In recent years,with the growing concerns on environmental protection and human health,new materials,such as lead-free piezoelectric materials,have received increasing attention.So far,three types of lead-free piezoelectric systems have been widely researched,i.e.,perovskites,bismuth layer-structured ferroelectrics,and tungsten-bronze type ferroelectrics.This article presents a new type of environmental friendly piezoelectric material with simple structure,the transition-metal(TM)-doped ZnO.Through substituting Zn2+ site with small size ion,we obtained a series of TM-doped ZnO with giant piezoresponse,such as Zn0.975V0.025O of 170 pC/N,Zn0.94Cr0.06O of 120 pC/N,Zn0.913Mn0.087O of 86 pC/N and Zn0.988Fe0.012O of 127 pC/N.The tremendous piezoresponses are ascribed to the introduction of switchable spontaneous polarization and high permittivity in TM-doped ZnO.The microscopic origin of giant piezoresponse is also discussed.Substitution of TM ion with small ionic size for Zn2+ results in the easier rotation of noncollinear TM-O1 bonds along the c axis under the applied field,which produces large piezoelectric displacement and corresponding piezoresponse enhancement.Furthermore,it proposes a general rule to guide the design of new wurtzite semiconductors with enhanced piezoresponses.That is,TM-dopant with ionic size smaller than Zn2+ substitutes for Zn2+ site will increase the piezoresponse of ZnO significantly.Finally,we discuss the improved performances of some TM-doped ZnO based piezoelectric devices.
