Soongeun Kwon

Experimental determination of the spring constant of an individual multiwalled carbon nanotube cantilever using fluorescence measurement

We report an experimental method to determine the spring constant of a multiwalled carbon nanotube (MWNT) cantilever as a mechanical piconewton force transducer. Electrostatic actuation was employed to investigate the mechanical properties of a MWNT cantilever. In order to measure nanotube’s deflection during actuation, fluorescent dyes were noncovalently attached to the end of the nanotubes. Also, the length dependence of the spring constant is studied by adjusting the length of MWNT via electrochemical etching. The results show that the spring constant of a MWNT cantilever is as small as 0.001 N/m and tunable in the range of 0.001–0.05 N/m.

Deterministic fabrication of carbon nanotube probes using the dielectrophoretic assembly and electrical detection

We report the controlled dielectrophoretic assembly for the deterministic fabrication of carbon nanotube (CNT) probes. Electrical detection was applied to the dielectrophoretic assembly of CNT probes. Dielectrophoretic manipulation with an ac electric field of 5 MHz was used to form the CNT bridge across oppositely aligned tungsten tips (W-tips). A dc electric field was simultaneously applied to monitor the direct current flowing through the gap. The detected nanocurrent reveals that the CNT bridge is formed between W-tips in real time. We compared current data with bundle diameter of CNT probes in field emission scanning electron microscopy (FE-SEM) images. As the number of assembled CNTs increased, current was increased. With the obtained linear relationship, the number of the attached CNTs can be estimated without confirmation of the FE-SEM image. This combined use of the current detection method with dielectrophoretic manipulation will provide a reliable process for the fabrication of CNT probes.

Dielectrophoretic assembly of semiconducting single-walled carbon nanotube transistor

Abstract A novel burning technique for making a semiconducting single-walled carbon nanotubes (SWNTs) transistor assembled by the dielectrophoretic force was suggested. The fabrication process consisted of two steps. First, to align and attach a bundle of SWNTs between the source and drain, the alternating (AC) voltage was applied to the electrodes. When a bundle of SWNTs was connected between two electrodes, some of metallic nanotubes and semi-conducing nanotubes existed together. The second step is to burn the metallic SWNTS by applying the voltage between two electrodes. With increasing the voltage, more current flowed through the metallic SWNTs, thus, the metallic SWNTs burnt earlier than the semiconducting one. This technique enables to obtain only semi-conducting SWNTs connection in the transistor. Through the I-V characteristic graph, the moment of metallic SWNTs burning and the characteristic of semi-conducing nanotubes were verified.

Nanometer displacement measurement of a multiwalled carbon nanotube cantilever under aqueous conditions

In this study, we report nanometer displacement measurement of an individual multiwalled nanotube (MWNT) in liquid, based on the high accuracy localization of individual fluorescent nanoparticles. In order to visualize a MWNT cantilever in liquid, a fluorescent polystyrene nanoparticle with an amine conjugate was selectively attached at the end of a nanotube by noncovalent hydrogen bonding between amine and carboxylic groups. Physical absorption of ethylenediamine gas vapor onto an as-fabricated MWNT cantilever renders the nanotube hydrophilic, enabling manipulation of the MWNT cantilever in liquid without bending or breaking. A fluorescent nanoparticle was localized by a two-dimensional Gaussian fit for the fluorescence intensity of the particle. During the manipulation of the nanotube cantilever in liquid, the displacement was determined by the positional change of the localized nanoparticle. The measurement technique was evaluated by measuring the displacement of a MWNT cantilever subjected to controlled manipulation such as a single line scan, step and stair response. The positional accuracy of the measurement was experimentally found to be 7 nm. The fluorescence measurement of a hydrophilic MWNT cantilever can be further used in biological applications such as biochemical sensors and single molecule force spectroscopy.

Quantitative displacement measurement of a nanotube cantilever with nanometer accuracy using epifluorescence microscopy

A method to measure the deflection of a nanotube cantilever with nanometer accuracy in an air or liquid environment is presented. We attached fluorescent dyes at the end of a nanotube to detect its deflection. The nanotube cantilever was fabricated with a multiwalled carbon nanotube that is attached to the end of an electrochemically etched tungsten tip, and it was imaged in an epifluorescence microscope system. The fluorescence intensity distribution of the fluorescent particles at the end of the nanotube was approximated with a Gaussian and fitted by least-squares method. Finally, we were able to measure the displacement of the nanotube cantilever during electrostatic actuation with positional accuracy of a few nanometers. This technique can be applied to a manipulator or a force transducer on related a few piconewton forces.

Distribution of electric field for carbon nanotube assembly: Experiments (II)

The distribution effect of electric field on the alignment and attachment of carbon nanotubes (CNTs) were investigated. The experimental results were compared with the simulation results according to three different shaped electrodes. In previous simulation, the round shaped electrodes were expected to be more effective for aligning and attaching a single CNT between two electrodes than conical or rectangular shaped electrodes. To verify the simulation results, three different shaped electrodes were introduced and a single multi-walled carbon nanotube (MWNT) was attached. The optimal conditions for aligning and attaching MWNTs such as the frequency, applied voltage and concentration of MWNTs solution were investigated. Through repeated experiments, frequency of 100 kHz-10 MHz, applied voltage of 0.3-1.3 Vrms/μm, concentration of 5 μg/mL in MWNTs solution were obtained as a possible condition range to attach MWNTs. Under these conditions, the yield of MWNTs attachment between two electrodes was up to 70%. In previous simulation, furthermore, it was verified that the size of the stable or quasi-stable region made CNTs aligned and attached on different shaped electrodes from the comparison of the experimental and simulation results. Most single MWNT attachment was accomplished on the round shaped electrodes.

Distribution of electric field for carbon nanotube assembly: Simulation (I)

Abstract The distribution of electric field for the alignment and attachment of carbon nanotubes (CNTs) was simulated. To be attached at the desired place, the aligned and attracted CNTs should be stayed in the desired area called the stable region or the quasi-stable region for an instant where the change of electric field is minimized. Since the conical electrode has the very narrow sized quasi-stable region, few CNTs can be attached. The rectangular electrodes have a wide stable region, so lots of CNTs can be attached. The results indicate that the round electrode which has a proper sized quasi-stable region is more effective for aligning and attaching a single CNT than the conical or rectangular shaped electrodes.

Efficient fabrication of wafer scale thin film of individualized single-walled carbon nanotubes by dual-nozzle spin casting

In this paper, a dual-nozzle spin casting method was proposed to form a thin film of individualized single-walled carbon nanotubes (SWNTs) at the wafer scale. Each nozzle simultaneously ejected the SWNT solution and methanol, respectively. During the ejection process, two solutions were mixed at the contacting end of the nozzles and then dropped onto the substrate. Functionalization of the wafer substrate with the amine group improved the uniformity of the SWNT thin film as well as the adhesion between the individualized SWNTs and the substrate. The best condition of the spin casting involved the substrate functionalization using 3-aminopropyltriethosilane aqueous solution with a concentration of approximately 10 mM and a deposition velocity of approximately 5000 rpm. The root-mean-square roughness of the fabricated SWNT layer over the wafer substrate was found to be 1.4-1.8 nm, which indicated that the resultant thin film was one or two layers of SWNTs. The wafer scale SWNT thin film formed by dual-nozzle spin casting can be further used for the mass production and high integration of the SWNT nanoelectronic devices.

Laser-assisted selective lithography of reduced graphene oxide for fabrication of graphene-based out-of-plane tandem microsupercapacitors with large capacitance

We present a laser lithography technique that uses a focused laser beam to fabricate out-of-plane tandem microsupercapacitors (MSCs) from reduced graphene oxide (rGO) with large areal capacitance. By controlling the depth of focus in a laser beam focused by an objective lens during laser lithography on a graphene oxide (GO) film, a rGO/GO/rGO structure is formed in the GO film, and subsequently, two independent interdigitated electrodes (IDEs) were fabricated on the top and bottom surfaces of the GO film. The out-of-plane tandem MSC with a parallel assembly of two rGO-IDEs showed two times larger areal capacitance than an in-plane single MSC with one rGO-IDE in the same MSC device footprint. The laser-assisted selective lithography technique using a focused laser beam developed in this study can be further applied to improve the energy density of MSCs without increasing the electrode area by vertically stacking multiple out-of-plane tandem IDEs.