I. Stensgaard

Properties of large organic molecules on metal surfaces

Abstract The adsorption of large organic molecules on surfaces has recently been the subject of intensive investigation, both because of the molecules’ intrinsic physical and chemical properties, and for prospective applications in the emerging field of nanotechnology. Certain complex molecules are considered good candidates as basic building blocks for molecular electronics and nanomechanical devices. In general, molecular ordering on a surface is controlled by a delicate balance between intermolecular forces and molecule–substrate interactions. Under certain conditions, these interactions can be controlled to some extent, and sometimes even tuned by the appropriate choice of substrate material and symmetry. Several studies have indicated that, upon molecular adsorption, surfaces do not always behave as static templates, but may rearrange dramatically to accommodate different molecular species. In this context, it has been demonstrated that the scanning tunnelling microscope (STM) is a very powerful tool for exploring the atomic-scale realm of surfaces, and for investigating adsorbate–surface interactions. By means of high-resolution, fast-scanning STM unprecedented new insight was recently achieved into a number of fundamental processes related to the interaction of largish molecules with surfaces such as molecular diffusion, bonding of adsorbates on surfaces, and molecular self-assembly. In addition to the normal imaging mode, the STM tip can also be employed to manipulate single atoms and molecules in a bottom–up fashion, collectively or one at a time. In this way, molecule-induced surface restructuring processes can be revealed directly and nanostructures can be engineered with atomic precision to study surface quantum phenomena of fundamental interest. Here we will present a short review of some recent results, several of which were obtained by our group, in which several features of the complex interaction between large organic molecules and metal surfaces were revealed. The focus is on experiments performed using STM and other complementary surface-sensitive techniques.

Enhancement of surface self-diffusion of platinum atoms by adsorbed hydrogen

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.

A high-pressure scanning tunneling microscope

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 gasses 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-rela...

Interaction of oxygen with Ni(110) studied by scanning tunneling microscopy

Abstract By means of scanning tunneling microscopy (STM), the dynamics of the oxygen (O)-induced reconstruction of the Ni(110) surface has been studied in real time and space by recording atom-resolved images of the clean and O-covered Ni(110) surfaces. It is found that oxygen adsorption at room temperature initiates the nucleation of two different coexisting structures. For low oxygen exposures, it is revealed that O induces a structure consisting of strings along the close-packed[11¯0] direction, growing either out from [001] step edges or in troughs created on large, flat terraces. STM reveals that this structure is indeed a reconstructed one, with O atoms plausibly located at a pseudo-threefold site of the rudimentary (111) face of the Ni rows (strings), rather than a disordered structure with O chemisorbed on the unreconstructed surface, as previously suggested in the literature. Coexisting with this string structure, the well known (3×1) and (2×1) reconstructions, corresponding to local O coverages of 1/3 and 1/2 and ML, respectively, develop locally. These reconstructions are stabilized by -Ni-O- rows running along the [001] direction. The nucleation and growth of these reconstructions prove that they are of the added-row rather than the missing-row type. At higher oxygen exposures, the (2×1) added-row structure is completed at the expense of the string structure, and subsequent oxygen exposure induces another added-row (3×1) structure corresponding to an O coverage of 2/3 ML. At even higher oxygen exposures, atom-resolved STM images and LEED reveal the existence of a (9×5) suboxide structure which has previously been identified as a (9×4) structure. The (9×5) structure is a two-layer structure, with Ni-Ni interdistances similar to those in a surface layer of NiO(100). Finally, an epitaxial NiO(001) oxide is formed. The nucleation and growth of these “oxide” phases are discussed.

An STM study of carbon-induced structures on Ni(111): evidence for a carbidic-phase clock reconstruction

The interaction of carbon with Ni(111) has been studied by scanning tunnelling microscopy. In the carbidic phase, formed by exposing the surface to ethylene at ∼ 500 K, the surface atoms are found to rearrange into an almost square surface mesh with a “clock” reconstruction which is quite similar to that observed for carbon on the Ni(100) surface. The square surface mesh is argued to be incommensurate although a |7227| cell is close to coincidence. The implications of the observed clock reconstruction for a possible structure sensitivity of the methanation reaction on Ni are discussed. Exposure of the Ni(111) surface to ethylene at higher temperatures leads to formation of single or multiple layers of graphite.

Oxidation of Cu(111): two new oxygen induced reconstructions

Abstract The chemisorption of oxygen on Cu(111) has been studied by scanning tunneling microscopy. Two hitherto unknown well ordered oxygen induced reconstructions with extremely large unit cells, 29 and 44 times the 1 × 1 surface unit, are presented. The reconstructions emerge as coincidence lattices between the 1 × 1 surface lattice and a hexagonal substructure, which is associated with the (111) plane of Cu 2 O.

Surface relaxation of Cu(110): An ion scattering investigation

The surface relaxation of Cu(110) has been investigated with High Energy Ion Scattering (HEIS). The interlayer distances in the surface show an oscillatory deviation from the bulk value of 1.278 A. The results indicate a first interlayer distance of 1.21±0.02 A and a second interlayer distance of 1.32±0.02 A, corresponding to a 5.3±1.6% contraction followed by a 3.3±1.6% expansion.

An investigation into the interactions between self-assembled adenine molecules and a Au(111) surface

Two molecular phases of the DNA base adenine (A) on a Au(111) surface are observed by using STM under ultrahigh-vacuum conditions. One of these phases is reported for the first time. A systematic approach that considers all possible gas-phase two-dimensional arrangements of A molecules connected by double hydrogen bonds with each other and subsequent ab initio DFT calculations are used to characterize and identify the two phases. The influence of the gold surface on the structure of A assemblies is also discussed. DFT is found to predict a smooth corrugation potential of the gold surface that will enable A molecules to move freely across the surface at room temperature. This conclusion remains unchanged if van der Waals interaction between A and gold is also approximately taken into account. DFT calculations of the A pairs on the Au(111) surface show its negligible effect on the hydrogen bonding between the molecules. These results justify the gas-phase analysis of possible assemblies on flat metal surfaces. Nevertheless, the fact that it is not the most stable gas-phase monolayer that is actually observed on the gold surface indicates that the surface still plays a subtle role, which needs to be properly addressed.

A fully automated, thimble-size scanning tunnelling microscope

A novel, fully automated high‐stability, high‐eigenfrequency scanning tunnelling microscope (STM) has been developed. Its key design feature is the application of two piezoelectric ceramic tubes, one for the x‐y‐z motion of the tip and one for a linear motor (‘nano‐worm’) used for the coarse positioning of the tip relative to the specimen. By means of the nano‐worm, the tip can be advanced in steps between 16 and 0·2 nm. The walking distance is >2 mm, with a maximum speed of 2000 steps/s. The nano‐worm positioning implies that this STM is fully controlled by electronic means, and that no mechanical coupling is needed, which makes operation of the STM extremely convenient. The axial‐symmetry construction is rigid, small and temperature‐compensated, yielding reduced sensitivity to mechanical and acoustic vibrations and temperature variations. The sample is simply placed on a piece of invar which surrounds the scanner tube and the nano‐worm and is held by gravity alone. This allows for easy sample mounting. The performance of the microscope has been tested in air by imaging a variety of surfaces, including graphite and biological samples.

Oxygen-adsorption induced reconstruction of Cu(110) studied by high energy ion scattering

Abstract The adsorption of oxygen on Cu(110) has been studied by High Energy Ion Scattering (HEIS). Analysis of the (2 × 1)-O structure indicated an absolute oxygen coverage of 0.51 ± 0.06 monolayer. Good agreement with experimental data was obtained for a buckled (110) surface model, in which every second [001] surface row is displaced outwards by 0.27 ± 0.05 A from its bulk-like position, with the remaining [001] surface rows shifted insignificantly inwards by 0.02 ± 0.03 A and the second layer displaced outwards by 0.06 ± 0.03 A . The c(6 × 2)-O structure was shown to involve lateral displacement components ⪆ 0.3 A for the first monolayer atoms.