Solid Surface Research division

  We are aiming at creating functional materials of clusters in the size range of <100 based on a specific ability to accumulate electric charges and energies in a sub-nano space. We have succeeded to create two-dimensional charge polarization in a subnano space [1,2] due to Schottky barrier junction at a subnano hetero interface between a platinum cluster disk and a silicon surface [3,4]. Novel dynamical behavior of the clusters has also been discovered; when a cluster is compressed in a period as short as 10 fs by impact onto a solid surface, one can gain ultrahigh particle- and energy-densities at the moment of the impact [5]. Guiding principles toward novel functional materials are derived from research studies on the specificities of these electronic- and geometric-structures and the dynamical behaviors

Functional materials based on charge accumulation in sub-nano space

  The high charge density provides significant extent of electric, magnetic and electromagnetic interactions for molecules (see Figure 1). Therefore, in combination with the space as small as a single molecule, one can educe novel functions of thermal- and photo-catalyses and photoelectric devices having both high efficiency and selectivity.

地図

             Figure 1: Scheme of subnano-space charge accumulation and
             functions expected. Due to Schottky barrier junction created
             at a subnano interface between a cluster and a semiconductor
             surface, electric charges are accumulated at the subnano
             interface (indicated as a blue doughnut in the figure).High
             functionality based on electric, magnetic and electromagnetic
             interactions are expected.


  The area of the subnano space is controlled with the size of the cluster, i.e. the number of the constituent atoms, and the charges are controlled by doping electron-donor and acceptor atoms in the cluster as sources of extra charges. They have been achieved by combination of the home-build cluster-ion source equipped with multi-magnetron-sputtering devices and a quadrupole mass filter having a high transmittance up to 16,000 amu [3,6,7]. The devices for the low-energy cluster impact have also been developed so as to remain the cluster size and atomic composition unchanged during the cluster deposition [3,6]. The clusters thus constructed are observed with a custom-built low-temperature UHV STM to elucidate the geometric and electronic structures with a ultra-high spatial resolution [1,3,4,8-10].
  It has been found that a platinum cluster supported on the silicon surface has a monatomic-layered geometry in a close-packed atomic arrangement in the size range between 20 and 40 [3,4]. The calculation [2] has supported the experimental finding [1] of the subnano-space two-dimensional charge polarization; the negative charge accumulation at the subnano interface between the cluster and the surface and the positive charging at the center-top of the cluster. Furthermore, this system has high thermal stability [8]. It has been also discovered that two-dimensional to three-dimensional transition of tungsten clusters supported on a graphite surface occurs at the size between 10 and 11 [9,10].
  Now, the catalytic activity of the supported clusters is measured with controlling the charge accumulated in the subnano space by doping electron donor atoms one by one [6,7]. TEM observation, X-ray spectroscopy and fs-laser spectroscopy are also employed.

Impulsive chemical reactions and subnano processing by cluster impact

  When a cluster is compressed at high energy- and atomic-density, impulsive chemical reactions such as four-center reactions, subnano processing, instantaneous heating and electronic excitation, etc. are expected to proceed in the cluster. This is driven by cluster impact , i.e. impact of a size-selected cluster onto a solid surface, through inducing impulsive forces at the center of the cluster and the top-most layers of the target surface [5]. Fundamental properties are obtained measuring the intensity and the velocity of the scattered species and observing the tracks constructed on the surface are observed. Molecular dynamics simulation is also employed.
  It has been discovered that by the impact of I2-(CO2)N and Br2-(CO2)N (N=1‐50) at the collision energy of 100 eV, a CO2 molecule solvated at a waist position of the halogen molecular anion is driven into the chemical bond of the halogen molecular anion so as to split the bond [11,12]. This novel process was named as ‘wedge effect’ [11]. The following many-body processes have also been studied; disproportionation of N3O3- by impact of N3O3-(NO)N [13], successive addition of S atoms by impact of (CS2)N- [14], redistribution processes of excess energies of the cluster impact [15,16], cluster effects in charge transfer from the cluster ions to the target surface [17], dissociation processes of clusters by low-energy impact [18].
  High-energy impact of a cluster brings subnano-scale processing. When (CO2)N+ (N=0‐25) is impinged on a graphite surface at an impact energy as high as 14 keV, the energy is concentrated in a volume as small as ~1 nm3 in the topmost layers of the graphite surface [19]. This results in construction of subnano structures such as cylindrical craters on the surface. The shape is controlled by the impact energy and the cluster size. These specific structures are developed as chemical reaction fields including metal clusters.

References

[1] H. Yasumatsu, T. Hayakawa and T. Kondow, Chem. Phys. Lett. 487, 279‐284 (2010).
[2] H. Yasumatsu, P. Murugan and Y. Kawazoe, Phys. Stat. Solidi B, 6, 1193‐1198 (2012).
[3] H. Yasumatsu, T. Hayakawa, S. Koizumi and T. Kondow, J. Chem. Phys. 123, 124709 (2005).
[4] H. Yasumatsu, T. Hayakawa and T. Kondow, J. Chem. Phys. 124, 014701 (2006).
[5] H. Yasumatsu and T. Kondow, Rep. Prog. Phys. 66, 1783‐1832 (2003).
[6] H. Yasumatsu, M. Fuyuki, T. Hayakawa and T. Kondow, J. Phys. Conf. Ser. 185, 012057 (2009).
[7] H. Yasumatsu, Euro. Phys. J. D, 63, 195‐200 (2011).
[8] N. Fukui and H. Yasumatsu, accepted, Euro. Phys. J. D (2013).
[9] T. Hayakawa, H. Yasumatsu and T. Kondow, Euro. Phys. J. D 52, 95‐98 (2009).
[10] T. Hayakawa and H. Yasumatsu, J. Nanoparticle Res. 14, 1022 (2012).
[11] H. Yasumatsu, A. Terasaki and T. Kondow, J. Chem. Phys. 106, 3806‐3812 (1997).
[12] U. Kalmbach, H. Yasumatsu, S. Koizumi, A. Terasaki and T. Kondow, J. Chem. Phys. 110, 7443‐7448 (1999).
[13] H. Yamaguchi, H. Yasumatsu and T. Kondow, Chem. Lett. 2001, 1166‐1167 (2001).
[14] S. Koizumi, H. Yasumatsu, S. Otani and T. Kondow, J. Phys. Chem. A 106, 267‐271 (2002).
[15] H. Yasumatsu, S. Koizumi, A. Terasaki and Tamotsu Kondow, J. Phys. Chem. A 102, 9581‐9585 (1998).
[16] H. Yasumatsu, S. Koizumi, A. Terasaki and T. Kondow, J. Chem. Phys. 105, 9509‐9514 (1996).
[17] H. Yasumatsu, A. Terasaki and T. Kondow, Int. J. Mass Spectrosc. Ion Proc. 174, 297‐303 (1998).
[18] S. Koizumi, H. Yasumatsu, S. Otani and T. Kondow, J. Chem. Phys. 121, 4833‐4838 (2004).
[19] H. Yasumatsu, Y. Yamaguchi and T. Kondow, Mol. Phys. 106, 509‐520 (2008). Erratum, 106, 1123‐1124 (2008).