Unusual nanoscale piezoelectricity-driven high current generation from a self S-defect-neutralised few-layered MoS₂...

We report the fabrication of a high-performance flexible piezoelectric nanogenerator based on S-defect-neutralised few-layered molybdenum disulphide (MoS₂) nanosheets. High-resolution transmission electron microscopy (HR-TEM) and Raman spectroscopy confirmed the number of stacked layers in the MoS₂ sheets to be 3-5. The defect, electronic and chemical states of the as-grown MoS₂ nanosheets were investigated by X-ray photoelectron spectroscopy (XPS). An as-fabricated MoS₂ nanogenerator with a CNT electrode generates an excellent high output voltage of 22 V and a record-high output current density of 9.00 μA cm^-2 under a small vertical compressive force of 1.5 kgf. The piezoelectric charge coefficient of the 2D MoS₂ nanosheets was investigated using piezoelectric force microscopy (PFM), and a very high piezoelectric charge coefficient (d₃₃) of 120 pm V^-1 was obtained. The energy conversion efficiency of the device was about 30%. Moreover, the MoS₂ nanosheets show a high dielectric constant of about 2649 at low frequency. The results suggest that the absence of S-defects can reduce the free charge carrier and screening effect, resulting in a high output voltage and current density. The performance of the nanogenerator is discussed in terms of its high d₃₃, high dielectric constant, the crystalline mixed phase of MoS₂ and the electronic state of the MoS₂ nanosheets.