Exciton-Selective Phonon Coupling in a Lead Halide Perovskite

Abstract

Exciton-phonon interactions govern the optical response of semiconductors, yet disentangling multiple coupling channels in lead halide perovskites remains challenging. We investigate CsPbBr3 microcrystals using photoluminescence, Raman and reflectance spectroscopy at low temperature, revealing the simultaneous presence of high-energy and Rashba excitons, each accompanied by distinct phonon replica series. High-energy exciton replicas are uniquely spaced by approximately 9 meV, whereas Rashba exciton replicas exhibit a characteristic approximately 6 meV spacing, indicating the specificity of the exciton-phonon coupling. Unsupervised machine learning applied to a large low-temperature photoluminescence dataset reveals these replica features are prevalent. With increasing temperature, replica features broaden and merge, evolving into a dominant longitudinal optical phonon coupling regime at room temperature. This work establishes direct spectroscopic evidence for concurrent, exciton-specific phonon coupling within a single material, offering new pathways to engineer light-matter interactions for optoelectronic and phonon-photon-based quantum device applications.

Figures (5)

• Figure 1: Morphological and optical characterization of CsPbBr3 sample. (a) Optical image and (b) Wide-field PL image of the CsPbBr3 sample, demonstrating strong emission. (c) Atomic force microscopy (AFM) image and corresponding height profile(inset), which shows a sample height of $\sim$0.83 $\mu\rm{m}$. (Scale bar: 5 $\mu$m.) (e) Raman spectra at 77 K, exhibiting phonon peaks at 8 meV (P$_1$), 9 meV (P$_2$), 10 (P$_3$) and 11 meV (P$_4$), with an additional weaker peaks at 16 meV (P$_5$) and 38 meV. Inset is a magnified view of the peak P$_6$.
• Figure 2: Low-temperature PL and power dependence. PL spectra at 77 K, multiple distinct emission peaks are observed, including the high energy first exciton (FX) and Rashba exciton (RX). (a) Shows the PL of the sample which exhibit single Rashba emission. (b) PL of the sample with double Rashba emission (RX$'$ and RX$"$), which appear as a peak and a hump in the spectra. (c) shows the PL of the sample where Rashba peak consists of three peaks (RX$'$, RX$"$ and RX$"'$ ). (d-e) The single, double(with energy separation $\Delta_1$) and triple(with energy separations $\Delta_2$ and $\Delta_3$) Rashba emission respectively in different samples. (g-i) $\Delta_1$, $\Delta_2$, and $\Delta_3$ distribution in different number of samples. (j) Colour map of PL intensity as a function of excitation power, showing no significant energy shift with varying power. (k) Normalised PL spectra at different excitation powers showing the low energy peaks scale like the highest intensity peak.
• Figure 3: Low-temperature (77 K) correlated PL, reflectance, and Raman spectra. (a) PL spectra showing equally separated phonon replica peaks of the FX and RX, with an additional peak F$_1'$. The inset shows the replica number (n) vs. $\Delta \rm{E}$ plot for the FX and the RX replicas. (b) Differential reflectance (DR) spectrum. The teal curve shows the simulated DR spectra. (c) First derivative of the differential reflectance (DDR) spectrum. The dips in the DDR correspond to a transition energy, where equally separated phonon replicas of FX and RX are observed. (d) Temperature-dependent PL energy separation, showing a constant energy separation across the given temperature range. (e) Temperature-dependent Raman spectra, exhibiting two strong peaks around 9 meV and 10 meV, exhibit nearly constant intensity and energy within the given temperature regime.
• Figure 4: Structural and recombination pathways, with samples comparison. (a) and (b) show normalised PL spectra of samples showing single split and triple split Rashba peaks, as a function of the energy separation of the RX and all replicas with respect to FX, respectively. (c) n vs mean $\Delta\rm{E}$ for FX and RX of the samples. The dotted lines indicate the mean phonon energy involved in the replica generation process ($\Delta\rm{E}_1$$\sim$9.4 meV and $\Delta\rm{E}_2$$\sim$5.6 meV). The shaded area indicates the available phonon energy bands. (d) Crystal structure of CsPbBr3. The slight displacement of the Cs$^+$ cation within the unit cell leads to its occupation at two distinct positions. The Pb-Br-Pb bond also tilts by an angle $\theta$ due to this displacement. (e) Schematic illustration of the various exciton levels, including the FX, RX, and their respective phonon replicas. The discrete replica states are depicted lying under their corresponding energy valleys. (f) Temperature evolution of PL spectra of the sample showing a double-split Rashba peak. With increasing temperature, sharp phonon replica features broaden and merge, evolving into a single peak with a hump at room temperature (enclosed by dashed grey lines). The dotted grey line (240-300 K) shows the emergence of the high-temperature phonon replica region.
• Figure 5: Statistical study of the emission characteristics for 260 samples. (a) Principal component scatter plot and k-means clustering of the emission lineshapes. The emission lineshape is classified into three clusters: C1 (blue), C2 (orange) and C3 (grey). C1 and C2 show the highest density in the scattered plot, referring to the most probable lineshapes. (b) Percentage distribution of the clusters. (c) Mean emission curve of clusters C1 (top panel), C2 (middle panel) and C3 (bottom panel) (in solid line). The highlighted region in each panel shows the variation of the spectra within the cluster across the mean spectra. (d) Two representative spectra from clusters C1 (top panel), C2 (middle panel) and C3 (bottom panel).