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RF-sputtering of gold on silica surfaces: Evolution from clusters to continuous films
Materials Science and Engineering C 25 (2005) 599 – 603 www.elsevier.com/locate/msec
RF-sputtering of gold on silica surfaces: Evolution from clusters to continuous films L. Armelao a, D. Barreca a,*, G. Bottaro a, G. Bruno b, A. Gasparotto c, M. Losurdo b, E. Tondello c a
ISTM-CNR and INSTM, Department of Chemistry, University of Padova, via Marzolo, 1-35131 Padova, Italy b IMIP-CNR and INSTM, via Orabona, 4-70126 Bari, Italy c Department of Chemistry and INSTM, University of Padova, via Marzolo, 1-35131 Padova, Italy Available online 2 August 2005
Abstract Au/SiO2 nanosystems were prepared by RF-sputtering of gold from Ar plasmas on amorphous silica substrates at temperatures as low as 60 -C. The interrelation between nanosystem properties and synthesis conditions was investigated in detail, with particular attention to the nucleation and coalescence processes of Au nanoparticles on the SiO2 surface. To this regard, special emphasis was given to the interplay between the system morphology and the resulting optical properties, as probed by both in-situ and ex-situ characterization techniques. In this context, Spectroscopic Ellipsometry (SE) enabled to gain important information on the energy dispersion of the complex dielectric function and, subsequently, on the evolution from dispersed clusters to continuous films. Such variations were tailored by proper combinations of the bias potential (V bias) and deposition time, indicating the possibility of a fine modulation of the optical response. D 2005 Elsevier B.V. All rights reserved. Keywords: Au/SiO2 nanosystems; Spectroscopic Ellipsometry; Clusters; Continuous films
1. Introduction Metal nanoparticles on oxide substrates have gained a markedly increasing consideration with regard to both scientific and technological purposes [1]. In particular, gold-silica nanosystems are among the most studied, thanks to their extensive applications in heterogeneous catalysis [2,3] and optics (non-linear devices, fiber optics chemical sensors) [1,4 – 6]. As a matter of fact, supported Au nanoparticles display size- and shape-dependent properties, which can further be tailored by varying their distribution on the substrate and interparticle spacing [4]. In particular, significant variations in the Au/SiO2 optical response are induced by modifications of the system morphology from cluster-like systems, where gold nanoparticles are dispersed on the silica surface, to island-like structures, where Au aggregates are partially interconnected between each other,
* Corresponding author. Tel.: +39 49 8275170; fax: +39 49 8275161. E-mail address: [email protected] (D. Barreca). 0928-4931/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2005.06.007
and ultimately, to continuous films [5,7]. The control of the Au particle distribution and concentration is therefore a keystep in order to develop nanosystems endowed with welltailored optical properties. To this regard, in-situ and ex-situ optical methods for real-time growth monitoring and feedback-control of the deposition process have attracted a great deal of attention due to their non-destructive and non-invasive nature [8]. In particular, Spectroscopic Ellipsometry (SE) is a powerful and versatile technique for the determination of the energy dispersion of the complex dielectric function, ((x) = ( 1(x) + i( 2(x) = (n + ik)2, and of the refractive index, n, and the extinction coefficient, k, of Au/SiO2 nanosystems with high accuracy, even in the spectral region of strong absorption. The use of a suitable modeling enables to obtain valuable information on the interrelations between structure and optical properties even for low-size nanosystems, whose detection and analysis is a hard task by means of conventional structural techniques alone [9]. Despite different SE studies on both Au island-like systems and continuous films on Si and SiO2 have appeared in the literature [1,8,10], most
600
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investigations have not been completely exhaustive. In particular, the system evolution from clusters to continuous films as a function of the preparation conditions has never been thoroughly examined. The possibility of obtaining precise material features by modulating Au/SiO2 nanostructure and morphology has motivated the investigation of several preparation methodologies [1,10]. Among the various synthetic approaches, RF-sputtering is one of the most feasible thanks to its inherent versatility and the capability of tailoring the metal content, as well as the particle dispersion, under soft and controlled conditions. In the present work, Au nanoparticles were deposited on SiO2 by RF-sputtering from Ar plasmas at temperatures as low as 60 -C, in order to minimize eventual thermal contributions with respect to plasma-dependent phenomena. The interrelations between nanosystem properties and processing parameters were investigated by means of a multi-technique characterization. While LRI (Laser Reflection Interferometry) was employed for the in-situ growth monitoring, ex-situ analyses were specifically focused on the Au/SiO2 nanostructure and morphology by GlancingIncidence X-ray Diffraction (GIXRD) and Atomic Force Microscopy (AFM). In particular, a comparative SE investigation of Au/SiO2 specimens prepared under different conditions (bias potential and sputtering time) was undertaken with the aim of obtaining an insight on the cluster-to-film evolution. To this aim, the energy dispersion of the optical constants was determined in the 0.75– 6.5 eV range and followed by a suitable modeling procedure, supported by results obtained from other characterization techniques.
2.2. Characterization In-situ LRI measurements were performed by a custombuilt apparatus, adopting a He – Ne diode laser (k = 670 nm) incident on the substrate at an angle of 70- from the normal. The reflected light was collected by a PIN diode and subsequently digitized and recorded vs. time. An interference filter centered at 670 nm with a 10-nm bandwidth was mounted before the detector diode. The silica substrates used for LRI measurements were ground on the back surface in order to prevent undesired reflections from the substrate/electrode interface. GIXRD patterns were recorded by a Bruker D8 Advance diffractometer equipped with a Go¨bel mirror and a CuKa source (40 kV, 40 mA), at a fixed incidence angle of 1.5-. The average crystallite dimensions were estimated by means of the Scherrer formula. SE spectra of the real, b( 1, and imaginary, b( 2, parts of the pseudodielectric function, b( = b( 1 + ib( 2 were measured in the 0.75 –6.5 eV energy range by a phase modulated spectroscopic ellipsometer (UVISEL-Jobin Yvon) at an incidence angle of 70.4-. The Bruggeman effective medium approximation (BEMA) combined with a standard regression method was used to analyze the measured SE spectra by a simple threephase model, i.e., SiO2 substrate/Au/air, the film thickness and/or clusters size and distribution being the fitting parameters. The optical constants of bulk Au were used for the SE spectra analysis. AFM measurements were performed in non-contact mode and in air using a Autoprobe CP Thermomicroscope instrument.
3. Results and discussion 2. Experimental 2.1. Synthesis Au depositions were performed in Ar plasmas by a custom-built RF-sputtering system (m = 13.56 MHz) consisting of a vacuum metal chamber with two vertical electrodes 5 cm apart. One of the two electrodes was coupled to an RF power generator (Cesar 133, Thin Films) through a matching box (VM 1000, Thin Films), while the other one, electrically grounded, was equipped with a resistive heater and its temperature was monitored by means of a thermocouple and an external controller. A 5-cm diameter gold target (0.1 mm thick; BAL-TEC AG, 99.99%) located on the RF electrode was used as gold source. The Herasil\ silica substrates (Heraeus) were placed on the grounded electrode. Control of RF-power and total pressure enabled tailored modifications of the bias potential (V bias), the Direct Current (DC) potential developed on the target [11], that, together with variations of the deposition time, strongly affected the properties of the obtained Au/SiO2 nanosystems.
The sputtering yield, which controls the nucleation and coalescence of Au clusters, is governed by the bias potential (V bias), the finger-print for sputtering experiments. This parameter, in turn, is related to the pressure ( p) and RFpower (W) of the process by the well-known relation [11]: jVbias j”
W p
1=2 :
ð1Þ
In the present work, combined variations of p and W resulted in bias potential values between 460 and 650 V. Fig. 1 shows typical LRI traces recorded during gold deposition on silica at different |V bias| values. As indicated in the caption, samples D and E were obtained at the same bias potential as B, but interrupting the sputtering process at 240 and 60 s. For samples AYC, obtained at 600 s deposition time, the final reflectance value increased with |V bias|, suggesting a concomitant rise of the overall deposited gold amount. The dependence of the growth rate on the bias potential is reported in Fig. 2, indicating that the sputtering yield, and
L. Armelao et al. / Materials Science and Engineering C 25 (2005) 599 – 603
601
9.4 A
18
9.2
Crystallite size (nm)
Vbias = -644 V 16 B
reflectance (%)
14
Vbias = -577 V
12 D 10
8.8 8.6 8.4 8.2 8.0
Vbias = -466 V
7.8
C
8
9.0
-650
-600
-550
-500
-450
V bias (V)
6
E Fig. 3. Dependence of the average gold crystallite size on V bias for Au specimens obtained at 600 s deposition time.
4 0
100
200
300
400
500
600
time (s)
hence, the deposited Au amount linearly increased with |V bias|. Irrespective of the synthesis conditions, GIXRD measurements indicated the presence of fcc gold [JCPDS #4-784, 2000]. The impact of the bias potential on the crystallite size determined by GIXRD is displayed in Fig. 3. As can be observed, higher sputtering yields resulted in the formation of bigger gold nanocrystallites. The observed linear dependence suggested that controlled V bias variations enabled to tailor the coherent diffraction domain size, which determines the electronic-optical properties of Au/SiO2 nanosystems. The interplay between the process parameters (V bias, deposition time), the structure/morphology of gold-silica specimens and the corresponding optical response have been determined by SE. Fig. 4 shows the measured spectra of the
(a) 5
5
k
n
k
3 2
2
2
2
3 4 Photon Energy (eV)
5
6
3 4 5 6 Photon Energy (eV)
2
C
D D
B
E E
A 1
1
2
3
4
5
6
5
6
Photon Energy (eV)
(b) 9
A
7
-3
6
< k>
70
Growth rate (nm/s)
11
n
3
8 80x 10
4
4
3
Fig. 1. LRI traces recorded during Au deposition on SiO2 for selected specimens. The vertical dotted line marks the end of deposition experiments for samples A, B, C. The bias potentials and sputtering times for the different specimens are as follows: (A) 644 V, 600 s; (B) 577 V, 600 s; (C) 466 V, 600 s; (D) 577 V, 240 s; (E) 577 V, 60 s.
60
5 4
50
3
40
2 1
30
B C
D E
0 1
20 -650
-600
-550
-500
-450
V bias (V ) Fig. 2. Dependence of the Au growth rate on the bias potential (V bias).
2
3
4
Photon Energy (eV) Fig. 4. Spectra of the refractive index bn (a) and extinction coefficient bk (b) for different Au/SiO2 nanosystems. The inset of (a) displays the spectrum of a bulk Au film used for comparison.
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L. Armelao et al. / Materials Science and Engineering C 25 (2005) 599 – 603
pseudo-refractive index, bn, and the pseudo-extinction coefficient, bk, for the specimens indicated in Fig. 1. The spectra at the B, D and E points, which correspond to specimens obtained at different sputtering time and fixed V bias (compare Fig. 1), describe the growth kinetics, while a comparison between A, B and C evidences the influence of the bias potential. It is worth noting that systems with a comparable final reflectivity, but obtained under different sputtering conditions (Fig. 1, points C, D), showed different SE spectra, indicating that the V bias value and sputtering times significantly affected the structural and optical properties of Au/SiO2 nanosystems. Therefore, SE analysis was specifically aimed at elucidating the effect of V bias and sputtering time on the system evolution. Samples A and B showed a similar behavior throughout the investigated photon energy range (Fig. 4), and their bn and bk spectra resembled those of a bulk reference Au film shown in the inset. The energy dependence of bk was characterized by the stronger interband 5dY6sp transition with an onset at 2.5 eV near the L point in the Brillouin zone [12]. The contribution of interband transitions was predominant at photon energies above 2.5 eV, while the bk increase observed below 2.5 eV was related to intraband transitions, that can be described by the Drude free-electron model. For energies higher than 2.5 eV, differences in the optical spectra were due to variations in surface roughness. As a whole, these data suggested that specimens A and B are optically thick nanostructured Au films. Conversely, spectra of samples D and E were drastically different from the former specimens in the 0.75 –2.5 eV range, where the measured bk showed a maximum at 1.9 eV, which was not observed in the spectra of A and B (Fig. 4b). Finally, the bk spectrum of sample C represented a transition between the previous categories (A, B and D, E). Fitting SE spectra to optical models enabled to derive the film thickness and/or cluster size and distribution. Some representative examples of the best-fit models are reported in Fig. 5 for B, C and D specimens. The obtained results showed that A- and B-like systems were continuous and nanostructured Au films, while C-like samples were characterized by the partial coalescence of gold nanoaggregates. Finally, D- and E-like specimens were formed by dispersed Au nanoclusters on
D
SiO2. Spectral modeling yielded a diameter of the hemispherical Au crystallites between 6 and 7 nm, with an average distance of å 17 nm and a 30% Au surface coverage for specimen D. The particle size and distribution affected the energy position of the peak observed in the bk spectra, while the relative distance and, hence, the surface coverage, influenced its amplitude. This peak could be identified as the Surface Plasmon Resonance (SPR) of gold nanoparticles, in agreement with previous literature results. For spherical aggregates with a radius of å 2 – 6 nm distributed with an average spacing of å 4– 12 nm, the SPR band is located at å 522 nm (2.4 eV) [13]. As determined by SE analysis, the larger cluster size and average separation obtained in the present case could explain the red-shift to å 1.9 eV of the SPR band [14]. The main difference between E and D spectra was the slightly higher silica coverage by gold particles in the latter case. Fig. 4 shows that in the bk spectrum of sample C the peak almost disappeared and shifted to lower energy, consistently with an increase of the average cluster size. This spectrum can be considered representative of the transition from nanostructured films to cluster-like systems by a progressive coalescence of Au nanoparticles, as already mentioned. In fact, the SE best-fit model for sample C indicated the presence of a very thin Au layer completely covering the surface characterized by hemispherical islands. As a whole, the obtained results suggested a significant influence of the sputtering yield and time on the morphology of the obtained Au/SiO2 systems. In particular, samples C and D had comparable final reflectivity values (see Fig. 1), but the difference in their SE spectra (Fig. 4) pointed out to different morphologies in the two cases. In order to elucidate this point, AFM measurements were performed on both specimens and representative micrographs are displayed in Fig. 6. As can be noticed, sample C was characterized by the presence of polygonal-like islands and a smaller aspect ratio (height/width) with respect to sample D. The latter displayed the presence of threedimensional Au particles, suggesting a predominance of vertical growth with respect to the lateral one. These results could be explained taking into account that larger Au
C
B Au+27 3%voids
Au+55 1%voids
17.5 .5 0.5 nm 6.5 0.5 nm
Au
10.5 2.0 nm 5.7 1.0 nm
Au
12.0
2.5 nm
34.0
2.8 nm
SiO2
SiO2
SiO2
Clusters
Coalescence
Continuous film+ Surface roughness
Fig. 5. Schematic representation of the best-fit models for Au/SiO2 nanosystems: cluster-like (D), island-like (C) and continuous films (B).
L. Armelao et al. / Materials Science and Engineering C 25 (2005) 599 – 603
Fig. 6. Representative AFM micrographs (200 200 nm2) of C and D samples. Vertical scale is from 0 to 6 nm.
crystallites were formed at higher |V bias|, and, hence, higher sputtering yields (compare Fig. 3). Their low surface mobility likely resulted in a low coverage of the SiO2 surface and in a minor density of nucleation sites. Due to the high sputtering yields, these nuclei grew vertically more than laterally, resulting in the formation of cluster-like Au/ SiO2 systems (sample D). Conversely, lower |V bias| and lower sputtering yields produced the initial deposition of crystallites with a smaller size (specimen C). Their higher surface mobility likely favoured the lateral growth and agglomeration that, under the present conditions, can be comparable or higher than the vertical one. These results were consistent with the initial formation of a thin layer completely covering the surface (Fig. 5) and with the surface morphology observed in Fig. 6. Conversely, a comparison of C, B and A samples, obtained at 600 s but progressively higher |V bias|, suggested that in this case gold mass transport onto SiO2 controlled the agglomeration of Au particles, and higher sputtering yields resulted in a progressive transformation of island-like systems (specimen C) into continuous gold coatings on silica (specimens A, B).
4. Conclusions The present work was focused on the interplay between nanostructure and optical properties of Au/SiO2 nanosystems, obtained by RF-sputtering of gold from Ar plasmas onto amorphous silica substrates at different sputtering
603
yields and deposition times. In particular, the attention was devoted to the nucleation and coalescence of Au nanoparticles, as probed from in-situ LRI measurements and exsitu SE analyses. The latter technique enabled to derive the dielectric function ((x) = ( 1(x) + i( 2(x) in the 0.75 – 6.5 eV energy range, and hence the bn and bk dispersion curves. The obtained results allowed to elucidate the evolution from dispersed clusters to island-like systems, and finally, to continuous gold films. This result was obtained by a detailed analysis of SE spectra, whose modeling provided valuable information on the cluster size and distribution on the silica surface. On the basis of the obtained data, the combined influence of the sputtering yield and time on the cluster nucleation and coalescence was taken into account. In summary, these results might be of interest for the design and preparation of Au/SiO2 nanosystems with a fine modulation of the optical response.
References [1] D. Dalacu, L. Martinu, J. Appl. Phys. 87 (2000) 228. [2] G.C. Bond, D.T. Thompson, Catal. Rev., Sci. Eng. 41 (1999) 319. [3] M. Haruta, Catal. Today 36 (1997) 153. [4] T. Ung, L.M. Liz-Marza´n, P. Mulvaney, J. Phys. Chem., B 105 (2001) 3441. [5] F. Meriaudeau, A. Wig, A. Passian, T. Downey, M. Buncick, T.L. Ferrell, Sens. Actuators, B, Chem. 69 (2000) 51. [6] H.B. Liao, W. Wen, G.K.L. Wong, J. Appl. Phys. 93 (2003) 4485. [7] H.B. Liao, R.F. Xiao, J.S. Fu, P. Yu, G.K.L. Wong, P. Sheng, Appl. Phys. Lett. 70 (1997) 1. [8] S. Lee, J. Hong, S.-G. Oh, Surf. Coat. Technol. 94 (1997) 368. [9] M. Losurdo, M.M. Giangregorio, P. Capezzuto, G. Bruno, M.F. Cerqueira, E. Alves, M. Stepikhova, Appl. Phys. Lett. 82 (2003) 2993. [10] A. Khabari, F.K. Urban III, P. Griffiths, I. Petrov, Y.-W. Kim, C. Bungay, J. Vac. Sci. Technol., B 21 (2003) 2313. [11] Y. Catherine, in: R.E. Clausing, L.L. Horton, J.C. Angus, P. Koidl (Eds.), Diamond and Diamond-Like Films and Coatings, NATO-ASI Series B: Physics, vol. 266, Plenum Press, New York, 1991, p. 193. [12] M. Guerrisi, R. Rosei, P. Winsemius, Phys. Rev., B 12 (1975) 557. [13] A. Tiwari, A. Chugh, C. Jin, J. Narayan, J. Nanosci. Nanotechnol. 3 (2003) 368. [14] K. Mu¨mmler, P. Wißmann, Thin Solid Films 313 – 314 (1998) 522.
RF-sputtering of gold on silica surfaces: Evolution from clusters to continuous films L. Armelao a, D. Barreca a,*, G. Bottaro a, G. Bruno b, A. Gasparotto c, M. Losurdo b, E. Tondello c a
ISTM-CNR and INSTM, Department of Chemistry, University of Padova, via Marzolo, 1-35131 Padova, Italy b IMIP-CNR and INSTM, via Orabona, 4-70126 Bari, Italy c Department of Chemistry and INSTM, University of Padova, via Marzolo, 1-35131 Padova, Italy Available online 2 August 2005
Abstract Au/SiO2 nanosystems were prepared by RF-sputtering of gold from Ar plasmas on amorphous silica substrates at temperatures as low as 60 -C. The interrelation between nanosystem properties and synthesis conditions was investigated in detail, with particular attention to the nucleation and coalescence processes of Au nanoparticles on the SiO2 surface. To this regard, special emphasis was given to the interplay between the system morphology and the resulting optical properties, as probed by both in-situ and ex-situ characterization techniques. In this context, Spectroscopic Ellipsometry (SE) enabled to gain important information on the energy dispersion of the complex dielectric function and, subsequently, on the evolution from dispersed clusters to continuous films. Such variations were tailored by proper combinations of the bias potential (V bias) and deposition time, indicating the possibility of a fine modulation of the optical response. D 2005 Elsevier B.V. All rights reserved. Keywords: Au/SiO2 nanosystems; Spectroscopic Ellipsometry; Clusters; Continuous films
1. Introduction Metal nanoparticles on oxide substrates have gained a markedly increasing consideration with regard to both scientific and technological purposes [1]. In particular, gold-silica nanosystems are among the most studied, thanks to their extensive applications in heterogeneous catalysis [2,3] and optics (non-linear devices, fiber optics chemical sensors) [1,4 – 6]. As a matter of fact, supported Au nanoparticles display size- and shape-dependent properties, which can further be tailored by varying their distribution on the substrate and interparticle spacing [4]. In particular, significant variations in the Au/SiO2 optical response are induced by modifications of the system morphology from cluster-like systems, where gold nanoparticles are dispersed on the silica surface, to island-like structures, where Au aggregates are partially interconnected between each other,
* Corresponding author. Tel.: +39 49 8275170; fax: +39 49 8275161. E-mail address: [email protected] (D. Barreca). 0928-4931/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2005.06.007
and ultimately, to continuous films [5,7]. The control of the Au particle distribution and concentration is therefore a keystep in order to develop nanosystems endowed with welltailored optical properties. To this regard, in-situ and ex-situ optical methods for real-time growth monitoring and feedback-control of the deposition process have attracted a great deal of attention due to their non-destructive and non-invasive nature [8]. In particular, Spectroscopic Ellipsometry (SE) is a powerful and versatile technique for the determination of the energy dispersion of the complex dielectric function, ((x) = ( 1(x) + i( 2(x) = (n + ik)2, and of the refractive index, n, and the extinction coefficient, k, of Au/SiO2 nanosystems with high accuracy, even in the spectral region of strong absorption. The use of a suitable modeling enables to obtain valuable information on the interrelations between structure and optical properties even for low-size nanosystems, whose detection and analysis is a hard task by means of conventional structural techniques alone [9]. Despite different SE studies on both Au island-like systems and continuous films on Si and SiO2 have appeared in the literature [1,8,10], most
600
L. Armelao et al. / Materials Science and Engineering C 25 (2005) 599 – 603
investigations have not been completely exhaustive. In particular, the system evolution from clusters to continuous films as a function of the preparation conditions has never been thoroughly examined. The possibility of obtaining precise material features by modulating Au/SiO2 nanostructure and morphology has motivated the investigation of several preparation methodologies [1,10]. Among the various synthetic approaches, RF-sputtering is one of the most feasible thanks to its inherent versatility and the capability of tailoring the metal content, as well as the particle dispersion, under soft and controlled conditions. In the present work, Au nanoparticles were deposited on SiO2 by RF-sputtering from Ar plasmas at temperatures as low as 60 -C, in order to minimize eventual thermal contributions with respect to plasma-dependent phenomena. The interrelations between nanosystem properties and processing parameters were investigated by means of a multi-technique characterization. While LRI (Laser Reflection Interferometry) was employed for the in-situ growth monitoring, ex-situ analyses were specifically focused on the Au/SiO2 nanostructure and morphology by GlancingIncidence X-ray Diffraction (GIXRD) and Atomic Force Microscopy (AFM). In particular, a comparative SE investigation of Au/SiO2 specimens prepared under different conditions (bias potential and sputtering time) was undertaken with the aim of obtaining an insight on the cluster-to-film evolution. To this aim, the energy dispersion of the optical constants was determined in the 0.75– 6.5 eV range and followed by a suitable modeling procedure, supported by results obtained from other characterization techniques.
2.2. Characterization In-situ LRI measurements were performed by a custombuilt apparatus, adopting a He – Ne diode laser (k = 670 nm) incident on the substrate at an angle of 70- from the normal. The reflected light was collected by a PIN diode and subsequently digitized and recorded vs. time. An interference filter centered at 670 nm with a 10-nm bandwidth was mounted before the detector diode. The silica substrates used for LRI measurements were ground on the back surface in order to prevent undesired reflections from the substrate/electrode interface. GIXRD patterns were recorded by a Bruker D8 Advance diffractometer equipped with a Go¨bel mirror and a CuKa source (40 kV, 40 mA), at a fixed incidence angle of 1.5-. The average crystallite dimensions were estimated by means of the Scherrer formula. SE spectra of the real, b( 1, and imaginary, b( 2, parts of the pseudodielectric function, b( = b( 1 + ib( 2 were measured in the 0.75 –6.5 eV energy range by a phase modulated spectroscopic ellipsometer (UVISEL-Jobin Yvon) at an incidence angle of 70.4-. The Bruggeman effective medium approximation (BEMA) combined with a standard regression method was used to analyze the measured SE spectra by a simple threephase model, i.e., SiO2 substrate/Au/air, the film thickness and/or clusters size and distribution being the fitting parameters. The optical constants of bulk Au were used for the SE spectra analysis. AFM measurements were performed in non-contact mode and in air using a Autoprobe CP Thermomicroscope instrument.
3. Results and discussion 2. Experimental 2.1. Synthesis Au depositions were performed in Ar plasmas by a custom-built RF-sputtering system (m = 13.56 MHz) consisting of a vacuum metal chamber with two vertical electrodes 5 cm apart. One of the two electrodes was coupled to an RF power generator (Cesar 133, Thin Films) through a matching box (VM 1000, Thin Films), while the other one, electrically grounded, was equipped with a resistive heater and its temperature was monitored by means of a thermocouple and an external controller. A 5-cm diameter gold target (0.1 mm thick; BAL-TEC AG, 99.99%) located on the RF electrode was used as gold source. The Herasil\ silica substrates (Heraeus) were placed on the grounded electrode. Control of RF-power and total pressure enabled tailored modifications of the bias potential (V bias), the Direct Current (DC) potential developed on the target [11], that, together with variations of the deposition time, strongly affected the properties of the obtained Au/SiO2 nanosystems.
The sputtering yield, which controls the nucleation and coalescence of Au clusters, is governed by the bias potential (V bias), the finger-print for sputtering experiments. This parameter, in turn, is related to the pressure ( p) and RFpower (W) of the process by the well-known relation [11]: jVbias j”
W p
1=2 :
ð1Þ
In the present work, combined variations of p and W resulted in bias potential values between 460 and 650 V. Fig. 1 shows typical LRI traces recorded during gold deposition on silica at different |V bias| values. As indicated in the caption, samples D and E were obtained at the same bias potential as B, but interrupting the sputtering process at 240 and 60 s. For samples AYC, obtained at 600 s deposition time, the final reflectance value increased with |V bias|, suggesting a concomitant rise of the overall deposited gold amount. The dependence of the growth rate on the bias potential is reported in Fig. 2, indicating that the sputtering yield, and
L. Armelao et al. / Materials Science and Engineering C 25 (2005) 599 – 603
601
9.4 A
18
9.2
Crystallite size (nm)
Vbias = -644 V 16 B
reflectance (%)
14
Vbias = -577 V
12 D 10
8.8 8.6 8.4 8.2 8.0
Vbias = -466 V
7.8
C
8
9.0
-650
-600
-550
-500
-450
V bias (V)
6
E Fig. 3. Dependence of the average gold crystallite size on V bias for Au specimens obtained at 600 s deposition time.
4 0
100
200
300
400
500
600
time (s)
hence, the deposited Au amount linearly increased with |V bias|. Irrespective of the synthesis conditions, GIXRD measurements indicated the presence of fcc gold [JCPDS #4-784, 2000]. The impact of the bias potential on the crystallite size determined by GIXRD is displayed in Fig. 3. As can be observed, higher sputtering yields resulted in the formation of bigger gold nanocrystallites. The observed linear dependence suggested that controlled V bias variations enabled to tailor the coherent diffraction domain size, which determines the electronic-optical properties of Au/SiO2 nanosystems. The interplay between the process parameters (V bias, deposition time), the structure/morphology of gold-silica specimens and the corresponding optical response have been determined by SE. Fig. 4 shows the measured spectra of the
(a) 5
5
k
n
k
3 2
2
2
2
3 4 Photon Energy (eV)
5
6
3 4 5 6 Photon Energy (eV)
2
C
D D
B
E E
A 1
1
2
3
4
5
6
5
6
Photon Energy (eV)
(b) 9
A
7
-3
6
< k>
70
Growth rate (nm/s)
11
n
3
8 80x 10
4
4
3
Fig. 1. LRI traces recorded during Au deposition on SiO2 for selected specimens. The vertical dotted line marks the end of deposition experiments for samples A, B, C. The bias potentials and sputtering times for the different specimens are as follows: (A) 644 V, 600 s; (B) 577 V, 600 s; (C) 466 V, 600 s; (D) 577 V, 240 s; (E) 577 V, 60 s.
60
5 4
50
3
40
2 1
30
B C
D E
0 1
20 -650
-600
-550
-500
-450
V bias (V ) Fig. 2. Dependence of the Au growth rate on the bias potential (V bias).
2
3
4
Photon Energy (eV) Fig. 4. Spectra of the refractive index bn (a) and extinction coefficient bk (b) for different Au/SiO2 nanosystems. The inset of (a) displays the spectrum of a bulk Au film used for comparison.
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pseudo-refractive index, bn, and the pseudo-extinction coefficient, bk, for the specimens indicated in Fig. 1. The spectra at the B, D and E points, which correspond to specimens obtained at different sputtering time and fixed V bias (compare Fig. 1), describe the growth kinetics, while a comparison between A, B and C evidences the influence of the bias potential. It is worth noting that systems with a comparable final reflectivity, but obtained under different sputtering conditions (Fig. 1, points C, D), showed different SE spectra, indicating that the V bias value and sputtering times significantly affected the structural and optical properties of Au/SiO2 nanosystems. Therefore, SE analysis was specifically aimed at elucidating the effect of V bias and sputtering time on the system evolution. Samples A and B showed a similar behavior throughout the investigated photon energy range (Fig. 4), and their bn and bk spectra resembled those of a bulk reference Au film shown in the inset. The energy dependence of bk was characterized by the stronger interband 5dY6sp transition with an onset at 2.5 eV near the L point in the Brillouin zone [12]. The contribution of interband transitions was predominant at photon energies above 2.5 eV, while the bk increase observed below 2.5 eV was related to intraband transitions, that can be described by the Drude free-electron model. For energies higher than 2.5 eV, differences in the optical spectra were due to variations in surface roughness. As a whole, these data suggested that specimens A and B are optically thick nanostructured Au films. Conversely, spectra of samples D and E were drastically different from the former specimens in the 0.75 –2.5 eV range, where the measured bk showed a maximum at 1.9 eV, which was not observed in the spectra of A and B (Fig. 4b). Finally, the bk spectrum of sample C represented a transition between the previous categories (A, B and D, E). Fitting SE spectra to optical models enabled to derive the film thickness and/or cluster size and distribution. Some representative examples of the best-fit models are reported in Fig. 5 for B, C and D specimens. The obtained results showed that A- and B-like systems were continuous and nanostructured Au films, while C-like samples were characterized by the partial coalescence of gold nanoaggregates. Finally, D- and E-like specimens were formed by dispersed Au nanoclusters on
D
SiO2. Spectral modeling yielded a diameter of the hemispherical Au crystallites between 6 and 7 nm, with an average distance of å 17 nm and a 30% Au surface coverage for specimen D. The particle size and distribution affected the energy position of the peak observed in the bk spectra, while the relative distance and, hence, the surface coverage, influenced its amplitude. This peak could be identified as the Surface Plasmon Resonance (SPR) of gold nanoparticles, in agreement with previous literature results. For spherical aggregates with a radius of å 2 – 6 nm distributed with an average spacing of å 4– 12 nm, the SPR band is located at å 522 nm (2.4 eV) [13]. As determined by SE analysis, the larger cluster size and average separation obtained in the present case could explain the red-shift to å 1.9 eV of the SPR band [14]. The main difference between E and D spectra was the slightly higher silica coverage by gold particles in the latter case. Fig. 4 shows that in the bk spectrum of sample C the peak almost disappeared and shifted to lower energy, consistently with an increase of the average cluster size. This spectrum can be considered representative of the transition from nanostructured films to cluster-like systems by a progressive coalescence of Au nanoparticles, as already mentioned. In fact, the SE best-fit model for sample C indicated the presence of a very thin Au layer completely covering the surface characterized by hemispherical islands. As a whole, the obtained results suggested a significant influence of the sputtering yield and time on the morphology of the obtained Au/SiO2 systems. In particular, samples C and D had comparable final reflectivity values (see Fig. 1), but the difference in their SE spectra (Fig. 4) pointed out to different morphologies in the two cases. In order to elucidate this point, AFM measurements were performed on both specimens and representative micrographs are displayed in Fig. 6. As can be noticed, sample C was characterized by the presence of polygonal-like islands and a smaller aspect ratio (height/width) with respect to sample D. The latter displayed the presence of threedimensional Au particles, suggesting a predominance of vertical growth with respect to the lateral one. These results could be explained taking into account that larger Au
C
B Au+27 3%voids
Au+55 1%voids
17.5 .5 0.5 nm 6.5 0.5 nm
Au
10.5 2.0 nm 5.7 1.0 nm
Au
12.0
2.5 nm
34.0
2.8 nm
SiO2
SiO2
SiO2
Clusters
Coalescence
Continuous film+ Surface roughness
Fig. 5. Schematic representation of the best-fit models for Au/SiO2 nanosystems: cluster-like (D), island-like (C) and continuous films (B).
L. Armelao et al. / Materials Science and Engineering C 25 (2005) 599 – 603
Fig. 6. Representative AFM micrographs (200 200 nm2) of C and D samples. Vertical scale is from 0 to 6 nm.
crystallites were formed at higher |V bias|, and, hence, higher sputtering yields (compare Fig. 3). Their low surface mobility likely resulted in a low coverage of the SiO2 surface and in a minor density of nucleation sites. Due to the high sputtering yields, these nuclei grew vertically more than laterally, resulting in the formation of cluster-like Au/ SiO2 systems (sample D). Conversely, lower |V bias| and lower sputtering yields produced the initial deposition of crystallites with a smaller size (specimen C). Their higher surface mobility likely favoured the lateral growth and agglomeration that, under the present conditions, can be comparable or higher than the vertical one. These results were consistent with the initial formation of a thin layer completely covering the surface (Fig. 5) and with the surface morphology observed in Fig. 6. Conversely, a comparison of C, B and A samples, obtained at 600 s but progressively higher |V bias|, suggested that in this case gold mass transport onto SiO2 controlled the agglomeration of Au particles, and higher sputtering yields resulted in a progressive transformation of island-like systems (specimen C) into continuous gold coatings on silica (specimens A, B).
4. Conclusions The present work was focused on the interplay between nanostructure and optical properties of Au/SiO2 nanosystems, obtained by RF-sputtering of gold from Ar plasmas onto amorphous silica substrates at different sputtering
603
yields and deposition times. In particular, the attention was devoted to the nucleation and coalescence of Au nanoparticles, as probed from in-situ LRI measurements and exsitu SE analyses. The latter technique enabled to derive the dielectric function ((x) = ( 1(x) + i( 2(x) in the 0.75 – 6.5 eV energy range, and hence the bn and bk dispersion curves. The obtained results allowed to elucidate the evolution from dispersed clusters to island-like systems, and finally, to continuous gold films. This result was obtained by a detailed analysis of SE spectra, whose modeling provided valuable information on the cluster size and distribution on the silica surface. On the basis of the obtained data, the combined influence of the sputtering yield and time on the cluster nucleation and coalescence was taken into account. In summary, these results might be of interest for the design and preparation of Au/SiO2 nanosystems with a fine modulation of the optical response.
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