Ellipsometric studies on uranyl arachidate Langmuir-Blodgett films

thin

ELSEVIER

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Thin SolidFilms 295 (1997) 210-213

Ellipsometric studies on uranyl arachidate Langmuir-Blodgett films H. K n o b l o c h "~'*, F. Pefiacorada -~,b, L. B r e h m e r ~ "~h~stlmte o3'Sohd State Phystcs, Univers,ty 03'Potsdam, Research Group "Thin Organic Fihns"; Kantstrafle 55, 14513 Teltow, Germany Departamento de Ffszca de la Materta Condensada, Universidad de Valladohd, Prado de la Magdalena s/n, 47011 Valladohd, Spare

Recetved29 January 1996; accepted30 September1996

Abstract

In the presence of counterlons, the quality of fatty acid monolayers, prepared by the Langmuir-Blodgett (LB) technique is highly influenced by the subphase pH value. In this paper, we present ethpsomemc studies on arachidic acid LB films, prepared at different subphase pH values, with uranyt (O=U=O)2 + as the countenon. In order to obtain the refractive indices and thickness, independently, the films were prepared as steps with bilayer step size. Refractive index, real and imaginary part, monolayer thlckness and surface roughness on a microscopic and macroscopic scale have been determined for three different pH values. The results show that films prepared at a pH value of 4.8 and 4.0 are of good optical quahty, whereas those prepared at pH 2.5 are mhomogeneous. © 1997 Elsevier Science S.A. Keywords: Elhpsometry:Langmmr-Blodgett,Surfaceroughness

1. Introduction In previous papers [ 1,2] we discussed the adsorption of uranyl ions [3-8] ( O = U = O ) ' + to Langmuir films and their influence on the Langmuir-Blodgett (LB) [9] film deposition process. We have shown, for example, the changes m the monolayer structure as a function of the subphase pH value due to the presence of uranyl ions. The results show a different behaviour of the films containing uranyl ions to those of most metal ion adsorptions, studied so far. As a result, the LB films show different structures, with respect to the subphase pH value at whlch they had been deposited. A crystalline and very rough film is obtained at low subphase pH values. Non-crystalhne LB films of high stability and homogenelty are obtained for subphases of higher pH value (e.g. pH 4.8). Drawing these results into account, we measured the refractive index and the film thickness of uranyl arachidate LB films deposited at different subphase pH values by applying ellipsometry. In order to find a variation of these optical properties as a function of the subphase pH, we prepared LB films in three different pH regions. As it was determined [ 1,2] the LB films deposited in each pH region have the same characteristics: at a subphase pH <3.5 the film consists mainly of arachldic acid; in the pH range of 3.5 < pH < 4.8 we obtained a mixture of arachidic acid and uranyl arachidate; * Corresponding author. 0040-6090/97/$17.00 © 1997 ElsevierScience S A. All rights reserved P H S 0 0 4 0 - 6 0 9 0 ( 9 6 ) 0940 1- 1

for subphase pH > 4.8 the film turned to be completely uranyl arachidate [ 1,2]. Samples have been prepared in each pH reglon to confirm the previous studies of this compound and to describe the optical properties of these LB films.

2. Materials and methods 2.1. L a n g m u i r - B l o d g e t t d e p o s i t i o n

Arachidic acid (Merck, 2.3 mM in CHCI3) and uranyt acetate ( > 99%, Merck) in aqueous solution (Millipore), 2 × 10-4 M, are the basic materials for uranyl arachidate LB film formation; the pH values were adjusted by applying HNO3. The formation of the monolayers in different pH regions is described in a previous paper [ 1 ]. The LB films have been deposited onto silicon substrates, hydrophobised by hexamethyldisilazane vapour in a closed chamber. Before LB film deposition and hydrophobisation, the silicon substrates were cleaned with hot chromic acid and rinsed with ultrapure water (Millipore). The roughness of the silicon substrates is lower than one nanometer as measured by scannmg force microscopy. We prepared LB films, forming steps of 10 to 40 monolayers with a step size of 2 monolayers, respectively. For all pH values, Y-type deposition was obtained, with an optimum transfer ratio of TR > 95%. The subphase pH for LB deposition has been selected with reference to the three characteristic regions of ion adsorption,

211

H Knobloch et al./Thm Sohd Fihns 295 (1997) 210-213

2.2. Ellipsometry

found in our previous work [ 1 ]. Three pH values corresponding to three adsorption regions of the monolayer were selected for our elhpsometric measurements: pH 2.5, 4.0 and 4.8, respectively. For all subphase pH values, the films were depomted at a surface pressure located in the condensed phase of the monolayer corresponding to each subphase pH [ 1] and for this reason the transfer pressure was higher for pH 4 and pH 4.8 (7r= 35 at p H = 2.5 and 7r=45 at higher pH).

Ellipsometry zs a well-suited technique for surface and thin film characterisatmn with a high sensitivity to refractive index and film thickness [ 10,11 ]. In general, ellipsometry measures the state of polarization of light, reflected from a surface. Normally, the reflected light is elhpUcally polarized, and the state of polarisation can be expressed by the angles k0"and A: tan ~ e "a =

elhpsometry

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P~ [d~ ] Fig. 2. Elhpsometry data of uranyl arachzdate Langmmr-Blodgett layers deposited on a mhcon substrate. The films were deposited at three subphase pH values: 2 5 (a), 4 (b) and 4 8 (c). The surface pressures dunng film transfer were or= 35 mN m - ~ (a) and -n'=45 mN m - ~ ((b) and (c)) The sohd hnes represent simulations as obtained by Fresnel calculations

H Knobloch et al /Thin Sohd Films 295 (1997) 210-213

212

x-y translation stage controlled by a stepping motor unit. The lateral resolution of the profile and mapping measurements corresponds to the diameter of the laser spot ( O = 1 mm). For data analysis, the measured ( ~A ) values were compared to Fresnel calculauons [ 13], on the layer system.

( ~A ) values for films prepared at pH 2.5 do not fit simulations performed at this refractive index. Assuming the refractive index would be the same in both cases, the measured and calculated data are in good agreement, when introducing an imaginary part of the refractive index (n = 1.514- 0.025i). Such a non-zero imaginary part in the refractive index indicates losses in a material. In any case this loss may occur, for example, when light is reflected from an inhomogeneous surface such as formed by an inhomogeneous ultra thin film structure. Since the LB deposition parameters, in general, do not affect the light absorption properties of a material, especially not of fatty acid films, we interpret this non-zero imaginary part as a contribution from scattering losses at film inhomogeneities such as micro-roughness due to crystallisation after the deposition process. In addition, the deposition parameters not do only affect the homogeneity on a microscopic scale (molecular order) of Langmmr-Blodgett films, but also have an influence on the macroscopic homogeneity. Fig. 3 (b) and 3 (c) show the calculated thickness of step-wise prepared uranyl arachidate LB films. In the case of subphase pH values of pH 4.8 and pH 4.0, the step structure of the LB film assembly is clearly pronounced and the plateau of each step is very flat, giving

3. Results and discussion

The uranyl arachldate films were characterised by ellipsometry with respect to thzckness and refractzve index as well as to the dependence of film homogeneity on the deposition parameters. In order to obtain samples of different thickness for our ellipsometric measurements, the uranyt arachldate LB films were transferred step-wise to hydrophobized silicon substrates as shown in Fig. 1. Fig. 2(a), 2(b) and 2(c) show typical gp-A[2 values of uranyl arachidate films prepared at different subphase pH and surface pressure values (pH=2.5, 7r=35 mN m -1 (a); pH=4.0, 7r=45 mN m -~ (b); pH=4.8, ~ = 4 5 mN m -~ (c)). For films prepared at pH 4.8 and pH 4.0, the ( ~ A ) values correspond qmte well to Fresnel calculations performed for a refractive index ofn = 1.514 of the LB film. The i

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Fig 3. Thickness profile of uranyl arachldate Langmmr-Blodgett films on a silicon substrate, the films were prepared step wise. The thickness is calculated from elhpsometry data, obtained by profile measurements for samples prepared at three subphase pH values: 2 5 (a), 4 (b) and 4.8 (c). The surface pressures were ~r= 35 mN m - 1 (a) and ~r=45 mN m - I ( ( b ) and ( c ) )

H Knobloch et al./Thm Solid Fdms 295 (1997) 210-213 1

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deposited at pH 4.8. The data thickness obtained by ellipsometry are in quite good agreement to those obtained by Xray scattering measurements described m our prewous work [2] In the case of films prepared at pH 2.4, we could not apply hnear regression, because no well-defined step structure can be detected in these films.

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4. Summary

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In summary, our results show that the optical quahty of uranyl arachidate Langmuir-Blodgett films strongly depends on the deposition parameters (subphase pH, subphase concentration, temperature, dipping speed, etc), especmlly on the pH value of the subphase which had been discussed m this work. The refractive index of the LB film containing uranyl counterions has not been measurably affected by counterion adsorption to the LB film. But, the crystallisatlon of the LB film may affect the optical properties of these films. Therefore, the presence of counterions avoiding thls crystallisatlon process is Important for obtaining LB films of fatty acid molecules with good optical quality. Since we applied a step-wise preparation of the LB films, we could obtain refractive index (real and imaginary part) and film thxckness independently, allowing for a comprehenswe charactensation of the LB films.

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No. of Dipping Cycles I

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Acknowledgements

No. of Dipping Cycles Fig 4 ThlcknessofuranylarachidateLangmulr-Blodgettfilmsasafunctmn of the number of layers for samples prepared at a subphase pH value of 4 and 4 8. The surface pressure in both cases was ~=45 mN m-

The authors wish thank Mr J Reache for interesting discussions

evidence of highly homogeneous film. In contrast, when prepared at a subphase pH value ofpH 2.8, there is no such welldefined step structure in the thickness profile (Fig. 3(a)). Therefore, these films are inhomogeneous even on a macroscopic scale. Fig. 4 shows the film thickness of the uranyl arachldate LB film structures as a function of the number of LB deposmon dipping cycles. Only for the two higher pH values of 4 and 4.8 can a defined layer structure with defined thickness be observed. Both samples are deposited at a surface pressure of 45 mN m - ~where the molecules are supposed to be almost perpendicular to the surface as characterised in uranyl arachidate study [ 1,2]. Since the data points lie on a straight line it is obvious that the transfer ratio is at about 100% up to a number of 20 dipping cycles. Assuming that each dipping cycle forms one complete uniform monolayer, which is justified with regard to the results m Fig. 3 (b), linear regression gives a thickness of 2.56 nm per monolayer, for the sample deposited at pH 4.0, and a thickness of 2.79 nm for that one

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