STM Study Research Articles

Squeezing and stretching Pd thin films: A high-resolution STM study of Pd/Au(111) and Pd/Cu(111) bimetallics

Pd bimetallic alloys are promising catalysts, especially for heterogeneous reactions involving hydrogen, as they exhibit increased activity and reduced demand for expensive precious metals. Using scanning tunneling microscopy, we examine the structure of Pd thin films on Cu(111) and Au(111) and demonstrate compression and expansion, respectively, of the bulk Pd lattice constant in the film. The relative binding strength of H to the two surfaces, inferred via tip-induced diffusion barriers, suggests that the strain in these systems may alter adsorbate binding and corroborates well-known trends in d-band shifts calculated by the density functional theory. Modification to the topography and activity of Pd films based on the choice of substrate metal illustrates the value of bimetallic systems for designing less expensive, tunable catalysts.

An STM and Monte Carlo study of the AlF3 thin film growth on Cu(1 1 1)

We report measurements of AlF3 thin film growth on Cu(1 1 1) at room temperature by means of scanning tunneling microscopy. The growth proceeds by the formation of fractal islands characterized by a very corrugated surface. Through uncovered zones and island density we determined a diffusion length of ~25 nm for the adsorbed molecules. Even with this large diffusion length the step-edges do not appear fully decorated. These experimental results are contrasted with simulations based on a limited diffusion aggregation model and Metropolis Monte Carlo.Additionally, the results of the AlF3 sub-monolayer growth on Cu(1 1 1) are compared with our previous results on Cu(1 0 0), finding that both systems show more differences than similarities. Thus, while the growth on Cu(1 0 0) shows fully decorated step-edges, on Cu(1 1 1), they present non-covered zones even at coverages as high as 0.7 monolayers. Supported on MC simulations we suggest that the qualitative difference between both faces is due to different step-edge behaviour.

STM study of C60F18 high dipole moment molecules on Au(111)

Scanning tunneling microscopy and spectroscopy studies of C60F18 molecules deposited on Au(111) are reported and compared to C60 molecules both at liquid helium temperature and room temperature (RT). Whereas adsorption and electronic properties of C60F18 single molecules were studied at low temperature (LT), self-assemblies were investigated at RT. In both cases, the fluorine atoms of the C60F18 molecules are pointed towards the surface. Individual C60F18 molecules on Au(111) have a HOMO–LUMO gap of 2.9eV. The self-assembled islands exhibit a close-packed hexagonal lattice with amorphous borders. The comparison with C60 molecules clearly demonstrates the influence of the C60F18 electric dipole moment (EDM) on the electronic properties of single molecules and on the thermodynamics of self-assembled islands. Besides, the apparent height value of a separate molecule increases in a self-assembly environment as a result of a depolarization phenomenon.

Structure and Electrical Properties of the Selected Crown Ether-Derivatized Bis(2,2'-bithienyl)Methanes in Langmuir-Blodgett Films

Physical properties of thin films of thiophene derivatives and their polymers are greatly affected by the structure and orientation of their molecules, as well as the local environment.1 In order to obtain thin films of such compounds for practical applications, from electronic devices to chemo- and biosensors.2-3, it is important to control orientation and order of the molecules in the formed functional materials. Interesting and efficient way to allow for such a control is Langmuir and Langmuir-Blodgett technique. In the presented contribution formation and properties of ultrathin films of the series of bis(2,2’-bithienyl)methane derivatives appended with crown ethers of varying size are presented. Uniform Langmuir films of liquid-expanded type were obtained at water-air interface for all of the used compounds. The influence of metal ions on the orientation of the molecules in the films was relatively small. Obtained Langmuir films were subsequently transferred onto the solid substrates by using Langmuir-Blodgett technique forming mono- and multilayers. In order to further elucidate structure of the formed layers AFM and STM techniques have been applied. The morphology of the multilayer films depended both on the size and orientation of the crown ether moiety in the molecules. However, in all cases formed multilayers were porous (Fig. 1). In case of monolayers, the STM studies helped to ascertain organization of the molecules in the Langmuir-Blodgett film. On the other hand the scanning tunnelling spectroscopy (STS) allowed acquisition of information about the electrical properties of the studied thiophene derivatives. Bibliography 1. D. Hu, J. Yu, K. Wong, B. Bagchi, P. J. Rossky, P. F. Barbara, Nature, 405, (2000), 1030. 2. A. G. MacDiarmid, Angew. Chem., Int. Ed., 40, (2001),2581. 3. R. Singhal, A. Chaubey, K. Kaneto, W. Takashima, B. D. Malhotra, Biotechnol. Bioeng., 85 (2004), 277. Figure 1

C3-Symmetric Pyridine and Bipyridine Derivatives: Simple Preparation by Cyclocondensation and 2D Self-Assembly at a Solution–Graphite Interface

The efficient preparation of four C3-symmetric (star-shaped) pyridine and bipyridine derivatives is reported. The key steps are Suzuki couplings of 4-pyridyl nonaflates with 4-acetyl-phenylboronic acid followed by an acid-promoted cyclocondensation reaction converting the methyl ketone moiety into the central benzene ring of the target compounds. Based on STM studies at a graphite–solution interface the two-dimensional arrangements of the compounds are discussed, showing the influence of the pyridine substitution pattern.

STM study of electrical transport properties of one dimensional contacts between MnSi∼1.7 nanowires and Si(111) and (110) substrates

We demonstrate the formation of contact barriers at the interfaces between MnSi1.7 nanowires (NWs) and Si substrates by the current–voltage (I–V) curves measured by scanning tunneling microscope with the tip contacting the NWs. The NWs on Si(110) exhibit linear reverse bias I–V curves, which suggests a parallel Ohmic surface state conductance of the Si(110) surface. The NWs on Si(111) exhibit nonlinear reverse bias I–V behavior, which indicates a considerable amount of minority carrier recombination-generation current. The NW length-dependence study of the forward bias current clearly shows that the quantitative change in NW length leads to a qualitative change in electrical transport properties. We derive a characteristic length LC ≈ 200 nm and the corresponding aspect ratio of ∼12–18 for MnSi1.7 NWs according to the variation of current density with the NW length.

23pAE-11 DFT and STM study of Si-rich graphene/SiC interface

23pAE-11 DFT and STM study of Si-rich graphene/SiC interface

Rh and Au deposited on ultrathin TiO~1.2 film formed on Rh(111) facets and the effects of CO exposure

Abstract This STM study deals with the structure and reactivity of Rh and Au deposited on a “pinwheel” TiO~ 1.2 ultrathin film (w-TiO-UTO) formed on Rh nanoparticles supported on TiO2(110). The deposition of a monolayer (ML) Au at 500 K disturbed the order of the w-TiO-UTO layer and led to the formation of 3D adparticles. The deposition of Rh (~ 1 ML) at 500 K and annealing up to 900 K led exclusively to the formation of 2D adparticles. It was found that 3D Au nanoparticles are gradually sintering in the range of 500–800 K. Upon a further raise of the temperature, a 3D→2D transformation takes place. The main difference is that on the top facet of the 2D Rh particles a w-TiO-UTO film forms, while the top facets of the 2D Au particles exhibit a clean gold metal structure. CO exposure of max. 10 min at 10 Pa causes no remarkable change of the encapsulated Rh particles, however, the 2D gold nanoparticles change significantly. The disintegration of Au particles and the spillover of Au atoms to the UTO surface accelerate at a CO pressure above 10− 1 Pa. This morphological process is reversible as an effect of annealing in UHV.

True perylene epitaxy on Ag(110) driven by site recognition effect.

We present a STM study of room temperature perylene adsorption on the Ag(110) surface. We have found a 2D perylene crystalline phase coexisting with the perylene liquid phase under thermal equilibrium. The reversible precipitation of the liquid phase at sub-monolayer coverage reveals the well ordered chiral crystalline phase existing in two enantiomorphic configurations of the ((-2)(3) (5)(2)) and ((2)(3) (5)(-2)) symmetry. This chiral phase is spatially separated into the 2D enantiopure islands of tens of nanometers size randomly distributed on the substrate and surrounded by the liquid medium. Analysis of surface registry of the crystalline phase combined with modeling of the intermolecular interactions indicates that its structure and symmetry is determined by a specific balance between the intermolecular attraction and intrinsic ability of the perylene aromatic board to recognize adsorption sites. The recognition effect was found to be strong enough to pin half of the perylene molecules into defined adsorption sites providing the structure skeleton. The attractive intermolecular interaction was found to be strong enough to bind another half of the molecules to the perylene skeleton shaping the true epitaxial structure.

Heat-induced formation of one-dimensional coordination polymers on Au(111): an STM study.

The formation of one-dimensional coordination polymers of cyano-substituted porphyrin derivatives on Au(111) induced by thermal annealing is demonstrated by means of scanning tunnelling microscopy. The polymer is stabilised by an unusual threefold coordination motif mediated between an Au atom and the cyano groups of the porphyrin derivatives.

Single-chiral-catalytic-surface-sites: STM and DFT study of stereodirecting complexes formed between (R)-1-(1-naphthyl)ethylamine and ketopantolactone on Pt(111)

Revealing the set of most stable bimolecular complexes formed by a prochiral molecule and a chiral modifier on Pt(111).

Interaction of ionic liquids with noble metal surfaces: structure formation and stability of [OMIM][TFSA] and [EMIM][TFSA] on Au(111) and Ag(111).

Aiming at a comprehensive understanding of the interaction of ionic liquids (ILs) with metal surfaces we have investigated the adsorption of two closely related ILs, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [EMIM][TFSA] and 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)imide [OMIM][TFSA], with two noble metal surfaces, Au(111) and Ag(111), under ultrahigh vacuum (UHV) conditions using scanning tunneling microscopy (STM). At room temperature, the ILs form a 2D liquid on either of the two surfaces, while at lower temperatures they condense into two-dimensional (2D) islands which exhibit ordered structures or a short-range ordered 2D glass structure. Comparison of the adlayer structures formed in the different adsorption systems and also with those determined recently for n-butyl-n-methylpyrrolidinium [TFSA](-) adlayers on Ag(111) and Au(111) (B. Uhl et al., Beilstein J. Nanotechnol., 2013, 4, 903) gains detailed insight into the adsorption geometry of the IL ions on the surface. The close similarity of the adlayer structures indicates that (i) the structure formation is dominated by the tendency to optimize the anion adsorption geometry, and that (ii) also in the present systems the cation adsorbs with the alkyl chain pointing up from the surface.

Ortho and para hydrogen dimers on G/SiC(0001): combined STM and DFT study.

The hydrogen (H) dimer structures formed upon room-temperature H adsorption on single layer graphene (SLG) grown on SiC(0001) are addressed using a combined theoretical-experimental approach. Our study includes density functional theory (DFT) calculations for the full (6√3 × 6√3)R30° unit cell of the SLG/SiC(0001) substrate and atomically resolved scanning tunneling microscopy images determining simultaneously the graphene lattice and the internal structure of the H adsorbates. We show that H atoms normally group in chemisorbed coupled structures of different sizes and orientations. We make an atomic scale determination of the most stable experimental geometries, the small dimers and ellipsoid-shaped features, and we assign them to hydrogen adsorbed in para dimers and ortho dimers configuration, respectively, through comparison with the theory.

Correction to “Effect of Substrate Chemistry on the Bottom-Up Fabrication of Graphene Nanoribbons: Combined Core-Level Spectroscopy and STM Study”

Correction to “Effect of Substrate Chemistry on the Bottom-Up Fabrication of Graphene Nanoribbons: Combined Core-Level Spectroscopy and STM Study”

Momentum-resolved STM studies of Rashba-split surface states on the topological semimetal Sb

Topological materials host protected surface states with locked spin and momentum degrees of freedom. The helical Dirac character of the surface states, of tremendous scientific interest, stems from the interplay of the bulk band structure and surface Rashba spin–orbit interaction. The semimetal Sb offers a pristine platform to examine the Rashba origins of the Dirac-like topological surface states. Here we present an overview of our momentum-resolved scanning tunneling spectroscopy studies of Sb, over an extended (300meV) energy range, revealing several features characteristic of the emergence of the Dirac-like surface states from a conventional Rashba-type parabolic dispersion. Our work provides a conceptual framework to create and investigate tunable Rashba states with topological properties.

Cathodic regeneration of a clean and ordered Cu(1 0 0)-(1×1) surface from an air-oxidized and disordered electrode: An operando STM study

In work related to the electrocatalysis of the CO2 reduction reactions, we recently reported in This Journal the structure and composition of a Cu(100) electrode surface, pre-dosed at low levels of O2(g) to simulate a Cu electrocatalyst unprotected from air, before and after immersion in alkaline electrolyte at fairly negative potentials to ascertain if an oxide-to-metal reduction reaction can be effected; experimental measurements were based upon ex situ techniques, low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES). It was found that the mildly oxidized surface remained ordered and could be cathodically reduced back to a well-ordered oxide-free Cu(100); the quality of the LEED pattern and AES spectrum was less than ideal, however, due to small amounts of base electrolyte remnant in the emersed layer. In this Short Communication, we present results from operando electrochemical scanning tunneling microscopy (EC-STM) that not only confirm the earlier observations but, more importantly, depict more accurately the actual electrocatalysis conditions. An as-received commercially oriented Cu(100) disk that had not been protected from air was observed to consist of narrow terraces encrusted with highly disordered oxides. Cyclic voltammetry and coulometry showed that the oxidized surface consisted of five monolayers of CuO and quarter of a monolayer of Cu2O. Upon complete cathodic reduction of the interfacial oxides, the surface was found to have reverted to a single-crystalline Cu(100)-(1×1) structure. It may thus be inferred that, under the conditions of alkaline electrochemical CO2 reduction, the Cu catalyst exists as a zerovalent metal.

Building blocks for studies of nanoscale magnetism: adsorbates on ultrathin insulating Cu2N

Scanning tunneling microscopy and spectroscopy were performed to study transition metal adatoms (Fe, Co, Cu) and individual metal-dithiol complexes on insulating Cu2N islands. Adsorption of metal adatoms on Cu2N is surprisingly complex and in the case of Fe, we find two distinct adsorption states for each of two distinct adsorption sites. Connection of these metal adatoms to dithiol molecules was pursued to model a single molecule junction, with the aim of understanding further details about the nature of metal/molecule electrical contact. Pronounced changes in local density of states, magnetic anisotropy and Kondo interactions were observed for Co adatoms connected to dithiol molecules. These results illustrate some of the challenges and opportunities for STM studies of nanoscale magnetism in complex systems.

High-pressure operando STM studies giving insight in CO oxidation and NO reduction over Pt(1 1 0)

Two catalytic systems have been studied at high pressures on the Pt(110) surface on an atomic level. The first system was the oxidation of CO by O2 towards CO2. In the framework of the second reaction, namely NO reduction, the effect of room temperature exposure of the surface to NO and H2 was investigated. To study these reaction systems at relevant pressures, the ReactorSTM has been used. This is a unique system which consists of a compact STM in which a flow reactor is integrated. The combined reactor with STM is housed inside a conventional vacuum system to allow for traditional surface science preparation and analysis techniques. The STM images obtained with the ReactorSTM under reaction conditions show the lifting of the (1x2) missing row reconstruction by high-pressure CO exposure. The lifting is followed by the formation of the (1x1) metallic Pt(110) structure for high CO/O2 ratios and a (1x2) lifted-row type surface oxide structure for more O2-rich conditions. The room temperature exposure of Pt(110) to H2 results in the formation of a (1x4) missing-row structure and deeper, nested missing rows. The exposure to high-pressure NO removes these missing-row structures.

Development and STM Study of Hydrogen based Deposition of Pt on Pd Thin Films

The key technological challenge for fuel-cells commercialization is design of nanomaterials with low-content of Pt and Pt-group noble-metals exhibiting high activity, improved selectivity and stability. New Pt-bi and tri-metallic catalysts of different shape, composition as well as geometrical configurations have been actively explored in recent years.1 Among different methods for design of Pt films, the surface limited redox replacement (SLRR) has an important place.2 This method is unique because it enables epitaxial deposition of: Pt films, monolayers and clusters, onto other noble metal surfaces with the atomic scale control. This has been achieved by utilization of underpotentially deposited (UPD) metal layer as sacrificial, in the galvanic replacement by more noble Pt.Several research groups demonstrated successful SLRR deposition of epitaxial Pt films using Cu UPD and Pb UPD for the design of nanopraticles as well as 2D-model surfaces.3-6 Our recent work demonstrated that using the SLRR protocol, a H-UPD layer can be exploited for homoepitaxial grow of Pt. 7 We extend further the concept of H mediated growth utilizing SLRR approach, onto Pd-thin film system. Pt thin films on Au are well studied and well known for their unique H-sorption behavior, and catalytic properties.8 Moreover, recent studies on the Pt-on-Pd9 and Pt-Pd-Au10 showed that this system is of interest for its electrocatalysis as well as H- storage potential.11 Here in this talk we will present the electrochemical and the in-situ STM study of Pt electrodeposition in the SLRR configuration. Our substrates were epitaxial Pd films (2-10 ML) electrodeposited on Au(111) films evaporated on mica or glass.8 High quality of the deposited epitaxial Pd films was confirmed electrochemically and by the in-situ STM. Surface morphology evolution of Pt films was studied in solution and conducted in the environmentally isolated STM system using custom designed set up for the solutions exchange. Furthermore, the Electrochemical Quartz Crystal Microbalance study of the growth kinetics showed that Pt structure and thickness can be controlled by the amount of absorbed hydrogen, solution pH, and Pt concentration. Electrochemical behavior of Pt films on 2ML and 10 ML Pd-Au show distinctly different H-sorption characteristics. Films on 2ML Pd/Au have characteristics of typical Pt surface, while films on the Pt-10ML Pd/Au showed uninhibited H absorption with more reversible behavior than on corresponding 10ML Pd/Au substrate.

STM Study on the (100) Surface of the Ag-In-Yb 1/1 Cubic Approximant

STM Study on the (100) Surface of the Ag In Yb 1/1 Cubic Approximant C. Cuia,b,∗, H. Sharma, P.J. Nugent, M. Shimoda and An Pang Tsai Department of Physics, Zhejiang Sci-Tech University, Hangzhou 310018, China Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan Surface Science Research Centre and Department of Physics, The University of Liverpool, Liverpool L69 3BX, UK National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan