qiang Li

High Performance Field‐Effect Ammonia Sensors Based on a Structured Ultrathin Organic Semiconductor Film

High performance organic field-effect transistor (OFET)-based ammonia sensors are demonstrated with ultrathin (4-6 molecular layers) dendritic microstripes of an organic semiconductor prepared via dip-coating. These sensors exhibit high sensitivity, fast response/recovery rate, good selectivity, low concentration detection ability, and reliable reversibility, as well as stability. Such a performance represents great progress in the field of OFET-based sensors.

Controllable Growth and Field-Effect Property of Monolayer to Multilayer Microstripes of an Organic Semiconductor

The controllable growth of partially aligned monolayer to multilayer micrometer stripes was demonstrated by adjusting the pulling speed in a dip-coating process. The number of molecular layers decreases with the increasing pulling speed. A lower pulling speed yields mixed multilayers (3-9 monolayers). It is noteworthy that pure monolayer and bilayer microstripes over large areas can be obtained at high pulling speeds. The stripe morphology strongly depends on the pulling speed or the number of molecular layers. XRD and confocal fluorescence measurements manifest that monolayer stripes are amorphous, while multilayer stripes (> or = 2) consist of crystalline states. FET devices were fabricated on these stripes. Monolayer stripes failed to reveal a field effect due to their amorphous state. In contrast, multilayer stripes exhibit good field-effect behavior. This study provides useful information for future molecular design in controlling molecular architectures. The controllable growth from monolayer to multilayer offers a powerful experimental system for fundamental research into the real charge accumulation and transporting layers for OFETs.

Molecular Orientation and Interface Compatibility for High Performance Organic Thin Film Transistor Based on Vanadyl Phthalocyanine

Organic thin film field-effect transistors (OTFTs) with mobility up to 1.0 cm2 V(-1) s(-1) and on/off ratio of 10(6)-10(8) as well as good environmental stability were demonstrated by using vanadyl phthalocyanine (VOPc), a pyramid-like compound with an ultra closely pi-stacked structure. The high performance, remarkable stability, low price, easy availability and nontoxicity of VOPc enabled it to be a promising candidate for OTFTs. Furthermore, we found that the mobility of the devices on OTS-modified Si/SiO2 substrates was 2 orders of magnitude higher than that of devices on Si/SiO2 substrates. Significantly, the relationship between field effect property and insulator surface property was explained from two new aspects of distribution of molecular orientation and interface compatibility, which might provide not only a useful model to explain why the surface modification with OTS could largely improve the field-effect performance but also a guide for rational optimization of device structure for higher performance. In addition, the field effect property of VOPc devices under vacuum, i.e., the oxygen doping effect on the VOPc devices, was measured. We found that the hole mobility decreased by several orders of magnitude with decreasing pressure. At a pressure below 10(-2) Pa, the device on OTS-modified substrates exhibited ambipolar conduction. These results indicated that the oxygen doping exerted essential effect on the field-effect property of VOPc, which was clearly distinct from that observed for pentacene-based OFETs.

Accurate electrical characterization of forward AC behavior of real semiconductor diode: giant negative capacitance and nonlinear interfacial layer

We have developed a new method to analyze the forward ac behavior of a semiconductor diode that uses a series mode. This method can accurately measure the dependence of series resistance, junction capacitance, junction voltage, ideality factor, and interfacial layer on forward bias voltages. Giant negative capacitance (NC) of the junction and the interfacial layer with nonlinear resistance and capacitance are confirmed in GaN Schottky diodes.

Organic thin-film transistors of phthalocyanines

Organic thin-film field-effect transistors (OTFTs) are emerging as attractive candidates for low-price, large-area, and flexible circuit applications. A variety of organic compounds have been utilized as active semiconductor materials for OTFTs, among which phthalocyanine compounds have attracted considerable attention owing to their remarkable chemical and thermal stability as well as good field-effect performance. Here, we review recent results on the phthalocyanine-based OTFTs. The correlation between the crystal packing structure and the charge transport property is discussed, and we conclude with a description of the future prospects for phthalocyanine-based OTFTs.

Dibenzothiophene derivatives as new prototype semiconductors for organic field-effect transistors

New prototype semiconductor materials based on dibenzothiophene (DBT) derivatives were successfully synthesized by a convergent approach using palladium catalyzed Stille coupling reactions. Thermogravimetric analysis, UV-vis spectra and electrochemistry results indicated these materials had good thermal and photooxidation stability. X-Ray diffraction measurements of the vacuum-evaporated films showed enhanced crystalline order with increasing substrate deposition temperature. The ordered vacuum-evaporated films with charge carrier mobility as high as 7.7 × 10−2 cm2 V−1 s−1 and an on/off ratio of nearly 1 × 107 had been achieved with 3,7-bis(5′-hexyl-thiophen-2′-yl)-dibenzothiophene (3,7-DHTDBTT). These results suggest that the 3,7-substituted DBT system is a good prototype for new type organic semiconductors and will play a more important role in organic semiconductors.

Patterning of polymer electrodes by nanoscratching.

2010 WILEY-VCH Verlag Gmb Growing scientific effort is being devoted to building electronic circuits entirely or partially of organic materials because of their attractive characteristics such as low cost, light weight, and mechanical flexibility. To realize low-cost and high-performance organic transistor circuits for practical applications, utilization of low-cost electrodes (such as conducting polymers) and downscaling the transistor critical feature to the sub-micro/ nanometer scale are two necessary concepts. However, integration of these two strategies, i.e., patterning polymer electrodes with sub-micro/nanometer resolution, remains a great challenge. Here we demonstrate the patterning of the conducting polymer poly(3,4-ethylenedioxythiophene):poly(4-styrenesulphonate) (PEDOT:PSS) (Fig. 1a), an excellent organic electrode material, with 50 nm resolution on both rigid and flexible substrates by atomic force microscopy (AFM) nanoscratching. The scratched grooves show good stability under solvent immersion, heat treatment, and long-term storage in air. This technique can realize small organic transistor electrode pairs (area of 0.5–0.6mm) and a high density electrode array (about 10 elements cm ). The small linewidth of the patterned PEDOT:PSS electrodes yields a parasitic overlap capacitance as low as 0.09 pF mm . The scratched sub-micro/nanometer channel shows excellent performance in organic transistors with high performance and low voltage. Downscaling the size of a single transistor not only allows the realization of high integration density and miniaturization of circuits, but also facilitates improved circuit performance and/or switching speed, as well as lower power consumption. Up to now, organic transistor-based circuits with a relatively fast switching speed (1 KHz to 20MHz) and large integration density ( 2000 transistors) have been demonstrated with metal electrodes defined by high resolution but complicated and expensive photolithography or e-beam lithography. It is widely recognized that the high cost of gold electrodes will overshadow the practical application of organic circuits, while polymer electrodes have a unique ability to fully embody the advantages of organic circuits such as low cost and mechanical flexibility, as well as the ability to achieve higher circuit performance because of their excellent compatibility with organic semiconductors. However, organic transistors/circuits with polymer electrodes such as PEDOT:PSS generally fabricated by volume printing techniques yield a much lower speed of 1–100Hz and require a relatively high voltage of 20–100V, mainly because of the poor resolution (10–50mm) of the direct-printing technique. In addition to mobility, the switching speed of a transistor is inversely proportional to channel length and the overlap linewidth between gate and source/drain electrodes (see Equation (3) in the Experimental section). Therefore, in order to achieve high-performance organic circuits with polymer electrodes, an effective but certainly challenging route is to define polymer electrode materials with submicro/nanometer resolution through a simple and low cost procedure. AFM lithography is a cost-effective and reliable technique for pattering submicro/nanometer-scale structures without complicated steps in comparison with other patterning techniques. To date, there is no report on the use of AFM nanoscratching to structure conducting polymers such as PEDOT:PSS. Figure 1b–d illustrate the schematic process of nanoscratching with a silicon tapping-mode cantilever (Supporting Information 1) in contact mode on a PEDOT:PSS film. Figure 1e shows an exemplified groove formed by nanoscratching, and the groove width is about 50 nm, which is the narrowest PEDOT:PSS groove reported to date. Although a sub-micrometer PEDOT:PSS groove has been achieved for transistor applications by a modified-printing method, this technique requires a prepatterning process based on an original patterningmethod such as photolithography, which will definitely increase the complexity and cost of the manufacturing process. The surface plot (Fig. 2a) and section analysis (Fig. 2b) clearly show the geometry of the multiple grooves. Figure 2c displays the

High-performance and stable organic transistors and circuits with patterned polypyrrole electrodes.

High performance p-/n-type transistors and complementary inverter circuits are demonstrated using patterned polypyrrole (PPY) as pure electrodes. Strikingly, these devices show good stability under continuous operation and long-term storage conditions. Furthermore, PPY electrodes also exhibit good applicability in solution-processed and flexible devices. All these results indicate the great potential of PPY electrodes in solution-processed, all-organic, flexible, transparent, and low-power electronics.

Polymer brush and inorganic oxide hybrid nanodielectrics for high performance organic transistors.

A novel covalence-linked PMMA-SiO(2) hybrid nanodielectrics was prepared by grafting approximately 10 nm PMMA brush onto the SiO(2) (approximately 9 nm) surface, which effectively combines the respective merits of PMMA and SiO(2). As a result, the hybrid nanodielectrics exhibit excellent dielectric performance (e.g., low leakage density (<10(-7) A/cm(2) at 6 MV/cm), high breakdown voltage (7 MV/cm), high capacitance (142 nF/cm(2)), good operational stability, and good compatibility with organic semiconductors), and enable organic field-effect transistors (OFETs) to work with high performance and low voltage. These results may open a way to build ultrathin dielectrics for high performance transistor and circuit, as well as for microelectronics, nanoelectronics, and organic electronics.

Structure Formation by Dynamic Self‐Assembly

This review summarizes the work conducted in the last decade on the fabrication of mesostructured patterns, which have lateral dimensions within the nano- and microscales, over a wafer-scaled size by means of dynamic self-assembly using Langmuir-Blodgett (LB) transfer or dip-coating. First, strategies to form mesostructures from a homogeneous Langmuir monolayer with controlled shape, size, and patterns alignment will be presented, followed by a detailed theoretical explanation of the pattern formation. In addition, the patterning of nanocrystals and other chemicals with LB transfer or other dynamic processes, such as dip-coating, will be summarized.