Namchul Jung

Gravimetric analysis of the adsorption and desorption of CO2 on amine-functionalized mesoporous silica mounted on a microcantilever array.

The kinetics of CO(2) adsorption and desorption over amine-functionalized mesoporous silica were investigated using silicon microcantilever arrays. Three types of mesoporous silica with different pore sizes were synthesized and functionalized with a variety of amine molecules. After depositing the silica sorbents onto the free end of each cantilever in an array, mass changes due to the adsorption and desorption of CO(2) were determined in situ with picogram sensitivity by measuring variations in the cantilever frequencies. The adsorption and desorption kinetics were found to be diffusion-controlled, and the kinetics were accelerated by increasing the temperature and pore size. The activation energies for adsorption and desorption of CO(2) were determined from Arrhenius plots.

Electronic Nose Based on Multipatterns of ZnO Nanorods on a Quartz Resonator with Remote Electrodes

An electrodeless monolithic multichannel quartz crystal microbalance (MQCM) sensor was developed via the direct growth of ZnO nanorod patterns of various sizes onto an electrodeless quartz crystal plate. The patterned ZnO nanorods acted as independent resonators with different frequencies upon exposure to an electric field. The added mass of ZnO nanostructures was found to significantly enhance the quality factor (QF) of the resonator in electrodeless QCM configuration. The QF increased with the length of the ZnO nanorods; ZnO nanorods 5 μm in length yielded a 7-fold higher QF compared to the QF of a quartz plate without ZnO nanorods. In addition, the ZnO nanorods offered enhanced sensitivity due to the enlarged sensing area. The developed sensor was used as an electronic nose for detection of vapor mixtures with impurities.

Effects of gold patterning on the bending profile and frequency response of a microcantilever

We have systematically investigated the effect of various gold patterns on the bending profile and frequency response of a microcantilever. The gold patterns were deposited on the cantilever arrays using four types of shadow mask. The local bending profile, slope, and curvature varied depending on the area and position of the gold pattern. Also, the variations in the first three modes of the flexural resonance frequencies of the gold patterned cantilevers were measured to understand the opposing effects of mass loading and flexural rigidity; both of these parameters are dependent on the position and area of the gold pattern. The experimental results validated the theoretical one-dimensional model introduced by Tamayo et al. [Appl. Phys. Lett. 89, 224104 (2006)] and our calculations using the finite element method. The gold patterns giving the maximum response of the mass loading and flexural rigidity change were determined by examining how the relative resonance frequency shifts as a function of the dista...

Evaporation of water droplets from hydrophobic and hydrophilic nanoporous microcantilevers

The evaporation dynamics of water droplets from the surfaces of well-defined nanoporous substrates, anodic aluminum oxide (AAO) cantilevers with various pore sizes, were investigated. The AAO cantilever surfaces were modified to be either hydrophilic or hydrophobic. After placing a water droplet on the cantilevers, variations in the resonance frequency and deflection during evaporation were related to the changes in mass and stress of the cantilever, respectively. The dynamics of water droplet evaporation on a hydrophilic AAO cantilever was found to be significantly different from that measured on a hydrophobic AAO cantilever due to the permeation of water into the hydrophilic nanopores.

Enhanced Photocatalytic Activity of Highly Crystallized and Ordered Mesoporous Titanium Oxide Measured by Silicon Resonators

Ordered mesoporous TiO(2) was synthesized using the combined assembly of soft and hard chemistries method and deposited as a film coating on a microcantilever sensor array along with two other types of TiO(2) film: one from nanoparticles and one prepared via a sol-gel reaction. After loading methylene blue molecules on the TiO(2) films, the films were exposed to ultraviolet radiation. The photocatalytic decomposition of methylene blue was monitored by measuring changes in the resonance frequency of each cantilever. The mesoporous TiO(2) film showed higher photocatalytic activity than conventional TiO(2) films fabricated from nanoparticles or via a sol-gel reaction; this difference is attributed to the purely anatase crystalline morphology of the mesoporous TiO(2) film as well as its well-organized pore structure. The three-dimensionally interconnected pore network facilitates the diffusion of methylene blue molecules to the photocatalytically active sites of the mesoporous TiO(2).

Enhanced mass sensitivity of stress-free, silicon nanowire-grown microcantilever sensors

A thin catalytic layer of nickel was evaporated onto one side of silicon microcantilevers and amorphous silicon nanowires were synthesized on the nickel surface. When nanowire-grown cantilevers were exposed to mercaptohexanol vapor, their mass sensitivity was almost two orders of magnitude greater than that of the corresponding plain cantilever due to the increase in surface area afforded by the nanowires, whereas adsorption of mercaptohexanol induced negligible deflection of the nanowire-grown cantilevers. This indicates that the gold-coated nanowire-grown cantilever can be used as a sensitive, stress-free sensor whose spring constant is not affected by the adsorption of alkanethiols.

Multi-modal characterization of nanogram amounts of a photosensitive polymer

Here, we demonstrate multi-modal approach of simultaneous characterization of poly(vinyl cinnamate) (PVCN) using a microcantilever sensor. We integrate nanomechanical thermal analysis with photothermal cantilever deflection spectroscopy for discerning ultraviolet (UV) exposure-induced variations in the thermodynamic and thermomechanical properties of the PVCN as a function of temperature and UV irradiation time. UV radiation-induced photo-cross-linking processes in the PVCN are verified with the increase of the Youngs modulus and cantilever deflection as well as the decrease in the hysteresis of deflection and the intensity of C=C peak in the nanomechanical infrared spectrum as a function of UV irradiation time.

Development of Galvanostatic Fourier Transform Electrochemical Impedance Spectroscopy

Here, we report development of the galvanostatic Fourier transform electrochemical impedance spectroscopy (FTEIS), which monitors impedance of electrochemical reactions activated by current steps. We first derive relevant relations for potential change upon application of a step current, obtain impedances theoretically from the relations by simulation, and verify them with experimental results. The validity of the galvanostatic FTEIS technique is demonstrated by measuring impedances of a semiconductive silicon wafer using the conventional frequency response analysis (FRA), the potentiostatic FTEIS, and the galvanostatic FTEIS methods, and the results are in excellent agreement with each other. This work is significant in that the galvanostatic FTEIS would allow one to record impedance changes during charge/discharge cycles of secondary batteries and fuel cells as well as electrochemically irreversible systems which may produce noise level chronoamperometric currents by potentiostatic techniques.

Suspended polymer nanobridge on a quartz resonator

A chemical vapor sensor based on a free-standing polystyrene (PS) nanofilm suspended between the tines of a quartz tuning fork (QTF) is demonstrated. Exposure to ethanol vapor decreased the modulus of the PS membrane, which resulted in a decrease in the resonance frequency of the QTF as a function of ethanol concentration. The suspended PS membrane structure on the QTF allowed gas molecules to diffuse into the membrane from both the top and bottom allowing faster response. The QTF response time was found to be 6.5 times faster than the response time of a conventional PS film-coated resonator sensor.

Photothermal cantilever deflection spectroscopy of a photosensitive polymer

The mechanical and chemical information of a poly(methyl methacrylate) (PMMA) film on a microcantilever were simultaneously acquired by photothermal cantilever deflection spectroscopy as a function of ultraviolet (UV) irradiation time. Nanomechanical infrared (IR) spectra from the PMMA-coated microcantilever agreed well with the Fourier transform infrared spectroscopy (FTIR) spectra of PMMA on gold-coated silicon wafer. The decreasing intensities of nanomechanical IR peaks represent chemical as well as mechanical information of UV radiation-induced photodegradation processes in the PMMA which cannot be obtained by a conventional FTIR technique. The observed decrease in the resonance frequency of the microcantilever is related to the change in the Young’s modulus of the PMMA under UV exposure.