Seungmuk Ji

Carbon nanotube film piezoresistors embedded in polymer membranes

We present carbon nanotube film (CNF) piezoresistors embedded in polymer membranes. CNFs by vacuum filtration are patterned on an Au-deposited Si-wafer and transferred onto the poly-dimethylsiloxane (PDMS) using the weak adhesion property between Au-layer and Si-wafer. Transmittance and I-V characteristic are measured to confirm transferred CNFs as transparent electrodes. The pressure sensor consists of CNF piezoresistors embedded in 130 μm thick circular PDMS membranes. The gauge factor of CNFs at different thickness is obtained around 10–20 when the resistance increases from 2.7 to 5.6 kΩ with applied pressure, which shows that CNFs can be used as transparent piezoresistors in polymer-based microelectromechanical systems.

Enhanced ultraviolet emission from hybrid structures of single-walled carbon nanotubes/ZnO films

We report interesting observation of strong enhancement of ultraviolet luminescence from hybrid structures of single-walled carbon nanotubes (SWNTs)/ZnO. SWNTs of 3–120 nm thickness (t) were deposited on top of 100 nm ZnO films/n-type Si (100) wafer by spin coating and vacuum filtration to form the hybrid structures. Photoluminescence (PL) intensity of the hybrid structures increases with increasing t up to 10 nm, becomes almost ten times larger at t=10 nm than that of the bare ZnO film and decreases with increasing t above 10 nm. This strong PL enhancement is also confirmed by PL mapping. These findings are discussed based on the surface-plasmon-mediated emission mechanism.

Effects of polymer coating on the adsorption of gas molecules on carbon nanotube networks

Functionalization of carbon nanotubes (CNTs) with polymers such as polyethyleneimine or nafion was found to not change the adsorption properties of gas molecules on CNTs, although functionalization can remarkably enhance the sensitivity of gas sensors. Charge transfer between adsorbed molecules and single-walled carbon nanotubes (SWCNTs) at defect sites causes a steep and nonlinear conductance change at low gas concentrations, while molecules physically adsorbed on the pristine surfaces result in the linear electrical responses at higher concentrations. In addition, the molecular binding energies at defect sites of SWCNTs were measured to be 0.61eV for NO2 and 0.53eV for NH3.

Fabrication of a Hybrid Superhydrophobic/superhydrophilic Surface for Water Collection: Gravure Offset Printing & Colloidal Lithography

We demonstrate the desert beetle back mimicking hybrid superhydrophilic/superhydrophobic patterned surface by using the combination method of colloidal lithography and gravure offset printing for nano and micro patterning, respectively. The two methods are cost-effective and industrially available techniques compared to the other nano/micro patterning methods. To verify the water collecting function of the hybrid surface, the water condensation behavior is investigated on the chilled surface in ambient temperature and high humidity. Due to the synergetic effect of drop and film wise condensation, the hybrid superhydrophobic/superhydrophilic surface shows the higher efficiency than one of single wettability surfaces. The work is underway to get the good patterns of hybrid surfaces for water collecting from the dew or fog.

Anti-reflective conducting indium oxide layer on nanostructured substrate as a function of aspect ratio

Antireflective conducting indium oxide layers were deposited using atomic layer deposition on a transparent nanostructured substrate grown using colloidal lithography. In order to explain the changes in the electrical resistivity and the optical transmittance of conducting indium oxide layers depending on various aspect ratios of the nanostructured substrates, we investigated the surface area and refractive index of the indium oxide layers in the film depth direction as a function of aspect ratio. The conformal indium oxide layer on a transparent nanostructured substrate with optimized geometry exhibited transmittance of 88% and resistivity of 7.32 × 10−4 Ω cm. The enhancement of electrical resistivity is strongly correlated with the surface area of the indium oxide layer depending on the aspect ratio of the nanostructured substrates. In addition, the improvement in transparency was explained by the gradual changes of the refractive index in the film depth direction according to the aspect ratio of the nanostructures.

Enhanced water droplet mobility on superhydrophobic rippled nanoshell array

A 3-dimensional rippled nanoshell structure (a hollow pillar with wriggly sidewall morphology) is demonstrated for superhydrophobicity. As a control group, a straight nanoshell structure without a rippled shape was also prepared. The rippled structure showed improved superhydrophobicity with a large contact angle and a small sliding angle compared to the straight nanoshell structure. These enhancements originate from the minimum of interfacial energy at the triple-phase contact line, which is located at the most protruded circular line along the rippled structure. Using a drop impingement test, the stabilization of a Cassie Baxter state on the rippled structure was also verified. The experimental observation of wetting transition from a Cassie-Baxter to a Wenzel state is well explained by a revamped capillary pressure model, which was customized for the rippled structure.

Controlled Growth of Multi-walled Carbon Nanotubes Using Arrays of Ni Nanoparticles

We have investigated the optimal growth conditions of carbon nanotubes (CNTs) using the chemical vapor deposition and the Ni nanoparticle arrays. The diameter of the CNT is shown to be controlled down to below 20 ㎚ by changing the size of Ni particle. The position and size of Ni particles are controlled continuously by using wafer-scale compatible methods such as lithography, ion-milling, and chemical etching. Using optimal growth conditions of temperature, carbon feedstock, and carrier gases, we have demonstrated that an individual CNT can be grown from each Ni nanoparticle with almost 100% probability over wide area of SiO₂/Si wafer. The position, diameter, and wall thickness of the CNT are shown to be controlled by adjusting the growth conditions.