Superconductivity of nanosized Pb-island structures whose radius is 0.8 to 2.5 times their coherence length was studied under magnetic fields using low-temperature scanning tunneling microscopy and spectroscopy. Spatial profiles of superconductivity were obtained by conductance measurements at zero-bias voltage. Critical magnetic fields for vortex penetration and expulsion and for superconductivity breaking were measured for each island. The critical fields depending on the lateral size of the islands and existence of the minimum lateral size for vortex formation were observed.

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 α-sexithiophene (6 T) 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-)terminated surfaces similar to those in the gas phase, whereas the nanoclusters destabilize at room temperature on 6 T-terminated surfaces owing to the loss of electronic closure via a change in the charge state.

The force sensor is key to the performance of atomic force microscopy (AFM). Nowadays, most atomic force microscopes use micromachined force sensors made from silicon, but piezoelectric quartz sensors are being applied at an increasing rate, mainly in vacuum. These self-sensing force sensors allow a relatively easy upgrade of a scanning tunneling microscope to a combined scanning tunneling/atomic force microscope. Two fundamentally different types of quartz sensors have achieved atomic resolution: the "needle sensor," which is based on a length-extensional resonator, and the "qPlus sensor," which is based on a tuning fork. Here, we calculate and measure the noise characteristics of these sensors. We find four noise sources: deflection detector noise, thermal noise, oscillator noise, and thermal drift noise. We calculate the effect of these noise sources as a factor of sensor stiffness, bandwidth, and oscillation amplitude. We find that for self-sensing quartz sensors, the deflection detector noise is independent of sensor stiffness, while the remaining three noise sources increase strongly with sensor stiffness. Deflection detector noise increases with bandwidth to the power of 1.5, while thermal noise and oscillator noise are proportional to the square root of the bandwidth. Thermal drift noise, however, is inversely proportional to bandwidth. The first three noise sources are inversely proportional to amplitude while thermal drift noise is independent of the amplitude. Thus, we show that the earlier finding that quoted an optimal signal-to-noise ratio for oscillation amplitudes similar to the range of the forces is still correct when considering all four frequency noise contributions. Finally, we suggest how the signal-to-noise ratio of the sensors can be improved further, we briefly discuss the challenges of mounting tips, and we compare the noise performance of self-sensing quartz sensors and optically detected Si cantilevers.

Chemical characterization was performed for an alkali-like superatom consisting of a Ta-encapsulating Si16 cage, Ta@Si16, deposited on a graphite substrate using X-ray photoelectron spectroscopy (XPS) to element-specifically clarify the local electronic structure of the cage atoms. The XPS spectra derived from Ta 4f and Si 2p core levels have been well modeled with a single chemical component, revealing the formation of a symmetric Si cage around the Ta atom in the deposited nanoclusters. On chemical treatments by heating or oxygen exposure, it is found that the deposited Ta@Si16 is thermally stable up to 700 K and is also exceptionally less reactive toward oxygen compared to other Ta-Si nanoclusters, although some heat degradation and oxidation accompany the treatments. These results show the promising possibility of applying Ta@Si16 as a building block to fabricate cluster-assembled materials consisting of naked nanoclusters.

Nanoclusters, aggregates of several to hundreds of atoms, have been one of the central issues of nanomaterials sciences owing to their unique structures and properties, which could be found neither in nanoparticles with several nanometer diameters nor in organometallic complexes. Along with the chemical nature of each element, properties of nanoclusters change dramatically with size parameters, making nanoclusters strong potential candidates for future tailor-made materials; these nanoclusters are expected to have attractive properties such as redox activity, catalysis, and magnetism. Alloying of nanoclusters additionally gives designer functionality by fine control of their electronic structures in addition to size parameters. Among binary nanoclusters, binary cage superatoms (BCSs) composed of transition metal (M) encapsulating silicon cages, M@Si, have unique cage structures of 16 silicon atoms, which have not been found in elemental silicon nanoclusters, organosilicon compounds, and silicon based clathrates. The unique composition of these BCSs originates from the simultaneous satisfaction of geometric and electronic shell-closings in terms of cage geometry and valence electron filling, where a total of 68 valence electrons occupy the superatomic orbitals of (1S)(1P)(1D)(1F)(2S)(1G)(2P)(2D) for M = group 4 elements in neutral ground state. The most important issue for M@Si BCSs is fine-tuning of their characters by replacement of the central metal atoms, M, based on one-by-one adjustment of valence electron counts in the same structure framework of Si cage; the replacement of M yields a series of M@Si BCSs, based on their superatomic characteristics. So far, despite these unique features probed in the gas-phase molecular beam and predicted by quantum chemical calculations, M@Si have not yet been isolated. In this Account, we have focused on recent advances in synthesis and characterizations of M@Si BCSs (M = Ti and Ta). A series of M@Si BCSs (M = groups 3 to 5) was found in gas-phase molecular beam experiments by photoelectron spectroscopy and mass spectrometry: formation of halogen-, rare-gas-, and alkali-like superatoms was identified through one-by-one tuning of number of total valence electrons. Toward future functional materials in the solid state, we have developed an intensive, size-selected nanocluster source based on high-power impulse magnetron sputtering coupled with a mass spectrometer and a soft-landing apparatus. With scanning probe microscopy and photoelectron spectroscopy, the structure of surface-immobilized BCSs has been elucidated; BCSs can be dispersed in an isolated form using C fullerene decoration of the substrate. The intensive nanocluster source also enables the synthesis of BCSs in the 100-mg scale by coupling with a direct liquid-embedded trapping method into organic dispersants, enabling their structure characterization as a highly symmetric "metal-encapsulating tetrahedral silicon-cage" (METS) structure with Frank-Kasper geometry.

The adsorption of two kinds of porphyrin (Cu-TBPP) and perylene (PTCDA) derived organic molecules deposited on KBr and Al 2 O 3 surfaces has been studied by non-contact force microscopy in ultra-high vacuum, our goal being the assembly of ordered molecular arrangements on insulating surfaces at room temperature. On a Cu(100) surface, well ordered islands of Cu-TBPP molecules were successfully imaged. On KBr and Al 2 O 3 surfaces, it was found that the same molecules aggregate in small clusters at step edges, rather than forming ordered monolayers. First measurements with PTCDA on KBr show that nanometre-scale rectangular pits in the surface can act as traps to confine small molecular assemblies.

The electronic states of three different sizes of compositionally precise thiolate-protected gold nanoclusters, Au 25 (SR) 18 , Au 38 (SR) 24 , and Au 144 (SR) 60 (R = C 12 H 25 ), have been evaluated by X-ray photoemission spectroscopy. The Au 4f corelevels of the nanoclusters are well reproduced by two spectral components derived from centered core-Au and positively charged shell-Au atoms, the numbers of which are determined based on the atomic structures of the nanoclusters. The spin−orbit splitting of Au 5d 5/2 and 5d 3/2 in the valence band becomes narrower than that for bulk Au, depending on the cluster size, which is quantitatively characterized by a reduction in the average coordination number of Au. The Au 5d valence-band spectra also show that the charge reorganization of 5d electrons induced by interaction with thiol molecules is more significant for the 5d 5/2 than the 5d 3/2 orbital.

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 (EF), 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 EF + 3.7 eV. The tunneling decay parameter, β, was fitted by β90 K = 0.097 Å(-1) and βRT = 0.13 Å(-1). These values were one order smaller than that at around EF by conventional contact probe methods.