Constraints in post-synthesis ligand exchange for hybrid organic (MEH-PPV)–inorganic (CdSe) nanocomposites

For an optimum charge/energy transfer performance of hybrid organic–inorganic colloidal nanocrystals for applications such as photonic devices and solar cells, the determining factors are the distance between the nanocrystal and polymer which greatly depends upon nanocrystal size/nanocrystal ligands. Short chain ligands are preferred to ensure a close contact between the donor and acceptor as a result of the tunnelling probability of the charges and the insulating nature of long alkyl chain molecules. Short distances increase the probability for tunnelling to occur as compared to long distances induced by long alkyl chains of bulky ligands which inhibit tunnelling altogether. The ligands on the as-synthesized nanocrystals can be exchanged for various other ligands to achieve desirable charge/energy transfer properties depending on the bond strength of the ligand on the nanocrystal compared to the replacement ligand. In this work, the constraints involved in post-synthesis ligand exchange process have been evaluated, and these factors have been tuned via wet chemistry to tailor the hybrid material properties via appropriate selection of the nanocrystal capping ligands. It has been found that both oleic acid and oleylamine (OLA)-capped cadmium selenide (CdSe) quantum dots (QDs) as compared with trioctylphosphine oxide (TOPO)-passivated CdSe QDs are of high quality, and they provide better steric stability against coagulation, homogeneity, and photostability to their respective polymer:CdSe nanocomposites. CdSe QDs particularly with OLA capping have relatively smaller surface energies, and thus, lesser quenching capabilities show dominance of photoinduced Forster energy transfer between donors (polymer) and acceptors (CdSe nanocrystals) as compared to charge transfer mechanism as observed in polymer:CdSe (TOPO) composites. It is conjectured that size quantization effects, stereochemical compatibility of ligands (TOPO, oleic acid, and oleyl amine), and polymer MEH-PPV stability greatly influence the photophysics and photochemistry of hybrid polymer–semiconductor nanocomposites.     

Charge transfer efficiency in hybrid bulk heterojunction composites

On the basis of recently published electrochemical measurements, the charge transfer efficiency within CdSe nanocrystal/conducting polymer heterojunction composites was investigated by means of luminescence interaction strength. It was found that poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene] and poly-9-vinylcarbazole luminescence was not totally quenched by nanocrystals, whereas poly-3-octylthiophene and polyvinylpyrrolidone was completely quenched. In case of poly-3-hexylthiophene, the nanocrystal luminescence was quenched. The results are in complete agreement with the electrochemical findings and thus, the CdSe nanocrystal/Polyvinylpyrrolidone composite should be a promising material for electroluminescent devices.     

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.     

Visible light excitation of CdSe nanocrystals triggers the release of coumarin from cinnamate surface ligands

The photochemical properties of organic ligands on the surface of nanocrystalline CdSe particles were examined. A number of thiols carrying a substituted cinnamate tail was synthesized. In solution, these cinnamate compounds undergo light-induced (374 nm) E-Z isomerization, followed by a nonphotolytic lactonization to give highly fluorescent coumarin. The cinnamate-thiols were successfully exchanged onto the CdSe nanocrystal, and the photochemical behavior of these conjugates was studied. Upon aerobic photolysis at 374 nm, the surface cinnamates released coumarin accompanied by rapid nanocrystal degradation. This degradation was not observed under similar anaerobic conditions or when the organic ligands did not contain the cinnamate group. Surprisingly, very similar results were obtained upon irradiation at visible wavelengths at which the cinnamate has no absorption. With the aid of UV-visible absorption spectroscopy, fluorescence spectroscopy, and electrochemistry, a unified theory for both the increased photoinstability of the nanocrystal as well as the coumarin release was proposed. It involves cinnamate radical anions on the CdSe surface, formed upon electron transfer from the excited nanocrystal to the surface cinnamate, undergoing E-Z isomerization. Practically, this results in the remarkable ability to release coumarin from nanocrystal ligands simply by exciting the nanocrystal with visible light. This new photorelease protocol not only aids in the understanding of fundamental nanocrystal-ligand interactions but may also offer new opportunities in the areas of drug delivery and imaging.     

Colloidal CdSe quantum dot electroluminescence: ligands and light-emitting diodes

We examine the effects of surface ligand exchange on the performance of hybrid organic/inorganic light emitting diodes (LEDs) that use colloidal nanocrystal quantum dots as emissive centers. Using a series of primary alkylamines with different alkane chain lengths, we exchange the native surface ligands on a series of CdSe/CdZnS/ZnS core/shell/shell nanocrystal quantum dots and compare the differences in photoluminescence and electroluminescence efficiency of the emissive quantum dot layer. We fabricate LEDs made with octadecylamine-, octylamine-, and butylamine-exchanged quantum dots. We find that the differences in electroluminescence efficiency of the devices are not always proportional to the photoluminescence quantum efficiency of the quantum dots. We discuss this trend both in terms of the competing needs of high photoluminescence efficiency and good charge injection and energy transfer, and also in terms of the different processability and film morphology arising from the use of nanoparticles passivated with shorter ligands. Correspondence: David S. Ginger, Department of Chemistry, University of Washington, Seattle, Washington 98195-1700, USA     

Organic-inorganic nanocomposites via directly grafting conjugated polymers onto quantum dots

Nanocomposites of poly(3-hexylthiophene)-cadmium selenide (P3HT-CdSe) were synthesized by directly grafting vinyl-terminated P3HT onto [(4-bromophenyl)methyl]dioctylphosphine oxide (DOPO-Br)-functionalized CdSe quantum dot (QD) surfaces via a mild palladium-catalyzed Heck coupling, thereby dispensing with the need for ligand exchange chemistry. The resulting P3HT-CdSe nanocomposites possess a well-defined interface, thus significantly promoting the dispersion of CdSe within the P3HT matrix and facilitating the electronic interaction between these two components. The photophysical properties of nanocomposites were found to differ from the conventional composites in which P3HT and CdSe QDs were physically mixed. Solid-state emission spectra of nanocomposites suggested the charge transfer from P3HT to CdSe QDs, while the energy transfer from 3.5 nm CdSe QD to P3HT was implicated in the P3HT/CdSe composites. A faster decay in lifetime further confirmed the occurrence of charge transfer in P3HT-CdSe nanocomposites.     

Engineering Sensitized Photon Upconversion Efficiency via Nanocrystal Wavefunction and Molecular Geometry

Triplet energy transfer from inorganic nanocrystals to molecular acceptors has attracted strong attention for high‐efficiency photon upconversion. Here we study this problem using CsPbBr₃ and CdSe nanocrystals as triplet donors and carboxylated anthracene isomers as acceptors. We find that the position of the carboxyl anchoring group on the molecule dictates the donor‐acceptor coupling to be either through‐bond or through‐space, while the relative strength of the two coupling pathways is controlled by the wavefunction leakage of nanocrystals that can be quantitatively tuned by nanocrystal sizes or shell thicknesses. By simultaneously engineering molecular geometry and nanocrystal wavefunction, energy transfer and photon upconversion efficiencies of a nanocrystal/molecule system can be improved by orders of magnitude.     

Semiconductor nanocrystals with multifunctional polymer ligands

In this letter, we describe the preparation of a versatile polymer ligand, which can be attached to CdSe/ZnS semiconductor nanocrystals via a phase transfer reaction. The ligand is based on a chain of reactive esters, which can, in principle, be substituted by any compound containing amino-functionalities. The polymer/nanocrystal complexes are characterized in terms of structure and photostability.     

Comparative study on coating CdSe nanocrystals with surfactants

We have synthesized CdSe nanocrystals (NCs) in sizes from 2.2 to 5.1 nm passivated with hydrophobic trioctylphosphine oxide (TOPO) in combination trioctylphosphine (TOP) or tributylphosphine (TBP) to obtain particles of the type CdSe/TOPO/TOP or CdSe/TOPO/TBP. These NCs were then dispersed in aqueous solution of ionic or non-ionic surfactants (such as stearate, oleic acid, Tween) using a biphase (water and chloroform or hexane) transfer method. It is found that both the structure of the surfactant and the native surface of the ligand govern the coating of the NCs with surfactants. More specifically, the hydrophobicity-hydrophilicity balance of the surfactant regulates the coating efficacy, thereby transferring the NC from the organic to the aqueous phase. The type of ligand on the NCs and the kind of coating surfactant also affect photoluminescence (PL). The ratio of PL and absorbance unit (defined as PL per 0.1 AU) was implemented as a tool to monitor changes in PL intensity and wavelength as a function of size, coatings and surface defects. Finally, the distribution of CdSe nanocrystals between pseudophases in cloud point extraction was discussed based on experimental results. It was concluded that the size of CdSe nanocrystal present in an appropriate pseudophase is correlated with the way in which the non-ionic surfactant coats CdSe nanocrystals.

Conjugated Polymer Composite Nanoparticles by Rapid Mixing

Composite nanoparticles from poly[(9,9‐di‐n‐octylfluoren‐2,7‐diyl)‐alt‐(benzo[2,1,3]thiadiazol‐4,8‐diyl)] (F8BT) and poly(9,9‐di‐n‐hexylfluoren‐2,7‐diyl) (PF) with embedded inorganic nanoparticles (TiO₂, CdSe, and CdSe/CdS) are prepared through kinetic trapping by rapid turbulent mixing in a multi‐inlet vortex mixer without the need for polymer functionalization. High contents of inorganic materials up to 50–60 wt% are realized for all composites. The influence of flow ratios, sodium dodecyl sulfate (SDS) concentration, and absolute flow rates on the particle size and morphology is studied. High water‐to‐THF ratios and high total flow rates around 2 m s⁻¹ yield particle sizes below 50 nm. By adjusting these parameters, controlled particle sizes between 30 to several hundred nanometers are obtained. Composite particles from CdSe/CdS and F8BT or PF show a strong quenching of the polymer emission and near exclusive emission from the inorganic nanocrystal, which indicates an efficient energy transfer with fluorescence quantum yields of 23% for the F8BT/CdSe/CdS composites and 21% for the PF/CdSe/CdS composites. The dispersions are colloidally stable for several months.

     

Temperature- and field-dependent energy transfer in CdSe nanocrystal aggregates studied by magneto-photoluminescence spectroscopy

The influence of temperature and applied magnetic fields on photoluminescence (PL) emission and electronic energy transfer (ET) of both isolated and aggregated CdSe nanocrystals was investigated. Following 400-nm excitation, temperature-dependent, intensity-integrated and energy-resolved PL measurements were used to quantify the emission wavelength and amplitude of isolated CdSe nanocrystals. The results indicated an approximately three-fold increase in PL intensity upon decreasing the temperature from 300 K to 6 K; this was attributed to a reduction of charge carrier access to nanocrystal surface trap states and suppression of thermal loss channels. Temperature-dependent PL measurements of aggregated CdSe nanocrystals, which included both energy-donating and -accepting particles, were analyzed using a modified version of F?rster theory. Temperature-dependent ET efficiency increased from 0.55 to 0.75 upon decreasing the sample temperature from 225 K to 6 K, and the ET data contained the same trend observed for the PL of isolated nanoclusters. The application of magnetic fields to increase nanocrystal ET efficiency was studied using magneto-photoluminescence measurements recorded at a sample temperature of 1.6 K. We demonstrated that the exciton fine structure population of the donor was varied using applied magnetic fields, which in turn dictated the PL yield and the resultant ET efficiency of the CdSe nanocrystal aggregate system. The experimental data indicated an ET efficiency enhancement of approximately 7%, which was limited by the random orientation of the spherical nanocrystals in the thin film.     

Hybrid organic-inorganic nanomaterials based on polythiophene dendronized nanoparticles

In this work, the synthesis, characterization, and applications of branched oligothiophene dendrons that act as electroactive surfactants for the capping of Au metal nanoparticles and CdSe quantum dots are described. Two distinct methods have been employed for synthesis: a ligand exchange process and a direct-capping synthesis approach. The coverage of the dendrons per nanocrystal, the nature of the surface coordination interactions, and energy transfer interactions were studied in detail using UV-vis absorbance, FT-IR, AFM, TEM, and photoluminescence spectroscopy. The competition/displacement in ligand metathesis is highlighted by the size of the dendron and nature of binding on semiconductor nanocrystals. In the other system using the direct capping method, the size of the Au nanoparticle is mediated by the dimensions of the ligand, i.e. alkyl chain spacer and dendron branching or size. These hybrid dendron/nanoparticle complexes are generally very soluble and stable in non-polar solvents. They exhibit energy transfer, surface plasmon resonance effects, and photoinduced charge transfer interactions between the metal/semiconductor and conjugated ligands. Adsorption on mica and graphite surfaces was observed. A one-layer photovoltaic cell was fabricated to demonstrate the potential for device applications.     

Chelating ligands for nanocrystals' surface functionalization

A new family of ligands for the surface functionalization of CdSe nanocrystals is proposed, namely alkyl or aryl derivatives of carbodithioic acids (R-C(S)SH). The main advantages of these new ligands are as follows: they nearly quantitatively exchange the initial surface ligands (TOPO) in very mild conditions; they significantly improve the resistance of nanocrystals against photooxidation because of their ability of strong chelate-type binding to metal atoms; their relatively simple preparation via Grignard intermediates facilitates the development of new bifunctional ligands containing, in addition to the anchoring carbodithioate group, a second function, which enables the grafting of molecules or macromolecules of interest on the nanocrystal surface. To give an example of this approach, we report, for the first time, the grafting of an electroactive oligomer from the polyaniline family-aniline tetramer-on CdSe nanocrystals after their functionalization with 4-formyldithiobenzoic acid. The grafting proceeds via a condensation reaction between the aldehyde group of the ligand and the terminal primary amine group of the tetramer. The resulting organic/inorganic hybrid exhibits complete extinction of the fluorescence of its constituents, indicating efficient charge or energy transfer between the organic and the inorganic semiconductors.     

Nanocrystal ligand exchange with 1,2,3,4-thiatriazole-5-thiolate and its facile in situ conversion to thiocyanate

The 1,2,3,4-thiatriazole-5-thiolate anion (TTT(-)) was found to be a strongly binding ligand for CdSe nanocrystals, quantitatively exchanging various long-chain ligands to yield stable colloidal suspensions in common polar solvents. The TTT(-) ligand thermolyzes at <100 °C to produce thiocyanate in situ, resulting in reduced quantum confinement in nanocrystal films. CdSe(TTT) possesses far higher colloidal stability than CdSe(SCN), and that, together with the facile synthesis of TTT(-), implies that this is a useful ligand for nanocrystal applications as a masked thiocyanate.     

Luminescent CdSe/CdS core/shell nanocrystals in dendron boxes: superior chemical,photochemical and thermal stability

The surface ligands, generation-3 (G3) dendrons, on each semiconductor nanocrystal were globally cross-linked through ring-closing metathesis (RCM). The global cross-linking of the dendron ligands sealed each nanocrystal in a dendron box, which yielded box-nanocrystals. Although the dendron ligands coated CdSe nanocrystals (CdSe dendron-nanocrystals) were already quite stable, the stability of CdSe box-nanocrystals against chemical, photochemical, and thermal treatments were dramatically improved in comparison to that of the original dendron-nanocrystals. Furthermore, the box structure of the ligands monolayer coupled with the stable inorganic CdSe/CdS core/shell nanocrystals resulted in a class of extremely stable nanocrystal/ligands complexes. The band edge photoluminescence of the core/shell dendron-nanocrystals and box-nanocrystals were partially remained, and could be further brightened through controlled chemical oxidation or photooxidation. Practically, the stability of the box-nanocrystals is sufficient for most fundamental studies and technical applications. The box-nanocrystals may represent a general solution for the commonly encountered instability for many types of colloidal nanocrystals. The size distribution of the empty dendron boxes formed by the dissolution of the inorganic nanocrystals in concentrated HCl was very narrow. The empty boxes as new types of polymer capsules are soluble in solution, mesoporous, and with a very thin but stable peripheral. Those nanometer-sized cavities should be of interest for many purposes in the field of solution host-guest chemistry.     

水溶性CdSe/CdS纳米晶合成条件的优化研究

以巯基乙醇为修饰剂,在水溶液中合成了稳定的CdSe/CdS纳米晶,应用单因素法和多目标单纯形法探索合成条件。通过透射电镜观察所合成的纳米晶的形貌和大小,用紫外-可见吸收光谱和荧光光谱对其光学特性进行了表征。并且以L-色氨酸荧光量子产率0.14为标准,测量了合成的CdSe/CdS纳米晶的荧光量子产率为0.37。     

Synthesis of CdSe/CdTe nanobarbells

Synthesis of monodisperse samples of CdSe nanorods with CdTe tips is achieved using the mechanism of rod nucleated growth to form CdSe/CdTe nanobarbells. This synthesis produces a nanocrystal displaying "type-II" behavior with a morphology that is particularly well suited for internal exciton separation and carrier transport.     

Aqueous synthesis of glutathione-capped CdTe/CdS/ZnS and CdTe/CdSe/ZnS core/shell/shell nanocrystal heterostructures

Here we demonstrate the aqueous synthesis of colloidal nanocrystal heterostructures consisting of the CdTe core encapsulated by CdS/ZnS or CdSe/ZnS shells using glutathione (GSH), a tripeptide, as the capping ligand. The inner CdTe/CdS and CdTe/CdSe heterostructures have type-I, quasi-type-II, or type-II band offsets depending on the core size and shell thickness, and the outer CdS/ZnS and CdSe/ZnS structures have type-I band offsets. The emission maxima of the assembled heterostructures were found to be dependent on the CdTe core size, with a wider range of spectral tunability observed for the smaller cores. Because of encapsulation effects, the formation of successive shells resulted in a considerable increase in the photoluminescence quantum yield; however, identifying optimal shell thicknesses was required to achieve the maximum quantum yield. Photoluminescence lifetime measurements revealed that the decrease in the quantum yield of thick-shell nanocrystals was caused by a substantial decrease in the radiative rate constant. By tuning the diameter of the core and the thickness of each shell, a broad range of high quantum yield (up to 45%) nanocrystal heterostructures with emission ranging from visible to NIR wavelengths (500-730 nm) were obtained. This versatile route to engineering the optical properties of nanocrystal heterostructures will provide new opportunities for applications in bioimaging and biolabeling.     

Ligand exchange on colloidal CdSe nanocrystals using thermally labile tert-butylthiol for improved photocurrent in nanocrystal films

As-prepared CdSe nanocrystals were ligand exchanged using tert-butylthiol, which yielded stable CdSe nanocrystal inks in the strong donor solvent tetramethylurea. The efficacy of ligand exchange was probed by thermogravimetric analysis (TGA) and FT-IR spectroscopy. By studying sequential exchanges of tetradecylphosphonic acid and then tert-butylthiol, TGA and energy dispersive X-ray spectroscopic evidence clearly demonstrated that the ligand exchange is essentially quantitative. The resulting tert-butylthiol-exchanged CdSe nanocrystals undergo facile thermal ligand expulsion (≤200 °C), which was studied by TGA-mass spectrometry. Mild thermal treatment of tert-butylthiol-exchanged CdSe nanocrystal films was found to induce loss of quantum confinement (as evidenced by UV-vis spectroscopy) and provided for increased electrochemical photocurrent, electron mobility, and film stability. Pyridine-exchanged CdSe nanocrystals were employed as a control system throughout to demonstrate the beneficial attributes of tert-butylthiol exchange; namely, lower organic content, better colloidal stability, improved interparticle coupling, and vastly increased electrochemical photocurrent response upon illumination.     

Single quantum dot-micelles coated with silica shell as potentially non-cytotoxic fluorescent cell tracers

The present study describes a stabilization of single quantum dot (QD) micelles by hydrophobic silica precursors and an extension of the silica layer to form a silica shell around the micelle. The obtained product consists of up to 92% of single nanocrystals (CdSe, CdSe/ZnS, or CdSe/ZnSe/ZnS quantum dots) in the silica micelles, coated with silica shell. The thickness of silica shell could vary, starting from 3 to 4 nm. Increasing the shell thickness increases the photoluminescent characteristics of QDs in aqueous solution. The silica-shelled single CdSe/ZnS QD micelles possess a high quantum yield in aqueous solution, a controlled small size, sharp photoluminescence spectra (fwhm approximately 30 nm), an absence of aggregation, and a high transparency. The presence of a hydrophobic layer between the QD and silica shell ensures an incorporation of other hydrophobic molecules (with interesting properties) in the close proximity of nanocrystal. Thus, it is possible to combine the characteristics of hybrid material with the priority of small size. The nanoparticles are amino functionalized and ready for conjugation. A comparatively good biocompatibility is demonstrated. The nanoparticles show ability for intracellular delivery and are noncytotoxic during long-term incubation with viable cells in the absence of light exposure, which makes them appropriate for cell tracing and drug delivery.