Plasma nitridation of Ge(100) surface studied by scanning tunneling microscopy

The geometric and electronic structures of the surface species on Ge(100) after plasma nitridation were investigated in this study. An electron cyclotron resonance (ECR) plasma source was used to directly nitride Ge(100), and scanning tunneling microscopy and spectroscopy (STM/STS) were employed to study the structures of the nitrided surface. Nitridation at room temperature generated a large diversity of adsorbate sites on the surface containing N, O, and displaced Ge atoms, differentiated by annealing between 200 °C and 450 °C. Conversely, nitridation at 500 °C produced Ge–N adsorbate sites which formed ordered and disordered structures on the surface free from oxygen. Density functional theory (DFT) simulations were performed focusing on the ordered nitride structure, and the simulated surface structure showed a good correspondence with the STM data. DFT calculations also found an increase of density of states near the Fermi level on the ordered nitride structure, which is consistent with the Fermi level pinning observed in the STS results. The DFT results predict H-passivation can unpin the Fermi level of the nitrided surface by reducing the dangling bonds and the bond strain, but the residual plasma damage and the low nitridation rate in UHV are challenges to obtain complementary experimental results.     

Atomic-scale surface science phenomena studied by scanning tunneling microscopy

Following the development of the scanning tunneling microscope (STM), the technique has become a very powerful and important tool for the field of surface science, since it provides direct real-space imaging of single atoms, molecules and adsorbate structures on surfaces. From a fundamental perspective, the STM has changed many basic conceptions about surfaces, and paved the way for a markedly better understanding of atomic-scale phenomena on surfaces, in particular in elucidating the importance of local bonding geometries, defects and resolving non-periodic structures and complex co-existing phases. The so-called “surface science approach”, where a complex system is reduced to its basic components and studied under well-controlled conditions, has been used successfully in combination with STM to study various fundamental phenomena relevant to the properties of surfaces in technological applications such as heterogeneous catalysis, tribology, sensors or medical implants. In this tribute edition to Gerhard Ertl, we highlight a few examples from the STM group at the University of Aarhus, where STM studies have revealed the unique role of surface defects for the stability and dispersion of Au nanoclusters on TiO₂, the nature of the catalytically active edge sites on MoS₂ nanoclusters and the catalytic properties of Au/Ni or Ag/Ni surfaces. Finally, we briefly review how reaction between complex organic molecules can be used to device new methods for self-organisation of molecular surface structures joined by comparatively strong covalent bonds.     

The decomposition of ammonia on an oxygen-precovered Ni(110) surface studied by scanning tunneling microscopy

The decomposition of ammonia on a Ni(110) surface with preadsorbed oxygen has been investigated in ultra-high vacuum at room temperature using scanning tunneling microscopy (STM). We propose a reaction model in which the high reactivity observed at low O coverage is ascribed to a direct interaction between the NH₃ molecules and the terminating atoms of the short, mobile -Ni-O- added rows which are observed on the surface under these conditions. This model is consistent with the observation that the surface becomes inert at high O coverage. We believe that the present reaction model can also explain results from some other experiments in which preadsorbed oxygen has been found to act as a promoter for dissociation of H-containing species, such as for NH₃ on Cu(110) and H₂O on Ni(110).     

Identifying the electronic properties of the Ge(111)-(2×1) surface by low-temperature scanning tunneling microscopy

We present the results of our low-temperature scanning tunneling microscopy (STM) investigation of the clean Ge(111) surface. Our experiments enable, for the first time, STM observation of one-dimensional surface screening around surface defects. We identify the dominating role of surface states in the low-temperature STM imaging as well as the important influence of nonequilibrium kinetics on the measured tunneling spectra.     

Thermal and kinetic surface roughening studied by scanning tunneling microscopy and atomic force microscopy

Recent studies of thermal roughening on Si surfaces and kinetic roughening of some growing films, copper and tungsten, by using scanning tunneling microscopy and atomic force microscopy are reviewed. A logarithmic divergence of the surface height fluctuations of Si(111) vicinal surfaces is confirmed, in agreement with the theoretical prediction of rough surface in thermal equilibrium. For the kinetically formed rough surfaces, power law dependences of the interface width on the system size are clearly observed. Furthermore, the tungsten films show a short-range scaling regime and a long-range “smooth” regime. The roughness exponents α are compared with theoretical predictions: for the typical Cu electrode position condition (α=1/2), the exponent appears to be close to that found for local growth models, and for tungsten films (0.7～0.8), it is consistent with recent predictions for growth where surface diffusion is predominant.     

Hydrogen-induced missing-row reconstructions of Pd(110) studied by scanning tunneling microscopy

The effect of hydrogen adsorption on the Pd(110) surface structure at room temperature has been studied by scanning tunneling microscopy. Depending on the partial pressure of hydrogen two different reconstructions of Pd(110) have been observed: a (1 × 3) phase at hydrogen pressures in the 10⁻⁹ mbar range and an additional (1 × 2) phase at p_H2 ≥ 5 × 10⁻⁸ mbar. Both reconstructions are found to be of the missing-row type. The evolution of the surface reconstructions has been followed in situ.     

When scanning tunneling microscopy gets the wrong adsorption site: H on Rh(100)

At low tunneling resistance, scanning tunneling microscopy (STM) images of a Rh(100) surface with adsorbed hydrogen reproducibly show protrusions in all bridge sites of the surface, leading to a naive interpretation of all bridge sites being occupied with H atoms. Using quantitative low-energy electron diffraction and temperature programmed desorption we find a much lower H coverage, with most H atoms in fourfold hollow sites. Density functional theory calculations show that the STM result is due to the influence of the tip, attracting the mobile H atoms into bridge sites. This demonstrates that STM images of highly mobile adsorbates can be strongly misleading and underlines the importance of additional analysis techniques.