Quantum Dot Solids Research Articles

Mid-infrared response of PbS colloidal quantum dot solids

Owing to their large surface-to-volume ratio, it easily introduces dopants to CQDs surface. Hence, by incorporating oxidation into the energy band using KNO2, PbS CQD photoconductors exhibit an extrinsic response under 3000 nm illumination.

Synthesis-on-substrate of quantum dot solids.

Perovskite light-emitting diodes (PeLEDs) with an external quantum efficiency exceeding 20% have been achieved in both green and red wavelengths1-5; however, the performance of blue-emitting PeLEDs lags behind6,7. Ultrasmall CsPbBr3 quantum dots are promising candidates with which to realize efficient and stable blue PeLEDs, although it has proven challenging to synthesize a monodispersed population of ultrasmall CsPbBr3 quantum dots, and difficult to retain their solution-phase properties when casting into solid films8. Here we report the direct synthesis-on-substrate of films of suitably coupled, monodispersed, ultrasmall perovskite QDs. We develop ligand structures that enable control over the quantum dots' size, monodispersity and coupling during film-based synthesis. A head group (the side with higher electrostatic potential) on the ligand provides steric hindrance that suppresses the formation of layered perovskites. The tail (the side with lower electrostatic potential) is modified using halide substitution to increase the surface binding affinity, constraining resulting grains to sizes within the quantum confinement regime. The approach achieves high monodispersity (full-width at half-maximum = 23 nm with emission centred at 478 nm) united with strong coupling. We report as a result blue PeLEDs with an external quantum efficiency of 18% at 480 nm and 10% at 465 nm, to our knowledge the highest reported among perovskite blue LEDs by a factor of 1.5 and 2, respectively6,7.

Superradiance and Exciton Delocalization in Perovskite Quantum Dot Superlattices.

Achieving superradiance in solids is challenging due to fast dephasing processes from inherent disorder and thermal fluctuations. Perovskite quantum dots (QDs) are an exciting class of exciton emitters with large oscillator strength and high quantum efficiency, making them promising for solid-state superradiance. However, a thorough understanding of the competition between coherence and dephasing from phonon scattering and energetic disorder is currently unavailable. Here, we present an investigation of exciton coherence in perovskite QD solids using temperature-dependent photoluminescence line width and lifetime measurements. Our results demonstrate that excitons are coherently delocalized over 3 QDs at 11 K in superlattices leading to superradiant emission. Scattering from optical phonons leads to the loss of coherence and exciton localization to a single QD at temperatures above 100 K. At low temperatures, static disorder and defects limit exciton coherence. These results highlight the promise and challenge in achieving coherence in perovskite QD solids.

Highly Orientated Perovskite Quantum Dot Solids for Efficient Solar Cells.

Perovskite quantum dots (PQDs) have emergedas competitive optoelectronic materials for photovoltaic applications due to their ideal bandgap energy, high defect tolerance, and solution processability. However, the highly dynamic surface and imperfect cubic structure of PQDs generally result in unfavorable charge-carrier transport within the PQD solids and serious nonradiative recombination. Herein, a highly orientated PQD solid is demonstrated using precursor engineering accompanied by a chemical stripping treatment (CST). A combination of systematic experimental studies and theoretical calculations is conducted to fundamentally understand the resurfacing of PQDs using the CST approach. The results reveal that the highly ordered PQDs can result in a high orientation of PQD solids, significantly promoting charge-carrier transport within the PQD solids. Meanwhile, the ideal cubic-structured PQD with an iodine-rich surface dramatically decreases surface trap states, thereby substantially diminishing trap-assisted nonradiative recombination. Consequently, the inorganic PQD solar cell delivers a power conversion efficiency of up to 16.25%. This work provides a feasible avenue to construct highly orientated PQD solids with improved photophysical properties for high-performance optoelectronic devices.

In Situ Growth of Strained Matrix on CsPbI3 Perovskite Quantum Dots for Balanced Conductivity and Stability.

Their nanoscale size endows perovskite quantum dots (QDs) with processing flexibility and high tunability of optoelectronic properties. The vast surface area also provides an opportunity for ligand engineering to offer QDs extra protection, which however, will impede charge transport in the QD array. Currently, the surface treatments that can balance both stability and conductivity of the perovskite QD array remain a huge challenge. Here, we report in situ growth of an atomic guanidinium lead iodide perovskite matrix on CsPbI3 QDs. In addition to the effect of trap passivation, the matrix can also provide substantial surface strain to improve the QD phase stability. Meanwhile, the ultrathin matrix allows efficient coupling and charge transport in the QD solids. As a result, the CsPbI3 QD solar cells can achieve both superior device stability and performance. We believe the development of a multifunctional surface matrix will become one of the future research focuses in perovskite QD-based devices.

Mid-Infrared Intraband Photodetector via High Carrier Mobility HgSe Colloidal Quantum Dots.

In this work, a room-temperature mixed-phase ligand exchange method is developed to obtain a relatively high carrier mobility (∼1 cm2/(V s)) on HgSe intraband colloidal quantum dot solids without any observable trap state. What is more, the doping from 1Se to 1Pe state in the conduction band could be precisely controlled by additional salts during this method, proved by optical and transport experiments. The high mobility and controllable doping benefit the mid-infrared photodetector utilizing the 1Se to 1Pe transition, with a 1000-fold improvement in response speed, which is several μs, a 55-fold increase in responsivity, which is 77 mA/W, and a 10-fold increase in specific detectivity, which is above 1.7 × 109 Jones at 80 K. The high-performance photodetector could serve as an intraband infrared camera for thermal imaging, as well as a CO2 gas sensor with a range from 0.25 to 2000 ppm.

Two-band optical gain and ultrabright electroluminescence from colloidal quantum dots at 1000 A cm−2

Colloidal quantum dots (QDs) are attractive materials for the realization of solution-processable laser diodes. Primary challenges towards this objective are fast optical-gain relaxation due to nonradiative Auger recombination and poor stability of colloidal QD solids under high current densities required to obtain optical gain. Here we resolve these challenges and achieve broad-band optical gain spanning the band-edge (1S) and the higher-energy (1P) transitions. This demonstration is enabled by continuously graded QDs with strongly suppressed Auger recombination and a current-focusing device design, combined with short-pulse pumping. Using this approach, we achieve ultra-high current densities (~1000 A cm−2) and brightness (~10 million cd m−2), and demonstrate an unusual two-band electroluminescence regime for which the 1P band is more intense than the 1S feature. This implies the realization of extremely large QD occupancies of up to ~8 excitons per-dot, which corresponds to complete filling of the 1S and 1P electron shells.

Optical fiber antenna employing quantum dots as gas molecule receptors

The discovery of odorant receptors inspiring researchers to refine the artificial olfactory system with the better design of receptor function and transducer function. Colloidal quantum dots (CQDs) offer an abundance of available surface area for chemical and electronic interactions, making them ideal receptors for gas molecules. In this paper, we propose optical fiber antenna (OFA) that mimic the structure and function of biological antenna by using CQDs as the artificial receptors for gas molecules. The charge transfer between the gas molecules and quantum dots is designed to modulate the interaction between the quantum dot solids and light through the variation of refractive index (RI) of quantum receptors. Specifically, lead sulfide (PbS) CQDs serving as NO2 receptors were deposited on the optical structure for comparison. The solution-processability of CQDs enable their uniform and conformal coating on the fiber. The OFA exhibited a sensitivity of 35 pm ppm−1 for NO2 sensing at room temperature with the detection limit estimated to be 1 ppb. The proof of OFA concept may pave the way to artificial olfactory networking and will benefit for the development of high precise, large scale, real time distributed gas monitoring application beyond human olfaction.

Anisotropic nanocrystal superlattices overcoming intrinsic light outcoupling efficiency limit in perovskite quantum dot light-emitting diodes

Quantum dot (QD) light-emitting diodes (LEDs) are emerging as one of the most promising candidates for next-generation displays. However, their intrinsic light outcoupling efficiency remains considerably lower than the organic counterpart, because it is not yet possible to control the transition-dipole-moment (TDM) orientation in QD solids at device level. Here, using the colloidal lead halide perovskite anisotropic nanocrystals (ANCs) as a model system, we report a directed self-assembly approach to form the anisotropic nanocrystal superlattices (ANSLs). Emission polarization in individual ANCs rescales the radiation from horizontal and vertical transition dipoles, effectively resulting in preferentially horizontal TDM orientation. Based on the emissive thin films comprised of ANSLs, we demonstrate an enhanced ratio of horizontal dipole up to 0.75, enhancing the theoretical light outcoupling efficiency of greater than 30%. Our optimized single-junction QD LEDs showed peak external quantum efficiency of up to 24.96%, comparable to state-of-the-art organic LEDs.

π-SnS Colloidal Nanocrystals with Size-Dependent Band Gaps

Colloidal quantum dot (QD) solids are materials that exploit the quantum confinement properties of the constituent nanocrystals. Significant progress in QD optoelectronic devices has been made mainly using compounds with concerning high degree of toxicity for practical applications. Sn-chalcogenides are among the possible environmentally benign alternatives the bulks of which have demonstrated intriguing properties for energy devices. Nanostructuring these Sn-chalcogenides may create variations in their crystal structures and properties. Here, we demonstrate robust quantum confinement effects in small-diameter cubic SnS colloidal nanocrystals (NCs) and their potential exploitation for high-performance optoelectronic devices. Well-controlled production of cubic SnS colloidal NCs with a 4–10 nm diameter is established by developing a one-pot synthesis procedure in an inert condition that allows Sn complexation with only a single type of ligand molecule. The analyses of the X-ray diffractogram and transmission electron micrograph revealed that the structure of NCs is the so-called π-type chiral and cubic structures. These NCs behave as quantum dots, exhibiting a quantum confinement effect with strong variations of energy band gaps by size. The photodetectors of the SnS NCs exhibit high responsivity comparable to the other established QD systems, although the channels are made only from a single or few layer(s). This new compound will open new pathways for establishing environmentally safe and high-performance QD optoelectronic and photocatalysis devices.

Ultrafast exciton transport at early times in quantum dot solids.

Quantum dot (QD) solids are an emerging platform for developing a range of optoelectronic devices. Thus, understanding exciton dynamics is essential towards developing and optimizing QD devices. Here, using transient absorption microscopy, we reveal the initial exciton dynamics in QDs with femtosecond timescales. We observe high exciton diffusivity (~102 cm2 s-1) in lead chalcogenide QDs within the first few hundred femtoseconds after photoexcitation followed by a transition to a slower regime (~10-1-1 cm2 s-1). QD solids with larger interdot distances exhibit higher initial diffusivity and a delayed transition to the slower regime, while higher QD packing density and heterogeneity accelerate this transition. The fast transport regime occurs only in materials with exciton Bohr radii much larger than the QD sizes, suggesting the transport of delocalized excitons in this regime and a transition to slower transport governed by exciton localization. These findings suggest routes to control the optoelectronic properties of QD solids.

The impact of chemical composition of halide surface ligands on the electronic structure and stability of lead sulfide quantum dot materials.

There is a high fundamental interest in the surface and bulk chemistry of quantum dot (QD) solids, as they have proven to be very promising materials in optoelectronic devices. The choice of surface ligands for quantum dots in solid devices determines many of the film properties, as the ligands influence for example the doping density, chemical stability and charge transport. Lead halide ligands have developed as the main ligand of choice for lead sulfide quantum dots, as they have been shown to passivate quantum dot surfaces and enhance the chemical stability. In this study, we successfully varied the ligand composition on the surface of PbS quantum dot films from pure lead iodide to pure lead bromide and investigated its influence on the chemical and electronic structure of the QD solids using hard X-ray photoelectron spectroscopy (HAXPES). Furthermore, we developed a surface treatment to prevent the surface oxidation of a bulk PbS reference sample. Through measurements of this sample and of lead halide reference samples, we were able to assign the contributions of different chemical bonding to the Pb 4f core level and of different atomic orbitals to the valence band spectral shape of the QD materials. Overall, we found that the valence band edge position was very similar for all different iodide:bromide ratios and that all investigated compositions were able to protect the quantum dot surfaces within solid films from oxidation. However, the ligand composition significantly influences the sample stability under X-rays. The iodide rich QD solids showed the highest stability with very little to no chemical changes over several hours of X-ray exposure, while the bromide rich QD solids changed already within the first hour of exposure.

Energetic Sulfide Vapor‐Processed Colloidal InAs Quantum Dot Solids for Efficient Charge Transport and Photoconduction

The energetic sulfide vapor process on colloidal indium arsenide (InAs) quantum dots (QDs) for integrated optoelectronic devices is demonstrated. X‐ray photoemission spectroscopy supports the presence of sulfur on QD surface, which increases the air stability, and ultraviolet photoemission spectroscopy and field‐effect transistor analysis confirm n‐type charge transport of corresponding QD films with higher electron mobilities. Photoconductivity of the same devices in near‐infrared wavelength shows gate bias‐dependent recombination of photogenerated carriers, coming from charge accumulation and depletion transition of QD channels. The synergetic approach of the vapor process on colloidal QD solids opens a promising opportunity for scalable and complementary metal‐oxide semiconductor‐compatible optoelectronic devices.

Reduction of Hydroxyl Traps and Improved Coupling for Efficient and Stable Quantum Dot Solar Cells.

Progress in quantum dot (QD)-based solar cells has been underpinned by the improvements in surface passivation and advancements in device engineering. Acute control over the surface properties is crucial to restrict the formation of in-gap trap states and improve the QD coupling in achieving conducting QD films. In this report, we demonstrate a solution-phase hybrid passivation strategy, which is beneficial in removing detrimental hydroxyl traps and improving the coupling between QDs by reducing the interdot distance. Advancement in surface passivation is translated to the long carrier lifetime, higher carrier mobility, and superior protection toward degradations in QD solids. The performance of solar cell devices is increased by 26% to reach an efficiency of 10.6%, compared to the state-of-the-art lead halide passivated solar cells. The improvement in solar cell performance is supported by the reduction of trap states and an 80 nm increase in thickness of the light-absorbing QD layer.

Charge Carrier Diffusion Dynamics in Multisized Quaternary Alkylammonium-Capped CsPbBr3 Perovskite Nanocrystal Solids.

CsPbBr3 quantum dots (QDs) are promising candidates for optoelectronic devices. The substitution of oleic acid (OA) and oleylamine (OLA) capping agents with a quaternary alkylammonium such as di-dodecyl dimethyl ammonium bromide (DDAB) has shown an increase in external quantum efficiency (EQE) from 0.19% (OA/OLA) to 13.4% (DDAB) in LED devices. The device performance significantly depends on both the diffusion length and the mobility of photoexcited charge carriers in QD solids. Therefore, we investigated the charge carrier transport dynamics in DDAB-capped CsPbBr3 QD solids by constructing a bi-sized QD mixture film. Charge carrier diffusion can be monitored by quantitatively varying the ratio between two sizes of QDs, which varies the mean free path of the carriers in each QD cluster. Excited-state dynamics of the QD solids obtained from ultrafast transient absorption spectroscopy reveals that the photogenerated electrons and holes are difficult to diffuse among small-sized QDs (4 nm) due to the strong quantum confinement. On the other hand, both photoinduced electrons and holes in large-sized QDs (10 nm) would diffuse toward the interface with the small-sized QDs, followed by a recombination process. Combining the carrier diffusion study with a Monte Carlo simulation on the QD assembly in the mixture films, we can calculate the diffusion lengths of charge carriers to be ∼239 ± 16 nm in 10 nm CsPbBr3 QDs and the mobility values of electrons and holes to be 2.1 (± 0.1) and 0.69 (± 0.03) cm2/V s, respectively. Both parameters indicate an efficient charge carrier transport in DDAB-capped QD films, which rationalized the perfect performance of their LED device application.

Prospects and challenges of colloidal quantum dot laser diodes

Colloidal quantum dots (QDs) combine the superior light-emission characteristics of quantum-confined semiconductors with the chemical flexibility of molecular systems. These properties could, in principle, enable solution-processable laser diodes with an ultrawide range of accessible colours. However, the realization of such devices has been hampered by fast optical gain decay due to non-radiative Auger recombination and poor stability of QD solids at the high current densities required for the lasing regime. Recently, these problems have been resolved, which resulted in the development of electrically pumped optical gain devices operating at ultrahigh current densities of around 1,000 A cm−2. The next step is the realization of a QD laser diode (QLD). Here we assess the status of the QD lasing field, examine the remaining challenges on the path to a QLD and discuss practical strategies for attaining electrically pumped QD lasing. Colloidal quantum dots may offer solution-processable lasers, with a wide range of colours available. Technical hurdles and progress towards realization of useful quantum dot laser diodes is reviewed.

Progress on the surface ligand engineering of lead sulfide colloidal quantum dots

<p indent=0mm>Colloidal quantum dots (CQDs) are zero-dimensional semiconductor materials with solution-processable properties. These materials have attracted considerable attention in the research and development of new photodetectors, photovoltaic cells, light-emitting diodes, and chemical sensors. The large exciton Bohr radius and Debye length and the considerable quantum size effect make lead sulfide one of the typical hotspots in CQDs. Owing to the large specific surface area and abundant surface dangling bonds, the surface ligand of CQDs greatly influences their physical and chemical properties. Surface ligand engineering can be used to realize the functional design and performance improvement of quantum dot semiconductor devices. This article reviews the research progress in the surface ligand engineering of lead sulfide (PbS) CQDs, focusing on the influence of surface ligands on their conductive properties and chemical activity. To improve the surface passivation and carrier mobility, PbS CQDs are developed from short-chain organic ligands to inorganic ligands represented by metal chalcogenide complexes (MCC), especially by the introduction of monovalent halogen atomic ligands, through surface ligand engineering. We further introduce the liquid-phase ligand-exchange technology. Compared with film-level ligand exchange, liquid-phase ligand exchange favors complete ligand replacement and one-step deposition of quantum dot solids using a colloidal stable nanoparticle ink. With further detailed research, the air stability of PbS CQDs can be improved. This improvement will not only lay a solid foundation for the application of PbS CQDs in optoelectronic devices but also provide opportunities for the development of room-temperature chemical sensors. In the last part, we discuss the chemical activity of PbS CQDs and their application for gas sensing. CQDs are ideal gas-sensitive materials owing to their large surface area and abundant active sites for gas adsorption, and the surface states formed by gas adsorption considerably affect the physical and chemical properties of the CQDs. Meanwhile, the CQDs have excellent film-forming properties at room temperature. Hence, they can be coated on a silicon substrate by simple and controllable methods such as spin coating or spraying. In summary, the surface ligand engineering of CQDs is an important strategy to develop new semiconductor functional devices. The optoelectronic properties and chemical activity of PbS CQDs indicate sufficient scope for design and regulation with various types, components, and introduction methods of the surface ligand. Great breakthroughs have been made with regard to PbS CQDs in both stability research and low-cost mass production over the last decade. Despite various challenges, the basic research on PbS CQDs for photoelectric and chemical sensing is ongoing. To improve the design ideas and fabrication methods of CQD functional devices, it is necessary to use methods based on theoretical calculations and microcharacterization techniques to reveal the effect of surface ligands on CQDs. The understanding of surface science and device physics of CQDs will drive the utilization of the semiconductor quantum effect. In the future, great breakthroughs are expected for the use of PbS quantum dots in the fields of infrared imaging, spectral analysis, gas sensing, and biochemical sensing.

A compact optical sensor for explosive detection based on NIR luminescent quantum dots

Detection of explosive traces in the vapor phase is of primary importance for safety and security in several environments. Different detection methods with high sensitivity are available in the market, but they are typically expensive and require specialized personnel to be operated. Here, we propose a compact, low-cost sensor for explosive detection based on the photoluminescence (PL) quenching of solid-state PbS quantum dot solids cast from the solution phase on a silicon substrate. We demonstrate the sensor capability to detect nitrobenzene vapor at a concentration as low as 445 ppb in air at room temperature, overcoming the performance of other state-of-the-art quantum dot-based PL sensors for nitroaromatic compounds. Moreover, the proposed system can be realized with off-the-shelf electronics and does not need any additional laboratory equipment to be operated, thus paving the way for its deployment in distributed sensor networks.

Facet-Oriented Coupling Enables Fast and Sensitive Colloidal Quantum Dot Photodetectors.

Charge carrier transport in colloidal quantum dot (CQD) solids is strongly influenced by coupling among CQDs. The shape of as-synthesized CQDs results in random orientational relationships among facets in CQD solids, and this limits the CQD coupling strength and the resultant performance of optoelectronic devices. Here, colloidal-phase reconstruction of CQD surfaces, which improves facet alignment in CQD solids, is reported. This strategy enables control over CQD faceting and allows demonstration of enhanced coupling in CQD solids. The approach utilizes post-synthetic resurfacing and unites surface passivation and colloidal stability with a propensity for dots to couple via (100):(100) facets, enabling increased hole mobility. Experimentally, the CQD solids exhibit a 10× increase in measured hole mobility compared to control CQD solids, and enable photodiodes (PDs) exhibiting 70% external quantum efficiency (vs 45% for control devices) and specific detectivity, D*>1012 Jones, each at 1550nm. The photodetectors feature a 7ns response time for a 0.01mm2 area-the fastest reported for solution-processed short-wavelength infrared PDs.

Enhancement of photoelectron emission efficiency from quantum dot solids, through electrical field biasing of interfaces

It is shown through comparison with experimental results that the efficiency of quantum dot (QD) film-based photoemission would be impacted by an inadequate supply of electrons from an electron source. An explanation for the related photocurrent droop, as arising from restricted electron transmission at the substrate-QD interface as well as between the QDs, is proposed. It is suggested that interfacial potential based biasing schemes could considerably enhance electronic coupling for improved transmission and quantum efficiency. Modeling indicates that electric fields of ∼450 MV/m would be necessary for ensuring electron transmission coefficients close to unity.