Dynamically Disordered Lattice in a Layered Pb-I-SCN Perovskite Thin Film Probed by Two-Dimensional Infrared Spectroscopy

Probing surface interactions in CdSe quantum dots with thiocyanate ligands

Surface chemistry dictates the optoelectronic properties of semiconductor quantum dots (QDs). Tailoring these properties relies on the meticulous selection of surface ligands for efficient passivation. While long-chain organic ligands boast a well-understood passivation mechanism, the intricacies of short inorganic ionic ligands remain largely unexplored. This study sheds light on the surface-passivation mechanism of short inorganic ligands, particularly focusing on SCN- ions on CdSe QDs. Employing steady-state and time-resolved infrared spectroscopic techniques, we elucidated the surface-ligand interactions and coordination modes of SCN--capped CdSe QDs. Comparative analysis with studies on CdS QDs unveils intriguing insights into the coordination behavior and passivation efficacy of SCN- ions on Cd2+ rich QD surfaces. Our results reveal the requirement of both surface-bound (strong binding) and weakly-interacting interfacial SCN- ions for effective CdSe QD passivation. Beyond fostering a deeper understanding of surface-ligand interactions and highlighting the importance of a comprehensive exploration of ligand chemistries, this study holds implications for optimizing QD performance across diverse applications.

Anionic Effects on Concentrated Aqueous Lithium Ion Dynamics

The dynamics, orientational anisotropy, diffusivity, viscosity, and density were measured for concentrated lithium salt solutions, including lithium chloride (LiCl), lithium bromide (LiBr), lithium nitrite (LiNO2), and lithium nitrate (LiNO3), with methyl thiocyanate as an infrared vibrational probe molecule, using two-dimensional infrared spectroscopy (2D IR), nuclear magnetic resonance (NMR) spectroscopy, and viscometry. The 2D IR, NMR, and viscosity results show that LiNO2 exhibits longer correlation times, lower diffusivity, and nearly 4 times greater viscosity compared to those of the other lithium salt solutions of the same concentration, suggesting that nitrite anions may strongly facilitate structure formation via strengthening water-ion network interactions, directly impacting bulk solution properties at sufficiently high concentrations. Additionally, the LiNO2 and LiNO3 solutions show significantly weakened chemical interactions between the lithium cations and the methyl thiocyanate when compared with those of the lithium halide salts.

Orientation of Thiocyanate Ions Tuning the Electron-Phonon Interactions in Pseudohalide Perovskites

Although the importance of electron-phonon interactions on the optoelectronic properties of perovskites has been well documented, the structural origin of electron-phonon interactions remains largely unexplored. In this study, using pseudohalide perovskites Cs2Pb(SCN)2I2(1-x)Br2x as a model, we have revealed how the orientation of SCN- anions tunes the electron-phonon interactions and the effective charge-carrier mobility by utilizing femtosecond sum frequency generation vibrational spectroscopy, supplemented by photoluminescence spectroscopy and femtosecond optical-pump terahertz-probe spectroscopy. The coupling between neighboring SCN- anions decreases as the Br content (x) increases but does not have a significant effect on the electron-phonon interactions. In contrast, the orientation angle of SCN- anions has a strong correlation with the electron-phonon interaction and effective charge-carrier mobility, that is, a more parallel orientation of SCN- anions leads to a higher electron-phonon interaction and lower effective charge-carrier mobility. This finding provides a molecule-level understanding of the inorganic lattice structure in tuning electron-phonon interactions and may offer valuable guidance for optimizing the optoelectronic properties of perovskites.

Spatial Heterogeneity of Biexcitons in Two-Dimensional Ruddlesden-Popper Lead Iodide Perovskites

Quantum confinement in two-dimensional (2D) Ruddlesden-Popper (RP) perovskites leads to the formation of stable quasi-particles, including excitons and biexcitons, the latter of which may enable lasing in these materials. Due to their hybrid organic-inorganic structures and the solution phase synthesis, microcrystals of 2D RP perovskites can be quite heterogeneous, with variations in excitonic and biexcitonic properties between crystals from the same synthesis and even within individual crystals. Here, we employ one- and two-quantum two-dimensional white-light microscopy to systematically study the spatial variations of excitons and biexcitons in microcrystals of a series of 2D RP perovskites BA2MAn-1PbnI3n+1 (n = 2-4, BA= butylammonium, MA = methylammonium). We find that the average biexciton binding energy of around 60 meV is essentially independent of the perovskite layer thickness (n). We also resolve spatial variations of the exciton and biexciton energies on micron length scales within individual crystals. By comparing the one-quantum and two-quantum spectra at each pixel, we conclude that biexcitons are more sensitive to their environments than excitons. These results shed new light on the ways disorder can modify the energetic landscape of excitons and biexcitons in RP perovskites and how biexcitons can be used as a sensitive probe of the microscopic environment of a semiconductor.

Interfacial engineering of halide perovskites and two-dimensional materials

Recently, halide perovskites (HPs) and layered two-dimensional (2D) materials have received significant attention from industry and academia alike. HPs are emerging materials that have exciting photoelectric properties, such as a high absorption coefficient, rapid carrier mobility and high photoluminescence quantum yields, making them excellent candidates for various optoelectronic applications. 2D materials possess confined carrier mobility in 2D planes and are widely employed in nanostructures to achieve interfacial modification. HP/2D material interfaces could potentially reveal unprecedented interfacial properties, including light absorbance with desired spectral overlap, tunable carrier dynamics and modified stability, which may lead to several practical applications. In this review, we attempt to provide a comprehensive perspective on the development of interfacial engineering of HP/2D material interfaces. Specifically, we highlight the recent progress in HP/2D material interfaces considering their architectures, electronic energetics tuning and interfacial properties, discuss the potential applications of these interfaces and analyze the challenges and future research directions of interfacial engineering of HP/2D material interfaces. This review links the fields of HPs and 2D materials through interfacial engineering to provide insights into future innovations and their great potential applications in optoelectronic devices.

Nanoscale heterogeneity of ultrafast many-body carrier dynamics in triple cation perovskites

In high fluence applications of lead halide perovskites for light-emitting diodes and lasers, multi-polaron interactions and associated Auger recombination limit the device performance. However, the relationship of the ultrafast and strongly lattice coupled carrier dynamics to nanoscale heterogeneities has remained elusive. Here, in ultrafast visible-pump infrared-probe nano-imaging of the photoinduced carrier dynamics in triple cation perovskite films, a ~20 % variation in sub-ns relaxation dynamics with spatial disorder on tens to hundreds of nanometer is resolved. We attribute the non-uniform relaxation dynamics to the heterogeneous evolution of polaron delocalization and increasing scattering time. The initial high-density excitation results in faster relaxation due to strong many-body interactions, followed by extended carrier lifetimes at lower densities. These results point towards the missing link between the optoelectronic heterogeneity and associated carrier dynamics to guide synthesis and device engineering for improved perovskites device performance.

The optoelectronic performance of lead halide perovskite in highfluence applications are hindered by heterogeneous multi-polaron interactions in the nanoscale. Here, Nishda et al. spatially resolve sub-ns relaxation dynamics on the nanometer scale by ultrafast infrared pumpprobe nanoimaging.

Visualizing Light-Induced Microstrain and Phase Transition in Lead-Free Perovskites Using Time-Resolved X-Ray Diffraction

Metal halide perovskites have emerged as promising materials for optoelectronic applications in the last decade. A large amount of effort has been made to investigate the interplay between the crystalline lattice and photoexcited charge carriers as it is vital to their optoelectronic performance. Among them, ultrafast laser spectroscopy has been intensively utilized to explore the charge carrier dynamics of perovskites, from which the local structural information can only be extracted indirectly. Here, we have applied a time-resolved X-ray diffraction technique to investigate the structural dynamics of prototypical two-dimensional lead-free halide perovskite Cs₃Bi₂Br₉ nanoparticles across temporal scales from 80 ps to microseconds. We observed a quick recoverable (a few ns) photoinduced microstrain up to 0.15% and a long existing lattice expansion (∼a few hundred nanoseconds) at mild laser fluence. Once the laser flux exceeds 1.4 mJ/cm², the microstrain saturates and the crystalline phase partially transfers into a disordered phase. This photoinduced transient structural change can recover within the nanosecond time scale. These results indicate that photoexcitation of charge carriers couples with lattice distortion, which fundamentally affects the dielectric environment and charge carrier transport.

Double-quantum-zero-quantum 2D coherent spectroscopy reveals quantum coherence between collective states in an atomic vapor

We report a novel, to the best of our knowledge, double-quantum-zero-quantum two-dimensional coherent spectroscopy (2DCS) that allows direct detection of the quantum coherence between multiparticle collective states. Through correlating the double-quantum coherence and the zero-quantum coherence, signatures for coherence between collective states can be well isolated as side peaks and readily identified in the 2D spectrum. The experiment is implemented in a vapor of rubidium atoms in a collinear 2DCS setup. Good agreement with a theoretical simulation using density matrix confirms the essential role of the interatomic correlation effect in generating the side peak signals. This 2D spectrum technique paves a new avenue for studying the coherent coupling of highly excited states and many-body properties.

Adsorbed Water Structure on Acrylate-Based Biocompatible Polymer Surface

The role of water in the excellent biocompatibility of the acrylate-based polymers widely used for antibiofouling coating material has been realized previously. Here, we report femtosecond mid-infrared pump-probe spectroscopy of the OD stretch band of HOD molecule adsorbed on highly biocompatible poly(2-methoxyethyl) acrylate [PMEA] and poorly biocompatible poly(2-phenoxyethyl) acrylate [PPEA], both of which reveal that there are two water species with significantly different vibrational lifetime. PMEA interacts more strongly with water than PPEA through the H-bonding interaction between carbonyl (C═O) and water. The vibrational lifetime of the OD stretch in PPEA is notably longer by factors of 3 and 7 than those in PMEA and bulk water, respectively. The IR-pump visible-probe photothermal imaging further unravels substantial spatial overlap between polymer CO group and water for hydrated PMEA and a significant difference in surface morphology than those in PPEA, which exhibits the underlying relationships among polymer-water interaction, surface morphology, and biocompatibility.

Vibrational exciton delocalization precludes the use of infrared intensities as proxies for surfactant accumulation on aqueous surfaces

Surface-sensitive vibrational spectroscopy is a common tool for measuring molecular organization and intermolecular interactions at interfaces. Peak intensity ratios are typically used to extract molecular information from one-dimensional spectra but vibrational coupling between surfactant molecules can manifest as signal depletion in one-dimensional spectra. Through a combination of experiment and theory, we demonstrate the emergence of vibrational exciton delocalization in infrared reflection–absorption spectra of soluble and insoluble surfactants at the air/water interface. Vibrational coupling causes a significant decrease in peak intensities corresponding to C–F vibrational modes of perfluorooctanoic acid molecules. Vibrational excitons also form between arachidic acid surfactants within a compressed monolayer, manifesting as signal reduction of C–H stretching modes. Ionic composition of the aqueous phase impacts surfactant intermolecular distance, thereby modulating vibrational coupling strength between surfactants. Our results serve as a cautionary tale against employing alkyl and fluoroalkyl vibrational peak intensities as proxies for concentration, although such analysis is ubiquitous in interface science.

Coupling between surfactant molecules at the air/water interface bleeds intensity into a diffuse background, such that single-wavelength vibrational intensity is effectively depleted at high surface coverage.

Structure and Optical Properties of Layered Perovskite (MA)2PbI2-xBrx(SCN)2 (0 ≤ x < 1.6)

The layered perovskite (MA)₂PbI₂(SCN)₂ (MA = CH₃NH₃⁺) is a member of an emerging series of compounds derived from hybrid organic-inorganic perovskites. Here, we successfully synthesized (MA)₂PbI_2-xBr_x(SCN)₂ (0 ≤ x < 1.6) by using a solid-state reaction. Despite smaller bromide substitution for iodine, 1% linear expansion along the a axis was observed at x ∼ 0.4 due to a change of the orientation of the SCN^- anions. Diffuse reflectance spectra reveal that the optical band gap increases by the bromide substitution, which is supported by the DFT calculations. Curiously, bromine-rich compounds where x ≥ 0.8 are light sensitive, leading to partial decomposition after ∼24 h. This study demonstrates that the layered perovskite (MA)₂PbI₂(SCN)₂ tolerates a wide range of bromide substitution toward tuning the band gap energy.

Fast phase cycling in non-collinear optical two-dimensional coherent spectroscopy

As optical two-dimensional coherent spectroscopy (2DCS) is extended to a broader range of applications, it is critical to improve the detection sensitivity of optical 2DCS. We developed a fast phase-cycling scheme in a non-collinear optical 2DCS implementation by using liquid crystal phase retarders to modulate the phases of two excitation pulses. The background in the signal can be eliminated by combining either two or four interferograms measured with a proper phase configuration. The effectiveness of this method was validated in optical 2DCS measurements of an atomic vapor. This fast phase-cycling scheme will enable optical 2DCS in novel emerging applications that require enhanced detection sensitivity.

Single Ensemble Non-exponential Photoluminescent Population Decays from a Broadband White-Light-Emitting Perovskite

The mechanism of white-light emission from layered Pb-X (X = Cl or Br) perovskites following UV excitation has generated considerable interest. Prior time-dependent studies indicated that the broadband photoluminescence (PL) from (110) perovskites arises from a distribution of self-trapped excitonic sites emitting in different regions of the visible spectrum with different decay dynamics. Here, using time-correlated single photon counting to study single crystals, we show that the white-light emission decay from the (110) perovskite (EDBE)PbBr₄ (EDBE = 2,2'-(ethylenedioxy)bis(ethylammonium)) behaves as a single ensemble. Following the rapid decay (0.6 ns) of a small spectral side band, the broad emission line shape is constant to 100 ns. We propose that rapid local structural fluctuations cause the self-trapped excitons (STEs) to experience a wide range of energies, resulting in the very broad PL. The STEs sample fluctuating local environments on time scales fast compared to the PL, which averages the PL decay at all emission wavelengths, yielding single ensemble PL dynamics. Although emission occurs from a very wide, inhomogeneously broadened spectral line with time-averaged single ensemble luminescence dynamics, the decay is tri-exponential. Two heuristic models for the tri-exponential decay involving defects are discussed. Spin-coated films show faster non-exponential decays with the slowest component of the crystal PL absent. Like the crystals, the film PL decays as a single ensemble. These results demonstrate that the broadband emission decay of (EDBE)PbBr₄ arises from a time-averaged single ensemble and not from a set of excited states emitting with distinct luminescence decays at different wavelengths.

Photoinduced Vibrations Drive Ultrafast Structural Distortion in Lead Halide Perovskite

The success of organic–inorganic perovskites in optoelectronics is dictated by the complex interplay between various underlying microscopic phenomena. The structural dynamics of organic cations and the inorganic sublattice after photoexcitation are hypothesized to have a direct effect on the material properties, thereby affecting the overall device performance. Here, we use ultrafast heterodyne-detected two-dimensional (2D) electronic spectroscopy to reveal impulsively excited vibrational modes of methylammonium (MA) lead iodide perovskite, which drive the structural distortion after photoexcitation. Vibrational analysis of the measured data allows us to monitor the time-evolved librational motion of the MA cation along with the vibrational coherences of the inorganic sublattice. Wavelet analysis of the observed vibrational coherences reveals the coherent generation of the librational motion of the MA cation within ∼300 fs complemented with the coherent evolution of the inorganic skeletal motion. To rationalize this observation, we employed the configuration interaction singles (CIS), which support our experimental observations of the coherent generation of librational motions in the MA cation and highlight the importance of the anharmonic interaction between the MA cation and the inorganic sublattice. Moreover, our advanced theoretical calculations predict the transfer of the photoinduced vibrational coherence from the MA cation to the inorganic sublattice, leading to reorganization of the lattice to form a polaronic state with a long lifetime. Our study uncovers the interplay of the organic cation and inorganic sublattice during formation of the polaron, which may lead to novel design principles for the next generation of perovskite solar cell materials.

Layered Perovskite (CH3NH3)2Pb(SCN)2I2 Single Crystals: Phase Transition and Moisture Stability

To study stability issues of three-dimensional perovskites, there is a strategy to introduce the thiocyanate ion (SCN^-) into CH₃NH₃PbI₃ (MAPbI₃) to solve these problems. Here, we report the bulk growth of layered perovskite MA₂Pb(SCN)₂I₂ single crystals by different growth methods in an ambient atmosphere. We also investigate the structural determination and refinements, band gap, and photoluminescence of MA₂Pb(SCN)₂I₂ single crystals. More importantly, the phase transition and stability of MA₂Pb(SCN)₂I₂ are systematically demonstrated. MA₂Pb(SCN)₂I₂ undergo the reversible single-crystal to single-crystal phase transition in the orthorhombic systems from the space group Pmmn (no. 59) to the space group Pmn2₁ (no. 31) at low temperature. Moreover, the temperature-dependent recovery behaviors of MA₂Pb(SCN)₂I₂ single crystals, powders, and thin films at high temperature are studied in detail. Besides, the moisture stability of MA₂Pb(SCN)₂I₂ is described when exposed to moisture condition by the experiment and theoretical calculations. It would be interesting to not only conduct a comprehensive study on the crystal structures and the phase transition processes of layered perovskites but also provide guidance for further optoelectronic applications of these perovskite materials.

Tunable exciton binding energy in 2D hybrid layered perovskites through donor-acceptor interactions within the organic layer

The strength of electrostatic interactions within semiconductors strongly affects their performance in optoelectronic devices. An important target is the tuning of a material's exciton binding energy-the energy binding an electron-hole pair through the electrostatic Coulomb force-independent of its electronic band gap. Here, we report on the doping of a family of two-dimensional hybrid perovskites, in which inorganic lead halide sheets alternate with naphthalene-based organic layers, with tetrachloro-1,2-benzoquinone (TCBQ). For four out of seven n = 1 perovskites, the incorporation of the electron-accepting TCBQ dopant into the organic sublattice containing the electron-donating naphthalene species enabled the tuning of the materials' 1s exciton binding energy. The naphthalene-TCBQ electron donor-acceptor interactions increased the electrostatic screening of the exciton, in turn lowering its binding energy relative to the undoped perovskite-by almost 50% in one system. Structural and optical characterization showed that the inorganic lattice is not significantly perturbed even though the layer-to-layer spacing increases upon molecular dopant incorporation.

Controlling the Dynamics of Ionic Liquid Thin Films via Multilayer Surface Functionalization

The structural dynamics of planar thin films of an ionic liquid (IL) 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BmimNTf₂) as a function of surface charge density and thickness were investigated using two-dimensional infrared (2D IR) spectroscopy. The films were made by spin coating a methanol solution of the IL on silica substrates that were functionalized with alkyl chains containing head groups that mimic the IL cation. The thicknesses of the ionic liquid films ranged from ∼50 to ∼250 nm. The dynamics of the films are slower than those in the bulk IL, becoming increasingly slow as the films become thinner. Control of the dynamics of the IL films can be achieved by adjusting the charge density on substrates through multilayer network surface functionalization. The charge density of the surface (number of positively charged groups in the network bound to the surface per unit area) is controlled by the duration of the functionalization reaction. As the charge density is increased, the IL dynamics become slower. For comparison, the surface was functionalized with three different neutral groups. Dynamics of the IL films on the functionalized neutral surfaces are faster than on any of the ionic surfaces but still slower than the bulk IL, even for the thickest films. These results can have implications in applications that employ ILs that have electrodes, such as batteries, as the electrode surface charge density will influence properties like diffusion close to the surface.

Does Dipolar Motion of Organic Cations Affect Polaron Dynamics and Bimolecular Recombination in Halide Perovskites?

The role of dipolar motion of organic cations in the A-sites of halide perovskites has been debated in an effort to understand why these materials possess such remarkable properties. Here, we show that the dipolar motion of cations such as methylammonium (MA) or formamidinium (FA) versus cesium (Cs) does not influence large polaron binding energies, delocalization lengths, formation times, or bimolecular recombination lifetimes in lead bromide perovskites containing only one type of A-site cation. We directly probe the transient absorption spectra of large polarons throughout the entire mid-infrared and resolve their dynamics on time scales from sub-100 fs to sub-μs using time-resolved mid-infrared spectroscopy. Our findings suggest that the improved optoelectronic properties reported of halide perovskites with mixed A-site cations may result from synergy among the cations and how their mixture modulates the structure and dynamics of the inorganic lattice rather than from the dipolar properties of the cations themselves.

Vibrational exciton nanoimaging of phases and domains in porphyrin nanocrystals

Much of the electronic transport, photophysical, or biological functions of molecular materials emerge from intermolecular interactions and associated nanoscale structure and morphology. However, competing phases, defects, and disorder give rise to confinement and many-body localization of the associated wavefunction, disturbing the performance of the material. Here, we employ vibrational excitons as a sensitive local probe of intermolecular coupling in hyperspectral infrared scattering scanning near-field optical microscopy (IR s-SNOM) with complementary small-angle X-ray scattering to map multiscale structure from molecular coupling to long-range order. In the model organic electronic material octaethyl porphyrin ruthenium(II) carbonyl (RuOEP), we observe the evolution of competing ordered and disordered phases, in nucleation, growth, and ripening of porphyrin nanocrystals. From measurement of vibrational exciton delocalization, we identify coexistence of ordered and disordered phases in RuOEP that extend down to the molecular scale. Even when reaching a high degree of macroscopic crystallinity, identify significant local disorder with correlation lengths of only a few nanometers. This minimally invasive approach of vibrational exciton nanospectroscopy and -imaging is generally applicable to provide the molecular-level insight into photoresponse and energy transport in organic photovoltaics, electronics, or proteins.

Tailoring Hot Exciton Dynamics in 2D Hybrid Perovskites through Cation Modification

We report a family of two-dimensional hybrid perovskites (2DHPs) based on phenethylammonium lead iodide ((PEA)₂PbI₄) that show complex structure in their low-temperature excitonic absorption and photoluminescence (PL) spectra as well as hot exciton PL. We replace the 2-position (ortho) H on the phenyl group of the PEA cation with F, Cl, or Br to systematically increase the cation's cross-sectional area and mass and study changes in the excitonic structure. These single atom substitutions substantially change the observable number of and spacing between discrete resonances in the excitonic absorption and PL spectra and drastically increase the amount of hot exciton PL that violates Kasha's rule by over an order of magnitude. To fit the progressively larger cations, the inorganic framework distorts and is strained, reducing the Pb-I-Pb bond angles and increasing the 2DHP band gap. Correlation between the 2DHP structure and steady-state and time-resolved spectra suggests the complex structure of resonances arises from one or two manifolds of states, depending on the 2DHP Pb-I-Pb bond angle (as)symmetry, and the resonances within a manifold are regularly spaced with an energy separation that decreases as the mass of the cation increases. The uniform separation between resonances and the dynamics that show excitons can only relax to the next-lowest state are consistent with a vibronic progression caused by a vibrational mode on the cation. These results demonstrate that simple changes to the cation can be used to tailor the properties and dynamics of the confined excitons without directly modifying the inorganic framework.

Ultrafast Carrier Dynamics of Dual Emissions from the Orthorhombic Phase in Methylammonium Lead Iodide Perovskites Revealed by Two-Dimensional Coherent Spectroscopy

The fundamental understanding of photoexcitation landscape and dynamics in hybrid organic-inorganic perovskites is essential for improving their performance in solar cells and other applications. The dual emission features from the orthorhombic phase in perovskites have been the focus of numerous recent studies, and yet their underlying molecular origin remains elusive. We use optical two-dimensional coherent spectroscopy to study the carrier dynamics and coupling of the dual emissions in a methylammonium lead iodide film at 115 K. The two-dimensional spectra reveal an ultrafast redistribution of the photoexcited carriers into the two emission resonances within 250 fs. The high-energy resonance is a short-lived transient state, and the low-energy emission state interacts with coherent phonons. The observed carrier dynamics provide important experimental evidence that can be compared with potential theoretical models and contribute to the understanding of the dual emissions as well as the overall energy level structure in hybrid organic-inorganic perovskites.

Ultrafast intramolecular vibrational energy transfer in carbon nitride hydrocolloid examined by femtosecond two-dimensional infrared spectroscopy

In this work, ultrafast vibrational and structural processes in a graphitic carbon nitride hydrocolloid system were studied using a combination of linear infrared and nonlinear two-dimensional infrared (2D IR) spectroscopies. The experimentally observed three IR line shapes in the C=N stretching vibration frequency region were analyzed and attributed to the rigid and conjugated molecular frame of the prepared g-CN molecular species, which is believed to be a dimeric tris-s-triazine, as well as attributed to insignificant solvent influence on the delocalized C=N vibrations. Vibrational transition density cubes were also computed for the proposed g-CN dimer, confirming the heterocyclic C=N stretching nature of the three IR absorption peaks. Intramolecular vibrational energy transfer dynamics and spectral diffusion of the g-CN system were characterized by examining a series of time-dependent 2D IR spectra. A picosecond intramolecular vibrational energy redistribution process was found to occur among these delocalized C=N stretching modes, acting as an efficient vibrational energy transfer channel. This work reasonably connects the experimentally observed IR signature to a specific g-CN structure and also provides the first report on the ultrafast intramolecular processes of such carbon nitride systems. The obtained results are fundamentally important for understanding the molecular mechanisms of such carbon-nitride based functional materials.

Longer Cations Increase Energetic Disorder in Excitonic 2D Hybrid Perovskites

We synthesize and characterize derivatives of the two-dimensional hybrid perovskite (2DHP) phenethylammonium lead iodide ((PEA)₂PbI₄) in which the para H on the cation is replaced with F, Cl, CH₃, or Br. These substitutions increase the length of the cation but leave the cross-sectional area unchanged, resulting in structurally similar PbI₄^2- frameworks with increasing interlayer spacing. Longer cations result in broader, blue-shifted excitonic absorption spectra with reduced or eliminated structure, indicating greater energetic disorder. Photoluminescence spectra are largely invariant and insensitive to cation length, suggesting polaron formation stabilizes a structural and electronic minimum. Temperature-dependent line width analysis reveals excitons couple to a vibration on the organic framework that is weakly sensitive to these cation substitutions, and Raman spectra and electronic structure calculations support the presence of such a cationic mode. Despite carriers being confined to the inorganic framework, the length of the organic cation alters the optical and electronic properties of 2DHPs.