Keanchuan Lee

Trapping effect of metal nanoparticle mono- and multilayer in the organic field-effect transistor

The effect of silver nanoparticles self-assembled monolayer (Ag NPs SAM) on charge transport in pentacene organic field-effect transistors (OFET) was investigated by both steady-state and transient-state methods, which are current-voltage measurements in steady-state and time-resolved microscopic (TRM) second harmonic generation (SHG) in transient-state, respectively. The analysis of electronic properties revealed that OFET with SAM exhibited significant charge trapping effect due to the space-charge field formed by immobile charges. Lower transient-state mobility was verified by the direct probing of carrier motion by TRM-SHG technique. It was shown that the trapping effect rises together with increase of SAM layers suggesting the presence of traps in the bulk of NP films. The model based on the electrostatic charge barrier is suggested to explain the phenomenon.

Modified transmission-line method for evaluation of the contact resistance: Effect of channel-length-dependent threshold voltage

A modified transmission-line method (TLM) for organic field-effect transistors (OFET) contact resistance extraction is proposed. It is shown that the standard TLM approach can provide even the apparent negative contact resistance due to assumption of linear channel-length-dependence of the channel resistance and constant threshold voltage. This can be corrected by the modified TLM, where effect of channel-length-dependent threshold voltage is included with taking into account the dielectric nature of the active layer of OFETs. Obtained results illustrate the need of the threshold voltage discussion for contact resistance evaluation and demonstrate modified TLM approach as more reliable extraction method.

Contact Resistance as an Origin of the Channel-Length-Dependent Threshold Voltage in Organic Field-Effect Transistors

Here we report a dielectric approach to verify the channel dependence of the threshold voltage in organic field-effect transistors (OFETs). This approach is based on dielectrics physics, and it shows that the potential drop on the injection electrode reduces the capability of applied voltage to accumulate charges that contribute to carrier transport along the channel, which is interpreted as a shift of the threshold voltage. That is, contact resistance is an origin of the channel-length-dependent threshold voltage.

Function of Interfacial Dipole Monolayer in Organic Field Effect Transistors

The effect of interfacial dipole monolayer on carrier injection property of pentacene OFET was studied to understand the origin of contact resistance, which was evaluated from a modified transmission line model. The results showed the contact resistance of pentacene OFET with the dipole monolayer is much higher. This large discrepancy was found due to a small potential drop difference at the interface generated by the dipole monolayer. In addition, the strong local electric field confirmed the reason for the large negative threshold voltage shift. These studies show the importance of local electric field at pentacene–SiO2 interface generated by the dipole monolayer.

Multiple-trapping in pentacene field-effect transistors with a nanoparticles self-assembled monolayer

A silver nanoparticles self-assembled monolayer (SAM) was incorporated in pentacene field-effect transistor and its effects on the carrier injection and transport were investigated using the current-voltage (I − V) and impedance spectroscopy (IS) measurements. The I − V results showed that there was a significant negative shift of the threshold voltage, indicating the hole trapping inside the devices with about two orders higher in the contact resistance and an order lower in the effective mobility when a SAM was introduced. The IS measurements with the simulation using a Maxwell-Wagner equivalent circuit model revealed the existence of multiple trapping states for the devices with NPs, while the devices without NPs exhibited only a single trap state.

Effects of Gold Nanoparticles on Pentacene Organic Field-Effect Transistors

The effect of gold nanoparticles (NPs) on pentacene organic field-effect transistors (OFETs) was being investigated by both DC and AC methods, which are current–voltage (I–V) measurements in steady-state and impedance spectroscopy (IS) respectively. Here poly(vinyl alcohol) (PVA) and PVA blended with Au NPs as composite are spin-coated on SiO2 as gate-insulator for top-contact pentacene OFET. The characteristics of the device were being investigated based on the contact resistance, trapped charges, effective mobility and threshold voltage based on transfer characteristics of OFET. Results revealed that OFET with NPs exhibited larger hysteresis and higher contact resistance at high voltage region. IS measurements were performed and the fitting of results by the Maxwell–Wagner equivalent circuit showed that for device with NPs a series of capacitance and resistance which represents trapping must be introduced in order to have agreeable fitting. The fitting had helped to clarify the reason behind the higher contact resistance and bigger hysteresis which was mainly caused by the space charge field formed by the traps when Au NPs were introduced into the device.

Dipolar electrostatic energy effect on relaxation process of monolayers at air-water interface: Analysis of thermodynamics and kinetics

In order to understand the effect of electrostatic energy on phase transition from monolayer to multilayer, isobaric relaxation process of Langmuir monolayers composed of stearic acid or ferroelectric polyvinylidene fluoride and trifluoroethylene copolymer with various vinylidene fluoride (VDF) ratios is investigated in terms of thermodynamic and kinetic analysis. A monotonous decreasing tendency of material loss with respect to temperature is observed for stearic acid monolayer, which is due to thermal activation effect on phase transition from monolayer to multilayer. In contrast, for the ferroelectric monolayer it presents a nonmonotonous behavior of losing materials with a peak position near the Curie temperature, which is not only owing to thermal activation but also dipole moment change. This observation is confirmed for the copolymer monolayers with other VDF content ratios. Amazingly, for the ferroelectric monolayers a good correspondence is found for critical temperatures evaluated from several independent methods including the analysis on slow collapse. This finding again tells the fact that the relaxation process, namely phase transition from monolayer to multilayer, is greatly influenced by dipolar electrostatic energy. Moreover, the study of time dependent relaxation process reveals a diffusionlike behavior of multilayer structure formation, which cannot be interpreted by classical models. Hence a new model based on diffusion-driven material transfer is proposed and diffusivity of the copolymer molecules is estimated with a value of 0.4x10(-5) cm(2)/s. As a whole, this research reflects the importance of dipolar electrostatic energy for phase transition of monolayers at air-water interface.

Conservation of the injection and transit time ratio in organic field-effect transistors: A thermally accelerated aging study

A thermally accelerated aging for degradation study was used to evaluate the degradation of organic transistor with shallow and deeps traps. A negative threshold voltage shift related to the increase of trapped charge density during the aging was only observed for device with initial shallow traps, while no shift was recorded for devices with deep traps. However, a decrease in the mobility was detected, and an almost constant contact resistance was estimated for both types of devices. Analysis of the potential distribution revealed the conservation of the potential drop on the injection electrode and across the channel region during the entire degradation process. As a result, the ratio of the injection and transit times was conserved and was independent from the shallow traps that were induced by accelerated aging.

Experimental study on electromagnetic-assisted ZnO nanofluid flooding for enhanced oil recovery (EOR)

Recently, nano-EOR has emerged as a new frontier for improved and enhanced oil recovery (IOR & EOR). Despite their benefits, the nanoparticles tend to agglomerate at reservoir conditions which cause their detachment from the oil/water interface, and are consequently retained rather than transported through a porous medium. Dielectric nanoparticles including ZnO have been proposed to be a good replacement for EOR due to their high melting point and thermal properties. But more importantly, these particles can be polarized under electromagnetic (EM) irradiation, which provides an innovative smart Nano-EOR process denoted as EM-Assisted Nano-EOR. In this study, parameters involved in the oil recovery mechanism under EM waves, such as reducing mobility ratio, lowering interfacial tensions (IFT) and altering wettability were investigated. Two-phase displacement experiments were performed in sandpacks under the water-wet condition at 95°C, with permeability in the range of 265–300 mD. A crude oil from Tapis oil field was employed; while ZnO nanofluids of two different particle sizes (55.7 and 117.1 nm) were prepared using 0.1 wt. % nanoparticles that dispersed into brine (3 wt. % NaCl) along with SDBS as a dispersant. In each flooding scheme, three injection sequential scenarios have been conducted: (i) brine flooding as a secondary process, (ii) surfactant/nano/EM-assisted nano flooding, and (iii) second brine flooding to flush nanoparticles. Compare with surfactant flooding (2% original oil in place/OOIP) as tertiary recovery, nano flooding almost reaches 8.5–10.2% of OOIP. On the other hand, EM-assisted nano flooding provides an incremental oil recovery of approximately 9–10.4% of OOIP. By evaluating the contact angle and interfacial tension, it was established that the degree of IFT reduction plays a governing role in the oil displacement mechanism via nano-EOR, compare to mobility ratio. These results reveal a promising way to employ water-based ZnO nanofluid for enhanced oil recovery purposes at a relatively high reservoir temperature.

Impact of pH on zinc oxide particle size by using sol-gel process

Zinc oxide (ZnO) nanoparticles were prepared and synthesized via sol-gel method, by using citric acid as a precursor. The annealing temperature was fixed at 600 °C. The impact of pH on the particle size was investigated. Based on the results from the Thermogravimetric Analysis (TGA), three different pH for the precursor which is 3.0, 5.0 and 1.01 were chosen followed by the characterization of the ZnO nanoparticle by using Powder X-Ray Diffraction (PXRD), Transmission Electron Microscopy (TEM) and Field Emission Scanning Electron Microscopy (FESEM). Results showed that the crystallite size estimated from PXRD increased with the pH value which was hexagonal structure for ZnO. TEM further revealed the same tendency which the Zn NPs size also increased with the alkalinity of the precursor.