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Detailed optical analysis of 100 MeV Ni7+ ion irradiated WO3 thin films using Surface Plasmon Resonance
Author’s Accepted Manuscript Detailed optical analysis of 100MeV Ni7+ ion irradiated WO3 thin films using Surface Plasmon resonance Savita Sharma, Ayushi Paliwal, Monika Tomar, Fouran Singh, Nitin K. Puri, Vinay Gupta www.elsevier.com/locate/radphyschem
PII: DOI: Reference:
S0969-806X(18)30542-5 https://doi.org/10.1016/j.radphyschem.2018.09.004 RPC7994
To appear in: Radiation Physics and Chemistry Received date: 8 June 2018 Revised date: 4 August 2018 Accepted date: 3 September 2018 Cite this article as: Savita Sharma, Ayushi Paliwal, Monika Tomar, Fouran Singh, Nitin K. Puri and Vinay Gupta, Detailed optical analysis of 100MeV Ni 7+ ion irradiated WO3 thin films using Surface Plasmon resonance, Radiation Physics and Chemistry, https://doi.org/10.1016/j.radphyschem.2018.09.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Detailed optical analysis of 100 MeV Ni7+ ion irradiated WO3 thin films using Surface Plasmon resonance Savita Sharma1,4, Ayushi Paliwal1, Monika Tomar2, Fouran Singh3, Nitin K. Puri4, Vinay Gupta1* 1
Department of Physics and Astrophysics, University of Delhi, Delhi 110007, India 2
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4
*
Physics Department, Miranda House, University of Delhi, Delhi 110007, India
Inter University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi, 110075, India
Department of Applied Physics, Delhi Technological University, Delhi 110042, India
Corresponding author E-mail address: [email protected]; Tel: +919811563101
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Abstract Modifications in the structural, optical properties and surface morphology of RF sputtered WO3 thin films after irradiation with 100 MeV Ni7+ ions have been reported in the present article. The WO3 thin films were deposited at different sputtering pressures (10 to 50 mTorr). Despite the grain growth due to irradiation, no structural phase transformation has been observed for the WO3 thin films deposited up to 30 mTorr growth pressure. However, complete amorphization post irradiation was detected for the orthorhombic WO3 thin film grown at at 50 mTorr sputtering pressure. AFM images indicated that the pristine WO3 thin film consists of uniform grains having smooth surface morphology. Upon irradiation, grain size increased consistently along with increase in surface roughness (Rq) at all sputtering pressures. The root mean square roughness (Rq) estimated from AFM analysis increased from 1.76 nm in pristine film to 9.89 nm in response to irradiation at a fluence of 1 × 1012 ions cm−2. A systematic decrease in optical band gap has been observed with irradiation, which can be correlated with Ni7+ ion induced midgap and near band edge defect states. Surface Plasmon Resonance (SPR) technique has been recognized as an appropriate tool to study the optical properties of pristine and irradiated WO3 thin films. SPR reflectance curves were recorded in angular interrogation mode at 633nm excitation wavelength for pristine and Ni7+ ions irradiated WO3 films deposited at varying deposition pressure.
Keywords : Irradiation, sputtering, band gap, fluence, Thin film.
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1. Introduction Swift Heavy Ion (SHI) irradiations which are characterized by ion energies above 1 MeVamu−1 range have been of great importance in studying material modifications. Several interesting and important properties of matter in the solid state are related to the presence of defects and impurities [1-4]. The imperfections in solids are exploited for employing them in various applications. For technological applications, the defects can be introduced either by varying the growth parameters (such as RF power, substrate temperature, sputtering pressure etc.) or by purposely doping the host material during the fabrication process. The defects can also be introduced in the crystalline solids by means of ion irradiation. In studying modification by SHI irradiation, there are two important parameters i.e. the electronic energy losses (Se) and nuclear energy losses (Sn) which occur due to inelastic collisions and elastic collisions respectively. The electronic energy loss (Se) is significantly higher as compared to Sn which is further responsible for producing significant excitation of the lattice in a very short interval of time (~ picoseconds). This causes the introduction of defect states in the lattice which give rise to elemental deviations in the electronic structure and material properties altogether. There are a number of reports on the study of oxide systems like ZnO, SiO2, CeO2 and Fe2O3, however only a few reports are available on the effect of ion beam-introduced modifications of tungsten trioxide (WO3) thin films [5-6]. WO3 illustrates intriguing optical properties, making it a useful candidate for applications in mirrors, solid-state micro-batteries, catalysts and filters [7-9]. WO3 is an important candidate for different applications including optical smart windows, flat panel displays, gas sensors etc [10-14]. Modifications developed in the surface morphology of materials by irradiation may be useful for many device applications [15-17]. In the case of WO3 films, most of the research work has been
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concentrated on irradiation induced enhanced electrical conductivity [18-21]. Further, Sivakumar et al reported the MeV N+ ion beam irradiation modification of WO3 thin films. This induce grain growth without any structural phase transition and also band gap reduces [22]. Structural and optical studies of ion irradiated WO3 thin films will be immensely beneficial in development of WO3 thin film based devices for various applications in outer space surrounded by various harmful radiations. Optical studies have shown momentous impact on the advancement of the infrastructure of optical communication system, information technology and also for the development of optical devices like solar cell, televisions, microscopes, LEDs, LASERS etc [23-24]. Surface Plasmon Resonance (SPR) is considered to be the most sensitive tool to explore the optical features of any material amongst all the available optical techniques. Also, it offers a flexibility to perform the optical measurements under varying ambient conditions. This is also advantageous in terms of sample preparation and data analysis. An electromagnetic wave incident on metal-dielectric interface having dielectric constants of opposite signs supports Surface plasmon waves (SPWs). Thus, SPWs can be utilized to study the metal-dielectric interfaces and metal surfaces in any ambient condition [25]. There are two configurations i.e. Kretschmann configuration and Otto configuration which are used to observed SPR phenomena. The major advantage of Otto configuration over Kretschmann configuration is that the glass prism is an absolute element for optical coupling. Another advantage is that there is no thickness constraint in case of Otto configuration. SPR spectrum exhibits a dip in reflectance corresponding to the energy transfer from the incident light to the SPW and its subsequent dissipation in the metal film. The SPR resonance condition for surface plasmon can be achieved in different modes in thin films which include (1) angular interrogation mode, (2) wavelength interrogation mode and (3) phase
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interrogation mode. The most common mode used to study the SPR reflectance curves is the angular interrogation mode where incident angle is varied in both Kretschmann and Otto configurations [25]. This paper presents the study of the modifications induced in tungsten oxide (WO3) thin films by swift heavy ion irradiation (SHI). The WO3 thin films were prepared using Rf magnetron sputtering at different growth pressures ranging from 10 to 50 mTorr at a fixed deposition temperature of 500oC. The pristine WO3 thin films were subjected to 100 MeV Ni7+ ion irradiation at a fluence of about 1×1012 ions cm−2. Variation in the film crystallinity, surface morphology and optical band gap of WO3 samples deposited at different sputtering pressures (10 mTorr to 50 mTorr) has been studied before and after 100 MeV Ni7+ irradiation. Furthermore, investigation of optical properties of both pristine and irradiated WO3 thin films was done using SPR technique. 2. Experimental details In the present work, rf-magnetron sputtering has been exploited for the deposition of WO3 thin films. Pure tungsten metal target (2” dia.) was sputtered at 40 W power in an Ar to O2 gas ratio of 60:40. The films were deposited on cleaned Corning glass substrate of 2 cm by 1 cm size at 500oC substrate temperature. The irradiation of the prepared WO3 thin films were performed at the Inter University Accelerator Centre (IUAC) in New Delhi, using Ni ions at a fixed fluence of 1 × 1012 ions cm−2 produced by 15 UD Pelletron accelerator. Deposition pressure is known to affect the film porosity and surface morphology to a great extent and hence the sputtering pressure was varied between 10 and 50 mTorr (the base pressure was approximately 6 × 10-6 Torr) keeping the composition of gas same (Ar :O2 = 60:40). Structural studies were observed using X-Ray Diffraction (XRD) technique (Make: Bruker D-8 X-Ray diffractometer). The
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optical properties of the samples were analyzed using UV-Visible (UV-Vis) spectroscopy (Perkin Elmer lambda-35 spectrophotometer). WO3 film thickness was measured using a surface profiler (DEKTAK 150). Surface morphology and topography were analyzed using contact mode Atomic force microscopy (AFM) technique (Bruker Dimension Icon). For SPR measurements, thickness of WO3 thin films prepared at different pressures has been fixed at 200 nm and Otto configuration is used to record their SPR reflectance curves. Thermal evaporation technique has been exploited for depositing Au thin film in order to excite the surface plasmons, where the film thickness is measured using an automated thickness monitor. The SPR measurement setup has been indigenously designed in our lab and its technical details are discussed in detail previously [26]. Also, the detailed schematic of the Otto configuration exploited in the present work to perform SPR studies for prism/air gap/Au/WO3 system at λ = 633 nm has been explained elsewhere [27]. The well known Fresnel’s equations have been used to estimate the complex dielectric constant and refractive index by fitting the SPR reflectance measurement data with incident angle [27]. 3. Results and discussion 3.1 Structural studies Figure 1 depicts the XRD spectra of the WO3 thin films grown at varying sputtering pressure (10 to 50 mTorr) before and after irradiation with 100 MeV Ni7+ ions. It may be clearly noted from Fig. 1 that the prepared pristine samples before irradiation exhibit XRD peaks corresponding to orthorhombic WO3 without formation of any secondary phase, thereby indicating the growth of polycrystalline thin films. Furthermore, all the films show preferred oriented growth of crystallites along (200) direction before and after 100 MeV Ni7+ ion irradiation. After irradiation, no transformation in structural phase could be observed in the WO3 thin films deposited up to 30
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mTorr pressure. However, complete amorphization post irradiation was observed for orthorhombic WO3 thin film deposited at 50 mTorr sputtering pressure. The thin film prepared at a higher growth pressure i.e. 50 mTorr has higher roughness and porosity, leading to the availability of a larger exposed area for the irradiation. This results in comparatively larger damage to WO3 film structure in the form of maximum amorphization as compared to WO3 films prepared at the lower growth pressures (10mTorr to 30mTorr). The crystallite size of the WO3 crystallites along dominant (200) peak was assessed by means of Scherrer’s formula [28]. Irradiated WO3 thin films exhibited larger crystallite size as compared to the corresponding pristine samples (Table 1) and was found to increase with increase in deposition pressure from 10 mTorr to 50 mTorr. The decrease in FWHM and in turn increase in crystallite size after irradiation indicates that the Ni7+ ion irradiation promoted the grain growth and thus crystallinity. Besides that, the narrowing of the peaks with enhancement in the intensity clearly signifies that Ni7+ irradiation activates the grain growth. In the present work, grain growth in WO3 thin films is induced by Ni ions irradiation even at lower fluence. This clearly indicates that the transfer of the energy from the incident ion to the target film gives the required energy for the improvement of the crystallinity. It may be noted that enhancement in crystallinity is drastic at the lower fluence (≤1×1012 ions/cm2) which is due to the effect of the energy deposited in electronic excitation/ionization in the films. Whereas at higher fluence, the degradation in crystallinity is subjected to high energy deposition with large defect concentration. When the growth of grain size is comparable to the spatial extension of thermal spike which is perpendicular to ion path, then the energy delivered by the lower fluence would favor the grain growth [29]. It is imperative to mention that the WO3 film deposited at a pressure of 50 mTorr
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resulted in drastic degradation in the film crystallinity after Ni7+ ion irradiation and hence the crystallite size could not be calculated. 3.2 Optical Studies 3.2.1 Characterization using UV-Visible technique Optical transmission and reflection spectra for the pristine WO3 and 100 MeV Ni7+ ion irradiated WO3 films have been measured over a wavelength range of 190–1100 nm and are shown in Fig.2 (a) and (b), respectively. It may be seen that the ion irradiation results in reduction of the film transmission in the visible region for all the deposited films. Figure 3 shows the Tauc plots of WO3 thin films (before and after) used to estimate the optical band gap [30]. Estimated values of the band gap for all the films are also tabulated in Table 1. Irrespective of the sputtering pressure maintained, the optical band gap of WO3 thin films was found to decrease in response to ion irradiation. The thickness of the WO3 thin films before irradiation was ~ 120 nm at different growth pressures (10 mTorr to 50 mTorr) as measured by DEKTAK. After irradiation, the thickness of WO3 thin films was found to be increased slightly ~133 nm. The presence of interference fringes indicates the existence of thin irradiated layer on the prepared WO3 thin films. The same trend has been reported by Kachhap et al. (2013) [31]. It is reported that the band gap reduces upon on irradiation, which may be due to enhancement in crystallinity or due to the formation of ion induced defect states in the forbidden energy band [32]. It is apparent that irradiation with swift heavy ions induces lattice imperfections or point defects such as vacancies, interstitials etc. The defects produced due to 100 MeV Ni7+ ions irradiation results in the fall of the optical band gap of WO3 thin film. Irradiation generates defects in WO3 film, producing localized states near the band edges in the forbidden gap. Vacancies created in these localized states are expected to be filled immediately by the outer
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electrons, which further create more holes in the valence band. Thus, it results in continuous increase in vacancies in WO3 thin film with ion irradiation. Donor states have been created by the vacancies compensated by the electrons and these states lying in the forbidden gap which forms a narrow donor band close to the conduction band. Thus, localized states are created near the band edges due to defects induced by Ni7+ ions which is responsible for the decrease in the optical band gap. The optical band gap was found to increase appreciably for WO3 thin film grown at 50 mTorr deposition pressure. The WO3 thin films grown at 50 mTorr deposition pressure showed a drastic degradation in the film crystallinity after Ni7+ ion irradiation. With amorphization in semiconductors, structural defects are formed leading to loss of normal coordination environments. This further deteriorate the optical properties and introduces the formation of defect states on irradiation. The defect states are formed in conduction band after Ni7+ ion irradiation finally leading to the increase in band gap for the films grown at 50 mTorr [33]. 3.3 Morphological studies The surface morphology of the prepared WO3 films have been analyzed using the AFM images. The two-dimensional AFM micrographs of WO3 thin films prepared at 500oC at varying deposition pressure from 10 to 50 nm, before and after irradiation, are shown in Fig. 4(a) to (h). Figure 4 indicates the fine morphology having homogenous distribution of grains for the pristine WO3 thin films prepared by varying pressure. It may be noted that the grain size increases with increasing sputtering pressure 10 mTorr to 30 mTorr for the unirradiated film, while it decreases for the sample grown at 50 mTorr pressure in accordance with the XRD studies. Furthermore, the surface roughness of WO3 film increases from 1 to 5 nm with increase in the sputtering pressure 10 to 50 mTorr along with the presence of voids. A highly porous morphology is
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obtained at high growth pressure (50 mTorr) which may be due to the arrival of the target species having less energy and the shadowing effects. During ion irradiation, the high energetic electrons transfers their kinetic energy to lattice resulting in rise of the temperature of the lattice sites above the melting of material. Therefore, heat gets restricted within the grains leading to grain agglomeration [34-35]. This justifies the process of grain size enhancement upon irradiation and also confirms the growth of grains after the incident high energy ion irradiation on WO3 films. The surface roughness value is evaluated on the basis of different parameters i.e. average roughness represented as Ra and root mean square surface roughness (Rq). Here, the root-meansquare (RMS) surface roughness, Rq has been estimated for clearly understanding the modifications on surface morphology of WO3 films post irradiation, as Ra does not give the exact picture of surface inhomogeneity. Rq illustrates the scanned roughness of the sample area that comprises Gaussian height distribution created by the defects [36-37]. The measured Rq values from the AFM analysis for WO3 thin films deposited at varying deposition pressures before and after 100 MeV Ni7+ ions irradiation have been listed in Table 1. It is evident from table 1 that an enhancement in surface roughness of all the films has been observed upon interaction with ion irradiation. 3.4 Optical characterization using Surface Plasmon resonance technique The SPR reflectance data (symbols) was acquired for the prism/air gap/metal system at incident light of = 633 nm using Otto configuration is already reported in our previous manuscript along with the theoretically fitted curve are shown in figure 5 [27]. Figure 5 depicts the SPR reflectance curves recorded for prism/air-gap/Au/WO3 (pristine and irradiated) system with WO3 thin films grown at varying deposition pressure. There are three crucial parameters describing the response of SPR reflectance curves with the change in dielectric media in contact with the metal layer. The
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resonance angle (θSPR) of the SPR reflectance curve corresponds to the real part of dielectric constant (or refractive index), whereas the full width at half maxima (FWHM) corresponds to the absorption losses [38]. Table 2 indicates the SPR angle and minimum reflectance for pristine and irradiated WO3 thin films at varying deposition pressure. As shown in figure 5 and table 2, θSPR for the prism/air-gap/Au/WO3 (irradiated) system is higher as compared to prism/air-gap/Au/WO3 (pristine) system. The increase in resonance angle in SPR curve is due to crystallization of WO3 thin films after irradiation. The enhancement in crystallinity indicates the increase in grain size increases resulting in reduction in grain boundaries and scattering centres for incident light. This results in the longer propagation length of SPW leading to shift in the resonance conditions to higher angle. As observed from figure 5 all the prepared WO3 samples shows the increase in SPR angle after irradiation. The width of SPR curve at half maximum which is generally termed as FWHM depicts the absorption losses in the dielectric film [38]. The WO3 thin film becomes rough and porous as observed from AFM analysis (figure 4), which leads to more scattering of incident light at WO3 film surface, which is reflected as the increase in the width of SPR reflectance curve. It is confirmed from figure 5 that SPR curve for WO3 thin film deposited at 50 mT sputtering pressure found to have highest FWHM. This may be due to highest roughness and porosity exhibited by 50 mT prepared WO3 thin film. 3.4.1 Determination of complex dielectric constant and refractive index: The continuous line in the SPR reflectance curves for WO3 thin films in figure 5 denotes the theoretically fitted curve using the well known Fresnel’s equations. Prior to the estimation of complex dielectric constant and complex refractive index of the samples, the thickness of the intermediate air gap and dielectric constant of metal (Au) are required. The reported values of dielectric constant of prism 0 = (1.517)2, dielectric constant of air 1 = 1.0 were also used in fitting
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the experimental data. The values of complex dielectric constant and refractive index for both asgrown and irradiated WO3 thin films are listed in Table 3. The gradual increase in the refractive index of WO3 thin films with increase in deposition pressure is evident from table 3 and also increasing trend is observed for WO3 thin films treated with irradiation. The observed increase in the value of refractive index for irradiated thin films can be attributed to the irradiation induced porous morphology and defects in the deposited thin films by swift ion beam irradiation [39]. 3. CONCLUSION In conclusion, WO3 thin films with orthorhombic structure were grown on corning glass substrates using RF magnetron sputtering technique under varying deposition pressures from 10 to 50 mTorr at 500oC substrate temperature. The modifications induced in the structural, optical and the surface morphological properties of the WO3 thin films have been studied prior to and post irradiation with 100 MeV Ni7+ ions at a fluence of about 1x1012 ions cm-2. The ion irradiation enhances the grain growth in WO3 thin films, however, there is no structural phase transition observed for the films grown at low pressure (10 to 30 mTorr). The complete amorphization with ion irradiation occurred for the films grown at higher pressure (>30 mTorr). A slight decrease in the optical transmission and a continuous decrease in the optical band gap is observed for WO3 thin films on ion irradiation. This decrease may be due to the incorporation of Ni7+ ions which forms localized states near the band edges with ion irradiation. AFM images show the agglomeration of grains and increase in roughness and porosity of the films on ion irradiation. The observed change in structural and optical property of WO3 thin film with Ni7+ ions are advantageous for various device applications including gas sensing and photodetection. The optical properties were studied for WO3 thin films and SPR reflectance curves were recorded for pristine and irradiated WO3 thin exploiting Otto configuration. The variation in 12
propagation constant of SPW at the metal–dielectric interface is responsible for a significant shift towards the higher angle after integrating WO3 thin film. The complex refractive index of WO3 thin films before and after irradiation have been estimated by theoretically fitting the experimental SPR data with the Fresnel’s equations. The defects and porosity induced by irradiation results in the rise in refractive index and extinction coefficient. 4. Acknowledgement Authors are thankful to University of Delhi for technical and financial support to carry out the work. 5. Conflicts of interest There is no conflict of interest in this manuscript.
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Table Captions : Table 1 Variation of grain size, surface roughness and band gap of WO3 thin film grown at varying pressures before and after irradiation Table 2 Value of SPR angle and minimum reflectance for WO3 thin films (pristine and irradiated) at varying deposition pressure
Table 3 Value of optical parameters i.e. refractive index and extinction coefficient for WO3 thin films at varying deposition pressure
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Figure Captions :
Figure 1 XRD patterns of pristine and Ni7+ ions irradiated WO3 films deposited at (a) 10 mTorr (b) 20 mTorr (c) 30 mTorr and (d) 50 mTorr deposition pressures. Figure 2 UV–Visible transmission and reflectance spectra of (a) pristine and (b) irradiated WO3 thin films deposited at varying deposition pressures. Figure 3 Tauc plots of the WO3 films deposited at different pressures showing variation in the energy band gap of WO3 films before and after Ni7+ ions irradiation. Figure 4 Atomic force microscope images (2 µm×2 µm) of WO3 thin films deposited at varying pressures from 10 mTorr to 50 mTorr before and after Ni7+ ion irradiation at a fluence of 1 x 1012 ions/cm2. Figure 5 Surface Plasmon Resonance reflectance curves for WO3 thin films deposited at varying pressures from 10 mTorr to 50 mTorr before and after Ni7+ ion irradiation at a fluence of 1 x 1012 ions/cm2.
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Table 1 Variation of grain size, surface roughness and band gap of WO3 thin film grown at varying pressures before and after irradiation Pressure (mTorr) 10 20 30 50
Grain Size (nm) Before After 10 13 16 17 21 25 23 ---
Surface Roughness (nm) Before After 1.8 2.1 2.0 3.2 2.2 5.6 4.7 9.9
Band gap (eV) before After 3.60 3.55 3.58 3.52 3.55 3.49 3.73 4.04
Table 2 Value of SPR angle and minimum reflectance for WO3 thin films (pristine and irradiated) at varying deposition pressure Pressure (mT)
SPR angle
Minimum reflectance
Pristine
Irradiated
Pristine
Irradiated
10
54.52
67.21
0.396
0.698
20
58.03
65.20
0.414
0.611
30
60.52
67.96
0.209
0.368
50
62.08
72.13
0.255
0.555
20
Table 3 Value of refractive index and extinction coefficient for WO3 thin films at varying deposition pressure Sputtering pressure (mT)
Pristine
Irradiated
Refractive index (ni)
Extinction coefficient (k)
Refractive index (ni)
Extinction coefficient (k)
10
2.21
0.020
2.31
0.027
20
2.22
0.019
2.25
0.029
30
2.24
0.007
2.35
0.011
50
2.27
0.025
2.33
0.028
21
Figure 1 XRD patterns of pristine and Ni7+ ions irradiated WO3 films deposited at (a) 10 mTorr (b) 20 mTorr (c) 30 mTorr and (d) 50 mTorr deposition pressures.
(a) 22
(b) Figure 2 UV–Visible transmission and reflectance spectra of (a) pristine and (b) irradiated WO3 thin films deposited at varying deposition pressures.
Figure 3 Tauc plots of the WO3 films deposited at different pressures showing variation in the energy band gap of WO3 films before and after Ni7+ ions irradiation
23
24
Figure 4 Atomic force microscope images (2 µm×2 µm) of WO3 thin films deposited at varying pressures from 10 mTorr to 50 mTorr before and after Ni7+ ion irradiation at a fluence of 1 x 1012 ions/cm2.
25
10 mTorr
0.9 0.8 0.7 0.6
50
55
60
65
70
Angle (o)
75
0.7 0.6
1.0
1.0
0.9
0.9
0.8 0.7 0.6 0.5
30 mTorr
0.4 0.3
46
48
50
52
50
55
60
65
Angle (o)
70
75
0.8 0.7 0.6 0.5 0.4 0.3
prism/Au/WO3(irradiated)/air
Theoretical prism/Au/WO3(pristine)/air
0.2
prism/Au/WO3(irradiated)/air
54
56
Angle (o)
58
80
50 mTorr
Theoretical prism/Au/WO3(pristine)/air
0.2 44
Theoretical prism/Au/WO3(pristine)/air prism/Au/WO3(irradiated)/air
45
80
Reflectance
Reflectance
45
0.8
0.4
prism/Au/WO3(irradiated)/air
0.4
0.9
0.5
Theoretical prism/Au/WO3(pristine)/air
0.5
20 mTorr
1.0
Reflectance
Reflectance
1.0
60
62
44 46 48 50 52 54 56 58 60 62 64 66
64
Angle (o)
Figure 5 Surface Plasmon Resonance reflectance curves for WO3 thin films deposited at varying pressures from 10 mTorr to 50 mTorr before and after Ni7+ ion irradiation at a fluence of 1 x 1012 ions/cm2.
26
Highlights
WO3 thin films RF sputtering technique at different pressures (10 to 50 mTorr).
100 MeV Ni7+ ions irradiation performed at fluence 1×1012 ions cm−2
Modifications in structural,optical properties,surface morphology after irradiation
AFM confirmed,on irradiation,grain size increase with increase in surface roughness
SPR technique to study optical properties of pristine and irradiated WO3 thin films
SPR reflectance curves in angular interrogation mode at 633nm excitation wavelength
Complete amorphization post irradiation for orthorhombic WO3 thin film at 50 mTorr
27
PII: DOI: Reference:
S0969-806X(18)30542-5 https://doi.org/10.1016/j.radphyschem.2018.09.004 RPC7994
To appear in: Radiation Physics and Chemistry Received date: 8 June 2018 Revised date: 4 August 2018 Accepted date: 3 September 2018 Cite this article as: Savita Sharma, Ayushi Paliwal, Monika Tomar, Fouran Singh, Nitin K. Puri and Vinay Gupta, Detailed optical analysis of 100MeV Ni 7+ ion irradiated WO3 thin films using Surface Plasmon resonance, Radiation Physics and Chemistry, https://doi.org/10.1016/j.radphyschem.2018.09.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Detailed optical analysis of 100 MeV Ni7+ ion irradiated WO3 thin films using Surface Plasmon resonance Savita Sharma1,4, Ayushi Paliwal1, Monika Tomar2, Fouran Singh3, Nitin K. Puri4, Vinay Gupta1* 1
Department of Physics and Astrophysics, University of Delhi, Delhi 110007, India 2
3
4
*
Physics Department, Miranda House, University of Delhi, Delhi 110007, India
Inter University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi, 110075, India
Department of Applied Physics, Delhi Technological University, Delhi 110042, India
Corresponding author E-mail address: [email protected]; Tel: +919811563101
1
Abstract Modifications in the structural, optical properties and surface morphology of RF sputtered WO3 thin films after irradiation with 100 MeV Ni7+ ions have been reported in the present article. The WO3 thin films were deposited at different sputtering pressures (10 to 50 mTorr). Despite the grain growth due to irradiation, no structural phase transformation has been observed for the WO3 thin films deposited up to 30 mTorr growth pressure. However, complete amorphization post irradiation was detected for the orthorhombic WO3 thin film grown at at 50 mTorr sputtering pressure. AFM images indicated that the pristine WO3 thin film consists of uniform grains having smooth surface morphology. Upon irradiation, grain size increased consistently along with increase in surface roughness (Rq) at all sputtering pressures. The root mean square roughness (Rq) estimated from AFM analysis increased from 1.76 nm in pristine film to 9.89 nm in response to irradiation at a fluence of 1 × 1012 ions cm−2. A systematic decrease in optical band gap has been observed with irradiation, which can be correlated with Ni7+ ion induced midgap and near band edge defect states. Surface Plasmon Resonance (SPR) technique has been recognized as an appropriate tool to study the optical properties of pristine and irradiated WO3 thin films. SPR reflectance curves were recorded in angular interrogation mode at 633nm excitation wavelength for pristine and Ni7+ ions irradiated WO3 films deposited at varying deposition pressure.
Keywords : Irradiation, sputtering, band gap, fluence, Thin film.
2
1. Introduction Swift Heavy Ion (SHI) irradiations which are characterized by ion energies above 1 MeVamu−1 range have been of great importance in studying material modifications. Several interesting and important properties of matter in the solid state are related to the presence of defects and impurities [1-4]. The imperfections in solids are exploited for employing them in various applications. For technological applications, the defects can be introduced either by varying the growth parameters (such as RF power, substrate temperature, sputtering pressure etc.) or by purposely doping the host material during the fabrication process. The defects can also be introduced in the crystalline solids by means of ion irradiation. In studying modification by SHI irradiation, there are two important parameters i.e. the electronic energy losses (Se) and nuclear energy losses (Sn) which occur due to inelastic collisions and elastic collisions respectively. The electronic energy loss (Se) is significantly higher as compared to Sn which is further responsible for producing significant excitation of the lattice in a very short interval of time (~ picoseconds). This causes the introduction of defect states in the lattice which give rise to elemental deviations in the electronic structure and material properties altogether. There are a number of reports on the study of oxide systems like ZnO, SiO2, CeO2 and Fe2O3, however only a few reports are available on the effect of ion beam-introduced modifications of tungsten trioxide (WO3) thin films [5-6]. WO3 illustrates intriguing optical properties, making it a useful candidate for applications in mirrors, solid-state micro-batteries, catalysts and filters [7-9]. WO3 is an important candidate for different applications including optical smart windows, flat panel displays, gas sensors etc [10-14]. Modifications developed in the surface morphology of materials by irradiation may be useful for many device applications [15-17]. In the case of WO3 films, most of the research work has been
3
concentrated on irradiation induced enhanced electrical conductivity [18-21]. Further, Sivakumar et al reported the MeV N+ ion beam irradiation modification of WO3 thin films. This induce grain growth without any structural phase transition and also band gap reduces [22]. Structural and optical studies of ion irradiated WO3 thin films will be immensely beneficial in development of WO3 thin film based devices for various applications in outer space surrounded by various harmful radiations. Optical studies have shown momentous impact on the advancement of the infrastructure of optical communication system, information technology and also for the development of optical devices like solar cell, televisions, microscopes, LEDs, LASERS etc [23-24]. Surface Plasmon Resonance (SPR) is considered to be the most sensitive tool to explore the optical features of any material amongst all the available optical techniques. Also, it offers a flexibility to perform the optical measurements under varying ambient conditions. This is also advantageous in terms of sample preparation and data analysis. An electromagnetic wave incident on metal-dielectric interface having dielectric constants of opposite signs supports Surface plasmon waves (SPWs). Thus, SPWs can be utilized to study the metal-dielectric interfaces and metal surfaces in any ambient condition [25]. There are two configurations i.e. Kretschmann configuration and Otto configuration which are used to observed SPR phenomena. The major advantage of Otto configuration over Kretschmann configuration is that the glass prism is an absolute element for optical coupling. Another advantage is that there is no thickness constraint in case of Otto configuration. SPR spectrum exhibits a dip in reflectance corresponding to the energy transfer from the incident light to the SPW and its subsequent dissipation in the metal film. The SPR resonance condition for surface plasmon can be achieved in different modes in thin films which include (1) angular interrogation mode, (2) wavelength interrogation mode and (3) phase
4
interrogation mode. The most common mode used to study the SPR reflectance curves is the angular interrogation mode where incident angle is varied in both Kretschmann and Otto configurations [25]. This paper presents the study of the modifications induced in tungsten oxide (WO3) thin films by swift heavy ion irradiation (SHI). The WO3 thin films were prepared using Rf magnetron sputtering at different growth pressures ranging from 10 to 50 mTorr at a fixed deposition temperature of 500oC. The pristine WO3 thin films were subjected to 100 MeV Ni7+ ion irradiation at a fluence of about 1×1012 ions cm−2. Variation in the film crystallinity, surface morphology and optical band gap of WO3 samples deposited at different sputtering pressures (10 mTorr to 50 mTorr) has been studied before and after 100 MeV Ni7+ irradiation. Furthermore, investigation of optical properties of both pristine and irradiated WO3 thin films was done using SPR technique. 2. Experimental details In the present work, rf-magnetron sputtering has been exploited for the deposition of WO3 thin films. Pure tungsten metal target (2” dia.) was sputtered at 40 W power in an Ar to O2 gas ratio of 60:40. The films were deposited on cleaned Corning glass substrate of 2 cm by 1 cm size at 500oC substrate temperature. The irradiation of the prepared WO3 thin films were performed at the Inter University Accelerator Centre (IUAC) in New Delhi, using Ni ions at a fixed fluence of 1 × 1012 ions cm−2 produced by 15 UD Pelletron accelerator. Deposition pressure is known to affect the film porosity and surface morphology to a great extent and hence the sputtering pressure was varied between 10 and 50 mTorr (the base pressure was approximately 6 × 10-6 Torr) keeping the composition of gas same (Ar :O2 = 60:40). Structural studies were observed using X-Ray Diffraction (XRD) technique (Make: Bruker D-8 X-Ray diffractometer). The
5
optical properties of the samples were analyzed using UV-Visible (UV-Vis) spectroscopy (Perkin Elmer lambda-35 spectrophotometer). WO3 film thickness was measured using a surface profiler (DEKTAK 150). Surface morphology and topography were analyzed using contact mode Atomic force microscopy (AFM) technique (Bruker Dimension Icon). For SPR measurements, thickness of WO3 thin films prepared at different pressures has been fixed at 200 nm and Otto configuration is used to record their SPR reflectance curves. Thermal evaporation technique has been exploited for depositing Au thin film in order to excite the surface plasmons, where the film thickness is measured using an automated thickness monitor. The SPR measurement setup has been indigenously designed in our lab and its technical details are discussed in detail previously [26]. Also, the detailed schematic of the Otto configuration exploited in the present work to perform SPR studies for prism/air gap/Au/WO3 system at λ = 633 nm has been explained elsewhere [27]. The well known Fresnel’s equations have been used to estimate the complex dielectric constant and refractive index by fitting the SPR reflectance measurement data with incident angle [27]. 3. Results and discussion 3.1 Structural studies Figure 1 depicts the XRD spectra of the WO3 thin films grown at varying sputtering pressure (10 to 50 mTorr) before and after irradiation with 100 MeV Ni7+ ions. It may be clearly noted from Fig. 1 that the prepared pristine samples before irradiation exhibit XRD peaks corresponding to orthorhombic WO3 without formation of any secondary phase, thereby indicating the growth of polycrystalline thin films. Furthermore, all the films show preferred oriented growth of crystallites along (200) direction before and after 100 MeV Ni7+ ion irradiation. After irradiation, no transformation in structural phase could be observed in the WO3 thin films deposited up to 30
6
mTorr pressure. However, complete amorphization post irradiation was observed for orthorhombic WO3 thin film deposited at 50 mTorr sputtering pressure. The thin film prepared at a higher growth pressure i.e. 50 mTorr has higher roughness and porosity, leading to the availability of a larger exposed area for the irradiation. This results in comparatively larger damage to WO3 film structure in the form of maximum amorphization as compared to WO3 films prepared at the lower growth pressures (10mTorr to 30mTorr). The crystallite size of the WO3 crystallites along dominant (200) peak was assessed by means of Scherrer’s formula [28]. Irradiated WO3 thin films exhibited larger crystallite size as compared to the corresponding pristine samples (Table 1) and was found to increase with increase in deposition pressure from 10 mTorr to 50 mTorr. The decrease in FWHM and in turn increase in crystallite size after irradiation indicates that the Ni7+ ion irradiation promoted the grain growth and thus crystallinity. Besides that, the narrowing of the peaks with enhancement in the intensity clearly signifies that Ni7+ irradiation activates the grain growth. In the present work, grain growth in WO3 thin films is induced by Ni ions irradiation even at lower fluence. This clearly indicates that the transfer of the energy from the incident ion to the target film gives the required energy for the improvement of the crystallinity. It may be noted that enhancement in crystallinity is drastic at the lower fluence (≤1×1012 ions/cm2) which is due to the effect of the energy deposited in electronic excitation/ionization in the films. Whereas at higher fluence, the degradation in crystallinity is subjected to high energy deposition with large defect concentration. When the growth of grain size is comparable to the spatial extension of thermal spike which is perpendicular to ion path, then the energy delivered by the lower fluence would favor the grain growth [29]. It is imperative to mention that the WO3 film deposited at a pressure of 50 mTorr
7
resulted in drastic degradation in the film crystallinity after Ni7+ ion irradiation and hence the crystallite size could not be calculated. 3.2 Optical Studies 3.2.1 Characterization using UV-Visible technique Optical transmission and reflection spectra for the pristine WO3 and 100 MeV Ni7+ ion irradiated WO3 films have been measured over a wavelength range of 190–1100 nm and are shown in Fig.2 (a) and (b), respectively. It may be seen that the ion irradiation results in reduction of the film transmission in the visible region for all the deposited films. Figure 3 shows the Tauc plots of WO3 thin films (before and after) used to estimate the optical band gap [30]. Estimated values of the band gap for all the films are also tabulated in Table 1. Irrespective of the sputtering pressure maintained, the optical band gap of WO3 thin films was found to decrease in response to ion irradiation. The thickness of the WO3 thin films before irradiation was ~ 120 nm at different growth pressures (10 mTorr to 50 mTorr) as measured by DEKTAK. After irradiation, the thickness of WO3 thin films was found to be increased slightly ~133 nm. The presence of interference fringes indicates the existence of thin irradiated layer on the prepared WO3 thin films. The same trend has been reported by Kachhap et al. (2013) [31]. It is reported that the band gap reduces upon on irradiation, which may be due to enhancement in crystallinity or due to the formation of ion induced defect states in the forbidden energy band [32]. It is apparent that irradiation with swift heavy ions induces lattice imperfections or point defects such as vacancies, interstitials etc. The defects produced due to 100 MeV Ni7+ ions irradiation results in the fall of the optical band gap of WO3 thin film. Irradiation generates defects in WO3 film, producing localized states near the band edges in the forbidden gap. Vacancies created in these localized states are expected to be filled immediately by the outer
8
electrons, which further create more holes in the valence band. Thus, it results in continuous increase in vacancies in WO3 thin film with ion irradiation. Donor states have been created by the vacancies compensated by the electrons and these states lying in the forbidden gap which forms a narrow donor band close to the conduction band. Thus, localized states are created near the band edges due to defects induced by Ni7+ ions which is responsible for the decrease in the optical band gap. The optical band gap was found to increase appreciably for WO3 thin film grown at 50 mTorr deposition pressure. The WO3 thin films grown at 50 mTorr deposition pressure showed a drastic degradation in the film crystallinity after Ni7+ ion irradiation. With amorphization in semiconductors, structural defects are formed leading to loss of normal coordination environments. This further deteriorate the optical properties and introduces the formation of defect states on irradiation. The defect states are formed in conduction band after Ni7+ ion irradiation finally leading to the increase in band gap for the films grown at 50 mTorr [33]. 3.3 Morphological studies The surface morphology of the prepared WO3 films have been analyzed using the AFM images. The two-dimensional AFM micrographs of WO3 thin films prepared at 500oC at varying deposition pressure from 10 to 50 nm, before and after irradiation, are shown in Fig. 4(a) to (h). Figure 4 indicates the fine morphology having homogenous distribution of grains for the pristine WO3 thin films prepared by varying pressure. It may be noted that the grain size increases with increasing sputtering pressure 10 mTorr to 30 mTorr for the unirradiated film, while it decreases for the sample grown at 50 mTorr pressure in accordance with the XRD studies. Furthermore, the surface roughness of WO3 film increases from 1 to 5 nm with increase in the sputtering pressure 10 to 50 mTorr along with the presence of voids. A highly porous morphology is
9
obtained at high growth pressure (50 mTorr) which may be due to the arrival of the target species having less energy and the shadowing effects. During ion irradiation, the high energetic electrons transfers their kinetic energy to lattice resulting in rise of the temperature of the lattice sites above the melting of material. Therefore, heat gets restricted within the grains leading to grain agglomeration [34-35]. This justifies the process of grain size enhancement upon irradiation and also confirms the growth of grains after the incident high energy ion irradiation on WO3 films. The surface roughness value is evaluated on the basis of different parameters i.e. average roughness represented as Ra and root mean square surface roughness (Rq). Here, the root-meansquare (RMS) surface roughness, Rq has been estimated for clearly understanding the modifications on surface morphology of WO3 films post irradiation, as Ra does not give the exact picture of surface inhomogeneity. Rq illustrates the scanned roughness of the sample area that comprises Gaussian height distribution created by the defects [36-37]. The measured Rq values from the AFM analysis for WO3 thin films deposited at varying deposition pressures before and after 100 MeV Ni7+ ions irradiation have been listed in Table 1. It is evident from table 1 that an enhancement in surface roughness of all the films has been observed upon interaction with ion irradiation. 3.4 Optical characterization using Surface Plasmon resonance technique The SPR reflectance data (symbols) was acquired for the prism/air gap/metal system at incident light of = 633 nm using Otto configuration is already reported in our previous manuscript along with the theoretically fitted curve are shown in figure 5 [27]. Figure 5 depicts the SPR reflectance curves recorded for prism/air-gap/Au/WO3 (pristine and irradiated) system with WO3 thin films grown at varying deposition pressure. There are three crucial parameters describing the response of SPR reflectance curves with the change in dielectric media in contact with the metal layer. The
10
resonance angle (θSPR) of the SPR reflectance curve corresponds to the real part of dielectric constant (or refractive index), whereas the full width at half maxima (FWHM) corresponds to the absorption losses [38]. Table 2 indicates the SPR angle and minimum reflectance for pristine and irradiated WO3 thin films at varying deposition pressure. As shown in figure 5 and table 2, θSPR for the prism/air-gap/Au/WO3 (irradiated) system is higher as compared to prism/air-gap/Au/WO3 (pristine) system. The increase in resonance angle in SPR curve is due to crystallization of WO3 thin films after irradiation. The enhancement in crystallinity indicates the increase in grain size increases resulting in reduction in grain boundaries and scattering centres for incident light. This results in the longer propagation length of SPW leading to shift in the resonance conditions to higher angle. As observed from figure 5 all the prepared WO3 samples shows the increase in SPR angle after irradiation. The width of SPR curve at half maximum which is generally termed as FWHM depicts the absorption losses in the dielectric film [38]. The WO3 thin film becomes rough and porous as observed from AFM analysis (figure 4), which leads to more scattering of incident light at WO3 film surface, which is reflected as the increase in the width of SPR reflectance curve. It is confirmed from figure 5 that SPR curve for WO3 thin film deposited at 50 mT sputtering pressure found to have highest FWHM. This may be due to highest roughness and porosity exhibited by 50 mT prepared WO3 thin film. 3.4.1 Determination of complex dielectric constant and refractive index: The continuous line in the SPR reflectance curves for WO3 thin films in figure 5 denotes the theoretically fitted curve using the well known Fresnel’s equations. Prior to the estimation of complex dielectric constant and complex refractive index of the samples, the thickness of the intermediate air gap and dielectric constant of metal (Au) are required. The reported values of dielectric constant of prism 0 = (1.517)2, dielectric constant of air 1 = 1.0 were also used in fitting
11
the experimental data. The values of complex dielectric constant and refractive index for both asgrown and irradiated WO3 thin films are listed in Table 3. The gradual increase in the refractive index of WO3 thin films with increase in deposition pressure is evident from table 3 and also increasing trend is observed for WO3 thin films treated with irradiation. The observed increase in the value of refractive index for irradiated thin films can be attributed to the irradiation induced porous morphology and defects in the deposited thin films by swift ion beam irradiation [39]. 3. CONCLUSION In conclusion, WO3 thin films with orthorhombic structure were grown on corning glass substrates using RF magnetron sputtering technique under varying deposition pressures from 10 to 50 mTorr at 500oC substrate temperature. The modifications induced in the structural, optical and the surface morphological properties of the WO3 thin films have been studied prior to and post irradiation with 100 MeV Ni7+ ions at a fluence of about 1x1012 ions cm-2. The ion irradiation enhances the grain growth in WO3 thin films, however, there is no structural phase transition observed for the films grown at low pressure (10 to 30 mTorr). The complete amorphization with ion irradiation occurred for the films grown at higher pressure (>30 mTorr). A slight decrease in the optical transmission and a continuous decrease in the optical band gap is observed for WO3 thin films on ion irradiation. This decrease may be due to the incorporation of Ni7+ ions which forms localized states near the band edges with ion irradiation. AFM images show the agglomeration of grains and increase in roughness and porosity of the films on ion irradiation. The observed change in structural and optical property of WO3 thin film with Ni7+ ions are advantageous for various device applications including gas sensing and photodetection. The optical properties were studied for WO3 thin films and SPR reflectance curves were recorded for pristine and irradiated WO3 thin exploiting Otto configuration. The variation in 12
propagation constant of SPW at the metal–dielectric interface is responsible for a significant shift towards the higher angle after integrating WO3 thin film. The complex refractive index of WO3 thin films before and after irradiation have been estimated by theoretically fitting the experimental SPR data with the Fresnel’s equations. The defects and porosity induced by irradiation results in the rise in refractive index and extinction coefficient. 4. Acknowledgement Authors are thankful to University of Delhi for technical and financial support to carry out the work. 5. Conflicts of interest There is no conflict of interest in this manuscript.
13
Table Captions : Table 1 Variation of grain size, surface roughness and band gap of WO3 thin film grown at varying pressures before and after irradiation Table 2 Value of SPR angle and minimum reflectance for WO3 thin films (pristine and irradiated) at varying deposition pressure
Table 3 Value of optical parameters i.e. refractive index and extinction coefficient for WO3 thin films at varying deposition pressure
14
Figure Captions :
Figure 1 XRD patterns of pristine and Ni7+ ions irradiated WO3 films deposited at (a) 10 mTorr (b) 20 mTorr (c) 30 mTorr and (d) 50 mTorr deposition pressures. Figure 2 UV–Visible transmission and reflectance spectra of (a) pristine and (b) irradiated WO3 thin films deposited at varying deposition pressures. Figure 3 Tauc plots of the WO3 films deposited at different pressures showing variation in the energy band gap of WO3 films before and after Ni7+ ions irradiation. Figure 4 Atomic force microscope images (2 µm×2 µm) of WO3 thin films deposited at varying pressures from 10 mTorr to 50 mTorr before and after Ni7+ ion irradiation at a fluence of 1 x 1012 ions/cm2. Figure 5 Surface Plasmon Resonance reflectance curves for WO3 thin films deposited at varying pressures from 10 mTorr to 50 mTorr before and after Ni7+ ion irradiation at a fluence of 1 x 1012 ions/cm2.
15
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Table 1 Variation of grain size, surface roughness and band gap of WO3 thin film grown at varying pressures before and after irradiation Pressure (mTorr) 10 20 30 50
Grain Size (nm) Before After 10 13 16 17 21 25 23 ---
Surface Roughness (nm) Before After 1.8 2.1 2.0 3.2 2.2 5.6 4.7 9.9
Band gap (eV) before After 3.60 3.55 3.58 3.52 3.55 3.49 3.73 4.04
Table 2 Value of SPR angle and minimum reflectance for WO3 thin films (pristine and irradiated) at varying deposition pressure Pressure (mT)
SPR angle
Minimum reflectance
Pristine
Irradiated
Pristine
Irradiated
10
54.52
67.21
0.396
0.698
20
58.03
65.20
0.414
0.611
30
60.52
67.96
0.209
0.368
50
62.08
72.13
0.255
0.555
20
Table 3 Value of refractive index and extinction coefficient for WO3 thin films at varying deposition pressure Sputtering pressure (mT)
Pristine
Irradiated
Refractive index (ni)
Extinction coefficient (k)
Refractive index (ni)
Extinction coefficient (k)
10
2.21
0.020
2.31
0.027
20
2.22
0.019
2.25
0.029
30
2.24
0.007
2.35
0.011
50
2.27
0.025
2.33
0.028
21
Figure 1 XRD patterns of pristine and Ni7+ ions irradiated WO3 films deposited at (a) 10 mTorr (b) 20 mTorr (c) 30 mTorr and (d) 50 mTorr deposition pressures.
(a) 22
(b) Figure 2 UV–Visible transmission and reflectance spectra of (a) pristine and (b) irradiated WO3 thin films deposited at varying deposition pressures.
Figure 3 Tauc plots of the WO3 films deposited at different pressures showing variation in the energy band gap of WO3 films before and after Ni7+ ions irradiation
23
24
Figure 4 Atomic force microscope images (2 µm×2 µm) of WO3 thin films deposited at varying pressures from 10 mTorr to 50 mTorr before and after Ni7+ ion irradiation at a fluence of 1 x 1012 ions/cm2.
25
10 mTorr
0.9 0.8 0.7 0.6
50
55
60
65
70
Angle (o)
75
0.7 0.6
1.0
1.0
0.9
0.9
0.8 0.7 0.6 0.5
30 mTorr
0.4 0.3
46
48
50
52
50
55
60
65
Angle (o)
70
75
0.8 0.7 0.6 0.5 0.4 0.3
prism/Au/WO3(irradiated)/air
Theoretical prism/Au/WO3(pristine)/air
0.2
prism/Au/WO3(irradiated)/air
54
56
Angle (o)
58
80
50 mTorr
Theoretical prism/Au/WO3(pristine)/air
0.2 44
Theoretical prism/Au/WO3(pristine)/air prism/Au/WO3(irradiated)/air
45
80
Reflectance
Reflectance
45
0.8
0.4
prism/Au/WO3(irradiated)/air
0.4
0.9
0.5
Theoretical prism/Au/WO3(pristine)/air
0.5
20 mTorr
1.0
Reflectance
Reflectance
1.0
60
62
44 46 48 50 52 54 56 58 60 62 64 66
64
Angle (o)
Figure 5 Surface Plasmon Resonance reflectance curves for WO3 thin films deposited at varying pressures from 10 mTorr to 50 mTorr before and after Ni7+ ion irradiation at a fluence of 1 x 1012 ions/cm2.
26
Highlights
WO3 thin films RF sputtering technique at different pressures (10 to 50 mTorr).
100 MeV Ni7+ ions irradiation performed at fluence 1×1012 ions cm−2
Modifications in structural,optical properties,surface morphology after irradiation
AFM confirmed,on irradiation,grain size increase with increase in surface roughness
SPR technique to study optical properties of pristine and irradiated WO3 thin films
SPR reflectance curves in angular interrogation mode at 633nm excitation wavelength
Complete amorphization post irradiation for orthorhombic WO3 thin film at 50 mTorr
27