Exciton Wave Function Research Articles

Many-body effects in an MXene Ti2CO2 monolayer modified by tensile strain: GW-BSE calculations.

MXenes, two-dimensional (2D) layered transition metal carbide/nitride materials with a lot of advantages including high carrier mobility, tunable band gap, favorable mechanical properties and excellent structural stability, have attracted research interest worldwide. It is imperative to accurately understand their electronic and optical properties. Here, the electronic and optical response properties of a Ti2CO2 monolayer, a typical member of MXenes, are investigated on the basis of first-principles calculations including many-body effects. Our results show that the pristine Ti2CO2 monolayer displays an indirect quasi-particle (QP) band gap of 1.32 eV with the conduction band minimum (CBM) located at the M point and valence band maximum (VBM) located at the Γ point. The optical band gap and binding energy of the first bright exciton are calculated to be 1.26 eV and 0.56 eV, respectively. Under biaxial tensile strains, the lowest unoccupied band at the Γ point shifts downward, while the lowest unoccupied band at the M point shifts upward. Then, a direct band gap appears at the Γ point in 6%-strained Ti2CO2. Moreover, the optical band gap and binding energy of the first bright exciton decrease continuously with the increase of the strain due to the increase of the lattice parameter and the expansion of the exciton wave function. More importantly, the absorbed photon flux of Ti2CO2 is calculated to be 1.76–1.67 mA cm−2 with the variation of the strain, suggesting good sunlight optical absorbance. Our work demonstrates that Ti2CO2, as well as other MXenes, hold untapped potential for photo-detection and photovoltaic applications.

The g-factor anisotropy of trapped excitons in CH<sub>3</sub>NH<sub>3</sub>PbBr<sub>3</sub> perovskite

Hybrid organic-inorganic perovskites show large potential applications in solar cells, light emitting diodes and low threshold lasers because of the high tolerance of defects compared with other semiconductor materials. Normally they have been synthesized by dilution method, generating a device with high performance, but they also introduce lots of defects. So far, investigations have been done intensively on ensemble defects both in theory and experiment, but single-defect related trapped excitons are yet to be explored. In this work, we prepared high-quality CH<sub>3</sub>NH<sub>3</sub>PbBr<sub>3</sub> perovskite nanowires with the length of about 1 μm and the width of several hundred nanometers by “reverse” ligand assisted reprecipitation method, and performed the magneto-photoluminescence measurement of different trapped excitons in single perovskite nanowires at a low temperature with a standard confocal microscopic system. The photoluminescence (PL) peak with narrow linewidth has been observed from trapped excitons with high luminescence intensity and the trapped excitons can be coupled with phonons in different ways. Both Zeeman splittings and diamagnetic effects have been observed in single trapped excitons under the magnetic field, and we found that the different trapped excitons have different Zeeman splittings and diamagnetic effects which is caused by the different defects near the trapped excitons. At the same time, we have extracted the g-factor of the trapped excitons under different magnetic field angles. The extracted exciton g-factors show anisotropic, which can be ascribed to the limitation of the lattice structure of the perovskite and the trapped exciton wave-function anisotropy under a vector magnetic field. Our results demonstrate that trapped excitons with narrow linewidth have very good luminescence properties and studying the magneto-optical properties from single trapped excitons can provide a deep understanding of trapped excitons in perovskites for applications in quantum light sources and spintronics. Furthermore, our results can also provide a possibility to control the electron spin in single-trapped-excitons-based hybrid organic-inorganic perovskites by manipulating the g-factor through an applied vector magnetic field, which promotes the application of the perovskite-based spintronics.

Defect-induced exciton localization in bulk gallium nitride from many-body perturbation theory

We present a many-body perturbation theory study of the excitonic properties of wurtzite GaN containing a single charged nitrogen vacancy. We determine that the lowest-energy exciton consists of a bulk to defect transition, resulting in a slight redshift ($<0.1$ eV) of the optical absorption onset and a 50 meV increase in the exciton binding energy when compared with pristine bulk. Furthermore, by analysis of the electron-hole correlation function, we quantify the defect-induced localization of the Wannier-Mott exciton in two ways. First, we show that the electron-hole separation is reduced, and that the exciton envelope wave function can be related to a simple model of a defect-bound exciton. Second, we show that the exciton center-of-mass does not display the periodicity of the lattice due to defect-induced localization. We anticipate that our approach, which quantitatively describes the influence of a point defect on the exciton wave function, will be generally applicable.

Anisotropic Two-Dimensional Screening at the Surface of Black Phosphorus.

Electronic screening can have direct consequences for structural arrangements on the nanoscale, such as on the periodic ordering of adatoms on a surface. So far, such ordering phenomena have been explained in terms of isotropic screening of free electronlike systems. Here, we directly illustrate the structural consequences of anisotropic screening, making use of a highly anisotropic two-dimensional electron gas (2DEG) near the surface of black phosphorous. The presence of the 2DEG and its filling is controlled by adsorbed potassium atoms, which simultaneously serve to probe the electronic ordering. Using scanning tunneling microscopy, we show that the anisotropic screening leads to the formation of potassium chains with a well-defined orientation and spacing. We quantify the mean interaction potential utilizing statistical methods and find that the dimensionality and anisotropy of the screening is consistent with the presence of a band bending-induced 2DEG near the surface. The electronic dispersion of the 2DEG inferred by electronic ordering is consistent with that measured by angle-resolved photoemission spectroscopy.

Electro-optical properties of a pressure-induced g−SiC7 sheet from many-body perturbation theory

Alongside a vast number of two-dimensional (2D) materials, $g\text{\ensuremath{-}}{\mathrm{SiC}}_{7}$ siligraphene has already been introduced to the 2D nanomaterial family with extraordinary sunlight absorption. In this work, we exploit the concept of the in-plane biaxial strain-induced electro-optical engineering in siligraphene from ab initio many-body perturbation theory calculations (GW + Bethe-Salpeter equation). While the semiconductor nature of the system is robust for both tensile and compressive strains, the direct GW band gap of 1.35 eV is decreased (increased) linearly with tensile (compressive) strain. This is accompanied by a variation in the optical transitions. The tensile strain in $g\text{\ensuremath{-}}{\mathrm{SiC}}_{7}$ shifts the optical absorption peaks to the lower photon energies, the so-called red shift, whereas the compressive strain causes a shift to the higher frequencies, the so-called blue shift. Eventually, the exciton binding energy trend and the real-space exciton distribution for the bright bound state is studied with strain, resulting in a alteration of the spatial distribution radius of exciton wave functions from tensile to compressive. Our findings extend the logic applications of siligraphene in photovoltaic devices, especially in tailoring the power conversion efficiency of excitonic solar cells.

Real-space exciton distribution in strained-siligraphene g-SiC7

Siligraphene belonging to the family of two-dimensional (2D) materials has great potential in optoelectronics due to its considerable excitonic effects. In this study, the strain effects on the electronic structure and the real-space exciton wave functions of g-SiC7 are investigated using the first-principles calculations based on the ab initio many-body perturbation theory. Alongside the increase (decrease) of the bandgap with compressive (tensile) strain, our results show that the exciton in the siligraphene monolayer under in-plane biaxial compressive strains is much more localized than that in the case of tensile one, leading to the higher and lower exciton binding energies, respectively. Moreover, the π↦π and π↦σ exciton state transition emerges when applying the compressive and tensile strains, respectively. Overall, our study reveals that a desirable way to dissociate the electron-hole coupling and to reduce the electron-hole recombination process is applying “in-plane biaxial tensile strain,” making g-SiC7 an excellent potential functional 2D semiconductor in optoelectronics.

Quantum Interference in a Single Perovskite Nanocrystal.

Coherent manipulation of the exciton wave function in a single semiconductor colloidal nanocrystal (NC) has been actively pursued in the past decades without any success, mainly due to the bothersome existence of the spectral diffusion and the photoluminescence (PL) blinking effects. Such optical deficiencies can be naturally avoided in the newly developed colloidal NCs of perovskite CsPbI3, leading to the PL spectrum with a stable intensity at the single-particle level. Meanwhile, from the first-order photon-correlation measurement, a PL line width smaller than 20 μeV is estimated for the emission state of the neutral exciton in a single CsPbI3 NC. Moreover, a dephasing time of about 10 ps can be extracted from the quantum interference measurement on the absorption state of the charged exciton. This stable demonstration of a coherent optical feature will advance single colloidal NCs into the quantum information regime, opening up an alternative yet prospective research direction beyond their traditional applications such as in optoelectronic devices and bioimaging.

Magneto-spectroscopy of exciton Rydberg states in a CVD grown WSe2 monolayer

The results of magneto-optical spectroscopy investigations of excitons in a CVD grown monolayer of WSe2 encapsulated in hexagonal boron nitride are presented. The emission linewidth for the 1s state is of 4.7 meV, close to the narrowest emissions observed in monolayers exfoliated from bulk material. The 2s excitonic state is also observed at higher energies in the photoluminescence spectrum. Magneto-optical spectroscopy allows for the determination of the g-factors and of the spatial extent of the excitonic wave functions associated with these emissions. Our work establishes CVD grown monolayers of transition metal dichalcogenides as a mature technology for optoelectronic applications.

Proposed Scheme to Generate Bright Entangled Photon Pairs by Application of a Quadrupole Field to a Single Quantum Dot.

Entangled photon sources are crucial for quantum optics, quantum sensing, and quantum communication. Semiconductor quantum dots generate on-demand entangled photon pairs via the biexciton-exciton cascade. However, the pair of photons are emitted isotropically in all directions, thus limiting the collection efficiency to a fraction of a percent. Moreover, strain and structural asymmetry in quantum dots lift the degeneracy of the intermediate exciton states in the cascade, thus degrading the measured entanglement fidelity. Here, we propose an approach for generating a pair of entangled photons from a semiconductor quantum dot by application of a quadrupole electrostatic potential. We show that the quadrupole electric field corrects for the spatial asymmetry of the excitonic wave function for any quantum dot dipole orientation and fully erases the fine-structure splitting without compromising the spatial overlap between electrons and holes. Our approach is compatible with nanophotonic structures such as microcavities and nanowires, thus paving the way towards a deterministic source of entangled photons with high fidelity and collection efficiency.

Plasmon-Assisted Suppression of Surface Trap States and Enhanced Band-Edge Emission in a Bare CdTe Quantum Dot.

Colloidal quantum dots have emerged as a versatile photoluminescent and optoelectronic material. Limitations like fluorescence intermittency, nonradiative Auger recombination, and surface traps are commonly addressed by growing a wide-band-gap shell. However, the shell isolates the excitonic wave function and reduces its interaction with the external environment necessary for different applications. Furthermore, their long emission lifetime hinders their use in high-speed optoelectronics. Here, we demonstrate a high degree of control on the photophysics of a bare core CdTe quantum dot solely by plasmon coupling, showing that more than 99% of the surface defect-state emission from a trap-rich quantum dot can be quenched. Moreover, the band-edge state excitonic and biexcitonic emission rates are Purcell enhanced by 1460- and 613-fold, respectively. Our findings show how plasmon coupling on bare quantum dots could make chemical approaches developed for improving their optical properties unnecessary, with implications for nanoscale lasers, light-emitting devices, solar cells, and ultrafast single-photon sources.

Electronic energy transfer studied by many-body Green's function theory.

We present a combination of many-body Green's function theory and Förster-Dexter theory to estimate the excitation energy transfer (EET) coupling in both the isolated and condensed systems. This approach employs the accurate wave functions of excitons, which are derived from the Bethe-Salpeter equation, in the donor and acceptor to set up the electronic coupling terms. Dexter coupling, which arises from the exchange-correlation effect, is evaluated based on the GW method which is a state-of-the-art ab initio theory for the description of self-energy. This approach is applicable to various situations, especially for periodic systems. The approach is tested on some model molecular dimers and compared with other high-level quantum chemistry methods together with the exact supermolecule scheme. Finally, we apply it to study the EET between periodic single-walled carbon nanotubes, exploring the dependence of EET on the chirality of nanotubes and the type of excitation transferred, finding that dark states play key roles in the EET between nanotubes. The EET rate falls as ∼D-12 approximately with the distance D between nanotubes for small D, much faster than the traditional Förster model.

Linewidth broadening and tunneling of excitons bound to N pairs in dilute GaAs:N

The exciton bound to a pair of nitrogen atoms situated at nearby lattice sites in dilute GaAs:N provides an energetically uniform electronic system, spectrally distinct from pairs with larger or smaller separations, and can even be grown with a uniform pair orientation in the crystal. We use photoluminescence excitation spectroscopy on an ensemble of N pairs to study the narrow continuous energy distribution within two of the individual exchange- and symmetry-split exciton states. Inhomogeneous linewidths of 50–60 μeV vary across the crystal on a mesoscopic scale and can be 30 μeV at microscopic locations indicating that the homogeneous linewidth inferred from previous time-domain measurements is still considerably broadened. While excitation and emission linewidths are similar, results show a small energy shift between them indicative of exciton transfer via phonon-assisted tunneling between spatially separated N pairs. We numerically simulate the tunneling in a spatial network of randomly distributed pairs having a normal distribution of bound exciton energies. Comparing the ensemble excitation-emission energy shift with the measured results shows that the transfer probability is higher than expected from the dilute pair concentration and what is known of the exciton wavefunction spatial extent. Both the broadening and the exciton transfer have implications for the goal of pair-bound excitons as a single- or multi-qubit system.

Electronic and optical properties of doped TiO2 by many-body perturbation theory

Doping is one of the most common strategies for improving the photocatalytic and solar energy conversion properties of TiO$_2$, hence an accurate theoretical description of the electronic and optical properties of doped TiO$_2$ is of both scientific and practical interest. In this work we use many-body perturbation theory techniques to investigate two typical n-type dopants, Niobium and Hydrogen, in TiO$_2$ rutile. Using the GW approximation to determine band edges and defect energy levels, and the Bethe Salpeter equation for the calculation of the absorption spectra, we find that the defect energy levels form non-dispersive bands %associated with localized states lying $\simeq 2.2 eV$ above the top of the corresponding valence bands ($\simeq 0.9 eV$ below the conduction bands of the {\it pristine} material). The defect states are also responsible for the appearance of low energy absorption peaks that enhance the solar spectrum absorption of rutile. The spatial distributions of the excitonic wavefunctions associated with these low energy excitations are very different for the two dopants, suggesting a larger mobility of photoexcited electrons in Nb-TiO$_2$.

Spectrum of exciton states in monolayer transition metal dichalcogenides: Angular momentum and Landau levels

A four-band exciton Hamiltonian is constructed starting from the single-particle Dirac Hamiltonian for charge carriers in monolayer transition metal dichalcogenides (TMDs). The angular part of the exciton wave function can be separated from the radial part, in the case of zero center of mass momentum excitons, by exploiting the eigenstates of the total exciton angular momentum operator with which the Hamiltonian commutes. We explain why this approach fails for excitons with finite center of mass momentum or in the presence of a perpendicular magnetic field and present an approximation to resolve this issue. We calculate the (binding) energy and average interparticle distance of different excited exciton states in different TMDs and compare these with results available in the literature. Remarkably, we find that the intervalley exciton ground state in the $\mp K$ valley has angular momentum $j=\pm1$, which is due to the pseudospin of the separate particles. The exciton mass and the exciton Landau levels are calculated and we find that the degeneracy of exciton states with opposite relative angular momentum is altered by a magnetic field.

Numerical modeling of indirect excitons in double quantum wells in an external electric field

The ground state of an exciton in the GaAs-based double quantum well structure in an external electric field is calculated by the direct numerical solution of the three-dimensional Schrödinger equation. A formation of the indirect exciton is demonstrated by the study of the localization of the exciton wave function in the double quantum well structure for different electric field strengths. A relatively slow radiative decay rate of the indirect exciton is observed.

Excitons in hexagonal boron nitride single-layer: a new platform for polaritonics in the ultraviolet

The electronic and optical properties of 2D hexagonal boron nitride are studied using first principle calculations. GW and BSE methods are employed in order to predict with better accuracy the excited and excitonic properties of this material. We determine the values of the band gap, optical gap, excitonic binding energies and analyse the excitonic wave functions. We also calculate the exciton energies following an equation of motion formalism and the Elliot formula, and find a very good agreement with the GW+BSE method. The optical properties are studied for both the TM and TE modes, showing that 2D hBN is a good candidate to polaritonics in the UV range. In particular it is shown that a single layer of h-BN can act as an almost perfect mirror for ultraviolet electromagnetic radiation.

Exciton spectroscopy of optical reflection from wide quantum wells

Optical spectroscopy of resonant reflection has been used for both experimental and theoretical studies of the exciton-light interaction in wide, high-quality quantum-well structures with the widths exceeding the exciton Bohr radius by an order of magnitude or more. The light mixes the low-lying confined exciton states which are captured by the generalized model developed by M. M. Voronov et al. [Phys. Solid State 49, 1792 (2007)]. We demonstrate that the high-energy confined states in the wide QWs can still be described by the standard model in which the amplitude reflection coefficient from the QW is a sum of individual size-quantized exciton resonances. The excitonic parameters extracted from fitting the experimental spectrum to the standard model agree with those obtained by the numerical solution of the two-particle Schr\odinger equation in a rectangular quantum well. The measured and microscopically calculated spectra are compared with those found with the widely used model of the center-of-mass exciton quantization and the polaritonic model. The comparison shows that the two approximate models considerably underestimate the interaction of confined excitons with light because they ignore the strong modification of the exciton wave function near the QW interfaces.

Two-dimensional excitons in monolayer transition metal dichalcogenides from radial equation and variational calculations

Exciton energy spectra of monolayer transition metal dichalcogenides (TMDs) in various dielectric environments are studied with an effective mass model using the Keldysh potential for the screened electron–hole interaction. Two-dimensional (2D) excitons are calculated by solving a radial equation (RE) with a shooting method, using boundary conditions that are derived by applying the asymptotic properties of the Keldysh potential. For any given main quantum number n, the exciton Bohr orbit shrinks as becomes larger (m is the orbital quantum number) resulting in increased strength of the electron–hole interaction and a decrease of the exciton energy. Further, both the exciton energy and its effective radius decrease linearly with . The screened hydrogen model (SHM) (Olsen et al 2016 Phys. Rev. Lett. 116 056401) is examined by comparing its exciton energy spectra with our RE solutions. While the SHM is found to describe the nonhydrogenic exciton Rydberg series (i.e. the energy’s dependence on n) reasonably well, it fails to account for the linear dependence of the exciton energy on the orbital quantum number. The exciton effective radius expression of the SHM can characterize the exciton radius’s dependence on n, but it cannot properly describe the exciton radius’s dependence on m, which is the cause of the SHM’s poor description of the exciton energy’s m-dependence. Analytical variational wave-functions are constructed with the 2D hydrogenic wave-functions for a number of strongly bound exciton states, and very close exciton energies and wave-functions are obtained with the variational method and the RE solution (exciton energies are within a 6% of deviation). The variational wave-functions are further applied to study the Stark effects in 2D TMDs, with an analytical expression derived for evaluating the redshift the ground state energy.

Dinitrogen Coupling to a Terpyridine-Molybdenum Chromophore Is Switched on by Fermi Resonance

Summary The traditional view of a chemical change is inherently local and classical, and such a change relies on a mix of thermodynamic and kinetic parameters to control reactivity. Often, the thermodynamic stability of chemical bonds necessitates significant energy input for activation. One fundamental question is potentially transformative: can quantum mechanics enable selective bond activation? A possible approach involves strategic input of energy to reaction-specific vibrational levels. Toward this goal, our work describes the coupling of vibrational motions in a terpyridine-molybdenum complex hosting a nonreactive substrate—dinitrogen. Ultrafast coherence spectroscopies revealed a Fermi-resonance coupling mechanism connecting in-plane breathing motion of the light-harvesting terpyridines with the stretching motion of the spatially disparate dinitrogen bridge. Notably, the coupling is significantly enhanced in the photoexcited state. This Fermi resonance indicates an energy conduit that drives the two motions in sync and thereby amplifies vibrational energy exchange. Achieving selective bond activation by bridging vibrations could present a quantum-inspired design principle in synthetic chemistry.

Classification of Energy States of the Exciton in Square Quantum Well

The energy states of the exciton in a single square GaAs-based quantum well are obtained from a numerical solution of the three-dimensional Schrodinger equation for the envelope of the exciton wave function. This equation is based on the exciton effective energy operator with a spherical approximation of the Luttinger Hamiltonian. The calculated states are classified based on the types of one-dimensional functions for the factorized form of the wave function. The upper limit for the energies of the exciton states in a quantum well is confirmed by the complex scaling method.