Tae-Youl Choi

Focused ion beam-assisted manipulation of single and double β-SiC nanowires and their thermal conductivity measurements by the four-point-probe 3-ω method

Control of one-dimensional (1D) nanostructures is demonstrated in this paper by selectively placing and aligning silicon carbide (beta-SiC) nanowires (NWs). We developed a reliable and highly reproducible way of placing a single or double SiC NW on pre-patterned electrodes by using a focused ion beam and a nanomanipulator. 3-omega signals obtained by the four-point-probe method were used in measuring the thermal conductivity of the NWs. The thermal conductivities of the placed single and double beta-SiC NWs were obtained at 82 +/- 6 W mK( - 1) and 73 +/- 5 W mK( - 1), respectively. The proposed technique offers new possibilities for manipulating and evaluating 1D nanoscale materials.

Individual carbon nanotube soldering with gold nanoink deposition

A method for soldering carbon nanotubes lying on microfabricated metal pads is presented. By employing the fountain-pen principle, the authors deposited a gold nanoparticle suspension (nanoink) film on the area where the carbon nanotube contacts the metal pad. The nanoink was deposited by using a capillary tube that was pulled into a micropipette with the tip outer diameter of 2μm. Individual carbon nanotubes were aligned selectively across the electrodes dielectrophoretically. After annealing and sintering of the nanoink pattern the four-point-probe resistance of the carbon nanotubes was measured, resulting in a good Ohmic or near-Ohmic contact (2–15kΩ).

A high-precision micropipette sensor for cellular-level real-time thermal characterization.

We report herein development of a novel glass micropipette thermal sensor fabricated in a cost-effective manner, which is capable of measuring steady thermal fluctuation at spatial resolution of ∼2 μm with an accuracy of ±0.01 °C. We produced and tested various micrometer-sized sensors, ranging from 2 μm to 30 μm. The sensor comprises unleaded low-melting-point solder alloy (Sn-based) as a core metal inside a pulled borosilicate glass pipette and a thin film of nickel coating outside, creating a thermocouple junction at the tip. The sensor was calibrated using a thermally insulated calibration chamber, the temperature of which can be controlled with an accuracy of ±0.01 °C, and the thermoelectric power (Seebeck coefficient) of the sensor was recorded from 8.46 to 8.86 μV/°C. We have demonstrated the capability of measuring temperatures at a cellular level by inserting our temperature sensor into the membrane of a live retinal pigment epithelium cell subjected to a laser beam with a focal spot of 6 μm. We measured transient temperature profiles and the maximum temperatures were in the range of 38–55 ± 0.5 °C.

Steady heat conduction-based thermal conductivity measurement of single walled carbon nanotubes thin film using a micropipette thermal sensor

In this paper, we describe the thermal conductivity measurement of single-walled carbon nanotubes thin film using a laser point source-based steady state heat conduction method. A high precision micropipette thermal sensor fabricated with a sensing tip size varying from 2 μm to 5 μm and capable of measuring thermal fluctuation with resolution of ±0.01 K was used to measure the temperature gradient across the suspended carbon nanotubes (CNT) film with a thickness of 100 nm. We used a steady heat conduction model to correlate the temperature gradient to the thermal conductivity of the film. We measured the average thermal conductivity of CNT film as 74.3 ± 7.9 W m(-1) K(-1) at room temperature.

Competition Between Resonant Plasmonic Coupling and Electrostatic Interaction in Reduced Graphene Oxide Quantum Dots

The light emission from reduced graphene oxide quantum dots (rGO-QDs) exhibit a significant enhancement in photoluminescence (PL) due to localized surface plasmon (LSP) interactions. Silver and gold nanoparticles (NPs) coupled to rGO nanoparticles exhibit the effect of resonant LSP coupling on the emission processes. Enhancement of the radiative recombination rate in the presence of Ag-NPs induced LSP tuned to the emission energy results in a four-fold increase in PL intensity. The localized field due to the resonantly coupled LSP modes induces n-π* transitions that are not observed in the absence of the resonant interaction of the plasmons with the excitons. An increase in the density of the Ag-NPs result in a detuning of the LSP energy from the emission energy of the nanoparticles. The detuning is due to the cumulative effect of the red-shift in the LSP energy and the electrostatic field induced blue shift in the PL energy of the rGO-QDs. The detuning quenches the PL emission from rGO-QDs at higher concentration of Ag NPs due to non-dissipative effects unlike plasmon induced Joule heating that occurs under resonance conditions. An increase in Au nanoparticles concentration results in an enhancement of PL emission due to electrostatic image charge effect.

Silver nanostructure sensing platform for maximum-contrast fluorescence cell imaging

We present herein a silver nanostructure-assisted sensing platform which consists of a combined structure of Ag nanowire (NW) and nanodot (ND) array. Highly enhanced fluorescence from fluorophore is attributed to a strongly coupled optical near-field interaction between proximately located Ag NW and NDs. We obtained enhanced fluorescence intensity with up to 140 folds, as contrasted from background intensity, reaching a theoretical maximum value. On the other hand, fluorescence lifetime was greatly reduced to 0.27 ns (from 2.17 ns for the same fluorophores without nanostructure). This novel platform can be a promising utility for optical imaging and labeling of biological systems with a great sensitivity.

Focused-ion-beam-assisted selective control of graphene layers: acquisition of clean-cut ultra thin graphitic film.

A focused-ion beam (FIB) and a nanomanipulator provide a novel way to selectively control and obtain a few layers of graphene. Because of its weak van der Waals force in the interlayer of graphite, the nanomanipulator could easily exfoliate a graphitic thin layer with no wrinkles on the surface from a highly oriented pyrolitic graphite (HOPG) by applying a shear force which exceeds the static interlayer shear force. Subsequently, a few layers of graphene were successfully obtained by applying a uniform shear force from a detached graphitic thin layer that had been transferred to a pre-determined site on an oxide wafer. The required shear force for clean cleavage of a graphitic thin layer was then estimated based upon experimental data. Raman scattering analysis was used to confirm the number of placed graphene layers and the placement of a few layers of graphene was projected to have about five atomic layers.

Numerical simulations of supersonic gas atomization of liquid metal droplets

Computational fluid dynamics simulations incorporating supersonic turbulent gas flow models and a droplet breakup model are performed to study supersonic gas atomization for producing micron-sized metal powder particles. Generally such atomization occurs in two stages: a primary breakup and a secondary breakup. Since the final droplet size is primarily determined by the secondary breakup, parent droplets of certain sizes (1 to 5 mm) typically resulting from the primary breakup are released at the corner of the nozzle and undergo the secondary breakup. A comparison of flow patterns with and without the introduction of a liquid melt clearly indicates that the mass loading effect is quite significant as a result of the gas–droplet interactions. The flow pattern change reasonably explains why the final droplets have a bimodal mass size distribution. The transient size changes of the droplets are well described by the behavior of the Weber number. The present results based on the 1 mm parent droplets best fit previous experimental results. Moreover, the effects of inlet gas pressure and temperature are investigated in an attempt to further reduce droplet size.

Micropipette-based thermal sensor for biological applications

We present herein a novel technique for the fabrication of a glass micropipette thermal sensor utilizing inexpensive thermocouple materials and we are capable of measuring thermal fluctuation with accuracy of ± 0.002 degree Celsius. We produced and tested various sensors in the sensing tip size, which ranges from 5 µm to 30 µm. The sensors comprise unleaded low melting point solder alloy (Sn) as a core metal inside a borosilicate glass pipette and thin film of nickel coated outside, creating a thermocouple junction at the tip. The sensor was calibrated by using a thermally insulated calibration chamber, the temperature of which can be controlled with precision of ± 0.1 °C, and the thermoelectric power (Seebeck coefficient) of the sensor was recorded from 8.46 to 8.86 µV/°C. The sensor we have produced is cost-effective and reliable in application for thermal conductivity measurement of micro-electro-mechanical systems (MEMS), and biological temperature sensing in micro level.

Focused Ion Beam in Thermal Science and Engineering

Laboratory of Thermodynamics, ETH Zurich CH-8092 Switzerland In this paper, utilizing a focused ion beam source for applications in thermal science is demonstrated. Focused-ion-beam (FIB) nanolithography has been used to make electrical contacts on nanometer-sized materials. However, a foreseeable damage occurring in these materials by highly energetic ion bombardment during metal deposition restrains its use. The appearance of the dual beam (conventional FIB with a scanning electron microscope) has facilitated the use of dual-beam FIB nanolithography owing to the possibility of acquiring electron beam images in situ and dissociating metalorganic compounds, giving rise to electron-beam-induced metal deposition [1]. Because interaction between electrons and the sample is less destructive than using ions, performing an electron-assisted deposition on the nanostructure can minimize undesired surface damage and structure modification of the nanostructure. The thermal properties of individual multi-walled carbon nanotubes was measured by utilizing the 4-point-probe 3-ω method, based on the fact that the third harmonic amplitude and phase as a response to applied alternate current at fundamental frequency, ω, can be expressed in terms of thermal conductivity and diffusivity. To this end, a microfabricated device composed of four metal electrodes was modified to manufacture nanometer-sized wires by using a focused ion beam source as shown in Fig. 1a. A carbon nanotube could then be suspended over a deep trench milled by the focused ion beam, preventing heat loss to the substrate (Fig. 1b). To deposit individual carbon nanotubes, we utilized the principle of dielectrophoresis (DEP). The dielectrophoresis has been considered as an excellent method for CNT manipulation. Herein, we applied a high-frequency (5 MHz) ac field combined with dc offset between the pads, 1 and 4. When a CNT touches the electrodes, it will stick to them and stays there by Van der Waals force [2]. Finally, to minimize the electrical contact resistance between carbon nanotubes and the metal electrodes, electron beam “soldering” was performed. The pads 1 and 4 have round shapes on the tip, which aids carbon nanotube deposition. Fig. 2 shows three different measurements of 3-ω signals for carbon nanotubes submerged in vacuum, air and deionized water. To this end, the 4-wire configuration (Fig. 1c) was utilized for measuring the 3