The 4th IEEE International NanoElectronics Conference, 2011
ABSTRACT Using STM, we have investigated early stages of nanostructures formation upon room tempe... more ABSTRACT Using STM, we have investigated early stages of nanostructures formation upon room temperature (RT) deposition of 0.12 ML Ni onto Ag/Ge(111)-√3×√3 surface. While RT growth mode is determined by cluster formation, annealing up to 200°C promotes cluster nucleation. Further annealing results in creation of hexagonal-shaped islands which conserve on the surface in the unchanged form even after 800°C annealing. Unusual thermal stability of these islands is explained in terms of Ni-Ge compounds formation. Keywords-Ni; Ge(111); Ag; STM; island growth
We have found that Co-2 × 2 islands grown on an Ag/Ge(111)-√3 × √3 surface have hcp structure wit... more We have found that Co-2 × 2 islands grown on an Ag/Ge(111)-√3 × √3 surface have hcp structure with the (11-20) orientation. The island evolution involves transformation of the unit cell shape from parallelogram into rectangular, which is accompanied by the island shape transformation from hexagonal into stripe-like. Identified are two crystallographic directions for the island growth, the pseudo-[0001] and the pseudo-[1-100]. We have observed the occurrence of a lateral shift between the topmost and the underlying bilayers in the case of the island growth along the pseudo-[0001] direction. In contrast, the topmost and the underlying bilayers are unshifted for the growth along the pseudo-[1-100] direction.
ABSTRACT Silicene, a two-dimensional honeycomb sheet consisting of Si atoms, has attracted much a... more ABSTRACT Silicene, a two-dimensional honeycomb sheet consisting of Si atoms, has attracted much attention as a new low-dimensional material because it gains various fascinating characteristics originating from the combination of Dirac fermion features with spin-orbit coupling. The novel properties such as the quantum spin Hall effect and the compatibility with the current Si device technologies have fueled competition to realize the silicene. This review article focuses on the geometric and electronic structures of silicene grown on Ag(1 1 1) investigated by scanning tunneling microcopy (STM), low energy electron diffraction (LEED) and density functional theory (DFT) calculations. The silicene on Ag(1 1 1) takes locally-buckled structure in which the Si atoms are displaced perpendicularly to the basal plane. As a result, several superstructures such as , and etc. emerge. The atomic arrangement of the 4 × 4 silicene has been determined by STM, DFT calculations and LEED dynamical analysis, while the other superstructures remain to be fully-resolved. In the 4 × 4 silicene, Si atoms are arranged to form a buckled honeycomb structure where six Si atoms of 18 Si atoms in the unit cell are displaced vertically. The displacements lead to the vertical shift of the substrate Ag atoms, indicating the non-negligible coupling at the interface between the silicene layer and the substrate. The interface coupling significantly modifies the electronic structure of the 4 × 4 silicene. No Landau level sequences were observed by scanning tunneling spectroscopy (STS) with magnetic fields applied perpendicularly to the sample surface. The DFT calculations showed that the π and π∗ bands derived from the Si 3pz are hybridized with the Ag electronic states, leading to the drastic modification in the band structure and then the absence of Dirac fermion features together with the two-dimensionality in the electronic states. These findings demonstrate that the strong coupling at the interface causes the symmetry breaking for the 4 × 4 silicene and as a result the disappearance of Dirac fermion features. The geometric and electronic structures of other superstructures are also discussed.
ABSTRACT We have investigated the structure of silicene forming into the (4 x 4) superstructure o... more ABSTRACT We have investigated the structure of silicene forming into the (4 x 4) superstructure on Ag(111) by using the low-energy electron diffraction. We found that Si atoms form a buckled honeycomb structure on Ag(111), which fully matches with the structure optimized with the density functional theory calculations. The Si - Si bond lengths range from 2.29 to 2.31 angstrom, which are close to the upper limit of Si=Si double bond and shorter than the bond length in the bulk diamond structure. The buckling in the silicene layer causes the displacement of Ag atoms in the first substrate layer with 0.3 angstrom perpendicular to the surface, which is a compelling evidence of the strong couplings at the interface between the silicene and the substrate.
ABSTRACT The shapes and structures of cobalt islands on Ag/Ge (1 1 1) &surd;3 × &... more ABSTRACT The shapes and structures of cobalt islands on Ag/Ge (1 1 1) &surd;3 × &surd;3 surfaces were studied using a scanning tunnelling microscope and low-energy electron diffraction techniques. Cobalt islands with periodic structures on their flat top layers were found after annealing at 200 300 °C. This new structure of the islands shows reflection symmetry and the mirror planes are along [-2 1 1], [1 1 -2], and [1 -2 1] axes of the Ge (1 1 1) surface. Annealing treatments at temperatures above 200 °C cause the cobalt atoms nucleate to form larger and higher islands. As a consequence, the island densities decrease rapidly as the temperature increases. In addition, an obvious structural and shape transformation of the cobalt islands occurs at 400 500 °C. These changes include the hexagonal shapes of islands, compact arrangement of the surface atoms with hexagonal symmetry, and looser layer separations. These phenomena can be explained by various atomic processes at different temperatures and the interactions between cobalt and the germanium substrate through the silver layer.
ABSTRACT Low energy electron diffraction (LEED), low temperature scanning tunneling microscopy (S... more ABSTRACT Low energy electron diffraction (LEED), low temperature scanning tunneling microscopy (STM) and density functional theory (DFT) based calculations were used to determine the evolution of the silicene structure on a Ag(111) surface. The phase diagram of the structure was obtained using LEED patterns. The corresponding atomic arrangements were confirmed using STM observations. Results show that the structure of silicene is controlled by the substrate temperature during deposition. Finally, we succeeded in synthesizing silicene on silicene/Ag(111), i.e. bilayer silicene.
Gundlach oscillation (or the standing-wave state) is a general phenomenon manifesting in the tunn... more Gundlach oscillation (or the standing-wave state) is a general phenomenon manifesting in the tunneling spectrum acquired from a metal surface using scanning tunneling spectroscopy. Previous studies relate the energy shift between peaks of the lowest-order Gundlach oscillation observed on the thin film and the metal substrate to the difference in their work functions. By observing Gundlach oscillations on Ag/Au(111), Ag/Cu(111), and Co/Cu(111) systems, we demonstrate that the work function difference is not the energy shift of the lowest order but the ones of higher order where a constant energy shift exhibits. Higher order Gundlach oscillations can thus be applied to determine the work function of thin metal films precisely.
The 4th IEEE International NanoElectronics Conference, 2011
ABSTRACT Using STM, we have investigated early stages of nanostructures formation upon room tempe... more ABSTRACT Using STM, we have investigated early stages of nanostructures formation upon room temperature (RT) deposition of 0.12 ML Ni onto Ag/Ge(111)-√3×√3 surface. While RT growth mode is determined by cluster formation, annealing up to 200°C promotes cluster nucleation. Further annealing results in creation of hexagonal-shaped islands which conserve on the surface in the unchanged form even after 800°C annealing. Unusual thermal stability of these islands is explained in terms of Ni-Ge compounds formation. Keywords-Ni; Ge(111); Ag; STM; island growth
We have found that Co-2 × 2 islands grown on an Ag/Ge(111)-√3 × √3 surface have hcp structure wit... more We have found that Co-2 × 2 islands grown on an Ag/Ge(111)-√3 × √3 surface have hcp structure with the (11-20) orientation. The island evolution involves transformation of the unit cell shape from parallelogram into rectangular, which is accompanied by the island shape transformation from hexagonal into stripe-like. Identified are two crystallographic directions for the island growth, the pseudo-[0001] and the pseudo-[1-100]. We have observed the occurrence of a lateral shift between the topmost and the underlying bilayers in the case of the island growth along the pseudo-[0001] direction. In contrast, the topmost and the underlying bilayers are unshifted for the growth along the pseudo-[1-100] direction.
ABSTRACT Silicene, a two-dimensional honeycomb sheet consisting of Si atoms, has attracted much a... more ABSTRACT Silicene, a two-dimensional honeycomb sheet consisting of Si atoms, has attracted much attention as a new low-dimensional material because it gains various fascinating characteristics originating from the combination of Dirac fermion features with spin-orbit coupling. The novel properties such as the quantum spin Hall effect and the compatibility with the current Si device technologies have fueled competition to realize the silicene. This review article focuses on the geometric and electronic structures of silicene grown on Ag(1 1 1) investigated by scanning tunneling microcopy (STM), low energy electron diffraction (LEED) and density functional theory (DFT) calculations. The silicene on Ag(1 1 1) takes locally-buckled structure in which the Si atoms are displaced perpendicularly to the basal plane. As a result, several superstructures such as , and etc. emerge. The atomic arrangement of the 4 × 4 silicene has been determined by STM, DFT calculations and LEED dynamical analysis, while the other superstructures remain to be fully-resolved. In the 4 × 4 silicene, Si atoms are arranged to form a buckled honeycomb structure where six Si atoms of 18 Si atoms in the unit cell are displaced vertically. The displacements lead to the vertical shift of the substrate Ag atoms, indicating the non-negligible coupling at the interface between the silicene layer and the substrate. The interface coupling significantly modifies the electronic structure of the 4 × 4 silicene. No Landau level sequences were observed by scanning tunneling spectroscopy (STS) with magnetic fields applied perpendicularly to the sample surface. The DFT calculations showed that the π and π∗ bands derived from the Si 3pz are hybridized with the Ag electronic states, leading to the drastic modification in the band structure and then the absence of Dirac fermion features together with the two-dimensionality in the electronic states. These findings demonstrate that the strong coupling at the interface causes the symmetry breaking for the 4 × 4 silicene and as a result the disappearance of Dirac fermion features. The geometric and electronic structures of other superstructures are also discussed.
ABSTRACT We have investigated the structure of silicene forming into the (4 x 4) superstructure o... more ABSTRACT We have investigated the structure of silicene forming into the (4 x 4) superstructure on Ag(111) by using the low-energy electron diffraction. We found that Si atoms form a buckled honeycomb structure on Ag(111), which fully matches with the structure optimized with the density functional theory calculations. The Si - Si bond lengths range from 2.29 to 2.31 angstrom, which are close to the upper limit of Si=Si double bond and shorter than the bond length in the bulk diamond structure. The buckling in the silicene layer causes the displacement of Ag atoms in the first substrate layer with 0.3 angstrom perpendicular to the surface, which is a compelling evidence of the strong couplings at the interface between the silicene and the substrate.
ABSTRACT The shapes and structures of cobalt islands on Ag/Ge (1 1 1) &surd;3 × &... more ABSTRACT The shapes and structures of cobalt islands on Ag/Ge (1 1 1) &surd;3 × &surd;3 surfaces were studied using a scanning tunnelling microscope and low-energy electron diffraction techniques. Cobalt islands with periodic structures on their flat top layers were found after annealing at 200 300 °C. This new structure of the islands shows reflection symmetry and the mirror planes are along [-2 1 1], [1 1 -2], and [1 -2 1] axes of the Ge (1 1 1) surface. Annealing treatments at temperatures above 200 °C cause the cobalt atoms nucleate to form larger and higher islands. As a consequence, the island densities decrease rapidly as the temperature increases. In addition, an obvious structural and shape transformation of the cobalt islands occurs at 400 500 °C. These changes include the hexagonal shapes of islands, compact arrangement of the surface atoms with hexagonal symmetry, and looser layer separations. These phenomena can be explained by various atomic processes at different temperatures and the interactions between cobalt and the germanium substrate through the silver layer.
ABSTRACT Low energy electron diffraction (LEED), low temperature scanning tunneling microscopy (S... more ABSTRACT Low energy electron diffraction (LEED), low temperature scanning tunneling microscopy (STM) and density functional theory (DFT) based calculations were used to determine the evolution of the silicene structure on a Ag(111) surface. The phase diagram of the structure was obtained using LEED patterns. The corresponding atomic arrangements were confirmed using STM observations. Results show that the structure of silicene is controlled by the substrate temperature during deposition. Finally, we succeeded in synthesizing silicene on silicene/Ag(111), i.e. bilayer silicene.
Gundlach oscillation (or the standing-wave state) is a general phenomenon manifesting in the tunn... more Gundlach oscillation (or the standing-wave state) is a general phenomenon manifesting in the tunneling spectrum acquired from a metal surface using scanning tunneling spectroscopy. Previous studies relate the energy shift between peaks of the lowest-order Gundlach oscillation observed on the thin film and the metal substrate to the difference in their work functions. By observing Gundlach oscillations on Ag/Au(111), Ag/Cu(111), and Co/Cu(111) systems, we demonstrate that the work function difference is not the energy shift of the lowest order but the ones of higher order where a constant energy shift exhibits. Higher order Gundlach oscillations can thus be applied to determine the work function of thin metal films precisely.
Uploads