Fundamental solution of the time-space bi-fractional diffusion equation with a kinetic source term for anomalous transport.

The purpose of this paper is to study the fundamental solution of the time-space bi-fractional diffusion equation incorporating an additional kinetic source term in semi-infinite space. The equation is a generalization of the integer-order model ∂ t ρ ( x , t ) = ∂ x 2 ρ ( x , t ) - ρ ( x , t ) (also known as the Debye-Falkenhagen equation) by replacing the first-order time derivative with the Caputo fractional derivative of order 0 < α < 1 , and the second-order space derivative with the Riesz-Feller fractional derivative of order 0 < ß < 2 . Using the Laplace-Fourier transforms method, it is shown that the parametric solutions are expressed in terms of the Fox's H-function that we evaluate for different values of α and ß .

Label-free DNA detection using silver nanoprism decorated silicon nanoparticles: Effect of silicon nanoparticle size and doping levels.

In the present work, we have fabricated silver nanoprism (AgNPrs)/silicon nanoparticle (SiNPs) hybrid arrays for highly sensitive detection of biomolecules via surface-enhanced Raman spectroscopy (SERS) technique. SiNPs having 7 to 37 nm in size and with phosphorous doping varying from 1 × 1019 to 1 × 1020 cm-3 were synthesized in nonthermal plasma synthesis. SiNPs were further immobilized on glass substrates using spin-coating, followed by deposition of AgNPrs using the drop-casting method. SERS studies showed that AgNPrs/SiNPs hybrid arrays exhibit substantial amplification of fingerprint bands of rhodamine 6G (R6G) compared to bare silicon as the reference. Raman signal intensity was found to be dependent on the size of SiNPs, with the largest nanoparticles exhibiting the highest SERS enhancement. In addition, an increase in phosphorous doping concentration was found to reduce R6G peak intensities. AgNPrs/SiNPs hybrid arrays showed excellent stability over time and high spot-to-spot reproducibility as well. Moreover, hybrid arrays enabled DNA detection through intense vibrational modes of human genomic DNA, with a lower detection limit of 1.5 pg/µL; indicating that AgNPrs/SiNPs sensors can serve as a reliable and cost-effective biosensing platform for rapid and label-free analysis of biomolecules.

Possibility of information encoding/decoding using the memory effect in fractional-order capacitive devices.

In this study, we show that the discharge voltage pattern of a supercapacitor exhibiting fractional-order behavior from the same initial steady-state voltage into a constant resistor is dependent on the past charging voltage profile. The charging voltage was designed to follow a power-law function, i.e. [Formula: see text], in which [Formula: see text] (charging time duration between zero voltage to the terminal voltage [Formula: see text]) and p ([Formula: see text]) act as two variable parameters. We used this history-dependence of the dynamic behavior of the device to uniquely retrieve information pre-coded in the charging waveform pattern. Furthermore, we provide an analytical model based on fractional calculus that explains phenomenologically the information storage mechanism. The use of this intrinsic material memory effect may lead to new types of methods for information storage and retrieval.

On the mechanistic pathways of exfoliation-and-deposition of graphene by bipolar electrochemistry.

Amongst the different graphene fabrication techniques, bipolar electrochemistry (BPE) has been recently reported as a simple, controllable, low cost, eco-friendly, and scalable method. It consists of a wirelessly placed carbon source between two feeding electrodes subjected to direct current (DC) voltage in a deionized water bath. Although the physicochemical characteristics of produced graphene have been evaluated, the exfoliation and deposition mechanisms are still unclear. In this study, a novel modified BPE system with an electrically-connected graphite-platinum couple acting as the bipolar electrode has been designed in order to decouple and investigate the contribution of anodic/cathodic exfoliation and deposition of graphene in the BPE process. Electron microscopy and Fourier transform infrared spectroscopy results indicate that both anodic and cathodic exfoliation of graphene could take place regardless of the type of polarization; however, the morphology and deposition rate highly depend on the polarization. Furthermore, the graphene fabricated by anodic exfoliation was found to show higher levels of oxidation compared to the graphene produced by cathodic exfoliation.

Atmospheric pressure air microplasma current time series for true random bit generation.

Generating true random bits of high quality at high data rates is usually viewed as a challenging task. To do so, physical sources of entropy with wide bandwidth are required which are able to provide truly random bits and not pseudorandom bits, as it is the case with deterministic algorithms and chaotic systems. In this work we demonstrate a reliable high-speed true random bit generator (TRBG) device based on the unpredictable electrical current time series of atmospheric pressure air microplasma (APAMP). After binarization of the sampled current time series, no further post-processing was needed in order for the bitstreams to pass all 15 tests of the NIST SP 800-22 statistical test suite. Several configurations of the system have been successfully tested at different sampling rates up to 100 MS/s, and with different inter-electrode distances giving visible/non-visible optical emissions. The cost-effectiveness, simplicity and ease of implementation of the proposed APAMP system compared to others makes it a very promising solution for portable TRBGs.

Structural effects of silver-nanoprism-decorated Si nanowires on surface-enhanced Raman scattering.

Surface enhanced Raman scattering (SERS) is an important analytical tool for the optochemical detection of molecules. The enhancement is commonly achieved by engineering (i) novel types and morphologies of plasmonic nanomaterials, and (ii) patterned or roughened supporting substrates of high surface area for increased light scattering and molecule adsorption. Si substrates can be easily and reproducibly textured for effective SERS applications. In this work, silver nanoprisms (AgNPr) coated silicon nanowire (SiNWs) of different morphologies have been prepared by metal-assisted chemical etching and tested for SERS detection of R6G dye. By varying the etching time from 5 to 30 min, the nanowires' lengths increased from 2.4 to 10.5 µm and resulted in a variable topological morphology of the substrates in terms of bundles and valleys. We found that an optimum of 10 min etching time led to the highest SERS enhancement of R6G on AgNPr/SiNWs at 612 cm-1 Raman shift (30× compared to R6G/Si and 2× compared to R6G/AgNPr/Si), with a detection limit comparable to that of state-of-the-art performances (down to 5×10-10 M of R6G). Such an enhancement is attributed to a middle ground between increased overall surface area of SiNWs, and the available bundle tops trapping the AgNPr and R6G molecules.

Acid-functionalized carbon nanofibers for high stability, thermoelectrical and electrochemical properties of nanofluids.

Carbon-based nanofluids are viewed as promising thermal fluids for heat transfer applications. However, other properties, such as electrical conductivity and electrochemical behavior, are usually overlooked and rarely investigated despite their importance for the overall performance characterization of a given application. In this study, we synthesized PAN-based carbon nanofibers (CNF) by electrospinning, and characterized them using electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, and thermogravimetric analysis. Thermoelectrical and electrochemical measurements were carried out on nanofluids. We found that, although CNF nanofluids exhibit good thermal and electrical properties with a negligible corrosive effect, the suspensions tend to sediment within a few days. However, acid treatment of CNF (F-CNF), which resulted in the shortening of the fibers and the appearance of surface-oxygenated species, made F-CNF-based nanofluids exhibit superior stability in water that extended for more than 90 days, with consistent and superior thermal and electrical properties.

Corrigendum: Reevaluation of Performance of Electric Double-layer Capacitors from Constant-current Charge/Discharge and Cyclic Voltammetry.

This corrects the article DOI: 10.1038/srep38568.

Reevaluation of Performance of Electric Double-layer Capacitors from Constant-current Charge/Discharge and Cyclic Voltammetry.

The electric characteristics of electric-double layer capacitors (EDLCs) are determined by their capacitance which is usually measured in the time domain from constant-current charging/discharging and cyclic voltammetry tests, and from the frequency domain using nonlinear least-squares fitting of spectral impedance. The time-voltage and current-voltage profiles from the first two techniques are commonly treated by assuming ideal RsC behavior in spite of the nonlinear response of the device, which in turn provides inaccurate values for its characteristic metrics [corrected]. In this paper we revisit the calculation of capacitance, power and energy of EDLCs from the time domain constant-current step response and linear voltage waveform, under the assumption that the device behaves as an equivalent fractional-order circuit consisting of a resistance Rs in series with a constant phase element (CPE(Q, α), with Q being a pseudocapacitance and α a dispersion coefficient). In particular, we show with the derived (Rs, Q, α)-based expressions, that the corresponding nonlinear effects in voltage-time and current-voltage can be encompassed through nonlinear terms function of the coefficient α, which is not possible with the classical RsC model. We validate our formulae with the experimental measurements of different EDLCs.

Reduced Graphene Oxide Thin Film on Conductive Substrates by Bipolar Electrochemistry.

Recent years have shown an increased interest in developing manufacturing processes for graphene and its derivatives that consider the environmental impact and large scale cost-effectiveness. However, today's most commonly used synthesis routes still suffer from their excessive use of harsh chemicals and/or the complexity and financial cost of the process. Furthermore, the subsequent transfer of the material onto a substrate makes the overall process even more intricate and time-consuming. Here we describe a single-step, single-cell preparation procedure of metal-supported reduced graphene oxide (rGO) using the principle of bipolar electrochemistry of graphite in deionized water. Under the effect of an electric field between two stainless steel feeder electrodes, grapheme layers at the anodic pole of the wireless graphite were oxidized into colloidal dispersion of GO, which migrated electrophoretically towards the anodic side of the cell, and deposited in the form of rGO (d(002) = 0.395 nm) by van der Waals forces. For substrates chemically more susceptible to the high anodic voltage, we show that the electrochemical setup can be adapted by placing the latter between the wireless graphite and the stainless steel feeder anode. This method is straightforward, inexpensive, environmentally-friendly, and could be easily scaled up for high yield and large area production of rGO thin films.

3D-nanoarchitectured Pd/Ni catalysts prepared by atomic layer deposition for the electrooxidation of formic acid.

Three-dimensionally (3D) nanoarchitectured palladium/nickel (Pd/Ni) catalysts, which were prepared by atomic layer deposition (ALD) on high-aspect-ratio nanoporous alumina templates are investigated with regard to the electrooxidation of formic acid in an acidic medium (0.5 M H2SO4). Both deposition processes, Ni and Pd, with various mass content ratios have been continuously monitored by using a quartz crystal microbalance. The morphology of the Pd/Ni systems has been studied by electron microscopy and shows a homogeneous deposition of granularly structured Pd onto the Ni substrate. X-ray diffraction analysis performed on Ni and NiO substrates revealed an amorphous structure, while the Pd coating crystallized into a fcc lattice with a preferential orientation along the [220]-direction. Surface chemistry analysis by X-ray photoelectron spectroscopy showed both metallic and oxide contributions for the Ni and Pd deposits. Cyclic voltammetry of the Pd/Ni nanocatalysts revealed that the electrooxidation of HCOOH proceeds through the direct dehydrogenation mechanism with the formation of active intermediates. High catalytic activities are measured for low masses of Pd coatings that were generated by a low number of ALD cycles, probably because of the cluster size effect, electronic interactions between Pd and Ni, or diffusion effects.