FORCES BETWEEN NOVEL SURFACES.
7 2 References.
1. M J. Spamaay, Physica 24, 751 (1958).
2. P.H.G.M. Van Blokland and J.Th.G. Overbeek, J. Chem. Soc. Faraday Trans. 1
74, 2637 (1978).
3. B.V. Deijaguin, T.N. Voropayeva, B.N. Kabanov and A.S. Titiyevskaya, J.
Colloid Sei. 19,113 (1964).
4. C.J. Coakley and D. Tabor, J. Phys. D 11.L78 (1978).
5. F. F. Fan and A. J. Bard, J. Amer. Chem. Soc. 109,6262(1987).
6. R. B. Morris, D. J. Franta and H. S. White, J. Phys. Chem. 91,3559(1987). 7. C. P. Smith, M. Maeda, L. Atanasoska, H. S. White and D. J. McClure, J. Phys.
Chem. 92,199(1988).
8. L. Holland, “Vacuum depsosition of thin films” John Wiley and Sons, New York 1960.
9. H. Jaeger, P. D. Mercer and R. G. Sherwood, Surface Sei. 11,265(1968). 10. H. Jaeger, P. D. Mercer and R. G. Sherwood, Surface Sei. 13,349(1969). 11. M. T. Clarkson, J. Phys. D: Appl. Phys. 22,475(1989).
12. J. N. Israelachvili, J. Colloid Interface Sei. 44,259(1973).
13. H.K. Christenson and C.E. Blom, J. Chem. Phys. 86, 419 (1987). 14. H.K. Christenson, J. Colloid Interface Sei. 104, 234 (1985).
15. H.K. Christenson, J. Phys. Chem. 90, 4 (1986).
16. V.A. Parsegian, G.H. Weiss and M.E. Schrader, J. Colloid Interface Sei. 61, 356 (1977).
73
5. Plasma modification o f mica.
C ovalent m odification of n a tu r a l m ica is ex trem ely difficult b ec a u se th ere are no accessible reactive sites on th e su rface. A te ch n iq u e w hich can create reactive sites on m ica w ould allow the p rep aratio n of a wide range of su rfa ce s u sin g w ell-know n coupling ag en t ch em istry . E x p o su re of m ica to w ater v apour p lasm a produces su c h a surface.
5.1 The plasma technique.
A low te m p e ra tu re or cold p la sm a is a g aseo u s com plex th a t is com posed of electrons, ions (positive an d negative), free ra d ic a ls, excited an d ground s ta te m olecules an d ato m s. The p la sm a is created by applying a stro n g radiofrequency electric field to a gas a t low p re ssu re . At su fficie n t field s tre n g th s electrical discharge occurs and a p lasm a is formed. O nce created th e p la sm a ca n be m aintained a t lower electric fields du e to its s e lf - s u s ta in in g c h a ra c te ris tic s . E le c tro n s in th is ty p e of n o n e q u ilib riu m p la sm a m ay have high energies (the electro n te m p e ra tu re is high) w hereas th e o th e r species are m u c h less energetic (their te m p e ra tu re is n e a r or slightly above am bient). All excited atom s, m olecules, ions an d free rad icals are referred to a s reactive species. These reactive species a n d high energy p h o to n s em itted d u rin g de-excitation of excited m olecules will in te ra c t w ith solid surfaces exposed to th e plasm a. The following processes m ay occur a t th e surface (see fig 1):
1) A reactive surface site m ay be created by charge tra n sfe r or by photolysis.
Incident energetic species Polymerization SURFACE Implantation - Q Reflection
O
Secondary o Electrons Sputtering Q AdsorptionFigure (1). The processes occurring at a surface after impact of energetic species are shown schematically.
7 4
2) Reactive species in the p lasm a m ay chem ically re a c t w ith the surface to form new functional groups.
3) E nergetic bom bardm ent m ay ca u se s tru c tu ra l ch an g es in th e surface of the target m aterial.
4) E nergetic b o m b ard m en t m ay lead to th e ejection of a ta rg e t atom , commonly referred to as sp u tterin g .
5) The reactive species m ay becom e b u ried or im p lan ted in th e su rfa c e . T his p ro c e ss is u s e d e x ten siv e ly in th e electronics in d u stry to dope silicon w afers.
6) T hin films m ay form th ro u g h p la sm a polym erization or re d e p o s itio n of s p u tte re d m a te ria l. In p a r tic u la r , th in polym eric films can be deposited on su rfa ce s by tre a tm e n t w ith th e p lasm a created from organic gases.
T he d esire d type of su rfa c e m o d ificatio n d ic ta te s th e choice of gas u se d to form th e p la sm a . In e rt g ases s u c h as h e liu m a n d arg o n s p u tte r an d a c tiv a te th e su rfa c e . P la sm a form ed from oxygen, w ater v ap o u r a n d am m o n ia s p u tte r an d a lte r th e ch em ical com position of th e s u rfa c e by c o v a len t in tro d u c tio n of fu n ctio n a l groups. T h in polym er film s c a n be fo rm e d b y u s in g o rg an ic g a s e s s u c h a s h y d ro c a rb o n s , flu o ro c arb o n s an d siloxanes. The e x te n t to w hich one p la sm a p ro c e ss o ccu rs w ith resp ect to th e o th e rs ca n be controlled by th e p ro cess p a ra m e te rs (power, gas flow ra te , a n d p re ssu re ). The reacto r design (the area to volum e ratio an d th e shape), an d th e p o sitio n of th e sam ple in th e re a c to r also in flu en ce th e ex ten t an d type of surface modification.
E x p lo itatio n of th e p lasm a p ro cess h a s proven to be a pow erful tech n iq u e for surface m odification of v arious m a terials in c lu d in g m in e ra ls, p ap e r, m etals a n d
polymers.
2 -5 . a m ajor7 5
advantage of the plasm a process is th a t it is a dry, relatively fast m ethod. U sually only a few m in u tes tre a tm e n t tim e is req u ired a n d no tim e co n su m in g drying of th e m odified s u rfa c e s is needed. P lasm a tre a tm e n t is ideal for p rep arin g novel su rfaces for direct force m e asu rem e n t b ec au se th e tre a tm e n t does n ot a lte r th e b u lk p ro p erties of th e s u b s tr a te . U n d er carefu lly cho sen conditions only m inor changes in surface topology occur a n d th e high degree of sm o o th n ess req u ired for su rfa c e force m e a s u re m e n t c a n be m a in ta in ed . M oreover, th e p ro c e s s is co n d u cted in a v acu u m environm ent a n d th u s co n tam in atio n of th e su rface is k ep t to a m inim um . The low te m p e ra tu re p la sm a technique h a s been u sed to modify m ica surfaces in th ree ways:
i) Mica su rfaces were exposed to a polym er-form ing p la sm a an d th in polym er films deposited.
ii) F unctional groups were introduced by exposing m ica surfaces to a non-polym er forming plasm a.
iii) P lasm a activated mica surfaces were reacted w ith chlorosilanes.
5.2 Materials and method.
The p la sm a re a c to r is show n sc h em a tica lly in fig. 2. It c o n sists of a large glass vessel w hich is evacuated w ith a ro tary v a c u u m p u m p to 10"4 to rr. Two co p p er b a n d s a ro u n d th e o u tsid e of th e re a c to r are co n n e cte d to a h ig h voltage RF g en e ra to r (ENI power system s m odel HP G-2 ). The flow ra te of th e gas is controlled w ith a flow controller (LH, HI-TEC). A large s h e e t of freshly cleaved m ica is placed in th e p la sm a re a c to r