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Adsorption of sulfur on a tungsten field emitter
Applications of Surface Science 5 (1980) 243—257 © North-Holland Publishing Company
ADSORPTION OF SULFUR ON A TUNGSTEN FIELD EMITTER M. WOHLMUTH and E. BECHTOLD Institut ffir Physikalische Chemie, Universität Innsbruck, A -6020 Innsbruck, Austria Received 20 March 1979
The adsorption of sulfur on a tungsten field emitter has been studied with a probe hole field emission microscope. If a beam of S 2 molecules impinges on the tip at a slanting angle from the rear a chemisorbed layer is formed at the observable part of the tip via mobile adsorbed molecules. The highest mobility of these adsorbed molecules is observed on the smooth regions near the (211) and (110) faces, whereas barriers hinder the diffusion onto the (110) and (111) faces. The diffusion of chemisorbed sulfur has been investigated by heating tips with spatially non-uniform sulfur deposits. These experiments indicate distinctly face specific and coverage dependent diffusion of adsorbed sulfur. On 4’~i1o) a fully sulfur tungsten tip after eV, heating = 0.4 covered eV, ~(1oo) = 0.45—0.5 ~ to 300°C 0.8the eVwork and functions increased ‘~4(31o) are 0.85—0.9 eV. by The~Fowler—Nordheim parameters and the FEM images obtained after further stepwise heating indicate face specific changes in the adlayer. Complete desorption of sulfur is achieved between 1300° and 1550°C depending on the substrate orientation. The Fowler—Nordheim data and the diffusion experiments suggest the existence of several distinct adsorption states.
1. Introduction In contrast to the extensively studied oxygen adsorption on tungsten only little effort has been expended on the homologous sulfur tungsten adsorption system. Investigations by field electron microscopy [1,2], and by field ionization mass spectrometry [3,41, have revealed complex phenomena if different orientations, coverages and thermal treatment are considered. It was the aim of the present work [5] to study the field electron emission from various sulfur covered tungsten single crystal planes under varying conditions. Although the conclusions which can be drawn from such experiments are somewhat limited, it may be expected that information on the formation of the adsorbed layer and on the existence of different adsorption states as well as on the conditions of their transformation can be obtained.
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2. Experimental 2.1. Apparatus The stainless steel vacuum system used in this work was pumped down after bake out to a base pressure of 2 X 10—10 mbar with an ion pump and a liquid nitrogen cooled Ti sublimation pump. The pressure of reactive gases, however, was in the 10—11 mbar range as judged by the rate at which the clean tungsten tips became contaminated. The probe hole field emission microscope is based on a design by Nieuwenhuys and Sachtler [6]. The basic arrangement consists of a tungsten emitter spotwelded onto a tungsten heating ioop with potential leads for temperature measurement, a screen with a probe hole and an electron collector which is surrounded by a shielding cylinder. The latter is connected with the shield of the rigid coaxial line from the electron collector to the Keithley 602 electrometer. An auxiliary electrOde between the screen and the collector, which was biased several hundred volts negative with respect to the collector and the screen, should prevent secondary or stray electrons from reaching the collector or escaping from it. The image of a selected crystal face can be positioned on the probe hole by moving the emitter by means of stainless steel bellows and by adjusting screws. Tungsten tips were formed by electrolytical etching of a 0,1 mm tungsten wire in concentrated aqueous NaOH solution at 3—4 V ac. They were thermally cleaned. Dosing of sulfur before the final heat treatment assisted the cleaning procedure. A beam of S2 molecules impinging on the tip under a slanting angle from the rear was produced in a solid state electrochemical source from Ag2 S. The construction of the electrochemical cell Pt/Ag/AgJ/Ag2 S/Pt and the conditions of operation have been described previously [2,3,7]. This arrangement allows shadow covering of the tip with negligible eontamination of the vacuum system. The density of the sulfur beam was always chosen in such a way that the corresponding vapor pressure was lower than the vapor pressure of solid sulfur at room temperature. The temperature of the S2 beam was 200 C. 2.2. Procedure Two types of experiments were carried out: First incremental dosing of sulfur at room temperature and secondly heating of fully or partially sulfur covered tips. Heating of the tip was always performed at zero field and the current—voltage data were subsequently measured at room temperature. The field emission current—voltage data were evaluated to the Fowler— 2 exp(—B çb3/2/1./) whereaccording i is the emission current, Nordheim (FN)voltage, equation i = Apre-exponential V V the applied A the term, ~ the work function and B a factor, which is proportional to the radius of curvature of the emitting area and therefore not exactly constant over the emitter. The variations of B were neglected and
M. Wohimuth, E. Bechtold /Adsorption of sulfur on tungsten field emitter
245
the work function of individual faces were calculated from the slope of FN plots of the probe hole current voltage data by using an average value of B [8]. This value was obtained from the FN plots of the total emitted current and an average work function of 4.5 eV [8]. For the (110), (100), (ill) and (310) orientations work functions of 5.8, 4.9, 4.6 and 4.3 eV were obtained, which are in reasonable agreement with published FEM work function data [8,9] for thermally cleaned tungsten tips. The reported work functions of the (110) face, however, vary over a wide range, a fact that has been discussed critically by Todd and Rhodin [101. The work function of the (110) face obtained under the assumption of a constant mean B factor seems to be too high, since the true value ofB for the (110) face will be higher than the mean value because of the reduced local curvature. The radii of the different tips used were calculated from the values of B derived from the total current voltage data. They were found to be between 1500and 2500 A. Two probe holes with diameters of I and 2 mm have been used. The diameters of the areas seen by the probe holes were between 70 and 250 A depending on the probe hole, on the distance tip to screen, and on the emitter. Therefore some contribution of the vicinals of the selected planes to the probe hole current cannot be excluded, especially in the case of the (111) and (310) orientations. —
3. Results and discussion 3.1. Incremental dosing of sulfur At room temperature the tip was exposed to several successive equal sulfur doses and after each dose current and voltage data were measured. Such experiments were carried out with the probe hole on the (110), (100), (111) and (310) orientations. The boundary of the region onto which sulfur molecules impinge directly from the molecular beam lies approximately at the upper left edge of the observable part ofthe tip (fig. 1). The dosinj experiments confirm and extend previous results [2]. At room temperature the impinging S2 molecules are adsorbed with high sticking probability forming a chernisorbed layer [2]. After saturation of the region of the tip, which is directly exposed to the beam, sulfur adsorbed on top of the chemisorbed layer diffuses to the edge of the latter, where it is trapped by transformation into an immobile state. As found by field ion mass spectroscopy [3,4] this mobile state consists of sulfur molecules S~(x = 4 to 8 and, in lower concentrations,up to x = 22 [4]), which are formed from the impinging S2 molecules. The mobile molecules are present only during and shortly after sulfur dosing. This is concluded from the observation that the spreading of sulfur declines rapidly after interruption ofthe sulfur beam. The region covered by the adsorption layer appears dark in the field emission image shown in fig. 1. The shape of its front is not that expected from the geometry of the emitter, but it is rather strongly influenced by the orientation of the substrate.
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-•
Fig. 1. (a)—(e) Shadow ~overing of a tungsten tip with successive equal sulfur doses. The sulfur beam impinges in the direction of the diagonal of the image from the upper left corner; (f)—(i) heating a tip with a non-uniform sulfur deposit (the tip is rotated counter clockwise by 45°as compared to fig. la); (f) after sulfur dosage at room temperature; (g) 1 mm at 60°C;(h) 1 mm at 110°C;(i) 1 mm at 250°C(the dark streak on the (111) region is due to the tip holder).
The propagation of the adsorption boundary may partly be interpreted by considering the roughness of the tip surface as derived from a marble model (e.g. [11]). Fig. la shows that the mobile sulfur molecules diffuse preferentially along smooth terraces of the vicinals of the (110) and {l 12} faces, but bypass at first the (110) and {l 11) faces (fig. Ib), which are surrounded by steps. Subsequently the (110) face is filled up by one additional sulfur dose. This is demonstrated by a sharp increase of its work function (fig. 2). It seems that a barrier hinders the diffusion onto
but not on the (110) face. On further dosing the adsorbed layer grows with a straight boundary towards the (010) face (fig. le). This may be due to the successive filling of terraces which are bound by steps acting as barriers. Besides this face specific mobility of the adsorbed sulfur molecules other effects
M. Wohlmuth, E. Bechtold /Adsorption of sulfur on tungsten field emitter
+ 1.0
247
-
.
e
Number
of S
2 Dq#.s
Fig. 2. Changes of work function and of the Fowler—Nordheim preexponential ofthe (110) face during sulfur covering. CA lgA = lgA/Ao, ~Ø = — q0A~,~,j andA, 0 are the pre-exponential and work function of the clean and adsorbate covered surface, respectively). The points correspond to successive equal sulfur doses. The 3th and 4th points (from the left) correspond to positions of the boundary of the adlayer as shown in fig. ic and id, respectively.
seem to influence the propagation of the adlayer. As is apparent from the heating experiments with a fully covered tip (section 3.4) the room temperature saturation coverages depend substantially on the substrate orientation. These different adsorption capacities of the faces to be covered will certainly affect the propagation of the adsorption layer but they can hardly account for the distinctly anisotropic growth of the adsorption layer around the (110) and {1 11 } faces. In addition the rate of the transformation of mobile into immobile adsorbed sulfur influences the distribution of sulfur on the tip during covering. This can be concluded fi°omthe slight darkening ahead of the edge of the dark appearing adsorption layer (fig. 1 a) which indicates some spreading of sulfur over nearly uncovered regions next to the (110) face. On most other regions, however, the well defined edge of the adlayer indicates trapping of the mobile molecules within the resolution length of the FEM (fig. 1).
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On the (111) face and similarly on the (100) and (310) orientations the final saturation values of both FN parameters are attained only by additional sulfur doses after the boundary of the layer which appears dark in the FEM has passed the probe hole area (fig. 3), indicating that the adsorbed layer is not completed in one step. The pre-exponential does not decrease monotonically but passes through a minimum during adsorption (fig. 3). In addition large current fluctuations which decline after sulfur dosing demonstrate an instability of the primarily formed adsorbed layer. These fluctuations can be affected by slight heating or by negative as well as positive fields. At some stages of shadow covering of the tip the fluctuations are superimposed on a current drift, whereby the current declines on the (111) face but increases on the (100) face. All these observations seem to indicate rearrangement and diffusion in the adlayer and perhaps desorption of weakly adsorbed sulfur.
+1.0 > 0
I + 0.5
0
-
A
1.0
.
I
I
I
I
-
—2.0I
I
I
0
I
5 Number
of S
2 Doses
Fig. 3. Changes of work function and of the Fowler—Nordheim pre-exponential of the (111) orientation during sulfur covering. The points correspond to subsequent equal sulfur doses which are the same as those of fig. 1. The five points belong to fig. la—e.
M. Wohlmuth, E. Bechtold / Adsorption of sulfur on tungsten field emitter
249
3.2. Heating tips with a non-uniform sulfur deposit The transport processes discussed in the previous section are due to the diffusion of weakly adsorbed sulfur molecules. Qualitative information on the mobility of more strongly adsorbed sulfur and on the relative stability of sulfur adsorbed on different oriented substrates can be obtained by heating tips with spatiallynon-uniform sulfur coverages. In the experiments shown in fig. if the regions which appear dark in the FEM image are heavily covered with sulfur (e.g. ~Ø(111)= 0.75 eV; (111) means the upper ~l1 1} face, the tip is rotated counter clockwise by 45U as compared to fig. la) whereas the other regions are only partly covered (e.g. ~~(l10) = 0.14 eV) or free of sulfur. On heating to 60°C(fig. ig) the dark layer proceeds slightly in the (310) (210) region and toward the (211) face but it retreats from the (110) face in the direction to the (lii) face. During this process the sulfur concentration on the (110) face attains nearly its saturation value as indicated by ~~(11O) = 0.37 eV. The dark layer extends probably due to diffusion of weakly adsorbed sulfur to the boundary where the diffusing species are trapped. The presence of weakly adsorbed sulfur on top of the chemisorbed layer up to 1S0~’Cwill be confirmed by the heating experiments described in the following section. The retreat of the boundary of the dark layer toward the (111) face, however, indicates the onset of diffusion of chemisorbed sulfur. This diffusion process can be seen more clearly by heating to 110°C (fig. lh). At this temperature the boundary retreats even in those regions where it had proceeded on heating to 60°C(e.g. near (210), fig. lh). Thereby sulfur diffuses from the retreating boundary of the high coverage region into the primarily sulfur free regions. Thus in this case the source of the diffusing sulfur as well as the whole diffusion region are known. Therefore the experiments demonstrate, in agreement with previous results [2], the onset of diffusion of chemisorbed sulfur on the (110), {2l 1), fl 1 1} vicinals. At 110°Cthe areas between the (110) and the (010) faces (fig. lh) remain free of sulfur since the mobility of adsorbed sulfur is still too low on these regions. Diffusion around the {l00} faces can be observed near 200°C. On heating to 110°Ca number of bright spots appear on the compl.etely covered regions but not on the regions with lower sulfur coverage (fig. lh). These spots are formed after prolonged sulfur exposure also at room temperature. As discussed in section 3.3 they are believed to be due to tungsten sulfur aggregates. After heating to 250°C(fig. ii) the dark regions disappeared as well as the bright spots and sulfur has spread over the whole tip. The work function of the (110) face remained constant at its saturation value whereas that of the (111) region has diminished (~Ø(111) = 0.45 eV) as compared to the value before heating (~Ø(i11)= 0.75 eV). In contrast to their behaviour on a partly covered tip the bright spots are present on a fully covered tip up to 700°C(see section 3.4). Thus it is concluded that they are stable only on a saturated adlayer and that in the experiment shown in fig. li they are decomposed by transformation into chemisorbed sulfur. This is accomplished because
250
M. Wohlmuth, E. Bechtold /Adsorption of sulfur on tungsten field emitter
the local sulfur concentration has decreased as a consequence of leveling of the sulfur concentration over the tip. The boundary of the dark layer is found at the same position as in fig. lh after an additional 2 mm heating to 110°Cindicating that the diffusion of sulfur from this boundary to areas of lower coverage has ceased after a certain sulfur concentration has been attained ahead of the boundary. By redosing of sulfur the boundary between regions of different coverage can be stabilised at 110°Con other locations of the (110), {l12}, ~111} vicinals. This boundary has been observed up to 300°C if the whole tip was precovered with a certain minimum sulfur concentration. After redosing and heating to 160°Cthe work function was increased by ~~(l11) = 0.8 eV if the dark layer covers the probe hole area, and by ~ 11) = 0.55 eV if the probe hole is just in front of its boundary. These different work functions indicate marked differences of the local sulfur concentrations at temperature where at lower coverages sulfur was spread. Analogous observations have been made for the regions between the (110) and (100) faces. The termination of the spreading of sulfur, after a certain sulfur concentration has been attained on the low coverage region, could be due to the higher stability of this inhomogeneous sulfur distribution as compared to a more uniform one or due to the blocking of diffusion paths at higher coverage. In the former case attractive lateral interactions should be present in the high coverage adlayer. For the platinum sulfur adsorption system similar phenomena were observed in the FEM [12] and it was shown that they are caused by the different structure and binding modes of the adlayer at low and high coverage, with attractive interactions at high sulfur concentration [7,121. On account of the sharp retreating boundary of the high coverage region on some locations an analogous interpretation appears plausible also for the sulfur adsorption on tungsten. In order to get preliminary information on the relative stability of sulfur adsorbed on differently oriented substrates a small amount of sulfur was deposited on the tip in such a way that the (ill) region was covered partly (~~(11l)= 0.14 eV) but the other regions of the visible part of the tip were free of sulfur. After heating to 300°C or to higher temperatures the (111) region was nearly free of sulfur (AØ(111) <0.05 eV) whereas the sulfur concentration on the regions betweenthe(l lO)andthe {lOO} faces increased. Since at this temperature the mobility of adsorbed sulfur suffices to establish diffusional equilibrium over the tip this distribution of sulfur reflects different stabilities of the adlayer on different orientations. Desorption experiments (section 3.6) confirm the lower stability of adsorbed sulfur on the { 111 } region as compared to the stability on the regions between the (110) and ~l00} faces also for the high temperature adlayer. 3.3. Saturated adsorbed layer at room temperature At saturation coverage irregularly distributed, sometimes flickering bright spots appear (fig. 4a) [1,2]. If these spots are of normal brightness the emission voltage is
M. Wohlmuth, E. Bechtold /Adsorption of sulfur on tungsten field emitter
—.
251
~
Fig. 4. Fully sulfur covered tungsten tip after heating to different temperatures: (a) 2 mm at
600°C,4 kV; (b) 5 mm at 800°C,4.5 kV; (c) 5 mm at 900°C,4.5 kV; (d) 5 mm at 1000°C, 5.2 kV; (e) 5 mm at 1100°C,5.2 kV; (f) 3 mm at 1200°C,5.2 kV; t~g)3 mm at 1300°C,4.8 kV; (h) 1 mm at 1400°C, 4.5 kV; (i) 1 mm at 1500°C,4.5 kV.
so low that the background is nearly dark. The increased emission of these spots is certainly due to field enhancement. In order to measure reliable emission currents these spots were removed by applying a positive voltage of the three- to four-fold value of the FEM voltage. Thereby some field desorption of weakly adsorbed sulfur cannot be excluded for some orientations. At full coverage and room temperature the exactness of the determination of the work functions suffers from the instability of the adlayer which seems to be due to the presence of weakly adsorbed sulfur (section 3.4) (fig. 5). Similar bright spots have been observed on an oxygen covered tip and interpreted to originate from tungsten oxide crystallites [13]. The thermal stability of these spots up to 700°Csuggests an analogous interpretation for the sulfur tungsten system [1,2].
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M. Wohlmuth, E. Bechtold /Adsorption of sulfur on tungsten field emitter
+0.5
hO 100
-
Ill 310—————
I’
“~ .4.0.5
~
p
.,I~~III
I
10
—0.5
I
0
I
500
I
I
I
I
I
1000
I
1500
°c
Fig. 5. Changes of work functions and of pre-exponentials of a fully sulfur covered tip after stepwise heating to increasing temperatures. Probe hole at (110), (100), (111) and (310) orientations. The curves are based on several experimental runs. The maximum deviations of single experimental values of ~0 from the given curve is about ±0.1eV at room temperature, and ±0.05eV between 200°Cand the desorption temperatures. The deviationsof single algA values from the given curve extend up to ±0.2. Between 800° and 980°C no values are given since in this range they are affected by field enhancement (see section 3.5).
The work function of all planes investigated is increased on adsorbing sulfur whereas the FN pre-exponential is decreased (fig. 5). Qualitatively the observed changes of the FN parameters are consistent with the assumption of an electronegative adsorbate which increases the work function and lowers the pre-exponential as a consequence of depolarisation [14] or anti-resonance effects [15]. 3.4. Adsorbed layer after stepwise heating up to 700°C The effects discussed in this and the subsequent sections were observed at room
M. Wohlmuth, E. Bechtold /Adsorpdon of sulfur on tungsten field emitter
253
temperature after heating the fully sulfur covered tip to different temperatures. The changes of the FN parameters caused by these heat treatments may be due to desorption of sulfur or due to changes in the type of binding and in the structure of the adlayer. In addition the face specificity of these processes may induce redistribution of sulfur between faces of different orientation. By this type of experiments changes of the adlayer can be detected which are not reverted during cooling down to room temperature. This can occUr because the state of the adlayer is frozen in at an intermediate temperature during cooling or because a more stable adsorbed layer is formed which. can only be attained by heating in order to overcome an activation barrier. The latter seems to prove right in most cases since it has been observed that the emission characteristics are fixed by the highest heating temperature and are not influenced by subsequent heating to lower temperatures. After heating the sulfur covered tip to 150 C the work functions of the (111) and (310) regions are decreased as compared to the room temperature values and the pre-exponentials are increased (fig. 5). These simultaneous changes in opposite directions of both FN parameters suggest desorption of weakly adsorbed sulfur whose existence is also inferred by observations during the formation of the adlayer and by heating experiments with a non-uniformly covered tip. On the (100) and (110) faces the work functions are decreased slightly after heating to 300°Cindicating minor changes of the adlayer. At room temperature the FEM image of the fully sulfur covered tip showsbrighter regions around the (110) and (100) faces. After slight heating (~ 100°C)the intensity of these regions remains nearly unchanged whereas that of the other regions has increased leading to a rather uniform brightness of the whole tip (except the {2l i} and the (110) faces). It appears that these changes of brightness on heating are due to desorption and that they indicate the orientations on the tip where weakly adsorbed sulfur can be formed at room temperature. On further heating up to 700°the FN parameters remain nearly constant on all faces investigated indicating a stable adsorbed layer (fig. 5). The bright spots reappeared after each heat treatment up to 700°Cprobably because of diffusion from the emitter shank and have been removed by field desorption before the current measurements were performed. The work function of the tip covered with this stable adlayer is increased by ~ø(l10) = 0.4 eV, ~~(100) = 0.45—0.5 eV, z~Ø(111)= 0.8 eV, = 0.85—0.9 eV. In similar manner for the oxygen adsorption on tungsten a lower work function increase has been found for the (110) face as compared to the less densely packed faces [16,17].
~~(31o)
3.5. Adsorbed layer after stepwise heating to temperatures between 800 and 1000°C Heating the tip to 800°C results in a concentration and in a more regular arrangement of the bright spots on the triangle formed by the {21 1} faces (fig. 4b). Further heating to 900°Creinforces the bright emission in this region and in addition the planes between the zone lines formed by the {211}, (110) and (110), {iOO} faces
254
M Wohlmuth, E. Bechtold /Adsorption of sulfur on tungsten field emitter
Fig. 6. Repeated sulfur dosing and heating. (a) After one sulfur dose and heating to 500°C(5kV) and (b) to 900°C (5 kV); (c) after another sulfur dose and heating to 500°C (5 kV) and (d) to 900°C (5 kV); (e) after a further sulfur dose and heating to 500°C (4.5 kV) and (f) to 900°C
(4.5 kV).
show higher granular emission. These features appear only if the tip was covered with a saturated adsorption layer but they are not observed on heating a partly sulfur covered tip. This is demonstrated by fig. 6, where images taken after successive sulfur dosing and heating are shown. Heating to 500°Cresults in equilibration of the sulfur concentration over the tip, but formation of the brightly emitting regions does not occur (fig. 6a, c). After heating to 900°Cthe bright emission is observed in cases where the sulfur concentration was above a critical value (fig. 6d) but not in cases of lower sulfur concentration (fig. 6b). After further sulfur doses and heating to 900°Cthe bright emission encompasses the entire area between the {21 l} poles (6e,f). If the bright emission is present the apparent work functions calculated from the FN equation are decreased on the (100) and (111) faces below the values of clean tungsten. Less distinct effects occur on the (110) face and in the (310) region. The
bright regions is certainly affected by field enhancement. Thus in the temperature range between 800°and 1000°Cthe work functions calculated under the assumption of an unchanged voltage field parameter cannot be considered to be true values and they are therefore not given in fig. 5. Heating to 1000°Cgradually removed the bright emission and increased the work functions (fig. 5). Around the {2 11 } planes an intense emission persists up to 1000°C (fig. 4d) and a large dark area surrounded by a bright edge appears in the region emission from the
M. Wohlmuth, E. Bechtold /Adsorption of sulfur on tungsten field emitter
255
from the (110) up to the (310) orientation. The observations in the temperature range of 800 to 1000°Csuggest rearrangement and redistribution in the adsorbed layer. Since the bright features can be removed by heating to 1100°Cand subsequently restored only after redosing of sulfur it is concluded that they disappear because of desorption. This low desorption temperature and the observation that the bright areas appear at saturation coverage only, demonstrate their lower stability as compared to the “normal” absorbed layer. One may suppose that the bright emission is due to field enhancement resulting from corrosive adsorption which could be the beginning of the formation of tungsten sulfide, or from face specific overgrowth caused by deposition of a tungsten sulfur compound, or from a combination of both. Although definite conclusions cannot be drawn from the present experiments there is evidence that face specific deposition takes place since the bright, irregularly distributed spots disappear simultaneously with the formation of the ordered high emission regions. The formation of these brightly emitting regions seems to be an activated process since after redosing of sulfur and in the presence of nuclei (fig. 6e) their reinforcement does not occur at 500°Cbut only at higher temperatures (fig. 6fl. These bright features disappear at temperatures where owing to its vapor pressure [18], decomposition of bulk tungsten sulfide, is expected to occur rapidly. 3.6. Adsorbed layer after stepwise heating above 1000°Cup to desorption temperature Further heating above 1000°Cproduces FEM images with typically enlarged dark areas and zone lines (fig. 4e—h). On further heating the work functions of the (Ill) and (310) regions decrease and the pre-exponentials increase more or less continuously toward their clean surface values (fig. 5), indicating desorption of sulfur. The cleaning-up of the {lll} regions can be seen in fig. 4g. On the (110) face the work function and the pre-exponential pass minima before the clean surface values are attained. On the (100) face a minimum of the pre-exponential but nqt pf the work function is observed. Although definite conclusions cannot be drawn from the present measurements it might be that, in analogy to the oxygen adsorption on (110) faces of tungsten, reconstruction [19] or geometric changes of the substrate surface [16] take place. In order to characterize the stability of the adlayer an attempt is made to estimate the activation energy of desorption. From the temperatures where the dif. ferent faces became clean (as judged from the FN parameters) after 10 s heating activation energies of 440 kJ/mole for the (111) face and 500 kJ/mole for the regions between the (110) and (100) faces were calculated. First order desorption with a normal pre-exponential was assumed for these estimations. Comparison of these activation energies with sulfur—sulfur bond energies [20—22] suggests atomic adsorption. No direct information on the desorbing species can be obtained from these ex-
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M. Wohlmuth, E. Bechtold /Adsorption of sulfur on tungsten field emitter
periments. If tungsten sulfur compounds are left out of consideration it seems that sulfur atoms and not molecules are desorbed. Supposing non-activated adsorption the activation energies of desorption may be equated to the binding energies of sulfur atoms to tungsten. Comparison of these values with bond energies of sulfur molecules [20—22] shows, that the activation energy of atomic desorption should be lower than that of molecular desorption, since the energy necessary to separate an additional sulfur atom from the surface is not compensated by the energy of formation of a sulfur—sulfur bond.
4. Conclusions (1) At room temperature ~2 molecules are chemisorbed on tungsten with high sticking probability via mobile adsorbed sulfur molecules. The highest mobility of these weakly adsorbed molecules is observed on smooth regions whereas steps hinder the diffusion. (2) Several distinct adsorbed states have been identified according to the temperature ranges where they exist: the mobile adsorbed molecules which are present only during sulfur dosing, sulfur which is desorbed between room temperature and 150°C,a sulfide like compound which is decomposed at 1000°C,and atomically adsorbed sulfur which is completely desorbed between 1300°and 1550°Cdepending on the substrate orientation. (3) The saturated adlayer which is stable in the temperature range from 300°to 800°Cincreases the work function of tungsten by ~~(110) = 0.4 eV, ~(1OO) = 0.45—0.5 eV, ~~(111) = 0.8 eV and ~ø(310) = 0.85—0.9 eV. Changes of work function and pre-exponentials on heating up to desorption temperature indicate face specific rearrangement in the adlayer.
Acknowledgement Support by the Fonds zur Forderung der wissenschaftlichen Forschung, Austria, is gratefully acknowledged. We also thank Dr. R. Abermann for experimental advice.
References [1] J.M. Saleh, MW. Roberts and C. Kemball, J. Catalysis 2 (1963) 189. [2] E. Bechtold, L. Wiesberg and J.H. Block, Z. Physik. Chem. Neue Folge 97 (1975) 97. [31 P.R. Davis, E. Bechtold and JR. Block, Surface Sri. 45 (1974) 585. [4] DL. Cocke, G. Abend and J.H. Block, J. Physical Chem. 80 (1976) 524. [5] M. Wohlmuth, Thesis, Innsbruck (1978). [6] B.E. Nieuwenhuys and W.M.H. Sachtler, Surface Sci. 34 (1973) 317.
M. Wohlmuth, E. Bechtold /Adsorption of sulfur on tungsten field emitter [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
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W. Heegemann, K.H. Meister, E. Bechtold and K. Hayek, Surface Sci. 49 (1975) 161. 1. Engel and R. Gomer, J. Chem. Phys. 50 (1969) 2428. 5. Hellwig and J.H. Block, Z. Physik. Chem. Neue Folge 83 (1973) 269. Ci. Todd and TN. Rhodin, Surface Sci. 36 (1973) 353. R. Gomer, Field Emission and Field Ionization (Harvard University Press, Cambridge, Mass., 1961). E. Bechtold and J.H. Block, Z. Physik. Chem. Neue Folge 90 (1974) 135. R. Corner and J.K. Hulm, J. Chern. Phys. 27 (1957) 1363. A.E. Bell and R. Gomer, J. fhem. Phys. 44 (1966) 1065. C.B. Duke and M.E. Alferieff, J. Chem. Phys. 46 (1967) 923. A.E. Bell, L.W. Swanson and L.C. Crouser, Surface Sci. 10 (1968) 254. T. Engel and R. Gomer, J. Chem. Phys. 52 (1970) 1832. L.S. Darken and R.W. Gurry, Physical Chemistry of Metals (McGraw-Hill, New York, 1953). T. Engel, H. Niehus and E. Bauer, Surface Sri. 52 (1975) 237. J. Berkowitz, in: Elemental Sulfur, ed. B. Meyer (lnterscience, New York, 1965) Chap. 7. 1. Drowart, P. Goldfinger, D. Detry, H. Rickert and H. Keller, Adv. Mass Spectry. 4(1967)
506. [22] J. Berkowitz and JR. Marquart, J. Chem. Phys. 39 (1963) 275.
ADSORPTION OF SULFUR ON A TUNGSTEN FIELD EMITTER M. WOHLMUTH and E. BECHTOLD Institut ffir Physikalische Chemie, Universität Innsbruck, A -6020 Innsbruck, Austria Received 20 March 1979
The adsorption of sulfur on a tungsten field emitter has been studied with a probe hole field emission microscope. If a beam of S 2 molecules impinges on the tip at a slanting angle from the rear a chemisorbed layer is formed at the observable part of the tip via mobile adsorbed molecules. The highest mobility of these adsorbed molecules is observed on the smooth regions near the (211) and (110) faces, whereas barriers hinder the diffusion onto the (110) and (111) faces. The diffusion of chemisorbed sulfur has been investigated by heating tips with spatially non-uniform sulfur deposits. These experiments indicate distinctly face specific and coverage dependent diffusion of adsorbed sulfur. On 4’~i1o) a fully sulfur tungsten tip after eV, heating = 0.4 covered eV, ~(1oo) = 0.45—0.5 ~ to 300°C 0.8the eVwork and functions increased ‘~4(31o) are 0.85—0.9 eV. by The~Fowler—Nordheim parameters and the FEM images obtained after further stepwise heating indicate face specific changes in the adlayer. Complete desorption of sulfur is achieved between 1300° and 1550°C depending on the substrate orientation. The Fowler—Nordheim data and the diffusion experiments suggest the existence of several distinct adsorption states.
1. Introduction In contrast to the extensively studied oxygen adsorption on tungsten only little effort has been expended on the homologous sulfur tungsten adsorption system. Investigations by field electron microscopy [1,2], and by field ionization mass spectrometry [3,41, have revealed complex phenomena if different orientations, coverages and thermal treatment are considered. It was the aim of the present work [5] to study the field electron emission from various sulfur covered tungsten single crystal planes under varying conditions. Although the conclusions which can be drawn from such experiments are somewhat limited, it may be expected that information on the formation of the adsorbed layer and on the existence of different adsorption states as well as on the conditions of their transformation can be obtained.
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2. Experimental 2.1. Apparatus The stainless steel vacuum system used in this work was pumped down after bake out to a base pressure of 2 X 10—10 mbar with an ion pump and a liquid nitrogen cooled Ti sublimation pump. The pressure of reactive gases, however, was in the 10—11 mbar range as judged by the rate at which the clean tungsten tips became contaminated. The probe hole field emission microscope is based on a design by Nieuwenhuys and Sachtler [6]. The basic arrangement consists of a tungsten emitter spotwelded onto a tungsten heating ioop with potential leads for temperature measurement, a screen with a probe hole and an electron collector which is surrounded by a shielding cylinder. The latter is connected with the shield of the rigid coaxial line from the electron collector to the Keithley 602 electrometer. An auxiliary electrOde between the screen and the collector, which was biased several hundred volts negative with respect to the collector and the screen, should prevent secondary or stray electrons from reaching the collector or escaping from it. The image of a selected crystal face can be positioned on the probe hole by moving the emitter by means of stainless steel bellows and by adjusting screws. Tungsten tips were formed by electrolytical etching of a 0,1 mm tungsten wire in concentrated aqueous NaOH solution at 3—4 V ac. They were thermally cleaned. Dosing of sulfur before the final heat treatment assisted the cleaning procedure. A beam of S2 molecules impinging on the tip under a slanting angle from the rear was produced in a solid state electrochemical source from Ag2 S. The construction of the electrochemical cell Pt/Ag/AgJ/Ag2 S/Pt and the conditions of operation have been described previously [2,3,7]. This arrangement allows shadow covering of the tip with negligible eontamination of the vacuum system. The density of the sulfur beam was always chosen in such a way that the corresponding vapor pressure was lower than the vapor pressure of solid sulfur at room temperature. The temperature of the S2 beam was 200 C. 2.2. Procedure Two types of experiments were carried out: First incremental dosing of sulfur at room temperature and secondly heating of fully or partially sulfur covered tips. Heating of the tip was always performed at zero field and the current—voltage data were subsequently measured at room temperature. The field emission current—voltage data were evaluated to the Fowler— 2 exp(—B çb3/2/1./) whereaccording i is the emission current, Nordheim (FN)voltage, equation i = Apre-exponential V V the applied A the term, ~ the work function and B a factor, which is proportional to the radius of curvature of the emitting area and therefore not exactly constant over the emitter. The variations of B were neglected and
M. Wohimuth, E. Bechtold /Adsorption of sulfur on tungsten field emitter
245
the work function of individual faces were calculated from the slope of FN plots of the probe hole current voltage data by using an average value of B [8]. This value was obtained from the FN plots of the total emitted current and an average work function of 4.5 eV [8]. For the (110), (100), (ill) and (310) orientations work functions of 5.8, 4.9, 4.6 and 4.3 eV were obtained, which are in reasonable agreement with published FEM work function data [8,9] for thermally cleaned tungsten tips. The reported work functions of the (110) face, however, vary over a wide range, a fact that has been discussed critically by Todd and Rhodin [101. The work function of the (110) face obtained under the assumption of a constant mean B factor seems to be too high, since the true value ofB for the (110) face will be higher than the mean value because of the reduced local curvature. The radii of the different tips used were calculated from the values of B derived from the total current voltage data. They were found to be between 1500and 2500 A. Two probe holes with diameters of I and 2 mm have been used. The diameters of the areas seen by the probe holes were between 70 and 250 A depending on the probe hole, on the distance tip to screen, and on the emitter. Therefore some contribution of the vicinals of the selected planes to the probe hole current cannot be excluded, especially in the case of the (111) and (310) orientations. —
3. Results and discussion 3.1. Incremental dosing of sulfur At room temperature the tip was exposed to several successive equal sulfur doses and after each dose current and voltage data were measured. Such experiments were carried out with the probe hole on the (110), (100), (111) and (310) orientations. The boundary of the region onto which sulfur molecules impinge directly from the molecular beam lies approximately at the upper left edge of the observable part ofthe tip (fig. 1). The dosinj experiments confirm and extend previous results [2]. At room temperature the impinging S2 molecules are adsorbed with high sticking probability forming a chernisorbed layer [2]. After saturation of the region of the tip, which is directly exposed to the beam, sulfur adsorbed on top of the chemisorbed layer diffuses to the edge of the latter, where it is trapped by transformation into an immobile state. As found by field ion mass spectroscopy [3,4] this mobile state consists of sulfur molecules S~(x = 4 to 8 and, in lower concentrations,up to x = 22 [4]), which are formed from the impinging S2 molecules. The mobile molecules are present only during and shortly after sulfur dosing. This is concluded from the observation that the spreading of sulfur declines rapidly after interruption ofthe sulfur beam. The region covered by the adsorption layer appears dark in the field emission image shown in fig. 1. The shape of its front is not that expected from the geometry of the emitter, but it is rather strongly influenced by the orientation of the substrate.
246
M. LVohlmuth, E. Bechtold /Adsorption of sulfur on tungsten field emitter
-•
Fig. 1. (a)—(e) Shadow ~overing of a tungsten tip with successive equal sulfur doses. The sulfur beam impinges in the direction of the diagonal of the image from the upper left corner; (f)—(i) heating a tip with a non-uniform sulfur deposit (the tip is rotated counter clockwise by 45°as compared to fig. la); (f) after sulfur dosage at room temperature; (g) 1 mm at 60°C;(h) 1 mm at 110°C;(i) 1 mm at 250°C(the dark streak on the (111) region is due to the tip holder).
The propagation of the adsorption boundary may partly be interpreted by considering the roughness of the tip surface as derived from a marble model (e.g. [11]). Fig. la shows that the mobile sulfur molecules diffuse preferentially along smooth terraces of the vicinals of the (110) and {l 12} faces, but bypass at first the (110) and {l 11) faces (fig. Ib), which are surrounded by steps. Subsequently the (110) face is filled up by one additional sulfur dose. This is demonstrated by a sharp increase of its work function (fig. 2). It seems that a barrier hinders the diffusion onto
but not on the (110) face. On further dosing the adsorbed layer grows with a straight boundary towards the (010) face (fig. le). This may be due to the successive filling of terraces which are bound by steps acting as barriers. Besides this face specific mobility of the adsorbed sulfur molecules other effects
M. Wohlmuth, E. Bechtold /Adsorption of sulfur on tungsten field emitter
+ 1.0
247
-
.
e
Number
of S
2 Dq#.s
Fig. 2. Changes of work function and of the Fowler—Nordheim preexponential ofthe (110) face during sulfur covering. CA lgA = lgA/Ao, ~Ø = — q0A~,~,j andA, 0 are the pre-exponential and work function of the clean and adsorbate covered surface, respectively). The points correspond to successive equal sulfur doses. The 3th and 4th points (from the left) correspond to positions of the boundary of the adlayer as shown in fig. ic and id, respectively.
seem to influence the propagation of the adlayer. As is apparent from the heating experiments with a fully covered tip (section 3.4) the room temperature saturation coverages depend substantially on the substrate orientation. These different adsorption capacities of the faces to be covered will certainly affect the propagation of the adsorption layer but they can hardly account for the distinctly anisotropic growth of the adsorption layer around the (110) and {1 11 } faces. In addition the rate of the transformation of mobile into immobile adsorbed sulfur influences the distribution of sulfur on the tip during covering. This can be concluded fi°omthe slight darkening ahead of the edge of the dark appearing adsorption layer (fig. 1 a) which indicates some spreading of sulfur over nearly uncovered regions next to the (110) face. On most other regions, however, the well defined edge of the adlayer indicates trapping of the mobile molecules within the resolution length of the FEM (fig. 1).
248
M. Wohlmuth, E. Bechtold /Adsorption of sulfur on tungsten field emitter
On the (111) face and similarly on the (100) and (310) orientations the final saturation values of both FN parameters are attained only by additional sulfur doses after the boundary of the layer which appears dark in the FEM has passed the probe hole area (fig. 3), indicating that the adsorbed layer is not completed in one step. The pre-exponential does not decrease monotonically but passes through a minimum during adsorption (fig. 3). In addition large current fluctuations which decline after sulfur dosing demonstrate an instability of the primarily formed adsorbed layer. These fluctuations can be affected by slight heating or by negative as well as positive fields. At some stages of shadow covering of the tip the fluctuations are superimposed on a current drift, whereby the current declines on the (111) face but increases on the (100) face. All these observations seem to indicate rearrangement and diffusion in the adlayer and perhaps desorption of weakly adsorbed sulfur.
+1.0 > 0
I + 0.5
0
-
A
1.0
.
I
I
I
I
-
—2.0I
I
I
0
I
5 Number
of S
2 Doses
Fig. 3. Changes of work function and of the Fowler—Nordheim pre-exponential of the (111) orientation during sulfur covering. The points correspond to subsequent equal sulfur doses which are the same as those of fig. 1. The five points belong to fig. la—e.
M. Wohlmuth, E. Bechtold / Adsorption of sulfur on tungsten field emitter
249
3.2. Heating tips with a non-uniform sulfur deposit The transport processes discussed in the previous section are due to the diffusion of weakly adsorbed sulfur molecules. Qualitative information on the mobility of more strongly adsorbed sulfur and on the relative stability of sulfur adsorbed on different oriented substrates can be obtained by heating tips with spatiallynon-uniform sulfur coverages. In the experiments shown in fig. if the regions which appear dark in the FEM image are heavily covered with sulfur (e.g. ~Ø(111)= 0.75 eV; (111) means the upper ~l1 1} face, the tip is rotated counter clockwise by 45U as compared to fig. la) whereas the other regions are only partly covered (e.g. ~~(l10) = 0.14 eV) or free of sulfur. On heating to 60°C(fig. ig) the dark layer proceeds slightly in the (310) (210) region and toward the (211) face but it retreats from the (110) face in the direction to the (lii) face. During this process the sulfur concentration on the (110) face attains nearly its saturation value as indicated by ~~(11O) = 0.37 eV. The dark layer extends probably due to diffusion of weakly adsorbed sulfur to the boundary where the diffusing species are trapped. The presence of weakly adsorbed sulfur on top of the chemisorbed layer up to 1S0~’Cwill be confirmed by the heating experiments described in the following section. The retreat of the boundary of the dark layer toward the (111) face, however, indicates the onset of diffusion of chemisorbed sulfur. This diffusion process can be seen more clearly by heating to 110°C (fig. lh). At this temperature the boundary retreats even in those regions where it had proceeded on heating to 60°C(e.g. near (210), fig. lh). Thereby sulfur diffuses from the retreating boundary of the high coverage region into the primarily sulfur free regions. Thus in this case the source of the diffusing sulfur as well as the whole diffusion region are known. Therefore the experiments demonstrate, in agreement with previous results [2], the onset of diffusion of chemisorbed sulfur on the (110), {2l 1), fl 1 1} vicinals. At 110°Cthe areas between the (110) and the (010) faces (fig. lh) remain free of sulfur since the mobility of adsorbed sulfur is still too low on these regions. Diffusion around the {l00} faces can be observed near 200°C. On heating to 110°Ca number of bright spots appear on the compl.etely covered regions but not on the regions with lower sulfur coverage (fig. lh). These spots are formed after prolonged sulfur exposure also at room temperature. As discussed in section 3.3 they are believed to be due to tungsten sulfur aggregates. After heating to 250°C(fig. ii) the dark regions disappeared as well as the bright spots and sulfur has spread over the whole tip. The work function of the (110) face remained constant at its saturation value whereas that of the (111) region has diminished (~Ø(111) = 0.45 eV) as compared to the value before heating (~Ø(i11)= 0.75 eV). In contrast to their behaviour on a partly covered tip the bright spots are present on a fully covered tip up to 700°C(see section 3.4). Thus it is concluded that they are stable only on a saturated adlayer and that in the experiment shown in fig. li they are decomposed by transformation into chemisorbed sulfur. This is accomplished because
250
M. Wohlmuth, E. Bechtold /Adsorption of sulfur on tungsten field emitter
the local sulfur concentration has decreased as a consequence of leveling of the sulfur concentration over the tip. The boundary of the dark layer is found at the same position as in fig. lh after an additional 2 mm heating to 110°Cindicating that the diffusion of sulfur from this boundary to areas of lower coverage has ceased after a certain sulfur concentration has been attained ahead of the boundary. By redosing of sulfur the boundary between regions of different coverage can be stabilised at 110°Con other locations of the (110), {l12}, ~111} vicinals. This boundary has been observed up to 300°C if the whole tip was precovered with a certain minimum sulfur concentration. After redosing and heating to 160°Cthe work function was increased by ~~(l11) = 0.8 eV if the dark layer covers the probe hole area, and by ~ 11) = 0.55 eV if the probe hole is just in front of its boundary. These different work functions indicate marked differences of the local sulfur concentrations at temperature where at lower coverages sulfur was spread. Analogous observations have been made for the regions between the (110) and (100) faces. The termination of the spreading of sulfur, after a certain sulfur concentration has been attained on the low coverage region, could be due to the higher stability of this inhomogeneous sulfur distribution as compared to a more uniform one or due to the blocking of diffusion paths at higher coverage. In the former case attractive lateral interactions should be present in the high coverage adlayer. For the platinum sulfur adsorption system similar phenomena were observed in the FEM [12] and it was shown that they are caused by the different structure and binding modes of the adlayer at low and high coverage, with attractive interactions at high sulfur concentration [7,121. On account of the sharp retreating boundary of the high coverage region on some locations an analogous interpretation appears plausible also for the sulfur adsorption on tungsten. In order to get preliminary information on the relative stability of sulfur adsorbed on differently oriented substrates a small amount of sulfur was deposited on the tip in such a way that the (ill) region was covered partly (~~(11l)= 0.14 eV) but the other regions of the visible part of the tip were free of sulfur. After heating to 300°C or to higher temperatures the (111) region was nearly free of sulfur (AØ(111) <0.05 eV) whereas the sulfur concentration on the regions betweenthe(l lO)andthe {lOO} faces increased. Since at this temperature the mobility of adsorbed sulfur suffices to establish diffusional equilibrium over the tip this distribution of sulfur reflects different stabilities of the adlayer on different orientations. Desorption experiments (section 3.6) confirm the lower stability of adsorbed sulfur on the { 111 } region as compared to the stability on the regions between the (110) and ~l00} faces also for the high temperature adlayer. 3.3. Saturated adsorbed layer at room temperature At saturation coverage irregularly distributed, sometimes flickering bright spots appear (fig. 4a) [1,2]. If these spots are of normal brightness the emission voltage is
M. Wohlmuth, E. Bechtold /Adsorption of sulfur on tungsten field emitter
—.
251
~
Fig. 4. Fully sulfur covered tungsten tip after heating to different temperatures: (a) 2 mm at
600°C,4 kV; (b) 5 mm at 800°C,4.5 kV; (c) 5 mm at 900°C,4.5 kV; (d) 5 mm at 1000°C, 5.2 kV; (e) 5 mm at 1100°C,5.2 kV; (f) 3 mm at 1200°C,5.2 kV; t~g)3 mm at 1300°C,4.8 kV; (h) 1 mm at 1400°C, 4.5 kV; (i) 1 mm at 1500°C,4.5 kV.
so low that the background is nearly dark. The increased emission of these spots is certainly due to field enhancement. In order to measure reliable emission currents these spots were removed by applying a positive voltage of the three- to four-fold value of the FEM voltage. Thereby some field desorption of weakly adsorbed sulfur cannot be excluded for some orientations. At full coverage and room temperature the exactness of the determination of the work functions suffers from the instability of the adlayer which seems to be due to the presence of weakly adsorbed sulfur (section 3.4) (fig. 5). Similar bright spots have been observed on an oxygen covered tip and interpreted to originate from tungsten oxide crystallites [13]. The thermal stability of these spots up to 700°Csuggests an analogous interpretation for the sulfur tungsten system [1,2].
252
M. Wohlmuth, E. Bechtold /Adsorption of sulfur on tungsten field emitter
+0.5
hO 100
-
Ill 310—————
I’
“~ .4.0.5
~
p
.,I~~III
I
10
—0.5
I
0
I
500
I
I
I
I
I
1000
I
1500
°c
Fig. 5. Changes of work functions and of pre-exponentials of a fully sulfur covered tip after stepwise heating to increasing temperatures. Probe hole at (110), (100), (111) and (310) orientations. The curves are based on several experimental runs. The maximum deviations of single experimental values of ~0 from the given curve is about ±0.1eV at room temperature, and ±0.05eV between 200°Cand the desorption temperatures. The deviationsof single algA values from the given curve extend up to ±0.2. Between 800° and 980°C no values are given since in this range they are affected by field enhancement (see section 3.5).
The work function of all planes investigated is increased on adsorbing sulfur whereas the FN pre-exponential is decreased (fig. 5). Qualitatively the observed changes of the FN parameters are consistent with the assumption of an electronegative adsorbate which increases the work function and lowers the pre-exponential as a consequence of depolarisation [14] or anti-resonance effects [15]. 3.4. Adsorbed layer after stepwise heating up to 700°C The effects discussed in this and the subsequent sections were observed at room
M. Wohlmuth, E. Bechtold /Adsorpdon of sulfur on tungsten field emitter
253
temperature after heating the fully sulfur covered tip to different temperatures. The changes of the FN parameters caused by these heat treatments may be due to desorption of sulfur or due to changes in the type of binding and in the structure of the adlayer. In addition the face specificity of these processes may induce redistribution of sulfur between faces of different orientation. By this type of experiments changes of the adlayer can be detected which are not reverted during cooling down to room temperature. This can occUr because the state of the adlayer is frozen in at an intermediate temperature during cooling or because a more stable adsorbed layer is formed which. can only be attained by heating in order to overcome an activation barrier. The latter seems to prove right in most cases since it has been observed that the emission characteristics are fixed by the highest heating temperature and are not influenced by subsequent heating to lower temperatures. After heating the sulfur covered tip to 150 C the work functions of the (111) and (310) regions are decreased as compared to the room temperature values and the pre-exponentials are increased (fig. 5). These simultaneous changes in opposite directions of both FN parameters suggest desorption of weakly adsorbed sulfur whose existence is also inferred by observations during the formation of the adlayer and by heating experiments with a non-uniformly covered tip. On the (100) and (110) faces the work functions are decreased slightly after heating to 300°Cindicating minor changes of the adlayer. At room temperature the FEM image of the fully sulfur covered tip showsbrighter regions around the (110) and (100) faces. After slight heating (~ 100°C)the intensity of these regions remains nearly unchanged whereas that of the other regions has increased leading to a rather uniform brightness of the whole tip (except the {2l i} and the (110) faces). It appears that these changes of brightness on heating are due to desorption and that they indicate the orientations on the tip where weakly adsorbed sulfur can be formed at room temperature. On further heating up to 700°the FN parameters remain nearly constant on all faces investigated indicating a stable adsorbed layer (fig. 5). The bright spots reappeared after each heat treatment up to 700°Cprobably because of diffusion from the emitter shank and have been removed by field desorption before the current measurements were performed. The work function of the tip covered with this stable adlayer is increased by ~ø(l10) = 0.4 eV, ~~(100) = 0.45—0.5 eV, z~Ø(111)= 0.8 eV, = 0.85—0.9 eV. In similar manner for the oxygen adsorption on tungsten a lower work function increase has been found for the (110) face as compared to the less densely packed faces [16,17].
~~(31o)
3.5. Adsorbed layer after stepwise heating to temperatures between 800 and 1000°C Heating the tip to 800°C results in a concentration and in a more regular arrangement of the bright spots on the triangle formed by the {21 1} faces (fig. 4b). Further heating to 900°Creinforces the bright emission in this region and in addition the planes between the zone lines formed by the {211}, (110) and (110), {iOO} faces
254
M Wohlmuth, E. Bechtold /Adsorption of sulfur on tungsten field emitter
Fig. 6. Repeated sulfur dosing and heating. (a) After one sulfur dose and heating to 500°C(5kV) and (b) to 900°C (5 kV); (c) after another sulfur dose and heating to 500°C (5 kV) and (d) to 900°C (5 kV); (e) after a further sulfur dose and heating to 500°C (4.5 kV) and (f) to 900°C
(4.5 kV).
show higher granular emission. These features appear only if the tip was covered with a saturated adsorption layer but they are not observed on heating a partly sulfur covered tip. This is demonstrated by fig. 6, where images taken after successive sulfur dosing and heating are shown. Heating to 500°Cresults in equilibration of the sulfur concentration over the tip, but formation of the brightly emitting regions does not occur (fig. 6a, c). After heating to 900°Cthe bright emission is observed in cases where the sulfur concentration was above a critical value (fig. 6d) but not in cases of lower sulfur concentration (fig. 6b). After further sulfur doses and heating to 900°Cthe bright emission encompasses the entire area between the {21 l} poles (6e,f). If the bright emission is present the apparent work functions calculated from the FN equation are decreased on the (100) and (111) faces below the values of clean tungsten. Less distinct effects occur on the (110) face and in the (310) region. The
bright regions is certainly affected by field enhancement. Thus in the temperature range between 800°and 1000°Cthe work functions calculated under the assumption of an unchanged voltage field parameter cannot be considered to be true values and they are therefore not given in fig. 5. Heating to 1000°Cgradually removed the bright emission and increased the work functions (fig. 5). Around the {2 11 } planes an intense emission persists up to 1000°C (fig. 4d) and a large dark area surrounded by a bright edge appears in the region emission from the
M. Wohlmuth, E. Bechtold /Adsorption of sulfur on tungsten field emitter
255
from the (110) up to the (310) orientation. The observations in the temperature range of 800 to 1000°Csuggest rearrangement and redistribution in the adsorbed layer. Since the bright features can be removed by heating to 1100°Cand subsequently restored only after redosing of sulfur it is concluded that they disappear because of desorption. This low desorption temperature and the observation that the bright areas appear at saturation coverage only, demonstrate their lower stability as compared to the “normal” absorbed layer. One may suppose that the bright emission is due to field enhancement resulting from corrosive adsorption which could be the beginning of the formation of tungsten sulfide, or from face specific overgrowth caused by deposition of a tungsten sulfur compound, or from a combination of both. Although definite conclusions cannot be drawn from the present experiments there is evidence that face specific deposition takes place since the bright, irregularly distributed spots disappear simultaneously with the formation of the ordered high emission regions. The formation of these brightly emitting regions seems to be an activated process since after redosing of sulfur and in the presence of nuclei (fig. 6e) their reinforcement does not occur at 500°Cbut only at higher temperatures (fig. 6fl. These bright features disappear at temperatures where owing to its vapor pressure [18], decomposition of bulk tungsten sulfide, is expected to occur rapidly. 3.6. Adsorbed layer after stepwise heating above 1000°Cup to desorption temperature Further heating above 1000°Cproduces FEM images with typically enlarged dark areas and zone lines (fig. 4e—h). On further heating the work functions of the (Ill) and (310) regions decrease and the pre-exponentials increase more or less continuously toward their clean surface values (fig. 5), indicating desorption of sulfur. The cleaning-up of the {lll} regions can be seen in fig. 4g. On the (110) face the work function and the pre-exponential pass minima before the clean surface values are attained. On the (100) face a minimum of the pre-exponential but nqt pf the work function is observed. Although definite conclusions cannot be drawn from the present measurements it might be that, in analogy to the oxygen adsorption on (110) faces of tungsten, reconstruction [19] or geometric changes of the substrate surface [16] take place. In order to characterize the stability of the adlayer an attempt is made to estimate the activation energy of desorption. From the temperatures where the dif. ferent faces became clean (as judged from the FN parameters) after 10 s heating activation energies of 440 kJ/mole for the (111) face and 500 kJ/mole for the regions between the (110) and (100) faces were calculated. First order desorption with a normal pre-exponential was assumed for these estimations. Comparison of these activation energies with sulfur—sulfur bond energies [20—22] suggests atomic adsorption. No direct information on the desorbing species can be obtained from these ex-
256
M. Wohlmuth, E. Bechtold /Adsorption of sulfur on tungsten field emitter
periments. If tungsten sulfur compounds are left out of consideration it seems that sulfur atoms and not molecules are desorbed. Supposing non-activated adsorption the activation energies of desorption may be equated to the binding energies of sulfur atoms to tungsten. Comparison of these values with bond energies of sulfur molecules [20—22] shows, that the activation energy of atomic desorption should be lower than that of molecular desorption, since the energy necessary to separate an additional sulfur atom from the surface is not compensated by the energy of formation of a sulfur—sulfur bond.
4. Conclusions (1) At room temperature ~2 molecules are chemisorbed on tungsten with high sticking probability via mobile adsorbed sulfur molecules. The highest mobility of these weakly adsorbed molecules is observed on smooth regions whereas steps hinder the diffusion. (2) Several distinct adsorbed states have been identified according to the temperature ranges where they exist: the mobile adsorbed molecules which are present only during sulfur dosing, sulfur which is desorbed between room temperature and 150°C,a sulfide like compound which is decomposed at 1000°C,and atomically adsorbed sulfur which is completely desorbed between 1300°and 1550°Cdepending on the substrate orientation. (3) The saturated adlayer which is stable in the temperature range from 300°to 800°Cincreases the work function of tungsten by ~~(110) = 0.4 eV, ~(1OO) = 0.45—0.5 eV, ~~(111) = 0.8 eV and ~ø(310) = 0.85—0.9 eV. Changes of work function and pre-exponentials on heating up to desorption temperature indicate face specific rearrangement in the adlayer.
Acknowledgement Support by the Fonds zur Forderung der wissenschaftlichen Forschung, Austria, is gratefully acknowledged. We also thank Dr. R. Abermann for experimental advice.
References [1] J.M. Saleh, MW. Roberts and C. Kemball, J. Catalysis 2 (1963) 189. [2] E. Bechtold, L. Wiesberg and J.H. Block, Z. Physik. Chem. Neue Folge 97 (1975) 97. [31 P.R. Davis, E. Bechtold and JR. Block, Surface Sri. 45 (1974) 585. [4] DL. Cocke, G. Abend and J.H. Block, J. Physical Chem. 80 (1976) 524. [5] M. Wohlmuth, Thesis, Innsbruck (1978). [6] B.E. Nieuwenhuys and W.M.H. Sachtler, Surface Sci. 34 (1973) 317.
M. Wohlmuth, E. Bechtold /Adsorption of sulfur on tungsten field emitter [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
257
W. Heegemann, K.H. Meister, E. Bechtold and K. Hayek, Surface Sci. 49 (1975) 161. 1. Engel and R. Gomer, J. Chem. Phys. 50 (1969) 2428. 5. Hellwig and J.H. Block, Z. Physik. Chem. Neue Folge 83 (1973) 269. Ci. Todd and TN. Rhodin, Surface Sci. 36 (1973) 353. R. Gomer, Field Emission and Field Ionization (Harvard University Press, Cambridge, Mass., 1961). E. Bechtold and J.H. Block, Z. Physik. Chem. Neue Folge 90 (1974) 135. R. Corner and J.K. Hulm, J. Chern. Phys. 27 (1957) 1363. A.E. Bell and R. Gomer, J. fhem. Phys. 44 (1966) 1065. C.B. Duke and M.E. Alferieff, J. Chem. Phys. 46 (1967) 923. A.E. Bell, L.W. Swanson and L.C. Crouser, Surface Sci. 10 (1968) 254. T. Engel and R. Gomer, J. Chem. Phys. 52 (1970) 1832. L.S. Darken and R.W. Gurry, Physical Chemistry of Metals (McGraw-Hill, New York, 1953). T. Engel, H. Niehus and E. Bauer, Surface Sri. 52 (1975) 237. J. Berkowitz, in: Elemental Sulfur, ed. B. Meyer (lnterscience, New York, 1965) Chap. 7. 1. Drowart, P. Goldfinger, D. Detry, H. Rickert and H. Keller, Adv. Mass Spectry. 4(1967)
506. [22] J. Berkowitz and JR. Marquart, J. Chem. Phys. 39 (1963) 275.