David Ehre

Water Freezes Differently on Positively and Negatively Charged Surfaces of Pyroelectric Materials

Freezing Supercool Water Under equilibrium conditions, water will freeze at 0°C, but, under certain conditions, it can be kept in a supercooled liquid form below this temperature. Ehre et al. (p. 672) present a careful and detailed study of the freezing of water drops on both positively and negatively charged pyroelectric surfaces using a combination of optical microscopy and x-ray diffraction: Supercooled water froze at different temperatures, depending on the charge of the substrate with the initial freezing occurring at the liquid-substrate interface on a positively charged substrate and at the air-water interface on a negatively charged substrate. Thus, freezing could be induced upon heating when the substrate charge also changed from negative to positive. Supercooled water on a surface can freeze upon heating in response to surface charge switching from negative to positive. Although ice melts and water freezes under equilibrium conditions at 0°C, water can be supercooled under homogeneous conditions in a clean environment down to –40°C without freezing. The influence of the electric field on the freezing temperature of supercooled water (electrofreezing) is of topical importance in the living and inanimate worlds. We report that positively charged surfaces of pyroelectric LiTaO3 crystals and SrTiO3 thin films promote ice nucleation, whereas the same surfaces when negatively charged reduce the freezing temperature. Accordingly, droplets of water cooled down on a negatively charged LiTaO3 surface and remaining liquid at –11°C freeze immediately when this surface is heated to –8°C, as a result of the replacement of the negative surface charge by a positive one. Furthermore, powder x-ray diffraction studies demonstrated that the freezing on the positively charged surface starts at the solid/water interface, whereas on a negatively charged surface, ice nucleation starts at the air/water interface.

Tetragonal CH3NH3PbI3 is ferroelectric

Significance Halide perovskite (HaP) semiconductors are revolutionizing the field of photovoltaic (PV) solar energy conversion by showing remarkable performance of solar cells made with HaPs. “Ferroelectrics” is one frequently suggested reason because it may allow the spatial separation of the flow of electrons from where they were generated (holes). Unlike common, electrically insulating, ferroelectric materials, HaPs [especially tetragonal methylammonium lead triiodide (MAPbI3)] are semiconducting, and to find out whether they are ferroelectric requires an approach that is different from what is done customarily. Using such an approach, we prove that tetragonal MAPbI3 is definitely ferroelectric. What still remains to be seen is whether this ferroelectric nature is important for how MAPbI3-based solar cells operate around room temperature. Halide perovskite (HaP) semiconductors are revolutionizing photovoltaic (PV) solar energy conversion by showing remarkable performance of solar cells made with HaPs, especially tetragonal methylammonium lead triiodide (MAPbI3). In particular, the low voltage loss of these cells implies a remarkably low recombination rate of photogenerated carriers. It was suggested that low recombination can be due to the spatial separation of electrons and holes, a possibility if MAPbI3 is a semiconducting ferroelectric, which, however, requires clear experimental evidence. As a first step, we show that, in operando, MAPbI3 (unlike MAPbBr3) is pyroelectric, which implies it can be ferroelectric. The next step, proving it is (not) ferroelectric, is challenging, because of the material’s relatively high electrical conductance (a consequence of an optical band gap suitable for PV conversion) and low stability under high applied bias voltage. This excludes normal measurements of a ferroelectric hysteresis loop, to prove ferroelectricity’s hallmark switchable polarization. By adopting an approach suitable for electrically leaky materials as MAPbI3, we show here ferroelectric hysteresis from well-characterized single crystals at low temperature (still within the tetragonal phase, which is stable at room temperature). By chemical etching, we also can image the structural fingerprint for ferroelectricity, polar domains, periodically stacked along the polar axis of the crystal, which, as predicted by theory, scale with the overall crystal size. We also succeeded in detecting clear second harmonic generation, direct evidence for the material’s noncentrosymmetry. We note that the material’s ferroelectric nature, can, but need not be important in a PV cell at room temperature.

Tetragonal CH3NH3PbI3is ferroelectric

Significance Halide perovskite (HaP) semiconductors are revolutionizing the field of photovoltaic (PV) solar energy conversion by showing remarkable performance of solar cells made with HaPs. “Ferroelectrics” is one frequently suggested reason because it may allow the spatial separation of the flow of electrons from where they were generated (holes). Unlike common, electrically insulating, ferroelectric materials, HaPs [especially tetragonal methylammonium lead triiodide (MAPbI3)] are semiconducting, and to find out whether they are ferroelectric requires an approach that is different from what is done customarily. Using such an approach, we prove that tetragonal MAPbI3 is definitely ferroelectric. What still remains to be seen is whether this ferroelectric nature is important for how MAPbI3-based solar cells operate around room temperature. Halide perovskite (HaP) semiconductors are revolutionizing photovoltaic (PV) solar energy conversion by showing remarkable performance of solar cells made with HaPs, especially tetragonal methylammonium lead triiodide (MAPbI3). In particular, the low voltage loss of these cells implies a remarkably low recombination rate of photogenerated carriers. It was suggested that low recombination can be due to the spatial separation of electrons and holes, a possibility if MAPbI3 is a semiconducting ferroelectric, which, however, requires clear experimental evidence. As a first step, we show that, in operando, MAPbI3 (unlike MAPbBr3) is pyroelectric, which implies it can be ferroelectric. The next step, proving it is (not) ferroelectric, is challenging, because of the material’s relatively high electrical conductance (a consequence of an optical band gap suitable for PV conversion) and low stability under high applied bias voltage. This excludes normal measurements of a ferroelectric hysteresis loop, to prove ferroelectricity’s hallmark switchable polarization. By adopting an approach suitable for electrically leaky materials as MAPbI3, we show here ferroelectric hysteresis from well-characterized single crystals at low temperature (still within the tetragonal phase, which is stable at room temperature). By chemical etching, we also can image the structural fingerprint for ferroelectricity, polar domains, periodically stacked along the polar axis of the crystal, which, as predicted by theory, scale with the overall crystal size. We also succeeded in detecting clear second harmonic generation, direct evidence for the material’s noncentrosymmetry. We note that the material’s ferroelectric nature, can, but need not be important in a PV cell at room temperature.

CH3NH3PbBr3 is not pyroelectric, excluding ferroelectric-enhanced photovoltaic performance

To experimentally (dis)prove ferroelectric effects on the properties of lead-halide perovskites and of solar cells, based on them, we used second-harmonic-generation spectroscopy and the periodic temperature change (Chynoweth) technique to detect the polar nature of methylammonium lead bromide (MAPbBr3). We find that MAPbBr3 is probably centrosymmetric and definitely non-polar; thus, it cannot be ferroelectric. Whenever pyroelectric-like signals were detected, they could be shown to be due to trapped charges, likely at the interface between the metal electrode and the MAPbBr3 semiconductor. These results indicate that the ferroelectric effects do not affect steady-state performance of MAPbBr3 solar cells.

Water-Induced Pyroelectricity from Nonpolar Crystals of Amino Acids†

Pyroelectricity, a property of certain crystalline materials, isthe creation of a temporary surface charge upon temperaturechange, resulting in an external electric current. Thermalmovement of the molecules alters their average positionsleading to changes in the crystal dipole moments. The Greekphilosopher Theophrastus discovered this property in themineral tourmaline already in 314 BC. Since then, thisphenomenon has led to a plethora of applications, such asnightvisiondevices,burglaralarms,andportablehigh-voltagegenerators.

Unusually Large Young's Moduli of Amino Acid Molecular Crystals.

Youngs moduli of selected amino acid molecular crystals were studied both experimentally and computationally using nanoindentation and dispersion-corrected density functional theory. The Young modulus is found to be strongly facet-dependent, with some facets exhibiting exceptionally high values (as large as 44 GPa). The magnitude of Youngs modulus is strongly correlated with the relative orientation between the underlying hydrogen-bonding network and the measured facet. Furthermore, we show computationally that the Young modulus can be as large as 70-90 GPa if facets perpendicular to the primary direction of the hydrogen-bonding network can be stabilized. This value is remarkably high for a molecular solid and suggests the design of hydrogen-bond networks as a route for rational design of ultra-stiff molecular solids.

Origin and structure of polar domains in doped molecular crystals

Doping is a primary tool for the modification of the properties of materials. Occlusion of guest molecules in crystals generally reduces their symmetry by the creation of polar domains, which engender polarization and pyroelectricity in the doped crystals. Here we describe a molecular-level determination of the structure of such polar domains, as created by low dopant concentrations (<0.5%). The approach comprises crystal engineering and pyroelectric measurements, together with dispersion-corrected density functional theory and classical molecular dynamics calculations of the doped crystals, using neutron diffraction data of the host at different temperatures. This approach is illustrated using centrosymmetric α-glycine crystals doped with minute amounts of different L-amino acids. The experimentally determined pyroelectric coefficients are explained by the structure and polarization calculations, thus providing strong support for the local and global understanding of how different dopants influence the properties of molecular crystals.

Source of Electrofreezing of Supercooled Water by Polar Crystals

Polar crystals, which display pyroelectricity, have a propensity to elevate, in a heterogeneous nucleation, without epitaxy, the freezing temperature of supercooled water (SCW). Upon cooling, such crystals accumulate an electric charge at their surfaces, which creates weak electric fields, <kV·cm(-1), that are thousands of times lower than necessary for inducing homogeneous ice nucleation. By performing comparative freezing experiments of SCW on the same surfaces of three different polar crystals of amino acids, we demonstrate that preventing the formation of charge at these surfaces, by linking the two hemihedral faces of the polar crystals with a conducting paint, reduces the temperature of freezing by 2-5 °C. The temperature of ice nucleation was found to be correlated with the amount of the surface charge, thus implying that the surface-charge-induced interactions affect the interfacial water molecules that trigger freezing at a higher temperature. This finding is in contrast to previous hypotheses, which attribute the enhanced SCW freezing to the effect of the electric field or capture of external ions or particles. Possible implications of this mechanism of freezing are presented.

Pyroelectric Measurement of Surface Layer: The Case of Thin Film on Dielectric Substrate

We propose an extension of the periodic temperature change (Chynoweth) technique for the measurement of pyroelectric coefficient in the case of a pyroelectric thin film on an insulating (non-conductive) substrate. The modified technique adequately determines the pyroelectric coefficient of the film if its thickness is known. The method determines the pyroelectric coefficient of the substrate even if it is much smaller than that of the film. The method overestimates the thickness of the pyroelectric film and can be used as an estimate only. If the thickness of the pyroelectric film is not known, the method gives a product of the pyroelectric coefficient of the film and its thickness.

The Contribution of Pyroelectricity of AgI Crystals to Ice Nucleation

The pyroelectricity of AgI crystals strongly affects the icing temperature of super-cooled water, as disentangled from that of epitaxy. This deduction was achieved by the design of polar crystalline ceramic pellets of AgI, with experimentally determined sense of polarity. These pellets are suitable for measuring both their pyroelectric properties as well as the icing temperature of super-cooled water, separately on each of the expressed Ag+ and I- hemihedral surfaces. The positive pyroelectric charge at the silver-enriched side elevates the icing temperature, whereas the negative charge at the iodide side decreases that temperature. Moreover, the effect of pyroelectric charge remains dominant despite the presence of contaminants on both the silver and the iodide-enriched surfaces. Consequently an electrochemical process for ice nucleation is suggested, which might be of relevance for understanding the role played by electric charges in heterogeneous icing processes in general.