SURFACE REFLECTION STUDIES AT ISIS ON THE

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Colloque C7, supplliment au nO1O, Tome 50, octobre 1989. SURFACE REFLECTION STUDIES AT ISIS ON THE REFLECTOMETER, CRISP. J. PENFOLD.
COLLOQUE DE PHYSIQUE C o l l o q u e C7, s u p p l l i m e n t a u n O 1 O , Tome 50, octobre 1989

SURFACE REFLECTION STUDIES AT ISIS ON THE REFLECTOMETER, CRISP J. PENFOLD

Neutron Science Division, Rutherford Appleton Laboratory. Chilton, Didcot, GB-Oxon OX11 OQX, Great-Britain

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Abstract The s u r f a c e reflectometer, CRISP, on t h e ISIS pulsed neutron source i s described. Recent developments i n t h e a n a l y s i s of r e f 1 e c t i . v i . t ~p r o f i l e s i s discussed. Some recent examples of t h e a p p l i c a t i o n of t h e technique from t h e extensive s c i e n t i f i c programme i n s u r f a c e chemistry, s u r f a c e magnetism and s o l i d f i l m s a r e presented. RBsumd - On dBcrit l e rdflectomhtre d e surface,CRISP i n s t a l l 6 s u r l a source d e neutrons pulsBs I S I S e t l ' o n d i s c u t e l e s dBveloppements r b c e n t s dans l ' a n a l y s e d e s p r o f i l s de r d f l e c t i v i t b . 0 n prdsente dgalement quelques exemples rbcents d e l ' a p p l i c a t i o n d e c e t t e technique 5 d e s programmes s c i e n t i f i q u e s importants en chirnie de surface, magnstisme d e s u r f a c e e t dans l ' b t u d e d e s couches minces. 1

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INTRODUCTION

Most of the common o p t i c a l phenomena, including r e f r a c t i o n , r e f l e c t i o n and i n t e r f e r e n c e , have been observed with slow neutrons (1). Since t h e f i r s t measurements of t o t a l r e f l e c t i o n by Fermi and coworkers (2), it has been e x t e n s i v e l y used i n neutron p o l a r i s e r s (3) and neutron guides ( 4 ) . I n recent years, however, a t t e n t i o n has focused on t h e a p p l i c a t i o n of specular r e f l e c t i o n of neutrons t o study s u r f a c e and i n t e r f a c i a l problems. It was shown by Thomas and coworkers (5) t h a t neutron r e f l e c t i o n experiments give information about t h e neutron r e f r a c t i v e index p r o f i l e normal t o t h e surface, and t h a t a judicious use of hydrogen/deuterium c o n t r a s t can provide unique information f o r a range of problems i n surface chemistry. Due t o t h e magnetic d i p o l e i n t e r a c t i o n , magnetic m a t e r i a l s e x h i b i t a neutron s p i n dependent r e f r a c t i v e index ( t h i s i s t h e b a s i s of neutron s p i n p o l a r i s e r s using c r i t i c a l r e f l e c t i o n ) and Felcher (6) has shown t h a t t h e specular r e f l e c t i o n of s p i n p o l a r i s e d neutrons i s a p a r t i c u l a r l y s e n s i t i v e probe of s u r f a c e magnetism. The advent of dedicated spectrometers (7,8) has been accompanied by a r a p i d expansion i n t h e s c i e n t i f i c a p p l i c a t i o n of t h e technique t o s u r f a c e chemistry (9) , surface magnetism (10) and s o l i d f i l m s (11). I n t h i s p r e s e n t a t i o n one of t h e s e (7) new breed of dedicated reflectometers, CRISP ( s i t u a t e d on t h e ISIS pulsed neutron source) w i l l be described. Recent advances i n t h e adaption of m u l t i l a y e r o p t i c a l methods t o t h e modelling and i n t e r p r e t a t i o n of neutron r e f l e c t i v i t i e s i s discussed. Finally, some recent examples of s t u d i e s i n surface chemistry, s o l i d f i l m s and s u r f a c e magnetism from t h e extensive s c i e n t i f i c programme on CRISP w i l l be presented. 2

- THEORETICAL DEVELOPMENTS

It has been shown that. t h e i n t e n s i t y of t h e r e f l e c t e d and t r a n s m i t t e d neutrons follows t h e same laws a s electromagnetic r a d i a t i o n with t h e e l e c t r i c vector perpendicular t o t h e plane of incidence (12). Hence t h e r e f r a c t i v e index a t t h e boundary between two media i s defined i n t h e usual way as,

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989709

where k ,k are t h e neutron wave vectors inside and outside t h e medium. index i h c8mmonly written as

The refractive

where = Nb/2n, C = Naa/4nl N i s t h e atomic number density, b i s t h e bound coherent scattering length, a, 1s the adsorption cross section and 1 i s t h e neutron wavelength.

A

Accordingly neutron reflection data has been analysed using standard o p t i c a l equations (for example, Fresnels law) and methods developed f o r multilayer o p t i c s (13). In particular, t h e matrix method of Born and Wolf ( 1 4 ) provides a convenient framework t o calculate r e f l e c t i v i t i e s f o r systems t h a t can be t r e a t e d a s a s e r i e s of discreet layers. However, f o r multilayer systems where t h e interfaces a r e non-ideal (diffuse or roughened) then t h i s approach rapidly becomes numerically unwieldly. A suitable a l t e r n a t i v e i s the method of Abeles (15) which i n conventional optics defines a c h a r a c t e r i s t i c matrix per layer i n terms of Fresnel coefficients, and phase factors from the relationship between t h e e l e c t r i c vectors i n successive layers such t h a t the c h a r a c t e r i s t i c matrix per layer i s ,

where, rm, i s t h e Fresnel reflection coefficient a t t h e m-1,mth interface such t h a t

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and

(21111) nm,lsin~m-l,

pm = nm sine,.

Following Cowley and Ryan (16) it i s now possible t o include a roughened o r diffuse interface a t each boundary (without dividing it i n t o a s e r i e s of discreet layers) by introducing a Gaussian roughness factor of t h e form described by Nevot and Crock ( 1 7 ) such t h a t r

m

=

pm-1 - pm exp Pm-l+Pm

- 0.5

(%-, % 2)

where
i s the root mean square roughness, For

N

layers t h e matrix elements Ml1,MI2

=

2k ~ i n 8 , - ~and

a, = 2k

sine,.

of t h e resultant matrix f o r the system give

the r e f l e c t i v i t y M21 M21

This method provides a convenient and closed form f o r calculating exactly r e f l e c t i v i t y p r o f i l e s f o r r e a l systems (13).

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EXPERIMENTAL

The essence of a neutron reflection experiment is to measure the specular reflection over a wide range of wave vector transfers (Q = 4n sin8/5, 8 is the glancing angle of incidence) perpendicular to the reflecting surface. The wide Q range can be achieved either by using monochromatic beam and scanning a larger number of angles, or by using the broad band neutron time-of-flight (TOF) method to determine h at fixed 8. As the critical glancing angles are small, narrow well collimated beams are required. To date, the majority of the reactor based measurements have been made using monochromatic long wavelength neutrons and a 8-28 angular scan. However, on a pulsed source, such as ISIS, the natural way to make the measurement is the white beam TOF method. The fixed sample geometry ensures a constant sample illurnination, and the Q resolution (dominated by the A8 contribution) is essentially constant over the wide Q range available.

Figure 1 Schematic diagram of the CRISP reflectometer. C : Chopper, B : Beryllium filter, J : Coarse collimating jaws, S1, S2 : Collimating slits, F : Frame overlap mirrors, R : Downstream collimation, M : Neutron beam monitor, S : Sample and D : Detector. The instrument views the 20K hydrogen moderator giving an effective wavelength range of 0.5 to 13A. The beam is inclined at 1.5' to the horizontal (specifically for liquid surfaces, and a horizontal slit geometry is used giving typical beam dimensions of 40 nun width and between 0.25 and 6 mm height; the beam size and divergence is variable and defined by two cadmium apertures (Sl,S2). A single disc chopper (c) defines the Additional wavelength band (A5) and provides some frame overlap suppression. suppression is provided by a series of frame overlap mirrors (F) which are set to reflect out of the main beam neutrons of wavelengths greater than 13A. The detector (D) (a single well shielded ~e~ detector, or a one dimensional multidetector with a positional resolution of < 1 mm) is located some 1.75 mm from the sample position. The experimental arrangement is extremely flexible and solid films can be studied over a range of angles from 0.25' to 3'; liquid surfaces can be studied at angles-jess than 1.1O by the insertion of a supermirror. The Q-gange available is - 4 x 10 to 0.65 A- , and the limiting reflectivity is S 10 A further demonstration of the flexibility of the spectrometer is that is can, at ease, be converted to a polarised neutron reflectometer (18). The incident beam is spin polarised by a cobalt-titanium supermirror, and the spin direction controlled by a static guide field and a Drabkin spin flipper: to give good polarising efficiency (>> 0.99) and flipping efficiency ( 2 80%) over the wavelength range 2-13A.

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- APPLICATIONS

An extensive s c i e n t i f i c programme (19) has emerged very quickly on CRISP, i n surface chemistry, s u r f a c e magnetism and s o l i d films. I n s u r f a c e chemistry much of t h e i n i t i a l i n t e r e s t has been i n t h e adsorption of s u r f a c t a n t s (9,20), polymers (21) and f a t t y a c i d s (22) and t h e a i r - l i q u i d i n t e r f a c e , and more r e c e n t l y has been successfully extended t o t h e l i q u i d - s o l i d i n t e r f a c e . I n surface magnetism t h e r e has been s t u d i e s on t h i n ferromagnetic films (23) and s u r f a c e s (24) and t h e determination of f l u x p e n e t r a t i o n i n superconductors (23) have been made. A range of s o l i d f i l m s has been (26,27), hard carbon films (28), investigated and include polymer films Langmuir-Blodgett f i l m s and some semiconductors f i l m s (11,29,30) The f i r s t example of t h e study of adsorption of s u r f a c t a n t s a t t h e a i r - l i q u i d i n t e r f a c e has been t h e study of s u r f a c e adsorption of d e c y l t r i m e t h y l m o n i u m bromide (DTAB). R e f l e c t i v i t y measurements on t h e d i f f e r e n t i s o t o p i c s p e c i e s f u l l y deuterated, protonated, chain deuterated and headgroup deuterated i n D20 and water c o n t r a s t matched t o a i r (cmw) enable a d e t a i l e d s t r u c t u r e model t o be determined. A t low s u r f a c t a n t concentration (< complete monolayer) a smooth s u r f a c e l a y e r i s formed. A t higher concentrations (L 0.05M) t h e l a y e r i s roughened t o minimise e l s c t r o s t a t i c c o n s t r a i n t s . The degree t o which t h e molecules stagger has been determined (- 10% of t h e hydrocarbon chain i n t h e headgroup region), a r e a per molecule evaluated and t h e e x t e n t of degree of hydration determined. s i m i l a r study has been c a r r i e d out i n t h e anionic s u r f a c t a n t tetramethyl ammonium dodecylsulphate (TMDS) (20) i n t h e concentration range 0.0004M t o 0.0118M (see Figure 2 ) . By s e l e c t i v e d e u t e r a t i o n of t h e tetramethyl ammonium counterion t h e r e is s t i l l s u f f i c i e n t r e f l e c t i v i t y (see Figure 3 ) t h a t a d d i t i o n a l information about t h e degree of counterion binding, and t h e extent of t h e d i f f u s e counterion layer, can be obtained. A

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Momentum transfer ( h-'1

~ e f l e c t i v i t yp r o f i l e s of hTMdDS i n n u l l r e f l e c t i n g water a t concentrations of 0.00095 (*) , 0.00285 (x) , 0.0057 (0) and 0.012M (t).

F i ure 2 %--0.0 0475 ( e r r o r b a r s ) ,

The s t r u c t u r e of t h e adsorbed l a y e r c o n s i s t s of t h r e e regions, t h e f i r s t , nearest t h e vapour, containing only hydrocarbon chains, t h e second containing a small f r a c t i o n of chains, t h e head groups, water and a f r a c t i o n of counterions, and t h e t h i r d containing water and a d i f f u s e atmosphere of counterions. The a r e a p e r molecule a t completion of t h e monolayer i s 60 f 2A. A t t h i s point t h e t h i c k n e s s of t h e chain region i s 14.5 f 1 A

Momentum transfer

Fiqure 3 R e f l e c t i v i t y p r o f i l e s of dTMhDS i n n u l l r e f l e c t i n g water a t concentrations of 0.0026M (0) and 0.011 (t)

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+

and t h e t h i c k n e s s of t h e headgroup region 10 1A. The f r a c t i o n of chains incorporated i n t o t h e headgroup region is 0.15, corresponding t o about 2 CH2 groups, and t h e f r a c t i o n of bound ions i s 0.25. Simple geometric c o n s t r a i n t s , i n conjunction with t h e r e f l e c t i v i t y r e s u l t s , i n d i c a t e t h a t t h e headgroups a r e arranged on a s i n g l e plane i n t h e complete TMDS monolayer, unlike t h e s i t u a t i o n found f o r DTAB. A t coverages down t o about half a monolayer t h e r e i s no s i g n i f i c a n t change i n t h e geometry of t h e l a y e r o t h e r than t h e i n c r e a s e i n t h e a r e a per molecule. For t h e i s o t o p i c composition dTMhDS i n n u l l r e f l e c t i n g water t h e d i f f u s e l a y e r of counterions c o n t r i b u t e s i g n i f i c a n t l y t o t h e r e f l e c t i v i t y . This i s t h e f i r s t time a d i r e c t measurement of t h e d i f f u s e double l a y e r has been made, and although t h e d i f f u s e l a y e r s c a l e s with Debye length (l/K i s 42A @ 0.0055M and 63A @ 0.0026M), t h e shape of t h e d i s t r i b u t i o n i s not c o n s i s t e n t with t h e S t e r n l a y e r model. o s p e c i e s i s c u r r e n t l y of much The nature of t h e i n t e r f a c e b e t ~ e e n ~ t w polymeric i n t e r e s t ; t h e degree of mixing between components i n a b i n a r y mixture has profound s i g n i f i c a n c e f o r t h e mechanical p r o p e r t i e s of t h e material, and t h e time dependence of intermixing can provide important information about i n t e r d i f f u s i o n c o e f f i c i e n t s of t h e polymer p a i r . Measurements have been carried out on inuniscible (26) (deuterated polymethymethacrylate, d-pnuna, and protonated polystyrene, h-ps) and miscible (27) d-pnuna and protonated s o l u t i o n chlorinated polyethylene, h-scpe) polymer p a i r s . Figure 4 shows t h e r e f l e c t i v i t y p r o f i l e before and a f t e r thermal annealing of t h e miscible polymer p a i r of d-pnuna and h-scpe. The low frequency f r i n g e s a r i s e s from t h e upper d-pma l a y e r (- 1000A) w h i l s t t h e higher frequency o s c i l l a t i o n s a r i s e from t h e lower h-scpe l a y e r (- 3000A). The damping of t h e f r i n g e s r e s u l t s from a d i f f u s e i n t e r f a c e between t h e two polymer films. Consistent with recent t h e o r i e s , on annealing, t h e width of t h e i n t e r f a c e does not change dramatically, but t h e i n t e r f a c e moves ( t h e d-pma l a y e r g e t s t h i n n e r ) through t h e film. For t h e immiscible (27) p a i r d-pnunalh-ps a sharp s t a t i o n a r y i n t e r f a c e i s observed, and t h e upper d-pmma l a y e r d e n s i f i e s on annealing. From t h i s and o t h e r measurements t h e r e is evidence t h a t t h e n a t u r e of t h i n polymer f i l m s (where f i l m t h i c k n e s s i s o r t h e order of t h e r a d i u s of g y r a t i o n of t h e polymer) may be d i f f e r e n t t o t h e bulk.

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unonnealed .0001l 2.0

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annealed at 120°C for 2 hrs

(L

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Neutron reflectivity from a deuterated polymethyl methacrylate (d-pm) film deposited on a hydrogenous solution chlorinate polyethylene (h-scpe) layer (- 30008) on a h/10 optical flat; (a) unannealed sample, pmma thickness 9728, scpe thickness - 27008, interface - 708, (b) sample annealed at 120°C for 2 hours, pmma thickness 9228, interface - 908.

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Undoubtedly the most sensitive neutron reflection measurements have been the measurement of the magnetisation in ultrathin epitaxial films (- 2 to 10 monolayers) of ferromagnetic materials. The spin dependence of the reflectivity in these measurements is enhanced by sandwiching the film between the substrate and a thicker overlayer. This enhanced sensitivity has been exploited in a series of investigations of ultrathin epitaxial layers of cobalt and iron grown on copper (co) single crystal substrate. The normally hexagonally close packed (hcp) cobalt and body centred cubic (bcc) iron are both grown as face centred cubic (fcc) onto the copper: and the resulting magnetisation state is of increasing theoretical and experimental interest. It has been shown in the most recent neutron reflection measurements (23) that fcc iron films can exist in a high moment magnetically ordered state (3 monolayers has a moment per atom close to bulk bcc iron, of 1.9 pB) (see 5a), but that this state is unstable with respect to increasing film thickness (moment at 3.6 monolayers has reduced to 0.5 pB) (see 5b) on growth temperature.

' " 8 3 monoloyers

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Figure 5 Spin reflectivity ratio Rt/R- (where Rt is the reflectivity with the neutron spin parallel to the applied magnetic field, and R- antiparallel) for (a) 3 monolayers and (b) 3.6 monolayers of iron grown epitaxially onto a Cu (100) single crystal surface, with a copper overlayer. The solid lines are model fits for the moments of

1.9 pB and 0.5 pB respectively. This implies that small alterations in the structure of fcc iron films can significantly alter their magnetic properties. In contrast cobalt films of thicknesses down to a single monolayer gave a moment close to that for bulk hcp cobalt. ACKNOWLEDGEMENTS R C Ward, C Shackleton, W G Williams and R Felici have contributed to the development of CRISP. R K Thomas, J S Higgins and W Schwarzacher are acknowledged for allowing their data to be included in this presentation. REFERENCES 1. A G Klein and S A Werner, Rep Prog Phys (1982) 5 259 2.

E Fermi and W Zinn, Phys Rev

2

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3. J B Hayter, J Penfold and W G Williams, J Phys E (1978) 2 454 4. H Maier-Leibnitz and T Springer, Reactor Sci and Tech, J of Nucl Energy Parts A and B (1963) 17 217 5. J B Hayter, R R Highfield, B J Pulman, R K Thomas, A I McMullen and J Penfold, J Chem Soc Faraday Trans 1 (1981) 97 1437 6. G P Felcher, Phys Rev B (1981) 24 1995 7. J Penfold, R C Ward and W G Williams, J Phys E (1987) 20 1411 P Felcher, R D Hilleke, R K Crawford, J Haumann, R Kleb and G Ostroski, Rev Sci Inst 3 609 (1987)

8. G

9. E M Lee, R K Thomas, J Penfold and R C Ward, J Phys Chem (1989) 2 381 10. G P Felcher, K E Gray, R T Kampwirth and M P Brodsky, Physica 136B (1986) 59 11. C D Ashworth, S Messoloras, R J Stewart and J Penfold, Semicond Sci Techno1 4 (1989) 1 12. J Lechner, "Theory of Reflection", Martinus Nijholf, Dortecht (1987) 13. J Penfold, Rutherford Appleton Laboratory internal report RAL-88-088 (1988) 14. M Born and E Wolf, "Principles of Optics" (Pergamon Press, Oxford) (1970) 15. 0 S Heavens "Optical Properties of Thin Films" Butterworths, London (1955) 16. R A Cowley and T W Ryan, J Phys D

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19. J Penfold, R K Thomas, in press 20. J Penfold, R K Thomas and E M Lee, submitted to Mol Phys 21. L Dai, J W White, J Kerr, R K Thomas J Penfold and M Aldissi, Proceedings of Int Conf on Synth Metals (1988) 22. M J Grundy, R M Richardson, S J Roser, J Penfold and R C Ward, Thin Solid Films 159 (1988) 43

23. W Scharzacher, W Allison, R F Willis, J Penfold, R C Ward and W F Egelhoff Jnr submitted to Solid State Corns

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24. P K Ivison, N Cowlam, M R J Gibbs, J Penfold and C Shackleton - submitted to J Phys 25. R Felici, J Penfold, R C Ward, E Olsi and C Matacotta, Nature

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26. M L Fernandez, J S Higgins, J Penfold, R C Ward, C Shackleton and D T Walsh, Polymer 2 (1988) 1923 27. M L Fernandez, J S Higgins, J Penfold and C Shackleton - in press 28. M J Grundy, R M Richardson, S J Roser, G Beamson, W J Bremman J Howard, M OrNeil, J Penfold, C Shackleton and R C Ward, Submitted to Thin Solid Films 29. N M Hardwood, S Messoloras, R J Stewart, J Penfold and R C Ward, Phil Mag B (1988) 58 217 -

30. C D Ashworth, S Messoloras, R J Stewart, J G Wilkes, I S Baldwin and J Penfold, Submitted to Phil Mag Letters