Thermal and ion-induced surface reactions of 1,1-difluoroethylene on Si(111)7 x 7 and vitreous SiO2

Thermal and ion-induced reactions of 1,1-difluoroethylene (1,1-C2H2F2 or iso-DFE) on Si(111)7 x 7 and vitreous SiO2 surfaces have been investigated by vibrational electron energy loss spectroscopy and thermal desorption spectrometry. Like ethylene, iso-DFE predominantly chemisorbs via a [2 + 2] cycloaddition mechanism onto the 7 x 7 surface as a di-sigma-bonded difluoroethane-1,2-diyl adstructure, which undergoes H abstraction and defluorination, producing hydrocarbon fragments and SiF(x) (x = 1-3) upon annealing to >700 K. Ion irradiation of Si(111)7 x 7 in iso-DFE at 50 eV impact energy appears to substantially enhance the production of hydrocarbon fragments and SiF(x)(), leading to stronger SiF4 desorption products over an extended temperature range (400-900 K). The observed SiC and SiF(x) produced on the 7 x 7 surface by ion irradiation in iso-DFE are found to be similar to those obtained by ion irradiation in the fluoromethane homologues, CF4 and CH2F2. The production of higher relative concentrations for the larger SiF(x) and C2-containing fragments is evidently favored on the 7 x 7 surface. On a vitreous SiO2 surface, ion irradiation in iso-DFE, unlike that in CF4 and CH2F2, appears to produce less SiF(x) than that on the 7 x 7 surface, which indicates that surface O does not interact strongly with the C2-containing fragments. The presence or absence of a C=C bond and the relative F-to-C ratio of the sputtering gas could therefore produce important effects on the resulting surface products obtained by low-energy ion irradiation.     

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.     

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.     

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)).     

The interfacial properties of MgCl(2) thin films grown on Si(111)7 x 7

Photoelectron spectroscopy with synchrotron radiation and low energy electron diffraction (LEED) were used in order to study the MgCl(2)Si(111) system. At submonolayer coverage of MgCl(2), a new LEED pattern was observed corresponding to a (sqr rt 3 x sqr rt 3)R30 degrees overlayer superimposed on the underlying reconstructed Si(111)7 x 7. The surface species at this stage are mainly molecular MgCl(2) and MgCl(x) (x<2) or MgO(x)Cl(y) attached to the Si substrate through Cl bridges coexisting with monodentate SiCl. The interfacial interaction becomes more pronounced when the submonolayer coverage is obtained by annealing thicker MgCl(2) layers, whereby desorption of molecular MgCl(2) is observed leaving on the nonreconstructed silicon surface an approximately 0.2 ML thick MgCl(x) layer which again forms the (sqr rt 3 x sqr rt 3 )R30 degrees superstructure.     

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.     

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.     

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.     

Cycloaddition reaction of 1,3-butadiene with a symmetric Si adatom pair on the Si(111)7x7 surface

We first found experimentally a cycloaddition reaction of a molecule on a symmetry Si pair, 1,3-butadiene on the Si adatom pair of Si(111)7x7, while up to now only asymmetric Si pairs were reported to be involved in cycloaddition reactions on Si surfaces. As the symmetry of a Si pair is expected to influence significantly a cycloaddition product and a reaction pathway, the [4+2]-like cycloaddition product of 1,3-butadiene on the Si adatom pair is suggested to form through a concerted reaction pathway in comparison to a stepwise reaction pathway, which is favorable in the formation of the [4+2]-like cycloaddition product on the asymmetric Si pair (the Si adatom-restatom pair).     

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.     

The chemisorption of coronene on Si(001)-2 x 1

Coronene (C24H12) adsorption on the clean Si(001)-2 x 1 surface was investigated by scanning tunneling microscopy and by density-functional calculations. The coronene adsorbed randomly at 25 degrees C on the surface and did not form two-dimensional islands. The scanning tunneling microscopy measurements revealed three adsorption sites for the coronene molecule on the Si(001) surface at low coverage. The major adsorption configuration involves coronene bonding to four underlying Si atoms spaced two lattice spacings apart in a dimer row. The two minor adsorption configurations involve asymmetrical bonding of a coronene molecule between Si dimer rows and form surface species with a mirror plane symmetry to their chiral neighbor species. The two minor bonding arrangements are stabilized by a type-C defect on the Si(001) surface.     

Attachment of styrene and phenylacetylene on Si(111)-7 x 7: the influence of substitution groups on the reaction mechanism and formation of pi-conjugated skeletons

The interactions of styrene and phenylacetylene and their isotope substitutions with a Si(111)-7 x 7 surface have been studied as model systems to mechanistically understand the chemical binding of conjugated pi-electron systems to di-radical-like silicon dangling bonds of the adjacent adatom-rest atom pair. Vibrational studies show that styrene mainly binds to the surface through a diradical reaction involving both the external C=C and its conjugated internal C=C of the phenyl ring with an adjacent adatom-rest atom pair, forming a 5-ethylidene-1,3-cyclohexadiene-like skeleton. On the other hand, phenylacetylene was shown to be covalently attached to Si(111)-7 x 7 through the external C[triple bond]C, forming a styrene-like conjugation system. These experimental results are consistent with density functional theory calculations. The different binding mechanisms for styrene and phenylacetylene clearly demonstrate that reaction channels for multifunctional organic molecules are strongly dependent on the chemical and physical properties of the functional groups. The resulting pi-electron conjugation structures may possibly be employed as intermediates for further organic syntheses and fabrication of multilayer organic films on semiconductor surfaces.     

Si-C(N) sigma linkages and N --> Si dative bonding at pyridine/Si(111)-7 x 7

We experimentally demonstrated that pyridine/Si(111)-7 x 7 can act as an electron donor/acceptor pair as a result of the charge transfer from the electron-rich N atom of pyridine to the electron-deficient adatom of the Si surface, evidenced by the upshift of 1.8 eV (state A) for the N(1s) core level upon the formation of a datively bonded complex compared to physisorbed molecules. Another state (B) whose N(1s) binding energy downshifts by 1.2 eV was assigned to an adduct through Si-C and Si-N covalent linkages, formed via a [4 + 2]-like addition mechanism on Si(111)-7 x 7. Binding molecules through the formation of the dative bond resulted from significant electron transfer opens a new approach for the creation of Si-based molecular architectures and modification of semiconductor interfacial properties with unsaturated organic molecules.     

Photon-stimulated desorption of F(-) ions from CF(3)Cl adsorbed on Si(111)-7x7

We report the photon-stimulated desorption of negative ions induced by direct dipolar dissociation and dissociative electron attachment. The photon-stimulated desorption of F(-) ions from CF(3)Cl physisorbed on a Si(111)-7x7 surface at 30 K in the photon energy range 12-35 eV was studied. The F(-) ion yield exhibits four resonances, at 12.8, 16.2, 19.5, and 22.3 eV, quite unlike the gas phase photodissociation cross section. The intensities of these resonances depend strongly on the CF(3)Cl coverage in a manner which varies from peak to peak. The resonances at 19.5 and 22.3 eV, which have a significant enhancement in the monolayer regime, are due to electron mediated dipolar dissociation of adsorbed CF(3)Cl molecules. The enhancement is attributed to surface electron attachment following molecular excitation. A significant enhancement in the monolayer regime has also been observed for the resonances at 12.8 and 16.2 eV. These two resonances are ascribable to a combination of electron mediated dipolar dissociation and dissociative electron attachment driven by photoelectrons generated in the neighboring molecules.     

STM study of the conformation and reaction of long-chain haloalkanes at Si(111)-7 x 7

Scanning tunneling microscopy (STM) has been used to study the adsorption of 1-fluoro-, 1-chloro-, and 1-bromo-substituted C(12) alkanes at the Si(111)-7 x 7 surface, at temperatures from 300 to 500 K. We report self-assembly of these physisorbed adsorbates, C(12)H(25)X, to form approximately circular corrals, (C(12)H(25)X)(2), with charge transfer to a corralled adatom in each case (cf. Dobrin et al. Surf. Sci. 2006, 600, L43). The corrals comprised pairs of semicircular horizontal long-chain molecules stable to approximately 100 degrees C. At > or =150 degrees C, the corrals desorbed or reacted locally to imprint a halogen atom, X-Si, and an adjacent alkane residue, R-Si. The corral height profiles, together with the location of the imprinted X-Si resulting from thermal or electron-induced surface reaction, led to a picture of the molecular configurations in these haloalkane corrals, (C(12)H(25)X)(2), X = F, Cl, Br, and the dichloro corrals, 1,12-dichlorododecane, (ClC(12)H(24)Cl)(2).     

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.     

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.     

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.     

Purely site-specific chemisorption and conformation of trimethylamine on Si(100)c(4 x 2)

One of the fundamental points of interest on the Si(100) surface is how the spatial localization of electron density on the buckled silicon dimer controls the site-specific reaction toward different Lewis acid and Lewis base molecules. We have investigated the adsorption of trimethylamine (TMA) on Si(100)c(4x2) using scanning tunneling microscopy (STM) at 80 K. The adsorbed TMA appears as a triangle-shaped bright protrusion in the occupied-state STM image. The triangle-shaped protrusion is ascribed to three methyl groups in the adsorbed TMA. The center of the protrusion is located on the down atom site, which indicates that the adsorption of TMA occurs only on the down dimer atom. Thus, TMA adsorption on Si(100)c(4x2) is found to be purely site-specific on the down dimer atom and can be categorized in Lewis acid-base reaction.