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.     

Excitonic Chemiluminescence in Si and CdSe Nanocrystals Induced by their Interaction with Ozone

The results of a systematic study of spectral and kinetic patterns of ozone‐adsorption‐induced luminescence (AL) in nanostructured Si and, for the first time, in colloidal CdSe/ZnS quantum dots (QDs) are reported and compared with photoluminescence (PL) of the same structures. The common excitonic nature of light emission under ozone chemisorption and photoexcitation is confirmed by excellent coincidence of AL and PL emission bands. This coincidence is maintained during correlated quenching of AL and PL emission caused by nonradiative defects generated under ozone adsorption. A possible mechanism for energy conversion is proposed in the framework of an exothermic oxidation reaction of core materials caused by ozone. The significant role of the quantum confinement effect differentiates the observed phenomenon from well‐known chemiluminescence in molecular systems. This research establishes a physicochemical basis for the development of a gas sensor with high selectivity to ozone. Also, our findings may be useful for testing core–shell colloidal QDs with ozone.     

Super Sensitization: Grand Charge (Hole/Electron) Separation in ATC Dye Sensitized CdSe,CdSe/ZnS Type‐I,and CdSe/CdTe Type‐II Core–Shell Quantum Dots

Ultrafast charge‐transfer dynamics has been demonstrated in CdSe quantum dots (QD), CdSe/ZnS type‐I core–shell, and CdSe/CdTe type‐II core–shell nanocrystals after sensitizing the QD materials by aurin tricarboxylic acid (ATC), in which CdSe QD and ATC form a charge‐transfer complex. Energy level diagrams suggest that the conduction and valence band of CdSe lies below the LUMO and the HOMO level of ATC, respectively, thus signifying that the photoexcited hole in CdSe can be transferred to ATC and that photoexcited ATC can inject electrons into CdSe QD, which has been confirmed by steady state and time‐resolved luminescence studies and also by femtosecond time‐resolved absorption measurements. The effect of shell materials (for both type‐I and type‐II) on charge‐transfer processes has been demonstrated. Electron injection in all the systems were measured to be <150 fs. However, the hole transfer time varied from 900 fs to 6 ps depending on the type of materials. The hole‐transfer process was found to be most efficient in CdSe QD. On the other hand, it has been found to be facilitated in CdSe/CdTe type‐II and retarded in CdSe/ZnS type‐I core–shell materials. Interestingly, electron injection from photoexcited ATC to both CdSe/CdTe type‐II and CdSe/ZnS type‐I core–shell has been found to be more efficient as compared to pure CdSe QD. Our observation suggests the potential of quantum dot core–shell super sensitizers for developing more efficient quantum dot solar cells.     

Interfacial Electron Transfer from CdSe/ZnS Quantum Dots to TiO2 Nanoparticles: Size Dependence at the Single‐Molecule Level

Electron transfer (ET) kinetics of CdSe/ZnS core/shell quantum dots (QDs) on bare coverslips and a TiO₂ nanoparticle‐coated thin film has been investigated at the single‐molecule level. The QDs prepared have three different diameters of 3.6, 4.6, and 6.4 nm. The trajectories of fluorescence intensity are acquired with respect to the arrival time. The on‐time events and subsequent fluorescence lifetimes are shorter with decreasing size. Given the lifetime measurements for QDs on glass and TiO₂, the rate constant of ET from QDs to TiO₂ may be determined to be 1.3×10⁷, 6.0×10⁶, and 4.7×10⁶ s^?1 for the increasing sizes of the QDs. The plot of on‐time probability density versus arrival time is characterized by power‐law statistics in the short time region and a bending tail in the long time region. Marcus’s ET model is employed to satisfactorily fit the bending tail behavior and to further calculate the ET rate constants. The theoretical counterparts for the different sizes are 1.4×10⁷, 6.4×10⁶, and 1.9×10⁶ s^?1, showing good agreement with the experimental results.     

表面对纳米微粒中稀土离子光谱性质的影响

对于稀土和过渡金属这样的离子中心,由于电子轨道的半径小，纳米尺度的限域对能级位置和跃迁速率的影响并不显著，表面效应成为这类材料与相应体材料光谱性质出现差异的主要因素。把使稀土离子光谱性质产生可测量变化的环境用一个以其为中心、半径为D的作用球表示，可以定义纳米材料厚度为D的表面层。用完整晶格的晶体场和一个补偿电荷的场近似描述表面层中稀土离子的晶体场。本文用这个模型分析了纳米微粒中稀土离子光谱的非均匀宽化，激光选择激发下发射光谱随激发波长的变化，非选择激发下发射光谱随温度的变化以及跃迁分支比的变化等实验现象。     

高质量CdSe量子点的水相制备与表征

以巯基丁二酸为稳定剂, 亚硒酸钠为硒源, 制备了高质量水溶性CdSe量子点. 研究了反应时间、 镉与硒的摩尔比及镉与巯基丁二酸的摩尔比等实验条件对CdSe量子点光谱性能的影响. 分别用紫外-可见光谱、 荧光光谱、 X射线粉末衍射和透射电子显微镜等对量子点进行表征. 结果表明, 采用这种方法制得的CdSe量子点为立方晶型, 量子点的荧光发射峰在518～562 nm范围内连续可调, 并且发射峰的半峰宽始终保持在35 nm左右, 荧光量子产率可达21%.     

Electronic Absorption Spectra and Redox Properties of C Type Cytochromes in Living Microbes

Shewanella is an electrogenic microbe that has significant content of c type cytochromes (ca. 0.5 mM ). This feature allows the optical absorption spectra of the cell‐membrane‐associated proteins to be monitored in vivo in the course of extracellular respiratory electron‐transfer reactions. The results show significant differences to those obtained in vitro with purified proteins.

     

Boron‐Doped Tri(9,10‐anthrylene)s: Synthesis,Structural Characterization,and Optoelectronic Properties

The reaction of 9,10‐dibromo‐9,10‐dihydro‐9,10‐diboraanthracene (9,10‐dibromo‐DBA, 3 ) with two equivalents of 9‐lithio‐2,6‐ or 9‐lithio‐2,7‐di‐tert‐butylanthracene gave the corresponding 9,10‐dianthryl‐DBAs featuring two ( 4 ) or four ( 5 ) inward‐pointing tert‐butyl groups. Compound 4 exists as two atropisomers, 4 and 4′ , due to hindered rotation about the exocyclic B? C bonds. X‐ray crystallography of 5 suggests that the overall interactions between facing tert‐butyl groups are attractive rather than repulsive. Even in solution, 4 / 4′ and 5 are stable toward air and moisture for several hours. Treatment of 3 with 10‐lithio‐9‐R‐2,7‐di‐tert‐butylanthracenes carrying phenyl (R=Ph), dimesitylboryl (R=Mes₂B), or N,N‐di(p‐tolyl)amino (R=Tol₂N) groups gave the corresponding 9,10‐dianthryl‐DBA derivatives 9 – 11 in moderate to good yields. In these molecules, all four solubilizing tert‐butyl groups are outward pointing. The solid‐state structures of 4 , 5 , 9 , and 10 reveal twisted conformations about the exocyclic B? C bonds with dihedral angles of 70–90°. A significant electron‐withdrawing character was proven for the Mes₂B moiety, but no appreciable +M effect was evident for Tol₂N. Compounds 5 , 9 , and 11 show two reversible DBA‐centered reduction waves in the cyclic voltammogram. In the case of 10 , a third reversible redox transition can be assigned to the Mes₂B–anthryl substituents. The UV/Vis absorption spectrum of 5 is characterized by a very broad band at λ_max=510 nm, attributable to a twisted intramolecular charge‐transfer interaction from the anthryl donors to the DBA acceptor. The corresponding emission band shows pronounced positive solvatochromism (λ_em=567 nm, C₆H₁₂; 680 nm, CH₂Cl₂) in line with a highly polar excited state. The charge‐transfer bands of 10 and 11 , as well as the emission bands of 9 and 10 , are redshifted relative to those of 5 . The Tol₂N derivative 11 is essentially nonfluorescent in solution, but emits bright wine‐red light in the solid state.     

Design and evaluation of polymer matrices for the encapsulation of CdSe/ZnS quantum dots in photonic nanocomposite thin films

A systematic approach and a new scheme for the evaluation of the as–is encapsulation of CdSe/ZnS core/shell quantum dots into polymer matrices is proposed, aiming to the implementation of thin film photonic integrated structures. Work focuses on quantum dots capped by hexadecylamine and trioctylphosphine oxide with no ligand exchange or other intermediate processing steps involved. The polymers studied include poly(methyl–methacrylate) (PMMA), polystyrene and acrylic polymers incorporating long alkyl chains, which are expected to promote the compatibility of the quantum dot ligands to that of the polymer chains. In this approach, the variation of photoluminescence properties of the nanocomposite thin films is measured versus increased concentration of the quantum dots, so as to evaluate the suitability of each polymer structure as an efficient host. Furthermore, the refractive index of the quantum dots/polymer nanocomposite thin films are also estimated using white light reflectance spectroscopy data, as appropriate for the integration of photonic devices. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 552–560     

Modulated Luminescence of a Stable Open‐Shell Triarylmethyl Radical: Effects of Chemical Modification on Its Electronic Structure and Physical Properties

The open‐shell luminescent (3,5‐dichloro‐4‐pyridyl)bis(2,4,6‐trichlorophenyl)methyl (PyBTM) radical contains a nitrogen atom that behaves as a stimulus‐responsive site. Chemical modification at this nitrogen atom, such as coordination of B(C₆F₅)₃ or methylation, shifts the emission maximum to the low‐energy region and increases the reduction potential. The emission colour may be regulated by the reversible Lewis acid–base reaction between B(C₆F₅)₃ and PyBTM. Comparison of the optical and electrochemical properties of the radicals with the electronic structures calculated by density functional theory has indicated that the chemical modification decreased the energy level of the β‐singly occupied molecular orbital, a key orbital in determining the optical and electrochemical properties of such systems.     

Role of the Polymer Matrix on the Photoluminescence of Embedded CdSe Quantum Dots

The photoluminescence (PL) of CdSe quantum dots (QDs) that form stable nanocomposites with polymer liquid crystals (LCs) as smectic C hydrogen‐bonded homopolymers from a family of poly[4‐(n‐acryloyloxyalkyloxy)benzoic acids] is reported. The matrix that results from the combination of these units with methoxyphenyl benzoate and cholesterol‐containing units has a cholesteric structure. The exciton PL band of QDs in the smectic matrix is redshifted with respect to QDs in solution, whereas a blueshift is observed with the cholesteric matrix. The PL lifetimes and quantum yield in cholesteric nanocomposites are higher than those in smectic ones. This is interpreted in terms of a higher order of the smectic matrix in comparison to the cholesteric one. CdSe QDs in the ordered smectic matrix demonstrate a splitting of the exciton PL band and an enhancement of the photoinduced differential transmission. These results reveal the effects of the structure of polymer LC matrices on the optical properties of embedded QDs, which offer new possibilities for photonic applications of QD–LC polymer nanocomposites.     

Improving the Photoinduced Charge Separation Parameters in Corrole–Perylene Carboximide Dyads by Tuning the Redox and Spectroscopic Properties of the Components

A couple of corrole–perylene carboximide dyads ( C2‐PIa and C2‐PIx ) have been synthesized and their photoreactivity has been evaluated. We aimed at obtaining better performances for photoinduced charge separation, both in terms of efficiency and in terms of lifetime, with respect to formerly studied systems. The energy level of the charge‐separated state was tuned by selecting perylene and corrole components with diverse redox and spectroscopic properties. High spectroscopic energy levels of the perylene carboximide derivatives (PIs) allow a fast charge separation to be maintained in competition with an energy‐transfer process from the PI to the corrole unit. Yields and lifetimes of charge separation in toluene are, respectively, 75 % and 2.5 μs for C2‐PIa and 65 % and 24 ns for C2‐PIx . The results and the effect of solvent polarity are discussed in the framework of current energy‐ and electron‐transfer theories.     

Biogenic Sulfur-Based Chalcogenide Nanocrystals: Methods of Fabrication,Mechanistic Aspects,and Bio-Applications

Bio-nanotechnology has emerged as an efficient and competitive methodology for the production of added-value nanomaterials (NMs). This review article gathers knowledge gleaned from the literature regarding the biosynthesis of sulfur-based chalcogenide nanoparticles (S-NPs), such as CdS, ZnS and PbS NPs, using various biological resources, namely bacteria, fungi including yeast, algae, plant extracts, single biomolecules, and viruses. In addition, this work sheds light onto the hypothetical mechanistic aspects, and discusses the impact of varying the experimental parameters, such as the employed bio-entity, time, pH, and biomass concentration, on the obtained S-NPs and, consequently, on their properties. Furthermore, various bio-applications of these NMs are described. Finally, key elements regarding the whole process are summed up and some hints are provided to overcome encountered bottlenecks towards the improved and scalable production of biogenic S-NPs.     

Multi-Color Fluorescent Carbon Dots: Graphitized sp2 Conjugated Domains and Surface State Energy Level Co-Modulate Band Gap Rather Than Size Effects

Four types of carbon dots (CDs) with various color (blue, green, yellow, and red) emissions have been synthesized under solvent-free conditions from citric acid and different nitrogen sources (DMF, urea, ethanamide, and formamide). By detailed characterization and comparison, it is confirmed that the graphitized sp² conjugated domain and surface functional groups such as C−O and C=N play synergetic roles in adjusting the fluorescence properties. Notably, the size effect is not the dominant mechanism to achieve multi-color fluorescence emissions in this work. The structural configuration of the carbon dots further influences the energy band structure, as demonstrated in simplified energy level diagrams. An absorption peak at approximately 560 nm appears in the visible light region for red-emitting CDs, assigned to an n→π* transition of the aromatic structure, thus introducing a new surface state energy level, resulting in a reduction in the energy of electron transition and the expansion into the visible region of the UV/Vis spectrum. Taking advantage of the diverse absorption and emission properties, different CDs/TiO₂ binary composites are obtained for photocatalytic degradation of organic dyes, and it is found that the absorption range in terms of visible light and the band gap of the carbon dots make a difference to the photocatalytic performance of the composites.     

Molecular Size and Electronic Structure Combined Effects on the Electrogenerated Chemiluminescence of Sulfurated Pyrene‐Cored Dendrimers

The electrochemistry, photophysics, and electrochemically generated chemiluminescence (ECL) of a family of polysulfurated dendrimers with a pyrene core have been thoroughly investigated and complemented by theoretical calculations. The redox and luminescence properties of dendrimers are dependent on the generation number. From low to higher generation it is both easier to reduce and oxidize them and the emission efficiency increases along the family, with respect to the polysulfurated pyrene core. The analysis of such data evidences that the formation of the singlet excited state by cation–anion annihilation is an energy‐deficient process and, thus, the ECL has been justified through the triplet–triplet annihilation pathway. The study of the dynamics of the ECL emission was achieved both experimentally and theoretically by molecular mechanics and quantum chemical calculations. It has allowed rationalization of a possible mechanism and the experimental dependence of the transient ECL on the dendrimer generation. The theoretically calculated Marcus electron‐transfer rate constant compares very well with that obtained by the finite element simulation of the whole ECL mechanism. This highlights the role played by the thioether dendrons in modulating the redox and photophysical properties, responsible for the occurrence and dynamics of the electron transfer involved in the ECL. Thus, the combination of experimental and computational results allows understanding of the dendrimer size dependence of the ECL transient signal as a result of factors affecting the annihilation electron transfer.     

Synthesis,Photophysical and Electrochemical Properties of Amide‐Linked Phthalocyanine‐Fullerene Dyad

A new amide‐linked phthalocyanine‐fullerene dyad ZnPc‐C₆₀ was synthesized and characterized. The photophysical and electrochemical properties of the ZnPc‐C₆₀ dyad were investigated. The fluorescence spectrum and quantum yield in different solvents showed the occurrence of photoinduced electron transfer (PET) from the singlet excited ZnPc to C₆₀, which was further confirmed by nanosecond transient absorption spectra and cyclic voltammetry data. The free energy change for charge separation (ΔG_CS) was estimated to be exothermic with ?0.51 eV, which favored the formation of charge‐separation state. The PET from ZnPc to C₆₀ in ZnPc‐C₆₀ made the dyad exhibit stronger reverse saturable absorption performance compared with C₆₀ and the control sample in the Z‐scan experiments, which indicated the synergistic effect of two active moieties in the dyad.     

Nitrogen-Doped Carbon Quantum Dots for Biosensing Applications: The Effect of the Thermal Treatments on Electrochemical and Optical Properties

The knowledge of the ways in which post-synthesis treatments may influence the properties of carbon quantum dots (CDs) is of paramount importance for their employment in biosensors. It enables the definition of the mechanism of sensing, which is essential for the application of the suited design strategy of the device. In the present work, we studied the ways in which post-synthesis thermal treatments influence the optical and electrochemical properties of Nitrogen-doped CDs (N-CDs). Blue-emitting, N-CDs for application in biosensors were synthesized through the hydrothermal route, starting from citric acid and urea as bio-synthesizable and low-cost precursors. The CDs samples were thermally post-treated and then characterized through a combination of spectroscopic, structural, and electrochemical techniques. We observed that the post-synthesis thermal treatments show an oxidative effect on CDs graphitic N-atoms. They cause their partially oxidation with the formation of mixed valence state systems, [CDs]⁰⁺, which could be further oxidized into the graphitic N-oxide forms. We also observed that thermal treatments cause the decomposition of the CDs external ammonium ions into ammonia and protons, which protonate their pyridinic N-atoms. Photoluminescence (PL) emission is quenched.