Excitation-Intensity-Dependent Color-Tunable Dual Emissions from Manganese-Doped CdS/ZnS Core/Shell Nanocrystals

Surface and Interface Engineering for Highly Stable CsPbBr3/ZnS Core/Shell Nanocrystals

Shelling with chalcogenides on the surface of lead halide perovskite (LHP) nanocrystals (NCs) is believed to be an effective approach to increase their stability under high-moisture/aqueous conditions, which is important for LHP NC-based optoelectronic devices. However, it is still a challenge to prepare high-quality LHP/chalcogenide core/shell NCs with moisture/aqueous stability. In this work, a surface-defect-induced strategy is carried out to facilitate the adsorption of Br- ions and subsequently Zn2+ ions to preform a bipolar surface, which reduces the energy barrier at the CsPbBr3/ZnS interface and promotes the epitaxial growth of the ZnS shell layer. The aqueous stability of the as-received NCs shows an increase of over 12 times compared to that of the original one. Likewise, Mn2+ ions are introduced to further reduce the geometric symmetry mismatch and defect density at the CsPbBr3/ZnS interface. Interestingly, aqueous stability characterizations illustrate negligible degradation even after 230 min of ultrasonication, suggesting their outstanding stability. This work proposes an effective approach to prepare high-quality LHP/chalcogenide core/shell NCs, which possess great potential in the fabrication of stable optoelectronic devices.

Type-I CdS/ZnS Core/Shell Quantum Dot-Gold Heterostructural Nanocrystals for Enhanced Photocatalytic Hydrogen Generation

Developing Type-I core/shell quantum dots is of great importance toward fabricating stable and sustainable photocatalysts. However, the application of Type-I systems has been limited due to the strongly confined photogenerated charges by the energy barrier originating from the wide-bandgap shell material. In this project, we found that through the decoration of Au satellite-type domains on the surface of Type-I CdS/ZnS core/shell quantum dots, such an energy barrier can be effectively overcome and an over 400-fold enhancement of photocatalytic H2 evolution rate was achieved compared to bare CdS/ZnS quantum dots. Transient absorption spectroscopic studies indicated that the charges can be effectively extracted and subsequently transferred to surrounding molecular substrates in a subpicosecond time scale in such hybrid nanocrystals. Based on density functional theory calculations, the ultrafast charge separation rates were ascribed to the formation of intermediate Au2S layer at the semiconductor-metal interface, which can successfully offset the energy confinement introduced by the ZnS shell. Our findings not only provide insightful understandings on charge carrier dynamics in semiconductor-metal heterostructural materials but also pave the way for the future design of quantum dot-based hybrid photocatalytic systems.

Highly Thermally Sensitive Cascaded Wannier-Mott Exciton Ionization/Carrier Localization in Manganese-Doped Perovskite Nanocrystals

Transition-metal doping in perovskite nanocrystals strongly alters the photophysical properties of these nanocrystals. However, the details of the underlying thermal and optical processes within such an intriguing symmetry-breaking nanosystem are far from clear. Herein, we study the sensitively temperature-dependent and highly competent delocalized exciton and transition-metal ion-captured carrier recombination processes in manganese-doped CsPbBr_0.6Cl_2.4 nanocrystals. The combined experimental and theoretical studies reveal that both the exciton ionization and capture of the band-edge carriers by the manganese ions play the dominant roles in determining the proportion of the manganese ions-dominated recombination process. A density functional theory calculation of the temporal fluctuation of the manganese ions-accommodated localized orbitals further confirms that the thermally enhanced nonadiabatic electron-phonon coupling promotes the probability of the carrier localization. These findings reveal the respective crucial roles of the exciton ionization and carrier capture in the localized recombination process in the transition-metal-doped semiconductor nanocrystals.

Dynamically tunable multicolor emissions from zero-dimensional Cs3LnCl6 (Ln: europium and terbium) nanocrystals with wide color gamut

This study demonstrates dynamically tunable multicolor emissions from a single component, zero-dimensional (0-D) cesium europium chloride (Cs₃EuCl₆) and cesium terbium chloride (Cs₃TbCl₆) nanocrystals (NCs). Highly uniform colloidal Cs₃EuCl₆ and Cs₃TbCl₆ NCs are synthesized via the heating-up method. Excitation-wavelength-dependent multicolor emissions from Cs₃EuCl₆ and Cs₃TbCl₆ NCs are observed. Under excitation of 330-400 nm, both NCs exhibit blue photoluminescence (PL). Under wavelengths shorter than 330 nm, characteristic red and green emissions are observed from Cs₃EuCl₆ and Cs₃TbCl₆, respectively, owing to the atomic emissions from the f-orbitals in trivalent europium (Eu³⁺) and terbium (Tb³⁺) ions. Cs₃EuCl₆ and Cs₃TbCl₆ NCs exhibit broadband excitation spectra and enhanced absorption properties. Particularly, Cs₃EuCl₆ NCs exhibit a very narrow full-width at half-maximum in both blue and red PL and no overlap between the two spectra. The photophysical properties of these NCs are further investigated to understand the multicolor PL origins by time-resolved and temperature-dependent PL measurements. Finally, the potential applications of Cs₃EuCl₆ and Cs₃TbCl₆ NCs as anti-counterfeiting inks for high-level security are demonstrated. Given their broadband excitation with enhanced absorption properties and dynamically tunable colors with a wide color gamut, Cs₃EuCl₆ and Cs₃TbCl₆ NCs have great potential as novel multicolor NC emitters for many emerging applications.

Blue and red upconversion color tuning of Tm3+ via pulse excitation based on the thermal effect

Color-tunable upconversion luminescence has wide prospects for anti-counterfeiting and disease diagnosis/treatment. To date, achieving high-quality tunable red and blue emissions using a single excitation wavelength remains a formidable challenge, due to the large energy difference between the red and blue photons. In this Letter, based on Tm³⁺ upconversion luminescence, blue dominant and red dominant emissions are generated upon 980-nm excitations using a short and long pulse, respectively. The corresponding color tuning mechanisms are investigated based on the spectral observations. The proposed color tuning strategy is particularly useful for in vivo applications as the red and blue lights play important roles in biological imaging and drug release, respectively.

Optical Dynamics of Copper-Doped Cadmium Sulfide (CdS) and Zinc Sulfide (ZnS) Quantum-Dots Core/Shell Nanocrystals

Recently, quantum-dot-based core/shell structures have gained significance due to their optical, optoelectronic, and magnetic attributes. Controlling the fluorescence lifetime of QDs shells is imperative for various applications, including light-emitting diodes and single-photon sources. In this work, novel Cu-doped CdS/ZnS shell structures were developed to enhance the photoluminescence properties. The objective was to materialize the Cu-doped CdS/ZnS shells by the adaptation of a two-stage high-temperature doping technique. The developed nanostructures were examined with relevant characterization techniques such as transmission electron microscopy (TEM) and ultraviolet–visible (UV–vis) emission/absorption spectroscopy. Studying fluorescence, we witnessed a sharp emission peak at a wavelength of 440 nm and another emission peak at a wavelength of 620 nm, related to the fabricated Cu-doped CdS/ZnS core/shell QDs. Our experimental results revealed that Cu-doped ZnS shells adopted the crystal structure of CdS due to its larger bandgap. Consequently, this minimized lattice mismatch and offered better passivation to any surface defects, resulting in increased photoluminescence. Our developed core/shells are highly appropriate for the development of efficient light-emitting diodes.

Power-Dependent Photoluminescence Efficiency in Manganese-Doped 2D Hybrid Perovskite Nanoplatelets

Substitutional metal doping is a powerful strategy for manipulating the emission spectra and excited state dynamics of semiconductor nanomaterials. Here, we demonstrate the synthesis of colloidal manganese (Mn²⁺)-doped organic-inorganic hybrid perovskite nanoplatelets (chemical formula: L₂[APb_1-xMn_xBr₃]_n-1Pb_1-xMn_xBr₄; L, butylammonium; A, methylammonium or formamidinium; n (= 1 or 2), number of Pb_1-xMn_xBr₆^4- octahedral layers in thickness) via a ligand-assisted reprecipitation method. Substitutional doping of manganese for lead introduces bright (approaching 100% efficiency) and long-lived (>500 μs) midgap Mn²⁺ atomic states, and the doped nanoplatelets exhibit dual emission from both the band edge and the dopant state. Photoluminescence quantum yields and band-edge-to-Mn intensity ratios exhibit strong excitation power dependence, even at a very low incident intensity (<100 μW/cm²). Surprisingly, we find that the saturation of long-lived Mn²⁺ dopant sites cannot explain our observation. Instead, we propose an alternative mechanism involving the cross-relaxation of long-lived Mn-site excitations by freely diffusing band-edge excitons. We formulate a kinetic model based on this cross-relaxation mechanism that quantitatively reproduces all of the experimental observations and validate the model using time-resolved absorption and emission spectroscopy. Finally, we extract a concentration-normalized microscopic rate constant for band edge-to-dopant excitation transfer that is ∼10× faster in methylammonium-containing nanoplatelets than in formamidinium-containing nanoplatelets. This work provides fundamental insight into the interaction of mobile band edge excitons with localized dopant sites in 2D semiconductors and expands the toolbox for manipulating light emission in perovskite nanomaterials.

Mn2+/Yb3+ Codoped CsPbCl3 Perovskite Nanocrystals with Triple-Wavelength Emission for Luminescent Solar Concentrators

Doping metal ions into lead halide perovskite nanocrystals (NCs) has attracted great attention over the past few years due to the emergence of novel properties relevant to optoelectronic applications. Here, the synthesis of Mn2+/Yb3+ codoped CsPbCl3 NCs through a hot-injection technique is reported. The resulting NCs show a unique triple-wavelength emission covering ultraviolet/blue, visible, and near-infrared regions. By optimizing the dopant concentrations, the total photoluminescence quantum yield (PL QY) of the codoped NCs can reach ≈125.3% due to quantum cutting effects. Mechanism studies reveal the efficient energy transfer processes from host NCs to Mn2+ and Yb3+ dopant ions, as well as a possible inter-dopant energy transfer from Mn2+ to Yb3+ ion centers. Owing to the high PL QYs and minimal reabsorption loss, the codoped perovskite NCs are demonstrated to be used as efficient emitters in luminescent solar concentrators, with greatly enhanced external optical efficiency compared to that of using solely Mn2+ doped CsPbCl3 NCs. This study presents a new model system for enriching doping chemistry studies and future applications of perovskite NCs.

A clustering-triggered emission strategy for tunable multicolor persistent phosphorescence

A clustering-triggered emission (CTE) strategy, namely the formation of heterogeneous clustered chromophores and conformation rigidification, for achieving tunable multicolor phosphorescence in single-component compounds is proposed. Non-conventional luminophores comprising just oxygen functionalities and free of π-bonding, i.e., d-(+)-xylose (d-Xyl), pentaerythritol (PER), d-fructose (d-Fru) and d-galactose (d-Gal), were adopted as a simple model system with an explicit structure and molecular packing to address the hypothesis. Their concentrated solutions and crystals at 77 K or under ambient conditions demonstrate remarkable multicolor phosphorescence afterglows in response to varying excitation wavelengths, because of the formation of diverse oxygen clusters with sufficiently rigid conformations. The intra- and inter-molecular O⋯O interactions were definitely illustrated by both single crystal structure analysis and theoretical calculations. These findings shed new light on the origin and simple achievement of tunable multicolor phosphorescence in single-component pure organics, and in turn, have strong implications for the emission mechanism of non-conventional luminophores.

Tetrahedral photoluminescent manganese(ii) halide complexes with 1,3-dimethyl-2-phenyl-1,3-diazaphospholidine-2-oxide as a ligand

Photoluminescent Mn(ii) tetrahedral complexes characterized by intense emission in the green region were isolated from the reaction of MnX₂ (X = Cl, Br, I) and the ligand 1,3-dimethyl-2-phenyl-1,3-diazaphospholidine-2-oxide.

Advanced Dynamic Photoluminescent Material for Dynamic Anticounterfeiting and Encryption

Anticounterfeiting is a vitally important issue in modern society. At present, the most commonly used luminescent anticounterfeiting technique is based on static photoluminescence (PL), which is easily counterfeited by certain substitutes. In this work, we report for the first time a dynamic PL material, Na₂CaGe₂O₆:Tb³⁺. Irradiated by a portable ultraviolet (254 nm) lamp, the PL color of the material due to Tb³⁺ changes from the initial red to yellow and, finally, green. The investigation reveals that the dynamic PL is due to the presence of appropriate traps and the cross-relaxation effect of Tb³⁺ in Na₂CaGe₂O₆. By employing this unique dynamic PL material, high-level dynamic luminescent anticounterfeiting and encryption devices can be fabricated. The dynamic PL features of the devices are easily detected using a cheap portable lamp, and at present, it is impossible for the features to be faked by any substitutes. In a virtual military scenario, the results demonstrate that the encryption device is safe and that a spy will be detected. Accordingly, this dynamic PL material could inspire more ingenious security designs.

Reversible Photo-Switching of Dual-Color Fluorescent Mn-Doped CdS-ZnS Quantum Dots Modulated by Diarylethene Molecules

Dynamic materials have been given an increased amount of attention in recent years with an expectation that they may exhibit properties on demand. Especially, the combination of fluorescent quantum dots (QDs) and light-responsive organic switches can generate novel photo-switchable materials for diverse applications. In this work, a highly reversible dynamic hybrid system is established by mixing dual-color emitting Mn-doped CdS-ZnS quantum dots (QDs) with photo-switchable diarylethene molecules. We show that the diarylethene 1,2-bis(5-(3,5-bis(trifluoromethyl)phenyl)-2-methylthiophen-3-yl)cyclopent-1-ene (switch molecule 1) performs fabulous photo-switching property (between its open, 1o and closed, 1c forms), and high fatigue resistance in this hybrid system. The emission color switching between blue and pink of the system can be induced mainly by selective quenching/recovering of the Mn- photoluminescence (PL) of the QDs due to the switchable absorbance of the molecule 1. Mechanistic studies show that quenching of QD emission following UV illumination was caused by both Förster resonance energy transfer (FRET) and reabsorption by surrounding 1c molecules in the case of the Mn-PL, and solely by reabsorption in the case of badngap- (BG-)PL. This photo-switchable system could be potentially used in applications ranging from self-erasing paper to super-resolution fluorescence imaging.

Three primary color emissions from single multilayered nanocrystals

The achievement of three-primary-color luminescence in a single material will lead to revolutionary developments of many advanced applications such as dynamic display with ultra-high resolution, and complex anti-counterfeiting. Here we report the realization of steady-state three-primary-color emission in single multilayered NaYF4 upconversion (UC) nanoparticles. In this core-shell structure, a novel design of a tri-sensitizer, i.e., Nd3+, Yb3+ and Er3+ ions, is utilized, which effectively absorbs the excitation photons of 808, 980 and 1550 nm, and then exhibits blue, red and green emissions, respectively. By simply combining the three primary color emissions, tunable full-color luminescence was achieved in this single material. These nanoparticles have demonstrated promising potential applications in dynamic display and multiple anti-counterfeiting.

Interface Engineering of Mn-Doped ZnSe-Based Core/Shell Nanowires for Tunable Host-Dopant Coupling

Transition metal ion doped one-dimensional (1-D) nanocrystals (NCs) have advantages of larger absorption cross sections and polarized absorption and emissions in comparison to 0-D NCs. However, direct synthesis of doped 1-D nanorods (NRs) or nanowires (NWs) has proven challenging. In this study, we report the synthesis of 1-D Mn-doped ZnSe NWs using a colloidal hot-injection method and shell passivation for core/shell NWs with tunable optical properties. Experimental results show optical properties of the NWs are controlled by the composition and thickness of the shell lattice. It was found that both the host-Mn energy transfer and Mn-Mn coupling are strongly dependent on the type of alloy at the interface of doped core/shell NWs. For Mn-doped type I ZnSe/ZnS core/shell NWs, the ZnS shell passivation can enhance florescence quantum yield with little effect on the location of the incorporated Mn dopant due to the identical cationic Zn²⁺ site available for Mn dopants throughout the core/shell NWs. However, for Mn-doped quasi type II ZnSe/CdS NWs and ZnSe/CdS/ZnS core/shell NWs, the cation alloying (Zn_1-xCd_xS(e)) can lead to metal dopant migration from the core to the alloyed interface and tunable host-dopant energy transfer efficiencies and Mn-Mn coupling. As a result, a tunable dual-band emission can be achieved for the doped NWs with the cation-alloyed interface. The interfacial alloying mediated energy transfer and Mn-Mn coupling provides a method to control the optical properties of the doped 1-D core/shell NWs.

Temporal and Remote Tuning of Piezophotonic-Effect-Induced Luminescence and Color Gamut via Modulating Magnetic Field

Light-emitting materials have been extensively investigated because of their widespread applications in solid-state lighting, displays, sensors, and bioimaging. In these applications, it is highly desirable to achieve tunable luminescence in terms of luminescent intensity and wavelength. Here, a convenient physical approach of temporal and remote tuning of light-emitting wavelength and color is demonstrated, which is greatly different from conventional methods. It is shown that by modulating the frequency of magnetic-field excitation at room temperature, luminescence from the flexible composites of ZnS:Al, Cu phosphors induced by the piezophotonic effect can be tuned in real time and in situ. The mechanistic investigation suggests that the observed tunable piezophotonic emission is ascribed to the tilting band structure of the ZnS phosphor induced by magnetostrictive strain under a high frequency of magnetic-field excitation. Furthermore, some proof-of concept devices, including red-green-blue full-color displays and tunable white-light sources are demonstrated simply by frequency modulation. A new understanding of the fundamentals of both luminescence and magnetic-optics coupling is thus provided, while offering opportunities in magnetic-optical sensing, piezophotonics, energy harvesting, novel light sources, and displays.

Chemically Tailoring the Dopant Emission in Manganese-Doped CsPbCl3 Perovskite Nanocrystals

Doping in perovskite nanocrystals adopts different mechanistic approach in comparison to widely established doping in chalcogenide quantum dots. The fast formation of perovskites makes the dopant insertions more competitive and challenging. Introducing alkylamine hydrochloride (RNH₃ Cl) as a promoting reagent, precise controlled doping of Mn^II in CsPbCl₃ perovskite nanocrystals is reported. Simply, by changing the amount of RNH₃ Cl, the Mn incorporation and subsequent tuning in the excitonic as well as Mn d-d emission intensities are tailored. Investigations suggested that RNH₃ Cl acted as the chlorinating source, controlled the size, and also helps in increasing the number of particles. This provided more opportunity for Mn ions to take part in reaction and occupied the appropriate lattice positions. Carrying out several reactions with varying reaction parameters, the doping conditions are optimized and the role of the promoting reagent for both doped and undoped systems are compared.

Mild synthesis of monodisperse tin nanocrystals and tin chalcogenide hollow nanostructures

A mild but robust synthetic strategy was developed to synthesize monodisperse Sn nanocrystals with tunable size by using tungsten hexacarbonyl as the reducing agent, and novel tin chalcogenide nanostructures have also been prepared using Sn nanocrystals as templates.

Core-shell interaction and its impact on the optical absorption of pure and doped core-shell CdSe/ZnSe nanoclusters

The structural, electronic, and optical properties of core-shell nanoclusters, (CdSe)(x)@(CdSe)(y) and their Zn-substituted complexes of x = 2-4 and y = 16-28, were studied with density functional theory calculations. The substitution was applied in the cores, the shells, and/or the whole clusters. All these clusters are characterized by their core-shell structures in which the core-shell interaction was found different from those in core or in shell, as reflected by their bondlengths, volumes, and binding energies. Moreover, the core and shell combine together to compose a new cluster with electronic and optical properties different from those of separated individuals, as reflected by their HOMO-LUMO gaps and optical absorptions. With the substitution of Cd by Zn, the structural, electronic, and optical properties of clusters change regularly. The binding energy increases with Zn content, attributed to the strong Zn-Se bonding. For the same core/shell, the structure with a CdSe shell/core has a narrower gap than that with a ZnSe shell/core. The optical absorption spectra also change accordingly with Zn substitution. The peaks blueshift with increasing Zn concentration, accompanying with shape variations in case large number of Cd atoms are substituted. Our calculations reveal the core-shell interaction and its influence on the electronic and optical properties of the core-shell clusters, suggesting a composition-structure-property relationship for the design of core-shell CdSe and ZnSe nanoclusters.

Förster resonance energy transfer mediated enhancement of the fluorescence lifetime of organic fluorophores to the millisecond range by coupling to Mn-doped CdS/ZnS quantum dots

Manganese-doped CdS/ZnS quantum dots have been used as energy donors in a Förster-like resonance energy transfer (FRET) process to enhance the effective lifetime of organic fluorophores. It was possible to tune the effective lifetime of the fluorophores by about six orders of magnitude from the nanosecond (ns) up to the millisecond (ms) region. Undoped and Mn-doped CdS/ZnS quantum dots functionalized with different dye molecules were selected as a model system for investigating the multiple energy transfer process and the specific interaction between Mn ions and the attached dye molecules. While the lifetime of the free dye molecules was about 5 ns, their linking to undoped CdS/ZnS quantum dots led to a long effective lifetime of about 150 ns, following a non-exponential transient. Manganese-doped core-shell quantum dots further enhanced the long-lasting decay time of the dye to several ms. This opens up a pathway to analyse different fluorophores in the time domain with equal spectral emissions. Such lifetime multiplexing would be an interesting alternative to the commonly used spectral multiplexing in fluorescence detection schemes.

Doped Semiconductor-Nanocrystal Emitters with Optimal Photoluminescence Decay Dynamics in Microsecond to Millisecond Range: Synthesis and Applications

Transition metal doped semiconductor nanocrystals (d-dots) possess fundamentally different emission properties upon photo- or electroexcitation, which render them as unique emitters for special applications. However, in comparison with intrinsic semiconductor nanocrystals, the potential of d-dots has been barely realized, because many of their unique emission properties mostly rely on precise control of their photoluminescence (PL) decay dynamics. Results in this work revealed that it would be possible to obtain bright d-dots with nearly single-exponential PL decay dynamics. By tuning the number of Mn(2+) ions per dot from ∼500 to 20 in Mn(2+) doped ZnSe nanocrystals (Mn:ZnSe d-dots), the single-exponential PL decay lifetime was continuously tuned from ∼50 to 1000 μs. A synthetic scheme was further developed for uniform and epitaxial growth of thick ZnS shell, ∼7 monolayers. The resulting Mn:ZnSe/ZnS core/shell d-dots were found to be essential for necessary environmental durability of the PL properties, both steady-state and transient ones, for the d-dot emitters. These characteristics combined with intense absorption and high PL quantum yields (70 ± 5%) enabled greatly simplified schemes for various applications of PL lifetime multiplexing using Mn:ZnSe/ZnS core/shell d-dots.

Intense photo- and tribo-luminescence of three tetrahedral manganese(II) dihalides with chelating bidentate phosphine oxide ligand

Three air-stable tetrahedral manganese(ii) dihalide complexes [MnX2(DPEPO)] (DPEPO = bis[2-(diphenylphosphino)phenyl]ether oxide; X = Cl, Br and I) were prepared. All of the obtained compounds were structurally characterized by single-crystal X-ray diffraction analyses, which reveal that they crystallize in centrosymmetric space groups and feature an isolated mononuclear structure with Mn(2+) in a tetrahedral environment. Interestingly, these complexes show excellent photoluminescent performance in neat solid form, with the highest total quantum yield (Φtotal) of up to 70% recorded for the dibromide complex. Intense green flashes of light could be observed by the naked eye when rubbing the manganese(ii) complexes.

Antiphase dual-color correlation in a reactant-product pair imparts ultrasensitivity in reaction-linked double-photoswitching fluorescence imaging

A pair of reversible photochemical reactions correlates their reactant and product specifically, and such a correlation uniquely distinguishes their correlated signal from others that are not linked by this reversible reaction. Here a nanoparticle-shielded fluorophore is photodriven to undergo structural dynamics, alternating between a green-fluorescence state and a red-fluorescence state. As time elapses, the fluorophore can be in either state but not both at the same time. Thus, the red fluorescence is maximized while the green fluorescence is minimized and vice versa. Such an antiphase dual-color (AD) corelationship between the red and green fluorescence maxima as well as between their minima can be exploited to greatly improve the signal-to-noise ratio, thus enhancing the ultimate detection limit. Potential benefits of this correlation include elimination of all interferences originating from single-color dyes and signal amplification of AD photoswitching molecules by orders of magnitude.

Temporal full-colour tuning through non-steady-state upconversion

Developing light-harvesting materials with tunable emission colours has always been at the forefront of colour display technologies. The variation in materials composition, phase and structure can provide a useful tool for producing a wide range of emission colours, but controlling the colour gamut in a material with a fixed composition remains a daunting challenge. Here, we demonstrate a convenient, versatile approach to dynamically fine-tuning emission in the full colour range from a new class of core-shell upconversion nanocrystals by adjusting the pulse width of infrared laser beams. Our mechanistic investigations suggest that the unprecedented colour tunability from these nanocrystals is governed by a non-steady-state upconversion process. These findings provide keen insights into controlling energy transfer in out-of-equilibrium optical processes, while offering the possibility for the construction of true three-dimensional, full-colour display systems with high spatial resolution and locally addressable colour gamut.

Mn-doped CdS/ZnS/CdS QD-based fluorescent nanosensor for rapid, selective, and ultrasensitive detection of copper(ii) ion

Energy levels of Mn-doped QD-based nanosensor and the quenching mechanism of the nanosensor by Cu²⁺.

Graphene activating room-temperature ferromagnetic exchange in cobalt-doped ZnO dilute magnetic semiconductor quantum dots

Control over the magnetic interactions in dilute magnetic semiconductor quantum dots (DMSQDs) is a key issue to future development of nanometer-sized integrated "spintronic" devices. However, manipulating the magnetic coupling between impurity ions in DMSQDs remains a great challenge because of the intrinsic quantum confinement effects and self-purification of the quantum dots. Here, we propose a hybrid structure to achieve room-temperature ferromagnetic interactions in DMSQDs, via engineering the density and nature of the energy states at the Fermi level. This idea has been applied to Co-doped ZnO DMSQDs where the growth of a reduced graphene oxide shell around the Zn(0.98)Co(0.02)O core turns the magnetic interactions from paramagnetic to ferromagnetic at room temperature, due to the hybridization of 2p(z) orbitals of graphene and 3d obitals of Co(2+)-oxygen-vacancy complexes. This design may open up a kind of possibility for manipulating the magnetism of doped oxide nanostructures.

Magnetically engineered semiconductor quantum dots as multimodal imaging probes

Light-emitting semiconductor quantum dots (QDs) combined with magnetic resonance imaging contrast agents within a single nanoparticle platform are considered to perform as multimodal imaging probes in biomedical research and related clinical applications. The principles of their rational design are outlined and contemporary synthetic strategies are reviewed (heterocrystalline growth; co-encapsulation or assembly of preformed QDs and magnetic nanoparticles; conjugation of magnetic chelates onto QDs; and doping of QDs with transition metal ions), identifying the strengths and weaknesses of different approaches. Some of the opportunities and benefits that arise through in vivo imaging using these dual-mode probes are highlighted where tumor location and delineation is demonstrated in both MRI and fluorescence modality. Work on the toxicological assessments of QD/magnetic nanoparticles is also reviewed, along with progress in reducing their toxicological side effects for eventual clinical use. The review concludes with an outlook for future biomedical imaging and the identification of key challenges in reaching clinical applications.

Yellow-light generation and engineering in zinc-doped cadmium sulfide nanobelts with low-threshold two-photon excitation

Through a simple doping route with zinc ion as a dopant in cadmium sulfide nanobelts, a bright yellow-colored light was obtained. The detailed chromaticity and brightness of the light can be engineered by the dopant concentration and the pumping power, which are used to control the dominant wavelength to any fine yellow color, and even cover the sodium-yellow-line of 589 nm. The nanobelts were synthesized through a chemical vapor deposition method. The peak shift of the XRD result proves that the zinc ions as a dopant exist in the nanobelts rather than in the ZnCdS alloy formation. Time-resolved photoluminescence of the nanobelt reveals the existence of the defect-related state, which induces a red band to further mix with green band-edge emission to form the yellow light. Moreover, low-threshold two-photon excitation was observed in the proper Zn-doped cadmium sulfide nanobelts. The dopant and pumping power-tuned generation and engineering of the yellow light makes it possible to use this kind of material as yellow light-emitting source.

Davydov splitting in cadmium vacancy emission, ferromagnetism and photosensitivity in manganese incorporated CdS nanocrystals

Surfactant and diamines-assisted CdS nanocrystals have Cd²⁺ vacancies with photoluminescence property. The neighbouring Cd²⁺–Cd²⁺ vacancies exhibit Davydov splitting. Addition of Mn²⁺ ions in these sites leads to ferromagnetism.

Visual monitoring of laser power and spot profile in micron region by a single chip of Zn-doped CdS nanobelts

On the basis of pumping-power-dependent emission property, Zn-doped CdS nanobelts are developed to monitor injected laser power and detect the profile of laser focal spots visually in the micron region through color changes.

Electrochemically controlled auger quenching of Mn²+ photoluminescence in doped semiconductor nanocrystals

Auger processes in colloidal semiconductor nanocrystals have been scrutinized extensively in recent years. Whether involving electron-exciton, hole-exciton, or exciton-exciton interactions, such Auger processes are generally fast and hence have been considered prominent candidates for interpreting fast processes relevant to photoluminescence blinking and multiexciton decay. With recent advances in the chemistries of nanocrystal doping, increasing attention is now being paid to analogous photophysical properties of colloidal-doped semiconductor nanocrystals. Here, we report the first investigation of the effects of electron-dopant exchange interactions on dopant luminescence in doped semiconductor nanocrystals. Using electrochemical techniques, electrical control of charge-carrier densities in films of colloidal Mn(2+)-doped CdS quantum dots has been achieved and used to demonstrate remarkably effective Auger de-excitation of photoexcited Mn(2+). The doped nanocrystals are found to be substantially more sensitive to Auger de-excitation than their undoped analogues, a result shown to arise primarily from the long Mn(2+) excited-state lifetime. This observation of exceptionally effective Auger quenching has broader implications in areas of high-power, single-particle, or electrically driven luminescence of doped semiconductor nanocrystals, and also suggests interesting opportunities for modulating Mn(2+) photoluminescence intensities on sublifetime time scales, or for imaging charge carriers in nanocrystal-based devices.

High-quality manganese-doped zinc sulfide quantum rods with tunable dual-color and multiphoton emissions

We report a simple, fast and green phosphine-free colloidal chemistry to synthesize high-quality wurtzite-type Mn-doped ZnS quantum rods (QRs) with tunable diameters (1.6-5.6 nm), high aspect ratios (up to 50), variable Mn doping levels (0.18-1.60%), and high quantum yields (up to 45%). The electron paramagnetic resonance spectra with modeling reveal the successful doping of paramagnetic Mn(2+) ions in the host ZnS QRs. The Mn-doped ZnS QRs demonstrate tunable dual-color (orange and blue) emissions by tuning the doping levels and UV excitation wavelengths. The orange emission with long decay lifetime (3.3 ms) originates from the doped Mn(2+) states, while the blue emission with fast decay lifetime (0.31 ns) is attributed to the QR surface states. The bright two- and three-photon excitation upconversion luminescence from the Mn-doped ZnS QRs have been observed using tunable near-infrared femtosecond laser. Our strategy provides a versatile route to programmably control the optical properties of anisotropic semiconductor nanomaterials, which may create new opportunities for photonic devices and bioimaging applications.