Dong-Weon Lee

Integrated microcantilevers for high-resolution sensing and probing

This topical review is focused on microcantilever-based sensing and probing functions that are realized by integrating a mechanically compliant cantilever with self-sensing and self-actuating elements, specific sensing materials as well as functionalized nano-tips. Such integrated cantilever devices have shown great promise in ultra-sensitive applications such as on-the-spot portable bio/chemical detection and in situ micro/nanoscale surface analysis and manipulation. The technical details of this review will be given in a sequence of cantilever sensors and, then, cantilever-tip probes. For the integrated cantilever sensors, the frequency-output style dynamic cantilevers are described first, with the contents including optimized resonance modes, sensing-group-modified nanostructures for specific bio/chemical mass adsorption and nanoscale sensing effects, etc. Thereafter, the static cantilever sensors for surface-stress detection are described in the sequence of the sensing mechanism, surface modification of the sensitive molecule layer and the model of specific reaction-induced surface-energy variation. After technical description of the cantilever sensors, the emphasis of the review moves to functionalized nano-tip equipped cantilever-tip probing devices. The probing functions are not only integrated on the cantilever but also integrated at the sharp apex of the tip. After description of single integrated cantilever probes and their applications in surface scanning and imaging, arrayed cantilever-tip devices and their simultaneous parallel operation for high throughput imaging and nanomechanical data storage are also addressed. With cantilever-tip probes as key elements, micro-analysis instruments are introduced that can be widely used for macro/nanoscale characterizations.

Microprobe array with electrical interconnection for thermal imaging and data storage

In this work, new novel methods for fabricating a thermal probe array with 32 /spl times/ 32 probes on one chip are proposed. It consists of silicon micromachined probe, AlN actuator, pyramidal SiO/sub 2/ tip on which the nano-scale metal-metal junction is formed using a self-alignment technique. The nano-junction can be used as a thermocouple to measure a local temperature on a sample surface or as a nano-heater to make a local deformation on a media. Using the fabricated thermal probe, temperature distribution is measured on a prepared sample surface and the local heating capability of the thermal probe is confirmed. Preliminary experiments for data writing and reading are performed on a phase change medium.

Micro-/Nanocombined Gas Sensors With Functionalized Mesoporous Thin Film Self-Assembled in Batches Onto Resonant Cantilevers

This paper reports a novel top-down/bottom-up combined resonant microcantilever chemical sensor, where the nanosensing material of a functionalized mesoporous thin film (MTF) is directly self-assembled on the sensing region of the integrated microcantilever. By using the batch-producible nano-on-micro construction technique, a large number of such sensors can be batch fabricated with uniform performance and low cost. More importantly, the sensing molecule terminals can be simultaneously constructed at the pore inner surface when the MTF is directly grown on the cantilever. With -NH2-group-functionalized MTF directly grown onto the surface of the cantilever free end, the micro-/nanocombined gravimetric sensor has experimentally exhibited quick response and highly sensitive detection of CO2 gas.

PANI and Graphene/PANI Nanocomposite Films — Comparative Toluene Gas Sensing Behavior

The present work discusses and compares the toluene sensing behavior of polyaniline (PANI) and graphene/polyaniline nanocomposite (C-PANI) films. The graphene–PANI ratio in the nanocomposite polymer film is optimized at 1:2. For this, N-methyl-2-pyrrolidone (NMP) solvent is used to prepare PANI-NMP solution as well as graphene-PANI-NMP solution. The films are later annealed at 230 °C, characterized using scanning electron microscopy (SEM) as well Fourier transform infrared spectroscopy (FTIR) and tested for their sensing behavior towards toluene. The sensing behaviors of the films are analyzed at different temperatures (30, 50 and 100 °C) for 100 ppm toluene in air. The nanocomposite C-PANI films have exhibited better overall toluene sensing behavior in terms of sensor response, response and recovery time as well as repeatability. Although the sensor response of PANI (12.6 at 30 °C, 38.4 at 100 °C) is comparatively higher than that of C-PANI (8.4 at 30 °C, 35.5 at 100 °C), response and recovery time of PANI and C-PANI varies with operating temperature. C-PANI at 50 °C seems to have better toluene sensing behavior in terms of response time and recovery time.

Cantilever with integrated resonator for application of scanning probe microscope

Abstract A silicon cantilever with a small torsional resonator is designed by finite element method (FEM) and fabricated by silicon micromachining technology. The mechanical elements of the probe consist of the cantilever beam for actuation and the small torsional resonator for force detection. The torsional resonator with small mass (width of 8 μm×length of 21 μm) is integrated at the end of the cantilever beam (width of 36 μm×length of 150 μm) and the resonator is suspended by thin two beams (width of 1 μm×length of 2.5 μm). High resonance frequency can be achieved by reduction of the resonator size and the smaller resonator with high resonance frequency is insensitive to thermo-mechanical noise that is inversely proportional to the resonance frequency. By flowing a current to the small metal lines on the resonator, the torsional resonator is vibrated. A force interaction between the tip and the sample is directly measured by the induced electromotive force. The high resonance of the cantilever beam is favorable for obtaining a high scanning speed in SPM operation instead of conventional piezo-tube scanner. In the vertical direction, the fabricated cantilever beams has a static deflection of 1 μm when flowing a dc current of 40 mA to the wire on the cantilever beam and applying magnetic field of 2000 G. The oscillation amplitude of the resonator is 28 nm when an ac current of 4 mA is applied to the wire on the resonator. The resonance frequency of the torsional resonator as measured in air was 3.4 MHz, with a quality factor of 203. The fabricated resonator has a thermal noise vibration amplitude of 6.3×10−15 m. At room temperature, the minimum detectable force of the resonator can reach 6.75×10−14 N.

A Super-Lyophobic 3-D PDMS Channel as a Novel Microfluidic Platform to Manipulate Oxidized Galinstan

We report a 3-D super-lyophobic polydimethylsiloxane (PDMS) microfluidic channel patterned with an array of multi-scale surface texture as a novel microfluidic platform to mobilize naturally oxidized Galinstan. Galinstan is a liquid metal that has multiple advantages over mercury such as non-toxicity, higher thermal conductivity, and lower electrical resistivity. However, Galinstan gets easily oxidized in an air environment and it becomes a viscoelastic liquid that wets almost any solid surface. We studied the feasibility of developing super-lyophobic surfaces against Galinstan, using various flat and textured surfaces including PDMS micropillar and microridge arrays by measuring static and dynamic contact angles. The highest advancing angle of 175 ° and receding angle of 163 ° were achieved on a surface patterned with micropillars, each of which was textured with additional roughness. Pitch distance between pillars was 175 μm. An extremely simple PDMS-PDMS bonding technique was used to fabricate a 3-D super-lyophobic channel structure as a microfluidic platform for oxidized Galinstan droplets. The driving force to actuate a ~ 3-μL Galinstan droplet in the 3-D super-lyophobic channel was 3.11±0.23 mN.

Graphene/polydimethylsiloxane nanocomposite strain sensor

The objective of this research is to fabricate graphene nanopowder composites based on polydimethylsiloxane (PDMS) and to characterize the gauge factor of the graphene/PDMS composites for the use of strain sensors. The fabrication of graphene/PDMS composites can be accomplished by simple sonication and micro molding processes. We found that the measured gauge factors strongly depend on the concentration of graphene flakes in the composites. Obtained gauge factor of the graphene/PDMS composite strain sensor reached about 233 at a graphene concentration of 8.33 vol.%, which was measured within a strain range of 2%.

A Seesaw-Structured Energy Harvester With Superwide Bandwidth for TPMS Application

In this paper, we introduce a novel seesaw-structured energy harvester for tire-pressure monitoring system (TPMS) applications. The unique design of the proposed energy harvester can effectively avoid the influence of enormous centrifugal force, which is attributed to the balance characteristic of seesaw structure. Two magnets placed on the seesaw end could swing the seesaw structure in every rotating cycle with the help of external noncontact magnetic repulsive force, and subsequently this unbalanced seesaw structure impacts the polyvinylidene difluoride (PVDF) cantilevers to generate the power. A miniature energy harvester prototype is fabricated using a 3-D printing technique. In the performance testing experiment, a peak voltage of 4.6 V and a constant peak output power of 36 μW with the optimized load resistance value of 0.6 MΩ are achieved within a superwide rotating frequency range of 215-965 r/min (25.7-115.4 km/h for a real car). Moreover, an average power per second is 5.625 μW at 750 r/min, which could be potential power supply for the TPMS application.

Fabrication of thermal microprobes with a sub-100 nm metal-to-metal junction

A technological approach for making a miniature thermocouple and nano-heater on a microprobe is presented. A non-uniform silicon dioxide (SiO2) layer is formed on a silicon V-groove by low-temperature oxidation, and then the non-uniform SiO2 layer is selectively etched in a wet solution to make a nano-hole at the apex of a SiO2 tip. A nano-junction of Pt/Cr (or Au/Cr) and Ni was exactly formed and situated through the nano-hole without an alignment process. The nano-junction acts as a thermocouple and nano-heater. The diameter of the fabricated nano-junction is below 100 nm. This technique has a high reproducibility and is suitable for mass production. Basic characteristics for application of the thermal microprobe are evaluated using a temperature-controlled oven and a photon counting CCD camera.

Note: high-efficiency energy harvester using double-clamped piezoelectric beams.

In this study, an improvement in energy conversion efficiency has been reported, which is realized by using a double-clamped piezoelectric beam, based on uniaxial stretching strain. The buckling mechanism is applied to maximize axial stress in the double-clamped beam. The voltage generated by using the double-clamped piezoelectric beam is higher than that generated by using other conventional structures, such as bending cantilevers coated/sandwiched with piezoelectric film, which is proven both theoretically and experimentally. The power generation efficiency is enhanced by further optimizing the double-clamped structure. The optimized high-efficiency energy harvester utilizing double-clamped piezoelectric beams generates a peak output power of 80 μW, under an acceleration of 0.1g.