Photon emission from adsorbed C60 molecules with sub-nanometer lateral resolution

We present the first experimental demonstration of spatially resolved photon emission of individual molecules on a surface. A scanning tunneling microscope (STM) was used as a local electron source to excite photon emission from hexagonal arrays of C₆₀ molecules on Au(110) surfaces. Specifically, we show that in maps of photon emission intensities, C₆₀ fullerenes appear as arrays of individual light emitters 4 Å in diameter and separated by 10 Å. Comparison with simultaneously recorded STM images reveals, that most intense emission is detected when the STM tip is centered above a molecule. The results demonstrate the highest spatial resolution of light emission to date using a scanning probe technique.     

Organic thin-film transistors of pentacene films fabricated from a supersonic molecular beam source

Top-contact organic thin-film transistors (OTFTs) of pentacene have been fabricated on bare SiO₂ and SiO₂ modified with hexamethyldisilazane (HMDS) and octadecyltrichlorosilane (OTS). The pentacene films were deposited from a supersonic molecular beam source with kinetic energy of incident molecules ranging from 1.5 to 6.7 eV. The field-effect mobility of OTFTs was found to increase systematically with increasing kinetic energy of the molecular beam. The improvements are more important on HMDS- and OTS-treated surfaces than on bare SiO₂. Tapping mode atomic force microscopy images reveal that pentacene thin films deposited at high kinetic energy form with significantly larger grains—independent of surface treatment—than films deposited using low-energy beams.     

Stability of two-dimensional nanostructures

We investigate atomic and molecular nanostructures on metal surfaces by variable low-temperature scanning tunnelling microscopy. In combination with molecular dynamics calculations we achieve a detailed understanding of the stability of these structures.?Atomic nanostructures in homoepitaxial metallic systems are thermodynamically only metastable. Two-dimensional islands on Ag(110) decay above a threshold temperature of T _l=175 K. Caused by the anisotropy of the surface, distinct decay behaviours exist above and below a critical temperature of T _c=220 K. Calculations based on effective medium potentials of the underlying rate limiting atomic processes allow us to identify the one-dimensional decay below T _c as well as the two-dimensional decay above T _c.?In contrast to atoms, the intermolecular electrostatic interaction of polar molecules leads to thermodynamically stable structures. On the reconstructed Au(111) surface, the pseudo-chiral 1-nitronaphthalin forms two-dimensional supermolecular clusters consisting predominantly of ten molecules. Comparison of images with submolecular resolution to local density calculations elucidates the thermodynamical stability as well as the internal structure of the decamers. Received: 25 March 1999 / Accepted: 17 August 1999 / Published online: 6 October 1999     

High resolution surface imaging of Co/ZrO2 catalyst by TOF-SIMS

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is a useful tool for the characterization of supported catalyst. Especially, the possibility of surface imaging appears very interesting. The images of not only elements distribution but also molecules or their fragments may be performed. Owing to the fast improvement of time-of-flight secondary ions mass spectrometers, obtaining the surface images with lateral resolution even below 100 nm is possible. The application of TOF-SIMS to the high resolution surface imaging of catalysts surfaces was shown. The influence of the preparation conditions of Co/ZrO₂ catalyst on the dispersion and composition of the active phase was investigated. The Co⁺/Zr⁺ and CoOH⁺/Zr⁺ intensity ratio values were calculated.     

Magnetic contrast from domain walls in BaFe12O19 by tunneling stabilized magnetic force microscopy

The magnetic structure of BaFe₁₂O₁₉ is imaged with a scanning tunneling microscope having a flexible, magnetic tip. We find that Fe thin films evaporated on a silicon tip, integrated with a cantilever, behave as magnetically soft tips. Therefore, we are able to image domain walls with high lateral resolution. A different contrast along the domain wall due to surface magnetic charges is observed. We explain the data using previously established models for wavy domain walls. The obtained images are the first experimental evidence of magnetic charges induced on the wavy domain walls in BaFe₁₂O₁₉.     

Surface potential images of polycrystalline organic semiconductors obtained by Kelvin probe force microscopy

P-type copper phthalocyanine (CuPc) and n-type hexadecafluorophthalocyanina-tocopper (F₁₆CuPc) polycrystalline films were investigated by Kelvin probe force microscopy (KPFM). Topographic and corresponding surface potential images are obtained simultaneously. Surface potential images are related with the local work function of crystalline facets and potential barriers at the grain boundaries (GBs) in organic semiconductors. Based on the spatial distribution of surface potential at GBs, donor- and acceptor-like trapping states in the grain boundaries (GBs) of p-CuPc and n-F₁₆CuPc films are confirmed respectively. In view of spatial energy spectrum in micro-scale provided by KPFM, it is going to be a powerful tool to characterize the local electronic properties of organic semiconductors.     

Determination of C60/C70 ratios in fullerene mixtures and film characterization by scanning tunneling microscopy

Fullerene powder mixtures with different C₆₀/C₇₀ ratios have been analyzed by a variety of techniques, and results have been compared. The fullerence mixtures have been characterized as solutions in n-hexane by high-pressure liquid chromatography (HPLC) and UV-VIS spectroscopy. Thin films of fullerenes on Au(111) have been prepared from the mixtures by sublimation. The sublimation process has been studied by simultaneous thermogravimetric and differential thermal analyses. Thin fullerene films on Au(111) have been investigated by scanning tunneling microscopy (STM). The STM images show primarily two types of ballshaped molecules arranged in a lattice with hexagonal symmetry (fcc(111) face, nearest neighbour distance: 1 nm). The two species differ in diameter. STM images of films made of mixtures of different C₆₀/C₇₀ ratios show that C₇₀ molecules display a larger apparent diameter (0.8 nm) and corrugation than C₆₀ molecules (0.7 nm). The C₆₀/C₇₀ ratios obtained by counting the corresponding molecular species in the STM images of the thin films are compared to the C₆₀/C₇₀ ratios determined by HPLC on hexane solutions of the mixtures. The observed differences might be explained by different rates of sublimation for the two species. The STM images reveal film defects (vacancies and boundaries) and dynamic processes (displacement of C₇₀ molecules and vacancies). In films prepared to have a C₆₀ coverage of less than one monolayer, stable structural units of the C₆₀(111) surface consisting of three or seven C₆₀ molecules are revealed by STM. Occasionally, substructure within individual fullerene molecules is observed.     

Ruthenium dioxide, a fascinating material for atomic scale surface chemistry

Over the past few years, RuO₂ has developed into one of the best-characterized late transition metal oxides in surface science, revealing unique and promising redox properties. The CO oxidation reaction over RuO₂ (110) was intensively studied by low-energy electron diffraction, scanning tunneling microscopy, high resolution core level spectroscopy, and density functional theory calculations, connecting structural and electronic properties with chemical properties. On the atomic scale the presence of one-fold coordinatively unsaturated Ru sites (1f-cus Ru) is the primary reason for the high activity of stoichiometric RuO₂ (110) towards the oxidation of CO and other small alcohols. On the stoichiometric RuO₂ (110) surface, CO molecules adsorb strongly (adsorption energy exceeding 1.2 eV) on top of the 1f-cus Ru atoms, from where the actual oxidation reaction step takes place via recombination with under-coordinated lattice oxygen to form CO₂ (the so-called Mars–van Krevelen mechanism); the conversion probability of this process is as high as 80%. This mechanism leads to a (partial) reduction of the RuO₂ (110) surface, producing two-fold coordinatively unsaturated Ru sites (2f-cus Ru) via the removal of bridging O atoms. Therefore, equally important for being a good catalyst is the facile re-oxidation of the mildly reduced RuO₂ (110) surface by oxygen supply from the gas phase. A weakly held oxygen species was found to adsorb on top of the 1f-cus Ru atoms and to actuate the restoration of the reduced RuO₂ (110) surface. On the reduced RuO₂ (110) surface, CO molecules adsorb in bridge sites above the 2f-cus Ru atoms by 1.85 eV, while the CO bond strength over 1f-cus Ru atoms is 1.61 eV. Received: 27 March 2001 / Accepted: 23 July 2001 / Published online: 3 April 2002     

Impact of organic contamination on laser-induced damage threshold of high reflectance coatings in vacuum

The influence of organic contamination in vacuum on the laser-induced damage threshold (LIDT) of coatings is studied. TiO₂/SiO₂ dielectric mirrors with high reflection at 1064 nm are deposited by the electron beam evaporation method. The LIDTs of mirrors are measured in vacuum and atmosphere, respectively. It is found that the contamination in vacuum is easily attracted to optical surfaces because of the low pressure and becomes the source of damage. LIDTs of mirrors have a little change in vacuum compared with in atmosphere when the organic contamination is wiped off. The results indicate that organic contamination is a significant reason to decrease the LIDT. N₂ molecules in vacuum can reduce the influence of the organic contaminations and prtectect high reflectance coatings.     

Enhanced cohesion of photo-oxygenated fullerene films: A new opportunity for lithography

The irradiation of sublimed fullerene (C₆₀ and C₇₀) thin films with ultraviolet light in an oxygen-rich ambient has been found to lead to a substantially increased cohesive energy in the fullerene solid. The decreased solubility and lower vapor pressure of the phototransformed material enables wet (organic solvents) or dry (thermal or photon-induced sublimation) development of photo-defined negative images. One micrometer wide lines with good edge definition are demonstrated. X-ray, infrared, optical absorption, and high performance liquid chromatography reveal that photo-oxygenated C₆₀ retains its fcc crystal structure but with a substantial fraction of the C₆₀ molecules modified with carbonyl (C=O) bonds.     

Improvement of depth resolution in XPS analysis of fluorinated layer using C60 ion sputtering

Depth profile of C₆₀ ion-used X-ray photoelectron spectroscopy (XPS) was studied on fluorinated organic layers with different thicknesses. We found that the depth resolution decreased, the sputtering rate went down and the surface turned rough as the layer thickness increased. This is because carbon-rich layer was formed on the surface by cross-linking reaction of the polymer and/or accumulation of degraded C₆₀ through continuous sputtering. Surprisingly, the high sputtering rate drastically improved the resolution of the analysis. The rate over 48.7 nm/min did not show any deterioration on the depth resolution, the sputtering rate and surface smoothness.     

Inelastic X-ray scattering studies of phonons propagating along the axial direction of a DNA molecule having different counter-ion atmosphere

Shear-aligned 40 wt% calf-thymus Na-DNA molecules in aqueous solutions are prepared in their liquid crystalline phases and studied by high resolution inelastic X-ray scattering (IXS). Measured IXS spectra are analyzed with the generalized three effective eignmode (GTEE) theory. The phonon dispersion relations along the axial direction of DNA molecules with different MgCl₂ concentrations are constructed and compared. It is found that the sound speed along the axial direction of DNA molecules varies only slightly, but the phonon dampening is greatly affected with the increase amount of MgCl₂ concentration. Using the GTEE theory, we are able to extract the longitudinal viscosity in the hydrodynamic limit from the Q-dependence of a fitted parameter. We make a comprehensive review of the GTEE theory and discuss detailed analyses of IXS spectra taking into account finite energy resolution of the instrument.     

Demonstration of positron re-emission microscopy using an immersion objective

Hulett et al. have suggested forming a magnified image of the positrons re-emitted from a surface having a negative positron affinity as a new kind of microscope, the positron re-emission microscope (PRM). We have built an immersion objective (cathode lens) for the PRM and obtained the first images using a Ni film back-illuminated with 5 keV positrons from a brightness enhanced slow positron beam. The magnification at the detector plane is 330x and the resolution is better than 1m. With the addition of a projector lens the magnification and resolution should be sufficient to observe defect structures and large molecules on the surface.     

Recent results in magnetic force microscopy

We present domain wall images obtained by using Magnetic Force Microscope (MFM) on magnetic samples like: double layer of permalloy alloy, magnetic hard disk, BaFe₁₂O₁₉ single crystal and YGdTmGa/YSmTmGa magnetic garnet. We have imaged topography and magnetic forces of the same area. The Fe double- and single-layer thin film tips have been prepared to achieve high sensitivity (10^–12N) and high resolution of MFM.     

Investigations of C60 molecules deposited on Si(111) by noncontact atomic force microscopy

We demonstrated the high resolution imaging of the organic molecules using noncontact atomic force microscopy in ultrahigh vacuum. The sample was C₆₀ molecules deposited on the Si(111)-7×7 reconstructed surface. When the thickness of the C₆₀ film was submonolayer, we could image some isolated C₆₀ molecules and the reconstructed Si surface simultaneously. However, the imaging was highly unstable not only because of the large structure but also due to the large difference between the interaction forces on the molecules and on the Si surface. On the other hand, when the thickness of the C₆₀ molecules was almost monolayer, individual molecules could be stably imaged.     

Ordered arrays of myoglobin adsorbed on the surfactant modified surface by scanning tunneling microscopy

Myoglobin molecules were deposited on a surfactant sodium dodecyl sulfate modified HOPG surface and imaged in air with a high resolution scanning tunneling microscope (STM) for the first time. STM images exhibit not only ordered arrays of the surfactant molecules but also regular two-dimensional arrays of myoglobin molecules. From STM images, the myoglobin molecule can be described as an ellipsoid-shaped pattern for the tertiary structure. In this study the dimensions of a myoglobin molecule were determined approximately as 43.0 × 36.2 × 8 ų, which are in good agreement with the known data from X-ray analysis, except for the height of a molecule which cannot be estimated from STM.     

Photostimulated chemistry at the metal-adsorbate interface

The ultraviolet photochemistry of molecules adsorbed on metallic surfaces has been studied using excimer lasers as radiation source. Dissociation with the fragments either ejected into the gas phase or retained on the surface is one prominent channel. The other is photodesorption of intact molecules. The desorbing molecules are characterized by time-of-flight mass spectroscopy and laser spectroscopy. The state of the adsorbate after irradiation is characterized by thermal desorption spectroscopy and high resolution electron energy loss spectroscopy. The methods and fundamental characteristics are exemplified using results from three studied systems: O₂, H₂O, and N₂O₄ adsorbed on Pd(111).     

Determination of the crystal structure of U6Fe5Al8Si9 by electron crystallography

The crystal structure of U₆Fe₅Al₈Si₉ was re-determined by electron crystallography, using selected area electron diffraction (SAED) and high resolution (HRTEM) images, taken along the [0 0 1] direction. The obtained results are very similar to those found previously by X-ray powder diffraction. The differences between the atomic positions found by SAED and HRTEM images and those found by X-ray powder diffraction were 0.11 and 0.08 Å, respectively.     

Fluorescence near-field microscopy of DNA at sub-10 nm resolution

We demonstrate apertureless near-field microscopy of single molecules at sub-10 nm resolution. With a novel phase filter, near-field images of single organic fluorophores were obtained with approximately sixfold improvement in the signal-to-noise ratio. The improvement allowed pairs of molecules separated by approximately 15 nm to be reliably and repeatedly resolved, thus demonstrating the first true Rayleigh resolution test for near-field images of single molecules. The potential of this technique for biological applications was demonstrated with an experiment that measured the helical rise of A-form DNA.