Optically Controlled TiO2-Embedded Supercapacitors: The Effects of Colloidal Size, Light Wavelength, and Intensity on the Cells' Performance.

Optically controlled supercapacitors (S-C) could be of interest to the sensor community, as well as set the stage for novel optoelectronic charging devices. Here, structures constructed of two parallel transparent current collectors (indium-tin-oxide, ITO films on glass substrates) were considered. Active-carbon (A-C) films were used as electrodes. Two sets of electrodes were used: as-is electrodes that were used as the reference and electrodes that were embedded with submicron- or micron-sized titanium oxide (TiO2) colloids. While immersed in a 1 M Na2SO4, the electrodes exhibited minimal thermal effects (<3 °C) throughout the course of experiments). The optically induced capacitance increase for TiO2-embedded S-C was large of the order of 30%, whereas S-C without the TiO2 colloids exhibited minimal optically related effects (<3%). Spectrally, the blue spectral band had a relatively larger impact on the light-induced effects. A lingering polarization effect that increased the cell capacitance in the dark after prolonged light exposure is noted; that effect occurred without an indication of a chemical reaction.

Optically Controlled Supercapacitors: Functional Active Carbon Electrodes with Semiconductor Particles.

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. The parallel plate cells were made of two transparent electrodes (ITO), each covered with a 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 a relatively flat optical absorption background, A-C exhibits two distinct absorption bands: in the near-infrared (near-IR and in the blue. The first may be attributed to absorption by the OH- group and the latter, by scattering, possibly through surface plasmons at the pore/electrolyte interface. Here, optical and thermal effects of sub-µm SiC particles that are embedded in A-C electrodes, are presented. Similar 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 capacitance change using CD experiments. The thermal effect was quite large when the SiC particles were replaced by CdSe/ZnS quantum dots; for the latter, the thermal effect was 35% compared to 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. A model suggests that the capacitance increase is related to an optically induced dipole effect.

The Effect of Periodic Spatial Perturbations on the Emission Rates of Quantum Dots near Graphene Platforms.

The quenching of fluorescence (FL) at the vicinity of conductive surfaces and, in particular, near a 2-D graphene layer has become an important biochemical sensing tool. The quenching is attributed to fast non-radiative energy transfer between a chromophore (here, a Quantum Dot, QD) and the lossy graphene layer. Increased emission rate is also observed when the QD is coupled to a resonator. Here, we combine the two effects in order to control the emission lifetime of the QD. In our case, the resonator was defined by an array of nano-holes in the oxide substrate underneath a graphene surface guide. At resonance, the surface mode of the emitted radiation is concentrated at the nano-holes. Thus, the radiation of QD at or near the holes is spatially correlated through the hole-array's symmetry. We demonstrated an emission rate change by more than 50% as the sample was azimuthally rotated with respect to the polarization of the excitation laser. In addition to an electrical control, such control over the emission lifetime could be used to control Resonance Energy Transfer (RET) between two chromophores.

Periodic Metallo-Dielectric Structures: Electromagnetic Absorption and its Related Developed Temperatures.

Multi-layer, metallo-dielectric structures (screens) have long been employed as electromagnetic band filters, either in transmission or in reflection modes. Here we study the radiation energy not transmitted or reflected by these structures (trapped radiation, which is denoted-absorption). The trapped radiation leads to hot surfaces. In these bi-layer screens, the top (front) screen is made of metallic hole-array and the bottom (back) screen is made of metallic disk-array. The gap between them is filled with an array of dielectric spheres. The spheres are embedded in a dielectric host material, which is made of either a heat-insulating (air, polyimide) or heat-conducting (MgO) layer. Electromagnetic intensity trapping of 97% is obtained when a 0.15 micron gap is filled with MgO and Si spheres, which are treated as pure dielectrics (namely, with no added absorption loss). Envisioned applications are anti-fogging surfaces, electromagnetic shields, and energy harvesting structures.

Lifetime and linewidth of individual quantum dots interfaced with graphene.

We report on luminescence lifetimes and linewidths from an array of individual quantum dots (QDs) that were either interfaced with graphene surface guides or dispersed on aluminum electrodes. The observed fluorescence quenching is consistent with screening by charge carriers. Fluorescence quenching is typically mentioned as a sign that chromophores are interfacing with a conductive surface (metal or graphene); we find that the QDs interfaced with the metal film exhibit shortened lifetime and line-broadening but not necessarily fluorescence quenching as the latter may be impacted by molecular concentration, reflectivity and conductor imperfections. We also comment on angle-dependent lifetime measurements, which we postulate depend on the specifics of the local density-of-states involved.

Gain and Raman line-broadening with graphene coated diamond-shape nano-antennas.

Using Surface Enhanced Raman Scattering (SERS), we report on intensity-dependent broadening in graphene-deposited broad-band antennas. The antenna gain curve includes both the incident frequency and some of the scattered mode frequencies. By comparing antennas with various gaps and types (bow-tie vs. diamond-shape antennas) we make the case that the line broadening did not originate from strain, thermal or surface potential. Strain, if present, further shifts and broadens those Raman lines that are included within the antenna gain curve.

Nonlinear behavior of vibrating molecules on suspended graphene waveguides.

Suspended graphene waveguides over micrometer-scale metal-mesh screens were used as platforms for Raman scattering. Raman signals of B. megaterium spores were found sensitive to in-plane rotations and tilt of the waveguides with respect to the incident linearly polarized pump beam. When at plasmonic resonance for the equivalent long wavelength of the vibration frequency, the Raman signal exhibited an additional quadratic effect.

Raman spectroscopy with graphenated anodized aluminum oxide substrates.

We measured the Raman scattering of graphene deposited nanohole arrays. As the sample was azimuthally rotated, periodicities of 7.5 degrees and 5 degrees were revealed for the 2700 cm(-1) and 1600 cm(-1) Raman lines of graphene, respectively. This is contrary to the scattered laser line azimuthal symmetry of 30 degrees for the hole array alone. When a reference dye (stilbene) was deposited on the graphenated platforms, its Raman peak shifted as a function of incident (tilt) angle; this was contrary to the unshifted 1600 cm(-1) peak of graphene itself. The data suggest strong coupling between the molecular vibrations as portrayed by Raman spectra and surface plasmon polariton waves propagating along the graphene surface.

Inductive cross-shaped metal meshes and dielectrics.

The Micro-Stripes program has been used to calculate resonance wavelengths and the bandwidth of inductive cross-shaped metal meshes in contact with dielectric layers. The shift of the resonance wavelength, depending on the thickness of the dielectric layers, has been studied for two refractive indices. The transmittance of two mesh filters with dielectric spacers or embedded in a dielectric has been calculated for specific alignment of the crosses of one mesh with respect to the other. Transmission line theory has been used to calculate the transmittance of two mesh filters with nonaligned crosses and dielectric layers. A coupled oscillator model has been used for interpretation of the interaction of resonance and Fabry-Perot modes.

Near-field effects in multilayer inductive metal meshes.

The transmittance of inductive single-layer and multilayer cross-shaped metal meshes has been calculated with the Micro-Stripes software program. The effect of symmetric and asymmetric alignment of the crosses of one mesh with respect to another was studied and compared with transmission line theory, which presents the nonaligned case. Significant differences are found for small spacing at approximately 1/5 the periodicity constant, whereas the differences disappear for large spacing. A pair of coupled surface waves is used to represent the mode of a single mesh. The resulting modes corresponding to the transmittance of multilayer metal meshes are interpreted by modes composed of resonance modes of a single mesh coupled by Fabry-Perot modes depending on the separation.