Award AccountsThe Chemical Society of Japan Award for Creative Work for 2008

Study on Electronic Properties of Composite Clusterstoward Nanoscale Functional Advanced Materials

Atsushi Nakajima

Department of Chemistry, Faculty of Science and Technology, Keio University,3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522

Received October 29, 2012; E-mail: nakajima@chem.keio.ac.jp

Clusters consisting of 10­1000 atoms size-dependently exhibit novel electronic and geometric properties. Inparticular, composite clusters composed of several elements and/or components provide a promising way for a bottom-upapproach for designing functional advanced materials, because the functionality of the composite clusters can beoptimized not only by the cluster size but also by their compositions. This account surveys recent investigations ofcomposite clusters focusing on the efforts to create a new functional composite cluster by a fine doping or hybridizationbased on their size-specific electronic properties. Organometallic clusters and caged clusters are demonstrated as arepresentative example of designing the functionality of magnetism and electronic state structures. In order to createfunctional nanomaterials, furthermore, a fine controlling methodology of the soft-landing technique has been developed tofix the composite clusters onto a surface decorated with a self-assembled monolayer. The embedded isolation mechanismon the substrate is discussed form the viewpoint of self-assembly phenomena with the molecular ordering of ·­·interaction, and also the electronic structures characterized by molecular ordering of ³­³ interaction is intrinsicallyrevealed by molecular clusters of ³-conjugate polyacenes as a model for self-assembled aggregates.

1. Introduction

Over several decades, nanometer scale (nanoscale) particleswith diameters less than 100 nm have attracted much interestas a new material phase because the downsizing to nanoscaleregime brings about significant changes in the physical andchemical properties of matter, such as optical, electrical, mag-netic, and catalytic properties.1­5 In particular, clusters consist-ing of 10­1000 atoms or molecules size-dependently exhibitnovel properties that differ greatly from those predicted byscaling laws;6,7 the cluster properties can be no longer scaledfrom those of the corresponding bulk metal. In 1962, Kubotheoretically predicted that reducing the size of bulk materialsinto nanoscale regime makes peculiar changes in the electronicheat capacity and magnetism, revealing that their electronicproperties become different from those in bulk.8 On the otherhand, with the development of electron microscopy, Uyeda hasfound a new type of nanoscale fine particles (nanoparticles;NPs) showing fivefold symmetry.9 Furthermore, Mackay andIno have independently discovered an icosahedral or multiple-twin NPs, and these discoveries reveal that NPs exhibit novelgeometric properties which are different from bulk.10,11 Thetwo representative factors of electronic and geometric struc-tures could promote the novelty and uniqueness of the nano-scale materials (nanomaterials) beyond the scaling laws, and inrecent years, nanomaterial science has been greatly developed

all over the world, together with impact toward many areasof science: chemistry, physics, biology, and so on. In particu-lar, the discoveries of fullerenes and giant magnetoresistiveeffects,12 and innovative development of biological enzymescience13 have demonstrated the wealth of nanomaterial scienceand technology.

Nanoscale systems less than the 10 nm regime deservegreater prominence in nanomaterials; palladium NPs (8 nm indiameter) exhibit ferromagnetism due to the peculiar surfacestructures, while solid palladium exhibits paramagnetism inbulk.14 Gold NPs (2­5 nm in diameter) supported on a metaloxide surface show catalytic activity for the oxidation ofcarbon monoxide.15 The charge state, adsorption site, and struc-tural fluctuation16 of gold NPs play a key role in the catalyticactivity. Such novel properties of the nanoscale species areoften governed by quantum confinement effects, where thequantized energy gaps comparably compete with the inter-atomic binding energy. In the nanoscale system, the competi-tion between the electronic state and packing structure ofthe nucleus results in unique characteristics as a new phasefor hierarchy in material. Since the quantum confinement effectsare significantly emphasized in the size range of 1 to 10 nm(10 to 104 atoms), the novel properties and functionalities ofthe nanoscale species emerge with unique geometric struc-ture that strongly depends on the number of atoms (i.e., clustersize).

© 2013 The Chemical Society of Japan

Published on the web April 15, 2013; doi:10.1246/bcsj.20120298

Bull. Chem. Soc. Jpn. Vol. 86, No. 4, 414­437 (2013)414

In the recent decade, however, the developments leadingto the applications of the nanomaterials have been very limit-ed only to carbon materials,17 i.e., fullerenes, graphene, andcarbon nanotubes. Several serious problems still remain togenerate nanoscale systems, for instance, (1) no accurate way topredict unique functional properties of the nanoscale species,and (2) few tuning parameters to optimize the physical andchemical properties of the nanoscale clusters (nanoclusters).Moreover, the technical issues for immobilization of nano-structures should be developed with the combination betweenthe bottom-up and top-down approaches. In order to establishnovel functional materials by using 1- to 10-nm nanostructures,the fabrication of well-assembled and finely controlled nano-structure formation are highly desired, along with the deepscientific understanding of the correlation between the func-tionality and assembled formation of the nanostructures. Theestablishment of the finely controlled and tunable nanoassem-bled functional systems will definitely contribute to unveilthe fundamental nature of the nanomaterial science with singlenanometer scale (1­10 nm). The research development in thisfield would initiate innovative fusion of nanoscience andnanotechnology to bring a new trend for advanced materialscience.

In this account, I would like to focus on composite clustersconsisting of multicomponents, because the composite clustersprovide a new parameter of the combinations of the constituentatoms or molecules as well as the cluster size.18 As apparentlydemonstrated in the chemistry field over many years, the com-binations of no less than two components can produce widevariety of useful chemical compounds, and then the compo-sition parameter enables us to design the electronic propertiesof the composite clusters and to optimize the cluster function-ality. Since composite clusters can be regarded as a minimumfunctional unit, the synthesis of functional composite clustersis a crucial fundamental to generate advanced nanomaterialwith the knowledge of cluster chemistry. In particular, in theformation of composite clusters, their geometric symmetry anddimensionality are emphasized to control the physical andchemical properties, because selective and anisotropic enhance-ments for optical, chemical, and magnetic properties can begenerally anticipated.18,19 In order to explore the synthesis andcharacterization of the composite clusters, several experimentalmethodologies have been developed as shown in Section 2.With the advent of the experimental methods, composite clus-ters of organometallic sandwich nanoclusters (1­5 nm length)were generated19­27 and characterized to show novel ferromag-netic electron spin ordering to be the smallest one-dimensionalmolecular magnets of vanadium­benzene nanoclusters,28,29 asdescribed in Sections 3.1 and 3.2. Furthermore, it has beenalso found that nanoclusters consisting of multicomponents,including silicon and aluminum, can act as novel functionalunits18,30­36 when the composite nanocluster is well stabilizedboth geometrically and electronically as a “superatom”33a,35a

as shown in Sections 3.3 and 3.4. In order to achieve surfacedecorations with the novel nanomaterials and to reveal the elec-tronic origin of cluster functionality, a new methodology forsoft-landing37 has been developed as described in Section 3.5,with a self-assembled monolayer (SAM) of alkanethiol onAu(111) surface.38­40 The isolation mechanism inside SAM on

the substrate will be further discussed form the viewpoint ofself-assembly phenomena with the molecular ordering of ·­·interaction between alkyl-chains.39 In Section 3.6, the self-assembled molecular ordering is discussed from the viewpointof the electronic structure evolution in molecular clusters of³-conjugate polyacenes,41,42 where ³­³ interaction betweenaromatic rings can provide the self-assembled molecularordering. Finally, Section 4 summarizes this work and dis-cusses future perspectives.

2. Experimental Section

2.1 Cluster Formation. 2.1.1 Composite BinaryClusters: Composite clusters consisting of metal­metal ormetal­semiconductor elements were conventionally generatedby laser vaporization of the corresponding elements with twoindependent frequency-doubled Nd3+:YAG lasers,18 as shownin Figure 1. Very intense supersonic helium gas pulses wereemployed to recombine and cool the laser-produced metalvapors, where a high pressure Even­Lavie pulsed valve (10Hz)was used at a stagnation pressure of 60­100 atm.43 Cations/neutrals/anions of the composite clusters were generated bythe dual laser vaporization method, and the optimum laserfluence required to maximize the cluster intensities variedaccording to the properties of the metal under investigation.The cluster temperature was apparently around room temper-ature due to the collisions with the He carrier gas in the 30mm long channel. The development of a vaporization targetwidened the combination of the elements for the compositeclusters. Indeed, the laser vaporization is applicable to thesoft solid of alkaline metal elements such as lithium (Li) andsodium (Na)18b and also to the powder C60 by molding them asa rod shape, where the C60 rod has been prepared by pressingpurchased C60 powder.24a

In this work, the cluster source was modified so that thecomposite clusters could be more effectively generated.

Pulsed Valve 1

Reaction Room

Sample Rod 1

He

Reactant Gas(H2O, F2 / He)

Vaporization Laser 2

Skimmer

Vaporization Laser 1

Lens

Lens

Sample Rod 2

Pulsed Valve 2

Figure 1. A schematic dual laser vaporization setup for themodified “face-to-face” cluster source combined with aflow-tube reactor (FTR). Pulsed Valve 1; high-pressureEven­Lavie pulsed valve for helium, Pulsed Valve 2;commercial General valve for reactant gas, and Vapor-ization Lasers 1 and 2; frequency-doubled Nd3+:YAGlasers are shown.

A. Nakajima Bull. Chem. Soc. Jpn. Vol. 86, No. 4 (2013) 415

Figure 1 shows the schematic diagrams of the modified “face-to-face” cluster source.35b In the previous source, two sam-ple rods were laterally separated by 4mm from each other,and their front surfaces were vaporized independently by thefollowing steps: (1) One sample rod was first laser-vaporized;(2) the generated vapor was carried downstream with the Hecarrier gas; (3) the other sample rod was laser-vaporized whenthe vapor of the first component arrived, and (4) the two vaporswere mixed. The two rods were laser-vaporized with a delay ofabout 5¯s in synchronization with the He carrier gas speed.While the vapor of the first element vaporized was carried tothe locations of the second target, it was cooled to formaggregates of itself. Hence, both individual clusters whichconsisted of one element and mixed clusters were generated,and the formation efficiency of binary mixed clusters wassignificantly lowered.

At that point, the focusing positions of the vaporizationlasers were modified, and were shifted toward each other by1.5mm and moved off the front of the sample rods, as shownin Figure 1. This shift of the focusing positions facilitated thevaporization of the curved surface of the sample rods and theeffective mixing of the hot sample plasmas. The improvementsoccurred because the atoms and ions in the plasma undergoa highly directional expansion perpendicular to the targetsurface.44 In fact, the mixed clusters were produced efficientlywhen the two rods were almost simultaneously vaporized,within a few hundreds nanoseconds (ns). This modificationresulted in the effective formation of binary metal­semi-conductor clusters particularly.

2.1.2 Composite Organometallic Clusters: Organo-metallic sandwich clusters were synthesized in the reactionbetween laser-vaporized metal atoms and the vapor of organicligand molecules around room temperature19,20 by means ofa flow-tube reactor (FTR).45 At the laser-vaporization area, ametal rod, such as vanadium (V) and europium (Eu) (purity99.9% typically), was vaporized by focusing second harmonic(532 nm) pulsed Nd3+:YAG laser (ca. 10mJ/pulse, 10Hz).Intense supersonic helium carrier gas pulses were employedusing an Even­Lavie pulsed valve to cool down the vaporizedhot metal species of atoms and these metal atoms wereentrained into the temperature-controlled FTR. In the reactionroom, ligand vapor diluted with helium gas (1.5­2 atm) wasinjected into the FTR in synchronization with the pulsed flowand the corresponding organometallic clusters were formed. Asmentioned later in Sections 3.1 and 3.2, depending on metalelements, an appropriate ligand molecule should be chosen todesign the properties of organometallic clusters formed. Whenbenzene (Bz) and 1,3,5,7-cyclooctatetraene (COT) vapor wasinjected for V and Eu vaporization, the organometallic clustersof Vn(Bz)m and Eun(COT)m were formed, respectively.19,20,25

2.1.3 Giant Molecular Clusters: By means of supersonicexpansion of a mixture of molecules diluted in a carrier gas(typically a noble gas), both an efficient cooling and formationof molecular clusters is easily realized in the gas phase. Toincrease the total number of collisions between seed moleculesduring expansion, it is necessary to obtain a reasonable vaporpressure of sample of interest by heating both the samplereservoir and the valve. Concurrently, the need for increasedpressure of carrier gas is evident to sufficiently remove the

condensation energy generated in clustering. Hence the effi-cient production of internally cold, large molecular clustersnecessitates the supersonic expansion at high seed ratios withhigher vapor pressure of sample as well as higher pressure ofcarrier gas.

Since the high operating pressure (up to 100 atm) andtemperature (up to 250 °C) of the Even­Lavie valve43 allowsfor producing cold molecular beams under adiabatic expansionconditions, this valve was incorporated with electron attach-ment sources in order to produce very large giant molecularcluster anions (cluster size n ² 100). Each sample of naph-thalene (Nph, melting point (mp): 80 °C), anthracene (Ac,mp 216 °C), tetracene (Tc, mp 357 °C), and five Ac alkyl-derivatives, namely 1-methylanthracene (1-MA; mp 87 °C),2-methylanthracene (2-MA; mp 205 °C), 9-methylanthracene(9-MA; mp 78 °C), 9,10-dimethylanthracene (9,10-DMA; mp183 °C), and 2-tert-butylanthracene (2-tBuA; mp 145 °C) wasplaced in a sample holder attached directly to a pulsed Even­Lavie valve. The sample container and the pulsed valve wereheated to around the melting temperature for each sample by asheath heater. The sample vapor was entrained in helium carriergas at a stagnation pressure of 5­100 atm. Cluster anions wereproduced by attachment of slow secondary electrons generatedby a high-energy electron impact ionization method (ca. 300eV, ca. 1mA) in the condensation zone in an expanding jet.41

2.2 Characterization of Composite Clusters in a GasPhase. 2.2.1 Mass Analysis and Adsorption Reactivity:The generated clusters were mass-analyzed with a time-of-flight (TOF) mass spectrometer: The neutral clusters thusformed were photoionized with an ArF laser (193 nm: 6.42 eV)or an F2 excimer laser (157 nm: 7.90 eV) and accelerated with aconstant electric field, while the cluster cations/anions weredirectly accelerated with a pulsed electric field.34b The laserpower dependence for the photoionization was measured toestablish the conditions for one-photon ionization, and the laserfluence typically used was below 1mJ cm¹2.

The adsorption reactivity measurements for composite clus-ters can be regarded as a chemical probe method in deducingthe geometric structure, when two elements exhibit contrastreactivity toward the reactant.34b The adsorption reactivitywas measured by means of a FTR combined with the clustersource: Reactant vapor diluted with 1.5 atm of He gas wasused as the reactant gas, and was injected into the FTR. Thebinary clusters were synchronously mixed with the reactantgas pulse at the FTR, and reacted with the reactant molecule.The products of the clusters were mass-analyzed by the TOFmass spectrometer, and the mass spectra of the clusters weremeasured before and after the reaction, so that the relativeadsorption reactivity might be measured. For metal­semi-conductor clusters, the reactivity toward water vapor provides auseful index to location of the doped metal atom, as mentionedin Section 3.3.

2.2.2 Photoionization Spectroscopy and PhotoelectronSpectroscopy: The ionization energies (Ei’s) of the clusterswere determined from the photoionization efficiency (PIE)curves measured by changing the ionization photon energy in0.06­0.09 eV intervals over the range of 5.3­6.0 eV with anoptical parametric oscillator (OPO) laser pumped by a Nd3+:YAG laser.19,46 The fluences of the tunable UV laser and the

AWARD ACCOUNTSBull. Chem. Soc. Jpn. Vol. 86, No. 4 (2013)416

fixed wavelength reference laser were both monitored by apyroelectric detector and were kept at 100­200¯J cm¹2 toavoid multiphoton processes. The normalized photoionizationintensity, Ieff, was evaluated by the ratio of the signal intensityIOPO to the reference signal intensity Iref including the normal-ization of the laser fluencies of both ionization lasers. Thereference laser was an ArF excimer laser. The normalizedIeff of a certain cluster was evaluated by the ratio of thesignal intensity IOPO to the signal intensity IArF including thenormalization of laser fluence of the probe and the referencelasers. The laser fluence was calculated from the outputs of thepyroelectric detectors (EOPO, EArF) and the wavelength of theionization lasers. Since a pyroelectric detector measures thetotal energy incident on it, this might be converted to a valueproportional to the number of photons using the wavelength.The abundances of photoions were obtained from the peakintensity of a certain cluster in a TOF spectrum (IOPO, IArF).Thus, Ieff can be written as:

Ieff ¼IOPO=EOPO

IArF=EArF¢­ArF

­OPOð1Þ

where ­OPO, ­ArF are the wavelengths of each laser.47 A PIEcurve was plotted against photon energy, and Ei was evaluatedby extrapolating the first linear rise in the PIE curve to thebaseline,1,48 and typical uncertainty of Ei’s was estimated to be«0.05 eV.

To record photoelectron (PE) spectra, anionic clustersproduced by the above procedure were sent into online TOFmass spectrometer to 3 keV for mass analysis and 900 eVfor PES (photoelectron spectroscopy) study,49 as shown inFigure 2. After being decelerated, the mass-selected anionsentered the photodetachment region of a magnetic-bottle-typeelectron spectrometer, and were photodetached with the thirdharmonic (355 nm, 3.49 eV) or the fourth harmonic (266 nm,

4.66 eV) or the fifth harmonic (213 nm, 5.82 eV) of anotherNd3+:YAG laser. The electrons were guided by a strong,inhomogeneous magnetic field, and subsequently with a weakguiding magnetic field, and detected by a microchannel plate(MCP). Their kinetic energy was analyzed by their TOF, andthe PE signal was typically accumulated to 30000 shots bya multichannel scaler/averager (Stanford Research System,SR430). Obtained energy resolution was about 70meV FWHMat 1 eV electron energy. The PE spectrometer was calibratedwith the 1S0 ¼ 2S1/2 and 1S0 ¼ 2D5/2 transitions of the Au¹

PE spectrum measured by the forth harmonic output (266 nm,4.661 eV) of the Nd3+:YAG laser.50,51 The laser power forphotodetachment was in the range of 10­20mJ cm¹2 for 355nm, 1­3mJ cm¹2 for 266 and 213 nm and no power dependentprocesses for the spectrum shape were observed.

2.3 Soft-Landing Experiments. Figure 3 schematicallyshows the soft-landing apparatus consisting of a cluster source,ion optics, a mass selection stage, and a deposition chamberwhere infrared reflection absorption spectroscopy (IRAS) andtemperature-programmed desorption (TPD) experiments areperformed.37,38 The organometallic cluster cations of Vn(Bz)mwere produced by the reaction between laser-vaporized Vatoms and Bz vapor in the expansion from a pulsed valve undera helium stagnation pressure of ca. 4 atm, and the cations wereguided into the deposition chamber by a series of ion optics,i.e., octapole ion guides, a quadrupole deflector, and electro-static lenses. Only the cluster cations were mass-selected bya quadrupole mass filter (4­4000 amu; Extrel); subsequently,the cluster cations were deposited onto a substrate with acollision energy of 20 « 10 eV under UHV conditions (� 2�10�10 Torr). The substrate could be cooled down to ca. 120Kby contact with a liquid nitrogen reservoir, and the substratetemperature was kept at 180­220K during the deposition bymeans of a heating element. The total amount of the deposited

Mass Spectrum

Inte

nsi

ty

Time of Flight

Photoelectron Spectrum

Electron Binding Energy

Inte

nsi

ty

AccelerationStages

Photodetachment LaserPermanent Magnet

Ion Detector

Electron Detector

Photoelectron

Magnetic FieldSolenoid Coil

Electron Gun

Even–Lavie Pulsed Valve

SampleContainer

He

Cluster Generation Time of Flight Mass Spectrometer

Magnetic BottlePhotoelectron Spectrometer

Figure 2. An experimental setup for anion photoelectron spectroscopy for composite cluster anions (source is not shown) and giantmolecular cluster anions. Even­Lavie valve for producing cold molecular beams was incorporated with electron attachment sourcesin order to produce giant molecular cluster anions (cluster size n ² 100). The cluster anions were mass-selected by online TOF massspectrometer. The mass-selected anions were photodetached with another Nd3+:YAG laser, and their kinetic energy of photo-electrons was analyzed by their TOF in a magnetic bottle photoelectron spectrometer.

A. Nakajima Bull. Chem. Soc. Jpn. Vol. 86, No. 4 (2013) 417

cluster ions was determined by integrating the ion current onthe substrate during the cluster deposition.

For the substrate, a commercially available 10 © 10mm2

gold substrate, Au (100 nm thickness)/Ti/Silica (TanakaPrecious Metals Co., Ltd.) having ca. 50 nm grains, was usedas a gold surface. To remove organic contaminants from thegold surface, the substrate was chemically cleaned by dipping ina piranha solution (3:1 concentrated H2SO4:H2O2)52 for about20min. A series of alkanethiolate-SAM (Cn-SAM: n = 8, 12,16, 18, and 22) substrates was prepared by immersing the goldsubstrates into a 2mM ethanolic solution of various alkane-thiols of octanethiol (C8H17SH), dodecanethiol (C12H25SH),hexadecanethiol (C16H33SH), octadecanethiol (C18H37SH), anddocosanethiol (C22H45SH) at room temperature for 20 h. Theformation of the SAM on the Au substrate was confirmed byIRAS and contact angle measurements at room temperature.

The IRAS measurements were performed with a FT-IRspectrometer: a collimated IR beam emerging through one ofthe side ports of the spectrometer was focused onto the samplesubstrate at a grazing incidence angle of ca. 80° from thesurface normal through a flat KBr window. After the reflectionfrom the substrate, the IR beam exiting through another KBrwindow at the opposite side of the apparatus was directed ontoan off-axis parabolic mirror, which refocused the beam onto theactive element of a liquid-nitrogen-cooled mercury cadmiumtelluride (MCT) detector. The IR optics and the detector weremounted in a vacuum chamber pumped to a pressure of about0.1 Torr to remove spectral background contributions due toatmospheric gases. All spectra were recorded with a spectralresolution of 2 cm¹1. Five hundred scans were accumulatedfor the background and sample spectra, which were recordedbefore and after the cluster deposition, respectively.

For the TPD measurements, the thermal desorption of thedeposited clusters was induced by heating the substrate with

a ceramic heater.38 The surface temperature was monitoredby a thermocouple and the heating rate was typically set atca. 1K s¹1. The desorbed clusters were ionized by electronimpact and then detected by another quadrupole mass spec-trometer. The entrance of the mass spectrometer was restrictedby a stainless steel skimmer (5mmº, placed ca. 1mm abovethe substrate) in order to detect only the species desorbed fromthe substrate surface.

3. Results and Discussion

3.1 Electronic Structure of Vn(Bz)n+1 Sandwich Clusters.Among extensive studies on organometallic clusters in the pastdecades,53­58 the characterization of organometallic clusterscomposed of transition-metal atoms and Bz molecules havebeen the focus of much of the research because they providebasic models for d­³ bonding interactions. In particular, themetal­ligand sandwich systems have attracted the attention ofscientists for more than a half century following the discoveryof ferrocene, Fe(η5-C5H5)2.53,54,59 Although a few examplesof the synthesis of triple-decker sandwich complexes composedof transition-metal atoms and cyclopentadienyl (C5H5; Cp)ligands such as Ni2Cp3+ (in the bulk)60 and Fe2Cp3+ (in the gasphase)61 have been reported, these complexes have attractedmuch attention for the detailed study of metal­ligand inter-actions in organometallic systems.

Many synthetic experiments have been conducted on novelorganometallics in the condensed phase, but various environ-mental factors such as oxidation or reduction of the prod-ucts make these approaches problematic. Since the adventof laser and molecular beam techniques, gas-phase experi-ments have offered ideal environments for detailed investiga-tions of chemical reactions of metal ions.62 Furthermore, gas-phase experiments enable us to make novel clusters and to useseveral powerful spectroscopic means.19,63 One of the advan-

Quadrupoleion deflector

Octapoleion guide

metaldisk

Nd+ YAG laser 532 nm, 100 Hz

LN2

cryostat

He pulsedvalve

benzene vapor

+

Cluster Generation Ion Guide & Mass Separation

Cluster Deposition

Reaction room Quadrupole mass filter

2.0 X 10-10

Torr

Collision energy20 ±± 10 eV

180-250 K

Deposition Substrates (SAM)

A

Ion current meter

+

Figure 3. The schematic soft-landing apparatus. Organometallic clusters are generated by the reaction between laser-vaporizedmetal atoms and ligand molecular vapor, and the cluster cations were guided by a series of ion optics, i.e., octapole ion guides,a quadrupole deflector, and electrostatic lenses. After mass selection with a quadrupole mass filter, target cluster cations weredeposited onto a SAM substrate with a collision energy of 20 « 10 eV in the deposition chamber, where infrared reflectionabsorption spectroscopy (IRAS) and temperature-programmed desorption (TPD) experiments are performed.

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tages of gas-phase synthesis is the exclusion of environmentalfactors such as counter ions and solvent molecules, whichinvolves the reactions through excited electronic states ofmetal atoms.64,65

Among the physical properties of the cluster, the photo-ionization behavior is one of the most important ones, becausethe cluster size dependence of Ei provides a quantitative mea-sure of the evolution of electronic structures.1,47,48 For V­Bzclusters, in particular, they preferably form a sandwich struc-ture of Vn(Bz)n+1 in which V atoms and Bz ligands are alter-nately piled up as illustrated in Figure 4,66 and the Ei of theclusters drop significantly with increasing cluster size.20 ThisEi tendency has been theoretically explained as the delocaliza-tion of d electrons along the molecular axis.23,67

In this section, the electronic structures of the multiple-decker V­Bz sandwich clusters are discussed by the measure-ments of PIE curves. The second ionization threshold is foundas a second onset in the PIE curve, and the values are com-pared with theoretical predictions on the quasi-band electronicstructure.

3.1.1 Measurements for the Photoionization Efficiency(PIE) Curves: Details of the experimental setup have beendescribed in Section 2.2.2.19,20 Figure 4 shows the photoioni-zation TOF mass spectrum of Vn(Bz)m clusters. The abbrevia-tion of (1, 2) means that this cluster consists of one V atomand two Bz molecules. The predominant peaks have magic-numbered compositions of (n, n + 1), and most of the minorpeaks are assigned to the (n, n) clusters. Together with otherexperimental and theoretical studies,19,20,23,66 the clusters havea sandwich structure at (n, n + 1) as shown in the inset ofFigure 4. Figure 5 shows the PIE spectra for (1, 2) to (5, 6),where the cluster PIE was plotted as a function of photonenergy. The PIE was normalized at the photon energy of 6.42eV. The first onset corresponds to the ionization threshold forremoving an electron from the HOMO, giving the lowest Ei.

As well as the first onset, a second ionization onset wasobserved as the change of slope in the PIE curves of (3, 4) and(4, 5). This second onset can be regarded as the opening ofanother ionization channel. The energies for the second onsetswere obtained to be 5.8 « 0.1, 4.9 « 0.1, and 4.7 « 0.1 eV for(3, 4), (4, 5), and (5, 6), respectively, although the second onsetfor (5, 6) is somewhat obscure. The energy for the second onsetdecreases with cluster size, which is similar to the trend in Ei.

The electronic features of the first and the second onsets foundin the PIE curves can be explained by the following quasi-bandelectronic structure; ionization processes from the HOMO andthe HOMO¹1.

When the theoretical analysis of the Ei’s was carried outby applying the simple Hückel method to the one-dimensionalBz­V­Bz­V­� � �­V­Bz structure (Bz: benzene),67 the exper-imental lowest Ei’s for n = 1, 2, and 3 can lead to that theHückel parameters U and ¢2 evaluated are 2.766 eV and0.9893. Having determined these parameters, all the remainingeigenvalues can be obtained for various cluster size. Interest-ingly, the above simple Hückel method predicts that the secondlowest Ei for (3, 4) is expected to be similar to the lowest Ei for(1, 2), because the next HOMO (HOMO¹1) of (3, 4) and theHOMO of (1, 2) have identical energy. Likewise, the secondlowest Ei for (5, 6) is expected to appear close to the lowest Ei

for (2, 3). The Hückel method can consistently predict not onlythe experimentally determined lowest Ei’s for (4, 5) and (5, 6)but also the second onsets in the PIE curves. This shows thatthe change in the slope of the curve can be reasonably ascribedto the second lowest ionization. Furthermore, this can beregarded as clear experimental evidence that the Vn(Bz)n+1

200 400 600Mass Number (m /z )

(n, m) = (1, 2)

(2, 3)

(3, 4)

(4, 5)(5, 6)(2, 2) (3, 3) (4, 4)

VnBzm

(5, 5)

Figure 4. Photoionization mass spectrum for VnBzm clus-ters. The predominant peaks have magic numbered compo-sitions of (n, m) = (n, n + 1), and most of the minor peaksare assigned to the (n, n) clusters.

0

0.4

0

0.4

Photon Energy / eV3.5 4.0 4.5 5.0 5.5 6.0 6.5

(4, 5)

(5, 6)

Pho

toio

niza

tion

Effi

cien

cy (

arb.

uni

ts)

0

1

0

0.3

1

(1, 2)

(2, 3)

(3, 4)

Ionization

from HOMO

4.9(1)

4.7(1)

[4.70]

[5.14]

5.8(1)

[5.75]

[6.58]from HOMO-1from HOMO

from HOMO

from HOMO

from HOMO

from HOMO-1

from HOMO-1

from HOMO-1

Figure 5. Photoionization efficiency (PIE) spectra ofVnBzm for (n, m) = (1, 2) to (5, 6). The first onsetcorresponds to the ionization energy, Ei, for removingan electron from the HOMO. As well as the first onset,a second ionization onset was observed as the changeof slope in the PIE curves of (3, 4), (4, 5), and (5, 6) tobe 5.8 « 0.1, 4.9 « 0.1, and 4.7 « 0.1 eV, respectively. Thesecond onsets can be explained by ionization processesfrom the next HOMO (HOMO¹1). The onsets predictedby the quasi-band electronic structure are also shown inbrackets.

A. Nakajima Bull. Chem. Soc. Jpn. Vol. 86, No. 4 (2013) 419

clusters have a quasi-band electronic structure with multideckersandwich structures.

3.2 Electronic Structure of Organolanthanide Multi-decker Sandwich Clusters. As discussed in the precedingSection 3.1, a multidecker sandwich clusters composed of Vand Bz could be synthesized in the gas phase.20,46 In practice,it has been found that the length of these one-dimensional(1-D) Vn(Bz)m clusters is limited to ca. 7 layers, because theV atoms must be prepared in electronically excited states tohave sufficient reaction probabilities. Similar 1-D stacking wasachieved in lanthanide (Ln) clusters using COT molecules asligands.68­71 As reported previously in bulk materials71­73 andin theoretical calculations,74 [Ln(COT)2] was proved to taketrivalent complexes except for divalent clusters of Ln = Euand Yb. Lanthanide ³-carbocyclic complexes were synthesizedas their salts of K[Ln(COT)2] by Hodgson et al.,72,75 wheretrivalency of Ln metals overcomes aromaticity of COT2¹,resulting in a complex denoted as [Ln3+(COT1.5¹)2]. An ioniccharacter well describes these phenomena rather than thecovalent one, where little overlap of 4f (Ln) and ³(COT) elec-trons exists and that directional bonding is relatively unim-portant. This predominantly ionic character results from whichCOT ligand can act as stable dianion.

In this section, the results on the gas-phase preparation ofthe multidecker Ln­COT sandwich clusters are presented. Theelectronic structure and bonding are discussed based on theresults of PES of Ln­COT anion and Ei of the neutral Ln­COTclusters. From the analysis of their electronic structures, theseLn­COT sandwiches were found to be highly ionic species,in which the Ln atoms are in most cases trivalent (Ln3+) andin some cases (e.g., europium (Eu)), divalent (Ln2+). Theelectronic properties of the Ln­COT are discussed in compar-ison with those of the sandwich clusters between transition-metal atoms and Bz molecules.20,46 In contrast to Vn(Bz)m,growth of Lnn(COT)m clusters can occur with abundantground state Ln atoms as reactants, thus enabling the efficientproduction of large clusters (n > 10).76

3.2.1 Mass Spectra of Lnn(COT)m Clusters:Figures 6a­6d show typical examples of the photoionizationmass spectra of Lnn(COT)m [Ln = lanthanide metals of Nd,Eu, Ho, and Yb; henceforth (n, m)]. Main peaks in each spec-trum showed almost the same compositions denoted as (n,n + 1). These (n, n + 1) species are indeed abundant and stableclusters formed at saturated concentrations of COT. The regularpattern in the mass spectra implies that these clusters consistof accumulation of a certain unit. The most probable structureis a multidecker sandwich cluster by analogy to the structure ofthe V­Bz clusters20,46 (See an inset in Figures 4 and 5).

3.2.2 Anion PE Spectra for [Ln(COT)2]¹ (Ln = Ce, Nd,Eu, Ho, and Yb): To elucidate further bonding nature of theseclusters, anion PE spectra were measured for [Ln(COT)2]¹,at 355 (3.49 eV) and 266 nm (4.66 eV) (Figure 7).25 In theformation of Ln­COT anions, larger clusters of (n, n + 1)¹ atn > 2 could be produced only for Eu and Yb, while (1, 2)¹ wasmainly produced for Ce, Nd, and Ho. This can be explainedby the electronic structure, which will be discussed later. In thePE spectra, the horizontal axis corresponds to electron bindingenergy, Eb, which is defined as Eb = h¯ ¹ Ek where Ek iskinetic energy of the photoelectron and h¯ is a photon energy

of the photodetachment laser. Arrows indicate the thresholdbinding energy (ET), which corresponds to the upper limit ofthe electron affinity (EA).

At first glance of the five PE spectra, striking similarity isreadily recognized and they are classified into two groups; oneis [Ce(COT)2]¹, [Nd(COT)2]¹, and [Ho(COT)2]¹ and the otheris [Eu(COT)2]¹ and [Yb(COT)2]¹. In the former group, twopeaks are located around the binding energy of 2.5 and 3.5 eV,and they have similar profiles with a sharp leading edge anda couple of shoulders at higher binding energy. In the latterone, the first two sharp peaks are located around 2.0 and 2.5 eV,and they are accompanied by a weaker broad band at higherbinding energy. The similarity in these two groups is ascribedto a common electronic feature; they are characterized as ahighly ionic cluster that depends not on the metal elements, buton the number of oxidation state. It is reasonable to assumethat all of Ce, Nd, and Ho take the oxidation state of +3, whileboth Eu and Yb take that of +2, by analogy to the reportedlanthanide complexes.72­74,77,78

In the oxidation states of 3+, Ce3+, Nd3+, and Ho3+ have1, 3, and 10 f electron(s), respectively. As shown in their PEspectra, this difference does not change the spectra. This isbecause the bands in the spectra come predominantly from themolecular orbital of the ligand molecules (COT2¹), in whichthe three anions of [Ln(COT)2]¹ (Ln = Ce, Nd, and Ho) aresatisfied with stable ionic configurations of [Ln3+(COT2¹)2].Since COT2¹ takes 10³ electronic configuration with the e2uHOMO as shown in the inset of Figure 7, two ligands ofCOT2¹ form two high-lying MOs of e2u and e2g with the levelordering e2u > e2g.79,80 Then, the bands of X and A are un-doubtedly associated with electron detachment from the highestoccupied molecular orbital (HOMO; e2u) and the HOMO¹1(e2g) molecular orbitals, respectively.

(a) Nd-COT

(b) Eu-COT

(c) Ho-COT

(d) Yb-COT

0 500 1000 1500 2000Mass number (m /z )

(1, 2)

(2, 3)

(3, 4)

(4, 5)(5, 6)

(1, 2)

(2, 3) (3, 4)

(4, 5)(5, 6)

(1, 2)(2, 3)

(3, 4)(4, 5)

(5, 6)

(1, 2)

(2, 3)(3, 4)

(4, 5)

(5, 6)

Figure 6. Photoionization mass spectra of Lnn(COT)m at193 nm (6.43 eV); (a) Nd, (b) Eu, (c) Ho, and (d) Yb. Thepredominant peaks have magic-numbered compositions of(n, n + 1).

AWARD ACCOUNTSBull. Chem. Soc. Jpn. Vol. 86, No. 4 (2013)420

On the other hand, Eu and Yb are typical examples for stableLn2+ complexes in bulk materials.70,71 This is because Eu andYb possess 4f7 and 4f14 configurations in the oxidation statesof +2, which corresponds to the half- and full-filled 4f orbitals,respectively, stabilized by the spin­spin exchange interaction.Therefore, their neutrals are considered to take [Eu2+(COT1¹)2]and [Yb2+(COT1¹)2] configurations. Difference of electronicand vibrational structures from Ln3+ clusters is ascribed to thechange of electronic configuration of COT from (COT1.5¹)2 to(COT1¹)2. In the PE spectra, two strong bands were observedand their gaps are 0.54 eV for neutral Eu(COT)2 and 0.52 eV forneutral [Yb(COT)2]. Based on the assignment for Ce(COT)2¹,Nd(COT)2¹, and Ho(COT)2¹, the two bands in Eu(COT)2¹ andYb(COT)2¹ are assigned to those from the HOMO (e2u) andHOMO¹1 (e2g), respectively, although it seems that the neutral[Eu(COT)2] and [Yb(COT)2] have lower symmetry than D8h

due to deformation of the 8-membered ring of COT1¹.3.2.3 Ionization Energies of Neutral Lnn(COT)n+1

Clusters: For neutral and cationic Ln­COT clusters, largerclusters having (n, n + 1) compositions were produced, and the

Ei’s of the neutral Ln­COT were measured by using photo-ionization spectroscopy to determine the electronic proper-ties.25,81 Figure 8 shows the size dependence of Ei for [Hon-(COT)n+1] and [Eun(COT)n+1]. Although Ei of [Ho3(COT)4]were determined from the final decline of the curve, the othersare shown by the lower and the upper limits of Ei’s, becausethese clusters cannot be photoionized by 6.00 eV photons, butcan be ionized by 6.42 eV photons of the ArF laser, wherethe energy of 6.00 eV corresponds to the limit of tunable rangeof the UV laser.

As shown in Figure 8, the size dependences of Ei arecontrast: For [Hon(COT)n+1] (Figure 8a), while Ei’s of (1, 2)and (2, 3) show similar values, Ei of (3, 4) largely drops by0.8 eV. For [Eun(COT)n+1] (Figure 8b), however, the Ei valuesare very similar for n = 1­3, although the values have rela-tively large uncertainty. The two size dependences of Ei’s for[Hon(COT)n+1] and [Eun(COT)n+1] are similarly observed inthose for [Ndn(COT)n+1] and [Ybn(COT)n+1], respectively, andthey can be explained by the counting of valence electrons,based on the multiply ionic states in the cluster. Consider-ing that Ln atoms favor Ln3+ or Ln2+ state in ligand field,allotment of valence electrons in neutral multidecker [Lnn-(COT)n+1] should be attributed as shown in Figure 9. In caseof Ho, Ln atoms can exist as Ln3+ ions interposed by COT forn = 1 and 2. For n = 3, however, one of the Ln atoms in themultiple-deck structure cannot become Ln3+ ion because ofthe lack of electron acceptability in the COT. Then the centralLn atom should result in a Ln2+ ion. Especially for Eu, clusterscan grow up with keeping Ln2+ ion, as shown in Figures 6band 6d. When we look at the tendency of Ei’s, drops of Ei’sat [Ho3(COT)4] can be reasonably explained by the change ofvalence electrons as follows; Since the central Ln atom in (3, 4)should take the Ln2+ ion in the neutral, large stabilization isexpected for the cationic (3, 4)+, by changing the charge fromLn2+ to Ln3+. In (1, 2) and (2, 3), on the other hand, no cationicstability is expected because the ionization process results inthe one either from Ln3+ to Ln4+ or from COT2¹ to COT¹. Ina larger sandwich cluster (n > 3), therefore, the oxidation statealways becomes +2 for the Ln atoms around the core of theneutral cluster, while terminal Ln atoms in both ends are +3.

In the preceding section on Vn(Bz)n+1 clusters, Ei’s showeddrastic decrement with the cluster size.20,46 This phenomenon

A

A

A

A

AX

X

X

X

X

0.0 1.0 2.0 3.0 0.0 1.0 2.0 3.0 4.0

(a) [Ce(COT)2] −

(b) [Nd(COT)2] −

(c) [Ho(COT)2]−

(d) [Eu(COT)2] −

(e) [Yb(COT)2] −X

X

X

X

X

* *

Electron binding energy/eV

at 355 nm at 266 nm

a2u

e1g

e2u

e3g

b2u

COT 2 -

Figure 7. Anion PE spectra for [Ln(COT)2]¹ at 355 (3.49eV) and 266 nm (4.66 eV); (a) [Ce(COT)2]¹, (b) [Nd-(COT)2]¹, (c) [Ho(COT)2]¹, (d) [Eu(COT)2]¹, and (e) [Yb-(COT)2]¹. Arrows indicate the threshold binding energy(ET), and asterisk * shows a hot band due to electronicallyexcited states of the anions. Electronic configuration forCOT2¹ is also shown in the inset.

(1,2) (2,3) (3,4)

6.5

6.0

5.5

5.0

Cluster size

Ioni

zatio

n en

ergy

/eV

(1,2) (2,3) (3,4)Cluster size

(a) Ho-COT (b) Eu-COT

Figure 8. Size dependence of ionization energies of [Lnn-(COT)n+1] (n = 1­3); (a) Ho­COT and (b) Eu­COT.

A. Nakajima Bull. Chem. Soc. Jpn. Vol. 86, No. 4 (2013) 421

of Ei is theoretically elucidated23,67 that the ionization occursfrom delocalized molecular orbital of vanadium­vanadiuminteraction interposed by ³* orbitals of benzene. In [Lnn-(COT)n+1], however, an orbital contributing to the ionizationprocess is considered to be discontinuously localized along themolecular axis, because the cluster is bonded through ionicbonds and the charge is localized at each component. Since thefirst ionization is expected to occur from 4f (Ln) orbitals in(1, 2),79,80 it is concluded that 4f orbital in the cluster islocalized and scarcely interacts with neighboring Ln atoms inthe cluster. According to the theoretical calculation of Dolgand co-workers,74c [Nd(COT)2] is a charge-transfer complex inwhich 3 of 6 electrons in Nd(6s24f4) transfer almost completelyto 2COT, resulting in a configuration of Nd3+(COT1.5¹)2.In case of Eu and Yb, although they are unaware of the extentof the charge transfer in the clusters, rather high Ei’s of [Eun-(COT)n+1] and [Ybn(COT)n+1] seem to ensure the completecharge transfer, because Ei’s of Eu and Yb clusters are ratherhigh (6.00 eV < Ei’s < 6.42 eV) compared to Ln3+ clusters.Then, it is concluded that bonding in [Lnn(COT)n+1] is ioniccharacter, in which Ln atoms exist as Ln3+ or Ln2+ ions.Moreover, the multiply ionic character for the Ln­COT clustersstrongly suggests that the binding energy between the Ln atomsand the ligand molecules are much larger that of the V­Bzcluster, which was estimated to be 1­2 eV.

This difference in size distributions for the two systems canbe traced to the difference in the bonding characteristics; themetal­ligand bonding in Eu­COT is highly ionic, while thatin V­Bz clusters is covalent. Growth processes that extend thelength of Eu­COT chains clearly involve a series of elementaryprocesses in which an alternating piling up of Eu atoms andCOT ligands occurs. Since the Eu­COT sandwiches consist ofpositively charged Eu ion and negatively charged COT, thesuccessive reaction likely proceeds with an electron transferfrom Eu to an Eu­COT intermediate or from an Eu­COT inter-

mediate to COT, satisfying the charge distribution. As seen inthe prototypical example of provided by the Li + F2 “harpoon”reaction,82 the reaction path is “downhill all the way” -nobarrier at all, when electron transfer takes place and an ion pair(e.g., Li+F2¹) is formed at the first stage of the reaction.

3.3 Anion PES of Metal­Silicon Clusters; MSin¹ (n =8­17). Composite clusters have clear advantages for designerchemistry, because the number of valence electrons and atomicradii can be finely tuned by suitable choices of the seconddoping element. Keeping the symmetry of the structure highcan be crucial for a large HOMO­LUMO gap, and two waysare conceivable to tune the properties of the binary clusterswhile retaining very symmetric structure; by the substitutionof the central atom or all of the caged atoms.18,33,83­85 Thesetwo methods of substitution were demonstrated in the magic-numbered behavior of the metal-encapsulated silicon clusters,ScSi16¹, TiSi16, and VSi16+,35 and in the systematic substitutionof the group 14 cage elements, that is for the MSi16¹, MGe16¹,MSn16¹, and MPb16¹ (M = Ti, Zr, and Hf ) clusters,36 wherethe former correspond to the substitution of the central atom,while the latter is to substitute the elements of the cage itself.

In this section, a set of systematic experiments are presentedto reveal the electronic properties of silicon clusters mixedwith a metal atom of group 3, 4, or 5 elements as a function ofthe number of Si atoms, using anion PES at 213 nm i.e., usingMSin, (M = Sc, Ti, V, Y, Zr, Nb, Lu, Tb, Ho, Hf, and Ta).34,35

Taking the results of the adsorption reactivity of H2O, thesize evolution of the electronic characteristics of MSin will bediscussed from the viewpoint of a cooperative effect betweengeometric and electronic structures.

3.3.1 Adsorption Reactivity of Anionic MSin¹: Figure 10shows mass spectra of the metal atom doped Sin clusters(MSin¹/0/+) measured with a time-of-flight mass spectrom-eter.34 In all of the mass spectra, bimodal distributions weretypically observed; clusters exhibit two distributions for n =6­11 and for n = 12­20.35,86 Magic-numbered behavior wasobserved at n = 16, depending on the group in the periodictable, the charge state, and a radius of a metal atom. Asdiscussed later in connection with adsorption reactivity andanion PE spectroscopy, some of MSi16¹/0/+ is stabilized as asuperatom both electronically and geometrically. In Figure 10the mass spectra containing a superatom is presented with acolored background. This magic numbers behavior becamemuch less prominent in other charged states; in Sc­Si com-pounds, for example, the magic number MSi16 appears onlyin the anions. As a neutral species, TiSi16 could be formed,and as cationic species, VSi16+, NbSi16+, and TaSi16+ could beproduced selectively by fine-tuning the source conditions,laser fluences, and flow rate of the He carrier. This selectiveformation is very similar to the initial finding of single-elementclusters of C60.17b Since the efficient mixing of hot vaporswith proper rapid cooling enhanced the selective formation, theTiSi16 neutral cluster itself was formed selectively. To obtainmagic clusters of MSin selectively at n = 16, the substitutionof a Ti atom for a neighboring Sc or V requires a change in thetotal charge of the cluster.

In order to investigate the structure of MSin clusters, thecluster size dependence of the adsorption reactivity toward H2Ovapor was measured,34,45 because a chemical probe method is

(b) Ln(II) - COT

(1, 2)

-1-1

+2

(2, 3)

(3, 4)

(4, 5)

-1-1-1 -1

+2 +2

-1-1-1 -1-1 -1

+2 +2+2

-1-1-1 -1-1 -1-1 -1

+2 +2+2 +2

(a) Ln(III) - COT

(1, 2)

(2, 3)

(3, 4)

(4, 5)

-1.5-1.5

+3

-1-1-2 -2

-1-1-2 -1-1 -1-1 -2

+3

-1-1-2 -1-1 -2

+3 +3+2

+3 +2+2 +3

+3

Figure 9. Allotment of valence electrons in neutral clustersof multiple deck sandwich [Lnn(COT)n+1]; (a) Ln3+­COTand (b) Ln2+­COT. The allotments are based on the as-sumption of the 2-electron COT acceptability up to COT2¹.As shown in (a), one of the Ln atoms should become +2oxidation state in (3,4), while in (b) all of Ln atoms areLn2+ at any sizes.

AWARD ACCOUNTSBull. Chem. Soc. Jpn. Vol. 86, No. 4 (2013)422

useful in deducing the structure of the MSin clusters. Since thereactivity of Sin is generally much lower than that of a metalatom, the reactivity is sensitive to location of the doped metalatom; high and low reactivities exhibit an exterior and aninterior metal atom, respectively.

Figure 11 shows the relative adsorption reactivity of anionicM1,2Sin¹ (M = Sc, Y, and Lu) clusters toward H2O vapor forn = 6­20. The reactivity decreases with an increasing numberof Si atoms, showing that the reactive metal atom site is hiddenby less reactive Si atoms. The threshold sizes of ScSin¹, YSin¹,and LuSin¹ were 15, 20, and 18, respectively. At a larger clus-ter size, the reactivity for M2Sin¹ is larger than that for M1Sin¹,because more Si atoms are required to form a metal-atom-encapsulated structure. The reactivity of MSin¹ exhibits com-mon local minima (or no reactivity) at n = 12 and 16, implying

200 300 400 500 600 700

(a) Sc (1.63 Å) (b) Ti (1.45 Å) (c) V (1.31 Å)

200 300 400 500 600 700

Si16Ti

Si16V+

200 300 400 500 600 700

Si16Zr

(d) Y (1.78 Å) (e) Zr (1.59 Å) (f) Nb (1.43 Å)

(g) Lu (1.72 Å) (h) Hf (1.56 Å) (i) Ta (1.43 Å)

Mass number (m /z )

200 300 400 500 600 700

Si16Y–

catio

nne

utra

lan

ion

200 300 400 500 600 700

Si16Sc–

catio

nne

utra

lan

ion

200 300 400 500 600 700

Si16Nb+

Si16Hf

200 300 400 500 600 700

Si16Lu–

catio

nne

utra

lan

ion

200 300 400 500 600 700 200 300 400 500 600 700

Si16Ta+

Figure 10. Time-of-flight mass spectra of the metal atomdoped Sin clusters (MSin¹/0/+); M = (a) Sc, (b) Ti, (c) V,(d) Y, (e) Zr, (f ) Nb, (g) Lu, (h) Hf, and (i) Ta. The arrowsshow the mass peak position ofMSi16 whether it is a magicnumber or not. Magic-numbered behavior was observedat n = 16 in some cases, depending the charge state and aradius of the metal atom. The mass spectra with a coloredbackground show that MSin¹/0/+ is stabilized at n = 16both electronically and geometrically.

Rel

ativ

e ad

sorp

tion

reac

tivity

(ar

b. u

nits

)

(a) Sc1,2Sin–

8 12 16 200.0

1.0

2.0

3.0

(b) Y1,2Sin–

8 12 16 200.0

1.0

2.0

(c) Lu1,2Sin–

The number of Si atoms8 12 16 20

0.0

1.0

2.0

3.0

Figure 11. Relative adsorption reactivity of anionic MSin¹

clusters toward H2O vapor; (a) Sc1,2Sin¹, (b) Y1,2Sin¹, and(c) Lu1,2Sin¹ for n = 6­20, where open and solid symbolsshows the plots for one and two metal-atom species,respectively. The reactivity decreases with an increase inthe number of Si atoms, showing that the metal atomcomprising the reactive site is hidden by less reactive Siatoms. At a larger cluster size, the reactivity for M2Sin¹ islarger than that for M1Sin¹, because more Si atoms arerequired to form a metal-atom-encapsulated structure. Forall the clusters, the reactivity exhibits local minima at n =12 and 16, implying that the metal atom is trapped in aSi cage.

A. Nakajima Bull. Chem. Soc. Jpn. Vol. 86, No. 4 (2013) 423

that the metal atom is trapped inside a Si cage. When thereactivity for n = 12 is compared with that for n = 16, thereactivity at n = 16 is lower than that at n = 12, which suggeststhat metal encapsulation approaches completion at n = 16. It isnoted that these clusters still exhibit some reactivity at n = 16,except for ScSi16¹. Thus, a Y or Lu atom is too large to beperfectly encapsulated by the anionic Si16 cage.

3.3.2 PE Spectra of MSin¹ Anions: To reveal theelectronic properties of MSin clusters, the PE spectra of theanionic clusters of MSin¹ (M = Sc, Y, Tb, Ho, Lu, Ti, Zr, Hf,V, Nb, and Ta) were measured at 213 nm (5.83 eV). Figure 12shows PE spectra of silicon cluster anions mixed with group 3,4, or 5 elements for n = 6­17 at 213 nm; (a) ScSin¹, (b) YSin¹,(c) LuSin¹, (d) TiSin¹, (e) ZrSin¹, (f ) HfSin¹, (g) VSin¹, (h)NbSin¹, and (i) TaSin¹. Arrows indicate the ET position, whichcorresponds to the upper limit of EA. For group 3 elements of(a) ScSin¹, (b) YSin¹, and (c) LuSin¹, all these clusters exhibitlocal threshold energy maxima at n = 10 and 16, although thelocal maximum at n = 16 is not prominent for YSi16¹. Thisresult implies that the excess electron completes the closure ofan electronic shell at n = 10 and 16.

For group 4 elements of (d) TiSin¹, (e) ZrSin¹, and (f )HfSin¹, a small bump labeled X appears at n = 16, exhibitinga local minimum of threshold energy, which is in contrast tothe local maximum of the threshold energy for the group 3MSi16¹ clusters. This unusual behavior indicates that the excesselectron occupies a singly occupied molecular orbital (SOMO),which was experimentally verified by our established techniqueof halogen atom doping,32,87 as mentioned later in Section 3.4.For group 5 elements of (g) VSin¹, (h) NbSin¹, and (i) TaSin¹,

the size dependence is relatively smooth in this size rangecompared to those of silicon cluster anions mixed with group 3or group 4 transition metal elements, although the thresholdenergy does appear to go through a local maximum at n = 14or 15 for all three clusters.

3.3.3 The Cooperative Stability of MSi16¹: The sizedependencies of the threshold energy for electron detachmentand adsorption reactivity can be ascribed to the electronicand geometric stabilities of metal atom-doped silicon clusteranions, MSin¹. Regarding the electronic and geometric stabilityof MSi16, their distinctive behaviors have been characterizedas a member of a class of superatoms;33a,35a anionic ScSi16¹,neutral TiSi16, and cationic VSi16+, NbSi16+, and TaSi16+ wereabundantly generated due to both the electronic and geometricclosings.35 At n = 16, as shown in Figure 12, the thresholdenergy for electron detachment exhibits local minima forMSin¹ clusters with group 4 elements, where the spectralfeatures of MSi16¹ doped with group 4 elements of Ti, Zr, andHf show a common small bump around 2.8­3.0 eV (labeled X).These features are markedly contrast to those for group 3 andgroup 5 element clusters, in which the detachment peaks startin the higher energy region without any such a separate peak.This spectral pattern suggests that neutral TiSi16 is a closed-shell molecule with a large HOMO­LUMO gap.

The PE spectra of Sc@Si16¹ yielded a vertical detachmentenergy (VDE) of 4.25 eV and an adiabatic detachment energyof 3.41 eV, values much higher than those of other metal-doped silicon clusters. This pronounced stability of Sc@Si16¹

is attributed to the cooperative effect between geometric andelectronic structures, producing a superatom.33a,35a As dis-

Electron Binding Energy/eV

(f) HfSin–

6

7

8

9

10

11

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13

14

15

16

17

(e) ZrSin–

6

7

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(d) TiSin–

6

7

8

9

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17

0 1 2 3 4 50 1 2 3 4 50 1 2 3 4 5

Electron Binding Energy/eV

(c) LuSin–

6

7

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9

10

11

12

13

14

15

16

17

(b) YSin–

6

7

8

9

10

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17

Inte

nsity

(ar

b. u

nits

)

(a) ScSin–

6

7

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9

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15

16

17

0 1 2 3 4 50 1 2 3 4 50 1 2 3 4 5

(g) VSin–

10

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15

16

17

(i) TaSin–

6

7

8

9

10

11

12

13

14

15

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17

(h) NbSin–

6

7

8

9

10

11

12

13

14

15

16

17

6

7

8

9

0 1 2 3 4 50 1 2 3 4 50 1 2 3 4 5

x x x

Figure 12. PE spectra of silicon cluster anions mixed with group 3, 4, or 5 elements for n = 6­17 at 213 nm; (a) ScSin¹, (b) YSin¹,(c) LuSin¹, (d) TiSin¹, (e) ZrSin¹, (f ) HfSin¹, (g) VSin¹, (h) NbSin¹, and (i) TaSin¹. Vertical arrows show an adiabatic detachmentenergy.

AWARD ACCOUNTSBull. Chem. Soc. Jpn. Vol. 86, No. 4 (2013)424

cussed above, there exists an optimum cavity size in which themetal atom can be encapsulated by the Si16 cage; a Sc atom(atomic radii; 1.63¡)88 can be encapsulated, while a Y (atomicradii; 1.72¡) or Lu atom is too large to be encapsulated bythe Si16 cage, which has been theoretically predicted as aFrank­Kasper structure.89­91 In addition, the cavity size of theSi16 cage apparently depends on the charge state as shown inFigure 10. Thus, the cavity size of the anionic Si16 cage can beestimated to be larger than 3.26¡ and smaller than 3.44¡ indiameter. The smaller threshold size of LuSin¹ (n = 18) thanthat of YSin¹ (n = 20) is explained by the respective atomicradii; 1.78¡ for Y and 1.72¡ for Lu.88

3.4 The Role of Cage Aromaticity of MX16¹ (M = Ti and

Zr, X = Si, Ge, Sn, and Pb). As mentioned in Section 3.3, amagic-numbered behavior was observed for the metal-encap-sulated silicon clusters, ScSi16¹, TiSi16, and VSi16+;35 of these,TiSi16 has been predicted theoretically as having a Frank­Kasper structure (Figure 12a).89 These clusters with transi-tion metal encapsulated by silicon are actually stabilized bycooperative electronic and geometric factors.33 In fact, exper-imental results consistently show the cooperative effect for theelectronic and geometric structures ofMSin clusters, when theirproperties were intensively studied by systematically changingthe central metal atoms among the atoms of groups 3, 4, and 5(M = Sc, Y, Lu, Ti, Zr, Hf, V, Nb, and Ta).35c This strategyretained the high symmetry cage geometry, while allowingsystematic electronic changes in the MSi16 clusters.36

Another way to tune the properties of binary caged clustersis to vary the elements of the cage itself. For the TiSi16¹,TiGe16¹, TiSn16¹, and TiPb16¹ clusters, their electronic sta-bility was examined by anion PE spectroscopy together withtheoretical calculations. To reveal the stability of these clustersquantitatively, anion PE spectra were measured with 213 nm(5.92 eV) radiation for the TiX16

¹ anions (X = Si, Ge, Sn, andPb). As shown in Figure 13, a small peak labeled X wasobserved around 2.4 eV in the PE spectra. Since the neutralTiX16 cluster possesses a closed electronic configuration withan even number of valence electrons, the small peak apparentlycorresponds to photodetachment from the SOMO of the clusteranion. In other words, the photoelectron observed was origi-nally an electron in the occupied LUMO of the neutral cluster,and then 2nd peak corresponds to photodetachment from theHOMO of the corresponding neutral cluster.

This assignment of single electron occupation can be indeedverified experimentally by our established technique of halo-gen-atom doping.32,87 Briefly, for the nondegenerate SOMO ingeometrically rigid clusters, a halogen atom can remove thesingle electron from the SOMO without any serious distortions.For Ti@Si16¹, indeed, in the 213-nm PE spectrum of the F-atom adduct Ti@Si16F¹ (not shown), the small peak X referredto above disappears, while the other spectral features are main-tained. Similarly to Ti@Si16F¹, the PE spectrum of Ti@Ge16F¹

provides verification of the presence of a SOMO, and it is con-cluded that the energy gap between the 1st and 2nd peak corre-sponds to the HOMO­LUMO gap of neutral MX16 clusters.

In the series of TiSi16, TiGe16, and TiSn16 clusters, the onespecies TiSi16 (1.90 eV) exhibits a large HOMO­LUMO gap,and TiGe16 (1.77 eV) also has a relatively large gap compared tothe other species. Since the diameter of the metal atom generally

increases with atomic number, this result implies that the geo-metric factor of the atomic radius of a metal atom plays animportant role in cooperation with the electronic factor; a largertetravalent metal atom better satisfies the larger caged X16. Onthe other hand, the HOMO­LUMO gap of TiPb16 (0.85 eV)is relatively small. For the electronic stability of TiX16 clusters,the possible aromatic character of Si164¹ in TiSi16 has beensuggested as the architects of the use of nucleus-independentchemical shifts (NICS) concepts to understand the occurrence ofstable clusters.92,93 As predicted previously, the TiX16 clustershave a metal-encapsulated Frank­Kasper polyhedron structure.To examine the aromatic character of X16

4¹, an NICS analysiswas conducted, and the NICS at the cage center of the optimizedX16

4¹ was calculated at the DFT level of theory, using the

(b) TiGe16-

(c) TiSn16-

1.90

Electron binding energy/eV

Inte

nsity

(ar

b. u

nits

)(d) TiPb16

-

0 1 2 3 4 5

(a) TiSi16-

1.77

1.47

0.85

X

X

X

X

Figure 13. PE spectra of the TiX16¹ anions (X = Si, Ge,

Sn, and Pb) at 213 nm; (a) TiSi16¹, (b) TiGe16¹, (c)TiSn16¹, and (d) TiPb16¹. Vertical arrows in the spectraindicate photodetachment from the singly occupied molec-ular orbital (SOMO; labeled X) and the highest occupiedmolecular orbital (HOMO) in a neutral cluster. Hence, theenergy difference between the arrows corresponds to theHOMO­LUMO gap of neutral clusters indicated in eV. Aninset in (a) shows the calculated Frank­Kasper typegeometry of TiSi16¹ clusters.

A. Nakajima Bull. Chem. Soc. Jpn. Vol. 86, No. 4 (2013) 425

B3PW91 functional built into the Gaussian 03 program pack-age.94,95 The NICS of Si164¹, Ge164¹, and Sn164¹ are negativevalues of ¹65.9, ¹30.5, and ¹12.3 ppm, respectively, and thusthese clusters exhibit aromatic character.

On the other hand, the Pb164¹ cage shows an antiaromaticcharacter with a positive NICS value of 8.2 ppm. Therefore,these results might explain the large HOMO­LUMO gap ofTiSi16, TiGe16, and TiSn16 as resulting from highly aromaticcharacters of Si164¹, Ge164¹, and Sn164¹, while TiPb164¹ has asmall HOMO­LUMO gap due to the antiaromaticity of Pb164¹.Furthermore, it is noted that the absolute values of NICS forSi164¹, Ge164¹, and Sn164¹ decrease monotonically with atomicnumber. This gradual decrease of the NICS means that theextent of aromaticity decreases with heavier elements. Hence, itis concluded that the stability of X16

¹ clusters can be associatedwith their aromatic characters.

When these results are combined the known superatom fami-ly of doped aluminum-based clusters of B@Al12¹, Si@Al12,and P@Al12+,18c,33a we can now say that the silicon-basedfamily of Sc@Si16¹, Ti@Si16, V@Si16+, Nb@Si16+, andTa@Si16+, can be included as a superatom family. With asystematic doping of transition-metal atoms of group-3, -4, and-5 (M = Sc, Y, Lu, Ti, Zr, Hf, V, Nb, and Ta) together withvariations in the charge states, it is found that the larger cavityof a Ge16 cage can lead to metal encapsulation structure ofanionic Y@Ge16¹, Lu@Ge16¹, and Tb@Ge16¹ which satisfyboth electronic and geometric closings, while the smaller Si16cage cannot encapsulate Y, Lu, and Tb atoms. All the membersof the series of M@Ge16 clusters (M = Ti, Zr, and Hf ) exhibita large HOMO­LUMO gap of around 1.8­1.9 eV, implying thata larger tetravalent metal atom better satisfies the Ge16 cage.

3.5 Room-Temperature Isolation of Sandwich Clustersvia Soft-Landing into n-Alkanethiol Self-Assembled Mono-layers. For gas-phase studies discussed above, some intri-guing functional clusters such as multidecker organometallicclusters and metal atom encapsulating silicon caged clustershave been successfully synthesized by combining laser-vapor-ization techniques with molecular beam methods. In particular,a wide variety of experimental and theoretical efforts have beendevoted to the characterization for a multidecker sandwichstructure,19,66,67,96­103 because it has been found that Mn(Bz)n+1

clusters possess unique size-dependent ferromagnetic proper-ties originating from their one-dimensional structures, whereVn(Bz)n+1 clusters exhibit the increase in their magneticmoments monotonically with the cluster size.28,29 Hence, it isanticipated that they become one of the most promising can-didates as building blocks in nanoscale spintronic devices. Inorder to utilize attractive functional clusters as building blocksin functional materials, however, matrix-isolation at temper-atures at or above room temperature is highly desired. The useof a self-assembled monolayer (SAM) as a room-temperatureisolation matrix is an effective approach to supporting thesandwich clusters on substrates.

In this section, the development of room-temperature matrixisolation is described for the neutralized V(Bz)2 clusters, i.e.,the SAM matrices support the clusters with their native sand-wich structure intact. Furthermore, the mechanism how theprojectile kinetic energy affects the resulting adsorption regimeof clusters on an n-alkanethiol SAM (Cn-SAM) matrix is

discussed, by comparing the physical vapor deposition (PVD)with the soft-landing of the chromium (Cr)­Bz sandwichclusters. Furthermore, the correlation between the thermaldesorption of the clusters and the phase transitions of the SAMmatrix is discussed.

3.5.1 Soft-Landing Isolation of Transition Metal­Ben-zene Clusters: Considerable experimental and theoreticalstudies have revealed that the characteristics of the transitionmetal­Bz clusters, including geometry,19,66,97 stability,67,99

optical98 and magnetic28,29,100,101 properties, are highly depend-ent on the nature of the transition metals in the clusters, inparticular, the change in the number of 3d electrons. Figure 14shows the IRAS spectra in the 650­1550 cm¹1 region after thesoft-landing ofM(Bz)2 cation clusters (M = Ti, V, and Cr) ontothe n-octadecanethiol SAM (C18-SAM) substrates using oursoft-landing apparatus as shown in Figure 3. In the spectra,three IR peaks were observed for Ti(Bz)2 and Cr(Bz)2 and fourpeaks for V(Bz)2 after the soft-landing of 2.0 © 1014 cationsonto the SAMs. Assuming that all the deposited ions aresuccessfully trapped on the substrate (i.e., sticking probabilityis equal to 1), the deposition of 2.0 © 1014 M(Bz)2 cations/cm2

seemingly provides an approximate monolayer coverage (1ML). For each sandwich cluster, the peak positions of these IRabsorption bands are in good agreement with the IR fundamen-tals104,105 for the neutral clusters in an Ar matrix or KBr pellet,as shown in Figure 14. These observations indicate that the soft-landed M(Bz)2 cluster cations lost their charge, and the result-ing neutral clusters adsorb on the SAM substrate with theirnative sandwich structure intact. Indeed, vibrational frequenciesfor the cation sandwich clusters of Ti(Bz)2+ and V(Bz)2+ aredistinctly different from the frequencies observed herein.

Further study of the IRAS spectra shows that the relativeintensity of the IR bands for the soft-landed M(Bz)2 clustersare rather different from those for the condensed-phase clusters.As a result of the IRAS surface selection rule,106 the relativeIR intensity of the A2u and E1u modes reflects the orientationalpreference of the D6h-symmetric M(Bz)2 clusters on the sub-strate, and the observation of strong E1u modes implies thatthe M(Bz)2 cluster tilts its molecular axis far from the surfacenormal on the SAM substrate.38 The orientation angle of thesoft-landed clusters can be semiquantitatively determined bythe RATIO method107 with the assumption that all the clustersare uniformly oriented on the SAM substrate. The averagedtilt angle (Θ) of the clusters with respect to the surface normalis given by comparing the intensity ratio of the A2u and E1u

modes as follow,36

sin2 � ¼ 2=f2þ ½ISAMðA2uÞ=ISAMðE1uÞ�=½IRDMðA2uÞ=IRDMðE1uÞ�g ð2Þ

where ISAM represents the IR absorption intensity in the orien-tated state (i.e., SAM substrate) and IRDM indicates the corre-sponding intensity in the random orientation (i.e., Ar matrix orKBr pellet). The averaged tilted angle (Θ) with respect to thesurface normal is estimated to be similar: Θ μ 74° for Ti(Bz)2,Θ μ 72° for V(Bz)2, and Θ μ 67° for Cr(Bz)2.

The orientational preferences may arise from chemical inter-actions between the sandwich clusters and the organothiolatesforming the SAM matrix. Since no frequency shift is seen inthe spectra of the clusters after the soft-landing, the origin of

AWARD ACCOUNTSBull. Chem. Soc. Jpn. Vol. 86, No. 4 (2013)426

the preferred orientation is most likely an extremely weakattractive CH­³ interaction between the capping benzene ringsof the cluster and the hydrogen atoms of methylene groups ofthe surrounding n-octadecanethiolate molecules. Indeed, it hasbeen reported that the CH­³ interaction energy is very small(ca. 6 kJmol¹1) in comparison to other hydrogen­³ interac-tions (e.g., OH­³ and NH­³),108­111 and then, such weak CH­³ interaction would only work to orient the sandwich clustersinside the SAM matrix but would not lead to any frequencyshift in the vibrations of the clusters.

3.5.2 Temperature-Programmed Desorption Study: Thethermal desorption kinetics of the M(Bz)2 clusters supportedon the SAM substrates can be evaluated in detail by a TPDstudy. The TPD measurements were performed after exposureof 1L for thermal deposition using the PVD technique andsoft-landing of 4 © 1013 ions/cm2 onto the C18-SAM substrate.Figure 15 shows the TPD spectra for parent-mass ions ofM(Bz)2+ supported on the C18-SAM substrates. In addition, theArrhenius plots for the TPD spectra are taken in order to extractthe apparent desorption activation energy and reaction order ofthe clusters.112

The top trace of Figure 15 depicts a typical TPD spectrum ofthermally deposited Cr(Bz)2 clusters. The thermal desorptionstarts ca. 230K and the desorption rate reaches maximum atca. 265K. The TPD curve displays a nearly symmetric shape

in which the ion intensity slowly decreases with a long tail athigher temperature. The presence of the long high-temperaturetail reflects surface diffusion of the clusters on the SAM sur-face,38,113,114 which provides a linear Arrhenius plot for n = 2(i.e., second-order desorption kinetics). Such surface diffusionwould bring about a random orientation prior to the desorptionprocess; in fact, the IRAS spectra for the Cr(Bz)2/C18-SAMafter the PVD show that the clusters are randomly oriented atany surface temperatures in the range 200­250K.

On the other hand, the TPD spectra of the Cr(Bz)2 clustersafter the soft-landing of 4 © 1013 ions/cm2 (= 0.2ML) ontoC18-SAM are also displayed in Figure 15b. A pronounceddifference in the TPD spectral feature was obtained between thesoft-landing and the PVD: For soft-landed clusters, the thermaldesorption starts at ca. 290K and the desorption rates reachmaxima at ca. 310K. The drastic increase in desorption tem-perature provides conclusive evidence that the soft-landedclusters are more strongly bonded on the SAM substrate thanthe clusters thermally deposited via the PVD method. Fur-thermore, the TPD curves for the soft-landed clusters illustrateasymmetric peak profiles in which the desorption rates rapidlydecrease after the peak maximum. The Arrhenius plots of theseTPD spectra demonstrates that the soft-landed clusters desorbfrom the C18-SAM substrate via first order desorption kinetics(n = 1); the thermal diffusion of the clusters in the desorptionprocess are inhibited. Thus, these TPD results clearly show thatthe soft-landed clusters do not physisorb on the surface of theSAM matrix.

The activation energy for desorption of the clusters sup-ported by the SAM substrates can be quantitatively determinedby taking Arrhenius plots of the measured TPD spectra.115,116

Temperature/K220 240 260 280 300 320 340

Inte

nsi

ty (

arb

. un

its)

(c) V(Bz)2 Soft-landing

(b) Cr(Bz)2 Soft-landing

(a) Cr(Bz)2

PVD

230 K

290 K

295 K

Figure 15. The TPD spectra for parent-mass ions ofM(Bz)2+ supported on the C18-SAM substrates; (a) Cr(Bz)2after the PVD (1L), (b) Cr(Bz)2 after soft-landing of 4.0 ©1013 clusters, and (c) V(Bz)2 after soft-landing of 4.0 ©1013 clusters onto C18-SAM substrate at 200K. Thresholdtemperatures for the thermal desorption are shown by thedownward arrows.

1400 1200 1000 800Wavenumber/cm–1

0.05

1418(E1u) 988

(E1u)

956(A2u)

747(A2u)

1419(E1u)

988(E1u)

956(A2u) 742

(A2u)

977(E1u)

945(A2u)

1410(E1u)

700(A2u)

947(A2u)

979(E1u)

0.05

1429(E1u)

996(E1u)

972(A2u)

0.05

Tran

smitt

ance

/%

1429(E1u)

996(E1u)

972(A2u) 796

(A2u)

(a) C18-SAM

Ar matrix

(c)

(b) C18-SAM

Ar matrix

C18-SAM

KBr pellet

Figure 14. IRAS spectra in the 650­1550 cm¹1 region afterthe soft-landing of 2.0 © 1014 M(Bz)2 cation clusters (M =Ti, V, and Cr) onto the C18-SAM substrate at 200K,along with their IR fundamentals in the condensed-phase(Ar matrix or KBr pellet): (a) Ti(Bz)2, (b) V(Bz)2, and(c) Cr(Bz)2. IR intensities for Ti(Bz)2/Ar matrix andV(benzene)2/Ar matrix are taken form Refs. 104 and 105,respectively, and Cr(benzene)2/KBr pellet is obtained inthis work.

A. Nakajima Bull. Chem. Soc. Jpn. Vol. 86, No. 4 (2013) 427

For the Cr(Bz)2 PVD clusters on C18-SAM substrate, thedesorption activation energy was evaluated to be 71 « 12kJmol¹1, which is comparable to the desorption activationenergy for V(Bz)2 clusters physisorbed on a gold surface of64 « 13 kJmol¹1.38b In contrast, the Cr(Bz)2 and V(Bz)2 clus-ters soft-landed on the C18-SAM matrix exhibit much largerdesorption activation energies of 130 « 40 kJmol¹1. The valueof desorption activation energy of ca. 1.5 eV provides anindication of threshold incident energy of the incoming clustersfor the penetration into the SAM matrix.

3.5.3 Correlation between the Thermal Desorption andPhase Transitions of SAM Matrix: The desorption acti-vation energy of the sandwich cluster depends on an alkyl-chain length of the Cn-SAM matrices. Figure 16 shows thatthe activation energy increases almost linearly with the chainlength of the SAM. This linear increment is quite analogousto the dependence of the enthalpy of phase transitions forcondensed phase n-alkanes on their chain length.117,118 Thisstriking similarity suggests that the thermal desorption of thematrix-isolated clusters may be associated with phase transi-tions of alkyl chains within the SAM matrix through themolecular ordering of ·­· interaction.

For n-alkanes there exists between the crystalline (solid)phases and liquid phase a series of weakly ordered plasticcrystalline phases, the “rotator phases,” so-called due to thelack of long-range order in the rotational degrees of freedomof their long alkyl-chains.119,120 Skinner and co-workers char-acterized the transition enthalpy at the crystal­rotator transi-tion (¦HT) as well as the fusion enthalpy at the rotator­liquidtransition (¦HF) of n-alkanes in the C17 to C36 range.118 Theyfound that both the ¦HT and ¦HF increase monotonically asthe chain length growth with an identical slope, while the valueof ¦HT is about one-half of that of ¦HF.

Below room temperature, the normal alkyl chains of the n-alkanethiol SAM are mostly linear (i.e., all-trans conformation)to enhance the chain­chain packing, whereas the rotator struc-ture of alkyl chains can be also observed, called gauche con-

formational defects, above room temperature.121­124 In fact, theadvent of the rotator phase can be followed by probing theinfluence of temperature on infrared spectra of the SAM.122,123

When the temperature-induced variations of the IRAS spectraare observed for the C18-SAM substrate, the integrated intensityof C­H asymmetric stretching at 2918 cm¹1 decreases with thesubstrate temperature, which is induced by a decrease in the tiltangle of the alkyl chains.124­126 Nonetheless, above 300K theintensity undergoes a further decrease caused by the appearanceof gauche conformational defects127 so that the phase tran-sitions of the highly ordered phase (crystal phase) to the rotatorphase takes place around 290K for the C18-SAM substrate.This phase-transition temperature is also fairly close to thethreshold desorption temperature of clusters supported on theC18-SAM substrate (ca. 290K). Thus, the crystal­rotator phasetransitions of the C18-SAM matrix probably assist in the releaseof the trapped sandwich clusters from the SAM matrix.

Figure 17 shows a schematic diagram of the soft-landingprocesses of V(Bz)2 clusters toward the SAM. When incomingV(Bz)2 causes substantial disordering of the SAM by thekinetic energy dissipation, the SAM through the molecularordering of ·­· interaction will be disordered. Since the SAMsubstrate is kept under the low-temperature of 200K, themolecular ordering of the SAM is recovered after the V(Bz)2cluster is embedded as a physisorbed trapping inside reorderedSAM. As discussed above, the embedded form apparentlyexhibits the orientational preferences arising from chemicalinteractions between the sandwich clusters and the organo-thiolates. In order to desorb the matrix-isolated clusters, it isnecessary to be associated with phase transitions of alkyl chainswithin the SAM matrix, and therefore, the activation energycontributes to isolate clusters up to above room temperature.Namely, the cluster soft-landing with SAM can provide theroom temperature matrix isolation without two-dimensional gasbehaviors of the clusters.

3.6 Electronic Structure Evolution of Oligoacene Mo-lecular Clusters. The n-alkanethiol SAM on the Au substrate

Des

orpt

ion

Act

ivat

ion

Ene

rgy/

kJ m

ol−

1

−1

The Number of Carbon in Alkyl Chain

Fusion E

nthalpy/kJ m

ol

4 8 12 16 20 240

50

100

150

200

0

50

100

150

200

Figure 16. Activation energy for desorption of the V(Bz)2clusters soft-landed on the n-alkanethiol SAM matricesplotted against the chain length (n = 6­20) of the SAM( ), along with the fusion enthalpy of correspondingchain-length n-alkanes ( ). The dashed line was providedby least-squares fitting of each set of data.

Fre

e en

erg

y

Disorderingof SAM

Gt

Re-orderingof SAM

V(Bz)2 + SAMGi

Activation Energyfor Desorption

Physisorbed Trapping inside SAM with Partial Disordering

desorptionadsorption Reaction coordinate

V(Bz)2 / SAM

Figure 17. Schematic diagram of the soft-landing ofV(Bz)2 clusters embedded inside the SAM. IncomingV(Bz)2 causes substantial disordering of the SAM by thekinetic energy dissipation, and the V(Bz)2 clusters areembedded as physisorbed trapping inside reordered SAM.

AWARD ACCOUNTSBull. Chem. Soc. Jpn. Vol. 86, No. 4 (2013)428

mentioned above is governed by molecular ordering of ·­·interaction between alkyl chains clipped at the surface throughAu­S bonding. The molecular ordering can be found also inorganic molecular solids, such as polycyclic aromatic hydro-carbons (PAH), and their electronic structure has attractedconsiderable interest for several decades because of its intimaterelationship with their optical, dielectric, and charge-transportproperties. Among the class of organic solids, crystalline linearoligoacenes, i.e., naphthalene, anthracene (Ac), tetracene (Tc),and pentacene, have investigated experimentally and theoret-ically since the 1960s and are a prototypical model systemfor the study of charge localization and transport.128­138 Addi-tionally, their promising potential as organic semiconductormaterials has generated a renewed interest in their intrinsicelectronic structure relevant to charge transport also to themolecular ordering at the interface.134­149

Since organic molecules aggregate into organic solidsthrough weak intermolecular interactions, the molecules gen-erally maintain their molecular properties in the solid state.While self-assembled hydrocarbons on a metal substrate aggre-gate via ·­· interaction, polyacene molecules do via ³­³interaction, where the latter is stronger than the former. Evenin a freely isolated cluster, therefore, polyacene clusters geo-metrically form a kind of ordered structure, a herringbonestructure. In general, this ³­³ interaction results in relativelynarrow HOMO/LUMO bandwidths, and hole/electron chargecarriers in organic solids are strongly localized on individualmolecules. Such a localized charge is stabilized by the elec-tronic, molecular, and lattice polarizations that play a key rolein determining the HOMO/LUMO levels in organic solids. Inorder to reveal the electronic structures in molecular clusters,anion PE spectroscopy is also a powerful method to observe theenergetics of the LUMO in ³-conjugated organic molecularclusters.41,42,150

In this section, a PE spectroscopic study of anionic clustersis presented for polyacene including naphthalene (Nph), anthra-cene (Ac), and tetracene (Tc). The key issues are to reveal thereason why the ordered crystal-like anion state appears in thelarge polyacene cluster anions, and also to unveil the micro-scopic views of electron localization in organic molecularcrystals by bridging isolated molecules to the bulk.

3.6.1 Mass Spectrometry for Anthracene and TetraceneCluster Anions: Figure 18 displays mass spectra of (Ac)n¹

and (Tc)n¹ expanding from the valve at a stagnation pressureof 70 atm. In Figure 18a, a smooth mass distributions of (Ac)n¹

is observed, while Figure 18b exhibits some striking features,including the even­odd alternation (3 < n < 14), the strongmagic numbers of n = 5, 14, and 19. The latter local maximasuggest that a plausible structure for its relatively small clustersis one with two-dimensional (2D) ordering, where all Tc mole-cules interact with their long molecular axes in parallel. The(Tc)14¹ cluster appears as the most persistent magic number,and then it presumably must possess a geometrically closedstructure. As illustrated in Figure 18b, it is proposed that then = 14 and 30 cluster adopts a 2D herringbone-type ordering,which is a finite-size analogue of the 2D herringbone layerfound in the Tc crystal. Indeed, ³-conjugated molecules withhigh structural anisotropy (e.g., rod-like ³-conjugated mole-cules such as linear oligoacenes and p-oligophenylenes) prefer

to aggregate themselves to maximize ³-orbital overlap betweenadjacent molecules (i.e., parallel to the a­b crystal plane).151

Furthermore, it is consistent with that the less magic-numbered behavior was observed for Ac and also for Nph,because the ³­³ interaction between adjacent moleculesbecomes weak. Thus, as shown in Figure 18b, it is presumedthat isomer II have 2D herringbone-type orderings that com-plete a geometric shell by 14 Tc molecules. In this structure,all the long molecular axes are parallel and the central fourmolecules are completely surrounded by the other 10 mole-cules. For the size range of n = 15­30, the characteristic massdistribution can be explained by a subshell closing,152 wherethe second 2D-shell is partly filled and this yields a more orless geometrically closed structure at the numbers of 16, 19, 21,24, 27, and 30.

3.6.2 PES for Ac Cluster Anions: Figure 19 shows thePE spectra of (Ac)n¹ (n = 1­100) measured at 532 nm. In thespectrum for n = 1, a bell-shaped envelope having an extensivevibrational progression of the C­C stretching Ag mode of 0.17eV (ca. 1370 cm¹1) is observed.153 The current spectra areanalogous with the previously reported spectra in whichirregular peak shifts and profiles have been observed,154,155

although the PE spectra in Figure 19 exhibit such anomalies

(a)

(b)

0 5000 10000 15000Mass Number (m /z )

1419

21

24

27 3033

36

10

5

816

12

n = 1 10 20 30 50 70

n = 14

n = 30

Figure 18. TOF mass spectra of (a) anthracene and (b)tetracene cluster anions expanding from the valve at astagnation pressure of 70 atm. In (a), a smooth mass distri-butions of (Ac)n¹ is observed, while in (b) some strikingfeatures are observed, including the even­odd alternation(3 < n < 14), the strong magic numbers of n = 5, 14, and19. For n = 14 and 30, proposed structures based on a 2Dherringbone-type ordering are shown.

A. Nakajima Bull. Chem. Soc. Jpn. Vol. 86, No. 4 (2013) 429

rather more clearly, probably due to the higher stagnationpressure (up to 100 atm).

The VDEs of the initial peak for n = 2­14 do not exhibita smooth size-dependence, especially above n = 4. For the(Ac)4¹ cluster, the shape of the spectral feature was signifi-cantly changed and a shoulder peak obviously appeared ata lower binding energy. Interestingly, a similar low energyfeature was evident for n = 5, 8, and 10, and was weakly seenfor n = 6, 7, 11, 12, and 14. The pronounced anomaly inthe spectral evolution suggests the coexistence of isomerswhose population varies drastically for n = 4­14. Hereafter,the higher and lower energy bands observed for n = 4­14 aredenoted as “I” and “II-1,” respectively, as shown in Figure 19.

For n = 15­38, band I is predominant under any ion sourceconditions, and their VDEs gradually shift toward a higherbinding energy with increasing n. In this size region, band II-1is weakly observed, suggesting that the anion state yieldingband II-1 becomes less stable than that of band I in the n =15­38 size range.

Above n μ 39, a new component (labeled II-2) appears at alower binding energy about 0.5 eV than the dominant band(band I). The intensity of band II-2 increased rapidly for n =39­44, and this band becomes the most prominent in thespectra for n ² 50. Interestingly, band II-2 exhibits two fea-tures; (1) a constant VDE at 1.47 eV for n = 50­100 and (2) arelatively sharp spectral profile having the vibrational structureof the C­C stretching ¯6 mode of a neutral Ac. These featuressuggest that a distinct isomeric form of isomer II-2 coexists inthe larger Ac cluster anions separately from an isomer assigned

to band I (i.e., isomer I). The coexistence of the isomers I and IIcan be observed similarly in the PE spectra of other polyacenecluster anions of (Nph)n¹ and (Tc)n¹, depending on the sourceconditions. Note that the isomer II is not photodissociationproducts, because no power dependent processes for the spec-trum shape were observed.

3.6.3 Coexitence of Isomers I and II: To clearly displaythis behavior, the effect of varying stagnation pressure is shownfor (Nph)100¹ in Figure 20. With increasing stagnation pres-sure, band II-2 becomes more dominant than band I and moreclearly exhibits the vibrational structure of the C­C stretchingmode of ¯4 and/or ¯5, indicating that more efficient coolingof the cluster ensemble occurs at higher stagnation pressure.The temperature variations on the cluster source suggest thatthe two anionic isomers I and II-2 at a given size are very closein (adiabatic) energy. These observations strongly suggest thepresence of two distinct anionic species (designated as isomersI and II-2).

As shown in Figures 19 and 20, there are four noticeablecharacteristics of the isomers I, II-1, and II-2 as follows: (1) theVDEs of isomer I consistently increase over the entire sizerange, (2) the VDEs of isomers II-1 (n ² ca. 14) and II-2 arealmost size-independent and are 0.5­0.6 eV lower than those ofisomer I, (3) the spectra of isomer II-2 exhibits a relatively sharpprofile showing intramolecular vibrational structure, whilethose of isomer I are broader and less structured, and (4) isomerII-2 (n ² 39) is favored in an intense supersonic expansion(² ca. 30 atm) that can produce a cold anion beam, whilewarmer ion source conditions (<10 atm) form only isomer I.

band I

404142434445

46

47

4948

506070

80

90

100

band II-2

39

0.0 0.5 1.0 1.5

3

6

7

89

10

5

4

2

n = 1

(Ac)n

0.0 0.5 1.0 1.5

band II-1

Electron binding energy/eV

30

35

111213

1415161718192025

363738

0.0 0.5 1.0 1.5

band II-1

band Iband I

Figure 19. PE spectra of (Ac)n¹ (n = 1­100) measured at532 nm. Bands labeled by I, II-1, and II-2 appear as differ-ent isomeric anions. Note that VDE value of band II-2(n μ 39) does not increase at all over the entire size rangeand remains constant at 1.47 eV and that vibrational struc-ture resembling that of (Ac)1¹ is observed for band II-2(n = 50­100) in the spectra.

Hot

Cold

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(f) - (a)

Electron binding energy/eV

0 21ν4,5

isomer I

isomer II-2

Figure 20. PE spectra of (Nph)100¹ measured at differentHe-stagnation pressures. As increasing the stagnation pres-sure (from top to bottom), band II-2 becomes more domi-nant than band I and displays the vibrational structureof the C­C stretching mode (v4 and/or v5) as shown atbottom. At a low stagnation pressure (top), only isomer I isobserved exclusively.

AWARD ACCOUNTSBull. Chem. Soc. Jpn. Vol. 86, No. 4 (2013)430

The constant VDEs signify that the stepwise stabilizationenergy added per molecule in the anion is the same as that in theneutral. Since the ion-neutral electrostatic interactions are muchstronger than the neutral­neutral interactions, it can be inferredthat there is a significant charge screening effect in isomers II-1and II-2. Moreover, the magnitude of VDEs is governed by notonly the energetics, but also by structural factors, meaning thatthe constant VDE values signify that the amount of structuralrelaxation incurred in the geometric change from the anion tothe neutral no longer depend on the cluster size. This is con-sistent with the magic-numbered behavior in the mass spectra(Figure 18) that isomer II has a solid-like ordered structure with2D herringbone-type orderings.

3.6.4 The Effect of Intermolecular Steric Hindrance andExtra Internal Degrees of Freedom: To gain further insightinto the nature of isomer II, the effect of geometric packingwas examined by two viewpoints of intermolecular sterichindrance and extra internal degrees of freedom, because bothof them should less favor the well-ordered isomer II. From theviewpoint of intermolecular steric hindrance, anion PE spec-troscopy at 532 nm was employed for alkyl-substituted Ac; 1-methylanthracene (1MA), 2-methylanthracene (2MA), 9-meth-ylanthracene (9MA), 9,10-dimethylanthracene (DMA), and 2-tert-butylanthracene (2TBA) (See Figure 21 for their struc-tures). For their monomer anions (n = 1), electron affinitiesare confined to the range from 0.47 to 0.59 eV, and are wellreproduced by density functional theory (DFT) calculations,showing the isoelectronic character of these molecules. Fortheir cluster anions, (2-MA)n¹, (1-MA)n¹, (9-MA)n¹, (9,10-DMA)n¹, and (2-tBuA)n¹ clusters for n = 1­100, the size-evolution of the PE spectra are similar to that of (Ac)n¹ only for(2-MA)n¹ recorded at high stagnation pressures 30 atm, whilethe PE spectra of the other alkyl-substituted Ac cluster anions

are clearly different from those of (Ac)n¹. For (1-MA)n¹,(9-MA)n¹, (9,10-DMA)n¹, and (2-tBuA)n¹ clusters, the PEspectra are rather simplified, where a single band, correspond-ing to band I (isomer I), shifts monotonically toward a higherbinding energy as the cluster grows in size. For these alkyl-substituted Ac cluster anions, neither isomer II-1 nor isomerII-2 is observed. Since the electronic characteristics of mono-mers of Ac and its alkyl derivatives are very similar, thisdifference can be ascribed to the steric effects of alkyl groupsupon the molecular ordering (i.e., intermolecular distances andorientations) in the cluster. Furthermore, it can be seen in thePE spectra for (Ac)n¹/(2-MA)n¹ that, if the isomer II-1 appearsin the PE spectra, then so does the isomer II-2. The correlationin the appearance of isomers II-1 and II-2 suggests that isomerII-1 can be a precursor form of isomer II-2.

The methyl-substitution effects on the formation of isomerII-1 consistently support the 2D structural morphology of iso-mer II-1. When the H atom in the C1 or C9 position of Ac issubstituted by a methyl group, it is conceivable that the effec-tive ³­³ interactions are substantially hindered and reducedwithin the 2D herringbone layer, giving rise to the instabilityof the 2D structure. Indeed, isomer II-1 was not produced in(1-MA)n¹, (9-MA)n¹, and (9,10-DMA)n¹ under any experi-mental conditions. On the other hand, the similarity of the be-havior of (2-MA)n¹ to (Ac)n¹ is likely because the methyl groupin the C2 position can be located outside the 2D layer and doesnot retard the ³­³ interactions within the 2D layer. In the caseof 2-tBuA, furthermore, isomer II-1 did not appear, because ofthe higher steric hindrance of a tert-butyl group, even at the C2position, that would strongly perturb the 2D structural motif.

From the viewpoint of extra internal degrees of freedom,anion PE spectroscopy was employed for molecular clustersconsisting of various PAH, as shown in Figure 21. Some

isomer II

(108)Acridine

200

100

(183)

(87)

(78)

(147)

(98)

(70)

(215)

(100)

(92)

(150)

9,10-DMA

2-t BuA

Phe

1-MA

9-MA

AcNph

Az

p-TP

m-TP

o-TP

BP

(86)

(56)

Py

300

(253)Chry

N

(204)

(216)

(81)

2-MA

Ac

Nph

Tc (357)meltingtemperature

Tm /°C

Figure 21. Coexistence of an ordered form, Isomer II: Along with isomer I, red (double frames)/pink (single frame) coloredmolecules exhibit the coexistence of isomers II-1 and -2/isomer II-1, respectively, while black ones do only isomer I. Sterichinderance of a substituted group and/or extra intramolecular degrees of freedom make the isomer II hardly formed. Melting pointfor each molecules in bulk is indicated in parenthesis.

A. Nakajima Bull. Chem. Soc. Jpn. Vol. 86, No. 4 (2013) 431

molecules exhibit lowered molecular symmetry and othershave extra intramolecular degrees of freedom. Except forpara-terphenyl (p-TP), other molecular clusters exhibit onlyisomer I in the anion PE spectra. For (p-TP)n¹, isomer II-1appear around n = 14, which can be well explained by 2Darrangements of p-TP molecules. Note that p-TP is expressedby pink color with single frame, because no isomer II-2 couldbe observed clearly at larger cluster size. Furthermore, melt-ing point (Tm) for each molecule in bulk is also shown inFigure 21. Interestingly, there is an apparent trend that thecoexistence of isomer II with isomer I correlates closely withTm; the isomer coexistence is observed in clusters consistingof high Tm molecules when they are compared within thesame number of aromatic rings. This is probably because themolecularly ordered isomer II is preferably found for moleculeswhich can form rigid molecular crystal exhibiting high Tm.Namely, as well as steric hindrance of a substituted group, extrainternal degrees of freedom make the isomer II hardly formed.

3.6.5 Bridging between Clusters and Thin Film in Bulk:To bridge between cluster and bulk, the cluster data obtainedherein are compared with the bulk data. When the VDE valuesof isomers I and II are plotted against n¹1/3, the VDEs ofisomer I with n ² 20 were fitted linearly and a straight lineextrapolates to a VDE(¨) value of 2.12 eV for Nph, 2.5 eVfor Ac, and 3.0 eV for Tc.42 In contrast to isomer I, the VDEsof isomer II-2 are not size-dependent, but are nearly constantwithin experimental uncertainties, i.e., 0.99 eV for (Nph)n¹,1.47 eV for Ac, and 2.00 eV for Tc with n = 60­100.42

Figure 22 shows an example of VDE plots for Ac togetherwith 2-MA and 1-MA against n¹1/3 for n = 1­100. Note thatfor Ac and 2-MA, isomers I, II-1, and II-2 appear, while for 1-MA only isomer I appears.

In molecular crystals, the adiabatic energy of the lowestunoccupied molecular orbital (LUMO) level with respect tothe vacuum level (hereafter referred to as “ELUMO”) is givenby ¹EAa + Peff

¹ where EAa is the adiabatic electron affinity ofthe gas-phase molecule and Peff

¹ is the effective polarizationenergy consisting of the charge-induced dipole interaction andthe charge­quadrupole interaction terms. Note that the con-tribution of lattice relaxation energy is ignored, because it isusually negligibly small (few tens of meV).134,143,156­159 TheEAa of a Nph molecule160 has been reported to be a negativevalue of ¹0.19 eV (Note that monomeric Nph¹ is not producedin the beam), while the EAa of Ac and Tc molecules determinedare 0.53 and 1.06 eV, respectively. By Munn and co-workers,the Peff

¹ of the Nph/Ac/Tc single crystal have been theoret-ically estimated as ¹1.05/¹1.01/¹0.92 eV,138 respectively. Onthe basis of these data, the ELUMO values of the Nph/Ac/Tcsingle crystal were estimated to be ¹0.86/¹1.54/¹1.98 eV,respectively. Also available in the literature concerning ELUMO

is the experimental result of inverse photoemission spectros-copy (IPES) for highly ordered thin film on Ag(111) reportedby Frank et al.;141 the energy of LUMO from the vacuum levelhas been reported to be ¹1.1, ¹1.7, and ¹1.8 eV for Nph, Ac,and Tc, where uncertainties are «0.15 eV. Because of imagecharges in the metal substrate, this energy may be a little largerthan the intrinsic energy of the bulk.143

Since these ELUMO values should correspond to ¹VDE forthe PE spectra,161 Figure 23 shows the comparison of these

ELUMO values with the present results of PE spectra for Nph/Ac/Tc at n = 100. Interestingly, the VDEs of isomer II-2are almost entirely in accord with those bulk values, showingthat electron localization in organic molecular crystals can bedescribed well by solid-like isomer II-2 with n ² 60. Thesefacts surprisingly demonstrate largely localized nature ofpolarization in a cluster analogue of polyacene single crystal,which has been generally believed to extend over many latticeconstants in OMCs.134,135 Namely, it can be concluded thatisomer II (with n ² 60) of polyacene clusters is the clusteranalogue of “polaron” in bulk crystal.

Finally, the VDE(¨) values of isomer I is discussed; theyare much larger than the predicted ELUMO values. The dis-crepancy indicates that isomer I does not have a solid-likeherringbone-type structure but has a reorganized structurehaving a monomeric anion core with surrounding neutrals. Infact, the cluster reorganization energy (RE) makes a consid-erable contribution to the VDE, i.e., VDE = ¹ELUMO + RE.Here RE is the total between the relaxation energy in the neutralstate and that in the anion state and consists of two parts: (1) thestructural rearrangement energy of the neutral, being the energydifference between the stable neutral and the neutral whosestructure is the same as that of the stable anion, and (2) thestructural rearrangement energy of the anion, which is theenergy difference between the stabilized anion and the anionwhose structure is the same as that of the stable neutral. The

110100∞Cluster size (n)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

−2.0

−1.5

−1.0

−0.5

0.0

1-MA (I)

Ac (I)

Ac (II-1)

Ac (II-2)

2-MA (I)

2-MA (II-1)

2-MA (II-2)

−VD

E/e

Vn −1/3

I

II-1II-2

IPES

− EAa

− P −eff

CAL.

1-MA

2-MA

Ac

Figure 22. VDE plots for Ac, 2-MA, and 1-MA againstn¹1/3 for n = 1­100. For Ac and 2-MA, isomers I, II-1, andII-2 appear, while for 1-MA only isomer I appears. On theleft axis, two bulk values are indicated; (solid line) theenergy of the LUMO level with respect to the vacuum level(ELUMO) of ¹1.7 eV, determined by IPES for a highly-ordered Ac thin film on Ag(111),141 and (dashed line) theenergy by adding the EAa value of an Ac molecule to thecalculated polarization energy (Peff

¹) for a monomer anionin the corresponding single crystal.161

AWARD ACCOUNTSBull. Chem. Soc. Jpn. Vol. 86, No. 4 (2013)432

results indicate that the equilibrium geometry of isomer I is farfrom the herringbone-type structure of the stable neutral.

4. Summary and Perspectives

This account describes the novelty and the importance ofcomposite clusters focusing on the efforts to create a newfunctional composite cluster by a fine doping or hybridizationbased on their size-specific electronic properties.18,30­36 In theviewpoint of symmetry and dimensionality of the compositeclusters, organometallic clusters19­29 and caged clusters34­36 aredemonstrated as an example of designing the functionality ofmagnetism and electronic state structure. In order to createfunctional nanomaterials, furthermore, a fine controlling meth-odology of the soft-landing technique has been established tofix the composite clusters onto a surface decorated witha SAM.38­40 The embedded isolation mechanism is closelyrelated to self-assembly phenomena with the molecular orderingof ·­· interaction between alkyl chains in the SAM. As a self-assembled aggregate model, molecular clusters of ³-conjugatepolyacenes provide detailed insight on molecular orderingthrough ³­³ interaction in an isolated cluster.41,42 As shownin Figure 24, depending on the intermolecular interaction,molecular ordering through ·­· interaction and ³­³ interac-tion appear as self-assembly phenomena. Since ·­· interactionis generally weaker than ³­³ interaction, a sulfur atom terminalis important for alkyl chains to form self-assembled monolayersthrough Au­S bonding. The fundamental insight related to theself assembly phenomena can be revealed as (1) 2D phase tran-sition of SAM and (2) conspicuous 2D stacking relevant topolaronic behavior, both of which strongly depend on chainlength or the number of aromatic rings.

In recent years, nanomaterial application has been verylimited to carbon based materials, C60, carbon nanotubes, andgraphenes, because of a lack of universal ways for designingfunctional nanomaterials and no synthetic routes for efficientnanocluster production. Furthermore, it is still hard to isolatea nanocluster selectively and also to fix it at a specific site,because both bottom-up and top-down approaches should becombined properly to develop the isolation and fixation of ananocluster. Since the properties of nanoclusters are changeddrastically with one atom addition or loss, it is necessary tocharacterize nanocluster assembled materials with specificcomposition and ordered aggregates.

As described in this account, organometallic sandwich nano-clusters19­29 and metal atom-encapsulated silicon cage clus-ters34­36 are synthesized through a gas phase reaction. Inparticular, nanoassembled materials consisting of the supera-tom families of caged silicon clusters with metal encapsulationwill be able to provide a bottom-up way to form a new type ofsemiconductor, which is completely different from conven-tional doping heterogeneous atoms into silicon materials. Onthe other hand, nanoassembled materials consisting of sand-wich organometallic nanoclusters will exhibit the functionalityof selective optical response and sensitive magnetic properties,and the organometallic nanocluster materials will open up anavenue of new optomagneto-materials. In order to realize the

anthracene tetracenenaphthalene

C18-SAMC8-SAM C12-SAM

SAM Isolation / 2D-meltingσ-σ

π-πLiquid-like Solid-likeOrdered

Polaronic behavior / high ordering

Figure 24. Molecular ordering through ·­· interactionbetween alkyl chains in the self-assembled monolayer(SAM) and thorough ³­³ interaction between polyacenemolecules in an isolated clusters. With increasing chainlength or the number of aromatic rings, higher molecularordering can be achieved, resulting in (1) thermally stabi-lized isolation of clusters inside the SAM and (2) conspicu-ous 2D stacking relevant to polaronic behavior. GraphicalAbstract Composite clusters composed of several elementsand/or components provide a promising way for a bottom-up approach for designing functional advanced materials.This account surveys recent investigations of compositeclusters focusing on the efforts to create organometallicclusters and caged clusters with the development of a soft-landing technique with a self-assembled monolayer. Self-assembly phenomena are also discussed using molecularclusters of ³-conjugate polyacenes.

n = 100

3.0Electron binding energy/eV

2.01.00.0

band II-2

Nph

Ac

Tc

band II-2

band II-2

Figure 23. PE spectra of (Nph)100¹, (Ac)100¹, and (Tc)100¹

measured at high He-stagnation pressures (70­100 atm),where band II-2 is dominant. The VDE indicated by a solidarrow is in agreement with the bulk values; (1) (solid line)the energy of the LUMO level with respect to the vacuumlevel (ELUMO) of ¹1.7 eV, determined by IPES for a highlyordered Ac thin film on Ag(111),141 and (dashed line) theenergy by adding the EAa value of a Nph/Ac/Tc moleculeto the calculated polarization energy (Peff

¹) for a monomeranion in the corresponding single crystal.161

A. Nakajima Bull. Chem. Soc. Jpn. Vol. 86, No. 4 (2013) 433

nanoassembled materials, two things should be explored;(1) the establishment of efficient isolation method of nano-clusters and (2) combination between bottom-up and top-downapproaches.

It is strongly desired to establish an efficient isolation methodsuitable for organometallic nanoclusters and caged siliconclusters. For industrial application to nanomaterial devices,indeed, both approaches of bottom-up and top-down shouldbe combined properly to develop the isolation and fixationof a nanoclusters, because bottom-up approach cannot controlthe arrangement of nanoclusters in large area, while top-downapproach of lithography cannot design a substrate with betterthan 10­30 nm accuracy. The application of self-assembledmonolayers and their related self assembled membrane, coupledwith lithography, is promising to realize higher uniformityof the designed substrate. Beyond molecular electronics, selfassembled ordering of the functional nanoclusters is a crucialissue to fabricate nanometer-scale functional clusters withreasonable handling procedures.

I would like to express to all the co-workers whose namesappear in the references my deep appreciation for their pro-ductive, energetic, and hard works. I also thank Professor Dr.Koji Kaya for his guidance and encouragements. This work ispartly supported by a program entitled “Research for the Future(RFTF)” of the Japan Society for the Promotion of Science(No. 98P01203), by a CREST program of Japan Science andTechnology Agency (JST), and by the Grant-in-Aids forScientific Research (A) (No. 19205004).

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Atsushi Nakajima is a Professor of Physical Chemistry at Keio University. He received his D.Sc.from the University of Tokyo in 1989 under the direction of Profs. Kozo Kuchitsu and TamotsuKondow. He became a Research Associate in 1989, an Assistant Professor in 1994, an AssociateProfessor in 1997, and a Professor in 2001 at Keio University. He received the Chemical Societyof Japan Award for Young Chemists in 1995 and the Chemical Society of Japan Award forCreative Work in 2008. His research interests include cluster science, molecular science, andsurface science as well as nanoscale material science.

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