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The surface microstructure optical properties relationship in solar absorbers: black chrome
Solar EnergyMaterials 1 (1979)69-79 ~)North-Holland PublishingCompany
T H E SURFACE M I C R O S T R U C T U R E O P T I C A L P R O P E R T I E S RELATIONSHIP IN SOLAR ABSORBERS: BLACK CHROME A. IGNATIEV, P. O ' N E I L L and G. ZAJAC Department of Physics, University of Houston, Houston, Texas 77004, USA
Received 6 September 1978
The relationshipbetweenthe surfacemicrostructureand the opticalpropertiesof solar absorbing electrodeposited black chrome films has been studied by scanning electron microscopy(SEM), X-ray photoemissionspectroscopy(XPS)and sputterdepth-profilingtechniques.The blackchrome films have been determinedto consist of a top layer of small ( =400 ,~)Cr203 particles with one or two sublayersof larger ( = 1000 ,~) closelypacked chromiumparticles. The optical propertiesof the solar absorbing films have been measured and analyzedvia the spheroid model and it has been shown that the optical response of the particulate black chrome films is significantlydetermined by their microstructure.
1. Introduction Electrodeposited black chrome films have recently been singled out for use as selective solar absorbing coatings in applications requiring temperatures up to 300°C [1-3]. The optical properties of the coatings have been well documented [1-4] with accompanying comments that the complex surface texture or surface microstructure of the films plays an extremely important role in defining the high solar absorptance of the coatings. There have been several tentative proposals on the coatings microstructure [2, 5, 6] however, there has been to date no direct determination of the black chrome microstructure nor have there been any attempts to interrelate the microstructure and optical properties of the coatings. We present here a combined scanning electron microscopy (SEM), X-ray photoemission spectroscopy (XPS) and sputter depth-profiling study of electro-deposited black chrome films which dearly indicates the microstructure of black chrome and defines the microstructure-optical properties relationship utilizing the effective medium approach of the spheroid model [7] developed for particulate media.
2. Experimental Black chrome coatings were electrodeposited on highly polished pure nickel substrates using Harshaw Chemical Co. "ChromOnyx" plating solution at a current density of 200 mA/cm 2. The coatings were approximately 0.1/~m thick as determined 69
70
A. lgnatiev et al. ,' Sur[ace microstructure optieal properties
Fig. 1. (a) SEM micrograph of an as prepared black chrome film ; (b) SEM micrograph of the black chrome film of (a) (same area) after an argon ion b o m b a r d m e n t dose of 1 × 1017 ion/cm 2 at 1.5 keV.
by large angle SEM measurements. Pure nickel was chosen for a substrate because a material with known optical constants is required for the determination of the solar absorptance of the black chrome coating via measurement of its reflectance in an integrating sphere spectrophotometer, and also because nickel has been the most widely used base for black chrome deposits on stainless steel since it provides a diffusion barrier against the steel constituents [8]. The nickel was highly polished (Buhler polishing paste # 3 - 0.03 p~ A1203) so that the effect of substrate surface imperfections on the black chrome microstructure would be minimized and so that an assumption of a smooth planar interface could be made in model calculations of the coating's optical properties.
A. Ignatiev et al./ Surface microstructure optical properties
71
3. Results
Previous proposals of black chrome microstructure relied principally on Auger electron spectroscopy (AES) and. X-ray diffraction measurements. Low resolution SEM micrographs indicated a porous, freely structured medium with X-ray diffraction indicating the presence of small metallic chromium particles [5]. AES studies indicated the presence of chromium and oxygen in the films and AES sputter-depth profiling data showed increasing chromium signal levels and decreasing oxygen signal levels in the AES spectrum with time as the black chrome was inert-gas ion bombarded [6]. This was thought to indicate an increase in the amount of metallic chromium with depth in the coating. It was therefore proposed that black chrome,consisted of small (less than the wavelength of light) metallic chromium-particles imbedded in a Cr203 dielectric with the chromium particle density increasing with depth [2, 5, 9]. The current work does not support this premise. Fig. la shows a high resolution SEM micrograph of a typical "as I~repared" electrodeposited black chrome film. Note the loosely-packed layer of 400 A particles on the surface (some with geometric shapes). Fig. lb shows an SEM micrograph of the same area as fig. la but after an argon ion bombardment dose of approximately 1 x 1017 ions/cm 2 at 1.5 keV. Note that the gross underlying structure observed in fig. la is retained with principally the top layer of small (~400 A) particles being removed by the bombardment. The underlying structure is now observed to be closely packed particles of ~ 1000 to 2000 ~ in size (with some clumping of particles). Figs. 2a through d show the dependence of the underlying structure on increasing ion bombardment dosage. It is seen that with a dose of 4 x 1017 ions/cm 2 (fig. 2d) most all of the black chrome layer has been removed by the bombardment process with the relatively smooth nickel substrate clearly visible. The important point to note here is that for this sample, the particulate structure underlying the top 400 A of the coating is a single layer of ~ 1000 ,~ closely packed particles with voids in between. The geometric microstructure of the chrome
Fig. 2(a)
72
A. 19natiev et a l / Sur[itce rnicrostructure optical properties
!?, i¸ ~
~b< ~
~
Fig. 2. SEM micrographs of the black chrome film of fig. 1 alter argon ion bombardment dose of: (a) l x 1017 ion/cm z, 1.5 keV, tb) 2 x 1017 ion/cm z. 1.5 keV. tc) 3 × 1017 ion/cm z, 1.5 keV, (d) 4 x 1017 ion/cm 2, 1.5 keV.
A. Ignatiev et al. / Surface microstructure optical properties
73
o >-
Lb
'
'
'
5', 5
'
BINOING ENERGY (eV)
Fig. 3: XPS spectra of "as prepared" black chrome (curve a) and Cr203 (curve b). The black chrome spectrum is curve resolved to indicate contributions to the chromium 2 P3/2 peak from metallic and oxidized chromium
black coating has therefore been determined by high-resolution SEM. Chemical composition of the film is now required, however, for a complete determination of the microstrueture. XPS along with inert-gas ion bombardment depth-profiling was subsequently utilized to determine the chemical constituency of the black chrome coatings which was then related to the SEM determined geometric structure. XPS measurements were carried out in a separate UHV chamber. Fig. 3 shows the XPS spectra of the oxygen level and the chromium 2 Pa/2 level for an "as prepared" film, and bulk Cr203. Note that the Cr 2 Pa/2 line of the "as prepared" film can be curve resolved into two peaks. One at the binding energy (B.E.) value of 580.5 eV-equivalent to that of the 2pa/2 level for Cr a + in Cr203, and the other at a B.E. of 578.4 eV equivalent to that of the level in metallic chromium. The mformaUon m fig. 3 indicates that the "as prepared" black chrome surface is principally composed of Cr2Oa with only 10 to 15% of the chromium present in the metallic state. Argon ion bombardment of an "as prepared" black chrome film for 20 min (total dose I x 1017 ions/era 2 at 3 keV) resulted in a surface with chromium primarily inthe metallic state, about 10% of the chromium in the Cr 3+ oxidized state and very little oxygen although in a binding state equivalent to that of oxygen in Cr203, i.e. little
74
A. 19natiev et al / Surface microstructure optical properties
../ .Q
CHROMIUM
.J BLACK CHROME-ETCH
, IJ
IJ,
,
.
.
L 'AcKc" ME,
~95 ~5 s~ 56~ s55 ~4s 53~ ~25 BINOING ENERGY (eV)
Fig. 4. XPS spectra of black chrome (curve a), black chrome after an argon ion bombardment dose of 1 x 1017 ions/cm2 at 3 keV (curve b) and metallic chromium (curve c).
compared to that in the "as prepared" surface (fig. 4). Further ionbombardment treatments resulted in little change in the XPS Cr a + and oxygen signal levels, but a marked increase in the metallic chromium signal (fig. 5). The oxygen and Cr 3+ signals indicate the presence of small amounts of Cr20 3 in the bulk of the black chrome film probably in the form of a thin layer or shell around the particles indicated by SEM which most probably are responsible for the metallic chromium signal. The latter increasing signal with sputtering time is similar to that previously reported in AES studies, however, it is extremely difficult to deconvolute contributions from the various chemical states of chromium in AES spectra. The fact that only the metallic chromium XPS signal changes with sputtering time is an indication that the originally proposed structure of black chrome (on the basis of the AES data) i.e., chromium particles dispersed in a Cr2Oa dielectric with chromium particle density increasing with depth into the film may be incorrect. Under the AES dictated model, a gradual decrease in the XPS Cr 3 + signal should be observed in addition to the increasing metallic chromium signal. This is not the case. In addition, and more direct are the results of the present SEM micrographs (figs. I and 2) which clearly indicate that the AES dictated model is not correct, i.e., beyond the top surface layer, the films are only one to two layers of ~ 1000 fk relatively uniformly distributed particles. The increasing metallic chromium signal is believed to be an artifact of the sputtering
C r 2 0 3 as
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\
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I ~A /
.... o
c,+~2~>---,,
. . . . . . . . o . . . . . . o-
0
20
40
60
TIME (min)
Fig. 5. Integrated XPS peak intensity vs. argon ion bombardment time at 3 keV for black chrome. 20 min exposure is equal to 1 x 1017 ions/cm 2.
process, related to both the preferential sputtering of the oxygen specie in the oxide and geometric effects in the sputtering of layered or shelled small (dia. ~<0.1 /~m) particles. Ion beam reduction of metal oxides has been previously observed in a number of oxide species [10] with only limited negative results on the reduction of Cr203 by low energy (400 eV) ion beams. The reduction of Cr20 3 by higher energy (1 to 3 keV) argon ion bombardment was, however, tested for in this study. Fig. 6 shows the dependence of the XPS Cr 3 + and metallic chromium 2 P3/2 levels in black chrome as a function of argon ion-bombardment time (3 keV), 5 x 10 is ions/cm 2 min). Fig. 6 indicates that a noticeable metallic contribution to the Cr 2 P3/2 level is observed after about 10 min bombardment time (5 × 1016 ions/cm 2 dose) with a subsequent steady increase of the metallic signal with bombardment time. The average slope of the increase is however, a factor of 3 to 5 less than that of the increased metallic chromium signal observed in black chrome. Therefore, an additional mechanism yielding an increasing metallic chrome XPS signal must be active in the argon ion bombardment of black chrome. The present SEM study indicates that the bulk of black chrome is composed of 1000 ~, particles which are probably covered with a layer or shell of Cr20 3. Recent studies of inert-gas ion bombardment of small multi-component shelled or layers of particles (dia. ~<0.1/~m) has shown that geometric effects in the sputtering can result in XPS depth profiles which exhibit a decrease to a nearly constant non-zero value of the outer shell component signal and a steady increase of the interior component signal with sputtering time [11, 12]. This is similar in behavior to the presently
76
A. lgnatiev et al. ,; Surface microstructure optical properties r',. . . . . . . . Z~. . . . . . . . A . . . . . . .
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-- --
~
_ . . . . .
A- --
--Z~
2P3/2
Cr
I iO
[]
2P3/2
t
I
20
30
t 40
TIME (mln)
Fig. 6. Integrated XPS peak intensities for curve resolved Cr 3 ÷ and metallic Cr components in Cr203 as a function of argon ion b o m b a r d m e n t time at 3 keV. 1 min is equal to a dose of 5 x 1015 ions/cm 2.
observed increase of metallic chromium XPS signal and constancy of C r 2 0 3 XPS signal with sputtering time and reinforces the current proposal that the bulk of black chrome is composed of chromium particles covered by a thin Cr203 shell. The conclusions on the microstructure of black chrome films are therefore: (1) The top surface of black chrome is a ~400 ~ thick layer of loosely packed particles of Cr203. (2) The remainder of the film is at most several layers of closely packed but uniformly dispersed ,~ 1000 ~ dia. chromium particles. (3) There is little C r 2 0 3 (10 to 15% of the amount in the top layer) in the bulk of the film, and it is probably found as a thin oxide on the chromium particles. The chromium particles are separated by voids in the film, making the film quite porous.
4. Optical properties The optical properties of black chrome can now be predicted from a first principles calculation incorporating the above determined film microstructure. Since it has been shown that the black chrome coating is essentially a two layer coating (a CrzO3 layer and a Cr layer) it is best to first examine the optical data in light of a single interference stack calculation. Fig. 7 shows the measured spectral hemispherical reflectance of a black chrome film compared to the reflectance calculated for the film stack composed of a solid structureless C r 2 0 3 layer (350 ~) a solid structureless Cr layer (650 ~) and a nickel substrate. The C r 2 0 3 and Cr layer thicknesses used in the calculation were those obtained from SEM measurements of the black chrome film with overall film thickness [(1000_ 100) ~] and the Cr2Oa overlayer mean particle size of 350 to 400 ~. The agreement in fig. 7 is quite poor for the structural model assuming pure C r 2 0 3 and Cr layers.
A. lgnatiev et al. Surface microstructure optical properties
77
tO0
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/,
;/ el,"
~ 60
,,'J/ \ ,-,./ / / i/. //
~ 40 20
0
0.1
0.5
1.0
5.0
I0.0
Wavelength (F.m)
Fig. 7. Measured spectral hemispherical reflectance for black chrome (curve a) compared to simple interference stack calculation of reflectance (carve b) and reduced dielectric constant interference stack calculation (curve c).
As was noted earlier, the layers are actually composed of relatively closely packed small particles of Cr203 and Cr respectively and not solid material and therefore it could be expected that the dielectric constants of the layers should be modified to reflect the packing fraction of the particles (assume spheres) in the layers. Fig. 7 also shows the effect of such a modification of the dielectric constants (dielectric constants reduced by the particle to void volume ratio) still assuming structureless films. There is some improvement of agreement, however, the agreement is still poor especially in the near infra-red wavelength region. What is clearly required here is to fully incorporate the microstructure of black chrome into the reflectance calculations. Recent interest in the optical properties of particulate media [5, 7, 14, 18] has resulted in a number of approaches toward the modelling of optical response in conjunction with particulate structure. Our previous work [7] has centered on effective medium theory and has stressed the importance not only of the shape of the particles in the medium, but also the distribution of particle shapes. These points have recently been corroborated [16] within the effective medium approach and we use here our recently proposed spheroid model [7] for particulate media to define the optical properties of black chrome within the confines of the determined microstructure. The spheroid model determines the reflectance of a particulate medium by generating depolarization factors (which can be related to the absorption coefficient) for the various shapes of particles (modeled as spheroids with varying semimajor to semiminor axis ratios) comprising the medium. Interband and Drude terms [19] are included in the model with standard thin film interference calculations [13] used to produce the theoretical reflectance of the composite film on an opaque-specular nickel substrate of known optical constants [20]. Fig. 8 shows the comparison between the spheroid model calculations and experiment for the optical response of black chrome. The structural model utilized in the calculations is depicted in fig. 9 and assumes the Cr203 overlayer to be composed of 350 ,~,spheres in a hexagonal close-packed structure and the 650/~ chromium sublayer
78
A, l,qnatiev e/al, ' SutJace microstructure optical properties
I00
_/ /
/'
80
6O
g -~40
// / //
20
, ~?" I ~ , , 1
0
OI
05
, 1.0
~ i
Wavelength(p.m)
~,~1 50
I0.0
Fig. 8. Comparison of black chrome measured reflectance (curve a) with that calculated within the spheroid model assuming the structure for the film depicted in fig. 9 and a log-normal distribution of chromium particles with r~=3 and a = 2
0.035/~
~ Crz 0 3
Nickel Substrate
Fig. 9. T h e s t r u c t u r a l m o d e l o f the b l a c k c h r o m e film utilized in the s p h e r o i d m o d e l c a l c u l a t i o n s o f the reflectance
to be composed of close-packed particles in a log-normal distribution of spheroid shapes with r~ = 3 and a = 2. Close observation of figs. 1 and 2 clearly indicates that there exists a distribution of chromium particle shapes with however only quite nominal deviations from spherical tending toward spheroid shape. The log-normal distribution of spheroid shapes was chosen to describe this system because log-normal distributions of particle shapes have been observed in inert-gas deposited chrome blacks [21]. The values of the median particle size r~ and the mean square deviation a were those obtained for inert-gas deposited chrome black. The agreement in fig. 8 is quite striking dearly indicating not only the applicability of our effective medium approach to the determination of the optical properties, b'lt that the proper optical response cannot be determined for a particulate medium unless the microscopic structure of the medium is considered.
5. Conclusions
In conclusion, the microstructure of electrodeposited black chrome films has been definitively determined and has been shown to play an important role in defining the
A. Ignatiev et al. / Surface microstructure optical properties
79
optical properties of black chrome. This result will probably be applicable to most highly solar absorbing materials and indicates avenues of study for understanding high temperature optical degradation of such materials, e.g., possible change in the microstructure of the absorber at elevated temperatures. In addition and possibly more important, this result now opens the way to possible tailoring of optical properties of a material by modification of its microstructure.
Acknowledgements The authors wish to acknowledge the extremely competent assistance of Mr. R. Keith with the SEM studies and to acknowledge support for this work by ERDA/ DOE.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [20]
G. E. McDonald, Solar Energy 17 (1975) 119. D. Mattox, J. Vac. Sci. Technol. 13 (1976) 127. R. B. Pettit and R. R. Sowell, J. Vac. Sci. Technol. 13 (1976) 596. K. D. Masterson and B. D. Seraphin, Report NSF-RANN-SE/GI-36731x/TR/75/1. (Unpublished). J. C. C. Fan and S. A, Spura, Appl. Phys. Lett. 30 (1977) 511. P. Braun, G. Betz, W. Farber and G. K. Wehner, Proc. 7th Int. Vac. Congr. and 3rd Int. Conf. Solid Surfaces (Vienna, 1977) p. 1825. P. O'Neill, A. Ignatiev and C. Doland, AlP Conf. Proc. 40 (1977) 288 ; and P. O'Neill and A. Ignatiev, Phys. Rev. B, in print. G. K. Wehner, Proc. Workshop on Selective Absorber Coatings (SERI, Golden, Colorado, 1977). P. M. Driver, R. W. Jones, C. L. Riddiford and R. J. Simpson, Solar Energy 19 (1977) 301. K. S. Kim, W. E. Baitinger, J. W. Amy and N. Winograd, J. Electron. Spectr. 5 (1974) 351. I. Matsuura and M. W. J. Wolfs, J. Cat. 37 (1975) 174. D. Briggs, to be published. O. S. Heavens, in:Physics of Thin Films, ed. G. Hass, vol. 2 (Academic Press, New York, 1964)p. 193. J. I. Gittleman, B. Abeles, P. Zanzucchi and Y. Arie, Thin Solid Films 45 (1977) 9. S. Norrman, T. Anderson, C. G. Granqvist and O. Hunderi, Solid State Commun. 23 (1977) 261. C. G. Granqvist and D. Hunderi, Appl. Phys. Lett. 32 (1978) 798. H. G. Craighead and R. A. Burhman, AlP Conf. Proc. 40 (1977) 193. W. Lamb, D. M. Wood and N. W. Ashcroft, AlP Conf. Proc. 40 (1977) 240. P. B. Johnson and R. W. Christy, Phys. Rev. B9 (1974) 5056. O. S. Heavens, Optical Properties of Thin Solid Films (Dover, NY, 1965) p. 63. P. O'Neill, C. Doland and A. Ignatiev, to be published.
T H E SURFACE M I C R O S T R U C T U R E O P T I C A L P R O P E R T I E S RELATIONSHIP IN SOLAR ABSORBERS: BLACK CHROME A. IGNATIEV, P. O ' N E I L L and G. ZAJAC Department of Physics, University of Houston, Houston, Texas 77004, USA
Received 6 September 1978
The relationshipbetweenthe surfacemicrostructureand the opticalpropertiesof solar absorbing electrodeposited black chrome films has been studied by scanning electron microscopy(SEM), X-ray photoemissionspectroscopy(XPS)and sputterdepth-profilingtechniques.The blackchrome films have been determinedto consist of a top layer of small ( =400 ,~)Cr203 particles with one or two sublayersof larger ( = 1000 ,~) closelypacked chromiumparticles. The optical propertiesof the solar absorbing films have been measured and analyzedvia the spheroid model and it has been shown that the optical response of the particulate black chrome films is significantlydetermined by their microstructure.
1. Introduction Electrodeposited black chrome films have recently been singled out for use as selective solar absorbing coatings in applications requiring temperatures up to 300°C [1-3]. The optical properties of the coatings have been well documented [1-4] with accompanying comments that the complex surface texture or surface microstructure of the films plays an extremely important role in defining the high solar absorptance of the coatings. There have been several tentative proposals on the coatings microstructure [2, 5, 6] however, there has been to date no direct determination of the black chrome microstructure nor have there been any attempts to interrelate the microstructure and optical properties of the coatings. We present here a combined scanning electron microscopy (SEM), X-ray photoemission spectroscopy (XPS) and sputter depth-profiling study of electro-deposited black chrome films which dearly indicates the microstructure of black chrome and defines the microstructure-optical properties relationship utilizing the effective medium approach of the spheroid model [7] developed for particulate media.
2. Experimental Black chrome coatings were electrodeposited on highly polished pure nickel substrates using Harshaw Chemical Co. "ChromOnyx" plating solution at a current density of 200 mA/cm 2. The coatings were approximately 0.1/~m thick as determined 69
70
A. lgnatiev et al. ,' Sur[ace microstructure optieal properties
Fig. 1. (a) SEM micrograph of an as prepared black chrome film ; (b) SEM micrograph of the black chrome film of (a) (same area) after an argon ion b o m b a r d m e n t dose of 1 × 1017 ion/cm 2 at 1.5 keV.
by large angle SEM measurements. Pure nickel was chosen for a substrate because a material with known optical constants is required for the determination of the solar absorptance of the black chrome coating via measurement of its reflectance in an integrating sphere spectrophotometer, and also because nickel has been the most widely used base for black chrome deposits on stainless steel since it provides a diffusion barrier against the steel constituents [8]. The nickel was highly polished (Buhler polishing paste # 3 - 0.03 p~ A1203) so that the effect of substrate surface imperfections on the black chrome microstructure would be minimized and so that an assumption of a smooth planar interface could be made in model calculations of the coating's optical properties.
A. Ignatiev et al./ Surface microstructure optical properties
71
3. Results
Previous proposals of black chrome microstructure relied principally on Auger electron spectroscopy (AES) and. X-ray diffraction measurements. Low resolution SEM micrographs indicated a porous, freely structured medium with X-ray diffraction indicating the presence of small metallic chromium particles [5]. AES studies indicated the presence of chromium and oxygen in the films and AES sputter-depth profiling data showed increasing chromium signal levels and decreasing oxygen signal levels in the AES spectrum with time as the black chrome was inert-gas ion bombarded [6]. This was thought to indicate an increase in the amount of metallic chromium with depth in the coating. It was therefore proposed that black chrome,consisted of small (less than the wavelength of light) metallic chromium-particles imbedded in a Cr203 dielectric with the chromium particle density increasing with depth [2, 5, 9]. The current work does not support this premise. Fig. la shows a high resolution SEM micrograph of a typical "as I~repared" electrodeposited black chrome film. Note the loosely-packed layer of 400 A particles on the surface (some with geometric shapes). Fig. lb shows an SEM micrograph of the same area as fig. la but after an argon ion bombardment dose of approximately 1 x 1017 ions/cm 2 at 1.5 keV. Note that the gross underlying structure observed in fig. la is retained with principally the top layer of small (~400 A) particles being removed by the bombardment. The underlying structure is now observed to be closely packed particles of ~ 1000 to 2000 ~ in size (with some clumping of particles). Figs. 2a through d show the dependence of the underlying structure on increasing ion bombardment dosage. It is seen that with a dose of 4 x 1017 ions/cm 2 (fig. 2d) most all of the black chrome layer has been removed by the bombardment process with the relatively smooth nickel substrate clearly visible. The important point to note here is that for this sample, the particulate structure underlying the top 400 A of the coating is a single layer of ~ 1000 ,~ closely packed particles with voids in between. The geometric microstructure of the chrome
Fig. 2(a)
72
A. 19natiev et a l / Sur[itce rnicrostructure optical properties
!?, i¸ ~
~b< ~
~
Fig. 2. SEM micrographs of the black chrome film of fig. 1 alter argon ion bombardment dose of: (a) l x 1017 ion/cm z, 1.5 keV, tb) 2 x 1017 ion/cm z. 1.5 keV. tc) 3 × 1017 ion/cm z, 1.5 keV, (d) 4 x 1017 ion/cm 2, 1.5 keV.
A. Ignatiev et al. / Surface microstructure optical properties
73
o >-
Lb
'
'
'
5', 5
'
BINOING ENERGY (eV)
Fig. 3: XPS spectra of "as prepared" black chrome (curve a) and Cr203 (curve b). The black chrome spectrum is curve resolved to indicate contributions to the chromium 2 P3/2 peak from metallic and oxidized chromium
black coating has therefore been determined by high-resolution SEM. Chemical composition of the film is now required, however, for a complete determination of the microstrueture. XPS along with inert-gas ion bombardment depth-profiling was subsequently utilized to determine the chemical constituency of the black chrome coatings which was then related to the SEM determined geometric structure. XPS measurements were carried out in a separate UHV chamber. Fig. 3 shows the XPS spectra of the oxygen level and the chromium 2 Pa/2 level for an "as prepared" film, and bulk Cr203. Note that the Cr 2 Pa/2 line of the "as prepared" film can be curve resolved into two peaks. One at the binding energy (B.E.) value of 580.5 eV-equivalent to that of the 2pa/2 level for Cr a + in Cr203, and the other at a B.E. of 578.4 eV equivalent to that of the level in metallic chromium. The mformaUon m fig. 3 indicates that the "as prepared" black chrome surface is principally composed of Cr2Oa with only 10 to 15% of the chromium present in the metallic state. Argon ion bombardment of an "as prepared" black chrome film for 20 min (total dose I x 1017 ions/era 2 at 3 keV) resulted in a surface with chromium primarily inthe metallic state, about 10% of the chromium in the Cr 3+ oxidized state and very little oxygen although in a binding state equivalent to that of oxygen in Cr203, i.e. little
74
A. 19natiev et al / Surface microstructure optical properties
../ .Q
CHROMIUM
.J BLACK CHROME-ETCH
, IJ
IJ,
,
.
.
L 'AcKc" ME,
~95 ~5 s~ 56~ s55 ~4s 53~ ~25 BINOING ENERGY (eV)
Fig. 4. XPS spectra of black chrome (curve a), black chrome after an argon ion bombardment dose of 1 x 1017 ions/cm2 at 3 keV (curve b) and metallic chromium (curve c).
compared to that in the "as prepared" surface (fig. 4). Further ionbombardment treatments resulted in little change in the XPS Cr a + and oxygen signal levels, but a marked increase in the metallic chromium signal (fig. 5). The oxygen and Cr 3+ signals indicate the presence of small amounts of Cr20 3 in the bulk of the black chrome film probably in the form of a thin layer or shell around the particles indicated by SEM which most probably are responsible for the metallic chromium signal. The latter increasing signal with sputtering time is similar to that previously reported in AES studies, however, it is extremely difficult to deconvolute contributions from the various chemical states of chromium in AES spectra. The fact that only the metallic chromium XPS signal changes with sputtering time is an indication that the originally proposed structure of black chrome (on the basis of the AES data) i.e., chromium particles dispersed in a Cr2Oa dielectric with chromium particle density increasing with depth into the film may be incorrect. Under the AES dictated model, a gradual decrease in the XPS Cr 3 + signal should be observed in addition to the increasing metallic chromium signal. This is not the case. In addition, and more direct are the results of the present SEM micrographs (figs. I and 2) which clearly indicate that the AES dictated model is not correct, i.e., beyond the top surface layer, the films are only one to two layers of ~ 1000 fk relatively uniformly distributed particles. The increasing metallic chromium signal is believed to be an artifact of the sputtering
C r 2 0 3 as
A. lonatiev et al. / Surface microstructure optical properties
t
75
\
r-
/",',,
y
0-:,:
I ~A /
.... o
c,+~2~>---,,
. . . . . . . . o . . . . . . o-
0
20
40
60
TIME (min)
Fig. 5. Integrated XPS peak intensity vs. argon ion bombardment time at 3 keV for black chrome. 20 min exposure is equal to 1 x 1017 ions/cm 2.
process, related to both the preferential sputtering of the oxygen specie in the oxide and geometric effects in the sputtering of layered or shelled small (dia. ~<0.1 /~m) particles. Ion beam reduction of metal oxides has been previously observed in a number of oxide species [10] with only limited negative results on the reduction of Cr203 by low energy (400 eV) ion beams. The reduction of Cr20 3 by higher energy (1 to 3 keV) argon ion bombardment was, however, tested for in this study. Fig. 6 shows the dependence of the XPS Cr 3 + and metallic chromium 2 P3/2 levels in black chrome as a function of argon ion-bombardment time (3 keV), 5 x 10 is ions/cm 2 min). Fig. 6 indicates that a noticeable metallic contribution to the Cr 2 P3/2 level is observed after about 10 min bombardment time (5 × 1016 ions/cm 2 dose) with a subsequent steady increase of the metallic signal with bombardment time. The average slope of the increase is however, a factor of 3 to 5 less than that of the increased metallic chromium signal observed in black chrome. Therefore, an additional mechanism yielding an increasing metallic chrome XPS signal must be active in the argon ion bombardment of black chrome. The present SEM study indicates that the bulk of black chrome is composed of 1000 ~, particles which are probably covered with a layer or shell of Cr20 3. Recent studies of inert-gas ion bombardment of small multi-component shelled or layers of particles (dia. ~<0.1/~m) has shown that geometric effects in the sputtering can result in XPS depth profiles which exhibit a decrease to a nearly constant non-zero value of the outer shell component signal and a steady increase of the interior component signal with sputtering time [11, 12]. This is similar in behavior to the presently
76
A. lgnatiev et al. ,; Surface microstructure optical properties r',. . . . . . . . Z~. . . . . . . . A . . . . . . .
Cr +3
-- --
~
_ . . . . .
A- --
--Z~
2P3/2
Cr
I iO
[]
2P3/2
t
I
20
30
t 40
TIME (mln)
Fig. 6. Integrated XPS peak intensities for curve resolved Cr 3 ÷ and metallic Cr components in Cr203 as a function of argon ion b o m b a r d m e n t time at 3 keV. 1 min is equal to a dose of 5 x 1015 ions/cm 2.
observed increase of metallic chromium XPS signal and constancy of C r 2 0 3 XPS signal with sputtering time and reinforces the current proposal that the bulk of black chrome is composed of chromium particles covered by a thin Cr203 shell. The conclusions on the microstructure of black chrome films are therefore: (1) The top surface of black chrome is a ~400 ~ thick layer of loosely packed particles of Cr203. (2) The remainder of the film is at most several layers of closely packed but uniformly dispersed ,~ 1000 ~ dia. chromium particles. (3) There is little C r 2 0 3 (10 to 15% of the amount in the top layer) in the bulk of the film, and it is probably found as a thin oxide on the chromium particles. The chromium particles are separated by voids in the film, making the film quite porous.
4. Optical properties The optical properties of black chrome can now be predicted from a first principles calculation incorporating the above determined film microstructure. Since it has been shown that the black chrome coating is essentially a two layer coating (a CrzO3 layer and a Cr layer) it is best to first examine the optical data in light of a single interference stack calculation. Fig. 7 shows the measured spectral hemispherical reflectance of a black chrome film compared to the reflectance calculated for the film stack composed of a solid structureless C r 2 0 3 layer (350 ~) a solid structureless Cr layer (650 ~) and a nickel substrate. The C r 2 0 3 and Cr layer thicknesses used in the calculation were those obtained from SEM measurements of the black chrome film with overall film thickness [(1000_ 100) ~] and the Cr2Oa overlayer mean particle size of 350 to 400 ~. The agreement in fig. 7 is quite poor for the structural model assuming pure C r 2 0 3 and Cr layers.
A. lgnatiev et al. Surface microstructure optical properties
77
tO0
c~N.~,/ ///t 80
tP//t
/,
;/ el,"
~ 60
,,'J/ \ ,-,./ / / i/. //
~ 40 20
0
0.1
0.5
1.0
5.0
I0.0
Wavelength (F.m)
Fig. 7. Measured spectral hemispherical reflectance for black chrome (curve a) compared to simple interference stack calculation of reflectance (carve b) and reduced dielectric constant interference stack calculation (curve c).
As was noted earlier, the layers are actually composed of relatively closely packed small particles of Cr203 and Cr respectively and not solid material and therefore it could be expected that the dielectric constants of the layers should be modified to reflect the packing fraction of the particles (assume spheres) in the layers. Fig. 7 also shows the effect of such a modification of the dielectric constants (dielectric constants reduced by the particle to void volume ratio) still assuming structureless films. There is some improvement of agreement, however, the agreement is still poor especially in the near infra-red wavelength region. What is clearly required here is to fully incorporate the microstructure of black chrome into the reflectance calculations. Recent interest in the optical properties of particulate media [5, 7, 14, 18] has resulted in a number of approaches toward the modelling of optical response in conjunction with particulate structure. Our previous work [7] has centered on effective medium theory and has stressed the importance not only of the shape of the particles in the medium, but also the distribution of particle shapes. These points have recently been corroborated [16] within the effective medium approach and we use here our recently proposed spheroid model [7] for particulate media to define the optical properties of black chrome within the confines of the determined microstructure. The spheroid model determines the reflectance of a particulate medium by generating depolarization factors (which can be related to the absorption coefficient) for the various shapes of particles (modeled as spheroids with varying semimajor to semiminor axis ratios) comprising the medium. Interband and Drude terms [19] are included in the model with standard thin film interference calculations [13] used to produce the theoretical reflectance of the composite film on an opaque-specular nickel substrate of known optical constants [20]. Fig. 8 shows the comparison between the spheroid model calculations and experiment for the optical response of black chrome. The structural model utilized in the calculations is depicted in fig. 9 and assumes the Cr203 overlayer to be composed of 350 ,~,spheres in a hexagonal close-packed structure and the 650/~ chromium sublayer
78
A, l,qnatiev e/al, ' SutJace microstructure optical properties
I00
_/ /
/'
80
6O
g -~40
// / //
20
, ~?" I ~ , , 1
0
OI
05
, 1.0
~ i
Wavelength(p.m)
~,~1 50
I0.0
Fig. 8. Comparison of black chrome measured reflectance (curve a) with that calculated within the spheroid model assuming the structure for the film depicted in fig. 9 and a log-normal distribution of chromium particles with r~=3 and a = 2
0.035/~
~ Crz 0 3
Nickel Substrate
Fig. 9. T h e s t r u c t u r a l m o d e l o f the b l a c k c h r o m e film utilized in the s p h e r o i d m o d e l c a l c u l a t i o n s o f the reflectance
to be composed of close-packed particles in a log-normal distribution of spheroid shapes with r~ = 3 and a = 2. Close observation of figs. 1 and 2 clearly indicates that there exists a distribution of chromium particle shapes with however only quite nominal deviations from spherical tending toward spheroid shape. The log-normal distribution of spheroid shapes was chosen to describe this system because log-normal distributions of particle shapes have been observed in inert-gas deposited chrome blacks [21]. The values of the median particle size r~ and the mean square deviation a were those obtained for inert-gas deposited chrome black. The agreement in fig. 8 is quite striking dearly indicating not only the applicability of our effective medium approach to the determination of the optical properties, b'lt that the proper optical response cannot be determined for a particulate medium unless the microscopic structure of the medium is considered.
5. Conclusions
In conclusion, the microstructure of electrodeposited black chrome films has been definitively determined and has been shown to play an important role in defining the
A. Ignatiev et al. / Surface microstructure optical properties
79
optical properties of black chrome. This result will probably be applicable to most highly solar absorbing materials and indicates avenues of study for understanding high temperature optical degradation of such materials, e.g., possible change in the microstructure of the absorber at elevated temperatures. In addition and possibly more important, this result now opens the way to possible tailoring of optical properties of a material by modification of its microstructure.
Acknowledgements The authors wish to acknowledge the extremely competent assistance of Mr. R. Keith with the SEM studies and to acknowledge support for this work by ERDA/ DOE.
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G. E. McDonald, Solar Energy 17 (1975) 119. D. Mattox, J. Vac. Sci. Technol. 13 (1976) 127. R. B. Pettit and R. R. Sowell, J. Vac. Sci. Technol. 13 (1976) 596. K. D. Masterson and B. D. Seraphin, Report NSF-RANN-SE/GI-36731x/TR/75/1. (Unpublished). J. C. C. Fan and S. A, Spura, Appl. Phys. Lett. 30 (1977) 511. P. Braun, G. Betz, W. Farber and G. K. Wehner, Proc. 7th Int. Vac. Congr. and 3rd Int. Conf. Solid Surfaces (Vienna, 1977) p. 1825. P. O'Neill, A. Ignatiev and C. Doland, AlP Conf. Proc. 40 (1977) 288 ; and P. O'Neill and A. Ignatiev, Phys. Rev. B, in print. G. K. Wehner, Proc. Workshop on Selective Absorber Coatings (SERI, Golden, Colorado, 1977). P. M. Driver, R. W. Jones, C. L. Riddiford and R. J. Simpson, Solar Energy 19 (1977) 301. K. S. Kim, W. E. Baitinger, J. W. Amy and N. Winograd, J. Electron. Spectr. 5 (1974) 351. I. Matsuura and M. W. J. Wolfs, J. Cat. 37 (1975) 174. D. Briggs, to be published. O. S. Heavens, in:Physics of Thin Films, ed. G. Hass, vol. 2 (Academic Press, New York, 1964)p. 193. J. I. Gittleman, B. Abeles, P. Zanzucchi and Y. Arie, Thin Solid Films 45 (1977) 9. S. Norrman, T. Anderson, C. G. Granqvist and O. Hunderi, Solid State Commun. 23 (1977) 261. C. G. Granqvist and D. Hunderi, Appl. Phys. Lett. 32 (1978) 798. H. G. Craighead and R. A. Burhman, AlP Conf. Proc. 40 (1977) 193. W. Lamb, D. M. Wood and N. W. Ashcroft, AlP Conf. Proc. 40 (1977) 240. P. B. Johnson and R. W. Christy, Phys. Rev. B9 (1974) 5056. O. S. Heavens, Optical Properties of Thin Solid Films (Dover, NY, 1965) p. 63. P. O'Neill, C. Doland and A. Ignatiev, to be published.