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TiO2 thin composite films
Ceramics International 45 (2019) 22336–22343
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Modified optical characteristics of TiO2/Au/TiO2 thin composite films a
b
b
a
Faiza Javed , Sofia Javed , Mohammad Mujahid , Fakhar ul Inam , Arshad Saleem Bhatti a b
T a,∗
Centre for Micro and Nano Devices (CMND), Department of Physics, COMSATS University Islamabad, Park Road, Islamabad, 44000, Pakistan School of Chemical and Materials Engineering (SCME), National University of Science and Technology, H-12, Islamabad, Pakistan
A R T I C LE I N FO
A B S T R A C T
Keywords: TiO2 ultrathin composite film Au nanoparticles Surface plasmon modes Spectroscopy ellipsometry Drude lorentz model Tauc lorentz model
We report the optical characteristics of the TiO2/Au-NPs/TiO2 composite thin films determined from the UV–Vis absorption spectroscopy and spectroscopic ellipsometry. The UV–Vis spectra indicated the enhanced optical absorption in the composite thin films in the range 1.6–3.0 eV. Spectroscopic ellipsometry was employed to determine the optical parameters of the composite films. The experimental results were fitted with hybrid model, which included Drude Lorentz model for Au layer and Urbach tail modified Tauc Lorentz model for TiO2 layers. The photoluminescence spectrum of the composite film clearly showed a weak emission band centered at 500 nm, which was absent in the pure TiO2 films. The extracted optical parameters manifested the generation of hot electrons at the Au/TiO2 interface due to Au surface plasmon (SP) modes, multiple interband transitions in Au in the energy range (1–4 eV) and a long Urbach tail (∼144 meV) in TiO2. These were direct indicators of hot electron scattering from the plasmon modes and their transfer to the TiO2 conduction band. The present study revealed that the plasmon exciton interactions in composite film can modify its optical properties.
1. Introduction There exists an immense interest in understanding the novel optical properties of metal/semiconductor nanostructured composites, which have potential applications in energy harvesting, manipulation and storage devices [1–4]. The presence of metallic nanoparticles induce various features in the host material, e.g., modulation of the band gap, enhanced absorption or emission and formation of charge trap centers [5–7]. TiO2 is a wide band gap semiconductor with large dielectric constant (∼70) and refractive index (∼3.2 @ 415 nm (3.1 eV)) It is considered as an ideal material for a number of applications [8,9], such as photocatalysis, photoconversion, energy storage and energy harvesting devices, etc. [10,11]. It has been demonstrated that incorporation of Au in TiO2 may lead to enhanced energy storage and harvesting capability. The metal nanoparticles (NPs) incorporated in a single or bilayer oxide thin films such as of ZnO, TiO2 and SnO2 have shown increased absorption effects in the visible part of the spectra [2,12,13]. Non-linear absorption and optical susceptibility (∼2 × 10−5 esu) for Au coated TiO2 dielectric matrix has been improved as compared to pure TiO2 or ZnS matrix [14]. A comparison of bare TiO2, Au coated TiO2 film as well as Au embedded TiO2 films for absorption and photochemical activity showed 35% enhanced absorption and photocatalytic activity in Au embedded TiO2 film [15]. Surface plasmon resonance (SPR) controlled absorption of TiO2 thin films has
∗
also been reported [16], which took into account the effects of dielectric media surrounding the Au-NPs on SPR peak position and its broadening. The SPR effects in TiO2 has also been studied by making periodic structures [17]. Theoretical studies of Au embedded TiO2 matrix have been performed to use it as a dielectric core in an optical fiber [18]. The Au–TiO2 composite grown using spray pyrolysis have been studied for phase modification [19]. The PL studies of Au embedded TiO2 revealed the plasmonic effects on the PL peak position and intensities [20]. Hybrid structures have also been used in other various applications like antibacterial activity, photocatalysis, photovoltaic fuel cells, for ethanol and methanol oxidation, etc. [21]. Thus, it is quite imperative to understand the electron dynamics and optical properties of hybrid or composite films formed by Au-layer and TiO2 film. Various techniques have been employed for the synthesis of TiO2–Au nanostructures, which are sol-gel, chemical vapor deposition, sputtering, atomic layer deposition (ALD) [22–25]. The embedded Au film or NPs in TiO2 introduces the defect states and reduces the band gap of the TiO2. In addition, intraband surface plasmon resonance and interband transition (5-d to 6-sp) in Au play a critical role by interacting with TiO2 band gap defect states. The plasmon scattering of electrons in Au-NPs results in the creation of hot electrons forming a non-thermalized distribution above the Fermi level and hence increases the probability of hot electrons transfer into TiO2 across the junction, thus affecting the density of conduction electrons in TiO2 by introducing new
Corresponding author. E-mail address: [email protected] (A.S. Bhatti).
https://doi.org/10.1016/j.ceramint.2019.07.262 Received 21 March 2019; Received in revised form 23 June 2019; Accepted 22 July 2019 Available online 24 July 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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energy states [1]. In addition, the decaying field of surface plasmon modes at the interface is reinforced in the presence of a high dielectric constant material, which leads to coupling of plasmon modes with excitons in the semiconductor [25,26]. The metal layer within a wide band gap semiconductor layered structures opens up new channels of absorption. This can be done by using monometal with mono-size or bimodal size distributions, which can create one or two absorption bands or using a bimetal layer configuration with two distinct plasmon modes. In addition, strong nonlinear optical effects can also be induced in the layered structures. For example, third order non-linear optical response and acousto-plasmonic sensing has been demonstrated by the selective excitation of resonant modes in Au–Pt bimetallic NPs embedded TiO2 composite thin film [27,28]. Spectroscopic ellipsometry is one of the best optical tools to extract optical parameters of thin films such as complex dielectric function. It is a sensitive technique and reveals the origin of optical processes. The ellipsometry data is usually fitted with various models for different layers to extract the optical parameters. For example, the Tauc-Lorentz model is used for amorphous and polycrystalline semiconductors and is based on single oscillator term for allowed transition in direct band gap semiconductors [29,30]. The metal layer (Au-layer) is best fitted with the Drude and Lorentz model as shown in Equ. (1), where the second term represents the contribution from free electrons and the third term gives the contribution from the inter band transitions. The Lorentz term is the sum of multiple oscillators, which takes into account all possible inter band transitions [31–33]:
ε=1+
ωp2 ω2
− iγω
5
+
∑
ω2 j = 1 o, j
ωp2, j − ω2 − iωγj
(1)
Where ‘ωo’ is the oscillation frequency, ‘ωp’ is the plasma frequency and ‘γ’ is the damping of the oscillator. ‘ε∞’ is the value of dielectric at high frequencies. Second term ‘εfree ’ donates the absorption due to free electron density oscillations. The summation in dielectric term due to interband transitions ‘εib ’ is to account for the number of oscillators. The Cauchy layer is represented by the glass slide and effective medium approximation (EMA) accounts for the presence of surface defects, roughness, etc. The modelling is done by employing theoretical models in the simplistic form with stacking of dielectric layers towards a polarized medium element presented to an external field to study the layered structures. In this paper, we present the optical characteristics of TiO2/Au-NPs/ TiO2 composite films deposited by RF and dc sputtering on glass substrates followed by annealing in air. The morphology of bare as deposited and annealed Au-NPs deposited on glass substrate was studied using atomic force microscopy (AFM). The UV–Vis absorption spectroscopy was employed to study the surface plasmon resonance modes of Au in the as grown and annealed composite films. Spectroscopic ellipsometry was employed in the range from 300 nm to 800 nm and many optical parameters, like s-and p-polarized reflectivity; complex dielectric function of the as grown and annealed composite films were determined. The ellipsometry data was fitted with various models, which included Drude Lorentz model for Au-film and Urbach tail modified Tauc Lorentz for TiO2 film. The results were best explained by taking into account the presence of hot carriers due to SPR and inter band transitions in Au. 2. Experiment The glass substrates were cleaned ultrasonically in acetone and IPA solutions for 20 min, respectively, rinsed with DI water after each step and dried with N2. RF and DC sputtering (DP650 by ALLIANCE) was used for the depositions of TiO2 and Au thin films, respectively. The temperature of the substrate was kept at 200 oC and the base and working pressure was maintained at 1.2 x 10−5 torr and 6.5 x 10−5 torr, respectively, for each deposition, while the power was 100 W and
30 W for the deposition of TiO2 and Au layers, respectively. The thickness of various layers was monitored by film thickness monitor, which showed the thicknesses of the deposited TiO2 and Au films were 20 nm and 2–4 nm, respectively. The thicknesses of various films were subsequently confirmed by the spectroscopic ellipsometry. Au was intentionally deposited in small amount to form Au-NPs, which would give large Au–TiO2 interface. The synthesized sandwich structure was subsequently annealed in air at 300 °C for 2 h in ambient conditions. The morphology of the bare Au-NPs film was studied using AGILENT’S PicoPlus atomic force microscope in the tapping mode in ambient conditions. Optical absorption spectroscopy was done by UV/Vis photospectrometer (UV-VIS BMS 2800) in transmission mode. Room temperature PL spectroscopy was carried out using LAB RAM III (by DongWoo Optron) with a laser emitting at 325 nm. The spectroscopic ellipsometry was done with SENTECH Instrument, GmbH (SE 850) in the range from 300 nm to 800 nm. The model fitting was done using SpectraRay to extract various optical parameters. 3. Results and discussion 3.1. Au film morphology The 2D and 3D images of the as grown and annealed bare Au film on glass substrates were acquired from the atomic force microscope are shown in Fig. 1 (a) and (b), respectively. The images showed that the Au was deposited in the particulate form. Gwyddion software was used to determine the density, size and average separation between the Au particles. Fig. 1 also shows the histogram of particle sizes. The average size of nanoparticles as extracted from the images was found to be 2.9 ± 0.5 nm and 4.5 ± 2 nm and particle density was 9.4 ± 0.6 x 1010 and 1.3 ± 0.3 x 1010 cm−2 in the as grown and annealed films, respectively. Thus annealing resulted in the formation of large Au-NPs with well-defined shapes with a broad size distribution. It is already known that the optical properties of layered structures are affected by the formation of Au islands or clusters on annealing [34]. The average inter-particle distance in the as grown and annealed Au-NPs film was found to be 8.5 ± 0.3 nm and 16.4 ± 0.2 nm, respectively. The coupling of Au-NPs in close proximity surrounded by a dielectric medium (TiO2) could also affect the plasmon resonance energy its line shape profile. 3.2. UV/Vis absorption and PL spectroscopy The UV/Vis absorption spectra shown in Fig. 2 are from the (a) as grown and (b) annealed Au-NPs film, TiO2 and TiO2/Au-NPs/TiO2 composite films, respectively. The absorption spectrum (black curve) of the as grown and annealed TiO2 films is also plotted for reference. The deposition of Au on TiO2 was confirmed from the absorption spectra of the simultaneously grown as grown and annealed Au/TiO2 bilayer films and is shown in the supporting information (SI_1). The absorption spectrum from the as grown Au-NPs showed a peak labeled as “A” at around 400 nm (∼3.1 eV) attributed to the interband transitions of Au, and a broad shoulder “B” at around 600 nm (∼2 eV) attributed to surface plasmon resonance (SPR) mode of the Au-NPs. The strong asymmetry in the SPR peak was due to irregular shapes and non-uniform distribution of sizes of Au-NPs [35]. The as grown TiO2 films showed no absorption peak in the spectral range of 500–800 nm. The as grown composite film showed enhanced absorption in the visible and two weak peaks “A” and “B” were also observed as marked in Fig. 2(a). The hump labeled “B” due to plasmon resonance mode was weak and attributed to irregular Au-NPs shapes [36]. Fig. 2 (b) shows a well-defined absorption spectral peak from the annealed Au-NPs films (red curve) due to plasmon resonance at 580 nm. There was no change in the absorption spectrum of the annealed TiO2 film. However, the absorption spectrum from the annealed composite film plotted in Fig. 2 (b) showed two bands, one around 450 nm and the
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Fig. 1. AFM images of bare (a) the as grown, and (b) annealed Au-NPs film on glass substrates.
covered with TiO2 thus creating a broader size distribution of Au-NPs in the composite film. The absorption spectrum of composite film was distinctively different than the pure TiO2 film and showed enhance absorption in the visible region. The energy band gap of the as grown and annealed TiO2 thin films was quite high ∼3.9 ± 0.1 eV as determined from Tauc's plots (details given in supporting information, SI_2). There have been very few reports, which have shown the band gap of TiO2 thin nanosheets and hierarchical nanostructures (NSs) as high as 3.8 eV [37,38]. However, the band gap of the annealed composite TiO2 films was reduced to 3.73 ± 0.07 eV. The decrease in the band gap was attributed to the creation of localized states near TiO2 band edges, a consequence of formation of Au/TiO2 interface. The extent of TiO2 band gap tail extended into the visible part of the spectra was determined using Urbach exponential expression: Fig. 2. UV/Vis absorption spectroscopy of (a) the as grown, and (b) annealed Au-NPs (red), TiO2 (black) and composite TiO2/Au/TiO2 (blue) thin films. Please notice the difference in the vertical scales in two Figures. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
second weak band at around 700 nm, already labeled as “A” and “B” in Fig. 2 (a). The absorption spectrum of the annealed composite films was much enhanced. The peak “B” due to SPR in the annealed composite film became much broader and was shifted towards the long wavelengths. This was possibly a manifestation of surrounding dielectric medium and interaction of hot electrons with TiO2. The observation of weak SPR mode was probably due to restricted coalescence of Au-NPs
α (ω) = α 0 exp ⎛⎜ ⎝
ℏω − Eo ⎞ ⎟ Eμ ⎠
(2)
where αo and Eo are the fundamental absorption coefficient and bandgap energy, respectively, and Eμ is the Urbach energy. Fitting of the absorption coefficient was done in the range of 3–4 eV with Equ. (2), the fitting results are given in supporting information SI_3. The absorption in this range is due to near band edge allowed transitions and participation of band edge tail states. The Urbach tail energies were 195 ± 5 and 255 ± 10 meV for the as grown and annealed composite thin films, respectively. Urbach energy was the measure of the localized density of states extended into the band gap primarily due to presence of defects, impurities, non-crystallinity.
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composite thin films from the amplitude and phase shift spectra. The amplitude ratio ‘Ψ’ and phase change ‘Δ’ is related to the s- and p-polarized light by:
ρ=
Rp Rs
= eiΔtanΨ
(3)
Where Rs and Rp are the Fresnel coefficients of the s- and p-polarized reflectivity components. The mean square error was minimized in each fitting by using regression algorithm as given:
⎧
1 MSE = ⎨ 2N − M ⎩
N
∑ I=1
2
2
Mod ⎡ ⎛ ψiMod − ψiExp ⎞ − ΔiExp ⎞ ⎛Δ +⎜ i ⎢⎜ Exp Exp ⎟ ⎟ σψ, i σΔ, i ⎢ ⎝ ⎠ ⎠ ⎣⎝
⎤⎫ ⎥ ⎬ ⎥ ⎦⎭
1 2
(4)
Where N is the total number of Ψ and Δ pairs, M is the number of total variables and σ is the standard deviation. The Mod and Exp refer to the theoretical and experimental data. The MSE for the glass substrates is usually less than 5 and in the present work the MSE for all samples was less than 1.5, except for annealed Au and TiO2/Au/TiO2 composite films. The thicknesses of various films were determined by using Cauchy's dispersion and Palik's model for TiO2 and Au-NPs layers, respectively, as given in Table 1 [39]. The thicknesses of Au and TiO2 films were found to be 20/2/30 nm in the as grown and 22/2.5/30 nm in the annealed Au sandwiched thin films. The error in the thicknesses was quite small (2/0.5/2), which resulted in small MSE in each fitting. The surface roughness and linear mixture in dielectric function were used to represent the inhomogeneities in Au and TiO2 films respectively, also shown in Table 1. For the as grown composite thin films the surface roughness of Au was too small (∼0.5 nm), which increased to 6 nm in the annealed composite films. The RMS value found using AFM was around 3 nm for annealed bare Au film, which accounted for the surface roughness in the structure due to spherical morphology of the nanoparticles. Meanwhile, linear mixture in dielectric function was used to represent different stoichiometric states of TiO2. In case when the dimensions are reduced, the dangling bonds at the surface may vary the oxidation state of Ti ions, like from Ti4+ to Ti3+. Resultantly, the interface may compose of mixture of TixOy phases. This is represented by a layer of linear mixture in dielectric function. The as grown TiO2 film possessed higher content to dielectric composition with thickness around 17 nm, but in the case of annealed composite film, it was reduced to mere 1 nm, which was attributed to decreased porosity of the film with effect to annealing [40]. The amplitude ratio ‘Ψ’ for the as grown and annealed Au, TiO2 and composite TiO2 thin films is plotted in Fig. 4 (a – c) in the spectral range of 300 nm–800 nm. The continuous and dashed lines represent the measured and fitted spectra of the as grown (black lines) and annealed (red lines) films, respectively. The coherent multiple reflections are already accounted during fitting of the experimental curve, where reflection from each component is combined using in built matrix formalisms (Jones and Mueller approach). The amplitude ratio of the reflected beam from the Au-NPs thin film (Fig. 4 (a)) shows a broad peak centered at 600 nm attributed to plasmon modes, which matched absorption spectra (Fig. 2 (a)). However, in the annealed Au film, two distinct peaks appeared at around 500 nm and 570 nm, which were tentatively ascribed to Au-NPs surface plasmon modes arising
Fig. 3. Photoluminescence spectra obtained from the as grown and annealed TiO2 (black and red) and Au embedded (blue and magenta) TiO2 thin films, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Photoluminescence spectroscopy of the as-grown and annealed TiO2 and Au sandwiched TiO2 thin films are shown in Fig. 3. The black and red curves represent TiO2, while Au embedded TiO2 are represented by blue and magenta curves for the as grown and annealed samples, respectively. A very interesting observation is the smearing of the band edge emission peak seen in pure TiO2 films to a much rounded/Gaussian peak in the composite films. This was a clear indication of introduction of tail states or near band edge defects in TiO2. Another observation is the emergence of a weak band in composite films at around 500 nm. There are three types of luminescence centers in TiO2 thin films and nanostructures: direct band to band allowed transitions in UV part (327 nm), oxygen defects at around 530 nm and Tin+ multivalent ionic states in IR range. All the three types of luminescent centers existed with large contribution from the direct band to band transitions and metal ionic states in the as-grown and annealed TiO2 samples. The Gaussian fitting of the Au embedded TiO2 thin films revealed that in annealed Au embedded films, the FWHM decreased to 45 meV from 52 meV in the as grown Au embedded film. The fitting parameters illustrated that involvement of sub-bandgap transitions is larger in the as grown Au embedded thin films as compared to their annealed counterpart and resonance of sub-bandgap states is more active in the as grown film. These findings were further probed by the spectroscopic ellipsometry measurements.
3.3. Spectroscopic ellipsometry Spectroscopic ellipsometry was employed to determine the thickness, reflectivity, and complex dielectric constants, of the Au, TiO2 and
Table 1 Modeled layered structures of thin films for spectroscopic ellipsometry for various thin films. Layer(s)
TiO2
Au
TiO2/Au-NPs/TiO2
Layered structure Models
Cauchy (Tauc Lorentz) Model (TiO2) Cauchy layer for absorption in Glass Glass SF11
EMA (surface roughness) Oscillator (Drude Lorentz Model) Cauchy layer for absorption in Glass Glass SF11
EMA (linear mixture in dielectric function) TiO2 (Tauc Lorentz Model) Au Oscollator (Drude Lorentz Model) EMA (surface roughness) TiO2 (Tauc Lorentz Model) Cauchy layer for absorption in Glass Glass SF11
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Fig. 4. Amplitude ratio plots of the as grown (black curves) and annealed (red curves) (a) Au-NPs, (b) TiO2 and (c) composite films. Phase change plots of the as grown (black solid curves) and annealed (red solid curves) (d) Au-NPs, (e) TiO2 and (f) composite films. The continuous and dashed lines are the experimental and fitted curves, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
due to a range of sizes (as seen in AFM histograms in Fig. 1 (b). The model however showed one broad peak centered at 530 nm. The appearance of two peaks was in close agreement with the absorption spectra, where a strong absorption was observed at 530 nm. In addition, fluctuations were also observed in the range 300–325 nm, which were ascribed to Au interband transitions, i.e., 6s-p conduction electrons to the holes in 5d-bands. Fig. 4 (b) shows the amplitude ratio spectrum from the TiO2 films, which showed a peak at around 325 nm attributed to band gap of TiO2 films. No significant effect of annealing was observed in TiO2 films. The spectrum of the amplitude ratio of the composite films structure was interesting and plotted in Fig. 4 (c), which showed a broad peak centered at 450 nm. The broad band was attributed to the plasmon modes of Au-NPs in the TiO2/Au-NPs/TiO2 composite films. The spectra were significantly different in the annealed composite film, where it showed strong fluctuation noise in the energy range from 300 nm to 425 nm. These fluctuations were tentatively ascribed to Fano type oscillations due to overlap of Au and TiO2 energy states [24], which overlapped with the band centered at 450 nm. The phase shift spectra of the three films plotted in Fig. 4(d–f) showed the same behavior, except the absorption in the composite film (Fig. 4 (f)) was much pronounced and was centered at 500 nm. For the as grown Au-NPs thin film in Fig. 4 (d), phase shift plot showed a dip first and then a peak in the range from 3.25 eV to 2.5 eV, which became much pronounced in the annealed Au-NPs film. As observed in amplitude ratio spectra, there was no significant feature or effect of annealing in the TiO2 films (Fig. 4 (e)). The phase shift in the as grown composite film showed a dip at around 340 nm and then a broad peak observed at 500 nm. However, in the annealed composite films, strong fluctuations in phase were observed below the TiO2 band gap and a strong broad peak centered at 500 nm was observed in the visible region (Fig. 4 (f)), which was ascribed to the presence of plasmon modes of Au-NPs. Thus, the incorporation of Au-NPs in the TiO2 films
resulted in strong absorption in the band gap of TiO2 due to surface plasmon modes of Au-NPs. 3.4. Optical parameters The effect of incorporation of Au-NPs layer on complex dielectric function was also studied by taking the polarization dependent reflectance spectra. The SPR mode is usually observed in the s-polarized wave and their propagating modes appear in the p-polarized wave. The optical parameters of the as grown and annealed Au-NPs, TiO2 and composite (TiO2/Au/TiO2) thin films were determined at an angle of 60°. The extracted dispersion profiles of the total reflectance ‘R’ as well as s- and p-polarized reflectivity Rs and Rp, and dielectric functions (real and imaginary) are plotted in Figs. 5 and 6, respectively. The total reflectance R and its s- (Rs) and p- (Rp) components from the as grown (solid curves) and annealed (dashed curves) Au-NPs film are plotted in Fig. 5 (a) (d) and (g), respectively. It was interesting to note that the total reflectance spectra from the as grown Au-NPs film showed signatures of both Rs and Rp as it showed two peaks, one at around 350 nm and other broad peak at around 680 nm. However, in the annealed film, the total reflectance spectra showed dominance of Rs at shorter wavelength up to 500 nm and then Rp dominated as it showed a sharp peak centered at 540 nm. In the case of TiO2 films (Fig. 5 (b), (e) and (h)), the total reflectance in the as grown film was dominated by Rp component of the reflectance, and Rs was dominant in the annealed TiO2 film. The effect of Au-NPs film incorporation in the as grown composite film was the most interesting to observe where Rs was large and Rp was small as shown in Fig. 5 (h) and (i), respectively. In the annealed composite film, Rs and Rp both increased and the total reflectance R showed contributions of both Rs and Rp (Fig. 5 (c)). The low reflectance intensity was tentatively attributed to the out of phase or incoherent oscillations, which were probably introduced due to scattering by the interface defects at the
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Fig. 5. (a–c) Reflectance spectra R, (d–f) Rs and (g–i) Rp of the as grown and annealed Au, TiO2 and sandwiched thin films.
Fig. 6. (a, c) Polarization density ‘Ɛ1’ and (b, d) polarizability ‘Ɛ2’ of the as grown, and annealed composite thin films.
Au/TiO2 interface. In the annealed films, improved coherence led to enhanced reflectance signal. The s- and p-polarized reflection curves can also be best explained by considering the variation in the sizes and orientation of of Au-NPs embedded as-grown and annealed composite films (shown in Supplementary Information Fig. 4). In bare Au-NPs, the broad total reflectance peaks as well as the enhanced s- and suppressed p-polarized reflectance was ascribed to the non-uniform sizes and shapes of the as-grown Au NPs. However, a broader and uniform size Au-NPs distribution resulted in a well-defined total reflected signal as well as enhanced parallel component of the reflection curve (Rp) in the annealed Au-NPs film. Similarly, in the annealed Au-NPs embedded TiO2 thin films, the increased reflectance along the s- and p-directions was due to the uniformly dispersed Au-NPs. However the effect of interface roughness scattering was observed in the reduced total reflectance. The dielectric function obtained from the amplitude and phase
change plots were then fitted with specific models for various layers that included Birchak law, Maxwell-Garnet and Lorentz & Lorenz formula, referred as equation (A-C) in SI_4. In these models Drude Lorentz and Urbach tail modified Tauc-Lorentz were employed to represent the Au-NPs and TiO2 layers, respectively [41]. The expression for Urbach tail modified Tauc Lorentz expression is provided in supplementary information SI_5. Birchak law gave the best optimized optical parameters with the structure factors (εeff =fa εa + fb εb ) fa = 0.05 for Au and fb = 0.95 for TiO2, respectively. The summary of parameters obtained from the fits of experimental curves are given in Table 2, where Apj and ωoj are the oscillator strengths and peak positions, while γj represents the damping of the respective oscillator, here ‘j’ refers to the number of oscillators. ‘Einf’ is the reference or fitting parameter added in real part to represent additional contributions at higher energies. For Tauc Lorentz model; A, E0 (described in text) and C represents the amplitude, Lorentz resonant frequency (peak energy in the joint density of states) and the broadening parameters, respectively. EU is the Urbach tail energy that comes because of disorder in the structure and is included as the width of the tail of localized states in the bandgap. Fig. 6(a–d) shows the experimental and fitted spectra along of the dielectric function for as grown and annealed composite thin films. Fig. 6 (a) and (b) are the real and Fig. 6 (c) and (d) are the imaginary parts of the dielectric function. In the as grown film, the spectrum is marked by a broad band from ∼340 nm to ∼550 nm. The band became a bit narrow and two sharp peaks superimposed at around 380 nm and 410 nm in the annealed composite film. Interband transitions can either dampen or prohibit the free electron resonant oscillations within the nanoparticles, therefore directly affect their optical properties. The interband transitions in Au occur in the range 1 eV–6 eV, with d subbands lying around 2 eV below the Fermi energy. It is observed that resonances in d-sub band to sp – conduction band occur in the range 2.6 eV–2.8 eV [42]. The energies of the interband transition basically exist at the Van Hove singularities present in the density of states near the symmetry points in the Brillouin zones, which depend on the shape of the electronic cloud and may vary with shape and size [43]. It is observed that for very small size of the nanoparticles interband resonant transitions are dominated. The as grown composite film showed a strong absorption approximately at 3.25 eV and a broad weak band
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Table 2 The parameters extracted from the fittings using Birchak law for the as grown and annealed composite thin film.
centered at 530 nm. The fitting parameters showed absorption was due to oscillator terms at 3.28 eV and 2.77 eV, respectively, which was obtained by the superposition of interband and intraband transitions of Au with TiO2. Their considerable oscillator strength indicated the resonance conditions that resulted in broad absorption band as shown in Fig. 6 (c). The annealed composite film displayed the strong peak split into a doublet at 3.25 and 3.10 eV and much stronger band centered at 530 nm in the ε2, observed in the fittings by multiple oscillator terms as many as three significant terms for the annealed composite film. UV/ Vis spectra have already shown improved free electron resonance effects, resulting in discrete interband absorption terms. The large values of Einf in the annealed sandwiched structure can be ascribed to highly localized interfacial electric field effects due to plasmon modes. Some values appeared very high due to the presence of critical points in joint density of states of TiO2 surrounding Au-NPs. It has been reported that optical properties of the Au-NPs are highly affected by the coupling between the interband transitions in Au and its plasmonic states [44,45]. The various levels and bands involved in the inter and intra band transitions in the Au/TiO2 hybrid system are shown in Fig. 6. The valence band states of pure TiO2 are represented by 2s and 2p states of O and 3d states of Ti [46,47]. The energy of O-2p states is around 5 eV, O-2s states is around 3.5 eV, and Ti-3d states is around 2.0 eV [46]. On the other hand, the inter band transitions in Au between 5d to 6sp are in the range from 3.05 eV to 4.32 eV and intraband transitions due to collective motion of conduction electrons or plasmons are in the range 1.0–2.5 eV as shown in Fig. 6 [45]. Resonance of interband transitions in Au are more readily achieved for Au-NPs as represented by two oscillator terms at 2.55 and 3.28 eV with a strength of 14% and 15%, respectively. In the annealed composite film, these energies are blue shifted significantly to 2.76 eV and 3.63 eV, respectively, with an oscillator strength of 14% and 5%. In the annealed composite film, another high energy term at 4.0 eV was introduced, which had a weak contribution of 2%. This was attributed to plasmon scattered hot electrons in the Au-NPs, which involved the O-2p states of Ti and the Au plasmonic/interband transition states corresponding to energies 3.0 eV and 2.5 eV as can be seen in Fig. 7. So far, the findings of Drude Lorentz model were discussed, the effect of Au incorporation was also observed from the value of Urbach tail energy of TiO2, which was extended into the visible part. The value of Urbach energy in the as grown was 110 ± 8 meV and it increased to 140 ± 10 meV in the annealed composite film, which accounted for the transfer of hot electrons from Au to TiO2. The difference in the values of Urbach energy determined from the ellipsometry and UV/Vis absorption spectroscopy was due to their distinct working principles [48] and polarization state on incident light. UV/Vis absorption records the sample at the normal incidence and with unpolarized light.
Fig. 7. A schematic of interband energy states involved in Au sandwiched TiO2 thin films.
Ellipsometry uses polarized light and is highly sensitive towards surface roughness, surface stoichiometry, various absorption/desorption effects taking place at the surfaces, etc.
4. Conclusion The optical properties of TiO2/Au-NPs/TiO2 composites films were explored using UV/Vis absorption, photoluminescence spectroscopy and spectroscopic ellipsometry. Incorporation of Au in TiO2 resulted in absorption in the band gap of TiO2. Spectroscopic ellipsometry was employed to obtain s- and p-polarized reflectance spectra, which were used to determine complex dielectric functions. Birchak law explained the variation of dielectric parameters of the as-grown and annealed composite film using Urbach Tail modified Tauc Lorentz model for TiO2 and Lorentz oscillator model for Au in the composite film. It showed the presence of as many as three oscillator terms and plasmon modes to explain the spectra in annealed composite films. The composite films showed strong absorption approximately around TiO2 band gap (3.25 eV and 3.10 eV) and a broad weak band centered at 530 nm. The models showed absorption was due to considerably strong oscillator terms at 3.28 eV and 2.77 eV, which was obtained by the superposition of interband and intraband transitions of Au with TiO2. The existence of interband gap energy states in TiO2 was determined from the width of the Urbach tail, which was due to interface states and participated in hot carrier transfer. The Urbach tail energy increased from 110 meV in the as-grown films to 140 meV in the annealed composite thin films. Thus, incorporation of Au-NPs in wide band gap semiconductors could become a key component of energy harvesting in the visible part of the spectrum.
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Acknowledgement [23]
FJ is funded by the Higher Education Commission of Pakistan for her PhD studies. The project is funded by the NRPU grants no. 261 and 1770.
[24]
[25]
Appendix A. Supplementary data
[26]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.07.262.
[27]
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Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Modified optical characteristics of TiO2/Au/TiO2 thin composite films a
b
b
a
Faiza Javed , Sofia Javed , Mohammad Mujahid , Fakhar ul Inam , Arshad Saleem Bhatti a b
T a,∗
Centre for Micro and Nano Devices (CMND), Department of Physics, COMSATS University Islamabad, Park Road, Islamabad, 44000, Pakistan School of Chemical and Materials Engineering (SCME), National University of Science and Technology, H-12, Islamabad, Pakistan
A R T I C LE I N FO
A B S T R A C T
Keywords: TiO2 ultrathin composite film Au nanoparticles Surface plasmon modes Spectroscopy ellipsometry Drude lorentz model Tauc lorentz model
We report the optical characteristics of the TiO2/Au-NPs/TiO2 composite thin films determined from the UV–Vis absorption spectroscopy and spectroscopic ellipsometry. The UV–Vis spectra indicated the enhanced optical absorption in the composite thin films in the range 1.6–3.0 eV. Spectroscopic ellipsometry was employed to determine the optical parameters of the composite films. The experimental results were fitted with hybrid model, which included Drude Lorentz model for Au layer and Urbach tail modified Tauc Lorentz model for TiO2 layers. The photoluminescence spectrum of the composite film clearly showed a weak emission band centered at 500 nm, which was absent in the pure TiO2 films. The extracted optical parameters manifested the generation of hot electrons at the Au/TiO2 interface due to Au surface plasmon (SP) modes, multiple interband transitions in Au in the energy range (1–4 eV) and a long Urbach tail (∼144 meV) in TiO2. These were direct indicators of hot electron scattering from the plasmon modes and their transfer to the TiO2 conduction band. The present study revealed that the plasmon exciton interactions in composite film can modify its optical properties.
1. Introduction There exists an immense interest in understanding the novel optical properties of metal/semiconductor nanostructured composites, which have potential applications in energy harvesting, manipulation and storage devices [1–4]. The presence of metallic nanoparticles induce various features in the host material, e.g., modulation of the band gap, enhanced absorption or emission and formation of charge trap centers [5–7]. TiO2 is a wide band gap semiconductor with large dielectric constant (∼70) and refractive index (∼3.2 @ 415 nm (3.1 eV)) It is considered as an ideal material for a number of applications [8,9], such as photocatalysis, photoconversion, energy storage and energy harvesting devices, etc. [10,11]. It has been demonstrated that incorporation of Au in TiO2 may lead to enhanced energy storage and harvesting capability. The metal nanoparticles (NPs) incorporated in a single or bilayer oxide thin films such as of ZnO, TiO2 and SnO2 have shown increased absorption effects in the visible part of the spectra [2,12,13]. Non-linear absorption and optical susceptibility (∼2 × 10−5 esu) for Au coated TiO2 dielectric matrix has been improved as compared to pure TiO2 or ZnS matrix [14]. A comparison of bare TiO2, Au coated TiO2 film as well as Au embedded TiO2 films for absorption and photochemical activity showed 35% enhanced absorption and photocatalytic activity in Au embedded TiO2 film [15]. Surface plasmon resonance (SPR) controlled absorption of TiO2 thin films has
∗
also been reported [16], which took into account the effects of dielectric media surrounding the Au-NPs on SPR peak position and its broadening. The SPR effects in TiO2 has also been studied by making periodic structures [17]. Theoretical studies of Au embedded TiO2 matrix have been performed to use it as a dielectric core in an optical fiber [18]. The Au–TiO2 composite grown using spray pyrolysis have been studied for phase modification [19]. The PL studies of Au embedded TiO2 revealed the plasmonic effects on the PL peak position and intensities [20]. Hybrid structures have also been used in other various applications like antibacterial activity, photocatalysis, photovoltaic fuel cells, for ethanol and methanol oxidation, etc. [21]. Thus, it is quite imperative to understand the electron dynamics and optical properties of hybrid or composite films formed by Au-layer and TiO2 film. Various techniques have been employed for the synthesis of TiO2–Au nanostructures, which are sol-gel, chemical vapor deposition, sputtering, atomic layer deposition (ALD) [22–25]. The embedded Au film or NPs in TiO2 introduces the defect states and reduces the band gap of the TiO2. In addition, intraband surface plasmon resonance and interband transition (5-d to 6-sp) in Au play a critical role by interacting with TiO2 band gap defect states. The plasmon scattering of electrons in Au-NPs results in the creation of hot electrons forming a non-thermalized distribution above the Fermi level and hence increases the probability of hot electrons transfer into TiO2 across the junction, thus affecting the density of conduction electrons in TiO2 by introducing new
Corresponding author. E-mail address: [email protected] (A.S. Bhatti).
https://doi.org/10.1016/j.ceramint.2019.07.262 Received 21 March 2019; Received in revised form 23 June 2019; Accepted 22 July 2019 Available online 24 July 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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energy states [1]. In addition, the decaying field of surface plasmon modes at the interface is reinforced in the presence of a high dielectric constant material, which leads to coupling of plasmon modes with excitons in the semiconductor [25,26]. The metal layer within a wide band gap semiconductor layered structures opens up new channels of absorption. This can be done by using monometal with mono-size or bimodal size distributions, which can create one or two absorption bands or using a bimetal layer configuration with two distinct plasmon modes. In addition, strong nonlinear optical effects can also be induced in the layered structures. For example, third order non-linear optical response and acousto-plasmonic sensing has been demonstrated by the selective excitation of resonant modes in Au–Pt bimetallic NPs embedded TiO2 composite thin film [27,28]. Spectroscopic ellipsometry is one of the best optical tools to extract optical parameters of thin films such as complex dielectric function. It is a sensitive technique and reveals the origin of optical processes. The ellipsometry data is usually fitted with various models for different layers to extract the optical parameters. For example, the Tauc-Lorentz model is used for amorphous and polycrystalline semiconductors and is based on single oscillator term for allowed transition in direct band gap semiconductors [29,30]. The metal layer (Au-layer) is best fitted with the Drude and Lorentz model as shown in Equ. (1), where the second term represents the contribution from free electrons and the third term gives the contribution from the inter band transitions. The Lorentz term is the sum of multiple oscillators, which takes into account all possible inter band transitions [31–33]:
ε=1+
ωp2 ω2
− iγω
5
+
∑
ω2 j = 1 o, j
ωp2, j − ω2 − iωγj
(1)
Where ‘ωo’ is the oscillation frequency, ‘ωp’ is the plasma frequency and ‘γ’ is the damping of the oscillator. ‘ε∞’ is the value of dielectric at high frequencies. Second term ‘εfree ’ donates the absorption due to free electron density oscillations. The summation in dielectric term due to interband transitions ‘εib ’ is to account for the number of oscillators. The Cauchy layer is represented by the glass slide and effective medium approximation (EMA) accounts for the presence of surface defects, roughness, etc. The modelling is done by employing theoretical models in the simplistic form with stacking of dielectric layers towards a polarized medium element presented to an external field to study the layered structures. In this paper, we present the optical characteristics of TiO2/Au-NPs/ TiO2 composite films deposited by RF and dc sputtering on glass substrates followed by annealing in air. The morphology of bare as deposited and annealed Au-NPs deposited on glass substrate was studied using atomic force microscopy (AFM). The UV–Vis absorption spectroscopy was employed to study the surface plasmon resonance modes of Au in the as grown and annealed composite films. Spectroscopic ellipsometry was employed in the range from 300 nm to 800 nm and many optical parameters, like s-and p-polarized reflectivity; complex dielectric function of the as grown and annealed composite films were determined. The ellipsometry data was fitted with various models, which included Drude Lorentz model for Au-film and Urbach tail modified Tauc Lorentz for TiO2 film. The results were best explained by taking into account the presence of hot carriers due to SPR and inter band transitions in Au. 2. Experiment The glass substrates were cleaned ultrasonically in acetone and IPA solutions for 20 min, respectively, rinsed with DI water after each step and dried with N2. RF and DC sputtering (DP650 by ALLIANCE) was used for the depositions of TiO2 and Au thin films, respectively. The temperature of the substrate was kept at 200 oC and the base and working pressure was maintained at 1.2 x 10−5 torr and 6.5 x 10−5 torr, respectively, for each deposition, while the power was 100 W and
30 W for the deposition of TiO2 and Au layers, respectively. The thickness of various layers was monitored by film thickness monitor, which showed the thicknesses of the deposited TiO2 and Au films were 20 nm and 2–4 nm, respectively. The thicknesses of various films were subsequently confirmed by the spectroscopic ellipsometry. Au was intentionally deposited in small amount to form Au-NPs, which would give large Au–TiO2 interface. The synthesized sandwich structure was subsequently annealed in air at 300 °C for 2 h in ambient conditions. The morphology of the bare Au-NPs film was studied using AGILENT’S PicoPlus atomic force microscope in the tapping mode in ambient conditions. Optical absorption spectroscopy was done by UV/Vis photospectrometer (UV-VIS BMS 2800) in transmission mode. Room temperature PL spectroscopy was carried out using LAB RAM III (by DongWoo Optron) with a laser emitting at 325 nm. The spectroscopic ellipsometry was done with SENTECH Instrument, GmbH (SE 850) in the range from 300 nm to 800 nm. The model fitting was done using SpectraRay to extract various optical parameters. 3. Results and discussion 3.1. Au film morphology The 2D and 3D images of the as grown and annealed bare Au film on glass substrates were acquired from the atomic force microscope are shown in Fig. 1 (a) and (b), respectively. The images showed that the Au was deposited in the particulate form. Gwyddion software was used to determine the density, size and average separation between the Au particles. Fig. 1 also shows the histogram of particle sizes. The average size of nanoparticles as extracted from the images was found to be 2.9 ± 0.5 nm and 4.5 ± 2 nm and particle density was 9.4 ± 0.6 x 1010 and 1.3 ± 0.3 x 1010 cm−2 in the as grown and annealed films, respectively. Thus annealing resulted in the formation of large Au-NPs with well-defined shapes with a broad size distribution. It is already known that the optical properties of layered structures are affected by the formation of Au islands or clusters on annealing [34]. The average inter-particle distance in the as grown and annealed Au-NPs film was found to be 8.5 ± 0.3 nm and 16.4 ± 0.2 nm, respectively. The coupling of Au-NPs in close proximity surrounded by a dielectric medium (TiO2) could also affect the plasmon resonance energy its line shape profile. 3.2. UV/Vis absorption and PL spectroscopy The UV/Vis absorption spectra shown in Fig. 2 are from the (a) as grown and (b) annealed Au-NPs film, TiO2 and TiO2/Au-NPs/TiO2 composite films, respectively. The absorption spectrum (black curve) of the as grown and annealed TiO2 films is also plotted for reference. The deposition of Au on TiO2 was confirmed from the absorption spectra of the simultaneously grown as grown and annealed Au/TiO2 bilayer films and is shown in the supporting information (SI_1). The absorption spectrum from the as grown Au-NPs showed a peak labeled as “A” at around 400 nm (∼3.1 eV) attributed to the interband transitions of Au, and a broad shoulder “B” at around 600 nm (∼2 eV) attributed to surface plasmon resonance (SPR) mode of the Au-NPs. The strong asymmetry in the SPR peak was due to irregular shapes and non-uniform distribution of sizes of Au-NPs [35]. The as grown TiO2 films showed no absorption peak in the spectral range of 500–800 nm. The as grown composite film showed enhanced absorption in the visible and two weak peaks “A” and “B” were also observed as marked in Fig. 2(a). The hump labeled “B” due to plasmon resonance mode was weak and attributed to irregular Au-NPs shapes [36]. Fig. 2 (b) shows a well-defined absorption spectral peak from the annealed Au-NPs films (red curve) due to plasmon resonance at 580 nm. There was no change in the absorption spectrum of the annealed TiO2 film. However, the absorption spectrum from the annealed composite film plotted in Fig. 2 (b) showed two bands, one around 450 nm and the
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Fig. 1. AFM images of bare (a) the as grown, and (b) annealed Au-NPs film on glass substrates.
covered with TiO2 thus creating a broader size distribution of Au-NPs in the composite film. The absorption spectrum of composite film was distinctively different than the pure TiO2 film and showed enhance absorption in the visible region. The energy band gap of the as grown and annealed TiO2 thin films was quite high ∼3.9 ± 0.1 eV as determined from Tauc's plots (details given in supporting information, SI_2). There have been very few reports, which have shown the band gap of TiO2 thin nanosheets and hierarchical nanostructures (NSs) as high as 3.8 eV [37,38]. However, the band gap of the annealed composite TiO2 films was reduced to 3.73 ± 0.07 eV. The decrease in the band gap was attributed to the creation of localized states near TiO2 band edges, a consequence of formation of Au/TiO2 interface. The extent of TiO2 band gap tail extended into the visible part of the spectra was determined using Urbach exponential expression: Fig. 2. UV/Vis absorption spectroscopy of (a) the as grown, and (b) annealed Au-NPs (red), TiO2 (black) and composite TiO2/Au/TiO2 (blue) thin films. Please notice the difference in the vertical scales in two Figures. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
second weak band at around 700 nm, already labeled as “A” and “B” in Fig. 2 (a). The absorption spectrum of the annealed composite films was much enhanced. The peak “B” due to SPR in the annealed composite film became much broader and was shifted towards the long wavelengths. This was possibly a manifestation of surrounding dielectric medium and interaction of hot electrons with TiO2. The observation of weak SPR mode was probably due to restricted coalescence of Au-NPs
α (ω) = α 0 exp ⎛⎜ ⎝
ℏω − Eo ⎞ ⎟ Eμ ⎠
(2)
where αo and Eo are the fundamental absorption coefficient and bandgap energy, respectively, and Eμ is the Urbach energy. Fitting of the absorption coefficient was done in the range of 3–4 eV with Equ. (2), the fitting results are given in supporting information SI_3. The absorption in this range is due to near band edge allowed transitions and participation of band edge tail states. The Urbach tail energies were 195 ± 5 and 255 ± 10 meV for the as grown and annealed composite thin films, respectively. Urbach energy was the measure of the localized density of states extended into the band gap primarily due to presence of defects, impurities, non-crystallinity.
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composite thin films from the amplitude and phase shift spectra. The amplitude ratio ‘Ψ’ and phase change ‘Δ’ is related to the s- and p-polarized light by:
ρ=
Rp Rs
= eiΔtanΨ
(3)
Where Rs and Rp are the Fresnel coefficients of the s- and p-polarized reflectivity components. The mean square error was minimized in each fitting by using regression algorithm as given:
⎧
1 MSE = ⎨ 2N − M ⎩
N
∑ I=1
2
2
Mod ⎡ ⎛ ψiMod − ψiExp ⎞ − ΔiExp ⎞ ⎛Δ +⎜ i ⎢⎜ Exp Exp ⎟ ⎟ σψ, i σΔ, i ⎢ ⎝ ⎠ ⎠ ⎣⎝
⎤⎫ ⎥ ⎬ ⎥ ⎦⎭
1 2
(4)
Where N is the total number of Ψ and Δ pairs, M is the number of total variables and σ is the standard deviation. The Mod and Exp refer to the theoretical and experimental data. The MSE for the glass substrates is usually less than 5 and in the present work the MSE for all samples was less than 1.5, except for annealed Au and TiO2/Au/TiO2 composite films. The thicknesses of various films were determined by using Cauchy's dispersion and Palik's model for TiO2 and Au-NPs layers, respectively, as given in Table 1 [39]. The thicknesses of Au and TiO2 films were found to be 20/2/30 nm in the as grown and 22/2.5/30 nm in the annealed Au sandwiched thin films. The error in the thicknesses was quite small (2/0.5/2), which resulted in small MSE in each fitting. The surface roughness and linear mixture in dielectric function were used to represent the inhomogeneities in Au and TiO2 films respectively, also shown in Table 1. For the as grown composite thin films the surface roughness of Au was too small (∼0.5 nm), which increased to 6 nm in the annealed composite films. The RMS value found using AFM was around 3 nm for annealed bare Au film, which accounted for the surface roughness in the structure due to spherical morphology of the nanoparticles. Meanwhile, linear mixture in dielectric function was used to represent different stoichiometric states of TiO2. In case when the dimensions are reduced, the dangling bonds at the surface may vary the oxidation state of Ti ions, like from Ti4+ to Ti3+. Resultantly, the interface may compose of mixture of TixOy phases. This is represented by a layer of linear mixture in dielectric function. The as grown TiO2 film possessed higher content to dielectric composition with thickness around 17 nm, but in the case of annealed composite film, it was reduced to mere 1 nm, which was attributed to decreased porosity of the film with effect to annealing [40]. The amplitude ratio ‘Ψ’ for the as grown and annealed Au, TiO2 and composite TiO2 thin films is plotted in Fig. 4 (a – c) in the spectral range of 300 nm–800 nm. The continuous and dashed lines represent the measured and fitted spectra of the as grown (black lines) and annealed (red lines) films, respectively. The coherent multiple reflections are already accounted during fitting of the experimental curve, where reflection from each component is combined using in built matrix formalisms (Jones and Mueller approach). The amplitude ratio of the reflected beam from the Au-NPs thin film (Fig. 4 (a)) shows a broad peak centered at 600 nm attributed to plasmon modes, which matched absorption spectra (Fig. 2 (a)). However, in the annealed Au film, two distinct peaks appeared at around 500 nm and 570 nm, which were tentatively ascribed to Au-NPs surface plasmon modes arising
Fig. 3. Photoluminescence spectra obtained from the as grown and annealed TiO2 (black and red) and Au embedded (blue and magenta) TiO2 thin films, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Photoluminescence spectroscopy of the as-grown and annealed TiO2 and Au sandwiched TiO2 thin films are shown in Fig. 3. The black and red curves represent TiO2, while Au embedded TiO2 are represented by blue and magenta curves for the as grown and annealed samples, respectively. A very interesting observation is the smearing of the band edge emission peak seen in pure TiO2 films to a much rounded/Gaussian peak in the composite films. This was a clear indication of introduction of tail states or near band edge defects in TiO2. Another observation is the emergence of a weak band in composite films at around 500 nm. There are three types of luminescence centers in TiO2 thin films and nanostructures: direct band to band allowed transitions in UV part (327 nm), oxygen defects at around 530 nm and Tin+ multivalent ionic states in IR range. All the three types of luminescent centers existed with large contribution from the direct band to band transitions and metal ionic states in the as-grown and annealed TiO2 samples. The Gaussian fitting of the Au embedded TiO2 thin films revealed that in annealed Au embedded films, the FWHM decreased to 45 meV from 52 meV in the as grown Au embedded film. The fitting parameters illustrated that involvement of sub-bandgap transitions is larger in the as grown Au embedded thin films as compared to their annealed counterpart and resonance of sub-bandgap states is more active in the as grown film. These findings were further probed by the spectroscopic ellipsometry measurements.
3.3. Spectroscopic ellipsometry Spectroscopic ellipsometry was employed to determine the thickness, reflectivity, and complex dielectric constants, of the Au, TiO2 and
Table 1 Modeled layered structures of thin films for spectroscopic ellipsometry for various thin films. Layer(s)
TiO2
Au
TiO2/Au-NPs/TiO2
Layered structure Models
Cauchy (Tauc Lorentz) Model (TiO2) Cauchy layer for absorption in Glass Glass SF11
EMA (surface roughness) Oscillator (Drude Lorentz Model) Cauchy layer for absorption in Glass Glass SF11
EMA (linear mixture in dielectric function) TiO2 (Tauc Lorentz Model) Au Oscollator (Drude Lorentz Model) EMA (surface roughness) TiO2 (Tauc Lorentz Model) Cauchy layer for absorption in Glass Glass SF11
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Fig. 4. Amplitude ratio plots of the as grown (black curves) and annealed (red curves) (a) Au-NPs, (b) TiO2 and (c) composite films. Phase change plots of the as grown (black solid curves) and annealed (red solid curves) (d) Au-NPs, (e) TiO2 and (f) composite films. The continuous and dashed lines are the experimental and fitted curves, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
due to a range of sizes (as seen in AFM histograms in Fig. 1 (b). The model however showed one broad peak centered at 530 nm. The appearance of two peaks was in close agreement with the absorption spectra, where a strong absorption was observed at 530 nm. In addition, fluctuations were also observed in the range 300–325 nm, which were ascribed to Au interband transitions, i.e., 6s-p conduction electrons to the holes in 5d-bands. Fig. 4 (b) shows the amplitude ratio spectrum from the TiO2 films, which showed a peak at around 325 nm attributed to band gap of TiO2 films. No significant effect of annealing was observed in TiO2 films. The spectrum of the amplitude ratio of the composite films structure was interesting and plotted in Fig. 4 (c), which showed a broad peak centered at 450 nm. The broad band was attributed to the plasmon modes of Au-NPs in the TiO2/Au-NPs/TiO2 composite films. The spectra were significantly different in the annealed composite film, where it showed strong fluctuation noise in the energy range from 300 nm to 425 nm. These fluctuations were tentatively ascribed to Fano type oscillations due to overlap of Au and TiO2 energy states [24], which overlapped with the band centered at 450 nm. The phase shift spectra of the three films plotted in Fig. 4(d–f) showed the same behavior, except the absorption in the composite film (Fig. 4 (f)) was much pronounced and was centered at 500 nm. For the as grown Au-NPs thin film in Fig. 4 (d), phase shift plot showed a dip first and then a peak in the range from 3.25 eV to 2.5 eV, which became much pronounced in the annealed Au-NPs film. As observed in amplitude ratio spectra, there was no significant feature or effect of annealing in the TiO2 films (Fig. 4 (e)). The phase shift in the as grown composite film showed a dip at around 340 nm and then a broad peak observed at 500 nm. However, in the annealed composite films, strong fluctuations in phase were observed below the TiO2 band gap and a strong broad peak centered at 500 nm was observed in the visible region (Fig. 4 (f)), which was ascribed to the presence of plasmon modes of Au-NPs. Thus, the incorporation of Au-NPs in the TiO2 films
resulted in strong absorption in the band gap of TiO2 due to surface plasmon modes of Au-NPs. 3.4. Optical parameters The effect of incorporation of Au-NPs layer on complex dielectric function was also studied by taking the polarization dependent reflectance spectra. The SPR mode is usually observed in the s-polarized wave and their propagating modes appear in the p-polarized wave. The optical parameters of the as grown and annealed Au-NPs, TiO2 and composite (TiO2/Au/TiO2) thin films were determined at an angle of 60°. The extracted dispersion profiles of the total reflectance ‘R’ as well as s- and p-polarized reflectivity Rs and Rp, and dielectric functions (real and imaginary) are plotted in Figs. 5 and 6, respectively. The total reflectance R and its s- (Rs) and p- (Rp) components from the as grown (solid curves) and annealed (dashed curves) Au-NPs film are plotted in Fig. 5 (a) (d) and (g), respectively. It was interesting to note that the total reflectance spectra from the as grown Au-NPs film showed signatures of both Rs and Rp as it showed two peaks, one at around 350 nm and other broad peak at around 680 nm. However, in the annealed film, the total reflectance spectra showed dominance of Rs at shorter wavelength up to 500 nm and then Rp dominated as it showed a sharp peak centered at 540 nm. In the case of TiO2 films (Fig. 5 (b), (e) and (h)), the total reflectance in the as grown film was dominated by Rp component of the reflectance, and Rs was dominant in the annealed TiO2 film. The effect of Au-NPs film incorporation in the as grown composite film was the most interesting to observe where Rs was large and Rp was small as shown in Fig. 5 (h) and (i), respectively. In the annealed composite film, Rs and Rp both increased and the total reflectance R showed contributions of both Rs and Rp (Fig. 5 (c)). The low reflectance intensity was tentatively attributed to the out of phase or incoherent oscillations, which were probably introduced due to scattering by the interface defects at the
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Fig. 5. (a–c) Reflectance spectra R, (d–f) Rs and (g–i) Rp of the as grown and annealed Au, TiO2 and sandwiched thin films.
Fig. 6. (a, c) Polarization density ‘Ɛ1’ and (b, d) polarizability ‘Ɛ2’ of the as grown, and annealed composite thin films.
Au/TiO2 interface. In the annealed films, improved coherence led to enhanced reflectance signal. The s- and p-polarized reflection curves can also be best explained by considering the variation in the sizes and orientation of of Au-NPs embedded as-grown and annealed composite films (shown in Supplementary Information Fig. 4). In bare Au-NPs, the broad total reflectance peaks as well as the enhanced s- and suppressed p-polarized reflectance was ascribed to the non-uniform sizes and shapes of the as-grown Au NPs. However, a broader and uniform size Au-NPs distribution resulted in a well-defined total reflected signal as well as enhanced parallel component of the reflection curve (Rp) in the annealed Au-NPs film. Similarly, in the annealed Au-NPs embedded TiO2 thin films, the increased reflectance along the s- and p-directions was due to the uniformly dispersed Au-NPs. However the effect of interface roughness scattering was observed in the reduced total reflectance. The dielectric function obtained from the amplitude and phase
change plots were then fitted with specific models for various layers that included Birchak law, Maxwell-Garnet and Lorentz & Lorenz formula, referred as equation (A-C) in SI_4. In these models Drude Lorentz and Urbach tail modified Tauc-Lorentz were employed to represent the Au-NPs and TiO2 layers, respectively [41]. The expression for Urbach tail modified Tauc Lorentz expression is provided in supplementary information SI_5. Birchak law gave the best optimized optical parameters with the structure factors (εeff =fa εa + fb εb ) fa = 0.05 for Au and fb = 0.95 for TiO2, respectively. The summary of parameters obtained from the fits of experimental curves are given in Table 2, where Apj and ωoj are the oscillator strengths and peak positions, while γj represents the damping of the respective oscillator, here ‘j’ refers to the number of oscillators. ‘Einf’ is the reference or fitting parameter added in real part to represent additional contributions at higher energies. For Tauc Lorentz model; A, E0 (described in text) and C represents the amplitude, Lorentz resonant frequency (peak energy in the joint density of states) and the broadening parameters, respectively. EU is the Urbach tail energy that comes because of disorder in the structure and is included as the width of the tail of localized states in the bandgap. Fig. 6(a–d) shows the experimental and fitted spectra along of the dielectric function for as grown and annealed composite thin films. Fig. 6 (a) and (b) are the real and Fig. 6 (c) and (d) are the imaginary parts of the dielectric function. In the as grown film, the spectrum is marked by a broad band from ∼340 nm to ∼550 nm. The band became a bit narrow and two sharp peaks superimposed at around 380 nm and 410 nm in the annealed composite film. Interband transitions can either dampen or prohibit the free electron resonant oscillations within the nanoparticles, therefore directly affect their optical properties. The interband transitions in Au occur in the range 1 eV–6 eV, with d subbands lying around 2 eV below the Fermi energy. It is observed that resonances in d-sub band to sp – conduction band occur in the range 2.6 eV–2.8 eV [42]. The energies of the interband transition basically exist at the Van Hove singularities present in the density of states near the symmetry points in the Brillouin zones, which depend on the shape of the electronic cloud and may vary with shape and size [43]. It is observed that for very small size of the nanoparticles interband resonant transitions are dominated. The as grown composite film showed a strong absorption approximately at 3.25 eV and a broad weak band
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Table 2 The parameters extracted from the fittings using Birchak law for the as grown and annealed composite thin film.
centered at 530 nm. The fitting parameters showed absorption was due to oscillator terms at 3.28 eV and 2.77 eV, respectively, which was obtained by the superposition of interband and intraband transitions of Au with TiO2. Their considerable oscillator strength indicated the resonance conditions that resulted in broad absorption band as shown in Fig. 6 (c). The annealed composite film displayed the strong peak split into a doublet at 3.25 and 3.10 eV and much stronger band centered at 530 nm in the ε2, observed in the fittings by multiple oscillator terms as many as three significant terms for the annealed composite film. UV/ Vis spectra have already shown improved free electron resonance effects, resulting in discrete interband absorption terms. The large values of Einf in the annealed sandwiched structure can be ascribed to highly localized interfacial electric field effects due to plasmon modes. Some values appeared very high due to the presence of critical points in joint density of states of TiO2 surrounding Au-NPs. It has been reported that optical properties of the Au-NPs are highly affected by the coupling between the interband transitions in Au and its plasmonic states [44,45]. The various levels and bands involved in the inter and intra band transitions in the Au/TiO2 hybrid system are shown in Fig. 6. The valence band states of pure TiO2 are represented by 2s and 2p states of O and 3d states of Ti [46,47]. The energy of O-2p states is around 5 eV, O-2s states is around 3.5 eV, and Ti-3d states is around 2.0 eV [46]. On the other hand, the inter band transitions in Au between 5d to 6sp are in the range from 3.05 eV to 4.32 eV and intraband transitions due to collective motion of conduction electrons or plasmons are in the range 1.0–2.5 eV as shown in Fig. 6 [45]. Resonance of interband transitions in Au are more readily achieved for Au-NPs as represented by two oscillator terms at 2.55 and 3.28 eV with a strength of 14% and 15%, respectively. In the annealed composite film, these energies are blue shifted significantly to 2.76 eV and 3.63 eV, respectively, with an oscillator strength of 14% and 5%. In the annealed composite film, another high energy term at 4.0 eV was introduced, which had a weak contribution of 2%. This was attributed to plasmon scattered hot electrons in the Au-NPs, which involved the O-2p states of Ti and the Au plasmonic/interband transition states corresponding to energies 3.0 eV and 2.5 eV as can be seen in Fig. 7. So far, the findings of Drude Lorentz model were discussed, the effect of Au incorporation was also observed from the value of Urbach tail energy of TiO2, which was extended into the visible part. The value of Urbach energy in the as grown was 110 ± 8 meV and it increased to 140 ± 10 meV in the annealed composite film, which accounted for the transfer of hot electrons from Au to TiO2. The difference in the values of Urbach energy determined from the ellipsometry and UV/Vis absorption spectroscopy was due to their distinct working principles [48] and polarization state on incident light. UV/Vis absorption records the sample at the normal incidence and with unpolarized light.
Fig. 7. A schematic of interband energy states involved in Au sandwiched TiO2 thin films.
Ellipsometry uses polarized light and is highly sensitive towards surface roughness, surface stoichiometry, various absorption/desorption effects taking place at the surfaces, etc.
4. Conclusion The optical properties of TiO2/Au-NPs/TiO2 composites films were explored using UV/Vis absorption, photoluminescence spectroscopy and spectroscopic ellipsometry. Incorporation of Au in TiO2 resulted in absorption in the band gap of TiO2. Spectroscopic ellipsometry was employed to obtain s- and p-polarized reflectance spectra, which were used to determine complex dielectric functions. Birchak law explained the variation of dielectric parameters of the as-grown and annealed composite film using Urbach Tail modified Tauc Lorentz model for TiO2 and Lorentz oscillator model for Au in the composite film. It showed the presence of as many as three oscillator terms and plasmon modes to explain the spectra in annealed composite films. The composite films showed strong absorption approximately around TiO2 band gap (3.25 eV and 3.10 eV) and a broad weak band centered at 530 nm. The models showed absorption was due to considerably strong oscillator terms at 3.28 eV and 2.77 eV, which was obtained by the superposition of interband and intraband transitions of Au with TiO2. The existence of interband gap energy states in TiO2 was determined from the width of the Urbach tail, which was due to interface states and participated in hot carrier transfer. The Urbach tail energy increased from 110 meV in the as-grown films to 140 meV in the annealed composite thin films. Thus, incorporation of Au-NPs in wide band gap semiconductors could become a key component of energy harvesting in the visible part of the spectrum.
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Acknowledgement [23]
FJ is funded by the Higher Education Commission of Pakistan for her PhD studies. The project is funded by the NRPU grants no. 261 and 1770.
[24]
[25]
Appendix A. Supplementary data
[26]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.07.262.
[27]
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