Strong Exciton-phonon Interaction Research Articles

Ultrafast intrinsic excited state localization m 2D layered As2S3 by interlayer bond formation

The family of two-dimensional (2D) layered materials with strong excitonic effect offers fascinating opportunities for studying excited state exciton behavior at 2D limit. While exciton dynamics in conventional 2D semiconductors (e.g. transition metal dichalcogenides) has been extensively studied, little is known about exciton properties and dynamics in 2D layered semiconductors with strong electron/exciton-phonon coupling. Here, by combining experimental and theoretical approaches, we reveal the intrinsic highly localized exciton (i.e. self-trapped exciton) in 2D layered As2S3, driven by strong exciton-phonon interaction. It is shown that photoexcited electron/hole charges in As2S3 localize spontaneously in ~110 fs, giving rise to large stokes-shifted and broad photoluminescence. An interlayer partial bond is formed between chalcogen atoms, triggering lattice distortion and carrier localization. Together with Urbach-Martienssen analysis, this study provides a comprehensive physical picture to understand the complex interplay between exciton and lattice dynamics in 2D semiconductors, which has strong implications to their optoelectronic properties and applications.

Epitaxial hexagonal boron nitride with high quantum efficiency

Two-dimensional (2D) hexagonal boron nitride (h-BN) is one of the few materials showing great promise for light emission in the far ultraviolet (UV)-C wavelength, which is more effective and safer in containing the transmission of microbial diseases than traditional UV light. In this report, we observed that h-BN, despite having an indirect energy bandgap, exhibits a remarkably high room-temperature quantum efficiency (∼60%), which is orders of magnitude higher than that of other indirect bandgap material, and is enabled by strong excitonic effects and efficient exciton-phonon interactions. This study offers a new approach for the design and development of far UV-C optoelectronic devices as well as quantum photonic devices employing 2D semiconductor active regions.

Incorporation sodium ions into monodisperse lead-free double perovskite Cs2AgBiCl6 nanocrystals to improve optical properties

Lead-free double perovskite nanocrystals (NCs) have emerged as a promising candidate in the optical field, owing to their non-toxic, good moist heat and chemical stability. However, their poor optical properties limited their application. To improve the optical properties of lead-free double perovskite NCs, metal ion doping or alloying had been suggested as a promising strategy. Here, we prepared monodisperse, uniformly sized, cubic morphology of Cs2AgBiCl6 NCs with different Na+ incorporation amounts via a simple hot-injection method. The Na+ incorporation broke the parity-forbidden transition by reducing the inversion symmetry of the electron wave function at the Ag site, which changed the parity of the self-trapped exciton wave function and thus allowed radiative recombination. As a result, the photoluminescence quantum yield (PLQY) of Na+-alloyed Cs2AgBiCl6 NCs (12.1%) was higher than that of Cs2AgBiCl6 NCs (2.4%), and the exciton lifetime of Na+-alloyed Cs2AgBiCl6 NCs increased to 36.98 ns from 17.58 ns for Cs2AgBiCl6 NCs. By adjusting the amount of Na+ incorporation, the band gap of Cs2AgBiCl6 NCs can be significantly tuned from ∼2.90 eV to ∼3.50 eV. Furthermore, the temperature-dependent photoluminescence spectra indicated that the Na+-alloyed Cs2AgBiCl6 NCs possessed higher longitudinal optical phonon energy and exciton binding energy compared to Cs2AgBiCl6 NCs. This suggested that there were strong exciton-phonon interactions during exciton recombination, a reduced probability of non-radiative processes, and excellent thermal stability. It offers a promising strategy for improving the optical properties of lead-free double perovskite NCs, and have the potential to replace traditional lead halide perovskite NCs in future optoelectronic applications.

The Bulk van der Waals Layered Magnet CrSBr is a Quasi-1D Material.

Correlated quantum phenomena in one-dimensional (1D) systems that exhibit competing electronic and magnetic order are of strong interest for the study of fundamental interactions and excitations, such as Tomonaga-Luttinger liquids and topological orders and defects with properties completely different from the quasiparticles expected in their higher-dimensional counterparts. However, clean 1D electronic systems are difficult to realize experimentally, particularly for magnetically ordered systems. Here, we show that the van der Waals layered magnetic semiconductor CrSBr behaves like a quasi-1D material embedded in a magnetically ordered environment. The strong 1D electronic character originates from the Cr-S chains and the combination of weak interlayer hybridization and anisotropy in effective mass and dielectric screening, with an effective electron mass ratio of mXe/mYe ∼ 50. This extreme anisotropy experimentally manifests in strong electron-phonon and exciton-phonon interactions, a Peierls-like structural instability, and a Fano resonance from a van Hove singularity of similar strength to that of metallic carbon nanotubes. Moreover, because of the reduced dimensionality and interlayer coupling, CrSBr hosts spectrally narrow (1 meV) excitons of high binding energy and oscillator strength that inherit the 1D character. Overall, CrSBr is best understood as a stack of weakly hybridized monolayers and appears to be an experimentally attractive candidate for the study of exotic exciton and 1D-correlated many-body physics in the presence of magnetic order.

Temperature-Dependent Exciton Dynamics in a Single GaAs Quantum Ring and a Quantum Dot.

Micro-photoluminescence was observed while increasing the excitation power in a single GaAs quantum ring (QR) at 4 K. Fine structures at the energy levels of the ground (N = 1) and excited (N = 2) state excitons exhibited a blue shift when excitation power increased. The excited state exciton had a strong polarization dependence that stemmed from the asymmetric localized state. According to temperature-dependence measurements, strong exciton–phonon interaction (48 meV) was observed from an excited exciton state in comparison with the weak exciton–phonon interaction (27 meV) from the ground exciton state, resulting from enhanced confinement in the excited exciton state. In addition, higher activation energy (by 20 meV) was observed for the confined electrons in a single GaAs QR, where the confinement effect was enhanced by the asymmetric ring structure.

Recombination dynamics of indirect excitons in hexagonal BN epilayers containing polytypic segments grown by chemical vapor deposition using carbon-free precursors

Hexagonal (h) BN is a semiconductor that crystallizes in layers of a two-dimensional honeycomb structure. Since hBN exhibits high quantum efficiency (QE) near-band edge emission at around 5.8 eV in spite of the indirect bandgap, hBN has a potential for the use in deep-ultraviolet light emitters. For elucidating the emission dynamics of indirect excitons (iXs) in hBN, spatially and temporally resolved luminescence measurements were carried out on hBN epilayers grown using carbon-free precursors. In addition to major μm-side flat-topped (0001) hBN columnar grains, sub-μm-scale polytypic segments were identified, which were likely formed by certain growth instabilities. The hBN domains exhibited predominant emissions of phonon-assisted fundamental iXs at 5.7–5.9 eV and a less-pronounced 4.0-eV emission band. The photoluminescence lifetime (τPL) for the iX emissions was 54 ps, which most likely represents the midgap recombination lifetime (τMGR) for an iX reservoir. Because τPL did not change while the cathodoluminescence (CL) intensity increased with temperature above 100 K, both the immobile character of iXs and strong exciton–phonon interaction seem significant for procreating the high QE. The CL intensity and τPL of the 5.5 eV band monotonically decreased with temperature, indicating that τPL represents τMGR, most probably a nonradiative lifetime, around the real states. Equally significant emissions at 6.035 eV at 12 K and 6.0–6.1 eV at 300 K were observed from the polytypic segments, most probably graphitic bernal BN, which also exhibited negligible thermal quenching property.

Temperature dependent activation energies of exciton states on a single GaAs laterally coupled-quantum-dot molecule and quantum dot

The objective of this study was to perform spectroscopy measurements of a single GaAs laterally coupled quantum dot molecule and quantum dot. Compared to photoluminescence spectra between a single GaAs laterally coupled quantum dot molecule and a single quantum dot, the presence of optical coupling was confirmed through dipole-dipole interactions in a single coupled quantum dot molecule. Based on temperature dependence measurements, weak exciton-phonon interactions (25 meV) were observed from the ground exciton state of a single GaAs laterally coupled quantum dot molecule in comparison with a strong exciton-phonon interaction (49 meV) from the ground exciton state of a single GaAs QD. Such weak interactions were due to reduced confinement at exciton states in a single laterally coupled quantum dot molecule. In addition, weaker activation energy (∼15 meV) was observed in terms of confined electrons in a single laterally coupled quantum dot molecule, where a confinement effect became reduced due to enlarged dimensional area in one direction along [1 1‾ 0].

Exciton-Phonon Coupling of Chiral One-Dimensional Lead-Free Hybrid Metal Halides at Room Temperature.

The interaction between organic cations and inorganic metal halide octahedral units strongly affects the properties of organic-inorganic hybrid metal halides. The "soft" property of the lattice provides the possibility of its strong exciton-phonon interaction. Here we report one-dimensional (1D) lead-free chiral organic-inorganic hybrid metal halide single crystals of (R/S)-methylbenzylamine bismuth iodide (R/S-MBA)2Bi2I8, which exhibits a high level of octahedral bond distortion. The introduction of chiral amines leads to a strong chiroptical response in the range of 200-600 nm. The strong exciton-phonon coupling can be observed through the coherent oscillation spectrum of transient absorption dynamics at room temperature. The coherent phonon oscillation frequencies are ∼97 and ∼130 cm-1, corresponding to the symmetrical stretching or bending of the Bi-I octahedron. Our work provides new insights for the study of exciton-phonon coupling in 1D chiral hybrid metal halides.

Phonon signatures in spectra of exciton polaritons in transition metal dichalcogenides

Embedding a monolayer of a transition metal dichalcogenide in a high-$Q$ optical cavity results in the formation of distinct exciton polariton modes. The polaritons are affected by the strong exciton-phonon interaction in the monolayer. We use a time convolutionless master equation to calculate the phonon influence on the spectra of the polaritons. We discuss the nontrivial dependence of the line shapes of both branches on temperature and detuning. The peculiar polariton dispersion relation results in a linewidth of the lower polariton being largely independent of the coupling to acoustic phonons. For the upper polariton, acoustic phonons lead to a low-energy shoulder of the resonance in the linear response. Furthermore, we analyze the influence of inhomogeneous broadening being the dominant contribution to the lower polariton linewidth at low temperatures. Our results point towards interesting phonon features in polariton spectra in transition metal dichalcogenides.

Activation of Self‐Trapped Emission in Stable Bismuth‐Halide Perovskite by Suppressing Strong Exciton–Phonon Coupling

AbstractAll‐inorganic bismuth‐halide perovskites are promising alternatives for lead halide perovskites due to their admirable chemical stability and optoelectronic properties; however, these materials deliver inferior photoluminescence (PL) properties, severely hindering their prospects in lighting applications. Here, a novel air‐stable but non‐emissive perovskite Rb3BiCl6 is synthesized, and the material is used as a prototype to uncover origin of the poor optical performance in bismuth‐halide perovskite. It is found that the extremely strong exciton–phonon interactions with a large coupling constant up to 693 meV leads to the seriously nonradiative recombination, which, however, can be effectively suppressed to 347 meV by introducing Sb3+ ions. As a result, Sb3+‐doped Rb3BiCl6 exhibits a stable yellow emission with unprecedented PL quantum yield up to 33.6% from self‐trapped excitons. Systematic spectroscopic characterizations and theoretical calculations are carried out to unveil the intriguing photophysical mechanisms. This work reveals the effect of exciton–phonon interaction, that is often underemphasized, on a material's photophysical properties.

Strong exciton-phonon interaction assisting simultaneous enhancement of photoluminescence and Raman scattering from suspended carbon nanotubes

We have observed intensity enhancement in exciton photoluminescence and $G$-band Raman scattering of suspended pristine semiconducting carbon nanotubes (CNTs) under resonant excitation while preventing surrounding dielectric Coulomb screening and emission quenching effects. Photoluminescence excitation (PLE) microspectroscopy for the isolated CNTs shows that the PL and Raman scattering are simultaneously enhanced when the excitation photon energy is equal to the sum of the energies of the lowest-energy bright exciton and a longitudinal optical (LO) phonon. This simultaneous enhancement is attributed to a large density of states of the lowest-energy bright exciton strongly interacting with the LO phonon at the \ensuremath{\Gamma} point. The resonant excitation is a key to understanding the excitation and emission processes of CNTs and to designing optimal photonic nanostructures applicable to efficient CNT-based light-emitting devices.

Suppressing Strong Exciton–Phonon Coupling in Blue Perovskite Nanoplatelet Solids by Binary Systems

AbstractQuasi‐two‐dimensional (2D) perovskites are promising candidates for light generation owing to their high radiative rates. However, strong exciton–phonon interactions caused by mechanical softening of the surface act as a bottleneck in improving their suitability for a wide range of lighting and display applications. Moreover, it is not easily available to tune the phonon interactions in bulk films. Here, we adopt bottom‐up fabricated blue emissive perovskite nanoplatelets (NPLs) as model systems to elucidate and as well as tune the phonon interactions via engineering of binary NPL solids. By optimizing component domains, the phonon coupling strength can be reduced by a factor of 2 driven by the delocalization of 2D excitons in out‐of‐plane orientations. It shows the picosecond energy transfer originated from the Förster resonance energy transfer (FRET) efficiently competes with the exciton–phonon interactions in the binary system.

Suppressing Strong Exciton-Phonon Coupling in Blue Perovskite Nanoplatelet Solids by Binary Systems.

Quasi-two-dimensional (2D) perovskites are promising candidates for light generation owing to their high radiative rates. However, strong exciton-phonon interactions caused by mechanical softening of the surface act as a bottleneck in improving their suitability for a wide range of lighting and display applications. Moreover, it is not easily available to tune the phonon interactions in bulk films. Here, we adopt bottom-up fabricated blue emissive perovskite nanoplatelets (NPLs) as model systems to elucidate and as well as tune the phonon interactions via engineering of binary NPL solids. By optimizing component domains, the phonon coupling strength can be reduced by a factor of 2 driven by the delocalization of 2D excitons in out-of-plane orientations. It shows the picosecond energy transfer originated from the Förster resonance energy transfer (FRET) efficiently competes with the exciton-phonon interactions in the binary system.

Theory of the absorption line shape in monolayers of transition metal dichalcogenides

The linear absorption spectra in monolayers of transition metal dichalcogenides show pronounced signatures of the exceptionally strong exciton-phonon interaction in these materials. To account for both exciton and phonon physics in such optical signals, we compare different theoretical methods to calculate the absorption spectra using the example of $\mathrm{MoSe_2}$. In this paper, we derive the equations of motion for the polarization either using a correlation expansion up to 4th Born approximation or a time convolutionless master equation. We show that the Born approximation might become problematic when not treated in high enough order, especially at high temperatures. In contrast, the time convolutionless formulation gives surprisingly good results despite its simplicity when compared to higher-order corrrelation expansion and therefore provides a powerful tool to calculate the lineshape of linear absorption spectra in the very popular monolayer materials.

Differential Absorption Features of CdSe QDs in the Case of Resonant and Nonresonant Excitons Excitation

Nanosecond differential absorption measurements of spherical CdSe QDs at the quasi-stationary resonant and nonresonant excitation of basic exciton transitions are reported. The significant decrease in absorption and saturation of the basic exciton transition 1Sh3/2–1Se are explained by the state filling effect. The observed blue shift of differential absorption minima with increasing pump intensities was explained by strong exciton–phonon interaction. In addition, the 2Sh3/2–1Se exciton transition bleaching and induced absorption of mixed 1Ph3/2–1Pe and 2Sh1/2–1Se exciton transitions were found.

Bright Free Exciton Electroluminescence from Mn-Doped Two-Dimensional Layered Perovskites.

Two-dimensional (2D) perovskites incorporating hydrophobic organic spacer cations show improved film stability and morphology compared to their three-dimensional (3D) counterparts. However, 2D perovskites usually exhibit low photoluminescence quantum efficiency (PLQE) owing to strong exciton-phonon interaction at room temperature, which limits their efficiency in light-emitting diodes (LEDs). Here, we demonstrate that the device performance of 2D perovskite LEDs can be significantly enhanced by doping Mn2+ in (benzimidazolium)2PbI4 2D perovskite films to suppress the exciton-phonon interaction. The distorted [PbI6]4- octahedra by Mn-doping and the rigid benzimidazolium (BIZ) ring without branched chains in the 2D perovskite structure lead to improved crystallinity and rigidity of the perovskites, resulting in suppressed phonon-exciton interaction and enhanced PLQE. On the basis of this strategy, for the first time, we report yellow electroluminescence from free excitons in 2D ( n = 1) perovskites with a maximum brightness of 225 cd m-2 and a peak EQE of 0.045%.

Effect of Phonons on Valley Depolarization in Monolayer WSe2

In this paper, temperature dependence of the excitonic bands in a mechanically exfoliated tungsten diselenide (WSe2) monolayer is studied using photoluminescence and circular dichroic photoluminescence (PL) in the temperature range between 8 and 300 K. The peak energies associated with the neutral exciton (A), charged exciton (trion) and localized excitons are extracted from the PL spectra revealing a trion binding energy of around 30 meV. The circular dichroic PL measured at 8 K shows about 45% valley polarisation that sharply reduces with increasing temperature to 5% at 300 K with photoexcitation energy of 1.96 eV. A detailed analysis of the emission line-width suggests that the rapid decrease of valley polarisation with the increase of temperature is caused by the strong exciton–phonon interactions which efficiently scatter the excitons into different excitonic states that are easily accessible due to the supply of excess photoexcitation energy. The emission line-width broadening with the increase of temperature indicate residual exciton dephasing lifetime < 100 fs, that correlates with the observed rapid valley depolarisation. Circular dichroic photoluminescence spectra in monolayer WSe2 and the influence of temperature on valley depolarization is shown.

Direct Evidence of Exciton-Exciton Annihilation in Single-Crystalline Organic Metal Halide Nanotube Assemblies.

Excitons in low-dimensional organic-inorganic metal halide hybrid structures are commonly thought to undergo rapid self-trapping following creation due to strong quantum confinement and exciton-phonon interaction. Here we report an experimental study probing the dynamics of these self-trapped excitons in the single-crystalline bulk assemblies of 1D organic metal halide nanotubes, (C6H13N4)3Pb2Br7. Through time-resolved photoluminescence (PL) measurements at different excitation intensities, we observed a marked variation in the PL decay behavior that is manifested by an accelerated decay rate with increasing excitation fluence. Our results offer direct evidence of the occurrence of an exciton-exciton annihilation process, a nonlinear relaxation phenomenon that takes place only when some of the self-trapped excitons become mobile and can approach either each other or those trapped excitons. We further identify a fast and dominant PL decay component with a lifetime of ∼2 ns with a nearly invariant relative area for all acquired PL kinetics, suggesting that this rapid relaxation process is intrinsic.

Enhanced Raman spectroscopy induced by surface defects in ripple-like CdS microbelts

Both normal and ripple-like CdS microbelts (MBs) were prepared and their optical properties were investigated by photoluminescent and Raman measurements. One emission at ∼510 nm for normal CdS MBs and two emissions around 513 and 725 nm for ripple-like one were observed. The longitudinal optical phonon mode (1LO) at ∼304 cm−1 and its overtone mode (2LO) at ∼605 cm−1, multiphonon processes and surface phonon peaked at 281 cm−1 were observed, the larger integrated intensities ratio of 2LO to 1LO indicates a strong exciton-phonon coupling interaction. Furthermore, more than 4 times larger enhanced Raman scattering for the ripple-like MBs than the normal one were observed, and the enhanced Raman scattering is mainly associated with the surface defects, which lead to a strong Fröhlich interaction through the charge transfer, not by the deformation potential through surface plasmon enhancement.

Exciton localization and phonon sidebands in polar ZnO/MgZnO quantum wells

Exciton localization is studied for polar ZnO/Mg${}_{0.27}$Zn${}_{0.73}$O quantum wells (QWs) grown by pulsed laser deposition. The samples exhibit both quantum confinement as well as the quantum-confined Stark effect. Emission energies between $3.46$ and $3.18\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.16em}{0ex}}\text{eV}$, depending on the QW thickness, are observed in wedge-shaped samples. Intense QW-phonon sidebands are observed as a result of a strong exciton-phonon interaction and the localization of excitons. To calculate the Huang-Rhys factor $S$ correctly, the concept of strongly and weakly localized excitons is applied. With increasing QW thickness from 3 to $9\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.16em}{0ex}}\text{nm}$, a decrease of $S$ from $0.67$ to $0.43$ and an increase of the fraction of strongly localized excitons from $0.2$ to $0.63$ is observed. The decrease of $S$ is explained by a strong increase of the in-plane pseudo-Bohr radius, whereas the increase of the fraction of strongly localized excitons is expected to result from the increasing strength of localization due to the internal electric field. In this regard, a theory based on an effective mass approximation is applied in calculations to determine the shape of the excitons under the appearance of an internal electric field.