Se-Jong Kahng

Bandgap modulation of carbon nanotubes by encapsulated metallofullerenes

Motivated by the technical and economic difficulties in further miniaturizing silicon-based transistors with the present fabrication technologies, there is a strong effort to develop alternative electronic devices, based, for example, on single molecules. Recently, carbon nanotubes have been successfully used for nanometre-sized devices such as diodes, transistors, and random access memory cells. Such nanotube devices are usually very long compared to silicon-based transistors. Here we report a method for dividing a semiconductor nanotube into multiple quantum dots with lengths of about 10 nm by inserting Gd@C82 endohedral fullerenes. The spatial modulation of the nanotube electronic bandgap is observed with a low-temperature scanning tunnelling microscope. We find that a bandgap of ∼0.5 eV is narrowed down to ∼0.1 eV at sites where endohedral metallofullerenes are inserted. This change in bandgap can be explained by local elastic strain and charge transfer at metallofullerene sites. This technique for fabricating an array of quantum dots could be used for nano-electronics and nano-optoelectronics.

Real space imaging of one-dimensional standing waves: direct evidence for a Luttinger liquid.

Electronic standing waves with two different wavelengths were directly mapped near one end of a single-wall carbon nanotube as a function of the tip position and the sample bias voltage with high-resolution position-resolved scanning tunneling spectroscopy. The observed two standing waves caused by separate spin and charge bosonic excitations are found to constitute direct evidence for a Luttinger liquid. The increased group velocity of the charge excitation, the power-law decay of their amplitudes away from the scattering boundary, and the suppression of the density of states near the Fermi level were also directly observed or calculated from the two different standing waves.

Phase separation in a two-dimensional Co–Cr alloy

Two-dimensional phase separation in a Co–Cr random binary alloy on a W(110) surface was confirmed with ultra-high-vacuum scanning tunneling microscopy. With 1.1 ML Co and 0.1 ML Cr co-deposited at room temperature, the film shows a kinetically limited structure. Upon annealing, phase separation between Co and Cr-rich phases of ∼100 A size was observed. Evidence of compositional inhomogeneity was observed in Cr-rich phases but not in Co-rich phases.

Intermixing in Stranski–Krastanov germanium overlayer on Si(100)

The growth behaviors of Stranski–Krastanov Ge overlayers on the Si(100)-(2×1) surface were studied with a combination of scanning tunneling microscopy and medium energy ion scattering spectroscopy. At the growth temperature of 350 °C, as Ge coverage increases, the Si surface peak in the channeling spectra decreases due to the shadowing of Ge atoms and can be fitted by a simple growth model at the Ge coverage less than 4 monolayers. At Ge coverages between 4 and 8 monolayers, the Si surface peak shows a broad enhancement, which can be explained with the growth of hut pits and that of three-dimensional hut clusters at the expense of Ge wetting layers.

Epitaxial Growth of Graphene by Surface Segregation and Chemical Vapor Deposition on Ru(0001) Studied with Scanning Tunneling Microscopy

Epitaxial graphene on metal substrates provides excellent platforms to study its atomic and electronic structures, and can be grown either by surface segregation of carbon or by chemical vapor deposition. The growth behaviors of the two methods, however, have not been directly compared each other. Here, we studied domain structures of graphene grown by three different methods, surface segregation, post-annealing with adsorbed ethylene, and high-temperature dose of ethylene, using scanning tunneling microscopy. The first two methods resulted in graphene regions with areas of 100 ㎚², whereas the third method showed large area graphene (＞10⁴ ㎚²) with regular hexagonal Moire patterns, implying that high-temperature dose of ethylene is preferable for further studies on graphene such as additional growth of organic molecules.

Surface Diffusion Coefficients of Adatoms on Strained Overlayers

Adatom kinetics on the surfaces of Co overlayers, prepared on the W(110) surface, was studied with scanning tunneling microscopy. By counting the number-density of the adatom-islands, we estimated the ratio of adatom diffusion coefficients. The ratio DW (110) : D 1ML Co : D 2ML Co was measured to be 1 : 125 : 33000 at room temperature, where D W(110) , D 1ML Co , and D 2ML Co are the diffusion coefficients on bare W(110) surface, on one-monolayer Co overlayer, and on two-monolayers Co overlayers, respectively. An increased diffusion coefficient on two-ML Co overlayers, relative to that on one-ML Co overlayers, was explained with the heteroepitaxial strain effect.

Kinetic processes in the growth and decomposition of a two-dimensional binary alloy

Abstract The growth and decomposition of a two-dimensional cobalt–chromium binary alloy on the W(110) surface were studied with scanning tunneling microscopy. At room temperature, co-deposited cobalt and chromium were found to grow in a homogeneous nucleation scheme. At the annealing temperature of 150°C, the co-deposited films decompose into two phases, cobalt-rich and chromium-rich phases, showing good agreement with the reported phase diagram. In the decomposed film with 0.9 monolayer of cobalt and 0.2 monolayer of chromium, the second-layer islands are cobalt-rich phases.