Yang Zhou

Ultrabroad Photoluminescence and Electroluminescence at New Wavelengths from Doped Organometal Halide Perovskites

Doping of semiconductors by introducing foreign atoms enables their widespread applications in microelectronics and optoelectronics. We show that this strategy can be applied to direct bandgap lead-halide perovskites, leading to the realization of ultrawide photoluminescence (PL) at new wavelengths enabled by doping bismuth (Bi) into lead-halide perovskites. Structural and photophysical characterization reveals that the PL stems from one class of Bi doping-induced optically active center, which is attributed to distorted [PbI6] units coupled with spatially localized bipolarons. Additionally, we find that compositional engineering of these semiconductors can be employed as an additional way to rationally tune the PL properties of doped perovskites. Finally, we accomplished the electroluminescence at cryogenic temperatures by using this system as an emissive layer, marking the first electrically driven devices using Bi-doped photonic materials. Our results suggest that low-cost, earth-abundant, solution-processable Bi-doped perovskite semiconductors could be promising candidate materials for developing optical sources operating at new wavelengths.

Unconventional Luminescent Centers in Metastable Phases Created by Topochemical Reduction Reactions

A low-temperature topochemical reduction strategy is used herein to prepare unconventional phosphors with luminescence covering the biological and/or telecommunications optical windows. This approach is demonstrated by using Bi(III)-doped Y2O3 (Y(2-x)Bi(x)O3) as a model system. Experimental results suggest that topochemical treatment of Y(2-x)Bi(x)O3 using CaH2 creates randomly distributed oxygen vacancies in the matrix, resulting in the change of the oxidation states of Bi to lower oxidation states. The change of the Bi coordination environments from the [BiO6] octahedra in Y(2-x)Bi(x)O3 to the oxygen-deficient [BiO(6-z)] polyhedra in reduced phases leads to a shift of the emission maximum from the visible to the near-infrared region. The generality of this approach was further demonstrated with other phosphors. Our findings suggest that this strategy can be used to explore Bi-doped or other classes of luminescent systems, thus opening up new avenues to develop novel optical materials.

Superbroad near-infrared photoluminescence covering the second biological window achieved by bismuth-doped oxygen-deficient gadolinium oxide

In this work, we demonstrated that bismuth-doped oxygen-deficient gadolinium oxides, produced through a low-temperature topochemical reduction strategy using CaH2 as a solid-state reducing agent, show superbroad near-infrared photoluminescence covering the second biological window. Structural analyses confirm that the topochemical reduction treatment of bismuth-doped gadolinium oxides creates oxygen vacancies in the gadolinium oxide matrix, which changes the coordination of Bi and makes it situate in a defective environment. Given the tantalizing superbroad near-infrared emission from the reduced phases, it is anticipated that this novel system may find applications for in vitro/in vivo near-infrared fluorescence imaging in the second biological window.

Ultrabroad near-infrared photoluminescence from bismuth doped CsPbI3: polaronic defects vs. bismuth active centers

Bismuth doped materials with near infrared (NIR) photoluminescence (PL) have recently attracted tremendous attention because of their great potential for photonic and optoelectronic devices that could find broad applications in modern optical telecommunications. However, the mechanistic studies of the NIR PL from these materials still significantly lag behind, which imposes substantial limitations in rationally discovering and designing new materials. In this contribution, we investigated the optical and structural properties of Bi doped CsPbI3 using a diverse range of experimental techniques. We proved that the observed ultrabroad NIR PL with lifetimes of tens of microseconds is not connected with a single-ion Bi active center. For the first time, we proposed a polaron model to rationally explain the observed PL from Bi doped CsPbI3. Finally, we demonstrate the first observation of NIR PL from nanowires among the Bi doped luminescent materials. The experimental results reported here not only deepen the understanding of NIR PL mechanisms in Bi doped materials, but also introduce a new family of solution-processed nanostructures operating in the NIR, which may stimulate further research in exploiting this technique to design a broad range of photonic and optoelectronic devices.

Cs4PbBr6/CsPbBr3 Perovskite Composites with Near-Unity Luminescence Quantum Yield: Large-Scale Synthesis, Luminescence and Formation Mechanism, and White Light-Emitting Diode Application

All-inorganic perovskites have emerged as a new class of phosphor materials owing to their outstanding optical properties. Zero-dimensional inorganic perovskites, in particular the Cs4PbBr6-related systems, are inspiring intensive research owing to the high photoluminescence quantum yield (PLQY) and good stability. However, synthesizing such perovskites with high PLQYs through an environment-friendly, cost-effective, scalable, and high-yield approach remains challenging, and their luminescence mechanisms has been elusive. Here, we report a simple, scalable, room-temperature self-assembly strategy for the synthesis of Cs4PbBr6/CsPbBr3 perovskite composites with near-unity PLQY (95%), high product yield (71%), and good stability using low-cost, low-toxicity chemicals as precursors. A broad range of experimental and theoretical characterizations suggest that the high-efficiency PL originates from CsPbBr3 nanocrystals well passivated by the zero-dimensional Cs4PbBr6 matrix that forms based on a dissolution-crystallization process. These findings underscore the importance in accurately identifying the phase purity of zero-dimensional perovskites by synchrotron X-ray technique to gain deep insights into the structure-property relationship. Additionally, we demonstrate that green-emitting Cs4PbBr6/CsPbBr3, combined with red-emitting K2SiF6:Mn4+, can be used for the construction of WLEDs. Our work may pave the way for the use of such composite perovskites as highly luminescent emitters in various applications such as lighting, displays, and other optoelectronic and photonic devices.

Ultra-broadband optical amplification at telecommunication wavelengths achieved by bismuth-activated lead iodide perovskites

It is extremely difficult to achieve hybrid halide perovskite semiconductors with luminescence at wavelengths longer than 1.0 μm, because of the inherent limitation of their band gaps. We show herein that solution-processable, Bi-activated, high-quality MAPbI3 films can be adopted as a new gain medium operating in the whole telecommunication window of 1260–1625 nm. Additionally, the structural and optical properties of Bi doped MAPbI3 have been investigated. The results indicate that the NIR PL originates from the structural defects induced by Bi. Finally, we accomplished optical amplification in the whole telecommunication window by using Bi-doped MAPbI3 films, which represents the first work where such a performance is attained among lead halide perovskites and Bi-doped photonic films. This work opens up exciting possibilities of using perovskite semiconducting materials as gain media for optical amplifiers and lasers operating in the telecommunication window.

Creation of near-infrared luminescent phosphors enabled by topotactic reduction of bismuth-activated red-emitting crystals

Bismuth-doped luminescent materials have gained significant attention in recent years owing to their huge potential for applications in telecommunications, biomedicine, and displays. However, controlled synthesis of these materials, in particular for those luminescing in the near-infrared (NIR) region, remains a challenging subject of continuous research efforts. Herein, we show that low-temperature topotactic reduction using Al metal powder as an oxygen getter can be adopted as a powerful technique for the conversion of bismuth-doped red-emitting systems into their NIR-emitting cousins. Thorough experimental characterization indicates that the framework oxygen of the hosts can be topotactically extracted, thus producing unique metal–oxygen–metal crystalline networks in reduced phases while preserving the crystalline structure of the precursor. For the first time, based on detailed analyses of X-ray absorption data, we identified that a minority of Bi atoms occupy Ba2+ sites, and most of the Bi atoms occupy the P5+ and/or B3+ sites in the as-synthesized Bi-doped BaBPO5. Subsequent topotactic treatment preferentially changes the local environment of Bi at the P5+/B3+ sites, which results in the occurrence of NIR emission owing to the birth of NIR-luminescent, defective Bi–O polyhedra in which Bi bears lower oxidation states with respect to that in the precursor. The site-specific topotactic reduction reaction reported here helps us create peculiar NIR-luminescent Bi–O units, and simultaneously does not seriously affect the red photoluminescence of Bi2+ situated at the Ba2+ sites. Given that the long-lived, ultrawide NIR emission covers the second biological window, the phosphors developed here hold great promise for in vivo luminescence and lifetime bioimaging. We anticipate that this low-temperature topotactic reduction strategy can be applied to the development of more novel Bi-doped luminescent materials in various forms that can find a broad range of functional applications.

Air-stable and highly luminescent bismuth complex nanoparticles

We report, for the first time, the preparation of air-stable, solution-processed, luminescent Bi complex nanoparticles (NPs) through a one-pot wet chemical reaction method. Strong visible photoluminescence (PL) from these Bi complex NPs, with a quantum yield as high as 5.9%, can be achieved. Interestingly, the observed PL is distinctly different from other materials containing Bi. Based on detailed analyses, a multi-stage formation and growth process for the Bi complex NPs has been proposed, and the observed PL is attributed to the intrinsic electronic transition inherent to the whole NPs. This work not only provides a synthetic method for luminescent NPs with sizes larger than 2 nm, but also can contribute significantly to a better understanding of the photophysical behaviors of materials containing the Bi element.

Ion-Exchangeable Microporous Polyoxometalate Compounds with Off-Center Dopants Exhibiting Unconventional Luminescence

The synthesis of luminescent polyoxometalates (POMs) typically relies on the assembly of POM ligands with rare earth or transition metals, placing significant constraints on the composition, structure, and hence the luminescence properties of the resultant systems. Herein, we show that the ion-exchange strategy can be used for the synthesis of novel POM-based luminescent materials. We demonstrate that introducing bismuth ions into an ion-exchangeable, microporous POM compound yields an unconventional system luminescing in the near-infrared region. Experimental characterization, coupled with quantum chemical calculations, confirms that bismuth ions site-specifically occupy an off-center site in the lattice, and have an asymmetric coordination geometry unattainable by other means, thus giving rise to peculiar emission. Our findings offer an effective strategy for the synthesis of POM-based luminescent materials, and the design concept may potentially be adapted to the creation of POM-based systems with other functionalities.

Superbroad near-infrared photoluminescence from bismuth-doped CsPbI_3 perovskite nanocrystals

Bismuth-doped materials show fascinating near-infrared (NIR) photoluminescence (PL) properties. However, synthesizing Bi-doped, NIR-luminescent, nanometer-sized materials with high PL quantum yields remains challenging. Here, Bi-doped CsPbI3 perovskite nanocrystals (NCs) with an average size less than 10 nm and showing a superbroad NIR PL covering the telecommunication and second biological optical windows were achieved. The NIR PL quantum yield of these NCs is up to 7.17% with the Bi doping concentration of 0.074%. Additionally, efficient energy transfer from the semiconducting CsPbI3 to bismuth-related active center can be realized. We anticipate that the developed systems may find applications in optoelectronic and photonic devices as well as biological imaging. This work enriches the bank of Bi-doped luminescent materials, and might stimulate research interest for synthesizing other classes of Bi-activated nanomaterials.