Strain tuning of ferroelectric polarization in hybrid organic inorganic perovskite compounds

Temperature and volumetric effects on structural and dielectric properties of hybrid perovskites

Three-dimensional organic-inorganic perovskites are rapidly evolving materials with diverse applications. This study focuses on their two representatives - acetamidinium manganese(II) formate (AceMn) and formamidinium manganese(II) formate (FMDMn) - subjected to varying temperature and pressure. We show that AceMn undergoes atypical pressure-induced structural transformations at room temperature, increasing the symmetry from ambient-pressure P21/n phase II to the high-pressure Pbca phase III. In turn, FMDMn in its C2/c phase II displays temperature- and pressure-induced ordering of cage cations that proceeds without changing the phase symmetry or energy barriers. The FMD+ cations do not order under constant volume across the pressure-temperature plane, despite similar pressure and temperature evolution of the unit-cell parameters. Temperature and pressure affect the cage cations differently, which is particularly pronounced in their relaxation dynamics seen by dielectric spectroscopy. Their motion require a rearrangement of the metal-formate framework, resulting in the energy and volumetric barriers defined by temperature-independent activation energy and activation volume parameters. As this process is phonon-assisted, the relaxation time is strongly temperature-dependent. Consequently, relaxation times do not scale with unit-cell volume nor H-bond lengths in formates, offering the possibility of tuning their electronic properties by external stimuli (like temperature or pressure) even without any structural changes.

Facile Control of Ferroelectricity Driven by Ingenious Interaction Engineering

Construction of ferroelectric and optimization of macroscopic polarization has attracted tremendous attention for next generation light weight and flexible devices, which brings fundamental vitality for molecular ferroelectrics. However, effective molecular tailoring toward cations makes ferroelectric synthesis and modification relatively elaborate. Here, the study proposes a facile method to realize triggering and optimization of ferroelectricity. The experimental and theoretical investigation reveals that orientation and alignment of polar cations, dominated factors in molecular ferroelectrics, can be controlled by easily processed anionic modification. In one respect, ferroelectricity is induced by strengthened intermolecular interaction. Moreover, ≈50% of microscopic polarization enhancement (from 8.07 to 11.68 µC cm-2 ) and doubling of equivalent polarization direction (from 4 to 8) are realized in resultant ferroelectric FEtQ2ZnBrI3 (FEQZBI, FEtQ = N-fluoroethyl-quinuclidine). The work offers a totally novel platform for control of ferroelectricity in organic-inorganic hybrid ferroelectrics and a deep insight of structure-property correlations.

Lead-free 2D MASnBr3 and Ruddlesden-Popper BA2MASn2Br7 as light harvesting materials

We have explored the structural, electronic, charge transport, and optical properties of lead-free 2D hybrid halide perovskites, MASnBr₃ and Ruddlesden-Popper perovskites, BAMASn₂Br₇ monolayers. Under density functional theory (DFT) calculation, we applied mechanical strain, i.e., tensile and compressive strain up to 10% in both cases. The mechanical strain engineering technique is useful for a tuned bandgap of 2D MASnBr₃ and 2D BAMASn₂Br₇. The calculated carrier mobility for the electron is 404 cm² V^-1 s^-1 and for the hole is up to 800 cm² V^-1 s^-1 for MASnBr₃. For BAMASn₂Br₇ the highest carrier mobility is up to 557 cm² V^-1 s^-1 for electrons and up to 779 cm² V^-1 s^-1 for the hole, which is 14% and 24% higher than the reported lead-iodide based perovskites, respectively. The calculated solar cell efficiency of 2D MASnBr₃ is 23.46%, which is 18% higher than the reported lead-based perovskites. Furthermore, the optical activity of the 2D MASnBr₃ and 2D BAMASn₂Br₇ shows a high static dielectric constant of 2.48 and 2.14, respectively. This is useful to show nanodevice performance. Also, 2D MASNBr₃ shows a high absorption coefficient of 15.25 × 10⁵ cm^-1 and 2D BAMASn₂Br₇ shows an absorption coefficient of up to 13.38 × 10⁵ cm^-1. Therefore our theoretical results suggest that the systems are under mechanical strain engineering. This is convenient for experimentalists to improve the performance of the 2D perovskites. The study supports these materials as good candidates for photovoltaic and optoelectronic device applications.

Ferroelectric hybrid organic-inorganic perovskites and their structural and functional diversity

Molecular ferroelectrics have gradually aroused great interest in both fundamental scientific research and technological applications because of their easy processing, light weight and mechanical flexibility. Hybrid organic-inorganic perovskite ferroelectrics (HOIPFs), as a class of molecule-based ferroelectrics, have diverse functionalities owing to their unique structure and have become a hot spot in molecular ferroelectrics research. Therefore, they are extremely attractive in the field of ferroelectrics. However, there seems to be a lack of systematic review of their design, performance and potential applications. Herein, we review the recent development of HOIPFs from lead-based, lead-free and metal-free perovskites, and outline the versatility of these ferroelectrics, including piezoelectricity for mechanical energy-harvesting and optoelectronic properties for photovoltaics and light detection. Furthermore, a perspective view of the challenges and future directions of HOIPFs is also highlighted.

Ferroelasticity in Organic-Inorganic Hybrid Perovskites

Molecular ferroelastics have received particular attention for potential applications in mechanical switches, shape memory, energy conversion, information processing, and solar cells, by taking advantages of their low-cost, light-weight, easy preparation, and mechanical flexibility. The unique structures of organic-inorganic hybrid perovskites have been considered to be a design platform for symmetry-breaking-associated order-disorder in lattice, thereby possessing great potential for ferroelastic phase transition. Herein, we review the research progress of organic-inorganic hybrid perovskite ferroelastics in recent years, focusing on the crystal structures, dimensions, phase transitions and ferroelastic properties. In view of the few reports on molecular-based hybrid ferroelastics, we look forward to the structural design strategies of molecular ferroelastic materials, as well as the opportunities and challenges faced by molecular-based hybrid ferroelastic materials in the future. This review will have positive guiding significance for the synthesis and future exploration of organic-inorganic hybrid molecular ferroelastics.

Negative Linear Compressibility in Organic-Inorganic Hybrid Perovskite [NH2NH3]X(HCOO)3 (X = Mn, Fe, Co)

Hybrid organic-inorganic perovskites [NH₂NH₃][X(HCOO)₃] (X = Mn, Fe, Co) have a so-called "wine-rack" type of geometry that could give origin to the rare property of negative linear compressibility, which is an exotic and highly desirable material response. We use first-principles density functional theory computations to probe the response of these materials to hydrostatic pressure and predict that, indeed, all three of them exhibit negative linear compressibility above a critical pressure of 1 GPa. Calculations reveal that, under pressure, XO₆ octahedra and -HCOO ligands remain relatively rigid while XO₆ octahedra tilt significantly, which leads to highly anisotropic mechanical properties and expansion along certain directions. These trends are common for the three materials considered.

Inorganic-Organic Hybrid Molecular Materials: From Multiferroic to Magnetoelectric

Inorganic-organic hybrid molecular multiferroic and magnetoelectric materials, similar to multiferroic oxide compounds, have recently attracted increasing attention because they exhibit diverse architectures, a flexible framework, fascinating physics, and potential magnetoelectric functionalities in novel multifunctional devices such as energy transformation devices, sensors, and information storage systems. Herein, the classification of multiferroicity and magnetoelectricity is briefly outlined and then the recent advances in the multiferroicity and magnetoelectricity of inorganic-organic hybrid molecular materials, particularly magnetoelectricity and the relevant magnetoelectric mechanisms and their categories are summarized. In addition, a personal perspective and an outlook are provided.

Thin Layer Buckling in Perovskite CsPbBr3 Nanobelts

Flexible semiconductor materials, where structural fluctuations and transformation are tolerable and have low impact on electronic properties, focus interest for future applications. Two-dimensional thin layer lead halide perovskites are hailed for their unconventional optoelectronic features. We report structural deformations via thin layer buckling in colloidal CsPbBr₃ nanobelts adsorbed on carbon substrates. The microstructure of buckled nanobelts is determined using transmission electron microscopy and atomic force microscopy. We measured significant decrease in emission from the buckled nanobelt using cathodoluminescence, marking the influence of such mechanical deformations on electronic properties. By employing plate buckling theory, we approximate adhesion forces between the buckled nanobelt and the substrate to be F_adhesion ∼ 0.12 μN, marking a limit to sustain such deformation. This work highlights detrimental effects of mechanical buckling on electronic properties in halide perovskite nanostructures and points toward the capillary action that should be minimized in fabrication of future devices and heterostructures based on nanoperovskites.

Indirect but Efficient: Laser-Excited Electrons Can Drive Ultrafast Polarization Switching in Ferroelectric Materials

To enhance the efficiency of next-generation ferroelectric (FE) electronic devices, new techniques for controlling ferroelectric polarization switching are required. While most prior studies have attempted to induce polarization switching via the excitation of phonons, these experimental techniques required intricate and expensive terahertz sources and have not been completely successful. Here, we propose a new mechanism for rapidly and efficiently switching the FE polarization via laser-tuning of the underlying dynamical potential energy surface. Using time-dependent density functional calculations, we observe an ultrafast switching of the FE polarization in BaTiO₃ within 200 fs. A laser pulse can induce a charge density redistribution that reduces the original FE charge order. This excitation results in both desirable and highly directional ionic forces that are always opposite to the original FE displacements. Our new mechanism enables the reversible switching of the FE polarization with optical pulses that can be produced from existing 800 nm experimental laser sources.

Out-of-Plane Mechanical Properties of 2D Hybrid Organic-Inorganic Perovskites by Nanoindentation

Two-dimensional (2D) layered hybrid organic-inorganic perovskites (HOIPs) have demonstrated improved stability and promising photovoltaic performance. The mechanical properties of such functional materials are both fundamentally and practically important to achieve both high performance and mechanically stable (flexible) devices. Here, we report the mechanical properties of a series of 2D layered lead iodide HOIPs and investigate the role of structural subunits (e.g., variation of the length of the organic spacer molecules, R and the number of inorganic layers, n) in the mechanical properties. Although 2D HOIPs have much lower nominal elastic modulus and hardness than 3D HOIPs, larger n number and shorter R lead to stiffer materials. Density functional theory simulations showed a trend similar to the experimental results. We compared these findings with other 2D layered crystals and shed light on routes to further tune the out-of-plane mechanical properties of 2D layered HOIPs.

Strain induced electronic structure variation in methyl-ammonium lead iodide perovskite

Methyl-ammonium lead iodide perovskite (CH₃NH₃PbI₃) has drawn great attention due to its excellent photovoltaic properties. Because of its loosely compacted structure, the structural, electronic and optical properties of CH₃NH₃PbI₃ are sensitive to external modulations. Strain effects on CH₃NH₃PbI₃ are fully investigated by the first principles calculations. The results indicate that the inorganic framework deforms under compression or stretch and the embedded organic CH₃NH₃+ molecules rotate correspondingly. A band gap oscillation and a new structural phase in response to the external strain were observed for the first time. These phenomena are explained with the nonlinear structural deformation and phase transition under the external strains. The semi-quantitative relationship between the band gap variation and geometry change under the external strain is obtained. We found that the shift of valence band maximum under the external strain is mostly determined by the most stretched or compressed Pb-I bond of CH₃NH₃PbI₃, and the shift of the conduction band minimum under the external strain is likely to be determined by the largest Pb-I-Pb bond angle in the system. These results are important for understanding of strain effects on semiconductors and guiding the experiments to improve the performance of the perovskite solar cells.

Effect of solvent, temperature and pressure on the stability of chiral and perovskite metal formate frameworks of [NH2NH3][M(HCOO)3] (M = Mn, Fe, Zn)

We report the synthesis, crystal structure, and thermal, Raman, infrared and magnetic properties of [NH₂NH₃][M(HCOO)₃] (HyM) compounds (M = Mn, Zn, Fe). Our results show that synthesis from methanol solution leads to perovskite polymorphs while that from 1-methyl-2-pyrrolidinone or its mixture with methanol allows obtaining chiral polymorphs. Perovskite HyFe, chiral HyFe and chiral HyMn undergo phase transitions at 347, 336 and 296 K, respectively, with symmetry changes from Pnma to Pna2₁, P6₃ to P2₁2₁2₁ and P6₃ to P2₁. X-ray diffraction and Raman studies show that the phase transitions are governed by dynamics of the hydrazinium ions. Low-temperature magnetic studies show that these compounds exhibit magnetic ordering below 9-12.5 K. Since the low-temperature structures of chiral HyMn and perovskite HyFe are polar, these compounds are possible multiferroic materials. We also report high-pressure Raman scattering studies of chiral and perovskite HyZn, which show much larger stiffness of the latter phase. These studies also show that the ambient pressure polar phases are stable up to at least 1.4 and 4.1 GPa for the chiral and perovskite phase, respectively. Between 1.4 and 2.0 GPa (for chiral HyZn) and 4.1 and 5.2 GPa (for perovskite HyZn) pressure-induced transitions are observed associated with changes in the zinc-formate framework. Strong broadening of Raman bands and the decrease in their number for the high-pressure phase of chiral HyZn suggest that this phase is disordered and has higher symmetry than the ambient pressure one.

Temperature-dependent studies of a new two-dimensional cadmium dicyanamide framework exhibiting an unusual temperature-induced irreversible phase transition into a three-dimensional perovskite-like framework

TPrACd can be grown in both layered and perovskite-like architectures. The layered polymorph transforms irreversibly to the perovskite one at 389 K.

Tuning the structure and properties of a multiferroic metal–organic-framework via growing under high magnetic fields

High magnetic field-induced synthesis has been demonstrated to tune the structure and properties of the multiferroic metal–organic framework [(CH₃)₂NH₂][Mn(HCOO)₃].

Lead-free Single-molecule Switching Material with Electric, Optical, Thermal Triple Controllable Multifunction Based on Perovskite-like Crystal and Flexible Thin Film

With the flourishing development of star molecule (CH₃NH₃)PbI₃, organic-inorganic perovskites with multifunction and flexibility have become a worldwide focus. However, the controllable photoelectric switchable material (especially electric, optical, thermal multifunctional switches) still face great challenges, and most of them are ceramic and toxic lead-based series. Herein a lead-free perovskite-like crystal and flexible thin film, ImMC (ImMC = (HIm)₆∙[MnCl₄∙MnCl₆]) (1), with many advantages over inorganic ceramics and lead-based perovskites, performs ideal optical and dielectric duple switching properties simultaneously. The order-disordered HIm (Im = imidazole) cations of α-type occupy two lattice sites corresponding to "Switch-ON/0" and "Switch-OFF/1" states, respectively. Interestingly, the optical and dielectric "ON/OFF or 0/1" switches can be integrated into one single-molecule single/duple channel module with high signal-noise ratio, in which the "ON/OFF" response can be precisely controlled by temperature or/and light wavelength signal to realize automatically multiple switching. In brief, the lead-free multifunctional switch opens up a brand new route and shows the mark of its real genius as a highly desirable material for its advanced applications in highly integrated circuit and ultrahigh-encrypted storage in flexible photoelectric devices.

Room temperature multiferroicity in hydrogenated triapentafulvalene and pentaheptafulvalene oligomers

To search for new organic multiferroics, we perform a systematic study on the magnetic and ferroelectric properties of fused triapentafulvalene and pentaheptafulvalene oligomers (n = 2-6), by using the density functional theory and quantum Monte Carlo method. It is found that the oligomers without hydrogenation always lie in the spin singlet (nonmagnetic) state, while a selective hydrogenation of carbon atoms at the ends of oligomers can result in the spin triplet (ferromagnetic) state, which is tens to hundreds meV lower than the nonmagnetic state. The formation of ferromagnetism can be attributed to the hydrogenation-induced near degeneracy between the highest occupied and lowest unoccupied molecular orbitals. Simultaneously, there exists a finite dipole moment in the ferromagnetic state, due to the breaking of the inversion symmetry of oligomers. Our results imply that the hydrogenated triapentafulvalene and pentaheptafulvalene oligomers could be promising candidates in the development of room temperature organic multiferroics.

Uniaxial Negative Thermal Expansion and Mechanical Properties of a Zinc-Formate Framework

The thermal expansion behavior of a metal-formate framework, Zn(HCOO)₂·2(H₂O) (1), has been systematically studied via variable temperature single-crystal X-ray diffraction. Our results demonstrate that this formate exhibits significant negative thermal expansion (NTE, -26(2) MK^-1) along its c-axis. Detailed structural analyses reveal that the large NTE response is attributed to the 'hinge-strut' like framework motion. In addition, the fundamental mechanical properties of framework 1 have been explored via nanoindentation experiments. The measured elastic modulus and hardness properties on the (00-2)/(100)/(110) facets are 35.5/35.0/27.1 and 2.04/1.83/0.47 GPa, respectively. The stiffness and hardness anisotropy can be correlated well with the underlying framework structure, like its thermoelastic behavior.

Growth of centimeter-sized [(CH3)2NH2][Mn(HCOO)3] hybrid formate perovskite single crystals and Raman evidence of pressure-induced phase transitions

The fewer the number of the nucleation sites formed in the vessel, the larger the size of the obtained crystals.

Synthesis and temperature-dependent studies of a perovskite-like manganese formate framework templated with protonated acetamidine

Acetamidinium manganese formate undergoes a phase transition from the Imma to the P2₁/n structure at 304 K and magnetic order below 9 K.

Structural, electronic and magnetic properties of metal–organic-framework perovskites [AmH][Mn(HCOO)3]: a first-principles study

Novel multiferroic Metal–Organic-Frameworks (MOFs) [AmH][M(HCOO)₃] are investigated in structural, electronic and magnetic properties using density functional theory.

Structural phase transition in perovskite metal–formate frameworks: a Potts-type model with dipolar interactions

A Monte Carlo study of a statistical model describing the order–disorder phase transition in perovskite-based [(CH₃)₂NH₂][M(HCOO)₃] dense metal–organic frameworks.