Hyungjoon Shim

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.

Direct observation of hopping and merging of single Au adatoms to form dimers on Si(111)-(7 × 7)

Au single adatoms and dimers were imaged on Si(111)-(7 × 7) at different temperatures and bias voltages using a variable-temperature scanning tunneling microscope. At room temperature (RT), a single Au adatom induces sharp highlighted triangular features in the half unit cells (HUCs) of Si(111)-(7 × 7). These triangular features become fuzzy at temperatures lower than 225 K, as a result of the reduced moving speed of the single Au adatoms inside the HUCs. The formation of an Au adatom dimer was directly observed at RT when a single Au adatom in a HUC jumped into a neighboring HUC that already contained a single Au adatom. The Au adatom dimer appears either as a noisy feature in the central area of the HUC defined by three Si center adatoms or as a bright protrusion located close to a corner Si adatom site at RT. It was observed that a noisy feature also can change into a bright protrusion, inducing charge redistribution in the nearby Si adatoms in both the occupied and neighboring HUCs.

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.

Analysis of Core-Level Spectra of the Li/Ge(111)-3× 1 and Na/Ge(111)-3× 1 Surfaces

By analyzing the Ge 3 d core-level photoelectron spectra, we studied the structures of the Na/Ge(111)-3× 1 and Li/Ge(111)-3× 1 surfaces. For both surfaces, two surface related components lying on either side of the main bulk peak were identified in the spectra. The similarity of the spectra from the two surfaces indicates that the surfaces do indeed share a common structure. The core-level photoelectron spectra of the Na- and Li-induced Ge(111)-3× 1 surfaces are well consistent with the honeycomb-chain-channel model, which was proposed as the structure of the Si(111)-3× 1 induced by alkali metals.

Homogeneous and heterogeneous nucleations in the surface phase transition: Si(111)4 × 1-In

Homogeneous and heterogeneous nucleations in a reduced-dimensional system undergoing a first-order structural phase transition were examined by using low electron energy diffraction and scanning tunneling microscopy. The high-temperature 4 × 1 phase of a Si(111)-In surface was supercooled at temperatures below the transition temperature () and evolved slowly into a low-temperature 8 × 2 phase with time. The transition rate decreased significantly as the temperature approached . The kinetics of the observed homogeneous nucleation was analyzed by classical nucleation theory. The introduction of oxygen atoms reduced the hysteresis and accelerated nucleation significantly, showing that the -raising oxygen impurity plays the role of a nucleation seed for heterogeneous nucleation.

Phase transition of In/Si(111)-4×1 surface studied with low-energy electron diffraction

The structural phase transition of an In/Si(111) surface was examined by low-energy electron diffraction (LEED). The transition temperature between the room-temperature 4×1 structure and low-temperature 8×2 structure was determined from the changes in the LEED intensity of the half-order (×2), eighth-order (8×), and fourth-order (4×) spots with temperature. The transition temperatures determined independently from three sets of LEED beam spots were within 1 K. The differently prepared In/Si(111)−4×1 surfaces, which had similar LEED quality at room temperature, showed variations of the transition temperature. The differences in the measured transition temperatures were attributed to the effects of the additional In adatoms remaining on the surface during the formation of a 4×1 reconstruction. The In adatoms suppress the condensation of the (8×2) phase, decreasing the phase transition temperature and rounding the otherwise sharp transition.

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.