Search Results (25)

We propose a new way to establish the existence of a superfluid phase in an exciton bilayer system by exploiting the properties of its collective modes. We focus on the density collective modes and treat them within Random Phase Approximation. By comparing results for the normal and superfluid states, we are able to identify unambiguous fingerprints of the exciton superfluid phase. We compare the collective modes of the exciton system and cold atom systems, and we discuss the collective modes of the exciton superfluid order parameter. Full article

In this study, we explore the exciton dynamics in a WS₂/MoS₂ van der Waals (vdW) heterostructure under varying pressures by integrating a laser-confocal photoluminescence (PL) spectroscope and an atomic force microscope (AFM). For the WS₂/MoS₂ heterostructure, the exciton emission belonging to MoS₂ is too weak to be distinguished from the PL spectra. However, upon contact with a Si probe, the emission intensity of WS₂ excitons significantly decreases from 34,234 to 6560, thereby matching the intensity level of MoS₂. This alteration substantially facilitates the exploration of interlayer excitonic properties within the heterostructures using PL spectroscopy. Furthermore, the Si probe can apply out-of-plane localized pressure to the heterostructure. With increasing pressure, the emission intensity of the WS₂ trions decreases at a rate twice that of other excitons, and the exciton energy increases at a rate of 0.1 meV nN⁻¹. These results elucidate that the WS₂ trions are particularly sensitive to the out-of-plane pressure within a WS₂/MoS₂ vdW heterostructure. Full article

The niobium oxide dihalides have recently been identified as a new class of van der Waals materials exhibiting exceptionally large second-order nonlinear optical responses and robust in-plane ferroelectricity. In contrast to second-order nonlinear processes, third-order optical nonlinearities can arise irrespective of whether a crystal lattice is centrosymmetric. Here, we report third harmonic generation (THG) in two-dimensional (2D) transition metal oxide iodides, namely NbOI₂ and TaOI₂. We observe a comparable THG intensity from both materials. By benchmarking against THG from monolayer WS₂, we deduce that the third-order susceptibility is approximately on the same order. THG resonances are revealed at different excitation wavelengths, likely due to enhancement by excitonic states and band edge resonances. The THG intensity increases for material thicknesses up to 30 nm, owing to weak interlayer coupling. After this threshold, it shows saturation or a decrease, due to optical interference effects. Our results establish niobium and tantalum oxide iodides as promising 2D materials for third-order nonlinear optics, with intrinsic in-plane ferroelectricity and thickness-tunable nonlinear efficiency. Full article

Many quantum dot light-emitting diodes (QLEDs) utilize ZnO nanoparticles (NPs) as an electron injection layer (EIL). However, the use of the ZnO NP EIL material often results in a charge imbalance within the quantum dot (QD) emitting layer (EML) and exciton quenching at the interface of the QD EML and ZnO NP EIL. To overcome these challenges, we introduced an arginine (Arg) interlayer (IL) onto the ZnO NP EIL. The Arg IL elevated the work function of ZnO NPs, thereby suppressing electron injection into the QD, leading to an improved charge balance within the QDs. Additionally, the inherent insulating nature of the Arg IL prevented direct contact between QDs and ZnO NPs, reducing exciton quenching and consequently improving device efficiency. An inverted QLED (IQLED) utilizing a 20 nm-thick Arg IL on the ZnO NP EIL exhibited a 2.22-fold increase in current efficiency and a 2.28-fold increase in external quantum efficiency (EQE) compared to an IQLED without an IL. Likewise, the IQLED with a 20 nm-thick Arg IL on the ZnO NP EIL demonstrated a 1.34-fold improvement in current efficiency and a 1.36-fold increase in EQE compared to the IQLED with a 5 nm-thick polyethylenimine IL on ZnO NPs. Full article

Semiconductor heterostructures have been the backbone of developments in electronic and optoelectronic devices. One class of structures of interest is the so-called type II band alignment, in which optically excited electrons and holes relax into different material layers. The unique properties observed in two-dimensional transition metal dichalcogenides and the possibility to engineer van der Waals heterostructures make them candidates for future high-tech devices. In these structures, electronic, optical, and magnetic properties can be tuned through the interlayer coupling, thereby opening avenues for developing new functional materials. We report the possibility of explicitly tuning the emission of interlayer exciton energies in the binary–ternary heterobilayer of Mo${}_{0.5}$W${}_{0.5}$Se${}_2$ with MoSe${}_2$ and WSe${}_2$. The respective interlayer energies of 1.516 eV and 1.490 eV were observed from low-temperature photoluminescence measurements for the MoSe${}_2$– and WSe${}_2$– based heterostructures, respectively. These interlayer emission energies are above those reported for MoSe${}_2$/WSe${}_2$ (≃1.30–1.45 eV). Consequently, binary–ternary heterostructure systems offer an extended energy range and tailored emission energies not accessible with the binary counterparts. Moreover, even though Mo${}_{0.5}$W${}_{0.5}$Se${}_2$ and MoSe${}_2$ have almost similar optical gaps, their band offsets are different, resulting in charge transfer between the monolayers following the optical excitation. Thus, confirming TMDs alloys can be used to tune the band-offsets, which adds another design parameter for application-specific optoelectronic devices. Full article

We investigated solution-grown single crystals of multidimensional 2D–3D hybrid lead bromide perovskites using spatially resolved photocurrent and photoluminescence. Scanning photocurrent microscopy (SPCM) measurements where the electrodes consisted of a dip probe contact and a back contact. The crystals revealed significant differences between 3D and multidimensional 2D–3D perovskites under biased detection, not only in terms of photocarrier decay length values but also in the spatial dynamics across the crystal. In general, the photocurrent maps indicate that the closer the border proximity, the shorter the effective decay length, thus suggesting a determinant role of the border recombination centers in monocrystalline samples. In this case, multidimensional 2D–3D perovskites exhibited a simple fitting model consisting of a single exponential, while 3D perovskites demonstrated two distinct charge carrier migration dynamics within the crystal: fast and slow. Although the first one matches that of the 2D–3D perovskite, the long decay of the 3D sample exhibits a value two orders of magnitude larger. This difference could be attributed to the presence of interlayer screening and a larger exciton binding energy of the multidimensional 2D–3D perovskites with respect to their 3D counterparts. Full article

Optimal design of a silicon nitride waveguide structure composed of resonant nanoantennas for efficient light coupling with interlayer exciton emitters in a MoSe${}_2$–WSe${}_2$ heterostructure is proposed. Numerical simulations demonstrate up to eight times coupling efficiency improvement and twelve times Purcell effect enhancement in comparison with a conventional strip waveguide. Achieved results can be beneficial for development of on-chip non-classical light sources. Full article

Excitonic condensation and superfluidity have recently received a renewed attention, due to the fabrication of bilayer systems in which electrons and holes are spatially separated and form stable pairs known as indirect excitons. Dichalcogenides- and graphene-based bilayers are nowadays built and investigated, giving access to systems with (i) only spin degeneracy and (ii) spin and valley degeneracy. Simulation studies performed in the last decades at $T=0$ for simple, model electron–hole bilayers, as function of the interlayer distance and in-layer carrier density, have revealed in case (i) the formation of biexcitons in a tiny region of the parameter space and in case (ii) the formation of stable compounds made of four electrons and four holes (quadriexcitons) in a sizable region of the parameter space. Of some interest is the relation of the properties of isolated biexcitons (quadriexcitons) and those of their finite-density counterpart. In fact, the isolated biexciton has been repeatedly studied in the last years with simulations and other techniques. No simulations, instead, are available to our knowledge for the isolated quadriexciton, for which we present here results of the first quantum Monte Carlo (QMC) study. Stability with respect to the dissociation into biexcitons and the pair correlations while varying the interlayer distance d are discussed. Full article

We consider the effect of the external magnetic field on the in-plane conductivity in the AA-stacked bilayer graphene system in the strong excitonic condensate regime. We include the effects of the applied inter-layer electric field and the Coulomb interactions. The on-site and inter-layer Coulomb interactions were treated via the bilayer Hubbard model. Using the solutions for the physical parameters in the system, we calculate the in-plane conductivity of the bilayer graphene. By employing the Green-Kubo formalism for the polarization function in the system, we show that the conductivity in the AA bilayer system is fully controlled by the applied magnetic field. For the partial filling in the layers, the electrical conductivity is different for different spin orientations, and, at the high values of the magnetic field, only one component remains with the given spin orientation. Meanwhile, for the half-filling limit, there is no spin-splitting observed in the conductivity function. The theory evaluated here shows the new possibility for spin-controlled electronic transport in the excitonic bilayer graphene system. Full article

Multilayered van der Waals heterostructures based on transition metal dichalcogenides are suitable platforms on which to study interlayer (dipolar) excitons, in which electrons and holes are localized in different layers. Interestingly, these excitonic complexes exhibit pronounced valley Zeeman signatures, but how their spin-valley physics can be further altered due to external parameters—such as electric field and interlayer separation—remains largely unexplored. Here, we perform a systematic analysis of the spin-valley physics in MoSe${}_2$/WSe${}_2$ heterobilayers under the influence of an external electric field and changes of the interlayer separation. In particular, we analyze the spin ($S_z$) and orbital ($L_z$) degrees of freedom, and the symmetry properties of the relevant band edges (at K, Q, and $\mathsf{\Gamma }$ points) of high-symmetry stackings at 0° (R-type) and 60° (H-type) angles—the important building blocks present in moiré or atomically reconstructed structures. We reveal distinct hybridization signatures on the spin and the orbital degrees of freedom of low-energy bands, due to the wave function mixing between the layers, which are stacking-dependent, and can be further modified by electric field and interlayer distance variation. We find that H-type stackings favor large changes in the g-factors as a function of the electric field, e.g., from $-5$ to 3 in the valence bands of the H${}_{\mathrm{h}}^{\mathrm{h}}$ stacking, because of the opposite orientation of $S_z$ and $L_z$ of the individual monolayers. For the low-energy dipolar excitons (direct and indirect in k-space), we quantify the electric dipole moments and polarizabilities, reflecting the layer delocalization of the constituent bands. Furthermore, our results show that direct dipolar excitons carry a robust valley Zeeman effect nearly independent of the electric field, but tunable by the interlayer distance, which can be rendered experimentally accessible via applied external pressure. For the momentum-indirect dipolar excitons, our symmetry analysis indicates that phonon-mediated optical processes can easily take place. In particular, for the indirect excitons with conduction bands at the Q point for H-type stackings, we find marked variations of the valley Zeeman (∼4) as a function of the electric field, which notably stands out from the other dipolar exciton species. Our analysis suggests that stronger signatures of the coupled spin-valley physics are favored in H-type stackings, which can be experimentally investigated in samples with twist angle close to 60°. In summary, our study provides fundamental microscopic insights into the spin-valley physics of van der Waals heterostructures, which are relevant to understanding the valley Zeeman splitting of dipolar excitonic complexes, and also intralayer excitons. Full article

The study of intra and interlayer excitons in 2D semiconducting vdW heterostructures is a very hot topic not only from a fundamental but also an applicative point of view. Due to their strong light–matter interaction, Transition Metal Dichalcogenides (TMD) and group-III nitrides are particularly attractive in the field of opto-electronic applications such as photo-catalytic and photo-voltaic ultra-thin and flexible devices. Using first-principles ground and excited-state simulations, we investigate here the electronic and excitonic properties of a representative nitride/TMD heterobilayer, the $AlN/W{S}_2$. We demonstrate that the band alignment is of type I, and low energy intralayer excitons are similar to those of a pristine $W{S}_2$ monolayer. Further, we disentangle the role of strain and AlN dielectric screening on the electronic and optical gaps. These results, although they do not favor the possible use of AlN/$W{S}_2$ in photo-catalysis, as envisaged in the previous literature, can boost the recently started experimental studies of 2D hexagonal aluminum nitride as a good low screening substrate for TMD-based electronic and opto-electronic devices. Importantly, our work shows how the inclusion of both spin-orbit and many-body interactions is compulsory for the correct prediction of the electronic and optical properties of TMD/nitride heterobilayers. Full article

The anion exchange of the halides, Br and I, is demonstrated through the direct mixing of two pure perovskite quantum dot solutions, CsPbBr₃ and CsPbI₃, and is shown to be both facile and result in a completely alloyed single phase mixed halide perovskite. Anion exchange is also observed in an interlayer printing method utilizing the pure, unalloyed perovskite solutions and a commercial inkjet printer. The halide exchange was confirmed by optical absorption spectroscopy, photoluminescent spectroscopy, X-ray diffraction, and X-ray photoemission spectroscopy characterization and indicates that alloying is thermodynamically favorable, while the formation of a clustered alloy is not favored. Additionally, a surface-to-bulk photoemission core level transition is observed for the Cs 4d photoemission feature, which indicates that the electronic structure of the surface is different from the bulk. Time resolved photoluminescence spectroscopy indicates the presence of multiple excitonic decay features, which is argued to originate from states residing at surface and bulk environments. Full article

MoS₂ nanoscrolls that have inner core radii of ∼250 nm are generated from MoS₂ monolayers, and the optical and transport band gaps of the nanoscrolls are investigated. Photoluminescence spectroscopy reveals that a MoS₂ monolayer, originally a direct gap semiconductor (∼1.85 eV (optical)), changes into an indirect gap semiconductor (∼1.6 eV) upon scrolling. The size of the indirect gap for the MoS₂ nanoscroll is larger than that of a MoS₂ bilayer (∼1.54 eV), implying a weaker interlayer interaction between concentric layers of the MoS₂ nanoscroll compared to Bernal-stacked MoS₂ few-layers. Transport measurements on MoS₂ nanoscrolls incorporated into ambipolar ionic-liquid-gated transistors yielded a band gap of ∼1.9 eV. The difference between the transport and optical gaps indicates an exciton binding energy of 0.3 eV for the MoS₂ nanoscrolls. The rolling up of 2D atomic layers into nanoscrolls introduces a new type of quasi-1D nanostructure and provides another way to modify the band gap of 2D materials. Full article

Two-dimensional (2D) vertical heterojunctions (HSs), which are usually fabricated by vertically stacking two layers of transition metal dichalcogenide (TMDC), have been intensively researched during the past years. However, it is still an enormous challenge to achieve controllable preparation of the TMDC trilayer or multilayered van der Waals (vdWs) HSs, which have important effects on physical properties and device performance. In this review, we will introduce fundamental features and various fabrication methods of diverse TMDC-based multilayered vdWs HSs. This review focuses on four fabrication methods of TMDC-based multilayered vdWs HSs, such as exfoliation, chemical vapor deposition (CVD), metal-organic chemical vapor deposition (MOCVD), and pulsed laser deposition (PLD). The latest progress in vdWs HS-related novel physical phenomena are summarized, including interlayer excitons, long photocarrier lifetimes, upconversion photoluminescence, and improved photoelectrochemical catalysis. At last, current challenges and prospects in this research field are provided. Full article

In 2H stacking bilayer MoS₂, the exciton with an interlayer nature has been evidenced due to the hybridization of hole states among both layers. The transition energy of this interlayer exciton is located between the A and B excitons. In this work, we investigate the evolution of optical properties in stacking MoS₂ bilayers with the twisted angles ranging from 0° to 60°, especially focusing on the interlayer exciton. The clear modulations of the exciton responses are observed by the room-temperature reflectance. The interlayer exciton transition is observed in the artificial stacking bilayer MoS₂ with the twisted angle around 60°. We found that the interlayer exciton is very sensitive to the twisted angle. Once the stacking angle deviates the 2H stacking by a few degrees, the interlayer transition is quenched. This is due to the bilayer symmetry and interlayer coupling of this unique system. Full article