MICROMECHANICAL PROPERTIES AND DOMAIN STRUCTURES OF PZN-PT PIEZOELECTRIC SINGLE CRYSTALS WONG MENG FEI NATIONAL UNIVERSITY OF SINGAPORE 2011 MICROMECHANICAL PROPERTIES AND DOMAIN STRUCTURES OF PZN-PT PIEZOELECTRIC SINGLE CRYSTALS WONG MENG FEI (B.Eng.(Hons.), Universiti Teknologi Malaysia) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF MECHANICAL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2011 Preface This dissertation is submitted for the degree of Doctor of Philosophy in the Department of Mechanical Engineering, National University of Singapore (NUS) under the supervision of Associate Professor Dr. Zeng Kaiyang. To the best of my knowledge, all of the results presented in this dissertation are original, and references are provided to the works by other researchers. The majority portions of this dissertation have been published in international journals or presented at various international conferences as listed below: The following journal papers are published based on the first part of the research: 1. M. F. Wong, X. Heng and K. Zeng, Domain characterization of Pb(Zn1/3Nb2/3)O3-(6-7%)PbTiO3 single crystals using scanning electron acoustic microscopy, J. Appl. Phys., 104 (2008) 074103. 2. M. F. Wong and K. Zeng, Deformation behavior of PZN-6%PT single crystal during nanoindentation, Philos. Mag., 88 (2008) 3105-3128. 3. M. F. Wong and K. Zeng, Elastic-plastic deformation of Pb(Zn1/3Nb2/3)O3-(67)%PbTiO3 single crystals during nanoindentation, Philos. Mag., 90 (2010) 1685-1700. 4. M. F. Wong and K. Zeng, Nanoscale domains and preferred cracking planes in Pb(Zn1/3Nb2/3)O3-(6-7)%PbTiO3 single crystals studied by piezoresponse force microscopy and fractography, J. Appl. Phys., 107 (2010) 124104. 5. M. F. Wong and K. Zeng, Mechanical polishing effects towards surface domain evolution in Pb(Zn1/3Nb2/3)O3-PbTiO3 single crystals, J. Am. Ceram. Soc., 94 (2011) 1079-1086. The following journal papers are published based on the second part of the research: 1. M. F. Wong, T. S. Herng, Z. Zhang, K. Zeng and J. Ding, Stable bipolar surface potential behavior of copper-doped zinc oxide films studied by Kelvin probe force microscopy, Appl. Phys. Lett., 97 (2010) 232103. i 2. T. S. Herng, M. F. Wong, D. Qi, J. Yi, A. Kumar, A. Huang, F. C. Kartawidjaja, S. Smadici, P. Abbamonte, C. Sánchez-Hanke, S. Shannigrahi, J. M. Xue, J. Wang, Y. P. Feng, A. Rusydi, K. Zeng and J. Ding, Mutual ferromagnetic-ferroelectric coupling in multiferroic copper doped ZnO, Adv. Mater., 23 (2011) 1635-1640. Conference Presentations (Oral): 1. M. F. Wong and K. Zeng, Deformation behavior of PZN-6%PT during nanoindentation, 20th International Symposium on Integrated Ferroelectrics, Singapore, Jun. – 12, 2008 (Presented by Meng Fei Wong). 2. M. F. Wong and K. Zeng, Mechanical properties and domain structures of Pb(Zn1/3Nb2/3)O3-PbTiO3 single crystals using nanoindentation and piezoresponse force microscopy, International Conference on Materials for Advanced Technology (ICMAT 2009), Symposium U: Mechanical Behavior of Micro- and Nano-scale Systems, Singapore, Jul. 28 – Jul. 2, 2009 (Presented by Meng Fei Wong). 3. M. F. Wong and K. Zeng, Nanoscale domains and preferred cracking planes in PZN-PT single crystals studied by fractography, Electronic Materials and Applications 2010 (EMA2010), S8: The Future of Electronic Ceramics: A New Investigator Symposium, Orlando, Florida, USA, Jan. 20 – 22, 2010 (Presented by Meng Fei Wong). 4. M. F. Wong and K. Zeng, Domain characterization of PZN-PT single crystals using scanning electron acoustic microscopy and piezoresponse force microscopy, Electronic Materials and Applications 2010 (EMA2010), S2: Symposium on Advanced Dielectric, Piezoelectric, and Ferroic Materials, and Emerging Fields in Electronics, Orlando, Florida, USA, Jan. 20 – 22, 2010 (Presented by Meng Fei Wong). Conference Presentations (Poster): 1. K. Zeng, M. F. Wong, T. S. Herng, A. Kumar, and J. Ding, Nanoscale ferroelectric, magnetic and surface potential behavior of multiferroic copperdoped ZnO using scanning probe microscopy technique, 2011 MRS Spring Meeting, Symposium WW: Multiferroic, Ferroelectric and Functional Materials, Interfaces and Heterostructures, San Francisco, USA, Apr. 28, 2011. 2. M. F. Wong, L. Shen and K. Zeng, Characterizing mechanical properties of piezoelectric single crystals PZN-PT using nanoindentation technique, 3rd MRS-S Conference on Advanced Materials, Singapore, Feb. 25 – 27, 2008. ii Acknowledgements Foremost, I would like to express my sincere appreciation to my supervisor, Associate Professor Dr. Zeng Kaiyang, for his guidance and motivation. His extensive discussions around my work from the initial to the final level have been very helpful for this study. I am also heartily thankful to the staff at Microfine Materials Technologies Pte. Ltd. and Institute of Materials Research and Engineering (IMRE) for providing samples and adequate characterization facilities. My special gratitude is due to all my family members. Without their continued support and interest, this thesis would not have been the same as presented here. Lastly, I am indebted to all officers and my fellow colleagues from Materials Laboratory and Experimental Mechanics Laboratory for their supports and assistance at various occasions. Unfortunately, it is not possible to list all of them in this limited space. I offer my regards and blessings to all of those who supported me in any respect during the completion of this project. iii Table of Contents Preface i Acknowledgements iii Table of Contents iv Summary viii List of Tables x List of Figures xi List of Symbols xix Chapter 1: Introduction 1.1 Piezoelectricity and Domains 1.2 Relaxor PZN-PT Single Crystals 1.3 Mechanical Polishing Effects 10 1.4 Research Objectives and Significance 12 1.5 Thesis Outline 14 References Chapter 2: Literature Review – Mechanical Properties and Their Relationships with Domains Structures in Piezoelectric Materials 15 17 2.1 Mechanical Properties and Nanoindentation 18 2.1.1 Elastic Modulus and Hardness 18 2.1.2 Anelastic Deformation 24 2.2 Domains Observation 29 2.2.1 Polarized Light Microscopy 29 2.2.2 Scanning Electron Acoustic Microscopy 32 iv 2.2.3 Piezoresponse Force Microscopy 2.3 Cracking Behaviors and Fractography References Chapter 3: Experimental Methodology 34 38 41 46 3.1 Sample Materials 46 3.2 Nanoindentation 49 3.2.1 Loading and Pop-In Events 49 3.2.2 Elastic-Plastic Deformation 50 3.3 Domains Observation 50 3.3.1 Scanning Electron Acoustic Microscopy 51 3.3.2 Piezoresponse Force Microscopy 53 3.3.3 Fractography 54 3.4 Mechanical Polishing Effects References Chapter 4: Nanoindentation 4.1 Deformation Behaviors 55 56 57 57 4.1.1 Load-Displacement Curve and Pop-In Event 58 4.1.2 Surface Layer 64 4.1.3 Elastic Modulus and Hardness Characterization 66 4.1.4 Indentation Size Effect 71 4.2 Elastic-Plastic Deformation 74 4.2.1 Effects of Crystal Orientation and Poling 78 4.2.2 Multiple Loading/Unloading 81 4.2.3 Effects of CSM Frequencies and Maximum Indentation Loads 83 v 4.3 Conclusion 87 References 89 Chapter 5: Domain Structures and Preferred Fracture Planes 5.1 Scanning Electron Acoustic Microscopy 5.1.1 92 92 Preliminary Imaging and Suitable Frequency Range 92 5.1.2 Poled and Unpoled (011)-Oriented Crystals 95 5.1.3 Poled and Unpoled (001)-Oriented Crystals 97 5.1.4 Stress-Induced Domain Switching 102 5.2 Piezoresponse Force Microscopy 107 5.2.1 Crystal Orientation and Polishing Effect 107 5.2.2 Poling Effect 115 5.2.3 Indentation Effect and Material Flow Directions 117 5.3 Fractography 121 5.3.1 Domains on Fracture Surfaces 121 5.2.2 Poling Effect 126 5.4 Preferred Fracture Planes 127 5.3 Conclusion 131 References 133 Chapter 6: Mechanical Polishing Effects 136 6.1 Structural Characterization 136 6.2 Localized Poling Effect 143 6.3 Nanoindentation 146 6.4 Conclusion 148 References 150 vi Chapter 7: Application of Scanning Probe Technique to Study the Properties of Ferroelectric and Multiferroic Materials 151 7.1 Film Material and Preparation 152 7.2 Switching Spectroscopy Piezoresponse Force Microscopy 153 7.3 Kelvin Probe Force Microscopy 159 7.4 Magnetic Force Microscopy 173 7.5 Conclusion 177 References 179 Chapter 8: Conclusions and Recommendations 182 8.1 Nanoindentation 182 8.2 Domains Observation and Preferred Fracture Planes 183 8.3 Mechanical Polishing Effects 185 8.4 Scanning Probe Technique Studies 185 8.5 Recommendations for Future Works 186 Appendix A: Anomaly in Nanoindentation Results 190 Appendix B: Mechanical Polishing Effects 191 vii Summary Relaxor piezoelectric single crystals of Pb(Zn1/3Nb2/3)O3-PbTiO3 (PZN-PT) solid solution have recently attracted considerable attention due to their superior dielectric and piezoelectric properties. Despite piezoelectric studies of these single crystal materials, the understanding of their deformation behavior and its correlation to domain orientation remains unclear. Since mechanical properties directly pertain to the crystals’ reliability in piezoelectric device applications, remarkable motivation has been spurred to investigate phenomena such as deformation behavior, cracking path, domain switching and evolution. In this thesis, the mechanical characterization of PZN-PT single crystals can be divided into three parts. In the first part, micromechanical properties and deformation characteristics of PZN-PT single crystals were examined using nanoindentation technique, including the investigation on crack initiation (pop-in events on the nanoindentation load-displacement curves), elastic modulus and hardness. Particular attention was also made to correlate the elastic recovery upon indentation unloading and domain activities. In the second part, domain structures were observed using Scanning Electron Acoustic Microscopy (SEAM) and Piezoresponse Force Microscopy (PFM) techniques. This provided information on macroscopic averaged domains and microscopic surface domains in PZN-PT single crystals. In addition, preferred fracture planes were investigated using fractography technique in conjunction with preferable domain switching directions. In the third part, the effects of mechanical polishing on domain reorientation were evaluated, leading to viii Chapter imaging. As shown in Fig. 7.7(e), the stripe-like structure completely disappears after the exposure to the in-plane magnetic field. This observation supports previous speculation of magnetic structures as the application of an external field is expected to cause a change in the MFM-imaged domains [34]. In the MFM imaging of the ZnO:Cu samples, it was difficult to obtain a highresolution image, probably due to the weak magnetic stray fields in the sample. In addition, since the MFM imaging is based on the magnetic interaction between the tip and the sample, it is an indirect imaging method and therefore potentially perturbative. If the lift height is too short, the topography effect and van der Waals forces may dominate, whereas the spatial resolution is bound to decrease with increasing lift height. Furthermore, the MFM has been proved to be much less efficient in the highresolution of soft magnetic materials, owing to their very specific ability to screen and spread magnetic charges. Therefore, an accurate MFM image interpretation relies on an estimation of a magnetization distribution and understanding the tip characteristics [35]. In this study, the observation of the magnetic domains is reported; however, this mapping, as a measure of the interaction force or force gradient, may be subjected to perturbation due to a complex interaction between the tip and the sample. For a comparison and procedure verification purposes, MFM mapping of a 1.44 MB floppy disk was conducted. The MFM images are shown in Fig. 7.8, taken at a lift height of 200 nm. Stripe-like feature can be easily identified from both the MFM amplitude and phase images in alternate contrast, which is apparently different from the topography image. This stripe-like feature is recognized as magnetic structures, with a domain width of ~ – µm. It should be noted that, the floppy disk, as a 176 Chapter typical magnetic material, demonstrates magnetic domain structures in both MFM amplitude and phase images, unlike the ZnO:Cu (8 at.%) film which shows vague magnetic structures only in the MFM amplitude image. This difference may be attributed to weaker magnetic nature in the ZnO:Cu (8 at.%) film to induce any phase difference during the magnetostatic force mapping in the second pass. However, further research is required for a better understanding in the magnetic behaviors of ZnO:Cu films. Fig. 7.8: MFM images of a floppy disk: (a) topography, (b) MFM amplitude, and (c) MFM phase images (field of view: 50 × 50 mm2). The scale bar in the images represents 10 µm. 7.5 Conclusion In summary, local piezoelectric, magnetic and surface potential properties of ZnO:Cu films has been investigated using various SPM techniques. SS-PFM on the film shows an inverse ferroelectric hysteresis loop within ±9.8 V, in which a negative bias increases the amplitude of the piezoresponse and vice versa. From this ferroelectric loop and localized poling effect, it is postulated that ZnO:Cu films may 177 Chapter have a predominantly upward ferroelectric domains, which is in-phase with the negative bias. Furthermore, magnetic domains, ~1 – µm in width, were successfully observed using the MFM technique. These results support ZnO:Cu films as a material with both intrinsic piezoelectric and magnetic properties. On the other hand, stable bipolar surface potential behavior of ZnO:Cu film at room temperature has been demonstrated in ZnO:Cu films with an appropriate amount of Cu doping (~8 at.%) and oxygen vacancies. Cu doping leads to localized hole trapping phenomenon, whereas oxygen vacancies promote electrons trapping stability in ZnO:Cu film. The bipolar charges can be effectively trapped and stored in the films for over 20 hours, pertaining to their reliability as a new type of transparent charge storage medium in potential storage applications. It is also observed that the charge dissipation process in ZnO:Cu film is significantly different from that in the PZN-PT crystal, indicating different dissipation mechanisms in these two materials. The PZN-PT crystal is a ferroelectric material, whereas ZnO:Cu is a semiconductor material which show somewhat both ferroelectric and ferromagnetic properties. 178 Chapter References [1] D. M. Bagnall, Y. F. Chen, Z. Zhu, T. Yao, S. Koyama, M. Y. Shen, T. Goto, Appl. Phys. Lett., 70 (1997) 2230-2232. [2] Ü. Özgür, Y. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Doğan, V. Avrutin, S.-J. Cho, H. Morkoç, J. Appl. Phys., 98 (2005) 041301. [3] C. Liu, F. Yun, H. Morkoç, J. Mater. Sci. - Mater. Electron., 16 (2005) 555597. [4] Y. C. Yang, C. Song, F. Zeng, F. Pan, Y. N. Xie, T. Liu, Appl. Phys. Lett., 90 (2007) 242903. [5] D. B. Buchholz, R. P. H. Chang, J. H. Song, J. B. Ketterson, Appl. Phys. Lett., 87 (2005) 082504. [6] T. S. Herng, S. P. Lau, S. F. Yu, H. Y. Yang, X. H. Ji, J. S. Chen, N. Yasui, H. Inaba, J. Appl. Phys., 99 (2006) 086101. [7] T. S. Herng, S. P. Lau, S. F. Yu, S. H. Tsang, K. S. Teng, J. S. Chen, J. Appl. Phys., 104 (2008) 103104. [8] J. B. Kim, D. Byun, S. Y. Ie, D. H. Park, W. K. Choi, J.-W. Choi, B. Angadi, Semicond. Sci. Technol., 23 (2008) 095004. [9] X. B. Wang, D. M. Li, F. Zeng, F. Pan, J. Phys. D: Appl. Phys., 38 (2005) 4104-4108. [10] A. Furukawa, N. Ogasawara, R. Yokozawa, T. Tokunaga, Jpn. J. Appl. Phys., 47 (2008) 8799-8801. [11] A. Hartmann, M. K. Puchert, R. N. Lamb, Surf. Interface Anal., 24 (1996) 671-674. [12] Q. Ma, D. B. Buchholz, R. P. H. Chang, Phys. Rev. B, 78 (2008) 214429. [13] S. V. Kalinin, D. A. Bonnell, Phys. Rev. B, 63 (2001) 125411. 179 Chapter [14] S. V. Kalinin, D. A. Bonnell, Nano Lett., (2004) 555-560. [15] Y. Kim, C. Bae, K. Ryu, H. Ko, Y. K. Kim, S. Hong, H. Shin, Appl. Phys. Lett., 94 (2009) 032907. [16] S. Jesse, H. N. Lee, S. V. Kalinin, Rev. Sci. Instrum., 77 (2006) 073702. [17] S. Jesse, A. P. Baddorf, S. V. Kalinin, Appl. Phys. Lett., 88 (2006) 062908. [18] J. S. Park, T. Minegishi, S. H. Lee, I. H. Im, S. H. Park, T. Hanada, T. Goto, M. W. Cho, T. Yao, S. K. Hong, J. H. Chang, J. Vac. Sci. Technol. A, 26 (2008) 90-96. [19] N. Balke, I. Bdikin, S. V. Kalinin, A. L. Kholkin, J. Am. Ceram. Soc., 92 (2009) 1629-1647. [20] H. Kawai, Jpn. J. Appl. Phys., (1969) 975-976. [21] Q. M. Zhang, V. Bharti, G. Kavarnos, Poly(vinylidene fluoride) (PVDF) and its copolymers, in: M. Schwartz (Ed.) Encyclopedia of Smart Materials, Vol. 2, John Wiley & Sons Inc., New York, 2002, pp. 807-825. [22] A. V. Bune, V. M. Fridkin, S. Ducharme, L. M. Blinov, S. P. Palto, A. V. Sorokin, S. G. Yudin, A. Zlatkin, Nature, 391 (1998) 874-877. [23] V. S. Bystrov, I. K. Bdikin, D. A. Kiselev, V. M. Fridkin, A. L. Kholkin, J. Phys. D: Appl. Phys., 40 (2007) 4571-4577. [24] B. J. Rodriguez, S. Jesse, S. V. Kalinin, J. Kim, S. Ducharme, V. M. Fridkin, Appl. Phys. Lett., 90 (2007) 122904. [25] M. Nonnenmacher, M. P. O’Boyle, H. K. Wickramasinghe, Appl. Phys. Lett., 58 (1991) 2921-2923. [26] X. Q. Chen, H. Hamada, T. Horiuchi, K. Matsushige, S. Watanabe, M. Kawai, P. S. Weiss, J. Vac. Sci. Technol. B, 17 (1999) 1930-1934. [27] A. Liscio, V. Palermo, P. Samori, Acc. Chem. Res., 43 (2010) 541-550. 180 Chapter [28] S. V. Kalinin, D. A. Bonnell, Phys. Rev. B, 62 (2000) 10419-10430. [29] S. Singh, P. Thiyagarajan, K. Mohan Kant, D. Anita, S. Thirupathiah, N. Rama, B. Tiwari, M. Kottaisamy, M. S. Ramachandra Rao, J. Phys. D: Appl. Phys., 40 (2007) 6312-6327. [30] A. Janotti, C. G. Van de Walle, Appl. Phys. Lett., 87 (2005) 122102. [31] S. Morita, T. Uchihashi, K. Okamoto, M. Abe, Y. Sugawara, Microscale contact charging on a silicon oxide, in: P. M. Vilarinho, Y. Rosenwaks, A. Kingon (Eds.), Scanning Probe Microscopy: Characterization, Nanofabrication and Device Application of Functional Materials, Springer Netherlands, Dordrecht, 2005, pp. 289-308. [32] Y. Kim, S. Buhlmann, J. Kim, M. Park, K. No, Y. K. Kim, S. Hong, Appl. Phys. Lett., 91 (2007) 052906. [33] Digital Instruments, Support Note: Magnetic Force Microscopy (MFM) Applicable to DimensionTM Series and MultimodeTM Systems, Santa Barbara, 1996. [34] R. D. Gomez, E. R. Burke, I. D. Mayergoyz, J. Appl. Phys., 79 (1996) 64416446. [35] J. Miltat, A. Thiaville, Magnetic Force Microscopy, in: K. H. J. Buschow, W. C. Robert, M. C. Flemings, B. Ilschner, E. J. Kramer, S. Mahajan, V. Patrick (Eds.), Encyclopedia of Materials: Science and Technology, Elsevier, Oxford, 2001, pp. 4772-4780. 181 Chapter 8: Conclusions and Recommendations This study mainly explored the micromechanical properties and deformation behavior of PZN-PT single crystals in both unpoled and poled states using nanoindentation technique. Attempts were made to correlate the mechanical deformation phenomena observed with domains contribution and their structures by using nanoindentation, Scanning Electron Acoustic Microscopy, fractography and Scanning Probe Microscopy techniques. Lastly, a controlled polishing procedure was introduced to eliminate the surface layer effects. General conclusions and recommendations for future works are summarized below. 8.1 Nanoindentation Nanoindentation experiments on PZN-(6-7)%PT revealed that pop-in events were observed at ultra-low loads, which could be related to the breaking of a surface deformed layer produced during the crystal fabrication process. Statistical analysis suggested that this surface layer was approximately 300 nm in thickness, and it may possess different mechanical properties from the interior. From a detailed nanoindentation analysis, it was fairly reasonable to conclude that unpoled PZN-PT single crystals have an elastic modulus of ~110 – 120 GPa and a hardness of ~5.5 – 6.0 GPa. These properties could be better enhanced by a poling process, especially for (011)-oriented crystals. 182 Chapter Apart from the loading behavior, the elastic-plastic deformation behavior of PZN-PT single crystals has also been studied using the nanoindentation technique. By isolating elastic contribution to the deformation using the unloading data, parameter r was defined to characterize the elastic-plastic deformation. The r-value increased with maximum indentation load due to a higher indentation stress induced, causing reduced elastic recovery of the material upon indentation unloading. However, the recovery was significantly improved after poling, implying the important role of domain reorientation towards elastic recovery. Nonetheless, with multiple loading/unloading cycles at a higher CSM frequency of 75 Hz, the domain contribution to the elastic recovery was significantly reduced and, as a consequence, the r-value was increased, indicating more elastic-plastic deformation. This CSM effect was, again, particularly pronounced for poled (011) surfaces. Therefore, the overall nanoindentation analysis suggested that the (011)-oriented crystals were more mechanically susceptible to frequency effects, despite a higher elastic modulus compared with their (001)-oriented counterparts. 8.2 Domains Observation and Preferred Fracture Planes In the attempt to correlate the mechanical properties and domains mobility investigated in the nanoindentation, domain structures PZN-(6-7)%PT have been studied using Scanning Electron Acoustic Microscope (SEAM), fractography and Piezoresponse Force Microscopy (PFM) techniques. From the SEAM results, the domain observation agreed reasonably well with the trigonometric projection of domain walls formed by rhombohedral and orthorhombic phases, respectively, on the 183 Chapter (001) and (011) surfaces. Similarly, the fractography results revealed cross-sectional domain structures that were consistent with the theoretical domain wall configuration of rhombohedral phase. In contrast, the domain structures observed using the PFM were significantly different. For example, the unpoled (001) surface was generally comprised of small patches of antiparallel domains, which were preferentially oriented along the polishing direction, while the unpoled (011) surface exhibited fingerprint-like nanodomain patterns with a preferential domain alignment direction along [01 ]. The difference between surface and internal domain patterns observed (PFM versus SEAM and fractography) could be related to the effect of the surface deformed layer and stress compensation between surface strain effect and the minimization of elastic energy. By applying indentations, localized stress-induced domain switching phenomenon has been studied. The critical shear stress to cause domain switching for PZN-PT crystal was estimated to be approximately 49 MPa based on theoretical analysis. When an indentation load was applied, domain switching took place in the form of hyperbolic butterfly shape to minimize the indentation-induced localized stress change. Plastic deformation and slip processes may then set in along {110} planes, causing material pile up in a cross-like pattern. This generated tensile hoop stress to form radial cracks along {100} special cleavage planes. With the observation of slip and fracture processes, preferred cracking planes of {100} and {110} were proposed. This finding of the fracture planes is of considerable importance in the device application, wherein the placement of the crystal’s cleavage planes along high stress directions should be avoided. 184 Chapter 8.3 Mechanical Polishing Effects In view of the effect of the surface deformed layer, a controlled fine polishing procedure using Al2O3 slurry of 0.3 µm-particle size was adopted to remove this layer. After polishing the surface to mirror finish, PFM observation revealed that the macroscopic orientations of the domain walls agreed well with permissible domain wall directions, instead of aligning along the polishing direction in as-polished state. The topography of the PZN-PTs was also altered analogous with the polarization direction. More specifically, the upward domains constituted a depression of ~10 nm compared with the downward domains, suggesting a different hardness for the head and tail domain sections. Furthermore, the structural distortion in the XRD profile and the pop-in event in the nanoindentation were successfully eliminated after this fine polishing procedure. The removal of the surface layer may also lower the coercive voltage of the crystals, thus enabling ferroelectric control with a smaller voltage. In view of the significance of mechanical polishing effects, a further mirror-finishing process is recommended in addition to the current sample preparation procedure. 8.4 Scanning Probe Technique Studies The second part of the research covered the explorative studies of using Scanning Probe Microscopy (SPM) technique to characterize ferroelectric and multiferroics materials. Due to geometrical and intrinsic constraints of PZN-PT single crystals, some variations in the SPM technique have been implemented on copperdoped zinc oxide (ZnO:Cu) film as the explorative studies. Switching Spectroscopy 185 Chapter Piezoresponse Force Microscopy (SS-PFM) showed that the piezoresponse in ZnO:Cu film increased with a negative bias and vice versa. Therefore, its ferroelectric domains may be weakly aligned in upward direction, in-phase with the negative bias. On the other hand, magnetic features with a domain width of ~1 – µm were observed using Magnetic Force Microscopy (MFM) technique. These observation results suggested that the ZnO:Cu film possessed both piezoelectric and magnetic properties, consistent with previous studies. Furthermore, surface potential behavior of ZnO:Cu film has been investigated using Kelvin probe force microscopy (KPFM) technique. In contrast to the pure ZnO with unipolar behavior, the ZnO:Cu film exhibited bipolar surface potential behavior upon the application of a dc bias. Therefore, it was concluded that Cu ions in the ZnO film promoted localized hole trapping phenomenon. With an appropriate amount of the Cu ions (~8 at.%), the charge trapping was reasonably stable, which could be associated with the presence of oxygen vacancies. In general, the results unveiled stable bipolar behavior in ZnO:Cu films as a potential charge storage medium in storage applications. This behavior was significantly different from what observed in PZN-PT crystal by using KPFM technique. 8.5 Recommendations for Future Works In this study of PZN-PT single crystals, PFM have been proven to be a powerful technique in characterizing nanoscale properties of static domain configuration and domain switching behaviors. However, the results presented in this 186 Chapter thesis are only a fundamental understanding of the crystals’ domains in as-prepared or mirror-finished form, without monitoring their in situ change due to an external effect. For example, if the PFM is conducted with the variation of the temperature, then the in situ domain transformation of the crystals can be observed upon heating to the Curie temperature. Likewise, the information on immediate domain evolution with the application of increasing biases can be obtained, if the poling facility is incorporated into the PFM. It may also be interesting to observe the in situ change in domain structures under tensile/compressive stress or electrical cycling effect. Furthermore, the PFM used in this study have several limitations due to the lack of certain attachment modules. First, the local hysteresis spectroscopy is restricted to an acquisition within ±9.8 V. This limits the PFM’s capability to quantitatively characterize the local polarization switching behavior of thick films or hard ferroelectric materials with a larger coercive voltage. Second, the PFM is only capable of detecting out-of-plane polarization response, while the in-plane component due to shear piezoresponse remains undetermined. Thus, an accurate vector of a ferroelectric domain cannot be deduced for a polarization with both the in-plane and out-of-plane components. This imaging of lateral piezoresponse is important for certain materials with interesting in-plane features, such as BiFeO3 films and perhaps, PZN-PT single crystals. Another remarkable aspect is that the elastic modulus and hardness obtained from nanoindentation experiments in this study may only be a representation of submicron properties. In fact, there was no difference between the nanoindentation properties for the positive and negative surface, although a later PFM observation 187 Chapter suggested that the downward domains (negative surface) constitute a higher hardness compared with the upward domains (positive surface). Therefore, detailed evaluation of nanoscale mechanical properties should be performed, probably using the AFMnanoindentation technique. Moreover, it may be interesting to have an in-depth fracture toughness analysis since it pertains to the cracking behavior critical to single crystal materials. For a further understanding of the mechanical properties, numerical solutions, such as Finite Element Method (FEM) with special elements, can be adopted to simulate the anisotropic response of the crystals under mechanical stress. However, due to the difficulties to establish constitutive laws or property matrices for single crystals with domain formation, this subject remains an open question and more research is required to gain better insight into domain behaviors in relaxor single crystals. On the other hand, mechanical degradation behavior of PZN-PT single crystals under simultaneous electromechanical loading has not been comprehensively explored. Therefore, for a workability assessment in practical applications, the crystal properties and domain structures could be investigated after the exposure to combined cyclic electrical and mechanical loadings. This assessment could be carried out using ferroelectric hysteresis testing and PFM techniques. A more valuable insight may be provided if the evaluation of the crystal performance is performed based on a working prototype. 188 APPENDIX 189 Appendix A: Anomaly in Nanoindentation Results (a) (b) This diagram showing the anomaly found in the (a) S-h and (b) S2-P curves during the preliminary nanoindentation testing of (011)-oriented PZN-(6-7)%PT single crystals. It seems that there are two stages of linear behavior, with a transition at an indentation depth of ~375 nm or a load of ~20 mN. These results were irreproducible and only one single stage of the linear behavior was found during subsequent tests. It is unclear of whether this phenomenon was due to errors in the experimental setup, the influence from the environment, or the phase instability when the crystals were “freshly” produced. 190 Appendix B: Mechanical Polishing Effects This figure showing PFM topography and phase images (field of view: 10 × 10 µm2) for the (a) as-polished and (b) mirror-finished (001)-oriented PZN-(6-7)%PT samples before and after applying a dc voltage of +10 V to the (5 × 5)-µm2 area in the middle (larger dotted area), followed by -10 V to a (2 × 2)-µm2 area within the positively poled region (smaller dotted area). The same dc writing has also been performed on the (c) as-polished and (d) mirror-finished (011)-oriented PZN-(6-7)%PT samples. For both orientations, the topography remains unchanged after biased, and the effect of the dc writing, with a bias of ±10 V, is more prevalent after polishing the crystals to mirror finish. The scale bar shown in the figure represents µm. 191 [...]... rotation and the presence of the metastable phases in the crystals [4-6] Therefore, PZN- PT single crystals have been considered as one of the most promising candidates for highperformance piezoelectric devices and systems In this chapter, an overview of piezoelectricity and PZN- PT single crystals will be briefly discussed 1 Chapter 1 1.1 Piezoelectricity and Domains Piezoelectricity is a phenomenon of coupling... domain orientation in the crystals remains unclear To gain insight into fundamental understanding of the mechanical properties of PZN- PT single crystals, the main purpose of this study is to investigate the deformation and domain characteristics of the PZN- PT single crystals The specific objectives of this research are: • to determine elastic modulus and hardness of PZN- PT single crystals using a detailed... surface of (001)-oriented PZN- (6-7) %PT single crystal Fig 4.6 Comparison of (a) P-h2, (b) S-h, and (c) S2-P curves for PZN- (67) %PT single crystals Fig 4.7 (a) Elastic modulus and (b) hardness as functions of indentation depth Fig 4.8 A plot of H2 versus 1/h for PZN- (6-7) %PT single crystals obtained from nanoindentation experiments using a Berkovich indenter xiii Fig 4.9 Isolation of elastic and elastic-plastic... analysis and isolating the effect from the distorted surface structure, • to determine elastic-plastic deformation of the crystals and its correlation with domain alignment in the crystals, • to study crack propagation and fracture behavior of PZN- PT single crystals using indentation and fractography techniques, 12 Chapter 1 • to observe domain structures and domain switching mechanisms of the crystals. .. concerning the micromechanical characterization and domain structures and evolution in PZN- PT single crystals Chapter 1 includes background information and research objectives Chapter 2 reviews literature findings on mechanical properties determined by nanoindentation analysis and domain structures of piezoelectric materials, with primary focus on ferroelectric single crystal materials Chapter 3 presents... effects of the surface and internal structures Therefore, detailed analysis is needed to characterize the mechanical properties of the PZN- PT crystals by isolating the contribution of the surface and internal bulk structures Furthermore, despite literature on the piezoelectric properties of these single crystal materials, the understanding of their deformation behavior and its correlation to domain. .. materials of PZN- PT single crystals and experimental setup Chapter 4 discusses the nanoindentation technique in characterizing mechanical properties of PZN- PT single crystals, including pop-in events, elastic modulus, hardness and elastic-plastic deformation Chapter 5 compares domain structures observed using various characterization techniques and deduces preferred fracture planes, whereas Chapter 6... Photoelastic images of a spherical indentation on PMN -PT single crystals along (a) and (b) directions and their stress intensity contours from ANSYS® simulation xii Fig 3.1 Schematic diagram showing various crystallographic orientations of the PZN- PT single crystals samples used in this study Fig 3.2 Positive and negative surfaces of the PZN- PT single crystals during poling process and possible... 1.1 Illustration of possible twin patterns in [001]/[010]/[100] and [001]/[110]/[1 1 0] oriented rhombohedral PZN- 4.5 %PT single crystals poled along [001] Table 4.1 Dielectric and piezoelectric properties of the poled (001)- and (011)oriented PZN- (6-7) %PT single crystals used in this study, poled at 0.8 and 1.2 kV/mm Table 4.2 Elastic modulus (E) and hardness (H) obtained from S-h and S2-P relationships... maximum indentation load of 20mN and 50 mN Table 5.1 Properties of flux-grown (001)- and (011)-oriented PZN- (6-7) %PT used in this study, poled at 0.8 and 0.4 kV/mm, respectively, along the thickness direction Table 6.1 Elastic modulus and hardness of PZN- PT single crystals, determined from nanoindentation experiments x List of Figures Fig 1.1 Schematic plots of (a) a hysteresis loop of electric polarization . MICROMECHANICAL PROPERTIES AND DOMAIN STRUCTURES OF PZN-PT PIEZOELECTRIC SINGLE CRYSTALS WONG MENG FEI NATIONAL UNIVERSITY OF SINGAPORE. characterization of PZN-PT single crystals can be divided into three parts. In the first part, micromechanical properties and deformation characteristics of PZN-PT single crystals were examined. NATIONAL UNIVERSITY OF SINGAPORE 2011 MICROMECHANICAL PROPERTIES AND DOMAIN STRUCTURES OF PZN-PT PIEZOELECTRIC SINGLE CRYSTALS WONG MENG FEI (B.Eng.(Hons.),