arXiv subjects

H. Grebel

Publications and source records attributed to H. Grebel.

12 recordsLinked to original sources

arXiv2025

Super-capacitors interfaced with quantum dots at the electrolyte/electrode interface: capacitance gain and fluorescence line-width narrowing

Quantum dots (QDs), embedded in supercapacitors (S-C) cells exhibited capacitance amplification that peaked at certain QD concentration. More than 2.5 peak amplification was demonstrated with cyclic voltammetry (C-V) at a scan rate of 0.1 V/s. The mass ratio of the dots to that of the active-carbon electrode (A-C) was less than 1:5000. Fluorescence signals from dry (without electrolyte) and wet (with electrolyte) samples, exhibited a correlated, substantial peak enhancement vs QD concentration, as well. In the case of wet samples, fluorescence line narrowing was demonstrated, which indicates a substantial fluorescence gain. Both effects are attributed to local field effects through formation of a colloidal array at the electrode/electrolyte interface. Embedding QDs at the electrolyte/electrode interface has an added value that it may be further enhanced by white light and indeed this is the case here. Probing the electrolyte/electrode interface with fluorescing materials adds to our basic knowledge of the interface and could be useful for light-sensitive S-C cells.

physics.chem-ph↗

arXiv2024

Linear and Nonlinear Ultra-Short Pulse Looped Antennas: Radiation and Parametric Oscillations

Modern optical systems send and receive ultra-short temporal pulses (USP). While ultra-broad band antennas do exist in the microwave region (e.g., log-periodic antennas), their short temporal response is typically limited by the antenna's large dispersion, hence, resulting in a substantial pulse broadening. The issue becomes more severe when one considers both the transmitted and received pulses. Through simulations and experiments one can show that properly designed loop antennas, either thick loops or 3-loop antennas, exhibit USP attributes, 280 ps upon transmission and 380 ps upon reception (or, an overall equivalent coherent channel exceeding 2.5 GHz). Finally, most parametric amplifiers are narrow band and one may ask if a broadband amplification is possible. A loop inside a loop system, coupled by a nonlinear impedance element exhibits a line narrowing and signal amplification with large bandwidth, which is inversely scalable with the loops' diameter. In all, these elements could be advantageous for applications such as ultra-wide bandwidth communication and non-linear quantum information systems.

physics.ins-det↗

arXiv2023

An Electrical Grid with Discrete Energy Levels

Minimizing both power fluctuations and energy waste in an electrical grid is a central challenge to energy policy. Any discrepancy between power production and loads may lead to inefficiencies and instability in the system. Right now, the electrical grid is an analog system that only retroactively reacts to power demands. The balancing act becomes even harder with the penetration of sustainable resources (e.g., wind turbines). Here, we consider the effect of random perturbations to the grid's steady states operation. A model is constructed and analyzed within the framework of a randomly perturbed Markovian chain. Instead of balancing continuous values for supply and demand, the model assumes that both the generators and the users adhere to discrete pattern energy levels which is supplemented by local, short-term energy storage units (STESU). Under reasonable assumptions, we show that this grid maintains stability (meaning, a constant difference between supply and demand) over long periods of time while subjected to randomly fluctuating energy conditions.

eess.SY↗

arXiv2023

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

We use surface enhanced Raman spectroscopy (SERS) in studying functionalized Au nanoparticles (AuNPs) when incorporated in active-carbon (A-C) based super-capacitor cells. We observe a resonance-like enhancement in the graphitic line (G-line) vs the D-line (defect line) of the A-C electrode. We also observed an enhancement in the specific capacitance of super-capacitor cell as a function of AuNPs concentration. All of these may be explained by the formation of a quasi-2D array of AuNPs at the interface between electrolyte and the electrode.

cond-mat.mes-hall↗

arXiv2022

Active-carbon based supercapacitors with Au colloids: the case for placing the colloids at the electrolyte/electrode interface

Supercapacitors (S-C) are short-term energy storage elements that find many applications, e.g., electronic charging devices and suppressors of power fluctuations in grids that are interfaced with sustainable sources. The capacitance of an ordinary capacitor increases when dispersing metallic colloids in its dielectric. A similar strategy for S-C means a deployment of nano-scale metal colloids (in our case, Au nano particles, or AuNPs) at the very narrow interface between an electrolyte and the porous electrode (here, active-carbon film on a grafoil current collector). This is achieved by making the ligand that is coating the AuNPs negatively charged. We demonstrated a very large specific capacitance increase with a minute addition of functionalized AuNPs to the slurry. For example, C-V data at a scan rate of 20 mV/s indicated a specific capacitance amplification by a factor of 10 when 30 micro-g of AuNPs were incorporated with 200 mg of active carbon while using a 1 M Na2SO4 electrolyte and a 5% cellulose acetate butyrate as a binder. We make the case that the adhesion of the AuNPs to the surface of the electrode was strong: upon replacing the electrolyte, from 1 M Na2SO4 to 1 M KOH and retaining the same set of electrodes, the enhancement capacitance factor decreased as compared to 1 M Na2SO4 electrolyte but remained large, ~3, as determined by C-V traces at the same scan rate of 20 mV/s.

physics.chem-ph↗

arXiv2021

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.

cond-mat.mes-hall↗

arXiv2020

Asymmetric supercapacitors: optical and thermal effects when active carbon electrodes are embedded with nano-scale semiconductor dots

Optical and thermal effects in asymmetric supercapacitors, whose active-carbon (AC) electrodes were embedded with nano-Si (n-Si) quantum dots (QD), are reported. We describe two structures: (1) p-n like, obtained by using a polyethylimine (PEI) binder for the n-like electrode and a polyvinylpyrrolidone (PVP) binder for the p-like electrode; (2) a single component binder, poly(methyl methacrylate) (PMMA). In general, AC appears black to the naked eye and one may assume that it acts as a black body absorber, namely, indiscriminately absorbing all light spectra. Yet, on top of a flat lossy spectra, AC (from two manufacturers) exhibited two distinct absorption bands: one in the blue (~ 400 nm) and the other one in the near IR (~ 840 nm). The n-Si material accentuated the absorption in the blue and bleached the IR absorption. Both bands contributed to capacitance increase: (a) when using aqueous solution and a PMMA binder, the optical related increased capacitance was 20% at low n-Si concentration and more than 100% for a high concentration dose; (b) when using IL electrolyte, the large, thermal capacitance increase (of ca 40%) was comparable to the optical effect (of ca 42%) and hence was assigned as an optically-induced thermal effect. The experimental data point to an optically induced capacitance increase even in the absence of the n-Si dots; this could be attributed to the absorption of AC in the blue.

physics.chem-ph↗

arXiv2020

The effect of periodic spatial perturbations on the emission rates of quantum dots near graphene platforms

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 and the lossy conductor. Increased emission rate is also observed when the chromophore is coupled to a resonator. Here we combine the two effects in order to control the emission lifetime of the chromophore. In our case, the resonator was defined by an array of nano-holes in the oxide substrate underneath a graphene surface guide. 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. Such control over the emission life-time could be used to control resonance energy transfer (RET) between two chromophores.

cond-mat.mes-hall↗

arXiv2019

Parametric Oscillation and amplification with gate controlled capacitor-within-capacitor

A Capacitor-within-Capacitor (CWC) is a nested electronic element that has two components: the cell (e.g., the outer capacitor) and the gate (e.g., the inner capacitor). The designations for the gate and cell may be interchanged. Here, an analysis and experimentation on a diode-interfaced parametric oscillations and amplification are described. By replacing the diode with a junction, made of doped nano-graphene films, we demonstrated a new structure whose doping may be electronically and chemically controlled.

physics.app-ph↗

arXiv2019

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.

physics.optics↗

arXiv2018

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.

physics.optics↗

arXiv2018

Transfer of Graphene with Protective Oxide Layers

Transfer of graphene, grown by Chemical Vapor Deposition (CVD), to a substrate of choice, typically involves deposition of a polymeric layer (typically, poly(methyl methacrylate, PMMA or polydimethylsiloxane, PDMS). These polymers are quite hard to remove without leaving some residues behind. Here we study a transfer of graphene with a protective thin oxide layer. The thin oxide layer is grown by Atomic Deposition Layer (ALD) on the graphene right after the growth stage on Cu foils. One can further aid the oxide-graphene transfer by depositing a very thin polymer layer on top of the composite (much thinner than the usual thickness) following by a more aggressive polymeric removal methods, thus leaving the graphene intact. We report on the nucleation growth process of alumina and hafnia films on the graphene, their resulting strain and on their optical transmission. We suggest that hafnia is a better oxide to coat the graphene than alumina in terms of uniformity and defects.

physics.app-ph↗