Strong Exciton-phonon Coupling Research Articles

Ultrafast reversible phase engineering in MoTe2 thin film via polaron formation

The emergence of various polymorphs in two-dimensional transition metal dichalcogenides provides an opportunity for robust phase engineering by temperature, strain, laser irradiation, and external charge doping (Keum in Nat. Phys. 11:482, 2015; Song in Nano Lett. 16:188, 2016; Cho in Science 349:625, 2015; Kim in Nano Lett. 17:3363, 2017). This provides means to develop homojunction of metal–semiconductor, enhance mobility, reduce contact resistance, and observe novel quantum critical phenomena in mesoscopic systems. The rich physics paves the way for ultrafast light-induced switching/memory devices and optical data processing in optoelectronics. However, the fundamental temporal evolution of the laser-driven phase transformation, in particular regarding heat and charge carriers, remains elusive. We report an ultrafast reversible structural transformation in MoTe2 by coherent phonon dynamics through polaron formation at room temperature. At a high photon density, the generated coherent phonons are coupled with excitons to form polarons. The strong exciton–phonon coupling disturbs and dephases the coherent phonons of the semiconducting 2H phase in MoTe2, and generates lattice distortions to further stabilize new coherent phonons of the metallic 1T’-phase, manifested by the emergence of the corresponding phonons in each phase. This structural transformation is fully reversible within a few picoseconds by switching on/off the laser. The nonlinear response of the phonon intensity to the excited carrier density in the intermediate region indicates a gradual structural transformation through coexisting 2H and 1T’ phases.

Erbium‐Induced Boost in Self‐Trapped Exciton Emission of Double Perovskites for Highly Sensitive Multimodal and Multiplexed Optical Thermography

AbstractSelf‐trapped excitons (STEs) of lead‐free perovskites have aroused tremendous interest in remote optical thermometry due to strong exciton–phonon coupling and large Stokes shifts. Herein, a bright multimodal and multiplexed optical thermometer is constructed with high sensitivity (Sr) and self‐calibrating ability based on Cs2NaInCl6:Er3+ double perovskite, allowing for fast and simplified reading via mobile devices. Intriguingly, Er3+ doping not only shows the characteristic green emissions but also introduces nanoelectronic domains through a new localized valance band maximum and breaks the symmetry of In3+ site, which facilitates the generation of more STEs. The temperature‐sensitive blue emission of STEs and temperature‐insensitive green emission of Er3+ endow multimodal optical thermometry including time‐resolved and ratiometric readout schemes with a maximum Sr of 3.8% K−1, where an upconversion primary thermometry serves as a reference to calibrate other modes. Meanwhile, the designed thermometers show robust photostability, repeatability, and structural stability for long‐time working and storage. In addition, the remarkable thermochromic phenomenon from blue to green enables a quick color‐multiplexed thermography on a smartphone, which is utilized to capture real‐time 2D thermal imaging of microelectronic devices. This work demonstrates the great potential of lead‐free double perovskite for low‐cost and portable optical thermometry.

Strong Exciton-Phonon Coupling as a Fingerprint of Magnetic Ordering in van der Waals Layered CrSBr.

The layered, air-stable van der Waals antiferromagnetic compound CrSBr exhibits pronounced coupling among its optical, electronic, and magnetic properties. As an example, exciton dynamics can be significantly influenced by lattice vibrations through exciton-phonon coupling. Using low-temperature photoluminescence spectroscopy, we demonstrate the effective coupling between excitons and phonons in nanometer-thick CrSBr. By careful analysis, we identify that the satellite peaks predominantly arise from the interaction between the exciton and an optical phonon with a frequency of 118 cm-1 (∼14.6 meV) due to the out-of-plane vibration of Br atoms. Power-dependent and temperature-dependent photoluminescence measurements support exciton-phonon coupling and indicate a coupling between magnetic and optical properties, suggesting the possibility of carrier localization in the material. The presence of strong coupling between the exciton and the lattice may have important implications for the design of light-matter interactions in magnetic semiconductors and provide insights into the exciton dynamics in CrSBr. This highlights the potential for exploiting exciton-phonon coupling to control the optical properties of layered antiferromagnetic materials.

Defects of perovskite semiconductor CsPbBr3 investigated via photoluminescence and thermally stimulated current spectroscopies

Halide perovskites are essential materials for hard radiation detectors at ambient temperature. To improve detector performance, charge transport must be investigated and optimized. Using photoluminescence (PL) and thermally stimulated current (TSC) spectroscopies, we investigate photogenerated charge carriers in Bridgman-grown CsPbBr3 single crystals to understand the nature of charge transport. PL spectroscopy of these halide perovskites revealed the presence of strong emission bands at the band edge, which were attributed to free or bound excitons. It is shown that a wide broadening of the excitonic linewidth in these halide perovskites arises from strong exciton–phonon coupling, which is substantially dominated by longitudinal optical phonons via Fröhlich interaction. An additional contribution due to the presence of ionized impurities was also observed. Crystals with a detectable sensitivity to high-energy gamma radiation are characterized by a higher intensity and a narrower linewidth of the principal PL peak at 2.326 eV. Defect states beyond 2.214 eV have a negative impact on detector sensitivity to high-energy gamma radiation. TSC spectroscopy reveals an array of trap levels spanning 0.15–0.70 eV, attributed to intrinsic point defects and multiple extrinsic defects involving dopants or impurities. Defects identified included Cs and Br vacancies, as well as Pb interstitials with concentrations in the 1011–1016 cm−3 range. Understanding how the synthesis process impacts the types and concentrations of the defects present is currently under investigation. Elimination or suppression of the defect/trap states should result in halide perovskite materials with longer carrier diffusion lengths and improved detector characteristics.

Observation of Multi-Phonon Emission in Monolayer WS2 on Various Substrates.

Transition metal dichalcogenides (TMDs) have unique absorption and emission properties that stem from their large excitonic binding energies, reduced-dielectric screening, and strong spin-orbit coupling. However, the role of substrates, phonons, and material defects in the excitonic scattering processes remains elusive. In tungsten-based TMDs, it is known that the excitons formed from electrons in the lower-energy conduction bands are dark in nature, whereas low-energy emissions in the photoluminescence spectrum have been linked to the brightening of these transitions, either via defect scattering or via phonon scattering with first-order phonon replicas. Through temperature and incident-power-dependent studies of WS2 grown by CVD or exfoliated from high-purity bulk crystal on different substrates, we demonstrate that the strong exciton-phonon coupling yields brightening of dark transitions up to sixth-order phonon replicas. We discuss the critical role of defects in the brightening pathways of dark excitons and their phonon replicas, and we elucidate that these emissions are intrinsic to the material and independent of substrate, encapsulation, growth method, and transfer approach.

Antimony Doping Enabled Photoluminescence Quantum Yield Enhancement in 0D Inorganic Bismuth Halide Crystals.

Owing to the exterior self-trapped excitons (STEs) with adjustable fluorescence beams, low-dimensional ns2-metal halides have recently received considerable attention in solid-state light-emitting applications. However, the photoluminescence (PL) mechanism in metal halides remains a major challenge in achieving high efficiency and controllable PL properties because the excited-state energy of ns2 conformational ions varies inhomogeneously with their coordination environments. Here, a novel zero-dimensional (0D) lead-free bismuth-based Rb3BiCl6·0.5H2O crystal was reported as a pristine crystal to modulate the optical properties. By doping Sb3+ ions with 5s2 electrons into Rb3BiCl6·0.5H2O crystals, bright orange emission at room temperature was obtained with a photoluminescence quantum yield of 39.7%. Optical characterizations and theoretical studies show that the Sb3+ doping can suppress the strong exciton-phonon coupling, optimize the electronic energy band structure, improve the thermal activation energy, soften the structural lattice of the host crystals, deepen the STE states, and ultimately lead to strong photoluminescence. This work manifests a fruitful manipulation in ripening bismuth-based halides with high-efficiency PL properties, and the PL enhancement mechanisms will guide future research in the exploration of emerging luminescent materials.

Subtractive Nanopore Engineered MXene Photonic Nanomedicine with Enhanced Capability of Photothermia and Drug Delivery for Synergistic Treatment of Osteosarcoma.

Two-dimensional (2D) nanomaterials as drug carriers and photosensitizers have emerged as a promising antitumor strategy. However, our understanding of 2D antitumor nanomaterials is limited to intrinsic properties or additive modification of different materials. Subtractive structural engineering of 2D nanomaterials for better antitumor efficacy is largely overlooked. Here, subtractively engineered 2D MXenes with uniformly distributed nanopores are synthesized. The nanoporous defects endowed MXene with enhanced surface plasmon resonance effect for better optical absorbance performance and strong exciton-phonon coupling for higher photothermal conversion efficiency. In addition, porous structure improves the binding ability between drug and unsaturated bonds, thus promoting drug-loading capacity and reducing uncontrolled drug release. Furthermore, the porous structure provides adhesion sites for filopodia, thereby promoting the cellular internalization of the drug. Clinically, osteosarcoma is the most common bone malignancy routinely treated with doxorubicin-based chemotherapy. There have been no significant treatment advances in the past decade. As a proof-of-concept, nanoporous MXene loaded with doxorubicin is developed for treating human osteosarcoma cells. The porous MXene platform results in a higher amount of doxorubicin-loading, faster near-infrared (NIR)-controlled doxorubicin release, higher photothermal efficacy under NIR irradiation, and increased cell adhesion and internalization. This facile method pioneers a new paradigm for enhancing 2D material functions and is attractive for tumor treatment.

Modulating Kerr nonlinearity and optical bistability in a monolayer graphene nanoribbon nanoresonator coupled to a metallic nanoshell

A scheme for modulating Kerr nonlinearity and optical bistability (OB) based on a monolayer graphene nanoribbon nanoresonator (NR) and a metallic nanoshell (MNS) is proposed. The MNS is made of an Au nanoshell and a dielectric core. Here, we not only demonstrate that the spectrum of the Kerr coefficient can switch from a four-peaked structure to a two-peaked structure by increasing the shell thickness of the MNS, but also plot bistability phase diagrams in the system’s parameter subspace, revealing that the bistable effect can be controlled via zero-channel, one-channel or two-channels by only changing the shell thickness in a strong exciton-phonon coupling regime. Additionally, the calculated results also show that the material of the core of MNS plays an essential role in tuning OB when the shell thickness is fixed. The findings pave a new way for developing optical Kerr switches and high-flexible bistable nanodevices.

Regulating Exciton De-Trapping of Te4+ -Doped Zero-Dimensional Scandium-Halide Perovskite for Fluorescence Thermometry with Record High Time-Resolved Thermal Sensitivity.

Fluorescence thermometry has been propelled to the forefront of scientific attention due to its high spatial resolution and remote non-invasive detection. However, recent generations of thermometers still suffer from limited thermal sensitivity (Sr ) below 10% change per Kelvin. Herein, this work presents an ideal temperature-responsive fluorescence material through Te4+ -doped 0D Cs2 ScCl5 ·H2 O, in which isolated polyhedrons endow highly localized electronic structures, and the strong electron-phonon coupling facilitates the formation of self-trapped excitons (STEs). With rising temperature, the dramatic asymmetric expansion of the soft lattice induces increased defects, strong exciton-phonon coupling, and low thermal activation energy, which evokes a rapid de-trapping process of STEs, enabling several orders of magnitude changes in the fluorescence lifetime over a narrow temperature range. After regulating the de-trapping process with different Te4+ doping, a record-high Sr (27.36% K-1 ) of fluorescence lifetime-based detection is achieved at 325 K. The robust stability against multiple heating/cooling cycles and long-term measurements enables a low temperature uncertainty of 0.067 K. Further, the developed thermometers are demonstrated for the remote local monitoring of operating temperature on internal electronic components. It is believed that this work constitutes a solid step towards building the next generation of ultrasensitive thermometers based on low-dimensional metal halides.

Impact of cation modification on phonon-dressed exciton dynamics in a prototype two-dimensional hybrid organic-inorganic perovskite system.

Organic-cation engineering has recently proven effective in flexibly regulating two-dimensional hybrid organic-inorganic perovskites (2D HOIPs) to achieve a diversity of newly emerging applications. There have been many mechanistic studies based on the structural tunability of organic cations; nevertheless, those with an emphasis on the effect solely caused by the organic cations remain lacking. To this end, here we deliberately design a set of 2D HOIPs in which the inorganic layers are kept nearly intact upon cation modification, i.e., the precursor phenethylammonium lead iodide and its four derivatives with the phenyl group's para-position H being replaced by CH3, F, Cl, and Br. By means of femtosecond time-resolved transient absorption spectroscopy and temperature-dependent/time-resolved photoluminescence spectroscopy, we interrogate the subtle impact of cation modification on phonon dynamics, coherent phonon modes, phonon-dressed exciton dynamics, and excitonic emissions. A concerted trend for phonon lifetimes and exciton relaxation lifetimes regulated by cation modification is revealed, evidencing the existence of strong exciton-phonon coupling in this 2D HOIP system. The observed mass effect can be ascribed to the change in moment of inertia of organic cations. In addition, we observe an interesting interplay of exciton kinetics pertinent to population transfers between two emissive states, likely linked to the subtle variation in crystal symmetry induced by cation modification. The mechanistic insights gained from this work would be of value for the 2D HOIPs-based applications.

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.

First-principles study of luminescence in hexagonal boron nitride single layer: Exciton-phonon coupling and the role of substrate

Hexagonal boron nitride (hBN) is a wide band gap material with both strong excitonic light emission in the ultraviolet and strong exciton-phonon coupling. Luminescence experiments performed on the recently synthesized monolayer form (m-hBN) present emission spectra that differ from one another, with some suggesting a coexistence between phonon-assisted and direct emission channels. Motivated by these results, we investigated the optical response of (m-hBN) using an ab initio approach that takes into account the effects of atomic vibrations on the luminescence spectra. We construct the dynamical exciton-phonon self-energy, then use it to perturbatively correct the optical response functions and test this approach on bulk hBN as a benchmark. Within our approach we are able to estimate the renormalization of the direct peak induced by phonon-assisted transitions, and this allows us to accurately describe spectra where both processes are present. We found that the emission signal of m-hBN is strongly dependent on its interaction with the substrate, which changes its nature from direct to indirect material and modifies the screening felt by the electrons. We attribute the m-hBN emission signal to the bright direct excitons and consider the likelihood of phonon replicas appearing.

Chemical Vapor Deposition of High-Optical-Quality Large-Area Monolayer Janus Transition Metal Dichalcogenides.

One-pot chemical vapor deposition (CVD) growth of large-area Janus SeMoS monolayers is reported, with the asymmetric top (Se) and bottom (S) chalcogen atomic planes with respect to the central transition metal (Mo) atoms. The formation of these 2D semiconductor monolayers takes place upon the thermodynamic-equilibrium-driven exchange of the bottom Se atoms of the initially grown MoSe2 single crystals on gold foils with S atoms. The growth process is characterized by complementary experimental techniques including Raman and X-ray photoelectron spectroscopy, transmission electron microscopy, and the growth mechanisms are rationalized by first principle calculations. The remarkably high optical quality of the synthesized Janus monolayers is demonstrated by optical and magneto-optical measurements which reveal the strong exciton-phonon coupling and enable an exciton g-factor of -3.3.

Tunable photoresponse in twisted lead iodide homostructures via exciton-phonon coupling

•A bottom-up-solution-based epitaxial method has been presented to synthesize tPHs •Twist-angle-triggered tunable exciton-phonon coupling in tPHs is clarified •The tPH-based photodetectors achieve tunable broadband photoresponse Twisted-angle-induced unusual electrical and optical properties in van der Waals homostructures have received extraordinary attention recently. However, these emerging properties have rarely been applied to optoelectronic devices. Here, we demonstrate broadband photoresponse capable of overcoming the band-gap limitation of semiconductors in the twisted lead iodide homostructures. The twist angle triggered strong and tunable exciton-phonon coupling leading to the formation of a self-trapped energy level within the band gap, which extends the photoresponse of lead iodide from the ultraviolet to the near-infrared region. The fabricated photodetectors based on twisted lead iodide homostructures exhibit an ultralow dark current (fA level) and a fast response (μs level) as well as a high responsivity from the ultraviolet to near-infrared range (398 mA/[email protected] nm and 0.511 mA/[email protected] nm). Our work presents a simple strategy to realize broadband photodetectors and demonstrates the potential of twisted two-dimensional (2D) structures for next-generation optoelectronic applications. Twisted-angle-induced unusual electrical and optical properties in van der Waals homostructures have received extraordinary attention recently. However, these emerging properties have rarely been applied to optoelectronic devices. Here, we demonstrate broadband photoresponse capable of overcoming the band-gap limitation of semiconductors in the twisted lead iodide homostructures. The twist angle triggered strong and tunable exciton-phonon coupling leading to the formation of a self-trapped energy level within the band gap, which extends the photoresponse of lead iodide from the ultraviolet to the near-infrared region. The fabricated photodetectors based on twisted lead iodide homostructures exhibit an ultralow dark current (fA level) and a fast response (μs level) as well as a high responsivity from the ultraviolet to near-infrared range (398 mA/[email protected] nm and 0.511 mA/[email protected] nm). Our work presents a simple strategy to realize broadband photodetectors and demonstrates the potential of twisted two-dimensional (2D) structures for next-generation optoelectronic applications.

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.

MoSi2N4: A 2D Regime with Strong Exciton–Phonon Coupling

AbstractThe exciton and phonon strong coupling in functional materials is rich of physics and has promising applications in modern optoelectronics and photonics. However, the direct realization of strong exciton–phonon coupling without complex and well‐designed resonator at room temperature is still a challenge in this topic associated with the Born–Oppenheimer approximation and constrained by the thermalized noise. Here, the enhancement of exciton–phonon coupling in 2D MoSi2N4 material is demonstrated by the near exciton resonance technique and first realization of the strong‐coupling states of exciton and phonon in MoSi2N4 at room temperature. With the involving of three‐band terms transition processes, the phonon scattering transition probability and exciton–phonon interaction are significantly enhanced. Then the strong coupling is reached which maintains high energy transfer efficiency between exciton and phonon supporting over ten phonon replicas at room temperature without introducing harsh resonant cavity. This work identifies that the 2D MoSi2N4 should be a promising strong‐coupling system for exploring many‐body quantum states with strong correlations and further applications in quantum systems, especially for exciton tailoring and room‐temperature tunable exciton lasers.

Exciton–phonon coupling in alanine-based hybrid lead bromide

We found the octahedral distortion in this compound drives its optical properties due to the STE formation. Also, a strong exciton–phonon coupling is observed and ascribed as the mechanism of temperature-induced PL broadening.

Temperature driven charge transfer process in quantum confined two-dimensional Mn-doped CsPbBr3 perovskite nanoplatelets.

A temperature dependent transient absorption study has been demonstrated for Mn2+ doped CsPbBr3 nanoplatelets. At 5 K, charge transfer is suppressed due to the trapping of charge carriers in defect states. By contrast, at 300 K, an efficient charge/energy transfer process is observed as the thermally active carriers become de-trapped from the defect states upon strong exciton-phonon coupling.

Tuning Exciton Recombination Pathways in Inorganic Bismuth-Based Perovskite for Broadband Emission

Single-component emitters with broadband emission are attractive but challenging for illumination and display applications. The two-dimensional organic-inorganic hybrid perovskites have exhibited outstanding broad emission property due to low electronic dimensionality and strong exciton-phonon coupling. However, few layered all-inorganic lead-free perovskites with broadband emission have been explored, and the explicit mechanism of exciton recombination in them also needs in-depth understanding. Herein, the inorganic bismuth-based perovskite Cs 3 Bi 2 Br 9 achieves the stable broadband emission under ambient temperature and pressure by tuning the exciton recombination pathways via antimony (Sb) doping, and the photoluminescence quantum yield (PLQY) realizes an enhancement from 2.9% to 15.9%. The photoluminescence excitation (PLE) spectra indicate that the doped Sb introduces newly extrinsic self-trapped states. The incorporation of Sb promotes the transfer of free excitons (FEs) to extrinsic self-trapped excitons (STEs) observed from Sb content-dependent steady-state PL spectra and, meanwhile, reduces the nonradiative recombination of the generated extrinsic STEs, which are primarily responsible for the remarkably enhanced broad emission. Furthermore, femtosecond transient absorption results elucidate a clear exciton dynamics, in which the transition from FEs to STEs might arise through the gradient energy levels, and finally extrinsic STEs at various energy states contribute to the broadband emission.

Lattice Dynamics and Optoelectronic Properties of Vacancy-Ordered Double Perovskite Cs2TeX6 (X = Cl–, Br–, I–) Single Crystals

The soft, dynamic lattice of inorganic lead halide perovskite CsPbX3 (X = Cl-, Br-, I-) leads to the emergence of many interesting photophysical and optoelectronic phenomena. However, probing their lattice dynamics with vibrational spectroscopy remains challenging. The influence of the fundamental octahedral building block in the perovskite lattice can be better resolved in zero-dimensional (0D) vacancy-ordered double perovskites of form A2BX6. Here we study Cs2TeX6 (X = Cl-, Br-, I-) single crystals to yield detailed insight into the fundamental octahedral building block and to explore the effect that its isolation in the crystal structure has on structural and electronic properties. The isolated [TeX6]2- octahedral units serve as the vibrational, absorbing, and emitting centers within the crystal. Serving as the vibrational centers, the isolated octahedra inform the likelihood of a random distribution of 10 octahedral symmetries within the mixed-halide spaces, as well as the presence of strong exciton-phonon coupling and anharmonic lattice dynamics. Serving as the absorbing and emitting centers, the isolated octahedra exhibit compositionally tunable absorption (1.50-3.15 eV) and emission (1.31-2.11 eV) energies. Due to greater molecular orbital overlap between neighboring octahedra with increasing halide anion size, there is a transition from a more molecule-like electronic structure in Cs2TeCl6 and Cs2TeBr6-as expected from the effective 0D nature of these single crystals-to a dispersive electronic structure in Cs2TeI6, typical of three-dimensional (3D) bulk single crystals.