Optically Controlled Supercapacitors with Semiconductor Embedded Active Carbon Electrodes

Supercapacitors, S-C - capacitors that take advantage of the large capacitance at the interface between an electrode and an electrolyte - have found many short-term energy applications. We concentrate here on optically induced, electrical and thermal effects. The parallel plate cells were made of two transparent electrodes (ITO), each covered with semiconductor-embedded, active carbon (A-C) layer. While A-C appears black, it is not an ideal blackbody absorber that absorbs all spectral light indiscriminately. In addition to relatively flat optical absorption background, A-C exhibits two distinct absorption bands: in the near-IR and in the blue. The first may be attributed to absorption by OH- group and the latter, by scattering, possibly by surface plasmons. Here, optical and thermal effects of sub-micron size SiC particles that are embedded in A-C electrode, are presented. Similarly to nano-Si particles, SiC exhibits blue band absorption, but it is less likely to oxidize. Using Charge-Discharge (CD) experiments, the relative optically related capacitance increase may be as large as ~34% (68% when the illuminated area is taken into account). Capacitance increase was noted as the illuminated samples became hotter. This thermal effect amounts to 20% of the overall relative change using CD experiments. The thermal effect was quite large when the SiC particles were replaced by CdSe/ZnS quatum dots; for the latter, the thermal effect was 35% compared with 10% for the optical effect. When analyzing the optical effect one may consider two processes: ionization of the semiconductor particles and charge displacement under the cell's terminals - a dipole effect. Our model suggests that the capacitance increase is related to an optically induced dipole

Raman spectroscopy of active-carbon electrodes when Au colloids are placed at the electrolyte/electrode interface

In the past, when Au nanoparticles (AuNPs) were placed just above (but not included in) active carbon (A-C) electrodes, a 10-fold amplification of gravitational specific capacitance were demonstrated, despite the small mass-ratio between the AuNPs and A-C; a ratio of 1:5000. We use surface enhance Raman spectroscopy (SERS) in further studying this system. Supercapacitors are volumetric elements while SERS is a surface interrogating method, whose signal could be impacted by many factors, including local colloid's preferential points (hot spots). Here, we use the ratio between the D- and G-lines of the A-C electrode, ID/IG, as a marker to eliminate the surface inconsistencies. At some concentration levels of AuNPs, the A-C Raman signature shows a clear preference towards the 1600 cm-1 vibration mode (G-line). Following that point, the cell exhibits a large specific capacitance

Optical cages made of graphitic frameworks

In pursuit of infrared (IR) radiation absorbers, we examine quasicrystal structures made of graphite wires. An array of graphitic cages and cage-within-cage, and whose overall dimensions is smaller than the radiation wavelength exhibit a flat absorption spectrum, A~0.83 between 10-30 microns and a quality loss factor of L~0.83 (L=A/Q, with Q, the quality factor). Simulations at microwave frequencies show multiple absorption lines. In the case of a cage within cage, energy is funneled towards the inner cage which result in a rather hot structure. Applications are envisioned as anti-fogging surfaces, EM shields and energy harvesting

Optical Cages

We examine array of metal-mesh frameworks for their wide-band absorption. These take the form of quasi-crystal optical cages. An array of cages tends to focus the incoming radiation within each framework. An array of cage-within-cage funnels the radiation from the outer cage to its inner core even further

Negative Differential Resistance: Gate Controlled and Photoconductance Enhancement in Carbon Nanotube Intraconnects

Intraconnects, as-grown single-walled carbon nanotubes bridging two metal electrodes, were investigated as gated structures. We show that even with a seemingly “ohmic” contact at zero gate voltage one observes negative differential resistance (NDR) at nonzero gate bias. Large differential photo conductance (DPC) was associated with the NDR effect raising hopes for the fabrication of novel high-speed optoelectronic devices