Light-emitting devices based on atomically thin MoSe_(2)

Atomically thin MoSe_(2) layers,as a core member of the transition metal dichalcogenides(TMDs)family,benefit from their appealing properties,including tunable band gaps,high exciton binding energies,and giant oscillator strengths,thus pro-viding an intriguing platform for optoelectronic applications of light-emitting diodes(LEDs),field-effect transistors(FETs),sin-gle-photon emitters(SPEs),and coherent light sources(CLSs).Moreover,these MoSe_(2) layers can realize strong excitonic emis-sion in the near-infrared wavelengths,which can be combined with the silicon-based integration technologies and further encourage the development of the new generation technologies of on-chip optical interconnection,quantum computing,and quantum information processing.Herein,we overview the state-of-the-art applications of light-emitting devices based on two-dimensional MoSe_(2) layers.Firstly,we introduce recent developments in excitonic emission features from atomically thin MoSe_(2) and their dependences on typical physical fields.Next,we focus on the exciton-polaritons and plasmon-exciton polaritons in MoSe_(2) coupled to the diverse forms of optical microcavities.Then,we highlight the promising applications of LEDs,SPEs,and CLSs based on MoSe_(2) and their heterostructures.Finally,we summarize the challenges and opportunities for high-quality emis-sion of MoSe_(2) and high-performance light-emitting devices.

Defect suppression and energy level alignment in formamidinium-based perovskite solar cells

The vast majority of high-performance perovskite solar cells(PSCs) are based on a formamidinium lead iodide(FAPbI_(3))-dominant composition. Nevertheless, the FA-based perovskite films suffer from undesirable phase transition and defects-induced non-ideal interfacial recombination, which significantly induces energy loss and hinders the improvement of device performance. Herein, we employed 4-fluorophenylmethylammonium iodide(F-PMAI) to modulate surface structure and energy level alignment of the FA-based perovskite films. The superior optoelectronic films were obtained with reduced trap density, pure α-phase FAPbI_(3) and favorable energy band bending. The lifetime of photogenerated charge carriers increased from 489.3 ns to 1010.6 ns, and a more “p-type” perovskite film was obtained by the post-treatment with F-PMAI. Following this strategy, we demonstrated an improved power conversion efficiency of 22.59% for the FA-based PSCs with an open-circuit voltage loss of 399 m V.

Probing angle-resolved reflection signatures of intralayer and interlayer excitons in monolayer and bilayer MoS_(2)

Strongly bound excitons in atomically thin transition metal dichalcogenides offer many opportunities to reveal the underlying physics of basic quasiparticles and many-body effects in the two-dimensional(2D)limit.Comprehensive reflection investigation on band-edge exciton transitions is essential to exploring wealthy light–matter interactions in the emerging 2D semiconductors,whereas angle-resolved reflection(ARR)characteristics of intralayer and interlayer excitons in 2D MoS_(2)layers remain unclear.Herein,we report ARR spectroscopic features of A,B,interlayer excitons in monolayer(ML)and bilayer(BL)MoS_(2)on three kinds of photonic substrates,involving distinct exciton–photon interactions.In a BL MoS_(2)on a protected silver mirror,the interlayer exciton with one-third amplitude of A exciton appears at 0.05 eV above the A exciton energy,exhibiting an angleinsensitive energy dispersion.When ML and BL MoS_(2)lie on a SiO_(2)-covered silicon,the broad trapped-photon mode weakly couples with the reflection bands of A and B excitons by the Fano resonance effect,causing the asymmetric lineshapes and the redshifted energies.After transferring MoS_(2)layers onto a one-dimensional photonic crystal,two high-lying branches of B-exciton polaritons are formed by the interactions between B excitons and Bragg photons,beyond the weak-coupling regime.This work provides ARR spectral benchmarks of A,B,interlayer excitons in ML and BL MoS_(2),gaining insights into the interpretation of light–matter interactions in 2D semiconductors and the design of their devices for practical photonic applications.

Strain-induced direct-indirect bandgap transition and phonon modulation in monolayer WS2

In situ strain photoluminescence （PL） and Raman spectroscopy have been employed to exploit the evolutions of the electronic band structure and lattice vibrational responses of chemical vapor deposition （CVD）-grown monolayer tungsten disulphide （WS2） under uniaxial tensile strain. Observable broadening and appearance of an extra small feature at the longer-wavelength side shoulder of the PL peak occur under 2.5% strain, which could indicate the direct-indirect bandgap transition and is further confirmed by our density-functional-theory calculations. As the strain increases further, the spectral weight of the indirect transition gradually increases. Over the entire strain range, with the increase of the strain, the light emissions corresponding to each optical transition, such as the direct bandgap transition （K-K） and indirect bandgap transition （F-K, ≥2.5%）, exhibit a monotonous linear redshift. In addition, the binding energy of the indirect transition is found to be larger than that of the direct transition, and the slight lowering of the trion dissociation energy with increasing strain is observed. The strain was used to modulate not only the electronic band structure but also the lattice vibrations. The softening and splitting of the in-plane E＇ mode is observed under uniaxial tensile strain, and polarization-dependent Raman spectroscopy confirms the observed zigzag-oriented edge of WS2 grown by CVD in previous studies. These findings enrich our understanding of the strained states of monolayer transition-metal dichalcogenide （TMD） materials and lay a foundation for developing applications exploiting their strain-dependent optical properties, including the strain detection and light-emission modulation of such emerging two-dimensional TMDs.

Thermal conductivity determination of suspended mono- and bilayer WS2 by Raman spectroscopy

We report the thermal conductivities of monolayer （1L） and bilayer （2L） WS2 grown by chemical vapor deposition （CVD）, which are determined by use of temperature and excitation dependences of E2g^1 and A1g Raman modes. The first-order temperature coefficients of E2g^1 and Alg modes in both supported and suspended WS2 layers were extracted. The frequency shift of the A3g mode with temperature is larger than that of the E1 mode for 1L-WS2, which is 2g attributed to stronger electron-phonon coupling for the A1g mode than that for the E12g mode. Moreover, by use of the shift of the phonon mode induced by laser heating, the thermal conductivities at room temperature were estimated to be 32 and 53 W/（m.K） for 1L- and 2L-WS2, respectively. Our results provide fundamental information about the thermal properties of WS2 layers, which is crucial for developing applications of atomically-thin WS2 devices.

Raman scattering investigation of twisted WS_(2)/MoS_(2) heterostructures: interlayer mechanical coupling versus charge transfer

Twisted van der Waals homo-and hetero-structures have aroused great attentions due to their unique physical properties,providing a new platform to explore the novel two-dimensional(2D)condensed matter physics.The robust dependence of phonon vibrations and electronic band structures on the twist angle has been intensively observed in transition metal dichalcogenide(TMD)homo-structures.However,the effects of twist angle on the lattice vibrational properties in the TMD heterostructures have not caused enough attention.Here,we report the distinct evolutions of Raman scattering and the underlying interlayer interactions in the twisted WS_(2)/MoS_(2) heterostructures.The shifts and linewidths of E_(2g)(Γ)and A_(1g)(Γ)phonon modes are found to be twist angle dependent.In particular,analogous to that of the twisted TMD homostructures,the frequency separations between E_(2g)(Γ)and A_(1g)(Γ)modes of MoS_(2) and WS_(2) in the twisted heterostructures varying with twist angle correlate with the interlayer mechanical coupling,essentially originating from the spacing-related repulsion between sulfur atoms.Moreover,the opposite shift behaviors and broadening of A_(1g)(Γ)modes caused by charge transfer are also observed in the twisted heterostructures.The calculated interlayer distances and band alignment of twisted WS_(2)/MoS_(2) through density functional theory further evidence our interpretations on the roles of the interlayer mechanical coupling and charge transfer in variations of Raman features.Such understanding and controlling of interlayer interaction through the stacking orientation are significant for future optoelectronic device design based on the newly emerged 2D heterostructures.

Monolayer tungsten disulfide in photonic environment:Angleresolved weak and strong light-matter coupling

Light-matter interactions in two-dimensional transition metal dichalcogenides(TMDs)are sensitive to the surrounding dielectric environment.Depending on the interacting strength,weak and strong exciton–photon coupling effects can occur when the exciton energy is resonant with the one of photon.Here we report angle-resolved spectroscopic signatures of monolayer tungsten disulfide(1L-WS2)in weak and strong exciton–photon coupling environments.Inherent optical response of 1L-WS_(2)in the momentum space is uncovered by employing a dielectric mirror as substrate,where the energy dispersion is angleindependent while the amplitudes increase at high detection angles.When 1L-WS_(2)sits on top of a dielectric layer on silicon,the resonant trapped photon weakly couples with the exciton,in which the minimum of reflection dip shifts at both sides of the crossing angle while the emitted exciton energy remains unchanged.The unusual shift of reflection dip is attributed to the presence of Fano resonance under white-light illumination.By embedding 1L-WS_(2)into a dielectric microcavity,strong exciton–photon coupling results in the formation of lower and upper polariton branches with an appreciable Rabi splitting of 34 meV at room temperature,where the observed blueshift of the lower polariton branch is indicative of the enhanced polaritonpolariton scattering.Our findings highlight the effect of dielectric environment on angle-resolved optical response of exciton–photon interactions in a two-dimensional semiconductor,which is helpful to develop practical TMD-based architectures for photonic and polaritonic applications.

Observation of Bragg polaritons in monolayer tungsten disulphide

Strong light-matter interactions involved with photons and quasiparticles are fundamentally interesting to access the wealthy many-body physics in quantum mechanics.The emerging two-dimensional(2D)semiconductors with large exciton binding energies and strong quantum confinement allow to investigate exciton-photon coupling at elevated temperatures.Here we report room-temperature formation of Bragg polaritons in monolayer semiconductor on a dielectric mirror through the exciton-Bragg photon coupling.With the negative detuning energy of -30 meV,angle-resolved reflection signals reveal anti-crossing behaviors of lower and upper polariton branches at±18°together with the Rabi splitting of 10 meV.Meanwhile,the strengthened photoluminescence appears in the lower polariton branch right below the anti-crossing angles,indicating the presence of the characteristic bottleneck effect caused by the slowing exciton-polariton energy relaxation towards the band minimum.The extracted coupling strength is between the ones of weak and distinct strong coupling regimes,where the eigenenergy splitting induced by the moderate coupling is resolvable but not large enough to fully separate two polaritonic components.Our work develops a simplified strategy to generate exciton-polaritons in 2D semiconductors and can be further extended to probe the intriguing bosonic characteristics of these quasiparticles,such as Bose-Einstein condensation,polariton lasing and superfluidity,directly at the material surfaces.

Tunable excitonic emission of monolayer WS2 for the optical detection of DNA nucleobases

Two-dimensional transition metal dichalcogenides （2D TMDs） possess a tunable excitonic light emission that is sensitive to external conditions such as electric field, strain, and chemical doping. In this work, we reveal the interactions between DNA nucleobases, i.e., adenine （A）, guanine （G）, cytosine （C）, and thymine （T） and monolayer WS2 by investigating the changes in the photoluminescence （PL） emissions of the monolayer WS2 after coating with nucleobase solutions. We found that adenine and guanine exert a clear effect on the PL profile of the monolayer WS2 and cause different PL evolution trends. In contrast, cytosine and thymine have little effect on the PL behavior. To obtain information on the interactions between the DNA bases and WS2, a series of measurements were conducted on adenine-coated WS2 monolayers, as a demonstration. The p-type doping of the WS2 monolayers on the introduction of adenine is clearly shown by both the evolution of the PL spectra and the electrical transport response. Our findings open the door for the development of label-free optical sensing approaches in which the detection signals arise from the tunable excitonic emission of the TMD itself rather than the fluorescence signals of label molecules. This dopant-selective optical response to the DNA nucleobases fills the gaps in previously reported optical biosensing methods and indicates a potential new strategy for DNA sequencing.

Spatial variations of valley splitting in monolayer transition metal dichalcogenide

In monolayer group-VI transition metal dichalcogenides(TMDs),valley splitting features have received a lot of attention since it can be potentially utilized for information storing and processing.Among the known two-dimensional(2D)TMDs,monolayer WSe2 or MoSe2 has been mostly selected for excitonic and valleytronic physics studies because of its sharp and well-resolved excitonic spectral features.Meanwhile,their high optical quality leads to a tremendous desire for developing promising WSe2-and MoSe2-based valleytronic devices.Toward this goal,exploring the uniformity of valley features crossing an entire piece of monolayer becomes necessary and critical.Here,we performed the systematic magnetophotoluminescence mapping measurements on mechanically exfoliated monolayer WSe2 and observed unconventional spatial variations of valley splitting.The observed nonuniformity is attributed to the modulated doping,which is probably due to the different distributions of unintentional absorbates across the sample.Such an unexpected doping effect shows the nonnegligible influence on the valley Zeeman splitting of the trion emission(XT)while affecting that of the neutral exciton emission(X0)trivially,evidencing for the large valleytronic sensitivity of the charged exciton.This work not only enriches the understanding of the doping effect on valley splitting but also is meaningful for developing 2D valleytronics.