In a crystal of the group 4 ( Ti, Zr, and Hf ) and group 5 ( V, Nb, and Ta ) transition-metal diboride, graphite-like ( honeycomb ) sheets of boron and close-packed metal layers stack alternately as shown below. The metal-metal direction parallel to the layer is taken as an a-axis, and the perpendicular one is taken as a c-axis. Here, the c-plane, namely the (0001) surfaces of ZrB2 and NbB2 have been studied.
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Making an atomically-clean flat surface is the first step for surface studies. After mechanically polishing to a mirror finish, the sample is cleaned by heating in an ultra-high vacuum (UHV) of 10−8 Pa, which is 13 orders of magnitude lower than the atmospheric pressure, and/or by ion-bombardment of accerelated rare-gas ions. The cleanliness is checked by reflection high-energy electron diffraction (RHEED) and Auger electron spectroscopy (AES).
In this case, we succeeded to obtain the clean ZrB2(0001) surface by repeatedly heating at 1900°C in the UHV, and the clean NbB2(0001) surface by repeated cycles of Xe+ ion bombardment and annealing at 1200°C in the UHV. The clean surface shows a sharp 1×1 RHEED pattern and no impurity is detected in AES. The RHEED picture is the example for the clean ZrB2(0001).
The atomic (lattice) vibration was investigated by means of high-resolution electron energy loss spectroscopy (HREELS). According to the quantum mechanics, the energy transfer of the vibration is quantized in a unit of the frequency times the Planck constant, which is called as "phonon". In the HREELS experiment, an energy-monochromated electron beam impinges on the surface. If the atomic vibration is excited, the electron loses its energy by the energy quantum of the vibrational frequency (phonon) according to the energy conservation law. By measureing the energy of scattered electron, we can know the vibrational frequency. As the interaction between an electron and an atom is very strong, the HREELS is so surface-sensitive that it can detect vibration in several layers at the surface.
The figure is the HREELS spectra for clean ZrB2(0001). By the momentum conservation law, the observed wavelength of the lattice vibration depends on the electron momentum transfer, which is changed with the detection angle. From the series of the spectra, we can determine the relationship between the wavelength ( the wave number ) and the frequency ( the energy), namely the phonon dispersion relation. By comparing the phonon dispersion with the model calculation, the force constants ( bond strength ) between the atoms can be known. [ Ref. T. Aizawa et al. Phys. Rev. B 65, 024303 (2002) ] From these data, the outermost layer of the ZrB2(0001) surface is revealed to be the metal layer, while that of the NbB2(0001) is the boron layer.
Exposing the clean surface to gas molecules results in chemical reactions. In order to understand complex chemical reactions on surfaces as catalysis, corrosion, etc., the basic knowledge how simple molecules react at the surface is important. The figure shows the HREELS spectra after exposing the ZrB2(0001) surface to H2, D2, O2, and CO at the room temperature. Each one shows one large loss peak in case of the H2, D2, and O2adsorption. This fact means that the molecule dissociates at the surface and adsorbs as an atom at the highly symmetric position, presumably at the three-fold hollow site ( the center of the triangle made by the surface metal atoms ). The CO adsorption makes the two loss peaks, one of which has the same frequency as the O adsorption. We can conclude that the CO adsorbs dissociatively and the other loss peak corresponds to the atomic C. [ Ref. T. Aizawa et al. J. Chem. Phys. 117, 11310 (2002) ]