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DOE OSTI2025

Spontaneous Formation of Single-Crystalline Spherulites in a Chiral 2D Hybrid Perovskite

In two-dimensional (2D) chiral metal-halide perovskites (MHPs), chiral organic spacers induce structural chirality and chiroptical properties in the metal-halide sublattice. This structural chirality enables reversible crystalline-glass phase transitions in (S-NEA) 2 PbBr 4 , a prototypical chiral 2D MHP where NEA + represents 1-(1-naphthyl)ethylammonium. Here, in this study, we investigate two distinct spherulite states of (S-NEA) 2 PbBr 4 , exhibiting either radial-like or stripe-like banded patterns depending on the annealing conditions of the amorphous film. Despite similarities in optical absorption and photoluminescence, the stripe-like, banded spherulite exhibits higher crystallinity and improved optical transparency compared to those of radial-like spherulite. X-ray nanoprobe measurements reveal tilting-angle modulations in the octahedral plane of stripe-like spherulites, correlating with the film’s surface geometry. Transfer matrix calculations indicate that the optical contrast in stripe-like patterns, seen in bright-field optical microscopy, arises from optical interference effects, differing from the contrast mechanism observed in polymer spherulites. Ultrafast carrier dynamics experiments suggest that the stripe-like spherulites resemble single crystals more closely than radial-like spherulites, while electrical conductivity measurements show enhanced charge carrier transport in stripe-like spherulites. These findings offer insights into MHP spherulite states with a single composition but different morphologies, previously observed only in polymers, highlighting their potential for optoelectronic applications.

36 MATERIALS SCIENCE↗
DOE OSTI2025

MXenoids: Generalization of MXene-Inspired Covalent Surface Modifications Across Two-Dimensional Materials

The ability to perform versatile covalent surface modifications in two-dimensional (2D) inorganic materials marks a significant advance in the functionalization of this broad family of materials. One particularly successful example of 2D materials with chemically modifiable surfaces are 2D transition metal carbides and nitrides (MXenes). MXenes' strong in-plane metal-carbon bonds and labile surface metal-halide bonds create altogether unprecedented opportunities for versatile postsynthetic modifications and assembling complex materials, including various organic-inorganic hybrids. Here, we demonstrate the general applicability of this surface modification strategy to non-MXene halide-terminated 2D materials, termed MXenoids. These surface modifications enable compositional and electronic structure engineering, introduce chiral hybrid organic-inorganic structures, and photoluminescence ranging from near-IR to blue. This study highlights the avenue of surface chemistry-driven materials design, enhancing the functional capabilities of 2D materials.

Zhou, Chenkun [University of Chicago, IL (United S↗
DOE OSTI2025

Multifunctional Polar 2D Lead Iodide Perovskites Exhibiting Persistent Spin Texture

Ferroelectric Rashba semiconductors are a rare class of multifunctional materials promising for spin–orbitronics due to the possibility of electrically switchable spin textures. Hybrid organic–inorganic metal halide perovskites offer exceptional tunability that can be harnessed to target noncentrosymmetry; however, the complex interactions between organic and inorganic components are not rationally understood. Here, we use an asymmetric spacer cation with a strong molecular dipole moment (2-fluorobenzylammonium, 2FBZ) and increase the intrinsic quantum well thickness to n > 1 to synthesize three new polar 2D lead iodide perovskites (2FBZ) 2 (A) n-1 Pb n I 3n+1 with A = methylammonium (MA) or formamidinium (FA) and n = 2, 3. Single-crystal structure analysis reveals the C 2v symmetry of these crystal structures and substantial structural distortions, which enable Rashba and Dresselhaus band splitting and persistent spin texture, confirmed by DFT calculations. Measurements of the resulting symmetry-dependent properties, such as second harmonic generation, switchable photovoltaic effect, ferroelectric polarization, and low temperature photoluminescence show switchable ferroelectric semiconducting properties. Furthermore, these results introduce a general chemical design strategy toward polar symmetry by exploiting the complex interplay between asymmetric spacer cations and A-site cations in quasi-2D (n > 1) halide perovskites to realize new classes of multifunctional materials for future spin–orbitronic applications.

Cations↗
DOE OSTI2026

Ultrafast Carrier Self-Trapping Driven by Strong Exciton–Phonon Coupling in 2D MA 3 Bi 2 I 6 Cl 3 Perovskite

Two-dimensional (2D) hybrid bismuth halide perovskites have emerged as promising lead-free materials for optoelectronic applications due to their solution processability and tunable structures. Here, in this study, we investigate 2D layered hybrid perovskite MA 3 Bi 2 I 6 Cl 3 using temperature-dependent photoluminescence (PL) and femtosecond transient absorption spectroscopy. Our results reveal strong coupling between excitons and phonons, evidenced by giant Huang–Rhys factors, coherent phonon oscillations, and ultrafast carrier self-trapping into small-polaron and self-trapped exciton (STE) states. These processes appear as time-dependent ground-state bleach and photoinduced absorption features, highlighting the influence of the lattice in carrier dynamics. Wavelength- and time-resolved measurements reveal that PL emission is dominated by STEs, while free exciton emission is weak and short-lived. By comparing 2D MA 3 Bi 2 I 6 Cl 3 with 0D MA 3 Bi 2 I 9 , which contains molecularly isolated [BiI 6 ] 3− octahedra and 2D MA 3 Bi 2 Br 9 perovskites, we demonstrate how halide composition and structural dimensionality influence the balance between free exciton populations and carrier localization. These insights uncover the intrinsic kinetic landscape of photoexcited states in MA 3 Bi 2 I 6 Cl 3 . Overall, our study contributes to a mechanistic understanding of exciton–phonon interactions in lead-free 2D perovskites.

Pradeep, Kodimana Ramakrishnan [Northwestern Univ.↗
DOE OSTI2017

Synthesis of Pt 3 Y and Other Early–Late Intermetallic Nanoparticles by Way of a Molten Reducing Agent

Early–late intermetallic phases have garnered increased attention recently for their catalytic properties. To achieve the high surface areas needed for industrially relevant applications, these phases must be synthesized as nanoparticles in a scalable fashion. Herein, Pt 3 Y—targeted as a prototypical example of an early–late intermetallic—has been synthesized as nanoparticles approximately 5–20 nm in diameter via a solution process and characterized by XRD, TEM, EDS, and XPS. The key development is the use of a molten borohydride (MEt 3 BH, M = Na, K) as both the reducing agent and reaction medium. Readily available halide precursors of the two metals are used. Accordingly, no organic ligands are necessary, as the resulting halide salt byproduct prevents sintering, which further permits dispersion of the nanoscale intermetallic onto a support. The versatility of this approach was validated by the synthesis of other intermetallic phases such as Pt 3 Sc, Pt 3 Lu, Pt 2 Na, and Au 2 Y.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Ligand Mediated Transformation of Cesium Lead Bromide Perovskite Nanocrystals to Lead Depleted Cs 4 PbBr 6 Nanocrystals

Lead halide perovskite nanocrystals (NCs) have emerged as attractive nanomaterials owing to their excellent optical and optoelectronic properties. Their intrinsic instability and soft nature enable a post-synthetic controlled chemical transformation. We studied a ligand mediated transformation of presynthesized CsPbBr 3 NCs to a new type of lead-halide depleted perovskite derivative nanocrystal, namely Cs 4 PbBr 6 . The transformation is initiated by amine addition, and the use of alkyl-thiol ligands greatly improves the size uniformity and chemical stability of the derived NCs. The thermodynamically driven transformation is governed by a two-step dissolution-recrystallization mechanism, which is monitored optically. Our results not only shed light on a decomposition pathway of CsPbBr 3 NCs but also present a method to synthesize uniform colloidal Cs 4 PbBr 6 NCs, which may actually be a common product of perovskite NCs degradation.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Intrinsic Instability of Cs 2 In(I)M(III)X 6 (M = Bi, Sb; X = Halogen) Double Perovskites: A Combined Density Functional Theory and Experimental Study

Recently, there has been substantial interest in developing double-B-cation halide perovskites, which hold the potential to overcome the toxicity and instability issues inherent within emerging lead halide-based solar absorber materials. Among all double perovskites investigated, In(I)-based Cs2InBiCl6 and Cs2InSbCl6 have been proposed as promising thin-film photovoltaic absorber candidates, with computational examination predicting suitable materials properties, including direct bandgap and small effective masses for both electrons and holes. In this study, we report the intrinsic instability of Cs2In(I)M(III)X6 (M = Bi, Sb; X = halogen) double perovskites by a combination of density functional theory and experimental study. Our results suggest that the In(I)-based double perovskites are unstable against oxidation into In(III)-based compounds. Further, the results show the need to consider reduction–oxidation (redox) chemistry when predicting stability of new prospective electronic materials, especially when less common oxidation states are involved.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Electronic Structural Analysis of Copper(II)–TEMPO/ABNO Complexes Provides Evidence for Copper(I)–Oxoammonium Character

Copper/aminoxyl species are proposed as key intermediates in aerobic alcohol oxidation. Several possible electronic structural descriptions of these species are possible, and here we probe this issue by examining four crystallographically characterized Cu/aminoxyl halide complexes by Cu K-edge, Cu L 2,3 - edge, and Cl K-edge X-ray absorption spectroscopy. The mixing coefficients between Cu, aminoxyl, and halide orbitals are determined via these techniques with support from density functional theory. The emergent electronic structure picture reveals that Cu coordination confers appreciable oxoammonium character to the aminoxyl ligand. The computational methodology is extended to one of the putative intermediates invoked in catalytic Cu/aminoxyl-driven alcohol oxidation reactions, with similar findings. On the whole, the results have important implications for the mechanism of alcohol oxidation and the underlying basis for cooperativity in this co- catalyst system.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Mechanistic Studies of Palladium-Catalyzed Aminocarbonylation of Aryl Chlorides with Carbon Monoxide and Ammonia

Mechanistic information on a reliable, palladium-catalyzed aminocarbonylation of aryl chlorides with ammonia is reported. The reaction occurs with ethylene complex 1 as catalyst, and mechanistic information was gained by isolation of catalytic intermediates and kinetic measurements, including the first mechanistic data on the oxidative addition of aryl chloride to a palladium(0) complex in the presence of CO. Arylpalladium and phenacylpalladium halide intermediates were synthesized, and kinetic measurements of the formation and reactions of these intermediates were undertaken to determine the mechanism of the oxidative addition of aryl bromides and chlorides to a Pd(0) dicarbonyl compound in the presence of CO and the mechanism of the reaction of ammonia with a Pd(II) phenacyl complex to form benzamide. The oxidative addition of aryl chlorides and aryl bromides was determined to occur with rate-limiting reaction of the haloarene with a three-coordinate Pd(0) species bearing a bidentate phosphine and one CO ligand. A primary 13 C kinetic isotope effect suggested that this step involves cleavage of the carbon-halogen bond. Here our data show that the formation of benzamide from the reaction of phenacylpalladium halide complexes with ammonia occurs by a pathway involving reversible displacement of chloride from a phenacylpalladium chloride complex by ammonia, deprotonation of the bound ammonia to form a phenacylpalladium amido complex, and reductive elimination to form the C-N bond. Consistent with this mechanism, the reaction of an aryl palladium amido complex with CO formed the corresponding primary benzamide. A catalyst deactivation pathway involving the formation of a Pd(I) dimer also was elucidated.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Excited-State Dynamics of a CsPbBr 3 Nanocrystal Terminated with Binary Ligands: Sparse Density of States with Giant Spin–Orbit Coupling Suppresses Carrier Cooling

Fully inorganic lead halide perovskite nanocrystals (NCs) are of interest for photovoltaic and light-emitting devices due to optoelectronic properties that can be tuned/optimized via halide composition, surface passivation, doping, and confinement. Compared to bulk materials, certain excited-state properties in NCs can be adjusted by electronic confinement effects such as suppressed hot carrier cooling and enhanced radiative recombination. In this paper, we use spinor Kohn–Sham orbitals (SKSOs) with spin–orbit coupling (SOC) interaction as a basis to compute excited-state dissipative dynamics simulations on a fully passivated CsPbBr 3 NC atomistic model. Redfield theory in the density matrix formalism is used to describe electron–phonon interactions which drive hot carrier cooling and nonradiative recombination ( k nonrad ). Radiative recombination ( k rad ) is calculated through oscillator strengths using SKSO basis. From k rad and k rad + k nonrad , we compute a theoretical photoluminescence quantum yield (PLQY) of 53%. Computed rates of hot carrier cooling ( k cooling ≈ 10 –1 1/ps) compare favorably with what has been reported in the literature. Interestingly, we observe that hot electron cooling slows down near the band edge, which we attribute to large SOC in the conduction band combined with strong confinement, which creates subgaps above the band edge. This slow carrier cooling could potentially impact hot carrier extraction before complete thermalization in photovoltaics (PVs). Implications of this work suggest that strong/intermediate confined APbX 3 NCs are better suited to applications in PVs due to slower carrier cooling near the conduction band edge, while intermediate/weak confined NCs are more appropriate for light-emitting applications, such as LEDs.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

High Li + and Mg 2+ Conductivity in a Cu-Azolate Metal–Organic Framework

A Cu-azolate metal–organic framework (MOF) uptakes stoichiometric loadings of Groups 1 and 2 metal halides, demonstrating efficient reversible release and reincorporation of immobilized anions within the framework. Ion-pairing interactions lead to anion-dependent Li + and Mg 2+ transport in Cu 4 (ttpm) 2 ·0.6CuCl 2 , whose high surface area affords a high density of uniformly distributed mobile metal cations and halide binding sites. The ability to systematically tune the ionic conductivity yields a solid electrolyte with a Mg 2+ ion conductivity rivaling the best materials reported to date. Finally, this MOF is one of the first in a promising class of frameworks that introduces the opportunity to control the identity, geometry, and distribution of the cation hopping sites, offering a versatile template for application-directed design of solid electrolytes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Asynchronous Photoexcited Electronic and Structural Relaxation in Lead-Free Perovskites

Vacancy-ordered lead free perovskites with more stable crystalline structure have been intensively explored as the alternatives for resolving the toxic and long-term stable issues of Lead halide perovskites (LHPs). The dispersive energy bands produced by the closely packed halide octahedral sublattice in these perovskites are meanwhile anticipated to facility the mobility of charge carriers. However, these perovskites suffer from unexpectedly poor charge carrier transport. To tackle this issue, we have employed the ultrafast, elemental-specific X-ray transient absorption (XTA) spectroscopy to directly probe the photoexcited electronic and structural dynamics of a prototypical vacancy-ordered lead-free perovskite (Cs 3 Bi 2 Br 9 ). We have discovered that the photogenerated holes quickly self-trapped at Br centers, simultaneously distorting the local lattice structure, likely forming small polarons in the configuration of V k center (Br 2 - dimer). More significantly, we have found a surprisingly long-lived, structural distorted state with a lifetime of ~ 59 μs, which is about three orders of magnitude slower than that of the charge carrier recombination. Such long-lived structural distortion may produce a transient “background” under continuous light illumination, influencing the charge carrier transport along the lattice framework.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Enhanced optoelectronic quality of perovskite thin films with hypophosphorous acid for planar heterojunction solar cells

Solution-processed metal halide perovskite semiconductors, such as CH 3 NH 3 PbI 3 , have exhibited remarkable performance in solar cells, despite having non-negligible density of defect states. A likely candidate is halide vacancies within the perovskite crystals, or the presence of metallic lead, both generated due to the imbalanced I/Pb stoichiometry which could evolve during crystallization. Herein, we show that the addition of hypophosphorous acid (HPA) in the precursor solution can significantly improve the film quality, both electronically and topologically, and enhance the photoluminescence intensity, which leads to more efficient and reproducible photovoltaic devices. We demonstrate that the HPA can reduce the oxidized I2 back into I-, and our results indicate that this facilitates an improved stoichiometry in the perovskite crystal and a reduced density of metallic lead.

14 SOLAR ENERGY↗
DOE OSTI2015

Perovskite-Fullerene Hybrid Materials Eliminate Hysteresis In Planar Diodes

Solution-processed planar perovskite devices are highly desirable in a wide variety of optoelectronic applications; however, they are prone to hysteresis and current instabilities. Here we report the first perovskite–PCBM hybrid solid with significantly reduced hysteresis and recombination loss achieved in a single step. This new material displays an efficient electrically coupled microstructure: PCBM is homogeneously distributed throughout the film at perovskite grain boundaries. The PCBM passivates the key PbI3 antisite defects during the perovskite self-assembly, as revealed by theory and experiment. Photoluminescence transient spectroscopy proves that the PCBM phase promotes electron extraction. We showcase this mixed material in planar solar cells that feature low hysteresis and enhanced photovoltage. Using conductive AFM studies, we reveal the memristive properties of perovskite films. We close by positing that PCBM, by tying up both halide-rich antisites and unincorporated halides, reduces electric field-induced anion migration that may give rise to hysteresis and unstable diode behaviour.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Controllable deuteration of halogenated compounds by photocatalytic D2O splitting

Deuterium labeling is of great value in organic synthesis and the pharmaceutical industry. However, the state-of-the-art C–H/C–D exchange using noble metal catalysts or strong bases/acids suffers from poor functional group tolerances, poor selectivity and lack of scope for generating molecular complexity. Herein, we demonstrate the deuteration of halides using heavy water as the deuteration reagent and porous CdSe nanosheets as the catalyst. The deuteration mechanism involves the generation of highly active carbon and deuterium radicals via photoinduced electron transfer from CdSe to the substrates, followed by tandem radicals coupling process, which is mechanistically distinct from the traditional methods involving deuterium cations or anions. Our deuteration strategy shows better selectivity and functional group tolerances than current C–H/C–D exchange methods. Extending the synthetic scope, deuterated boronic acids, halides, alkynes, and aldehydes can be used as synthons in Suzuki coupling, Click reaction, C–H bond insertion reaction etc. for the synthesis of complex deuterated molecules.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Transformation from crystalline precursor to perovskite in PbCl2-derived MAPbI3

Abstract Understanding the formation chemistry of metal halide perovskites is key to optimizing processing conditions and realizing enhanced optoelectronic properties. Here, we reveal the structure of the crystalline precursor in the formation of methylammonium lead iodide (MAPbI 3 ) from the single-step deposition of lead chloride and three equivalents of methylammonium iodide (PbCl 2 + 3MAI) (MA = CH 3 NH 3 ). The as-spun film consists of crystalline MA 2 PbI 3 Cl, which is composed of one-dimensional chains of lead halide octahedra, coexisting with disordered MACl. We show that the transformation of precursor into perovskite is not favored in the presence of MACl, and thus the gradual evaporation of MACl acts as a self-regulating mechanism to slow the conversion. We propose the stable precursor phase enables dense film coverage and the slow transformation may lead to improved crystal quality. This enhanced chemical understanding is paramount for the rational control of film deposition and the fabrication of superior optoelectronic devices.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

How lasing happens in CsPbBr 3 perovskite nanowires

Lead halide perovskites are emerging as an excellent material platform for optoelectronic processes. There have been extensive discussions on lasing, polariton formation, and nonlinear processes in this material system, but the underlying mechanism remains unknown. Here we probe lasing from CsPbBr 3 perovskite nanowires with picosecond (ps) time resolution and show that lasing originates from stimulated emission of an electron-hole plasma. We observe an anomalous blue-shifting of the lasing gain profile with time up to 25 ps, and assign this as a signature for lasing involving plasmon emission. The time domain view provides an ultra-sensitive probe of many-body physics which was obscured in previous time-integrated measurements of lasing from lead halide perovskite nanowires.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

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