Faceting and steady-state shape of Rh- and Pd-covered W field emitter tips

Pelhos, K.; Madey, Theodore E.; Błaszczyszyn, R.

doi:10.1016/s0039-6028(99)00196-x

Cited by 27 publications

(15 citation statements)

References 31 publications

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“…To provide guidance for future faceting studies of planar surfaces, we have been collaborating with the group of R. Blaszczyszyn to conduct field-emission microscopy studies (15). In this work, a nearly hemispherical emitter tip, coated with metallic overlayers, develops facets on annealing.…”

Section: Resultsmentioning

confidence: 99%

Nanoscale surface chemistry

Madey

Pelhos

et al. 2002

Proc. Natl. Acad. Sci. U.S.A.

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We report evidence in several experiments for nanometer-size effects in surface chemistry. The evidence concerns bimetallic systems, monolayer films of Pt or Pd on W(111) surfaces. Pyramidal facets with {211} faces are formed on annealing on physical monolayer of Pt, Pd on a W(111) substrate, and facet sizes increase with annealing temperature. We used synchrotron radiation-based soft x-ray photoemission to show that monolayer films of Pt, Pd, on W ''float'' on the outer surface, whereas multilayer films form alloys on annealing. Acetylene reactions over bimetallic planar and faceted Pd͞W surfaces exhibit size effects on the nanometer scale, that is, thermal desorption spectra of reactively formed benzene and ethylene (after acetylene adsorption) change systematically with facet size. In the second case, the decomposition of C 2H2 over planar and faceted Ir(210) surfaces also exhibits structure sensitivity; temperature programmed desorption of H 2 from C2H2 dissociation depends on the nanoscale surface structure. Finally, we have characterized interactions of Cu with the highly ordered S(4 ؋ 4)͞W(111) surface. The substrate is a sulfur-induced nanoscale reconstruction of W(111) with (4 ؋ 4) periodicity, having broad planar terraces (Ϸ30 nm in width). Fractional monolayers of vapordeposited Cu grow as threedimensional clusters on the S(4 ؋ 4) surface over a wide coverage range. At low Cu coverage (< 0.1 ML), Cu nanoclusters nucleate preferentially at characteristic 3-fold hollow sites; we find a clear energetic preference for one type of site over others, and evidence for self-limiting growth of nanoclusters.A n important issue in surface chemistry and catalysis is how surface structures and features with nanometer dimensions affect reactivity in heterogeneous systems (1-3). The focus of our work has been on several aspects of nanoscale phenomena that influence surface chemistry, including faceting of metallic and model bimetallic catalyst surfaces, and nucleation of subnanometer metallic clusters on sulfided surfaces. We study atomically rough substrates [bcc (111) surface of W, and fcc (210) surface of Ir] that are morphologically unstable, that is, the initially planar substrate becomes covered with nanoscale facets when covered with monolayer films of gases or other metals, and heated to elevated temperature. Major objectives of this work have been (i) to determine how the surface transition from planar to faceted affects the surface reactivity of metallic and bimetallic systems, and (ii) to characterize the nucleation and growth of metals on sulfided W surfaces. The three main components of this effort are surface structure, surface chemistry, and surface electronic properties.The importance of bimetallic catalysts based on Pt-group metals has been increasing in recent decades (4, 5). These catalysts display important advantages over classical reforming catalysts, including better stability, as well as improved activity and selectivity. In particular, refractory metals (W, Mo, Re, . . .) in combination with ...

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Section: Resultsmentioning

confidence: 99%

Nanoscale surface chemistry

Madey

Pelhos

et al. 2002

Proc. Natl. Acad. Sci. U.S.A.

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“…In contra st to the adsorpti on of Pd on W [18 ] and Mo [19] m i crocrysta l s, the Pd l ayer on the T a emi tter ti p do es no t i nÛuence the faceti ng tra nsiti on of the substra te surf ace. If any, onl y a weak rearra ngement of the regi on aro und the (1 10) po le coul d be indi cated in the FE i m ages for È > 1 ML after heati ng the ti p i n the tem p erature rang e 995{ 1340 K. Form ati on of a surf a ce al l oy i n som e crysta l l ogra phi c regi ons [12] was deduced i n an FIM study [13].…”

Section: R Esu L T S An D Discussionmentioning

confidence: 99%

“…Such m easurem ents pro vi de i nform atio n concerni ng the di˜usi on pa ra meters f or vari ous crysta l l ogra phi c di recti ons a nd regi ons of the surf ace. R ecentl y FEM has been used to study the faceti ng pro cess for adsorpti on system s such as Pd/ W , Rh/ W [18 ], Pt/ W , Pd/ Mo and Pt/ Ir [19 , 12] as wel l as di˜usi on under condi ti ons when faceti ng pro cess occurs [12,20]. The presented wor k wa s i ni ti al l y underta ken to study the faceti ng pro cess.…”

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confidence: 99%

Diffusion of Palladium on Tantalum Microcrystal

Antczak

Błaszczyszyn

2003

Acta Phys. Pol. A

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“…Works were carried out both on plane W(1 1 1) crystals (e.g. [1][2][3][4][5]) and curved surfaces of W emitter tips [5][6][7][8][9][10]. Accordingly, the growth, alloying and segregation of transition metals (including Pt) deposited on tungsten (1 1 1) and (1 1 2) planes were studied [11][12][13][14].…”

Section: Introductionmentioning

confidence: 99%