Deciphering the anisotropic energy harvesting responses of an above room temperature molecular ferroelectric copper(ii) complex single crystal

Abstract

The mechanical/piezoelectric and/or thermal/pyroelectric energy harvesting efficiency is observed to be extremely good in multi-component ferroelectric inorganic oxides in their single-crystal form rather than in their polycrystalline counterparts (pellets and thick/thin films). However, growing such multi-component single crystals is a challenging and cost-intensive process besides the difficulty in tuning their long-range ferroic ordering and the involvement of toxic heavy elements. Instead, discrete inorganic metal complexes can be potential alternatives for which one can overcome these caveats by an appropriate design strategy. Herein, we report a biocompatible and an above room temperature (T_c > 380 K) molecular ferroelectric [Cu₂(L-phe)₂(bpy)₂(H₂O)](ClO₄)₂·2H₂O single crystal (1) with profound anisotropic piezo- and pyro-electric responses along different unit cell axes. Energy harvesting data at room temperature reveal that the highest possibility of scavenging mechanical energy (∼30 μW m⁻²) is preferentially along the b-axis. This is attributed to the large spontaneous polarization (P_s = 2.5 μC cm⁻²) and piezoelectric coefficient (d₃₃ = 23.5 pm V⁻¹) observed along the b-axis, compared to those along the other two axes. The highest output voltage (7.4 V cm⁻²) and pyroelectric coefficient (29 μC m⁻² K⁻¹) obtained for the single-crystal device are impressively higher than those of most of the reported materials. Such a molecular anisotropic single-crystal piezo-/pyro-electric nanogenerator (SC-PENG) with excellent mechanical and thermal energy harvesting competence is reported for the first time.