The surface structure of silver-coated gold nanocrystals and its influence on shape control
Abstract not provided
Padmos, J. Daniel↗
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Non-equilibrium anisotropic colloidal single crystal growth with DNA
Abstract Anisotropic colloidal crystals are materials with novel optical and electronic properties. However, experimental observations of colloidal single crystals have been limited to relatively isotropic habits. Here, we show DNA-mediated crystallization of two types of nanoparticles with different hydrodynamic radii that form highly anisotropic, hexagonal prism microcrystals with AB 2 crystallographic symmetry. The DNA directs the nanoparticles to assemble into a non-equilibrium crystal shape that is enclosed by the highest surface energy facets (AB 2 (10 $$\overline 1$$ 1 ¯ 0) and AB 2 (0001)). Simulations and theoretical arguments show that this observation is a consequence of large energy barriers between different terminations of the AB 2 (10 $$\overline 1$$ 1 ¯ 0) facet, which results in a significant deceleration of the (10 $$\overline 1$$ 1 ¯ 0) facet growth rate. In addition to reporting a hexagonal colloidal crystal habit, this work introduces a potentially general plane multiplicity mechanism for growing non-equilibrium crystal shapes, an advance that will be useful for designing colloidal crystal habits with important applications in both optics and photocatalysis.
59 BASIC BIOLOGICAL SCIENCES↗
A model for the fragmentation kinetics of crumpled thin sheets
As a confined thin sheet crumples, it spontaneously segments into flat facets delimited by a network of ridges. Despite the apparent disorder of this process, statistical properties of crumpled sheets exhibit striking reproducibility. Experiments have shown that the total crease length accrues logarithmically when repeatedly compacting and unfolding a sheet of paper. Here, we offer insight to this unexpected result by exploring the correspondence between crumpling and fragmentation processes. We identify a physical model for the evolution of facet area and ridge length distributions of crumpled sheets, and propose a mechanism for re-fragmentation driven by geometric frustration. This mechanism establishes a feedback loop in which the facet size distribution informs the subsequent rate of fragmentation under repeated confinement, thereby producing a new size distribution. We then demonstrate the capacity of this model to reproduce the characteristic logarithmic scaling of total crease length, thereby supplying a missing physical basis for the observed phenomenon.
42 ENGINEERING↗
Anisotropic surface potentials induced by competitive ion adsorption enable the synthesis of branched cubic Pt mesocrystals
Creation of complex nanostructured materials through oriented attachment (OA) requires the manipulation of interparticle forces, including electrostatic repulsion, which depends strongly on surface potentials and can be modified through the effect of solution environment on interfacial chemistry. Here we show that time-dependent anisotropies in surface potential driven by competitive ion adsorption can alter facet-selectivity during OA. This phenomenon enables the synthesis of branched cubic Pt mesocrystals. Initially, Pt nanoparticles attach preferentially at their {100} facets to form a well-defined cubic core. Over time, changes in ion adsorption shift the attachment preference to the {111} facets, promoting branch formation. In both stages, anisotropic surface potentials generate electrostatic torques that align the particles prior to attachment. These findings demonstrate a generalizable strategy for directing the architecture of nanomaterials through time-resolved control of interfacial chemistry during OA, offering new pathways for the design of complex mesoscale structures.
Bae, Yuna [Pacific Northwest National Laboratory (↗
Visualizing the bidirectional optical transfer function for near-field enhancement in waveguide coupled plasmonic transducers
Here, we report visualizations of the bidirectional near-field optical transfer function for a waveguide-coupled plasmonic transducer as a metrology technique essential for successful development for mass-fabricated near-field devices. Plasmonic devices have revolutionized the observation of nanoscale phenomena, enabling optical excitation and readout from nanoscale regions of fabricated devices instead of as limited by optical diffraction. Visualizations of the plasmonic transducer modes were acquired both by local near-field excitation of the antenna on the front facet of a waveguide using the focused electron beam of a scanning electron microscope as a probe of the near-field cathodoluminescence during far-field collection from the back facet of the waveguide, and by local mapping of the optical near-field for the same antenna design using scattering scanning near-field optical microscopy as a probe of the near-field optical mode density for far-field light focused into the back facet of the waveguide. Strong agreement between both measurement types and numerical modeling was observed, indicating that the method enables crucial metrological comparisons of as fabricated device performance to as-modeled device expectations for heat-assisted magnetic recording heads, which can be extended to successful development of future near-field-on-chip devices such as optical processor interconnects.
42 ENGINEERING↗
Hydrogen adsorption and desorption at the Pt(110)-(1×2) surface: experimental and theoretical study
The interaction of hydrogen with the Pt(110)-(1×2) surface is studied using temperature programmed desorption (TPD) measurements and density functional theory (DFT) calculations. The ridges in this surface resemble edges between micro-facets of Pt nano-particle catalysts used for hydrogen evolution (HER) and hydrogen oxidation reactions (HOR). The binding energy and activation energy for desorption are found to depend strongly on hydrogen coverage. At low coverage, the strongest binding sites are found to be the low coordination bridge sites at the edge and this is shown to agree well with the He-atom interaction and work function change which have been reported previously. At higher hydrogen coverage, the higher coordination sites on the micro-facet and in the trough get populated. The simulated TPD spectra based on the DFT results are in close agreement with our experimental spectra and provide microscopic interpretation of the three measured peaks. The lowest temperature peak obtained from the surface with highest hydrogen coverage does not correspond to desorption directly from the weakest binding sites, the trough sites, but is due to desorption from the ridge sites, followed by subsequent, thermally activated rearrangement of the H-adatoms. The reason is low catalytic activity of the Pt-atoms at the trough sites and large reduction in the binding energy at the ridge sites at high coverage. The intermediate temperature peak corresponds to desorption from the micro-facet. The highest temperature peak again corresponds to desorption from the ridge sites, giving rise to a re-entrant mechanism for the thermal desorption.
Gudmundsdóttir, Sigrídur↗
Sulfur-resistant and regenerable Ni/Co spinel-based catalysts for methane dry reforming
Oxide-supported metal catalysts were prepared in this study by thermal impregnation of highly crystalline and highly-faceted starting spinels of the form (M 0.75 Mg 0.25 )Al 2 O 4 , where M = Ni, Co, or Cu and mixtures thereof. In situ reduction at 900°C extracts the transition metals from the oxide, and the resulting catalysts contain metal crystallites with particle sizes of ~100 nm and exceedingly low dispersion, but show high activity for dry reforming of methane with turnover frequencies as large as 3.9 at 850°C. The Ni 0.37 5Cu 0.375 Mg 0.25 Al 2 O 4 catalyst shows stable methane conversion out to 12 hours on stream without performance-degrading coking. For the Ni/Co catalysts, the reforming activity and sulfur tolerance are both functions of the Ni/Co ratio and the synthesis temperature of the starting spinel, with Ni 0.375 Co 0.375 Mg 0.25 Al 2 O 4 synthesized at 1500°C displaying fast reaction kinetics even in the presence of 20 ppm H 2 S. High reforming activity is attributed to long linear lengths of high-perfection facet edges and corners on the metal crystallites. Sulfur tolerance seems to be improved by a combination of the oxygen storage capacity of the defective spinel support and its faceting that provides additional reaction sites for activation of CO 2 .
36 MATERIALS SCIENCE↗
Growth and characterization of epitaxially stabilized ceria(001) nanostructures on Ru(0001)
We have studied (001) surface terminated cerium oxide nanoparticles grown on a ruthenium substrate using physical vapor deposition. Their morphology, shape, crystal structure, and chemical state are determined by low-energy electron microscopy and micro-diffraction, scanning probe microscopy, and synchrotron-based X-ray absorption spectroscopy. Square islands are identified as CeO 2 nanocrystals exhibiting a (001) oriented top facet of varying size; they have a height of about 7 to 10 nm and a side length between about 50 and 500 nm, and are terminated with a p(2 × 2) surface reconstruction. Micro-illumination electron diffraction reveals the existence of a coincidence lattice at the interface to the ruthenium substrate. The orientation of the side facets of the rod-like particles is identified as (111); the square particles are most likely of cuboidal shape, exhibiting (100) oriented side facets. Lastly, the square and needle-like islands are predominantly found at step bunches and may be grown exclusively at temperatures exceeding 1000 °C.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Nature of peptide wrapping onto metal nanoparticle catalysts and driving forces for size control
Colloidal metal nanocrystals find many applications in catalysis, energy conversion devices, and therapeutics. However, the nature of ligand interactions and implications on shape control have remained uncertain at the atomic scale. Large differences in peptide adsorption strength and facet specificity were found on flat palladium surfaces versus surfaces of nanoparticles of 2 to 3 nm size using accurate atomistic simulations with the Interface force field. Folding of longer peptides across many facets explains the formation of near-spherical particles with local surface disorder, in contrast to the possibility of nanostructures of higher symmetry with shorter ligands. The average particle size in TEM correlates inversely with the surface coverage with a given ligand and with the strength of ligand adsorption. The role of specific amino acids and sequence mutations on the nanoparticle size and facet composition is discussed, as well as the origin of local surface disorder that leads to large differences in catalytic reactivity.
Chemistry↗
Enhancing Electrocatalysis for Hydrogen Production over CoP Catalyst by Strain: A Density Functional Theory Study
The facet-dependent strain effects on the hydrogen evolution reaction catalyzed by CoP were studied using density functional theory methods. We show that both atomic and electronic effects need to be taken into consideration to understand the strain effects. For the (111) surface, tensile strain promotes hydrogen evolution at all the levels of hydrogen coverage. For the (101) surface, a moderate tensile strain of ~3% endows this facet with optimum performance.
Liu, Fanghan↗
Mechanisms of alumina growth via atomic layer deposition on nickel oxide and metallic nickel surfaces
We aim at elucidating the mechanism of the trimethyl aluminum (TMA) decomposition on oxidized nickel (NiO) and metallic nickel (Ni) facets in the absence of a source of hydroxyl groups. This TMA decomposition mechanism constitutes the earliest stage of growth of Al 2 O 3 coatings with the atomic layer decomposition (ALD) method, which stabilizes nickel catalysts in energy-intensive processes such as the dry reforming of methane. Our first-principles calculations suggest thermodynamic favorability for the TMA decomposition on metallic nickel compared to oxidized nickel. Moreover, the decomposition of TMA on metallic nickel showed almost no differences in terms of energy barriers between flat and stepped surfaces. Regarding the impact of the CH 3 radicals formed after TMA decomposition, we calculated stronger adsorption on metallic nickel facets than on oxidized nickel, and these adsorption energies are comparable to the adsorption energies calculated in earlier works on Al 2 O 3 ALD growth on palladium surfaces. These results lead us to believe in the growth of porous Al 2 O 3 coatings triggered by CH 3 contamination rather than due to preferential TMA decomposition on stepped and/or defective facets. The CH 3 radicals are likely to be thermally stable at temperatures used during Al 2 O 3 ALD processes, partially passivating the surface towards further TMA decomposition.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Formation of open ruthenium branched structures with highly exposed active sites for oxygen evolution reaction electrocatalysis
The formation of exposed active sites that have high activity and stability for oxygen evolution reaction (OER) catalysis is a significant opportunity for improving water electrolysers. Low-index facets surface Ru can achieve both high activity and stability for OER. Here, we present a new catalyst design where low-index faceted Ru branches are grown off the corners of Pt nanocubes, forming open Ru branched nanoparticles. This open branched structure, exposing low-index facets on its length-tunable branch, enables a high electrochemically active surface area (ECSA), achieving high activity and stability for OER. This design strategy and synthetic control provide a principle for achieving high-performance OER nanocatalysts.
Xiao, Sa [The University of New South Wales, Sydne↗
High peak power (≥10 mW) quantum cascade superluminescent emitter
We report room temperature and milliwatt range mid-infrared superluminescent emission at 5 μm from Quantum Cascade (QC) devices. To achieve high power superluminescence, we utilize an ultrastrong coupling QC laser design, and employ a cavity formed by the combination of a 17° tilted cleaved facet and a wet etched rounded and sloped facet to introduce additional mirror loss. For pulsed mode operation, a 8 mm long and 15 μm wide device achieves ∼1.3 mW peak power at 300 K and a 25 μm wide device with Si{sub 3}N{sub 4} anti-reflection coated rounded facet achieves ∼10.2 mW peak optical output power at 250 K.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Indium segregation during III–V quantum wire and quantum dot formation on patterned substrates
We report a model for metalorganic vapor-phase epitaxy on non-planar substrates, specifically V-grooves and pyramidal recesses, which we apply to the growth of InGaAs nanostructures. This model—based on a set of coupled reaction-diffusion equations, one for each facet in the system—accounts for the facet-dependence of all kinetic processes (e.g., precursor decomposition, adatom diffusion, and adatom lifetimes) and has been previously applied to account for the temperature-, concentration-, and temporal-dependence of AlGaAs nanostructures on GaAs (111)B surfaces with V-grooves and pyramidal recesses. In the present study, the growth of In{sub 0.12}Ga{sub 0.88}As quantum wires at the bottom of V-grooves is used to determine a set of optimized kinetic parameters. Based on these parameters, we have modeled the growth of In{sub 0.25}Ga{sub 0.75}As nanostructures formed in pyramidal site-controlled quantum-dot systems, successfully producing a qualitative explanation for the temperature-dependence of their optical properties, which have been reported in previous studies. Finally, we present scanning electron and cross-sectional atomic force microscopy images which show previously unreported facetting at the bottom of the pyramidal recesses that allow quantum dot formation.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Preparation of atomically flat rutile TiO 2 (001) surfaces for oxide film growth
The availability of low-index rutile TiO 2 single crystal substrates with atomically flat surfaces is essential for enabling epitaxialgrowth of rutile transition metal oxide films. The high surface energy of the rutile (001) surface often leads to surface faceting, which precludes the sputter and annealing treatment commonly used for the preparation of clean and atomically flat TiO 2 (110) substrate surfaces. In this work, we reveal that stable and atomically flat rutile TiO 2 (001) surfaces can be prepared with an atomically ordered reconstructedsurface already during a furnace annealing treatment in air. We tentatively ascribe this result to the decrease in surface energy associated with the surface reconstruction, which removes the driving force for faceting. Despite the narrow temperature window where this morphology can initially be formed, we demonstrate that it persists in homoepitaxialgrowth of TiO 2 (001) thin films. The stabilization of surface reconstructions that prevent faceting of high-surface-energy crystal faces may offer a promising avenue towards the realization of a wider range of high quality epitaxial transition metal oxide heterostructures.
36 MATERIALS SCIENCE↗
Explaining the structure sensitivity of Pt and Rh for aqueous-phase hydrogenation of phenol
Phenol is an important model compound to understand the thermocatalytic (TCH) and electrocatalytic hydrogenation (ECH) of biomass to biofuels. Although Pt and Rh are among the most studied catalysts for aqueous-phase phenol hydrogenation, the reason why certain facets are active for ECH and TCH is not fully understood. Herein, we identify the active facet of Pt and Rh catalysts for aqueous-phase hydrogenation of phenol and explain the origin of the size-dependent activity trends of Pt and Rh nanoparticles. Phenol adsorption energies extracted on the active sites of Pt and Rh nanoparticles on carbon by fitting kinetic data show that the active sites adsorb phenol weakly. We predict that the turnover frequencies (TOFs) for the hydrogenation of phenol to cyclohexanone on Pt(111) and Rh(111) terraces are higher than those on (221) stepped facets based on density functional theory modeling and mean-field microkinetic simulations. The higher activities of the (111) terraces are due to lower activation energies and weaker phenol adsorption, preventing high coverages of phenol from inhibiting hydrogen adsorption. We measure that the TOF for ECH of phenol increases as the Rh nanoparticle diameter increases from 2 to 10 nm at 298 K and -0.1 V vs the reversible hydrogen electrode, qualitatively matching prior reports for Pt nanoparticles. The increase in experimental TOFs as Pt and Rh nanoparticle diameters increase is due to a larger fraction of terraces on larger particles. These findings clarify the structure sensitivity and active site of Pt and Rh for the hydrogenation of phenol and will inform the catalyst design for the hydrogenation of bio-oils.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Fabrication and field emission properties of vertical, tapered GaN nanowires etched via phosphoric acid
The controlled fabrication of vertical, tapered, and high-aspect ratio GaN nanowires via a two-step top-down process consisting of an inductively coupled plasma reactive ion etch followed by a hot, 85% H 3 PO 4 crystallographic wet etch is explored. The vertical nanowires are oriented in the $[0001]$ direction and are bound by sidewalls comprising of $\{33\bar{6}2\}$ semipolar planes which are at a 12° angle from the [0001] axis. High temperature H3PO4 etching between 60 °C and 95 °C result in smooth semipolar faceting with no visible micro-faceting, whereas a 50 °C etch reveals a micro-faceted etch evolution. High-angle annular dark-field scanning transmission electron microscopy imaging confirms nanowire tip dimensions down to 8–12 nanometers. The activation energy associated with the etch process is 0.90 ± 0.09 eV, which is consistent with a reaction-rate limited dissolution process. The exposure of the $\{33\bar{6}2\}$ type planes is consistent with etching barrier index calculations. The field emission properties of the nanowires were investigated via a nanoprobe in a scanning electron microscope as well as by a vacuum field emission electron microscope. Finally, the measurements show a gap size dependent turn-on voltage, with a maximum current of 33 nA and turn-on field of 1.92 V nm -1 for a 50 nm gap, and uniform emission across the array.
77 NANOSCIENCE AND NANOTECHNOLOGY↗
Phase-field model of freeze casting
Directional solidification of water-based solutions has emerged as a versatile technique for templating hierarchical porous materials. However, the underlying mechanisms of pattern formation remain incompletely understood. In this work, we present a detailed derivation and analysis of a quantitative phase-field model for simulating this nonequilibrium process. The phase-field model extends the thin-interface formulation of dilute binary alloy solidification with antitrapping to incorporate the highly anisotropic energetic and kinetic properties of the partially faceted ice-water interface. This interface is faceted in the basal plane normal to the ⟨0001⟩ directions and atomically rough in other directions within the basal plane. On the basal plane, the model reproduces a linear or nonlinear relationship between the interface growth rate and the kinetic undercooling that can be linked to experimental measurements. In both cases, spontaneous parity breaking of the solidification front is observed when the preferred growth direction is aligned with the temperature gradient. This phenomenon leads to the formation of partially faceted ice lamellae that drift laterally in one of the ⟨0001⟩ directions. Here, we demonstrate that the drifting velocity of the ice lamellae is controlled by the kinetics on the basal plane and converges as the thickness of the diffuse solid-liquid interface decreases. Furthermore, we examine the effect of the form of the kinetic anisotropy, which is chosen here such that the inverse of the kinetic coefficient varies linearly from a finite value in the ⟨0001⟩ directions to zero in all other directions within the basal plane, consistent with the assumption that the interface grows in local thermodynamic equilibrium in this plane. Our results indicate that the drifting velocity of ice lamellae is not affected by the slope of this linear relation, and the radius and undercooling at the tip of an ice lamella converge at relatively small slope values. Consequently, the phase-field simulations remain quantitative with computationally tractable choices of both the interface thickness and the slope assumed in the form of the kinetic anisotropy.
Materials science↗