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Strain-dependent electrical and optical properties of atomically thin transition metal dichalcogenides may be useful in sensing applications. However, the question of how strain relaxes in atomically thin materials remains not well understood. Herein, the strain relaxation of triangular WS2 deposited on polydimethylsiloxane substrate is investigated. The photoluminescence of trions (X–) and excitons (X0) undergoes linear redshifts of ≈20 meV when the substrate tensile strain increases from 0 to 0.16. However, when the substrate strain further increases from 0.16 to 0.32, the redshifts cease due to strain relaxation in WS2. The strain relaxation occurs through formation of wrinkles in the WS2 crystal. The pattern of wrinkles is found to be dependent on the relative angle between an edge of the triangular WS2 crystal and tensile strain direction. Finite element simulations of the strain distribution inside the WS2 crystals explain the experimental observations.

A specially designed n-type semiconductor consisting of Ca-doped ZnO (CZO) nanoparticles is used as the electron transport layer (ETL) in high-performance multicolor perovskite light-emitting diodes (PeLEDs) fabricated using an all-solution process. The band structure of the ZnO is tailored via Ca doping to create a cascade of conduction energy levels from the cathode to the perovskite. This energy band alignment significantly enhances conductivity and carrier mobility in the CZO ETL and enables controlled electron injection, giving rise to sub-bandgap turn-on voltages of 1.65 V for red emission, 1.8 V for yellow, and 2.2 V for green. The devices exhibit significantly improved luminance yields and external quantum efficiencies of, respectively, 19 cd A−1 and 5.8% for red emission, 16 cd A−1 and 4.2% for yellow, and 21 cd A−1 and 6.2% for green. The power efficiencies of these multicolor devices demonstrated in this study, 30 lm W−1 for green light-emitting PeLED, 28 lm W−1 for yellow, and 36 lm W−1 for red are the highest to date reported. In addition, the perovskite layers are fabricated using a two-step hot-casting technique that affords highly continuous (>95% coverage) and pinhole-free thin films. By virtue of the efficiency of the ETL and the uniformity of the perovskite film, high brightnesses of 10 100, 4200, and 16,060 cd m−2 are demonstrated for red, yellow, and green PeLEDs, respectively. The strategy of using a tunable ETL in combination with a solution process pushes perovskite-based materials a step closer to practical application in multicolor light-emitting devices.

Graphene is a novel two-dimensional material composed of a one-atom-thick planar sheet of sp2-bonded carbon atoms perfectly arranged in a honeycomb lattice that has exceptional photonic and electronic properties. We believe that the true potential of graphene lies in optical sensors, especially for biochemical sensing in the diagnostics and health care sector. Graphene has extraordinary properties, such as a one-atom thickness, extremely high surface-to-volume ratio, large surface area, ability to quench fluorescence, excellent biocompatibility, broadband light absorption, ultrafast response time, high mechanical strength, outstanding robustness, and flexibility. The working principle is based on whenever the biomolecules come into contact with graphene, the Fermi level shifts to either p-type or n-type, changing the opto-electronic properties. The most important factors in graphene-based optical sensing are lowering the limit of detection and increasing the specificity of label-free biochemical sensing. This article comprehensively and critically reviews emerging graphene optical biochemical sensors. We first elaborate on their opto-electronic properties, fabrication, numerical modeling and simulation, and then review various sensing applications, such as single-cell detection, neural imaging and optogenetics, colorimetric multifunctional sensors, cancer diagnosis, protein and DNA sensing, and gas sensing. Finally, the roadmap of current and future trends in graphene-based optical biochemical sensors is discussed.