Molecular organization in MAPLE‐deposited conjugated polymer thin films and the implications for carrier transport characteristics

The morphological structure of poly(3‐hexylthiophene) (P3HT) thin films deposited by both Matrix Assisted Pulsed Laser Evaporation (MAPLE) and solution spin‐casting methods are investigated. The MAPLE samples possessed a higher degree of disorder, with random orientations of polymer crystallites along the side‐chain stacking, π–π stacking, and conjugated backbone directions. Moreover, the average molecular orientations and relative degrees of crystallinity of MAPLE‐deposited polymer films are insensitive to the chemistries of the substrates onto which they were deposited; this is in stark contrast to the films prepared by the conventional spin‐casting technique. Despite the seemingly unfavorable molecular orientations and the highly disordered morphologies, the in‐plane charge carrier transport characteristics of the MAPLE samples are comparable to those of spin‐cast samples, exhibiting similar transport activation energies (56 vs. 54 meV) to those reported in the literature for high mobility polymers. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 39–48     

Direct investigations of the interface impedance of organic field-effect transistors with self-assembled-monolayer-modified electrodes

It has been demonstrated that the modification of electrodes with self-assembled monolayers (SAMs) reduces the contact resistance and improves the device performances of organic field-effect transistors (OFETs). However, it has been difficult to judge if the contact resistance was reduced by the change in the electronic properties or by the change in the morphology of the metal–organic interface caused by the SAM modification because they have been difficult to be separately assessed. We have directly investigated the local impedance and the potential difference at the electrode–channel interfaces of the OFETs with and without modification of the electrodes by a pentafluorobenzenethiol SAM using frequency-modulation scanning impedance microscopy (FM-SIM). The potential profile measurement and the FM-SIM measurement at the interface showed that the improvement of the field-effect mobility in the SAM-modified OFET was caused by the reduction of the energy level mismatch, namely, the hole injection barrier at the source–channel interface, presumably with the reduction of the hole trap sites at the source–channel interface.     

Fast detection of blood gases by solution gated organic field effect transistors

We characterize the electrochemical stability of the organic semiconductor Dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) in aqueous solutions. Electrochemical stability of DNTT in solution is validated by cyclic voltammetry and demonstrated by solution gating of DNTT organic field effect transistors (OFETs). Then, we investigate the response time of DNTT OFETs to ammonia, a common blood gas. For bare OFETs, the response time to ammonia is 1–2s only. The exact response time depends on the DNTT film morphology; the fastest response is obtained for pronounced 3D (Volmer-Weber) growth. By comparing OFETs with and without a semipermeable parylene-C encapsulation layer, the influence of the capping on the response time is investigated. An encapsulation layer of 86 nm prolongs the response time to 100s, indicating that parylene-C acts as an efficient diffusion barrier for ammonia.     

A single-molecule diode with significant rectification and negative differential resistance behavior

A series of ferrocenylalkanethiol (HSCnFc) single molecular junctions are modeled and their rectification ratios (RRs) are up to 100 (for HSC11Fc), which agrees with the experiments of Whitesides et al. Not only explanation to the origin of the remarkable large RR is given, but also the reason why one order deviation of RR between HSC11Fc and HSC9Fc is discussed and depicted, which was not pointed out by previous researchers. The single asymmetric accessible molecular orbital (MO) model is evaluated, which is different from the Donor (D)–Acceptor (A) models reported before and a clear negative differential resistance (NDR) behavior is found and explained in the HSC11Fc based device.     

Structural properties and electrical characteristics of high-k Dy2O3 gate dielectrics

This paper describes the structural properties and electrical characteristics of thin Dy₂O₃ dielectrics deposited on silicon substrates by means of reactive sputtering. The structural and morphological features of these films after postdeposition annealing were studied by X-ray diffraction and X-ray photoelectron spectroscopy. It is found that Dy₂O₃ dielectrics annealed at 700 °C exhibit a thinner capacitance equivalent thickness and better electrical properties, including the interface trap density and the hysteresis in the capacitance-voltage curves. Under constant current stress, the Weibull slope of the charge-to-breakdown of the 700 °C-annealed films is about 1.6. These results are attributed to the formation of well-crystallized Dy₂O₃ structure and the reduction of the interfacial SiO₂ layer.     

EDOT‐diketopyrrolopyrrole copolymers for high bulk hole mobility and near infrared absorption

To obtain novel low‐bandgap materials with tailored hole‐transport properties and extended absorption, electron rich 3,4‐ethylenedioxythiophene is introduced as a comonomer in diketopyrrolo[3,4‐c]pyrrole copolymers with different aryl flanking units. The polymers are characterized by absorption and photoluminescence spectroscopy, dynamic scanning calorimetry, cyclic voltammetry, and X‐ray diffraction. The charge transport properties of these new materials are studied carefully using an organic field effect transistor geometry where the charge carriers are transported over a narrow channel at the semiconductor/dielectric interface. These results are compared to bulk charge carrier mobilities using space‐charge limited current (SCLC) measurements, in which the charge carrier is transported through the complete film thickness of several hundred nanometers. Finally, charge carrier mobilities are correlated with the electronic structure of the compounds. We find that in particular the thiophene‐flanked copolymer PDPP[T]₂‐EDOT is a very promising candidate for organic photovoltaics, showing an absorption response in the near infrared region with an optical bandgap of 1.15 eV and a very high bulk hole mobility of 2.9 × 10^?4 cm² V^?1 s^?1 as measured by SCLC. This value is two orders of magnitudes higher than SCLC mobilities reported for other polydiketopyrrolopyrroles and is in the range of the well‐known hole transporting polymer poly(3‐hexylthiophene). © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 639–648     

Potassium Ion-Selective Field-Effect Transistor (ISFET) Sensors: Studies on Gate Dimensions and Ion-Implantation Levels

Abstract

Silicon nitride membrane ISFET sensor chips have been produced with varying gate dimensions. A series of width/length (W/L) aspect ratios have been examined, combined with three levels of boron ion-implant. The level of ion-implantation affects the threshold voltage; this is important as a low threshold voltage allows the use of low noise operating conditions. Gate dimensions are also important factors for they determine the level of drain current for a given gate and drain voltages. A novel design feature, aimed at achieving wide gates, is the use of folded gates as well as having a straight structure. The evaluation of devices with gates covered with poly(vinyl chloride) (PVC)-valinomycin-dioctyl adipate was based on their response to potassium chloride standards when it was shown that there may be a maximum width of gate above which there is no improvement of response. Also, the effect of folding the gate structure is discussed and shown to be tenable, thus permitting greater miniaturisation.     

A 2D Transconductance and Sub-threshold behavior model for triple material surrounding gate (TMSG) MOSFETs

A 2D analytical model for transconductance, Sub-threshold current and Sub-threshold swing for Triple Material Surrounding Gate MOSFET (TMSG) is presented in this paper. Based on the solution of two dimensional Poisson equation, the physics based model of sub-threshold current of the device is derived. The model also includes the effect of gate oxide thickness and silicon thickness on the sub-threshold swing characteristics. Transconductance to drain current ratio of the triple material surrounding gate is calculated since it is a better criterion to access the performance of the device. The effectiveness of TMSG design was scrutinized by comparing with other triple material and dual material gate structures. Moreover the effect of technology parameter variations is also studied and proposed. This proposed model offers basic guidance for design of TMSG MOSFETs. The results of the analytical model are compared with the MEDICI simulation results thus providing validity of the proposed model.     

Off-resonant activation of optical emission

Recent reports have identified a three-wave optically parametric mechanism for the active enhancement of fluorescence using off-resonant radiation. In this Letter it is shown by numerical simulation that the output of a laser system optically pumped just below threshold, can be strongly enhanced by this mechanism, using an ancillary beam of moderate intensity. The electrodynamics and kinetics of the nonlinear optical mechanism are analyzed, model calculations performed, and the output is illustrated graphically. The response demonstrates a novel method for achieving all-optical transistor action.     

Enzymatic sensing with laccase-functionalized textile organic biosensors

Herein we present a textile wearable electrochemical transistor by functionalizing a single cotton yarn with semiconducting polymer. The organic electrochemical transistor (OECT), which is low cost and completely integrated e-textile, is decorated by adsorption of the fungal laccase POXA1b, and is used as biosensor for the direct detection of Tyrosine (L-Tyr) without the use of electron mediators. The detection of Tyr in real-case scenario such as human physiological fluids would own a paramount importance in noninvasive analysis of the patient's condition, monitoring and preventing several pathologies. To assess the reaction progression, the redox process is studied by UV–visible absorption with test reference molecule of 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonate) (ABTS): the results confirmed that the oxidation reaction is driven by the presence of laccase enzyme and direct electron transfer occurred. The modulation of the signal response and the kinetic of the signal is used to detect Tyr molecule in aqueous solution and the role of the enzyme adsorption on the textile is analyzed. A kinetic analysis of the characteristic modulation times of the sensing curves, confirm the sensing properties of the textile device. The textile-based biosensor is demonstrated to monitor human health biomarkers through wearable applications in a non-invasive way, finding potential application in sport, healthcare and working safety.