Pressure-induced semiconductor-to-metal phase transition of a charge-ordered indium halide perovskite

Exciton engineering of 2D Ruddlesden-Popper perovskites by synergistically tuning the intra and interlayer structures

Designing two-dimensional halide perovskites for high-performance optoelectronic applications requires deep understanding of the structure-property relationship that governs their excitonic behaviors. However, a design framework that considers both intra and interlayer structures modified by the A-site and spacer cations, respectively, has not been developed. Here, we use pressure to synergistically tune the intra and interlayer structures and uncover the structural modulations that result in improved optoelectronic performance. Under applied pressure, (BA)2(GA)Pb2I7 exhibits a 72-fold boost of photoluminescence and 10-fold increase of photoconductivity. Based on the observed structural change, we introduce a structural descriptor χ that describes both the intra and interlayer characteristics and establish a general quantitative relationship between χ and photoluminescence quantum yield: smaller χ correlates with minimized trapped excitons and more efficient emission from free excitons. Building on this principle, we design a perovskite (CMA)2(FA)Pb2I7 that exhibits a small χ and an impressive photoluminescence quantum yield of 59.3%.

Time-Resolved Dynamics of Metal Halide Perovskite under High Pressure: Recent Progress and Challenges

Metal halide perovskites have garnered significant attention in the scientific community for their promising applications in optoelectronic devices. The application of pressure engineering, a viable technique, has played a crucial role in substantially improving the optoelectronic characteristics of perovskites. Despite notable progress in understanding ground-state structural changes under high pressure, a comprehensive exploration of excited-state dynamics influencing luminescence remains incomplete. This Perspective delves into recent advances in time-resolved dynamics studies of photoexcited metal halide perovskites under high pressure. With a focus on the intricate interplay between structural alterations and electronic properties, we investigate electron-phonon interactions, carrier transport mechanisms, and the influential roles of self-trapped excitons (STEs) and coherent phonons in luminescence. However, significant challenges persist, notably the need for more advanced measurement techniques and a deeper understanding of the phenomena induced by high pressure in perovskites.

Pressure-Modulated Anomalous Organic-Inorganic Interactions Enhance Structural Distortion and Second-Harmonic Generation in MHyPbBr3 Perovskite

Organic-inorganic halide perovskites possess unique electronic configurations and high structural tunability, rendering them promising for photovoltaic and optoelectronic applications. Despite significant progress in optimizing the structural characteristics of the organic cations and inorganic framework, the role of organic-inorganic interactions in determining the structural and optical properties has long been underappreciated and remains unclear. Here, by employing pressure tuning, we realize continuous regulation of organic-inorganic interactions in a lead halide perovskite, MHyPbBr3 (MHy+ = methylhydrazinium, CH3NH2NH2+). Compression enhances the organic-inorganic interactions by strengthening the Pb-N coordinate bonding and N-H···Br hydrogen bonding, which results in a higher structural distortion in the inorganic framework. Consequently, the second-harmonic-generation (SHG) intensity experiences an 18-fold increase at 1.5 GPa, and the order-disorder phase transition temperature of MHyPbBr3 increases from 408 K under ambient pressure to 454 K at the industrially achievable level of 0.5 GPa. Further compression triggers a sudden non-centrosymmetric to centrosymmetric phase transition, accompanied by an anomalous bandgap increase by 0.44 eV, which stands as the largest boost in all known halide perovskites. Our findings shed light on the intricate correlations among organic-inorganic interactions, octahedral distortion, and SHG properties and, more broadly, provide valuable insights into structural design and property optimization through cation engineering of halide perovskites.

Theoretical and Experimental Advances in High-Pressure Behaviors of Nanoparticles

Using compressive mechanical forces, such as pressure, to induce crystallographic phase transitions and mesostructural changes while modulating material properties in nanoparticles (NPs) is a unique way to discover new phase behaviors, create novel nanostructures, and study emerging properties that are difficult to achieve under conventional conditions. In recent decades, NPs of a plethora of chemical compositions, sizes, shapes, surface ligands, and self-assembled mesostructures have been studied under pressure by in-situ scattering and/or spectroscopy techniques. As a result, the fundamental knowledge of pressure-structure-property relationships has been significantly improved, leading to a better understanding of the design guidelines for nanomaterial synthesis. In the present review, we discuss experimental progress in NP high-pressure research conducted primarily over roughly the past four years on semiconductor NPs, metal and metal oxide NPs, and perovskite NPs. We focus on the pressure-induced behaviors of NPs at both the atomic- and mesoscales, inorganic NP property changes upon compression, and the structural and property transitions of perovskite NPs under pressure. We further discuss in depth progress on molecular modeling, including simulations of ligand behavior, phase-change chalcogenides, layered transition metal dichalcogenides, boron nitride, and inorganic and hybrid organic-inorganic perovskites NPs. These models now provide both mechanistic explanations of experimental observations and predictive guidelines for future experimental design. We conclude with a summary and our insights on future directions for exploration of nanomaterial phase transition, coupling, growth, and nanoelectronic and photonic properties.

Pressure-Induced Band Gap Engineering of Nontoxic Lead-Free Halide Perovskite CsMgI3 for Optoelectronic Applications

Recently, lead halide perovskites have gained considerable attention by dint of their predominant physiochemical features and potential use in various applications with an improved power conversion efficiency. Despite the incredible technological and research breakthroughs in this field, most of those compounds present an obstacle to future commercialization due to their instability and extreme poisonousness. Because of this, it is preferable to replace lead with alternative stable elements to produce eco-friendly perovskites with equivalent optoelectronic qualities similar to lead-based perovskites. However, Pb-free perovskite-based devices have relatively low power conversion efficiency. Pressure might be considered an effective way for modifying the physical characteristics of these materials to enhance their performance and reveal structure-property correlations. The present study has been done to investigate the structural, electronic, optical, elastic, mechanical, and thermodynamic properties of nontoxic perovskite CsMgI₃ under hydrostatic pressure by using density functional theory (DFT). At ambient pressure, the present findings are in excellent agreement with the available experimental data. Pressure causes the Mg-I and Cs-I bonds to shorten and become stronger. The absorption coefficient in the visible and ultraviolet (UV) zones grows up with the increase in pressure. Additionally, we have observed low reflectivity, a high-intensity conductivity peak, and a dielectric constant in the visible region of the electromagnetic spectrum. As pressure rises, the band gap keeps narrowing, facilitating an electron from the valence band to get excited easily at the conduction band. Furthermore, we analyze the mechanical, elastic, and thermodynamic properties under pressure, which suggests that this compound exhibit ductile behavior. The shrunk band gap and improved physical properties of CsMgI₃ under hydrostatic pressure suggest that this material may be used in solar cells (for photovoltaic applications) and optoelectronic devices more frequently than at ambient pressure. In addition, this paper emphasizes the feasibility of hydrostatic pressure in the systematic modification of the optoelectronic and mechanical characteristics of lead-free halide perovskites.

Tuning band gap and enhancing optical functions of AGeF3 (A = K, Rb) under pressure for improved optoelectronic applications

The current study diligently analyzes the physical characteristics of halide perovskites AGeF3 (A = K, Rb) under hydrostatic pressure using density functional theory. The goal of this research is to reduce the electronic band gap of AGeF3 (A = K, Rb) under pressure in order to improve the optical characteristics and assess the compounds' suitability for optoelectronic applications. The structural parameters exhibit a high degree of precision, which correlates well with previously published work. In addition, the bond length and lattice parameters decrease significantly leading to a stronger interaction between atoms. The bonding between K(Rb)-F and Ge-F reveal ionic and covalent nature, respectively, and the bonds become stronger under pressure. The application of hydrostatic pressure demonstrates remarkable changes in the optical absorption and conductivity. The band gap becomes lower with the increment of pressure, resulting in better conductivity. The optical functions also predict that the studied materials might be used in a variety of optoelectronic devices operating in the visible and ultraviolet spectrum. Interestingly, the compounds become more suitable to be used in optoelectronic applications under pressure. Moreover, the external pressure has profound dominance on the mechanical behavior of the titled perovskites, which make them more ductile and anisotropic.

Pressure-Induced Amorphization and Crystallization of Heterophase Pd Nanostructures

Control of structural ordering in noble metals is very important for the exploration of their properties and applications, and thus it is highly desired to have an in-depth understanding of their structural transitions. Herein, through high-pressure treatment, the mutual transformations between crystalline and amorphous phases are achieved in Pd nanosheets (NSs) and nanoparticles (NPs). The amorphous domains in the amorphous/crystalline Pd NSs exhibit pressure-induced crystallization (PIC) phenomenon, which is considered as the preferred structural response of amorphous Pd under high pressure. On the contrary, in the spherical crystalline@amorphous core-shell Pd NPs, pressure-induced amorphization (PIA) is observed in the crystalline core, in which the amorphous-crystalline phase boundary acts as the initiation site for the collapse of crystalline structure. The distinct PIC and PIA phenomena in two different heterophase Pd nanostructures might originate from the different characteristics of Pd NSs and NPs, including morphology, amorphous-crystalline interface, and lattice parameter. This work not only provides insights into the phase transition mechanisms of amorphous/crystalline heterophase noble metal nanostructures, but also offers an alternative route for engineering noble metals with different phases.

Theoretical Prediction of Mixed-Valence Layered Halide Perovskites Cs4M(IV)M(II)2X12 (M = Ge, Sn; X = Cl, Br)

Charge-ordered compounds (i.e., Cu⁺/Cu²⁺, Au⁺/Au³⁺, In⁺/In³⁺, Tl⁺/Tl³⁺, Sb³⁺/Sb⁵⁺, and Bi³⁺/Bi⁵⁺) have been widely explored because of their unique physical properties. Here, a new class of ⟨111⟩-oriented mixed-valence layered halide perovskites Cs₄M(IV)M(II)₂X₁₂ (M = Ge, Sn; X = Cl, Br) with C2/m, R-3m, and I41/amd space groups was predicted by first-principles calculations. Based on the decomposition enthalpy, the phonon spectrum, and the mechanical stability criteria, we found that Cs₄GeGe₂Cl₁₂ (C2/m and R-3m), Cs₄GeGe₂Br₁₂ (R-3m), and Cs₄GeGe₂Br₆Cl₆ (R-3m) exhibit thermodynamic, dynamical, and mechanical stability. The electronic structure calculations show that the predicted band gap of stable Cs₄Ge(IV)Ge(II)₂X₁₂ varies from 1.16 to 2.25 eV. And an isolated intermediate conduction band contributed by the Ge(IV) 4s states below the Ge(II)/Ge(IV) 4p states is observed in these compounds, which is similar to previously reported Cs₄CuSb₂Cl₁₂ but different from Cs₄CdM(III)₂Cl₁₂ (M = Sb, Bi). In addition, the calculated static dielectric constant and optical absorption coefficient of Cs₄GeGe₂Br₁₂ are close to those of typical double perovskites (e.g., Cs₂AgBiBr₆). We believe that our work enriches the family of mixed-valence halide perovskites and provides a new platform for potential optoelectronic semiconductor design.

Photoinduced Ultrafast Symmetry Switch in SnSe

Layered tin selenide (SnSe) has recently emerged as a high-performance thermoelectric material with the current record for the figure of merit (ZT) observed in the high-temperature Cmcm phase. So far, access to the Cmcm phase has been mainly obtained via thermal equilibrium methods based on sample heating or application of external pressure, thus restricting the current understanding only to ground-state conditions. Here, we investigate the ultrafast carrier and phononic dynamics in SnSe. Our results demonstrate that optical excitations can transiently switch the point-group symmetry of the crystal from Pnma to Cmcm at room temperature in a few hundreds of femtoseconds with an ultralow threshold for the excitation carrier density. This nonequilibrium Cmcm phase is found to be driven by the displacive excitation of coherent A_g phonons and, given the absence of low-energy thermal phonons, exists in SnSe with the status of 'cold lattice with hot carriers'. Our findings provide an important insight for understanding the nonequilibrium thermoelectric properties of SnSe.

Blue-Violet Emission with Near-Unity Photoluminescence Quantum Yield from Cu(I)-Doped Rb3InCl6 Single Crystals

Low-dimensional metal halides have attracted considerable attention due to their unique optoelectronic properties. In this study, we report a solid-state synthesis of air-stable all-inorganic Pb-free zero-dimensional (0D) Rb₃InCl₆ single crystals (SCs). By a heterovalent doping of Cu⁺ ions, the Rb₃InCl₆:Cu⁺ SCs featured an efficient blue-violet emission with a greatly enhanced photoluminescence (PL) quantum yield (95%) and an ultralong PL lifetime (13.95 μs). Combined with temperature-dependent PL and density functional theory calculations, we conclude that the efficient electronic isolation, enhanced exciton-phonon coupling, and electronic structure modulation after doping lead to bright blue-violet emission. Furthermore, the SCs exhibited excellent stability, maintaining 90% of the initial PL intensity after being stored in ambient conditions for more than two months. The results provide a new strategy for improving the optoelectronic properties of 0D all-inorganic metal halides, which is promising for potential light-emitting applications.

A New Perspective and Design Principle for Halide Perovskites: Ionic Octahedron Network (ION)

The metal halide ionic octahedron, [MX₆] (M = metal cation, X = halide anion), is considered to be the fundamental building block and functional unit of metal halide perovskites. By representing the metal halide ionic octahedron in halide perovskites as a super ion/atom, the halide perovskite can be described as an extended ionic octahedron network (ION) charge balanced by selected cations. This new perspective of halide perovskites based on ION enables the prediction of different packing and connectivity of the metal halide octahedra based on different solid-state lattices. In this work, a new halide perovskite Cs₈Au_3.5In_1.5Cl₂₃ was discovered on the basis of a BaTiO₃-lattice ION {[InCl₆][AuCl₅][Au/InCl₄]₃}^8-, which is assembled from three different ionic octahedra [InCl₆], [AuCl₆], and [Au/InCl₆] and balanced by positively charged Cs cations. The success of this ION design concept in the discovery of Cs₈Au_3.5In_1.5Cl₂₃ opens up a new venue for the rational design of new halide perovskite materials.

Band-Edge Orbital Engineering of Perovskite Semiconductors for Optoelectronic Applications

Lead (Pb) halide perovskites have achieved great success in recent years because of their excellent optoelectronic properties, which is largely attributed to the lone-pair s orbital-derived antibonding states at the valence band edge. Guided by the key band-edge orbital character, a series of ns²-containing (i.e., Sn²⁺, Sb³⁺, and Bi³⁺) Pb-free perovskite alternatives have been explored as potential photovoltaic candidates. On the other hand, based on the band-edge orbital components (i.e., M²⁺ s and p/X^- p orbitals), a series of strategies have been proposed to optimize their optoelectronic properties by modifying the atomic orbitals and orbital interactions. Therefore, understanding the band-edge electronic features from the recently reported halide perovskites is essential for future material design and device optimization. This Perspective first attempts to establish the band-edge orbital-property relationship using a chemically intuitive approach and then rationalizes their superior properties and explains the trends in electronic properties. We hope that this Perspective will provide atomic-level guidance and insights toward the rational design of perovskite semiconductors with outstanding optoelectronic properties.

Pressure-Triggered Blue Emission of Zero-Dimensional Organic Bismuth Bromide Perovskite

Understanding the structure-property relationships in Zero-dimensional (0D) organic-inorganic metal halide perovskites (OMHPs) is essential for their use in optoelectronic applications. Moreover, increasing the emission intensity, particularly for blue emission, is considerably a challenge. Here, intriguing pressure-induced emission (PIE) is successfully achieved from an initially nonluminous 0D OMHP [(C6H11NH3)4BiBr6]Br·CH3CN (Cy4BiBr7 ) upon compression. The emission intensity increases significantly, even reaching high-efficiency blue luminescence, as the external pressure is increased to 4.9 GPa. Analyses of the in situ high-pressure experiments and first-principle calculations indicate that the observed PIE can be attributed to the enhanced exciton binding energy associated with [BiBr6]3- octahedron distortion under pressure. This study of Cy4BiBr7 sheds light on the relationship between the structure and optical properties of OMHPs. The results may improve potential applications of such materials in the fields of pressure sensing and trademark security.

First-Principles Study on Optic-Electronic Properties of Charge-Ordered Indium Halide Perovskite Cs2In(I)In(III)Cl6 at High Pressure

Using the first principle method we studied, theoretically and in detail, the structural, optical, and electronic properties of a charge-ordered indium halide perovskite Cs2In(I)In(III)Cl6 at high pressure. In this structure, In1, In2, and In3 are octahedrally coordinated, whereas In4 is at the center of a pentagonal bipyramid. The charge of In on In1 and In2 sites can be assigned to 3+, while In+ occupies In3 and In4 sites. The results indicated that the band gap decreases, and the electron excitation produces the red-shift of peak value of optical absorption coefficient in visible and infrared regions with increasing pressure, and the reflectivity decreases in visible and infrared regions with increasing pressure. These theoretical results provide a basis for designing related inorganic halide perovskites.

Isosymmetric compression of cubic halide perovskites $$\mathrm{ABX}_{3}$$ ($$A=K, Rb, Cs$$; $$B=Ge, Sn, Pb$$ and $$X=Cl,Br,I$$)-influence of cation–anion exchange: a first principle study

Halide perovskites are potential candidates for their use in solar cells. In this study, an investigation of the structural, mechanical and electro-optical properties of $$\mathrm{ABX}_{3}$$ ABX 3 ($$A=K, Rb, Cs$$ A = K , R b , C s ; $$B=Ge, Sn, Pb$$ B = G e , S n , P b and $$X=Cl,Br,I$$ X = C l , B r , I ), under isosymmetric compression, is presented. All the calculations are performed with generalized gradient approximation schemes. We have examined the effect of cation and anion substitution on their overall properties.

Pressure-Suppressed Carrier Trapping Leads to Enhanced Emission in Two-Dimensional Perovskite (HA)2 (GA)Pb2 I7

A remarkable PL enhancement by 12 fold is achieved using pressure to modulate the structure of a recently developed 2D perovskite (HA)₂ (GA)Pb₂ I₇ (HA=n-hexylammonium, GA=guanidinium). This structure features a previously unattainable, extremely large cage. In situ structural, spectroscopic, and theoretical analyses reveal that lattice compression under a mild pressure within 1.6 GPa considerably suppresses the carrier trapping, leading to significantly enhanced emission. Further pressurization induces a non-luminescent amorphous yellow phase, which is retained and exhibits a continuously increasing band gap during decompression. When the pressure is released to 1.5 GPa, emission can be triggered by above-band gap laser irradiation, accompanied by a color change from yellow to orange. The obtained orange phase could be retained at ambient conditions and exhibits two-fold higher PL emission compared with the pristine (HA)₂ (GA)Pb₂ I₇ .

Mixed Redox-Couple-Involved Chalcopyrite Phase CuFeS2 Quantum Dots for Highly Efficient Cr(VI) Removal

Iron-based nanosized ecomaterials for efficient Cr(VI) removal are of great interest to environmental chemists. Herein, inspired by the "mixed redox-couple" cations involved in the crystal structure and the quantum confinement effects resulting from the particle size, a novel type of iron-based ecomaterial, semiconducting chalcopyrite quantum dots (QDs), was developed and used for Cr(VI) removal. A high removal capacity up to 720 mg/g was achieved under optimal pH conditions, which is superior to those of the state-of-the-art nanomaterials for Cr(VI) removal. The mechanism of Cr(VI) removal was elucidated down to an atomic scale by combining comprehensive characterization techniques with adsorption kinetic experiments and DFT calculations. The experimental results revealed that the material was a good electron donor semiconductor attributed to the existence of "mixed redox couple of Cu(I)-S-Fe(III)" in the crystal structure. With the size-dependent quantum confinement effect and the high surface area, the semiconducting chalcopyrite QDs could effectively remove Cr(VI) from aqueous solution through a syngenetic photocatalytic reduction and adsorption mechanism. This study not only reports the design histogram of the iron-based CuFeS₂ QD ecomaterial for efficient Cr(VI) removal but also paves the way for understanding the atomic-scale mechanism behind the syngenetic effects of using the QD semiconducting material for Cr(VI) removal.

Effect of B-site cation ordering on high temperature thermoelectric behavior of Ba x Sr2-x TiFeO6 double perovskites

Here, we have reported the detailed structural analysis in correlation with thermoelectric properties of Ba doped Sr₂TiFeO₆ (BSTF) double perovskites in the temperature range from 300 K to 1100 K. BSTF compositions exhibit single phase cubic structure with [Formula: see text] crystal symmetry from room temperature to 523 K and also at temperature beyond 923K. Rietveld refinement of high temperature XRD data suggests the coexistence of two cubic phases with [Formula: see text] space group having same composition in the intermediate temperature region. Correlation of the phase-fraction with electrical conductivity data posits the possibility of high temperature cubic phase being conductive compared to the insulator-like cubic phase observed at room temperature. The experimental analysis alone seems insufficient to explain the conductivity behavior demonstrating semiconductor [Formula: see text] to metal like [Formula: see text] transition. Hence DFT framework has been adopted for computational analysis coupled with the Boltzmann transport equations to understand their thermoelectric properties based on the electronic restructuring occurred due to octahedral arrangements in these double perovskites. It has been shown that clustering of FeO₆ octahedra may lead to the formation of a conduction path in the cubic phase of BSTF, which induces metallic behavior in these double perovskites.

Pressure-Induced Remarkable Enhancement of Self-Trapped Exciton Emission in One-Dimensional CsCu2I3 with Tetrahedral Units

Self-trapped exciton (STE) emissions derived from inorganic octahedral units make metal halide perovskites promising photoluminescence materials for light-emitting applications. However, there is still little understanding of the intrinsic STE emissions in metal halide perovskites or derivatives with nonoctahedral units. In this work, via high pressure compression, remarkable STE emission enhancement is, for the first time, realized in one-dimensional CsCu₂I₃ crystals with {CuCl₄} tetrahedral units. The intertetrahedral distortion is believed to induce the slight emission enhancement of the ambient phase under initial compression. Notably, the obvious structural distortions of both inter- and intratetrahedra are responsible for the significant emission enhancement of the high pressure phase. This work not only sheds light on the structure-optical property relationships of tetrahedron-based halide complexes, but also may provide guidance for the design and fabrication of highly luminescent metal halides.

Organic–inorganic hybrid perovskite electronics

Organic–inorganic hybrid perovskite is a leading successor for the next generation of (opto)electronics.