Localized Excitons Research Articles

Singlet–triplet splitting energy management for thermally activated delayed fluorescence emitters: up-conversion of acceptor triplet into charge transfer singlet state

Energetic proximity between charge transfer singlet (1CT) and a local exciton triplet (3LE) is critical to facilitate reverse intersystem crossing rate and thus ultimately TADF.

Analysis of localized excitons in strained monolayer WSe2 by first principles calculations.

Although there is growing interest in enhancement of single-photon emission (SPE) in two-dimensional transition metal dichalcogenides, particularly when introducing strain by the generation of wrinkles in monolayer WSe2, understanding the effects of the wrinkle type on the response has been lacking. In this work, by investigating the electronic and optical properties of monolayer WSe2 with wrinkles by first principles calculations, we gain insight into the tunability of the response, where the band gap is found to be modulated by the wrinkle type. Our detailed analyses of the local electronic structures show that the strain distribution from regions with different local strain magnitudes and types affect the band alignments. We demonstrate that introducing a wrinkle in the monolayer results in a red-shift, including the bright A exciton and the lowest energy dark exciton, and the dependence on strain is consistent with available measurements. The energy difference between the A exciton and the lowest mid-gap energy for a single Se vacancy in the wrinkled WSe2 monolayer as a function of strain is consistent with a suggestion on the origin of SPE in monolayer WSe2. Our results will encourage engineering of wrinkle types for enhanced SPE at specific wavelengths, which could potentially originate from hybridization of the localized strained dark exciton and a mid-gap point-defect exciton.

Revealing Excitonic Complexes in Monolayer WS2 on Talc Dielectric

We report on a detailed study of low-temperature photoluminescence (PL) and magnetophotoluminescence under perpendicular magnetic fields (up to 30 T) and circularly polarized excitation on high-quality monolayer (ML) ${\mathrm{WS}}_{2}$ on a thick layer (120 nm) of talc dielectric. Remarkably, we obtained high-quality samples without any evidence of localized exciton emission at low temperature and a PL linewidth comparable to that of ${\mathrm{WS}}_{2}/h$-$\mathrm{BN}$ samples. As a consequence, we observe well-resolved emission peaks at low temperature due to the formation of excitonic complexes, including a dark-trion (DT) state and phonon replicas of the DT without the application of an in-plane magnetic field. The nature of the emission peaks, the magnetic field dependence of the degree of polarization, and g factors are discussed in detail and compared with the corresponding results obtained for $h$-$\mathrm{BN}$ encapsulated transition-metal dichalcogenide (TMD) samples. We observe that under ${\ensuremath{\sigma}}^{+}$-polarized excitation the sign of the circular polarization of biexcitons is reversed under higher magnetic fields. In addition, the dark-trion polarization increases considerably with increasing perpendicular magnetic field, demonstrating different behavior compared with previous studies of dark trions on monolayer ${\mathrm{WSe}}_{2}$. Our results suggest that talc is indeed a promising layered material for the surface protection of ML TMDs and to explore fundamental physics in view of applications in optoelectronic devices.

Deoxyribonucleic Acid Encoded and Size-Defined π-Stacking of Perylene Diimides.

Natural photosystems use protein scaffolds to control intermolecular interactions that enable exciton flow, charge generation, and long-range charge separation. In contrast, there is limited structural control in current organic electronic devices such as OLEDs and solar cells. We report here the DNA-encoded assembly of π-conjugated perylene diimides (PDIs) with deterministic control over the number of electronically coupled molecules. The PDIs are integrated within DNA chains using phosphoramidite coupling chemistry, allowing selection of the DNA sequence to either side, and specification of intermolecular DNA hybridization. In this way, we have developed a “toolbox” for construction of any stacking sequence of these semiconducting molecules. We have discovered that we need to use a full hierarchy of interactions: DNA guides the semiconductors into specified close proximity, hydrophobic–hydrophilic differentiation drives aggregation of the semiconductor moieties, and local geometry and electrostatic interactions define intermolecular positioning. As a result, the PDIs pack to give substantial intermolecular π wave function overlap, leading to an evolution of singlet excited states from localized excitons in the PDI monomer to excimers with wave functions delocalized over all five PDIs in the pentamer. This is accompanied by a change in the dominant triplet forming mechanism from localized spin–orbit charge transfer mediated intersystem crossing for the monomer toward a delocalized excimer process for the pentamer. Our modular DNA-based assembly reveals real opportunities for the rapid development of bespoke semiconductor architectures with molecule-by-molecule precision.

Nanoscale Modification of WS2 Trion Emission by Its Local Electromagnetic Environment

Structural, electronic, and chemical nanoscale modifications of transition metal dichalcogenide monolayers alter their optical properties. A key missing element for complete control is a direct spatial correlation of optical response to nanoscale modifications due to the large gap in spatial resolution between optical spectroscopy and nanometer-resolved techniques. Here, we bridge this gap by obtaining nanometer-resolved optical properties using electron spectroscopy at cryogenic temperatures, specifically electron energy loss spectroscopy for absorption and cathodoluminescence for emission, which are then directly correlated to chemical and structural information. In an h-BN/WS2/h-BN heterostructure, we observe local modulation of the trion (X-) emission due to tens of nanometer wide dielectric patches. Trion emission also increases in regions where charge accumulation occurs, close to the carbon film supporting the heterostructures. The localized exciton emission (L) detected here is not correlated to strain above 1%, suggesting point defects might be involved in their formation.

Probing the carrier recombination mechanism of Cr-doped CsPbCl3 via temperature-dependent PL and TR-PL

In this study, the carrier recombination mechanism of Cr-doped CsPbCl3 is investigated by temperature-dependent photoluminescence (PL) and time-resolved PL (TR-PL). Because of non-uniform doping with Cr, exciton localization is observed. This behavior can be demonstrated not only by temperature-dependent PL, where emission intensity rapidly increases at a temperature of less than 100 K, but also by temperature-dependent TR-PL, where temperature dependency of the radiative lifetime exhibits a linear dependence at temperatures ranging from 20 K to 100 K. Furthermore, PL decay lifetimes of Cr-doped CsPbCl3 at 20 K as a function of monitored emission energies are investigated. Results reveal that the excitons of Cr-doped CsPbCl3 exhibit a large localization depth (Eloc). The methods employed herein are anticipated to open new avenues with immense characterization ability for examining the emission properties of inorganic lead trihalide perovskites.

Single photon generation from AlGaN exciton localization centers exhibiting narrow spectral linewidths

We report the discovery and characterization of single-photon-emitting carrier localization centers that are spontaneously formed along misfit dislocations in AlGaN. The emitters exhibit extremely narrow linewidths, which are in some cases narrower than our resolution limit of 35 µeV. Spectral analysis reveals a record-low inhomogeneous broadening (smaller than 20 µeV), which can be characterized as almost spectral-diffusion free. Such narrow linewidths allow for an unprecedented discussion of the homogeneous linewidths of quantum emitters in the III-nitrides and, in the current case, provide a lower bound on the pure-dephasing time T2 of ∼200 ps. These experimental results will pave the way to further improve the performance of III-nitride low-dimensional nanostructure-based quantum emitters.

The Role of Subsurface Valence Band Localization in the Passivation of Perovskite Nanocrystals

AbstractTraditional colloidal quantum dots rely on larger‐bandgap semiconductor shells for the localization of excitons in their emissive core. Unshelled quantum dots are typically weakly luminescent due to severe quenching by deep electronic traps. Lead halide perovskite nanocrystals deviate from this convention and require only common monofunctional aliphatic ligands on their surfaces to attain near‐unity luminescence quantum yields. Here, a combination of density functional theory calculations and experimental studies is employed to elucidate the mechanistic role of the ligands. The attachment of a surface ligand leads to a distortion of the surface, which results in the localization of the valence band deeper into the subsurface layers. This creates an energetic profile that is similar to a conventional core–shell quantum dot and renders the valence electrons unavailable for interaction with adventitious chemical species. It is further shown that a phosphonate moiety offers significantly stronger binding and leads to the long‐term preservation of this beneficial subsurface localized band structure. As experimental proof, a red‐emitting mixed‐halide perovskite nanocrystal, which is usually susceptible to degradation and spectral shifts, exhibits remarkable stability under a phosphonate ligand shell. These findings and mechanistic insights may guide the future engineering of robust perovskite nanocrystals for color displays and light‐emitting applications.

Different optical characteristics between monolayer and bilayer WS2 due to interlayer interaction

In this research Raman spectroscopy and photoluminescence (PL) spectroscopy measurements reveal different optical characteristics between monolayer (1 L) and bilayer (2 L) WS2 due to the interlayer interaction. First, ultralow frequency (ULF) Raman spectra indicate that 2 L WS2 shows additional shear mode (S) and layer-breathing (LB) mode in comparison with 1 L WS2. The 2 L WS2 has the in-plane vibration mode E2g1 red-shifted while the out-plane vibration mode A1g blue-shifted. Then, both direct transition (DT) and indirect transition (IT) emission are observed in the PL spectra for 2 L WS2 while only DT for 1 L WS2. The PL signal corresponding to localized excitons, that is measured as a low energy side tail with the DT emission, is more prominent for 1 L WS2 at low temperature of 7 K. The DT emission and IT emission show opposite variation trend as the temperature increases from 7 K to 300 K, while the average phonon energy in 2 L WS2 is estimated to be almost twice of that for 1 L WS2. These observations illustrate that the interlayer interaction not only change the phonon characteristics for WS2, but also impact the energy band structure and the exciton dynamics, including exciton radiative recombination and exciton-phonon interaction.

Resonance energy transfer from moiré-trapped excitons in MoSe2/WSe2 heterobilayers to graphene: Dielectric environment effect

Moir\'e excitons in twisted heterobilayer (HBL) transition metal dichalcogenides (TMDs) exhibit an interesting platform for engineering optical properties. The screening from the dielectric environment surrounding the HBL and the twist angle, arising from vertically stacking two layered TMD materials, allow the possibility to control and tune the interlayer exciton states. In this paper, we show in the first part that interlayer excitons in $\mathrm{Mo}{\mathrm{Se}}_{2}/\mathrm{W}{\mathrm{Se}}_{2}$ HBL localize due to the moir\'e potential arising within the $\text{TMD} \text{HBL}$ and discuss the conditions for which each localization site can be viewed as a local quantum emitter. In the second part, we show that the trapped exciton energy can be transferred, via the F\"orster resonance energy transfer (FRET) mechanism, to an adjacent two-dimensional sheet of doped graphene (Gr). The analysis of the localized exciton quenching rate is provided within the random-phase approximation, which allows one to access the collective electronic behavior within the Gr layer. Our model demonstrates the high sensitivity of FRET efficiency to external parameters, particularly the dielectric environment and the distance between the donor---here the HBL---and the acceptor (Gr). These properties show that FRET can be quite a useful tool for applications in biological systems and nanoscale sensors.

Exciton luminescence and many-body effect of monolayer WS2 at room temperature

Monolayer transition metal dichalcogenides favor the formation of a variety of excitonic quasiparticles, and can serve as an ideal material for exploring room-temperature many-body effects in two-dimensional systems. Here, using mechanically exfoliated monolayer WS2 and photoluminescence (PL) spectroscopy, exciton emission peaks are confirmed through temperature-dependent and electric-field-tuned PL spectroscopy. The dependence of exciton concentration on the excitation power density at room temperature is quantitatively analyzed. Exciton concentrations covering four orders of magnitude are divided into three stages. Within the low carrier concentration stage, the system is dominated by excitons, with a small fraction of trions and localized excitons. At the high carrier concentration stage, the localized exciton emission from defects coincides with the emission peak position of trions, resulting in broad spectral characteristics at room temperature.

Co-Doping of Cerium and Bismuth into Lead-Free Double Perovskite Cs2AgInCl6 Nanocrystals Results in Improved Photoluminescence Efficiency.

Lead-free double cation metal halide perovskites have recently attracted considerable attention, with continuing research efforts focusing on the improvement of their stability and photoluminescence quantum yield (PL QY). In this study, Ce3+ has been co-doped together with Bi3+ into lead-free double perovskite Cs2AgInCl6 nanocrystals (NCs) in order to improve their crystallinity and PL QY. Both uncoordinated chloride ions and silver vacancies could be eliminated using this co-doping strategy, and the resulting Ce3+,Bi3+-co-doped Cs2AgInCl6 NCs showed adjustable PL emission peaks in the range of 589 to 577 nm by varying the doping amount of Ce3+ with a fixed feeding ratio of bismuth precursor set at 1%. Cs2AgInCl6 NCs doped with 1% Bi alone reached a PL QY of 10% for the PL peak centered at 591 nm, while those co-doped with 1% Bi and 2% Ce together achieved the highest PL QY of 26% for the PL peak centered at 580 nm. The use of Ce3+ as a dopant promoted the localization of self-trapped excitons to prevent PL quenching, although the ion's 5d excited state may potentially provide an energetically favorable indirect route for the radiative relaxation process. This also resulted in a blue shift of the PL maximum and increased the exciton binding energy, thus promoting the radiative recombination of self-trapped excitons.

Phonon scattering and exciton localization: molding exciton flux in two dimensional disorder energy landscape

Two dimensional excitonic devices are of great potential to overcome the dilemma of response time and integration in current generation of electron or/and photon based systems. The ultrashort diffusion length of exciton arising from ultrafast relaxation and low carrier mobility greatly discounts the performance of excitonic devices. Phonon scattering and exciton localization are crucial to understand the modulation of exciton flux in two dimensional disorder energy landscape, which still remain elusive. Here, we report an optimized scheme for exciton diffusion and relaxation dominated by phonon scattering and disorder potentials in WSe2 monolayers. The effective diffusion coefficient is enhanced by > 200% at 280 K. The excitons tend to be localized by disorder potentials accompanied by the steadily weakening of phonon scattering when temperature drops to 260 K, and the onset of exciton localization brings forward as decreasing temperature. These findings identify that phonon scattering and disorder potentials are of great importance for long-range exciton diffusion and thermal management in exciton based systems, and lay a firm foundation for the development of functional excitonic devices.

Forming a dielectric exciton phase in strongly correlated systems with spin crossover

Formation of the magnetic structure and exciton condensate of local magnetic excitons in strongly correlated systems near the spin crossover under high pressure is considered in terms of the effective Hamiltonian obtained from a two-band Hubbard model in the regime of strong electron correlations. The coexistence of the long-range antiferromagnetic order and exciton condensate and the occurrence of the magnetization caused by the condensation of local magnetic excitons are demonstrated. Transformation of the electronic structure of the antiferromagnetic high-spin insulator into the paramagnetic two-band correlated metal via a narrow-gap antiferromagnetic excitonic semiconductor is obtained.

Imaging Charged Exciton Localization in van der Waals WSe2/MoSe2 Heterobilayers.

Exciton localization in transition-metal dichalcogenide monolayers is behind a variety of interesting phenomena and applications, including broad-spectrum solar cells and single-photon emissions. Strain fields at the periphery of topographically distinct features such as nanoscopic bubbles were recently associated with localized charge-neutral excitons. Here, we use tip-enhanced photoluminescence (PL) to visualize excitons in WSe2/MoSe2 heterobilayers (HBL). We find strong optical emission from charged excitons, particularly positively charged trions, in HBL supported by interlayer charge transfer. Our results reveal strong trion confinement, with a localization length scale comparable to the trion size, at the apex region inside individual nanoscopic bubbles. Nano-PL mapping also shows sub-10-nm spatial variations in the localized trion emission spectra, which stem from atomic-scale potential energy fluctuations. These findings demonstrate the possibility of confining charged exciton complexes that are electrically tunable, opening up further opportunities to probe many-body exciton physics and to explore additional possible sites for strong exciton localization that can lead to quantum emission.

Abnormal temperature-dependent photoluminescence characteristics of ReS2 nanowalls

Two samples with [001] orientated rhenium disulfide (ReS2) nanowalls (NWs) grown above and in front of precursor (NH4ReO4) by chemical vapor deposition were investigated. The temperature-dependent photoluminescence (PL) indicated that the PL peak exhibited linear blue-shift at a rate of ∼0.24 meV K−1 with increasing the temperature from 10 to 300 K, while the linewidth monotonically increased due to the exciton–phonon interaction. This abnormal blue-shift of PL emission energy, which is explained by a competition between the band gap shrinkage and the energy level degeneracy with respect to the increase of temperature and lattice constant, enables ReS2 NWs to possess great potential for development of thermal sensors. In addition, exciton localization effect in the ReS2 NWs from abundant edges and weak interlayer interaction was also observed to be related to the height and density of ReS2 NWs. These results not only enrich the understanding for exciton dynamics in ReS2 NWs, but also help to exploit ReS2 NWs for device applications.

Energy diagram and parameters regarding localized states in InGaN/GaN nanocolumns

The parameters related to the localized states in green-emitting indium gallium nitride (InGaN) have been evaluated by considering the energy diagrams derived by five different methods: (1) the exponential tail of the low-energy side of photoluminescence (PL) spectra, (2) the photon energy dependence of PL decay time, (3) excitation energy dependence of the PL peak energy, (4) the PL excitation spectrum, and (5) the temperature dependence of PL peak energy. The results indicate that the energy diagram of InGaN is divided into four regions: deep localized states, migration region, transition region, and extended states. It is suggested that wider localized states and a narrower transition region are preferable in order to achieve higher PL efficiency. In addition, the dependence of carrier density on PL properties supports the fact of photo-generated carriers forming localized excitons in green-emitting InGaN, although the carriers do not form localized excitons in orange-emitting InGaN and instead exist as localized electrons and holes.

Influence of direct deposition of dielectric materials on the optical response of monolayer WS2

We investigate the effects of direct deposition of different dielectric materials (AlOx, SiOx, SiNx) onto atomically thin TMDC WS2 on its optical response using atomic layer deposition (ALD), electron beam evaporation (EBE), plasma-enhanced chemical vapor deposition (PECVD), and magnetron sputtering. The photoluminescence measurements reveal quenching of the excitonic emission after all deposition processes, which is linked to the increased level of charge doping and associated rise of the trion emission and/or the localized (bound) exciton emission. Furthermore, Raman spectroscopy allows us to clearly correlate the observed changes in excitonic emission with the increased levels of lattice disorder and defects. In particular, we show that the different doping levels in a monolayer WS2 capped by a dielectric material are strongly related to the defects in the WS2 crystal introduced by all capping methods, except for ALD. The strong charge doping in the ALD-capped sample seems to be caused by other factors, such as deviations in the dielectric layer stoichiometry or chemical reactions on the monolayer surface, which makes ALD distinct from all other techniques. Overall, the EBE process results in the lowest level of doping and defect densities and in the largest spectral weight of the exciton emission in the PL. Sputtering is revealed as the most aggressive dielectric capping method for WS2, fully quenching its optical response. Our results demonstrate and quantify the effects of direct deposition of dielectric materials onto monolayer WS2, which can provide valuable guidance for the efforts to integrate monolayer TMDCs into functional optoelectronic devices.

Localized polariton states in a photonic crystal intercalated by a transition metal dichalcogenide monolayer

Beyond the extensively studied microcavity polaritons, which are coupled modes of semiconductor excitons and microcavity photons, nearly 2D semiconductors placed in a suitable environment can support spatially localized exciton–polariton modes. We demonstrate theoretically that two distinct types of such modes can exist in a photonic crystal with an embedded transition metal dichalcogenide (TMD) monolayer and derive an equation that determines their dispersion relations. The localized modes of two types occur in the zeroth- and first-order stop bands of the crystal and have substantially different properties. The latter type of localized modes, which appear inside the light cone, can be described as a result of coupling of the TMD exciton and an optical Tamm state of the TMD-intercalated photonic crystal. We suggest an experiment for detecting these modes and simulate it numerically.

Defect-Enhanced Exciton–Exciton Annihilation in Monolayer Transition Metal Dichalcogenides at High Exciton Densities

The exciton–exciton annihilation (EEA) process easily occurs in monolayer transition metal dichalcogenides (TMDs) because of the strong Coulomb interaction and quantum confinement effect, which enhance the many-body interaction of excitons. This process can affect the performance of the optoelectronic devices. It is crucial to examine the effect of defect states on the EEA process and determine whether it is comparable to that at the low excitation intensities, particularly when applied to laser devices at a high exciton density. In this study, femtosecond transient absorption spectroscopy was used to explore the EEA process of four types of CVD-grown monolayer TMDs (i.e., WS2, WSe2, MoS2, and MoSe2). We demonstrated that the defect-assisted EEA process of local excitons is enhanced and plays a key role in the exciton relaxation process at high exciton densities of approximately 1012 cm–2 below a Mott density of approximately 1013 cm–2. The measured EEA rates for WS2, WSe2, MoSe2, and MoS2 were 0.016, 0.026, 0.049, and 0.102 cm2/s, respectively, implying that EEA is enhanced as defect states increase in monolayer TMDs. Our results provide a profound insight into the effect of defect states on the EEA process in monolayer TMDs at high exciton densities.