F. A. Flaherty

Investigation of the morphology of the initial growth of the aromatic molecule p-quaterphenyl on NaCl (001)

Investigations of the structural properties of the initial layers of p-quaterphenyl (p-4P) vapor deposited onto NaCl (001) have been carried out using atomic force microscopy (AFM), grazing incidence x-ray diffraction (GIXRD), and helium atom scattering (HAS). A series of AFM images reveal accumulations of p-4P molecules around surface defects. Film thicknesses determined from height analyses of these images are in reasonable agreement with those found using GIXRD. The GIXRD studies indicate that p-4P films vapor-deposited at ambient temperature are composed of crystallites oriented with the long molecular axis nearly perpendicular to the NaCl (001) surface. For a nominally three-monolayer film, the b axis of the crystallites appears preferentially oriented along the substrate’s [110] azimuth, but with increasing thickness the x-ray features resemble those from two-dimensional (2D) powders. The diffraction peaks found in the HAS investigation at ∼50 K for a thick film grown at ∼200 K are similarly consist...

Surface structure of niobium-doped potassium tantalate KTa1 − xNbxO3 obtained from helium atom scattering studies

Helium atom scattering experiments have been carried out on the (001) surface of KTaO(3) doped with 6, 10 and 15% Nb, produced by cleaving in situ single crystal samples. Several phenomena were observed through He atom diffraction measurements, including metastable behavior immediately after cleaving, reconstruction of a fraction of the surface to (2 × 1) domains after thermally cycling the surface temperatures, and the distribution of step heights of terraces at the surface as multiples of the unit cell dimension of ∼ 4 Å. In addition, a large hysteresis effect was found in the (100) azimuth in the variation of surface reflectivity with surface temperature; a much smaller hysteresis effect was found in the (110) azimuth. The hysteresis effects appear for all Nb doping concentrations. Finally, small oscillations in the specular and Bragg diffraction intensities near the surface temperature 80 K were also observed.

Polar catastrophe and the structure of KTa1-xNbxO₃ surfaces: Results from elastic and inelastic helium atom scattering

The structure and dynamics of cleaved (001) surfaces of potassium tantalates doped with niobium, KTa1-xNbxO₃ (KTN), with x ranging from 0% to 30%, were measured by helium atom scattering (HAS). Through HAS time-of-flight (TOF) experiments, a dispersionless branch (Einstein phonon branch) with energy of 13-14 meV was observed across the surface Brillouin zone in all samples. When this observation is combined with the results from earlier experimental and theoretical studies on these materials, a consistent picture of the stable surface structure emerges: After cleaving the single-crystal sample, the surface should be composed of equal areas of KO and TaO₂/NbO₂ terraces. The data, however, suggest that K⁺ and O²⁻ ions migrate from the bulk to the surface, forming a charged KO lattice that is neutralized primarily by additional K⁺ ions bridging pairs of surface oxygens. This structural and dynamic modification at the (001) surface of KTN appears due to its formally charged KO(-1) and TaO₂/NbO₂(+1) layers and avoids a “polar catastrophe.” This behavior is contrasted with the (001) surface behavior of the fluoride perovskite KMnF₃ with its electrically neutral KF and MnF₂ layers.