Augmentation of thermoelectric performance of VO2 thin films irradiated by 200 MeV Ag9+-ions

Radiation Physics and Chemistry 123 (2016) 55–62

Contents lists available at ScienceDirect

Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem

Augmentation of thermoelectric performance of VO2 thin films irradiated by 200 MeV Ag9 þ -ions G.R. Khan a,n, A. Kandasami b, B.A. Bhat a a b

Nanotech Research Lab, Department of Physics, National Institute of Technology, Srinagar 190006, Kashmir, India Material Science Division, Inter University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110067, India

H I G H L I G H T S

    

Thermoelectric properties of VO2 thin films enhance upon SHI irradiation. Structural properties show that crystallite size decrease upon SHI irradiation. Metal–insulator phase transition temperature of films diminish upon SHI irradiation. Seebeck coefficient remains constant but electrical conductivity increases manifold. Mobility at room temperature and hysteresis width reduce on increasing ion fluence.

art ic l e i nf o

a b s t r a c t

Article history: Received 9 December 2015 Received in revised form 11 February 2016 Accepted 12 February 2016 Available online 13 February 2016

Swift Heavy Ion (SHI) irradiation with 200 MeV Ag9 þ -ion beam at ion fluences of 1E11, 5E11, 1E12, and 5E12 for tuning of electrical transport properties of VO2 thin films fabricated by so–gel technique on alumina substrates has been demonstrated in the present paper. The point defects created by SHI irradiation modulate metal to insulator phase transition temperature, carrier concentration, carrier mobility, electrical conductivity, and Seebeck coefficient of VO2 thin films. The structural properties of the films were characterized by XRD and Raman spectroscopy and crystallite size was found to decrease upon irradiation. The atomic force microscopy revealed that the surface roughness of specimens first decreased and then increased with increasing fluence. Both resistance as well as Seebeck coefficient measurements demonstrated that all the samples exhibit metal–insulator phase transition and the transition temperatures decreases with increasing fluence. Hall effect measurements exhibited that carrier concentration increased continuously with increasing fluence which resulted in an increase of electrical conductivity by several orders of magnitude in the insulating phase. Seebeck coefficient in insulating phase remained almost constant in spite of an increase in the electrical conductivity by several orders of magnitude making SHI irradiation an alternative stratagem for augmentation of thermoelectric performance of the materials. The carrier mobility at room temperature decreased up to the beam fluence of 5E11 and then started increasing whereas Seebeck coefficient in metallic state first increased with increasing ion beam fluence up to 5E11 and thereafter decreased. Variation of these electrical transport parameters has been explained in detail. & 2016 Elsevier Ltd. All rights reserved.

Keywords: Swift Heavy Ion Silver ion irradiation Sol–gel technique Metal–insulator phase transition Thermoelectric properties VO2 thin films

1. Introduction With the beginning of the third millennium of the Gregorian calendar, Swift Heavy Ion (SHI) irradiation has become a versatile technique in the material science to fine tune quite a few physical properties of materials that are considered crucial for contemporary device fabrication. In SHI irradiation, particles are

n

Corresponding author. E-mail address: [email protected] (G.R. Khan).

http://dx.doi.org/10.1016/j.radphyschem.2016.02.020 0969-806X/& 2016 Elsevier Ltd. All rights reserved.

accelerated to very high energies (ZMeV/u) in particle accelerators to create a stream of ions piercing through a solid which release energy in the material during electron–electron interaction in the course of inelastic electron excitation process. During this process, various changes transpire in the material such as phase transition, amorphization, defect creation, dynamic annealing, dimensional alteration, deep implantation, macroscopic deformation, nanostructure formation, and surface modification [1,2]. Due to ample choice of ion-species and extensive range of ion-energies, SHI irradiation provides exceptional opportunities for optimization of physical properties via ion-implantation of materials for

56

G.R. Khan et al. / Radiation Physics and Chemistry 123 (2016) 55–62

fabrication of novel devices. Vanadium dioxide (VO2) being an emblematical strongly correlated electron system that undergoes a temperature-driven firstorder reversible semiconductor to metal phase transition (SMT) at a transition temperature (Tt) of 340 K [3]. The SMT is accompanied by a polymorphic transformation from low temperature monoclinic phase (P21/c) to high temperature tetragonal rutile phase (P42/mnm) [4]. Subsequently, SMT induces drastic changes in the structural and electric properties of VO2. These stimulating characteristics make VO2 as a prospective material for diverse technological applications. However, in most of the modern applications, it has become enviable that Tt of VO2 be diminished to room temperature or below. In order to attain this target, numerous attempts have been made to reduce Tt through a number of techniques. In thin film technology, Tt of VO2 has been reduced to the ambient due to quantum size effect by miniaturizing their film thickness to the nanoscale [5] and by doping VO2 thin films with high valence metal ions that generate a donor-like defect to decrease Tt [6]. Electrical transport properties of VO2 offer an important but challenging problem of fundamental nature. Till now only a few thermoelectric measurements of pristine VO2 [7–10] and tungsten-doped VO2 [11] have been reported. Further, carrier concentration is a key parameter and, nonetheless, data available for this parameter is insufficient, particularly for thin films. Furthermore, the Hall effect measurements in VO2 thin films have been a strenuous task due to a lot of complexities cropping up in the form of low Hall mobility, high carrier concentration (low Hall voltage) and large noise because of irregular lattice transformation across SMT. Moreover, electrons are majority carriers on either side of the SMT and the reported results for electron density differ as much as 2 orders of magnitude both in low-temperature semiconducting phase as well as in high-temperature metallic phase [12–16]. The effect of SHI irradiation on thermoelectric measurements of VO2 thin films has not been reported by any research group till date. The irradiation on Cr-doped vanadium sesquioxide [(V1  xCrx )2O3] by 6 GeV Pb-ions has demonstrated that prominent changes do take place in the metal-to-insulator transition (MIT) temperature and electrical properties of VO2 [17–18]. The irradiation of VO2 thin films fabricated on heavily-doped n-type Si by 1 GeV 238 U-ions have revealed that a shift of the MIT temperature takes place toward lower values [19]. A controlled modification of SMT of VO2 single-crystal thin films irradiated by 200 MeV Au-ions have also been reported [20]. No work has been reported on carrier concentration, and thermoelectric measurements of SHI irradiated VO2 thin films so far to the best knowledge of the authors. The present work reports the effect of 200 MeV Ag9 þ -ions at different ion beam fluences on electrical transport properties of VO2 thin films fabricated by sol–gel technique on alumina substrates. Herein, it has been demonstrated that by SHI irradiation, carrier concentration and hence conductivity of VO2 thin films can be enhanced in insulating phase by few orders of magnitude devoid of any appreciable change in Seebeck coefficient that augments thermoelectric performance of VO2 thin films.

placed on magnetic stirrer with vigorous stirring till clear yellow solution gets formed. 5.40 g of H2C2O4  2H2O were added to the above solution in small amounts with continuous stirring till clear blue solution gets formed due to reduction of V5 þ into V2 þ as per the reaction mechanism, V2O5+3H2 C2O4→2VOC2O4 + 2CO2 +3H2 O. 10 ml of freshly prepared polyvinyl alcohol (PVA, 0.2 g) solution was added drop wise to the above solution and stirred for next 30 minutes continuously at room temperature. After aging for one day, the solution was available for coating. Prior to deposition, the alumina substrates were first treated by ultrasonic cleaning in dilute sulfuric acid for 10 minutes and then thoroughly rinsed with ethanol, acetone and de-ionized water. The films were coated on substrates at a rate of 3000 rpm for 30 seconds using spin coater (Spin-NXG-P1:Apex Instruments, India). After each coating, the wet films were subsequently dried at 80 °C for 15–20 min on a hot plate to remove excessive water and organics. Eight successive coatings of VO2 gel were performed on each alumina substrate. Finally, these VO2 gel films were crystallized by annealing at 400 °C for 4 h under ambient atmosphere in a programmable tubular furnace (Nabertherm GmbH Tube furnace: RHTC80) with a heating and cooling rate of 3 °C per min. 2.2. Swift Heavy Ion irradiation VO2 thin films deposited on alumina substrates were irradiated by 200 MeV Ag9 þ -ions at the 15 UD Pelletron Accelerator Facility, IUAC, New Delhi at ion beam fluences of 1E11, 5E11, 1E12, and 5E12 ions/cm2. The Ag9 þ -ion was chosen due to its higher mass and the energy regime was selected after simulation using SRIM2013 code [21]. Fig. 1 shows nuclear (En) and electronic (Ee) energy loss of Ag9 þ -ions in VO2 matrix. In nuclear stopping, ions collide with the target atoms elastically and lose their energy to the target lattice causing atomic displacements which result in the creation of Frenkel defects (vacancy and interstitial pairs). However, electronic stopping is inelastic in nature that induce dense electronic excitations at high ion energies. The energy stored in electronic excitations gets rapidly transferred to the phonons as the electron–phonon relaxation times are less than picoseconds. According to thermal spike model, these dense electronic excitations can raise the temperature to several thousand degrees within a narrow cylindrical zone of a few nm around the ion path for short time durations [22]. The electronic excitations also cause breaking of chemical bonds via Coulomb explosion which can lead to significant disorder in the VO2 matrix around the ion-tracks [23]. Fig. 2 shows SRIM simulation projected depth of  12,000 nm at 200 MeV for Ag9 þ -ion which is much larger than  184 nm, the

2. Experimental details 2.1. Fabrication of VO2 thin films Thin films of VO2 were fabricated by inorganic sol–gel method on alumina substrates using vanadium pentaoxide (V2O5) (SigmaAldrich, purity 499.9%) and H2C2O4  2H2O (Sigma-Aldrich) as primary precursors. 3.7 g of V2O5 powder were melted in a ceramic crucible at 750 °C in muffle furnace for 30 minutes and then poured quickly in a beaker containing 30 ml of deionized water

Fig. 1. SRIM simulation electronic and nuclear energy loss versus energy for Ag9 þ -ion in VO2 matrix. At 200 MeV, electronic stopping is clearly the dominant energy loss mechanism.

G.R. Khan et al. / Radiation Physics and Chemistry 123 (2016) 55–62

57

Fig. 3. RBS spectra along with SIMNRA simulation results of unirradiated VO2 thin film. Fig. 2. SRIM simulation projected depth of 12,000 nm at 200 MeV for Ag9 þ -ion in VO2 matrix. Inset shows energy losses are almost constant throughout the VO2 film thickness.

thickness of VO2 films. Inset of the Fig. 2 shows variation of nuclear and electronic energy loss of 200 MeV verses depth in the depth range of 0–300 nm indicating that energy losses are almost constant throughout the VO2 film thickness. Therefore, the effects of electronic excitations on the structural and electronic properties of VO2 thin films are anticipated to be uniform throughout the thickness of the films.

3. Characterizations Rutherford backscattering spectrometry (RBS) was performed using 2 MeV He2 þ -ions for compositional studies of the SHI irradiated VO2 thin films. Rutherford simulation program (SIMNRA) was used to simulate the RBS spectra. The phase purity and crystal structures of the samples were characterized by X-ray diffractometery (XRD) using Bruker D8 Advance diffractometer with Cu-Kα (λ ¼1.5406 Å) X-ray source at a scan speed of 0.5°/min at room temperature in the diffraction angle (2θ) range of 20–50°. Surface analysis of the samples were performed by Atomic Force Microscopy (AFM). The temperature (T) dependence of electrical resistance (R) and Seebeck coefficient (s) of the films were determined using DC standard four probe technique and bridge method systems fitted with lakeshore temperature controller and Keithley 2612 A system sourcemeter. The Hall effect measurement was done at room temperature using Ecopia HMS-3000 Hall Probe System to evaluate carrier concentration (n) and carrier mobility (μ). 3.1. RBS analysis Since the oxides of vanadium can exist in a wide range of stochiometries, to check that all the samples exhibit VO2 stoichiometry before irradiation, the composition of thin films were confirmed by RBS measurements and SIMNRA simulation code [24]. Fig. 3 shows RBS spectra of unirradiated VO2 thin film along with SIMNRA simulation results. The simulation results suggest V/ O ratio as 1/2 within simulation error thereby indicating good stoichiometry of the samples. The thickness of sample determined by simulation was found to be  184 nm. 3.2. XRD analysis Fig. 4 plots XRD spectra of unirradiated and 200 MeV Ag9 þ -ion irradiated VO2 thin films at different fluences in the 2θ range of 25–60°. Diffraction pattern of the samples can be indexed to a monoclinic VO2 phase [JCPDS card no 82-0661, space group P21/C].

Fig. 4. XRD spectra of unirradiated and 200 MeV Ag9 þ -ion irradiated VO2 thin films at different fluences. Inset shows broadening and shifting towards lower 2θ value of (011) peak indicating decrease in crystallite size with increasing beam fluence.

Full range 2θ scan clearly indicates that there exist no other vanadium oxide phases in any of the samples thereby signifying good phase purity of VO2 thin films. The sharp peak at 2θ ¼41.71° corresponds to the reflection from the (0006) alumina substrate whereas peaks at 2θ ¼27.88°, 37.70°, 48.42° and 55.51 °correspond to reflections from (011), (210), (302) and (211/220) planes of the monoclinic phase of VO2, respectively. High intensity peak at 27.88° indicates that the films are highly (011) oriented with the lowest energy of the monoclinic phase which is in agreement with earlier reported growth orientations [25–26]. Inset of Fig. 4 presents magnified view of (011) peak which clearly shows that the peak intensity decreases with increasing fluence. The inset also depicts broadening and shifting of (011) peak towards lower 2θ value signifying decrease in crystallite size upon increasing fluence. This implies that SHI irradiation causes disordered regions in the films which increase with increasing fluence. At fluence 5E12 ions/cm2, no sharp peak exists indicating amorphous behavior of this film which may be due to the overlapping of ion-tracks (defective disordered regions) that occur at this fluence causing neardisappearance of the VO2 peaks. Also a slight shift of (011) peak towards lower 2θ was observed for the irradiated films in comparison to unirradiated film indicating enhancement of adjacent interplanar space (D-spacing) upon irradiation as per Bragg’s law. Since unirradiated films are under compressive strain along the out of plane direction [20], this strain relaxes due to creation of defects in disordered ion-track regions leading to a shift in the peak positions. The average crystallite size apparently decreases with increasing fluence as calculated by using Scherrer’s formula [27]. The increase in D-spacing induces tensile stress along the (000Z) direction which shifts the MIT temperature towards the

58

G.R. Khan et al. / Radiation Physics and Chemistry 123 (2016) 55–62

Fig. 5. Raman spectra of unirradiated and 200 MeV Ag9 þ -ion irradiated VO2 thin films at different fluences.

lower value as observed both in R–T as well as in Seebeck coefficient measurements. This result is in agreement with the well known observation that during the transition from semiconducting to metal phase, VO2 volume expands by 1% of its original volume. 3.3. Raman analysis Raman scattering measurements have been performed to study vibrational modes of VO2 thin films. The low-temperature monoclinic phase of VO2 has a distorted rutile structure with space group C53V (P21/c). Group theory predicts that in the low-temperature monoclinic insulating phase, there are nine Ag and nine Bg Raman active modes for which the Raman tensors are: [28]

⎛0 0 e⎞ ⎛ a d 0⎞ ⎜ ⎟ ⎜ ⎟ A g=⎜ d b 0⎟ Bg =⎜ 0 0 f ⎟. ⎜ ⎟ ⎝0 0 c⎠ ⎝ e f 0⎠ Fig. 5 displays Raman spectra of unirradiated and 200 MeV Ag9 þ -ion irradiated VO2 thin films in the shift range of 100–700 cm  1. The Raman spectrum of unirradiated sample is dominated by the peaks at 191, 224, 260, 308, 341, 394, 451, 491 and 614 cm  1 which match with Stokes lines assigned to VO2 monoclinic phase [29–30]. None of the peak associated with other phases of vanadium oxide are observed indicating high purity of VO2 phase of the samples. Table 1 compares Raman peak values of seven Ag and two Bg modes identified in the low temperature monoclinic phase with other works [31–33]. Raman active modes at low energy region (  191 and 224 cm  1) correspond to characteristic Ag-symmetry vibrational modes of the monoclinic (lowtemperature) structure of VO2 which vanish upon transition into the tetragonal (high-temperature) phase [30,33–35]. These phonon modes play a crucial role in the structural transition of VO2 as they are associated with the pairing and tilting motions of V–V dimmers that map the monoclinic into the tetragonal lattice Table 1 Comparison of Raman data of three referred works with the present one. Reference [34]

Reference [35]

Reference [36]

Present work

Peak

Mode

Peak

Mode

Peak

Mode

Peak

194 226 260 311 337 X X 493 615

Ag Ag Ag Ag Ag X X Ag Ag

194 225 258 308 339 395 453 489 618

Ag Ag Bg Ag Bg Bg Bg Bg Ag

X 226 262 311 339 395 454 483 618

X Ag Ag Bg and Ag Ag Bg Bg Bg Ag

191 224 260 308 341 394 451 491 614

configuration [36]. A broad mode at 614 cm  1 is associated with V–O vibration [37]. Phonons in the intermediate and high-energy regions are assigned to different V–O vibrations. The absence of any significant peak around 700 cm  1 independently confirm the lack of V2O5 phase, the most common occurring secondary phase during the VO2 film growth. SHI irradiation causes broadening and shifting of Raman peaks towards the lower wavenumber region due to creation of defects in disordered ion-track regions. Similar phenomenon of Raman peak shift has been observed in PbTiO3 which is attributed to increase in concentration of oxygen vacancies [38]. The shifting of Raman peaks towards lower wavenumber due to creation of oxygen vacancies upon SHI irradiation is supported by shifts of peaks towards higher wavenumber for oxygen enhanced VO2 samples [39]. Furthermore, peak intensities and FWHM increase upon increasing fluence which may be consequently due to decrease in crystallinity of films with increasing fluence. The disappearance of distinct Raman peaks at 5  1012 ions/cm2 fluence confirm that the 5  1012 ions/cm2 fluence is above the threshold of overlapping fluence where ion-tracks overlapped with each other. Therefore, for the irradiated films at this fluence, Raman peaks corresponding to VO2 vibrational modes disappear due to a high degree of disorder in overlapping of ion-tracks throughout the thickness of the VO2 thin film. 3.4. Surface analysis The surface properties of the films prior and post irradiation have been investigated using AFM. Fig. 6(a–e) shows the two-dimensional surface topography of the unirradiated and irradiated films at different fluences for 1 mm2 scan area. AFM micrographs clearly show that the irradiation has notable impact on grain size. Grain size decreases whereas density increases with increasing fluence which is in conformity with XRD results. Films irradiated at the fluence of 1E12 were partially amorphous whereas films irradiated at the fluence of 5E12 were completely amorphous in nature. The decline in the grain size is ascribed to the irradiationinduced grain splitting effect. Fig. 6(a*–e*) shows three-dimensional surface topography for 1 mm2 scan area of unirradiated and irradiated VO2 thin films. Fig. 6 demonstrates that surface roughness first increases and then decreases with the increase of fluence. Fig. 7 displays the variation of root mean square (RMS) roughness of samples determined from AFM micrographs. The roughness of unirradiated film is 27.92 nm. However, upon SHI irradiation, RMS roughness decreases to 11.21 and 10.42 nm values at fluences 1E11 and 5E11 ions/cm2, and thereafter increases monotonically to 33.69 and 51.22 nm values at fluences 1E12 and 5E12 ions/cm2, respectively. Similar roughness behavior have been reported by other investigators [1,40–44]. Smoothening of the surface at low fluences may be due to irradiation induced viscous flow, volume diffusion, or surface diffusion. Variation of surface roughness with different dose of ion beam irradiation can be explained on the basis of interplay between the dynamics of surface roughening that occurs during sputtering and smoothening induced by the material transport. The collision surge as an outcome of ion–solid interaction can enable target atoms to acquire enough kinetic energy to escape from the solid surface (sputtering). At low fluences, surface smoothening occurs because of those atoms which are ejected to surface due to volume diffusion mechanism with too low energy to escape the energy barrier but can drift parallel to the surface. At higher fluences, the dominant mechanism is evaporation–condensation which increases surface roughness because of the evaporation of atoms from a hot surface heated by an inelastic thermal spike. The results indicate that at higher fluence of 1E12 ions/cm2, the surface evaporation mechanism (sputtering) is

G.R. Khan et al. / Radiation Physics and Chemistry 123 (2016) 55–62

59

Fig. 6. (a–e). 2-D AFM images of unirradiated and 200 MeV Ag9 þ -ion irradiated VO2 thin films at different ion fluences. (a) unirradiated (b) 1E11 (c) 5E11 (d) 1E12 (e) ) 5E12 ions/cm2. a*–e* shows corresponding 3-D AFM micrographs.

responsible for increase in surface roughness. Further, at higher fluences, hillocks appear on the surface of thin films which can be explained as follows. In the case of low fluence regime of ion irradiation, the damaged regions do not overlap with each other.

Beyond certain critical fluence, damaged zones start overlapping due to the cumulative effect of electronic excitation induced shear motion of atoms towards the surface, which leads to the formation of hillocks.

60

G.R. Khan et al. / Radiation Physics and Chemistry 123 (2016) 55–62

3.5. Transport studies

Fig. 7. Variation of RMS roughness of the samples calculated from AFM images.

Fig. 8(a) shows variation of resistance as a function of temperature for unirradiated and 200 MeV Ag9 þ -ion irradiated VO2 thin films at different ion fluences. Unirradiated VO2 thin films exhibit MIT at a temperature of 340 K during heating and at 329 K during cooling with a hysteresis width of 11 K. After irradiation with Ag9 þ -ions, transition temperature decreases to 335, 332, 329 and 325 K corresponding to ion fluences 1E11, 5E11, 1E12, and 5E12 ions/cm2, respectively. However, simultaneously the sharpness of transition decreases and the breadth of transition increases continuously with increasing fluence. Hysteresis width reduces to 8, 5, 3 and 1 K for films when irradiated at fluences 1E11, 5E11, 1E12, and 5E12 ions/cm2, respectively. Further, resistance in the insulating state decreases by large amount of magnitude and increases slightly in metallic state. Hall measurements demonstrate that all the samples are n-type semiconductors. Variation of carrier concentration and mobility of the films measured at room temperature with increasing fluence is shown in Fig. 8(b). Carrier concentration increases continuously with increasing fluence whereas mobility decreases first with increasing fluence from 1  1012 ions/cm2 and then increases with further increase in

Fig. 8. Variation thermoelectric parameters of unirradiated and irradiated VO2 thin films: (a) resistance as a function of temperature (b) carrier concentration and mobility at room temperature as a function of irradiation fluence (c) Seebeck coefficient as a function of temperature (d) schematic representation of electronic band structure of VO2 modified by oxygen vaccancies created by SHI irradiation. The short lines show energy level positions due to oxygen vaccancies within the gap and dots show electron occupancies.

G.R. Khan et al. / Radiation Physics and Chemistry 123 (2016) 55–62

fluence. Fig. 8(c) reveals variation of Seebeck coefficient of unirradiated and irradiated VO2 thin films. Negative value of Seebeck coefficient also confirms that all the samples are n-type semiconductors. MIT transition obviously appears in the variation of Seebeck coefficient as a function of temperature and shift of transition temperature with irradiation at different fluences. In insulating phase around 300 K, Seebeck coefficient increases linearly with temperature whereas in the metallic state, it remains constant as temperature increases. In the semiconducting phase, there is no appreciable variation of Seebeck coefficient with increasing fluence in spite of increase in electrical conductivity by few orders; however, in the metallic phase it first increases with increase of fluence up to 1E12 and then decreases with further increase of fluence. The value of Seebeck coefficient at 400 k for unirradiated film is  23 mV/K, and irradiated films at the fluences 1E11, 5E11, 1E12, and 5E12 ions/cm2 are  57,  95,  86, and 43 mV/K, respectively. Similar behavior of Seebeck coefficient with temperature was reported by a number of investigators [7– 10,45]. Variation of the foregoing transport parameters upon SHI irradiation is explained as follows. SHI irradiation creates defects (oxygen vacancies) in disordered ion-track regions and reduces grain size as observed in XRD and Raman spectra. The induction of these defects modifies the band structure of the film in semiconducting phase. The creation of oxygen vacancies can be expressed by the chemical equilibrium as Oox ↔Vox þ 1 (O2), where Oox represents the neutral oxygen atom in an 2

oxide site and Vox represents the neutral oxygen vacancy with two trapped electrons [46]. Thus a missing oxygen atom in VO2 results in one or two electrons localized in an oxygen vacancy state. The driving force for the localization of the electrons in the oxygen vacancy state is the Madelung potential of the highly ionic crystal [7]. In this way, the place occupied by the O−2 -anion in the regular lattice is taken by one or two “free” electrons in the defective crystal [47]. These electrons located on the oxygen vacancy states have a direct effect on the electronic structure of VO2 by forming a donor level below the conduction band as shown in Fig. 8(d). Moreover, the removal of neutral oxygen atoms to form oxygen vacancies can also cause the redistribution of the excess electrons among the nearest neighboring V atoms around the oxygen vacancy site, and form shallow donor states below the conduction band originating from V 3d orbits [48]. These donor states increase with increasing oxygen vacancies leading to increase in carrier concentration and shift in the Fermi level of VO2 toward conduction band thereby narrowing the band gap and hence lowering the resistance in the insulating state [49,50]. Furthermore, these defect states act as nucleation centers and nucleation of phase transition occurs more likely at the position of defects as less thermal energy is required to overcome the barrier of transformation [51,52]. The calculation of the change in free energy due to the formation of a new phase and the experimental results suggest that more potent defects can lead to an earlier occurrence of phase transition [53]. So the defect plays a significant role in the process of phase transition and reduces the phase transition temperature as well as hysteresis width of the transition. In the present work, defects are produced in the films by 200 MeV Ag9 þ -ion irradiation and these defect sites along with grain boundaries scatter the charge carriers which decreases carrier mobility due to irradiation up to ion fluence 5E11. With further increase in fluence, the defect states overlap with each other causing partially or fully amorphous behavior of the VO2 thin films which reduces grain boundary scattering of charge carriers and hence increase in carrier mobility. Variation of Seebeck coefficient with irradiation is based on the phenomenon of filtering of charge carriers along the grain boundaries and defect states. The irradiation creates defects in the VO2 thin films and also reduces the

61

grain size as observed in XRD spectra, Raman spectra and AFM images and with increasing fluence crystallite size goes on decreasing continuously. These defect states and grain boundaries act as filtering centers that allow only charge carriers with high energy to transport from high temperature region to lower temperature region and trap low energy charge carriers. As Seebeck coefficient depends upon the average energy of charge carriers transporting from higher temperature region to lower temperature region, its value goes on increasing upon irradiation of VO2 thin films. At fluence 5E12, an overlapping of ion tracks occur which shows amorphous nature with no grain boundaries. Thus no trapping of low energy charge carriers takes place at this fluence and hence decreases the Seebeck coefficient.

4. Conclusions The SHI irradation is a viable method for controlling electrical and thermoelectric properties of VO2 thin films by introducing point defects. SHI irradiation with 200 MeV Ag9 þ -ion beam at ion fluences of 1E11, 5E11, 1E12, and 5E12 for tuning of electrical transport properties of VO2 thin films fabricated by so–gel technique on alumina substrates has been demonstrated in the present paper. The point defects (oxygen vacancies) created by SHI irradiation tune metal to insulator phase transition temperature, carrier concentration, carrier mobility, electrical conductivity, and Seebeck coefficient of VO2 thin films. Structural studies of the films show decrease in crystallite size upon irradiation and surface roughness first decreases and then increases with increasing fluence. All the samples exhibit metal–insulator phase transition and the transition temperature decreases marginally towards room temperature with the increase of ion beam fluence. Unirradiated VO2 thin films exhibit MIT at a temperature of 340 K during heating and at 329 K during cooling with a hysteresis width of 11 K. After irradiation with Ag9 þ -ions, transition temperature decreases to 335, 332, 329 and 325 K corresponding to ion fluences 1E11, 5E11, 1E12, and 5E12 ions/cm2, respectively. However, simultaneously the sharpness of transition decreases and the breadth of transition increases continuously with increasing fluence. Hysteresis width reduces to 8, 5, 3 and 1 K for films when irradiated at fluences 1E11, 5E11, 1E12, and 5E12 ions/cm2, respectively. As a result of creation of oxygen vaccancies, the carrier concentration increases continuously upon irradation which inturn enhances electrical conductivity of VO2 thin films in the insulating phase by several orders of magnitude. Upon irradation, the carrier mobility decreases due to scattering of charge carriers by these point defects and the grain boundarires at low fluence but increases due to amorphization of VO2 thin films at higher fluence. Seebeck coefficient in insulating phase remains almost constant in spite of increase in electrical conductivity by several orders of magnitude which results in augmentation of thermoelectrical performance in VO2 thin films. In the high tempreture insulating phase, Seebeck coefficient increases at lower beam fluence and decreases at higher beam fluence which has been explained on the basis of filtering of charge carriers along grain boundaries and defect sites in VO2 thin film. The results discussed herein, demonstrate that irradiation of VO2 thin films by 200 MeV Ag9 þ -ions provides an excellent and exceptional technique for augmentation of thermoelectric properties that are considered essential for modern device fabrication. References Thomas, H., Thomas, S., Ramanujan, R.V., Avasthi, D.K., Al-Omari, I.A., Al-Harthi, S., Anantharaman, M.R., 2012. Swift heavy ion induced surface and

62

G.R. Khan et al. / Radiation Physics and Chemistry 123 (2016) 55–62

microstructural evolution in metallic glass thin films. Nucl. Instrum. Methods Phys. Res. Sect. B: Beam Interact. Mater. Atoms 287, 85–90. Mehta, G.K., 2000. High energy accelerators and materials science. Proc. Indian Natl. Sci. Acad. Part A 66, 653–674. Morin, F.J., 1959. Oxides which show a metal-to-insulator transition at the Neel temperature. Phys. Rev. Lett. 3, 34. McWhan, D.B., Marezio, M., Remeika, J.P., Dernier, P.D., 1974. X-ray diffraction study of metallic VO2. Phys. Rev. B 10, 490. Khan, G.R., Bhat, B.A., 2015. Quantum size effect across semiconductor-to-metal phase transition in vanadium dioxide thin films. Micro Nano Lett. 10, 607–612. Zhang, Y., Zhang, J., Zhang, X., Huang, C., Zhong, Y., Deng, Y., 2013. The additives W, Mo, Sn and Fe for promoting the formation of VO2 (M) and its optical switching properties. Mater. Lett. 92, 61–64. Berglund, C.N., Guggenheim, H.J., 1969. Electronic Properties of VO2 near the semiconductor-metal transition. Phys. Rev. 185, 1022. Wu, C., Feng, F., Feng, J., Dai, J., Peng, L., Zhao, J., Xie, Y., 2011. Hydrogen-incorporation stabilization of metallic VO2 (R) phase to room temperature, displaying promising low-temperature thermoelectric effect. J. Am. Chem. Soc. 133, 13798–13801. Cao, J., Fan, W., Zheng, H., Wu, J., 2009. Thermoelectric effect across the metalinsulator domain walls in VO2 microbeams. Nano Lett. 9, 4001–4006. Fu, D., Liu, K., Tao, T., Lo, K., Cheng, C., Liu, B., Wu, J., 2013. Comprehensive study of the metal-insulator transition in pulsed laser deposited epitaxial VO2 thin films. J. Appl. Phys. 113, 043707. Katase, T., Endo, K., Ohta, H., 2014. Thermopower analysis of the electronic structure around the metal-insulator transition in V1  xWxO2. Phys. Rev. B 90, 161105. Barker Jr, A.S., Verleur, H.W., Guggenheim, H.J., 1966. Infrared optical properties of vanadium dioxide above and below the transition temperature. Phys. Rev. Lett. 17, 1286. Kitahiro, I., Ohashi, T., Watanabe, A., 1966. Hall effect of vanadium dioxide powder. J. Phys. Soc. Jpn. 21, 2422. Hensler, D.H., 1968. Transport properties of sputtered vanadium dioxide thin films. J. Appl. Phys. 39, 2354–2360. Kwan, C.C.Y., Griffiths, C.H., Eastwood, H.K., 1972. Transport and structural properties of VO2 films. Appl. Phys. Lett. 20, 93–95. Rosevear, W.H., Paul, W., 1973. Hall Effect in VO2 near the semiconductor-to-metal transition. Phys. Rev. B 7, 2109. Kokabi, H.R., Studer, F., Toulemonde, M., 1996. Damage induced by high energy lead irradiation in (V0.997Cr0.003)2O3 ceramics. Nucl. Instrum. Methods Phys. Res. Sect. B: Beam Interact. Mater. Atoms 111, 75–83. Kokabi, H.R., Studer, F., 1997. Influence of high energy lead irradiation on the low and high temperature metal-semiconductor transitions in (V1  xCrx)2O3 ceramics. Nucl. Instrum. Methods Phys. Res. Sect. B: Beam Interact. Mater. Atoms 124, 47–54. Hofsäss, H., Ehrhardt, P., Gehrke, H.G., Brötzmann, M., Vetter, U., Zhang., K., Krauser, J., Trautmann, C., Ko, C., Ramanathan, S., 2011. Tuning the conductivity of vanadium dioxide films on silicon by swift heavy ion irradiation. AIP Adv. 1, 032168. Gupta, A., Singhal, R., Narayan, J., Avasthi, D.K., 2011. Electronic excitation induced controlled modifications of semiconductor-to-metal transition in epitaxial VO2 thin films. J. Mater. Res. 26, 2901. Ziegler, J. F., SRIM  2013, Retrievable from website: Interactions of ions with matter. Toulemonde, M., Costantini, J.M., Dufour, C., Meftah, A., Paumier, E., Studer, F., 1996. Track creation in SiO2 and BaFe12O19 by swift heavy ions: a thermal spike description. Nucl. Instrum. Methods Phys. Res. Sect. B: Beam Interact. Mater. Atoms 116, 37–42. Avasthi, D.K., Assmann, W., Notle, H., Mieskes, H.D., Ghosh, S., Mishra, N.C., 2000. Transport of oxygen atoms mediated by electronic excitation. Nucl. Instrum. Methods Phys. Res. B 166, 345–349. M. Mayer, SIMNRA, a simulation program for the analysis of NRA, RBS and ERDA, in: AIP Conference Proceedings, 1999, pp. 541–544. Yan, J., Huang, W., Zhang, Y., Liu, X., Tu, M., 2008. Characterization of preferred orientated vanadium dioxide film on muscovite (001) substrate. Phys. Status Solidi (A) 205, 2409–2412. Zhang, C., Cao, W., Adedeji, A.V., Elsayed-Ali, H.E., 2014. Preparation and properties of VO2 thin films by a novel sol–gel process. J. Sol–Gel Sci. Technol. 69, 320–324. Scherrer, P., 1918. Determination of the size and internal structure of colloidal

particles using X-rays. Math. Phys. Kl. 1918, 98–100. Kuzmany, H., 2009. Solid-State Spectroscopy – An introduction, 2nd Ed. Springer, Berlin. Zhang, S., Chou, J.Y., Lauhon, L.J., 2009. Direct correlation of structural domain formation with the metal insulator transition in a VO2 nanobeam. Nano Lett. 9, 4527–4532. Schilbe, P., 2002. Raman scattering in VO2. Phys. B: Condens. Matter 316, 600–602. Vikhnin, V.S., Goncharuk, I.N., Davydov, V.Y., Chudnovskii, F.A., Shadrin, E.B., 1995. Raman spectra of the high-temperature phase of vanadium dioxide and model of structural transformations near the metal-semiconductor phase transition. Phys. Solid State 37, 1971–1978. Chen, X.B., 2011. Assignment of the Raman modes of VO2 in the monoclinic insulating phase. J. Korean Phys. Soc. 58, 100. Pan, M., Liu, J., Zhong, H., Wang, S., Li, Z.F., Chen, X., Lu, W., 2004. Raman study of the phase transition in VO2 thin films. J. Cryst. Growth 268, 178–183. Kim, H.T., Chae, B.G., Youn, D.H., Kim, G., Kang, K.Y., Lee, S.J., Lim, Y.S., 2005. Raman study of electric-field-induced first-order metal-insulator transition in VO2-based devices. Appl. Phys. Lett. 86, 242101. Srivastava, R., Chase, L.L., 1971. Raman Spectrum of Semiconducting and Metallic VO2. Phys. Rev. Lett. 27, 727. Cavalleri, A., Dekorsy, T., Chong, H.H., Kieffer, J.C., Schoenlein, R.W., 2004. Evidence for a structurally-driven insulator-to-metal transition in VO2: a view from the ultrafast timescale. Phys. Rev. B 70, 161102. Marini, C., Arcangeletti, E., Di Castro, D., Baldassare, L., Perucchi, A., Lupi, S., Postorino, P., 2008. Optical properties of V1  xCrxO2 compounds under high pressure. Phys. Rev. B 77, 235111. Nishida, K., Osada, M., Takeuchi, H., Yosiaki, I., Sakai, J., Ito, N., Katoda, T., 2008. Raman spectroscopy study of oxygen vacancies in PbTiO3 thin films generated heat-treated in hydrogen atmosphere. Jpn. J. Appl. Phys. 47, 7510. Parker, J.C., 1990. Raman scattering from VO2 single crystals: a study of the effects of surface oxidation. Phys. Rev. B 42, 3164. Dash, P., Mallick, P., Rath, H., Tripathi, A., Prakash, J., Avasthi, D.K., Mazumder, S., Varma, S., Satyam, P.V., Mishra, N.C., 2009. Surface roughness and power spectral density study of SHI irradiated ultra-thin gold films. Appl. Surf. Sci. 256, 558. Gupta, A., Chauhan, R.S., Agarwal, D.C., Kumar, S., Khan, S.A., Tripathi, A., Avasthi, D. K., 2008. Smoothing, roughening and sputtering: the complex evolution of immiscible Fe/Bi bilayer system. J. Phys. D: Appl. Phys. 41, 215306. Kanjilal, A., Kanjilal, D., 2006. Surface roughening in Si1  xGex alloy films by 100 MeV Au: composition dependency. Solid State Commun. 139, 531. Goswami, D.K., Dev, B.N., 2003. Nanoscale self-affine surface smoothing by ion bombardment. Phys. Rev. B 68, 033401. Mayr, S.G., Averback, R.S., 2001. Surface smoothing of rough amorphous films by irradiation-induced viscous flow. Phys. Rev. Lett. 87, 196106. Katase, T., Endo, K., Ohta, H., 2015. Thermopower analysis of metal–insulator transition temperature modulations in vanadium dioxide thin films with lattice distortion. Phys. Rev. B 92, 035302. Gillet, M., Lemire, C., Gillet, E., Aguir, K., 2003. The role of surface oxygen vacancies upon WO3 conductivity. Surf. Sci. 532, 519–525. Pacchioni, G., 2003. Oxygen vacancy: the invisible agent on oxide surfaces. Chem. Phys. Chem. 4, 1041–1047. Liu, G., Li, F., Wang, D.W., Tang, D.M., Liu, C., Ma, X., Cheng, H.M., 2007. Electron field emission of a nitrogen-doped TiO2 nanotube array. Nanotechnology 19, 025606. Khan, S.A., Kalathil, M.M., Nisar, S., Lee, J., A., Cho, M.H., 2013. Oxygen vacancy induced band gap narrowing of ZnO nanostructure by electrochemically active biofilm. Nanoscale 5, 9238. Hasnip, P.J., Refson, K., Probert, M.I.J., Yates, J.R., Clark, S.J., Pickard, C.J., 2014. Density functional theory in the solid state. Philos. Trans. R. Soc. A 372, 20130270. Frenzel, A., Qazilbash, M.M., Brehm, M., Chae, B.G., Kim, B.J., Kim, H.T., Basov, D.N., 2009. Inhomogeneous electronic state near the insulator-to-metal transition in the correlated oxide VO2. Phys. Rev. B 80, 115115. Appavoo, K., Lei, D.Y., Sonnefraud, Y., Wang, B., Pantelides, S.T., Maier, S.A., Haglund Jr, R.F., 2012. Role of defects in the phase transition of VO2 nanoparticles probed by plasmon resonance spectroscopy. Nano Lett. 12, 780–786. Lopez, R., Haynes, T.E., Boatner, L.A., Feldman, L.C., Haglund Jr, R.F., 2002. Size effects in the structural phase transition of VO2 nanoparticles. Phys. Rev. B 65, 224113.