Chalcogenide Quantum Dots Research Articles

Heat reflux sonochemical synthesis of Cu3BiS3 quantum dots: Experimental and first-principles investigation of spin–orbit coupling on structural, electronic, and optical properties

Semiconductor quantum dots (QD) exhibit promising electrical and optical properties in photovoltaic cells by improving their stability and efficiency enhancement. Therefore, in this study, novel Cu3BiS3 (CBS) ultrafine QDs were synthesized to function as effective solar absorbers, using a facile heat reflux sonochemical synthesis process under inert (N2) atmosphere. Using high-resolution transmission electron microscopy (HRTEM) analysis, the size of CBS QDs was found to be in the range of 3–8 nm. Furthermore, the crystallite size (D), strain (ε), and dislocation density (δ) of the QDs were investigated in detail by confirming the orthorhombic wittichenite structure. The photocurrent density (PCD) of CBS QDs obtained at a voltage range of 0.5–1 V was found between 0.2 and 0.6 mA/cm2 under illumination; otherwise, it has a linear nature and good ohmic behavior in the range of − 0.1 V to + 0.1 V. In additionally, the spin–orbit coupling (SOC) interaction was computed using the Perdew–Burke–Ernzerhof (PBE) and Tran-Blaha-modified Becke–Johnson (TB-mBJ) functionals in the density functional theory (DFT) framework. The results showed a bandgap transition from direct to indirect with the inclusion of SOC, as confirmed by the electronic band structure. Moreover, the DFT-calculated bandgap using the TB-mBJ functional (∼1 eV) achieved near consistency with the experimentally obtained bandgap (∼1.4 eV). Therefore, based on the experimental and computational results, this study can pave the way toward developing novel copper chalcogenide QDs for application in future photovoltaic cells.

Transition Metal Chalcogenides Quantum Dots: Emerging Building Blocks toward Solar-to-Hydrogen Conversion

Transition metal chalcogenides quantum dots (TMCs QDs) nanocrystals are featured by large absorption coefficient, abundant active sites, and quantum confinement effect, rendering them emerging light-harvesting antennas for solar energy conversion. Although diverse TMCs QDs-based photosystems have been explored for multifarious photoredox catalysis, an elaborate and comprehensive summary on TMCs QDs-based solar-to-hydrogen conversion has been rarely reviewed. In this Review, we elucidate the fundamental physicochemical properties of TMCs QDs (e.g., CdS QDs, CdSe QDs, CdTe QDs, etc.) currently being extensively investigated along with synthesis methodologies. Modification strategies for boosting photocatalytic water splitting performances of TMCs QDs, such as cocatalyst loading, heterojunction engineering, element doping, and plasmonic metal photosensitization, are then specifically introduced. Besides, we present detailed information on the latest advances on the photocatalytic hydrogen generation involving sacrificial reagents, Z-scheme photosystems, one-step excitation photosystems for overall water splitting, and photosensitizers endowed by molecular catalysts, metal–organic frameworks, or conjugated polymers. Finally, future outlook and challenge on the development of TMCs QDs-dominated photocatalysis are highlighted. It is anticipated that our Review would inspire ongoing interest in unleashing the great potential of TMCs QDs nanocrystals for solar energy conversion.

A novel strategy to construct activatable silver chalcogenide quantum dots nanoprobe for NIR-Ⅱ fluorescence imaging of hypochlorous acid in vivo

It is necessary to develop sensitive and selective probes for real-time in vivo monitoring of hypochlorous acid (HClO) which plays a significant role in physiological and pathological processes. The second near-infrared (NIR-Ⅱ) luminescent silver chalcogenide quantum dots (QDs) have shown great potential in developing activatable nanoprobe for HClO in terms of their outstanding imaging performance in the living organism. However, the limited strategy for the construction of activatable nanoprobes severely restricts their widespread applications. Herein, we proposed a novel strategy for developing an activatable silver chalcogenide QDs nanoprobe for NIR-Ⅱ fluorescence imaging of HClO in vivo. The nanoprobe was fabricated by mixing an Au-precursor solution with Ag2Te@Ag2S QDs to allow cation exchange and release Ag ions and then reducing the released Ag ions on the QDs surface to form an Ag shell for quenching of the emission of QDs. The Ag shell of QDs was oxidized and etched in the presence of HClO, resulting in the disappearance of their quenching effect on QDs and the activation of the QDs emission. The developed nanoprobe enabled highly sensitive and selective determination of HClO and imaging of HClO in arthritis and peritonitis. This study provides a novel strategy for the construction of activatable nanoprobe based on QDs and a promising tool for NIR-Ⅱ imaging of HClO in vivo.

Surface Engineering of MoS2 Quantum Dots via Cyclodextrin-Based Host-Guest Chemistry as Versatile Platforms for Enhanced Cell Imaging Application.

Transition metal chalcogenide quantum dots (QDs), especially MoS2 QDs, are an emerging class of novel optical probes for versatile bioanalytical applications owing to their distinct physicochemical properties. However, the reasonable use of these QDs for biological imaging has been largely restricted due to the challenge of controllable surface functionalization. In this work, we report a new strategy to engineer the surface of MoS2 QDs by taking advantage of cyclodextrin (CD)-based host-guest chemistry. The prepared β-CD-modified QDs (β-CD-MoS2 QDs) exhibit enhanced fluorescence properties, excellent biocompatibility, and good stability, making them promising as novel optical probes for bioimaging. Cellular imaging experiments revealed that these β-CD-MoS2 QDs can enter living cells through multiple internalization pathways, which differs significantly from pristine QDs. Particularly, we observed that the intracellular accumulation of MoS2 QDs in lipid droplets was enhanced owing to the specific binding of β-CD to cholesterol, which was then harnessed for monitoring the lipid metabolism in living cells via fluorescence imaging. Furthermore, we also demonstrated the potential use of β-CD-MoS2 QDs for targeted cell imaging and microplate-based cell recognition, which can be easily achieved via bioconjugation with functional motifs (e.g., folate acid) through host-guest chemistry. Altogether, these results illustrate the great potential of engineering the surface of MoS2 QDs and other analogous materials via CD-based host-guest chemistry for advancing their cell imaging applications.

Electrochemical Gelation of Metal Chalcogenide Quantum Dots: Applications in Gas Sensing and Photocatalysis.

ConspectusMetal chalcogenide quantum dots (QDs) are prized for their unique and functional properties, associated with both intrinsic (quantum confinement) and extrinsic (high surface area) effects, as dictated by their size, shape, and surface characteristics. Thus, they have considerable promise for diverse applications, including energy conversion (thermoelectrics and photovoltaics), photocatalysis, and sensing. QD gels are macroscopic porous structures consisting of interconnected QDs and pore networks in which the pores may be filled with solvent (i.e., wet gels) or air (i.e., aerogels). QD gels are unique because they can be prepared as macroscale objects while fully retaining the size-specific quantum-confined properties of the initial QD building blocks. The extensive porosity of the gels also ensures that each QD in the gel network is accessible to the ambient, leading to high performance in applications that require high surface areas, such as (photo)catalysis and sensing.Metal chalcogenide QD gels are conventionally prepared by chemical approaches. We recently expanded the toolbox for QD gel synthesis by developing electrochemical gelation methods. Relative to conventional chemical oxidation approaches, electrochemical assembly of QDs (1) enables the use of two additional levers for tuning the QD assembly process and gel structure: electrode material and potential, and (2) allows direct gel formation on device substrates to simplify device fabrication and improve reproducibility. We have discovered two distinct electrochemical gelation methods, each of which enables the direct writing of gels on an active electrode surface or the formation of free-standing monoliths. Oxidative electrogelation of QDs leads to assemblies bridged by dichalcogenide (covalent) linkers, whereas metal-mediated electrogelation proceeds via electrodissolution of active metal electrodes to produce free ions that link QDs by binding to pendant carboxylate functionalities on surface ligands (non-covalent linkers). We further demonstrated that the electrogel composition produced from the covalent assembly could be modified by controlled ion exchange to form single-ion decorated bimetallic QD gels, a new category of materials. The QD gels exhibit unprecedented performance for NO2 gas sensing and unique photocatalytic reactivities (e.g., the "cyano dance" isomerization and the reductive ring-opening arylation). The chemistry unveiled during the development of electrochemical gelation pathways for QDs and their post-modification has broad implications for guiding the design of new nanoparticle assembly strategies and QD gel-based gas sensors and catalysts.

How to minimize voltage and fill factor losses to achieve over 20% efficiency lead chalcogenide quantum dot solar cells: Strategies expected through numerical simulation

Lead chalcogenide colloidal quantum dot solar cells (CQDSCs) have the potential to revolutionize the field of light-to-electricity conversion with their exceptional optoelectronic properties. Unfortunately, realizing their full potential has been hindered by persistent and poorly understood limitations in fill factor (FF) and open-circuit voltage (Voc) losses. In this study, we performed a systematic numerical analysis of practical PbS CQDSCs to identify the root causes of FF and Voc losses in the current development stage, and to provide a clear and feasible roadmap for achieving a PCE of more than 20% in future development stages. Our analysis revealed that the highly effective route for enhancing the current 10% device is to initially modify the internal resistances, resulting in a significant reduction in FF losses to 15%, followed by systematic optimization of surface recombination velocities in the absorber layer and the absorber/hole transfer layer (HTL) interface, which can generate a Voc improvement of 3.76%, ultimately leading to a near-15% PCE. To further elevate PCE to unprecedented heights, we identified the precise regulation of surface excess charge densities at the absorber/HTL interface and the HTL/back contact interface as critical factors. By finely tuning these performance-limiting factors, we demonstrated the feasibility of achieving over 20% PCE, with minimal Voc loss of 318.10 mV and almost negligible FF loss of 6.08% in PbS CQDSCs. Our investigation provides crucial insights into the causes of FF and Voc losses in PbS CQDSCs and offers a clear pathway for future progress in this rapidly evolving field.

Observable Hole-State Kinetics and Its Implications for Optical Gain in Hole-Engineered Quantum Dots

Disentangling carrier kinetics is of both fundamental and technological importance for optoelectronics, while the hole-state dynamics in Cd chalcogenide quantum dots (QDs) remains far from understood. Herein, we take the CdZnS-based alloyed QDs as the model system to examine the hole-state dynamics by transient absorption (TA) spectroscopy. The spectroscopic signature of the hole states could be directly manifested from the TA spectra, even in low-quality CdZnS QDs, which arises from the distinctive features of alloyed QDs. By completely passivating the hole traps, the resulting CdZnS/ZnS QDs feature a significant hole contribution of 17% to the TA bleach and associated bleach buildup kinetics. These merits endow the CdZnS-based QDs with excellent optical gain properties, where an ultralong gain lifetime (1.1 ns) and a broad gain bandwidth (>300 meV) have been observed thanks to the positive trion-suppressed Auger recombination and the retarded hot hole cooling under high excitation intensities. Such a slow hot carrier cooling (200 ps) in the alloyed QDs further indicates their potential in other optoelectronic devices, such as the hot carrier solar concentrators. These findings clarify the characters of the valence band hole and offer insights into the manipulation of hole states for optimizing device performances.

Defect Passivation Results in the Stability of Cesium Lead Halide Perovskite Nanocrystals

Nanoheterostructures (NHSs) based on lead halide perovskites (LHPs) and chalcogenide quantum dots have proved to be promising candidates for photovoltaic device applications. However, understanding the defect chemistry at the interfaces of LHPs and chalcogenides is essential to stabilize them and further tune their optoelectronic properties. Here, we demonstrate a route for designing CsPbBr3–PbSe NHSs and other derivatives of LHP-based NHSs using defect-rich MoSe2 nanosheets (NSs) and study the effect of the size of PbSe NPs on their optical properties. In this synthesis route, PbSe nanoparticles (NPs) are formed at an early stage of the reaction through a unique cation displacement reaction, over which CsPbBr3 nanocrystals (NCs) are epitaxially grown. Using this methodology, a nearly 3-fold enhancement in photoluminescence (PL) is achieved, whereas other selenium precursors, which form larger PbSe NPs, result in negligible PL enhancement with respect to the pure CsPbBr3 NCs. Detailed density functional theory (DFT) calculations suggest that the PbSe NPs are responsible for passivating the surface defects that consequently enhance the PL intensity. However, in the case of larger PbSe NPs, the associated valence and conduction bands lie within the band-gap region of CsPbBr3, creating a type-I heterostructure between the two materials, thereby affecting the luminescence properties. Strong passivation of surface defects in CsPbBr3–PbSe NHSs is also evidenced from low-temperature PL studies. Furthermore, the resulting CsPbBr3–PbSe NHSs demonstrate enhanced stability in the presence of water and do not degrade under ambient conditions for several months.

Precipitation and optical properties of PbSexS1-x quantum dots in glasses

Lead chalcogenide quantum dots have particular optoelectronic properties induced by the quantum confinement effects and have promising potentials towards applications in infrared spectral range. Modulation of the optoelectronic properties of lead chalcogenide quantum dots is mainly based on the thermal treatment. In this work, precipitation and optical properties of alloyed PbSexS1-x quantum dots in glasses are investigated. Composition, size, absorption, and photoluminescence properties of PbSexS1-x quantum dots are modulated by chalcogen elements in glasses and thermal treatment. By adjusting the concentration of ZnSe and ZnS in the glasses, average diameters of the PbSexS1-x quantum dots are tuned in the range of 8.1–16.1 nm, their absorption peak wavelengths are tuned in the range of 724–2497 nm, and photoluminescence bands are tuned in the range of 1016–2165 nm with full width at half maximum in the range of 167–517 nm. Fraction of Se in the PbSexS1-x quantum dots precipitated in glasses is dependent on the concentrations of ZnSe and ZnS in the glass and heat-treatment temperature. Increase in concentration of S in the glasses results in the reduction of relative fraction of Se in the PbSexS1-x quantum dots formed in glasses. More importantly, high heat-treatment temperature promotes the enrichment of Se in PbSexS1-x quantum dots, and Se fraction in PbSexS1-x quantum dots increases from 0.69 to 0.89, from 0.53 to 0.85, from 0.47 to 0.79, and from 0.45 to 0.76 for those precipitated in glasses with ZnSe/(ZnSe+ZnS) molar ratios of 0.545, 0.375, 0.286, and 0.231, respectively. Increase in ZnSe/(ZnSe+ZnS) molar ratios in glasses also leads to reduction in diameter of PbSexS1-x quantum dots formed in glasses. These alloyed PbSexS1-x quantum dots also exhibit improved photoluminescence quantum yields and enhanced resistance to thermal quenching of photoluminescence.

Ligand and local surface strain-assisted doping for cadmium chalcogenide II-VI quantum dots

Different specific preparation methods of Ag-doped cadmium chalcogenide nanocrystals (NCs) in the post-synthetic stage were proposed by many groups. These methods usually emphasize the Ag precursors but ignore other factors that can influence the Ag-doping process. We used Ag film to dope the CdSe and CdS quantum dots to achieve the Ag-dopant photoluminescence (PL). That Ag-doping process can be modified effectively via tuning the surface stoichiometry, ligands, and morphology of cadmium chalcogenide quantum dots. Ag source is not the key factor influencing the doping process. We attributed those phenomena to that the local surface strain caused by those factors can lower the barrier of the Ag–Cd cation exchange. We also found that Ag+ can passivate the chalcogenide-rich QDs to enhance the band edge emission. This conclusion can explain the diversity of Ag-doping in various cadmium chalcogenide nanocrystals. The pure Ag-dopant PL was obtained by artificially constructing a strain-rich surface on CdSe QDs.

Short‐Wave Infrared Photodetectors and Imaging Sensors Based on Lead Chalcogenide Colloidal Quantum Dots

AbstractLead chalcogenide quantum dots (QDs) are one of the next generations of ideal narrow bandgap infrared semiconductors, due to their succinct solution processing, low‐cost fabrication, size‐tunable infrared bandgap, and excellent optoelectronic properties. Tremendous efforts including synthesis methods, surface ligand engineering, and device architecture engineering, drastically contribute to the significant improvement of the performance of the photodetectors based on QDs. In recent years, with the rapid development of consumer electronics, short‐wave infrared (SWIR) imaging sensors are in urgent demand. Thanks to the flexible manipulation of the QD thin film deposition process, a variety of QD‐based imaging technologies have been studied, including single‐pixel imaging sensors, integrated imaging sensors with readout circuit, and upconversion imaging sensors, which can effectively reduce the cost of SWIR imaging sensors and promote the commercial application in the consumer electronics. Herein, recent advances of QD‐based photodetectors and imaging sensors are summarized, emphatically focusing on the synthesis of QDs, surface ligand engineering, device architecture engineering, and imaging technology.

(Invited) Ligand Exchange at Chalcogenide and Perovskite Nanocrystal Surfaces Examined Via Isothermal Titration Calorimetry

I will describe our group’s efforts to quantify ligand association and exchange reactions on semiconductor nanocrystal (NC) surfaces via isothermal titration calorimetry (ITC). ITC provides a direct thermal measurement of reaction progress. As shown in earlier work by our group and others examining ligand exchange on chalcogenide and pnictide quantum dots, ITC is able to access the enthalpy change of reaction, equivalency, and effective equilibrium constants, including in cases that are challenging to resolve by NMR alone. Here, we will describe the extension of this approach to ligand exchange reactions on halide perovskite NCs, and to ligand exchange on chalcogenide NCs in aqueous solution. Such advances are needed to contribute a quantitative description of NC surface chemistry toward applications such as advanced optoelectronic devices, bioimaging probes, and photocatalysis.

Ecotoxicity and Sustainability of Emerging Pb‐Based Photovoltaics

Emerging Pb‐based photovoltaic (PV) technologies, including in particular solution processed halide perovskite solar cells (PSCs) and Pb chalcogenide quantum dot solar cells (QDSCs), are among the most promising next‐generation PV technologies for a range of disruptive energy and electronic applications. However, the potential toxicity and leakage of hazardous Pb species have become one of the main barriers to their large‐scale application. When solar cells are subject to physical damage or failure of encapsulation, rapid leakage of Pb may occur, which can be accelerated by exposure to external environmental weathering conditions such as rainfall and elevated temperature. Herein, an in‐depth investigation on the essential role of Pb in PSCs and QDSCs, as well as common causes of Pb leakage, is undertaken. The hazardous effects of Pb toxicity on soil plants, bacteria, animals, and human cells are also evaluated. Recent progress in developing effective strategies for Pb leakage reduction, such as Pb‐free or Pb‐less perovskite materials, device architecture design, encapsulation absorbers for PSCs, and core–shell structure and ligand exchange method for QDSCs, in addition to Pb recycling strategies of end‐of‐life solar cells are summarized. This review provides quantitative insights into the future development of eco‐friendly emerging Pb‐based PV technologies.

How to get high-efficiency lead chalcogenide quantum dot solar cells?

How to get high-efficiency lead chalcogenide quantum dot solar cells?

Optical transitions, exciton radiative decay, and valley coherence in lead chalcogenide quantum dots

We propose the concept of valley coherence and superradiance in the reciprocal space and show that it leads to an $N$-fold decrease of the bright exciton radiative lifetime in quantum dots (QDs) of an $N$-valley semiconductor. Next we explain why, despite this, the exciton radiative lifetimes in PbX (X = S, Se, Te) QDs, measured from the photoluminescence decay, are in the microsecond range. We also address peculiarities of the light-matter interaction in nanostructures made of narrow-gap materials with strong inter-band coupling.

Fabrication of a dye-based random laser using ZnS:Mn quantum dots and investigating the effects of their concentration

Zinc chalcogenide quantum dots (QDs) doped with paramagnetic transition metal ions (particularly ZnS:Mn QDs) are new attractive but rarely examined semiconductor nanocrystals that have excellent optical properties and enhanced thermal and environmental stability compared to Cd-based QDs. In this paper, we demonstrate a dye-based random laser (RL) with nonresonant feedback using ZnS:Mn QDs as the scattering medium that are dispersed in a Rhodamine B (RhB) dye solution. The nonlinear variation of the emission spectrum as a function of the excitation energy implies a random lasing threshold. Moreover, we observe a blue-shift of the emission wavelength by 10.3 nm and a 5.3 times decrease in the RL threshold by increasing the scatterer concentration. We also provide a theoretical discussion based on the diffusion theory for explaining the observed experimental results.

Recent Advances in Metal Chalcogenide Quantum Dots: From Material Design to Biomedical Applications

AbstractWith the rapid progress in nanomaterials and biochemistry, there has been an explosion of interest in biomolecule‐modified quantum dots (QDs) for biomedical applications. Metal chalcogenide quantum dots (MCQDs), as the most widely studied QDs, have attracted tremendous attention in the biomedical field on account of their unique and excellent optical properties and the ease of biomolecular modifications. Herein, important advances in MCQDs over recent years are reviewed, from materials design to biomedical applications. Especially, this review focuses on the challenges encountered in the applications of MCQDs in biomedical fields and how these problems can be solved by rational design of synthesis methods and modifications, which have opened a universal route to develop the functionalized MCQDs. Moreover, recent processes in bioimaging, biosensing, and cancer therapy based on MCQDs are examined, including the rapid detection and diagnosis of severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2). This review provides broad insights into MCQDs in the biomedical field and will inspire material researchers to develop MCQDs in the future.

Photoemission of the Upconverted Hot Electrons in Mn-Doped CsPbBr3 Nanocrystals.

Hot electrons play a crucial role in enhancing the efficiency of photon-to-current conversion or photocatalytic reactions. In semiconductor nanocrystals, energetic hot electrons capable of photoemission can be generated via the upconversion process involving the dopant-originated intermediate state, currently known only in Mn-doped cadmium chalcogenide quantum dots. Here, we report that Mn-doped CsPbBr3 nanocrystals are an excellent platform for generating hot electrons via upconversion that can benefit from various desirable exciton properties and the structural diversity of metal halide perovskites (MHPs). Two-dimensional Mn-doped CsPbBr3 nanoplatelets are particularly advantageous for hot electron upconversion due to the strong exciton-dopant interaction mediating the upconversion process. Furthermore, nanoplatelets reveal evidence for the hot electron upconversion via long-lived dark excitons in addition to bright excitons that may enhance the upconversion efficiency. This study establishes the feasibility of hot electron upconversion in MHP hosts and demonstrates the potential merits of two-dimensional MHP nanocrystals in the upconversion process.

Cadmium Chalcogenide (CdS, CdSe, CdTe) Quantum Dots for Solar‐to‐Fuel Conversion

Photocatalysis is considered a promising technology to alleviate the pressure of environmental pollution and energy shortage. Among various photocatalysts, cadmium chalcogenide quantum dots (CdX QDs) stand out due to their excellent visible light absorption ability and efficient charge separation properties. This review presents the atomic and electronic structures of CdX QDs and their synthetic strategies. In addition, the reaction mechanism of CdX QDs in photocatalytic solar‐to‐fuel conversion is discussed. Another focus of this review is the optimization strategies for CdX QDs to improve their photocatalytic performance. Finally, the challenges and the perspectives on CdX QDs are briefly proposed.

Shaping Nanoscale Ribbons into Microhelices of Controllable Radius and Pitch.

We report fabrication of highly flexible micron-sized helices from nanometer-thick ribbons. Building upon the helical coiling of such ultrathin ribbons mediated by surface tension, we demonstrate that the enhanced creep properties of highly confined materials can be leveraged to shape helices into the desired geometry with full control of the final shape. The helical radius, total length, and pitch angle are all freely and independently tunable within a wide range: radius within ∼1-100 μm, length within ∼100-3000 μm, and pitch angle within ∼0-70°. This fabrication method is validated for three different materials: poly(methyl methacrylate), poly(dimethylaminoethyl methacrylate), and transition metal chalcogenide quantum dots, each corresponding to a different solid-phase structure: respectively a polymer glass, a cross-linked hydrogel, and a nanoparticle array. This demonstrates excellent versatility with respect to material selection, enabling further control of the helix mechanical properties.