Single and twinned plates of 2D layered BiI3 for use as nanoscale pressure sensors

Hexagonal plates of layered bismuth tri-iodide (BiI3) at the micro-to-nanometer scale were fabricated at room temperature via a simple solution phase synthesis without any stabilizer. Tuning the reaction conditions yielded single-nanoplate, twinned-nanoplate and dumbbell-shaped morphologies of BiI3. The top surface, examined by scanning electron microscopy, possesses a hexagonal shape, while transmission electron microscopy indicates that they form into shapes such as dumbbells. These structures are highly crystalline with strong c-axis orientation. This was confirmed by X-ray diffraction, selected area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM) studies. The nanoplates exhibit optical band gaps of between 1.7 and 1.78 eV. The transport properties investigated by conductive atomic force microscopy show that the conductivity is either Schottky type or Poole–Frenkel defect related, depending on whether they are single or twinned nanoplates. To understand the carrier transport under applied pressure, we carried out conducting AFM studies on these nanoplates, which revealed that these BiI3 plates are piezoelectric and can be used as a nanoscale pressure sensor. The highest pressure sensitivities of the single and twinned nanoplates were found to be 4.47 meV MPa−1 and 4.53 meV MPa−1, respectively.