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Register a new userStructure of graphene and a surface carbide grown on the (0001) surface of rhenium
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Transition metal surfaces catalyse a broad range of thermally-activated reactions involving carbon-containing-species -- from atomic carbon to small hydrocarbons or organic molecules, and polymers. These reactions yield well-separated phases, for instance graphene and the metal surface, or, on the contrary, alloyed phases, such as metal carbides. Here, we investigate carbon phases on a rhenium (0001) surface, where the former kind of phase can transform into the latter. We find that this transformation occurs with increasing annealing time, which is hence not suitable to increase the quality of graphene. Our scanning tunneling spectroscopy and reflection high-energy electron diffraction analysis reveal that repeated short annealing cycles are best suited to increase the lateral extension of the structurally coherent graphene domains. Using the same techniques and with the support of density functional theory calculations, we next unveil, in real space, the symmetry of the many variants (two six-fold families) of a rhenium surface carbide observed with diffraction since the 1970s, and finally propose models of the atomic details. One of these models, which nicely matches the microscopy observations, consists of parallel rows of eight aligned carbon trimers with a so-called $(7timessqrt{mathrm{19}})$ unit cell with respect to Re(0001).
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We demonstrate a method for synthesizing large scale single layer graphene by thermal annealing of ruthenium single crystal containing carbon. Low energy electron diffraction indicates the graphene grows to as large as millimeter dimensions with good long-range order, and scanning tunneling microscope shows perfect crystallinity. Analysis of Moire pattern augmented with first-principles calculations shows the graphene layer is incommensurate with the underlying Ru(0001) surface forming a N by N superlattice with an average lattice strain of ~ +0.81%. Our findings offer an effective method for producing high quality single crystalline graphene for fundamental research and large-scale graphene wafer for device fabrication and integration.
MnAs has been grown by means of MBE on the GaN(0001)-(1x1) surface. Two options of initiating the crystal growth were applied: (a) a regular MBE procedure (manganese and arsenic were delivered simultaneously) and (b) subsequent deposition of manganese and arsenic layers. It was shown that spontaneous formation of MnAs dots with the surface density of 1$cdot 10^{11}$ cm$^{-2}$ and $2.5cdot 10^{11}$ cm$^{-2}$, respectively (as observed by AFM), occurred for the layer thickness higher than 5 ML. Electronic structure of the MnAs/GaN systems was studied by resonant photoemission spectroscopy. That led to determination of the Mn 3d - related contribution to the total density of states (DOS) distribution of MnAs. It has been proven that the electronic structures of the MnAs dots grown by the two procedures differ markedly. One corresponds to metallic, ferromagnetic NiAs-type MnAs, the other is similar to that reported for half-metallic zinc-blende MnAs. Both system behave superparamagnetically (as revealed by magnetization measurements), but with both the blocking temperatures and the intra-dot Curie temperatures substantially different. The intra-dot Curie temperature is about 260 K for the former system while markedly higher than room temperature for the latter one. Relations between growth process, electronic structure and other properties of the studied systems are discussed. Possible mechanisms of half-metallic MnAs formation on GaN are considered.
Most spectroscopic methods for studying the electronic structure of metal surfaces have the disadvantage that either only occupied or only unoccupied states can be probed, and the signal is cut at the Fermi edge. This leads to significant uncertainties, when states are very close to the Fermi level. By performing low-temperature scanning tunneling spectroscopy and ab initio calculations, we study the surface-electronic structure of La(0001) and Lu(0001), and demonstrate that in this way detailed information on the surface-electronic structure very close to the Fermi energy can be derived with high accuracy.
We present a structural analysis of the graphene-4HSiC(0001) interface using surface x-ray reflectivity. We find that the interface is composed of an extended reconstruction of two SiC bilayers. The interface directly below the first graphene sheet is an extended layer that is more than twice the thickness of a bulk SiC bilayer (~1.7A compared to 0.63A). The distance from this interface layer to the first graphene sheet is much smaller than the graphite interlayer spacing but larger than the same distance measured for graphene grown on the (000-1) surface, as predicted previously by ab intio calculations.
Interfaces of sapphire are of technological relevance as sapphire is used as a substrate in electronics, lasers, and Josephson junctions for quantum devices. In addition, its surface is potentially useful in catalysis. Using first principles calculations, we show that, unlike bulk sapphire which has inversion symmetry, the (0001) sapphire surface is piezoelectric. The inherent broken symmetry at the surface leads to a surface dipole and a significant response to imposed strain: the magnitude of the surface piezoelectricity is comparable to that of bulk piezoelectrics.
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