Taekyung Kim

Crystalline and amorphous structures of Ge–Sb–Te nanoparticles

We report effects of thermal annealing on the structures of Ge–Sb–Te (GST) nanoparticles synthesized by pulsed laser ablation deposition. The average diameter of the GST nanoparticles is an order of 10nm. The as-prepared sample contains nanocrystals surrounded by an amorphous phase. Further crystallization occurs during annealing. The structures of the nanocrystals and amorphous phase were studied by electron diffraction and radial distribution function analyses. The results show that the nanoparticles annealed at 100°C are crystalline, consisting of a mixture of face centered cubic (fcc) and hexagonal Ge2Sb2Te5 (dominant). In comparison, the nanoparticles annealed at 200°C are mostly fcc. The surrounding amorphous phase has similar atomic arrangements to the previously reported amorphous GST thin films.

Electrical transport in small bundles of single-walled carbon nanotubes: Intertube interaction and effects of tube deformation

We report a combined electronic transport and structural characterization study of small carbon nanotube bundles in field-effect transistors (FETs). The atomic structures of the bundles are determined by electron diffraction using an observation window built in the FET. The electrical transport of single-walled nanotube bundles depends on the structure of individual tubes, deformation due to intertube interaction, and the orientation with respect to the electric field. Ab initio simulations show that tube deformation in the bundle induces a band gap opening in a metallic tube. These results show the importance of intertube interaction in electrical transport of bundled carbon nanotubes.

Growth modes of carbon nanotubes on metal substrates

Growth temperature induced changes in Al∕Fe thin film catalysts are examined for chemical vapor deposition of carbon nanotubes directly on metal substrates. The film thickness, growth temperature, and supporting substrate affect the size and the density of Fe catalyst nanoparticles which in turn control the diameter, length, and single versus multiwalled nature of carbon nanotubes. Growths on two metal substrates, Au and Mo, using sputter deposited Al∕Fe thin films are compared by transmission and scanning electron microscopy, and Raman analyses. Striking differences in the growth modes are observed with Au substrate enhancing multiwalled nanotube growth with metal catalyst particles at the tip away from the substrate and Mo substrate promoting single-walled nanotube growth with the catalyst nanoparticles remaining on the substrate. Oxidative treatment of Mo underlayer (i.e., relatively thick layer Mo sputtered on Au prior to Al∕Fe catalyst deposition) can also induce nanotube growth with catalyst particl...

Quantitative structural analysis of individual nanotubes by electron diffraction

A general method for quantitative structure analysis of individual, cylindrical, carbon nanotubes is described here. The method is based on electron diffraction of individual nanotubes and analysis using a combination of helical diffraction theory and diffraction geometry of the underlying lattice. Experimental recording of nanotube diffraction is achieved using a nanometer-sized electron beam. Procedures are developed for 1) the measurement of chiral angles in both single- and multi-wall nanotubes and 2) structure determination based on Bessel function fitting of layer line intensity oscillations. The accuracy of the method is demonstrated for the structure determination of a single- and double-wall carbon nanotubes and partial structural analysis of a multiwall carbon nanotube. The results show that the single-, double- and incommensurate multi-wall tubes are well described by the cylindrical tube model. However, a large Debye-Waller factor in the radial direction is obtained. The method developed here is general and can be applied to other cylindrical nanotubes.

Imaging suspended carbon nanotubes in field-effect transistors configured with microfabricated slits for transmission electron microscopy

Field-effect transistors with carbon nanotubes (CNTs) suspended across etched slits and fabricated by chemical vapor deposition have been characterized by electrical measurements and transmission electron microscopy. Two devices are examined here: One is semiconducting from two single-wall CNTs, and the other is semiconducting and metallic, with a large off current, that comes from multiple nanotubes. The study highlights the importance of structural characterization in understanding the performance of CNT devices.

Optical manipulation of silicon microparticles in biological environments

Manipulation of micron-scale silicon particles has been investigated with optical tweezers implemented using a two-dimensional scanning trap driven with acousto-optic modulators. Spheres of latex, Poly(methyl methacrylate) (PMMA), silica, and silver-coated PMMA have been utilized to calibrate transverse trapping forces. The goal of this work is to non-invasively manipulate 10-20 μm silicon-based devices in and around cells.