AFM Measurements of DNA
around, in the range from less than 10 hertz to about 30 hertz,” says Cohen. “Te lab has three separate AFM systems, each with several different modules that require very precise vibration isolation for all of our research, including our DNA electron transport studies. We have opted for negative- stiffness vibration isolation to provide the necessary low-noise environment.” [3] Negative-stiffness mechanism (NSM) isolators have the
flexibility of custom tailoring resonant frequencies vertically and horizontally. Tey employ a completely mechanical concept in low-frequency vibration isolation. Vertical-motion isolation is provided by a stiff spring that supports a weight load, combined with an NSM. Te net vertical stiffness is made very low without affecting the static load-supporting capability of the spring. Beam-columns connected in series with the vertical-motion isolator provide horizontal-motion isolation. Te horizontal stiffness of the beam-columns is reduced by the “beam-column” effect. A beam-column behaves as a spring combined with an NSM. Te result is a compact passive isolator capable of very low vertical and horizontal natural frequencies and very high internal structural frequencies (Figure 4). “We tried air tables, but they did not do very well for us
with the horizontal vibrations,” continues Cohen. “Ten we compared active systems to the negative-stiffness isolator, measuring the frequency spectrum up to about 100 hertz, and
the active systems did not perform as well as the negative- stiffness isolator.” [3] Transmissibility with negative-stiffness is substantially
improved over air systems, which can make vibration isolation problems worse since they have a resonant frequency that can match that of floor vibrations. Transmissibility is a measure of the vibrations that transmit through the isolator relative to the input vibrations. Te NSM isolators, when adjusted to 0.5 Hz, achieve 93 percent isolation efficiency at 2 Hz, 99 percent at 5 Hz, and 99.7 percent at 10 Hz. NSM transmissibility is also improved over active systems.
Conclusion Te measurement of electron transport through DNA
molecules by AFM is one more step toward DNA-related electronic devices. Negative-stiffness vibration isolation was an essential component of the AFM system.
References [1] C Nogues et al., Phys Chem Chem Phys 6 (2004) 4459–66. [2] SR Cohen and A Bitler, Curr Opin Colloid In 13 (2008) 316–25.
[3] SR Cohen, interviewed by Jim McMahon, tape recording, May 16, 2007.
[4] DK Ferry, Laser Focus World 43(10) (2007) 107–10. [5] DL Platus, Proc SPIE 3786 (2009) 98–105.
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