Jongbaeg Kim

A highly sensitive hydrogen sensor with gas selectivity using a PMMA membrane-coated Pd nanoparticle/single-layer graphene hybrid.

A polymer membrane-coated palladium (Pd) nanoparticle (NP)/single-layer graphene (SLG) hybrid sensor was fabricated for highly sensitive hydrogen gas (H2) sensing with gas selectivity. Pd NPs were deposited on SLG via the galvanic displacement reaction between graphene-buffered copper (Cu) and Pd ion. During the galvanic displacement reaction, graphene was used as a buffer layer, which transports electrons from Cu for Pd to nucleate on the SLG surface. The deposited Pd NPs on the SLG surface were well-distributed with high uniformity and low defects. The Pd NP/SLG hybrid was then coated with polymer membrane layer for the selective filtration of H2. Because of the selective H2 filtration effect of the polymer membrane layer, the sensor had no responses to methane, carbon monoxide, or nitrogen dioxide gas. On the contrary, the PMMA/Pd NP/SLG hybrid sensor exhibited a good response to exposure to 2% H2: on average, 66.37% response within 1.81 min and recovery within 5.52 min. In addition, reliable and repeatable sensing behaviors were obtained when the sensor was exposed to different H2 concentrations ranging from 0.025 to 2%.

Improvement of Gas-Sensing Performance of Large-Area Tungsten Disulfide Nanosheets by Surface Functionalization

Semiconducting two-dimensional (2D) transition metal dichalcogenides (TMDCs) are promising gas-sensing materials due to their large surface-to-volume ratio. However, their poor gas-sensing performance resulting from the low response, incomplete recovery, and insufficient selectivity hinders the realization of high-performance 2D TMDC gas sensors. Here, we demonstrate the improvement of gas-sensing performance of large-area tungsten disulfide (WS2) nanosheets through surface functionalization using Ag nanowires (NWs). Large-area WS2 nanosheets were synthesized through atomic layer deposition of WO3 followed by sulfurization. The pristine WS2 gas sensors exhibited a significant response to acetone and NO2 but an incomplete recovery in the case of NO2 sensing. After AgNW functionalization, the WS2 gas sensor showed dramatically improved response (667%) and recovery upon NO2 exposure. Our results establish that the proposed method is a promising strategy to improve 2D TMDC gas sensors.

Lithography-Free Fabrication of Large Area Subwavelength Antireflection Structures Using Thermally Dewetted Pt/Pd Alloy Etch Mask

We have demonstrated lithography-free, simple, and large area fabrication method for subwavelength antireflection structures (SAS) to achieve low reflectance of silicon (Si) surface. Thin film of Pt/Pd alloy on a Si substrate is melted and agglomerated into hemispheric nanodots by thermal dewetting process, and the array of the nanodots is used as etch mask for reactive ion etching (RIE) to form SAS on the Si surface. Two critical parameters, the temperature of thermal dewetting processes and the duration of RIE, have been experimentally studied to achieve very low reflectance from SAS. All the SAS have well-tapered shapes that the refractive index may be changed continuously and monotonously in the direction of incident light. In the wavelength range from 350 to 1800 nm, the measured reflectance of the fabricated SAS averages out to 5%. Especially in the wavelength range from 550 to 650 nm, which falls within visible light, the measured reflectance is under 0.01%.

Microfabricated torsional actuators using self-aligned plastic deformation of silicon

In this paper, we describe angular vertical-comb-drive torsional microactuators made in a new process that induces residual plastic deformation of single-crystal-silicon torsion bars. Critical dimensions of the vertically interdigitated moving-and fixed-comb actuators are self-aligned in the fabrication process and processed devices operate stably over a range of actuation voltages. We demonstrate MEMS scanning mirrors that resonate at 2.95kHz and achieve optical scan angles up to 19.2 degrees with driving voltages of 40V/sub dc/ plus 13V/sub pp/. After continuous testing of five billion cycles at the maximum scanning angle, we do not observe any signs of degradation in the plastically deformed silicon torsion bars.

Ultrasonic Bonding for MEMS Sealing and Packaging

The feasibility of ultrasonic bonding for hermetic microelectromechanical systems (MEMS) packaging has been demonstrated utilizing the solid phase vibration and welding process to bond two elements rapidly at low temperature. Two different approaches have been developed including lateral and vertical ultrasonic bonding setups with three sets of material bonding systems: In-to-Au, Al-to-Al, and plastics-to-plastics. The process utilizes purely mechanical vibration energy to enable low temperature bonding between similar or dissimilar materials without precleaning of the bonding surfaces. In these prototype demonstrations, the typical bonding process used tens of watts at room temperature environment and the bonds were accomplished within seconds for bonding cavities with areas of a few mm2 . Preliminary tests show that packaged MEMS cavities can survive gross leakage tests by immersing the bonded chip into liquids. As such, ultrasonic bonding could potentially be broadly applied for hermetic MEMS sealing and packaging especially where temperature limitation is a critical issue. Ultrasonic polymeric bonding could be applied for capping polymer-based microfluidic chips. This paper describes the ultrasonic bonding and hermetic sealing processes as well as the characterizations of bonding tools and equipment setups.

Aligned Carbon Nanotube Arrays for Degradation-Resistant, Intimate Contact in Micromechanical Devices

Figure 1 . a) Schematic of the three-terminal micromechanical switch with aligned CNT arrays as the contact material. b) Onand off-states of the contact according to the actuation of the shuttle. c) Optical microscopy images of the contact region of CNT arrays in the onand offstates according to the applied gate voltage. The CNT arrays are in contact under V G = 0 V, and the current fl ows through the contact (the on-state). At V G = 52 V, the shuttle is driven in the direction of the gate electrode and the CNT arrays are separated (the off-state). The inset shows the corresponding relationship between I SD and V G . The scale bar is 10 μ m. The emergence of and advances in nano/ microelectromechanical systems (N/ MEMS) [ 1 , 2 ] have enabled the realization of miniaturized mechanical switches with high functionality in terms of low power consumption, fast operating speed, and low cost of fabrication. Various solid contact materials have been investigated for nanoand micromechanical contacts, including dopedSi, [ 3 , 4 ] single-crystal SiC and diamond, [ 5 , 6 ] and diverse metals and their alloys. [ 7 ] However, regardless of the contact material, these solidto-solid contacts suffer from diverse failure mechanisms such as stiction by adhesion, electrical short by welding and melting, unavoidable surface degradation by mechanical wear and abrasion, and a small real area of contact due to surface asperity. Therefore, the low reliability and durability of contacts are major challenges to overcome in nanoand micromechanical switching devices. Espe-

Fabrication of a multi-walled carbon nanotube-deposited glass fiber air filter for the enhancement of nano and submicron aerosol particle filtration and additional antibacterial efficacy.

We grew multi-walled carbon nanotubes (MWCNTs) on a glass fiber air filter using thermal chemical vapor deposition (CVD) after the filter was catalytically activated with a spark discharge. After the CNT deposition, filtration and antibacterial tests were performed with the filters. Potassium chloride (KCl) particles (<1 μm) were used as the test aerosol particles, and their number concentration was measured using a scanning mobility particle sizer. Antibacterial tests were performed using the colony counting method, and Escherichia coli (E. coli) was used as the test bacteria. The results showed that the CNT deposition increased the filtration efficiency of nano and submicron-sized particles, but did not increase the pressure drop across the filter. When a pristine glass fiber filter that had no CNTs was used, the particle filtration efficiencies at particle sizes under 30 nm and near 500 nm were 48.5% and 46.8%, respectively. However, the efficiencies increased to 64.3% and 60.2%, respectively, when the CNT-deposited filter was used. The reduction in the number of viable cells was determined by counting the colony forming units (CFU) of each test filter after contact with the cells. The pristine glass fiber filter was used as a control, and 83.7% of the E. coli were inactivated on the CNT-deposited filter.

Development of a flexible three-axis tactile sensor based on screen-printed carbon nanotube-polymer composite

A flexible, three-axis carbon nanotube (CNT)–polymer composite-based tactile sensor is presented. The proposed sensor consists of a flexible substrate, four sensing cells, and a bump structure. A CNT–polydimethylsiloxane (PDMS) composite is produced by a solvent evaporation method, and thus, the CNTs are well-dispersed within the PDMS matrix. The composite is directly patterned onto a flexible substrate using a screen printing technique to fabricate a sensor with four sensing cells. When a force is applied on the bump, the magnitude and direction of force could be detected by comparing the changes in electrical resistance of each sensing cell caused by the piezoresistive effect of the composite. The experimentally verified sensing characteristics of the fabricated sensor exhibit a linear relationship between the resistance change and the applied force, and the measured sensitivities of the sensor for the normal and shear forces are 6.67 and 86.7%/N for forces up to 2.0 and 0.5 N, respectively. Experiments to verify the load-sensing repeatability show a maximum 2.00% deviation of the resistance change within the tested force range.

Bi-directional electrothermal electromagnetic actuators

A new breed of in-plane bi-directional MEMS actuators based on controlled electrothermal buckling and electromagnetic Lorentz force has been demonstrated under both dc and ac operations. Experimentally, bi-directional actuators made by the standard surface-micromachining process have a lateral actuation range of several microns and can exert forces over 100 ?N, while those made by SOI and MetalMUMPs processes have an operation range up to several tens of microns and can exert more than 20 mN of force. Reliability tests show that SOI/MetalMUMPs and surface-micromachined actuators can operate for more than 1 and 100 million cycles, respectively, with no signs of degradation. As such, these micro-actuators could be used for MEMS devices that require a bi-directional movement with a large force output such as bi-directional micro-relays.

Monolithic 2-D scanning mirror using self-aligned angular vertical comb drives

We have demonstrated microfabricated, monolithic two degrees of freedom (two-dimensional) electrostatic torsional mirrors using a three-mask process on silicon-on-insulator wafer with a single plastic deformation step. The mirror operated independently in two orthogonal directions as controlled by two sets of self-aligned angular vertical combs. The measured dynamic performance showed resonant frequencies of 10.56 and 1.54 kHz with optical scanning angles up to 27/spl deg/ and 20/spl deg/ in the two orthogonal axes, respectively, under driving voltages of 20 V/sub dc/ plus 15 V/sub pp/. A 90-day continuous mirror operation at peak resonance, in equivalent to 80 and 12.1 billion cycles on the two orthogonal axes, showed negligible performance variations.