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Two-dimensional (2D) hybrid organic–inorganic perovskites (HOIPs) offer an outstanding opportunity for spin-related technologies owing in part to their tunable structural symmetry breaking and distortions driven by organic–inorganic hydrogen (H) bonds. However, understanding how H-bonds tailor inorganic symmetry and distortions and therefore enhance spin splitting for more effective spin manipulation remains imprecise due to challenges in measuring H atom positions using X-ray diffraction. Here, we report a thermally induced structural transition (at ∼209 K) for a 2D HOIP, (2-BrPEA)2PbI4[2-BrPEA = 2-(2-bromophenyl)ethylammonium], which induces inversion asymmetry and a strong spin splitting (ΔE> 30 meV). While X-ray diffraction generally establishes heavy atom coordinates, we utilize neutron diffraction for accurate H atom position determination, demonstrating that the structural transition-induced rearrangement of H-bonds with distinct bond strengths asymmetrically shifts associated iodine atom positions. Consequences of this shift include an increased structural asymmetry, an enhanced difference between adjacent interoctahedra distortions (i.e., Pb–I–Pb bond angles), and therefore significant spin splitting. We further show that H-only density-functional theory (DFT) relaxation of the X-ray structure shifts H atoms to positions that are consistent with the neutron experimental data, validating a convenient pathway to more generally improve upon HOIP H-bonding analyses derived from quicker/less-expensive X-ray data.

Two-dimensional organic semiconductor-incorporated perovskites are a promising family of hybrid materials for optoelectronic applications, owing in part to their inherent quantum well architecture. Tuning their structures and properties for specific properties, however, has remained challenging. Here we report a general method to tune the dimensionality of phase-pure organic semiconductor-incorporated perovskite single crystals during their synthesis, by judicious choice of solvent. The length of the conjugated semiconducting organic cations and the dimensionality (nvalue) of the inorganic layers can be manipulated at the same time. The energy band offsets and exciton dynamics at the organic–inorganic interfaces can therefore be precisely controlled. Furthermore, we show that longer and more planar π-conjugated organic cations induce a more rigid inorganic crystal lattice, which leads to suppressed exciton–phonon interactions and better optoelectronic properties as compared to conventional two-dimensional perovskites. As a demonstration, optically driven lasing behaviour with substantially lower lasing thresholds was realized.

Chiral organic building blocks can be incorporated into hybrid organic–inorganic metal-halide crystalline semiconductors so as to control and impact the interconversion between light, charge, and spin. Here, we report a series of hybrid antimony and bismuth halide materials of the general formula MBA4B2X10(B = Sb3+, Bi3+; X = Br–, I–) and study how chiral symmetry breaking imposed by the templating organic chiral methylbenzylammonium (MBA) cations induces symmetry breaking within the inorganic sublattice and leads to a unique spin-texture. The chiral MBA cations introduce two structural modifications to the metal-halide sublattice that consists of isolated edge-sharing dimers of B2X10octahedra: (1) the dimers have a chiral spatial arrangement with respect to one another and (2) there is an asymmetric distortion of the two subunits within the individual dimers with a higher distortion caused by strong stereochemical activity of the Sb 5s2lone pair electrons. The structural distortions and chiral arrangement result in circular dichroism (CD) at the band edge of the inorganic framework with dissymmetry factors in the range of 10–4. Chiral spin-splitting of the inorganic states, caused by breaking of the inversion symmetry and large spin–orbit coupling, is studied via density functional theory (DFT), and a multiband effective mass theory was developed that links the DFT-derived spin-splitting of helical character (i.e., spin expectation value not perpendicular to the crystal momentum) to the observed CD. We also find broad red photoluminescence from the MBA4Sb2Br10compounds, which we attribute to self-trapped excitonic emission driven by the large distortion due to the lone pair expression.

Two-dimensional hybrid organic–inorganic perovskite (HOIP) semiconductors with pronounced spin splitting, mediated by strong spin–orbit coupling and inversion symmetry breaking, offer the potential for spin manipulation in future spintronic applications. However, HOIPs exhibiting significant conduction/valence band splitting are still relatively rare, given the generally observed preference for (near)centrosymmetric inorganic (especially lead-iodide-based) sublattices, and few approaches are available to control this symmetry breaking within a given HOIP. Here, we demonstrate, using (S-2-MeBA)2PbI4(S-2-MeBA = (S)-(−)-2-methylbutylammonium) as an example, that a temperature-induced structural transition (at ∼180 K) serves to change the degree of chirality transfer to and inversion symmetry breaking within the inorganic layer, thereby enabling modulation of HOIP structural and electronic properties. The cooling rate is shown to dictate whether the structural transition occurs─i.e., slow cooling induces the transition while rapid quenching inhibits it. Ultrafast calorimetry indicates a minute-scale structural relaxation time at the transition temperature, while quenching to lower temperatures allows for effectively locking in the metastable room-temperature phase, thus enabling kinetic control over switching between distinct states with different degrees of structural distortions within the inorganic layers at these temperatures. Density functional theory further highlights that the low-temperature phase of (S-2-MeBA)2PbI4shows more significant spin splitting relative to the room-temperature phase. Our work opens a new pathway to use kinetic control of crystal-to-crystal transitions and thermal cycling to modulate spin splitting in HOIPs for future spintronic applications, and further points to using such “sluggish” phase transitions for switching and control over other physical phenomena, particularly those relying on structural distortions and lattice symmetry.

We present an accurate computational approach to calculate absolute K-edge core electron excitation energies as measured by X-ray absorption spectroscopy. Our approach employs an all-electron Bethe–Salpeter equation (BSE) formalism based on GWquasiparticle energies (BSE@GW) using numeric atom-centered orbitals (NAOs). The BSE@GWmethod has become an increasingly popular method for the computation of neutral valence excitation energies of molecules. However, it was so far not applied to molecular K-edge excitation energies. We discuss the influence of different numerical approximations on the BSE@GWcalculation and employ in our final setup (i) exact numeric algorithms for the frequency integration of the GWself-energy, (ii) G0W0and BSE starting points with ∼50% of exact exchange, (iii) the Tamm–Dancoff approximation and (iv) relativistic corrections. We study the basis set dependence and convergence with common Gaussian-type orbital and NAO basis sets. We identify the importance of additional spatially confined basis functions as well as of diffuse augmenting basis functions. The accuracy of our BSE@GWmethod is assessed for a benchmark set of small organic molecules, previously used for benchmarking the equation-of-motion coupled cluster method [Peng et al., J. Chem. Theory Comput., 2015, 11, 4146], as well as the medium-sized dibenzothiophene (DBT) molecule. Our BSE@GWresults for absolute excitation energies are in excellent agreement with the experiment, with a mean average error of only 0.63 eV for the benchmark set and with errors <1 eV for the DBT molecule.

Quaternary chalcogenide semiconductors are promising materials for energy conversion and nonlinear optical applications, with properties tunable primarily by varying the elemental composition and crystal structure. Here, we first analyze the connections among several cubic crystal structure types, as well as the orthorhombic Ag2PbGeS4-type structure, reported for select members within the Ag–BII–MIV–X (BII= Sr, Pb; MIV= Si, Ge, Sn; X = S, Se) compositional space. Focusing on the Ag–Pb–Si–S and Ag–Sr–Sn–S systems, we show that one structure type, with the formulas Ag2Pb3Si2S8and Ag2Sr3Sn2S8, is favored. We have prepared powder and single-crystal samples of Ag2Pb3Si2S8and Ag2Sr3Sn2S8, showing that each takes on the noncentrosymmetric cubic space group I4̅3dand is isostructural to the previously reported compound Ag2Sr3Ge2Se8. Through hybrid density functional theory calculations, these cubic compounds are demonstrated to be (quasi-)direct band gap semiconductors with high densities of states at the band maxima. The band-gap energies are measured by reflectance spectroscopy as 1.95(3) and 2.66(4) eV for Ag2Pb3Si2S8and Ag2Sr3Sn2S8, respectively. We further measure the optical properties and show the electronic band structures of three other isostructural AI–BII–MIV–X-type materials, i.e., Ag2Sr3Si2S8, Ag2Sr3Ge2S8, and Ag2Sr3Ge2Se8, showing that the band gaps can be predictably tuned by element substitution. Detailed visual analyses of the different structures and of their relationships with other members of the Ag–BII–MIV–X compositional family provide a basis for a broader understanding of the structure formation and optoelectronic properties within the quaternary chalcogenide semiconductor family.

Quaternary chalcogenide materials have long been a source of semiconductors for optoelectronic applications. Recent studies on I2–II–IV–X4(I = Ag, Cu, Li; II = Ba, Sr, Eu, Pb; IV = Si, Ge, Sn; X = S, Se) materials have shown particular versatility and promise among these compounds. These semiconductors take advantage of a diverse bonding scheme and chemical differences among cations to target a degree of antisite defect resistance. Within this set of compounds, the materials containing both Ag and Sr have not been experimentally studied and leave a gap in the full understanding of the family. Here, we have synthesized powders and single crystals of two Ag- and Sr-containing compounds, Ag2SrSiS4and Ag2SrGeS4, each found to form in the tetragonal I4̅2mstructure of Ag2BaGeS4. During the synthesis targeting the title compounds, two additional materials, Ag2Sr3Si2S8and Ag2Sr3Ge2S8, have also been identified. These cubic compounds represent impurity phases during the synthesis of Ag2SrSiS4and Ag2SrGeS4. We show through hybrid density functional theory calculations that Ag2SrSiS4and Ag2SrGeS4have highly dispersive band-edge states and indirect band gaps, experimentally measured as 2.08(1) and 1.73(2) eV, respectively. Second-harmonic generation measurements on Ag2SrSiS4and Ag2SrGeS4powders show frequency-doubling capabilities in the near-infrared range.

Perovskite solar cells have received substantial attention due to their potential for low-cost photovoltaic devices on flexible or rigid substrates. Thiocyanate (SCN)-containing additives, such as MASCN (MA = methylammonium), have been shown to control perovskite film crystallization and the film microstructure to achieve effective room-temperature perovskite absorber processing. Nevertheless, the crystallization pathways and mechanisms of perovskite formation involved in MASCN additive processing are far from clear. Using in situ X-ray diffraction and photoluminescence, we investigate the crystallization pathways of MAPbI3and reveal the mechanisms of additive-assisted perovskite formation during spin coating and subsequent N2drying. We confirm that MASCN induces large precursor aggregates in solution and, during spin coating, promotes the formation of the perovskite phase with lower nucleation density and overall larger initial nuclei size, which forms upon reaching supersaturation in solution, in addition to intermediate solvent–complex phases. Finally, during the subsequent N2drying, MASCN facilitates the dissociation of these precursor aggregates and the solvate phases, leading to further growth of the perovskite crystals. Our results show that the nature of the intermediate phases and their formation/dissociation kinetics determine the nucleation and growth of the perovskite phase, which subsequently impact the film microstructure. These findings provide mechanistic insights underlying room-temperature, additive-assisted perovskite processing and help guide further development of such facile room-temperature synthesis routes.

Incorporating chiral organic molecules into organic/inorganic hybrid 2D metal-halide perovskites results in a novel family of chiral hybrid semiconductors with unique spin-dependent properties. The embedded chiral organic moieties induce a chiroptical response from the inorganic metal–halide sublattice. However, the structural interplay between the chiral organic molecules and the inorganic sublattice, as well as their synergic effect on the resulting electronic band structure need to be explored in a broader material scope. Here we present three new layered tin iodide perovskites templated by chiral (R/S-)methylbenzylammonium (R/S-MBA), i.e., (R-/S-MBA)2SnI4, and their racemic phase (rac-MBA)2SnI4. These MBA2SnI4compounds exhibit the largest level of octahedral bond distortion compared to any other reported layered tin iodide perovskite. The incorporation of chiral MBA cations leads to circularly polarized absorption from the inorganic Sn–I sublattice, displaying chiroptical activity in the 300–500 nm wavelength range. The bandgap and chiroptical activity are modulated by alloying Sn with Pb, in the series of (MBA)2Pb1–xSnxI4. Finally, we show that vertical charge transport through oriented (R-/S-MBA)2SnI4thin films is highly spin-dependent, arising from a chiral-induced spin selectivity (CISS) effect. We demonstrate a spin-polarization in the current–voltage characteristics as high as 94%. Our work shows the tremendous potential of these chiral hybrid semiconductors for controlling both spin and charge degrees of freedom.

Recent work on quaternary semiconductors Cu2BaSn(S,Se)4and Ag2BaSnSe4for photovoltaic and thermoelectric applications, respectively, has shown the promise of exploring the broader family of defect-resistant I2-II-IV-X4materials (where I, II, and IV refer to the formal oxidation state of the metal cations and X is a chalcogen anion) with tetrahedrally coordinated I/IV cations and larger II cations (i.e., Sr, Ba, Pb, and Eu) for optoelectronic and energy-related applications. Chemical dissimilarity among the II and I/IV atoms represents an important design motivation because it presents a barrier to antisite formation, which otherwise may act as electronically harmful defects. We herein show how all 31 experimentally reported I2-II-IV-X4examples (with large II cations and tetrahedrally coordinated smaller I/IV cations), which form within five crystal structure types, are structurally linked. Based on these structural similarities, we derive a set of tolerance factors that serve as descriptors for phase stability within this family. Despite common usage in the well-studied perovskite system, Shannon ionic radii are found to be insufficient for predicting metal–chalcogen bond lengths, pointing to the need for experimentally derived correction factors as part of an empirically driven learning approach to structure prediction. We use the tolerance factors as a predictive tool and demonstrate that four new I2-II-IV-X4compounds, Ag2BaSiS4, Ag2PbSiS4, Cu2PbGeS4, and Cu2SrSiS4, can be synthesized in correctly predicted phases. One of these compounds, Ag2PbSiS4, shows potentially promising optoelectronic properties for photovoltaic applications.

Halide argyrodite solid-state electrolytes of the general formula Li6PS5Xexhibit complex static and dynamic disorder that plays a crucial role in ion transport processes. Here, we unravel the rich interplay between site disorder and dynamics in the plastic crystal argyrodite Li6PS5CN and the impact on ion diffusion processes through a suite of experimental and computational methodologies, including temperature-dependent synchrotron powder X-ray diffraction, AC electrochemical impedance spectroscopy, 7Li solid-state NMR, and machine learning-assisted molecular dynamics simulations. Sulfide and (pseudo)halide site disorder between the two anion sublattices unilaterally improves long-range lithium diffusion irrespective of the (pseudo)halide identity, which demonstrates the importance of site disorder in dictating bulk ionic conductivity in the argyrodite family. Furthermore, we find that anion site disorder modulates the presence and time scales of cyanide rotational dynamics. Ordered configurations of anions enable fast, quasi-free rotations of cyanides that occur on time scales of 1011Hz at T= 300 K. In contrast, we find that cyanide dynamics are slow or frozen in Li6PS5CN when site disorder between the cyanide and sulfide sublattices is present at T= 300 K. We rationalize the observed differences in cyanide dynamics in the context of elastic dipole interactions between neighboring cyanide anions and local strain induced by the configurations of site disorder that may impact the energetic landscape for cyanide rotational dynamics. Through this study, we find that anion disorder plays a decisive role in dictating the extent and time scales of both lithium ion and cyanide dynamics in Li6PS5CN.

Recently, bournonite (CuPbSbS3) has been identified as a potential ferroelectric photovoltaic (PV) material with a 1.3 eV band gap, which falls in an appropriate range for single-junction PV devices. Progress in applying bournonite has yielded several studies reporting successful thin-film processing, the most recent of which demonstrated a 2.65% power conversion efficiency for a bournonite-based PV device. In an effort to explore bournonite band gap engineering for PV and other prospective applications (e.g., thermoelectric, spintronic), we here consider the solid-state synthesis of selenium-alloyed bournonite, CuPbSb(S1–xSex)3, across the full range of x(0.0 ≤ x≤ 1.0) and report phase purity for 0.0 ≤ x≤ 0.5. We characterize the crystal structure and band gap of the samples using X-ray diffraction and diffuse reflectance spectroscopy, determining a band gap decrease for single-phase samples from 1.25 eV at x= 0.0 to 1.06 eV at x= 0.5. Formation energies determined using dispersion-corrected hybrid density functional theory (DFT) support experimental findings and are consistent with the stability (instability) of the x= 0.5 (x= 1.0) structure relative to starting materials. The computed band structures show a decreasing trend in the band gap with increasing x, again consistent with experiment, with the states at the conduction (valence) band minima (maxima) being principally derived from Pb (S/Se) states. Spin texture analysis and detailed comparison of spin splitting parameters show that Se alloying modifies the band gap while maintaining Rashba spin splitting character within the electronic structure. Rashba splitting is most pronounced for the conduction band minimum along the Γ–Xpath in the Brillouin zone. Energy separation between spin states is maintained at ΔE∼0.2 eV for the various Se contents, with σzas the spin direction. A smaller spin splitting occurs along Γ–Z(decreasing value with increasing x), with σxas the spin direction.

Semiconductor quantum-well structures and superlattices are key building blocks in modern optoelectronics, but it is difficult to simultaneously realize defect-free epitaxial growth while fine tuning the chemical composition, layer thickness and band structure of each layer to achieve the desired performance. Here we demonstrate the modulation of the electronic structure—and consequently the optical properties—of organic semiconducting building blocks that are incorporated between the layers of perovskites through a facile solution processing step. Self-aggregation of the conjugated organic molecules is suppressed by functionalization with sterically demanding groups and single crystalline organic–perovskite hybrid quantum wells (down to one-unit-cell thick) are obtained. The energy and charge transfers between adjacent organic and inorganic layers are shown to be fast and efficient, owing to the atomically flat interface and ultrasmall interlayer distance of the perovskite materials. The resulting two-dimensional hybrid perovskites are very stable due to protection given by the bulky hydrophobic organic groups.

Two-dimensional (2D) hybrid organic–inorganic perovskites (HOIPs) represent diverse quantum well heterostructures composed of alternating inorganic and organic layers. While 2D HOIPs are nominally periodic in three dimensions for X-ray scattering, the inorganic layers can orient quasi-randomly, leading to rotational stacking disorder (RSD). RSD manifests as poorly resolved, diffuse X-ray scattering along the stacking direction, limiting the structural description to an apparently disordered subcell. However, local ordering preferences can still exist between adjacent unit cells and can considerably impact the properties, particularly the electronic structure. Here, we elucidate RSD and determine the preferred local ordering in the 2D [AE2T]PbI4HOIP (AE2T: 5,5′-bis(ethylammonium)-[2,2′-bithiophene]). We use first-principles calculations to determine energy differences between a set of systematically generated supercells with different order patterns. We show that interlayer ordering tendencies are weak, explaining the observed RSD in terms of differing in-plane rotation of PbI6octahedra in neighboring inorganic planes. In contrast, the ordering preference within a given organic layer is strong, favoring a herringbone-type arrangement of adjacent AE2T cations. The calculated electronic level alignments of proximal organic and inorganic frontier orbitals in the valence band vary significantly with the local arrangement of AE2T cations; only the most stable AE2T configuration leads to an interfacial type-Ib band alignment consistent with observed optical properties. The present study underscores the importance of resolving local structure arrangements in 2D HOIPs for reliable structure–property prediction.

Three-dimensional (3D) hybrid organic–inorganic lead halide perovskites (HOIPs) feature remarkable optoelectronic properties for solar energy conversion but suffer from long-standing issues of environmental stability and lead toxicity. Associated two-dimensional (2D) analogues are garnering increasing interest due to superior chemical stability, structural diversity, and broader property tunability. Toward lead-free 2D HOIPs, double perovskites (DPs) with mixed-valent dual metals are attractive. Translation of mixed-metal DPs to iodides, with their prospectively lower bandgaps, represents an important target for semiconducting halide perovskites, but has so far proven inaccessible using traditional spacer cations due to either intrinsic instability or formation of competing non-perovskite phases. Here, we demonstrate the first example of a 2D Ag–Bi iodide DP with a direct bandgap of 2.00(2) eV, templated by a layer of bifunctionalized oligothiophene cations, i.e., (bis-aminoethyl)bithiophene, through a collective influence of aromatic interactions, hydrogen bonding, bidentate tethering, and structural rigidity. Hybrid density functional theory calculations for the new material reveal a direct bandgap, consistent with the experimental value, and relatively flat band edges derived principally from Ag-d/I-p (valence band) and Bi-p/I-p (conduction band) states. This work opens up new avenues for exploring specifically designed organic cations to stabilize otherwise inaccessible 2D HOIPs with potential applications for optoelectronics.

Hybrid perovskites incorporating conjugated organic cations enable unusual charge carrier interactions among organic and inorganic structural components, but are difficult to prepare as films due to disparate component chemical/physical characteristics (e.g., solubility, thermal stability). Here we demonstrate that resonant infrared matrix-assisted pulsed laser evaporation (RIR-MAPLE) mitigates these challenges, enabling facile deposition of lead-halide-based perovskite films incorporating variable-length oligothiophene cations. Density functional theory (DFT) predicts suitable organic and inorganic moieties that form quantum-well-like structures with targeted luminescence or exciton separation/quenching. RIR-MAPLE-deposited films enable confirmation of these predictions by optical measurements, which further display excited state behavior transcending traditional quantum-well models—i.e., with appropriate selection of specially synthesized organic/inorganic moieties, intercomponent carrier transfer efficiently converts excitons from singlet to triplet states in organics for which intersystem crossing cannot ordinarily compete with recombination. These observations demonstrate the uniquely versatile excited-state behavior in hybrid perovskite quantum wells, and the value of integrating DFT, organic synthesis, RIR-MAPLE and spectroscopy for screening/preparing rationally devised complex structures.

Recently, the I2–II–IV–VI4(I = Cu, Ag; II = Ba, Sr; IV = Ge, Sn; VI = S, Se) materials family was identified as a promising source of potential new photovoltaic (PV) and photoelectrochemical (PEC) absorbers. These materials avoid the pitfalls of the successful photovoltaic semiconductors Cu(In,Ga)(S,Se)2and CdTe, as they do not contain scarce (In, Te) or toxic (Cd) elements. Furthermore, ionic sizes and coordination preferences are very different for the I, II, and IV cations in the I2–II–IV–VI4family, providing an intriguing avenue to avoid intrinsic antisite disordering that limits efficiency improvement in Cu2ZnSn(S,Se)4(where Cu and Zn can easily substitute for one another). Here, we experimentally and computationally explore alloys Cu2BaGe1–xSnxSe4(CBGTSe, 0 ≤ x≤ 1) to fine-tune the structural, optical, and electronic properties for the relatively large band gap (Eg= 1.91(5) eV) unalloyed compound Cu2BaGeSe4(CBGSe). We show that CBGTSe maintains the P31crystal structure type of the parent CBGSe up to x≤ 0.70. A minimum band gap value of 1.57(5) eV can be reached at x= 0.70 before the structure transforms to the Ama2 structure type. The experimental and theoretical investigations demonstrate the potential of CBGTSe for thin-film PV and PEC absorbers.

With the advent of efficient electronic structure methods, effective continuum solvation methods have emerged as a way to, at least partially, include solvent effects into simulations without the need for expensive sampling over solvent degrees of freedom. The multipole moment expansion (MPE) model, while based on ideas initially put forward almost 100 years ago, has recently been updated for the needs of modern electronic structure calculations. Indeed, for an all-electron code relying on localized basis sets andmore importantlya multipole moment expansion of the electrostatic potential, the MPE method presents a particularly cheap way of solving the macroscopic Poisson equation to determine the electrostatic response of a medium surrounding a solute. In addition to our implementation of the MPE model in the FHI-aims electronic structure theory code [Blum, V.; Comput. Phys. Commun.2009, 180, 2175−2196, DOI: 10.1016/j.cpc.2009.06.022], we describe novel algorithms for determining equidistributed points on the solvation cavitydefined as a charge density isosurfaceand the determination of cavity surface and volume from just this collection of points and their local density gradients. We demonstrate the efficacy of our model on an analytically solvable test case, against high-accuracy finite-element calculations for a set of ≈140000 2D test cases, and finally against experimental solvation free energies of a number of neutral and singly charged molecular test sets [Andreussi, O.; J. Chem. Phys.2012, 136, 064102, DOI: 10.1063/1.3676407; Marenich, A. V.; Minnesota Solvation Database, Version 2012; University of Minnesota: Minneapolis, MN, USA,2012.]. In all test cases we find that our MPE approach compares very well with given references at computational overheads < 20% and sometimes much smaller compared to a plain self-consistency cycle.

The development of metal halide perovskite/perovskite heterostructures is hindered by rapid interfacial halide diffusion leading to mixed alloys rather than sharp interfaces. To circumvent this outcome, we developed an ion-blocking layer consisting of single-layer graphene (SLG) deposited between the metal halide perovskite layers and demonstrated that it effectively blocks anion diffusion in a CsPbBr3/SLG/CsPbI3heterostructure. Spatially resolved elemental analysis and spectroscopic measurements demonstrate the halides do not diffuse across the interface, whereas control samples without the SLG show rapid homogenization of the halides and loss of the sharp interface. Ultraviolet photoelectron spectroscopy, DFT calculations, and transient absorbance spectroscopy indicate the SLG has little electronic impact on the individual semiconductors. In the CsPbBr3/SLG/CsPbI3, we find a type I band alignment that supports transfer of photogenerated carriers across the heterointerface. Light-emitting diodes (LEDs) show electroluminescence from both the CsPbBr3and CsPbI3layers with no evidence of ion diffusion during operation. Our approach provides opportunities to design novel all-perovskite heterostructures to facilitate the control of charge and light in optoelectronic applications.

Recently, there has been extensive research into photovoltaic, thermoelectric, and nonlinear optical applications of chalcogenide semiconductors within the large set of defect-resistant I2-II-IV-X4(I = Li, Cu, Ag; II = Ba, Sr, Eu, Pb; IV = Si, Ge, Sn; X = S, Se) compounds. Five Eu-including compounds have previously been reported within this family, but a comparative study of possible structures and electronic properties of all 18 Eu-based combinations is still absent. Herein, we use hybrid density functional theory to study rare-earth-including I2-II-IV-X4semiconductors with Eu on the II site, in order to further understand this family and test the geometric tolerance factor approach (reported in our previous work) as a tool for predicting potential stable structures. We investigate how the exchange mixing parameter of the HSE06 density functional, α, affects the energetic positions of electronic levels, especially of the localized f-electron orbitals near the band edges of the extended semiconductor structures, using literature photoemission and band gap data of EuS for comparison. Lowest-energy quaternary structure candidates, energy band structures, and densities of states are computationally predicted for all 18 materials. Based on its predicted photovoltaics-relevant band gap, the previously unknown compound Cu2EuSnSe4was selected and synthesized. The experimental structure, lattice parameters, and band gap of Cu2EuSnSe4are consistent with the computational predictions, confirming a 1.55 eV band gap.

Hybrid perovskites have emerged as a group of semiconductors that can solve key problems involving efficiency and production cost in optoelectronics and spintronics research. However, several challenges still lie ahead for the successful adoption of these materials at an industrial scale.An abundance of data on novel hybrid perovskites and their properties is emerging in the literature. Capturing this treasure trove of data is virtually impossible without organized, curated data collection efforts.Large, curated, and targeted databases for hybrid perovskites may be especially promising as a foundation of machine learning efforts for tackling material design problems.Construction of such large databases would likely require a hybrid approach involving automated extraction (using fine-tuned language processing models and computer vision models), but careful validation by domain experts.

Cation mixing in two-dimensional (2D) hybrid organic–inorganic perovskite (HOIP) structures represents an important degree of freedom for modifying organic templating effects and tailoring inorganic structures. However, the limited number of known cation-mixed 2D HOIP systems generally employ a 1:1 cation ratio for stabilizing the 2D perovskite structure. Here, we demonstrate a chiral–chiral mixed-cation system wherein a controlled small amount (<10%) of chiral cation S-2-MeBA (S-2-MeBA = (S)-(−)-2-methylbutylammonium) can be doped into (S-BrMBA)2PbI4(S-BrMBA = (S)-(−)-4-bromo-α-methylbenzylammonium), modulating the structural symmetry from a higher symmetry (C2) to the lowest symmetry state (P1). This structural change occurs when the concentration of S-2-MeBA, measured by solution nuclear magnetic resonance, exceeds a critical level─specifically, for 1.4 ± 0.6%, the structure remains as C2, whereas 3.9 ± 1.4% substitution induces the structure change to P1 (this structure is stable to ∼7% substitution). Atomic occupancy analysis suggests that one specific S-BrMBA cation site is preferentially substituted by S-2-MeBA in the unit cell. Density functional theory calculations indicate that the spin splitting along different k-paths can be modulated by cation doping. A true circular dichroism band at the exciton energy of the 3.9% doping phase shows polarity inversion and a ∼45 meV blue shift of the Cotton-effect-type line-shape relative to (S-BrMBA)2PbI4. A trend toward suppressed melting temperature with higher doping concentration is also noted. The chiral cation doping system and the associated doping-concentration-induced structural transition provide a material design strategy for modulating and enhancing those emergent properties that are sensitive to different types of symmetry breaking.

Interfaces and subsurface layers are critical for the performance of devices made of 2D materials and heterostructures. Facile, nondestructive, and quantitative ways to characterize the structure of atomically thin, layered materials are thus essential to ensure control of the resultant properties. Here, we show that contact-resonance atomic force microscopywhich is exquisitely sensitive to stiffness changes that arise from even a single atomic layer of a van der Waals-adhered materialis a powerful experimental tool to address this challenge. A combined density functional theory and continuum modeling approach is introduced that yields sub-surface-sensitive, nanomechanical fingerprints associated with specific, well-defined structure models of individual surface domains. Where such models are known, this information can be correlated with experimentally obtained contact-resonance frequency maps to reveal the (sub)surface structure of different domains on the sample.