We report on the interface between graphene and 4H-SiC͑0001͒ as investigated by scanning tunneling microscopy ͑STM͒ and low energy electron diffraction ͑LEED͒. It is characterized by the so-called ͑6 ͱ 3 ϫ 6 ͱ 3͒R30°reconstruction, whose structural properties are still controversially discussed but at the same time are crucial for the controlled growth of homogeneous high-quality large-terrace graphene surfaces. We discuss the role of three observed phases with periodicities ͑6 ͱ 3 ϫ 6 ͱ 3͒R30°, ͑6 ϫ 6͒, and ͑5 ϫ 5͒. Their LEED intensity levels and spectra strongly depend on the surface preparation procedure applied. The graphitization process imprints distinct features in the STM images as well as in the LEED spectra. The latter have the potential for an easy and practicable determination of the number of graphene layers by means of LEED.
The SiC͑111͒-͑333͒ phase was analyzed by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED) holography, density functional theory (DFT), and conventional LEED. A single adatom per unit cell found in STM acts as a beam splitter for the holographic inversion of discrete LEED spot intensities. The resulting 3D image guides the detailed analyses by LEED and DFT which find a Si tetramer on a twisted Si adlayer with cloverlike rings. This twist model with one dangling bond left per unit cell represents a novel ͑n3n͒-reconstruction mechanism of group-IV (111) surfaces. [S0031-9007(97)
Using temperature-programmed desorption, supported by X-ray photoelectron spectroscopy and scanning tunneling microscopy, a comprehensive overview of the main reactions of 5,10,15,20-tetraphenyl-21H,23H-porphyrin (2HTPP) on Cu(111) as a function of coverage and temperature is obtained. Three reactions were identified: metalation with Cu substrate atoms, stepwise partial dehydrogenation, and finally complete dehydrogenation. At low coverage the reactions are independent of coverage, but at higher coverage metalation becomes faster and partial dehydrogenation slower. This behavior is explained by a weaker interaction between the iminic nitrogen atoms and the Cu(111) surface in the high-coverage checkerboard structure, leading to faster metalation, and the stabilizing effect of T-type interactions in the CuTPP islands formed at high coverage after metalation, leading to slower dehydrogenation. Based on the amount of hydrogen released and the appearance in STM, a structure of the partially dehydrogenated molecule is suggested.
We show that both single-crystalline and nanostructured MgO surfaces convert free-base tetraphenyl porphyrin (2HTPP) into magnesium tetraphenyl porphyrin (MgTPP) at room temperature. The reaction can be viewed as an ion exchange between the two aminic protons of the 2HTPP molecule with a Mg(2+) ion from the surface. The driving force for the reaction is the strong stability of the formed hydroxyl groups along the steps and at defects on the MgO surface. We have used an integrated characterization approach that includes UV/Vis diffuse reflectance measurements on nanostructured powders, X-ray photoelectron spectroscopic investigation of atomically clean MgO(100) single-crystalline thin films, and density functional theory (DFT) calculations on model systems. The DFT calculations demonstrate that MgTPP formation is strongly exothermic at the corners, edges and steps, but slightly endothermic on terrace sites. This agrees well with the UV/Vis diffuse reflectance, which upon adsorption of 2HTPP shows a decrease in the absorption band associated with corner and edge sites on MgO nanocube powders.
Promoted by Si enrichment during the formation of the reconstructed ͑ p 3 3 p 3 ͒R30 ± phase on hexagonal SiC(0001) a cubic stacking sequence develops at the surface. The reconstruction is ultimately resolved to consist of Si adatoms in T 4 sites as found by quantitative LEED crystallography. Prior to the ͑ p 3 3 p 3 ͒R30 ± phase evolution mesalike structures with various atomic periodicities are observed by STM. Smoothening of this rough and Si enriched state provides the material for the formation of the modified stacking sequence which could serve as seed for preparation of SiC polytype heterostructures. [S0031-9007(99)08644-5]
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