A New 2H-2H′/1T Cophase in Polycrystalline MoS₂ and MoSe₂ Thin Films

We report on 2H-2H′/1T phase conversion of MoS₂ and MoSe₂ polycrystalline films grown by thermally assisted conversion. The structural conversion of the transition metal dichalcogenides was successfully carried out by organolithium treatment on chip. As a result we obtained a new 2H-2H′/1T cophase system of the TMDs thin films which was verified by Raman spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The conversion was successfully carried out on selected areas yielding a lateral heterostructure between the pristine 2H phase and the 2H′/1T cophase regions. Scanning electron microscopy and atomic force microscopy revealed changes in the surface morphology and work function of the cophase system in comparison to the pristine films, with a surprisingly sharp lateral interface region

High-Performance Hybrid Electronic Devices from Layered PtSe₂ Films Grown at Low Temperature

Layered two-dimensional (2D) materials display great potential for a range of applications, particularly in electronics. We report the large-scale synthesis of thin films of platinum diselenide (PtSe₂), a thus far scarcely investigated transition metal dichalcogenide. Importantly, the synthesis by thermally assisted conversion is performed at 400 °C, representing a breakthrough for the direct integration of this material with silicon (Si) technology. Besides the thorough characterization of this 2D material, we demonstrate its promise for applications in high-performance gas sensing with extremely short response and recovery times observed due to the 2D nature of the films. Furthermore, we realized vertically stacked heterostructures of PtSe₂ on Si which act as both photodiodes and photovoltaic cells. Thus, this study establishes PtSe₂ as a potential candidate for next-generation sensors and (opto-)­electronic devices, using fabrication protocols compatible with established Si technologies

Direct Observation of Degenerate Two-Photon Absorption and Its Saturation in WS₂ and MoS₂ Monolayer and Few-Layer Films

The optical nonlinearity of WS₂ and MoS₂ monolayer and few-layer films was investigated using the <i>Z</i>-scan technique with femtosecond pulses from the visible to the near-infrared range. The nonlinear absorption of few- and multilayer WS₂ and MoS₂ films and their dependences on excitation wavelength were studied. WS₂ films with 1–3 layers exhibited a giant two-photon absorption (TPA) coefficient as high as (1.0 ± 0.8) × 10⁴ cm/GW. TPA saturation was observed for the WS₂ film with 1–3 layers and for the MoS₂ film with 25–27 layers. The giant nonlinearity of WS₂ and MoS₂ films is attributed to a two-dimensional confinement, a giant exciton effect, and the band edge resonance of TPA

A Commercial Conducting Polymer as Both Binder and Conductive Additive for Silicon Nanoparticle-Based Lithium-Ion Battery Negative Electrodes

This work describes silicon nanoparticle-based lithium-ion battery negative electrodes where multiple nonactive electrode additives (usually carbon black and an inert polymer binder) are replaced with a single conductive binder, in this case, the conducting polymer PEDOT:PSS. While enabling the production of well-mixed slurry-cast electrodes with high silicon content (up to 95 wt %), this combination eliminates the well-known occurrence of capacity losses due to physical separation of the silicon and traditional inorganic conductive additives during repeated lithiation/delithiation processes. Using an <i>in situ</i> secondary doping treatment of the PEDOT:PSS with small quantities of formic acid, electrodes containing 80 wt % SiNPs can be prepared with electrical conductivity as high as 4.2 S/cm. Even at the relatively high areal loading of 1 mg/cm², this system demonstrated a first cycle lithiation capacity of 3685 mA·h/g (based on the SiNP mass) and a first cycle efficiency of ∼78%. After 100 repeated cycles at 1 A/g this electrode was still able to store an impressive 1950 mA·h/g normalized to Si mass (∼75% capacity retention), corresponding to 1542 mA·h/g when the capacity is normalized by the total electrode mass. At the maximum electrode thickness studied (∼1.5 mg/cm²), a high areal capacity of 3 mA·h/cm² was achieved. Importantly, these electrodes are based on commercially available components and are produced by the standard slurry coating methods required for large-scale electrode production. Hence, the results presented here are highly relevant for the realization of commercial LiB negative electrodes that surpass the performance of current graphite-based negative electrode systems

Basal-Plane Functionalization of Chemically Exfoliated Molybdenum Disulfide by Diazonium Salts

Although transition metal dichalcogenides such as MoS₂ have been recognized as highly potent two-dimensional nanomaterials, general methods to chemically functionalize them are scarce. Herein, we demonstrate a functionalization route that results in organic groups bonded to the MoS₂ surface <i>via</i> covalent C–S bonds. This is based on lithium intercalation, chemical exfoliation and subsequent quenching of the negative charges residing on the MoS₂ by electrophiles such as diazonium salts. Typical degrees of functionalization are 10–20 atom % and are potentially tunable by the choice of intercalation conditions. Significantly, no further defects are introduced, and annealing at 350 °C restores the pristine 2H-MoS₂. We show that, unlike both chemically exfoliated and pristine MoS₂, the functionalized MoS₂ is very well dispersible in anisole, confirming a significant modification of the surface properties by functionalization. DFT calculations show that the grafting of the functional group to the sulfur atoms of (charged) MoS₂ is energetically favorable and that S–C bonds are formed