Colloidal Quantum Dots Research Articles

Cu-Doped InP/ZnS Core/Shell Quantum Dots:Charge Carrier Dynamics and Solar Hydrogen Production

Photocatalytic hydrogen production is a clean and sustainable method to convert solar energy into chemical fuel. Among the different structural types of photocatalysts, colloidal quantum dots (QDs) with narrow bandgap and tunable band structure have demonstrated particularly effective utility in solar hydrogen production. In the present study, InP/ZnS core/shell QDs were prepared and employed as photocatalyst backbone for hydrogen production. By introducing Cu dopants into the ZnS shell of InP/ZnS, the interfacial charge dynamics between InP and ZnS can be manipulated to achieve an activity optimization for solar hydrogen production. Time-resolved photoluminescence spectroscopy was utilized to study the quantitative effect of Cu dopants on the interfacial charge dynamics of the samples. This work provides a fresh hint to manipulate the charge transfer dynamics of QDs by means of dopant introduction towards highly efficient solar hydrogen production.

Lead Sulfide Quantum Dots for Synaptic Transistors: Modulating the Learning Timescale with Ligands

One of the emerging paradigms to resolve pressing issues of modern computing electronics, such as limits in miniaturization and excessive energy consumption, takes inspiration from the biological brain and is therefore expected to display some of its properties, such as energy efficiency and effective learning. Moreover, one of the brain's remarkable properties is its ability to process complex information by resolving it on different timescales. In synapse‐emulating artificial devices, some form of memory (e.g., hysteresis in current–voltage characteristics) is required. One of the important characteristics of biological synapses is the coexistence of short‐ and long‐term memory, also called plasticity. However, a broader exploration of memory at multiple timescales in materials remains limited. Herein, the first example of synaptic transistors utilizing colloidal quantum dots (CQDs) as active material is reported. It is demonstrated that PbS‐CQDs, with metal halide and perovskite‐like ligands, are ideal as an active material for synaptic transistors exhibiting both short‐ and long‐term plasticity. Most interestingly, by changing the chemistry of the quantum dot outer monolayer, a drastic change in the temporal response of the learning is observed, demonstrating the possibility of engineering materials exhibiting learning at multiple timescales, similar to the biological synapses.

Tailoring Eco-Friendly Colloidal Quantum Dots for Photoelectrochemical Hydrogen Generation.

A photoelectrochemical (PEC) cell is able to realize effective solar-to-hydrogen energy conversion from water by using the semiconductor photoelectrode. Semiconducting colloidal quantum dots (QDs) with captivating features of size-tunable optoelectronic properties and broad light absorption are regarded as promising photosensitizers in solar-driven PEC systems. Up to now, different types of QDs have been developed to achieve high-efficiency PEC H2 generation, while the majority of state-of-the-art QDs-PEC systems are still fabricated from QDs consisting of heavy metals (e.g., Cd and Pb), which are extremely harmful to the human health and natural environment. In this context, substantial efforts have been made to mitigate the usage of highly toxic heavy metals and concurrently promote the development of alternative environment-friendly QDs with comparable features. This review presents recent advances of solar-driven PEC devices based on several typical environment-friendly QDs (e.g., carbon QDs, I-III-VI QDs and III-V QDs). A variety of techniques (e.g., shell thickness tuning, alloying/doping, and ligands exchange, etc.) to engineer these QD's optoelectronic properties and achieve high-efficiency PEC H2 production are thoroughly discussed. Furthermore, the critical challenges and future perspectives of advanced eco-friendly QDs-PEC systems in terms of QDs' synthesis, photo-induced charge kinetics, and operation stability/efficiency are briefly proposed.

Dipolar Ligands Tune Plasmonic Properties of Tin-Doped Indium Oxide Nanocrystals.

Surface functionalization with dipolar molecules is known to tune the electronic band alignment in semiconductor films and colloidal quantum dots. Yet, the influence of surface modification on plasmonic nanocrystals and their properties remains little explored. Here, we functionalize tin-doped indium oxide nanocrystals (ITO NCs) via ligand exchange with a series of cinnamic acids with different electron-withdrawing and -donating dipolar characters. Consistent with previous reports on semiconductors, we find that withdrawing (donating) ligands increase (decrease) the work function caused by an electrostatic potential shift across the molecular layer. Quantitative analyses of the plasmonic extinction spectra reveal that varying the ligand molecular dipole affects the near-surface depletion layer, with an anticorrelated trend between the electron concentration and electronic volume fraction, factors that are positively correlated in as-synthesized NCs. Electronic structure engineering through surface modification provides access to distinctive combinations of plasmonic properties that could enable optoelectronic applications, sensing, and hot electron-driven processes.

Lead Chalcogenide Colloidal Quantum Dots for Infrared Photodetectors.

Infrared detection technology plays an important role in remote sensing, imaging, monitoring, and other fields. So far, most infrared photodetectors are based on InGaAs and HgCdTe materials, which are limited by high fabrication costs, complex production processes, and poor compatibility with silicon-based readout integrated circuits. This hinders the wider application of infrared detection technology. Therefore, reducing the cost of high-performance photodetectors is a research focus. Colloidal quantum dot photodetectors have the advantages of solution processing, low cost, and good compatibility with silicon-based substrates. In this paper, we summarize the recent development of infrared photodetectors based on mainstream lead chalcogenide colloidal quantum dots.

Modeling and simulation of > 19% highly efficient PbS colloidal quantum dot solar cell: A step towards unleashing the prospect of quantum dot absorber

We used the SCAPS-1D simulation tool to maximize the performance of lead sulfide (PbS) solar cells. A PbS solar cell was first modeled and then experimentally verified from past research. The ZnO electron-transport layer (ETL) was then replaced with ZnO:Al ETL material. Additionally, the fluorine-doped tin oxide work function, PbS-TBAI, ZnO:Al layer thicknesses, ZnO:Al/PbS-TBAI, PbS-TBAI/PbS-EDT defect density, PbS-TBAI defect density, ZnO:Al, and PbS-TBAI doping concentration were optimized. Results showed a greater alignment of the absorber-layer valence band with the HOMO and LUMO of ZnO:Al than that of ZnO ETL. The PbS solar cell exhibited the greatest efficiency at the optimum values of 0.4 µm, 0.04 µm, 1019 cm–3, 4.0 eV, 1017 cm–3, 1014 cm–2, 1018 cm–2, and 3.8 eV for absorber thickness, ETL thickness, ETL doping concentration, work function of front contact, PbS-TBAI doping concentration, and electron affinity of ZnO:Al, respectively. The PbS solar cell also performed best at interface defect densities of 1014 and 1018 cm–2 for ZnO:Al/PbS-TBAI and PbS-TBAI/PbS-EDT, respectively. The PCE of ZnO:Al ETL-based device also increased from 15.036% before optimization to 19.169% after. The optimized result was found to be affected by temperature. The PbS-optimized device with inorganic hole-transport layers (HTLs) demonstrated higher performance than the device with organic HTL. This finding was attributed to the low hole mobility in the organic HTL than those of inorganic ones. Therefore, the ZnO:Al ETL-based device was more efficient than the ordinary ZnO ETL-based device.

Observation of amplified spontaneous emission in Cd1-xZnxSe/ZnSe alloyed quantum dots through CW pumping

Colloidal quantum dots (QDs) have attracted much attention due to their optical tunability and application as a unique solution processable gain medium in lasers. Many research groups over the world have successfully attained amplification from Cd-based QDs through femtosecond (fs) and nanosecond (ns) pumping. However, for the wide application of QD-based lasers, an operation in the CW regime is much more favorable. Here, we present a critical step towards this goal by demonstrating CW laser (532 nm) pumped amplified spontaneous emission (ASE) in Cd1-xZnxSe/ZnSe alloyed core QDs dispersed in toluene at room temperature. Alloyed core QDs show lower non radiative transitions, which has been demonstrated by TA dynamics in this report. The threshold level for CW ASE is observed to be 31 W cm-2. These results indicate that these QDs can sustain CW-stimulated emission at room temperature, and can be widely applicable by further optimizing the QD material quality in order to have a low amplification threshold.

Colloid Quantum Dots and Solar Cell Applications

Colloid Quantum Dots and Solar Cell Applications

Correction: Tian et al. Fluorescence Resonance Energy Transfer Properties and Auger Recombination Suppression in Supraparticles Self-Assembled from Colloidal Quantum Dots. Inorganics 2023, 11, 218

Error in Figure [...]

Ultranarrow Mid-infrared Quantum Plasmon Resonance of Self-Doped Silver Selenide Nanocrystal.

The infrared quantum plasmon resonance (IR QPR) of nanocrystals (NCs) exhibits the combined properties of classical and quantum mechanics, potentially overcoming the limitations of conventional optical features. However, research on the development of localized surface plasmon resonance (LSPR) from colloidal quantum dots has stagnated, owing to the challenge of increasing the carrier density of semiconductor NCs. Herein, we present the mid-IR QPR of a self-doped Ag2Se NC with an exceptionally narrow bandwidth. Chemical modification of the NC surface with chloride realizes this narrow QPR bandwidth by achieving a high free-carrier density in the NC. The mid-IR QPR feature was thoroughly analyzed by using various experimental methods such as Fourier transform (FT) IR spectroscopy, X-ray photoelectron spectroscopy, and current-voltage measurements. In addition, the optical properties were theoretically analyzed using the plamon-in-a-box model and a modified hydrodynamic model that revealed the effect of coupling with the intraband transition and the limited nature of electron density in semiconductor NCs. Integrating the quantum effect into the plasmonic resonance reduces the peak bandwidth to 19.7 meV, which is an extremely narrow bandwidth compared with that of the LSPR of conventional metal oxide or metal chalcogenide NCs. Our results demonstrate that self-doped silver selenide quantum dots are excellent systems for studying mid-IR QPR.

Multiple-beam colloidal quantum dot lasers in a waveguide-grating-waveguide microcavity

In this work, multiple-beam colloidal quantum dot lasers are achieved in a double waveguide-grating (waveguide-grating-waveguide, W-G-W) microcavity. The grating is fabricated simply using interference lithography, and CdSe/CdS/ZnS colloidal quantum dots (CQDs) are spin-coated as the gain to form an active waveguide-grating structure. The photoresist film, which is not completely etched to the bottom, provides another waveguide-grating structure. In the W-G-W structure, low-threshold multiple-beam laser output is realized under optical pumping with emission peaks at 664.6 and 645.2 nm. The oblique laser is derived from a quasi-propagation mode. The thresholds of the two laser modes are 22.7 and 28.3 μJ/cm2, respectively, and both laser modes are TE0 modes. In addition, the emission wavelengths of the two modes of the designed distributed feedback laser can be flexibly tuned by changing the thickness of the CQDs' waveguide layer. The presence of quasi-propagation modes provides another method for designing compact laser sources, which could help in the design of wireless communication networks, hyperspectral 3D sensing, and color laser displays.

Promise to electrically pumped colloidal quantum dot lasers

Promise to electrically pumped colloidal quantum dot lasers

Controlled mixing during colloidal quantum dot synthesis: A proxy-concept based on equivalent parameters

Mixing is a key process parameter during liquid phase particle synthesis that requires special attention, in particular for scalable process and reactor development. This issue is especially pressing for advanced materials where on the one hand multifunctional properties require products of utmost quality, while on the other hand only few strategies for process design are yet developed. One material class in this context is quantum dot (QD) nanocrystals. By making use of automation and high throughout (HT) experimentation, we could recently demonstrate qualitatively the role of mixing on focusing and defocusing during QD synthesis by hot injection. In this work, we demonstrate the application of HT methodologies to investigate the effect of mixing as an important process parameter during process optimization and scale up. We performed reaction optimization by screening a multi-parameter design space using design of experiments (DoE) and machine learning (ML). We developed a so-called cold model as proxy that is based on equivalent mixing times (EMTs) derived from a competing parallel reaction system. Experimentally, we realized a fully automated approach using a HT platform for particle synthesis. The final dataset (128 experiments for model development including repeats and 29 experiments for prediction testing) was combined with DoE and complimented with machine learning. By doing so, with DoE, we could reduce the number of experiments required to develop a predictive, statistical knowledge base for EMTs at ambient conditions. Finally, we demonstrated the applicability of EMTs for two defined stirrer geometries to fine tune the widths of the particle size distribution and photoluminescence of QDs at increasing reaction volumes from 20 mL to 80 mL. This opens the door towards scalable processes for QDs at increased reaction volumes while maintaining product quality.

Unlocking the Potential of Colloidal Quantum Dot/Organic Hybrid Solar Cells: Band Tunable Interfacial Layer Approach.

Hybrid colloidal quantum dot (CQD)/organic architectures are promising candidates for emerging optoelectronic devices having high performance and inexpensive fabrication. For unlocking the potential of CQD/organic hybrid devices, enhancing charge extraction properties at electron transport layer (ETL)/CQD interfaces is crucial. Hence, we carefully adjust the interface properties between the ETL and CQD layer by incorporating an interfacial layer for the ETL (EIL) using several types of cinnamic acid ligands. The EIL having a cascading band offset (ΔEC) between the ETL and CQD layer suppresses the potential barrier and the local charge accumulation at ETL/CQD interfaces, thereby reducing the bimolecular recombination. An optimal EIL effectively expands the depletion region that facilitates charge extraction between the ETL and CQD layer while preventing the formation of shallow traps. Representative devices with an EIL exhibit a maximum power conversion efficiency of 14.01% and retain over 80% of initial performances after 300 h under continuous maximum power point operation.

Interface-Induced Crystallinity Enhancement of Perovskite Quantum Dots for Highly Efficient Light-Emitting Diodes.

Perovskite quantum dot light-emitting diodes (QLEDs) with high color purity and wide color gamut have good application prospects in the next generation of display technology. However, colloidal perovskite quantum dots (PQDs) may introduce a large number of defects during the film-forming process, which is not conducive to the luminous efficiency of the device. Meanwhile, the disordered film formation of PQDs will form interfacial defects and reduce the device performance. Here, we report an interface-induced crystallinity enhancement (IICE) strategy to increase the crystallinity of PQDs at the hole transport layer (HTL)/PQD interface. As a result, both the Br- vacancies in the PQD film and the interfacial defects were well passivated and the leakage current was also suppressed. We achieved QLEDs with a maximum external quantum efficiency (EQE) of 16.45% and current efficiency (CE) of 61.77 cd/A, showing improved performance to more than twice that of the control devices. The IICE strategy paves a new way to enhance the crystallinity of PQD films, so as to improve the performance of QLEDs for application in the future display field.

Time-Resolved Line Shapes of Single Quantum Emitters via Machine Learned Photon Correlations.

Solid-state single-photon emitters (SPEs) are quantum light sources that combine atomlike optical properties with solid-state integration and fabrication capabilities. SPEs are hindered by spectral diffusion, where the emitter's surrounding environment induces random energy fluctuations. Timescales of spectral diffusion span nanoseconds to minutes and require probing single emitters to remove ensemble averaging. Photon correlation Fourier spectroscopy (PCFS) can be used to measure time-resolved single emitter line shapes, but is hindered by poor signal-to-noise ratio in the measured correlation functions at early times due to low photon counts. Here, we develop a framework to simulate PCFS correlation functions directly from diffusing spectra that match well with experimental data for single colloidal quantum dots. We use these simulated datasets to train a deep ensemble autoencoder machine learning model that outputs accurate, noiseless, and probabilistic reconstructions of the noisy correlations. Using this model, we obtain reconstructed time-resolved single dot emission line shapes at timescales as low as 10ns, which are otherwise completely obscured by noise. This enables PCFS to extract optical coherence times on the same timescales as Hong-Ou-Mandel two-photon interference, but with the advantage of providing spectral information in addition to estimates of photon indistinguishability. Our machine learning approach is broadly applicable to different photon correlation spectroscopy techniques and SPE systems, offering an enhanced tool for probing single emitter line shapes on previously inaccessible timescales.

Electric-field-induced colour switching in colloidal quantum dot molecules at room temperature.

Colloidal semiconductor quantum dots are robust emitters implemented in numerous prototype and commercial optoelectronic devices. However, active fluorescence colour tuning, achieved so far by electric-field-induced Stark effect, has been limited to a small spectral range, and accompanied by intensity reduction due to the electron-hole charge separation effect. Utilizing quantum dot molecules that manifest two coupled emission centres, we present a unique electric-field-induced instantaneous colour-switching effect. Reversible emission colour switching without intensity loss is achieved on a single-particle level, as corroborated by correlated electron microscopy imaging. Simulations establish that this is due to the electron wavefunction toggling between the two centres, induced by the electric field, and affected by the coupling strength. Quantum dot molecules manifesting two coupled emission centres may be tailored to emit distinct colours, opening the path for sensitive field sensing and colour-switchable devices such as a novel pixel design for displays or an electric-field-induced colour-tunable single-photon source.

Efficient PbSe Quantum Dot Infrared Photovoltaic Applying MXene Modified ZnO Electron Transport Layer

AbstractInfrared (IR) solar cells are potential optoelectronic devices for boosting the power conversion efficiency (PCE) of conventional photovoltaics (such as pervoskite and silicon solar cells) by broadening the utilization range of the sunlight spectrum to short‐wavelength infrared region. PbSe colloidal quantum dots (QDs) are one of the optimal candidates for IR solar cells because of their tunable bandgap in the IR region and flexible solution processibility. At present, the best PbSe QD IR photovoltaics generally adopt ZnO as an electron transport layer (ETL). However, the intrinsic drawbacks and surface defects of ZnO can potentially deteriorate the PCE of devices. Herein, Ti3C2Tx, a representative 2D transition carbide, is combined with sol‐gel ZnO to develop a new hybrid ETL for fabricating high‐performance IR solar cells. This combination effectively suppresses the defects within ZnO by forming new bondings and simultaneously enhances the crystalline of ZnO film. Meanwhile, the introduction of Ti3C2Tx into ZnO film accelerates the transport and collection of photo‐generated carriers by constructing a new electron transport pathway. Consequently, compared to the bare devices, the infrared PCE of PbSe QD solar cells increases by 19.5% to 1.04%. These results demonstrate that this hybrid ETL can offer a bright approach for developing high‐performance optoelectronic devices.

Photonic Characterization and Modeling of Highly Efficient Color Conversion Layers With External Reflectors

We propose a new method for fabricating arrays of colloidal quantum dots for color conversion layers (CCLs) with atomic layer deposited (ALD) encapsulation. Semiconductor processes were used to transfer the CCL onto a transparent glass substrate. This method enables pixels as small as 5 μm, and the pixel's size and pitch can be controlled through the semiconductor process and mask layout. A dual-color array with a red-and-green CCL was also demonstrated to be suitable for microdisplay applications in experiments. The best color conversion efficiency of a single color CCL reached as high as 20.7%. A set of distributed Bragg reflector mirrors was used to increase photon recycling, and these mirrors yielded an increase in peak intensity of 35.7%. We formulated a model of incoherent reflection and transmission to calculate the changes in CCL properties when mirrors are added; this model had close fit with results from experiments in which mirrors of different reflection coefficients were used. Through the use of ALD, the CCL can be stored at a normal room-temperature environment for more than 9000 hours, and the projected lifetime is linearly extrapolated to be 44,041 hours.

Recent Progress of Quantum Dot Infrared Photodetectors

AbstractInfrared (IR) technology plays a significant role in many application scenarios, such as satellite remote sensing, environmental monitoring, biomedical engineering, imaging, communications, and so on. Demands of the next generation IR photodetectors, including ease of fabricaation, low cost, high flexibility, and so on, greatly drive the development of colloidal IR quantum dots (QDs) as the most competitive candidate material for IR photodetectors. In this article, the progress of IR QD materials and photodetectors, the fundamentals of IR QDs, the integration of IR QDs with other materials for hybrid photodetectors, and the applications of the QD IR photodetectors are reviewed.