Formation and optical properties of prolate silver particles in glasses

NANoSTRUCTURED MATERIALS VOL. 3, PP. 53-59, 1993 COPYRGHT @1993 PERGAMONPRESSL'ro. ALL RIGHTSRESERVED.

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FORMATION AND OPTICAL PROPERTIES OF PROLATE SILVER PARTICLES IN GLASSES A. Berger I and H. Hofmeister 2

lCenter for Research in Electro-Optics and Lasers, University of Central Florida, 12424 Research Parkway, Suite 400, Orlando, FI 32826, USA 2MPI Ftir Mikrostrukturphysik, Weinbergweg, 0-4050 Halle, Germany

Abstract--Spherical crystalline silver particles produced by ion-exchange and following annealing process in a soda-lime glass are deformed by stretching the glass. Results concerning the size dependence of particle deformation are presented. Optical spectra as well as electron microscopical results are given for a wide range of particle sizes (10 nm -100 nm) within one sample, i.e. for a constant glass deformation.

INTRODUCTION

Small crystalline silver particles suspended in glasses show very interesting optical properties. The color and appearance of the glasses depend on the size, shape and number of the precipitated particles. Usually these particles are isolated (non-interacting) and spherical shaped so that their absorption and scattering can be described by Mie's theory (1). If the particles are small enough (less than about 20 nm diameter) the glasses tend to be transparent (very little light scattering) and are yellow or brown colored. The absorption, when plotted against the wavelength, gives a Lorentzian line band with the maximum near 410 nm. A higher concentration of the particles usually causes an aggregation resulting in a transparent dark red coloration (2). In the literature there are some articles (3-9) discussing the deformation of originally spherical metal particles in glasses at temperatures below the melting point of bulk silver (Tin = 962°C (11)). Stookey and Araujo (3) first mentioned the possibility of stretching glasses containing homogeneously distributed small silver metal particles at temperatures near the glass softening point in such a manner as to elongate the particles to prolate spheroids and align them along a common axis. They observed a split of the silver absorption band into two broad bands as well as polarizing effects. The split of the absorption band depends on the elongation of the particles. Whereas the maximum of the short-wavelength absorption band is in the blue or ultraviolet, the long-wavelength band can be shifted throughout the whole visible region, resulting in different colorations of the glass. Polarization is caused by different absorption of the vector components of light oriented parallel and perpendicular, respectively, to the long axis of the spheroids. Subsequent work (4-7), resulted in the development ofCorning's dichroic near-infrared 53

54

A BERGERAND H HOFMEISTER

polarizer POLARCOR (Coming 8612), with the absorption maximum of the long-wavelength band in the near infrared. Mennig (8) showed that similar results can be found for silver ionexchanged glasses, where the particles are distributed only in a surface layer, when elongating these glasses near the glass transition temperature Tg. This paper presents experimental results of the deformation of small crystalline spherical silver particles in a soda-lime glass by stretching the glass. The silver particles were produced by an ion-exchange and a following annealing process. Whereas former investigations and discussions are limited to certain particle sizes usually smaller than 20 nm, this paper includes results for particle sizes ranging from about 10 nm to 100 nm, i.e. particles showing in the spherical case strong light scattering effects. The different particle sizes are produced within one glass sample, i.e. the glass deformation was the same for all particles, After describing the experimental devices and conditions, results of optical spectroscopy and transmission electron microscopy are given.

EXPERIMENTAL For all the investigations a sheet of commercial soda-lime glass (composition see Table 1) with a size of 130 x 9.5 x 1.8 mm 3 was used. The glass was subjected to an ion exchange in a salt melt of 2 weight-% AgNO3 - 98 weight-% NaNO3 at 400°C for 2 h. The ion exchange was followed by a subsequent annealing process of 100 h at 650°C resulting in the formation of spherical silver particles (see (11)). The deformation of the glass sheet was realized by using an electrical furnace (length 34 mm, diameter of the circular opening 11 ram) and applying a constant force F = 29.4 N to the end of the sheet (see Figure 1; a detailed description is given in (9)). The maximum temperature within the furnace was 595°C, the axial temperature gradient can be described by a Gaussian curve, with a full width of half maximum of 34 mm, equal the furnace length. The move-in velocity vi was regulated so that the ratio vdvi, move-in to move-out velocity, was kept at the constant value v d vi = 90, with Vo= lcm/min. Because of the slow velocities, temperature differences in the glass perpendicular to the drawing direction can be neglected. By cutting, grinding and polishing, cross-sectional samples with a thickness of 40 p.m were produced of both the de formed and undeformed glass. On these samples optical extinction spectra could be measured with a microspectrophotometer (13) at different penetration depths (the local resolution power in this direction is about 10 Bm). For carrying out electron-microscopical investigations on the effect of penetration depth two thin sections of the glass (thickness - 40 Bm) were cut perpendicular to the stained surface. These two thin sections were glued between TABLE 1 Glass Composition (measured by x-ray fluorescence analysis (12)) content

SiO2

Na20

CaO

MgO

A1203

K20

503

Fe203

weight-%

72.5%

14.4%

6.1%

4.0%

1.5%

0.7%

0.37%

0.13%

PROLATESILVERPARTICLESINGLASSES

55

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Figure 2. (a) Optical extinction spectra of undeformed glass after subtraction on the extinction of untreated glass for different penetration depths. (b) Dependence of silver particle radius in the undeformed glass on the penetration depth; solid line theoretical description according to (11).

56

A BERGERAND H HOFMEISTER

two aluminum supporting rings in such a way that their ion exchanged surfaces were contiguous along a ring diameter. The specimen was further thinned by ion beam etching until there was a hole. Afterwards the sample was covered with a thin carbon film by vacuum evaporation to avoid charging effects in the electron microscope. Ion beam thinning shaped the edge of the hole to a wedge allowing electron transmission at the thin parts. Due to the specimen geometry, electronmicroscopical investigations along the edge of the hole yield results for different penetration depths (14).

RESULTS After the ion-exchange the silver in the glass is only in ionic state, within a surface layer of 55 Ixm. The following annealing process results in the formation of silver particles as described in (11). Electron microscopy shows that there are spherical particles which electron diffraction identifies as crystalline silver. The particles are predominantly monocrystaUine. In each volume element a gaussian distribution of the particle sizes occurs. The mean particle size changes with the penetration depth drastically, whereas their volume concentration is nearly constant (11,14,15). In Figure 2a the change of the optical extinction spectra with the depth is shown. Using the method described in (11), the dependence of particle radius (Figure 2b) on the relative penetration depth was determined. (Because of the gaussian distribution this method yields mean particle sizes.) The diameters, estimated thus, are in very good agreement with the electron microscopical mean particle sizes. The silver particles with a size greater than about 15 nm show strong visible light scattering. The deformation process results in a constant stretching of the glass with a thickness of 0.19 mm and a height of 1.0 mm, i.e. the original ratio height/thickness of the rectangular cross section is maintained. The stretching factor is 9.5. The extinction spectra after deformation (Figure 3) show a drastic change with the relative penetration depth, i.e. with particle size. (The radius Ro, radius of the undeformed particle, given in the figures was evaluated using the theoretical model of (11)). The polarization independent extinction band of the spherical silver particles are split off in two bands which are dependent on the polarization. The extinction bands measured with a polarization perpendicular to the stretching direction are Lorentzian line shaped bands with a slightly in the UV shifted maximum. With greater penetration, i.e. increasing particle size and deformation (see Figure 3), the extinction maximum is further shifted in the ultraviolet region, resulting in a visible clear glass (with this light polarization). The spectra measured with a polarization parallel to the stretching direction, however, are much more complex. With increasing penetration depth the extinction maximum is shifted from 410 nm through the whole visible region to the near infrared. Parallel to the shift the half-width of the bands is growing. At penetration depths of x = 0.22 and x = 0.25, (i.e. particle sizes which in the spherical case show distinct scattering effects), there is an increase of the extinction in the near infrared. As the upper wavelength limit for the extinction measurement was 850 nm, it could not be proved if an increase can be found for still greater particles sizes also. In both polarizations a visible light scattering in the VIS-region could not be observed.

PROLATESILVERPARTICLESIN GLASSES

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57

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'

'

'

'

450 550 650 750 wavelength (nm)

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.350

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/ /

\ °IA

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"-

"- 0 ~ - ~ , . , , , . z . , 850 350 450 550 650 750 850 wGvelength (nm)

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550

r

450 550 650 750 wavelength (nm)

850

Figure 3. Optical extinction spectra of deformed glass after subtraction of untreated glass for different penetration depths x; Ro - radius of undeformed particle determined with model given in (11).

58

A BERGER ANO H HOFMEISTER

20011m

20Ohm 1

Figure 4. TEM micrographs of deformed glass at the relative penetration depths x = 0.2 and x = 0.5.

6 ÷

÷

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, 4

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60

70

Figure 5. Dependence of ration a/b on radius of undeformed particle Ro.

PROLATESILVERPARTICLESIN GLASSES

59

Transmission electron microscopical images (see Figure 4) show prolate, mainly monocrystalline silver particles. With greater penetration depth, i.e. growing particle size, a/b the ratio of the length of the major axis to the minor axis, is increasing. Figure 5 shows the dependence of a/ b on R. Because of the low number of electron microscopically imaged particles a statistic was not possible. But already these results show that there is a distribution of a/b for a particle size. CONCLUSIONS Stretching of a soda-lime glass containing spherical silver particles results in the deformation of the spheres. The eccentricity of the spheroids depends strongly on the particle size. The measured optical extinction spectra deviate from spectra calculated on the basis of Gans' theory (16). Further investigations should include the measurement of the extinction spectra in the near infrared as well as of the light scattering and calculations of the optical properties on the basis of theories taking into account higher multipoles. A theoretical description of the particle deformation will be given in (16). ACKNOWLEDGEMENTS When the experimental work was doneA. Berger was with Martin-Luther-Universitlit HalleWittenberg, Fachbereich Physik, Germany. We thank S. Thiel for realizing the glass deformation. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

G. Mie, Ann. Phys. 25,377,(1908). A. Berger, K.J. BergandH. Hofmeister, Z. Phys. D-Atoms, Molecules and Clusters 20,313, (1991). S.D. Stookey and R. J. Araujo, Appl. Opt. 7, 777 (1968). T.P. Seward III, Proc. SPIE - Solid State Optical Control Devices 464, 96 (1984). D.N. Grim, Proc. SPIE - Infrared Systems and Components 750, 18 (1987). M. Taylor, Proc. SPIE - Glasses for Optoelectmnics 1128, 186 (1989). M. Taylor, G. Bucher and K. Jones, Proc. SPIE - Polarization Considerations for optical Systems 1166, 446 (1989). M. Mennig, Dissertation A, Halle (1986). W.G. Drost, Dissertation A, Halle (1992). Handbook of Physics and Chemistry (1983-1984). Boca Raton: CRC Press 1984. A. Berger, J. Non-Cryst. solids, in press. S. Thiel, Praktikumsbeleg, Torgau (1989). K.H. Brauer and F. FrOhlich, Exp. Techn. Physik 6, 216 (1958). M. Dubiel, H. Hofmeister, K.-J.Berg and A.Berger, VerOffenflichungenzur 12.Tagung Elektronenmikroskopie, pp. 424. Dresden (1988). K.-J. Berg, A. Berger and H. Hofmeister, Z. Phys. D - Atoms, Molecules and Clusters 20, 309 (1991). A. Berger, Glastechn. Ber. (in preparation).