Photoluminescence Full-width At Half-maximum Research Articles

Ultranarrow Deep-Blue Luminescence of Perovskite Nanocrystals by A-Site Cation Control.

Metal-halide perovskite nanocrystals (NCs) are one of the most promising emitters for the application of display and nanolight sources. The full width at half-maximum (FWHM) of photoluminescence (PL) emission is essential for color purity, which however remains a difficulty to further reduce the FWHM of the perovskite NCs at room temperature. Here, we show the quasi-sphere perovskite NCs with narrow PL emission at a deep-blue wavelength of ∼430 nm; its PL FWHM reaches ∼11 nm at room temperature, owing to the monodispersion in size distribution as well as the symmetric quasi-sphere morphology of NCs releasing the fine structure splitting-induced inhomogeneous broadening. Through regulating A cations with respect to the ratio of FA (or MA)-to-Cs and Cs-to-Pb, the PL emission of the NCs could be tuned from ∼505 to ∼430 nm combined with varied morphologies from large cube to small quasi-sphere. Such spectroscopic and morphological discrepancies are supposed to be attributed to the different crystalline kinetics that is strongly dependent on the synthetic condition. To be specific, in the case of increasing FA (or MA)-to-Cs, the growth rate of CsPbBr3 and FAPbBr3 (or MAPbBr3) perovskites is determined by the reactivity of transient species, while in the case of decreasing the Cs-to-Pb ratio, the growth rate of perovskites is slowed down by the serious reduction of Cs+ in the precursor. This study provides an effective strategy to adjust the emission across from green to deep-blue color and promotes the perovskite NCs with a narrow FWHM, and tunable PL emission facilitates in application of optoelectronic devices.

Temperature-dependent photoluminescence characterization of compressively strained AlGaInAs quantum wells

A series of compressively strained AlGaInAs quantum well (QW) structures with different annealing treatment durations at 170 °C were investigated by temperature-dependent photoluminescence (PL). An abnormal S-shaped behavior in emission energy and a non-monotone evolution in spectral band width were observed at low temperature. A significant negative linear correlation between full width at half maximum (FWHM) and emission energy at different test temperature was exhibited in samples with different heat treatment durations. The highly linear relation demonstrated that the dependencies and relationships between PL peak energy and FWHM vs temperature are consistent. The anomalous blue shift and concomitant narrowing of FWHM in AlGaInAs QW were affected by lattice strain fluctuations due to the difference of thermal expansion coefficients between adjacent layers and strong carriers’ localization at low temperature.

High Stable Perovskite-Quantum-Dot Using Ligand Engineering for Liquid-Crystals-Display Applications

In recent years, semiconducting metal halide perovskite quantum dots have become attractive for use in many applications such as photovoltaics, photodetectors, light emitting diodes (LEDs), and lasers due to their outstanding electrical and optical properties.1 Particularly, metal halide perovskite materials are suitable for application in white LEDs (WLEDs) and displays as well as solar energy conversion materials due to their wide color tunability (400-800 nm), narrow photo-luminescence full width at half maximum (PL-FWHM) (~20 nm), and high photoluminescence quantum yields (PLQYs) (>80%), as well as their low temperature and cost-effective synthesis process, compared with inorganic luminescence materials such as phosphors.2,3 However, the instability of perovskite QDs has always been considered as the main hindrance significantly deteriorating the reliability and stability of their optoelectronic device and potential future applications.4 To overcome this issue of the instability of perovskite QDs, a few number of research work embedding QDs in organic polymer matrices, e.g., poly(methyl methacrylate) (PMMA), has been conducted to enable good processability and compatibility with typical packaging approaches.5 However, they exhibited limited stability improvement under harsh conditions like ultraviolet light excitation.6 Because of PMMA has a relatively high oxygen diffusion coefficient of 3.3 × 10–9 cm2 s−1 at 22 °C,7 it cannot completely prevent the photo-oxidation of QDs. Here, we proposed an efficient and simple method to improve the stability of perovskite QDs using effective ligand engineering. This proposed ligand Engineering was applied during the synthesis of perovskite QDs to modify and passivate their surface using organic ligand materials such as perfluorooctanoic acid (PFOA) as shown in Fig 1. PFOA belongs to fluorine functional group. The fluorine group is considered as promising ligand materials for perovskite QDs due to its outstanding optical properties and high stability against environmental and chemical factors. In order to synthesize CsPbBr3-XIX QDs capping with PFOA ligand, 5 mL of octadecene (ODE) and 0.188 mmol of PbX2 (such as PbI2 and PbBr2) were loaded into 25 mL 3-neck flask and then dried under vacuum for 1h at 120 ºC. 0.5 mL of dried oleylamine (OAm) and different ratios of oleic acid (OA) to PFOA (OA:PFOA). The selected OA:PFOA ratios are 10:0, 6:4, 5:5, 4:6, and 3:7. The mixture of OA:PFOA was injected at 120 ºC under N2 gas ambient. After solutionization of a PbX2 salt in ODE, the temperature raised to 165 ºC then Cs-oleate solution (0.4 mL, 0.125 M in ODE) was quickly injected and maintained for 5 sec. later, the reaction mixture was cooled using an ice-water bath. The final crude solution was purified by centrifuging process. For smaller QDs, the synthesis process should be carried out at a reaction temperature of below 160 ºC. A centrifugation of below 0 ºC or/and an addition of tert-butanol to the crude solution (ODE:tBuOH=1:1 by volume) were found to be helpful to achieve a complete precipitation.of QDs. After centrifugation, the supernatant discarded and the precipitated particles were redispersed in toluene. Both perovskite CsPbBr2I1 QDs capped and uncapped with PFOA ligand showed a cubical-like shape, high crystallinity, and well dispersity, as shown in HR-TEM images of Figs. 2(a) and (b). In addition, the diameters of perovskite CsPbBr2I1 QDs capped and uncapped with PFOA ligand were 10.2±2.39 and 11.5±2.61, respectively. The PFOA capped perovskite CsPbBr2I1 QDs showed a higher absorption in visible region (400-800 nm) with increasing the mole fraction of PFOA in comparing to uncapped QDs, and PL peaks shifted to longer wavelengths (from 534 to 551 nm) due to the Ostwald ripening, as shown in Figs 3(a) and (b). The QDs had a broader PL-FWHM ranging from 24.4 to 35.1 nm and PL intensity ranging from 32,000 to 15,000 a.u. by increasing the mole fraction of PFOA as shown clearly in Figure 3 (c). However, It is worth to note that the PL intensity of perovskite CsPbBr2I1 QDs capped with PFOA ligand decreased 15 % after 7 days, while uncapped QDs decreased 57 % after the same period, as shown in Fig 3 (d). Finally, high stability of perovskite QDs capped with PFOA ligands can be a promising candidate material for QD-based display applications such as QD-LCD. Figure 1

Excitation dependence of the photoluminescence lineshape in Ga(NAsP)/GaP multiple quantum well: experiment and Monte-Carlo simulation

Peculiarities of the excitation-dependent photoluminescence (PL) lineshape in Ga(NAsP)/GaP multiple quantum well structures are studied both experimentally and theoretically. A peculiar behavior of the PL peak energy and PL full width at half maximum (FWHM) at low temperatures is revealed experimentally. The PL peak energy increases with the increasing excitation intensity and tends to saturation at high excitation intensities, reflecting the gradual filling of the disorder-induced localized states. The PL FWHM initially decreases with the increasing excitation intensity, reaching a minimum at a relatively high excitation intensity, and then increases for higher excitation intensities. Such a non-monotonous behavior of the PL lineshape is explained by the hopping relaxation of excitons in the tail of localized states, assuming a non-monotonous density of localized states, consisting of exponential and Gaussian components.

Photoluminescence properties of MgxZn1−xO films grown by molecular beam epitaxy

The optical properties of MgxZn1−xO films with x=0.03, 0.06, 0.08, and 0.11 grown by molecular beam epitaxy (MBE) have been studied by temperature-dependent photoluminescence (PL) measurement. It is presented that the full-width at half-maximum (FWHM) of the 12K PL spectrum of MgZnO films increases with increasing Mg concentration and would deviate significantly from the simulation curve of Schubert model with higher Mg contents. The abnormal broader PL FWHM is inferred from larger compositional fluctuation by incorporating higher Mg contents, which results in larger effect of excitonic localization to induce more significant S-shaped behavior of the PL peak energy with temperature dependence. Additionally, the degree of localization increases as the linear proportion of the PL FWHM, indicating that the excitonic behavior in MgZnO films belong to the strong localization effect.

Effective improvement in optical properties of colloidal CdTe@ZnS quantum dots synthesized from aqueous solution

Efficient synthesis of high-quality quantum dots (QDs) with excellent optical properties by aqueous synthesis is still of great significance for extended optical applications. Herein we highlight the advantages in optical properties of colloidal CdTe@ZnS QDs prepared by a facile and highly effective aqueous synthesis method. These achievements were realized by delicate manipulation of the conditions involved in nucleation and the growth process. Transmission electron microscopy (TEM) images indicated the QDs were uniform size and well dispersible. The emission peaks of the as-prepared QDs could shift from 496 to 698 nm with narrow full width at half maximum (FWHM), and the corresponding fluorescent color changed from green to red. Moreover, the emission could even reach to the near-infrared (NIR) region (706–796 nm) by extending the reaction time. The highest photoluminescence (PL) quantum yield (QY) of the QDs could reach to 60%, and the average of FWHM was about 55 nm. To address the problem of wide size-distribution in PL QY decrease and FWHM broadening, the colloids of QDs prepared at long reaction time (above 3 h) were centrifuged (12 000 r min−1). In addition, the assessment of QD cytotoxicity indicated the CdTe@ZnS QDs were much less cytotoxic and showed good biocompatibility. Compared with organic synthesis, our aqueous synthesis of QDs could be carried out efficiently on a large scale and showed good batch-to-batch reproducibility. The as-prepared CdTe@ZnS QDs exhibited excellent optical properties and hold a good potential to be applied in optoelectronic and biological applications.

Temperature Dependence of Emission Properties of Self-Assembled InGaN Quantum Dots

Emission properties of self-assembled green-emitting InGaN quantum dots (QDs) grown on sapphire substrates by using metal organic chemical vapor deposition are studied by temperature-dependent photoluminescence (PL) measurements. As temperature increases (15–300 K), the PL peak energy shows an anomalous V-shaped (redshift—blueshift) variation instead of an S-shaped (redshift—blueshift—redshift) variation, as observed typically in green-emitting InGaN/GaN multi-quantum wells (MQWs). The PL full width at half maximum (FWHM) also shows a V-shaped (decrease—increase) variation. The temperature dependence of the PL peak energy and FWHM of QDs are well explained by a model similar to MQWs, in which carriers transferring in localized states play an important role, while the confinement energy of localized states in the QDs is significantly larger than that in MQWs. By analyzing the integrated PL intensity, the larger confinement energy of localized states in the QDs is estimated to be 105.9 meV, which is well explained by taking into account the band-gap shrinkage and carrier thermalization with temperature. It is also found that the nonradiative combination centers in QD samples are much less than those in QW samples with the same In content.

Luminescent high temperature sensor based on the CdSe/ZnS quantum dot thin film

A high temperature sensor based on the multi-parameter temperature dependent characteristic of photoluminescence (PL) of quantum dot (QD) thin film is demonstrated by depositing the CdSe/ZnS core/shell QDs on the SiO2 glass substrates. The variations of the intensity, the peak wavelength and the full width at half maximum (FWHM) of PL spectra with temperature are studied experimentally and theoretically. The results indicate that the peak wavelength of the PL spectra changes linearly with temperature, while the PL intensity and FWHM vary exponentially for the temperature range from 30 °C to 180 °C. Using the obtained temperature dependent optical parameters, the resolution of the designed sensor can reach 0.1 nm/°C.

Nonpolar growth and characterization of a-plane InGaN/GaN quantum well structures with different indium compositions

Characteristics of nonpolar (1 1–2 0) a-plane GaN template on r-plane sapphire substrate and subsequently grown InGaN/GaN quantum well (QW) structures were investigated. The crystal orientation and the defect evolution behavior in a-plane GaN template were confirmed by using selected area diffraction and transmittance electron microscopy. The values of full width at half maximum (FWHM) of (1 1–2 0) X-ray rocking curves along the c- and m-directions were 415 and 595 arcsec respectively. The optical characteristics of a-plane InGaN/GaN QW samples with different indium compositions were intensively studied by using temperature-dependent photoluminescence (PL) spectra. The thermal activation energy for localized states increased from 26.5 to 41 meV with increase of indium compositions. Also their relative PL efficiencies and FWHM values showed a potential for nonpolar-based light emitters.

Structural and optical properties of Zn(Mg,Cd)O alloy films grown by remote-plasma-enhanced MOCVD

We report the structural and optical properties of wurtzite-structure Zn(Mg,Cd)O ternary alloys. Wurtzite (0 0 0 1) Zn 1− x Cd x O and Mg y Zn 1− y O films were grown on (11–20) sapphire substrates using remote-plasma-enhanced metalorganic chemical vapor deposition. The large bowing parameters of Zn 1− x Cd x O and Mg y Zn 1− y O ternary alloys are 3.0 and 3.5, respectively, which reflects the large difference of each binary’s electronegativity. We have analyzed the broadening of photoluminescence (PL) in Zn(Mg,Cd)O alloys on alloy content by taking into account the statistical alloy fluctuation and the localization of the exciton, and have clarified that the localization of the exciton strongly affects to PL full-width at half-maximum (FWHM) in Zn(Mg,Cd)O alloys. The alloy broadenings in steady-state PL of Zn(Mg,Cd)O alloys are in good agreement with the calculated tendency by the theoretical model based on the statistical alloy fluctuation, while PL FWHM of Zn 1− x Cd x O is three times larger than the calculated results. Moreover, as another way to confirm alloy broadening, we also have done time-resolved PL measurements and derived the localized depth of the exciton in ZnO-based system, indicating a good agreement with the tendency of PL FWHM broadening.

Red Emission from ZnO-Based Double Heterojunction Diode

We have successfully realized high-quality red-band-gap Zn1-xCdxO films by remote-plasma-enhanced metalorganic chemical vapor deposition (RPE-MOCVD). By using the in-situ thermal annealing process, the compositional homogeneity of the Zn1-xCdxO films has been markedly improved. Proper annealing in vacuum provides a full-width at half-maximum (FWHM) in photoluminescence (PL) of 126 meV, which is almost comparable to that of typical ZnO films. ZnO-based double heterojunction (DH) system with in-situ post-annealed Zn1-xCdxO film is grown on p-4H-SiC substrate, which shows red electroluminescence (EL) with improved PL FWHM of 213 meV.

Luminescence and surface properties of MgxZn1−xO thin films grown by pulsed laser deposition

We investigated the surface roughness and the luminescence properties of MgxZn1−xO thin films (0⩽x⩽0.19). The thin films were grown on a-plane sapphire substrates by pulsed laser deposition. The root-mean-square surface roughness depends on the oxygen partial pressure p(O2) applied during deposition and takes a minimal value of 0.6nm for p(O2)=1×10−3mbar. Deposition of the thin films on a ZnO buffer layer further diminishes the surface roughness. The photoluminescence maximum peak from the MgxZn1−xO thin films is due to free excitons, which are localized in potential minima at low temperatures, and shows a linear high-energy shift with increasing x. The full width at half maximum (FWHM) of the photoluminescence depends on p(O2). Samples with the smallest FWHM values were grown at p(O2)=1.6×10−2mbar. We analyze the contributions of alloy broadening and of the lateral Mg-concentration inhomogeneity to the photoluminescence FWHM. In optimized samples FWHM is limited by random alloy broadening. Scanning cathodoluminescence shows that the lateral inhomogeneity of the Mg distribution decreases with p(O2) if the samples are grown directly on the sapphire substrates. Deposition of the MgxZn1−xO thin films on a ZnO buffer layer leads to a laterally homogeneous Mg distribution in the samples.

Comparison between multilayered InAs quantum dot lasers with different dot densities

This letter describes the dependence of the dot density per layer on laser performance for 1.3-μm range multilayered InAs quantum dot (QD) lasers having 12-QD layer in order to increase the maximum modal gain of the ground state (GS). Twelve-QD lasers with almost identical photoluminescence (PL) intensities and full width at half maximum (FWHM) were prepared, with a dot density per layer in the range of (1.4–3.0)×1010cm−2. Results for the undoped 12-QD lasers reveal that the lasers with almost the same PL intensity and FWHM have an almost identical relationship between the threshold current density (Jth) and the mirror loss and have much the same maximum mirror loss from which lasers can oscillate from the GS. This is probably due to the fact that the Jth of dot lasers fabricated by the self-assembled method are strongly dominated by the gain width that is governed by the nonuniformity of QDs.

Growth of AlAsSb/InGaAs MBE-layers for all-optical switches

AlAsSb- and InGaAs(:Si)-layers have been grown by molecular beam epitaxy lattice matched to InP-substrates and characterized by X-ray diffraction, photoluminescence (PL) and capacitance–voltage profiling. Multi-quantum-well structures with 10 periods of 6 nm InGaAs-wells and 15 nm of AlAsSb-barriers were investigated in terms of interface abruptness and Si-doping. The PL full-width at half-maximum (PL-FWHM) at room temperature of undoped structures was reduced from 137 to 84 nm due to growth interruptions at interfaces under As-flow and with In and Sb segregation compensation. The Si doping by co-deposition of the InGaAs wells was compared to δ-doping centered to the wells. Uniformly doped InGaAs-wells with a Si concentration of 1×10 19 atoms/cm 3 showed an increased PL-FWHM of 178 nm due to degradation of interfaces. By δ-doping with 0.05 monolayers of Si in the middle of the wells the PL-FWHM could be reduced to 122 nm and the intensity of the signal could be increased compared to the uniformly doped quantum wells.

Structural properties of strained piezoelectric [1 1 1]A-oriented InGaAs/GaAs quantum well structures grown by MOVPE

We report the structural properties of strained InGaAs/GaAs double confinement quantum well (QW) structures grown on (1 1 1)A GaAs by metalorganic vapor phase epitaxy. The structures were extensively analyzed by photoluminescence (PL) spectroscopy, High-resolution X-ray diffractometry (HRXRD) and transmission electron microscopy (TEM). Excellent interfacial characteristics and uniformly strained structures with no compositional modulation were obtained for [1 1 1]A-oriented QWs with well widths of 65–100 Å containing up to ∼20% In grown over a range of growth temperatures and V/III ratios. HRXRD analyses also show that the InGaAs well layers are pseudomorphic. A narrow PL full-width-at-half-maximum (FWHM) of 7 meV at 12 K was achieved for a QW structure grown at 630°C with a V/III ratio of 260. A monolayer (ML) analysis of PL FWHM data indicates a well width fluctuation of ±1 ML, which is consistent with TEM observations showing a well width fluctuation of less than ±2 ML. The well width fluctuation values of [1 1 1]A QWs are in the range of ±(1–2) ML for growth temperatures of 630°C and 680°C with V/III ratios between 105 and 260, while for simultaneously grown [1 0 0]-oriented structures the fluctuations are in the range ±2 to over ±4 ML, depending on the V/III ratio.

Laser-MBE growth of high-quality ZnO thin films on Al 2 O 3 (0001) and SiO 2 /Si(100) using the third harmonics of a Nd:YAG laser

High-quality ZnO thin films were grown on single-crystalline Al2O3(0001) and amorphous SiO2/Si(100) substrates at 400–640 °C using laser molecular beam epitaxy. For film growth, the third harmonics of a pulsed Nd:YAG laser were illuminated on a ZnO target. The ZnO films were epitaxially grown on Al2O3(0001) with the narrow X-ray diffraction full width at half maximum (FWHM) of 0.04° and the films on SiO2/Si(100) exhibited a preferred c-axis orientation. Furthermore, the films exhibited excellent optical properties in photoluminescence (PL) measurements with very sharp excitonic and weak deep-level emission peaks. At 15 K, PL FWHM values of the films grown on Al2O3(0001) and SiO2/Si(100) were 3 and 18 meV, respectively.

Optoelectronic device performance on reduced threading dislocation density GaAs/Si

A technique for the heteroepitaxy of GaAs/Si films having reduced threading dislocation density is presented. The important attribute of this technique is the suppression of three-dimensional Volmer–Weber island formation during initial deposition. This suppression is achieved by deposition of a stoichiometric GaAs buffer layer by a migration-enhanced epitaxy technique on silicon at 348 K to a thickness greater than the “monolithic thickness,” hm. Subsequent GaAs films deposited by conventional molecular beam epitaxy on buffer layers of thickness greater than hm possess structural and optical characteristics that exceed those for state-of-the-art GaAs/Si layers: an x-ray full width at half maximum (FWHM) of 110 arcsec with a dislocation density at the film surface of 3×106 cm−2 and a concomitant 4 K photoluminescence FWHM of 2.1 meV. The p-i-n structures suitable for use as light-emitting diodes (LEDs) that were grown on the reduced threading dislocation density GaAs/Si and by means of forward- and reverse-bias measurements, demonstrated an ideality factor of n=1.5, an increased reverse-bias breakdown electric field of 2.1×107 V/m, and an intrinsic region resistivity of 4×107 Ω cm for LEDs of increasingly smaller mesa size.

ALE growth and optical characterization of ZnSe/ZnS strained quantum well structures on Si substrate

The ZnSe/ZnS strained single quantum well (SSQW) and strained multiple quantum well (SMQW) structures were grown on (1 0 0)-oriented Si substrates at 150°C in a horizontal cold-wall quartz reactor by low-pressure metalorganic atomic layer epitaxy (ALE) at a pressure of 30 Torr. Dimethylzinc (DMZn), hydrogen sulfide (H 2S) and hydrogen selenide (H 2Se) were used as the reactants. X-ray diffraction (XRD), secondary ion mass spectromery (SIMS) and photoluminescence (PL) were used to characterize the strained quantum wells. The high-quality crystalline ZnS(1000 Å)/ZnSe(7l Å)/ZnS(1000 Å)/Si sub. SSQW structure exhibits a 32 meV full-width at half-maximum (FWHM) for the 7 K PL spectra. The PL peak emission energy dependence on the well thickness for the ZnSe/ZnS SSQW structure is in good agreement with predictions based on a parabolic well-strain mode. In addition, the well-thickness dependence of the PL peak energy and FWHM of SSQW structures are described in detail. The temperature dependence of the PL peak energy and FWHM of SMQW structures are also measured from 7 to 156 K using a He–Cd laser operated at a wavelength of 325 nm as an excitation source. Better understanding of the temperature-dependent behavior would be helpful in the optoelectronic device development.

Effect of In xGa 1− xAs (0⩽ x⩽0.4) capping layer on self-assembled 1.3 μm wavelength InAs/GaAs quantum islands

We report the effect of In x Ga 1− x As (0⩽ x⩽0.4) capping layer on photoluminescence (PL) properties of 1.3 μm wavelength self-assembled InAs quantum islands, which are formed via depositing 3.5 monolayers (ML) InAs on GaAs (1 0 0) substrate by molecular beam epitaxy (MBE). Compared with the In x Ga 1− x As capping layer containing a larger In mole fraction x⩾0.2 and the GaAs capping layer ( x=0), the InAs islands covered by the In 0.1Ga 0.9As layer show PL with lower emission energy, narrower full-width at half-maximum (FWHM), and quite stronger intensity. The PL peak energy and FWHM become more temperature dependent with the increase of In content in the In x Ga 1− x As capping layer ( x⩾0.2), while the InAs islands covered by the In 0.1Ga 0.9As layer is much less temperature sensitive. In addition, the InAs islands covered by the In 0.1Ga 0.9As capping layer show room temperature PL wavelength at about 1.3 μm.

Growth of GaN layer by metal-organic chemical vapor deposition system with a novel three-flow reactor

For the first time, this paper reports a novel kind of three-flow reactor which can be developed for metal-organic chemical vapor deposition (MOCVD) system to grow GaN-based structures. Two parallel carrier gas-flow for Group-III sources and NH 3 separated by a quartz plate which are also parallel to the substrate are used. The N 2, which is used as a sub-flow, flows through a quartz filter perpendicular to the substrate where it meets the two parallel group III and group V source flows. This new flow arrangement enhances the mixture of group III–V sources at the contacting substrate, compared with the previous three parallel laminar gas MOCVD systems due to the perpendicular direction of the N 2 sub-flow. Based on this MOCVD system with this special configuration, a mirror-like epitaxial GaN layer on sapphire substrate is obtained with an excellent uniformity. Also, the quality of the GaN layers are characterized by room temperature photoluminescence (PL) and X-ray diffraction (XRD) measurements, where the full-width at half-maximum (FWHM) of 41.12 meV for PL spectra and FWHM of about 220 arcsec for XRD rocking curve were measured, respectively. In addition, the carrier concentration and Hall mobility are found to be 2.6×10 17/cm 3 and 300 cm 2/V s, respectively.