(3.0 nm range) NC-AFM image of clean, unreconstructed surface. Line profile shown in lower panel is along the white line in the upper panel. The terraces are 250–300 nm wide and the steps are 0.4 nm high. Vertical height represented by gray scale as indicated in the bar at left (same range as in profile); lighter shading represents high regions of the surface and darker shading corresponds to lower regions. Image has been smoothed using a 2D Gaussian filter with standard deviation 3.8 nm. (Scan conditions: , ).
Dots: The Pd LEIS intensity, extrapolated to 300 K from TP-LEIS data (see Fig. 3), vs Pd coverage dosed to the oxide surface at 300 K. Since the Pd LEIS intensity here is normalized to that of a continuous Pd film, it equals the fraction of the oxide area covered by Pd, . Solid line: Result expected for layer-by-layer growth of Pd. Dashed line: best fit of (growth of 3D particles of a fixed shape; ) to the data points in the coverage range 0–2.0 ML. The inset shows the coverage range 0–1.0 ML expanded in scale. The data show that Pd grows as 2D islands up to a coverage of , after which it grows as 3D particles.
(a) TPD (3.3 K/s) after dosing 1.0 ML Pd on . Peaks were seen at 370 and 520 K, attributed to and , respectively, from and adsorbed on the surface from the chamber background gas (mainly due to outgassing from the electron flood gun used for charge neutralization during TP-LEIS). (b) TP-LEIS (1 K/s) of different Pd coverages on . When and desorb from the Pd (as shown in top figure) at 350 and 490 K, the Pd LEIS signal grows. [A minor temperature decrease in desorption peak temperature arises from a decrease in heating rate (Ref. 51)]. The Pd signal decreases between the two desorption peaks with nearly the same slope as at higher temperature, and suggests that sintering is fast below 400 K and is significant at room temperature. Dashed line shows estimated TP-LEIS signal that would be seen if no and impurity were present.
(0.6 nm range) NC-AFM images of (a) the clean alumina surface in an area containing one step and (b)–(d) 0.8 ML Pd in areas between step edges on the alumina surface. Pd particles shown (b) as dosed at room temperature, (c) after annealing to 680 K, and (d) after annealing to 1000 K. Before imaging (c) and (d), the sample was allowed to cool to room temperature. Height represented by same gray scale in each panel. Paired with each image is a line profile taken along the white line indicated in the image. Some of the apparent roughness in (b) and (d) is due to pixelation. Images have pixel sizes of (a) 0.55 nm, (b) 1.3 nm, (c) 0.75 nm, and (d) 1.3 nm; each was smoothed with a 2D Gaussian filter of standard deviation 0.63 nm. [Scan conditions: (a) , ; (b) , ; (c) , ; and (d) , ].
Histogram of apparent Pd particle diameters for 0.8 ML Pd at room temperature (empty bars) and after annealing to 680 K (lined bars) and 1000 K (solid bars). These data were compiled from the images shown in Figs. 4(b)–4(d), respectively. The three data sets were taken on consecutive days using the same Si AFM tip on the same sample. The mean and standard deviation of each set (see Table I) were used to calculate the position and width of the three Gaussian curves shown as dashed lines.
(0.8 nm range) NC-AFM images of (a) 0.4 ML and (b) 0.8 ML of Pd, dosed at 300 K and annealed briefly to 920 and 930 K, respectively. Line profiles along the white lines are shown below each image. The islands on the higher coverage surface are larger than those of the lower coverage, while the total particle density is similar at these two coverages, in (a) and in (b). Images have pixel sizes of (a) 0.25 nm and (b) 0.44 nm; each was smoothed with a 2D Gaussian filter of standard deviation 0.37 nm. [Scan conditions: (a) , and (b) , ].
Summary of Pd particle average thickness, diameter, and number density for several Pd coverages and anneal temperatures. Thickness was derived from LEIS data for particle preparation conditions similar to those of the AFM images presented in this paper (see references in third column). Average apparent diameter measured by AFM, , as well as particle number density from AFM, , are presented. The apparent Pd coverage is calculated from , , and , and the particle diameter estimate is made from , , and , as described in the text.
Article metrics loading...
Full text loading...