Hyung Cheoul Shim

Continuous extraction of highly pure metallic single-walled carbon nanotubes in a microfluidic channel

Highly pure metallic single-walled carbon nanotubes were continuously extracted from a mixture of semiconducting and metallic species using a nondestructive, scalable method. Two laminar streams were generated in an H-shaped microfluidic channel with two inlets and two outlets. The flow conditions were carefully controlled to minimize diffusive and convective transport across the boundary between the two flows. Dielectrophoretic force from the embedded electrode at the junction extracted metallic nanotubes from a stream of nanotube suspension toward the other stream of buffer solution without nanotubes. The highly pure metallic and enriched semiconducting nanotubes were obtained simultaneously at each outlet using this novel approach. Excellent selectivity was verified by electrical transport measurement, absorption, and Raman spectroscopic analysis.

Efficient electron transfer in functional assemblies of pyridine-modified NQDs on SWNTs

Nanocrystal quantum dot (NQD)/single-walled carbon nanotube (SWNT) hybrid nanomaterials were synthesized, assembled into field effect transistors (FETs) via dielectrophoresis (DEP), and characterized optically and electronically. The pyridine moiety functioned as a short, noncovalent linker between the NQDs and SWNTs and allowed more efficient carrier transfer through the assemblies without deleteriously altering electronic structures. Photoluminescence studies of the resulting assemblies support an efficient carrier transfer process in CdSe-py-SWNTs unlike that of CdSe/ZnS-py-SWNTs. The use of DEP as a means of controlling the assembly process allowed the creation of a SWNT array containing densely packed CdSe NQDs across a 2 mum gap between electrodes. Observations and characterization of the photocurrent, resistivity, gate dependence, and optical properties of these systems suggest efficient electron transfer from photoexcited NQDs to SWNTs.

Fabrication of probe-typed carbon nanotube point emitters

Probe-typed carbon nanotube (CNT) point emitter was fabricated by attaching single-walled CNT (SWCNT) and double-walled CNT (DWCNT) bundles onto the atomic force microscope tip using dielectrophoresis method. The field emission current from SWCNT point emitter was 4.9μA at 750V, which is corresponding to the emission current density of at least 1.2×103A∕cm2. The Fowler-Nordheim plots for the SWCNT and DWCNT point emitters revealed that the SWCNT bundle consists of more individual SWCNTs than DWCNT bundle and, as a result, the field emission performance of the SWCNT point emitter is better than the DWCNT point emitter. It is suggested that the probe-typed CNT point emitters can be used for microwave amplifiers and high-resolution electron-beam instruments.

Preferential elimination of metallic single-walled carbon nanotubes using microwave irradiation

This study presents a simple, easy and rapid technique for the preferential destruction of metallic single-walled carbon nanotubes (m-SWNTs) using microwave irradiation. The proportion of m-SWNTs in a randomly networked film that were made of pristine SWNTs was gradually reduced with microwave irradiation of 1000 W at 2.45 GHz ranging from 0 to 20 min. Additionally it was observed that the m-SWNTs with a higher chiral angle were destroyed first. The Raman spectra and drain current-gate voltage characteristics curve show that this method facilitated the selective removal of m-SWNTs.

One-Step Deposition of Photovoltaic Layers Using Iodide Terminated PbS Quantum Dots.

We present a one-step layer deposition procedure employing ammonium iodide (NH4I) to achieve photovoltaic quality PbS quantum dot (QD) layers. Ammonium iodide is used to replace the long alkyl organic native ligands binding to the QD surface resulting in iodide terminated QDs that are stabilized in polar solvents such as N,N-dimethylformamide without particle aggregation. We extensively characterized the iodide terminated PbS QD via UV-vis absorption, transmission electron microscopy (TEM), thermogravimetric analysis (TGA), FT-IR transmission spectroscopy, and X-ray photoelectron spectroscopy (XPS). Finally, we fabricated PbS QD photovoltaic cells that employ the iodide terminated PbS QDs. The resulting QD-PV devices achieved a best power conversion efficiency of 2.36% under ambient conditions that is limited by the layer thickness. The PV characteristics compare favorably to similar devices that were prepared using the standard layer-by-layer ethandithiol (EDT) treatment that had a similar layer thickness.

Controlled assembly of CdSe/MWNT hybrid material and its fast photoresponse with wavelength selectivity

We report the novel assembly method of CdSe quantum dot (QD)/pyridine/multi-walled carbon nanotube (CdSe-py-MWNT) hybrid material between electrodes using two-step dielectrophoresis (DEP). At the first step, we assembled the individual MWNT between electrodes by the DEP method. At the second step, the CdSe-py materials were assembled onto the MWNT by DEP method again, which enables site specific and density controlled assembly of QDs. As the photoresponse results, the recovery time of the device fabricated was about 250 times faster than that of a similar CdSe-py-SWNT device using a single-walled carbon nanotube (SWNT) instead of a MWNT. Moreover, it was demonstrated that the optoelectronic property of the device could be modulated by the size of CdSe NQD assembled on a MWNT. We characterized the material and the device by using SEM, TEM, absorption spectroscopy, and optoelectronic instruments.

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.

Purification of carbon nanotubes through an electric field near the arranged microelectrodes

In this work, we attempt to purify multi-walled carbon nanotubes (MWNTs) using electrophoresis induced by the application of an AC electric field to a set of microelectrodes in a microliquid channel. This purifying method is different from conventional methods based on chemical processes. It was observed that most of the MWNTs could pass along the microliquid channel without attaching to the electrode under specific conditions of 1 kHz, at 0.2 Vrms μm−1. On the other hand, the majority of the carbon impurities attached to the electrodes under identical conditions. Field emission scanning electron microscopy (FESEM) images and Raman spectra confirm that this condition is beneficial for removing carbon impurities. The proposed approach has potential applicability in the development of microdevices that can simultaneously perform the purification and fabrication of MWNTs.

Atomic layer deposition effect on the electrical properties of Al2O3-passivated PbS quantum dot field-effect transistors

We have investigated the effect of atomic layer deposition (ALD) on the electrical properties of colloidal PbS quantum dot field-effect transistors (PbS QD-FETs). Low-temperature Al2O3 ALD process was used to fill up the pore spaces of PbS QD films containing 1, 2-ethanedithiol ligands. Upon deposition of Al2O3 on PbS film, the PbS QD-FETs showed ambipolar behavior. The treated film retained this property for over 2 months, despite of exposure to air. This change in the electrical properties of the PbS QD-FETs is attributed to the formation of electron channels in the Al2O3-passivated PbS film. We conclude that these electron transport channels in the AlxOy-PbS film are formed due to substitution of the Pb sites by Al metal and chemical reduction of Pb2+ ions, as determined by an analysis of the depth profile of the film using secondary ion mass spectrometry and X-ray photoelectron spectroscopy.

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.