Highly ordered C60 monolayer self-assembled by using an iodine template on an Au(111) surface in solution

An iodine-modified Au(111) surface, (I/Au(111)), was used as a substrate to prepare a C 60 adlayer by self-organization in a benzene solution. A highly ordered C 60 adlayer was successfully prepared due to the moderate C 60-I/Au(111) interaction. Two lattice structures, (2 square root 3 x 2 square root 3) R30 degrees and p(2 x 2), were imaged for this C 60 adlayer. For the first structure, a featureless ball-like molecular shape was imaged, ascribed to the molecular rotation resulting from a symmetrical location between C 60 and iodine atoms. For the p(2 x 2) structure, the asymmetrical location of C 60 with respect to the iodine atoms freezes the C 60 molecules on the substrate, leading to a clear image of intramolecular structure. The intermediate iodine atoms in the C 60/I/Au(111) adlayer can be desorbed by electrochemically reduction without significantly affecting the ordering of the C 60 adlayer. However, the internal pattern of C 60 disappears in the absence of iodine.     

Electrostatically controlled nanostructure of cationic porphyrin diacid on sulfate/bisulfate adlayer at electrochemical interface

Two different cationic tetraphenyl porphyrins, one with two carboxyphenyl groups in cis-position and the other in trans-position (cis- and trans-H(4)DCPP(2+)), have been examined to control the structure of their 2D supramolecular assemblies in 0.05 M H(2)SO(4) at electrochemical interfaces. Electrochemical scanning tunneling microscopy (EC-STM) images revealed the formation of supramolecularly organized nanostructures of cis-H(4)DCPP(2+) such as dimer, trimer, and tetramer on the (square root(3) x square root(7)) sulfate/bisulfate adlayer, suggesting the importance of both electrostatic interaction between cationic porphyrin core and sulfate/bisulfate adlayer and the hydrogen bond formation between carboxyl groups of the nearest neighbor cationic porphyrins. Trans-H(4)DCPP(4+) ions were also found to be aligned in the square root(3) direction of the sulfate/bisulfate adlayer. The structure of these cationic porphyrin adlayers was found to depend upon the electrode potential; i.e., when the potential was changed in the negative direction, the (square root(3) x square root(7)) sulfate/bisulfate adlayer disappeared, and no ordered arrays were formed. In contrast, when 0.1 M HClO(4) was used as an electrolyte solution, only a disordered array was observed. The results of the present study indicate that the (square root(3) x square root(7)) sulfate/bisulfate adlayer formed on Au(111) in 0.05 M H(2)SO(4) plays a significant role as a nanorail template in the control of electrostatically assembled diacid porphyrin dicarboxylic acid derivative. In addition, the high-resolution STM clearly distinguished between cis-H(4)DCPP(2+) ion and cis-H(2)DCPP molecule. The cis-H(2)DCPP molecules on Au(111) provided an adlayer structure and an electrochemical behavior which are different from those of cis-H(4)DCPP(2+) ions.     

In situ stress and nanogravimetric measurements during underpotential deposition of bismuth on (111)-textured Au

The surface stress associated with the underpotential deposition (upd) of bismuth on (111)-textured Au is examined, using the wafer curvature method, in acidic perchlorate and nitrate supporting electrolyte. The surface stress is correlated to Bi coverage by independent nanogravimetric measurements using an electrochemical quartz crystal nanobalance. The mass increase measured in the presence of perchlorate is consistent with the (2 x 2) and (p x square root 3)-2Bi adlayers reported in the literature. ClO(4)(-) does not play a significant role in the upd process. The complete Bi monolayer causes an overall surface stress change of about -1.4 N m(-1). We attribute this compressive stress to the formation of Bi-Au bonds which partially satisfy the bonding requirements of the Au surface atoms, thereby reducing the tensile surface stress inherent to the clean Au surface. At higher Bi coverage, an additional contribution to the compressive stress is due to the electrocompression of the (p x square root 3)-2Bi adlayer. In nitric acid electrolyte, NO(3)(-) coadsorbs with Bi over the entire upd region but has little fundamental impact on adlayer structure and stress.     

Adlayers of C60-C60 and C60-C70 fullerene dimers formed on au(111) in benzene solutions studied by STM and LEED

Scanning tunneling microscopy (STM) and low-energy electron diffraction were used to reveal the structures of ordered adlayers of [2+2]-type C60-C60 fullerene dimer (C120) and C60-C70 cross-dimer (C130) formed on Au(111) by immersingit in abenzene solution containing C120 or C130 molecules. High-resolution STM images clearly showed the packing arrangements and the electronic structures of C120 and C130 on the Au(111) surface in ultrahigh vacuum. The (2 square root3 x 4square root3)R30 degrees, (2square root3 x 5square root3)R30 degrees, and (7 x 7) structures were found for the C120 adlayer on the Au(111) surface, whereas C130 molecules were closely packed on the surface. Each C60 or C70 monomer cage was discerned in the STM image of a C130 molecule.     

Two-dimensional supramolecular organization of copper octaethylporphyrin and cobalt phthalocyanine on Au(111): molecular assembly control at an electrochemical interface

Mixed adlayers of 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine copper(II) (CuOEP) and cobalt(II) phthalocyanine (CoPc) were prepared by immersing Au(111) substrate in a benzene solution containing CuOEP and CoPc molecules, and they were investigated in 0.1 M HClO(4) by cyclic voltammetry (CV) and in-situ scanning tunneling microscopy (STM). The composition of the mixed adlayer consisting of CuOEP and CoPc molecules was found to vary depending on the immersion time. CoPc molecules displaced CuOEP molecules during the modification process with increasing immersion time, and the CuOEP molecules were completely replaced with CoPc molecules in the mixed solution after a long modification time. The two-component adlayer consisting of CuOEP and CoPc, which has a structure with the constituent molecules arranged alternately, was found to form either a p(9 x 3(square root)7R - 40.9 degrees) or a p(9 x 3(square root)7R - 19.1 degrees) structure, each involving two molecules on the Au(111) surface. The surface mobility and the molecular reorganization of CuOEP and CoPc were accelerated by modulation of the electrode potential. Different surface structures were produced at different electrode potentials, and hence potential modulation should allow a precisely controllable phase separation to take place in aqueous HClO(4).     

Comparative investigation of underpotential deposition of Ag from aqueous and ionic electrolytes: An electrochemical and in situ STM study

Underpotential deposition (UPD) of Ag on Au(111) has been studied with two different electrolytes: aqueous 0.1 M H2SO4 solution in comparison with the ionic liquid 1-butyl-3-methylimidazolium chloride BMICl + AlCl3. Of particular interest is the distinct behavior of 2D phase formation at both interfaces, which has been investigated by cyclic and linear sweep voltammetry in combination with in situ electrochemical scanning tunneling microscopy (STM). It is found that one monolayer (ML) of Ag is formed in the UPD region in both electrolytes. In aqueous solution, atomically resolved STM images at 500 mV versus Ag/Ag+ show a (3 x 3) adlayer of Ag, whereas after sweeping the potential just before the commencement of the bulk Ag deposition, a transition from expanded (3 x 3) to pseudomorphic ML of Ag on Au(111) occurs. In BMICl-AlCl3, the first UPD process of Ag exhibits two peaks at 410 and 230 mV indicating that two distinct processes on the surface take place. For the first time, STM images with atomic resolution reveal a transition from an inhomogeneous to an ordered phase with a (square root of 3 x square root of 3)R30 degrees structure and an adsorption of AlCl4- anions having a superlattice of (1.65 x square root of 3)R30 degrees preceding the deposition of Ag.     

Adsorbate-geometry specific subsurface relaxation in the CO/Pt(111) system

A dramatic multilayer substrate relaxation is observed for the (square root 19 x square root 19)-13CO adlayer phase on a Pt(111) electrode by surface X-ray scattering. Within the (square root 19 x square root 19) unit cell, a vertical expansion of 0.28 A was determined for the Pt atoms under near-top-site CO molecules, whereas only 0.04 A was found under near-bridge-site CO molecules. The lateral displacements involve small rotations toward more symmetric bonding. Both the expansions and rotations extend into the bulk with a decay length of 1.8 Pt layers. This nonuniform layer expansion, hitherto unseen, appears to be a manifestation of the differential stress induced by CO adsorption at different sites.     

Surface phase transitions in one-dimensional channels arranged in a triangular cross-sectional structure: theory and Monte Carlo simulations

Monte Carlo simulations and finite-size scaling analysis have been carried out to study the critical behavior in a submonolayer lattice-gas of interacting monomers adsorbed on one-dimensional channels arranged in a triangular cross-sectional structure. Two kinds of lateral interaction energies have been considered: (1) w(L), interaction energy between nearest-neighbor particles adsorbed along a single channel and (2) w(T), interaction energy between particles adsorbed across nearest-neighbor channels. We focus on the case of repulsive transverse interactions (w(T)>0), where a rich variety of structural orderings are observed in the adlayer, depending on the value of the parameters k(B)Tw(T) (being k(B) the Boltzmann constant) and w(L)w(T). For w(L)w(T)=0, successive planes are uncorrelated, the system is equivalent to the triangular lattice, and the well-known ([square root] 3 x [square root] 3) [([square root] 3 x ([square root] 3)(*)] ordered phase is found at low temperatures and a coverage, theta, of 13. In the more general case (w(L)/w(T) not equal 0), a competition between interactions along a single channel and a transverse coupling between sites in neighboring channels leads to a three-dimensional adsorbed layer. Consequently, the ([square root] 3 x ([square root] 3) and (([square root] 3 x ([square root] 3)(*) structures "propagate" along the channels and new ordered phases appear in the adlayer. Each ordered phase is separated from the disordered state by a continuous order-disorder phase transition occurring at a critical temperature, T(c), which presents an interesting dependence with w(L)/w(T). The Monte Carlo technique was combined with the recently reported free energy minimization criterion approach (FEMCA) [F. Roma et al., Phys. Rev. B 68, 205407 (2003)] to predict the critical temperatures of the order-disorder transformation. The excellent qualitative agreement between simulated data and FEMCA results allows us to interpret the physical meaning of the mechanisms underlying the observed transitions.     

Electrodeposition of Au monolayer on Pt(111) mediated by self-assembled monolayers

In-situ scanning tunneling microscopy (STM), cyclic voltammetry (CV), and infrared reflection-adsorption spectroscopy (IRRAS) have been used to examine the electrodeposition of gold onto Pt(111) electrodes modified with benzenethiol (BT) and benzene-1,2-dithiol (BDT) in 0.1 M HClO4 containing 10 microM HAuCl4. Both BT and BDT were attached to Pt(111) via one sulfur headgroup. STM and IRRAS results indicated that the other SH group of BDT was pendant in the electrolyte. Both BT and BDT formed (2 x 2) structures at the coverage of 0.25, and they were transformed into (square root(3) x square root(3))R30 degrees as the coverage was raised to 0.33. These two organic surface modifiers resulted in 3D and 2D gold islands at BT- and BDT-coated Pt(111) electrodes, respectively. The pendant SH group of BDT could interact specifically with gold adspecies to immobilize gold adatoms on the Pt(111) substrate, which yields a 2D growth of gold deposition. Molecular resolution STM revealed an ordered array of (6 x 2 square root(13)) after a full monolayer of gold was plated on the BDT/Pt(111) electrode. Since BDT was strongly adsorbed on Pt(111), gold adatoms only occupied free sites between BDT admolecules on Pt(111). This is supported by a stripping voltammetric analysis, which reveals no reductive desorption of BDT admolecules at a gold-deposited BDT/Pt(111) electrode. It seems that the BDT adlayer acted as the template for gold deposit on Pt(111). In contrast, a BT adlayer yielded 3D gold deposit on Pt(111). This study demonstrates unambiguously that organic surface modifiers could contribute greatly to the electrodeposition of metal adatoms.     

Oriented organic islands and one-dimensional chains on a Au(111) surface fabricated by electrodeposition: an STM study

Organic islands and oriented one-dimensional (1D) chains are fabricated on a Au(111) surface by electrodeposition. The cyclic voltammograms (CVs) of Au(111) in solutions containing nitrobenzene and picric acid show an electrochemical reaction in a negative potential region, which results in irreversible reductive deposition. The deposition process is monitored by in situ electrochemical scanning tunneling microscopy (ECSTM). At the double layer potential region, for example, nitrobenzene molecules form a well-defined adlayer in a (square root(3) x square root(3)) structure. With potential shifting negative to the reductive region, nitrobenzene is reduced to hydroxyaminobenzene. Organic islands were formed first and then aggregated into ordered 1D chains. The formation of these organic islands and 1D chains is completely potential-dependent. Intriguingly, the so-prepared islands and 1D chains are well-oriented along the reconstructed lines of the underlying Au(111) substrate and stable under ambient conditions even if the sample was removed from electrolyte solution. The results reported here provide a simple and effective method to fabricate oriented organic nanodots and nanowires on a solid surface by an electrochemical technique.     

On the Spreading of Glycerol Trioleate

The benzoquinone/hydroquinone (Q/H(2)Q) redox reaction has been studied by electrochemical-scanning tunneling microscopy (EC-STM) at a Pd(111)-(square3xsquare3)R30 degrees -I electrode surface in a solution that contained 10(-4) M H(2)Q in 0.05 M H(2)SO(4); iodine-pretreatment of the Pd(111) surface was to prevent chemisorption of organic-derived species. The molecule-resolved EC-STM images indicated that: (i) at a potential where only H(2)Q is present in solution, a self-assembled (square21xsquare21)R10.9 degrees -eta(6)-H(2)Q monolayer is produced in which the H(2)Q molecules are oriented parallel to the surface; (ii) at a potential where partial oxidation (to Q) occurs, a self-assembled (square21xsquare21)R10.9 degrees -eta(6)-QH adlayer is generated, where QH represents quinhydrone, an equimolar mixture of Q and H(2)Q; in this structure, the Q and H(2)Q molecules are oriented vertically, face-to-face, and arranged alternately along a given row, reminiscent of the crystal structure of quinhydrone. The partial oxidation-induced molecular reorientation, which is reversible, most likely arises from favorable Q-H(2)Q face-to-face interactions; that is, complete oxidation would yield only flat-oriented Q species. Unfortunately, at potentials where only Q would be present in solution, I-catalyzed corrosion of the Pd starts to occur, which leads to noise-laden EC-STM images. Copyright 2001 Academic Press.     

Scanning tunneling microscopy of sulfur and benzenethiol chemisorbed on Ru(0001) in 0.1 M HClO4

In situ scanning tunneling microscopy (STM) combined with linear sweep voltammetry was used to examine spatial structures of sulfur adatoms (SA) and benzenethiol (BT) molecules adsorbed on an ordered Ru(0001) electrode in 0.1 M HClO4. The Ru(0001) surface, prepared by mechanical polishing and electrochemical reduction at -1.5 V (vs RHE) in 0.1 M HClO4, contained atomically flat terraces with an average width of 20 nm. Cyclic voltammograms obtained with an as-prepared Ru(0001) electrode in 0.1 M HClO4 showed characteristics nearly identical to those of Ru(0001) treated in high vacuum. High-quality STM images were obtained for SA and BT to determine their spatial structures as a function of potential. The structure of the SA adlayer changed from (2 x mean square root of 3)rect to domain walls to (mean square root of 7 x mean square root of 7)R19.1 degrees and then to disordered as the potential was scanned from 0.3 to 0.6 V. In contrast, molecules of BT were arranged in (2 x mean square root of 3)rect between 0.1 and 0.4 V, while they were disordered at all other potentials. Adsorption of BT molecules was predominantly through the sulfur headgroup. Sulfur adatoms and adsorbed BT molecules were stable against anodic polarization up to 1.0 V (vs RHE). These two species were adsorbed so strongly that their desorption did not occur even at the onset potential for the reduction of water in 0.1 M KOH.     

Scanning tunneling microscopy, spectroscopy, and nanolithography of epitaxial graphene chemically modified with aryl moieties

The reduction of diazonium salts has recently been proposed as a robust covalent modification scheme for graphene surfaces. While preliminary studies have provided indirect evidence that this strategy decorates graphene with aryl moieties, the molecular ordering and conformation of the resulting adlayer have not been directly measured. In this Article, we report molecular-resolution characterization of the adlayer formed via the spontaneous reduction of 4-nitrophenyl diazonium (4-NPD) tetrafluoroborate on epitaxial graphene on SiC(0001) using ultrahigh vacuum (UHV) scanning tunneling microscopy (STM) and spectroscopy (STS). An atomically flat inhomogeneous layer of covalently bonded organic molecules is observed after annealing the chemically treated surface at ～500 °C in UHV. STM and STS results indicate that the adlayer consists predominantly of aryl oligomers that sterically prevent uniform and complete covalent modification of the graphene surface. The adsorbed species can be selectively desorbed by the STM tip above a threshold sample bias of -5 V and tunneling current of 1 nA, thus enabling the fabrication of a diverse range of graphene nanopatterns at the sub-5 nm length scale.     

The reactive chemisorption of alkyl iodides at Cu(110) and Ag(111) surfaces: a combined STM and XPS study

The chemisorption of methyl and phenyl iodide has been studied at Cu(110) and Ag(111) surfaces at 290 K with STM and XPS. At both surfaces dissociative adsorption of both molecules leads to chemisorbed iodine, with the STM showing c(2 x 2) and (square root 3 x square root 3)R30 structures at the Cu(110) and Ag(111) surfaces, respectively. At the Cu(110) surface a comparison of coexisting c(2 x 2) I(a) and p(2 x 1) O(a) domains shows the iodine adatoms to be chemisorbed in hollow sites with evidence at low coverage for diffusion in the (110) direction. In the case of methyl iodide no carbon adsorption is observed at either the silver or the copper surfaces, but chemisorbed phenyl groups are imaged at the Cu(110) surface after exposure to phenyl iodide. The STM images show the phenyl groups as bright features approximately 0.7 nm in diameter and 0.11 nm above the iodine adlayer, reaching a maximum surface concentration after approximately 6 Langmuir exposure. However, the phenyl coverage decreases with subsequent exposures to PhI and is negligible by approximately 1000 L exposure, consistent with the formation and desorption of biphenyl. The adsorbed phenyls are located above hollow sites in the substrate, they are stabilized at the top and bottom of step edges and in paired chains (1.1 nm apart) on the terraces with a regular interphenyl spacing within the chains of 1.0 nm in the (110) direction. The interphenyl ring spacing and diffusion of individual phenyls from within the chains shows that the chains do not consist of biphenyl species but may be a precursor to their formation. Although the XPS data shows carbon present at the Ag(111) surface after exposure to PhI, no features attributable to phenyl groups were observed by STM.     

Redox transformations of adsorbed NO molecules on a Pt(100) electrode

The electrochemical behavior of adsorbed NO molecules on a Pt(100) electrode has been studied in perchloric acid solutions by means of cyclic voltammetry. According to the literature data, a saturated NO adlayer with a coverage of ～0.5 monolayers (MLs) is formed under open circuit conditions in an acidic nitrite solution as a result of a disproportionation reaction. The saturated adlayer is stable in the potential range of 0.4–0.9 V vs. a reversible hydrogen electrode in 0.1 M HClO₄. NO molecules are oxidized at 0.9–1.1 V with the formation of adsorbed nitrite anions, and they can be reduced to ammonia at potentials less than 0.4 V. In this paper it has been shown that the adlayer stability depends on the surface coverage and extent of ordering. An unsaturated NO adlayer demonstrates NO ? NH₃ redox transformations at 0.5–0.8 V.     

Diamond surface conductivity under atmospheric conditions: theoretical approach

The electron transfer from an H-terminated diamond (100)-2 x 1 surface to a neutral or acidic water adlayer has been theoretically investigated, using quantum mechanical DFT calculations under periodic boundary conditions. A surface conductivity of p-type was found to be induced by the acidic environment. An electron transfer of 1.8 electrons per surface unit cell was observed to take place from the upper part of the diamond valence band to the lowest unoccupied molecular level of the aqueous adlayer that contains one H(3)O(+) ion. The result is a hole delocalized over the whole diamond model slab. Also, a pronounced weakening of the H(3)O(+) bonds by the interaction with the diamond surface is observed.     

Surface reorganization accompanying the formation of sulfite and sulfate by reaction of sulfur dioxide with oxygen on Ag111

On the Ag(111)-p(4x4)-O surface SO2(g) reacts with oxygen according to SO2(g)+O(a)-->SO3(a). Sulfite forms in a (2 radical3x2 radical3)R30 degrees structure. The restructuring of the surface atoms during sulfite formation is indicative of the deconstruction of the p(4x4)-O structure. Heating the sulfite-covered surface to 700 K affects the disproportionation of SO3 to SO4 in a (4 square root of 3 x square root of 3)R30 degrees structure accompanied by the desorption of SO2(g) and smoothing of the surface. Continued heating beyond 700 K affects the complete decomposition of sulfate to SO2(g) and O2(g).     

X-ray emission spectroscopy of (2 square root of 3 x 2 square root of 3)R30 degrees CO/Ru(0001): comparison to c(2x2)CO/Ni(100) and c(2x2)CO/Cu(100)

The atom specific electronic structure of (2 square root of 3 x 2 square root of 3)R30 degrees CO on hcp Ru(0001) has been determined with resonantly excited x-ray emission spectroscopy. We find that the general features of the local adsorbate electronic structure are similar to the situation of CO adsorbed on the fcc metals Ni(100) and Cu(100). The interpretation of the surface chemical bond of (2 square root of 3 x 2 square root of 3)R30 degrees CO/Ru(0001) based on the direct application of the local, allylic model from on-top adsorption on the fcc(100) surfaces Ni(100) and Cu(100) explains many aspects of the surface chemical bond. However, also nonlocal contributions like adsorbate-adsorbate interaction and the deviation from upright on-top adsorption on the Ru(0001) surface influence observables like the heat of adsorption and the Me-CO bond strength.     

Surface order-disorder phase transitions and percolation

In the present paper, the connection between surface order-disorder phase transitions and the percolating properties of the adsorbed phase has been studied. For this purpose, four lattice-gas models in the presence of repulsive interactions have been considered. Namely, monomers on honeycomb, square, and triangular lattices, and dimers (particles occupying two adjacent adsorption sites) on square substrates. By using Monte Carlo simulation and finite-size scaling analysis, we obtain the percolation threshold theta(c) of the adlayer, which presents an interesting dependence with w/k(B)T (w, k(B), and T being the lateral interaction energy, the Boltzmann constant, and the temperature, respectively). For each geometry and adsorbate size, a phase diagram separating a percolating and a nonpercolating region is determined.     

Square, hexagonal, and row phases of PTCDA and PTCDI on Ag-Si(111)square root(3) x square root(3)R30 degrees

We have investigated the ordered phases of the perylene derivatives perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA) and the imide analogue PTCDI on the Ag-Si(111)square root(3) x square root(3)R30 degrees surface using scanning tunneling microscopy. We find that PTCDA forms square, hexagonal, and herringbone phases, which coexist on the surface. The existence of a square phase on a hexagonal surface is of particular interest and is a result of a near commensurability between the molecular dimensions and the surface lattice. Contrast variations across the square islands arise from PTCDA molecules binding to different sites on the surface. PTCDI on Ag-Si(111)square root(3) x square root(3)R30 degrees forms extended rows, as well as two-dimensional islands, both of which are stabilized by hydrogen bonding mediated by the presence of imide groups. We present models for the molecular arrangements in all these phases and highlight the role of hydrogen bonding in controlling this order.