Low-temperature Scanning Tunneling Microscopy Study Research Articles

Structural paradox in submonolayer chlorine coverage on Au(111)

In this work, we present a combined low-temperature scanning tunneling microscopy (STM) and density functional theory (DFT) study of chlorine adsorption on Au(111) at low coverages. Our STM study of Cl/Au(111) system has shown that at submonolayer coverages ($\ensuremath{\theta}<$ 0.1 ML) chlorine atoms form chainlike structures with abnormally short distances of 3.8 \AA{} between them. Our DFT calculations have shown that chlorine atoms can interact with each other through distortion of the substrate and this indirect elastic interaction is strong enough to affect their arrangement in the chainlike structures.

Elementary Process for CVD Graphene on Cu(110): Size-selective Carbon Clusters

Revealing the graphene growth mechanism at the atomic-scale is of great importance for achieving high quality graphene. However, the lack of direct experimental observation and density functional theory (DFT) verification hinders a comprehensive understanding of the structure of the carbon clusters and evolution of the graphene growth on surface. Here, we report an in-situ low-temperature scanning tunneling microscopy (LT-STM) study of the elementary process of chemical vapor deposition (CVD) graphene growth via thermal decomposition of methane on Cu(110), including the formation of monodispersed carbon clusters at the initial stage, the graphene nucleation and the ripening of graphene islands to form continuous graphene film. STM measurement, supported by DFT calculations, suggests that the carbon clusters on the surface are C2H5. It is found that graphene layers can be joined by different domains, with a relative misorientation of 30°. These graphene layers can be decoupled from Cu(110) through low temperature thermal cycling.

Molecular Ordering and Dipole Alignment of Vanadyl Phthalocyanine Monolayer on Metals: The Effects of Interfacial Interactions

We present an in situ low-temperature scanning tunneling microscopy (LT-STM) study to elucidate the effects of interfacial interactions on the molecular ordering and dipole alignment of dipolar vanadyl phthalocyanine (VOPc) monolayer on metal surfaces, including Cu(111), Ag(111), Au(111), and graphite. The adsorption of VOPc on the relatively inert graphite surface leads to the formation of well-ordered molecular dipole monolayer with unidirectionally aligned O-up configuration. In contrast, VOPc on Cu(111), Ag(111), and Au(111) adopts both O-up and O-down configurations. The VOPc strongly chemisorbs on Cu(111), leading to the formation of one-dimensional molecular chains, and two-dimensional molecular islands comprising pure O-down adsorbed VOPc molecules at low and high coverage, respectively. In contrast, VOPc physisorbs on Au(111) and results in an orientation transition from flat-lying to inclined molecular islands. Regarding the interfacial interaction strength, the Ag(111) represents an intermediat...

Substrate Reconstruction Mediated Unidirectionally Aligned Molecular Dipole Dot Arrays

We report the fabrication of unidirectionally aligned molecular dipole dot arrays of vanadyl phthalocyanine (VOPc) on Au(111). Low-temperature scanning tunneling microscopy (LT-STM) studies reveal that isolated VOPc molecules adsorb predominantly at the elbow sites of the 22 × √3 herringbone reconstructed Au(111) surface, and adopt an O-down configuration with the phthalocyanine plane parallel to the substrate. It is proposed that the low-coordinated Au atom at the elbow sites on the reconstructed Au(111) can interact strongly with the O-down oriented VOPc molecule, and hence leads to the formation of the unidirectionally aligned VOPc molecular dipole dot arrays.

Evidence of multiple nodeless energy gaps in superconductingBa0.6K0.4Fe2As2single crystals from scanning tunneling spectroscopy

We report on low-temperature scanning tunneling microscopy and spectroscopy (STM) studies of the electronic structure of single-crystalline ${\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$. Multiple superconducting gaps are observed in the density of states (DOS) and the sizes of the two dominant gaps ${\ensuremath{\Delta}}_{L}$ and ${\ensuremath{\Delta}}_{S}$ are 7.6 and 3.3 meV, respectively. The flat bottom of the DOS spectra near zero bias indicates the nodeless feature of the gaps, while the global fitting to the spectra definitely requires the anisotropy. The nodeless gaps with finite anisotropy revealed in our STM data agree well with the expectations of an extended $s$-wave superconductivity.

Orientationally Ordered C60 on p-Sexiphenyl Nanostripes on Ag(111)

Long range orientational ordering within C(60) monolayers is observed on p-sexithiophene (6P) monolayer nanostripes on Ag(111) at 77 K. Low-temperature scanning tunneling microscopy studies reveal that the C(60)-6P intermolecular interaction constrains all C(60) molecules to adsorb on their hexagons atop 6P molecules. The orientation-dependent bond-to-bond Coulomb interaction between charge deficient single bonds and double bonds with excess charge in neighboring C(60) molecules results in the in-plane orientational ordering and contributes to the lowering of the total energy of the orientationally ordered C(60) islands.

Evidence of Sn Adatoms Quantum Tunneling at theα−Sn/Si(111)Surface

We present a low-temperature scanning tunneling microscopy study of the alpha-Sn/Si(111) surface that demonstrates the fluctuating behavior of the Sn adatoms. The dynamical fluctuation model, successfully applied in describing the alpha-Sn/Ge(111) surface, is proposed for the related alpha-Sn/Si(111) surface too, although with a much lower transition temperature. In addition, a new phenomenon appears responsible for the unexpected evidence that the average oscillation frequency remains constant at temperatures lower than 15 K, in contradiction to the Arrhenius law. We explain this phenomenon as quantum tunneling of Sn adatoms.

Conformational Adaptation and Selective Adatom Capturing of Tetrapyridyl-porphyrin Molecules on a Copper (111) Surface

We present a combined low-temperature scanning tunneling microscopy and near-edge X-ray adsorption fine structure study on the interaction of tetrapyridyl-porphyrin (TPyP) molecules with a Cu(111) surface. A novel approach using data from complementary experimental techniques and charge density calculations allows us to determine the adsorption geometry of TPyP on Cu(111). The molecules are centered on "bridge" sites of the substrate lattice and exhibit a strong deformation involving a saddle-shaped macrocycle distortion as well as considerable rotation and tilting of the meso-substituents. We propose a bonding mechanism based on the pyridyl-surface interaction, which mediates the molecular deformation upon adsorption. Accordingly, a functionalization by pyridyl groups opens up pathways to control the anchoring of large organic molecules on metal surfaces and tune their conformational state. Furthermore, we demonstrate that the affinity of the terminal groups for metal centers permits the selective capture of individual iron atoms at low temperature.

Adsorption, Interaction, and Manipulation of Dibutyl Sulfide on Cu{111}

This paper describes a low-temperature scanning tunneling microscopy (STM) study of a simple thioether, dibutyl sulfide, on a Cu{111} surface. The literature is full of data about thiol-based monolayers; however, relatively little is known about thioether self-assembly. Thioethers are more resilient to oxidation than thiols and offer the potential for control over nanoscale assembly in two dimensions parallel to the surface. Therefore, robust assembly schemes derived from thioethers may offer a new class of self-assembled systems with novel and useful properties. At a medium surface coverage and a temperature of 78 K, dibutyl sulfide grows in small, highly ordered islands in which the ordering is driven by both the molecule-surface dative bonds and intermolecular van der Waals bonding. Annealing to around 120 K allows diffusion and reordering of the molecules and the formation of large, very well ordered domains with little or no defects. We show high-resolution images of the molecular arrays and propose a model for their packing structure. These data suggest the potential use of thioethers for a variety of self-assembly applications that require control over molecular spacing parallel to the surface. We also show how the STM tip can be used to manipulate individual molecules within the ordered structures and that the arrays can act as a nanoscale abacus. The range of motion of the manipulated molecules inside a regular array reflects the potential imposed upon them by their neighbors.

Crossover from Three-Dimensional to Two-Dimensional Geometries of Au Nanostructures on Thin MgO(001) Films: A Confirmation of Theoretical Predictions

In this Letter, we report a low-temperature scanning tunneling microscopy study aiming to explore the adsorption properties of Au with respect to the thickness of supported MgO films. For different MgO film thicknesses (3 ML and 8 ML), we find significant differences in the distribution of Au adsorption sites and in the Au cluster geometry, in line with recent calculations and electron paramagnetic resonance experiments. On the surface of thick MgO films or unsupported MgO, Au adsorbs on O sites [Phys. Rev. Lett. 96, 146804 (2006)], and the equilibrium cluster geometry is three-dimensional. In contrast, on thin MgO films, the calculations predicted (i) a change of the preferred Au nucleation site [Phys. Rev. Lett. 94, 226104 (2005)] and (ii) a stabilization of two-dimensional Au cluster geometries [Phys. Rev. Lett. 97, 036106 (2006)].

Uniform Pb nanowires of magic thickness on Si(111) controlled by elastic interaction and quantum size effects

An ordered array of uniform single crystal Pb nanowires with a magic thickness of 9 monolayers (ML) was obtained on Si(111) by a two-step method. The coverage of 6.4 ML is critical for the formation of uniform nanowires at the deposition temperature of 200 K and the flux rate of 1.2 ML/min, while at other coverages elongated islands or interconnected stripes with various thicknesses are formed. The width ratio of the nanowires to the Si(111) substrate terraces is found to be a characteristic value of 0.54. In situ low-temperature scanning tunneling microscopy study reveals that the magic thickness and uniform distribution of the Pb nanowires are governed by the interplay between the electronic quantum size effects along the normal direction and the lateral elastic interaction induced by strain.

Self-Assembly of Extended Polycyclic Aromatic Hydrocarbons on Cu(111)

We present a low-temperature scanning tunneling microscopy study on the self-assembly of extended polycyclic aromatic hydrocarbons with different symmetries on the Cu111 surface. All molecules show a commensurate monolayer structure, with significant structural differences with respect to the unit cell of the molecular lattice and the orientational ordering. We find that the molecular lattice and the molecular orientation are largely dominated by molecule-substrate interactions, whereas molecule-molecule interactions determine the molecular packing density via steric repulsion. Moreover, we show that the structure of the monolayer is transferred to the second layer via molecule-molecule interaction.

Self-assembly and conformation of tetrapyridyl-porphyrin molecules on Ag(111)

We present a low-temperature scanning tunneling microscopy (STM) study on the supramolecular ordering of tetrapyridyl-porphyrin (TPyP) molecules on Ag(111). Vapor deposition in a wide substrate temperature range reveals that TPyP molecules easily diffuse and self-assemble into large, highly ordered chiral domains. We identify two mirror-symmetric unit cells, each containing two differently oriented molecules. From an analysis of the respective arrangement it is concluded that lateral intermolecular interactions control the packing of the layer, while its orientation is induced by the coupling to the substrate. This finding is corroborated by molecular mechanics calculations. High-resolution STM images recorded at 15 K allow a direct identification of intramolecular features. This makes it possible to determine the molecular conformation of TPyP on Ag(111). The pyridyl groups are alternately rotated out of the porphyrin plane by an angle of 60 degrees.

Visualization of the Molecular Jahn-Teller Effect in an Insulating K 4 C 60 Monolayer

We present a low-temperature scanning tunneling microscopy (STM) study of K(x)C60 monolayers on Au(111) for 3 < or = x < or = 4. The STM spectrum evolves from one that is characteristic of a metal at x = 3 to one that is characteristic of an insulator at x = 4. This electronic transition is accompanied by a dramatic structural rearrangement of the C60 molecules. The Jahn-Teller effect, a charge-induced mechanical deformation of molecular structure, is directly visualized in the K4C60 monolayer at the single-molecule level. These results, along with theoretical analyses, provide strong evidence that the transition from metal to insulator in K(x)C60 monolayers is caused by the Jahn-Teller effect.

Tailoring molecular self-organization by chemical synthesis: Hexaphenylbenzene, hexa-peri-hexabenzocoronene, and derivatives on Cu (111)

A low-temperature scanning tunneling microscopy (LT-STM) study of four slightly different hydrocarbons based on hexaphenylbenzene (HPB) and hexa-peri-hexabenzocoronene (HBC) on the Cu(111) surface at submonolayer coverage is presented. All molecules show a commensurate monolayer structure, with significant differences in structure and growth mode. We find that the long range order and the size of defect free domains increase when the intermolecular interaction becomes stronger with respect to the molecule-substrate interaction. Moreover, we can assign adsorption and self-ordering properties to the different chemical groups within the molecules.

Low-Temperature Mobility and Structure Formation of a Prochiral Aromatic Thiol (2,5-Dichlorothiophenol) on Cu(111)

We present a low-temperature scanning tunneling microscopy study of increasing coverages of 2,5-dichlorothiophenol, an asymmetrically halo-substituted aromatic thiol, on Cu(111). At low coverage, deprotonation of the thiol occurs spontaneously upon adsorption at 80 K. Albeit the low deposition temperature, we find the formation of adsorbate islands at low coverage, which coalesce into a well-ordered film of horizontally adsorbed molecules at increasing coverage. This behavior indicates (i) significant mobility of the thiols on Cu(111) even at low temperatures and (ii) attractive adsorbate-adsorbate interactions. At higher coverages intermolecular interactions prevent long-range diffusion of adsorbates and thermal activation of the S-H bond becomes necessary. A close analysis of the molecular films reveals chiral recognition between neighboring molecules, which leads to the formation of enantiopure areas on the surface. Upright orientation of individual molecules starts at the boundaries between such phases and can be induced by scanning tunneling microscopy.

Site determination and thermally assisted tunneling in homogenous nucleation.

A combined low-temperature scanning tunneling microscopy and density functional theory study on the binding and diffusion of copper monomers, dimers, and trimers adsorbed on Cu(111) is presented. Whereas atoms in trimers are found in fcc sites only, monomers as well as atoms in dimers can occupy the fcc as well as the metastable hcp site. In fact the dimer fcc-hcp configuration is only 1.3 meV less favorable with respect to the fcc-fcc configuration. This enables a confined intracell dimer motion, which at temperatures below 5 K is dominated by thermally assisted tunneling.

Structure of Linear Double-Stranded Deoxyribonucleic Acid Adsorbed on Cu(111) Surfaces: a Low-Temperature Scanning Tunneling Microscopy Study

We have obtained high-resolution scanning tunneling microscopy (STM) images of poly(dA-dT)·poly(dA-dT) and poly(dG-dC)·poly(dG-dC) adsorbed on Cu(111) substrates. STM revealed that the two deoxyribonucleic acid molecules have different adsorbed structures and topographic heights. We concluded that the observed structural differences originate from the inherent differences in the stability of the adenine–thymine (A–T) and guanine–cytosine (G–C) base pairs.

Low-temperature orientationally ordered structures of two-dimensional C60.

Orientationally ordered structures of two-dimensional (2D) C(60) at low temperature have been investigated theoretically and experimentally. Using total energy optimization with a phenomenological potential, we find the ground state is a close packed hexagonal lattice in which all the molecules have the same orientation. Several local minima of the potential energy surface are found to be associated with other 1 x 1 lattices as well as 2 x 2 lattices. The energies of the orientational domain boundaries of the 1x1 lattices are also computed, and two kinds of which yield negative values. A majority of these theoretical findings are confirmed by our low-temperature scanning tunneling microscopy study of a 2D C(60) array supported on a self-assembled monolayer.

Low-temperature scanning tunneling microscopy of YBa 2Cu 3O 7− δ

Low-temperature scanning tunneling microscopy studies have been performed on YBa 2Cu 3O 7− δ single crystals. We present two types of atomic images corresponding to BaO- and CuO-chain layers, which vary with applied bias voltage and temperature. At low bias voltage, the electronic modulation images of the CuO-chain layer tend to be emphasized. Warming the sample up to about 45 K alters the atomic images of the BaO layers but does not alter those of the CuO-chain layers. These results suggest that the electronic modulation images are closely related to the electronic states near the Fermi level and are attributed to the charge density wave.