Evolution of structural and magnetic properties of Ar²⁺ ion irradiated TiO₂ thin films annealed under argon atmosphere

TiO₂ is an essential and resourceful material with extensive range of applications.^1–5 In general TiO₂ crystallizes in three different forms i.e. anatase, brookite and rutile with the crystal structure, tetragonal (⁠$D4h19$⁠), orthorhombic (⁠$D4h15$⁠) and tetragonal (⁠$D4h14$⁠), respectively. The anatase, brookite and rutile phase shows band gap 3.19, 3.11 and 3.03 eV, respectively.^6,7 In addition to these phases of TiO₂, titanium sub-oxides (Magneli phase compounds, Ti_nO_2n-1, 4≤n≤10) have been explored for its electrochemical applications. Out of all the Magneli phase compounds Ti₄O₇ is having highest electrical conductivity.^8,9 It is generally derived from the reduction of TiO₂.^8,9 Ion irradiation has been used as a unique tool in material modification.¹⁰ Two types of energy loss processes, electronic energy loss (S_e) and nuclear energy loss (S_n) occurs during interaction of energetic ions with the material. In the keV range, S_n is dominant that generates cluster of defects and atomic size point defects in the objective.¹¹ The post irradiation effects of energetic ion beams on the structural properties are of recent interest.^12–14 In our earlier report, we have shown irradiation outcome of 500 keV Ar²⁺ ion in oxygen environment annealed TiO₂ thin films.¹⁵ In another report, we have carried out irradiation of 100 MeV Ag⁷⁺ ions on undoped and Co doped TiO₂ thin films which are deposited through pulsed laser deposition (PLD) technique.¹⁶ While anatase phase to brookite transformation was observed after irradiating 500 keV Ar²⁺ ions, followed by amorphization, 100 MeV Ag⁷⁺ ion beam irradiation makes the films amorphous. Transformation of crystalline TiO₂ nano-rod to amorphous one is reported on irradiating 50 keV Ar⁺ ions by Saini et al.¹⁷ Low energy ion irradiation on the solid surface give rise self-organized periodic patterns of nanostructures for example ripple, dots, facet, pits or holes in certain experimental circumstances.^18–23 Besides structural transformation these energetic ion irradiation brings alteration in magnetic properties as well. Thakur et al.^24,25 demonstrates paramagnetism to ferromagnetism transformation in the films of TiO₂ on irradiating Ag¹⁵⁺ ions of 200 MeV. Mohanty et al.¹⁶ has studied the structural and magnetic state after irradiating Ag⁷⁺ ions in cobalt doped TiO₂ thin films. Bharati et al.,^11,15 has observed alteration in magnetic properties after irradiating with Ar²⁺ ions of 500 keV in TiO₂ thin films. Though there are some reports in structural transformation in TiO₂ caused by irradiation,^26,27 but not enough studies have reported on effects of low energy in TiO₂.

In this report, we have observed the role of low energy ion beam (LEIB) irradiation on the structural as well as magnetic state of 900 °C annealed TiO₂ films in Ar atmosphere.

Thin films of TiO₂ were deposited using electron beam evaporation process on Si substrate. TiO₂ target which was used for the evaporation has 99.99% purity (STREM Chemicals, USA). The deposited films were annealed at 900 °C under Ar gas for 1 h.

Films were irradiated by Ar²⁺ ions of 500 keV under high vacuum at IUAC, New Delhi, India. Thereafter, the pristine film named as A, film irradiated with ion fluence, 1 × 10¹⁴ ions/cm² as B and 5 × 10¹⁶ ions/cm² as C. Further, films were characterized with X-ray diffraction (XRD, Bruker D8 Advance) and Micro-Raman spectroscopy, scanning probe microscope (SPM) and SQUID VSM (Quantum Design, USA).

To study the post irradiation effect by creating defects, the films are irradiated by LEIB. Using SRIM code, the S_e, S_n and R_p (projected range) has calculated.^11,15 The obtained values for S_e, S_n are 918.7, 364.7 eV nm^-1, respectively, and R_p is 3634 Å for 500 keV Ar²⁺ ion. As film depth is less than R_p, the projectile ion will get entered deep within the substrate. Fig. 1 show the GAXRD (glancing angle X-ray diffraction) patterns of the film A, B and C. However, Film A shows diffraction peaks at 26.44, 29.38, 35.38 and 37.90 correspond to the plane (-121), (1-21), (204) and (-220) of Ti₄O₇ (JCPDS PDF# 771392). Peak at 25.33 in film B match to the plane (101) of anatase phase (JCPDS PDF# 89-4921) and no other peak observed, whereas in the film C a peak is observed at 29.39 and 35.01 relates to the planes (1-21) and (204) of Ti₄O₇ phase after irradiation of 500 keV Ar²⁺ ions. The suppression of peak (-121) and (-220) takes place after irradiation in film C. It indicates that the film get oriented along the (1-21) direction. In film annealed under oxygen atmosphere at 900 °C shows radiation resistant behavior for Ti₄O₇. However, a film annealed in argon atmosphere at 900 °C does not show radiation resistant behavior for Ti₄O₇. In this case phase conversion takes from Ti₄O₇ to anatase phase of TiO₂ at the lowest fluence and at highest fluence again Ti₄O₇ is appeared with orientation of film along the (1-21) direction.

The phase of the films is further confirmed through Raman spectroscopy as shown in Fig. 2. The film A, shows a broad peak at 145 cm^-1, due to the presence of the Magneli phase Ti₄O₇.³⁴ Film B shows a pointed peak at 144 cm^-1 along with one more peaks at 639 cm^-1 of anatase phase, whereas film C exhibits only a broad peak at 146 cm^-1 which corresponds to Ti₄O₇. Peak related to Si appears at 521 cm^-1 in line with earlier reports^11,15 and that gets suppressed with irradiation, indicating amorphisation of Si substrate. Nowadays Magneli phase has been explored for electrochemical applications because of its chemical inertness and high electrical conductivity.⁸ Out of these Magneli phase compounds Ti₄O₇ has highest electrical conductivity 1000 S/cm, which in general comparable to graphite.⁹ Chemical synthesis of Ti₄O₇ phase is a difficult process. Still no report shows pure Ti₄O₇ phase formation through ion irradiation. This is the first report that shows the Ti₄O₇ phase formation through ion irradiation. Further, surface topography of the films have been studied.

All the figures, having scanned surface area 1 μm × 1 μm (Fig. 3). The non-uniform surface is result from film topography demonstration. However, it can be noted that films show structure like hillock having variable dimensions are spread disproportionately over surface of the film. The roughness of the films is represented by means of R_rms (root mean square roughness). The obtained values of R_rms are 0.061, 0.063 and 0.079 nm for the sample A, B and C, respectively. Here, we observe increase in R_rms with irradiation. Previously it has been reported that films annealed at 900 °C in oxygen environment exhibits higher roughness compared to the Ar annealed films at the ion fluence 1 × 10¹⁵ ions/cm².³⁵ In oxygen environment it has observed that 900 °C annealed films show decrease in roughness with irradiation, which is just opposite to this case.¹⁵ Khanam et al.²¹ demonstrated that when low energy ion irradiated on the given material, the ion get incorporated in the material, results into swelling of material which brings van der Waals crystals formation. At higher fluence, excessive swelling of particles observed with thermodynamically instability. These swollen particles split into isolated particle to get the stability with change in roughness. The surface roughness deviation mostly occurs due to the consistent alteration of the hillock size obtained by the ion implantation.²¹ Ramana et al.²² and Atuchin et al.²³ have explained that low energy ion disturbs the bonding in the material and brings structural transformation. The observed roughness deviation might be due to structural variation as observed from XRD, Raman and alteration of hillock size. Grain analysis, exhibits the grain structure in the pristine as well as irradiated films. The grain size is 28 nm for pristine and lowest fluence irradiated film, whereas, at highest fluence, the average grain size is 29 nm. Films annealed in oxygen atmosphere at 500 °C shows increase grain size with irradiation.¹¹ However, 900 °C annealed films does not show much effect on grain size with irradiation, similar to the present case.¹⁵ It indicates that grain size might not effect much by irradiation if the film annealed at higher temperature. The energy deposited by the ion beam enhances the lattice temperature so that transient temperature enhancement gives rise to structural transformation, as observed from XRD. The deposited heat by the ions, get localized and due to thermal conductivity, dissipates to the neighboring atom which brings structural changes in the material.

Magnetic properties of these films have been studied using the field (H) dependent magnetisation (M) analysis, shown in Fig. 4. The selected range of magnetic field for measurement is 0 to 10 kOe. Diamagnetic part of the Si substrate has been eliminated out from the measured data. The signature of ferromagnetism in all the films obtained from the hysteresis loop with the finite coercivity and remanence. The inset of Fig. 4 represents zoomed portion of hysteresis loops. The magnetization saturates at about 3 kOe in pristine film with saturation magnetization 11.49 emu/cc, coercivity is 66.87 Oe, remanence is 1.11 emu/cc. Film B saturates at 5 kOe and the saturation magnetization is 72.68 emu/cc with coercivity, 20.58 Oe and remanence is 1.87 emu/cc. However, film C saturates at 4 kOe with the saturation magnetization is 27.78 emu/cc, coercivity, 46.23 Oe and remanence, 1.54 emu/cc. Previously different reports demonstrate RTFM in the TiO₂ films fabricated through different process like sputtering, sol–gel, spray pyrolysis, PLD etc.^24–33 Still now there are very few reports on RTFM in the TiO₂ films obtained from e-beam evaporation technique. The causes after the ferromagnetism within TiO₂ are moreover explained by different researcher.

Rumaiz et al.³³ ascribe the magnetism in TiO₂ films to the oxygen vacancies, whereas, Wei et al.³¹ demonstrated that 2p electron in the oxygen performing a significant job in exchange interaction as well as ferromagnetic ordering. Either oxygen vacancies linked with two electrons can confine the adjacent Ti ions to get change into Ti³⁺ or possibly delocalized in the matrix of TiO₂. RTFM in TiO₂ films obtained from e-beam evaporation process, annealed in O₂ and Ar environment has been studied formerly where the phase was anatase.²⁸ Mohanty et al., demonstrated RTFM and the magnitude of the moment in Co-doped TiO₂ films based on oxygen vacancies and the crystallinity of the film.²⁹ RTFM in TiO₂ doped with Mn is demonstrated using bound magnetic polarons concept.³⁰ Yoon et al. established that ferromagnetism in TiO₂ stems from anionic defects.³² The higher moment has been accredited to higher oxygen defects.²⁵ Therefore, here occurrence of RTFM is attribute to factors like crystallinity, oxygen vacancies and Ti³⁺ in the films.

Considering our reported results and the present data it is observed that when the energetic ion beam bombards with a target material, structural as well as magnetic properties transformation occurs. The transformation in structural and magnetic property varies from material to material. This variation depends on its initial physical properties as well as its synthesis conditions along with its heat dissipation capacity. In our earlier report, where the films are annealed in O₂ atmosphere at 500 °C, films get crystallize in anatase phase. On irradiating with the lower fluence ion beam, amorphisation of the film takes place, whereas, at highest fluence, it recrystailise into brookite phase.¹¹ Upon varying the annealing temperature to 900 °C in O₂ environment, result crystalline film stabilized in anatase phase with an additional Ti₄O₇ phase. Irradiating with the ion fluence 1 × 10¹⁴ ions/cm², anatase film become amorphous and at the fluence, 5 × 10¹⁶ ions/cm², it again get crystalised into brookite phase. In these films the Ti₄O₇ phase shows radiation resistant behavior.¹⁵ However, in the present case, where the films are annealed in Ar environment at 900 °C. Film crystallizes to the oxygen deficient Ti₄O₇ phase due to the oxygen deficient environment (Ar). Here it is observed that film stabilized in anatase phase at the lowest fluence. On irradiating with the enhanced fluence, it shows the Ti₄O₇ phase. Crystalline TiO₂ nano-rods amorphises by irradiating with Ar⁺ ions 50 keV.¹⁷ Low energy ion beam irradiation give self-organized periodic patterns on solid surface under definite experimental circumstances.^18–23 Compared to the erlier reports of oxygen annealed films,^11,15 here, in this case saturation magnetization tremendously increases after irradiation. This is one of the new achievements obtained in this case. Pure Ti₄O₇ is highly conducting material.⁹ Still now there is no report of low energy ion beam irradiation in TiO₂ films show pure Ti₄O₇, as a result of irradiation, for the first time it is reported in this case. As the structural transformation varies from case to case, the change in roughness and grain structure also varies accordingly. All the films show RTFM due to the existence of oxygen vacancies, but variation in the magnetic properties is due to the variation in the quantity of oxygen vacancies in addition to the crystallinity of the films.

TiO₂ films fabricated by means of e-beam evaporation procedure then annealed at 900 °C in Ar environment followed by irradiation of Ar²⁺ ions having energy 500 keV. Evolution of structure was deliberated from GAXRD and Raman spectroscopy. The pristine film stabilized in Ti₄O₇ phase. Upon irradiating with ion fluence 1 × 10¹⁴ ions/cm², phase transformation takes place and film get crystallized with anatase phase. However, at 5 × 10¹⁶ ions/cm², film exhibited Ti₄O₇ phase. From AFM result, roughness found higher in the irradiated films compared to pristine ones. From the magnetic studies RTFM was observed in all the films no matter what the phase or crystallinity. The RTFM of the films was imputing to the complex interplay between oxygen vacancies, Ti³⁺ and the crystallinity of the films.

All the authors are thankful to CIFC, IIT (BHU) and scientists of LEIBF, IUAC, New Delhi, for given the measurement facilities.

The data that support the findings of this study are available from the corresponding author upon reasonable request.