Surface Chemistry Controls Magnetism in Cobalt Nanoclusters

Magnetic properties of Co₁₃ and Co₅₅ nanoclusters, passivated by surface ligand shells that exhibit varying electronic interactions with the metal, are studied using density functional theory. The calculations show that the chemical nature of the bond between the ligand and the metal core (X-type or L-type) impacts the total magnetic moment of Co nanoclusters dramatically. Furthermore, the chemical identity of the ligand within each binding motif then provides a fine handle on the exhibited magnetic moment of the cluster. Thus, ligand shell chemistry is predicted to not only stabilize Co nanoclusters, but provide a powerful approach to control their magnetic properties, which combined enable a variety of magnetism-based applications

Seedless Initiation as an Efficient, Sustainable Route to Anisotropic Gold Nanoparticles

Seedless initiation has been used as a simple and sustainable alternative to seed-mediated production of two canonical anisotropic gold nanoparticles: nanorods and nanoprisms. The concentration of reducing agent during the nucleation event was found to influence the resulting product morphology, producing nanorods with lengths from 30 to 630 nm and triangular or hexagonal prisms with vertex-to-vertex lengths ranging from 120 to over 700 nm. The seedless approach is then used to eliminate several chemical reagents and reactions steps from classic particle preparations while achieving almost identical nanoparticle products and product yields. Our results shed light on factors that influence (or do not influence) the evolution of gold nanoparticle shape and present a dramatically more efficient route to obtaining these architectures. Specifically, using these methods reduces the total amount of reagent needed to produce nanorods and nanoprisms by as much as 90 wt % and, to the best of our knowledge, has yielded the first report of spectroscopically discernible, colloidal gold nanoplates synthesized using a seedless methodology

Impacts of Copper Position on the Electronic Structure of [Au_25‑xCu_<i>x</i>(SH)₁₈]⁻ Nanoclusters

Here, we use density functional theory to model the impact of heteroatom position on the optoelectronic properties of mixed metal nanoclusters. First, we consider the well-described [Au₂₅(SH)₁₈]⁻ motif, and substitute Cu atoms at the three geometrically unique positions within the cluster. These clusters are atomically precise and show an electronic structure that is a function of both composition and heteroatom position. We then model clusters containing Cu substitutions at two positions, and demonstrate an additional and significant impact from heteroatom proximity with respect to one another. For each system, we report the formation energy, HOMO–LUMO gap, and energy level structure, and suggest how trends in these parameters may be explained using classic atomic descriptors such as electronegativity, analogous to design principles widely used in the field of organic electronics. Further, we use linear response time-dependent density functional theory to model the absorption behavior of each system in order to correlate these electronic properties with a convenient experimental readout

Correlating Carrier Density and Emergent Plasmonic Features in Cu_2–<i>x</i>Se Nanoparticles

Recently, a wide variety of new nanoparticle compositions have been identified as potential plasmonic materials including earth-abundant metals such as aluminum, highly doped semiconductors, as well as metal pnictides. For semiconductor compositions, plasmonic properties may be tuned not only by nanoparticle size and shape, but also by charge carrier density which can be controlled via a variety of intrinsic and extrinsic doping strategies. Current methods to quantitatively determine charge carrier density primarily rely on interpretation of the nanoparticle extinction spectrum. However, interpretation of nanoparticle extinction spectra can be convoluted by factors such as particle ligands, size distribution and/or aggregation state which may impact the charge carrier information extracted. Therefore, alternative methods to quantify charge carrier density may be transformational in the development of these new materials and would facilitate previously inaccessible correlations between particle synthetic routes, crystallographic features, and emergent optoelectronic properties. Here, we report the use of ⁷⁷Se solid state nuclear magnetic resonance (NMR) spectroscopy to quantitatively determine charge carrier density in a variety of Cu_2–<i>x</i>Se nanoparticle compositions and correlate this charge carrier density with particle crystallinity and extinction features. Importantly, we show that significant charge carrier populations are present even in nanoparticles without spectroscopically discernible plasmonic features and with crystal structures indistinguishable from fully reduced Cu₂Se. These results highlight the potential impact of the NMR-based carrier density measurement, especially in the study of plasmon emergence in these systems (i.e., at low dopant concentrations)

Impact of As-Synthesized Ligands and Low-Oxygen Conditions on Silver Nanoparticle Surface Functionalization

Here, we compare the ligand exchange behaviors of silver nanoparticles synthesized in the presence of two different surface capping agents: poly­(vinyl­pyrrolidone) (MW = 10 or 40 kDa) or trisodium citrate, and under either ambient or low-oxygen conditions. In all cases, we find that the polymer capping agent exhibits features of a weakly bound ligand, producing better ligand exchange efficiencies with an incoming thiolated ligand compared to citrate. The polymer capping agent also generates nanoparticles that are more susceptible to reactions with oxygen during both synthesis and ligand exchange. The influence of the original ligand on the outcome of ligand exchange reactions with an incoming thiolated ligand highlights important aspects of silver nanoparticle surface chemistry, crucial for applications ranging from photocatalysis to antimicrobials

Decoupling Mechanisms of Platinum Deposition on Colloidal Gold Nanoparticle Substrates

Nanoscale platinum materials are essential components in many technologies, including catalytic converters and fuel cells. Combining Pt with other metals can enhance its performance and/or decrease the cost of the technology, and a wide range of strategies have been developed to capitalize on these advantages. However, wet chemical synthesis of Pt-containing nanoparticles (NPs) is challenging due to the diverse metal segregation and metal–metal redox processes possible under closely related experimental conditions. Here, we elucidate the relationship between Pt­(IV) speciation and the formation of well-known NP motifs, including frame-like and core–shell morphologies, in Au–Pt systems. We leverage insights gained from these studies to induce a controlled transition from redox- to surface chemistry-mediated growth pathways, resulting in the formation of Pt NPs in epitaxial contact and linear alignment along a gold nanoprism substrate. Mechanistic investigations using a combination of electron microscopy and ¹⁹⁵Pt NMR spectroscopy identify Pt­(IV) speciation as a crucial parameter for understanding and controlling the formation of Pt-containing NPs. Combined, these findings point toward fully bottom-up methods for deposition and organization of NPs on colloidal plasmonic substrates

Photoluminescent Gold–Copper Nanoparticle Alloys with Composition-Tunable Near-Infrared Emission

Discrete gold nanoparticles with diameters between 2 and 3 nm show remarkable properties including enhanced catalytic behavior and photoluminescence. However, tunability of these properties is limited by the tight size range within which they are observed. Here, we report the synthesis of discrete, bimetallic gold–copper nanoparticle alloys (diameter ≅ 2–3 nm) which display photoluminescent properties that can be tuned by changing the alloy composition. Electron microscopy, X-ray photoelectron spectroscopy, inductively coupled plasma mass spectrometry, and pulsed-field gradient stimulated echo ¹H NMR measurements show that the nanoparticles are homogeneous, discrete, and crystalline. Upon varying the composition of the nanoparticles from 0% to 100% molar ratio copper, the photoluminescence maxima shift from 947 to 1067 nm, with excitation at 360 nm. The resulting particles exhibit brightness values (molar extinction coefficient (ε) × quantum yield (Φ)) that are more than an order of magnitude larger than the brightest near-infrared-emitting lanthanide complexes and small-molecule probes evaluated under similar conditions

Ligand-Mediated Deposition of Noble Metals at Nanoparticle Plasmonic Hotspots

We report the use of gold nanoparticle surface chemistry as a tool for site-selective noble metal deposition onto colloidal gold nanoparticle substrates. Specifically, we demonstrate that partial passivation of the gold nanoparticle surface using thiolated ligands can induce a transition from linear palladium island deposition to growth of palladium selectively at plasmonic hotspots on the edges or vertices of the underlying particle substrate. Further, we demonstrate the broader applicability of this approach with respect to substrate morphology (e.g., prismatic and rod-shaped nanoparticles), secondary metal (e.g., palladium, gold, and platinum), and surface ligand (e.g., surfactant molecules and <i>n</i>-alkanethiols). Taken together, these results demonstrate the important role of metal–ligand surface chemistry and ligand packing density on the resulting modes of multimetallic nanoparticle growth, and in particular, the ability to direct that growth to particle regions of impact such as plasmonic hotspots

Efficient Energy Transfer from Near-Infrared Emitting Gold Nanoparticles to Pendant Ytterbium(III)

Here, we demonstrate efficient energy transfer from near-infrared-emitting <i>ortho</i>-mercaptobenzoic acid-capped gold nanoparticles (AuNPs) to pendant ytterbium­(III) cations. These functional materials combine the high molar absorptivity (1.21 × 10⁶ M^–1 cm^–1) and broad excitation features (throughout the UV and visible regions) of AuNPs with the narrow emissive properties of lanthanides. Interaction between the AuNP ligand shell and ytterbium is determined using both nuclear magnetic resonance and electron microscopy measurements. In order to identify the mechanism of this energy transfer process, the distance of the ytterbium­(III) from the surface of the AuNPs is systematically modulated by changing the size of the ligand appended to the AuNP. By studying the energy transfer efficiency from the various AuNP conjugates to pendant ytterbium­(III) cations, a Dexter-type energy transfer mechanism is suggested, which is an important consideration for applications ranging from catalysis to energy harvesting. Taken together, these experiments lay a foundation for the incorporation of emissive AuNPs in energy transfer systems

Imaging Energy Transfer in Pt-Decorated Au Nanoprisms via Electron Energy-Loss Spectroscopy

Driven by the desire to understand energy transfer between plasmonic and catalytic metals for applications such as plasmon-mediated catalysis, we examine the spatially resolved electron energy-loss spectra (EELS) of both pure Au nanoprisms and Pt-decorated Au nanoprisms. The EEL spectra and the resulting surface-plasmon mode maps reveal detailed near-field information on the coupling and energy transfer in these systems, thereby elucidating the underlying mechanism of plasmon-driven chemical catalysis in mixed-metal nanostructures. Through a combination of experiment and theory we demonstrate that although the location of the Pt decoration greatly influences the plasmons of the nanoprism, simple spatial proximity is not enough to induce significant energy transfer from the Au to the Pt. What matters more is the spectral overlap between the intrinsic plasmon resonances of the Au nanoprism and Pt decoration, which can be tuned by changing the composition or morphology of either component