Noble-metal nanostructures on carburized W(110)

Abstract

Noble metal nanostructures of Au, Ag and Cu were prepared on two types of carbon-modified W(110) surfaces—R(15 × 12) and R(15 × 3)—and investigated by means of scanning tunneling microscopy. For all deposited metals qualitatively the same behaviour is observed: On the R(15 × 12)-template always isotropic clusters are formed. In contrast, on the R(15 × 3)-substrate the anisotropy of the nanostructures can be tuned from clusters at low temperatures via thin nanowires to thicker nanobars at high deposition temperatures. At intermediate temperatures on the R(15 × 3) the anisotropic Au nanowires arrange themselves into straight lines along domain boundaries induced by deposition of the Au metal. Similarities and differences to Au nanostructures as recently reported by Varykhalov et al. [A. Varykhalov, O. Rader, W. Gudat. Physical Review B 77, 035412 (2008).] are discussed.

Adsorption and Reactions of Carbon Monoxide and Oxygen on Bare and Au-Decorated Carburized W(110)

“…The 420 K peak, resulting from CO molecules only weakly affected by the presence of carbon, is reduced most strongly, indicating preferential Au nucleation in these surface regions. Our STM findings concerning the growth behavior of Au 36 corroborate this assumption: Preferentially, Au nucleates on carbon-poor regions of the unit cell; at elevated deposition temperatures (i.e., 700 K) Au atoms in “excess” of the optimum coverage (≈0.12 ML) diffuse to W(110) terraces always coexisting with the broader carbon-modified ones. 29,36 Hence, tungsten-rich areas on the surface (clean terraces as well as carbon-poor regions within the unit cell) are preferentially covered with Au atoms—leading to a selective decrease of the signal attributed to CO desorption from these areas.…”

“…The surface was cleaned from carbon impurities by heating in oxygen as described in refs and − . Tungsten oxides formed in course of the procedure were removed by flashing the sample to 2300–2600 K. The carbon superstructures were generated following the preparation routines as described in refs , , , and . Carbon was provided by thermolysis of ethene (C 2 H 4 ) (heating for 10 min at 1250 K in 5 × 10 –8 mbar).…”

Thirty per cent contrast in secondary-electron imaging by scanning field-emission microscopy

“…Terraces ‘2’, ‘3’ and ‘6’ have a clearly visible (albeit faint) fine structure, consisting of thin rows running along the direction indicated by the arrows. This fine structure is an indication that terraces are carbonized by an incomplete cleaning procedure which left some residual carbon impurities behind [14]. The contrast in the secondary-electron image figure 9 b distinguishes precisely the carbonized terraces from the non-carbonized ones, while the existence of a monoatomic step dividing terraces ‘4’ and ‘5’ is not registered by the secondary-electron image.…”

“…The contrast in the brightness cannot be a Fe–W contrast, as the W is completely buried beneath the Fe film. However, the STM image (figure 10 b ) of the region marked by a red square window in figure 10 a reveals that the origin of the selective secondary-electron production is the actual Fe morphology, the brighter regions corresponding to a finer structured Fe films (middle terrace) that typically grows on carbonized terraces, as opposed to the platelets-like morphology of Fe films grown on ‘clean’ W surfaces (the terraces on the left and on the right of the central one) [14]. It appears that the secondary-electron production is also very sensitive to the morphology of the films on the atomic level.…”

“…However, when secondary electrons (green in figure 1 b ) are used for imaging, the figures of contrast and horizontal resolution typical of STM are, surprisingly, (almost) recovered. Specifically, we have imaged a submonolayer Fe film grown, by Molecular Beam Epitaxy, on top of a W(110)-surface [14,15]. We observe that the secondary-electron contrast, when going across a monoatomic thick step dividing an Fe-terrace from the W-surrounding, can be as high as 30%.…”