J . Phys. Chem. 1992,96, 8998-9001
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(2) Brandt, N. B.; Chudinov, S. M.; Ponomarev, Ya. G. Semimetals, Graphite and its Compounds. Modern Problems in Condensed Matter Sciences; North-Holland: Amsterdam, 1988; Vol. 20.1. (3) Davis, S. M.; Somorjai, G. A. In The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis; King, D. A., Woodruff, D. P., Eds.; Elsevier: Amsterdam, 1983; Vol. 4. (4) Horsley, J. A. In Chemistry and Physics of Solid Surfaces; Vanselow, R., Howe, R., Eds.;Springer: Berlin, 1990; Vol. 8. (5) Binnig, G.;Fuchs, H.; Gerber, Ch.; Rohrer, H.; Stoll, E.; Tosatti, E. Europhys. Lett. 1986, I, 31. (6) Park, S.-I.; Quate, C. F. Appl. Phys. Lett. 1986, 48, 112. (7) Soler, A. M.; Baro, N.; Garcia, J. M.; Rohrer, H. Phys. Rev. Lett. 1986, 57, 444. (8) Hansma. P. K. Bull. Am. Phvs. Soc. 1985.30. 251. Kuwabara. M.: Clarke, D. R.; Smith, D. A. Appl. bhys. Lett. 1990, 56, 2396. (9) Sugawara, Y.; Ishizaka, T.; Morita, S. Jpn. J . Appl. Phys. 1990, 29, 1539.
(IO) Tersoff, J. Phys. Rev. Letr. 1986, 57, 440.
Slonczewski, J. C.; Weiss, P. R. Phys. Reu. 1958, 109, 272. McClure, J. W. Phys. Rev. 1957, 108, 612. Tomanek, D.; Louie, S.G.Phys. Reu. B 1988, 37, 8327. Tomanek, D.; Louie, S.G.; Mamin, H. J.; Abraham, D. W. Thomson, R. E.; Clarke, J. Phys. Reu. B 1987, 35, 7790. (15) Land, T. A,; Michely, T.; Behm, R. J.; Hemminger, J. C.; Comsa, G.SurF. Sci. 1992. 264.261. (16j Ashcroft, N. W.; Mermin, N. D. Solid State Physics; W. B. Saunders Co.: Philadelphia, 1976. (17) Hoffmann, R. Solids and Surfaces: A Chemist’s View of Bonding in Extended Strucrures; VCH: New York, 1988. (18) Misurkin, I. A.;Ovchinnikov, A. A. Teor. Eksp. Khim. 1968,4,3 (in Russian); Theor. Exp. Chem. 1968, 4, 3 (in English). (19) Hayashi, H.; Nasu, K. Phys. Reo. B 1985,32, 5295. Belinskii, A. E.; Tchougreeff, A. L.; Misurkin, I. A. Teor. Eksp. Khim. 1989, 25, 513 (in (11) (12) (13) (14)
Photoinduced Changes in the Se-Ag-I
Russian); Theor. Exp. Chem. 1989, 25, 475 (in English). Tchougreeff, A. L.; Misurkin, I. A. Zh. Strukt. Khim. 1989, 30, No 3, 24 (in Russian); J. Srrucr. Chem. 1989, 30, 377 (in English). (20) HGckel, E. 2.Phys. 1931, 70, 204; fbid. 1931, 72, 301; fbid. 1932, 76, 628. (21) Dewar, M. J. S. The PMO Theory of Organic Chemistry; Plenum: New York, 1975. (22) Misurkin, I. A.; Ovchinnikov, A. A. Vsp. Khim. 1977,46, 1835 (in Russian); Russ. Chem. Reu. 1977, 46, 967 (in English). (23) Solyom, J. Adu. Phys. 1979, 28, 201. (24) Misurkin, I. A.; Ovchinnikov, A. A. Teor. Eksp. Khim. 1976,12,291 (in Russian); Theor. Exp. Chem. 1976, 12, 225 (in English). (25) Peacock, T. E. Electronic Properties of Aromatic and Heterocyclic Molecules; A P London, 1965. (26) Coulson, C. A.; Taylor, R. Proc. Phys. Soc. 1952, A65, 815. (27) Weiss, C.; Kobayashi, H.; Gouterman, M. J . Mol. Spectrosc. 1965, 16, 415. (28) Schug, J.; Reid, R. D.; Lilly, A. C.; Dwyer, R. W. Chem. Phys. Lerr. 1986, 128, 5. (29) Gutfreund, H.; Little, W. A. Phys. Reu. 1%9, 183, 68. (30) Wiesendanger, R.; Guntherodt, H. J.; Guntherodt, G.;Gambino, R. J.; Rut, R. Phys. Rev. Lett. 1990, 65, 247. (31) Molotkov, S.N. Surf. Sei. 1992, 261, 7. (32) Tersoff, J.; Hamann, D. R. Phys. Rev. Lett. 1983, 50, 1998. 133) Tersoff. J.: Hamann. D. R. Phvs. Rev. B 1985. 31. 805. (34) Tsukada, M.; Kobayashi, K.; Isshuki, N.; Kagkhima, H. Surf. Sci. Rep. 1991, 13, 265. (35) Himpsel, F. J. J. Magn. Magn. Mater. 1991, 102, 261. (36) Fu, C. L.; Freeman, A. J.; Oguchi, T. Phys. Reu. Lett. 1985,54,2700. (37) Richter, R.; Gay, J. G.; Smith, J. R. Phys. Reu. Lett. 1985,54, 2704. (38) Kelty, S. P.; Leiber, C. M. J . Phys. Chem. 1989, 93, 5983. (39) Kelty, S. P.; Lieber, C. M. Phys. Rev. B 1989, 40, 5856.
System
M. Mitkova,* T. Petkova, P. Markovski, and V. Mateev Central Laboratory of Optical Storage and Processing of Information, Bulgarian Academy of Sciences, Sofia I I 13, P.O. Box 95, Bulgaria (Received: May 12, 1992; In Final Form: July 14, 1992) Photoinduced changes in thin films from the SeAg-I system are investigated firstly, employing various methods-holography, microscopy, electron microscopy, X-ray analysis, and Auger electron spectroscopy. The influence of the composition on the photoinduced changes, as well as the possibility of inducing considerable photoanisotropy, is demonstrated. The mechanism of the processes is discussed on the basis of the experimental data, the phenomena being related to Weigert effect in silver halide systems, photocrystallization,and orientation of defects in chalcogenide systems.
Introduction The amorphous semiconductors are a very attractive material because of their great potential applicability.’ The photoinduced changes, especially typical of chalcogenideglassy materials, allow reversible2and irreversible3optical recording in them. Alongside with the photoinduced changes of the absorption coefficient and the refractive index, photoinduced anisotropy is also ~bserved.~ Optical recording in these materials does not need subsequent processing. Besides, they possess good chemical resistance in aggressive ambients and high transmittance in the near-IR region. In recent years the researchers’ interest has been drawn to the synthesis and investigationsof a new chalcogenide-halide family of materials for fiber optic^.^ The investigations on chalcogenide-halide glasses of the SeAg-I system6give grounds to expect that they are suitable for holographic recording combining the good transmittance in the IR region of the halide materials with the photosensitivity of the chalcogenides. The objective of the present work is to investigate the photoinduced changes in this system and the possibilitiesfor applying them as an optical recording medium, using various investigation methods-holography, microscopy, electron microscopy, X-ray analysis, and Auger electron spectroscopy.
Experimental Section The investigated films with a thickness of 500-1000 nm were prepared by vacuum thermal evaporation.’ The specimens were
left in darlaress for 24 h in order to eliminate the possible influence of their thermal history. The chemical composition of the thin films was investigated by Auger electron spectroscopy. The transmission spectra of Ar+ laser irradiated and nonirradiated samples were studied in the spectral range 400-2500 nm. The experimental setup for holographic recording is shown in Figure 1. A continuous Ar+ laser with a wavelength A = 488 nm in a standard interferometric configuration produces fringes with a spatial frequency 200 mm-I. The intensity of the interfering beams is controlled with a gradual attenuator. The substrate holder temperature is maintained constant by a thermostat at 20 f 1 OC. During recording the diffraction efficiency is measured with a linearly polarized H e N e laser beam with a wavelength X = 632.8 nm. The local heating at the area of recording is measured with a copper-constantan thermocouple. The holographic scalar recording is camed out with two equal circular (left-hand or right-hand rotating) polarizations. The recording beam’s intensity is 2.5 W/cm2. The resulting interference pattern has a circular polarization and intensity, varying by a sinusoidal law. The polarization holographic recording permits the reconstruction of both the intensity distribution of the wavefront and the recording wave polarization? The polarization recording with two orthogonal circularly (left-hand and right-hand rotating) polarized Ar+ laser beams is accomplished by the arrangement in Figure 1, removing the X/2 phase plate. In this case the
0022-3654/92/2096-8998$03.00/00 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 22, 1992 8999
Photoinduced Changes in the StAg-I System I
PR
W
M ?hX!L
S
I
A
0,5
0.6
Or6 1,O lFl+ ( p m )
L5
Figure 1. Holographic recording setup: L, Ar+ laser; PR, polarization rotator; A, gradual attenuator; W, Wollastone prism; M, mirror; X/2, X/4, phase plates; S, specimen; T, thermostat; D, detector.
II
’”1
2o A a
s
Y
c
V (cm-1) Figure 3. Transmission spectra of films of (1) %&3&5, (2) before irradiation; (1’) S e g 5 1 s ,(2’) Sq,,AglSIIs, after irradiation.
wg&
t(min)
Fnspln 2. Composition profilogram of a film with composition !3q&,,IS.
resulting interference field has a periodically modulated linear polarization and constant intensity over the specimen ~urface.~ The recording beam’s intensity is varied-0.5, 1.3,2.1,2.5 W/cm*. The morphology and structure of the films is investigated with an electron microscope in the SEI mode and diffraction mode. Besides, the film structure before and after irradiation is investigated by X-ray diffraction. The small area of the spot, irradiated with the laser beam (because of the relatively low sensitivity of the material), is the cause for using the Debye-Sherer-Hull method for three ranges of the irradiation angle with X-rays5-30°, 30-70°, and 70-90°.
Results and Discussion Investigated are specimens containing 5-1 5 atom % silver and iodine in which selenium ranges between 70 and 90 atom %. The investigation by Auger electron spectroscopy showed that the composition of the thin films has a satisfactory similarity to that of the starting material. Figure 2 illustrates the behavior of a film with composition w g l S I ssputtered in depth as a function of time. A typical spectrum of the specimens with compositions Sc&gsIs and S q ~ g l s I I(1s and 2) before and after irradiation (1’ and 2’) is shown in Figure 3. From this the conclusion can be drawn that the most appropriate wavelengths for holographic “ d i n g are in the blue and green spectral region. At a recording wavelength X = 488 nm the optical density of the specimens is >1. After exposure, the optical density increases in the visible and near-IR regions. It is Seen that, on irradiation with an Ar+ laser photoinduced changes in the optical properties take place, the absorption edge is shifted to the longer wavelengths, and the transmission in the visible region diminishes. These changes can be attributed to both the changes in the chalcogenide matrix and the formation of silver atoms when light causes breakage of the silver-iodine bonds. The holographic scalar grating, recorded with an Ar+ laser, has an amplitudephasecharacter. It does not quire additional processing after exposure. The value of the diffraction efficiency, measured periodically for 6 months after the recording, did not show any changes. The multiple heating and cooling of the material in the range 20-50 “C,following the holographic recording, does not change the efficiency either. A micrograph of the spatial modulation of such a grating with magnification XlOO is shown in Figure 4. The change in the diffraction efficiency ( q ) with time in a scalar recording depending on the composition of the system at an intensity of 2.5 W/cm2 is illustrated in Figure 5. The calculations show that the recorded
Figure 4. Photograph of a diffraction grating in a film S Q ~ A ~ , . , I ~ . ~ .
U
2
600
900
4
6
1200
E(J/cm2)
8
10
t(min)
Figure 5. Relationship between the diffraction efficiency of scalar recording and the composition: ( 1 ) S e g 5 1 5 ;(2) S ~ & J A & ~ , & (3) Se80&5115; (4) S%Ag12,S112,5; ( 5 ) S%5Ag7.517.5; (6) s ~ ~ l O 1 l O .
grating has an amplitudephasecharacter. Despite the high values of the intensity, measurement with a thermocouple reveals that the local heating caused by the irradiation does not lead to an increase above the crystallization temperature of the material. The temperature of the specimens rises up to 10 OC at the most above the temperature of the thermostat. It is evident that the highest diffraction efficiency is reached in compositions with the highest selenium composition. The investigations show that the increase in the contents of silvex and iodine contributes to a substantial increase in the sensitivity of the system and makes possible the photoinduction of anisotropy.
,
9ooo The Journal of Physical Chemistry, Vol. 96, No. 22, 1992
-s
156 , 468
0.31
,
780
I
1092
I
Mitkova et al.
1404Er/Cm2)
I
1
Y
e
18 20
t (min)
Figure 6. Relationship between the diffraction efficiency of polarization recording and the composition: (1) Se,,,AglsIIs; (2) Se,sAg12,sI,zs; (3) Se8dg10110; (4) SC82.5A87,J110. ,
3
Figure 8. (a, top) Electron microscope photograph of a film with composition Se8,,AgloIl,. (b, bottom) Electron microscope photograph of scalar recording in a Se8,,AgloIlo.
0
5
10
15
20
t (min) Figure 7. Diffraction efficiency dependence of polarization recording on the intensity for a Se,,,AglSIIsfilm at (1) I = 1.3 W/cm2; (2) I = 2.1 W/cm2; (3) I = 2.5 W/cm2.
The investigation on birefringence at X = 632.8 nm shows that the maximum value of the birefringence change (An), measured holographically and polarimetrically, is An = 0.0 16. Figure 6 illustrates the diffraction efficiency dependence on t h e at an intensity of 1.3 W/cm2 as a function of the composition in polarization recording. More than 20 specimens with different contents of selenium, silver, and iodine were investigated. The highest diffraction efficiency is measured in the composition Se7~gl,I!, at X = 632.8 nm, 7 0.25%. It may be suggested that in this composition silver and iodine are linked completely into silver iodide, as proposed in the structural model of these glasses6 The diffraction efficiency dependenceon the exposure at various intensities is shown in Figure 7. The diffraction efficiency increases with intensities up to 1.3 W/cm2. At higher intensities, the difference between the change in the refractive index in parallel and perpendicular direction of the electrical vector of the induced light begins to diminish, leading to a decrease in the diffraction efficiency. The gratings of this type have only two diffraction orders whose intensities IL and IR are proportional to the left-hand and right-hand circular components of the irradiating light, Le., v(+lJ?(-ll = IR/IL. Thus, polarization beam splitters, hamg no match m classical optics, can be recorded holographically in photoanisotropic materials. Electron microscope investigations were carried out in order to clarify the character of the observed phenomena. Initially, the films are homogeneous with a very uniform morphology (Figure sa). After scalar recording a crystalline phase is formed (Figure 8b) whose X-ray analysis shows the presence of selenium and silver
-
iodide. Probably, to a great extent, the photoinduced phenomena are the consequenceof microstructural changes, resulting eventually in material crystallization, as we have observed also in other silver chalcogenide systems.I0 As far as the character of the polarization recording is concerned, it can be considered on the basis of the mechanisms of photoinduced anisotropy in silver halides and chalcogenide ma‘terials,known hitherto in the literature. Probably, the essence of polarization recording in thin films of silver halides is explained by the induction of the Weigert effect. The investigations so far do not explain unambiguously its nature. The photoinduced anisotropy in thin films of silver halides is supposed to be related to the formation of “chains”,consisting of closely positioned spherical silver granules with a diameter much smaller than the wavelength of the irradiating light.” On the other hand, in our case, when the concentration of the selenium atoms exceeds considerably that of the other two elements in the system, it may be suggested that they play an important role in the occurrence of the investigated phenomena. On light irradiation, a number of defects appear related mainly to the lonepair selenium electrons. Thus, C3+and CI- defects, according to the reactions I2 + 2c20.= 21- 2c3+
+ Ag + C20 = Ag+ + 2Cl’
with dipole character are created that orient in a certain direction under the action of light. These defects, which according to Dembovsky are quasi-molecular12or in accordance with Klinger’s theory form soft configuration^,'^ feature a potential with two minima, and they may be in either one of the two states. The interaction with polarized light leads to dipole moment orientation predominantly in one direction and this results in the appearance of a corresponding macroscopic optical axis. One comparison in the holographic parameters of the polarization recording of mixed chalcogenide glasses of silver Io and the investigated glasses shows that the latter possess analogous diffraction efficiency but they
J. Phys. Chem. 1992,96,9001-9007 have several times higher sensitivity especially at higher temp e r a t u r e ~which ~ ~ suggests that the halogen atoms play an important role for the effects observed. The participation of those two mechanisms brings about the photoinduced anisotropy in the investigated glasses and this will be an object of future investigations.
Conclusions As a result of the investigations on the specimens from the system, it may be concluded that the photoinduced changes in these glasses are due to macrostructural changes, leading mainly to photocrystallization, Weigert effect, and defect orientation. The influence of the composition on the photoinduced changes, as well as the possibility of inducing considerablephotoanisotropy, is demonstrated and discussed. On the basis of these changes, holographic optical elements, such as polarization beamsplitters for the red and near-IR region, possessing good diffraction efficiency and having no analogues in classical optics, can be fabricated.
9001
Registry No. Se, 7782-49-2; Ag, 7440-22-4; I*, 7553-56-2.
References and Notes (1) (2) (3) (4) 133.
Tanaka, K. J. Non-Crysr. Solids 1991, 137-138, 1. Owen, A. E.; Firth, A. P.; Ewen. A. P. Philos. Mag. 1985, E52, 347. Tanaka, K. J . Non-Cryst. Solids 1980, 35-36, 1023. Lyubin, V. M.; Tikchomirov, V. K. J . Non-Cryst. Solids 1989, 114,
(5) Lucas, J.; Zhang, H. J . Non-Cryst. Solids 1990, 125, I. ( 6 ) Mitkova, M.; Petkova, T.; Janakiev, A. Mater. Chem. Phys. 1991, 30(l), 5 5 . (7) Petkova, T.; Mitkova, M. Thin Solid Films 1991, 205, 25. (8) Kakichashvili, Sh. D. Opr. Spectrosc. 1972, 33, 324. (9) Nikolova, L.; Todorov, T. Opt. Acta 1984, 31(5), 579. (10) Mitkova, M.; Vateva, E.; Skordeva, E. J . Non-Crysr. Solids 1991, 137-138, 1013. (1 1) Ageev, L. A.; Miloslavsky, V. K.; Shklyarev, I. N. UFZ 1976,2110, 1681. (12) Dembovsky, S. A.; Chechetkina, E. A. Mater. Res. Bull. 1981, 16, (107), 1437. (13) Klinger, M. I. Phys. Rep. 1988. 165(5-6), 276. (14) Mateev, V.; Petkova, T.; Markovsky, P.; Mitkova, M. Thin Solid
Films, submitted for publication.
Template Synthesis of Infrared-Transparent Metal Microcylinders: Comparison of Optical Properties with the Predictions of Effective Medium Theory Colby A. FOSS,Jr., Michael J. Tierney: and Charles R. Martin* Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523 (Received: May 18, 1992; In Final Form: July 22, 1992)
Metal-insulator composites of varying metal volume fraction have been prepared by electrochemical deposition of gold into porous aluminum oxide membranes. The cylindrical pore array structureof the host oxide serves as a template for the formation of Au particles ca. 0.26 pm in diameter with lengths ranging from 0.3 to 3 Mm depending on the deposition time. The composites display a significant transparency in the infrared spectrum between 2000 and 4OOO cm-I. The Au volume fraction and effective medium theory screening parameter K were estimated from scanning electron microscopic analyses of cylinder dimensions and orientations in the composite membranes. Comparison of experimental spectra with those calculated using Maxwell-Gamett or Bruggeman theories indicates that neither approach is entirely satisfactory. The spectra are consistent, however, with retarded polarization effects due to nonnegligible Au particle separation distances in the composites.
I. Introduction Recently, Aspnes et al. described the microstructural limits for metal-insulator composites that are both electronically conductive and optically transparent.’ They predicted that the optimal metal particle geometry for transparency in unpolarized light consists of long narrow cylinders arranged with their principal axes parallel to the direction of light incidence (see Figure 1). We have recently shown that microporous alumina membranes can be used as templates to prepare arrays of metal microcylinders which have this optimal microgeometry.2 The cylindrical pores of anodically grown A1203membrane3 serve as templates for the electrochemical deposition of parallel arrays of metal microcylinders. The resulting Au-A1203 composites were shown to display significant transparency in the infrared region of the spectrum.2 The optical and dielectric properties of compasite materials are often discussed in the context of effective medium theory (EMT)! Effective medium theory attempts to predict the optical properties of a composite material from (1) the optical constants of the components in their pure form, (2) the volume fraction of each component, and (3) the shape and orientation of the component particles in the composite. The Au-A1203 composites investigated here are ideal model systems for evaluating theoretical models because the host oxide structure ensures a parallel alignment of the metal particles. Also, the aspect ratios of these particles can *Author to whom correspondence should be addressed. ‘Present address: Teknekron Sensor Development Corp., 1080 Marsh Rd, Menlo Park, CA 94025.
be controlled by varying the amount of metal deposited. In this paper, we compare experimental transmittance spectra of Au-A1203 composites containing different amounts of Au with those calculated using EMT. We focus primarily on the Maxwell-Gamett (MG) treatment because it is appropriate for metal particles that are isolated from each other by a layer of the insulating component (in this case, the pore walls of the host oxide). However, we also discuss the self-consistent Bruggeman (BR) approach, which treats the composite as a random mixture of both metallic and insulating particles.6 Both the MG and BR models are based on the ClausiusMossotti equation? which is formally valid only for static electric fields.’ In an optical experiment, where we are dealing with electromagnetic radiation (i.e., photon fields), there is the complication of photon ~cattering.~J””~ However, if the particle dimensions and their mutual separation distances are small relative to the wavelength of light, scattering effects can be considered negligible. This situation is often referred to as the ‘quasi-static” or infinite wavelength limit of effective mediud In the recent literature, the practical limits of quasi-static models are often discussed with regard to particle size. An often cited criterion for the applicability of quasi-static models is that the ratio of the particle diameter d to the wavelength of light employed (d/X) should be less than 0.1.4J0In this report, we demonstrate that while our composites fulfill this criterion over most of the spectral range considered, they are not completely amenable to quasi-static treatments. The infrared transmittance spectra are, however, amenable to a modified MG approach that takes into
0022-3654/92/2096-9001$03.00/00 1992 American Chemical Society
