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Polymer–metal optical nanocomposites with tunable particle plasmon resonance prepared by vapor phase co-deposition
Materials Letters 58 (2004) 1530 – 1534 www.elsevier.com/locate/matlet
Polymer–metal optical nanocomposites with tunable particle plasmon resonance prepared by vapor phase co-deposition A. Biswas a,*, O.C. Aktas a, J. Kanzow a, U. Saeed a, T. Strunskus b, V. Zaporojtchenko a, F. Faupel a a
Nanocomposite Research Laboratory, Lehrstuhl fu¨r Materialverbunde, Technische Fakulta¨t der CAU, Kaiserstr. 2, D-24143, Kiel, Germany b Lehrstuhl fu¨r Physikalische Chemie I, Ruhr-Universita¨t Bochum, Universita¨tsstr. 150, NC 5/28 D-44780, Bochum, Germany Received 28 July 2003; accepted 10 October 2003
Abstract A simple one-step vapor phase co-deposition at elevated target temperature is employed to generate optical nanocomposites (~100 – 300 nm) of polymer-based metallic nanoparticles with uniform size and shape. Homogeneously, three dimensionally distributed isolated Ag nanoparticles are produced with appropriate volume filling (~10 – 30%) in Teflon AF matrix at various co-deposition target temperatures of 120 – 400 jC. An extremely narrow particle size distribution of ~5 – 7 nm based on TEM observations is demonstrated. Different ultrathin nanocomposite color filters as a result of plasmon resonance shifting in the UV – VIS wavelength region are also generated. D 2003 Elsevier B.V. All rights reserved. Keywords: Nanocomposites; Nanomaterials; Optical materials and properties; Deposition; Electron microscopy; Polymers; Thin films
1. Introduction The optical properties of discontinuous metallic films or granular composite films, consisting of metal nanoparticles embedded in a dielectric, have been of interest since the beginning of the 20th century when MaxwellGarnett [1] described the first theoretical framework to explain the resonant absorption or dielectric anomaly which characterizes such systems. Metallic nanoparticle optical properties are determined by a collective oscillation of the free electrons in the particles described by socalled plasmon-resonance absorption and can be observed by conventional spectroscopy. The ability to tune such particle plasmon resonances over a spectral range is highly desirable in various applications of metallic nanoparticles [2]. Moreover, as optical material applications are expanding, the need for novel optically functional and transparent materials increases. Polymer – metal nanocomposite materials show great promise as they can provide the necessary stability and easy processability with interesting optical properties. The general principles in the
* Corresponding author. Tel.: +49-431-880-6230; fax: +49-431-8806229. E-mail address: [email protected] (A. Biswas). 0167-577X/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2003.10.037
construction of optical composites involve the intimate mixing of optically functional materials within a processable matrix. Secondly, optical scattering must be avoided in these types of composites, resulting in a tradeoff between particle size and refractive index (RI) mismatch (the difference in RI between the matrix and the particles). However, this RI mismatch is not important for smaller particles ( < 25 nm). Using such a composite structure, nanocomposites have been formed basically using complex multiple chemical synthesis routes with nonlinear optical and laser amplification properties, among others [3]. Further, as optical properties of nanoparticles depend largely on the particle size, the color of nanoparticles should also depend on the size, so do nanocomposites, and hence upon changing the particle size, these materials offer interesting optical color filter properties [4]. For a majority of technological applications of such nanocomposites, a narrow size distribution of isolated nanoparticles in a specific size regime is of vital importance and requirement. However, due to the strong nanoparticles aggregation tendency during the production of such nanostructures, one of the major technological challenge is the control of the particles size distribution while generating a high particle volume filling in the quantum size range. Although, the choice of organic media to
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embed metallic nanoparticles is helpful for isolation during the generation process besides controlling the particle size, it is quite challenging to optimize such nanostructures for desired properties. The existing state of the art to prepare such interesting optical functional nanocomposites involves mostly multiple or tedious fabrication steps, and obviously, a single-step or straightforward method is highly desired. Here, we introduce an easy and single-step relatively new method of vapor phase elevated target temperature co-deposition of polymer and metal to produce optical functional nanocomposites. The aim of this paper is to show the effectiveness of the technique to generate an extremely narrow particle size distribution which also allows easy tunning of particle plasmon resonance over a wide wavelength range to produce different ultrathin color filters.
2. Experimental procedures Teflon AFR was chosen as the polymer matrix due to its known properties of a high degree of resistance to any chemical attack and excellent transparency. The chemical structure of the polymer can be found in our earlier publication [5]. Vapor phase co-deposition of Teflon AFR (granulates, Dupont) and Ag (wires, Good Fellow Industries) was carried-out using a homemade evaporation chamber consisting of separate heatable boats at various target temperatures of 120 –400 jC. Nanocomposite films were prepared with thicknesses of 100 –300 nm. Deposition rates of polymer and metal were power controlled with a nearly constant value of 4 P/min for metal and 10 – 20 P/min for polymer to prepare different metal volume filling in the polymer matrix. Relatively low rate of metal evaporation was chosen in order to control nanoparticle growth process in a better way. Estimation of the particle filling was done using quartz microbalance thickness informations for the two components. Vapor phase deposition of polymers has a range of advantageous features. Some of these are excellent conformality over complex topography, the possibility of good film uniformity on large diameter wafers, and environmentally safe processing due to the absence of solvents [6]. For TEM measurements (Philips CM 30), samples were collected on carbon-covered (~10 nm) Cu grids. Optical characterization of the nanocomposites was carried out using a Perkin-Elmer UV –VIS Lambda 900 spectrometer.
3. Results and discussion Fig. 1a and b shows a TEM planar view of the dispersed Ag nanoparticles in the Teflon AF matrix along with an elemental profile of the nanocomposite (~100 nm, target temperature: 120 jC) as shown by TEM EDX. In view of the co-deposition, the vapor phase generated
Fig. 1. (a) TEM planar view of Ag nanoparticles 3D distributed in Teflon AF matrix (Nanoparticle volume filling: ~10%) along with SAED pattern on the generated nanoparticles showing fcc nature, in the inset. (b) EDX of the nanocomposite (~100 nm) showing elemental profile with C, O, F due to Teflon AF matrix along with Ag and Cu lines originating from the nanoparticles and backing TEM Cu grid, respectively.
nanoparticles are believed to be distributed three-dimensionally in the polymer matrix. It is clear from the TEM image (Fig. 1a) that the nanoparticles have almost uniformly spherical shape along with the size. A good control on the particle size distribution is evident, as a size distribution profile shows an extremely narrow particle size distribution of nearly 80% of the visible particles in the range of 5 – 7 nm, (Fig. 2). This is quite promising as compared to the existing large volume of the literature [7] showing a mostly uncontrollable particle size distribution due to the preparation conditions. The crystal structure of the so produced nanoparticles is observed to be of fcc, as analysed by the selected area electron diffraction measurements (Fig. 1a, inset).
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Fig. 2. Size distribution profile of the generated Ag nanoparticles distributed in Teflon AF matrix showing almost 80% particles in the size range of 5 – 7 nm.
During room temperature co-deposition process, a significant fraction of arriving metal atoms on low-surface energy polymers such as Teflon AF is expected to desorbed back to the vacuum due to a very weak metal –polymer interaction [8]. However, co-deposition of polymer and metal atoms at high-substrate temperatures (>100 jC) has proven to be quite effective in the present case to enhance significantly metal sticking on the polymer surface. In this case, reactive radicals generated during thermal decomposition in the vapor phase are expected to become highly mobile upon reaching the substrate maintained at elevated temperatures and thereby offering additional nucleation centres to the metal atoms. Hence, simultaneously arriving metal atoms, which gain additional thermal energy upon landing at the hot substrate are trapped in these available nucleation sites to finally settle down and grow into nanoparticles with a certain preferred size/structure, as suggested by the present observations. The effective role of additional nucleation centres for cluster growth, which eventually enhances metal-sticking coefficients, was also evident in our earlier work on irradiation-induced defect centres generated in polymers [9]. As described in the beginning, metal nanoparticles show characteristic plasmon resonance modes during interaction with electromagnetic waves as a result of collective oscillations of free electrons and local enhancement of the electromagnetic field. This phenomenon largely depends on the particle size, shape, and the surrounding dielectric matrix material. Particle plasmon resonances occur through absorption energies in the intra-band transitions and can be either of dipolar (excitation of one surface plasmon) in the case of spherical particles or multipolar excitation when the particles are nonspherical in geometry. Further, in the later case, according to Mie’s theory [10], the corresponding resonance should split into transverse and longitudinal modes giving rise to longer
Fig. 3. Tunning of the plasmon absorption resonances (dipolar excitation) at various deposition temperatures showing a red shift of the wavelength from 413 to 455 nm. Nanoparticles are assumed to be spherical in shape with 10% filling.
wavelengths (red shift) and lower wavelengths (blue shift) characteristic particle plasmon absorption peaks. In the present work, vapor phase co-deposition was applied at various deposition temperatures to produce nanocomposites with different particle size, filling in order to generate spherical and nonspherical shapes. This attempt was aimed at tuning the particle plasmon resonance in a range of wavelength by a single-step technique. Fig. 3 shows particle plasmon resonance absorption peaks in the visible region of different wavelengths from Teflon AF/Ag nanocomposites (~200 and 300 nm thicknesses) prepared at various target temperatures (120 – 220 jC). At the deposition temperature of 120 jC, nanocomposites with thickness ~200 nm and filled with nearly 10% volume Ag nanoparticles showed an optical signature of a typical particle plasmon resonance at the wavelength around 413 nm. The
Fig. 4. Splitting of plasmon absorption resonances at the deposition temperature of 400 jC and originating from multipolar excitation of nonspherical nanoparticles (with filling ~30%) at the wavelengths 630 nm (red shift) and 380 nm (blue shift).
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Table 1 An overview of the different ultrathin nanocomposite color filters of thicknesses ~ 200 – 300 nm, produced at different co-deposition temperatures Co-deposition temperatures (jC)
Plasmon resonance wavelength (nm)
Ag nanoparticle filling in the nanocomposites (~ 200 – 300 nm) (%)
Color of the nanocomposite filters
120
413 (dipolar) 455 (dipolar) 380, 630 (multipolar)
10
yellow
10
golden yellow
30
golden brown
220 400
target temperature in the next set of preparation was then increased to 220 jC with a nanocomposite thickness of ~ 300 nm while maintaining nearly the same particle filling (~ 10%) in order to alter only the particle size. This resulted in easy tunning of the particle plasmon resonance from 413 to 455 nm in the visible wavelength region in the form of a distinct red shift (Fig. 3). Teflon AF showed an excellent transparency. In this case, nanoparticles can be assumed to be generally spherical in view of the below percolation threshold (~15%) particle volume filling, and hence single dipolar resonances were observed (Fig. 3). In order to examine the particle shape, i.e., nonspherical case, and to equally shift the plasmon resonance further, particle filling was increased to nearly 30% and deposition was carried out at a higher temperature of 400 jC. Fig. 4 demonstrates a distinct particle plasmon red shift at around the wavelength 630 nm from the nanocomposite prepared at 400 jC. It is noteworthy here that this wavelength region corresponds to the nonlinear optical properties of nanoparticles [3]. Clearly, due to the apparent nonspherical shape (particle aggregation) in this case, the particle plasmon resonance is associated with two absorption peaks apparently showing multipolar excitation and the resonance splits into red shift (~ 630 nm) and blue shift (~ 380 nm) (Fig. 4). However, the resonance splitting and the observed broad band could also be attributed to the large particle size distribution at higher volume filling and in the high aggregation regime. The physical origin of these multipolar excitations might have two different sources, one due to the shape and other due to the size of the particle. It was recently reported based on theoretical simulations [11] that the Ag plasmon absorption peaks at wavelengths around 410 –450 nm correspond to the dipolar resonance of nanospheres, whereas the peaks observed at smaller and higher wavelengths are due to high multipolar excitations originating from nonspherical shapes. This is in general agreement with our observations here. Concerning the polymer matrix produced at such high temperature, it still shows up to almost 100% transparency (Fig. 4). Further, XPS measurements carried out on the nanocomposites produced at different temperatures indicated the survival of the polymer matrix at the highest
Fig. 5. Diferent colors of Teflon AF/Ag nanocomposites (~ 200 nm – 300 nm thicknesses) generated at various deposition temperatures. (a) Yellow color filter produced at 120 jC. (b) Golden yellow color filter at 220 jC. (c) Golden brown color filter at 400 jC.
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temperature used (400 jC) with gradual increase in the C/F ratio in the polymer matrix with deposition temperature. An overview of the ultrathin nanocomposite color filters produced at different co-deposition temperatures is given in Table 1. Fig. 5 gives a direct impression of the vapor phase generated different polymer – metal nanocomposite color filters showing different colors produced at various deposition temperatures. The apparent inhomogeneous color distribution in the nanocomposites could be attributed to the nonuniform heat transfer to the sample due to the carbon adhesive attached to the substrate holder. The used technique opens up the possibility to generate multicolor filters or multiwavelength functional properties from one nanocomposite system by simply applying layered composite structures.
4. Conclusions In conclusion, an easy and single-step elevated temperature vapor phase co-deposition has been employed to produce Teflon AF/Ag optical nanocomposites with narrow particle size distribution of 5 –7 nm. The nanocomposites produced at various deposition temperatures show shifting of plasmon resonance wavelengths in the UV – VIS region and show promise to function as ultrathin color filters.
Acknowledgements The authors are highly thankful to Mr. Stefan Rehders for his invaluable technical assistance in developing the deposition chamber.
References [1] J.C. Maxwell-Garnett, Philos. Trans. R. Soc. London 203 (1904) 385; 205 (1906) 237. [2] G. Xu, M. Tazawa, P. Jin, S. Nakao, K. Yoshimura, Appl. Phys. Lett. 82 (2003) 3811. [3] L.L. Beecroft, C.K. Ober, Chem. Mater. 9 (1997) 1302. [4] W. Caseri, Macromol. Rapid Commun. 21 (2000) 705. [5] A. Biswas, Z. Marton, J. Kanzow, J. Kruse, V. Zaporojtchenko, F. Faupel, T. Strunskus, Nano Lett. 3 (2003) 69. [6] W.N. Gill, S. Rogojevic, T. Lu, Low dielectric constant materials for IC applications, in: P.S. Ho, J. Leu, W.W. Lee (Eds.), Springer Series in Advanced Microelectronics, Springer, Heidelberg, vol. 95, 2003. [7] A. Heilmann, Polymer Films with Embedded Metal Nanoparticles, Springer, Berlin, 2003. [8] A. Thran, M. Keine, V. Zaporojtchenko, F. Faupel, Phys. Rev. Lett. 82 (1999) 1903. [9] V. Zaporojtchenko, T. Strunskus, K. Behnke, C. von Bechtolsheim, M. Keine, F. Faupel, J. Adhes. Sci. Technol. 14 (2000) 467. [10] G. Mie, Phys. Z. 8 (1907) 769. [11] I.O. Sosa, C. Noguez, R.G. Barrera, J. Phys. Chem., B 107 (2003) 6 269.
Polymer–metal optical nanocomposites with tunable particle plasmon resonance prepared by vapor phase co-deposition A. Biswas a,*, O.C. Aktas a, J. Kanzow a, U. Saeed a, T. Strunskus b, V. Zaporojtchenko a, F. Faupel a a
Nanocomposite Research Laboratory, Lehrstuhl fu¨r Materialverbunde, Technische Fakulta¨t der CAU, Kaiserstr. 2, D-24143, Kiel, Germany b Lehrstuhl fu¨r Physikalische Chemie I, Ruhr-Universita¨t Bochum, Universita¨tsstr. 150, NC 5/28 D-44780, Bochum, Germany Received 28 July 2003; accepted 10 October 2003
Abstract A simple one-step vapor phase co-deposition at elevated target temperature is employed to generate optical nanocomposites (~100 – 300 nm) of polymer-based metallic nanoparticles with uniform size and shape. Homogeneously, three dimensionally distributed isolated Ag nanoparticles are produced with appropriate volume filling (~10 – 30%) in Teflon AF matrix at various co-deposition target temperatures of 120 – 400 jC. An extremely narrow particle size distribution of ~5 – 7 nm based on TEM observations is demonstrated. Different ultrathin nanocomposite color filters as a result of plasmon resonance shifting in the UV – VIS wavelength region are also generated. D 2003 Elsevier B.V. All rights reserved. Keywords: Nanocomposites; Nanomaterials; Optical materials and properties; Deposition; Electron microscopy; Polymers; Thin films
1. Introduction The optical properties of discontinuous metallic films or granular composite films, consisting of metal nanoparticles embedded in a dielectric, have been of interest since the beginning of the 20th century when MaxwellGarnett [1] described the first theoretical framework to explain the resonant absorption or dielectric anomaly which characterizes such systems. Metallic nanoparticle optical properties are determined by a collective oscillation of the free electrons in the particles described by socalled plasmon-resonance absorption and can be observed by conventional spectroscopy. The ability to tune such particle plasmon resonances over a spectral range is highly desirable in various applications of metallic nanoparticles [2]. Moreover, as optical material applications are expanding, the need for novel optically functional and transparent materials increases. Polymer – metal nanocomposite materials show great promise as they can provide the necessary stability and easy processability with interesting optical properties. The general principles in the
* Corresponding author. Tel.: +49-431-880-6230; fax: +49-431-8806229. E-mail address: [email protected] (A. Biswas). 0167-577X/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2003.10.037
construction of optical composites involve the intimate mixing of optically functional materials within a processable matrix. Secondly, optical scattering must be avoided in these types of composites, resulting in a tradeoff between particle size and refractive index (RI) mismatch (the difference in RI between the matrix and the particles). However, this RI mismatch is not important for smaller particles ( < 25 nm). Using such a composite structure, nanocomposites have been formed basically using complex multiple chemical synthesis routes with nonlinear optical and laser amplification properties, among others [3]. Further, as optical properties of nanoparticles depend largely on the particle size, the color of nanoparticles should also depend on the size, so do nanocomposites, and hence upon changing the particle size, these materials offer interesting optical color filter properties [4]. For a majority of technological applications of such nanocomposites, a narrow size distribution of isolated nanoparticles in a specific size regime is of vital importance and requirement. However, due to the strong nanoparticles aggregation tendency during the production of such nanostructures, one of the major technological challenge is the control of the particles size distribution while generating a high particle volume filling in the quantum size range. Although, the choice of organic media to
A. Biswas et al. / Materials Letters 58 (2004) 1530–1534
1531
embed metallic nanoparticles is helpful for isolation during the generation process besides controlling the particle size, it is quite challenging to optimize such nanostructures for desired properties. The existing state of the art to prepare such interesting optical functional nanocomposites involves mostly multiple or tedious fabrication steps, and obviously, a single-step or straightforward method is highly desired. Here, we introduce an easy and single-step relatively new method of vapor phase elevated target temperature co-deposition of polymer and metal to produce optical functional nanocomposites. The aim of this paper is to show the effectiveness of the technique to generate an extremely narrow particle size distribution which also allows easy tunning of particle plasmon resonance over a wide wavelength range to produce different ultrathin color filters.
2. Experimental procedures Teflon AFR was chosen as the polymer matrix due to its known properties of a high degree of resistance to any chemical attack and excellent transparency. The chemical structure of the polymer can be found in our earlier publication [5]. Vapor phase co-deposition of Teflon AFR (granulates, Dupont) and Ag (wires, Good Fellow Industries) was carried-out using a homemade evaporation chamber consisting of separate heatable boats at various target temperatures of 120 –400 jC. Nanocomposite films were prepared with thicknesses of 100 –300 nm. Deposition rates of polymer and metal were power controlled with a nearly constant value of 4 P/min for metal and 10 – 20 P/min for polymer to prepare different metal volume filling in the polymer matrix. Relatively low rate of metal evaporation was chosen in order to control nanoparticle growth process in a better way. Estimation of the particle filling was done using quartz microbalance thickness informations for the two components. Vapor phase deposition of polymers has a range of advantageous features. Some of these are excellent conformality over complex topography, the possibility of good film uniformity on large diameter wafers, and environmentally safe processing due to the absence of solvents [6]. For TEM measurements (Philips CM 30), samples were collected on carbon-covered (~10 nm) Cu grids. Optical characterization of the nanocomposites was carried out using a Perkin-Elmer UV –VIS Lambda 900 spectrometer.
3. Results and discussion Fig. 1a and b shows a TEM planar view of the dispersed Ag nanoparticles in the Teflon AF matrix along with an elemental profile of the nanocomposite (~100 nm, target temperature: 120 jC) as shown by TEM EDX. In view of the co-deposition, the vapor phase generated
Fig. 1. (a) TEM planar view of Ag nanoparticles 3D distributed in Teflon AF matrix (Nanoparticle volume filling: ~10%) along with SAED pattern on the generated nanoparticles showing fcc nature, in the inset. (b) EDX of the nanocomposite (~100 nm) showing elemental profile with C, O, F due to Teflon AF matrix along with Ag and Cu lines originating from the nanoparticles and backing TEM Cu grid, respectively.
nanoparticles are believed to be distributed three-dimensionally in the polymer matrix. It is clear from the TEM image (Fig. 1a) that the nanoparticles have almost uniformly spherical shape along with the size. A good control on the particle size distribution is evident, as a size distribution profile shows an extremely narrow particle size distribution of nearly 80% of the visible particles in the range of 5 – 7 nm, (Fig. 2). This is quite promising as compared to the existing large volume of the literature [7] showing a mostly uncontrollable particle size distribution due to the preparation conditions. The crystal structure of the so produced nanoparticles is observed to be of fcc, as analysed by the selected area electron diffraction measurements (Fig. 1a, inset).
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Fig. 2. Size distribution profile of the generated Ag nanoparticles distributed in Teflon AF matrix showing almost 80% particles in the size range of 5 – 7 nm.
During room temperature co-deposition process, a significant fraction of arriving metal atoms on low-surface energy polymers such as Teflon AF is expected to desorbed back to the vacuum due to a very weak metal –polymer interaction [8]. However, co-deposition of polymer and metal atoms at high-substrate temperatures (>100 jC) has proven to be quite effective in the present case to enhance significantly metal sticking on the polymer surface. In this case, reactive radicals generated during thermal decomposition in the vapor phase are expected to become highly mobile upon reaching the substrate maintained at elevated temperatures and thereby offering additional nucleation centres to the metal atoms. Hence, simultaneously arriving metal atoms, which gain additional thermal energy upon landing at the hot substrate are trapped in these available nucleation sites to finally settle down and grow into nanoparticles with a certain preferred size/structure, as suggested by the present observations. The effective role of additional nucleation centres for cluster growth, which eventually enhances metal-sticking coefficients, was also evident in our earlier work on irradiation-induced defect centres generated in polymers [9]. As described in the beginning, metal nanoparticles show characteristic plasmon resonance modes during interaction with electromagnetic waves as a result of collective oscillations of free electrons and local enhancement of the electromagnetic field. This phenomenon largely depends on the particle size, shape, and the surrounding dielectric matrix material. Particle plasmon resonances occur through absorption energies in the intra-band transitions and can be either of dipolar (excitation of one surface plasmon) in the case of spherical particles or multipolar excitation when the particles are nonspherical in geometry. Further, in the later case, according to Mie’s theory [10], the corresponding resonance should split into transverse and longitudinal modes giving rise to longer
Fig. 3. Tunning of the plasmon absorption resonances (dipolar excitation) at various deposition temperatures showing a red shift of the wavelength from 413 to 455 nm. Nanoparticles are assumed to be spherical in shape with 10% filling.
wavelengths (red shift) and lower wavelengths (blue shift) characteristic particle plasmon absorption peaks. In the present work, vapor phase co-deposition was applied at various deposition temperatures to produce nanocomposites with different particle size, filling in order to generate spherical and nonspherical shapes. This attempt was aimed at tuning the particle plasmon resonance in a range of wavelength by a single-step technique. Fig. 3 shows particle plasmon resonance absorption peaks in the visible region of different wavelengths from Teflon AF/Ag nanocomposites (~200 and 300 nm thicknesses) prepared at various target temperatures (120 – 220 jC). At the deposition temperature of 120 jC, nanocomposites with thickness ~200 nm and filled with nearly 10% volume Ag nanoparticles showed an optical signature of a typical particle plasmon resonance at the wavelength around 413 nm. The
Fig. 4. Splitting of plasmon absorption resonances at the deposition temperature of 400 jC and originating from multipolar excitation of nonspherical nanoparticles (with filling ~30%) at the wavelengths 630 nm (red shift) and 380 nm (blue shift).
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Table 1 An overview of the different ultrathin nanocomposite color filters of thicknesses ~ 200 – 300 nm, produced at different co-deposition temperatures Co-deposition temperatures (jC)
Plasmon resonance wavelength (nm)
Ag nanoparticle filling in the nanocomposites (~ 200 – 300 nm) (%)
Color of the nanocomposite filters
120
413 (dipolar) 455 (dipolar) 380, 630 (multipolar)
10
yellow
10
golden yellow
30
golden brown
220 400
target temperature in the next set of preparation was then increased to 220 jC with a nanocomposite thickness of ~ 300 nm while maintaining nearly the same particle filling (~ 10%) in order to alter only the particle size. This resulted in easy tunning of the particle plasmon resonance from 413 to 455 nm in the visible wavelength region in the form of a distinct red shift (Fig. 3). Teflon AF showed an excellent transparency. In this case, nanoparticles can be assumed to be generally spherical in view of the below percolation threshold (~15%) particle volume filling, and hence single dipolar resonances were observed (Fig. 3). In order to examine the particle shape, i.e., nonspherical case, and to equally shift the plasmon resonance further, particle filling was increased to nearly 30% and deposition was carried out at a higher temperature of 400 jC. Fig. 4 demonstrates a distinct particle plasmon red shift at around the wavelength 630 nm from the nanocomposite prepared at 400 jC. It is noteworthy here that this wavelength region corresponds to the nonlinear optical properties of nanoparticles [3]. Clearly, due to the apparent nonspherical shape (particle aggregation) in this case, the particle plasmon resonance is associated with two absorption peaks apparently showing multipolar excitation and the resonance splits into red shift (~ 630 nm) and blue shift (~ 380 nm) (Fig. 4). However, the resonance splitting and the observed broad band could also be attributed to the large particle size distribution at higher volume filling and in the high aggregation regime. The physical origin of these multipolar excitations might have two different sources, one due to the shape and other due to the size of the particle. It was recently reported based on theoretical simulations [11] that the Ag plasmon absorption peaks at wavelengths around 410 –450 nm correspond to the dipolar resonance of nanospheres, whereas the peaks observed at smaller and higher wavelengths are due to high multipolar excitations originating from nonspherical shapes. This is in general agreement with our observations here. Concerning the polymer matrix produced at such high temperature, it still shows up to almost 100% transparency (Fig. 4). Further, XPS measurements carried out on the nanocomposites produced at different temperatures indicated the survival of the polymer matrix at the highest
Fig. 5. Diferent colors of Teflon AF/Ag nanocomposites (~ 200 nm – 300 nm thicknesses) generated at various deposition temperatures. (a) Yellow color filter produced at 120 jC. (b) Golden yellow color filter at 220 jC. (c) Golden brown color filter at 400 jC.
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temperature used (400 jC) with gradual increase in the C/F ratio in the polymer matrix with deposition temperature. An overview of the ultrathin nanocomposite color filters produced at different co-deposition temperatures is given in Table 1. Fig. 5 gives a direct impression of the vapor phase generated different polymer – metal nanocomposite color filters showing different colors produced at various deposition temperatures. The apparent inhomogeneous color distribution in the nanocomposites could be attributed to the nonuniform heat transfer to the sample due to the carbon adhesive attached to the substrate holder. The used technique opens up the possibility to generate multicolor filters or multiwavelength functional properties from one nanocomposite system by simply applying layered composite structures.
4. Conclusions In conclusion, an easy and single-step elevated temperature vapor phase co-deposition has been employed to produce Teflon AF/Ag optical nanocomposites with narrow particle size distribution of 5 –7 nm. The nanocomposites produced at various deposition temperatures show shifting of plasmon resonance wavelengths in the UV – VIS region and show promise to function as ultrathin color filters.
Acknowledgements The authors are highly thankful to Mr. Stefan Rehders for his invaluable technical assistance in developing the deposition chamber.
References [1] J.C. Maxwell-Garnett, Philos. Trans. R. Soc. London 203 (1904) 385; 205 (1906) 237. [2] G. Xu, M. Tazawa, P. Jin, S. Nakao, K. Yoshimura, Appl. Phys. Lett. 82 (2003) 3811. [3] L.L. Beecroft, C.K. Ober, Chem. Mater. 9 (1997) 1302. [4] W. Caseri, Macromol. Rapid Commun. 21 (2000) 705. [5] A. Biswas, Z. Marton, J. Kanzow, J. Kruse, V. Zaporojtchenko, F. Faupel, T. Strunskus, Nano Lett. 3 (2003) 69. [6] W.N. Gill, S. Rogojevic, T. Lu, Low dielectric constant materials for IC applications, in: P.S. Ho, J. Leu, W.W. Lee (Eds.), Springer Series in Advanced Microelectronics, Springer, Heidelberg, vol. 95, 2003. [7] A. Heilmann, Polymer Films with Embedded Metal Nanoparticles, Springer, Berlin, 2003. [8] A. Thran, M. Keine, V. Zaporojtchenko, F. Faupel, Phys. Rev. Lett. 82 (1999) 1903. [9] V. Zaporojtchenko, T. Strunskus, K. Behnke, C. von Bechtolsheim, M. Keine, F. Faupel, J. Adhes. Sci. Technol. 14 (2000) 467. [10] G. Mie, Phys. Z. 8 (1907) 769. [11] I.O. Sosa, C. Noguez, R.G. Barrera, J. Phys. Chem., B 107 (2003) 6 269.