Johannes Svensson

Electrochromic hydrated nickel oxide coatings for energy efficient windows: Optical properties and coloration mechanism

Durable electrochromic coatings of hydrated nickel oxide were produced by reactive rf magnetron sputtering of Ni followed by treatment in KOH. Spectrophotometry was used to assess the achievable modulation of luminous and solar transmittance and to verify that the studied material is interesting for ‘‘smart window’’ applications. 15N nuclear reaction analysis suggested that coloration occurred upon hydrogen extraction.

Modulated transmittance and reflectance in crystalline electrochromic WO3 films: Theoretical limits

A quantitative theory for the optical properties of crystalline electrochromic materials is introduced. It is based on the double injection model and includes ionized impurity scattering of free electrons. Solar and luminous properties are evaluated versus electron density for doped WO3 films. Our results point to the possibility of creating efficient ‘‘smart windows’’ with dynamic control of the inflow of radiant energy.

Carbon nanotube bolometers

A cryogenic bolometer has been fabricated using a bundle of single-walled carbon nanotubes as absorber. A bolometric response was observed when the device was exposed to radiation at 110 GHz. The temperature response was 0.4 mV/K, with an intrinsic electrical responsivity at low frequency up to 109 V/W and noise equivalent power of 3×10−16 W/Hz1/2 at 4.2 K. The response is largest at input power levels of a few femtowatts and decreases inversely proportional to the input power. Low frequency noise shows a 1/f dependence.

The dependence of the Schottky barrier height on carbon nanotube diameter for Pd?carbon nanotube contacts

Direct measurements are presented of the Schottky barrier (SB) heights of carbon nanotube devices contacted with Pd electrodes. The SB barrier heights were determined from the activation energy of the temperature-dependent thermionic emission current in the off-state of the devices. The barrier heights generally decrease with increasing diameter of the nanotubes and they are in agreement with the values expected when assuming little or no influence of Fermi level pinning.

Carbon nanotube based bolometer

The contacts of single carbon nanotubes and bundles of carbon nanotubes with superconducting and metallic electrodes are investigated in order to create bolometers and electron coolers. Tunneling contacts of the carbon nanotubes with aluminum electrodes are obtained. The current-voltage characteristics of junctions are analyzed for temperatures from room temperature to 300 mK. The resistance of individual nanotubes is primarily determined by defects and is too large for applications. The use of the bundles of carbon nanotubes makes it possible to considerably reduce the resistance of the bolometer, which is determined by a small number of conducting tubes with good tunneling contacts with the electrodes. The energy gap is equal to hundreds and tens of millivolt in the former and latter cases, respectively. Structures containing bundles of carbon nanotubes can be described in a model with a Schottky barrier. The samples with bundles of carbon nanotubes exhibit the bolometric response to external high-frequency radiation at a frequency of 110 GHz with an amplitude up to 100 μV and a temperature voltage response to 0.4 mV/K.

TOWARDS SUPERSENSITIVE BOLOMETERS AND ELECTRON COOLERS BASED ON CARBON NANOTUBES

Carbon nanotubes (CNT) are being intensively developed for novel electronics. Electron cooling by superconductor-insulator-nanotube (SIN) tunnel junctions could be extremely effective due to the small volume of the CNT. A novel concept of a Cold-Electron Bolometer with a CNT as absorber should demonstrate record sensitivity due to the very low temperature that is predicted to be reached in the CNT (less than the phonon temperature). Objectives of this work is to demonstrate effective electron cooling in superconducting nanostructures comprising a Carbon Nanotube; and develop a supersensitive Cold-Electron Bolometer based on a cooled carbon nanotube as absorber.