Changes In Charge Transport Research Articles

Exploring ion mobility mechanisms in poly indolequinone polymers: a case study on black soldier fly melanin

Black soldier fly (BSF) melanin is a new supply of the brown-black pigment eumelanin. Given that eumelanin is a model bioelectronic material for applications such as medical devices and sensors, understanding BSF melanin’s electrical properties is important to confirm its viability as an advanced material. Presented here is a systematic, hydration dependent alternating current study of BSF melanin utilising both H2O and D2O vapours. There is a clear difference between the vapours, enabling a thorough analysis including Nyquist plots with model circuit analysis, broad band dielectric spectroscopic modelling as well as applying the Trukhan model to understand free ion concentration and mobility changes as a function of hydration. We find that BSF melanin behaves similarly to previous reports on synthetic systems, and the analysis here sheds additional light on potential charge transport changes. Significantly, a key finding is that there are two different mobility mechanisms for ion transport depending on hydration.

Design of Promising Uranyl(VI) Complexes Thin Films with Potential Applications in Molecular Electronics.

In this work, it is proposed the development of organic semiconductors (OS) based on uranyl(VI) complexes. The above by means of the synthesis and the characterization of the complexes by Infrared spectroscopy, Nuclear magnetic resonance spectroscopy, mass spectrometry, and X-ray diffraction. Films of these complexes were deposited and subsequently, topographic and structural characterization was carried out by Scanning Electron Microscopy, X-ray diffraction, and Atomic Force Microscopy. Additionally, the nanomechanical evaluation was performed to know the stiffness of uranyl films using their modulus of elasticity. Also, the optical characterization took place in the devices and their bandgap value ranges between 2.40 and 2.93 eV being the minor for the film of the uranyl complex with the N on pyridine in position 4 (2 c). Finally, the electrical behavior of the uranyl(VI) films was evaluated, and important differences were obtained: the uranyl complex with the N on pyridine in position 2 (2 a) film is not influenced by changes in lighting and its current density is in the order of 10-3 A/cm2. The film with uranyl complex with the N on pyridine in position 3 (2 b) and 2 c presents a greater current flow under lighting conditions and two orders of magnitude larger than in film 2 a. In these films 2 b and 2 c, ohmic behavior occurs at low voltages, while at high voltages the charge transport changes to space-charge limited current behavior.

Humidity Sensing with Supramolecular Nanostructures.

Precise monitoring of the humidity level is important for the living comfort and for many applications in various industrial sectors. Humidity sensors have thus become one among the most extensively studied and used chemical sensors by targeting a maximal device performance through the optimization of the components and working mechanism. Among different moisture-sensitive systems, supramolecular nanostructures are ideal active materials for the next generation of highly efficient humidity sensors. Their noncovalent nature guarantees fast response, high reversibility, and fast recovery time in the sensing event. Herein, the most enlightening recent strategies on the use of supramolecular nanostructures for humidity sensing are showcased. The key performance indicators in humidity sensing, including operation range, sensitivity, selectivity, response, and recovery speed are discussed as milestones for true practical applications. Some of the most remarkable examples of supramolecular-based humidity sensors are presented, by describing the finest sensing materials, the operating principles, and sensing mechanisms, the latter being based on the structural or charge-transport changes triggered by the interaction of the supramolecular nanostructures with the ambient humidity. Finally, the future directions, challenges, and opportunities for the development of humidity sensors with performance beyond the state of the art are discussed.

Synergistic charge transfer on n-WO3/p-WS2/Au ternary heterojunction material for solar energy-driven sustainable H2 production

This study investigated the effects of changes in charge transport and photo-quantity on photocatalytic water splitting by a new ternary heterojunction with the structure n-WO3/p-WS2/Au, in which gold (Au), an electron transport medium, was inserted between negative (n)-type tungsten oxide (WO3) plates and a positive (p)-type tungsten sulfide (WS2) sheet. When two particles (n-WO3 and p-WS2) were used individually as catalysts, no water splitting occurred, even after a sufficient lapse of time. However, the decomposition of methyl orange on the n-WO3 single particle and the reduction of Ag+ ions on the p-WS2 single particle occurred favorably. In contrast to the single particle results, with the n-WO3/p-WS2 heterojunction particle, hydrogen was generated at 10.30 μmol g–1 h–1 by water splitting, and with the n-WO3/p-WS2/Au ternary heterojunction particle, more than double this amount of hydrogen was generated. A lower photoluminescence intensity, higher photocurrent density, longer electron/hole recombination time, and faster electron transport rates were possible with the ternary heterojunction particle. The interparticle junction resulted in synergy in the photochemical properties, with the formation of effective redox reaction sites. In particular, the loaded Au acted as an electron trapper that attracted electrons, rather than the surface plasmon resonance effect, thereby expanding the sites for reduction reactions in the particle and improving hydrogen generation. Eventually, it was found that the charge transfer on the n-WO3/p-WS2/Au ternary heterojunction particles progressed by an M-scheme.

Molecular Design of Stretchable Polymer Semiconductors: Current Progress and Future Directions.

Stretchable polymer semiconductors have advanced rapidly in the past decade as materials required to realize conformable and soft skin-like electronics become available. Through rational molecular-level design, stretchable polymer semiconductor films are now able to retain their electrical functionalities even when subjected to repeated mechanical deformations. Furthermore, their charge-carrier mobilities are on par with the best flexible polymer semiconductors, with some even exceeding that of amorphous silicon. The key advancements are molecular-design concepts that allow multiple strain energy-dissipation mechanisms, while maintaining efficient charge-transport pathways over multiple length scales. In this perspective article, we review recent approaches to confer stretchability to polymer semiconductors while maintaining high charge carrier mobilities, with emphasis on the control of both polymer-chain dynamics and thin-film morphology. Additionally, we present molecular design considerations toward intrinsically elastic semiconductors that are needed for reliable device operation under reversible and repeated deformation. A general approach involving inducing polymer semiconductor nanoconfinement allows for incorporation of several other desired functionalities, such as biodegradability, self-healing, and photopatternability, while enhancing the charge transport. Lastly, we point out future directions, including advancing the fundamental understanding of morphology evolution and its correlation with the change of charge transport under strain, and needs for strain-resilient polymer semiconductors with high mobility retention.

Flexible self-assembly carbon nanotube/polyimide thermal film endowed adjustable temperature coefficient of resistance

The development of multi-role flexible thermal films embedded with single-walled carbon nanotubes (SWCNTs) exhibiting an adjustable temperature coefficient of resistance (TCR) is presented. The composite film is prepared by an alternating electric field to assembling CNTs on Ni conductive layer and polyimide. Modified vacuum thermal treatment is then conducted to adjust the TCR behavior of films, thereby gaining the positive, negative and near-zero TCR ranging from −1.5% °C−1 to nearly 1.0% °C−1 at different annealing conditions, respectively. The changes of morphologies, tube crystallinity and chemical elements in films are investigated. The enhanced intertube couplings in bundles of CNTs, formations of chemical bonds and recrystallization in heat-treated films, resulting in the change of charge transport, play a dominant role in the evolution of the TCR behavior. Heat-treated films also exhibit linear temperature dependence and high stability while operating at wide ambient temperature, leading to broad prospects in flexible electronic thermal applications.

Efficient carbon nanotube/polyimide composites exhibiting tunable temperature coefficient of resistance for multi-role thermal films

Thermal films based on carbon nanotube (CNT) composite film have attracted focus in practical applications. This paper develops a highly flexible and multi-role CNT/Polyimide (PI) composite film by employing the dielectrophoresis technique to incorporate bundles of SWCNT into patterned PI substrates. The CNT composite film with a nickel conductive layer exhibits a tunable temperature coefficient of resistivity (TCR) while vacuum thermally treated at diverse temperature. The evolution of the TCR could be controlled from the negative correlation (reached −15000 ppm/°C) to the positive (up to nearly 10000 ppm/°C), which is mainly determined by the reform of the tube connection and covalent bonds in composites due to accumulated thermal energy of annealing, resulting in the change of charge transport. Moreover, the transient response of film devices annealed at different temperature could remain on the order of microseconds proving the robustness of the technology. Demonstrations in human body temperature sensing and micro wind tunnel flow measuring represent excellent sensitivity and reproducibility. The proposed thermal film based on CNT/PI composite with a tunable resistance-temperature characteristic gives a great application potential in intelligent protection electronics, sensing elements and other related thermal applications.

Redox Control of Charge Transport in Vertical Ferrocene Molecular Tunnel Junctions

Summary Controlling charge transport through molecular tunnel junctions is of crucial importance for exploring basic physical and chemical mechanisms at the molecular level and realizing the applications of molecular devices. Here, through a combined experimental and theoretical investigation, we demonstrate redox control of cross-plane charge transport in a vertical gold/self-assembled monolayer (SAM)/graphene tunnel junction composed of a ferrocene-based SAM. When an oxidant/reductant or electrochemical control is applied to the outside surface of the neutral single-layer graphene top electrode, reversible redox reactions of ferrocene groups take place with charges crossing the graphene layer. This leads to counter anions on the outer surface of graphene, which balance the charges of ferrocene cations in the oxidized state. Correspondingly, the junctions switch between a high-conductance, neutral state with asymmetrical characteristics and a low-conductance, oxidized state with symmetrical characteristics, yielding a large on/off ratio (>100).

Ambipolarity and Air Stability of Silicon Phthalocyanine Organic Thin‐Film Transistors

AbstractSilicon phthalocyanines (SiPcs) are a class of conjugated, planar molecule that have recently been investigated for use in organic photovoltaics (OPVs), organic light‐emitting diodes (OLEDs), and organic thin‐film transistors (OTFTs) due to their variable structure and ease of synthesis. Bottom‐gate, bottom‐contact OTFTs with four SiPc derivatives used as the semiconducting layers are prepared using physical vapor deposition. Devices using bis(pentafluorophenoxy) silicon phthalocyanine (F10‐SiPc) deposited on 140 °C substrates demonstrate electron field‐effect mobilities (μ) of up to 0.54 cm2 V−1 s−1, among the best currently reported for N‐type phthalocyanine‐based transistors. All materials show dramatic changes in charge transport when characterized under vacuum (P < 0.1 Pa) compared to in air at atmospheric pressure, typically switching from electron majority charge carriers to holes, with the change dependent on material structure and energetics. F10‐SiPc is close to balanced ambipolar in air, with μ around 5 × 10−3 cm2 V−1 s−1 for both holes and electrons. These results demonstrate SiPcs' potential as N‐type semiconductors in OTFTs as well as their adjustable charge transport as affected by operation environment.

Effect of Alkyl Chain Branching Point on 3D Crystallinity in High N‐Type Mobility Indolonaphthyridine Polymers

AbstractHerein, this study investigates the impact of branching‐point‐extended alkyl chains on the charge transport properties of three ultrahigh n‐type mobility conjugated polymers. Using grazing incidence wide‐angle X‐ray scattering, analysis of the crystallinity of the series shows that while π–π interactions are increased for all three polymers as expected, the impact of the side‐chain engineering on polymer backbone crystallinity is unique to each polymer and correlates to the observed changes in charge transport. With the three polymers exhibiting n‐type mobilities between 0.63 and 1.04 cm2 V−1 s−1, these results ratify that the indolonaphthyridine building block has an unprecedented intrinsic ability to furnish high‐performance n‐type organic semiconductors.

Molecular diodes with rectification ratios exceeding 105 driven by electrostatic interactions.

Molecular diodes operating in the tunnelling regime are intrinsically limited to a maximum rectification ratio R of ∼103. To enhance this rectification ratio to values comparable to those of conventional diodes (R ≥ 105) an alternative mechanism of rectification is therefore required. Here, we report a molecular diode with R = 6.3 × 105 based on self-assembled monolayers with Fc-C≡C-Fc (Fc, ferrocenyl) termini. The number of molecules (n(V)) involved in the charge transport changes with the polarity of the applied bias. More specifically, n(V) increases at forward bias because of an attractive electrostatic force between the positively charged Fc units and the negatively charged top electrode, but remains constant at reverse bias when the Fc units are neutral and interact weakly with the positively charged electrode. We successfully model this mechanism using molecular dynamics calculations.

End-of-Waste SiC-Based Flexible Substrates with Tunable Electrical Properties for Electronic Applications.

We demonstrated the suitability of polymer composites filled with silicon carbide (SiC) powders derived from a recycling process for applications in electronic devices manufacturing. SiC powders have been synthesized from the process byproducts and used as fillers in the formulation of polystyrene (PS)/SiC composites, which have been used in the preparation of substrates using the solution-casting technique. Different substrates have been prepared by changing the concentration of SiC in the composite in the range from 6.7 to 67 wt % and used in simple electronic devices by performing gold contacts in both planar and stacked configurations. The electrical behaviors of both stacked and planar devices were investigated in direct current (DC) and alternate current (AC) regimes. The experimental results showed that charge percolation could be considered an explanation for the abrupt change in the differential conductivity observed around 30 wt %. Fowler-Nordheim tunneling at high fields has been found to be compatible with static characteristics and with high-frequency AC measurements and, therefore, charge tunneling between SiC islands has been proposed as the physical mechanism provoking the changes in charge transport in the substrates investigated. From this first experimental analysis, it appears that SiC/PS composites could suit their use in tunneling-gate dielectrics (i.e., in transistors suitable for their applications in nonvolatile random-access memory) for low concentrations or as a continuous semiconducting media when SiC is dispersed in high-concentration composites.

Suppression of contact-induced spin dephasing in graphene/MgO/Co spin-valve devices by successive oxygen treatments

By successive oxygen treatments of graphene nonlocal spin-valve devices we achieve a gradual increase of the contact-resistance--area products $({R}_{c}A)$ of Co/MgO spin injection and detection electrodes and a transition from linear to nonlinear characteristics in the respective differential $dV\text{\ensuremath{-}}dI$ curves. With this manipulation of the contacts, both spin lifetime and the amplitude of the spin signal can significantly be increased by a factor of seven in the same device. This demonstrates that contact-induced spin dephasing is the bottleneck for spin transport in graphene devices with small ${R}_{c}A$ values. With increasing ${R}_{c}A$ values, we furthermore observe the appearance of a second charge neutrality point (CNP) in gate-dependent resistance measurements. Simultaneously, we observe a decrease of the gate voltage separation between the two CNPs. The strong enhancement of the spin-transport properties as well as the changes in charge transport are explained by a gradual suppression of a Co-graphene interaction by improving the oxide barrier during oxygen treatment.

Organic vapor discrimination with chemiresistor arrays of temperature modulated tin-oxidenanowires and thiolate-monolayer-protected gold nanoparticles

This paper explores the discrimination of organic vapors with arrays of chemiresistors (CRs)employing interface layers of tin-oxide nanowires (NWs) and thiolate-monolayer-protectedgold nanoparticles (MPNs). The former devices use contact-printed mats of NWs onmicro-hotplate membranes to bridge a pair of metal electrodes. Oxidation at the NWsurface causes changes in charge transport, the temperature dependence of which differsamong different vapors, permitting vapor discrimination. The latter devices use solvent castfilms of MPNs on interdigital electrodes operated at room temperature. Sorption into theorganic monolayers causes changes in film tunneling resistance that differ among differentvapors and MPN structures, permitting vapor discrimination. Here, we comparethe performance and assess the ‘complementarity’ of these two types of sensors.Calibrated responses from an NW CR operated at two different temperaturesand from a set of four different MPN CRs were generated for three test vapors:n-hexane, toluene, and nitromethane. This pooled data set was then analyzed usingprincipal components regression classification models with varying degrees ofrandom error superimposed on the responses via Monte Carlo simulation in order toestimate the rates of recognition/discrimination for arrays comprising differentcombinations of sensors. Results indicate that the diversity of most of the dualMPN-CR arrays exceeds that of the dual NW-CR array. Additionally, in assessing allpossible arrays of 4–6 CR sensors, the recognition rates of the hybrid arrays (i.e. MPN + NW) were no better than that of the 4-sensor array containing only MPN CRs.

Surface and subsurface morphology of operating nanowire:fullerene solar cells revealed by photoconductive-AFM

The 3D nanometer scale phase separated morphology of organic solar cells crucially affects performance. We demonstrate that photoconductive atomic force microscopy can provide both surface and subsurface information in operating organic solar cells providing direct correlation between 3D film morphology, local nanoscale optoelectronic properties and device characteristics. P3HT nanowire:PCBM bulk-heterojunction working devices were investigated. The macroscopic solar cell performance improvements upon thermal annealing, such as an increase in the short circuit current, the open circuit voltage and the fill factor, are consistent with observed enrichment of PCBM at the air interface and increased nanowire crystallinity. PC-AFM is able to directly resolve the associated changes in charge transport and collection at the local scale, with an estimated depth resolution of at least 20 nm inside the film.

Controlling charge transport in blue organic light-emitting devices by chemical functionalization of host materials

We report the photophysical characterization, computational results, and device properties for ambipolar phosphine oxide-based host materials that were chemically functionalized to control the charge transport. We study the effects of structural modifications of phosphine oxide hosts on the charge balance in the emissive zone of organic light-emitting devices (OLEDs). Significant changes in charge transport within the emissive layer are observed upon introduction of functional groups, such as pyridine and carbazole, into the organic phosphine oxide host structure. We demonstrate that rational design of host materials allows for the control of charge balance in the emissive zone of OLEDs.

Charge transport in cancer-related genes and early carcinogenesis

The electronic transmission properties of DNA molecules are believed to play a significant role in many physical phenomena taking place in living organisms (Chakraborty, 2007) [1]. Here we study the charge transport (CT) properties of cancer-related genes, including some of the most important tumor suppressors. We find that the changes in averaged CT around the sites of pathogenic and cancerous mutations are statistically smaller than those on sites where pathogenic mutations have not been observed. The results suggest that CT might be an indicator to discriminate between pathogenic and non-pathogenic mutations at an early stage. Mutations which cause little change in CT may be more likely to occur, or more likely to be missed by damage-repair enzymes which probe CT, and are therefore more likely to persist and cause disease.

Au-TiB x -n-6H-SiC Schottky barrier diodes: Specific features of charge transport in rectifying and nonrectifying contacts

Mechanism of charge transport in a diode of a silicon carbide’s Schottky barrier formed by a quasi-amorphous interstitial phase TiBx on the surface of n-6H-SiC (0001) single crystals with an uncompensated donor (nitrogen) concentration of ∼1018 cm−3 and dislocation density of ∼(106–108) cm−2 has been studied. It is demonstrated that, at temperatures T ≲ 400 K, the charge transport is governed by the tunneling current along dislocations intersecting the space charge region. At T > 400 K, the mechanism of charge transport changes to a thermionic mechanism with a barrier height of ∼0.64 eV and ideality factor close to 1.3.

The Changing Face of PEDOT:PSS Films: Substrate, Bias, and Processing Effects on Vertical Charge Transport

Poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) is widely used as a semitransparent anode layer in organic light-emitting diodes and polymer photovoltaics. We use conductive atomic force microscopy (c-AFM) to map the electronic properties of PEDOT:PSS films during a variety of processing steps to better explain how the observed changes in macroscopic electronic properties arise from local changes in charge transport. We observe only small conductive regions, ∼20 nm in size, surrounded by more insulating regions in all of the films studied. We confirm that these features dominate the c-AFM measurements, independent of the substrate. We observe a marked increase in the density of the conductive regions with increasing annealing times, increasing applied bias (independent of polarity), and decreasing PSS concentration (achieved by altering PEDOT:PSS grades). We also find an increase in current flow following a chlorobenzene wash, suggesting the solvents used in processing the active semicon...

Boron Incorporation and Its Effect on Electronic Properties of Ge:H Films Deposited by LF Plasma

ABSTRACTBoron (B) doping of plasma deposited silicon films have been widely studied and applied in many devices, while B-doping of germanium has been poorly reported in literature. We have reported previously about Ge:H films with low density of localized states deposited by LF plasma with optimal hydrogen dilution.This work is devoted to a study of boron incorporation and its effect on electronic properties in Ge:H films. The films were obtained by low frequency (LF) plasma deposition from GeH4+SiH4 +B2H6 mixture diluted with hydrogen. The deposition parameters were as follow: substrate temperature Ts = 300 oC, discharge frequency f= 110 kHz, pressure P= 0.6 Torr, power W= 300 W, germane flow QGeH4= 50 sccm, silane flow, hydrogen flow QH2=3500 sccm, diborane flow was varied in the range of QB2H6=0 to 20 sccm providing boron concentration in gas phase in the range of Y=0 to 4%. Composition of the films was characterized by SIMS profiling. Hydrogen bonding was studied by FTIR. Temperature dependence of conductivity measured in DC regime in vacuum thermostat was employed to study carrier transport. Optical measurements provided optical gap, sub-gap absorption and refraction index. Boron incorporation in solid film demonstrated fast increase in the range of Y = 0 to 1.4% and then increase became slower. Hydrogen concentration in the films was determined by absorption of Ge-H stretching mode at k ≈ 1870 cm−1 and it showed weak increase with change of Y from 0 to 4%. Activation energy of conductivity increased in the range of Y = 0 to 1.5% suggesting a compensation of electron conductivity, reaching maximum value Ea =0.5 eV (corresponding approximately to Eg/2) at Y= 1.5%. Then Ea reduced to minimum value Ea = 0.27 eV at Y= 3.5% showing a trend to saturation with further Y increase. This behavior is related to change of charge transport from electron to intrinsic at Y= 1.5% and further to hole transport.