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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...

Stress waveforms applied to microbatteries induce current transients with a power-law decay that signals stress., Energy storage science calls for techniques to elucidate ion transport over a range of conditions and scales. We introduce a new technique, pascalammetry, in which stress is applied to a solid-state electrochemical device and induced faradaic current transients are measured and analyzed. Stress-step pascalammetry measurements are performed on operando microbattery probes (Li2O/Li/W) and Si cathodes, revealing stress-assisted Li+ diffusion. We show how non-Cottrellian lithium diffusional kinetics indicates stress, a prelude to battery degradation. An analytical solution to a diffusion/activation equation describes this stress signature, with spatiotemporal characteristics distinct from Cottrell’s classic solution for unstressed systems. These findings create an unprecedented opportunity for quantitative detection of stress in solid-state batteries through the current signature. Generally, pascalammetry offers a powerful new approach to study stress-related phenomena in any solid-state electrochemical system.

A carbon-nanotube-enabling scanning probe technique/nanotechnology for manipulating and measuring lithium at the nano/mesoscale is introduced. Scanning Li-nanopipette and probe microscopy (SLi-NPM) is based on a conductive atomic force microscope (AFM) cantilever with an open-ended multi-walled carbon nanotube (MWCNT) affixed to its apex. SLi-NPM operation is demonstrated with a model system consisting of a Li thin film on a Si(111) substrate. By control of bias, separation distance, and contact time, attograms of Li can be controllably pipetted to or from the MWCNT tip. Patterned surface Li features are then directly probed via noncontact AFM measurements with the MWCNT tip. The subsequent decay of Li features is simulated with a mesoscale continuum model, developed here. The Li surface diffusion coefficient for a four (two) Li layer thick film is measured as D=8(±1.2)×10(-15) cm(2) s(-1) (D=1.75(±0.15)×10(-15) cm(2) s(-1)). Dual-Li pipetting/measuring with SLi-NPM enables a broad range of time-dependent Li and nanoelectrode characterization studies of fundamental importance to energy-storage research.

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...

The recently discovered large nonsaturating magnetoresistance in semimetal $\mathrm{WT}{\mathrm{e}}_{2}$ may result from near-perfect electron-hole compensation, however recent reports question whether the compensation is adequate to explain the observations. Experiments on significantly uncompensated $\mathrm{WT}{\mathrm{e}}_{2}$ are needed. We measure magnetoresistance ${\ensuremath{\rho}}_{xx}(H)$, Hall effect ${\ensuremath{\rho}}_{xy}(H)$, and an electrolyte gating effect in thin (100 nm) exfoliated $\mathrm{WT}{\mathrm{e}}_{2}$. We observe ${\ensuremath{\rho}}_{xy}(H)$ linear in $H$ at low $H$ consistent with near-perfect compensation, however ${\ensuremath{\rho}}_{xy}(H)$ becomes nonlinear and changes sign with increasing $H$, implying a breakdown of compensation. We break compensation more significantly by using an electrolytic gate for highly electron-doped $\mathrm{WT}{\mathrm{e}}_{2}$ with Li. In gated $\mathrm{WT}{\mathrm{e}}_{2}$ the nonsaturating ${\ensuremath{\rho}}_{xx}(H)$ persists to $H=14\phantom{\rule{0.16em}{0ex}}\mathrm{T}$, even with significant deviation from perfect electron-hole compensation $(p/n=0.84)$ where the two-band model predicts a saturating ${\ensuremath{\rho}}_{xx}(H)$. Our results indicate electron-hole compensation is not the mechanism for extremely large magnetoresistance in $\mathrm{WT}{\mathrm{e}}_{2}$; alternative explanations are needed.

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.

Surface self-assembly process of 9-anthracene carboxylic acid (AnCA) on Ag(111) was investigated using STM. Depending on the molecular surface density, four spontaneously formed and one annealed AnCA ordered phases were observed, namely a straight belt phase, a zigzag double-belt phase, two simpler dimer phases, and a kagome phase. The two high-density belt phases possess large unit cells on the scale length of 10 nm, which are seldom observed in molecular self-assembled structures. This structural diversity stems from a complicated competition of different interactions of AnCA molecules on metal surface, including intermolecular and molecular-substrate interactions, as well as the steric demand from high molecular surface density.

Nanoscale structure–electric potential relations in films of the organic molecular semiconductors C60 and titanyl phthalocyanine (TiOPc) on Ag(111) have been measured under UHV conditions. Noncontact force methods were utilized to image domain structures and boundaries with molecular resolution, while simultaneously quantifying the local surface electric potential. Sensitivity and spatial resolution for the local potential measurement were first established on Ag(111) through direct observation of the electrical dipole and potential step, φstep = 10 ± 3 mV, of monatomic crystallographic steps. A local surface potential increase of 27 ± 11 mV occurs upon crossing the boundary between the neat Ag(111) surface and C60 islands. Potential steps in binary C60–TiOPc films, nanophase-separated into crystalline C60 and TiOPc domains, were then mapped quantitatively. The 207 ± 66 mV potential step across the C60-to-TiOPc domain boundary exhibits a 3.6 nm width that reflects the spatial resolution for electric poten...

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 nonlocal Hall bar geometry is used to detect neutral-current Hall effects in graphene on silicon dioxide. Disorder is tuned by the addition of $\mathrm{Au}$ or $\mathrm{Ir}$ adatoms in ultrahigh vacuum. A reproducible neutral-current Hall effect is found in both as-fabricated and adatom-decorated graphene. The Hall angle exhibits a complex but reproducible dependence on gate voltage and disorder, and notably breaks electron-hole symmetry. An exponential dependence on length between Hall and inverse Hall probes indicates a neutral-current relaxation length of approximately $300\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$. The short relaxation length and lack of precession in a parallel magnetic field suggest that the neutral currents are valley currents. No signature of the spin-orbit coupling induced spin Hall effect is observed in the $\mathrm{Au}$- or $\mathrm{Ir}$-decorated graphene. The near lack of temperature dependence from 7 to 300 K is unprecedented among reports of valley Hall effect in graphene, and promising for using controlled disorder for room temperature neutral-current electronics.

Graphene decorated with 5d transitional metal atoms is predicted to exhibit many intriguing properties; for example iridium adatoms are proposed to induce a substantial topological gap in graphene. We extensively investigated the conductivity of single-layer graphene decorated with iridium deposited in ultra-high vacuum at low temperature (7 K) as a function of Ir concentration, carrier density, temperature and annealing conditions. Our results are consistent with the formation of Ir clusters of ~100 atoms at low temperature, with each cluster donating a single electronic charge to graphene. Annealing graphene increases the cluster size, reducing the doping and increasing the mobility. We do not observe any sign of an energy gap induced by spin-orbit coupling, possibly due to the clustering of Ir.

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 importance of a solvent in regulating the adhesion forces between surfaces is studied quantitatively with scanning force microscopy. Both samples and tips are coated with alkyl thiolate monolayers of type HS(CH2)10Y and force measurements are conducted as a function of terminal group Y (Y = CH2CH3, CH2OCH3, CO2CH3, CO(NH2), CO2H, and CH2OH) and solvent (water, ethanol, and n-hexadecane). Adhesive forces in water span the greatest range (0.30−12.5 nN), with hydrophobic surfaces adhering most strongly and hydrophilic surfaces most weakly. In ethanol the adhesive forces are substantially smaller and in n-hexadecane they are negligible. In water, these adhesive forces are consistent with the work required to exclude solvent from the tip−sample interface, indicating that solvent exclusion dominates adhesion. Such macroscopic solvent exclusion cannot fully explain the adhesive forces in ethanol. This force data is used to evaluate the tip−sample interfacial energies (γts) of like CH3- and CH2OCH3-terminated...

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.

The structure of w-hydroxyalkanethiol monolayers containing an intemal ether group is probed by traditional surface infrared reflection-absorption spectroscopy and a novel electrochemical method based on the measured shifts in the potential of zero charge (pzc) upon introduction of the ether group. The analyses of the infrared and electrochemical data give complementary structural information. Vibrational bandwidth measurements indicate that these monolayers exhibit crystalline packing. From a comparison of the integrated infrared absorption intensities between monolayer and bulk samples of the thiols, an average tilt angle of 29" and an average twist angle of 40" are determined for the ether modified w-hydroxyalkanethiol monolayers. The pzc of the ether modified monolayer coated Au electrodes is observed to shift by 130 i 40 mV from that characteristic of an unmodified w-hydroxyalkanethiol monolayer. A simple dipole analysis of the magnitude of these pzc shifts, assuming a twist angle of 40", gives an average tilt angle of 23 & 8", in reasonable agreement with the infrared determination. In addition, the monolayers modified by an oxygen located at an even methylene site (from the S atom) are observed to give a positive pzc shift, while those located at odd sites give strictly negative pzc shifts. Such an evedodd effect on the sign of the pzc shift indicates that the S-C bond is directed away from the surface normal. This electrochemical approach to structural analysis is generally applicable to crystalline monolayers which contain a net static internal dipole.

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.

Pulsed molecular beam‐surface infrared measurements of the kinetics of CO populating step sites on Ni(9,1,1) are reported and interpreted in terms of elementary surface rate processes. An analytic model is developed to describe the distribution of CO between step and terrace sites in the equilibrium limit, and refine our previous determination of the binding energy difference between these sites to ΔEs‐t=0.6±0.2 kcal/mol. Time‐resolved surface infrared measurements indicate that the equilibrium step coverage is reached within 100 ms of the chemisorption event. This rapid migration across the (100) terraces to step sites implies a barrier to surface hopping of

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.

The unoccupied electronic structure of C${}_{60}$ layers on Ag(111) was investigated with two-photon photoemission (2PPE) and scanning tunneling spectroscopy (STS). The focus, in addition to providing unoccupied level assignments for C${}_{60}$ monolayers, is on identifying signatures of C${}_{60}$ superatom molecular orbitals (SAMOs), diffuse unoccupied levels weakly bound by the spherical potential of the fullerene shell, in the two complementary techniques. The s-SAMO is identified 3.3 eV above the Fermi level in 2PPE and at a bias of 3.5 eV in STS. Possible contributions from $p$-SAMO levels are also discussed for both measurement techniques. The results are compared with recent low-temperature scanning tunneling microscopy (LT-STM) work that investigated C${}_{60}$ SAMOs on Cu(111) and oxygen-covered Cu(110) surfaces and with previous 2PPE measurements for C${}_{60}$ monolayers on Au(111) and Cu(111).

Controlled deposition of titanyl phthalocyanine (TiOPc) on Ag(111) produces a honeycomb monolayer phase consisting of TiOPc molecules with two distinctive tilt angles. This periodic arrangement of polar molecules is used to direct C(70) growth into low-density 3D films with novel C(70) kagome lattice arrangements. Structural models for the C(70) kagome lattice are determined from layer-by-layer scanning tunneling microscopy images and related to the dipolar TiOPc template and C(70)'s anisotropic polarizability. Molecular templates with designed electrostatic features offer a practical method to control 3D film organization on the nanoscale by harnessing anisotropic molecular interactions at the growth interface.

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.

The chemisorption properties of carbon monoxide on two vicinal Ni(100) surfaces have been studied with surface infrared reflection–absorption spectroscopy and low energy electron diffraction. For coverages ≤0.50 monolayer, equilibrium adlayers are formed in which CO populates atop sites on the low‐index (100) terrace, as well as twofold bridging sites along both the highly‐kinked and close‐packed step edges of the Ni[(100)‐1.4°(010)] and Ni[(100)‐9°(011)] surfaces investigated. Low energy electron diffraction (LEED) measurements confirm that all three long‐range structures established on the (100) surface—c(2×2) at 0.50 ML, hexagonal at 0.61 ML, and compressed‐hexagonal at 0.69 ML—are also formed on the Ni [(100)‐1.4°(010)] surface. On the Ni [(100)‐9°(011)] surface, however, only the ordered c(2×2) structure appears. A simple Arrhenius analysis of the relative population of step and terrace sites estimates a small binding energy preference for populating step sites. This weak preference is of compa...

The ultrafast dynamics at a well-characterized $\text{CuPc}/{\text{C}}_{60}$ organic photovoltaic heterojunction have been directly measured with time-resolved two-photon photoemission (TR-2PPE). Phthalocyanine/${\text{C}}_{60}$ donor-acceptor interfaces, characterized in detail via scanning tunneling microscopy, provide model systems for studies of critical charge separation and recombination processes that determine device performance. TR-2PPE studies of copper phthalocyanine $(\text{CuPc})/{\text{C}}_{60}$ interfaces reveal ultrafast charge separation and provide evidence for recombination via low-lying CuPc triplet levels.

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.

We report experiments highlighting the mechanistic role of mobile pentacene precursors in the formation of a network C(60)-pentacene co-crystalline structure on Ag(111). This co-crystalline arrangement was first observed by low temperature scanning tunneling microscopy (STM) by Zhang et al. (Zhang, H. L.; Chen, W.; Huang, H.; Chen, L.; Wee, A. T. S. J. Am. Chem. Soc. 2008, 130, 2720-2721). We now show that this structure forms readily at room temperature from a two-dimensional (2-D) mixture. Pentacene, evaporated onto Ag(111) to coverages of 0.4-1.0 ML, produces a two-dimensional (2-D) gas. Subsequently deposited C(60) molecules combine with the pentacene 2-D gas to generate a network structure, consisting of chains of close-packed C(60) molecules, spaced by individual C(60) linkers and 1 nm x 2.5 nm pores containing individual pentacene molecules. Spontaneous formation of this stoichiometric (C(60))(4)-pentacene network from a range of excess pentacene surface coverage (0.4 to 1.0 ML) indicates a self-limiting assembly process. We refine the structure model for this phase and discuss the generality of this co-crystallization mechanism.

Low-dimensional boundaries between phases and domains in organic thin films are important in charge transport and recombination. Here, fluctuations of interfacial boundaries in an organic thin film, acridine-9-carboxylic acid (ACA) on Ag(111), have been visualized in real time, and measured quantitatively, using Scanning Tunneling Microscopy. The boundaries fluctuate via molecular exchange with exchange time constants of 10-30 ms at room temperature, yielding length mode fluctuations that should yield characteristic f-1/2 signatures for frequencies less than ~100 Hz. Although ACA has highly anisotropic intermolecular interactions, it forms islands that are compact in shape with crystallographically distinct boundaries that have essentially identical thermodynamic and kinetic properties . The physical basis of the modified symmetry is shown to arise from significantly different substrate interactions induced by alternating orientations of successive molecules in the condensed phase. Incorporating this additional set of interactions in a lattice gas model leads to effective multi-component behavior, as in the Blume-Emery-Griffiths (BEG) model, and can straightforwardly reproduce the experimentally observed isotropic behavior. The general multi-component description allows the domain shapes and boundary fluctuations to be tuned from isotropic to highly anisotropic in terms of the balance between intermolecular interactions and molecule-substrate interactions. Key words: Organic thin film, fluctuations, STM, molecular interactions, diffusion kinetics, phase coexistence, 32 pages, 7 figures

The microscopic diffusion of CO on stepped Pt(111) crystal surfaces has been investigated with pulsed molecular beam–time‐resolved surface infrared methods. Following a rapid exposure to CO, we record the time evolution of the CO surface vibrational spectra as the CO diffuse from the initial random distribution to the thermodynamically favored step sites. The data are simulated with a model that describes the kinetics of diffusion across the terraces, as well as the filling of step sites. We critically evaluate this model and the general experimental approach by extending our previous measurements of CO diffusion on Pt(28(111)–(110)) to a surface with higher step density, Pt(l2(111)–(110)), with varying coverages. The model describes both sets of data with the same parameters, confirming the original determination of the diffusion barrier (ΔET) and prefactor (AT) for microscopic surface hopping of CO/Pt(111). This further provides a quantitative means to estimate systematic errors. We report ΔET=4.0±0.7...

Serpentine chain C60 phases were observed in scanning tunneling microscopy (STM) images of C60 layers on zinc phthalocyanine (ZnPc) or pentacene covered Ag(111) and Au(111) surfaces. This low-density, quasi-one-dimensional organization contrasts starkly with the close-packed hexagonal phases observed for C60 layers on bare metal substrates. STM was employed to perform a detailed investigation of these chain structures for C60/ZnPc/Ag(111) heterolayers. Motivated by the similarity of these chain phases, and the chain and stripe organization occurring in dipole-fluid systems, we investigated a model based on competing van der Waals attractions and electrostatic repulsions between C60 molecules as an explanation for the driving force behind these monolayer phases. Density functional theory (DFT) calculations revealed significant charge transfer to C60 from the Ag(111) substrate, through the intervening ZnPc layer, inducing electrostatic interactions between C60 molecules. Molecular dynamics simulations performed with attractive van der Waals interactions plus repulsive dipole-dipole interactions reproduced the C60 chain phases with dipole magnitudes consistent with DFT calculations.

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 dependence of supramolecular structure on fractional molecular coverage has been investigated for acridine-9-carboxylic acid (ACA) and the C(60):ACA binary molecular system. The coverage-dependent phase diagram for ACA is first determined from room-temperature STM imaging. At low molecular coverages (theta0.4 ML, ML = monolayer), ACA forms a 2-D gas phase. Ordered ACA structures appear with increasing coverage: first a chain structure composed of ACA molecules linked by consecutive O-H...N hydrogen bonds (theta0.4 ML), then a dimer structure composed of ACA dimers linked by paired carboxyl-carboxyl hydrogen bonds (theta approximately equal to 1.0 ML). Structures of the C(60):ACA binary system depend on the coverage of predeposited ACA. At intermediate (0.4 ML approximately 0.8 ML) ACA coverages, C(60) deposition results in a hexagonal cooperative structure with the C(60) periodicity nearly 3 times that of the normal C(60) 2-D packing of 1 nm and exists in enantiopure domains. At higher ACA coverages, a C(60) quasi-chain structure is formed in which parallel C(60) chains are spaced by ACA dimer domains. The mechanistic role of the initial ACA phase in the formation of C(60):ACA supramolecular structures is described. Chemically intuitive molecular packing models are presented based on the observed STM images.

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.

We have demonstrated a method for fabricating C60 overlayers with controlled spacing and chirality by reactive coadsorption with the aromatic molecule acridine-9-carboxylic acid (ACA). Structural control is achieved by the mismatched symmetries of the coadsorbates, as well as specific intermolecular and adsorbate-substrate interactions. The resulting supramolecular structure has a C60 period nearly three times as large as the normal C60 2D packing of 1 nm and exists in enantiopure domains with robust chirality.

Continuous silver films of nanometer thickness, grown at room temperature on $(7\ifmmode\times\else\texttimes\fi{}7)\ensuremath{-}\mathrm{Si}(111),$ display a wormlike morphology and thermally decay into isolated three-dimensional crystallites. We have tracked the dewetting process in real time using variable-temperature scanning tunneling microscopy. In this ultrathin regime, a range of transitional morphologies are observed, dependent on the substrate step density, annealing time and temperature. Initial film morphology is modulated by the underlying step structure and the growing crystallites show a corresponding step-induced deformation. During the initial stages of dewetting, the Ag-denuded areas transform to the $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}\ensuremath{-}\mathrm{A}\mathrm{g}/\mathrm{S}\mathrm{i}(111)$ surface, and spatially nonuniform denuded zones appear around the growing crystallites. Enhanced dewetting rates are observed in areas of lower step density, largely because steps act as kinetic obstacles. A simple form to relate the energy gain in dewetting to the density of crystallographic steps is provided. On preprepared $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}\ensuremath{-}\mathrm{A}\mathrm{g}/\mathrm{S}\mathrm{i}(111)$ substrates, dewetting occurs spontaneously even at room temperature.

Growth of an ultrathin lead oxide layer causes massive changes in the shape of lead crystallites. The dynamics of this process was investigated with time-lapsed scanning tunneling microscopy. Pure lead crystallites proved extremely resistant to oxidation. Once nucleated by surface impurities, monolayer films of lead oxide grew readily on lead (111) microfacets in an autocatalytic process. The anisotropic growth of orthorhombic lead oxide films (massicot structure) was most rapid along the direction of weakest lead-oxygen bonding, which suggests that the growth edge autocatalyzes oxygen dissociation by providing proximal sites for oxygen dissociation and attachment.

Three-dimensional relaxation of small crystallites was imaged in real time using variable-temperature scanning tunneling microscopy. The micron-sized Pb crystallites, supported on Ru(0001), were equilibrated at 500– 550 K, and the volume-preserving shape relaxation was induced by a rapid temperature decrease to 353– 423 K. The (111) facet at the top of the crystallite grows by sequential peeling of single atomic layers, which shrink like circular islands. The rate of layer peeling slows dramatically as a new final state is reached.

By controlled deposition, TiOPc, a molecular semiconductor with anisotropic interactions can generate a molecular film with a characteristic pattern repeat size of 15 nm. This structure then served as a nanotemplate for a superlattice of C(60) clusters with characteristic diameters of 7 nm. As a result, C(60) deposition on the TiOPc film template forms a pattern of nanophase-separated C(60) and TiOPc domains with a characteristic domain size of 7 nm. This feature size is matched to the exciton diffusion length in photovoltaic materials composed of small organic molecules. A dislocation network in a molecular film provides a promising method for generating pattern features on the several-nanometer length scale, bridging the practical limits of "bottom up" and "top down" strategies.

Surface chemical processes generally involve the transfer of mass within a complex adlayer structure. Relating mass transfer in such complex systems to the underlying microscopic events is a formidable problem in surface chemical physics. We have used scanning tunneling microscopy (STM) to image mass transfer on Ag(110) and its vicinals in a variety of surface chemical processes. By investigating a broad range of phenomena on these surfaces, including self diffusion, adsorption, faceting, and reaction, we test and obtain a more comprehensive understanding of how mass is transferred within the complex surface chemical milieu. For these vicinal Ag(110) surfaces we find that mass exchange is highly efficient event at room temperature. The mechanisms for mass transfer do not appear unique, but new channels open, as needed, to satisfy the chemical potential balance and sustain mass transfer. For oxidation reactions on Ag(110), crystallographic steps and substrate reconstruction are key elements to the mass-exchange mechanisms.

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.