Photo-Controlled Ferroelectric-Based Nanoactuators - ACS Applied

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Photo-Controlled Ferroelectric-Based Nanoactuators Fernando Rubio-Marcos,*,‡ David Páez-Margarit,§ Diego A. Ochoa,§ Adolfo Del Campo,‡ José F. Fernández,‡ and José E. García*,§ ‡

Department of Electroceramics, Instituto de Cerámica y Vidrio (CSIC), Madrid 28049, Spain Department of Physics, Universitat Politècnica de Catalunya−BarcelonaTech, Barcelona 08034, Spain

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S Supporting Information *

ABSTRACT: Finding a feasible principle for a future generation of nanooptomechanical systems is a matter of intensive research, because it may provide new device prospects for optoelectronics and nanomanipulation techniques. Here we show that the strain of a ferroelectric crystal can be manipulated to achieve macroscopic, stable, and reproducible dimensional changes using illumination with photon energy below the material bandgap. The photoresponse can be activated without direct light incidence on the actuation area, because the cooperative nature of the phenomenon extends the photoinduced strain to the whole material. These results may be useful for developing the next generation of high-efficiency photocontrolled ferroelectric devices.

KEYWORDS: photoferroelectrics, barium titanate, ferroelectric crystals, photostrain, light-induced phenomena

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chose a (100)-oriented BaTiO3 (BTO) ferroelectric crystal, which exhibits a polydomain state constituted by in-plane polarization a domains and out-of-plane polarization c domains with a head-to-head (H−H) configuration of the polarization vectors (see Supporting Information 2). According to previous studies, the H−H configuration between a and c domains in BTO produces a large accumulation of the local stress at the domain wall12−14 and, as a consequence, the b domains are formed to produce stress release.5 The H−H domain configuration enables the stabilization of the charged domain walls (CDWs), which are intimately linked to the lightactivated change in macroscopic polarization.6 Atomic force microscopy (AFM) is used to investigate lightinduced strain at a local scale, as illustrated in Figure 1a,b. The surface features of the BTO crystal in dark conditions are shown in Figure 1c. The AFM scan reveals the topography of parallel domains, which is associated with soft transitions (Figure 1d), thus confirming that the c domain protrudes accordingly with out-of-plane distortion while the a domain is depleted. Moreover, the CDWs are asymmetric, as denoted by the appearance of the b domains pinned at a−c domain walls (see insert in Figure 1d). In order to prove the ability to produce a net strain by applying a coherent light, a light source with 65 mW of power and 532 nm of wavelength is used. The reproducibility and reversibility of the process is investigated by means of in situ sequential AFM scans for a set of off−on−

erroelectric materials develop a spontaneous electric polarization that can be switched by applying an external electric field. Ferroelectrics are outstanding as piezoelectric materials, particularly for their ability to transform electrical energy into mechanical energy and vice versa. This fundamental property is at the core of many applications, such as high-precision actuation, medical ultrasonic imaging, green electric power generation, and so on. A critical functionality of piezoelectric materials for their use as actuators is the electrostrain; that is, the ability to produce a strain by applying an external electric field. In order to better understand this phenomenon, it is important to take into account that the prime mechanism is associated with domain switching,1 not only in inorganic polydomain ferroelectric but also in organic− inorganic ones.2−4 In general, the electrostrain improvement requires effective methodologies capable of modulating and controlling domain switching. The photocontrol of ferroelectric properties has recently attracted notable attention as an effective alternative methodology for electric polarization switching.5−11 Specifically, it has been reported that macroscopic polarization, and consequently its related properties, can be tuned by means of visible light in a ferroelectric crystal.6−8 This finding implies lower power consumption and an external control of the functional properties without contact. It is important to point out that no spatial confinement of light is needed to tune the macroscopic polarization of the material, and that the effect emerges with relatively low light intensity.6,7 In this work, we go a step further and demonstrate that a macroscopic strain can be reversibly driven by light power control in ferroelectrics. To illustrate this proof-of concept, we © XXXX American Chemical Society

Received: January 25, 2019 Accepted: March 28, 2019

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DOI: 10.1021/acsami.9b01628 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces

Figure 1. Revealing the ability to induce controllable strain by light at the nanoscale. (a, b) Schematic illustration of the experimental setup for the atomic force microscopy (AFM) measurements. The net elongation induced by the illumination is illustrated. (c) AFM image of the crystal under dark conditions, showing the domain topography inside the marked black box in Figure S1a. Scale bar, 25 μm. (d) AFM topography scan along the white dashed arrow of c. The inset shows a detail of the domain boundary topography, which corresponds to the presence of a b domain located close to the a−c domain wall (see Supporting Information 2). (e) AFM image under illumination conditions, displaying a sequential scan for a set composed of off−on−off illumination cycle for the same region previously studied under dark conditions. Scale bar, 25 μm. The numbers next to the AFM image delimit the light switching, obtaining three regions in the image of 150 (length) × 25 (width) μm. (f) AFM topography scan along the green dashed arrow marked in e, showing a net elongation of 150 nm under illumination conditions (65 mW).

Supporting Information 5). This basic experiment confirms that light-induced strain can be reversibly controlled by varying the light power (Figure 2a). A linear trend between the photoinduced elongation and the light power is shown in Figure 2b for the studied light power range. As may be observed, this phenomenon does not present a light power threshold, which strengthens the case that ferroelectrics could be used as an active optical material for high-performance photoactuation systems. A similar set of experiments is performed, this time using a BTO crystal with a thickness of 50 μm (see Supporting Information 6 for topographical and domain structure characterizations of this sample). The results show that the photostrain in the BTO crystal with a reduced thickness shows a similar linear trend (Figure 2b). The light effect generates a reversible strain with a net elongation of ∼20 nm under a light power of 65 mW, which corresponds to a photoinduced strain value of 0.04%. While the net elongation is an order of magnitude lower than the obtained elongation for the 500 μmthick sample, the strain slightly increases because the higher density of CWDs in the 50 μm thick sample as a consequence of the higher b domain density revealed in this sample (see Figures S1 and S6). The functionality of a material for actuator applications is subject to its ability to be strained in a stable and reversible fashion and, especially, to be able to produce a macroscopic response that can be easily monitored. In this respect, a new experiment was designed in which the light source was placed

off illumination cycles in the same region previously studied under dark conditions (Figure 1e). As demonstrated in Figure 1f, the illumination generates a reversible strain with a net elongation of ca. 150 nm, which implies a noticeable photoinduced strain value of 0.03% comparable with the electrostrain values recently reported for PMN−PT based ceramics under an electric field of 2 kV cm−1.15 Nonferroelectric substrates with similar geometric characteristics to the BTO crystal are measured in order to prove that the AFM signal results from the elongation of the BTO crystal and not from an artifact in the measurements (see Supporting Information 3). The thermal drift of the BTO crystal was simultaneously monitored in the laser-induced displacement experiments (see Supporting Information 4) to clarify the role that thermal processes might play in the observed effect. Thus, the maximum thermal drift obtained under extreme illumination conditions (that is, under a light power of 65 mW) was less than 2.0 °C. This increase in temperature produces no remarkable changes in the elongation of the BTO crystal (Supporting Information 4), thereby providing irrefutable proof that local optical heating is not at the core of the lightinduced strain response. A reliable photoactuator should be able to control its lightinduced strain response. In this respect, a set of experiments are carried out to evaluate the strain dependency with the light power, the result of which is shown in Figure 2 (see also B


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less due to the decrease in light power density. This fact confirms a significant finding: the effect propagates along the sample, which means that the light-induced strain is not just a local response of the irradiated area. Once it is proved that the photoresponse propagates along the sample, the following step consists in confirming macroscopically that there exists a light-induced nanoactuation. Contact profilometry is used in detect the light-induced deformation in a millimeter-range area (Figure 4). The laser beam is focused far from the scanning area (Figure 4a), which is selected far from the previously studied zone in order to obtain information from another high-density domain region of the sample. The surface scanning is carried out in a similar way to that previously performed by AFM (Figure 4b). A first scan is performed without illumination to obtain the surface topography, which in this case provides relevant information, given the possible large-scale nonflat surface of the crystal. The topography image (Figure 4c) shows a low rough surface where the domain structure can be easily identified. An extracted line profile confirms a rather flat shape of the BTO surface in dark conditions (Figure 4d). The resulting topography image of the light-induced displacement experiment is shown in Figure 4e, where a clear change in the color scale may be observed; this indicates the occurrence of a net elongation when the light is on, which disappears when the light is off. It is important to note that the light-induced crystal elongation is of the order of 130 nm (Figure 4f), which is a value similar to that obtained from the local response measurements for the same light power (see Figure 2b). This result corroborates that the nanoactuation effect can be macroscopically controlled since it is not a local response of the irradiated area of the material. Thus, the appearance of charge regions at the domain walls that may affect the AFM measurement is discarded, thereby proving the macroscopic response. The use of photonic concepts such as evanescent coupling, optical forces, photonic crystals, and plasmons to achieve nanoactuation based on light triggering require complex architectures at the micro- and nanoscale fabrication in order to obtain the desired effect.16,17 Thus, so far, we have shown that it is possible to control reversible light-induced strain in ferroelectrics. The next step is to consider the scope of our findings for future technologies. The discovered light-driven nanoactuation phenomenon may constitute the basic principle of simple ferroelectric-based devices that transduce optical energy into mechanical deformation in a controllable fashion, thereby providing a real alternative for noncontact devices such as nano-optomechanical systems (NOMS). Future applications in terms of an optical noncontact nanoactuator include nanomanipulation techniques in environments where electrical contacts are unsuitable, such as liquids and ultrahigh vacuum, or whether electromagnetic-noise free environments are required. Indeed, light-driven nanoactuation should be considered as a macroscopic phenomenon enabling a net nano elongation with tunable amplitude by light power control, thereby giving rise to a new generation of noncontact ferroelectric-based nanoactuators driven by light. As regards the physical origin of the phenomenon, the photovoltaic effect can be discarded. Photopolarization and photostriction phenomena as a consequence of photovoltaic effect in ferroelectric materials have been reported for a long time.18−20 However, the photovoltaic effect requires illumination with photon energy above the BTO bandgap (i.e., 3.2 eV).

Figure 2. Tunable photostrain through light power control and its dependence with the crystal thickness. (a) Light power dependence of the net elongation (Δheight) under different light power. A set of topography line profiles (extracted from the AFM images) as a function of the light power is shown. A simplified experimental scheme is shown in the upper part of the image. The light power is controlled by a linear variable filter, ranging from 0 (dark condition) to 65 mW (maximum power of the light source). (b) Evolution of the photoinduced strain as a function of the light power and its dependence with the thickness of the BTO crystal. In both cases, the phototunable strain shows a linear trend with light power. The net elongation depends on the thickness of the BTO crystal. The error bars of the photoinduced strain are not shown because they are lower than the size of the dots.

in eight different positions, thereby illuminating regions of the crystal far from the scanned area (Figure 3a). Figure 3b−d shows the AFM mapping of the same region of the crystal when the light spot illuminates different regions. As may be observed, a net elongation is light-generated but without a direct light incidence (i.e., the monitored region is dark). The measured elongation is almost the same for any position of the laser beam on the sample surface, illuminating a region far from the monitored area (Figure 3e). However, when the light beam does not fall completely on the sample the effect is C


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Figure 3. Extension of the light-induced strain at dark regions. (a) Optical microscopy image of the BTO crystal under study. A pattern (millimeter rule) indicating the points where the light beam is focused is shown inside the optical image. A simplified scheme representing the measurement sequence is shown below. The region marked as a black square (located in the light beam between the positions 1 and 2) indicates the monitored region (i.e., the place where the light-induced strain is monitored). (b−d) Sequence of AFM topography images displaying the reversible lightinduced strain for some positions of the light beam under the same illumination conditions (65 mW). Scale bar, 25 μm. The topography line profile extracted from the AFM image is plotted on the right of each image, revealing the light-dependent strain, according to the off-on−off light cycle. (e) Evolution of the photoinduced strain as a function of the light beam position. The light green regions represent the positions where the light beam falls partially outside the crystal surface, whereas the green region indicates the positions where the light beam falls completely on the surface.

effect, which is well-known in solid-state systems.27−29 In the presence of this potential, an alternating external force induces a net motion of domain walls in a certain direction determined by the asymmetry of the potential. In summary, the illumination causes nonequilibrium at the CDWs, which modifies the pressure of the wall that then leads to the motion of domain walls. Lastly, regarding the fact that the effect propagates along the material, the cooperative nature of domain wall motion could be considered. Moreover, the semitransparent nature of the sample may contribute to the light diffusion across it. The BTO is a negative uniaxial crystal with relatively high refractive index n = 2.42.30 This may produce a near-total light reflection on the major faces of the crystal. The BTO thus behaves as a waveguide that confines the light, allowing its propagation along the crystal, which could also account for the fact that the ferroelectric crystal extends the light-induced strain state to the dark region of the material. In any event, triggering a net elongation in zones far from where the illumination occurs could pave the way to new applications of optical devices.

In this work, a green 532 nm laser diode is used as light source, which involves a photon energy below the crystal bandgap (i.e., 2.3 eV). It has been reported that the BTO bandgap is somehow reduced at domain walls in ∼0.1−0.2 eV,21 but this reduction could not yield a photovoltaic effect in the experiments reported herein either. Therefore, the lightinduced domain rearrangement of the BTO crystal should be considered as the mechanism responsible for the photostrain. The a−c CDWs are hindered by b domains to stress release.5,22 The inclusion of b domain leads to a symmetry breaking, which is related to the appearance of long-range strain fields.23 The light-induced local electric field engenders a reversible transformation of the domain structure so that the volume of c and b domains increases at the expense of the a domains volume (Supporting Information 7). As a consequence of this light-induced ferroelectric domain switching, a net elongation is obtained. The reversible rearrangement of domains may originate by a light-driven domain wall motion, as was recently hypothesized.6 The accumulated charge at CDWs appears to compensate the bound polarization charges from the domains to make the net pressure over the wall zero.12 This charge exhibits metallic conductivity and consequently can be regarded as “locally free charges” in a way similar to a 2D electron gas at interfaces.13 The charge accumulation leads to a modification of the energy bands in the BTO,24 thus creating an asymmetric saw-teeth potential.11,25,26 This type of asymmetric potential is known to produce the so-called ratchet

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b01628. Materials and methods, domain structure characterization of the (100)-oriented BaTiO3 crystal, AFM D


Letter

ACS Applied Materials & Interfaces Author Contributions

F.R.-M., J.E.G., and J.F.F. planned the research and designed the experiments. D.P.-M., D.A.O., A.D.C., and F.R.-M. performed the experiments. Data processing was carried out by D.P.-M., A.D.C., and F.R.-M. All authors contributed to the discussion of the results. The manuscript was written by F.R.M. and J.E.G. with input from D.P.-M. and J.F.F. The work was supervised by J.E.G. and J.F.F. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the MINECO (Spanish Government) project MAT2017-86450-C4-1-R. F.R-M is indebted to MINECO for a “Ramon y Cajal” contract (ref: RyC-201518626), which is cofinanced by the European Social Fund. F.RM also acknowledges support from a 2018 Leonardo Grant for Researchers and Cultural Creators (BBVA Foundation). The authors thank Dr. Trifon Trifonov (Barcelona Research Center in Multiscale Science and Engineering) for technical support in the profilometry measurements.

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(1) Narayan, B.; Malhotra, J. S.; Pandey, R.; Yaddanapudi, K.; Nukala, P.; Dkhil, B.; Senyshyn, A.; Ranjan, R. Electrostrain in Excess of 1% in Polycrystalline Piezoelectrics. Nat. Mater. 2018, 17, 427− 431. (2) You, Y.-M.; Liao, W.-Q.; Zhao, D.; Ye, H.-Y.; Zhang, Y.; Zhou, Q.; Niu, X.; Wang, J.; Li, P.-F.; Fu, D.-W.; Wang, Z.; Gao, S.; Yang, K.; Liu, J.-M.; Li, J.; Yan, Y.; Xiong, R.-G. An Organic-Inorganic Perovskite Ferroelectric with Large Piezoelectric Response. Science 2017, 357, 306−309. (3) Liao, W.-Q.; Tang, Y.-Y.; Li, P.-F.; You, Y.-M.; Xiong, R.-G. Large Piezoelectric Effect in a Lead-Free Molecular Ferroelectric Thin Film. J. Am. Chem. Soc. 2017, 139, 18071−18077. (4) Liao, W.-Q.; Zhang, Y.; Hu, C.-L.; Mao, J.-G.; Ye, H.-Y.; Li, P.F.; Huang, S. D.; Xiong, R.-G. A Lead-Halide Perovskite Molecular Ferroelectric Semiconductor. Nat. Commun. 2015, 6, 7338. (5) Rubio-Marcos, F.; Del Campo, A.; Marchet, P.; Fernández, J. F. Ferroelectric Domain Wall Motion Induced by Polarized Light. Nat. Commun. 2015, 6, 6594. (6) Rubio-Marcos, F.; Ochoa, D. A.; Del Campo, A.; García, M. A.; Castro, G. R.; Fernández, J. F.; García, J. E. Reversible Optical Control of Macroscopic Polarization in Ferroelectrics. Nat. Photonics 2018, 12, 29−32. (7) Páez-Margarit, D.; Rubio-Marcos, F.; Ochoa, D. A.; Del Campo, A.; Fernández, J. F.; García, J. E. Light-Induced Capacitance Tunability in Ferroelectric Crystals. ACS Appl. Mater. Interfaces 2018, 10, 21804−21807. (8) Yang, M.-M.; Alexe, M. Light-Induced Reversible Control of Ferroelectric Polarization in BiFeO3. Adv. Mater. 2018, 30, 1704908. (9) Akamatsu, H.; Yuan, Y.; Stoica, V. A.; Stone, G.; Yang, T.; Hong, Z.; Lei, S.; Zhu, Y.; Haislmaier, R. C.; Freeland, J. W.; Chen, L.-Q.; Wen, H.; Gopalan, V. Light-Activated Gigahertz Ferroelectric Domain Dynamics. Phys. Rev. Lett. 2018, 120, 096101. (10) McGilly, L. J.; Yudin, P.; Feigl, L.; Tagantsev, A. K.; Setter, N. Controlling Domain Wall Motion in Ferroelectric Thin Films. Nat. Nanotechnol. 2015, 10, 145−150. (11) Yang, S. Y.; Seidel, J.; Byrnes, S. J.; Shafer, P.; Yang, C.-H.; Rossell, M. D.; Yu, P.; Chu, Y.-H.; Scott, J. F.; Ager, J. W.; Martin, L. W.; Ramesh, R. Above-Bandgap Voltages from Ferroelectric Photovoltaic Devices. Nat. Nanotechnol. 2010, 5, 143−147. (12) Mokrý, P.; Tagantsev, A. K.; Fousek, J. Pressure on Charged Domain Walls and Additional Imprint Mechanism in Ferroelectrics. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 75, 094110.

Figure 4. Evidence of the light-activated nanoactuation at the macroscopic scale. (a) Optical image of the surface of a 500 μm thick BTO crystal. The 1.0 mm × 0.5 mm region indicated by a red square shows the area where x−y profilometry images are taken. The light spot is drawn at the place where the crystal is illuminated when the laser is on. Scale bar = 1 mm. (b) Closer optical image of the scanned area, indicating the slow and fast, x−y, scan directions of the profilometry measurement. Scale bar = 250 μm. (c) Contact profilometry mapping of the scanned area without illumination. (d) The x−z extracted profile along the dashed line, showing a relatively flat surface. (e) Contact profilometry mapping of the scanned area in a sequence of off−on−off illumination. The light is switched on and off at the positions indicated by the dotted lines. (f) The x−z extracted profile along the dashed line shows a net elongation of the region as compared with the off region, thereby evidencing a lightactivated deformation phenomenon.

measurements in nonferroelectric samples, thermal characterization, light-activated strain tunability, domain structure of the (100)-oriented BaTiO3 single crystal with reduced thickness, and light-induced self-organization of symmetry breaking of domain (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (F.R-M.). *E-mail: [email protected] (J.E.G.). ORCID

Fernando Rubio-Marcos: 0000-0002-2479-3792 José F. Fernández: 0000-0001-5894-9866 José E. García: 0000-0002-1232-1739 E


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Photo-Controlled Ferroelectric-Based Nanoactuators - ACS Applied

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