Core-shell Colloidal Quantum Dots Research Articles

InSb/InP Core-Shell Colloidal Quantum Dots for Sensitive and Fast Short-Wave Infrared Photodetectors.

Colloidal quantum dot (CQD) technology is considered the main contender toward a low-cost high-performance optoelectronic technology platform for applications in the short-wave infrared (SWIR) to enable 3D imaging, LIDAR night vision, etc. in the consumer electronics and automotive markets. In order to unleash the full potential of this technology, there is a need for a material that is environmentally friendly, thus RoHS compliant, and possesses adequate optoelectronic properties to deliver high-performance devices. InSb CQDs hold great potential in view of their RoHS-compliant nature and─in principle─facile access to the SWIR. However, to date progress in realizing high-performance optoelectronic devices, including photodetectors (PDs), has been limited. Here, we have developed a synthesis method for producing size-tunable InSb CQDs with distinct excitonic peaks spanning a wide range from 900 to 1750 nm. To passivate the surface defects and enhance the photoluminescence (PL) efficiency of InSb CQDs, we further designed an InSb/InP core-shell structure. By employing the InSb/InP core-shell CQDs in a photodiode device stack, we report on robust InSb CQD SWIR photodetectors that exhibit an external quantum efficiency (EQE) of 25% at 1240 nm, a wide linear dynamic range exceeding 128 dB, a photoresponse time of 70 ns, and a specific detectivity of 4.4 × 1011 jones.

Interface defects repair of core/shell quantum dots through halide ion penetration.

The interface defects of core-shell colloidal quantum dots (QDs) affect their optoelectronic properties and charge transport characteristics. However, the limited available strategies pose challenges in the comprehensive control of these interface defects. Herein, we introduce a versatile strategy that effectively addresses both surface and interface defects in QDs through simple post-synthesis treatment. Through the combination of fine chemical etching methods and spectroscopic analysis, we have revealed that halogens can diffuse within the crystal structure at elevated temperatures, acting as "repairmen" to rectify oxidation and significantly reducing interface defects within the QDs. Under the guidance of this protocol, InP core/shell QDs were synthesized by a hydrofluoric acid-free method with a full width at half-maximum of 37.0 nm and an absolute quantum yield of 86%. To further underscore the generality of this strategy, we successfully applied it to CdSe core/shell QDs as well. These findings provide fundamental insights into interface defect engineering and contribute to the advancement of innovative solutions for semiconductor nanomaterials.

Efficient Short-Wave Infrared Light-Emitting Diodes Based on Heavy-Metal-Free Quantum Dots.

Short-wave infrared (SWIR) light emission is important for a diverse range of modern applications, such as eye-safe depth sensing, light detection and ranging (LiDAR), facial recognition, eye tracking, optical communication, and health-monitoring technologies. However, there are a very limited number of known semiconductors that can emit efficiently in the SWIR spectral range. Presently, SWIR light-emitting diodes (LEDs) based on colloidal quantum dots (CQD) are dominated by lead chalcogenide systems, despite the presence of heavy metal and modest efficiencies. Here, a highly efficient SWIR LED based on heavy-metal-free indium arsenide (InAs) core-shell CQDs is presented. In the LED design, the implementation of an otherwise hole-transporting poly(vinylcarbazole) (PVK) layer on the electron-injecting side of the device stack leads to a surprising enhancement in device performance, giving remarkably high external quantum efficiencies (EQEs) of 13.3% at 1006 nm. Single-carrier device and optical investigations reveal the origins of enhancement to be the electronic decoupling of the CQD layer with the electron-injecting zinc oxide (ZnO) layer, which mitigates luminescence quenching and improves charge balance. This work marks one of the highest efficiencies reported for heavy-metal-free solution-processed LEDs in the SWIR spectral region, and can find significant applications in emerging consumer electronic technologies.

Complete Mapping of Interacting Charging States in Single Coupled Colloidal Quantum Dot Molecules

Colloidal quantum dots (CQDs), major building blocks in modern optoelectronic devices, have so far been synthesized with only one emission center where the exciton resides. Recent development of coupled colloidal quantum dots molecules (CQDM), where two core–shell CQDs are fused to form two emission centers in close proximity, allows exploration of how charge carriers in one CQD affect the charge carriers in the other CQD. Cryogenic single particle spectroscopy reveals that while CQD monomers manifest a simple emission spectrum comprising a main emission peak with well-defined phonon sidebands, CQDMs exhibit a complex spectrum with multiple peaks that are not all spaced according to the known phonon frequencies. Based on complementary emission polarization and time-resolved analysis, this is assigned to fluorescence of the two coupled emission centers. Moreover, the complex peak structure shows correlated spectral diffusion indicative of the coupling between the two emission centers. Utilizing Schrödinger-Poisson self-consistent calculations, we directly map the spectral behavior, alternating between neutral and charged states of the CQDM. Spectral shifts related to electrostatic interaction between a charged emission center and the second emission center are thus fully mapped. Furthermore, effects of moving surface charges are identified, whereby the emission center proximal to the charge shows larger shifts. Instances where the two emission centers are negatively charged simultaneously are also identified. Such detailed mapping of charging states is enabled by the coupling within the CQDM and its anisotropic structure. This understanding of the coupling interactions is progress toward quantum technology and sensing applications based on CQDMs.

Nonblinking Colloidal Quantum Dots via Efficient Multiexciton Emission.

Nonblinking colloidal quantum dots (QDs) are significant to their applications as single-photon sources or light-emitting materials. Herein, a simple heat-up method was developed to synthesize high-qualityWZ-CdSe/CdS core-shell colloidal QDs, which achieved a near-unity photoluminescence quantum yield (PLQY). It was found that the blinking behavior of such QDs was completely suppressed at high excitation intensities, and ultra-stable PL emission was observed. For this reason, a systematic investigation was conducted, revealing that the complete blinking suppression was attributed mainly to the efficient multiexciton emission at high excitation intensities. Such high-quality QDs with nonblinking behaviors and nearly ideal PL properties at high excitation intensities have massive potential applications in various robust conditions, including QD display screens, single-particle tracks, and single-photon sources.

Nonlinear Optical Characterization of InP@ZnS Core-Shell Colloidal Quantum Dots Using 532 nm, 10 ns Pulses.

InP@ZnS core-shell colloidal quantum dots (CQDs) were synthesized and characterized using the z-scan technique. The nonlinear refraction and nonlinear absorption coefficients (γ = −2 × 10−12 cm2 W−1, β = 4 × 10−8 cm W−1) of these CQDs were determined using 10 ns, 532 nm pulses. The saturable absorption (β = −1.4 × 10−9 cm W−1, Isat = 3.7 × 108 W cm−2) in the 3.5 nm CQDs dominated at small intensities of the probe pulses (I ≤ 7 × 107 W cm−2) followed by reverse saturable absorption at higher laser intensities. We report the optical limiting studies using these CQDs showing the suppression of propagated nanosecond radiation in the intensity range of 8 × 107–2 × 109 W cm−2. The role of nonlinear scattering is considered using off-axis z-scan scheme, which demonstrated the insignificant role of this process along the whole range of used intensities of 532 nm pulses. We discuss the thermal nature of the negative nonlinear refraction in the studied species.

Third order nonlinear optical properties of CdTe/CdSe Quasi Type-II Colloidal Quantum Dots

Core/shell Colloidal Quantum Dots (CQDs) are of interest for nonlinear photonic applications due to their unique properties. In this work, firstly CdTe/CdSe Quasi Type-II Colloidal Quantum Dots with an average core size of 3.54 nm and four different shell thicknesses (1.8 nm, 2.7 nm, 3.2 nm and 3.8 nm) were synthesized and characterized using UV–Vis spectroscopy and Transmission electron microscopy (TEM). Secondly nonlinear optical properties of these structures were determined using Femtosecond Z-scan method at the wavelength of 800 nm. Measured values for two photons absorption coefficients ( β ) and nonlinear refractive indices ( n 2 ) were found to be on the orders of 10 −11 cm/W and 10 −16 cm 2 /W, respectively. In addition, the third order nonlinear optical susceptibility values for these structures were calculated to be on the order of 10 −10 esu. Quantum dots with thicker shells exhibited faintly higher nonlinear absorption coefficient and slightly lower third order nonlinear optical susceptibility values compared with that of thinner shells. Furthermore, pump power dependant nonlinear optical properties of core/shell nano-structures were investigated and it is observed that two photons absorption coefficient and the third order nonlinear optical susceptibility are pump dependant, while nonlinear refractive indices does not respond to the pump power. • CdTe/CdSe Quasi Type-II Colloidal Quantum Dots (CQD) show strong nonlinearity. • Nonlinear absorption coefficients, nonlinear refraction indices and third order susceptibilities were measured. • CQDs with thick shells showed slightly higher nonlinear absorption coefficient. • Nonlinear absorption coefficients and third order susceptibilities were found to be power dependent.

Design of Core-Shell Quantum Dots-3D WS2 Nanowall Hybrid Nanostructures with High-Performance Bifunctional Sensing Applications.

Transition metal dichalcogenides (TMDCs) have recently attracted a tremendous amount of attention owing to their superior optical and electrical properties as well as the interesting and various nanostructures that are created by different synthesis processes. However, the atomic thickness of TMDCs limits the light absorption and results in the weak performance of optoelectronic devices, such as photodetectors. Here, we demonstrate the approach to increase the surface area of TMDCs by a one-step synthesis process of TMDC nanowalls from WOx into three-dimensional (3D) WS2 nanowalls. By utilizing a rapid heating and rapid cooling process, the formation of 3D nanowalls with a height of approximately 150 nm standing perpendicularly on top of the substrate can be achieved. The combination of core-shell colloidal quantum dots (QDs) with three different emission wavelengths and 3D WS2 nanowalls further improves the performance of WS2-based photodetector devices, including a photocurrent enhancement of 320-470% and shorter response time. The significant results of the core-shell QD-WS2 hybrid devices can be contributed by the high nonradiative energy transfer efficiency between core-shell QDs and the nanostructured material, which is caused by the spectral overlap between the emission of core-shell QDs and the absorption of WS2. Besides, outstanding NO2 gas-sensing performance of core-shell QDs/WS2 devices can be achieved with an extremely low detection limit of 50 ppb and a fast response time of 26.8 s because of local p-n junctions generated by p-type 3D WS2 nanowalls and n-type core-shell CdSe-ZnS QDs. Our work successfully reveals the energy transfer phenomenon in core-shell QD-WS2 hybrid devices and shows great potential in commercial multifunctional sensing applications.

Synthesis and characterization of In1−xGaxP@ZnS alloy core-shell type colloidal quantum dots

We report synthesis of colloidal quantum dots (CQDs), indium gallium phosphide with zinc sulphide shell (In1−xGaxP@ZnS) by a hot injection method and investigate effects of In/Ga ratio on the optical properties of the obtained CQDs. The bandgap of CQDs (1.34–3.54eV) is controlled by the In/Ga ratio without the quantum confinement effect. The photoluminescence (PL) quantum yield is also dependent on the In/Ga ratio, and temperature dependent PL results show that the randomness of crystal lattice is the most influential parameter. These results show that the In1−xGaxP alloy CQDs can be useful for absorbing the whole range of visible light. In addition, this article can provide background knowledge regarding effect of core metal composition on the optical properties of produced CQDs.

Ultrafast Charge Carrier Dynamics and Nonlinear Optical Absorption of InP/ZnS Core–Shell Colloidal Quantum Dots

Understanding of ultrafast carrier dynamics in InP/ZnS colloidal quantum dots (QDs) is essential for their optoelectronic applications. In this paper, we have successfully fabricated high-quality I...

The shell thickness and surface passivation dependence of fluorescence decay kinetics in CdSe/ZnS core-shell and CdSe core colloidal quantum dots

Femtoseconds, ultrafast, time-resolved, fluorescence upconversion spectroscopy was utilized to investigate the photo-induced emission decay dynamics of CdSe and CdSe/ZnS colloidal quantum dots whose surfaces were stabilized with octadecyl amine (ODA). The studies focused on understanding the influence of the surface stabilizing group on the fluorescence decay dynamics of CdSe and CdSe/ZnS and the effect of shell thickness on the photo-physical properties of core-shell quantum dots. The Core-shell materials showed excellent photostability, longer rise times, and lengthy decay times.

Kennard's detailed balance relation for photoluminescence: General proof and experimental results for InP/ZnS core-shell nanocrystals

According to the principle of detailed balance, the luminescence intensity at a wavelength λ1 as a result of excitation at a wavelength λ2 must be related in some way to the luminescence intensity at λ2 as a result of excitation at λ1. In 1926, E. H. Kennard derived such a relation by applying the concept of the Einstein coefficients to some model system. Our study provides a general thermodynamic proof that Kennard's relation is valid for arbitrary macroscopic photoluminescent bodies. An experimental test of this relation is also given for InP/ZnS core-shell colloidal quantum dots synthesized by the conventional high-temperature organometallic procedure. For these particles, the emission spectra at various excitation wavelengths and the photoluminescence excitation spectra at various emission wavelengths have been obtained. It was found that Kennard's relation does hold for the obtained spectra.

Influence of Interfacial Strain on Optical Properties of PbSe/PbS Colloidal Quantum Dots

The interface in PbSe/PbS core/shell colloidal quantum dots (CQDs) is subject to strain forces due to a 3% crystallographic mismatch between the constituents. The strain profile in PbSe/PbS CQDs was simulated using the classical linear elasticity model, under the assumption of spherical-symmetric dot and isotropic materials. The derived strain profile was incorporated into a band structure calculation to evaluate the influence on the electronic band-edges of the core/shell CQDs. The electronic energy states evaluated were in close agreement with the absorption edges of various core/shell CQDs with different core diameters and shell thicknesses. Furthermore, the synthesized CQDs underwent thermal annealing at various temperatures, thereby creating the alloying interface; consequently, their absorption and photoluminescence spectra exhibited spectral red-shift compared with the untreated samples. The band gap energy red-shift was simulated by the theoretical model, including smoothing potential at the inter...

Ultrafast Saturable Absorption of Core/Shell Colloidal Quantum Dots

Ultrafast saturable absorption (SA) materials that are capable of blocking the optical absorption under strong excitation have extensive applications in photonic devices. This work presents core/shell colloidal quantum dots (CQDs) which have the quantized energy levels, excellent band gap tunability, and possess significant SA performance. When the band gap is close to the pump pulse energy, the CQDs show significant resonant SA response. At the same excitation conditions, the core/shell CQDs dispersions show better SA response than graphene dispersions, and comparable to the recently reported molybdenum disulfide. The carrier dynamics of the SA of the CQDs is analyzed systematically. The research has also found that the two-photon absorption of the CQDs show nearly cubic power law of the band gap, while the SA performance keeps almost the same in the nonresonant regime. Further, superior passive Q-switched laser behavior is observed using the CQDs as a saturable absorber. The results directly reveal the physical processes of this basic problem and broaden the applications of CQDs in photonic devices.

Atomistic Design of CdSe/CdS Core-Shell Quantum Dots with Suppressed Auger Recombination.

We design quasi-type-II CdSe/CdS core-shell colloidal quantum dots (CQDs) exhibiting a suppressed Auger recombination rate. We do so using fully atomistic tight-binding wave functions and microscopic Coulomb interactions. The recombination rate as a function of the core and shell size and shape is tested against experiments. Because of a higher density of deep hole states and stronger hole confinement, Auger recombination is found to be up to six times faster for positive trions compared to negative ones in 4 nm core/10 nm shell CQDs. Soft-confinement at the interface results in weak suppression of Auger recombination compared to same-bandgap sharp-interface CQDs. We find that the suppression is due to increased volume of the core resulting in delocalization of the wave functions, rather than due to soft-confinement itself. We show that our results are consistent with previous effective mass models with the same system parameters. Increasing the dot volume remains the most efficient way to suppress Auger recombination. We predict that a 4-fold suppression of Auger recombination can be achieved in 10 nm CQDs by increasing the core volume by using rodlike cores embedded in thick shells.

Random lasing in a colloidal quantum dot-doped disordered polymer.

We report random lasing in colloidal quantum dots (CQDs) doped disordered polymer. The CdSe/ZnS core-shell CQDs are dispersed in hybrid polymer including two types of monomers with different rates of polymerization. After UV curing, spatially localized random resonators are formed owing to long range refractive-index fluctuations in inhomogeneous polymer with gain. Upon the optical excitation, random lasing action is triggered above the threshold of 7mJ/cm2. Through the investigation on the spectral characteristics of random laser, the wavelengths of random lasers strongly depend on pump position, which confirms that random laser modes originate from spatially localized resnonators. According to power Fourier transform of emission spectrum, the average size of equivalent micro resonators is attributed to be 50 μm. The proposed method provides a facile route to develop random lasers based on CQDs, showing potential applications on random fiber laser and laser displays.

Air-Stable, Near- to Mid-Infrared Emitting Solids of PbTe/CdTe Core-Shell Colloidal quantum dots.

Light emitters and detectors operating in the near- and mid-infrared spectral regions are important to many applications, such as telecommunications, high-resolution gas analysis, atmospheric pollution monitoring, medical diagnostics, and night vision. Various lead chalcogenides (binary, ternary, and quaternary alloys) in the form of quantum dots (QDs) or quantum wells provide narrow bandgap energies that cover the broad infrared region corresponding to wavelengths of 1-30 μm. Here, we report an inexpensive, all-solution-based synthesis strategy to thin-film solids consisting of 5-16 nm PbTe QDs encapsulated by CdTe shells. Colloidally synthesized PbTe QDs were first converted into core-shell PbTe/CdTe QDs, and then deposited as thin films. The subsequent fusion of the CdTe shells is achieved by ligand removal and annealing in the presence of CdCl2 . Contrary to highly unstable bare PbTe QDs, PbTe/CdTe QD solids exhibit bright and stable near- to mid-infrared emission at wavelengths of 1-3 μm, which is also retained upon prolonged storage at ambient conditions for one year.

Temperature-Dependent Recombination Processes in Small-Sized PbSe/PbS Core/Shell Colloidal Quantum Dots

The work focuses on the optical investigation of temperature-dependent recombination processes in small-sized PbSe/PbS core/shell colloidal quantum dots (CQDs) with core diameter of (2 – 2.5) nm and the shell thickness of (0.5 – 1.0) nm under air-free conditions and after air exposure. These CQDs have a tunable absorption edge around 1 μm and a rather narrow photoluminescence linewidth, their emission recombination process is characterized by the μs time-scale and by longer radiative lifetimes in the entire temperature range and especially at low temperatures (a 2.6-fold increase) as compared to the corresponding PbSe samples. The PbSe/PbS core/shell CQDs are oxidation-stable towards time-limited air exposure. DOI: http://dx.doi.org/10.5755/j01.ms.20.2.6321

Negative differential resistance phenomena in colloidal quantum dots-based organic light-emitting diodes

The influence of ligands on the electrical behavior of CdSe/ZnS core-shell colloidal quantum dots (CQDs)-based organic light-emitting diodes is presented. Negative differential resistance (NDR) phenomena at room temperature are observed from single-layer device ITO/poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS)/CQDs/Al in which the original capping ligand tri-n-octylphosphine oxide (TOPO) of CQDs is exchanged with oleylamine, as well as in both bilayer device ITO/PEDOT:PSS/CQDs/BCP(10 nm)/Al and trilayer device ITO/PEDOT:PSS/CQDs/BCP(10 nm)/Alq3(10 nm)/Al. However, such a kind of NDR phenomenon disappears if TOPO is exchanged with hexadecylamine. Therefore, NDR phenomenon depends greatly on the ligands of the CQDs, and the origin of NDR from these devices is discussed.

Enhancement of the power conversion efficiency of polymer solar cells by functionalized single-walled carbon nanotubes decorated with CdSe/ZnS core–shell colloidal quantum dots

In this paper, we demonstrated an enhanced performance of polymer solar cells by incorporating functionalized single-walled carbon nanotubes (SWCNTs) decorated with CdSe/ZnS core–shell colloidal quantum dots (CQDs) into copolymers of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as active layer. Short-circuit current density and power conversion efficiency of the ITO/PEDOT:PSS/P3HT:PCBM:(CdSe/ZnS-SWCNTs)/Al solar cells can be enhanced by more than 31 and 23 %, respectively, as compared with the control device ITO/PEDOT:PSS/P3HT:PCBM/Al. This enhancement is due to the high electron-transporting ability of SWCNTs and the increased absorption of CdSe/ZnS CQD in visible region. It shows an applicable way to improve the efficiency of polymer solar cells by incorporating suitable quantity of CQDs-decorated SWCNTs with suitable kinds of CQDs and suitable acid treatment to the SWCNTs.