No abstract

The structural characteristics of biological specimens, such as wet proteins and fixed living cells, can be conveniently probed in their host aqueous media using soft X‐rays in the water window region (200–600 eV). Conventional X‐ray detectors in this area exhibit low spatial resolution, have limited sensitivity, and require complex fabrication procedures. Here, many of these limitations are overcome by introducing a direct soft X‐ray detector based on ultrathin tin mono‐sulfide (SnS) nanosheets. The distinguishing characteristic of SnS is its high photon absorption efficiency in the soft X‐ray region. This factor enables the fabricated soft X‐ray detectors to exhibit excellent sensitivity values on the order of 104 μCGyVac−1 cm−2 at peak energies of ≈600 eV. The peak signal is found to be sensitive to the number of stacked SnS layers, with thicker SnS nanosheet assemblies yielding a peak response at higher energies and with peak sensitives of over 2.5 × 104 μCGyVac−1 cm−2 at 1 V. Detailed current–voltage and temporal characteristics of these detectors are also presented. These results showcase the excellent performance of SnS nanosheet‐based soft X‐ray detectors compared to existing direct soft X‐ray detectors, including that of the emerging organic–inorganic perovskite class of materials.

Tungsten disulfide (WS 2 ) exhibits intriguing tribological properties and has been explored as an excellent lubricious material in thin-film and solid lubricants. However, the poor dispersibility of WS 2 has been a major challenge for its utilization in liquid lubricant applications. Herein, a top-down integrated approach is presented to synthesize oxygenated WS 2 (WS 2 −O) nanosheets via strong acid-mediated oxidation and ultrasound-assisted exfoliation. The ultrathin sheets of WS 2 −O, comprising 4−7 molecular lamellae, exhibit oxygen/hydroxyl functionalities. The organosilanes having variable surface-active leaving groups (chloro and ethoxy) are covalently grafted, targeting the hydroxyl/oxygen functionalities on the surface of WS 2 −O nanosheets. The grafting of organosilanes is governed by the reactivity of chloro and ethoxy leaving groups. The DFT calculations further support the covalent interaction between the WS 2 −O nanosheets and organosilanes. The alkyl chain-functionalized WS 2 −O nanosheets displayed excellent dispersibility in mineral lube base oil. A minute dose of chemically functionalized-WS 2 (0.2 mg.mL −1 ) notably enhanced the tribological properties of mineral lube oil by reducing the friction coefficient (52%) and wear volume (79%) for a steel tribopair. Raman analysis of worn surfaces revealed WS 2 -derived lubricious thin film formation. The improved tribological properties are attributed to ultralow thickness, stable dispersion, and low shear strength of chemically functionalized WS 2 nanosheets, along with protective thin film formation over the contact interfaces of a steel tribopair. The present work opens a new avenue toward exploiting low-dimensional nanosheets for minimizing energy losses due to high friction.

Nanoelectronic devices that are self-powered through the conversion of mechanical energy into electronic energy are of great interest for the fields of wearables and medical implants. The ability to find nanomaterials that have piezoelectric properties suitable for nanogenerators can be a challenge. Zinc oxide (ZnO) is a well-known material that has one of the highest piezoelectric tensors among tetrahedrally bonded semiconductors, due to its buckled noncentrosymmetric structure on the (0001) and (0001̅)] surfaces. However, this piezoelectric response is diminished in few-layered two-dimensional (2D) ZnO, as it loses it buckled structure and becomes graphitic. As a consequence, ultrathin 2D ZnO sheets are not suitable for nanoscale devices that rely on a piezoelectric response. In this work, we show that defects, specifically, zinc and oxygen vacancies, and adsorbed oxygen, can restore the wurtzite structure and piezoelectric response of free standing 2D ZnO sheets at specific thicknesses. Using density functional theory calculations, we show that both a 1 monolayer (ML) surface coverage of oxygen and a 25% concentration of zinc vacancies can restore the buckling in few-layer 2D ZnO nanosheets. For a 1.48 nm thick sheet with a 25% Zn vacancy concentration, an e 33 value of 1.26 C/m2 can be achieved, which is 120% enhancement compared to the bulk value. These findings provide a methodology for screening piezoelectric properties of other materials and demonstrate that defects can be used to restore the wurtzite structure and dielectric properties of ultrathin 2D ZnO sheets that would allow them to be used in nanoscale piezoelectric devices.