J. Phys. Chem. 1995,99, 13834-13838
13834
Metal Clusters in Plasma Polymer Matrices: Gold Clusters Ryszard Lamber,* Stefan Wetjen, and Giinter Schulz-EMoff Institut f i r Angewandte und Physikalische Chemie, Universitat Bremen, FB 2, 0-28334 Bremen, Germany Alfred Baalmann Fraunhofer-Institut f i r Angewandte Materialjorschung, 0-28717 Bremen, Germany Received: May I , 1995; In Final Form: July 6, 1995@
By means of an inert gas evaporation technique in combination with a simultaneous plasma polymerization process, gold clusters with mean diameters ranging from 1.4 to 5 nm have been prepared within plasma polymer matrices from vinyltrimethylsilane and tetraethoxysilane monomers. For the characterization of these materials high-resolution electron microscopy (HRTEM), electron diffraction (ED), X-ray photoelectron spectroscopy ( X P S ) , Fourier transform infrared (FTIR) spectroscopy, and UV-vis spectrophotometry were used. Electron diffraction has shown a decrease of the Au lattice parameter with decreasing size of the gold cluster. The experimental data were used to deduce the value of the surface stress coefficient f = 6.3 f 1 N/m. UV-vis spectrophotometry revealed a strong blue shift of the plasmon resonance frequency from A = 578 nm (DA,= 5 nm) to A = 532 nm (DA"= 2 nm). In the case of the 1.4 nm clusters, a distinct absorption peak could not be observed. In the case of the plasma polymer matrix from a tetraethoxysilane monomer, a strong chemical damping of the surface plasmon resonance has been observed.
1. Introduction The physical and chemical properties of metal clusters entrapped in a solid state matrix are of considerable interest both from a fundamental point of view and for potential applications. For the quantitative analysis of size-dependent structural, optical, and electronic properties, isolated and fixed clusters with uniform size and shape have been prepared. There is a variety of methods for the preparation of metal clusters trapped in matrices such as solid gases, zeolites, and polymers.'-7 Plasma polymers as matrices for metal clusters were used for the first time by Kay et aL7 The deposition technique developed by Kay relies on the simultaneous plasma polymerization and the deposition of the metal by ~puttering.~An altemative is the simultaneous deposition of the metal by evaporation and plasma polymerization.*-12 These deposition techniques are well-suited in the case of nonreactive metal/ plasma polymer systems. The aim of this study was to develop a deposition technique for the preparation of metal clusters in inert and thermally stable plasma polymer matrices. Due to the chemical inertness and thermal stability of silicon-containing plasma polymers, the vinyltrimethylsilane (VTMS) and tetraethoxysilane (TEOS) monomers have been chosen for the preparation of plasma polymer matrices. In order to avoid the formation of intermetallic compounds during the deposition of transition metal/ silicon-containing plasma polymer composites, a novel deposition technique has been developed, separating the metal cluster source from the plasma discharge region and leading to metal clusters uniform in size embedded in an amorphous dielectric matrix. I
2. Experimental Procedures The experimental setup for the deposition of metal clusters in plasma polymer films is shown in Figure 1. Metal clusters were produced using an inert gas evaporation technique. The
* Corresponding author. FAX: +49(421)218-4918. @Abstractpublished in Advance ACS Absrrucrs, August 15, 1995.
Source
Figure 1. Schematic diagram of the experimental setup for the deposition of metal clusters in plasma polymer films.
plasma polymer films were prepared in a microwave discharge from VTMS or TEOS monomers. The metal cluster source was separated from the plasma discharge region. Due to the pressure difference between the evaporation chamber (metal cluster source) and the plasma polymerization region, metal clusters formed in the evaporation chamber were injected into the plasma zone. The size of the clusters was varied by controlling the temperature of the vapor source (evaporation rate). Gold clusters were prepared by evaporation of a Au wire (Alfa Products, 15 ppm) from a resistance-heated tungsten boat. The experimental method used was described in detail in a previous paper.I3 For the characterization of the metal clusterlplasma polymer composites, high-resolution transmission electron microscopy (HRTEM), electron diffraction (ED), X-ray photoelectron spectroscopy (XPS),Fourier transform infrared (FTIR) spectroscopy, and UV-vis spectrophotometry were used. For the characterization with different methods the material was deposited on NaCl (TEM, ED), PEEK foils ( X P S ) ,thick aluminum films on glass (FTIR), and quartz plates (UV-vis spectrophotometry). The conventional TEM and electron diffraction study
0022-3654/9512099-13834$09.00/0 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 38, 1995 13835
Gold Clusters in Plasma Polymer Matrices
f = -3/4(Aa/a)D/K
Figure 2. Electron micrographs of the plasma polymer-Au specimens with Au cluster size equal to (left) 1.4 nm and (right) 5 nm.
TABLE 1: Mean Particle Size (0)and Standard Deviation (a)of Gold Clusters in Plasma Polymer Films specimen mean particle size (nm) standard deviation (nm) no. of atoms
1
2
3
4
5
1.4 0.36 85
2.0 0.36 248
2.5 0.5 484
3.7 0.5 1571
5.0 0.5 3876
was carried out with a Philips EM 420 T electron microscope operated at 120 kV. High-resolution electron microscopy examinations (HRTEM) were performed in a Philips CM 20 Ultra-Twin electron microscope (200 kV), which provided a 0.19 nm point resolution. XPS measurements were carried out using an ESCALAB MK I1 (VG Instruments/Fisons). FTIR spectra were recorded on a Biorad FTS 60a spectrometer in diffuse reflectance (resolution 2 cm-'). UV-vis spectra were acquired in transmission at wavelengths between 800 and 200 nm employing a Cary 4 spectrophotometer.
3. Results 3.1. Structural Characterization. A series of gold-containing plasma polymer films have been prepared. The mean size of the Au clusters was varied in the range 1.4-5 nm (Table 1). Particle size distributions were determined by visual counting of the particles on the EM micrographs for a given size interval. At least 200 particles were counted. Figure 2 shows typical electron micrographs taken from specimen 1 (D =.1.4 nm) and specimen 5 (D = 5 nm). The mean size of the Au particles was calculated using the equation
D =x n i c / C n i
+
were ni is the number of crystallites with diameter Di ALl, while 0,= Di (AD/2). The corresponding size histograms are shown in Figure 3. The HRTEM study showed that smaller (1.4-2.5 nm) Au clusters are mostly single crystals. Larger Au particles are often twinned but have a common (1 11) plane. Electron diffraction showed a strong variation in the lattice constant as a function of the Au cluster size. Figure 4 shows the dependence of the relative lattice constant of gold clusters as a function of the reciprocal diameter of Au particles embedded in the VTMS plasma polymer. The straight line was found by the method of least squares. Following Vermaak, Mays, and Kuhlmann-Wilsdorf, l4 the observed lattice contraction can be interpreted in terms of surface stress. It has been shown that, for spherical particles with a cubic structure and lattice constant a, the surface stress can be calculated according to the following relation:
+
where Au is the change in the lattice constant due to the surface stress, u is the lattice constant of bulk material, D is the mean particle size, and K is the compressibility of the bulk material. Using the value of K = 6.23 x (N/m2)-' I5 and a = 0.407 86 nm the value off= 6.3 f 1 N/m has been obtained for gold clusters within the size range 1.4-5 nm. Since there are no reports on surface stress coefficients of metal clusters embedded in plasma polymer matrices, this value can only be compared with results reported for gold particles supported on carbon. Mays et a1.16 reported for Au particles within the size range 3.5-12.5 nm a value off= 1.2 N/m. Soliard and Flueli17 obtained for gold particles with mean sizes ranging from 3.0 to 40 nm a value o f f = 3.2 N/m. The higher value of the surface stress coefficient for gold clusters in a plasma polymer matrix is probably due to the embedding in the matrix. It should be noted that a similar result for the surface stress coefficient has been obtained for palladium clusters embedded in a VTMS matrix, f = 6.0 f 0.9 N/m.18 3.2. Spectroscopic Characterization (FTIR and XPS). FTIR study revealed that the presence of gold clusters has no influence on the structure of the plasma polymerized matrix. Figure 5 shows a typical XPS spectrum obtained from a A d VTMS composite film containing gold clusters with D = 2 nm. The value of the binding energy of Au 4f712, 83.92 f 0.1 eV, corresponds to that of metallic gold Au 4f712, 84.0 eV. 3.3. Optical Properties (UV-vis). Small metal particles show a strong absorption peak in the UV-vis region due to surface plasmon e x c i t a t i ~ n . ~ ?A ' ~series - ~ ~ of UV-vis extinction spectra of AuNTMS composites are shown in Figure 6. A strong shift of the plasmon absorption in dependence upon the gold cluster size from A = 578 nm for 5 nm clusters to A = 532 nm for 2 nm clusters is visible. The shift of the peak position is accompanied by an increase of the half-width of the plasmon band. In the case of 1.4 nm clusters a distinct absorption peak could not be observed. The clusters consist of around 85 atoms and should already possess properties of the bulk material.27 The vanishing of the plasmon band in this case can be ascribed to the 5d-6sp interband transitions.28 P e r ~ s o n ~ ~ has shown theoretically that a strong influence of matrix environments on the optical absorption of small metallic particles can be expected. The influence of the chemical interface, Le. damping of cluster plasmon resonances, has been recently shown experimentally by Kreibig et al.30 A similar strong influence of the plasma polymer matrix on the width of cluster plasmon resonances has been observed in the case of the tetraethoxysilane plasma polymer matrix. Figure 7 shows two typical UV-vis extinction spectra taken from AdTEOS specimens containing 2 and 5 nm Au clusters. In the case of 2 nm gold clusters only a very weak and broad extinction band at 449 nm is visible. The presence of larger Au clusters with D = 5 nm leads to a distinct absorption peak A = 545.5 nm (half-width = 155.4 nm) as observed for the VTMS matrix (Figure 6). 4. Discussion
4.1. Structural Characterization. Due to the amorphous nature of the plasma polymer matrix, the crystalline character of the metal phase can be observed by electron diffraction or direct lattice plane imaging. Careful inspection of highresolution electron micrographs has shown that Au clusters up to 2.5 nm are mostly single crystals. Larger Au particles are often twinned and have a common (1 11) plane. A number of particles consist of two or three parts connected via an incoherent boundary. Electron diffraction study has shown a
Lamber et al.
13836 J. Phys. Chem., Vol. 99,No. 38, 1995
0
2
D(nm)
4
E
Figure 3. Particle size distributions of the plasma polymer-Au cluster specimens with diameter equal to (a) 1.4, (b) 2, (c) 2.5, (d) 3.7, and (e) 5 nm.
T
2.5nm
\
-z
,
-3 5
0
10
20
30
40
50
Bo
\* 70
l W D [nm"]
Figure 4. Variation of the relative lattice parameter A d a as a function of the reciprocal diameter of the gold clusters.
strong variation in the lattic parameter as a function of gold cluster size. From the relative change of the lattice parameter as a function of the reciprocal diameter of the particles, a value of the surface stress coefficient f = 6.3 f 1 N/m has been obtained. This value is much higher than results reported for
gold particles supported on ~arbon.'~J'If the data on the lattice contraction of the ligand-stabilized Au55 cluster3' are used for calculation of the surface stress coefficient, a value off = 7.2 N/m is obtained. This result is comparable with 6.3 N/m for gold clusters embedded in a VTMS plasma polymer matrix and supports our assumption that the high value of the surface stress coefficient is due to the embedding of the gold clusters in a plasma polymer matrix. The lattice parameter of gold cluster was calculated from the (111) Au reflections. Due to the amorphous nature of the plasma polymer matrix, it was possible to detect the (111) reflection even in the case of the 1.4 nm ~ ~ shown that metal cluster.'* Wasserman and V e r m a a l ~have the lattice constant determination of small Ag particles from the (1 11) reflections can lead to a considerable apparent decrease in the observed lattice constant and consequently to a higher value of the surface stress coefficient. It is concluded that the (220) reflections are the most suitable for reliable measurements of the lattice contraction. Due to the very small size of the gold clusters used in our study, the determination of the lattice parameter from the (220) reflections was not possible. In the case of the 5 nm particles a fairly good correspondence between lattice constants from the (1 11) and (220) reflections has been
J. Phys. Chem., Vol. 99,No. 38, 1995 13837
Gold Clusters in Plasma Polymer Matrices I
I
I
I
I
I
I
0,30 0'35
P
F
0,25
1
0,20 0,15
0,lO 0.05
'
C
0,oo 200
S
,
400
1
600
800
Wavelength [nm]
Figure 7. UV-vis extinction spectra taken from gold clusters embedded in the plasma polymer matrix prepared from tetraethoxysilane.
I
I
I
92
90
Y
I
m
e
I
4
I 82
1
no
Birding E m q y lev)
Figure 5. XPS spectrum taken from a AuNTMS composite film (D = 2 nm).
0,35 OI4O
A
5,O nm
3.7 nm
0,30 0,25
0,15
0,lO
0,05 0,oo 1
200
,
400
800
800
Wavelength [nm]
Figure 6. UV-vis extinction spectra taken from specimens a-e of Figure 3.
obtained. This indicates that similar to small platinum particles33 the (1 11) reflections of Au can be used for a reliable determination of the lattice constant of small gold particles. 4.2. Spectroscopic Characterization. As revealed by FTIR spectra, the presence of the gold clusters has no influence on the structure of the plasma-polymerized matrix. X-ray photoelectron study was performed on specimens with gold cluster sizes above 2 nm. (In the case of smaller clusters no reliable gold signal could be achieved.) In the case of 2 nm Au clusters spectra typical for metallic gold have been obtained (Figure 5). 4.3. Optical Properties. The size dependence of surface plasmon resonances of small metallic particles was the subject of a number of papers.2,20,24*34-46 In alkali metal clusters the average dipole resonance frequency moves to lower energies with decreasing particle size (red shift).26 In the case of noble metal clusters both b 1 ~ e ~ , ~ and~ red , ~shifts2*20*24,42-46 ~ . ~ ~ - ~ have been reported. Similar to results for Ag35-40and Au cluster^,^.^^ a strong blue shift of the plasmon resonance frequency has been
observed in the case of gold clusters embedded in a VTMS plasma polymer matrix. A similar behavior was observed for silver clusters in a VTMS matrix.47 The strongest effect (blue shift) occurs in the size range 2-5 nm.37-39,41Wilcoxon et al?I reported for gold colloids a shift of ca. 40 kn in the range of cluster sizes between 20 and 2.4 nm. The shift of the plasmon resonance from 578 to 532 nm over the size region 5-2 nm is comparable with that reported by Kreibig.34 It is generally accepted that the calculation of the shift in the plasmon resonance as a function of particle size is a difficult task, and a meaningful comparison with the experimental results is somewhat u n ~ e r t a i n . ~ ~First-principle -~* quantum theoretical calculations give adequate descriptions of the spectral positions of the resonance for small alkali clusters.26 For noble metal clusters the Mie theory for the scattering and absorption of electromagnetic radiation by a small metallic particle is generally applied. The size dependence of the plasmon peak position is explained by a size dependence of the dielectric function of a small metallic particle, which is usually described by a Drude free-electron mode120930or by a quantum mechanical mode1.20,30,35,38 The strong shift of the resonance frequency observed in our experiments for the AuNTMS specimens is probably not only due to the change of the size of the clusters from 5 to 2 nm. The present technique leads to a simultaneous increase of the filling factor with increasing size of the Au clusters, which can also be responsible for a blue shift as has been shown by Granqvist and H ~ n d e r i . 4 ~Teo 9 ~ ~et aL5' have proposed to use the appearance of surface plasmon resonance in metal clusters as evidence for the beginning of collective phenomena characteristic of the bulk metal. They interpreted the presence of a distinct absorption in the case of Au18Ag20 clusters with a size of ca. 0.15 nm as evidence for their metallic properties. On the other hand, other optical measurements on gold clusters with similar sizes52or largeP941showed no obvious plasmon feature typical for metallic gold. The vanishing of the Au plasmon band in the case of 1.4 nm gold clusters can be ascribed to the 5d-6sp interband transition.28 The influence of the plasmon polymer matrix on the electronic properties of the smallest Au cluster should also be taken into c~nsideration.~~ The strong broadening of the surface plasmon excitation in the case of 2 nm Au clusters in a TEOS plasma polymer (Figure 7) matrix can be explained by strong chemical damping as reported by Kreibig et aL30 for silver cluster in a Si02 matrix. As revealed by RE?, the plasma polymer films prepared from a tetraethoxysilane monomer exhibit a very strong Si-0-Si band similar to that observed for a Si02 matrix.
13838 J. Phys. Chem., Vol. 99, No. 38, 1995 5. Conclusions 1. By combining the metal evaporation in an inert gas atmosphere with the simultaneous plasma polymerization process, gold clusters with narrow size distributionembedded within an amorphous, dielectric matrix can be reproducibly prepared. This system is well-suited for the structural and optical characterization of the clusters. 2. From the size dependence of the Au lattice parameter a surface stress coefficient off = 6.3 f 1 N/m for gold clusters embedded in a vinyltrimethylsilane matrix has been calculated. 3. Optical measurements on gold clusters showed a strong dependence of the surface plasmon resonance upon the size of the Au clusters 1 = 578 nm for 5 nm gold particles and 1 = 532 nm for 2 nm gold clusters. 4. SiO2-like matrices (plasma polymer films from a tetraethoxysilane monomer) are not well-suited as embedding medium for optical applications of the gold clusters due to very strong damping of surface plasmon resonance. 5. This technique can also be applied for the preparation of other metal clusters (Pd,’* Pt, and Ag).
Acknowledgment. We thank Professor Dr. N. I. Jaeger (Universitat Bremen) for valuable contributions and critical reading of the manuscript. We are very grateful to Dr.V. Schlett (Fraunhofer-Institut f i r Angewandte Materialforschung, Bremen) for the XPS measurements on the mewplasma polymer specimens. Financial support by the Stiftung Volkswagenwerk (AZ:U67837) is gratefully acknowledged. References and Notes (1) Davis, S. C.; Klabunde, K. J. Chem. Rev. 1982, 82, 153. (2) Halperin, W. P. Rev. Mod. Phys. 1986, 58, 533 and references therein. (3) Halicioglu, T.; Bauschlicher, C. W., Jr. Rep. Prog. Phys. 1988, 51, 883. (4) Schmid, G. Chem. Rev. 1992, 92, 1709 and references therein. (5) Sachtler, W. M. H.; Zhang, Z. Adv. C a d 1993, 39, 129. (6) Tonscheid, A,; Ryder, P. L.; Jaeger, N. I.; Schulz-Ekloff, G. Surf. Sci. 1993, 281, 513. (7) Kay, E. Z. Phys. D 1986, 3, 251 and references therein. (8) Biederman, H.; Martinu, L.; Slavinska, D.; Chudacek, I. Pure Appl. Chem. 1988, 60, 607 and references therein. (9) Kampfrath, G.; Heilmann, A,; Hamann, C. Vacuum 1988, 38, 1. (10) Heilmann, A.; Hamann, C. Progr. Colloid Polym. Sci. 1991, 85, 102. (11) Heilmann, A.; Werner, J.; Hopfe, V. Supplement to Z. Phys. D. 1993, 39. (12) Hamack, J. T.; Benndorf, C. Mater. Sci. Eng. 1991, A140, 764. (13) Lamber, R.; Baalmann, A.; Jaeger, N. I.; Schulz-Ekloff, G.; Wetjen, S . Adv. Mater. 1944, 6, 223.
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