Exploring the formation of gold/silver nanoalloys with gas-phase synthesis and machine-learning assisted simulations.

While nanoalloys are of paramount scientific and practical interest, the main processes leading to their formation are still poorly understood. Key structural features in the alloy systems, including the crystal phase, chemical ordering, and morphology, are challenging to control at the nanoscale, making it difficult to extend their use to industrial applications. In this contribution, we focus on the gold/silver system that has two of the most prevalent noble metals and combine experiments with simulations to uncover the formation mechanisms at the atomic level. Nanoparticles were produced using a state-of-the-art inert-gas aggregation source and analyzed using transmission electron microscopy and energy-dispersive X-ray spectroscopy. Machine-learning-assisted molecular dynamics simulations were employed to model the crystallization process from liquid droplets to nanocrystals. Our study finds a preponderance of nanoparticles with five-fold symmetric morphology, including icosahedra and decahedra which is consistent with previous results on mono-metallic nanoparticles. However, we observed that gold atoms, rather than silver atoms, segregate at the surface of the obtained nanoparticles for all the considered alloy compositions. These segregation tendencies are in contrast to previous studies and have consequences on the crystallization dynamics and the subsequent crystal ordering. We finally showed that the underpinning of this surprising segregation dynamics is due to charge transfer and electrostatic interactions rather than surface energy considerations.

Machine-Learning Accelerated First-Principles Accurate Modeling of the Solid-Liquid Phase Transition in MgO under Mantle Conditions.

While accurate measurements of MgO under extreme high-pressure conditions are needed to understand and model planetary behavior, these studies are challenging from both experimental and computational modeling perspectives. Herein, we accelerate density functional theory (DFT) accurate calculations using deep neural network potentials (DNPs) trained over multiple phases and study the melting behavior of MgO via the two-phase coexistence (TPC) approach at 0-300 GPa and ≤9600 K. The resulting DNP-TPC melting curve is in excellent agreement with existing experimental studies. We show that the mitigation of finite-size effects that typically skew the predicted melting temperatures in DFT-TPC simulations in excess of several hundred kelvin requires models with ∼16 000 atoms and >100 ps molecular dynamics trajectories. In addition, the DNP can successfully describe MgO metallization well at increased pressures that are captured by DFT but missed by classical interatomic potentials.

Development and Validation of Versatile Deep Atomistic Potentials for Metal Oxides.

Machine learning interatomic potentials powered by neural networks have been shown to readily model a gradient of compositions in metallic systems. However, their application to date on ionic systems tends to focus on specific compositions and oxidation states owing to their more heterogeneous chemical nature. Herein we show that a deep neural network potential (DNP) can model various properties of metal oxides with different oxidation states without additional charge information. We created and validated DNPs for AgxOy, CuxOy MgxOy, PtxOy, and ZnxOy, whereby each system was trained without any limitations on oxidation states. We illustrate how the database can be augmented to enhance the DNP transferability for a new polymorph, surface energies, and thermal expansion. In addition, we show that these potentials can correctly interpolate significant pressure and temperature ranges, exhibit stability over long molecular dynamics simulation time scales, and replicate nonharmonic thermal expansion, consistent with experimental results.

Tuning electrical and interfacial thermal properties of bilayer MoS2via electrochemical intercalation.

Layered two-dimensional (2D) materials such as MoS2have attracted much attention for nano- and opto-electronics. Recently, intercalation (e.g. of ions, atoms, or molecules) has emerged as an effective technique to modulate material properties of such layered 2D films reversibly. We probe both the electrical and thermal properties of Li-intercalated bilayer MoS2nanosheets by combining electrical measurements and Raman spectroscopy. We demonstrate reversible modulation of carrier density over more than two orders of magnitude (from 0.8 × 1012to 1.5 × 1014cm-2), and we simultaneously obtain the thermal boundary conductance between the bilayer and its supporting SiO2substrate for an intercalated system for the first time. This thermal coupling can be reversibly modulated by nearly a factor of eight, from 14 ± 4.0 MW m-2K-1before intercalation to 1.8 ± 0.9 MW m-2K-1when the MoS2is fully lithiated. These results reveal electrochemical intercalation as a reversible tool to modulate and control both electrical and thermal properties of 2D layers.

Optimization and validation of a deep learning CuZr atomistic potential: Robust applications for crystalline and amorphous phases with near-DFT accuracy.

We show that a deep-learning neural network potential (DP) based on density functional theory (DFT) calculations can well describe Cu-Zr materials, an example of a binary alloy system, that can coexist in as ordered intermetallic and as an amorphous phase. The complex phase diagram for Cu-Zr makes it a challenging system for traditional atomistic force-fields that cannot accurately describe the different properties and phases. Instead, we show that a DP approach using a large database with ∼300k configurations can render results generally on par with DFT. The training set includes configurations of pristine and bulk elementary metals and intermetallic structures in the liquid and solid phases in addition to slab and amorphous configurations. The DP model was validated by comparing bulk properties such as lattice constants, elastic constants, bulk moduli, phonon spectra, and surface energies to DFT values for identical structures. Furthermore, we contrast the DP results with values obtained using well-established two embedded atom method potentials. Overall, our DP potential provides near DFT accuracy for the different Cu-Zr phases but with a fraction of its computational cost, thus enabling accurate computations of realistic atomistic models, especially for the amorphous phase.

In situ environmental TEM observation of two-stage shrinking of Cu2O islands on Cu(100) during methanol reduction.

The structural dynamics of Cu catalyst regeneration from Cu2O under methanol is poorly understood. In situ Environmental TEM on Cu(100)-supported Cu2O islands reveals a transition from anisotropic to isotropic shrinking during reduction. Two-stage reduction is statistically supported and explained by preferential methanol reactivity on Cu2O nano-islands with DFT simulations.

Transition of surface phase of cobalt oxide during CO oxidation.

In situ/operando studies of a heterogeneous catalyst are particularly valuable for achieving a fundamental understanding of catalytic mechanisms at a molecular level by establishing a correlation between the observed catalytic performance and the corresponding surface chemistry during catalysis. Herein, CO oxidation on cobalt oxides was studied via ambient pressure X-ray photoelectron spectroscopy (AP-XPS). During CO oxidation on CoO in the temperature range of 140-180 °C, the active surface phase of CoO progressively transforms to Co3O4. Kinetic studies of CO oxidation on the surface phase CoO at 80-120 °C and on the formed Co3O4 at 160-220 °C show that CoO and Co3O4 exhibit different activation barriers: 49.3 kJ mol-1 for CoO and 36.9 kJ mol-1 for Co3O4. This study demonstrates the transition of the active surface phase of a transition metal oxide-based catalyst under catalytic conditions with no change in the bulk phase of the catalyst.

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

Here, we demonstrate efficient energy transfer from near-infrared-emitting ortho-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.

Structure and Function of Iron-Loaded Synthetic Melanin.

We describe a synthetic method for increasing and controlling the iron loading of synthetic melanin nanoparticles and use the resulting materials to perform a systematic quantitative investigation on their structure-property relationship. A comprehensive analysis by magnetometry, electron paramagnetic resonance, and nuclear magnetic relaxation dispersion reveals the complexities of their magnetic behavior and how these intraparticle magnetic interactions manifest in useful material properties such as their performance as MRI contrast agents. This analysis allows predictions of the optimal iron loading through a quantitative modeling of antiferromagnetic coupling that arises from proximal iron ions. This study provides a detailed understanding of this complex class of synthetic biomaterials and gives insight into interactions and structures prevalent in naturally occurring melanins.

Polymeric Gd-DOTA amphiphiles form spherical and fibril-shaped nanoparticle MRI contrast agents.

A Gd3+-coordinated polymerizable analogue of the MRI contrast agent Gd-DOTA was used to prepare amphiphilic block copolymers, with hydrophilic blocks composed entirely of the polymerized contrast agent. The resulting amphiphilic block copolymers assemble into nanoparticles (NPs) of spherical- or fibril-shape, each demonstrating enhanced relaxivity over Gd-DOTA. As an initial examination of their behavior in vivo, intraperitoneal (IP) injection of NPs into live mice was performed, showing long IP residence times, observed by MRI. Extended residence times for particles of well-defined morphology may represent a valuable design paradigm for treatment or diagnosis of peritoneal malignances.

Effects of Ligand Geometry on the Photophysical Properties of Photoluminescent Eu(III) and Sm(III) 1-Hydroxypyridin-2-one Complexes in Aqueous Solution.

A series of 10 tetradentate 1-hydroxy-pyridin-2-one (1,2-HOPO) ligands and corresponding eight-coordinated photoluminescent Eu(III) and Sm(III) complexes were prepared. Generally, the ligands differ by the linear (nLI) aliphatic linker length, from 2 to 8 methylene units between the bidentate 1,2-HOPO chelator units. The photoluminescent quantum yields (Φtot) were found to vary with the linker length, and the same trend was observed for the Eu(III) and Sm(III) complexes. The 2LI and 5LI bridged complexes are the brightest (Φtotxε). The change in ligand wrapping pattern between 2LI and 5LI complexes observed by X-ray diffraction (XRD) is further supported by density functional theory (DFT) calculations. The bimodal Φtot trends of the Eu(III) and Sm(III) complexes are rationalized by the change in ligand wrapping pattern as the bridge (nLI) is increased in length.

Polycatechol Nanoparticle MRI Contrast Agents.

Amphiphilic triblock copolymers containing Fe(III) -catecholate complexes formulated as spherical- or cylindrical-shaped micellar nanoparticles (SMN and CMN, respectively) are described as new T1-weighted agents with high relaxivity, low cytotoxicity, and long-term stability in biological fluids. Relaxivities of both SMN and CMN exceed those of established gadolinium chelates across a wide range of magnetic field strengths. Interestingly, shape-dependent behavior is observed in terms of the particles' interactions with HeLa cells, with CMN exhibiting enhanced uptake and contrast via magnetic resonance imaging (MRI) compared with SMN. These results suggest that control over soft nanoparticle shape will provide an avenue for optimization of particle-based contrast agents as biodiagnostics. The polycatechol nanoparticles are proposed as suitable for preclinical investigations into their viability as gadolinium-free, safe, and effective imaging agents for MRI contrast enhancement.

Description and Role of Bimetallic Prenucleation Species in the Formation of Small Nanoparticle Alloys.

We report the identification, description, and role of multinuclear metal-thiolate complexes in aqueous Au-Cu nanoparticle syntheses. The structure of these species was characterized by nuclear magnetic resonance spectroscopy, mass spectrometry, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy techniques. The observed structures were found to be in good agreement with thermodynamic growth trends predicted by first-principles calculations. The presence of metal-thiolate complexes is then shown to be critical for the formation of alloyed Au-Cu architectures in the small nanoparticle regime (diameter ∼2 nm). In the absence of mixed metal-thiolate precursors, nanoparticles form with a Cu-S shell and a Au-rich interior. Taken together, these results demonstrate that prenucleation species, which are discrete molecular precursors distinct from both initial reagents and final particle products, may provide an important new synthetic route to control final metal nanoparticle composition and composition architectures.

Ligand-Mediated "Turn On," High Quantum Yield Near-Infrared Emission in Small Gold Nanoparticles.

Small gold nanoparticles (∼1.4-2.2 nm core diameters) exist at an exciting interface between molecular and metallic electronic structures. These particles have the potential to elucidate fundamental physical principles driving nanoscale phenomena and to be useful in a wide range of applications. Here, we study the optoelectronic properties of aqueous, phosphine-terminated gold nanoparticles (core diameter = 1.7 ± 0.4 nm) after ligand exchange with a variety of sulfur-containing molecules. No emission is observed from these particles prior to ligand exchange, however the introduction of sulfur-containing ligands initiates photoluminescence. Further, small changes in sulfur substituents produce significant changes in nanoparticle photoluminescence features including quantum yield, which ranges from 0.13 to 3.65% depending on substituent. Interestingly, smaller ligands produce the most intense, highest energy, narrowest, and longest-lived emissions. Radiative lifetime measurements for these gold nanoparticle conjugates range from 59 to 2590 µs, indicating that even minor changes to the ligand substituent fundamentally alter the electronic properties of the luminophore itself. These results isolate the critical role of surface chemistry in the photoluminescence of small metal nanoparticles and largely rule out other mechanisms such as discrete (Au(I)-S-R)n impurities, differences in ligand densities, and/or core diameters. Taken together, these experiments provide important mechanistic insight into the relationship between gold nanoparticle near-infrared emission and pendant ligand architectures, as well as demonstrate the pivotal role of metal nanoparticle surface chemistry in tuning and optimizing emergent optoelectronic features from these nanostructures.

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 (195)Pt 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 (1)H 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.

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.

Analysis of Lanthanide Complex Dendrimer Conjugates for Bimodal NIR and MRI Imaging.

Advances in clinical diagnostic instrumentation have enabled some imaging modalities to be run concurrently. For diagnostic purposes, multimodal imaging can allow for rapid location and accurate identification of a patient's illness. The paramagnetic and near Infra-red (NIR) properties of Dy(III) and Yb(III) are interesting candidates for the development of bimodal NIR and magnetic resonance imaging (MRI) contrast agents. To enhance their intrinsic bimodal properties, these lanthanides were chelated using the hexadentate-all-oxygen-donor-ligand TREN-bis-(1-Me)-3,2-HOPO-TAM-NX (NX, where X = 1, 2 or 3) and subsequently conjugated to the esteramide dendrimer (EA), to improve bioavailability, solubility, and relaxivity. Of these new complexes synthesized and evaluated, DyN1-EA had the largest ionic T(1) relaxivity, 7.60 mM(-1) s(-1), while YbN3-EA had the largest ionic T(2) relaxivity with a NIR quantum yield of 0.17 % when evaluated in mouse serum. This is the first Yb(III) bimodal NIR/T(2) MRI contrast agent of its kind evaluated.

Circularly polarized luminescence of curium: a new characterization of the 5f actinide complexes.

A key distinction between the lanthanide (4f) and the actinide (5f) transition elements is the increased role of f-orbital covalent bonding in the latter. Circularly polarized luminescence (CPL) is an uncommon but powerful spectroscopy which probes the electronic structure of chiral, luminescent complexes or molecules. While there are many examples of CPL spectra for the lanthanides, this report is the first for an actinide. Two chiral, octadentate chelating ligands based on orthoamide phenol (IAM) were used to complex curium(III). While the radioactivity kept the amount of material limited to micromole amounts, spectra of the highly luminescent complexes showed significant emission peak shifts between the different complexes, consistent with ligand field effects previously observed in luminescence spectra.

Spectroscopic investigations of lanthanide ion binding to nucleic acids.

Luminescent lanthanide (Ln(III)) ions are valuable spectroscopic probes for metal ion binding sites in nucleic acids. In this chapter, we briefly review Ln(III) luminescence and the information available from these experiments. An emphasis is placed on direct excitation Eu(III) spectroscopy as a tool. Eu(III) excitation spectroscopy is used to show that solutions containing micromolar Eu(III), 100 mM NaCl, and 20 mM MES buffer contain predominantly a mononuclear Eu(III) aqua complex and an Eu(III) hydroxide complexes. The binding of these species to various RNA and DNA sequences are monitored by using Eu(III) excitation spectroscopy. Eu(III) luminescence lifetime data shows that the Eu(III) ion typically loses 1-3 water molecules to form innersphere complexes with RNA and DNA that contain tandem base pair mismatches or hairpin loops. In addition, early studies that used nucleobase-sensitized Eu(III) or Tb(III) luminescence within transfer RNA or in the hammerhead ribozyme are presented. Luminescence resonance energy transfer studies are shown to be useful for determining distances between bound Ln(III) ion and organic fluorophores or between two different Ln(III) ions. To supplement luminescence data, the binding sites of paramagnetic Ln(III) ions are determined by monitoring the chemical shifts of nucleotide protons. Binding sites are identified by following the protons that are influenced by the Ln(III) pseudo-contact shift.