Physica B 180 & 181 (1992) 462-464

North-Holland PWSICA I!

The application of specular neutron reflection to the study of surfaces and interfaces

J. Penfold ISIS Science Division. Rutherford Appleton Luboratory, Chilton. Didcor. Oxon OX1 1 OQX, UK

In recent years the specular reflection of neutrons has proved to be a valuable new technique for determining the

structure of surfaces and interfaces, and a widespread application to a range of problems in surface chemistry. solid films

and surface magnetism has emerged. The white beam time of flight (TOF) method for reflection measurements, exploited

on a pulsed neutron source, has proved to be particularly important. Recent developments on the instrumentation and

analysis methods at ISIS are presented and recent experimental results, covering a range of scientific applications are discussed

1. Introduction

Since the advent of dedicated instrumentation, the specular reflection of neutrons has been widely applied as a technique for the study of surface and interfaces 11, 21 in surface chemistry. solid films and surface magnetism.

The essence of a neutron reflection experiment is to measure the specular reflection as a function of wave vector transfer. Q. perpendicular to the reflecting surface. This can be related to the neutron refractive index profile normal to the surface; which is often simply related to the scattering length density, giving information about the composition and density gra- dient at surfaces and interfaces.

The variation of neutron scattering length with iso- tope makes it possible to vary the scattering length density, and hence refractive index by isotopic substi- tution. Hydrogen-deuterium substitution is particular- ly important for chemical systems, and gives a great deal of flexibility in the manipulation of the refractive index.

2. Developments at 1SIS

Recent developments and improvements in in- strumentation on CRISP [3] have improved the quality and range of scientific applications on the spectrome- ter. Optimisation of the incident beam collimation has improved the incident neutron flux by a factor of 2.5; the integrated flux between 0.5 and 6.5 I& is now -I()“Ncm~‘s~’ with ISIS at 100 +A. The addition of

a nimonic T,, chopper means that the spectrometer can operate at 25 Hz (half the source frequency), and reliable data can be obtained over the extended wave- length range of 0.5-l 1.0 A.

The installation of a position sensitive detector [4] with a 0.7mm resolution has given rise to increased interest in the non-specular scattering for roughened

surfaces and other lateral surface unhomogeneities; and in addition it is especially valuable where an accurate estimate of the glancing angle of incidence is required.

The multidetector, in combination with further op- timisation of alignment procedures, has been particu- larly important for the study of the liquid-solid inter- face (see for example, ref. [5]).

The increasing complexity of the systems studied is particularly highlighted by the increasing interest in the study of adsorption at the liquid-liquid interface.

There have been significant developments in analy- sis methods; and in particular model fitting procedures have been much extended [6]. Detailed models have been applied to the study of surfactant adsorption [7], and most recently Thomas et al. [SJ have developed a method based on the kinematic approximation and contrast variation to determine the partial structure factors for the different components in an adsorbed layer.

3. Recent scientific highlights

Extensive use of the technique has been made in the study of surfactant of polymer adsorption at the air- liquid interface, and some recent examples are de- scribed elsewhere in these proceedings [9].

The penetrating power of thermal neutrons allows reflection measurements to be made at the liquid-solid interface, by glancing incidence through the solid

phase. Roser et al. [5] have exploited this advantage in their recent pioneering study of the in situ electro- chemical deposition of a polymer film. They were able to determine the thickness and density profile of a polybithiophene film at an electrode surface; where the undoped films are found to be dense and contain little solvent or electrolyte (see fig. 1).

Stewart et al. [I] have used neutron reflection to

0921.4526/92/$05.00 0 1992 - Elsevier Science Publishers B.V. All rights reserved

J. Penfold I Specular neutron reflection in the study of surfaces and interfaces 463

I 1

0.02 0.03

Wavevector transfer, Q (A-‘)

I

O.OL

Fig. 1. Neutron reflectivity data for a polybithophene film on a Pt electrode. The points are experimental data and the solid line is

a model fit for the profile shown in the inset.

investigate the effect of varying MBE growth condi- tions and IniGa composition in InGaAs thin films on GaAs. Figure 2 shows the reflectivity profile for an InAs film of the order of 1OOOA on GaAs. Above a critical concentration of In, an unusual effect is ob- served in the region of total reflection (see inset in fig. 2), and is a maximum for the pure InAs film on GaAs. This is attributed to SANS of the evanescent wave from indium clusters in the film.

block copolmers. Jones et al. [ll] have recently dem- onstrated lameller ordering in thin polymer film spun from a blend of d-polystyrene and polybutadiene (see fig. 3). These films can, however, give rise to consider- able off-specular scattering which is associated with

lateral inhomogeneities.

There has been much recent interest in the structure Acknowledgements

of thin polymer films by neutron reflection, including the nature of interfaces between miscible and immisc- ible polymers, surface enrichment and ordering in

I wish to thank Richard Jones, Steve Roser and Roger Stewart for allowing the use of their CRISP data prior to publication.

1.0 -

0 l-

x z > 0.01 - c

z

h o-001 -

0 OOOl-

0~00001 I

. -T IS

0 do95 0.02 O-03 O-O&

Wavevector transfer, Q ( In A-‘)

0 06

Fig. 2. Reflectivity profile for a 1000 8, InAs film on GaAs. The inset shows the region of total reflection on an expanded scale.

464 J. Penfold I Specular neutron reflection in the study of surfaces and interfaces

0~020 O-030 0.0.45

Wovevector transfer. Q (in 6-‘)

Fig, 3. Reflectivity profile for a spin film of a blend of deuterated polystyrene and polybutadiene in silicon. The inset shows the

off-specular scattering from the film.

References

[l] J. Penfold and R.K. Thomas. J. Phys. Condens. Matter

2 (1990) 1369.

[2] T.P. Russell, Mater. Res. Rep. 5 (1990) No. 4.

[3] .I. Penfold, R.C. Ward and W.G. Williams, J. Phys. E.

Sci. Instrum. 20 (1987) 1411.

[4] Ordela Inc., Oak Ridge, Tennessee, USA, Model

1202N position sensitive detector.

[S] S.J. Roser, R.M. Richardson, M.J. Swann and A.R.

Hillman, J. Chem. Sot. Faraday Trans. (1991) in press.

[6] G.J. Herdman, J. Penfold and C. Shackleton, Ruther-

ford Appleton Laboratory Report, RAL-90 (1990).

[7] J. Penfold, R.K. Thomas, E. Simister, E. Lee. A.

Rennie, J. Phys. Condens. Matter 2 (1990) SA 411.

[8] E.A. Simister, E.M. Lee, R.K. Thomas and J. Penfold.

J. Phys. Chem., submitted.

[9] P.G. Cummins, J. Penfold, R.K. Thomas, E. Simister

and E. Staples, Physica B I80 & 181 (1992) 483 (these

Proceedings).

[lo] Stewart et al., unpublished data.

[ 111 Jones et al.. unpublished data.