- Email: [email protected]
Incomplete wetting of uniform surfaces by sulfur hexafluoride
426
Surface Science 162 (1985)426-431 North-Holland, Amsterdam
I N C O M P L E T E W E T r i N G OF U N I F O R M S U R F A C E S BY S U L F U R HEXAFLUORIDE M. B O U C H D O U G ('NRS. Laboratoire Maurice Letort. associb it I'unieer~itb de Namer 1. BP 104. F- 54600 Villers- les- Namer. France
T. C E V A a n d C. M A R T I Groupe de Physique des Sohdes de I'Ecole Normale Supbrieure. l,ahoratoire associb au ('N RS, Tour 23. 2 Place Jussieu. F-75251 Paris ('&dex 05. France
and J. M E N A U C O U R T
a n d A. T H O M Y
('NRS. Laboratoire Maurtce Letort. associi, it I'Unu,ersttO de Nam T 1. BP 104. F- 546(X) Villers- les- Nano'. France
Received 1 April 1985: accepted for publication 4 April 1985
SF~, film adsorbed on the (0001) face of graphite has been studied by a volumetric technique between 120 and 180 K. It is characterized by a single monomolecular layer as it was also observed on the densest face of boron nitride and several metals (('u, Cd, Zn). It is shown by X-ray diffraction that the monolayer is a 2D solid, the structure of which differs appreciably from that of any plane of the 31) crystal of St~. These results confirm that, due to this mismatch, the 3D crystal cannot grow layer by layer over the whole uniform part of the surface.
i. Introduction S u l f u r h e x a f l u o r i d e b e i n g a highly stable m o l e c u l e , n o n - p o l a r a n d v e r y s y m m e t r i c a l , is e s p e c i a l l y i n t e r e s t i n g for the s t u d y of i n t e r m o l e c u l a r forces for i n s t a n c e in critical p h e n o m e n a (cf. ref. [1]). It is to be n o t e d that a l t h o u g h S F 6 is a g l o b u l a r a n d n e a r l y s p h e r i c a l m o l e c u l e . It d o e s n o t crystallize in the face c e n t r e d c u b i c system, c o n t r a r y to m e t h a n e a n d rare gases, but in the c e n t r e d c u b i c s y s t e m a b o v e 93 K o r in the o r t h o r h o m b i c s y s t e m b e l o w 93 K. M o r e o v e r , the r a t i o of the critical to the t r i p l e p o i n t t e m p e r a t u r e ( T i T t = 1.4) is a p p r e c i a b l y s m a l l e r t h a n for m e t h a n e 0 0 3 9 - 6 0 2 8 / 8 5 / $ 0 3 . 3 0 ',~(':Elsevier S c i e n c e P u b l i s h e r s B.V. ( N o r t h - H o l l a n d Physics P u b l i s h i n g D i v i s i o n )
M. Bouchdoug et aL / Incomplete wetting of uniform surfaces by SF~
e(coverage) . . . .
D
BI
~C(3D) . . . . . . . . .
A -4
P(Torr)
I -3
427
-2
-I
0
Fig. 1. ~ h e m a t i c SF6 adsorption isotherms on graphite (after ref. [6]); (1) 125 K; (2) 130 K; (3) 145 K; (4) 160 K; (5) 180 K. 8: fractional surface coverage (8 = 1 at B1). C ( 3 D ) o r Po axis: 3D condensation under the saturating vapour pressure P0- Isotherms with the same characteristics have been obtained on powdered boron nitride, cupper, cadmium and zinc, the surface of which exposes mainly faces of highest density [4-6].
(2.2) or rare gases without quantum effect (1.8 for argon, krypton and xenon). It is therefore interesting to study the adsorption behaviour of SF6 as compared to the above mentioned gases. We are here especially concerned with epitaxy and wetting phenomena which have been a subject of increasing interest since a few years (see refs. [2,3]) for a recent bibliography). Adsorption isotherms have been determined on several crystallized solids: copper, cadmium, zinc [4], graphite [5], boron nitride [6]. Their surfaces are very homogeneous, exposing essentially the densest face with hexagonal structure [(111) face in the case of copper, (0001) face in the other cases]. All the isotherms have been determined below the 3D triple point of SF6. Their common feature is to show a single step (fig. 1) at any temperature (120-.180 K in the case of graphite and boron nitride). This was interpretated as follows. Since SF6 crystallizes in the centred cubic system at the isotherm temperatures, there is no crystal plane favoured on the surfaces considered, their structure being hexagonal. In other words, the monolayer structure should be different from that of any plane of the 3D crystal. Consequently, the 3D crystal cannot grow layer by layer contrary to argon, krypton and xenon below their 3D triple point [7] and presumably also to methane. In order to confirm this interpretation, we tried to get known the structure of the monolayer in the case of graphite. The determination of isotherms by a volumetric technique on exfoliated graphite of homogeneous surface allowed us to get known the density of the film corresponding to given stages of its formation. Its structure has been obtained by X-ray diffraction using Papyex as adsorbent. Before giving the results, some relevant data concerning SF6 are recalled as well as a brief description of the adsorbents and the techniques is given.
428
M. Bouchdoug et aL / Incomplete wetting of untform surfaces hv Sty,
2. Thermodynamical and structural data of SI,~,
Triple point and critical temperatures: T~(3D) = 222.3 K; T~.(3D) = 318.7 K. Saturating vapour pressure (P.): Dependence on temperature between 120 and 180K: Ioglo l ~ ( T o r r ) = - 1 2 6 5 / ' I ' + 8 . 9 5 5 5
(after ref. [13]).
Crystalline structure: between 93 and 222.3 K, SF~, crystallizes in the centred cubic system with a = 5.915 A; cross section of a molecule in the densest plane: os~,(110)= 24.75 A 2 (of. ref. [8]). 3. Gas puri~'
Better than 99.9% for the two types of experiments (volumetry and X-ray diffraction). 4. Graphites
For the determination of isotherms: exfoliated graphite of homogeneous surface (specific surface area: about 50 m -~ g - l ; sample surface area: 1 2 m:) (of. ref. [91). For the determination of spectra: compressed exfoliated graphite provided by " L e Carbonc Lorraine" (Papyex, specific surface area: 20 m 2 per gram or cm 3 (of. ref. [10]). The samples (ca. 50 m 2) were prepared and set as described in refs. [11,12]. It is noteworthy that SF~, isotherms obtained on Papycx are nearly the same as those obtained on exfoliated graphite [6]. ()utgassing: The samples were baked at 800 or 14000( ` and outgassed until reaching a dynamical pressure lower than 1 0 4 Torr. 5. Apparatus and methods
Adsorption isotherms have been determined with a classical apparatus by discontinuous introduction of the adsorbate [9]. Spectra have been obtained with an apparatus derived fro those described in refs. [11,12]. They are given after subtracting the graphite background. The gas was introduced in the adsorption cell, 1/6th of its total volume (16 cm ~) being at the temperature of the isotherm. 6. SF 6 cross section in the monolayer as deduced from the isotherms
Let us call B I the point of inflection of the plateau between the vertical part of the isotherms and the /~ axis (fig. 1). This point has been assumed [5] to
M. Bouchdoug et al. / Incomplete wetting of uniform surfaces b.v SF~
429
correspond to the monolayer completion (0 = 1). Point B~ is well defined by its ordinate for temperatures of about 125 K. The adsorbed quantity corresponding to this point on the 124.6 K isotherm - Nn,(SF 6, 125 K) - compared to the krypton quantity adsorbed at the B~ point on the 77,3 K isotherm-- Nn.(Kr, 77 K) on the same graphite sample, leads to: Nm(SF 6, 125 K ) / N B , ( K r , 77 K) = 0.60 _+ 0.01 (the value found with Papyex is the same within the experimental uncertainty). Then: o m ( K r , 77 K ) / o B , ( S F 6, 125 K) = 0.60 + 0.01, o being the molecular cross section. Given o~.(Kr, 77 K ) = 14.7 + 0.35 A 2 molecule t (cf. ref. [9]), we obtain: on~(SF6, 125 K) = 24.5 +_ 1.0 A,2 molecule -1, compared to Os~,(ll0)= 24.75 A2 (section 2), this value would lead to the conclusion that B I actually corresponds to the monolayer completion as assumed in ref. [5]. In fact, it will be shown that the isotherm region corresponding to the monolayer completion is probably located nearer to the saturating vapor pressure P0.
7. Structure of the monolayer as deduced from X-ray diffraction
7.1. Spectra for 0 = 1.14 (23.0 + 0.4)× 1019 SF6 molecules have been introduced in the adsorption cell corresponding to 0 = 1.14 + 0.05. Given the large value of the surface area of the sample, this quantity corresponds to a fractional coverage constant within 0.5% below 140 K. From 95 to 140 K (which is the highest temperature considered) spectra are similar to curve a in fig. 2: they exhibit a main peak ( a peak) and a smaller second one (/3 peak) corresponding respectively to X = 1.45 _+ 0.02 ,~, 1 and X = 1.51 +_ 0.02 ~,- I at 124 K. Such spectra could be considered as characteristic of a 2D solid in which three neighbour admolecules would form an isosceles triangle with 4.8, 4.8 and 5.0 ~, sides at 124 K, the admolecule cross section being 20.5 ,~2. But in fact, the/3 peak certainly corresponds to SF6 3D crystallites formed on the surface in the neighbourhood of defects. Such crystallites could effectively form due to the 0 value considered. Moreover it has been shown by recent experiments that the /3 peak becomes more and more important with increasing O, while the a peak does not change. Consequently, when saturated, the monolayer is certainly a 2D solid with
M. Bouchdoug et al. / Incomplete wetting of uniform surface~ hi" Sl'~,
430
C0001) cjra phile face ,f o~
,3 4.92A
2) "0
2
3
0
(b)
71 1.0
1.5
2.0
Fig. 2. X-ray diffraction profiles corresponding to the SF¢, film adsorbed on graphite: (a) 0 :- 1.14 at T = 1 2 4 K; (b) 0 = 0 . 6 3 at I " = 1 2 0 K; (c) 0 ,- 0.63 at T = 1 3 0 K. x ( d i f f u s i o n v e c t o r } = (47r/)~) sin c< *k = 1.5418 A: a: B r a g g angle. In the case of a 2 D h e x a g o n a l structure, d = 4 ~ r / x j ' 3 and o = d 2 ~ / 3 / 2 , d being the distance between two nearest neighbour molecules and o the corresponding cross section assuming that there are no vacancies.
hexagonal structure in which the distance between two neighbour admolecules is: d]14 = 5.00 _+ 0.05 ,& around 125 K, the corresponding cross section being: o1.14 = 21.7 _+ 0.5 ,~2 per admolecule. Thus, whatever the interpretation of a-spectra the complete monolayer has to be considered as an incommensurate 2D solid, the structure of which is different from that of any plane in the SF6 2D crystal, namely from a (110) plane. It is to be noted that the difference between the admolecule cross section values deduced respectively from spectra and from isotherms is at least 6% This difference may be explained as follows. Either there are at least 6% vacancies at B l, or this point does not exactly correspond to the monolayer completion. The examination of spectra relative to 0.63 will enable us to favour the last explanation.
M. Bouchdoug et al. / Incomplete wetting of uniform surfaces by SF~
431
7. 2. Spectra f o r 0 = O. 63
In this case (12.7 + 0.2) 1019 S F6 molecules have a d s o r p t i o n cell. A single a s y m m e t r i c a l peak is observed which is hexagonal structure (curves b and c in fig. 2). W h e n ture, this peak shifts t o w a r d s small angles (decreasing shape. At a b o u t 125 K, it c o r r e s p o n d s to: X = 1 . 4 0 ± 0 . 0 5 , h t -1
been i n t r o d u c e d in the characteristic of a 2 D increasing the t e m p e r a X), without a c h a n g e in
d063=5.lSq--0.05A,
and if a s s u m i n g that there are no vacancies: o0.6.~ --- 23.2 + 0.5 A,2. This 00.63 value overlaps oB, within the e x p e r i m e n t a l uncertainties. This tends to show that the isotherm part between D and B t c o r r e s p o n d s to the filling of vacancies in the solid which forms in the transition (part A D in fig. 1), and that the m o n o l a y e r s a t u r a t i o n has to be located at higher 0. nearer to the P0 axis.
8. Conclusion W e think that o u r results o b t a i n e d with g r a p h i t e can be e x t e n d e d to the o t h e r m e n t i o n e d substrates (BN, Cu, Cd, Zn) for which such a detailed study c a n n o t be w o r k e d out. It is likely that for all these cases, the structure of the single m o n o l a y e r is different from that of any p l a n e of the 3D crystal of S F 6 which prevents its growth layer by layer over the whole u n i f o r m part of the surface.
References [1] S.N. Biswas, N.J. Trappeniers and J.H.B. Hoogland, Physica 126A (1984) 384. [21 J.M. Gay, M. Bienfait and J. Suzanne, J. Physique 45 (1984) 1497. [3] 1. Bassignana and Y. Larher, Surface Sci. 147 (1984) 48. [4] B. Genot and X. Duval, J. Chim. Physique 69 (1972) 1238; 70 (1973) 134. [5] M. Bouchdoug, J. Menaucourt and A. Thomy, J. Chim. Physique 81 (1984) 381. [61 M. Bouchdoug, unpublished results. [71 J.A. Venables, J.L. Seguin. J. Suzanne and M. Bienfait, Surface Sci. 145 (1984) 345. [8] J.C. Taylor and A.B. Waugh, J. Solid State Chem. 18 (1976) 241. [9] A. Thomy, X. Duval and J. Regnier, Surface Sci. Rept. 1 (1981) 1. [10] C. BCxzkel,J.P. Coulomb and N. Dupont-Pavlovsky, Surface Sci. 116 (1982) 369. I11] T. Ceva and C. Marti, J. Physique Lenres 39 (1978) L22. 112] M. Goldmann, Th~se de 3e Cycle, Universit~ de Paris VII (1983). [13] B. Genot, J. Chim. Physique 68 (1971) 111.
Surface Science 162 (1985)426-431 North-Holland, Amsterdam
I N C O M P L E T E W E T r i N G OF U N I F O R M S U R F A C E S BY S U L F U R HEXAFLUORIDE M. B O U C H D O U G ('NRS. Laboratoire Maurice Letort. associb it I'unieer~itb de Namer 1. BP 104. F- 54600 Villers- les- Namer. France
T. C E V A a n d C. M A R T I Groupe de Physique des Sohdes de I'Ecole Normale Supbrieure. l,ahoratoire associb au ('N RS, Tour 23. 2 Place Jussieu. F-75251 Paris ('&dex 05. France
and J. M E N A U C O U R T
a n d A. T H O M Y
('NRS. Laboratoire Maurtce Letort. associi, it I'Unu,ersttO de Nam T 1. BP 104. F- 546(X) Villers- les- Nano'. France
Received 1 April 1985: accepted for publication 4 April 1985
SF~, film adsorbed on the (0001) face of graphite has been studied by a volumetric technique between 120 and 180 K. It is characterized by a single monomolecular layer as it was also observed on the densest face of boron nitride and several metals (('u, Cd, Zn). It is shown by X-ray diffraction that the monolayer is a 2D solid, the structure of which differs appreciably from that of any plane of the 31) crystal of St~. These results confirm that, due to this mismatch, the 3D crystal cannot grow layer by layer over the whole uniform part of the surface.
i. Introduction S u l f u r h e x a f l u o r i d e b e i n g a highly stable m o l e c u l e , n o n - p o l a r a n d v e r y s y m m e t r i c a l , is e s p e c i a l l y i n t e r e s t i n g for the s t u d y of i n t e r m o l e c u l a r forces for i n s t a n c e in critical p h e n o m e n a (cf. ref. [1]). It is to be n o t e d that a l t h o u g h S F 6 is a g l o b u l a r a n d n e a r l y s p h e r i c a l m o l e c u l e . It d o e s n o t crystallize in the face c e n t r e d c u b i c system, c o n t r a r y to m e t h a n e a n d rare gases, but in the c e n t r e d c u b i c s y s t e m a b o v e 93 K o r in the o r t h o r h o m b i c s y s t e m b e l o w 93 K. M o r e o v e r , the r a t i o of the critical to the t r i p l e p o i n t t e m p e r a t u r e ( T i T t = 1.4) is a p p r e c i a b l y s m a l l e r t h a n for m e t h a n e 0 0 3 9 - 6 0 2 8 / 8 5 / $ 0 3 . 3 0 ',~(':Elsevier S c i e n c e P u b l i s h e r s B.V. ( N o r t h - H o l l a n d Physics P u b l i s h i n g D i v i s i o n )
M. Bouchdoug et aL / Incomplete wetting of uniform surfaces by SF~
e(coverage) . . . .
D
BI
~C(3D) . . . . . . . . .
A -4
P(Torr)
I -3
427
-2
-I
0
Fig. 1. ~ h e m a t i c SF6 adsorption isotherms on graphite (after ref. [6]); (1) 125 K; (2) 130 K; (3) 145 K; (4) 160 K; (5) 180 K. 8: fractional surface coverage (8 = 1 at B1). C ( 3 D ) o r Po axis: 3D condensation under the saturating vapour pressure P0- Isotherms with the same characteristics have been obtained on powdered boron nitride, cupper, cadmium and zinc, the surface of which exposes mainly faces of highest density [4-6].
(2.2) or rare gases without quantum effect (1.8 for argon, krypton and xenon). It is therefore interesting to study the adsorption behaviour of SF6 as compared to the above mentioned gases. We are here especially concerned with epitaxy and wetting phenomena which have been a subject of increasing interest since a few years (see refs. [2,3]) for a recent bibliography). Adsorption isotherms have been determined on several crystallized solids: copper, cadmium, zinc [4], graphite [5], boron nitride [6]. Their surfaces are very homogeneous, exposing essentially the densest face with hexagonal structure [(111) face in the case of copper, (0001) face in the other cases]. All the isotherms have been determined below the 3D triple point of SF6. Their common feature is to show a single step (fig. 1) at any temperature (120-.180 K in the case of graphite and boron nitride). This was interpretated as follows. Since SF6 crystallizes in the centred cubic system at the isotherm temperatures, there is no crystal plane favoured on the surfaces considered, their structure being hexagonal. In other words, the monolayer structure should be different from that of any plane of the 3D crystal. Consequently, the 3D crystal cannot grow layer by layer contrary to argon, krypton and xenon below their 3D triple point [7] and presumably also to methane. In order to confirm this interpretation, we tried to get known the structure of the monolayer in the case of graphite. The determination of isotherms by a volumetric technique on exfoliated graphite of homogeneous surface allowed us to get known the density of the film corresponding to given stages of its formation. Its structure has been obtained by X-ray diffraction using Papyex as adsorbent. Before giving the results, some relevant data concerning SF6 are recalled as well as a brief description of the adsorbents and the techniques is given.
428
M. Bouchdoug et aL / Incomplete wetting of untform surfaces hv Sty,
2. Thermodynamical and structural data of SI,~,
Triple point and critical temperatures: T~(3D) = 222.3 K; T~.(3D) = 318.7 K. Saturating vapour pressure (P.): Dependence on temperature between 120 and 180K: Ioglo l ~ ( T o r r ) = - 1 2 6 5 / ' I ' + 8 . 9 5 5 5
(after ref. [13]).
Crystalline structure: between 93 and 222.3 K, SF~, crystallizes in the centred cubic system with a = 5.915 A; cross section of a molecule in the densest plane: os~,(110)= 24.75 A 2 (of. ref. [8]). 3. Gas puri~'
Better than 99.9% for the two types of experiments (volumetry and X-ray diffraction). 4. Graphites
For the determination of isotherms: exfoliated graphite of homogeneous surface (specific surface area: about 50 m -~ g - l ; sample surface area: 1 2 m:) (of. ref. [91). For the determination of spectra: compressed exfoliated graphite provided by " L e Carbonc Lorraine" (Papyex, specific surface area: 20 m 2 per gram or cm 3 (of. ref. [10]). The samples (ca. 50 m 2) were prepared and set as described in refs. [11,12]. It is noteworthy that SF~, isotherms obtained on Papycx are nearly the same as those obtained on exfoliated graphite [6]. ()utgassing: The samples were baked at 800 or 14000( ` and outgassed until reaching a dynamical pressure lower than 1 0 4 Torr. 5. Apparatus and methods
Adsorption isotherms have been determined with a classical apparatus by discontinuous introduction of the adsorbate [9]. Spectra have been obtained with an apparatus derived fro those described in refs. [11,12]. They are given after subtracting the graphite background. The gas was introduced in the adsorption cell, 1/6th of its total volume (16 cm ~) being at the temperature of the isotherm. 6. SF 6 cross section in the monolayer as deduced from the isotherms
Let us call B I the point of inflection of the plateau between the vertical part of the isotherms and the /~ axis (fig. 1). This point has been assumed [5] to
M. Bouchdoug et al. / Incomplete wetting of uniform surfaces b.v SF~
429
correspond to the monolayer completion (0 = 1). Point B~ is well defined by its ordinate for temperatures of about 125 K. The adsorbed quantity corresponding to this point on the 124.6 K isotherm - Nn,(SF 6, 125 K) - compared to the krypton quantity adsorbed at the B~ point on the 77,3 K isotherm-- Nn.(Kr, 77 K) on the same graphite sample, leads to: Nm(SF 6, 125 K ) / N B , ( K r , 77 K) = 0.60 _+ 0.01 (the value found with Papyex is the same within the experimental uncertainty). Then: o m ( K r , 77 K ) / o B , ( S F 6, 125 K) = 0.60 + 0.01, o being the molecular cross section. Given o~.(Kr, 77 K ) = 14.7 + 0.35 A 2 molecule t (cf. ref. [9]), we obtain: on~(SF6, 125 K) = 24.5 +_ 1.0 A,2 molecule -1, compared to Os~,(ll0)= 24.75 A2 (section 2), this value would lead to the conclusion that B I actually corresponds to the monolayer completion as assumed in ref. [5]. In fact, it will be shown that the isotherm region corresponding to the monolayer completion is probably located nearer to the saturating vapor pressure P0.
7. Structure of the monolayer as deduced from X-ray diffraction
7.1. Spectra for 0 = 1.14 (23.0 + 0.4)× 1019 SF6 molecules have been introduced in the adsorption cell corresponding to 0 = 1.14 + 0.05. Given the large value of the surface area of the sample, this quantity corresponds to a fractional coverage constant within 0.5% below 140 K. From 95 to 140 K (which is the highest temperature considered) spectra are similar to curve a in fig. 2: they exhibit a main peak ( a peak) and a smaller second one (/3 peak) corresponding respectively to X = 1.45 _+ 0.02 ,~, 1 and X = 1.51 +_ 0.02 ~,- I at 124 K. Such spectra could be considered as characteristic of a 2D solid in which three neighbour admolecules would form an isosceles triangle with 4.8, 4.8 and 5.0 ~, sides at 124 K, the admolecule cross section being 20.5 ,~2. But in fact, the/3 peak certainly corresponds to SF6 3D crystallites formed on the surface in the neighbourhood of defects. Such crystallites could effectively form due to the 0 value considered. Moreover it has been shown by recent experiments that the /3 peak becomes more and more important with increasing O, while the a peak does not change. Consequently, when saturated, the monolayer is certainly a 2D solid with
M. Bouchdoug et al. / Incomplete wetting of uniform surface~ hi" Sl'~,
430
C0001) cjra phile face ,f o~
,3 4.92A
2) "0
2
3
0
(b)
71 1.0
1.5
2.0
Fig. 2. X-ray diffraction profiles corresponding to the SF¢, film adsorbed on graphite: (a) 0 :- 1.14 at T = 1 2 4 K; (b) 0 = 0 . 6 3 at I " = 1 2 0 K; (c) 0 ,- 0.63 at T = 1 3 0 K. x ( d i f f u s i o n v e c t o r } = (47r/)~) sin c< *k = 1.5418 A: a: B r a g g angle. In the case of a 2 D h e x a g o n a l structure, d = 4 ~ r / x j ' 3 and o = d 2 ~ / 3 / 2 , d being the distance between two nearest neighbour molecules and o the corresponding cross section assuming that there are no vacancies.
hexagonal structure in which the distance between two neighbour admolecules is: d]14 = 5.00 _+ 0.05 ,& around 125 K, the corresponding cross section being: o1.14 = 21.7 _+ 0.5 ,~2 per admolecule. Thus, whatever the interpretation of a-spectra the complete monolayer has to be considered as an incommensurate 2D solid, the structure of which is different from that of any plane in the SF6 2D crystal, namely from a (110) plane. It is to be noted that the difference between the admolecule cross section values deduced respectively from spectra and from isotherms is at least 6% This difference may be explained as follows. Either there are at least 6% vacancies at B l, or this point does not exactly correspond to the monolayer completion. The examination of spectra relative to 0.63 will enable us to favour the last explanation.
M. Bouchdoug et al. / Incomplete wetting of uniform surfaces by SF~
431
7. 2. Spectra f o r 0 = O. 63
In this case (12.7 + 0.2) 1019 S F6 molecules have a d s o r p t i o n cell. A single a s y m m e t r i c a l peak is observed which is hexagonal structure (curves b and c in fig. 2). W h e n ture, this peak shifts t o w a r d s small angles (decreasing shape. At a b o u t 125 K, it c o r r e s p o n d s to: X = 1 . 4 0 ± 0 . 0 5 , h t -1
been i n t r o d u c e d in the characteristic of a 2 D increasing the t e m p e r a X), without a c h a n g e in
d063=5.lSq--0.05A,
and if a s s u m i n g that there are no vacancies: o0.6.~ --- 23.2 + 0.5 A,2. This 00.63 value overlaps oB, within the e x p e r i m e n t a l uncertainties. This tends to show that the isotherm part between D and B t c o r r e s p o n d s to the filling of vacancies in the solid which forms in the transition (part A D in fig. 1), and that the m o n o l a y e r s a t u r a t i o n has to be located at higher 0. nearer to the P0 axis.
8. Conclusion W e think that o u r results o b t a i n e d with g r a p h i t e can be e x t e n d e d to the o t h e r m e n t i o n e d substrates (BN, Cu, Cd, Zn) for which such a detailed study c a n n o t be w o r k e d out. It is likely that for all these cases, the structure of the single m o n o l a y e r is different from that of any p l a n e of the 3D crystal of S F 6 which prevents its growth layer by layer over the whole u n i f o r m part of the surface.
References [1] S.N. Biswas, N.J. Trappeniers and J.H.B. Hoogland, Physica 126A (1984) 384. [21 J.M. Gay, M. Bienfait and J. Suzanne, J. Physique 45 (1984) 1497. [3] 1. Bassignana and Y. Larher, Surface Sci. 147 (1984) 48. [4] B. Genot and X. Duval, J. Chim. Physique 69 (1972) 1238; 70 (1973) 134. [5] M. Bouchdoug, J. Menaucourt and A. Thomy, J. Chim. Physique 81 (1984) 381. [61 M. Bouchdoug, unpublished results. [71 J.A. Venables, J.L. Seguin. J. Suzanne and M. Bienfait, Surface Sci. 145 (1984) 345. [8] J.C. Taylor and A.B. Waugh, J. Solid State Chem. 18 (1976) 241. [9] A. Thomy, X. Duval and J. Regnier, Surface Sci. Rept. 1 (1981) 1. [10] C. BCxzkel,J.P. Coulomb and N. Dupont-Pavlovsky, Surface Sci. 116 (1982) 369. I11] T. Ceva and C. Marti, J. Physique Lenres 39 (1978) L22. 112] M. Goldmann, Th~se de 3e Cycle, Universit~ de Paris VII (1983). [13] B. Genot, J. Chim. Physique 68 (1971) 111.