Strong Excitonic Effects Research Articles

Prediction of a two-dimensional S3N2 solid for optoelectronic applications

Two-dimensional materials have attracted tremendous attention for their fascinating electronic, optical, chemical, and mechanical properties. However, the band gaps of most reported two-dimensional (2D) materials are smaller than 2.0 eV, which has greatly restricted their optoelectronic applications in the blue and ultraviolet range of the spectrum. Here, we propose a stable trisulfur dinitride (${\mathrm{S}}_{3}{\mathrm{N}}_{2}$) 2D crystal that is a covalent network composed solely of S-N $\ensuremath{\sigma}$ bonds. The ${\mathrm{S}}_{3}{\mathrm{N}}_{2}$ crystal is dynamically, thermally, and chemically stable, as confirmed by the computed phonon spectrum and ab initio molecular dynamics simulations. GW calculations show that the ${\mathrm{S}}_{3}{\mathrm{N}}_{2}$ crystal is a wide, direct band-gap (3.92 eV) semiconductor with a small-hole effective mass. In addition, the band gap of ${\mathrm{S}}_{3}{\mathrm{N}}_{2}$ structures can be tuned by forming multilayer ${\mathrm{S}}_{3}{\mathrm{N}}_{2}$ crystals, ${\mathrm{S}}_{3}{\mathrm{N}}_{2}$ nanoribbons, and ${\mathrm{S}}_{3}{\mathrm{N}}_{2}$ nanotubes, expanding its potential applications. The anisotropic optical response of the 2D ${\mathrm{S}}_{3}{\mathrm{N}}_{2}$ crystal is revealed by GW--Bethe-Salpeter-equation calculations. The optical band gap of ${\mathrm{S}}_{3}{\mathrm{N}}_{2}$ is 2.73 eV and the exciton binding energy of ${\mathrm{S}}_{3}{\mathrm{N}}_{2}$ is 1.19 eV, showing a strong excitonic effect. Our result not only marks the prediction of a 2D crystal composed of nitrogen and sulfur, but also underpins potential innovations in 2D electronics and optoelectronics.

Optoelectronic properties in 2D GeC and SiC hybrids: DFT and many body effect calculations

This work investigates optoelectronic properties of SiC- and GeC-hybrids that are still unexplored. Using DFT and many body effect calculations, we study, among others, charge density, dielectric functions, reflectivity and electron energy loss spectroscopy. The charge transfer determines the bonds nature between the atoms constituting the hybrids. A comparative study of gap and binding energy using different approximations is presented. The dielectric functions reveal strong excitonic effects. Two peaks related to p and p+s electron plasmons are observed in the spectra of loss energy. Moreover, the analysis of reflectivity shows that hybrids are transparent in the visible region. Other interesting findings are also detailed.

Density of states in the bilayer graphene with the excitonic pairing interaction

In the present paper, we consider the excitonic effects on the single particle normal density of states (DOS) in the bilayer graphene (BLG). The local interlayer Coulomb interaction is considered between the particles on the non-equivalent sublattice sites in different layers of the BLG. We show the presence of the excitonic shift of the neutrality point, even for the noninteracting layers. Furthermore, for the interacting layers, a very large asymmetry in the DOS structure is shown between the particle and hole channels. At the large values of the interlayer hopping amplitude, a large number of DOS at the Dirac’s point indicates the existence of the strong excitonic coherence effects between the layers in the BLG and the enhancement of the excitonic condensation. We have found different competing orders in the interacting BLG. Particularly, a phase transition from the hybridized excitonic insulator phase to the coherent condensate state is shown at the small values of the local interlayer Coulomb interaction.

Excitons in atomically thin 2D semiconductors and their applications

AbstractThe research on emerging layered two-dimensional (2D) semiconductors, such as molybdenum disulfide (MoS2), reveals unique optical properties generating significant interest. Experimentally, these materials were observed to host extremely strong light-matter interactions as a result of the enhanced excitonic effect in two dimensions. Thus, understanding and manipulating the excitons are crucial to unlocking the potential of 2D materials for future photonic and optoelectronic devices. In this review, we unravel the physical origin of the strong excitonic effect and unique optical selection rules in 2D semiconductors. In addition, control of these excitons by optical, electrical, as well as mechanical means is examined. Finally, the resultant devices such as excitonic light emitting diodes, lasers, optical modulators, and coupling in an optical cavity are overviewed, demonstrating how excitons can shape future 2D optoelectronics.

Salt assisted sonochemical exfoliation and synthesis of highly stable few-to-monolayer WS2 quantum dots with tunable optical properties

As a 2D material, few to monolayer WS2 have seen much research interest due to its strong excitonic effects, which will affect the optical and optoelectronic properties. In this study, we report synthesis of highly stable few-to-monolayer quantum dots of tungsten disulfide (WS2) and its analysis. A facile method in which salts such as sodium hydroxid, sodium chloride, ammonium hydroxide and lithium hydroxide assisted sonochemical exfoliation of bulk WS2 powder has been employed in N-Methyl-2-pyrrolidone for 2 h. Resultant WS2 quantum dot nanosheets were characterized by the XRD, DLS, TEM, HRTEM, Raman and UV–Visible Spectroscopy to amass structural, morphological and optical properties. Salts promoted the exfoliation processes and the prepared few-to-monolayer WS2 quantum dot have a lateral size of 3 ± 0.5 nm and better yield in comparison to WS2 quantum dots in N-Methyl-2-pyrrolidone. Further the Raman and UV–Vis. analysis reiterate the successful exfoliation mechanism which consumes less time and the WS2 quantum dots show blue shift with an excitonic band gap of 2.7 eV.

Frequency-dependent dielectric function of semiconductors with application to physisorption

The dielectric function is one of the most important quantities that describes the electrical and optical properties of solids. Accurate modeling of the frequency-dependent dielectric function has great significance in the study of the long-range van der Waals (vdW) interaction for solids and adsorption. In this work, we calculate the frequency-dependent dielectric functions of semiconductors and insulators using the $GW$ method with and without exciton effects, as well as efficient semilocal density functional theory (DFT), and compare these calculations with a model frequency-dependent dielectric function. We find that for semiconductors with moderate band gaps, the model dielectric functions, $GW$ values, and DFT calculations all agree well with each other. However, for insulators with strong exciton effects, the model dielectric functions have a better agreement with accurate $GW$ values than the DFT calculations, particularly in high-frequency region. To understand this, we repeat the DFT calculations with scissors correction, by shifting DFT Kohn-Sham energy gap to match the experimental band gap. We find that scissors correction only moderately improves the DFT dielectric function in low-frequency region. Based on the dielectric functions calculated with different methods, we make a comparative study by applying these dielectric functions to calculate the vdW coefficients ($C_3$ and $C_5$) for adsorption of rare-gas atoms on a variety of surfaces. We find that the vdW coefficients obtained with the nearly-free electron gas-based model dielectric function agree quite well with those obtained from the $GW$ dielectric function, in particular for adsorption on semiconductors, leading to an overall error of less than 7% for $C_3$ and 5% for $C_5$. This demonstrates the reliability of the model dielectric function for the study of physisorption.

Structure Dependence of Excitonic Effects in Chiral Graphene Nanoribbons**Supported by the National Key Scientific Research Projects of China under Grant No 2015CB932400, the National Natural Science Foundation of China under Grant Nos 11504158, 61474059, and U1432129, the Program for New Century Excellent Talents in University of Ministry of Education of China under Grant No NCET-11-1003, and the Jiangxi Provincial ‘Ganpo

We explore the excitonic effects in chiral graphene nanoribbons (cGNRs), whose edges are composed alternatively of armchair-edged and zigzag-edged segments. For cGNRs dominated by armchair edges, their energy gaps and exciton energies decrease with increasing chirality angles, and they, as functions of widths, oscillate with the period of three, while the exciton binding energies do not have such distinct oscillation. On the other hand, for cGNRs dominated by zigzag edges, all the energy gaps, exciton energies, and exciton binding energies show oscillation properties with their widths, due to the interactions between the edge states localized at the opposite zigzag edges. In addition, the triplet excitons are energy degenerate when the electrons are spin-unpolarized, while the degeneracy split when the electrons are spin-polarized. All the studied cGNRs show strong excitonic effects with the exciton binding energies of hundreds of meV.

Two‐Dimensional Single‐Layer Organic–Inorganic Hybrid Perovskite Semiconductors

The electronic properties of single-layer hybrid perovskite BA2MI4 (M = Ge, Sn, and Pb) are theoretically investigated. The single-layer perovskites not only show tunable bandgaps, ranging from ≈1.5 to ≈2.0 eV, but also exhibit strong exciton effect. The desired properties of these atomically thin perovskites will motivate future experimental studies of 2D perovskites for improved functionality.

Layer-dependent properties ofSnS2andSnSe2two-dimensional materials

The layer dependent structural, electronic and vibrational properties of SnS2 and SnSe2 are investigated using first-principles density functional theory (DFT). The in-plane lattice constants, interlayer distances and binding energies are found to be layer-independent. Bulk SnS2 and SnSe2 are both indirect band gap semiconductors with Eg = 2.18 eV and 1.07 eV, respectively. Few-layer and monolayer 2D systems also possess an indirect band gap, which is increased to 2.41 eV and 1.69 eV for single layers of SnS2 and SnSe2. The effective mass theory of 2D excitons, which takes into account the combined effect of the anisotropy, non-local 2D screening and layer-dependent 3D screening, predicts strong excitonic effects. The binding energy of indirect excitons in monolayer samples, Ex~0.9 eV, is substantially reduced to Ex = 0.14 eV in bulk SnS2 and Ex = 0.09 eV in bulk SnSe2. The layer-dependent Raman spectra display a strong decrease of intensities of the Raman active A1g mode upon decreasing the number of layers down to a monolayer, by a factor of 7 in the case of SnS2 and a factor of 20 in the case of SnSe2 which can be used to identify number of layers in a 2D sample.

A many-body GW + BSE investigation of electronic and optical properties of C2N

A recently synthesized layered material C2N was investigated based on many-body perturbation theory using the GW plus Bethe-Salpeter equation approach. The electronic band gap was determined to be ranging from 3.75 to 1.89 eV from the monolayer to the bulk. Significant GW quasiparticle corrections, of more than 0.9 eV, to the Kohn-Sham band gaps from the local density approximation calculations are found. Strong excitonic effects play a crucial role in optical properties. We found large binding energies of greater than 0.6 eV for bound excitons in few-layer C2N, while it is only 0.04 eV in bulk C2N. All the structures exhibit strong and broad optical absorption in the visible light region, which makes C2N a promising candidate for solar energy conversion, such as photocatalytic water splitting.

On the Study of Exciton Binding Energy with Direct Charge Generation in Photovoltaic Polymers

Overcoming the strong excitonic effect in organic photovoltaics (OPVs) is one of the key strategies for the design of materials and device structures. Approaches to measure the binding energy in organics are, however, not readily available. Here, the electrical and optical quantum efficiencies of a series of conjugated photovoltaic polymers are systematically studied. It is found that there is generally a secondary onset in external quantum efficiency (EQE) spectrum at high excitation energy, independent of optical absorbance of the materials. Combining photoluminescence quantum yield measurement and adding a small amount (1 wt%) of fullerene acceptor in polymer blends, it further confirms that the pristine polymer EQE spectrum has a transition from exciton to free‐charge generation with increased excitation energy. Strikingly, by comparing the reported photoemission spectroscopy results, the secondary onsets in the EQE spectra have excellent agreement with the transport gap (Eg) in corresponding materials. Consequently, the exciton binding energies of the polymers can be obtained with the smallest 0.6 eV in PCE10 to the largest 1.2 eV in MEHPPV. The results of this study demonstrate a facile access to the transport gap and exciton binding energies in organic semiconductors. This provides valuable information for further understanding the strong excitonic effect in photovoltaic cells.

Excitons in boron nitride single layer

Boron nitride single layer belongs to the family of 2D materials whose optical properties are currently receiving considerable attention. Strong excitonic effects have already been observed in the bulk and still stronger effects are predicted for single layers. We present here a detailed study of these properties by combining \textit{ab initio} calculations and a tight-binding-Wannier analysis in both real and reciprocal space. Due to the simplicity of the band structure with single valence ($\pi$) and conduction ($\pi^*$) bands the tight-binding analysis becomes quasi quantitative with only two adjustable parameters and provides tools for a detailed analysis of the exciton properties. Strong deviations from the usual hydrogenic model are evidenced. The ground state exciton is not a genuine Frenkel exciton, but a very localized "tightly-bound" one. The other ones are similar to those found in transition metal dichalcogenides and, although more localized, can be described within a Wannier-Mott scheme.

Photonics and optoelectronics of 2D semiconductor transition metal dichalcogenides

The electronic and optical properties and the recent progress in applications of 2D semiconductor transition metal dichalcogenides with emphasis on strong excitonic effects, and spin- and valley-dependent properties are reviewed.

Strong excitonic effects in hydrogen-graphene-fluorine janus graphene

Abstractauthoren We present a first-principles many-body Green's function method (GW approximation and Bethe–Salpeter equation) of the electronic and optical properties of recently predicted hydrogen–graphene–fluorine janus graphene. Significant self-energy corrections, of more than 50%, to the Kohn–Sham bandgap from the local density approximation (LDA) calculations are found. Moreover, the optical absorption spectrum of this janus graphene is dominated by enhanced excitonic effects with formation of a bound exciton with considerable binding energy. The reduced spatial separation of excited electrons and holes gives rise to extremely short radiative lifetimes, preventing condensation.

Nonlinear optical selection rule based on valley-exciton locking in monolayer ws2

Optical selection rule fundamentally determines the optical transition between energy states in a variety of physical systems from hydrogen atoms to bulk crystals such as GaAs. It is important for optoelectronic applications such as lasers, energy-dispersive X-ray spectroscopy and quantum computation. Recently, single layer transition metal dichalcogenide (TMDC) exhibits valleys in momentum space with nontrivial Berry curvature and excitons with large binding energy. However, it is unclear how the unique valley degree of freedom combined with the strong excitonic effect influences the optical excitation. Here we discover a new set of optical selection rules in monolayer WS2,imposed by valley and exciton angular momentum. We experimentally demonstrated such a principle for second harmonic generation (SHG) and two-photon luminescence (TPL). Moreover, the two-photon induced valley populations yield net circular polarized photoluminescence after a sub-ps interexciton relaxation (2p->1s) and last for 8 ps. The discovery of this new optical selection rule in valleytronic 2D system not only largely extend information degrees but sets a foundation in control of optical transitions that is crucial to valley optoeletronic device applications such as 2D valley-polarized light emitting diodes (LED), optical switches and coherent control for quantum computing.

Exciton energy-momentum map of hexagonal boron nitride

Understanding and controlling the way excitons propagate in solids is a key for tailoring materials with improved optoelectronic properties. A fundamental step in this direction is the determination of the exciton energy-momentum dispersion. Here, thanks to the solution of the parameter-free Bethe- Salpeter equation (BSE), we draw and explain the exciton energy-momentum map of hexagonal boron nitride (h-BN) in the first three Brillouin zones. We show that h-BN displays strong excitonic effects not only in the optical spectra at vanishing momentum $\mathbf{q}$, as previously reported, but also at large $\mathbf{q}$. We validate our theoretical predictions by assessing the calculated exciton map by means of an inelastic x-ray scattering (IXS) experiment. Moreover, we solve the discrepancies between previous experimental data and calculations, proving then that the BSE is highly accurate through the whole momentum range. Therefore, these results put forward the combination BSE and IXS as the tool of choice for addressing the exciton dynamics in complex materials.

Thickness-Dependent Electronic and Optical Properties of Bernal-Stacked Few-Layer Germanane

Few-layer germanane has been successfully fabricated experimentally very recently and shown great potential applications in electronic and optoelectronic devices. In this work, we investigate thickness-dependent electronic and optical properties of Bernal-stacked few-layer germanane within the framework of many-body perturbation theory. Few-layer and bulk germanane are all direct band gap semiconductors, and the band gaps are tunable in a broad range. Strong excitonic effects are observed in few-layer germanane, leading to a distinct red shift in optical absorption spectra, and the exciton binding energy in monolayer germanane can be as large as 750 meV, which is 15 times larger than that of bulk germanane. More importantly, the quasi-particle band gaps, optical gaps, and exciton binding energies rapidly decrease with the increase of the layer and follow a power law of A + B/Nβ (0 < β < 2) with the stacking layer.

First-Principles Investigation of Strong Excitonic Effects in Oxygen 1s X-ray Absorption Spectra.

We calculated the oxygen 1s X-ray absorption spectra (XAS) of acetone and acetic acid molecules in vacuum by utilizing the first-principles GW+Bethe-Salpeter method with an all-electron mixed basis. The calculated excitation energies show good agreement with the available experimental data without an artificial shift. The remaining error, which is less than 1% or 2-5 eV, is a significant improvement from those of time-dependent (TD) density functional methods (5% error or 27-29 eV for TD-LDA and 2.4-2.8% error or 13-15 eV for TD-B3LYP). Our method reproduces the first and second isolated peaks and broad peaks at higher photon energies, corresponding to Rydberg excitations. We observed a failure of the one-particle picture (or independent particle approximation) from our assignment of the five lowest exciton peaks and significant excitonic or state-hybridization effects inherent in the core electron excitations.

Quasi-particle energies and optical excitations of hydrogenated and fluorinated germanene.

Using density functional theory, the G0W0 method and Bethe-Salpeter equation calculations, we systematically explore the structural, electronic and optical properties of hydrogenated and fluorinated germanene. The hydrogenated/fluorinated germanene tends to form chair and zigzag-line configurations and its electronic and optical properties show close geometry dependence. The chair hydrogenated/fluorinated and zigzag-line fluorinated germanene are direct band-gap semiconductors, while the zigzag-line hydrogenated germanene owns an indirect band-gap. Moreover, the quasi-particle corrections are significant and strong excitonic effects with large exciton binding energies are observed. Moreover, the zigzag-line hydrogenated/fluorinated germanene shows highly anisotropic optical responses, which may be used as a good optical linear polarizer.

Deep‐UV exciton emission from Mg1 − xCaxO—A first‐principle investigation

AbstractIn order to investigate the ultraviolet and electron emission properties of rock‐salt Mg1 − xCaxO, first‐principle calculations are carried out with x ranging from 0 to 0.5. The electron–hole interaction is taken into account by solving the Bethe–Salpeter equation. In pure MgO, the calculated exciton binding‐energy value of 83 ± 3 meV is quite close to the published experimental values. The electronic properties of doped MgO are investigated based on the super‐cell model. Both the optical bandgap and the exciton binding energy of the super cell of Mg1 − xCaxO are calculated for five discrete compositions, with an increasing amount x of CaO. The results show that there are strong excitonic effects in all of these materials. A rapid reduction in bandgap value is observed for Mg1 − xCaxO, when increasing the x value. The exciton binding energy of Mg1 − xCaxO shows a minimum value of about 40 meV around x = 0.2. In all of these cases, the exciton is stable at room temperature. The lowest excitation levels in Mg1 − xCaxO are determined by dark excitons. The occurrence of these dark excitons might explain why Mg1 − xCaxO is a strong source of delayed electron emission, after being bombarded by ions in a plasma.