Correlation between Cr3+ Luminescence and Oxygen Vacancy

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Correlation Between Cr Luminescence and Oxygen Vacancy Disorder in Strontium Titanate Under MeV Ion Irradiation Miguel L. Crespillo, Joseph Turner Graham, Fernando Agullo-Lopez, Yanwen Zhang, and William J. Weber J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04352 • Publication Date (Web): 25 Aug 2017 Downloaded from http://pubs.acs.org on August 27, 2017

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The Journal of Physical Chemistry

Correlation between Cr3+ Luminescence and Oxygen Vacancy Disorder in Strontium Titanate under MeV Ion Irradiation M. L. Crespillo1*, J. T. Graham2,1, F. Agulló-López3, Y. Zhang4 and W. J. Weber1,4* 1

Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996, USA

2

Department of Mining and Nuclear Engineering, Missouri University of Science and Technology, Rolla, Missouri 65409, USA

3

Centro de Microanálisis de Materiales, CMAM-UAM, Cantoblanco, Madrid 28049, Spain

4

Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA

*Corresponding authors: Email address: [email protected] (M. L. Crespillo)

Tel: +1-865-360-2287

Email address: [email protected] (W. J. Weber)

Tel: +1-865-974-0415

Abstract Strontium titanate (SrTiO3), a model system with a strongly correlated electronic structure, has attracted much attention recently because of its outstanding physicochemical properties and considerable potentials for technological applications. The capability to control oxygen vacancy profiles and their effect on valence states of cations will increase significantly the functionality of devices based on transition metal oxides. This work presents new insights into the near-infrared luminescence emission of Cr3+ centers in

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stoichiometric SrTiO3 induced using 3 MeV protons at temperatures of 100 K, 170 K and room temperature. The study covers a wide spectral range, including near-infrared, visible and near-UV regions. Our main purpose is to investigate the role of the oxygen vacancies introduced by energetic charged particles on the shape and yield of induced luminescence spectra, in particular to explore the interplay between the Cr3+ luminescence at 1.55 eV and oxygen disorder. A clear correlation is found between the decay of the Cr luminescence yield during irradiation and the growth of a band at 2.0 eV, well-resolved below 170 K, which has been very recently attributed to d-d transitions of electrons self-trapped as Ti3+ in the close vicinity of oxygen vacancies. This correlation suggests irradiation-induced oxidation of the Cr3+ (Cr3+ → Crn+, n>3) via trapping of irradiation-induced holes, while the partner electrons are self-trapped as Ti3+. These new results provide effective guidelines for further understanding the electronic and photocatalytic behavior of STO:Cr3+.

*Corresponding authors: Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996, United States. Ph: +1-865-360-2287. Email address: [email protected] (M. L. Crespillo). Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996, United States. Ph: +1-865-974-0415. Email address: [email protected] (W. J. Weber).


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1.

Introduction Strontium titanate (SrTiO3, STO) is an oxide crystal with cubic perovskite structure

at room temperature (RT) that exhibits a paraelectric-ferroelectric transition at ≈ 35 K and another one at around 100 K, corresponding to a cubic-tetragonal transition.1 It shows a rich physicochemical behavior, including a variety of electrical properties, that modify the response of the crystal from insulating to semiconducting, metallic and even superconducting. Moreover, it manifests intense photocatalytic activity for hydrogen generation from water decomposition.2 This later property is closely related to doping with transition metal impurities, such as chromium, as well as to thermal treatments and ion irradiation that introduce oxygen vacancies.3-10 In particular, chromium doping appears to play a key role on all these processes, and allows for a reversible insulator-to-conductor transition or resistive memory effect.11-13 It is generally accepted that the Cr dopant ions in STO substitute for Ti4+ and occupy an octahedral site as Cr4+,14 but shift to Cr3+ under electronic excitation. The possibility of other valence states, such as Cr6+, as well as the occupancy of Sr sites has been also discussed,15-17 mostly in connection with the photocatalytic response of STO, since this activity is very dependent on the charge state of chromium.18-21 One should remark that the charge state is strongly influenced by the location of the Fermi level in the STO crystal, which can be modified through impurity doping, thermo-chemical treatments and irradiation. Luminescence is a main, and often unique, optical technique to investigate the properties of transition metal impurities in dielectric materials and their relation to the lattice structure. Photoluminescence (PL), as a key optical technique, is widely used to identify and characterize excited states of defect centers.22 The luminescence behavior of



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Cr in STO under a variety of excitation sources has been extensively investigated.23-28 The reported experiments include excitation by electrons (cathodoluminescence, CL), UV, Xrays (radioluminescence, RL) and laser pulses. In particular, using photoluminescence (PL) excitation above the band gap (3.27 eV), a well-resolved near-infrared (NIR) band at 1.55 eV (often called the B-band), together with a set of vibronic side-bands, has been observed in STO. It has been associated with spin-forbidden optical transitions 2Eg →4A2g of Cr3+ centers incorporated in the crystal during growth.23-26 The vibronic side-bands, dominant on the low-energy side, have been attributed to the contribution of lattice modes and possibly a local mode. The emission yield of the NIR bands has been shown to increase with temperature between 50 and 100 K and then decrease above such temperature up to RT.29 The decrease in this temperature range has been attributed to non-radiative decay processes, in accordance with the temperature dependence of the fluorescence lifetime of Cr3+ in STO. The yield at or near RT is very low, making the PL difficult to observe. It is worthwhile mentioning that the photoconductivity of the material presents a quite similar excitation spectrum to that for the PL.29 Although the 1.55 eV fluorescence of STO has been attributed to electronic transitions of a Cr3+ ion, there is little work currently available for excitation with ion beams (ionoluminescence, IL). This technique provides remarkable advantages over those using other excitation sources.30-33 First, the very high electronic excitation rate (electronic stopping power, Se) for energetic ions offers a higher sensitivity to detect low Cr concentrations. Moreover, in comparison to monochromatic light (laser) sources, IL presents a very broad excitation energy range, involving in-gap as well as conduction and valence band levels. On the other hand, the IL data may serve to better portray the features


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and dynamics of the irradiation-induced structural damage. Finally, the irradiation-induced lattice defects, mainly oxygen vacancies, allow real-time investigation of their synergistic interaction with chromium dopants. Consequentially, the IL results will provide a more complete picture of the Cr3+ luminescence behavior in perovskites and may provide new insights into their electronic structure and photocatalytic response. Novel luminescence response of SrTiO3:Cr to 3 MeV proton irradiation is investigated in relation to the generated oxygen disorder. Irradiations have been performed at temperatures of 100 K, 170 K and RT (i.e. on the cubic phase of the crystal), and the spectra cover a wide wavelength range from 1.3 eV to 3.3 eV. Under 3 MeV proton irradiation, isolated vacancies and interstitials are expected resulting from either electronic excitation or elastic ion-atom scattering (nuclear energy loss, Sn). The paper presented here is in the line of previous luminescence experiments on SrTiO334-39 and essentially follows the same methodology used in our previous work.39 In this reference, a well-resolved emission band peaked at 2.0 eV has been observed at 100 K, and is partially resolved at 170 K. It has been attributed to electrons self-trapped (polarons) as Ti3+ adjacent to an oxygen vacancy,27-29, 34-39 and it is, therefore, considered to be an indicator of the generated oxygen disorder. In the present work, a close correlation has been found between the evolution of this band and the Cr3+ luminescence yield. Specifically, a noticeable decay of the NIR Cr luminescence on increasing fluence has been correlated with the growth of the 2.0 eV luminescence emission, which should correspond to the formation of oxygen vacancies during irradiation. Thus, the two luminescence emissions correspond to the two final fates of the e-h (electron-hole) pairs excited by the irradiation: holes trapped at Cr3+ (leading to Crn+), whereas the electron partners become self-trapped at oxygen vacancies. We expect



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that these new data provide a more complete understanding of the role of oxygen vacancies on the electronic structure and luminescence behavior of STO. 2

Experimental details High-purity stoichiometric STO (001) single crystals, provided by MTI Corporation

(Richmond, CA), were used in this study. The crystals were not intentionally doped, and the Cr concentration (Cr is lower than 0.5 ppm wt) was lower than in previous papers, such as those by Stokowski et al.24 and Kim et al.26 Due to the relevance of possible charge transfer processes between transition metal impurities on the light emission features observed in our work, Table 1 lists the nominal content of main impurities as provided by the manufacturer. One-side polished samples, with dimensions of 12.5×12.5×0.5 mm, were irradiated at temperatures of 100 K, 170 K and RT under a vacuum of 5×10-5 Pa, at the Ion Beam Materials Laboratory (IBML) at the University of Tennessee, Knoxville.40 The samples were tilted a few degrees with respect to the incident ion beam to avoid ion channeling. Adjustable beam slits were used to define irradiation areas of 3×3 mm2 (RT) and 2×2 mm2 (100 K, 170 K) on the sample surface. The ion beam was defocused and wobbled in the horizontal and vertical directions over a wider area with the aim of defining a homogeneous irradiated region. Beam homogeneity was within 10% throughout the irradiated area, which was validated by checking the IL on Al2O3 (used as a scintillator) monitored with a CCD camera. Low beam current densities in the range of 3.2-4.4 nA/mm2 (ion flux in the range of (2.0-2.8)×1012 cm-2s-1) were used to avoid beam heating and charge accumulation on the samples.41 Stopping powers for 3 MeV protons in STO were calculated using the Stopping and Range of Ions in Matter (SRIM) binary collision approximation (BCA) software using full-cascade simulations (version 2012).42, 6 ACS Paragon Plus Environment

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The

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calculated stopping powers and atomic displacements including all the host ions, i.e. Sr, Ti and O, are shown in Table 2, assuming threshold displacement energies of 45 eV, 70 eV and 80 eV for the O,44, 45 Ti46 and Sr46 atoms, respectively,47 and a density of 5.12 g cm-3 for STO. The ionizing dose rate is considered to be relevant and it has also been included in Table 2. A schematic diagram of the in-situ luminescence experimental setup is shown in Fig. 1.48 The light emitted from the samples is transmitted through a sapphire window port at 150 with respect to the ion beam direction, collected into a silica optical fiber of 1 mm diameter located outside of the vacuum chamber and coupled to an imaging spectrometer, which consists of a monochromator (Acton Research Corp. SpectraPro-2556 with 500 mm focal length and fitted with a 150 lines/mm grating blazed at 500 nm) and a liquid nitrogencooled back illuminated UV enhanced CCD camera multichannel array detector (Princeton Instruments, Spec-10: 2KBUV). The spectral resolution is better than 0.2 nm. More details regarding the optical setup, spectrometer and detector can be found elsewhere.40, 48 Time evolution of the IL spectra was acquired using an integration time per spectrum between 0.2 and 1.0 s at LT and RT, respectively. The ion-induced luminescence intensity was normalized to take into account the ion energy, flux, the acquisition time and the irradiated spot area (luminescent area), with the aim of quantitatively comparing the luminescent yields coming from different ion beam irradiations. Furthermore, as has been described previously,48 the IL corrected spectra have been converted from wavelength () (originally in units of I()d) to energy space (I(E)dE) by taking into account the factor 2).



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The steady state temperature of the sample surface was measured via a K-type (chromel-alumel) thermocouple. More details on the experimental apparatus are provided elsewhere.40, 41 3.

Results and analysis

3.1.

Near infrared luminescence (NIR) spectra of Cr3+ and kinetics Figure 2 (a-c) shows the IL spectra of STO obtained under irradiation with 3 MeV

protons at temperatures of RT (a), 170 K (b) and 100 K (c) to different ion fluences. At the lowest irradiation temperatures (particularly 100 K), the spectra clearly show the R (zerophonon) line (often designated as B-line) at 1.555 eV corresponding to the 2Eg → 4A2g transition of the Cr3+ ion in a substitutional position for Ti4+ (Figure 3 shows in detail the sharp structure of the NIR emission bands at 100 K). No splitting of this sharp line was observed in our experiments, even at the lowest temperature (100 K), which is close or even below that for the cubic-tetragonal phase transition. The vibronic structure of the emission band is in good accordance with that measured in the PL experiments.16,

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As

expected from the theory of vibronic coupling,49 the ratio of the intensity of the vibronic side-bands to that for the more intense (central) NIR peak (B-line) decreases on lowering the temperature. On the other hand, the overall intensity of the spectra is strongly enhanced with decreasing temperature due to an increase of free carrier lifetime. The shape of the NIR spectra is independent of irradiation fluence over an extended range, indicating that the defects introduced by the irradiation do not modify the environment of the emitting chromium impurities. However, the luminescence yield at the zero-phonon line (1.55 eV) is strongly dependent on irradiation fluence, as illustrated in Figure 4 for the three investigated irradiation temperatures. The kinetics shows an initial


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fast growing stage as a function of fluence (difficult to appreciate and evaluate in the figures) that saturates after a fluence of 4×1011 cm-2 at 100 K and 3×1012 cm-2 at 170 K. This is due to the establishment of a steady-state concentration of electron-hole (e-h) pairs under ion beam excitation and a corresponding steady-state concentration of emitting Cr3+ centers. The behavior shown in Figure 4 is consistent with similar data reported for the luminescence and conductivity in pure and doped sapphire crystals;50-52 and the dose ranges appear reasonable (i.e. the fluence of 4×1011 cm-2 corresponds to an ionizing dose rate of about 10 kGy, a reasonable value for the e-h concentration to reach steady state in nominally pure oxide crystals). As in the present work, a relation between the impurity and oxygen vacancies was indicated. After this initial process, a slowly decreasing stage of the yield appears, which extends well into the 1014 cm-2 fluence range and should be associated with the structural disorder generated by the irradiation. We postulate that the oxygen vacancy defects introduced by irradiation through nuclear scattering collisions tend to reduce the concentration of Cr3+ and the resulting NIR emission. At a qualitative level, one observes that the overall intensity of the spectra is strongly enhanced with decreasing temperature because of an increase of free carrier lifetime. However, the quantitative effect of temperature on the measured IL yield cannot be easily evaluated from our experiments due to the combined effects of temperature and fluence. One can try to evaluate the role of temperature on the kinetics of the initial IL growth stage before the effects of irradiation damage become significant. An estimate can be determined from the maximum values of the emission yields obtained in the kinetic curves of Figure 4. From a qualitative analysis, one concludes that the dependence of the NIR emission yield on irradiation temperature follows a similar decreasing trend as observed in PL experiments under UV excitation at 365 nm (above the band gap).23, 29 As 9 ACS Paragon Plus Environment


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an example, our data for proton irradiations show decreasing emission yields by factors of around 7 when increasing the temperature from 100 K to 170 K. For comparison, the PL yield measured in another study29 decreases by a factor of 4.5 upon a temperature increase from 100 K to 170 K. The effect, which is consistent with the temperature dependence of the photoconductivity, has been associated to the increasing importance of the non-radiative processes in the Cr3+ center emission.29 The same explanation very likely applies to the IL data, decrease of luminescence lifetime with increasing temperature. 3.2.

Visible and near-UV spectra under irradiation In addition to the Cr3+ emissions, other spectral bands at 2.8 eV, 2.5 eV and 2.0 eV

are observed, as illustrated in Figure 5, in accordance with previous reports.3-6, 9, 10, 34-39, 53, 54

These bands have been attributed to different decay channels for the electronic

excitations,39 and they will serve as a reference to discuss the effect of irradiation on the chromium luminescence. The kinetic behavior of all these bands is quite different, as illustrated in Figure 6. The 2.5 eV and 2.8 eV bands show a two-stages behavior with fluence, including a very fast initial growth up to an approximate steady level (Figure 6(a)). This level is maintained up to rather high fluences, in the range a few times 1014 cm-2. This is the fluence scale for the lattice damage caused by the irradiation, in which the 1.55 eV emission of Cr experiences a rapid decay (Figure 4). On the other hand, the 2.0 eV band, sometimes observed as an unresolved shoulder to the 2.5 eV band in heavily disordered/strained or amorphous STO,3-6 shows a quite different behavior (Figure 6(b)). It grows slowly from the start of irradiation and, at variance with the other bands, appears clearly related to lattice damage. Therefore, we have focused our attention on 2.0 eV band, which is dominant and well-resolved at low temperatures (100 and 170 K). One should note


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that the peak energy for this band experiences a slight but noticeable shift to higher energy at lower temperatures, from 1.93 eV at 170 K to 2.04 eV at 100 K. The observation may have, indeed, physical relevance but its analysis would require additional study of the polaron model of the center that it is out of the scope of the present paper. The important point for this study is that the band exhibits (in the 1014 cm-2 fluence range) a growth rate similar to the decay rate found for the Cr3+ emission.39 The correlation will be discussed in the following section. 4

Discussion: Correlation between the Cr3+ and the 2.0 eV emission bands under irradiation The basic model used to describe the IL in STO is similar to that proposed for band-

to-band excitation in PL, as well as for ion-beam induced luminescence (IL) in other dielectric materials, such as silica.55 Under irradiation with energetic ion beams, a high density of e-h pairs are produced until a steady-state concentration is reached, which is determined by the excitation rate and carrier lifetime. The data indicate that a fluence range of around or below 1011-1012 cm-2 is typically associated with the establishment of a steadystate e-h cloud under ion-beam excitation. During their lifetime, the e and h will move through the lattice until they become trapped and/or recombine at suitable recombination centers. Such processes involve interaction with holes. The Cr3+ ions placed at Ti4+ sites can trap a hole to become Cr4+, which finally can recombine with an electron and yield the characteristic 1.55 eV emission (zero-phonon line). The detailed process is schematically represented in Figure 7(a). To investigate the alternative processes involving electrons, we focus on the band at 2.0 eV, which is dominant at low temperatures and appears to be related to irradiation-



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induced oxygen disorder.39 Its kinetics behavior, as a function of fluence, can be compared to that found for the Cr3+ emission. As commented in the previous section, the growth of the 2.0 eV band can be correlated with the decay observed for the second (slow) stage of the Cr luminescence. For a better appraisal of the situation, the 1.55 eV and 2.0 eV yield kinetics at 170 and 100 K, shown previously in Figure 4 and Figure 6(b), are plotted together in Figure 8. A clear anti-correlation between the two emission yields is well illustrated for irradiations at 170 K (Figure 8(a)) and 100 K (Figure 8(b)). The relationships observed in Figure 8(b) may be quantified by evaluating the dependency of the intensity of the 2.0 eV emission (associated to oxygen vacancy concentration) on the square root of ion fluence and the dependency of the inverse of the NIR chromium yield on ion fluence. As is illustrated in the inset of Figure 8(b), an approximate linear behaviour with similar slope is obtained as a function of ion fluence. It enhances the evidence that the actual reduction of the Cr3+ centres will be proportional to the square of the O vacancy type centres. The assumed square-root kinetics for the 2.0 eV band with fluence may suggest the role of statistical clustering of vacancies during irradiation and, consequently, the formation of vacancy-chromium pairs. We note that a similar correlation cannot be found for either of the two other bands (2.5 eV and 2.8 eV in the spectra), which are considered to be purely electronic in nature.39 In order to understand this correlative behavior, a model for the origin of the 2.0 eV emission band is needed. Ab initio theoretical (DFT) calculations56-58 suggest that the 2.0 eV band is attributed to electrons trapped as Ti3+ in the vicinity of an oxygen vacancy. As a result, a new state is created below the conduction band edge having mainly a 3d(z2) character. This model for the oxygen vacancy center is different from that of the well-accepted F-type center in many other oxides, which has the electrons localized inside the oxygen vacancy.59 However, the situation is not so different, since the 12 ACS Paragon Plus Environment

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wavefunction for the new in-gap level mixes with the s-levels and extends into the vacancy. With respect to this model, the 2.0 eV emission is assigned to the localized Ti3+ d-d transitions from conduction band (CB) 3d(t2g) levels to new in-gap 3d(eg) levels induced by the oxygen vacancy.56-58, 60 Interestingly, ab initio calculations56 have been also performed on the role of n- and p-type doping and associated introduction of O vacancies on the electronic structure of STO, and the results reveal that the new Ti3+ 3d(eg) levels are generated close, but below the conduction band edge. The present luminescence data provide support to the interpretation, schematically illustrated in Figure 7(b), in which the electron partner of the hole involved in Figure 7(a) becomes trapped at the excited 3d(t2g) level of the cation next to an oxygen vacancy, and decays to the ground 3d(eg) state. Moreover, the model may be consistent with the absorption data for STO irradiated with Xrays at low temperatures.34 One should note that polaron models for vacancy centers in other oxides, such as sapphire and even LiNbO3, have been sometimes invoked.60,

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However, the case of STO is particularly relevant since the model for the oxygen vacancy center relies on ab-initio simulations using DFT theory57 that provides detailed quantitative predictions on its electronic structure. More quantitative comparative analyses of that electronic structure would be desirable but are not easy to perform since most characterization techniques (X-ray absorption near-edge spectroscopy (XANES), X-ray photoelectron spectroscopy (XPS), Electron energy-loss spectra (EELS)) have to be performed after irradiation. Such post-irradiation analyses may not be representative of the true sample state under irradiation, as observed in the in-situ IL experiments. Although independent determinations of vacancy concentration have not been made, we have performed some SRIM simulations of the irradiation induced damage. These simulations indicate that at the maximum yield level for the 2.0 eV emission, occurring above a fluence 13 ACS Paragon Plus Environment


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of ≈ 3×1014 cm-2, the O-vacancy concentration reaches a value of around 4.5×10-5, which would allow for significant luminescence emission. The simulations assume elastic collision damage and use the threshold values for atomic displacements derived from ab initio molecular dynamics calculations.47 A more quantitative correlation between oxygen concentrations and luminescence yields is not easy, since the quantum efficiency of the optical transitions responsible for the 2.0 eV band is not known. One should also consider previous Rutherford Backscattering Spectroscopy in Channeling configuration (RBS/C) experiments,62 which clearly show the progressive amorphization of STO crystals with fluence up to a fully amorphous state as the damage dose achieves a value from 0.1 to 0.4 dpa (displacements per atom), depending on the temperature. These experiments do not distinguish the type of defects; thus, the specific role of oxygen vacancies cannot be separately identified. 5

Summary and outlook Spectral and kinetic data on the Cr3+ luminescence induced by high energy

irradiation with 3 MeV protons at 100 K, 170 K and RT have been presented and discussed. At the lowest temperatures (100 K and 170 K) over an extended fluence range (up to around 3×1014 cm-2), the shape of the Cr3+ spectra is well resolved and quite comparable to those obtained by PL. However, the emission yields are much higher in comparison to photon excitation. On the other hand, the role of irradiation temperature closely resembles that measured in PL experiments and inhibits the IL yield of Cr3+ above 100 K. At all investigated temperatures, the evolution of the NIR emissions yields with irradiation fluence exhibits a rapid initial growth. It is ascribed to the rise of the electronic carrier population up to a rough steady-state, being essentially determined by the dynamics and


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lifetime of the excited carriers. That initial stage is followed by a decreasing stage of the Cr3+ emission, which should be related to the production of defects and damage during the irradiation. A main outcome is that the decay of the Cr3+ luminescence is accompanied by the correlated growth of the 2.0 eV emission band attributed to d-d transitions for electrons trapped as Ti3+ in the vicinity of oxygen vacancies. The analyses discussed in this paper offer a simple explanation for the anti-correlation found between the 1.55 eV and 2.0 eV luminescence emissions. It essentially represents the competition between the two routes corresponding to the trapping of e and h partners, resulting from the excitation of STO by irradiation, as Ti3+ and Cr4+, respectively. As a consequence, the valence states of Cr3+ become Crn+ (n  3), thus inhibiting (quenching) the typical vibronic emission of Cr3+ centers. Interaction of energetic charged particles with oxides offers a high excitation rate and an extremely broad excitation spectrum in comparison to PL. Moreover, the damage to the lattice caused by either electronic excitation or elastic ion-atom scattering allows introducing controlled amounts of lattice defects, mainly vacancies and interstitials. Our results have clearly illustrated the correlation between impurity and defect levels induced during irradiation, and highlight the role of IL as a tool for studying luminescence and radiation damage, as well as their synergy. In particular, ion-beam irradiation, affecting a micrometer depth region below the surface, may offer a novel strategy to modify the distribution of Cr valence states, through changes in the electronic structure and Fermi level. This generation of oxygen vacancies and the electron self-trapping as Ti3+ should play a key role for bistable resistance memories13 and for controlling the photocatalytic response of STO. In particular, the electronic effects reported in this paper can occur not far from the sample surface especially when low-energy ions are used, which should strongly influence the access of carriers to the sample surface and thus the photocatalytic behavior. 15 ACS Paragon Plus Environment


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In fact, ion-beam irradiation may offer a strategy to modify the distribution of Cr valence states and control the electronic and possibly the photocatalytic response. Using the proposed oxygen vacancy model for the 2.0 eV emission, it would be possible to monitor the evolution with dose of the vacancy concentration. This would provide a very useful tool for radiation damage studies in STO. The capability to control oxygen vacancy profiles and their effect on the valence states of cations will increase significantly the functionality and potential for technological applications of devices based on transition metal oxides.


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Acknowledgements This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. M. L. Crespillo and J. T. Graham acknowledge support from the University of Tennessee Governor’s Chair program.



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20. Kvyatkovskii, O. E. Ab initio calculations of neutral and charged impurity centers of manganese and chromium in strontium titanate. Phys. Solid State 2012, 54, 1397-1407. 21. Liu, J. W.; Chen, G.; Li, Z. H; Zhang, Z. G. Electronic structure and visible light photocatalysis water splitting property of chromium-doped SrTiO3. J. Solid State Chemistry 2006, 179, 3704-3708. 22. Ropp, R. C. Luminescence and the Solid State; Elsevier: Amsterdam, 2004. 23. Sihvonen, Y. T. Photoluminescence, photocurrent and phase-transition correlations in SrTiO3. J. Appl. Phys. 1967, 38, 4431-4435. 24. Stokowski, S. E.; Schawlow, A. L. Spectroscopic studies of SrTiO3 using impurity-ion probes. Phys. Rev. 1969, 178, 457-464. 25. Grabner, L. Photoluminescence in SrTiO3. Phys. Rev. 1969, 177, 1315-1323. 26. Kim, Q.; Powell, R. C.; Mostoller, M.; Wilson, T. M. Analysis of the vibronic spectrum of chromium doped strontium titanate. Phys. Rev. B 1975, 12, 5627-5642. 27. Mackor, A.; Blasse, G. Visible-light induced photocurrents in SrTiO3-LaCrO3 singlecrystalline electrodes. Chem. Phys. Lett. 1981, 77, 6-8. 28. Basun, S. A.; Bianchi, U.; Bursian, V. E.; Kaplyanskii, A. A.; Kleemann, W.; Markovin, P. A.; Sochava, L. S.; Vikhnin, V. S. Photoinduced phenomena in Sr1-xCaxTiO3, 0 ≤ x ≤ 0.12. Ferroelectrics 1996, 183, 255-264. 29. Feng, T. Anomalous photoelectronic processes in SrTiO3. Phys. Rev. B 1982, 25, 627-642. 30. Townsend, P. D.; Khanlary, M.; Hole, D. E. Information obtainable from ion beam luminescence. Surface & Coatings Technology 2007, 201, 8160–8164. 31. Townsend, P. D. Variations on the use of ion beam luminescence. Nucl. Instrum. Methods B. 2012, 286, 35-39. 32. Townsend, P. D.; Wang, Y. Defect studies using advances with ion beam excited luminescence. Energy Procedia 2013, 41, 64-79. 33. Townsend, P. D.; Crespillo, M. L. An ideal system for analysis and interpretation of ion beam induced luminescence. Physics Procedia 2016, 66, 345-351. 34. Aguilar, M.; Agulló-López F. X-ray induced processes in SrTiO3. J. Appl. Phys. 1982, 53, 9009-9014. 35. Leonelli, R.; Brebner, J. L. Time-resolved spectroscopy of the visible emission band in strontium titanate. Phys. Rev. B 1986, 33, 8649-8656. 36. Kan, D.; Terashima, T.; Kanda, R.; Masuno, A.; Tanaka, K.; Chu, S.; Kan, H.; Ishizumi, A.; Kanemitsu, Y.; Shimakawa, Y.; Takano, M. Blue-light emission at room temperature from Ar+irradiated SrTiO3. Nature Mater. 2005, 4, 816-819. 37. Rubano, A.; Paparo, D.; Miletto, F.; Scotti di Uccio, U.; Marrucci, L. Recombination kinetics of a dense electron-hole plasma in strontium titanate. Phys. Rev. B 2007, 76, 125115. 38. Rubano, A.; Paparo, D.; Miletto Granozio, F.; Scotti di Uccio, U.; Marrucci, L. Blue luminescence of SrTiO3 under intense optical excitation. J. Appl. Phys. 2009, 106, 103515. 39. Crespillo, M. L.; Graham, J. T.; Agulló-López, F.; Zhang, Y.; Weber, W. J. Role of oxygen vacancies on light emission mechanisms in SrTiO3 induced by high-energy particles. J. Physics D: Appl. Phys. 2017, 50, 155303. 40. Zhang, Y.; Crespillo, M. L.; Xue, H.; Jin, K.; Chen, C. -H.; Fontana, C. L.; Graham, J. T.; Weber, W. J. New ion beam materials laboratory for effective investigation of materials modification and irradiation effects. Nucl. Instrum. Meth. B. 2014, 338 19-30. 41. Crespillo, M. L.; Graham, J. T.; Zhang, Y.; Weber, W. J. Temperature measurements during high flux ion beam irradiations. Rev. Sci. Instrum. 2016, 87, 024902. 42. Ziegler, J. F.; Ziegler, M. D.; Biersack, J. P. SRIM - The stopping and range of ions in matter. Nucl. Instrum. Methods B 2010, 268, 1818-1123.



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43. Ziegler, J. F. Software code. SRIM -The Stopping and Range of Ions in Matter SRIM v2012; 2012. 44. Cooper, R.; Smith, K. L.; Colella, M.; Vance, E. R.; Phillips, M. Optical emission due to ionic displacement in alkaline earth titanates. J. Nucl. Mater. 2001, 289, 199-203. 45. Smith, K. L.; Collela, M.; Cooper, R.; Vance, E. R. Measured displacement energies of oxygen ions in titanates and zirconates. J. Nucl. Mater. 2003, 321, 19-28. 46. Smith, K. L.; Zaluzec, N. J. The displacement energies of cations in perovskite (CaTiO3). J. Nucl. Mater. 2005, 336, 261-266. 47. Zhang, Y.; Lian, J.; Zhu, Z.; Bennett, W. D.; Saraf, L. V.; Rausch, J. L.; Hendricks, C. A.; Ewing, R. C.; Weber, W. J. Response of strontium titanate to ion and electron irradiation. J. Nucl. Mater. 2009, 389, 303-310. 48. Crespillo, M. L.; Graham, J. T.; Zhang, Y.; Weber, W. J. In-situ luminescence monitoring of ion-induced damage evolution in SiO2 and Al2O3. J. Lumin. 2016, 172, 208-218. 49. Di Bartolo, B. Optical Interactions in Solids; John Wiley and Sons, Inc.: New York, 1968. 50. Jardin, C.; Canut, B.; Ramos, S. M. M. The luminescence of sapphire subjected to the irradiation of energetic hydrogen and helium ions. J. Phys. D: Appl. Phys. 1996, 29, 2066-2070. 51. Inouye, A.; Nagata, S.; Toh, K.; Tsuchiya, B.; Yamamoto, S.; Shikama, T. Luminescence of Crdoped alumina induced by charged particle irradiation. J. Nucl. Mater. 2007, 367-370, 1112-116. 52. Malo, M.; Moroño, A.; Hodgson, E. R. Radioluminescence monitoring of radiation induced surface electrical degradation in aluminas. J. Nucl. Mater. 2013, 442, S520-S523. 53. Yang, B.; Townsend, P. D.; Fromknecht, R. Low temperature detection of phase transitions and relaxation processes in strontium titanate by means of cathodoluminescence. J. Phys.: Condens. Matter. 2004, 16, 8377-8386. 54. Wang, Y.; Zhao, Y.; Zhang, Z.; Zhao, C.; Wu, X.; Finch, A. A.; Townsend, P. D. Energy dependence of radioluminescence spectra from strontium titanate. J. Lumin. 2015, 166, 17-21. 55. Bachiller-Perea, D.; Jiménez-Rey, D.; Muñoz-Martín, A.; Agulló-López, F. Exciton mechanisms and modeling of the ionoluminescence in silica. J. Phys. D: Appl. Phys. 2016, 49, 085501. 56. Luo, W.; Duan, W.; Louie, S. G.; Cohen, M. L. Structural and electronic properties of ndoped and p-doped SrTiO3. Phys. Rev. B 2004, 70, 214109. 57. Ricci, D.; Bano, G.; Pacchioni, G.; Illas, F. Electronic structure of a neutral oxygen vacancy in SrTiO3. Phys. Rev. B 2003, 68, 224105 58. Buban, J. P.; Iddir, H.; Ogüt, S. Structural and electronic properties of oxygen vacancies in cubic and antiferrodistortive phases of SrTiO3. Phys. Rev. B 2004, 69, 180102 (R). 59. Agulló-López, F.; Catlow, C. R.; Townsend, P. D. Point defects in materials; Academic Press: London, 1984. 60. Janotti, A.; Varley, J. B.; Choi, M.; Van de Walle, C. G. Vacancies and small polarons in SrTiO3. Phys. Rev. B 2014, 90, 085202. 61. Zhang, J.; Walsh, S.; Brooks, C.; Schlom, D. G.; Brillson, L. J. Depth-resolved cathodoluminescence spectroscopy study of defects in SrTiO3. J. Vac. Sci. Technol. B 2008, 26, 1466-1471. 62. Zhang, Y.; Lian, J.; Wang, C. M.; Jiang, W.; Ewing, R. C.; Weber, W. J. Ion-induced damage accumulation and electron-beam-enhanced recrystallization in SrTiO3. Phys. Rev. B 2005, 72, 094112.


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Tables Table 1. List of impurities concentration of STO (100) single crystals from MTI Corporation (Purity: 99.99742%) according to the manufacturer analyses.

Element

NRM (ppm wt)

Li

< 0.05

B

0.17

Na

0.36

Mg

0.16

Al

1.60

Si

5.20

P

0.53

S

7.40

Cl

 7.00

K

< 1.00

V

< 0.05

Cr

< 0.50

Fe

1.10

Co

< 0.05

Ni

< 0.05

Nb

1.30

Ba

0.81

Pb

0.21



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1 2 3 Table 2. Irradiation parameters for 3 MeV protons in STO calculated using Stopping and 4 5 Range of Ions in Matter (SRIM) full-cascade simulations (version 2012),42, 43 where Se is 6 7 8 the electronic stopping power at the surface, Se max is the maximum value of the electronic 9 10 stopping power, Sn is the maximum value of the nuclear stopping power and Rp is the 11 12 projected ion range. Eioniz stands for energy deposited into the electronic system per incident 13 14 15 ion (integrating Se along Rp), Ne-h, average number of electron-hole (e-h) pairs generated by 16 17 single ion impact along Rp, e-h, e-h pairs density per unit of volume along Rp considering 18 19 20 an excitation radius of 10 nm. (dpa)max represents the total displacements (including all the 21 22 host ions, i.e. Sr, Ti and O) per atom rate at the damage peak, [Ox. vac.] is the Oxygen 23 24 vacancy concentration rate at the damage peak, and IDR stands for the ionizing dose rate. 25 26 27 The rates have been calculated considering an average flux value of 2.4×1012 cm-2s-1. 28 29 30 Table 2. Irradiation Parameters for 3 MeV Protons in STO Calculated using SRIM (2012) 31 Full-Cascade Simulations. 32 33 34 Ion E Se Se max Sn Rp Eioniz Ne-h (dpa)max [Ox. vac.] e-h 35 3 (MeV) (keV/nm) (keV/nm) (keV/nm) (m) (MeV) (e-h/ion) (e-h/cm ) (dpa/s) (s-1) 36 37 3.00 0.04 0.14 4.01×10-5 54.00 2.98 3.75×105 2.21×1019 5.42×10-7 3.56×10-7 38 H 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 22 60 ACS Paragon Plus Environment

IDR (kGy/s) 42.00

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Figures

Figure 1



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RT

(a)

170 K

B-peak

(b)

B-peak

100 K

(c)

Figure 2


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100 K

Figure 3



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1.55 eV

Figure 4


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(a)

(d)

(b)

(e)

B-peak

B-peak

(c)

(f)

Figure 5



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(a)

(b)

Figure 6


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(a)

(b)

O Sr Ti Cr

Figure 7



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(a)

(b)

Figure 8


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TOC Graphic

Conduction Band (CB)

Conduction Band (CB) 3d (t2g)

1.55 eV

2.0 eV 3d (eg)

Cr

4+

Cr

3+

3+

Ti

(a) Valence Band (VB)

(b) Valence Band (VB)



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Figure captions Figure 1. Schematic of the experimental setup for in-situ IL measurements adopted from Crespillo et al.48 Figure 2. IL spectra (a.u. equals “arbitrary units”) obtained under 3 MeV proton irradiation at (a) room temperature (RT), (b) 170 K and (c) 100 K to various fluences. Note the zerophonon line (B-peak) emission band and the vibronic side-bands components in the NIR region can be clearly identified. Insets in (b, c) show in detail the evolution of the B-line intensity as a function of fluence at LT. Figure 3. Characteristic NIR IL spectra in detail of STO at 100 K under 3 MeV proton irradiation for the initial irradiation fluence (4×1011 cm-2), where the maximum intensity for zero-phonon line is reached. Note that the zero-phonon line (B-peak) emission band and the vibronic side-bands components (associated primarily with lattice phonons) can be clearly identified in very good agreement with those obtained by Sihvonen et al. under light excitation.23 The dashed curves represent the emission bands used to decompose the spectra. The solid green line represents the global fit of the spectrum. The values on the right side show the exact energy values for each spectral component. Figure 4. Evolution of the 1.55 eV (B-line) emission band yield as a function of 3 MeV proton irradiation fluence at different temperatures. Figure 5. (a-c) NIR and (d-f) Visible-UV IL spectra obtained under 3 MeV proton irradiation at (a, d) room temperature (RT), (b, e) 170 K and (c, f) 100 K to various fluences. The Visible-UV spectra can be decomposed into three Gaussian bands peaked at 2.0 eV, 2.5 eV and 2.8 eV, as discussed in Crespillo et al.39 (a-c) show a wide spectral range where the evolution of the band peaked at 2.0 eV can be compared with the zero32 ACS Paragon Plus Environment

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phonon line (B-peak) and vibronic side-bands in the NIR region. Insets show in detail the evolution of the B-line intensity as a function of fluence at LT. Figure 6. Comparison of the yield intensity evolution as a function of 3 MeV proton irradiation fluences at different temperatures for: (a) the 2.5 eV and 2.8 eV emission bands; and (b) the 2.0 eV emission band. This figure has been adopted from Crespillo et al.39 Figure 7. Schematic for possible mechanisms of the electronic processes involved in the IL for STO. The conduction band (CB) is mainly composed of 3d orbitals of Ti and the valence band (VB) is associated to O 2p states. White and grey circles represent the excited holes (h) and electrons (e), respectively. (a) Ionization of Cr3+ ions placed at Ti4+ sites produces Cr4+ through h trapping, followed by the subsequent capture of an e and recombination, leading to the characteristic 1.55 eV emission (zero-phonon line) (b) Electrons are self-trapped as Ti3+ at Ti4+ sites adjacent to oxygen vacancies.57 The generated electronic levels are mostly d-type. The transition between the excited and ground levels gives rise to the 2.0 eV luminescence band. STO (001) 1×1×2 unit cell structures are included close to the schematics as representative examples of pristine in (a), and damaged structure with oxygen vacancies in (b). Figure 8. Anti-correlative behavior between the emission kinetics of the 1.55 eV (B-line) and 2.0 eV bands as a function of 3 MeV proton irradiation fluences at different temperatures: (a) 170 K and (b) 100 K. The inset in (b) evidences a clear correlation between the square of the yield at 2.0 eV and the inverse of the yield at 1.55 eV as a function of fluence (see text for further details).



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Figure 1

ACS Paragon Plus Environment

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Figure 2

RT

(a)

170 K

B-peak

(b) B-peak

100 K

(c)



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Figure 3

100 K


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Figure 4

1.55 eV



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Page 38 of 41

(a)

(d)

(b)

(e)

B-peak

B-peak

(c)

(f)

Figure 5


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Figure 6

(a)

(b)



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Figure 7

(a)

(b)

O Sr Ti Cr


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(a)

(b)

Figure 8


Correlation between Cr3+ Luminescence and Oxygen Vacancy

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