FIELD ION- AND ELECTRON-EMISSION MEASUREMENTS ON SINGLE METAL CLUSTERS : Re ON W(110)

Transcription

1 FIELD ION- AND ELECTRON-EMISSION MEASUREMENTS ON SINGLE METAL CLUSTERS : Re ON W(110) N. Ernst, G. Ehrlich To cite this version: N. Ernst, G. Ehrlich. FIELD ION- AND ELECTRON-EMISSION MEASUREMENTS ON SINGLE METAL CLUSTERS : Re ON W(110). Journal de Physique Colloques, 1986, 47 (C2), pp.c2-47-c2-51. < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1986 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 JOURNAL DE PHYSIQUE Colloque C2, supplement au n03, Tome 47, mars 1986 page c2-47 FIELD ION- AND ELECTRON-EMISSION MEASUREMENTS ON SINGLE METAL CLUSTERS: Re ON ~(110)* N. ERNST and G. EHRLICH' Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, Berlin 33, F.R.G. + Coordinated Science Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL 61801, U.S.A. Abstract - Using a combined probe hole FIM-FEM, Fowler-Nordheim studies have been carried out on tungsten emitters prepared by sputterina and field evaporation. The projection of the iris probe hole onto a W(110) plane could be made as small as the unit cell of W(110), mapped out by adatom diffusion. A dependence of (110) work function values on plane size as well as on tip radius has been observed, arising from variations in the local field strength and from contributions to the probe hole current from surrounding low work function areas. A distinct difference in electron emission behaviour between Re5/W(110) and Rel/W(llO) was detected. Rel slightly enhances the local field emission current; Re5 causes a current decrease. Changes of FN preexponential factors and of local work functions caused by adsorption of a single Re atom and a Re cluster will be presented. I - Introduction Last year we described a combined field electron and field ion microscope, with which Fowler-Noraheim (FN) data on clean W(110) as well as on W(110) covered with a single Re adatom were obtained /l/. The local field electron emission current was found to increase by about 20 % following the deposition of a single rheniur atom, in agreement with previous studies /2,3/. The effect of a single adatom on field- electron emission has recently been discussed in terms of physical quantities like the local density of electronic states as \.:ell as the local electron-potential bar- rier /4/, and it appears that the nature of the adspecies, that is chemical dif- ferences or clustering of adatoms, should significantly influence field-emission measurements, especially electron-energy distributions /4,5/. b!e have carried out introductory current-voltage measurements on a single Re adatom and a Re pentamer (Re5) on W(110), which reveal marked differences between the two. In addition, field-electron emission was examined from clean Gl(110) surfaces prepared by neon sputtering and field evaporation to assess the effects of plane size as well as tip radius on our measurements. I1 - Experimental Method The combined field ion and field electron probe hole microscope is equipped with an * Work performed at the University of Illinois under NSF Grant DFlR Article published online by EDP Sciences and available at

3 JOURNAL DE PHYSIQUE adjustable iris and a computer aided data acquisition system /l/. Single particle de- tection is made possible through the use of a chevron channel plate assembly coupled to a conductive phosphor screen. The position and size of the probe hole, projected on a field evaporated W(110) plane, was determined by superposing a helium field ion micrograph on the electron-emission through the probe hole, as shown in Fig. l(a). The bright spots of the FIM pattern in Fig. l(a), photographed with the iris probe- hole completely open, indicate the position of edge-atoms at the steps of the first three layers for a field evaporated (110) oriented tungsten emitter. A nlesh of the surface unit-cells of the first atomic layer was derived by recording the binding sites for a single Re adatom diffusing over the surface. It is displayed on the FIM image together with the edge of the field electron emission spot photographed with small iris diameter, shown as a circle in the center of Fig. l(a). In this particular case, the probe-hole projection covers approximately three unit-cells. This area cor- responds to a circle with radius. The probe-hole position could additionally be checked by measuring the local minima of the helium field ion and electron-emission currents. This feature turned out to be essential for the performance of experi~ents on the clean W(110) surface. I11 - Results and Discussion (a) Clean W(Z1O) Fig. l(a) - Superposition of electron evission Fin. l(b) - Results of FN measurethrough probe-hole on a field-ion micrograph. ments on clean W(110) obtained at The mesh on the W(110) plane was derived from different field evaporation voltsingle Re adatom diffusion and allows direct aaes (tip radii) and maximum (110) deterpination of the projected area of the iris plane sizes. probe-hole, shown as a circle in the center. Field-electron emission originating from W(110), as well as from the tip as a whole, was analysed according to FN theory (see e.g. Ref. /4/). Extrerely sharp tips were prepared by neon sputtering and surfaces were characterized by FEV and FIM. Values

4 for the slopes mave and mpro, obtained from FN plots of total and probe-hole currents, are plotted in Fig. l(b) as a function of field evaporation voltage i.e. increasing tip radius. A significant reduction of the voltage-to-field strength conversion factor B,,,, derived from mave data in the usual way 141, is apparent in Fig. l(b) as the tip radius increases. Data for the work function of the (110) plane were calculated from measured FN slopes, assuming the value fbr the tip as a whole to be 4.5 ev. In Fig. 2(a), values are plotted versus the d.c. field evaporation voltage required for the removal of one W(110) layer within about five seconds. The derived work function increases from 4.9 ev to 5.1 ev as the FEV voltape is raised from 5.7 kv to 14.1 kv. This is expected As the tip radius increases the central (110) plane increases in size, and the contribution of roupher low work function regions to the probe-hole current diminishes. However, a pronounced scatter is also evident in the data, well beyond the error limits for indi- vidual measurements. This scatter becomes stronger as the tip radius becomes laroer. To Probe Hole F~eld Evaporation Voltage (kv) for One W(110) Layer/Five Seconds.,.,a, Fig. 2(a) - Work function data of clean, field evaporated W(110) obtained from FN analysis of f ield-el ectron emission. lpro ~ F~~,rllo,R,~~l...,.-. Fig. 2(b) - Scherratic section through the apex of a field ewitter and quantities determining the current actually detected in a probe hole experiment. As outlined schematically in Fig. 2(b), the probe-hole current ipro, measured during field-electron emission, especially from relatively sharp tips, is poverned by at least four physical quantities, such as the local field strength distribution, the geometry of the tip apex and the tangential velocity of those electrons originating from surroundin? low work function areas 151. Computer simulations of the probe-hole current have been performed following earlier work by Liu /10/. The calculations sugcest a significant influence not only of plane size, but also of tip radius on FN data. Efforts were made in the experiments to create (110) planes of maximum diameter by means of FIM controlled FEV, but plane sizes were not reproduced exactly, and it is presumably this effect that causes the scatter. For the present the important

5 CZ-50 JOURNAL DE PHYSIQUE conclusion is that absolute work function values cannot be safely derived from these experiments, but that work function changes due to adsorption are accessible. (b) Rhenium on W(11O) Fig. 3(a) shovrs a sequence of field-ion micronraphs with five Re adatoms initially located on the flat (110) plane. A rhenium pentamer (Re5) was created after several heating cycles at 400 K following a recently reported procedure /11/. As demonstrat- ed in Fig. 3(a), the ion beam emitted from adsorbed Re5 was aligned with the iris probe-hole covering the center of the rhenium cluster. After pumping off the helium gas, the field-electron currents through the probe hole as well as from the tip as a whole were recorded as a function of tip voltage. Following this, FIM of the sur- face showed the cluster in the same position and orientation as observed prior to electron-emission measurements. In an attempt to field desorb the rhenium pentamer, the tip voltage was then carefully raised while watching the FIM pattern. One spe- cies, appearinq as a dim image spot (Fig. 3(a)), remained at the surface after field desorption of part of the cluster. Some of the FN data, taken thereafter, are plot- ed in Fig. 3(b) and displayed the same features as measured after deposition of a single rhenium atom /l/: At a tip voltage of 1 kv the actual probe-hole current ex- After Heat~ng After FD. Re, Fig. 3(a) - FIN of Re/!.!(llO). Fig. 3(b) - Section of the FN plot of the fieldemission current from W(110), covered with a rhenium pentamer (lower data), one rhenium atom (upper data) and clean surface (data in the middle). ceeds the emission-current measured for the clean surface by about 20 %. The presence of a rhenium pentamer, however, causes a decrease of the probe-hole current by about 20 % relative to the emission from the clean surface. It should be noted that FN data, measured for the tip as a whole, did not show any significant difference before and after rhenium field desorption.

6 Since the measured current-voltage curves obey the ordinary FN behaviour over sev- eral orders of magnitude, we have subnitted the present data to an FN analysis, even Table 1 Summary FN analysis : Re/W(110) - A+= workfunction change, B=ln(bc,/bRe), change of preexponential factor and ~ i ~ ~ probe-hole current change relative to data obtained for clean t!(110). though FN theory cannot rigorously account for electron-emission changes due to a single adatom /4,5/. As one vight expect, local work function changes are noderate, but they are different in sign for the two adspecies. A wore dramatic effect is not- ed in the change in the preexponential factor and the local field emission current. More data, such as electron-energy distributions, are required before any final con- clusions can be drawn about the physical foundations of these observations. However, the present measurements already serve to demonstrate a quite unusual effect of rhenium clusters on the emission properties of the surface. Acknowledgements We are indebted to many in this laboratory for their help and advice during this investigation. Special thanks are due to Robert B. Bales, Thomas L. Gilbertson, Donald J. Holmgren, W i l l iam L. Lawrence, and R. Liu, now of the Bell Labs. REFERENCES /l/ Ernst, N. and Ehrlich, G., J. de Physique - 45(C9) (1984) 293. /2/ Plummer, E.W. and Rhodin, T.N., Appl. Phys. Lett. 11 (1967) 194. /3/ Kellogg, G.L. and Tsonci, T.T., Surf. Sci (1977) 343. /4/ Modinos, A., in: 'Field, Thermionic, and Secondary Electron Emission Spectroscopy" (Plenum Press, New York 1984) and references therein. /5/ Plummer, E.W. and Gadzuk, J.W., Rev. Mod. Phys (1973) 487. /6/ Young, R.D. and Miller, E.W., 3. Appl. Phys (1962) 91. /7/ Swanson, L.W., and Be1 l, A.E., Adv. Electron. Phys (1973) 193. /8/ Todd, C.J. and Rhodin, T.N., Surf. Sci. 36 (1973) /g/ Polizzotti, R.S. and Ehrlich, G., Surf. Sci (1980) 24. /10/Liu, R., Ph.D. Thesis, University of Illinois at Urbana-Champaign (1977); Liu, R. and Ehrlich, G., Surf. Sci (1982) 207. /ll/fink, H.-kl. and Ehrlich, G., Surf. Sci. 150 (1985) 419.