Imaging Ferroelectric Domains and Domain Walls Using Charge

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Imaging Ferroelectric Domains and Domain Walls Using Charge Gradient Microscopy: Role of Screening Charges Sheng Tong, Il Woong Jung, Yoon-Young Choi, Seungbum Hong, and Andreas Karl Roelofs ACS Nano, Just Accepted Manuscript • Publication Date (Web): 11 Jan 2016 Downloaded from http://pubs.acs.org on January 11, 2016

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Imaging Ferroelectric Domains and Domain Walls Using Charge Gradient Microscopy: Role of Screening Charges Sheng Tong†, Il Woong Jung†, Yoon-Young Choi‡, Seungbum Hong‡,*, and Andreas Roelofs†,* †

Nanoscience and Technology Division, Argonne National Laboratory, Lemont, IL

60439, USA, ‡Materials Science Division, Argonne National Laboratory, Lemont, IL 60439, USA

ABSTRACT Advanced scanning probe microscopies (SPMs) open up the possibilities of the next-generation ferroic devices that utilize both domains and domain walls as active elements. However, current SPMs lack the capability of dynamically monitoring the motion of domains and domain walls in conjunction with the transport of the screening charges that lower the total electrostatic energy of both domains and domain walls. Charge gradient microscopy (CGM) is a strong candidate to overcome these shortcomings because it can map domains and domain walls at high speed and mechanically remove the screening charges. Yet the underlying mechanism of the CGM signals is not fully understood due to the complexity of the electrostatic interactions.

*

Corresponding authors: [email protected] (S.H.), [email protected] (A.R.)

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Here, we designed a semiconductor-metal CGM tip, which can separate and quantify the ferroelectric domain and domain wall signals by simply changing its scanning direction. Our investigation reveals that the domain wall signals are due to the spatial change of polarization charges, while the domain signals are due to continuous removal and supply of screening charges at the CGM tip. In addition, we observed asymmetric CGM domain currents from the up and down domains, which are originated from the different debonding energies and the amount of the screening charges on positive and negative bound charges. We believe that our findings can help design CGM with high spatial resolution and lead to breakthroughs in information storage and energy harvesting devices. KEYWORDS: atomic force microscopy, focused ion beam, periodically poled lithium niobate, electrostatic force


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Ferroelectric materials are being intensively investigated because of their many possible commercial applications, such as capacitors, sensors and transducers, filters, pyroelectric devices, buzzers, gas igniters, ultrasonic motors, and memories.1-15 In these materials, screening charges shield the electric field from the bound charges until they are completely compensated. The presence of the chemical adsorbates (usually water molecules) from the surrounding environment and multi-domains/domain walls can efficiently reduce the depolarization field, and lower the overall electrostatic energy to the ground state.1,2,4,7-12,15-20 Kinetics of the formation of domains/domain walls and buildup of screening charges have been explored extensively using various modes of scanning probe microscopy (SPM) at the nanoscale. However, the slow scan velocity (usually below 0.1 mm/s) used for the ferroelectric domain mapping, e.g. piezoresponse force microscopy (PFM), electrostatic force microscopy (EFM), and Kelvin probe force microscopy (KPFM), has often limited the in-situ observation of such phenomena to the quasi-static regime.1,8,15-17 Recently developed charge gradient microscopy (CGM) provides a solution to this challenge by mapping the polarization charges at high speed and tuning the screening condition from equilibrium to non-equilibrium on the surface of ferroelectrics.21,22 For unscreened ferroelectric surfaces, when a grounded conductive atomic force microscopy (AFM) tip (hereafter “CGM tip”) passes over the domain walls, it measures the current spike, which is proportional to the potential gradient, because the near surface polarization charges attract charges of opposite sign and repel those of the same sign. Therefore, the CGM current polarity from the domain walls in ferroelectrics is opposite in left to right and right to left scans. Such a “complementary charge”


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mechanism in utilizing the charges of opposite polarity is the same as that of the cloud charge monitors used for thunderstorms and lightning, but at the nanoscale.22 CGM is attractive for two main reasons. First, CGM current signals are proportional to the scan speed, in contrast to other common measurement techniques where integration time depends on the signal-to-noise ratio. As a consequence, faster scanning increases the measurement signals. For example, CGM imaging on periodically poled lithium niobate (PPLN) was achieved at a scan velocity of ~5 mm/s or scan rate of 78 Hz in the fast scan direction, only limited by the resonance frequency of the X-Y actuators in AFM stage.21 Thus, CGM can be used for nanoscale “slow” dynamic imaging of polarization charges. Secondly, the CGM tip can remove the local screening charges on the sample surface by a mechanical scraping, with little influence to the polarization charges underneath.21-24 Thus, CGM can be used for studying the effect of remanent polarization and tuning the degree of screening on the surface without influence from screening charges. This capability allows for control of the surface chemistry of ferroelectrics by tuning the degree of screening on the surface.24 Because of the complexity of the current signals, we have not fully understood the mechanisms behind CGM. The “complementary charge” mechanism based on the interactions of ferroelectric charges and grounded CGM tip as we described above does explain the signal for the domain wall current. However, Hong et al.21 discovered the presence of a domain current signal when CGM measurements are conducted at high speed (> 50 µm/s) in ambient condition. They proposed additional mechanisms that take into consideration the local movements of screening charges during CGM scans. In one mechanism, “accumulated external screening charges” explain continuous CGM current 4 ACS Paragon Plus Environment

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flows on domains with opposite charge polarity to the screening charges. Another mechanism, “separation and refill of external screening charges,” explains the continuous CGM current on domains with flow of the same polarity charges as the screening charges to the CGM tip. As a result of the above-mentioned mechanisms, the current contrast in CGM images consists of strong domain wall signals but weak domain signals. Here we designed a hybrid semiconductor-metal CGM tip to understand mechanisms of domain and domain wall signals by displacing the external screening charges with the silicon before the metal part scans the surface.

Results and Discussion We began with a semiconductor-metal CGM tip composed of Pt/Cr-coated Si in quadratic pyramid geometry. Focused ion beam (FIB) etching removed the Pt/Cr coating on three facets of the CGM tip, leaving only one side with a conductive film. Figure 1 shows a schematic of the CGM tip on top of a PPLN sample. The scanning electron microscopy (SEM) images in Figure 1b–1d clearly show that the Pt/Cr coating was milled on three sides. When both the Pt/Cr and Si are in contact with the PPLN surface at a scan angle ~40–50°, we traced a PPLN sample with silicon in front and metal at back (SFMB) to the scan direction and retraced it with metal in front and silicon at back (MFSB). During the SFMB scan, the removed screening charges that may accumulate on the silicon are shielded from electrostatic interaction with the Pt/Cr because the screening length of silicon (1–2 nm) is much smaller than the thickness of the silicon (see Supporting Information).


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The hybrid CGM tip showed interestingly different CGM current signals in the trace and retrace scans. As seen in Figure 2, the SFMB scan exhibits strong domain patterns, while domain wall current signals dominate in the MFSB scan. With a grounded CGM tip, a positive current is defined as charges flowing from the CGM tip to the ground, and the negative current in the opposite direction. As shown by the graphs in Figure 2, the negative and positive currents in the SFMB scan correspond to the down and up domains, respectively, while the negative and positive current spikes across the domain wall in the MFSB correspond to the transitions from up to down and from down to up domains, respectively. The current profiles obtained in trace and retrace for the first scan are repeatable in different areas of the PPLN. However, multiple CGM scanning at the same area leads to a change in the measured trace signal. Figure 3 displays multiple scans obtained with the FIB-milled CGM tip. During the SFMB scans (Figures 3a–3c), the signal from the domain current diminishes and is gradually replaced by that from the domain wall current. After the three consecutive scans, the CGM images were comparable with those measured using a non-FIB milled CGM tip, namely, strong signals from the domain wall current of opposite polarity at the same domain wall boundary, without significant signals from the domain current, as shown in the line profiles of CGM current (Figure 3g). The rate of charge removal from external surface screening via mechanical means is faster than that of the rescreening.23 As such, we hypothesized that the “complementary charge” mechanism for an unscreened surface will dominate after repetitive scanning results in a lower degree of screening. To validate this hypothesis, as shown in Figure 4, we scanned the same area three times to acquire clear domain wall signals (Figs. 4a–4c) 6 ACS Paragon Plus Environment

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and left the sample in air for ~20 min, during which the change of the surface net potential was determined by EFM after the three CGM scans (Figs. 4d–4f). After 20 min, the EFM images show relatively little contrast, indicating a fully rescreened PPLN surface. Then, we performed the CGM imaging again. The first CGM scan (Figure 4g) shows mainly domain signals. However, the subsequent two scans (Figs. 4h, 4i) exhibited more of the domain wall signal. These images suggest that, once the surface is fully rescreened, the domain signals prevail in the CGM image using the FIB-milled CGM tip in the SFMB scan. Based on the above results, we will discuss the physical mechanism of the domain current signals, i.e., the charge transport and the electrostatic interactions among the CGM tip, the surface screening charges, and the ferroelectric charges. In ambient, a freshly ferroelectric surface with unpaired ionic bonds tends to lower the surface energy via short-range adsorption of water molecules.16-20,25-28 Both physisorption and chemisorption occur on the ferroelectric surface at room temperature, as illustrated in Figure S3.16-19,29 Finally, a monolayer of –OH groups, which are negative in charge, is formed on the ferroelectric surface.19,20 Consequently, their adsorption on up and down domains can introduce a different termination structure on the ferroelectric surface due to the long-range forces. On the up domains, the underneath ions can be screened by their rumpling. On the down domains, where the –OH ions cannot screen the negative bound charges, surface layers of ions reconstruct to electroneutralize the polarization charges, then an extra layer of H3O+ ions forms above the M–(OH)n layer to complete the screening, as illustrated in Figure 5.19,20,30 We control the applied force to the cantilever during the CGM carefully so as not to wear either the ferroelectric surface or the CGM 7 ACS Paragon Plus Environment

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tip. The prime debonded ions are OH- rather than H+ or H3O+ because the hydroxyl bond dissociation energy of the O-H bond (4.27 eV)31 is stronger than that of the M–O bond (e.g., 1.95 eV for Li–O in LiNbO5).32 The debonded OH- ions have three possible routes. First, they may pass through to the back of the CGM tip. Second, they may be pushed or kicked away further from the tip. These ion transport paths do not directly result in domain current signals because the flow of the OH- ions does not disturb the electrostatic equilibrium around the tip. In the third route, the debonded OH- ions accumulate at the tip end. As shown in the MFSB scan (Figure 5a), in regular CGM or MFSB scans, above the down domains, continuous current will be supplied when the free OH- ions accumulate on the metal coating on the tip, the same as the mechanism of “accumulated external screening charges”.21 Meanwhile, the unscreened ferroelectric surface tends to attract electrons from the tip as the CGM tip mechanically removes external screening charges, the same as the mechanism of “separation and refill of external screening charges”.21 The competition between these two mechanisms can cancel each other out and leave zero domain signals because of their opposite current polarity. Atop the down domain, because H3O+ ions above the –OH group may neutralize the debonded OH- ions, the other H3O+ ions accumulate on the tip and supply the screening charges for the surface bonds. Therefore, the CGM domain current may be also measured as zero (Figure 5a). As shown in Figure 5b, in the SFMB scan, domain signals prevail due to the mechanism of “separation and refill of external screening charges” because the accumulated debonded OH- ions are shielded by the Si layer in electrostatically interacting with the Pt/Cr film. In this condition, as the metal part of the tip contacts the 8 ACS Paragon Plus Environment

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up domains, electrons are injected from the tip and screen the positive polarization charges. Furthermore, as the metal part of the tip contacts the down domains, electrons in the tip are repelled by the negative polarization, creating positively charged region near the tip that locally screens the negative polarization. We can measure the domain current signals in the SFMB scan. Note in the current profile for Figure 2a the peaks in the current amplitude for positive polarity are larger than those for negative polarity. This difference may be due to the electrons injection to the bound charges on the up domains while no positive charges can flow from the CGM tip to screen the down domains. In this scenario, as shown in Figure 5c, the net potential above the up and down domains will be zero and negative after CGM scan, respectively. The asymmetric current flow may also be due to different desorption energies of the OH- ions above different polarization charges, because the binding energy is larger for the up than the down domains. For example, Garrity et al. reported that the binding energy of OH- to the down-domain surface is larger than that of the up-domain surface by 0.5 eV–0.9 eV on BaTiO3 (100).33 Therefore, more OH- ions are removed on the down domains than on the up domains. To further investigate the asymmetric CGM domain currents, we obtained a PFM image and a series of EFM images immediately after the CGM scan (Figure 6). A CGM scan had been conducted within the dashed square region in Figure 6a. The initial PFM and EFM phase images exhibit clear domain contrast within the square region. The line profile in Figure 6f shows that the down domains undergo a larger change in EFM phase (~12.5°) than that of the up domains (~2.5°), where the positive (negative) EFM phases indicate negative (positive) net potentials on the down (up) domains. As the change in 9 ACS Paragon Plus Environment

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EFM phase is proportional to the surface potential change, it supports the validation of the two scenarios proposed previously. After the 18 min rescreening, the overall EFM phase within the CGM scanned area is higher than that of the surroundings (Figure 6e), which further supports the electron injection scenario. From the time-resolved EFM phase profiles in Figure 6f, we extracted the peaks for the up and down domains and fitted them with the first-order adsorption equation:23,25

 t ∆φ = ∆φ0 exp −   τ

(1)

where ∆φ0 is the EFM phase contrast between the peak EFM phase and the baseline, ∆φ0 is a prefactor, t is time, and τ is the adsorption half-life time. The fitting results are given in Figure 6g, with different τ in the down domain (8.49 min) and the up domain (7.03 min). As smaller τ values are associated with smaller adsorption energy,34 the adsorption energy of the external ions on the up domains in the PPLN is smaller than that on the down domains. This finding is in agreement with reported literature. For example, Garra et al. measured the desorption energy of the water on PPLN, namely, 86 kJ/mol in down domains and 82 kJ/mol in up domains.35 The inferred greater binding energy and the smaller adsorption energy on the up domains agrees with reports that the surface energy is similar in the up and down domains,33,36 and that the adsorption energy is equal to the ferroelectric energy plus the total energy of the free water molecule minus the binding energy.37


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Conclusion We imaged and manipulated the charges compensating the polarization underneath the PPLN surface using a semiconductor-metal CGM tip. The decay of the domain current signals after repeated CGM scans and their retention indicate that these signals can be measured when the ferroelectric surface is mechanically unscreened and the screening charges continuously flow to the ferroelectric surface from the grounded CGM tip. This effect is not seen in regular CGM or MFSB scans because it is weakened by the electrostatic interactions between the accumulated external screening charges and screening charges in the metal part of the CGM tip. The separation and refill of external screening charges becomes apparent only when a semiconductor part is used to shield the metal part in the CGM tip. Based on the results in this paper, we can design a CGM tip for either domain or domain wall mapping of ferroelectric surfaces at a high line scan speed. Such a specialized CGM tip with short-circuit current density of ~55 A/m2 can be scaled up for energy harvesting of kinetic energy, for example, by employing an array of CGM tips or multiple razor-blade type electrodes directly engaged on the ferroelectric surfaces. In addition, CGM might be applicable to dynamic imaging during the charge removal process on polar materials, ferroelectric switching with high spatial resolution, and data reading for ferroelectric probe storage memories.


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Methods and Materials Materials. A commercial periodically poled lithium niobate (AR-PPLN test sample, Asylum Research, USA) was used. It consists of a 3 mm × 3 mm × 0.5 mm LiNbO3 transparent die, with a 10-µm domain pitch. FIB Milled CGM Tip. The Pt/Cr-coated Si probe (ElectricTAP300-G, BudgetSensors, USA) was milled with a Nova 600 NanoLab focused ion beam microscopy (FEI Company, USA). PFM, CGM, and EFM Imaging. Atomic force microscopy (AFM) experiments were carried out with Asylum MFP-3D scanning probe microscopy (Asylum Research., USA). Piezoresponse force microscopy (PFM) was performed at a modulation voltage of 1 Vpp, and frequency of 1.53 MHz. Charge gradient microscopy (CGM) was performed with a conductive cantilever holder (ORCA, gain of 5 × 108 V/A, Asylum Research, USA) while the bottom electrode of AR-PPLN was grounded. The scan speed was measured at 10 Hz over 50 µm. The applied force to the probe was ~ 2 µN in all CGM measurements. Double-pass electrostatic force microscopy (EFM) was conducted with a lift height of 50 nm and probe ac bias of 3 V at -10 % of the probe resonance frequency. SEM Imaging. Scanning electron microscopy (SEM) and energy dispersive Xray spectroscopy (EDS) were carried out on a field emission scanning electron microscope (JEOL JSM-7500F, JEOL USA Inc., USA) with EDS (Thermo Scientific) and a Nova 600 NanoLab FIB-SEM dual-beam microscope (FEI Company, USA). Experimental Sequence. Before patterning, a Pt/Cr-coated Si probe (CGM tip) was used to conduct AFM, PFM, and CGM measurements on the PPLN sample. The CGM signals consisted of domain wall current spikes that match the PFM phase images.


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The current spikes are of opposite polarity in trace and retrace scans. We conducted CGM scans more than twenty times in order to blunt the tip end. We used the SEM to examine the flat hybrid tip end of both the metal and silicon in the tip (Figure S1). After that, FIB was used to mill off the three sides of Pt/Cr, leaving only one side with a metal lead to the tip, achieving a semiconductor-metal patterned tip end (Figure 1). Both CGM and EFM images were obtained and are shown in Figures 2 – 4. After these measurements, SEM images and EDS analysis (Figure S2) were undertaken to illustrate the CGM tip degrading and the debris. The PFM image and the time-resolved EFM images in Figure 6 after the CGM scan were obtained with an Ir-coated Si probe (ASYELEC-02, Asylum Research, USA). The humidity and temperature were kept at ~ 18 % and 28 ℃ during the experiments, in order to monitor the rescreening charge dynamics at slow rate and to minimize the influence from the neighboring screening charges. It is reported that low humidity decreases the thickness of water layer and the size of the water meniscus around the CGM tip, and reduces the rescreening rate due to the low diffusion rate of the water gas. 12,16,38


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Associated Content Support Information Detailed surface adsorption kinetics and SEM images of the CGM probe before and after the experiments. This material is available free of charge via the Internet at http://pubs.acs.org. Author Information Corresponding authors *Email: [email protected] (S.H.) *Email: [email protected] (A.R.) Notes The authors declare no competing financial interest. Acknowledgements This work was performed, in part, at the Center for Nanoscale Materials, a U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences User Facility under Contract No. DE-AC02-06CH11357, and at the U.S. Department of Energy, Office of Science, Materials Science and Engineering Division under contract No. DOE-BESDMSE. The work of CGM, PFM and EFM was performed at the Materials Science Division, and the work of SEM and FIB was performed at the Center for Nanoscale Materials.


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28. Verniani, F. The Total Mass of the Earth's Atmosphere, J. Geophys. Res. 1966, 71, 385-391. 29. Morimoto, T.; Nagao, M.; Tokuda, F. Relation Between the Amounts of Chemisorbed and Physisorbed Water on Metal Oxides, J. Phys. Chem. 1969, 73, 243-246. 30. Shin, J.; Nascimento, V. B.; Geneste, G.; Rundgren, J.; Plummer, E. W.; Dkhil, B.; Kalinin, S. V.; Baddorf, A. P. Atomistic Screening Mechanism of Ferroelectric Surfaces: an in-situ Study of the Polar Phase in Ultrathin BaTiO3 Films Exposed to H2O, Nano Lett. 2009, 9, 3720-3725. 31. Darwent, B., Bond dissociation energies in simple molecules. Nat. Stand. Ref. Data Ser., Nat. Bur. of Stand. (U.S.): WASHINGTON, D. C., 1970; Vol. 31, pp 52-55. 32. He, Y. L.; Xue, D. F. Bond-energy Study of Photorefractive Properties of Doped Lithium Niobate Crystals, J. Phys. Chem. C 2007, 111, 13238-13243. 33. Garrity, K.; Kakekhani, A.; Kolpak, A.; Ismail-Beigi, S. Ferroelectric Surface Chemistry: First-principles Study of the PbTiO3 Surface, Phys. Rev. B 2013, 88, 045401. 34. Adamson, A. W.; Gast, A. P., Physical chemistry of surfaces. Wiley-Interscience: University of Michigan, 1997; pp 601-603 35. Garra, J.; Vohs, J. M.; Bonnell, D. A. The Effect of Ferroelectric Polarization on the Interaction of Water and Methanol with the Surface of LiNbO3 (0001), Surf. Sci. 2009, 603, 1106-1114.


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Figures

Figure 1. (a) Schematic of FIB-milled CGM tip on top of a periodically poled lithium niobate (PPLN) crystal, where red indicates the Pt/Cr-coated area and black, the uncoated Si area; (b–d) SEM images of the FIB-milled tip, imaging from the top (b), back (c), and right (d) with respect to the position shown in (a).


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Figure 2. CGM images scanned from (a) left to right (SFMB) and (b) right to left (MFSB). Graphs below show the current profiles taken from the dashed lines in the CGM images. The notation ⊙ and ⊗ represent the up and down polarization direction on the PPLN crystal.


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Figure 3. CGM images measured continuously on the PPLN in three SFMB (a–c) and MFSB (d–f) scans. (g) Line profiles of CGM current in (c) and (f).


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Figure 4. Influence of screening charges on the CGM signal patterns. (a–c) Three consecutive CGM SFMB scan images; (d–f) continuous EFM phase images at (d) 0– 2 min, (e) 4– 6 min, and (f) 12 – 14 min after the three CGM scans; (g–i) three consecutive CGM SFMB scan images after 20-min rescreening.


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Figure 5. Schematic of the electrostatic interactions between the screening charges (OHand H3O+) and the CGM tip during the (a) MFSB and (b) SFMB scans. (c) Schematic of the surface chemistry and the spatial overall potential instantly after CGM scan. 24 ACS Paragon Plus Environment

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Figure 6. PFM image and continuous EFM images measured after CGM scan. (a) PFM phase; (b–e) EFM phase obtained 0 – 18 min after CGM; (f) line profile of EFM phases as a function of time elapsed after CGM scans with pattern as shown in (b). The line profiles between the two blue dashed lines represent the region scanned by the CGM tip. (g) Exponential decay of EFM phase contrast for up and down domains.


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TOC


Imaging Ferroelectric Domains and Domain Walls Using Charge

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