Narrow Photoluminescence Linewidth Research Articles

Chemically Tailored Growth of 2D Semiconductors via Hybrid Metal-Organic Chemical Vapor Deposition.

Two-dimensional (2D) semiconducting transition-metal dichalcogenides (TMDCs) are an exciting platform for excitonic physics and next-generation electronics, creating a strong demand to understand their growth, doping, and heterostructures. Despite significant progress in solid-source (SS-) and metal-organic chemical vapor deposition (MOCVD), further optimization is necessary to grow highly crystalline 2D TMDCs with controlled doping. Here, we report a hybrid MOCVD growth method that combines liquid-phase metal precursor deposition and vapor-phase organo-chalcogen delivery to leverage the advantages of both MOCVD and SS-CVD. Using our hybrid approach, we demonstrate WS2 growth with tunable morphologies─from separated single-crystal domains to continuous monolayer films─on a variety of substrates, including sapphire, SiO2, and Au. These WS2 films exhibit narrow neutral exciton photoluminescence line widths down to 27-28 meV and room-temperature mobility up to 34-36 cm2 V-1 s-1. Through simple modifications to the liquid precursor composition, we demonstrate the growth of V-doped WS2, MoxW1-xS2 alloys, and in-plane WS2-MoS2 heterostructures. This work presents an efficient approach for addressing a variety of TMDC synthesis needs on a laboratory scale.

Enhanced optical properties of MoSe2 grown by molecular beam epitaxy on hexagonal boron nitride

Transition metal dichalcogenides (TMDs) like MoSe2 exhibit remarkable optical properties such as intense photoluminescence (PL) in the monolayer form. To date, narrow-linewidth PL is only achieved in micrometer-sized exfoliated TMD flakes encapsulated in hexagonal boron nitride (hBN). In this work, we develop a growth strategy to prepare monolayer MoSe2 on hBN flakes by molecular beam epitaxy in the van der Waals regime. It constitutes the first step toward the development of large area single crystalline TMDs encapsulated in hBN for potential integration in electronic or optoelectronic devices. For this purpose, we define a two-step growth strategy to achieve monolayer-thick MoSe2 grains on hBN flakes. The high quality of MoSe2 allows us to detect very narrow PL linewidth down to 5.5 meV at 13 K, comparable to the one of encapsulated exfoliated MoSe2 flakes. Moreover, sizeable PL can be detected at room temperature as well as clear reflectivity signatures of A, B, and charged excitons.

InAs quantum dots with a narrow photoluminescence linewidth for a lower threshold current density in 1.55 µm lasers

Uniform quantum dots (QDs) with a narrowed linewidth of photoluminescence (PL) are crucial for developing high-performance QD lasers. This study focuses on optimizing the growth conditions of InAs QDs on (001) InP substrates using metal-organic chemical vapor deposition (MOCVD), targeting applications in 1.55 µm QD lasers. By fine-tuning growth parameters such as the V/III ratio, deposition thickness, and growth temperature, we attained a QD density of 4.13 × 1010 cm−2. Further, a narrowed PL full width at half maximum (FWHM) of 40.1 meV was achieved in a five-stack InAs QD layer. This was accomplished using the double-cap technique, which reduced the height dispersion of QDs and shifted the emission wavelength to 1577 nm. Broad-area lasers incorporating a five-stack optimized InAs/InAlGaAs structure demonstrated a low threshold current density of 80 A/cm2 per QD layer, and a saturation power of 163 mW in continuous-wave (CW) mode at room temperature.

Excitons in (Al,Ga)N quantum dots and quantum wells grown on (0001)-oriented AlN templates: Emission diagrams and valence band mixings

The luminescence efficiency of AlxGa1−xN quantum dots (QDs) and quantum wells (QWs), buried in AlN cladding layers and emitting in the ultraviolet range between 234 and 310 nm, has been investigated. The growth and optical properties have been done using similar aluminum composition (varying from 0.4 to 0.75) for both QDs and QWs. In order to compare as much as possible the optical properties, the QWs were fabricated with a growth time tuned such that the QW width is similar to the average height of the QDs. The photoluminescence (PL) showed emission ranging from 4 to 5.4 eV, putting into evidence differences in terms of full width at half maximum, PL intensity, and asymmetry of the line shape between QDs and QWs. The results show shorter wavelengths and a slightly narrower PL linewidth for QWs. To determine the light emission dependence with the electric field direction in the crystal, the evolutions of the emission diagrams for all samples were recorded along two orthogonal directions, namely, the “in-plane” (growth) and the “on-side” directions, from which the light emission was collected. For the whole QDs and QWs samples' series, the shapes of the emission diagram indicate emission in both in-plane and on-side directions, as evidenced by intra-valence band mixings caused by strain effects combined with the anisotropic Coulomb interactions that are particularly contributing to the polarization at wavelengths below 260 nm. Furthermore, the degree of polarization, determined for each sample, showed good agreement with results from the literature.

Prolonged dephasing time of ensemble of moiré-trapped interlayer excitons in WSe2-MoSe2 heterobilayers

The moiré superlattices of transition metal dichalcogenide heterobilayers have a pronounced effect on the optical properties of interlayer excitons (IXs) and have been intensively studied in recent years. However, the impact of moiré potentials on the temporal coherence of the IXs has not yet been investigated in detail. Here, we systematically investigate the coherence properties of both the ensemble of delocalized and the ensemble of localized IXs trapped in moiré potentials of the hexagonal boron nitride encapsulated WSe2-MoSe2 heterostructures. Our low-temperature first-order correlation measurements show that prolonged T2 dephasing times with values up to 730 fs can be obtained from the ensemble of localized IXs under moderate pump powers. We observed up to almost a five-fold increase over the values we obtained from the delocalized IXs, while more than two-fold over the previously reported values of T2 ~ 300 fs from the delocalized IXs. The prolonged values of T2 dephasing times and narrow photoluminescence (PL) linewidths for the ensemble of moiré-trapped IXs compared to delocalized one indicate that dephasing mechanisms caused by exciton-low energy acoustic phonon and exciton-exciton scattering are significantly suppressed due to the presence of localization potentials. Our pump power-dependent T2 results show that ultra-long dephasing times can be expected if the dephasing time measurements are performed with the narrow photoluminescence emission line of a single moiré-trapped IX at a low pump power regime. The prolonged values of IX dephasing times would be critical for the applications of quantum information science and the development of two-dimensional material-based nanolasers.

Design Rules for Obtaining Narrow Luminescence from Semiconductors Made in Solution.

Solution-processed semiconductors are in demand for present and next-generation optoelectronic technologies ranging from displays to quantum light sources because of their scalability and ease of integration into devices with diverse form factors. One of the central requirements for semiconductors used in these applications is a narrow photoluminescence (PL) line width. Narrow emission line widths are needed to ensure both color and single-photon purity, raising the question of what design rules are needed to obtain narrow emission from semiconductors made in solution. In this review, we first examine the requirements for colloidal emitters for a variety of applications including light-emitting diodes, photodetectors, lasers, and quantum information science. Next, we will delve into the sources of spectral broadening, including "homogeneous" broadening from dynamical broadening mechanisms in single-particle spectra, heterogeneous broadening from static structural differences in ensemble spectra, and spectral diffusion. Then, we compare the current state of the art in terms of emission line width for a variety of colloidal materials including II-VI quantum dots (QDs) and nanoplatelets, III-V QDs, alloyed QDs, metal-halide perovskites including nanocrystals and 2D structures, doped nanocrystals, and, finally, as a point of comparison, organic molecules. We end with some conclusions and connections, including an outline of promising paths forward.

Narrow Near-Infrared Emission from InP QDs Synthesized with Indium(I) Halides and Aminophosphine.

Nonpyrophoric aminophosphines reacted with indium(III) halides in the presence of zinc chloride have emerged as promising phosphorus precursors in the synthesis of colloidal indium phosphide (InP) quantum dots (QDs). Nonetheless, due to the required P/In ratio of 4:1, it remains challenging to prepare large-sized (>5 nm), near-infrared absorbing/emitting InP QDs using this synthetic scheme. Furthermore, the addition of zinc chloride leads to structural disorder and the formation of shallow trap states inducing spectral broadening. To overcome these limitations, we introduce a synthetic approach relying on the use of indium(I) halide, which acts as both the indium source and reducing agent for aminophosphine. The developed zinc-free, single-injection method gives access to tetrahedral InP QDs with an edge length > 10 nm and narrow size distribution. The first excitonic peak is tunable from 450 to 700 nm by changing the indium halide (InI, InBr, InCl). Kinetic studies using phosphorus NMR reveal the coexistence of two reaction pathways, the reduction of transaminated aminophosphine by In(I) and via redox disproportionation. Etching the surface of the obtained InP QDs at room temperature with in situ-generated hydrofluoric acid (HF) leads to strong photoluminescence (PL) emission with a quantum yield approaching 80%. Alternatively, surface passivation of the InP core QDs was achieved by low-temperature (140 °C) ZnS shelling using the monomolecular precursor zinc diethyldithiocarbamate. The obtained InP/ZnS core/shell QDs that emit in a range of 507-728 nm exhibit a small Stokes shift (110-120 meV) and a narrow PL line width (112 meV at 728 nm).

Ytterbium-Doped Lead-Halide Perovskite Nanocrystals: Synthesis, Near-Infrared Emission, and Open-Source Machine Learning Model for Prediction of Optical Properties.

Lead-halide perovskite nanocrystals are an attractive class of materials since they can be easily fabricated, their optical properties can be tuned all over the visible spectral range, and they possess high emission quantum yields and narrow photoluminescence linewidths. Doping perovskites with lanthanides is one of the ways to widen the spectral range of their emission, making them attractive for further applications. Herein, we summarize the recent progress in the synthesis of ytterbium-doped perovskite nanocrystals in terms of the varying synthesis parameters such as temperature, ligand molar ratio, ytterbium precursor type, and dopant content. We further consider the dependence of morphology (size and ytterbium content) and optical parameters (photoluminescence quantum yield in visible and near-infrared spectral ranges) on the synthesis parameters. The developed open-source code approximates those dependencies as multiple-parameter linear regression and allows us to estimate the value of the photoluminescence quantum yield from the parameters of the perovskite synthesis. Further use and promotion of an open-source database will expand the possibilities of the developed code to predict the synthesis protocols for doped perovskite nanocrystals.

An alloyed mid-shell strategy assisted realization of thick-shell violet-blue ZnSe/ZnSexS1-x/ZnS quantum dots with high color purity

Environmentally friendly zinc selenide (ZnSe) based core/shell quantum dots (QDs) are promising emitting materials for special light sources and displays. However, ZnSe-based core/shell QDs with violet-blue emission that simultaneously possess thick shells and narrow emission linewidths are still challenging thus far. We herein use a gradient ZnSexS1-x/ZnS shell strategy to target this goal. The ZnSexS1-x mid shell helps the lattice strain relaxation during the growth, whereas the wide bandgap ZnS outer shell benefits the optical properties by passivating the emitting cores and improving the quantum confinement effect. These ZnSe core QDs show a unity zinc blende structure and high crystallinity. The optimized ZnSe/ZnSexS1-x/ZnS QDs exhibit tunable emission wavelengths (415–445 nm), narrow photoluminescence linewidths (10.2–14.2 nm), thick shells (>3.9 nm), and high photoluminescence quantum yields (>50%). We postulate that these excellent features are due to the use of alloyed ZnSexS1-x layer at the core/shell interfaces. This study provides a feasible idea for the development of blue ZnSe-based QDs with perfect emitting properties.

Low-defect-density ZnO homoepitaxial films grown by low-temperature ALD

We report atomic layer deposition (ALD) of ZnO thin films on O-polar surface crystalline ZnO substrates at the relatively low temperatures of 120, 150, and 200 °C. The as-grown ZnO films are studied with aberration-corrected transmission electron microscopy and diffraction contrast, photoluminescence (PL), and surface photovoltage (SPV) spectroscopy. We find that the homoepitaxial films have a monocrystalline structure with the density of basal stacking faults comparable to that of the substrate (∼1011 cm−2) and that the stacking faults can induce high lattice strain due to their interaction with the inversion domain boundaries. The narrow excitonic PL linewidth (2 meV at 8 K) and the sharp SPV bandgap transition confirm the high quality of the ZnO films. Despite similarities in the film properties, the growth temperature has an effect on the density and spatial distribution of intrinsic defects. Our results demonstrate a considerable potential of ALD ZnO homoepitaxy for fabricating high-quality ZnO nanostructures and attaining viable p-type ZnO.

Quantum-Dot Based Vertical External-Cavity Surface-Emitting Lasers With High Efficiency

We demonstrate an optically pumped vertical external-cavity surface-emitting laser (VECSEL) based on quantum dots (QDs) with a high optical gain. The photoluminescence (PL) emission of the QDs at the operating wavelength of 1030 nm has been optimized towards a high integrated intensity by using a dot density in the range of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> and a narrow PL linewidth of around 30 meV due to an improved size homogeneity. Additionally, post-growth rapid thermal annealing at 650 °C for 45 s was found to reduce the non-radiative recombination centers and further improve the output power efficiency. During this study, the number of QD layers, the heat sink temperature and the heat spreader material were analyzed. The best results with a record slope-efficiency of 35% and a maximum continuous-wave output power of 5.7 W could be achieved with an RTA treated QD based VECSEL containing 24 QD layers in the gain section. The laser was processed with a solder-based flip-chip bonding technique on a diamond heat sink and temperature stabilized at 4 °C. Furthermore, an excellent symmetric beam profile with M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> = 1.12 was achieved.

Highly anisotropic excitons and multiple phonon bound states in a van der Waals antiferromagnetic insulator.

Two-dimensional (2D) semiconductors enable the investigation of light-matter interactions in low dimensions1,2. Yet, the study of elementary photoexcitations in 2D semiconductors with intrinsic magnetic order remains a challenge due to the lack of suitable materials3,4. Here, we report the observation of excitons coupled to zigzag antiferromagnetic order in the layered antiferromagnetic insulator NiPS3. The exciton exhibits a narrow photoluminescence linewidth of roughly 350 μeV with near-unity linear polarization. When we reduce the sample thickness from five to two layers, the photoluminescence is suppressed and eventually vanishes for the monolayer. This suppression is consistent with the calculated bandgap of NiPS3, which is highly indirect for both the bilayer and the monolayer5. Furthermore, we observe strong linear dichroism (LD) over a broad spectral range. The optical anisotropy axes of LD and of photoluminescence are locked to the zigzag direction. Furthermore, their temperature dependence is reminiscent of the in-plane magnetic susceptibility anisotropy. Hence, our results indicate that LD and photoluminescence could probe the symmetry breaking magnetic order parameter of 2D magnetic materials. In addition, we observe over ten exciton-A1g-phonon bound states on the high-energy side of the exciton resonance, which we interpret as signs of a strong modulation of the ligand-to-metal charge-transfer energy by electron-lattice interactions. Our work establishes NiPS3 as a 2D platform for exploring magneto-exciton physics with strong correlations.

Electrical and optical properties of transition metal dichalcogenides on talc dielectrics.

Advanced van der Waals (vdW) heterostructure devices rely on the incorporation of high quality dielectric materials which need to possess a low defect density as well as being atomically smooth and uniform. In this work we explore the use of talc dielectrics as a potentially clean alternative substrate to hexagonal boron nitride (hBN) for few-layer transition metal dichalcogenide (TMDC) transistors and excitonic TMDC monolayers. We find that talc dielectric transistors show small hysteresis which does not depend strongly on sweep rate and show negligible leakage current for our studied dielectric thicknesses. We also show narrow photoluminescence linewidths down to 10 meV for different TMDC monolayers on talc which highlights that talc is a promising material for future van der Waals devices.

Polarized emission from single perovskite FAPbBr3 nanocrystals

Owing to the superior performance of lead-halide perovskites in various optoelectronic devices, their low-dimensional counterparts of quantum-confined nanocrystals (NCs) have started drawing intensive research attention. Here we report a systematic study on the optical properties of single FAPbBr3 NCs, which demonstrate high-purity single-photon emission, large absorption cross-section and narrow photoluminescence linewidth. Interestingly, linearly-polarized emission can be partially observed in single FAPbBr3 NCs at the room temperature, the degree of which is significantly enhanced at the cryogenic temperature. The above polarization phenomenon is attributed to the large energy-level splitting of the bright-exciton states, leading to efficient exciton recombination from the lowest state with a 1D dipole moment. This unique feature of linear polarization in the optical emission of single FAPbBr3 NCs has not only provided a deep understanding of their exciton energy-level structures, but also suggested potential polarization-oriented applications such as in photo-detectors, light-emitting diodes and lasers.

Observation of Electron Shakeup in CdSe/CdS Core/Shell Nanoplatelets.

While ensembles of CdSe nanoplatelets (NPLs) show remarkably narrow photoluminescence line widths at room temperature, adding a CdS shell to increase their fluorescence efficiency and photostability causes line width broadening. Moreover, ensemble emission spectra of CdSe/CdS core/shell NPLs become strongly asymmetric at cryogenic temperatures. If the origin of these effects were understood, this could potentially lead to stable core/shell NPLs with narrower emission, which would be advantageous for applications. To move in this direction, we report time-resolved emission spectra of individual CdSe/CdS core/shell NPLs at 4 K. We observe surprisingly complex emission spectra that contain multiple spectrally narrow emission features that change during the experiment. With machine-learning algorithms, we can extract characteristic peak energy differences in these spectra. We show that they are consistent with electron "shakeup lines" from negatively charged trions. In this process, an electron-hole pair recombines radiatively but gives part of its energy to the remaining electron by exciting it into a higher single-electron level. This "shakeup" mechanism is enabled in our NPLs due to strong exciton binding and weak lateral confinement of the charge carriers. Time-resolved single-photon-counting measurements and numerical calculations suggest that spectral jumps in the emission features originate from fluctuations in the confinement potential caused by microscopic structural changes on the NPL surface (e.g., due to mobile surface charges). Our results provide valuable insights into line width broadening mechanisms in colloidal NPLs.

Influence of Pb vs Ga solvents during liquid phase epitaxy on the optical and electrical properties of GaSbBi layers

We have investigated the influence of Pb vs Ga solvents during the liquid phase epitaxial growth of GaSbBi layers on the overall physical, optical, and electrical properties of the material. 10 K photoluminescence (PL) measurements show narrower PL linewidths in layers grown from the Pb solvent compared to that obtained for layers grown under similar conditions using Ga as the solvent. This is an indication of the improved crystalline quality of the material grown from the Pb solvent. GaSbBi layers grown from Pb solvents show a reduction of the carrier concentration by over one order of magnitude than that measured in the corresponding materials grown from Ga solvents. Detailed analysis of the core level transitions, revealed by X-ray photoelectron spectroscopy, indicates the generation of a lesser number of VGa and antisite GaSb defects responsible for the residual hole concentration in GaSb and GaSbBi layers grown from Pb solvents than in the same materials grown using Ga as the solvent. This is suggested as the main reason for the observed reduction of carrier concentration in the layers grown from the Pb solvent.

Tuning and Probing the Distribution of Cu+ and Cu2+ Trap States Responsible for Broad-Band Photoluminescence in CuInS2 Nanocrystals.

The processes that govern radiative recombination in ternary CuInS2 (CIS) nanocrystals (NCs) have been heavily debated, but recently, several research groups have come to the same conclusion that a photoexcited electron recombines with a localized hole on a Cu-related trap state. Furthermore, it has been observed that single CIS NCs display narrower photoluminescence (PL) line widths than the ensemble, which led to the conclusion that within the ensemble there is a distribution of Cu-related trap states responsible for PL. In this work, we probe this trap-state distribution with in situ photoluminescence spectroelectrochemistry. We find that Cu2+ states result in individual “dark” nanocrystals, whereas Cu+ states result in “bright” NCs. Furthermore, we show that we can tune the PL position, intensity, and line width in a cyclic fashion by injecting or removing electrons from the trap-state distribution, thereby converting a subset of “dark” Cu2+ containing NCs into “bright” Cu+ containing NCs and vice versa. The electrochemical injection of electrons results in brightening, broadening, and a red shift of the PL, in line with the activation of a broad distribution of “dark” NCs (Cu2+ states) into “bright” NCs (Cu+ states) and a rise of the Fermi level within the ensemble trap-state distribution. The opposite trend is observed for electrochemical oxidation of Cu+ states into Cu2+. Our work shows that there is a direct correlation between the line width of the ensemble Cu+/Cu2+ trap-state distribution and the characteristic broad-band PL feature of CIS NCs and between Cu2+ cations in the photoexcited state (bright) and in the electrochemically oxidized ground state (dark).

Approaching the intrinsic photoluminescence linewidth in transition metal dichalcogenide monolayers

Excitonic states in monolayer transition metal dichalcogenides (TMDCs) have been the subject of extensive recent interest. Their intrinsic properties can, however, be obscured due to the influence of inhomogeneity in the external environment. Here we report methods for fabricating high quality TMDC monolayers with narrow photoluminescence (PL) linewidth approaching the intrinsic limit. We find that encapsulation in hexagonal boron nitride (h-BN) sharply reduces the PL linewidth, and that passivation of the oxide substrate by an alkyl monolayer further decreases the linewidth and also minimizes the charged exciton (trion) peak. The combination of these sample preparation methods results in much reduced spatial variation in the PL emission, with a full-width-at-half-maximum as low as 1.7 meV. Analysis of the PL line shape yields a homogeneous width of 1.43 ± 0.08 meV and inhomogeneous broadening of 1.1 ± 0.3 meV.

Growth of 18O isotopically enriched ZnO nanorods by two novel VPT methods

We have developed two novel vapour phase transport methods to grow ZnO nanorod arrays isotopically enriched with 18O. Firstly, a three-step process used to grow natural and Zn-enriched ZnO nanorods has been further modified, by replacing the atmospheric O2 with enriched 18O2, in order to grow 18O-enriched ZnO nanorods using this vapour-solid method on chemical bath deposited buffer layers. In addition, 18O-enriched ZnO nanorods were successfully grown using 18O isotopically enriched ZnO source powders in a vapour-liquid-solid growth method. Scanning electron microscopy studies confirmed the success of both growth methods in terms of nanorod morphology, although in the case of the vapour-liquid-solid samples, the nanorods’ c-axes were not vertically aligned due to the use of a non-epitaxial substrate. Raman and PL studies indicated clearly that O-enrichment was successful in both cases, although the results indicate that the enrichment is at a lower level in our samples compared to previous reports with the same nominal enrichment levels. The results of our studies also allow us to comment on both levels of enrichment achieved and on novel effects of the high temperature growth environment on the nanorod growth, as well as suggesting possible mechanisms for such effects. Very narrow photoluminescence line widths, far narrower than those reported previously in the literature for isotopically enriched bulk ZnO, are seen in both the vapour-solid and vapour-liquid-solid nanorod samples demonstrating their excellent optical quality and their potential for use in detailed optical studies of defects and impurities using low temperature photoluminescence.

Crystal Phase Quantum Dots in the Ultrathin Core of GaAs-AlGaAs Core-Shell Nanowires.

Semiconductor quantum dots embedded in nanowires (NW-QDs) can be used as efficient sources of nonclassical light with ultrahigh brightness and indistinguishability, needed for photonic quantum information technologies. Although most NW-QDs studied so far focus on heterostructure-type QDs that provide an effective electronic confinement potential using chemically distinct regions with dissimilar electronic structure, homostructure NWs can localize excitons at crystal phase defects in leading to NW-QDs. Here, we optically investigate QD emitters embedded in GaAs-AlGaAs core-shell NWs, where the excitons are confined in an ultrathin-diameter NW core and localized along the axis of the NW core at wurtzite (WZ)/zincblende (ZB) crystal phase defects. Photoluminescence (PL)-excitation measurements performed on the QD-emission reveal sharp resonances arising from excited electronic states of the axial confinement potential. The QD-like nature of the emissive centers are suggested by the observation of a narrow PL line width, as low as ~300 μeV, and confirmed by the observation of clear photon antibunching in autocorrelation measurements. Most interestingly, time-resolved PL measurements reveal a very short radiative lifetime <1 ns, indicative of a transition from a type-II to type-I band alignment of the WZ/ZB crystal interface in GaAs due to the strong quantum confinement in the ultrathin NW core.