Probing liquid/solid interfaces at the molecular level.

Molecular dynamics simulation studies of the structure and the differential capacitance (DC) for the ionic liquid (IL) N-methyl-N-propylpyrrolidinium bis(trifluoromethane)sulfonyl imide ([pyr13][TFSI]) near a graphite electrode have been performed as a function temperature and electrode potential. The IL exhibits a multilayer structure that extends 20−30 A from the electrode surface. The composition and ion orientation in the innermost layer were found to be strongly dependent on the electrode potential. While at potentials near the potential of zero charge (PZC), both cations and anions adjacent to the surface are oriented primarily perpendicular to the surface, the counterions in first layer orient increasingly parallel to the surface with increasing electrode potential. A minimum in DC observed around −1 VRPZC (potential relative to the PZC) corresponds to the point of highest density of perpendicularly aligned TFSI near the electrode. Maxima in the DC observed around +1.5 and −2.5 VRPZC are associated...

Molecular Insights into the Potential and Temperature Dependences of the Differential Capacitance of a Room-Temperature Ionic Liquid at Graphite Electrodes

Investigations of long-chain hydrocarbon and substituted hydrocarbon materials physisorbed at the liquid−solid interface have provided a particularly fruitful area of investigation for the scanning tunneling microscope (STM). Studies of this kind provide considerable information concerning surface−adsorbate interactions, the nature of the STM contrast mechanism for molecules adsorbed on surfaces, the effect of the chemical nature of the liquid solvent on the rearrangement and bonding of adsorbates, and the role played by the underlying structure of the surface itself (defects, domains, etc.) in determining the molecular arrangement of these adsorbates. All of these studies are aimed at the elucidation and control of surface and adsorbate structures and their relationship to the electronic and chemical properties of materials. The successful imaging of numerous physisorbed organic films at the solution−solid interface has demonstrated that the STM is sensitive enough to differentiate chemical functional gr...

STM Investigations of Organic Molecules Physisorbed at the Liquid−Solid Interface

The adsorption of benzenethiol on Au(111) from aqueous solution has been investigated by using surface-enhanced infrared absorption spectroscopy (SEIRAS), scanning tunneling microscopy (STM), and cyclic voltammetry (CV). This molecule is dissociatively adsorbed on the surface as a Au benzenethiolate and forms a well-ordered monolayer with a commensurate (√13×√13)R13.9° symmetry. The ordered phase is formed within a few minutes in 0.1 mM aqueous solution, and further immersion of the substrate results in the deposition of multilayers. The phenyl ring is tilted about 30° from the surface normal in the monolayer. The coverage of this molecule is calculated to be 0.31 (or 0.71 nmol cm-2) from the (√13×√13)R13.9° symmetry, but it was estimated to be 0.23 (or 0.53 nmol cm-2) from the reductive desorption of the monolayer in alkaline solution. The discrepancy between the two measurements may be ascribed to the coadsorption of sulfur atoms and the presence of vacancies.

Molecular Orientation and Ordered Structure of Benzenethiol Adsorbed on Gold(111)

A series of functionalized alkanes and/or alkyl alcohols have been prepared and imaged by scanning tunneling microscopy (STM) methods on graphite surfaces. The stability of these ordered overlayers has facilitated reproducible collection of STM images at room temperature with submolecular resolution, in most cases allowing identification of individual hydrogen atoms in the alkane chains, but in all cases allowing identification of molecular length features and other aspects of the image that can be unequivocally related to the presence of functional groups in the various molecules of concern. Functional groups imaged in this study include halides (X = F, Cl, Br, I), amines, alcohols, nitriles, alkenes, alkynes, ethers, thioethers, and disulfides. Except for −Cl and −OH, all of the other functional groups could be distinguished from each other and from −Cl or −OH through an analysis of their STM metrics and image contrast behavior. The dominance of molecular topography in producing the STM images of alkanes and alkanols was established experimentally and also was consistent with quantum chemistry calculations. Unlike the contrast of the methylene regions of the alkyl chains, the STM contrast produced by the various functional groups was not dominated by topographic effects, indicating that variations in local electronic coupling were important in producing the observed STM images of these regions of the molecules. For molecules in which electronic effects overwhelmed topographic effects in determining the image contrast, a simple model is presented to explain the variation in the electronic coupling component that produces the contrast between the various functional groups observed in the STM images. Additionally, the bias dependence of these STM images has been investigated and the contrast vs bias behavior is related to factors involving electron transfer and hole transfer that have been identified as potentially being important in dominating the electronic coupling in molecular electron transfer processes.

Source of Image Contrast in STM Images of Functionalized Alkanes on Graphite: A Systematic Functional Group Approach

Monolayer films of several primary substituted hydrocarbons CH3(CH2)nCH2X (X = CH3, OH, NH2, SH, Cl, Br, I; n = 16−30) have been imaged on graphite at the solution−substrate interface using a scanning tunneling microscope (STM) The straight chain hydrocarbons form well-ordered 2-D films on graphite and physisorb with their molecular axes parallel to the surface The NH2, SH, Br, and I end groups are observed as bright spots in the STM image corresponding to an enhancement in the tunnel current in the vicinity of the functional group, relative to the remainder of the carbon chain On the other hand, the OH and Cl substituents were not distinguishable from the alkyl chain in the STM images Comparison of the relative “brightness” of the functional groups with respect to the carbon chain reveals an empirical relationship between increasing relative brightness and increasing molecular polarizability A model is proposed to describe the STM imaging mechanism for these insulating, physisorbed films in which th

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Functional Group Identification in Scanning Tunneling Microscopy of Molecular Adsorbates

Scanning Tunneling Microscopy Studies of Solvent Effects on the Adsorption and Mobility of Triacontane/Triacontanol Molecules Adsorbed on Graphite

Scanning tunneling microscopy (STM) images of l-docosanol, 1-docosanethiol, didocosyl disulfide, and 1-chlorooctadecane on graphite are compared. The images of the I -docosanethiol and didocosyl disulfide show bright spots which are attributed to the positions of the S-H and S-S functional groups. The STM images of the l-docosanol and l-chlorooctadecane do not show such bright spots. Thb fact that both the S-H and S-S groups appear bright in the STM images indicates that the presence of an S atom on graphite results in a higher tunneling current when the tip scans over it compared to the tunneling current over a C, O. or Cl atom. The diff'erent behavior of the S atoms conrpiired to the O. C. and Cl atoms is discussed in terrns of the interactions between these atoms and the underlving graphite substrate. The persistent brightness of S atoms in the images of molecular adsorbates sugsests that sult'ur nrav serve as a useful "chromophore" for molecules imased bv STM.

Differentiating Functional Groups with the Scanning Tunneling Microscope

Mixtures of triacontanol/triacontane adsorbed on graphite surfaces have been investigated using STM in order to understand the relative importance of functional groups in determining the adsorbate/surface sticking probability. The images observed for these mixtures indicate that the alcohol and alkanes form separate regions on the surface and that the sticking probability is dependent on the liquid phase environment above the graphite surface. The mixtures exhibit interesting reorganization dynamics during which the alcohols and alkanes separate out on the surface.

Scanning Tunneling Microscopy Studies of Alcohol/Alkane Mixtures Adsorbed on Graphite Surfaces

Scanning tunneling microscopy images of n-alkanes, n-alcohols, n-alkylthiols and n-alkylchlorides have been obtained to determine if the STM is capable of differentiating between molecules where the only difference is the functional group at the ends of a molecule. These studies indicate that there are two possible ways of differentiating between these molecules: (1) by observing differences in the relative orientation of the molecules with respect to each other, and (2) by the magnitude of the tunneling current in the vicinity of the functional group. Using the tunneling current to differentiate between functional groups also permits identification of the position of the functional group in the molecule.

Bhawani Venkataraman

Papers

Functional Group Identification in Scanning Tunneling Microscopy of Molecular Adsorbates

Scanning Tunneling Microscopy Studies of Solvent Effects on the Adsorption and Mobility of Triacontane/Triacontanol Molecules Adsorbed on Graphite

Differentiating Functional Groups with the Scanning Tunneling Microscope

Scanning Tunneling Microscopy Studies of Alcohol/Alkane Mixtures Adsorbed on Graphite Surfaces

Scanning Tunneling Microscopy Studies of Hydrocarbons Adsorbed on Graphite Surfaces