Effect of Au clustering on ferromagnetism in Au doped TiO2 films: theory and experiments investigation

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Highlights

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    Magnetic properties of the TiO2 films and Au cluster doped TiO2 films fabricated by sol-gel and sputtering methods.

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    Saturation magnetizations are in the order of Au-TiO2 (annealed in N2) > undoped TiO2 (in air) > Au-TiO2 (in air).

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    Formation energy of Au cluster doped TiO2 films is lower than the oxygen vacancy and Au cluster codoped TiO2 films.

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    Codoping of oxygen vacancies and Au clusters led to the spin-split of Ti 3d and O 2p in Au cluster doped TiO2 films.

Abstract

In this paper, we investigated the physical properties especially the magnetic properties of the TiO2 films and Au cluster doped TiO2 films fabricated by sol-gel and sputtering methods combined experiments and first-principles calculations. All the samples annealed under air and N2 atmosphere respectively exhibit room temperature ferromagnetism with the crystal phase of anatase. The values of the saturation magnetizations are in the order of Au δ-doped TiO2 (annealed in N2)>undoped TiO2 (annealed in air)>Au δ-doped TiO2 (annealed in air). The first principles calculation results show that the formation energy of Au cluster doped TiO2 films is lower than that of the oxygen vacancy and Au cluster codoped TiO2 films. The effects of the Au cluster dopant are the retard of the formation of surface oxygen vacancy and the electrons transfer from 3d states of Ti atoms to Au 5d states in Au cluster doped TiO2 films. The codoping of surface oxygen vacancies, bulk oxygen vacancies and Au clusters led to the spin-split of Ti 3d and O 2p in Au cluster doped TiO2 films (annealed in N2) which yield the highest saturation magnetization.

Introduction

Dilute magnetic semiconductors (DMS), which are desirable for spintronic applications, have been extensively investigated combing magnetic and transport properties [1]. However, the Curie temperature (Tc) is not high enough and the density of spin polarized carriers is not large enough for those confirmed DMS materials for applications. In recent decades, more and more attention has been focused on DMS such as transition metals doped ZnO [2], [3], SnO2 [4], and TiO2 [5] which are often referred as dilute magnetic oxide semiconductor (DMOS), due to realization of room temperature (RT) ferromagnetism (FM). There are various models to explain the magnetism in DMOS such as bound magnetic polaron (BMP) model [6], charge transfer based Storner type model [7], Ruderman-Kittel-Kasuya-Yosida (RKKY) model [8], and double exchange model [9]. However, the mechanism of the magnetic order is not yet clear partly due to the inconsistent results reported in this field [10], [11].

In TiO2-based DMOS, the observed ferromagnetism is attributed to the following factors: the transition metal dopants, Ti3+ ions, cation vacancies, and oxygen vacancies (VOs) [12]. On the one hand, the transition metal elements have unpaired d-electrons that can provide magnetic moment to DMOS, but the cluster dopants of the transition metal cannot be ruled out (yet possible) since they are hardly detected by X-ray diffraction (XRD), and it remains a unsolved issue whether the FM is intrinsic or extrinsic in TiO2-based DMOS [12]. On the other hand, the presence of VO has been pointed out as a possible factor influencing the FM behavior of the TiO2 films [13]. It is not clear that the VO induces transition metal cluster and/or it causes magnetic ordering yet. For transition metal clusters such as Au clusters, the magnetic moments display a pronounced odd-even oscillation with the number of gold atoms [14]. Moreover, even undoped TiO2−x nanotube films or TiO2−x nanorod particles reported have RT FM [15]. Yoon et al. show that the magnetization in oxygen-deficient TiO2 films increases as VOs increase. [16].

Whether the observed FM has anything to do with the transition metal cluster doping or to be solely related to intrinsic defects remains an unresolved issue in TiO2-based DMOS. Here, we synthesized the undoped TiO2 films (annealed in air) and Au δ-doped TiO2 films (annealed in air and in N2 respectively). The focus of the present work is to bring further insight about the origin of the RT FM observed in TiO2-based materials by studying the relation between VOs and Au cluster doped in TiO2 films. The experimental results are consistent with the results of the first principles calculations, and the magnetism is associated with the VO defects and Au clusters. The strict connections between Au cluster and VO were discussed to explain the ferromagnetism observed in these materials.

Section snippets

Experimental details

Fig. 1 shows the flow chart of the synthesis process for the undoped TiO2 films, Au δ-doped TiO2 films (annealed in air) and Au δ-doped TiO2 films (annealed in N2) prepared by sol–gel and sputtering process. A clear solution is prepared by reacting tetrabutyl titanate (C16H36O4Ti) with a mixture of critical amount of water and hydrochloric acid (HCl) in an ethanol (C2H5OH) diluted medium. The fluorine doped tin oxide (FTO) substrates were cleaned in de-ion water and in absolute ethanol solution

Results and discussion

Fig. 2 shows XRD pattern of undoped TiO2 films and Au δ-doped TiO2 films on FTO substrates. The peaks appearing around 25.3°, 36.9°, 37.8°, 48.1°, 53.9°, 55°, 62.1°, 62.9°, 68.7°, 70.2° and 75.0° are indexed to TiO2 anatase (101), (103), (004), (200), (105), (211), (213), (204), (116), (220) and (215), where the peaks presented at about 26.7°, 34.0°, 51.7°, and 65.8° correspond to FTO (110), (101), (211), and (301) for the specimens. It means that all the samples of undoped and Au δ-doped TiO2

Conclusions

In summary, we have investigated the magnetic properties of undoped TiO2 and Au cluster doped TiO2 films with experiments and first-principles calculations. The experimental results are consistent with the first principles calculations. The values of the magnetic moment are in the order: Au cluster doped TiO2<undoped TiO2 films (annealed in air)<Au cluster doped TiO2 films (annealed in N2). The Vos and Au cluster play an important role in the origin of the RT FM. On the one hand, the doping of

Acknowledgement

This work is supported by the National Nature Science Foundation of China Nos. 11404100, 11474086, 11175135, 10904116 and 11304083, the Postdoctoral Research Sponsorship in Henan Province No. 2014062, the Post-Doctoral Research Foundation of Henan Normal University No. 01026500204, and the Start-up Foundation for Doctors of Henan Normal University Nos. 01026500257 and 01026500121. This work is also supported by the High Performance Computing Center of Henan Normal University. The authors would

References (34)

  • M. Wang et al.

    Appl. Surf. Sci.

    (2014)
  • J. Yu et al.

    Appl. Catal. B: Environ.

    (2009)
  • Z. Zhou et al.

    J. Alloy. Compd.

    (2016)
  • P.M. Kumar et al.

    Thin Solid Films

    (2000)
  • F.B. Dejene et al.

    Physica B

    (2016)
  • M. Hamadanian et al.

    Mater. Sci. Semicond. Process.

    (2016)
  • G.A. Prinz

    Science

    (1998)
  • K. Ueda Tabata et al.

    Appl. Phys. Lett.

    (2001)
  • B. Pal et al.

    J. Appl. Phys.

    (2010)
  • X. Liu et al.

    J. Phys. Chem. C

    (2010)
  • R.K. Singhal et al.

    J. Appl. Phys.

    (2010)
  • J.M.D. Coey et al.

    Nat. Mater.

    (2005)
  • J.M.D. Coey et al.

    New J. Phys.

    (2010)
  • Z.J. Wang et al.

    Appl. Phys. Lett.

    (2003)
  • J. Chen et al.

    Phys. Rev. B

    (2006)
  • K. Bapna et al.

    Appl. Phys. Lett.

    (2011)
  • A.R. Barman et al.

    AIP Adv.

    (2012)
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