Unisized two-dimensional platinum clusters on silicon(111)-7x7 surface observed with scanning tunneling microscope

Uni-sized platinum clusters (size range of 5-40) on a silicon(111)-7 x 7 surface were prepared by depositing size-selected platinum cluster ions on the silicon surface at the collision energy of 1.5 eV per atom at room temperature. The surface thus prepared was observed by means of a scanning tunneling microscope (STM) at the temperature of 77 K under an ambient pressure less than 5 x 10(-9) Pa. The STM images observed at different cluster sizes revealed that (1) the clusters are flattened and stuck to the surface with a chemical-bond akin to platinum silicide, (2) every platinum atom occupies preferentially the most reactive sites distributed within a diameter of approximately 2 nm on the silicon surface at a cluster size up to 20, and above this size, the diameter of the cluster increases with the size, and (3) the sticking probability of an incoming cluster ion on the surface increases with the cluster size and reaches nearly unity at a size larger than 20.     

Dissociation mechanism of 2-propanol on a Si(111)-(7x7) surface studied by scanning tunneling microscopy

Adsorption of 2-propanol, (CH3)2CHOH, on a Si(111)-7x7 surface was studied by scanning tunneling microscopy. (CH3)2CHOH adsorbs equally on the faulted and unfaulted half unit cells by forming Si-OCH(CH3)2 and Si-H on an adatom and rest atom pair. Si-OCH(CH3)2 is consecutively increased in each half unit cell, and the adsorption is saturated when every half unit cell has three Si-OCH(CH3)2, which corresponds to 0.5 of the adatom coverage. The sticking probability for the dissociation of (CH3)2CHOH is independent of the adatom coverage from 0 to 0.4, but it depends on coverage at higher than 0.4. By counting the darkened adatoms, Si-OCH(CH3)2 on the center adatom (m) and that on the corner adatom (n), it was found the m/n ratio is ca. 4 for the first dissociation of (CH3)2CHOH in virgin half unit cell, but it becomes ca. 1.9 and 1.8 when two and three Si-OCH(CH3)2 are contained in a half unit cell. This result reveals that the dissociation probability of (CH3)2CHOH at the adatom-rest atom pair site is influenced by the nearest Si-OCH(CH3)2 in the half unit cell.     

Intact size-selected Au(n) clusters on a TiO2(110)-(1 x 1) surface at room temperature

Experiments in which mass-selected gold clusters were deposited on a surface have found that the catalytic properties depend strongly on cluster size. However, these experiments have not established definitively that the clusters maintain their size after deposition. We report here work in which we deposit low kinetic energy, mass-selected Aun+ (n = 1-8) clusters on a rutile TiO2(1 x 1) surface and use ultrahigh vacuum scanning tunneling microscopy (UHV-STM) to determine their size and shape.     

Secondary oxidation product on Si(111)-(7 x 7) characterized by isotope-labeled vibrational spectroscopy

The reaction of O(2) with Si(111)-(7 x 7) has been studied by electron energy-loss spectroscopy at 82 K. In addition to the losses due to Si-O-Si configurations, we observed two Si-O stretch modes depending on the coverage. A 146-meV peak appears at the initial reaction stage and was ascribed to a metastable product with one oxygen atom bonding on top of Si adatom and the other inserted into the backbond. The initial product is further oxidized to produce the second Si-O stretch peak at 150 meV. The secondary product was partially substituted with isotopes and analyzed with a simple model of coupled oscillators. The vibrational spectra reflect dynamical couplings between the isotopes, which is consistent with those predicted from the tetrahedral SiO(4) structure with one on top and three inserted oxygen atoms.     

Landing of size-selected Agn+ clusters on single crystal TiO2 (110)-(1x1) surfaces at room temperature

Mass-selected Ag(n) (+) (n=1,2,3) clusters with impact energy less than 2 eV per atom were deposited from the gas phase onto rutile titania (110)-(1x1) single crystal surfaces at room temperature and imaged using ultra-high vacuum scanning tunneling microscopy. Upon reaching the surface, Ag monomers sintered to form three-dimensional islands of approximately 50 atoms in size, with an average measured height of 7.5 A and diameter of 42 A. This suggests that the monomers are highly mobile on the titania surface at room temperature. Dimers also sintered to form large clusters upon deposition, approximately 30 atoms in size, with an average height of 6.2 A and diameter of 33 A. Clusters formed from monomer deposition appeared approximately three times more frequently at step edges than clusters formed from dimer deposition, indicating that the surface mobility of deposited monomers is higher than that of deposited dimers. In sharp contrast to the deposition of monomers and dimers, the deposition of trimers resulted in a high density of very small clusters on the order of a few atoms in size, indicative of intact trimers on the surface, implying that deposited trimers have very limited mobility on the surface at room temperature.     

Molecular anchor Cu-S formed on a thiophene mediated Si(111)-(7x7) surface

Thiophene molecule selectively binds to the adjacent adatom-rest atom pair on the Si(111)-(7x7) surface through its alpha-carbon atoms, leading to the covalent attachment of a C-S-C linkage and remaining C=C (beta-carbon) bond onto the surface. Photoemission studies show that Cu atom readily adsorbs onto the S atom of the functional group to form the Cu-S molecular anchor in two forms: one points away from the thiophene C=C group; the other points toward the C=C group.     

Adsorption kinetics and patterning of a Si(111)-7 x 7 surface by dissociation of methanol

CH(3)OH undergoes dissociation on a Si(111)-7 x 7 surface via a two dimensionally free precursor. The sticking probability attained by the STM (scanning tunneling microscopy) was entirely coverage independent, where the observed image represented the final state of the adsorption. CH(3)OH dissociates equally on the faulted and unfaulted halves at room temperature. However, the dissociation at the center adatom-rest atom site is four times preferential to that at the corner adatom-rest atom site in each half unit cell. Such site selectivity, center/corner, changes with the occupation of adatoms in corresponding half unit cell, that is, center/corner=4 for the half unit cell with one reacted adatom, but 2.6 and 1.8 for the half unit cells with two and three reacted adatoms, respectively. Such site selectivity is well rationalized by the dissociation depending on the local conformation of the site instead of the local density of states (LDOS). The site selectivity of center/corner is well reproduced by considering the occurrence probability of the whole dissociation pattern. As the STM image represents the final state of the adsorption, if the final step of adsorption involves dissociation of molecule or precursor, the STM image reflects the dissociation probability depending on the local structure. On the other hand, if no dissociation of molecule or precursor is involved at the final step, the adsorption probability might depend on the LDOS. The adsorption of H(2)S, H(2)O, and NH(3) is also discussed from this general viewpoint of adsorption. The concept of a two dimensionally free precursor will be important to understand the kinetics of heterogeneous catalysis.     

Molecular dynamics of haloalkane corral formation and surface halogenation at Si(111)-7 x 7

Long-chain organic molecules, 1-halododecane, RX (X = Cl,Br), adsorbed on Si(111)-7 x 7 were shown to form stable dimeric corrals; type I around corner holes and type II around corner adatoms S. Dobrin et al. [Surf. Sci. Lett. 600, L43 (2006)]. Here we examine the molecular dynamics of corral formation, in which mobile physisorbed adsorbates spontaneously convert to immobile. At high coverage the mechanism gives evidence of involving collisions between mobile vertical monomers, giving types I and II immobile horizontal dimers, vD +vD -->h2 (I, II). At low coverage mobile vertical monomers collide with immobile horizontal ones to form largely type-II corrals, vD + h-->h2 (II). Thermal reaction of corrals with X = Br brominates the surface by two distinct molecular pathways, thought to have more general applicability: "daughter-mediated" reaction of vertical v(A) with a low activation energy (here Ea approximately 5 kcal mol(-1)) and "parent-mediated" reaction of horizontal h or h2 with high activation energy (here Ea = 29 kcal mol(-1)).     

Dissociation of N2 on a Si(111)-7x7 Surface at Room Temperature

We demonstrate that the strong N₂ bond can be efficiently dissociated at low pressure and ambient temperature on a Si(111)-7x7 surface. The reaction was experimentally investigated by scanning tunnelling microscopy and X-ray photoemission spectroscopy. Experimental and density functional theory results suggest that relatively low thermal energy collision of N₂ with the surface can facilitate electron transfer from the Si(111)-7x7 surface to the π*-antibonding orbitals of N₂ that significantly weaken the N₂ bond. This activated N₂ triple bond dissociation on the surface leads to the formation of a Si₃N interface.     

Electronic charging of non-metallic clusters: size-selected Mo(x)S(y) clusters supported on an ultrathin alumina film on NiAl(110)

Two photon photoemission was used to investigate the interfacial charge transfer for size-selected Mo(x)S(y) (x/y: 2/6, 4/6, 6/8, 7/10) clusters deposited on an ultrathin alumina film prepared on a NiAl(110) surface. The local work function of the surface increases with increasing cluster coverage, which is unexpected for charge transfer resulting from the formation of Mo-O bonds between the clusters and the alumina surface. By analogy with Au atoms and clusters on metal-supported ultrathin oxide films, we invoke electron tunneling from the NiAl substrate to explain the charge transfer to the Mo(x)S(y) clusters. Electron tunneling is favored by the large electron affinities of the Mo(x)S(y) clusters and the relatively low work function induced by the presence of the alumina film. The interfacial dipole moments derived from coverage-dependent measurements are cluster dependent and reflect differences in Mo(x)S(y) cluster structure and surface bonding. These results extend previous observations of electronic charging to non-metallic clusters, specifically, metal sulfides, and suggest a novel way to modify the electronic structure and reactivity of nanocatalysts for heterogeneous chemistry.     

Electron correlation effects at semiconductor surfaces and interfaces: Si(111)-5x5, Si(111)-7x7 and Sn/Ge(111)

Electron correlation effects associated with the dangling bond surface states of Si(111)-5×5, Si(111)-7×7 and Sn/Ge(111)-3×3 are analyzed. In all the cases, extensive LDA-calculations are performed and effective two-dimensional Hamiltonians are deduced. Our analysis of these Hamiltonians shows that: (a) the Si(111)-5×5 surface states exhibits a metal-insulator transition; (b) the Si(111)-7×7 surface shows important similarities with the Si(111)-5×5 case, but it has a dangling bond surface band having a metallic character; (c) finally, the Sn/Ge(111)-3×3 dangling bond surface bands also shows important correlation effects that are found, however, not to affect the metallic character of the surface bands.     

Studies of chemisorbed tetracene on si(111)-7x7

The chemisorption of tetracene on the Si(111)-7x7 surface was studied using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. On the basis of the STM results and dimension analysis, two types of binding configurations were proposed. One of the configurations involves the di-sigma reaction between two C atoms of an inner ring with an adatom-rest atom pair on the substrate to give rise to an unsymmetrical butterfly structure. Tetracene in another configuration possesses four C-Si bonds that are formed via di-sigma reactions between the C atoms at the terminal rings with two center adatom-rest atom pairs within one-half of the surface unit cell. Besides, two other binding modes were proposed based on the dimension compatibility between the tetracene C and the substrate Si dangling bonds even though their identifications through the STM images are nonexclusive. Structural modeling and adsorption energies calculations were carried out using the DFT method. Factors affecting the relative thermodynamic stabilities based on the calculation results and the relative populations of tetracene in the different binding configurations as observed experimentally were discussed.     

Reaction of 1,2-dibromobenzene with the Si(111)-7x7 surface, a DFT study

Reaction between 1,2-dibromobenzene and the Si(111)-7x7 surface has been studied theoretically on the DFT(B3LYP/6-31G(d)) level. A 12-atom silicon cluster, representing two adatoms and one rest atom of the faulted half of the unit cell, was used to model the silicon surface. The first step of the reaction was a covalent attachment (chemisorption) of an intact 1,2-dibromobenzene molecule to the silicon cluster. Binding energies were calculated to be between 1.04 and 1.14 eV, depending on the orientation of the molecule. A second step of the reaction was the transfer of the Br atom to the silicon cluster. Activation energies for the transfer of the Br atom were calculated to be between 0.4 and 0.6 eV, suggesting that the thermal bromination reaction occurs on a microsecond time scale at room temperature. A third step of the reaction could be the transfer of the second Br atom of the molecule, the desorption of the organic radical, or the change of the adsorption configuration of the radical, depending on the original orientation of the adsorbed intact molecule. A novel, aromatic, two-sigma-bound adsorbed configuration of the C6H4 radical, in which a carbon ring of the radical is perpendicular to the silicon surface, has been introduced to explain previous experimental observations (Surf. Sci. 2004, 561, 11).     

Binding of glycine and L-cysteine on Si(111)-7 x 7

The adsorption of glycine and l-cysteine on Si(111)-7 x 7 was investigated using high-resolution electron energy loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS). The observation of the characteristic vibrational modes and electronic structures of NH3+ and COO- groups for physisorbed glycine (l-cysteine) demonstrates the formation of zwitterionic species in multilayers. For chemisorbed molecules, the appearance of nu(Si-H), nu(Si-O), and nu(C=Omicron) and the absence of nu(O-H) clearly indicate that glycine and l-cysteine dissociate to produce monodentate carboxylate adducts on Si(111)-7 x 7. XPS results further verified the coexistence of two chemisorption states for each amino acid, corresponding to a Si-NH-CH2-COO-Si [Si-NHCH(CH2SH)COO-Si] species with new sigma-linkages of Si-N and Si-O, and a NH2-CH2-COO-Si [NH2CH(CH2SH)COO-Si] product through the cleavage of the O-H bond, respectively. Glycine/Si(111)-7 x 7 and l-cysteine/Si(111)-7 x 7 can be viewed as model systems for further modification of Si surfaces with biological molecules.     

Adsorption-induced desorption of benzene on Si(111)-7 x 7 by substrate-mediated electronic interactions

The process of benzene adsorption on an adjacent adatom-rest atom pair on Si(111)-7 x 7 at room temperature was studied using in-situ scanning tunneling microscopy (STM). Both adsorption and desorption of benzene were observed to take place mostly at adjacent sites during the process. DFT calculation results show that the bond length between the rest atom and the carbon atom in a pre-adsorbed benzene molecule increases due to the charge transfer from a neighboring rest atom in response to an approaching benzene molecule. Such increase in the bond length, when coupled resonantly to the C-Si thermal vibration, could result in bond breakage and desorption of the adsorbate. The studies provide evidence for the desorption of a chemisorbed benzene caused by an adsorbing benzene at a neighboring site through a substrate-mediated electronic interaction.     

Selective attachment of 1,4-benzenedimethanethiol on the copper mediated Si(111)-(7 x 7) surface through S-Cu linkage

The well-defined and patterned copper clusters formed on the Si(111)-(7 x 7) surface have been employed as a template for selective binding of 1,4-benzenedimethanethiol (HS-CH2-C6H4-CH2-SH, 1,4-BDMT), to form ordered molecular nanostructures. Scanning tunneling microscopic (STM) studies showed that each 1,4-BDMT molecule preferentially binds to two neighboring copper atoms within one copper cluster through the S-Cu interaction with its molecular plane parallel to the surface, whereas some 1,4-BDMT bond to individually adsorbed copper atoms, resulting in an upright configuration. Large-scale two-dimensional molecular nanostructures can be obtained using this patterned assembly technique. Our experiments demonstrate the feasibility for controllable growth of ordered molecular nanostructures on the Si(111)-(7 x 7) surface.     

Controlled growth of Zn nano-dots on a Si(111)-7x7 surface saturated with C2H5OH

Metal atoms bonded with Si adatoms on the Si(111)-(7x7) surface undergo migration by hopping adjacent Si-rest atoms with dangling bond. By saturated adsorption of Si(111)-(7x7) surface with C(2)H(5)OH, the whole Si-rest atoms and a half of Si adatoms are occupied with Si-H and Si-OC(2)H(5), so that the Zn atoms adsorbed on this surface cannot migrate by hopping. When Zn atoms were deposited on this surface, ca. 5 nm Zn dots were grown in the hexagonal spacing of ca. 5.4 nm width around the corner holes, which work as a mold. This is quite different from the growth of honeycomb layers composed of Zn(3) clusters on the clean Si(111)-(7x7) surface. The dots grow up to nine (1.97 nm) to 13 layers (2.64 nm) by keeping their size, which implies a layer-by-layer growth of dots in the mold, where the growth is controlled by the kinetics instead of energetic feasibility.     

Scanning tunneling microscopy and spectroscopy studies of 4-methyl- 4'-(n-mercaptoalkyl)biphenyls on Au(111)-(1x1).

4-methyl-4'-(n-mercaptoalkyl)biphenyl (CH3-C6H4-C6H4-(CH2)n-SH, n=3-6, BPn) monolayers assembled on Au(111)-(1x1) in 1,3,5,-trimethylbenzene (TMB) at various temperatures are studied by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). High resolution STM images reveal that BP3 and BP5 form a (sqrt 3x2sqrt 3) repeating motif superimposed on a temperature-dependent Moire pattern. BP4 and BP6 adlayers are characterized by a coexisting (2sqrt 3x5sqrt 3) majority phase and a temperature-dependent (3xpsqrt 3) minority phase. Assembly at 60 degrees C or 90 degrees C leads to p=5. Compression of the adlayer was found at higher temperatures. Combined with high-resolution structure experiments, the electronic characteristics of BP3 and BP4 self-assembled monolayers (SAMs) were studied by monitoring current-distance (iT-Deltaz) and current-voltage (iT-Ebias) characteristics in TMB employing a gold STM tip|BPn|Au(111)-(1x1) configuration. The semilogarithmic (iT-Deltaz) plots yielded three linear regions in the range 10 pA相似文献