Experimental modeling of the temperature dependence of the transfer function of rotational motion sensors based on electrochemical transducers

The most important advantage of electrochemical transducers of motion parameters (ECT) is the high steepness of conversion of the mechanical signal to electric current. However, the working characteristics of ECT strongly depend on the temperature so that the use of ECT in precision measuring instruments requires the accurate compensation of temperature variations of its characteristics. A method of temperature measurements is developed aimed at modeling the temperature behavior of the ECT transfer function. A model of temperature behavior of ECT-based rotational motion transducers is elaborated. Results of tests which demonstrate good applicability of this model are shown.     

Synthetic principles directing charge transport in low-band-gap dithienosilole-benzothiadiazole copolymers

Given the fundamental differences in carrier generation and device operation in organic thin-film transistors (OTFTs) and organic photovoltaic (OPV) devices, the material design principles to apply may be expected to differ. In this respect, designing organic semiconductors that perform effectively in multiple device configurations remains a challenge. Following "donor-acceptor" principles, we designed and synthesized an analogous series of solution-processable π-conjugated polymers that combine the electron-rich dithienosilole (DTS) moiety, unsubstituted thiophene spacers, and the electron-deficient core 2,1,3-benzothiadiazole (BTD). Insights into backbone geometry and wave function delocalization as a function of molecular structure are provided by density functional theory (DFT) calculations at the B3LYP/6-31G(d,p) level. Using a combination of X-ray techniques (2D-WAXS and XRD) supported by solid-state NMR (SS-NMR) and atomic force microscopy (AFM), we demonstrate fundamental correlations between the polymer repeat-unit structure, molecular weight distribution, nature of the solubilizing side-chains appended to the backbones, and extent of structural order attainable in p-channel OTFTs. In particular, it is shown that the degree of microstructural order achievable in the self-assembled organic semiconductors increases largely with (i) increasing molecular weight and (ii) appropriate solubilizing-group substitution. The corresponding field-effect hole mobilities are enhanced by several orders of magnitude, reaching up to 0.1 cm(2) V(-1) s(-1) with the highest molecular weight fraction of the branched alkyl-substituted polymer derivative in this series. This trend is reflected in conventional bulk-heterojunction OPV devices using PC(71)BM, whereby the active layers exhibit space-charge-limited (SCL) hole mobilities approaching 10(-3) cm(2) V(-1) s(-1), and yield improved power conversion efficiencies on the order of 4.6% under AM1.5G solar illumination. Beyond structure-performance correlations, we observe a large dependence of the ionization potentials of the polymers estimated by electrochemical methods on polymer packing, and expect that these empirical results may have important consequences on future material study and device applications.     

Influence of magnetic forces on electrochemical mass transport

Enhancements of the order of 100% in the mass transport limited current for electrodeposition have been observed in magnetic fields of order 1 T. The effect of the field is to induce convection in the solution and it is equivalent to rotating the electrode or stirring the solution. In this communication, a quantitative comparison is made of the magnitude of various body forces which have been proposed to account for the experimentally observed effects, with a view to identifying the likely source of the field enhancement. When the magnetic field is uniform, the Lorentz force and the electrokinetic force both contribute significantly to the field enhancement.     

Mechanism of electrochemical charge transport in individual transition metal complexes

We used electrochemical scanning tunneling microscopy (STM) and spectroscopy (STS) to elucidate the mechanism of electron transport through individual pyridyl-based Os complexes. Our tunneling data obtained by two-dimensional electrochemical STS and STM imaging lead us to the conclusion that electron transport occurs by thermally activated hopping. The conductance enhancement around the redox potential of the complex, which is reminiscent of switching and transistor characterics in electronics, is reflected both in the STM imaging contrast and directly in the tunneling current. The latter shows a biphasic distance dependence, in line with a two-step electron hopping process. Under conditions where the substrate/molecule electron transfer (ET) step is dominant in determining the overall tunneling current, we determined the conductance of an individual Os complex to be 9 nS (Vbias = 0.1 V). We use theoretical approaches to connect the single-molecule conductance with electrochemical kinetics data obtained from monolayer experiments. While the latter leave some controversy regarding the degree of electronic coupling, our results suggest that electron transport occurs in the adiabatic limit of strong electronic coupling. Remarkably, and in contrast to established ET theory, the redox-mediated tunneling current remains strongly distance dependent due to the electronic coupling, even in the adiabatic limit. We exploit this feature and apply it to electrochemical single-molecule conductance data. In this way, we attempt to paint a unified picture of electrochemical charge transport at the single-molecule and monolayer levels.     

DNA-mediated charge transport requires conformational motion of the DNA bases: elimination of charge transport in rigid glasses at 77 K

We have proposed that DNA-mediated charge transport (CT) is gated by base motions, with only certain base conformations being CT-active; a CT-active conformation can be described as a domain, a transiently extended pi-orbital defined dynamically by DNA sequence. Here, to explore these CT-active conformations, we examine the yield of base-base CT between photoexcited 2-aminopurine (Ap*) and guanine in DNA in rigid LiCl glasses at 77 K, where conformational rearrangement is effectively eliminated. Duplex DNA assemblies (35-mers) were constructed containing adenine bridges Ap(A)nG (n = 0-4). The yield of CT was monitored through fluorescence quenching of Ap* by G. We find, first, that the emission intensity of Ap* in all DNA duplexes increases dramatically upon cooling and becomes comparable to free Ap*. This indicates that all quenching of Ap* in duplex DNA is a dynamic process that requires conformational motion of the DNA bases. Second, DNA-mediated CT between Ap* and G is not observed at 77 K; rather than hindering the ability of DNA to transport charge, conformational motion is required. Moreover, the lack of DNA-mediated CT at 77 K, even through the shortest bridge, suggests that the static structures adopted upon cooling do not represent optimum CT-active conformations. These observations are consistent with our model of conformationally gated CT. Through conformational motion of the DNA bases, CT-active domains form and break-up transiently, both facilitating and limiting CT.     

Ionic and mass transport in micro-nanofluidic devices: a matter of volumic surface charge

The shape and the surface charge of microchannels are critical parameters for ionic and mass transport in microfluidic systems. A great number of studies and developments have been carried out in order to optimize these features separately. We propose to consider them together within a new fundamental parameter for microfluidics, that we named the Volumic Surface Charge (VSC), which is the ratio of the surface charge to the section height in planar microchannels. The non-linear effects induced by rapid VSC variations can result in selective preconcentration processes, which can be used for a simultaneous preconcentration and separation of biomolecules within simple straight channels. In this review, we first present 3 different techniques that we developed to tune the VSC either by surface chemical patterning, integration of polarisable interfaces or geometrical constrictions. The proof of concept of the selective preconcentration using VSC variations will be presented on the basis of experimental results obtained with fluorescent probes and numerical simulations.     

Correlation between impedance spectroscopy and bubble-induced mass transport in the electrochemical reduction of carbon dioxide

In the electrochemical conversion of carbon dioxide, high currents need to be employed to obtain large production rates, thus implying that mass transport of reactants and products is of crucial importance.This aspect can be investigated by employing a model that depicts the local environment for the reduction reactions. Simultaneously, electrochemical impedance spectroscopy, despite being a versatile technique, has rarely been adopted for studying the mass transport features during the carbon d...     

Low molecular mass organogelator based gel electrolyte with effective charge transport property for long-term stable quasi-solid-state dye-sensitized solar cells

Stable quasi-solid-state dye-sensitized solar cells (DSC) were fabricated using 12-hydroxystearic acid as a low molecular mass organogelator (LMOG) to form gel electrolyte. TEM image of the gel exhibited the self-assembled network constructed by the LMOG, which hindered flow and volatilization of the liquid. The formation of less-mobile polyiodide ions such as I 3 (-) and I 5 (-) confirmed by Raman spectroscopy increased the conductivity of the gel electrolytes by electronic conduction process, which should be rationalized by the Grotthuss-type electron exchange mechanism caused by rather packed polyiodide species in the electrolytes. The results of the accelerated aging tests showed that the gel electrolyte based dye-sensitized solar cell could retain over 97% of its initial photoelectric conversion efficiency value after successive heating at 60 degrees C for 1000 h and device degradation was also negligible after one sun light soaking with UV cutoff filter for 1000 h.     

A new parameter to control heat transport in nanofluids: surface charge state of the particle in suspension

Although various conjectures have been proposed to explain the abnormal increase in thermal conductivity of nanofluids, the detailed mechanism has not been fully understood and explained. The main reason is due to the lack of knowledge of the most fundamental factor governing the mechanisms such as Brownian motion, liquid layering, phonon transport, surface chemical effects, and agglomeration. Applying a surface complexation model for the measurement data of hydrodynamic size, zeta potential, and thermal conductivity, we have shown that surface charge states are mainly responsible for the increase in the present condition and may be the factor incorporating all the mechanisms as well.     

A fluorescence aptasensor based on DNA charge transport for sensitive protein detection in serum

A novel fluorescence aptasensor based on DNA charge transport for sensitive protein detection has been developed. A 15nt DNA aptamer against thrombin was used as a model system. The aptamer was integrated into a double strand DNA (dsDNA) that was labeled with a hole injector, naphthalimide (NI), and a fluorophore, Alexa532, at its two ends. After irradiation by UV light, the fluorescence of Alexa532 was bleached due to the oxidization of Alexa532 by the positive charge transported from naphthalimide through the dsDNA. In the presence of thrombin, the binding of thrombin to the aptamer resulted in the unwinding of the dsDNA into ssDNA, which led to the blocking of charge transfer and the strong fluorescence emission of Alexa532. By monitoring the fluorescence signal change, we were able to detect thrombin in homogeneous solutions with high selectivity and high sensitivity down to 1.2 pM. Moreover, as DNA charge transfer is resistant to interferences from biological contexts, the aptasensor can be used directly in undiluted serum with similar sensitivity as that in buffer. This new sensing strategy is expected to promote the exploitation of aptamer-based biosensors for protein assays in complex biological matrixes.     

Structural,ion transport parameter and electrochemical properties of plasticized polymer composite electrolyte based on PVA: A novel approach to fabricate high performance EDLC devices

In this study a novel polymer composite electrolytes (PCEs) based on poly (vinyl alcohol) (PVA): Ce(III)-complex:NH₄SCN plasticized with glycerol are prepared by solution cast technique. XRD and FTIR routes are used to study the film structure. The crystalline and amorphous areas are determined through the deconvolution of XRD spectra and their values were used to calculate the degree of crystallinity. The deconvolutions of the FTIR of asymmetric C≡N stretching mode are carried out to establish the bands coupled with free ions, contact ion pairs and ion aggregates. The maximum ambient temperature DC conductivity of 2.07 × 10⁻³ S cm⁻¹ is recorded for the sample with the lowest degree of crystallinity. It was found that the number density (n), mobility (μ) and diffusion coefficient (D) of ions are increased with the glycerol concentration. Field emission scanning electron microscopy (FESEM) is used to examine the effect of plasticizer on film morphology. The DC conductivity trend is interpreted in detail with the help of dielectric properties. It is found that the transference numbers of ions (t_ion) and electrons (t_el) are 0.965 and 0.035, respectively. It is shown by the linear sweep voltammetry (LSV) that the potential window of the PCE is 2.1 V. A shape, which is nearly rectangular at lower scan rates, is identified from cyclic voltammetry (CV). Specific capacitance and energy density are exhibited by EDLC with average of 161.5 F/g and 18.17 Wh/kg, respectively within 400 cycles. The initial power density is shown by EDLC to be 2.825 × 10³ W/kg.     

Frontiers in first principles modelling of electrochemical simulations

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Generation and transport of charge carriers in polyvinylcarbazole

The processes of generation and transport of excess charge carriers in polyvinylcarbazole at room temperature and at 353 K were experimentally studied. The polymer was charged with pulses of electrons at energies of 7 and 50 keV, which differed in both linear energy transfer and track structure. A universal method of investigation based on the combination of the time-of-flight technique in the both (surface and bulk) modes with the measurement of radiation-induced conductivity was used. The radiation-chemical yield of free charges was measured using two independent procedures. It was shown that the radiation chemical yield of free charges at 293 K was somewhat smaller for 7-keV than for 50-keV electrons; in the latter case, G = 1.1 in an electric field of 2 × 10⁷ V/m. The parameters of the generalized physical model were determined for polyvinylcarbazole.     

Photogeneration and transport of charge carriers in polysilylenes

The attachment of π-conjugated chromophores that absorb the radiation with long wavelengths to poly(methylphenylsilylene) ( 1 ) via reactions of its formylated derivative is described. Some of the polymers obtained show improved photostability and higher quantum photogeneration efficiency in comparison with the parent polymer. Photoconductive ultra-thin layers can be prepared from polar derivatives of ( 1 ) by the Langmuir–Blodgett technique.     

Effects of the photooxidant on DNA-mediated charge transport

A direct comparison of DNA charge transport (CT) with different photooxidants has been made. Photooxidants tested include the two metallointercalators, Rh(phi)(2)(bpy')(3+) and Ru(phen)(bpy')(dppz)(2+), and three organic intercalators, ethidium (Et), thionine (Th), and anthraquinone (AQ). CT has been examined through a DNA duplex containing an A(6)-tract intervening between two 5'-CGGC-3' sites with each of the photooxidants covalently tethered to one end of the DNA duplex. CT is assayed both through determination of the yield of oxidative guanine damage and, in derivative DNA assemblies, by analysis of the yield of a faster oxidative trapping reaction, ring opening of N(2)-cyclopropylguanine (d(CP)G) within the DNA duplex. We find clear differences in oxidative damage ratios at the distal versus proximal 5'-CGGC-3' sites depending upon the photooxidant employed. Importantly, nondenaturing gel electrophoresis data demonstrate the absence of any DNA aggregation by the DNA-bound intercalators. Hence, differences seen with assemblies containing various photooxidants cannot be attributed to differential aggregation. Comparisons in assemblies using different photooxidants thus reveal characteristics of the photooxidant as well as characteristics of the DNA assembly. In the series examined, the lowest distal/proximal DNA damage ratios are obtained with Ru and AQ, while, for both Rh and Et, high distal/proximal damage ratios are found. The oxidative damage yields vary in the order Ru > AQ > Rh > Et, and photooxidants that produce higher distal/proximal damage ratios have lower yields. While no oxidative DNA damage is detected using thionine as a photooxidant, oxidation is evident using the faster cyclopropylguanosine trap; here, a complex distance dependence is found. Differences observed among photooxidants as well as the complex distance dependence are attributed to differences in rates of back electron transfer (BET). Such differences are important to consider in developing mechanistic models for DNA CT.     

Theoretical study on charge transport of quinacridone polymorphs

The charge carrier transporting ability in the polymorphism of quinacridone (QA) has been studied using density‐functional theory and Marcus charge transport theory. The theoretical results indicated quinacridone has good electron transport ability and electron mobilities of all the polymorphism are at 10^?2 magnitude. But its hole mobility, which varied with the different molecular packing, is at range of 10^?1–10^?3 magnitude. The difference of charge carrier mobilities among the polymorphism is originated from the different packing mode. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Electrochemistry and charge transport in sporopollenin particle arrays

Electrochemical oxidation of hollow sporopollenin particles immobilised on an electrode surface is investigated in aqueous acidic solution. Redox activity is demonstrated to occur via a mixture of 2e^?–2H⁺ and 2e^?–1H⁺ processes, likely due to the oxidation of conjugated phenolics embedded within the surface-shell of the polymer particles. Charge transport over the surface is suggested to be fast based on comparison with an approximate physicochemical model.     

Substituent effects on charge transport in films of Au nanocrystals

Charge transport (CT) in films of arylthiol-capped Au nanocrystals (NCs) exhibits strong substituent effects, with electron-donating substituents markedly decreasing conductivity. Films suited for measurements were obtained by ligand-exchange reactions on AuNCs grown at the water/toluene interface. Detailed analysis suggests the NCs interact with the ligands by resonance rather than inductive effects. The films were characterized by TEM, SEM, XPS, UV/vis, and AFM. CT characteristics were studied between 15 and 300 K.     

Influence of functional groups on charge transport in molecular junctions

Using density functional theory (DFT), we analyze the influence of five classes of functional groups, as exemplified by NO(2), OCH(3), CH(3), CCl(3), and I, on the transport properties of a 1,4-benzenedithiolate (BDT) and 1,4-benzenediamine (BDA) molecular junction with gold electrodes. Our analysis demonstrates how ideas from functional group chemistry may be used to engineer a molecule's transport properties, as was shown experimentally and using a semiempirical model for BDA [Nano Lett. 7, 502 (2007)]. In particular, we show that the qualitative change in conductance due to a given functional group can be predicted from its known electronic effect (whether it is sigma/pi donating/withdrawing). However, the influence of functional groups on a molecule's conductance is very weak, as was also found in the BDA experiments. The calculated DFT conductances for the BDA species are five times larger than the experimental values, but good agreement is obtained after correcting for self-interaction and image charge effects.     

Zero-point fluctuations in naphthalene and their effect on charge transport parameters

We calculate the effect of vibronic coupling on the charge transport parameters in crystalline naphthalene, between 0 and 400 K. We find that nuclear fluctuations can cause large changes in both the energy of a charge on a molecule and on the electronic coupling between molecules. As a result, nuclear fluctuations cause wide distributions of both energies and couplings. We show that these distributions have a small temperature dependence and that, even at high temperatures, vibronic coupling is dominated by the effect of zero-point fluctuations. Because of the importance of zero-point fluctuations, we find that the distributions of energies and couplings have substantial width, even at 0 K. Furthermore, vibronic coupling with high energy modes may be significant, even though these modes are never thermally activated. Our results have implications for the temperature dependence of charge mobilities in organic semiconductors.