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Application of Carbon Nanotubes as Electromechanical Sensors

Researchers have used Materials Studio's DMol3 to examine bonding differences between two types of nanotube deformation: (1) bending, and (2) pushing with atomically sharp AFM tips.

Such an understanding should lead to a better design of ultrasensitive sensors that could detect even the smallest mechanical perturbations, and also the design of new types of strain gauges based on carbon nanotubes.

Carbon nanotubes have recently turned into a hot area of research activity, fuelled by experimental breakthroughs that have led to realistic possibilities of using them in a host of commercial applications: Field emission-based flat panel displays, novel semiconducting devices in microelectronics, hydrogen storage devices, chemical sensors, and most recently in ultra-sensitive electromechanical sensors.

An important experiment with regards to developing electromechanical sensors involved a metallic nanotube suspended over a 600 nm long trench. When the middle part of such a suspended nanotube was pushed with the tip of an atomic force microscope (AFM), the conductivity was found to decrease by almost two orders of magnitude [1]. This drop in conductance was much higher than previously predicted values for tubes bent under mechanical duress. An interesting explanation was put forward by O(N) tight-binding calculations, which show that beyond a critical deformation several C-atoms close to the AFM tip become sp3-coordinated. This leads to the tying up of π-electrons into localized σ-states, which would explain the large drop in electrical conductance.

In order to check the above idea, the first-principles DFT code DMol3 was used to examine bonding differences between two types of nanotube deformation: (1) bending, and (2) pushing with atomically sharp AFM tips. The smallest models of nanotubes necessary in such simulations typically involve a few thousand atoms, which make a pure DFT simulation unfeasible. Therefore, the researchers took recourse to a combination of DFT and classical molecular mechanics [2, 3]. Bond reconstruction, if any, is likely to occur only in the highly deformed, middle part of the tube. For such atoms (~ 100-150 atoms including AFM-tip atoms) a DFT description was used, while the long and essentially straight part away from the middle was described accurately using the Universal Forcefield (UFF).

What was found from such calculations was very interesting: for AFM-pushed tubes, no sp3 coordination occurs (see Fig. 1). Rather the tube stretches, which might open up an energy gap for tubes of certain chiralities, e.g., the metallic zigzag tubes [3, 4]. The interpretation was supported by electronic tight-binding transport calculations based on the non-equilibrium Green's function (NEGF) formalism [3].

Subsequent experiments [5] have confirmed the above theoretical interpretation, and have prompted nanotechnologists to explore the design of new types of strain gauges and pressure sensors based on carbon nanotubes [6].

Ongoing collaboration with the nanotechnology group of NASA Ames Research Center, Moffet Field, USA, is expected to yield more interesting results in the near future.

The computational work (reference [3]) received NASA-CSC's best paper award in Applied Science for the year 2002.

References

  1. T. W. Tombler et al., Nature, 2000, 405, 769.
  2. A. Maiti, Phys. Stat. Sol. B, 2001, 226, 87.
  3. A. Maiti, A. Svizhenko, and M. P. Anantram, Phys. Rev. Lett., 2002, 88, 126805.
  4. A. Maiti, Nature Materials (London), 2003, 2, 440.
  5. E. D. Minot et al., Phys. Rev. Lett., 2003, 90, 156401.
  6. J. Cao, Q. Wang, and H. Dai, Phys. Rev. Lett., 2003, 90, 157601.