Published papers

One end open V(n)Bz(n)(-) (n = 1-5; Bz = benzene) and both ends open V(n)Bz(n-1)(-) (n = 2-5) vanadium-benzene cluster anions were studied using anion photoelectron spectroscopy and density functional calculations. The smaller (n ≤ 3) V(n)Bz(n) and V(n)Bz(n-1) clusters and corresponding anions were found to have structural isomers, whereas full-sandwiched V(n)Bz(n+1) clusters preferred to form multipledecker sandwich structures. Several isomeric V(2)Bz(2) structures were identified theoretically and the anion photoelectron spectra of V(2)Bz(2)(0/-) were explained well by the coexistence of two isomeric structures: (1) a V-2-core structure sandwiched between benzene molecules and (2) an alternating sandwich structure with the spin state strongly dependent on the structure. The adiabatic electron affinity of both V(n)Bz(n) and V(n)Bz(n-1) was found to increase with the cluster size at larger sizes (n = 4 or 5) and approaches to that of V(n)Bz(n+1). The evolution of the structural and electronic properties of V(n)Bz(m) and V(n)Bz(m)(-) (m = n and n - 1) with size is discussed in comparison with V(n)Bz(n+1) and V(n)Bz(n+1)(-).

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 Si-16 cage nanoclusters (Ta@Si-16) behaving as rare-gas-like superatoms are synthesized in the gas phase and deposited on conductive surfaces terminated with acceptor-like C-60 and donor-like alpha-sexithiophene (6T) molecules. Scanning tunneling microscopy and spectroscopy have demonstrated that Ta@Si-16 cations can be densely immobilized onto C-60-terminated surfaces while retaining their cage shape and positive charge, which is realized by creating binary charge transfer complexes (Ta@Si-16(+)-C-60(-)) on the surfaces. The Ta@Si-16 nanoclusters exhibit excellent thermal stability on C-60-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.

The formation of metallic layers on ultrathin molecular films via a well-controlled interface is essential for constructing organic nanodevices composed of metal/molecule/metal sandwich junctions. The scanning tunneling microscopy and spectroscopy studies demonstrate that an ultrasharp metal/molecule interface is realizable by depositing size-selected Ag nanoclusters (Agn) from the gas phase on few-layer films of C 60 molecules. It is also demonstrated that Agn nanoclusters can be immobilized on monolayer films of oligothiophene molecules via C60 molecules, although they three-dimensionally aggregate on bare oligothiophene films. It is also shown that electrons and holes are injected into the topmost layer of C60 films via the Ag n/C60 interface. Moreover, the barrier height for carrier injection at the Agn/C60 interface can be modified depending on the size of Agn nanoclusters and the kinetic energy during the deposition. The present results demonstrate that the controlled immobilization of metallic nanoclusters on molecular films can be used as a fabrication technology for metal/molecule/metal junctions. Metal/molecule heterojunctions are constructed by the controlled immobilization of size-selected Ag nanoclusters (Agn) from the gas phase on ultrathin C60 films. In such heterojunctions, electrons and holes can be precisely injected into the topmost C60 molecules in films via an atomically abrupt metal/molecule interface. The barrier height for carrier injection can be modified.

In this study, we investigate the photoexcited electronic states of ferrocene (Fc) molecules adsorbed on an organic insulating surface by two-photon photoemission spectroscopy. This insulating layer, composed of a decanethiolate self-assembled monolayer formed on an Au(111) substrate, enables us to probe the electronically excited states localized at the adsorbed Fc molecules. The adsorbate-specific state is resonantly excited by photons at 4.57 eV, which is 0.5 eV smaller than the energy of the first molecular Rydberg state of free Fc in the gas phase. This result indicates that the electrons are bound to both the excited hole formed in the adsorbate and the positive image charge induced in the substrate. The hybridized electronic characteristics of the adsorbate-specific state are responsible for the strong transition selectivity and short lifetime of the excited state.

A new ion source based on modulated pulse power (MPP) magnetron sputtering (MSP) was developed and demonstrated for cluster production. By employing MPP-MSP, both silver and silicon cluster beams were produced and analyzed by a quadrupole mass spectrometer. It is found that the maximum intensity of silver cluster anions reaches 500 pA, which is considerably higher than that with a conventional DC-MSP. For silicon cluster cation, the overall ion intensity of the cluster beam by MPP-MSP is about three times higher than that generated by DC-MSP.

Clusters consisting of 101000 atoms size-dependently exhibit novel electronic and geometric properties. In particular, composite clusters composed of several elements and/or components provide a promising way for a bottom-up approach for designing functional advanced materials, because the functionality of the composite clusters can be optimized not only by the cluster size but also by their compositions. This account surveys recent investigations of composite clusters focusing on the efforts to create a new functional composite cluster by a fine doping or hybridization based on their size-specific electronic properties. Organometallic clusters and caged clusters are demonstrated as a representative example of designing the functionality of magnetism and electronic state structures. In order to create functional nanomaterials, furthermore, a fine controlling methodology of the soft-landing technique has been developed to fix the composite clusters onto a surface decorated with a self-assembled monolayer. The embedded isolation mechanism on the substrate is discussed form the viewpoint of self-assembly phenomena with the molecular ordering of σ σ interaction, and also the electronic structures characterized by molecular ordering of π- π interaction is intrinsically revealed by molecular clusters of -conjugate polyacenes as a model for self-assembled aggregates.

Charge separation dynamics relevant to an electron transfer have been revealed by time- and angle-resolved two-photon photoemission spectroscopy for an n-alkanethiolate self-assembled monolayer (SAM) on a Au(111) surface fabricated by a chemical-wet process. The electron was photoexcited into an image potential state located at 3.7 eV above the Fermi level (E-F), and it survived well for more than 100 Ps on dodecanethiolate (C12)-SAM. The degree of electron separation is precisely controlled by selecting the length of the alkyl chain (C10-C18). We have also evaluated molecular conductivity at the specific electron energy of E-F + 3.7 eV. The tunneling decay parameter, beta, was fitted by beta(90K) = 0.097 angstrom(-1) and beta(RT) = 0.13 angstrom(-1). These values were one order smaller than that at around EF by conventional contact probe methods.

Soft- and reactive-landing of gas-phase synthesized cationic Cr(aniline)(2) complexes onto self-assembled monolayers of methyl-terminated (CH3-SAM) and carboxyl-terminated (COOH-SAM) organothiolates coated on gold were performed at hyperthermal collision energy (5-20 eV). The properties of the Cr(aniline)(2) complexes on the SAM surfaces were characterized using infrared reflection absorption spectroscopy (IRAS) and temperature-programmed desorption (TPD), together with theoretical calculations based on density functional theory (DFT). For the CH3-SAM, the Cr(aniline)(2) complexes were embedded inside the SAM matrix in a neutral charge state, keeping a sandwich structure. For the COOH-SAM, the IRAS and TPD study revealed that the amine-containing Cr(aniline)(2) complexes were bound to the SAM surface in two forms of physisorption and chemical linking through an amide bond. In the desorption, the latter form appeared as the reaction product between organothiolates and Cr(aniline)(2) above 400 K, where the organothiolate molecules, forming the SAM, were desorbed from the gold surface. The results show that the hyperthermal depositions onto a COOH-SAM bring about reactive-landing followed by covalent linking of an amide bond between the amine-containing Cr(aniline)(2) complexes to the carboxyl-terminated SAM surface, in which the binding sites can be separated from the functional sites of the d-pi interaction.

Metal-rich K1-5(KI)(n)(+) (n similar to 27) clusters were produced by the supersonic cooling expansion of laser-induced KI plasma. The obtained mass spectroscopic peaks show the unique magic numbers at n = 9, 13, and 22 for K-1(KI)(n); n = 8, 12, and 21 for K-2(KI)(n); n = 8, 12, 16, and 21 for K-3(KI)(n); n 7, 9, 11, and 13 for K-4(KI)(n); and n = 12 and 21 for K-5(KI)(n). These magic numbers are fairly different from those of other reported metal-rich Na1-9(NaF)(n)(+) and Cs1-5(CsI)(n)(+) clusters. It is interesting to note that K-2.3,K-5(KI)(n)(+) clusters have the same magic numbers at n = 12 and 21 and possess structures that are closely related to the 3 x 3 x 3 and 3 x 3 x 5 cubic structures with the excess electron at the side anion vacancy or the center anion vacancy. (C) 2007 Elsevier B.V. All rights reserved.

Nanometer- and micrometer-sized particles (NPs and MPs) of gas-phase Co, Fe, and Fe-Nd-Dy-B were magnetically trapped on a substrate. For the magnetic trapping of NPs, two stepwise evolutions were investigated: (1) the formation of NP aggregates above a critical diameter of 8 nm and (2) increase in the length to a few micrometers, which is the result of the equilibrium between the magnetostatic and thermal energies of NPs and the granular linear chains such as granular nanorods and nanofibers. The magnetic trapping of MPs or MPsNPs is particularly interesting because of the sequential pearl-necklace-like growth and magnetic separation in the linear chains.

Highly reliable lines and arrays comprising the smallest metallic nanoparticles (NPs) were produced by using a combination of techniques such as lithography, lift-off, size selection, aerosol deposition, and NP colloids. The narrowest 80 nm Au lines were produced by the deposition of only 4 nm Au NPs, driven by two factors-NP kinetics and collision-induced surface melting. An array of unsupported free 30-100 Ni NPs and one of 20-40 nm Ni NPs supported in holes were produced by the principles of hole-size-induced filtering and the binding of thiol groups.

Cluster anions of 2,2'- bithiophene, (2T)(n)(-), were produced up to n similar to 500 in the gas-phase. The energetics of the excess electron in the (2T)(n)(-) clusters with n = 1- 100 were explored by negative ion photoelectron spectroscopy. When the vertical detachment energies (VDEs) obtained from the photoelectron spectra were analyzed by a plot against n(-1/3), it has been revealed that the excess electron trapping level thus extrapolated is located at similar to 0.8 eV below the conduction band minimum (i.e. LUMO) of the 2T thin film. The large slope of the VDEs vs. n(-1/3) plot suggests that the neutral 2T molecules surrounding the anion core take non-planar twisted conformations with permanent dipole moments, resulting in the exceedingly deep trapping of the excess electron in the 2T cluster anions.

Electrochemical behavior of gold-nanoparticle-dispersed diamond electrodes have been studied for the oxidation of oxalic acid in phosphate buffer solution. Comparison is made with gold, as-deposited diamond and gold-implanted diamond. The electrodes combine the high catalytic activity of gold and the low background current of diamond electrodes. Moreover, the electrodes can overcome the problem of stability as it is found when using other electrodes.

Optical properties of the deposited germanium-oxide (Ge-O) prepared from precursors of the Ge-O clusters were investigated. The strong visible photoluminescence peaked at 500 nm was observed under the excitation of 325 nm laser light, when the GenOn clusters were deposited onto a substrate as a precursor. The XPS spectrum of the deposited Ge-O on the substrate shows the contribution from the oxidation state of Ge2+, which shows that 1:1 composition of Ge2+-O2- is a key factor to exhibit the visible emission. Correspondingly, the electronic structures of the germanium-oxide cluster precursors produced by a laser vaporization were studied by using photoelectron spectroscopy (PES) for GenOm- and GenOmF- (n=2-6, m=1-5) in the gas phase. Our development of the halogen atom doping method enables us to determine the HOMO-LUMO gap of the corresponding neutral GenOn clusters experimentally, and it was found that the gap of the GenOn (n=2-5) clusters reasonably corresponds to the energy of the visible light. These results indicate that the GenOn clusters can be ascribed to the center of the visible photoluminescence.

In gas phase, electronic and geometric properties of iron-sulfur cluster (FenSm, n = 1-8, m = 2-6) have been investigated using photoelectron spectroscopy (PES) of their anions and a chemical probe method for their cations. The electron affinities of FenSm greatly change at the composition of n = m, and the reactivity of FenSm+ toward NH3 and C2H2 is enhanced around equally mixed compositions. These results show that the FenSm clusters produced in gas phase seemingly take the structure having alternate bonds between Fe and S atoms, likewise that of an active site of iron-sulfur proteins. (C) 1997 Elsevier Science B.V.