The impact of Cr adhesion layer on CNFET electrical characteristics.

The effect of a Cr adhesion layer on the transfer characteristics of Cr/Au-contacted carbon nanotube field-effect transistors (CNFETs) based on individual single-walled carbon nanotubes (SWNTs) is presented in this paper. We show that a very thin Cr layer (≈0.4 nm) already has an impact on the carrier transport in Schottky-barrier-modulated CNFETs. The ratio of the p- and n-branch current is reduced by eight times when the Cr adhesion layer thickness is increased from 0 to 8 nm. We suggest a change in Schottky barrier height at the contact as the determining mechanism for this result. Additionally, superior lifetime of devices is observed even for non-passivated CNFETs with preserved clean SWNT/Cr/Au-contacts using Cr layer thinner than 2 nm. Our experiments show that the role of the adhesion layer in metal/nanotube contacts should be explicitly considered when designing CNTFET-based circuits, developing CNFET fabrication processes, and analyzing the corresponding properties of the electrical contacts.

Electron diffraction of an in situ strained double-walled carbon nanotube.

The strain-induced change in a carbon-nanotube diffraction pattern is found after applying strain, using a microelectromechanical tensile stage, to the outer shell of a double-walled carbon nanotube, while the inner shell provides an unstrained reference pattern. The nanotube is found to have chirality (63,21)@(65,32) with 16-20° tilt and strain up to 1% in the outer shell.

Axially tunable carbon nanotube resonators using co-integrated microactuators.

Tuning of the mechanical resonance frequency of single-walled carbon nanotubes (SWCNTs) is achieved by application of uniaxial strain by purely mechanical means, utilizing both directly grown and dry-transferred SWCNTs. The induction of a beam-to-string transition is achieved, resulting in an axial tension sensitivity of 9.4 × 10(10) Hz/ε in the vibrating string regime. Increases in the resonant Q-factor, removal of residual slack, and resonance frequency changes from 10 to 60 MHz are affected.

Photomask-based integration process of low-defect suspended carbon nanotubes into SOI MEMS.

A fully-developed photomask-based integration process is reported. The process can integrate suspended carbon nanotubes (CNTs) into micro-structures on silicon-on-insulator chips. The process features batch-compatible fabrication and post-growth metallization of suspended CNTs, which has never been demonstrated by any other processes. The post-growth metallization avoids deterioration of the metals at the elevated CNT growth temperature and enables mechanically robust double-clamped configuration. Two key steps ensure a significant reduction of the risk for damage or contamination of the CNTs during post-growth processing. SiO2 bridges were fabricated to physically support CNTs during the wet processing, and a protective Al2O3 layer (∼40 nm) was deposited to prevent resist contamination during lithography. The combination of these two steps enables the removal of the unprotected suspended segments of unwanted CNTs by oxygen plasma ashing, improving device yield by a factor of six. The electrically interfaced suspended CNT device possessed high CNT quality (D/G(+) intensity ratio of 1/224 in Raman spectroscopy) and good electrical properties, such as low device resistances as low as 105 kΩ and reduced gate hysteresis as low as 65 mV in ambient air. Measurements of eights devices indicate that the release step did not have a systematic influence on the device resistances.

Advances in NO2 sensing with individual single-walled carbon nanotube transistors.

The charge carrier transport in carbon nanotubes is highly sensitive to certain molecules attached to their surface. This property has generated interest for their application in sensing gases, chemicals and biomolecules. With over a decade of research, a clearer picture of the interactions between the carbon nanotube and its surroundings has been achieved. In this review, we intend to summarize the current knowledge on this topic, focusing not only on the effect of adsorbates but also the effect of dielectric charge traps on the electrical transport in single-walled carbon nanotube transistors that are to be used in sensing applications. Recently, contact-passivated, open-channel individual single-walled carbon nanotube field-effect transistors have been shown to be operational at room temperature with ultra-low power consumption. Sensor recovery within minutes through UV illumination or self-heating has been shown. Improvements in fabrication processes aimed at reducing the impact of charge traps have reduced the hysteresis, drift and low-frequency noise in carbon nanotube transistors. While open challenges such as large-scale fabrication, selectivity tuning and noise reduction still remain, these results demonstrate considerable progress in transforming the promise of carbon nanotube properties into functional ultra-low power, highly sensitive gas sensors.

Atomically precise bottom-up fabrication of graphene nanoribbons.

Graphene nanoribbons-narrow and straight-edged stripes of graphene, or single-layer graphite-are predicted to exhibit electronic properties that make them attractive for the fabrication of nanoscale electronic devices. In particular, although the two-dimensional parent material graphene exhibits semimetallic behaviour, quantum confinement and edge effects should render all graphene nanoribbons with widths smaller than 10 nm semiconducting. But exploring the potential of graphene nanoribbons is hampered by their limited availability: although they have been made using chemical, sonochemical and lithographic methods as well as through the unzipping of carbon nanotubes, the reliable production of graphene nanoribbons smaller than 10 nm with chemical precision remains a significant challenge. Here we report a simple method for the production of atomically precise graphene nanoribbons of different topologies and widths, which uses surface-assisted coupling of molecular precursors into linear polyphenylenes and their subsequent cyclodehydrogenation. The topology, width and edge periphery of the graphene nanoribbon products are defined by the structure of the precursor monomers, which can be designed to give access to a wide range of different graphene nanoribbons. We expect that our bottom-up approach to the atomically precise fabrication of graphene nanoribbons will finally enable detailed experimental investigations of the properties of this exciting class of materials. It should even provide a route to graphene nanoribbon structures with engineered chemical and electronic properties, including the theoretically predicted intraribbon quantum dots, superlattice structures and magnetic devices based on specific graphene nanoribbon edge states.

Narrowing SWNT diameter distribution using size-separated ferritin-based Fe catalysts.

Sensors and devices made from single-walled carbon nanotubes (SWNTs) are most often electrically probed through metal leads contacting the semiconducting SWNTs (s-SWNTs). Contact barriers in general and Schottky barriers (SBs) in particular are usually obtained at a metal-semiconductor interface. The unique one-dimensional structure (1D) of SWNTs allows tailoring of the SB heights through the contact metal type and the size of the s-SWNT bandgap. A large workfunction reduces the SB height (e.g. using Pd as the metal contact material). The bandgap of an SWNT is inversely proportional to its diameter. Ohmic contacts--the preferable choice--are achieved for s-SWNTs with diameters greater than 2 nm on Pd metal leads. SWNT device reproducibility, on the other hand, requires a narrow distribution of the SWNT diameters. Here, we present a method to fabricate SWNTs with a large and adjustable mean diameter (1.9-2.4 nm) and very narrow diameter distribution (+/- 0.27 nm at mean diameter 1.9 nm). The results are achieved through a size separation of the ferritin catalyst particles by sedimentation velocity centrifugation prior to their use in the chemical vapor deposition (CVD) formation of SWNTs.

Aqueous dispersion and dielectrophoretic assembly of individual surface-synthesized single-walled carbon nanotubes.

The successful dispersion and large-scale parallel assembly of individual surface-synthesized large-diameter (1-3 nm) single-walled carbon nanotubes (SWNTs), grown by chemical vapor deposition (CVD), is demonstrated. SWNTs are removed from the growth substrate by a short, low-energy ultrasonic pulse to produce ultrapure long-term stable surfactant-stabilized solutions. Subsequent dielectrophoretic deposition bridges individual, straight, and long SWNTs between two electrodes. Electrical characterization on 223 low-resistance devices (R(average) approximately 200 kOmega) evidences the high quality of the SWNT raw material, prepared solution, and contact interface. The research reported herein provides an important framework for the large-scale industrial integration of carbon nanotube-based devices, sensors, and applications.