Ricky J. Tseng

Patterning organic single-crystal transistor arrays

Field-effect transistors made of organic single crystals are ideal for studying the charge transport characteristics of organic semiconductor materials. Their outstanding device performance, relative to that of transistors made of organic thin films, makes them also attractive candidates for electronic applications such as active matrix displays and sensor arrays. These applications require minimal cross-talk between neighbouring devices. In the case of thin film systems, simple patterning of the active semiconductor layer minimizes cross-talk. But when using organic single crystals, the only approach currently available for creating arrays of separate devices is manual selection and placing of individual crystals—a process prohibitive for producing devices at high density and with reasonable throughput. In contrast, inorganic crystals have been grown in extended arrays, and efficient and large-area fabrication of silicon crystalline islands with high mobilities for electronic applications has been reported. Here we describe a method for effectively fabricating large arrays of single crystals of a wide range of organic semiconductor materials directly onto transistor source–drain electrodes. We find that film domains of octadecyltriethoxysilane microcontact-printed onto either clean Si/SiO2 surfaces or flexible plastic provide control over the nucleation of vapour-grown organic single crystals. This allows us to fabricate large arrays of high-performance organic single-crystal field-effect transistors with mobilities as high as 2.4 cm2 V-1 s-1 and on/off ratios greater than 107, and devices on flexible substrates that retain their performance after significant bending. These results suggest that our fabrication approach constitutes a promising step that might ultimately allow us to utilize high-performance organic single-crystal field-effect transistors for large-area electronics applications.

Digital memory device based on tobacco mosaic virus conjugated with nanoparticles

Nanostructured viruses are attractive for use as templates for ordering quantum dots to make self-assembled building blocks for next-generation electronic devices. So far, only a few types of electronic devices have been fabricated from biomolecules due to the lack of charge transport through biomolecular junctions. Here, we show a novel electronic memory effect by incorporating platinum nanoparticles into tobacco mosaic virus. The memory effect is based on conductance switching, which leads to the occurrence of bistable states with an on/off ratio larger than three orders of magnitude. The mechanism of this process is attributed to charge trapping in the nanoparticles for data storage and a tunnelling process in the high conductance state. Such hybrid bio–inorganic nanostructures show promise for applications in future nanoelectronics.

Charge transfer effect in the polyaniline-gold nanoparticle memory system

A composite system comprised of polyaniline nanofibers bonded with gold nanoparticles is shown to possess a memory effect via a charge transfer mechanism. The charge transfer occurs between the imine nitrogen in the polyaniline and the gold nanoparticles as confirmed by x-ray photoelectron spectroscopy and Raman spectroscopy. This charge transfer enables a bistable electrical conductivity, allowing the material system to be used as a digital memory device. The charge transfer is further confirmed by the elimination of the conductance switching when the fully reduced form of polyaniline, leucoemeraldine, which possesses no imine nitrogens, is used in place of the emeraldine form.

Size Control of Gold Nanoparticles Grown on Polyaniline Nanofibers for Bistable Memory Devices

Controlling reaction temperature for a set time enables the size of gold nanoparticles autoreduced on the surface of polyaniline nanofibers to be controlled. The size of the gold nanoparticles can be used to tune the electrical bistable memory effect in gold/polyaniline nanofiber composite devices. Turn-on voltages and on/off ratios improve with decreasing nanoparticle size, making this a promising method to enhance performance and create smaller devices. Long-term stability of the composites can be improved by the addition of stabilizers following autoreduction of the gold nanoparticles.

Organic Memory Device Fabricated Through Solution Processing

Novel organic memory devices including nonvolatile and write-once-read-many-times memory devices are reported. These devices were fabricated through a simple solution processing technique. Programmable electrical bistability was observed on a device made from a polymer film containing metal nanoparticles capped with saturated alkanethiol and small conjugated organic compounds sandwiched between two metal electrodes. The pristine device, which was in a low-conductivity state, exhibited an abrupt increase of current when the device was scanned up to a few volts. The high-conductivity state can be returned to the low-conductivity state by applying a certain voltage in the reverse direction. The device has a good stability in both states, and the transition from the low- to the high-conductivity state takes place in nanoseconds, so that the device can be used as a low-cost, high-density, high-speed, and nonvolatile memory. The electronic transition is attributed to the electric-field-induced charge transfer between the metal nanoparticles and small conjugated organic molecule. The electrical behavior of the device is strongly dependent on the materials in the polymer film. When gold nanoparticles capped with aromatic thiol were used, the device exhibited a transition from low- to high-conductivity state at the first voltage scan, and the device in the high-conductivity state cannot be returned to the low-conductivity state. This device can be used as a write-once-read-many-times memory device.

Nanoparticle-induced negative differential resistance and memory effect in polymer bistable light-emitting device

Recently, electrical bistability was demonstrated in polymer thin films incorporated with metal nanoparticles [J. Ouyang, C. W. Chu, C. R. Szmanda, L. P. Ma, and Y. Yang, Nat. Mater. 3, 918 (2004)]. In this letter, we show the evidence that electrons are the dominant charge carriers in these bistable devices. Direct integration of bistable polymer layer with a light-emitting polymer layer shows a unique light-emitting property modulated by the electrical bistability. A unique negative differential resistance induced by the charged gold nanoparticles is observed due to the charge trapping effect from the nanoparticles when interfaced with the light-emitting layer.

Highly efficient 7,8,10-triphenylfluoranthene-doped blue organic light-emitting diodes for display application

We have demonstrated an organic light-emitting diode based on blue-fluorescent dopant 7,8,10-triphenylfluoranthene in a host of dipyrenylfluorene derivatives. The device shows pure blue emission with a peak wavelength of 456 nm and Commission International de L’Eclairage coordinate at (0.164, 0.188). An electroluminescence efficiency as high as 3.33cd∕A and external quantum efficiency of 2.48% can be achieved. Comparison of the photoluminescence and electroluminescence spectra reveals a nearly identical exciton relaxation and efficient energy transfer from the host to the dopant.

Organic single-crystal complementary inverter

The authors demonstrate the operation of an organic single-crystal complementary circuit in the form of a simple inverter. The device is constructed from a high mobility p-type organic single-crystal transistor of tetramethylpentacene (TMPC) and a n-type single-crystal transistor of N,N′-di[2,4-difluorophenyl]-3,4,9,10-perylenetetracarboxylic diimide (PTCDI). Field-effect mobilities of up to 1.0cm2∕Vs are reported for TMPC devices, while a mobility of 0.006cm2∕Vs is reported for a n-type PTCDI single-crystal device. Considering that organic single-crystal inverters have not yet been explored, they are representative of potential candidates for use in high-performance complementary circuits.

Microscale memory characteristics of virus-quantum dot hybrids

An electrical multi stability effect was observed for a single layer device fabricated, comprising a hybrid virus-semiconducting quantum dot (CdSe∕ZnS core/shell Qds) assembled onto icosahedral-mutant-virus template (CPMV-T184C). A substrate based bottom-up pathway was used to conjugate two different color emitting Qds for fluorescence visualization and to insert a charging/decharging factor. Pulsed wave measurements depicted distinct conductive states with repeatable and nonvolatile behavior as a functioning memory element.

Multi-layer stackable polymer memory devices

Multi-layer stackable polymer memory architecture is an interesting new direction for polymer memory. The memory density can be increased by increasing the number of stacked layers without reducing the minimum feature size. To achieve multi-level stacking, the polymer used must be able to be cross-linked so that it will not be dissolved upon deposition of additional layers. This requirement also makes the polymer robust enough to withstand conventional lithographic processes. In this paper, the various approaches to achieve cross-linkable polymer memory are discussed. Device fabrication and performance are also reported.