We present a comprehensive investigation of the Land & eacute; g-factor of the exciton Rydberg series and band-edge electron-hole pairs in two-dimensional phenethylammonium lead iodide (PEPI) films using magnetic circular dichroism (MCD) spectroscopy. At low magnetic field (B < 0.5 T), we observe a sizable difference of 15%-20% between the effective g-factors of the 1s exciton and that of the higher energy Rydberg excitons, which overlap with the interband (IB) electron-hole (e-h) pair transitions at the band-edge (labeled here as the "2s+" band). At T = 3 K, we obtained g(1s) = 1.86 +/- 0.15 and g(2s +) = 2.33 +/- 0.15. These results demonstrate that the exciton g-factor is smaller than the sum of the individual electron and hole band edge g-factors, namely gexciton < ge + gh = gIB . The experimental results are rationalized by theoretical calculations of the g-factors using a multiband effective-mass model that includes the electron-hole interaction for the different exciton states. It is shown that with the decreasing spatial extent of the exciton wavefunction, the exciton g-factor also decreases. At B > 10 T, the interband Landau level transition (N = 1) extrapolates to the bandgap value in PEPI at Eg = 2.62 +/- 0.016 eV, providing further evidence of the formation of Rydberg excitons.

Chiral materials are important tools for transducing circularly polarized light within many emerging opto-electronic and spin-based technologies. Here, we demonstrate that thin films of a bismuth iodide-based 0D chiral hybrid organic-inorganic semiconductor (HOIS) exhibit large anisotropy values in circularly polarized light emission (CPLE) that approach 50%, with mirror-image responses from front-and back-side measurements. A comprehensive analysis of light-wave propagation, absorption, emission, and scattering is constructed on the basis of a symmetry-derived exciton fine structure model, which accurately describes the direction and polarization dependence of the observed excitonic circular dichroism and CPLE, including contributions from both photoluminescence and resonant Raman scattering. Our analysis indicates that molecular chirality drives preferential film alignment with respect to the out-of-plane lattice vector direction, producing the observed anisotropies. This first demonstration in an HOIS system provides a unique route for enhancing polarization-dependent emission, and circularly polarized light transduction more broadly, in self-assembled HOIS.

Chiral halide perovskites (c-HPs) merge the chirality of organic cations with the semiconducting properties of metal halide frameworks, creating a family of chiral semiconductors with tunable chiroptoelectronic behavior. Here, we describe the impact of periodic banded morphologies of textured c-HP (R/S-NEA)2PbBr4 films (NEA = 1-(1-naphthyl)ethylammonium) on their chiroptical behavior. Due to the interplay between the crystalline and glassy phases, the c-HP film growth is driven by rhythmic precipitation, producing a distinctive controllable radial banded pattern with the (R/S-NEA)2PbBr4 inorganic planes oriented parallel to the substrate. The banded morphology can be controlled, as evidenced by the growth temperature dictating the ridge-to-ridge spacing as well as the density of banded regions. The resulting circular dichroism (CD) spectral shape, intensity, and polarity vary in a seemingly random manner across processing conditions. However, these spectral features can be explained by considering key features of the banded morphology, such as refraction of the incident light due to surface morphology, birefringence, and stacked, rotated crystallites. These effects cannot be canceled by averaging front and back CD spectra of c-HP films, and our model incorporating these effects reproduces all observed CD spectra remarkably well. The control over the c-HP morphology and prediction capabilities of our CD modeling leads to further understanding of this class of semiconductors and the possibility of exploiting structural features for light polarization control akin to enhanced metamaterials.

The expression of metal lone-pair electrons is hypothesized to underpin many of the interesting properties of inorganic halide perovskite semiconductors. Recently, a stable low-temperature monoclinic polar phase was predicted for CsSnBr3 and CsSnI3, opening the possibility of direct investigation of a ferroelectric distorted structure compared to the undistorted structure. To date, there have been no experimental reports of such a structure in CsSnI3, and the low-temperature optical properties of CsSnI3 nanocrystals have remained unexplored. Here we report optical and structural evidence of a phase transition around 240 K in 8.9 nm CsSnI3 nanocrystals. Several changes in optical behavior occur below this transition point, including high-energy photoluminescence (PL) that emits concurrently with the exciton PL. The emergence of this high-energy PL is correlated with X-ray diffraction (XRD) and differential scanning calorimetry (DSC) supporting a phase transition from the orthorhombic structure between 240-200 K. Transient absorption measurements show an increase in the excited state lifetimes, i.e., slowed carrier cooling, at 200 K when photoexciting with photon energies above the high-energy state, consistent with slowed carrier cooling and emergence of high-energy PL. We hypothesize that the slowed carrier cooling is distinctive to this phase transition that modifies both the electronic and phonon structures that dictate excited-state carrier dynamics, and we discuss these changes.

The combination of semiconducting properties and synthetically tunable chirality in chiral metal halide semiconductors (CMHS) offer a compelling platform for room temperature control over electronic spin properties, leveraging effects such as chirality-induced spin selectivity (CISS) for the development of new opto-spintronic functionalities. We report room-temperature CISS-induced magnetoresistance (CISS-MR) exceeding 100

Circularly polarized light generation and detection are critical for future spin‐based technologies that inter‐convert circularly polarized photons and electron spins. However, detailed mechanisms in such spin‐photon interfaces are often either poorly understood or operate at cryogenic temperatures since typically small energies separating spin‐split electronic bands facilitate thermally driven spin depolarization. Recently, several 2D hybrid perovskites with polar achiral cations were theoretically demonstrated to exhibit conduction and valence band spin‐splitting energies greatly exceeding room‐temperature thermal energy, suggesting their utility as spin‐photon interfaces with practical operating temperatures. Here, a strong “spin memory” effect is reported in such a polar achiral layered perovskite that enables large room‐temperature circularly polarized emission anisotropy following excitation with circularly polarized light. The polarization anisotropy depends strongly on temperature (thermally activated), excitation energy, and crystal orientation with respect to the excitation source. Temperature‐dependent photoconductance measurements reveal similar thermally activated carrier generation. These observations suggest a mechanism whereby giant in‐plane splitting of single‐particle levels protects spin‐polarization of photogenerated electrons and holes before recombination. Although polarized light emission is explored in greater detail in chiral perovskites, these results reveal that even without chirality, large spin memory in polar achiral perovskites can enable spin‐photon interfaces that operate at elevated temperatures.

Recent activity in the area of chiroptical phenomena has been focused on the connection between structural asymmetry, electron spin configuration and light/matter interactions in chiral semiconductors. In these systems, spin-splitting phenomena emerge due to inversion symmetry breaking and the presence of extended electronic states, yet the connection to chiroptical phenomena is lacking. Here, we develop an analytical effective mass model of chiral excitons, parameterized by density functional theory. The model accounts for parity mixing of the band edge Bloch functions resulting from polar distortions, resulting in allowed magnetic dipole transitions. Through the study of a prototypical chiral 2D hybrid perovskite semiconductor, we show that circular dichroism of the chiral exciton and its interband continuum emerges from spin-splitting via cross-coupling of Rashba-like and chiral/helical spin-texture components. To demonstrate the generality of our approach, and as a counterpoint, we apply our model to describe chiroptical properties of three-dimensionally confined excitons in perovskite nanocrystals that occur without chiral lattice distortions.

We present evidence for the magnetic field dependence of the electron and hole Lande g-factors in films of the hybrid organic-inorganic perovskite formamidinium lead iodide HC(NH2)2PbI3 (FAPI). We perform transient pump-probe Kerr spectroscopy on FAPI in magnetic fields up to 25 T, which reveals a significant deviation from the expected linear dependence of the quantum beating frequency on the applied magnetic field. We also found a decrease of the quantum beating frequencies with increasing pump intensity for a fixed magnetic field. These observations are discussed in terms of the dependence of the electron and hole Lande g-factors on the band gap, within a simple model in which the effective band gap increases at high magnetic field due to Landau level formation and increases with increasing pump intensity by virtue of shifts in the quasi-Fermi levels.

Chirality-induced spin selectivity (CISS) phenomena arise from an interplay among structural chirality, electron spin orientation, and charge current. Steady-state observations such as magnetoresistance offer little insight into the timescales that govern the spin-charge interconversion and often conflate interfacial and bulk phenomena. By contrast, inverse CISS involves the conversion of spin to a charge current. Using terahertz (THz) emission spectroscopy, we directly measured an ultrafast charge current due to inverse CISS with picosecond time resolution. Polarity and polarization analysis of the THz emission map the induced charge current direction upon spin injection. We found that a charge current is generated along the spin orientation that changes direction with stereochemical configuration. These observations directly demonstrate the inherent coupling between spin and charge currents in chiral systems, offering key insights into their fundamental dynamics.

Picosecond time-resolved and cw magneto-optical methods have been used for studying the spin-related properties of excitons and photocarriers in methyl-ammonium lead bromide (MAPbBr(3)) thin film, single crystal, and light-emitting diodes (LED), focusing on the Lande g values of these species. Using the transient circularly polarized photoinduced quantum beatings (QB) under an applied magnetic field, B in MAPbBr(3) single crystal, we obtained the anisotropic g values of electrons for B field along [010] and [001]: |g([001])(e)| = 2.15 and |g([010])(e)| = 1.75, and for holes |g([001])(h)| = 0.42 and |g([010])(h)| = 0.60. We also used the magnetic circular dichroism method for measuring the bright excitons' g value, g(ex) = g(e) + g(h) = 2.5. From these two types of measurements we conclude that g(h) > 0 in MAPbBr(3). This conclusion was corroborated by measuring the magnetoelectroluminescence response of LED based on MAPbBr(3) active layer. The g values in single crystal and their average in films are in excellent agreement with a k center dot p model that shows similarity and difference to those of MAPbI3. We also observed the influence of the Overhauser field on the QB frequencies that is induced by the dynamic nuclear polarization generated by the spin-aligned electrons using circularly polarized pump or probe beams.

While semiconductor nanocrystals provide versatile fluorescent materials for light-emitting devices, their brightness suffers from the "dark exciton"─an optically inactive electronic state into which nanocrystals relax before emitting. Recently, a theoretical mechanism, the Rashba effect, was discovered that can overcome this limitation by inverting the lowest-lying levels and creating a bright excitonic ground state. However, no methodology is available to systematically identify materials that exhibit this inversion, hindering the development of superbright nanocrystals and their devices. Here, based on a detailed understanding of the Rashba mechanism, we demonstrate a procedure that reveals previously unknown "bright-exciton" nanocrystals. We first define physical criteria to reduce over 500,000 known solids to 173 targets. Higher-level first-principles calculations then refine this list to 28 candidates. From these, we select five with high oscillator strength and develop effective-mass models to determine the nature of their lowest excitonic state. We confirm that four of the five solids yield bright ground-state excitons in nanocrystals. Thus, our results provide a badly needed roadmap for experimental investigation of bright-exciton nanomaterials.

Semiconductors with large energetic separation ΔE± of energy sub-bands with distinct spin expectation values (spin textures) represent a key target to enable control over spin transport and spin-optoelectronic properties. While the paradigmatic case of symmetry-dictated Rashba spin splitting and associated spin textures remains the most explored pathway toward designing future spin-transport-based quantum information technologies, controlling spin physics beyond the Rashba paradigm by accessing strategically targeted crystalline symmetries holds significant promise. In this paper, we show how breaking the traditional paradigm of octahedron-rotation based structure distortions in 2D organic-inorganic perovskites (2D-OIPs) can facilitate exceptionally large spin splittings (ΔE± > 400 meV) and spin textures with extremely short spin helix lengths (lPSH ∼ 5 nm). A simple bond angle difference captures the distortion-driven global asymmetry and correlates quantitatively with first-principles computed spin-splitting magnitudes. A multiband effective mass model that accounts for interlayer coupling provides a unified understanding of how specific symmetry elements dictate layer- and state-dependent spin polarizations within these multi-quantum-well structures. The general symmetry analysis methodology presented here, together with the potential for rationally creating 2D-OIPs with unique symmetry patterns, opens a pathway to design semiconductors with outstanding spin properties for next generation opto-spintronics.

The brightness of an emitter is ultimately described by Fermi’s golden rule, with a radiative rate proportional to its oscillator strength times the local density of photonic states. As the oscillator strength is an intrinsic material property, the quest for ever brighter emission has relied on the local density of photonic states engineering, using dielectric or plasmonic resonators 1 , 2 . By contrast, a much less explored avenue is to boost the oscillator strength, and hence the emission rate, using a collective behaviour termed superradiance. Recently, it was proposed 3 that the latter can be realized using the giant oscillator-strength transitions of a weakly confined exciton in a quantum well when its coherent motion extends over many unit cells. Here we demonstrate single-photon superradiance in perovskite quantum dots with a sub-100 picosecond radiative decay time, almost as short as the reported exciton coherence time 4 . The characteristic dependence of radiative rates on the size, composition and temperature of the quantum dot suggests the formation of giant transition dipoles, as confirmed by effective-mass calculations. The results aid in the development of ultrabright, coherent quantum light sources and attest that quantum effects, for example, single-photon emission, persist in nanoparticles ten times larger than the exciton Bohr radius.

Introducing chirality into organic/inorganic hybrid materials can impart chiroptical properties such as circular dichroism. The ability to tune chiroptical properties in self-assembled materials can have important implications for spintronic and optoelectronic applications. Here, a chiral organic cation, (R/S)-4-methoxy-alpha-methylbenzylammonium, is incorporated to synthesize the bismuth-based hybrid organic-inorganic metal halide semiconductor, (R/S-MeOMePMA)BiI4. Thin films of this Bi-based compound demonstrate large chiroptical responses, with circular dichroism anisotropy (gCD) values up to approximate to 0.1, close to the highest value observed in another chiral metal-halide semiconductor, (R-MBA2CuCl4). Detailed investigation reveals that this large gCD in (R/S-MeOMePMA)BiI4 is caused by the apparent CD effect. Careful selection of deposition conditions and the concomitant thin-film orientation enables the control of gCD, with maximum value observed when its thin film has a well-crystallized preferred (001) orientation parallel to the substrate. The results support a growing body of evidence that low symmetry plays an important role in achieving unusually large gCD in these chiral metal-halide materials and provides design rules for achieving large chiroptical response via morphology control. 0D bismuth-based hybrid organic-inorganic metal halide semiconductor, (R/S-MeOMePMA)BiI4 demonstrates large chiroptical responses caused by the apparent CD effect. Careful manipulation of deposition conditions and the concomitant thin-film orientation enables the control of gCD and apparent CD effect, with maximum gCD and apparent CD effect observed when its thin film has a preferred (001) orientation parallel to the substrate.image

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 MBA(4)B(2)X(10) (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 B2X10 octahedra: (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 5s(2) lone 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 MBA(4)Sb(2)Br(10) compounds, which we attribute to self-trapped excitonic emission driven by the large distortion due to the lone pair expression.

A comprehensive study of the exciton fine structure (EFS) is presented in 2D‐phenethylammonium lead iodide films using magnetic field‐induced polarization of photoluminescence (PL) in both Faraday and Voigt configurations at fields up to 25 Tesla. Three exciton bands are identified in the PL spectrum associated with bound, dark, and bright excitons, respectively. Under a high magnetic field in Faraday/Voigt configuration, large field‐induced circular/linear polarization is observed in the PL band related to the dark exciton, which is magnetically activated. Furthermore, it is found that the dark exciton has an anomalous field‐induced circular polarization, which cannot be explained by the classical Boltzmann distribution of spin‐polarized species. These findings are well explained by an effective mass model that includes exchange terms unique to the monoclinic symmetry as a perturbation of the EFS in the approximate tetragonal symmetry. It is also confirmed that the field‐induced linear polarization is sensitive to the monoclinic exchange term, whereas the field‐induced circular polarization is immune to such term.

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.

We have shown theoretically that giant magnetic circular dichroism (MCD) should be observed at the absorption band edge of two-dimensional (2D) semiconductors exhibiting Rashba splitting in the presence of a magnetic field perpendicular to the 2D layers. This feature has been found in calculated interband sigma+ and sigma- light absorption coefficients for transitions between the Landau levels (LLs) of the conduction and valence bands. The giant MCD signal is shown to be connected with an unusual magnetic field dependence of the lowest LL in the presence of the Rashba term. A second unique signature of Rashba splitting is a reduction of the exciton diamagnetic shift created by the Rashba terms. We show that these terms lead to the characteristic Rashba exciton dispersion with minima at nonzero momentum. The analysis of the exciton dispersion at nonzero momentum conducted by Gor'kov and Dzaloshinskii shows that variable separation leads to mixed magneto-impulse terms. These terms lead to a negative diamagnetic shift, which should be observed in the exciton photoluminescence that occurs from the dispersion minimum.

While nanoscale semiconductor crystallites provide versatile fluorescent materials for light-emitting devices, such nanocrystals suffer from the "dark exciton"$\unicode{x2014}$an optically inactive electronic state into which the nanocrystal relaxes before emitting. Recently, a theoretical mechanism was discovered that can potentially defeat the dark exciton. The Rashba effect can invert the order of the lowest-lying levels, creating a bright excitonic ground state. To identify materials that exhibit this behavior, here we perform an extensive high-throughput computational search of two large open-source materials databases. Based on a detailed understanding of the Rashba mechanism, we define proxy criteria and screen over 500,000 solids, generating 173 potential "bright-exciton" materials. We then refine this list with higher-level first-principles calculations to obtain 28 candidates. To confirm the potential of these compounds, we select five and develop detailed effective-mass models to determine the nature of their lowest-energy excitonic state. We find that four of the five solids (BiTeCl, BiTeI, Ga$_2$Te$_3$, and KIO$_3$) can yield bright ground-state excitons. Our approach thus reveals promising materials for future experimental investigation of bright-exciton nanocrystals.