Dynamical properties of physisorbed monolayers: Mössbauer studies

Vacuum~volume41/numbers 1-3/pages 401 to 403/1990 Printed in Great Britain

0042-207X/9053.00+.00 © 1990 Pergamon Press plc

Dynamical properties of physisorbed monolayers: M6ssbauer studies H S h e c h t e r a n d R B r e n e r , Solid State Institute and Department of Physics, Technion-lsrael Institute of

Technology, Haifa 32 000, Israel and J S u z a n n e , CRMC2, Departement de Physique, Facult~ des Sciences de Luminy, Case 901, 13288 Marseille

Cedex 9, France

Applications of M6ssbauer spectroscopy to study the dynamical properties of condensed physisorbed films are discussed. MSssbauer spectroscopy has been proven to be a sensitive probe for observing changes in the neighbourhood of phase transitions. The angular and temperature dependence of M5ssbauer spectral intensity makes it possible to distinguish the dynamical behavior of adsorbed molecules such as diffusion in the film plane from that in the direction perpendicular to it. Examples of 2D melting and edge melting which occur in , submonolayers of Sn(CH3) 4 and Fe(CO)5 physisorbed on graphite and MgO are presented.

1. Introduction

Monolayers of gases adsorbed on high area uniform substrates show structures and phase transitions characteristic of twodimensional (2D) phases ~. Extensive studies by numerous experimental techniques on distinct monolayer phases have presented microscopic views of the local structural arrangements of the adsorbed atoms and their positions relative to the substrate. Relatively few studies on dynamical properties of physisorbed monolayers have been published so far. In particular, diffusive motions of adsorbed molecules have been measured by quasielastic neutron scattering 2. Debye-Waller factors along directions parallel and perpendicular to the film plane have been measured by M6ssbauer spectroscopy 3. For gamma rays directed perpendicular to the film, the Debye-Waller factor decreases with increasing temperature and the change is associated with the perpendicular mean-squared displacement of the adsorbed molecule 4. These M6ssbauer measurements yield the adsorption binding energy. In the parallel direction, a sudden drop in the M6ssbauer spectral intensity to undetectable levels was observed when the 2D melting temperature, T,~(2D), was approached 5. Singwi and Sjolander 6 have suggested that M6ssbauer technique can be used to study the nature of diffusion of atoms in a bulk solid. They showed that the M6ssbauer resonance crosssection is a product of the Debye-Waller factor and a diffusion dependent function which was confirmed experimentally 7. For monolayer films adsorbed on aligned substrates, the diffusion in the film plane can be detected and the sensitivity to diffusion may be attenuated by geometrical factors when the observation is done along other directions 5,s. This permits the M6ssbauer spectroscopy to probe the mobile states of the adsorbed film in the neighbourhood of 2D phase transitions 5's-1°. Premelting effects have been under current interest. Recent experiments have actually demonstrated the existence of surface melting in solids ~ and in adsorbed multilayer films 12. During the last year evidence has been presented for the existence of edge

melting~ 3. ~4, the 'one-dimensional' analog of the surface melting in 2D physisorbed islands. Edge melting occurs very close to Tm(2D) when peripheral molecules of the 2D islands become mobile. In this report we present evidence from M6ssbauer experiments that edge melting occurs in the islands of Sn(CH3) 4 and Fe(CO)5 adsorbed on graphite (0001) and MgO(100) planes. 2. Experimental details

Fe(CO)~ (IPC) and Sn(CH3) 4 (TMT) are liquids at room temperature and display convenient vapor pressures for adsorption isotherm measurements. M6ssbauer absorbers were prepared for submonolayer surface densities of IPC and TMT molecules adsorbed on the hexagonal (0001) basal planes of exfoliated graphite (Grafoil) 15 sheets and the cubic (100) planes of MgO powder 16. A 10 g stack of parallel Grafoil sheets was prepared which allowed two configurations of M6ssbauer measurements: one with the direction of the gamma rays parallel to the common plane of the sheets (graphite basal planes), and the other perpendicular to it. The Grafoil stack was first cleaned by heating in vacuum and then transferred under helium atmosphere to the M6ssbauer transmission cell 9. The submonolayer film samples were prepared by following the adsorption vapor pressure isotherms of either IPC or TMT to the desired surface coverageg'~°.Basically the procedure for preparing the MgO substrate was similar 2'5. The temperature of the sample was controlled by liquid nitrogen flow. A constant acceleration spectrometer and either 5'7 Co:Rh(14keV) or Batlgsno3 (23.9 keV) gamma M6ssbauer sources were used. The accumulated M6ssbauer velocity spectra were analysed for the spectral parameters from which the background-corrected spectral intensity was determined. 3. Experimental results

In Figure 1 we show the temperature variation of the M6ssbauer spectral intensity for several coverages of IPC adsorbed on 401

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graphite basal planes. The intensity measured in the perpendicular direction (Figure 1 (A)) can be described by a simple Einstein model 4. The solid lines through the experimental points are results of a fit to a simple harmonic model of small vibrations of the adsorbed molecule. Such a fit yields the adsorption binding energy. Figure I(B) illustrates the temperature variation of the spectral intensity in the parallel direction. The broken lines indicate the exponential Debye-Waller behavior ~7 extrapolated from lower temperatures. The experimental results in the parallel direction (Figure I(B)) are remarkably different from those shown in Figure I(A): The intensity decreases more rapidly in the parallel direction becoming practically zero at ~ 175 K. From previous neutron diffraction experiments la we identify the temperature where Mfssbauer intensity vanishes as the Tm(2D). Furthermore, as the temperature increases, gradual deviations from the exponential variation start as T~0.9Tm(2D). Any deviation from the harmonic behavior is generally associated with dynamic effects, particularly diffusion s . Recently, we have interpreted these deviations of spectral intensity from exponential behavior at temperatures close to T,(2D) as edge-melting ~4. The results for T M T submonolayer films adsorbed on graphite (Figure 2) and MgO (Figure 3) are similar to those obtained for IPC on graphite. The perpendicular and parallel configurations could not be distinguished for the randomly oriented crystallites of the MgO powder substrate. The randomness of the (100) adsorption planes orientation is the reason that the spectral intensity in Figure 3 does not vanish at a certain temperature as observed in Figures I(B) and 2(b).

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0.5, 0.7 and 0.9 layer films of Sn(CHa)4 adsorbed on MgO powder. The solid lines through the experimental points are a result of power law fit and the model of edge melting as described in the text. The residual intensity observed for T > T,, (2D) is due to the perpendicular component of the randomly oriented MgO crystallites.

diffraction 9 techniques. In both systems, as T ~ TIn(D) the structure becomes incommensurate with the substrate. From the available molecular dimensions of T M T molecules we assume that the structure of T M T submonolayers adsorbed on MgO(100) planes is also incommensurate. The diffraction experimental results indicate that these incommensurate structures remain unchanged until Tm (2D) is reached. It is then reasonable to assume that the submonolayer solid islands do not break up and that their size remains with sufficient coherent length for diffraction peaks to appear. We believe that the anomalous reduction of the M6ssbauer spectral intensity (see Figures 1-3) is associated with edge dynamical instability: The M6ssbauer intensity is then assumed to be the contribution of the molecules in the 2D island which remain harmonically stable. It has been shown that the M6ssbauer resonance absorption cross-section for a mobile molecule adsorbed on a 2D substrate is s

trr=s(O,T) = (2hK/F)[1 + (2hDk2/F)sin 2 0]-1/2

(1)

H Shechter et al: M6ssbauer studies of monolayer properties where K is the regular exponential on-resonance Debye-Waller factor, F is the natural linewidth, 0 is the angle of the gamma vector k with respect to the surface normal, and D is the diffusion coefficient. Equation (1) can be extended to the case where an adsorbed island consists of N O molecules. Suppose that as a consequence of a premelting process, n < N o molecules become mobile and N o - n remain in the island. In this case the observed spectral intensity I(0, 7) is proportional to N o - n(73:

represented reasonably well by a simple mean field theory, with no substrate effects, no registry or intrinsic structure of the 2D solid. The thermodynamics of edge melting in such 2D solid islands is identical to that of surface melting, the only difference being that in edge melting a thin peripheral band of solid liquifies and becomes the 2D solid-2D vapor interface.

I(O,T) oc [ N O - n ( T ) ] ( 2 h K / F ) + rl(T)o'res(0 , T).

We have studied several incommensurate solids and found that the temperature and configurational dependence of the M6ssbauer spectral intensity can be described by a power law. The behavior of these films of different structure is surprisingly similar, indicating edge melting. The discovery of edge melting by M6ssbauer spectroscopy in incommensurate 2D solids is supported by the signature of premelting of the edge of a incommensurate 2D neon solid adsorbed on graphite on the specific heat t3. From these results we strongly believe that at this stage the incommensurability is a necessary condition for the substrate to be unimportant when 2D mobility traditions are considered. Consequently, the edge melting and 2D melting must be an inherent property of the 2D solid as the surface and 3D melting are in bulk solids.

(2)

Equation (2) can be used to describe the experimental results in Figures I(B)-3(B). We assume that n(73 represent the edge molecules which become mobile as T--* T m (2D) and form what is probably the melting of the edge. A similar effect of edge melting has been actually predicted by computer simulations 19. The temperature dependence of the thickness/.(73 of the edge liquid layer depends on the nature of the forces (logarithmic for short range interactions), the width and shape of the solid-liquid interface, roughness, and finite size effects 2°'2 ~. In our discussion we assume that the edge liquid layer follows a power law, L oc t - l/p, where t = (T,, - 73/Tm and p is the interaction exponent. In 2D systems with smooth edges and non-retarded dispersion forces p = 4. Thermal fluctuations, roughening and other effects z°'2~ can reduce p. It is sufficient to show that as T--* T,,(2D), 3 < p = const < 4 in order to be able to describe the results by edge melting. Now the islands can be in form of strips 13 formed by decorating linear defects, or closer in shape to a circle. We discuss the last model but the physics is independent of the island shape. Let the island be of radius Ro at T < T~,(2D). The temperature dependence of the spectral intensity l(n/2, 73 will be proportional to N O - n ( T ) o c S(T), where S(T) is the average island area of radius R < R o. At temperature T~ the island has a radius R o -- L i, where L./is the width of the peripherial liquid band and Sj = 7t(R o -- Lj) 2. The contribution of the Debye-Waller factor, K# < K o, to the effect can be added and thus / ( ~ / 2 , T j ) o c S2(T2)/S o = ( K i / K o ) ( 1 - L~/Ro) 2,

(3)

where So is the island area at some temperature T O < Tm at which a deviation from harmonic behavior is observed. In order to fit the experimental results we use the expression

l(rt/2, T~) = (K~/Ko)(1 - at]-~') 2,

(4)

where a and ~b = 1/p are fitting parameters. The model produces a reasonable fit* to the experimental results (the solid lines in Figures I(B)-3(B). F o r the IPC-graphite system this analysis yields ~b = 0.25 for 0.9 layer coverage. This value is reduced for lower coverages up to ff = 0.20, 0.11 and 0.11 for the coverages 0.7, 0.5 and 0.3 respectively. F o r the TMT-graphite system, ~J = 0.29-I-0.03 for all coverages, and for T M T adsorbed on MgO, ¢ = 0.25. The reasons for deviations of ~b from the predicted value of 0.25 have been discussed elsewhere ~3't4'2t. The results presented in Figures I(B)-3(B) show that the temperature dependence of the M6ssbauer spectral intensity can be

* These empirical fits of ~ may be slightly different if the strip-model is selected, or on the ability to determine T=(2D). In our case, we measured T= (2D) by X-ray and neutron diffraction techniques.

5. Summary

Acknowledgements We thank D K a t m o r for valuable assistance in sample preparation. This work was supported by the US-Israel Science F o u n d a tion, G r a n t No 86-00294.

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