Growth and Characterization of Crystalline Silica Films on Pd(100)

Abstract

Silica films grown on Pd(100) were characterized by Auger electron spectroscopy, low-energy electron diffraction (LEED), and scanning tunneling microscopy (STM). While no evidence of long-range order could be detected for films grown below 600 K, STM images of these films nevertheless revealed flat surfaces through which the step-terrace structure of the substrate could be seen. Annealing the films in 10 −6 Torr of O 2 above 975 K resulted in crystalline bilayers that produced hexagonal LEED patterns with a periodicity twice that of the substrate and with one of the overlayer close-packed directions paralleling Pd [011]. The extent of the crystalline domains was limited to typically five repeat units along two of the three close-packed directions of the film but was tens of repeat units long along the third. The lattice matching to the substrate expands the spacing in the bilayer on Pd(100) compared to bulk crystalline SiO 2 and bilayers observed on other substrates; as a consequence, it is suggested that the regular domain boundaries that form help relieve stress. The dominant features in high-resolution STM images were dark pores surrounded by six other pores; consistent with prior studies, these features are assigned to six-membered rings of corner-sharing SiO 4 tetrahedra. Elongation of the pores at the domain boundaries is attributed to insertion of edge-sharing tetrahedra into the rings. Ab initio calculations on freestanding bilayers were performed to understand the effect of the substantial strain on the growth and structure of the film. The results indicate that relaxation orthogonal to the commensurate direction can greatly reduce the strain energy; as a consequence, the square substrate promotes epitaxial growth of crystalline SiO 2 by providing an incommensurate direction along which the film can relax.

Structural and Electronic Heterogeneity of Two Dimensional Amorphous Silica Layers

“…Previously we found that three-dimensionally disordered silica fi lms were diffi cult to image without the tip changing the surface, indicating a diffi culty in establishing stable tunneling. [ 11 ] Inspired by this earlier fi nding, it is suggested that the dramatic decrease in tip-sample distance as the imaging bias was raised toward the Fermi level at this location may be associated with the breakdown of the bilayer structure weakening the coupling to the metal surface.…”

“…[ 21 ] More prolonged annealing of multilayer fi lms led to additional silica loss and eventually the appearance of crystalline diffraction patterns. The crystalline phase could also be prepared by depositing ≈2 ML SiO 2 under the above conditions followed by annealing above 970 K. [ 11 ] The position and shape of the Si LVV AES peak indicated that the as grown and annealed fi lms were SiO 2 . The STM images were collected at room temperature with W tips at tunneling currents between 0.1 and 0.55 nA; over this range the tunneling current did not noticeably affect the images.…”

“…In contrast, images of crystalline silica bilayer domains show only six-fold coordinated holes. [ 11,12 ] Finally, as highlighted by the black line in Figure 1 a, linear rows of holes could be detected, but these only run short distances before they bend. The line in Figure 1 b shows more clearly that these are rows of sixfold coordinated holes that bend when they run into or pass a different sized hole.…”

“…[ 14 ] The height-height correlation functions yielded a 0.58 nm average distance between the holes, which is expanded compared to crystalline bilayers on Pd(100) where the periodicity is 0.55 nm. [ 11 ] Structural disorder as observed in these fi lms may be expected to induce variations in the local electronic properties. [ 5,7 ] To characterize this relationship, STM videos were recorded as the magnitude of the bias voltage was decremented.…”

Oxidation of the Ru(0001) surface covered by weakly bound, ultrathin silicate films

“…It is well established that the thinnest silica film forms a hexagonal layer of corner-sharing [SiO 4 ] tetrahedra (referred to as "silicatene" in analogy to graphene [5]) which is strongly bound to a metal support through Si-O-Metal linkages [6]. On noble metals such as Ru(0001), Pd(100) and Pt(111), double-layer (or bilayer) silicate films may be grown, which are weakly bound to the support via dispersive forces [7][8][9][10]. In this case, a relatively large space between the silicate sheet and the metal support allows, in principle, small molecules to intercalate the interface, diffuse and ultimately react on the metal surface.…”