ion bombardment effect on the band gap of anatase TiO2 ultrathin films
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 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 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 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 ion exposure time during 5 min, 10 min and 20 min. The conservation of the anatase TiO2 crystalline structure after bombarding by 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.
References (42)
Surf. Sci. Rep.
(2003)- et al.
J. Photochem. Photobiol. Chem.
(2018) - et al.
Opt. Mater.
(2018) - et al.
Opt. Mater.
(2015) - et al.
J. Photochem. Photobiol. Chem.
(2005) - et al.
Superlattice. Microst.
(2017) - et al.
J. Photochem. Photobiol. Chem.
(2010) - et al.
J. Cryst. Growth
(1993) - et al.
J. Alloy. Comp.
(2006) - et al.
Appl. Surf. Sci.
(2017)
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.
Cited by (16)
Controlled formation of Ag-Ag<inf>x</inf>O nanoparticles on the surface of commercial TiO<inf>2</inf> based composites for enhanced photocatalytic degradation of oxalic acid and phenol
2023, Catalysis TodayCitation Excerpt :Ag2O/TiO2 composites were investigated in several photocatalytic processes because Ag2O has relatively low band gap energy, and usually, it is easy to prepare [31]. Ag2O/TiO2 composites are p-n type semiconductors, where the band gap energy of Ag2O is ≈1.3 eV [32], while this value is 3.2 eV for anatase [33] and 3.0 eV in the case of rutile [34]. The growing interest towards p-n type semiconductors can be attributed to their improved e−/+ charge separation properties [34].
Design of thin DLC/TiO<inf>2</inf> film interference coatings on glass screen protector using a neon–argon-based gas injection magnetron sputtering technique
2022, Diamond and Related MaterialsCitation Excerpt :This structure exhibits also an equal to zero value of k, throughout the visible region, meaning that the light from that spectrum is not absorbed. As a result, an unusually high optical band gap (Eg = 3.64 eV) was observed for the synthesized amorphous TiO2 (Fig. 6b), consistent with that reported by others [65,66]. This blue shift within the optical band gap confirms that the disordered structure of TiO2 is likely to show the confinement of delocalized states, resulting in even more dielectric nature of the TiO2, than that of DLC layer.
Thickness and ion irradiation induced structural phase changes in the thin films of titanium dioxide
2021, Thin Solid FilmsCitation Excerpt :Moreover, they are perfect systems for fundamental understanding of the phase change, charge state dynamics and related optical and electrical properties, and studies in this direction are emerging [35–37]. Recently, the effect of thickness and ion irradiation on the electronic structure and defects in ultra-thin TiO2 films of thickness in the range of 20–30 nm was reported [33]. The fabrication of ultra-thin films generally employed atomic layer deposition technique [31,32,34,35], and sol-gel methods were regularly used for relatively thicker films (> 20 nm) [12].