Hybrid Organic-inorganic Metal Halide Research Articles

Anisotropic Electron-Phonon Interactions in 2D Lead-Halide Perovskites.

Two-dimensional (2D) hybrid organic-inorganic metal halide perovskites offer enhanced stability for perovskite-based applications. Their crystal structure's soft and ionic nature gives rise to strong interaction between charge carriers and ionic rearrangements. Here, we investigate the interaction of photogenerated electrons and ionic polarizations in single-crystal 2D perovskite butylammonium lead iodide (BAPI), varying the inorganic lamellae thickness in the 2D single crystals. We determine the directionality of the transition dipole moments (TDMs) of the relevant phonon modes (in the 0.3-3 THz range) by the angle- and polarization-dependent THz transmission measurements. We find a clear anisotropy of the in-plane photoconductivity, with a ∼10% reduction along the axis parallel with the transition dipole moment of the most strongly coupled phonon. Detailed calculations, based on Feynman polaron theory, indicate that the anisotropy originates from directional electron-phonon interactions.

Anisotropic Electron-Phonon Interactions in 2D Lead-Halide Perovskites.

Two-dimensional (2D) hybrid organic-inorganic metal halide perovskites offer enhanced stability for perovskite-based applications. Their crystal structure's soft and ionic nature gives rise to strong interaction between charge carriers and ionic rearrangements. Here, we investigate the interaction of photogenerated electrons and ionic polarizations in single-crystal 2D perovskite butylammonium lead iodide (BAPI), varying the inorganic lamellae thickness in the 2D single crystals. We determine the directionality of the transition dipole moments (TDMs) of the relevant phonon modes (in the 0.3-3 THz range) by the angle- and polarization-dependent THz transmission measurements. We find a clear anisotropy of the in-plane photoconductivity, with a ∼10% reduction along the axis parallel with the transition dipole moment of the most strongly coupled phonon. Detailed calculations, based on Feynman polaron theory, indicate that the anisotropy originates from directional electron-phonon interactions.

Strain in Hybrid Organic-Inorganic Metal Halide Perovskites Microstructures by Numerical Simulations.

Hybrid organic-inorganic metal halide perovskites (HOIPs) are promising materials for optoelectronics applications. Their optical and electrical properties can be controlled by strain engineering, that results from application of local elastic deformation or deposition on pre-patterned substrates acquiring a conformal 3D shape. Most interesting, their mechanical properties depend on their crystal structure, composition and dimensionality. We explore by numerical simulations the deformation of a selection of HOIPs comprising a broad range of elastic properties. We consider an axial symmetry with the formation of microdomes on flakes. Radial and vertical forces are considered, finding that the radial force is more effective to obtain large deformation. Large vertical displacement and strain is obtained for HOIPs with low stiffness. The layered nature of HOIPs, that are formed by inorganic layers of different thickness and organic spacers, is also investigated, revealing a non-monotonous trend with the proportion of inorganic to organic part.

Nonlinear Optical Effects of Hybrid Antimony(III) Halides Induced by Stereoactive 5s2 Lone Pairs and Trimethylammonium Cations.

Organic-inorganic hybrid metal halides have unique optical and electronic properties, which are advantageous in the study of nonlinear optical materials. To investigate the effect of stereoactive lone pair electrons and the induction of organic cations on the structure of hybrid antimony(III) halides on nonlinear optics, we synthesize two noncentrosymmetric hybrid antimony(III)-based halide single crystals (TMA)3Sb2X9 (TMA = NH(CH3)3+, X = Cl, Br) by a room-temperature slow evaporation method, and their single-crystal structures, phase transition, X-ray photoelectron spectroscopy, and energy-band structure calculations are studied. More importantly, second-harmonic generation results of (TMA)3Sb2X9 (X = Cl, Br) are about 0.7 and 0.8 × KH2PO4(KDP), respectively. Interestingly, (TMA)3Sb2Cl9 single crystals undergo a reversible structural transition from Pc (No. 7) at room temperature to P21/c (No. 14) at 400 K, while the (TMA)3Sb2Br9 single crystals belong to the noncentrosymmetric space group R3c (No. 161), which clarifies the previous results. This work not only deepens the understanding of the role in lone pair electrons and organic cations in the structural induction in antimony-based halide perovskite materials but also provides guidance for subsequent nonlinear optical explorations.

Synthesis, structure, and luminescence properties of a 0D organic-inorganic cadmium iodide: Combined experimental and theoretical approach

Hybrid organic-inorganic metal halide materials with zero-dimensional (0D) structures have emerged as a captivating field of research due to their distinctive electronic properties and remarkable broadband emission characteristics. In this study, we successfully synthesized a novel hybrid cadmium iodide compound, (BZA)2CdI4.H2O (BZA+: benzylammonium), in the form of single crystals employing the solvent-evaporation method. The room temperature single-crystal X-ray diffraction analysis revealed that (BZA)2CdI4.H2O possesses a typical 0D crystal structure, where (CdI4)2− anions are spatially isolated and surrounded by (BZA)+ cations and H2O molecules. The crystal packing is intricately governed by various non-covalent intermolecular interactions, including π–π stacking interactions, NH…I, NH…O, and OH…I hydrogen bonds, quantified through Hirshfeld surface analysis. Thermal analysis further demonstrated that the removal of water molecules in the crystal lattice yields a dehydrated phase of (BZA)2(CdI4). Upon 330 nm irradiation, the title compound displayed a broad bluish-white light emission, featuring a primary band at 430 nm and a secondary band at 500 nm. Based on photoluminescence spectra measurements and density functional theory (DFT) calculations, the dual-band emission is assigned to the emission of (BZA)+ organic cations and to excitons confined in the [CdI4]2– isolated inorganic tetrahedron, respectively. Interestingly, the presence of isolated molecular units in the 0-D structure results in strongly localized charges and bluish-white light emission with Commission International de l'Eclairage (CIE) colour coordinates (0.23, 0.28). These results confirm the promise of 0D metal-halide hybrids with dual-band emission as luminescent materials for solid-state lighting.

Suprahydrostable ultrasmall hybrid cuprous halide with pseudo-net protector for wireless light-triggered rewritable transient display and dynamically recoverable encryption

Zero-dimensional (0D) organic-inorganic hybrid metal halides (OIHMHs) are receiving significant attention due to their exceptional photovoltaic properties. However, their applications are restricted by their inferior water stability and undesired particle size. Herein, we synthesized suprahydrostable ultrasmall 0D OIHMHs with an average size of ∼3.4 nm, named (TOA)CuBr2 for the first time. (TOA)CuBr2 nanocrystals exhibit bright fluorescence attributed to self-trapping excitons and demonstrated by density functional theory calculations. It is worth emphasizing that (TOA)CuBr2 displays excellent reversible photoluminescence (PL) response to UV light, temperature, and acid/alkali simultaneously, especially to acid/alkali. The underlying mechanism of the reversibility properties is explained and the reversible PL switch is developed by utilizing the acid/alkali responsive property. Moreover, it is the first demonstration of a real-time rewritable transient display based on 0D OIHMHs with “writing/self-erasing/rewriting” capability under wireless near-infrared light. In addition, encrypted strips prepared by in-situ growth of (TOA)CuBr2 in polydimethylsiloxane show quite stable fluorescence properties in various scenarios. Dynamically recoverable encryption systems are also designed by taking full advantage of the material’s multi-response characteristics. Our work provides a strategy for synthesizing suprahydrostable ultrasmall OIHMHs, which will lay a solid foundation for developing their multi-scenario applications in the future.

Boosted Second Harmonic Generation of a Chiral Hybrid Lead Halide Resonant to Charge Transfer Exciton from Metal Halide Octahedra to Ligand.

Chiral hybrid organic-inorganic metal halides (HOMHs) offer an ideal platform for the advancement of second-order nonlinear optical (NLO) materials owing to their inherent noncentrosymmetric structures. The enhancement of optical nonlinearity of chiral HOMHs could be achieved by matching the free exciton and/or self-trapped exciton energy levels with desired NLO frequencies. However, the current scarcity of resonance modes and low resonance ratio hamper the further improvements of NLO performance. Herein, we propose a new resonant channel of charge transfer (CT) excited states from metal halide polyhedra to organic ligand to boost the second-order optical nonlinearity of chiral HOMHs. The model lead halide (C7H10N)PbBr3 (C7H10N=1-ethylpyridinium) exhibits a drastically enhanced second harmonic generation in resonance to the deep CT exciton energy, with intensity of up to 111.0 times that of KDP and 10.9 times that of urea. The effective NLO coefficient has been determined to be as high as ~40.2 pm V-1, balanced with a large polarization ratio and high laser damage threshold. This work highlights the contribution of organic ligands in the construction of a resonant channel for enhancing second-order NLO coefficients of metal halides, and thus provides guidelines for designing new chiral HOMHs materials for advanced nonlinear photonic applications.

Boosted Second Harmonic Generation of a Chiral Hybrid Lead Halide Resonant to Charge Transfer Exciton from Metal Halide Octahedra to Ligand

AbstractChiral hybrid organic–inorganic metal halides (HOMHs) offer an ideal platform for the advancement of second‐order nonlinear optical (NLO) materials owing to their inherent noncentrosymmetric structures. The enhancement of optical nonlinearity of chiral HOMHs could be achieved by matching the free exciton and/or self‐trapped exciton energy levels with desired NLO frequencies. However, the current scarcity of resonance modes and low resonance ratio hamper the further improvements of NLO performance. Herein, we propose a new resonant channel of charge transfer (CT) excited states from metal halide polyhedra to organic ligand to boost the second‐order optical nonlinearity of chiral HOMHs. The model lead halide (C7H10N)PbBr3 (C7H10N=1‐ethylpyridinium) exhibits a drastically enhanced second harmonic generation in resonance to the deep CT exciton energy, with intensity of up to 111.0 times that of KDP and 10.9 times that of urea. The effective NLO coefficient has been determined to be as high as ~40.2 pm V−1, balanced with a large polarization ratio and high laser damage threshold. This work highlights the contribution of organic ligands in the construction of a resonant channel for enhancing second‐order NLO coefficients of metal halides, and thus provides guidelines for designing new chiral HOMHs materials for advanced nonlinear photonic applications.

A novel halide hybrid white light emitter with remarkable structural and emission stability

Addressing the issues of self-absorption effect among different components in traditional white light-emitting diodes (WLEDs) and the high cost associated with rare earth elements, we propose the potential utility of zero-dimensional organic–inorganic metal halide hybrids (0D OIMHs) in white WLEDs fabrication. In this study, we synthesized a novel 0D OIMH, (C5N2H14)ZnCl4, characterized by broadband white light emission. The (C5N2H14)ZnCl4 crystal features a short ultraviolet cutoff situated at 190 nm and emits cold white light with a Color Rendering Index (CRI) of 73 under an excitation wavelength of 260 nm. Furthermore, (C5N2H14)ZnCl4 demonstrates exceptional structural and emission stability in an air atmosphere over a span of three months.

Three new low-dimensional organic inorganic hybrid metal halides: Syntheses, structures and properties

It is well known that organic inorganic hybrid metal halides have been excellent materials in the field of luminescence. So far, organic-inorganic Pb(Ⅱ) halide hybrids materials have been extensively studied for their excellent optical properties. However, the toxicity of lead may generate additional pollution. It is important to develop new lead-free halide hybrids. In this work, three new organic inorganic hybrid metal halides, namely [(C4H10NO)Ag2I3] (1), [(C4H10NO)2CuI2.5]·0.5I·0.5H2O (2) and [(C4H9NO)2CdI2] (3) were synthesized under mild conditions. Compounds 1 and 2 display different one dimensional inorganic chains surrounded by organic amine cations, while compound 3 shows a zero dimensional structure. Their luminescent and optical properties were investigated in detail. The result suggests that these three compounds are potential optical materials.

Fine-Tuning Optoelectronic Features in Organic-Inorganic Hybrid Metal Halides: A Focus on the Phenylbutanammonium Series.

Extensive research has been dedicated to exploring the potential applications of organic-inorganic hybrid metal halides in optoelectronics. This study presents findings on three metal halides based on phenylbutanammonium (PBA). Specifically, (PBA)2MnBr4(H2O)2 and (PBA)2Sn(IV)Cl6 exhibit zero-dimensional structures with P21/c and Pnma space groups, respectively, while (PBA)2Sn(II)Br4 features a two-dimensional structure with P1̅ space group. Under UV excitation, (PBA)2MnBr4(H2O)2 exhibits double emission arising from the 4T1 → 6A1 transitions of Mn2+ in two distinct coordination environments. The emission spectrum of (PBA)2SnCl6 aligns with that of PBACl, suggesting that the luminescence originates from the organic component. The yellow emission of (PBA)2SnBr4 is attributed to the self-trapped excitons. This study introduces the PBA series of compounds, revealing that varying metal ions and halogen combinations can adjust the structural dimensions and influence optical properties. The insights gained from this work serve as a guide for the preparation of efficient white light-emitting diodes.

Unveiling heavy heterovalent doping-modulated microstructure and thermoelectric performance in bulk hybrid perovskite single crystals

Organic-inorganic hybrid metal halide perovskites have emerged as a promising candidate for low-temperature thermoelectric conversion. However, challenges remain in heavily doping bulk perovskite single crystals without phase segregation or self-doping compensation issues to increase the carrier concentration. Previous efforts to enhance their thermoelectric performance rely on photoexcitation or surface/interface doping, which is not suitable for bulk devices to work in dark. This study achieves high monovalent and trivalent doping concentrations of up to 5 % in bulk single crystals of MAPbI3 and MAPbBr3 without phase segregation or defect formation. For monovalent doping, K+ and Li+ are incorporated into the perovskite lattice as interstitials. The resulting lattice distortion/microstrain enhances the electrical conductivity by 10 ∼ 100 times of undoped samples at 420 K. Trivalent doping using SbCl3, BiBr3, and BiI3 significantly enhances the carrier concentration and electrical conductivity, up to 1012 cm−3 and 0.038 S m−1 for MAPbBr3 with 10 % BiI3 at 465 K, respectively, which is 105 times of the undoped samples and 10 times of the best reports on doped MAPbBr3. Interestingly, the Bi3+ dopant oxidizes the deep defect state Pb0, resulting in Bi+ and Pb2+ formation and contributing to the enhanced electrical conductivity. Enhanced thermoelectric figure of merit (ZT) is achieved in MAPbBr3 with 5 % BiI3 at 315 K, almost 100 times of the best reports on doped MAPbBr3. This study provides fundamental understanding of doping-modulated microstructural, charge carrier, and thermoelectric properties of hybrid halide perovskites, offering potential doping strategies to realize the theoretically predicted promising thermoelectric properties in their bulk single crystals for practical thermoelectric devices. The doping-modulated carrier transport will also guide the optimization of optoelectronic applications of halide perovskites for solar cells, light emitters, and photodetectors.

The effect of solvent on the formation of low-dimensional metal halides and their self-trapped exciton emission

Low-dimensional organic–inorganic hybrid metal halides, with broadband luminescence, have attracted much attention for optoelectronic applications due to their rich in structural diversity and solution processibility. However, it is still unclear about how the solvent molecules influence the optoelectronic properties of the solution processed low-dimensional metal halides. Here, we prepared five different antimony-based crystal structures, [SbCl6]3- as the metal halide octahedron and 4, 4-difluoropiperidine (DFPD+) as organic cation, by using different solvents: hydrochloric acid (HCl) aqueous solution and four organic solvents (dimethylformamide (DMF), methanol (MeOH), acetonitrile (ACN) and dimethylacetamide (DMAC)). We revealed the relation between their crystal structures and optical properties, and we found the participation of organic molecules in the crystal structure causes significant lattice distortions, which is beneficial for achieving self-trapped exciton (STE) emission. Among them, (DFPD)6SbCl9·2DMAC exhibits a remarkable photoluminescence (PL) quantum yield of approximately 90 %. The STE dynamics in (DFPD)6SbCl9·2DMAC were characterized by femtosecond transient absorption and time-resolved PL spectroscopies. Simultaneously, this study also provides new directions for expanding the application of low-dimensional luminescent metal halides: in addition to UV-LEDs, efficient and rapid detection of methanol or acetonitrile can be achieved using the raw materials, while also realizing the potential application of multi-level optical anti-counterfeiting.

Guanidinium‐Based Manganese(II) Bromide with High Glass‐Forming Ability for Thermoplastic Curved X‐ray Imaging

AbstractThe development of large‐area transparent organic‐inorganic hybrid metal halide (OIMH) scintillation screens is restricted by the anisotropic single‐crystal growth, numerous grain boundaries in polycrystalline wafers, and inhomogeneous dispersion in perovskite‐polymer composite films. The crystal‐glass phase transition of OIMH materials may provide a promising solution for the above significant challenges. Herein, a new class of transparent amorphous guanidinium‐based manganese bromide glasses, (DPG)2MnBr4 and (DOTG)2MnBr4 (DPG = 1,3‐diphenylguanidinium, DOTG = 1,3‐di‐o‐tolylguanidinium), are synthesized through a low‐temperature melt‐quenching process. The (DOTG)2MnBr4 shows impressive glass‐forming ability because of large viscosity (η) at the melting temperature (Tm) (η(Tm) = 3426 mPa·s) and small fragility index (m = 52.35), which can be a potential glass scintillator. The large‐area (e.g., 13 cm × 13 cm) transparent amorphous (DOTG)2MnBr4 glass scintillator shows high light transmittance of > 80%, a low detection limit of 237.3 nGy s−1 and high X‐ray imaging spatial resolution of 12 lp mm−1. Interestingly, the glass transition temperature of < 40 °C gives (DOTG)2MnBr4 glass unique thermoplastic properties, allowing it to conform to irregularly shaped objects and reduce the distortion in X‐ray imaging.

Synthesis, Structure, and Optical Properties of a 0D Hybrid Organic-Inorganic Metal Halide (C5N2H14Cl)GeCl3.

Due to the flexible structural tunability and excellent photoelectric performance, hybrid organic-inorganic metal halides (OIMHs) have attracted intensive attention and become a hot topic in the field of materials. It is important and necessary to explore new OIMHs and study their structure-property relationship. In this work, a new lead-free OIMH, (C5N2H14Cl)GeCl3, is synthesized by the combination of hydrothermal and solution methods. This compound features a zero-dimensional structure composed of inorganic [GeCl3]- trigonal pyramids surrounded by isolated Cl- anions and organic (C5N2H14)2+ cations. Preliminary characterization and first-principles calculations are performed to study its basic optical properties. Interestingly, (C5N2H14Cl)GeCl3 shows weak blue emission under ultraviolet excitation, and the intrinsic mechanism is discussed.

White light emission in 0D halide perovskite [(CH3)3S]2SnCl6·H2O crystals through variation of doping ns2 ions

With the rapid development of white LEDs, the research of new and efficient white light emitting materials has attracted increasing attention. Zero dimensional (0D) organic–inorganic hybrid metal halide perovskites with superior luminescent property are promising candidates for LED application, due to their abundant and tailorable structure. Herein, [(CH3)3S]2SnCl6·H2O is synthesized as a host for dopant ions Bi3+ and Sb3+. The Sb3+ doped, or Bi3+/Sb3+ co-doped, [(CH3)3S]2SnCl6·H2O has a tunable optical emission spectrum by means of varying dopant ratio and excitation wavelength. As a result, we can achieve single-phase materials suitable for emission ranging from cold white light to warm white light. The intrinsic mechanism is examined in this work, to clarify the dopant effect on the optical properties. The high stability of title crystalline material, against water, oxygen and heat, makes it promising for further application.Graphical

1D Tin(II)‐Based Chiral Hybrid Perovskite Single Crystals with Extremely Distorted Inorganic Chains for Second Harmonic Generation

AbstractThe organic–inorganic hybrid metal halides with chiral structures have attracted great attentions in the field of optoelectronic functional materials, especially for nonlinear optics. To date, second harmonic generation (SHG) properties of several classes of chiral hybrid halides with different metal ions have been reported. However, SHG‐active tin(II)‐based chiral materials are rarely explored, hindering deep understandings of the relationship between their structures and SHG properties. Herein, Sn2+ ions are successfully introduced into chiral hybrid perovskites, and four new species of chiral hybrid perovskites (R/S‐α‐PEA)SnX3 (X = Cl and Br, PEA = phenylethylammonium) are obtained. The as‐grown crystals are in P212121 space group, with highly distorted [SnX6] octahedral units forming 1D chain structures in the lattice, due to the effect of stereochemically active lone pair 5s2 electrons in Sn2+. Such crystals have an SHG response of ≈1.4× Y‐cut α‐quartz for (S‐α‐PEA)SnBr3 and ≈0.9× Y‐cut α‐quartz for (S‐α‐PEA)SnCl3 at 1020 nm excitation, with high polarization ratio of SHG signals as well as wide bandgaps and transparency windows in the meanwhile. This work opens up a new platform for discovering more novel tin(II)‐based chiral perovskites with superior nonlinear optical properties in the future.

Mn-Doped M2CdCl4 (M = CH3NH3+, C2H8N+, and C3H10N+) Layered Hybrid Perovskite and Its Flexible Film Based on Simple Mechanochemical Synthesis.

Layered hybrid perovskites show significant advantages in the field of optoelectronics. However, the low quantum efficiency and complex preparation methods limit their applications. In this work, we developed a series of perovskite powders with a two-dimensional (2D) layered structure of organic-inorganic hybrid metal halides M2CdCl4:x%Mn (M = CH3NH3+, C2H8N+, C3H10N+) via facile mechanochemical methods. The prepared manganese Mn-doped MA2CdCl4 produces orange emission at 605 nm under both 254 and 420 nm excitation, which originates from a dual excitation channel competition mechanism, and its excitation channel could be changed with the increase of Mn2+ ion concentration. Typically, MA2CdCl4:20%Mn powder exhibits high photoluminescence quantum yield (PLQY) close to 90% at 605 nm due to the organic amine ions enlarging the Mn-Mn interlayer distances. In addition, we prepared MA2CdCl4:x%Mn@PVA flexible films, which also exhibit good luminescence at 254 nm excitation and were unexpectedly found to have a better response to Cs+, which could be a candidate for anticounterfeiting applications.

Near-Unity Green Luminescent Hybrid Manganese Halides as X-ray Scintillators.

The increasing demands in optoelectronic applications have driven the advancement of organic-inorganic hybrid metal halides (OIMHs), owing to their exceptional optical and scintillation properties. Among them, zero-dimensional (0D) low-toxic manganese-based scintillators have garnered significant interest due to their exceptional optical transparency and elevated photoluminescence quantum yields (PLQYs), making them promising for colorful light-emitting diodes and X-ray imaging applications. In this study, two OIMH single crystals of (Br-PrTPP)2MnBr4 (Br-PrTPP = (3-bromopropyl) triphenylphosphonium) and (Br-BuTPP)2MnBr4 (Br-BuTPP = (4-bromobutyl) triphenylphosphonium) were prepared via a facile saturated crystallization method. Benefiting from the tetrahedrally coordinated [MnBr4]2- polyhedron, both of them exhibited strong green emissions peaked at 517 nm owing to the d-d electron transition of Mn2+ with near-unity PLQYs of 99.33 and 86.85%, respectively. Moreover, benefiting from the high optical transparencies and remarkable luminescence properties, these manganese halides also exhibit excellent radioluminescent performance with the highest light yield of up to 68,000 photons MeV-1, negligible afterglow (0.4 ms), and linear response to X-ray dose rate with the lowest detection limit of 45 nGyair s-1. In X-ray imaging, the flexible film made by the composite of (Br-PrTPP)2MnBr4 and PDMS shows an ultrahigh spatial resolution of 12.78 lp mm-1, which provides a potential visualization tool for X-ray radiography.

From C4H7N2Ge0.4Sn0.6Br3 to C6H11N2Ge0.4Sn0.6Br3: Effective Modulation of the Second Harmonic Generation Effect and Optical Band Gap by Planar π-Conjugated Organic Cation Size.

In the search for nonlinear optical (NLO) materials with excellent overall performance, we have devoted ourselves to organic-inorganic hybrids consisting of anionic groups containing stereochemically active lone-pair (SCALP) electron cations and organic planar π-conjugated group cations. Accordingly, in this paper, two novel organic-inorganic hybrid metal halides, C4H7N2Ge0.4Sn0.6Br3 (I) and C6H11N2Ge0.4Sn0.6Br3 (II), have been synthesized. The powder second-harmonic technique shows that both C4H7N2Ge0.4Sn0.6Br3 and C6H11N2Ge0.4Sn0.6Br3 have moderately strong second-order nonlinear optical effects, which are about 2.03 (I) and 1.16 (II) times that of KH2PO4 (KDP), respectively. They also have different optical band gaps of 2.75 (I) and 2.88 eV (II) due to the different sizes of the organic cations, and their photoluminescent and thermal properties were also investigated. This work provides new structural insights for the design and modulation of organic-inorganic hybrid halide materials with multiple excellent optical properties.