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Optical enhancement of Au doped ZrO2 thin films by sol–gel dip coating method
Physica B 457 (2015) 182–187
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Optical enhancement of Au doped ZrO2 thin films by sol–gel dip coating method I. John Berlin n, K. Joy Thin film Lab, Post Graduate and Research Department of Physics, Mar Ivanios College, Thiruvananthapuram 695015, India
art ic l e i nf o
a b s t r a c t
Article history: Received 2 August 2014 Received in revised form 13 October 2014 Accepted 15 October 2014 Available online 22 October 2014
Homogeneous and transparent Au doped ZrO2 thin films were prepared by sol–gel dip coating method. The films have mixed phase of tetragonal, monoclinic and face centered cubic with crack free surface. Due to the increase in Au doping concentration many-body interaction occurs between free carriers and ionized impurities causing decrease in optical band gap from 5.72 to 5.40 eV. Localized surface plasmon resonance peak of the Au doped films appeared at 610 nm. Conversion of photons to surface plasmons allows the sub-wavelength manipulation of electromagnetic radiation. Hence the prepared Au doped ZrO2 thin films can be applied in nanoscale photonic devices such as lenses, switches, waveguides etc. Moreover the photoluminescence (PL) intensity of Au doped ZrO2 thin films decrease due to decrease in the radiative recombination, life time of the excitons and suppression of grain growth of ZrO2 with increasing Au dopant. & Elsevier B.V. All rights reserved.
Keywords: Zirconia Au doped ZrO2 Thin film Sol–gel Photoluminescence Localized surface plasmon resonance
1. Introduction In recent years, metal oxides and their nanostructures have emerged as an important class of materials with a rich spectrum of properties and great potential for device applications, e.g., transparent electrodes, high-electron mobility transistors, gas sensors, photovoltaic and photonic devices and hydrogen storage applications. Of this class, wide-band gap oxide semiconductor of zirconia (ZrO2) has occupied the forefront in the past decade [1–3] because of large binding energy of ZrO2 has an ideal choice for inorganic passivation shells in a variety of semiconductor core/ shell nanoparticles. It is now well known that the metal doped ZrO2 has the high potential for fabricating novel nanoelectronics and optical devices with enhanced performance. It is due to the intentional defects generated by doping ions, which may have a profound effect on the electronic and optical properties of the materials. [4– 8] Further when the noble metals are doped with wide band gap semiconductor (ZrO2) it can create new opportunities for plasmonic manipulation of light because of the plasmon resonance frequency of noble metals [9,10]. The presence of noble metals such as Au and Ag nanoparticles (NPs) in the ZrO2 matrix, cause strong light absorption band in the visible region known as Localized surface plasmon resonance (LSPR). This LSPR is formed due to coherent oscillation of the conduction band electrons n
Corresponding author. Fax: þ91 471 2530023. E-mail address: [email protected] (I. John Berlin).
http://dx.doi.org/10.1016/j.physb.2014.10.013 0921-4526/& Elsevier B.V. All rights reserved.
(surface plasmon oscillation) induced by interaction with an electromagnetic field [11]. The near-field coupling of localized surface plasmon resonance is increasingly being exploited for the construction of nanoscale optical and photonic devices (lenses, switches, linear optical devices and waveguides) because Au particles convert photons to electromagnetic modes of radiation (localized surface plasmons) [12]. The LSPR peaks for Au NPs have been reported to be located at 556 nm [13,14] and the optical responses of metallic clusters reveal information about their electronic structure. The resonance frequency depends on size, morphology, shape and distribution, as well as on the particular dielectric characteristic of the surrounding medium in which the Au NPs are dispersed [15,16]. Recently, the sol–gel process has emerged as a promising technique for the synthesis of noble metal–metal oxide compounds with enhanced nonlinear optical properties [13,17]. Few reports have been published about the studies on Au doped ZrO2 by sol–gel method [13,17,18] but no information is available about the effect of Au on the Photoluminescence (PL) of ZrO2 films. Moreover, the structural, optical and manipulation of electromagnetic radiation of Au doped zirconia thin films by sol– gel dip coating method have not been systematically studied yet. In this paper, we have investigated the effect of Au on the photoluminescence and optical properties of ZrO2 thin films prepared by sol–gel dip coating technique. In addition, the crystalline structure, surface morphology and film composition of the films were also determined. The basis of the technique is to coat a substrate with a precursor solution containing the requisite metal
I. John Berlin, K. Joy / Physica B 457 (2015) 182–187
183
components in the required proportion, which transforms to a gel layer because of solvent evaporation and/or chemical reactions [1–3].
2. Experimental procedure In this study, inorganic precursor route was chosen for the fabrication of nanocrystalline transparent zirconia thin films. Zirconium oxychloride octahydrate (ZrOCl2 8H2O) (Sigma-Aldrich 99.5%) was used for the preparation of the precursor solution [19]. The mixture of 2-butanol and ethanol ( in the ratio 1:1) was used as the solvent. A homogeneous solution of zirconium oxychloride octahydrate (2 wt%) was prepared by mixing 1 mol of zirconium oxychloride octahydrate in 1/3 of the total volume of mixed 2-butanol and ethanol. The solution was stirred for 45 min using magnetic stirrer. The water for hydrolysis and nitric acid for oxidation of ratio, Water: HNO3: Acetlyeacetone¼ 20: 0.4: 3 were then added to the salt–alcohol solution. Gold as a dopant was added in the form of tetrachloroaurate (III) trihydrate with five different Au/Zr ratio: 0 (undoped), 4, 8, 10 and 12 mol% respectively. The prepared precursor solution was heated at 60 °C and deposited on clean quartz substrate using the dip coating apparatus. The dip coated films were dried at room temperature and prefired at 150 °C for 1 h. This process of coating and drying was repeated to obtain films of appropriate thickness suitable for XRD analysis. The doped ZrO2 films were annealed at 500 °C for 1 h in air, for crystallization. The dip coated films were then cooled down to room temperature. Structural and optical characterizations of these annealed films were then performed. Crystalline phase of the Zirconia thin films were characterized by XRD using the X-ray diffractometer (Model-PW 1710 PHILIPS) have K-Alpha 1 wavelength of 1.54056 and continuous scan type with scan step size of 0.0668.The scanning electron microscope study has done with the equipment of the model number (JEOL JSM6390LA). Optical transmittance was studied using a spectrophotometer (Model – JASCO-V550). Emission spectra were recorded by using a Perkin-Elmer Fluorescence Spectrometer (Model-LS55) with a 40 W Xenon Lamp as the excitation source and 2.5 nm excitation and emission slit width.
Fig. 1. XRD pattern of (a) undoped (b) 4 (c) 8 (d) 10 and (e) 12% Au doped ZrO2 thin films.
Table1 Diffraction angle, crystalline phase, FWHM and grain size of ZrO2 and Au doped zirconia thin films. Thin films samples
Diffraction angle 2θ (deg)
Crystalline phase
FWHM (rad)
Grain size (nm)
Undoped
30.21 28.65
ZrO2 T(101) ZrO2 M(-111)
0.0080 0.0050
17.021 29.762
Au 4%
30.40 28.30 38.00
ZrO2 T(101) ZrO2 M(-111) Au FCC(111)
0.0087 0.0089 0.0174
16.441 16.066 8.997
Au 8%
30.44 28.56 38.32
ZrO2 T(101) ZrO2 M(-111) Au FCC(111)
0.0100 0.0090 0.0087
14.369 15.897 16.871
Au 10%
30.46 28.62 38.24
ZrO2 T(101) ZrO2 M(-111) Au FCC(111)
0.0119 0.0111 0.0043
12.507 13.208 33.612
Au 12%
30.77 28.76 38.25
ZrO2T(101) ZrO2 M(-111) Au FCC(111)
0.0125 0.0120 0.0020
12.024 12.402 64.623
3. Result and discussion 3.1. Structural studies The X-ray diffraction pattern of undoped and Au doped ZrO2 thin films annealed at 500 °C are shown in Fig.1. The X-ray diffraction patterns revealed a mixed phase of tetragonal and monoclinic ZrO2 with a preferred orientations along 2θ E30.21 for T(111) and 28.65 for M(-111) [1,5]. Moreover, the Au doped films consist of an additional peak from 2θ E 38.2 to 38.38 which is due to Au atom of FCC(111) crystalline structure [JCPDS file no: 652870]. The peak became well defined for higher concentration of Au, indicating good crystallinity of the noble NPs. The diffraction angle, FWHM and grain size of the ZrO2 and Au are shown in the Table.1. It was observed that, when the Au dopant concentration increased from 4% to 12%, the grain size of ZrO2 decreased from 16.444 to 12.024 nm and 16.066 to 12.402 nm for tetragonal and monoclinic respectively. This may be due to the incorporation of Au in ZrO2 gel prepared by sol–gel method. As the concentration of Au increased in the gel, the particle density of Au or number of Au particles around the ZrO2 particle increased. The similar result was observed by Sonawane, Chan and Rosli et al. [23] in their Au doped films. This restricts the contact between ZrO2 particles with each other causing the inhibition in particle growth. The kinetics of this process is influenced by the dopant atoms, since the mobility of Au
particles gets agglomerated in the grain boundaries due to impurity drag [9]. The grain size of the Au increased from 8.997 to 64.623 nm (Table 1). This may be due to the fact that, particles grow by the condensation of gold atoms available in the neighborhood of each nucleating seed. The size of ionic Au is large (ionic radius1.37 Å and 1.44 Å for Au þ and Au°), compared to ionic Zr (ionic radius 0.80 Å for Zr4 þ ) and it is difficult to replace the Au þ and Au0 with Zr 4 þ . When the film is annealed at 500 °C, the Au particles were diffused towards the surface of the films because of low chemical affinity between the host and the Au atoms [20–23]. 3.2. Surface morphology studies Fig. 2 shows the surface morphology of undoped and Au doped ZrO2 thin films annealed at 500 °C. Surface of all films were continuous and without microcracks. The undoped ZrO2 film (Fig. 2.a) showed that the particles are almost uniformly distributed all over
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Fig. 2. Surface morphology of (a) undoped (b) 4 (c) 8 and (d) 12% Au doped ZrO2 thin films.
the surface of the film. The morphology of the films has been changed with increasing the Au dopant (Fig. 7.2.b–d). The surface of 4% Au doped films (Fig. 2.b) showed the uniform distribution of the grains with the presence of a few bright elongated shape particles on the surface of the film. The earlier reports agrees the above observation [20–23]. As the Au dopant increases to 8 and 12 Mol% (Fig. 2c and d), it was observed that the grains grow as bigger aggregates on the surface of the films with non-uniform distribution. Increasing the Au concentration will shift the gold reduction equilibrium toward the formation of metallic gold and will decrease the solubility of gold species [17]. Hence, as the gold dopant increases, the Au NPs get retained on the surface and aggregate, due to the higher kinetic energy and mobility at 500 °C [22,23]. Moreover, the addition of Au suppresses the growth of ZrO2 grains. 3.3. XPS studies To determine the composition of the films, XPS analyzes were carried out. XPS measurements of Zr3d, O1s, and Au 4d3 lines core level peaks of Au (8%) doped ZrO2 thin film annealed at 500 °C is shown in Fig. 3. The doublets of Zr3d5/2 and Zr3d3/2 levels were
measured at 184.6 and 185.6 eV, respectively. The O1s peak can be fitted with two peaks; at 527.81 and a shoulder at 537.07 eV. The first peak at lower energy is attributed to Zr–O bond [24], while the second one located at higher energy, and corresponds to O–H bonds from adsorbed water molecules which are weakened at 500 °C. The XPS peak of Au 4d3 was observed at 355.5 eV due to the presence of Au in the thin film sample (Fig. 3.c) [25]. 3.4. Optical studies Fig. 4 depicts the optical transmittance spectra of undoped and Au doped ZrO2 thin films annealed at 500 °C. The undoped, 4% and 8% of Au doped ZrO2 thin films have E75% transmittance in the UV–visible range. When the doping amount of Au increased to 10%, the average transmittance of the films decreased and surface plasmon resonance (SPR) peak appeared at 610 nm (inset of Fig. 4). Huang et al. [13] have observed the SPR peak at 595 nm in their Au doped ZrO2 thin films. When the doping amount of Au increased to 12%, the SPR peak shift towards higher wavelength (654 nm) and broadens. The broadening and shift of SPR peak is due to the change in host environments of the wide band gap metal oxides with the increase of grain size of Au.
183
184
186
187
537.07
527.81
Intensity (a.u)
Zr 3d 3/2 185
185
O 1s
185 .66
184 .53
Intensity (a.u)
Zr 3d5/2
I. John Berlin, K. Joy / Physica B 457 (2015) 182–187
520
525
530
Binding Energy (eV)
535
540
545
550
Intensity (a.u)
355.5
Au 4d3
Binding Energy (eV)
340
345
350
355
360
365
370
Binding Energy (eV) Fig. 3. XPS spectra of ZrO2 thin film annealed at 500˚C (a) Zr 3d(b) O1s and (c) Au 4d3.
100
a 80
b
60
d e
40
20
e (d) 10 %Au (e) 12 %Au
0.3
500
0 200
a b c d e
d
0.6
Absorption
Transmittance %
c
600
700
Wavelength (nm)
300
400
500
600
700
800
900
wave length (nm) Fig. 4. Transmittance spectra of (a) undoped (b) 4 (c) 8 (d) 10 and (e) 12% Au doped ZrO2 thin films (Inset: absorption vs wavelength of SPR peak).
The optical properties (plasmon (SPR)) of nanometer-sized noble metal particles dispersed in metal oxides can be related to Mie–Drude equation [14]. Kreibig et al. [14] reported that the broadening of SPR peaks is due to the damping, which is determined by the distance traveled by electron scattering, which in turn depends on the size of noble metal particles. This study is in agreement with the Mie–Drude equation in the case of broadening and peak shift of plasmon spectra for Au -doped films. The size dependence of plasmon has been investigated in several studies [14,26,27], and observed both blue and red shifts, depending on the host matrix. It is indicated that red shift can be explained by taking into account, the spilling out of delocalized valence electrons beyond the positive ion region and this phenomenon is affected by the matrix [26]. When the free electron transfers from the matrix to the particle, the electron density is increased in the outer particle surface. This outer layer interface between noble metal and matrix play an important role in optical enhancement. The near-field coupling of localized surface plasmon resonance is increasingly being exploited for the construction of nanoscale optical and photonic devices (lenses, switches, and waveguides) because Au particles convert photons to electromagnetic modes of radiation [12]. Conversion of photons to surface plasmons allows
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Energy (eV)
Table -2 Thickness, energy band gap and refractive index of undoped and Au doped ZrO2 thin films. Thickness (nm)
Energy band gap (eV)
Refractive index
Undoped Au 4% Au 8% Au 10% Au 12%
270 180 185 167.70 143.60
5.72 5.71 5.70 5.64 5.40
2.11 2.13 2.16 2.18 2.30
the sub-wavelength manipulation of electromagnetic radiation. Optically this conversion can reach 100% efficiency and it has wide applications in waveguides below the diffraction limit of light. The undoped and less Au doped films [Fig. (4.a–c)] display a typical interference behavior. When the films were doped with 10–12% of Au, the interference behavior was affected due to the influence of Au nanoparticles in the film, which changes the medium and the interference conditions. The thickness and refractive index of the films were calculated using Swanepoel method [28] and shown in Table 2. The bandgap (Eg) values of films were obtained by extrapolating the linear portion of (αhυ)2 vs hυ plots intercept the photon energy axis shown in Fig. 5. From the figure, the optical band gap varied from 5.72 to 5.40 eV with increase in Au doping concentration (Table 2). Due to the increase in Au doping concentration many-body interaction occurs between free carriers and ionized impurities causing decrease in optical band gap. It is wellknown that when optical transition or carrier transport for semiconductors occurs in a band positioned far from the conduction band bottom or valence band top, the non parabolicity effects should be taken into account [29]. However, due to many-body effects of free carriers on the conduction and valence bands, which are known as bandgap renormalization, bandgap narrowing usually occurs [30]. 3.5. Photoluminescence studies Fig. 6 shows the PL spectra of undoped and Au doped ZrO2 thin films. All the films were excited at 243 nm. The emission peaks were observed at 389 nm (3.18 eV), 306 nm (4.05 eV), 602 nm
(αhν)2(eV/cm)2
a b c d e
5.0
5.5 Energy, hν (eV)
6.0
Fig. 5. Direct band gap of the (a) undoped (b) 4 (c) 8 (d)10 and (e) 12% Au doped ZrO2 thinfilms.
Intensity (arb.units)
Thin film samples
4.14
3.10
2.48
200
a b
150
c
2.07
1.77
1.24
600
700
800
d
100
50
e 0 300
400
500
wavelength (nm) Fig. 6. PL spectra of (a) undoped (b) 4 (c) 8 (d) 10 (e) 12% Au doped ZrO2 thin films.
(2.05 eV) and 767 nm (1.61 eV). The energy gap of tetragonal ZrO2 phase is greater than 5.5 eV [1–3]. At 243 nm (5.11 eV) excitation a large intensity emission band of 390 nm is produced. The excitation band at 243 nm corresponds to energy near the energy gap of ZrO2 tetragonal phase and has been assigned to grain boundaries defect states, which are an inherent aspect of the nanocrystallinity [2–4]. The intense zirconia emission peak at E389 nm in the ZrO2 thin film can be due to the ionized oxygen vacancies (F and F_centers) from the conduction band. Generally UV emission can arise as a result of the radiative recombination of a photo generated hole with an electron occupying the oxygen vacancy [3]. The PL intensity of Au doped ZrO2 thin films decreased with increase in Au concentration. Anna et al. [31] have discussed the mechanism of quenching effect of gold on the photoluminescence of Si nanocrystal. They proposed three mechanisms such as (i) decrease in the radiative recombination (ii) decrease in the life time of the excitons and (iii) suppression of grain growth of ZrO2 by dopants, for the decrease of PL intensity. Gold particles produce an efficient deep recombination trap state in crystalline materials [32] which in turn decrease in the recombination rate. Thus, the PL intensity of Au doped ZrO2 thin films were observed to decrease. The other explanation for the observed decrease in PL intensity is the decrease in lifetime of excitons. The decrease in lifetime of exciton may be the effect of coupling (energy transfer) between the densely packed nanocrystals. The Au dopants lead to recombination paths with rates faster than the excitonic decay rate. In this case, a quenched nanocrystal would also affect the lifetime of neighboring nanocrystals, leading to a reduction (but not full quenching) of the latter. The lifetime reduction with increasing Au concentration is due to an increase in non-radiative decay rate and is at least partially responsible for the decrease in the PL intensity. This proves that Au is an effective quencher of excited nanocrystals. The third reason for the reduction of PL intensity was due to the reduction of the crystallization of ZrO2. In addition, when the Au dopant increases, the Au particles were aggregated on the surface of the ZrO2 films and scattered the incident radiations on the surface of the films. 4. Conclusion The Au doped ZrO2 thin films were prepared by sol–gel dip coating method. XRD pattern revealed that Au doped films have
I. John Berlin, K. Joy / Physica B 457 (2015) 182–187
mixed phase of tetragonal and monoclinic of ZrO2 and Au atom of FCC (111) crystalline structure along with the grain size of ZrO2 decreased with increase in Au concentration. The SPR peak appeared at 610 nm and the optical band gap decreased with increasing Au dopant. The near-field coupling of localized surface plasmon resonance is increasingly being exploited for the construction of nanoscale optical and photonic devices (lenses, switches, and waveguides). Formations of recombination trap state, decrease in the life time of excitons and suppression of the grain growth of host matrix (ZrO2) have played vital roles for the decrease of PL intensity.
Acknowledgment The authors are grateful for the financial assistance of UGC, Government of India, Major Research Project (2009–2012) and are grateful Mr. R.S.Vinod Mar Ephraem College of Engineering and Technology.
Reference [1] I. John Berlin, J.S. Lakshmi, S. Sujatha Lekshmy, G.P. Daniel, P.V. Thomas, K. Joy, J. Sol–Gel Sci. Technol. 58 (2011) 669. [2] I. John Berlin, V.S. Anitha, P.V. Thomas, K. Joy, J. Sol–Gel Sci. Technol. 64 (2012) 289. [3] J.S. Lakshmi, I. John Berlin, P.D. Georgi, P.V. Thomas, K. Joy, Physica B 406 (3050) (2011) L56. [4] I. John Berlin, L.V. Maneeshya, K.T. Jijimon, P.V. Thomas, K. Joy, J. Lumin. 132 (2012) 3077. [5] I. John Berlin, S. Sujatha lekshmy, P.V.V. Thomas, Ganesan, K. Joy, Thin Solid films 550 (2014) 199. [6] K. Joy, J. Sol–Gel Sci. Technol. 66 (2013) 293. [7] H.Q. Cao, X.Q. Qiu, B. Luo, Y. Liang, Y.H. Zhang, R.Q. Tan, M.J. Zhao, Q.M. Zhu, Adv. Funct. Mater. 14 (2004) 243.
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[8] J. Xingtao, Y. Wei, Q. Minghui, L.J. Jianping, Magn. Magn. Mater. 321 (2009) 2354. [9] P. Kumar, J. Singh, V. Parashar, K. Singh, R.S. Tiwari, O.N. Srivastava, K. Ramam, A.C. Pandey, CrystEngComm 14 (2012) 1653. [10] G. Garcia, R. Buonsanti, E.L. Runnerstrom, R.J. Mendelsberg, A.L. lordes, A. Anders, T.J. Richardson, D.J. Milliron, Nano Lett. 11 (2011) 4415. [11] T.A.E. Brolossy, T. Abdallah, M.B. Mohamed, S. Abdallah, K. Easawi, S. Negm, H. Talaat, Eur. Phys. J. Spec. Top. 153 (2008) 361. [12] E. Hutter, H.F. Janos, Adv. Mater. 16 (2004) 1685. [13] W. Huang, J. Shi, H.A. Chavez, E.Z. Ma, G. Ma, B.F.J. Espinoza, Superficies y Vacio 17 (2004) 13. [14] U. Kreibig, L. Genzel, Surf. Sci. 156 (1985) 678. [15] D. Dalacu, L. Martinu, J. Appl. Phys. 87 (2000) 228. [16] C.F. Bohren, D.R. Huffman, Absorption and Scattering of Light by Small Particles, Wiley- VCH Verlag GmbH & Co, Germany, 1998. [17] M. Epifani, C. Giannini, L. Tapfer, L. Vasanelli, J. Am. Ceram. Soc. 83 (2000) 2385. [18] A. Eremenko, N. Smirnova, I. Gnatiuk, O. Linnik, N. Vityuk, Y. Mukha, A. Korduban, Nanocomposites and Polymers with Analytical Methods, 2011. [19] A.K. Atta, P.K. Biswas, D. Ganguli, Thin Solid Films 197 (1991) 187. [20] R.S. Sonawane, M.K. Dongare, J. Mol. Catal. A: Chem. 243 (2006) 68. [21] S.A. Guillen, S.A. Mayéna, D.G. Torres, P.R. Castanedo, Maldonadob, Mater Sci. Eng. B 174 (2010) 84. [22] K. Chan, B.T. Goh, S.A. Rahman, M.R. Muhamad, C.F. Dee, Z. Aspanut, Vacuum 86 (2012) 1367. [23] N. Rosli, K.W. Chan, I.P. Jamal, S.A. Rahman, B.T. Goh, Z. Aspanut, Adv. Mater. Res. 501 (2012) 221. [24] J.C. Dupin, D. Gonbeau, P. Vinatier, A. Levasseur, Phys. Chem. Chem. Phys. 2 (2000) 1319. [25] C. Vincent, Handbooks of Monochromatic XPS Spectra, XPS International, LLC California, 1999. [26] S. Fedrigo, W. Harbich, J. Buttet, Phys. Rev. B 47 (1993) 10706. [27] H. Hovel, S. Fritz, A. Hilger, V. Kreibig, M. Vollmer, Phys. Rev. B 48 (1993) 18178. [28] R. Swanepoel, J. Phys. E: Sci. Instrum. 16 (1983) 1214. [29] T. Minami, MRS. Bull. 25 (2000) 38. [30] Z. Abdolahzadeh, S.M. Rozati, Physica B 407 (2012) 4512. [31] L. Anna, J.A. Tchebotareva, S.B. Michiel de Dood, A. Julie, Harry Atwater, J. Albert Polman, Lumin 114 (2005) 137. [32] S.M. Sze, K.K. Ng, Physics of Semiconductor Devices, John Wiley & Sons, Inc., New Jersey, 2007.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Optical enhancement of Au doped ZrO2 thin films by sol–gel dip coating method I. John Berlin n, K. Joy Thin film Lab, Post Graduate and Research Department of Physics, Mar Ivanios College, Thiruvananthapuram 695015, India
art ic l e i nf o
a b s t r a c t
Article history: Received 2 August 2014 Received in revised form 13 October 2014 Accepted 15 October 2014 Available online 22 October 2014
Homogeneous and transparent Au doped ZrO2 thin films were prepared by sol–gel dip coating method. The films have mixed phase of tetragonal, monoclinic and face centered cubic with crack free surface. Due to the increase in Au doping concentration many-body interaction occurs between free carriers and ionized impurities causing decrease in optical band gap from 5.72 to 5.40 eV. Localized surface plasmon resonance peak of the Au doped films appeared at 610 nm. Conversion of photons to surface plasmons allows the sub-wavelength manipulation of electromagnetic radiation. Hence the prepared Au doped ZrO2 thin films can be applied in nanoscale photonic devices such as lenses, switches, waveguides etc. Moreover the photoluminescence (PL) intensity of Au doped ZrO2 thin films decrease due to decrease in the radiative recombination, life time of the excitons and suppression of grain growth of ZrO2 with increasing Au dopant. & Elsevier B.V. All rights reserved.
Keywords: Zirconia Au doped ZrO2 Thin film Sol–gel Photoluminescence Localized surface plasmon resonance
1. Introduction In recent years, metal oxides and their nanostructures have emerged as an important class of materials with a rich spectrum of properties and great potential for device applications, e.g., transparent electrodes, high-electron mobility transistors, gas sensors, photovoltaic and photonic devices and hydrogen storage applications. Of this class, wide-band gap oxide semiconductor of zirconia (ZrO2) has occupied the forefront in the past decade [1–3] because of large binding energy of ZrO2 has an ideal choice for inorganic passivation shells in a variety of semiconductor core/ shell nanoparticles. It is now well known that the metal doped ZrO2 has the high potential for fabricating novel nanoelectronics and optical devices with enhanced performance. It is due to the intentional defects generated by doping ions, which may have a profound effect on the electronic and optical properties of the materials. [4– 8] Further when the noble metals are doped with wide band gap semiconductor (ZrO2) it can create new opportunities for plasmonic manipulation of light because of the plasmon resonance frequency of noble metals [9,10]. The presence of noble metals such as Au and Ag nanoparticles (NPs) in the ZrO2 matrix, cause strong light absorption band in the visible region known as Localized surface plasmon resonance (LSPR). This LSPR is formed due to coherent oscillation of the conduction band electrons n
Corresponding author. Fax: þ91 471 2530023. E-mail address: [email protected] (I. John Berlin).
http://dx.doi.org/10.1016/j.physb.2014.10.013 0921-4526/& Elsevier B.V. All rights reserved.
(surface plasmon oscillation) induced by interaction with an electromagnetic field [11]. The near-field coupling of localized surface plasmon resonance is increasingly being exploited for the construction of nanoscale optical and photonic devices (lenses, switches, linear optical devices and waveguides) because Au particles convert photons to electromagnetic modes of radiation (localized surface plasmons) [12]. The LSPR peaks for Au NPs have been reported to be located at 556 nm [13,14] and the optical responses of metallic clusters reveal information about their electronic structure. The resonance frequency depends on size, morphology, shape and distribution, as well as on the particular dielectric characteristic of the surrounding medium in which the Au NPs are dispersed [15,16]. Recently, the sol–gel process has emerged as a promising technique for the synthesis of noble metal–metal oxide compounds with enhanced nonlinear optical properties [13,17]. Few reports have been published about the studies on Au doped ZrO2 by sol–gel method [13,17,18] but no information is available about the effect of Au on the Photoluminescence (PL) of ZrO2 films. Moreover, the structural, optical and manipulation of electromagnetic radiation of Au doped zirconia thin films by sol– gel dip coating method have not been systematically studied yet. In this paper, we have investigated the effect of Au on the photoluminescence and optical properties of ZrO2 thin films prepared by sol–gel dip coating technique. In addition, the crystalline structure, surface morphology and film composition of the films were also determined. The basis of the technique is to coat a substrate with a precursor solution containing the requisite metal
I. John Berlin, K. Joy / Physica B 457 (2015) 182–187
183
components in the required proportion, which transforms to a gel layer because of solvent evaporation and/or chemical reactions [1–3].
2. Experimental procedure In this study, inorganic precursor route was chosen for the fabrication of nanocrystalline transparent zirconia thin films. Zirconium oxychloride octahydrate (ZrOCl2 8H2O) (Sigma-Aldrich 99.5%) was used for the preparation of the precursor solution [19]. The mixture of 2-butanol and ethanol ( in the ratio 1:1) was used as the solvent. A homogeneous solution of zirconium oxychloride octahydrate (2 wt%) was prepared by mixing 1 mol of zirconium oxychloride octahydrate in 1/3 of the total volume of mixed 2-butanol and ethanol. The solution was stirred for 45 min using magnetic stirrer. The water for hydrolysis and nitric acid for oxidation of ratio, Water: HNO3: Acetlyeacetone¼ 20: 0.4: 3 were then added to the salt–alcohol solution. Gold as a dopant was added in the form of tetrachloroaurate (III) trihydrate with five different Au/Zr ratio: 0 (undoped), 4, 8, 10 and 12 mol% respectively. The prepared precursor solution was heated at 60 °C and deposited on clean quartz substrate using the dip coating apparatus. The dip coated films were dried at room temperature and prefired at 150 °C for 1 h. This process of coating and drying was repeated to obtain films of appropriate thickness suitable for XRD analysis. The doped ZrO2 films were annealed at 500 °C for 1 h in air, for crystallization. The dip coated films were then cooled down to room temperature. Structural and optical characterizations of these annealed films were then performed. Crystalline phase of the Zirconia thin films were characterized by XRD using the X-ray diffractometer (Model-PW 1710 PHILIPS) have K-Alpha 1 wavelength of 1.54056 and continuous scan type with scan step size of 0.0668.The scanning electron microscope study has done with the equipment of the model number (JEOL JSM6390LA). Optical transmittance was studied using a spectrophotometer (Model – JASCO-V550). Emission spectra were recorded by using a Perkin-Elmer Fluorescence Spectrometer (Model-LS55) with a 40 W Xenon Lamp as the excitation source and 2.5 nm excitation and emission slit width.
Fig. 1. XRD pattern of (a) undoped (b) 4 (c) 8 (d) 10 and (e) 12% Au doped ZrO2 thin films.
Table1 Diffraction angle, crystalline phase, FWHM and grain size of ZrO2 and Au doped zirconia thin films. Thin films samples
Diffraction angle 2θ (deg)
Crystalline phase
FWHM (rad)
Grain size (nm)
Undoped
30.21 28.65
ZrO2 T(101) ZrO2 M(-111)
0.0080 0.0050
17.021 29.762
Au 4%
30.40 28.30 38.00
ZrO2 T(101) ZrO2 M(-111) Au FCC(111)
0.0087 0.0089 0.0174
16.441 16.066 8.997
Au 8%
30.44 28.56 38.32
ZrO2 T(101) ZrO2 M(-111) Au FCC(111)
0.0100 0.0090 0.0087
14.369 15.897 16.871
Au 10%
30.46 28.62 38.24
ZrO2 T(101) ZrO2 M(-111) Au FCC(111)
0.0119 0.0111 0.0043
12.507 13.208 33.612
Au 12%
30.77 28.76 38.25
ZrO2T(101) ZrO2 M(-111) Au FCC(111)
0.0125 0.0120 0.0020
12.024 12.402 64.623
3. Result and discussion 3.1. Structural studies The X-ray diffraction pattern of undoped and Au doped ZrO2 thin films annealed at 500 °C are shown in Fig.1. The X-ray diffraction patterns revealed a mixed phase of tetragonal and monoclinic ZrO2 with a preferred orientations along 2θ E30.21 for T(111) and 28.65 for M(-111) [1,5]. Moreover, the Au doped films consist of an additional peak from 2θ E 38.2 to 38.38 which is due to Au atom of FCC(111) crystalline structure [JCPDS file no: 652870]. The peak became well defined for higher concentration of Au, indicating good crystallinity of the noble NPs. The diffraction angle, FWHM and grain size of the ZrO2 and Au are shown in the Table.1. It was observed that, when the Au dopant concentration increased from 4% to 12%, the grain size of ZrO2 decreased from 16.444 to 12.024 nm and 16.066 to 12.402 nm for tetragonal and monoclinic respectively. This may be due to the incorporation of Au in ZrO2 gel prepared by sol–gel method. As the concentration of Au increased in the gel, the particle density of Au or number of Au particles around the ZrO2 particle increased. The similar result was observed by Sonawane, Chan and Rosli et al. [23] in their Au doped films. This restricts the contact between ZrO2 particles with each other causing the inhibition in particle growth. The kinetics of this process is influenced by the dopant atoms, since the mobility of Au
particles gets agglomerated in the grain boundaries due to impurity drag [9]. The grain size of the Au increased from 8.997 to 64.623 nm (Table 1). This may be due to the fact that, particles grow by the condensation of gold atoms available in the neighborhood of each nucleating seed. The size of ionic Au is large (ionic radius1.37 Å and 1.44 Å for Au þ and Au°), compared to ionic Zr (ionic radius 0.80 Å for Zr4 þ ) and it is difficult to replace the Au þ and Au0 with Zr 4 þ . When the film is annealed at 500 °C, the Au particles were diffused towards the surface of the films because of low chemical affinity between the host and the Au atoms [20–23]. 3.2. Surface morphology studies Fig. 2 shows the surface morphology of undoped and Au doped ZrO2 thin films annealed at 500 °C. Surface of all films were continuous and without microcracks. The undoped ZrO2 film (Fig. 2.a) showed that the particles are almost uniformly distributed all over
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Fig. 2. Surface morphology of (a) undoped (b) 4 (c) 8 and (d) 12% Au doped ZrO2 thin films.
the surface of the film. The morphology of the films has been changed with increasing the Au dopant (Fig. 7.2.b–d). The surface of 4% Au doped films (Fig. 2.b) showed the uniform distribution of the grains with the presence of a few bright elongated shape particles on the surface of the film. The earlier reports agrees the above observation [20–23]. As the Au dopant increases to 8 and 12 Mol% (Fig. 2c and d), it was observed that the grains grow as bigger aggregates on the surface of the films with non-uniform distribution. Increasing the Au concentration will shift the gold reduction equilibrium toward the formation of metallic gold and will decrease the solubility of gold species [17]. Hence, as the gold dopant increases, the Au NPs get retained on the surface and aggregate, due to the higher kinetic energy and mobility at 500 °C [22,23]. Moreover, the addition of Au suppresses the growth of ZrO2 grains. 3.3. XPS studies To determine the composition of the films, XPS analyzes were carried out. XPS measurements of Zr3d, O1s, and Au 4d3 lines core level peaks of Au (8%) doped ZrO2 thin film annealed at 500 °C is shown in Fig. 3. The doublets of Zr3d5/2 and Zr3d3/2 levels were
measured at 184.6 and 185.6 eV, respectively. The O1s peak can be fitted with two peaks; at 527.81 and a shoulder at 537.07 eV. The first peak at lower energy is attributed to Zr–O bond [24], while the second one located at higher energy, and corresponds to O–H bonds from adsorbed water molecules which are weakened at 500 °C. The XPS peak of Au 4d3 was observed at 355.5 eV due to the presence of Au in the thin film sample (Fig. 3.c) [25]. 3.4. Optical studies Fig. 4 depicts the optical transmittance spectra of undoped and Au doped ZrO2 thin films annealed at 500 °C. The undoped, 4% and 8% of Au doped ZrO2 thin films have E75% transmittance in the UV–visible range. When the doping amount of Au increased to 10%, the average transmittance of the films decreased and surface plasmon resonance (SPR) peak appeared at 610 nm (inset of Fig. 4). Huang et al. [13] have observed the SPR peak at 595 nm in their Au doped ZrO2 thin films. When the doping amount of Au increased to 12%, the SPR peak shift towards higher wavelength (654 nm) and broadens. The broadening and shift of SPR peak is due to the change in host environments of the wide band gap metal oxides with the increase of grain size of Au.
183
184
186
187
537.07
527.81
Intensity (a.u)
Zr 3d 3/2 185
185
O 1s
185 .66
184 .53
Intensity (a.u)
Zr 3d5/2
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520
525
530
Binding Energy (eV)
535
540
545
550
Intensity (a.u)
355.5
Au 4d3
Binding Energy (eV)
340
345
350
355
360
365
370
Binding Energy (eV) Fig. 3. XPS spectra of ZrO2 thin film annealed at 500˚C (a) Zr 3d(b) O1s and (c) Au 4d3.
100
a 80
b
60
d e
40
20
e (d) 10 %Au (e) 12 %Au
0.3
500
0 200
a b c d e
d
0.6
Absorption
Transmittance %
c
600
700
Wavelength (nm)
300
400
500
600
700
800
900
wave length (nm) Fig. 4. Transmittance spectra of (a) undoped (b) 4 (c) 8 (d) 10 and (e) 12% Au doped ZrO2 thin films (Inset: absorption vs wavelength of SPR peak).
The optical properties (plasmon (SPR)) of nanometer-sized noble metal particles dispersed in metal oxides can be related to Mie–Drude equation [14]. Kreibig et al. [14] reported that the broadening of SPR peaks is due to the damping, which is determined by the distance traveled by electron scattering, which in turn depends on the size of noble metal particles. This study is in agreement with the Mie–Drude equation in the case of broadening and peak shift of plasmon spectra for Au -doped films. The size dependence of plasmon has been investigated in several studies [14,26,27], and observed both blue and red shifts, depending on the host matrix. It is indicated that red shift can be explained by taking into account, the spilling out of delocalized valence electrons beyond the positive ion region and this phenomenon is affected by the matrix [26]. When the free electron transfers from the matrix to the particle, the electron density is increased in the outer particle surface. This outer layer interface between noble metal and matrix play an important role in optical enhancement. The near-field coupling of localized surface plasmon resonance is increasingly being exploited for the construction of nanoscale optical and photonic devices (lenses, switches, and waveguides) because Au particles convert photons to electromagnetic modes of radiation [12]. Conversion of photons to surface plasmons allows
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Energy (eV)
Table -2 Thickness, energy band gap and refractive index of undoped and Au doped ZrO2 thin films. Thickness (nm)
Energy band gap (eV)
Refractive index
Undoped Au 4% Au 8% Au 10% Au 12%
270 180 185 167.70 143.60
5.72 5.71 5.70 5.64 5.40
2.11 2.13 2.16 2.18 2.30
the sub-wavelength manipulation of electromagnetic radiation. Optically this conversion can reach 100% efficiency and it has wide applications in waveguides below the diffraction limit of light. The undoped and less Au doped films [Fig. (4.a–c)] display a typical interference behavior. When the films were doped with 10–12% of Au, the interference behavior was affected due to the influence of Au nanoparticles in the film, which changes the medium and the interference conditions. The thickness and refractive index of the films were calculated using Swanepoel method [28] and shown in Table 2. The bandgap (Eg) values of films were obtained by extrapolating the linear portion of (αhυ)2 vs hυ plots intercept the photon energy axis shown in Fig. 5. From the figure, the optical band gap varied from 5.72 to 5.40 eV with increase in Au doping concentration (Table 2). Due to the increase in Au doping concentration many-body interaction occurs between free carriers and ionized impurities causing decrease in optical band gap. It is wellknown that when optical transition or carrier transport for semiconductors occurs in a band positioned far from the conduction band bottom or valence band top, the non parabolicity effects should be taken into account [29]. However, due to many-body effects of free carriers on the conduction and valence bands, which are known as bandgap renormalization, bandgap narrowing usually occurs [30]. 3.5. Photoluminescence studies Fig. 6 shows the PL spectra of undoped and Au doped ZrO2 thin films. All the films were excited at 243 nm. The emission peaks were observed at 389 nm (3.18 eV), 306 nm (4.05 eV), 602 nm
(αhν)2(eV/cm)2
a b c d e
5.0
5.5 Energy, hν (eV)
6.0
Fig. 5. Direct band gap of the (a) undoped (b) 4 (c) 8 (d)10 and (e) 12% Au doped ZrO2 thinfilms.
Intensity (arb.units)
Thin film samples
4.14
3.10
2.48
200
a b
150
c
2.07
1.77
1.24
600
700
800
d
100
50
e 0 300
400
500
wavelength (nm) Fig. 6. PL spectra of (a) undoped (b) 4 (c) 8 (d) 10 (e) 12% Au doped ZrO2 thin films.
(2.05 eV) and 767 nm (1.61 eV). The energy gap of tetragonal ZrO2 phase is greater than 5.5 eV [1–3]. At 243 nm (5.11 eV) excitation a large intensity emission band of 390 nm is produced. The excitation band at 243 nm corresponds to energy near the energy gap of ZrO2 tetragonal phase and has been assigned to grain boundaries defect states, which are an inherent aspect of the nanocrystallinity [2–4]. The intense zirconia emission peak at E389 nm in the ZrO2 thin film can be due to the ionized oxygen vacancies (F and F_centers) from the conduction band. Generally UV emission can arise as a result of the radiative recombination of a photo generated hole with an electron occupying the oxygen vacancy [3]. The PL intensity of Au doped ZrO2 thin films decreased with increase in Au concentration. Anna et al. [31] have discussed the mechanism of quenching effect of gold on the photoluminescence of Si nanocrystal. They proposed three mechanisms such as (i) decrease in the radiative recombination (ii) decrease in the life time of the excitons and (iii) suppression of grain growth of ZrO2 by dopants, for the decrease of PL intensity. Gold particles produce an efficient deep recombination trap state in crystalline materials [32] which in turn decrease in the recombination rate. Thus, the PL intensity of Au doped ZrO2 thin films were observed to decrease. The other explanation for the observed decrease in PL intensity is the decrease in lifetime of excitons. The decrease in lifetime of exciton may be the effect of coupling (energy transfer) between the densely packed nanocrystals. The Au dopants lead to recombination paths with rates faster than the excitonic decay rate. In this case, a quenched nanocrystal would also affect the lifetime of neighboring nanocrystals, leading to a reduction (but not full quenching) of the latter. The lifetime reduction with increasing Au concentration is due to an increase in non-radiative decay rate and is at least partially responsible for the decrease in the PL intensity. This proves that Au is an effective quencher of excited nanocrystals. The third reason for the reduction of PL intensity was due to the reduction of the crystallization of ZrO2. In addition, when the Au dopant increases, the Au particles were aggregated on the surface of the ZrO2 films and scattered the incident radiations on the surface of the films. 4. Conclusion The Au doped ZrO2 thin films were prepared by sol–gel dip coating method. XRD pattern revealed that Au doped films have
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mixed phase of tetragonal and monoclinic of ZrO2 and Au atom of FCC (111) crystalline structure along with the grain size of ZrO2 decreased with increase in Au concentration. The SPR peak appeared at 610 nm and the optical band gap decreased with increasing Au dopant. The near-field coupling of localized surface plasmon resonance is increasingly being exploited for the construction of nanoscale optical and photonic devices (lenses, switches, and waveguides). Formations of recombination trap state, decrease in the life time of excitons and suppression of the grain growth of host matrix (ZrO2) have played vital roles for the decrease of PL intensity.
Acknowledgment The authors are grateful for the financial assistance of UGC, Government of India, Major Research Project (2009–2012) and are grateful Mr. R.S.Vinod Mar Ephraem College of Engineering and Technology.
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