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Iwasa T.,Nakajima Designer Nanocluster Assembly Project | Iwasa T.,Keio University | Nakajima A.,Nakajima Designer Nanocluster Assembly Project | Nakajima A.,Keio University
Journal of Physical Chemistry C | Year: 2013

Density functional computations are used to evaluate the geometric, electronic, and optical properties of endohedral aluminum clusters X@Al 12 (X = B, Al, Si, P) and their assemblies. All X@Al 12+/0/- clusters are perfect or slightly distorted icosahedral structures, with the exception of Al13+, which is highly distorted. The projected density of states (PDOS) onto the spherical harmonics of monomers clearly reveals superatom behavior and electron shell closings of F orbitals in a 40-electron species. The electronic absorption spectrum of SiAl12 is analyzed in terms of the superatom orbitals. The optimized structures of X@Al12-Y@Al12 (X-Y = Si-Si, B-P, Al-P) dimers are constructed by facing the sides of the monomers in a staggered fashion. The PDOS of the dimers mostly exhibit five hybridizations: S, P, SD, PF, and SDG. The exceptions are HOMO, which possesses a DFG hybridized character and lies between the PF and SDG regions, and LUMO, which possesses a DG hybridized character. By analyzing the simulated absorption spectra of the B@Al12-P@Al12 and Al13-P@Al12 heterodimers, charge transfers from B/Al@Al12 to P@Al12 are found in the visible region, weakly accompanying the opposite charge transfer. The heterodimers have substantial charge carriers, estimated as the difference in electron counts from the closed-shell Si@Al12, with slight charge depletions (∼0.2). The charge distributions in B@Al 12 and P@Al12 are essentially unaltered by the insertion of Si@Al12 into the heterodimer, resulting in that the heterotrimer possesses a larger dipole moment than the heterodimer. © 2013 American Chemical Society.

Zhang C.,Nakajima Designer Nanocluster Assembly Project | Zhang C.,Keio University | Tsunoyama H.,Nakajima Designer Nanocluster Assembly Project | Tsunoyama H.,Keio University | And 5 more authors.
Journal of Physical Chemistry A | Year: 2013

We developed a new nanocluster (NC) ion source based on the high-power impulse magnetron sputtering (HiPIMS) technique coupled with a gas flow cell reactor. Silver NC anions (Agn-) with a maximum intensity of 5.5 nA (Ag11-) are generated with the size ranging from the atomic anion to the 70-mer, which is well-controlled by simply adjusting the peak power and repetition rate of the HiPIMS. By time-resolved density profiles of Agn-, we find that the ion beam generated by HiPIMS is characterized by individual 100 ms duration "bunches" below a repetition rate of 10 Hz, which is well-Thermalized with a group velocity of 5 m/s. The high intensity of the NCs is attributable to the high ionization fraction by this HiPIMS ion source, while the underlying mechanism of the flexible size tuning of the ion source is understood by time-resolved mass spectrometry coupled with the sequential growth mechanism; the increment of the density of the target species in the bunches with the peak power and the overlapping of the bunches with the repetition rate cause the formation of large NCs. © 2013 American Chemical Society.

Nakaya M.,Nakajima Designer Nanocluster Assembly Project | Nakaya M.,Keio University | Iwasa T.,Nakajima Designer Nanocluster Assembly Project | Iwasa T.,Keio University | And 6 more authors.
Nanoscale | Year: 2014

The controlled assembly of superatomic nanocluster ions synthesized in the gas phase is a key technology for constructing a novel series of functional nanomaterials. However, it is generally difficult to immobilize them onto a conductive surface while maintaining their original properties owing to undesirable modifications of their geometry and charge state. In this study, it has been shown that this difficulty can be overcome by controlling the donor-acceptor interaction between nanoclusters and surfaces. Cations of Ta-atom-encapsulated Si16 cage nanoclusters (Ta@Si16) behaving as rare-gas-like superatoms are synthesized in the gas phase and deposited on conductive surfaces terminated with acceptor-like C60 and donor-like α-sexithiophene (6T) molecules. Scanning tunneling microscopy and spectroscopy have demonstrated that Ta@Si16 cations can be densely immobilized onto C60-terminated surfaces while retaining their cage shape and positive charge, which is realized by creating binary charge transfer complexes (Ta@Si16+-C60-) on the surfaces. The Ta@Si16 nanoclusters exhibit excellent thermal stability on C60-teminated surfaces similar to those in the gas phase, whereas the nanoclusters destabilize at room temperature on 6T-terminated surfaces owing to the loss of electronic closure via a change in the charge state. This journal is © The Royal Society of Chemistry.

Nakaya M.,Nakajima Designer Nanocluster Assembly Project | Nakaya M.,Keio University | Shikishima M.,Keio University | Shibuta M.,Nakajima Designer Nanocluster Assembly Project | And 7 more authors.
ACS Nano | Year: 2012

The electronic properties of alkanethiol self-assembled monolayers (alkanethiolate SAMs) associated with their molecular-scale geometry are investigated using scanning tunneling microscopy and spectroscopy (STM/STS). We have selectively formed the three types of alkanethiolate SAMs with standing-up, lying-down, and lattice-gas phases by precise thermal annealing of the SAMs which are conventionally prepared by depositing alkanethiol molecules onto Au(111) surface in solution. The empty and filled states of each SAM are evaluated over a wide energy range covering 6 eV above/below the Fermi level (E F) using two types of STS on the basis of tunneling current-voltage and distance-voltage measurements. Electronic states originating from rigid covalent bonds between the thiol group and substrate surface are observed near E F in the standing-up and lying-down phases but not in the lattice-gas phase. These states contribute to electrical conduction in the tunneling junction at a low bias voltage. At a higher energy, a highly conductive state stemming from the alkyl chain and an image potential state (IPS) formed in a vacuum gap appear in all phases. The IPS shifts toward a higher energy through the change in the geometry of the SAM from the standing-up phase to the lattice-gas phase through the lying-down phase. This is explained by the increasing work function of alkanethiolate/Au(111) with decreasing density of surface molecules. © 2012 American Chemical Society.

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