Don Walker

Single-ion adsorption and switching in carbon nanotubes

Single-ion detection has, for many years, been the domain of large devices such as the Geiger counter, and studies on interactions of ionized gasses with materials have been limited to large systems. To date, there have been no reports on single gaseous ion interaction with microelectronic devices, and single neutral atom detection techniques have shown only small, barely detectable responses. Here we report the observation of single gaseous ion adsorption on individual carbon nanotubes (CNTs), which, because of the severely restricted one-dimensional current path, experience discrete, quantized resistance increases of over two orders of magnitude. Only positive ions cause changes, by the mechanism of ion potential-induced carrier depletion, which is supported by density functional and Landauer transport theory. Our observations reveal a new single-ion/CNT heterostructure with novel electronic properties, and demonstrate that as electronics are ultimately scaled towards the one-dimensional limit, atomic-scale effects become increasingly important.

Single Event Effects in Carbon Nanotube-Based Field Effect Transistors Under Energetic Particle Radiation

We present results from proton radiation experiments with carbon nanotube field effect transistors. Single event effects were observed consisting of drops in current, with very long durations (100 s of ms), and sudden, discrete switching events between quantized current levels. These studies are important for the development and understanding of advanced nano-electronic devices operating in the space radiation environment.

Dark current characterization of irradiated solar cells

A decrease in ideality factor of GaAs solar cells due to irradiation with 1MeV electron radiation resulted in decreased open circuit voltage and maximum power. Dark current-voltage measurements suggest that 1 MeV electron radiation primarily affects dark current produced at voltages greater than 0.5 V. The dark saturation current of irradiated solar cells increased but a simultaneous decrease in ideality factor caused a reduction of the open circuit voltage. The reduced ideality factor further indicates a change in the primary recombination mechanism.

Proton irradiation of metallic single-walled carbon nanotubes

Metallic single-walled carbon nanotube (SWNT) films on space qualified solar cell coverglass were irradiated with 100 keV protons at doses of 1 × 10¹³ and 1 × 10¹⁴ protons/cm². The samples were analyzed using UV-vis-NIR spectroscopy, Raman spectroscopy, and four-point probe measurements. The results indicate that defects are not introduced into the metallic carbon nanotubes after irradiation.

On-orbit characterization of space solar cells on nano-satellites

The Aerospace Corporation has been building, testing, and flying miniature satellites in the pico-and nano-satellite class for over a decade. Significant advances have been made to the bus avionics unit and other satellite subsystems during this time. The advances have enabled various on-orbit tests and experiments, one of which has been to host space solar cell experiment payloads. Recent solar cell flight experiments on Aerospaces CubeSats (AeroCubes) demonstrated several subsystems can simultaneously operate to obtain precise measurements of space solar cell performance. Low cost, rapid return CubeSat missions can be valuable development tools for advancing the readiness level of space technologies.

Stability of isotype reference cells used for multi-source solar simulator calibration

Calibration of individual zones in a multi-source solar simulator using isotype reference cells presents unique challenges. Higher bandgap materials are used as filter layers to match the spectral response of an active isotype cell to its corresponding response in a full multijunction cell. Experimental results suggest that luminescence from these filter layers could occur during calibration of isotype references, thereby artificially increasing the current output. We show that proton irradiation of the filter layers can quench radiative recombination within these layers thereby reducing the luminescent coupling. We also take a look at transient responses from some isotype cells.

Increasing the limiting efficiency of space solar cells at end-of-life

An absolute increase of 3.73% in the end-of-life efficiency of a six junction, series connected, monolithic solar cell can be achieved as determined by detailed balance calculations. Multijunction solar cells were designed and optimized for three solar spectra: AM 0, AM 0 under a MgF2/ITO coated cerium-doped borosilicate glass, and AM0 under MgF2/ITO cerium-doped borosilicate glass that was irradiated with proton energies and fluences equivalent to 15 years in a GEO orbit. Also, the difference between end-of-life and beginning-of-life efficiency decreases for multi-junction solar cells designed for end-of-life versus those designed for beginning-of-life conditions.