Ligand Exchange Process Research Articles

Surface Decomposition and Healing in Solution-Phase Ligand Exchange for Efficient Colloidal Quantum Dot Solar Cells

The efficiency of solution-processed colloidal quantum dot (CQD) thin-film solar cells has been significantly improved recently. However, there is still potential for efficiency improvements, which can be realized through a deeper understanding of surface chemical treatment for CQDs. In this study, we developed CQD thin-film solar cells with an improved power conversion efficiency (PCE) of 11.97%. We accomplished this by simply controlling the species and their molar concentrations used in the surface chemical treatment in the solutionphase ligand exchange process. Iodine treatment that generates conductive CQDs induces surface defects via cationic decomposition of the CQD surface; healing the CQD surface requires an excess of lead cations in the solution-phase ligand exchange process. Accordingly, we found that manufacturing CQD thin-film solar cells that have high efficiencies requires well-controlled surface treatment to effectively exchange surface ligands and simultaneously minimize surface defects on the CQD surface.

Alkane C-H activation and ligand exchange on silica supported d0 metal alkylidenes: relevance to alkane metathesis.

In this work, we study the ligand exchange process between an alkane and a series of silica supported metal alkylidenes, which may occur by different pathways: C-H addition, σ-bond metathesis, and α-H abstraction. The results indicate that the α-H abstraction pathway is the preferred one, regardless of the catalyst and ligands. This is in contrast to the expected preference for the C-H addition route. When looking for the origin of this preference, our calculations revealed that the α-H abstraction pathway is driven by entropy, which favors the initial dissociation of the alkyl ligand from the catalyst.

Enhancement on charge transfer properties of Cu12Sb4S13 quantum dots hole transport materials by surface ligand modulation in perovskite solar cells

A two-step surface ligand-exchange process has been introduced for Cu12Sb4S13 QDs hole transport materials, which effectively enhanced the photovoltaic conversion efficiency of perovskite solar cells from 14.65% to 15.43%.

Chiral amine ligands at rhenium(V) pentahydride complexes allow for characterization of an energetically accessible and reversible steric inversion of diastereotopic phosphorus atoms

Three new rhenium(V) pentahydride complexes stabilized by a pair of triphenylphosphine ligands and a chiral amine ligand were prepared. Dynamic 31P-{1H} NMR spectroscopy of the chiral complexes demonstrates an unusual steric inversion for the diastereotopic pair of phosphorus atoms in each chiral complex. One chiral ReH5(PPh3)2(amine) complex is supported by an aromatic amine ligand and demonstrates unambiguously that such ligands participate in a steric inversion dynamic process rather than a simple rotation about the Re-N bond. The activation parameters and rate constants for the steric inversion can be determined directly or indirectly for non-hydride atom dynamic processes in two of the three chiral complexes from simulations of dynamic NMR spectra. Characterization of the steric inversion step by atoms other than hydride ligands allows for a means to test whether any hydride ligand exchange processes are manifestations of the steric inversion process. Under the experimental conditions in this report, a pairwise exchange of hydride ligands that reside in pseudododecahedral A sites appears to be a separate manifestation of the steric inversion dynamic process. A change in the nominal coordination number seems likely to cause the steric inversion that is reported.

PbTe Kuantum Noktalar ve Fotovoltaik Uygulamalarda Bant Enerji Hizalamasının Yapılması

Lead telluride (PbTe) quantum dots, despite being considered as one of the most promising candidates for future photovoltaics owing to their higher multiple exciton generation yields, have received limited attention in solar cell designs due their less explored surface chemistry and high air sensitivity. This study demonstrates the synthesis and characterization of highly crystalline PbTe QDs and their utilization in solution processed solar cells through band alignment engineering. Ultraviolet photoelectron spectroscopy studies showed that the conduction and valence band levels depend strongly on the type of surface ligand utilized for the ligand exchange process. Conduction and valence band levels of tetrabutylammonium iodide (TBAI) and 1,2-ethanedithiol (EDT) treated PbTe QDs with respect to vacuum were measured as -3.73 eV/-4.83 eV and -3.48 eV/-4.45 eV, respectively. The presence of a band offset between the conduction and valence band levels of TBAI and EDT treated layers allowed us to engineer the band alignment in the light absorbing layer. As a result, solar cells where TBAI and EDT ligand treated QDs were utilized in a bilayer cell architecture reached a photo conversion efficiency of 0.65%.

Suppressing the Dark Current in Quantum Dot Infrared Photodetectors by Controlling Carrier Statistics

AbstractLead sulfide colloidal quantum dot photodiodes (PbS QDPDs) exhibit a high energy conversion efficiency for infrared detection. Despite the high photoinduced current, the performance of PbS QDPDs is limited by the high dark current which is rarely investigated. Understanding the dark current in PbS QDPDs is critical to improving the detectivity of PbS QDPDs. Herein, it is demonstrated that minority carriers of I‐passivated PbS films and trap sites of EDT‐passivated PbS films are related to the dark current of PbS QDPD. Utilizing annealing and low‐temperature ligand exchange processes, the dark current density can be decreased almost tenfold by suppressing the minority carrier diffusion in the PN junction and trap‐assisted charge injection from the electrode. PN junction simulation, space charge limited current measurements, as well as structural, optical, and chemical characterizations are conducted to elucidate the origins of the dark current suppression. The authors achieve the lowest dark current density of 2.9 × 10−5 mA cm−2 at −1 V among PbS‐based QDPDs and a high detectivity of 6.7 × 1012 Jones at 980 nm. It is believed that this work provides fundamental understanding of carrier statistics in nanomaterials and device performance as well as a technological basis for realizing low‐cost high‐performance optoelectronic devices.

Quantum dot thin film imaging enables in situ, benchtop analysis of ligand exchange at the solution-film interface

This paper discusses experiments using benchtop video imaging to track the contraction of PbS quantum dot (QD) thin films supported on a liquid subphase during ligand exchange of the native oleate (C 17 H 33 O 2 − ) ligands with various primary carboxylic acids (C n H 2n+1 CO 2 H; n = 0–5), thiols (C n H 2n+1 SH; n = 2–6), and amines (C n H 2n+1 NH 2 ; n = 2–6). While widely used in the preparation of research-grade photovoltaics and thin-film transistors, this ligand exchange process at the thin film/solution interface has not been studied in situ and in real time with sufficient time resolution to distinguish fast ligand exchange processes from slower, destructive processes such as surface etching. Effective ligand shell lengths L e calculated from film areas were consistent with previous TEM studies, so film area measurements determined from video images effectively track changes in interparticle spacing on the nanoscale. L e values distinguished ligands that generated high degrees of oleate displacement (carboxylic acids and thiols) from those that generated low degrees of oleate displacement (amines). Displacement of the native ligand was found to limit the rate of film contraction, so contraction rates serve as a probe of underlying ligand exchange dynamics. When the entering ligand was in excess, the film contraction rate displayed a sublinear dependence on its concentration. This was explained in terms of a two-step mechanism involving the pre-equilibrium association of the entering ligand with the QD surface followed by dissociation of oleate. Thiols and carboxylic acids produced faster oleate displacement than did amines, but ligand length was not observed to have any consistent effect on the oleate displacement rate. The ability of carboxylic acids and thiols to promote oleate dissociation through proton transfer was proposed as an explanation of this rate difference as 1 H NMR spectra of colloidal-phase samples showed carboxylic acids and thiols generated protonated oleic acid while amines did not. These experiments demonstrate the utility of video imaging of QD thin films suspended on a liquid subphase as a simple and rapid benchtop technique for monitoring chemical and structural variables relevant for QD-based device preparation in situ . • PbS quantum dot thin film contraction is limited by the rate of oleate displacement. • Benchtop Video imaging can track dot reorganization and ligand exchange in situ. • The rate of oleate exchange increases with increasing entering ligand concentration. • Carboxylic acids and thiols displace oleate faster from PbS dots than do amines. • Amines displace less oleate from PbS dot films than do carboxylic acids or thiols.

Multidimensional Mass Spectrometry Assisted Metallo-Supramolecular Chemistry

Multidimensional Mass Spectrometry Assisted Metallo-Supramolecular Chemistry

Metallic fusion of nanocrystal thin films for flexible and high-performance electromagnetic interference shielding materials

Low-cost flexible electromagnetic interference (EMI) shielding materials are attracting considerable attention because of the rapid development of wearable smart electronics, such as wearable health monitoring systems, and flexible energy storage and harvesting systems. In the present study, we developed ultrathin, flexible, and high-performance EMI shielding materials using Ag nanocrystals (NCs) through low-cost, room-temperature wet chemical processes conducted at atmospheric pressure. Sequential ligand-exchange and reduction processes not only substantially reduced the distance between the Ag NCs but also induced intensive metallic fusion of the Ag NCs, resulting in large-scale three-dimensional interconnected conductive pathways. The fused Ag NC thin films exhibited high electrical conductivity (∼24,000 S/cm for a film ∼800 nm thick) and outstanding mechanical stability, offering stable EMI shielding performance over 1000 cycles of various mechanical deformations, including twisting, crumpling, and folding. In addition, compared with most previously reported solution-based materials, our Ag NC thin films demonstrated an excellent EMI shielding effectiveness value of ∼60 dB in the X-band range 8.2–12.2 GHz at much thinner thickness (∼1.3 μm). With the advantages of easy processing, good mechanical flexibility, and high-performance EMI shielding, the fused NC thin films developed in the present work represent innovative potential EMI shielding materials for next-generation wearable smart electronics.

Selective ligand exchange synthesis of Au16(2-PET)14 from Au15(SG)13.

Replacement of protecting ligands of gold nanoclusters by ligand exchange has become an established post-synthetic tool for selectively modifying the nanoclusters' properties. Several Au nanoclusters are known to additionally undergo size transformations upon ligand exchange, enabling access to cluster structures that are difficult to obtain by direct synthesis. This work reports on the selective size transformation of Au15(SG)13 (SG: glutathione) nanoclusters to Au16(2-PET)14 (2-PET: 2-phenylethanethiol) nanoclusters through a two-phase ligand exchange process at room temperature. Among several parameters evaluated, the addition of a large excess of exchange thiol (2-PET) to the organic phase was identified as the key factor for the structure conversion. After exchange, the nature of the clusters was determined by UV-vis, electrospray ionization-time of flight mass spectrometry, attenuated total reflection-Fourier transform infrared, and extended x-ray absorption fine-structure spectroscopy. The obtained Au16(2-PET)14 clusters proved to be exceptionally stable in solution, showing only slightly diminished UV-vis absorption features after 3 days, even when exposed to an excess of thiol ligands.

Designing a nanocrystal-based temperature and strain multi-sensor with one-step inkjet printing

Wearable multi-sensors based on nanocrystals have attracted significant attention, and studies on patterning technology to implement such multi-sensors are underway. Conventional patterning processes may affect material properties based on high temperatures and harsh chemical conditions. In this study, we developed an inkjet printing technique that can overcome these drawbacks through the application of patterning processes at room temperature and atmospheric pressure. Nanocrystal-based ink is used to adjust properties efficiently. Additionally, the viscosity and surface tension of the solvents are investigated and optimized to increase patterning performance. In the patterning process, the electrical, electrothermal, and electromechanical properties of the nanocrystal pattern are controlled by the ligand exchange process. Experimental results demonstrate that a multi-sensor with a temperature coefficient of resistance of 3.82 × 10-3 K-1 and gauge factor of 30.6 can be successfully fabricated using one-step inkjet printing.

A Ligand Exchange Process for the Diversification of Palladium Oxidative Addition Complexes.

Palladium oxidative addition complexes (OACs) have recently emerged as useful tools to enable challenging bond connections. However, each OAC can only be formed with one dative ligand at a time. As no one ligand is optimal for every cross-coupling reaction, we herein disclose a ligand exchange protocol for the preparation of a series of OACs bearing a variety of ancillary ligands from one common complex. These complexes were further applied to cross-coupling transformations.

Formation of the Inorganic and Organic Shells on the Surface of CdSe Quantum Dots.

Embedding quantum dots (QDs) into an organic matrix of controllable order requires the identification of their structural characteristics. This analysis is necessary for the creation of anisotropic composites that are sensitive to external stimuli. We have studied the QD structures formed during the single-step synthesis of CdSe/ZnS QDs and their transformations after the initial ligand's substitution for another ligand. This single-step process leads to the formation of the core/shell structure. We detect the presence of two oleic acid residues ionically connected to Zn and Cd. At the same time, the amount of Cd oleate at the surface is very small. We observe the ligand exchange process at the surface of the core/shell QDs. The oleic acid residues are substituted by terphenyl-containing (TERPh-COOH) aromatic acid residues. The reaction between CdSe/ZnS carrying TOP and oleic acid residues ionically bound with QDs and terphenyl-containing acid leads to the coexistence of multiple ligands on the QD surface at a ratio of 11:6:33 for TOP/OA/TERPh-COOH.

Water-Dispersible Fe3O4 Nanoparticles Modified with Controlled Numbers of Carboxyl Moieties for Magnetic Induction Heating

Monodisperse Fe3O4 nanoparticles (NPs) with excellent water dispersibility and stability were prepared by the introduction of the carboxyl group (−COOH) on the surface of the as-prepared oleyl-capped Fe3O4 NPs. Controlled introduction of the COOH moieties on the NP surface was carried out by a simple ligand exchange reaction using two types of phosphonic acid ligands with dodecyl moiety and the terminal COOH group. The degree of modification of the COOH group on the particle surface was controlled by tuning the molar ratios of the two phosphonic acids. During the ligand exchange reaction, no change was observed in the crystal structure and morphology of the Fe3O4 NPs. The results of Fourier transform infrared (FT-IR) spectroscopy confirmed that the two ligands were bound to the Fe3O4 NP surface through their phosphoric acid functional groups. The surface coverage and molar ratios of the two ligands were evaluated by thermal gravimetric analysis (TGA) and via proton nuclear magnetic resonance (1H NMR) measurements, respectively. Results obtained from the thermal analyses were consistent with the initial molar ratios of the ligands used in the reaction, which reflect on the efficiency of the developed ligand exchange process. Our COOH-modified Fe3O4 NPs could be dispersed in water by deprotonation for over 6 months and exhibit a typical ferrofluid behavior without the addition of other surfactants and dispersants, showing high dispersion stability. Furthermore, the magnetic induction heating performance of the Fe3O4 NPs in aqueous dispersions was evaluated and the specific absorption rate (SAR) value was estimated to be 38.3 W/g Fe. These results suggest that our COOH-modified Fe3O4 NPs with the desired number of COOH surface moieties can be advantageous for applications such as precise surface design, water-based ferrofluid, and hyperthermia treatment.

Mechanistic understanding of antimony(V) complexation on montmorillonite surfaces: Insights from first-principles molecular dynamics

Clay minerals play a vital role in retaining antimony (Sb) by surface complexation, which can largely attenuate its mobility and availability. However, a profound understanding of the environmental process of Sb on clay-water interface is insufficient. In this study, aiming to quantitatively revealing the complexation mechanism of Sb(V) onto montmorillonite at the molecular scale, we performed first-principles molecular dynamics (FPMD) simulations to systematically investigate the related structural and energetic information. In interlayer space, [Sb(OH)6]- anion can bind with Ca2+/Na+/Al3+/Fe3+ as stable inner-sphere complexes, and the Sb-Al/Fe complex is much more favorable than Sb-Na/Ca complex. On edge surfaces, the ligand exchange processes on possible sites in a wide range of pH have been examined. At (010) edge, [Sb(OH)6]− was adsorbed as bidentate complex on Al-(H2O)(H2O) and Mg-(H2O)(H2O) and monodentate complex on Al-(H2O)(OH), and at (110) edge [Sb(OH)6]− formed monodentate complex on Al-(H2O) and Mg-(H2O). The desorption free energies indicated that the Sb-Al bidentate complex is the most stable and favorable edge species, followed by the Sb-Mg bidentate complex. Overall, this work provides the molecular-scale insights into the fate of antimony in natural environments, and the derived fundamental data contribute to improving the experiment-based surface complexation models (SCM).

Ligand‐Free MAPbI3Quantum Dot Solar Cells Based on Nanostructured Insulating Matrices

The stability, either chemical or thermal, and performance of colloidal quantum dot (CQD) devices are typically limited by the presence of surface‐bonded organic ligands required to stabilize the nanocrystals. In addition, optimization of charge transport implies lengthy ligand exchange processing. Herein, evidence of efficient charge transport through a network of ligand‐free perovskite quantum dots (PQDs) embedded in an insulating porous matrix made of monodisperse SiO2nanoparticles is shown. Methylammonium lead iodide (CH3NH3PbI3or MAPbI3) QDs are prepared in situ by infiltration of precursors within the matrix pores, which act both as nanoreactors for the synthetic reaction and as supporting scaffolds, hence reducing the number of synthetic and postprocessing steps usually required in CQD solar cells. Above a certain nanocrystal load, charge percolation is reached and dot‐to‐dot transport achieved without compromising quantum confinement effects. Solar cells based on MAPbI3QDs prepared in this way present a 9.3% efficiency, the highest reported for a scaffold‐supported PQD solar cell, and significantly improved stability under solar illumination with respect to their bulk counterparts. Therefore, adequately designed networks of ligand‐free PQDs can be used as both light harvesters and photocarrier conductors, in an alternative configuration to that used in previously developed QD solar cells.

Single-Crystal-to-Single-Crystal Transformations of Metal-Organic-Framework-Supported, Site-Isolated Trigonal-Planar Cu(I) Complexes with Labile Ligands.

Transition-metal complexes bearing labile ligands can be difficult to isolate and study in solution because of unwanted dinucleation or ligand substitution reactions. Metal-organic frameworks (MOFs) provide a unique matrix that allows site isolation and stabilization of well-defined transition-metal complexes that may be of importance as moieties for gas adsorption or catalysis. Herein we report the development of an in situ anion metathesis strategy that facilitates the postsynthetic modification of Cu(I) complexes appended to a porous, crystalline MOF. By exchange of coordinated chloride for weakly coordinating anions in the presence of carbon monoxide (CO) or ethylene, a series of labile MOF-appended Cu(I) complexes featuring CO or ethylene ligands are prepared and structurally characterized using X-ray crystallography. These complexes have an uncommon trigonal planar geometry because of the absence of coordinating solvents. The porous host framework allows small and moderately sized molecules to access the isolated Cu(I) sites and displace the "place-holder" CO ligand, mirroring the ligand-exchange processes involved in Cu-centered catalysis.

Multidentate ligand approach for conjugation of perovskite quantum dots to biomolecules

Building compatible surface on perovskite quantum dots (PQDs) for applications like sensing analytes in aqueous medium is highly challenging and if achieved by simple means can revolutionize disease diagnostics. The present work reports the surface engineering of CsPbBr3 QDs via “simple ligand exchange process” to achieve water-compatible QDs towards detection of biomolecules. The monodentate oleic acid ligand in CsPbBr3 QDs is exchanged with dicarboxylic acid containing (bidentate) ligands such as folic acid (FA), ethylenediamine tetra-acetic acid (EDTA), succinic acid (SA) and glutamic acid (GA) to develop an efficient water-compatible PQD-ligand system. optical and theoretical studies showed the existence of a stronger binding between the perovskite and succinic acid ligand as compared to oleic acid (OA) and all other ligands. Replacement of OA with SA and retention of crystal structure is validated using spectroscopic and microscopic tools. It is observed that SA ligands facilitate better electronic coupling with PQDs and show significant improvement in fluorescence and stability. Further N-Hydroxy succinimide (NHS), which is a well-known compound to activate carboxyl groups, is used to bind onto SA PQDs as multidentate ligand, to form water stable PQDs. SA PQDs react with NHS (in water) to form multidentate ligand passivated PQDs that show very high photoluminescence (PL) as compared to OA PQDs in toluene. This also results in the formation of an NHS ester that allows bioconjugation with PQDs. This simple probe in water is further utilized for sensing a highly hydrophilic bovine serum albumin (BSA) protein as a model target to demonstrate the potential and effectiveness of this process to create compatible QDs for the successful conjugation of biomolecules. Although the focus of this work is to demonstrate bioconjugation and not achieving higher sensitivity levels, the intrinsic sensing level of these compatible QDs towards BSA shows a detection limit of 51.47 nM, which is above par with other reports in literature.

Open‐Circuit Voltage Loss in Lead Chalcogenide Quantum Dot Solar Cells

Lead chalcogenide colloidal quantum dot solar cells (CQDSCs) have received considerable attention due to their broad and tunable absorption and high stability. Presently, lead chalcogenide CQDSC has achieved a power conversion efficiency of ≈14%. However, the state-of-the-art lead chalcogenide CQDSC still has an open-circuit voltage (Voc ) loss of ≈0.45V, which is significantly higher than those of c-Si and perovskite solar cells. Such high Voc loss severely limits the performance improvement and commercialization of lead chalcogenide CQDSCs. In this review, the Voc loss is first analyzed via detailed balance theory and the origin of Voc loss from both solar absorber and interface is summarized. Subsequently, various strategies for improving the Voc from the solar absorber, including the passivation strategies during the synthesis and ligand exchange are overviewed. The great impact of the ligand exchange process on CQD passivation is highlighted and the corresponding strategies to further reduce the Voc loss are summarized. Finally, various strategies are discussed to reduce interface Voc loss from charge transport layers. More importantly, the great potential of achieving performance breakthroughs via various organic hole transport layers is highlighted and the existing challenges toward commercialization are discussed.

Switchable Binding Energy of Ionic Compounds and Application in Customizable Ligand Exchange for Colloid Nanocrystals.

The ability to engineer the surface ligands or adsorbed molecules on colloid nanocrystals (NCs) is important for various applications, as the physical and chemical properties are strongly affected by the surface chemistry. Here, we develop a facile and generalized ionic compound-mediated ligand-exchange strategy based on density functional theory calculations, in which the ionic compounds possess switchable bonding energy when they transfer between the ionized state and the non-ionized state, hence catalyzing the ligand-exchange process. By using an organic acid as the intermediate ligand, ligands such as oleylamine, butylamine, polyvinylpyrrolidone, and poly(vinyl alcohol) can be freely exchanged on the surface of Au NCs. Benefiting from this unique ligand-exchange strategy, the ligands with strong bonding energy can be replaced by weak ones, which is hard to realize in traditional ligand-exchange processes. The ionic compound-mediated ligand exchange is further utilized to improve the catalytic properties of Au NCs, facilitate the loading of nanoparticles on substrates, and tailor the growth of colloid NCs. These results indicate that the mechanism of switchable bonding energy can be significantly expanded to manipulate the surface property and functionalization of NCs that have applications in a wide range of chemical and biomedical fields.