Tailoring the properties of spray deposited V2O5 thin films using swift heavy ion beam irradiation

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Original Article

Tailoring the properties of spray deposited V

₂

O

₅

thin

ﬁlms using swift

heavy ion beam irradiation

R. Rathika

a

, M. Kovendhan

b,*

, D. Paul Joseph

c,**

, Rekha Pachaiappan

d

,

A. Sendil Kumar

e

, K. Vijayarangamuthu

f

, C. Venkateswaran

b

, K. Asokan

g

,

S. Johnson Jeyakumar

a,***

a_{Department of Physics, TBML College, Porayar, 609307, India}

b_{Department of Nuclear Physics, University of Madras, Chennai, 600025, India} c_{Department of Physics, National Institute of Technology, Warangal, 506004, India}

d_{Department of Physics, Adhiyaman Arts and Science College for Women, Uthangarai, 635207, Tamilnadu, India}

e_{Department of Physics, Koneru Lakshmaiah Education Foundation, Vaddeswaram, Guntur District, Andhra Pradesh, 522502, India} f_{Center for Nanoscience and Technology, Pondicherry University, Pondicherry, 605014, India}

g_{Inter University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi, 110 067, India}

a r t i c l e i n f o

Article history:

Received 20 November 2019 Received in revised form 23 March 2020 Accepted 10 April 2020 Available online 17 April 2020

Keywords: Ion beam irradiation V2O5thinﬁlms

Spray coatings

Optical materials and properties Radiation damage

a b s t r a c t

Swift heavy ion (SHI) beam irradiation can generate desirable defects in materials by transferring suf-ﬁcient energy to the lattice that favours huge possibilities in tailoring of materials. The effect of Ag15þ_ion

irradiation with energy 200 MeV on spray deposited V2O5thinﬁlms of thickness 253 nm is studied at

various ion doses from 5 1011_{to 1} ₁₀13 _ions/cm2_{. The XRD results of pristine}_{ﬁlm conﬁrmed}

orthorhombic structure of V2O5and its average crystallite size was found to be 20 nm. The peak at

394 cm1in Raman spectra conﬁrmed OeVeO bonding of V2O5, whereas 917 cm1arise because of

distortion in stoichiometry by a loss of oxygen atoms. Raman peaks vanished completely above the ion ﬂuence of 5 1012_ions/cm2_{. Optical studies by UV}_{eVis spectroscopy shows decrement in transmittance}

with an increase in ionﬂuence up to 5 1012_ions/cm2_{. The red shift is observed both in the direct and}

indirect band gaps until 5 1012_ions/cm2_{. The surface topography of the pristine}_{ﬁlm revealed sheath}

like structure with randomly distributed spherical nano-particles. The roughness offilm decreased and the density of spherical nanoparticles increased upon irradiation. Irradiation improved the conductivity significantly for fluence 5 1011_ions/cm2_{due to band gap reduction and grain growth.}

1. Introduction

Vanadium pentoxide (V2O5) is one of the exciting transition

metal oxides, highly abundant in nature, and low cost electro-chromic material with amazing physical and chemical properties [1]. V2O5thinﬁlms can be integrated for a large number of

appli-cations such as in electrochromic devices [2], solar cells [3], gas sensors [4], and solid state batteries [5]. Vanadium based alloys are potential candidate for nuclear fusion reactor applications due to its

superior mechanical strength and ductility even at elevated tem-peratures [6]. Generation of hydrogen and helium ions will be much higher and nuclear heating will be lesser in theﬁrst wall by using vanadium instead of niobium [7].

Several methods including spray deposition, sol-gel, sputtering, thermal evaporation and chemical vapour deposition (CVD) are generally utilized to synthesize V2O5thinﬁlms [8e10]. We used the

spray pyrolysis method since it is a low cost, simple and versatile method for obtaining good quality thin films [11]. The physical properties of thefilms deposited by this method depends on the composition of precursor in addition to other spray parameters. The thickness of thefilm can be easily altered by changing the rate of spraying, substrate temperature and nozzle to the substrate distance [12]. V2O5 having better photoactivity, can be used to

replace the expensive platinum electrode in dye sensitized solar cell (DSSC). Kovendhan et al., synthesized lithium doped V2O5thin

* Corresponding author. ** Corresponding author. *** Corresponding author.

E-mail addresses:[email protected] (M. Kovendhan),[email protected]

(D.P. Joseph),[email protected](S.J. Jeyakumar).

Contents lists available atScienceDirect

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https://doi.org/10.1016/j.net.2020.04.013

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ﬁlm by spray pyrolysis technique and explored its applicability as photo-anode in devices [13].

Ion beams create nanostructures that are easy to tune for the properties of material. Modification of particular properties at nanoscale level of a material is possible with an ion beam irradia-tion [14]. Swift heavy ion (SHI) irradiation can melt a zone of ma-terial around an ion trajectory by interaction of high energy ion with the solid target. In this way, the solid phase transformation takes place at a lower temperature with SHI irradiation which can produce amorphization or recrystallization in the material [15]. Such modifications/enhancement in material properties will defi-nitely open a new way to novel applications. The influence of irradiation induced defects and grain growth in working of nano-electronic devices, play a vital role in emerging technology [16]. Improved conductivity is observed in vanadium dioxidefilms while irradiating it with a 1 GeV238U swift heavy ion beam [17]. Mitdank et al., demonstrated an analytic model using ion beams to explain the phase transition in porous TiO2/V2O5[18].

Electrochromic devices ﬁnd applications in smart windows, display units, self-dimming rear mirrors, electronic papers etc. For example, smart windows are energy saving technology that can modulate the amount of heat and light entering the building in an efﬁcient way. V2O5 exhibit both cathodic and anodic

electro-chromism, and it can modulate between reflecting state and absorbing state depending on intercalation [19]. The process of developing simple, low cost and scalable methods for synthesizing electrochromic coatings and transparent conductors are main areas of interest worldwide [20,21]. Further, property modification of electrochromic materials for high performance and extending their application in various fields are real challenge for industrial de-velopers and researchers [4]. We believe that property modi fica-tions of electrochromic material could be effectively achieved by swift heavy ion beam irradiation. In this research work, V2O5thin

ﬁlms have been irradiated with Ag15þ_{ion beam of energy 200 MeV}

in order to study its effect on the properties at various ionﬂuences. 2. Experimental procedure

2.1. Spray deposition

The V2O5thinﬁlms were deposited onto ITO coated glass

sub-strate at a temperature of 450C using the spray pyrolysis tech-nique. The precursor solution contained a stoichiometric amount of vanadium trichloride dissolved in 250 ml of bi-distilled water with a few drops of sulfuric acid (to avoid precipitation). The solution was sprayed through spray nozzleﬁxed at a distance of 35 cm from the substrate. The reaction involved in formation of V2O5ﬁlm is as

follows [13],

2VCl3þ 2H2Oþ H2SO4 / V2O5þ 4HCl [ þ1₂O2[ þ S [ þ Cl2[

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2.2. Irradiation experiments

Fig. 1shows the schematics of the V2O5thinﬁlm deposition by

spray pyrolysis and 200 MeV Ag15þion beam irradiation. Irradia-tion experiments were carried out at high energy ion beam facility using the 15 UD Pelletron accelerator available at Inter University Accelerator Centre (IUAC), New Delhi. V2O5 thinﬁlms are loaded

onto a copper ladder and were positioned vertically for the ion beam to incident normally. Irradiation at room temperature with

106mbar and 1 pnA beam current was carried out using 200 MeV Ag15þions. An electromagnetic scanner was used to scan over the sample of area 1 1 cm2_{uniformly. Ion}_{ﬂuence was varied from}

5 1011_{to 1}₁₀13_ions/cm2_{by varying the duration of ion beam}

exposure. The Stopping Range of Ions in Matter (SRIM) software is used to simulate irradiation parameters like range,ﬂuence, angle of incidence of silver ion in V2O5prior to irradiation [22]. The

elec-tronic energy loss (Se), nuclear energy loss (Sn) and projected range

of Ag15þion in V2O5were found to be 1757 eV/Å, 4.376 eV/Å and

17.60

m

m respectively (Fig. 2(a)). The higher Sevalue indicates that

the electronic stopping is stronger than the nuclear stopping for Ag15þion slowing down in V2O5at 200 MeV energy. The calculated

Se/Snratio value is ~401.

2.3. Characterization methods

Various characterization methods were employed to investigate the properties of pristine and irradiated samples. The X-ray Diffraction (XRD) data were collected using X'pert High scorer, PANalytical X-ray diffractometer with CuKaradiation. The Raman analysis were done using Renishaw system in back scattering ge-ometry at an excitation wavelength of 514.5 nm with an argon ion laser source. The transmittance spectrum were taken with UVeVis double beam spectrometer of JASCO type V-570 in normal inci-dence. The morphology of the V2O5thinﬁlms were characterized

with Atomic Force Microscopy (AFM-SPI 3800 N) in non-contact mode. The thickness of the V2O5ﬁlm is determined to be 253 nm

by using XP-1 surface proﬁler (Ambios Technology Inc.). Hall effect measurements were performed at room temperature to estimate the electrical transport properties with the Ecopia HMS 3000 Hall system.

3. Results and discussion 3.1. XRD analysis

The X-ray diffraction patterns of pristine and irradiated V2O5

thin ﬁlms at various ﬂuences 5 1011_{, 1} ₁₀12_{, 5}₁₀12 _and

1 1013_ions/cm2_{are shown in}_{Fig. 2}_{(b). The observed XRD pattern}

of the pristine sample is indexed using the reference pattern of orthorhombic V2O5 (JCPDS No.: 89e2482) [13,23]. Mrigal et al.,

synthesized V2O5 thin ﬁlms by the spray pyrolysis method also

reported similar orthorhombic structure [8].

It was observed that at the substrate temperature of 300C, the pulsed laser deposited V2O5 thin ﬁlms grow with orthorhombic

symmetry [24]. XRD peaks of irradiatedﬁlm (110) and (012) exactly matches with that of JCPDS: 89e2482 which conﬁrms V2O5. The

XRD peak at 23.7belongs to VO2phase observed in addition to the

standard V2O5 peaks. The substrate temperature 450C used in

V2O5 ﬁlm deposition was too high which distorted the VeO

bondings [25]. The oxygen vacancies created in the V2O5leads to

the emergence of this VO2peak [26]. The average crystallite size of

various planes of pristine sample estimated using Scherrer formula is 20 nm. Texture coefficient; TChkl¼ Ihkl I0hkl 1 N PN N¼1II0hklhkl (2)

where Ihklis the measured intensity of the h k l plane; I0hklis the

standard intensity of the h k l plane from the JCPDS, and N is the number of reﬂections observed in the XRD pattern of the ﬁlms.

From the above equation(2)preferred orientation of pristine ﬁlm is found to be along the (421) plane. The crystal structural parameters of pristine sample such as crystallite size (D),

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dislocation density (

d

), the number of crystallites per unit surface area (N) and micro strain (ε) are calculated using below equations [27], D¼ 0:9

l

b

cos

q

(3)

d

¼ 1 D2 (4) N¼ d D2 (5) ε ¼

b

cos

q

4 (6)

Where d is the thickness of_ﬁlm,

l

is X-ray wavelength (1.5406 Å),

q

is the Bragg angle, and

b

is the FWHM in radians. The parameters are as follows: D¼ 20 nm,

d

¼ 0.29, E ¼ 0.0003 and the number of crystallites per unit surface area (N) is 6.46 107_m2_.

The suppression of XRD peaks is observed upon irradiation, however very few weak peaks sustained even after irradiation indicating partial amorphization of the ﬁlm possibly with point defects [28,29]. At lowerﬂuence (5 1011_{and 1}₁₀12_ions/cm2_),

the XRD peaks (110) and (012) remained resistant to ion beam irradiation [30]. Our case seems to follow two stages, in which Fig. 1. Schematics of the V2O5thinﬁlm deposition by spray pyrolysis and 200 MeV Ag15þion beam irradiation.

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crystalline V2O5under SHI irradiation transforms into, amorphous

material as the radiation damage increases and restored crystalline structure asﬂuence increases. At the ion ﬂunce 5 1012_ions/cm2_,

the XRD peaks vanished completely and the ﬁlm became amor-phous. The reappearance of the XRD peaks at 1 1013_ions/cm2

fluence confirm the SHI beam irradiation induced recrystallization in material as a result of electronic energy loss [31,32]. This shows that the material transforms gradually from crystalline to amor-phous and recrystallized without any phase variations. The amorphization and recrystallization are related to the process that is attributed to defect creation and energy deposition by ion beam irradiation. Generally the number of defects increases with an in-crease in irradiationfluence at room temperature [33]. At lower fluence, atoms do not possess sufficient mobility and hence could not get over the energy barrier for recrystallization [34]. At higher fluence, mobility of atoms improved significantly, and dynamic defect annealing became prominent [34]. Therefore recrystalliza-tion of orthorhombic V2O5 from amorphous film was due to

nucleation and crystallization of localized area by the dynamic annealing effect. Due to irradiation induced defects, both XRD and Raman peak intensity decreased signiﬁcantly and only few peaks remained (due to partial amorphization) after irradiation. 3.2. Raman analysis

Raman spectroscopic investigation can be used to establish the structural modiﬁcations induced in V2O5due to SHI beam

irradia-tion.Fig. 3(a) represents the Raman spectra of irradiated V2O5thin

films at various fluences along with the pristine film. The structure of

a

-V2O5(orthorhombic) belongs to the Pmmn space group [35].

The V2O5layers are formed from packing of edge shared VO5square

pyramids linked in the‘ab’ plane. The Raman active modes in the low frequency range 200 cm1 and 500 cm1 belongs to the external deformation mode. The Raman mode at 394 cm1arise from OeVeO bending vibrations [36]. The oxygen non-stoichiometry in V2O5is the reason for the absence of 994 cm1

peak which corresponds to the characteristic strong bending mode of vanadyl peak [9]. Because of this non-stoichiometry a new peak was observed at 917 cm1[37].

The Raman band around 917 cm1is assigned to the short V]O bond which appears when V5þreduced to V4þdue to the presence of lattice vacancy defect in V2O5. This observed shift in vanadyl

peak is because of change in its bond length and weak bonding [24,38]. The high substrate temperature used in ﬁlm deposition

might have created such variations in V2O5 structure that is

attributed to the shifting. Intensity of all the Raman bands begins to decline due to irradiation induced breakage of V2O5bondings by

track formation [39,40]. However, no additional peaks or band shifting was observed after irradiation. The thermal spike model is applicable for a variety of irradiated materials to explain the gen-eration of latent tracks [33,41]. According to which, energy of an incident ion is depositedfirst on electrons within a short span of time (1015 to 1014 s) along the ion beam path, which is then dispersed to other electrons through electron-electron coupling and then to the lattice in short duration (1013to 1012s) through electron-phonon coupling. When the temperature is above the threshold value Seth, the material inside a cylindrical zone around the ion path of few nanometer in diameter melts and gets modi_fied forming track [40,42,43]. Asfluence increases the amount of defects increased and hence peak intensity decreased [44]. The fluence 5 1012_ions/cm2_{seems to be critical}_{fluence at which irradiation}

degraded the sample's surface completely and the Raman peaks fully vanished. Further increasing ﬂuence makes no change in Raman modes except for broadening of the peaks at 1 1013_ions/

cm2. At higher ﬂuence, generally cylindrical tracks overlap with each other and generate lots of ion tracks in the material [45]. This results in complete amorphization of V2O5thinﬁlm above the ion

ﬂunce of 1 1012_ions/cm2_.

3.3. UV-Vis spectra

The transmittance spectra of pristine and irradiatedfilms with variousfluences in UVeVis range are presented inFig. 3(b). This provides the details concerning electronic levels of the material in the higher energy range of the optical spectrum. Apart from studying the absorption coefficient, more information is evident about the band structure and band gap.

The expression for absorption coefﬁcient (

a

) is [13].

a

¼ 1 1 d ln 100 T (7) where, T is transmittance and d is the thickness of the deposited ﬁlm.

The pristineﬁlm exhibits more than 80% transparency in the visible range. Due to non-stoichiometry/oxygen vacancy the optical absorption edge fall was observed at around 300 nm which is in consistent with the XRD and Raman results [13]. After irradiating

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with an ionﬂuence of 5 1011_ions/cm2

,transmittance begins to

decrease [46].

The optical transparency decreased gradually up to to 50% as a function ofﬂuence except for 1 1013_ions/cm2_{. The above}

dis-cussion clearly shows the reduction in optical transmission of irradiated samples compared to pristine one. Upon irradiation, transmittance loss arises due to the several reasons. Earlier re-searchers reported that SHI irradiation creates various point defects like interstitials, vacancies, etc., [47]. Transmittance of ion irradi-ated films depends on concentration of point defects that can absorb light [48]. The influence of SHI beam irradiation on optical properties of V2O5thinfilms is mainly reduction in transmittance

up to 5 1012_ions/cm2_{and signi}_{ﬁcant shift in absorption edge. The}

optical absorption band edge (Fig. 3(b)) shifted towards higher wavelength with increasingﬂuence around 430 nm for 5 1012

ions/cm2. This red shift indicates the band gap shrinkage as a function offluence which is detailed in the next section [49]. Also, absorption edge falls gradually after irradiation, due to decrease in crystallinity of material [50]. The absorption coefficient value in-creases with an increase influence due to absorbing nature of samples irradiated up to 5 1012 _ions/cm2 _{fluence [}₅₁_{]. Beyond}

5 1012 _ions/cm2 _{transmittance once again begins to increase}

instead of decreasing which may be due to recrystallization by SHI irradiation, however it is lesser than the pristineﬁlm.

3.4. Optical properties

The Tauc relation for inter band transitions is given by the relation [52,53],

a

h

n

¼

a

0(hv- Eg)n (8)

where, h

n

is the incident photon energy,

a

is the absorption coef-ﬁcient,

a

0is the band tailing parameter and Egis the optical band

gap. In the above equation, n¼ ½ for direct allowed transitions and n¼ 2 for indirect allowed transitions. The direct band gap (Eg) value

is obtained by linear_{ﬁtting and extrapolating the straight line to}

the abscissa by plotting (

a

h

n

)2and h

n

(Fig. 4(a)). Similarly, the in-direct band gap (Egind) value is obtained by linear ﬁtting and

extrapolating the straight line to the abscissa (

a

h

n

)½ and h

n

(Fig. 4(b)) [54,55].

The indirect transitions are promoted by phonon assisted pro-cess. The direct and indirect band gap values of pristine sample was found to be 3.5 eV and 2.7 eV respectively (Fig. 4(a)). For the pris-tineﬁlm the higher value of optical band gaps might be due to the non-stoichiometric nature of V2O5ﬁlms. Both the direct band gap

and the indirect band gap showed a red shift upon irradiation [56]. The optical band gap reduced systematically with ﬂuence up to 5 1012_ions/cm2 _{and there after it begins to increase. A slight}

increase in band gap value for 1 1013_ions/cm2_{ﬂuence was due to}

amorphization and recrystallization which corroborates well with the XRD results. Comparing direct and indirect band gap, it is obviously clear that the direct transition dominates andﬁts well.

Generally amorphous or low crystalline materials have Urbach tail as they are associated with extra localized levels that extend into the band gap [56,57]. Here irradiated ﬁlms are partially amorphous as observed from the XRD and Raman spectra. The Tauc model established on electronic transitions between localized (distorted) energy levels of amorphous solids is therefore valid in this case [58]. In the band tail, near the optical band edge, ab-sorption coefﬁcient (

a

) varies exponentially with photon energy (h

n

) as [59],

a

¼

a

0exp (h

n

/EU) (9)

where,

a

0is an optical constant and EUis the Urbach energy. By

plotting ln(

a

) against the incident photon energy (h

n

) andﬁnding the slope of the linear region provides the value of Urbach energy. The Urbach energy corresponds to the band tail width from localized energy states due to structural disorder [60]. It is observed that the Urbach energy (Fig. 5(b)) increased withﬂuence due to increase in irradiation induced defects. The SHI beam irradiation induced defects which created extra defect levels between the valence and conduction bands [54]. Since the sample transforms

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from crystalline to amorphous structure, the width of the band tail increases and depend on the band gap energy [52,57]. The band gap reduction is attributed to the band tailing effect which is due to greater density of band tail states in the forbidden gap. The varia-tion in direct band gap, indirect band gap and phonon values for different ionﬂuences are clearly shown inFig. 5a and b.

3.5. AFM analysis

The evolution of surface morphology with various ionfluences were examined by AFM analysis. The 2D AFM images of pristine and few selected irradiated samples are shown inFig. 6. The surface of the pristine film was found to have sheath like structure with randomly distributed nearly spherical-shaped particles. After Fig. 5. Variation of the (a) direct and indirect band gap, (b) phonon energy and Urbach energy of pristine and 200 MeV Ag15þSHI ion beam irradiated V2O5thinfilms with various

ﬂuence.

Fig. 6. The 2D AFM images of (a) pristine, (b) 5 1011_ions/cm2_{, (c) 5}₁₀12_ions/cm2_{, and (d) 1}₁₀13_ions/cm2_{irradiated V}

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irradiation sheath like structure gets converted into spherical agglomerated granular particles.

The root mean square surface roughness (Rrms) is calculated

using the equation given below,

Rrms¼_N1 " XN i¼1 Zi_Z2 #1=2 (10)

Where N is the number of surface height data and Z is the mean height distance [61].

The Rrmsand the average grain size extracted from AFM images

of pristine and irradiatedﬁlms (Fig. 6) are shown inTable 1. The surface of the pristine ﬁlm was rough, with a value 23e37 nm, however, it reduced after irradiation. When irradiated with 5 1011

ions/cm2, the Rrmsvalue drastically reduced to 0.72 and the surface

became smoother (Table 1) [62].

When swift heavy ion passes through the material, simulta-neously smoothening and roughening action takes place on the surface of the materials [63]. Atoms of the target material acquires enormous energy due to lattice heating induced by SHI ion beam irradiation [43]. The kinetic energy of such atoms is lesser to over-come the binding energy of the surface. Even though they could not leave the surface, they drift in a parallel direction to the surface with the gained energy [64]. This mass transport was the main reason for partial amorphization as also observed from the XRD and Raman results. The grain size of pristine ﬁlm was 22 nm, whereas it increased to 37 nm after irradiation with 5₁₀11_ions/cm2_ﬂuence.

Incident high energy ions melted smaller sized particles, immediate quenching effect generated grain growth in the material. When ion ﬂuence increased, the grain size also increased until the sample became amorphous [65,66]. Irradiation induced grain growth observed in V2O5can be explained by the thermal spike model.

Sudden rise in temperature of lattice in a very short interval of time (<1012_{s) leads to}_{“thermal spike” along the ion track [}₆₇_].

Thermal spike created point defects may occur in grain boundary or in its vicinity. Thermally activated atoms migrate to vacancies that occur on grain boundary whereas vacancies created near grain boundary are occupied by interstitials from collision. For the atomic migration to take place the driving forces are the chemical con-centration gradients and grain boundary curvature [41,68]. The grain boundary migrates outwards as the total atomic jump was towards its centre of curvature. Mobility of grain boundary increased proportionally with an increase in generated defects and hence the grain growth. At higherﬂuences, coalescence of neigh-boring nano particles or grain splitting takes place depending on location of grains from ion incident position [69]. Whenﬂuence increased above 5 1011_ions/cm2_{, under heavy pressure, grains of}

V2O5 starts to split into smaller grains [70,71]. The grain size

eventually reduced to 30 nm for 5 1012 _ions/cm2 _{ﬂuence and}

further to 27 nm for 1 1013_ions/cm2_ﬂuence.

3.6. Hall effect measurements

The electrical transport parameters of pristine and irradiated ﬁlms measured using the Hall effect at room temperature is given

in Table 2. The electrical conductivity depends on both carrier concentration and mobility of charge carriers in a material. In polycrystalline materials, generally grain boundaries and conﬁned interface charges produce inter-grain band bending and potential barriers [72]. Electrons may be trapped into these potential wells that exist between grain boundaries and could not contribute to the conduction mechanism. Upon irradiation, there is a signiﬁcant rise in carrier concentration and hence conductivity is improved dras-tically [73].

The AFM analysis showed that grain size of the irradiated samples is bigger compared to the pristine one. When grain size being greater than the mean free path of charge carriers, grain boundaries have no signiﬁcant effect on conductivity [74]. Electrons can cross several grains before being scattered by phonons in the absence of grain boundary scattering. In addition, the band gap decreases which means lesser energy is enough to excite electrons from the valence band to conduction band. The band gap reduction and grain growth leads to maximum conductivity of 98.25

U

1cm1 for 5 1011_ions/cm2_{ﬂuence. Irradiated V}

2O5showed improved

electrical conductivity which may exhibit better electrochemical action compared to pristine ones. The changes in conductivity, re-sistivity, mobility, and carrier concentration of pristine and irradi-ated samples withﬂuence are depicted inFig. 7a and b and values are given inTable 2.

However, further increasing the ionﬂuence, conductance begins to decrease. Even though mobility increased, conductivity has not increased due to a considerable decrease in carrier concentration. Domination of the grain size variation over the band gap reduction was the reason for the observed fall in conductivity for 1 1012

ions/cm2 ﬂuence [44]. Electron scattering arises from i) external surfaces ii) phonons, point defects and iii) grain boundaries [75]. When grain size decreases electron scattering by grain-boundaries would be the main contribution to the observed resistivity [76]. As a consequence of grain size reduction, there was an increase in resistivity above 5 1011_ions/cm2 _{that restricts the number of}

charge carriers leading to decrease in conductivity. However, the value is higher than the pristine sample (Table 2).

4. Conclusion

In summary, the effect of 200 MeV Ag15þion irradiation on the properties of spray deposited V2O5thinﬁlms is investigated. The

XRD patterns reveal that the irradiated films undergo fluence dependent amorphization and recrystallization. Optical property investigation reveals that, there is an absorption edge shift toward the lower wavelengths as thefluence increases. In addition, the SHI beam irradiation induced a decrease in the direct and indirect band gaps of the V2O5film up to 5 1012ions/cm2which is due to

in-crease of the structural disorder leading to inin-creased defect density levels. The width of band tail or Urbach energy was also estimated. The band gap energy from direct allowed transitions dominated and its values decreased from 3.5 eV to 2.9 eV by increasing the ion ﬂuence to 1 1012_ions/cm2_{. The reduction in bandgap indicates}

that the irradiated V2O5ﬁlm is a potential candidate for solar

en-ergy applications. The irradiation of theﬁlms produced signiﬁcant grain growth and an improvement in the surface smoothness. The

Table 1

Surface parameters of pristine and irradiated V2O5thinﬁlms.

Fluence (ions/cm2₎ _{Average surface roughness (R}

rms) (nm) Average grain size (nm)

Pristine 23.37± 1.16 22± 1.10

5 1011 _0.720_{± 0.03} ₃₇_{± 1.85}

5 1012 _1.077_{± 0.05} ₃₀_{± 1.50}

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significant improvement in electrical transport properties at 5 1011_ions/cm2_{fluence is due to grain growth with fluence of}

ions. The Hall results demonstrate that irradiation with Ag15þion remarkably reduce the electrical resistivity of the V2O5 ﬁlms in

comparison to pristine. These irradiated_{films can be used as an} electrode in electrochromic devices that may show fast coloration/ bleaching transitions than EC devices fabricated using the pristine V2O5film. Thus irradiation effects on the property of V2O5thinfilm

implies their potential for extending its usage in a wide range of applications.

Declaration of competing interest

The authors declare that they have no known competing ﬁnancial interests or personal relationships that could have appeared to inﬂuence the work reported in this paper.

Acknowledgements

The authors thank the Inter University Accelerator Centre (IUAC), New Delhi, for providing access to the irradiation facility. References

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Fluence (ions/cm2₎ _r₍_U_cm) _s₍_U_cm)1 _N b(/cm3) m(cm2/Vs) Pristine 6.276 105 _1.593₁₀6 _2.478₁₀13 _4.014₁₀1 5 1011 _1.018₁₀2 _9.825₁₀1 _1.966₁₀18 _3.119₁₀2 1 1012 _1.574₁₀2 _6.353₁₀1 _3.784₁₀17 _1.048₁₀3 5 1012 _7.818₁₀2 _1.279₁₀3 _8.005₁₀11 _9.974₁₀3 1 1013 _2.339₁₀2 _4.274₁₀3 _2.377₁₀14 _1.122₁₀2

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