Well-Defined Colloidal 2‑D Layered Transition-Metal Chalcogenide Nanocrystals via Generalized Synthetic Protocols

While interesting and unprecedented material characteristics of two dimensionality (2-D) layered nanomaterials are emerging, their reliable synthetic methodologies are not well developed. In this study we demonstrate general applicability of synthetic protocols to a wide range of colloidal 2-D layered transition-metal chalcogenide (TMC) nanocrystals. As distinctly different from other nanocrystals, we discovered that 2-D layered TMC nanocrystals are unstable in the presence of reactive radicals from elemental chalcogen during the crystal formation. We first introduce the synthesis of titanium sulfide and selenide where well-defined single crystallinity and lateral size controllability are verified, and then such synthetic protocols are extended to all of group IV and V transition-metal sulfide (TiS₂, ZrS₂, HfS₂, VS₂, NbS₂, and TaS₂) and selenide (TiSe₂, ZrSe₃, HfSe₃, VSe₂, NbSe₂, and TaSe₂) nanocrystals. The use of appropriate chalcogen source is found to be critical for the successful synthesis of 2-D layered TMC nanocrystals. CS₂ is an efficient chalcogen precursor for metal sulfide nanocrystals, whereas elemental Se is appropriate for metal selenide nanocrystals. We briefly discuss the effects of reactive radical characteristics of elemental S and Se on the formation of 2-D layered TMC nanocrystals

Chemical Synthetic Strategy for Single-Layer Transition-Metal Chalcogenides

A solution-phase synthetic protocol to form two-dimensional (2D) single-layer transition-metal chalcogenides (TMCs) has long been sought; however, such efforts have been plagued with the spontaneous formation of multilayer sheets. In this study, we discovered a solution-phase synthetic protocol, called “diluted chalcogen continuous influx (DCCI)”, where controlling the chalco­gen source influx (e.g., H₂S) during its reaction with the transition-metal halide precursor is the critical parameter for the formation of single-layer sheets as examined for the cases of group IV TMCs. The continuous influx of dilute H₂S throughout the entire growth period is necessary for large sheet formation through the exclusive <i>a-</i> and <i>b-</i>axial growth processes. By contrast, the burst influx of highly concentrated H₂S in the early stages of the growth process forms multilayer TMC nanodiscs. Our DCCI protocol is a new synthetic concept for single-layer TMCs and, in principle, can be operative for wide range of TMC nanosheets

Magnetic Tandem Apoptosis for Overcoming Multidrug-Resistant Cancer

Multidrug resistance (MDR) is a leading cause of failure in current chemotherapy treatment and constitutes a formidable challenge in therapeutics. Here, we demonstrate that a nanoscale magnetic tandem apoptosis trigger (m-TAT), which consists of a magnetic nanoparticle and chemodrug (e.g., doxorubicin), can completely remove MDR cancer cells in both in vitro and in vivo systems. m-TAT simultaneously activates extrinsic and intrinsic apoptosis signals in a synergistic fashion and downregulates the drug efflux pump (e.g., P-glycoprotein) which is one of the main causes of MDR. The tandem apoptosis strategy uses low level of chemodrug (in the nanomolar (nM) range) to eliminate MDR cancer cells. We further demonstrate that apoptosis of MDR cancer cells can be achieved in a spatially selective manner with single-cell level precision. Our study indicates that nanoscale tandem activation of convergent signaling pathways is a new platform concept to overcome MDR with high efficacy and specificity

Magnetic Nanoparticles for Ultrafast Mechanical Control of Inner Ear Hair Cells

We introduce cubic magnetic nanoparticles as an effective tool for precise and ultrafast control of mechanosensitive cells. The temporal resolution of our system is ∼1000 times faster than previously used magnetic switches and is comparable to the current state-of-the-art optogenetic tools. The use of a magnetism-gated switch reported here can address the key challenges of studying mechanotransduction in biological systems. The cube-shaped magnetic nanoparticles are designed to bind to components of cellular membranes and can be controlled with an electromagnet to exert pico-Newtons of mechanical force on the cells. The cubic nanoparticles can thus be used for noncontact mechanical control of the position of the stereocilia of an inner ear hair cell, yielding displacements of tens of nanometers, with sub-millisecond temporal resolution. We also prove that such mechanical stimulus leads to the influx of ions into the hair cell. Our study demonstrates that a magnetic switch can yield ultrafast temporal resolution, and has capabilities for remote manipulation and biological specificity, and that such magnetic system can be used for the study of mechanotransduction processes of a wide range of sensory systems

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

Effects of Direct Solvent-Quantum Dot Interaction on the Optical Properties of Colloidal Monolayer WS₂ Quantum Dots

Because of the absence of native dangling bonds on the surface of the layered transition metal dichalcogenides (TMDCs), the surface of colloidal quantum dots (QDs) of TMDCs is exposed directly to the solvent environment. Therefore, the optical and electronic properties of TMDCS QDs are expected to have stronger influence from the solvent than usual surface-passivated QDs due to more direct solvent-QD interaction. Study of such solvent effect has been difficult in colloidal QDs of TMDC due to the large spectroscopic heterogeneity resulting from the heterogeneity of the lateral size or (and) thickness in ensemble. Here, we developed a new synthesis procedure producing the highly uniform colloidal monolayer WS₂ QDs exhibiting well-defined photoluminescence (PL) spectrum free from ensemble heterogeneity. Using these newly synthesized monolayer WS₂ QDs, we observed the strong influence of the aromatic solvents on the PL energy and intensity of monolayer WS₂ QD beyond the simple dielectric screening effect, which is considered to result from the direct electronic interaction between the valence band of the QDs and molecular orbital of the solvent. We also observed the large effect of stacking/separation equilibrium on the PL spectrum dictated by the balance between inter QD and QD-solvent interactions. The new capability to probe the effect of the solvent molecules on the optical properties of colloidal TMDC QDs will be valuable for their applications in various liquid surrounding environments

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 Synthesis of Single-Layer MSe₂ (M = Mo, W) Nanosheets via Anisotropic Solution-Phase Growth Approach

The generation of single-layer 2-dimensional (2D) nanosheets has been challenging, especially in solution-phase, since it requires highly anisotropic growth processes that exclusively promote planar directionality during nanocrystal formation. In this study, we discovered that such selective growth pathways can be achieved by modulating the binding affinities of coordinating capping ligands to the edge facets of 2D layered transition-metal chalcogenides (TMCs). Upon changing the functional groups of the capping ligands from carboxylic acid to alcohol and amine with accordingly modulated binding affinities to the edges, the number of layers of nanosheets is controlled. Single-layer MSe₂ (M = Mo, W) TMC nanosheets are obtained with the use of oleic acid, while multilayer nanosheets are formed with relatively strong binding ligands such as oleyl alcohol and oleylamine. With the choice of appropriate capping ligands in the 2D anisotropic growth regime, our solution-based synthetic method can serve a new guideline for obtaining single-layer TMC nanosheets

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

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