ute to the electronic transitions lower in energy. [17] This brings the expected reverse-bias turn-on potential into the range of 2.3±1.6 V, which is in reasonable agreement with the experimentally observed potential of 2.0 V. Hence, in reverse bias no current was observed until the bias overcame a potential approximately equal to the difference between the electron affinity of TiO 2 and the ionization potential of PAN, which suggests that a p±n junction was formed at the PAN/TiO 2 interface. shown to exhibit the characteristics of a built-in p±n junction at the TiO 2 /PAN interface. These concentric shell structures were grown on metal nanowires by a simple, low-temperature, adsorption method using a combination of ªin-membraneº and ªon-wireº layer-by-layer assembly. A rectification ratio > 120 has been observed at 3 V, which represents a significant improvement over simpler, ªon-wireº, semiconductor-particle diodes.[8] The characteristics we observed are close to those found in p±n junctions formed by crossing p-and n-type single-crystal Si nanowires [3b] and InP nanowires, [3e] although the rectification ratio is still inferior to that of single-crystal p-Si/n-GaN nanowire cross-point diodes.[3a] A strong similarity in the thickness, morphology, and electrical properties of films grown on metal nanowires and on planar substrates was found. This emphasizes the versatility of the wet, layer-bylayer, adsorption method. In particular, the availability of rectifying tube-shells on metal nanowires, which can be assembled into various kinds of cross-point structures, opens up the possibility of constructing logic gates and more complex circuits by means of electrofluidic, microfluidic, or chemically driven nanowire assembly. One Volt Organic Transistor** By Leszek Artur Majewski, Raoul Schroeder, and Martin Grell* Organic field-effect transistors (OFETs) promise cheap, flexible, and disposable ªplasticº electronics. Performance of organic electronics will be limited by the lower carrier mobility compared to inorganic semiconductors; however, there are a large number of potential OFET applications that require only moderate computing power, but demand low price.A major problem that hinders the development of practical OFET devices is that current devices require rather high voltages to operate, while in a typical low-end application, the COMMUNICATIONS 192
Organic field-effect transistors (OFETs) are one of the current hot topics in the materials science and materials chemistry field.[1±3] Of similar performance, but cheaper than amorphous silicon, [4] OFETs are aimed at disposable, mobile, flexible low-duty applications. Memory elements for OFET applications, such as display driver logic [5,6] or radio frequency identification (RFID) tags are, [7,8] however, limited: capacitor elements for active matrix display logic are volatile and refreshment is current-intensive. First attempts at non-volatile organic memories involve electrets [9] and capacitors integrated in the gate electrode (ªfloating gatesº).[10] These transistor memories, however, have to be charged at high voltages for seconds, and the memory is not truly permanent (timescale: minutes). Inorganic permanent single transistor memories using ferroelectrics [11±15] overcome this for high-quality inorganic electronics, but are not suitable for OFETs due to poor performance of organic semiconductors on them, [16] cost, and incompatibility with flexible substrates, an important advantage of organic transistors. [17,18] In this communication, we present the ªFerrOFETº, an entirely organic memory device. In what results in a huge simplification of the production process, we rely on the discovery by Murata et al. [19] of amorphous polymers that exhibit ferroelectric-like properties. Surprisingly, this class of polymers, a cheap, commercially available nylon poly(m-xylylene adipamide) (MXD6) (see Fig. 1a, with n = 4), is ferroelectric-like in its amorphous state, not in its crystalline state.[20] Ferroelectric-like means that the displacement versus electric field (D±E) hysteresis exhibits the same shape as typical ferroelectric materials but, as an amorphous material, it does not exhibit the thermodynamic phase or crystalline structure associated with ferroelectricity. The polymer does not exhibit a Curie temperature, above which the remanent polarization ceases; instead ferroelectric-like behavior is only observed up COMMUNICATIONS
We report on our latest improvements in organic field‐effect transistors (OFETs) using ultra‐thin anodized gate insulators. Anodization of titanium (Ti) is an extremely cheap and simple technique to obtain high‐quality, very thin (∼ 7.5 nm), pinhole‐free, and robust gate insulators for OFETs. The anodized insulators have been tested in transistors using pentacene and poly(triarylamine) (PTAA) as active layers. The fabricated devices display low‐threshold, normally “off” OFETs with negligible hysteresis, good carrier mobility, high gate capacitance, and exceptionally low inverse subthreshold slope. Device performance is improved via chemical modification of TiO2 with an octadecyltrichlorosilane (OTS) self‐assembled monolayer (SAM). As the result of this combination of favorable properties, we have demonstrated OFETs that can be operated with voltages well below 1 V.
electrospinning station. The details of the electrospinning setup have been described previously [3]. A voltage of 22.5 kV was applied to the solution to start the spinning process, and the electrospun fibers were collected in a random mat of approximately 12 cm 12 cm. A large quantity of PAN nanofibers could be produced within several minutes of processing time.Synthesis of SiC Nanowires from PAN-fiber Templates: The PAN nanofibers were stabilized in air for 30 min at 200 C and carbonized in nitrogen for 1 h at 700 C. Then, the nanofibers were further heated to 1600 C (1.4 C min ±1) and held for 1 h in an alumina tube furnace in Ar. The nanofibers were placed in an alumina crucible together with a mixture of silica and graphite power, which could produce SiO and CO vapors upon heating. After the thermal treatment, SiC fibers with a graphite coating were synthesized, which were then etched in aqua regia for 12 h at room temperature to remove the graphite shell and obtain the SiC nanowires.Characterization: Transmission electron microscopy (TEM) analysis was performed with a JEOL 2010F TEM with a FasTEM control system, operated at 200 kV and with a point-to-point resolution of 0.23 nm. Electron energy loss spectroscopy (EELS) for phase identification was performed with a Gatan parallel EELS spectrometer. The EELS analysis was conducted in the scanning transmission electron microscopy (STEM) mode of the JEOL 2010F microscope with a probe size of 1 nm. X-ray diffraction (XRD) analysis was done on a Geigerflex diffractometer (Rigaku, Japan) using Cu Ka radiation with an accelerating voltage of 35 kV. In recent years, organic field-effect transistor (OFET) technology has evolved from a fairly small field of academic research [1] to a widespread development effort in industry [2±4] and academia. [5,6] While originally the semiconductor, and most importantly, its mobility, were the center of research efforts, more and more efforts are now aimed at overcoming charge-injection problems [7,8] and obtaining better gate insulators [9,10] for the ultimate goalÐlow-voltage, flexible, and cheap chip technology. Already, active-matrix logic circuits, [11] radiofrequency identification, [5] and non-volatile memories [12] have been demonstrated in the prototype state. Now finally, with the introduction of new gate insulators, operation voltages are being reduced.[10]Thus far, reduction of the transistor driving voltages has been linked to the use of high-k (k: dielectric constant), inorganic, metal-oxide films deposited either by sputtering [13] or by using the low-cost, highly viable solution of anodization.[10] With dielectric constants 10 £ k < 300, metal oxides are ideal candidates for making high-capacitance organic transistors capable of lowvoltage operation. However, sputtering is prohibitively expensive, and although anodization is excellent for making largearea, low-leakage, metal oxide gate insulators cheaply, patterning is limited to resist etching, which has the difficulty that some oxides are very resistant to...
The threshold voltage and carrier mobilities were characterized in pentacene-based organic field-effect transistors with gold top-contact electrodes for different thickness of the pentacene film. The thickness of the semiconductor layer influences the values of the threshold voltage and, to a lesser extent, the saturation current. In this letter, we show that the thickness-dependent part of the threshold voltage results from the presence of an injection barrier at the gold–pentacene contact. We also show how the ratio between the gate insulator thickness and the semiconductor layer thickness alter the value for the saturation current, and therefore produces values for the field-effect mobility that are too low.
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