Sharpened VO2 Phase Transition via Controlled Release of Epitaxial Strain.

Phase transitions in correlated materials can be manipulated at the nanoscale to yield emergent functional properties, promising new paradigms for nanoelectronics and nanophotonics. Vanadium dioxide (VO2), an archetypal correlated material, exhibits a metal-insulator transition (MIT) above room temperature. At the thicknesses required for heterostructure applications, such as an optical modulator discussed here, the strain state of VO2 largely determines the MIT dynamics critical to the device performance. We develop an approach to control the MIT dynamics in epitaxial VO2 films by employing an intermediate template layer with large lattice mismatch to relieve the interfacial lattice constraints, contrary to conventional thin film epitaxy that favors lattice match between the substrate and the growing film. A combination of phase-field simulation, in situ real-time nanoscale imaging, and electrical measurements reveals robust undisturbed MIT dynamics even at preexisting structural domain boundaries and significantly sharpened MIT in the templated VO2 films. Utilizing the sharp MIT, we demonstrate a fast, electrically switchable optical waveguide. This study offers unconventional design principles for heteroepitaxial correlated materials, as well as novel insight into their nanoscale phase transitions.

Injectable Flexible Subcutaneous Electrode Array Technology for Electrocardiogram Monitoring Device.

Implantable cardiac monitors have undergone considerable miniaturization. However, they continue to be associated with complications such as infection, bleeding/bruising, and device extrusion or migration. In this paper, we demonstrate the feasibility of using a small, flexible, injectable, subcutaneous microelectrode-based device to record electrocardiograms (ECGs). We describe the fabrication process and demonstrate the ease of insertion of the injectable ECG device in vivo swine model. We also demonstrate our device's high-density channel microelectrode array's ability to detect the P, R, and T waves. The amplitude of these waves showed excellent correlation with distance of the bipolar electrodes used to detect them. Given the success of our initial studies, this device has the potential to improve the safety profile of implantable cardiac monitors and simplify the implantation procedure to allow for placement in a primary care setting.

Single-neuronal cell culture and monitoring platform using a fully transparent microfluidic DEP device.

Dielectrophoresis using multi-electrode arrays allows a non-invasive interface with biological cells for long-term monitoring of electrophysiological parameters as well as a label-free and non-destructive technique for neuronal cell manipulation. However, experiments for neuronal cell manipulation utilizing dielectrophoresis have been constrained because dielectrophoresis devices generally function outside of the controlled environment (i.e. incubator) during the cell manipulation process, which is problematic because neurons are highly susceptible to the properties of the physiochemical environment. Furthermore, the conventional multi-electrode arrays designed to generate dielectrophoretic force are often fabricated with non-transparent materials that confound live-cell imaging. Here we present an advanced single-neuronal cell culture and monitoring platform using a fully transparent microfluidic dielectrophoresis device for the unabated monitoring of neuronal cell development and function. The device is mounted inside a sealed incubation chamber to ensure improved homeostatic conditions and reduced contamination risk. Consequently, we successfully trap and culture single neurons on a desired location and monitor their growth process over a week. The proposed single-neuronal cell culture and monitoring platform not only has significant potential to realize an in vitro ordered neuronal network, but also offers a useful tool for a wide range of neurological research and electrophysiological studies of neuronal networks.

Band-Bending of Ga-Polar GaN Interfaced with Al2O3 through Ultraviolet/Ozone Treatment.

Understanding the band bending at the interface of GaN/dielectric under different surface treatment conditions is critically important for device design, device performance, and device reliability. The effects of ultraviolet/ozone (UV/O3) treatment of the GaN surface on the energy band bending of atomic-layer-deposition (ALD) Al2O3 coated Ga-polar GaN were studied. The UV/O3 treatment and post-ALD anneal can be used to effectively vary the band bending, the valence band offset, conduction band offset, and the interface dipole at the Al2O3/GaN interfaces. The UV/O3 treatment increases the surface energy of the Ga-polar GaN, improves the uniformity of Al2O3 deposition, and changes the amount of trapped charges in the ALD layer. The positively charged surface states formed by the UV/O3 treatment-induced surface factors externally screen the effect of polarization charges in the GaN, in effect, determining the eventual energy band bending at the Al2O3/GaN interfaces. An optimal UV/O3 treatment condition also exists for realizing the "best" interface conditions. The study of UV/O3 treatment effect on the band alignments at the dielectric/III-nitride interfaces will be valuable for applications of transistors, light-emitting diodes, and photovoltaics.

High-performance green flexible electronics based on biodegradable cellulose nanofibril paper.

Today's consumer electronics, such as cell phones, tablets and other portable electronic devices, are typically made of non-renewable, non-biodegradable, and sometimes potentially toxic (for example, gallium arsenide) materials. These consumer electronics are frequently upgraded or discarded, leading to serious environmental contamination. Thus, electronic systems consisting of renewable and biodegradable materials and minimal amount of potentially toxic materials are desirable. Here we report high-performance flexible microwave and digital electronics that consume the smallest amount of potentially toxic materials on biobased, biodegradable and flexible cellulose nanofibril papers. Furthermore, we demonstrate gallium arsenide microwave devices, the consumer wireless workhorse, in a transferrable thin-film form. Successful fabrication of key electrical components on the flexible cellulose nanofibril paper with comparable performance to their rigid counterparts and clear demonstration of fungal biodegradation of the cellulose-nanofibril-based electronics suggest that it is feasible to fabricate high-performance flexible electronics using ecofriendly materials.