Exciton Band Research Articles

Detection of dielectric screening effect by excitons in two-dimensional semiconductors and its application

Atomically thin transition metal dichalcogenides (TMDCs) like MX<sub>2</sub> (M = W or Mo, X = S or Se) are well-known examples of two-dimensional (2D) semiconductors. They have attracted wide and long-lasting attention due to the strong light-matter interaction and unique spin-valley locking characteristics. In the 2D limit, the reduced dielectric screening significantly enhances the Coulomb interaction. The optical properties of monolayer TMDCs are thus dominated by excitons, the tightly bound electron-hole pairs. In this work, we briefly overview the history and recent research progress of optical spectroscopy studies on TMDCs. We first introduce the layer-dependent band structure and the corresponding modifications on optical transitions, and briefly mention the effects of external magnetic fields and the charge doping on excitons. We then introduce a novel sensing technique enabled by the sensitivity of excitons to the dielectric environment. The exciton excited states (Rydberg states) observed in monolayer TMDCs have large Bohr radii (> few nm), where the electric field lines between electron-hole pairs well extends out of the material. Hence the Coulomb interaction (which affects the quasiparticle band gap and exciton binding energies) in the monolayer TMDC is sensitive to the dielectric environment, making the excitons in 2D semiconductor an efficient quantum sensor in probing dielectric properties of the surroundings. The method is of high spatial resolution and only diffraction limited. We enumerate the applications of monolayer WSe<sub>2</sub> dielectric sensor in detecting the secondary Dirac point of graphene induced by the graphene-hBN superlattice potential, as well as the fractional correlated insulating states emerging in WS<sub>2</sub>/WSe<sub>2</sub> moiré superlattices. Meanwhile, a unified framework for describing the many-body interactions and dynamical screenings in the system is still lacking. Future theoretical and experimental efforts are needed for a complete understanding. Finally, we further discuss the perspectives and potential applications of this non-destructive and efficient dielectric sensing method.

Efficient Photoluminescence of Manganese-Doped Two-Dimensional Chiral Alloyed Perovskites.

In this work, we introduced chiral cations into the achiral two-position layered perovskite system for the first time to form an alloyed system that still retains a clear layered structure. In addition, in order to explore the potential photoelectric properties of the alloyed system, manganese ions were doped into the alloyed system. The XRD pattern shows that the steady-state absorption and emission spectra of the alloyed system have a large structural distance, while the doped manganese system exhibits a two-color photoluminescence phenomenon. In addition, combined with time-resolved fluorescence and testing, the photoluminescence characteristics and ultralong lifetime of Mn-doped samples were further characterized. The exciton band structure of the lead halide perovskite framework can be adjusted through this design strategy. Mn2+ ions can form characteristic energy levels in the host system and then energy transfer of excitons occurs, which is of great significance for the development of new functional and high-efficiency photoluminescent materials.

Photonic bound states in the continuum in nanostructured transition metal dichalcogenides for strong photon-exciton coupling.

Optical cavities supporting the optical bound states in the continuum (BICs) with infinite radiative lifetime have recently been widely reported. Here, we theoretically investigate BICs in an ultrathin photonic structure consisting of a multilayer, nanostructured transition metal dichalcogenide (TMD). Using finite element simulations, we demonstrate two distinct groups of BICs, namely symmetry-protected BICs and interference-based BICs, in the photonic system. Under normal incidence, their dispersion can be mediated by altering the grating pitch, which makes it possible to explore the strong coupling of these two photonic modes with the TMD exciton band in the same structure. This work expands not only the library of traditional nanophotonic approaches, but also provides more possibilities for optoelectronic devices toward miniaturization.

Evaluating the Electronic Structure of Coexisting Excitonic and Multiexcitonic States in Periodic Systems: Significance for Singlet Fission.

Singlet fission (SF) in organic molecular solids is an example of a process that is challenging to describe with the most common electronic structure approaches. It involves optically bright singlet excited states delocalized over many molecules, which could be efficiently treated by density functional theory, and multiexcitonic localized states that have to be studied with wavefunction methods, usually with small clusters considering their expensive computational costs. In this work, we propose a methodology to combine multiconfigurational wavefunction calculations with reduced Hamiltonian to investigate the electronic structure of large clusters or fully periodic systems. The method is applied to the prototypical SF materials tetracene and pentacene. The results allow one to study how states of different natures (excitonic, charge-transfer, and multiexcitonic) coexist and are contaminated by their couplings in large or periodic systems. Novel insights are therefore possible. For example, because the excitonic bands are relatively broad with respect to the multiexcitonic states, there are limited regions of the crystal momentum space where the transition between the two is more likely.

Quasiparticle band structure, exciton, and optical properties of few-layer blue phosphorus

The evolution of quasiparticle electronic structure, exciton, and optical properties in the few-layer blue phosphorus has been studied with the many-body perturbation method. The quasiparticle band gap decreases gradually with increasing the layer number and the system will finally evolve to metallic state in the bulk form. In spite of the increasing thickness in these indirect-band-gap semiconductors, the exciton-dominated optical properties with large binding energies are still found in the few-layer systems. With increasing layer number, the overall dielectric screening gets enhanced. However, the persistent large joint density of states associated with the parallel band structure in blue phosphorus gives rise to a much larger binding energy than the value predicted in an effective screening model. Our studies have not only provided a deep understanding of excitons in 2D semiconductors but also provided solid data for further experimental exploration of the application of few-layer blue phosphorus in optoelectronics.

Influence of RE (Pr3+, Er3+, Nd3+) doping on structural, vibrational and enhanced persistent photocatalytic properties of ZnO nanostructures

Rare earth (RE- Pr, Er and Nd) doped ZnO nanostructures were prepared through simple wet chemical precipitation route. The RE doping induced interesting morphological transition from spherical to flower like structures were analyzed. The X-ray diffraction (XRD) measurements revealed that the prepared materials were of highly crystalline in nature and RE dopant ions did not altered the crystal structure of ZnO. The microstrain of ZnO was altered with respect to the nature of dopants. In the case of the Pr doped ZnO, X-ray photoelectron spectroscopy (XPS) analysis confirmed that the dopant (Pr) ions successfully substituted in the ZnO lattice. Raman spectra revealed RE doping induced lower energy side shift and variation in intensity of the peaks related to the characteristic phonon modes of ZnO. In the case of Nd doped ZnO nanostructures, dopant induced suppression in classical Raman modes and evolution of multiphonon related modes were identified. Optical diffuse reflectance spectral (DRS) measurements, along with the characteristic excitonic band of ZnO, other bands associated to the transitions of 4f energy levels related to the RE ions were observed. The partially filled 4f orbitals led to the enhanced photocatalytic activity in RE doped ZnO nanostructures. The observed enhanced photocatalytic activity in RE doped ZnO when compared to bare ZnO was discussed. The decolorization efficiency of MB ensued the following order 96 > 94 > 86 > 78% for ZnErO, ZnNdO, ZnPrO and ZnO, respectively.

Modulated photoluminescence and photodynamic efficiency of hydroxyapatite-methylene blue@carbon-ions by ion-π coupling interactions

As a noninvasive methodology, image-guided photodynamic therapy is promising to treat diseases without inducing drug resistance with chemotherpy in clinical applications. The photo-related properties of methylene blue (MB) as flurophore/photosensitizer are closely related with energy and charge transformations in multifunctional drug delivery systems. Herein, MB monomers are combined by citrate- and β-cyclodexin-modified hydroxyapatite (HAp) with ions-doped carbon shells prepared by hydrothermal carbonization of glucose. Photoluminescence and photodynamic efficiency are tuned by ion-π coupling interactions between ions (M) and MB molecules in HAp-MB@C-M. With increased addition of Zn2+ ions during carbonization, narrowed absorbance peaks to low excitonic band without additional decay channels are observed for HAp-MB@C-Zn with increased emission intensities. As energy decay pathways, ring vibrations of MB are quite weak with small infrared peaks for these fluorochromes with increased luminescence emissions. High oscillate strength contributes to the increased generation efficiency of singlet oxygen for MB released from HAp-MB@C-Zn. The present studies would shed light on the modulations of aggregation states of photosensitizers and their photo-related properties of luminescence emission and therapy efficiency to meet the simultaneous requirements of image-guided photodynamic therapy.

Density-matrix renormalization group study of optical conductivity of the Mott insulator for two-dimensional clusters

The real part of optical conductivity $\text{Re}\sigma(\omega)$ of the Mott insulators has a large amount of information on how spin and charge degrees of freedom interact with each other. By using the time-dependent density-matrix renormalization group, we study $\text{Re}\sigma(\omega)$ of the two-dimensional Hubbard model on a square lattice at half filling. We find an excitonic peak at the Mott-gap edge of $\text{Re}\sigma(\omega)$ not only for the two-dimensional square lattice but also for two- and four-leg ladders. For the square lattice, however, we do not clearly find a gap between an excitonic peak and continuum band, which indicates that a bound state is not well defined. The emergence of an excitonic peak in $\text{Re}\sigma(\omega)$ implies the formation of a spin polaron. Examining the dependence of $\text{Re}\sigma(\omega)$ on the on-site Coulomb interaction and next-nearest neighbor hoppings, we confirm that an excitonic peak is generated from a magnetic effect. Electron scattering due to an electron-phonon interaction is expected to easily suppress an excitonic peak since spectral width of an excitonic peak is very narrow. Introducing a large broadening in $\text{Re}\sigma(\omega)$ by modeling the electron-phonon coupling present in La$_{2}$CuO$_{4}$ and Nd$_{2}$CuO$_{4}$, we obtain $\text{Re}\sigma(\omega)$ comparable with experiments.

Dielectric Confinement and Exciton Fine Structure in Lead Halide Perovskite Nanoplatelets.

Owing to their flexible chemical synthesis and the ability to shape nanostructures, lead halide perovskites have emerged as high potential materials for optoelectronic devices. Here, we investigate the excitonic band edge states and their energies levels in colloidal inorganic lead halide nanoplatelets, particularly the influence of dielectric effects, in a thin quasi-2D system. We use a model including band offset and dielectric confinements in the presence of Coulomb interaction. Short- and long-range contributions, modified by dielectric effects, are also derived, leading to a full modelization of the exciton fine structure, in cubic, tetragonal and orthorhombic phases. The fine splitting structure, including dark and bright excitonic states, is discussed and compared to recent experimental results, showing the importance of both confinement and dielectric contributions.

Dependence of the nonlinear optical response of CdSe nanoscrolls on coating with oleic or acetic acid

The photoluminescence and nonlinear transmission of atomically thin population of colloidal CdSe rolled-up nanosheets were investigated at the one-photon stationary excitations of excitons by the third harmonic of Nd3+:YAP laser pulses. It was revealed that the nonlinear optical response of the nanoscrolls is strongly influenced by multiwalled rolled-up structure. The broadening of exciton band and the red shift of the photoluminescence were observed and explained by coupling effect within the folded scroll-layers.

High Anisotropic Optoelectronics in Two Dimensional Layered PbSnX2 (X = S/Se).

We systematically study the giant anisotropic optoelectronics in layered PbSnX2 (X = S/Se). The highly anisotropic optoelectronics mainly originates from the asymmetric sublattices SnX, resulting in the anisotropy of photoelectronic properties with fascinating visible light absorption range in single-layer and bilayer PbSnX2. We employ uniaxial strain in both the x and y directions and find an indirect-to-direct band gap transition, while the quasiparticle indirect band gap presents excellent linear scaling with biaxial strain in monolayer PbSnX2. We also demonstrate ultrahigh anisotropic mobilities of electrons (μy > μx) and holes (μx > μy) in both single-layer and bilayer PbSnX2 (X = S/Se), and spin-orbit coupling effects and the increase of layer number significantly reduce exciton binding energies and band gaps. Finally, the strong layer dependence of the band structure is clearly seen when the film thickness is less than 4 layers. Our results provide a fundamental understanding of highly anisotropic PbSnX2 (X = S/Se) and show two potential candidates in photoelectric applications.

Methylhydrazinium lead iodide – one dimensional chain phase with excitonic absorption and large energy band gap

In this work the crystal structure of the new hybrid organic-inorganic compound has been studied using single-crystal X-ray diffraction (SCXRD). Methylhydrazinium lead iodide (MHyPbI3) crystallizes in monoclinic, centrosymmetric P21/c space group with a = 4.5067(4) Å, b = 19.1892(16) Å, c = 12.0029(12) Å and β = 98.178(8)°. The structure is built of edge-sharing [PbI6]4− octahedra forming parallel double chains along the [100] direction. MHy+ cations are placed between the chains. The motif is stabilized via two kinds of NH···I hydrogen bonds. RT absorption studies reveal the excitonic absorption band at 413 nm (3.00 eV). The estimated energy band gap value Eg is 3.11 eV (400 nm). At 367 K MHyPbI3 transforms to the new HT polymorph of P21/m symmetry with a = 8.895(1) Å, b = 18.185(1) Å and c = 13.197(1) Å and β = 99.10(1)°, that corresponds to the 2a monoclinic superstructure of the RT phase. The HT polymorph is metastable and returns to the pristine RT phase after ca. 50 min at room temperature.

ZnO core-triggered nitrogen-deficient carbonaceous g-C3N4 shell enhances the visible-light-driven disinfection

• A solid-state synthesis of ZnO@g-C 3 N 4 core-shell nanocomposites is reported. • A ZnO-triggered nitrogen-deficient shell is formed up to 20 wt.% g-C 3 N 4 loading. • This shell imparts stability and enhanced photocatalytic efficacy to the core. • The 5 wt.% g-C 3 N 4 loading exhibits high valence band intensity and exciton lifetime. • This optimal catalyst displays high disinfection performance under visible light. We report a solid-state synthesis of ZnO@g-C 3 N 4 core-shell nanocomposites, in which melamine—a precursor of graphitic carbon nitride (g-C 3 N 4 )—is thermally decomposed over a pre-formed ZnO core. The thermolysis has led to the formation of a nitrogen-deficient carbonaceous shell over the ZnO core—reported for the first time—when the g-C 3 N 4 loading has been sequentially increased up to 20 wt.%. With a further increase in loading to 50 and 80 wt.%, the signature of the g-C 3 N 4 has been observed in the shell. The quantitative studies using TGA have revealed the excessive decomposition of melamine over the surface of ZnO, whilst the nitrogen-deficiency of the carbonaceous shell has been unearthed by the XPS studies. The core-shell nanocomposite with 5 wt.% g-C 3 N 4 loading has been found to possess enhanced photocatalytic performance towards methylene blue degradation and visible-light-driven water disinfection. The enhanced photocatalytic activity has been attributed to the thin carbonaceous shell imparting high valence band intensity and exciton lifetime to the ZnO core. The nanocomposites have been found to generate an excess of hydroxyl radicals in comparison to g-C 3 N 4 or ZnO. Furthermore, the toxicity studies performed with the fibroblast feeder cells have revealed the biocompatibility of the materials for practical applications.

Atomistic tight-binding calculations of CdSe/CdS core/shell dot-in-hexagonal platelet nanocrystals with interesting electronic structures and optical properties

I theoretically determine the electronic and optical signatures of CdSe/CdS core/shell dot-in-hexagonal platelet nanocrystals with experimentally synthesized dimensions exploiting the atomistic tight-binding theory. With the increasing lateral sizes and thicknesses of the CdS hexagonal platelets, the reduction of excitonic band gaps is ascribed to the quantum confinement consequence. The excitonic band gaps emitting the visible spectra are carried out by CdS shell sizes. The optical characteristic of CdSe/CdS dot-in-hexagonal platelet nanocrystals is advanced when comparing with bare CdSe nanocrystal. The reduction of oscillator strengths, excitonic binding energies and stokes shift with the increasing sizes of CdSe shell is ascribed to the electron-hole wave function overlaps. The key findings from the tight-binding analysis agree well with the experimental observation. Finally, this theoretical study delivers the essential direction to predesign of core/shell dot-in-hexagonal platelet nanocrystals with specific characteristics for various optoelectronic technologies.

Finite-momentum excitons in rubrene single crystals

Excitons with nonzero momenta and their energy dependence are important for time dependent phenomena such as transport properties or the coupling to external or internal fields, for example in electron energy loss spectroscopy. In this paper we calculate the momentum dependent energy landscape of excitons in rubrene single crystals. We show that singlet excitons exhibit a dispersion that is qualitatively similar to the electronic valence and conduction bands, namely a relatively large bandwidth along $\mathrm{\ensuremath{\Gamma}}$--Y and much flatter bands along $\mathrm{\ensuremath{\Gamma}}$--X and $\mathrm{\ensuremath{\Gamma}}$--Z. However, the absolute value of the bandwidths is significantly weaker than for both electronic bands. Triplet excitons, on the other hand, show much less dispersion and the exciton bands are much flatter than their singlet counterparts.

Excitons in CsPbBr3 Halide Perovskite.

Excitons in Bridgman grown halide perovskite CsPbBr3 single crystals were examined using photoluminescence (PL) spectroscopy to determine the nature of the electronic states. The photoluminescence intensity was strongly temperature-dependent and depended upon the specific exciton band. At low temperatures intrinsic disorder and its related shallow below bandgap tail states determine the emission properties. Photoluminescence at low temperature revealed the presence of several strong bands at the band edge that is attributed to free or trapped/bound excitons. This PL emission results from strong electron-phonon coupling with an average phonon energy Eph of 6.5 and 27.4 meV for the emissions, comparable to that observed in other perovskites. The Huang-Rhys parameter S was calculated to be 3.81 and 1.51, indicating strong electron-phonon coupling. The interactions between electrons and phonons produce small polarons that tend to bind charge carriers and result in trapped/bound excitons. The transient photoluminescence response of each specific band was studied, and the results indicated a multiphonon recombination process. Average PL lifetimes of ∼17 ns for free excitons and ∼38 ns for trapped/bound excitons were determined. The observed edge states could be associated with native defects such as vacancies and interstitials, as well as twinning due to the cubic-to-tetragonal phase transition in CsPbBr3. Elimination of the trapping sites for binding excitons could lead to improved charge transport mobilities, carrier lifetimes, and detector properties in this system.

Signatures of Dimensionality and Symmetry in Exciton Band Structure: Consequences for Exciton Dynamics and Transport

Exciton dynamics, lifetimes, and scattering are directly related to the exciton dispersion or band structure. Here, we present a general theory for exciton band structure within both ab initio and model Hamiltonian approaches. We show that contrary to common assumption, the exciton band structure contains nonanalytical discontinuities—a feature which is impossible to obtain from the electronic band structure alone. These discontinuities are purely quantum phenomena, arising from the exchange scattering of electron–hole pairs. We show that the degree of these discontinuities depends on materials’ symmetry and dimensionality, with jump discontinuities occurring in 3D and different orders of removable discontinuities in 2D and 1D, whose details depend on the exciton degeneracy and material thickness. We connect these features to the early stages of exciton dynamics, radiative lifetimes, and diffusion constants, in good correspondence with recent experimental observations, revealing that the discontinuities in the band structure lead to ultrafast ballistic transport and suggesting that measured exciton diffusion and dynamics are influenced by the underlying exciton dispersion.

Zitterbewegung of Moiré Excitons in Twisted MoS_{2}/WSe_{2} Heterobilayers.

The moiré pattern observed in stacked noncommensurate crystal lattices, such as heterobilayers of transition metal dichalcogenides, produces a periodic modulation of their band gap. Excitons subjected to this potential landscape exhibit a band structure that gives rise to a quasiparticle dubbed the moiré exciton. In the case of MoS_{2}/WSe_{2} heterobilayers, the moiré trapping potential has honeycomb symmetry and, consequently, the moiré exciton band structure is the same as that of a Dirac-Weyl fermion, whose mass can be further tuned down to zero with a perpendicularly applied field. Here we show that, analogously to other Dirac-like particles, the moiré exciton exhibits a trembling motion, also known as Zitterbewegung, whose long timescales are compatible with current experimental techniques for exciton dynamics. This promotes the study of the dynamics of moiré excitons in van der Waals heterostructures as an advantageous solid-state platform to probe Zitterbewegung, broadly tunable by gating and interlayer twist angle.

Coupling between Emissive Defects on Carbon Nanotubes: Modeling Insights.

Covalent functionalization of single-walled carbon nanotubes (SWCNTs) with organic molecules results in red-shifted emissive states associated with sp3-defects in the tube lattice, which facilitate their improved optical functionality, including single-photon emission. The energy of the defect-based electronic excitations (excitons) depends on the molecular adducts, the configuration of the defect, and concentration of defects. Here we model the interactions between two sp3-defects placed at various distances in the (6,5) SWCNT using time-dependent density functional theory. Calculations reveal that these interactions conform to the effective model of J-aggregates for well-spaced defects (>2 nm), leading to a red-shifted and optically allowed (bright) lowest energy exciton. H-aggregate behavior is not observed for any defect orientations, which is beneficial for emission. The splitting between the lowest energy bright and optically forbidden (dark) excitons and the pristine excitonic band are controlled by the single-defect configurations and their axial separation. These findings enable a synthetic design strategy for SWCNTs with tunable near-infrared emission.

Trion Binding Energy Variation on Photoluminescence Excitation Energy and Power during Direct to Indirect Bandgap Crossover in Monolayer and Few-Layer MoS2

The behavior of the trion and exciton photoluminescence (PL) under various laser excitation conditions, that is, energy and power, is studied in monolayer and few-layer (FL) chemical vapor deposition (CVD)-MoS2 flakes grown on SiO2/Si. The room-temperature μ-PL spectroscopy of the flakes shows that the A exciton at 1.86 eV is overlapped by the trion component. First, the thickness-dependent characterization of the flakes has been carried out. The trion spectral weight and dissociation energy are observed to increase with the number of layers, which is correlated with an increase in nonequilibrium electron density. We propose the presence of many-body effects explaining this unusual dependence in CVD-MoS2 on n-type SiO2/Si. In the power-dependent μ-PL measurements, the deconvolution of spectra for the exciton and trion bands shows a faster intensification of the trion component compared to the A exciton with increasing laser power. The trion binding (dissociation) energy varied from 28 to 33 meV in the monolayer and from 32 to 46 meV in FL MoS2, when increasing the power of excitation light. The phenomenon is found to be more prominent when increasing the energy of excitation from 2.33 to 3.06 eV. The enhancement of the trion binding energy leads to a strong intensification of the trion component. Thereby, the intense A exciton/trion PL band redshifts and becomes more asymmetric when increasing the power of excitation at both energies of 2.33 or 3.06 eV. Simultaneously, the B exciton contribution to the spectrum also increases. Such an enhanced formation of the trions and B excitons is explained by a rate of photogenerated electron–hole pairs, leading to a higher population of nonequilibrium electrons. In the measurements under a higher excitation energy of 3.06 eV, the PL redshift is more prominent because the trion component is more intense compared to the A band. This is explained by considering a higher absorption of MoS2 at higher energies.