Optoelectronic Properties of CuInS2 Nanocrystals and Their Origin

Synthesis and Optical Properties of CdSeTe/CdZnS/ZnS Core/Shell Nanorods

Semiconductor nanorods (NRs) have great potential in optoelectronic devices for their unique linearly polarized luminescence which can break the external quantum efficiency limit of light-emitting diodes (LEDs) based on spherical quantum dots. Significant progress has been made for developing red, green, and blue light-emitting NRs. However, the synthesis of NRs emitting in the deep red region, which can be used for accurate red LED displays and promoting plant growth, is currently less explored. Here, we report the synthesis of deep red CdSeTe/CdZnS/ZnS dot-in-rod core/shell NRs via a seeded growth method, where the doping of Te in the CdSe core can extend the NR emission to the deep red region. The rod-shaped CdZnS shell is grown over CdSeTe seeds. By growing a ZnS passivation shell, the CdSeTe/CdZnS/ZnS NRs exhibit a photoluminescence emission peak at 670 nm, a full width at a half maximum of 61 nm and a photoluminescence quantum yield of 45%. The development of deep red NRs can greatly extend the applications of anisotropic nanocrystals.

Enhancement mechanism of quantum yield in core/shell/shell quantum dots of ZnS-AgIn5S8/ZnIn2S4/ZnS

To achieve a high quantum yield (QY) of nanomaterials suitable for optical applications, we improved the optical properties of AgIn5S8 (AIS) quantum dots (QDs) by employing an alloyed-core/inner-shell/outer-shell (ZAIS/ZIS/ZnS) structure. We also investigated the mechanism of optical transitions to clarify the improvement of QYs. In AIS, the low-energy absorption near the band edge region is attributed to the weakly allowed band gap transition, which gains oscillator strength through state intermixing and electron-phonon coupling. The main photoluminescence is also ascribed to the weakly allowed band gap transition with characteristics of self-trapped excitonic emission. With alloying/shelling processes, the weakly allowed transition is enhanced by the evolution of the electronic structures in the alloyed core, which improves the band gap emission. In shelled structures, the nonradiative process is reduced by the reconstructed lattice and passivated surface, ultimately leading to a high QY of 85% in ZAIS/ZIS/ZnS. These findings provide new insights into the optical transitions of AIS because they challenge previous conclusions. In addition, our work elucidates the mechanism behind the enhancement of QY accomplished through alloying/shelling processes, providing strategies to optimize nontoxic QDs for various applications using a green chemistry approach.

Emergence of CuInS2 derived photocatalyst for environmental remediation and energy conversion

Hydrogen production, catalytic organic synthesis, carbon dioxide reduction, environmental purification, and other major fields have all adopted photocatalytic technologies due to their eco-friendliness, ease of use, and reliance on sunlight as the driving force. Photocatalyst is the key component of photocatalytic technology. Thus, it is of utmost importance to produce highly efficient, stable, visible-light-responsive photocatalysts. CIS stands out among other visible-light-response photocatalysts for its advantageous combination of easy synthesis, non-toxicity, high stability, and suitable band structure. In this study, we took a brief glance at the synthesis techniques for CIS after providing a quick introduction to the fundamental semiconductor features, including the crystal and band structures of CIS. Then, we discussed the ways doping, heterojunction creation, p-n heterojunction, type-II heterojunction, and Z-scheme may be used to modify CIS's performance. Subsequently, the applications of CIS towards pollutant degradation, CO2 reduction, water splitting, and other toxic pollutants remediation are reviewed in detail. Finally, several remaining problems with CIS-based photocatalysts are highlighted, along with future potential for constructing more superior photocatalysts.

Synthesis and hybridization of CuInS2 nanocrystals for emerging applications

Copper indium sulfide (CuInS2) is a ternary A(I)B(III)X(VI)2-type semiconductor featuring a direct bandgap with a high absorption coefficient. In attempts to explore their practical applications, nanoscale CuInS2 has been synthesized with crystal sizes down to the quantum confinement regime. The merits of CuInS2 nanocrystals (NCs) include wide emission tunability, a large Stokes shift, long decay time, and eco-friendliness, making them promising candidates in photoelectronics and photovoltaics. Over the past two decades, advances in wet-chemistry synthesis have achieved rational control over cation-anion reactivity during the preparation of colloidal CuInS2 NCs and post-synthesis cation exchange. The precise nano-synthesis coupled with a series of hybridization strategies has given birth to a library of CuInS2 NCs with highly customizable photophysical properties. This review article focuses on the recent development of CuInS2 NCs enabled by advanced synthetic and hybridization techniques. We show that the state-of-the-art CuInS2 NCs play significant roles in optoelectronic and biomedical applications.

Transition Metal Ions Influence the Performance of Photodetector of Two-Dimensional CdS Nanoplatelets

Transition metal-doped two-dimensional (2D) semiconductor nanoplatelets (NPLs) with atomically precise thickness have attracted much research interest due to their inherent photo-physical properties. In this work, we have synthesized 2D Cu-doped CdS NPLs, investigated the charge transfer dynamics using ultrafast transient absorption spectroscopy, and fabricated an efficient photodetector device. A large Stoke's shifted emission at ~685 nm with an average lifetime of about ~1.45 μs is observed in Cu-doped CdS NPLs. Slower bleach recovery kinetics leads to large charge carrier separation in Cu-doped NPLs which is beneficial for photodetector applications. Cu-doped NPLs-based photodetectors exhibit high photocurrent, fast response (~120 ms), ~600 times higher photoresponsivity, and ~300 times higher detectivity (~4.1×1013 Jones) than undoped CdS NPLs. These excellent properties of Cu-doped CdS NPLs make this material an efficient alternative for next-generation optoelectronic devices.

Building in biologically appropriate multifunctionality in aqueous copper indium selenide-based quantum dots

Advanced nanoplatforms equipped with different functional moieties for theranostics hold appealing promise for reshaping precision medicine. The reliable construction of an individual nanomaterial with intrinsic near-infrared (NIR) photofunction and magnetic domains is much desired but largely unexplored in a direct aqueous synthesis system. Herein, we develop an aqueous phase synthetic strategy for Mn2+ doping of ZnS shell grown on Zn-Cu-In-Se core quantum dots (ZCISe@ZnS:Mn QDs), providing the optimal NIR fluorescence quantum efficiency of up to 18.9% and meanwhile efficiently introducing paramagnetic domains. The relaxometric properties of the water-soluble Mn-doped QDs make them desirable for both the longitudinal and transverse (T1 and T2) magnetic resonance (MR) contrast enhancement due to the shell lattice-doped Mn2+ ions with slow tumbling rates and favoured spin-proton dipolar interactions with surrounding water molecules. Surprisingly, the incorporation of Mn2+ ions into the shell is found to significantly enhance the production of reactive oxygen species (ROS) by combining both the chemodynamic and photodynamic processes upon NIR light irradiation, showing great potential for efficient photo-assisted ablation of cancer cells. Furthermore, a broad-spectrum excitation range beneficial for bright NIR fluorescence imaging of breast cancer has been proven and offers high flexibility in the choice of incident light sources. Multiparametric MR imaging of the brain has also been successfully demonstrated in vivo.

Copper indium sulfide quantum dots in photocatalysis

Since the advent of photocatalytic technology, scientists have been searching for semiconductor materials with high efficiency in solar energy utilization and conversion to chemical energy. Recently, the development of quantum dot (QD) photocatalysts has attracted much attention because of their unique characteristics: small size, quantum effects, strong surface activity, and wide photoresponse range. Among ternary chalcogenide semiconductors, CuInS₂ QDs are increasingly examined in the field of photocatalysis due to their high absorption coefficients, good matching of the absorption range with sunlight spectrum, long lifetimes of photogenerated electron-hole pairs and environmental sustainability. In this review paper, the structural and electronic properties, synthesis methods and various photocatalytic applications of CuInS₂ QDs are systematically expounded. The current research status on the photocatalytic properties of materials based on CuInS₂ QD is discussed combined with the existing modification approaches for the enhancement of their performances. Future challenges and new development opportunities of CuInS₂ QDs in the field of photocatalysis are then prospected.

Dye-induced photoluminescence quenching of quantum dots: role of excited state lifetime and confinement of charge carriers

We investigate the role of quantum confinement and photoluminescence (PL) lifetime of photoexcited charge carriers in semiconductor core/shell quantum dots (QDs) via PL quenching due to surface modification. Surface modification is controlled by varying the number of dye molecules adsorbed onto the QD shell surface forming QD-dye nanoassemblies. We selected CuInS₂/ZnS (CIS) and InP/ZnS (InP) core/shell QDs exhibiting relatively weak (664 meV) and strong (1194 meV) confinement potentials for the conduction band electron. Moreover, the difference in the emission mechanism gives rise to a long and short excited state lifetime of CIS (ca. 290 ns) and InP (ca. 37 ns) QDs. Dye molecules of different ionic characters (rhodamine 575: zwitterionic and rhodamine 560: cationic) are used as quenchers. A detailed analysis of Stern-Volmer data shows that (i) quenching is generally more pronounced in CIS-dye assemblies as compared to InP-dye assemblies, (ii) dynamic quenching is dominating in all QD-dye assemblies with only a minor contribution from static quenching and (iii) the cationic dye shows a stronger interaction with the QD shell surface than the zwitterionic dye. Observations (i) and (ii) can be explained by the differences in the amplitude of the electronic component of the exciton wavefunction near the dye binding sites in both QDs, which results in the breaking up of the electron-hole pair and favors charge trapping. Observation (iii) can be attributed to the variations in electrostatic interactions between the negatively charged QD shell surface and the cationic and zwitterionic dyes, with the former exhibiting a stronger interaction. Moreover, the long lifetime of CIS QDs facilitates us to easily probe different time scales of the trapping processes and thus differentiate the origins of static and dynamic quenching components that appear in the Stern-Volmer analysis.

Luminescent Quantum Dots: Synthesis, Optical Properties, Bioimaging and Toxicity

Luminescent nanomaterials such as semiconductor nanocrystals (NCs) and quantum dots (QDs) attract much attention to optical detectors, LEDs, photovoltaics, displays, biosensing, and bioimaging. These materials include metal chalcogenide QDs and metal halide perovskite NCs. Since the introduction of cadmium chalcogenide QDs to biolabeling and bioimaging, various metal nanoparticles (NPs), atomically precise metal nanoclusters, carbon QDs, graphene QDs, silicon QDs, and other chalcogenide QDs have been infiltrating the nano-bio interface as imaging and therapeutic agents. Nanobioconjugates prepared from luminescent QDs form a new class of imaging probes for cellular and in vivo imaging with single-molecule, super-resolution, and 3D resolutions. Surface modified and bioconjugated core-only and core-shell QDs of metal chalcogenides (MX; M = Cd/Pb/Hg/Ag, and X = S/Se/Te,), binary metal chalcogenides (MInX₂; M = Cu/Ag, and X = S/Se/Te), indium compounds (InAs and InP), metal NPs (Ag, Au, and Pt), pure or mixed precision nanoclusters (Ag, Au, Pt), carbon nanomaterials (graphene QDs, graphene nanosheets, carbon NPs, and nanodiamond), silica NPs, silicon QDs, etc. have become prevalent in biosensing, bioimaging, and phototherapy. While heavy metal-based QDs are limited to in vitro bioanalysis or clinical testing due to their potential metal ion-induced toxicity, carbon (nanodiamond and graphene) and silicon QDs, gold and silica nanoparticles, and metal nanoclusters continue their in vivo voyage towards clinical imaging and therapeutic applications. This review summarizes the synthesis, chemical modifications, optical properties, and bioimaging applications of semiconductor QDs with particular references to metal chalcogenide QDs and bimetallic chalcogenide QDs. Also, this review highlights the toxicity and pharmacokinetics of QD bioconjugates.

I-III-VI Quantum Dots and Derivatives: Design, Synthesis, and Properties for Light-Emitting Diodes

Quantum dots (QDs) are important frontier luminescent materials for future technology in flexible ultrahigh-definition display, optical information internet, and bioimaging due to their outstanding luminescence efficiency and high color purity. I-III-VI QDs and derivatives demonstrate characteristics of composition-dependent band gap, full visible light coverage, high efficiency, excellent stability, and nontoxicity, and hence are expected to be ideal candidates for environmentally friendly materials replacing traditional Cd and Pb-based QDs. In particular, their compositional flexibility is highly conducive to precise control energy band structure and microstructure. Furthermore, the quantum dot light-emitting diodes (QLEDs) exhibits superior prospects in monochrome display and white illumination. This review summarizes the recent progress of I-III-VI QDs and their application in LEDs. First, the luminescence mechanism is illustrated based on their electronic-band structural characteristics. Second, focusing on the latest progress of I-III-VI QDs, the preparation mechanism, and the regulation of photophysical properties, the corresponding application progress particularly in light-emitting diodes is summarized as well. Finally, we provide perspectives on the overall current status and challenges propose performance improvement strategies in promoting the evolution of QDs and QLEDs, indicating the future directions in this field.

Non-Toxic CuInS2 /ZnS Colloidal Quantum Dots for Near-Infrared Light-Emitting Diodes

Ternary CuInS₂ quantum dots (QDs) with photoluminescence that is tunable from the visible to the near-infrared (NIR) region are promising light-emitters for consumer electronics due to the absence of toxic elements such as Pb, Cd, or As. Despite the compelling performance of visible-light-emitting CuInS₂ QDs, reports on NIR emission remain limited, with modest efficiencies at wavelengths beyond 900 nm. In this work, the facile synthesis of NIR-emitting CuInS₂ /ZnS QDs is reported. A combination of two sulfur precursors w as used in the synthesis, comprising 1-dodecanethiol (DDT) and hexamethyldisilathiane (HMDS). The reactive HMDS facilitates faster nucleation and leads to a higher density of emissive Cu-deficiency sites. The resulting QDs exhibit high photoluminescence quantum efficiency (PLQE) of 65% at a long emission wavelength of 920 nm. Using these QDs, NIR light-emitting diodes (LED) are fabricated, which attain an external quantum efficiency (EQE) of 8.2%. This efficiency is comparable to the best reported PbS and InAs QD LEDs, and the emission wavelength exceeds that of lead iodide perovskites. This work thus marks one of the first reports of efficient NIR LEDs based on environmentally benign CuInS₂ QDs and may open up promising new applications in consumer electronic products.

Nontoxic Fluorescent Nanoprobes for Multiplexed Detection and 3D Imaging of Tumor Markers in Breast Cancer

Multiplexed fluorescent immunohistochemical analysis of breast cancer (BC) markers and high-resolution 3D immunofluorescence imaging of the tumor and its microenvironment not only facilitate making the disease prognosis and selecting effective anticancer therapy (including photodynamic therapy), but also provides information on signaling and metabolic mechanisms of carcinogenesis and helps in the search for new therapeutic targets and drugs. The characteristics of imaging nanoprobe efficiency, such as sensitivity, target affinity, depth of tissue penetration, and photostability, are determined by the properties of their components, fluorophores and capture molecules, and by the method of their conjugation. Regarding individual nanoprobe components, fluorescent nanocrystals (NCs) are widely used for optical imaging in vitro and in vivo, and single-domain antibodies (sdAbs) are well established as highly specific capture molecules in diagnostic and therapeutic applications. Moreover, the technologies of obtaining functionally active sdAb–NC conjugates with the highest possible avidity, with all sdAb molecules bound to the NC in a strictly oriented manner, provide 3D-imaging nanoprobes with strong comparative advantages. This review is aimed at highlighting the importance of an integrated approach to BC diagnosis, including the detection of biomarkers of the tumor and its microenvironment, as well as the need for their quantitative profiling and imaging of their mutual location, using advanced approaches to 3D detection in thick tissue sections. The existing approaches to 3D imaging of tumors and their microenvironment using fluorescent NCs are described, and the main comparative advantages and disadvantages of nontoxic fluorescent sdAb–NC conjugates as nanoprobes for multiplexed detection and 3D imaging of BC markers are discussed.

Tunable 0D/2D/2D Nanocomposite Based on Green Zn-Doped CuInS2 Quantum Dots and MoS2/rGO as Photoelectrodes for Solar Hydrogen Production

Charge separation, transmission, and light absorption properties are critical to determining the performance of photoelectrochemical (PEC) devices. An important strategy to control such properties is based on using heterostructured materials. Herein, a tunable zero-dimensional (0D)/two-dimensional (2D) heterostructure is designed based on quantum dots (QDs) and 2D nanosheets (NSs). Specifically, eco-friendly Zn-doped CuInS₂ QDs prepared by hot injection were anchored on hierarchical (2D/2D) MoS₂/rGO (MG) NSs through a facile sonication-assisted method to develop a 0D/2D/2D heterojunction-based photoelectrode for solar hydrogen production. The interfacial structure and band alignment between the proposed 0D QDs and 2D/2D MG NSs were engineered by modulating the Zn molar ratio during the QD synthesis. As proof of concept, the optimized 0D/2D/2D photoanode exhibits almost five times higher PEC activity than MG/CuInS₂ and MoS₂/Zn-CuInS₂ NSs due to the enhanced light absorption, efficient charge separation, and transmission. Zn doping and the presence of graphene are essential in enhancing performance in the proposed heterostructure, reducing recombination of charge carriers, and improving sunlight absorption. This work shows how optimal band alignment control and carbon addition can facilitate charge transfer, enabling the development of highly efficient PEC devices based on 0D/2D/2D heterostructure nanocomposites.

Optically Detected Magnetic Resonance Spectroscopy of Cu-Doped CdSe/CdS and CuInS2 Colloidal Quantum Dots

Copper-doped II-VI and copper-based I-III-VI₂ colloidal quantum dots (CQDs) have been at the forefront of interest in nanocrystals over the past decade, attributable to their optically activated copper states. However, the related recombination mechanisms are still unclear. The current work elaborates on recombination processes in such materials by following the spin properties of copper-doped CdSe/CdS (Cu@CdSe/CdS) and of CuInS₂ and CuInS₂/(CdS, ZnS) core/shell CQDs using continuous-wave and time-resolved optically detected magnetic resonance (ODMR) spectroscopy. The Cu@CdSe/CdS ODMR showed two distinct resonances with different g factors and spin relaxation times. The best fit by a spin Hamiltonian simulation suggests that emission comes from recombination of a delocalized electron at the conduction band edge with a hole trapped in a Cu²⁺ site with a weak exchange coupling between the two spins. The ODMR spectra of CuInS₂ CQDs (with and without shells) differ significantly from those of the copper-doped II-VI CQDs. They are comprised of a primary resonance accompanied by another resonance at half-field, with a strong correlation between the two, indicating the involvement of a triplet exciton and hence stronger electron-hole exchange coupling than in the doped core/shell CQDs. The spin Hamiltonian simulation shows that the hole is again associated with a photogenerated Cu²⁺ site. The electron resides near this Cu²⁺ site, and its ODMR spectrum shows contributions from superhyperfine coupling to neighboring indium atoms. These observations are consistent with the occurrence of a self-trapped exciton associated with the copper site. The results presented here support models under debate for over a decade and help define the magneto-optical properties of these important materials.

Spectral analysis on the acceptor concentration-dependent fluorescence resonance energy transfer process in CuInS2@ZnS-SQ complexes

Owing to the broad spectral response and flexible choices of donors and acceptors, fluorescence resonance energy transfer (FRET) system based on quantum dots (QDs) is a potential candidate for enhancing performance of solar cells and other optoelectronic devices. Thus it is necessary to develop such FRET systems with high efficiency and understand the involved photophysical dynamics. Here, with type I CuInS₂@ZnS core-shell quantum dots as the energy donor, series of CuInS₂@ZnS-SQ complexes are synthesized by adjusting the acceptor (squaric acid, SQ) concentration. The FRET dynamics of the samples is systematically investigated by virtue of steady-state emission, time-resolved fluorescence decay, and transient absorption measurements. The experimental results display a positive correlation between the energy transfer efficient (η). The best energy transfer efficient achieved from experimental data is 52%. This work provides better understanding of the photophysical dynamics in similar complexes and facilitates further development of new photoelectronic devices based on relevant FRET systems.

Highly Efficient and Stable CdZnSeS/ZnSeS Quantum Dots for Application in White Light-Emitting Diode

Semiconductor quantum dots (QDs) are a promising luminescent phosphor for next-generation lightings and displays. In particular, QD-based white light-emitting diodes (WLEDs) are considered to be the candidate light sources with the most potential for application in displays. In this work, we synthesized quaternary/ternary core/shell alloyed CdZnSeS/ZnSeS QDs with high bright emission intensity. The QDs show good thermal stability by performing high temperature-dependent experiments that range from 295 to 433 K. Finally, the WLED based on the CdZnSeS/ZnSeS QDs exhibits a luminous efficiency (LE) of 28.14 lm/W, an external quantum efficiency (EQE) of 14.86%, and a warm bright sunlight close to the spectrum of daylight (Commission Internationale de l'éclairage (CIE) coordinates 0.305, 0.371). Moreover, the photoluminescence (PL) intensity, LE, EQE, and correlated color temperature (CCT) of as-prepared QD WLED remained relatively stable with only slight changes in the luminescence stability experiment.

Light-harvesting antennae based on copper indium sulfide (CIS) quantum dots

Copper indium sulfide quantum dots (CIS QDs) and their core-shell analogues (CIS@ZnS QDs) were functionalized with pyrene chromophores via a dihydrolipoamide bifunctional binding moiety: UV excitation of the pyrene chromophores resulted in sensitized emission of the CIS core because of an efficient energy transfer process; the core-shell hybrid system exhibits a 50% increased brightness when excited at 345 nm.

Tunable structural and optical properties of AgxCuyInS2 colloidal quantum dots

Herein is discussed tunable absorption and emission properties of AgxCuyInS2 quantum dots involving wurtzite to chalcopyrite phase transformation dependent on Ag+ dopant content.

Synthetic Ligand Selection Affects Stoichiometry, Carrier Dynamics, and Trapping in CuInSe2 Nanocrystals

CuInSe₂ nanocrystals exhibit tunable near-infrared bandgaps that bolster utility in photovoltaic applications as well as offer potential as substitutes for more toxic Cd- and Pb-based semiconductor compositions. However, they can present a variety of defect states and unusual photophysics. Here, we examine the effects of ligand composition (oleylamine, diphenylphosphine, and tributylphosphine) on carrier dynamics in these materials. Via spectroscopic measurements such as photoluminescence and transient absorption, we find that ligands present during the synthesis of CuInSe₂ nanocrystals impart nonradiative electronic states which compete with radiative recombination and give rise to low photoluminescence quantum yields. We characterize the nature of these defect states (hole vs electron traps) and investigate whether they exist at the surface or interior of the nanocrystals. Carrier lifetimes are highly dependent on ligand identity where oleylamine-capped nanocrystals exhibit rapid trapping (<20 ps) followed by diphenylphosphine (<500 ps) and finally tributylphosphine (>2 ns). A majority of carrier population localizes at indium copper antisites (electrons), copper vacancies (holes), or surface traps (electrons and/or holes), all of which are nonemissive.

Unraveling the Emission Pathways in Copper Indium Sulfide Quantum Dots

Semiconductor copper indium sulfide quantum dots are emerging as promising alternatives to cadmium- and lead-based chalcogenides in solar cells, luminescent solar concentrators, and deep-tissue bioimaging due to their inherently lower toxicity and outstanding photoluminescence properties. However, the nature of their emission pathways remains a subject of debate. Using low-temperature single quantum dot spectroscopy on core-shell copper indium sulfide nanocrystals, we observe two subpopulations of particles with distinct spectral features. The first class shows sharp resolution-limited emission lines that are attributed to zero-phonon recombination lines of a long-lived band-edge exciton. Such emission results from the perfect passivation of the copper indium sulfide core by the zinc sulfide shell and points to an inversion in the band-edge hole levels. The second class exhibits ultrabroad spectra regardless of the temperature, which is a signature of the extrinsic self-trapping of the hole assisted by defects in imperfectly passivated quantum dots.

ZnIn2 S4 -Based Photocatalysts for Energy and Environmental Applications

As a fascinating visible-light-responsive photocatalyst, zinc indium sulfide (ZnIn₂ S₄ ) has attracted extensive interdisciplinary interest and is expected to become a new research hotspot in the near future, due to its nontoxicity, suitable band gap, high physicochemical stability and durability, ease of synthesis, and appealing catalytic activity. This review provides an overview on the recent advances in ZnIn₂ S₄ -based photocatalysts. First, the crystal structures and band structures of ZnIn₂ S₄ are briefly introduced. Then, various modulation strategies of ZnIn₂ S₄ are outlined for better photocatalytic performance, which includes morphology and structure engineering, vacancy engineering, doping engineering, hydrogenation engineering, and the construction of ZnIn₂ S₄ -based composites. Thereafter, the potential applications in the energy and environmental area of ZnIn₂ S₄ -based photocatalysts are summarized. Finally, some personal perspectives about the promises and prospects of this emerging material are provided.

Ag-In-Zn-S Quantum Dot-Dominated Interface Kinetics in Ag-In-Zn-S/NiFe LDH Composites toward Efficient Photoassisted Electrocatalytic Water Splitting

Photoassisted electrocatalysis (P-EC) emerges as a rising star for hydrogen production by embedding photoactive species in electrocatalysts, for which the interfacial structure design and charge transfer kinetics of the multifunctional catalysts remain a great challenge. Herein, Zn-AgIn₅S₈ quantum dots (ZAIS QDs) were embedded into 2D NiFe layered double hydroxide nanosheets through a simple hydrothermal treatment to form 0D/2D composite catalysts for P-EC. With evidence from transient photovoltage spectroscopy, we acquired a clear and fundamental understanding on the kinetics of charge extraction time and extraction amount in the 0D/2D heterojunctions that was proved to play a key role in P-EC. Upon light illumination, for HER, the optimized NiFe-ZAIS exhibits obviously reduced overpotentials of 129 and 242 mV at current densities of 10 and 50 mA cm^-2, which are 22 and 33 mV lower than those of dark electrocatalysis, respectively. For OER, the NiFe-ZAIS electrode also shows low overpotentials of 220 and 268 mV at current densities of 10 and 50 mA cm^-2, respectively, under light illumination, which were able to almost double the intrinsic activity. Finally, with NF@NiFe-ZAIS as both the cathode and the anode, the assembled electrolyzer only requires 1.62 V to reach the overall water splitting current density of 10 mA cm^-2 under P-EC. This work provides a useful example for the profound understanding of the design and the kinetics study of multifunctional P-EC catalysts.

Luminescent copper indium sulfide (CIS) quantum dots for bioimaging applications

Copper indium sulfide (CIS) quantum dots are ideal for bioimaging applications, by being characterized by high molar absorption coefficients throughout the entire visible spectrum, high photoluminescence quantum yield, high tolerance to the presence of lattice defects, emission tunability from the red to the near-infrared spectral region by changing their dimensions and composition, and long lifetimes (hundreds of nanoseconds) enabling time-gated detection to increase signal-to-noise ratio. The present review collects: (i) the most common procedures used to synthesize stable CIS QDs and the possible strategies to enhance their colloidal stability in aqueous environment, a property needed for bioimaging applications; (ii) their photophysical properties and parameters that affect the energy and brightness of their photoluminescence; (iii) toxicity and bioimaging applications of CIS QDs, including tumor targeting, time-gated detection and multimodal imaging, as well as theranostics. Future perspectives are analyzed in view of advantages and potential limitations of CIS QDs compared to most traditional QDs.

Photoluminescent Nanoparticles for Chemical and Biological Analysis and Imaging

Research related to the development and application of luminescent nanoparticles (LNPs) for chemical and biological analysis and imaging is flourishing. Novel materials and new applications continue to be reported after two decades of research. This review provides a comprehensive and heuristic overview of this field. It is targeted to both newcomers and experts who are interested in a critical assessment of LNP materials, their properties, strengths and weaknesses, and prospective applications. Numerous LNP materials are cataloged by fundamental descriptions of their chemical identities and physical morphology, quantitative photoluminescence (PL) properties, PL mechanisms, and surface chemistry. These materials include various semiconductor quantum dots, carbon nanotubes, graphene derivatives, carbon dots, nanodiamonds, luminescent metal nanoclusters, lanthanide-doped upconversion nanoparticles and downshifting nanoparticles, triplet-triplet annihilation nanoparticles, persistent-luminescence nanoparticles, conjugated polymer nanoparticles and semiconducting polymer dots, multi-nanoparticle assemblies, and doped and labeled nanoparticles, including but not limited to those based on polymers and silica. As an exercise in the critical assessment of LNP properties, these materials are ranked by several application-related functional criteria. Additional sections highlight recent examples of advances in chemical and biological analysis, point-of-care diagnostics, and cellular, tissue, and in vivo imaging and theranostics. These examples are drawn from the recent literature and organized by both LNP material and the particular properties that are leveraged to an advantage. Finally, a perspective on what comes next for the field is offered.

Hydrazine Hydrate and Dissolved Oxygen-Triggered Near-Infrared Chemiluminescence from CuInS2@ZnS Nanocrystals for Bioassays

The overwhelming majority of commercially available chemiluminescence (CL) assays are conducted in the eye-visible region. Herein, a near-infrared (NIR) aqueous CL strategy was proposed with CuInS₂@ZnS nanocrystals (CIS@ZnS NCs) as emitters. Hydrazine hydrate (N₂H₄·H₂O) could inject electrons into the conduction band of the CIS@ZnS NCs and simultaneously transformed to the intermediate radical N₂H₃^•. N₂H₃^• reduced dissolved oxygen (O₂) to O₂^-•, while the O₂^-• could inject holes into the valence band of the CIS@ZnS NCs. The recombination of electrons and holes at Cu⁺ defects in CIS@ZnS NCs eventually yielded efficient NIR CL at around 824.1 nm, which is the longest waveband for NCs CL to the best of our knowledge. The NIR CL could be conveniently performed in the neutral aqueous medium (pH 7.0) with a quantum yield of 0.0155 Einstein/mol and was successfully employed for constructing a signal-off CL biosensor with ascorbic acid as the analyte as well as a signal-on CL biosensor for determining ascorbate oxidase, which indicates that this NIR CL system has a promising potential for bioassays in diverse ways.

Ultrafast photophysical process of bi-exciton Auger recombination in CuInS2 quantum dots studied by transient-absorption spectroscopy

Auger recombination is an ultrafast and unnegligible photophysical process in colloidal semiconductor quantum dots (QDs) due to competition with charge separation or radiative recombination processes, pivotal for their applications ranging from bio-labeling, light-emitting diodes, QD lasing to solar energy conversion. Among diverse QDs, ternary chalcopyrite is recently receiving significant attention for its heavy-metal free property and remarkable optical performance. Given deficient understanding of the Auger process for ternary chalcopyrite QDs, CuInS₂ QDs with various sizes are synthesized as a representative and the bi-exciton lifetime (τ_BX) is derived by virtue of ultrafast time resolved absorption spectrum. The trend of τ_BX varying with size is consistent with the universal scaling of τ_BX versus QD volume (V): τ_BX = γV. The scaling factor γ is 6.6 ± 0.5 ps·nm^-3 for CuInS₂ QDs, and the bi-exciton Auger lifetime is 4-5 times slower than typical CdSe QDs with the same volume, suggesting reduced Auger recombination rate in ternary chalcopyrite. This work facilitates clearer understanding of Auger process and provides further insight for rational design of light-harvesting and emitting devices based on ternary chalcopyrite QDs.

Unusual Spectral Diffusion of Single CuInS2 Quantum Dots Sheds Light on the Mechanism of Radiative Decay

The luminescence of CuInS₂ quantum dots (QDs) is slower and spectrally broader than that of many other types of QDs. The origin of this anomalous behavior is still under debate. Single-QD experiments could help settle this debate, but studies by different groups have yielded conflicting results. Here, we study the photophysics of single core-only CuInS₂ and core/shell CuInS₂/CdS QDs. Both types of single QDs exhibit broad PL spectra with fluctuating peak position and single-exponential photoluminescence decay with a slow but fluctuating lifetime. Spectral diffusion of CuInS₂-based QDs is qualitatively and quantitatively different from CdSe-based QDs. The differences reflect the dipole moment of the CuInS₂ excited state and hole localization on a preferred site in the QD. Our results unravel the highly dynamic photophysics of CuInS₂ QDs and highlight the power of the analysis of single-QD property fluctuations.

Recent research on the luminous mechanism, synthetic strategies, and applications of CuInS2 quantum dots

We discuss the synthesis and luminescence mechanisms of CuInS₂ QDs, the strategies to improve their luminous performance and their potential application in light-emitting devices, solar energy conversion, and the biomedical field.

FRET-Based Analysis of AgInS2/ZnAgInS/ZnS Quantum Dot Recombination Dynamics

Ternary quantum dots (QDs) are very promising nanomaterials with a range of potential applications in photovoltaics, light-emitting devices, and biomedicine. Despite quite intensive studies of ternary QDs over the last years, the specific relaxation channels involved in their emission mechanisms are still poorly understood, particularly in the corresponding core-shell nanostructures. In the present work, we have studied the recombination pathways of AgInS₂ QDs stabilized with the ZnAgInS alloy layer and the ZnS shell (AIS/ZAIS/ZnS QDs) using time-resolved fluorescence spectroscopy. We have also investigated FRET in complexes of AIS/ZAIS/ZnS QDs and cyanine dyes with the absorption bands overlapping in the different regions of the QD emission spectrum, which allowed us to selectively quench the radiative transitions of the QDs. Our studies have demonstrated that FRET from QDs to dyes results in decreasing of all QD PL decay components with the shortest lifetime decreasing the most and the longest one decreasing the least. This research presents important approaches for the investigation of ternary QD luminescence mechanisms by the selective quenching of recombination pathways. These studies are also essential for potential applications of ternary QDs in photodynamic therapy, multiplex analysis, and time-resolved FRET sensing.

Water-soluble ZnCuInSe quantum dots for bacterial classification, detection, and imaging

Bacteria are everywhere and pose severe threats to human health and safety. The rapid classification and sensitive detection of bacteria are vital steps of bacterial community research and the treatment of infection. Herein, we developed optical property-superior and heavy metal-free ZnCuInSe quantum dots (QDs) for achieving rapid discrimination of Gram-positive/Gram-negative bacteria by the naked eye; driven by the structural differences of bacteria, ZnCuInSe QDs are effective in binding to Gram-positive bacteria, especially Staphylococcus aureus (S. aureus), in comparison with Gram-negative bacteria and give discernable color viewed by the naked eye. Meanwhile, based on its distinctive fluorescence response, the accurate quantification of S. aureus was investigated with a photoluminescence system in the concentration ranges of 1 × 10³ to 1 × 10¹¹ CFU/mL, with a limit of detection of 1 × 10³ CFU/mL. Furthermore, we demonstrated the feasibility of ZnCuInSe QDs as a fluorescence probe for imaging S. aureus. This simple strategy based on ZnCuInSe QDs provides an unprecedented step for rapid and effective bacterial discrimination, detection, and imaging.

Cu-In-S/ZnS@carboxymethylcellulose supramolecular structures: Fluorescent nanoarchitectures for targeted-theranostics of cancer cells

Although the field of oncology nanomedicine has shown indisputable progress in recent years, cancer remains one of the most lethal diseases, where the early diagnosis plays a pivotal role in the patient's prognosis and therapy. Herein, we report for the first time, the synthesis of biocompatible nanostructures composed of Cu-In-S and Cu-In-S/ZnS nanoparticles functionalized with carboxymethylcellulose biopolymer produced by a green aqueous process. These inorganic-organic colloidal nanohybrids developed supramolecular architectures stabilized by chemical functional groups of the polysaccharide shell with the fluorescent semiconductor nanocrystal core, which were extensively characterized by several morphological and spectroscopical techniques. Moreover, these nanoconjugates were covalently bonded with folic acid via amide bonds and electrostatically conjugated with the anticancer drug, producing functionalized supramolecular nanostructures. They demonstrated nanotheranostics properties for bioimaging and drug delivery vectorization effective for killing breast cancer cells in vitro. These hybrids offer a new nanoplatform using fluorescent polysaccharide-drug conjugates for cancer theranostics applications.

Cytotoxicity, fluorescence tagging and gene-expression study of CuInS/ZnS QDS - meso (hydroxyphenyl) porphyrin conjugate against human monocytic leukemia cells

The toxicity of heavy metals present in binary semiconductor nanoparticles also known as quantum dots (QDs) has hindered their wide applications hence the advent of non-toxic ternary quantum dots. These new group of quantum dots have been shown to possess some therapeutic action against cancer cell lines but not significant enough to be referred to as an ideal therapeutic agent. In this report, we address this problem by conjugating red emitting CuInS/ZnS QDs to a 5,10,15,20-tetrakis(3-hydroxyphenyl)porphyrin -photosensitizer for improved bioactivities. The glutathione capped CuInS/ZnS QDs were synthesized in an aqueous medium using a kitchen pressure cooker at different Cu: In ratios (1:4 and 1:8) and at varied temperatures (95 °C, 190 °C and 235 °C). Optical properties show that the as-synthesized CuInS/ZnS QDs become red-shifted compared to the core (CuInS) after passivation with emission in the red region while the cytotoxicity study revealed excellent cell viability against normal kidney fibroblasts (BHK21). The highly fluorescent, water-soluble QDs were conjugated to 5,10,15,20-tetrakis(3-hydroxyphenyl)porphyrin (mTHPP) via esterification reactions at room temperature. The resultant water-soluble conjugate was then used for the cytotoxicity, fluorescent imaging and gene expression study against human monocytic leukemia cells (THP-1). Our result showed that the conjugate possessed high cytotoxicity against THP-1 cells with enhanced localized cell uptake compared to the bare QDs. In addition, the gene expression study revealed that the conjugate induced inflammation compared to the QDs as NFKB gene was over-expressed upon cell inflammation while the singlet oxygen (1O2) study showed the conjugate possessed large amount of 1O2, three times than the bare porphyrin. Thus, the as-synthesized conjugate looks promising as a therapeutic agent for cancer therapy.

A novel strategy to design a multilayer functionalized Cu2S thin film counter electrode with enhanced catalytic activity and stability for quantum dot sensitized solar cells

As the essential component of a quantum dot-sensitized solar cell (QDSC), the counter electrode (CE) plays an important role in electron transfer and catalytic reduction acquisition throughout the device. A novel route to design multilayer functionalized Cu₂S thin films as CEs with high catalytic activity and enhanced stability, as well as large specific surface area and high conductivity, is presented. Firstly, Mo-based films were prepared by magnetron sputtering on a glass substrate, and then porous CuZnMo conductive films were formed by etching with hydrochloric acid. Secondly, indium tin oxide (ITO) film was sputtered onto the porous structure to act as a protective layer, and a porous ITO/CuZnMo structured film was obtained after optimization. In the third step, multilayer Cu(x)/ITO/CuZnMo structured films were acquired by sputtering Cu films. Finally, multilayer Cu₂S(t)/ITO/CuZnMo functionalized film CEs were obtained via in situ sulfidation of sputtered Cu films. The functions of conduction and resistance to electrolyte corrosion were produced and enhanced by annealing an ITO layer at high temperature prior to Cu deposition, while catalytic activity enabled by Cu₂S was realized from Cu film sulfidation. The multilayer Cu₂S/ITO(500 °C)/CuZnMo functionalized films exhibit high catalytic activity and enhanced stability for resistance to electrolyte corrosion. Taking multilayer Cu₂S/ITO(500 °C)/CuZnMo films as CEs, the QDSCs demonstrated good stability of power conversion efficiency (PCE) after 500 h of irradiation, from an initial 4.21% to a final 4.00%. Furthermore, the thickness of Cu₂S film modulated by the duration of Cu sputtering was investigated. It was found that the QDSCs using multilayer Cu₂S(40 min)/ITO/CuZnMo functionalized film with a Cu₂S thickness of 1.2 μm as CE exhibit the best performance, and the R _ct value was 0.57 Ω. The best photovoltaic performance with a PCE of 5.21% (V _oc = 533.1 mV, J _sc = 18.80 mA cm^-2, FF = 52.84%) was achieved under AM 1.5 radiation with an incident power of 100 mW cm^-2. This design of a multilayer functionalized CE introduces potential alternatives to the common brass-based CE for long-term QDSCs with high performance.

Ternary Metal Chalcogenides: Into the Exciton and Biexciton Dynamics

Intra-band-gap state-induced low-toxicity colloidal I-III-VI ternary metal chalcogenide nanocrystals (NCs) have emerged as promising alternatives to the toxic Cd- and Pb-chalcogenides for different optoelectronic and bioimaging applications. In this Perspective, we provide the primary understanding of the intra-band-gap state-induced photoluminescence (PL) of I-III-VI NCs, specifically CuInS₂ and AgInS₂, as a function of particle size and composition and correlated with time-resolved PL measurements. The intra-band-gap state-induced ultrafast exciton and biexciton dynamics are discussed in detail to unravel the subpicosecond carrier relaxation dynamics through transient absorption measurement. Furthermore, ultrafast dissociation of the biexciton on Au@CuInS₂ hybrid NCs has been revealed to be due to the presence of Au, which has direct relevance to the improvement of the solar cell efficiency. The proper fundamental insight of the ultrafast exciton and biexciton dynamics of these materials will enable utilization of ternary metal chalcogenides in photovoltaic as well as light-emitting devices.

Quaternary quantum dots with gradient valence band for all-inorganic perovskite solar cells

The severe interface charge recombination caused by the large energy difference between perovskite material and carbon electrode significantly limits the further performance improvement of the all-inorganic perovskite solar cells (PSCs). We apply innovatively multilayer of quaternary Ag-In-Ga-S (AIGS) quantum dots (QDs) with cascade-like valence bands as hole-transport materials to assemble all-inorganic PSCs, and the resultant all-inorganic PSCs exhibit a power conversion efficiency (PCE) of 8.46%, which is enhanced by 20.9% in comparison with 7% for the pristine device. The high performance of the PSCs indicates that sequential layers of AIGS QDs with cascade-like energy levels can facilitate the charge separation, reduce the barrier the holes crossing and suppress the charge recombination. Stack of QDs with cascade-like energy levels provide solution-processed PSCs with a new method to enhance device performance.

Eco-Friendly Colloidal Quantum Dot-Based Luminescent Solar Concentrators

Luminescent solar concentrators (LSCs) have attracted significant attention as promising solar energy conversion devices for building integrated photovoltaic (PV) systems due to their simple architecture and cost-effective fabrication. Conventional LSCs are generally comprised of an optical waveguide slab with embedded emissive species and coupled PV cells. Colloidal semiconductor quantum dots (QDs) have been demonstrated as efficient emissive species for high-performance LSCs because of their outstanding optical properties including tunable absorption and emission spectra covering the ultraviolet/visible to near-infrared region, high photoluminescence quantum yield, large absorption cross sections, and considerable photostability. However, current commonly used QDs for high-performance LSCs consist of highly toxic heavy metals (i.e., cadmium and lead), which are fatal to human health and the environment. In this regard, it is highly desired that heavy metal-free and environmentally friendly QD-based LSCs are comprehensively studied. Here, notable advances and developments of LSCs based on unary, binary, and ternary eco-friendly QDs are presented. The synthetic approaches, optical properties of these eco-friendly QDs, and consequent device performance of QD-based LSCs are discussed in detail. A brief outlook pointing out the existing challenges and prospective developments of eco-friendly QD-based LSCs is provided, offering guidelines for future device optimizations and commercialization.

Optoelectronic Properties of Ternary I-III-VI2 Semiconductor Nanocrystals: Bright Prospects with Elusive Origins

Colloidal nanocrystals of ternary I-III-VI₂ semiconductors are emerging as promising alternatives to Cd- and Pb-chalcogenide nanocrystals because of their inherently lower toxicity, while still offering widely tunable photoluminescence. These properties make them promising materials for a variety of applications. However, the realization of their full potential has been hindered by both their underdeveloped synthesis and the poor understanding of their optoelectronic properties, whose origins are still under intense debate. In this Perspective, we provide novel insights on the latter aspect by critically discussing the accumulated body of knowledge on I-III-VI₂ nanocrystals. From our analysis, we conclude that the luminescence in these nanomaterials most likely originates from the radiative recombination of a delocalized conduction band electron with a hole localized at the group-I cation, which results in broad bandwidths, large Stokes shifts, and long exciton lifetimes. Finally, we highlight the remaining open questions and propose experiments to address them.

Doping as a Strategy to Tune Color of 2D Colloidal Nanoplatelets

Among colloidal nanocrystals, 2D nanoplatelets (NPLs) made of II-VI compounds appear as a special class of emitters with an especially narrow photoluminescence signal. However, the PL signal in the case of NPLs is only tunable by a discrete step. Here, we demonstrate that doping is a viable path to finely tune the color of these NPLs from green to red, making them extremely interesting as phosphors for wide-gamut display. In addition, using a combination of luminescence spectroscopy, tight-binding simulation, transport, and photoemission, we provide a consistent picture for the Ag⁺-doped CdSe NPLs. The Ag-activated state is strongly bound and located 340 meV above the valence band of the bulk material. The Ag dopant induces a relative shift of the Fermi level toward the valence band by up to 400 meV but preserves the n-type nature of the material.

Copper's Role in the Photoluminescence of Ag1- xCu xInS2 Nanocrystals, from Copper-Doped AgInS2 ( x ∼ 0) to CuInS2 ( x = 1)

A series of Ag_{1- x}Cu _xInS₂ nanocrystals (NCs) spanning from 0 ≤ x ≤ ∼1 was synthesized by partial cation exchange to identify copper's contributions to the electronic structure and spectroscopic properties of these NCs. Discrete midgap states appear above the valence band upon doping AgInS₂ NCs with Cu⁺ (small x). Density functional theory calculations confirm that these midgap states are associated with the 3d valence orbitals of the Cu⁺ impurities. With increasing x, these impurity d levels gradually evolve to become the valence-band edge of CuInS₂ NCs, but the highest-occupied orbital's description does not change significantly across the entire range of x. In contrast with this gradual evolution, Ag_{1- x}Cu _xInS₂ NC photoluminescence shifts rapidly with initial additions of Cu⁺ (small x) but then becomes independent of x beyond x > ∼0.20, all the way to CuInS₂ ( x = 1.00). Data analysis suggests small but detectable hole delocalization in the luminescent excited state of CuInS₂ NCs, estimated by Monte Carlo simulations to involve at most about four copper ions. These results provide unique insights into the luminescent excited states of these materials and they reinforce the description of CuInS₂ NCs as "heavily copper-doped NCs" in which photogenerated holes are rapidly localized in copper 3d-based orbitals.