nm with incorporation of N via nitridation of TiO₂ nanowires in NH₃ flow. In addition, we demonstrate a synergistic effect involving hydrogenation and nitridation cotreatment of TiO₂ nanowires that further redshift the active spectrum of TiO₂ to 570 nm. The Ta and N co-incorporated TiO₂ nanowires were also prepared and showed significant enhancement in photoelectrochemical performance compared to mono-incorporation of Ta or N. This enhancement is due to fewer recombination centers from charge compensation effects and suppression of the formation of an amorphous layer on the nanowires during the nitridation process. Finally, we have developed hydrothermal synthesis of single crystalline TiO₂ nanoplatelet arrays on virtually all substrates and demonstrated their applications in water photo-oxidation and dye sensitized solar cells.Item New platforms for electronic devices: n-channel organic field-effect transistors, complementary circuits, and nanowire transistors(2007-05) Yoo, Byungwook, 1975-; Dodabalapur, Ananth, 1963-This work focused on the fabrication and electrical characterization of electronic devices and the applications include the n-channel organic field-effect transistors (OFETs), organic complementary circuits, and the germanium nanowire transistors. In organic devices, carbonyl-functionalized [alpha],[omega]-diperfluorohexyl quaterthiophenes (DFHCO-4T) and N,N' --bis(n-octyl)-dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDI-8CN2) are used as n-type semiconductors. The effect of dielectric/electrode surface treatment on the response of bottom-contact devices was also examined to maximize the device performance. Some of innovative techniques that employ the conducting polymer, poly(3,4-ethylenedioxythiophene) / poly(styrene sulfonate) (PEDOT/PSS) for the fabrication of OFETs, were compared and investigated. The device performance and the fabrication yield were also considered. Organic complementary ring oscillators and D flip-flops were demonstrated with PDI-8CN2 and pentacene as the n-type and ptype material, respectively. Both circuits recorded the highest speed that any organic transistor-based complementary circuit has achieved to date. The speed of these complementary circuits will be enhanced by increasing the mobility of n-channel further as well as reducing channel lengths and overlap capacitances between the source/drain electrodes and the gate. The semiconductors should be solution processible to be compatible with the inexpensive fabrication techniques envisioned for printed electronic circuits. PDI-8CN2 was used for solution-processed n-channel OFETs and the various parameters are compared for the optimization of devices. Utilizing optimized process parameters and surface treatments for solution-deposited PDI-8CN2 OFETs, we have successfully shown the first fabrication of complementary organic ring oscillators and Dflip flops by the micro-injection of the solution of both p-type and n-type materials in air. One of the potential platforms for low cost fabrication on flexible substrates is the use of inorganic semiconductor nanowires. Accordingly, the germanium nanowire FETs were fabricated and characterized. Conductivity enhanced PEDOT/PSS was employed as the electrode material for nanowire transistors to improve the electrical contacts to the source and drain.Item Photoemission study of stepped surface, thin film and nanowire growth(2008-12) Zhou, Xubing; Erskine, James L.Steps on a high index metal or semiconductor surface may play a fundamental role for electronic structure, adsorption, film growth, chemical reaction and catalysis. The surface atomic and electronic structures of stepped W(110) surfaces have been investigated by a few research groups during the past 20 years. But there is still a lot of controversy. We use high resolution core level photoemission to study several different stepped tungsten surfaces. Curve fittings of the spectra permit tests of core-level binding- energy shift models that relate local atomic coordination to binding -energy differences associated with terrace and step-edge atoms. For the first time we find a well resolved W4f₂/₇ peak associated with step edge atoms. We attribute previous failure to directly detect the step-edge effects in core level photoemission to contamination by hydrogen. The well resolved peaks for surface atoms with different coordinations can serve as a “finger print” for specific atoms. Experiments in which stepped surfaces are systematically dosed by H₂ clarify the role played by H contamination. We also grow Ag nanowires on the stepped W(110) surface and use angle resolved photoemission to study the band structure. We find distinct dispersion for the nanowires along the step edge direction while there is only little dispersion perpendicular to the wires. The second part of the research is core level photoemission study on Cesium film growth on Cu(100) surface. We study the phonon broadening effect for Cs at different temperatures. We compare our data with previous theoretical models and get good results on surface and bulk Debye temperatures and zero temperature phonon broadening. The binding energy shifts for the Cs 5p₂/₇ at different temperatures have also been investigated. The results fit the lattice expansion model very well except at temperature higher than 200 K. The higher temperature deviation is caused by thermal evaporation of Cs films. This conclusion is checked by the following coverage dependent core level peaks study on the Cs/Cu(100) system.Item Porous silicon nanoneedles for intracellular delivery of small interfering RNA(2011-05) Chiappini Dottore, Ciro; Liu, Xuewu; Ferrari, Mauro, 1959-; Tasciotti, Ennio; Markey, Mia; Ruoff, Rodney; Zhang, XiaojingThe rational and directed delivery of genetic material to the cell is a formidable tool to investigate the phenotypic effects of gene expression regulation and a promising therapeutic strategy for genetic defects. RNA interference constitutes a versatile approach to gene silencing. Despite the development of numerous strategies the transfection of small interfering RNA (siRNA) is highly dependent on cell type and conditions. Direct physical access to the intracellular compartment is a promising path for high efficiency delivery independently of cell type and conditions. Silicon nanowires grant such access with minimal toxic effects, and allow intracellular delivery of DNA when actuated by atomic force microscope. These findings reveal the potential for porous silicon nanostructures to serve as delivery vectors for nucleic acids due to their porous nature, elevated biocompatibility, and biodegradability. This dissertation illustrates the development a novel platform for efficient siRNA transfection based on an array of porous silicon nanoneedles. The synthesis of biodegradable and biocompatible porous nanowires was accomplished by a novel strategy for electroless etch of silicon that allows anisotropic etch simultaneously with porosification. An ordered array of cone shaped porous silicon nanoneedles with tunable tip size, array density and aspect ratio was obtained coupling this strategy with patterned metal deposition and selective reactive ion etch. This process also granted control over porosity, nanopore size and flexural modulus. The combination of these parameters was appropriately optimized to ensure cell penetration, maximize siRNA loading and minimize cytotoxic effects. Loading of the negatively charged siRNA molecules was optimized by applying an external electric field to the nanoneedles under appropriate voltage conditions to obtain a tenfold increase over open circuit loading, and efficient penetration of the siRNA within the porous volume of the needles. Alternative surface chemistry modification provided a means for effective siRNA loading and sustained release. siRNA transfection was achieved by either imprinting the nanoneedles array chip over a culture of MDA-MB-231 cells or allowing the cells to self-impale over the needles. The procedures allowed the needles to penetrate across the cell membrane without influencing cell proliferation. siRNA was successfully transfected and was effective at suppressing gene expression.Item Semiconductor nanowires : from a nanoscale system to a macroscopic material(2011-12) Holmberg, Vincent Carl; Korgel, Brian Allan, 1969-Semiconductor nanowires are one-dimensional nanoscale systems that exhibit many unique properties. Their nanoscale size can lead to defect densities and impurity populations different than bulk materials, resulting in altered diffusion behavior and mechanical properties. Synthetic methods now support the large-scale production of semiconductor nanowires, enabling a new class of materials and devices that use macroscopic quantities of nanowires. These advances have created an opportunity to fabricate bulk structures which exhibit the unique physical properties of semiconductor nanowires, bridging the properties of a nanoscale system with macroscopic materials. High aspect ratio germanium nanowires were synthesized in supercritical organic solvents using colloidal gold nanocrystal seeds. The nanowires were chemically passivated inside the reactor system using in situ thermal hydrogermylation and thiolation. The chemical stability of the passivated nanowires was studied by exposure to highly corrosive and oxidative environments. Chemical surface functionalization of germanium nanowires was investigated by covalently tethering carboxylic acid groups to the surface, as a general platform for the further functionalization of nanowire surfaces with molecules such as polyethylene glycol. Surface functionalization with dopant-containing molecules was also explored as a potential route for doping nanowires. In addition, static charging was exploited in the development of an electrostatic deposition method for semiconductor nanowires. In situ transmission electron microscopy experiments were conducted on gold-seeded germanium nanowires encapsulated within a volume-restricting carbon shell. A depressed eutectic melting temperature was observed, along with strong capillary effects, and the solid-state diffusion of gold into the crystalline stem of the germanium nanowire, occurring at rates orders of magnitude slower than in the bulk. Copper, nickel, and gold diffusion in silicon nanowires were also investigated. The rate of gold diffusion was found to be a strong function of the amount of gold available to the system. Finally, germanium nanowires were found to exhibit exceptional mechanical properties, with bending strengths approaching that of an ideal, defect-free, perfect crystal, and strength-to-weight ratios greater than either Kevlar or carbon fiber. Macroscopic quantities of nanowires were used to fabricate large sheets of free-standing semiconductor nanowire fabric, and the physical, morphological, and optical properties of the material were investigated.Item Silicon and germanium nanostructures : synthesis and in situ TEM study(2015-08) Lu, Xiaotang; Korgel, Brian Allan, 1969-; Ekerdt, John G; Chelikowsky, James R; MacDonald, Allan H; Yu, GuihuaA variety of chemical routes exist for a wide range of nanomaterials with tunable size, shape, composition and surface chemistry. Of these materials, silicon (Si) and germanium (Ge) nanomaterials have been some of the most challenging to synthesize. Solution-liquid-solid (SLS) growth of Si was studied using tin (Sn) as the seeding metal. Si nanorods with narrow diameters can be grown by the decomposition of trisilane in hot squalane in the presence of Sn nanocrystals. Photoluminescence could be obtained from the Si nanorods by thermal hydrosilylation passivation. This colloidal synthesis could be further simplified to a single-step reaction procedure by the in situ formation of Sn seed particles. In addition to trisilane as a Si source, isotetrasilane, neopentasilane and cyclohexasilane were studied for Si nanorod growth: all three reactants enabled nanorod formation at lower growth temperatures. A monophenylsilane (MPS) enhanced growth was discovered for supercritical fluid-liquid-solid (SFLS) growth of Ge nanowires that enables the Ge conversion of ~100%. A variety of metalorganic compounds were studied for replacing pre-synthesized metal nanoparticles to induce Ge nanowire growth. Si and Ge nanowires are some of the most promising anode materials in lithium ion batteries (LIBs) because of their high lithium storage capacity. However, the significant chemical and physical changes that occur during cycling hamper their practical uses. In situ transmission electron microscopy (TEM) techniques were conducted to observe and understand structural and interfacial changes of the Si and Ge nanowires during electrochemical cycling; and, therefore, resolving the problems with current anodes by materials modification. The in situ TEM experiments showed that the incorporation of Sn into Si nanowires can enhance their rate capability. But the enhanced Li diffusion leads to the premature pore formation in Si nanowires. Ge nanowires has been discovered the potential as sodium ion battery anodes after an initial activation with a lithiation step to amorphize the nanowires.Item Solution phase synthesis and characterization of III-V, II-VI and CdSe.₀₈Te.₉₂ semiconductor nanowires(2008-05) Fanfair, Dayne Dustan, 1978-; Korgel, Brian Allan, 1969-There are many advantages to the solution phase synthesis of semiconductor nanowires, the most notable of which are the ease of scalability and the production of nanowires in higher yields than those typically obtained in chemical vapor deposition (CVD) based processes. The solution phase synthesis of high quality, high aspect ratio (>100) narrow diameter semiconductor nanowires depends sensitively on three parameters: the diameter of the nanocrystals utilized to promote (seed) nanowire growth, molecular precursor decomposition kinetics and the choice of solvent in which the nanowires are grown. Bismuth is a low melting point (270 °C) semimetal and thus an ideal candidate for the solution-liquid-solid (SLS) growth of nanowires. A bismuth nanocrystal synthesis was developed that affords nanocrystals with average diameters from 4 - 20 nm. The nanocrystal diameter is controlled by varying the capping ligand (TOPO) to bismuth molar ratio. The synthesis of Au2Bi nanocrystals was also studied as it also affords small diameter (~ 2 nm) nanocrystals that are suitable for SLS nanowire growth. Molecular precursor decomposition kinetics can have a significant impact on nanowire yield and quality. Precursors that decompose too quickly can produce relatively large diameter nanowires, while precursors that decompose too slowly can produce nanowires with a highly tortuous morphology as a result of a high density of crystallographic defects. The choice of molecular precursor for the synthesis of III-V and II-VI nanowires was investigated. The solvent in which nanowires are grown can also have a significant effect on nanowire yield, quality and morphology. Coordinating solvents such as alkylphosphine oxides and alkylamines can interact with the atoms, or atomic complexes, that constitute nanowires and thus mediate the nanowire growth rate. In some instances, for example InAs nanowires grown in TOPO, this interaction can completely quench nanowire growth. This solvent effect has been investigated for the growth of III-V and II-VI nanowires. Solvents can also affect nanowire morphology. Branched ZnSe nanowires, i.e. hybrid nanostructures in which ZnSe nanorods grow epitaxially from the surface of ZnSe nanowires, are synthesized in trioctylamine whereas TOPO suppresses this branched growth. Finally, a mechanism which allows for the synthesis of narrow diameter nanowires seeded by much larger diameter nanocrystals is investigated. Bismuth nanocrystals with an average diameter of ~ 20 nm are utilized to promote the growth of narrow diameter (~ 6 nm) CdSe.₀₈Te.₉₂ nanowires.Item Synthesis and characterization of III-V semiconductor nanowires and fabrication of colloidal nanorod solar cells(2006) Davidson, Forrest Murray; Korgel, Brian A.Nanowires have attracted intensive research efforts due to their one-dimensional quantum confinement and their ability to serve as the building blocks for and functional components of future semiconductor devices. Widespread use of nanowires in bottom-up device fabrication will require a general method for the controllable synthesis of nanowires with regards to shape, size, composition, and interfacial properties. Gallium arsenide and gallium phosphide nanowires as small as 8 nm in diameter were synthesized in supercritical hexane and seeded by alkanethiol-stabilized 7 nm gold nanocrystals. The wires are single crystal with a zinc-blende structure and grow exclusively in the <111> direction. The importance of precursor degradation kinetics was explored. Multiple lamellar {111} twins are observed in GaAs, GaP and InAs nanowires synthesized by supercritical fluid-liquid-solid (SFLS) and solution-liquid-solid (SLS) approaches. All of these nanowires have zinc blende (cubic) crystal structure and were grown in the <111> direction. The twins cross-section the nanowires to give them a “bamboo”-like appearance in TEM images. In contrast, Si and Ge nanowires with <111> growth direction do not exhibit {111} twins, even though this is a common twin plane with relatively low twin energy in diamond cubic Ge and Si. However, Si and Ge nanowires with <112> growth directions typically have several {111} twins extending down the length of the nanowires. A semi-quantitative model that explains the observed twinning in III-V and IV nanowires is presented. Heterojunction solar cells were fabricated from colloidal solutions of CdTe and CdSe nanorods by sequential spin casting onto ITO coated glass substrates. A broad range of factors impacting the success of the solar cell fabrication were explored; including the method of nanorod synthesis, choice of capping ligand, method of active layer application, and use of hydrazine treatment. It is necessary to maximize the stability of the nanords in solution to ensure even application into thin films. However, electrical properties of the nanorod films are improved by using weaker stabilizing agents. The devices exhibit good diode behavior.