16 research outputs found
Nanotubes from the Misfit Layered Compounds MS–TaS<sub>2</sub>, Where M = Pb, Sn, Sb, or Bi: Synthesis and Study of Their Structure
Tubular structures of the MS–TaS<sub>2</sub> with (M = Pb,
Sn, Sb, Bi) misfit layered compounds are reported. The lattice mismatch
between the alternating MS and TaS<sub>2</sub> layers leads to a variety
of chiral tubular structures. Such tubular structures are studied
via scanning electron microscopy (SEM), high resolution transmission
electron microscopy (HRTEM), and selected area electron diffraction
(SAED). For the PbS–TaS<sub>2</sub> and SnS–TaS<sub>2</sub> tubules, relative in-plane orientations as well as folding
vectors of the two subsystems can be determined. However, almost ring-like
SAED patterns are obtained for SbS–TaS<sub>2</sub> nanotubes
precluding exact determination of the relative in plane orientation.
Also, very complex diffraction patterns were obtained for BiS–TaS<sub>2</sub> nanotubes
Study of Tubular Structures of the Misfit Layered Compound SnS<sub>2</sub>/SnS
Tubular structures of the SnS<sub>2</sub>/SnS misfit
compound,
which are currently prepared in large amounts, are reported. The lattice
mismatch between the two alternating sublayers of SnS<sub>2</sub> and
SnS leads to a variety of chiral tubular structures. Such tubular
structures are studied via high resolution transmission electron microscopy
(HRTEM) and selected area electron diffraction (SAED). The diversity
of the structures manifests itself through different stacking orders
of SnS<sub>2</sub> and SnS layers along their common <i>c</i>-axis and their relative in-plane orientation. Folding vectors and
chiral angles of both subsystems can be determined
Quaternary Chalcogenide-Based Misfit Nanotubes LnS(Se)-TaS(Se)<sub>2</sub> (Ln = La, Ce, Nd, and Ho): Synthesis and Atomic Structural Studies
We
have synthesized quaternary chalcogenide-based misfit nanotubes LnS(Se)-TaS<sub>2</sub>(Se) (Ln = La, Ce, Nd, and Ho). None of the compounds described
here were reported in the literature as a bulk compound. The characterization
of these nanotubes, at the atomic level, has been developed via different
transmission electron microscopy techniques, including high-resolution
scanning transmission electron microscopy, electron diffraction, and
electron energy-loss spectroscopy. In particular, quantification at
sub-nanometer scale was achieved by acquiring high-quality electron
energy-loss spectra at high energy (∼between 1000 and 2500
eV). Remarkably, the sulfur was found to reside primarily in the distorted
rocksalt LnS lattice, while the Se is associated with the hexagonal
TaSe<sub>2</sub> site. Consequently, these quaternary misfit layered
compounds in the form of nanostructures possess a double superstructure
of La/Ta and S/Se with the same periodicity. In addition, the interlayer
spacing between the layers and the interatomic distances within the
layer vary systematically in the nanotubes, showing clear reduction
when going from the lightest (La atom) to the heaviest (Ho) atom.
Amorphous layers, of different nature, were observed at the surface
of the nanotubes. For La-based NTs, the thin external amorphous layer
(inferior to 10 nm) can be ascribed to a Se deficiency. Contrarily,
for Ho-based NTs, the thick amorphous layer (between 10 and 20 nm)
is clearly ascribed to oxidation. All of these findings helped us
to understand the atomic structure of these new compounds and nanotubes
thereof
Atomic Structural Studies on Thin Single-Crystalline Misfit-Layered Nanotubes of TbS-CrS<sub>2</sub>
Various nanotubes
from ternary misfit compounds have been reported
in recent years. In the present work, the detailed atomic structure
and chemical configuration of misfit-layered nanotubes based on the
TbS-CrS<sub>2</sub> are reported. These analyses have been developed
via different transmission electron microscopy techniques, including
high-resolution scanning transmission electron microscopy, electron
diffraction, and electron energy loss spectroscopy. These structural
analyses show that two different kinds of nanotubes can be produced:
a “regular” nanotube and a “wavy” one.
Both kinds of nanotubes show the alternating arrangements of the TbS
and CrS<sub>2</sub> subsystems; however, the wavy ones present a nearly
periodically deficiency in terbium. In addition to the structural
investigation, the chemical analyses have proved that the outer layer
of both kinds of nanotubes is composed of the elements Cr and S. All
these findings helped to understand the growth mechanism during the
sulfurization reaction taking place in the synthesis process
Gold Nanoparticles as Surface Defect Probes for WS<sub>2</sub> Nanostructures
WS<sub>2</sub> inorganic nanotubes (INT) and inorganic fullerene-like nanoparticles (IF) are well-known for their high mechanical strength and as superior solid lubricants. The outermost WS<sub>2</sub> layer is considered to be fully bonded; thus, it was suggested that the interactions of these WS<sub>2</sub> nanostructures with their surroundings are governed by purely van der Waals (vdW) interactions. However, in the case of IF-WS<sub>2</sub> nanoparticles, the faceted surface may contain sites with nonsaturated coordination, which, in turn, react with the surrounding media. Gold nanoparticles (GNP) were used as probes for the IF-WS<sub>2</sub> surface defects, mapped by both scanning and transmission electron microscopy. The interaction between the GNP and the reactive surface was investigated using INT-WS<sub>2</sub> as a model and was characterized by atomic force microscopy (AFM)
Strontium Cobalt Oxide Misfit Nanotubes
Low-dimensional
misfit layered compounds have been found to have ultralow thermal
conductivity, which is attributed to their unique structure and the
low dimensionality. There are a few studies reporting the preparation
of sulfide-based misfit nanotubes but only one study on oxide-based
analogs. In this investigation, we report a new oxide-based misfit
nanotube derived from misfit layered strontium cobaltite. Thorough
structural investigation by electron microscopy techniques, including
electron diffraction, aberration corrected high-resolution (scanning)
transmission electron microscopy, and electron energy-loss spectroscopy
along with density functional theory calculations show that these
nanotubes consist of alternating layers of SrCoO<sub>2</sub> and CoO<sub>2</sub>. We have studied systematically the effect of base concentration
on the structure and composition of the nanotubes, which reveals the
importance of misfit stress to tightly roll the structure into tubular
form and thus control the synthesis. Electronic structure calculations
find that the structures are semiconducting with a ferrimagnetic ground
state. Our studies further extend the family of bulk misfit layered
oxides into the 1D realm with potential applications in thermoelectric
and electronic devices
Field-Effect Transistors Based on WS<sub>2</sub> Nanotubes with High Current-Carrying Capacity
We
report the first transistor based on inorganic nanotubes exhibiting
mobility values of up to 50 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> for an individual WS<sub>2</sub> nanotube. The current-carrying
capacity of these nanotubes was surprisingly high with respect to
other low-dimensional materials, with current density at least 2.4
× 10<sup>8</sup> A cm<sup>–2</sup>. These results demonstrate
that inorganic nanotubes are promising building blocks for high-performance
electronic applications
Nanotubes from Oxide-Based Misfit Family: The Case of Calcium Cobalt Oxide
Misfit
layered compounds (MLCs) have generated significant interest
in recent years as potential thermoelectric materials. MLC nanotubes
could reveal behavior that is entirely different from the bulk material.
Recently, new chemical strategies were exploited for the synthesis
of nanotubular forms of chalcogenide-based MLCs, which are promising
candidates for thermoelectric materials. However, analogous synthesis
of oxide-based MLC nanotubes has not been demonstrated until now.
Here, we report a chemical strategy for synthesis of cobalt-oxide-based
misfit nanotubes. A combination of high-resolution (scanning) transmission
electron microscopy (including image simulations), spatially resolved
electron energy-loss spectroscopy, electron diffraction, and density
functional theory (DFT) calculations is used to discover the formation
of a phase within these nanotubes that differs significantly from
bulk calcium cobaltite MLCs. Furthermore, DFT calculations show that
this phase is semiconducting with a band gap in excess of 1 eV, unlike
bulk calcium cobaltite MLCs, which are known to be metallic. Through
systematic experiments, we propose a formation mechanism for these
nanotubes that could also apply more generally to realizing other
oxide-based MLC nanotubes
Nanotubes from Chalcogenide Misfit Compounds: Sn–S and Nb–Pb–S
Carbon fullerenes and nanotubes revolutionized understandingof the reactivity of nanoscale compounds. Subsequently, our group and others discovered analogous inorganic compounds with hollow, closed nanostructures. Such inorganic nanostructures offer many applications, particularly in the energy and electronics industries.One way to create inorganic nanostructures is via misfit layer-ed compounds (MLC), which are stacks of alternating two-dimensional molecular slabs, typically held together via weak van der Waals forces. They contain “misfits” in their <i>a</i>–<i>b</i> plane structures that can make them unstable, leading to collapse of the slabs into tubular nanostructures. For example, metal chalcogenide MLCs of the general formula (MX)<sub>1+<i>y</i></sub>/TX<sub>2</sub> (M = Sn, Pb, Bi, Sb, and other rare earths; T = Sn, Ti, V, Cr, Nb, Ta, etc.; X = S or Se) consist of a superstructure of alternating layers where the MX unit belongs to a (distorted NaCl) orthorhombic symmetry group (O), the TX<sub>2</sub> layer possesses trigonal (T) or octahedral symmetry, and the two layers are held together via both van der Waals and polar forces. A misfit in the <i>a</i> axis or both <i>a</i> and <i>b</i> axes of the two sublattices may lead to the formation of nanostructures as the lattices relax via scrolling. Previous research has also shown that the abundance of atoms with dangling bonds in the rims makes nanoparticles of compounds with layered structure unstable in the planar form, and they tend to fold into hollow closed structures such as nanotubes.This Account shows that combining these two triggers, misfits and dangling bond annihilation in the slab rims, leads to new kinds of nanotubes from MLCs. In particular, we report the structure of two new types of nanotubes from misfits, namely, the SnS/SnS<sub>2</sub> and PbS/NbS<sub>2</sub> series. To decipher the complex structures of these nanotubes, we use a range of methods: high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDS), selected area electron diffraction (SAED) analyses, scanning electron microscopy (SEM), and Cs-corrected scanning transmission electron microscopy (STEM) in the high-angle annular dark-field mode (HAADF). In both new types, the lattice mismatch between the two alternating sublayers dictates the relative layer-stacking order and leads to a variety of chiral tubular structures. In particular, the incommensuration (a type of misfit) of the SnS<sub>2</sub>/SnS system in both the (in plane) <i>a</i> and <i>b</i> directions leads to a variety of relative in-plane orientation and stacking orders along the common <i>c</i>-axis. Thus the SnS/SnS<sub>2</sub> nanotubes form superstructures with the sequence O–T and O–T–T, and mixtures thereof. We also report nanotubes of the misfit layered compound (PbS)<sub>1.14</sub>NbS<sub>2</sub>, and of NbS<sub>2</sub> intercalated with Pb atoms, with the chemical formula PbNbS<sub>2</sub>. Thus, the possibility to use two kinds of folding mechanisms jointly offers a new apparatus for the synthesis of unique 1-D nanostructures of great complexity and a potentially large diversity of physicochemical properties
Spectroscopic Determination of Phonon Lifetimes in Rhenium-Doped MoS<sub>2</sub> Nanoparticles
We
investigated the infrared vibrational properties of pristine and Re-substituted
MoS<sub>2</sub> nanoparticles and analyzed the extracted phonon lifetimes
in terms of multiple scattering events. Our measurements reveal both
size- and doping-dependent changes that we attribute to grain boundary
scattering and charge and mass effects, respectively. By contrast,
Born charge is affected only by size. These findings illustrate the
utility of reaching beyond traditional bulk semiconductors and quantum
dots to explore how doping and confinement impact carrier-phonon interactions
in low-dimensional semiconducting nanomaterials