Article pubs.acs.org/JPCC
Heterodimerization via the Covalent Bonding of Ta@Si16 Nanoclusters and C60 Molecules Masato Nakaya,†,‡ Takeshi Iwasa,†,‡ Hironori Tsunoyama,†,‡ Toyoaki Eguchi,†,‡ and Atsushi Nakajima*,†,‡,§ †
Nakajima Designer Nanocluster Assembly Project, ERATO, JST, KSP, 3-2-1 Sakado, Takatsu-ku, Kawasaki 213-0012, Japan Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan § Keio Institute of Pure and Applied Sciences (KiPAS), Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan ‡
S Supporting Information *
ABSTRACT: Initial products prepared via the surface immobilization of Ta-atom-encapsulated Si16 cage (Ta@Si16) nanoclusters on solid surfaces terminated with monolayer films of C60 molecules were investigated using scanning tunneling microscopy (STM). The STM results indicated that marked aggregation and desorption of surface-immobilized Ta@Si16 nanoclusters were not induced, even after thermal annealing at ∼500 K, whereas the local vertical and lateral positions of the Ta@Si16 nanoclusters with respect to neighboring adsorption sites in the C60 film were modified. This local positional transition occurred on C60 monolayer films weakly bonded (via van der Waals forces) to substrates such as highly oriented pyrolytic graphite (HOPG) but did not occur on C60 monolayer films covalently bonded to substrates such as Si(111)7 × 7. These results indicated that the heterodimer consisting of a Ta@Si16 nanocluster and a C60 molecule, Ta@Si16−C60, was formed as an initial product via covalent bonding, which inhibited wide-range surface migration of the Ta@Si16 nanoclusters but allowed them to locally change their positions via thermally activated precessional motion. In addition, the transition temperature of the local positional shift was found to decrease as the area density of the surface-immobilized Ta@Si16 nanoclusters increased, indicating that the barrier height of the precessional motion of the Ta@Si16−C60 heterodimer was decreased due to accumulation of the elastic strain energy generated in the C60 films. exhibit chemical and thermal stability,11,12 which is important for creating functional materials via the hierarchical assembly of superatomic nanoclusters on solid surfaces without losing the original properties of each nanocluster. In particular, the fabrication of M@Si16-based assemblies on conductive surfaces is indispensable, not only for characterizing their atomic-scale geometry and electronic properties using conventional surface analysis tools, such as scanning tunneling microscopy/scanning tunneling spectroscopy (STM/STS) and photoelectron spectroscopy (PES), but also for applying them as active elements in electronic devices consisting of electrode/ nanocluster/electrode heterojunctions, although it is known that in most cases the original properties of gas-phasesynthesized nanoclusters are undesirably modified on con-
1. INTRODUCTION The synthesis of molecules, complexes, and bulk crystals from multiple atomic elements has been an essential technology in recent science and engineering fields. The controlled assembly of superatomic nanoclusters composed of multiple atoms is expected to be a novel material-processing technology providing hierarchical nanostructures with tailored dimensionality and functionalities.1−7 Cage-shaped Si16 nanoclusters encapsulating a single metal atom in the center of the cage (M@Si16)8−16 have received attention as potential building blocks for superatomic assemblies. M@Si16 nanocluster ions synthesized in the gas phase using laser vaporization and magnetron sputtering sources11,12,17 have been shown to behave as superatoms, exhibiting halogen-, rare-gas-, and alkali-like physicochemical properties depending on the type of metal atom and the charge state.11−16 In particular, rare-gaslike M@Si16 nanoclusters, such as neutral, anionic, and cationic Si16 cages encapsulating metal atoms of group-4 (e.g., Ti@Si16 and Zr@Si16), group-3 (e.g., Sc@Si16− and Lu@Si16−), and group-5 (e.g., V@Si16+ and Ta@Si16+), respectively, are selectively synthesized as magic number nanoclusters. They © XXXX American Chemical Society
Special Issue: Current Trends in Clusters and Nanoparticles Conference Received: November 7, 2014 Revised: January 25, 2015
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DOI: 10.1021/jp511157n J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 1. Surface immobilization of Ta@Si16 nanoclusters and their geometrical changes due to thermal annealing. (a) Left and right panels, respectively, show wide-scale and magnified STM images taken at RT after depositing Ta@Si16 cations onto C60-terminated HOPG surface at ∼90 K. The imaging conditions (Vtip and It) were −2.2 V and 5 pA for the left panel and −2.3 V and 2 pA for the right panel, respectively. (b) STM line profile taken along the line segment A−B in panel a. (c) Histogram of the dot heights measured on a Ta@Si16-deposited C60/HOPG surface such as that shown in panel a. (d,e) Histograms of dot heights measured on the Ta@Si16-deposited C60/HOPG surfaces after thermal annealing at 433 and 508 K, respectively. The histograms shown in panels c−e were constructed by measuring the heights of 971, 1140, and 1196 dots, respectively. The imaging conditions (Vtip and It) for the inset images in panels c−e were −2.3 V and 2 pA, respectively.
Si(111)7 × 7 substrates were used. These substrates were functionalized with C60 monolayer films by depositing C60 molecules at room temperature (RT) in an ultrahigh vacuum (UHV).21 The HOPG surfaces were cleaned via thermal annealing at 770 K in an UHV prior to deposition of the C60 molecules. The Si(111)7 × 7 surfaces were prepared via annealing of Si(111) substrates at 923 K for 10 h followed by repeated annealing at 1500 K for 4 s. The C60 molecules were deposited at RT via thermal evaporation of C60 powder (purity: 99.95%) from a Ta crucible while maintaining a deposition rate of 0.03 ML/min, where 1 ML of C60 corresponds to 1.15 × 106 molecules/μm2. TanSim nanoclusters with various charge states were produced in a gas aggregation apparatus with a direct current magnetron sputtering source22,23 from a Ta−Si mixed target. Ta@Si16 cations were selectively created by fine-tuning the synthesis conditions. The Ta@Si16 cations were then massselected from the cationic TanSim species using a quadrupole mass filter and deposited on the substrates at a deposition temperature (Td) of ∼90 K with a typical deposition rate of ∼2.6 × 103 ions/μm2·min. The kinetic energy (Ek) for the Ta@ Si16 cations was maintained at the lowest level by applying an appropriate positive voltage to the substrate during deposition (typically ∼0.01 eV per atom). The samples were transferred into an analysis chamber while under vacuum and were evaluated using STM at RT in an UHV.
ductive surfaces due to changes in their geometries and charge states.17−20 Recently, we experimentally and theoretically demonstrated that monolayers of rare-gas-like Ta@Si16 cations can be created by densely immobilizing cations onto conductive surfaces terminated with C60 molecules while maintaining the symmetrical cage shape and charge state of the Ta@Si16 cations. This successful immobilization was realized by controlling the donor−acceptor interactions between the nanoclusters and the surfaces.17 In this system, the Ta@Si16 cation is once neutralized immediately after adsorption by accepting electrons from the C60-terminated conductive surface and then cationized again via electron transfer from a neutral Ta@Si16 nanocluster to acceptor-like C60 molecules. In addition, it is considered that another key factor determining the charge states of surfaceimmobilized Ta@Si16 nanoclusters is the adsorption structure of the nanoclusters on C60-terminated surfaces. Previous experimental results have suggested that each Ta@Si 16 nanocluster is chemically adsorbed on top of each C60 molecule via covalent Si−C bonds while maintaining its cage shape.17 This has been supported by theoretical calculations showing that there are some stable bonding structures between single Ta@Si16 nanoclusters and single C60 molecules. Herein, we have investigated the dependence of the changes in the geometry of surface-immobilized Ta@Si16 nanoclusters on the sample temperature and density of nanoclusters using STM, from which the dynamics in the thermally activated local positional transitions of Ta@Si16 nanoclusters initially immobilized onto C60-teminated surfaces was revealed. This result also indicates that Ta@Si16−C60 heterodimers are initial products in the deposition of Ta@Si16 cations onto C60terminated surfaces.
3. RESULTS AND DISCUSSION 3.1. STM Analysis of Ta@Si16 Deposited on C60/HOPG. The left and right panels in Figure 1a show wide-scale and magnified STM images of a C60-terminated HOPG surface taken at RT after depositing Ta@Si16 cations at a Td of 90 K, in which dot-shaped structures were sparsely created on the surface. Most of the dots had heights (hd) of ∼0.6 nm (measured from the top of the surrounding C60 molecules), as shown in the line profile taken along line segment A−B (Figure 1b). The distribution of the dot height was also confirmed from
2. EXPERIMENTAL SECTION All experiments were performed under vacuum conditions. Highly oriented pyrolytic graphite (HOPG), MoS2, and B
DOI: 10.1021/jp511157n J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 2. Geometrical models of Ta@Si16 nanoclusters immobilized on a C60-terminated HOPG surface. The left and right sides of the Figure show standing up and tilted Ta@Si16−C60 heterodimers, respectively.
Figure 3. Ta@Si16 nanoclusters immobilized on C60-terminated Si(111)7 × 7 surfaces. The left and right panels in panel a, respectively, show widescale and magnified STM images taken at RT after depositing Ta@Si16 cations onto the Si(111)7 × 7 surface at ∼90 K. The imaging conditions (Vtip and It) were 2.2 V and 5 pA, respectively. (b,c) Histograms of the dot heights measured on Ta@Si16-deposited C60/Si(111)7 × 7 surfaces such as that shown in panel a before and after thermal annealing at 773 K. The histograms shown in panels b and c were constructed by measuring the heights of 1720 and 1625 dots, respectively.
the height histogram (Figure 1c) constructed by measuring hd for about 1000 dots; a single peak clearly appeared at hd of ∼0.6 nm. This value is less than the theoretically predicted sizes for Ta@Si16 cations (0.89 to 0.95 nm).17 The height distribution of the dots changed depending on the postdeposition annealing temperature (Ta). Figure 1d,e shows histograms of hd measured after annealing the surface shown in Figure 1a at 433 and 508 K for 10 min and 30 s, respectively. With increasing Ta, an additional peak appeared at hd of ∼0.8 nm and became dominant in the height histograms. Note that the density of the dots was nearly unchanged after annealing, as shown in the insets in Figure 1c−e, indicating that the observed hd shift was not caused by aggregation of the surface-immobilized nanoclusters via surface migration. In addition, it was found that the adsorption sites for the dots on the C60/HOPG surfaces changed after thermal annealing (Supporting Information); the ratio of dots positioned on the atop sites of the C60 film was
