Insulating 6,6-Phenyl-C61-butyric Acid Methyl Ester on Transition-Metal Dichalcogenides: Impact of the Hybrid Materials on the Optical and Electrical Properties.

In this study we develop a strategy to insulate 6,6 -Phenyl C61 butyric acid methyl ester (PCBM) on the basal plane of transition metal dichalcogenides (TMDs). Concretely single layers of MoS2, MoSe2, MoTe2, WS2, WSe2 and WTe2 and ultrathin MoO2 and WO2 were grown via chemical vapor deposition (CVD). Then, the thiol group of a PCBM modified with cysteine reacts with the chalcogen vacancies on the basal plane of TMDs, yielding PCBM-MoS2, PCBM-MoSe2, PCBM-WS2, PCBM-WSe2, PCBM-WTe2, PCBM-MoO2 and PCBM-WO2. Afterwards, all the hybrid materials were characterized using several techniques, including XPS, Raman spectroscopy, TEM, AFM, and cyclic voltammetry. Furthermore, PCBM causes a unique optical and electrical impact in every TMDs. For MoS2 devices, the conductivity and photoluminescence (PL) emission achieve a remarkable enhancement of 1700 % and 200 % in PCBM-MoS2 hybrids. Similarly, PCBM-MoTe2 hybrids exhibit a 2-fold enhancement in PL emission at 1.1 eV. On the other hand, PCBM-MoSe2, PCBM-WSe2 and PCBM-WS2 hybrids exhibited a new interlayer exciton at 1.29-1.44, 1.7 and 1.37-154 eV along with an enhancement of the photo-response by 2400, 3200 and 600 %, respectively. Additionally, PCBM-WTe2 and PCBM-WO2 showed a modest photo-response, in sharp contrast with pristine WTe2 or WO2 which archive pure metallic character.

Unveiling the Photoinduced Electron-Donating Character of MoS2 in Covalently Linked Hybrids Featuring Perylenediimide.

The covalent functionalization of MoS2 with a perylenediimide (PDI) is reported and the study is accompanied by detailed characterization of the newly prepared MoS2 -PDI hybrid material. Covalently functionalized MoS2 interfacing organic photoactive species has shown electron and/or energy accepting, energy reflecting or bi-directional electron accepting features. Herein, a rationally designed PDI, unsubstituted at the perylene core to act as electron acceptor, forces MoS2 to fully demonstrate for the first time its electron donor capabilities. The photophysical response of MoS2 -PDI is visualized in an energy-level diagram, while femtosecond transient absorption studies disclose the formation of MoS2 .+ -PDI.- charge separated state. The tunable electronic properties of MoS2 , as a result of covalently linking photoactive organic species with precise characteristics, unlock their potentiality and enable their application in light-harvesting and optoelectronic devices.

Observation of Drastic Electronic-Structure Change in a One-Dimensional Moiré Superlattice.

We report the first experimental observation of a strong-coupling effect in a one-dimensional moiré superlattice. We study one-dimensional double-wall carbon nanotubes (DWCNTs) in which van der Waals-coupled two single nanotubes form a one-dimensional moiré superlattice. We experimentally combine Rayleigh scattering spectroscopy and electron beam diffraction on the same individual DWCNTs to probe the optical transitions of the structure-identified DWCNTs in the visible spectral range. Among more than 30 structure-identified DWCNTs examined, we experimentally observed and identified a drastic change of the optical transition spectrum in a DWCNT with chirality (12,11)@(17,16). The origin of the marked change is attributed to the strong intertube coupling effect in the moiré superlattice formed by two nearly armchair nanotubes. Our numerical simulation is consistent with the experimental findings.

Stabilization of metallic phases through formation of metallic/semiconducting lateral heterostructures.

In this study, we develop a new approach for stabilization of metallic phases of monolayer MoS2 through the formation of lateral heterostructures composed of semiconducting/metallic MoS2. The structure of metallic (a mixture of T and T') and semiconducting (2H) phases was unambiguously characterized by Raman spectroscopy, x-ray photoelectron spectroscopy, photoluminescence imaging, and transmission electron microscope observations. The amount of NaCl, reaction temperature, reaction time, and locations of substrates are essential for controlling the percentage of metallic/semiconducting phases in lateral heterostructures; loading a large amount of NaCl at low temperatures with short reaction times prefers metallic phases. The existence of the semiconducting phase in MoS2 lateral heterostructures significantly enhances the stability of the metallic phases through passivation of reactive edges. The same approach can be applied to other transition metal dichalcogenides (TMDs), such as WS2, leading to boosting of basic research and application of TMDs in metallic phases.

Systematic Study of Photoluminescence Enhancement in Monolayer Molybdenum Disulfide by Acid Treatment.

Monolayer molybdenum disulfide (MoS2) is an atomically thin semiconducting material with a direct band gap. This physical property is attributable to atomically thin optical devices such as sensors, light-emitting devices, and photovoltaic cells. Recently, a near-unity photoluminescence (PL) quantum yield of a monolayer MoS2 was demonstrated via a treatment with a molecular acid, bis(trifluoromethane)sulfonimide (TFSI); however, the mechanism still remains a mystery. Here, we work on PL enhancement of monolayer MoS2 by treatment of Brønsted acids (TFSI and sulfuric acid (H2SO4)) to identify the importance of the protonated environment. In TFSI as an acid, different solvents-1,2-dichloroethane (DCE), acetonitrile, and water-were studied, as they show quite different acidity in solution. All of the solvents showed PL enhancement, and the highest was observed in DCE. This behavior in DCE would be due to the higher acidity than others have. Acids from different anions can also be studied in water as a common solvent. Both TFSI and H2SO4 showed similar PL enhancement (∼4-8 enhancement) at the same proton concentration, indicating that the proton is a key factor to enhance the PL intensity. Finally, we considered another cation, Li+ from Li2SO4, instead of H2SO4, in water. Although Li and H atoms showed similar binding energy on MoS2 from theoretical calculations, Li2SO4 treatment showed little PL enhancement; only coexisting H2SO4 reproduced the enhancement. This study demonstrated the importance of a protonated environment to increase the PL intensity of monolayer MoS2. The study will lead to a solution to achieve high optical quality and to implementation for atomically thin optical devices.

Construction of Covalent Organic Nanotubes by Light-Induced Cross-Linking of Diacetylene-Based Helical Polymers.

Organic nanotubes (ONTs) are tubular nanostructures composed of small molecules or macromolecules that have found various applications including ion sensor/channels, gas absorption, and photovoltaics. While most ONTs are constructed by self-assembly processes based on weak noncovalent interactions, this unique property gives rise to the inherent instability of their tubular structures. Herein, we report a simple "helix-to-tube" strategy to construct robust, covalent ONTs from easily accessible poly(m-phenylene diethynylene)s (poly-PDEs) possessing chiral amide side chains that can adopt a helical conformation through hydrogen-bonding interactions. The helically folded poly-PDEs subsequently undergo light-induced cross-linking at longitudinally aligned 1,3-butadiyne moieties across the whole helix to form covalent tubes (ONTs) both in solution and solid phases. The structures of poly-PDEs and covalent ONTs were characterized by spectroscopic analyses, diffraction analysis, and microscopic analyses. We envisage that this simple yet powerful "helix-to-tube" strategy will generate a range of ONT-based materials by introducing functional moieties into a monomer.

Isolation and Structure Determination of a Missing Endohedral Fullerene La@C70 through In†Situ Trifluoromethylation.

D5h-symmetric fullerene C70 (D5h-C70) is one of the most abundant members of the fullerene family. One longstanding mystery in the field of fullerene chemistry is whether D5h-C70 is capable of accommodating a rare-earth metal atom to form an endohedral metallofullerene M@D5h-C70, which would be expected to show novel electronic properties. The molecular structure of La@C70 remains unresolved since its discovery three decades ago because of its extremely high instability under ambient conditions and insolubility in organic solvents. Herein, we report the single-crystal X-ray structure of La@C70(CF3)3, which was obtained through in situ exohedral functionalization by means of trifluoromethylation. The X-ray crystallographic study reveals that La@C70(CF3)3 is the first example of an endohedral rare-earth fullerene based on D5h-C70. The dramatically enhanced stability of La@C70(CF3)3 compared to La@C70 can be ascribed to trifluoromethylation-induced bandgap enlargement.

Core-level spectroscopy to probe the oxidation state of single europium atoms.

The valence of individual europium atoms confined in carbon nanotubes is successfully measured by using core-level electron energy loss spectroscopy. Changes in the oxidation state at the atomic scale have been observed in Eu atomic chains exposed to oxygen. A transitory behavior has been identified where multiple atoms show a signal consistent with a sum of Eu^{2+} and Eu^{3+}. This indicates that single atoms change their valence state multiple times during the reaction, suggesting that oxidation in confined spaces and with extra energy input (from the electron beam) might not be a simple one step electron transfer event.

Minimal inflammogenicity of pristine single-wall carbon nanotubes.

Carbon nanotubes (CNTs) are a novel synthetic material comprising only carbon atoms. Based on its rigidity, its electrical and heat conductivity and its applicability to surface manufacturing, this material is expected to have numerous applications in industry. However, due to the material's dimensional similarity to asbestos fibers, its carcinogenicity was hypothesized during the last decade, and indeed, we have shown that multi-wall CNTs (MWCNTs) of 50 nm in diameter are potently carcinogenic to mesothelial cells after intraperitoneal injection. Additionally, we suggested that inflammogenicity after intraperitoneal injection can predict mesothelial carcinogenesis. However, few data have been published on the intraperitoneal inflammogenicity of single-wall CNTs (SWCNTs). Here, we conducted a series of studies on SWCNTs using both intraperitoneal injection into rats and MeT5A mesothelial cells. Intraperitoneal injection of 10 mg SWCNTs caused no remarkable inflammation in the abdominal cavity, and the exposure of MeT5A cells to up to 25 µg/cm(2) SWCNTs did not alter proliferation. MWCNTs of 50 nm in diameter were used as a positive control, and tangled MWCNTs of 15 nm in diameter were used as a negative control. The results suggest that SWCNTs are a low-risk material with respect to mesothelial carcinogenesis.

Template Synthesis of Linear-Chain Nanodiamonds Inside Carbon Nanotubes from Bridgehead-Halogenated Diamantane Precursors.

A simple method for the synthesis of linear-chain diamond-like nanomaterials, so-called diamantane polymers, is described. This synthetic approach is primarily based on a template reaction of dihalogen-substituted diamantane precursors in the hollow cavities of carbon nanotubes. Under high vacuum and in the presence of Fe nanocatalyst particles, the dehalogenated radical intermediates spontaneously form linear polymer chains within the carbon nanotubes. Transmission electron microscopy reveals the formation of well-aligned linear polymers. We expect that the present template-based approach will enable the synthesis of a diverse range of linear-chain polymers by choosing various precursor molecules. The present technique may offer a new strategy for the design and synthesis of one-dimensional nanomaterials.

Size-selective complexation and extraction of endohedral metallofullerenes with cycloparaphenylene.

A new strategy for the non-chromatographic extraction of metallofullerenes from solutions of arc-processed raw soot is based on the size-selective complexation with cycloparaphenylene (CPP). [11]CPP has a high affinity for Mx @C82 (x=1, 2); for example, Gd@C82 can be selectively extracted from a fullerene mixture by the addition of [11]CPP. This approach should open new opportunities in metallofullerene chemistry, including for the bulk extraction of metallofullerenes.

Synthesis and Characterization of Transition Metal Dichalcogenide Nanoribbons Based on a Controllable O2 Etching.

Although the synthesis of monolayer transition metal dichalcogenides has been established in the last decade, synthesizing nanoribbons remains challenging. In this study, we have developed a straightforward method to obtain nanoribbons with controllable widths (25-8000 nm) and lengths (1-50 µm) by O2 etching of the metallic phase in metallic/semiconducting in-plane heterostructures of monolayer MoS2. We also successfully applied this process for synthesizing WS2, MoSe2, and WSe2 nanoribbons. Furthermore, field-effect transistors of the nanoribbons show an on/off ratio of larger than 1000, photoresponses of 1000%, and time responses of 5 s. The nanoribbons were compared with monolayer MoS2, highlighting a substantial difference in the photoluminescence emission and photoresponses. Additionally, the nanoribbons were used as a template to build one-dimensional (1D)-1D or 1D-2D heterostructures with various transition metal dichalcogenides. The process developed in this study offers simple production of nanoribbons with applications in several fields of nanotechnology and chemistry.

Intermolecular Interaction between Single-Walled Carbon Nanotubes and Encapsulated Molecules Studied by Polarization Resonance Raman Microscopy.

In the present study, we investigated the intermolecular interactions between single-walled carbon nanotubes (SWCNTs) and encapsulated molecules by polarization resonance Raman microscopy. C70 encapsulated in SWCNTs is investigated under incident laser polarization parallel and perpendicular to the tube axis. We employed two excitation laser wavelengths 442 and 532 nm, which are in resonance with different electronic states of C70. Under 532 nm excitation, no distinct polarization dependence is found in the Raman spectral pattern, while under 442 nm excitation, a peak not previously seen for this excitation wavelength was clearly observed for parallel excitation. This result can be explained by the modulation of the resonance Raman process via a charge transfer contribution between C70 and the SWCNTs, which is sensitive to the incident polarization as well as the excitation wavelength. The intensity of the local electronic field inside a SWCNT is higher for the parallel excitation than the perpendicular excitation when the nanotubes are in a bundle. The results can be explained by field localization effects at the nanotube walls, qualitatively supported by finite-difference time-domain simulations.

Field-effect transistor antigen/antibody-TMDs sensors for the detection of COVID-19 samples.

We fabricated sensors by modifying the surface of MoS2 and WS2 with COVID-19 antibodies and investigated their characteristics, including stability, reusability, sensitivity, and selectivity. Thiols and disulfanes in antibodies strongly interact with vacant Mo or W sites of MoS2 or WS2, yielding durable devices that are stable for several days in the air or water. More importantly, detachment of the antibodies is suppressed even during the aggressive cleaning process of the devices at pH 3, which allows reusing the same device in several experiments without appreciable loss of sensitivity. Therefore, the nanodevice may be employed in samples of different patients. Further, we found a limit of detection (LOD) of 1 fg ml-1 at room temperature, time responses of 1 second, and selectivity against interferences such as KLH protein or Albumin.

Observation of the photovoltaic effect in a van der Waals heterostructure.

van der Waals (vdW) heterostructures, which can be assembled with various two-dimensional materials, provide a versatile platform for exploring emergent phenomena. Here, we report an observation of the photovoltaic effect in a WS2/MoS2 vdW heterostructure. Light excitation of WS2/MoS2 at a wavelength of 633 nm yields a photocurrent without applying bias voltages, and the excitation power dependence of the photocurrent shows characteristic crossover from a linear to square root dependence. Photocurrent mapping has clearly shown that the observed photovoltaic effect arises from the WS2/MoS2 region, not from Schottky junctions at electrode contacts. Kelvin probe microscopy observations show no slope in the electrostatic potential, excluding the possibility that the photocurrent originates from an unintentionally formed built-in potential.

Topological difference in 2D layers steers the formation of rigid and flexible 3D supramolecular isomers: impact on the adsorption properties.

Starting with the same precursors, pyridine-2,3-dicarboxylate (pyrdc) and 4,4'-bipyridyl (bipy), two 3D porous coordination polymers, {[Cu(bipy)(0.5)(pyrdc)]·3H(2)O} (1) and {[Cu(bipy)(0.5)(pyrdc)]·0.5bipy·3H(2)O} (2), have been synthesized by changing the solvent system from MeOH/H(2)O to EtOH/H(2)O. Single-crystal structure analysis revealed that 1 and 2 are supramolecular isomers with 3D pillared-layer structures having 1D channel systems. Isomer 1 has a flexible structure and shows gated adsorption behavior, while framework 2 has a rigid backbone and exhibits the adsorption properties of typical microporous materials.

Rock-salt-type crystal of thermally contracted C60 with encapsulated lithium cation.

Rock solid: fullerene-encapsulated Li(+) (Li(+)@C(60)) is an alkaline cation owing to the spherical shape and positive charge. Li(+)@C(60) crystallizes as a rock-salt-type crystal in the presence of PF(6)(-). The orientations of C(60) and PF(6)(-) (orange) are perfectly ordered below 370 K, and Li(+) (purple) hops within the cage. At temperatures below 100 K two Li(+) units are localized at two polar positions within each C(60) .

Interlayer Interactions in 1D Van der Waals Moiré Superlattices.

Studying two-dimensional (2D) van der Waals (vdW) moiré superlattices and their interlayer interactions have received surging attention after recent discoveries of many new phases of matter that are highly tunable. Different atomistic registry between layers forming the inner and outer nanotubes can also form one-dimensional (1D) vdW moiré superlattices. In this review, experimental observations and theoretical perspectives related to interlayer interactions in 1D vdW moiré superlattices are summarized. The discussion focuses on double-walled carbon nanotubes (DWNTs), a model 1D vdW moiré system, and the authors highlight the new optical features emerging from the non-trivial strong interlayer coupling effect and the unique physics in 1D DWNTs. Future directions and questions in probing the intriguing physical phenomena in 1D vdW moiré superlattices such as, correlated physics in different 1D moiré systems beyond DWNTs are proposed and discussed.

Highly controlled acetylene accommodation in a metal-organic microporous material.

Metal-organic microporous materials (MOMs) have attracted wide scientific attention owing to their unusual structure and properties, as well as commercial interest due to their potential applications in storage, separation and heterogeneous catalysis. One of the advantages of MOMs compared to other microporous materials, such as activated carbons, is their ability to exhibit a variety of pore surface properties such as hydrophilicity and chirality, as a result of the controlled incorporation of organic functional groups into the pore walls. This capability means that the pore surfaces of MOMs could be designed to adsorb specific molecules; but few design strategies for the adsorption of small molecules have been established so far. Here we report high levels of selective sorption of acetylene molecules as compared to a very similar molecule, carbon dioxide, onto the functionalized surface of a MOM. The acetylene molecules are held at a periodic distance from one another by hydrogen bonding between two non-coordinated oxygen atoms in the nanoscale pore wall of the MOM and the two hydrogen atoms of the acetylene molecule. This permits the stable storage of acetylene at a density 200 times the safe compression limit of free acetylene at room temperature.

An ion-selective crown ether covalently grafted onto chemically exfoliated MoS2 as a biological fluid sensor.

We describe the basal plane functionalization of chemically exfoliated molybdenum disulfide (ce-MoS2) nanosheets with a benzo-15-crown-5 ether (B15C5), promoted by the chemistry of diazonium salts en route to the fabrication and electrochemical assessment of an ion-responsive electrode. The success of the chemical modification of ce-MoS2 nanosheets was investigated by infrared and Raman spectroscopy, and the amount of the incorporated crown ether was estimated by thermogravimetric analysis. Raman spatial mapping at on-resonance excitation allowed us to disclose the structural characteristics of the functionalized B15C5-MoS2 nanosheets and the impact of basal plane functionalization to the stabilization of the 1T phase of ce-MoS2. Morphological investigation of the B15C5-MoS2 hybrid was implemented by atomic force microscopy and high-resolution transmission electron microscopy. Furthermore, fast-Fourier-transform analysis and in situ energy dispersive X-ray spectroscopy revealed the crystal lattice of the modified nanosheets and the presence of crown-ether addends, respectively. Finally, B15C5-MoS2 electrodes were constructed and evaluated as ion-selective electrodes for sodium ions in aqueous solution and an artificial sweat matrix.