All-Perovskite Multicomponent Nanocrystal Superlattices

Nanocrystal superlattices (NC SLs) have long been sought as promising metamaterials, with nanoscale-engineered properties arising from collective and synergistic effects among the constituent building blocks. Lead halide perovskite (LHP) NCs come across as outstanding candidates for SL design, as they demonstrate collective light emission, known as superfluorescence, in single- and multicomponent SLs. Thus far, LHP NCs have only been assembled in single-component SLs or coassembled with dielectric NC building blocks acting solely as spacers between luminescent NCs. Here, we report the formation of multicomponent LHP NC-only SLs, i.e., using only CsPbBr3 NCs of different sizes as building blocks. The structural diversity of the obtained SLs encompasses the ABO6, ABO3, and NaCl structure types, all of which contain orientationally and positionally locked NCs. For the selected model system, the ABO6-type SL, we observed efficient NC coupling and Förster-like energy transfer from strongly confined 5.3 nm CsPbBr3 NCs to weakly confined 17.6 nm CsPbBr3 NCs, along with characteristic superfluorescence features at cryogenic temperatures. Spatiotemporal exciton dynamics measurements reveal that binary SLs exhibit enhanced exciton diffusivity compared to single-component NC assemblies across the entire temperature range (from 5 to 298 K). The observed coherent and incoherent NC coupling and controllable excitonic transport within the solid NC SLs hold promise for applications in quantum optoelectronic devices.

Persistent enhancement of exciton diffusivity in CsPbBr3 nanocrystal solids

In semiconductors, exciton or charge carrier diffusivity is typically described as an inherent material property. Here, we show that the transport of excitons among CsPbBr3 perovskite nanocrystals (NCs) depends markedly on how recently those NCs were occupied by a previous exciton. Using transient photoluminescence microscopy, we observe a striking dependence of the apparent exciton diffusivity on excitation laser power that does not arise from nonlinear exciton-exciton interactions or thermal heating. We interpret our observations with a model in which excitons cause NCs to transition to a long-lived metastable configuration that markedly increases exciton transport. The exciton diffusivity observed here (>0.15 square centimeters per second) is considerably higher than that observed in other NC systems, revealing unusually strong excitonic coupling between NCs. The finding of a persistent enhancement in excitonic coupling may help explain other photophysical behaviors observed in CsPbBr3 NCs, such as superfluorescence, and inform the design of optoelectronic devices.

Bright Excitonic Fine Structure in Metal-Halide Perovskites: From Two-Dimensional to Bulk

The optical response of two-dimensional (2D) perovskites, often referred to as natural quantum wells, is primarily governed by excitons, whose properties can be readily tuned by adjusting the perovskite layer thickness. We have investigated the exciton fine structure splitting in the archetypal 2D perovskite (PEA)2(MA)n-1PbnI3n+1 with varying numbers of inorganic octahedral layers n = 1, 2, 3, and 4. We demonstrate that the in-plane excitonic states exhibit splitting and orthogonally oriented dipoles for all confinement regimes. The evolution of the exciton states in an external magnetic field provides further insights into the g-factors and diamagnetic coefficients. With increasing n, we observe a gradual evolution of the excitonic parameters characteristic of a 2D to three-dimensional transition. Our results provide valuable information concerning the evolution of the optoelectronic properties of 2D perovskites with the changing confinement strength.

Dipole-Dependent Waveguiding in an Anisotropic Metal-Organic Framework

The interaction between excitons and photons underlies a range of emergent technologies, such as directional light emission, molecular lasers, photonic circuits, and polaritonic devices. Two of the key parameters that impact exciton-photon coupling are the binding energy of excitons and the relative orientations between the exciton dipole and photon field. Tightly bound excitons are typically found in molecular crystals, where nevertheless the angular relationship of excitons with photon fields is difficult to control. Here, we demonstrate directional exciton dipoles and photon fields, anchored by metal-ligand coordination. In a pyrene-porphyrin bichromophoric metal-organic framework (MOF), we observe that the perpendicular arrangement of the pyrene- and porphyrin-based exciton dipoles engenders orthogonal polarizations of their respective emissions. The alignment of the directional exciton and photon fields gives rise to an anisotropic waveguide effect, where the pyrene- and the porphyrin-based emissions show distinct spatial distribution within microplate-shaped MOF crystals. This capability to simultaneously host heterogenous excitonic states and anisotropic photon fields points toward MOFs' yet-to-be-realized potential as a platform for advancing the frontier in the field of exciton-photonics, which centers around engineering emergent properties from the interplay between excitons and photons.

Discovery of enhanced lattice dynamics in a single-layered hybrid perovskite

Layered hybrid perovskites exhibit emergent physical properties and exceptional functional performances, but the coexistence of lattice order and structural disorder severely hinders our understanding of these materials. One unsolved problem regards how the lattice dynamics are affected by the dimensional engineering of the inorganic frameworks and their interaction with the molecular moieties. Here, we address this question by using a combination of spontaneous Raman scattering, terahertz spectroscopy, and molecular dynamics simulations. This approach reveals the structural dynamics in and out of equilibrium and provides unexpected observables that differentiate single- and double-layered perovskites. While no distinct vibrational coherence is observed in double-layered perovskites, an off-resonant terahertz pulse can drive a long-lived coherent phonon mode in the single-layered system. This difference highlights the dramatic change in the lattice environment as the dimension is reduced, and the findings pave the way for ultrafast structural engineering and high-speed optical modulators based on layered perovskites.

Giant Nonlinear Optical Response via Coherent Stacking of In-Plane Ferroelectric Layers

Thin ferroelectric materials hold great promise for compact nonvolatile memory and nonlinear optical and optoelectronic devices. Herein, an ultrathin in-plane ferroelectric material that exhibits a giant nonlinear optical effect, group-IV monochalcogenide SnSe, is reported. Nanometer-scale ferroelectric domains with ≈90°/270° twin boundaries or ≈180° domain walls are revealed in physical-vapor-deposited SnSe by lateral piezoresponse force microscopy. Atomic structure characterization reveals both parallel and antiparallel stacking of neighboring van der Waals ferroelectric layers, leading to ferroelectric or antiferroelectric ordering. Ferroelectric domains exhibit giant nonlinear optical activity due to coherent enhancement of second-harmonic fields and the as-resulted second-harmonic generation was observed to be 100 times more intense than monolayer WS₂ . This work demonstrates in-plane ferroelectric ordering and giant nonlinear optical activity in SnSe, which paves the way for applications in on-chip nonlinear optical components and nonvolatile memory devices.

Fine Structure Splitting of Phonon-Assisted Excitonic Transition in (PEA)2PbI4 Two-Dimensional Perovskites

Two-dimensional van der Waals materials exhibit particularly strong excitonic effects, which causes them to be an exceptionally interesting platform for the investigation of exciton physics. A notable example is the two-dimensional Ruddlesden-Popper perovskites, where quantum and dielectric confinement together with soft, polar, and low symmetry lattice create a unique background for electron and hole interaction. Here, with the use of polarization-resolved optical spectroscopy, we have demonstrated that the simultaneous presence of tightly bound excitons, together with strong exciton-phonon coupling, allows for observing the exciton fine structure splitting of the phonon-assisted transitions of two-dimensional perovskite (PEA)2PbI4, where PEA stands for phenylethylammonium. We demonstrate that the phonon-assisted sidebands characteristic for (PEA)2PbI4 are split and linearly polarized, mimicking the characteristics of the corresponding zero-phonon lines. Interestingly, the splitting of differently polarized phonon-assisted transitions can be different from that of the zero-phonon lines. We attribute this effect to the selective coupling of linearly polarized exciton states to non-degenerate phonon modes of different symmetries resulting from the low symmetry of (PEA)2PbI4 lattice.

1D Hybrid Semiconductor Silver 2,6-Difluorophenylselenolate

Organic-inorganic hybrid materials present new opportunities for creating low-dimensional structures with unique light-matter interaction. In this work, we report a chemically robust yellow emissive one-dimensional (1D) semiconductor, silver 2,6-difluorophenylselenolate─AgSePhF₂(2,6), a new member of the broader class of hybrid low-dimensional semiconductors, metal-organic chalcogenolates. While silver phenylselenolate (AgSePh) crystallizes as a two-dimensional (2D) van der Waals semiconductor, introduction of fluorine atoms at the (2,6) position of the phenyl ring induces a structural transition from 2D sheets to 1D chains. Density functional theory calculations reveal that AgSePhF₂ (2,6) has strongly dispersive conduction and valence bands along the 1D crystal axis. Visible photoluminescence centered around λ_p ≈ 570 nm at room temperature exhibits both prompt (110 ps) and delayed (36 ns) components. The absorption spectrum exhibits excitonic resonances characteristic of low-dimensional hybrid semiconductors, with an exciton binding energy of approximately 170 meV as determined by temperature-dependent photoluminescence. The discovery of an emissive 1D silver organoselenolate highlights the structural and compositional richness of the chalcogenolate material family and provides new insights for molecular engineering of low-dimensional hybrid organic-inorganic semiconductors.

Lead Halide Perovskite Nanocrystals with Low Inhomogeneous Broadening and High Coherent Fraction through Dicationic Ligand Engineering

Lead halide perovskite nanocrystals (LHP NCs) are an emerging materials system with broad potential applications, including as emitters of quantum light. We apply design principles aimed at the structural optimization of surface ligand species for CsPbBr₃ NCs, leading us to the study of LHP NCs with dicationic quaternary ammonium bromide ligands. Through the selection of linking groups and aliphatic backbones guided by experiments and computational support, we demonstrate consistently narrow photoluminescence line shapes with a full-width-at-half-maximum below 70 meV. We observe bulk-like Stokes shifts throughout our range of particle sizes, from 7 to 16 nm. At cryogenic temperatures, we find sub-200 ps lifetimes, significant photon coherence, and the fraction of photons emitted into the coherent channel increasing markedly to 86%. A 4-fold reduction in inhomogeneous broadening from previous work paves the way for the integration of LHP NC emitters into nanophotonic architectures to enable advanced quantum optical investigation.

Nanocrystal phononics

Colloidal nanocrystals are successfully used as nanoscale building blocks for creating hierarchical solids with structures that range from amorphous networks to sophisticated periodic superlattices. Recently, it has been observed that these superlattices exhibit collective vibrations, which stem from the correlated motion of the nanocrystals, with their surface-bound ligands acting as molecular linkers. In this Perspective, we describe the work so far on collective vibrations in nanocrystal solids and their as-of-yet untapped potential for phononic applications. With the ability to engineer vibrations in the hypersonic regime through the choice of nanocrystal and linker composition, as well as by controlling their size, shape and chemical interactions, such superstructures offer new opportunities for phononic crystals, acoustic metamaterials and optomechanical systems.

Light Emission in 2D Silver Phenylchalcogenolates

Silver phenylselenolate (AgSePh, also known as "mithrene") and silver phenyltellurolate (AgTePh, also known as "tethrene") are two-dimensional (2D) van der Waals semiconductors belonging to an emerging class of hybrid organic-inorganic materials called metal-organic chalcogenolates. Despite having the same crystal structure, AgSePh and AgTePh exhibit a strikingly different excitonic behavior. Whereas AgSePh exhibits narrow, fast luminescence with a minimal Stokes shift, AgTePh exhibits comparatively slow luminescence that is significantly broadened and red-shifted from its absorption minimum. Using time-resolved and temperature-dependent absorption and emission microspectroscopy, combined with subgap photoexcitation studies, we show that exciton dynamics in AgTePh films are dominated by an intrinsic self-trapping behavior, whereas dynamics in AgSePh films are dominated by the interaction of band-edge excitons with a finite number of extrinsic defect/trap states. Density functional theory calculations reveal that AgSePh has simple parabolic band edges with a direct gap at Γ, whereas AgTePh has a saddle point at Γ with a horizontal splitting along the Γ-N₁ direction. The correlation between the unique band structure of AgTePh and exciton self-trapping behavior is unclear, prompting further exploration of excitonic phenomena in this emerging class of hybrid 2D semiconductors.

Uniaxial Strain Engineering via Core Position Control in CdSe/CdS Core/Shell Nanorods and Their Optical Response

Anisotropic strain engineering has emerged as a powerful strategy for enhancing the optoelectronic performance of semiconductor nanocrystals. Here, we show that CdSe/CdS dot-in-rod structures offer a platform for fine-tuning the optical response of CdSe quantum dots through anisotropic strain. By controlling the spatial position of the CdSe core within a growing CdS nanorod shell, varying degrees of uniaxial strain can be introduced. Placing CdSe cores at the end of the CdS nanorod induces strong asymmetric compression along the c-axis of the wurtzite CdSe core, dramatically altering its absorption and emission characteristics, whereas CdSe cores located near the middle of the nanorod experience a comparatively weak uniaxial strain field. The change in absorption and emission spectra and dynamics for highly strained end-position CdSe/CdS nanorods is explained by (1) relative shifting of the valence band light hole and heavy hole levels and (2) introduction of a strong piezoelectric potential, which spatially separates the electron and hole wave functions. The ability to tune the degree of uniaxial strain through core position control in a nanorod structure creates opportunities for precisely modulating the electronic properties of CdSe nanocrystals while simultaneously taking advantage of dielectric and optical anisotropies intrinsic to 1D nanostructures.

Robust estimation of charge carrier diffusivity using transient photoluminescence microscopy

Transient microscopy has emerged as a powerful tool for imaging the diffusion of excitons and free charge carriers in optoelectronic materials. In many excitonic materials, extraction of diffusion coefficients can be simplified because of the linear relationship between signal intensity and local excited state population. However, in materials where transport is dominated by free charge carriers, extracting diffusivities accurately from multidimensional data is complicated by the nonlinear dependence of the measured signal on the local charge carrier density. To obtain accurate estimates of charge carrier diffusivity from transient microscopy data, statistically robust fitting algorithms coupled to efficient 3D numerical solvers that faithfully relate local carrier dynamics to raw experimental measurables are sometimes needed. Here, we provide a detailed numerical framework for modeling the spatiotemporal dynamics of free charge carriers in bulk semiconductors with significant solving speed reduction and for simulating the corresponding transient photoluminescence microscopy data. To demonstrate the utility of this approach, we apply a fitting algorithm using a Markov chain Monte Carlo sampler to experimental data on bulk CdS and methylammonium lead bromide (MAPbBr3) crystals. Parameter analyses reveal that transient photoluminescence microscopy can be used to obtain robust estimates of charge carrier diffusivities in optoelectronic materials of interest, but that other experimental approaches should be used for obtaining carrier recombination constants. Additionally, simplifications can be made to the fitting model depending on the experimental conditions and material systems studied. Our open-source simulation code and fitting algorithm are made freely available to the scientific community.

Quantification of Exciton Fine Structure Splitting in a Two-Dimensional Perovskite Compound

Applications of two-dimensional (2D) perovskites have significantly outpaced the understanding of many fundamental aspects of their photophysics. The optical response of 2D lead halide perovskites is dominated by strongly bound excitonic states. However, a comprehensive experimental verification of the exciton fine structure splitting and associated transition symmetries remains elusive. Here we employ low temperature magneto-optical spectroscopy to reveal the exciton fine structure of (PEA)₂PbI₄ (here PEA is phenylethylammonium) single crystals. We observe two orthogonally polarized bright in-plane free exciton (FX) states, both accompanied by a manifold of phonon-dressed states that preserve the polarization of the corresponding FX state. Introducing a magnetic field perpendicular to the 2D plane, we resolve the lowest energy dark exciton state, which although theoretically predicted, has systematically escaped experimental observation (in Faraday configuration) until now. These results corroborate standard multiband, effective-mass theories for the exciton fine structure in 2D perovskites and provide valuable quantification of the fine structure splitting in (PEA)₂PbI₄.

Morphological Control of 2D Hybrid Organic-Inorganic Semiconductor AgSePh

Silver phenylselenolate (AgSePh) is a hybrid organic-inorganic two-dimensional (2D) semiconductor exhibiting narrow blue emission, in-plane anisotropy, and large exciton binding energy. Here, we show that the addition of carefully chosen solvent vapors during the chemical transformation of metallic silver to AgSePh allows for control over the size and orientation of AgSePh crystals. By testing 28 solvent vapors (with different polarities, boiling points, and functional groups), we controlled the resulting crystal size from <200 nm up to a few μm. Furthermore, choice of solvent vapor can substantially improve the orientational homogeneity of 2D crystals with respect to the substrate. In particular, solvents known to form complexes with silver ions, such as dimethyl sulfoxide (DMSO), led to the largest lateral crystal dimensions and parallel crystal orientation. We perform systematic optical and electrical characterizations on DMSO vapor-grown AgSePh films demonstrating improved crystalline quality, lower defect densities, higher photoconductivity, lower dark conductivity, suppression of ionic migration, and reduced midgap photoluminescence at low temperature. Overall, this work provides a strategy for realizing AgSePh films with improved optical properties and reveals the roles of solvent vapors on the chemical transformation of metallic silver.

Dipole-mediated exciton management strategy enabled by reticular chemistry

Selectively blocking undesirable exciton transfer pathways is crucial for utilizing exciton conversion processes that involve participation of multiple chromophores. This is particularly challenging for solid-state systems, where the chromophores are fixed in close proximity. For instance, the low efficiency of solid-state triplet–triplet upconversion calls for inhibiting the parasitic singlet back-transfer without blocking the flow of triplet excitons. Here, we present a reticular chemistry strategy that inhibits the resonance energy transfer of singlet excitons. Within a pillared layer metal–organic framework (MOF), pyrene-based singlet donors are situated perpendicular to porphyrin-based acceptors. High resolution transmission electron microscopy and electron diffraction enable direct visualization of the structural relationship between donor and acceptor (D–A) chromophores within the MOF. Time-resolved photoluminescence measurements reveal that the structural and symmetry features of the MOF reduce the donor-to-acceptor singlet transfer efficiency to less than 36% compared to around 96% in the control sample, where the relative orientation of the donor and acceptor chromophores cannot be controlled.

A strategy is designed to selectively block undesirable pathways in photophysical processes that consist of a mixture of Förster and Dexter energy transfer steps.

Power-Dependent Photoluminescence Efficiency in Manganese-Doped 2D Hybrid Perovskite Nanoplatelets

Substitutional metal doping is a powerful strategy for manipulating the emission spectra and excited state dynamics of semiconductor nanomaterials. Here, we demonstrate the synthesis of colloidal manganese (Mn²⁺)-doped organic-inorganic hybrid perovskite nanoplatelets (chemical formula: L₂[APb_1-xMn_xBr₃]_n-1Pb_1-xMn_xBr₄; L, butylammonium; A, methylammonium or formamidinium; n (= 1 or 2), number of Pb_1-xMn_xBr₆^4- octahedral layers in thickness) via a ligand-assisted reprecipitation method. Substitutional doping of manganese for lead introduces bright (approaching 100% efficiency) and long-lived (>500 μs) midgap Mn²⁺ atomic states, and the doped nanoplatelets exhibit dual emission from both the band edge and the dopant state. Photoluminescence quantum yields and band-edge-to-Mn intensity ratios exhibit strong excitation power dependence, even at a very low incident intensity (<100 μW/cm²). Surprisingly, we find that the saturation of long-lived Mn²⁺ dopant sites cannot explain our observation. Instead, we propose an alternative mechanism involving the cross-relaxation of long-lived Mn-site excitations by freely diffusing band-edge excitons. We formulate a kinetic model based on this cross-relaxation mechanism that quantitatively reproduces all of the experimental observations and validate the model using time-resolved absorption and emission spectroscopy. Finally, we extract a concentration-normalized microscopic rate constant for band edge-to-dopant excitation transfer that is ∼10× faster in methylammonium-containing nanoplatelets than in formamidinium-containing nanoplatelets. This work provides fundamental insight into the interaction of mobile band edge excitons with localized dopant sites in 2D semiconductors and expands the toolbox for manipulating light emission in perovskite nanomaterials.

Size and Quality Enhancement of 2D Semiconducting Metal-Organic Chalcogenolates by Amine Addition

The use of two-dimensional (2D) materials in next-generation technologies is often limited by small lateral size and/or crystal defects. Here, we introduce a simple chemical strategy to improve the size and overall quality of 2D metal-organic chalcogenolates (MOCs), a new class of hybrid organic-inorganic 2D semiconductors that can exhibit in-plane anisotropy and blue luminescence. By inducing the formation of silver-amine complexes during a solution growth method, we increase the average size of silver phenylselenolate (AgSePh) microcrystals from <5 μm to >1 mm, while simultaneously extending the photoluminescence lifetime and suppressing mid-gap emission. Mechanistic studies using ⁷⁷Se NMR suggest dual roles for the amine in promoting the formation of a key reactive intermediate and slowing down the final conversion to AgSePh. Finally, we show that amine addition is generalizable to the synthesis of other 2D MOCs, as demonstrated by the growth of single crystals of silver 4-methylphenylselenolate (AgSePhMe), a novel member of the 2D MOC family.

Revealing the Brønsted-Evans-Polanyi relation in halide-activated fast MoS2 growth toward millimeter-sized 2D crystals

Achieving large-size two-dimensional (2D) crystals is key to fully exploiting their remarkable functionalities and application potentials. Chemical vapor deposition growth of 2D semiconductors such as monolayer MoS2 has been reported to be activated by halide salts, for which various investigations have been conducted to understand the underlying mechanism from different aspects. Here, we provide experimental evidence showing that the MoS2 growth dynamics are halogen dependent through the Brønsted-Evans-Polanyi relation, based on which we build a growth model by considering MoS2 edge passivation by halogens, and theoretically reproduce the trend of our experimental observations. These mechanistic understandings enable us to further optimize the fast growth of MoS2 and reach record-large domain sizes that should facilitate practical applications.

Temperature-Independent Dielectric Constant in CsPbBr3 Nanocrystals Revealed by Linear Absorption Spectroscopy

Fundamental photophysical behavior in CsPbBr₃ nanocrystals (NCs), especially at low temperatures, is under active investigation. While many studies have reported temperature-dependent photoluminescence, comparatively few have focused on understanding the temperature-dependent absorption spectrum. Here, we report the temperature-dependent (35-300 K) absorption and photoluminescence spectra of zwitterionic ligand-capped CsPbBr₃ NCs with four different edge lengths (d = 4.9, 7.2, 8.1, and 13.2 nm). The two lowest-energy excitonic transitions are quantitatively modeled over the full temperature range within the effective mass approximation considering the quasi-cubic NC shape and nonparabolicity of the electronic bands. Significantly, we find that the effective dielectric constant determined from the best fit model parameters is independent of temperature. Moreover, we observe a temperature-dependent Stokes shift that saturates at a finite value of Δ ≈ 10 meV at low temperatures for d = 7.2 nm NCs, which is absent in bulk CsPbBr₃ films. Overall, these observations highlight differences between the temperature-dependent dielectric behavior of NC and bulk perovskites and point to the need for a more unified theoretical understanding of absorption and emission in halide perovskites.

State of the Art and Prospects for Halide Perovskite Nanocrystals

Metal-halide perovskites have rapidly emerged as one of the most promising materials of the 21st century, with many exciting properties and great potential for a broad range of applications, from photovoltaics to optoelectronics and photocatalysis. The ease with which metal-halide perovskites can be synthesized in the form of brightly luminescent colloidal nanocrystals, as well as their tunable and intriguing optical and electronic properties, has attracted researchers from different disciplines of science and technology. In the last few years, there has been a significant progress in the shape-controlled synthesis of perovskite nanocrystals and understanding of their properties and applications. In this comprehensive review, researchers having expertise in different fields (chemistry, physics, and device engineering) of metal-halide perovskite nanocrystals have joined together to provide a state of the art overview and future prospects of metal-halide perovskite nanocrystal research.

Resonance-Enhanced Excitation of Interlayer Vibrations in Atomically Thin Black Phosphorus

The strength of interlayer coupling critically affects the physical properties of 2D materials such as black phosphorus (BP), where the electronic structure depends sensitively on layer thickness. Rigid-layer vibrations reflect directly the interlayer coupling strength in 2D van der Waals solids, but measurement of these characteristic frequencies is made difficult by sample instability and small Raman scattering cross sections in atomically thin elemental crystals. Here, we overcome these challenges in BP by performing resonance-enhanced low-frequency Raman scattering under an argon-protective environment. Interlayer breathing modes for atomically thin BP were previously unobservable under conventional (nonresonant) excitation but became strongly enhanced when the excitation energy matched the sub-band electronic transitions of few-layer BP, down to bilayer thicknesses. The measured out-of-plane interlayer force constant was found to be 10.1 × 10¹⁹ N/m³ in BP, which is comparable to graphene. Accurate characterization of the interlayer coupling strength lays the foundation for future exploration of BP twisted structures and heterostructures.

Tuning the Excitonic Properties of the 2D (PEA)2(MA)n-1PbnI3n+1 Perovskite Family via Quantum Confinement

In atomically thin two-dimensional (2D) crystals, the excitonic properties and band structure scale strongly with the thickness, providing a new playground for the investigation of exciton physics in the ultimate confinement regime. Here, we demonstrate the evolution of the fundamental excitonic properties, such as reduced mass, wave function extension, and exciton binding energy, in the 2D perovskite (PEA)₂(MA)_n-1Pb_nI_3n+1, for n = 1, 2, 3. These parameters are experimentally determined using optical spectroscopy in a high magnetic field up to 65 T. The observation of the interband Landau level transitions provides direct access to the reduced effective mass μ and band gap E_g. We show that μ increases with the number of inorganic sheets n, reaching the value of three-dimensional (3D) MAPbI₃ already for n = 3. Our experimental observations contradict the general expectation that quantum confinement leads to an enhanced carrier mass, showing another aspect of the unprecedented flexibility in the design of the electronic properties of 2D perovskites.

Two Origins of Broadband Emission in Multilayered 2D Lead Iodide Perovskites

Broadband emission in lead iodide 2D perovskites has been alternately attributed to self-trapped excitons (STEs) or permanent structural defects and/or impurities. Here, we investigate six different multilayered (n > 1) 2D lead iodide perovskites as a function of sample temperature from 5 to 300 K. We distinguish shallow defect-associated emission from a broad near-infrared (NIR) spectral feature, which we assign to an STE through subgap photoexcitation experiments. When we varied the thickness (n = 2, 3, 4), A-site cation (methylammonium vs formamidinium), and organic spacer (butylammonium vs hexylammonium vs phenylethylammonium), we found that the temperature dependence of broad NIR emission was strongly correlated with both the strength of electron-phonon coupling and the extent of structural deformation of the ground-state lattice, strongly supporting the assignment of this spectral feature to an STE. However, the extent to which formation of these STEs is intrinsic versus defect-assisted remains open to debate.

Low-frequency Raman spectrum of 2D layered perovskites: Local atomistic motion or superlattice modes?

We report the low-frequency Raman spectrum (ω = 10 cm^-1-150 cm^-1) of a wide variety of alkylammonium iodide based 2D lead halide perovskites (2D LHPs) as a function of A-site cation (MA = methylammonium and FA = formamidinium), octahedral layer thickness (n = 2-4), organic spacer chain length (butyl-, pentyl-, hexyl-), and sample temperature (T = 77 K-293 K). Using density functional theory calculations under the harmonic approximation for n = 2 BA:MAPbI, we assign several longitudinal/transverse optical phonon modes between 30 cm^-1 and 100 cm^-1, the eigendisplacements of which are analogous to that observed previously for octahedral twists/distortions in bulk MAPbI. Additionally, we propose an alternative assignment for low-frequency modes below this band (<30 cm^-1) as zone-folded longitudinal acoustic phonons corresponding to the periodicity of the entire layered structure. We compare measured spectra to predictions of the Rytov elastic continuum model for zone-folded dispersion in layered structures. Our results are consistent across the various 2D LHPs studied herein, with energetic shifts of optical phonons corresponding to microscopic structural differences between materials and energetic shifts of acoustic phonons according to changes in the periodicity and elastic properties of the perovskite/organic subphases. This study highlights the importance of both the local atomic order and the superlattice structure on the vibrational properties of layered 2D materials.

A time-domain view of charge carriers in semiconductor nanocrystal solids

The movement of charge carriers within semiconductor nanocrystal solids is fundamental to the operation of nanocrystal devices, including solar cells, LEDs, lasers, photodetectors, and thermoelectric modules. In this perspective, we explain how recent advances in the measurement and simulation of charge carrier dynamics in nanocrystal solids have led to a more complete picture of mesoscale interactions. Specifically, we show how time-resolved optical spectroscopy and transient photocurrent techniques can be used to track both equilibrium and non-equilibrium dynamics in nanocrystal solids. We discuss the central role of energetic disorder, the impact of trap states, and how these critical parameters are influenced by chemical modification of the nanocrystal surface. Finally, we close with a forward-looking assessment of emerging nanocrystal systems, including anisotropic nanocrystals, such as nanoplatelets, and colloidal lead halide perovskites.

Multi-cation perovskites prevent carrier reflection from grain surfaces

The composition of perovskite has been optimized combinatorially such that it often contains six components (A_xB_yC_1-x-yPbX_zY_3-z) in state-of-art perovskite solar cells. Questions remain regarding the precise role of each component, and the lack of a mechanistic explanation limits the practical exploration of the large and growing chemical space. Here, aided by transient photoluminescence microscopy, we find that, in perovskite single crystals, carrier diffusivity is in fact independent of composition. In polycrystalline thin films, the different compositions play a crucial role in carrier diffusion. We report that methylammonium (MA)-based films show a high carrier diffusivity of 0.047 cm² s^-1, while MA-free mixed caesium-formamidinium (CsFA) films exhibit an order of magnitude lower diffusivity. Elemental composition studies show that CsFA grains display a graded composition. This curtails electron diffusion in these films, as seen in both vertical carrier transport and surface potential studies. Incorporation of MA leads to a uniform grain core-to-edge composition, giving rise to a diffusivity of 0.034 cm² s^-1 in CsMAFA films. A model that invokes competing crystallization processes allows us to account for this finding, and suggests further strategies to achieve homogeneous crystallization for the benefit of perovskite optoelectronics.

Spatially Resolved Photogenerated Exciton and Charge Transport in Emerging Semiconductors

We review recent advances in the characterization of electronic forms of energy transport in emerging semiconductors. The approaches described all temporally and spatially resolve the evolution of initially localized populations of photogenerated excitons or charge carriers. We first provide a comprehensive background for describing the physical origin and nature of electronic energy transport both microscopically and from the perspective of the observer. We introduce the new family of far-field, time-resolved optical microscopies developed to directly resolve not only the extent of this transport but also its potentially temporally and spatially dependent rate. We review a representation of examples from the recent literature, including investigation of energy flow in colloidal quantum dot solids, organic semiconductors, organic-inorganic metal halide perovskites, and 2D transition metal dichalcogenides. These examples illustrate how traditional parameters like diffusivity are applicable only within limited spatiotemporal ranges and how the techniques at the core of this review,especially when taken together, are revealing a more complete picture of the spatiotemporal evolution of energy transport in complex semiconductors, even as a function of their structural heterogeneities.

Setting an Upper Bound to the Biexciton Binding Energy in CsPbBr3 Perovskite Nanocrystals

Cesium lead halide perovskite nanocrystals are promising emissive materials for a variety of optoelectronic applications. To fully realize the potential of these materials, we must understand the energetics and dynamics of multiexciton states which are populated under device relevant excitation conditions. We utilized time-resolved and spectrally-resolved photoluminescence studies to investigate the biexciton binding energy as well as a red-shifted emission feature previously reported under high-flux excitation conditions. We determine that this red-shifted emission feature can be ascribed to sample sintering induced by air-exposure and high-flux irradiation. Furthermore, we determine that the biexciton binding energy at room temperature is at most ±20 meV, providing a key insight toward understanding many-body interactions in the lead halide perovskite lattice.

Melting Transitions of the Organic Subphase in Layered Two-Dimensional Halide Perovskites

We investigate the phase behavior of two-dimensional (C _xH_{2 x+1}NH₃)₂[(MA,FA)PbI₃] _n-1PbI₄ layered perovskites near room temperature (-20 °C to +100 °C) as a function of the octahedral layer thickness ( n = 1, 2, 3, 4), alkylammonium chain length (butyl, pentyl, and hexyl), and identity of the small organic cation (methylammonium and formamidinium). Using differential scanning calorimetry and X-ray diffraction, we observe a reversible first-order phase transition corresponding to a partial melting transition of the alkylammonium chains separating the perovskite layers. The melting temperature, T_m, increases from 10 to 77.9 to 95.9 °C as the carbon chain length increases from C₄ to C₅ to C₆, but it is insensitive to octahedral layer thickness, n. The latent heat of melting, Δ H_m, was in the range of 3-5 kJ/mol-spacer, indicating only partial disordering of the carbon chain. We discuss these findings and their implications in the context of melting in other two-dimensional molecular systems.

Inverse Temperature Dependence of Charge Carrier Hopping in Quantum Dot Solids

In semiconductors, increasing mobility with decreasing temperature is a signature of charge carrier transport through delocalized bands. Here, we show that this behavior can also occur in nanocrystal solids due to temperature-dependent structural transformations. Using a combination of broadband infrared transient absorption spectroscopy and numerical modeling, we investigate the temperature-dependent charge transport properties of well-ordered PbS quantum dot (QD) solids. Contrary to expectations, we observe that the QD-to-QD charge tunneling rate increases with decreasing temperature, while simultaneously exhibiting thermally activated nearest-neighbor hopping behavior. Using synchrotron grazing-incidence small-angle X-ray scattering, we show that this trend is driven by a temperature-dependent reduction in nearest-neighbor separation that is quantitatively consistent with the measured tunneling rate.

Phase-Modulated Degenerate Parametric Amplification Microscopy

Second-order nonlinear optical interactions, including second-harmonic generation (SHG) and sum-frequency generation (SFG), can reveal a wealth of information about chemical, electronic, and vibrational dynamics at the nanoscale. Here, we demonstrate a powerful and flexible new approach, called phase-modulated degenerate parametric amplification (DPA). The technique, which allows for facile retrieval of both the amplitude and phase of the second-order nonlinear optical response, has many advantages over conventional or heterodyne-detected SHG, including the flexibility to detect the signal at either the second harmonic or fundamental field wavelength. We demonstrate the capabilities of this approach by imaging multigrain flakes of single-layer MoS₂. We identify the absolute crystal orientation of each MoS₂ domain and resolve grain boundaries with high signal contrast and sub-diffraction-limited spatial resolution. This robust all-optical method can be used to characterize structure and dynamics in organic and inorganic systems, including biological tissue, soft materials, and metal and semiconductor nanostructures, and is particularly well-suited for imaging in media that are absorptive or highly scattering to visible and ultraviolet light.

Ultrafast Charge Transfer at a Quantum Dot/2D Materials Interface Probed by Second Harmonic Generation

Hybrid quantum dot (QD)/transition metal dichalcogenide (TMD) heterostructures are attractive components of next generation optoelectronic devices, which take advantage of the spectral tunability of QDs and the charge and exciton transport properties of TMDs. Here, we demonstrate tunable electronic coupling between CdSe QDs and monolayer WS₂ using variable length alkanethiol ligands on the QD surface. Using femtosecond time-resolved second harmonic generation (SHG) microscopy, we show that electron transfer from photoexcited CdSe QDs to single-layer WS₂ occurs on ultrafast (50 fs to 1 ps) time scales. Moreover, in the samples exhibiting the fastest charge transfer rates (≤50 fs) we observed oscillations in the time-domain signal corresponding to an acoustic phonon mode of the donor QD, which coherently modulates the SHG response of the underlying WS₂ layer. These results reveal surprisingly strong electronic coupling at the QD/TMD interface and demonstrate the usefulness of time-resolved SHG for exploring ultrafast electronic-vibrational dynamics in TMD heterostructures.

High repetition-rate femtosecond stimulated Raman spectroscopy with fast acquisition

Time-resolved femtosecond stimulated Raman spectroscopy (FSRS) is a powerful tool for investigating ultrafast structural and vibrational dynamics in light absorbing systems. However, the technique generally requires exposing a sample to high laser pulse fluences and long acquisition times to achieve adequate signal-to-noise ratios. Here, we describe a time-resolved FSRS instrument built around a Yb ultrafast amplifier operating at 200 kHz, and address some of the unique challenges that arise at high repetition-rates. The setup includes detection with a 9 kHz CMOS camera and an improved dual-chopping scheme to reject scattering artifacts that occur in the 3-pulse configuration. The instrument demonstrates good signal-to-noise performance while simultaneously achieving a 3-6 fold reduction in pulse energy and a 5-10 fold reduction in acquisition time relative to comparable 1 kHz instruments.

A Nanobionic Light-Emitting Plant

The engineering of living plants for visible light emission and sustainable illumination is compelling because plants possess independent energy generation and storage mechanisms and autonomous self-repair. Herein, we demonstrate a plant nanobionic approach that enables exceptional luminosity and lifetime utilizing four chemically interacting nanoparticles, including firefly luciferase conjugated silica (SNP-Luc), d-luciferin releasing poly(lactic-co-glycolic acid) (PLGA-LH₂), coenzyme A functionalized chitosan (CS-CoA) and semiconductor nanocrystal phosphors for longer wavelength modulation. An in vitro kinetic model incorporating the release rates of the nanoparticles is developed to maximize the chemiluminescent lifetimes to exceed 21.5 h. In watercress (Nasturtium officinale) and other species, the nanoparticles circumvent limitations such as luciferin toxicity above 400 μM and colocalization of enzymatic reactions near high adenosine triphosphate (ATP) production. Pressurized bath infusion of nanoparticles (PBIN) is introduced to deliver a mixture of nanoparticles to the entire living plant, well described using a nanofluidic mathematical model. We rationally design nanoparticle size and charge to control localization within distinct tissues compartments with 10 nm nanoparticles localizing within the leaf mesophyll and stomata guard cells, and those larger than 100 nm segregated in the leaf mesophyll. The results are mature watercress plants that emit greater than 1.44 × 10¹² photons/sec or 50% of 1 μW commercial luminescent diodes and modulate "off" and "on" states by chemical addition of dehydroluciferin and coenzyme A, respectively. We show that CdSe nanocrystals can shift the chemiluminescent emission to 760 nm enabling near-infrared (nIR) signaling. These results advance the viability of nanobionic plants as self-powered photonics, direct and indirect light sources.

CdSe Nanoplatelet Films with Controlled Orientation of their Transition Dipole Moment

Using liquid-liquid interfacial assembly, we control the deposition of CdSe nanoplatelets into face-down or edge-up configurations. Controlled assembly, combined with back focal plane imaging, enabled unambiguous determination of the transition dipole orientation. The transition dipole moment of the emissive band-edge exciton in CdSe nanoplatelets was found to be isotropically oriented within the plane of the nanoplatelet with no measurable out-of-plane component and no preference for the long- or short-axis of the nanoplatelet. Importantly, CdSe nanoplatelet films in the face-down configuration exhibited unity dipole orientation within the plane of the film, which could improve the external efficiency of nanoplatelet LEDs, lasers, photodetectors, and photovoltaic cells beyond that which is possible with isotropic emitters. We also show that the two self-assembled configurations have different Förster energy transfer rates, as a result of different dipole orientation and internanoplatelet distance.

Charge Carrier Hopping Dynamics in Homogeneously Broadened PbS Quantum Dot Solids

Energetic disorder in quantum dot solids adversely impacts charge carrier transport in quantum dot solar cells and electronic devices. Here, we use ultrafast transient absorption spectroscopy to show that homogeneously broadened PbS quantum dot arrays (σ_hom²:σ_inh² > 19:1, σ_inh/k_BT < 0.4) can be realized if quantum dot batches are sufficiently monodisperse (δ ≲ 3.3%). The homogeneous line width is found to be an inverse function of quantum dot size, monotonically increasing from ∼25 meV for the largest quantum dots (5.8 nm diameter/0.92 eV energy) to ∼55 meV for the smallest (4.1 nm/1.3 eV energy). Furthermore, we show that intrinsic charge carrier hopping rates are faster for smaller quantum dots. This finding is the opposite of the mobility trend commonly observed in device measurements but is consistent with theoretical predictions. Fitting our data to a kinetic Monte Carlo model, we extract charge carrier hopping times ranging from 80 ps for the smallest quantum dots to over 1 ns for the largest, with the same ethanethiol ligand treatment. Additionally, we make the surprising observation that, in slightly polydisperse (δ ≲ 4%) quantum dot solids, structural disorder has a greater impact than energetic disorder in inhibiting charge carrier transport. These findings emphasize how small improvements in batch size dispersity can have a dramatic impact on intrinsic charge carrier hopping behavior and will stimulate further improvements in quantum dot device performance.

Modulation of Low-Frequency Acoustic Vibrations in Semiconductor Nanocrystals through Choice of Surface Ligand

Recent experimental and theoretical results have highlighted the surprisingly dominant role of acoustic phonons in regulating dynamic processes in nanocrystals. While it has been known for many years that acoustic phonon frequencies in nanocrystals depend on their size, strategies for tuning acoustic phonon energy at a given fixed size were not available. Here, we show that acoustic phonon frequencies in colloidal quantum dots (QDs) can be tuned through the choice of the surface ligand. Using low-frequency Raman spectroscopy, we explore the dependence of the l = 0 acoustic phonon resonance in CdSe QDs on ligand size, molecular weight, and chemical functionality. On the basis of these aggregated observations, we conclude that the primary mechanism for this effect is mass loading of the QD surface and that interactions between ligands and with the surrounding environment play a comparatively minor yet non-negligible role.

Highly Tunable Colloidal Perovskite Nanoplatelets through Variable Cation, Metal, and Halide Composition

Colloidal perovskite nanoplatelets are a promising class of semiconductor nanomaterials-exhibiting bright luminescence, tunable and spectrally narrow absorption and emission features, strongly confined excitonic states, and facile colloidal synthesis. Here, we demonstrate the high degree of spectral tunability achievable through variation of the cation, metal, and halide composition as well as nanoplatelet thickness. We synthesize nanoplatelets of the form L2[ABX3]n-1BX4, where L is an organic ligand (octylammonium, butylammonium), A is a monovalent metal or organic molecular cation (cesium, methylammonium, formamidinium), B is a divalent metal cation (lead, tin), X is a halide anion (chloride, bromide, iodide), and n-1 is the number of unit cells in thickness. We show that variation of n, B, and X leads to large changes in the absorption and emission energy, while variation of the A cation leads to only subtle changes but can significantly impact the nanoplatelet stability and photoluminescence quantum yield (with values over 20%). Furthermore, mixed halide nanoplatelets exhibit continuous spectral tunability over a 1.5 eV spectral range, from 2.2 to 3.7 eV. The nanoplatelets have relatively large lateral dimensions (100 nm to 1 μm), which promote self-assembly into stacked superlattice structures-the periodicity of which can be adjusted based on the nanoplatelet surface ligand length. These results demonstrate the versatility of colloidal perovskite nanoplatelets as a material platform, with tunability extending from the deep-UV, across the visible, into the near-IR. In particular, the tin-containing nanoplatelets represent a significant addition to the small but increasingly important family of lead- and cadmium-free colloidal semiconductors.

Kinetics of the self-assembly of nanocrystal superlattices measured by real-time in situ X-ray scattering

On solvent evaporation, non-interacting monodisperse colloidal particles self-assemble into a close-packed superlattice. Although the initial and final states can be readily characterized, little is known about the dynamic transformation from colloid to superlattice. Here, by using in situ grazing-incidence X-ray scattering, we tracked the self-assembly of lead sulfide nanocrystals in real time. Following the first appearance of an ordered arrangement, the superlattice underwent uniaxial contraction and collective rotation as it approached its final body-centred cubic structure. The nanocrystals became crystallographically aligned early in the overall self-assembly process, showing that nanocrystal ordering occurs on a faster timescale than superlattice densification. Our findings demonstrate that synchrotron X-ray scattering is a viable method for studying self-assembly in its native environment, with ample time resolution to extract kinetic rates and observe intermediate configurations. The method could be used for real-time direction of self-assembly processes and to better understand the forces governing self-organization of soft materials.