Probing nanoscale functionalities of metal-organic framework nanocrystals

Experimental investigation of functional properties of metal-organic frameworks (MOFs) at nanoscale precision is challenging and rarely reported. In this study, we report the piezo- and ferroelectric properties of NUS-6 MOF nanocrystals using dual AC resonance tracking piezoresponse force microscopy and piezoresponse force spectroscopy for the first time. Both NUS-6-(Hf) and NUS-6-(Zr) nanocrystals displayed anomalous piezoelectricity with the calculated piezoelectric coefficient d_zz constants of 2.0-3.5 pm V^-1 and 1.5-2.5 pm V^-1, respectively. Moreover, NUS-6-(Hf) showed much better polarization-switching behaviors (ferroelectricity) than NUS-6-(Zr), featured by very low coercive biases in the ferroelectric hysteresis (PR) loop. Furthermore, elasticity and thermal stability of the NUS-6 nanocrystals have been presented. The results have opened a realm of probing piezo- and ferro-electric properties as well as mechanical properties of MOF nanocrystals, which are promising materials for applications in integrated microelectromechanical systems (MEMS).