Passivated Nanocrystals Research Articles

Interpretation of the photoluminescence decay kinetics in metal halide perovskite nanocrystals and thin polycrystalline films

In this paper we present critical analysis of different points of view on interpretation of the photoluminescence (PL) decay kinetics in lead halide perovskites prepared in the form of well passivated nanocrystals (PNCs) or thin polycrystalline layers. In addition to the literature data, our own measurements are also considered. For PNCs, a strong dependence of the PL lifetimes on the type of passivating ligand was observed with a consistently high PL quantum yield. It is shown that such ligand effects, as well as a decrease in the PL lifetime with decreasing temperature, are well qualitatively explained by the phenomenological model of thermally activated delayed luminescence, in which the extension of the PL decay time with temperature occurs due to the participation of shallow non-quenching traps. In the case of thin perovskite layers, we conclude that the PL kinetics under sufficiently low excitation intensity is determined by the excitation quenching on the layer surfaces. We demonstrate that a large variety of possible PL decay kinetics for thin polycrystalline perovskite films can be modelled by means of one-dimensional diffusion equation with use of the diffusion coefficient D and surface recombination velocity S as parameters and conclude that long-lived PL kinetics are formed in case of low D and/or S values.

Efficient and Spectrally Stable Blue Perovskite Light‐Emitting Diodes Based on Potassium Passivated Nanocrystals

AbstractMetal halide perovskites have aroused tremendous interest in the past several years for their promising applications in display and lighting. However, the development of blue perovskite light‐emitting diodes (PeLEDs) still lags far behind that of their green and red cousins due to the difficulty in obtaining high‐quality blue perovskite emissive layers. In this study, a simple approach is conceived to improve the emission and electrical properties of blue perovskites. By introducing an alkali metal ion to occupy some sites of peripheral suspended organic ligands, the nonradiative recombination is suppressed, and, consequently, blue CsPb(Br/Cl)3 nanocrystals with a high photoluminescence quantum efficiency of 38.4% are obtained. The introduced K+ acts as a new type of metal ligand, which not only suppresses nonradiative pathways but also improves the charge carrier transport of the perovskite nanocrystals. With further engineering of the device structure to balance the charge injection rate, a spectrally stable and efficient blue PeLED with a maximum external quantum efficiency of 1.96% at the emission peak of 477 nm is fabricated.

Surface passivation effects on the electronic and optical properties of 3C-SiC nanocrystals

Using time-dependent density functional theory the optical absorption spectrum of SiC nanocrystals was studied with changes in their surface structures as they are passivated with some functional groups such as amine (–NH2), amide (–CONH2), hydroxyl (–OH), carboxylic acid (–CO2H) and thiol (–SH) groups. The variations of some quantities such as formation energy, optical gap, single particle energy gap and lowest exciton binding energy of SiC nanocrystals are discussed with changes in their surface structure. Results indicate that in all cases the silicon dangling bonds on the surface of SiC nanocrystals are favorable for constructing stable structures with attached functional groups as in some cases their stability is more than pure hydrogenated SiC nanocrystal as an ideal structure. Also the functional groups attached to the surface of SiC nanocrystals change the optical properties of the SiC nanocrystals as the changes are remarkable for amide and thiol passivated nanocrystals.

(Invited) Nanocrystalline Materials: Optimization of Thin Film Properties

The approach of superlattices to enable the size control of different nanocrystalline materials was investigated. In case of size controlled Si nanocrystals synthesized by SiO/SiO2 superlattices this approach enables a precise bandgap engineering of well passivated nanocrystals. Which results in a tunable luminescence signal, a resonant energy transfer to Er3+ ions and the confirmation of the quantum confined origin of the luminescene and the prove of optical amplification. In case of Zr based dielectrics the size control by the superlattice approach enables the stabilization of the tetragonal phase, which is favorable for high capacitance applications. For HfO2 an additional doping of the films, e.g. by Si atoms for the phase stabilization is necessary. Doping or interlayer additionally improves the electrical performance of the dielectrics.

Optical study of CdS- and ZnS-passivated CdSe nanocrystals in gelatin films

CdSe nanocrystals were synthesized in aqueous solutions using gelatin as a stabilizer. Anappreciable improvement of photoluminescence efficiency of nanocrystal-containingpolymeric films was achieved by passivation of as-formed nanocrystals in solutionswith CdS and ZnS. The passivation-induced variation of the photoluminescenceefficiency and its maximum position was found to be dependent on the totalvolume of the passivating material and the sequence of the reagents involved. Thephotoluminescence of both unpassivated and passivated nanocrystals was noticeably(∼200 meV) red-shifted from the first absorption maxima. A possible origin of the emissionobserved is discussed.

Folate-Receptor-Mediated Delivery of InP Quantum Dots for Bioimaging Using Confocal and Two-Photon Microscopy

A novel method for the synthesis of highly monodispersed hydrophillic InP-ZnS nanocrystals and their use as luminescence probes for live cell imaging is reported. Hydrophobic InP-ZnS nanocrystals are prepared by a new method that yields high-quality, luminescent core-shell nanocrystals within 6-8 h of total reaction time. Then by carefully manipulating the surface of these passivated nanocrystals, aqueous dispersions of folate-conjugated nanocrystals (folate-QDs) with high photostability are prepared. By use of confocal microscopy, we demonstrate the receptor-mediated delivery of folic acid conjugated quantum dots into folate-receptor-positive cell lines such as KB cells. These folate-QDs tend to accumulate in multi-vescicular bodies of KB cells after 6 h of incubation. Receptor-mediated delivery was confirmed by comparison with the uptake of these particles in folate-receptor-negative cell lines such as A549. Efficient two-photon excitation of these particles and two-photon imaging using these particles are also demonstrated. The use of these InP-ZnS nanoparticles and their efficient two-photon excitation can be potentially useful for deep tissue imaging for future in vivo studies.

Drying-mediated self-assembly of nanoparticles.

Systems far from equilibrium can exhibit complex transitory structures, even when equilibrium fluctuations are mundane. A dramatic example of this phenomenon has recently been demonstrated for thin-film solutions of passivated nanocrystals during the irreversible evaporation of the solvent. The relatively weak attractions between nanocrystals, which are efficiently screened in solution, become manifest as the solvent evaporates, initiating assembly of intricate, slowly evolving structures. Although certain aspects of this aggregation process can be explained using thermodynamic arguments alone, it is in principle a non-equilibrium process. A representation of this process as arising from the phase separation between a dense nanocrystal 'liquid' and dilute nanocrystal 'vapour' captures some of the behaviour observed in experiments, but neglects entirely the role of solvent fluctuations, which can be considerable on the nanometre length scale. Here we present a coarse-grained model of nanoparticle self-assembly that explicitly includes the dynamics of the evaporating solvent. Simulations using this model not only account for all observed spatial and temporal patterns, but also predict network structures that have yet to be explored. Two distinct mechanisms of ordering emerge, corresponding to the homogeneous and heterogeneous limits of evaporation dynamics. Our calculations show how different choices of solvent, nanoparticle size (and identity) and thermodynamic state give rise to the various morphologies of the final structures. The resulting guide for designing statistically patterned arrays of nanoparticles suggests the possibility of fabricating spontaneously organized nanoscale devices.

Integral Equation Theory for the Interactions between Passivated Nanocrystals in Supercritical Fluids: Solvophobic and Solvophilic Cases

A study of the interactions between nanocrystals in supercritical fluids for the solvophobic and solvophilic cases is presented. The model for the nanocrystal consists of a highly polarizable core and a passivation layer, both of which are uniformly composed of Lennard-Jones particles. A nonlocal integral equation theory is applied to study the excess and total potential of mean force between the nanocrystals. The effects of solvent density, and thickness and density of passivation layer, on the total potential of mean force and on solubility of nanocrystals in solutions is discussed. Important differences observed between solvophobic and solvophilic cases are analyzed in terms of the local solvent density profile around the nanocrystals. Interesting characteristic features, such as the disappearance of anomalous behavior previously reported by us for nonpassivated attractive solutes, are predicted. Certain qualitative feature observed in experimental studies of nanocrystal interactions in supercritical s...

Self-Assembled Cobalt Oxide Nanocrystals with Tetrahedral Shape

Abstract Nanocrystal materials are an emerging research field of chemistry, physics and materials science. The size and shape specificity of nanocrystals suggests them as building blocks for constructing selfassembly passivated nanocrystals superlattices (NCS's) or nanocrystals arrays (NCA) [1-6]. In this paper, NCAs of CoO with controlled tetrahedral shape are reported and their structural stability is examined by in-situ TEM. Cobalt oxide nanocrystals were synthesized by chemical decomposition of Co2(CO)8 in toluene under oxygen atmosphere, as given in detail elsewhere [1].Sodium bis(2-ethylhexyl) sulfosuccinate (Na(AOT)) was added as a surface active agent, forming an ordered monolayer passivation (called the thiolate) over the nanocrystal surface. The particle size was controlled by adjusting the wt.% ratio between the precursor and Na(AOT). The as-prepared solution contained Co, CoO and possibly C03O4 nanoparticles, and pure CoO nanoparticles were separated by applying a small magnetic field, which is generated by a horseshoe permanent magnet in vertical direction.

In Situ Structural Evolution of Self-Assembled Oxide Nanocrystals

Self-assembling of organic passivated nanocrystals has attracted a lot of interest recently. In this paper, the structural evolution of tetrahedral CoO nanocrystals is studied in situ using transmission electron microscopy. The as-prepared superlattices are Na(AOT)-passivated CoO nanocrystals, packed into monolayer and multilayer arrays on an amorphous carbon film. As the specimen temperature increased from 170 to 200 °C, the passivation layer is gradually evaporated/decomposed. For temperatures higher than 200 °C, the CoO nanocrystals experience a solid-state reaction, while the monolayer packing configuration is still preserved, but not the multilayers. The replacement reaction finishes at ∼600 °C, and the final product is identified as mixed carbide nanocrystals of Co2C and Co3C.

Cluster beams from passivated nanocrystals

Stable, intense beams of large metal clusters (from 102 to 103 Au or Ag atoms) can be produced by laser desorption of molecular films of passivated nanocrystals that have undergone fractionation by size and a separate structural characterization. The mass onset of the desorbed species corresponds directly to the dimensions of the nanocrystal core, indicating that only the surfactant shell is lost during desorption. Photofragmentation, photoionization, and photodetachment from beams of large metal clusters, generated in this way, have all been observed.