NASA Tech Briefs Index 1980

Tech Briefs are short announcements of new technology derived from the research and development activities of the National Aeronautics and Space Administration. These briefs emphasize information considered likely to be transferrable across industrial, regional, or disciplinary lines and are issued to encourage commercial application. This Index to NASA Tech Briefs contains abstracts and four indexes -- subject,. personal author, originating Center, and Tech Brief number -- for 1980 Tech Briefs

CREATION AND CHARACTERIZATION OF SILICON NANOWIRE ELECTRON RATCHETS FOR USE AS GEOMETRIC DIODES AT ROOM TEMPERATURE

Ratcheting effects, like that of a socket wrench, convert a fluctuating, unbiased force into unidirectional motion. If these effects could be applied to electrons, it may unlock advances in high speed signal processing and energy harvesting. However, to create an electron ratchet, control over the geometry must be achieved on the order of the mean-free-path of the electron. This level of precision was achieved using vapor-liquid-solid (VLS) growth of nanowires (NW) followed by wet chemical dopant-selective etching. Modulation of the nanowire diameter created a cylindrical sawtooth geometry with broken inversion symmetry on a nanometer length scale. In a two-terminal device, the NW structures responded as a geometric diode that funnels electrons preferentially in one direction through specular reflection of quasi-ballistic electrons at the NW surface. This ratcheting effect manifests itself in an asymmetry in current when comparing forward and reverse bias currents. Generally it was shown that wires with deeper etching, leading to steeper funnel shapes, yielded higher asymmetries. With properly tuned surfaces, maximum asymmetries of >103 were achieved. To fully demonstrate the electron ratcheting mechanism the devices were measured with alternating current and showed charge rectification up to an instrument limited frequency of 40 GHz. Because the devices have ultra-low capacitance, their frequency response is believed to be ~1 THz and limited not by resistance-capacitance time, but by the electron flight time through the geometry. To gain insight through simulation, a finite-element model was created to measure the electrostatics of the device, but its neglect for ballistic transport of charges, means that it did not predict the experimental current asymmetry. An analytical model was created to measure the transmission probability of a particle through a diode shape and a Monte Carlo model was made to consider charge carrier scattering and semiconductor physics as particles transport through the geometry. Both models agree with experiment, providing further evidence for the ballistic nature of the electrons in the geometry. The findings presented in this work demonstrate the creation of the structurally-tunable NW geometric diodes that have applications in THz sensing, data processing, and energy harvesting.Doctor of Philosoph

NASA Tech Briefs, February 1998

Topics: Test Tools; Electronic Components and Circuits; Electronic Systems; Physical Sciences; Materials; Computer Software; Mechanics; Machinery/Automation; Manufacturing/Fabrication; Life Sciences

Catch the sun: new ancillary ligands in Copper(I) dye-sensitized solar cells lead to panchromatic light harvesting

The world population is increasing. More people demand better living conditions and the boom in the developing countries continues apace. The story of human progress is also the story of energy. Energy was and will be the key feature of progress. Nowadays, three energy transitions have taken place. These have developed our society from wood burning to coal-powered steam engines to a dependence on electricity which involved burning fossil fuels. All energy transitions up to now have included burning materials that produce not only energy but also air pollution and so-called greenhouse gases. The CO2 concentration in Earth's atmosphere is not at an acceptable level and the effects affect everyone on Earth. However, since the oil crisis in the 1970s and nuclear disasters such as Chernobyl and Fukushima, public awareness has been raised and the demand for alternative energy sources has increased. Renewable energy sources that disappeared over a long period, such as wind power, have made their comeback and a topical example of new renewable resources is photovoltaics. Photovoltaics produce renewable, sustainable and eco-friendly energy. Most commercial solar cells are based on silicon. Since their development in 1953, new generations of solar cells have been investigated in order to make them cheaper, more environmentally friendly and more efficient. Dye-sensitized solar cells are one cutting edge technology. They had their breakthrough in 1991 with the developments of Michael Grätzel and Brain O'Regan. A dye is adsorbed onto a semiconductor surface that is adhered to a conducting glass substrate. The electrical circuit is closed with a counter electrode and an electrolyte. Several types of dyes have been investigated but only a few have shown promising results. A new and exciting area encompasses copper(I)-based dyes. They consist of copper(I) complexes which incorporate a ligand with functional groups to anchor to the semiconductor surface and an ancillary ligand which can be structurally tuned to optimize light harvesting. Most of the ancillary ligands are based on a 2,2'-bipyridine core. Investigations aimed at improving cell performance had, in 2015, "got stuck" at photoconversions of around 2%. This thesis describes the path to new types of simple ancillary ligands that surpass the performance of the most optimized 2,2'-bipyridine-based ligands. The path includes the development of general methods that improve the economical part of the fabrication of the solar cells and the regeneration of destroyed dye. The new families of ancillary ligand lead to the development of panchromatic co-sensitized copper(I) dye-sensitized solar cells. For the first time, a copper(I) dye has been combined with a commercially available and cheap organic dye and the remarkable performance has shown the exciting potential of copper(I) dye-sensitized solar cells. Catch the Sun