Neil M. Tucker

Low-voltage organic thin-film transistors with π-σ-phosphonic acid molecular dielectric monolayers

Pentacene-based organic thin-film transistors (OTFTs) have been fabricated using π-σ-phosphonic acid self-assembled monolayers (SAMs) on top of aluminum oxide as the gate dielectrics. With ultrathin dielectrics, high capacitances up to 760nF∕cm2 and low leakage current densities of 10−8A∕cm2 at 2V could be obtained, allowing operation of OTFTs within −3V. Vast improvements in the gate leakage current (∼2 orders), on/off current ratio (1 order), and subthreshold slope down to 85mV∕decade are achieved compared to control devices without SAMs. The OTFTs with pentacene vapor deposited at room temperature on SAM dielectrics-modified substrates exhibit mobilities of 0.14–0.30cm2∕Vs, on/off current ratios of 105, and threshold voltages of −(1.3–1.5)V.

Recent progress in developing highly efficient and thermally stable nonlinear optical polymers for electro-optics

Recent development of high-performance nonlinear optical polymers for electro-optics (E-O) is reviewed in this paper. A highly efficient and thermally stable nonlinear optical (NLO) chromophore, namely 2-[4-(2-{5-[2-(4-{Bis-(tert-butyl-dimethyl-silanyloxy)-ethyl]-amino}-phenyl)-vinyl]-thiophen-2-yl}-vinyl)-3-cyano-5-trifluoromethyl-5H-furan-2-ylidene]-malononitrile, has been prepared and incorporated in amorphous polycarbonate (APC) composites. The result from high electric field poling shows a very large E-O coefficient (r33 = 94 pm/V at 1.3 μm), ~80% of which can be maintained at 85 °C for more than 500 hours. In addition to this guest/host sysytem, a high Tg side-chain polymer, derived from a 3-D cardo-type polimide with dendron-encapsulated chromophores as pendent groups has also been synthesized and characterized. A high degree of chromophore dipole orientation and a large r33 of 71 pm/V at 1.3 μm can be achieved in this poled polyimide. More than 90% of its E-O activity can be maintained at 85 °C for more than 600 hours. To access the full potential of poled polymers for device applications, we have developed a new lattice-hardening approach to overcome the “nonlinearity-stability-tradeoff” of conventional thermoset methods. By using the Diels-Alder lattice-hardening process, we can achieve the same high poling efficiency and large r33value as in a guest-host system while maintaining good thermal stability seen in densely-crosslinked polymers. By modifying the electronic properties of the crosslinking reagents, we can fine-tune the processing temperature window of the Diels-Alder reactions to achieve hardened materials with optimal properties.

Solvent-dispersed benzothiadiazole-tetrathiafulvalene single-crystal nanowires and their application in field-effect transistors.

A new organic semiconductor (BT-TTF) based on molecular moieties of benzothiadiazole and tetrathiafulvalene was designed and synthesized, and its structure, molecular packing and charge-transporting properties were determined. Thermal properties, electrochemical behaviors, and optical absorption of this molecule were studied by using differential scanning calorimetry/thermal gravimetric analysis, cyclic voltammetry, and ultraviolet-visible spectroscopy, respectively. Its bulk and nanowire single crystals were prepared and characterized by X-ray crystallography, scanning electron microscopy, transmission electron microscopy, and field-effect transistors. It is found that short intermolecular S···S (3.41 Å), S···C (3.49 Å), and S···N (3.05 Å) contacts define the solid-state structure of BT-TTF single crystals which π-stack along the [100] with interplanar distances of 3.49 Å. Solvent-cast single-crystal nanowire transistors showed mobilities as large as 0.36 cm2/(V s) with current on/off ratios of 1×10(6). This study further illustrates the impact of molecular design and a demonstration of high-performance single-crystal nanowire transistors from the resulting semiconductor.

A novel approach to achieve highly efficient nonlinear optical polymers from guest-host systems

A series of side-chain electrooptic (E-O) polymers have been prepared by Diels-Alder reaction in a solid state and characterized for their nonlinear optical properties. A synthesized chromophore were easily attached to a pendent anthracenyl moiety functionalized on the poly(methylmethacrylate-co-anthrylmethylmethacrylate) thermally in the bulk films during the poling process without compromising E-O performances. We have also controlled a chromophore concentration to determine its critical loading density at which chromophore-chromophore electrostatic interaction occurs in the polymer matrix. The highest E-O coefficient was 110 pm/V for the 34 wt% of the doped chromophore in the polymer at the wavelength of 1.3 μm. A high loading density of chromophore was obtained without observing a severe phase separation in the polymer matrix by an AFM morphology study. This novel approach provides to demonstrate a strategy for developing highly efficient E-O materials with the full potential of a chromophore.

Low-voltage organic electro-optic modulators using transparent conducting oxides as electrodes

In this paper, we present a novel device structure for organic electro-optic modulators using transparent conducting oxides (TCOs) as electrodes to substantially reduce the switching voltage, and describe their fabrication. We report two different types of device geometry, a top conducting and a side conducting geometry, and discuss their strengths and weaknesses. We discuss how the voltage and speed performance of such modulators are dependant on the conductivity/optical loss ratio of the TCO electrodes. Our device simulation shows that by appropriately engineering the high TCO conductivity/optical loss ratio, 4-6x lower switching voltage can be achieved while still maintaining high modulation frequencies and low optical loss. We show that certain new TCO materials are capable of achieving the high conductivity/optical loss required for efficient modulation in the 1300-1550 nm wavelength range. We summarize the optical loss characteristics at 1300 nm of different types of thin-film TCO materials grown using different deposition techniques. TCO electrodes based on different types of materials, such as In2O3, ZnO, and ITO have been investigated for our device structures. Fabrication issues associated with the deposition of TCO electrodes directly on organic EO materials and our approach to addressing them are discussed. Initial results for organic EO modulators fabricated with TCOs as electrodes are presented, and the performance of these modulators are compared with theoretical modeling results. The new device structures presented here will enable next generation low-voltage organic EO modulators targeting RF photonics applications.

Transparent Conducting Oxide (TCO) Electrode Based High-speed Organic Electro-optic (EO) Modulator

A novel organic Mach Zehnder EO modulator using TCO as electrodes has been demonstrated. The simulation results show such structure is suitable for high frequency operation by engineering TCO material conductivity and loss.