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.     

Organic superconductors revisited

The surfaces of a ten years aged crystal and a freshly prepared κ-(BEDT-TTF)₂Cu(NCS)₂ crystal were compared by scanning tunneling microscopy (STM). The molecularly-resolved STM images of the bc plane of the crystals agree with each other and with the electronic contrast obtained by new density functional theory (DFT) based simulations. Even after ten years STM images of the molecular stacking of BEDT-TTF display a variation in brightness at the positions of different molecules. We attribute this symmetry breaking concerning the brightness in the STM images of the otherwise equivalent BEDT-TTF dimers to the electronic states of a relaxed surface.     

Scanning tunneling microscopy of fluorinated fullerene molecules on silicon surfaces

Scanning tunneling microscopy (STM) under ultra-high vacuum conditions is used to study the initial stages of adsorption of C₆₀F₁₈ and C₆₀F₃₆ fluorofullerene molecules on Si(111)-7 x 7 and Si(100)-2 x 1 surfaces. Spatially resolved STM images of individual molecules and ab initio calculations show that the fluorofullerene molecules interact with an Si surface, with the F atoms oriented toward the surface. The large electric dipole moment of the molecules induces strong polarization on the surface, but the charge transfer is weak. The presence of C₆₀F₃₆ isomers with different symmetry—T, C ₃, and C ₁—is revealed in STM images for the first time.     

Monolayered adatom aggregation induced by metallofullerene molecules on Cu(100)

The adsorption and self-assembly of Gd@C₈₂ molecules on Cu(100) surface have been investigated using scanning tunneling microscopy (STM). The metallofullerene molecules in the assemblies showed two characteristic apparent heights in the STM images. STM manipulation and spectroscopy was performed and revealed the formation of Cu adatom islands underneath the Gd@C₈₂ molecules. The monolayered Cu aggregates were resulted from the adatom–molecule complexation, which is supported by density functional theory (DFT) calculations that show charge transfer and electrostatic interactions between Gd@C₈₂ and adatoms. In addition, sub-molecularly resolved STM images demonstrated the structural and orientational ordering of Gd@C₈₂ assemblies upon thermal annealing. DFT calculations demonstrated that Gd atom located at the lower part of the carbon cage is a favored adsorption configuration for Gd@C₈₂ molecules adsorbed on Cu(100).     

Scanning tunneling microscopy/spectroscopy study of adsorption of C<Subscript>60</Subscript>F<Subscript>36</Subscript> molecules on the 7 × 7-Si(111) surface

Spatially resolved images of an individual C₆₀F₃₆ fluorofullerene molecules on Si(111)-7 × 7 surface have been obtained by means of scanning tunneling microscopy/spectroscopy (STM/STS). The presence of isomers with different symmetry (T, C ₃, C ₁) has been revealed in STM investigation of initial adsorption stage of C₆₀F₃₆ on silicon surface Si(111)-(7 × 7). The adsorbed fluorofullerene molecule can occupy any adsorption site of silicon surface (corner site, faulted half, unfaulted half) that indicates for strong molecule-substrate interaction. The HOMO-LUMO gap of the adsorbed C₆₀F₃₆ molecules have been estimated from current image tunneling spectroscopy (CITS) and z(V) with engaged feedback measurements. The value of HOMO-LUMO gap observed experimentally was 3 eV. The C₆₀F₃₆ molecules adsorption on Si(111)-(7 × 7) surface was stable and kept equilibrium configuration during several hours.     

第一过渡金属酞菁分子的电子结构的第一性原理计算

对第一过渡金属酞菁化合物(Metal Phthalocyanine,MPc,M=Sc,Ti,V,Cr,Mn,Fe,Co,Ni和Cu)的电子结构和基本物理化学性质进行了第一性原理计算.理论模拟出来的STM图像表现出亚分子结构,与已有的实验观察结果相当吻合,且跟金属原子的d电子组态明显有关.在小偏压条件下,第一过渡金属首尾端ScPc,NiPc和CuPc分子的中央金属离子在STM图像表现为空洞,其他所有金属酞菁分子的中央金属离子均为亮斑.同时还研究了ScPc和NiPc分子的STM图像与偏压的关系,当针尖偏压分别 关键词： STM图像模拟 金属酞菁 电子结构     

Insight into STM image contrast of n-tetradecane and n-hexadecane molecules on highly oriented pyrolytic graphite

Two-dimensional ordered patterns of n-tetradecane (n-C₁₄H₃₀) and n-hexadecane (n-C₁₆H₃₄) molecules at liquid/graphite interface have been directly imaged using scanning tunneling microscope (STM) under ambient conditions. STM images reveal that the two different kinds of molecules self-organize into ordered lamellar structures in which alkane chains of the molecules extend along one of three equivalent lattice axes of highly oriented pyrolytic graphite (HOPG) basal plane. For n-C₁₄H₃₀ molecules, the molecular axes are observed to tilt by 60° with respect to inter-lamellar trough lines and the carbon backbones of the alkane chains are perpendicular to the HOPG basal plane in an all-trans conformation. However, for n-C₁₆H₃₄ molecules, the molecular axes are perpendicular to lamellar borders (90°) and the planes of the all-trans carbon skeletons are parallel to the graphite basal plane. The results clearly indicate that outmost hydrogen atoms of the alkane chains dominate atom-scaled features of the STM images. That is, in the case of long-chain alkane molecules, topographic effects dominantly determine STM image contrast of the methylene regions of the alkane chains that are adsorbed on HOPG.     

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.     

Tip-induced surface polarization: a new mechanism for contrast in the scanning tunnelling microscope

We propose a new mechanism influencing the contrast in STM images. It involves the polarization of the surface by the electric field from the tip; this affects the contrast when different parts of the surface have different polarizabilities. We demonstrate its importance for the case of ethene (C₂H₄) molecules adsorbed on the Si(001) surface; for this system we are able to obtain qualitative agreement between experiments and ab initio calculations by including the tip-induced field.     

The chemisorption of pentacene on Si(0 0 1)-2 × 1

Adsorption structures of the pentacene (C₂₂H₁₄) molecule on the clean Si(0 0 1)-2 × 1 surface were investigated by scanning tunneling microscopy (STM) in conjunction with density functional theory calculations and STM image simulations. The pentacene molecules were found to adsorb on four major sites and four minor sites. The adsorption structures of the pentacene molecules at the four major sites were determined by comparison between the experimental and the simulated STM images. Three out of the four theoretically identified adsorption structures are different from the previously proposed adsorption structures. They involve six to eight Si-C covalent chemical bonds. The adsorption energies of the major four structures are calculated to be in the range 67-128 kcal/mol. It was also found that the pentacene molecule hardly hopped on the surface when applying pulse bias voltages on the molecule, but was mostly decomposed.     

Phase transition of thiophene molecules on Au(1 1 1) in solution

The orientation and packing arrangement of thiophene molecules on a well-defined Au(1 1 1) surface in 0.1 M HClO₄ solution have been investigated as a function of applied potentials by in situ scanning tunneling microscopy (STM) and electrochemical method. Thiophene molecules are found to form highly ordered adlayers in the double layer region. High-resolution STM images reveal different adlayer structures. Thiophene molecules take flat-lying and vertical orientation at 0.3 and 0.6 V, respectively. Compared with the results of electrochemical measurement, we concluded that the phase transition of thiophene on Au(1 1 1) occurs as the potential is changed between 0.1 and 0.65 V.     

STM imaging of a model surface of Ru(4,4′-dicarboxy-2,2′-bipyridine)2(NCS)2 dye-sensitized TiO2 photoelectrodes

A TiO₂(1 1 0)-(1 × 1) surface was prepared in an ultra-high vacuum, transported in laboratory air, and observed with a scanning tunneling microscope (STM) operated in a vacuum of 10⁻⁴ Pa. Empty state images showed atomically flat terraces separated by single-height steps, on which 5-fold-coordinated surface Ti atoms were observed as spots arranged in a rectangular lattice. The Ru(4,4′-dicarboxy-2,2′-bipyridine)₂(NCS)₂ (N3) dye was adsorbed on the TiO₂ surface by immersing the TiO₂ wafer into an acetonitrile solution of the dye. In the empty state images, individual N3 molecules were observed as oval particles protruding by 0.6 nm from the TiO₂ surface. The oval shape elongated to the [1  0] directions was attributed to electron tunneling from tip to unoccupied states localized at the two carboxyl groups bound to the TiO₂ surface.     

Scanning tunneling microscopy study of molecular order at liquid-solid interfaces

Adsorbates of normal alkane C₃₆H₇₄, cycloalkanes (CH₂)₄₈ and (CH₂)₇₂, decanol C₁₀H₂₁OH, 4-hexyl-4-CyanoBiphenyl (6CB) and 4-octyl-4t-CyanoBiphenyl (8CB) on graphite and -Nb₃I₈ were studied by Scanning Tunneling Microscopy (STM), and the molecular arrangements at the liquid-solid interface were examined. Large-scale STM images show that the adsorbates possess complex multilayered structures, and that molecular ordering at the liquid-solid interfaces occurs primarily in the immediate vicinity of the substrate. Molecular-scale STM images are primarily determined by the electronic contributions of the most protruded atoms of the topmost overlayer. The underlying overlayers and the substrate affect the images indirectly by perturbing the topography of the topmost overlayer. The STM images of the adsorbates on graphite show that the atomically flat surface of graphite leads organic molecules to form lamella-like structures, while on the grooved surface of -Nb₃I₈, long chain-like molecules are trapped in the grooves. We were unable to image the cycloalkanes on -Nb₃I₈, which suggests that the cycloalkanes cannot assemble on the grooved surface due to a mismatch between the molecular shape and surface topography. The layers of 6CB and 8CB adsorbed on -Nb₃I₈ exhibit two types of domains, which may be related to how the grooves of the -Nb₃I₈ surface are occupied by the organic molecules. The STM images of decanol adsorbed on -Nb₃I₈ show two domains of different brightness. The relative brightness of these domains switches reversibly as the gap resistance is changed in the region around –60 M.     

Scanning tunneling microscopy of surface-adsorbed fullerenes: C60, C70, and C84

Scanning tunneling microscopy (STM) is used to characterize partial monolayers of C₆₀, C₇₀, and C₈₄ adsorbed on the Au(1 1 1) surface at room temperature and under ambient conditions. A high degree of structural polymorphism is observed for monolayers of each of these fullerenes. For C₆₀, three lattice packings are observed, including a previously unreported 7 × 7 R21.8° structure that is stabilized by adjacent surface step defects. For C₇₀, two lattice packings are observed, and analysis of molecular features in STM images allows molecular binding geometry to be determined. In one of the two observed lattice structures, C₇₀ molecules align their long axis along the surface normal, while in the other, molecules align parallel to the surface and along a gold lattice direction. The parallel geometry is also preferred for isolated and loosely packed molecules on the surface. C₈₄ exhibits a large number of lattice orientations and no long-range order, and likely binds incommensurately on Au(1 1 1). Time series of images of partial C₇₀ monolayers show progressive surface modification as a result of perturbation by the STM tip; this is in contrast to the behavior of C₆₀, where alterations in surface structure at room temperature are thermally driven.     

STM characterisation of organic molecules on H-terminated Si(111)

We present the first high resolution STM images of organic molecules on the technological important hydrogen terminated silicon surface. Ordered layers of PTCDA and PTCDI were prepared on this surface by organic molecular beam epitaxy. The submolecular contrast of these molecules on Si(111)/H obtained in the high resolution images agrees with the corresponding images on HOPG and MoS₂ substrates.     

甘氨酸在Cu(001)表面的吸附结构

结合低能电子衍射(LEED)及其消失斑点规律和扫描隧道显微镜(STM)手段确定了室温下甘氨酸在Cu(001)表面能形成c(2×4)和两种(2×4)吸附结构((2×4)₁和(2×4)₂),并推断出在两种(2×4)结构单胞中两甘氨酸分子的羧基相对于衬底的吸附取向一致,而它们的氨基则不同.实验中还观察到c(2×4)与(2×4)₂结构能相互转变成窄条相互穿插共存,这说明几种吸附结构能量相近. 关键词：     

Morphology of pentacene films deposited on Cu(1 1 9) vicinal surface

We investigate the morphology of a pentacene (C₂₂H₁₄) film adsorbed on the Cu(1 1 9) vicinal surface by scanning tunnelling microscopy (STM). Thermal treatment of a thick film of molecules generates a long-range ordered structure. Series of molecular rows are alternated with areas where the molecules assume two equivalent orientations. STM data analysis suggests that the ordered structure can be described by a rippled morphology. The behaviour of the film at different annealing temperatures suggests a possible explanation of the film structure as due to an adsorbate-induced modification of the substrate.     

Dimerization of three xanthene dyes on Au(1 1 1) surface

The adsorption structures of three xanthene dyes (rhodamine B (Rh B), fluorescein and eosin) on Au(1 1 1) in HClO₄ solution, have been investigated by in situ scanning tunneling microscopy (STM) and cyclic voltammetry. High-resolution STM images reveal the molecular orientation and packing arrangement in the ordered adlayers. A (5 × 10) structure is found on Rh B adlayer. (5 × 8) structures are observed on fluorescein and eosin adlayers, respectively. An intriguing aspect of this work is that three xanthene molecules form dimeric structures on Au(1 1 1) surface. The electrostatic interaction and van der Waals force are responsible to the dimeric formation of Rh B, while the interaction between Br atoms and hydrogen bond correspond to the dimerization of eosin and fluorescein, respectively. The structural models are tentatively proposed for the three ordered adlayers. The results obtained will be helpful to understand the interaction mechanism of dimerization and the degradation mechanism of dye pollutant.     

Internal structure of C60 fullerence molecules as revealed by low-temperature STM

We have performed Scanning Tunneling Microscopy (STM) in ultra-high vacuum at low temperatures (5 and 50K) of unordered and ordered C₆₀ layers adsorbed on a Au(110) surface. STM topographs of the frozen C₆₀ molecules reveal four symmetric patterns within single molecules, which may be associated with different orientations of the fullerenes on a highly corrugated gold substrate.     

Theoretical simulation of STM image of C60 molecules on Si(100)-(2 × 1) surface

Theoretical simulation of STM image of C₆₀ molecules on Si(100)-(2 × 1) surface was performed by the DV-Xα-LCAO method. The results excellently reproduced the internal stripe pattern of the STM image of C₆₀ observed by experiment. We confirm that it is the interaction between C₆₀ and the Si substrate which causes this internal structure.