FIM OBSERVATION OF MONOLAYER Pd ADSORBED ON W AND Mo SURFACES

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1 FIM OBSERVATION OF MONOLAYER Pd ADSORBED ON W AND Mo SURFACES K. Okuno, H. Kim To cite this version: K. Okuno, H. Kim. FIM OBSERVATION OF MONOLAYER Pd ADSORBED ON W AND Mo SURFACES. Journal de Physique Colloques, 1989, 50 (C8), pp.c8-291-c < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1989 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 COLLOQUE DE PHYSIQUE Colloque C8, supplement au no 11, Tome 50, novembre 1989 FIM OBSERVATION OF MONOLAYER pd ADSORBED ON W AND Mo SURFACES K. OKUNO and H. KIM Department of Electrical Engineering, Nagasaki Institute of Applied Science 536 Abamachi, Nagasaki Japan Abstract - Field ion microscopic observation of the monolayer Pd adsorbed on the W and Mo substrate surfaces has been investigated. The adsorbed structure of the observed monolayer Pd was found to be a pseudomorph on (011) and (112) planes, respectively.the pseudomorphic structure on the Mo(Ol1) and (112) planes was also formed even at liquid nitrogen temperature. The desorption field strengths of the monolayer Pd are extremely higher than that of the evaporation field of bulk Pd, and are close to the evaporation field strengths of the substrate surfaces. Field desorgtion behavior in submonolayer level differs on the W(Ol1) and Mo(Ol1) planes. Although the adsorbed Pd on the Mo(Ol1) plane was found isolated atomic arrangement, no such atomic behavior is observed in Pd/M(O11) system. These results seem that the substrate effect is intrinsic. I - INTRODUCTION In recent years, the interactive physical and electrical properties at an a- tomic level on the substrate surface have become of increasing interest. Field ion microscopy(f1m) has ultimately an atomic resolution/l/. Hence, the technique is very attractive to understand the characteristic behaviarr of a- tomic events induced on the substrate surface. Therefore, many works using FIM method have been performed concerning studies of atom-atom interactions on the substate surface, of interfacial interactions at metal-metal interactions and the nucleation and epitaxial growth etc/2-5/, To obtain further crystallographkcal informations additionally to the electrical properties of.pd covered W surface, which were investigated by field electron microscopy/6-7/, FIM observation has been performed for the Pd adlayer and adatom on the W and Mo substrate surfaces, in monolayer and submonolayer level. I1 - EXPERIMENTAL A metal system field ion microscope which was constructed with turbo pump (120 l/s), ion pump(60 l/s) and Ti sublimation pump used in this ex eriments. The background pressure of the apparatus is of the order of the lo-? Pa. To select arbitrary field ion image sizes, it is possible to move a MCP image intensifier some distance from the tip. The Pd wire of 0.5 mrn diam&ter(purity of %) was used, and the Pd was vapour deposited on the W and Mo sub- Article published online by EDP Sciences and available at

3 strate surfaces by resistive heating of the Pd wire. After each DC field desorption of Pd adlayer and adatom, heon field ion images were respectively observed at various desorption field strengths. A local field strength for the desorption was estimated from that evaporttion field for the substrate surface W(Ol1) and No (011) are 5.5 v/& and 4.6 V/A at liquid ni5rogen tempegature/c/. The atomic radii of the Pd,W and Mo are A, A and A, respectively. The mismatch of sizes between adsorbate(pd) and substrates(w,mo) is therefore very small. I11 - RESULT AND DISCUSSION ADSORBED MONOLAYER Pd ON W SURFACE After substrate W surface was cleaned,pd was vapour deposited at liquid nitrogen temperature and the Pd covered W tip was heated at s500 K for 2 minutes. Neon field ion images of the clean W and the Pd covered W surfaces age shown in Fig.la-b. Fig.lb was taken at lowest imaging voltage Fdes=4.10 V/A which is possibleforthe observation. At this field strength, Pd overlayer can not be observed already. This image is covered with a monolaver Pd on the (011) and (1121 planes. Especially, the Pd on C112) planes which are marked with A in Fig.lb can be reall recognized as a monolayer from the bright. rows on the channels of the W For the further field desorption, the sub- Fig.1. Neon field ion images of the substrate clean W surface(a) and the Pd covered W surface(g) were taken. Fig-lb was observed at Fdes=4.10 V/A. This image can really be recognized that adsorbed Pd on the channels of 1112) plane is covered with a monolayer(marked with A), and the adsorbed layer forms a pseudomorph. Pd on (011) plane is also same structure.

4 strate W surface is revealedby the field desorption of the monolaver Pd at Pd-W interfaces. This field strength which was observed a monolayer Pd adsorbeg on the W surface Fdes~4.10 v/& is higher than that of the bulk Pd (3.7 V/A), andits value corresponds to 0.745Feva of the evaporation field strength of the W(Ol1) plane. This result indicates that the desorption field strength of a monolayer Pd is strongly affected by the substrate surface atom. The atomic structure of the monolaver Pd adsorbed on W(Ol1) and (1121 planes can be recognized as a pseudomorphic monolayer Pd, which is same atomic arrangement completely to that of the substrate surface.atom/9/. A sequence of field ion images of the desorption processes of the monolayer Pd adsorbed on substrate W surface with increment of the desorption field strengths are shown in Fig.2. Fig.2a-b are still covered with uniform monolayer Pd on the W(Oll), (1121 planes and (011) steps, respectively. Fig.2d is shown that only the Pd on three step rings of W(Ol1) still covered with a monolayer Pd. The desorption field strength of the pseudomorph Pd on the VJ(011) Fig.2. Neon field ion image was observed at various field strengths for adsorbed Pd on the W surgace: (a) at Fdes= 3.93 v/&, (b) at Fdes=4.23 V/A, the substrate W surface is covered with monolayer Pd, and (1121 plane can really be recognized to cover with P$ of the pseudomorphicostructure, (c) at Fdes=4.36 V/A, (d) at Fdes=4.79 V/A.

5 Fig.3. Neon field ion images of the Pd covered Mo surface were observed at various fiel; strengths: (a) clean Mo surface, (b) at Fdes=3.68 V/A, this image is covered with a monolayer Pd. The adsorbed Pd on Mo(Ol1) and (1121 planes forms a pseudomgrphic layer. (c) at Fdes=3.86 v/a, (dl at Fdes=3.95 V/A, the pseudomorphic layer was field desorbed(mark&d with A). (e) at Fdes= 4.36 v/a, ( 1 at Fdes=4.47 V/A, isolated Pd was induced in the field desorption of the monolayer Pd adsorbed on the Mo (011). This behaviour is not observed in the Pd on W(Oll), which is formed the same pseudomorphic layer

6 is extremely close to the evaporation field strength of the substrate W(Ol1) ADSORBED MONOLAYER Pd ON Mo SURFACE An observation of the adsorbed Pd on the Mo substrate surface was investigated with the same procedure as that was done on the W surface. After Pd vapour deposited on the Mo substrate surface at liquid nitrogen temperature, Pd covered Mo tip was heated at Q500 R. A series of the field desorption processes of field ion images of Pd/Mo system are shown in Fig.3. Fig.3b-c also show monolayer Pd adsorbed on the Mo surface. The monolayer Pd on Mo(Ol1) and planes is also the same pseudomorph as that was found on the W surface. The pseudomorphic Pd layer on the Mo surface found to form even at liquid nitrogen temperatgre. The pseudomorphic layer on the ~0{112) plane was desorbed at ~ des=4.4~~/~ in comparison with that of evaporation field of the substrate Mo 4.6 V/A/8/. From these results observed on the W and Mo surfaces, the desorption field strengths of the pseudomorph Pd does not depend upon the evaporation field of bulk Pd. It depend on the binding of the substrate W and Mo. These results also imply that Pd-W interactions at the Pd-W interfaces are very strong, and the substrate effect for the binding of the monolayer is intrinsic. In the field desorption processes of the pseudomorphic Pd on the Mo(Ol1) plane, isolated Pd was found as shown in Fig.3f. This phenomenon was not observed on Fig.4. Neon field ion images at the various desorption field strengths of thg adsorbed Pd layer on the Mo substrate surface: (a) at Fdes=4.72 V/A, it is pseudornorphic Pd layer on the Mo(011) surface. (b) at Fdes=4.36 v/w, a critical field desorption of the pseudomorphic Pd layer. (c) at Fdes=4.38 V/A, isolated Pd on the blo(011) plane was induced rearrangement due to the desorption field effect. (d) at Fdes= 4.40 v/&, the substrate Mo surface appeared at this field strength.

7 on the W(Ol1) plane which was observed the same pseudomorphic gd monolayer. This desorption field strength of the isolated Pd Fde~=4.47~V/A is extremely close to the evaporation field of the substrate Mo 4.5 V/A. Well resolved field ion images of the field desorption processes of the same pseudomorphic Pd layer formed on Mo(Ol1) surface are shown in Fig.4. The isolated Pd structure in Fig.4~ imply that it is induced the rearrangement due to the field effects in the submonolayer level. This behaviour implies that Pd-Pd interaction on the Mo(Ol1) surface is repulsive interactions and interaction on the W(Ol1) surface is attractive. IV - CONCLUSIONS Neon field ion images of a monolayer adsorbed Pd on the W and Mo substrate surface were observed with careful field desorption. An adsorbed Pd on (011) and (112) of the both substrates(w,mo) found to be pseudomorphic 1ayer.In the field desorption processes of the pseudomorph Pd, isolated Pd was observed on the Mo(Ol1) plane. This phenomenon seems that it formed with the field assisted rearrangement. But the same phenomenon was not observed on the W(Oll), which forms the same pseudomorph. V - REFERENCES /1/ ~.~.fiuller and T.T.Tsong: Field Ion Microscopy, Principles and Applications(America Elsevier, New York, 1596). /2/ T.T.Tsong: Progress in Surface Science. lo(1980) 165. /3/ D.W.Bassett: Thin Solid Films. 48(1978) 237. /4/ H.W.Fink and Gert Ehrlich: Surf-Sci. llo(1981) L611. /5/ Q.J.Gao and T.T.Tsong: Surf.Sci. 191(1987) L737. /6/ K.Okuno and H.Kim: Jpn.J Appl.Phys. 28(1989) /7/ H.Roux and M.Drechsler: Surf-Sci. 71(1978) 375. /8/ T.T.Tsong: Surf-Sci: 70(1978) 211. /9/ 0.Nishikawa and A.R.Saadat: Surf.Sci. 60(1976) 301.