Photoluminescence Linewidth Research Articles

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).

Variations in the Composition of the Phases Lead to the Differences in the Optoelectronic Properties of MAPbBr3 Thin Films and Crystals

Photoluminescence (PL) spectra from thin films (TFs) and bulk crystals (BCs) of hybrid organo-halide perovskites are significantly different, the origin of which and their impact on the efficiency of the perovskite-based photoactive devices have been debated. We have used two-photon PL to study the temperature-dependent changes in the spectra of the TFs and the BCs of methylammonium lead bromide (MAPbBr3) perovskites in order to clarify the origin of the differences. Our results show that the differences in the spectra are due to the variation in the phase composition. At room temperature, the tetragonal (TE) phase is dominant in the BCs, while the orthorhombic (OR) phase is dominant in the TFs. The PL spectra of the TFs also show discernible contributions from the TE and the cubic phases. At lower temperatures, the increase in excitonic recombination causes a red shift of the PL spectra from the TFs, while a phase transition from the TE to the OR phase results in a blue shift of the PL from the BCs. The temperature-dependent narrowing of the PL linewidths shows a stronger coupling between the longitudinal optical phonons and the free carriers in the OR phase as compared to the TE phase, implying a reduced carrier mobility. However, as the OR phase is metastable at the room temperature, the slow phase transition to the TE phase should improve the photocurrent yield in the TFs, provided that the sample is shielded from other types of degradation.

Narrow band-edge photoluminescence from AgInS2 semiconductor nanoparticles by the formation of amorphous III\u2013VI semiconductor shells

Nanoparticles of I–III–VI semiconductors are promising candidates for novel non-toxic fluorescent materials. However, removal of defect levels responsible for their broad-band emission has not been successful to date. The present study demonstrates, for the first time, the coating of core AgInS2 nanoparticles—one of the I–III–VI group semiconductors with a bandgap in the visible region—with III–VI group semiconductors. The AgInS2/InSx and AgInS2/GaSx (x = 0.8–1.5) core/shell structures generate intense narrow-band photoluminescence originating from a band-edge transition at a wavelength shorter than that of the original defect emission. Microscopic analyses reveal that the GaSx shell has an amorphous nature, which is unexpected for typical shell materials such as crystalline lattice-matching ZnS. Single-particle spectroscopy shows that the average linewidth of the band-edge photoluminescence is as small as 80.0 meV (or 24 nm), which is comparable with that of industry-standard II–VI semiconductor quantum dots. In terms of photoluminescence quantum yield, a value of 56% with nearly single-band emission has been achieved as a result of several modifications to the reaction conditions and post-treatment to the core/shell nanoparticles. This work indicates the increasing potential of AgInS2 nanoparticles for use as practical cadmium-free quantum dots.

Phonon Coupling with Excitons and Free Carriers in Formamidinium Lead Bromide Perovskite Nanocrystals.

Organometal halide perovskites in the form of nanocrystals (NCs) have attracted enormous attention due to their unique optoelectronic and photoluminescence (PL) properties. Here, we examine the phase composition and the temperature dependence of emission line width broadening in formamidinium lead bromide (FAPbBr3) perovskite nanocrystals (NCs) for light-emitting applications and identify different charge-carrier scattering mechanisms. Our results show most of the emission is from the orthorhombic phase. The PL line width broadening at high temperature is dominated by the Fröhlich interaction between the free charge carriers and the optical phonons. At low temperatures, the peak of the PL spectrum exhibits a continuous red shift indicating an increase of excitons contribution at lower temperatures, and concurrently the line width also narrows down due to the inhibition of the optical phonons. From the temperature-dependent measurements, the coupling strength of both the charge phonon interaction and the exciton phonon interaction have been determined. The obtained results indicate that the charge phonon coupling strengths are higher compared to the exciton phonon coupling.

Radiative recombination and electron-phonon coupling in lead-free CH3NH3SnI3 perovskite thin films

Solution-processed metal halide perovskites are attracting significant attention as high-performance photovoltaic materials. The electron-phonon interactions in lead halide perovskites are currently in the focus of interest because these interactions play an important role to the observed superior material properties and high device performance. Here, we investigated the electron-phonon coupling in lead-free $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Sn}{\mathrm{I}}_{3}$ thin films by analyzing the temperature-dependence of the photoluminescence (PL) spectra. The PL linewidth shows a superlinear dependence on the temperature, which suggests that the electron--longitudinal-optical (LO) phonon interaction is the dominating broadening mechanism of the optical transitions in $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Sn}{\mathrm{I}}_{3}$ near room temperature. Furthermore, from the analysis of the low-energy tail of the PL spectra by using the van Roosbroeck-Shockley relation, we determined the Urbach energy below the band gap and the optical-phonon energy that contributes to the optical absorption. This phonon energy coincides with the LO phonon energy determined from the PL width analysis. Our work quantitatively verifies that the electrons in tin halide perovskites efficiently couple with optical phonons, and demonstrates that their intrinsic optical properties at room temperature are comparable to those of lead halide perovskites implemented in high-efficiency solar cells.

Effect of a pick-and-drop process on optical properties of a CVD-grown monolayer tungsten disulfide

We systematically investigated changes of a photoluminescence (PL) spectral shape by picking a tungsten disulfide monolayer (1L-${\mathrm{WS}}_{2}$) grown on a sapphire (SA) substrate and dropping it on various substrates such as SA, ${\mathrm{SiO}}_{2}$, and hexagonal boron nitride (hBN). An inhomogeneous contribution to the exciton PL linewidth was reduced by picking the CVD-grown 1L-${\mathrm{WS}}_{2}$, meaning potential energy disorder lowering in the 1L-${\mathrm{WS}}_{2}$. In addition, a defect-bound exciton peak was not seen at low temperature for all the samples with the pick-and-drop process although it was clearly observed for the as-grown sample. It was thought that the as-grown 1L-${\mathrm{WS}}_{2}$ was covalently bonded on the SA substrate surface due to the existence of crystal imperfections in the ${\mathrm{WS}}_{2}$ and the covalent bonds formed midgap states. A PL spectrum had to be carefully treated in order to fairly evaluate a crystal quality in a CVD-grown 1L-${\mathrm{WS}}_{2}$ since PL spectral shapes were determined by complexes of the defect density with other mechanisms such as strain fluctuation and a connection with the substrate.

Temperature dependence of excitonic transitions in Al0.60Ga0.40N/Al0.70Ga0.30N multiple quantum wells from 4 to 750 K

The excitonic optical properties of an Al0.60Ga0.40N/Al0.70Ga0.30N multiple quantum well structure were studied by photoluminescence (PL) spectroscopy at various temperatures. An analysis of the temperature dependence of PL peak energy revealed the stronger localization of excitons than biexcitons, consistent with the trend predicted from the difference between the spatial extents of excitons and biexcitons. The PL linewidth of the excitons increased linearly with increasing temperature up to 300 K and then more rapidly above 300 K. This result indicated that the exciton-longitudinal optical phonon interaction became prominent as the broadening mechanism of the exciton linewidth above 300 K. The luminescence of both excitons and biexcitons exhibited low thermal quenching, with their intensities at 300 K remaining at 37% and 47%, respectively, of their intensities at 4 K. These observations reflected the high thermal stability of the excitons and biexcitons in this multiple quantum well structure. In addition, the ratio of the PL intensity of the exciton-biexciton scattering to the product of the exciton and biexciton luminescence intensities was independent of temperature up to 200 K and increased rapidly with increasing temperature above 250 K. The temperature-independent behavior of this ratio up to 200 K originated from the localization of excitons and biexcitons, and the rapid increase in this ratio with temperature above 250 K reflected an increase in the frequency of interactions between the excitons and biexcitons due to the gradual delocalization of the excitons and biexcitons with increasing temperature.

Narrowing the Photoluminescence of Aqueous CdTe Quantum Dots via Ostwald Ripening Suppression Realized by Programmed Dropwise Precursor Addition

Ostwald ripening is the dominant particle growth mechanism in traditional routes to synthesize aqueous CdTe quantum dots (QDs), which results in a broadening of their size distribution and also their photoluminescence line width, in the case of prolonged reactions. We introduce a method to suppress the Ostwald ripening through the replenishment of precursors before size defocusing occurs in the aqueous synthetic system, which is realized through a programmed dropwise precursor (source) addition (denoted as PDPA) by a syringe pump arrangement that can control the precursor feed volume and rate. As a result, we have obtained a series of highly monodisperse CdTe QDs, with the emission full width at half-maximum in the red region (106 meV) being even narrower than that in the green region (160 meV). We monitored the conductivity of the reaction solution to follow the consumption of precursors, providing feedback as to whether the replenishment volume was matched. Correlations between the emission quantum yiel...

Thermal induced carrier’s transfer in bimodal size distribution InAs/GaAs quantum dots

This work reports on the investigation of the thermal induced carriers’ transfer mechanism in vertically stacked bimodal size distribution InAs/GaAs quantum dots (QD). A model treating the QD as a localized states ensemble (LSE) has been employed to fit the atypical temperature dependence of the photoluminescence (PL) emission energies and linewidth. The results suggest that thermally activated carriers transfer within the large size QD family occurs through the neighboring smaller size QD as an intermediate channel before direct carriers redistribution. The obtained activation energy suggests also the possible contribution of the wetting layer (WL) continuum states as a second mediator channel for carriers transfer.

Growth and optical characteristics of InAs quantum dot structures with tunnel injection quantum wells for 1.55 [formula omitted]m high-speed lasers

InP based lattice matched tunnel injection structures consisting of a InGaAs quantum well, InAlGaAs barrier and InAs quantum dots designed to emit at 1.55 μm were grown by molecular beam epitaxy and investigated by photoluminescence spectroscopy and atomic force microscopy. The strong influence of quantum well and barrier thicknesses on the samples emission properties at low and room temperatures was investigated. The phenomenon of a decreased photoluminescence linewidth of tunnel injection structures compared to a reference InAs quantum dots sample could be explained by the selection of the emitting dots through the tunneling process. Morphological investigations have not revealed any effect of the injector well on the dot formation and their size distribution. The optimum TI structure design could be defined.

Nano-“Squeegee” for the Creation of Clean 2D Material Interfaces

Two-dimensional (2D) materials exhibit many exciting phenomena that make them promising as materials for future electronic, optoelectronic, and mechanical devices. Because of their atomic thinness, interfaces play a dominant role in determining material behavior. In order to observe and exploit the unique properties of these materials, it is therefore vital to obtain clean and repeatable interfaces. However, the conventional mechanical stacking of atomically thin layers typically leads to trapped contaminants and spatially inhomogeneous interfaces, which obscure the true intrinsic behavior. This work presents a simple and generic approach to create clean 2D material interfaces in mechanically stacked structures. The operating principle is to use an AFM tip to controllably squeeze contaminants out from between 2D layers and their substrates, similar to a "squeegee". This approach leads to drastically improved homogeneity and consistency of 2D material interfaces, as demonstrated by AFM topography and significant reduction of photoluminescence line widths. Also, this approach enables emission from interlayer excitons, demonstrating that the technique enhances interlayer coupling in van der Waals heterostructures. The technique enables repeatable observation of intrinsic 2D material properties, which is crucial for the continued development of these promising materials.

Midgap States in PbS Quantum Dots Induced by Cd and Zn Enhance Photon Upconversion

Exciton exchange between semiconductor quantum dots (QDs) and organic chromophores may find application in next-generation photovoltaics, probes, or photocatalysts. We study the effect of shell composition and thickness on triplet energy transfer (TET) from PbS QDs to rubrene in a hybrid organic–inorganic photon upconverison system and show that defect states introduced by surface adsorbed Zn and Cd result in up to 700 and 325 fold enhancements in the photon upconverison quantum yield (QY). Time-correlated single-photon counting, photoluminescence (PL), and photon upconversion QY measurements on PbS QD light absorbers with submonolayer shells of 0.37–1.60 A of CdS and 0.04–0.22 A of ZnS reveal these core–shell QDs have reduced radiative and nonradiative rates relative to the core. Their broadened absorption and PL line widths suggest TET may occur from thermally accessible surface defects, in addition to the dark excitonic states at the band-edge.

Rethinking the theoretical description of photoluminescence in compound semiconductors

Semiconductor compounds, such as Ga(NAsP)/GaP or GaAsBi/GaAs, are in the focus of intensive research due to their unique features for optoelectronic devices. The optical spectra of compound semiconductors are strongly influenced by the random scattering potentials caused by compositional and structural disorder. The disorder potential is responsible for the red-shift (Stokes shift) of the photoluminescence (PL) peak and for the inhomogeneous broadening of the PL spectra. So far, the anomalous broadening of the PL spectra in Ga(NAsP)/GaP has been explained assuming two coexisting length scales of disorder. However, this interpretation appears in contradiction to the recently observed dependence of the PL linewidth on the excitation intensity. We suggest an alternative approach that describes the PL characteristics in the framework of a model with a single length scale of disorder. The price is the assumption of two types of localized states with different, temperature-dependent non-radiative recombination rates.

Temperature-dependent inhomogeneous optical behavior of AlInGaN quaternary alloys

The temperature-dependent anomalous optical properties of AlInGaN quaternary alloys have been studied using Monte Carlo simulation of phonon-assisted exciton hopping. The simulation results agreed well with the experimental photoluminescence linewidths and the peak energy positions while considering the additional inhomogeneous broadening with band potential fluctuation and the phonon-induced radiative lifetime of the exciton. The incorporation of band-gap shrinkage into the conventional Monte Carlo simulation shows a good fit for the photoluminescence peak energy positions in AlInGaN quaternary alloys with temperature-induced experimental unusual behavior (redshift-blueshift-redshift). The W-shaped temperature-dependent inhomogeneities are observed for the PL linewidths. The temperature-induced S-shaped photoluminescence band peaks and W-shaped linewidth have been attributed to the change of exciton dynamics due to the indium aggregated potentials and the exciton localization effects. These results could be important to understanding the realistic optical properties of AlInGaN quaternary alloy-based electronic and optoelectronic devices.

Influence of Bi on morphology and optical properties of InAs QDs

We study the tsurface morphology and photoluminescence (PL) property of InAs quantum dots (QDs) on GaAs using bismuth (Bi) in the layer prior to or after the growth of QDs. Incorporating Bi in the layer prior to the QD deposition delays the onset of InAs QD formation resulting in a decrease in QD height and density. As a surfactant, adding Bi in the GaAs capping layer at a high growth temperature reduces the In surface diffusion length leading to uniform and well preserved InAs QDs in terms of height and density. The incorporation of 3% Bi at a low growth temperature, which forms a GaAsBi capping layer, can effectively lower the PL transition energy up to 163 meV and reduce the PL linewidth, leading to an emission wavelength of 1.365 μm at 77 K.

High-Mobility and High-Optical Quality Atomically Thin WS2

The rise of atomically thin materials has the potential to enable a paradigm shift in modern technologies by introducing multi-functional materials in the semiconductor industry. To date the growth of high quality atomically thin semiconductors (e.g. WS2) is one of the most pressing challenges to unleash the potential of these materials and the growth of mono- or bi-layers with high crystal quality is yet to see its full realization. Here, we show that the novel use of molecular precursors in the controlled synthesis of mono- and bi-layer WS2 leads to superior material quality compared to the widely used direct sulfidization of WO3-based precursors. Record high room temperature charge carrier mobility up to 52 cm2/Vs and ultra-sharp photoluminescence linewidth of just 36 meV over submillimeter areas demonstrate that the quality of this material supersedes also that of naturally occurring materials. By exploiting surface diffusion kinetics of W and S species adsorbed onto a substrate, a deterministic layer thickness control has also been achieved promoting the design of scalable synthesis routes.

Structural and optical properties of diluted magnetic $$\hbox {Ga}_{{1-x}}\hbox {Mn}_{{x}}$$ Ga 1 - x Mn x As–AlAs quantum wells grown on high-index GaAs planes

We report on the structural and optical properties of $$\hbox {Ga}_{{1-x}}\hbox {Mn}_{{x}}\hbox {As}$$ –AlAs quantum wells (QWs) with $$x = 0.1\%$$ grown by molecular beam epitaxy (MBE) on semi-insulating GaAs substrates with orientations (100), (110), (311)B and (411)B. Atomic force microscopy (AFM), X-ray diffraction (XRD) and photoluminescence (PL) techniques were used to investigate these QWs. AFM results have evidenced the formation of Mn-induced islands, which are randomly distributed on the surface. These islands tend to segregate for samples grown on (110) and (411)B planes, while no clear segregation was observed for samples grown on (100) and (311)B orientations. Results show that the PL line width increases with Mn segregation. XRD measurements were used to determine $$2\theta , d$$ and cell parameters.

Influence of well width fluctuations on recombination properties in semipolar InGaN quantum wells studied by time- and spatially-resolved near-field photoluminescence

Scanning near-field photoluminescence spectroscopy has been applied to distinguish the relevance of quantum well (QW) alloy composition and well width fluctuations on emission linewidth and recombination times in semipolar (202¯1) plane InGaN QWs. It has been found that well width fluctuations, compared to variations of InGaN alloy composition, play a negligible role in defining the photoluminescence linewidth. However, the well width strongly affects the recombination times. Prolonged radiative and nonradiative carrier lifetimes in wide QWs have been attributed to electron and hole separation by in-plane electric fields caused by nonplanarity of QW interfaces.

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.

Effect of carrier confinement on effective mass of excitons and estimation of ultralow disorder in AlxGa1\u2212xAs/GaAs quantum wells by magneto-photoluminescence

Effect of charge carrier confinement and ultra-low disorder acquainted in AlGaAs/GaAs multi-quantum well system is investigated via Magneto-photoluminescence spectroscopy. Significant increase of effective mass is observed for the confined exciton in narrow QWs. The foremost reason behind such an observation is due to the induced non-parabolicity in bands. Moreover, as the thickness of the QW are reduced, confined excitons in QW experience atomic irregularities at the hetero-junctions and their effects are prominent in the photoluminescence linewidth. Amount of photoluminescence line-broadening caused by the atomic irregularities at the hetero-junctions is correlated with average fluctuation (δ1) in QW thickness. The estimated δ1 for Al0.3Ga0.7As/GaAs QWs are found to be ±(0.14 − 1.6)× ‘one monolayer thickness of GaAs layer’. Further, the strong perturbations due to magnetic field in a system helps in realizing optical properties of exciton in QWs, where magnetic field is used as a probe to detect ultralow defects in the QW. Additionally, the influence of magnetic field on the free and bound exciton luminescence is explained by a simple model. The proposed approach for measuring the interface and volume defects in an ultra-low disordered system by Magneto-PL spectroscopy technique will be highly beneficial in high mobility devices for advanced applications.