Kinetic and Thermodynamic Modified Wulff Constructions for Twinned Nanoparticles

Wulff constructions are a powerful tool to predict the shape of nanoparticles, which strongly influences their performance in catalysis, sensing, and surface-enhanced spectroscopies. Previous Wulff models focused on energy minimization and included contributions from the surface energy, interface energy, twin boundaries, and segregation-induced bulk energy changes. However, a large number of shapes cannot be understood by such thermodynamic approaches, in particular many of the twinned late transition metal (Ag, Au, Pt, Pd, etc.) particles of interest in catalysis and plasmonics. A review of the modified Wulff (i.e., twinned) construction is presented here, followed by the development of a modified kinetic Wulff model, which, by including kinetic parameters, explains the emergence of commonly observed shapes such as bitetrahedra, truncated bitetrahedra, thin triangular platelets, perfect decahedra, and decahedral rods

Tip-Enhanced Raman Imaging of Plasmon-Driven Coupling of 4‑Nitrobenzenethiol on Au-Decorated Magnesium Nanostructures

Magnesium nanoparticles (MgNPs) exhibit localized surface plasmon resonances across the ultraviolet, visible, and near-infrared parts of electromagnetic spectrum and are attracting increasing interest due to their sustainability and biocompatibility. In this study, we used tip-enhanced Raman spectroscopy (TERS) to examine the photocatalytic properties of MgNP protected by a thin native oxide layer and their Au-modified bimetallic analogs produced by partial galvanic replacement, Au-MgNPs. We found no reduction of 4-nitrobenzenethiol (4-NBT) to p,p′-dimercaptoazobisbenzene (DMAB) when a Au-coated tip was placed in contact with a self-assembled monolayer of 4-NBT molecules adsorbed on MgNPs alone. However, decorating Mg with Au made these bimetallic structures catalytically active. The DMAB signal signature of photocatalytic activity was more delocalized around AuNPs attached to Mg than around AuNPs on a Si substrate, indicating coupling between the Mg core and Au decorations. This report on photocatalytic activity of a bimetallic structure including plasmonic Mg paves the way for further catalyst architectures benefiting from Mg’s versatility and abundance

MOESM1 of Micro-Extinction Spectroscopy (MExS): a versatile optical characterization technique

Additional file 1. Additional figures

Kinetic and Thermodynamic Modified Wulff Constructions for Twinned Nanoparticles

Wulff constructions are a powerful tool to predict the shape of nanoparticles, which strongly influences their performance in catalysis, sensing, and surface-enhanced spectroscopies. Previous Wulff models focused on energy minimization and included contributions from the surface energy, interface energy, twin boundaries, and segregation-induced bulk energy changes. However, a large number of shapes cannot be understood by such thermodynamic approaches, in particular many of the twinned late transition metal (Ag, Au, Pt, Pd, etc.) particles of interest in catalysis and plasmonics. A review of the modified Wulff (i.e., twinned) construction is presented here, followed by the development of a modified kinetic Wulff model, which, by including kinetic parameters, explains the emergence of commonly observed shapes such as bitetrahedra, truncated bitetrahedra, thin triangular platelets, perfect decahedra, and decahedral rods

Ba₂An(S₂)₂S₂ (An = U, Th): Syntheses, Structures, Optical, and Electronic Properties

The compounds Ba₂An­(S₂)₂S₂ (An = U, Th) have been synthesized by reactions of the elements with BaS and S at 1273 and 1173 K, respectively. These isostructural compounds crystallize in a new structure type in the tetragonal space group <i>D</i>_4<i>h</i>¹⁵-<i>P</i>4₂/<i>nmc</i>. The structure comprises Ba²⁺ cations and _∞²[An­(S₂)₂(S)₂^4–] layers. The An⁴⁺ cations in these layers are arranged linearly and are bridged by S^2– anions. Coordination about the An center, which has symmetry 4̅<i>m</i>2, consists of two S₂^2– ions and four S^2– ions. Thus, the compounds are charge-balanced with An⁴⁺. No other alkali-metal actinide chalcogenides are known that contain chalcogen–chalcogen bonds. Optical measurements on Ba₂Th­(S₂)₂S₂ indicate a direct band gap of 2.46(5) eV. Density functional theory calculations, performed with the HSE exchange-correlation potential, lead to band gaps of 2.2 and 1.8 eV for Ba₂Th­(S₂)₂S₂ and Ba₂U­(S₂)₂S₂, respectively, thus demonstrating the utility of applying this functional to 5f-electron systems

Single-Crystal Structures, Optical Absorptions, and Electronic Distributions of Thorium Oxychalcogenides ThOQ (Q = S, Se, Te)

The compounds ThOS, ThOSe, and ThOTe have been synthesized, and their structures have been determined by means of single-crystal X-ray diffraction methods. All three compounds adopt the PbFCl structure type in the tetragonal space group <i>D</i>_4<i>h</i>⁷ – <i>P</i>4/<i>nmm</i>. More precise crystallographic data have been obtained for ThOS and ThOSe, which had previously only been known from X-ray powder diffraction data. ThOS, ThOSe, and ThOTe are yellow-, orange-, and black-colored, respectively. From single-crystal optical absorption measurements the band gaps are 2.22, 1.65, and 1.45 eV, respectively. Optical band gaps, ionic charges, and densities of states were calculated for the three compounds with the use of Density Functional methods

Plasmonic Properties of Self-Assembled Gold Nanocrescents: Implications for Chemical Sensing

A bottom-up approach, the Langmuir–Blodgett technique, is used for the preparation of composite thin films of gold nanoparticles and polymers: poly(styrene-b-2-vinylpyridine), poly-2-vinylpyridine, and polystyrene. The self-assembly of poly(styrene-b-2-vinylpyridine) at the air–water interface leads to the formation of surface micelles, which serve as a template for the organization of gold nanoparticles into ring assemblies. By using poly-2-vinylpyridine in conjunction with low surface pressure, the distance between nanostructures can be increased, allowing for optical characterization of single nanostructures. Once deposited on a solid substrate, the preorganized gold nanoparticles are subjected to further growth by the reduction of additional gold, leading to a variety of nanostructures which can be divided into two categories: nanocrescents and circular arrays of nanoparticles. The optical properties of individual structures are investigated by optical dark-field spectroscopy and numerical calculations. The plasmonic behavior of the nanostructures is elucidated through the correlation of optical properties with structural features and the identification of dominant plasmon modes. Being based on a self-assembly approach, the reported method allows for the formation of interesting plasmonic materials under ambient conditions, at a relatively large scale, and at low cost. These attributes, in addition to the resonances located in the near-infrared region of the spectrum, make nanocrescents candidates for biological and chemical sensing

Single-Crystal Structures, Optical Absorptions, and Electronic Distributions of Thorium Oxychalcogenides ThOQ (Q = S, Se, Te)

The compounds ThOS, ThOSe, and ThOTe have been synthesized, and their structures have been determined by means of single-crystal X-ray diffraction methods. All three compounds adopt the PbFCl structure type in the tetragonal space group <i>D</i>_4<i>h</i>⁷ – <i>P</i>4/<i>nmm</i>. More precise crystallographic data have been obtained for ThOS and ThOSe, which had previously only been known from X-ray powder diffraction data. ThOS, ThOSe, and ThOTe are yellow-, orange-, and black-colored, respectively. From single-crystal optical absorption measurements the band gaps are 2.22, 1.65, and 1.45 eV, respectively. Optical band gaps, ionic charges, and densities of states were calculated for the three compounds with the use of Density Functional methods

Plasmonic Near-Electric Field Enhancement Effects in Ultrafast Photoelectron Emission: Correlated Spatial and Laser Polarization Microscopy Studies of Individual Ag Nanocubes

Electron emission from single, supported Ag nanocubes excited with ultrafast laser pulses (λ = 800 nm) is studied via spatial and polarization correlated (i) dark field scattering microscopy (DFM), (ii) scanning photoionization microscopy (SPIM), and (iii) high-resolution transmission electron microscopy (HRTEM). Laser-induced electron emission is found to peak for laser polarization aligned with cube diagonals, suggesting the critical influence of plasmonic near-field enhancement of the incident electric field on the overall electron yield. For laser pulses with photon energy below the metal work function, coherent multiphoton photoelectron emission (MPPE) is identified as the most probable mechanism responsible for electron emission from Ag nanocubes and likely metal nanoparticles/surfaces in general

Optoplasmonic Effects in Highly Curved Surfaces for Catalysis, Photothermal Heating, and SERS

Surface curvature can be used to focus light and alter optical processes. Here, we show that curved surfaces (spheres, cylinders, and cones) with a radius of around 5 μm lead to maximal optoplasmonic properties including surface-enhanced Raman scattering (SERS), photocatalysis, and photothermal processes. Glass microspheres, microfibers, pulled fibers, and control flat substrates were functionalized with well-dispersed and dense arrays of 45 nm Au NP using polystyrene-block-poly-4-vinylpyridine (PS-b-P4VP) and chemically modified with 4-mercaptobenzoic acid (4-MBA, SERS reporter), 4-nitrobenzenethiol (4-NBT, reactive to plasmonic catalysis), or 4-fluorophenyl isocyanide (FPIC, photothermal reporter). The various curved substrates enhanced the plasmonic properties by focusing the light in a photonic nanojet and providing a directional antenna to increase the collection efficacy of SERS photons. The optoplasmonic effects led to an increase of up to 1 order of magnitude of the SERS response, up to 5 times the photocatalytic conversion of 4-NBT to 4,4′-dimercaptoazobenzene when the diameter of the curved surfaces was about 5 μm and a small increase in photothermal effects. Taken together, the results provide evidence that curvature enhances plasmonic properties and that its effect is maximal for spherical objects around a few micrometers in diameter, in agreement with a theoretical framework based on geometrical optics. These enhanced plasmonic effects and the stationary-phase-like plasmonic substrates pave the way to the next generation of sensors, plasmonic photocatalysts, and photothermal devices