Published papers

Metal-encapsulating silicon cage (M@Si-16) nanoclusters (NCs) are promising superatoms (SAs) as function-tunable nanomaterials, which exhibit a superior chemical stability owing to electronic and geometric closures. Here, we examine how the superatomic nature of an alkali-like Ta@Si-16 SA is staggered by the variation of the number of Si atoms in TaSin NCs (n = 6, 8, 12, 15, 17, and 18) and subsequent immobilization on a C-60 fullerene substrate. Using X-ray photoelectron spectroscopy, the size dependence of chemical robustness of TaSin NCs on C-60 against O-2 exposures is quantitatively evaluated: In addition to the most outstanding stability of a Ta@Si-16 SA as compared to TaSi15, some enhanced stability is also observed at Ta@Si-17, which couples in a Si-adatom structure of (Ta@Si-16)-Si to the support. While oxidative reactivities of TaSin are gradually suppressed from small NCs (n = 6) to larger ones (n = 18) with increasing number of Si atoms, the results show that (1) an enclosing Si cage around a Ta atom is completed at Ta@Si-16 on C-60 and (2) Ta@Si-17 NCs are robust against O-2 oxidation with assistance from the stability of Ta@Si-16 SA despite the Si-adatom.

Metal-encapsulating Si-16 cage clusters (M@Si-16) are promising superatoms (SAs) for designing tunable properties for their assembled materials by changing the central metal atom: halogen-like, rare-gas-like, and alkali-like characteristics appear for the central metal atom of groups 3, 4, and 5, respectively. To fabricate SA assemblies, metal-encapsulating M@Si16 SAs (M = Lu, Hf, and Ta) must be controllably immobilized on a substrate. Substrates decorated with organic molecules can facilitate optimization of a cluster-surface interaction because the molecular local interactions between SAs and predeposited organic molecules govern the electronic properties through molecular complexation. In this study, M@Si-16 SAs are size-selectively soft-landed on organic substrates deposited with n-type fullerene (C-60) and p-type hexa-tert-butyl-hexa-peri-hexabenzocoronene (HB-HBC, C66H66), and the electronic states of M@Si-16 on the organic substrates are characterized by X-ray and ultraviolet photoelectron spectroscopy. On the C-60 substrate, all M@Si-16 are fixed to be cationic, forming M@Si-16(+) C-60(-) via a charge transfer interaction, while on an HB-HBC substrate, M@Si16-HB-HBC+ (M = Lu and Hf) is formed with anionic M@Si-16(-). Together with density functional theory calculations, the charge preference of the M@Si-16 SA is examined based on its chemical stability against O-2 gas exposure; Lu@Si-16 on HB-HBC is more robust toward O-2 than that on C-60, while Ta@Si-16 on HB-HBC is less robust than that on C-60. Depending on the SA properties, an appropriate selection of organic molecules for deposition provides a molecular designer concept for forming SA-assembled nanomaterials through the cluster-surface interaction.

Functional surfaces decorated with organic molecules and/or nanoclusters (NCs) composed of several tens of atoms are promising for use in future photoelectronic substrates, whose functionalities are governed by molecular local electronic/plasmonic excitations at the interfaces. Here, we combine two-photon photoemission spectroscopy (2P-PES) and microscopy (2P-PEEM) to investigate the local excited-state dynamics at organic surfaces functionalized with NCs. The 2P-PES and 2P-PEEM for organic fullerene (C60) layers on graphite and Au substrates demonstrated photophysical characterization of electronic and plasmonic properties, including propagating surface plasmon polaritons (SPPs). The SPP propagation at the Au interface buried by overlayered C60 can be visualized by Agn NC deposition, which enhances plasmon-induced hot electrons, where the threshold number of Ag atoms (n >= 9) for the plasmonic response is revealed by the size dependence of 2P-PES for Agn NCs on C60 layers.

© 2020 American Chemical Society. Visualization of surface plasmon polariton (SPP) propagation at dielectric/metal interfaces is indispensable in providing opportunities for the precise designing and controlling of the functionalities of future plasmonic nanodevices. Here, we report the visualization of SPPs propagating along the buried organic/metal interface of fullerene (C60)/Au(111), through dual-colored two-photon photoemission electron microscopy (2P-PEEM) which precisely visualizes the SPP propagation of plasmonic metal nanostructures. Although SPPs excited by near-infrared photons at the few monolayer C60/Au(111) interface are clearly visualized as interference beat patterns between the SPPs and incident light, faithfully reflecting SPP properties modulated by the overlayer, photoemission signals are suppressed for thicker C60 films, due to less valence electrons participating in 2P-photoemission processes. With the use of silver (Agn (n = 21 and 55)) nanoclusters, which exhibit enhancement of overall photoemission intensities due to localized surface plasmons functioning as SPP sensitizers, it is revealed that the 2P-PEEM is applicable to the imaging of SPPs for thick C60/Au(111) interfaces, where SPP properties are hardly modulated by the added small amount (∼0.1 monolayer) of Agn sensitizers.

The development of efficient fuel cells greatly promotes reducing the consumption of fossil energy, and it is crucial to enhance the platinum (Pt) catalytic activity by optimizing both the nanoparticle size and support effect. In this study, we generate a smaller and uniform size of naked Pt nanocluster (NC) catalystsviaa dry process in the gas phase, and using the direct powder embedded trapping method, the Pt NCs arespatioselectivelysupported on Marimo carbon (MC) that comprises a high density of carbon nanofilaments. At a minimum Pt loading of 0.05 mgPtcm−2for both the anode and cathode in a single cell configuration, a membrane-electrode assembly (MEA) formed by the dry-deposition Pt-NC(d)/MC exhibits superior power density (rated) of 0.105 gPtkW−1at a current density of 1.2 A cm−2, which is better output power density than the conventional MEA formed by Pt catalystsviaa wet process. The origin of the improved performance is investigated using transmission electron microscopy; dry-deposition Pt catalysts that are monodispersely loaded on the outer surface of MC can facilitate not only the gas reaction but also leaving the generated water. The present results demonstrate that the dry deposition of Pt NCs on MC can be used as a scalable catalyst synthesis method to reduce the Pt loading.

The local photoexcited surface states of a polycrystalline-copper (Cu) substrate were observed via two-photon photoelectron emission microscopy (2P-PEEM) using a femtosecond laser. The 2P-PEEM results revealed local distributions of the photoexcited electrons, strongly depending on the crystal domains. With the energy-filtered images, the local 2P-photoemission spectra were selectively obtained for (1 1 1)- and (1 1 0)/(1 0 0)-crystal domains. Furthermore, the 2P-PEEM could visualize propagating Cu surface plasmon polaritons with wavevector of 7.61 x 10(6) m(-1). These results indicate that the 2P-PEEM technique is a promising tool for studying the distribution of nonequivalent photoexcited surface states for various inhomogeneous functional nanomaterials.

2PPE spectroscopy for plasmonic response reveals the deformation to flattened geometries of Agn nanoclusters on C60 above n = 55.

The formation and molecular structure of a self-assembled monolayer (SAM) of anthracene-substituted alkanethiol on Au(111) have been investigated by scanning tunneling microscopy and infrared reflection absorption spectroscopy. A clean and well-ordered SAM composed of densely packed "standing-up" molecules is formed by a wet-chemical process in air, followed by thermal annealing in vacuum. In the SAM, anthracene moieties are arranged in a bulklike in-plane herringbone structure and keep their crystalline ordering above room temperature, which is in contrast to the less ordering in Au nanoclusters ligated by the anthracene-alkanethiolates. In addition, hexagonal arrangement of anthracene dimer is found on the surface of the SAM. These structures remain after annealing at 400 K. Scanning tunneling spectroscopy performed on the surface shows an energy gap of similar to 3.6 eV, similar to the band gap of bulk anthracene. The above results demonstrate that the highly ordered semiconducting anthracene monolayer is successfully formed on the insulating alkanethiolate thin film and has high thermal stability due to a strong chemical bonding across the interface.

Bimetallic cobalt (Co)-vanadium (V) clusters, ConVm- (n = 4-14, m = 0-2), are investigated using anion photoelectron spectroscopy at 4.66 eV photon energy. Electron affinities (EA) and vertical detachment energies (VDE) are determined, and the electronic structures of V- and V2-doped Con clusters are compared to those of pure Con clusters. Bonding and hybridization effects can be explained using a charge-transfer model. A peak shape analysis confirms the high stability of the Co12V cluster with a most plausible icosahedral structure with the V atom in the cage center. Together with ionization potentials (IP), the electronic structures of the clusters are compared to H2 chemisorption rate coefficients reported previously. Thus, this study completes the investigation of electronic properties and reactivity of the bimetallic ConVm clusters as a function of charge, size, and distribution. Although the reactivity of ConVm reveals a better correlation toward the EA rather than toward IP, no improved correlation is found between promotion energies and reactivity. This result clearly shows that the chemisorption reactivity of the heterogeneous clusters is largely governed by geometric factors.

Adiabatic electron affinities (EA(a)'S) of seven oligophenyls: biphenyl (BP), 4-cyanobiphenyl (CNBP), o-terphenyl (o-TP), m-terphenyl (m-TP), p-terphenyl (p-TP), p-quaterphenyl (p-QP), and 1,3,5-triphenylbenzene (TPB), have been evaluated by anion photoelectron spectroscopy and density functional theory calculations. The result provides the direct determination of the EA(a) values of the isolated CNBP, p-TP, p-QP, and TPB molecules, while EA(a) of BP, o-TP, and m-TP were estimated from the extrapolation of EA(a) vs cluster size. The excellent agreement between the experimental and calculated EA(a) were obtained.