Perovskite Crystals Reveal Distinct Light-Matter Interactions

Scientists are increasingly focused on understanding exciton-phonon interactions within lead halide perovskites, crucial for optimising their optoelectronic properties. Pradeepa H. L., Sagnik Chatterjee, and Sayantan Patra, working with colleagues from the Department of Physics at the Indian Institute of Science Education and Research (IISER), Pune, and Hardeep and Akshay Singh from the Department of Physics at the Indian Institute of Science, Bangalore, report detailed spectroscopic analysis of CsPbBr3 microcrystals. Their collaborative research, also involving Swapneswar Bisoi, Saqlain Mushtaq, Ashish Arora, and Atikur Rahman from IISER Pune, reveals the simultaneous presence of high-energy and Rashba excitons, each coupling to phonons in a distinct manner. This discovery of exciton-selective phonon coupling, demonstrated through photoluminescence, Raman and reflectance spectroscopy, provides direct evidence for controlling light-matter interactions and opens new avenues for designing advanced optoelectronic and phonon-photon-based devices.

Understanding how light and vibration interact within materials has long been a complex problem. Now, detailed measurements reveal that different forms of light within perovskites couple to vibrations in distinct ways. This discovery opens up new possibilities for designing materials that control both light and sound. Scientists are increasingly focused on understanding how semiconductors interact with light and sound, a field with implications for future technologies.

Controlling these interactions within lead halide perovskites, a class of material exhibiting remarkable optical properties, presents a considerable challenge. Recent investigations into cesium lead bromide (CsPbBr3) microcrystals have revealed a surprising complexity in how these materials respond to excitation. Researchers have demonstrated the simultaneous presence of two distinct types of excitons, high-energy and Rashba excitons, each coupled to unique vibrational modes of the crystal lattice, known as phonons.

These excitons, bound pairs of electrons and holes, each induce a series of ‘replica’ emissions in the photoluminescence spectrum, spaced by specific energy values. In particular, high-energy exciton replicas are consistently spaced by approximately 9 meV, while those associated with Rashba excitons exhibit a spacing of roughly 6 meV. This difference suggests a specificity in how each exciton interacts with the surrounding crystal vibrations.

Applying unsupervised machine learning techniques to a large dataset of low-temperature photoluminescence spectra statistically confirmed the prevalence of these replica features across multiple microcrystals. The behaviour of these excitons changes with temperature. As temperature increases, the distinct replica features broaden and eventually merge, transitioning into a dominant coupling regime involving longitudinal optical phonons at room temperature.

Beyond fundamental materials science, these findings open avenues for engineering light-matter interactions with greater precision. For instance, the ability to control exciton-phonon coupling could prove valuable in designing more efficient optoelectronic devices, such as solar cells, and even in developing novel phonon-photon-based quantum technologies.

Researchers employed a combination of low-temperature photoluminescence, Raman spectroscopy, and reflectance measurements to probe the behaviour of CsPbBr3 microcrystals. By synthesizing these perovskite samples using a near-room-temperature solvothermal method, they achieved controlled growth of large-area, highly crystalline structures. At 77 K, detailed analysis of the emitted light revealed multiple peaks, attributable to different excitonic species and their recombination pathways. The spectra displayed a complex pattern, including single, double, or triple splitting in the lower energy peaks, a characteristic previously linked to Rashba excitons.

Exciton-specific phonon coupling and temperature-dependent replica behaviour in CsPbBr3 microcrystals

At 77 K, analysis of CsPbBr3 microcrystals revealed distinct high-energy and Rashba excitons, each accompanied by a series of phonon replica peaks. These replicas exhibited characteristic energy spacings of approximately 9 meV for the high-energy exciton and around 6 meV for the Rashba exciton, demonstrating exciton-specific phonon coupling. Unsupervised analysis of a large photoluminescence dataset confirmed the prevalence of these replica features at low temperatures.

Single phonon Huang-Rhys (HR) factors ranged from 1.6 to 3.6 for the high-energy exciton and from 0.75 to 0.94 for the Rashba exciton, values that contrast with reports of decreasing HR factors in larger crystals. Increasing the temperature to 300 K caused these sharp, low-energy phonon replicas to broaden and merge. Once reaching approximately 150 K, the distinct replica features coalesced into a single peak. Then, this merged with the Rashba exciton peak near 190 K.

A broad peak persisted at room temperature, alongside the emergence of an additional, lower energy peak starting at 240 K, its intensity increasing with further temperature rises. The observed phonon modes within the 8-12 meV band are directly associated with the high energy exciton, contributing to the prominent high energy exciton phonon replicas seen in both differential reflectance and photoluminescence spectra.

Inside this range, a particularly strong mode at approximately 9.4 meV was identified as key to generating the high-energy exciton replicas. Lower-energy phonon modes between 5-7 meV are important in the formation of Rashba exciton replicas, linked to bending of the Pb-Br-Pb bonds. The Rashba splitting within the photoluminescence spectrum increased to approximately 24 meV at high temperatures.

Raman spectroscopy detected two strong peaks around 9 meV and 10 meV at 77 K, exhibiting nearly constant intensity and energy across the measured temperature range. These findings establish direct spectroscopic evidence for concurrent, exciton-specific phonon coupling within a single material, offering potential for engineering light-matter interactions.

Exciton-phonon coupling characterisation in solvothermally grown CsPbBr3 microcrystals

CsPbBr3 perovskite microcrystals underwent detailed spectroscopic analysis to explore exciton-phonon interactions. Samples were synthesised via a near-room-temperature solvothermal method, utilising an alcohol-based solvent to promote controlled growth of large-area, highly crystalline structures. Optical and wide-field photoluminescence (PL) imaging, performed at room temperature with 405nm excitation, confirmed strong emission across the samples.

Atomic force microscopy (AFM) characterised the crystal height, revealing dimensions ranging from 200nm to 900nm. Then, low-temperature (77 K) Raman spectroscopy was employed to identify characteristic CsPbBr3 single crystal peaks. Strong phonon peaks appeared at 9 meV, 10 meV, 16 meV, and 38 meV, corresponding to octahedral distortions and Pb-Br bond stretching, alongside a longitudinal optical (LO) phonon mode.

Excitation at 77 K revealed multiple resolved emission peaks in PL spectra, attributed to differing excitonic species and recombination pathways. Careful observation of numerous samples showed the highest intensity peak often comprised one, two, or three closely spaced peaks. To analyse these features, k-means clustering was applied to a dataset of 260 low-temperature PL spectra, statistically confirming the prevalence of replica features.

PL and reflectance contrast spectra revealed that peaks grouped into distinct sets with equally spaced characteristics. These equally spaced peaks were assigned as phonon replicas of both the high-energy and Rashba excitons, with the high-energy exciton replicas spaced by approximately 9-10 meV and the Rashba exciton replicas exhibiting a characteristic 6 meV spacing. This spacing indicates specificity in the exciton-phonon coupling.

Exciton types display differing interactions with atomic vibrations in perovskites

The complex dance between light and vibration within lead halide perovskites is now coming into sharper focus. For years, understanding how excitons, bound electron-hole pairs, interact with the material’s atomic vibrations, or phonons, has been hampered by the sheer complexity of these interactions. Researchers have long suspected that different types of excitons couple to phonons in distinct ways, but proving this experimentally proved elusive.

Detailed spectroscopic analysis of cesium lead bromide microcrystals reveals that high-energy and Rashba excitons each exhibit unique ‘fingerprints’ in the way they interact with lattice vibrations. Identifying these exciton-specific phonon couplings opens up possibilities for controlling the flow of energy within the material. By selectively enhancing or suppressing certain vibrational modes, it may become possible to engineer more efficient light-emitting diodes or even create entirely new devices that exploit the interaction between light and sound, phonon-photon interactions.

The work doesn’t resolve all questions. The observed effects are prominent at low temperatures, and how these specific couplings evolve at higher temperatures, and in different perovskite compositions, requires further investigation. This work suggests a level of control previously unseen. Unlike many materials where exciton-phonon coupling is treated as a general disturbance, the ability to target specific excitons with specific phonons could allow for a degree of ‘tuning’ of material properties.

The long-term implications remain speculative, but the potential to design materials with tailored optical and vibrational characteristics is a compelling prospect. Beyond this specific perovskite, the techniques employed here could be applied to a wider range of materials, potentially unlocking similar levels of control in other optoelectronic systems.