8 research outputs found
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 surfaceāPbāoleate 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 coreāshell 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 coreāshell
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 coreāshell 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 coreāshell
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<sub>1ā<i>x</i></sub>M<sub><i>x</i></sub>Br<sub>3</sub> Perovskite Nanocrystals through Cation Exchange
Colloidal
CsPbX<sub>3</sub> (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 CsPbX<sub>3</sub> NCs. Here, we present
a method that allows partial cation exchange in colloidal CsPbBr<sub>3</sub> NCs, whereby Pb<sup>2+</sup> is exchanged for several isovalent
cations, resulting in doped CsPb<sub>1ā<i>x</i></sub>M<sub><i>x</i></sub>Br<sub>3</sub> NCs (M= Sn<sup>2+</sup>, Cd<sup>2+</sup>, and Zn<sup>2+</sup>; 0 < <i>x</i> ā¤ 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 Pb<sup>2+</sup> for M<sup>2+</sup> 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 CsPbBr<sub>3</sub> NCs. The blue-shift
in the optical spectra is attributed to the lattice contraction that
accompanies the Pb<sup>2+</sup> for M<sup>2+</sup> 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<sub>2(1ā<i>x</i>)</sub>Eu<sub>2<i>x</i></sub>(MoO<sub>4</sub>)<sub>4(1ā<i>y</i>)</sub>(WO<sub>4</sub>)<sub>4<i>y</i></sub> (0 ā¤ <i>x ā¤</i> 1, 0 ā¤ <i>y ā¤</i> 1) Red Phosphors
Scheelite related compounds (<i>A</i>ā²,<i>A</i>ā³)<sub><i>n</i></sub>[(<i>B</i>ā²,<i>B</i>ā³)ĀO<sub>4</sub>]<sub><i>m</i></sub> with <i>B</i>ā², <i>B</i>ā³
= 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<sub>2(1ā<i>x</i>)</sub>Eu<sub>2<i>x</i></sub>(MoO<sub>4</sub>)<sub>4(1ā<i>y</i>)</sub>(WO<sub>4</sub>)<sub>4<i>y</i></sub> (0
ā¤ <i>x ā¤</i> 1, 0 ā¤ <i>y ā¤</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)ĀD incommensurately modulated structures with superspace
group <i>I</i>4<sub>1</sub>/<i>a</i>(Ī±,Ī²,0)Ā00Ā(āĪ²,Ī±,0)Ā00,
while the structures of all tungstates are (3 + 1)ĀD incommensurately
modulated with superspace group <i>I</i>2/<i>b</i>(<i>Ī±Ī²</i>0)Ā00. 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
[1Ģ
10] direction of the scheelite subcell. The replacement of
the smaller Gd<sup>3+</sup> by the larger Eu<sup>3+</sup> at the <i>A</i>-sublattice does not affect the nature of the incommensurate
modulation, but an increasing replacement of Mo<sup>6+</sup> by W<sup>6+</sup> switches the modulation from (3 + 2)ĀD to (3 + 1)ĀD 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
compoundsā luminescent properties were measured, and the optical
properties were related to the structural properties of the materials.
CaGd<sub>2(1ā<i>x</i>)</sub>Eu<sub>2<i>x</i></sub>(MoO<sub>4</sub>)<sub>4(1ā<i>y</i>)</sub>(WO<sub>4</sub>)<sub>4<i>y</i></sub> phosphors emit intense red
light dominated by the <sup>5</sup>D<sub>0</sub>ā<sup>7</sup>F<sub>2</sub> transition at 612 nm, along with other transitions
from the <sup>5</sup>D<sub>1</sub> and <sup>5</sup>D<sub>0</sub> excited
states. The intensity of the <sup>5</sup>D<sub>0</sub>ā<sup>7</sup>F<sub>2</sub> transition reaches a maximum at <i>x</i> = 0.5 for <i>y</i> = 0 and 1
Incommensurate Modulation and Luminescence in the CaGd<sub>2(1ā<i>x</i>)</sub>Eu<sub>2<i>x</i></sub>(MoO<sub>4</sub>)<sub>4(1ā<i>y</i>)</sub>(WO<sub>4</sub>)<sub>4<i>y</i></sub> (0 ā¤ <i>x ā¤</i> 1, 0 ā¤ <i>y ā¤</i> 1) Red Phosphors
Scheelite related compounds (<i>A</i>ā²,<i>A</i>ā³)<sub><i>n</i></sub>[(<i>B</i>ā²,<i>B</i>ā³)ĀO<sub>4</sub>]<sub><i>m</i></sub> with <i>B</i>ā², <i>B</i>ā³
= 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<sub>2(1ā<i>x</i>)</sub>Eu<sub>2<i>x</i></sub>(MoO<sub>4</sub>)<sub>4(1ā<i>y</i>)</sub>(WO<sub>4</sub>)<sub>4<i>y</i></sub> (0
ā¤ <i>x ā¤</i> 1, 0 ā¤ <i>y ā¤</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)ĀD incommensurately modulated structures with superspace
group <i>I</i>4<sub>1</sub>/<i>a</i>(Ī±,Ī²,0)Ā00Ā(āĪ²,Ī±,0)Ā00,
while the structures of all tungstates are (3 + 1)ĀD incommensurately
modulated with superspace group <i>I</i>2/<i>b</i>(<i>Ī±Ī²</i>0)Ā00. 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
[1Ģ
10] direction of the scheelite subcell. The replacement of
the smaller Gd<sup>3+</sup> by the larger Eu<sup>3+</sup> at the <i>A</i>-sublattice does not affect the nature of the incommensurate
modulation, but an increasing replacement of Mo<sup>6+</sup> by W<sup>6+</sup> switches the modulation from (3 + 2)ĀD to (3 + 1)ĀD 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
compoundsā luminescent properties were measured, and the optical
properties were related to the structural properties of the materials.
CaGd<sub>2(1ā<i>x</i>)</sub>Eu<sub>2<i>x</i></sub>(MoO<sub>4</sub>)<sub>4(1ā<i>y</i>)</sub>(WO<sub>4</sub>)<sub>4<i>y</i></sub> phosphors emit intense red
light dominated by the <sup>5</sup>D<sub>0</sub>ā<sup>7</sup>F<sub>2</sub> transition at 612 nm, along with other transitions
from the <sup>5</sup>D<sub>1</sub> and <sup>5</sup>D<sub>0</sub> excited
states. The intensity of the <sup>5</sup>D<sub>0</sub>ā<sup>7</sup>F<sub>2</sub> transition reaches a maximum at <i>x</i> = 0.5 for <i>y</i> = 0 and 1