Scanning Tunneling Microscopy Study Research Articles

Ordering of Zn-centered porphyrin and phthalocyanine on TiO2(011): STM studies.

Zn(II)phthalocyanine molecules (ZnPc) were thermally deposited on a rutile TiO2(011) surface and on Zn(II)meso-tetraphenylporphyrin (ZnTPP) wetting layers at room temperature and after elevated temperature thermal processing. The molecular homo- and heterostructures were characterized by high-resolution scanning tunneling microscopy (STM) at room temperature and their geometrical arrangement and degree of ordering are compared with the previously studied copper phthalocyanine (CuPc) and ZnTPP heterostructures. It was found that the central metal atom may play some role in ordering and growth of phthalocyanine/ZnTPP heterostructures, causing differences in stability of upright standing ZnPc versus CuPc molecular chains at given thermal annealing conditions.

Covalent Functionalization by Cycloaddition Reactions of Pristine Defect-Free Graphene.

Based on a low-temperature scanning tunneling microscopy study, we present a direct visualization of a cycloaddition reaction performed for some specific fluorinated maleimide molecules deposited on graphene. Up to now, it was widely admitted that such a cycloaddition reaction can not happen without pre-existing defects. However, our study shows that the cycloaddition reaction can be carried out on a defect-free basal graphene plane at room temperature. In the course of covalently grafting the molecules to graphene, the sp2 conjugation of carbon atoms was broken, and local sp3 bonds were created. The grafted molecules perturbed the graphene lattice, generating a standing-wave pattern with an anisotropy which was attributed to a (1,2) cycloaddition, as revealed by T-matrix approximation calculations. DFT calculations showed that while both (1,4) and (1,2) cycloadditions were possible on free-standing graphene, only the (1,2) cycloaddition could be obtained for graphene on SiC(0001). Globally averaging spectroscopic techniques, XPS and ARPES, were used to determine the modification in the elemental composition of the samples induced by the reaction, indicating an opening of an electronic gap in graphene.

Polymorphic Layered MoTe2 from Semiconductor, Topological Insulator, to Weyl Semimetal

Large size (∼2 cm) single crystals of layered MoTe2 in both 2H- and 1T′-types were synthsized using TeBr4 as the source of Br2 transport agent in chemical vapor transport growth. The crystal structures of the as-grown single crystals were fully characterized by X-ray diffraction, Raman spectroscopy, scanning transmission electron microscopy, scanning tunneling microscopy (STM), and electrical resistivity (ρ) measurements. The resistivity ρ(T), magnetic susceptibility χ(T), and heat capacity Cp(T) measurement results reveal a first order structural phase transition near ∼240 K for 1T′-MoTe2, which has been identified to be the orthorhombic Td-phase of MoTe2 as a candidate of Weyl semimetal. The STM study revealed different local defect geometries found on the surface of 2H- and Td-types of MoTe6 units in trigonal prismatic and distorted octahedral coordination, respectively.

Water and CO (co-)adsorption on pseudomorphic Pt films on Ru(0001) - a low-temperature scanning tunneling microscopy study.

Coadsorption of CO and water under ultrahigh vacuum (UHV) conditions can be considered as a model system for the interaction of metal surfaces with CO in an aqueous electrochemical environment. Nevertheless, this has rarely been investigated, and in particular for catalytically relevant bimetallic systems, there is hardly any information available. Here we report results of a low-temperature scanning tunneling microscopy (STM) study on the adsorption and coadsorption of CO and water on a Ru(0001) surface covered with a pseudomorphic Pt film of 2 or 3 monolayers thickness. The role of kinetic effects introduced by the sequence of adsorption, either pre-adsorption of CO followed by water adsorption or pre-adsorption of water followed by CO adsorption, on the adlayer structure formation will be demonstrated and discussed. Furthermore, the data show a distinct influence of the thickness of the Pt film, reflecting changes in the chemistry of the Pt surface due to electronic interactions with the underlying Ru(0001) substrate ('vertical ligand effects'). Implications of the present findings on the interaction of CO with these bimetallic PtRu surfaces under electrochemical conditions will be discussed.

Incommensurate charge ordered states in the t–t′–J model

We study the incommensurate charge ordered states in the model using the Gutzwiller mean field theory on large systems. In particular, we explore the properties of incommensurate charge modulated states referred to as nodal pair density waves (nPDW) in the literature. nPDW states intertwine site and bond charge order with modulated d-wave pair order, and are characterized by a nonzero amplitude of uniform pairing; they also manifest a dominant intra-unit cell d-density wave form factor. To compare with a recent scanning tunneling microscopy (STM) study (Hamidian et al 2015 Nat. Phys. 12 150) of the cuprate superconductor BSCCO-2212, we compute the continuum local density of states (LDOS) at a typical STM tip height using the Wannier function based approach. By Fourier transforming Cu and O sub-lattice LDOS we also obtain bias-dependent intra-unit cell form factors and spatial phase difference. We find that in the nPDW state the behavior of form factors and spatial phase difference as a function of energy agrees remarkably well with the experiment.This is in contrast to commensurate charge modulated states, which we show do not agree with experiment. We propose that the nPDW states are good candidates for the charge density wave phase observed in the superconducting state of underdoped cuprates.

Chemistry at the square nanometer: reactivity at liquid/solid interfaces revealed with an STM.

For more than three decades the scanning tunnelling microscope (STM) has proven to be an indispensable tool to image molecules adsorbed at a surface at the highest detail possible. In addition to simply imaging molecules, STM can also be applied to monitor dynamic surface phenomena, including chemical reactions. By studying reactions at a surface at the single molecule level, unique information about reaction mechanisms can be obtained which remains hidden when conventional ensemble techniques are used. Many STM studies of chemical reactions have been performed in extreme environments like ultrahigh vacuum or high pressure chambers, but these are far removed from conditions in which most chemical and biological processes take place, i.e., in a liquid at ambient atmospheres. This feature paper highlights the developments in the relatively unexplored research area of investigating chemical reactions with an STM at a liquid/solid interface under ambient conditions. Covalent couplings between molecules, light-induced isomerisations, reactions under electrochemical control, and complex multistep processes and catalysis are discussed.

Single-layer ZnS supported on Au(111): A combined XPS, LEED, STM and DFT study

Single-layer of ZnS, consisting of one atomic layer of ZnS(111) plane, has been grown on Au(111) and characterized using X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). While the LEED measurement indicates a coincidence structure of ZnS-(3×3)/Au(111)-(4×4), high resolution STM images reveal hexagonal unit cells of 6.7×6.7Å2 and 11.6×11.6Å2, corresponding to √3 and 3 times the unit cell of the ideal zincblende ZnS-(1×1), respectively, depending on the tunneling conditions. Calculations based on density functional theory (DFT) indicate a significantly reconstructed non-planar structure of ZnS single-layer on Au(111) with 2/3 of the S anions being located nearly in the plane of the Zn cations and the rest 1/3 of the S anions protruding above the Zn plane. The calculated STM image shows similar characteristics to those of the experimental STM image. Additionally, the DFT calculations reveal the different bonding nature of the S anions in ZnS single-layer supported on Au(111).

Tracking Invisible Transformations of Physisorbed Monolayers: LDI-TOF and MALDI-TOF Mass Spectrometry as Complements to STM Imaging.

Triphenyleneethynylene (TPEE) derivatives bearing one long aliphatic chain on each terminal aryl ring and two short aliphatic chains on the central aryl ring (core chains) self-assemble single component and 1-D patterned, two-component, crystalline monolayers at the solution-graphite interface. The monolayer morphology directs the core chains off the graphite, making them accessible for chemical reactions but invisible to imaging by scanning tunneling microscopy (STM). This precludes using STM to monitor transformations of the core chains, either by reaction or solution-monolayer exchange of TPEE molecules. Laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF MS) successfully identifies TPEE compounds within physisorbed monolayers. The LDI-TOF spectra of TPEE monolayer-graphite samples exhibit strong molecular ion peaks and minimal fragmentation or background. LDI-TOF and STM techniques are combined to evaluate monolayer composition and morphology, track solution-monolayer exchange, to identify reaction products and to measure kinetics of chemical reactions at the solution-monolayer interface. LDI-TOF MS provides rapid qualitative evaluation of monolayer composition across a graphite substrate. Challenges to quantitative composition evaluation by LDI-TOF include compound-specific light absorption, surface desorption/ionization and fragmentation characteristics. For some, but not all, compounds, applying matrix onto a self-assembled monolayer increases molecular ion intensities and affords more accurate assessment of monolayer composition via matrix assisted laser desorption/ionization (MALDI) MS. Matrix addition precludes subsequent chemical or STM studies of the monolayer, whereas reactions and STM may be performed at nonirradiated regions following LDI-TOF measurements. LDI- and MALDI-TOF MS are useful complements to STM and are easily implemented tools for study of physisorbed monolayers.

Growth of silicon on tungsten diselenide

Here, we report a scanning tunneling microscopy and spectroscopy study of the growth of silicon on a tungsten diselenide (WSe2) substrate. We have found convincing experimental evidence that silicon does not remain on the WSe2 substrate but rather intercalates between the top layers of WSe2. Upon silicon deposition, the flat WSe2 surface converts into a surface with a hill-and-valley structure. The lattice constant of the hill-and-valley structure is identical to the lattice constant of WSe2 and the transition from hills to valleys is very gradual, suggesting that the top layer is composed of pristine WSe2. In order to verify this conjecture, we have removed the height information from our scanning tunneling microscopy signal and obtained chemical contrast of the surface by recording dI/dz, rather than the conventional regulation voltage of the z-piezo. The spatially resolved dI/dz maps provide compelling evidence that the deposited silicon does indeed not reside on top of the WSe2 substrate.

The Nanostructuring of Atomically Flat Ru(0001) upon Oxidation and Reduction.

The O/Ru(0001) system is widely studied due to its rich phase variety of various stoichiometry and atomic arrangements, including the formation of a RuO2/Ru(0001) oxide layer. Apart from homogeneous ruthenium surfaces in certain oxidation states, also strongly heterogeneous surfaces can exist due to oxidation state’s variation at the nanoscale. We report on a scanning tunneling microscopy (STM) study of the nanostructuring of the oxidized Ru(0001) surface as a result of its interaction with molecular oxygen at elevated temperatures and subsequent reduction of a resulting RuO2 film by CO or HCl molecules from the gas phase in high-vacuum environment.

Structural and electronic properties of ultrathin FeSe films grown on Bi2Se3(0 0 0 1) studied by STM/STS

We report scanning tunnelling microscopy and spectroscopy (STM/STS) studies on one and two unit cell (UC) high FeSe thin films grown on Bi2Se3(0 0 0 1). In our thin films, we find the tetragonal phase of FeSe and dumb-bell shaped defects oriented along Se–Se bond directions. In addition, we observe striped moiré patterns with a periodicity of (7.3 ± 0.1) nm generated by the mismatch between the FeSe lattice and the Bi2Se3 lattice. We could not find any signature of a superconducting gap in the tunneling spectra measured on the surface of one and two UC thick islands of FeSe down to 6.5 K. The spectra rather show an asymmetric behavior across and a finite density of states at the Fermi level (EF) resembling those taken in the normal state of bulk FeSe.

Scanning tunneling microscopy study of the early stages of epitaxial growth of CoSi2 and CoSi films on Si(111) substrate: Surface and interface analysis

We report scanning tunneling microscopy observations of solid-phase epitaxial growth of ultra-thin CoSi2 and CoSi films at heating temperatures in the range of 350–600°C using a disordered ‘(1×1)’ phase of Co/Si(111) as a pre-existing template. It is found that depending on the growth conditions two types of epitaxial cobalt silicide films can be formed. At coverages below ~2 ML (1 ML=7.83×1014atoms/cm2) of Co, CoSi2 phase grows in the form of flat two-layer patches spreading over the surface at elevation of the temperature. It consists of precisely two the completely filled Co layers and four Si layers above the interface. At coverages higher than ~2 ML of Co, the layer-by-layer growth of CoSi phase with the CsCl-type crystal structure also takes place. The CoSi phase consists of one or more Co-Si double layers, which are located on top of the second Si-Co-Si triple layer of CoSi2, i.e., it has а coherent double interface CoSi/CoSi2/Si(111). According to experimental findings and ab initio total-energy calculations, the structures with the eightfold coordination of interfacial Co atoms and lateral unit cells of the silicide layer and substrate mutually rotated by 180° prove to be the most stable ones.

Perspective: Structure and dynamics of water at surfaces probed by scanning tunneling microscopy and spectroscopy.

The detailed and precise understanding of water-solid interaction largely relies on the development of atomic-scale experimental techniques, among which scanning tunneling microscopy (STM) has proven to be a noteworthy example. In this perspective, we review the recent advances of STM techniques in imaging, spectroscopy, and manipulation of water molecules. We discuss how those newly developed techniques are applied to probe the structure and dynamics of water at solid surfaces with single-molecule and even submolecular resolution, paying particular attention to the ability of accessing the degree of freedom of hydrogen. In the end, we present an outlook on the directions of future STM studies of water-solid interfaces as well as the challenges faced by this field. Some new scanning probe techniques beyond STM are also envisaged.

Temperature-Induced Four-Fold-on-Six-Fold Symmetric Heteroepitaxy, Rocksalt SmN on Hexagonal AlN

We report on the occurrence of rotational domains involving 4-fold symmetric epitaxy on a hexagonal net, a rare case in the literature. A temperature-driven crossover from fully (111)- to (001)-oriented samarium nitride grown by molecular beam epitaxy on hexagonal (0001) aluminum nitride is observed by means of in situ reflection high energy electron diffraction, scanning tunnelling microscopy studies, and ex situ X-ray diffraction. Using an especially rich set of growth conditions and nitrogen precursors, we observe the key role played by the growth kinetics, with a strong thermal cracking of the ammonia source and a re-evaporation of Sm adatoms occurring in the same temperature range of the orientation crossover.

Step-Type Selective Oxidation of Platinum Surfaces

We report on a combined TPD and STM study of O2 adsorption and dissociation on various Pt surfaces with varying (111) terrace widths and either (110) or (100) step geometries. Our quantitative TPD results show that (110) stepped surfaces adsorb considerably more oxygen at 100 K, regardless of terrace width, than either (100) stepped surfaces or planar Pt(111). These results suggest that O2 dissociates on the (110) stepped surfaces at 100 K, well lower than required for temperature-induced dissociation on (111) planes. The amount of oxygen desorbing from recombinative desorption of adsorbed oxygen atoms is also greater on (110) stepped surfaces. In addition, the partitioning of adsorbed oxygen between molecular and dissociative states depends on the step geometry; (110) stepped surfaces show an uptake plateau indicative of a threshold surface concentration for low-temperature dissociation, whereas (100) stepped surfaces do not. Scanning tunneling microscope (STM) images for various O coverages and surface ...

Manipulation of the surface density of states of Ag(111) by means of resonators: Experiment and theory

We show that the density of surface Shockley states of Ag(111) probed by the differential conductance $G(V)=dI/dV$ by a scanning-tunneling microscope (STM) can be enhanced significantly at certain energies and positions introducing simple arrays of Co or Ag atoms on the surface, in contrast to other noble-metal surfaces. Specifically we have studied resonators consisting of two parallel walls of five atoms deposited on the clean Ag(111) surface. A simple model in which the effect of the adatoms is taken into account by an attractive local potential and a small hybridization between surface and bulk at the position of the adatoms explains the main features of the observed $G(V)$ and allows us to extract the proportion of surface and bulk states sensed by the STM tip. These results might be relevant to engineer the surface spectral density of states, to study the effects of surface states on the Kondo effect, and to separate bulk and surface contributions in STM studies of topological surface states.

1,3,5-Benzenetribenzoic Acid on Cu(111) and Graphene/Cu(111): A Comparative STM Study.

The self-assembly of 1,3,5-benzenetribenzoic acid (BTB) molecules on both Cu(111) and epitaxial graphene grown on Cu(111) were studied by scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) under ultrahigh vacuum conditions. On Cu(111), the BTB molecules were found to mainly arrange in close-packed structures through H-bonding between the (partially) deprotonated carboxylic acid groups. In addition, porous structures formed by intact BTB molecules-and also based on H-bonding-were observed. On graphene grown on Cu(111) the BTB molecules mainly form porous structures accompanied by small patches of disordered close-packed structures. Upon annealing, BTB adsorbed on Cu(111) is fully deprotonated and arranges in the close-packed structure while in contrast on graphene/Cu(111) the porous network is exclusively formed. This shows that the molecular self-assembly behavior is highly dependent on the first substrate layer: one graphene layer is sufficient to considerably alter the interplay of molecule substrate and intermolecular interactions in favor of the latter interactions.

Growth of ordered molecular layers of PTCDA on Pb/Si(111) surfaces: a scanning tunneling microscopy study

The growth of well-ordered layers of PTCDA (3,4,9,10-perylene-tetracarboxylic-dianhydride) molecules on Pb/Si(111) surfaces has been investigated by scanning tunneling microscopy (STM) under ultra-high vacuum conditions. These Pb/Si(111) substrates, which present several distinct phases with different reconstructions, have allowed the exploration of new passivation schemes for the growth of ordered organic layers on Si(111) surfaces. According to our STM measurements, the higher Pb coverage phases (namely the so-called hexagonal incommensurate and reconstructions) present rather inert surfaces that allow easy diffusion of PTCDA molecules at room temperature and the formation of a well ordered first molecular layer which displays a herringbone reconstruction. For multilayer PTCDA coverage on these Pb/Si(111) phases, the formation of three-dimensional crystallites, with structure similar to that of the bulk PTCDA crystal, has been observed, indicating that a Stranski–Krastanov growth mode is dominant. On lower Pb coverage substrates (presenting the defective and mosaic reconstructions) no long range PTCDA order has been obtained. The systematic variation of the substrate reconstruction has allowed in the present work the relation of the surface reactivity of each reconstruction to the formation of ordered layers of PTCDA on Pb/Si(111) substrates.

Plasmonics—the interaction of light with metal surface electrons

The realization of light amplification by stimulated emission of radiation opened up an enormous wealth of potential new research and technologies in a broad wavelength range of electromagnetic waves. One of the new fields is plasmonics, based on the special properties of some materials with negative refractive index. In this case surface electromagnetic waves, coupled to surface electrons, the so-called surface plasmons can be generated. These waves among others represent a large enhancement of the EM field near the surface of the materials. The present paper illustrates some of the consequences of this phenomenon for a broad range of phenomena from ‘lasing’ to electron pairing. The latter is the basic condition for superconductivity, in our case found at room temperature. Measurements with a scanning tunneling microscope, furthermore electron and photon emission studies are the source of the presented experimental data.

Visualizing anisotropic propagation of stripe domain walls in staircaselike transitions ofIrTe2

We present a scanning tunneling microscopy (STM) study of the domain evolution across two first-order phase transitions of stripe modulations in ${\mathrm{IrTe}}_{2}$ that occur at ${T}_{\mathrm{C}}\ensuremath{\approx}275$ K and ${T}_{\mathrm{S}}\ensuremath{\approx}180$ K, respectively. Phase coexistence of the hexagonal $(1\ifmmode\times\else\texttimes\fi{}1)$ structure and the $(5\ifmmode\times\else\texttimes\fi{}1)$ stripe modulation is observed at ${T}_{\mathrm{C}}$, while various $(p\ifmmode\times\else\texttimes\fi{}1)$ modulations ($p=3n+2$ with $2\ensuremath{\le}n\ensuremath{\in}\mathbb{N}$) are observed below ${T}_{\mathrm{S}}$. Using STM atomic resolution, we observe anisotropic propagation of domain boundaries along different directions, indicating significantly different kinetic energy barriers. These results are consistently explained by a theoretical analysis of the energy barrier for domain wall propagation as obtained by density functional theory. Individual switching processes observed by STM indicate that the wide temperature range of the transition from the $(5\ifmmode\times\else\texttimes\fi{}1)$ stripes to the $(6\ifmmode\times\else\texttimes\fi{}1)$-ordered ground state is probably caused by the numerically limited subset of switching processes that are allowed between a given initial and the final state. The observations on ${\mathrm{IrTe}}_{2}$ are discussed in terms of a ``harmless staircase'' with a finite number of first-order transitions between commensurate phases and within a ``dynamical freezing'' scenario.