bending nanotubes for electronic modification

from the society-for-imposition-of-cruelty-to-nanotubes dept.
Two groups of researchers have measured electronic effects of mechanical deflection in nanotubes. A group mostly at Clemson permanently bent multi-walled nanotubes (MWNTs) and saw "local metallic character" at the kink. They have an abstract online. A group mostly at Stanford reversibly bent single-walled nanotubes (SWNTs) with an AFM tip and saw hundredfold drops in conductivity in their experiments. They have an abstract and a press release online. In the (primarily) Clemson group, D. Tekleab, R. Czerw, D.L. Carroll, & P.M. Ajayan, writing in [Appl.Phys.Let. 76:3594-3596 12Jun2000], bent their tubes ultrasonically. They dispersed their MWNTs in tetrahydrofuran, then applied 200 watts of ultrasound to the dispersion. They found that tubes with diameters <5nm "are more easily kinked while a majority of the larger tubes remained undeformed." The authors performed STM imaging and spectroscopy in UHV with a Pt-Ir tip on a HOPG substrate.

Spectroscopy measurements from I(V) measurements from one tube with a 25 degree kink were presented. The tube was originally semiconducting, with a 0.2 eV band gap far from the kink on both sides. Near the kink "the band gap close to the Fermi energy has become narrower by the addition of interface states to the gap."

The authors interpret these states as being locally "metallic", "analogous to a quantum dot". They consider local strain, topological defects, and interlayer interactions to be possible causes for the interface states. They find that the changes to the electron energy spectra of the tube fade out beyond ~1.7 nm from the kink, confirming that these states are local.

In the (primarily) Stanford group, Thomas W. Tombler, Chongwu Zhou, Leo Alexseyev, Jing Kong, Hongjie Dai, Lel Liu, C.S. Jayanthi, Meijie Tang, & Shi-Yu Wu writing in [Nature 405:769-772 15Jun2000] measured conductivity in a SWNT while bending it with an AFM tip. This group suspended SWNTs over trenches etched into a SiO2 surface. They describe experiments on a 3.1 nm diameter SWNT, suspended over a trench 605 nm wide and 175 nm deep. The authors grew the SWNTs using patterned catalyst islands. They contacted them electrically by depositing micron-wide electrodes of "20 nm thick Ti and 60 nm thick Au" on them, one on each side of the trench.

The authors measured conductance through their SWNTs while pressing on the portion of the tube that crossed the trench. The tube was repeatedly pressed with an AFM tip, and both force and conductance were measured as a function of the SWNT's deflection down into the trench. For small deflections the force was proportional to deflection3, as was expected for an initially unstressed elastic string.

The resistance rose dramatically for large deflections, from 200 kohms for the undeflected SWNT to 25 megohms for a deflection producing a 14 degree bend and an average strain of 3%.

Both the forces and the resistance changes were almost perfectly reversible, and repeatable from cycle to cycle. The authors use this to rule out damage to the metal-SWNT contacts and motion of the SWNT along the substrate as contributors to these effects.

The authors did quantum mechanical calculations for a (5,5) SWNT subjected to pressure from a tip while supported from its ends. It also showed a drop in conductivity, which was interpreted as due to the formation of additional carbon-carbon bonds between the front and back of the tube. These bonds tied up some of the orbitals that form the conducting pi states in the unstressed SWNT. The local hybridization of the carbon shifted from conducting, unsaturated sp2 towards insulating sp3. These are different from previous calculated results, "in which the nanotube structure is more or less uniformly bent or strained" (because the bending was produced by tilting the ends, rather than applying force from a local tip). The previous results predicted smaller changes in hybridization, and therefore smaller changes in conductivity than those in this paper.

The sharper changes in hybridization than previously expected should assisted in selective mechanochemistry, perhaps allowing a sequence of substituents to be chemisorbed onto a SWNT surface.

This work is also a strong indication of the potential value of multi-tip scanning probe experiments. Even the most straightforward dual tip setup, one where the tips approach from diametrically opposed directions, would be a great boon to this work. One could measure changes in geometries, in local strain, and in I(V) spectra on the side of the tube opposite the AFM tip. One might be able to directly demonstrate the local hybridization changes from spatially resolved I(V) spectra, perhaps with atom-by-atom resolution.

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