Jiyoung Chang

Polysilicon microstructures to characterize static friction

The design, fabrication process, and initial experiment results for a test microstructure for measuring static friction are described. A normal force is applied to a displaced suspended structure by an underlying electrode. The tangential force to measure the frictional force is produced by a restoring force from the displaced suspension. The spring constant is found empirically by resonating the structure. The mu for coarse-grained polysilicon-polysilicon interfaces is found to be 4.9+or-1.0. Silicon nitride-polysilicon surfaces exhibit less friction with a mu of 2.5+or-0.5. Friction is found to be independent of apparent area. Tests were conducted on a piezoelectric crystal resulting in a reduction of friction and improved reliability of data extraction.<<ETX>>

Micromechanical structures for thin film characterization

Micromechanical structures designed for material characterization through analysis of their lateral vibrations are reported. The structures are made of doped LPCVD (low-pressure chemical vapor deposited) polycrystalline silicon and consist of beams supporting a rigid mass which is used to drive as well as sense the oscillatory motion. Amplitude and phase responses with respect to frequency are measured by an amplitude modulation technique and are used with the corresponding mechanical model to estimate various electromechanical properties. Youngs modulus for this material is estimated to be 174+or-10 GPa, with the film supporting a tensile residual stress of 10+or-2 MPa. It is shown that some structures exhibit significant nonlinearities for moderate lateral displacements. The observed response curves are well modeled by Duffings equation for a stiffening spring.<<ETX>>

High quality Mn-doped (Na,K)NbO3 nanofibers for flexible piezoelectric nanogenerators.

Enhanced piezoelectric and energy-harvesting characteristics of Mn-doped (Na0.5K0.5)NbO3 (NKN) nanofibers have been investigated with actual fabrication of potential flexible nanogenerators. The electrospinning process of nanofibers has been initially optimized with the proper level of chelating agent and annealing temperature. High quality nanofibers are successfully obtained only by means of a certain level of doped-Mn, which incorporates into the NKN perovskite structure and facilitates significant grain growth. A single-particle-stacked structure along the direction of fiber length becomes more evident with increasing Mn content. An XPS analysis confirms that Mn exists in multivalent states of Mn(2+)/Mn(3+). The effective piezoelectric coefficient of the nanofibers is found to be enhanced by 5 times with Mn-doping up to 3 mol % as characterized by piezoelectric force microscopy. The resultant flexible nanogenerators on PES films have exhibited ∼0.3 V output voltage and ∼50 nA output current under a bending strain.

A Near-Infrared Mechano Responsive Polymer System

A new type of crosslinked polyarylamide based film with a fibrillar morphology that is photomechanically responsive has been created. Without any additive or need of pre-alignment, these films can actuate under NIR stimulation. By constructing the photoresponsive film on top of a piezoelectric poly(vinylidene difluoride) film, electrical energy can be generated under NIR excitation. This is the first polymer system that exhibits NIR response without any conventional photosensitive moieties.

Electrostatically actuated carbon nanowire nanotweezers

Nanotweezers based on two carbon nanowires by means of localized chemical vapor deposition using a focused ion beam (FIB-CVD) have been successfully demonstrated. The nanotweezers have been constructed on fixed microelectrodes made from a heavily doped SOI wafer using a single-mask and deep reactive ion etching (DRIE) process. The location, dimension and gap between the two nanowires are precisely controlled such that the tweezing motion and the operation voltage can be easily adjusted. Both bent type and straight type nanotweezers with parallel nanowires of 300 nm in diameter and 15–19.6 µm in length have been built and tested. Experimental results show continuous gap closing movements of 0.6–1.2 µm achieved with the operation voltage down to 30 V for the prototype devices. The modulus of elasticity of FIB-CVD carbon nanowires also has been measured to be 84.5 GPa from the tweezing motion. Potential applications of these nanotweezers include manipulation of nanoparticles and nanoscale objects.

Large array electrospun PVDF nanogenerators on a flexible substrate

This paper presents PVDF (Polyvinylidene fluoride) based electrospun nanofibers as nanogenerators in series and/or in parallel to amplify harvested energy due to mechanical strain on a flexible substrate. A chemically resistive flexible polymer substrate has been used with patterned comb-shape gold electrodes by means of a conventional lithography process. A total of 500 parallel nanofibers have been fabricated and connected to amplify current outputs under repeated mechanical straining tests. Peak current of 35nA has been collected with a 0.2mV of peak voltage.

Real-Time Observation of Water-Soluble Mineral Precipitation in Aqueous Solution by In Situ High-Resolution Electron Microscopy

The precipitation and dissolution of water-soluble minerals in aqueous systems is a familiar process occurring commonly in nature. Understanding mineral nucleation and growth during its precipitation is highly desirable, but past in situ techniques have suffered from limited spatial and temporal resolution. Here, by using in situ graphene liquid cell electron microscopy, mineral nucleation and growth processes are demonstrated in high spatial and temporal resolution. We precipitate the mineral thenardite (Na2SO4) from aqueous solution with electron-beam-induced radiolysis of water. We demonstrate that minerals nucleate with a two-dimensional island structure on the graphene surfaces. We further reveal that mineral grains grow by grain boundary migration and grain rotation. Our findings provide a direct observation of the dynamics of crystal growth from ionic solutions.

Pick, break, and placement of one-dimensional nanostructures for direct assembly and integration

A direct, simple, and versatile assembly method for the manipulation of one-dimensional nanostructures and their integration with microscale devices has been demonstrated. Using a probe station with an unbiased tungsten probe, the facile process has been employed to accurately pick, break, and place individual titanium dioxide nanoswords and zinc oxide nanowires under a room-temperature, dry environment. The surface morphology of the nanostructures, probe tips, and adhesion forces were characterized. As such, the technique could enable the rapid assembly of individual nanostructures with complementary metal-oxide-semiconductor-compatible or complex microscale devices.

A flexible graphene FET gas sensor using polymer as gate dielectrics

We have successfully demonstrated a Graphene Field Effect Transistor (GFET) gas sensor on a flexible plastic substrate with a sensitivity of 0.00428ppm-1 (ΔR/R0) for ammonia. Compared with the state-of-art technologies, four distinctive advancements have been achieved: (1) first demonstration of a flexible graphene FET gas sensor; (2) a new fabrication process to achieve embedded-gate GFET on a flexible substrate; (3) proof of utilizing polymeric materials of parylene and polyethylenimine (PEI) as gate dielectrics and channel dopant for graphene FETs, respectively; and (4) validation of a gas sensing mechanism utilizing the real-time, n-type graphene doping process induced by the exposure of graphene FET to the targeted gases. As such, the proposed sensing scheme/platform could open up a new class of research in graphene-based, flexible gas sensing systems.

Facile electron-beam lithography technique for irregular and fragile substrates

A facile technique is presented which enables high-resolution electron beam lithography on irregularly-shaped, non-planar or fragile substrates such as the edges of a silicon chip, thin and narrow suspended beams and bridges, or small cylindrical wires. The method involves a spin-free dry-transfer of pre-formed uniform-thickness polymethyl methacrylate, followed by conventional electron beam writing, metal deposition, and lift-off. High-resolution patterning is demonstrated for challenging target substrates. The technique should find broad application in micro- and nano-technology research arenas.