Atomic Structure of Quantum Gold Nanowires: Quantification of the Lattice Strain

Theoretical studies exist to compute the atomic arrangement in gold nanowires and the influence on their electronic behavior with decreasing diameter. Experimental studies, <i>e.g.</i>, by transmission electron microscopy, on chemically synthesized ultrafine wires are however lacking owing to the unavailability of suitable protocols for sample preparation and the stability of the wires under electron beam irradiation. In this work, we present an atomic scale structural investigation on quantum single crystalline gold nanowires of 2 nm diameter, chemically prepared on a carbon film grid. Using low dose aberration-corrected high resolution (S)TEM, we observe an inhomogeneous strain distribution in the crystal, largely concentrated at the twin boundaries and the surface along with the presence of facets and surface steps leading to a noncircular cross section of the wires. These structural aspects are critical inputs needed to determine their unique electronic character and their potential as a suitable catalyst material. Furthermore, electron-beam-induced structural changes at the atomic scale, having implications on their mechanical behavior and their suitability as interconnects, are discussed

Atomic Structure of Quantum Gold Nanowires: Quantification of the Lattice Strain

Theoretical studies exist to compute the atomic arrangement in gold nanowires and the influence on their electronic behavior with decreasing diameter. Experimental studies, <i>e.g.</i>, by transmission electron microscopy, on chemically synthesized ultrafine wires are however lacking owing to the unavailability of suitable protocols for sample preparation and the stability of the wires under electron beam irradiation. In this work, we present an atomic scale structural investigation on quantum single crystalline gold nanowires of 2 nm diameter, chemically prepared on a carbon film grid. Using low dose aberration-corrected high resolution (S)TEM, we observe an inhomogeneous strain distribution in the crystal, largely concentrated at the twin boundaries and the surface along with the presence of facets and surface steps leading to a noncircular cross section of the wires. These structural aspects are critical inputs needed to determine their unique electronic character and their potential as a suitable catalyst material. Furthermore, electron-beam-induced structural changes at the atomic scale, having implications on their mechanical behavior and their suitability as interconnects, are discussed

Ligand-Induced Shape Transformation of PbSe Nanocrystals

We present a study of the relation between the surface chemistry and nanocrystal shape of PbSe nanocrystals with a variable Pb-to-Se stoichiometry and density of oleate ligands. The oleate ligand density and binding configuration are monitored by nuclear magnetic resonance and Fourier transform infrared absorbance spectroscopy, allowing us to quantify the number of surface-attached ligands per NC and the nature of the surface–Pb–oleate configuration. The three-dimensional shape of the PbSe nanocrystals is obtained from high-angle annular dark field scanning transmission electron microscopy combined with an atom counting method. We show that the enhanced oleate capping results in a stabilization and extension of the {111} facets, and a crystal shape transformation from a truncated nanocube to a truncated octahedron

Three-Dimensional Elemental Mapping at the Atomic Scale in Bimetallic Nanocrystals

A thorough understanding of the three-dimensional (3D) atomic structure and composition of core–shell nanostructures is indispensable to obtain a deeper insight on their physical behavior. Such 3D information can be reconstructed from two-dimensional (2D) projection images using electron tomography. Recently, different electron tomography techniques have enabled the 3D characterization of a variety of nanostructures down to the atomic level. However, these methods have all focused on the investigation of nanomaterials containing only one type of chemical element. Here, we combine statistical parameter estimation theory with compressive sensing based tomography to determine the positions and atom type of each atom in heteronanostructures. The approach is applied here to investigate the interface in core–shell Au@Ag nanorods but it is of great interest in the investigation of a broad range of nanostructures

Three-Dimensional Elemental Mapping at the Atomic Scale in Bimetallic Nanocrystals

A thorough understanding of the three-dimensional (3D) atomic structure and composition of core–shell nanostructures is indispensable to obtain a deeper insight on their physical behavior. Such 3D information can be reconstructed from two-dimensional (2D) projection images using electron tomography. Recently, different electron tomography techniques have enabled the 3D characterization of a variety of nanostructures down to the atomic level. However, these methods have all focused on the investigation of nanomaterials containing only one type of chemical element. Here, we combine statistical parameter estimation theory with compressive sensing based tomography to determine the positions and atom type of each atom in heteronanostructures. The approach is applied here to investigate the interface in core–shell Au@Ag nanorods but it is of great interest in the investigation of a broad range of nanostructures

Highly Emissive Divalent-Ion-Doped Colloidal CsPb_1–<i>x</i>M_<i>x</i>Br₃ Perovskite Nanocrystals through Cation Exchange

Colloidal CsPbX₃ (X = Br, Cl, and I) perovskite nanocrystals (NCs) have emerged as promising phosphors and solar cell materials due to their remarkable optoelectronic properties. These properties can be tailored by not only controlling the size and shape of the NCs but also postsynthetic composition tuning through topotactic anion exchange. In contrast, property control by cation exchange is still underdeveloped for colloidal CsPbX₃ NCs. Here, we present a method that allows partial cation exchange in colloidal CsPbBr₃ NCs, whereby Pb²⁺ is exchanged for several isovalent cations, resulting in doped CsPb_1–<i>x</i>M_<i>x</i>Br₃ NCs (M= Sn²⁺, Cd²⁺, and Zn²⁺; 0 < <i>x</i> ≤ 0.1), with preservation of the original NC shape. The size of the parent NCs is also preserved in the product NCs, apart from a small (few %) contraction of the unit cells upon incorporation of the guest cations. The partial Pb²⁺ for M²⁺ exchange leads to a blue-shift of the optical spectra, while maintaining the high photoluminescence quantum yields (>50%), sharp absorption features, and narrow emission of the parent CsPbBr₃ NCs. The blue-shift in the optical spectra is attributed to the lattice contraction that accompanies the Pb²⁺ for M²⁺ cation exchange and is observed to scale linearly with the lattice contraction. This work opens up new possibilities to engineer the properties of halide perovskite NCs, which to date are demonstrated to be the only known system where cation and anion exchange reactions can be sequentially combined while preserving the original NC shape, resulting in compositionally diverse perovskite NCs

Incommensurate Modulation and Luminescence in the CaGd_{2(1–<i>x</i>)}Eu_2<i>x</i>(MoO₄)_{4(1–<i>y</i>)}(WO₄)_4<i>y</i> (0 ≤ <i>x ≤</i> 1, 0 ≤ <i>y ≤</i> 1) Red Phosphors

Scheelite related compounds (<i>A</i>′,<i>A</i>″)_<i>n</i>[(<i>B</i>′,<i>B</i>″)­O₄]_<i>m</i> with <i>B</i>′, <i>B</i>″ = W and/or Mo are promising new light-emitting materials for photonic applications, including phosphor converted LEDs (light-emitting diodes). In this paper, the creation and ordering of A-cation vacancies and the effect of cation substitutions in the scheelite-type framework are investigated as a factor for controlling the scheelite-type structure and luminescent properties. CaGd_{2(1–<i>x</i>)}Eu_2<i>x</i>(MoO₄)_{4(1–<i>y</i>)}(WO₄)_4<i>y</i> (0 ≤ <i>x ≤</i> 1, 0 ≤ <i>y ≤</i> 1) solid solutions with scheelite-type structure were synthesized by a solid state method, and their structures were investigated using a combination of transmission electron microscopy techniques and powder X-ray diffraction. Within this series all complex molybdenum oxides have (3 + 2)­D incommensurately modulated structures with superspace group <i>I</i>4₁/<i>a</i>(α,β,0)­00­(−β,α,0)­00, while the structures of all tungstates are (3 + 1)­D incommensurately modulated with superspace group <i>I</i>2/<i>b</i>(<i>αβ</i>0)­00. In both cases the modulation arises because of cation-vacancy ordering at the <i>A</i> site. The prominent structural motif is formed by columns of <i>A</i>-site vacancies running along the <i>c</i>-axis. These vacant columns occur in rows of two or three aligned along the [1̅10] direction of the scheelite subcell. The replacement of the smaller Gd³⁺ by the larger Eu³⁺ at the <i>A</i>-sublattice does not affect the nature of the incommensurate modulation, but an increasing replacement of Mo⁶⁺ by W⁶⁺ switches the modulation from (3 + 2)­D to (3 + 1)­D regime. Thus, these solid solutions can be considered as a model system where the incommensurate modulation can be monitored as a function of cation nature while the number of cation vacancies at the <i>A</i> sites remain constant upon the isovalent cation replacement. All compounds’ luminescent properties were measured, and the optical properties were related to the structural properties of the materials. CaGd_{2(1–<i>x</i>)}Eu_2<i>x</i>(MoO₄)_{4(1–<i>y</i>)}(WO₄)_4<i>y</i> phosphors emit intense red light dominated by the ⁵D₀–⁷F₂ transition at 612 nm, along with other transitions from the ⁵D₁ and ⁵D₀ excited states. The intensity of the ⁵D₀–⁷F₂ transition reaches a maximum at <i>x</i> = 0.5 for <i>y</i> = 0 and 1

Incommensurate Modulation and Luminescence in the CaGd_{2(1–<i>x</i>)}Eu_2<i>x</i>(MoO₄)_{4(1–<i>y</i>)}(WO₄)_4<i>y</i> (0 ≤ <i>x ≤</i> 1, 0 ≤ <i>y ≤</i> 1) Red Phosphors

Scheelite related compounds (<i>A</i>′,<i>A</i>″)_<i>n</i>[(<i>B</i>′,<i>B</i>″)­O₄]_<i>m</i> with <i>B</i>′, <i>B</i>″ = W and/or Mo are promising new light-emitting materials for photonic applications, including phosphor converted LEDs (light-emitting diodes). In this paper, the creation and ordering of A-cation vacancies and the effect of cation substitutions in the scheelite-type framework are investigated as a factor for controlling the scheelite-type structure and luminescent properties. CaGd_{2(1–<i>x</i>)}Eu_2<i>x</i>(MoO₄)_{4(1–<i>y</i>)}(WO₄)_4<i>y</i> (0 ≤ <i>x ≤</i> 1, 0 ≤ <i>y ≤</i> 1) solid solutions with scheelite-type structure were synthesized by a solid state method, and their structures were investigated using a combination of transmission electron microscopy techniques and powder X-ray diffraction. Within this series all complex molybdenum oxides have (3 + 2)­D incommensurately modulated structures with superspace group <i>I</i>4₁/<i>a</i>(α,β,0)­00­(−β,α,0)­00, while the structures of all tungstates are (3 + 1)­D incommensurately modulated with superspace group <i>I</i>2/<i>b</i>(<i>αβ</i>0)­00. In both cases the modulation arises because of cation-vacancy ordering at the <i>A</i> site. The prominent structural motif is formed by columns of <i>A</i>-site vacancies running along the <i>c</i>-axis. These vacant columns occur in rows of two or three aligned along the [1̅10] direction of the scheelite subcell. The replacement of the smaller Gd³⁺ by the larger Eu³⁺ at the <i>A</i>-sublattice does not affect the nature of the incommensurate modulation, but an increasing replacement of Mo⁶⁺ by W⁶⁺ switches the modulation from (3 + 2)­D to (3 + 1)­D regime. Thus, these solid solutions can be considered as a model system where the incommensurate modulation can be monitored as a function of cation nature while the number of cation vacancies at the <i>A</i> sites remain constant upon the isovalent cation replacement. All compounds’ luminescent properties were measured, and the optical properties were related to the structural properties of the materials. CaGd_{2(1–<i>x</i>)}Eu_2<i>x</i>(MoO₄)_{4(1–<i>y</i>)}(WO₄)_4<i>y</i> phosphors emit intense red light dominated by the ⁵D₀–⁷F₂ transition at 612 nm, along with other transitions from the ⁵D₁ and ⁵D₀ excited states. The intensity of the ⁵D₀–⁷F₂ transition reaches a maximum at <i>x</i> = 0.5 for <i>y</i> = 0 and 1