Keisuke Sagisaka

Production of extended single-layer graphene.

Graphene has attracted an enormous amount of interest recently because of its unique electronic, optical, mechanical, and other properties. We report a promising method for producing single-layer graphene fully covering an entire substrate at low temperature. Single-layer graphene sheets have been synthesized on a whole 2 cm ×2 cm nickel (Ni) film deposited on a highly oriented pyrolytic graphite (HOPG) substrate by heating the Ni/HOPG in a vacuum. The carbon atoms forming our graphene are diffused from the graphite substrate through the nickel template. Our results demonstrate how to control the amount of carbon atoms for graphene formation to yield graphene films with a fine controlled thickness and crystal structure. Our method represents a significant step toward the scalable synthesis of high-quality graphene films with predefined thickness and toward realizing the unique properties of graphene films.

Graphene growth on a Pt(111) substrate by surface segregation and precipitation

We report on the fabrication of a sizable graphene sheet on a carbon-doped Pt(111) substrate through surface segregation and precipitation. Scanning Auger electron spectroscopy (AES) reveals that the graphene covered more than 98% of the substrate surface. Our graphene consists of single-layer graphene across the substrate with fractions of several micrometer wide bi- and tri-layer graphene islands. We also show that the number of graphene layers can be precisely determined by analyzing AES data. While Raman spectroscopy is usually used to study graphene on SiO₂, we show that AES is a powerful tool to characterize graphene grown on metal substrates.

AFM observations of self-assembled lambda DNA network on silanized mica

Atomic force microscopy (AFM) was used to explore self-assembly behavior of λ-DNA molecules adsorbed to bare mica and aminosilanized mica surfaces. AFM experiments show that λ-DNA molecules can be hardly adsorbed on bare mica surface, but can be strongly adsorbed and can be self-assembled into DNA network on silanized mica surface. Interestingly, a new neural-like DNA network structure was observed on the mica surface derivatized by N-(2-aminoethyl)-3-aminopropyl-trimethoxysilane, differing quite from the mesh network structure on the mica surface modified by aminopropylytriethoxy silane (APS). Our experimental results can offer the possibility for constructing DNA-based nanostructures on solid surface. The formation of the neural-like network was also discussed in this article.

Active nanocharacterization of nanofunctional materials by scanning tunneling microscopy

Abstract Recent developments in the application of scanning tunneling microscopy (STM) to nanofabrication and nanocharacterization are reviewed. The main focus of this paper is to outline techniques for depositing and manipulating nanometer-scale structures using STM tips. Firstly, the transfer of STM tip material through the application of voltage pulses is introduced. The highly reproducible fabrication of metallic silver nanodots and nanowires is discussed. The mechanism is thought to be spontaneous point-contact formation caused by field-enhanced diffusion to the apex of the tip. Transfer through the application of z-direction pulses is also introduced. Sub-nanometer displacement pulses along the z-direction form point contacts that can be used for reproducible nanodot deposition. Next, the discovery of the STM structural manipulation of surface phases is discussed. It has been demonstrated that superstructures on Si(001) surfaces can be reverse-manipulated by controlling the injected carriers. Finally, the fabrication of an atomic-scale one-dimensional quantum confinement system by single-atom deposition using a controlled point contact is presented. Because of its combined nanofabrication and nanocharacterization capabilities, STM is a powerful tool for exploring the nanotechnology and nanoscience fields.

An atomic resolution scanning tunneling microscope that applies external tensile stress and strain in an ultrahigh vacuum

We have developed an ultrahigh vacuum scanning tunneling microscope with an in situ external stress application capability in order to determine the effects of stress and strain on surface atomistic structures. It is necessary to understand these effects because controlling them will be a key technology that will very likely be used in future nanometer-scale fabrication processes. We used our microscope to demonstrate atomic resolution imaging under external tensile stress and strain on the surfaces of wafers of Si(111) and Si(001). We also successfully observed domain redistribution induced by applying uniaxial stress at an elevated temperature on the surface of a wafer of vicinal Si(100). We confirmed that domains for which an applied tensile stress is directed along the dimer bond become less stable and shrink. This suggests that it may be feasible to fabricate single domain surfaces in a process that controls surface stress and strain.

Scanning tunnelling microscopy in extreme fields: very low temperature, high magnetic field, and extreme high vacuum

We present the performance of our newly developed very-low-temperature scanning tunnelling microscope (VLT-STM). This system can operate with high spatial and energy resolution at temperatures down to 350 mK, and in a magnetic field up to 11 T. The uniqueness of our VLT-STM is that the system possesses extreme-high-vacuum chambers (XHV) ( Pa). System operation ranges from sample preparation, such as cleaning and deposition, to observations in an extremely clean environment. XHV will have a significant impact within material sciences, particularly when treating a semiconductor surface. Test results have revealed STM images obtained below 1 K and with atomic resolution of highly oriented pyrolytic graphite (HOPG), Si(100) dimers, and Au(111) surfaces. Our Si(100) experiments are the first atomically-resolved STM images of the semiconductor surface obtained below 1 K. The results of those tests have conclusively determined its true ground state structure—a subject under debate for many years. Some of the STM images acquired in a high magnetic field are included in this paper. The XHV-VLT-STM system is state-of-the-art and a very powerful instrument for exploration of the nano-sciences.

Quasi-one-dimensional quantum well on Si(100) surface crafted by using scanning tunneling microscopy tip

We fabricated quasi-one-dimensional (1D) quantum wells on the Si(100) surface by using a scanning tunneling microscopy (STM) tip. Electron waves were confined to a single silicon dimer row by two tungsten nanodots that were separated by several nanometers. The tungsten dots were deposited by point contact between the STM tip and the sample. The size of the dots we created on the Si(100) surface was as small as the width of a single dimer. Differential conductance mapping and scanning tunneling spectroscopy detected different quantum states confined to the quasi-1D quantum well as changing bias voltage.

Ethanol adsorption on rutile TiO2(110)

We investigated ethanol adsorption on TiO2(110) surfaces with scanning tunneling microscopy (STM) at 78 K. The ethanol was deposited by pulse injection. The STM images show that the ethanol molecules adsorbed above Ti4+ sites formed rows in the [001] direction even in the case of multilayer adsorption. Ethanol desorbed from the surface when the sample was annealed to room temperature, and the multilayer was reduced to a monolayer. Mainly ethoxy groups remained attached to the surface, and the majority of them were bound to the Ti rows. Furthermore, electron-stimulated desorption experiments revealed that only a few molecules desorbed from the surface when the scans were conducted with a sample bias ≥+3 V. In contrast, when scans were conducted at ≥+4 V, the majority of the hydrogen and covalent bonds between the adsorbates and the rutile were broken and only a small amount of species remained adsorbed on the surface. In the case of the cold dose without further annealing, the remaining ethoxy groups preferentially adsorbed on the Ti rows, whereas in the case of the annealed surface, many of the ethoxys shifted to oxygen bridging sites. This change in the adsorption behavior can be explained by the different oxidation states of the surface, which play an important role in the adsorption-desorption process.

Precise scanning tunneling microscopy images of Si(100) surface dimers at 4.2 K

The atomic geometry of the Si(100) surface has been under debate for the past four decades. Symmetric dimer in 2?1 unit and asymmetric dimer in c(4?2) and p(2?2) units have been suggested as surface models. Our low-temperature scanning tunneling microscopy (STM) images reveal that the surface is comprised of only asymmetric dimers at 4.2 K. In order to precisely observe the surface configuration with atomic resolution, we have employed a combination of low sample bias and low tunneling current settings. The area of p(2?2) structure has increased at 4.2 K on our sample in contrast to the surface at 77 K where c(4?2) is widely distributed.

Local density of electronic states in MgB2 studied by low temperature STM and STS: direct evidence for a multiple-gap superconductor

Abstract Scanning tunneling microscopy and spectroscopy (STM/STS) experiments on a high-density sintered MgB 2 surface were performed at 4.2 K, using a low temperature STM. The intrinsic superconducting density of states (DOS) was obtained in the tunneling spectra exhibiting the BCS-shaped characteristic with a metallic background. A double-gap structure, with the gap values of Δ L =9.5 meV and Δ S =4.0 meV, was clearly observed in the local tunneling spectra. These values give a BCS ratio (2 Δ / k B T C ) of 5.6 and 2.4, respectively, well above and well below the BCS limit. The result supports the multiple-gap model in MgB 2 in the clean limit. The proximity effect observed in the conductance spectra and the origin of the varied reported energy gap values of MgB 2 were also discussed in this study.