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The influence of gas phase composition on the process of Au–Hg amalgam formation
Applied Surface Science 206 (2003) 78±89
The in¯uence of gas phase composition on the process of Au±Hg amalgam formation Tomasz Kobiela*, Bogdan Nowakowski, Ryszard Dus Institute of Physical Chemistry, Polish Academy of Sciences, al. Kasprzaka 44/52, 01-224 Warsaw, Poland Received 29 May 2002; accepted 9 October 2002
Abstract Interaction of thin Au ®lms deposited on a glass substrate with Hg vapor at 298 K was studied. The in¯uence of gas phase composition on the kinetics of the amalgamation process was examined. Several gas atmospheres were tried: vacuum (residual gases pressure 1 10 7 Pa, with Hg vapor pressure 0.24 Pa), H2, Ar and air at pressure around 101 kPa. The electrical resistance measurements in the process of Au±Hg amalgam formation were correlated with the atomic force microscopy investigations of amalgam surface structure and X-ray diffraction spectra of the bulk. The results of these studies clearly demonstrate the in¯uence of gas phase composition on the kinetics of the amalgamation process. The experimental data are compared with theoretical calculations of the kinetic processes occurring at the thin Au ®lm surface. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Thin gold ®lm; Au±Hg alloys; Reaction studies by AFM and XRD methods
1. Introduction The signi®cance of the mercury amalgamation process is related to practical applications of gold amalgams [1±4]. Various experimental methods have been applied to study the interaction of thin gold ®lms with mercury vapor [5±8]. There have been several studies using electrical resistance measurements to determine the change of the properties of thin Au ®lms due to mercury adsorption and absorption [9±12]. In order to better understand the transducing mechanism of Au ®lms used as Hg sensors, George and Glausinger [12] have studied the interaction of Hg with gold surfaces using a variety of methods. Resistivity measurements were correlated with surface morphology studies of * Corresponding author. Fax: 48-3912-0238/48-22-632-5276. E-mail address: [email protected] (T. Kobiela).
gold ®lms deposited on various substrates. It was found that polycrystalline, unannealed gold ®lms deposited on alumina demonstrated the largest increase in resistivity due to interaction with mercury. The Au ®lms deposited on silica did saturate after Hg vapor exposure and next showed signi®cantly lower increase in resistance. This was interpreted as the result of the sticking coef®cient of mercury on gold decreasing with coverages greater than one monolayer [11,13]. Recently Levlin et al. [14] investigated the in¯uence of temperature and mercury concentration in the gas phase on the adsorption ef®ciency and gold surface saturation level. They observed a variation of the saturation level depending on the concentration of mercury in the carrier gas. In our previous report [15] we applied atomic force microscopy (AFM) imaging to polycrystalline Au
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 1 1 9 0 - X
T. Kobiela et al. / Applied Surface Science 206 (2003) 78±89
®lms exposed to Hg vapor in an ambient atmosphere. The studies of amalgam formation on various thin gold ®lm structures showed the fundamental importance of surface defects on the rate of amalgamation. In the present work, the in¯uence of gas phase composition on the kinetics of amalgam process is investigated. This study is motivated by the recent report concerning Ag±Hg alloy formation [16]. It has been found that the Ag2Hg3 compound is formed within approximately 10 h when Ag interacts with Hg vapor under vacuum conditions, but a much longer period of several days is required to obtain this compound under ambient atmosphere. In the present paper, we compare resistance measurements of polycrystalline Au ®lms exposed to Hg vapor in the vacuum chamber and in several different gases such as H2, Ar and air, kept at pressure of around 101 kPa. On the basis of the above-mentioned literature it is assumed that the rate of the resistance changes correspond to the rate of amalgam formation in thin gold ®lm deposited on the glass substrate. It has been established that molecular hydrogen and argon does not adsorb on annealed gold surface [17]. Thus these gases do not in¯uence surface processes in the reaction of amalgam formation, but they in¯uence solely Hg diffusion in the gas phase. We cannot exclude some traces of carbon and sulfur compounds in the air in our laboratory. Thus, amalgamation in air could be in¯uenced to some extent by the adsorption of these compounds on the Au surface. The surface structure of amalgams was studied by the AFM method, while their bulk structure and composition was determined by means of the X-ray diffraction (XRD) technique. The gas phase composed of Hg vapor (under pressure 0.1 Pa) and H2 or Ar (under pressure 100 kPa) corresponds to the well-known theoretical model of diluted gas [18]. This allows comparing calculations of kinetic processes occurring at the thin ®lm surface with the experimental data. 2. Experimental 2.1. Thin gold ®lm preparation Thin Au ®lm preparation was performed in an ultra-high vacuum glass system routinely reaching 10 8 Pa. Glass microscope plates from Menzel Glaser
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(Germany) were used as the substrate for Au ®lm deposition. The substrate was prepared by cutting of the plates into 1 cm2 pieces, degreasing them by means of a standard surfactant, and rinsing in a chromic acid mixture and distilled water. Next the substrate was placed in a vacuum chamber. The chamber was equipped with tungsten feedthroughs connected via platinum wire to two pieces of thin platinum foil semi-melted onto the edges of the substrate. This arrangement allowed the measurement of resistance R of the thin Au ®lm during the deposition. Deposition of gold was carried out by means of evaporation of Au wire (Johnson±Mattey grade I, diameter 0.1 mm) from the tungsten heater under pressure below 1 10 7 Pa. The external chamber diameter was 7 cm, and the distance between the gold source and the substrate was 3 cm. To reduce both the surface diffusion of Au on glass and gold coalescence, the vacuum chamber was maintained at 78 K. Deposition was completed when the resistance of the thin Au ®lm at 78 K was 9 O. Other evaporation parameters including the temperature of the chamber, pressure, and heating current (3 A, tungsten wire diameter 0.35 mm) were kept constant during all experiments. Immediately after evaporation all samples were annealed at 370 K (water bath) for a period of 15 min under UHV conditions. Next the chamber was cooled down to 298 K. The resistance of the annealed Au ®lm was 2 O at 298 K. The resistance of gold ®lm enables the standardization of thin Au layer thickness on the surfaces of our substrates. The average thickness of the evaporated Au layer determined by the AFM method was 50 nm. 2.2. Au±Hg formation in the vacuum chamber Immediately after sample preparation the vacuum chamber was connected with the Hg source maintained at 298 K. The residual gas pressure was around 10 7 Pa. The chamber temperature was constant (298 K) for 40 h. At this temperature, the equilibrium Hg vapor pressure reaches 0.24 Pa. The changes of electrical resistance of the thin Au ®lm were continuously recorded. Next the obtained Au±Hg amalgams were moved for further examination (AFM and XRD). After the AFM and XRD measurements the samples of amalgams were placed back in the vacuum chamber and heated at 500 K for 1 h. Finally the
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samples were re-examined by means of the AFM and XRD methods. 2.3. Au±Hg formation in H2, Ar and air atmospheres In order to determine the in¯uence of gas phase composition on the kinetics of the amalgamation process the reaction was carried out in several selected atmospheres of different gases. In the case of the ambient air atmosphere, the thin Au ®lm was placed in a glass reactor, immediately after being removed from the vacuum system. The volume of the glass reactor was similar to that of the vacuum chamber. A drop of mercury was placed at the bottom of the reactor in a small Petri dish. The distance between the sample and the mercury source was around 2 cm. In the case of the other atmospheres the reactor was connected to the gas dosing system. Next it was evacuated using a commercial rotary pump and back-®lled with one of the two carrier gases: argon (5.0 quality) or hydrogen (5.0) (AGA Gas GmbH). To minimize pump oil contamination, the reactor was connected to the gas dosing system through a liquid nitrogen cryotrap. The general scheme of the gas line has been described previously [19]. The construction of the reactor allowed measurement of the ®lm resistance during mercury adsorption. As a reference, thin gold ®lm was placed in the reactor ®lled with each of the above-mentioned gases in turn, however, without the mercury drop. No change of resistance was observed (Fig. 6e). The reproducibility of the electrical resistance data reported in this paper was tested several times by replacing the mercurydosed Au ®lms with new Au samples and repeating the Hg deposition. The agreement of the experimental data from different runs was very good. In all the cases the mercury deposition was continued for 40 h at 298 K. Next the samples were moved for AFM examination. 2.4. Atomic force microscopy The SPM used in these studies is a commercial instrument, model TMX 2000 `Discoverer' (TopoMetrix, California). The topography of the obtained Au and Au±Hg alloy ®lms was studied by means of AFM in contact, and constant force modes. Two scanners
(70, 25 mm in the XY plane) and standard TopoMetrix Si3N4 tips were applied. All scans were performed under an ambient atmosphere. In the case of amalgam ®lms, samples were ®rst moved to the storage vessel and kept for 1±2 h in air without contact with mercury vapor. This procedure was used in order to reach a stable image. For each sample several images at various positions were taken to gain better knowledge of the variations of local structures [20]. 2.5. X-ray diffraction The XRD studies, for the same ®lms as used in the AFM study, were performed on a commercial X-ray Diffractometer Geiger¯ex (Rigaku-Denki) in standard Bragg±Brentano y±2y geometry. The Ni ®ltered CuKa radiation (18 mA, 40 kV) was applied. The step size was set at 0.058 in 2y, and scanning time was 10 s for each step. 3. Results and discussion 3.1. Au±Hg alloy formation in the vacuum chamber For this study, thin Au ®lms were examined by means of the above-mentioned methods: (i) before the amalgamation process, (ii) after 40 h exposition to mercury vapor ``in situ'' in the UHV apparatus, (iii) after storing the ®lm under ambient conditions at 298 K, and ®nally (iv) after thermal decomposition of amalgam under UHV conditions. The AFM image of typical thin gold ®lm used in our study is shown in Fig. 1a. The surface is characterized by a pebble-type structure. The gold grains with mean diameters of about 30 nm and heights of 5 nm are randomly distributed. The XRD pattern (Fig. 2a) exhibits all the peaks expected for gold [21] occurring at about 38.198, 44.398, 64.588, 77.558 and 81.728 in 2y. They correspond to gold (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) planes, respectively. A very pronounced increase in the peak intensity of the (1 1 1) plane parallel to the substrate surface was registered. This is the preferred orientation for polycrystalline fcc metals deposited onto amorphous substrates [22]. When the gold ®lm, after deposition under the UHV conditions, was exposed ``in situ'' to Hg vapor, very dramatic changes in topography occurred (Fig. 1b). The
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Fig. 1. AFM images and surface pro®les obtained during amalgamation of thin Au ®lm in the vacuum chamber: (a) thin gold ®lm; (b) after 40 h interaction of Au with Hg vapor in the vacuum chamber; (c) the same ®lm as in (b) but stored 6 months under ambient conditions without contact with mercury; (d) after thermal decomposition of the amalgam.
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Fig. 2. XRD patterns obtained during amalgamation of thin Au ®lm in the vacuum chamber: (a) thin gold ®lm; (b) after 40 h interaction of Au with Hg vapor in the vacuum chamber; (c) the same ®lm as in (b) but stored 6 months under ambient conditions without contact with mercury; (d) after thermal decomposition of the amalgam.
continuous gold ®lm (Fig. 1a) was transformed into the isolated amalgam islands (Fig. 1b). The islands of 400± 800 nm height were separated by non-covered areas of glass. The distance between the edges of the amalgam
islands varied from 100 nm to 1 mm. In the course of the amalgamation the optical and electrical properties of the ®lm changed drastically. Light re¯ecting, shiny thin gold ®lm was transformed into transparent ®lm similar
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Fig. 3. Relative electrical resistance changes during amalgamation of Au ®lm in an UHV chamber.
to a frosted bulb of milky color. The change of relative resistance registered during mercury exposure is demonstrated in Fig. 3. The electric resistance increases from R0 2 O to 7 1013 O. This ®nal resistance corresponds to that characteristic of glass. Very pronounced differences in the XRD pro®les from thin gold ®lm (Fig. 2a) and the ®lm of Au±Hg alloy obtained after 40 h exposure to Hg vapor in the vacuum chamber (Fig. 2b) are clearly seen. In the latter case new peaks appeared at about 37.648, 38.648, 39.898, 42.838, and 77.628 in 2y, and the intensity of the gold peaks decreased. A mercury-rich amalgam AuHg2 was identi®ed [23]. In the course of further studies the AuHg2 ®lm was stored at 298 K for 6 months under ambient conditions without contact with mercury and then re-scanned. A dramatic change in the structure of the amalgam occurred. The XRD results show that a phase transition from AuHg2 to a gold-rich compound Au3Hg [24] is observed (Fig. 2c). Such a phase transition was also observed by Gomes et al. [25]. They used a quartz crystal microbalance to detect different phases of the Au± Hg system. Surprisingly, the AFM surface topography remained unchanged within the resolution of our experiment (Fig. 1c). Fig. 1d demonstrates the surface
topography after heating of Au3Hg at 500 K for 1 h. The XRD results con®rmed complete amalgam decomposition under these conditions (Fig. 2d). When comparing Fig. 2a and d it is evident that in the latter case the XRD pattern is more like that known for the polycrystalline phase, whereas in Fig. 2a it is clearly from a highly textured ®lm. 3.2. Au±Hg alloy formation under the selected gas atmospheres In the course of this study, Au ®lms were examined: (a) before the amalgamation process and after 40 h of exposure to Hg vapor at 298 K in (b) ambient air, (c) an argon atmosphere and ®nally in (d) a hydrogen atmosphere. In order to determine the composition of these mercury-dosed Au ®lms, an ``ex situ'' XRD study was undertaken. Fig. 4 shows the diffraction patterns of thin Au ®lms obtained before amalgamation (Fig. 4a) and after exposure to Hg vapor under above-mentioned conditions (Fig. 4b±d). It is clearly visible that new peaks developed in patterns: (b) at about 37.478, and (c±d) at about 35.638, 37.478, 40.458, and 69.658 in 2y. These peaks are associated
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Fig. 4. XRD patterns of thin Au ®lms obtained (a) before amalgamation and after 40 h of exposure to Hg vapor at 298 K in (b) ambient air, (c) argon atmosphere and ®nally in (d) hydrogen atmosphere.
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Fig. 5. AFM images and surface pro®les of thin Au ®lms obtained (a) before amalgamation and after 40 h of exposure to Hg vapor at 298 K in (b) ambient air, (c) argon atmosphere and ®nally in (d) hydrogen atmosphere.
with the formation of Au3Hg [24]. In addition, there was a decrease of intensity of (1 1 1) and (2 2 2) gold peaks for pro®les (c) and (d), while the intensities of other gold peaks increased. This is direct evidence that
the amalgamation process induced changes in the Au ®lm structure. The changes of Au ®lm surface morphology caused by amalgamation were studied by means of AFM
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(Fig. 5). We presume that similarly as observed under vacuum conditions, the appearance of pronounced protrusions on the thin Au ®lm surface corresponds to local Au±Hg alloy formation. The surface density of the protrusions then corresponds to the progress of amalgamation. Forty hours exposition of the gold to Hg vapor under the selected gas atmospheres did not result in the formation of the discontinuous, milky Au±Hg alloy ®lms. However, as demonstrated in Fig. 5a±d, there are pronounced differences in surface morphology depending on the gas atmosphere. In Fig. 5a, a typical image of clean gold ®lm is shown. The changes on the surface caused by amalgam formation in an ambient atmosphere are demonstrated in Fig. 5b. Amalgam islands (of mean diameter 300 nm and height 50 nm) with a characteristic protruding structure, seen as bright areas on a dark background corresponding to the smooth gold surface can be observed. The detailed study of gold amalgam formation in an ambient atmosphere was carried out in our laboratory and previously reported [15]. Similar exposure of the gold to mercury vapor in an argon atmosphere (Fig. 5c) led to the formation of larger islands of amalgam (of mean diameter 500 nm and height 75 nm). The amalgam islands are surrounded by pits in the nearest vicinity of the protrusions. The amalgamation process in a hydrogen atmosphere is shown in Fig. 5d. A signi®cantly higher fraction of the sample surface is now covered by Au±Hg alloy. The topography is dominated by decidedly larger amalgam islands. The average height of the structures is around two to three times higher than that observed in an argon atmosphere (compare Fig. 5c). Pits occur in the nearest vicinity of some of the protrusions, similarly as the in argon atmosphere. The pits could be interpreted as the ®rst step in continuous Au ®lm disintegration. A comparison of topography changes observed in Fig. 5a±d suggests that the amalgamation process in argon atmosphere is slower that in a hydrogen atmosphere. However in ambient air the rate of amalgamation seems to be even lower. This can be a result of competitive adsorption of contaminants from the ambient atmosphere such as hydrocarbons or compounds of sulfur. In the case of clean hydrogen or argon atmosphere such co-adsorption did not occur. The same trend was observed in the relative resistance (DR/R0) changes of thin gold ®lms. As mentioned above, it has been well established that
amalgam formation is accompanied by an increase of thin gold ®lm resistance [9±12]. At the beginning of amalgamation the increase of the resistance is almost linearly dependent on mercury uptake [9]. Thus the rate of the resistance changes is proportional to the progress of Au±Hg alloy formation. Our experimental conditions assure the same rate of Hg atoms' reaction with gold for Ar and H2 atmosphere, from the moment when mercury adatoms appear on the Au surface. Therefore the only reason for the difference in the progress of amalgamation is the different rate of Hg atoms' diffusion from the mercury source through the gas phase to the Au ®lm. Fig. 6 presents the resistance curves due to Hg vapor interactions with thin gold ®lms under different gas phases. In three cases corresponding to amalgamation in H2, Ar and air, interaction of mercury with thin gold ®lms results in an increase of electrical resistance. The relative resistance curves have a linear region followed by the onset of saturation. However the saturation level is different in each case. Moreover the big variation in the net increase of gold ®lm relative resistance depending on the gas phase composition during mercury adsorption was observed. In a hydrogen atmosphere it undergoes a net increase of resistance of around 6 O, while only about 2 O in argon atmosphere and 1 O in ambient air. This is clear evidence of the in¯uence of gas phase composition on the kinetics of the amalgamation process. The relative resistance curve observed under vacuum conditions is different. No traces of the onset of saturation were observed. The reproducibility of the described phenomena was very good. The experimental results are compared with the kinetic model. We consider a gaseous mixture consisting of mercury vapor dissolved in the carrier gas: hydrogen, argon and air, respectively. In this system mercury vapor is the inhomogenously distributed trace gas which diffuses through the carrier gas. It can be realistically assumed that at carrier gas pressure 100 kPa the number of Hg atom collisions with the Au surface and consequently the rate of adsorption will be determined by the ¯ux of mercury. The ¯ux of mercury atoms diffusing down a concentration gradient from the mercury source through the carrier gas to the gold ®lm is JHg
D
Dn l
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Fig. 6. Comparison of relative electrical resistance changes during amalgamation of Au ®lm carried out in (a) the vacuum chamber, (b) hydrogen atmosphere, (c) argon atmosphere and (d) ambient air. As a reference (e) the baseline resistivity of thin gold ®lm without contact with mercury.
where D is the diffusion coef®cient, Dn/l the concentration gradient (l is the diffusion path). The diffusion coef®cient of the Hg vapor is calculated from the solution of the Boltzmann equation by the Chapman±Enskog method [18]: mHg 1=2 2kT 1=2 3 1 D 1 16 ncg d2 pmHg mcg where ncg is the carrier gas concentration, k the Boltzmann constant, T the temperature, mHg the mass of mercury atoms, mcg the mass of carrier gas atoms, and d the collisional diameter. The following expression shows the derived ratio of the calculated diffusion coef®cient of mercury in a hydrogen atmosphere to the calculated diffusion coef®cient of mercury in an argon atmosphere: r 2 DHg H2 dHg Ar mAr 2 4:5 DHg Ar dHg H2 mH2 where dHg Ar 12 dHg dAr ;
dHg H2 12 dHg dH2
where mAr, mH2 is the mass of argon atoms and hydrogen molecules, respectively. This result is in good agreement with observed changes in electrical resistance for the Ar and H2 atmosphere. The best match with the mean relative resistance ratio before saturation indicates the predominance of the in¯uence of the mercury atom collision frequency with the gold ®lm surface at the beginning of the amalgamation process. Subsequent changes of the resistance can be in¯uenced by the amalgamation occurring in the bulk. The rate of amalgam formation in air does not ful®l our expectation. The in¯uence of carbon and sulfur compounds certainly present in the air in our laboratory could in¯uence this process. 3.3. Comparison of Au±Hg formation in the vacuum chamber and gas atmosphere At this point it is important to compare the formation of Au±Hg amalgams in selected gas atmospheres and in the vacuum chamber. In spite of the same mercury vapor partial pressure in the reactor and the vacuum chamber, different amalgamation ef®ciency is observed.
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The ¯ux of mercury atoms in the vacuum chamber can be expressed as follows [18]: JHg vac 14 nHgn where n is the mean thermal velocity: 8kT 1=2 n pmHg and nHg is the Hg concentration. Calculations show that the ratio of the ¯ux of mercury atoms in the vacuum chamber to the ¯ux of mercury atoms diffusing through argon is higher than 104: JHg vac 104 JHg Ar A big difference of this ¯ux can also be concluded on the basis of the comparison of thin gold ®lm topography changes after the process of mercury amalgamation performed in the vacuum chamber and in argon. Au±Hg interaction in vacuum leads to the formation of highly discontinuous ®lm consisting of large, isolated islands (Fig. 1b). This discontinuity has been con®rmed by electrical resistance measurements. Moreover there is a dramatic departure from the shape of Au ®lm relative resistance due to mercury adsorption in the vacuum chamber and in the gaseous atmospheres (Fig. 6). In the ®rst case disintegration of the thin Au ®lm leads to a very high increase of relative resistance. The intensive ¯ux of mercury atoms onto the gold ®lm surface results in high density of adsorbate formation, and rapid surface amalgamation. Because of the different structure of gold and Au± Hg alloy, the amalgamation can induce stress leading to rearrangement within the thin Au ®lm. Such a phenomenon was observed recently by Hu et al. [26]. They found that interaction between mercury vapor and thin gold ®lm adsorbed on one side of a bimaterial microcantilever produces stress, resulting in readily measurable curvatures of the cantilever de¯ection. In our case adsorption-induced surface rearrangement creates new defects which are active sites in further Au±Hg alloy formation. The fast and strong increase of density of the active surface sites leads to the rapid amalgamation of gold, changes of the ®lm structure leading to the appearance of pronounced protrusions and the disintegration of the continuous gold ®lm.
4. Conclusions The signi®cance of this article is that it provides a comparative study of 40 h interaction of thin Au ®lms with Hg vapor at 298 K in the vacuum chamber and in several different gases such as H2, Ar and air, kept in a reactor at pressure of around 101 kPa. The results of this investigation clearly demonstrate the in¯uence of gas phase composition on the kinetics of the amalgamation process and can be summarized as follows: (1) Interaction of thin Au ®lms with Hg vapor at 298 K under all the above-mentioned conditions leads to a change in the thin ®lm topography and an increase of electrical resistance. (2) In spite of the same mercury vapor partial pressure in the reactor and vacuum chamber, different amalgamation ef®ciency is observed. Exposure of thin Au ®lm placed in the vacuum chamber to Hg vapor led to very dramatic changes in topography and electrical resistance. Continuous gold ®lm is transformed into isolated amalgam islands and the electric resistance increases from R0 2 O to 7 1013 O. This discontinuous structure of the ®lm remains unchanged after the Au±Hg alloy's thermal decomposition. Thus a continuous thin Au ®lm can be transformed into a discontinuous ®lm consisting of separated Au islands of signi®cant height. When the amalgam formation was carried out in the selected gas atmospheres the discontinuous, milky Au±Hg alloy ®lms were not formed as a result of 40 h exposition of gold to Hg vapor. However there are pronounced differences in the ®lm surface morphology and electrical resistance depending on the gas atmosphere.
Acknowledgements The authors are obliged to Dr. Z. Kaszkur and Dr. B. Mierzwa for their help in XRD measurements.
References [1] M. Grimwade, Interdiscipl. Sci. Rev. 4 (1992) 371. [2] F. Habashi, Principles of Extractive Metallurgy, Gordon and Breach, New York, 1969.
T. Kobiela et al. / Applied Surface Science 206 (2003) 78±89 [3] R. Lappalainen, A. Yli-Upro, Scand. J. Dent. Res. 95 (4) (1987) 364. [4] A. Oden, M. Tullberg, Acta Odontol. Scand. 43 (1) (1985) 15. [5] M.A. Butler, A.J. Ricco, R.J. Baughman, J. Appl. Phys. 67 (9) (1990) 4320. [6] E.P. Scheide, J.K. Taylor, Environ. Sci. Technol. 8 (1974) 1097. [7] C. Battistoni, E. Bemparad, A. Galdikas, S. Kaciulis, G. Mattogano, S. Mickevicius, V. Olevano, Appl. Surf. Sci. 103 (1996) 107. [8] M. Levlin, H.E.M. Niemi, P. Hautojavari, E. Ikavalko, T. Latinen, Fresen. J. Anal. Chem. 355 (1996) 2. [9] J.J. McNerney, P.R. Buseck, Science 178 (1972) 611. [10] M.A. George, W.S. Glausinger, T. Thundat, S.M. Lindsay, Thin Solid Films 189 (1990) 59. [11] M.A. George, Electrical, spectroscopic and morphological investigation of mercury adsorption on thin gold ®lms, Ph.D. Thesis, Arizona State University, USA, 1991. [12] M.A. George, W.S. Glausinger, Thin Solid Films 245 (1994) 215. [13] T.T. Mercer, Anal. Chem. 51 (1979) 1026.
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[14] M. Levlin, E. Ikavalko, T. Latinen, Fresen. J. Anal. Chem. 365 (1999) 577. [15] R. Nowakowski, T. Kobiela, Z. Wolfram, R. DusÂ, Appl. Surf. Sci. 115 (1997) 217. [16] R. Nowakowski, J. Pielaszek, R. DusÂ, Appl. Surf. Sci. 199 (2002) 40. [17] L. StobinÂski, R. DusÂ, Surf. Sci. 298 (1993) 101. [18] S. Chapman, T.G. Cowling, The Mathematical Theory of Nonuniform Gases, Cambridge University Press, London, 1953. [19] T. Kobiela, R. DusÂ, Vacuum 63 (2001) 267. [20] S.J. Fang, S. Haplepete, W. Chen, C.R. Helms, J. Appl. Phys. 82 (1997) 5891. [21] JCPDS Data File No. [04-0784]. [22] K.L. Chopra, Thin Film Phenomena, McGraw-Hill, New York, 1969. [23] JCPDS Data File No. [04-0780]. [24] JCPDS Data File No. [04-0808]. [25] M.T.S.R. Gomes, M.O. Oliveira, J.A.B.P. Oliveira, Langmuir 15 (1999) 8780. [26] Z. Hu, T. Thundat, R.J. Warmack, J. Appl. Phys. 90 (2001) 427.
The in¯uence of gas phase composition on the process of Au±Hg amalgam formation Tomasz Kobiela*, Bogdan Nowakowski, Ryszard Dus Institute of Physical Chemistry, Polish Academy of Sciences, al. Kasprzaka 44/52, 01-224 Warsaw, Poland Received 29 May 2002; accepted 9 October 2002
Abstract Interaction of thin Au ®lms deposited on a glass substrate with Hg vapor at 298 K was studied. The in¯uence of gas phase composition on the kinetics of the amalgamation process was examined. Several gas atmospheres were tried: vacuum (residual gases pressure 1 10 7 Pa, with Hg vapor pressure 0.24 Pa), H2, Ar and air at pressure around 101 kPa. The electrical resistance measurements in the process of Au±Hg amalgam formation were correlated with the atomic force microscopy investigations of amalgam surface structure and X-ray diffraction spectra of the bulk. The results of these studies clearly demonstrate the in¯uence of gas phase composition on the kinetics of the amalgamation process. The experimental data are compared with theoretical calculations of the kinetic processes occurring at the thin Au ®lm surface. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Thin gold ®lm; Au±Hg alloys; Reaction studies by AFM and XRD methods
1. Introduction The signi®cance of the mercury amalgamation process is related to practical applications of gold amalgams [1±4]. Various experimental methods have been applied to study the interaction of thin gold ®lms with mercury vapor [5±8]. There have been several studies using electrical resistance measurements to determine the change of the properties of thin Au ®lms due to mercury adsorption and absorption [9±12]. In order to better understand the transducing mechanism of Au ®lms used as Hg sensors, George and Glausinger [12] have studied the interaction of Hg with gold surfaces using a variety of methods. Resistivity measurements were correlated with surface morphology studies of * Corresponding author. Fax: 48-3912-0238/48-22-632-5276. E-mail address: [email protected] (T. Kobiela).
gold ®lms deposited on various substrates. It was found that polycrystalline, unannealed gold ®lms deposited on alumina demonstrated the largest increase in resistivity due to interaction with mercury. The Au ®lms deposited on silica did saturate after Hg vapor exposure and next showed signi®cantly lower increase in resistance. This was interpreted as the result of the sticking coef®cient of mercury on gold decreasing with coverages greater than one monolayer [11,13]. Recently Levlin et al. [14] investigated the in¯uence of temperature and mercury concentration in the gas phase on the adsorption ef®ciency and gold surface saturation level. They observed a variation of the saturation level depending on the concentration of mercury in the carrier gas. In our previous report [15] we applied atomic force microscopy (AFM) imaging to polycrystalline Au
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 1 1 9 0 - X
T. Kobiela et al. / Applied Surface Science 206 (2003) 78±89
®lms exposed to Hg vapor in an ambient atmosphere. The studies of amalgam formation on various thin gold ®lm structures showed the fundamental importance of surface defects on the rate of amalgamation. In the present work, the in¯uence of gas phase composition on the kinetics of amalgam process is investigated. This study is motivated by the recent report concerning Ag±Hg alloy formation [16]. It has been found that the Ag2Hg3 compound is formed within approximately 10 h when Ag interacts with Hg vapor under vacuum conditions, but a much longer period of several days is required to obtain this compound under ambient atmosphere. In the present paper, we compare resistance measurements of polycrystalline Au ®lms exposed to Hg vapor in the vacuum chamber and in several different gases such as H2, Ar and air, kept at pressure of around 101 kPa. On the basis of the above-mentioned literature it is assumed that the rate of the resistance changes correspond to the rate of amalgam formation in thin gold ®lm deposited on the glass substrate. It has been established that molecular hydrogen and argon does not adsorb on annealed gold surface [17]. Thus these gases do not in¯uence surface processes in the reaction of amalgam formation, but they in¯uence solely Hg diffusion in the gas phase. We cannot exclude some traces of carbon and sulfur compounds in the air in our laboratory. Thus, amalgamation in air could be in¯uenced to some extent by the adsorption of these compounds on the Au surface. The surface structure of amalgams was studied by the AFM method, while their bulk structure and composition was determined by means of the X-ray diffraction (XRD) technique. The gas phase composed of Hg vapor (under pressure 0.1 Pa) and H2 or Ar (under pressure 100 kPa) corresponds to the well-known theoretical model of diluted gas [18]. This allows comparing calculations of kinetic processes occurring at the thin ®lm surface with the experimental data. 2. Experimental 2.1. Thin gold ®lm preparation Thin Au ®lm preparation was performed in an ultra-high vacuum glass system routinely reaching 10 8 Pa. Glass microscope plates from Menzel Glaser
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(Germany) were used as the substrate for Au ®lm deposition. The substrate was prepared by cutting of the plates into 1 cm2 pieces, degreasing them by means of a standard surfactant, and rinsing in a chromic acid mixture and distilled water. Next the substrate was placed in a vacuum chamber. The chamber was equipped with tungsten feedthroughs connected via platinum wire to two pieces of thin platinum foil semi-melted onto the edges of the substrate. This arrangement allowed the measurement of resistance R of the thin Au ®lm during the deposition. Deposition of gold was carried out by means of evaporation of Au wire (Johnson±Mattey grade I, diameter 0.1 mm) from the tungsten heater under pressure below 1 10 7 Pa. The external chamber diameter was 7 cm, and the distance between the gold source and the substrate was 3 cm. To reduce both the surface diffusion of Au on glass and gold coalescence, the vacuum chamber was maintained at 78 K. Deposition was completed when the resistance of the thin Au ®lm at 78 K was 9 O. Other evaporation parameters including the temperature of the chamber, pressure, and heating current (3 A, tungsten wire diameter 0.35 mm) were kept constant during all experiments. Immediately after evaporation all samples were annealed at 370 K (water bath) for a period of 15 min under UHV conditions. Next the chamber was cooled down to 298 K. The resistance of the annealed Au ®lm was 2 O at 298 K. The resistance of gold ®lm enables the standardization of thin Au layer thickness on the surfaces of our substrates. The average thickness of the evaporated Au layer determined by the AFM method was 50 nm. 2.2. Au±Hg formation in the vacuum chamber Immediately after sample preparation the vacuum chamber was connected with the Hg source maintained at 298 K. The residual gas pressure was around 10 7 Pa. The chamber temperature was constant (298 K) for 40 h. At this temperature, the equilibrium Hg vapor pressure reaches 0.24 Pa. The changes of electrical resistance of the thin Au ®lm were continuously recorded. Next the obtained Au±Hg amalgams were moved for further examination (AFM and XRD). After the AFM and XRD measurements the samples of amalgams were placed back in the vacuum chamber and heated at 500 K for 1 h. Finally the
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samples were re-examined by means of the AFM and XRD methods. 2.3. Au±Hg formation in H2, Ar and air atmospheres In order to determine the in¯uence of gas phase composition on the kinetics of the amalgamation process the reaction was carried out in several selected atmospheres of different gases. In the case of the ambient air atmosphere, the thin Au ®lm was placed in a glass reactor, immediately after being removed from the vacuum system. The volume of the glass reactor was similar to that of the vacuum chamber. A drop of mercury was placed at the bottom of the reactor in a small Petri dish. The distance between the sample and the mercury source was around 2 cm. In the case of the other atmospheres the reactor was connected to the gas dosing system. Next it was evacuated using a commercial rotary pump and back-®lled with one of the two carrier gases: argon (5.0 quality) or hydrogen (5.0) (AGA Gas GmbH). To minimize pump oil contamination, the reactor was connected to the gas dosing system through a liquid nitrogen cryotrap. The general scheme of the gas line has been described previously [19]. The construction of the reactor allowed measurement of the ®lm resistance during mercury adsorption. As a reference, thin gold ®lm was placed in the reactor ®lled with each of the above-mentioned gases in turn, however, without the mercury drop. No change of resistance was observed (Fig. 6e). The reproducibility of the electrical resistance data reported in this paper was tested several times by replacing the mercurydosed Au ®lms with new Au samples and repeating the Hg deposition. The agreement of the experimental data from different runs was very good. In all the cases the mercury deposition was continued for 40 h at 298 K. Next the samples were moved for AFM examination. 2.4. Atomic force microscopy The SPM used in these studies is a commercial instrument, model TMX 2000 `Discoverer' (TopoMetrix, California). The topography of the obtained Au and Au±Hg alloy ®lms was studied by means of AFM in contact, and constant force modes. Two scanners
(70, 25 mm in the XY plane) and standard TopoMetrix Si3N4 tips were applied. All scans were performed under an ambient atmosphere. In the case of amalgam ®lms, samples were ®rst moved to the storage vessel and kept for 1±2 h in air without contact with mercury vapor. This procedure was used in order to reach a stable image. For each sample several images at various positions were taken to gain better knowledge of the variations of local structures [20]. 2.5. X-ray diffraction The XRD studies, for the same ®lms as used in the AFM study, were performed on a commercial X-ray Diffractometer Geiger¯ex (Rigaku-Denki) in standard Bragg±Brentano y±2y geometry. The Ni ®ltered CuKa radiation (18 mA, 40 kV) was applied. The step size was set at 0.058 in 2y, and scanning time was 10 s for each step. 3. Results and discussion 3.1. Au±Hg alloy formation in the vacuum chamber For this study, thin Au ®lms were examined by means of the above-mentioned methods: (i) before the amalgamation process, (ii) after 40 h exposition to mercury vapor ``in situ'' in the UHV apparatus, (iii) after storing the ®lm under ambient conditions at 298 K, and ®nally (iv) after thermal decomposition of amalgam under UHV conditions. The AFM image of typical thin gold ®lm used in our study is shown in Fig. 1a. The surface is characterized by a pebble-type structure. The gold grains with mean diameters of about 30 nm and heights of 5 nm are randomly distributed. The XRD pattern (Fig. 2a) exhibits all the peaks expected for gold [21] occurring at about 38.198, 44.398, 64.588, 77.558 and 81.728 in 2y. They correspond to gold (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) planes, respectively. A very pronounced increase in the peak intensity of the (1 1 1) plane parallel to the substrate surface was registered. This is the preferred orientation for polycrystalline fcc metals deposited onto amorphous substrates [22]. When the gold ®lm, after deposition under the UHV conditions, was exposed ``in situ'' to Hg vapor, very dramatic changes in topography occurred (Fig. 1b). The
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Fig. 1. AFM images and surface pro®les obtained during amalgamation of thin Au ®lm in the vacuum chamber: (a) thin gold ®lm; (b) after 40 h interaction of Au with Hg vapor in the vacuum chamber; (c) the same ®lm as in (b) but stored 6 months under ambient conditions without contact with mercury; (d) after thermal decomposition of the amalgam.
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Fig. 2. XRD patterns obtained during amalgamation of thin Au ®lm in the vacuum chamber: (a) thin gold ®lm; (b) after 40 h interaction of Au with Hg vapor in the vacuum chamber; (c) the same ®lm as in (b) but stored 6 months under ambient conditions without contact with mercury; (d) after thermal decomposition of the amalgam.
continuous gold ®lm (Fig. 1a) was transformed into the isolated amalgam islands (Fig. 1b). The islands of 400± 800 nm height were separated by non-covered areas of glass. The distance between the edges of the amalgam
islands varied from 100 nm to 1 mm. In the course of the amalgamation the optical and electrical properties of the ®lm changed drastically. Light re¯ecting, shiny thin gold ®lm was transformed into transparent ®lm similar
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Fig. 3. Relative electrical resistance changes during amalgamation of Au ®lm in an UHV chamber.
to a frosted bulb of milky color. The change of relative resistance registered during mercury exposure is demonstrated in Fig. 3. The electric resistance increases from R0 2 O to 7 1013 O. This ®nal resistance corresponds to that characteristic of glass. Very pronounced differences in the XRD pro®les from thin gold ®lm (Fig. 2a) and the ®lm of Au±Hg alloy obtained after 40 h exposure to Hg vapor in the vacuum chamber (Fig. 2b) are clearly seen. In the latter case new peaks appeared at about 37.648, 38.648, 39.898, 42.838, and 77.628 in 2y, and the intensity of the gold peaks decreased. A mercury-rich amalgam AuHg2 was identi®ed [23]. In the course of further studies the AuHg2 ®lm was stored at 298 K for 6 months under ambient conditions without contact with mercury and then re-scanned. A dramatic change in the structure of the amalgam occurred. The XRD results show that a phase transition from AuHg2 to a gold-rich compound Au3Hg [24] is observed (Fig. 2c). Such a phase transition was also observed by Gomes et al. [25]. They used a quartz crystal microbalance to detect different phases of the Au± Hg system. Surprisingly, the AFM surface topography remained unchanged within the resolution of our experiment (Fig. 1c). Fig. 1d demonstrates the surface
topography after heating of Au3Hg at 500 K for 1 h. The XRD results con®rmed complete amalgam decomposition under these conditions (Fig. 2d). When comparing Fig. 2a and d it is evident that in the latter case the XRD pattern is more like that known for the polycrystalline phase, whereas in Fig. 2a it is clearly from a highly textured ®lm. 3.2. Au±Hg alloy formation under the selected gas atmospheres In the course of this study, Au ®lms were examined: (a) before the amalgamation process and after 40 h of exposure to Hg vapor at 298 K in (b) ambient air, (c) an argon atmosphere and ®nally in (d) a hydrogen atmosphere. In order to determine the composition of these mercury-dosed Au ®lms, an ``ex situ'' XRD study was undertaken. Fig. 4 shows the diffraction patterns of thin Au ®lms obtained before amalgamation (Fig. 4a) and after exposure to Hg vapor under above-mentioned conditions (Fig. 4b±d). It is clearly visible that new peaks developed in patterns: (b) at about 37.478, and (c±d) at about 35.638, 37.478, 40.458, and 69.658 in 2y. These peaks are associated
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Fig. 4. XRD patterns of thin Au ®lms obtained (a) before amalgamation and after 40 h of exposure to Hg vapor at 298 K in (b) ambient air, (c) argon atmosphere and ®nally in (d) hydrogen atmosphere.
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Fig. 5. AFM images and surface pro®les of thin Au ®lms obtained (a) before amalgamation and after 40 h of exposure to Hg vapor at 298 K in (b) ambient air, (c) argon atmosphere and ®nally in (d) hydrogen atmosphere.
with the formation of Au3Hg [24]. In addition, there was a decrease of intensity of (1 1 1) and (2 2 2) gold peaks for pro®les (c) and (d), while the intensities of other gold peaks increased. This is direct evidence that
the amalgamation process induced changes in the Au ®lm structure. The changes of Au ®lm surface morphology caused by amalgamation were studied by means of AFM
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(Fig. 5). We presume that similarly as observed under vacuum conditions, the appearance of pronounced protrusions on the thin Au ®lm surface corresponds to local Au±Hg alloy formation. The surface density of the protrusions then corresponds to the progress of amalgamation. Forty hours exposition of the gold to Hg vapor under the selected gas atmospheres did not result in the formation of the discontinuous, milky Au±Hg alloy ®lms. However, as demonstrated in Fig. 5a±d, there are pronounced differences in surface morphology depending on the gas atmosphere. In Fig. 5a, a typical image of clean gold ®lm is shown. The changes on the surface caused by amalgam formation in an ambient atmosphere are demonstrated in Fig. 5b. Amalgam islands (of mean diameter 300 nm and height 50 nm) with a characteristic protruding structure, seen as bright areas on a dark background corresponding to the smooth gold surface can be observed. The detailed study of gold amalgam formation in an ambient atmosphere was carried out in our laboratory and previously reported [15]. Similar exposure of the gold to mercury vapor in an argon atmosphere (Fig. 5c) led to the formation of larger islands of amalgam (of mean diameter 500 nm and height 75 nm). The amalgam islands are surrounded by pits in the nearest vicinity of the protrusions. The amalgamation process in a hydrogen atmosphere is shown in Fig. 5d. A signi®cantly higher fraction of the sample surface is now covered by Au±Hg alloy. The topography is dominated by decidedly larger amalgam islands. The average height of the structures is around two to three times higher than that observed in an argon atmosphere (compare Fig. 5c). Pits occur in the nearest vicinity of some of the protrusions, similarly as the in argon atmosphere. The pits could be interpreted as the ®rst step in continuous Au ®lm disintegration. A comparison of topography changes observed in Fig. 5a±d suggests that the amalgamation process in argon atmosphere is slower that in a hydrogen atmosphere. However in ambient air the rate of amalgamation seems to be even lower. This can be a result of competitive adsorption of contaminants from the ambient atmosphere such as hydrocarbons or compounds of sulfur. In the case of clean hydrogen or argon atmosphere such co-adsorption did not occur. The same trend was observed in the relative resistance (DR/R0) changes of thin gold ®lms. As mentioned above, it has been well established that
amalgam formation is accompanied by an increase of thin gold ®lm resistance [9±12]. At the beginning of amalgamation the increase of the resistance is almost linearly dependent on mercury uptake [9]. Thus the rate of the resistance changes is proportional to the progress of Au±Hg alloy formation. Our experimental conditions assure the same rate of Hg atoms' reaction with gold for Ar and H2 atmosphere, from the moment when mercury adatoms appear on the Au surface. Therefore the only reason for the difference in the progress of amalgamation is the different rate of Hg atoms' diffusion from the mercury source through the gas phase to the Au ®lm. Fig. 6 presents the resistance curves due to Hg vapor interactions with thin gold ®lms under different gas phases. In three cases corresponding to amalgamation in H2, Ar and air, interaction of mercury with thin gold ®lms results in an increase of electrical resistance. The relative resistance curves have a linear region followed by the onset of saturation. However the saturation level is different in each case. Moreover the big variation in the net increase of gold ®lm relative resistance depending on the gas phase composition during mercury adsorption was observed. In a hydrogen atmosphere it undergoes a net increase of resistance of around 6 O, while only about 2 O in argon atmosphere and 1 O in ambient air. This is clear evidence of the in¯uence of gas phase composition on the kinetics of the amalgamation process. The relative resistance curve observed under vacuum conditions is different. No traces of the onset of saturation were observed. The reproducibility of the described phenomena was very good. The experimental results are compared with the kinetic model. We consider a gaseous mixture consisting of mercury vapor dissolved in the carrier gas: hydrogen, argon and air, respectively. In this system mercury vapor is the inhomogenously distributed trace gas which diffuses through the carrier gas. It can be realistically assumed that at carrier gas pressure 100 kPa the number of Hg atom collisions with the Au surface and consequently the rate of adsorption will be determined by the ¯ux of mercury. The ¯ux of mercury atoms diffusing down a concentration gradient from the mercury source through the carrier gas to the gold ®lm is JHg
D
Dn l
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Fig. 6. Comparison of relative electrical resistance changes during amalgamation of Au ®lm carried out in (a) the vacuum chamber, (b) hydrogen atmosphere, (c) argon atmosphere and (d) ambient air. As a reference (e) the baseline resistivity of thin gold ®lm without contact with mercury.
where D is the diffusion coef®cient, Dn/l the concentration gradient (l is the diffusion path). The diffusion coef®cient of the Hg vapor is calculated from the solution of the Boltzmann equation by the Chapman±Enskog method [18]: mHg 1=2 2kT 1=2 3 1 D 1 16 ncg d2 pmHg mcg where ncg is the carrier gas concentration, k the Boltzmann constant, T the temperature, mHg the mass of mercury atoms, mcg the mass of carrier gas atoms, and d the collisional diameter. The following expression shows the derived ratio of the calculated diffusion coef®cient of mercury in a hydrogen atmosphere to the calculated diffusion coef®cient of mercury in an argon atmosphere: r 2 DHg H2 dHg Ar mAr 2 4:5 DHg Ar dHg H2 mH2 where dHg Ar 12 dHg dAr ;
dHg H2 12 dHg dH2
where mAr, mH2 is the mass of argon atoms and hydrogen molecules, respectively. This result is in good agreement with observed changes in electrical resistance for the Ar and H2 atmosphere. The best match with the mean relative resistance ratio before saturation indicates the predominance of the in¯uence of the mercury atom collision frequency with the gold ®lm surface at the beginning of the amalgamation process. Subsequent changes of the resistance can be in¯uenced by the amalgamation occurring in the bulk. The rate of amalgam formation in air does not ful®l our expectation. The in¯uence of carbon and sulfur compounds certainly present in the air in our laboratory could in¯uence this process. 3.3. Comparison of Au±Hg formation in the vacuum chamber and gas atmosphere At this point it is important to compare the formation of Au±Hg amalgams in selected gas atmospheres and in the vacuum chamber. In spite of the same mercury vapor partial pressure in the reactor and the vacuum chamber, different amalgamation ef®ciency is observed.
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The ¯ux of mercury atoms in the vacuum chamber can be expressed as follows [18]: JHg vac 14 nHgn where n is the mean thermal velocity: 8kT 1=2 n pmHg and nHg is the Hg concentration. Calculations show that the ratio of the ¯ux of mercury atoms in the vacuum chamber to the ¯ux of mercury atoms diffusing through argon is higher than 104: JHg vac 104 JHg Ar A big difference of this ¯ux can also be concluded on the basis of the comparison of thin gold ®lm topography changes after the process of mercury amalgamation performed in the vacuum chamber and in argon. Au±Hg interaction in vacuum leads to the formation of highly discontinuous ®lm consisting of large, isolated islands (Fig. 1b). This discontinuity has been con®rmed by electrical resistance measurements. Moreover there is a dramatic departure from the shape of Au ®lm relative resistance due to mercury adsorption in the vacuum chamber and in the gaseous atmospheres (Fig. 6). In the ®rst case disintegration of the thin Au ®lm leads to a very high increase of relative resistance. The intensive ¯ux of mercury atoms onto the gold ®lm surface results in high density of adsorbate formation, and rapid surface amalgamation. Because of the different structure of gold and Au± Hg alloy, the amalgamation can induce stress leading to rearrangement within the thin Au ®lm. Such a phenomenon was observed recently by Hu et al. [26]. They found that interaction between mercury vapor and thin gold ®lm adsorbed on one side of a bimaterial microcantilever produces stress, resulting in readily measurable curvatures of the cantilever de¯ection. In our case adsorption-induced surface rearrangement creates new defects which are active sites in further Au±Hg alloy formation. The fast and strong increase of density of the active surface sites leads to the rapid amalgamation of gold, changes of the ®lm structure leading to the appearance of pronounced protrusions and the disintegration of the continuous gold ®lm.
4. Conclusions The signi®cance of this article is that it provides a comparative study of 40 h interaction of thin Au ®lms with Hg vapor at 298 K in the vacuum chamber and in several different gases such as H2, Ar and air, kept in a reactor at pressure of around 101 kPa. The results of this investigation clearly demonstrate the in¯uence of gas phase composition on the kinetics of the amalgamation process and can be summarized as follows: (1) Interaction of thin Au ®lms with Hg vapor at 298 K under all the above-mentioned conditions leads to a change in the thin ®lm topography and an increase of electrical resistance. (2) In spite of the same mercury vapor partial pressure in the reactor and vacuum chamber, different amalgamation ef®ciency is observed. Exposure of thin Au ®lm placed in the vacuum chamber to Hg vapor led to very dramatic changes in topography and electrical resistance. Continuous gold ®lm is transformed into isolated amalgam islands and the electric resistance increases from R0 2 O to 7 1013 O. This discontinuous structure of the ®lm remains unchanged after the Au±Hg alloy's thermal decomposition. Thus a continuous thin Au ®lm can be transformed into a discontinuous ®lm consisting of separated Au islands of signi®cant height. When the amalgam formation was carried out in the selected gas atmospheres the discontinuous, milky Au±Hg alloy ®lms were not formed as a result of 40 h exposition of gold to Hg vapor. However there are pronounced differences in the ®lm surface morphology and electrical resistance depending on the gas atmosphere.
Acknowledgements The authors are obliged to Dr. Z. Kaszkur and Dr. B. Mierzwa for their help in XRD measurements.
References [1] M. Grimwade, Interdiscipl. Sci. Rev. 4 (1992) 371. [2] F. Habashi, Principles of Extractive Metallurgy, Gordon and Breach, New York, 1969.
T. Kobiela et al. / Applied Surface Science 206 (2003) 78±89 [3] R. Lappalainen, A. Yli-Upro, Scand. J. Dent. Res. 95 (4) (1987) 364. [4] A. Oden, M. Tullberg, Acta Odontol. Scand. 43 (1) (1985) 15. [5] M.A. Butler, A.J. Ricco, R.J. Baughman, J. Appl. Phys. 67 (9) (1990) 4320. [6] E.P. Scheide, J.K. Taylor, Environ. Sci. Technol. 8 (1974) 1097. [7] C. Battistoni, E. Bemparad, A. Galdikas, S. Kaciulis, G. Mattogano, S. Mickevicius, V. Olevano, Appl. Surf. Sci. 103 (1996) 107. [8] M. Levlin, H.E.M. Niemi, P. Hautojavari, E. Ikavalko, T. Latinen, Fresen. J. Anal. Chem. 355 (1996) 2. [9] J.J. McNerney, P.R. Buseck, Science 178 (1972) 611. [10] M.A. George, W.S. Glausinger, T. Thundat, S.M. Lindsay, Thin Solid Films 189 (1990) 59. [11] M.A. George, Electrical, spectroscopic and morphological investigation of mercury adsorption on thin gold ®lms, Ph.D. Thesis, Arizona State University, USA, 1991. [12] M.A. George, W.S. Glausinger, Thin Solid Films 245 (1994) 215. [13] T.T. Mercer, Anal. Chem. 51 (1979) 1026.
89
[14] M. Levlin, E. Ikavalko, T. Latinen, Fresen. J. Anal. Chem. 365 (1999) 577. [15] R. Nowakowski, T. Kobiela, Z. Wolfram, R. DusÂ, Appl. Surf. Sci. 115 (1997) 217. [16] R. Nowakowski, J. Pielaszek, R. DusÂ, Appl. Surf. Sci. 199 (2002) 40. [17] L. StobinÂski, R. DusÂ, Surf. Sci. 298 (1993) 101. [18] S. Chapman, T.G. Cowling, The Mathematical Theory of Nonuniform Gases, Cambridge University Press, London, 1953. [19] T. Kobiela, R. DusÂ, Vacuum 63 (2001) 267. [20] S.J. Fang, S. Haplepete, W. Chen, C.R. Helms, J. Appl. Phys. 82 (1997) 5891. [21] JCPDS Data File No. [04-0784]. [22] K.L. Chopra, Thin Film Phenomena, McGraw-Hill, New York, 1969. [23] JCPDS Data File No. [04-0780]. [24] JCPDS Data File No. [04-0808]. [25] M.T.S.R. Gomes, M.O. Oliveira, J.A.B.P. Oliveira, Langmuir 15 (1999) 8780. [26] Z. Hu, T. Thundat, R.J. Warmack, J. Appl. Phys. 90 (2001) 427.