N2+ ion bombardment effect on the band gap of anatase TiO2 ultrathin films

doi:10.1016/j.optmat.2018.11.045

Optical Materials

Volume 88, February 2019, Pages 282-288

https://doi.org/10.1016/j.optmat.2018.11.045 Get rights and content

Highlights

•
Determination of optical response of TiO₂ ultrathin films after bombarding by $N_{2}^{+}$ ion beam.
•
The Tauc-Lorentz and Urbach models were used for analyzing of the optical constants of TiO₂ with $N_{2}^{+}$ ion exposure time.
•
The widening of the band gap of TiO₂ with $N_{2}^{+}$ ion exposure time was attributed to the thickness effect.
•
Raman and ellipsometry measurements show the formation of defects in TiO₂ after the $N_{2}^{+}$ ion bombardment.

Abstract

We report a study of the effect of nitrogen ion bombardment on the optical properties of anatase TiO₂ ultrathin films, particularly the band gap energy. The TiO₂ films were prepared by a sol-gel method and dip-coating process. The as-prepared TiO₂ films were then exposed to a $N_{2}^{+}$ low-energy ion beam from a microwave electron cyclotron resonance (ESR) ion source. Raman and spectroscopic ellipsometry (SE) analysis were performed on TiO₂ films prepared at different $N_{2}^{+}$ exposure times. The Raman measurements reveal the conservation of the anatase TiO₂ crystalline structure after the ion beam exposure. From a detailed ellipsometric study, the thickness of layers, the dielectric function, the band gap and the Urbach energies were determined. The obtained results show an increase of the TiO₂ band gap with the decrease of thickness of films during $N_{2}^{+}$ exposure time. The band gap energy was blue shifted from 20 meV to 140 meV as the exposure time was increased from 5 min to 20 min when the thickness was decreased from 30 nm to 21 nm. This increasing of band gap energy could be explained by the thickness effect. From the band tail, the Urbach energy was also affected by $N_{2}^{+}$ ion beam. These results are in good agreement with the observed broadending of the Raman band the O single bond TiO bending vibration mode, as the exposure time increases.

Introduction

Titanium dioxide (TiO₂) semiconductor has attracted great interest from researcher due to its optical, chemical and electronic properties [1]. These characteristics are fueled and fanned by its prospects for several applications in photocatalysis [2], dye-sensitized solar cells [3], blue lighting [4] and self-cleaning surfaces [5]. However, the anatase phase of TiO₂ has large band gap (3.2 eV) traducing limit efficiency in ultra violet range. A More efficient TiO₂ material can be achieved by TiO₂ to absorb light in the visible region. Thereby, the incorporation of nitrogen (N) in TiO₂ matrix is suitable for enhancing TiO₂ photo-activity under the sunlight [6]. Despite the absorption in visible range, there are several optical studies relating to the effect of N atoms on the band gap of anatase TiO₂ [[5], [6], [7], [8], [9], [10], [11], [12], [13]]. For example, some authors [[7], [8], [9]] have founded a band gap narrowing due to the modification of the valence band by the hybridization of the two levels O 2p and N 2p, while other researchers [10,11] have shown that the band gap is unchanged and the N 2p states are localized just above the O 2p valence band maximum. Whereas in recent publications [12,13], the authors have founded a decrease in the band gap for relatively high N concentration using transmission measurements [12,13], while for low N concentration, the band gap is conserved [12,13] and a localized N 2p states are created [13]. In these previous works, the band gap was estimated by transmission measurements and by theoretical calculations of the density of states. However, spectroscopic ellipsometry (SE) technique is a powerful tool to determine the optical properties of materials such as the optical band gap [14].

Recently, SE was used to study the optical constants and band gap evolution with phase transition in sub-20-nm-thick TiO₂ films prepared by atomic layer deposition [15]. Deducing from band gap evolution, the authors have found that the anatase TiO₂ transforms to rutile TiO₂ at high annealing temperature around 800 °C [15]. Consequently, SE can deduce the crystalline structure of deposited and annealed films. For this, in this paper, we propose to study by SE the $N_{2}^{+}$ ion bombardment effect on the band gap energy of TiO₂ ultrathin films with thickness varying between 21 nm and 30 nm. The Raman spectroscopy was used to study the effect of $N_{2}^{+}$ ion bombardment on the crystalline structure of the TiO₂ films. SE was also used to extract the dielectric function, the band gap and the Urbach energies of the films from the numerical analysis using constrained cubic splines method, Tauc-Lorentz and Urbach models. These results will be helpful to understand the thickness effect of ultrathin films on the optical responses.

Section snippets

Preparation of the TiO₂ ultrathin films

The sol-gel method was used to elaborate the TiO₂ films. The preparation procedure of the sol is detailed elsewhere [16]. Briefly, titanium tetraisopropoxide (Aldrich, 99%) was used as a precursor to synthesize the titania sol via an acid catalyzed sol-gel process at room temperature and under argon atmosphere. Propan-2-ol (Aldrich, 99.8%) and hydrochloric acid (ARCOS Organics) were used as solvent and catalyst, respectively.

The sol was deposited onto quartz microscope slides. Prior to use, the

Raman analysis

Fig. 1 presents the Raman spectra of the TiO₂ after 0, 5, 10 and 20 min exposure time to the $N_{2}^{+}$ ion beam. The observed vibration modes at 142 cm⁻¹,197 cm⁻¹, 395 cm⁻¹, 514 cm⁻¹ and 637 cm⁻¹ are attributed to the anatase phase [19]. These modes are also observed in the spectra of irradiated samples traducing the conservation of the anatase crystalline structure. Other authors have found that the anatase TiO₂ transforms to rutile TiO₂ at high annealing temperature around 800 °C [15]. The low

Conclusion

In summary, the optical properties of TiO₂ ultrathin films were investigated by using ellipsometry over the $N_{2}^{+}$ ion exposure time during 5 min, 10 min and 20 min. The conservation of the anatase TiO₂ crystalline structure after bombarding by $N_{2}^{+}$ ion beam and the presence of N bounded to Ti was established by Raman measurements. This study highlights both the effects of the thickness of ultrathin films and the nitrogen ion bombardment on the optical responses, particularly on the band gap

Declaration of interest statement

None.

Acknowledgment

The authors would like to acknowledge Pascal Franchetti (LCP-A2MC) for technical assistance in Raman spectroscopy measurements.

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N2+ ion bombardment effect on the band gap of anatase TiO2 ultrathin films

Highlights

Abstract

Introduction

Section snippets

Preparation of the TiO2 ultrathin films

Raman analysis

Conclusion

Declaration of interest statement

Acknowledgment

Surf. Sci. Rep.

J. Photochem. Photobiol. Chem.

Opt. Mater.

Opt. Mater.

J. Photochem. Photobiol. Chem.

Superlattice. Microst.

J. Photochem. Photobiol. Chem.

J. Cryst. Growth

J. Alloy. Comp.

Appl. Surf. Sci.

Thin Solid Films

Appl. Surf. Sci.

Superlattice. Microst.

Mater. Chem. Phys.

Mater. Chem. Phys.

Sol. Energy Mater. Sol. Cells

Thin Solid Films

Chem. Rev.

Appl. Phys. Lett.

Science

J. Phys. Chem.

$N_{2}^{+}$ ion bombardment effect on the band gap of anatase TiO₂ ultrathin films

Preparation of the TiO₂ ultrathin films