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The interaction of solvent molecules with metallic surfaces impacts many interfacial chemical processes. We investigate the chemical and structure evolution that follows adsorption of the polar solvent dimethylformamide (DMF) on Ag(111). An Ag(DMF)2 coordination complex forms spontaneously by DMF etching of Ag(111), yielding mixed films of the complexes and DMF. Utilizing ultrahigh vacuum scanning tunneling microscopy (UHV–STM), in combination with X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) computations, we map monolayer phases from the 2-D gas regime, consisting of a binary mixture of DMF and Ag(DMF)2, through the saturation monolayer limit, in which these two chemical species phase separate into ordered islands. Structural models for the near-square DMF phase and the chain-like Ag(DMF)2 phase are presented and supported by DFT computation. Interface evolution is summarized in a surface pressure–composition phase diagram, which allows structure prediction over arbitrary ex...

The desorption and interactions of ethylene carbonate (EC) and dimethyl carbonate (DMC) with clean and lithiated graphite substrates were measured by temperature-programmed desorption (TPD) and reaction (TPR) methods under UHV conditions. Both EC and DMC interact weakly with the clean C(0001) surface with adsorption energies of 0.60 ± 0.06 and 0.64 ± 0.05 eV, respectively. Addition of Li+ to the C(0001) substrate significantly increases the binding energies of molecular carbonates, and the range of measured values is indicative of EC solvation of lithium ions. EC undergoes complete decomposition on metallic Li films. Organolithium products were quantified by TPR, and the amount of lithium carbonate product was determined by detailed mass balance analysis. Decomposition of 1.5 L of EC resulted in the formation of 0.64 ± 0.12 L of lithium ethylene dicarbonate, 0.40 ± 0.05 L of lithium ethylene glycolate, and 0.5 ± 0.2 L of lithium carbonate. The branching ratio at the immediate EC–metallic lithium interface...

We report an ultrahigh vacuum scanning tunneling microscopy study of thermally driven interface rearrangement in binary films of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and zinc phthalocyanine (ZnPc), a model electron acceptor–electron donor system for organic solar cells. Neat PCBM films have been previously shown to undergo a transition from a disordered (glassy) phase to a crystalline hexagonal close-packed (hcp) arrangement above a critical packing density of 0.9 molecules/nm2. We now show how local PCBM density has a critical impact on binary film structure evolution. Bilayer films of PCBM and ZnPc undergo a spontaneous vertical phase separation to PCBM/ZnPc/Au(111) stacking at lower (

The molecular organization of a polar phthalocyanine, titanyl phthalocyanine (TiOPc), on the Ag(110) surface was studied with scanning tunneling microscopy (STM). In addition to the intermolecular hydrogen bonding and electrostatic interactions that guide structure for nonpolar Pc’s, we find evidence for the impact of dipole interactions on layer organization for TiOPc/Ag(110). TiOPc molecules adsorb on Ag(110) with a tilted orientation that contrasts with the flat-lying orientation typically observed for nonpolar phthalocyanines. At the highest TiOPc coverages, the layer consists of neighboring TiOPc molecules with opposing molecular tilts. Close inspection reveals a higher level organization, consisting of “zigzag” TiOPc molecular chains with molecular spacings along the chain that are smaller than the average within the layer. We suggest an explanation for this behavior based on the dipole–dipole interactions within the chain. At reduced coverage, variable densities of molecular vacancies form, dependi...

Molecular interfaces have been prepared in titanyl phthalocyanine (TiOPc)–C60 monolayer films on Ag(111) and characterized with ultra high vacuum–scanning tunneling microscopy. Two distinct TiOPc monolayer phases, namely, a hexagonal phase and a honeycomb phase, were first generated as molecular-film substrates for C60 growth. Both TiOPc monolayer structures rearrange upon C60 adsorption, yielding films with nanophase-separated TiOPc and C60 domains and a new cocrystalline TiOPc2(C60)1 honeycomb network. Kinetic access to the cocrystalline network is most facile from the hexagonal TiOPc monolayer, producing extended domains. Detailed models for emergent structural phases and molecular interfaces are presented. Mechanisms for C60-induced rearrangements are developed from coverage-dependent measurements.

Pentacene deposited onto a Ag(111) surface at 300 K is studied using scanning tunneling microscopy at temperatures of 300 and 50 K, providing structural insight into its unusual growth habit. At room temperature, an unexpectedly high pentacene coverage is needed to nucleate ordered pentacene islands, which appear surrounded by a disordered pentacene phase. These room temperature pentacene nuclei are revealed as bilayer structures from their coverage-dependent size evolution and molecularly resolved images of domain boundaries, recorded at 50 K. At this reduced temperature, two different monolayer phases with long-range order and commensurate with the Ag(111) surface lattice further emerge. These two monolayer phases exhibit comparable (0.7 vs 0.8 molecule/nm2) packing densities, but distinct intermolecular registration and alignment with respect to the silver sublattice.

The adsorption and molecular architecture of titanyl phthalocyanine (TiOPc) monolayer films on Ag (111) have been studied with scanning tunneling microscopy. Depending on deposition flux, TiOPc selectively forms three distinct ordered monolayer structures. At lower (

Molecular self-assembly of isonicotinic acid (INA) on Ag(111) was studied using STM and XPS. INA molecules organize into an ordered layer structure commensurate with the Ag(111) surface. A head-to-tail hydrogen-bond configuration was determined for the self-assembled structure based on STM observations and corroborated by XPS core-level shifts. The monolayer structure is closely related to the (100) surface of the INA crystalline solid. Monolayer domains, commensurate with the Ag(111) substrate, are sorted into two distinct enantiomeric domains, indexed as (1 −2, 3 6) and (3 2, 3 6) with respect to the Ag(111) surface lattice. Corresponding rotational domains are observed for each of the enantiomeric domains.

A small fraction of Ru-supported Pb crystallites, prepared by quenching from just below the melting point (∼600 K), is found in a nearly spherical metastable configuration that relaxes after an STM tip interaction. The metastable structure could not be reproduced by exposure to hydrogen, water, carbon monoxide or oxygen. However, AES measurements demonstrate the segregation of oxygen from the Pb/Ru interface to the surface of the continuous Pb film upon post-deposition annealing, suggesting surface oxygen as a possible origin of the metastability. The rapid relaxation of the metastable crystallites has been measured using STM, providing access to the early stages of relaxation where crystalline layers are removed on the time scale of seconds, in comparison with the time scales of minutes to hours for quenched structures. The shape evolution (facet radius vs. time) for different crystallites is consistent with predictions of shape-preserving relaxation from an initial Pokrovsky–Talapov shape for T

Using variable-temperature STM we studied the response of facetted 3D crystallites to temperature changes under volume preserving conditions. Defect-free micron-sized Pb crystallites preserve their initial facet size after a temperature increase from 0.64Tm to 0.8Tm (the melting point). In contrast, a temperature drop from Tm to 0.6Tm results in an increase in facet size, where material is transferred from the facets to the rounded regions of the crystal. A simple model suggests that the corresponding facet expansion stops due to a nucleation barrier, when the facet radius reaches approximately 75% of the equilibrium value. Use of the model to determine the energetic parameters governing the equilibrium crystal shape from non-equilibrium measurements is discussed.

Equilibrium crystal shapes exhibit flat facets and rough vicinal surfaces, with transitions between them being either continuous or discontinuous, the latter recognizable by a sharp edge. In general, mixed repulsive/attractive step–step interactions may lead to continuous or discontinuous facet-to-vicinal transitions. In can be shown that the contact angle at the facet for a discontinuous transition is directly related to the ratio of the step interaction strengths. Alternatively, surface reconstruction of facets can also be responsible for sharp edges at the facet boundary. In this case the contact angle is related to the difference between surface free energies of the reconstructed and unreconstructed facet as well as the corresponding difference of step interaction energies. Fitting the experimental shapes by theoretical expressions can be used to extract the relevant surface and step free energies and also step interaction energies. Experimental examples of Pb and Au equilibrium shapes are evaluated and discussed. Step free energies of vicinal Au(1 1 1) and Au(1 0 0) surfaces, evaluated by both models, are 30 and 10 meV/A 2 , respectively.

Scanning tunneling microscopy has proven to be a very powerful method for real space imaging of molecules on surfaces. Here we demonstrate how the morphological and chemical structure of the tunneling tip alters the apparent topography of STM images. When imaged with conventional W tips, isolated aluminum hydride (alane) oligomers appear as 2.8 A protrusions on Al(111). When imaged with alane-terminated tips, STM image contrast is both enhanced and inverted, with alane oligomers appearing as depressions. We attribute this image reversal to the electronic levels of tip-adsorbed alanes, whose distance from the surface Fermi levels reduces the coherence in the tunneling junction. The absence of a bias dependence in these images suggests predominant tunneling through tip-alane LUMO states.

Complementary scanning tunneling microscopy and surface infrared measurements show that H reacts strongly with Al(111), producing a variety of new alane (aluminum hydride) surface species. Alane oligomers, ranging in size from the monomer to 30-mers, form through a sequence of surface etching and condensation reactions. Atomic hydrogen initiates production by extracting aluminum atoms from the surface lattice to create mobile monohydride monomers (ad-Al H), which predominate in the low-coverage regime. At higher hydrogen coverages, multihydride oligomers form in coexistence with the ad-Al H. These alane oligomers are more thermally stable, remaining on the surface at room temperature, where they are directly imaged. The mass transfer of aluminum to surface alanes is discussed in relationship to alane stoichiometry.

On low index nickel surfaces, repulsive interactions between atomic oxygen and CO drive the phase separation of these species into oxygen-rich and CO-rich islands. Because these adsorbates interact differently with crystallographic steps, the size and the structure of these islands are modified on stepped surfaces. We have monitored coadsorption-induced changes in the distribution of CO with IRRAS, observing six different CO stretching bands which are assigned to distinct local chemisorption environments. When oxygen fully saturates sites along the step edge, the steps are completely blocked from CO adsorption and virtually all the CO population on the terraces shifts from atop to bridge sites. This terrace site shift is similarly accomplished by atomic oxygen chemisorbed at terrace sites. From these coadsorption-induced changes in CO site distributions, constrained by the 10 A terrace width, we conclude the through-metal OCO interaction responsible for this CO site shift must be operative over a range of 5 A. At θo = 0.18 ML, when oxygen occupies, but does not fully saturate the step edge, a new CO adsorption site is created, with a characteristic frequency of 1750 cm−1. This new site is assigned to CO bonded to kinks along the step edge based upon its low intensity (∼ geometric kink density), enhanced binding strength and sensitivity to oxygen coverage. At higher oxygen coverages, compression of the CO adlayer is observed, with CO shifting to asymmetric bridge sites. As saturation coverage is approached, CO occupies weakly bound sites in close proximity ( ∼ 3 A ) to O adatoms, with high characteristic frequencies of 2100 cm−1.

The presence of diffuse intensity in the LEED pattern of clean Ni(119), a stepped Ni(100) surface, indicates a disorder in the distribution of (100) terrace lengths. This diffuse intensity is attenuated upon adsorption of small amounts of oxygen, indicating an adsorbate-induced ordering of the surface, apparent at oxygen concentrations as low as θO = 0.03 ML. The physical origin of the oxygen-induced ordering is explored by considering two alternatives: an increase in repulsive interactions between surface steps and an inhibition of kink formation along step edges. The degree of surface ordering is found consistent with an increase in surface stress of 200 dyne cm or an increase in kink formation energy of just 40 meV. At an oxygen coverage matching the number of available step sites (θO ≈ 0.20 ML), a distinctive (9 × 1) oxygen overlayer pattern emerges which, upon additional oxygen exposure, transforms into the p(2 × 2) pattern typical of oxygen adsorption on Ni(100) at θO ≈ 0.25 ML. LEED “fingerprinting” of the I-V characteristics of both the (9 × 1)-O Ni (119) and p (2 × 2)-Ni (119) patterns was used to deduce the local oxygen chemisorption geometry. We conclude that oxygen in the (9 × 1) overlayer occupies 4-fold hollow sites near the step edge. Based upon this adsorption site, an oxygen-induced increase in the kink formation energy is the likely physical origin of the step ordering.

Surface chemical reactions often require a ready supply of substrate atoms to occur. In principle, steps serve as an efficient source of these atoms, provided that detachment rates from the step edges are sufficiently large. In this paper, we characterize atomic detachment rates from steps on clean Ag(110) by examining step fluctuations. We show that these rates are sufficient to supply atoms to form the added-row reconstruction of oxidized Ag(110) when the oxygen partial pressure is low. For high oxygen pressures, however, we find that step detachment rates are slow compared with oxidation rates, and the step source of Ag is supplemented by vacancy-island generation on the terraces. These results are compared to those obtained for the similar O/Cu(110) and O/Ni(110) systems.

The distribution of terrace widths on vicinal Ag(110) surfaces is measured with scanning tunneling microscopy as a function of mean terrace width 〈l〉 and interpreted in terms of a step-step interaction potential. As previously observed on vicinal Cu(100) surfaces, but in contrast to reports involving semiconductor surfaces, the distribution does not scale simply with 〈l〉. For vicinal Ag(110) surfaces with 〈l〉 = 22 A , the distribution resembles that expected for noninteracting steps. For 〈l〉 = 30 A , the distribution narrows significantly, suggesting repulsive interactions. For 〈l〉 = 40 A , the distribution has a form expected for attractive interactions. The absence of simple scaling of this distribution reveals that the decay of step-step interactions is more complicated than the l−2 behavior expected for elastic or dipolar repulsions. A nonmonotonic potential is found to reproduce the observed terrace-width distributions, as demonstrated with Monte Carlo simulations of a terrace-step-kink model. According to this trial potential, step interactions are dominated by a repulsive l−2 term at short l and by an RKKY-like oscillatory term at longer l. The form of the latter term is reminiscent of indirect interactions between adsorbates.

Step configurations on Ag(110) near pinning sites have been measured under ultrahigh vacuum conditions with scanning tunneling microscopy (STM). At isolated pinning sites the surface structure is generally characterized by a high density of monatomic steps (step bunch) on the “lower” side of the pinning site and a very large (> 1000 A) terrace on the “upper” side of the pinning site. The step bunch partially envelopes the pinning site, resulting in a curved interface between the step bunch and the upper terrace. We propose that the shape of the step bunches near these pinning sites reflects the local thermodynamic equilibrium between the surface tension of the curved step front and the pressure due to step-step interactions. These interactions are evaluated for steps with the same net orientation as the pinned steps via STM measurements of terrace width distributions. The distributions indicate that step-step interactions are predominantly entropie with a step stiffness of ~ kT /(5 A). The Gibbs-Thomson relation, together with these measurements of step-step interactions, are used to predict the curvature of the step bunch-upper terrace interface at the pinning site. The predicted radius of curvature of 3000 A for 50 A mean step separation agrees with experimental observations.

Using a combination of calculations from first principles and scanning tunneling microscopy, we have investigated file interplay between substrate-mediated and intermolecular interactions in the formation of high-density checkerboard binary superstructures from the codeposition of phenyl and perfluorophenyl Zn-phthalocyanines (ZnPc-F(16)ZnPc) on the Ag(111) Surface. The analysis of the electronic structure of the interface shows the essential role played by the Substrate in the formation of the molecular layer and opens the way toward the development of tailored surfaces for advanced supramolecular design.

Photoelectron spectroscopy was used to explore changes in Fermi level alignment, within the pi-pi* gap, arising from modifications to the coupling chemistry of conjugated phenylene ethynylene oligomers to the Au surface. Self-assembled monolayers were formed employing either thiol (4,4'-ethynylphenyl-1-benzenethiol or OPE-T) or isocyanide (4,4'-ethynylphenyl-1-benzeneisocyanide or OPE-NC) coupling. The electronic density of states in the valence region of the two systems are nearly identical with the exception of a shift to higher binding energy by about 0.5 eV for OPE-NC. Corresponding shifts appear in C(1s) spectra and in the threshold near E(F). The lack of change in the optical absorption suggests that a rigid shift of the Fermi level within the pi-pi* gap is the major effect of modifying the coupling chemistry. Qualitative consideration of bonding in each case is used to suggest the influence of chemisorption-induced charge transfer as a potential explanation. Connections to other theoretical and experimental work on the effects of varying coupling chemistries are also discussed.

The molecular architecture of acridine-9-carboxylic acid (ACA) grown on Ag (111) by physical vapor deposition was characterized by using UHV-STM and XPS. At lower coverage, ACA molecules exist in a 2-d gas phase on the surface at room temperature. With increased coverage (0.4 ML), ACA molecules self-organize into distinctive adlayer structures that are correlated with underlying substrate morphology. On step-free Ag (111) regions, ACA molecules form large islands in coexistence with the 2-d ACA gas. These islands are commensurate with the Ag (111) substrate, indexed as (4 0, 2 4) in matrix notation, and can exceed 100 nm in size. There are two nonequivalent ACA molecules in each unit cell. XPS core level measurements reveal a hydrogen-bonding interaction between ACA molecules, with the ring nitrogen acting as the H-bond acceptor and the carboxyl proton acting as the H-bond donor. A structural model for this phase consists of chains of ACA molecules linked by head-to-tail hydrogen bonds along the substrate [10] direction. Alternating ACA tilting angles account for the two nonequivalent ACA molecules and the observed high packing density. Completely different molecular arrangements are observed on Ag (111) surface regions roughened by a higher density of crystallographic steps (terrace widthsor = 6 nm). Pairs of ACA molecules arrange in a zigzag pattern in a (12 2, 6 5) overlayer structure with a diluted packing density. The structural model for this lower density phase consists of carboxyl-carboxyl linked ACA dimers in a flat-lying molecular orientation.

Thin films of metal phthalocyanine (MPc) are grown on an Au(111) support with a newly developed aerosol molecular beam deposition source and characterized in situ via ultrahigh vacuum scanning tunneling microscopy. MPcs are delivered to Au(111) in a series of N2-entrained microsized solvent droplets of variable surface residence time. Phthalocyanine film registration to the herringbone reconstruction of the Au(111) surface, indicative of thermodynamically favored structure, is observed at submonolayer coverages for aromatic solvents with long residence times. Aerosol-deposited monolayer film structures are noncrystalline with tilted MPc orientations and vacancy nanocavities. Upon annealing, MPc molecules adopt flat-lying orientations with respect to the substrate and vacancies are eliminated. Film morphologies indicate solvation-mediated film nucleation and growth, with less long-range ordering that in vapor-generated films.