Highly Efficient Electrochemiluminescence of MnS:CdS@ZnS Core-Shell Quantum Dots for Ultrasensitive Detection of MicroRNA

In this work, a novel electrochemiluminescence (ECL) biosensor was developed for ultrasensitive detection of microRNA let-7a (miRNA let-7a) based on MnS:CdS@ZnS core-shell quantum dots (QDs) as ECL luminophores with high ECL efficiency. Impressively, compared to the CdS:Mn@ZnS QDs prepared by ionic doping with ECL efficiency of 0.87%, MnS:CdS@ZnS QDs synthesized by bimetallic clusters (Cd₂Mn₂O₄) doping exhibited high ECL efficiency of up to 15.84% with S₂O₈^2- as cathodic coreactant due to the elimination of the dopants size mismatch and "self-purification" effect, which could achieve the surface defect passivation of MnS:CdS@ZnS QDs for effectively improving the ECL emission. Furthermore, with the help of strand displacement amplification (SDA), the trace target miRNA let-7a was able to be converted to a number of output DNA labeled with ferrocene (Fc) to construct an ultrasensitive ECL biosensor. The well-designed ECL biosensor for miRNA let-7a exhibited high stability and excellent sensitivity of a concentration variation from 10 aM to 1 nM and a low detection limit of 4.1 aM, which was further applied to the analysis of miRNA let-7a from cancer cell (MCF-7) lysate. Thus, this strategy provides a novel method to prepare high-efficient ECL emitters for the construction of ECL biosensing platforms in biological fields and clinical diagnosis.

Permanent Electrochemical Doping of Quantum Dot Films through Photopolymerization of Electrolyte Ions

Quantum dots (QDs) are considered for devices like light-emitting diodes (LEDs) and photodetectors as a result of their tunable optoelectronic properties. To utilize the full potential of QDs for optoelectronic applications, control over the charge carrier density is vital. However, controlled electronic doping of these materials has remained a long-standing challenge, thus slowing their integration into optoelectronic devices. Electrochemical doping offers a way to precisely and controllably tune the charge carrier concentration as a function of applied potential and thus the doping levels in QDs. However, the injected charges are typically not stable after disconnecting the external voltage source because of electrochemical side reactions with impurities or with the surfaces of the QDs. Here, we use photopolymerization to covalently bind polymerizable electrolyte ions to polymerizable solvent molecules after electrochemical charge injection. We discuss the importance of using polymerizable dopant ions as compared to nonpolymerizable conventional electrolyte ions such as LiClO₄ when used in electrochemical doping. The results show that the stability of charge carriers in QD films can be enhanced by many orders of magnitude, from minutes to several weeks, after photochemical ion fixation. We anticipate that this novel way of stable doping of QDs will pave the way for new opportunities and potential uses in future QD electronic devices.

Observation of tunable surface plasmon resonances and surface enhanced infrared absorption (SEIRA) based on indium tin oxide (ITO) nanoparticle substrates

The application of surface enhanced infrared absorption (SEIRA) is severely restricted in many fields due to the SEIRA substrates are constructed mainly from expensive noble metals. Therefore, the development of new SEIRA substrates other than the noble metallic ones is very valuable. Here we introduced a new semiconductor SEIRA substrate, the indium tin oxide (ITO) nanoparticles (NPs), to study the SEIRA property. The results demonstrate that the ITO NPs show the SEIRA property and the enhancement is dependent to the doping ratio of the heteroatoms of tin. The ITO NPs with the 5% atomic doping ratio show the highest SEIRA enhancement factor (EF), which is about 24. The limit of detection (LOD) of the 1,1'-dicarboxyferrocene (dcFc) molecule was as low as 10^-5 mol/L. The present study proves that the tin-doped indium oxide can be used as a new and inexpensive semiconductor SEIRA substrate. It also proves that the doped semiconductor NPs have strong potentials for being used as emerging SEIRA substrates.

Interfacial Manganese-Doping in CsPbBr3 Nanoplatelets by Employing a Molecular Shuttle

Mn-doping in cesium lead halide perovskite nanoplatelets (NPls) is of particular importance where strong quantum confinement plays a significant role towards the exciton-dopant coupling. In this work, we report an immiscible bi-phasic strategy for post-synthetic Mn-doping of CsPbX3 (X=Br, Cl) NPls. A systematic study shows that electron-donating oleylamine acts as a shuttle ligand to transport MnX2 through the water-hexane interface and deliver it to the NPls. The halide anion also plays an essential role in maintaining an appropriate radius of Mn2+ and thus fulfilling the octahedral factor required for the formation of perovskite crystals. By varying the thickness of parent NPls, we can tune the dopant incorporation and, consequently, the exciton-to-dopant energy transfer process in doped NPls. Time-resolved optical measurements offer a detailed insight into the exciton-to-dopant energy transfer process. This new approach for post-synthetic cation doping paves a way towards exploring the cation exchange process in several other halide perovskites at the polar-nonpolar interface.

Doping Colloidal Quantum Dot Materials and Devices for Photovoltaics

Colloidal semiconductor nanocrystals have generated tremendous interest because of their solution processability and robust tunability. Among such nanocrystals, the colloidal quantum dot (CQD) draws the most attention for its well-known quantum size effects. In the last decade, applications of CQDs have been booming in electronics and optoelectronics, especially in photovoltaics. Electronically doped semiconductors are critical in the fabrication of solar cells, because carefully designed band structures are able to promote efficient charge extraction. Unlike conventional semiconductors, diffusion and ion implantation technologies are not suitable for doping CQDs. Therefore, researchers have creatively developed alternative doping methods for CQD materials and devices. In order to provide a state-of-the-art summary and comprehensive understanding to this research community, we focused on various doping techniques and their applications for photovoltaics and demystify them from different perspectives. By analyzing two classes of CQDs, lead chalcogenide CQDs and perovskite CQDs, we compared different working scenarios of each technique, summarized the development in this field, and raised our own future perspectives.

Synthesis, characterization and evaluation of aqueous Zn-based quantum dots for bioapplications

Semiconducting nanoparticles called quantum dots (Qds) present unique optoelectronic properties based on their extremely small size, composition, and spherical shape, which make them suitable for use as diagnostic and theranostic agents in biological samples. The main scope of the fabrication of Qds is real-time diagnosis, therapy, drug delivery, and in vitro and in vivo tracking, presenting strong resistance to photobleaching. In this work, quantum dots such as ZnO, ZnSe, ZnS, and doped ZnS : Mn and ZnS : Cd were developed via a simple sol-gel synthesis in an aqueous solution. Morphological, structural, and optical characterizations were investigated. Moreover, an in vitro biological evaluation of Qds was performed. The results indicate that the photoluminescence is enhanced after doping ZnS Qds with Mn2+ and Cd2+. Qds have been synthesized for use as fluorescent agents for real-time monitoring in bio-applications.

Hydrothermal synthesis of water-soluble Mn- and Cu-doped CdSe quantum dots with multi-shell structures and their photoluminescence properties

Optical properties of semiconductor quantum dots (QDs) can be tuned by doping with transition metal ions. In this study, water-soluble CdSe/ZnS:Mn/ZnS QDs with the core/shell/shell structure were synthesized through a hydrothermal method, in which the surface of the CdSe core was coated with a ZnS:Mn shell and ZnS capping shell. Herein, the CdSe core QDs were prepared first and then doped with Mn2+; therefore, the QD size and doping level could be controlled independently and interference from the self-purifying effect could be avoided. When CdSe cores with diameters less than 1.9 nm were used, Mn-related photoluminescence (PL) was observed as the main PL band, whereas the band-edge PL was mainly observed when larger CdSe cores were used. Furthermore, using ZnS:Cu as the doping shell layer, CdSe/ZnS:Cu/ZnS and ZnSe/ZnS:Cu/ZnS nanoparticles were successfully synthesized, and Cu-related PL was clearly observed. These results indicate that the core/shell/shell QD structure with doping in the shell layer is a versatile method for synthesizing doped QDs.

A paper-based visualization chip based on nitrogen-doped carbon quantum dots nanoprobe for Hg(Ⅱ) detection

Hg(II) is one of the most toxic heavy metal ions. The bioconcentration and degradation-resistant of Hg(II) bring about serious harm to the ecosystem and humans. Therefore, the establishment of an accurate and effective method for detecting mercury ions is of great significance to environmental protection, food safety and human health. In this work, a new fluorescent nanoprobe was presented using nitrogen-doped carbon quantum dots (N-CQDs) for Hg(II) sensing with high stability and selectivity. On this basis, a paper-based chip was innovatively developed for visualization detection of Hg(II). The N-CQDs were prepared through a one-step hydrothermal reaction using catechol and ethylenediamine as carbon and nitrogen sources, respectively. As-prepared N-CQDs exhibit the strong green fluorescence at the excitation/emission wavelength of 370/511 nm. In aqueous solution, a rapid and highly sensitive detection method of Hg(II) was established by the joint of dynamic and static quenching effect of Hg(II) on N-CQDs fluorescence. Under the optimized conditions, there was a stable correlation between the fluorescence intensity change of N-CQDs and the concentrations of Hg(II) in the range of 15 ∼ 10⁴ nM, and the detection limit was down to 8 nM (S/N = 3). The recoveries of water, sorghum and rice were 91.60 to 102.46%, which was consistent with ICP-MS. More importantly, the N-CQDs nanoprobe was further integrated in nitrocellulose membrane to develop paper-based chip for Hg(II) visualization detection, and the detection performance was also excellent. This strategy had significant implications for achieving low-cost, on-site real-time monitoring of mercury (II) in the environment and food.

Defects and impurities in colloidal Ga2O3 nanocrystals: new opportunities for photonics and lighting

Defects, both native and extrinsic, critically determine functional properties of metal oxides. Gallium oxide has recently gained significant attention for its promise in microelectronics, owing to the unique combination of conductivity and high breakdown voltage, and solid-state lighting, owing to the strong photoluminescence in the visible spectral region. These properties are associated with the presence of native defects that can form both donor and acceptor states in Ga2O3. Recently, Ga2O3 nanocrystal synthesis in solution and optical glasses has been developed, allowing for a range of new applications in photonics, lighting, and photocatalysis. This review focuses on the structure and properties of Ga2O3 nanocrystals with a particular emphasis on the electronic structure and interaction of defects in reduced dimensions and their role in the observed photoluminescence properties. In addition to native defects, the effect of selected external impurities, including lanthanide and aliovalent dopants, and alloying on the emission properties of Ga2O3 nanocrystals are also discussed.

Colloidal Inorganic Ligand-Capped Nanocrystals: Fundamentals, Status, and Insights into Advanced Functional Nanodevices

Colloidal nanocrystals (NCs) are intriguing building blocks for assembling various functional thin films and devices. The electronic, optoelectronic, and thermoelectric applications of solution-processed, inorganic ligand (IL)-capped colloidal NCs are especially promising as the performance of related devices can substantially outperform their organic ligand-capped counterparts. This in turn highlights the significance of preparing IL-capped NC dispersions. The replacement of initial bulky and insulating ligands capped on NCs with short and conductive inorganic ones is a critical step in solution-phase ligand exchange for preparing IL-capped NCs. Solution-phase ligand exchange is extremely appealing due to the highly concentrated NC inks with completed ligand exchange and homogeneous ligand coverage on the NC surface. In this review, the state-of-the-art of IL-capped NCs derived from solution-phase inorganic ligand exchange (SPILE) reactions are comprehensively reviewed. First, a general overview of the development and recent advancements of the synthesis of IL-capped colloidal NCs, mechanisms of SPILE, elementary reaction principles, surface chemistry, and advanced characterizations is provided. Second, a series of important factors in the SPILE process are offered, followed by an illustration of how properties of NC dispersions evolve after ILE. Third, surface modifications of perovskite NCs with use of inorganic reagents are overviewed. They are necessary because perovskite NCs cannot withstand polar solvents or undergo SPILE due to their soft ionic nature. Fourth, an overview of the research progresses in utilizing IL-capped NCs for a wide range of applications is presented, including NC synthesis, NC solid and film fabrication techniques, field effect transistors, photodetectors, photovoltaic devices, thermoelectric, and photoelectrocatalytic materials. Finally, the review concludes by outlining the remaining challenges in this field and proposing promising directions to further promote the development of IL-capped NCs in practical application in the future.

Highly Concentrated, Zwitterionic Ligand-Capped Mn2+:CsPb(Br x Cl1-x )3 Nanocrystals as Bright Scintillators for Fast Neutron Imaging

Fast neutron imaging is a nondestructive technique for large-scale objects such as nuclear fuel rods. However, present detectors are based on conventional phosphors (typically microcrystalline ZnS:Cu) that have intrinsic drawbacks, including light scattering, γ-ray sensitivity, and afterglow. Fast neutron imaging with colloidal nanocrystals (NCs) was demonstrated to eliminate light scattering. While lead halide perovskite (LHP) FAPbBr₃ NCs emitting brightly showed poor spatial resolution due to reabsorption, the Mn²⁺-doped CsPb(BrCl)₃ NCs with oleyl ligands had higher resolution because of large apparent Stokes shift but insufficient concentration for high light yield. In this work, we demonstrate a NC scintillator that features simultaneously high quantum yields, high concentrations, and a large apparent Stokes shift. In particular, we use long-chain zwitterionic ligand capping in the synthesis of Mn²⁺-doped CsPb(BrCl)₃ NCs that allows for attaining very high concentrations (>100 mg/mL) of colloids. The emissive behavior of these ASC18-capped NCs was carefully controlled by compositional tuning that permitted us to select for high quantum yields (>50%) coinciding with Mn-dominated emission for minimal self-absorption. These tailored Mn²⁺:CsPb(BrCl)₃ NCs demonstrated over 8 times brighter light yield than their oleyl-capped variants under fast neutron irradiation, which is competitive with that of near-unity FAPbBr₃ NCs, while essentially eliminating self-absorption. Because of their rare combination of concentrations above 100 mg/mL and high quantum yields, along with minimal self-absorption for good spatial resolution, Mn²⁺:CsPb(BrCl)₃ NCs have the potential to displace ZnS:Cu as the leading scintillator for fast neutron imaging.

Fast-Response Metal-Semiconductor-Metal Junction Ultraviolet Photodetector Based on ZnS:Mn Nanorod Networks via a Cost-Effective Method

In this work, Mn2+-doped ZnS nanorods were synthesized by a facile hydrothermal method. The morphology, structure, and composition of the as-prepared samples were investigated. The temperature-dependent photoluminescence of ZnS:Mn nanorods was analyzed, and the corresponding activation energies were calculated by using a simple two-step rate equation. Mn2+-related orange emission (4T1 → 6A1) demonstrates high stability and is comparatively less affected by the temperature variations than the defect-related emission. A metal-semiconductor-metal junction ultraviolet photodetector based on the nanorod networks has been fabricated by a cost-effective method. The device exhibits visible blindness, superior ultraviolet photodetection with a responsivity of 1.62 A/W, and significantly fast photodetection response with the rise and decay times of 12 and 25 ms, respectively.

Enhanced charge-transfer induced by conduction band electrons in aluminum-doped zinc oxide/molecule/Ag sandwich structures observed by surface-enhanced Raman spectroscopy

In the semiconductor/molecule/metal system, enhancing the efficiency of the charge-transfer (CT) plays a pivotal role in improving the sensitivity of semiconductor-based surface-enhanced Raman scattering (SERS). In this work, use of SERS for detection of an enhanced CT in a chemically-etched Al-doped ZnO (AZO), 4-mercaptopyridine (MPy) molecule, and Ag nanoparticles (NPs) (AZO/MPy/Ag) sandwich structure is reported. A series of CT routes are proposed in the energy level diagram of AZO/MPy/Ag assemblies under the excitation line at 633 nm. Very interestingly, for the first of its kind, a significant CT route from the conduction band (CB) of AZO to the lowest unoccupied molecular orbital (LUMO) of MPy molecule is detected. This route can remarkably improve the degree of CT in the AZO/MPy/Ag system by about 48% compared with that of the ZnO/MPy/Ag system. Furthermore, the uniquely enhanced CT route is also further confirmed by alternative probe molecules like p-aminothiophenol (PATP) and 4-mercaptobenzoic acid (MBA). The discovery of this extra CT route will inevitably play an irreplaceable role in SERS enhancement through its participating in the CT enhancement mechanism.

II-VI core/shell quantum dots and doping with transition metal ions as a means of tuning the magnetoelectronic properties of CdS/ZnS core/shell QDs: A DFT study

This paper examines the alterations in the properties of II-VI Quantum Dots (QDs) when these are coated with a shell made of another material of the same family and investigates the structural, electronic and magnetic properties of doped CdS/ZnS core/shell QDs. The core/shell QDs have been constructed by building the shell over the bare core QD and it is found that this construction of a shell over the bare QD can bring about dramatic changes in its optical properties. On changing the shell by varying either the cation or the anion, substantial variations are brought about in the band gap and electrophilicity. The trend of Fermi energies is more negative for core/shell QDs than for the QDs without a shell, and the value is almost the same for core/shell QDs with the same core. Swapping of the core and the shell materials brings greater stability in the case of shells of the wider band gap materials. Binding energy data demonstrates that the CdS/ZnS, CdSe/ZnSe, CdSe/CdS core/shell systems are more stable than ZnS/CdS, ZnSe/CdSe, CdS/CdSe core/shell systems, respectively. An augmentation in the properties is found on doping the QD with transition metal ions. The binding energies are found to be functions of the kind of dopant as well as the spin multiplicity and account for the stability of one spin state over the other at a specific site of the QD. The most fascinating property that plays a decisive role in the extant work is the introduction of magnetism in core/shell QDs as a result of the entry of unpaired electrons within the CdS/ZnS QDs on doping with transition metal ions. The deviation of the observed magnetic moments from the expected values increases as the dopant is varied from Mn²⁺ to Fe²⁺ to Co²⁺ to Ni²⁺ to Cu²⁺. Hirshfeld charge analysis shows that the doped ion accepts negative charge from the sulfide ions in the core, with the smallest charge transfer seen in the case of Hg²⁺ ions. As we move from Mn²⁺ to Hg²⁺, the trend followed for the Hirshfeld charges indicates that the overall charge on the core is lower and that on the shell is higher for all the doped cases in comparison to the undoped CdS/ZnS core/shell QD. The band gap values reveal that the Fe²⁺ doped CdS/ZnS core/shell structures have the smallest band gaps. Hence, we expect that this paper will help researchers to develop a strategy to produce QDs of the anticipated properties for various applications, and transition metal ions can be successfully employed for modification of various magnetoelectronic properties of the host semiconductor for future applications in nanotechnology.

Recent Progress on the Scanning Tunneling Microscopy and Spectroscopy Study of Semiconductor Heterojunctions

The band alignment, interface states, interface coupling, and carrier transport of semiconductor heterojunctions (SHs) need to be well understood for the design and fabrication of various important semiconductor structures and devices. Scanning tunneling microscopy (STM) with high spatial resolution and scanning tunneling spectroscopy (STS) with high energy resolution are significantly contributing to the understanding on the important properties of SHs. In this work, the recent progress on the use of STM and STS to study lateral, vertical and bulk SHs is reviewed. The spatial structures of SHs with atomically flat surface have been examined with STM. The electronic band structures (e. g., the band offset, interface state, and space charge region) of SHs are measured with STS. Combined with the spatial structures and the tunneling spectra features, the mechanism for the carrier transport in the SH may be proposed.

Recent developments in germanium containing clusters in intermetallics and nanocrystals

Multimetallic clusters can be described as building blocks in intermetallics, compounds prepared from all metals and/or semi-metals, and in Zintl phases, a subset of intermetallics containing metals with large differences in electronegativity. In many cases, these intermetallic and Zintl phases provide the first clue for the possibilities of bond formation between metals and semi-metals. Recent advances in multimetallic clusters found in Zintl phases and nanoparticles focusing on Ge with transition metals and semi-metals is presented. Colloidal routes to Ge nanocrystals provide an opportunity for kinetically stabilized Ge-metal and Ge-semi-metal bonding. These routes provide crystalline nanoclusters of Ge, hereafter referred to as nanocrystals, that can be structurally characterized. Compositions of Ge nanocrystals containing transition metals, and the semi-metals, Sb, Bi, and Sn, whose structures have recently been elucidated through EXAFS, will be presented along with potential applications.

Perovskite energy funnels for efficient white emission

Doping Mn²⁺ into CsPbCl₃ nanocrystals (NCs) yields strong orange emission, while the related emission in Mn²⁺ doped CsPbBr₃ NCs is impaired seriously. This is mainly ascribed to back energy transfer from the Mn²⁺ dopant to the host. Doping Mn²⁺ into perovskites with multiple-quantum-well (MQW) structures may address this issue, where the energy funnels ensure a rapid energy transfer process, and thus resulting in a high photoluminescence quantum yield (PLQY). Here, we have developed an Ag⁺ assisted Mn²⁺ doping method in which Mn²⁺ can be easily doped into Br-based MQW perovskites. In this MQW perovskites, both nanoplatelets (NPLs) and NCs were formed simultaneously, where efficient energy transfer occurred from the NPLs with a higher energy bandgap to the NCs with a smaller energy bandgap, and then to the Mn²⁺ dopants. White lighting solution with a PLQY up to 98% has been acquired by altering the experimental parameters, such as reaction time and the Pb-to-Mn feed ratio. The successful doping of Mn²⁺ into CsPbBr₃ host has great significance and shows promising application for next-generation white lighting.

Optical Imaging in the Second Near Infrared Window for Vascular Bioimaging

Optical imaging in the second near infrared region (NIR-II, 1000-1700 nm) provides higher resolution and deeper penetration depth for accurate and real-time vascular anatomy, blood dynamics, and function information, effectively contributing to the early diagnosis and curative effect assessment of vascular anomalies. Currently, NIR-II optical imaging demonstrates encouraging results including long-term monitoring of vascular injury and regeneration, real-time feedback of blood perfusion, tracking of lymphatic metastases, and imaging-guided surgery. This review summarizes the latest progresses of NIR-II optical imaging for angiography including fluorescence imaging, photoacoustic (PA) imaging, and optical coherence tomography (OCT). The development of current NIR-II fluorescence, PA, and OCT probes (i.e., single-walled carbon nanotubes, quantum dots, rare earth doped nanoparticles, noble metal-based nanostructures, organic dye-based probes, and semiconductor polymer nanoparticles), highlighting probe optimization regarding high brightness, longwave emission, and biocompatibility through chemical modification or nanotechnology, is first introduced. The application of NIR-II probes in angiography based on the classification of peripheral vascular, cerebrovascular, tumor vessel, and cardiovascular, is then reviewed. Major challenges and opportunities in the NIR-II optical imaging for vascular imaging are finally discussed.

Intense photoluminescence from Cu-doped CdSe nanotetrapods triggered by ultrafast hole capture

Brightly photoluminescent Cu-doped CdSe nanotetrapods (NTPs) have been prepared by a modified hot injection method. Their photoluminescence (PL) has a quantum yield of 38% and decays slowly over a few microseconds, while the PL in undoped NTPs has a rather small quantum yield of 1.7% and decays predominantly in tens of picoseconds, with a minor component in the nanosecond time regime. PL spectra of doped NTPs are significantly Stokes shifted compared to the band edge (BE). Efficient PL quenching by a hole scavenger confirms the oxidation state of +I for the dopant ion and establishes hole capture by this ion to be the primary event that leads to the Stokes shifted PL. A fast decay of the photoinduced absorption band, along with a similar decay in PL, observed in a femtosecond optical gating experiment, yields a time constant of about a picosecond for the hole capture from the valence band (VB) by Cu⁺. The remarkably long PL lifetime in the doped NTPs is ascribed to the decrease in the overlap between the wavefunctions of the photogenerated electrons and the captured hole. Hot carrier relaxation processes, triggered by excitation at energies greater than the band gap, leave their signature in a rise time of few hundreds of femtoseconds, in the ground state bleach recovery kinetics. Hence, a complete picture of exciton dynamics in the doped NTPs has been obtained using ultrafast spectroscopic techniques working in tandem.

Evidence of band filling in PbS colloidal quantum dot square superstructures

PbS square superstructures are formed by the oriented assembly of PbS quantum dots (QDs), reflecting the facet structures of each QD. In the square assembly, the quantum dots are highly oriented, in sharp contrast to the conventional hexagonal QD assemblies, in which the orientation of QDs is highly disordered, and each QD is connected through ligand molecules. Here, we measured the transport properties of the oriented assembly of PbS square superstructures. The combined electrochemical doping studies by electric double layer transistor (EDLT) and spectroelectrochemistry showed that more than fourteen electrons per quantum dot are introduced. Furthermore, we proved that the lowest conduction band is formed by the quasi-fourth degenerate quantized (1S_e) level in the PbS QD square superstructures.

Semiconductor quantum dots: Technological progress and future challenges

In quantum-confined semiconductor nanostructures, electrons exhibit distinctive behavior compared with that in bulk solids. This enables the design of materials with tunable chemical, physical, electrical, and optical properties. Zero-dimensional semiconductor quantum dots (QDs) offer strong light absorption and bright narrowband emission across the visible and infrared wavelengths and have been engineered to exhibit optical gain and lasing. These properties are of interest for imaging, solar energy harvesting, displays, and communications. Here, we offer an overview of advances in the synthesis and understanding of QD nanomaterials, with a focus on colloidal QDs, and discuss their prospects in technologies such as displays and lighting, lasers, sensing, electronics, solar energy conversion, photocatalysis, and quantum information.

Determinants of crystal structure transformation of ionic nanocrystals in cation exchange reactions

Changes in the crystal system of an ionic nanocrystal during a cation exchange reaction are unusual yet remain to be systematically investigated. In this study, chemical synthesis and computational modeling demonstrated that the height of hexagonal-prism roxbyite (Cu_1.8S) nanocrystals with a distorted hexagonal close-packed sulfide anion (S^2-) sublattice determines the final crystal phase of the cation-exchanged products with Co²⁺ [wurtzite cobalt sulfide (CoS) with hexagonal close-packed S^2- and/or cobalt pentlandite (Co₉S₈) with cubic close-packed S^2-]. Thermodynamic instability of exposed planes drives reconstruction of anion frameworks under mild reaction conditions. Other incoming cations (Mn²⁺, Zn²⁺, and Ni²⁺) modulate crystal structure transformation during cation exchange reactions by various means, such as volume, thermodynamic stability, and coordination environment.

Interaction of Folic Acid with Mn2+ Doped CdTe/ZnS Quantum Dots: In Situ Detection of Folic Acid

To utilize the nanomaterials as an effective carrier for the drug delivery applications, it is important to study the interaction between nanomaterials and drug or biomolecules. In this study GSH functionalized Mn2+-doped CdTe/ZnS QDs has been utilized as a model nanomaterial due to its high luminescence property. Folic acid (FA) gradually quenches the FL of GSH functionalized Mn2+ - doped CdTe/ZnS QDs. The Stern-Volmer quenching constant (Ksv), binding constant (Ks) and effective quenching constant (Ka) for the FA-QDs system is calculated to be 1.32 × 105 M-1, 1.92 × 105 and 0.27 × 105 M-1, respectively under optimized condition (Temp. 300 K, pH 8.0, incubation time 40 min.). The effects of temperature, pH, and incubation time on FA-QDs system have also been studied. Statistical analysis of the quenched FL intensity versus FA concentration revealed a linear range from 1 × 10-7 to 5.0 × 10-5 for FA detection. The LOD of the current nano-sensor for FA was calculated to be 0.2 μM. The effect of common interfering metal ions and other relevant biomolecules on the detection of FA (12.0 μM) have also been investigated. L-cysteine and glutathione displayed moderate effect on FA detection. Similarly, the common metal ions (Na+, K+, Ca2+ and Mg2+) produced minute interference while Zn2+ Cu2+ and Fe3+ exert moderate interference. Toxic metal ions (Hg2+ and Pb2+) produced severe interferences in FA detection.Graphical abstract GSH-Mn2+ CdTe/ZnS QDs based Fluorescence Nanosensor for Folic acid.

Continuously Graded Quantum Dots: Synthesis, Applications in Quantum Dot Light-Emitting Diodes, and Perspectives

Colloidal quantum dot (QD) light-emitting diodes (QLEDs) hold the promise of next-generation displays and illumination owing to their excellent color saturation, high efficiency, and solution processability. For achieving high-performance light-emitting diodes (LEDs), engineering the fine compositions and structures of QDs is of paramount importance and attracts tremendous research interest. The recently developed continuously graded QDs (cg-QDs) with gradually altered nanocompositions and electronic band structures present the most advanced example in this area. In this Perspective, we summarize the current progress in LEDs based on cg-QDs, mainly concentrating on their synthesis and advantages in addressing the great challenges in QLEDs, like efficiency roll-off at high current densities, short operation lifetimes at high brightness, and low brightness near the voltage around the bandgap. In addition, we propose accessible approaches exploiting the cutting-edge mechanisms and techniques to further optimize and improve the performance of QLEDs.

Quantized doping of CdS quantum dots with twelve gold atoms

Through a bottom-up strategy, CdS quantum dots (QDs) doped with 12 gold atoms in each nanocrystal (NC) were prepared by cation exchange reactions. The (Au12) dopants inside the CdS matrix were directly observed using Cs-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images and quantitatively confirmed using the inductively coupled plasma atomic emission spectroscopy (ICP-AES) data. With a photoluminescence quantum yield (PLQY) of 37.5%, the as-prepared (Au12)@CdS QDs emitted light at 635 nm. Due to the injection of excited electrons from the lowest unoccupied molecular orbital (LUMO) of dopants to the conduction band (CB) of CdS, multiple fine peaks were observed in the photoluminescence excitation (PLE) spectra. By using clusters as starting materials, we demonstrate a universal approach for the precise tailoring of dopants and provide a pathway for band energy engineering of doped QDs.

Charge Carrier Localization in Doped Perovskite Nanocrystals Enhances Radiative Recombination

Nanocrystals based on halide perovskites offer a promising material platform for highly efficient lighting. Using transient optical spectroscopy, we study excitation recombination dynamics in manganese-doped CsPb(Cl,Br)3 perovskite nanocrystals. We find an increase in the intrinsic excitonic radiative recombination rate upon doping, which is typically a challenging material property to tailor. Supported by ab initio calculations, we can attribute the enhanced emission rates to increased charge carrier localization through lattice periodicity breaking from Mn dopants, which increases the overlap of electron and hole wave functions locally and thus the oscillator strength of excitons in their vicinity. Our report of a fundamental strategy for improving luminescence efficiencies in perovskite nanocrystals will be valuable for maximizing efficiencies in light-emitting applications.

Turning Low-Nanoscale Intrinsic Silicon Highly Electron-Conductive by SiO2 Coating

Impurity doping in silicon (Si) ultra-large-scale integration is one of the key challenges which prevent further device miniaturization. Using ultraviolet photoelectron spectroscopy and X-ray absorption spectroscopy in the total fluorescence yield mode, we show that the lowest unoccupied and highest occupied electronic states of ≤3 nm thick SiO₂-coated Si nanowells shift by up to 0.2 eV below the conduction band and ca. 0.7 eV below the valence band edge of bulk silicon, respectively. This nanoscale electronic structure shift induced by anions at surfaces (NESSIAS) provides the means for low-nanoscale intrinsic Si (i-Si) to be flooded by electrons from an external (bigger, metallic) reservoir, thereby getting highly electron- (n-) conductive. While our findings deviate from the behavior commonly believed to govern the properties of silicon nanowells, they are further confirmed by the fundamental energy gap as per nanowell thickness when compared against published experimental data. Supporting our findings further with hybrid density functional theory calculations, we show that other group IV semiconductors (diamond, Ge) do respond to the NESSIAS effect in accord with Si. We predict adequate nanowire cross-sections (X-sections) from experimental nanowell data with a recently established crystallographic analysis, paving the way to undoped ultrasmall silicon electronic devices with significantly reduced gate lengths, using complementary metal-oxide-semiconductor-compatible materials.

Manganese Doping Promotes the Synthesis of Bismuth-based Perovskite Nanocrystals While Tuning Their Band Structures

The doping of halide perovskite nanocrystals (NCs) with manganese cations (Mn²⁺ ) has recently enabled enhanced stability, novel optical properties, and modulated charge carrier dynamics of the NCs host. However, the influence of Mn doping on the synthetic routes and the band structures of the host has not yet been elucidated. Herein, it is demonstrated that Mn doping promotes a facile, safe, and low-hazard path toward the synthesis of ternary Cs₃ Bi₂ I₉ NCs by effectively inhibiting the impurity phase (i.e., CsI) resulting from the decomposition of the intermediate Cs₃ BiI₆ product. Furthermore, it is observed that the deepening of the valence band level of the host NCs upon doping at Mn concentration levels varying from 0 to 18.5% (atomic ratio) with respect to the Bi content. As a result, the corresponding Mn-doped NCs solar cells show a higher open-circuit voltage and longer electron lifetime than those employing the undoped perovskite NCs. This work opens new insights on the role of Mn doping in the synthetic route and optoelectronic properties of lead-free halide perovskite NCs for still unexplored applications.

Identification of Point Defects in Atomically Thin Transition-Metal Dichalcogenide Semiconductors as Active Dopants

Selective doping in semiconductors is essential not only for monolithic integrated circuity fabrications but also for tailoring their properties including electronic, optical, and catalytic activities. Such active dopants are essentially point defects in the host lattice. In atomically thin two-dimensional (2D) transition-metal dichalcogenides (TMDCs), the roles of such point defects are particularly critical in addition to their large surface-to-volume ratio, because their bond dissociation energy is relatively weaker, compared to elemental semiconductors. In this Mini Review, we review recent advances in the identifications of diverse point defects in 2D TMDC semiconductors, as active dopants, toward the tunable doping processes, along with the doping methods and mechanisms in literature. In particular, we discuss key issues in identifying such dopants both at the atomic scales and the device scales with selective examples. Fundamental understanding of these point defects can hold promise for tunability doping of atomically thin 2D semiconductor platforms.

Nanocrystal Quantum Dots: From Discovery to Modern Development

This review traces nanocrystal quantum dot (QD) research from the early discoveries to the present day and into the future. We describe the extensive body of theoretical and experimental knowledge that comprises the modern science of QDs. Indeed, the spatial confinement of electrons, holes, and excitons in nanocrystals, coupled with the ability of modern chemical synthesis to make complex designed structures, is today enabling multiple applications of QD size-tunable electronic and optical properties.

Triskelion Structured Colloidal Quantum Dots

We show, using density functional theory and ab initio molecular dynamics, that certain small colloidal quantum dots with a mixed nanocrystal core capped with achiral surface ligands spontaneously form a triskelion (from the Greek, three-legged) structure with (approximate) C₃ symmetry that can be dynamically stable at room temperature when additionally capped with small amine ligands. Furthermore, the nanocrystal core also forms a triskelion structure. The focus of our study is a colloidal quantum dot with a Cd₁₆Se₇Te₃ core (and a charge of +12) capped with negatively charged surface ligands to achieve charge neutrality-in the simplest instance, 12 Cl^--to form the colloidal quantum dot Cd₁₆Se₇Te₃Cl₁₂. The small size of the core (for which almost all atoms are surface atoms), the high positive charge that destabilizes the core, the mixed (Cd/Te) composition that creates mechanical strain in the core, and the inclusion of precisely three Te atoms in the predominantly Se core all play critical roles in the spontaneous formation of the triskelion structure.

All-Solution-Processed Quantum Dot Electrical Double-Layer Transistors Enhanced by Surface Charges of Ti3C2Tx MXene Contacts

Fully solution-processed, large-area, electrical double-layer transistors (EDLTs) are presented by employing lead sulfide (PbS) colloidal quantum dots (CQDs) as active channels and Ti₃C₂T_x MXene as electrical contacts (including gate, source, and drain). The MXene contacts are successfully patterned by standard photolithography and plasma-etch techniques and integrated with CQD films. The large surface area of CQD film channels is effectively gated by ionic gel, resulting in high performance EDLT devices. A large electron saturation mobility of 3.32 cm² V^-1 s^-1 and current modulation of 1.87 × 10⁴ operating at low driving gate voltage range of 1.25 V with negligible hysteresis are achieved. The relatively low work function of Ti₃C₂T_x MXene (4.42 eV) compared to vacuum-evaporated noble metals such as Au and Pt makes them a suitable contact material for n-type transport in iodide-capped PbS CQD films with a LUMO level of ∼4.14 eV. Moreover, we demonstrate that the negative surface charges of MXene enhance the accumulation of cations at lower gate bias, achieving a threshold voltage as low as 0.36 V. The current results suggest a promising potential of MXene electrical contacts by exploiting their negative surface charges.

Atomically precise vanadium-oxide clusters

Polyoxovanadate (POV) clusters are an important subclass of polyoxometalates with a broad range of molecular compositions and physicochemical properties. One relatively underdeveloped application of these polynuclear assemblies involves their use as atomically precise, homogenous molecular models for bulk metal oxides. Given the structural and electronic similarities of POVs and extended vanadium oxide materials, as well as the relative ease of modifying the homogenous congeners, investigation of the chemical and physical properties of pristine and modified cluster complexes presents a method toward understanding the influence of structural modifications (e.g. crystal structure/phase, chemical makeup of surface ligands, elemental dopants) on the properties of extended solids. This review summarises recent advances in the use of POV clusters as atomically precise models for bulk metal oxides, with particular focus on the assembly of vanadium oxide clusters and the consequences of altering the molecular composition of the assembly via organofunctionalization and the incorporation of elemental "dopants".

Exploiting the Lability of Metal Halide Perovskites for Doping Semiconductor Nanocomposites

Cesium lead halides have intrinsically unstable crystal lattices and easily transform within perovskite and nonperovskite structures. In this work, we explore the conversion of the perovskite CsPbBr₃ into Cs₄PbBr₆ in the presence of PbS at 450 °C to produce doped nanocrystal-based composites with embedded Cs₄PbBr₆ nanoprecipitates. We show that PbBr₂ is extracted from CsPbBr₃ and diffuses into the PbS lattice with a consequent increase in the concentration of free charge carriers. This new doping strategy enables the adjustment of the density of charge carriers between 10¹⁹ and 10²⁰ cm^-3, and it may serve as a general strategy for doping other nanocrystal-based semiconductors.

Synthesis, Bottom up Assembly and Thermoelectric Properties of Sb-Doped PbS Nanocrystal Building Blocks

The precise engineering of thermoelectric materials using nanocrystals as their building blocks has proven to be an excellent strategy to increase energy conversion efficiency. Here we present a synthetic route to produce Sb-doped PbS colloidal nanoparticles. These nanoparticles are then consolidated into nanocrystalline PbS:Sb using spark plasma sintering. We demonstrate that the introduction of Sb significantly influences the size, geometry, crystal lattice and especially the carrier concentration of PbS. The increase of charge carrier concentration achieved with the introduction of Sb translates into an increase of the electrical and thermal conductivities and a decrease of the Seebeck coefficient. Overall, PbS:Sb nanomaterial were characterized by two-fold higher thermoelectric figures of merit than undoped PbS.

Advances in Perovskite Light-Emitting Diodes Possessing Improved Lifetime

Recently, perovskite light-emitting diodes (PeLEDs) are seeing an increasing academic and industrial interest with a potential for a broad range of technologies including display, lighting, and signaling. The maximum external quantum efficiency of PeLEDs can overtake 20% nowadays, however, the lifetime of PeLEDs is still far from the demand of practical applications. In this review, state-of-the-art concepts to improve the lifetime of PeLEDs are comprehensively summarized from the perspective of the design of perovskite emitting materials, the innovation of device engineering, the manipulation of optical effects, and the introduction of advanced encapsulations. First, the fundamental concepts determining the lifetime of PeLEDs are presented. Then, the strategies to improve the lifetime of both organic-inorganic hybrid and all-inorganic PeLEDs are highlighted. Particularly, the approaches to manage optical effects and encapsulations for the improved lifetime, which are negligibly studied in PeLEDs, are discussed based on the related concepts of organic LEDs and Cd-based quantum-dot LEDs, which is beneficial to insightfully understand the lifetime of PeLEDs. At last, the challenges and opportunities to further enhance the lifetime of PeLEDs are introduced.

Stable near white light emission in CsPbCl3 perovskite quantum dots by incorporating Al3+/Mn2+ ions

All inorganic cesium lead halides (CsPbX3, X = Cl, Br, I) are promising materials and have been developed in recent years for various optoelectronic devices and applications because of their excellent optoelectronic properties. Regardless of their excellent characteristics their stability is still uncertain and it is challenging task to obtain stable emission in CsPbX3 perovskite quantum dots (PQDs), hence limiting their practical optoelectronic application. In this context, several approaches have been used like an-ion exchange, ion doping, and core–shell structure to enhance PQDs stability. Herein, we synthesized dual ion co-doped Al3+/Mn2+ CsPbCl3 PQDs for stable light emitting diodes through traditional hot injection method for the first time. By adjusting molar concentration of Al3+/Mn2+ CsPbCl3, co-doped PQDs were successfully prepared. The co-doped PQDs exhibit tunable emission, covering a wide range under UV excitation. Moreover, these high luminescent co-doped PQDs were used to fabricate WLEDs, displaying stable near white light emission with the chromaticity coordination at (0.35, 0.28). Some new evidence has emerged, although some aspects of Mn2+ and Al3+ doping are considered to be consistent with previous conclusions. This viewpoint incorporates all of these details and focuses on the path of transition metal ion doping to perovskite nanostructures and offers an overview for possible potential studies.

Giant Zeeman Splitting for Monolayer Nanosheets at Room Temperature

Giant Zeeman splitting and zero-field splitting (ZFS) are observed in 2D nanosheets that have monolayers of atomic thickness. In this study, single-crystalline CdSe(ethylenediamine)_0.5 and Mn²⁺-doped nanosheets are synthesized via a solvothermal process. Tunable amounts of Mn²⁺(0.5-8.0%) are introduced, resulting in lattice contraction as well as phosphorescence from five unpaired electrons. The exciton dynamics are dominated by spin-related electronic transitions (⁴T₁ → ⁶A₁) with long lifetimes (20.5, 132, and 295 μs). Temperature-varied EPR spectroscopy with spectral simulation reveals large ZFS (D = 3850 MHz) due to axial distortion of substituted Mn²⁺ (S = 5/2). In the magnetic circular dichroism (MCD) measurements, we observed giant Zeeman splitting with large effective g values (up to 231 ± 21), which implies huge sp-d exchange interactions in 2D monolayer regimes, leading to diluted magnetic semiconductor (DMS) materials.