Defect-related Photoluminescence Research Articles

Distinct Valley Polarization in Vertical Heterobilayers: Difference between Edge- and Center-Nucleated WS2/MoS2.

Structural imperfections can cause both beneficial and detrimental consequences on the excitonic characteristics of transition metal dichalcogenides (TMDs). Regarding valley selection, structural defects typically promote valley depolarization in monolayer TMDs, but defect healing via an additional growth process can restore valley polarization in vertical heterobilayers (VHs). In this study, we analyzed the valley polarization of center-nucleated and edge-nucleated VHs (WS2/MoS2) grown using a controlled growth process and discovered that defect-related photoluminescence (PL) is strongly suppressed in the center-nucleated VHs due to defect healing. Additionally, we demonstrated that the valley polarization of lower-lying intralayer excitons is more sensitive to the defect density of the sample than to higher-lying intralayer excitons. Despite defect healing in the center-nucleated VHs, the temperature-dependent PL study indicated that valley depolarization of the lower-lying intralayer excitons becomes significant below 100 K because of stronger hybridization of defect states. Also, we conducted a comprehensive study on the excitation intensity dependence to investigate the electron-doping-induced Auger recombination mechanism, which also contributes to valley depolarization of intralayer excitons via regeneration of intervalley trions. Our findings provide valuable insight into the development of VH-based valleytronic devices.

The spectrally resolved photoluminescence decay in YAG:Er, ZnO and SiO2 crystals

We present an optical setup for measuring spectrally resolved photoluminescence (PL) mean decay time using conventional UV LED with sinusoidal excitation and a phase shift method. The phase sensitive detection was applied on the Er-doped yttrium aluminium garnet single crystal (YAG:Er, grown by the micro-pulling-down method) and hydrothermally grown zinc oxide micro-crystallites (ZnO). Commercial ZnO and silicon oxide (SiO2) microcrystalline powders were measured for comparison. SiO2 powder was annealed in O2 plasma at 500 °C for cleaning. While YAG:Er shows well resolved greenish PL peaks with mean decay time about 15 μs related to the Stark splitting of Er3+ (4f11) states and ZnO micro-powder broad reddish PL with similar PL decay time, the plasma treated SiO2 powder shows weak bluish defect-related PL with significantly faster mean decay times below 100 ns.

Non-toxic arsenic nanoparticles for surface-enhanced Raman spectroscopy applications

In this study, we present green synthesis of Arsenic nanoparticles (As NPs) using different plant extracts. The resultant As NPs exhibit diverse sizes, shapes and assembly of nanotubules. In addition, As NPs have large band gap that ranges from 3.09 to 3.88 eV. They are enriched with defect states and exhibit multiple defect related photoluminescence. Owing to the aggregation and nanotubule assembly of As NPs, they are utilized for surface-enhanced Raman spectroscopy (SERS) sensing application. Under the illumination of continuous wave 785 nm laser, these As NPs demonstrate the detection of Methylene blue dye molecules via non-plasmonic chemical enhancement of SERS signal with an enhancement factor of 10 3 . Intriguingly, As NPs exhibit non-toxicity towards specific microbes, suggesting safety in handling during experiments and potential applications in SERS imaging and biomedicines.

Tailoring the structure and self-activated photoluminescence of carbonated amorphous calcium phosphate nanoparticles for bioimaging applications.

Self-activated luminescent calcium phosphate (CaP) nanoparticles, including hydroxyapatite (HA) and amorphous calcium phosphate (ACP), are promising for bioimaging and theragnostic applications in nanomedicine, eliminating the need for activator ions or fluorophores. In this study, we developed luminescent and stable citrate-functionalized carbonated ACP nanoparticles for bioimaging purposes. Our findings revealed that both the CO32- content and the posterior heating step at 400 °C significantly influenced the composition and the structural ordering of the chemically precipitated ACP nanoparticles, impacting the intensity, broadness, and position of the defect-related photoluminescence (PL) emission band. The heat-treated samples also exhibited excitation-dependent PL under excitation wavelengths typically used in bioimaging (λexc = 405, 488, 561, and 640 nm). Citrate functionalization improved the PL intensity of the nanoparticles by inhibiting non-radiative deactivation mechanisms in solution. Additionally, it resulted in an increased colloidal stability and reduced aggregation, high stability of the metastable amorphous phase and the PL emission for at least 96 h in water and supplemented culture medium. MTT assay of HepaRG cells, incubated for 24 and 48 h with the nanoparticles in concentrations ranging from 10 to 320 μg mL-1, evidenced their high biocompatibility. Internalization studies using the nanoparticles self-activated luminescence showed that cellular uptake of the nanoparticles is both time (4-24 h) and concentration (160-320 μg mL-1) dependent. Experiments using confocal laser scanning microscopy allowed the successful imaging of the nanoparticles inside cells via their intrinsic PL after 4 h of incubation. Our results highlight the potential use of citrate-functionalized carbonated ACP nanoparticles for use in internalization assays and bioimaging procedures.

Leveraging plasmonic hot electrons to quench defect emission in metal-semiconductor nanostructured hybrids.

Modeling light-matter interactions in hybrid plasmonic materials is vital to their widening relevance from optoelectronics to photocatalysis. Here, we explore photoluminescence (PL) from ZnO nanorods (ZNRs) embedded with gold nanoparticles (Au NPs). A progressive increase in Au NP concentration introduces significant structural disorder and defects in ZNRs, which paradoxically quenches defect related visible PL while intensifying the near band edge (NBE) emission. Under UV excitation, the simulated semi-classical model realizes PL from ZnO with sub-bandgap defect states, eliciting visible emissions that are absorbed by Au NPs to generate a non-equilibrium hot carrier distribution. The photo-stimulated hot carriers, transferred to ZnO, substantially modify its steady-state luminescence, reducing NBE emission lifetime and altering the abundance of ionized defect states, finally reducing visible emission. The simulations show that the change in the interfacial band bending at the Au-ZnO interface under optical illumination facilitates charge transfer between the components. This work provides a general foundation to observe and model the hot carrier dynamics and strong light-matter interactions in hybrid plasmonic systems.

Photoluminescence of high energy xenon ion irradiated magnesium aluminate spinel single crystals

Laser confocal microscopy technique (60 ps laser pulse excitation at 445 nm) and a time-correlated single photon counting (TCSPC) technique have been used to study the photoluminescence (PL) in 152 MeV Xe ion irradiated MgAl2O4 single crystals. It was shown that radiation defects produced by swift xenon ions give rise to broad luminescence band positioned in the 470–800 nm wavelength range. The measurements of the PL decay curves have evidenced that lifetime of defect-related PL signal gradually increases from 6 to 10 ns in 500–620 nm spectral range and decreases with ion fluence increasing. The decrease of the luminescence intensity observed after fluences ∼1013 cm−2 is attributed to overlapping of individual ion track core regions.

Defect structures and (ferro)magnetism in Zn1-xFexO nanoparticles with the iron concentration level in the dilute regime (x = 0.001 – 0.01) prepared from acetate precursors

Zn1-xFexO nanoparticles with the iron concentrations level in the dilute regime (x = 0.001–––0.01) were produced by a sol–gel route from acetate precursors along with an un-doped and 3 at.% Fe-doped reference. The X-ray diffraction of the un-doped and 0.1–1 at.% Fe-doped samples reveal the reflections for only the ZnO wurtzite structure. Fe doping enhances the a-axis lattice constant, the unit cell volume and the microstrain. Iron doping reduces the average crystallite/particle size (confirmed by Scanning Electron Microscopy), improving the surface-to-volume ratio or the concentration of defective surface sites. XPS identifies the iron in both Fe3+ and Fe2+ states. XPS and 57Fe Mossbauer spectroscopy indicate a broad distribution (distortion) of Fe3+ sites on the surface of ZnO nanoparticles. The blue shift and broadening of the UV emission, and quenching of defect-related photoluminescence in the Fe-doped samples verify the presence of iron in the ZnO lattice and surface intrinsic defects. 0.1–1 at.% Fe-doped ZnO show room temperature ferromagnetism, RTFM, characteristic of dilute magnetic semiconductors, DMS. The magnetization measurements with temperature evidence an antiferromagnetic alignment and an increase of ferromagnetic contribution with Fe doping up to 1 at.%. Zn0.97Fe0.3O reference is a superparamagnetic ZnO/ZnFe2O4 nanocomposite with a blocking temperature of 20 K; HRTEM shows (ultra)fine ZnFe2O4 particles at the surface of ZnO nanoparticles. The analysis of experimental data of 0.1–1 at.% Fe-doped ZnO was done in terms of iron coupling with intrinsic defects, which can generate surface Fe3+ states with geometries similar to the Fe3+ in inverse spinel ZnFe2O4. The superexchange interaction (resembling that in the inverse spinel ZnFe2O4) between the Fe3+ sites with distorted configuration resulting in ferrimagnetism was hypothesised as a possible mechanism of RTFM. Experimental (structural, local chemical, magnetic, optical) and interpretation results can be used to optimize the processing conditions for Fe-doped ZnO to serve as an effective DMS, e.g. for spintronic applications.

The effect of Ag doping on the structure, optical, and electronic properties of ZnO nanostructures deposited by atmospheric pressure MOCVD on Ag/Si substrates

Atmospheric pressure metal–organic chemical vapour deposition was used to synthesize Ag-containing ZnO nanostructures of different morphology on Si substrates coated by Ag. Ag from Ag/Si substrates and Ag from silver acetylacetonate after its decomposition were used as a catalyst for ZnO nanocrystal growth for deposition of ZnO nanostructures with different morphologies. We investigated the relation of the structural parameters and chemical composition probed by x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), and Raman spectroscopy with the photoluminescence (PL) properties and electron–phonon coupling (EPC) reflected in the multi-phonon Raman spectra. The Raman and PL spectra were studied at different powers of the exciting laser radiation (P exc). The spectral position and width of the phonon Raman peaks and the near bandgap PL band at low P exc are supposed to be determined by the structural quality of the surfaces/boundaries of the crystallites. The intensity of the near-bandgap and defect-related PL and the magnitude of the EPC are additionally affected by the dopant concentration. Because of the large crystallite size (>30 nm, determined from XRD), the effects of phonon or electron confinement are negligible in these nanostructures. The behaviour of the position and width of phonon and PL bands with increasing P exc indicates that the heat dissipation in the film, which is dependent on the nanostructure morphology and Ag content, plays an important role. In addition, the cytotoxicity of ZnO:Ag nanostructures was investigated by using monolayer cell cultures of epithelioid origin Madin-Darby bovine kidney and Madin-Darby canine kidney cells at a MTT assay revealing that the level of silver doping of ZnO nanostructures, their morphology, and geometric dimensions determine their toxic effects.

Modulation of photoluminescence and optical limiting properties of spray-coated tin oxide thin film through Eu doping

This work reports the effect of Eu doping on the structural, morphological, linear, and nonlinear optical properties of Sn1−xEuxO2 (x = 0 to 0.10) thin films prepared using spray pyrolysis. The films crystallized in tetragonal structure and the crystallite size reduced due to Eu incorporation. The film morphology consisted of evenly distributed dense grains with trapezohedron or bipyramidal shape. The Eu doping reduced the transmittance and energy band gap of the deposited films. The photoluminescence (PL) emission spectra comprised the contributions from both the transitions related to the Eu3+ ions and the defects, such as oxygen vacancies and tin interstitials in the host lattice. The films also showed a near white light emission due to the different defect-related PL emissions. The Z-scan technique revealed the third-order optical nonlinearity shown by the films. Amongst the doped films, Sn0.92Eu0.08O2 films exhibited least optical limiting threshold.Graphical abstract

Effects of Na2O on the optical properties of CdSe QDs embedded in glasses

In this work, effects of Na2O concentration on the precipitation of CdSe QDs in glasses is investigated. It is found that increase in concentration of Na2O reduces the connectivity of the network of glass and decreases the solubility of Se in the glass, leading to the formation of Se-related clusters and lower temperature for the precipitation of CdSe QDs. For glasses containing lower concentration of Na2O, defect-related photoluminescence (PL) is suppressed and the overall PL changes from dual band to narrow band with tunable wavelength in the visible range. For glasses containing higher concentration of Na2O, only dual band PL is observed due to the phase separation upon high temperature heat-treatment. Results reported in this work demonstrate that modulation of the PL properties of QDs in glasses can be accessed by adjusting the composition and structure of glasses.

Impact of synthesis temperature and precursor ratio on the crystal quality of MOCVD WSe2 monolayers

Structural defects in transition metal dichalcogenide (TMDC) monolayers (ML) play a significant role in determining their (opto)electronic properties, triggering numerous efforts to control defect densities during material growth or by post-growth treatments. Various types of TMDC have been successfully deposited by MOCVD (metal-organic chemical vapor deposition), which is a wafer-scale deposition technique with excellent uniformity and controllability. However, so far there are no findings on the extent to which the incorporation of defects can be controlled by growth parameters during MOCVD processes of TMDC. In this work, we investigate the effect of growth temperature and precursor ratio during MOCVD of tungsten diselenide (WSe2) on the growth of ML domains and their impact on the density of defects. The aim is to find parameter windows that enable the deposition of WSe2 ML with high crystal quality, i.e. a low density of defects. Our findings confirm that the growth temperature has a large influence on the crystal quality of TMDC, significantly stronger than found for the W to Se precursor ratio. Raising the growth temperatures in the range of 688 °C to 791 °C leads to an increase of the number of defects, dominating photoluminescence (PL) at low temperatures (5.6 K). In contrast, an increase of the molar precursor ratio (DiPSe/WCO) from 1000 up to 100 000 leads to less defect-related PL at low temperatures.

Ultrawide bandgap willemite-type Zn2GeO4 epitaxial thin films

Willemite-type Zn2GeO4 is a promising ultrawide bandgap semiconductor material. To date, experimental results on growth and physical properties of epitaxial thin films of willemite-type Zn2GeO4 are not available. Here, we report the heteroepitaxial growth of (00.1)-oriented Zn2GeO4 thin films on c-plane sapphire substrates using pulsed laser deposition. The in-plane orientation relationships are [11.0] Zn2GeO4//[11.0] Al2O3 and [11¯.0] Zn2GeO4//[11¯.0] Al2O3. A 450 nm thick epitaxial film with a surface roughness of 2.5 nm deposited under 0.1 mbar oxygen partial pressure exhibits a full width at half maximum (FWHM) of rocking curve of (00.6) reflex of 0.35°. The direct bandgap is evaluated to be 4.9 ± 0.1 eV. The valence band maximum is determined to be 3.7 ± 0.1 eV below the Fermi level. Together with the density-functional theory band structure calculation, it is suggested that the O 2p orbital and Zn 3d orbital dominantly contribute to the valence band of Zn2GeO4. The steady-state photoluminescence (PL) spectra of the films under 266 nm excitation at room temperature exhibit a broad defect-related emission band centered at 2.62 eV with a FWHM of 0.55 eV. The origin of this native defect-related PL is suggested to correlate with Zn interstitials. This work advances the fundamental study on willemite-type Zn2GeO4 epitaxial thin films for potential device application.

Significance of Hydroxyl Groups on the Optical Properties of ZnO Nanoparticles Combined with CNT and PEDOT:PSS.

We report on the synthesis of ZnO nanoparticles and their hybrids consisting of carbon nanotubes (CNT) and polystyrene sulfonate (PEDOT:PSS). A non-aqueous sol–gel route along with hydrated and anhydrous acetate precursors were selected for their syntheses. Transmission electron microscopy (TEM) studies revealed their spherical shape with an average size of 5 nm. TEM also confirmed the successful synthesis of ZnO-CNT and ZnO-PEDOT:PSS hybrid nanocomposites. In fact, the choice of precursors has a direct influence on the chemical and optical properties of the ZnO-based nanomaterials. The ZnO nanoparticles prepared with anhydrous acetate precursor contained a high amount of oxygen vacancies, which tend to degrade the polymer macromolecule, as confirmed from X-ray photoelectron spectroscopy and Raman spectroscopy. Furthermore, a relative increase in hydroxyl functional groups in the ZnO-CNT samples was observed. These functional groups were instrumental in the successful decoration of CNT and in producing the defect-related photoluminescence emission in ZnO-CNT.

Band gap tuning of Ga2O3–Al2O3 ceramics

Translucent ceramics comprising monoclinic Ga 2 O 3 and non-cubic Ga 2 O 3 –Al 2 O 3 solid solution with a composition of Ga 1.5 Al 0.5 O 3 , GaAlO 3 and Ga 0.4 Al 1.6 O 3 were fabricated through two-step spark plasma sintering by using calcinated commercial mixture oxide ceramic powders. All sintered ceramics were nearly fully densified, with a relative density exceeding 99.2%. The average grain sizes decreased with increasing Al content. Band gap was tuned from 4.08 to 6.37 eV. The highest total transmittance (73.6% at 1100 nm) was measured in Ga 0.4 Al 1.6 O 3 ceramics with both hexagonal (rhombohedral) and monoclinic crystal structures and a minimum average grain size of 0.288 ± 0.067 μm. Defect related photoluminescence, measured under excitation by a 254 nm UV lamp in the GaAlO 3 and Ga 0.4 Al 1.6 O 3 ceramics, lasted more than 15 s after removal of the UV source. These translucent ceramics from Ga 2 O 3 –Al 2 O 3 solid solution provide an alternative form of a potential transparent conducting material.

Electrical and optical properties of hydrogen plasma treated β-Ga2O3 thin films

The behavior of H in β-Ga2O3 is of substantial interest because it is a common residual impurity that is present in β-Ga2O3, regardless of the synthesis methods. Herein, we report the influences of H-plasma exposure on the electric and optical properties of the heteroepitaxial β-Ga2O3 thin films grown on sapphire substrates by chemical vapor deposition. The results indicate that the H incorporation leads to a significantly increased electrical conductivity, a greatly reduced defect-related photoluminescence emission, and a slightly enhanced transmittance, while it has little effect on the crystalline quality of the β-Ga2O3 films. The significant changes in the electrical and optical properties of β-Ga2O3 may originate from the formation of shallow donor states and the passivation of the defects by the incorporated H. Temperature dependent electrical properties of the H-incorporated β-Ga2O3 films are also investigated, and the dominant scattering mechanisms at various temperatures are discussed.

Material characterizations and light/gas dual-sensing properties of ZnO nanorods incorporating buried carbon nanotube in the seed layer

In this study, ZnO nanorods (NRs) are grown on the ZnO seed layer incorporating carbon nanotubes (CNTs). Field emission scanning electron microscope (FESEM) and transmission electron microscope (TEM) images show that CNTs are well distributed in the seed layer. X-ray photoelectron spectra (XPS) reveal the enhancement of oxygen defect-related chemical bindings and photoluminescence (PL) spectra show the increase of defect-related PL intensity. The enhanced oxygen defects for ZnO NRs on CNT-incorporated seed layer improve the electron concentration and the conductivity after light/gas sensing; as a result, the light/gas dual-sensing behaviors for CNT-incorporated ZnO NRs sensor are boosted. Owing to steady response, simple fabrication, and compact size, light/gas dual-sensing chips with ZnO NRs on CNT-incorporated seed layer are promising for ultraviolet (UV) light and gas detection.

Optical Properties of Hydrothermally Grown ZnO Nanoflowers

Abstract: A simple hydrothermal method has been successfully employed to synthesize flower-like ZnO nanostructure. X-ray diffraction data confirm the formation of ZnO with a Wurtzite structure. FESEM images show the flower-like morphology of the synthesized nanostructures. The energy dispersive X-ray spectroscopic analysis confirms the stoichiometric composition.. X-ray fluorescence spectrum shows no impurity element in the synthesized ZnO. The synthesized ZnO exhibits low absorption in the visible region of wavelength. Band gap enhancement was also observed owing to the quantum confinement effect. The synthesized ZnO nanoflowers exhibit strong room-temperature photoluminescence with a broad emission peak at 429 nm arising due to the recombination of electrons at zinc interstitials and holes in the valence band. This defect-related photoluminescence is very important in the context of understanding the defect dynamics in ZnO. Background: Zinc oxide (ZnO) is a well-known multifunctional material possessing unique structural, electrical, and optical properties that are very useful in various device applications. Being a high and direct band gap semiconductor, it is potentially being used in various UV light sources and detectors fabrication. However, the emission and absorption properties strongly depend on the size of the ZnO nanoparticles which in turn depends on the morphology of the nanostructure. Therefore, it is very much important to understand the structure-property relationship for a predictable device performance. Objectives: Our objective of this work is to synthesize flower-like ZnO nanostructures using a simple hydrothermal method. The flower-like ZnO morphology offers a large surface area that will be very suitable for designing gas and chemical sensor devices. Another objective of this work is to study the crystallography of ZnO. Next, the optical properties (emission and absorption) have been investigated to understand the defect-related photoluminescence mechanism. Method: A simple hydrothermal method has been deployed to synthesize flower-like ZnO nanostructures. A chloride decomposition scheme has been used to produce zinc hydroxide ions that will produce ZnO nuclide. At the onset of saturation, ZnO nanocrystals start to grow. The entire reaction was performed inside a Teflon cell stainless steel autoclave. The autoclave was placed in a horizontal tube furnace and maintained at 150 °C for 2 hr. resulting in the formation of white powder-like material. Results: The X-ray diffraction data confirm the formation of polycrystalline ZnO having a Wurtzite structure. Flower-like morphology was clearly observed in FESEM images. The EDS data confirm the composition of ZnO with proper stoichiometry. Gibb’s free energy calculation favors the reaction under the experimental condition. The absorption spectrum was used to calculate the band gap of the synthesized ZnO nanoflowers. The Tauc plot revealed the band gap of the synthesized ZnO to be~ 3.69 eV. This enhancement of band gap compared to bulk ZnO occurs due to the quantum confinement effect. The synthesized ZnO nanoflowers exhibit broad photoluminescence peaked at 429 nm owing to the presence of interstitial zinc. Conclusion: A hydrothermal method has been successfully used to synthesize well-crystalline ZnO nanoflowers of proper stoichiometry. The flower-like nanostructure exhibits band gap enhancement due to the quantum confinement effect. Room temperature visible photoluminescence was observed from the ZnO nanoflowers with a board emission peak at 429 nm. This emission arises due to the presence of deep-level zinc interstitial states. This finding will be very useful in understanding the role of defects in the visible emission from ZnO nanostructures.

In situ-Synthesized cadmium sulfide quantum dots in pore-forming protein and polysaccharide matrices for optical biosensing applications

The main limitation for practical implementation of quantum dots-based sensors and biosensors is the possible contamination of sensing media with quantum dots (QDs) moved out from the sensor structure, being critical for living systems measurements. Numerous efforts have addressed the challenge of pre-synthesized QDs incorporation into porous matrix provide, on the one hand, proper fixation of quantum dots in its volume and preserving a free analyte transfer from the sensing media to them - on the other hand. Here, we propose an alternative insight into this problem. Instead of using preliminary synthesized particles for doping a matrix, we have in situ synthesized cadmium sulfide QDs in porous biopolymeric matrices, both in an aqueous solution and on a mica substrate. The proposed technique allows obtaining QDs in a matrix acting simultaneously as a ligand passivating surface defects and preventing QDs aggregation. The conjugates were used as a photoluminescence sensor for the metal ions and glutathione detection in an aqueous media. Different kinds of sensor responses have been found depending on the analyte nature. Zinc ions' presence initiates the intraband QDs emission increases due to the reduction of non-radiative processes. The presence of copper ions, in contrast, leads to a gradual photoluminescence decrease due to the formation of the non-luminescent copper-based alloy in the QDs structure. Finally, the presence of glutathione initiates a ligand exchange process followed by some QDs surface treatment enhancing defect-related photoluminescence. As a result, three different kinds of sensor responses for three analytes allow claiming development of a new selective QD-based sensor suitable for biomedical applications.

Point defects in beryllium-doped GaN

Detailed analysis of the defect-related photoluminescence (PL) in semi-insulating and $n$-type Be-doped GaN samples grown by molecular beam epitaxy is presented. The dominant PL bands in these samples are the ultraviolet $({\mathrm{UVL}}_{\mathrm{Be}})$, yellow $({\mathrm{YL}}_{\mathrm{Be}})$, red $({\mathrm{RL}}_{\mathrm{Be}})$, and green (GL2) bands. The ${\mathrm{UVL}}_{\mathrm{Be}}$ band is caused by electron transitions via the $0/\ensuremath{-}$ level of the ${\mathrm{Be}}_{\mathrm{Ga}}$ acceptor located at 113 meV above the valence band. The ${\mathrm{UVL}}_{\mathrm{Be}}$ band is quenched above 60 K, and the quenching is tunable by excitation intensity. The quenching of the ${\mathrm{YL}}_{\mathrm{Be}}$ above 200 K is likely related to an acceptor level at 0.3 eV. In semi-insulating samples, the quenching temperature is tunable with the intensity of the excitation laser. Based on the first-principles calculations, the ${\mathrm{YL}}_{\mathrm{Be}}$ band is preliminarily attributed to the ${\mathrm{Be}}_{\mathrm{Ga}}\text{\ensuremath{-}}{\mathrm{V}}_{\mathrm{N}}\text{\ensuremath{-}}{\mathrm{Be}}_{\mathrm{Ga}}$ complex. The new ${\mathrm{RL}}_{\mathrm{Be}}$ band is attributed to the internal transition in the ${\mathrm{Be}}_{\mathrm{Ga}}\text{\ensuremath{-}}{\mathrm{V}}_{\mathrm{N}}$ complex with a deep $+/0$ transition level at about 0.95 eV.

Activation energy distribution in thermal quenching of exciton and defect-related photoluminescence of InP/ZnS quantum dots

Thermal quenching is one of the essential factors in reducing the efficiency of radiative processes in luminophores of various nature. The emission activity of low-dimensional structures is influenced also by multiplicity of parameters that are related to synthesis processes, treatment regimes, etc. In the present work, we have investigated the temperature dependence of photoluminescence caused by exciton and defect-related transitions in ensembles of biocompatible InP/ZnS core/shell nanocrystals with an average size of 2.1 and 2.3 nm. The spread in the positions of energy levels is shown to be due to size distribution of quantum dots in the ensembles under study. For a quantitative analysis of the experimental data, we have proposed a band model accounting for the Gaussian distribution of the thermally activated barriers in the photoluminescence quenching processes. The model offers the thermal escape of an electrons from the core into the shell as the main mechanism for non-radiative decay of excitons. In turn, the quenching of defect-related emission is predominantly brought about through the emptying of the hole capture centers based on dangling phosphorus bonds. We have revealed the correlation between size distributions of quantum dots and scatter of the activation energy of exciton luminescence quenching. The developed approach will give further the possibility to optimize technological regimes and methods for band engineering of indium phosphide-based type-I quantum dots.