Carbon Nanotube

Figure 13.10. Illustration of a single-walled carbon nanotube mounted on an STM tip attached to a cantilever arm of an atomic force microscope. [Adapted from H. Dai et a!., Nature 384, 147(1996).]

break. The MWNT tip can also be used in the tapping mode. When the nanotube bends on impact, there is a coherent deexcitation of the cantilever oscillation. The MWNT serves as a compliant spring, which moderates the impact of each tap on the surface. Because of the small cross section of the tip, it can reach into deep trenches on the surface that are inaccessible to normal tips. Since the MWNTs are electrically conducting, they may also be used as probes for an STM.

The azobenzene molecule, shown in Fig. 13.1 la, can change from the trans isomer to the cis isomer by subjecting it to 313-nm light. Isomers are molecules having the same kind of atoms and the same number of bonds but a different equilibrium geometry. Subjecting the cis isomer to light of wavelength greater than 380 nm causes the cis form to return to the original trans form. The two forms can be distinguished by their different optical absorption spectra. Notice that the cis isomer is shorter than the trans isomer. Azobenzene can also form a polymer consisting of a chain of azobenzene molecules. In die polymer form it can also undergo the trans-to-cis transformation by exposure to 365-nm light. When this occurs, the length of the polymer chain decreases. A group at the University of Munich have constructed a molecular machine based on die photoisomerization of the azobenzene polymer. They attached the trans form of the polymer to the cantilever of an atomic force microscope as shown in Fig. 13.11b and then subjected it to light of 365-nm wavelength, causing the polymer to contract and the beam to bend. Exposure to 420-nm light causes the polymer to return to the trans form, allowing the beam to return to its original position. By alternately exposing the polymer to pulses of420-and 365-nm light, the beam could be made to oscillate. This is the first demonstration of an artificial single-molecule machine that converts light energy to physical work.

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