- Email: [email protected]
The role of surface roughness in total internal reflection ellipsometry of hybrid systems
Applied Surface Science 256 (2009) 640–644
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
The role of surface roughness in total internal reflection ellipsometry of hybrid systems Z. Balevicius a,*, V. Vaicikauskas a,1, G.-J. Babonas b,2 a b
Institute of Physics, Savanoriu ave. 231, LT-02300 Vilnius, Lithuania Semiconductor Physics Institute, A. Gostauto 11, LT-01108 Vilnius, Lithuania
A R T I C L E I N F O
A B S T R A C T
Article history: Received 17 June 2009 Received in revised form 6 August 2009 Accepted 8 August 2009 Available online 14 August 2009
Total internal reflection ellipsometry (TIRE) technique was used to investigate the role of surface roughness in the hybrid system composed of octadecanethiole layer on Au thin film. The samples with Au films of different microstructure were explored. The experimental results were interpreted in the model, which took into account the surface roughness of Au film in the hybrid system. It was shown that optical parameters of octadecanethiole were in correspondence for samples of different microstructure in the case of adequate models used for interpretation of TIRE data. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Total internal reflection ellipsometry Spectroscopic ellipsometry Surface plasmons Self-assembled monolayers Octadecanethiole
1. Introduction Nowadays, surface plasmon resonance (SPR)-based biosensors [1], as non-destructive, label free, in situ applicable tool for diagnostics and control, are widely used in biotechnology, medicine and food industry. This type of biosensors needs a thin gold film of thickness ca. 50 nm to excite efficiently the surface plasma waves. The SPR-effect is observed as a sharp dip in reflectance. The presence of dielectric material on Au surface results in a shift of the reflectance minimum angle or wavelength depending on the interrogation mode. Hence, the changes in SPR-parameters provide a sensitive technique for detection of thin monolayer-thick surface layers of organic materials deposited on Au. The SPR-technique can be also applied in ellipsometry increasing the sensitivity in the investigations of surface layers [2]. In fact, these measurements represent the spectroscopic ellipsometry that is carried out in conditions of total internal reflection. The combined technique is called as total internal reflection ellipsometry (TIRE) [3,4]. A large sensitivity of TIRE enables one to analyze in detail the structure and properties of thin
* Corresponding author at: Institute of Physics, Nonlinear Optics and Spectroscopy Lab., Savanoriu ave. 231, LT-02300 Vilnius, Lithuania. Tel.: +370 52644866; fax: +370 52644866. E-mail addresses: zbalevicius@ar.fi.lt (Z. Balevicius), [email protected] (V. Vaicikauskas), jgb@pfi.lt (G.-J. Babonas). 1 Tel.: +370 52644866; fax: +370 52644866. 2 Tel.: +370 52619402; fax: +370 52627123. 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.08.033
organic layers. For this purpose an adequate model [5] of the structure under investigation is needed. Thus, the surface roughness and microstructure of Au film should be taken into account in the model of hybrid system. On the one hand, it is known [6] that the properties of Au films depend on the deposition technique. Moreover, as followed from single wavelength SPR studies [7], the model of two layers with significantly different optical constants should be constructed for interpretation of the data obtained for Au film on substrate measured from both sides. On the other hand, the thiol-groups most frequently used for deposition of organic compounds are chemically bond to Au film surface leading to the formation of a 0.12 nm thick S/Au interface layer [8], which could be taken into account at the construction of structural model for interpretation of ellipsometric data. The aim of this work is twofold. Firstly, TIRE was utilized to demonstrate the influence of roughness of the Au layer on the optical response and, as a result, on the determination of optical constants of thin octadecanethiole CH3(CH2)17SH (ODT) layer. Secondly, it was shown that using the adequate model, the analysis of experimental TIRE data has given close optical parameters of ODT layers deposited on Au layers with different microstructure. 2. Materials and methods The experimental set-up (Fig. 1) used for present investigations consisted of a spectral ellipsometer SOPRA GES-5 with rotating analyzer and 458 BK7 glass prism connected with glass slide by the refraction index matching fluid (Cargille, USA). The glass prism was
Z. Balevicius et al. / Applied Surface Science 256 (2009) 640–644
641
Fig. 1. The scheme of TIRE experimental set-up.
mounted on a holder. The ellipsometer operated in the spectral region 0.25–2.0 mm, in which the principle ellipsometric angles c and D could be determined. A 75 W collimated Xenon lamp mounted in an air-cooled module was used as the light source. High quality Rochon prisms mounted on hollow-axis step motors served as polarizer and analyzer. Collimating optics ensures that the beam divergence was about 1 mrad. Both arms of goniometer were computer controlled. The repeatability of the position of both arms was 0.018. The experimental ellipsometric data were analyzed by means of the SOPRA program Winelli [9]. Two samples of glass slide with deposited Au films of different surface roughness were investigated. In the first sample (Au1) the Au film was prepared by thermal evaporation technique in vacuum (105 Torr). In the second sample (Au2), the Au film was produced by magnetron sputtering on glass substrate coated with adhesion layer. The microstructure of the samples under investigation was different because of various technology used (see, e.g., [10,11]). Both Au layers under investigation were polycrystalline. The surface morphology of Au layers was characterized by atomic force microscopy (AFM) (Fig. 2) and profilometry techniques. The micrographs were obtained making use of a scanning probe microscope D3100, Nanoscope IVa (Veeco) and profilometer DESKTAK 32. TIRE measurements have been carried out in the spectral range from 400 to 850 nm (Figs. 3 and 4). The optical constants of Au films for both samples were determined (Fig. 5) from the analysis of TIRE data. The Au-coated glass slides were immersed for 18 h into ethanol solution of octadecanethiole (1 mM C18H37SH/95% ethanol) at room temperature. It is known [12] that rapid chemisorption of the ODT molecules occurs in 20 min followed by slower adsorption stabilized within 2 h. Therefore, the immersion time was long enough for the formation of self-assembled monolayers (SAMs) of ODT on the surfaces of Au films in both samples. After immersion procedure, the samples were washed in ethanol and dried at room temperature. The optical response of composite samples ODT/Au/BK7 was measured in Kretschmann configuration (Fig. 1). The ellipsometric data were analyzed in a multi-layer model and the spectra of optical constants for ODT were determined (Fig. 6). 3. Results and discussion The morphology studies have shown (Fig. 2) that the surface roughness of Au films differed significantly in both samples under investigation. The RMS values of surface roughness were 1.87 and 0.4 nm with the peak values 7 and 2 nm for Au films in samples Au1 and Au2, which have been prepared by thermal evaporation and magnetron sputtering, respectively. According to profilometric
Fig. 2. The AFM micrographs of Au layer surface for samples Au1 (a) and Au2 (b).
data, the thicknesses of Au layers in Au1 and Au2 were correspondingly 35.6 and 44.6 nm. AFM studies of microstructure have shown that Au layers consisted of crystallites of size 50 and 20 nm for two samples, respectively. TIRE measurements have shown that the surface plasmon waves were excited at external angles of incidence equal to 448 and 43.58 for samples Au1 and Au2, respectively. In SPR spectra the dip was observed correspondingly at 624 and 650 nm (Figs. 3 and 4). Numerous publications (see, e.g., [13,14]) were dedicated for studies of the dependence of electrical resistivity in metal thin films due to electron scattering from grain boundaries and film surfaces. The structural features influence also on the optical constants [15,16] and should be taken into account in developing the adequate ellipsometric model and including the corresponding parameters in the description of mechanisms contributing to the optical response.
Z. Balevicius et al. / Applied Surface Science 256 (2009) 640–644
642
included (see, e.g., [19]). However, in the present work a narrow spectral range was analyzed and the contribution of one Lorentz oscillator was enough to achieve a reasonable agreement between calculated and experimental data. The inverse problem in TIRE was solved by a standard procedure. According to experimental geometry used in Winelli software, in model calculations the glass prism was treated as ambient, and air was considered as a substrate, whereas the Au film with ODT layer represented the multi-layer system under investigation. The internal angles of light incidence at the glass–Au film interface were 44.108 and 43.688 for Au1 and Au2 samples, respectively. The calculated ellipsometric spectra were fitted to experimental data for bare Au films assuming the optical parameters for free carriers and interband transitions (1) as adjustable parameters at fixed values of thickness which have been determined from structural studies. The model applied was good enough to describe experimental data with the MSE of 2.0 103. As noted above, the composition of the ‘‘surface’’ Au layers L2 was estimated in the Bruggeman model in effective media approximation with the parameters obtained for ‘‘bulk’’ layers L1. From the fitting procedure it followed that the layers L2 were composed of 0.63Au + 0.37voids and 0.90Au + 0.10voids for samples Au1 and Au2, respectively. A larger gold amount in sample Au2 is in agreement with a smaller surface roughness as compared to that in sample Au1. The refraction index of Au films in samples Au1 and Au2 were in a good agreement with reference data [19,20] (Fig. 5). The analysis of TIRE data on hybrid samples with ODT layer on the top of Au films has been performed by analogues fitting procedure. However, several assumptions have been accepted.
Fig. 3. Spectra of ellipsometric parameters C (a) and D (b) for bare Au layer (1) and Au layer coated with SAM of ODT (2) in sample Au1.
TIRE data were analyzed making use of multi-layer model. In the case of sample Au1 with a larger surface roughness, a two-layer model for ‘‘bulk’’ and ‘‘surface’’ was accepted for Au film. As a rule, effective media approximation (EMA) is used to model the contribution of surface roughness (see, e.g., [17]) and microstructure of mesoporous Au films [18] to the optical response of real surfaces. The thicknesses of surface layers of Au film were assumed to be equal to the RMS values determined from AFM studies. The composition of the surface layer was estimated by the Bruggeman EMA considering the contributions due to Au and voids. The presence of Cr layer of thickness 1.2 nm at the glass slide was also taken into account in the calculations of optical response for sample Au2. In order to model the spectral dependence of ellipsometric parameters, the contributions of Drude and Lorentz oscillators were taken into account [9]: "ðlÞ ¼ "1 þ
X
Bk l 2
k
2
2
½ðl=lLk Þ 1iðl=G Lk Þ
ðl=l p Þ ; 1 þ iðl=g p Þ
(1)
where e1 is the dielectric function due to all higher energy interband transitions, Bk, lLk and GLk is the amplitude parameter, spectral position and damping constant of kth Lorentz line, respectively, lp and gp is the wavelength corresponding to the plasma frequency and scattering parameter of free carriers, respectively. It should be noted that two interband transitions at 450 and 300 nm are taken into account in several models to describe the refraction index or dielectric function of Au in a wider spectral range and both the phase and dimension of critical points can be
Fig. 4. Spectra of ellipsometric parameters C (a) and D (b) for bare Au layer (1) and Au layer coated with SAM of ODT (2) in sample Au2.
Z. Balevicius et al. / Applied Surface Science 256 (2009) 640–644
643
The Cauchy function for refraction index: n¼Aþ
Fig. 5. The optical constants of gold for sample Au1 and Au2 obtained from ellipsometric measurements along with reference data [20].
B
l2
þ
C
l4
(2)
was used in the regression analysis to obtain the optical constants of ODT layer in both samples. The values A = 1.4342 4.9 103, B = 1.8026 103 mm2 and A = 1.4346 8.7 103, B = 1.53 103 mm2 were determined for samples Au1 and Au2, respectively (Fig. 6). It should be noted that the value of refraction index n = 1.45 is most frequently used for ODT in evaluation the thickness of layers on solid surfaces by single wavelength (l = 633 nm) ellipsometry (see, e.g., [21]). This value is close to that (n = 1.46) which follows from molar refractivity [12]. The spectral dependence of ODT refraction index was not widely studied. The reference data from [23] are compared with present TIRE results in Fig. 6. As seen, the present data are close to average value of refraction index used in null-ellipsometry [21] and dispersion agrees well with data in [23]. The refraction index values mentioned above are higher than n 1.12–1.13 determined for ODT in the range 0.4–0.8 mm for ODT SAM used as a very thin photo resists [24]. As seen from Fig. 6, the spectra of refraction index for ODT were quite similar for samples Au1 and Au2 confirming hence the validity of the accepted model. 4. Conclusions TIRE experiments have been carried out on hybrid samples of ODT layer deposited on Au films of different microstructure. The analysis of the optical response has shown that the surface roughness play a significant role in determination of the optical constants of both Au and ODT layers. The adequate model of the hybrid structure allows one to determine the optical constants of ODT layer which are in a good correspondence for the samples of different microstructure.
Fig. 6. Spectra of ODT optical constants obtained for TIRE results along with reference data [23].
Firstly, as is well known [21], the length of ODT chain is 2.2 nm. The head groups of ODT molecules with sulfur are covalently bonded to Au on the surface with the tail groups oriented at the angle of 20–308 from the surface normal [22]. As determined in [23], the ellipsometric thickness 2.0 0.2 nm of ODT monolayer was stable for more than 10 days. Therefore, in the present analysis it was assumed that the thickness of ODT SAM was 2 nm. From this point of view, particular features of the structure and properties of ODT SAM should manifest themselves in the optical constants determined from the optical response of the hybrid samples. Secondly, it is reasonable to expect that the behavior and properties of ODT molecules in SAM are very similar in two samples whereas the difference in surface roughness is to be taken into account by modeling the different structure for two hybrid samples. The regression analysis has shown that the calculated spectra for hybrid sample Au1 can be well fitted to experimental TIRE data only when some part of ODT material was included in the composition of the surface layer L2. The inclusion of interface in the effective media approximation is a usual technique in the fitting procedure to model the optical response of nanoporous Au films (see, e.g., [18]). The best fitting of experimental TIRE data was obtained for the following composition of surface layer L2 in sample Au1: 0.63(Au) + 0.23(voids) and 0.14(ODT). In contrast, for smooth hybrid sample Au2 the modeled spectra were well fitted to TIRE experiment without insertion of ODT material into the modeled thin surface layer.
Acknowledgements The work was partially supported by Agency for International Science and Technology Development Programmes in Lithuania COST program action MP0702. The authors thank Dr. V. Stankevicˇ for preparation of gold layers and V. Bukauskas for experimental assistance in AFM investigations. We are also grateful to R. Budvytyte for octadecanethiole preparation. References [1] J. Homola, S.S. Yee, G. Gaulitz, Surface plasmon resonance sensors: review, Sens. Actuators B 54 (1999) 3–15. [2] H. Arwin, M. Poksinski, K. Johansen, Enhancement in ellipsometric thin film sensitivity near surface plasmon resonance conditions, Phys. Stat. Sol. (a) 205 (2008) 817–820. [3] H. Arwin, M. Poksinski, K. Johansen, Total internal reflection ellipsometry— principles and applications, Appl. Optics 43 (2004) 3028–3036. [4] M. Poksinski, H. Arwin, Protein monolayers monitored by internal reflection ellipsometry, Thin Solid Films 455–456 (2004) 716–721. [5] R.H. Muller, J.C. Farmer, Macroscopic optical model for ellipsometry of an underpotential deposit: lead on copper and silver, Surf. Sci. 135 (1983) 521–531. [6] E.D. Palik, Handbook of Optical Constants of Solids III, Academic, New York, 1998. [7] A. Rueda, N. Vogel, M. Kreiter, Characterization of gold films by surface plasmon spectroscopy: large errors and small consequences, Surf. Sci. 603 (2009) 491–497. [8] J. Shi, B. Hong, A.N. Parikh, R.W. Collins, D.L. Allara, Optical characterization of electronic transitions arising from the Au/S interface of self-assembled n-alkanethiolate monolayers, Chem. Phys. Lett. 246 (1995) 90–94. [9] http://www.sopra-sa.com. [10] O. Hunderi, Optics of rough surfaces, discontinuous films and heterogeneous materials, Surf. Sci. 96 (1980) 1–31. [11] R.H. Muller, C.G. Smith, Use of film-formation models for interpretation of ellipsometric observations, Surf. Sci. 96 (1980) 375–400. [12] M. Sastry, A note on the use of ellipsometry for studying the kinetics of formation of self-assembled monolayers, Bull. Mater. Sci. 23 (2000) 159–163.
644
Z. Balevicius et al. / Applied Surface Science 256 (2009) 640–644
[13] J.M. Camacho, A.I. Oliva, Surface and grain boundary contributions in the electrical resistivity of metallic nanofilms, Thin Solid Films 515 (2006) 1881–1885. [14] R. Dus, E. Nowicka, R. Nowakowski, Electron scattering cross section on the surface of thin Ag and Au films induced by atomic deuterium adsorption, Langmuir 20 (2004) 9138–9143. [15] J. Szczyrbowski, K. Schalzbauer, H. Hoffmann, Optical transmittance and reflectance and dynamic current density for thin metallic films, Phys. Rev. B 32 (1985) 763–770. [16] H.G. Tompkins, S. Tasic, J. Baker, D. Convey, Spectroscopic ellipsometry measurements of thin metal films, Surf. Interf. Anal. 29 (2000) 179–187. [17] S. D’Elia, N. Scaramuzza, F. Ciuchi, C. Versace, G. Strangi, R. Bartolino, Ellipsometry investigation of the effects of annealing temperature on the optical properties of indium tin oxide thin films studied by Drude–Lorentz model, Appl. Surf. Sci. 255 (2009) 7203–7211. [18] G.B. Smith, A.I. Maaroof, A. Gentle, Homogenized Lorentz–Drude optical response in highly nanoporous conducting gold layers produced by de-alloying, Opt. Commun. 271 (2007) 263–268.
[19] P.G. Etchegoin, E.C. Le Ru, M. Meyer, An analytical model for the optical properties of gold, J. Chem. Phys. 125 (2006) 164705. [20] P.B. Johnson, R.W. Christy, Optical constants of the noble metals, Phys. Rev. B 6 (1972) 4370–4379. [21] K. Adlkofer, M. Tanaka, Stable surface coating of gallium arsenide with octadecylthiol monolayers, Langmuir 17 (2001) 4267–4273. [22] M.D. Porter, T.B. Bright, D.L. Allara, C.E.D. Chidsey, Spontaneously organized molecular assemblies. 4. Structural characterization of an n-alkyl thiol monolayers on gols by optical ellipsometry, infrared spectroscopy, and electrochemistry, J. Am. Chem. Soc. 109 (1987) 3559–3568. [23] H.A. Budz, R.R. LaPierre, Properties of octadecanethiol self-assembled monolayers deposited on GaAs from liquid and vapor phases, J. Vac. Sci. Technol. A 26 (2008) 1425–1431. [24] W.-S. Chang, J. Kim, S.-H. Cho, K.-H. Whang, Femtosecond-laser-coupled nearfield scanning optical microscopy pattering using self-assembled monolayers, Jpn. J. Appl. Phys. 45 (2006) 2082–2086.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
The role of surface roughness in total internal reflection ellipsometry of hybrid systems Z. Balevicius a,*, V. Vaicikauskas a,1, G.-J. Babonas b,2 a b
Institute of Physics, Savanoriu ave. 231, LT-02300 Vilnius, Lithuania Semiconductor Physics Institute, A. Gostauto 11, LT-01108 Vilnius, Lithuania
A R T I C L E I N F O
A B S T R A C T
Article history: Received 17 June 2009 Received in revised form 6 August 2009 Accepted 8 August 2009 Available online 14 August 2009
Total internal reflection ellipsometry (TIRE) technique was used to investigate the role of surface roughness in the hybrid system composed of octadecanethiole layer on Au thin film. The samples with Au films of different microstructure were explored. The experimental results were interpreted in the model, which took into account the surface roughness of Au film in the hybrid system. It was shown that optical parameters of octadecanethiole were in correspondence for samples of different microstructure in the case of adequate models used for interpretation of TIRE data. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Total internal reflection ellipsometry Spectroscopic ellipsometry Surface plasmons Self-assembled monolayers Octadecanethiole
1. Introduction Nowadays, surface plasmon resonance (SPR)-based biosensors [1], as non-destructive, label free, in situ applicable tool for diagnostics and control, are widely used in biotechnology, medicine and food industry. This type of biosensors needs a thin gold film of thickness ca. 50 nm to excite efficiently the surface plasma waves. The SPR-effect is observed as a sharp dip in reflectance. The presence of dielectric material on Au surface results in a shift of the reflectance minimum angle or wavelength depending on the interrogation mode. Hence, the changes in SPR-parameters provide a sensitive technique for detection of thin monolayer-thick surface layers of organic materials deposited on Au. The SPR-technique can be also applied in ellipsometry increasing the sensitivity in the investigations of surface layers [2]. In fact, these measurements represent the spectroscopic ellipsometry that is carried out in conditions of total internal reflection. The combined technique is called as total internal reflection ellipsometry (TIRE) [3,4]. A large sensitivity of TIRE enables one to analyze in detail the structure and properties of thin
* Corresponding author at: Institute of Physics, Nonlinear Optics and Spectroscopy Lab., Savanoriu ave. 231, LT-02300 Vilnius, Lithuania. Tel.: +370 52644866; fax: +370 52644866. E-mail addresses: zbalevicius@ar.fi.lt (Z. Balevicius), [email protected] (V. Vaicikauskas), jgb@pfi.lt (G.-J. Babonas). 1 Tel.: +370 52644866; fax: +370 52644866. 2 Tel.: +370 52619402; fax: +370 52627123. 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.08.033
organic layers. For this purpose an adequate model [5] of the structure under investigation is needed. Thus, the surface roughness and microstructure of Au film should be taken into account in the model of hybrid system. On the one hand, it is known [6] that the properties of Au films depend on the deposition technique. Moreover, as followed from single wavelength SPR studies [7], the model of two layers with significantly different optical constants should be constructed for interpretation of the data obtained for Au film on substrate measured from both sides. On the other hand, the thiol-groups most frequently used for deposition of organic compounds are chemically bond to Au film surface leading to the formation of a 0.12 nm thick S/Au interface layer [8], which could be taken into account at the construction of structural model for interpretation of ellipsometric data. The aim of this work is twofold. Firstly, TIRE was utilized to demonstrate the influence of roughness of the Au layer on the optical response and, as a result, on the determination of optical constants of thin octadecanethiole CH3(CH2)17SH (ODT) layer. Secondly, it was shown that using the adequate model, the analysis of experimental TIRE data has given close optical parameters of ODT layers deposited on Au layers with different microstructure. 2. Materials and methods The experimental set-up (Fig. 1) used for present investigations consisted of a spectral ellipsometer SOPRA GES-5 with rotating analyzer and 458 BK7 glass prism connected with glass slide by the refraction index matching fluid (Cargille, USA). The glass prism was
Z. Balevicius et al. / Applied Surface Science 256 (2009) 640–644
641
Fig. 1. The scheme of TIRE experimental set-up.
mounted on a holder. The ellipsometer operated in the spectral region 0.25–2.0 mm, in which the principle ellipsometric angles c and D could be determined. A 75 W collimated Xenon lamp mounted in an air-cooled module was used as the light source. High quality Rochon prisms mounted on hollow-axis step motors served as polarizer and analyzer. Collimating optics ensures that the beam divergence was about 1 mrad. Both arms of goniometer were computer controlled. The repeatability of the position of both arms was 0.018. The experimental ellipsometric data were analyzed by means of the SOPRA program Winelli [9]. Two samples of glass slide with deposited Au films of different surface roughness were investigated. In the first sample (Au1) the Au film was prepared by thermal evaporation technique in vacuum (105 Torr). In the second sample (Au2), the Au film was produced by magnetron sputtering on glass substrate coated with adhesion layer. The microstructure of the samples under investigation was different because of various technology used (see, e.g., [10,11]). Both Au layers under investigation were polycrystalline. The surface morphology of Au layers was characterized by atomic force microscopy (AFM) (Fig. 2) and profilometry techniques. The micrographs were obtained making use of a scanning probe microscope D3100, Nanoscope IVa (Veeco) and profilometer DESKTAK 32. TIRE measurements have been carried out in the spectral range from 400 to 850 nm (Figs. 3 and 4). The optical constants of Au films for both samples were determined (Fig. 5) from the analysis of TIRE data. The Au-coated glass slides were immersed for 18 h into ethanol solution of octadecanethiole (1 mM C18H37SH/95% ethanol) at room temperature. It is known [12] that rapid chemisorption of the ODT molecules occurs in 20 min followed by slower adsorption stabilized within 2 h. Therefore, the immersion time was long enough for the formation of self-assembled monolayers (SAMs) of ODT on the surfaces of Au films in both samples. After immersion procedure, the samples were washed in ethanol and dried at room temperature. The optical response of composite samples ODT/Au/BK7 was measured in Kretschmann configuration (Fig. 1). The ellipsometric data were analyzed in a multi-layer model and the spectra of optical constants for ODT were determined (Fig. 6). 3. Results and discussion The morphology studies have shown (Fig. 2) that the surface roughness of Au films differed significantly in both samples under investigation. The RMS values of surface roughness were 1.87 and 0.4 nm with the peak values 7 and 2 nm for Au films in samples Au1 and Au2, which have been prepared by thermal evaporation and magnetron sputtering, respectively. According to profilometric
Fig. 2. The AFM micrographs of Au layer surface for samples Au1 (a) and Au2 (b).
data, the thicknesses of Au layers in Au1 and Au2 were correspondingly 35.6 and 44.6 nm. AFM studies of microstructure have shown that Au layers consisted of crystallites of size 50 and 20 nm for two samples, respectively. TIRE measurements have shown that the surface plasmon waves were excited at external angles of incidence equal to 448 and 43.58 for samples Au1 and Au2, respectively. In SPR spectra the dip was observed correspondingly at 624 and 650 nm (Figs. 3 and 4). Numerous publications (see, e.g., [13,14]) were dedicated for studies of the dependence of electrical resistivity in metal thin films due to electron scattering from grain boundaries and film surfaces. The structural features influence also on the optical constants [15,16] and should be taken into account in developing the adequate ellipsometric model and including the corresponding parameters in the description of mechanisms contributing to the optical response.
Z. Balevicius et al. / Applied Surface Science 256 (2009) 640–644
642
included (see, e.g., [19]). However, in the present work a narrow spectral range was analyzed and the contribution of one Lorentz oscillator was enough to achieve a reasonable agreement between calculated and experimental data. The inverse problem in TIRE was solved by a standard procedure. According to experimental geometry used in Winelli software, in model calculations the glass prism was treated as ambient, and air was considered as a substrate, whereas the Au film with ODT layer represented the multi-layer system under investigation. The internal angles of light incidence at the glass–Au film interface were 44.108 and 43.688 for Au1 and Au2 samples, respectively. The calculated ellipsometric spectra were fitted to experimental data for bare Au films assuming the optical parameters for free carriers and interband transitions (1) as adjustable parameters at fixed values of thickness which have been determined from structural studies. The model applied was good enough to describe experimental data with the MSE of 2.0 103. As noted above, the composition of the ‘‘surface’’ Au layers L2 was estimated in the Bruggeman model in effective media approximation with the parameters obtained for ‘‘bulk’’ layers L1. From the fitting procedure it followed that the layers L2 were composed of 0.63Au + 0.37voids and 0.90Au + 0.10voids for samples Au1 and Au2, respectively. A larger gold amount in sample Au2 is in agreement with a smaller surface roughness as compared to that in sample Au1. The refraction index of Au films in samples Au1 and Au2 were in a good agreement with reference data [19,20] (Fig. 5). The analysis of TIRE data on hybrid samples with ODT layer on the top of Au films has been performed by analogues fitting procedure. However, several assumptions have been accepted.
Fig. 3. Spectra of ellipsometric parameters C (a) and D (b) for bare Au layer (1) and Au layer coated with SAM of ODT (2) in sample Au1.
TIRE data were analyzed making use of multi-layer model. In the case of sample Au1 with a larger surface roughness, a two-layer model for ‘‘bulk’’ and ‘‘surface’’ was accepted for Au film. As a rule, effective media approximation (EMA) is used to model the contribution of surface roughness (see, e.g., [17]) and microstructure of mesoporous Au films [18] to the optical response of real surfaces. The thicknesses of surface layers of Au film were assumed to be equal to the RMS values determined from AFM studies. The composition of the surface layer was estimated by the Bruggeman EMA considering the contributions due to Au and voids. The presence of Cr layer of thickness 1.2 nm at the glass slide was also taken into account in the calculations of optical response for sample Au2. In order to model the spectral dependence of ellipsometric parameters, the contributions of Drude and Lorentz oscillators were taken into account [9]: "ðlÞ ¼ "1 þ
X
Bk l 2
k
2
2
½ðl=lLk Þ 1iðl=G Lk Þ
ðl=l p Þ ; 1 þ iðl=g p Þ
(1)
where e1 is the dielectric function due to all higher energy interband transitions, Bk, lLk and GLk is the amplitude parameter, spectral position and damping constant of kth Lorentz line, respectively, lp and gp is the wavelength corresponding to the plasma frequency and scattering parameter of free carriers, respectively. It should be noted that two interband transitions at 450 and 300 nm are taken into account in several models to describe the refraction index or dielectric function of Au in a wider spectral range and both the phase and dimension of critical points can be
Fig. 4. Spectra of ellipsometric parameters C (a) and D (b) for bare Au layer (1) and Au layer coated with SAM of ODT (2) in sample Au2.
Z. Balevicius et al. / Applied Surface Science 256 (2009) 640–644
643
The Cauchy function for refraction index: n¼Aþ
Fig. 5. The optical constants of gold for sample Au1 and Au2 obtained from ellipsometric measurements along with reference data [20].
B
l2
þ
C
l4
(2)
was used in the regression analysis to obtain the optical constants of ODT layer in both samples. The values A = 1.4342 4.9 103, B = 1.8026 103 mm2 and A = 1.4346 8.7 103, B = 1.53 103 mm2 were determined for samples Au1 and Au2, respectively (Fig. 6). It should be noted that the value of refraction index n = 1.45 is most frequently used for ODT in evaluation the thickness of layers on solid surfaces by single wavelength (l = 633 nm) ellipsometry (see, e.g., [21]). This value is close to that (n = 1.46) which follows from molar refractivity [12]. The spectral dependence of ODT refraction index was not widely studied. The reference data from [23] are compared with present TIRE results in Fig. 6. As seen, the present data are close to average value of refraction index used in null-ellipsometry [21] and dispersion agrees well with data in [23]. The refraction index values mentioned above are higher than n 1.12–1.13 determined for ODT in the range 0.4–0.8 mm for ODT SAM used as a very thin photo resists [24]. As seen from Fig. 6, the spectra of refraction index for ODT were quite similar for samples Au1 and Au2 confirming hence the validity of the accepted model. 4. Conclusions TIRE experiments have been carried out on hybrid samples of ODT layer deposited on Au films of different microstructure. The analysis of the optical response has shown that the surface roughness play a significant role in determination of the optical constants of both Au and ODT layers. The adequate model of the hybrid structure allows one to determine the optical constants of ODT layer which are in a good correspondence for the samples of different microstructure.
Fig. 6. Spectra of ODT optical constants obtained for TIRE results along with reference data [23].
Firstly, as is well known [21], the length of ODT chain is 2.2 nm. The head groups of ODT molecules with sulfur are covalently bonded to Au on the surface with the tail groups oriented at the angle of 20–308 from the surface normal [22]. As determined in [23], the ellipsometric thickness 2.0 0.2 nm of ODT monolayer was stable for more than 10 days. Therefore, in the present analysis it was assumed that the thickness of ODT SAM was 2 nm. From this point of view, particular features of the structure and properties of ODT SAM should manifest themselves in the optical constants determined from the optical response of the hybrid samples. Secondly, it is reasonable to expect that the behavior and properties of ODT molecules in SAM are very similar in two samples whereas the difference in surface roughness is to be taken into account by modeling the different structure for two hybrid samples. The regression analysis has shown that the calculated spectra for hybrid sample Au1 can be well fitted to experimental TIRE data only when some part of ODT material was included in the composition of the surface layer L2. The inclusion of interface in the effective media approximation is a usual technique in the fitting procedure to model the optical response of nanoporous Au films (see, e.g., [18]). The best fitting of experimental TIRE data was obtained for the following composition of surface layer L2 in sample Au1: 0.63(Au) + 0.23(voids) and 0.14(ODT). In contrast, for smooth hybrid sample Au2 the modeled spectra were well fitted to TIRE experiment without insertion of ODT material into the modeled thin surface layer.
Acknowledgements The work was partially supported by Agency for International Science and Technology Development Programmes in Lithuania COST program action MP0702. The authors thank Dr. V. Stankevicˇ for preparation of gold layers and V. Bukauskas for experimental assistance in AFM investigations. We are also grateful to R. Budvytyte for octadecanethiole preparation. References [1] J. Homola, S.S. Yee, G. Gaulitz, Surface plasmon resonance sensors: review, Sens. Actuators B 54 (1999) 3–15. [2] H. Arwin, M. Poksinski, K. Johansen, Enhancement in ellipsometric thin film sensitivity near surface plasmon resonance conditions, Phys. Stat. Sol. (a) 205 (2008) 817–820. [3] H. Arwin, M. Poksinski, K. Johansen, Total internal reflection ellipsometry— principles and applications, Appl. Optics 43 (2004) 3028–3036. [4] M. Poksinski, H. Arwin, Protein monolayers monitored by internal reflection ellipsometry, Thin Solid Films 455–456 (2004) 716–721. [5] R.H. Muller, J.C. Farmer, Macroscopic optical model for ellipsometry of an underpotential deposit: lead on copper and silver, Surf. Sci. 135 (1983) 521–531. [6] E.D. Palik, Handbook of Optical Constants of Solids III, Academic, New York, 1998. [7] A. Rueda, N. Vogel, M. Kreiter, Characterization of gold films by surface plasmon spectroscopy: large errors and small consequences, Surf. Sci. 603 (2009) 491–497. [8] J. Shi, B. Hong, A.N. Parikh, R.W. Collins, D.L. Allara, Optical characterization of electronic transitions arising from the Au/S interface of self-assembled n-alkanethiolate monolayers, Chem. Phys. Lett. 246 (1995) 90–94. [9] http://www.sopra-sa.com. [10] O. Hunderi, Optics of rough surfaces, discontinuous films and heterogeneous materials, Surf. Sci. 96 (1980) 1–31. [11] R.H. Muller, C.G. Smith, Use of film-formation models for interpretation of ellipsometric observations, Surf. Sci. 96 (1980) 375–400. [12] M. Sastry, A note on the use of ellipsometry for studying the kinetics of formation of self-assembled monolayers, Bull. Mater. Sci. 23 (2000) 159–163.
644
Z. Balevicius et al. / Applied Surface Science 256 (2009) 640–644
[13] J.M. Camacho, A.I. Oliva, Surface and grain boundary contributions in the electrical resistivity of metallic nanofilms, Thin Solid Films 515 (2006) 1881–1885. [14] R. Dus, E. Nowicka, R. Nowakowski, Electron scattering cross section on the surface of thin Ag and Au films induced by atomic deuterium adsorption, Langmuir 20 (2004) 9138–9143. [15] J. Szczyrbowski, K. Schalzbauer, H. Hoffmann, Optical transmittance and reflectance and dynamic current density for thin metallic films, Phys. Rev. B 32 (1985) 763–770. [16] H.G. Tompkins, S. Tasic, J. Baker, D. Convey, Spectroscopic ellipsometry measurements of thin metal films, Surf. Interf. Anal. 29 (2000) 179–187. [17] S. D’Elia, N. Scaramuzza, F. Ciuchi, C. Versace, G. Strangi, R. Bartolino, Ellipsometry investigation of the effects of annealing temperature on the optical properties of indium tin oxide thin films studied by Drude–Lorentz model, Appl. Surf. Sci. 255 (2009) 7203–7211. [18] G.B. Smith, A.I. Maaroof, A. Gentle, Homogenized Lorentz–Drude optical response in highly nanoporous conducting gold layers produced by de-alloying, Opt. Commun. 271 (2007) 263–268.
[19] P.G. Etchegoin, E.C. Le Ru, M. Meyer, An analytical model for the optical properties of gold, J. Chem. Phys. 125 (2006) 164705. [20] P.B. Johnson, R.W. Christy, Optical constants of the noble metals, Phys. Rev. B 6 (1972) 4370–4379. [21] K. Adlkofer, M. Tanaka, Stable surface coating of gallium arsenide with octadecylthiol monolayers, Langmuir 17 (2001) 4267–4273. [22] M.D. Porter, T.B. Bright, D.L. Allara, C.E.D. Chidsey, Spontaneously organized molecular assemblies. 4. Structural characterization of an n-alkyl thiol monolayers on gols by optical ellipsometry, infrared spectroscopy, and electrochemistry, J. Am. Chem. Soc. 109 (1987) 3559–3568. [23] H.A. Budz, R.R. LaPierre, Properties of octadecanethiol self-assembled monolayers deposited on GaAs from liquid and vapor phases, J. Vac. Sci. Technol. A 26 (2008) 1425–1431. [24] W.-S. Chang, J. Kim, S.-H. Cho, K.-H. Whang, Femtosecond-laser-coupled nearfield scanning optical microscopy pattering using self-assembled monolayers, Jpn. J. Appl. Phys. 45 (2006) 2082–2086.