Orientational Control of Colloidal 2D-Layered Transition Metal Dichalcogenide Nanodiscs <i>via</i> Unusual Electrokinetic Response

We report an unusual response of colloidal layered transition metal dichalcogenide (TMDC) nanodiscs to the electric field, where the orientational order is created transiently only during the time-varying period of the electric field while no orientational order is created by the DC field. This result is in stark contrast to the typical electrokinetic response of various other colloidal nanoparticles, where the permanent dipole or (and) anisotropic-induced dipole creates a sustaining orientational order under the DC field. This indicates the lack of a sizable permanent dipole or (and) anisotropic-induced dipole in colloidal TMDC nanodiscs, despite their highly anisotropic lattice structure. While the orientational order is created only transiently by the time-varying field, a near-steady-state orientational order can be obtained by using an AC electric field. We demonstrate the utility of this method for the controlled orientation of colloidal nanoparticles that cannot be controlled <i>via</i> the usual interaction of the electric field with the nanoparticle dipole

Transformative Two-Dimensional Layered Nanocrystals

Regioselective chemical reactions and structural transformations of two-dimensional (2D) layered transition-metal chalcogenide (TMC) nanocrystals are described. Upon exposure of 2D TiS2 nanodiscs to a chemical stimulus, such as Cu ion, selective chemical reaction begins to occur at the peripheral edges. This edge reaction is followed by ion diffusion, which is facilitated by interlayer nanochannels and leads to the formation of a heteroepitaxial TiS2–Cu2S intermediate. These processes eventually result in the generation of a single-crystalline, double-convex toroidal Cu2S nanostructure. Such 2D regioselective chemical reactions also take place when other ionic reactants are used. The observations made and chemical principles uncovered in this effort indicate that a general approach exists for building various toroidal nanocrystals of substances such as Ag2S, MnS, and CdS

Synthesis and Magnetic Properties of Gd Doped EuS Nanocrystals with Enhanced Curie Temperatures

EuS nanocrystals (NCs) were doped with Gd resulting in an enhancement of their magnetic properties. New EuS and GdS single source precursors (SSPs) were synthesized, characterized, and employed to synthesize Eu1−xGdxS NCs by decomposition in oleylamine and trioctylphosphine at 290 °C. The doped NCs were characterized using X-ray diffraction, transmission electron microscopy, and scanning transmission electron microscopy, which support the uniform distribution of Gd dopants through electron energy loss spectroscopy (EELS) mapping. X-ray absorption spectroscopy (XAS) revealed the dopant ions in Eu1−xGdxS NCs to be predominantly Gd3+. NCs with a variety of doping ratios of Gd (0 ≤ x x) as quantified with ICP-AES. Enhancement of the Curie temperature (TC) was observed for samples with low Gd concentrations (x ≤ 10%) with a maximum TC of 29.4 K observed for NCs containing 5.3% Gd. Overall, the observed TC, Weiss temperature (θ), and hysteretic behavior correspond directly to the doping level in Eu1−xGdxS NCs and the trends qualitatively follow those previously reported for bulk and thin film samples

Synthesis and Magnetic Properties of Gd Doped EuS Nanocrystals with Enhanced Curie Temperatures

EuS nanocrystals (NCs) were doped with Gd resulting in an enhancement of their magnetic properties. New EuS and GdS single source precursors (SSPs) were synthesized, characterized, and employed to synthesize Eu1−xGdxS NCs by decomposition in oleylamine and trioctylphosphine at 290 °C. The doped NCs were characterized using X-ray diffraction, transmission electron microscopy, and scanning transmission electron microscopy, which support the uniform distribution of Gd dopants through electron energy loss spectroscopy (EELS) mapping. X-ray absorption spectroscopy (XAS) revealed the dopant ions in Eu1−xGdxS NCs to be predominantly Gd3+. NCs with a variety of doping ratios of Gd (0 ≤ x x) as quantified with ICP-AES. Enhancement of the Curie temperature (TC) was observed for samples with low Gd concentrations (x ≤ 10%) with a maximum TC of 29.4 K observed for NCs containing 5.3% Gd. Overall, the observed TC, Weiss temperature (θ), and hysteretic behavior correspond directly to the doping level in Eu1−xGdxS NCs and the trends qualitatively follow those previously reported for bulk and thin film samples

Unveiling Chemical Reactivity and Structural Transformation of Two‑Dimensional 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 (¹O₂) or base (H₂O)) of external stimuli, TiS₂ nanocrystal is respectively transformed to either a TiO₂ nanodisc through a “compositional metathesis” or a TiO₂ toroid through multistage “edge-selective structural transformation” processes. These chemical reactions can serve as the new design concept for functional 2D layered nanostructures. For example, TiS_2(disc)-TiO_2(shell) nanocrystal constitutes a high performance type II heterojunction which not only a wide range solar energy coverage (∼80%) 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 WSe₂ 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 WSe₂ 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 2‑D Layered TiS₂ 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 TiS₂ 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 TiS₂ 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

Facet-Dependence of Electron Storage in Gold-Decorated Titania Nanocrystals

A variety of materials have been developed over the last two decades with the goal of extending the function of light-absorbing devices to low-light or nighttime conditions. Typically, this requires storage of photogenerated charges. The capacity of a material to store charges depends on a range of physicochemical features, including the crystallographic nature of materials’ interfaces, which we investigate here. We implemented a model system consisting of gold nanoparticles (AuNPs) supported on titanium dioxide (TiO2) anatase nanocrystals with predominantly (101), (100), or (001) facets. Cyclic voltammetry in dark, anaerobic conditions showed that all three materials exhibited increased current densities with increasing illumination time, with the highest increase observed for Au/TiO2(001). We further employed photocharged Au/TiO2 particles for catalytic reactions in the dark and found a consistent trend of Au/TiO2(001) being the most active. Using density functional theory, we calculated the Bader charge and the partial density of states, revealing that the presence of additional oxygen atoms at the Au/TiO2 interface leads to charge depletion from Au, providing more accessible vacant states to accept electrons from TiO2, with calculations showing the greatest charge depletion for Au/TiO2(001). Our results suggest that crystal surface engineering can be used as a powerful tool to optimize materials for electron storage

Anisotropic Electron–Phonon Coupling in Colloidal Layered TiS₂ 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 TiS₂ nanodiscs. The transient absorption signal from the diameter- and thickness-controlled TiS₂ 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 MoS₂. 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 TiS₂, 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