[Ag₂₅(SR)₁₈]⁻: The “Golden” Silver Nanoparticle

Silver nanoparticles with an atomically precise molecular formula [Ag₂₅(SR)₁₈]⁻ (−SR: thiolate) are synthesized, and their single-crystal structure is determined. This synthesized nanocluster is the only silver nanoparticle that has a virtually identical analogue in gold, i.e., [Au₂₅(SR)₁₈]⁻, in terms of number of metal atoms, ligand count, superatom electronic configuration, and atomic arrangement. Furthermore, both [Ag₂₅(SR)₁₈]⁻ and its gold analogue share a number of features in their optical absorption spectra. This unprecedented molecular analogue in silver to mimic gold offers the first model nanoparticle platform to investigate the centuries-old problem of understanding the fundamental differences between silver and gold in terms of nobility, catalytic activity, and optical property

Switching a Nanocluster Core from Hollow to Nonhollow

Modulating the structure–property relationship in atomically precise nanoclusters (NCs) is vital for developing novel NC materials and advancing their applications. While promising biphasic ligand-exchange (LE) strategies have been developed primarily to attain novel NCs, understanding the mechanistic aspects involved in tuning the core and the ligand-shell of NCs in such biphasic processes is challenging. Here, we design a single phase LE process that enabled us to elucidate the mechanism of how a hollow NC (e.g., [Ag₄₄(SR)₃₀]^4–, SR: thiolate) converts into a nonhollow NC (e.g., [Ag₂₅(SR)₁₈]⁻) and vice versa. Our study reveals that the complete LE of the hollow [Ag₄₄(SPhF)₃₀]^4– NCs (SPhF: 4-fluorobenzenethiolate) with incoming 2,4-dimethylbenzenethiol (HSPhMe₂) induced distortions in the Ag₄₄ structure forming the nonhollow [Ag₂₅(SPhMe₂)₁₈]⁻ by a disproportionation mechanism, while the reverse reaction of [Ag₂₅(SPhMe₂)₁₈]⁻ with HSPhF prompted an unusual dimerization of Ag₂₅, followed by a rearrangement step that reproduces the original [Ag₄₄(SPhF)₃₀]^4–. Remarkably, both the forward and the backward reactions proceed through similar size intermediates that seem to be governed by the boundary conditions set by the thermodynamic and electronic stability of the hollow and nonhollow metal cores. Furthermore, the resizing of NCs highlights the surprisingly long-range effect of the ligands which are felt by atoms far deep in the metal core, thus opening a new path for controlling the structural evolution of nanoparticles

Neat and Complete: Thiolate-Ligand Exchange on a Silver Molecular Nanoparticle

Atomically precise thiolate-protected noble metal molecular nanoparticles are a promising class of model nanomaterials for catalysis, optoelectronics, and the bottom-up assembly of true molecular crystals. However, these applications have not fully materialized due to a lack of ligand exchange strategies that add functionality, but preserve the properties of these remarkable particles. Here we present a method for the rapid (<30 s) and complete thiolate-for-thiolate exchange of the highly sought after silver molecular nanoparticle [Ag₄₄(SR)₃₀]⁻⁴. Only by using this method were we able to preserve the precise nature of the particles and simultaneously replace the native ligands with ligands containing a variety of functional groups. Crucially, as a result of our method we were able to process the particles into smooth thin films, paving the way for their integration into solution-processed devices

Effect of Precursor Ligands and Oxidation State in the Synthesis of Bimetallic Nano-Alloys

The characteristics of bimetallic nanomaterials are dictated by their size, shape, and elemental distribution. Solution synthesis is widely utilized to form nanomaterials, such as nanoparticles, with controlled size and shape. However, the effects of variables on the characteristics of bimetallic nanomaterials are not completely understood. In this study, we used a continuous-flow synthetic strategy to explore the effects of the precursor ligands and the precursor oxidation state in the shape-controlled synthesis of platinum alloy nano-octahedra and show their effect on the nanoparticle size and the elemental distribution within the alloy nanoparticle. We demonstrate that this strategy can tune the size of monodisperse PtM (M = Ni or Cu) alloy nanocrystals ranging from 3 to 16 nm with an octahedral shape using acetylacetonate or halide precursors of Pt­(II), Pt­(IV), and Ni­(II) or Cu­(II). The nanoparticles formed from halide precursors showed an enrichment of platinum on their surfaces, and the use bromide ligands in the presence of air showed the formation of concave and uneven surface facets. The two nanocrystal precursors can be utilized independently and can control the size with a trend of Pt­(acac)₂ < PtCl₂ < PtCl₄ < PtBr₂ < PtBr₄ and M­(acac)₂ < MCl₂ < MBr₂ for the secondary metal (copper or nickel). These results open up avenues to understand the effect of the ligand shell of a precursor during the synthesis of alloy nanoparticles as well as to control, in a scalable manner, the nanomaterial size and surface chemistry

Room-Temperature Engineering of All-Inorganic Perovskite Nanocrsytals with Different Dimensionalities

Room-Temperature Engineering of All-Inorganic Perovskite Nanocrsytals with Different Dimensionalitie

Synthesis and Optical Properties of a Dithiolate/Phosphine-Protected Au₂₈ Nanocluster

While monothiols and simple phosphines are commonly exploited for size-controlled synthesis of atomically precise gold nanoclusters (NCs), dithiols or dithiol-phosphine combinations are seldom applied. Herein, we used a dithiol (benzene-1,3-dithiol, BDT) and a phosphine (triphenylphosphine, TPP) together as ligands and synthesized an atomically precise gold NC with the formula [Au₂₈(BDT)₄(TPP)₉]²⁺. This NC exhibited multiple absorption features and a charge of +2, which are distinctly different from the reported all-thiolated [Au₂₈(SR)₂₀]⁰ NC (SR: monothiolate). The composition of [Au₂₈(BDT)₄(TPP)₉]²⁺ NC was deduced from high-resolution electrospray ionization mass spectrometry (ESI MS) and it was further corroborated by thermogravimetric analysis (TGA). Differential pulse voltammetry (DPV) revealed a HOMO–LUMO gap of 1.27 eV, which is in good agreement with the energy gap of 1.3 eV obtained from its UV–vis spectrum. The successful synthesis of atomically precise, dithiol-protected Au₂₈ NC would stimulate theoretical and experimental research into bidentate ligands as a new path for expanding the library of different metal NCs, which have so far been dominated by monodentate thiols

Ligand-Free Nanocrystals of Highly Emissive Cs₄PbBr₆ Perovskite

Although ligands of long carbon chains are very crucial to form stable colloidal perovskite nanocrystals (NCs), they create a severe barrier for efficient charge injection or extraction in quantum-dot-based optoelectronics, such as light emitting diode or solar cell. Here, we report a new approach to preparing ligand-free perovskite NCs of Cs₄PbBr₆, which retained high photoluminescence quantum yield (44%). Such an approach involves a polar solvent (acetonitrile) and two small molecules (ammonium acetate and cesium chloride), which replace the organic ligand and still protect the nanocrystals from dissolution. The successful removal of hydrophobic long ligands was evidenced by Fourier transform infrared spectroscopy, ζ potential analysis, and thermogravimetric analysis. Unlike conventional perovskite NCs that are extremely susceptible to polar solvents, the ligand-free Cs₄PbBr₆ NCs show robust resistance to polar solvents. Our ligand-free procedure opens many possibilities not only from a material hybridization perspective but also in optimizing charge injection and extraction in semiconductor quantum-dot-based optoelectronics applications

Quantum Confinement-Tunable Ultrafast Charge Transfer at the PbS Quantum Dot and Phenyl‑C₆₁-butyric Acid Methyl Ester Interface

Quantum dot (QD) solar cells have emerged as promising low-cost alternatives to existing photovoltaic technologies. Here, we investigate charge transfer and separation at PbS QDs and phenyl-C₆₁-butyric acid methyl ester (PCBM) interfaces using a combination of femtosecond broadband transient absorption (TA) spectroscopy and steady-state photoluminescence quenching measurements. We analyzed ultrafast electron injection and charge separation at PbS QD/PCBM interfaces for four different QD sizes and as a function of PCBM concentration. The results reveal that the energy band alignment, tuned by the quantum size effect, is the key element for efficient electron injection and charge separation processes. More specifically, the steady-state and time-resolved data demonstrate that only small-sized PbS QDs with a bandgap larger than 1 eV can transfer electrons to PCBM upon light absorption. We show that these trends result from the formation of a type-II interface band alignment, as a consequence of the size distribution of the QDs. Transient absorption data indicate that electron injection from photoexcited PbS QDs to PCBM occurs within our temporal resolution of 120 fs for QDs with bandgaps that achieve type-II alignment, while virtually all signals observed in smaller bandgap QD samples result from large bandgap outliers in the size distribution. Taken together, our results clearly demonstrate that charge transfer rates at QD interfaces can be tuned by several orders of magnitude by engineering the QD size distribution. The work presented here will advance both the design and the understanding of QD interfaces for solar energy conversion