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Structure and optical properties of Ag–Al2O3 nanocermet solar selectivecoatings prepared using unbalanced magnetron sputtering

Harish C. Barshilia a,n, Prashant Kumar a, K.S. Rajam a, A. Biswas b

a Surface Engineering Division, National Aerospace Laboratories (CSIR), HAL Airport Road, Kodihalli, Bangalore 560 017, Karnataka, Indiab Spectroscopy Division, Bhabha Atomic Research Center, Mumbai 400 085, India

a r t i c l e i n f o

Article history:

Received 17 November 2010

Accepted 10 January 2011Available online 23 February 2011

Keywords:

Nanocermet

Surface plasmon resonance

Solar selective coating

Sputtering

a b s t r a c t

Ag–Al2O3 nanocermet spectrally selective solar absorber coatings were prepared at different Ag

contents on copper, silicon and glass substrates using unbalanced magnetron sputtering technique.

Asymmetric bipolar pulsed direct current power supply and radio frequency power supply were used to

sputter Ag and Al2O3 targets, respectively. The optimized coating exhibited high absorptance (a¼0.93)

in the visible region and low emittance (e¼0.04–0.05 at 82 1C) in the infrared region of the solar

spectrum. Presence of the strong absorption band in the absorber coating is due to the surface plasmon

resonance, i.e., collective oscillation of the conduction band electrons under the influence of the optical

excitation. Atomic force microscopy, field emission scanning electron microscopy (FESEM), X-ray

diffraction, micro-Raman spectroscopy, spectroscopic ellipsometry and spectrophotometer were used

to characterize the nanostructure, composition and optical properties of these coatings. The face

centered cubic crystalline structure of Ag nanoparticles inclusion in the amorphous alumina dielectric

matrix was confirmed using X-ray diffraction. The size distribution and concentration of Ag nanopar-

ticles embedded in Al2O3 dielectric matrix was studied using FESEM image analysis. The variations of

refractive index and extinction coefficient with wavelength were obtained using phase modulation

spectroscopic ellipsometry. The variation of absorption with wavelength in the UV–vis region was

characterized using spectrophotometer. In order to study the thermal stability of the absorber coatings,

they were annealed in vacuum at different temperatures (i.e., 200–400 1C) for 2 h. For the vacuum

annealed coatings (heated up to 400 1C), chemical/micro-structural changes were studied using micro-

Raman spectroscopy and FESEM. No shift in the Raman peaks for the Al2O3 was observed, confirming its

structural stability in the absorber coatings with annealing in vacuum up to 400 1C. However, FESEM

image analysis confirmed that the degradation in the vacuum annealed coatings was due to

defragmentation of the Ag nanoparticles.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

Solar collector is an environmental friendly and nonpollutingtechnique to convert the everlasting solar radiation into thermalenergy to fulfill the ever increasing demand of the future greenenergy supply [1]. In a solar collector for the efficient photo-thermalconversion applications, a solar absorber coating, designed usingsuitable solar radiation absorbing materials composition must havethe following properties: (1) a very high absorptance (in the visibleregion) and (2) a very low emittance (in the infrared region) [2,3].Previously, solar absorbers based on cermet coatings: Pt–Al2O3 [4],Ni–Al2O3 [5], Mo–Al2O3 [6], W–AlN [7], Au–MgO [8] and Cr–Cr2O3 [9]have been reported by various authors. Recently use of nanocermet(i.e., Ag or Au metal nanoparticles embedded in a dielectric matrix)have attracted considerable attention for their potential use in

different applications like solar energy conversion, optical wave-guides, photo-chromic, photo-electrochemical applications and gassensor [10–13].

Nanocermets are the potential candidates for solar energy con-version because they tend to exhibit a strong absorption of solarradiation (usually, in the visible region of solar spectrum), whichoccurs due to the quantum confinement effect: called surfaceplasmon resonance phenomenon (SPR), i.e., collective oscillations ofthe conduction electrons of noble metal nanoparticles (e.g., Ag andAu) embedded in a dielectric matrix [14,15]. Different matrices ofSiO2, ZnO, Bi2O3, DLC, Si3N4, BN and WO3�x as dielectric materials forthe Ag metal inclusion have been reported by various research-ers [15–22]. Optical properties of these nanocermets strongly dependon the particle size, shape, concentration of particles in the matrix,particle distribution and the local dielectric environment of the hostmatrix [14–28]. To our knowledge, nanocermet coating based onAl2O3 has not been studied.

In this paper, we describe an Ag–Al2O3 nanocermet solar selectivecoating, which is deposited on copper, silicon and glass substrates

Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/solmat

Solar Energy Materials & Solar Cells

0927-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved.

doi:10.1016/j.solmat.2011.01.034

n Corresponding author. Tel.: +91 80 2508 6494; fax: +91 80 2521 0113.

E-mail address: harish@nal.res.in (H.C. Barshilia).

Solar Energy Materials & Solar Cells 95 (2011) 1707–1715

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using unbalanced magnetron co-sputtering technique. Asymmetricbipolar-pulsed direct current (DC) generator and radio frequency (RF)generator were used to deposit the solar selective coatings. To gethigh absorptance and low emittance, the concentration of Ag inclu-sion in the Al2O3 matrix was adjusted by varying the Ag target powerwhile the Al2O3 target power was kept constant. To further enhancethe absorptance value (40.90) of the absorber coating, an Aginterlayer and an Al2O3 antireflection coating were deposited.

The first layer (i.e., Ag interlayer) has high reflectance in theinfrared region and low reflectance in the visible region. Thesecond layer (i.e., Ag–Al2O3 nanocermet) is the main absorbinglayer. The third layer (i.e., Al2O3), a dielectric in nature, acts as anantireflection coating. For the best optimized process parameters(i.e., target power and deposition time), high absorptance (0.93)in the visible region and low emittance (0.04–0.05) in the infraredregion have been achieved for this absorber coating. Thermalstability tests have also been carried out for the developedcoatings. Solar spectrum reflectometer and emissometer, X-raydiffraction (XRD), atomic force microscope (AFM), field emissionscanning electron microscope (FESEM), phase-modulated spectro-scopic ellipsometry (SE), spectrophotometer and micro-Ramanspectroscopy techniques were used to characterize the coatings.

2. Experimental details

The Ag–Al2O3 nanocermet solar selective absorbers were depos-ited on copper (dimension: 0.035�0.035�0.002 m3), silicon(dimension: 0.010�0.010�0.0005 m3) and glass (dimension:0.0154�0.008�0.001 m3) substrates using an unbalanced magne-tron sputtering system at 25 1C. Fig. 1 shows the block diagram of

the sputtering system. Highly pure, Ag target (purity¼99.999%) andAl2O3 target (purity¼99.999%) were used for the deposition of thesecoatings. The diameter of both the targets was 0.075 m. All thesubstrates, before putting them into the vacuum chamber, weremetallographically polished and then ultrasonically cleaned in anultrasonic agitator using isopropyl alcohol and acetone, each for15 min, respectively. Prior to deposition of the absorber coatings, thevacuum chamber was pumped down to a base pressure of 5.5�10–

4 Pa. An asymmetric bipolar pulsed DC generator (f¼100 kHz, pulsewidth¼2976 ns, positive pulse bias¼ +37 V) was used to sputter Agtarget while a RF power supply (frequency¼13.56 MHz) was usedto sputter the Al2O3 target. The Ag interlayer and Ag–Al2O3

nanocermet layer were deposited from the non-reactive sputteringof Ag and Al2O3 targets in Ar-plasma at the deposition pressure of3.2 Pa. Al2O3 antireflection coating was deposited at the depositionpressure of 5.2 Pa from the reactive sputtering of Al2O3 target in theAr+O2 plasma. For the deposition of the bottom Ag interlayer, thepower density of Ag target was 0.11 W/cm2 and the argon flow ratewas 30 SCCM (SCCM denotes cubic centimeter per minute at STP).For the co-sputtered deposition of the middle Ag–Al2O3 nanocermetlayer, the power densities of Ag target and Al2O3 target were 0.039–0.083 and 2.15 W/cm2, respectively and the argon flow rate was30 SCCM. Sputtering of Al2O3 target was carried out at a powerdensity of 2.14 W/cm2 for the deposition of Al2O3 antireflectioncoating as a top layer, while argon and oxygen flow rates were keptfixed at 30 and 10 SCCM, respectively. To prepare highly uniformcoatings, during the deposition of all the three layers, the sampleholder was rotated at 25 rpm using a DC stepper motor.

Solar spectrum reflectometer (model SSR) and emissometer(model AE) of M/s. Devices and Services, USA, were used tomeasure the optical properties (i.e., total hemispherical

Cu SAMPLE

END HALL ION SOURSE

Heater

CATHODECATHODE

Al2O3 -TARGET

DC-MOTOR

SS Sample Holder

BIPOLARPULSEDDC POWER SUPPLY

RF POWER SUPPLY

VACUUMGAUGES

ROTARY PUMP

TURBOPUMP

VALVE

VACUUM CHAMBER

INSULATOR

VIEWPORT

MFC

MFC

Ar

O2

WATERWATER

Ag - TARGET

Fig. 1. Block diagram of the unbalanced magnetron co-sputtering system showing the arrangement of the sample holder between the targets.

H.C. Barshilia et al. / Solar Energy Materials & Solar Cells 95 (2011) 1707–17151708

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absorptance ‘a’ and total hemispherical emittance ‘e’) of theabsorber coatings [29]. For the solar spectrum reflectometer, atungsten–halogen lamp was used as a source of illumination.Measurements of the radiation reflected by the samples weredone at an angle of 201 from the normal, with four filtereddetectors (UV, blue, red and infrared). A solar spectrum measure-ment was achieved by summing the four outputs in the appro-priated proportions. Air mass 2 was used for the calibration ofsolar reflectometer. The emissometer which is used to measurethe emittance of absorber coatings for flat plate solar thermalcollector was heated to 82 1C, so that the sample to be measuredneed not be heated. At 82 1C, the spectral range of the thermalradiation emitted from the surface is in the range 3–30 mm [2].Calibration of both the instruments was done using standardsamples. The accuracies of the measured a-values are 72% with adrift of 71% +0.003/h and the emissometer has a repeatability of70.01 units. Measurements of the absorptance and the emittancevalues were taken at four different places for the absorber coat-ings coated on copper substrates, and the values reported hereinare the average of four measurements. The crystalline structure ofthe absorber coating was studied using X-ray diffraction (Mod-el—XPERT PRO) in the thin film mode at a glancing angle of 21with X-ray Ka radiation (l¼0.15406) nm. The root mean squareroughness values of the as-deposited samples were measuredusing atomic force microscopy (surface imaging systems) incontact mode. For AFM, the maximum scan ranges in the X, Y

and Z axes were 40, 40 and 4 mm, respectively. Along the X, Y andZ axes, the AFM resolution was o1 nm. The 2-dimentional sur-face images of the Ag–Al2O3 solar selective absorbers coatingwere taken using Carl Zeiss SUPRA 40VP, field emission scanningelectron microscope. Values of the optical constants (i.e., refrac-tive index, n and extinction coefficient, k) for the Ag–Al2O3

nanocermet solar selective coatings were measured using spec-troscopic phase modulated ellipsometry (Model—UVISELTM 460,ISA JOBIN-YVON-SPEX) in the wavelength range 350–1200 nm.Spectrophotometers, i.e., Reflectometer (Spellar Inc. USA ModelNo. C-23) and Photospectrometer (Model No. 119), were used tomeasure the absorption, i.e., A(l) in the wavelength range from300 to 750 nm.

For the thermal stability test characterization, the Ag–Al2O3

nanocermet solar selective absorber coatings deposited on copperand silicon substrates were annealed in vacuum (8�10–4 Pa) inthe temperature (TA) range 250–400 1C for 2 h. The temperatureof the samples was increased from room temperature to thedesired temperature at a slow heating rate of 3 1C/min andmaintained the desired temperature for 2 h. Subsequently, thesamples were cooled down at a rate of 3 1C/min. In the presentstudy, a DILOR-JOBIN-YVON-SPEX integrated micro-Raman spec-trometer was used to check the changes in the chemical composi-tion of the surface of the vacuum annealed solar selective coatingsdeposited on silicon substrates [30]. Furthermore, for the absor-ber coatings (deposited on silicon substrates) heat treated in thevacuum (8�10–4 Pa) for 2 h, FESEM images of the coatings weretaken to check changes in Ag nanoparticles morphologyembedded in the alumina dielectric matrix.

3. Results and discussion

The nanocermet coating consisted of three layers. Total thick-ness of the absorber coating having all the three layers wasapproximately 120 nm. The coating consists of an Ag–Al2O3

nanocermet main absorber layer (hereafter called layer no. 2)sandwiched between the other two layers: Ag interlayer (here-after layer no.1) and Al2O3 antireflection (hereafter layer no.3).For the optimized process parameters, as given in Table 1, the

absorber coating deposited on copper substrate exhibited a highabsorptance (0.93) and a low emittance (0.04–0.05). The nano-cermet coatings were prepared at different Ag contents byvarying the power to Ag target while keeping the Al2O3 targetpower constant. Fig. 2 shows the variations of the absorptanceand emittance values with respect to varying Ag target power. Itis confirmed from Fig. 2 that as we increase the Ag target power(i.e., 7–10 W), absorptance value increases from 0.84 to a max-imum value of 0.936. Further increase in the Ag target power upto 15 W resulted in a decrease in the absorptance. On the otherhand, increase in the Ag target power (i.e., 7–11 W), the emittanceincreased from 0.04 to 0.05, while for high power of the Ag targetup to 15 W a drastic increase in emittance value up to 0.10 wasobserved. It is due to the fact that, if the metal content inside thedielectric matrix is increased to high value, optical properties ofthe coating tend close to bulk silver and, consequently, a highreflectance instead of a high absorptance in the visible range isobserved. Furthermore, absorption edge shifts to longer wave-length with an increase in the metal content in the dielectricmatrix and at the same time, emittance also increases with anincrease in the metal content to a higher value [15].

3.1. Structural and micro-structural characterization

Fig. 3 shows a typical XRD pattern of the Ag–Al2O3 nanocermetcoating deposited on Si substrate at an Ag power of 10 W. TheXRD data shows peaks at 2y¼38.181, 44.221 and 64.591, whichcorrespond to (1 1 1), (2 0 0) and (2 2 0) reflections of fcc Ag,

Table 1Optimized process parameters for the Ag–Al2O3 nanocermet solar selective

coating.

Layers Ag target

power (W)

Al2O3 target

power (W)

Ar flow rate

(SCCM)

O2 flow rate

(SCCM)

Po

(Pa)

Ag 20 – 30 – 3.2

Ag–Al2O3 10 435 30 – 3.2

Al2O3 – 390 30 10 5.2

6 8 10 12 14 16

0.84

0.87

0.90

0.93

0.04

0.06

0.08

0.10

Ag target power (W)

� �

Fig. 2. Variations of the absorptance (a) and the emittance (e) values with respect

to Ag target power.

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respectively. No peaks for Al2O3 were observed, indicating itsamorphous nature. Further, estimation of the crystallite size ofthe Ag nanoparticles from the XRD data was done using theDebye–Scherrer equation. The average crystallite size of thecoating was of the order of 8 nm.

Fig. 4 shows the three-dimensional AFM topographic images ofAg–Al2O3 nanocermet coatings, which were deposited on Sisubstrates for the varying Ag target powers. For the absorbercoatings, an increase in the root mean square surface roughnessvalue (Rrms) was observed with increasing Ag target power. TheRrms values were 7, 18 and 23 nm for the absorber coatingsprepared at Ag target power of 7, 10 and 13 W, respectively.The increase in the surface roughness of the absorber coatingswith increasing Ag target power was accompanied by an increasein the emittance as shown in Fig. 2. This is because emittancewhich is a surface property depends upon the surface condition ofthe material, i.e., surface roughness of the coating [1].

Two-dimensional FESEM micrographs of the Ag–Al2O3 nano-cermet coatings deposited on Si substrates for the varying Agtarget power are shown in Fig. 5(a)–(c). The FESEM images wereanalyzed by Scanning Probe Image Processor software (ImageMetrology A/S) to calculate the surface coverage of Ag nanoparti-cles and their size distribution. The plane view statistical analysisindicated that the Ag nanoparticles surface coverage was approxi-mately 18%, 37% and 45% for the coatings prepared at 7, 10 and13 W, respectively. In all the three coatings, nearly isolated Agnanoparticles having a broad size and shape distribution werepresent in the Al2O3 dielectric matrix. Fig. 5(d)–(f) shows the sizedistribution of the Ag nanoparticles corresponding to Fig. 5(a)–(c),respectively. In Fig. 5(a)–(c), the average size of the Ag nanopar-ticles was 15, 49 and 100 nm, respectively. It is confirmed from theFESEM images and their size histograms that on increasing

Ag target power, formation of bigger particles having shorterinter-particles spacing and improved uniformity in size distribu-tion of Ag nanoparticles inside the alumina matrix took place.Absorber coating shown in Fig. 5(b) exhibited the highest absorp-tance of 0.93 and the lowest emittance of 0.04–0.05.

3.2. Optical characterization

3.2.1. Ellipsometry studies

Phase-modulated spectroscopic ellipsometry has been used forthe optical characterization (i.e., n and k measurements) of threelayer samples. Fig. 6(a)–(c) shows the experimental c (amplituderatio of parallel and perpendicular components of the reflectedwaves) and D (relative phase change) spectra over a wavelengthrange 350–1200 nm for three different Ag–Al2O3 nanocermetcoatings on Cu substrates deposited at Ag target power of 7, 10and 13 W, respectively. The above experimental spectra werefitted with a three layer theoretical model. The bottom Ag-layer ismodeled using the well known Drude model used for metal, thetop Al2O3 layer is modeled using Tauc–Lorentz model [31] forgenerating refractive indices and extinction coefficients spectra.The interlayer, which is mixture of Ag and Al2O3 and is the mainabsorber layer, is modeled using effective medium approximation(EMA) theory [14,32]. The fitting details are described elsewherein our previous work [33,34].

The theoretically generated spectra were fitted with experi-mentally measured ellipsometric spectra by varying thicknessand parameters of dispersion relation of every layer for extractingthe thickness, refractive indices and extinction coefficients ofdifferent layers. The measured ellipsometric spectra were fittedby minimizing the squared difference (w2) between the measuredand calculated values of the ellipsometric parameters. Maximumnumber of iteration allowed was 100 and criterion for conver-gence used was dw2

¼0.000001. In Fig. 6, the best fitted theoreticalspectra are also shown with the experimental spectra.

In Fig. 7(a) the variations of refractive indices and extinctioncoefficients of the Ag–Al2O3 nanocermets with Ag target powerare shown. It is seen that the value of extinction coefficient for the10 W sample is more than the 7 and 13 W samples. This resultsupports the absorptance data shown in Fig. 3. So 10 W is theoptimum Ag target power for the deposition of Ag–Al2O3 nano-cermet coating prepared under present conditions. In Fig. 7(b) therefractive indices and extinction coefficients of all three layers areshown. The n and k values of Ag metal layer increased withincreasing wavelength, which confirms the metallic behavior ofsilver layer. Also the values of it are in good agreement with thevalues reported by other workers [35]. The extinction coefficientof second layer, i.e., Ag–Al2O3 first increases then decreases withincreasing wavelength and value of it is less than the Ag-layer.This signifies it as a semi-metallic layer. The extinction coefficientof top Al2O3 layer is zero in this wavelength range and the

30 40 50 60 700

500

1000

1500

(220)

(200)

(111)

Inte

nsity

(ar

b. u

nits

)

2θ (deg.)

Fig. 3. Typical XRD pattern of an Ag–Al2O3 nanocermet absorber coating depos-

ited on Si substrate.

nm

0

5000

10000 05000

10000

100

250

nm

nm

0

5000

10000 05000

10000

nm

250

100

nm

0

5000

100005000

10000

100

250

nm

0

Fig. 4. (a)–(c) Three-dimensional AFM topographic images of the Ag–Al2O3 nanocermet absorber coatings deposited on Si substrate for the varying Ag target powers (i.e., 7,

10 and 13 W, respectively).

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refractive index is also less than the Ag–Al2O3 layer. So this layeracts as an anti-reflecting layer.

3.2.2. Spectrophotometer studies

Spectrophotometry was used to measure the absorption spec-tra of the Ag–Al2O3 nanocermet coating (deposited on glasssubstrate for the best optimized process parameters as givenin Table 1) in the near-ultraviolet and visible range 300–750 nm.Values of reflectance and transmittance of the Ag–Al2O3 nano-cermet coatings were measured using two different models of

spectrophotometer, i.e., Reflectometer and Photospectrometer,respectively. Both these instruments were facilitated with adouble beam detection technique. Uncoated glass sample of samequality was used as a reference sample for the measurement. Theabsorption spectra for the absorber coating were calculated fromthe measured reflectance R(l) and transmittance T(l) spectra as

A¼ 1�ðRþTÞ:

Fig. 8 shows the absorption spectrum A(l) for the absorbercoating prepared at the optimized process conditions. The broad

200 nm

25 75 125 225175

Cou

nt

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400

1200

2000

2800

40 120 200 280 360C

ount

Diameter (nm)

100

300

500

40 120 200 280

Cou

nt

Diameter (nm)

20

60

140

100 Mean diameter = 100 nm

Mean diameter = 49 nm

Mean diameter = 15 nm

Fig. 5. (a)–(c) Two-dimensional FESEM micrographs of the Ag–Al2O3 nanocermet coatings deposited on Si substrates for the varying Ag target powers (i.e., 7, 10 and 13 W,

respectively) and (d)–(f) size distribution of the Ag nanoparticles corresponding to (a)–(c), respectively.

400 600 800 1000 120020

30

40

50

0

100

200

300

Wavelength (nm)

400 600 800 1000 1200

20

30

40

50

60

0

100

200

300

Wavelength (nm)

400 600 800 1000 1200

36

39

42

45

0

100

200

300

Wavelength (nm)

Ψ (d

eg.)

Δ (d

eg.)

Fig. 6. Variations of experimentally determined c and D values with wavelength along with best-fit theoretical curves for the Ag–Al2O3 nanocermet absorber coatings deposited on

Cu substrate for the varying Ag target powers (i.e., 7, 10 and 13 W, respectively). Here, (—) and (o) lines represent the theoretical and experimental curves, respectively.

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absorption peak in the visible region of solar radiation, which isdue to surface plasmon resonance, occurred at the wavelength,lRE610 nm. Further, the absorption spectrum exhibited anabsorption edge at 326 nm, which is due to the interbandtransition [14]. The absorption peak was not sharp but broad innature, which is expected due to the inhomogeneous Ag nano-particles shape and size distribution of Ag nanoparticles, asshown in Fig. 5 and explained by Mandal et al. [16]. It is alsoreported that the radiative damping of the collective electronexcitation, generally for the nanoparticles larger than 50 nm,significantly broadens the line-width of the SPR (i.e., extrinsicsize effect) [14]. Further, for the nanoparticles smaller than20 nm, intrinsic size effects become predominant and due to this,the additional surface damping modifies the dielectric constant,thus showing a strong broadening of the surface plasmon line-width with decreasing particle size [14]. The optical absorptionspectrum was dominated by a single absorption peak only whichis due to the dipolar excitation (i.e., dipole particle plasmonresonance) between the Ag nanoparticles. While no multipleabsorption peaks are observed in the present work, which occurdue to higher order multi-poles excitation (i.e., quadrupoleplasmon resonance) of Ag nanoparticles inside the dielectric

400 600 800 1000 1200

1.2

1.6

2.0

2.4

2.8

n

Wavelength (nm)

n # (7 W)

n #(10 W)

n # (13 W)

0.6

0.8

1.0

k # (7 W)

k # (10 W)

k # (13 W)

400 600 800 1000 1200

0.0

0.8

1.6

2.4

3.2

Wavelength (nm)

n of Agn of Ag-Al2O3n Al2O3

0

3

6

9

k

k of Agk of Ag-Al2O3k of Al2O3

Fig. 7. (a) Variations of experimentally determined n and k values for the Ag–Al2O3 nanocermet absorber coating deposited on Cu substrate for the varying Ag target

powers (i.e., 7, 10 and 13 W, respectively) and (b) variations of n and k of all the three layers with wavelength.

300 450 600 7500.6

0.7

0.8

0.9

1.0

326 nm

A =

1-(

R+

T)

Wavelength (nm)

λR = 610 nm

Fig. 8. A typical absorption spectrum A(l) for the Ag–Al2O3 nanocermet absorber

coating prepared at the optimized process parameters.

Table 2Effect of 2 h annealing (in vacuum) on the absorptance and emittance values of the

Ag–Al2O3 nanocermet solar selective coatings deposited on Cu substrates.

Annealing

temperature

(1C)

Absorptance (a) Emittance (e)

As-

deposited

Annealed Da As-

deposited

Annealed De

200 0.930 0.924 �0.006 0.05 0.05 0.00

250 0.932 0.914 �0.018 0.05 0.04 �0.00

350 0.936 0.888 �0.048 0.05 0.05 0.00

400 0.931 0.887 �0.044 0.04 0.06 +0.02

300 450 600 750 900

329

Raman Shift (cm-1)

641

826

545

473

395

350 °C

250 °C

400 °C

As-deposited

Inte

nsity

(ar

b. u

nit)

730

Fig. 9. Composite Raman spectra of the as-deposited absorber coating and the

absorber coatings heat treated at 250, 350 and 400 1C for 2 h in vacuum.

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matrix. The higher order multi-polar excitations are inherent tothe geometry of the particles as explained well in Refs. [20,25,26].

3.3. Thermal stability tests

At an operating temperature, thermal stability of solar absor-bers is an important point of concern because at higher tempera-ture the absorbers get degrade, which results in a decrease in thesolar selectivity [34]. So for this, Ag–Al2O3 nanocermet coatingsdeposited on copper and silicon substrates were heat treated atdifferent temperatures (i.e., TA¼200, 250, 350 and 400 1C) underhigh vacuum conditions (8�10–4 Pa) for 2 h. After heat treatmentup to 400 1C, the measured values of the absorptance and theemittance of the absorber coatings (deposited on Cu substrate)are summarized in Table 2. At 400 1C, a drastic decrease in thesolar selectivity was observed, which was due to the decreasein the absorptance value while the emittance value increased.For the absorber coatings vacuum annealed for 250 and 350 1Cfor 2 h, only the absorptance decreased while the emittancewas almost constant. At 200 1C, very less degradation in the solarselectivity was observed, confirming that the absorber coatingwas stable at 200 1C. At TA¼400 1C, the emittance increaseddrastically, which could be attributed due to two reasons:

(1) shifting of the thermal re-radiating spectrum to lower wave-lengths and gradually overlapping with the spectrum of solarradiation [36] and (2) diffusion of the Cu into the coating [34].Even, at TA¼400 1C, change in color of the coating from deepbrown to dull black was observed, which could be due to diffusionof Cu into the coating as has been reported previously [34].

The structural stability of the Ag–Al2O3 nanocermet absorbers athigher temperatures was studied using micro-Raman spectroscopy.The absorber coatings deposited on Cu substrates were used for theRaman characterization. Fig. 9 shows the composite Raman spectraof the as-deposited absorber coating and the absorber coatings heattreated at 250, 350 and 400 1C for 2 h in vacuum. As-depositedcoating exhibited broad Raman peaks centered at 329, 395, 473,545, 641, 730 and 826 cm�1, which correspond to various phases ofAl2O3 [37,38]. After heat treatment up to 400 1C, the absorbercoatings exhibited the same Raman peaks as mentioned above forthe as-deposited absorber coating, indicating no Raman shift in theheat treated coatings. Hence the chemical composition of Al2O3 inthe vacuum annealed samples was very stable. As Al2O3 is believedto be stable at 400 1C, the degradation in the optical properties ofthe Ag–Al2O3 nanocermet is expected due to defragmentation ofthe Ag nanoparticles as discussed below.

Fig. 10(a)–(d) shows the FESEM images of the as-depositedabsorber coating and the absorber coatings annealed in vacuum

200 nm40 120 200 280 360

Cou

nt

Diameter (nm)

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Cou

nt

25 75 125 175 225

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Diameter (nm)

2500

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1500

25 75 125 175 225

Diameter (nm)

Cou

nt

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2000

400

1200

2800

Diameter (nm)

25 75 125 175 225 225

Cou

nt

3600

2000

400

1200

2800

Mean diameter = 49 nm

Mean diameter = 18 nm

Mean diameter = 18 nm

Mean diameter = 18 nm

Fig. 10. (a)–(d) FESEM images of the as-deposited absorber coating and the absorber coatings annealed for 2 h in vacuum at 250, 350 and 400 1C, respectively, and (e)–(h)

size distribution of the Ag nanoparticles corresponding to (a)–(d), respectively.

H.C. Barshilia et al. / Solar Energy Materials & Solar Cells 95 (2011) 1707–1715 1713

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at 250, 350 and 450 1C, respectively. The corresponding sizedistributions are given in Fig. 10(e)–(g), respectively. From theplane view statistical analysis of the FESEM images, it is con-firmed that in all the three annealed samples, Ag nanoparticlesgot defragmented into smaller Ag nanoparticles inside the aluminamatrix and a significant reduction in the Ag surface coverage wasconfirmed compared to the Ag surface coverage in the as-depositedsample. For the defragmented Ag nanoparticles in the annealedcoatings in Fig. 10(b)–(d), the average nanoparticle size wasapproximately 18 nm as compared to the average size of 49 nmin the as-deposited coating in Fig. 10(a). Further, as compared to37% of Ag surface coverage in the as-deposited coating in Fig. 10(a),the reduction in the Ag surface coverage was 35%, 31% and 30% forthe annealed coatings in Fig. 10(b)–(d). Hence for the absorbercoatings vacuum annealed at 250, 350 and 400 1C, the degradationin the optical properties was due to defragmentation of Agnanoparticles inside the alumina matrix.

4. Conclusions

Ag–Al2O3 nanocermet solar selective coatings (a¼0.93 ande¼0.04–0.05) have been deposited on Cu, Si and glass substratesusing an unbalanced magnetron sputtering system. By adjustingthe Ag content in the Ag–Al2O3 nanocermet, it is possible toachieve a high solar selectivity of 18.6–23.25 in the absorbercoatings. AFM study indicated an increase in the surface rough-ness with an increase in the Ag content in Ag–Al2O3 nanocermetand hence the increase in the emittance value. FESEM and X-raydiffraction analysis confirmed that the Ag nanoparticles, having abroad size and shape distribution in the Al2O3 dielectric matrix,were present in the fcc crystalline phase. Spectrophotometermeasurements confirmed the maximum absorption peak positionat 610 nm in the visible region, which was due to surface plasmonresonance of the Ag nanoparticles embedded in the Al2O3 dielec-tric matrix. The effect of vacuum annealing on thermal stability ofthe absorber coatings showed that these coatings were stable upto 200 1C for 2 h. Micro-Raman spectroscopy and field emissionscanning electron microscopy studies of the vacuum annealedcoatings confirmed that the degradation in these coating at highertemperature (250–400 1C) was due to defragmentation of the Agnanoparticles in the Al2O3 dielectric matrix.

Acknowledgements

The authors would like to thank Director, NAL, for givingpermission to publish these results. Mr. N. Selvakumar is thankedfor absorptance and emittance measurements. Mr. N.T. Manikanda-nath, Mr. Praveen and Mr. Siju are thanked for help in themeasurements of Raman, AFM and FESEM data. Work supportedin part by CSIR Networked Project No. FAC-00-01-11.

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