Three-Electron Dynamics of the Interparticle Coulombic Decay in Doubly Excited Clusters with One-Dimensional Continuum Confinement

A detailed analysis of the electronic structure and decay dynamics in a symmetric system with three electrons in three linearly aligned binding sites representing quantum dots (QDs) is given. The two outer A QDs are two-level potentials and can act as (virtual) photon emitters, whereas the central B QD can be ionized from its one level into a continuum confined on the QD axis upon absorbing virtual photons in the inter-Coulombic decay (ICD) process. Two scenarios in such an ABA array are explored. One ICD process is from a singly excited resonance state, whose decay releasing one virtual photon we find superimposed with resonance energy transfer among both A QDs. Moreover, the decay-process manifold for a doubly excited (DE) resonance is explored, in which collective ICD among all three sites and excited ICD among the outer QDs engage. Rates for all processes are found to be extremely low, although ICD rates with two neighbors are predicted to double compared to ICD among two sites only. The slowing is caused by Coulomb barriers imposed from ground or excited state electrons in the A sites. Outliers occur on the one hand at short distances, where the charge transfer among QDs mixes the possible decay pathways. On the other hand, we discovered a shape resonance-enhanced DE-ICD pathway, in which an excited and localized B* shape resonance state forms, which is able to decay quickly into the final ICD continuum.     

Size Effect of Graphene Quantum Dots on Photoluminescence

High-photoluminescence (PL) graphene quantum dots (GQDs) were synthesized by a simple one-pot hydrothermal process, then separated by dialysis bags of different molecular weights. Four separated GQDs of varying sizes were obtained and displayed different PL intensities. With the decreasing size of separated GQDs, the intensity of the emission peak becomes much stronger. Finally, the GQDs of the smallest size revealed the most energetic PL intensity in four separated GQDs. The PL energy of all the separated GQDs shifted slightly, supported by density functional theory calculations.     

Study and Comparison on Purification Methods of Multicolor Emission Carbon Dots

Recent years have witnessed a rapid development of carbon dots (CDs), due to their outstanding luminescence properties and excellent biocompatibility. However, the internal structure and photoluminescent (PL) mechanism of CDs are still the subject of considerable debate, which is due to the fact that reaction products usually contain mixtures of several CD fractions as well as molecular intermediate and side products. Therefore, careful purification of the CDs is significant for analysis of structure and luminescence mechanism. Here, multicolor emission CDs were prepared by a one-pot pyrolysis of citric acid in formamide. Then, the precipitation method, dialysis and gel permeation chromatography (GPC) are successively employed to purify the multicolor emission CDs. This post-treatment allowed us to compare the optical properties of CDs obtained by different separation methods and provide a valuable guidance for the purification of CDs.     

High-Efficiency FRET Processes in BODIPY-Functionalized Quantum Dot Architectures

Efficient FRET systems are developed combining colloidal CdSe quantum dots (QDs) donors and BODIPY acceptors. To promote effective energy transfer in FRET architectures, the distance between the organic fluorophore and the QDs needs to be optimized by a careful system engineering. In this context, BODIPY dyes bearing amino-terminated functionalities are used in virtue of the high affinity of amine groups in coordinating the QD surface. A preliminary QD surface treatment with a short amine ligand is performed to favor the interaction with the organic fluorophores in solution. The successful coordination of the dye to the QD surface, accomplishing a short donor–acceptor distance, provides effective energy transfer already in solution, with efficiency of 76 %. The efficiency further increases in the solid state where the QDs and the dye are deposited as single coordinated units from solution, with a distance between the fluorophores down to 2.2 nm, demonstrating the effectiveness of the coupling strategy.     

Long-Range Transport in an Assembly of ZnO Quantum Dots: The Effects of Quantum Confinement,Coulomb Repulsion and Structural Disorder

We have studied the storage and long-range transport of electrons in a porous assembly of weakly coupled ZnO quantum dots permeated with an aqueous and a propylene carbonate electrolyte solution. The number of electrons per ZnO quantum dot is controlled by the electrochemical potential of the assembly; the charge of the electrons is compensated by ions present in the pores. We show with optical and electrical measurements that the injected electrons occupy the S, P, and D type conduction electron levels of the quantum dots; electron storage in surface states is not important. With this method of three-dimensional charge compensation, up to ten electrons per quantum-dot can be stored if the assembly is permeated with an aqueous electrolyte. The screening of the electron charge is less effective in the case of an assembly permeated with a propylene carbonate electrolyte solution. Long-range electron transport is studied with a transistor set-up. In the case of ZnO assemblies permeated with an aqueous electrolyte, two quantum regimes are observed corresponding to multiple tunnelling between the S orbitals (at a low occupation) and P orbitals (at a higher occupation). In a ZnO quantum-dot assembly permeated with a propylene carbonate electrolyte solution, there is a strong overlap between these two regimes.     

Nonadiabatic relaxation dynamics of water anion cluster and its isotope effects by ring‐polymer molecular dynamics simulation

We have applied a recently developed hybrid quantum ring‐polymer molecular dynamics method to the nonadiabatic p → s relaxation dynamics in water anion clusters to understand the isotope effects observed in previous experiments. The average relaxation times for (H₂O)₅₀^? and (D₂O)₅₀^? were calculated at 120 and 207 fs, respectively, and are comparable to the experimental results. Therefore, we conclude that nuclear quantum effects play an essential role in understanding the observed isotope effects for water anion cluster nonadiabatic dynamics. The nonadiabatic relaxation mechanisms are also discussed in detail. © 2014 Wiley Periodicals, Inc.     

Side chain mediated electronic contact between a tetrahydro-4H-thiopyran-4-ylidene-appended polythiophene and CdTe quantum dots

The properties of a mixed CdTe quantum dot/tetrahydro-4H-thiopyran-4-ylidene-functionalized polythiophene system are reported. This system was prepared by exposing trioctylphosphine (TOP)-capped CdTe quantum dots to the polythiophene in solution. Strong fluorescence emission quenching and shortening of the fluorescence emission lifetimes of both the polythiophene and the quantum dots occur when they are mixed, indicating the occurrence of photoinduced charge separation. Photoinduced absorption spectroscopy reveals a considerable decrease in the population of the polythiophene triplet excited state in the mixed system. These results demonstrate that between the quantum dots and the polythiophene there is both physical and electronic contact, which is mediated by the tetrahydro-4H-thiopyran-4-ylidene side chains.     

不同环境因子作用下水溶性CdTe量子点的生长动力学研究

以硫普罗宁为稳定剂,水热法制备了水溶性CdTe量子点,系统研究了回流时间、反应物配比、pH值、反应温度和电解质种类等环境因子对量子点生长动力学及光物理性质的影响。结果表明,溶液的pH值及反应物配比对CdTe量子点的光物理性质均有重要影响。优化条件后,回流5 h可得到发射峰位于550 nm的CdTe量子点,其荧光量子效率高达52%;在氯化钠作用下,量子点生长加快,即高浓度氯化钠会减弱溶液中粒子间静电排斥,促进离子扩散,有利于量子点的生长;添加苯磺酸钠会抑制量子点的生长,有利于制备高荧光量子效率的小尺寸CdTe量子点。     

Small-size ZnO/MgO quantum systems with zinc oxide nanoparticles of controlled size

A method is proposed for the preparation of nanosized ZnO/MgO systems by the deposition of previously formed zinc oxide nanoparticles onto a support from a colloidal solution and for determination of the size of the deposited ZnO particles from the electronic spectra. The morphology of composites containing zinc oxide nanoparticles was investigated by electronic spectroscopy and transmission electron microscopy. The possibility of synthesizing solid-phase ZnO/MgO quantum systems with controllable size characteristics was demonstrated. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 43, No. 3, pp. 183–188, May–June, 2007.     

Structural and Size Effects on the Spectroscopic and Redox Properties of CdSe Nanocrystals in Solution: The Role of Defect States

Two series of CdSe quantum dots (QDs) with different diameters are prepared, according to frequently used protocols of the same synthetic procedure. For each sample the photophysical properties and the potentials for the first reduction and oxidation processes in organic solution are determined. The band gap obtained from electrochemical experiments is compared with that determined from the absorption and luminescence spectra. While the optical band gap decreases upon increasing the nanocrystal diameter, as expected on the basis of quantum confinement, the redox potentials and the electrochemical band gap are not monotonously related to the QD size. For both series, the smallest and largest QDs are both easier to oxidize and reduce than mid‐sized QDs. In fact, the latter samples exhibit very broad voltammetric profiles, which suggests that the heterogeneous electron‐transfer processes from/to the electrode are kinetically hindered. Conversely, the electrochemical band gap for the smallest and largest particles of each series is somewhat smaller than the optical band gap. These results indicate that, while the optical band gap depends on the actual electron–hole recombination within the nanocrystal, and therefore follows the size dependence expected from the particle‐in‐a‐box model, the electrochemical processes of these QDs are strongly affected by other factors, such as the presence of surface defects. The investigations suggest that the influence of these defects on the potential values is more important for the smallest and largest QDs of each series, as confirmed by the respective luminescence bands and quantum yields. An interpretation for the size‐dependent evolution of the surface defects in these nanocrystals is proposed based on the mechanism of their formation and growth.     

Electrochemical determination of mesoscopic phenomena, defect states in CdSe nanocrystals and charge carrier manipulability

The review gives an overview of cyclic voltammetry measurements performed on CdSe nanocrystals (so called Quantum Dots). Data of relevant publications is gathered and newly interpreted to give complete insights in charge transfer processes at QD surfaces. In specific, it is focused on the size-dependent electronic properties (Quantum-Size-Effect), the characterisation of surface defects, and the characterisation of hybrid nanocrystal/conducting polymer composites. First two authors contribute equally to this work Correspondence: Thomas Nann, School of Chemical Sciences and Pharmacy, University of East Anglia (UEA), Norwich NR4 7TJ, U.K.     

Carbon Dots for Carbon Dummies: The Quantum and The Molecular Questions Among Some Others

Carbon Dots (CDs) are carbon nanoparticles which were discovered in 2004. Despite two decades of intensive work from the scientific community and a colossal amount of gathered experimental data, no definitive consensus exists to date on several key aspects such as the actual definition of CDs and the origin of their emissive properties. This review proposes a critical evaluation of these fundamental questions. Lay persons will also find here an alternative introduction to the CDs domain, including synthetic strategies, photophysical properties, as well as challenges and outlook of this exciting new area.     

量子化尺寸纳米颗粒及其在生物体系中的作用

本文从胶体化学与物理化学的角度介绍了纳米颗粒的尺寸量子化效应。介绍了作者所进行的纳米颗粒在生物体系中的应用工作。纳米颗粒金和二氧化硅能显著地提高葡萄糖氧化酶的生物活性。纳米颗粒金能提高视黄醛仿生膜的光电响应电流和寿命。作者用纳米颗粒的吸附浓集效应、吸附定向效应、和量子尺寸效应来解释这些效应     

CdX(X=Se,Te,Te/ZnS)量子点脂质体的制备及毒性比较

在水相中以巯基乙酸(mercaptoacetic acid, MA)为稳定剂合成了CdSe、CdTe、CdTe/ZnS量子点及谷胱甘肽(glutathione, GSH)为稳定剂合成了CdTe量子点,然后通过卵磷脂和胆固醇修饰制得相应的量子点脂质体。溶血实验证实GSH修饰量子点的溶血率低于MA修饰的量子点45%;脂质体修饰后,量子点的溶血率＜5%,达到生物医用材料要求。不同表面修饰的量子点对小鼠毒性存在明显差异,荧光显微镜观察组织切片证实量子点在小鼠体内主要分布在肺、肾、胸腺等组织中,而脂质体量子点在脑组