Mark E. Roberts

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.

Self-Sorted, Aligned Nanotube Networks for Thin-Film Transistors

To find use in electronics, single-walled carbon nanotubes need to be efficiently separated by electronic type and aligned to ensure optimal and reproducible electronic properties. We report the fabrication of single-walled carbon nanotube (SWNT) network field-effect transistors, deposited from solution, possessing controllable topology and an on/off ratio as high as 900,000. The spin-assisted alignment and density of the SWNTs are tuned by different surfaces that effectively vary the degree of interaction with surface functionalities in the device channel. This leads to a self-sorted SWNT network in which nanotube chirality separation and simultaneous control of density and alignment occur in one step during device fabrication. Micro-Raman experiments corroborate device results as a function of surface chemistry, indicating enrichment of the specific SWNT electronic type absorbed onto the modified dielectric.

Water-stable organic transistors and their application in chemical and biological sensors.

The development of low-cost, reliable sensors will rely on devices capable of converting an analyte binding event to an easily read electrical signal. Organic thin-film transistors (OTFTs) are ideal for inexpensive, single-use chemical or biological sensors because of their compatibility with flexible, large-area substrates, simple processing, and highly tunable active layer materials. We have fabricated low-operating voltage OTFTs with a cross-linked polymer gate dielectric, which display stable operation under aqueous conditions over >104 electrical cycles using the p-channel semiconductor 5,5′-bis-(7-dodecyl-9H-fluoren-2-yl)-2,2′-bithiophene (DDFTTF). OTFT sensors were demonstrated in aqueous solutions with concentrations as low as parts per billion for trinitrobenzene, methylphosphonic acid, cysteine, and glucose. This work demonstrates of reliable OTFT operation in aqueous media, hence opening new possibilities of chemical and biological sensing with OTFTs.

Fabrication of low-cost electronic biosensors

The fabrication of miniaturized, low-cost, flexible sensors based on organic electronics via high-throughput techniques (e.g. printing) is expected to provide important benefits for applications in chemical and biological detection. The rapid maturation of synthetic methodology in the field of organic electronics has lead to the creation of new materials at an incredible rate and an increased understanding of semiconductor-analyte interactions. Owing to these advances, we have seen steady improvements in sensitivity, stability, and specificity, in addition to the detection of a wide range of chemical analytes. In this review, we address the fabrication, challenges, and sensor performance of organic transistor-based detection devices with an outlook toward developing sensors capable of operating in biologically relevant media.

Material and device considerations for organic thin-film transistor sensors

The rapid development of the field of organic electronics has sparked great interest in the use of organic thin-film transistors (OTFTs) as low-cost electronic sensors. The direct coupling of the electronic and the sensor media to provide real time electrical output has already demonstrated high sensitivity to a variety of chemical species. The synthetic versatility of organic materials also provides endless routes to impart functionality for specifically targeted chemical interactions. Owing to their compatibility with flexible materials and simple fabrication methods, OTFTs are poised to have a tremendous impact on future portable detection technology. This article reviews recent progress made toward improved sensitivity, selectivity and stability of OTFT sensors through material and device engineering. Specific consideration is paid to the interaction of the electronic materials with the analytes as a means of providing insight into mechanistic principles as well as the future direction of OTFTs.

In Situ, Label-Free DNA Detection Using Organic Transistor Sensors

Rapid and highly sensitive PNA-DNA hybridization assays have attracted enormous attention for a wide variety of applications ranging from genotyping to molecular diagnosis. [ 1 , 2 ] Conventional optical detection systems based on microarrays and real-time PCR involve expensive detection protocols, typically requiring a fl uorescent dye and optical sources/detectors; however, this method has become the standard technique for quantifying the extent of hybridization between surface immobilized probes and fl uorophore-labeled DNA targets. Recent advances in chemical detection research, in part benefi ting from the overwhelming progress made in organic electronics, have shown great promise for a viable, low-cost alternative to current optical detection systems. [ 3 , 4 ] The utilization of organic transistor technology in chemical sensors is particularly encouraging. This simple platform allows for the fabrication of low-cost, large-area, and fl exible devices with air stability, low-power consumption, biocompatibility, and facile surface modifi cation for the detection of a wide range of analyte species. [ 5 , 6 ] Many examples exist for the detection of analyte vapors using an OTFT platform, with numerous reports addressing the ability to identify particular analytes either through the use of a fi ngerprint response [ 7 , 8 ] or by incorporating selective detection layers on functional OTFTs. [ 4 , 9 ] Few examples of chemical detection in aqueous systems have been demonstrated; however, these devices were not selective toward a particular analyte. [ 3 , 10 ] Selective in situ detection with OTFTs requires a versatile method for the immobilization of various selective molecular probes within proximity to the active transport channel. Here, we report a real-time, in situ selective detection scheme for short-chain DNA targets by employing organic transistors as the electrical read-out platform. The surfaces of the OTFTs were modifi ed with a thin maleic anhydride (MA) polymer layer

Sorted and aligned single-walled carbon nanotube networks for transistor-based aqueous chemical sensors.

Detecting trace amounts of analytes in aqueous systems is important for health diagnostics, environmental monitoring, and national security applications. Single-walled carbon nanotubes (SWNTs) are ideal components for both the sensor material and active signal transduction layer because of their excellent electronic properties and high aspect ratio consisting of entirely surface atoms. Submonolayer arrays, or networks of SWNTs (SWNTnts) are advantageous, and we show that topology characteristics of the SWNT network, such as alignment, degree of bundling, and chirality enrichment strongly affect the sensor performance. To enable this, thin-film transistor (TFT) sensors with SWNTnts were deposited using a one-step, low-cost, solution- based method on a polymer dielectric, allowing us to achieve stable low-voltage operation under aqueous conditions. These SWNT-TFTs were used to detect trace concentrations, down to 2 ppb, of dimethyl methylphosphonate (DMMP) and trinitrotoluene (TNT) in aqueous solutions. Along with reliable cycling underwater, the TFT sensors fabricated with aligned, sorted nanotube networks (enriched with semiconductor SWNTs) showed a higher sensitivity to analytes than those fabricated with random, unsorted networks with predominantly metallic charge transport.

High-performance microscale single-crystal transistors by lithography on an elastomer dielectric

Organic single crystals have emerged as powerful tools for the exploration of the intrinsic charge transport properties of organic materials. To date, however, the limited number of fabrication techniques has forced a steep compromise between performance, reproducibility, range of feature sizes, gentle treatment of the single crystal, and facility of construction. Here the authors present a materials-general technique for the fabrication of single-crystal field-effect transistors with the use of a spin-coated elastomer gate dielectric and photolithographically defined source and drain electrodes. This allows the production of feature sizes and patterns previously impossible with reported elastomeric techniques yet yields devices with performance far superior to those fabricated on nonconformal dielectrics. The authors measure saturation-regime mobilities of 19.0 and 1.9cm2∕Vs for the semiconductors rubrene and pentacene, comparable to the best published values, and 2.4cm2∕Vs for tetracene, nearly double t...

Solution Assembly of Organized Carbon Nanotube Networks for Thin-Film Transistors

Ultrathin, transparent electronic materials consisting of solution-assembled nanomaterials that are directly integrated as thin-film transistors or conductive sheets may enable many new device structures. Applications ranging from disposable autonomous sensors to flexible, large-area displays and solar cells can dramatically expand the electronics market. With a practical, reliable method for controlling their electronic properties through solution assembly, submonolayer films of aligned single-walled carbon nanotubes (SWNTs) may provide a promising alternative for large-area, flexible electronics. Here, we report SWNT network TFTs (SWNTntTFTs) deposited from solution with controllable topology, on/off ratios averaging greater than 10(5), and an apparent mobility averaging 2 cm(2)/V.s, without any pre- or postprocessing steps. We employ a spin-assembly technique that results in chirality enrichment along with tunable alignment and density of the SWNTs by balancing the hydrodynamic force (spin rate) with the surface interaction force controlled by a chemically functionalized interface. This directed nanoscale assembly results in enriched semiconducting nanotubes yielding excellent TFT characteristics, which is corroborated with mu-Raman spectroscopy. Importantly, insight into the electronic properties of these SWNT networks as a function of topology is obtained.

Induced Sensitivity and Selectivity in Thin‐Film Transistor Sensors via Calixarene Layers

The field of organic electronics holds tremendous potential for applications in solar cells (organic photovoltaics, OPV), radiofrequency identification tags (RFID), lighting or displays (organic light-emitting diodes, OLED), as well as sensors. These applications benefit from the use of organic materials, which are amendable to very low cost, flexible, and large-area processing techniques (e.g., roll-to-roll printing). Specifically, the design and development of sensors that take advantage of these benefits can lead to manufacturing of cheap, disposable electronic units for medicinal, food storage, and environmental monitoring applications. The ability to couple the sensory electrical output with on-chip signal processing can overcome the need for bulky, expensive equipment typically required for most optical detection methods. In order to attain commercial viability, chemical sensors based on organic electronics must continue to address the remaining issues in repeatability, reproducibility, stability, and selectivity. It is well known that organic thin-film transistors (OTFTs) can experience a modulation in current upon exposure of the active organic semiconductor layer to the ambient environment or an analyte stream. This modulation occurs as the analytes interact with the transport region of a field-effect transistor (FET), typically the first few layers of the film, or within the grain boundaries formed during thin-film growth. The analytes may influence thin-film conductivity as either dopants or traps for charge carriers, through bulk resistive effects on inter-grain transport, or through accumulated electrostatic dipole interactions. Materials typically associated with high-performance electronic devices are based on simple polyaromatic structures and do not possess inherent sites that promote specific sensory response. Additionally, the sensor response critically depends on the film properties (e.g., functional groups, grain boundaries). Thus far, two strategies for enhancing the selectivity of FET sensors have been investigated. The first is based on formation of non-specific chemical-fingerprint arrays and the second uses side-group functionality within the semiconductor backbone to elicit selective responses to a target analyte. The chains generally diminish the semiconductor performance; therefore, the use of bilayer architectures, or a combination of two semiconductor materials has been investigated to circumvent the performance limitation. Organic electronics face many challenges that must be addressed before achieving commercial viability, with reproducibility and stability especially critical for chemical sensors. While excellent device reproducibility can be obtained through careful optimization of process conditions in large-scale production, this can be costly for each individually functionalized sensor layer. To overcome this limitation, we have developed a general strategy that induces analyte sensitivity into any existing OTFT. Our strategy involves incorporating calixarene container molecules onto the surface of an existing OTFT. High-purity single crystals of calix[n]arene architectures have been grown via sublimation to provide materials for selective molecular uptake. The molecules also show high environmental and aqueous stability as demonstrated via gasand analyte-sorption studies. The tunable nature of the container cavity serves to trap desired molecular shapes and functionalities with typically high selectivity. The burgeoning interest in host-guest materials has given rise to exceptionally complex singleand multicomponent self-assembled structures, and such materials have been incorporated into sensors based on fluorescence, surface acoustic wave (SAW), and quartz crystal microbalance (QCM) and polymer-coated chemiresistors. In this article, we report the use of ultra-thin films of container molecules as sensory layers on an existing OTFT, resulting in both enhanced sensitivity as well as shape discrimination between vapors of small-molecule analytes. The container molecules evaluated in this report for thin-film modification were calix[8]arene (C[8]A) and C-methylcalix[4]resorcinarene (CM[4]RA) (Fig. 1 a,b). Molecules based on