Jeffrey B. Chou

Extremely Elastic Wearable Carbon Nanotube Fiber Strain Sensor for Monitoring of Human Motion

The increasing demand for wearable electronic devices has made the development of highly elastic strain sensors that can monitor various physical parameters an essential factor for realizing next generation electronics. Here, we report an ultrahigh stretchable and wearable device fabricated from dry-spun carbon nanotube (CNT) fibers. Stretching the highly oriented CNT fibers grown on a flexible substrate (Ecoflex) induces a constant decrease in the conductive pathways and contact areas between nanotubes depending on the stretching distance; this enables CNT fibers to behave as highly sensitive strain sensors. Owing to its unique structure and mechanism, this device can be stretched by over 900% while retaining high sensitivity, responsiveness, and durability. Furthermore, the device with biaxially oriented CNT fiber arrays shows independent cross-sensitivity, which facilitates simultaneous measurement of strains along multiple axes. We demonstrated potential applications of the proposed device, such as strain gauge, single and multiaxial detecting motion sensors. These devices can be incorporated into various motion detecting systems where their applications are limited to their strain.

Design of wide-angle selective absorbers/emitters with dielectric filled metallic photonic crystals for energy applications

The design and simulation of a wide angle, spectrally selective absorber/emitter metallic photonic crystal (MPhC) is presented. By using dielectric filled cavities, the angular, spectrally selective absorption/emission of the MPhC is dramatically enhanced over an air filled design by minimizing diffraction losses. Theoretical analysis is performed and verified via rigorous coupled wave analysis (RCWA) based simulations. An efficiency comparison of the dielectric filled designs for solar thermophotovoltaic applications is performed for the absorber and emitter which yields a 7% and 15.7% efficiency improvement, respectively, compared to air filled designs. The converted power output density is also improved by 33.5%.

Direct Insulation‐to‐Conduction Transformation of Adhesive Catecholamine for Simultaneous Increases of Electrical Conductivity and Mechanical Strength of CNT Fibers

Increase in conductivity and mechanical properties of a carbon nanotube (CNT) fiber inspired by mussel-adhesion chemistry is described. Infiltration of polydopamine into an as-drawn CNT fiber followed by pyrolysis results in a direct insulation-to-conduction transformation of poly(dopamine) into pyrolyzed-poly(dopamine) (py-PDA), retaining the intrinsic adhesive function of catecholamine. The py-PDA enhances both the electrical conductivity and the mechanical strength of the CNT fibers.

Hafnia-plugged microcavities for thermal stability of selective emitters

Two-dimensional arrays of micro-cavities effectively control photon motion and selectively emit radiation tailored to the preferred bandgap of photovoltaic (PV) cells, thus enhancing the efficiency of thermophotovoltaic energy conversion. At the high operating temperatures, however, the micro- and nano-patterned structures of the selective emitters quickly lose their integrity––obliterating the tight tolerances required for precise spectral control. Even if oxidation, recrystallization, and grain growth could be avoided with single-crystal tungsten (W) selective emitters with vacuum packaging, surface diffusion, evaporation, and re-condensation are not avoidable in long-term operation at high temperatures. The concept of a planar array of plugged micro-cavities to suppress the curvature-dependent thermal degradation modes is proposed and tested. Based on scale-accelerated failure tests of silicon devices, the lifetime of W selective emitters operating at 1100 K is estimated to be at least 30 yr.

Global optimization of omnidirectional wavelength selective emitters/absorbers based on dielectric-filled anti-reflection coated two-dimensional metallic photonic crystals

We report the design of dielectric-filled anti-reflection coated (ARC) two-dimensional (2D) metallic photonic crystals (MPhCs) capable of omnidirectional, polarization insensitive, wavelength selective emission/absorption. Using non-linear global optimization methods, optimized hafnium oxide (HfO2)-filled ARC 2D Tantalum (Ta) PhC designs exhibiting up to 26% improvement in emittance/absorptance at wavelengths λ below a cutoff wavelength λc over the unfilled 2D TaPhCs are demonstrated. The optimized designs possess high hemispherically average emittance/absorptance εH of 0.86 at λ < λc and low εH of 0.12 at λ > λc.

Integrating Optoelectronic Tweezers for Individual Particle Manipulation with Digital Microfluidics Using Electrowetting-On-Dielectric (EWOD)

This paper presents the integration of two powerful technologies: manipulation of droplets (i.e., digital microfluidics) using electrowettingon-dielectric (EWOD) and manipulation of individual particle inside the droplets using optoelectronic tweezers (OET). A novel platform for maintaining a viable cell culture environment is proposed as an application example, in which EWOD operations bring droplets containing cells, medium and waste into and out of the cell environment and OET operations address and manipulate the individual cells in coordination with the fluidic operations. Functions of EWOD and OET required to realize the concept are demonstrated.

Surface plasmon assisted hot electron collection in wafer-scale metallic-semiconductor photonic crystals

Plasmon assisted photoelectric hot electron collection in a metal-semiconductor junction can allow for sub-bandgap optical to electrical energy conversion. Here we report hot electron collection by wafer-scale Au/TiO2 metallic-semiconductor photonic crystals (MSPhC), with a broadband photoresponse below the bandgap of TiO2. Multiple absorption modes supported by the 2D nano-cavity structure of the MSPhC extend the photon-metal interaction time and fulfill a broadband light absorption. The surface plasmon absorption mode provides access to enhanced electric field oscillation and hot electron generation at the interface between Au and TiO2. A broadband sub-bandgap photoresponse centered at 590 nm was achieved due to surface plasmon absorption. Gold nanorods were deposited on the surface of MSPhC to study localized surface plasmon (LSP) mode absorption and subsequent injection to the TiO2 catalyst at different wavelengths. Applications of these results could lead to low-cost and robust photo-electrochemical applications such as more efficient solar water splitting.

Low friction liquid bearing mems micromotor

This paper examines the performance of rotating microdevices incorporating a liquid bearing to couple a rotating element to a fixed substrate. Liquid bearing technology promises to significantly improve the durability and lifetime of micromechanical motors. Here, the fluid is confined between the rotor and stator using surface patterning of a hydrophobic layer. Magnetic actuation of 10 mm diameter silicon rotor is used to characterize the liquid bearing motor at rotation rates up to 1800 rpm. Bearings with fluid thickness from 20–200 microns are characterized. A minimum torque of 0.15 µN-m is required to overcome static friction and initiate rotation. At rotation rates above 720 rpm, the rotor wobble is less than ±1 mrad and the bearing exhibits viscous friction with a drag coefficient of 1.2 × 10−3 µN-m/rpm.

Electrothermally Actuated Lens Scanner and Latching Brake for Free-Space Board-to-Board Optical Interconnects

The design, fabrication, and characterization of an electrothermally actuated lens scanner with bistable mechanical brakes, for the application of free-space board-to-board optical interconnects, are presented. An electrothermally actuated stepper motor is used to scan a 2.8-mm-diameter lens shuttle by a maximum of ±170 μm (±1.57° scanning angle). Bistable mechanical brakes, toggled with U-shaped thermal actuators, will grip and hold the lens shuttle in place while dissipating zero power. The minimum frictional braking force and maximum actuator force are both measured to be 0.75 mN. A position sensing detector is used to accurately measure the dynamics of the stepper motor and lens system, from which we verify our analytical model. Long-term testing results and solutions are also presented. We demonstrate a robust 10-Gb/s optical link capable of maintaining connection despite a board tilting of 0.45° while dissipating zero power.

Electrothermally actuated free space board-to-board optical interconnect with zero power hold

We present an electrothermally actuated MEMS lens scanner for board-to-board free-space optical interconnect systems capable of zero standby power. U-shaped thermal brakes, and lens frames are designed, fabricated, and characterized with a vertical cavity surface emitting laser (VCSELs) for dynamic beam steering. We demonstrate up to 170 µm displacement, at a maximum speed of 350 µm/s. The mechanical brakes have a holding force of 1.6 mN with switching speeds of up to 100Hz. A telecentric (4-f) optical interconnect operating at 1.25 Gb/s is demonstrated. The microlens scanner increases the tilt tolerance of the board by 2.5°.