Probe platinum-iridium

It is well known that contact between the bottom of the tip and the sample surface will not be between two smooth, regular surfaces. In particular, the bottom of the tip may contain many asperities, and one of these asperities will serve as the probe. In STM experiments the most common tip is made from a tungsten poly-crystalline wire, and other tip materials are commonly transition metals (platinum, iridium, alloys) [21]. It is generally agreed today that only a very sharp tip with a single atom at its pinnacle is suitable to obtain atomic resolution on close-packed surfaces. But, such a tip is highly unstable. Therefore, the fabrication and characterization of defined tips, e.g. by field ion microscopy, have not been achieved, nor can it... 

STM is based on the principle that a probe, made of tungsten (W) or platinum-iridium (Pt-Ir) alloy, scans the surface of a sample with the help of a piezoelectric device (Figure 15.2) at a distance 5 to 10 A (0.5 to 1 nm). 

In Table 4.1 chemisorption data on alumina-supported platinum-iridium catalysts and related catalysts containing platinum or iridium alone show the effect of varying the temperature of calcination of the catalyst (in air or oxygen-helium mixture) on the metal dispersion (40,41). Data are presented for chemisorption of carbon monoxide, hydrogen, and oxygen. The final three catalysts in the table contained more metal than the first three. They also contained 0.1 wt% Fe (enriched with 57Fe) incorporated as a probe for Moss-bauer spectroscopy experiments (41). The presence of the iron is ignored in the discussion of the chemisorption results. 

The use of the ethane hydrogenolysis reaction as a chemical probe in the characterization of platinum-iridium on alumina catalysts is illustrated by the data in Figure 4.21. Rates of hydrogenolysis per iridium atom are shown over a range of temperatures for a series of catalysts containing 0.3 wt% iridium... 

Various types of physical probes have been employed in our studies of platinum-iridium clusters. The probes include X-ray diffraction, extended X-ray absorption fine structure (EXAFS), and Mossbauer effect spectroscopy. All of the probes have provided useful information for the characterization of the bimetallic clusters. 

Mossbauer Effect Spectroscopy Studies (41). Another physical probe which has been used in the characterization of platinum-iridium catalysts is Mossbauer effect spectroscopy (3,4), 52). It can be applied to catalysts in which virtually all of the metal atoms are surface atoms. In Mossbauer effect spectroscopy one is concerned with a transition between a ground state and an excited state of a nucleus (53). 

Figure 4.32 Mossbauer spectra at 25°C on alumina-supported platinum, iridium, and platinum-iridium catalysts (samples B, C, and D, respectively) containing a small amount of Fe (0.1 wt%) enriched with 57Fe as a Mossbauer probe (3,41). (Sample A is a reference material containing only the enriched Fe on alumina.) (Reprinted with permission from Academic Press, Inc.)...

Figure 3 Scanning electron micrograph of (A) mechanically sharpened platinum-iridium STM probe, (B) silicon nitride AFM cantilevers, and (C) carbon nanotube tip on silicon nitride AFM cantilever tip. (Images (A) and (B) are courtesy of Veeco Instruments Santa Barbara, CA, USA, with permission.)...

The ruthenium(II) polypyridyl complexes are also popular but the brightnesses do not exceed 15,000 and thermal quenching is rather significant. This property can be utilized to design temperature-sensitive probes providing that the dyes are effectively shielded from oxygen (e.g., in polyacrylonitrile beads). Despite often very high emission quantum yields the visible absorption of cyclometallated complexes of iridium(III) and platinum(II) is usually poor (e < 10,000 M-1cm-1), thus,... 

Abstract Pressure-sensitive paint (PSP) is applied to the areodynamics measurement. PSP is optical sensor based on the luminescence of dye probe molecules quenching by oxygen gas. Many PSPs are composed of probe dye molecules, such as polycyclic aromatic hydrocarbons (pyrene, pyrene derivative etc.), transition metal complexes (ruthenium(II), osumium(II), iridium(III) etc.), and metalloporphyrins (platinum (II), palladium(II), etc.) immobilized in oxygen permeable polymer (silicone, polystyrene, fluorinated polymer, cellulose derivative, etc.) film. Dye probe molecules adsorbed layer based PSPs such as pyrene derivative and porphyrins directly adsorbed onto anodic oxidised aluminium plat substrate also developed. In this section the properties of various oxygen permeable polymer for matrix and various dye probes for PSP are described. 

The data indicate that platinum clusters associated with iron are altered by addition of iridium to the sample. We conclude that the added iridium is incorporated in the platinum clusters to give Ptlr clusters containing the iron probe atoms. If the added iridium was present as separate iridium clusters... 

In this field of luminescent organometallic probes, rhenium and iridium complexes have played so far the major role, thus explaining their predominance in this chapter. The lower number of platinum, rhodium, and gold fluorescent complexes could be explained by demanding synthesis, low luminescence efficiency, poor operational stability, unsuitability for biological applications, or even simply by the lack of systematic studies. 

---