ABSTRACT
The nanometer-scale size, excellent mechanical and electrical properties of CNTs offer opportunities to couple the mechanical and electrical domains at nanoscale device level, making CNTs an ideal material for preparing NEMS. Among many CNT NEMS, mechanical oscillators hold bright future. Their wide range applications, including ultrasensitive mass/force detection, have trigged extensive study of CNTs’ vibration properties. Acoustoelectric effect [29] and thermal vibrations [30] have been first studied in suspended doubly clamped SWCNTs. Since these nanometer-sized resonators oscillate at very high frequencies but with very small vibration amplitudes, it is desirable to study mechanical vibration driven and detected by electrical signals. Static and dynamic mechanical deflections were then electrically activated and studied in cantilevered MWCNTs in an in situ transmission electron microscope (TEM) [31]. When a static potential is applied, CNTs become electrically charged and will be bent toward the counter electrode. Once a time-dependent voltage is applied, the CNTs receive a time-dependent force and are then resonantly excited at the fundamental frequency and higher harmonics. The deflection and resonation could be observed in TEM by monitoring CNT’s deflected contours. This electrically excited mechanical resonance of CNTs opens up applications in nanoresonant devices such as sensors, oscillator circuits, and nanoballances [ 32]. It was also found that CNT’s resonant frequency could be tuned electrically. In a field emission microscope, Purcell et al. [33] have observed that the resonant frequency varies linearly with applied voltages. Their experimental setup is shown in Figure 15.1. Anode voltage Vs was used to excite resonance, while voltage VA was used to generate electron emission. When resonance is excited, the pattern on screen will become larger and emission current will become smaller. Using this setup, the resonant frequency can be exactly measured by monitoring the emission current. And more interestingly, the voltage VA also creates a longitudinal tension on the CNTs, and thus the resonance could be tuned by changing the voltage VA, similar to
the tuning of a guitar string. In this study, the frequency could be tuned by over 10 times.
