Saleem G. Rao

Nanotube electronics: Large-scale assembly of carbon nanotubes

Nanoscale electronic devices made from carbon nanotubes, such as transistors and sensors, are much smaller and more versatile than those that rely on conventional microelectronic chips, but their development for mass production has been thwarted by difficulties in aligning and integrating the millions of nanotubes required. Inspired by biomolecular self-assembly processes, we have created chemically functionalized patterns on a surface, to which pre-grown nanotubes in solution can align themselves in huge numbers. This method allows wafer-scale fabrication of millions of carbon-nanotube circuits with single-nanotube precision, and may enable nanotube-based devices, such as computer chips and high-density sensor arrays, to be produced industrially.

High-performance, hysteresis-free carbon nanotube field-effect transistors via directed assembly

High-performance, single-wall carbon nanotube field-effect transistors (SWCNT-FETs) are fabricated using directed assembly and mass-produced carbon nanotubes (CNTs). These FETs exhibit operating characteristics comparable to state-of-the-art devices, and the process provides a route to large-scale functional CNT circuit assembly that circumvents problems inherent in processes relying on chemical vapor deposition. Furthermore, the integration of hydrophobic self-assembled monolayers in the device structure eliminates the primary source of gating hysteresis in SWCNT-FETs; this leads to hysteresis-free FET operation while exposing unmodified nanotube surfaces to ambient air.

Directed-assembly of single-walled carbon nanotubes using self-assembled monolayer patterns comprising conjugated molecular wires

Self-assembled monolayer (SAM) patterns on electrodes are often utilized to guide the assembly of single-walled carbon nanotubes (SWCNTs) onto the electrodes to form desired device structures. In this case, the SWCNTs are in contact with the electrodes through the SAM which comprises molecular wires. Presumably, it is desirable to use conjugated molecular wires for a low contact resistance because they have been reported as a better electric conductor than non-conjugated ones. However, until now, the directed-assembly of SWCNTs has been driven mostly via molecular wires with alkane backbones which are known to be relatively poor conductors. Herein, we report large-scale directed-assembly of SWCNTs utilizing SAM patterns comprising conjugated molecular wires. We achieved highly selective adsorption and precision alignment of SWCNTs utilizing polar SAM patterns comprising conjugated molecular wires, while SAM patterns with non-polar terminal groups efficiently prevented adsorption of SWCNTs. Furthermore, we developed a process for assembling a SWCNT across two electrodes coated with conjugated molecular wires, and the electrical conduction through the SWCNT was measured via a conducting atomic force microscope. This result could be an important guideline for large-scale directed-assembly of SWCNT-based devices in the future.

Tuning morphology and magnetic properties of sputtered permalloy by organic underlayers

We report the effect of polarity of self-assembled monolayers on magnetic properties and morphology of a deposited magnetic material. Sputtering of permalloy (Ni79Fe21) on a patterned structure of self-assembled monolayers (SAMs), with −COOH and −CH3 terminal groups, results in the formation of a film on −COOH regions and clusters on −CH3 regions. Further investi-gations reveal that the cluster formation gives rise to superparamagnetism, while the film shows usual ferromagnetic behavior. The observed contrast in morphology and magnetism of permalloy is attributed to different growth mechanisms arising from the difference in reactive nature of the terminal functional groups of the SAMs.

Conductance Microscopy for Electric Conduction Study of Bio-Inspired Hybrid Nanostructures under Ambient Conditions

Electrical conductance of single stranded DNA (5′-TTT TTT TTT T/3 Thio MC3-D/-3′) monolayer patterns on Au surface is compared with those of various organic molecular patterns via the conductance microscope (CM) technique that allows one to take nanoscale conductance images utilizing a conducting AFM tip in contact mode AFM. In the experiment, reference molecules and ssDNA are patterned on the same substrate via direct deposition methods such as dip-pen nanolithography and microcontact printing. Then, conductance microscope image is recorded revealing the relative conductivity of ssDNA patterns relative to various reference molecules. 16-mercaptohexadecanoic acid and 2-mercaptobenzimidazole patterns are found conducting better than the ssDNA patterns. This result indicates that the ssDNA with 10T bases is a relatively poor electrical conductor. The capabilities of CM technique are also tested on various nanostructures including the single wall carbon nanotube junction.