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Cola,
Baratunde A.
Cola,
Baratunde A.
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ItemAn Optical Rectenna(Georgia Institute of Technology, 2015-08-25) Cola, Baratunde A.An optical rectenna – that is, a device that directly converts free‐propagating electromagnetic waves at optical frequencies to d.c. electricity – was first proposed over 40 years ago, yet this concept has not been demonstrated experimentally due to fabrication challenges at the nanoscale. Realizing an optical rectenna requires that an antenna be coupled to a diode that operates on the order of 1 petahertz (switching speed on the order of a femtosecond). Ultralow capacitance, on the order of a few attofarads, enables a diode to operate at these frequencies; and the development of metal‐insulator‐metal tunnel junctions with nanoscale dimensions has emerged as a potential path to diodes with ultralow capacitance, but these structures remain extremely difficult to fabricate and couple to a nanoscale antenna reliably. Here we demonstrate an optical rectenna by engineering metal‐insulator‐metal tunnel diodes, with ultralow junction capacitance of approximately 2 attofarads, at the tips of multiwall carbon nanotubes, which act as the antenna and metallic electron field emitter in the diode. This demonstration is achieved using very small diode areas based on the diameter of a single carbon nanotube (about 10 nanometers), geometric field enhancement at the carbon nanotube tips, and a low work function semi‐transparent top metal contact. Using vertically‐aligned arrays of the diodes, we measure d.c. open‐circuit voltage and short‐circuit current at visible and infrared electromagnetic frequencies that is due to a rectification process, and quantify minor contributions from thermal effects. In contrast to recent reports of photodetection based on hot electron decay in plasmonic nanoscale antenna, a coherent optical antenna field is rectified directly in our devices, consistent with rectenna theory. Our devices show evidence of photon‐assisted tunneling that reduces diode resistance by two orders of magnitude under monochromatic illumination. Additionally, power rectification is observed under simulated solar illumination. Numerous current‐voltage scans on different devices, and between 5‐77 degrees Celsius, show no detectable change in diode performance, indicating a potential for robust operation.
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ItemPhoto-thermal enhanced carbon nanotube rectenna arrays for solar energy conversion(Georgia Institute of Technology, 2012-09) Cola, Baratunde A. ; Sibble, DanThis project was aimed at fabricating dense ordered arrays of vertical carbon nanotube (CNT) rectenna with diode components exhibiting high asymmetry and nonlinearity. Its broad objectives were to explore and demonstrate the devices with solar energy conversion at high solar frequencies and with efficient power conversion that can be used in a variety of defense applications.
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ItemConformally coating vertically aligned carbon nanotube arrays using thermal decomposition of iron pentacarbonyl(Georgia Institute of Technology, 2012-05) Hildreth, Owen ; Cola, Baratunde A. ; Graham, Samuel ; Wong, C. P.Conformally coating vertically aligned carbon nanotubes (v-CNT) with metals or oxides can be difficult because standard line-of-sight deposition methods, such as dc sputter coating and electron-beam evaporation, are hindered by the low mean-free-path with
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ItemNitrogen- and Boron-Doped Carbon Nanotube Electrodes in a Thermo-Electrochemical Cell(Georgia Institute of Technology, 2012) Salazar, Pablo F. ; Kumar, Satish ; Cola, Baratunde A.We explore the prospects of using doped carbon nanotube (CNT) electrodes to increase the output power of thermo-electrochemical cells (TECs). CNT buckypaper electrodes doped with nitrogen and boron were characterized using cyclic voltammetry, impedance spectroscopy, and TEC test with potassium ferri/ferrocyanide electrolyte. Both doping states increased the electrochemically active surface area of CNT electrodes. Electrostatic interactions with potassium ions altered the charge transfer kinetics for doped CNT electrodes; yet, the symmetry of the charge transfer remained approximately equal to that of pristine CNTs. In TEC test, accumulation of potassium ions at doped CNT electrodes was found to reduce short-circuit current.
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ItemHighly specular carbon nanotube absorbers(Georgia Institute of Technology, 2010-10) Wang, X. J. ; Wang, L. P. ; Adewuyi, O. S. ; Cola, Baratunde A. ; Zhang, ZhuominSpecular black materials have important applications, such as in absolute cryogenic radiometers, space-borne spectroradiometers, and some energy conversion devices. While vertically aligned carbon nanotubes (VACNT) can have close-to-unity absorptance, so far the reported reflection has been essentially diffuse. This letter describes a highly specular black absorber made of VACNT. Both the bidirectional reflectance distribution function and specular reflectance were measured at the wavelength λ = 635 nm using a laser scatterometer. The ordinary and extraordinary optical constants were obtained by fitting the specular reflectance, calculated from modified reflectance formulae for light incident from air to a uniaxial medium, considering surface roughness. Furthermore, the absorptance at λ = 635 nm was shown to be 0.994±0.002, based on the measured directional-hemispherical reflectance.
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ItemEmerging Solar Thermal Technologies(Georgia Institute of Technology, 2010-04-15) Cola, Baratunde A. ; Jeter, Sheldon M.Sheldon Jeter's presentation is an overview of solar thermal power concepts, technology, and deployments from historical to modern to current times.