Evidence of One-Dimensional Channels in Hydrogen-Bonded Organic Porous Thin Films Fabricated at the Air/Liquid Interface.

Visualization of periodically aligned pores in organic frameworks is a key to the understanding of their structural control. Comparing to monolayer-thick self-assembled molecular networks, real-space nanoscale characterization of thicker films, especially obtaining information on the stacking manner of molecules is challenging. Here, we report an atomic force microscopy study of hydrogen-bonded thin films fabricated at the air/liquid interface. The presence of one-dimensional channels is evidenced by resolving honeycomb structures over the films with the thickness variation of more than several nanometers. We also demonstrate that the film thickness can be controlled by the ratio of mixed solvent rather than the surface pressure during the fabrication at the air/liquid interface.

Aromatic Azide Transformation on the Ag(111) Surface Studied by Scanning Probe Microscopy.

Chemical transformation of 9-azidophenanthrene on the Ag(111) surface was studied by nc-AFM in UHV. High-resolution imaging supported by first-principle calculations revealed the structure of the final products that originated from a common and elusive 9-phenanthryl nitrenoid intermediate chemisorbed on the Ag(111) surface. A formal nitrene insertion into the C-H bond along with its dimerisation and hydrogenation were identified as main reaction channels. Thus, the ability of aryl azides to form covalent σ- and π-bonds between their transformation products on a solid surface was demonstrated at a single-molecule level.

Pentacene/TiO2 Anatase Hybrid Interface Study by Scanning Probe Microscopy and First Principles Calculations.

The understanding and control of the buried interface between functional materials in optoelectronic devices is key to improving device performance. We combined atomic resolution scanning probe microscopy with first-principles calculations to characterize the technologically relevant organic/inorganic interface structure between pentacene molecules and the TiO2 anatase (101) surface. A multipass atomic force microscopy imaging technique overcomes the technical challenge of imaging simultaneously the corrugated anatase substrate, molecular adsorbates, monolayers, and bilayers at the same level of detail. Submolecular resolution images revealed the orientation of the adsorbates with respect to the substrate and allowed direct insights into interface formation. Pentacene molecules were found to physisorb parallel to the anatase substrate in the first contact layer, passivating the surface and promoting bulk-like growth in further organic layers. While molecular electronic states were not significantly hybridized by the substrate, simulations predicted localized pathways for molecule-surface charge injection. The localized states were associated with the molecular lowest unoccupied molecular orbital inside the oxide conduction band, pointing to efficient transfer of photo-induced electron charge carriers across this interface in prospective photovoltaic devices. In uncovering the atomic arrangement and favorable electronic properties of the pentacene/anatase interface, our findings testify to the maturity and analytic power of our methodology in further studies of organic/inorganic interfaces.

Atomic-scale luminescence measurement and theoretical analysis unveiling electron energy dissipation at a p-type GaAs(110) surface.

Luminescence of p-type GaAs was induced by electron injection from the tip of a scanning tunnelling microscope into a GaAs(110) surface. Atomically-resolved photon maps revealed a significant reduction in luminescence intensity at surface electronic states localized near Ga atoms. Theoretical analysis based on first principles calculations and a rate equation approach was performed to describe the perspective of electron energy dissipation at the surface. Our study reveals that non-radiative recombination through the surface states (SS) is a dominant process for the electron energy dissipation at the surface, which is suggestive of the fast scattering of injected electrons into the SS.

Atomic species identification at the (101) anatase surface by simultaneous scanning tunnelling and atomic force microscopy.

Anatase is a pivotal material in devices for energy-harvesting applications and catalysis. Methods for the accurate characterization of this reducible oxide at the atomic scale are critical in the exploration of outstanding properties for technological developments. Here we combine atomic force microscopy (AFM) and scanning tunnelling microscopy (STM), supported by first-principles calculations, for the simultaneous imaging and unambiguous identification of atomic species at the (101) anatase surface. We demonstrate that dynamic AFM-STM operation allows atomic resolution imaging within the material's band gap. Based on key distinguishing features extracted from calculations and experiments, we identify candidates for the most common surface defects. Our results pave the way for the understanding of surface processes, like adsorption of metal dopants and photoactive molecules, that are fundamental for the catalytic and photovoltaic applications of anatase, and demonstrate the potential of dynamic AFM-STM for the characterization of wide band gap materials.

Imaging three-dimensional surface objects with submolecular resolution by atomic force microscopy.

Submolecular imaging by atomic force microscopy (AFM) has recently been established as a stunning technique to reveal the chemical structure of unknown molecules, to characterize intramolecular charge distributions and bond ordering, as well as to study chemical transformations and intermolecular interactions. So far, most of these feats were achieved on planar molecular systems because high-resolution imaging of three-dimensional (3D) surface structures with AFM remains challenging. Here we present a method for high-resolution imaging of nonplanar molecules and 3D surface systems using AFM with silicon cantilevers as force sensors. We demonstrate this method by resolving the step-edges of the (101) anatase surface at the atomic scale by simultaneously visualizing the structure of a pentacene molecule together with the atomic positions of the substrate and by resolving the contour and probe-surface force field on a C60 molecule with intramolecular resolution. The method reported here holds substantial promise for the study of 3D surface systems such as nanotubes, clusters, nanoparticles, polymers, and biomolecules using AFM with high resolution.

Supramolecular assembly through interactions between molecular dipoles and alkali metal ions.

Establishing a way to fabricate well-ordered molecular structures is a necessary step toward advancement in organic optoelectronic devices. Here, we propose to use interactions between electric dipoles of molecules and alkali metal ions to form a well-developed homogeneous monolayer of diarylethene molecules on the Cu(111) surface with the aid of NaCl co-deposition. Scanning tunneling microscopy and density functional theory calculation results indicate that the formation of a row-type structure occurs as a result of interactions between the Na(+) ions and the diarylethene molecular dipoles, drastically changing the adsorption configuration from that without Na(+).

Adsorption-induced stability reversal of photochromic diarylethene on metal surfaces.

A scanning tunneling microscopy study of a photochromic diarylethene adsorbed on Au and Cu reveals the reversal of thermodynamic stability between its two isomers compared to the gas phase, in solution, and its single crystal, which could result from the molecule-substrate interaction such as charge transfer and hybridization.

Two-dimensional superstructure formation of fluorinated fullerene on Au(111): a scanning tunneling microscopy study.

A two-dimensional fluorinated fullerene (C(60)F(36)) superstructure has been successfully formed on Au(111) and was investigated using scanning tunneling microscopy (STM) and density functional theory calculations. Although there exist three isomers (C(3), C(1), and T) in our molecular source, STM images of the molecules in the well-ordered region all appear identical, with 3-fold symmetry. This observation together with the differences in the calculated lowest unoccupied molecular orbital (LUMO) distribution among the three isomers suggests that a well-ordered monolayer consists of only the C(3) isomer. Because of the strong electron-accepting ability of C(60)F(36), the adsorption orientation can be explained by localized distribution of its LUMO, where partial electron transfer from Au(111) occurs. Intermolecular C-F···π electrostatic interactions are the other important factor in the formation of the superstructure, which determines the lateral orientation of C(60)F(36) molecules on Au(111). On the basis of scanning tunneling spectra obtained inside the superstructure, we found that the LUMO is located at 1.0 eV above the Fermi level (E(F)), while the highest occupied molecular orbital (HOMO) is at 4.6 eV below the E(F). This large energy gap with the very deep HOMO as well as uniform electronic structure in the molecular layer implies a potential for application of C(60)F(36) to an electron transport layer in organic electronic devices.

Structure and reactions of carbon and hydrogen on Ru(0001): a scanning tunneling microscopy study.

The interaction between carbon and hydrogen atoms on a Ru(0001) surface was studied using scanning tunneling microscopy (STM), density functional theory (DFT) and STM image calculations. Formation of CH species by reaction between adsorbed H and C was observed to occur readily at 100 K. When the coverage of H increased new complexes of the form of CH+nH (n=1, 2, and 3) were observed. These complexes, never observed before, might be precursors for further hydrogenation reactions. DFT analysis reveals that a considerable energy barrier exists for the CH+H-->CH(2) reaction.