Haizheng Zhong

Brightly Luminescent and Color-Tunable Colloidal CH3NH3PbX3 (X = Br, I, Cl) Quantum Dots: Potential Alternatives for Display Technology

Organometal halide perovskites are inexpensive materials with desirable characteristics of color-tunable and narrow-band emissions for lighting and display technology, but they suffer from low photoluminescence quantum yields at low excitation fluencies. Here we developed a ligand-assisted reprecipitation strategy to fabricate brightly luminescent and color-tunable colloidal CH3NH3PbX3 (X = Br, I, Cl) quantum dots with absolute quantum yield up to 70% at room temperature and low excitation fluencies. To illustrate the photoluminescence enhancements in these quantum dots, we conducted comprehensive composition and surface characterizations and determined the time- and temperature-dependent photoluminescence spectra. Comparisons between small-sized CH3NH3PbBr3 quantum dots (average diameter 3.3 nm) and corresponding micrometer-sized bulk particles (2-8 μm) suggest that the intense increased photoluminescence quantum yield originates from the increase of exciton binding energy due to size reduction as well as proper chemical passivations of the Br-rich surface. We further demonstrated wide-color gamut white-light-emitting diodes using green emissive CH3NH3PbBr3 quantum dots and red emissive K2SiF6:Mn(4+) as color converters, providing enhanced color quality for display technology. Moreover, colloidal CH3NH3PbX3 quantum dots are expected to exhibit interesting nanoscale excitonic properties and also have other potential applications in lasers, electroluminescence devices, and optical sensors.

Noninjection Gram-Scale Synthesis of Monodisperse Pyramidal CuInS2 Nanocrystals and Their Size-Dependent Properties

CuInS2 nanocrystals are viewed as very good candidates for solar harvesting and light emitting applications. Here we report an optimized noninjection method for the synthesis of monodisperse pyramidal CuInS2 nanocrystals with sizes ranging from 3 to 8 nm. This synthetic route is able to yield large amounts of high quality nanoparticles, usually in the gram scale for one batch experiment. The structure and surface studies showed that the resulting nanocrystals are pyramids of CuInS2 tetragonal phase with well-defined facets, while their surface is functionalized with dodecanethiol capping ligands. Spectroscopic and electrochemical measurements revealed size-dependent optical and electrical properties of CuInS2 nanocrystals, demonstrating quantum confinement effects in these systems. The size-dependent optical bandgaps of CuInS2 nanocrystals were found to be consistent with the finite-depth well effective mass approximation (EMA) calculations, which provide a convenient method to estimate the diameter of CuInS2 pyramids. Additionally we have also determined some important physical parameters, including bandgaps and energy levels, for this system, which are crucial for the integration of CuInS2 nanocrystals in potential device applications.

Tuning the Luminescence Properties of Colloidal I-III-VI Semiconductor Nanocrystals for Optoelectronics and Biotechnology Applications.

In the past 5 years, colloidal I-III-VI nanocrystals such as CuInS2, CuInSe2, and AgInS2 have been intensively investigated for the potential to replace commonly available colloidal nanocrystals containing toxic elements in light-emitting and solar-harvesting applications. Many researchers from different disciplines are working on developing new synthetic protocols, performing spectroscopic studies to understand the luminescence mechanisms, and exploring various applications. To achieve enhanced performance, it is very desirable to obtain high-quality materials with tunable luminescence properties. In this Perspective, we highlight the current progress on tuning the luminescence properties of I-III-VI nanocrystals, especially focusing on the advances in the synthesis, spectroscopic properties, as well as the primary applications in light-emitting devices and bioimaging techniques. Finally, we outline the challenges concerning luminescent I-III-VI NCs and list a few important research tasks in this field.

A facile route to synthesize chalcopyrite CuInSe2 nanocrystals in non-coordinating solvent

A facile route for synthesizing CuInSe2 nanocrystals was developed using an alkanethiol as ligand and noncoordinating octadecene as solvent. The CuInSe2 nanoparticles and nanoplates were obtained, depending on the reaction temperature and time. When the reaction temperature was set at 180 °C and reaction for an hour, CuInSe2 nanoparticles with an average size of 6.2 nm were produced. As the reaction time was prolonged to 3 h, nanoplates appeared inside the nanoparticles. When the reaction temperature was elevated to 210 °C and reaction for 3 h, triangular or hexagonal CuInSe2 nanoplates with sharp edges were synthesized. It was proposed that the Ostwald ripening induced the morphology evolution of the CuInSe2 nanoparticles into nanoplates at the higher temperature. The nanoparticles and nanoplates were characterized by transmission electron microscopy (TEM), selected area electron diffraction (SAED), x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS). The UV–vis absorption spectra of the CuInSe2 nanoparticles were significantly blue-shifted in comparison with the bulk material due to the quantum confinement.

In Situ Fabrication of Halide Perovskite Nanocrystal-Embedded Polymer Composite Films with Enhanced Photoluminescence for Display Backlights.

A simple and versatile in situ fabrication of MAPbX3 nanocrystal-embedded polymer composite films is developed by controlling the crystallization process from precursor solutions. The composite films exhibit enhanced photoluminescence properties, improved stability, and excellent piezoelectric and mechanical properties. Applications of these composite films as color converters in liquid-crystal-display backlights are demonstrated, showing bright potential in display technology.

Shape Tuning of Type II CdTe-CdSe Colloidal Nanocrystal Heterostructures through Seeded Growth

Type II CdTe-CdSe colloidal nanocrystal heterostructures in a variety of shapes (fat rods, Y-shape rods, and tetrapods) were prepared using a seeded growth method based solely on different sized CdTe seeds. The tailoring of the shape of nanocrystal heterostructures provides a way to modify interfacial charge-transfer and radiative recombination in the near-infrared region. In particular, this advance sets the scene for studying shape-dependent charge-transfer reactions on the nanoscale.

Red emissive CuInS 2 -based nanocrystals: a potential phosphor for warm white light-emitting diodes

We here report the integration of red emissive CuInS(2) based nanocrysals as a potential red phosphor for warm light generation. By combining red emissive CuInS(2) based nanocrysals with commercial yellow emissive YAG:Ce and green emissive Eu(2+) doped silicate phosphors, we fabricated warm white light-emitting diodes with high color rendering index up to ~92, high luminous efficiency of 45~60 lm/W and color temperature less than 4000K.

Controlled synthesis of 3D nanostructured Cd4Cl3(OH)5 templates and their transformation into Cd(OH)2 and CdS nanomaterials

Cd4Cl3(OH)5 is a good precursor compound that can be easily converted into Cd(OH)2 or functional compounds CdS and CdSe. Novel 3D nanostructured Cd4Cl3(OH)5 nanoflower arrays were controllably synthesized through a simple low temperature hydrothermal process. The morphology was characterized by scanning electron microscopy, x-ray diffraction (XRD), transmission electron microscopy (TEM) and selected area electron diffraction (SAED). The cadmium precursor, solution concentration, solvent, reaction temperature and reaction time are of great importance on the flower-like structure. The formation mechanism of the flower-like structure is also discussed. The Cd4Cl3(OH)5 nanoflowers as template compounds were further transformed into Cd(OH)2 and CdS nanoflowers by an anion-exchange reaction.

Ray-trace simulation of CuInS(Se)(2) quantum dot based luminescent solar concentrators

To enhance the performance of luminescent solar concentrator (LSC), there is an increased need to search novel emissive materials with broad absorption and large Stokes shifts. I-III-VI colloidal CuInS2 and CuInSe2 based nanocrystals, which exhibit strong photoluminescence emissions in the visible to near infrared region with large Stokes shifts, are expected to improve performance in luminescent solar concentrator applications. In this work, the performance of CuInS(Se)2 quantum dots in simple planar LSC is evaluated by applying Monte-Carlo ray-trace simulation. A systematic parameters study was conducted to optimize the performance. An optimized photon concentration ratio of 0.34 for CuInS2 nanocrystals and 1.25 for CuInSe2 nanocrystals doping LSC are obtained from the simulation. The results demonstrated that CuInSe2 based nanocrystals are particularly interesting for luminescent solar concentrator applications, especially to combine with low price Si solar cells.

Small GSH-Capped CuInS2 Quantum Dots: MPA-Assisted Aqueous Phase Transfer and Bioimaging Applications

An efficient ligand exchange strategy for aqueous phase transfer of hydrophobic CuInS2/ZnS quantum dots was developed by employing glutathione (GSH) and mercaptopropionic acid (MPA) as the ligands. The whole process takes less than 20 min and can be scaled up to gram amount. The material characterizations show that the final aqueous soluble samples are solely capped with GSH on the surface. Importantly, these GSH-capped CuInS2/ZnS quantum dots have small size (hydrodynamic diameter <10 nm), moderate fluorescent properties (up to 34%) as well as high stability in aqueous solutions (stable for more than three months in 4 °C without any significant fluorescence quenching). Moreover, this ligand exchange strategy is also versatile for the aqueous phase transfer of other hydrophobic quantum dots, for instance, CuInSe2 and CdSe/ZnS quantum dots. We further demonstrated that GSH-capped quantum dots could be suitable fluorescence markers to penetrate cell membrane and image the cells. In addition, the GSH-capped CuInS2 quantum dots also have potential use in other fields such as photocatalysis and quantum dots sensitized solar cells.