Scanning tunneling microscopy and transmission electron microscopy studies of Au and Pd clusters grown on a clean graphite surface

Abstract

Scanning tunneling microscopy (STM) has been used to image gold and palladium clusters deposited under ultrahigh vacuum (UHV) conditions on clean cleaved surfaces of natural graphite single crystals. Extensive transmission electron microscopy (TEM) studies were performed to determine the growth conditions. Moreover each sample was checked by ex situ TEM after each in situ STM study. The overall shape of the largest isolated clusters imaged by STM (100–200 Å) is consistent with TEM observations: triangular or hexagonal outlines and clusters sitting along surface steps are elongated by coalescence. STM also reveals that they have a height versus diameter ratio about 0.1. Smallest clusters (10–20 Å) are imaged with atomic resolution simultaneously with the graphite lattice. They are close packed and 2–3 atomic layers high. On occasion, two-dimensional Pd islands are observed. They have an hexagonal non-close-packed arrangement.

Scanning Tunneling Microscopy and Scanning Tunneling Spectroscopy Studies of Powdery Palladium/Graphite Model Catalysts

“…In this case the clusters have an average size of 2.76 nm (see also Figure , which shows the corresponding size distribution) and the average width-to-height ratio is shifted to 1.6, with 2 / 3 of the particles having width-to-height ratios between 0.6 and 1.6 (black bars in Figure ), representing once again a nearly spherical shape. It should be mentioned that the particle dimensions measured here are comparable to those of Pd clusters which were deposited on an HOPG crystal from a colloid solution by an electrophoretical process but quite different from the sizes of vapor-deposited clusters 9 Histogram showing the size distribution of clusters on RFL graphite with 0.1% palladium.

10 Histogram of the width-to-height ratio of the same clusters as used for the histogram in Figure .

11 Histogram showing the size distribution of clusters on RFL graphite with 1.0% palladium.

12 Distribution of width-to-height ratios of clusters on RFL graphite with 1.0% palladium.

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An ultrahigh vacuum sputter source forin situdeposition of size-selected clusters: Ag on graphite

“…Amorphous carbon [29][30][31][32][33] and HOPG [21][22][23][33][34][35][36][37][38][39][40][41][42][43][44][45][46] have often been used as substrates for the deposition of evaporated atoms or supported clusters. For almost all experiments made at room or higher temperature, metals were found to be mobile on amorphous carbon [31,33] and on HOPG [22,23,[33][34][35][36][37][38][39][40]. Furthermore, low sticking coefficients S have been observed (0.1 < S < 0.3) [29,34,39].…”

Growth and photoemission spectroscopic studies of ultrathin noble metal films on graphite

“…Notably, Ag and Au on MoS 2 [16,17], Cu, Ag, and Au on WSe 2 [15,18,19], Ag and Au on highly oriented pyrolitc graphite (HOPG) [5,20] form epitaxial overlayers despite a considerable lattice mismatch between the metal and the substrate. There are many reports on the growth of noble metals on HOPG, MoTe 2 , WTe 2 , WSe 2 [5,14,15] and on single-crystalline graphite surface [13,[21][22][23][24]. However, so far enough attention has not been paid to the study of surface electronic structure of these noble metal-covered van der Waals' surfaces.…”