Partial Ligand Exchange Research Articles

Improving photovoltaic performance of light-responsive double-heterojunction nanorod light-emitting diodes.

Double heterojunction nanorods enable both electroluminescence and light harvesting capabilities within the same device structure, providing a promising platform for energy-scavenging displays and related applications. However, the efficiency of the photovoltaic mode remains modest for useful power conversion and may be challenging to improve without sacrificing performance in electroluminescence. Through a facile on-film partial ligand exchange with benzenethiol integrated into the device fabrication step, we achieve an average of more than threefold increase in power conversion efficiency while maintaining the maximum external quantum efficiency and the maximum luminance in the LED mode. The improved photovoltaic performance is mainly due to the increase in the short circuit current, which we attribute to the enhanced charge separation afforded by the partial ligand exchange. The recovery of the photoluminescence lifetime under the forward bias suggests that the hole traps introduced by benzenethiols are filled prior to reaching the voltage at which light emission begins, allowing LED performance to be maintained and possibly improved.

The double cluster compound [Nb6Cl14(MeCN)4][Nb6Cl14(pyz)4]·6MeCN (Me: methyl, pyz: pyrazine) with a layered structure resulting from weak intermolecular interactions

Abstract The synthesis and the crystal structure of the double cluster compound [Nb6Cl14(MeCN)4][Nb6Cl14(pyz)4]·6CH3CN are described. The synthesis is based on a partial ligand exchange reaction, which proceeds upon dissolving [Nb6Cl14(pyz)4]·2CH2Cl2 in acetonitrile. The compound is built up of two discrete neutral cluster units, which consist of octahedra of Nb6 atoms coordinated by 12 edge-bridging chlorido and two terminal chlorido ligands, and four acetonitrile ligands on one and four pyrazine ligands on the other cluster unit. Co-crystallized acetonitrile molecules are also present. The single-crystal structure determination has revealed a cluster arrangement in which the [Nb6Cl14(pyz)4] units are connected by (halogen) lone-pair–(pyrazine) π interactions. These lead to chains of [Nb6Cl14(pyz)4] clusters. These chains are further connected to cluster layers by (nitrile-halogen) dipole–dipole interactions, in which the [Nb6Cl14(MeCN)4] and co-crystallized MeCN molecules are also involved. These cluster layers are arranged parallel to the crystallographic {011} plane.

Exploring the Generality of PVP‐Assisted Phase Transfer of Nanoparticles

AbstractFor most nanoparticles, the synthetic environment usually differs from the application environment, making phase transfer a critical bottleneck in the exploration of nanotechnology. Herein, a general phase transfer method by using polyvinyl pyrrolidone (PVP) and an intermediate solvent is demonstrated. A variety of nanoparticles can be transferred into different media, such as water, polar organic solvent, nonpolar solvent, and polymer precursor, while maintaining their colloidal stability. In the traditional methods of two‐phase transfer, the initial solvent is a powerful competitor, which greatly limits the window of applicable conditions. A two‐step approach where the incubation with the intermediate solvent removes the initial solvent and allows sufficient time for ligand exchange is designed. This avoids the complications caused by partial ligand exchange at the limited area of the interface. Such a method of general phase transfer to different solvents and the understanding of the underlying mechanism would broaden the application of nanomaterials.

Photoluminescence Enhancement of NIR‐II Emissive Ag2S Quantum Dots via Chloride‐Mediated Growth and Passivation

AbstractAg2S quantum dots (QDs) with high photoluminescence quantum yield (PLQY) in the second near‐infrared (NIR‐II, 1000–1700 nm) window are urgently pursued. Halide atoms (especially chloride) have been studied as inorganic capping ligands on improving the PLQY of Cd‐, Pb‐based chalcogenide QDs; however, it remains a challenge to well replace the organic ligand with chloride ligand from silver chalcogenide QDs. Herein, a novel strategy to prepare Ag2S QDs with significant photoluminescence (PL) enhancement via chloride‐mediated growth and passivation is demonstrated. The oleylamine and oleate ligands on the surface of silver‐rich Ag2S QDs are readily replaced by chloride ligands due to the smaller size and higher binding affinity with Ag+. Upon partial ligand exchange and chloride passivation, the surface trap states in Ag2S QDs are reduced through a decrease of unsaturated surface Ag atoms. Therefore, Ag2S QDs with approximately 300‐fold enhancement of PL intensity, and a PLQY of 47.6% at 1220 nm emission, and a lifetime of 256 ns are obtained because the nonradiative recombination is inhibited. This work reveals that the passivation of Ag2S QDs with chloride ligands is fully feasible, providing new insights into improving the PLQY of silver chalcogenide QDs.

Chitosan Matrix Encapsulation of α‐Lipoic Acid (LA) Anchored Gold Nanoparticles: A Combined Experimental and Theoretical Study

AbstractIn this article the competitive binding of citrate (2‐hydroxypropane‐1,2,3‐tricarboxylate), chitosan and α‐lipoic acid ((R)‐5‐(1,2‐dithiolan‐3‐yl) pentanoic acid, (LA) are tested on the surface of Gold Nanoparticle (AuNP). The chitosan matrix encapsulated gold nanoparticle resulted in colour reversibility in presence of α‐lipoic acid through partial ligand exchange mechanism. Different experimental analysis including TEM, EDX, spectrophotometry and FTIR were spectrometry performed to check the interaction mechanism and explain the colour reversibility. Binding and colour reversibility mechanism was also supported through the Density functional Theory calculations.

Reversible Diels–Alder Reactions with a Fluorescent Dye on the Surface of Magnetite Nanoparticles

Diels–Alder reactions on the surface of nanoparticles allow a thermoreversible functionalization of the nanosized building blocks. We report the synthesis of well-defined magnetite nanoparticles by thermal decomposition reaction and their functionalization with maleimide groups. Attachment of these dienophiles was realized by the synthesis of organophosphonate coupling agents and a partial ligand exchange of the original carboxylic acid groups. The functionalized iron oxide particles allow a covalent surface attachment of a furfuryl-functionalized rhodamine B dye by a Diels–Alder reaction at 60 °C. The resulting particles showed the typical fluorescence of rhodamine B. The dye can be cleaved off the particle surface by a retro-Diels–Alder reaction. The study showed that organic functions can be thermoreversibly attached onto inorganic nanoparticles.

Improved Stability and Photodetector Performance of CsPbI3 Perovskite Quantum Dots by Ligand Exchange with Aminoethanethiol

AbstractA surface engineering strategy aimed at improving the stability of CsPbI3 perovskite quantum dots (QDs) both in solution and as films is demonstrated, by performing partial ligand exchange with a short chain ligand, 2‐aminoethanethiol (AET), in place of the original long chain ligands, oleic acid (OA) and oleylamine (OAm), used in synthesis. This results in the formation of a compact ligand barrier around the particles, which prevents penetration of water molecules and thus degradation of the films and, in addition, at the same time improves carrier mobility. Moreover, the AET ligand can passivate surface traps of the QDs, leading to an enhanced photoluminescence (PL) efficiency. As a result, AET‐CsPbI3 QDs maintain their optical performance both in solution and as films, retaining more than 95% of the initial PL intensity in water after 1 h, and under ultraviolet irradiation for 2 h. Photodetectors based on the AET‐CsPbI3 QD films exhibit remarkable performance, such as high photoresponsivity (105 mA W−1) and detectivity (5 × 1013 Jones at 450 nm and 3 × 1013 Jones at 700 nm) without an external bias. The photodetectors also show excellent stability, retaining more than 95% of the initial responsivity in ambient air for 40 h without any encapsulation.

Control of Energy Flow Dynamics between Tetracene Ligands and PbS Quantum Dots by Size Tuning and Ligand Coverage.

We have prepared a series of samples with the ligand 6,13-bistri(iso-propyl)silylethynyl tetracene 2-carboxylic acid (TIPS-Tc-COOH) attached to PbS quantum dot (QD) samples of three different sizes in order to monitor and control the extent and time scales of energy flow after photoexcitation. Fast energy transfer (∼1 ps) to the PbS QD occurs upon direct excitation of the ligand for all samples. The largest size QD maintains the microsecond exciton lifetime characteristic of the as-prepared oleate terminated PbS QDs. However, two smaller QD sizes with lowest exciton energies similar to or larger than the TIPS-Tc-COO- triplet energy undergo energy transfer between QD core and ligand triplet on nanosecond to microsecond timescales. For the intermediate size QDs in particular, energy can be recycled many times between ligand and core, but the triplet remains the dominant excited species at long times, living for ∼3 μs for fully exchanged QDs and up to 30 μs for partial ligand exchange, which is revealed as a method for controlling the triplet lifetime. A unique upconverted luminescence spectrum is observed that results from annihilation of triplets after exclusive excitation of the QD core.

Ligand Exchange-Mediated Activation and Stabilization of a Re-Based Olefin Metathesis Catalyst by Chlorinated Alumina.

Extensive chlorination of γ-Al2O3 results in the formation of highly Lewis acidic surface domains depleted in surface hydroxyl groups. Adsorption of methyltrioxorhenium (MTO) onto these chlorinated domains serves to activate it as a low temperature, heterogeneous olefin metathesis catalyst and confers both high activity and high stability. Characterization of the catalyst reveals that the immobilized MTO undergoes partial ligand exchange with the surface, whereby some Re sites acquire a chloride ligand from the modified alumina while donating an oxo ligand to the support. More specifically, Re LIII-edge EXAFS and DFT calculations support facile ligand exchange between MTO and Cl-Al2O3 to generate [CH3ReO2Cl+] fragments that interact with a bridging oxygen of the support via a Lewis acid-base interaction. According to IR and solid-state NMR, the methyl group remains intact, and does not evolve spontaneously to a stable methylene tautomer. Nevertheless, the chloride-promoted metathesis catalyst is far more active and productive than MTO/γ-Al2O3, easily achieving a TON of 100 000 for propene metathesis in a flow reactor at 10 °C (compared to TON < 5000 for the nonchlorinated catalyst). Increased activity is a consequence of both a larger fraction of active sites and a higher intrinsic activity for the new sites. Increased stability is tentatively attributed to a stronger interaction between MTO and chlorinated surface regions, as well as extensive depletion of the Brønsted acidic surface hydroxyl population. The reformulated catalyst represents a major advance for Re-based metathesis catalysts, whose widespread use has thus far been severely hampered by their instability.

Establishing Porosity Gradients within Metal-Organic Frameworks Using Partial Postsynthetic Ligand Exchange.

Crystalline 3-D materials bearing interlinked domains of differential porosity and functionality offer the potential for organizing and shuttling molecular and nanoscale matter to specific locations within 3-D space. Here, we present methods for creating prototype MOF materials that have such structural features. Specifically, the process of pore expansion via ligand exchange was studied for an isoreticular series of mesoporous MOFs based on bMOF-100. It was found that pore expansion occurs incrementally in small steps and that it proceeds gradually in an "outside→in" fashion within individual crystals. The ligand exchange reaction can be terminated prior to complete crystal conversion to yield intermediate product MOFs, denoted bMOF-100/102 and bMOF-102/106, which bear descending porosity gradients from the crystal periphery to the crystal core. As a proof of concept, size-sensitive incorporation of a gold-thiolate nanocluster, Au133(SR)52, selectively in the bMOF-102/106 crystal periphery region was accomplished via cation exchange. These new methods open up the possibility of controlling molecular organization and transport within porous MOF materials.

Improved electroluminescence of quantum dot light-emitting diodes enabled by a partial ligand exchange with benzenethiol

In this study, benzenethiol ligands were applied to the surface of CdSe@ZnS core@shell quantum dots (QDs) and their effect on the performance of quantum dot light-emitting diodes (QD-LEDs) was investigated. Conventional long-chained oleic acid (OA) and trioctylphosphine (TOP) capping ligands were partially replaced by short-chained benzenethiol ligands in order to increase the stability of QDs during purification and also improve the electroluminescence performance of QD-LEDs. The quantum yield of the QD solution was increased from 41% to 84% by the benzenethiol ligand exchange. The mobility of the QD films with benzenethiol ligands approximately doubled to 2.42 × 10−5 cm2 V−1 s−1 from 1.19 × 10−5 cm2 V−1 s−1 compared to the device consisting of OA/TOP-capped QDs, and an approximately 1.8-fold improvement was achieved over QD-LEDs fabricated with bezenethiol ligand-exchanged QDs with respect to the maximum luminance and current efficiency. The turn-on voltage decreased by about −0.6 V through shifting the energy level of the QDs with benzenethiol ligands compared to conventional OA and TOP ligands.

Partial ligand exchange as a critical approach to the synthesis of transparent ytterbium fluoride–polymer nanocomposite monoliths for gamma ray scintillation

A partial ligand exchange route to the preparation of transparent polymer nanocomposites with a high nanoparticle loading in bulk size for gamma-ray detection.

SnS44– Metal Chalcogenide Ligand, S2– Metal Free Ligand, and Organic Surface Ligand Toward Efficient CdSe Quantum Dot- Sensitized Solar Cells

Inorganic surface ligands such as metal chalcogenides ligand SnS44– and metal free ligand S2– were introduced for CdSe quantum dot sensitized solar cell (QDSSC) applications. The SnS44– ligand QDs were successfully deposited onto TiO2 photoanode through metal ion coordination. In solution, metal–ammonia complexes of Zn2+, Cd2+, and Cu2+ can be coordinated by the SnS44– ligands and reverse the zeta potential. Similarly, the metal ions can be sandwiched by SnS44– ligands and the photoanode. Using the metal ion bridged SnS44– ligand QDs as the sensitizer, photovoltaic properties of the QDSSCs have been studied. Cd2+ mediated deposition case showed better photovoltaic performance than the cases of Zn2+ or Cu2+. To further investigate the surface ligand effect on QDSSC, organic/inorganic mixed surface CdSe QDs were introduced using partial ligand exchange after the deposition onto the TiO2 photoanode. The postdeposition surface ligand exchange with inorganic ligands such as SnS44– and S2– is thought to retain ...

Synthesis of Multifunctional Fe3O4–CdSe/ZnS Nanoclusters Coated with Lipid A toward Dendritic Cell-Based Immunotherapy

We demonstrate a novel route to synthesize Fe3O4-CdSe/ZnS multifunctional nanoclusters (MNCs) with excellent optical and magnetic properties and biocompatibility. The successful fabrication of highly fluorescent and magnetic MNCs is achieved via a coupling process based on a partial ligand exchange reaction at the aqueous-organic solution interface. In addition, we show that dendritic cells (DCs), the sentinel of the immune system, can uptake the MNCs without significant change in cell viability. The MNCs uptaken by the DCs can be used for imaging, tracking, and separating the DCs. Furthermore, the MNCs can be loaded with a pathogen-associated molecular pattern, lipid A, via a hydrophobic-hydrophobic interaction. Ex vivo labeling of DCs with the MNC-lipid A complex enhances the DC migration to draining lymph nodes and tumor antigen-specific T cell responses in vivo. Our work may contribute to the development of synthetic routes to various multifunctional nanoclusters and DC-based cancer immunotherapies.

Effect of ligand exchange in optical and morphological properties of CdTe nanoparticles/P3HT blend

Luminescent CdTe nanoparticles were synthesized in aqueous medium at low temperature under the inert atmosphere using water soluble precursors. Potassium tellurite was employed as the tellurium source for the synthesis. As synthesized CdTe nanoparticles were phase transferred into organic medium via partial ligand exchange method through long chain organic ligand 1-dodecanethiol in the presence of acetone. The phase transferred CdTe nanoparticles were blended homogeneously with P3HT polymer in a common solvent (chloroform) for possible application as the active layer in hybrid solar cell structure. The prepared blends were characterised with UV–Vis, Photoluminescence, SEM and AFM analysis. The XRD patterns of the particles in two phases confirm the uniformity of the cubic structure. The size distribution of the synthesized particles was confirmed through TEM analysis. The effective interactions of the donor and acceptor components were confirmed through UV–Visible spectroscopy. The efficient charge transfer processes of the blends were confirmed through photoluminescence analysis of the nanoparticles various volume additions with polymer. The morphological analysis of the blends was carried out using the Scanning Electron Microscopy which reveals the distribution of the nanoparticles in the polymer. AFM analysis of the coated blend film explores the phase separation of the nanoparticles when blended with the polymer in chloroform. Advantages of these nanoparticles for solar cell applications were discussed.

Synthesis of thiol modified CdSe nanoparticles/P3HT blends for hybrid solar cell structures

Thioglycolic acid (TGA) capped cadmium selenide (CdSe) nanoparticles were synthesised in aqueous medium through wet chemical method using sodium selenite as the selenium source. The synthesised particles were transformed into organic medium using 1-dodecanethiol through efficient partial ligand exchange strategy. The UV–visible spectra recorded for the particles reveal the size distribution in water as well as in organic solvent. TEM analysis of the synthesised particles shows the size and the nature of distribution of the particles in solution. The phase transferred particles were blended with the polymer poly-3-hexyl thiophene (P3HT) in chloroform. UV–visible and photoluminescence spectra of the polymer with various volumes of the nanoparticles clearly indicate the effective coupling and the efficient charge transfer process in the blend. SEM analysis of the CdSe–P3HT blends confirms the effective distribution of the nanoparticles in the polymeric matrix. AFM studies reveal the morphology of the nanoparticles and the phase separation process in the blends.

Polyamine ligand-mediated self-assembly of gold and silver nanoparticles into chainlike structures in aqueous solution: towards new nanostructured chemosensors.

Polyamine ligands are very versatile compounds due to their water solubility and flexibility. In the present work, we have exploited the binding ability of a polyamine molecular linker (L2−) bearing different functional groups, which favors the self-assembling of silver nanoparticles (AgNPs) and gold nanoparticles (AuNPs) into 1D nanochains in aqueous solution. The chainlike assemblies of AuNPs and AgNPs were structurally stable for a long period of time, during which their characteristic optical properties remained unchanged. The mechanism of AuNPs and AgNPs chain assembly associated with the induction of electric dipole–dipole interactions arising from the partial ligand exchange of surface-adsorbed citrate ions by (L2−) was investigated. UV/Vis spectrophotometry and transmission electron microscopy (TEM) were used to determine timedependent structural changes associated with formation of the 1D nanoparticle structures. Finally, the sensing of Hg2+ in aqueous solution using AgNPs@(L)2− and AuNPs@(L)2− assemblies was also carried out in aqueous solution.

Nonmonotonic Size Dependence in the Hole Mobility of Methoxide-Stabilized PbSe Quantum Dot Solids

We present a facile procedure to fabricate p-type PbSe-based quantum dot solids with mobilities as large as 0.3 cm(2) V(-1)s(-1). Upon partial ligand exchange of oleate-capped PbSe quantum dots with the methoxide ion, we observe a pronounced red shift in the excitonic transition in conjunction with a large increase in conductivity. We show that there is little correlation between these two phenomena and that the electronic coupling energy in PbSe quantum dot solids is much smaller than often assumed. However, we observe for the first time a nonmonotonic size dependence of the hole mobility, illustrating that coupling can nonetheless be dominant in determining the transport characteristics. We attribute these effects to a decrease in charging energy and interparticle spacing, leading to enhanced electronic coupling on one hand and enhanced dipole interactions on the other hand, which is held responsible for the majority of the red shift.

Synthesis and Efficient Phase Transfer of CdSe Nanoparticles for Hybrid Solar Cell Applications

Colloidal cadmium selenide (CdSe) nanoparticles capped by a short chain thiol ligand thioglycolic acid (TGA) were prepared in aqueous medium at 100∘C using sodium selenite as the selenium source. The prepared particles were efficiently transferred into organic phases through the partial ligand exchange process using 1-dodecanethiol (1-DDT). The FT-IR spectrum analysis shows the existence of the ligands on the surface of the nanoparticles in aqueous and organic phase through their characteristic modes. TEM analysis of the prepared samples reveals the size and crystalline nature of the particles. EDX analysis indicates the presence of respective elements in the resultant powdered sample. The possible mechanism of the phase transfer process was analysed. The resultant phase-transferred particles can efficiently be blended with a low band gap semiconducting polymer in a common solvent for active layer in hybrid solar cell fabrication. The results are discussed in detail.

Amphiphilic, cross-linkable diblock copolymers for multifunctionalized nanoparticles as biological probes

Nanoparticles (NPs) play an increasingly important role in biological labeling and imaging applications. However, preserving their useful properties in an aqueous biological environment remains challenging, even more as NPs therein have to be long-time stable, biocompatible and nontoxic. For in vivo applications, size control is crucial in order to route excretion pathways, e.g. renal clearance vs. hepato-biliary accumulation. Equally necessary, cellular and tissue specific targeting demands suitable linker chemistry for surface functionalization with affinity molecules, like peptides, proteins, carbohydrates and nucleotides. Herein, we report a three stage encapsulation process for NPs comprised of (1) a partial ligand exchange by a multidentate polyolefinic amine ligand, PI-N3, (2) micellar encapsulation with a precisely tuned amphiphilic diblock PI-b-PEG copolymer, in which the PI chains intercalate to the PI-N3 prepolymer and (3) radical cross-linking of the adjacent alkenyl bonds. As a result, water-soluble NPs were obtained, which virtually maintained their primal physical properties and were exceptionally stable in biological media. PEG-terminal functionalization of the diblock PI-b-PEG copolymer with numerous functional groups was mostly straightforward by chain termination of the living anionic polymerization (LAP) with the respective reagents. More complex affinity ligands, e.g. carbohydrates or biotin, were introduced in a two-step process, prior to micellar encapsulation. Advantageously, this pre-assembly approach opens up rapid access to precisely tuned multifunctional NPs, just by using mixtures of diverse functional PI-b-PEG polymers in a combinatorial manner. All constructs showed no toxicity from 0.001 to 1 μM (particle concentration) in standard WST and LDH assays on A549 cells, as well as only marginal unspecific cellular uptake, even in serum-free medium.