This Materials World Network award supports an experimental program to investigate whether the mechanical quality factor, Q, of graphene resonators can be made sufficiently large so that the thermal motion of these inherently low mass structures can be observed at low temperatures e.g. 50 mK. Our goal is to demonstrate a Q of 10,000 or discover why it is not possible to reach this Q value. New techniques for measuring the amplitude of motion will have to be implemented. Methods to obtain high Q such as tension, defect reduction and elimination of surface artifacts will have to be invented to achieve the goal of observing thermal motion. The technology to fabricate synthetic few-layer graphite has been developed at Cornell and the NRL, and will allow comparison to natural graphene. The Materials World team funded by this research program brings together expertise in nano-mechanics, synthetic growth of graphene (and few layer-graphite), high frequency signal recovery and low temperature physics, the latter two areas being important to probe the potential quantum nature of the material. Understanding non-linear behavior will also play a role in the success of the program, and non-linearities usually regarded as being undesirable, may actually be beneficial for future quantum information applications. The research program will be integrated with partner programs at Helsinki University and NRL. Graduate students will have the opportunity to work with their counterparts by spending a semester in Finland and by hosting counterparts at Cornell.
Graphene is a unique material - a self supporting two-dimensional material that can be completely defect free. What are its mechanical properties? Particularly at low temperatures where the non-classical behavior is expected to become most apparent will graphene display mechanical behavior that is classical or quantum-mechanical? The Materials World team funded by this research program brings together expertise in nano-mechanics, synthetic growth of graphene (and few layer-graphite), high frequency signal recovery and low temperature physics. Quantum mechanics is usually associated with small objects (e.g. atoms) and classical behavior with large objects (e.g. baseballs). Somewhere between lies a no man's land, and the team at Cornell University, Helsinki University and the US Naval Research Laboratory seeks to answer whether a graphene sheet, consisting of approximately 10 to 100 billion atoms is quantum or classical in nature. This understanding will help to determine if the material properties will be useful for new types of electro-mechanical devices, which has implications for the future communications and electronics industries. This research provides a demanding experimental environment that will educate and train graduate students for successful careers in the Nation's scientific and technological infrastructure.
The project advanced the state of the art in several important areas. In the technology arena, we conceived of, designed and actualized graphene (single layer of graphite) resonators with varied geometries. These were fabricated from synthetically grown graphene. They were in the form of doubly clamped beams, square or rectangular and circular membranes (Fig 1). The devices show much sharper resonance frequency (an indication of lower energy loss), and also the resonant frequency can be "tuned" by applying a voltage that alters the tension of the device. The challenges that we faced will likely impact technology development for applications that require integration into conventional silicon based electronics. A second technology development was in the area of opto mechanics where we showed that the performance of these graphene devices can be altered by their interaction with laser light in an optical cavity where the light intensity is spatially varying. The use of this technology can find numerous applications in sensing. Our scientific activities were broad. At Cornell, experiments conducted in our labs showed that as devices are made larger, the resulting losses at the point of attachment become less important. This lays out the route to achieving high performance high frequency devices. We also found that we can fabricate large scale nearly identical arrays of these devices with high performance (Fig 2). These arrays could be the basis of future detectors. The opto-mechanical behaviors were also unexpected. We found that because of the relatively strong absorption of graphene, that the devices mechanical behavior can be tuned in a number of ways by placing them precisely in a cavity formed by the device and a mirror and illuminating the cavity with laser light. By varying the wavelength of light, the characteristics of the cavity can be tuned (Fig 3). Similar effects can also be seen by attracting the graphene to the mirror using a voltage. The result is that the narrowness of the resonance can be adjusted at will. Experiments at Helsinki showed that the graphene’s mechanics was highly non-linear and we can take advantage of these behaviors in future applications. Results from Helsinki also examined the behavior of aluminum resonators at low temperatures where the use of a focused ion beam led to extraordinarily strong coupling between a mechanical device and its electrical environment. The broader impacts of the research were considerable. Our students have benefited from international collaborations and have taken up positions in university research (at LMU in Munich and at Columbia University in NY). The PIs mentored REU students and Cornell undergraduates and developed presentations to propagate the concepts of order-of-magnitude estimation to encourage students to carry out simple estimations using basic scientific principles – thus thinking creatively to examine and solve complex problems that they may encounter in all spheres of activity. Research material was also incorporated into a course taught at Cornell. Ph.D. dissertations submitted as part of the project are listed below: Microelectromechanical Sensors and Measurements Darren Southworth, Ph.D. Thesis, Cornell University, October 2010. Engineering Atomically Thin Mechanical Systems Robert A. Barton, Ph.D. Thesis, Cornell University, September 2012.
