4 research outputs found
Unveiling Chemical Reactivity and Structural Transformation of TwoâDimensional Layered Nanocrystals
Two-dimensional
(2D) layered nanostructures are emerging fast due
to their exceptional materials properties. While the importance of
physical approaches (e.g., guest intercalation and exfoliation) of
2D layered nanomaterials has been recognized, an understanding of
basic chemical reactions of these materials, especially in nanoscale
regime, is obscure. Here, we show how chemical stimuli can influence
the fate of reaction pathways of 2D layered nanocrystals. Depending
on the chemical characteristics (Lewis acid (<sup>1</sup>O<sub>2</sub>) or base (H<sub>2</sub>O)) of external stimuli, TiS<sub>2</sub> nanocrystal
is respectively transformed to either a TiO<sub>2</sub> nanodisc through
a âcompositional metathesisâ or a TiO<sub>2</sub> toroid
through multistage âedge-selective structural transformationâ
processes. These chemical reactions can serve as the new design concept
for functional 2D layered nanostructures. For example, TiS<sub>2(disc)</sub>-TiO<sub>2(shell)</sub> nanocrystal constitutes a high performance
type II heterojunction which not only a wide range solar energy coverage
(âŒ80%) with near-infrared absorption edge, but also possesses
enhanced electron transfer property
Colloidal Single-Layer Quantum Dots with Lateral Confinement Effects on 2D Exciton
Controlled lateral
quantum confinement in single-layer transition-metal
chalcogenides (TMCs) can potentially combine the unique properties
of two-dimensional (2D) exciton with the size-tunability of exciton
energy, creating the single-layer quantum dots (SQDs) of 2D TMC materials.
However, exploring such opportunities has been challenging due to
the limited ability to produce well-defined SQDs with sufficiently
high quality and size control, in conjunction with the commonly observed
inconsistency in the optical properties. Here, we report an effective
method to synthesize high-quality and size-controlled SQDs of WSe<sub>2</sub> via multilayer quantum dots (MQDs) precursors, which enables
grasping a clear picture of the role of lateral confinement on the
optical properties of the 2D exciton. From the single-particle optical
spectra and polarization anisotropy of WSe<sub>2</sub> SQDs of varying
sizes in addition to their ensemble data, we reveal how the properties
of 2D exciton in single-layer TMCs evolve with increasing lateral
quantum confinement
Photoinduced Separation of Strongly Interacting 2âD Layered TiS<sub>2</sub> Nanodiscs in Solution
Colloidal 2-D layered transition
metal dichalcogenide (TMDC) nanodiscs synthesized with uniform diameter
and thickness can readily form the vertically stacked assemblies of
particles in solution due to strong interparticle cohesive energy.
The interparticle electronic coupling that modifies their optical
and electronic properties poses a significant challenge in exploring
their unique properties influenced by the anisotropic quantum confinement
in different directions taking advantage of the controlled diameter
and thickness. Here, we show that the assemblies of 2-D layered TiS<sub>2</sub> nanodiscs are efficiently separated into individual nanodiscs
via photoexcitation of the charge carriers by pulsed laser light,
enabling the characterization of the properties of noninteracting
TiS<sub>2</sub> nanodiscs. Photoinduced separation of the nanodiscs
is considered to occur via transient weakening of the interparticle
cohesive force by the dense photoexcited charge carriers, which facilitates
the solvation of each nanodisc by the solvent molecules
Anisotropic ElectronâPhonon Coupling in Colloidal Layered TiS<sub>2</sub> Nanodiscs Observed via Coherent Acoustic Phonons
Atomically
thin layered transition metal dichalcogenides with highly anisotropic
structure exhibit strong anisotropy in various material properties.
Here, we report the anisotropic coupling between the interband optical
transition and coherent acoustic phonon excited by ultrashort optical
excitation in a colloidal solution of multilayered TiS<sub>2</sub> nanodiscs. The transient absorption signal from the diameter- and
thickness-controlled TiS<sub>2</sub> nanodiscs dispersed in solution
exhibited an oscillatory feature, which is attributed to the modulation
of the interband absorption peak by the intralayer breathing mode.
However, the signature of the interlayer acoustic phonon was not observed,
while it has been previously observed in noncolloidal exfoliated sheets
of MoS<sub>2</sub>. The dominance of the intralayer mode in modulating
the interband optical transition was supported by the density functional
theory (DFT) calculations of the optical absorption spectra of TiS<sub>2</sub>, which showed the stronger sensitivity of the interband absorption
peak in the visible region to the in-plane strain than to the out-of-plane
strain