The fundamental investigation of energy dissipation from surface forces (i.e., surface dissipation) in nanomechanical resonators is proposed, especially in regards to how it may be used to detect and study biological or chemical reactions. In the past decade, micro- and nano-mechanical resonators have been thoroughly studied for use as chemical and biological sensors by detecting mass-induced change of resonant frequency that results from specific reactions. However, the dissipation of vibrational energy due to surface forces, one reason that resonators eventually "ring down," has until now not been utilized as a sensing mechanism. Previous work demonstrated that surface forces can have a more powerful effect on surface dissipation than on resonant frequency, opening up the possibility that surface dissipation could be used as a new sensing metric for resonating biological or chemical sensors. The parameter quality factor (Q) is a measure of energy dissipation in resonators, with energy lost to surface forces (Qsurface) being the particular loss mechanism of interest in this work. Toward this end, three objectives will be pursued: 1) Fabrication of resonators with Qsurface as the limiting energy dissipation mechanism, 2) Measurement of Qsurface dependence on different gases, and 3) Tracking the change of Qsurface over time to determine if temporal information about Qsurface can give insight into the occurrence or progress of biological or chemical reactions.
The intellectual merit of the proposed work is as follows: First, a fundamental question - "Why do resonators not ring forever?" - will be addressed. While some forms of energy dissipation (e.g., air damping) are reasonably well understood, others remain elusive. Surface dissipation is especially difficult to understand, because its dependence on a high surface-to-volume ratio makes it difficult to see above the nanoscale. Next, the use of Qsurface as a new tool for measuring the occurrence or progress of chemical or biological reactions will be explored, revealing insight into surface forces that would not be measureable with a purely mass-induced frequency-shift method. Finally, the use of time-resolved measurements will be investigated to reveal differences between chemical or biological reactions that occur during the reaction process, as opposed to looking at only the before-and-after snapshot views.
This work will have broader impact in several ways. In engineering, the use of surface dissipation could allow for a new platform for measuring biological and chemical reactions. The time-resolution of these measurements could lead to discovery of surface force-related phenomena during these reactions. In education, this program will enable four undergraduate students from underrepresented groups to each receive one year of research training. This will prepare these students to be the technical leaders and role models for the next generation of engineers. The undergraduate researchers will serve as role models and mentors to high school students by teaching them about how engineering benefits society.
The research component of this program has been to investigate energy dissipation in miniature resonating structures. Essentially, the researchers are trying to answer the questions, "Why does a tuning fork not ring forever? Where does the energy of vibration go as the tuning fork stops ringing?" These questions have practical implications because miniature resonating devices are being used in everyday life. Resonators are in laptop computers, mobile phones, and nearly every other electronic device. Understanding how these devices dissipate energy is a key step toward improving their performance. Improving the performance of resonators will lead to reduced power consumption (i.e., longer batter life) and better performance (e.g., clearer mobile phone calls). In order to answer these questions about energy dissipation, the researchers have been designing, building, and testing miniature resonators that are less than 1000 atomic layers thick. To date, structures have been designed that can change the amount of surface charge on a resonating device by 1,000,000,000 times. Controlling the surface charge is critical, because the researchers hypothesize that charge on the surface of these resonators is the cause of energy dissipation. Understanding this surface-induced dissipation could allow for its use as a sensing mechanism. The major education activity funded under this grant has been the design of an outreach demonstration that integrates circuit design and music. The undergraduate students funded by this project, have developed an outreach demonstration. One undergraduate student has presented this demonstration to a several groups of high school students that were involved in a summer program at UCLA. He has also gone to several high schools to present his demonstration. More than 100 high school students have seen this demonstration in total.
