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INVESTIGATION OF SULPHUR ADSORPTION AND GROWTH ON TUNGSTEN BY FIELD
EMISSION MICROSCOPY
J. Saleh
To cite this version:
J. Saleh. INVESTIGATION OF SULPHUR ADSORPTION AND GROWTH ON TUNGSTEN BY FIELD EMISSION MICROSCOPY. Journal de Physique Colloques, 1987, 48 (C6), pp.C6-475-C6-480.
�10.1051/jphyscol:1987678�. �jpa-00226886�
Colloque C6, suppl6ment au nO1 1, Tome 48, novembre 1987
INVESTIGATION O F SULPHUR ADSORPTION AND GROWTH O N TUNGSTEN BY FIELD EMISSION MICROSCOPY
J.M. Saleh
Department of Chemistry, College of Science, University of Baghdad, Baghdad, Jadiriya, Republic of Iraq
Abstract
e interaction of sulphur vapour with tungsten at 295 K and pressures 10'~ to
?m-2 has bey stud
4
ed by field emission microscopy. Sulphur adsorption at a pressure of 10 N m' produced a large number of brightly emitting spots which moved continually. The emitting region was shown to be distributed at random, but with a noticeable preference for the (011) plane and its vicinals. A field-induced process resulting in a sudden enlargement of the emitting area and consequent mechanical failure of the tip occurred at 295 K provided the sulphur pressure > 10'4 N m-2; the beh viour was ascribed to the growth of sulphide whisker.. With sulphur pressures<
10-5 N mm2, a steady sulphur adsorption took place on tungsten resulting in the formation of a smooth chemisorbed sulphur layer on the surface. Complete removal of the chemisorbed sulphur was not possible below 1850 K. Activation energies for both sulphur migration and desorption could be determined. Surface potential changes throughout adsorption, migration and desorption processes could be estimated from the corresponding values of the work functions.INTRODUCTION
There have been very few field emission investigations (1-4) on the adsorption of sulphur on tungsten in contrast to the extensively studied oxygen adsorption by this technique. Ihe previous work (1-4) in this field ignored the influence of pressure on the adsorption and growth of sulphur on tungsten. Moreover, the kinetic aspects of sulphur interaction with tungsten have entirely been disregarded.
It was the aim of the present work to fill this gap through studying the field electron emission from different sulphur covered tungsten tips obtained under
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987678
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varying sulphur pressures. Activation energies for the removal of sulphur from such surfaces could also be determined.
EXPWIMENTAL
lhe apparatus and the experimental technique have already been described (2,5).
Recrystallized sulphur was obtained from Hopkin and Williams Ltd., it was stored in a sealed off ampoule, one end of which had been drawn to a fine point., and then placed in the appropriate section of the apparatus. The latter could be evacuated independently to 10'~ N mm2 in order to avoid the contamination of the field emission tube. ?he section containing the sulphur ampoule was cooled in liquid nitrogen before the fine point of the ampoule was broken, it was then subjected to thorough pumping before the sulphur vapour was introduced into the field emission system. Since the vapour pressure of sulphur is 10'~ N me2 at 295 K, it was necessary to maintain the sulphur ampoule at various temperatures higher or lower than 295 K in order to adjust the sulphur vapour at pressures in the desired range of lom4 to N m-2.
RESULTS AND DISCUSSION
--
Sulphur adsorption on tungsten tip was followed at 295 K under pressures ranging from to lom7 N .~'M The latter was divided into low, intermediate and high pressures corresponding respectively to the pressures lom4, 10'~ and ,< 10'~ N m . -2
Low Pressures (Plate I)
-
lhe initial adsorption of sulphur at 295 K under pressures 10'~ N m-2 within 2 minutes of the tip exposure to sulphur vapour occurred along the lines connecting the central (011) plane to those of (111) and (112) as indicated in the patterns 1-4 of plate I. With increasing adsorption and the applied potential, patterns 5-8 were obtained involving the increase of emission around the (111) planes and the edges of the (011) plane. Surface saturation of the tip (pattern 8) was attained in a period of 5 minutes; the surface potential at this stage decreased by 0.33 volt. lhe distribution and stability of pattern 8 did not alter when the tip was heated under pressures of 10'~ N m-2 to 850 K as shown in the patterns 9-10.
On heating the tip to 900 K in the absence of sulphur, emission disappeared from all regions except those surrounding (111) as shown in patterns 11-12. At 950- 1050 K, each of the (111) regions is shown (patterns 11-12) to split into three identical frag~nents arranged in a triangular form confining a nonemissive (ill) plane. Further heating of the sulphur-covered tungsten to 1400 K gave rise to the image 15-21. Surface images 11-21 reflect migration of sulphur which occurs over the temperatures 950-1400 K; an activation energy of 164 k J mol-' and an over-all surface potential increase of 0.19 V have been estimated for this process. Removal
resulting in patterns 27-30. An activation energy of 420 k J mol-' was calculated (2,5) for the removal of sulphur from the tip surface.
It was possible to obtain the images 22-26 when the tungsten tip at a stage corresponding to pattern 21 has been exposed to sulphur vapour at 850 K and a pressure of N m-2. 'Ihese patterns (22-26) resembled those of 13-14 and were produced under similar applied potentials.
Intermediate Pressures (Plate 11)
Adsorption of sulphur at 295 K under pressures of loW5 N m-' occurred within 3 minutes (patterns 1-8) on the edges of the (112) and (011) planes; the latter extended towards the (112) planes. Adsorption was accompanied by the decrease of surface potential by 0.4 V. 'Ihe surface phase remained stable over the temperatures 295-850 K as soon in patterns 9-10 which have been obtained subsequent to the heating of the tip to such temperatures.
'Ihe emission increased from the (112) planes and the (001) regions became more uniform when the tungsten tip was heated to 950 K (patterns 11-12). At temperatures 1000-1150 K, the (111) region acquired a triangular or pear-like shape, the emission increased around the (011) plane and the (001) region underwent some contraction as indicated in the patterns 13-14. When the temperature of the tip was raised to 1400 K, the (011) plane became emissive and the pear-shaped pattern of the (111) region spread toward the three neighbouring (112) planes as shown in the patterns 15-16.
An activation energy of 178 k J mol" was derived for sulphur migration on tungsten (patterns 11-16) over the temperatures 900-1400 K; this also corresponded to an increase in surface potential by 0.22 V. Gradual removal of sulphur took place at temperatures 1450-1850 K (patterns 17-18). An activation energy of 415 k J mol"
was derived for the removal of sulphur.
Patterns 19-21 were obtained on exposing the tip at a stage corresponding to the image 18 to sulphur vapour at a pressure of 10'~ N m-2 at 950 K. Further
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adsorption of sulphur on the tip at this stage caused a complete disappearance of emission from the (111) and (112) regions while only the (001) regions on the surface remained emissive.
H&& Pressures (Plate 111)
In a series of experiments, the tungsten tip at 295 K was exposed to sulphur vapour under a pressure of 10'~ N m-2, the images obtained are those numbered 1-15 of plate 111 as well as those which are given the letters A-E of plate 11. Sulphur attacked all regions of the tungsten surface causing the emission to disappear from regions surrounding (OOl), (111) and (011) as in the patterns 5-9* 'Ihe adsorption of sulphur, thereafter, resulted in the appearance of small, usually bright, images distributed somewhat at random but with noticeable preference for the (011) plane and their vicinals (patterns 10-12). 'Ihe images are observed to rotate, to liberate, to change from singlets to doublets or to q&druplets or vice versa, particularly when the residual sulphur pressure in the tube was kept at 6 N m -2
.
Pattern I1 remained often stable under the applied field for as long as 30 minutes; in the presence of the sulphur stream, some of the emitting spots began to grow into ball-shaped images. Suddenly, one of the images continued growing, the others disappearing, until it occupied within 30 sec. the whole area of the screen;patterns A to E of plate I1 represent few such stages.. If the applied field is maintained, the electron emission from the ball-shaped spot ceases due to the deformation of the tip. Mechanical rupture of the tip only occurs in the presence of the applied voltage. The work function increase by this stage (pattern 12) is 0.48 V.
There is a strong evidence for a field-induced surface process (6) for the interaction of sulphur with tungsten at pressures
>,
N m-2, since if the field is turned off at the stage shown in pattern E of plate I1 the same pattern can be obtained 30 minutes later. Such a field-induced process resulting in the sudden enlargement of the emitting area is likely to reflect a nucleation process, possibly the growth of sulphide whisker, which would account for the mechanical failure of the tip due to an enhancement in the field strength, Mercury whiskers are reported (7) to grow under pressures 10"-
10'~ N mW2.Increases in the pattern size can arise only from lessened compression of the lines of force at the whisker tip (8-10). This indicates that changes result from the increase in whisker length, which lessens the effect of the substrate on the field at the end. Needle-like growths have been observed, with electron microscope, to form when a thin oxide layer on aluminum is heated in vacuum at 873 K (11).
If, however, the tip is quickly heated to 750 K, before sufficient time is allowed for the tip failure to occur, the ball-shaped \image is seen to break up into several fragments as indicated in the patterns 13-15 of plate 111. Desorption of sulphur from such a surface produces, thereafter, images which are similar to those of plate 11.
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3. Bechtold, E., Wiesberg, L. and Block, J. H., Z. Physik. Chem. Neue Folge, 1975, 97, 97.
4. Wohlmuth, M. and Bechtold, E., Appl. Surf. Sci., 1980, 5, 343.
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6. Ehrlich, G. and Hudde, F. G., J. Chem. Phys., 1959, 30, 493.
7. Sears, G. W., Acta Met., 1955, 3, 361.
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