Ligand Shell Research Articles

Zwitterionic Carbazole Ligands Enhance the Stability and Performance of Perovskite Nanocrystals in Light-Emitting Diodes.

We introduce a new carbazole-based zwitterionic ligand (DCzGPC) synthesized via Yamaguchi esterification which enhances the efficiency of lead halide perovskite (LHP) nanocrystals (NCs) in light-emitting diodes (LED). A facile ligand exchange of the native ligand shell, monitored by nuclear magnetic resonance (NMR), ultraviolet-visible (UV-vis), and photoluminescence (PL) spectroscopy, enables more stable and efficient LHP NCs. The improved stability is demonstrated in solution and solid-state LEDs, where the NCs exhibit prolonged luminescence lifetimes and improved luminance, respectively. These results represent a promising strategy to enhance the stability of LHP NCs and to tune their optoelectronic properties for further application in LEDs or solar cells.

Surface Chemistry of Lead Halide Perovskite Colloidal Nanocrystals.

ConspectusThe surface chemistry of lead halide perovskite nanocrystals (NCs) plays a major role in dictating their colloidal and structural stability as well as governing their optical properties. A deep understanding of the nature of the ligand shell, ligand-NC, and ligand-solvent interactions is therefore of utmost importance. Our recent studies have revealed that such knowledge can be harnessed following a multidisciplinary approach comprising chemical, structural, and spectroscopic analyses coupled with atomistic modeling. We show that specific surface terminations can be produced only by employing flexible and versatile syntheses that enable to work under desired conditions. In this Account, we first describe our studies aimed at synthesizing CsPbBr3 NCs with various surface terminations. These include CsPbBr3 NCs prepared under Br- and oleylamine-rich conditions, which feature a ligand shell composed of alkylammonium-Br species and a photoluminescence quantum yield (PLQY) of ∼90%. On the other hand, taking advantage of the inability of secondary amines to bind to the perovskite NCs surface, we could prepare cuboidal CsPbBr3 NCs bearing a Cs-oleate surface termination and a PLQY of 70% by employing oleic acid and secondary alkylamines. In the quest to identify ligands that can bind more strongly than oleates or primary alkylammonium ions to the surface of NCs already in the synthesis step, we used phosphonic acids as the sole ligands in the CsPbBr3 NCs synthesis, which yielded NCs with a truncated octahedron shape, high PLQY (∼100%), and a PbBr2-terminated surface passivated by hydrogen phosphonates and phosphonic acid anhydride. The surface chemistry and the stability of perovskite NCs were investigated via ad-hoc postsynthesis treatments. We found, for example, that reacting oleylammonium-Br-terminated NCs with stoichiometric amounts of neutral primary alkylamines (or their conjugated acids) led to a partial replacement of oleylammonium ions with new alkylammonium ions (following a deprotonation/protonation mechanism), which resulted in a boost of the PLQY (up to 100%) and of the NCs' colloidal stability. Similar results in terms of optical properties were achieved by treating Cs-oleate-terminated NCs with alkylammonium-carboxylate or quaternary ammonium-Br (namely, didodecyldimethylammonium-Br, DDA-Br) couples. Interestingly, when the native NCs are ligand exchanged with DDA-Br, the ligand shell is then composed of species not bearing any proton. This, in turn, enabled us to study the interaction of such NCs with a variety of ligands under completely aprotic conditions wherein these DDA-Br-capped NCs were basically inert. The only exceptions were carboxylic, phosphonic, and sulfonic acids that were capable of stripping surface DDA-Br couples. As a note, most studies on CsPbBr3 NCs to date have focused primarily on choosing ligands with specific anchoring groups rather than on tuning the length and type of alkyl chains, as this is time-consuming and requires a large number of syntheses. Our recent developments in the computational chemistry of colloidal NCs are expected to provide a pivotal role in this direction since they can be integrated with machine learning models to investigate with greater details the ligand-NC, ligand-ligand, and ligand-solvent interactions and ultimately find optimal candidates through the prediction of surfactant properties using high-throughput data sets.

Covalent Modification of Nanoparticle‐Imprinted Matrices for Selective Nanoparticle Recognition

AbstractNanoparticle‐imprinted matrices (NAIMs) were prepared using ammonium‐terminated SiO2 nanoparticles (NPs) as templates initially attached to activated glassy carbon. After electrodeposition of a matrix of polyplumbagin and polyphenol, the selectivity of the polymer matrix was tuned towards the ligand shell of the Au analyte NPs by conjugate addition of substituted thiols to the plumbagin moiety within the matrix. The reactions were followed by cyclic voltammetry (CV) and the presence of thiols and amino groups in the film was verified by X‐ray photoelectron spectroscopy (XPS). After dissolution of the template NPs, the matrix had complementary cavities for the uptake of Au NPs. The amount of Au NP inside the cavities was quantified by the charge transferred during electrodissolution of the Au NPs in 3 M KCl in linear sweep voltammetry, which depended on the covalent modification of the polyplumbagin matrix, the pH during uptake and the size of the Au analyte NPs. The NAIM system modified with 1,2‐ethanedithiol showed the best uptake with 9.6×107 NPs cm−2 at pH 10, which is five times higher than the uptake of the same matrix at pH 5. In contrast, the cysteamine‐modified NAIM exhibited the highest uptake at pH 5.

Surface Engineering of Metal and Semiconductor Nanocrystal Assemblies and Their Optical and Electronic Devices.

ConspectusColloidal nanocrystals (NCs) are composed of inorganic cores and organic or inorganic ligand shells and serve as building blocks of NC assemblies. Metal and semiconductor NCs are well known for the size-dependent physical properties of their cores. The large NC surface-to-volume ratio and the space between NCs in assemblies places significant importance on the composition of the NC surface and ligand shell. Nonaqueous colloidal NC syntheses use relatively long organic ligands to control NC size and uniformity during growth and to prepare stable NC dispersions. However, these ligands create large interparticle distances that dilute the metal and semiconductor NC properties of their assemblies. In this Account, we describe postsynthesis chemical treatments to engineer the NC surface and design the optical and electronic properties of NC assemblies. In metal NC assemblies, compact ligand exchange reduces the interparticle distance and drives an insulator-to-metal transition tuning the dc resistivity over a 1010 range and the real part of the optical dielectric function from positive to negative across the visible-to-IR region. Juxtaposing NC and bulk metal thin films in bilayers allows the differential chemical and thermal addressability of the NC surface to be exploited in device fabrication. Ligand exchange and thermal annealing densifies the NC layer, creating interfacial misfit strain that triggers folding of the bilayers and is used to fabricate, with only one lithography step, large-area 3D chiral metamaterials. In semiconductor NC assemblies, chemical treatments such as ligand exchange, doping, and cation exchange control the interparticle distance and composition to add impurities, tailor stoichiometry, or make entirely new compounds. These treatments are employed in longer studied II-VI and IV-VI materials and are being developed as interest in III-V and I-III-VI2 NC materials grows. NC surface engineering is used to design NC assemblies with tailored carrier energy, type, concentration, mobility, and lifetime. Compact ligand exchange increases the coupling between NCs but can introduce intragap states that scatter and reduce the lifetime of carriers. Hybrid ligand exchange with two different chemistries can enhance the mobility-lifetime product. Doping increases carrier concentration, shifts the Fermi energy, and increases carrier mobility, creating n- and p-type building blocks for optoelectronic and electronic devices and circuits. Surface engineering of semiconductor NC assemblies is also important to modify device interfaces to allow the stacking and patterning of NC layers and to realize excellent device performance. It is used to construct NC-integrated circuits, exploiting the library of metal, semiconductor, and insulator NCs, to achieve all-NC, solution-fabricated transistors.

Isolation and Structures of Polyarene Palladium Nanoclusters.

We report that surrounding coordination of neutral six-membered arene rings affords molecularly well-defined organotransition metal nanoclusters. With the use of [2.2]paracyclophane as the face-capping arene ligand, we have isolated two polyarene palladium nanoclusters, one consisting of a hexakis-arene ligand shell and a hexagonal close-packed Pd13 anticuboctahedron trichloride core, and the other consisting of an octakis-arene ligand shell and a non-close-packed Pd17 square gyrobicupola dichloride core, both with Pd-Pd direct bonding. The μ4-facial coordination mode of arene was discovered through the structural characterization of the Pd13 cluster. Their Pd13 and Pd17 cores, which are distinct from the previously identified face-centered-cubic Pd13 core surrounded by seven-membered cycloheptatrienyl, are explained by stereochemical and theoretical analyses.

Photocrosslinkable Zwitterionic Ligands for Perovskite Nanocrystals: Self‐Assembly and High‐Resolution Direct Patterning

AbstractPerovskite nanocrystals (PNCs) are attractive photoactive materials in various optoelectronic devices including light‐emitting diodes, solar cells, and photodetectors. However, the weakly bound surface ligands on PNCs reduce colloidal stability and cause film formation and patterning difficulties, severely restricting their practical applications. Herein, a rationally designed photocrosslinkable zwitterionic (PZ) ligand is introduced to obtain directly patternable CsPbBr3 PNCs with enhanced colloidal stability, optical properties, and self‐assembly propensity. The PZ ligands strongly interact with the pre‐synthesized PNCs in solution, substantially replacing the original capping ligands and effectively passivating surface defects. This surface engineering induces strong electrostatic interactions between the PNCs, enabling the fabrication of densely packed CsPbBr3 PNC films. Furthermore, the methacrylate group of the PZ ligands serves as a bridge for active radical propagation in the ligand shells around the PNCs upon UV exposure. Accordingly, high‐resolution direct photopatterning can be achieved through ligand crosslinking, and the resulting PNC patterns (minimum line spacing of 4 µm) maintain optical stability for over 2 weeks. Therefore, this study demonstrates that a tailored ligand design strategy enables the simultaneous achievement of high colloidal stability, optical properties, photopatternability, and self‐assembly propensity and has considerable potential to be extended to other PNC materials.

Ligands on Nanocrystal Surfaces, the 1H Nuclear Magnetic Resonance Fingerprint.

ConspectusSurfaces are an integral part of colloidal nanocrystals (NCs). Hence, understanding the binding and packing to NC surfaces of organic ligands, which are often used to stabilize NC colloids, is an essential aspect of the formation of NCs with desired chemical or physical properties. Since NCs lack a unique structure, not a single analytical technique can provide a complete description of the chemistry of NC surfaces. Even so, solution 1H nuclear magnetic resonance spectroscopy stands out as a unique method to study the organic ligand shell for its capability to distinguish between surface bound species and surface inactive residues from NC synthesis and purification.In this account, we first set the stage by highlighting the fingerprints of ligands bound to NCs in solution 1H NMR, which are broadened and shifted resonances, slow diffusion, and pronounced transfer of spin polarization between nearby protons. These characteristics enable bound ligands to be identified and quantified by 1D 1H NMR spectroscopy, diffusion-ordered spectroscopy (DOSY), and nuclear Overhauser effect spectroscopy (NOESY). Even so, we argue in a second part that much more insight in surface chemistry can be obtained from the in situ monitoring of ligand exchange processes. The chemical analysis of released compounds and the thermodynamic study of exchange equilibria provide a surprisingly detailed picture of the chemistry of the NC-ligand bond, the heterogeneity of binding sites, and the bunching of ligands on the NC surface. Multiple case studies are discussed to illustrate these different aspects of NC surface chemistry, where work on CdSe NCs in particular indicates that binding sites at facet edges are most vulnerable for ligand loss. While such weak binding sites are a liability for optoelectronic applications, they could offer an opportunity for catalysis. Moreover, the general character of the methodology introduced calls for realizing a broad, quantitative survey of NC-ligand interactions, well beyond the extensively studied case of CdSe NCs.In a third part, we address in more detail the line broadening that characterizes ligands bound to NCs, which results from a combination of reduced mobility and a diversity of chemical environments. Hence, chemical shift and line shape, or rates of transversal relaxation and interligand cross-relaxation, can all convey information on the ligand environment, especially when solvents are used that are chemically distinct from the ligand chain, such as aromatic versus aliphatic. Two examples that illustrate this point are the relation between line width and ligand solvation, where better solvated ligands yield more narrow resonances, and the possibility to identify different parts of the inhomogeneously broadened resonance with ligands bound on different locations at the NC surface. Interestingly, such results question the limits of NC size and ligand packing density at which the current bound-ligand paradigm, modest inhomogeneous broadening, will break down. Building on this question, we summarize in a final part the current status of NC ligand analysis by solution 1H NMR and outline directions for further research.

Localizing Oleylamine Ligands on Amine–Halide Copassivated Indium Phosphide Nanocrystals

The surface capping of organic ligands is a full part of a colloidal nanocrystal (NC) that provides colloidal stability, helps passivating electronic trap-states, or governs charge hopping between adjacent NCs. A central aspect to understanding this ligand shell is its inherently heterogeneous nature caused by, for example, site- or facet-dependent adsorption energies. Even so, the identification of ligands bound to different surface sites remains highly challenging. In this work, we propose a case study on oleylamine/chloride copassivated InP NCs, for which we show that the heterogeneous broadening of 1H resonances in the nuclear magnetic resonance spectrum enables ligands adsorbed to facets to be distinguished from edge- or corner-bound ligands. More specifically, we demonstrate by means of the recovery of spectral holes burned in the alkene resonance of surface-bound oleylamine that the ligand shell consists of spatially distinct pools of oleylamine characterized by a different average chemical shift. Combining this observation with classical molecular dynamics simulations, we assign the downfield and upfield side of the alkene resonance to oleylamine bound to facets, or edges and corners, respectively. Importantly, we show that this proposed spectral fingerprint of surface-localization of ligands agrees with the change in shape of the alkene resonance when increasing the NC size and when exchanging part of the oleylamine for hexadecanethiol. Given the general applicability of the methodology introduced here, we believe this work offers a starting point to assess ligand localization for a broad range of colloidal NCs and develop a better understanding of the binding and packing of ligands on NC surfaces.

The Colloidal Stability of Apolar Nanoparticles in Solvent Mixtures

Solvent engineering is a powerful and versatile method to tune colloidal stability. Here, we link the molecular structure of apolar ligand shells on gold nanoparticles with their colloidal stability in solvent mixtures. The agglomeration temperature of the particles was measured with small-angle X-ray scattering. It depended on solvent composition and changed linearly for hexane-hexadecane mixtures, but nonlinearly for cyclohexane-hexadecane and hexanol-hexadecane mixtures. Molecular dynamics (MD) simulations indicate that agglomeration is dominated by temperature-dependent ligand order in the alkane mixtures and that the temperature at which the ligand shell orders depends on the solvent composition near the ligands, which can differ substantially from the bulk composition. Small-angle neutron scattering confirmed that, at intermediate solvent compositions above the agglomeration temperature, the fraction of cyclohexane near the ligands was larger than in the bulk. The enrichment of cyclohexane near the ligands stabilized their disordered state, which, consequently, led to the experimentally observed nonlinear trend of the agglomeration temperature. In contrast, hexanol was depleted from the ligand shell at all temperatures. This again stabilized the disordered state. Furthermore, we found that agglomeration at high hexanol fractions was driven by a solvophobic effect that exceeded the influence of ligand order. The results show that strong nonlinearities in the colloidal stability of nanoparticle dispersions in solvent mixtures are directly linked to the molecular details of ligand-solvent and solvent-solvent interactions, which can be used to precisely tune stability.

Effect of a redox-mediating ligand shell on photocatalysis by CdS quantum dots.

Semiconductor quantum dots (QDs) are efficient organic photoredox catalysts due to their high extinction coefficients and easily tunable band edge potentials. Despite the majority of the surface being covered by ligands, our understanding of the effect of the ligand shell on organic photocatalysis is limited to steric effects. We hypothesize that we can increase the activity of QD photocatalysts by designing a ligand shell with targeted electronic properties, namely, redox-mediating ligands. Herein, we functionalize our QDs with hole-mediating ferrocene (Fc) derivative ligands and perform a reaction where the slow step is hole transfer from QD to substrate. Surprisingly, we find that a hole-shuttling Fc inhibits catalysis, but confers much greater stability to the catalyst by preventing a build-up of destructive holes. We also find that dynamically bound Fc ligands can promote catalysis by surface exchange and creation of a more permeable ligand shell. Finally, we find that trapping the electron on a ligand dramatically increases the rate of reaction. These results have major implications for understanding the rate-limiting processes for charge transfer from QDs and the role of the ligand shell in modulating it.

Triple Ligand Engineered Gold Nanoclusters with Enhanced Fluorescence and Device Compatibility for Efficient Electroluminescence Light-Emitting Diodes.

Gold nanoclusters (Au NCs) are potential emitters for electroluminescent light-emitting diodes (EL-LEDs) but restricted by the limited photoluminescence quantum yield (PLQY) and poor device compatibility. Herein, triple ligand engineered Au NCs enable the fabrication of Au NC-based LEDs with improved EL efficiency. Rigidified triple ligand shells greatly reduce the nonradiative transition and thus increase the PLQY of Au NCs from 2.1 to 73.4%. Most importantly, this strategy significantly improves the compatibility between Au NCs and charge transport materials in EL-LED fabrication. As a result, the EL-LEDs reach a maximum brightness of 1104 cd/m2 and an external quantum efficiency of 5.1%, which is the highest recorded for any reported Au NC-based EL-LEDs.

Patterning of Hydrophilic and Hydrophobic Gold and Magnetite Nanoparticles by Dip Pen Nanolithography (Small 18/2023)

Dip Pen Nanolithography In article number 2208069, Bart Jan Ravoo and co-workers report the surface patterning of nanoparticles using dip pen nanolithography. Metal and metal oxide nanoparticles functionalized with various organic ligands can be patterned in nanosized clusters. Depending on the polarity of the ligand shell, water or toluene is used as ink solvent. No modification of atomic force microscope (AFM) tip or substrate surface is required.

Spontaneous Patterning of Binary Ligand Mixtures on CdSe Nanocrystals: From Random to Janus Packing.

Binary compositions of surface ligands are known to improve the colloidal stability and fluorescence quantum yield of nanocrystals (NCs), due to ligand-ligand interactions and surface organization. Herein, we follow the thermodynamics of a ligand exchange reaction of CdSe NCs with alkylthiol mixtures. The effects of ligand polarity and length difference on ligand packing were investigated using isothermal titration calorimetry (ITC). The thermodynamic signature of the formation of mixed ligand shells was observed. Correlating the experimental results with thermodynamic mixing models has allowed us to calculate the interchain interactions and to infer the final ligand shell configuration. Our findings demonstrate that, in contrast to macroscopic surfaces, the small dimensions of the NCs and the subsequent increased interfacial region between dissimilar ligands allow the formation of a myriad of clustering patterns, controlled by the interligand interactions. This work provides a fundamental understanding of the parameters determining the ligand shell structure and should help guide smart surface design toward NC-based applications.

Bringing Selectivity in H/D Exchange Reactions Catalyzed by Metal Nanoparticles through Modulation of the Metal and the Ligand Shell

Ru and Rh nanoparticles catalyze the selective H/D exchange in phosphines using D2 as the deuterium source. The position of the deuterium incorporation is determined by the structure of the P-based substrates, while activity depends on the nature of the metal, the properties of the stabilizing agents, and the type of the substituent on phosphorus. The appropriate catalyst can thus be selected either for the exclusive H/D exchange in aromatic rings or also for alkyl substituents. The selectivity observed in each case provides relevant information on the coordination mode of the ligand. Density functional theory calculations provide insights into the H/D exchange mechanism and reveal a strong influence of the phosphine structure on the selectivity. The isotope exchange proceeds via C-H bond activation at nanoparticle edges. Phosphines with strong coordination through the phosphorus atom such as PPh3 or PPh2Me show preferred deuteration at ortho positions of aromatic rings and at the methyl substituents. This selectivity is observed because the corresponding C-H moieties can interact with the nanoparticle surface while the phosphine is P-coordinated, and the C-H activation results in stable metallacyclic intermediates. For weakly coordinating phosphines such as P(o-tolyl)3, the interaction with the nanoparticle can occur directly through phosphine substituents, and then, other deuteration patterns are observed.

Re-Examination of the Polymer Encapsulation of Quantum Dots for Biological Applications

The application of semiconductor core–shell quantum dots in biology requires the dispersion of these nanoparticles in water. A widely used phase transfer strategy is their encapsulation with amphiphilic polymers, a method that is believed to preserve the original ligand shell of quantum dots. In this contribution, we test this belief for the first time using a common encapsulation protocol based on poly(styrene-co-maleic anhydride) and ethanolamine and demonstrate that the original quantum dot ligand shell is in fact altered during the polymer encapsulation. In particular, our results show that primary amines, which are typically added to increase the hydrophilicity of the encapsulating polymer, displace the original ligands as a necessary step in the phase transfer process. We demonstrate that the surface of quantum dots synthesized using phosphinic acid as a ligand is readily modified by ethanolamine and hence shows high phase transfer efficiency, whereas quantum dots synthesized using oleic acid ligands did not show reactivity toward the primary amine and failed to transfer in the same conditions. Instead, a thiol, mercaptoethanol, was employed to displace the original oleic acid ligands and promote efficient poly(styrene-co-maleic anhydride) encapsulation. This work also provides practical guidelines for how the quantum dot community can better design and implement phase transfer protocols for a given composition, in order to maximize the efficiency and durability of the resulting water-dispersible nanoparticles for biological applications.

Lyotropic Nematic Phases of Isotropic Nanoparticles via Semiflexible Polymer Ligands.

Lyotropic liquid crystalline(LC) nanomaterials are normally achieved through particle shape anisotropy. Herein, it is shown that lyotropic nematic rather than cubic phases are produced from sphericalnanoparticles (NPs) with semi-flexible polymer ligands. ZrO2 nanocrystals (4nm dia.) are coated with a dense shell of poly(hexyl isocyanate) (PHIC), a helical rod-like polymer that forms lyotropic LC phases in a range of organic solvents. Solvent casted NPs with PHIC ligands above the persistence length form linear assemblies, separated by a characteristic distance related to the chain length while NPs with shorter, rigid rod PHIC ligands pack hexagonally. Concentrated NP-PHIC dispersions present nematic textures similar to the free PHIC nematic solutions but at lower critical concentrations, widening the isotropic-nematic biphasic region. 2 H NMR spectra of the NPs dispersed in a deuterated solvent display quadrupolar splittings that increase with NP concentration, showing that the PHIC ligands are magnetically aligned. The high degree of orientation order is evidence that splaying of the ligand shell transforms the spherical NPs to rod-like shapes that assemble to produce nematic lyotropic LC phases and linear NP arrays. This approach to creating anisotropic assemblies can be extended to other types of spherical NPs and semiflexible polymers.

Regulation of Chirality Transfer and Amplification from Chiral Cysteine to Gold Nanorod Assemblies using Nonchiral Surface Ligands

AbstractOwing to large chiroptical activity and high tunability, dynamic chiral superstructures of plasmonic nanoparticles have attracted great attention. A useful strategy has been developed to obtain strong plasmonic chiroptical responses (plasmonic circular dichroism, PCD) by adding small chiral thiols on achiral side‐by‐side assemblies of gold nanorods (AuNRs). Despite the critical role of chiral thiols, the impact of achiral surface ligands is yet to be revealed. Herein, the effects of achiral surface ligands are systematically investigated from the viewpoints of counter ion, hydrocarbon chain length, and head group of surfactants. A well‐packed ligand shell is found to be beneficial for the chirality transfer and amplification from adsorbed chiral thiols to rod assemblies. The PCD temperature amplification effect is observed by using different surfactant ligands and reflects the energy cost of rod‐twisting in the assemblies. This study deepens the understanding of chiral plasmonic assemblies driven by small chiral molecules and provides rational design to fabricate such assembly structures.

Zr Oxo Cluster for Cascade Conversion of Furfural to Alkyl Levulinates

AbstractA novel dodecanuclear Zr oxo cluster [Zr6O4(OH)4 (HSCH2CH2COO)12]2 (ZrO‐SH‐10) has been constructed under the room temperature, followed by oxidation of the sulfhydryl group with H2O2 to achieve a bifunctional catalyst with Lewis acid and Brönsted acid sites. The characterization of catalysts indicated that {Zr6O4} cluster core can be stabilized with a shell of carboxylate ligands, and the resulting ZrO‐SO3H was formed as a discrete molecular catalyst, exhibiting superior activity and recyclability for the cascade conversion of furfural to alkyl levulinate by the integration of transfer hydrogenation and alcoholysis in n‐butanol. The yield of n‐butyl levulinate can achieve as high as 91 % under the optimum conditions. Meanwhile, no leaching of the active species was found, which confirmed the structure of the Zr oxo cluster was air and moisture‐stable. On the basis of the studies on the reaction kinetics and isotope tracking, the reaction mechanism was proposed accordingly.

Langmuir and Langmuir-Blodgett Films of Gold and Silver Nanoparticles.

Recently the focus of the Langmuir-Blodgett technique as a method of choice to transfer monolayers from the air/water interface onto solid substrates in a controllable fashion has been shifting toward purely hydrophobic gold and silver nanoparticles. The fundamental interactions between particles that become relevant in the absence of polar groups range from dispersive attractions from the metal cores and repulsions between ligand shells to weaker entropic factors. The layer evolution is explored, starting with interfacial self-assembly upon solution spreading and domain and circular island formation, which subsequently merge into a complete monolayer and finally form multilayers or macroscopic wrinkles. Moreover, structural properties such as the core:ligand size ratio are investigated in the context of dispersive forces, whereby the nanoparticles with small cores and long ligands tend not to aggregate sufficiently to produce continuous films, those with large cores and short ligands were found to aggregate irreversibly, and those in between the two extremes were concluded to be able to form highly organized crystalline films. Similarly, the characteristics of the spreading solution such as the concentration and the solvent type crucially influence the film crystallinity, with the deciding factor being the degree of affinity between the capping ligand and the solvent used for spreading. Finally, the most common strategies employed to enhance the mechanical stability of the metal nanoparticle films along with the recent attempts to functionalize the particles in attempts to improve their applicability in the industry are summarized and evaluated in relation to their future prospects. One of the objectives of this feature article is to elucidate the differences between hydrophobic metal nanoparticles and typical amphiphilic molecules that the majority of the literature in the field describes and to familiarize the reader with the knowledge required to design Langmuir-Blodgett nanoparticle systems as well as the strategies to improve existing ones.

Binding Affinity of Monoalkyl Phosphinic Acid Ligands toward Nanocrystal Surfaces

We recently introduced monoalkyl phosphinic acids as a ligand class for nanocrystal (NC) synthesis. Their metal salts have interesting reactivity differences compared to metal carboxylates and phosphonates and provide a cleaner work-up compared to phosphonates. However, little is known about the surface chemistry of NCs with monoalkyl phosphinate ligands. Here, we probe the relative binding affinity of monoalkyl phosphinate ligands with respect to other X-type ligands. We perform competitive ligand exchange reactions with carboxylate and phosphonate ligands at the surface of hafnia, cadmium selenide, and zinc sulfide NCs. We monitor the ligand shell composition by solution 1H and 31P NMR spectroscopy. Using a monoalkyl phosphinic acid with an ether functionality, we gain an additional NMR signature, apart from the typical alkene resonance in oleic acid and oleylphosphonic acid. We find that carboxylate ligands are easily exchanged upon exposure to monoalkyl phosphinic acids, whereas an equilibrium is reached between monoalkyl phosphinates and phosphonates, slightly in the favor of phosphonate (K = 2). Phosphinic acids have thus an intermediate binding affinity between carboxylic acids and phosphonic acids for all the NCs studied. These results enable the sophisticated use of monoalkyl phosphinic acids for NC synthesis and for post-synthetic surface engineering.