Synthesis of Ultrasmall and Magic-Sized CdSe Nanocrystals

Nanocrystals exhibit useful properties not found in their bulk counterparts; however, a subclass of nanocrystals that consist of diameters on the order of 2 nm or less further exhibit unique properties. As synthetic methodologies of nanocrystals have matured, greater emphasis has been made on controlling the early stages of the reaction in order to gain access to these sub-2 nm species. This review provides an overview of ultrasmall and magic-sized nanocrystals, and the diverse chemical means to obtain them. Due to their small size and their resultant properties, these ultrasmall and magic-sized nanocrystals have a distinct advantage in many applications including achieving renal clearance for the purpose of biological imaging, producing simple and high-quality white LEDs, and controlling the growth of nanocrystals to produce various morphologies

Synthesis of Ultrasmall and Magic-Sized CdSe Nanocrystals

Nanocrystals exhibit useful properties not found in their bulk counterparts; however, a subclass of nanocrystals that consist of diameters on the order of 2 nm or less further exhibit unique properties. As synthetic methodologies of nanocrystals have matured, greater emphasis has been made on controlling the early stages of the reaction in order to gain access to these sub-2 nm species. This review provides an overview of ultrasmall and magic-sized nanocrystals, and the diverse chemical means to obtain them. Due to their small size and their resultant properties, these ultrasmall and magic-sized nanocrystals have a distinct advantage in many applications including achieving renal clearance for the purpose of biological imaging, producing simple and high-quality white LEDs, and controlling the growth of nanocrystals to produce various morphologies

Efficient Diffusive Transport of Hot and Cold Excitons in Colloidal Type II CdSe/CdTe Core/Crown Nanoplatelet Heterostructures

Cadmium chalcogenide colloidal quantum wells or nanoplatelets (NPLs), a class of new materials with atomically precise thickness and quantum confinement energy, have shown great potential in optoelectronic applications. Short exciton lifetimes in two-dimensional (2D) NPLs can be improved by the formation of type II heterostructures, whose properties depend critically on the mechanism of exciton transport. Herein, we report a study of room-temperature exciton in-plane transport mechanisms in type-II CdSe/CdTe core/crown (CC) colloidal NPL heterostructures with the same CdSe core and different CdTe crown sizes. Photoluminescence excitation measurements show unity quantum efficiency for transporting excitons created at the crown to the CdSe/CdTe interface (to form lower-energy charge-transfer excitons). At near band edge excitation, the crown-to-core transport time increases with crown size (from 2.7 to 5.6 ps), and this size-dependent transport can be modeled well by 2D diffusion of thermalized excitons in the crown with a diffusion constant of 2.5 ± 0.3 cm²/s (about a factor of 1.6 times smaller than the bulk value). With excitation energy above the band edge, there is an increased contribution of hot exciton transport (up to 7% of the total excitons at 400 nm excitation with diffusion constant that is over twice that of cold excitons). The percentage of hot exciton transport decreases with increasing NPL sizes and decreasing excess excitation photon energy. The observed ultrafast and efficient hot and cold exciton crown-to-core transport suggests their potential applications as light-harvesting and light-emitting materials

Low Threshold Multiexciton Optical Gain in Colloidal CdSe/CdTe Core/Crown Type-II Nanoplatelet Heterostructures

Colloidal cadmium chalcogenide core/crown type-II nanoplatelet heterostructures, such as CdSe/CdTe, are promising materials for lasing and light-emitting applications. Their rational design and improvement requires the understanding of the nature of single- and multiexciton states. Using pump fluence and wavelength-dependent ultrafast transient absorption spectroscopy, we have identified three spatially and energetically distinct excitons (in the order of increasing energy): interface-localized charge transfer exciton (X_CT, with electron in the CdSe core bound to the hole in the CdTe crown), and CdTe crown-localized X_CdTe and CdSe core-localized X_CdSe excitons. These exciton levels can be filled sequentially, with each accommodating two excitons (due to electron spin degeneracy) to generate one to six exciton states (with lifetimes of ≫1000, 209, 43.5, 11.8, 5.8, and 4.5 ps, respectively). The spatial separation of these excitons prolongs the lifetime of multiexciton states. Optical gain was observed in tri- (XX_CTX_CdTe) and four (XX_CTXX_CdTe) exciton states. Because of the large absorption cross section of nanoplatelets, an optical gain threshold as low as ∼43 μJ/cm² can be achieved at 550 nm excitation for a colloidal solution sample. This low gain threshold and the long triexciton (gain) lifetime suggest potential applications of these 2D type-II heterostructures as low threshold lasing materials

Eu³⁺-Doped ZnB₂O₄ (B = Al³⁺, Ga³⁺) Nanospinels: An Efficient Red Phosphor

This paper describes the synthesis of Eu­(III)-doped ZnB₂O₄ (B = Al­(III) or Ga­(III)) nanospinels with Eu­(III) concentrations varying between 1% and 15.6%. The synthesis was achieved through a microwave (MW) synthetic methodology producing 3 nm particles by the thermal decomposition of zinc undecylenate (UND) and a metal 2,4-pentanedionate (B­(acac)₃, B = Al³⁺ or Ga³⁺) in oleylamine (OAm). The nanospinels were then ligand exchanged with the β-diketonate, 2-thenoyltrifluoroacetone (tta). Using tta as a ligand on the surface of the particles resulted in soluble materials that could be embedded in lens mimics, such as poly­(methyl methacrylate) (PMMA). Through a Dexter energy transfer mechanism, tta efficiently sensitized the Eu­(III) doped within the nanospinels, resulting in red phosphors with intrinsic quantum efficiencies (QEs) and QEs in PMMA as high as 50% when excited in the UV. Optical measurements on the out of batch and tta-passivated nanospinels were done to obtain absorption, emission, and lifetime data. The structural properties of the nanospinels were evaluated by ICP-MS, pXRD, TEM, FT-IR, EXAFS, and XANES

Encoding Abrupt and Uniform Dopant Profiles in Vapor–Liquid–Solid Nanowires by Suppressing the Reservoir Effect of the Liquid Catalyst

Semiconductor nanowires (NWs) are often synthesized by the vapor–liquid–solid (VLS) mechanism, a process in which a liquid dropletsupplied with precursors in the vapor phasecatalyzes the growth of a solid, crystalline NW. By changing the supply of precursors, the NW composition can be altered as it grows to create axial heterostructures, which are applicable to a range of technologies. The abruptness of the heterojunction is mediated by the liquid catalyst, which can act as a reservoir of material and impose a lower limit on the junction width. Here, we demonstrate that this “reservoir effect” is not a fundamental limitation and can be suppressed by selection of specific VLS reaction conditions. For Au-catalyzed Si NWs doped with P, we evaluate dopant profiles under a variety of synthetic conditions using a combination of elemental imaging with energy-dispersive X-ray spectroscopy and dopant-dependent wet-chemical etching. We observe a diameter-dependent reservoir effect under most conditions. However, at sufficiently slow NW growth rates (≤250 nm/min) and low reactor pressures (≤40 Torr), the dopant profiles are diameter independent and radially uniform with abrupt, sub-10 nm axial transitions. A kinetic model of NW doping, including the microscopic processes of (1) P incorporation into the liquid catalyst, (2) P evaporation from the catalyst, and (3) P crystallization in the Si NW, quantitatively explains the results and shows that suppression of the reservoir effect can be achieved when P evaporation is much faster than P crystallization. We expect similar reaction conditions can be developed for other NW systems and will facilitate the development of NW-based technologies that require uniform and abrupt heterostructures

Ferroelectric Particles Generated through a Simple, Room-Temperature Treatment of CdSe Quantum Dots

Ferroelectric Particles Generated through a Simple, Room-Temperature Treatment of CdSe Quantum Dot

Ferroelectric Particles Generated through a Simple, Room-Temperature Treatment of CdSe Quantum Dots

Ferroelectric Particles Generated through a Simple, Room-Temperature Treatment of CdSe Quantum Dot

The Possibility and Implications of Dynamic Nanoparticle Surfaces

Understanding the precise nature of a surface or interface is a key component toward optimizing the desired properties and function of a material. For semiconductor nanocrystals, the surface has been shown to modulate fluorescence efficiency, lifetime, and intermittency. The theoretical picture of a nanocrystal surface has included the existence of an undefined mixture of trap states that arise from incomplete passivation. However, our recent scanning transmission electron microscope movies and supporting theoretical evidence suggest that, under excitation, the surface is fluctuating, creating a dynamic population of surface and subsurface states. This possibility challenges our fundamental understanding of the surface and could have far-reaching ramifications for nanoparticle-based technologies. In this Perspective, we discuss the current theories behind the optical properties of nanocrystals in the context of fluxionality

Elimination of Hole–Surface Overlap in Graded CdS_<i>x</i>Se_1–<i>x</i> Nanocrystals Revealed by Ultrafast Fluorescence Upconversion Spectroscopy

Interaction of charge carriers with the surface of semiconductor nanocrystals plays an integral role in determining the ultimate fate of the excited state. The surface contains a dynamic ensemble of trap states that can localize excited charges, preventing radiative recombination and reducing fluorescence quantum yields. Here we report quasi-type-II band alignment in graded alloy CdS_<i>x</i>Se_1–<i>x</i> nanocrystals revealed by femtosecond fluorescence upconversion spectroscopy. Graded alloy CdS_<i>x</i>Se_1–<i>x</i> quantum dots are a compositionally inhomogeneous nano-heterostructure designed to decouple the exciton from the nanocrystal surface. The large valence band offset between the CdSe-rich core and CdS-rich shell separates the excited hole from the surface by confining it to the core of the nanocrystal. The small conduction band offset, however, allows the electron to delocalize throughout the entire nanocrystal and maintain overlap with the surface. Indeed, the ultrafast charge carrier dynamics reveal that the fast 1–3 ps hole-trapping process is fully eliminated with increasing sulfur composition and the decay constant for electron trapping (∼20–25 ps) shows a slight increase. These findings demonstrate progress toward highly efficient nanocrystal fluorophores that are independent of their surface chemistry to ultimately enable their incorporation into a diverse range of applications without experiencing adverse effects arising from dissimilar environments