Geunseop Lee

Control of phase transition in quasi-one-dimensional atomic wires by electron doping

We report on the controlled change in the phase transition in In atomic chains on a Si(111) surface by introducing Na as impurity atoms. The Na-induced changes in the transition temperature (Tc) from a metallic room-temperature 4×1 structure into an insulating low-temperature 8×2 structure were determined by using low-energy electron diffraction. The Tc decreased almost linearly when the amount of deposited Na atoms was increased. The decrease in Tc with the increase in the amount of adsorbed Na atoms is suggested to be due to the doping of electrons from adsorbate to the substrate.

Influence of substrate temperature on submonolayer Au adsorption on an Si(111)-(7 × 7) surface studied by scanning tunneling microscopy

The influence of substrate temperature on the adsorption of submonolayer Au on an Si(111)-(7 × 7) surface was investigated using a scanning tunneling microscope. With a small amount of Au deposited at room temperature, Au atoms form small clusters located inside the half unit cells (HUCs) of the Si(111)-(7 × 7) surface while most parts of the surface are undisturbed. When the substrate temperature during deposition of the same Au amounts increases from room temperature up to 565 °C, the small Au clusters grow, coalesce into larger clusters, incorporate Si atoms, and finally form a (5 × 2) reconstruction. The Si(111)-(5 × 2)–Au reconstruction takes place in the form of island–hole pairs on a terrace, whereas only holes are formed in the upper terrace at a step. Both the islands and the holes display the (5 × 2) reconstruction. The occurrence of the island–hole pairs of the (5 × 2) reconstruction results from the fact that the Si atom density in the (5 × 2) reconstruction is lower than in the (7 × 7) reconstruction.

Room-temperature growth of Mg on Si(111): stepwise versus continuous deposition

Using low-energy electron diffraction and scanning tunnelling microscopy, we studied the formation of Mg silicide and metallic Mg islands on a Si(111)-7 × 7 surface at room temperature as a function of Mg coverage. We found that the mechanism by which Mg islands grew on the Si(111)-7 × 7 surface, and the morphology of the islands that formed, depended on whether the Mg deposition was performed in a stepwise or continuous manner. When Mg was deposited in a stepwise manner, with 1xa0h between deposition events, an amorphous Mg silicide overlayer formed on the Si(111)-7 × 7 surface during the initial stage of deposition (up to 2.0xa0ML Mg coverage), as shown by the observation of δ7 × 7 and 1 × 1 low-energy electron diffraction patterns. Upon further stepwise Mg deposition, round-shaped Mg islands grew on the amorphous Mg silicide layer, as shown by scanning tunnelling microscopy and the emergence of a 1 × 1 low-energy electron diffraction pattern. If, on the other hand, the Mg was deposited continuously in a single step, hexagonal Mg islands formed on the flat Mg silicide layers, and a [Formula: see text] and 1 × 1 mixed phase was observed. Moreover, using scanning tunnelling spectroscopy, we confirmed the semiconducting and metallic nature of the Mg silicide layer and hexagonal Mg islands on the Si(111)-7 × 7 surface depending on their Mg coverage, respectively.

Cooperative interplay between impurities and charge density wave in the phase transition of atomic wires

Impurities interact with a charge density wave (CDW) and affect the phase transitions in low-dimensional systems. By using scanning tunneling microscopy, we visualize the interaction between oxygen impurities and the CDW in indium atomic wires on Si(111), a prototypical one-dimensional electronic system, and unveil the microscopic mechanism of the intriguing O-induced increase of the transition temperature (Tc). Driven by the fluctuating CDW, the O atoms adopt an asymmetric structure. By adjusting the asymmetry, a pair of O impurities in close distance can pin the one-dimensional CDW, which develops into the two-dimensional domains. First-principles calculations showed that the asymmetric interstitially-incorporated O defects induce shear strains, which assists the formation of hexagon structure of the CDW phase. The cooperative interplay between the O impurities and the CDW is responsible for the enhancement of the CDW condensation and the consequent increase in Tc.

Guided self-assembly of unidirectionally oriented quasi-one-dimensional 3,4,9,10–perylene-tetracarboxylic-dianhydride chains using surface nanowires

A quasi-one-dimensional chains of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) molecules are grown by using nanowire arrays on a In/Si(111)-(4×1) surface as a template. The PTCDA molecules self-assemble in a “head-and-tail” configuration with their long axis aligned with the direction of the surface nanowires. This unidirectional orientation is unique and indicates that the molecular arrangement is not governed by hydrogen bonding which prevails in the self-assembly of the PTCDA molecules on two-dimensional surfaces. Existence of weak and short-ranged interchain molecular interactions is also deduced, revealing the quasi-one-dimensional character of the molecular chains.

Reinvestigation of the alkali-metal-induced Ge(111)3 × 1 reconstruction on the basis of boundary structure observations.

We have investigated the surface atomic structure of boundary area of Li- and Na-induced Ge(111)3 × 1 reconstruction using scanning tunneling microscope. On Li/Ge(111)3 × 1, the 3 × 1 phase was found to be terminated with a single row in the filled-state image and with dimer-like features in the empty-state image. The images of both interior and boundary of the Li/Ge(111)3 × 1 surface are compatible with the honeycomb-chain-channel (HCC) model, which has substrate atoms with double bonds and is well established as the structure of AM/Si(111)3 × 1 surfaces. In contrast, termination with zigzag double rows at the domain boundary edges was observed in the filled-state images of the Na/Ge(111)3 × 1 phase, which is not reconcilable with the HCC structure. The filled-state STM feature of the boundary region of the Na/Ge(111)3 × 1 phase supports a structural model not having Ge = Ge double bonds, which was proposed to interpret its empty-state images. The trend of bondings between atoms in the surface layer of the AM-induced 3 × 1 reconstruction of Si and Ge is discussed in terms of electronegativity differences.