Situ Scanning Tunneling Microscopy Images Research Articles

Effects of Anion Coadsorption on the Self-Assembly of 11-Acryloylamino Undecanoic Acid on an Au(111) Electrode.

11-acryloylamino undecanoic acid (AAUA) is a versatile polymerizable surfactant that has been applied to coat medical devices, and these applications can benefit from a fundamental understanding of its interaction with a metal substrate. Cyclic voltammetry and in situ scanning tunneling microscopy (STM) were used to examine the adsorption configuration of AAUA molecules on an ordered Au(111) electrode and their mutual interactions, as AAUA was adsorbed from a methanol dosing solution. In addition to the van der Waals force between the aliphatic groups, the hydrogen bonding between the carboxylic acid and acrylamide groups was also important to guide the spatial arrangement of AAUA admolecules on the Au electrode. The -COOH group of AAUA admolecule likely dissociated in neutral media to -COO-, which formed hydrogen bonds with H2PO4 - in phosphate buffer solution (PBS). This interaction between the AAUA admolecules and ions in the electrolyte resulted in different electrochemical characteristics observed in phosphate buffer solution (PBS) and potassium sulfate (K2SO4). Molecular-resolution STM imaging revealed distinctly different AAUA spatial structures on the Au electrode in PBS and K2SO4. Shifting the potential positively to 0.5 V (versus Ag/AgCl) led to lifting of the reconstructed Au(111) to the (1 × 1) phase and the dissolution of the ordered AAUA film, suggesting that the orientation of the AAUA admolecule was altered. The ordered AAUA adlayer could be partially recovered by shifting the potential negatively.

The Deposition of Lead on a Pt(100) Electrode and the Effects on the Oxidation of Formic Acid

The electrodeposition of foreign metals on a platinum (Pt) electrode can be used to alter the electrochemical properties of the modified electrode. The current study employed in situ scanning tunneling microscopy (STM) to characterize the spatial structures of a monolayer and multilayer lead (Pb) electrodeposited on an ordered Pt(100) electrode in 0.1 M perchloric acid (HClO4) with and without formic acid under potential control. The Pt(100) bead crystal used in this study was pretreated by a modified annealing and quenching method, producing a long-range ordered, unreconstructed (1 × 1) surface. The underpotential deposition (UPD) of Pb on the Pt(100) electrode resulted in a pair of sharp and reversible peak at 0.5 V (vs. Ag/AgCl), prior to the bulk deposition at E < -0.45 V in 0.1 M HClO4 + 1 mM Pb(ClO4)2. Cyclic potential sweeping in the UPD regimes resulted in a stable voltammogram, but bulk Pb deposition led to varying morphologies of the profiles. Atomic resolution STM images were obtained to reveal a well-ordered Pt(100)-(√2 × √2)R45°-Pb structure and elongated rows of Pb aggregates and local (√2 × 2√2) R45° in the initial and final stages of Pb UPD, respectively. Bulk Pb deposition at E < -0.5 V led to a crystalline Pb film, but a roughened Pt(100) surface was imaged after the Pb deposit was anodically stripped. The activity of the Pt(100) electrode toward formic acid oxidation was nearly tripled after being modified with a sub-monolayer of Pb, and increased by another 20 % when it was loaded with multilayer Pb.

In Situ Characterization of Atomic-Scale Surface Restructuring of Copper Electrodes Under CO2 Electroreduction Conditions

The electrochemical CO2 reduction reaction (CO2RR) by copper-based heterogeneous catalysts has attracted widespread attention due to its potential social, economic, and environmental benefits, as it utilizes electricity from renewable sources to transform CO2 into value-added chemicals, thus closing the carbon cycle[1]. However, because the energy efficiency, selectivity and lifetime of CO2RR on Cu are still far away from those required at industrially conditions, a knowledge-based catalyst design to optimization of the catalytic system is urgently needed. The reactivity and selectivity of CO2RR depends on the catalyst material as well as on surface and interfacial structure/composition and the nanoscale morphology of the catalyst[2]. With the development of in situ and operando characterization techniques, the fundamental understanding of the structure-reactivity/selectivity relationships of catalysts under reaction conditions can be obtained. Here we monitor the CO-induced surface reconstruction of Cu(100) electrodes during CO2RR in 0.1M KHCO3 solution by in situ scanning tunneling microscopy (STM), surface X-ray diffraction (SXRD), and Raman spectroscopy. In situ STM images during step-wise potential changes show reversible cluster formation. The nanosized clusters start to appear at a potential ≤ -0.2 V vs. RHE and slowly disappear once the potential is changed back to more positive potentials, the clusters slowly disappear. However, the formation of Cu nanocluster leads to an irreversible modification of the surface structure on the molecular level. According to our previous research [3],freshly prepared Cu(100) electrodes are covered by a well-ordered (bi)carbonate adlayer in the double layer regime. After a potential excursion to the range of cluster formation, a disordered adlayer phase with only short-range order is observed in STM images, which can be explained by the presence of Cu adatoms and surface vacancies that remain on the electrode after dissolution of the nanoclustersIn the in situ SXRD studied, crystal truncation rods (CTRs) were recorded at different potentials from the double layer (0 V) to the CO2RR (-1.1 V) regime and then back to 0 V. Quantitative analysis based on a surface adatom/vacancy pairs model exhibits a clear increase in the surface defects density at potentials ≤ -0.2 V and a change in the defects coverage of ≈ 4%. After increasing the potential back to 0 V, the coverage of these defects decreases only slightly, indicating that some Cu adatoms remain on the surface. These results are in good agreement with our STM studies. The vertical expansion of the topmost atomic Cu layer exhibits a distinctly different potential dependence. This surface relaxation is not related to cluster formation, but can be attributed to changes in the type of chemisorbed species on the surface.To identify the adsorbate species on copper surface, in situ surface-enhanced Raman was employed. In the double layer range, bidentate carbonates and (bi)carbonate anions are dominant surface species. Below -0.04 V, the characteristic bands of carbonate become weak or disappear. In the meantime, new bands assigned to adsorbed carboxylate or carboxyl groups emerge, which can be attributed to the first reduction intermediates of CO2[4]. These bands disappear at potential below -0.34 V. Adsorbed CO appears on the surface starting from -0.24 V in the same potential range as the Cu nanocluster. In the reverse potential scan back to more positive values, only weak carbonate bands reappear and formate bands remain constant up to 0.26 V. These observed behavior suggests that some intermediates are formed irreversibly, in accordance with the irreversible changes in the molecular structure of the adlayer.[1] S. Nitopi et. al., Chem. Rev. 2019, 119(12), 7610–7672[2] Y. Y. Birdja at. al, Nature Energy 2019, 4, 732–745[3] R. Amirbeigiarab et. al., Angew. Chemie Int. Ed. 2022, 61 (46), No. e202211360[4] W. Shan et. al., ACS Nano 2020, 14(9), 11363–11372

The spatial structures of iron oxide formed on an ordered Pt(111) electrode in pH 3 sulfate and chloride media

Platinum (Pt) electrodes modified with iron (Fe) have been used as electrocatalysts to split water, to reduce oxygen, and to oxidize methanol. The current study employed cyclic voltammetry and in situ scanning tunneling microscopy (STM) to probe the redox chemistry and spatial structure of an iron hydroxide thin film adsorbed on an ordered Pt(111) electrode in pH 3 sulfate solution (1 mM H2SO4 + 0.1 M K2SO4) containing 10 mM FeSO4. The Pt(111) electrode was pretreated with the conventional annealing-and-quenching method, which resulted in a sub-monolayer native oxide. After this native oxide was stripped off, the adsorption of Fe2+ ions and the reduction of oxygen concurred to yield an ordered FeOH film on the Pt(111) electrode. In situ STM imaging revealed a unique spoke – wheel (SW) structure between 0.1 and -0.1 V (vs. Ag/AgCl). As opposed to the hexagonal FeO(111) bilayer structure observed on Pt(111) in the vacuum, a distorted hexagonal adlattice was seen with this SW pattern. Shifting the potential positively and negatively resulted in transformations of the SW structure into distorted moiré patterns. The same STM experiment performed with Fe2(SO4)3 led to a disordered film, indicating that ferrous, not ferric, ions were orderly adsorbed on the Pt(111) electrode. The role of anion in the formation of the FeOH film on the Pt(111) electrode was also conducted in pH 3 chloride solution (1 mM HCl + 0.1 M KCl + FeCl2).

Adsorption of 2-Mercapto-1-methylimidazole on the (√3 × 22) Reconstructed and (1 × 1) Phases of an Ordered Au(111) Electrode under Potential Control

The self-assembled monolayers (SAMs) of organosulfur compounds bearing an imidazole (Im) functional group can form hydrogen bonds with others and tailor specific interfacial structures used for CO2 reduction, biosensing, and corrosion prevention. The current study employed cyclic voltammetry and in situ scanning tunneling microscopy (STM) to address the adsorption and structure of 2-mercapto-1-methylimidazole (MMI) on the reconstructed and unreconstructed Au(111) electrodes as a function of potential. The adsorbed MMI molecule assumed the unprotonated form, allowing its S- and N-ends to bind with the Au electrode at a positive potential. Molecular resolution STM images showed an ordered Au(111)–(√21 × √21)R10.9°–MMI structure. The Au–N bond could weaken and be broken at a negative potential, leading to a disordered MMI adlayer. This restructuring event was coupled with the protonation of the Im group. These processes resulted in a sharp peak, which shifted negatively by 60 mV with an increase of 1 pH unit. MMI admolecules were desorbed at a more negative potential to unveil a pitted Au surface, resulting from etching of MMI adsorption. This feature supports the S–Au–S motif for MMI adsorbed to the Au(111) electrode, irrespective of the initial Au surface structure. The effect of MMI as an additive to the deposition bath of Ni was also explored.

The adsorption and electropolymerization of terthiophene on Au(1 1 1) electrode – Probed by in situ STM

• In situ STM imaging terthiophene molecule adsorbed on an ordered Au(1 1 1) electrode in sulfuric and perchloric acid. • Dominant effect of potential control on the adsorption and oxidation of terthiophene admolecule. • Terthiophene admolecules accompanied by bisulfate anions to form highly ordered adlattices. • Revelation of adsorption configuration and spatial arrangement of TT on Au(1 1 1) electrode. • STM visualization of oligothiophenes produced by anodization. In situ scanning tunneling microscopy (STM) was used to examine terthiophene (TT) adsorbed on an ordered Au(1 1 1) electrode in 0.1 M H 2 SO 4 and HClO 4 , as a model system to gain a fundamental understanding of the effects of potential control on the coadsorption of anion and the oxidation of a thiophene - bearing entity. TT was irreversibly adsorbed onto an Au(1 1 1) bead crystal from a dosing solution made of ethanol at room temperature. The cyclic voltammogram (CV) recorded with this sample in 0.1 M H 2 SO 4 showed a series of well-defined peaks, resulting from the restructuring of the TT adlayer, coupled with anion coadsorption and the phase transition of the Au(1 1 1) electrode. In contrast, the CV obtained in 0.1 M HClO 4 was mostly featureless before TT admolecules were irreversibly oxidized at E > 0.7 V (vs Ag/AgCl). TT molecule was adsorbed most horizontally on Au(1 1 1) electrode in H 2 SO 4 , but displaced by bisulfate anion at positive potential. Anodization of this TT/Au(1 1 1) electrode triggered the oxidation and subsequent coupling of TT monomers to yield oligothiophenes, whose internal structures and dimensions were revealed by STM in pH1 and 5 sulfate media. The as – produced oligothiophenes appeared as winding ribbons spanning ∼ 200 and ∼ 40 Å in pH 5 and 1 sulfate media. Their internal structures, particularly the trans and cis conformations of neighboring thiophenes units, were visualized by molecular resolution STM.

Adstructures of platinum-complex precursors and platinum nanoparticles formed on low-index single-crystal Au surfaces for oxygen reduction reaction

Abstract We investigated the adlayer structures of Pt complexes, PtCl42− and PtBr42−, by using in situ scanning tunneling microscopy (STM) and the electrochemical behavior on the Pt nanoparticles deposited from the PtCl42− and PtBr42− precursors on Au(100), Au(111), and Au(110) surfaces in 0.1 M HClO4. The STM investigation revealed that the PtCl42− and the PtBr42− adlayers had very similar structures on Au(111) and Au(110) surfaces: the (√7 × √7)R19.1° structure on Au(111), and the c(2 × 4) structure on Au(110). On Au(100), PtCl42− formed a (√5 × √5)R26.7° structure, whereas PtBr42− was adsorbed randomly in a flat configuration. High-resolution STM images also revealed pinwheel features of the Pt complexes, resulting from the flat-lying adsorption configurations. The electrochemical reduction of the PtBr42− adlayer proceeded at a more negative potential than the PtCl42− adlayer to form metallic Pt nanoparticles. The average size of the Pt particles varied on the different Au surfaces. The cyclic voltammograms and the catalytic activities for the oxygen reduction reaction were different on the Pt complexes and the three Pt-modified Au(hkl) electrodes. The structures of deposited Pt nanoparticles depended on the substrate Au electrodes. The order of the ORR activity of Pt nanoparticles on Au substrates, Pt/Au(111) > Pt/Au(100) ≥ Pt/Au(110), corresponded to the relative ease of specific adsorption of anions on Pt single-crystal electrodes, Pt(111) > Pt(110) > Pt(100).

Identically Sized Co Quantum Dots on Monolayer WS2 Featuring Ohmic Contact

Identically sized $\mathrm{Co}$ quantum dots (QDs) are constructed on monolayer tungsten disulfide (${\mathrm{WS}}_{2}$), forming coupled heterostructures. Topographical images investigated by means of in situ scanning tunneling microscopy (STM) show a bias-dependent feature. First-principles-calculated binding energies combined with the simulated STM images identify that the $\mathrm{Co}$ QDs possess a unique magic number, with a tetrahedral ${\mathrm{Co}}_{4}$ configuration. Numerical differential conductance measured by means of scanning tunneling spectroscopy indicates a p-type doping and an Ohmic contact property for the ${\mathrm{Co}}_{4}/{\mathrm{WS}}_{2}$ system. The mechanism of the transport and conduction properties of the coupled heterostructures is further revealed by analyzing the work functions and interfacial interaction. Our findings offer some references for the controlled fabrication of identically sized zero-dimensional and two-dimensional heterostructures, and we propose a feasible strategy for Ohmic interface contact in nanodevices.

Organic Molecular Layer with High Electrochemical Bistability: Synthesis, Structure, and Properties of a Dynamic Redox System with Lipoate Units for Binding to Au(111).

Biphenyl-2,2'-diylbis(10-methyl-9-methyleneacridan)-type electron donor 1, which has two tethered cyclic disulfide units at the 6,6'-positions, was designed and synthesized as the first member of a dynamic redox (dyrex) system that can form molecular layers on a Au(111) electrode. Upon the two-electron (2 e) oxidation of 1, the persistent dicationic dye 22+ was generated with the formation of a new C-C bond, which is reversibly cleaved upon 2 e reduction to regenerate 1 (dyrex behavior). Similar dyrex interconversion occurs in the molecular layer of 1 on gold. The chemical identities of 1/Au and electrochemically generated 22+ /Au were unambiguously determined by in situ IR spectroscopy in the attenuated total reflection mode. In situ scanning tunneling microscopy (STM) was conducted under electrochemical conditions to examine the surface structure of 1 adsorbed on a Au(111) electrode. Although no long-range-ordered morphology was found in the STM image of 1, an in situ STM study of the potential-induced dyrex reaction of 1 to 22+ showed that the grained spots in the image became slightly brighter.

In situ STM imaging of polyethylene glycol adsorbed on an Au(111) electrode in pH 3

In situ scanning tunneling microscopy (STM) is used to examine the electrified interface of Au(111) immersed in a pH3 sulfate medium containing polyethylene glycol (PEG) with an average molecular weight of 6000. The cyclic voltammograms thus obtained show two sharp peaks at −0.35 and −0.38V (vs. reversible hydrogen electrode), which correlates with the STM observation of a highly ordered (2×2√3)rect structure and the adsorption of PEGs. X-ray photoelectron spectroscopy is used to examine the film formed at −0.4V, revealing prominent C–C and C–O–C structures, thereby supporting the view of adsorption and reduction PEGs on the Au(111) electrode. STM imaging at the initial stage of PEG's adsorption reveals winding linear segments 0.6nm wide and 20–40nm long, implying helical conformations of PEGs. The PEG film dissolves and yields a high density of nanoclusters, as the potential is switched stepwise from −0.4 to 0.9V.

Structural Changes of 4,4′-(Dithiodibutylene)dipyridine SAM on a Au(111) Electrode with Applied Potential and Solution pH and Influence of Alkyl Chain Length of Pyridine-Terminated Thiolate SAMs on Cytochrome c Electrochemistry

From the standpoint of cytochrome c electrochemistry, it is important to understand the relationship between the electron transfer reaction and alkyl chain length of pyridine-terminated self-assembled monolayers (SAMs) at the molecular level. In the present study, SAMs of 4,4′-(dithiodibutylene)dipyridine ((PyC4S)2) formed on a Au(111) surface were investigated in aqueous electrolyte solutions by cyclic voltammetry (CV), surface-enhanced infrared absorption spectroscopy (SEIRAS), and in situ scanning tunneling microscopy (STM). Structural changes in the PyC4S-SAM were driven by changes in the applied electrode potential or pH of the electrolyte solution. In 0.05 M HClO4, a structural change in PyC4S-SAM was drastically induced by the potential manipulation from the open circuit potential to 0.10 V versus a reversible hydrogen electrode (RHE). A high-resolution STM image showed that the PyC4S-SAMs formed a p(12 × √3 R–30°) adlattice at ∼0.15 V versus RHE. A semisquarely arranged adlattice was newly formed ...

Dysprosium electrodeposition from a hexaalkylguanidinium-based ionic liquid.

The rare-earth element dysprosium (Dy) is an important additive that increases the magnetocrystalline anisotropy of neodymium magnets and additionally prevents from demagnetizing at high temperatures. Therefore, it is one of the most important elements for high-tech industries and is mainly used in permanent magnetic applications, for example in electric vehicles, industrial motors and direct-drive wind turbines. In an effort to develop a more efficient electrochemical technique for depositing Dy on Nd-magnets in contrast to commonly used costly physical vapor deposition, we investigated the electrochemical behavior of dysprosium(iii) trifluoromethanesulfonate in a custom-made guanidinium-based room-temperature ionic liquid (RTIL). We first examined the electrodeposition of Dy on an Au(111) model electrode. The investigation was carried out by means of cyclic voltammetry (CV) and X-ray photoelectron spectroscopy (XPS). The initial stages of metal deposition were followed by in situ scanning tunneling microscopy (STM). CV measurements revealed a large cathodic reduction peak, which corresponds to the growth of monoatomic high islands, based on STM images taken during the initial stages of deposition. XPS identified these deposited islands as dysprosium. A similar reduction peak was also observed on an Nd-Fe-B substrate, and positively identified as deposited Dy using XPS. Finally, we varied the concentration of the Dy precursor, electrolyte flow and temperature during Dy deposition and demonstrated that each of these parameters could be used to increase the thickness of the Dy deposit, suggesting that these parameters could be tuned simultaneously in a temperature-controlled flow cell to enhance the thickness of the Dy layer.

In situ scanning tunneling microscopy of the poisoning of a Co(0 0 0 1) Fischer–Tropsch model catalyst by sulfur

The poisoning of a Co(0001) model catalyst for the Fischer–Tropsch reaction by sulfur was investigated by in situ scanning tunneling microscopy (STM). Experiments were performed under syngas at pressures around 10mbar and at sample temperatures around 500K under methanation conditions. After the reaction experiments, the surface was analyzed by X-ray photoelectron spectroscopy. Mass spectrometry showed that the sample was poisoned when hydrogen with <0.5ppm of sulfur was used in the syngas. STM images recorded under these conditions showed a strong restructuring of the surface into small terraces. The terraces were fully covered by a stable surface phase, displaying small ordered areas with (2×2) and (√19×√19)R23.4° structures. The observations contrast with the flat terraces covered by a mobile adsorption layer when sulfur-free hydrogen was used. The poisoned surface state is interpreted as a mixed sulfur/carbon layer that additionally involves a reconstruction of the Co(0001) surface.

Nucleation and growth of primary nanostructures in SrTiO3 homoepitaxy.

SrTiO3 nanoislands on SrTiO3 (001) in a diffusion-limited growth regime were studied using in situ scanning tunneling microscopy (STM). The STM images revealed two characteristic features of nucleation stages. First, the minimum lateral size of the one-unit-cell (uc)-high SrTiO3 islands was 4 × 4 uc2. Second, one-dimensional SrTiO3 islands of a 4 uc width grew along the crystal symmetry directions. These observations suggest that 4 × 4-uc2 islands act as a minimum nucleation seed, and the addition of SrTiO3 molecular species of the same width is the primary and dominant growth process in SrTiO3 homoepitaxy. A close inspection of the surface of the substrate during the deposition process revealed possible connections between surface reconstruction and energetically favorable nucleation of SrTiO3 islands.

In situ scanning tunneling microscopy study of Ca-modified rutile TiO2(110) in bulk water.

Despite the rising technological interest in the use of calcium-modified TiO2 surfaces in biomedical implants, the Ca/TiO2 interface has not been studied in an aqueous environment. This investigation is the first report on the use of in situ scanning tunneling microscopy (STM) to study calcium-modified rutile TiO2(110) surfaces immersed in high purity water. The TiO2 surface was prepared under ultrahigh vacuum (UHV) with repeated sputtering/annealing cycles. Low energy electron diffraction (LEED) analysis shows a pattern typical for the surface segregation of calcium, which is present as an impurity on the TiO2 bulk. In situ STM images of the surface in bulk water exhibit one-dimensional rows of segregated calcium regularly aligned with the [001] crystal direction. The in situ-characterized morphology and structure of this Ca-modified TiO2 surface are discussed and compared with UHV-STM results from the literature. Prolonged immersion (two days) in the liquid leads to degradation of the overlayer, resulting in a disordered surface. X-ray photoelectron spectroscopy, performed after immersion in water, confirms the presence of calcium.

Scanning Tunneling Microscopy Examination of Rubrene Deposited on Au(111) in Aqueous Solution

Rubrene has received much attention for its potential in the fabrication of organic thin film transistors (OTFTs), and the interface between rubrene and metal leads, such as those made of gold, is important in this context. In this study in situ scanning tunneling microscopy (STM) was used to examine rubrene molecules deposited on a Au(111) electrode from a 8 or 80 μM benzene dosing solution under potential control in 0.1 M perchloric acid. Immersion in either dosing solution for 10 min or longer yielded ordered structures featuring parallel stripes aligned in the ⟨121⟩ direction of the Au(111)-(1×1) surface. Unusual surface features of ragged step edges and one-atom-deep pits were observed on the heavily dosed Au(111) sample, which suggests unexpected rubrene-induced restructuring of the gold substrate. Roughly 0.1 monolayer gold adatoms were codeposited with rubrene molecules on the Au(111) electrode. Molecular-resolution STM imaging suggests an upright adsorption configuration of rubrene molecules anchored with two phenyl groups on the Au(111) electrode. Ordered rubrene adlattices were seen between 0.1 and 1.0 V (vs reversible hydrogen electrode), but disappeared at E < 0 V, leading to aggregates of gold atoms. These results lend support to the proposed restructuring of the gold surface.

Effect of the Formation of Highly Ordered Platinum(II) Octaethylporphyrin Adlayer on the Surface Reconstruction of Gold and Supramolecular Assembly of Fullerenes

The structures of 2D molecular assemblies of platinum(II) octaethylporphyrin (PtOEP) on Au(111) and Au(100) surfaces were examined using in situ scanning tunneling microscopy (STM) in 0.05 M HClO4 solutions. The in situ STM observations revealed that the reconstructed rows of Au(111) and Au(100) were stabilized through the formation of highly ordered arrays by adsorption of PtOEP in a wide potential range of the electric double-layer region and that the PtOEP overlayer expanded the stable potential range for the reconstructed phase. Furthermore, the supramolecular assembly of C60 and C84 on a highly ordered PtOEP adlayer formed on Au(111) surfaces was investigated. STM images revealed site-selective supramolecular assembly of fullerenes, especially C84, which exhibited a low coverage on the PtOEP adlayer. C60 molecules exhibited a full coverage and formed highly ordered adlayers on PtOEP, whereas C84 exhibited disordered arrays. The results of this study showed that the strong π-electron donation by the PtOEP molecules stabilized the reconstructed rows of single-crystal Au planes and that the PtOEP adlayer acted as a strong electron-donating layer, aiding the formation of supramolecularly assembled C60 molecules.

Controlling the stereochemistry and regularity of butanethiol self-assembled monolayers on au(111).

The rich stereochemistry of the self-assembled monolayers (SAMs) of four butanethiols on Au(111) is described, the SAMs containing up to 12 individual C, S, or Au chiral centers per surface unit cell. This is facilitated by synthesis of enantiomerically pure 2-butanethiol (the smallest unsubstituted chiral alkanethiol), followed by in situ scanning tunneling microscopy (STM) imaging combined with density functional theory molecular dynamics STM image simulations. Even though butanethiol SAMs manifest strong headgroup interactions, steric interactions are shown to dominate SAM structure and chirality. Indeed, steric interactions are shown to dictate the nature of the headgroup itself, whether it takes on the adatom-bound motif RS(•)Au(0)S(•)R or involves direct binding of RS(•) to face-centered-cubic or hexagonal-close-packed sites. Binding as RS(•) produces large, organizationally chiral domains even when R is achiral, while adatom binding leads to rectangular plane groups that suppress long-range expression of chirality. Binding as RS(•) also inhibits the pitting intrinsically associated with adatom binding, desirably producing more regularly structured SAMs.

Reaction Mechanism and Regioselectivity of Methyl Oxirane on Si(111)-(7 × 7)

The reaction mechanism and regioselectivity of methyl oxirane (C3H6O) adsorbed on Si(111)-(7 × 7) have been studied using high-resolution electron energy loss spectroscopy (HREELS), in situ scanning tunneling microscopy (STM), and density functional theory (DFT) calculations. The experimental results demonstrate that methyl oxirane chemically binds to Si(111)-(7 × 7) through both dative-bonded addition and ring-opening reaction via cleavage of C–C or one C–O bond within the epoxy group. The STM images also reveal that the adsorption is site-selective with a preference for center adatoms on the faulted half of the (7 × 7) unit cell. The DFT calculations further show that breaking the C(CH3)–O bond is most kinetically favorable with a relatively small barrier of ∼3 kcal/mol. The dative-bonded states are observable on the surface only after three adjacent adatom–rest atom pairs within one-half unit cell are fully reacted with the methyl oxirane molecules during the ring-opening reaction.

In situ scanning tunneling microscopy of the adsorption and polymerization of aniline on Au(1 1 1) electrode in nitric acid

In situ scanning tunneling microscopy (STM) is used to study the morphology and molecular structure of polyaniline (PAN) produced electrochemically on Au(111) electrode in 0.5M nitric acid containing 30mM aniline. Aniline molecules are coadsorbed with nitrate anions in zigzagged chains running parallel to the 〈110〉 directions of the Au(111) electrode. A highly ordered (3×2√3)rect adlattice is observed between 0.5 and 0.8V (vs. reversible hydrogen electrode), which restructures into a (3×2√21) phase at E>0.85V. Guided by the ordered aniline adlayer, the second aniline adlayer is oxidized and polymerizes at 0.92V, yielding one-dimensional PAN stripes aligned along the 〈110〉 directions of the Au(111) substrate. Molecular-resolution STM imaging discerns benzoidal and quinoidal rings in the backbone of PAN. The first two PAN layers grow as a smooth film, but the subsequent layers are deposited in 3D to give a rough morphology. Switching the potential from 0.8 to 0.6V reconfigures PAN molecules from linear to winding form.