Scanning Tunneling Microscopy Image Contrast Research Articles

Strain-Relief Patterns and Kagome Lattice in Self-Assembled C60 Thin Films Grown on Cd(0001).

We report an ultra-high vacuum low-temperature scanning tunneling microscopy (STM) study of the C60 monolayer grown on Cd(0001). Individual C60 molecules adsorbed on Cd(0001) may exhibit a bright or dim contrast in STM images. When deposited at low temperatures close to 100 K, C60 thin films present a curved structure to release strain due to dominant molecule–substrate interactions. Moreover, edge dislocation appears when two different wavy structures encounter each other, which has seldomly been observed in molecular self-assembly. When growth temperature rose, we found two forms of symmetric kagome lattice superstructures, 2 × 2 and 4 × 4, at room temperature (RT) and 310 K, respectively. The results provide new insight into the growth behavior of C60 films.

Desymmetrised pentaporphyrinic gears mounted on metallo-organic anchors.

Mastering intermolecular gearing is crucial for the emergence of complex functional nanoscale machineries. However, achieving correlated motion within trains of molecular gears remains highly challenging, due to the multiple degrees of freedom of each cogwheel. In this context, we designed and synthesised a series of star-shaped organometallic molecular gears incorporating a hydrotris(indazolyl)borate anchor to prevent diffusion on the surface, a central ruthenium atom as a fixed rotation axis, and an azimuthal pentaporphyrinic cyclopentadienyl cogwheel specifically labelled to monitor its motion by non-time-resolved Scanning Tunneling Microscopy (STM). Desymmetrisation of the cogwheels was first achieved sterically, i.e. by introducing one tooth longer than the other four. For optimal mechanical interactions, chemical labelling was also investigated as a preferential way to induce local contrast in STM images, and the electronic properties of one single paddle were modulated by varying the porphyrinic scaffold or the nature of the central metal. To reach such a structural diversity, our modular synthetic approach relied on sequential cross-coupling reactions on a penta(p-halogenophenyl)cyclopentadienyl ruthenium(ii) key building block, bearing a single pre-activated p-iodophenyl group. Chemoselective Sonogashira or more challenging Suzuki–Miyaura reactions allowed the controlled introduction of the tagged porphyrinic tooth, and the subsequent four-fold cross-couplings yielded the prototypes of pentaporphyrinic molecular gears for on-surface studies, incorporating desymmetrised cogwheels over 5 nm in diameter.

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.

Scanning tunneling microscopy contrast of isovalent impurities on the GaAs (110) surface explained with a geometrical model

Theoretical scanning tunneling microscopy (STM) images for all group-III and -V dopants on the GaAs (110) surface are calculated using density functional theory (DFT). In addition, a geometrical model based on the covalent radii of the dopants and substrate atoms is used to interpret the images. We find that the covalent radius of the dopant determines the geometry of the surface, which in turn determines the contrast seen in the STM images. Our model allows bond lengths to be predicted with an error of less than 4.2% and positions to be predicted with an average deviation of only 0.19 \AA{} compared to positions from fully relaxed DFT. For nitrogen we demonstrate good qualitative agreement between simulated and experimental STM images for dopants located in the first three surface layers. We are able to explain differences in both the contrast and positions of bright and dark features in the STM image based on our geometrical model. We then provide a detailed quantitative analysis of the positions of the bright features for nitrogen dopants in the second layer. The agreement of the DFT calculation with experiment is excellent, with the positions of the peaks in simulated and experimental STM scans differing by less than 2% of the lattice constant. For dopants other than nitrogen, we compare the calculated STM image contrast with the available experimental data and again find good agreement.

Bias voltage dependence of molecular orientation of dialkyl ketone and fatty acid alkyl ester at the liquid–graphite interface

Molecular orientations of self-assembled 18-pentatriacontanone (as ketone) and stearyl stearate (as ester) monolayers adsorbed on a graphite surface were studied by scanning tunneling microscopy (STM) at the liquid–solid interface. At a positive sample bias, the central areas of the dialkyl ketone and fatty acid alkyl ester molecules in the STM images appeared as two bright regions on both sides of a dim spot and a bright region on one side of a dim spot, whereas at a negative sample bias, the areas appeared dim. This contrast variation indicates that a perpendicular carbon skeleton-plane orientation with the CO pointing down on the surface is favorable for a substrate with positive charge and vice versa because of the greater electronegativity of the oxygen atom. Upon the bias voltage reversal, the delay time for the STM image contrast change in the region was observed on a time scale of minutes. The difference between the delay time lengths for the direction of bias polarity change indicates that the perpendicular configuration with CO pointing up is more stable than that with CO pointing down. These results indicate that the use of an electric field along a direction vertical to the monolayer on the substrate provides control over the orientations of the molecules between two stable states at the liquid–solid interface.

Direct Probing of the Structure and Electron Transfer of Fullerene/Ferrocene Hybrid on Au(111) Electrodes by in Situ Electrochemical STM

The electron donor-acceptor dyads are an emerging class of materials showing important applications in nonlinear optics, dye-sensitized solar cells, and molecular electronics. Investigation of their structure and electron transfer at the molecular level provides insights into the structure-property relationship and can benefit the design and preparation of electron donor-acceptor dyad materials. Herein, the interface adstructure and electron transfer of buckyferrocene Fe(C60Me5)Cp, a typical electron donor-acceptor dyad, is directly probed using in situ electrochemical scanning tunneling microscopy (STM) combined with theoretical simulations. It is found that the adsorption geometry and assembled structure of Fe(C60Me5)Cp is significantly affected by the electrochemical environments. In 0.1 M HClO4 solution, Fe(C60Me5)Cp forms well-ordered monolayers and multilayers on Au(111) surfaces with molecular dimer as the building block. In 0.1 M NaClO4 solution, typical six-fold symmetric close-packed monolayer with vertically adsorbed Fe(C60Me5)Cp is formed. Upon electrochemical oxidation, the oxidized Fe(C60Me5)Cp shows higher brightness in an STM image, which facilitates the direct visualization of the interfacial electrochemical electron transfer process. Theoretical simulation indicates that the electrode potential-activated, one-electron transfer from Fe(C60Me5)Cp to the electrode leads to the change of the delocalization character of the frontier orbital in the molecule, which is responsible for the STM image contrast change. This result is beneficial for understanding the structure and property of single electron donor-acceptor dyads. It also provides a direct approach to study the electron transfer of electron donor-acceptor compounds at the molecular level.

Scanning tunneling microscopy investigation of nonanuclear[3 × 3] MnII supramolecular grids

A scanning tunneling microscopy (STM) study of[3 × 3] MnII supramolecular grids on Au(111) substrates is presented. Two-dimensional ordering viaself-assembly is observed at various coverages. Submolecular resolution is attained withsolvent deposition of low concentration samples upon freshly exposed substrates. Acomparison of submolecular contrast in STM images is conducted and the importance ofsuitable image processing techniques is demonstrated in resolving the layer structure ofobtained high coverage data.

Scanning tunneling microscopy image contrast variation on a copper (II) complex adlayer with tip–sample separation

Contrast variation of scanning tunneling microscopy (STM) image has been investigated as a function of tip–sample separation on a bis[1,3-di( p- n-tetradecyloxyphenyl)propane-1,3-dionato]copper(II) (abbreviated as C 14O–Cu) adlayer on highly oriented pyrolytic graphite with STM. Submolecular features of the molecular core and interdigitated alkyl chain are clearly revealed. A preliminary theoretical calculation is carried out to simulate the image variation. The simulated results are in agreement with STM observation.

Coadsorption and interactions of O and H on Pd([formula omitted

The interactions between oxygen and hydrogen coadsorbed on Pd(111) have been studied by scanning tunneling microscopy (STM) in the temperature range from 25 to 230 K. Atomic oxygen without coadsorbed hydrogen forms a (2×2) structure in a continuous layer or in the form of islands. Individual oxygen atoms are also observed between the islands at low temperature. Coadsorbed hydrogen modifies the STM image contrast and enhances the diffusion of the isolated oxygen atoms. Above 120 K hydrogen modifies the structure of the oxygen islands, transforming them into rows of oxygen atom pairs at nearest neighbor distances. Above 150 K, hydrogen causes all (2×2)-O islands to convert into a (√3×√3) structure, which is stable up to 210 K. Above 210 K the (√3×√3) structure transforms back to (2×2) due to dissolution of the surface hydrogen into the bulk. During these transformations the number of oxygen atoms on the surface remains unchanged. Above 220 K the oxygen population decreases by reaction with dissolved hydrogen to form H2O which desorbs from surface.

Structure and electronic properties of CH 3- and CF 3-terminated alkanethiol monolayers on Au( [formula omitted]): a scanning tunneling microscopy, surface X-ray and helium scattering study

The structure and the electronic properties of a series of CH 3- and CF 3-terminated alkanethiol monolayers on Au(1 1 1) have been studied by scanning tunneling microscopy (STM) and surface X-ray and helium scattering. At full coverage, the CH 3-terminated monolayers form long-range ordered domains of a (√3×√3) R30° and a (2√3×3) R30° standing-up phase. By thermal desorption, distinct lying-down phases of intermediate density as well as the ( p×√3) lying-down phase were generated. In contrast, the CF 3-terminated monolayers at full coverage form a standing-up phase of hexagonal symmetry that exhibits no long-range order at room temperature. Even after annealing, the domain sizes are smaller by more than one order of magnitude compared to the CH 3-terminated thiol monolayers. A comparison of the low-density lying-down phases suggests no measurable influence of the CF 3-group on the ordering. The current–voltage dependence ( I– V-curves) measured by scanning tunneling spectroscopy (STS) shows no voltage gap for CH 3-terminated decanethiols. In contrast, in the I– V-curves for CF 3-terminated decanethiol monolayers, an asymmetric voltage gap of about 2 V can be clearly observed. The latter results are discussed in terms of a microscopic model that includes the formation of an interfacial Coulomb barrier at the CF 3/vacuum boundary. In addition, the effects of the tunneling conditions on the STM image contrast were examined. These studies demonstrate that the nature of the STM images and thus, the respective apparent lateral order of the films, strongly depends on the choice of the tunneling parameters.

Functional Group Contrast in Scanning Tunneling Microscopy Images of Substituted Phenylethers

Numerous scanning tunneling microscopy (STM) studies of adsorbates at gas/solid and liquid/solid interfaces have been reported. Although early STM experiments of these systems were concerned primarily with visualizing molecules at the atomic level, the focus has shifted to extracting chemical information from STM images, including the identity of atoms or of functional groups within an adsorbed molecule. However, STM image interpretation continues to be an immense challenge and one currently debated issue of critical importance is the mechanism(s) by which the image contrast reveals atomic and molecular structure. Recently, a combination of electronic and geometric factors was proposed. The electronic factor addresses the coupling between the energy levels of the adsorbate and the Fermi level of the surface whereas the geometric factor is related to the spatial overlap between the STM tip and the functional group.A previous study in our laboratory of a homologous series of para-halogenated phenyloctadecyl ethers (X-POEs, where X = H, CI, Br, I), physisorbed onto highly oriented pyrolytic graphite (HOPG), revealed a bias-dependent contrast in STM images resembling calculated (HyperChem) electron density contours of bonding molecular orbitals.

Scanning tunneling microscopy image contrast of monolayer platinum on graphite

Platinum particles supported on graphite have been investigated by scanning tunneling microscopy (STM). For one monolayer thick Pt particles the individual Pt atoms form a characteristic intensity pattern due to a mismatch between the Pt and graphite lattice. Based on density functional theory calculations and model structures of Pt on graphite it is argued that the observed STM imaging contrast has its origin in the tip induced elastic deformation of the graphite underneath the Pt particle. The Pt–graphite potential is much stiffer than the graphite–graphite potential. The calculations furthermore indicate that Pt adsorption is favored over top rather than hole sites and that the barrier for diffusion is very low.

Interpretation of GaAs(110) scanning tunneling microscopy image contrast by the symmetry of the surface Bloch wave functions

A simple qualitative correlation between the corrugation anisotropy observed in scanning tunneling microscope (STM) images of GaAs(110) surfaces and the symmetry properties of the surface states is presented. We show that as a function of bias, tunneling from different electronic states near high-symmetry points of the surface Brillouin zone gives rise to a distinct corrugation along [11̄0] and [001] in STM images. Existing models of the surface band structure are used to identify these states. We show that at small bias, due to band bending effects, the same surface state near the conduction-band edge determines the image corrugation in both filled and empty states images of n-type GaAs.

Effects of Molecular Geometry on the STM Image Contrast of Methyl- and Bromo-Substituted Alkanes and Alkanols on Graphite

Scanning tunneling microscopy (STM) images have been collected for a series of substituted alkanes and alkanols that form ordered overlayers at room temperature on highly ordered pyrolytic graphite surfaces. Molecules that have been imaged possess an internal bromide, with or without terminal alcohol groups (HO(CH2)9CHBr(CH2)10OH and H3C(CH2)16CHBr(CH2)16CH3), an internal −OH group (H3C(CH2)16CHOH(CH2)16CH3), and an internal methyl group (H3C(CH2)16CHCH3(CH2)16CH3). These data allow comparison to the STM image contrast reported previously for molecules in which −OH, −Br, and −CH3 groups were located in terminal positions of alkane chains adsorbed onto graphite surfaces. When the functional groups were in gauche positions relative to the alkyl chain, and thus produced molecular features that protruded toward the tip, the functional groups were observed to produce bright regions in a constant current STM image, regardless of the STM contrast behavior observed for these same functional groups when they were in terminal positions of adsorbed alkyl chains. These observations are in excellent agreement with theoretical predictions of the STM behavior of such systems. Additionally, several interesting packing structures have been observed that have yielded insight into the intermolecular forces that control the packing displayed by these overlayers.

STM study of oxygen-adsorbed TiC(111) surface

The surfaces of TiC(111) and TiC(111)-( 3 × 3 )R30-O were investigated using scanning tunneling microscopy (STM), low-energy electron diffraction (LEED) and DV-Xα calculation. Atomically resolved STM images of an oxygen-adsorbed surface were observed to have bright spots with a ( 3 × 3 )R30 periodicity. The STM image contrast was invariant with respect to the polarity of the sample bias voltage. The STM images and the calculation results are discussed in terms of surface geometry and local electronic state.

Influence of the Substrate on Order and Image Contrast for Physisorbed, Self-Assembled Molecular Monolayers: STM Studies of Functionalized Hydrocarbons on Graphite and MoS2

Scanning tunneling microscopy (STM) has been used to investigate the influence of different substrates on monolayer ordering and molecular image contrast for singly and doubly substituted long-chain hydrocarbons. Self-assembly at the phenyloctane−graphite and phenyloctane−MoS2 interfaces can be mildly or strongly affected by the underlying surface. In the most extreme cases, changes in the ordering of two-dimensional adlayers involve both rotation and translation of the molecules. Differences in the electronic characteristics of the underlying surface also have important implications for the STM image contrast exhibited by many molecules. Careful inspection of functional group contrast on both substrates provides useful information about the relative importance of geometric/topological versus electronic coupling factors in the STM tunneling mechanism for physisorbed molecules.

Bias-Dependent STM Image Contrast Study of Phenyloctadecyl Ethers Physisorbed onto Highly Oriented Pyrolytic Graphite

A homologous series of para-substituted phenyloctadecyl ethers (X-POEs, X = H, Cl, Br, and I) was prepared using Williamson's ether synthesis. The structure and purity of the ethers were confirmed using 1H NMR and GC/MS. Scanning tunneling microscopy (STM) images were acquired from monolayers of the ethers which formed at the surface of highly oriented pyrolytic graphite from a 2.5 wt % solution in phenylhexane. For all four ethers, the monolayers displayed a contrast which varied as a function of tip−sample bias. A comparison of STM images of the adsorbed ether molecules with electron density contours calculated using HyperChem suggested a bias-dependent participation to tunneling of individual bonding molecular orbitals (MOs). For example, at biases of −0.26 to −0.70 V, STM images of I-POE resembled the highest occupied molecular orbital (HOMO) exhibiting two bright spots for the pair of lobes of the phenyl ring and one bright spot for the halogen atom. At higher biases, contrast was observed for the phenyl ring alone (−0.8 to −1.2 V) and for the alkyl tail (−1.0 to −1.8 V) which was similar to that of the HOMO-1 and HOMO-4 contours, respectively. Owing to the measurement of an enhanced tunneling current simultaneous with the acquisition of atomically resolved bias-dependent STM images of the X-POE adsorbates, a resonance tunneling mechanism between the tip and substrate via MOs of molecular adsorbates adjacent to and including the HOMO is proposed.

Scanning Tunneling Microscopy Image Contrast as a Function of Scan Angle in Hydrogen-Bonded Self-Assembled Monolayers

Scanning tunneling microscopy (STM) has been performed on monolayers of the nucleic acid base, adenine, adsorbed to molybdenum disulfide (MoS2) surfaces. Analysis of the real space images of the adsorbates, together with molecular mechanics simulations suggest that the molecules form monolayers stablized by cyclic hydrogen bonds. We present data showing the effect on image contrast of varying scan angles of the STM tip. These indicate an anisotropic response of components of the monolayer to the scan direction and support a structural model implicating cyclic hydrogen bonds in both the stability of the monolayer and in the STM image contrast mechanism.

Highly ordered nanoscale surface alloy formed through Cr-induced Pt(111) reconstruction

We present scanning tunneling microscopy (STM) results of an ordered Cr/Pt surface alloy formed upon annealing 1.5--3 ML Cr/Pt(111) to 800 K. This alloy exhibits a nanoscale pattern with remarkable features: (i) it appears only along surface step edges; (ii) it forms a highly symmetric hexagonal network of one-atom-wide dislocation lines, with a unit-cell dimension of 17.3 \AA{}; and (iii) the contrast in STM images is sensitive to the STM bias voltage, reflecting a strong local variation of the electronic structure due to the presence of a regular array of two-dimensional Cr clusters containing ten atoms.

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

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.