My research program is part of the field of nano- and micro-electromechanical systems. Most of my research is focused on applications at the periphery of information technology. Sensors and actuators have been the primary motivators for MEMS from its beginnings in the 1980s. Breakthroughs in materials, lower-cost manufacturing technology, micro- and nano-structure design, sophisticated electromechanical control of arrays of active elements, and encapsulation and packaging have built a powerful platform for new applications in areas such as health care, biotechnology, ubiquitous sensing, and energy conversion. My research is part of the BioInterfaces and the Internet of Things thrusts in SystemX.
Thermionic energy conversion is based on the emission of electrons from a very hot cathode and their collection on a cooler anode that can be made of a material with a different workfunction. The concept dates to the early 20th century and was realized using vacuum tube technology for space applications in the 1960s – 1980s. My group is working to realize a wafer-scale technology for thermionic converters and is contributing to computational modeling for predicting the work function-lowering effects of surface coatings, new approaches for achieving ultra-low work functions, as well as the design and fabrication of thermally isolated, suspended micro-cathodes. I am collaborating with Nick Melosh (MSE), Piero Pianetta (EE/SLAC), Jens Nørskov (ChemE/SLAC), Z.-X. Shen (Physics/Applied Physics/SLAC) to demonstrate a new kind of thermionic converter, in which a semiconductor photocathode replaces the conventional metal cathode. A related project uses thermionic emission of electrons into a microfabricated cavity for vacuum microelectronics. Tom Lee, Amin Arbabian, and I are collaborating to demonstrate a micro Barkhausen-Kurz oscillator, which promises to be an efficient THz source.
My research in bio-NEMS is two areas: the label-free identification and sorting of circulating tumor cells and the label-free, electronic detection of biomolecules. This first project is part of the Stanford Genome Technology Center, with Ron Davis (BioChem and SGTC Director) and Stefanie Jeffrey (Oncology) as collaborators. Our approach is based on generating biomechanical shear and normal stresses on cells in emulsion droplets in a ferrofluid, as the droplets move through a microfluidic channel. Those droplets containing cells with anomalous responses are actuated downstream by magnetophoresis. The second project is to design and fabricate a nano electrochemical system that mimics the olfactory receptor that gives us our sense of smell. Using a feedback voltage to suppress noise at the electrochemical interface, the tunneling current from an electrode through a self-assembled monolayer (SAM) film to electronic states in the ionic solution is found to contain information on the vibrational spectra of adsorbed molecular species. The sensor front-end is fabricated on the end of a metallic probe using electrochemical etching, functionalization, and nanoparticle self-assembly in the Nanosystems Shared Test Facilities (NSTF), atomic layer deposition (ALD) in SNF, and focused ion beam (FIB) etching in SNC. Sample preparation is a critical component of label-free sensing; we have developed a three-stage microfluidic system that can extract and orient biomolecules starting from a blood sample. My collaborators on this project are Boris Murmann (EE) and Ron Davis.
Nano electromechanical relays are devices that have very attractive characteristics for implementing digital logic. In contrast to nanoscale CMOS transistors, they have infinite subthreshold slope and no leakage currents. The target application is the programming fabric for field-programmable gate arrays (FPGAs) and the approach is a lateral electrostatically actuated switch. Contact degradation is a serious challenge, which is being addressed by compliant contact design and the use of robust contact surfaces made from ALD TiN and ruthenium oxide. Philip Wong (EE) and Subhasish Mitra (EE/CS) are collaborating on this effort. In addition, Tom Kenny (ME) is collaborating on the use of NEM relays for charging floating electrodes in vacuum encapsulated resonators and sensors.