Scanning tunneling microscopy observations of benzene molecules on the Rh(111)-(3 x 3) (C6H6+2CO) surface

Unveiling Adatoms in On-Surface Reactions: Combining Scanning Probe Microscopy with van't Hoff Plots

Scanning probe microscopy has become an essential tool to not only study pristine surfaces but also on-surface reactions and molecular self-assembly. Nonetheless, due to inherent limitations, some atoms or (parts of) molecules are either not imaged or cannot be unambiguously identified. Herein, we discuss the arrangement of two different nonplanar molecular assemblies of para-hexaphenyl-dicarbonitrile (Ph₆(CN)₂) on Au(111) based on a combined theoretical and experimental approach. For deposition of Ph₆(CN)₂ on Au(111) kept at room temperature, a rhombic nanoporous network stabilized by a combination of hydrogen bonding and antiparallel dipolar coupling is formed. Annealing at 575 K resulted in an irreversible thermal transformation into a hexagonal nanoporous network stabilized by native gold adatoms. However, the Au adatoms could neither be unequivocally identified by scanning tunneling microscopy nor by noncontact atomic force microscopy. By combining van't Hoff plots derived from our scanning probe images with our density functional theory calculations, we were able to confirm the presence of the elusive Au adatoms in the hexagonal molecular network.

Visualization of the Borazine Core of B3N3-Doped Nanographene by STM

Electronic interface properties and the initial growth of hexa-peri-hexabenzocoronene with a borazine core (BN-HBC) on Au(111) have been studied by using X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), and scanning tunneling microscopy (STM). A weak, but non-negligible, interaction between BN-HBC and Au(111) was found at the interface. Both hexa-peri-hexabenzocoronene (HBC) and BN-HBC molecules form well-defined monolayers. The different contrast in STM images of HBC and BN-HBC at different tunneling voltages with submolecular resolution can be ascribed to differences in the local density of states (LDOS). At positive and negative tunneling voltages, STM images reproduce the distribution of the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) as determined by density functional theory (DFT) calculations very well.

In Situ Variable-Temperature Scanning Tunneling Microscopy Studies of Graphene Growth Using Benzene on Pd(111)

Using a combination of in situ ultrahigh-vacuum variable-temperature scanning tunneling microscopy, ex situ Raman spectroscopy, and scanning electron microscopy, we investigated the growth of graphene using benzene on Pd(111) at temperatures up to 1100 K. Benzene adsorbs readily on Pd(111) at room temperature and forms an ordered superstructure upon annealing at 473 K. Exposure to benzene at 673 K enhances Pd step motion and yields primarily amorphous carbon upon cooling to room temperature. Monolayer graphene domains, 10-30 nm in size, appear during annealing this sample at 873 K. Dosing benzene at 1100 K results in graphene domains with varying degrees of crystallinity, while post-deposition annealing at 1100 K for 1200 s yields monolayer graphene domains larger than 150 × 150 nm². Our results, which indicate that graphene growth on Pd(111) using benzene requires deposition/annealing temperatures higher than 673 K, are in striking contrast with the reported growth of graphene using benzene at temperatures as low as 373 K on relatively inert Cu surfaces.

Photochromic Diarylethenes Designed for Surface Deposition: From Self-Assembled Monolayers to Single Molecules

The efficient switching that can occur between two stable isomers of diarylethenes makes them particularly promising targets for opto- and molecular electronics. To examine these classes of molecules for electronics applications, they have been subjected to a series of scanning tunneling microscopy (STM) experiments, which are the focus of this Review. A brief introduction to the chemical design of diarylethenes in terms of their switching capabilities along with the basics of STM are presented. Next, initial STM studies on these compounds under ambient conditions are discussed. An overview of how molecular design affects the isomerization and self-assembly of diarylethenes at the solid-liquid interface as investigated by STM is then presented, as well as single-molecule studies under ultrahigh vacuum. The last section presents further prospects for molecular design in the field.

Multi-Component Organic Layers on Metal Substrates

Increasingly high hopes are being placed on organic semiconductors for a variety of applications. Progress along these lines, however, requires the design and growth of increasingly complex systems with well-defined structural and electronic properties. These issues have been studied and reviewed extensively in single-component layers, but the focus is gradually shifting towards more complex and functional multi-component assemblies such as donor-acceptor networks. These blends show different properties from those of the corresponding single-component layers, and the understanding on how these properties depend on the different supramolecular environment of multi-component assemblies is crucial for the advancement of organic devices. Here, our understanding of two-dimensional multi-component layers on solid substrates is reviewed. Regarding the structure, the driving forces behind the self-assembly of these systems are described. Regarding the electronic properties, recent insights into how these are affected as the molecule's supramolecular environment changes are explained. Key information for the design and controlled growth of complex, functional multicomponent structures by self-assembly is summarized.

Chlorhexidine: beta-cyclodextrin inhibits yeast growth by extraction of ergosterol

Chlorhexidine (Cx) augmented with beta-cyclodextrin (β-cd) inclusion compounds, termed Cx:β-cd complexes, have been developed for use as antiseptic agents. The aim of this study was to examine the interactions of Cx:β-cd complexes, prepared at different molecular ratios, with sterol and yeast membranes. The Minimal Inhibitory Concentration (MIC) against the yeast Candida albicans (C.a.) was determined for each complex; the MICs were found to range from 0.5 to 2 μg/mL. To confirm the MIC data, quantitative analysis of viable cells was performed using trypan blue staining. Mechanistic characterization of the interactions that the Cx:β-cd complexes have with the yeast membrane and assessment of membrane morphology following exposure to Cx:β-cd complexes were performed using Sterol Quantification Method analysis (SQM) and scanning electron microscopy (SEM). SQM revealed that sterol extraction increased with increasing β-cd concentrations (1.71 ×10³; 1.4 ×10³; 3.45 ×10³, and 3.74 ×10³ CFU for 1:1, 1:2, 1:3, and 1:4, respectively), likely as a consequence of membrane ergosterol solubilization. SEM images demonstrated that cell membrane damage is a visible and significant mechanism that contributes to the antimicrobial effects of Cx:β-cd complexes. Cell disorganization increased significantly as the proportion of β-cyclodextrin present in the complex increased. Morphology of cells exposed to complexes with 1:3 and 1:4 molar ratios of Cx:β-cd were observed to have large aggregates mixed with yeast remains, representing more membrane disruption than that observed in cells treated with Cx alone. In conclusion, nanoaggregates of Cx:β-cd complexes block yeast growth via ergosterol extraction, permeabilizing the membrane by creating cluster-like structures within the cell membrane, possibly due to high amounts of hydrogen bonding.

Laser Deposition, Vibrational Spectroscopy, NMR Spectroscopy and Stm Imaging of C60 and C70

We recently demonstrated that C60 and C70, as well as other fullerenes, can be deposited and accumulated on surfaces using laser ablation of graphite in an Inert gas atmosphere. After learning of the work of Krätschmer et al. indicating the presence of C60 in carbon soot, we showed that samples consisting almost exclusively of C60 and C70 can be sublimed from such soot. Vibrational Raman spectra of C60 and C70 were obtained from these samples. The C60 spectrum Is consistent with the calculated spectrum of Buckmlnsterfullerene, and the strongest three lines can be assigned on the basis of frequency and polarization. The NMR spectrum of dissolved C60 was then obtained, and found to consist of a single resonance, establishing the icosahedral symmetry of this molecule. STM images of the C60 molecules on a Au(111) crystal face show that these clusters form hexagonal arrays with an intercluster spacing of 11.0 Å and are mobile at ambient temperature. Distinctly taller species evident in the arrays are believed to be C70 clusters. Vibrational Raman and infrared spectra have also been obtained for separated C60 and C70.

Molecular Dynamics in Two-Dimensional Supramolecular Systems Observed by STM

Since the invention of scanning tunneling microscopy (STM), 2D supramolecular architectures have been observed under various experimental conditions. The construction of these architectures arises from the balance between interactions at the medium-solid interface. This review summarizes molecular motion observed in 2D-supramolecular structures on surfaces using nanospace resolution STM. The observation of molecular motion on surfaces provides a visual understanding of intermolecular interactions, which are the major driving force behind supramolecular arrangement.

NO-Induced Reorganization of Porphyrin Arrays

We studied the interaction of a highly ordered array of Co-tetraphenylporphyrin (CoTPP) with NO on Ag(111) by in situ scanning tunneling microscopy and X-ray photoelectron spectroscopy. Upon NO exposure, the initially quadratically ordered CoTPP layer reorganizes, showing a wealth of highly ordered NO+CoTPP coadsorbate phases with increasing size of the unit cell, interpreted as due to attractive lateral dipole-dipole interactions between the two species. The findings not only suggest a novel approach to control the arrangement of adsorbed porphyrins in particular but also should generally be considered in the production of functional layers from large organic molecules under ambient conditions or after exposure to small electronegative molecules.

Chiral recognition of PVBA on Pd(111) and Ag(111) surfaces

An asymmetric planar molecule, 4-trans-2-(pyrid-4-yl-vinyl) benzoic acid (PVBA), has been used to establish the organic chiral recognition on fcc(111) metal surfaces. The strong correlation between the orientation and chiral recognition of PVBA on both Ag(111) and Pd(111) guides the choice of a model potential, which determines the relative binding energy of PVBA on fcc(111). An angle-dependent calculation of relative binding energy reproduces the experimental observation of the chiral recognition of PVBA on Ag(111) but not on Pd(111).

Time-Lapse STM Studies of Diastereomeric Cinchona Alkaloids on Platinum Metals

The adsorption of cinchonidine (CD) and cinchonine (CN) on Pt(111) and Pd(111) single crystals has been investigated by means of scanning tunneling microscopy (STM) in an ultrahigh vacuum system. In time-lapse series the mobilities of different adsorption species have been determined on a single molecule basis and with varying hydrogen background pressures in the system. The diastereomeric cinchona alkaloids, CD and CN, which are widely used as chiral modifiers of platinum group metals in catalytic enantioselective hydrogenation, showed similar adsorption modes and diffusion behavior on Pt(111), except that the flatly adsorbed CN molecules which were free (not in a dimer/cluster) were significantly more mobile than their CD analogues. CD adsorbed on Pd(111) showed similar adsorption modes as observed on Pt(111) but at considerably higher mobility of the flatly absorbed species already in the low-pressure region. The observed adsorption behaviors are discussed in the context of independent ATR-IR measurements and theoretical calculations. Special emphasis is put on the nonlinear effect observed in hydrogenation reactions with CD/CN mixtures. Our observations corroborate that this effect is mainly a consequence of the different adsorption strengths of CD and CN on Pt.

The Liquid Water−Benzene System

The 500 MHz NMR spectra of water-benzene solution near saturation at 303.15, 323.15, and 343.15 K indicate that there is a proton-proton exchange between the water and benzene molecules. In the solution water appears to be present as a dimer attached to the benzene pi cloud on one side of each of the two (initially degenerate) fundamental energy levels, as predicted by the Jahn-Teller effect. This view is reinforced by the fact that one of its hydrogen atoms hovers above one of the carbon atoms and the other three are spread upward around the C6 axis of the benzene molecule. It is also supported by the calculated NMR spectra. Both effects are responsible for the change in the NMR spectra of the water molecules from a single line into four AB signals.

Image Contrast Analysis of STM Images of Self-Assembled Dioctadecyl Chalcogenides on Graphite at the Liquid-Solid Interface

The structures of self-assembled monolayers of dioctadecyl selenide (CH3(CH2)17)2Se and dioctadecyl telluride (CH3(CH2)17)2Te, as well as dioctadecyl ether (CH3(CH2)17)2O and dioctadecyl sulfide (CH3(CH2)17)2S, on graphite at the liquid-solid interface were systematically investigated by scanning tunneling microscopy (STM). Both dioctadecyl selenide and telluride formed monolayer structures in which the tilt angle between the molecular axis of the alkyl chain and the lamellae axis was 90 degrees , while dioctadecyl ether assembled with a tilt angle of 60 degrees . Dioctadecyl sulfide was found to make two different self-assembled structures having tilt angles of 60 and 90 degrees . When selenide was embedded in ether compounds in mixed self-assembled monolayers, the alkyl chains of the selenide became blurred, implying that the alkyl chains in the monolayers were unstable. This is in contrast with the structure of co-adsorbed monolayers of the ether and sulfide compounds, where the images of all alkyl chains had high spatial resolution. For the co-adsorbed monolayers, the image contrast of chalcogen atoms was normalized compared with that of alkyl chains of the ether compound in the same image frame. The normalized image contrast was found to be independent of the measurement conditions involving tip shapes, having the following trend, Te>Se>S>C>O. The difference in the normalized image contrast among chalcogen atoms are discussed based on fundamental parameters like polarizability and atomic radii.

Molecular assembly and self-assembly: molecular nanoscience for future technologies

In this review the emerging science of single molecules is discussed in the perspective of nanoscale molecular functions and devices. New methods for the controlled assembly of well-defined molecular nanostructures are presented: self assembly and single molecular positioning. The observation and selective modification of conformation, electronics, and molecular mechanics of individual molecules and molecular assemblies by scanning probes is demonstrated. To complement this scientific review, some of the possible consequences and visions for future developments are discussed, as far as they derive from the presented systems. The prospects of nanoscale science to stimulate technological evolution are exemplified.

Ferroelectric molecular films for nanoscopic ultrahigh-density memories

The formation and visualization of nanometer-scale polarized domains in ultrathin ferroelectric molecular films by scanning-probe microscopy are described. These operations to the ferroelectric domains correspond to the "writing" and the "reading" process, respectively, for the data-storage application. In addition, nanometer-scale structures and the local electrical properties of the local domains, including the interface effect, are discussed. The achieved minimum diameter of the written ferroelectric domains was 30 nm. The size of the "recording" dot corresponds to the recording density of about 230 Gbit/in.(2). The "erasing" process by switching domains was also demonstrated. Furthermore, nanometer-scale ferroelectric domains using VDF oligomer molecular films were successfully formed, which has opened the way to the control of single molecular dipoles.