Quantum Dot Layers Research Articles

Friction-Induced Clustered Rearrangement at a PbS Quantum Dot Nanocoating via Long-Term Lubrication under an Atmosphere Environment.

We report a long-term lubrication for a PbS QD nanocoating sliding against bearing steel balls in the air. Through tribo-physchemical interactions, ultralow friction (μ ≈ 0.078 ± 0.0026) is achieved for the system tested under 1 N for 60 min. During the rubbing process, the tribo-film of the counterfacing ball is covered by a degraded PbS QD layer and amorphous mixed phase. Meanwhile, the disc track surface is composed of degraded PbS QD layers, clustered rearranged PbS QD districts, induced decomposed Pb-enriched multilayers, and an amorphous mixed phase via friction-induced structural transformation. The PbS QDs are transferred onto the sliding contacts to form a robust tribo-film, which is the key to realizing ultralow friction. Consequently, a long-term lubrication mechanism is attributed to the synergetic tribo-physchemical interaction along sliding interfaces upon shift, redirection, and decomposition of nanoparticles. These discoveries reveal QD-based nanolubricants in common working conditions for mechanical engineering.

Multi‐Level Anti‐Counterfeiting and Optical Information Storage Based on Luminescence of Mn‐Doped Perovskite Quantum Dots

AbstractTraditional fluorescent anti‐counterfeiting labels can be easily cloned due to the lack of sufficient security. Therefore, multilevel anti‐counterfeiting techniques with higher security need to be constantly explored. This study reports a multilayer composite structure containing Mn doped CsPbCl3 (Mn:CsPbCl3) perovskite quantum dots (PQDs) layer and CsPbBr3 PQDs layer. The encrypted information is patterned on the Mn:CsPbCl3 PQDs layer with weak orange fluorescence, which is concealed by the brighter green fluorescence of CsPbBr3 PQDs at the upper layer. Based on the well‐separated photoluminescence (PL) peak positions, inverse PL temperature‐dependences and distinct PL lifetimes of these two PQDs, the encrypted information can be read out by adding cut‐off filter, heating and using ultrafast camera (UCA)/pulsed excitation (PE). This multilevel anti‐counterfeiting label significantly increases the difficulties of imitation meanwhile no expensive equipment is required for the verification of authenticity. Moreover, two bits of binary numbers that can be represented by dual emissions of the Mn:CsPbCl3 PQDs by changing the doping content, which significantly increases the information storage capacity, are also demonstrated.

Transient Dynamics of Charges and Excitons in Quantum Dot Light-Emitting Diodes.

Wide interest in quantum dot (QD) light-emitting diodes (QLEDs) for potential application to display devices and light sources has led to their rapid advancement in device performance. Despite such progress, detailed operation mechanisms of QLEDs, which are necessary for the fundamental understanding and further improvements, have been still uncertain because of the intricate interaction between charges and excitons in electrical operation. In this work, the transient electroluminescence (TREL) signals of dichromatic QLEDs which are purposely designed to consist of two different color-emitting QD layers are analyzed. As a result, not only can the charge injection and exciton recombination processes be visualized but the electron mobility of the QD layer can also be estimated. Furthermore, the effects of Förster resonant energy transfer between two QDs and exciton quenching near the QD layer are quantitatively measured in QLED operation. The authors believe that their results based on TREL analyses will contribute to the understanding and development of high-performance QLEDs.

Modeling of Relative Intensity Noise in QD-VCSEL

In this paper, the dynamics and statics properties of a 1.3 m InAs/InGaAs Quantum-Dot (QD) vertical-cavity surface-emitting laser (VCSEL) have been investigated using a self-consistent coupled rate-thermal equations model. Results demonstrate that self-heating caused by current transition through the Bragg mirrors and active area of the laser produces temperature rise and deteriorates the performance of the laser where it is most apparent at higher temperatures. Although the minimum value of Relative intensity noise (RIN) and the turn-on delay and maximum value of the output power occurs at the same bias current, the maximum achievable 3-dB modulation bandwidth is obtained at lower bias currents. In addition, the larger aperture size and the greater number of QD layers modify the self-heating and enhance the efficiency and output characteristics of the laser where the rolled-off of the laser happens at higher bias currents.

MoS2/WSe2 vdW Heterostructures Decorated with PbS Quantum Dots for the Development of High-Performance Photovoltaic and Broadband Photodiodes.

van der Waals heterostructures (vdWHs) overcoming the lattice and processing limitations of conventional heterostructures provide an opportunity to develop high-performance 2D vdWH solar cells and photodiodes. However, it is challenging to improve the sensitivity and response speed of 2D vdWH photovoltaic devices due to the low light absorption efficiency and electron/hole traps in heterointerfaces. Here, we design a PbS/MoS2/WSe2 heterostructure photodiode in which a light-sensitive PbS quantum dot (QD) layer combined with a MoS2/WSe2 heterostructure significantly enhances the photovoltaic response. The electron current in the heterostructure is increased by the effective collection of photogenerated electrons induced by PbS QDs. The device exhibits a broadband photovoltaic response from 405 to 1064 nm with a maximum responsivity of 0.76 A/W and a specific detectivity of 5.15 × 1011 Jones. In particular, the response speed is not limited by multiple electron traps in the PbS QDs/2D material heterointerface, and a fast rising/decaying time of 43/48 μs and a -3 dB cutoff frequency of over 10 kHz are achieved. The negative differential capacitance and frequency dependence of capacitance demonstrate the presence of interface states in the MoS2/WSe2 heterointerface that hamper the improvement of the response speed. The scheme to enhance photovoltaic performance without sacrificing response speed provides opportunities for the development of high-performance 2D vdWH optoelectronic devices.

P‐81: Enhanced Color‐Conversion Efficiency of Quantum‐Dot Layer Using Low‐Refractive‐Index Layer

In displays that use QD as color conversion materials, the QD layer must re‐absorb the blue light that passes through it to achieve high‐luminance red and green pixels. In this study, a low refractive index layer was introduced between the color filter and the QDs layer to reuse the light that passes through the QDs layer. This result shows that there is an 8% increase in luminescence compared to that in Ref.

26‐3: Optical Modeling of Quantum Dot‐OLED (QD‐OLED) Color Conversion

Quantum Dots (QDs) as a color converting layer paired with a blue OLED emitter are investigated through a ray‐tracing model. By adjusting various properties of the QD layer, trends in performance such as the color of emitted light, green photon output, blue leakage, and loss mechanisms are investigated. This simple model and associated data analysis provide insight into important design considerations for QD color converted pixels for future display technologies such as QD‐OLED and microLEDs.

SWIR imaging using PbS QD photodiode array sensors.

We fabricated a 1 × 10 PbS QD photodiode array with multiple stacked QD layers with high-resolution patterning using a customized photolithographic process. The array showed the average responsivity of 5.54 × 10-3 A/W and 1.20 × 10-2 A/W at 0 V and -1 V under 1310- nm short-wavelength infrared (SWIR) illumination. The standard deviation of the pixel responsivity was under 10%, confirming the uniformity of the fabrication process. The response time was 2.2 ± 0.13 ms, and the bandwidth was 159.1 Hz. A prototype 1310-nm SWIR imager demonstrated that the QD photodiode-based SWIR image sensor is a cost-effective and practical alternative for III-V SWIR image sensors.

Tunability of solar cell with double quantum dot structure

This work introduces a double quantum dot (QD) structure as an intermediate band for high-performance solar cells. The density matrix equations, coupled with the continuity-current equation, are solved numerically to address the interaction between all the states, which is impossible with the rate equation modeling. This modeling allowed us to obtain quantum efficiency (QE). Using the orthogonalized plane wave assumption for WL-QD, the momentum matrix elements of QD-QD, QD-wetting layer (WL), WL-barrier transitions are calculated. Results are simulated in both the excitonic and non-excitonic (electron-hole, eh) cases and exhibit the importance of adding the QD layer.The results confirm the importance of adding the intermediate band (QD layer). Both band-to-band recombination rates and relaxation rates in the DQD structure are modulated according to the energy difference between DQD states. Compared to the eh model, the excitonic model reduces the QD-QD relaxation rates and all band-to-band recombination rates. The quantum efficiency (QE) is attaining at longer recombination and relaxation times that are confirmed depending on the wider energy difference by manipulating the transitions of the DQD structure.

Charge‐Selective, Narrow‐Gap Indium Arsenide Quantum Dot Layer for Highly Stable and Efficient Organic Photovoltaics

AbstractThe past decade has seen a dramatic surge in the power conversion efficiency (PCE) of next‐generation solution‐processed thin‐film solar cells rapidly closing the gap in PCE of commercially‐available photovoltaic (PV) cells. Yet the operational stability of such new PVs leaves a lot to be desired. Specifically, chemical reaction with absorbers via high‐energy photons transmitted through the typically‐adapted metal oxide electron transporting layers (ETLs), and photocatalytic degradation at interfaces are considered detrimental to the device performance. Herein, the authors introduce a device architecture using the narrow‐gap, Indium Arsenide colloidal quantum dots (CQDs) with discrete electronic states as an ETL in high‐efficiency solution‐processed PVs. High‐performing PM6:Y6 organic PVs (OPVs) achieve a PCE of 15.1%. More importantly, as the operating stability of the device is significantly improved, retaining above 80% of the original PCE over 1000 min under continuous illumination, a Newport‐certified PCE of 13.1% is reported for nonencapsulated OPVs measured under ambient air. Based on operando studies as well as optical simulations, it suggested that the InAs CQD ETLs with discrete energy states effectively cut‐off high‐energy photons while selectively collecting electrons from the absorber. The findings of this works enable high‐efficiency solution‐processed PVs with enhanced durability under operating conditions.

Electrical Stimulation of Neurons with Quantum Dots via Near-Infrared Light.

Photovoltaic biointerfaces offer wireless and battery-free bioelectronic medicine via photomodulation of neurons. Near-infrared (NIR) light enables communication with neurons inside the deep tissue and application of high photon flux within the ocular safety limit of light exposure. For that, nonsilicon biointerfaces are highly demanded for thin and flexible operation. Here, we devised a flexible quantum dot (QD)-based photovoltaic biointerface that stimulates cells within the spectral tissue transparency window by using NIR light (λ = 780 nm). Integration of an ultrathin QD layer of 25 nm into a multilayered photovoltaic architecture enables transduction of NIR light to safe capacitive ionic currents that leads to reproducible action potentials on primary hippocampal neurons with high success rates. The biointerfaces exhibit low in vitro toxicity and robust photoelectrical performance under different stability tests. Our findings show that colloidal quantum dots can be used in wireless bioelectronic medicine for brain, heart, and retina.

Beyond a Linker: The Role of Photochemistry of Crosslinkers in the Direct Optical Patterning of Colloidal Nanocrystals

AbstractSurface chemistry mediated direct optical patterning represents an emerging strategy for incorporating colloidal nanocrystals (NCs) in integrated optoelectronic platforms including displays and image sensors. However, the role of photochemistry of crosslinkers and other photoactive species in patterning remains elusive. Here we show the design of nitrene‐ and carbene‐based photocrosslinkers can strongly affect the patterning capabilities and photophysical properties of NCs, especially quantum dots (QDs). Their role beyond physical linkers stems from structure‐dictated electronic configuration, energy alignment and associated reaction kinetics and thermodynamics. Patterned QD layers with designed carbene‐based crosslinkers fully preserve their photoluminescent and electroluminescent properties. Patterned light emitting diodes (QLEDs) show a maximum external quantum efficiency of ≈12 % and lifetime over 4800 h, among the highest for reported patterned QLEDs. These results would guide the rational design of photoactive species in NC patterning and create new possibilities in the monolithic integration of NCs in high‐performance device platforms.

Beyond a Linker: The Role of Photochemistry of Crosslinkers in the Direct Optical Patterning of Colloidal Nanocrystals.

Surface chemistry mediated direct optical patterning represents an emerging strategy for incorporating colloidal nanocrystals (NCs) in integrated optoelectronic platforms including displays and image sensors. However, the role of photochemistry of crosslinkers and other photoactive species in patterning remains elusive. Here we show the design of nitrene- and carbene-based photocrosslinkers can strongly affect the patterning capabilities and photophysical properties of NCs, especially quantum dots (QDs). Their role beyond physical linkers stems from structure-dictated electronic configuration, energy alignment and associated reaction kinetics and thermodynamics. Patterned QD layers with designed carbene-based crosslinkers fully preserve their photoluminescent and electroluminescent properties. Patterned light emitting diodes (QLEDs) show a maximum external quantum efficiency of ≈12 % and lifetime over 4800 h, among the highest for reported patterned QLEDs. These results would guide the rational design of photoactive species in NC patterning and create new possibilities in the monolithic integration of NCs in high-performance device platforms.

Single Conjugated Polymer with Four Stepwise HOMO Levels for Effective Hole Injection Across Large Barrier 1.4 eV to Core–Shell Quantum Dot Layer for Electroluminescence in Inverted QLED

AbstractA novel graded‐HOMO‐level hole transport polymer (g‐HTP) is proposed for the first time with spiro‐polyfluorene (sPF) as the main chain and the three hole‐transporting moieties triphenyl amine (TPA), carbazole (Cz), and N,N′‐dicarbazolyl‐3,5‐benzene (mCP) as side chains for effective hole injection across the large barrier 1.4 eV into the green‐emission core–shell quantum dot (QD) CdSe/ZnS emission layer in inverted QD light‐emitting diode (i‐QLED): ITO/ZnO/QD/HTP/MoO3/Al, in which both oleophilic ligands (OA and TOP) in the QD are partially removed from QD surface by annealing at 270 °C for preventing dissolution by solvent in subsequent coating of hole transport material (HTM) solutions atop and also improving electron and hole mobilities, especially the former. The proposed g‐HTP with various m:n mole ratios of mCP to TPA/Cz moieties along with various reported HTMs (PVK, Poly‐TPD and TFB) are investigated. Among them, the proposed g‐HTP with the comonomer mole ratio 1:1 gives the best performance ηmax 36.1 cd A−1 and Bmax 208 600 cd m−2, which is the highest performance among the reported i‐QLEDs with single hole‐transport layer ever documented. In addition, its efficiency roll off is low from 35.8 cd A−1 at 10 000 cd m−2 to 31.1 cd A−1 at 100 000 cd m−2.

Full-Color Quantum Dot Light-Emitting Diodes Based on Microcavities

Full-color display is a primary challenge for the commercialization of quantum dots (QDs). In this study, we utilize the spectral narrowing phenomenon of microcavities to fabricate the red, green and blue quantum dot light-emitting diodes (QLEDs) with a single QD layer. This work theoretically analyses the role of microcavities in adjusting the emitting color of QLEDs. By enhanced microcavity and properly chosen spacer thickness, the spectral selectivity shifts, realizing the full-color-tunability of QLEDs. The tunable experimental spectra of microcavity QLEDs are observed, in excellent agreement with our theoretical design. Benefiting from the spectral narrowing of microcavity and the narrow spectra of QDs, a high color purity with full width at half maximum (FWHM) of 18 to 25 nm is realized, leading to a color gamut ratio of 104.8% compared to National Television System Committee (NTSC) standard. The light extraction is also enhanced by constructive interference and the Purcell effect in the microcavity. Moreover, in the fabrication of red, green, and blue pixels, patterning the transparent cathode has better feasibility and lower damage relative to patterning the light-emitting layer.

Synthesis and characterization of InP/ZnSe/ZnS quantum dots for photo-emissive color conversion

In this paper, a InP/ZnSe/ZnS quantum dots photoresist (QDPR) film has been successfully prepared after the as-synthesized InP/ZnSe/ZnS quantum dots (QDs) were mixed with the photoresist, where the molar ratio of P3- : In3+: Se2- was 6:1:3 and the reaction time the ZnS shell was 60 min. The influence of the thickness of the film and the mass ratio of InP/ZnS QDs to photoresist on the photoluminescence quantum yield (PLQY) was investigated. The results show that the PLQY changes from 39.9% to 52.6% and the CIE color coordinates could vary from (0.28, 0. 10) to (0.65, 0.32) with the assistance of 5.5 pairs of DBR structures when the thickness of the InP/ZnSe/ZnS QDPR films is 7.82 µm and the mass ratio (wt%) of the InP/ZnSe/ZnS QDs to the photoresist is 25%. Compared with the InP/ZnSe/ZnS QDs film, the PLQY of the patterned InP/ZnSe/ZnS QDs layer can reach to be 47.2% and the CIE color coordinates of is varied from (0.28, 0.10) to (0.35, 0.14). Furthermore, the PLQY of the patterned InP/ZnSe/ZnS QDs layer can be improved to 65.1% at 5.5 pairs of DBR structures. Based on these results, the InP/ZnSe/ZnS QDs films, especially patterned QD layer, show great potential for the fabrication of high-quality QD color filter and full-color displays.

Dual-step photocarrier injection by mixture layer of ZnO QDs and MoS2 NPs on hybrid PdAu NPs

Hybrid UV-photodetector (PD) design consisted of various compatible nano-material systems are drawing significant research interests due to their improved device tunability and functionality. A hybrid ZnO–MoS2/HNP UV-PD design is demonstrated by the mixture layer of zinc oxide (ZnO) quantum dot (QD) and molybdenum di-sulfate (MoS2) nanoparticles (NPs) on the plasmonic hybrid PdAu NP template. The proposed design exhibits an excellent photocurrent of 1.45×10−3 A under the 54.9 mW/mm2 at ± 10 V due to a dual step enhancement of photo-carrier injection. It then leads to the significantly improved performance factors of photo-responsivity 1430 mA/mm2 and detectivity 4.50 × 1010 jones at 0.34 mW/mm2. The rise time (Tr) and fall time (Tf) of 3.37 and 0.35 s are obtained for the ZnO–MoS2/HNP PD. The demonstrated performance factors indicate that the hybrid ZnO–MoS2/HNP UV-PD platform is one of the best UV-PDs as summarized in Table 1.

Investigation of Nonlinear Optical Properties of Quantum Dots Deposited onto a Sample Glass Using Time-Resolved Inline Digital Holography.

We report on the application of time-resolved inline digital holography in the study of the nonlinear optical properties of quantum dots deposited onto sample glass. The Fresnel diffraction patterns of the probe pulse due to noncollinear degenerate phase modulation induced by a femtosecond pump pulse were extracted from the set of inline digital holograms and analyzed. The absolute values of the nonlinear refractive index of both the sample glass substrate and the deposited layer of quantum dots were evaluated using the proposed technique. To characterize the inhomogeneous distribution of the samples’ nonlinear optical properties, we proposed plotting an optical nonlinearity map calculated as a local standard deviation of the diffraction pattern intensities induced by noncollinear degenerate phase modulation.

Efficient Radiative Enhancement in Perovskite Light-Emitting Devices through Involving a Novel Sandwich Localized Surface Plasmon Structure.

In recent years, CsPbX3 (X = Cl, Br, I) perovskite quantum dots (QDs) have been considered as the most promising materials for light-emitting diodes (LEDs). However, the advances of CsPbX3 quantum dot-based light emitting diodes (QLEDs) still lagged behind inorganic III-V LEDs and other organic LEDs. Herein, a strategy to improve the performances of perovskite QLEDs is reported by utilizing the localized surface plasmon resonance (LSPR) effects of Au nanospheres (NSs). It is accomplished by introducing a Au NS layer into the electron transport layer of Ca2+ -CsPbBr3 QLEDs, where the diameter and spacing of Au NSs and the interaction distance between the Ca2+ -CsPbBr3 QD and Au NS layers are modulated, according to the theoretical simulation of Finite-difference time-domain. As a result, the photoluminescence quantum yield of Ca2+ -CsPbBr3 QD layer is improved from 31.5% to 73.3%. Finally, the luminance of Ca2+ -CsPbBr3 QLEDs is improved from 16824 to 63931cd m-2 and external quantum efficiency (EQE) is improved from 4.2% to 10.5%. The radiative transition rate can be remarkably modulated from 0.7 × 107 to 6.6 × 107 s-1 . The enhancement in luminance and EQE are the best values in the LSPR modified perovskite QLEDs and the strategy offered in this work fits with other LEDs and optoelectrical devices.

Direct Imaging of the Carrier Capture into Individual InP Quantum Dots of a Semiconductor Disk Laser Membrane.

We report on nanoscopic exploration of the luminescence from individual InP quantum dots (QDs) by means of highly spatially resolved cathodoluminescence (CL) spectroscopy directly performed in a scanning transmission electron microscope (STEM). A 7-fold layer stack with high-density InP quantum dots is embedded as an active medium membrane in an external-cavity surface-emitting laser. We characterize the vertical transfer of carriers within the periodic separate confinement heterostructure and determine the capture efficiency of carriers from the cladding layer into the quantum dot layers. Benefiting from the nanoscale resolution of our STEM-CL, we perform single-dot spectroscopy on single isolated QDs in the STEM lamella resolving the details of the excitonic structure of individual quantum dots. Executing highly spatially resolved spectrum line scans within the QD layers, we directly visualize the lateral transport, i.e., the efficient lateral capture of carriers into an individual QD. We observe a characteristic change of the spectral fingerprint during this line scan, while the electron beam is approaching and subsequently receding from the quantum dot position. This directly correlates to the increase and decrease of the numbers of excess carriers reaching the dot, i.e., altering the quantum dot population. The characteristic shift of emission energies visualize the renormalization of the ground-state energy of the single dot, and the intensity ratio of the excitonic recombinations verifies this change of the occupation and the state-filling.