Layered Lead Iodide Research Articles

Solar-Driven Conversion of CO2 to C2 Products by the 3d Transition Metal Intercalates of Layered Lead Iodides.

Photocatalytic CO2 reduction to high-value-added C2+ products presents significant challenges, which is attributed to the slow kinetics of multi-e- CO2 photoreduction and the high thermodynamic barrier for C-C coupling. Incorporating redox-active Co2+/Ni2+ cations into lead halide photocatalysts has high potentials to improve carrier transport and introduce charge polarized bimetallic sites, addressing the kinetic and thermodynamic issues, respectively. In this study, a coordination-driven synthetic strategy is developed to introduce 3d transition metals into the interlamellar region of layered organolead iodides with atomic precision. The resultant bimetallic halide hybrids exhibit selective photoreduction of CO2 to C2H5OH using H2O vapor at the evolution rates of 24.9-31.4µmol g-1h-1 and high selectivity of 89.5-93.6%, while pristine layered lead iodide yields only C1 products. Band structure calculations and photoluminescence studies indicate that the interlayer Co2+/Ni2+ species greatly contribute to the frontier orbitals and enhance exciton dissociation into free carriers, facilitating carrier transport between adjacent lead iodide layers. In addition, Bader charge distribution calculations and in situ experimental spectroscopic studies reveal that the asymmetric Ni-O-Pb bimetallic catalytic sites exhibit intrinsic charge polarization, promoting C-C coupling and leading to the formation of the key *OC-CHO intermediate.

Large Phase-Transition Temperature Enhancement Achieved in a Layered Lead Iodide Hybrid Crystal by H/F Substitution.

Organic-inorganic hybrid metal halides with structural flexibility and solution processability have been widely investigated for different application scenarios. However, the effective construction of phase-transition materials with a high phase-transition temperature (Ttr) for potential practical applications remains a great challenge, and reports on the regulation of Ttr with significant enhancement have been rare. In this manuscript, we have realized a large Ttr increase of 148 K in a layered hybrid lead iodide crystal (4-FTMBA)4Pb3I10 (4-FTMBA = 4-fluoro-N,N,N-trimethylbenzenaminium) by the H/F substitution strategy. Compared to the parent (TMBA)4Pb3I10 (TMBA = N,N,N-trimethylbenzenaminium), H/F substitution preserves the structural framework and crystal symmetry in (4-FTMBA)4Pb3I10. The introduction of heavier fluorine will significantly increase the motion barrier for the order-disorder transition, resulting in the remarkably improved Ttr. Temperature-dependent crystal structures, Raman spectra, and dielectric analyses well support the phase-transition behavior. In addition, evident thermochromism with a tunable direct band gap in (4-FTMBA)4Pb3I10 has been observed using UV-vis spectra. To the best of our knowledge, the achieved Ttr enhancement of 148 K by H/F substitution is the highest among the organic-inorganic hybrid lead halide phase-transition materials. This finding would greatly inspire the rational design of functional materials with high performance.

Solvent‐Free Preparation and Moderate Congruent Melting Temperature of Layered Lead Iodide Perovskites for Thin‐Film Formation

AbstractLead halide 2D hybrid perovskites (HP) have emerged as promising materials for photovoltaic and lighting applications. Solvent‐free preparations offer greener route, but require congruent melting of halide perovskite for thin films, which has been demonstrated only for monolayered HP (n=1 of the (A)2(MA)n−1PbnI3n+1 series) at the quite high temperature Tm (Tmelting) of 171 °C. Here, we report on the solvent‐free preparation (n=2, 4), thermal behaviour and melt‐processed thin films of a series of HP (GABA)2(MA)n−1PbnI3n+1 (n=1, 2, 4; GABA+: 4‐ammoniumbutyric acid cation). The n=1 and n=2 HP have an exceptional low congruent Tm of 126 °C and 136 °C, respectively, and, for n=1, a very good stability in the molten state. The liquid–solid reaction of molten (GABA)2PbI4 or (GABA)2(MA)Pb2I7 with MAPbI3 in 1/1 (T=130 °C) or 1/2 (T=135 °C) ratio leads to pure phases of the n=2 HP and (GABA)2(MA)3Pb4I11 (n=4, Tm=185 °C), respectively. Melt‐processed thin films of n=1, n=2 and n=4 HP have also been prepared.

Solvent-Free Preparation and Moderate Congruent Melting Temperature of Layered Lead Iodide Perovskites for Thin-Film Formation.

Lead halide 2D hybrid perovskites (HP) have emerged as promising materials for photovoltaic and lighting applications. Solvent-free preparations offer greener route, but require congruent melting of halide perovskite for thin films, which has been demonstrated only for monolayered HP (n=1 of the (A)2 (MA)n-1 Pbn I3n+1 series) at the quite high temperature Tm (Tmelting ) of 171 °C. Here, we report on the solvent-free preparation (n=2, 4), thermal behaviour and melt-processed thin films of a series of HP (GABA)2 (MA)n-1 Pbn I3n+1 (n=1, 2, 4; GABA+ : 4-ammoniumbutyric acid cation). The n=1 and n=2 HP have an exceptional low congruent Tm of 126 °C and 136 °C, respectively, and, for n=1, a very good stability in the molten state. The liquid-solid reaction of molten (GABA)2 PbI4 or (GABA)2 (MA)Pb2 I7 with MAPbI3 in 1/1 (T=130 °C) or 1/2 (T=135 °C) ratio leads to pure phases of the n=2 HP and (GABA)2 (MA)3 Pb4 I11 (n=4, Tm =185 °C), respectively. Melt-processed thin films of n=1, n=2 and n=4 HP have also been prepared.

Pressure-induced two-dimensional to three-dimensional structural phase transition in 2H-type layered lead iodide PbI2

2H-type lead iodide (PbI2) is a two-dimensional (2D) layered semiconductor and has recently received considerable attention as a precursor for lead halide perovskite solar cells. Although the pressure-induced structural phase transitions of PbI2 have been investigated for several decades, the detailed crystalline structure of PbI2 at high pressure is still elusive because of lacking in situ x-ray diffraction (XRD) studies. In this report, in situ Raman spectra and XRD patterns of 2H-PbI2 under pressure have been investigated in a diamond-anvil cell. Both Raman spectra and XRD patterns provide consistent evidence for two structural phase transitions upon compression at approximately 0.58 and 2.6 GPa. The XRD patterns suggest that the complete pressure-driven phase transition sequence of PbI2 up to 20 GPa is two-dimensional hexagonal 2H-P3¯m1→polytype 4H-P3m1→three-dimensional (3D) orthorhombic Pnma. The uncovered 2D to 3D structural transition in 2H-PbI2 advances the understanding of structural stability of 2H-type layered semiconductors and lead halide perovskite.

Effects of Defect on Work Function and Energy Alignment of PbI2: Implications for Solar Cell Applications

Two-dimensional (2D) layered lead iodide (PbI2) is an important precursor and common residual species during the synthesis of lead-halide perovskites. There currently exist some debates and uncertainties about the effect of excess PbI2 on the efficiency and stability of the solar cell with respect to its energy alignment and energetics of defects. Herein, by applying the first-principles calculations, we investigate the energetics, changes of work function and the defective levels associated with the iodine vacancy (VI) and interstitial iodine (II) defects of monolayer PbI2 (ML-PbI2). We find that the PbI2 has a very low formation energy of VI of 0.77 and 0.19 eV for dilute and high concentration, respectively, reflecting coalescence tendency of isolated VI, much lower than that of vacancies in other 2D materials like phosphorene. Similar to VI, a low formation energy of II of 0.65 eV is found, implying a high population of such defects. Both defects generate in-gap defective levels which are mainly due to the unsaturated chemical bonds of p-orbitals of exposed Pb or inserted I. Such rich defective levels allow the VI and II as the reservoir or sinks of electron/hole carriers in PbI2. Our results suggest that the remnant PbI2 in perovskite MAPbI3 (or FAPbI3) would play dual opposite roles in affecting the efficiency of the perovskite: (1) Forming Schottky-type interface with MAPbI3 (or FAPbI3) in which the built-in potential would facilitate the electron-hole separation and prolong the carrier lifetime; (2) Acting as the recombination centers due to the deep defective levels. To promote the efficiency by the Schottky effect, our work reveals that the II defect is favored, and to reduce the recombination centers the VI defect should be suppressed. Our results shall be beneficial in improving strategies for the related optoelectronics applications.

Confinement-Driven Ferroelectricity in a Two-Dimensional Hybrid Lead Iodide Perovskite.

Organic-inorganic hybrid perovskites (OIHPs) hold a great potential for scientific and technological endeavors in the field of ferroelectrics, solar cells, and electroluminescent devices, because of their structural diversity, low cost of manufacture, and ease of fabrication. However, lead iodide perovskite ferroelectrics with narrow band gap have rarely been reported. Here, we present a new two-dimensional (2D) layered lead iodide perovskite ferroelectric, (4,4-DFHHA)2PbI4 (4,4-DFHHA = 4,4-difluorohexahydroazepine), with a spontaneous polarization (Ps) of 1.1 μC/cm2 at room temperature, a direct bandgap of 2.32 eV, and a high Curie temperature Tc of 454 K (beyond that of BaTiO3, 393 K). On the basis of the nonferroelectrics (HHA)I, (4-FHHA)I, and (4,4-DFHHA)I (HHA = hexahydroazepine, 4-FHHA = 4-fluorohexahydroazepine), we assembled them with PbI2 to form lead iodide perovskites. Because the space between adjacent one-dimensional (1D) chains is relatively large and the confinement effect is not obvious, the cations are still in a disordered state, and 1D OIHPs (HHA)PbI3 and (4-FHHA)PbI3 are also nonferroelectrics at room temperature. In the confined environment of the 2D PbI42- framework for (4,4-DFHHA)2PbI4, the 4,4-DFHHA cations become ordered, and their asymmetric distribution leads to the spontaneous polarization. This work offers an efficient strategy for enriching the family of lead iodide perovskite ferroelectrics through the confinement effect and should inspire further exploration of the interplay between ferroelectricity and photovoltaics.

Layered Lead Iodide of [Methylhydrazinium]2PbI4 with a Reduced Band Gap: Thermochromic Luminescence and Switchable Dielectric Properties Triggered by Structural Phase Transitions

Layered lead halides offer potential for light-emitting and photovoltaic devices. Here, we report the synthesis of two-dimensional (2D) lead iodide with single-octahedra slabs separated by the smal...

Chiral 2D Perovskites with a High Degree of Circularly Polarized Photoluminescence.

Chiral materials are of particular interest and have a wide range of potential applications in life science, material science, spintronic, and optoelectronic devices. Two-dimensional (2D) hybrid organic-inorganic lead halide perovskites have attracted increasing attention. Incorporating the chiral organic ligands into the layered lead iodide frameworks would introduce strong chirality in pure 2D perovskites for potential applications in circularly polarized light (CPL) emission and detection; nonetheless, studies on those aspects are still in their infancy. Here, we report on the strong CPL emission and sensitive CPL detection in the visible-wavelength range in pure chiral ( R-/ S-MBA)2PbI4 (MBA = C6H5C2H4NH3) 2D perovskites, which are successfully synthesized with a needle shape and millimeter size by incorporating the chiral molecules. The chiral 2D perovskites ( R-MBA)2PbI4 and ( S-MBA)2PbI4 exhibit an average degree of circularly polarized photoluminescence (PL) of 9.6% and 10.1% at 77 K, respectively, and a maximum degree of the circularly polarized PL of 17.6% is achieved in ( S-MBA)2PbI4. The degree of circularly polarized PL dramatically decreases with increasing temperature, implying that the lattice distortion induced by the incorporated chiral molecules and/or temperature-dependent spin flipping might be the origin for the observed chirality. Finally, CPL detection has been achieved with decent performance in our chiral 2D perovskite microplate/MoS2 heterostructural devices. The high degree of the circularly polarized PL and excellent CPL detection together with the layered nature of pure chiral 2D perovskites enables them to be a class of very promising materials for developing and exploring spin associated electronic devices based on the chiral 2D perovskites.

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.

Perovskite-Like Organic-Inorganic Hybrid Lead Iodide with a Large Organic Cation Incorporated within the Layers.

A great effort has been made to investigate 2D perovskites to improve the stability and controllability in the fabrication of photoelectronic devices. As far as we know, only small organic cations such as methylammonium can incorporate into the multilayered perovskite structure except the cations sandwiched between the inorganic layers. We report here a new layered lead iodide, (H2Aepz)3Pb4I14 (1), where larger organic cations, bis-protonated 2-(2-aminoethyl)pyrazole (Aepz), not only were sandwiched between the inorganic layers but also were incorporated within the perovskite-like PbI layered structure. Another 2D compound, (H2Aepz)PbI4 (2), was also prepared that was a one-layer perovskite. A simple Schottky device was prepared to investigate the photoelectroresponsive properties of the compounds in comparison with that of a typical organic-inorganic hybrid perovskite. In general, the energy gap is decreased with an increase in the perovskite layers, but the band gap of two-layered 1 is larger than that of one-layered 2. The photocurrent densities of the compounds are in the order of 1 < 2 < (CH3NH3)PbI3, which is discussed based on the crystal structures and band energy gaps.

(TTF)Pb2I5: A Radical Cation-Stabilized Hybrid Lead Iodide with Synergistic Optoelectronic Signatures

(TTF)Pb 2 I 5 : a Radical Cation-Stabilized Hybrid Lead Iodide with Synergistic Optoelectronic Signatures Hayden A. Evans, †,‡,§ Anna J. Lehner, § John G. Labram, ∥ Douglas H. Fabini, §,⊥ Omar Barreda, § Sara R. Smock, ‡ Guang Wu, ‡ Michael L. Chabinyc, ∥ , ⊥ * Ram Seshadri, †,‡,§,⊥ * and Fred Wudl ‡,§,⊥ * Mitsubishi Chemical Center for Advanced Materials, University of California, Santa Barbara, CA 93106 Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA 93106 Materials Research Laboratory, University of California, Santa Barbara CA 93106 California NanoSystems Institute, University of California, Santa Barbara CA 93106 ⊥ Materials Department, University of California, Santa Barbara, CA 93106, USA. ABSTRACT: Hybrid organic inorganic materials offer the opportunity to combine functionalities of two distinct chemis- tries. Tetrathiafulvalene (TTF) is the precursor to some of the first organic metals, and perovskite lead iodides have re- cently proved their efficacy as photovoltaic materials. Here we describe a hybrid material that incorporates both, compris- ing TTF radical cations stabilizing an anionic layered lead–iodide network. Preparation, property analysis, and DFT calcu- lations are reported, verifying that (TTF)Pb 2 I 5 is a narrow gap semiconductor with optical properties synergistically influ- enced by the TTF +• radical cation. This radical cation contributes to states in the electronic gap formed by orbitals in the inorganic framework. Hybrid organic-inorganic materials can potentially combine the properties of highly tunable functional or- ganics with those of complementary inorganic networks in a single material. Distinct control over what is desired from each component becomes possible in such materi- als, allowing the functions of the organic and inorganic components to work synergistically. Tetrathiafulvalene (TTF), the organic component in the new hybrid material reported here, is a precursor to some of the early organic metals and was a clear choice when choosing a functional organic. It was first reported by Wudl et al. in the early 1970's, 1,2 and has been studied for its impressive electron donating abilities, 3,4 including derivatives displaying su- perconductivity. 5 Hybrid compounds containing TTF have also shown novel electronic and magnetic properties. Devic et al. 6,7 reported a ethylenedithio-1,2-diiodotetrathiafulvalene (EDT-TTF-I 2+• ) hybrid with metallic character, and showed its metal-insulator transition to be preserved within a hybrid compound with PbI 2 . Kondo et al. 8 simi- larly reported TTF +• hybrids containing Sn and Cl/Br, all which display semiconducting behavior due to interac- tions through the extended TTF networks. Magnetic compounds with TTF derivatives have also been made with transition metal halides, and found to be composed of two kinds of donor columns which magnetically couple with the anions through close contact between metal hal- ide interactions. 9 Metal-organic-frameworks (MOFs), a related class of hybrid materials, generally possess poor electronic functionality, but attempts to prepare electri- cally conductive MOFs have recently been made. Exam- ples include incorporating a conjugated molecule within the framework directly, 10 as a guest that electronically couples metal atoms in the framework using (TCNQ), 11 or post-synthetic modification of an insulating MOF via chemical leaching. 12,13 Stavila et al. 14 have recently re- viewed this topic. Layered lead halides, in their own right, are an attractive set of materials with rich optoelectronic and semiconducting behavior. The extensive work of Mitzi and coworkers 15,16 in the late 90’s first highlighted how amines and diammines combine with main group halides to form unique layered perovskite structures, which moti- vated the chemistry of organic ammonium cation “spac- ers” to flourish. Following the work of Kojima et al. 17 demonstrating the use of perovskite CH 3 NH 3 PbI 3 materi- als as solution deposited photovoltaics, there has been a huge flurry of research in this space to enhance the stabil- ity of the air unstable compound. This has included the use of ammonium spacer chemistry by Smith et al. 18 which improved stability of a layered perovskite incorpo- rating phenylethylammonium ions into the structure. Cao et al. 19 also explored this idea with butylammonium in- stead of phenylethylammonium, and reported that the extent of ammonium spacer incorporation as well as growth direction of subsequent films had significant im-

Whispering gallery mode lasing from hexagonal shaped layered lead iodide crystals.

We report on the synthesis and optical gain properties of regularly shaped lead iodide (PbI2) platelets with thickness ranging from 10-500 nm synthesized by chemical vapor deposition methods. The as-prepared single crystalline platelets exhibit a near band edge emission of ∼ 500 nm. Whispering gallery mode (WGM) lasing from individual hexagonal shaped PbI2 platelets is demonstrated in the temperature-range of 77-210 K, where the lasing modes are supported by platelets as thin as 45 nm. The finite-difference time-domain simulation and the edge-length dependent threshold confirm the planar WGM lasing mechanism in such hexagonal shaped PbI2 platelet. Through a comprehensive study of power-dependent photoluminescence (PL) and time-resolved PL spectroscopy, we ascribe the WGM lasing to be biexcitonic in nature. Moreover, for different thicknesses of platelet, the lowest lasing threshold occurs in platelets of ∼ 120 nm, which attributes to the formation of a good Fabry-Pérot resonance cavity in the vertical direction between the top and bottom platelet surfaces that enhances the reflection. Our present study demonstrates the feasibility of planar light sources based on layered semiconductor materials and that their thickness-dependent threshold characteristic is beneficial for the optimization of layered material based optoelectronic devices.

Publisher's Note: Excitons and biexcitons bound to a positive ion in a bismuth-doped inorganic-organic layered lead iodide semiconductor [Phys. Rev. B 70, 205330 (2004)

Received 11 January 2005DOI:https://doi.org/10.1103/PhysRevB.71.049903©2005 American Physical Society

Excitons and biexcitons bound to a positive ion in a bismuth-doped inorganic-organic layered lead iodide semiconductor

We have studied optical properties of an inorganic-organic layered lead iodide semiconductor doped with bismuth ions $({\mathrm{Bi}}^{3+})$. A new absorption band was observed at $2.15\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ that is by $0.21\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ lower than two-dimensional $1s$ excitons $(2.36\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ and is assigned to excitons bound to the bismuth ions. In the photoluminescence spectra with high excitation density $(\ensuremath{\sim}0.4\phantom{\rule{0.3em}{0ex}}\mathrm{MW}∕{\mathrm{cm}}^{2})$, a novel emission band was observed at the energy of $2.51\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ that is by $0.15\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ higher than the exciton resonance, and is attributed to biexcitons bound to the bismuth ions. On the basis of the experimental results, the binding energies of the bismuth-bound exciton and biexciton were estimated approximately as 210 and $10\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, respectively. It is found that the binding energy of the positive-ion $({\mathrm{Bi}}^{3+})$ bound biexciton is by far smaller than that of the corresponding bound exciton. The much lower stability of the positive-ion bound biexciton is discussed.

Chemical and Size Characterization of Layered Lead Iodide Quantum Dots via Optical Spectroscopy and Atomic Force Microscopy

Lead iodide (PbI2) clusters were synthesized from the chemical reaction of NaI (or KI) with Pb(NO3)2 in H2O, D2O, CH3OH, and C3H7OH solvents. The observation of absorption features between the 550 and 350 nm region obtained with an integrating sphere strongly suggests PbI2 quantum dot formation in solution. Comparison of spectra of PbI2 clusters in solution with PbI2 clusters formed by impregnation of PbI2 in four different pore-sized porous silica substrates indicates that the PbI2 cluster size in solution is less than 2.5 nm in the lateral dimension. Atomic force microscopy (AFM) measurements of PbI2 solutions deposited on mica and highly oriented pyrolytic graphite surfaces indicate that the clusters are single layered. The measured height is 1.0 ± 0.1 nm, which is ∼0.3 nm larger than the layer thickness observed for the bulk materials. The swollen layer thickness can be attributed to the intralayer contraction from the strong lateral interaction among PbI2 molecules, which is supported by ab initio calculations. Raman scattering measurements of the LO and TO modes of PbI2 in bulk and in the confined state were also conducted in 50−150 cm-1 region. Three bands observed at 74, 96, and 116 cm-1 for the confined materials are assigned to the TO2, LO2, and LO1 modes, respectively. The relatively small red shift in the LO modes for PbI2 in the porous hosts may be caused by the surface phonon of PbI2 nanoparticles confined in the porous silica.