Polaron Binding Energy Research Articles

Electrical properties and evaluation of band tail states in Mg doped p-type multilayer hBN

A series of Mg doped p-type multi-layered hBN films were prepared by atmospheric pressure chemical vapor deposition. Temperature dependent conductivity measurements were performed from 0.1 Hz to 10 MHz to analyze the characteristics of tail states close to the valence band edge. Jonscher's power law (Aωs) is successfully applied to understand charge carrier transport through these states. In this work, exponent S increases from 0.6 → 0.8, 0.8 → 0.995, and 1.4 → 1.6 for samples B (precursor temperature, 750 °C), D (850 °C), and E (900 °C), indicating that non-overlapping small polaron tunneling dominates to 548 K. Polaron binding energies of 0.2–0.40 eV and tunneling distances <4.9 Å are calculated, confirming transport through localized states. The density of states near the Fermi level N(EF) was extracted from a fit to the AC conductivity data, yielding values of 1015 and 1017eV−1cm−3 as the precursor temperature increases. Singular Mg acceptor levels of 74, 30, and 17 meV are identified for each sample. A hole concentration from 6.5 × 1017 to 1 × 1018 cm−3 and carrier mobility from 18 to 25 cm2/V s is measured at 300 K. From RC fitting, carrier recombination lifetimes of 1.2, 0.4, and 0.35 μs are determined. Fermi's golden rule is used to determine an optical joint density of states of 1.1 × 1021 eV−1 cm−3 at a band edge. Overall, we show that AC conductivity is an effective method to evaluate midgap states in 2D (two-dimensional) materials at EF and p-type hBN possesses sufficient electrical properties to be integrated into a wide range of semiconductor applications.

Elucidating phonon dephasing mechanisms in layered perovskites with coherent Raman spectroscopies.

Organic-inorganic hybrid perovskite quantum wells exhibit electronic structures with properties intermediate between those of inorganic semiconductors and molecular crystals. In these systems, periodic layers of organic spacer molecules occupy the interstitial spaces between perovskite sheets, thereby confining electronic excitations to two dimensions. Here, we investigate spectroscopic line broadening mechanisms for phonons coupled to excitons in lead-iodide layered perovskites with phenyl ethyl ammonium (PEA) and azobenzene ethyl ammonium (AzoEA) spacer cations. Using a modified Elliot line shape analysis for the absorbance and photoluminescence spectra, polaron binding energies of 11.2 and 17.5meV are calculated for (PEA)2PbI4 and (AzoEA)2PbI4, respectively. To determine whether the polaron stabilization processes influence the dephasing mechanisms of coupled phonons, five-pulse coherent Raman spectroscopies are applied to the two systems under electronically resonant conditions. The prominence of inhomogeneous line broadening mechanisms detected in (AzoEA)2PbI4 suggests that thermal fluctuations involving the deformable organic phase broaden the distributions of phonon frequencies within the quantum wells. In addition, our data indicate that polaron stabilization primarily involves photoinduced reorganization of the organic phases for both systems, whereas the impulsively excited phonons represent less than 10% of the total polaron binding energy. The signal generation mechanisms associated with our fifth-order coherent Raman experiments are explored with a perturbative model in which cumulant expansions are used to account for time-coincident vibrational dephasing and polaron stabilization processes.

(Invited) A Polaron Paradigm for Perovskite Nanocrystal Emitting States

Perovskite NCs such as CsPbBr3 have numerous uses. Most involve exploiting their light emission and are motivated by their defect tolerance and large, as-made emission quantum yields. Absent, though, is a fundamental understanding of perovskite NC emitting states, central to these applications. Of particular note are observations of near-universal, size-, temperature-, and composition-dependent absorption/emission Stokes shifts where observed energy differences are between absorbing and emitting states.Very little is known about the origin of these shifts. Moreover, little is known about the exact identity of the emitting state. This is to be contrasted to more conventional NCs such as CdSe where band edge exciton fine structure quantitatively accounts for both global and resonant Stokes shifts, with emission emerging from a dark exciton. Although similar perovskite NC fine structure might account for their shifts, both theory and experiment predict bright/dark fine structure splittings easily an order of magnitude too small to account for experiment.We now propose that perovskite NC emitting states are polarons, which result from the lattice accommodation of photogenerated charges. Polaron binding energies and lifetimes are, in turn, suggested to be the origin of observed size-, temperature-, and composition-dependent absorption/emission Stokes shifts and excited state lifetimes. This represents a significant departure from more conventional descriptions of NC band edge states, which exclusively involve exciton fine structure and dark exciton emitting states.

Theory of acoustic polarons in the two-dimensional SSH model applied to the layered superatomic semiconductor Re6Se8Cl2.

Layered superatomic semiconductors, whose building blocks are atomically precise molecular clusters, exhibit interesting electronic and vibrational properties. In recent work [Tulyagankhodjaev et al., Science 382, 438 (2023)], transient reflection microscopy revealed quasi-ballistic exciton dynamics in Re6Se8Cl2, which was attributed to the formation of polarons due to coupling with acoustic phonons. Here, we characterize the electronic, excitonic, and phononic properties with periodic density functional theory. We further parameterize a polaron Hamiltonian with nonlocal (Su-Schrieffer-Heeger) coupling to an acoustic phonon to study the polaron ground state binding energy and dispersion relation with variational wavefunctions. We calculate a polaron binding energy of about 10meV at room temperature, and the maximum group velocity of our polaron dispersion relation is 1.5 km/s, which is similar to the experimentally observed exciton transport velocity.

Polaron Vibronic Progression Shapes the Optical Response of 2D Perovskites.

The optical response of 2D layered perovskites is composed of multiple equally-spaced spectral features, often interpreted as phonon replicas, separated by an energy Δ ≃ 12 - 40meV, depending upon the compound. Here the authors show that the characteristic energy spacing, seen in both absorption and emission, is correlated with a substantial scattering response above ≃200cm-1 (≃25meV) observed in resonant Raman. This peculiar high-frequency signal, which dominates both Stokes and anti-Stokes regions of the scattering spectra, possesses the characteristic spectral fingerprints of polarons. Notably, its spectral position is shifted away from the Rayleigh line, with a tail on the high energy side. The internal structure of the polaron consists of a series of equidistant signals separated by 25-32cm-1 (3-4meV), depending upon the compound, forming a polaron vibronic progression. The observed progression is characterized by a large Huang-Rhys factor (S >6) for all of the 2D layered perovskites investigated here, indicative of a strong charge carrier - lattice coupling. The polaron binding energy spans a range ≃20-35meV, which is corroborated by the temperature-dependent Raman scattering data. The investigation provides a complete understanding of the optical response of 2D layered perovskites via the direct observation of polaron vibronic progression. The understanding of polaronic effects in perovskites is essential, as it directly influences the suitability of these materials for future opto-electronicapplications.

Polaron effects created by interface phonons in thin film on ionic substrates

The effective parameter of the electron-phonon interaction in a semiconductor film on ionic substrates has been determined. It is shown that interface phonons “transfer” the polarization of the medium from the substrate to the film. As a result, the polaron binding energy and the effective mass of carriers in the same film on different substrates can vary by tens or even hundreds of percent. Conditions for the realization of strong interaction for various films are found.

Large Exciton Polaron Formation in 2D Hybrid Perovskites via Time-Resolved Photoluminescence.

We find evidence for the formation and relaxation of large exciton polarons in 2D organic-inorganic hybrid perovskites. Using ps-scale time-resolved photoluminescence within the phenethylammonium lead iodide family of compounds, we identify a red shifting of emission that we associate with exciton polaron formation time scales of 3-10 ps. Atomic substitutions of the phenethylammonium cation allow local control over the structure of the inorganic lattice, and we show that the structural differences among materials strongly influence the exciton polaron relaxation process, revealing a polaron binding energy that grows larger (up to 15 meV) in more strongly distorted compounds.

Adiabatic theory of the polaron spectral function

An analytic theory for the spectral function for electrons coupled with phonons is formulated in the adiabatic limit. In the case when the chemical potential is large and negative μ → − ∞ the ground state does not have the adiabatic deformation and the spectral function is defined by the standard perturbation theory. In this limit we use the diagram technique in order to formulate an integral equation for the renormalized vertex. The spectral function was evaluated by solving the Dyson’s equation for the self-energy with the renormalized vertex. The moments of the spectral function satisfy the exact sum rules up to the 7th moment. In the case when the chemical potential is pinned at the polaron binding energy the spectral function is defined by the ground state with a nonzero adiabatic deformation. We calculate the spectral function with the finite polaron density in the adiabatic limit. We also demonstrate how the sum rules for higher moments may be evaluated in the adiabatic limit. Contrary to the case of zero polaron density the spectral function with the finite polaron concentration has some contributions which are characteristic for polarons.

Polarons in Conjugated Polyelectrolytes: A First‐Principles Perspective

AbstractConjugated polyelectrolytes (CPEs) comprised of conjugated backbones and pendant ionic functionalities are versatile organic semiconductors with myriad optoelectronic applications. Although polarons are dominant charge carriers in conducting CPEs owing to strong electron‐phonon coupling, their properties are not well understood, especially from a first‐principles perspective. In this study, a comprehensive study on the stability, structural deformation, electronic structure, and optical absorption of positive (or hole)/negative (or electron) polarons and bi‐polarons in CPEs is conducted. It is explored how these properties depend on the electrostatic interaction between the polarons and ionic functionalities, including alkyl chains, ionic groups, and counterions. It is then examined how the bandgap, polaron binding energy, and optoelectronic structure of CPEs can be tuned by various combinations of the donor and acceptor units in their backbones. Finally, electrochemical stability of CPEs is studied and light is shed on the absence of negative polarons in CPEs. The strategy to improve the electrochemical stability of n‐doped CPEs is also discussed.

Inclusion of infrared dielectric screening in the GW method from polaron energies to charge mobilities

We introduce in the many-body GW scheme the modulation of the screened Coulomb interaction W arising from the macroscopic dielectric response in the infrared. We derive expressions for the polaron binding energies, the renormalization of the effective masses and for the electron and hole relaxation times. Electron and hole mobilities are then obtained from the incorporation of appropriate scattering rules. Zinc-blende GaN and orthorhombic MAPbI3 are used as test beds finding fair agreement with results from rigorous electron-phonon coupling approaches. Although limited to polar phonons, our method has a negligible computational cost.

The mechanism behind SnO metallization under high pressure

SnO is known to undergo metallization at ∼ 5 GPa while retaining its tetragonal symmetry. However, the mechanism of this metallization remains speculative. We present a combined experimental and computational study including pressure-dependent infrared spectroscopy, resistivity, and neutron powder diffraction measurements. We show that, while the excess charge mobility increases with pressure, the lattice distortion, in terms of the z-position of Sn, is reduced. Both processes follow a similar trend that consists of two stages, a moderate increment up to ∼ 3 GPa followed by a rapid increase at higher pressure. This behavior is discussed in terms of polaron delocalization. The pressure-induced delocalization is dictated by the electron–phonon coupling and related local anisotropic lattice distortion at the polaron site. We show that these polaronic states are stable at 0 GPa with a binding energy of ∼ 0.35 eV. Upon increasing the pressure, the polaron binding energy is reduced with the electron–phonon coupling strength of Γ and M modes, enabling the electrical phase transition to occur at ∼ 3.8 GPa. Further compression increases the total electron–phonon coupling strength up to a maximum at 10 GPa, which is a strong evidence of dome-shaped superconductivity transition with Tc = 1.67 K.

Flexural deformations and collapse of bilayer two-dimensional crystals by interlayer excitons

We develop a consistent theory of the interlayer exciton-polaron formed in atomically-thin bilayers. Coulomb attraction between an electron and a hole situated in the different layers results in their flexural deformation and provides an efficient mechanism of the exciton coupling with flexural phonons. We study the effect of layers tension on the polaron binding energy and effective mass leading to suppression of polaron formation by the tension both in the weak and strong coupling regimes. We also consider the role of the nonlinearity related to the interaction between the out- and in-plane lattice displacements and obtain the criterion of the layer sticking, where the exciton collapses, due to the Coulomb attraction between the charge carriers.

Coherent interaction and action–counteraction theory in small polaron systems, and ground state properties

Based on the coherent interaction and action–counteraction principles, we investigate the ground state properties for small polaron systems, the coherent-squeezed fluctuation correction, and the anomalous lattice quantum fluctuation, with the new variational generator containing correlated squeezed-coherent coupling and quantum entanglement. Noting that −2t is the T.B.A. energy, for the coherent interaction effect, we find the ground-state energy E 0 to be −2.428t, in which the coherent squeezed fluctuation correction −A 0 t is −0.463t (where t is the hopping integral, ω is the phonon frequency), with the electron–one-phonon coupling constant g = 1 and the electron–two-phonon coupling constant g 1 = −0.1. However, as a result of the action–counteraction effect, is −2.788t, but is −0.735t. As to the polaron binding energy (E P), for the coherent interaction effect, E P is −1.38ω, but for the action–counteraction effect, is −1.88ω. In particular, the electron–two-phonon interaction noticeably enlarges the coherent interaction and the coherent squeezed quantum fluctuation correction. By intervening with the quantum entanglement, the evolutions of the squeezed coherent state and the lattice quantum fluctuation begin to take control. At that time, we encounter a new quantum phase coherence phenomenon — the collapse and revival of inversion repeatedly for the coherent state in the entangled evolution.

Reaction Mechanism of Photocatalytic Hydrogen Production at Water/Tin Halide Perovskite Interfaces

While instability in aqueous environment has long impeded employment of metal halide perovskites for heterogeneous photocatalysis, recent reports have shown that some particular tin halide perovskites (THPs) can be water-stable and active in photocatalytic hydrogen production. To unravel the mechanistic details underlying the photocatalytic activity of THPs, we compare the reactivity of the water-stable and active DMASnBr3 (DMA = dimethylammonium) perovskite against prototypical MASnI3 and MASnBr3 compounds (MA = methylammonium), employing advanced electronic–structure calculations. We find that the binding energy of electron polarons at the surface of THPs, driven by the conduction band energetics, is cardinal for photocatalytic hydrogen reduction. In this framework, the interplay between the A-site cation and halogen is found to play a key role in defining the photoreactivity of the material by tuning the perovskite electronic energy levels. Our study, by elucidating the key steps of the reaction, may assist in development of more stable and efficient materials for photocatalytic hydrogen reduction.

Low Exciton Binding Energies and Localized Exciton–Polaron States in 2D Tin Halide Perovskites

AbstractAside from band gap reduction, little is understood about the effect of the tin‐for‐lead substitution on the fundamental optical and optoelectronic properties of metal halide perovskites (MHPs), especially when transitioning from 3D to lower dimensional structures. Herein, we take advantage of the spectroscopic isolation of excitons in 2D MHPs to study the intrinsic differences between lead and tin MHPs. The exciton's spectral fine structure indicates a larger polaron binding energy in tin MHPs. Additionally, the electroabsorption responses of the 2D MHPs demonstrates that tin MHPs have exciton binding energies 1.5–2× lower than that of their lead counterparts. Despite the lower binding energy, the excitons in tin MHPs are more Frenkel‐like with small radii, small polarizabilities, and large dipole moments. These results are interpreted as consequences of small polaron formation and disorder‐induced dipole moments. This work highlights the wide range of intrinsic differences between lead and tin MHPs as well as the complexity of excited states in these systems.

Charge Carrier Transport Mechanism in Ta2 O5 , TaON, and Ta3 N5 Studied by Applying Polaron Hopping and Bandlike Models.

TaON and Ta3 N5 are considered promising materials for photocatalytic and photoelectrochemical water splitting. In contrast, their counterpart Ta2 O5 does not exhibit good photocatalytic performance. This may be explained with the different charge carrier transport mechanisms in these materials, which are not well understood yet. Herein, we investigate the charge transport properties in Ta2 O5 , TaON, and Ta3 N5 by polaron hopping and bandlike models. First, the polaron binding energies were calculated to evaluate whether the small polaron occurs in these materials. Then we performed calculations to localize the excess carriers as small polarons using a hybrid density functional. We find that the small polaron hopping is the charge transfer mechanism in Ta2 O5, whereas our calculations indicate that this mechanism may not occur in TaON and Ta3 N5 . We also investigated the bandlike model mechanism by calculating the charge carrier mobility of these materials using the effective mass approximation, but the calculated mobility is not consistent with experimental results. This study is a first step towards understanding charge transport in oxynitrides and nitrides and furthermore establishes a simple rule to determine whether a small polaron occurs in a material.

Finite-range effects in the unitary Fermi polaron

Quantum Monte Carlo techniques are employed to study the properties of polarons in an ultracold Fermi gas, at $T=0$, and in the unitary regime using both a zero-range model and a square-well potential. For a fixed density, the potential range is varied and results are extrapolated and compared against a zero-range model. A discussion regarding the choice of an interacting potential with a finite range is presented. We compute the polaron effective mass, the polaron binding energy, and the effective coupling between them. The latter is obtained using the Landau-Pomeranchuk's weakly interacting quasiparticle model. The contact parameter is estimated by fitting the pair distribution function of atoms in different spin states.

Polarons and their induced interactions in highly imbalanced triple mixtures

We unravel the polaronic properties of impurities immersed in a correlated trapped one-dimensional Bose-Bose mixture. This setup allows the impurities to couple either attractively or repulsively to a specific host, thus offering a highly flexible platform for steering the emergent polaronic properties. Specifically, the impurity residue peak and strength of induced interactions can be controlled by varying the coupling of the impurities to the individual bosonic components. In particular, it is possible to maintain the quasiparticle character for larger interaction strengths as compared to the case of impurities immersed in a single bosonic species. We explicate a hierarchy of the polaron binding energies in terms of the impurity-medium interactions, thereby elucidating the identification of the polaronic resonances in recent experimental radio-frequency schemes. For strong attractive impurity-medium couplings, bipolaron formation is captured. Our findings pave the way for continuously changing the quasiparticle character, under the impact of trap effects, while exposing the role of correlations in triple-mixture settings.

Phenomenological model for long-wavelength optical modes in transition metal dichalcogenide monolayer

Transition metal dichalcogenides (TMDs) are an exciting family of 2D materials; a member of this family, ${\mathrm{MoS}}_{2}$, became the first studied monolayer semiconductor. In this paper, a generalized phenomenological continuum approach for the optical vibrations of the monolayer TMDs valid in the long-wavelength limit is developed. The equation of motions for nonpolar and polar oscillations include the phonon dispersion up to a quadratic approximation in the phonon wave vector. On the other hand, the polar modes satisfy coupled equations for the displacement vector and the inner electric field. The two-dimensional phonon dispersion curves for in-plane and out-of-plane oscillations are thoroughly analyzed. The model parameters are fitted from density functional perturbation theory calculations. The current formalism provides an effective tool to describe the phonon dispersion curves around the $\mathrm{\ensuremath{\Gamma}}$ point of the Brillouin zone for a large group of members of the TMD monolayers. A detailed evaluation of the intravalley Pekar-Fr\"ohlich and the ${A}_{1}$-homopolar mode deformation potential coupling mechanisms is performed. The effects of metal ions and chalcogen atoms on polaron mass and binding energy are studied. It is argued that both mechanisms should be considered for a correct analysis of the properties of the polaron or of any process that involves the intraband transitions assisted by the electron-phonon interaction.

Semiclassical model for calculating exciton and polaron pair energetics at interfaces

Exciton and polaron pair dissociation is a key functional aspect of photovoltaic devices. To improve upon the current state of interfacial transport models, we augment the existing classical models of dielectric interfaces by incorporating results from ab initio calculations, allowing us to calculate exciton and polaron binding energies more accurately. We demonstrate the predictive capabilities of this new model using two interfaces: (i) the boron subphthalocyanine chloride (SubPc) and C60 interface, which is an archetype for many organic photovoltaic devices; and (ii) pentacene and silicon (100), which represents a hybrid between organic and inorganic semiconductors. Our calculations predict that the insertion of molecular dipoles at interfaces can be used for improving polaron pair dissociation and that sharp transitions in dielectric permittivity can have a stronger effect on the polaron pair dissociation than even the electron-hole Coulomb interaction.