First detailed in 1987 and since 2009 has been applied successfully in photovoltaic (PV) devices,1,2 Methylammonium lead iodide (MAPbI3) crystallizes in a tetragonal perovskite structure at room temperature.3
Due to the fact that perovskite materials typically feature ferroic properties and MAPbI3 has an additional polar methylammonium center-ion (2.3 D),4 researchers have extensively debated a potential ferroelectric and ferroelastic nature of the material.5–7
Ferroelectricity stems from a change in crystal symmetry when a ferroelectric phase transition occurs; this is where the loss of the center symmetry leads to a sudden electrical polarization.
To reduce the electrostatic energy, the crystal form parallelly aligned domains of polarization. Depending on the orientation of the domain polarization with respect to the walls between various domains, the walls can conduct a bound charge.
This charge can be compensated by free charge carriers fitted to conducting 2D sheets.8 For MAPbI3, theoreticians indicated that domain walls with alternating charges could act as charge-selective carrier pathways, reducing the recombination in the PV material.5
Ferroelasticity is attributed to a change in the crystal system, for example, from a cubic to a tetragonal structure during a ferroelastic phase transition.
The distortion of the unit cell establishes internal stress in the material that decreases through the formation of domains with fluctuating unit cell orientations.8
The structural anomalies at the domain walls among ferroelastic domains can create a local loss of center symmetry and, therefore, produce a domain wall polarization.9 For MAPbI3, theoreticians suggested a similar effect which is advantageous for directed charge carrier extraction.10
The classification and characterization of ferroelectric domains necessitate a contact mode Atomic Force Microscopy (AFM) technique - Piezoelectric Force Microscopy (PFM) - that resolves local electromechanical responses with a nanometer resolution.11,12
In PFM, the surface of a ferroelectric sample is scanned using a conductive tip fitted to a cantilever while introducing an AC voltage between the tip and a back‑electrode beneath the sample. Since all ferroelectric materials are piezoelectric, the applied AC voltage applies a periodic deformation of the sample, known as piezoresponse (Figure 1a).
Dependent on the polarization orientation in the domains below the tip, for domains with a polarization perpendicular to the sample surface, the piezoresponse is either in-phase or 180° out‑of‑phase with the applied AC voltage.11
PFM introduces low frequency AC voltages in its basic configuration. This technique referred to as off‑resonance PFM, is less vulnerable to topographic crosstalk since it functions far away from the cantilever’s contact resonance (3 - 5 times of the free resonance).
Materials with frail inherent piezoresponse or thin films, however, may need an increase of the signal-to-noise ratio by selecting an AC voltage close or at the contact resonance of the cantilever.11
Although this advanced single frequency resonance PFM technique enhances the sensitivity to the piezoresponse, the signal detected becomes more vulnerable to crosstalk, for example, from topography and sample mechanics.
The frequency of the contact resonance is heavily reliant on a stable and unmodified tip‑sample contact, which is difficult to attain while scanning, especially on samples with a lot of topography or heterogeneous nanomechanics as anticipated on domains with a ferroelastic nature.13
Figure 1. a) Schematic working principle of Piezoelectric Force Microscopy (PFM), where an AC voltage introduces a periodic sample oscillation, i.e., the piezoresponse, during a contact mode scan. The piezoresponse is detected via optical beam deflection using a superluminscent diode and a position sensitive photodetector. b) Amplitude of the contact resonance CR1 at frequency f1 in the frequency spectrum of the AC excitation with the two sidebands SB generated at f1±f1m used for the Dual Frequency Resonance Tracking (DFRT). Image Credit: Park Systems Europe
Dual Frequency Resonance Tracking (DFRT), available by combining a Park Systems NX10 AFM with a Zurich Instruments HF2 Lock‑In Amplifier (see Application Note 56), facilitates the differentiation between true electromechanical signal contributions and mechanical or topographic crosstalk utilizing supplementary feedback that tracks the contact resonance.
The HF2 produces sidebands left and right of the contact resonance at frequencies determined by the bandwidth at half maximum of the contact resonance for DFRT (Figure 1b).
The feedback contrasts the amplitudes of both sidebands continuously and modifies the frequency of the AC voltage to maintain a constant amplitude ratio during the PFM scan.
Thus, DFRT not only adds another signal channel for local visualization of the contact resonance distribution, which enables qualitative resolution of mechanical heterogeneities,13 but also allows a significant reduction of crosstalk
The piezoresponse was imaged on a MAPbI3 film via DFRT and resolved an out-of-plane domain with specific PFM amplitude and phase signals as well as a corresponding shift in the contact resonance frequency for this application note.
The frequency signal demonstrates a mechanical comparison between the domain and the bulk grain, indicating a ferroelastic character of the domain.
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- Poglitsch, A. & Weber, D. Dynamic disorder in methylammoniumtrihalogenoplumbates (II) observed by millimeter‐wave spectroscopy. J. Chem. Phys. 87, 6373–6378 (1987).
- Kojima, A., Teshima, K., Shirai, Y. & Miyasaka, T. Organometal halide perovskites as visible-light sensitizers for photovoltaic cells. J. Am. Chem. Soc. 131, 6050–6051 (2009).
- Stoumpos, C. C., Malliakas, C. D. & Kanatzidis, M. G. Semiconducting tin and lead iodide perovskites with organic cations: phase transitions, high mobilities, and near-infrared photoluminescent properties. Inorg. Chem. 52, 9019–9038 (2013).
- Wilson, J. N., Frost, J. M., Wallace, S. K. & Walsh, A. Dielectric and ferroic properties of metal halide perovskites. APL Mater. 7, 10901 (2019).
- Frost, J. M. et al. Atomistic origins of high-performance in hybrid halide perovskite solar cells. Nano Lett. 14, 2584–2590 (2014).
- Röhm, H., Leonhard, T., Hoffmann, M. J. & Colsmann, A. Ferroelectric domains in methylammonium lead iodide perovskite thin-films. Energy Environ. Sci. 10, 950–955 (2017).
- Hermes, I. M. et al. Ferroelastic Fingerprints in Methylammonium Lead Iodide Perovskite. J. Phys. Chem. C 120, (2016).
- Tagantsev, A. K., Cross, L. E. & Fousek, J. Domains in ferroic crystals and thin films. 13, (Springer, 2010).
- Zubko, P., Catalan, G., Buckley, A., Welche, P. R. L. & Scott, J. F. Strain-gradient-induced polarization in SrTiO 3 single crystals. Phys. Rev. Lett. 99, 167601 (2007).
- Warwick, A. R., Íñiguez, J., Haynes, P. D. & Bristowe, N. C. First-principles study of ferroelastic twins in halide perovskites. J. Phys. Chem. Lett. 10, 1416–1421 (2019).
- Soergel, E. Piezoresponse force microscopy (PFM). J. Phys. D Appl. Phys. 44, 464003 (2011).
- Güthner, P. & Dransfeld, K. Local poling of ferroelectric polymers by scanning force microscopy. Appl. Phys. Lett. 61, 1137–1139 (1992).
- Rodriguez, B. J., Callahan, C., Kalinin, S. V & Proksch, R. Dual-frequency resonance-tracking atomic force microscopy. Nanotechnology 18, 475504 (2007).
This information has been sourced, reviewed and adapted from materials provided by Park Systems Europe.
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