Detection of Nanoscale Structural Defects in Degraded Fe-Doped

Subscriber access provided by UNIVERSITY OF TOLEDO LIBRARIES

C: Surfaces, Interfaces, Porous Materials, and Catalysis

Detection of Nanoscale Structural Defects in Degraded Fe-Doped SrTiO by Ultrafast Photoacoustic Waves 3

Ying Zhang, Onur Kurt, David Ascienzo, Qian Yang, Tony Le, Steven G. Greenbaum, Thorsten J. M. Bayer, Clive A. Randall, and Yuhang Ren J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03240 • Publication Date (Web): 29 May 2018 Downloaded from http://pubs.acs.org on May 29, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Detection of Nanoscale Structural Defects in Degraded Fe-Doped SrTiO3 by Ultrafast Photoacoustic Waves Ying Zhang1, 2, 3,┼, Onur Kurt,2, 4,┼, David Ascienzo2, 3, Qian Yang2, 3, Tony Le2, 3, Steve Greenbaum 2,3, Thorsten J. M. Bayer5, 6, Clive A. Randall5, 6 and Yuhang Ren2, 3 * 1

School of Instrumentation Science & Opto-electronics Engineering, Beihang University, No. 37 Xueyuan Road,

Haidian District, Beijing, 100191, China 2

Department of Physics and Astronomy, Hunter College, The City University of New York, 695 Park Avenue, New

York, New York, 10065, USA 3

The Graduate Center, The Graduate Center, The City University of New York, 365 5th Ave., New York, NY 10016

4

Electrical Engineering, The City College of New York, The City University of New York, 160 Convent Avenue,

New York, NY 10031 5

Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802

6

Materials Research Institute, The Pennsylvania State University, University Park, PA 16802

┼

Ying Zhang and Onur Kurt contributed equally to this work.

*Corresponding author: [email protected]

Strontium titanate, SrTiO3, has been intensively investigated as a model material in defect chemistry research. The underlying mechanism of the effects associated with a large variety of defects often require microstructure imaging. In the present work, the distribution of nanoscale structural defects in electrodegraded Fe-doped SrTiO3 (Fe:STO) single crystals is directly revealed by ultrafast photoacoustic waves. We utilized time-resolved reflectance spectra to nondestructively characterize local structural distortions near the degraded anode and cathode interfaces in both the reduced and oxidized crystals along with transmission electron microscopy to image these defects. We show that an accumulation of oxygen vacancies resulted in significant structural deformations near the degraded cathode interface of the reduced crystal. The defect distribution shows a strong dependence on oxygen vacancy concentration and diffusion within the crystals.

ACS Paragon Plus Environment

1

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 19

1. INTRODUCTION Perovskite-based titanates have been essential components in building various electroceramic devices such as dielectric capacitors, thermistors, actuators, and sensors,1-3 due to their unique dielectric and temperature properties.4,5 The desire to improve material and device performance requires detailed information to be gathered on the voltage-induced resistance degradation and temperature stress which causes failure and breakdown of SrTiO3 (STO) as a model material.6-10 STO is a well-known and widely used electroceramic single crystal for dielectric applications. Fe is commonly used as a transition metal dopant in dielectric materials since it is cheap and readily available.11 In Fe-doped SrTiO3 (Fe:STO), Fe substitutes Ti ions and oxygen vacancies are created in the first coordination shell of Fe3+ centers to conserve charge neutrality due to the mixed valence (Fe3+/Fe4+) states of the acceptor dopant, Fe.12-14 As a result of this, Fe doping in the STO increases concentration of oxygen vacancy as compared to undoped STO. For this reason, Fe:STO becomes an excellent candidate for examining field-induced structural changes due to the migration of oxygen vacancies.8,15 Fe:STO single crystals were annealed at high temperature under different partial pressures of oxygen in order to determine the effect of oxygen vacancy on the conductivity of the samples. In the presence of dc-voltage and temperature stress during the degradation process, oxygen ions and oxygen vacancies undergo a so-called demixing process where oxygen ions migrate to the anode interface and oxygen vacancies migrate to cathode interface.8,16-18 As a consequences of this, defect complexes including oppositely charged defects or large amount space charges are formed when oxygen vacancies migrate across the material and pile up at an electrode interface.19,20 The migration of oxygen vacancies to the interfaces under an applied dc-voltage and thermal stress are major reason to failure and breakdown in electroceramic devices.6-9 However, interactions of oxygen vacancies with


2

Page 3 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


immobile point defect centers in the lattice are often neglected when modeling defect mobility. This is simply because there is not a proven method to directly characterize the local distribution of oxygen vacancies and clusters in an oxide lattice, especially in a dilute concentration. Recent advances in cutting-edge femtosecond laser systems make it possible for ultrasonic measurements of nanoscale structural distortions in bulk and thin film samples.21-24 Acoustic wave propagation in the GHz-THz frequency range can be generated by ultrashort optical pump pulses and then detected by time-delayed probe pulses.25,26 This method, called laser picosecond acoustics, has been applied to a wide variety of metallic and semiconductor thin films, heterojunctions,24,27 and most recently, complex systems.28 A quantitative analysis of the photoacoustic signals significantly improved the spatial resolution of defects by two orders of magnitude compared to conventional optical measurements, especially in the dilute regime well below 1%.29 This is extremely valuable for understanding the nature of structural defects due to migration of oxygen vacancies, particularly near the surface and interface region in titanate oxides. In this paper, we report on a direct detection of the distribution of nanoscale structural defects in the electrodegraded Fe doped SrTiO3 (Fe:STO) single crystals using ultrafast photoacoustic waves. The photoinduced reflectance oscillations are clearly observed and they are attributed to the excitation of coherent longitudinal acoustic phonons in both STO and Fe:STO single crystals. We utilized time-resolved reflectance spectra to non-destructively characterize the distribution of local structural distortions near the degraded anode and cathode interfaces in both reduced and oxidized Fe:STO single crystals along with the transmission electron microscopy (TEM) to image these defects. The transient signals in the degraded reduced crystal exhibit many kinks at a time delay corresponding to the travel of the normal longitudinal acoustic (LA) wave across the


3


Page 4 of 19

local distorted structures. We reveal that accumulation of oxygen vacancies result in significant structural changes near the degraded cathode interface. The strain distribution also shows a strong dependence on oxygen vacancy concentration and diffusion within the crystals. To understand the degradation mechanisms which lead to breakdown and failure in electroceramic devices, we used a diluted Fe doping of 0.01 wt% that introduces enough oxygen vacancy concentrations to enable us to detect the migration process. In our study, the oxygen vacancy concentration needs to be well controlled and over doped oxygen vacancies may turn the sample into a conductor. The electrochemical conditions (8 kV/cm, 40 V, 210 C) were chosen to get observable defect dynamics in the sample bulk and interface structures. 2. EXPERIMENTAL METHODS For the present study, Verneuil-grown, 10 x 10 x 0.5 mm3, (100) SrTiO3 single crystals doped with 0.01 wt% Fe were cut into 5 x 5 x 0.5 mm3 pieces and epi-polished on both sides (MTI, Richmond, CA). Fe:STO single crystals were then annealed in a tube furnace at a temperature of 900 °C under a pO2 = 2x10-5 bar and a pO2 = 0.2 bar for reduction and oxidation treatment, respectively. Afterwards, reduced crystals were quenched in argon (pO2 = 2x10-5 bar) whereas oxidized crystals were quenched in air (pO2 = 0.2 bar) at 25 °C to freeze-in defect concentrations. As a result of the annealing and quenching treatment process, the reduced crystal has higher oxygen vacancy concentration than the oxidized crystal since oxidation treatment leads to lower oxygen vacancy concentrations in the oxidized crystal.14,30 Finally, to generate the migration of oxygen vacancy in the crystals, the 10 nm thick planar, amorphous platinum electrodes were sputtered onto the polished sample surfaces for longitudinal dc-field application. Degradation of Fe-doped STO was done separately with Pt electrodes on both sides.31 During the degradation process, two crystals were individually placed in a temperature bath of 210 oC under


4

Page 5 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


an applied dc voltage of 40 V (Edc = 0.8 kV/cm for ~2.8 hours) to induce oxygen ion and vacancy migrations toward the anode and cathode interfaces.12,20 During the electrochemical oxidation/reduction (redox) reactions in the high temperature bath at 40 V, the oxidation and reduction processes take place at the Fe-site. The redox-type reactions affect the balance between Fe3+ and Fe4+, and therefore the oxygen vacancy concentration in the material.12 Based on a change in oxygen vacancy concentration, an adjustment of the electron and hole concentration takes place to maintain local charge neutrality: [Fe]total = [Fe3+] + [Fe4+]; 2[Vo] + [h] = [Fe3+] + [e]. The oxidation process mainly happens at the anode region, while the reduction reaction is introduced at the cathode side. The time-resolved measurements reported here used a femtosecond pump-probe setup. We used 100-fs laser pulses of energy density ~ 0.1 mJ/cm2 and ~ 0.03 mJ/cm2 for the pump and probe beams, respectively. The photons were from a modelocked Ti-sapphire amplifier laser (RegA 9000, Coherent) and a 250-kHz optical parametric amplifier (OPA9400, Coherent) operating at 800 nm and a wavelength between 400 nm and 800 nm, respectively. The TEM images were collected at 200 kV using the FEI Titan Themis 200a located at the Advance Science Research Center (ASRC) at the City University of New York. 3. RESULTS AND DISCUSSION Fig. 1 illustrates the time evolution of ∆R/R from the reduced Fe:STO single crystal at room temperature. The trace of transient reflectivity change, ∆R/R, explicitly shows a clear oscillatory component due to the excitation of coherent longitudinal acoustic phonons (CLAP). As shown in the inset of Fig. 1, the Fourier transforms of the fits reveal two frequencies: 45.6 GHz and 58.6 GHz, for longitudinal acoustic phonon oscillations probed by 800 nm and 630 nm, respectively. We performed time-resolved reflectivity experiments for the reduced Fe:STO single crystal by changing either the pump or the probe wavelength. We note that the frequency and amplitude of


5


Page 6 of 19

these oscillations show a strong dependence on the probe but not the pump wavelength. The oscillatory component of the transient reflectance at = 800 is larger by at least an order of magnitude than that at = 630 . Moreover, we observe a clear dispersive behavior. The excitation and detection of the coherent longitudinal acoustic phonons can be understood in the backscattering geometry. A schematic diagram of the picosecond ultrasonic experiment is shown in the inset of Fig. 2. When the “pump” pulse has a pulse duration shorter than the thermal and stress confinement times of the Pt layer, no thermal energy is exchanged with the surroundings. The energy deposition on timescales shorter than those for mechanical displacements raises the pressure within the Pt layer by Δ = , where is the Grüneisen parameter, is the optical absorption coefficient, and is the incident laser fluence. This pressure rise induces a thermoelastic expansion and a normal strain flow across the Pt layer. The strain flow generates a broadband LA wave and propagates through the Fe:STO crystal. The LA wave is encoded with information pertaining to the geometrical properties of the crystal structures and is governed by the wave equation in onedimension:32,33

where

−

= −

,

(1)

is the thermal expansion coefficient of the sample, !" is the specific heat capacity, and #

is a function which describes the heating of the sample in time and space. As the wave propagates into the crystal, we send a “probe” pulse which undergoes a Doppler shift following the interaction with the moving pump-generated LA wave, as in stimulated Brillouin scattering (SBS).25,34 The transient reflectivity responses exhibit quasi-periodic Brillouin oscillations arising from the acoustic pulses during their propagation in the three-dimensional crystals. The frequency of


6

Page 7 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the LA oscillations depends on the central wavelength of the probe, , but not the pump pulses. The LA wave vector was determined from the SBS phase-matching condition as $ = 4& / , where is the index of refraction of the sample. The frequency of the oscillations can be expressed as35 ) = ( +" ,-./&)0

(2)

where ) is the sound velocity in the crystal. The incident angle of the probe beam, . , is also taken into account where 1 . = (1/ )1 . , the probe beam wave vector is 0 = 2&/ , and LA wave vector can be rewritten as $ = 2 0 ,-.. In our experiments, the measured value of . = 15° and = [5.197 + 0.1600 ; /(39.5 − 0.07840 ; )]/; .35,36 Figure 2 shows the dependence of the frequency of the oscillatory component on the probe wave vector for the STO and reduced Fe:STO single crystals. The oscillation frequency increases monotonically with the wave vector of the excited LA mode. The slope gives a phase velocity for the CLAP mode of +" = (7.8 ± 0.1%) × 10@ / normal to the sample surface. Our experimental data of reduced Fe:STO shows a very good agreement (within ~ 1 %) with the theoretical and experimental values on STO.25 This is consistent with the diluted nature of Fe doping in the crystal. The propagation length of the LA wave can be described by: L = +" B, with t as the corresponding time coordinate on the oscillatory curves. We estimate the spatial resolution from the temporal resolution of our probe pulses. In our experiment, the temporal resolution △ B = 0.1 ps , and thus the spatial resolution is 0.78 nm in our time-resolved measurements. It is important to note that the shortest wavelength of coherent acoustic waves we achieved in our ultrafast measurements is ~ 100 nm. Such ultrashort acoustic waves enabled us to directly


7


Page 8 of 19

probe nanoscale structural distortions in the degraded Fe:STO crystals. We performed the timeresolved pump-probe measurements on the fresh and degraded oxidized (B045) and reduced (B042) Fe:STO single crystals. Figure 3 shows the oscillatory components of ∆R/R in the reduced (R), the degraded oxidized anode (DOA), the degraded reduced anode (DRA) and cathode (DRC) of Fe:STO crystals. Both the wavelengths of pump and probe beams are 800 nm. We show that the amplitudes of oscillatory components decrease by at least 80% in three degraded measurements, compared to those in the fresh reduced crystals. At the elevated temperature (210°C) under an applied dc voltage (40 V) during the degradation process, oxygen ions and vacancies migrate towards the anode and cathode of the Fe:STO crystals, respectively, and accumulate at their interfaces.18-20,37 Since the oscillation signal is directly proportional to the energy of strain flow transferring from the Pt layer to Fe:STO crystal, we realize that a great portion of the strain flow has been blocked by the restructured anode and cathode interfaces with sufficient oxygen ions and vacancies during the degradation process, respectively. In the meantime, the strain field distribution near the anode and cathode show a strong dependence on oxygen vacancy concentration and diffusion within the degraded crystals as we previously discussed.10,16-18 It is evident from our time-resolved spectra that the deformed structure is highly localized and is almost undetectable in the degraded oxidized anode interface. Only one small distortion of ~2 ps (~15 nm) of the propagated LA wave near 220 ps (about 1.7 µm into the anode) is observed. In contrast, the deformations are shown to be more scattered in the degraded reduced crystal. The reflectivity traces exhibit many more small kinks, as indicated by red circles in Fig. 3. These kinks are extremely short (1~ 5 ps) in temporal width. They can be translated to a spatial extent from 8 nm to 40 nm, corresponding to the structural deformations near the anode interface of the degraded reduced crystal. For the degraded reduced cathode,


8

Page 9 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


substantial distortions of the oscillations are identified: the oscillations last only two cycles and then a significant suppression of oscillations occurs in a wide range of time delays, shown as in the red rectangle box from 52.6 ps to 215.1 ps. A relatively strong strain region is shown to be spread across the cathodic regions (between 410 nm and 1677 nm) of the degraded reduced crystal. In order for the structural strain to relax from demixing process, oxygen vacancies need to undergo an energetically favorable reordering across the cathodic bulk. Empty 3d-oribitals of Fe are filled by electrons due to the migration of oxygen vacancies under imposed dc-voltages and Fe4+ is reduced to Fe3+ at the degraded cathode interface.12,13,38 As a consequence of this migration, immobile Fe3+ center and oxygen vacancy interactions result in significant structural changes, particularly near the degraded cathode interface of reduced Fe:STO single crystal. This introduces an extended network of defect complexes along diffusion pathways. The relatively strong strain regions can be therefore related to oxygen vacancy clustering, or defect ordering (Fe3+ - Oxygen Vacancy), due to the migration of oxygen vacancies across the cathodic bulk. As shown in Fig. 4a and 4b, the TEM images in both degraded reduced cathode (a) and degraded oxidized anode (b) do not show a clear re-structuring of the lattice associated with changes to the ultrafast LA waves. This is consistent with the diluted Fe doping (0.01 wt.%) nature in the STO crystals. The oxygen vacancies occupy much less than 1/1000 unit cells even in the very reduced condition, as estimated in the previous calculations. It is therefore quite challenging to observe structural defects due to the migration of oxygen vacancy using TEM. The local structure at the cathode is affected significantly by the presence of oxygen vacancies since these defects contribute to oxygen bond bending out of the ab plane as well as in-plane compression of the crystal unit cell. The strain distribution is splotchy and non-uniform since the crystal thicknesses (~ 0.5 mm) is on the order of the width of the bulk regions.


9


Page 10 of 19

4. CONCLUSIONS In summary, photoacoustic waves have been utilized to directly detect the distributions of nanoscale structural defects in the electrodegraded Fe:STO single crystals. The transient reflectivity data reveal that the accumulation of oxygen vacancies causes significant structural changes near the degraded cathode interface. The strain distribution shows a strong dependence on oxygen vacancy concentration and diffusion within the crystals. Our characterization of structural and electrochemical nanoscale changes due to electric field-induced strain and oxygen vacancy migration advances the knowledge of electrodegradation in perovskite-based titanate single crystals.

ACKNOWLEDGMENTS Research at Hunter was supported by AFOSR grants (grant no. FA9550-17-1-0342 and FA955017-1-0339). Research at PSU was supported by an AFOSR grant (grant no. FA9550-14-1-0067).


10

Page 11 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


REFERENCES 1. 68.

Buchanan, R. C. Ceramic Materials for Electronics. CRC press: New York, 2004; Vol.

2. Schrott, A.; Misewich, J.; Nagarajan, V.; Ramesh, R. Ferroelectric Field-Effect Transistor with a SrRuxTi1-XO3 Channel. Appl. Phys. Lett. 2003, 82, 4770. 3. Tao, S.; Irvine, J. T. A Redox-Stable Efficient Anode for Solid-Oxide Fuel Cells. Nat. Mater. 2003, 2 (5), 320-323. 4. Homes, C.; Vogt, T.; Shapiro, S.; Wakimoto, S.; Ramirez, A. Optical Response of HighDielectric-Constant Perovskite-Related Oxide. Science 2001, 293 (5530), 673-676. 5. Hao, X. A Review on the Dielectric Materials for High Energy-Storage Application. J. Adv. Dielectr. 2013, 3 (01), 1330001. 6. Liu, W.; Yang, G.-Y.; Randall, C. A. Evidence for Increased Polaron Conduction near the Cathodic Interface in the Final Stages of Electrical Degradation in SrTiO3 Crystals. Jpn. J. Appl. Phys. 2009, 48 (5R), 051404. 7. Waser, R.; Baiatu, T.; Härdtl, K. H. Dc Electrical Degradation of Perovskite‐Type Titanates: I, Ceramics. J. Am. Ceram. Soc. 1990, 73 (6), 1645-1653. 8. Baiatu, T.; Waser, R.; Härdtl, K. H. Dc Electrical Degradation of Perovskite‐Type Titanates: III, a Model of the Mechanism. J. Am. Ceram. Soc. 1990, 73 (6), 1663-1673. 9. Yang, G.; Dickey, E.; Randall, C.; Randall, M.; Mann, L. Modulated and Ordered Defect Structures in Electrically Degraded Ni–BaTiO3 Multilayer Ceramic Capacitors. J. Appl. Phys. 2003, 94 (9), 5990-5996. 10. Ascienzo, D.; Greenbaum, S.; Bayer, T.; Maier, R.; Randall, C.; Ren, Y. Observation of Structural Inhomogeneity at Degraded Fe-Doped SrTiO3 Interfaces. Appl. Phys. Lett. 2016, 109 (3), 031602. 11. Baker, J. N.; Bowes, P. C.; Long, D. M.; Moballegh, A.; Harris, J. S.; Dickey, E. C.; Irving, D. L. Defect Mechanisms of Coloration in Fe-Doped SrTiO3 from First Principles. Appl. Phys. Lett. 2017, 110 (12), 122903.


11


Page 12 of 19

12. Lenser, C.; Kalinko, A.; Kuzmin, A.; Berzins, D.; Purans, J.; Szot, K.; Waser, R.; Dittmann, R. Spectroscopic Study of the Electric Field Induced Valence Change of Fe-Defect Centers in SrTiO3. Phys. Chem. Chem. Phys. 2011, 13 (46), 20779-20786. 13. Vračar, M.; Kuzmin, A.; Merkle, R.; Purans, J.; Kotomin, E.; Maier, J.; Mathon, O. JahnTeller Distortion around Fe4+ in Sr(FexTi1-X)O3-∆ from X-Ray Absorption Spectroscopy, X-Ray Diffraction, and Vibrational Spectroscopy. Phys. Rev. B 2007, 76 (17), 174107. 14. Wang, J.-J.; Huang, H.-B.; Bayer, T. J.; Moballegh, A.; Cao, Y.; Klein, A.; Dickey, E. C.; Irving, D. L.; Randall, C. A.; Chen, L.-Q. Defect Chemistry and Resistance Degradation in FeDoped SrTiO3 Single Crystal. Acta. Mater. 2016, 108, 229-240. 15. Hölbling, T.; Söylemezoğlu, N.; Waser, R. A Mathematical-Physical Model for the Charge Transport in P-Type SrTiO3 Ceramics under Dc Load: Maxwell-Wagner Relaxation. Journal of electroceramics 2002, 9 (2), 87-100. 16. Kurt, O.; Ascienzo, D.; Greenbaum, S.; Bayer, T.; Randall, C.; Madamopoulos, N.; Ren, Y. Nonlinear Optical Detections of Structural Distortions at Degraded Fe-Doped SrTiO3 Interfaces. Mater. Chem. Phys. 2017. 17. Ascienzo, D.; Yuan, H.; Greenbaum, S.; Bayer, T. J.; Maier, R. A.; Wang, J.-J.; Randall, C. A.; Dickey, E. C.; Zhao, H.; Ren, Y. Investigation of Electric Field–Induced Structural Changes at Fe-Doped SrTiO3 Anode Interfaces by Second Harmonic Generation. Materials 2016, 9 (11), 883. 18. Ascienzo, D.; Kurt, O.; Greenbaum, S.; Bayer, T.; Maier, R.; Randall, C.; Ren, Y. Formation of Structural Defects and Strain in Electrodegraded Fe-Doped SrTiO3 Crystals Due to Oxygen Vacancy Migration. J. Am. Ceram. Soc. 19. Bayer, T. J.; Wang, J.-J.; Carter, J. J.; Chen, L.-Q.; Randall, C. A. In Impedance Spectroscopy Utilized to Study the Spatial Distribution of Conductivity within Capacitors During Operation, Applications of Ferroelectrics, European Conference on Application of Polar Dielectrics, and Piezoelectric Force Microscopy Workshop (ISAF/ECAPD/PFM), 2016 Joint IEEE International Symposium on the, IEEE: 2016; pp 1-4. 20. Bayer, T. J.; Wang, J.-J.; Carter, J. J.; Moballegh, A.; Baker, J.; Irving, D. L.; Dickey, E. C.; Chen, L.-Q.; Randall, C. A. The Relation of Electrical Conductivity Profiles and Modulus Data Using the Example of STO:Fe Single Crystals: A Path to Improve the Model of Resistance Degradation. Acta. Mater. 2016, 117, 252-261. 21. Li, B.; Zhang, W.; Liu, X.; Deng, H.; Lan, T.; Liu, B.; Liu, J.; Wang, X.; Zeng, Z.; Zhang, L. Chromatic Effect in a Novel THz Generation Scheme. New J. Phys. 2017, 19 (11), 113025. 22. Sum, T. C.; Mathews, N.; Xing, G.; Lim, S. S.; Chong, W. K.; Giovanni, D.; Dewi, H. A. Spectral Features and Charge Dynamics of Lead Halide Perovskites: Origins and Interpretations. Acc. Chem. Res. 2016, 49 (2), 294-302.


12

Page 13 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


23. Khafizov, M.; Pakarinen, J.; He, L.; Henderson, H.; Manuel, M.; Nelson, A.; Jaques, B.; Butt, D.; Hurley, D. H. Subsurface Imaging of Grain Microstructure Using Picosecond Ultrasonics. Acta. Mater. 2016, 112, 209-215. 24. Pollock, K. L.; Doan, H. Q.; Rustagi, A.; Stanton, C. J.; Cuk, T. Detecting the Photoexcited Carrier Distribution across Gaas/Transition Metal Oxide Interfaces by Coherent Longitudinal Acoustic Phonons. J. Phys. Chem. Lett. 2017, 8 (5), 922-928. 25. Ren, Y.; Trigo, M.; Merlin, R.; Adyam, V.; Li, Q. Generation and Detection of Coherent Longitudinal Acoustic Phonons in the La0.67Sr0.33MnO3 Thin Films by Femtosecond Light Pulses. Appl. Phys. Lett. 2007, 90 (25), 251918. 26. Ishioka, K.; Rustagi, A.; Beyer, A.; Stolz, W.; Volz, K.; Höfer, U.; Petek, H.; Stanton, C. J. Sub-Picosecond Acoustic Pulses at Buried GaP/Si Interfaces. Appl. Phys. Lett. 2017, 111 (6), 062105. 27. Strohm, E. M.; Moore, M. J.; Kolios, M. C. Single Cell Photoacoustic Microscopy: A Review. IEEE J. Sel. Topics Quantum Electron. 2016, 22 (3), 137-151. 28. Paltauf, G.; Dyer, P. E. Photomechanical Processes and Effects in Ablation. Chem. Rev. 2003, 103 (2), 487-518. 29. Maier, R. A. Dynamics of Oxygen Vacancies and Defect Complexes in the Perovskite Oxide Structure. Ph.D. Dissertation, The Pennsylvania State University, PA, 2014. 30. Denk, I.; Münch, W.; Maier, J. Partial Conductivities in SrTiO3: Bulk Polarization Experiments, Oxygen Concentration Cell Measurements, and Defect‐Chemical Modeling. J. Am. Ceram. Soc. 1995, 78 (12), 3265-3272. 31. Bayer, T.; Carter, J.; Wang, J.-J.; Klein, A.; Chen, L.-Q.; Randall, C. Determination of Electrical Properties of Degraded Mixed Ionic Conductors: Impedance Studies with Applied Dc Voltage. J. Appl. Phys. 2017, 122 (24), 244101. 32.

Wang, L.; Wu, H.-i. Biomedical Optics: Principles and Imaging Wiley. New York 2007.

33. Diebold, G.; Westervelt, P. The Photoacoustic Effect Generated by a Spherical Droplet in a Fluid. J. Acoust. Soc. Am. 1988, 84 (6), 2245-2251. 34. Brivio, S.; Polli, D.; Crespi, A.; Osellame, R.; Cerullo, G.; Bertacco, R. Observation of Anomalous Acoustic Phonon Dispersion in Srtio 3 by Broadband Stimulated Brillouin Scattering. Appl. Phys. Lett. 2011, 98 (21), 211907. 35. Bozovic, I.; Schneider, M.; Xu, Y.; Sobolewski, R.; Ren, Y.; Lüpke, G.; Demsar, J.; Taylor, A.; Onellion, M. Long-Lived Coherent Acoustic Waves Generated by Femtosecond Light Pulses. Phys. Rev. B 2004, 69 (13), 132503. 36. Cardona, M. Optical Properties and Band Structure of SrTiO3 and BaTiO3. Phys. Rev. 1965, 140 (2A), A651.


13


Page 14 of 19

37. Ascienzo, D.; Greenbaum, S.; Bayer, T.; Randall, C.; Ren, Y. Probing Electrocolored FeDoped SrTiO3 Bulks Using Optical Second Harmonic Generation. Acta. Mater. 2017, 126, 520527. 38. Enriquez, E.; Chen, A.; Harrell, Z.; Dowden, P.; Koskelo, N.; Roback, J.; Janoschek, M.; Chen, C.; Jia, Q. Oxygen Vacancy-Tuned Physical Properties in Perovskite Thin Films with Multiple B-Site Valance States. Sci. Rep. 2017, 7, 46184.


14

Page 15 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. Reflectance oscillations induced by femtosecond light pulses in the reduced Fe:SrTiO3 single crystal. The pump wavelength is 800 nm and the probe wavelength is either 800 nm or 630 nm. Inset: The frequency spectrum is obtained by applying FFT without any filtering or smoothing. 82x134mm (300 x 300 DPI)



Figure 2. Frequency of oscillations as a function of the probe wave vector. Solid red circles and square gray dots are our experimental data in the reduced Fe:SrTiO3 and SrTiO3 single crystals, respectively. The solid line is the predicted theoretical dependence between ν and kprobe fitted for the sound velocities. Inset: A schematic diagram of the ultrafast acoustic generation and detection. 82x86mm (300 x 300 DPI)


Page 16 of 19

Page 17 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. Reflectance oscillations at 800nm for the reduced (R), the degraded oxidized anode (DOA), the degraded reduced anode (DRA) and the degraded reduced cathode (DRC) of Fe:SrTiO3 crystals. The small open circles and rectangle indicate the defect induced kinks and the suppressed oscillation range in the time-resolved LA spectra. 82x83mm (300 x 300 DPI)



Figure 4. TEM images for the degraded reduced cathode (a) and the degraded oxidized anode (b) of Fe:SrTiO3. 170x86mm (300 x 300 DPI)


Page 18 of 19

Page 19 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


50x50mm (300 x 300 DPI)


Detection of Nanoscale Structural Defects in Degraded Fe-Doped

Recommend Documents