Exciton Photoluminescence Intensity Research Articles

Scalable Large-Area 2D-MoS2/Silicon-Nanowire Heterostructures for Enhancing Energy Storage Applications.

Two-dimensional (2D) transition-metal dichalcogenides have shown great potential for energy storage applications owing to their interlayer spacing, large surface area-to-volume ratio, superior electrical properties, and chemical compatibility. Further, increasing the surface area of such materials can lead to enhanced electrical, chemical, and optical response for energy storage and generation applications. Vertical silicon nanowires (SiNWs), also known as black-Si, are an ideal substrate for 2D material growth to produce high surface-area heterostructures, owing to their ultrahigh aspect ratio. Achieving this using an industrially scalable method paves the way for next-generation energy storage devices, enabling them to enter commercialization. This work demonstrates large surface area, commercially scalable, hybrid MoS2/SiNW heterostructures, as confirmed by Raman spectroscopy, with high tunability of the MoS2 layers down to the monolayer scale and conformal MoS2 growth, parallel to the silicon nanowires, as verified by transmission electron microscopy (TEM). This has been achieved using a two-step atomic layer deposition (ALD) process, allowing MoS2 to be grown directly onto the silicon nanowires without any damage to the substrate. The ALD cycle number accurately defines the layer number from monolayer to bulk. Introducing an ALD alumina (Al2O3) interface at the MoS2/SiNW boundary results in enhanced MoS2 quality and uniformity, demonstrated by an order of magnitude reduction in the B/A exciton photoluminescence (PL) intensity ratio to 0.3 and a reduction of the corresponding layer number. This high-quality layered growth on alumina can be utilized in applications such as for interfacial layers in high-capacity batteries or for photocathodes for water splitting. The alumina-free 100 ALD cycle heterostructures demonstrated no diminishing quality effects, lending themselves well to applications that require direct electrical contact with silicon and benefit from more layers, such as electrodes for high-capacity ion batteries.

Mitigating substrate effects of van der Waals semiconductors using perfluoropolyether self-assembled monolayers.

The properties of transition metal dichalcogenides (TMDCs) are critically dependent on the dielectric constant of substrates, which significantly limits their application. To address this issue, we used a perfluorinated polyether (PFPE) self-assembled monolayer (SAM) with low surface energy to increase the van der Waals (vdW) gap between TMDCs and the substrate, thereby reducing the interaction between them. This resulted in a reduction in the subthreshold swing value, an increase in the photoluminescence intensity of excitons, and a decrease in the doping effect by the substrate. This work will provide a new way to control the TMDC/dielectric interface and contribute to expanding the applicability of TMDCs.

Anisotropic Excitons Reveal Local Spin Chain Directions in a van der Waals Antiferromagnet.

A long-standing pursuit in materials science is to identify suitable magnetic semiconductors for integrated information storage, processing, and transfer. Van der Waals magnets have brought forth new material candidates for this purpose. Recently, sharp exciton resonances in antiferromagnet NiPS3 have been reported to correlate with magnetic order, that is, the exciton photoluminescence intensity diminishes above the Néel temperature. Here, it is found that the polarization of maximal exciton emission rotates locally, revealing three possible spin chain directions. This discovery establishes a new understanding of the antiferromagnet order hidden in previous neutron scattering and optical experiments. Furthermore, defect-bound states are suggested as an alternative exciton formation mechanism that has yet to be explored in NiPS3 . The supporting evidence includes chemical analysis, excitation power, and thickness dependent photoluminescence and first-principles calculations. This mechanism for exciton formation is also consistent with the presence of strong phonon side bands. This study shows that anisotropic exciton photoluminescence can be used to read out local spin chain directions in antiferromagnets and realize multi-functional devices via spin-photon transduction.

Directional Exciton-Energy Transport in a Lateral Heteromonolayer of WSe2-MoSe2.

Controlling the direction of exciton-energy flow in two-dimensional (2D) semiconductors is crucial for developing future high-speed optoelectronic devices using excitons as the information carriers. However, intrinsic exciton diffusion in conventional 2D semiconductors is omnidirectional, and efficient exciton-energy transport in a specific direction is difficult to achieve. Here we demonstrate directional exciton-energy transport across the interface in tungsten diselenide (WSe2)-molybdenum diselenide (MoSe2) lateral heterostructures. Unidirectional transport is spontaneously driven by the built-in asymmetry of the exciton-energy landscape with respect to the heterojunction interface. At excitation positions close to the interface, the exciton photoluminescence (PL) intensity was substantially decreased in the WSe2 region and enhanced in the MoSe2 region. In PL excitation spectroscopy, it was confirmed that the observed phenomenon arises from lateral exciton-energy transport from WSe2 to MoSe2. This directional exciton-energy flow in lateral 2D heterostructures can be exploited in future optoelectronic devices.

Relaxation and darkening of excitonic complexes in electrostatically doped monolayer WSe2 : Roles of exciton-electron and trion-electron interactions

We present photoluminescence measurements in monolayer WSe$_2$, which point to the importance of the interaction between charged particles and excitonic complexes. The theoretical analysis highlights the key role played by exchange scattering, referring to cases wherein the particle composition of the complex changes after the interaction. For example, exchange scattering renders bright excitonic complexes dark in monolayer WSe$_2$ on accounts of the unique valley-spin configuration in this material. In addition to the ultrafast energy relaxation of hot excitonic complexes following their interaction with electrons or holes, our analysis sheds light on several key features that are commonly seen in the photoluminescence of this monolayer semiconductor. In particular, we can understand why the photoluminescence intensity of the neutral bright exciton is strongest when the monolayer is hole-doped rather than charge neutral or electron-doped. Or the reason for the dramatic increase of the photoluminescence intensity of negatively-charged excitons (trions) as soon as electrons are added to the monolayer. To self-consistently explain the findings, we further study the photoluminescence spectra at different excitation energies and analyze the behavior of the elusive indirect exciton.

Highly Strain-Tunable Interlayer Excitons in MoS2/WSe2 Heterobilayers

Interlayer excitons in heterobilayers of transition-metal dichalcogenides (TMDCs) have generated enormous interest due to their permanent vertical dipole moments and long lifetimes. However, the effects of mechanical strain on the optoelectronic properties of interlayer excitons in heterobilayers remain relatively uncharacterized. Here, we experimentally demonstrate strain tuning of Γ-K interlayer excitons in molybdenum disulfide and tungsten diselenide (MoS2/WSe2) wrinkled heterobilayers and obtain a deformation potential constant of ∼107 meV/% uniaxial strain, which is approximately twice that of the intralayer excitons in the constituent monolayers. We further observe a nonmonotonic dependence of the interlayer exciton photoluminescence intensity with strain, which we interpret as being due to the sensitivity of the Γ point to band hybridization arising from the competition between in-plane strain and out-of-plane interlayer coupling. Strain engineering with interlayer excitons in TMDC heterobilayers offers higher strain tunability and new degrees of freedom compared to their monolayer counterparts.

Controllable Magnetic Proximity Effect and Charge Transfer in 2D Semiconductor and Double-Layered Perovskite Manganese Oxide van der Waals Heterostructure.

Optically generated excitonic states (excitons and trions) in transition metal dichalcogenides are highly sensitive to the electronic and magnetic properties of the materials underneath. Modulation and control of the excitonic states in a novel van der Waals (vdW) heterostructure of monolayer MoSe2 on double-layered perovskite Mn oxide ((La0.8 Nd0.2 )1.2 Sr1.8 Mn2 O7 ) is demonstrated, wherein the Mn oxide transforms from a paramagnetic insulator to a ferromagnetic metal. A discontinuous change in the exciton photoluminescence intensity via dielectric screening is observed. Further, a relatively high trion intensity is discovered due to the charge transfer from metallic Mn oxide under the Curie temperature. Moreover, the vdW heterostructures with an ultrathin h-BN spacer layer demonstrate enhanced valley splitting and polarization of excitonic states due to the proximity effect of the ferromagnetic spins of Mn oxide. The controllable h-BN thickness in vdW heterostructures reveals a several-nanometer-long scale of charge transfer as well as a magnetic proximity effect. The vdW heterostructure allows modulation and control of the excitonic states via dielectric screening, charge carriers, and magnetic spins.

Charge‐Accumulation Effect in Transition Metal Dichalcogenide Heterobilayers

Charge transfer in transition-metal-dichalcogenides (TMDs) heterostructures is a prerequisite for the formation of interlayer excitons, which hold great promise for optoelectronics and valleytronics. Charge accumulation accompanied by a charge-transfer process can introduce considerable effect on interlayer exciton-based applications; nevertheless, this aspect has been rarely studied up to date. This work demonstrates how the charge accumulation affects the light emission of interlayer excitons in van der Waals heterobilayers (HBs) consisting of monolayer WSe2 and WS2 . As excitation power increases, the photoluminescence intensity of interlayer excitons increases more rapidly than that of intralayer excitons. The phenomenon can be explained by charge-accumulation effect, which not only increases the recombination probability of interlayer excitons but also saturates the charge-transfer process. This scenario is further confirmed by a careful examination of trion binding energy of WS2 , which nonlinearly increases with the increase of the excitation power before reaching a maximum of about 75 meV. These investigations provide a better understanding of interlayer excitons and trions in HBs, which may provoke further explorations of excitonic physics as well as TMDs-based devices.

Thermally driven reversible photoluminescence modulation in WS2/VO2 heterostructure

We demonstrate reversible modulation of photoluminescence in chemical vapor deposition (CVD) grown WS2 monolayer transferred on VO2 thin films that undergo insulator-metal transition. Thermally driven phase transition in VO2 thin film involving structural change as well as thermal expansion mismatch at interfaces induce strain and able to reversibly tune photoluminescence (PL) of atomic thin WS2 layer. PL intensity is increased with enhancement factor of 1.83 which corresponds to ~250% enhancement when heated from 30 °C to 70 °C. With further increase of temperature, PL intensity is found to be decreasing due to thermal quenching. While interference effect arising from metallic VO2/WS2 interface may also contribute for luminescence enhancement in metallic state, the observed increase in photoluminescence intensity during heating within the insulating state reveals the role of thermal strain. Systematic in-situ Raman and PL measurements revealed role of phase transition in controlling the PL intensity of excitons in WS2 monolayer. Single crystalline VO2 microplates also have been used to confirm the PL intensity modulation across phase transition. The observed direct correlation between photoluminescence and phase change interaction of VO2 with atomic thin layer of WS2 provide novel platform to tune the optical properties for diverse smart photonic applications.

Evidence of exciton dissociation into carriers induced by electric field in rubrene crystals

We investigated an optical carrier injection mechanism in an organic semiconductor rubrene crystal. We simultaneously measured exciton photoluminescence (PL) and photocurrent under an electric field higher than 1 kV/cm at room temperature. An anti-correlation between the exciton PL intensity and photocurrent was observed. This directly indicates that carriers are generated through an exciton dissociation induced by the electric field. The estimated exciton dissociation efficiency is on the order of 10−2 for an excitation wavelength of 442 nm under an electric field of 12 kV/cm at room temperature. In addition, we revealed that both carriers and excitons are trapped at oxidation sites at the surface area of the crystal and more efficiently emit a modified luminescence.

Excitonic States and Photoluminescence Spectra of a Quantum Dot Molecule Exposed to the External Static Electric Field

The influence of a static electric field on the energy levels, wave functions, and optical properties of excitons in a quantum dot molecule is analyzed. Taking into account the cylindrical symmetry of the molecule, the wave functions and excitonic energies are calculated when the external constant electric field is applied along the symmetry axis. Two dots separated by a narrow barrier are considered as one system of three‐dimensional potential wells for an electron and a hole. The six‐dimensional Schrödinger equation is transformed into the equivalent eigenvalue problem given by the system of the coupled two‐dimensional second order differential equations as the separation of the relative‐ and center‐of‐mass motion of an electron and a hole is not possible in this case. The differential equations are solved numerically using finite differences method. Having the eigenfunctions and eigenvalues, the excitonic photoluminescence intensity is calculated and analyzed. As an example, the configuration indicating on the possibility of non‐monotonic behavior of the photoluminescence intensity versus electric field is discussed. The comparison of numerical results with experimental data for the GaInAs quantum dot molecule is presented. Satisfactory agreement with the available experimental data is obtained.

(Invited) Strain and Electromechanical Coupling at the Nanoscale in Atomically Thin Transition Metal Dichalcogenides

The introduction of mechanical strain is a well-established technique for engineering electronic and optical properties in a broad range of semiconductor materials. In atomically thin materials such as graphene and mono- or few-layer transition metal dichalcogenides (TMDCs), the deliberate introduction and engineering of strain becomes an even more powerful approach for controlling material properties due to the high levels of elastic strain that can be accommodated and the feasibility of achieving precise, highly inhomogeneous strain distributions at the nanoscale. Full realization of and control over these possibilities will require the ability to characterize and control strain, and accompanying changes in electronic, optical, electromechanical, or other properties, in such materials at the nanoscale. To this end, we have used a variety of proximal probe microscopy and spectroscopy techniques to characterize strain and associated material properties in TMDC materials with nanoscale spatial resolution. We first describe the use of tip-enhanced Raman spectroscopy (TERS) and tip-enhanced photoluminescence (TEPL) to characterize strain and optical properties of atomically thin MoS2 to which local strain has been applied via transfer onto a nanopatterned substrate. In these studies, plasmonic modes at the apex of a metal-coated scanning probe tip are excited by laser illumination at or near the plasmon resonance wavelength, enabling Raman scattering and photoluminescence signals to be detected from nanoscale volumes with precise positional control. In tip-induced resonant Raman spectroscopy of monolayer and bilayer MoS2, we observe large enhancements in Raman signal levels measured for MoS2 associated with excitation of plasmonic gap modes between an Au-coated probe tip and Au substrate surface onto which MoS2 has been transferred. Transitions in exciton photoluminescence intensity between monolayer and bilayer regions of MoS2 are observed and discussed. Significant differences in nanoscale Raman spectra between monolayer and bilayer MoS2 are also observed. The origins of specific resonant Raman peaks, their dependence on MoS2 layer thickness, and spatial resolution associated with the transition in Raman spectra between monolayer and bilayer regions are described. In TERS and TEPL studies of bilayer MoS2 with inhomogeneous strain created by transfer to a nanopatterned Au-coated substrate, we observe clear shifts in Raman peak positions and intensities which we correlate with MoS2 phonon deformation potentials and strain-induced changes in electronic structure and phonon dispersion. We then present an example of previously unobserved physical behavior that can arise in connection with nanoscale variations in strain in atomically thin TMDC materials, specifically in the area of electromechanical coupling – the interplay between strain and dielectric polarization in materials. Piezoelectricity, in which there is a linear relationship between dielectric polarization (or an applied electric field) and strain in materials lacking inversion symmetry, is the most well-known of such phenomena. Flexoelectricity, in which strain gradients induce a dielectric polarization field (or its converse, in which gradients in an applied electric field induce strain), is present in all materials but has been much less thoroughly studied, in large part due to the difficulty of achieving sufficiently large strain gradients in conventional crystalline materials. Atomically thin TMDCs offer unique opportunities to explore these and related phenomena. For example, in-plane piezoelectricity was theoretically predicted and subsequently observed experimentally to occur in monolayer and odd-layer MoS2, arising due to the reduction in crystal symmetry in the atomically thin limit. Here, we discuss piezoresponse force microscopy measurements in which out-of-plane electromechanical response, which we have tentatively attributed to flexoelectricity, is observed in monolayer MoS2 and other TMDCs. The estimated effective flexoelectric coefficients are found to be very consistent with those predicted by a simple physical model, and lead to electromechanical coupling response comparable in size to piezoelectric effects in the same materials.

Eu modified Cu2O thin films: Significant enhancement in efficiency of photoelectrochemical processes through suppression of charge carrier recombination

We propose a simple way to increase incident photon-to-current conversion efficiency (IPCE, Y) for electrodeposited p-type Cu2O films through addition of Eu(III) to the electrodeposition bath. This is the first reported enhancement of photocurrent for Cu2O modified with a rare-earth element. Our study is based on hypothesis that a large ionic radius of Eu(III) promotes its precipitation in form of inclusions of another phase, which act as getter centers leading to purification of host material from detrimental impurities and, correspondingly, to increase in lifetime of non-equilibrium charge carriers. SEM, EDX and XRD analyses indicate that addition of Eu(III) results in some increase of Cu2O crystallite size and growth of a secondary Eu containing phase without changing the Cu2O lattice parameters. Electrochemical impedance spectroscopy indicates invariance of acceptor concentration and flat band potential for Eu modified films. Remarkable increase of charge carriers’ lifetime, which manifests in the growth of Cu2O exciton photoluminescence (PL) intensity and PL decay constant, leads to increased IPCE for the Eu modified films. Optimization of Eu(III) concentration in the electrodeposition bath allows attaining the cathodic photocurrent as high as 3.2 mA cm−2 (35 mW cm−2 Xe lamp illumination), which is as ≈40% greater than that for the pure Cu2O film and corresponds to Y ≈ 100% at wavelength λ < 450 nm.

Photoluminescence characteristics of CdSe quantum dots: role of exciton–phonon coupling and defect/trap states

In this paper, we report temperature dependent photoluminescence (PL) characteristics of CdSe colloidal QDs with average diameter ~2.8 nm. Temperature dependence of strongly confined exciton PL peak position, linewidth and intensity were investigated in 30 K to 300 K temperature range. Our studies reveal nearly four times weaker exciton-LO phonon coupling than bulk CdSe crystal. Theoretically, it should be vanishingly small due to near identical electron and hole charge distributions in strongly confined QDs. On the other hand, exciton-acoustic phonon coupling is an order of magnitude larger than its bulk counterpart. Observed finite value of exciton-LO phonon coupling and enhanced exciton-acoustic phonon coupling are due to piezoelectric strain fields. PL intensity exhibits anomalous behavior in the temperature range 100–230 K. This has been explained by thermally activated detrapping of the charge carriers trapped in the potential wells formed at the interface adjoining dislocations/stacking faults developed during the synthesis process. Above 230 K, PL is partially quenched by thermal escape of charge carriers from luminescing exciton state to higher lying nonluminescing states.

Robust high-temperature trion emission in monolayers of Mo(SySe1−y)2 alloys

Atomically thin semiconducting transition-metal dichalcogenides enable new insight into physics of many-body effects mediated by Coulomb and electron-phonon interactions. We report photoluminescence (PL) and reflectivity contrast (RC) measurements of excitons ($X$) and trions ($T$) and Raman spectra of phonons in monolayers (MLs) of $\mathrm{Mo}{({\mathrm{S}}_{y}{\mathrm{Se}}_{1\ensuremath{-}y})}_{2}$ alloys with sulfur mole content up to $y=0.5$. We find the phonon energy crossing the trion binding energy with the sulfur mole content. Binary ${\mathrm{MoSe}}_{2}$ and ternary $\mathrm{Mo}{({\mathrm{S}}_{y}{\mathrm{Se}}_{1\ensuremath{-}y})}_{2}$ alloys exhibit contrasting behavior in the temperature evolution of excitons and trions PL intensity from $T=7$ to 295 K. In ${\mathrm{MoSe}}_{2}$, the trion dominates PL spectra at low temperatures but the exciton dominates PL at higher temperature. In contrast, in ternary $\mathrm{Mo}{({\mathrm{S}}_{y}{\mathrm{Se}}_{1\ensuremath{-}y})}_{2}$ alloys trions dominate PL spectra at all measured temperatures, with the trion to exciton PL intensity ratio increasing with sulfur content. We attribute the strong increase of the trion PL intensity in $\mathrm{Mo}{({\mathrm{S}}_{y}{\mathrm{Se}}_{1\ensuremath{-}y})}_{2}$ MLs with increase of sulfur mole content to two effects: (i) strong increase of exciton-trion coupling mediated by the optical phonon, which is realized by tuning phonon energy through trion binding energy, and (ii) significant increase of two-dimensional electron gas concentration. We also demonstrate that increasing sulfur content in $\mathrm{Mo}{({\mathrm{S}}_{y}{\mathrm{Se}}_{1\ensuremath{-}y})}_{2}$ alloys increases total PL intensity at high temperature. With the temperature growth from 7 to 295 K, the total PL in ${\mathrm{MoSe}}_{2}$ decreases about three orders of magnitude more than in $\mathrm{Mo}{({\mathrm{S}}_{0.5}{\mathrm{Se}}_{0.5})}_{2}$.

Manifestation of oxygen desorption in photoluminescence spectra of ZnO

The influence of oxygen desorption on the photoluminescence of zinc oxide has been investigated. Photodesorption and thermal desorption of oxygen have been controlled using mass spectrometry. It has been found that the removal of oxygen from the ZnO surface leads to a manifold increase in the exciton photoluminescence intensity. The process has a reversible character. The mechanism of increase in the exciton luminescence intensity of ZnO during the oxygen desorption has been discussed.

Chemical doping modulation of nonlinear photoluminescence properties in monolayer MoS2

We demonstrate a simple modulation technique of nonlinear optical properties in monolayer (1L) MoS2 via chemical doping. The strong nonlinear behavior of the exciton photoluminescence (PL) intensity is observed with increasing excitation power density for low-electron-density 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ)-doped 1L-MoS2, whereas the exciton PL intensity of as-prepared, heavily electron-doped 1L-MoS2 exhibits weak sublinear behavior. These results are attributable to an enhanced exciton–exciton annihilation rate for the excitons in F4TCNQ-doped 1L-MoS2 as the dominant decay pathway under strong optical excitation conditions.

Transforming bilayer MoS2 into single-layer with strong photoluminescence using UV-ozone oxidation

The use of single-layer MoS2 in light emitting devices requires innovative methods to enhance its low photoluminescence (PL) quantum yield. In this work, we report that single-layer MoS2 with a strong PL can be prepared by oxidizing bilayer MoS2 using UV-ozone oxidation. We show that as compared to mechanically-exfoliated single-layer MoS2, the PL intensity of the single-layer MoS2 prepared by UV-ozone oxidation is enhanced by 20–30 times. We demonstrate that the PL intensity of both neutral excitons and trions (charged excitons) can be greatly enhanced in the oxidized MoS2 samples. These results provide novel insights into the PL enhancement of single-layer MoS2.

Effect of Rare-Earth Doping on Structural and Luminescent Properties of Screen-Printed ZnO Films

The effect of Sm3+ and/or Ho3+ doping on structural and luminescent properties of screen-printed ZnO films sintered at 1000°C was investigated by photoluminescence (PL), PL excitation and Raman scattering methods. For all the films, ultraviolet excitonic and visible defect-related PL bands of ZnO were detected. The doping with Ho3+ ions produced an enhancement of PL in ZnO films, the excitonic PL intensity being increased prominently, while the co-doping with Sm3+ and Ho3+ ions resulted in PL decrease in ZnO films. Only for (Sm,Ho)- co-doped ZnO films, the rare-earth PL bands were detected. The reduction of Sm3+ to Sm2+ was observed demonstrating 5D0→7FJ radiative transitions. The mechanism of PL and PL excitation is discussed in terms of the formation of rare-earth complexes as well as energy transfer towards them from ZnO host.

Influence of the ZnTe barrier width on photoluminescence spectra of CdTe/ZnTe superlattices with layers of quantum dots

The photoluminescence and Raman spectra of CdTe/ZnTe superlattices with layers of CdTe quantum dots have been measured in the temperature range of 5–85 K. It has been found that, in the photoluminescence spectra, there are three emission bands, which are attributed to the luminescence of two-dimensional wetting layers, direct excitons, and spatially indirect excitons in quantum dots, respectively. The influence of the width of the ZnTe barrier layer in the range of 2–50 ML on the energy and width of the emission lines of quantum dots and wetting layer has been analyzed. The temperature dependences of the photoluminescence intensity of direct and indirect excitons have been investigated and the corresponding activation energies of temperature quenching of luminescence have been determined.