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We report a method for the fabrication of high structural quality CrN nanoislands on Cu(001). The CrN nanoislands can be fabricated in ultrahigh vacuum conditions by means of Cr atoms deposition on saturated Cu(001)c(2 × 2)-N surface and subsequent annealing at 500 °C. Existence of two types of nanoislands is shown by scanning tunneling microscopy investigations with atomic resolution for different CrN nanoislands formed on surface with 0.35 monolayer Cr coverage. The measured in-plane lattice constant (a = 0.39 ± 0.01 nm) of the CrN nanoislands is 8% larger than the Cu substrate lattice. X-ray photoemission spectroscopy investigations reveal chromium nitride phase transition from CrN to Cr 2 N associated with changes of Cr concentration.

Nanostructures and nanopattern formation by mixing Rh atoms were studied on the N-saturated Cu(001) surface using scanning tunneling microscopy and X-ray photoemission spectroscopy. Most of the Rh atoms are in the topmost layer of the surface, and make a line structure of two atomic width in the direction. Each Rh atom in the line is bonded to one N atom. The density of trench-like structures at the N-saturated surface decreases with increasing the density of the lines. This indicates that the line structure reduces the surface stress induced by N adsorption. When the Rh atoms cover more than a quarter of surface on average, the lines form a rectangular pattern at the surface. The average separation decreases with the increase of the Rh density, and is 4 nm for the 30% Rh coverage. The pattern formation is attributed to the efficient reduction of lattice strain as the grid pattern on the partially N-adsorbed Cu(001) surface.

Structures of monolayer nickel nitride (NiN) on Cu(0 0 1) surface are studied by X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). Formations of Ni–N chemical bonds and NiN monolayer at the surface are confirmed by XPS on the N-adsorbed Cu(0 0 1) surfaces after Ni deposition and subsequent annealing to 670 K. A c(2 × 2) structure is always observed in the LEED patterns, which is a quite contrast to the (2 × 2)p4g structure observed usually at the N-adsorbed Ni(0 0 1) surface. Atomic images by STM indicate the mixture of Ni–N and Cu–N structures at the surface. Density of the trenches on the N-saturated surface decreases and the grid pattern on partially N-covered surfaces becomes disordered with increasing the Ni coverage. These results are attributed to the decrease of the surface compressive stress at the N-adsorbed Cu surface by mixing Ni atoms.

The initial stage of molecular oxygen adsorption on Cu(001) surface is investigated by scanning tunneling microscopy and first principles calculations. The molecule always dissociates on the surface between 5 and 300 K. The dissociated oxygen atoms adsorb at the two 4-fold hollow sites separated by twice of the nearest-neighbor distance of the surface Cu atoms in the close-packed directions, [110] and [110]. This is the most stable structure evaluated for the pair adsorbed at hollow site by the calculation. The adsorbed oxygen atoms stay at the same site below 80 K, and thermally migrate on the surface above 300 K.

The growth of MnN overlayers on Cu(0 0 1) has been investigated by low energy electron diffraction and scanning tunneling microscopy with emphasis on the morphology evolution with coverage. A striking feature in the growth is the formation of the self-assembled two-dimensional superstructure at the coverage of 0.9 ML. At this coverage, the MnN nano-islands show a square shape and a well-defined size. They are regularly arrayed with a periodicity of (3.5 ± 0 . 1) nanometer and form a two-dimensional square superstructure. The MnN island superstructure is stabilized by an unconventional short-range mechanism. A structural model has been proposed to explain the self-assembly and the high-quality of the superstructure. To explore possible wide applications of the new self-assembly mechanism, the growths of MnN on Pd(0 0 1) and calcium nitride on Cu(0 0 1) have also been investigated. MnN overlayers on Pd(0 0 1) present a layer-by-layer-like growth behavior, and highly ordered calcium nitride nanostructure is formed on Cu(0 0 1). Simple discussion is made on the conditions under which the novel mechanism leading to the self-assembly of MnN islands can be applied to other systems. We propose that a large lattice misfit between the overlayer and the substrate, low vacancy formation energy for the substrate, moderate interfacial bonding strength, and a large elastic constant for the overlayer compared with the substrate could be important factors for the split growth and self-assembly.

Strain-induced nanopatterns formed by the coadsorption of nitrogen and oxygen atoms are studied on the Cu(0 0 1) surface by scanning tunneling microscopy. A square grid pattern similar to that on the N-adsorbed surface appears, and consists of square c(2 × 2) areas with adsorbed N and O atoms when the total density of the adsorbates is around 30% of the Cu atom density on the clean surface. We evaluated the surface strain using a first-principles calculation for a coadsorbed surface and compared it with those on the clean and N-adsorbed surfaces. The strain on the coadsorbed surface is smaller than that of the N-adsorbed surface. The observed size of the square c(2 × 2) area on the coadsorbed surface is larger than that on the N-adsorbed surface with increasing the density of the adsorbates on average as expected by the strain reduction. On the other hand, there is no significant difference in the period of the grid pattern.

Nonlocal one-dimensional motions of a topological defect are induced by electron tunneling through the dangling-bond states on the clean Ge(001) surface using scanning tunneling microscopy below 80 K. The direction of the motion depends both on the energy of the carriers in the surface state and on the distance between the defect and the tunneling point. The results are interpreted using an electronic excitation model by hot carriers injected to the surface states. The critical distance of the motion is anisotropic and consistent with the band structure of the surface states., Comment: 4 pages, 4 figures, to be published in J. Phys. Soc. Japan

Strain-induced phenomena on Cu(0 0 1)-c(2 × 2)N surfaces are reviewed on the basis of the observations by scanning tunneling microscopy. On this surface, 5 nm × 5 nm square patches with the c(2 × 2)N surface can be arranged regularly and separated by a few nm wide clean Cu surfaces. An inhomogeneous lattice deformation in a nanometer scale is directly imaged on this surface, and nanopattern formations on vicinal surfaces are discussed. Both the growth of the magnetic metal nanodots and the dissociative adsorption of oxygen molecules are dependent on the local surface strain at the clean Cu area of the nitrogen-induced nanopattern. Strain dependent atom processes such as nanocrystal growths and molecular adsorptions are crucial for the further nanostructure formation using surface nanopatterns as templates.

The stable structure of clean Si(0 0 1) surface around 100 K is the c(4 × 2) arrangement constructed by buckled dimers. This structure was widely accepted as the ground state in 1990’s. The view was challenged at the beginning of 2000’s by the observations of a p(2 × 1) structure below 20 K with scanning tunneling microscopy (STM). Recent experimental studies confirm that the dimer is buckled below 30 K. Large tip–surface interaction, and/or tunneling current induced dynamical effect are now experimentally evident in the STM images at low temperatures. Moreover, a current induced structure transformation is discovered below 40 K even in the study by low energy electron diffraction. Dynamical electronic and vibrational effects are theoretically studied for accounting the observation of a p(2 × 1) structure below 20 K.

Adsorption of oxygen molecules on an inhomogeneously strained Cu(0 0 1) surface is studied at room temperature by scanning tunneling microscopy using a grid pattern of the partly nitrogen-covered surface. On this surface, nitrogen-adsorbed patches with average size of 5 × 5 nm 2 were separated by 1–2 nm wide clean Cu surface. Oxygen molecules are selectively dissociated on the clean Cu area, where the nitrogen-adsorbed patches inhomogeneously induce the compressive surface stress. The oxygen adsorbates are located at the fourfold hollow sites of the clean Cu surface, and migrate only on the clean surface without changing the nitrogen-adsorbed patches. Both the dissociation probability of oxygen molecule and the diffusion barrier of the oxygen adsorbates are small on the highly compressed area. When the local density of oxygen adsorbates exceeds 1/3 of the Cu atom density at (0 0 1) surface, the surface becomes disordered.

In-plane elastic lattice strain on the Cu(0 0 1)–c(2 × 2)N surfaces is investigated by scanning tunneling microscopy on the surface where nitrogen-adsorbed patches with average size of 5 × 5 nm 2 (c(2 × 2)N patches) are well separated by wide clean Cu surface. The lattice distortion on clean Cu surface is recognized in the vicinity of the boundary to a c(2 × 2)N patch. The positions of the protrusions observed on the c(2 × 2)N patch are compared with the surrounding undistorted (1 × 1) lattice of the clean Cu surface. Most of the protrusions on the c(2 × 2)N patches locate on the fourfold hollow sites of the undistorted Cu lattice. The lattice distortion is significant only near the boundary to the surrounding clean Cu surface.

We study submonolayer Ag deposited Ge(001) surfaces at 90 K by scanning tunneling microscopy (STM). Silver atoms and their small aggregates on the surface are found as bright dots on the surface at 78 K. The Ag aggregates form small clusters, and move with the change of the buckling phase in the adjacent Ge dimer row at 180 K as observed in successive STM images. These are transient processes to the formation of a surface alloy between Ag and Ge.

Growth of iron is studied on the nitrogen adsorbed Cu(0 0 1) surface, where 5×5 nm2 patches of N-adsorbed c(2×2) structure are squarely arranged and separated by a few nm wide clean Cu(0 0 1) surfaces. Mono-atomic layer Fe dots initially grow at the intersections of the clean Cu lines when the widths of the lines are smaller than 3 nm. The results are compared with the previous results on the growth of Ni and Co on the same surface. The position-selective nucleation of iron dots is ascribed to the difference in the surface strain caused by the presence of c(2×2)N patches; strain at the intersection is smaller than that at the middle of the narrow line. Similar explanation is applicable for Ni and Co growth although the detail of the selective growth depends on the species. On nitrogen-saturated Cu(0 0 1) surfaces as well, position-selective nucleation of iron dots is observed and the origin is also ascribed to the inhomogeneous surface strain.

Atomic-size contacts of iron were made in a scanning tunneling microscope (STM) at 4.2 K, and their electronic conduction was studied. We measured the conductance of the contact between the metal substrate and the STM tip as a function of the displacement between them. The observed conductance curves were quantized, and some steps in the conductance curves have hysteresis for repeated displacements. The unit of the quantization is e2/h as expected in ferromagnetic materials without spin degeneracy. We also measured the magnetic field dependence of the conductance up to 150 mT. These results are discussed in terms of the ballistic transport in metallic wires accompanied with the continuous deformation, the sudden atomic rearrangements and the change of the magnetic states around the wires.

We studied the conductance of atomic-sized wires of iron made in a scanning tunneling microscope (STM) at 4.2 K. The conductance was measured as a function of the displacement between the iron substrate and the STM tip during the elongation and compression of the wires. The observed conductance curves were quantized, and some of the steps in the conductance curves display hysteresis for repeated displacements. The unit of the quantization is e 2 /h as expected in ferromagnetic materials. A magnetic field of up to 150 mT hardly affects the conductance. These results are discussed in terms of the ballistic transport accompanied with the sudden atomic rearrangements and the continuous deformation around the wires.

Geometrical structures of the Sn-adsorbed Cu(0 0 1) surfaces are studied with scanning tunneling microscopy. There are four phases in the Sn coverage range between 0.2 and 0.5 mono-atomic layer (ML). On the basis of the observed atomic images in this range, we propose structural models for the phases with 0.33 and 0.375 ML of Sn. All the phases consist of embedded Sn atoms in the Cu surface, forming two-dimensional surface alloy structures. On the surface with ∼0.4 ML of Sn, a novel one-dimensional structure is observed.

We present rewritable, nanometer-scale patterns formed on Ge(001) at 80 K, which are based on the transition between c(4×2) and p(2×2) surface reconstructions induced by a scanning tunneling microscope (STM). We have found that a negative (−0.8 V and 0.5 s) sample bias voltage pulse creates a c(4×2)-reconstructed domain of ∼1.6×2.0 nm2 in a p(2×2)-reconstructed region. Applying the negative pulses at approriate positions, we form an intended pattern of the c(4×2) reconstruction. The course of patterning can be monitored by STM with a small bias voltage (−0.2 V) without affecting the written pattern. The whole region can be initialized to the p(2×2) by a scan with the bias voltage of +0.8 V.

Self-assembled MnN nanoislands have been prepared on Cu(001) substrate. The nanoislands show a square shape and a well-defined size. They are regularly arrayed with a periodicity of (3.5+/-0.1) nanometer and form a two-dimensional square superstructure. The MnN island superstructure is stabilized by a short-range mechanism. A structural model has been proposed to explain the self-assembly and the high quality of the superstructure.

We have studied initial growth of Sn atoms on Ge(001) surfaces at room temperature and 80 K by scanning tunneling microscopy. For Sn deposition onto the Ge(001) substrate at room temperature, the Sn atoms form two kinds of one-dimensional structures composed of ad-dimers with different alignment, in the and the directions, and epitaxial structures. For Sn deposition onto the substrate at 80 K, the population of the dimer chains aligning in the direction increases. The diffusion barrier of the Sn adatom on the substrate kinetically determines the population of the dimer chain. We propose that the diffusion barrier height depends on surface strain induced by the adatom. The two kinds of dimer chains appearing on the Ge(001) and Si(001) surfaces with adatoms of the group-IV elements are systematically interpreted in terms of the surface stain., 12 figures

The reconstruction on Ge(001) surface is locally and reversibly changed between c(4x2) and p(2x2) by controlling the bias voltage of a scanning tunneling microscope (STM) at 80K. It is c(4x2) with the sample bias voltage V_b =< -0.7V. This structure can be kept with V_b =< 0.6V. When V_b is higher than 0.8V during the scanning, the structure changes to p(2x2). This structure is then maintained with V_b >= - 0.6V. The observed local change of the reconstruction with hysteresis is ascribed to inelastic scattering during the electron tunneling in the electric field under the STM-tip., 4 pages, 4 figures, submitted to Phys. Rev. Lett

Atomic motion through excitation of extended surface electronic states on Ge(001) is studied using extraction of electrons by scanning tunneling microscopy and density functional theory. Single-electron excitation into the surface states nonlocally alters the tilting orientation of the surface Ge dimer, and the change rate depends on the excitation energy. Theoretical investigations identify the excited electronic states for the dimer motion, and clarify the strong coupling between the surface state electrons and a local vibrational mode of the dimer for changing the tilting orientation.

On the Ge(001) surface, it has previously been shown that a single-electron excitation process induces a local and reversible superstructure transformation between c (4×2) and p (2×2) by scanning tunneling microscopy. A structure transformation in the direction opposite to that of the previous result is demonstrated by increasing the tunneling current in the electron injection to the surface. In addition, the increased tunneling current shortens the range of hot electron propagation in the surface state. The observed results are attributed to the local increase in surface lattice temperature by a multiple electronic excitation process.

The electronic states of cobalt nanoislands on a nitrogen-saturated Cu 001 c 2 2 -N surface have been investigated by means of x-ray absorption spectroscopy. In the case of low coverages, when the surface is covered by small Co islands, a shoulder structure in the Co L-edge absorption spectrum is observable at an energy about 3 eV higher than the absorption edge. By considering the unoccupied Co 3d state calculated by first-principles method, x-ray photoelectron spectra and the multiple scattering during the absorption process, the shoulder is attributed to the correlation among 3d electrons. The satellite intensity decreases with the increase in the Co island size. The perimeter Co atoms of the island are assigned as the origins of the correlation satellite by comparing its intensity with the morphology of the islands observed using scanning tunneling microscopy. The polarization dependent satellite intensity suggests that the electron correlation in the in-plane 3d orbital of the perimeter atoms is larger than that in the out-of-plane one.

Scattering potentials for ${\ensuremath{\pi}}^{\ensuremath{\ast}}$ electrons at Si-Ge and Sn-Ge dimers on a Ge(001) surface are studied by scanning tunneling microscopy and ab initio calculations. Phase-shift analysis of standing waves in $dI/dV$ images reveals that Si and Sn atoms located in the conduction path of ${\ensuremath{\pi}}^{\ensuremath{\ast}}$ electrons form potentials with the sign opposite to each other. Density-functional calculations and simple calculations based on the nearly-free-electron model explain the observed potential structures. These results are qualitatively understood by relative $p$-orbital energy of the Si, Sn, and Ge atoms.

A topological defect, which is formed at the boundary between the two stable superstructures on the clean Ge(001) surface, $c(4\ifmmode\times\else\texttimes\fi{}2)$ and $p(2\ifmmode\times\else\texttimes\fi{}2)$, can be laterally manipulated at $80\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ by injection of carriers into the surface states from a metal tip of the scanning tunneling microscope (STM). We measured the probability of the defect motion as functions of the direction from the defect to the injection point, the distance between them, and the bias voltage during the carrier injection. Observed anisotropy of the probability on the lateral directions is attributed to the anisotropic propagation of the carriers in the surface electron and hole bands. For the electron propagation, the probability at the fixed distance in the directions both parallel and perpendicular to the Ge dimer axis is larger on the $p(2\ifmmode\times\else\texttimes\fi{}2)$ surface than on the $c(4\ifmmode\times\else\texttimes\fi{}2)$ surface. The observed decay of the standing waves of the surface ${\ensuremath{\pi}}^{*}$ electrons indicates that the difference is caused by the energy relaxation of electrons due to inelastic scattering. The most stable position of the defect depends on the sample bias voltage during the carrier injection. This suggests that the electric field due to the STM tip mainly determines the direction of the defect motion.

Electronic states of the clean Ge(001) surface are studied by angle-resolved photoemission spectroscopy and standing wave observation with scanning tunneling microscopy. The bottom of the surface conduction band is $0.3\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ above the top of the valence band, and thermally filled with increasing temperature above $470\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Its dispersion in the direction perpendicular to the dimer axis is much larger than that in the Ge dimer-axis direction. The valence band top at $\overline{\ensuremath{\Gamma}}$ is a bulk state, and surface resonances exist near this point. These features are consistent with the band calculation based on the density functional theory for the $c(4\ifmmode\times\else\texttimes\fi{}2)$ surface.

Dependence of the initial growth of Fe nano-islands on the deposition rate and the substrate temperature during the deposition is studied on Cu(001)-c(2×2)N surfaces, in which 5×5 nm2 patches of nitrogen-covered c(2×2) structures are squarely arranged and separated by 2-nm-wide lines of a clean Cu surface on average. Incorporated Fe clusters on the surface are formed on both the clean Cu surface and nitrogen-covered surface. Those at the intersections of the clean Cu lines most effectively act as nucleation centers of the monolayer islands at very low coverage. The sticking process of mobile Fe adatoms to the edge of the islands dominates the island growth for the increased coverage at 300 K and 100 K. The site dependences both for the Fe inclusions in the surface and for the sticking probability of mobile Fe adatoms to the edge of the islands are attributed to the difference of the surface strain.

Self-organized nanopatterns on vicinal N-adsorbed Cu(001) surfaces have been studied by scanning tunneling microscopy. A stripe pattern appears on the Cu(001)-c(2×2)N surface vicinal to the direction. The surface consists of 1-nm-wide lines of clean Cu(001) surface and 2-nm-wide c(2×2)N lines. There are two stripe domains, in which the Cu lines are parallel or perpendicular to the step edges. On the surface vicinal to the direction, there appears a one-dimensional array of 5×5 nm2 square patches of c(2×2)N surface separated by a 1-nm-wide clean Cu surface.