High performance thermoelectric materials in a wide range of temperatures are essential to broaden the application
spectrum of thermoelectric devices. This paper presents experiments on the power and efficiency characteristics of lowand
mid-temperature thermoelectric materials. We show that as long as an appreciable temperature difference can be
created over a short thermoelectric leg, good power output can be achieved. For a mid-temperature n-type doped
skutterudite material an efficiency of over 11% at a temperature difference of 600 °C could be achieved. Besides the
improvement of thermoelectric materials, device optimization is a crucial factor for efficient heat-to-electric power
conversion and one of the key challenges is how to create a large temperature across a thermoelectric generator
especially in the case of a dilute incident heat flux. For the solar application of thermoelectrics we investigated the
concept of large thermal heat flux concentration to optimize the operating temperature for highest solar thermoelectric
generator efficiency. A solar-to-electric power conversion efficiency of ~5% could be demonstrated. Solar
thermoelectric generators with a large thermal concentration which minimizes the amount of thermoelectric
nanostrucutured bulk material shows great potential to enable cost-effective electrical power generation from the sun.
In terms of operational bandwidth and speed, photonic components are superior to electronic ones. However, it is
difficult to control photons on nanoscale structures for data processing and interconnection. Nanophotonic device using
surface plasmon (SP) offers an ideal solution to combine the superior technical advantages of both photonics and
electronics on the same chip. The SP wavelength is much shorter than that of the exciting light, allowing the use of SP in
various techniques that overcome diffraction limits. In this paper, we report an interesting plasmonic effect, enhanced
backward scattering, by using a periodically-aligned carbon nanotube (CNT) array. The CNTs are grown on a
transparent glass substrate with an average diameter of 50 nm and a length of about 1 μm. To enhance the conductivity,
the CNTs are also coated with 10-nm Au layer by using E-beam CVD technique. By shining a laser beam to the CNT
array, we found that the scattering intensity is maximally enhanced at the backward incident direction. The enhanced
backward incident scattering is observed by using both periodic and nonperiodic CNT samples. The experimental results
suggest that the backward scattering effect is due to the SP excitation and coupling. The proposed technique exploiting
aligned carbon-nanotube arrays to manipulate surface plasmon will lead to useful optical features such as optical
antennae effects, retro-reflection, switching, wavelength add/drop multiplexing, and may be particularly useful for
optical sensing, smart target identification and optical wireless secure communication applications.
Nanowires are interesting candidates for thermoelectric applications because of their potentially low thermal
conductivity and high power factor. However, measurements at the single-wire level are challenging and tend to lack
detailed information about the atomic-level structure of the sample and contacts. We are modifying a high-resolution
transmission electron microscope (HRTEM) with integrated scanning tunneling microscope (STM) for in-situ
measurements of the thermoelectric properties of individual nanowires and nanotubes. A slender hot-wire probe is used
to make electrical and thermal contact to the free end of a nanowire or nanotube. The electrical conductance of the
nanowire/nanotube can be measured with the usual STM mode of operation. The Seebeck coefficient can be extracted
from the transient response to a step change in the joule heating of the hot-wire probe. The thermal conductance can be
calculated from the temperature and heat leakage of the hot-wire probe. These measurements are combined with
detailed HRTEM observations.
The optical properties of periodic and nonperiodic arrays of aligned multiwalled carbon nanotubes are presented.
Experimental analysis indicates a complex optical response that is attributed to both the individual carbon nanotube
scatterers and also to the array ensembles. These studies indicate that by controlling the geometry and spacing of the
arrays, it is possible to create structures that respond very strongly to specific wavelengths or bands of wavelengths.
Structures such as these may form the basis for numerous applications in energy conversion. This would include highly
efficient solar energy conversion as well as sensitive, finely tuned detectors that can respond to predetermined
wavelength bands ranging from the ultraviolet to the infrared region. Experimental, theoretical and modeled results are
discussed as they apply to these applications. Challenges and issues are discussed.
We have studied growth of carbon nanotubes by chemical vapor deposition and zinc oxide nanowires by physical vapor deposition on carbon cloth with iron sulfate or stainless steel as a catalyst, and various combinations of carbon source and carrier gases. The field emission of these nanostructures shows a combined result of field enhancement from carbon nanotubes or Zinc oxide nanowires and carbon cloth. An emission current density of 1 mA/cm2 was obtained at 0.4 V/μm and 0.7 V/μm for nanotubes and nanowires, respectively. Field enhancement factor of ~4x104 has been observed. Moreover, electron transport characteristics and structural studies of carbon nanotubes have been investigated. Microscopic observations of electric wall-by-wall breakdown imply that transport in the nanotubes is not ballistic and that a significant scattering occurs as carriers traverse nanotubes length.
Vertically-aligned carbon nanotubes/nanofibers grown on various substrates by a direct-current plasma-enhanced chemical vapor deposition method have been shown experimentally to function as classical low-loss dipole antenna arrays at optical frequencies. Two fundamental antenna effects, e.g., the polarization effect and length matching effect, directly observed on large-scale CNT arrays in visible frequency range, hold them promising for industry-level fabrication of devices including linear/beam-splitting polarizers, solar energy converters, THz demodulators, etc., some of which will, however, require or prefer a flexible and/or transparent conducting substrate to be compatible for multi-level integration and low-cost manufacturing process. A low-energy dark discharge fabrication technique is therefore devised which successfully yields CNT antennas directly on polyimide films and transparent conducting oxides (ITO, ZnO) with the absence of a buffer layer.
Large-scale, two-dimensional arrays of periodic particles were prepared by nanosphere lithography. We modified the fabrication technique based on a self-assembly of latex particles on water surface in order to improve mask quality and size. Modifications of particles arrangement in an array were also practicable by using double-layered masks and mask transfer method. Such particle arrays were used for catalytic growth of aligned carbon nanotubes and ZnO nanorods with various configurations, length, and diameter. These exhibit interesting phenomena - antenna effects, photonic bandgap behavior, subwavelength lensing, and enhanced field emission. Therefore, they can be used in variety of future optoelectronic devices, such as THz and IR detectors.
Spectroscopic observations are presented for carbon nanotubes grown on silicon and quartz substrates in a hexagonal honeycomb configuration using self-assembly nanosphere lithography and plasma enhanced chemical vapor deposition method. A white light source is used as an incident beam and light reflected from the surface of the carbon nanotubes results in a distinctive signature in the reflected spectrum. A comparison of non-periodic arrays and periodic arrays of
carbon nanotubes show that the reflectance signature is only observed when the carbon nanotubes are oriented in a periodic array. Further observations regarding the light antenna effect observed in nonperiodic arrays are also reported. Theoretical curves show good agreement to experimentally observed phenomena. The unique optical properties of the arrays combined with the excellent mechanical and electrical properties of carbon nanotubes indicate that these materials may find many uses in the field of optoelectronics.
he fabrication, electrochemical characterization, and sensing applications of low-site density carbon nanotubes based nanoelectrode arrays (CNT-NEAs) are reported in this work. Spin-coating of an epoxy resin provides a new way to create the electrode passivation layer that effectively reduces the current leakage and eliminates the electrode capacitance by sealing the side-wall of CNTs. The CNT-NEAs fabricated in our work effectively use the open ends of CNTs for electrochemical sensing. The open ends of the CNTs have fast electron transfer rates similar to a graphite edge-plane electrode, while the side-walls present very slow electron transfer rates similar to the graphitic basal plane. Cyclic voltammetry showed the sigmoidal shape curves with low capacitive current and scan-rate-independent limiting current. The successful development of a glucose biosensor based on CNT-NEAs for the selective detection of glucose is also described. Glucose oxidase was covalently immobilized on the CNTs tips via carbodiimide chemistry by forming amide linkages between the amine residues and carboxylic acid groups on the open ends of CNTs. The biosensor effectively performs selective electrochemical detections of glucose in the presence of common interferences.
Carbon nanotubes were grown on silicon and quartz substrates in a honeycomb configuration using self-assembly nanosphere lithography and plasma enhanced chemical vapor deposition methods. Photonic nanoarrays were fabricated with varying spacing and carbon nanotube height. Both periodic and nonperiodic arrays were produced and evaluated. Optical properties of the arrays were studied and related to array geometry. Three dimensional diffraction maps were created that reveal the manner in which the nanoarrays interact with visible light. The unique optical properties of the arrays combined with the excellent mechanical and electrical properties of carbon nanotubes indicates that these materials may find many uses in the field of optoelectronics.
We report about our progress in developing a process for the manufacture of carbon nanotube (CNT) atomic force microscopy (AFM) cantilevers. Due to their exceptional mechanical properties, CNT are among the most promising materials for high aspect ratio critical dimension metrology (CDM) AFM probes. Our goal is to produce well-defined long lasting CNT probes for CDM measurements in the <100 nm pitch range. Our efforts currently focus on manufacturing precisely aligned CNT having defined locations, diameters and lengths. The CNT are grown using plasma enhanced chemical vapor deposition (PECVD). The CNT growth process is enabled by the presence of a catalyst, which allows precise definition of the growth location. Experimental data from CNT grown on Si AFM probes and catalyst patterns prepared by focused ion beam (FIB) and electron beam lithography (EBL) is being shown. Furthermore, first results from scanning experiments with CNT-AFM cantilevers are shown.
Carbon nanotubes (CNT) have been grown in a honeycomb configuration on silicon substrates using nanosphere self-assembly and plasma enhanced chemical vapor deposition. The optical properties of the arrays were also studied. Diffraction efficiency was found to be a function of the wavelength, angle of incidence and state of polarization of incident light. The unique optical properties of the arrays combined with the excellent mechanical and electrical properties of carbon nanotubes indicates that these materials may find many uses in the field of optoelectronics. In addition to their optical properties, periodic CNT arrays have a host of other unique electromagnetic and mechanical properties that may be exploited for numerous applications. Polarization measurements indicate that the intensity of both the diffracted light and diffusely scattered light is dependent on wavelength and angle of incidence. These arrays not only reflect and diffract light, but can also have a photonic band gap in, or around, the visible frequency range. The precise frequency location and size of this gap can be controlled by the structural and material parameters of the arrays.
A variety of novel ZnO nanostructures such as nanowires, nanowalls, hierarchical nanostructures with 6-, 4-, and 2-fold symmetries, nanobridges, nanonails have been successfully grown by a vapor transport and condensation technique. Doping both In and Sn into ZnO hierarchical nanostructures can be created. The 2-fold eutectic ZnO structures can also be created without any doping in the source. It was found that the hierarchical nanostructures can be divided into two
categories: homoepitaxial and heteroepitaxial where heteroepitaxy creates the multifold nanostructures. The novel ZnO nanowalls and aligned nanowires on a-plane of sapphire substrate have also been synthesized and the photoluminescence is studied. The ZnO nanowires also demonstrated very good field emission properties, comparable to carbon nanotubes. These nanostructures may find applications in a variety of fields such as field emission, photovoltaics, transparent EMI shielding, supercapacitors, fuel cells, high strength and multifunctional nanocomposites, etc. that require not only high
surface area but also structural integrity.
Carbon nanotubes (CNT) have exceptional mechanical strength at small diameters needed for measuring high aspect ratio features. Manually attached carbon nanotube atomic force microscopy probes have demonstrated exceptional longevity. Unfortunately, due to the manual attachment process, and the usually arbitrary diameter and length of the used CNT, such probes are not suitable for high aspect ratio critical dimension metrology (CDM). For reproducible and accurate CDM measurements precisely defined CNT probes are necessary. We are reporting about the progress made growing carbon nanotubes (CNT) directly on top of standard Si probes. The goal is to produce well-defined long lasting probes for CDM measurements in the <100 nm pitch range. Our efforts currently focus on manufacturing precisely aligned CNT having defined locations, diameters and lengths. This is accomplished by using plasma assisted chemical vapor deposition in combination with focused ion beam (FIB) patterned catalyst films. Our results demonstrate that it is possible to manufacture 1:10 aspect ratio CNT probes at <100 nm diameters.
Rudy Schlaf, Yusuf Emirov, Jay Bieber, Arun Sikder, J. Kohlscheen, Deron Walters, Mohammed Islam, Bhavik Metha, Z. Ren, Terri Shofner, Benjamin Rossie, Michael Cresswell
Carbon nanotubes (CNT) are among the candidates for atomic force microscopy probes for use in high aspect ratio critical dimension metrology (CDM). Their mechanical strength at small diameters makes them ideal probes for narrow and deep features. The synthesis of CNT has been making great progress in recent years. The use of CNT in scanning probe microscopy, however, has been limited due to a number of problems. While the CNT probes generally appear to be long lasting, the manufacture of precisely aligned CNT of defined length, diameter and number of walls poses a number of challenges. Yet, such precisely defined CNT probes seem to be required if the cantilevers are to be used for CDM. Our result demonstrate, for example, that the attachment angle of CNT with respect to the cantilever beam is crucial for their application in CDM. We report about our efforts to overcome these problems by growing well-defined CNT on standard Si cantilevers using chemical vapor deposition in combination with focused ion-beam machining techniques.
The c-axis optical properties of cuprate superconductors have been studied by grazing angle reflectivity technique. We tested the interlayer tunneling model of high temperature superconductivity and showed that the basic relation between the condensation energy of the superconducting state and the interlayer Josephson coupling does not hold for Tl2Ba2CuO6. Measuring the reflectivity at an oblique angle of incidence on the ab-surface of Tl2Ba2CuO6, we found the c-axis plasma resonance at 28 cm-1. The corresponding Josephson coupling energy is at least an order of magnitude lower than the condensation energy determined for the specific heat measurements. In the double layer compound Tl2Ba2CaCu2O8 the c-axis plasma resonance was observed at 26 cm-1. The c-axis penetration depth and the c-axis conductivity follow the Ambegaokar-Baratoff or 'dirty limit' superconductor relation if a very small energy gap is assumed.
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