Aarhus

The interface between metal and reducible oxide has attracted increasing interest in catalysis. The FeO_x–Pt interface has been a typical example, which showed remarkable activity for the preferential oxidation of CO (PROX) at low temperatures. However, model catalytic studies under vacuum conditions or in high-pressure O-rich environment at 450 K have reported two different active phases with iron in two different valence states, invoking a possible pressure gap. To identify the active phase for low-temperature CO oxidation and PROX, it is necessary to investigate the stability and activity of FeO/Pt(111) under the realistic reaction conditions. We thus conducted an in situ study on FeO/Pt(111) from ultrahigh vacuum to the atmospheric pressure of reactant gases. Our study shows FeO islands were easily oxidized in 1 Torr O₂ to form the trilayer FeO₂ islands. However, the presence of 2 Torr CO could prevent the oxidation of FeO islands and lead to CO oxidation at the FeO/Pt(111) interface. The FeO/Pt(111) surface exhibits an excellent activity for CO₂ production with an initial reaction rate measured to be ∼1 × 10¹⁴ molecules·cm^–2·s^–1 at 300 K. FeO islands supported on Pt(111) were further investigated in the PROX gas, i.e., the mixture of 98.5% H₂, 1% CO, and 0.5% O₂, at elevated pressures up to 1 bar. Our results thus bridged the pressure gap and identified the bilayer FeO islands on Pt(111) as the active phase for PROX under the realistic reaction conditions.

The growth and structural properties of ZnO thin films on both Au(111) and Cu(111) surfaces were studied using either NO₂ or O₂ as oxidizing agent. The results indicate that NO₂ promotes the formation of well-ordered ZnO thin films on both Au(111) and Cu(111). The stoichiometric ZnO thin films obtained on these two surfaces exhibit a flattened and non-polar ZnO(0001) structure. It is shown that on Au(111), the growth of bilayer ZnO nanostructures (NSs) is favored during the deposition of Zn in presence of NO₂ at 300 K, whereas both monolayer and bilayer ZnO NSs could be observed when Zn is deposited at elevated temperatures under a NO₂ atmosphere. The growth of bilayer ZnO NSs is caused by the stronger interaction between two ZnO layers than between ZnO and Au(111) surface. In contrast, the growth of monolayer ZnO NSs involves a kinetically controlled process. ZnO thin films covering the Au(111) surface exhibits a multilayer thickness, which is consistent with the growth kinetics of ZnO NSs. Besides, the use of O₂ as oxidizing agent could lead to the formation of sub-stoichiometric ZnOx structures. The growth of full layers of ZnO on Cu(111) has been a difficult task, mainly because of the interdiffusion of Zn promoted by the strong interaction between Cu and Zn and the formation of Cu surface oxides by the oxidation of Cu(111). We overcome this problem by using NO2 as oxidizing agent to form well-ordered ZnO thin films covering the Cu(111) surface. The surface of the well-ordered ZnO thin films on Cu(111) displays mainly a moiré pattern, which suggests a (3 × 3) ZnO superlattice supported on a (4 × 4) supercell of Cu(111). The observation of this superstructure provides a direct experimental evidence for the recently proposed structural model of ZnO on Cu(111), which suggests that this superstructure exhibits the minimal strain. Our studies suggested that the surface structures of ZnO thin films could change depending on the oxidation level or the oxidant used. The oxidation of Cu(111) could also become a key factor for the growth of ZnO. When Cu(111) is pre-oxidized to form copper surface oxides, the growth mode of ZnO_x is altered and single-site Zn could be confined into the lattice of copper surface oxides. Our studies show that the growth of ZnO is promoted by inhibiting the diffusion of Zn into metal substrates and preventing the formation of sub-stoichiometric ZnO_x. In short, the use of an atomic oxygen source is advantageous to the growth of ZnO thin films on Au(111) and Cu(111) surfaces.

Graphene-like ZnO (g-ZnO) nanostructures (NSs) and thin films were prepared on Au(111), and their reactivities toward CO and H₂ were compared with that of wurtzite ZnO (w-ZnO) (0001) single crystals. The interaction and reaction between CO/H₂ and the different types of ZnO surfaces were studied using near-ambient-pressure scanning tunneling microscopy (NAP-STM), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. The reactivity of the w-ZnO(0001) surface toward CO and H₂ was found to be more prominent than those on the surfaces of g-ZnO/Au(111). CO oxidation took place primarily at the edge sites of w-ZnO(0001) and the interface between g-ZnO NSs and Au(111), while g-ZnO thin films on Au(111) appeared to be inert below 600 K. Similarly, the w-ZnO(0001) surface could dissociate H₂ at 300 K, accompanied by a substantial surface reconstruction, while g-ZnO on Au(111) appeared inert for H₂ activation at 300 K. DFT calculations showed that the reactivities of ZnO surfaces toward CO could be related to the formation energy of oxygen vacancy (EOvf), which could be related to the charge transfer to lattice oxygen atoms or surface polarity.

Near surface alloy (NSA) model catalyst Pt/Cu/Pt(1 1 1) was prepared on Pt(1 1 1) through a controlled vapor deposition of Cu atoms. Different coordination environments of Pt atoms of the topmost Pt layer with the underneath Cu atoms in the subsurface result in different local electronic structures of surface Pt atoms. Surface structure and chemistry of the NAS model catalyst in Torr pressure of CO were studied with high pressure scanning tunneling microscopy (HP-STM) and ambient pressure X-ray photoelectron spectroscopy (AP-XPS). In Torr pressure of CO, the topmost Pt layer of Pt/Cu/Pt(1 1 1) is restructured to thin nanoclusters with size of about 1 nm. Photoemission feature of O 1s of CO on Pt/Cu/Pt(1 1 1) suggests CO adsorbed on both edge and surface of these formed nanoclusters. This surface is active for CO oxidation. Atomic layers of carbon are formed on Pt/Cu/Pt(1 1 1) at 573 K in 2 Torr of CO.

Catalysis science has emerged as one of the crucial fields in energy science responsible for a sustainable energy world. Fundamental study of surface structures of catalysts at atomic scale under reaction conditions or during catalysis is critical in understanding catalytic mechanisms because a single catalysis event is performed on a specific site comprising one or several atoms with appropriate geometric and electronic structures. Ambient pressure high temperature scanning tunneling microscopy (APHT-STM) can identify structural details of catalyst surfaces at nano or atomic level when catalysts are under reaction conditions or during catalysis. By using APHT-STM, structures of the step edge of Pt(111) and the surface of Ni(557) were studied under reaction conditions. For Pt(111) model catalyst in a CO environment at a pressure of 0.1 Torr or higher, Pt atoms at step edges exhibit a dynamic restructuring. They form kink sites at 0.1 Torr and create nanoclusters near the step edges at 1–10 Torr within 1–2 min. A pressure-dependent restructuring of step edges of Pt(111) was revealed. In in situ studies of the vicinal surface, Ni(557) shows a pressure-dependent restructuring in CO, resulting from reorganization of all surface atoms. Nickel nanoclusters are formed on the whole surface, consistent with the increased coverage of CO chemisorbed on the Ni surface at a relatively higher pressure. Restructuring of atoms at a step edge of terraces of a flat surface and all atoms of a vicinal surface suggest the dynamic nature of model metal catalyst surfaces. Essentially, the surface structure of a metal catalyst in a reactive environment is determined by its reaction or catalysis condition.

The initial oxidation of Ni₃Al(111) was imaged by in situ scanning tunneling microscopy (STM) at 700–750 K. At 740 K ± 10 K a moiré structure is formed as the major surface phase: high resolution STM data atomically resolve a top hexagonal lattice with a lattice constant of 2.93 ± 0.01 Å aligned or slightly rotated with respect to the substrate. Auger electron spectra acquired from the surface phase identify Al atoms in an oxidic environment together with Ni atoms unaffected by the oxidation of the Ni₃Al(111) surface. A special mass balance analysis applied to STM images recorded during formation of the moiré structure allowed to extract the metal content of the surface phase. The moiré phase can be attributed to a single O/Al double layer of α-Al₂O₃ ontop of the Ni₃Al(111) crystal. The surface double layer is laterally expanded by ∼7% with respect to α-Al₂O₃ and, relating to the next nearest neighbor distance of the substrate of 2.52 Å, it contains 0.73 ML oxygen and 0.49 ML aluminium atoms. The building principle of the surface phase is almost identical to the one of the reported O_i/Al_i interface layer of the so called (√67 x √67)RI 2.2° surface oxide, except for its rotational alignment with respect to the substrate as shown in a careful moiré analysis. It could be shown that this thinnest possible surface aluminum oxide layer is formed due to kinetic restrictions: the oxide grows within the first layer of the Ni₃Al(111) surface ejecting 0.5 ML surface metal atoms, which are then converted into the surface oxide laterally separated at the ascending step edge of the same terrace. While the formation of the surface oxide is kinetically hindered most likely by the availability of Al adatoms, all rearrangement processes required for the surface oxide formation on each terrace are not rate limiting as identified by in situ STM. Instead, the local oxide growth rather follows the kinetics driven by the adsorption probability of the impinging oxygen molecules and provides the possibility to entirely cover whole Ni₃Al(111) surface.

Active catalytic sites have traditionally been analyzed based on static representations of surface structures and characterization of materials before or after reactions. We show here by a combination of in situ microscopy and spectroscopy techniques that, in the presence of reactants, an oxide catalyst’s chemical state and morphology are dynamically modified. The reduction of Cu2O films is studied under ambient pressures (AP) of CO. The use of complementary techniques allows us to identify intermediate surface oxide phases and determine how reaction fronts propagate across the surface by massive mass transfer of Cu atoms released during the reduction of the oxide phase in the presence of CO. High resolution in situ imaging by AP scanning tunneling microscopy (AP-STM) shows that the reduction of the oxide films is initiated at defects both on step edges and the center of oxide terraces.

The reducibility of metal oxides is of great importance to their catalytic behavior. Herein, we combined ambient‐pressure scanning tunneling microscopy (AP–STM), X‐ray photoemission spectroscopy (AP–XPS), and DFT calculations to study the CO titration of CuxO thin films supported on Cu(1 1 1) (CuxO/Cu(1 1 1)) aiming to gain a better understanding of the roles that the Cu(1 1 1) support and surface defects play in tuning catalytic performances. Different conformations have been observed during the reduction, namely, the 44 structure and a recently identified (5–7–7–5) Stone–Wales defects (5–7 structure). The DFT calculations revealed that the Cu(1 1 1) support is important to the reducibility of supported CuxO thin films. Compared with the case for the Cu2O(1 1 1) bulk surface, at the initial stage CO titration is less favorable on both the 44 and 5–7 structures. The strong CuxO↔Cu interaction accompanied with the charge transfer from Cu to CuxO is able to stabilize the oxide film and hinder the removal of O. However, with the formation of more oxygen vacancies, the binding between CuxO and Cu(1 1 1) is weakened and the oxide film is destabilized, and Cu2O(1 1 1) is likely to become the most stable system under the reaction conditions. In addition, the surface defects also play an essential role. With the proceeding of the CO titration reaction, the 5–7 structure displays the highest activity among all three systems. Stone–Wales defects on the surface of the 5–7 structure exhibit a large difference from the 44 structure and Cu2O(1 1 1) in CO binding energy, stability of lattice oxygen, and, therefore, the reduction activity. The DFT results agree well with the experimental measurements, demonstrating that by adopting the unique conformation, the 5–7 structure is the active phase of CuxO, which is able to facilitate the redox reaction and the Cu2O/Cu(1 1 1)↔Cu transition.

Copper has excellent initial activity for the oxidation of CO, yet it rapidly deactivates under reaction conditions. In an effort to obtain a full picture of the dynamic morphological and chemical changes occurring on the surface of catalysts under CO oxidation conditions, a complementary set of in situ ambient pressure (AP) techniques that include scanning tunneling microscopy, infrared reflection absorption spectroscopy (IRRAS), and X-ray photoelectron spectroscopy were conducted. Herein, we report in situ AP CO oxidation experiments over Cu(111) model catalysts at room temperature. Depending on the CO:O2 ratio, Cu presents different oxidation states, leading to the coexistence of several phases. During CO oxidation, a redox cycle is observed on the substrate’s surface, in which Cu atoms are oxidized and pulled from terraces and step edges and then are reduced and rejoin nearby step edges. IRRAS results confirm the presence of under-coordinated Cu atoms during the reaction. By using control experiments to isolate individual phases, it is shown that the rate for CO oxidation decreases systematically as metallic copper is fully oxidized.

The transformation of CO₂ into alcohols or other hydrocarbon compounds is challenging because of the difficulties associated with the chemical activation of CO₂ by heterogeneous catalysts. Pure metals and bimetallic systems used for this task usually have low catalytic activity. Here we present experimental and theoretical evidence for a completely different type of site for CO₂ activation: a copper-ceria interface that is highly efficient for the synthesis of methanol. The combination of metal and oxide sites in the copper-ceria interface affords complementary chemical properties that lead to special reaction pathways for the CO₂→CH₃OH conversion.

Adsorbate-driven morphological changes of pitted-Cu(111) surfaces have been investigated following the adsorption and desorption of CO and H. The morphology of the pitted-Cu(111) surfaces, prepared by Ar⁺ sputtering, exposed a few atomic layers deep nested hexagonal pits of diameters from 8 to 38 nm with steep step bundles. The roughness of pitted-Cu(111) surfaces can be healed by heating to 450–500 K in vacuum. Adsorption of CO on the pitted-Cu(111) surface leads to two infrared peaks at 2089–2090 and 2101–2105 cm^-1 for CO adsorbed on under-coordinated sites in addition to the peak at 2071 cm^-1 for CO adsorbed on atop sites of the close-packed Cu(111) surface. CO adsorbed on under-coordinated sites is thermally more stable than that of atop Cu(111) sites. Annealing of the CO-covered surface from 100 to 300 K leads to minor changes of the surface morphology. In contrast, annealing of a H covered surface to 300 K creates a smooth Cu(111) surface as deduced from infrared data of adsorbed CO and scanning tunnelling microscopy (STM) imaging. The observation of significant adsorbate-driven morphological changes with H is attributed to its stronger modification of the Cu(111) surface by the formation of a sub-surface hydride with a hexagonal structure, which relaxes into the healed Cu(111) surface upon hydrogen desorption. These morphological changes occur ∼150 K below the temperature required for healing of the pitted-Cu(111) surface by annealing in vacuum. In contrast, the adsorption of CO, which only interacts with the top-most Cu layer and desorbs by 200 K, does not significantly change the morphology of the pitted-Cu(111) surface.

In situ X-ray photoelectron spectroscopy (XPS), infrared reflection absorption spectroscopy (IRRAS), and scanning tunneling microscopy (STM) were used to study the reduction of potassium-modified Cu₂O/Cu(111) by CO. By following the time evolution of the O _1s peak of Cu₂O, we determined that the apparent activation energy for Cu₂O reduction by 2 × 10^–4 Torr CO is decreased by ∼30% in the presence of K. On the K-modified surface, both XPS and IRRAS data show the formation of a surface species identified by IRRAS as carbonate (CO₃^2–), likely forming a K⁺-CO₃^2– complex, which is stable up to 500 K. STM images show that K+-CO₃^2– complexes form chains around reduced Cu islands, thereby hindering the mass transfer of Cu atoms and preventing the reconstruction of the surface. Theoretical calculations show that the formation of carbonate on the K-modified “44” Cu₂O structure is thermodynamically favorable compared to the formation of CO₂ on either the bare or K-modified surfaces.

Here, we present the design of a new reactor-like high-temperature near ambient pressure scanning tunneling microscope (HT-NAP-STM) for catalysis studies. This HT-NAP-STM was designed for exploration of structures of catalyst surfaces at atomic scale during catalysis or under reaction conditions. In this HT-NAP-STM, the minimized reactor with a volume of reactant gases of ∼10 ml is thermally isolated from the STM room through a shielding dome installed between the reactor and STM room. An aperture on the dome was made to allow tip to approach to or retract from a catalyst surface in the reactor. This dome minimizes thermal diffusion from hot gas of the reactor to the STM room and thus remains STM head at a constant temperature near to room temperature, allowing observation of surface structures at atomic scale under reaction conditions or during catalysis with minimized thermal drift. The integrated quadrupole mass spectrometer can simultaneously measure products during visualization of surface structure of a catalyst. This synergy allows building an intrinsic correlation between surface structure and its catalytic performance. This correlation offers important insights for understanding of catalysis. Tests were done on graphite in ambient environment, Pt(111) in CO, graphene on Ru(0001) in UHV at high temperature and gaseous environment at high temperature. Atom-resolved surface structure of graphene on Ru(0001) at 500 K in a gaseous environment of 25 Torr was identified.
ACKNOWL

Fundamental understanding of chemistry confined to nanospace remains a challenge since molecules encapsulated in confined microenvironments are difficult to be characterized. Here, we show that CO adsorption on Pt(111) confined under monolayer hexagonal boron nitride (h-BN) can be dynamically imaged using near ambient pressure scanning tunneling microscope (NAP-STM) and thanks to tunneling transparency of the top h-BN layer. The observed CO superstructures on Pt(111) in different CO atmospheres allow to derive surface coverages of CO adlayers, which are higher in the confined nanospace between h-BN and Pt(111) than those on the open Pt surface under the same conditions. Dynamic NAP-STM imaging data together with theoretical calculations confirm confinement-induced molecule enrichment effect within the 2D nanospace, which reveals new chemistry aroused by the confined nanoreactor.
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Pt–Fe bimetallic alloys are important model catalysts for a number of catalytic reactions. Combining scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS), we have studied the structures of Pt–Fe surface alloys prepared on Pt(111) under a variety of conditions. Although the surface and subsurface structures of the Pt–Fe surface alloy could be varied with the deposition amount of Fe atoms and the annealing temperature, a characteristic alloy surface with a bright striped pattern could be identified, which consists of a Pt-dominant surface layer with a small percentage of Fe atoms in the form of isolated atoms or clusters in the surface lattice and a subsurface layer with an ordered Pt₃Fe alloy structure. The bright stripes observed in STM were surface dislocations caused by stress relaxation owing to the lattice mismatch between the surface and subsurface layers. This characteristic alloy surface could be prepared on Pt(111) by depositing sub-monolayer Fe at ∼460 K to facilitate Fe diffusion in the near-surface region, or annealing multilayer Fe at ∼700 K, to enhance bulk diffusion of Fe atoms. The synthesis of this Pt–Fe alloy surface with well-defined structures could allow for further model catalytic studies.

We investigate the decay of Ag islands on Cu(111) by variable low temperature scanning tunneling microscopy
between 195 and 250 K. Such islands exhibit a misfit dislocation pattern forming (8 × 8) to (10 × 10)
superstructures because of a major lattice mismatch between silver and copper. The decay of islands smaller than
200 nm² alternates between a slower and a faster decay. It is slower for specific island sizes, in particular those
with magic numbers of superstructure unit cells.We relate these changes to the complexity of the heteroepitaxial
decay, involving a deconstruction of the misfit dislocation pattern and a simultaneous diffusion of several
adspecies during decay.

We follow the decay of two-dimensional Ag nanoclusters, called islands, on Cu–Ag core–shell supports
by variable low temperature scanning tunneling microscopy in the temperature range between 160 and
260 K. We reveal two qualitatively different types of decay mechanisms, either linear in time, indicative of
an interface-limited decay, or non-linear in time, indicative of diffusion-limited decay. In contrast to
conventional decay on monometallic supports, the decay exponent of the diffusion-limited decay
depends on temperature; it varies by one order of magnitude. Moreover, the decay rate decreases
with increasing temperature. This unusual behaviour is traced back to the temperature-dependent shell of
the core–shell support.

Metal adatoms play a key role in surface diffusion, adsorption conformation, and self-assembly of porphyrin molecules on metal surfaces. Herein, we study the specific influence of coadsorption of Fe, Co, and Pd atoms on the behavior of 2H-tetrakis(p-cyano)phenylporphyrin (2H-TCNPP) on Cu(111) using scanning tunneling microscopy. Upon co-deposition of Fe and Co, the molecules form one-dimensional (1D) linear chains after mild annealing on Cu(111) driven by the interaction of its cyano groups with metal adatoms. A similar behavior has been observed previously on Cu(111), mediated by Cu adatoms, where the functional CN groups were also found to lower the reaction rate of the so-called porphyrin self-metalation reaction with Cu atoms significantly, in comparison to the non-cyano-functionalized porphyrin. Upon co-deposition of Pd and mild annealing, we find a remarkably different behavior, that is, a massive reorganization from 1D molecular chains to a peculiar rectangular 2D (two-dimensional) network. The molecular appearance changes to a clover shape, which is attributed to a Pd-induced dehydrogenation and subsequent ring closure reaction of the phenyl and pyrrole groups.
 

Monolayer hexagonal boron–nitrogen–carbon (h-BNC) is considered a prominent candidate for the next generation of semiconductor electronic devices. Nevertheless, experimental evidence of h-BNC formation is limited, including a detailed study of its morphological and electronic properties. Here, successful growth of h-BNC from an unexplored single molecular precursor (hexamethyl borazine, C₆H₁₈B₃N₃) using a conventional CVD approach on Ir(111) is reported. The conformation structure of the monolayer and its correlation with the local electronic properties are discussed based on scanning tunneling microscopy/spectroscopy (STM/STS) and X-ray photoelectron spectroscopy (XPS) results. The results show an h-BNC structure that can be described as BN-doped graphene since the moiré lattice parameter is preserved along with the alloy. This BN-doped cluster, renamed as h-BN “nanodonuts” according to the electronic density exhibited in STM images, have a tendency to place specific positions within the moiré superstructure, and it is constituted by at least (BN)₈ units arranged in a 6-fold BN rings conformation, as evidenced by simulation of STM images based on density functional theory (DFT). For a BN concentration of about 17%, a band gap between 1.4 and 1.6 eV was determined. The versatility of the novel molecular precursor is proven by the growth of a high-quality h-BN monolayer on Rh(111).
 

On-surface coupling reactions and molecular conformation are essential processes for building tailored functional molecular nanostructures. Here, we study the thermal debromination and reactivity of free-base tetra(4-bromophenyl)porphyrin (H₂TBrPP) on Cu(111) as a function of the substrate temperature. It has been previously reported in the literature that C–Br bonds remain intact at room temperature (RT) and that the Br···Cu(111) interaction induces a drastic surface reconstruction around the molecule periphery and a distortion in the adsorbate itself. However, based on a combination of STM and XPS experiments, supported by density functional theory (DFT) calculations, we instead demonstrate that debromination readily occurs at RT, leading to a new interpretation of both the adsorption behavior and the molecular coupling of H₂TBrPP on Cu(111). For the molecules deposited on the metallic substrate held above RT, our STM measurements show the growth of ordered 2D metal–organic frameworks (MOFs).
 

Polyethylene production through catalytic ethylene polymerization is one of the most common processes
in the chemical industry. The popular Cossee-Arlman mechanism hypothesizes that the ethylene be
directly inserted into the metal–carbon bond during chain growth, which has been awaiting microscopic
and spatiotemporal experimental confirmation. Here, we report an in situ visualization of ethylene
polymerization by scanning tunneling microscopy on a carburized iron single-crystal surface. We
observed that ethylene polymerization proceeds on a specific triangular iron site at the boundary
between two carbide domains. Without an activator, an intermediate, attributed to surface-anchored
ethylidene (CHCH3), serves as the chain initiator (self-initiation), which subsequently grows by
ethylene insertion. Our finding provides direct experimental evidence of the ethylene polymerization
pathway at the molecular level.

Polyethylene is a common plastic with a production rate of ∼100 million tons every year, which corresponds to about one-third of all plastics (1). Most of the polyethylene is created with the help of solid catalysts. These catalysts help link together ethylene molecules, the monomers that make up polyethylene. To better understand the complex reactions that connect the monomers, researchers have mainly focused on the measurements of the formation rate and the composition of the resulting polymer. They have also used spectroscopy to analyze the catalysts, but extracting information about the processes on the individual connection sites, known as active centers, can be challenging. On page 1188 of this issue, Guo et al. (2) present a way to view the processes on the active centers microscopically by using scanning tunneling microscopy (STM) and a model catalyst.

Methanol formation over Cu/ZnO catalysts is linked with a catalytically active phase created by contact between Cu nanoparticles and Zn species whose chemical and structural state depends on reaction conditions. Herein, we use variable-temperature scanning tunneling microscopy at elevated pressure conditions combined with X-ray photoelectron spectroscopy measurements to investigate the surface structures and chemical states that evolve when a CuZn/Cu(111) surface alloy is exposed to reaction gas mixtures. In CO₂ hydrogenation conditions, Zn stays embedded in the CuZn surface, but once CO gas is added to the mixture, the Zn segregates onto the Cu surface. The Zn segregation is CO-induced, and establishes a new dynamic state of the catalyst surface where Zn is continually exchanged at the Cu surface. Candidates for the migrating few-atom Zn clusters are further identified in time-resolved imaging series. The findings point to a significant role of CO affecting the distribution of Zn in the multiphasic ZnO/CuZn/Cu catalysts.

We report the influence of lithium ions on binding and structure of water nanoislands on Au(111) by
temperature-programmed desorption and variable-temperature scanning tunneling microscopy. Water coverages
between a fraction and full bilayer and two lithium coverages (<0.15% ML) are explored. Lithium enhances selectively the
binding of some of the water molecules on precovered Au(111) as compared to water on pristine Au(111), which is
revealed by an increase of the water desorption temperature by approx. 10 K. Surprisingly, the effect of lithium on the
structure of water is much more extended than expected from these desorption experiments. A small amount of lithium changes
the structure of water nanoislands drastically compared to those on pristine Au(111). On pristine Au(111), water ice grows in
the form of crystalline islands that are two or three bilayers high. On Li precovered Au(111), the islands are more corrugated, at
a 5 times broader apparent height distribution and much smaller, at a 4 times smaller area distribution. These changes reflect the
influence of lithium as a structure maker, or kosmotrope, on water. Our study provides unprecedented real-space information of
the influence of a kosmotrope on the water structure at the nanoscale. We utilize its kosmotropic behavior to provide real-space
images of desorption.

If a material grows on another material with a largely different lattice constant, which of
the two adapts for an energetically favorable growth? To tackle this question, we investigate the
growth of Ag on Cu(111) by variable temperature scanning tunneling microscopy. The structures
grown between 120 and 170 K are remarkably different from those grown between 200 and 340 K.
The low-temperature structure is rectangular-like and consists of stacked rods, 7 to 8 Ag atoms long,
which form a superstructure without long-range order. This structure covers the whole surface prior
to nucleation of further layers. The high-temperature structure is hexagonal and consists of misfit
dislocations forming 8  8 to 10  10 superstructures. For this structure, second layer nucleation sets
in far before the closure of the first monolayer. While both structures are driven by the large lattice
misfit between the two materials, the growing Ag layer adapts to the Cu surface at low temperature,
while the Cu surface adapts to the growing Ag layer at higher temperature.

We investigate the shape of monatomic high Cu islands on a Cu(111) surface by
variable-temperature scanning tunneling microscopy between 110 K and 240 K. Low temperature
dendrites evolve towards more compact shapes at increasing temperature; finally reaching the
equilibrium shape of a hexagon with rounded corners. Time-lapsed imaging at increasing
temperature reveals the onset of shape change to be at ≈170 K, corresponding to the onset of edge
and corner diffusion of atoms along the island’s borders. Despite a substantial variation for
individual islands at each temperature, the mean fractal dimension increases monotonously
between 170 K up to 240 K, from the smallest to the largest values feasible for islands grown on
surfaces.

We report on strategies for characterizing hexagonal coincidence phases by analyzing the involved spatial moiré beating frequencies of the pattern. We derive general properties of the moiré regarding its symmetry and construct the spatial beating frequency  as the difference between two reciprocal lattice vectors  of the two coinciding lattices. Considering reciprocal lattice vectors , with lengths of up to n times the respective (1, 0) beams of the two lattices, readily increases the number of beating frequencies of the nth-order moiré pattern. We predict how many beating frequencies occur in nth-order moirés and show that for one hexagonal lattice rotating above another the involved beating frequencies follow circular trajectories in reciprocal-space. The radius and lateral displacement of such circles are defined by the order n and the ratio x of the two lattice constants. The question of whether the moiré pattern is commensurate or not is addressed by using our derived concept of commensurability plots. When searching potential commensurate phases we introduce a method, which we call cell augmentation, and which avoids the need to consider high-order beating frequencies as discussed using the reported  moiré of graphene on SiC(0001). We also show how to apply our model for the characterization of hexagonal moiré phases, found for transition metal-supported graphene and related systems. We explicitly treat surface x-ray diffraction-, scanning tunneling microscopy- and low-energy electron diffraction data to extract the unit cell of commensurate phases or to find evidence for incommensurability. For each data type, analysis strategies are outlined and avoidable pitfalls are discussed. We also point out the close relation of spatial beating frequencies in a moiré and multiple scattering in electron diffraction data and show how this fact can be explicitly used to extract high-precision data.

Despite tremendous importance in catalysis, the design and improvement of the oxide- metal interface has been hampered by the limited understanding on the nature of interfacial sites, as well as the oxide-metal interaction (OMI). Through the construction of well-defined Cu₂O-Pt, Cu₂O-Ag, Cu₂O-Au interfaces, we found that Cu₂O Nanostructures (NSs) on Pt exhibit much lower thermal stability than on Ag and Au, although they show the same surface and edge structures, as identified by element-specific scanning tunneling microscopy (ES-STM) images. The activities of the Cu₂O-Pt and Cu₂O-Au interfaces for CO oxidation were further compared at the atomic scale and showed in general that the interface with Cu₂O NSs could annihilate the CO-poisoning problem suffered by Pt group metals and enhance the interaction with O₂, which is a limiting step for CO oxidation catalysis on group IB metals. While both interfaces could react with CO at room temperature, the OMI was found to determine the reactivity of supported Cu₂O NSs by 1) tuning the activity of interfacial oxygen atoms and 2) stabilizing oxygen vacancies or vice versa, the dissociated oxygen atoms at the interface. Our study provides new insight for OMI and for the development of Cu-based catalysts for low temperature oxidation reactions.

The formation of interfacial metal–oxide structures on the Pt₃Ni(111) bimetallic surface was investigated using scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) techniques at near-ambient pressure (NAP). Direct observation of surface images clearly shows the occurrence of surface segregation of the sub-surface Ni depending on the surrounding gas-phase conditions. Especially, the prepared topmost Pt-skin layer of the Pt₃Ni(111) is altered by Ni oxide segregation that makes an interfacial Pt-NiO_1−x nanostructure with dissociated oxygen. This metal–oxide interface could provide active sites for more-efficient carbon monoxide (CO) conversion processes under mixed CO/O₂ gas environments; the associated specific chemical binding energy was identified using NAP-XPS. The combined operando observations from the NAP-STM and NAP-XPS on the Pt₃Ni(111) surface reveal that the interfacial metal–oxide structure is strongly correlated with the origin of the enhanced catalytic activity at thermodynamic equilibrium.

Chemical reactions that convert sp2 to sp3 hybridization have been demonstrated to be a fascinating yet challenging route
to functionalize graphene. So far it has not been possible to precisely control the reaction sites nor their lateral order at the
atomic/molecular scale. The application prospects have been limited for reactions that require long soaking, heating, electric
pulses or probe-tip press. Here we demonstrate a spatially selective photocycloaddition reaction of a two-dimensional
molecular network with defect-free basal plane of single-layer graphene. Directly visualized at the submolecular level, the
cycloaddition is triggered by ultraviolet irradiation in ultrahigh vacuum, requiring no aid of the graphene Moiré pattern. The
reaction involves both [2+2] and [2+4] cycloadditions, with the reaction sites aligned into a two-dimensional extended and
well-ordered array, inducing a bandgap for the reacted graphene layer. This work provides a solid base for designing and engineering
graphene-based optoelectronic and microelectronic devices.

We present a mechanistic study on the formation and dynamic changes of a ligand-based heterogeneous Pd catalyst for chemoselective hydrogenation of α,β-unsaturated aldehyde acrolein. Deposition of allyl cyanide as a precursor of a ligand layer renders Pd highly active and close to 100 % selective toward propenol formation by promoting acrolein adsorption in a desired configuration via the C=O end. Employing a combination of real-space microscopic and in-operando spectroscopic surface-sensitive techniques, we show that an ordered active ligand layer is formed under operational conditions, consisting of stable N-butylimine species. In a competing process, unstable amine species evolve on the surface, which desorb in the course of the reaction. Obtained atomistic-level insights into the formation and dynamic evolution of the active ligand layer under operational conditions provide important input required for controlling chemoselectivity by purposeful surface functionalization.

Co growth on Cu(111) was investigated at several temperatures between 120 K and 300 K by variabletemperature
fast-scanning tunneling microscopy at submonolayer coverage. Islands nucleate heterogeneously
at step edges and homogeneously on terraces. The height and area distribution difference between these two
types of differently nucleated islands is attributed to a step edge alloy. Furthermore, the transformation from
one-monolayer high islands to two-monolayer high islands is followed in time-lapsed sequences between 145 and
165 K. A surprising low-energy barrier for upward mass transport of E_upward ≈ (0.15 ± 0.04) eV is determined
for islands on terraces. At 120 and 150 K, the terrace islands are pure Cu; in contrast, at room temperature, terrace
islands larger than ≈120 nm² alloy at their border.

We investigate the induced growth of a Ag layer on a Cu(111) surface by variable low-temperature scanning
tunneling microscopy between 100 and 140 K at submonolayer coverage. Without any interference by the scanning process, the
Ag atoms form a two-dimensional gas on the Cu(111) surface.Imaging the surface at elevated voltage induces nucleation and
growth of one-dimensional Ag stripes of monolayer height, eventually filling the surface of the imaged area completely. The stripes consist of rods of atoms with a preferential length of (1.88 ± 0.10) nm, corresponding to approx. seven or eight Ag
atoms on eight to nine Cu hollow sites. At a ratio of approximately 1:3, rods of double length are the second most
observed species. The rods stack in the ⟨112⟩ directions at the √3 distance of Cu(111). Although all equivalent three surface directions are observed, their abundance is not equally distributed, such that the rod direction aligned with the fast scanning direction predominates. At slow growth rates, it is possible to create a striped pattern with one surface direction only.

Deposition of a porphyrin onto metallic copper followed by heating leads to an unprecedented type of linking of the molecules giving a mixture of covalent multiporphyrin nanostructures at the surface.

A new design of a noncontact atomic force microscope (AFM) is introduced in this paper, based on a piezoelectric oscillator sensor (PEOS) for ambient and liquid environments. Because of the recent development of quartz technology, the PEOS sensor operates independently from conventional laser alignments. The sensor is based on the length extension resonator, which has high force sensitivity and can deliver high resolution AFM images in ultrahigh vacuum. The oscillator design was tested in different gas compositions and liquids to determine its oscillation stability. The scan performance was investigated in both air and liquid on the topography of an inorganic hard material, graphite. The usability of PEOS for soft organic materials was further proven by imaging biological samples of DNA origami.

We have investigated the structure and morphology of nanosized palladium clusters supported by a thin Al2O3 film on NiAl(110) using scanning tunneling microscopy. Well-ordered clusters with a diameter above ≈40Å were imaged with atomic resolution, allowing a crystallographic identification of the cluster facets. A new method to obtain quantitative information on the work of adhesion (adhesion energy) of metal clusters deposited on oxides is introduced. For Pd on Al2O3 we obtain a value of Wadh=2.8±0.2J/m2. This result is at variance with values recently derived on the basis of ab initio density-functional theory.

Defects such as oxygen vacancies play a crucial role in the surface properties of transition metal oxides. By means of time-resolved, high-resolution scanning tunneling microscopy, we unraveled an adsorbate-mediated diffusion mechanism of oxygen vacancies on rutile TiO2(110). Adsorbed oxygen molecules mediate vacancy diffusion through the loss of an oxygen atom to a vacancy and the sequential capture of an oxygen atom from a neighboring bridging oxygen row, leading to an anisotropic oxygen vacancy diffusion pathway perpendicular to the bridging oxygen rows.

Through an interplay between scanning tunneling microscopy experiments and density functional theory calculations, we determine unambiguously the active surface site responsible for the dissociation of water molecules adsorbed on rutile TiO2(110). Oxygen vacancies in the surface layer are shown to dissociate H2O through the transfer of one proton to a nearby oxygen atom, forming two hydroxyl groups for every vacancy. The amount of water dissociation is limited by the density of oxygen vacancies present on the clean surface exclusively. The dissociation process sets in as soon as molecular water is able to diffuse to the active site.

The reaction of O2 with an adlayer of the oligopyridine 2-phenyl-4,6-bis(6-(pyridine-2-yl)-4-(pyridine-4-yl)-pyridine-2-yl)pyrimidine (2,4′-BTP), adsorbed on the (111) surfaces of silver and gold and on HOPG – which can be considered as a model system for inorganic|organic contacts – was investigated by fast scanning tunneling microscopy (video STM) and dispersion corrected density functional theory (DFT-D) calculations. Only on Ag(111), oxidation of the 2,4′-BTP adlayer was observed, which is related to the fact that under the experimental conditions O2 adsorbs dissociatively on this surface leading to reactive O adatoms, but not on Au(111) or HOPG . There is a distinct regiospecifity of the oxidation reaction caused by intermolecular interactions. In addition, the oxidation leads to a chiral ordering. The relevance of these findings for reactions involving organic monolayers is discussed.

The electronic connection of single molecules to nanoelectrodes on a surface is a basic, unsolved problem in the emerging field of molecular nanoelectronics. By means of variable temperature scanning tunneling microscopy, we show that an organic molecule (C90H98), known as the Lander, can cause the rearrangement of atoms on a Cu(110) surface. These molecules act as templates accommodating metal atoms at the step edges of the copper substrate, forming metallic nanostructures (0.75 nanometers wide and 1.85 nanometers long) that are adapted to the dimensions of the molecule.

We present a combined experimental and theoretical study of the adsorption of water on the rutile TiO2(011)-2×1 surface, whose “brookite (001)-like” reconstruction has been recently elucidated. By using scanning tunneling microscopy and density functional theory calculations, we provide evidence that water adsorbs weakly on the stoichiometric surface, while hydroxyls resulting from water dissociation at surface O vacancies act as nucleation centers for the growth of H-bonded water clusters that are confined in one dimension.

The graphene moiré structures on 4d and 5d metals, as they demonstrate both long (moiré) and short (atomic) scale ordered structures, are the ideal systems for the application of scanning probe methods. Taking graphene–Ir(111) as an example, we present the complex studies of this graphene–metal moiré–structure system by means of 3D scanning tunnelling and atomic force microscopy/spectroscopy as well as Kelvin-probe force microscopy. The results clearly demonstrate variation of the moiré and atomic scale contrast as a function of the bias voltage as well as the distance between the scanning probe and the sample, allowing one to discriminate between topographic and electronic contributions in the imaging of a graphene layer on metals. The presented results are compared with the state-of-the-art density functional theory calculations demonstrating excellent agreement between theoretical and experimental data.

We report on the conformation and self-assembly properties of meso-tetramesitylporphyrin on Cu(100). The results show that the presence of the mesityl groups limits the interaction between the porphyrin ring and the copper surface, contributing to the high porphyrin mobility at room temperature. At low temperatures it is the substrate which determines the molecule orientation. The intermolecular interaction is also very weak, and only for high coverages do the porphyrins self-assemble to form large islands with two different mirror symmetric unit cells. The porphyrins can be Fe metalated by sublimation of Fe at room temperature on a porphyrin overlayer deposited on the copper surface.

The interaction of water with metal oxide surfaces is of fundamental importance to various fields of science, ranging from geophysics to catalysis and biochemistry. In particular, the discovery that TiO2 photocatalyses the dissociation of water has triggered broad interest and intensive studies of water adsorption on TiO2 over decades. So far, these studies have mostly focused on the (110) surface of the most stable polymorph of TiO2, rutile, whereas it is the metastable anatase form that is generally considered photocatalytically more efficient. The present combined experimental (scanning tunnelling microscopy) and theoretical (density functional theory and first-principles molecular dynamics) study gives atomic-scale insights into the adsorption of water on anatase (101), the most frequently exposed surface of this TiO2 polymorph. Water adsorbs as an intact monomer with a computed binding energy of 730 meV. The charge rearrangement at the molecule–anatase interface affects the adsorption of further water molecules, resulting in short-range repulsive and attractive interactions along the [010] and directions, respectively, and a locally ordered (2×2) superstructure of molecular water.

The relationship between the structural/morphological and electronic properties of size-selected gold nanoparticles was investigated using scanning tunneling microscopy and spectroscopy. The nanoparticles were synthesized by inverse micelle encapsulation and were dip-coated on TiO2∕Ti(15nm)∕Si(111). Annealing in vacuum to 500 °C resulted in the removal of the polymer and the formation of an ultrathin TiC support. Significant changes in the electronic local density of states (LDOS) of the nanoparticles, in particular, the onset of nonmetallic behavior, were observed with decreasing particle size. The nanoparticle-support interactions were studied and evidence for substrate-induced modifications in the LDOS of interfacial gold atoms is found.

Large area chiral supramolecular self-assembly of rubrene has been achieved on Au(111) surface at room temperature. The basic building block of such self-assembled layers consists of two double-layered Y-shape supramolecular structures containing eight twisted rubrene molecules. Chirality is maintained and transferred from the bottom layer to the top layer in the successive molecular layers for up to five layers. Such chiral multilayers can be potential candidates in enantioselective catalysis and chiral separations.

Functionalization of thin-film heterostructures on the basis of their electrical, optical and magnetic properties, requires precise control of the film stresses that develop during the growth process. By using real-time in situ stress measurements, the present study reveals strikingly that the in-plane film stress oscillates with increasing film thickness at the initial stage of epitaxial Al(111) film growth on a Si(111)−√3×√3−Al surface, with a periodicity of 2 times the Fermi wavelength of bulk Al and a stress variation from maximum to minimum as large as 100 MPa. Such macroscopic stress oscillations are shown to be caused by quantum confinement of the free electrons in the ultrathin epitaxial metal film. The amplitude, period, and phase of the observed stress oscillations are consistent with predictions based on the free electron model and continuum elasticity.

Scanning tunneling microscopy (STM) and density-functional theory are used to reexamine the structure of the renowned p(4×4)−O/Ag(111) surface oxide. The accepted structural model [C. I. Carlisle et al., Phys. Rev. Lett. 84, 3899 (2000)] is incompatible with the enhanced resolution of the current STM measurements. An “Ag6 model” is proposed that is more stable than its predecessor and accounts for the coexistence of the p(4×4) and a novel c(3×5√3)rect phase. This coexistence is an indication of the dynamic complexity of the system that until now has not been appreciated.

A mechanism for switching the chirality of molecules adsorbed on gold surfaces offers a thermally activated approach to growing two-dimensional crystals of a single enantiomer.

The role of the configuration of metal surface atoms in the interaction between individual large, planar organic molecules and a metal substrate was investigated by low-temperature scanning tunneling microscopy and density functional theory calculations, including a semi-empirical correction scheme to account for dispersion effects. As test case, we used the adsorption of the oligopyridine derivative 2-phenyl-4,6-bis(6-(pyridine-2-yl)-4-(pyridine-4-yl)pyridine-2-yl)pyrimidine (2,4′-BTP) on a stepped Ag(100) surface. Both experiment, via statistical evaluation of the adsorption site and orientation of 2,4′-BTP admolecules, and theory indicate distinct structural effects. The results are compared with the adsorption behavior of pyridine derivatives and benzene on metal surfaces. Consequences on the understanding of the interaction between heteroatoms or functional groups in large organic adsorbates and metal atoms in typical nano-scaled surface defects and hence of the interaction with more realistic metal surfaces are discussed.

We report on a method to fabricate a porous two dimensional (2D) array of porphyrins on c(2×2)N∕Cu(001) with pore sizes larger than 5nm, larger than the reported sizes for hydrogen-bonded or coordination porous organic networks. When deposited on the square nanopattern created by partial nitridation of the Cu(001) surface, the porphyrin molecules prefer to adsorb on clean copper instead of adsorbing on the CuN islands, forming a porous 2D array. This nanopatterning technique can be straightforwardly extended to other molecular species to form the pore walls since its working principle only depends marginally on the nature of the intermolecular interactions.

The preparation of Si(1 0 0) surfaces in chemical vapor environments suitable for subsequent epitaxial III–V integration by chemical vapor deposition (CVD) involving metal-organic precursors was investigated by surface sensitive instruments accessible through a dedicated sample transfer to ultra high vacuum (UHV). Using X-ray photoelectron spectroscopy for inspection of the chemical surface composition, we verified the ability to obtain clean Si(1 0 0) free of oxygen or other contaminants. Annealing for 30 min in a pure hydrogen atmosphere of 950 mbar pressure was found to be sufficient if the surface temperature reached at least 950 °C. We characterized the crucial annealing step comprehensively regarding reliability, dependency on essential process parameters (such as annealing time and surface temperature). Our results verified significant differences to established Si(1 0 0) UHV preparation routines and therefore indicated a major influence of the process gas in the SiO2 removal process, hence we also considered a chemically active role of the hydrogen ambient in the deoxidation reaction. A complementary assessment of the general structure and the atomic configuration of our CVD-prepared Si(1 0 0) surfaces included low energy electron diffraction and scanning tunnelling microscopy and confirmed atomically flat surfaces with two-domain (2×1)/(1×2) reconstruction typical for completely deoxidized Si(1 0 0) as well as for a monohydride termination.

High-quality Sn-doped In2O3 (ITO) films were grown epitaxially on yttria stabilized zirconia (111) with oxygen-plasma assisted molecular beam epitaxy (MBE). The 12 nm thick films, containing 2–6% Sn, are fully oxidized. Angle-resolved x-ray photoelectron spectroscopy (ARXPS) confirms that the Sn dopant substitutes In atoms in the bixbyite lattice. From XPS peak shape analysis and spectroscopic ellipsometry measurements it is estimated that, in a film with 6 at.% Sn, ~1/3 of the Sn atoms are electrically active. Reflection high energy electron diffraction (RHEED) shows a flat surface morphology and scanning tunneling microscopy (STM) shows terraces several hundred nanometers in width. The terraces consist of 10 nm wide orientational domains, which are attributed to the initial nucleation of the film. Low energy electron diffraction (LEED) and STM results show a bulk-terminated (1 × 1) surface, which is supported by first-principles density functional theory (DFT) calculations. Atomically resolved STM images are consistent with Tersoff–Hamann calculations that show that surface In atoms are imaged bright or dark, depending on the configuration of their O neighbors. The coordination of surface atoms on the In2O3(111)–1×1 surface is analyzed in terms of their possible role in surface chemical reactions.

The one-dimensional diffusion of Pt adatoms in the missing row troughs of the (1×2) reconstructed Pt(110) surface is monitored directly from atomically resolved time-lapsed scanning tunneling microscopy images. For this self-diffusion system, it is surprisingly found that not only jumps between nearest neighbor sites but also long jumps, i.e., jumps between next nearest neighbor sites, participate. The hopping rate for these long jumps is found to follow an Arrhenius dependence with an activation barrier for diffusion (Ed2=0.89eV) slightly larger than that for single jumps (Ed1=0.81eV).

We present a statistic evaluation of the azimuth orientations of flat‐adsorbed oligopyridine molecules in disordered adlayers on Au(111) and (111) oriented Ag‐adlayers on Ru(0001). On both surfaces, we find a strong preference for a set of twelve angles, which belong to one specific, unsymmetrical alignment and its symmetry equivalents. These angles are also those that exclusively occur in more densely packed, ordered structures on the same surfaces. We describe a geometric fitting algorithm, which correctly predicts these angles, and which only requires the substrate lattice and the positions of the nitrogen atoms within the flat‐adsorbed molecule as input parameters. Such predictions are particularly valuable to reduce the parameter space in structure simulations [C. Rohr, M. Balbás Gambra, K. Gruber, E. C. Constable, E. Frey, T. Franosch, B. A. Hermann, Nano Lett. 2009, 10, 833].

We present a combined experimental and theoretical study of the surface structure of single crystal Bi(100) via scanning tunneling microscopy (STM), low-energy electron diffraction intensity versus energy (LEED-IV) analysis and density functional theory (DFT). We find that the surface is unreconstructed and shows an unusually large oscillatory multilayer relaxation down to the sixth layer. This unexpected behavior will be explained by a novel mechanism related to the deeply penetrating electronic surface states. STM reveals wide (100) terraces, which are separated by two-layer high steps in which the shorter of the two interlayer spacings is terminating this surface, consistent with the LEED structural analysis and DFT.

Investigating the dynamics in an adlayer of the oligopyridine derivative 2‐phenyl‐4,6‐bis(6‐(pyridine‐2‐yl)‐4‐(pyridine‐4‐yl)pyridine‐2‐yl)pyrimidine (2,4′‐BTP) on Ag(111) by fast scanning tunneling microscopy (video‐STM), we found that rotating 2,4′‐BTP adsorbates coexist in a two‐dimensional (2D) liquid phase (β‐phase) in a dynamic equilibrium with static adsorbate molecules. Furthermore, exchange between an ordered phase (α‐phase) and β‐phase leads to fluctuations of the domain boundary on a time scale of seconds. Quantitative evaluation of the temperature‐dependent equilibrium between rotating and static adsorbates, evaluated from a large number of STM images, gains insight into energetic and entropic stabilization and underlines that the rotating adsorbate molecules are stabilized by an entropy contribution, which is compatible with that derived by using statistical mechanics. The general validity of the concept of entropic stabilization of rotating admolecules, favoring rotation already at room temperature, is tested for other typical small, mid‐size and large adsorbates.

Scanning tunnelling microscopy (STM) has proved to be a fascinating and powerful technique in the field of surface science. The fact that sets the STM apart from most other surface sensitive techniques is its ability to resolve the structure of surfaces on an atomic scale, that is atom-by-atom, and furthermore its ability to study the dynamics of surface processes. This article presents a survey of recent STM studies of well characterized single crystal metal surfaces under ultra-high vacuum conditions. It particularly addresses STM investigations of clean metal surfaces, adsorbates on metal surfaces, adsorbate-induced restructuring of metal surfaces, chemical reactions on metal surfaces, metal-on-metal growth and finally studies of electron confinement and quantum size effects on metal surfaces.

Organic molecules adsorbed on solid surfaces display a fascinating variety of new physical and chemical phenomena ranging from self-assembly and molecular recognition to nonlinear optical properties and current rectification. Both the fundamental interest in these systems and the promise of technological applications have motivated a strong research effort in understanding and controlling these properties. Scanning tunneling microscopy (STM) and, in particular, its ability to manipulate individual adsorbed molecules, has become a powerful tool for studying the adsorption geometry and the conformation and dynamics of single molecules and molecular aggregates. Here we review selected case studies demonstrating the enormous capabilities of STM manipulations to explore basic physiochemical properties of adsorbed molecules. In particular, we emphasize the role of STM manipulations in studying the coupling between the multiple degrees of freedom of adsorbed molecules, the phenomenon of molecular molding, and the possibility of creating and breaking individual chemical bonds in a controlled manner, i.e., the concept of single-molecule chemistry.

Graphene‐metal interfaces have been the subject of surface science since the beginning of 1960s when the studies of the catalytic properties of the metallic surfaces with low‐energy electron diffraction methods were started. After discovery of the unique transport properties of graphene defined by its electronic structure, i.e. the linear dispersion of the electronic states E(k) in the vicinity of the Fermi level, many applications of graphene were proposed and recently realised. These findings renew the interest in graphene on metals as it was realised that synthesis of graphene on metals with its further transfer on insulating or polymer support is the most perspective way to move this technology from lab to industry. Recent applications of scanning probe microscopy and spectroscopy methods to graphene‐ metal systems, from complete layers to nanostructures, shed light on the mechanism of interaction at the interface between graphene and metal that defines the electronic properties of the system and its transport properties (see the Feature Article by Dedkov et al. on pp. 451–468). Besides, fascinating quantum phenomena inherent in graphene nano‐objects open a door for the application of graphene in future nanotechnology.

Adsorption structures formed upon vapor deposition of the natural amino acid l-cysteine onto the (111) surface of gold have been investigated by scanning tunneling microscopy under ultrahigh vacuum conditions. Following deposition at room temperature and at cysteine coverages well below saturation of the first monolayer, we found coexistence of unordered molecular islands and extended domains of a highly ordered molecular overlayer of quadratic symmetry. As the coverage was increased, a number of other structures with local hexagonal order emerged and became dominant. Neither of the room temperature, as-deposited, ordered structures showed any fixed rotational relationship to the underlying gold substrate, suggesting a comparatively weak and nonspecific molecule−substrate interaction. Annealing of the cysteine-covered substrate to 380 K lead to marked changes in the observed adsorption structures. At low coverages, the unordered islands developed internal order and their presence started to perturb the appearance of the surrounding Au(111) herringbone reconstruction. At coverages beyond saturation of the first monolayer, annealing led to development of a (√3 × √3)R30° superstructure accompanied by the formation of characteristic monatomically deep etch pits, i.e., the behavior typically observed for alkanethiol self-assembled monolayers on Au(111). The data thus show that as-deposited and thermally annealed cysteine adsorption structures are quite different and suggest that thermal activation is required before vacuum deposited cysteine becomes covalently bound to single crystalline Au(111).

Structure formation in an ionic liquid adlayer: First molecularly resolved scanning tunneling microscopy images of an ionic liquid adlayer ([Py1,4]+ [FAP]− (see image)), evaporated on a Au(111) surface, resolve a molecular pattern at 210 K with a distinct short range order, indicating a 2D solid, while at room temperature, the mobility of the adlayer is too high to resolve molecular features, as expected for a 2D liquid.

We employed temperature-controlled fast-scanning tunneling microscopy to monitor the diffusion of tetrapyridylporphyrin molecules on the Cu(111) surface. The data reveal unidirectional thermal migration of conformationally adapted monomers in the 300−360 K temperature range. Surprisingly equally oriented molecules spontaneously form dimers that feature a drastically increased one-dimensional diffusivity. The analysis of the bonding and mobility characteristics indicates that this boost is driven by a collective transport mechanism of a metallosupramolecular complex.

Self-assembly of adsorbed organic molecules is a promising route towards functional surface nano-architectures, and our understanding of associated dynamic processes has been significantly advanced by several scanning tunnelling microscopy (STM) investigations. Intramolecular degrees of freedom are widely accepted to influence ordering of complex adsorbates, but although molecular conformation has been identified8 and even manipulated by STM, the detailed dynamics of spontaneous conformational change in adsorbed molecules has hitherto not been addressed. Molecular surface structures often show important stereochemical effects as, aside from truly chiral molecules, a large class of so-called prochiral molecules become chiral once confined on a surface with an associated loss of symmetry. Here, we investigate a model system in which adsorbed molecules surprisingly switch between enantiomeric forms as they undergo thermally induced conformational changes. The associated kinetic parameters are quantified from time-resolved STM data whereas mechanistic insight is obtained from theoretical modelling. The chiral switching is demonstrated to enable an efficient channel towards formation of extended homochiral surface domains. Our results imply that appropriate prochiral molecules may be induced (for example, by seeding) to assume only one enantiomeric form in surface assemblies, which is of relevance for chiral amplification and asymmetric heterogenous catalysis.

Stereochemistry plays a central role in controlling molecular recognition and interaction: the chemical and biological properties of molecules depend not only on the nature of their constituent atoms but also on how these atoms are positioned in space. Chiral specificity is consequently fundamental in chemical biology and pharmacology and has accordingly been widely studied. Advances in scanning probe microscopies now make it possible to probe chiral phenomena at surfaces at the molecular level. These methods have been used to determine the chirality of adsorbed molecules, and to provide direct evidence for chiral discrimination in molecular interactions and the spontaneous resolution of adsorbates into extended enantiomerically pure overlayers. Here we report scanning tunnelling microscopy studies of cysteine adsorbed to a (110) gold surface, which show that molecular pairs formed from a racemic mixture of this naturally occurring amino acid are exclusively homochiral, and that their binding to the gold surface is associated with local surface restructuring. Density-functional theory calculations indicate that the chiral specificity of the dimer formation process is driven by the optimization of three bonds on each cysteine molecule. These findings thus provide a clear molecular-level illustration of the well known three-point contact model for chiral recognition in a simple bimolecular system.

Nanostructures often have unusual properties that are linked to their small size. We report here on extraordinary chemical properties associated with the edges of two-dimensional MoS2 nanoclusters, which we show to be able to hydrogenate and break up thiophene (C4H4S) molecules. By combining atomically resolved scanning tunnelling microscopy images of single-layer MoS2 nanoclusters and density functional theory calculations of the reaction energetics, we show that the chemistry of the MoS2 nanoclusters can be associated with one-dimensional metallic states located at the perimeter of the otherwise insulating nanoclusters. The new chemistry identified in this work has significant implications for an important catalytic reaction, since MoS2 nanoclusters constitute the basis of hydrotreating catalysts used to clean up sulfur-containing molecules from oil products in the hydrodesulfurization process.

Nanostructured Nb-doped SrTiO3(001) surfaces were investigated with STM, and the surface patterns for observed nanostructures were assigned. Sequential C60 deposition onto these nanostructured templates reveals distinct growth modes, including discrete small C60 islands on the c(4 × 2) reconstruction surface, parallel one-dimensional C60 chains on (6 × 2) dilines, C60 double chains on (8 × 2) trilines, epitaxial C60 close packed adlayers over (11 × 2) tetralines, and two-dimensional ordered C60 dimer arrays on (7 × 6) waffles. These structural diversities mainly stem from the relatively strong adsorbate−substrate interactions as well as the surface topography demands. The nanostructured oxide surfaces as templates thus have great potential in molecular nanoarchitecture.

The structural response of the Cu(110) surface to H2 gas pressures ranging from 10−13 to 1 bar is studied using a novel high-pressure scanning tunneling microscope (HP-STM). We find that at H2 pressures larger than 2 mbar the Cu(110) surface reconstructs into the ( 1×2) “missing-row” structure. From a quantitative analysis of the pressure dependence of the surface reconstruction, we conclude that Cu(110) responds identically to hydrogen at ultrahigh vacuum conditions and at atmospheric pressures. From the HP-STM data, we extract refined values for the adsorption and desorption rate constants

Azobenzene and its derivatives can undergo reversible trans−cis isomerizations when irradiated with light, making them potential candidates for optically sensitive materials and devices. The adsorption and diffusion of azobenzene on the Cu(110) surface was investigated with a variable-temperature scanning tunneling microscope. The trans-isomer was observed and found to occupy two adsorption geometriesan energetically stable and a metastable state. Diffusion occurred along the closed-packed [1 −1 0] direction of the surface, and the diffusivity for the two adsorption states was found to differ by approximately 1 order of magnitude.

The intersection between dislocations and a Ag(111) surface has been studied using an interplay of scanning tunneling microscopy (STM) and molecular dynamics. Whereas the STM provides atomically resolved information about the surface structure and Burgers vectors of the dislocations, the simulations can be used to determine dislocation structure and orientation in the near-surface region. In a similar way, the subsurface structure of other extended defects can be studied. The simulations show dislocations to reorient the partials in the surface region leading to an increased splitting width at the surface, in agreement with the STM observations. Implications for surface-induced cross slip are discussed.

By means of scanning tunneling microscopy (STM), it has been possible to obtain the first atomic-scale images of the Co–Mo–S structure present in hydrodesulfurization (HDS) catalysts. Information on the catalytically important edge structures has been obtained by synthesizing single-layer Co–Mo–S nanoclusters using the Au(111) herringbone structure as a template. It is observed that the presence of the Co promoter atoms causes the shape of the MoS2 nanoclusters to change from triangular to hexagonally truncated. This change in morphology appears to be driven by a preference for Co to be located at the S-edge of MoS2. The results also directly show that the presence of the Co atoms perturbs the local electronic environment of neighboring S atoms and this provides further insight into the effect of the promoter atoms.

The interaction of largish molecules with metal surfaces has been studied by combining the imaging and manipulation capabilities of the scanning tunneling microscope (STM). At the atomic scale, the STM results directly reveal that the adsorption of a largish organic molecule can induce a restructuring of a metal surface underneath. This restructuring anchors the molecules on the substrate and is the driving force for a self-assembly process of the molecules into characteristic molecular double rows.

The microstructural development of thin (thickness < 10 nm) oxide layers grown on Zr surfaces by thermal oxidation was investigated by in-vacuo STM and XPS. To this end, single-crystalline Zr(0001) and Zr(100) surfaces were prepared under UHV conditions by a cyclic treatment of ion-sputtering and in-vacuo annealing steps and then exposed to dry O2(g) in the temperature range of 300–450 K (at pO2 = 1×10− 4 Pa). Oxidation proceeds by the fast formation of a dense arrangement of tiny oxide nuclei, which cover the entire Zr surface. The initial oxide cluster size is about 1.2 ± 0.1 nm. The transport processes on the oxidizing surface become promoted with increasing temperature and thereby the oxide clusters rearrange into bigger agglomerates with increasing oxidation time. At the same time, a long-range atomic order develops in the oxide overgrowths, as evidenced from the emergence of a bonding/non-bonding fine structure in the resolved oxide-film upper valence band, as measured in-situ by XPS.

We present the design and performance of a high-pressure scanning tunneling microscope (HP–STM), which allows atom-resolved imaging of metal surfaces at pressures ranging from ultrahigh vacuum (UHV) to atmospheric pressures (1×10−10–1000 mbar) on a routine basis. The HP–STM is integrated in a gold-plated high-pressure cell with a volume of only ∼0.5 l, which is attached directly to an UHV preparation/analysis chamber. The latter facilitates quick sample transfer between the UHV chamber and the high-pressure cell, and allows for in situ chemical and structural analysis by a number of analytical UHV techniques incorporated in the UHV chamber. Reactant gases are admitted to the high-pressure cell via a dedicated gas handling system, which includes several stages of gas purification. The use of ultrapure gases is essential when working at high pressures in order to achieve well-defined experimental conditions. The latter is demonstrated in the case of H/Cu(110) at atmospheric H2 pressures where impurity-related structures were observed.

Site‐specific chiral recognition in a two‐dimensional heterochiral structure leading to highly enantiospecific substitution by a chiral guest was observed by scanning tunneling microscopy (STM). Thus, in the case of succinic acid (SU) on Cu(110), which forms two chiral motifs denoted D‐ and L‐SU, (R,R)‐tartaric acid ((R,R)‐TA=R) only substitutes L sites (see picture; in the STM image R appears as “slots” aligned −20° to the [001] direction).

Two anthracene carboxylic acid (AnCA) self assembly structures on Ag(111) were employed to investigate the template effects on sequentially deposited C60 molecules using scanning tunneling microscope. The initial AnCA structures execute strong modulations on C60 growth. Either laterally separated AnCA–C60 elongated domain arrays or an “epitaxial” C60 dimer structure over AnCA can be formed depending on the selected AnCA template. These distinct C60 growth modes are closely related to the structural stability of the AnCA templates. Our studies suggest a pathway of molecular nanostructure fabrication through the choice of suitable template.

Understanding the interaction of O2 with ketones on metal oxide surfaces is important for the photo-oxidation of toxic organic molecules. The consecutive reaction steps of acetone molecules with oxygen adatoms (Oa's) on partially oxidized TiO2(110) surfaces have been studied using high-resolution scanning tunneling microscopy (STM) at 300 K. The sequential isothermal STM images reveal two types of acetone–Oa species as a result of reactions of acetone with an oxygen adatom and a bridging bound oxygen vacancy (VO). One such species is the Ti5c-bound acetone–Oa diolate formed from Ti5c-bound acetone reacting with Oa. The diolate is mobile at 300 K and can assist the diffusion of surface Oa by exchanging the acetone oxygen with the Oa. The second acetone–Oa species is the VO-bound acetone–Oa complex formed from a VO-bound acetone reacting with an Oa located on the neighboring Ti row. The VO-bound complex is stationary at 300 K. This species has not been reported previously.

Direct studies of how organic molecules diffuse on metal oxide surfaces can provide insights into catalysis and molecular assembly processes. We studied individual catechol molecules, C6H4(OH)2, on a rutile TiO2(110) surface with scanning tunneling microscopy. Surface hydroxyls enhanced the diffusivity of adsorbed catecholates. The capture and release of a proton caused individual molecules to switch between mobile and immobile states within a measurement period of minutes. Density functional theory calculations showed that the transfer of hydrogen from surface hydroxyls to the molecule and its interaction with surface hydroxyls substantially lowered the activation barrier for rotational motion across the surface. Hydrogen bonding can play an essential role in the initial stages of the dynamics of molecular assembly.

Defining pathways to assemble long-range-ordered 2D nanostructures of specifically designed organic molecules is required in order to optimize the performance of organic thin-film electronic devices. We report on the rapid fabrication of a nearly perfect self-assembled monolayer (SAM) composed of a single-domain 6,13-dichloropentacene (DCP) brick-wall pattern on Au(788). Scanning tunneling microscopy (STM) results show the well-ordered DCP SAM extends over hundreds of nanometers. Combining STM results with insights from density functional theory, we propose that a combination of unique intermolecular and molecule-step interactions drives the DCP SAM formation.

The growth behaviour of the oligopyridine derivative 2-phenyl-4,6-bis(6-(pyridine-2-yl)-4-(pyridine-4-yl)pyridine-2-yl)pyrimidine (2,4′-BTP) on Ag(100) in the sub-monolayer regime was investigated by variable temperature scanning tunneling microscopy under ultra-high vacuum conditions. Over the entire coverage range, the molecules are adsorbed in a flat lying configuration, with preferential orientations with respect to the 〈110〉 direction of the surface. The azimuth angles are derived using a previously introduced algorithm that fits the positions of the intramolecular N atoms geometrically to the underlying surface lattice (“points-to-lattice fit”) [H.E. Hoster et al., Langmuir 2007, 23, 11570], indicating that the orientation of the admolecules and thus of the adllayer structure with respect to the Ag(100) surface lattice is determined by the 2,4′-BTP−Ag(100) interaction, while intermolecular interactions are decisive for the structure of the adlayer. The results will be compared to other adsorption systems.

The electronic and crystallographic structure of the graphene/Rh(111) moiré lattice is studied via combination of density-functional theory calculations and scanning tunneling and atomic force microscopy (STM and AFM). Whereas the principal contrast between hills and valleys observed in STM does not depend on the sign of applied bias voltage, the contrast in atomically resolved AFM images strongly depends on the frequency shift of the oscillating AFM tip. The obtained results demonstrate the perspectives of application atomic force microscopy/spectroscopy for the probing of the chemical contrast at the surface.

The surface composition of isolated Au0.5Fe0.5 nanoparticles (NPs) synthesized by micelle encapsulation and supported on TiO2(110) has been investigated. The study reveals that phase-segregated structures are present after annealing at 300 °C. A subsequent thermal treatment at 700 °C resulted in the formation of a AuFe alloy. At this temperature, a state characteristic of Fe was identified at the NPs’ surface. Annealing at 900 °C resulted in the disappearance of the Fe surface state, which is attributed to Au segregation to the surface. The initial hexagonal NP arrangement on the TiO2(110) surface was preserved up to 900 °C. At 1000 °C, Au desorption was observed.

Scanning tunneling microscopy (STM) images taken on a freshly cleaved anatase TiO2(101) sample show an almost perfect surface with very few subsurface impurities and adsorbates. Surface oxygen vacancies are not typically present but can be induced by electron bombardment. In contrast, a reduced anatase (101) crystal shows isolated as well as ordered intrinsic subsurface defects in STM, consistent with density functional theory (DFT) calculations which predict that O vacancies (VO’s) at subsurface and bulk sites are significantly more stable than on the surface.

Epitaxial bcc-Fe(001) ultrathin films have been grown at ∼50 °C on reconstructed GaAs(001)−(4×6) surfaces and investigated in situ in ultrahigh vacuum (UHV) by reflection high-energy electron diffraction, scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy (XPS), and 57Fe conversion electron Mössbauer spectroscopy (CEMS). For tFe = 1 ML (monolayer) Fe coverage, isolated Fe nanoclusters are arranged in rows along the [110] direction. With increasing tFe the Fe clusters first connect along the [−110], but not along the [110] direction at 2.5 ML, then consist of percolated Fe clusters without a preferential orientation at 3 ML, and finally form a nearly smooth film at 4 ML coverage. Segregation of Ga atoms within the film and on the Fe surface appears to occur at tFe = 4 ML, as evidenced by XPS. For coverages below the magnetic percolation, temperature-dependent in situ CEMS measurements in zero external field provided superparamagnetic blocking temperatures TB of 62 ± 5, 80 ± 10, and 165 ± 5 K for tFe = 1.9, 2.2, and 2.5 ML, respectively. At T

Surface diffusion of atoms is an important phenomenon in areas of materials processing such as thin-film growth and sintering. Self-diffusion (that is, diffusion of the atoms of which the surface is comprised) has been much studied on clean metal and semiconductor surfaces. But in most cases of practical interest the diffusion happens on surfaces partly covered by atoms and molecules adsorbed from the gas phase. Adsorbed hydrogen atoms are known to be capable of both promoting and inhibiting self-diffusion, offering the prospect of using adsorbed gases to control growth or sintering processes. Here we derive mechanistic insights into this effect from observations, using the scanning tunnelling microscope, of hydrogen-promoted self-diffusion of platinum on the Pt(110) surface. We see an activated Pt–H complex which has a diffusivity enhanced by a factor of 500 at room temperature, relative to the other Pt adatoms. Our density-functional calculations indicate that the Pt–H complex consists of a hydrogen atom trapped on top of a platinum atom, and that the bound hydrogen atom decreases the diffusion barrier.

This letter reports (i) the enhanced thermal stability (up to 1060 °C) against coarsening and/or desorption of self-assembled Pt nanoparticles synthesized by inverse micelle encapsulation and deposited on TiO2(110) and (ii) the possibility of taking advantage of the strong nanoparticle/support interactions present in this system to create patterned surfaces at the nanoscale. Following our approach, TiO2 nanostripes with tunable width, orientation, and uniform arrangement over large surface areas were produced.

Enantiospecific adsorption of cysteine molecules onto chiral kink sites on the Au(110)-(1×2) surface was observed by scanning tunneling microscopy. l- and d-cysteine dimers were found to adopt distinctly different adsorption geometries at S kinks, which can be understood from the need to reach specific, optimum molecule−substrate interaction points. Extended, homochiral domains of l/d-cysteine were furthermore observed to grow preferentially from R/S kinks. The results constitute the first direct, microscopic observation of enantiospecific molecular interaction with chiral sites on a metal single-crystal surface.

Realization of graphene moiré superstructures on the surface of 4d and 5d transition metals offers templates with periodically modulated electron density, which is responsible for a number of fascinating effects, including the formation of quantum dots and the site selective adsorption of organic molecules or metal clusters on graphene. Here, applying the combination of scanning probe microscopy/spectroscopy and the density functional theory calculations, we gain a profound insight into the electronic and topographic contributions to the imaging contrast of the epitaxial graphene/Ir(111) system. We show directly that in STM imaging the electronic contribution is prevailing compared to the topographic one. In the force microscopy and spectroscopy experiments we observe a variation of the interaction strength between the tip and high-symmetry places within the graphene moiré supercell, which determine the adsorption sites for molecules or metal clusters on graphene/Ir(111).

Amino acids find their feet: Scanning tunneling microscopy of racemic (R,S)‐proline on Cu(110) reveals rows of random chiral amino acid sequences, showing that organization is not governed by molecular chirality. Instead, the system is dictated by a strict heterochiral adsorption‐footprint template, in which each adsorption position can be occupied by either enantiomer (see picture), resulting in a random solid solution in 2D.