Shape memory alloys (SMAs) have the potential to be used for a wide variety of microelectromechanical systems (MEMS) applications, providing a unique combination of large deflections and high work output. A major drawback for SMAs in many applications has been the low frequency response, which is typically on the order of 100 Hz or lower, even in microscale SMA actuators. In MEMS applications, the higher surface-to-volume ratios have enabled responses to be improved by an order or magnitude or more. By further shrinking the SMA film/device dimensions, the frequency response may be improved even further. In this paper, we present a new, simplified process for fabricating sputtered, thin film SMA MEMS actuators based on nickel-titanium alloy (NiTi or, aka, NITINOL) that consisted of only one photo step to pattern the actuators using SU8. When heated through its solid–solid phase transition from low-temperature martensite to high-temperature austenite, the NiTi alloy undergoes changes in associated physical properties, such as Young’s modulus, resistivity, and surface roughness, that are critical to controlling MEMS performance. For example, these material property changes allow for the design of active or passive microscale sensors and actuators. In the new process, we are able to fabricate ultrathin films of NiTi with nanoscale thickness, which can be thermally cycled through two stable positions very rapidly, making it an intriguing thermal sensor and actuator material for high frequency applications. Additionally, NiTi can be used as an active thermal switch through resistive (i.e. joule) heating. We demonstrated a greatly improved frequency response of up to 3000 Hz with turn on voltages as low as 0.5 V (corresponding to only 1 mW power consumption) for devices exhibiting microns of cantilever tip deflection over millions of cycles, indicating these new SMA MEMS actuators have potential application for low voltage switching, modulation and tuning.

Direct ultrasonication of graphite particles dispersed in water, in the presence of a surfactant, is a promising way to produce pristine nanoscale graphene platelets (NGPs) without graphite intercalation or oxidation. We investigate possible exfoliation mechanisms, specifically those involving sodium dodecylbenzenesulfonate (SDBS) surfactant, and compare their corresponding energies. The model includes interlayer van der Waals interactions and a force-field approach capable of treating charged surfactant and solvent. Our calculations reveal the significant role of SDBS in liquid-phase NGP production, through a locking mechanism that prevents restacking.

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Adsorption of functional groups on graphene nanoribbons has fundamental impacts on their applications in areas as diverge as energy storage, nanoelectronics, drug delivery, and sensors. To reveal adsorption geometries, energy barriers, and room temperature rate constants for assessing reaction kinetics, we study interactions of NO 2 molecules with ultra-narrow ($1 nm) hydrogen terminated armchair graphene nanoribbons (AGNRs), using first principles. We show that formation of hydrogen bonded NO 2 at the edge and physisorbed NO 2 at the center are processes without barriers, whereas chemisorption at center or edge are activated processes. Nonbonding and weak sp 3 hybridization at the edge of AGNR are shown to be more favorable than center adsorptions, revealing increased reactivity compared to graphene. Resultant modulations of quantum transport are calculated for sensing extremely low gas concentrations. Detectable current decrease is predicted for two hydrogen-bonded or chemisorbed molecules. We discuss possible measures to enhance sensitivity of GNRs for detecting extremely low concentrations of nitrogen dioxide and similar molecules.