Strain tunable excitonic optical properties in monolayer Ga_(2)O_(3)

Two-dimensional(2D)Ga_(2)O_(3)has been confirmed to be a stable structure with five atomic layer thickness configuration.In this work,we study the quasi-particle electronic band structures and then access the excitonic optical properties through solving the Bethe-Salpeter equation(BSE).The results reveal that the exciton dominates the optical absorption in the visible light region with the binding energy as large as~1.0 eV,which is highly stable at room temperature.Importantly,both the dominant absorption P_(1)and P_(2)peaks are optically bright without dark exciton between them,and thus is favorable for luminescence process.The calculated radiative lifetime of the lowest-energy exciton is 2.0×10^(-11)s at 0 K.Furthermore,the radiative lifetime under+4%tensile strain is one order of magnitude shorter than that of the strainfree case,while it is less insensitive under the compressive strain.Our findings set the stage for future theoretical and experimental investigation on monolayer Ga_(2)O_(3).

Excitonic optical properties in monolayer SnP_(2)S_(6)

Quantum confinement effect and reduced dielectric screening in two-dimensional(2D)dramatically enhance theelectron-hole interactions.In this work,we use many-body perturbation theory and Bethe-Salpeter equation(BSE)toinvestigate the electronic and excitonic optical properties of monolayer SnP_(2)S_(6).Our findings reveal that the excitoniceffect dominates the optical absorption spectra in the visible light range,and the lowest-energy exciton X0 in monolayerSnP_(2)S_(6)is optically bright with the binding energy of 0.87 eV and the radiative lifetime of~10^(-11)s,which is highly advantageousto the photo-luminescence.Most importantly,the absence of optically forbidden states below the bright statesX0 would give rise to a high quantum efficiency of 2D SnP_(2)S_(6).We also find that applied biaxial strain can further shortenthe radiative lifetime of the bright states.These results imply that 2D SnP_(2)S_(6)is a promising candidate for the optoelectronicdevices.

Pseudohalide induced tunable electronic and excitonic properties in two-dimensional single-layer perovskite for photovoltaics and photoelectronic applications

Two-dimensional(2D) layered organic-inorganic hybrid perovskites have attracted much more attention for some applications than their three-dimensional(3D) perovskite counterparts due to their promising thermal and moisture stabilities.In particular, the 2D perovskite devices have shown better promise for optoelectronic applications.However, tunability of optoelectronic properties is often demanded to improve the device performance.Herein, we adopt a newly method to tune the electronic properties of 2D perovskite by introducing pseudohalide into the structure.In this work, we designed a pseudohalidesubstituted 2D perovskite by substituting the out-of-plane halide with pseudohalide and studied the electronic and excitonic properties of 2D-BA2MX4 and 2D-BA2MX2Ps2(M=Ge^(2+), Sn^(2+), and Pb^(2+);X=I;Ps=NCO, NCS, OCN, SCN, Se CN).We revealed the dependence of electronic properties including band gaps, composition of band edges, bonding characteristics, work functions, effective masses, and exciton binding energies on different pseudohalides substituted in 2D perovskite.Our results indicate that the substitution of pseudohalide in 2D perovskites is energetically favorable and can significantly affect the bonding characteristics as well as the CBM and VBM that often play major role in determining their performance in optoelectronic devices.It is expected that the pseudohalide substitution will be helpful in developing more advanced optoelectronic device based on 2D perovskite by optimizing band alignment and promoting charge extraction.

Impact of Structural Disorder on Excitonic Behaviors and Dynamics in 2D Organic-Inorganic Hybrid Perovskites

Two thin-film 2 D organic-inorganic hybrid perovskites,i.e.,2-phenylethylammonium lead iodide(PEPI)and 4-phenyl-1-butylammonium lead iodide(PBPI)were synthesized and investigated by steady-state absorption,temperature-dependent photoluminescence,and temperature-dependent ultrafast transient absorption spectroscopy.PBPI has a longer organic chain(via introducing extra ethyl groups)than PEPI,thus its inorganic skeleton can be distorted,bringing on structural disorder.The comparative analyses of spectral profiles and temporal dynamics revealed that the greater structural disorder in PBPI results in more defect states serving as trap states to promote exciton dynamics.In addition,the fine-structuring of excitonic resonances was unveiled by temperature-dependent ultrafast spectroscopy,suggesting its correlation with inorganic skeleton rather than organic chain.Moreover,the photoexcited coherent phonons were observed in both PEPI and PBPI,pointing to a subtle impact of structural disorder on the low-frequency Raman-active vibrations of inorganic skeleton.This work provides valuable insights into the optical properties,excitonic behaviors and dynamics,as well as coherent phonon effects in 2 D hybrid perovskites.

Melting of electronic/excitonic crystals in 2D semiconductor moirépatterns:A perspective from the Lindemann criterion

Using the Lindemann criterion,we analyzed the quantum and thermal melting of electronic/excitonic crystals recently discovered in two-dimensional(2D)semiconductor moirépatterns.We show that the finite 2D screening of the atomically thin material can suppress(enhance)the inter-site Coulomb(dipolar)interaction strength,thus inhibits(facilitates)the formation of the electronic(excitonic)crystal.Meanwhile,a strong enough moiréconfinement is found to be essential for realizing the crystal phase with a wavelength near 10 nm or shorter.From the calculated Lindemann ratio which quantifies the fluctuation of the site displacement,we estimate that the crystal will melt into a liquid above a critical temperature ranging from several tens Kelvin to above 100 K(depending on the system parameters).

Dynamic Variation of Excitonic Coupling in Excited Bilayer Graphene Quantum Dots

The intermolecular interaction determines the photophysical properties of the organic aggregates,which are critical to the performance of organic photovoltaics.Here,excitonic coupling,an important intermolecular interaction in organic aggregates,between theπ-stacking graphene quantum dots is studied by using transient absorption spectroscopy.We find that the spectral evolution of the ground state bleach arises from the dynamic variation of the excitonic coupling in the excitedπ-stacks.According to the spectral simulations,we demonstrate that the kinetics of the vibronic peak can be exploited as a probe to measure the dynamics of excitonic coupling in the excitedπ-stacks.

Influence of Polar Pressure Transmission Medium on the Pressure Coefficient of Excitonic Interband Transitions in Monolayer WSe_2

The influence of the pressure transmission medium（PTM）on the excitonic interband transitions in monolayer tungsten diselenide（WSe2）is investigated using photoluminescence（PL）spectra under hydrostatic pressure up to 5GPa.Three kinds of PTMs,condensed argon（Ar）,1：1 n-pentane and isopentane mixture（PM）,and4：1 methanol and ethanol mixture（MEM,a PTM with polarity）,are used.It is found that when either Ar or PM is used as the PTM,the PL peak of exciton related to the direct K-K interband transition shows a pressure-induced blue-shift at a rate of 32±4 or 32±1 meV/GPa,while it turns to be 50±9meV/GPa when MEM is used as the PTM.The indirect A-K interband transition presents almost no shift with increasing pressure up to approximatel.y 5 GPa when Ar and PM are used as the PTM,while it shows a red-shift at the rate of-17±7meV/GPa by using MEM as the PTM.These results reveal that the optical interband transitions of monolayer WSe2 are very sensitive to the polarity of the PTM.The anomalous pressure coefficient obtained using the polar PTM of MEM is ascribed to the existence of hydrogen-like bonds between hydroxyl in MEM and Se atoms under hydrostatic pressure.

Structure Dependence of Excitonic Effects in Chiral Graphene Nanoribbons

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.

Thickness-dependent excitonic properties of atomically thin 2H-MoTe2

Two-dimensional(2D)2H-MoTe2 is a promising semiconductor because of its small bandgap,strong absorption,and low thermal conductivity.In this paper,we systematically study the optical and excitonic properties of atomically thin 2H-MoTe2(1–5 layers).Due to the fact that the optical contrast and Raman spectra of 2H-MoTe2 with different thicknesses exhibit distinctly different behaviors,we establish a quantitative method by using optical images and Raman spectra to directly identify the layers of 2H-MoTe2 thin films.Besides,excitonic states and binding energy in monolayer/bilayer 2H-MoTe2 are measured by temperature-dependent photoluminescence(PL)spectroscopy.At temperature T=3.3 K,we can observe an exciton emission at^1.19 eV and trion emission at^1.16 eV for monolayer 2H-MoTe2.While at room temperature,the exciton emission and trion emission both disappear for their small binding energy.We determine the exciton binding energy to be 185 meV(179 meV),trion binding energy to be 20 meV(18 me V)for the monolayer(bilayer)2H-MoTe2.The thoroughly studies of the excitonic states in atomically thin 2H-MoTe2 will provide guidance for future practical applications.

Non-perturbative multiphoton excitation studies in an excitonic coupled quantum well system using high-intensity THz laser fields

Multiphoton excitations and nonlinear optical properties of exciton states in GaAs/AlxGa1-xAs coupled quantum well structure have been theoretically investigated under the influence of a time-varying high-intensity terahertz(THz)laser field.Non-perturbative Floquet theory is employed to solve the time-dependent equation of motion for the laser-driven excitonic quantum well system.The response to the field parameters,such as intensity and frequency of the laser electric field on the state populations,can be used in various optical semiconductor device applications,such as photodetectors,sensors,all-optical switches,and terahertz emitters.

Excitonic optical absorption in semiconductors under intense terahertz radiation

The excitonic optical absorption of GaAs bulk semiconductors under intense terahertz （THz） radiation is investigated numerically. The method of solving initial-value problems, combined with the perfect matched layer technique, is used to calculate the optical susceptibility. In the presence of a driving THz field, in addition to the usual exciton peaks, 2p replica of the dark 2p exciton and even-THz-photon-sidebands of the main exciton resonance emerge in the continuum above the band edge and below the main exciton resonance. Moreover, to understand the shift of the position of the main exciton peak under intense THz radiation, it is necessary to take into consideration both the dynamical Franz-Keldysh effect and ac Stark effect simultaneously. For moderate frequency fields, the main exciton peak decreases and broadens due to the field-induced ionization of the excitons with THz field increasing. However, for high frequency THz fields, the characteristics of the exciton recur even under very strong THz fields, which accords with the recent experimental results qualitatively.

Evaluating First-Order Molecular Properties of Delocalized Ionic or Excited States in Molecular Aggregates by Renormalized Excitonic Method

In the past few years,the renormalized excitonic model(REM)approach was developed as an efficient low-scaling ab initio excited state method,which assumes the low-lying excited states of the whole system are a linear combination of various single monomer excitations and utilizes the effective Hamiltonian theory to derive their couplings.In this work,we further extend the REM calculations for the evaluations of first-order molecular properties(e.g.charge population and transition dipole moment)of delocalized ionic or excited states in molecular aggregates,through generalizing the effective Hamiltonian theory to effective operator representation.Results from the test calculations for four different kinds of one dimensional(1D)molecular aggregates(ammonia,formaldehyde,ethylene and pyrrole)indicate that our new scheme can efficiently describe not only the energies but also wavefunction properties of the low-lying delocalized electronic states in large systems.

Valley dynamics of different excitonic states in monolayer WSe_(2)grown by molecular beam epitaxy

Monolayer transition-metal dichalcogenides possess rich excitonic physics and unique valley-contrasting optical selection rule,and offer a great platform for long spin/valley lifetime engineering and the associated spin/valleytronics exploration.Using two-color time-resolved Kerr rotation and time-resolved reflectivity spectroscopy,we investigate the spin/valley dynamics of different excitonic states in monolayer WSe_(2)grown by molecular beam epitaxy.With fine tuning of the photon energy of both pump and probe beams,the valley relaxation process for the neutral excitons and trions is found to be remarkably different-their characteristic spin/valley lifetimes vary from picoseconds to nanoseconds,respectively.The observed long trion spin lifetime of>2.0 ns is discussed to be associated with the dark trion states,which is evidenced by the photon-energy dependent valley polarization relaxation.Our results also reveal that valley depolarization for these different excitonic states is intimately connected with the strong Coulomb interaction when the optical excitation energy is above the exciton resonance.

Excitonic Doppler-Rabi Oscillations in a Moving Organic Slab

It is theoretically shown that excitonic Doppler-Rabi oscillations can occur in an organic slab moving along the axis of a high-Q cavity. Due to the N enhancement of the vacuum Rabi frequency, this effect can be more eas ily observed than that in a moving two-level atom.

Excitonic transitions in Be-doped GaAs/AlAs multiple quantum well

A series of GaAs/A1As multiple-quantum wells doped with Be is grown by molecular beam epitaxy. The photolu- minescence spectra are measured at 4, 20, 40, 80, 120, and 200 K, respectively. The recombination transition emission of heavy-hole and light-hole free excitons is clearly observed and the transition energies are measured with different quantum well widths. In addition, a theoretical model of excitonic states in the quantum wells is used, in which the symmetry of the component of the exciton wave function representing the relative motion is allowed to vary between the two- and three- dimensional limits. Then, within the effective mass and envelope function approximation, the recombination transition energies of the heavy- and light-hole excitons in GaAs/A1As multiple-quantum wells are calculated each as a function of quantum well width by the shooting method and variational principle with two variational parameters. The results show that the excitons are neither 2D nor 3D like, but are in between in character and that the theoretical calculation is in good agreement with the experimental results.

Effects of Quantum well Size Alteration on Excitonic Population Oscillation Slow Light Devices Properties

This paper investigates the effects of quantum well size changes on center frequency and slow down factor of an slow light device. In this way, we consider the quantum well size alteration effects on oscillator strength and binding energy of exciton. First, we investigate the variations in oscillator strength of exciton due to different quantum well size. Second, exciton binding energy level shift due to size of quantum well is investigated. According to this analysis, we have developed a new method for tuning slow light device bandwidth center frequency and slow down factor. Analysis and simulation of a basic GaAs/AlGaAs quantum wells optical slow light device based on excitonic population oscillation shows that size of quantum wells could tune both of the frequency properties and slow down factor of an optical slow light device. In our simulation with 34 quantum wells each with the width of 60?, we have received the slow down factor of more than 60,000. These achievements are useful in optical nonlinearity enhancements, all-optical signal processing applications and optical communications.

Mechanistic insights into excitonic photoluminescence in hybrid organic-inorganic perovskites with targeted regulation of organic cations

We report a mechanistic study of excitonic photoluminescence in predesigned hybrid organic-inorganic perovskite(HOIP)systems,i.e.,(DMAEA)Pb_(2)I_(6),(DMAPA)PbI_(4),(DEAEA)Pb_(2)I_(6),and(DEAPA)_(4)Pb_(5)I_(18),featuring targeted regulation of organic cations.Starting from the prototype DMAEA(i.e.,2-N,N-dimethylamino-l-ethylamine)for(DMAEA)Pb_(2)I_(6),the other three HOIPs differ only in the extensions with CH_(2)group(s)at the“head”or/and“tail”of DMAEA that is an“alkylated ammonia”.Their crystal structures are constructed and structural distortions are evaluated.The steady-state/transient absorption and emission spectroscopic characterizations,combined with the band-structure calculations,are conducted.The two different photoluminescence(PL)mechanisms are identified,i.e.,PL emissions dominated by free excitons for(DMAPA)PbI_(4)and by self-trapped excitons for(DMAEA)Pb_(2)I_(6),(DEAEA)Pb_(2)I_(6),and(DEAPA)_(4)Pb_(5)I_(18).The self-trapped excitonic effect involved in the latter three HOIPs is quantitatively analyzed.This work would be of guiding value for the design of HOIP systems based on organic-cation engineering,beneficial for the pertinent performance optimization in light-emitting applications.

Achieving Intrinsic Dual-Band Excitonic Luminescence from a Single Three-Dimensional Perovskite Nanoparticle Through Ni^(2+)-Mediated Halide Anion Exchange

Rapid halide anion exchange easily occurring in metal-halide perovskite nanoparticles(NPs)makes it nearly impossible to create a single three-dimensional(3D)CsPbX_(3)(X=Cl,Br,I)NP that simultaneously comprises two separate perovskite components.To circumvent this problem,we first propose a Ni^(2+)-mediated halide anion-exchange strategy in zero-dimensional(0D)Ni^(2+)-doped Cs_(4)PbBr_(6)(Cs_(4)PbBr_(6):Ni)perovskites to achieve ultra-stable 3D CsPbX_(3)NPs with two coexisting different perovskite individuals of CsPbCl_(3)and/or CsPbBr_(3).By combining the experimental results with first-principles calculations,we confirm that the completely isolated[PbBr_(6)]4−octahedra in 0D Cs_(4)PbBr_(6):Ni NPs can restrict rapid halide anion exchange and the anion diffusion preferentially proceeds in the proximity of substitutional NiPb centers,namely[NiBr_(6)]4−octahedra in a meta-stable state,rather than in the 0D Cs_(4)PbBr_(6)and residual 3D CsPbBr_(3)regions,thereby delivering intrinsic dual-band excitonic luminescence from a single 3D CsPbX_(3)NP with a more stable and efficient CsPbCl_(3)component as compared to its counterparts synthesized using conventional methods.These new insights regarding the precise control of halide anion exchange enable the preparation of a new type of high-efficiency perovskite materials with suppressed anion interdiffusion for prospective optoelectronic devices.

Excitonic devices based on two-dimensional transition metal dichalcogenides van der Waals heterostructures

Excitonic devices are an emerging class of technology that utilizes excitons as carriers for encoding, transmitting, and storing information. Van der Waals heterostructures based on transition metal dichalcogenides often exhibit a type II band alignment, which facilitates the generation of interlayer excitons. As a bonded pair of electrons and holes in the separation layer, interlayer excitons offer the chance to investigate exciton transport due to their intrinsic out-of-plane dipole moment and extended exciton lifetime. Furthermore, interlayer excitons can potentially analyze other encoding strategies for information processing beyond the conventional utilization of spin and charge. The review provided valuable insights and recommendations for researchers studying interlayer excitonic devices within van der Waals heterostructures based on transition metal dichalcogenides. Firstly, we provide an overview of the essential attributes of transition metal dichalcogenide materials, focusing on their fundamental properties, excitonic effects, and the distinctive features exhibited by interlayer excitons in van der Waals heterostructures. Subsequently, this discourse emphasizes the recent advancements in interlayer excitonic devices founded on van der Waals heterostructures, with specific attention is given to the utilization of valley electronics for information processing, employing the valley index. In conclusion, this paper examines the potential and current challenges associated with excitonic devices.

Disentangling the electron-lattice dichotomy of the excitonic insulating phase in Ta_(2)Ni(Se_(1-x)S_(x))_(5) with sulfur substitution and potassium deposition

Ta_(2)NiSe_(5) is a promising candidate for hosting an excitonic insulator(EI)phase,a novel electronic state driven by electron-hole Coulomb attraction.However,the role of electron-lattice coupling in the formation of the EI phase remains controversial.Here,we use angle-resolved photoemission spectroscopy(ARPES)to study the band structure evolution of Ta_(2)Ni(Se_(1-x)S_(x))_(5) with sulfur substitution and potassium deposition,which modulate the band gap and the carrier concentration,respectively.We find that the Ta 5d states originating from the bottom of the conduction band persist at the top of the valence band in the low-temperature monoclinic phase,indicating the importance of exciton condensation in opening the gap in the semi-metallic band structure.We also observe that the characteristic overlap between the conduction and valence bands can be restored in the monoclinic lattice by mild carrier injection,suggesting that the lattice distortion in the monoclinic phase is not the main factor for producing the insulating gap,but rather the exciton condensation in the electronic system is the dominant driving force.Our results shed light on the electron-lattice decoupling and the origin of the EI phase in Ta_(2)Ni(Se_(1-x)Sx)_(5).