Published papers

Evolution of electronic structures of nanoscale biphenyl cluster anions, (BP)(n)(-) (n = 2-100), have been studied by photoelectron spectroscopy in a size-selective fashion. The energetics of vertical detachment energy (VDE) revealed negative electron affinity of a biphenyl molecule and substantial cluster reorganizations via electron attachment. Comparison of the VDE energetics with naphthalene cluster anions implied that the freezing (and/or melting) temperatures of the BP clusters may be much lower than those of naphthalene clusters.

Vacuum-synthesized Ag nanoparticles were dispersed and immobilized on an organosilane-functionalized glass surface in water. Nearly spherical particles with diameters of ≈ 200 nm were used for a substrate for surface enhanced Raman scattering (SERS). Molecules deposited on this substrate in both gas and liquid phases were detected by SERS. The SERS efficiency of this substrate was compared with that of Ag colloidal substrate and the morphological difference of these SERS substrates was studied by scanning electron microscopy. The aggregates of 5-10 Ag nanoparticles formed at the spread area exhibited prominent SERS activity.

The magnetic properties of gas-phase terbium-cyclooctatetraene multi-decker sandwich complexes, Tbn(C8H8)n+1 were measured using a Stern-Gerlach type magnetic deflection approach. Beams of Tbn(C8H8)n+1 complexes displayed one-sided deflection toward high field - indicating that fast spin relaxation occurs within the complexes as they pass through the magnetic field. The magnetic moment for Tbn(C8H8)n+1 (n = 1-5) was evaluated using the Langevin model. Evolution of magnetic moment with the complex size is discussed with electronic structures for oxidation states of Tb3+/2+ ions, implying the possibility of antiferromagnetic interaction of two adjacent Tb2+ ions.

Gas-phase synthesized vanadium-benzene 1:2 (VBz2) sandwich clusters were size-selectively deposited onto bare gold and long-chain n-alkanethiolate [-S-(CH2)n-1-CH3; n = 16, 18, and 22] self-assembled monolayer (SAM)-coated gold substrates under ultrahigh vacuum (UHV) conditions. Investigation of the resulting deposited clusters was performed by infrared reflection absorption spectroscopy (IRAS) and thermal desorption spectroscopy (TDS). The IR frequencies of the soft-landed VBz 2 clusters show excellent agreement with the fundamentals reported in IR data of VBz2 in an argon matrix. The analysis of IRAS spectra reveals that while there was no orientational preference of the VBz2 clusters on a bare gold substrate, the VBz2 clusters deposited onto the SAM substrates were highly oriented with the molecular axis 70-80° tilted off the surface normal. In addition, analysis of TDS spectra revealed unusually large adsorption heats of the physisorbed VBz2 clusters. The present results are explained by cluster penetration into the long-chain alkanethiolate SAM and for the first time demonstrate the matrix isolation of gas-phase organometallic clusters around room temperature.

Ultrafine gold and nickel nanoparticles (NPs) were fabricated by pulsed laser ablation in helium gas, and the NPs 2-15 nm in diameter were selectively classified by an electrostatic size-selection technique employing a low-pressure differential mobility analyzer (LP-DMA). In the classification of the NPs, a chain of several gold NPs or doubly charged NPs were produced as well as singly charged spherical NPs. The morphological selection was performed with the LP-DMA, using a relationship in which the cross section of the spherical NP chains was governed by the product of the diameter and the length, instead of by the square of the diameter. Below a blocking temperature of about 100 K, the Ni NPs that were 3.1 nm in diameter exhibited ferromagnetism.

Amorphous iron or cobalt carbide nanoparticles with a carbon onion structure were produced using a pulsed laser ablation of metallocene powders at room temperature. While they are ferromagnetic at room temperature, they exhibit a drastic change in magnetic spin moments at 20-30 K. These magnetic properties are ascribed to a considerable magnetic contribution of superparamagnetism for the amorphous iron or cobalt carbide nanoparticles with a carbon onion and the origin of the magnetic properties is discussed.

The effect of Cs atom doping on metallic aluminum clusters and covalent silicon clusters, AlnCsm (n = 5-14, m = 0-3) and SinCsm (n = 5-16, m = 0-3), was examined by mass spectrometry and anion photoelectron spectroscopy. For clusters containing Cs atom(s), the electron affinities of both clusters are generally decreased and the following characteristic features are observed: for AlnCsm Cs-atom doping causes (1) electron filling into the electronic shell structure of the Al-n, clusters and (2) geometrical packing of icosahedral 13-atoms, while for SinCsm Cs-atom doping enhances electronic stability to be ascribed to pure Si-n clusters, particularly at (n, m) = (10, 3) and (13, 1). (c) 2006 Elsevier B.V. All rights reserved.

The electronic structures of binary gold cluster anions: AunM- (M = Pd, Ni, Zn, Cu, Mg) were investigated by anion photoelectron spectroscopy at the photon energy of 4.66 eV. The spectra exhibit even-odd alternation of electron affinities and characteristic spectral changes with n, which are accountable in terms of the interactions between the Au clusters and doped metal atoms. For the binary clusters containing Pd, Ni, and Cu. their valence electrons are shared with those of the Au clusters, whereas for Mg the doped atom is weakly bound to Au clusters. A size-dependent spectral change is clearly observed for AunPd- at n approximate to 5. (c) 2006 Elsevier B.V. All rights reserved.

The adsorption state and thermal stability of V( benzene)(2) sandwich clusters soft-landed onto a self- assembled monolayer of different chain-length n-alkanethiols (C-n-SAM, n) 8, 12, 16, 18, and 22) were studied by means of infrared reflection absorption spectroscopy (IRAS) and temperature-programmed desorption (TPD). The IRAS measurement confirmed that V(benzene)(2) clusters are molecularly adsorbed and maintain a sandwich structure on all of the SAM substrates. In addition, the clusters supported on the SAM substrates are oriented with their molecular axes tilted 70-80 off the surface normal. An Arrhenius analysis of the TPD spectra reveals that the activation energy for the desorption of the supported clusters increases linearly with the chain length of the SAMs. For the longest chain C-22-SAM, the activation energy reaches similar to 150kJ/mol, and the thermal desorption of the supported clusters can be considerably suppressed near room temperature. The clear chain-length-dependent thermal stability of the supported clusters observed here can be explained well in terms of the cluster penetration into the SAM matrixes.

The geometric and electronic structures of aluminum binary clusters, AlnX (X) Si and P), have been investigated, using mass spectrometry, anion photoelectron spectroscopy, photoionization spectroscopy, and theoretical calculations. Both experimental and theoretical results show that Al12Si has a high ionization energy and low electron affinity and Al12P has a low ionization energy, both with the icosahedral structure having a central Si or P atom, revealing that Al12Si and Al12P exhibit rare- gas- like and alkali superatoms, respectively. Experiments confirmed the possibility that the change in the total number of valence electrons on substitution could produce ionically bound binary superatom complexes, the binary cluster salts Al12P+F- and Al12B-Cs+.

The present account describes our efforts to understand the microscopic aspects of condense phase phenomena, such as ion solvation in molecular liquids and charge carrier localization in molecular solids, by viewing finite-size molecular clusters as their embryonic forms. In this effort, we efficiently prepared supersonically cooled, isolated molecular cluster anions with up to more than 100 constituent molecules and size-selectively investigated their electronic structures using anion photoelectron spectroscopy. Large anionic clusters of two different types of organic molecule; acetonitrile and naphthalene reported as examples of the noteworthy results obtained in our studies. In these two systems, we found that energetically close anionic isomers coexist over a broad range in size, and their contribution in the photoelectron spectra could be separated using anion beam hole-burning technique. A detailed inspection into the electronic states and size-dependent energetics of each isomer has enabled us to establish a link from the large finite cluster to the infinite bulk system.

Silicon clusters mixed with a transition metal atom, MSia, were generated by a double-laser vaporization method, and the electronic and geometric stabilities for the resulting clusters with transition metal encapsulated by silicon were examined experimentally. By means of a systematic doping with transition metal atoms of groups 3, 4, and 5 (M = Sc, Y, Lu, Ti, Zr, Hf, V, Nb, and Ta), followed by changes of charge states, we explored the use of an electronic closing of a silicon caged cluster and variations in its cavity size to facilitate metal-atom encapsulation. Results obtained by mass spectrometry, anion photoelectron spectroscopy, and adsorption reactivity toward H2O show that the neutral cluster doped with a group 4 atom features an electronic and a geometric closing at n = 16. The MSi16 cluster with a group 4 atom undergoes an electronic change in (i) the number of valence electrons when the metal atom is substituted by the neighboring metals with a group 3 or 5 atom and in (ii) atomic radii with the substitution of the same group elements of Zr and Hf. The reactivity of a halogen atom with the MSi 16 clusters reveals that VSi16F forms a superatom complex with ionic bonding.

Electronic properties of silicon and germanium atom doped indium clusters, InnSim and InnGem, were investigated by photoionization spectroscopy of the neutrals and photoelectron spectroscopy of the anions. Size dependence of ionization energy and electron affinity for InnSi1 and InnGe1 exhibit pronounced even-odd alternation at cluster sizes of n = 10-16, as compared to those for pure In-n clusters. This result shows that symmetry lowering with the doped atom of Si or Ge results in undegeneration of electronic states in the 1d shell formed by monovalent In atoms.

Gas-phase synthesized multidecker V-2(benzene)(3) sandwich complexes were soft-landed onto a self-assembled monolayer of an n-octadecanethiol (C-18-SAM). The infrared absorption spectrum of this complex supported on the C-18-SAM was obtained by infrared reflection absorption spectroscopy (IRAS). The vibrational assignment of the IRAS spectrum for the V-2(benzene)(3) sandwich was based on a harmonic frequency analysis that employed density functional theory. The close similarity between the experimental and calculated results for the IR absorption frequencies demonstrates that the V-2(benzene)(3) complexes have a multidecker sandwich structure on the SAM substrate. Moreover, our thermal desorption study established that the sandwich complexes can be isolated on the SAM substrate up to a high temperature of similar to 350 K.

Ultrafine gold and platinum nanoparticles (NPs) were fabricated by pulsed laser ablation in helium gas and the NPs 2-15 nm in diameter were selectively classified by an electrostatic size-selection technique employing a low-pressure differential mobility analyzer (LP-DMA). The spherical NPs obtained showed a narrower distribution of diameters of anionic NPs over cationic NPs. With this knowledge, the anionic NPs were deposited onto silicon substrates designed by electron beam lithography processing, and designable patterns of arrayed NPs were produced by removing the photoresist layer.

Uniform-sized silver nanoparticles with average diameter of 13.7 nm have been prepared in the gas-phase by combining a pulsed laser ablation method with a low pressure-differential mobility analyzer (LP-DMA). By depositing the silver nanoparticles onto a silicon substrate, a granular film consisting of size-selected silver nanoparticles has been fabricated and its morphology and electronic properties have been examined using transmission electron microscopy (TEM) and UV-visible absorption spectroscopy. This granular silver film serves as a highly active substrate for surface-enhanced Raman scattering (SERS).

A molecular beam of multilayer metal-benzene organometallic clusters M-n(C6H6)(m) (M = Al, Sc, Ti, and V) was produced by a laser vaporization synthesis method, and their magnetic deflections were measured. Multidecker sandwich clusters of transition-metal atoms and benzene Sc-n(C6H6)(n+1) (n = 1, 2) and V-n(C6H6)(n+1) (n = 1-4) possess magnetic moments that increase monotonously with n. The magnetic moments of Al(C6H6), Sc-n(C6H6)(n+1), and V-n(C6H6)(n+1) are smaller than that of their spin-only values as a result of intracluster spin relaxation, an effect that depends on the orbital angular momenta and bonding characters of the orbitals containing electron spin. While Ti(C6H6)(2) was found to be nonmagnetic, Ti-n(C6H6)(n+1) (n = 2, 3) possess nonzero magnetic moments. The mechanism of ferromagnetic spin ordering in M-2(C6H6)(3) (M = Sc, Ti, V) is discussed qualitatively in terms of molecular orbital analysis. These sandwich species represent a new class of one-dimensional molecular magnets in which the transition-metal atoms are formally zerovalent.

Photoelectron spectroscopy of tetracene cluster anions, (tetracene), (n = 1 - 100), reveals the coexistence of two types of isomers, designated as isomers I and II-1 (n = 10-50) or isomers I and II-2 (n > 60), in a wide size range. The vertical detachment energies (VDEs) of isomer I increase persistently due to polarization and structural relaxation effects, where a monomeric anion core is encompassed with geometrically reorganized neutral molecules. Conversely, a characteristic ion distribution in the mass spectrum of (tetracene),, ensues from the two-dimensional (2D) herringbone-type ordering of isomer II-1, whose VDEs remain constant at 1.80 eV for n >= 14. Also, isomer II-2, presumably adopting multilayered structural motifs, exhibits invariable VDEs of 2.0 eV, a manifestation of significant charge screening effects in these isomers. The invariable nature of the VDEs of isomers II-1 and II-2 unambiguously demonstrates a largely localized nature of polarization induced by the excess charge residing in microscopic crystal-like environments. Surprisingly, only 14 tetracene molecules within a 2D herringbone-type layer including an excess charge can provide the charge stabilization energy corresponding to similar to 80% of that of the crystal, and the rest of the energy is provided by polarization of neutral molecules in adjacent layers.

The evolution of the electronic structure of molecular aggregates is investigated using anion photoelectron (PE) spectroscopy for anionic clusters of anthracene (Ac) and its alkyl derivatives: 1-methylanthracene (1MA), 2-methylanthracene (2MA), 9-methylanthracene (9MA), 9,10-dimethylanthracene (DMA), and 2-tert-butylanthracene (2TBA). For their monomer anions (n=1), electron affinities are confined to the range from 0.47 to 0.59 eV and are well reproduced by density functional theory calculations, showing the isoelectronic character of these molecules. For cluster anions (n=2-100) of Ac and 2MA, two types of isomers I and II coexist over a wide size range: isomers I and II-1 (4 <= n < 30) or isomers I and II-2 (n >=similar to 40 for Ac and n >=similar to 55 for 2MA). However, for the other alkyl-substituted Ac cluster anions (i.e., 1MA, 9MA, DMA, and 2TBA), only isomer I is exclusively formed, and neither isomer II-1 nor II-2 is observed. The vertical detachment energies (VDEs) of isomer I in all the anionic clusters depend almost linearly on n(-1/3). In contrast, the VDEs of isomers II-1 (n >= 14) and II-2 (n=40-100), appeared only in Ac and 2MA cluster anions, remain constant with n and are similar to 0.5 eV lower than those of isomer I. The PE spectra revealed the characteristics of each isomer: isomer I possesses a monomeric anion core that is gradually embedded into the interior of the cluster with increasing n. On the other hand, isomers II-1 and II-2 possess a multimeric (perhaps tetrameric) anion core, but they differ in the number of layers from which they are made up; monolayer (isomer II-1) and multilayers (isomer II-2) of a two-dimensionally ordered, finite herringbone-type structure, in which electron attachment produces only little geometrical rearrangement. Moreover, the agreement of the constant VDEs of isomer II-2 with the bulk data demonstrates the largely localized nature of the electronic polarization around the excess charge in a crystal-like environment, where about 50 molecules provide a charge stabilization energy comparable to the bulk.

Nanogranular films and lines, which are composed of size-selected Au, Ni, and Pt nanoparticles (NPs) and which exhibit an intermediate structure between that of the individual NPs and that of polycrystalline ones, were produced by a fully physical process. By using a combination of techniques such as lithography, lift-off, size selection, and aerosol deposition, well-designed Au lines (linewidth W=80 nm) and thickness-controlled rectangular patterns could be produced, while Ni and Pt NPs did not form continuous lines if the linewidth was less than W=150 nm. This difference might be explained by the extent of collision-induced surface melting during NP deposition onto the substrate.