Open Access
Add to BibSonomy
Abstract
TiO2:Yb,Er films with different concentrations of Mn2+ are grown on SiO2 glass substrates by pulsed laser deposition. It is found that the introduction of Mn2+ enhanced the intensity of upconversion emission. In particular, TiO2:Yb,Er thin film with 5% Mn2+ ions exhibits the brightest upconversion emission. The upconversion red emission intensity is increased by 2.5-fold than that of a TiO2:Yb,Er thin film without Mn2+ ions, which is ascribed to the multi-photon absorption and efficient exchange-energy transfer process between Er3+ and Mn2+. The high transmittance and good conductivity of the films made them possible to act as electron transport layer in solar cells.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The upconversion (UC) luminescence of rare earth ion-doped semiconductors has garnered significant attention owing to its extensive applications, such as color displays, sensors, photocatalysis, biomedical applications and solar cells [1–6]. In particular, spectral UC has been proposed as a strategy for improving photovoltaic energy conversion in solar cells [7–10]. To date, Yb3+ typically serve as a sensitizer due to its two-level energy structure, and Er3+ is one of the most popular activators for the plentiful meta-stable energy structure [11]. Titanium dioxide (TiO2) is regarded as a candidate host material owing to its low phonon energy, thermal stability and wide band gap energy (3.0-3.2 eV) [12,13]. Moreover, TiO2 can be used as an electron transport layer in perovskite solar cells. Currently, the electron transport layer of perovskite solar cells with high efficiency is based primarily on TiO2 materials [14–16]. Significant effort has been expended to enhance UC emission in perovskite solar cells [17–19]. Zhou et al. synthesized a UC layer to improve the power efficiency of perovskite solar cells; green and red UC emissions were detected under the excitation of near-infrared light [17]. Recently, Er3+ and Yb3+ co-doped TiO2 nanorod arrays were fabricated using a one-pot hydrothermal method. An infrared response and an improved current density were observed. Moreover, a lower electron recombination rate compared with undoped TiO2 in perovskite solar cell [18]. Zhang et al. synthesized UC Er-doped rutile TiO2 nanorod arrays and applied them to perovskite solar cells. Compared with the undoped device, both the photocurrent density and power conversion efficiency were improved. High UC is regarded as one of the reasons that contributed to the performance improvement [19]. To apply doped TiO2 films to photovoltaic devices, more investigation are necessitated to improve their UC luminescence efficiency and conductivity.
In this work, TiO2:Yb,Er films with controllable red and green UC emissions have been grown by pulsed laser deposition (PLD). The energy transfer process in Mn2+-doped TiO2:Yb,Er thin films has been extensively investigated. It was found that the UC red light emissions enhanced significantly via 5% Mn2+ doping, and that multi-photon absorption and exchange-energy transfer between Mn2+ and Er3+ ions contributed to UC emission improvement. In addition, high transmittance performance and good electrical conductivity are obtained in 5% Mn2+ doped TiO2:Yb,Er thin films, which is helpful for designing solar cells with controllable UC emission properties.
2. Experimental section
The samples were grown on SiO2 glass substrates by PLD using a KrF excimer laser operating at a wavelength and pulse energy of 148 nm and 200 mJ, respectively. Doped TiO2 targets were prepared by solid-state synthesis of TiO2 powder mixed with pure Yb2O3, Er2O3, and MnO2. These materials were purchased from Heifei Kejing Materials Technology Co., Ltd. The concentrations of Yb and Er in the targets were fixed at 10% and 2%, respectively. The concentration of Mn was between 0% and 20%. During deposition, the oxygen pressure and substrate temperature were set to be 0.01 Pa and 350°C, respectively. The doped TiO2 ceramic targets and substrates were rotated at a speed of 10 rpm to prevent possible structural deterioration and to facilitate the synthesis of homogeneous films. The laser rate was maintained at 15 Hz, and the number of applied laser pulses was 30,000, resulting in the film thickness of approximately 200 nm, as measured using a Bruker Dektak-XT profilometer. Finally, a thermal evaporator with designed plastic masks was applied to deposit 90 nm gold electrodes for electrical property measurements.
The crystalline structure of the samples was analyzed by X-ray diffraction (XRD, SmartLab, Rigaku) with Cu Kα in 2θ-ω scan mode. The surface morphology and root mean square roughness were studied by atomic force microscope (AFM, NX-20, Park Systems). The composition and elements distribution were determined using field emission scanning electron microscopy (SEM, Sigma 500 VP, Carl Zeiss AG) with energy-dispersive X-ray spectroscopy (EDX). Optical properties were measured using an ultraviolet-visible absorption spectrometer (Lambda 1050, PerkinElmer). UC emission spectra were recorded using a fluorescence spectrophotometer (FLS1000, Edinburgh) equipped with a 980 nm laser with tunable power. The temperature-dependent electrical resistivity was measured in the van der Pauw geometry using a Hall setup (8404, LakeShore).
3. Results and discussion
Figure 1(a) shows the XRD 2θ-ω scan of Mn-doped TiO2:Yb,Er thin film. The diffraction peaks of the film were consistent with the results for standard diffraction data of rutile TiO2 (JCPDS No. 21-1276). No Yb2Ti2O7 or Er2Ti2O7 phases appeared in the XRD spectra when the films were annealed at temperatures as high as 800 °C. This indicates that the Yb and Er elements were fully doped into the TiO2 host. The typical surface morphology of the doped TiO2 film is shown in Fig. 1(b). The film has a smooth surface with root mean square roughness of less than 1.5 nm. In addition, the elemental components of the Mn2+-doped TiO2:Yb,Er thin films were measured via energy dispersive spectrometer analysis. As shown in Fig. 1(c-f), the results of SEM equipped with EDX show the elemental distributions in the order of Ti, Mn, Yb, and Er. It is evident that all doped elements have been found and distributed homogeneously throughout the film.
Fig. 1. (a) XRD 2θ-ω scan of TiO2:Yb,Er thin film doped with 5% Mn. (b) AFM image of same sample shown in (a). (c)-(f) show distributions of Ti, Mn, Yb, and Er obtained by EDX.
The UC emission spectra of the samples are shown in Fig. 2(a) under a 980 nm laser excitation. Three major emission bands were observed, i.e., at 526 nm (green light), 550 (green light), and 660 nm (red light), which originated from the 2H11/2 to 4I15/2, 4S3/2 to 4I15/2, and 4F9/2 to 4I15/2 transitions of Er3+ ions, respectively. It was evident that the UC emission intensities first increased with the incorporation of Mn2+ ions and then decreased with further the addition of Mn2+ ions. In particular, the film with 5% Mn2+ showed the most intense red emission which enhanced by 2.5- folds compared with that of the film without Mn2+ ions. The green emissions are shown in Fig. 2(a) as an inset. The change of green emission intensity with Mn doping concentration is consistent with the red emission. The optical transparency of Mn2+-doped TiO2 films was investigated based on optical transmittance spectra. As shown in Fig. 2(b), all Mn2+-doped TiO2:Yb,Er films exhibited high transmittance. The average transmittance of all samples was higher than 81%. Clear diffraction fringes were observed in the transmittance spectra, which indicated the high quality of the film. This result is significant for the electron transport layer in perovskite solar cells, for which high transmittance allows more light to propagate through the electron transport layer to the perovskite absorber layer.
Fig. 2. (a) PL spectra of TiO2:Yb,Er thin film doped with Mn concentrations of 0%, 2.5%, 5%, 10%, and 20% under 980 nm excitation with 1 W pump power. Inset show green upconversion light of TiO2:Yb,Er thin film doped with Mn concentrations of 0%, 2.5%, 5%, 10%, and 20%. (b) Transmittance spectra of TiO2:Yb,Er thin film with Mn concentrations of 0%, 2.5%, 5%, 10%, and 20%.
In order to understand the UC mechanisms of Mn-doped TiO2:Yb,Er thin films, the pumping power dependence of the UC emission of the samples was measured in this work. As shown in Fig. S1 (see Supplement 1), the slopes of the linear fitting were about 1-2, which can be explained by a two-photon energy transfer process [20,21]. The energy level structures of Yb3+, Er3+, and Mn2+ and the possible energy transfer processes are shown in Fig. 3. Generally, Yb3+ ions with two-level structures function as sensitizers, while Er3+ ions are used as activators owing to their rich meta-stable level structure. For the UC emissions at 526 nm (Er3+: 2H11/2 → 4I15/2), 550 nm (Er3+: 4S3/2 → 4I15/2), and 660 nm (Er3+: 4F9/2 → 4I15/2), the following two energy transfers and nonradiative relaxation processes are regarded as the regular routes: Firstly, the electrons of Yb3+ absorb energy from excitation power of the 980 nm laser and jump from the ground state 2F7/2 to the excited state 2F5/2. Secondly, energy transfer occurs between Yb3+ and Er3+; consequently, the electrons of Er3+ are excited to 4I11/2 and then to the 4F7/2 state. Finally, the nonradiative relaxation processes of Er3+ relax electrons from the 4F7/2 state to the 2H11/2, 4S3/2, and 4F9/2 states. Subsequently, green and red emissions occur when electrons located at these levels return to the ground state 4I15/2.
In the present work, the introduction of Mn2+ into TiO2:Yb,Er thin films enhanced both the UC red and green luminescence when the concentration of Mn2+ was less than 10%. However, Bai et al. compared the UC luminescence spectra of NaYF4 and MnF2 host nanocrystals doped with Er3+ and Yb3+ ions. Green and red mixture emissions were detected in NaYF4:Er3+/Yb3+ sample, whereas only a stronger red emission was observed in the MnF2:Er3+/Yb3+ sample due to the introduction of Mn2+. Such a red-emission enhancement is ascribed to the cross-relaxation of energy between the Mn3+ and Er2+ ions [22]. Tian et al. doped Mn2+ ions into NaYF4: Yb/Er to change the transition possibilities of Er3+ and promote the transition of red emission. The color emissions are tuned from green to red [23].
In this paper, it is clear that both green and red luminescence spectra have been improved due to the introduction of Mn2+. The explanations above imply that other mechanisms are likely to contribute significantly to the improvement in the UC luminescence. The multi-photon absorption and the cross-relaxation energy between Mn2+ and Er3+ ions might be responsible for the enhancement in the UC red emission. As shown in Fig. 3, the electrons of Er3+ jumped from the 4I15/2 to 2H9/2 level through three energy transfer processes between Yb3+ and Er3+, with the electrons of Er3+ relaxing from the 4I11/2 to 4I13/2 energy level. Subsequently, nonradiative energy transfer occurred from the 2H9/2 level of Er3+ to the 4T1 level of Mn2+, followed by back-energy transfer to the 4F9/2 level of Er3+. The efficient exchange-energy transfer between Er3+ and Mn2+ increased the probability of red luminescence owing to the resonances between the Mn2+ absorption and the plentiful meta-stable levels of Er3+ in the crystal host lattice [23–25].
Besides the mechanism for promoting UC red emission described above, the energy transfer between Yb3+ and Mn2+ might pump electrons of Mn2+ from the 6A1 to 4T1 levels [26–28]. Subsequently, the red UC emission was promoted by energy exchange from Mn2+ (4T1) to Er3+ (2F5/2). For green UC luminescence, it was clear that 5% Mn2+ doping contributed to some improvements as well. Similar to previous green UC emissions in materials including Yb3+ and Mn2+, there are two possible mechanisms for promoting the UC green luminescence. One is the energy transfer mechanism of Yb3+-Mn2+ dimer. The other is the green UC emission of Mn2+ (4T1→6A1), which is ascribed to the cooperative luminescence of excited Yb3+ pairs, where energy transfer from two Yb3+ ions to Mn2+ ion [29–31].
To investigate the transport properties, the sheet resistance was measured at room temperature. The values are 1.494 × 106 Ω/□, 1.082 × 106 Ω/□, 1.935 × 107 Ω/□, and 3.235 × 108 Ω/□ for 2.5%, 5%, 10% and 20% Mn doped TiO2:Yb,Er films, respectively. The sheet resistance of pure TiO2:Yb,Er film was over the range of instrument. It was evident that the sample with a Mn doping concentration of 5% has the smallest sheet resistance. Combine sheet resistance with transmittance, the change of band gap caused by Mn doping is considered as one of the possible reasons. Further, The temperature-dependent resistivity of TiO2:Yb,Er thin films with 5% Mn doping was measured from 300 to 200 K. The conductive behavior of semiconductor is presented in Fig. 4(a), it is evident that the in-plane resistivity of Mn2+ doped TiO2:Yb,Er thin film increased with temperature decreased. To investigate the transport properties, temperature-dependent conductivity data of TiO2:Yb,Er film are analyzed using the thermal activation model [32,33],
where $\sigma$ is the conductivity, $W$ is the activation energy, and $k_{B}$ is Boltzmann’s constant. As shown in Fig. 4(b), the temperature-dependent conductivity is well described by the thermal activation model in the given temperature range. The value of activation energy $W$ is approximately 346 meV, based on the linear fitting conductivity data, which is consistent with the theoretical and experimental values in the previous work [34].Fig. 4. (a) Temperature-dependent resistivity of TiO2:Yb,Er thin film with 5% Mn2+. (b) Arrhenius plot of electrical conductivity for Mn-doped TiO2:Yb,Er thin film.
4. Conclusion
In summary, TiO2:Yb,Er with red and green emission thin films were fabricated by PLD. The intensity of UC red emission is successfully improved by introducing 5% Mn2+ in to TiO2 host crystal lattice. It is discovered that multi-photon absorption and the cross-relaxation between Mn2+ and activator Er3+ ions might have contributed to the enhanced red emission. High transmittance in the visible region was detected in the samples. The appearance of interference fringes indicates the high quality of Mn doped TiO2:Yb,Er films. In addition, the electrical properties were studied by thermal activation model. Temperature-dependent conductivity data exhibit a semiconducting behavior of Mn2+ doped TiO2:Yb,Er thin film with a activation energy of 346 meV. This work might be helpful for further understanding the mechanism of upconversion luminescence, and the experimental results demonstrated the potential application of doped-TiO2 films for designing solar cells with controllable upconversion emission property.
Funding
National Natural Science Foundation of China (11904198, 51872161, 51902179); Natural Science Foundation of Shandong Province (ZR2018BA029, ZR2019BF019); Project of Shandong Province Higher Educational Science and Technology Program (J18KA244).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
References
1. X. Zheng, R. K. Kankala, C-G. Liu, S-B. Wang, A-Z. Chen, and Y. Zhang, “Lanthanides-doped near-infrared active upconversion nanocrystals: upconversion mechanisms and synthesis,” Coordination Chemistry Reviews 438(1), 213870 (2021). [CrossRef]
2. P. Li, M. Jia, G. Liu, A. Zhang, Z. Sun, and Z. Fu, “Investigation on the fluorescence intensity ratio sensing thermometry based on nonthermally coupled levels,” ACS Appl. Bio Mater. 2(4), 1732–1739 (2019). [CrossRef]
3. Y. Ma and S. Li, “NaYF4:Yb,Tm@TiO2 core@shell structures for optimal photocatalytic degradation of ciprofloxacin in the aquatic environment,” RSC Adv. 9(57), 33519–33524 (2019). [CrossRef]
4. M. S. Sebag, Z. Hu, K. O. Lima, H. Xiang, P. Gredin, M. Mortier, L. Billot, L. Aigouy, and Z. Chen, “Microscopic evidence of upconversion-induced near-infrared light harvest in hybrid perovskite solar cells,” ACS Appl. Energy Mater. 1(8), 3537–3543 (2018). [CrossRef]
5. W. Bi, Y. Wu, C. Chen, D. Zhou, Z. Song, D. Li, G. Chen, Q. Dai, Y. Zhu, and H. Song, “Dye sensitization and local surface plasmon resonance-enhanced upconversion luminescence for efficient perovskite solar cells,” ACS Appl. Mater. Interfaces 12(22), 24737–24746 (2020). [CrossRef]
6. A. Khare, “A critical review on the efficiency improvement of upconversion assisted solar cells,” J. Alloys Compd. 821(25), 153214 (2020). [CrossRef]
7. F. Xu, Y. Sun, H. Gao, S. Jin, Z. Zhang, H. Zhang, G. Pan, M. Kang, X. Ma, and Y. Mao, “High-performance perovskite solar cells based on NaCsWO3@NaYF4:Yb, Er upconversion nanoparticles,” ACS Appl. Mater. Interfaces 13(2), 2674–2684 (2021). [CrossRef]
8. H. Zhang, Y. Xiao, F. Qi, P. Liu, Y. Wang, F. Li, C. Wang, G. Fang, and X. Zhao, “Near-infrared light-sensitive hole-transport-layer free perovskite solar cells and photodetectors with hexagonal NaYF4:Yb3+,Tm3+@SiO2 upconversion nanoprism-modified TiO2 scaffold,” ACS Sustainable Chem. Eng. 7(9), 8236–8244 (2019). [CrossRef]
9. H. Cho, N. Lee, Y. J. Song, J. Hong, H. Kang, S. Y. Lim, J. Lee, H. M. Kim, N. Lee, S. H. Cho, and S. M. Lee, “Strategically manipulated polymer solar cells to incorporate plasmonically enhanced spectral upconversion backplane,” Adv. Optical Mater. 8(16), 2000466 (2020). [CrossRef]
10. M. wang, Y. Wu, F. Juan, Y. Li, B. Shi, F. Xu, J. Jia, H. Wei, and B. Cao, “Enhanced photocurrent of perovskite solar cells by dual-sensitized β-NaYF4: Nd3+/Yb3+/Er3+ up-conversion nanoparticles,” Chem. Phys. Lett. 763(16), 138253 (2021). [CrossRef]
11. J. Zhou, Q. Liu, W. Feng, Y. Sun, and F. Y. Li, “Upconversion luminescent materials: advances and applications,” Chem. Rev. 115(1), 395–465 (2015). [CrossRef]
12. W. L. Wang, Q. K. Shang, W. Zheng, H. Yu, X. J. Feng, Z. D. Wang, Y. B. Zhang, and G. Q. Li, “A novel near-infrared antibacterial material depending on the upconverting property of Er3+-Yb3+-Fe3+ tridoped TiO2 nanopowder,” J. Phys. Chem. C 114(32), 13663–13669 (2010). [CrossRef]
13. Y. Zhang, J. Chen, G. Hou, D. Li, and K. Chen, “Low power consumption light emitting device containing TiO2:Er3+ thin film prepared by sol-gel method,” Opt. Express 28(5), 6064–6070 (2020). [CrossRef]
14. C. Tao, S. Neutzner, L. Colella, S. Marras, A. R. Kandada, M. Gandini, M. D. Bastiani, G. Pace, L. Manna, M. Caironi, C. Bertarelli, and A. Petrozza, “17.6% steady state efficiency in low-temperature processed standard-architecture planar solar cells with the perovskite absorber grown onto a PCBM-based electron extraction layer,” Energy Environ. Sci 8(8), 2365–2370 (2015). [CrossRef]
15. H. Tan, A. Jain, O. Voznyy, X. Lan, F. P. García de Arquer, J. Z. Fan, R. Quintero-Bermudez, M. Yuan, B. Zhang, Y. Zhao, F. Fan, P. Li, L. N. Quan, Y. Zhao, Z. H. Lu, Z. Yang, S. Hoogland, and E. H. Sargent, “Efficient and stable solution-processed planar perovskite solar cells via contact passivation,” Science 355(6326), 722–726 (2017). [CrossRef]
16. H. P. Zhou, Q. Chen, G. Li, S. Luo, T. B. Song, H. S. Duan, Z. R. Hong, J. B. You, and Y. S. Liu, ang Y. Yang, “Inerface engineering of highly efficient perovskite solar cells,” Science 345(6196), 542–546 (2014). [CrossRef]
17. X. Zhou, H. Wang, H. P. Xia, H. W. Song, X. Chen, J. X. Hu, and B. J. Chen, “Efficient up-conversion Yb3+, Er3+ co-doped Na2Lu9F32 single crystal for photovoltaic application under solar cell spectrum excitation,” Chin. Chem. Lett. 17(9), 091601 (2019). [CrossRef]
18. L. Wang, K. C. Chen, H. Tong, K. Wang, L. Tao, Y. X. Zhang, and X. F. Zhou, “Inverted pyramid Er3+ and Yb3+ co-doped TiO2 nanorod arrays based perovskite solar cell: infrared response and improved current density,” Ceram. Int. 46(8), 12073–12079 (2020). [CrossRef]
19. H. Zhang, Q. S. Zhang, Y. Q. Lv, C. Yang, H. H. Chen, and X. F. Zhou, “Upconversion Er-doped TiO2 nanorod arrays for perovskite solar cells and the performance improvement,” Mater. Res. Bull. 106, 346–352 (2018). [CrossRef]
20. Q. Tian, W. Yao, W. Wu, and C. Jiang, “NIR light-activated upconversion semiconductor photocatalysts,” Nanoscale Horiz. 4(1), 10–25 (2019). [CrossRef]
21. D. W. Goodwin, “Spectra and energy levels of rare earth ions in crystals,” Phys. Bull. 20(12), 525 (1969). [CrossRef]
22. Z. Bai, H. Lin, J. Johnson, S. C. Rong. Gui, K. Imakita, R. Monzatami, M. Fujii, and N. Hashemi, “The single-band red upconversion luminescence from morphology and size controllable Er3+/Yb3+ doped MnF2 nanostructures,” J. Mater. Chem. C 2(9), 1736–1741 (2014). [CrossRef]
23. G. Tian, Z. Gu, L. Zhou, W. Yin, X. Liu, L. Yan, S. Jin, W. Ren, G. Xing, S. Li, and Y. Zhao, “Mn2+ dopant-controlled synthesis of NaYF4:Yb/Er upconversion nanoparticles for in vivo imaging and drug delivery,” Adv. Mater. 24(9), 1226–1231 (2012). [CrossRef]
24. S. Zeng, Z. Yi, W. Lu, C. Qian, H. Wang, L. Rao, T. Zeng, H. Liu, H. Liu, B Fei, and J. Hao, “Simultaneous realization of phase/size Manipulation, upconversion luminescence enhancement, and blood vessel imaging in multifunctional nanoprobes through transition metal Mn2+ doping,” Adv. Funct. Mater. 24(26), 4051–4059 (2014). [CrossRef]
25. F. Wang and X. G. Liu, “Recent advances in the chemistry of lanthanide-doped upconversion nanocrystals,” Chem. Soc. Rev. 38(4), 976–989 (2009). [CrossRef]
26. R. Martín-Rodríguez, R. Valiente, and M. Bettinelli, “Room-temperature green upconversion luminescence in LaMgAl11O19:Mn2+,Yb3+ upon infrared excitation,” Appl. Phys. Lett. 95(9), 091913 (2009). [CrossRef]
27. C. N. Xie, S. K. Xie, R. X. Yi, R. P. Cao, H. Li Yuan, and F. Xiao, “Site-Selective occupation for broadband upconversion luminescence in Ca3Y(GaO)3(BO3)4:Yb3+,Mn2+ phosphors,” J. Phys. Chem. C 124(12), 6845–6852 (2020). [CrossRef]
28. E. Song, Z. Chen, M. Wu, S. Ding, S. Ye, S. Zhou, and Q. Zhang, “Room-temperature wavelength-tunable single-band upconversion luminescence from Yb3+/Mn2+ codoped fluoride perovskites ABF3,” Adv. Opt. Mater. 4(5), 798–806 (2016). [CrossRef]
29. C. Reinhard, P. Gerner, F. Rodríguez, S. García-Revilla, R. Valiente, and H. U. Güdel, “Near-infrared to green photon upconversion in Mn2+ and Yb3+ doped lattices,” Chem. Phys. Lett. 386(1-3), 132–136 (2004). [CrossRef]
30. J. L. Wang, E. H. Song, M. Wu, W. B. Dai, X. F. Jiang, B. Zhou, and Q. Y. Zhang, “Room-temperature green to orange color-tunable upconversion luminescence from Yb3+/Mn2+ co-doped CaO,” J. Mater. Chem. C 4(42), 10154–10160 (2016). [CrossRef]
31. P. Gerner, O. S. Wenger, R. Valiente, and H. U. Güdel, “Green and red light emission by upconversion from the near-IR in Yb3+ doped CsMnBr3,” Inorg. Chem. 40(18), 4534–4542 (2001). [CrossRef]
32. M. Kneiß, M. Jenderka, K. Brachwitz, M. Lorenz, and M. Grundmann, “Modeling the electrical transport in epitaxial undoped and Ni-, Cr-, and W-doped TiO2 anatase thin films,” Appl. Phys. Lett. 105(6), 062103 (2014). [CrossRef]
33. C. Aruta, M. Angeloni, G. Balestrino, N. G. Boggio, P. G. Medaglia, A. Tebano, B. Davidson, M. Baldini, D. Di Castro, P. Postorino, P. Dore, A. Sidorenko, G. Allodi, and R. De Renzi, “Preparation and characterization of LaMnO3 thin films grown by pulsed laser deposition,” J. Appl. Phys. 100(2), 023910 (2006). [CrossRef]
34. N. A. Deskins and M. Dupuis, “Electron transport via polaron hopping in bulk TiO2:A density functional theory characterization,” Phys. Rev. B 75(19), 195212 (2007). [CrossRef]
