Lithium-Induced Reorientation of Few-Layer MoS2 Films.

Molybdenum disulfide (MoS2) few-layer films have gained considerable attention for their possible applications in electronics and optics and also as a promising material for energy conversion and storage. Intercalating alkali metals, such as lithium, offers the opportunity to engineer the electronic properties of MoS2. However, the influence of lithium on the growth of MoS2 layers has not been fully explored. Here, we have studied how lithium affects the structural and optical properties of the MoS2 few-layer films prepared using a new method based on one-zone sulfurization with Li2S as a source of lithium. This method enables incorporation of Li into octahedral and tetrahedral sites of the already prepared MoS2 films or during MoS2 formation. Our results discover an important effect of lithium promoting the epitaxial growth and horizontal alignment of the films. Moreover, we have observed a vertical-to-horizontal reorientation in vertically aligned MoS2 films upon lithiation. The measurements show long-term stability and preserved chemical composition of the horizontally aligned Li-doped MoS2.

Selective Tumor Hypoxia Targeting Using M75 Antibody Conjugated Photothermally Active MoOx Nanoparticles.

Photothermal therapy (PTT) mediated at the nanoscale has a unique advantage over currently used cancer treatments, by being spatially highly specific and minimally invasive. Although PTT combats traditional tumor treatment approaches, its clinical implementation has not yet been successful. The reasons for its disadvantage include an insufficient treatment efficiency or low tumor accumulation. Here, we present a promising new PTT platform combining a recently emerged two-dimensional (2D) inorganic nanomaterial, MoOx, and a tumor hypoxia targeting element, the monoclonal antibody M75. M75 specifically binds to carbonic anhydrase IX (CAIX), a hypoxia marker associated with many solid tumors with a poor prognosis. The as-prepared nanoconjugates showed highly specific binding to cancer cells expressing CAIX while being able to produce significant photothermal yield after irradiation with near-IR wavelengths. Small aminophosphonic acid linkers were recognized to be more effective over the combination of poly(ethylene glycol) chain and biotin-avidin-biotin bridge in constructing a PTT platform with high tumor-binding efficacy. The in vitro cellular uptake of nanoconjugates was visualized by high-resolution fluorescence microscopy and label-free live cell confocal Raman microscopy. The key to effective cancer treatment may be the synergistic employment of active targeting and noninvasive, tumor-selective therapeutic approaches, such as nanoscale-mediated PTT. The use of active targeting can streamline nanoparticle delivery increasing photothermal yield and therapeutic success.

Large-Area MoS2 Films Grown on Sapphire and GaN Substrates by Pulsed Laser Deposition.

In this paper, we present the preparation of few-layer MoS2 films on single-crystal sapphire, as well as on heteroepitaxial GaN templates on sapphire substrates, using the pulsed laser deposition (PLD) technique. Detailed structural and chemical characterization of the films were performed using Raman spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction measurements, and high-resolution transmission electron microscopy. According to X-ray diffraction studies, the films exhibit epitaxial growth, indicating a good in-plane alignment. Furthermore, the films demonstrate uniform thickness on large areas, as confirmed by Raman spectroscopy. The lateral electrical current transport of the MoS2 grown on sapphire was investigated by temperature (T)-dependent sheet resistance and Hall effect measurements, showing a high n-type doping of the semiconducting films (ns from ~1 × 1013 to ~3.4 × 1013 cm-2 from T = 300 K to 500 K), with a donor ionization energy of Ei = 93 ± 8 meV and a mobility decreasing with T. Finally, the vertical current injection across the MoS2/GaN heterojunction was investigated by means of conductive atomic force microscopy, showing the rectifying behavior of the I-V characteristics with a Schottky barrier height of ϕB ≈ 0.36 eV. The obtained results pave the way for the scalable application of PLD-grown MoS2 on GaN in electronics/optoelectronics.

Nanofriction Properties of Mono- and Double-Layer Ti3C2Tx MXenes.

Unique structure and ability to control the surface termination groups of MXenes make these materials extremely promising for solid lubrication applications. Due to the challenging delamination process, the tribological properties of two-dimensional MXenes particles have been mostly investigated as additive components in the solvents working in the macrosystem, while the understanding of the nanotribological properties of mono- and few-layer MXenes is still limited. Here, we investigate the nanotribological properties of mono- and double-layer Ti3C2Tx MXenes deposited by the Langmuir-Schaefer technique on SiO2/Si substrates. The friction of all of the samples demonstrated superior lubrication properties with respect to SiO2 substrate, while the friction force of the monolayers was found to be slightly higher compared to double- and three-layer flakes, which demonstrated similar friction. The coefficient of friction was estimated to be 0.087 ± 0.002 and 0.082 ± 0.003 for mono- and double-layer flakes, respectively. The viscous regime was suggested as the dominant friction mechanism at high scanning velocities, while the meniscus forces affected by contamination of the MXenes surface were proposed to control the friction at low sliding velocities.

Optical Characterization of Few-Layer PtSe2 Nanosheet Films.

Thin films of transition-metal dichalcogenides are potential materials for optoelectronic applications. However, the application of these materials in practice requires knowledge of their fundamental optical properties. Many existing methods determine optical constants using predefined models. Here, a different approach was used. We determine the sheet conductance and absorption coefficient of few-layer PtSe2 in the infrared and UV-vis ranges without recourse to any particular model for the optical constants. PtSe2 samples with a thickness of about 3-4 layers were prepared by selenization of 0.5 nm thick platinum films on sapphire substrates at different temperatures. Differential reflectance was extracted from transmittance and reflectance measurements from the front and back of the sample. The film thickness, limited to a few atomic layers, allowed a thin-film approximation to calculate the optical conductance and absorption coefficient. The former has a very different energy dependence in the infrared, near-infrared, and visible ranges. The absorption coefficient exhibits a strong power-law dependence on energy with an exponent larger than three in the mid-infrared and near-infrared regions. We have not observed any evidence for a band gap in PtSe2 thin layers down to an energy of 0.4 eV from our optical measurements.