Shinya Ohno

Arrays of magnetic nanodots on nitrogen-modified Cu(001) surfaces

A self-organized nanometre-scale pattern on a nitrogen-adsorbed Cu(001)-c(2 × 2) surface is used as a template for the fabrication of a magnetic nanodot array. By adjusting the coverage of nitrogen on this surface, 5 nm × 5 nm square patches of the c(2 × 2)–N surface can be arranged squarely and separated by 2 nm wide strips of clean Cu surface. Magnetic transition metals grow selectively on the clean Cu surface, and two-monolayers thick dots are dominantly formed at the intersections of the 2 nm wide Cu strips. Consequently, magnetic dots of a few nanometres in diameter are squarely organized with 7 nm intervals. Both the formation of the nitrogen-adsorbed nanopattern and the growth of the magnetic nanodots have been studied in detail by using scanning tunnelling microscopy and electron diffraction. Strain relief of the partly nitrogen-covered surface is the driving force producing the square arrangement of the nitrogen-adsorbed patches. The organization of the magnetic dots is ascribed both to selective nucleation of the magnetic metal dots on the clean Cu surface and to the large growth rate at the intersections. The magneto-optical Kerr effect is employed to examine the magnetic properties of the Co dot array. A long-range order among double-layer dots is established before their percolation occurs with increasing size. The origin of the novel magnetism is discussed.

Growth of ferromagnetic dot arrays on Cu(001)–c(2×2)N surfaces

Growth of iron is studied on the nitrogen adsorbed Cu(001) surface, where 5 x 5 nm 2 patches of N-adsorbed c(2 x 2) structure are squarely arranged and separated by a few nm wide clean Cu(001) 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 x 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(001) surfaces as well, position-selective nucleation of iron dots is observed and the origin is also ascribed to the inhomogeneous surface strain.

The local electronic properties and formation process of titanium silicide nanostructures on Si(001)-(2 × 1)

Titanium silicide island formation on an Si(001)-(2 × 1) surface was studied by means of scanning tunneling microscopy (STM) in situ at high temperature. Just after the start of annealing at 873 K, homogeneous nucleation occurs on the terrace, while preferential growth at the step edges was observed upon prolonged annealing. As the titanium silicide islands grow, multiple steps are formed nearby. The island size distribution was analyzed at several temperatures. Two types of TiSi2 structures, namely C49 and C54, were identified from the scanning tunneling spectroscopy (STS) spectra, in accordance with first-principles calculations. There was a critical island size for the transformation of C49–C54.

One-Dimensional Self-Organized Patterns on Vicinal Cu(001)-c(2 × 2)N Surfaces

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.

Electron impact effects on the oxidation of Si(111) at 90 K

The Si(111)-7×7 surface has been subjected to oxidation by molecular O2 at 90 K and the kinetics of this process have been studied by x-ray photoelectron spectroscopy (XPS). In the midst of the oxidation process, the thin oxide layer was electronically excited in ultrahigh vacuum using 100 eV electron bombardment. No charging of the oxide layer was observed. It was found that excitation of the oxide layer by electron bombardment led to almost no change in the oxidation kinetics, measured following bombardment. XPS studies showed that two oxygen-containing surface species are produced by oxidation (in the absence of electrons) with O(1s) binding energies of 533.1 and 535.1 eV. Upon electron bombardment, the higher binding energy species is converted to the lower binding energy species. Continued oxidation after electron bombardment showed that the higher binding energy species was replenished again. This result suggests that adsorption at 90 K leads to highly strained Si–O–Si species and that electron bombardment of these species produces the stable oxidized structure. The results are compared to similar experiments on Al2O3 where, in contrast to a SiO2 film, it was found that surface charging of a thin Al2O3 film on Al(111) leads to a greatly enhanced oxidation rate.The Si(111)-7×7 surface has been subjected to oxidation by molecular O2 at 90 K and the kinetics of this process have been studied by x-ray photoelectron spectroscopy (XPS). In the midst of the oxidation process, the thin oxide layer was electronically excited in ultrahigh vacuum using 100 eV electron bombardment. No charging of the oxide layer was observed. It was found that excitation of the oxide layer by electron bombardment led to almost no change in the oxidation kinetics, measured following bombardment. XPS studies showed that two oxygen-containing surface species are produced by oxidation (in the absence of electrons) with O(1s) binding energies of 533.1 and 535.1 eV. Upon electron bombardment, the higher binding energy species is converted to the lower binding energy species. Continued oxidation after electron bombardment showed that the higher binding energy species was replenished again. This result suggests that adsorption at 90 K leads to highly strained Si–O–Si species and that electron bomb...

Photoinduced charge transfer from vacuum-deposited molecules to single-layer transition metal dichalcogenides

Variations of photoluminescence (PL) and Raman spectra of single-layer MoS2, MoSe2, WS2, and WSe2 due to the vacuum deposition of C60 or copper phthalocyanine (CuPc) molecules have been investigated. PL spectra are decomposed into two competitive components, an exciton and a charged exciton (trion), depending on carrier density. The variation of PL spectra is interpreted in terms of charge transfer across the interfaces between transition metal dichalcogenides (TMDs) and dopant molecules. We find that deposited C60 molecules inject photoexcited electrons into MoS2, MoSe2, and WS2 or holes into WSe2. CuPc molecules also inject electrons into MoS2, MoSe2, and WS2, while holes are depleted from WSe2 to CuPc. We then propose a band alignment between TMDs and dopant molecules. Peak shifts of Raman spectra and doped carrier density estimated using a three-level model also support the band alignment. We thus demonstrate photoinduced charge transfer from dopant molecules to single-layer TMDs.

SiO2/Si interfaces on high-index surfaces: Re-evaluation of trap densities and characterization of bonding structures

Interface trap densities, Dit, at the thermally oxidized Si surfaces were investigated for the (001), (111), (110), (120), (331), and (113) orientations. The oxides were formed by dry or wet oxidation in the temperature range of 700–950 °C. Dit took a maximum not only on the (111) surfaces but also on (110). Low Dit values were obtained on wet-oxidized high-index surfaces. Correlation between Dit and the interface anisotropy observed by reflectance difference spectroscopy suggests preferential oxidation of the Si atoms with the (111)-like bonding geometry on the (113) surfaces.

Time Courses and Time‐Resolved Spectra of Firefly Bioluminescence Initiated by Two Methods of ATP Injection and Photolysis of Caged ATP

The time‐dependent characteristics of firefly bioluminescence initiated by manual injection of adenosine triphosphate (ATP) into buffer solution containing luciferin (Ln), luciferase (Luc) and Mg2+ were measured with a resolution of 10 ms, and compared with those obtained by photolysis of caged ATP. The time course depends on pH; both rise and decay rates decrease when pH is lowered from 7.8 to 6.8. In contrast, the parameter λ in the kinetic formula related to diffusion of ATP is almost independent of pH. The pH dependence of the time course of bioluminescence can be explained by the same pH tendency as the rate of ATP binding at the active site of Luc. The time‐resolved spectra can be decomposed into two Gaussian components with maxima at 2.2 and 2.0 eV. At pH 7.8, the band at 2.2 eV is more intense than that at 2.0 eV for all three concentration conditions. At lower pH, the band at 2.2 eV becomes weaker than that at 2.0 eV. The intensity ratio of the 2.0 and 2.2 eV bands is constant for duration time of 600 s for both injection and photolysis experiments, and the above conclusions are unaffected by the concentration ratio [Ln]/[Luc].

Reaction kinetics in the rapid oxide growth on Si(001)-(2×1) probed with reflectance difference spectroscopy

We investigated the NO adsorption process on Si(001)-(2×1) and the oxygen adsorption process on potassium-covered Si(001)-(2×1) by reflectance difference spectroscopy (RDS). In both cases, the time courses that deviated from a simple Langmuirian kinetics could not be well fitted with a single exponential function, indicating the involvement of two different processes. In NO adsorption, a highly coordinated nitrogen adsorption site (N≡Si3) might play a role in the initial reaction process, producing an inhomogeneous strain on the first layer of oxynitride. In potassium-assisted oxidation, a sudden decrease in RD intensity just after oxygen exposure is associated with a reaction of oxygen with a potassium film, and the subsequent oxidation is slightly enhanced by the potassium–oxygen complex.

Initial Growth of Fe Nano-Islands on Cu(001)-c(2×2)N Surfaces

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.