Effects of Water and Different Solutes on Carbon‐Nanotube Low‐Voltage Field‐Effect Transistors

Semiconducting single‐walled carbon nanotubes (swCNTs) are a promising class of materials for emerging applications. In particular, they are demonstrated to possess excellent biosensing capabilities, and are poised to address existing challenges in sensor reliability, sensitivity, and selectivity. This work focuses on swCNT field‐effect transistors (FETs) employing rubbery double‐layer capacitive dielectric poly(vinylidene fluoride‐co‐hexafluoropropylene). These devices exhibit small device‐to‐device variation as well as high current output at low voltages (<0.5 V), making them compatible with most physiological liquids. Using this platform, the swCNT devices are directly exposed to aqueous solutions containing different solutes to characterize their effects on FET current–voltage (FET I–V) characteristics. Clear deviation from ideal characteristics is observed when swCNTs are directly contacted by water. Such changes are attributed to strong interactions between water molecules and sp²‐hybridized carbon structures. Selective response to Hg²⁺ is discussed along with reversible pH effect using two distinct device geometries. Additionally, the influence of aqueous ammonium/ammonia in direct contact with the swCNTs is investigated. Understanding the FET I–V characteristics of low‐voltage swCNT FETs may provide insights for future development of stable, reliable, and selective biosensor systems.     

High‐Performance Partially Aligned Semiconductive Single‐Walled Carbon Nanotube Transistors Achieved with a Parallel Technique

Single‐walled carbon nanotubes (SWNTs) are widely thought to be a strong contender for next‐generation printed electronic transistor materials. However, large‐scale solution‐based parallel assembly of SWNTs to obtain high‐performance transistor devices is challenging. SWNTs have anisotropic properties and, although partial alignment of the nanotubes has been theoretically predicted to achieve optimum transistor device performance, thus far no parallel solution‐based technique can achieve this. Herein a novel solution‐based technique, the immersion‐cum‐shake method, is reported to achieve partially aligned SWNT networks using semiconductive (99% enriched) SWNTs (s‐SWNTs). By immersing an aminosilane‐treated wafer into a solution of nanotubes placed on a rotary shaker, the repetitive flow of the nanotube solution over the wafer surface during the deposition process orients the nanotubes toward the fluid flow direction. By adjusting the nanotube concentration in the solution, the nanotube density of the partially aligned network can be controlled; linear densities ranging from 5 to 45 SWNTs/μm are observed. Through control of the linear SWNT density and channel length, the optimum SWNT‐based field‐effect transistor devices achieve outstanding performance metrics (with an on/off ratio of ~3.2 × 10⁴ and mobility 46.5 cm²/Vs). Atomic force microscopy shows that the partial alignment is uniform over an area of 20 × 20 mm² and confirms that the orientation of the nanotubes is mostly along the fluid flow direction, with a narrow orientation scatter characterized by a full width at half maximum (FWHM) of <15° for all but the densest film, which is 35°. This parallel process is large‐scale applicable and exploits the anisotropic properties of the SWNTs, presenting a viable path forward for industrial adoption of SWNTs in printed, flexible, and large‐area electronics.     

Cover Picture: High‐Performance Organic Single‐Crystal Transistors on Flexible Substrates (Adv. Mater. 17/2006)

The cover shows a comparison of thin and thick rubrene single crystals where the flexibility of the thin rubrene crystals is clearly illustrated. On p. 2320, Yang, Bao, and co‐workers report that high performance flexible transistors on plastic substrates fabricated by using these rubrene “thin‐film” single‐crystals demonstrate mobility as high as 4.6 cm² Vs^–1 and ON/OFF ratios of approximately 10⁶.     

Carbon Nanotube Field‐Effect‐Transistor‐Based Biosensors

There is an explosive interest in 1D nanostructured materials for biological sensors. Among these nanometer‐scale materials, single‐walled carbon nanotubes (SWNTs) offer the advantages of possible biocompatibility, size compatibility, and sensitivity towards minute electrical perturbations. In particular, because of these inherent qualities, changes in SWNT conductivity have been explored in order to study the interaction of biomolecules with SWNTs. This Review discusses these interactions, with a focus on carbon nanotube field‐effect transistors (NTFETs). Recent examples of applications of NTFET devices for detection of proteins, antibody–antigen assays, DNA hybridization, and enzymatic reactions involving glucose are summarized. Examples of complementary techniques, such as microscopy and spectroscopy, are covered as well.