Published papers

The electronic structures and structural morphologies of naphthalene cluster anions, (naphthalene) n- (n=3-150), and its related aromatic cluster anions, (acenaphthene) n- (n=4-100) and (azulene) n- (n=1-100), are studied using anion photoelectron spectroscopy. For (naphthalene) n- clusters, two isomers coexist over a wide size range: isomers I and II-1 (28≤n≤60) or isomers I and II-2 (n∼60). Their contributions to the photoelectron spectra can be separated using an anion beam hole-burning technique. In contrast, such an isomer coexistence is not observed for (acenaphthene) n- and (azulene) n- clusters, where isomer I is exclusively formed throughout the whole size range. The vertical detachment energies (VDEs) of isomer I (7≤n≤100) in all the anionic clusters depend linearly on n-13 and their size-dependent energetics are quite similar to one another. On the other hand, the VDEs of isomers II-1 and II-2 produced in (naphthalene) n- clusters with n∼30 remain constant at 0.84 and 0.99 eV, respectively, 0.4-0.6 eV lower than those of isomer I. Based upon the ion source condition dependence and the hole-burning photoelectron spectra experiments for each isomer, the energetics and characteristics of isomers I, II-1, and II-2 are discussed: isomer I is an internalized anion state accompanied by a large change in its cluster geometry after electron attachment, while isomers II-1 and II-2 are crystal-like states with little structural relaxation. The nonappearance of isomers II-1 and II-2 for (acenaphthene) n- and (azulene) n- and a comparison with other aromatic cluster anions indicate that a highly anisotropic and symmetric π -conjugated molecular framework, such as found in the linear oligoacenes, is an essential factor for the formation of the crystal-like ordered forms (isomers II-1 and II-2). On the other hand, lowering the molecular symmetry makes their production unfavorable.

Multilayer lanthanide-cyclooctatetraene organometallic clusters, Ln n(C8H8)m, (Ln = Eu, Tb, Ho, Tm; n = 1 - 7; m = n - 1, n, n + 1) were produced by a laser vaporization synthesis method. The magnetic deflections of these organometallic sandwich clusters were measured by a molecular beam magnetic deflection technique. Most of the sandwich species displayed one-sided deflection, while some of smaller Ln-C 8H8 clusters showed symmetric broadening without or with only very small (or absent) net high-field deflection. In general, the total magnetic moments, calculated from the magnitude of the beams deflections, increase with the number of lanthanide atoms (i.e., with increasing sandwich layers); however for Tb-, Ho-, and Tm-C8H8 clusters with n > 3, the suppression of the magnetic moments was observed, possibly through antiferromagnetic interactions. For Eu-C8H8 clusters, we observe a linear increase of the magnetic moments with the number of Eu atoms up to n = 7, with average magnetic moment per Eu atom around 7 μB - similar to that displayed by conventionally synthesized mononuclear Eu IIC8H8 complexes, indicating that Eu atoms exist as Eu2+ ions in the full sandwich Eun(C 8H8)n+1 clusters. These results suggest that Eun(C8H8)n+1 is a promising candidate for a high-spin, one-dimensional building block in organometallic magnetic materials.

We have investigated the nanosurface-confined anisotropic growth of ordered-ellipsoidal Fe nanogranules when an Fe plume was deposited at a slanting angle onto an anodized aluminum oxide (AAO) film. Layer-by-layer growth was also investigated. This growth is driven by two critical factors: (1) a new rhombic AAO cell and (2) the slanting deposition of the Fe plume. During slanting deposition, the rhombic AAO cell induces strong restrictions in the nucleation site, growth direction, and granular size; therefore, the degree of freedom during growth is restricted. The magnetic dipoles of the ordered Fe nanogranules are placed along the long axis of the ellipsoid at an angle of 180° (antiparallel) due to the demagnetizing field, shape anisotropy, and magnetic dipole-to-dipole interactions.

Gas-phase synthesized transition metal-benzene sandwich complexes of M(benzene)2 (M = Ti, V, and Cr) are soft-landed onto a self-assembled monolayer of n-octadecanethiol (C18-SAM) at a collision energy of 10-20 eV. The resulting adsorption states and thermal desorption kinetics of the soft-landed complexes are studied with infrared reflection absorption spectroscopy and temperature-programmed desorption. The complexes keep their native sandwich structure intact on the SAM substrate even after the "hyperthermal" deposition event. The soft-landed complexes are oriented with their molecular axes largely tilted off the surface normal of the SAM substrate and exhibit unusually large desorption activation energies (E d = ∼130 kJ/mol). For comparison, thermal deposition (∼25 meV) of Cr(benzene)2 vapor onto the C18-SAM, carried out using a physical vapor deposition technique, showed that the complexes are weakly physisorbed (Ed = ∼70 kJ/mol) on the SAM with a random orientation. Only a hyperthermal collision event allows the incident complexes to penetrate into the SAM matrix. The desorption of the embedded complexes in the SAM is then suppressed to around room temperature and may be associated with the crystal-rotator phase transitions of the SAM matrix.

Mass spectrometry and photoelectron spectroscopy of o-, m-, and p-terphenyl cluster anions, (o-TP)(n)(-) (n = 2-100), (m-TP)(n)(-) (n = 2-100), and (p-TP)(n)(-) (n = 1-100), respectively, are conducted to investigate the effect of molecular shape on the molecular aggregation form and the resultant ion core character of the clusters. For (o-TP)(n)(-) and (m-TP)(n)(-), neither magic numbers nor discernible isomers are observed throughout the size range. Furthermore, their vertical detachment energies (VDEs) increase up to large n and depend linearly on n(-1/3), implying that they possess a three-dimensional (3D), highly reorganized structure encompassing a monomeric anion core. For (p-TP)(n)(-), in contrast, prominent magic numbers of n = 5, 7, 10, 12, and 14 are observed, and the VDEs show pronounced irregular shifts below n = 10, while they remain constant above n = 14 (isomer A). These results can be rationalized with two-dimensional (2D) orderings of p-TP molecules and different types of 2D shell closure at n = 7 and 14, the monomeric and multimeric anion core, respectively. Above n = 16, the new feature (isomer B) starts to appear at the higher binding side of isomer A, and it becomes dominant with n, while isomer A gradually disappears for larger sizes. In contrast to isomer A, the VDEs of isomer B continuously increase with the cluster size. This characteristic size evolution suggests that the transition to modified 2D aggregation forms from 2D ones occurs at around n = 20.

Silicon (Si), germanium (Ge), tin (Sn), and lead (Pb) clusters mixed with a group-4 transition metal atom [M= titanium (Ti), zirconium (Zr), and hafnium (Hf)] were generated by a dual- laser vaporization method, and their properties were analyzed by means of time- of- flight mass spectroscopy and anion photoelectron spectroscopy together with theoretical calculations. In the mass spectra, mixed neutral clusters of MSi(16), MGe(16), and MSn(16) were produced specifically, but the yield of MPb(16) was low. The anion photoelectron spectra revealed that MSi(16), MGe(16), and MSn(16) neutrals have large highest occupied molecular orbital - lowest unoccupied molecular orbital gaps of 1.5 - 1.9 eV compared to those of MPb(16) (0.8 - 0.9 eV), implying that MSi(16), MGe(16), and MSn(16) are evidently electronically stable clusters. Cage aromaticity appears to be an important determinant of the electronic stability of these clusters: Calculations of nucleus- independent chemical shifts (NICSs ) show that Si(16)(4-), Ge(16)(4-), and Sn(16)(4-) have aromatic characters with negative NICS values, while Pb(16)(4-) has an antiaromatic character with a positive NICS value. (c) 2008 American Institute of Physics.

Gas-phase-synthesized chromium-benzene 1:2 sandwich cation complexes [Cr+(benzene)2] were soft-landed on a self-assembled monolayer (SAM) of fluorinated alkanethiol (C10F-SAM) at a hyperthermal collision energy of ∼20 eV. The adsorption properties and thermal stability of the soft-landed complexes were studied with infrared reflection absorption spectroscopy (IRAS) and temperature-programmed desorption (TPD). The landed complexes were neutralized due to charge transfer from the SAM substrate, but their native sandwich structure remained intact. The hyperthermal collision event resulted in the penetration of the incoming complexes into the C10F-SAM matrix. The embedded complexes then tended to orient their molecular axes approximately along the surface normal. This orientational preference is measurably different from that of complexes isolated in alkanethiol SAM matrices, a discrepancy that might be caused by a repulsive interaction between the n cloud of the capping benzene rings of the complex and the side-chain CF2 groups of the fluorocarbon chains in the C10F-SAM matrix. The thermal desorption study showed that the complexes supported inside the C10F-SAM could resist thermal desorption until the high-temperature region of ∼320 K, a persistence revealing a large desorption activation energy (∼190 kJ/mol).

Gas-phase synthesized Cr(benzene)(2) sandwich complexes were soft-landed onto a carboxyl-terminated self-assembled monolayer (C15COOH-SAM) on a gold substrate at hyperthermal collision energy (5-20 eV). The deposition process and adsorption properties of the soft-landed complexes were investigated by means of infrared reflection absorption spectroscopy (IRAS) and temperature programmed desorption (TPD) measurements. With the soft-landing, the Cr(benzene)(2) complexes are adsorbed at the SAM surface while keeping their native sandwich structure intact even after the hyperthermal collision event. Moreover, the complexes tend to orient their molecular axes reasonably or comparatively close to the surface normal direction to the SAM surface. This orientation might arise from chemical interactions between the benzene ring in the sandwich complexes and carboxyl groups on the SAM surface. An Arrhenius analysis of the TPD spectra reveals the desorption activation energy of soft-landed complexes as 133 +/- 10 kJ/mol, a value which is comparable to typical adsorption heat of chemisorptions. Chemical interactions between the Cr(benzene)(2) complexes and carboxyl groups would provide firm trapping of the complexes at the SAM surface, preventing the supported complexes from being desorbed until near room temperature.

Multiple-decker vanadium-benzene sandwich clusters Vn(benzene) n+1 produced by a laser-vaporization synthesis method were soft-landed onto self-assembled monolayers of alkanethiol (C18H-SAM) and fluorinated alkanethiol (C10F-SAM) at 200 K. Noncontact atomic force microscopy has been used to examine the resulting adsorption states of the clusters landed on the SAMs at room temperature. For each SAM substrate, the aggregates of the deposited clusters were observed at the vacancy islands and near the steps of the SAM surface. The result indicates that, at room temperature, the clusters landed on the SAM substrate thermally diffuse on the surface to form columnar-shape islands around the defect sites of the SAM surface.

A hexadecyl monolayer covalently attached to Si(111) surfaces (C 16-Si(111)) was prepared at 200 °C from 1-hexadecene. Formation of the monolayer was characterized by water contact angle measurement, attenuated total reflection infrared (ATR-IR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). Gas phase synthesized vanadium (V)-benzene (Bz) 1:2 (VBz2) sandwich clusters were size-selectively deposited onto the C16-Si(111) substrate thus prepared and an oxidized Si substrate. Investigation of the resultant clusters was implemented by thermal desorption spectroscopy (TDS). About 30 K increase in threshold desorption temperature of the landed clusters was observed on going from the oxidized Si to the C16-Si(111) substrate, a result indicating that the clusters are more strongly bound to the C16-Si(111) than to the oxidized Si. This result was explained by the penetration of the landed clusters into the hexadecyl monolayer.

Ligand-free naked and oleylamine-capped nickel nanoparticles (Ni NPs) were size-selectively prepared in the size ranges of 5-17 nm (naked) and 13-19 nm (capped) in diameter through gas- and liquid-phase syntheses, respectively. As for the magnetic properties of both the naked and oleylamine-capped Ni NPs, the blocking temperatures, TB, increase with the size, where TB for the naked Ni NPs are significantly higher than that for the capped Ni NPs. The results indicate that the ferromagnetic core region of Ni NPs is reduced by the formation of nonmagnetic layers at the surface of the capped NPs, where their thickness is estimated to be 1-2 nm.

The surface and bulk electronic states of hydrogen-terminated Si(111)-(1 × 1) [H-Si(111)] were studied by two-photon photoemission (2PPE) spectroscopy. With the polarization and emission-angle dependence of the 2PPE spectra, five series of electronic structures for H-Si(111) can be identified as A, B, C, D, and E in the 2PPE spectra by fine tuning the exciting photon energy, hv, from 4.23 to 4.90 eV. For hv < 4.6 eV, peak A arises from a surface resonance at a 1.01 eV from EF, and at the photon energy higher than 4.6 eV, peak A indicates photoemission from an intermediate state, corresponding to an image-potential state at 3.61 eV from EF. Other structures, B, C, and D arise from bulk states, while structure E occurs from an electronhole excitation process. This work has revealed the surface resonance and the image-potential state on H-Si(111) for the first time. © Springer-Verlag 2009.

Two-photon photoemission (2PPE) spectroscopy was employed for size-selected silver nanoparticles (Ag NPs) deposited on a hydrogen-terminated Si(1 1 1) [H-Si(1 1 1)] to reveal photoemission enhancement through local surface plasmon resonance (LSPR). At low coverage of Ag NPs around 0.005 monolayer equivalents (MLE), the photoemission from the H-Si(1 1 1) substrate was enhanced by similar to 2 x 10(2) times. Above 0.10 MLE, in contrast, photoemission from the Ag NPs themselves was observed. From the experimental results of polarization and the photoemission angle dependence, the photoemission enhancement, depending on the coverage of Ag NPs, is attributed to near-fields from LSPR of Ag NPs. (C) 2010 Elsevier B.V. All rights reserved.

The photoemission enhancement with local surface plasmon resonance (LSPR) was studied by two-photon photoemission (2PPE) spectroscopy for size-selected silver (Ag) and gold (Au) metal nanoparticles (NPs) deposited on a hydrogen-terminated Si(111)-(1 × 1) [H-Si(111)] surface. At 0.0015 monolayer equivalents (MLE) of Ag NPs, the photoemission enhancement was observed at a photon energy of 3.10 eV, which corresponds to the peak energy for the LSPR of isolated Ag NPs. A surface with around 0.005 MLE LSPR of size-selected Ag NPs exhibited three- and four-photon photoemission processes, implying monodispersed Ag NPs on H-Si(111). This enhancement could not be observed for Au NP deposition, even at 1.0 MLE in the photon energy range of 2.90-3.23 eV. Taking into account the polarization and photoemission-angle dependences, the photoemission enhancement could be accounted for by a mechanism involving the near-fields induced by the LSPR of Ag NPs. This mechanism is consistent with an analysis based on the effect of the interparticle distance between Ag NPs on the near-field intensity and polarization.

Density functional theory computations are employed to investigate the geometric, electronic, and vibrational properties of a neutral Ti(aniline) experimentally soft-landed onto an alkanethiol self-assembled monolayer (CH-SAM) matrix. Five optimized structures of a bare Ti(aniline) are obtained: Three of the five optimized structures show Ti-phenyl ring bonds with singlet, triplet, and quintet spin states and the other two show Ti-N bonds with triplet and quintet states. For their calculated IR spectra in the range 0-2000 cm -1, the peak position and intensity of the NH2 scissor mode (∼1600 cm-1) are insensitive to the Ti binding and spin states, whereas those of the NH2-C6H5 intergroup stretching (∼1200 cm-1) and NH2 inversion (500-1000 cm-1) modes are sensitive to these factors. To study Ti(aniline) in a CH-SAM matrix, geometric structures of a modeled system of Ti(aniline)-C3H8 are optimized and their infrared reflection absorption spectra are directly calculated by projecting the normal mode derivatives of the dipole moment of the system onto the surface normal. C3H8 stabilizes Ti(aniline) by about 8-10 kcal/mol but causes little change in the IR spectrum in the range 1300-1700 cm-1, except for the quintet state. From a comparison of the predicted IRAS spectra with the experiment, Ti(aniline) in CH-SAM is concluded to be in a triplet state with a Ti-phenyl ring bond.

The physical properties of surface modified indium-tin oxide (ITO) and SnO2(110) with an adsorbed aliphatic chain monolayer were studied by soft-landing experiments coupled with infrared (IR) spectroscopy, temperature programmed desorption (TPD) spectroscopy, and scanning probe microscopy. The ER spectra showed that the aliphatic chains of palmitic acid on both the metal oxide surfaces form a crystalline-phase monolayer with an all-trans conformation at room temperature. Gas-phase synthesized Cr(benzene)(2) cations deposited with a hyperthermal energy (20 eV) were used to probe the surface morphology. The IR spectrum after deposition showed that the cations soft-landed after being neutralized and oriented to cant the molecular axes far from the surface normal direction through penetration into the monolayer matrix The TPD spectrum showed that the desorption activation energy of Cr(benzene)2 from an aliphatic monolayer on ITO is much lower than that from the aliphatic monolayer on SnO2 or a self-assembled monolayer of alkanethiol on Au(111). The lower thermal stability of Cr(benzene)(2) on the aliphatic monolayer on ITO can be ascribed to considerable surface roughness of the ITO itself, which can be characterized by scanning probe microscopy.

Lanthanide nanoclusters ligated by planar organic molecules have one-dimensional sandwich structures and are air/moisture sensitive. To investigate their optical properties, originating from the lanthanide elements, for novel nanodevices, we preserved the nanoclusters with a cycloolefin polymer as a gas barrier. The technique enabled us to conduct various spectroscopic analyses without oxidation of the samples, such as fluorescence microscopy, photoluminescence, and X-ray absorption fine structure determination, thus revealing that lanthanide sandwich nanoclusters containing europium (Eu) atoms exhibit orange fluorescence attributed to divalent Eu 2+.

Two-photon photoemission (2PPE) spectroscopy has been employed to probe the electronic states of n-alkanethiolate self-assembled monolayers (SAMs) on an Au(111) substrate fabricated in a wet chemical process. Electronic states newly formed in the SAM formation were observed below and above the Fermi level (E F) for various alkyl chain lengths of C12-, C18-, and C22-SAMs. At 3.5-3.7 eV above E F, an unoccupied state originating from a bond between gold and sulfur atoms appears, a state that shows little dispersion with parallel to the surface. The peak position of the unoccupied state depends on the substrate temperature, and it was stabilized with increasing temperature; E F +3.7 eV at 85 K and E F +3.5 eV at 330 K for C18-SAM. The stabilization of the state is attributed to the increase of intermolecular interaction at sulfur atoms with their neighboring S atoms, which is caused by a change in the tilt angle of the alkyl chains in the SAM: on increasing the temperature, the interaction between S atoms in the SAM is promoted by the more upright alkyl chains.

The geometric, electronic, and optical properties of a heterodimer and trimer consisting of metal-encapsulating silicon cage clusters, M@Si-16 (M = Sc, Ti, V) with D-4d symmetry, are studied using density functional computations to explore the possibility of using these clusters as building blocks for a nanometer scale heteroassembly. In this study, among the possible low-lying geometries, the linear-form of the hetero-oligomers is adopted as a model system, where the D-4d monomers are covalently bonded by facing their squares in an eclipsed fashion. The heterodimer consisting of halogen-like Sc@Si-16 and alkaline-like V@Si-16 has a dipole moment of 7.63 D, and its occupied and virtual frontier orbitals are localized to V@Si-16 and to Sc@Si-16, respectively. Some of the inner molecular orbitals exhibit superatomic bonding and antibonding character. The electronic excitations involve charge-transfer states mainly from V@Si-16 to Sc@Si-16 in the optical energy region. The linear heterotrimer of Sc@Si-16-Ti@Si-16-V@Si-16, formed by inserting the rare-gas-like Ti@Si-16, has a larger dipole moment of 15.6 D and one or more localized frontier orbitals compared to the dimer. We propose possible formation routes to realize the present hetero-oligomers using photoexcitation or energy-selective electron injection into several LUMOs of the monomers that are suitable for linear-oligomerization.

The electronic properties of germanium and tin clusters containing a transition-or lanthanide-metal atom from group 3, 4, or 5, MGen (M = Sc, Ti, V, Y, Zr, Nb, Lu, Hf, and Ta) and MSnn (M = Sc, Ti, Y. Zr, and Hf), were investigated by anion photoelectron spectroscopy at 213 nm. In the case of the group 3 elements Sc, Y, and Lu, the threshold energy of electron detachment of MGen- exhibits local maxima at n = 10 and 16, while in the case of the group 4 elements Ti, Zr, and Hf, it exhibits a local minimum only at n = 16, associated with the presence of a small bump in the spectrum. A similar behavior is observed for MSnn- around n = 16, and these electronic characteristics of MGen and MSnn are closely related to those of MSin. Compared to MSin, however, the larger cavity size of a Ge-n cage allows metal atom encapsulation at a smaller size n. A cooperative effect between the electronic and geometric structures of clusters with a large cavity of Ge-16 or Sn-16 is discussed together with the results of experiments that probe their geometric stability via their reactivity to H2O adsorption.