Quantum Dot Layers Research Articles

Advanced dry etching of GaAs/AlGaAs multilayer wafer with InAs quantum dot for circular defect in photonic crystal laser

We previously proposed the circular defect in two-dimensional photonic crystal (CirD) laser based on a GaAs/AlGaAs multilayer wafer, which is fabricated by dry etching. This CirD laser uses InAs quantum dot (QD) layers as the gain medium, but this inhibits vertical dry etching. We improved the dry etching process by introducing three QD layers and three-step dry etching for fabricating the CirD laser. As a result, CirD structures with good etching profiles could be fabricated and excellent optical properties were obtained. We observed lasing by a CirD laser fabricated by deep etching under photoexcitation measurement for the first time.

Enhanced optical properties and dark I-V characteristics of silicon nanowire-carbon quantum dots heterostructures

Herein, we report a silicon nanowire (SiNW) array-carbon quantum dot (CQD) heterostructure photovoltaic device via direct coating of CQD on chemically-etched SiNW arrays aided by Ag. By using carbon quantum dots layer as a competent element for surface passivation and modification for SiNWs, the solar cells efficiency is improved. The 1.6 times absorption enhancement has been recorded for nitrogen doped CQD decorated pyramidal SiNW heterostructure in comparison to that of CQDs coated silicon nanowires on the planar surfaces. Inclusion of nitrogen doped CQDs into the pyramidal SiNW arrays gives enhanced absorption intensity which can act as a good absorber layer in solar cell. The heterostructure also displays a significant photoluminescence in the blue region as probed by using time-resolved photoluminescence (TRPL) technique which provide the insight into the recombination mechanism in the synthesized heterostructures and is discussed in detail.

Multi-step photon upconversion in quantum-dot-based solar cells with a double-heterointerface structure

Photon upconversion (PU) is a process where an electron is excited from the valence band to the conduction band of a wide-gap semiconductor by the sequential absorption of two or more photons via real states. For example, two-step PU can generate additional photocurrent in the so-called intermediate-band solar cells. In this work, we consider two- and three-step processes; we study multi-step PU in a quantum dot (QD)-based single-junction solar cell with a double-heterointerface structure. The solar cell consists of three different absorber layers: Al0.7Ga0.3As, Al0.3Ga0.7As, and GaAs, which form two heterointerfaces. Just beneath each heterointerface, an InAs/GaAs QD layer was inserted. After band-to-band excitation, electrons accumulate at each heterointerface, and then, below-bandgap photons excite a certain fraction of these electrons above the barrier energy. The photoluminescence spectra of the InAs QDs reveal slightly different QD size distributions at the two heterointerfaces. We show that the external quantum efficiency is improved by additional irradiation with below-bandgap infrared photons, which suggests a multi-step PU process that involves the two heterointerfaces. The dependence of the photocurrent on the infrared excitation power density only shows a superlinear behavior when the GaAs layer is excited but the Al0.3Ga0.7As layer is not. These data demonstrate a multi-step PU process that consists of one intraband transition at each of the two heterointerfaces and one interband transition in GaAs.

Ultrasensitive and simultaneous monitoring of multiple small-molecule pollutants on an immunochromatographic strip with multilayered film-like fluorescent tags

A wide variety of small-molecule pollutants is harmful to human health, and their highly sensitive universal and rapid detection in complex environments remains a challenge. Herein, a multiplexed and ultrasensitive immunochromatographic strip (ICS) was developed for the universal analysis of three kinds of different pollutants based on multilayered fluorescent nanofilm-guided signal amplification. Flexible three-dimensional nanofilms (GO–MQD) with large surface areas, high quantum dot (QD) loading, superior luminescence, and good stability were synthesized through the electrostatic adsorption-mediated layer-by-layer assembly of three layers of small QDs onto two-dimensional graphene oxide (GO) nanosheets, modified with specific antibodies, and utilized as enhanced fluorescent tags in the ICS method for quantitative target detection. By combining the GO–MQD nanofilms and multiplexed ICS, the proposed assay can rapidly and sensitively detect aflatoxin B1, clenbuterol, and kanamycin in actual samples/environmental samples (pork extract, milk, river water, and lake water) with low detection limit (0.87, 2.04, and 0.81 pg/mL), fast testing time (15 min), good stability and high reproducibility (RSD < 8.71 %). The GO–MQD–ICS method developed here exhibits great potential to meet the demands of the on-site and practical detection of small-molecule pollutants.

Far-Red Interlayer Excitons of Perovskite/Quantum-Dot Heterostructures.

Interlayer excitons (IXs) at the interface of heterostructures (HSs) with a staggered band alignment are fascinating quantum quasi-particles with light-emitting and long-lifetime characteristics. In this study, the energy band alignments (EBAs) of the HS of MAPbI3 perovskite thin sheets with CdSe-ZnS core-shell quantum dot (QD) layers are modulated by using different diameters of the QDs. Far-red IX emission is observed at 1.42eV from the HS of MAPbI3 /CdSe-ZnS-QD (λem =645nm) with type-II EBA owing to charge transfer. The lifetime of the far-red IXs is estimated to be 5.68µs, which is considerably longer than that (0.715ns) of the intralayer excitons from CdSe-ZnS-QD. With increasing incident excitation power, the PL peak and its intensity of IXs are blue-shifted and linearly increased, respectively, indicating a strong dipole alignment of far-red IXs at the heterojunction. Back focal plane imaging suggests that the directions of dipole moments of the IXs are relatively out-of-plane compared to those of the intralayer excitons (MAPbI3 and CdSe-ZnS-QD). Notably, the abnormal behavior of the optical characteristics is observed near the phase transition temperature (90K) of MAPbI3 . MAPbI3 /CdSe-ZnS-QD HS photodetectors show the increase in photocurrent and detectivity compared to MAPbI3 at IX excitation.

Modeling charge transport mechanism in inorganic quantum dot light-emitting devices through transport layer modification strategies

Quantum dot light-emitting devices (QLEDs) are potential candidates for lighting and display applications. The charge transport mechanism which plays an essential part in the performance of these devices, however, needs to be explored and analyzed for further improvement. The imbalance of the injection and transport of charge carriers within the device adversely affects the efficiency and stability of the device. Charge balance can be improved by better charge injection of holes while suppressing the excessive electrons. A simple and effective strategy to achieve this is using double transport layers or doped transport layers to modulate the band alignment and injection of charge carriers. Here, we propose a new structure and investigate the physical processes within a QLED with a double hole transport layer for improved charge injection of holes and a doped electron transport layer for controlled charge injection of electrons. We find that the process of charge injection, tunneling, and recombination is significantly improved within the quantum dot layer and a better charge balance is achieved in the emissive layer. Through the theoretical simulation model, useful results are obtained which pave the way for designing high-performing QLEDs.

Modified Zinc Magnesium Oxide for Optimal Charge‐Injection Balance in InP Quantum Dot Light‐Emitting Diodes

AbstractBalanced charge injection into the emissive layer is a prerequisite for achieving highly efficient quantum dot light‐emitting diodes (QLEDs). The similar energy distribution of charge transport layers and indium phosphide (InP) quantum dots (QDs) facilitates excess electron injection to the InP QD layer. In this study, magnesium‐doped ZnO nanoparticles (ZMO NPs) are modified to suppress the electron injection to the InP QD layer. Particularly, hydroxyl groups are effectively replaced by electrically stable states through the passivation of ZMO NPs, retarding electron injection through the ZMO NP layer. The modified ZMO‐NP‐based InP QLEDs exhibit a maximum external quantum efficiency of 9.6% with a substantially enhanced operational lifetime compared with that of devices made with unmodified ZMO NPs.

High performance flexible photodetector based on 0D-2D perovskite heterostructure

Flexible photodetectors (PDs) comprised of low-dimensional organic-inorganic hybrid perovskites having perovskite quantum dots are the next generation wearable optoelectronic devices. Here, a flexible Vis-NIR PD is designed containing 2D DJ perovskite (4AMP)(MA) 2 Pb 3 I 10 (4AMP = 4-(aminomethyl)piperidinium, MA = methylammonium) (n3) and the micro concentration of CsPbI 3 perovskite quantum dots (QDs) layered heterostructures. The device response under 660 nm light is increased to 615% controlled by the optimal concentration of QDs. The device combination as per the mass of QDs depicts strong photosensitivity and high-power output. The band gap between the two is minimal forming a matching structure which lowers the energy barrier of carrier transport process. QDs layer fills the gap of perovskite film forming an almost defect-free heterostructure. QDs layer isolates water and passivates the perovskite layer for the high-performance of optoelectronic devices. The degradation of PDs with the optimal concentration of QDs under up to 5000 bending cycles and different bending angles can be ignored, and the devices show a self-healing phenomenon with the increase of bending cycles. The optimized strategy will develop flexible, wearable, high-performance and low-cost PDs. QDs-concentration-dependent based on 0D-2D perovskite heterostructure was applied to enhance the performance of flexible photodetector.

Direct Photo-Patterning of Efficient and Stable Quantum Dot Light-Emitting Diodes via Light-Triggered, Carbocation-Enabled Ligand Stripping.

Next generation displays based on quantum dot light-emitting diodes (QLEDs) require robust patterning methods for quantum dot layers. However, existing patterning methods mostly yield QLEDs with performance far inferior to the state-of-the-art individual devices. Here, we report a light-triggered, carbocation-enabled ligand stripping (CELS) approach to pattern QLEDs with high efficiency and stability. During CELS, photogenerated carbocations from triphenylmethyl chlorides remove native ligands of quantum dots, thereby producing patterns at microscale precision. Chloride anions passivate surface defects and endow patterned quantum dots with preserved photoluminescent quantum yields. It works for both cadmium-based and heavy-metal-free quantum dots. CELS-patterned QLEDs show remarkable external quantum efficiencies (19.1%, 17.5%, 12.0% for red, green, blue, respectively) and a long operation lifetime (T95 at 1000 nits up to 8700 h). Both are among the highest for patterned QLEDs and approach the records for nonpatterned devices, which makes CELS promising for building high-performance QLED displays and related integrated devices.

Introduction of Multilayered Dual-Signal Nanotags into a Colorimetric-Fluorescent Coenhanced Immunochromatographic Assay for Ultrasensitive and Flexible Monitoring of SARS-CoV-2.

Timely, accurate, and rapid diagnosis of SARS-CoV-2 is a key factor in controlling the spread of the epidemic and guiding treatments. Herein, a flexible and ultrasensitive immunochromatographic assay (ICA) was proposed based on a colorimetric/fluorescent dual-signal enhancement strategy. We first fabricated a highly stable dual-signal nanocomposite (SADQD) by continuously coating one layer of 20 nm AuNPs and two layers of quantum dots onto a 200 nm SiO2 nanosphere to provide strong colorimetric signals and enhanced fluorescence signals. Two kinds of SADQD with red and green fluorescence were conjugated with spike (S) antibody and nucleocapsid (N) antibody, respectively, and used as dual-fluorescence/colorimetric tags for the simultaneous detection of S and N proteins on one test line of ICA strip, which can not only greatly reduce the background interference and improve the detection accuracy but also achieve a higher colorimetric sensitivity. The detection limits of the method for target antigens via colorimetric and fluorescence modes were as low as 50 and 2.2 pg/mL, respectively, which were 5 and 113 times more sensitive than those from the standard AuNP-ICA strips, respectively. This biosensor will provide a more accurate and convenient way to diagnose COVID-19 in different application scenarios.

Tracing spatial confinement in semiconductor quantum dots by high-order harmonic generation

We report here on results of systematic experimental-theoretical investigation of high-order harmonic generation (HHG) in layers of CdSe semiconductor quantum dots of different sizes and a reference bulk CdSe thin film. We observe a strong decrease in the efficiency, up to complete suppression of HHG with energies of quanta above the band gap for the smallest dots, whereas the intensity of below band gap harmonics remains weakly affected by the dot size. In addition, it is observed that the suppression of the above gap harmonics is enhanced with increasing the driving wavelength. These systematic investigations allow us to develop a simple physical picture explaining the observed suppression of the highest harmonics: the discretization of electronic energy levels seems to be not the predominant contribution to the observed suppression but rather the confined dot size itself, causing field-driven electrons to scatter off the dot's walls. The reduction in the dot size below the classical electron oscillatory radius and the corresponding scattering limits the maximum acceleration by the laser field. Moreover, this scattering leads to a chaotization of motion, causing dephasing and a loss of coherence, therefore suppressing the efficiency of the emission of highest-order harmonics. Our results demonstrate a regime of intense laser-nanoscale solid interaction, intermediate between the bulk and single-molecule response, and are crucial for nanophotonic platforms aiming at control over high-order harmonic properties and efficiency.

Solvent Modulation of Chiral Perovskite Films Enables High Circularly Polarized Luminescence Performance from Chiral Perovskite/Quantum Dot Composites.

Materials with circularly polarized luminescence (CPL) activity are promising in many chiroptoelectronics fields, such as for biological probes, asymmetric photosynthesis, information storage, spintronic devices, and so on. Promoting the value of the dissymmetry factor (glum) for the CPL-active materials based on chiral perovskite draws increasing attention since a higher glum value indicates better CPL. In this work, we find that, after being treated with a facile solvent modulation strategy, the chirality of 2D chiral perovskite films has been enhanced a lot, which we attribute to an increased lattice distortion degree. By forming chiral perovskite/quantum dot (QD) composites, the CPL-active material is successfully obtained. The calculated maximum |glum| of these composites increased over 4 times after solvent modulation treatment (1.53 × 10-3 for the pristine sample of R-DMF and 6.91 × 10-3 for R-NMP) at room temperature. Moreover, the enhancement of the CPL intensity is ascribed to two aspects: one is the generation and transportation of spin-polarized charge carriers from chiral perovskite films to combine in the QD layer, and the other is the solvent modulation strategy to enlarge the lattice distortion of chiral perovskite films. This facile route provides an effective way to construct CPL-active materials. More importantly, this kind of composite material (chiral perovskite film/QD layer) can be easily applied for fabricating circularly polarized light-emitting diode devices for electroluminescence.

Multiple Function Synchronous Optimization by PbS Quantum Dots for Highly Stable Planar Perovskite Solar Cells with Efficiency Exceeding 23%

AbstractColloidal lead sulfide (PbS) quantum dots (QDs), which possess quantum confinement effect and processing compatibility with perovskite, are regarded as an excellent material for optimizing perovskite solar cells (PSCs). However, the existing PSCs optimized by PbS QDs are still facing the challenges of poor performance of the charge transport layers, low utilization in the near‐infrared (NIR) region, and unsuitable energy level alignment, which limit the improvement of power conversion efficiency (PCE). Herein, a synchronous optimization strategy is realized via simultaneously introducing PbS QDs into SnO2 electron transport layer and employing rare‐earth‐doped PbS QDs (Eu:PbS QDs) film with hydrophobic chain ligands as the NIR light‐absorping layer and hole transport layer (HTL) of devices. PbS QDs effectively decrease the density of trap states by passivating defects. Eu:PbS QDs film with adjustable bandgap is employed as an absorption layer to broaden the NIR spectral absorption. The well‐matched energy level between Eu:PbS QDs layer and perovskite layer implies efficient hole transfer at the interface. The successful synchronous optimization greatly elevates all photovoltaic parameters, reaching a maximum PCE of 23.27%. This PCE is the highest for PSCs utilizing PbS QDs material in recent years. The optimized PSCs retain long‐term moisture and light stability.

Long-Wavelength Luminescence of InSb Quantum Dots in Type II Broken-Gap Heterostructure

The features of the electroluminescence spectra of narrow-gap type II InAs/InSb/InAs heterostructures containing a single layer of InSb quantum dots placed into the p-n-InAs junction were studied. The luminescent properties of the heterostructures under a forward and reverse bias in the temperature range of 77–300 K were investigated as a function of the surface density of nano-objects buried in the narrow-gap matrix. When applying the reverse bias to the heterostructures under study, the suppression of negative interband luminescence and the dominance of interface recombination transitions at the InSb/InAs type II heterojunction were observed at room temperature. The radiation, which corresponded to recombination transitions involving localized electron-hole states of the InSb quantum dots, was revealed and recorded at low temperatures.

The Photonic Atom Probe as a Tool for the Analysis of the Effect of Defects on the Luminescence of Nitride Quantum Structures.

By collecting simultaneously optical and chemical/morphological data from nanoscale volumes, the Photonic Atom Probe (PAP) can be applied not only to the study of the relationship between optical and structural properties of quantum emitter but also to evaluate the influence of other factors, such as the presence of point defects, on the photoluminescence. Through the analysis of multiple layers of InGaN/GaN quantum dots (QDs), grown so that the density of structural defects is higher with increasing distance from the substrate, we establish that the light emission is higher in the regions exhibiting a higher presence of structural defects. While the presence of intrinsic point defects with non-radiative recombination properties remains elusive, our result is consistent with the fact that QD layers closer to the substrate behave as traps for non-radiative point defects. This result demonstrates the potential of the PAP as a technique for the study of the optical properties of defects in semiconductors.

Stable Quantum Dot Light-Emitting Diodes Employing TiO2 Nanoparticles as a Mixed Emission Layer

Colloidal quantum dots (QDs) have attractive optical and electrical characteristics for various applications in the display industry. They have superb properties including color tunability, which is achieved by controlling the particle size and solution processes used for quantum dot light-emitting diodes (QLEDs). Here we demonstrate high efficiency QLEDs that consist of a mixed layer of TiO2 nanoparticles (NPs) and QDs. ZnO is the key material responsible for the promising results of the electroluminescence devices, which exhibited well-matched energy levels and robustness. In this report, Li-doped TiO2 NPs were synthesized under ambient conditions as an alternative electron transport layer (ETL) for QLEDs. A stable mixture of TiO2 NPs and QDs was prepared in chlorobenzene and then applied to QLEDs without a conventional ETL. QLEDs with such a simplified structure produce a maximum luminance of 123,311 cd/m&lt;sup&gt;2&lt;/sup&gt; with a current efficiency of 40.5 cd/A. These results are comparable to those of conventional QLEDs. Additionally, the predicted T50 at 100 cd/m&lt;sup&gt;2&lt;/sup&gt; was 1,420 h, based on the T50 at 1,600 cd/m&lt;sup&gt;2&lt;/sup&gt;. These clearly indicate not only the promising results of the TiO2 NPs as an inorganic ETL, but also the remarkable performance of the simplified device with less layers. The reduction in fabrication steps using this solution-based process is also advantageous for next-generation display technology.

Исследование фотолюминесценции в системе InGaAs/GaAs с квантовыми точками спектрального диапазона 1100 нм

The results of studying the optical properties of InGaAs quantum dots are presented. Single-layer InGaAs quantum dots with a height of 5.3, 3.6 and 2.6 monolayers, as well as three-stacked layers of tunnel-uncoupled quantum dots with a height of 2.6 monolayers were formed by molecular–beam epitaxy according to the Stransky-Krastanov mechanism on GaAs substrates, using the partial capping and annealing technique. A decrease in the size of quantum dots makes it possible to carry out a blueshift of the photoluminescence spectrum maximum from 1200 nm to 1090 nm, and an increase in the number of QD layers makes it possible to compensate for the decrease in the peak intensity. It is shown that this type of quantum dots is suitable for creating the lasers active regions with a vertical microcavity for neuromorphic computing.

Interplay of Purcell effect and extraction efficiency in CsPbBr3 quantum dots coupled to Mie resonators.

Inorganic halide perovskite quantum dots have risen in recent years as efficient active materials in numerous optoelectronic applications ranging from solar cells to light-emitting diodes and lasers, and have lately been tested as quantum emitters. Perovskite quantum dots are often coupled to photonic structures either to enhance their emission properties, by accelerating their emission rate thanks to the Purcell effect, or to increase light extraction. From a theoretical point of view, the first effect is often considered at the single-dipole level while the latter is often treated at the mesoscopic level, except possibly for quantum emitters. In this work we employ a layer of perovskite quantum dots coupled to dielectric Mie resonators to exploit both effects simultaneously and achieve an 18-fold increase in luminescence. Our numerical simulations, combined with spatially- and time-resolved photoluminescence measurements, reveal how the macroscopic response of the perovskite-on-Mie resonator structure results from the interplay of the two effects averaged over the whole spatial distribution of emitters. Our work provides thus guiding principles for maximizing the output intensity of quantum emitters embedded into photonic resonators as well as classical emitters integrated in perovskite-based optoelectronic devices.

Излучательная рекомбинация в разъединенном гетеропереходе II типа InAs/InSb с квантовыми точками на интерфейсе

The electroluminescent properties of narrow-gap type II InAs/InSb/InAs heterostructures containing a single layer of InSb quantum dots placed at the interface of the p-n junction in InAs were studied. The features of the electroluminescence spectra depending on the surface density of nanoobjects at a broken-gap type II heterointerface were investigated both at forward and reverse bias. When applying a reverse bias to the heterostructures under study, the suppression of negative interband luminescence and the dominance of interface recombination transitions at the InSb/InAs type II heterojunction were observed at room temperature. The radiation, which corresponded to recombination transitions involving localized states of InSb quantum dots, was recorded at low temperature.

Investigation of photoluminescence in the InGaAs/GaAs system with 1100-nm range quantum dots

The results of studying the optical properties of InGaAs quantum dots are presented. Single-layer InGaAs quantum dots with a height of 5.3, 3.6 and 2.6 monolayers, as well as three-stacked layers of tunnel-uncoupled quantum dots with a height of 2.6 monolayers were formed by molecular-beam epitaxy according to the Stransky--Krastanov mechanism on GaAs substrates, using the partial capping and annealing technique. A decrease in the size of quantum dots makes it possible to carry out a blueshift of the photoluminescence spectrum maximum from 1200 nm to 1090 nm, and an increase in the number of QD layers makes it possible to compensate for the decrease in the peak intensity. It is shown that this type of quantum dots is suitable for creating the lasers active regions with a vertical microcavity for neuromorphic computing. Keywords: molecular-beam epitaxy, gallium arsenide, InGaAs, Stranski--Krastanow mode.