Surface Capping Ligands Research Articles

Uncovering the Role of Trioctylphosphine on Colloidal and Emission Stability of Sb-Alloyed Cs2NaInCl6 Double Perovskite Nanocrystals.

Doping and compositional tuning of Cs2AInCl6 (A = Ag, Na) double perovskite nanocrystals (PNCs) is considered a promising strategy toward the development of light-emitting sources for applications in solution-processed optoelectronic devices. Oleic acid and oleylamine are by far the most often used surface capping ligands for PNCs. However, the undesirable desorption of these ligands due to proton-exchange reaction during isolation and purification processing results in colloidal and structural instabilities. Thus, the improvement of colloidal and optical stability of PNCs represents one of the greatest challenges in the field. Here, we report a trioctylphosphine-mediated synthesis and purification method toward Sb-alloyed Cs2NaInCl6 PNCs with excellent stability and optical features. Nuclear magnetic resonance spectroscopy enabled one to explain the role of trioctylphosphine and to reveal the reaction mechanism during crystal nucleation and growth. Under the optimized reaction conditions, in situ-generated trioctylphosphonium chloride and benzoyl trioctylphosphonium chloride serve as highly reactive halide sources, while benzoyl trioctylphosphonium and oleylammonium cations together with the oleate anion serve as surface capping ligands, which are bound strongly to the PNC surface. The tightly bound ionic pair of oleylammonium oleate and benzoyl trioctylphosphonium chloride/oleate ligands allows one to obtain monodispersed bright-blue-emitting PNCs with high photoluminescence quantum yields exceeding 50% at an optimum Sb content (0.5%), which also exhibit long-term colloidal stability. The approach based on dual cationic ligand passivation of double PNCs opens the doors for applications in other systems with a potential to achieve higher stability along with superior optical properties.

Single-parameter-tuned synthesis for shape-controlled gold nanocrystals stimulated by iron carbonyl

Shape-controlled synthesis is essential for functional nanomaterials, allowing deeper insights intothe relationship between the structures and the catalytic properties. Synthesis of nanocrystals with particular morphologies are usually studied independently among various synthetic methods, those underline that different surface capping ligands or shape-directing agents bring about disparate shapes. However, a single quantitative parameter method is still lacking to realize precise control of well-defined morphology nanocrystals, especially anisotropic structures, which is essential to understanding the growth process of nanocrystals. Herein, we proposed a single-parameter-tuned synthesis strategy for preparation of shape-controlled gold nanocrystals by regulating the amount of iron carbonyl, by which we produced highly monodisperse Au nanocrystals with various shapes in organic phase including nanoplates (diameter of 16.02 ± 1.13 nm and thickness of 5.35 ± 0.58 nm), nanorods (length of 37.53 ± 3.73 nm and width of 5.26 ± 0.37 nm) and nanospheres (diameter of 8.26 ± 0.38 nm). The single-parameter-tuned method reveals the dual roles of iron carbonyl for controlling the shapes of gold nanocrystals including reductant and oxidative etchant and empowers versatility in synthetic methodology for other noble metals. Moreover, catalytic activity shifting in shapes of nanocrystals was revealed based on the reduction of 4-nitrophenol, showing that the as-synthesized Au nanoplates displayed the enhanced catalytic performance with the lowest activation energy. Our work provides a brand-new pathway for shape-controlled synthesis of noble-metal nanocrystals and has a strong practical value in application fields.

Repulsive, Densely Packed Ligand-Shells Mediate Interactions between PbS Nanocrystals in Solution

The surface capping ligands on nanocrystals (NCs) impart colloidal stability and determine the organization into ordered solids when cast from solution. Here, we use small-angle neutron scattering (SANS) to measure the solution structure factor in dispersions of fully passivated, oleate-capped PbS NCs up to concentrations of 16 v/v%. Fitting the data by assuming an attractive square well interaction potential between NCs yields a repulsive core diameter larger than the physical NC core diameter. This repulsive core stems from a densely packed region of the ligand-shell near the NC surface. The spatial extent of the repulsive core increases with temperature as ligand motion increases, becoming repulsive farther out in the ligand-shell, while the portion of ligand-shell outside of the repulsive region remains attractive with a strength of ∼1 kBT. SANS provides a molecular interpretation of the fundamental interparticle interactions in colloidal nanocrystal suspensions, ultimately relevant to understanding the self-organization of nanoscale materials.

Amine‐Free Synthesis of Colloidal Cesium Lead Halide Perovskite Nanocrystals

AbstractColloidal cesium lead halide (CsPbX3) perovskite nanocrystals (PNCs) are of great interest in research because of their excellent optical properties and exceptional colour tunability. Oleic acid and oleylamine are the most commonly used surface capping ligands in colloidal CsPbX3 synthesis. The oleylamine plays an important role in surface passivation and maintaining colloidal stability. But in a long run, it is responsible for poor colloidal stability because of the facile proton exchange leading to the formation of labile oleylammonium halide, which pulls the halide ions out of the NC surface. By leveraging the synthetic toolboxes developed from decades of research into more traditional semiconductor nanocrystals, very recently researchers focused on developing various amine‐free synthesis approaches to improve the colloidal and photostability of CsPbX3 perovskite NCs. In this review, we have summarized various amine‐free based synthetic routes of CsPbX3 (X=Cl, Br, I) PNCs that are reported to date. We reviewed this progress in terms of their underlying synthetic approaches, and post‐synthetic treatment steps. Furthermore, we reviewed and analyzed the prospects of these perovskite nanocrystals in terms of their photo‐luminescence properties and device performances. In going forward, a deeper understanding of the role of precursors and ligands will significantly bolster the versatility of these amine‐free PNC materials.

Can surface capping ligands probe cation exchange in inorganic nanoparticles?

Structural reorganization of surface capping ligands can be used to track cation exchange reactions in inorganic nanoparticles.

Improved stability and performance of all inorganic perovskite quantum dots synthesized directly with N-alkylmonoamine ligands for light-erasable transistor memory

All inorganic perovskite quantum dots (PeQDs) of CsPbX3 (X = Cl, Br, I, or their mixture) are regarded as promising candidates for photovoltaics and optoelectronic devices owing to their excellent and unique optical and electronic properties. However, their inherent poor stability severely limits their practical applications. Herein, aiming at improving their stability and photoluminescence (PL) properties, a novel surface capping ligand strategy was performed via directly synthesizing CsPbX3 quantum dots (QDs) by hot injection method with straight-chain n-alkylmonoamine instead of conventional organic amine (oleylamine) as ligands. With stronger intermolecular interaction and denser alignment, n-alkylmonoamine ligands would form a stable protective layer on the surface of PeQDs. As a result, the n-alkylmonoamine capped CsPbBr3 QDs exhibited remarkable long-term storage with 70% of PLQY retaining after 30 days in the open air, impressive water resistance for over 1 h in water, significant photostability with hour-scale laser exposure, and high thermal stability with 60% of PLQY retaining after heating at 80 °C for over 60 min. In addition, by blending the CsPbBr3 QDs with polymer as the gate dielectric, novel light-erasable transistor memories were fabricated based on the device architecture of a floating-gate transistor memory (FGOTM). Acting as dense charge tunneling layers for charge trapping materials (PeQDs) in light-erasable transistor memory, the alkylmonoamine ligand layers led to significant broadening of memory windows, presenting various charge capture abilities of the PeQDs with the different capping ligands.

Close-Packed Ultrasmooth Self-assembled Monolayer of CsPbBr3 Perovskite Nanocubes.

The use of colloidal self-assembly to form the complex multiscale patterns in many optoelectronic devices has been a long-standing dream of the nanoscience community. While great progress has been made using charged colloids in polar solvents, controlled assembly from nonpolar solvents is much more challenging. The major challenge is colloidal clustering caused by strong van der Waals (vdW) attraction between long-chain surface capping ligands passivating the surface of nanocrystals. Such clustering degrades ordering in packing during the self-assembly process. While ligand exchange to provide colloidal stability in polar phases is often an option, this is not the case for the exciting new class of halide perovskites due to the material’s solubility in essentially all polar solvents. Here, we report surface-functionalized self-assembly of luminescent CsPbBr3 perovskite nanocubes by partially replacing long-chain oleyl groups (18 carbon chain) with short-chain thiocyanate (SCN–). This enables the fabrication of ultrasmooth monolayer thin films of nanocubes with a root-mean-square (RMS) roughness of around 4 Å. This ultrasmooth large area self-assembled layer could act as high-efficiency optoelectronic devices like solar cells, light-emitting diodes (LEDs), transistors, etc. We correlate our experimental results with simulations, providing detailed predictions for lattice constants with chain conformations showing reduced free energy for cubes grafted with short-chain thiocyanate compared to long-chain oleyl groups, thus facilitating better self-assembly.

Ligand engineering of mid-infrared Ag2Se colloidal quantum dots

Abstract Silver selenide colloidal quantum dots exhibit distinct optical absorption in the mid-wavelength infrared spectral region, which arises from the intraband transition between the first and the second quantum-confined energy levels. The optical absorption coefficient, carrier mobility, and carrier lifetime, which are the three primary parameters that determine the ultimate performance of a photodetector, are expected to be heavily dependent on the surface capping ligands. Herein, we characterize these parameters on silver selenide colloidal quantum dots films chemically treated with ligands selected from literatures that report high-performance quantum dot-based solar cells, photodetectors, transistors, and thermoelectrics. We correlate these results with the mid-infrared responsivities measured from photoconductive photodetectors fabricated from respective ligand-exchanged films. The insights gained from this study may serve as a foundation for enabling future intraband CQD-based MWIR sensing and imaging technologies.

Surface Ligand-Controlled Wavelength-Tunable Luminescence of Gold Nanoclusters: Cellular Imaging and Smart Fluorescent Probes for Amyloid Detection

Gold nanoclusters (Au NCs) are an emerging class of fluorescent nanomaterials due to their fascinating chemical or physical properties and atomically precise structures; hence, they have been widely used in the field of biosensing and bioimaging. In this article, we demonstrate the green synthesis of orange, yellow, green, and cyan emitting Au NCs by core etching and ligand exchange methodology. Our investigation reveals that the chain length of the mercaptan acids, which are present on the surface of the Au NCs, controls the optical and electronic properties of the synthesized NCs. The steady-state and time-resolved spectroscopic data suggest that the emission properties of Au NCs mainly originate from the ligand to metal charge transfer (LMCT) transition. Alterations of the optical properties of these Au NCs can be proposed due to the difference in the core size of the Au NCs, which is strongly influenced by the surface-capping ligands. These NCs are highly biocompatible and nontoxic as evidenced by the cell viability and cellular uptake studies. By virtue of this, our as-synthesized NCs have been successfully used as excellent intracellular fluorescent imaging probes. Interestingly, fluorescence properties of Au NCs can efficiently probe the protein amyloids associated with several neurodegenerative diseases. To facilitate research in the field of amyloidosis, we have demonstrated fluorescence lifetime imaging microscopy (FLIM) and fluorescence correlation spectroscopy (FCS) as two advanced tools to probe the aggregation of proteins and to monitor the physical interactions between proteins and NCs. It has been observed that the hydrophobicity of the NC surface can trigger the amyloid detection capability of Au NCs. Owing to these unique optical and attractive biological properties coupled with the imaging capability, these ultrasmall-sized Au NCs may enable in vivo detection of amyloids in the near future.

Electrocatalytic Metallic Nanostructures Prepared By Electrorefining and Cathodic Corrosion

Metallic nanostructures (NS) due to their unique properties are often used as electrocatalytic electrode materials in energy conversion and storage as well as in sensing applications. Their common methods of preparation involve homogeneous synthesis and electrodeposition from solutions containing appropriate precursor and often reducing agent and capping agent (surfactant). The main drawbacks of these methods are restricted yield, waste production and often intrinsic contamination of the surface of obtained NS. Moreover, surface capping ligands affect electrocatalytic activity by hindrance of reactant access to the active sites.We propose a versatile method of preparation of bare (non-capped) mono- and multi-metallic (Au,1 Cu,2 Ag, Fe) NS by localized electrorefining of polycrystalline raw metals at high efficiency in a controlled manner. Catalytic activities of obtained NS towards oxygen reduction reaction (ORR) in alkaline media, hydrogen evolution (HER) and carbon dioxide reduction (CO2RR) to easily electrooxidizable fuels (formic acid and carbon monooxide) were analyzed by feedback mode scanning electrochemical microscopy (SECM). The morphology and specific activity of obtained NS depend on used support material (indium tin oxide (ITO) and glassy carbon) and other electrorefining parameters such as electrodeposition potential, source velocity and the composition of the electrolyte. The last parameter determines stoichiometry of Faradaic processes. Both gold and copper electrooxidation and electroreduction can follow one electron stoichiometry. ORR in alkaline media and CO2RR occur effectively on NS with highly developed surfaces, containing large number of defects, corners and edges, whereas HER occurs efficiently at crystallographic Cu facets. Bare gold NS also exhibit outstanding plasmonic properties. They were applied as support for surface-enhanced Raman spectroscopy (SERS) of single molecules providing enhancement factor >106.1 Another method of preparation of metallic nanostructures is cathodic corrosion. This approach for platinum was studied by Koper group.3 We used this procedure for platinum ultramicroelectrode treatment. Obtained Pt NS exhibit unique catalytic properties towards electrooxidation of glucose.

Impact of Enantiomeric Ligand Composition on the Photophysical Properties of Chiral Ag29 Nanoclusters

Abstract Chiral surface ligands have often been employed to impart optical activity to metal nanoclusters, metal nanoparticles and semiconductor nanocrystals. They are considered to form a chiral structure to a certain degree whereas the effect of such chiral structure on the global physicochemical properties apart from chiroptical ones has been unexplored. We herein demonstrate the impact of optical purity of chiral surface ligand on the emission property of silver nanocluster (NCs). Chiral bidentate α-dihydrolipoic acid (DHLA) with varied enantiomeric excess (ee) values was employed as a surface capping ligand to prepare a series of silver NCs, displaying identical absorption and emission profiles typical for Ag29 NCs. Interestingly, the emission quantum yields exhibited a clear dependence on the enantiopurity of DHLA. The more enantiopure DHLA afforded more emissive NCs. This |ee|-dependent emission efficiency was discussed in association with the orientation of ligands on the Ag29 NCs. The surface structures of Ag29(dithiolate)12 models composed of enantiopure and racemic ligand systems were compared with the aid of DFT calculations, suggesting that the enantiopure surface is more stable with one-handed ligand orientation. Two-dimensional NMR technique also supported the observation that well-defined ligand orientations depend on the enantiomeric composition of chiral ligand.

Seeing subtle nanoparticle differences

Nanomaterials A challenge in the fabrication of nanoparticles is that even for particles of uniform size, there will still be a distribution in the atomic arrangements and surface capping ligands from one particle to the next. Using liquid-cell transmission electron microscopy, Kim et al. reconstructed the structure of individual nanocrystals synthesized in one batch while they were still in solution. A comparison of multiple particles showed structural heterogeneity and differences between the interior and the outer shell of the individual nanoparticles, as well as nanoparticles containing extended defects and thus differences in internal strain, all of which can affect the physical and chemical properties of each particle. Science , this issue p. [60][1] [1]: /lookup/doi/10.1126/science.aax3233

Critical differences in 3D atomic structure of individual ligand-protected nanocrystals in solution

Precise three-dimensional (3D) atomic structure determination of individual nanocrystals is a prerequisite for understanding and predicting their physical properties. Nanocrystals from the same synthesis batch display what are often presumed to be small but possibly important differences in size, lattice distortions, and defects, which can only be understood by structural characterization with high spatial 3D resolution. We solved the structures of individual colloidal platinum nanocrystals by developing atomic-resolution 3D liquid-cell electron microscopy to reveal critical intrinsic heterogeneity of ligand-protected platinum nanocrystals in solution, including structural degeneracies, lattice parameter deviations, internal defects, and strain. These differences in structure lead to substantial contributions to free energies, consequential enough that they must be considered in any discussion of fundamental nanocrystal properties or applications.

Diorganyl Dichalcogenides as Surface Capping Ligands for Germanium Nanocrystals

Indirect ligand exchange methods have been demonstrated to replace the oleylamine capping with dodecanethiol for germanium nanocrystals (Ge NCs). In these methods, hydrazine is employed to effectiv...

What Role Can Surface Capping Ligand Play To Control Dopant Emission in Semiconductor Nanoparticles?

The role of surface capping ligands in controlling dopant photoluminescence in semiconductor nanoparticles is examined by monitoring emission in terbium cation incorporated zinc sulfide [Zn(Tb)S] n...

Luminescent switch of polysaccharide-peptide-quantum dot nanostructures for targeted-intracellular imaging of glioblastoma cells

Abstract Glioblastoma multiforme (or GBM) remains one of the deadliest types of brain cancers. Nanomedicine can offer new strategies for fighting against GBM by combining the earliest possible diagnosis with multiple options of therapy. Hence, in this work, cysteine (Cys) and Poly-L-Arginine (PA) moieties were grafted to carboxymethyl cellulose (CMC) to produce biofunctional hybridized macromolecules (CMC_Cys and CMC_PA). These polymer-peptide conjugates were used simultaneously as surface capping ligands and biofunctional modifiers for the synthesis of ternary Ag-In-S (AIS) quantum dots (AIS QDs) via a green chemical process in aqueous medium and room temperature. These core-shell supramolecular nanostructures (AIS@CMC, AIS@CMC_Cys, and AIS@CMC_PA) were tested as fluorescent nanoprobes (“OFF-ON”) for targeted bioimaging and in vitro intracellular tracking of glioblastoma cells (GBM, U-87 MG). The nanosystems were characterized for physicochemical, structural, and morphological properties by NMR, UV–Vis, PL, FTIR, TEM/EDX/SAED, zeta potential, and DLS. Cytocompatibility was evaluated by mitochondrial activity assay, and confocal laser scanning microscopy was performed for investigating the kinetics of cellular uptake. The grafting caused a noticeable reduction of surface charges, associated with a drastic photoluminescence quenching (i.e., “OFF-state”) of AIS@CMC_Cys and AIS@CMC_PA compared to unmodified AIS@CMC. This effect was smartly applied for bioimaging GBM cells and for monitoring the internalization process by intracellular tracking, which underwent strong “de-quenching” at very early incubation times (~5 min). Thus, these novel hybrid nanocolloids produced via eco-friendly scalable aqueous process show potential as responsive fluorescent bioprobes for bioimaging and tracking intracellular pathways and mechanisms as a powerful weapon for fighting against brain cancer cells.

Probing the role of oscillator strength and charge of exciton forming molecular J-aggregates in controlling nanoscale plasmon-exciton interactions.

In this study, we probe into the roles of exciton oscillator strength and charge of J-aggregates as well as nanoparticle's surface capping ligands in dictating the plasmon-exciton interaction. We systematically compare the plasmon-exciton coupling strengths of two hybrid plexcitonic systems involving CTAB-capped hollow gold nanoprisms (HGNs) and two different cyanine dyes, TDBC and PIC, having very similar J-band spectral positions and linewidths, but different oscillator strengths and opposite charges. Both HGN-PIC and HGN-TDBC systems display large Rabi splitting energies which are found to be extremely dependent on dye-concentrations. Interestingly, for our plexciton systems we find that there is interplay between the exciton oscillator strength and the electrostatic interaction amid dyes and HGN-surfaces in dictating the coupling strength. The oscillator strength dominates at low dye-concentrations resulting in larger Rabi splitting in the HGN-PIC system while at high concentrations, a favorable electrostatic interaction between TDBC and CTAB-capped HGN results in larger exciton population of the HGN-surface and in turn larger Rabi splitting for the HGN-TDBC system than the HGN-PIC system even though TDBC has a lower oscillator strength than PIC. The trend in Rabi splitting is just reversed when the HGN surface is modified with a negatively charged polymer, confirming the role of electrostatic interactions in influencing the plasmon-exciton coupling strength.

Exploiting kinetics for assembly of multicomponent nanoparticle networks with programmable control of heterogeneity.

Kinetic control of metal chalcogenide nanoparticle oxidative assembly is realized by varying the redox potential of the chalcogenide, structure (wurtzite vs. zinc blende), and ligand chain length. This knowledge is exploited to form two-component (ZnS + CdSe) hybrid aerogels with minimal heterobonding (phase-segregated) or maximal heterobonding (intimately mixed).

Inorganic clusters as metalloligands: ligand effects on the synthesis and properties of ternary nanopropeller clusters.

Redox-active multimetallic platforms with synthetically addressable and hemilabile active sites are attractive synthetic targets for mimicking the reactivity of enzymatic co-factors toward multielectron transformations. To this end, a family of ternary clusters featuring three edge metal sites anchored on a [Co6Se8] multimetallic support via amidophosphine ligands are a promising platform. In this report, we explore how small changes in the stereoelectronic properties of these ligands alter [Co6Se8] metalloligand formation, but also substrate binding affinity and strength of the edge/support interaction in two new ternary clusters, M3Co6Se8L6 (M = Zn, Fe; L(-) = Ph2PN(-)iPr). These clusters are characterized extensively using a range of methods, including single crystal X-ray diffraction, electronic absorption spectroscopy and cyclic voltammetry. Substrate binding studies reveal that Fe3Co6Se8L6 resists coordination of larger ligands like pyridine or tetrahydrofuran, but binds the smaller ligand CNtBu. Additionally, investigations into the synthesis of new [Co6Se8] metalloligands using two aminophosphines, Ph2PN(H)iPr (LH) and iPr2PN(H)iPr, led to the synthesis and characterization of Co6Se8LH6, as well as the smaller clusters Co4Se2(CO)6LH4, Co3Se(μ2-PPh2)(CO)4LH3, and [Co(CO)3(iPr2PN(H)iPr)]2. Cumulatively, this study expands our understanding on the effect of the stereoelectronic properties of aminophosphine ligands in the synthesis of cobalt chalcogenide clusters, and, importantly on modulating the push-pull dynamic between the [Co6Se8] support, the edge metals and incoming coordinating ligands in ternary M3Co6Se8L6 clusters.

Stable Ultraconcentrated and Ultradilute Colloids of CsPbX3 (X = Cl, Br) Nanocrystals Using Natural Lecithin as a Capping Ligand.

Attaining thermodynamic stability of colloids in a broad range of concentrations has long been a major thrust in the field of colloidal ligand-capped semiconductor nanocrystals (NCs). This challenge is particularly pressing for the novel NCs of cesium lead halide perovskites (CsPbX3; X = Cl, Br) owing to their highly dynamic and labile surfaces. Herein, we demonstrate that soy lecithin, a mass-produced natural phospholipid, serves as a tightly binding surface-capping ligand suited for a high-reaction yield synthesis of CsPbX3 NCs (6–10 nm) and allowing for long-term retention of the colloidal and structural integrity of CsPbX3 NCs in a broad range of concentrations—from a few ng/mL to >400 mg/mL (inorganic core mass). The high colloidal stability achieved with this long-chain zwitterionic ligand can be rationalized with the Alexander–De Gennes model that considers the increased particle–particle repulsion due to branched chains and ligand polydispersity. The versatility and immense practical utility of such colloids is showcased by the single NC spectroscopy on ultradilute samples and, conversely, by obtaining micrometer-thick, optically homogeneous dense NC films in a single spin-coating step from ultraconcentrated colloids.