Shintaro Fujii

Cutting of Oxidized Graphene into Nanosized Pieces

We report a simple approach to producing nanosized graphene on the basis of chemical oxidation of a graphene sheet followed by cutting of the sheet using a scanning probe microscopic (SPM) manipulation technique. The structural and electronic properties of the oxidized sheet are characterized by noncontact atomic force microscopic (NC-AFM) imaging and SPM spectroscopy under ultrahigh vacuum conditions. Regularly spaced linear defects with a spacing of 5-10 nm and a length of >100 nm were found on the sheet, which can be attributed to the result of linear arrangement of epoxide functional groups. The cutting experiments are performed on sheets in which the linear defects were observed in advance. Cutting is initiated by a point contact between the preoxidized sheet and the AFM probe. The local mechanical stress caused by the point contact leads to rupture of the sheet, which proceeds linearly along the linear defect of the epoxide groups. We propose that the linear defect structures can be used as a template to determine the cutting direction of the sheet. According to recently proposed theoretical predictions, the linear epoxide groups have preferential alignment along a zigzag direction in the graphene lattice, and therefore, the cut edge shape could have well-defined alignment along the zigzag direction. This cutting procedure of the graphene sheet could be a useful method for the production of nanosized graphene with well-defined edges.

Rectifying Electron-Transport Properties through Stacks of Aromatic Molecules Inserted into a Self-Assembled Cage.

Aromatic stacks formed through self-assembly are promising building blocks for the construction of molecular electronic devices with adjustable electronic functions, in which noncovalently bound π-stacks act as replaceable modular components. Here we describe the electron-transport properties of single-molecule aromatic stacks aligned in a self-assembled cage, using scanning probe microscopic and break junction methods. Same and different modular aromatic pairs are noncovalently bound and stacked within the molecular cage holder, which leads to diverse electronic functions. The insertion of same pairs induces high electronic conductivity (10(-3)-10(-2) G0, G0 = 2e(2)/h), while different pairs develop additional electronic rectification properties. The rectification ratio was, respectively, estimated to be 1.4-2 and >10 in current-voltage characteristics and molecular orientation-dependent conductance measurements at a fixed bias voltage. Theoretical calculations demonstrate that this rectification behavior originates from the distinct stacking order of the internal aromatic components against the electron-transport direction and the corresponding lowest unoccupied molecular orbital conduction channels localized on one side of the molecular junctions.

Single molecular resistive switch obtained via sliding multiple anchoring points and varying effective wire length.

A single molecular resistive (conductance) switch via control of anchoring positions was examined by using a molecule consisting of more than two same anchors. For this purpose, we adopted the covered quaterthiophene (QT)-based molecular wire junction. The QT-based wire consisted of two thiophene ring anchors on each side; thus, shift of anchors was potentially possible without a change in the binding modes and distortion of the intramolecular structure. We observed three distinct conductance states by using scanning tunneling microscope-based break junction technique. A detailed analysis of the experimental data and first-principles calculations revealed that the mechanism of the resistive switch could be explained by standard length dependence (exponential decay) of conductance. Here, the length is the distance between the anchoring points, i.e., length of the bridged π-conjugated backbone. Most importantly, this effective tunneling length was variable via only controlling the anchoring positions in the same molecule. Furthermore, we experimentally showed the possibility of a dynamic switch of anchoring positions by mechanical control. The results suggested a distinct strategy to design functional devices via contact engineering.

Currents through single molecular junction of Au/hexanedithiolate/Au measured by repeated formation of break junction in STM under UHV: Effects of conformational change in an alkylene chain from gauche to trans and binding sites of thiolates on gold

The currents through single molecular bridges of 1,6-hexanedithiolate sandwiched between two gold protruded electrodes were measured by scanning tunneling microscopy (STM) under ultrahigh vacuum. The currents through the single molecules were measured by repeating formation of break junction between an Au(111) substrate covered with 1,6-hexanedithiolate and a gold STM tip, while current-separation (i-s) curves were repeatedly recorded. The gradual increase in the tunneling currents through the single molecules was observed almost every time (ca. 80%) during stretching of the molecular bridges. The increase in the tunneling currents can be attributed to the increase in the single molecular conductivity caused by the change in alkylene chains of 1,6-hexanedithiolate from gauche to trans conformations. The change from the gauche rich (4-5 gauche content in a single hexylene chain) to all-trans conformation resulted in one order of magnitude increase in the observed currents. Between the extreme gauche rich (5 gauche content) and all-trans (0 gauche content) conformations, there are many kinds of conformers (i.e., rotamers) with different gauche contents having different single molecular conductivities. Complexity of the observed currents due to such conformational changes made the study of the effect of Au-S contacts on single molecular conductivities difficult, although the effect was observed clearly for single molecular bridges with a previous rigid pi-conjugated system without the conformational effect (K. Ishizuka et al., Jpn. J. Appl. Phys., 2006, 45, 2037). To solve this problem, new methods are proposed and their usefulness is demonstrated.

Self-Assembly of Nanometer-Sized Boroxine Cages from Diboronic Acids

By use of the reversible trimerization of boronic acids, the series of boroxine cages 3-mer, 6-mer, and 12-mer were constructed from rationally designed diboronic acids whose bond angles between two C-B bonds are 60°, 84°, and 117°, respectively. Boroxine cages 6-mer and 12-mer have 1.5 and 2.5 nm sized cavities, respectively.

Measurements of Currents through Single Molecules of Alkanedithiols by Repeated Formation of Break Junction in Scanning Tunneling Microscopy under Ultrahigh Vacuum

We measured currents through the single molecules of 1,6-hexanedithiol and 1,8-octanedithiol using a scanning tunneling microscope (STM) under ultrahigh vacuum (UHV). Self-assembled monolayers (SAMs) of alkanedithiols were formed on Au(111) surfaces of gold films deposited on mica by dipping them in each 1 mM ethanol solution of alkanedithiols for 1 min. The sample with each SAM was introduced into the UHV-STM chamber. The current was recorded as a bare Au STM tip was repeatedly pushed against and drawn away from the sample surface. In this way, the single molecules of each alkanedithiol compound bridged a gap between the Au(111) surface and the Au STM tip apex by forming S–Au bonds at both ends of the molecules. The single-molecular conductivities Gm6 and Gm8 of alkanedithiols (N=6, 8) in this break junction were found to be approximately 9.4×10-5G0 and 1.3×10-5G0 (G0: quantum conductance), respectively.

Direct Imaging of Monovacancy-Hydrogen Complexes in a Single Graphitic Layer

Understanding how foreign chemical species bond to atomic vacancies in graphene layers can advance our ability to tailor the electronic and magnetic properties of defective graphenic materials. Here we use ultra-high vacuum scanning tunneling microscopy (UHV-STM) and density functional theory to identify the precise structure of hydrogenated single atomic vacancies in a topmost graphene layer of graphite and establish a connection between the details of hydrogen passivation and the electronic properties of a single atomic vacancy. Monovacancy-hydrogen complexes are prepared by sputtering of the graphite surface layer with low energy ions and then exposing it briefly to an atomic hydrogen environment. High-resolution experimental UHV-STM imaging allows us to determine unambiguously the positions of single missing atoms in the defective graphene lattice and, in combination with the ab initio calculations, provides detailed information about the distribution of low-energy electronic states on the periphery of the monovacancy-hydrogen complexes. We found that a single atomic vacancy where each sigma-dangling bond is passivated with one hydrogen atom shows a well-defined signal from the non-bonding pi-state which penetrates into the bulk with a (\sqrt 3 \times \sqrt 3)R30^ \circ periodicity. However, a single atomic vacancy with full hydrogen termination of sigma-dangling bonds and additional hydrogen passivation of the extended pi-state at one of the vacancys monohydrogenated carbon atoms is characterized by complete quenching of low-energy localized states. In addition, we discuss the migration of hydrogen atoms at the periphery of the monovacancy-hydrogen complexes which dramatically change the vacancys low-energy electronic properties, as observed in our low-bias high-resolution STM imaging.

Molecular dynamics simulation of non-contact atomic force microscopy of self-assembled monolayers on Au(111)

We attempted molecular dynamics (MD) simulation of non-contact atomic force microscopy (nc-AFM) of a self-assembled monolayer (SAM) on Au(111) by modifying a previously developed MD simulation method. In the previous simulation, the interaction between a single gold atom tip and a SAM sample consisting of the united atoms (CH3) with a mass (m0) and a continuum gold substrate was treated explicitly in terms of microscopic potentials. On the other hand, the probe tip with an artificial reduced mass (msl) was bound to a cantilever spring in order to describe its macroscopic nc-AFM behaviour. In the present study, we treated explicitly the interaction between a gold probe tip (the radius = R) having a single gold atom at the apex and the same SAM on gold. By using this new MD method, we succeeded in interpreting our recent experimental nc-AFM observation. Namely, the local atom?atom interactions in terms of Lennard-Jones potentials together with the long-range sphere?substrate interaction are responsible for inversion and molecular-dependent imaging of van der Waals (i.e.?chemically inert) molecules bound on gold substrates.

Triphosphasumanene Trisulfide: High Out-of-Plane Anisotropy and Janus-Type π-Surfaces

A triphosphasumanene trisulfide was designed and synthesized as an out-of-plane anisotropic π-conjugated molecule. Incorporating three anisotropic phosphine sulfide moieties into a sumanene skeleton induced a cumulative anisotropy with a large dipole moment (12.0 D), which is aligned in perpendicular direction with respect to the π-framework and more than twice as large as those of conventional out-of-plane anisotropic molecules. In the crystal, the molecules align to form columnar structures, in which electron-rich and electron-deficient sides of the π-framework face each other. The interactions between the electron-rich surfaces, which contain three sulfur atoms, and Au(111) were examined by X-ray photoelectron spectroscopy.

Dependence of tunneling current through a single molecule of phenylene oligomers on the molecular length

The electrical properties of single phenylene oligomers were studied in terms of the dependence of the tunneling current on the length of the oligomers using self-assembling techniques and scanning tunneling microscopy (STM). It is important to isolate single molecules in an insulating matrix for the measurement of the conductivity of the single molecule. We demonstrate here a novel self-assembled monolayer (SAM) matrix appropriate for isolation of the single molecules. A bicyclo[2.2.2]octane derivative was used for a SAM matrix, in which the single molecules were inserted at molecular lattice defects. The isolated single molecules of phenylene oligomers inserted in the SAM matrix were observed as protrusions in STM topography using a constant current mode. We measured the topographic heights of the molecular protrusions using STM and estimated the decay constant, beta, of the tunneling current through the single phenylene oligomers using a bilayer tunnel junction model.