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.
Single crystal Fe/Ag(001) superlattices with various periodicities were fabricated using ultrahigh vacuum evaporation deposition. It was found that single crystal bcc Fe layers and single crystal fcc Ag layers can epitaxially grow on a single crystal Ag buffer layer alternately,which was deposited on NaCl single crystal chips by ion beam assisted deposition. The magnetic measurements of the superlattices reveal an oscillation coupling between ferromagnetism and antiferromagnetism as a function of the Ag layer thickness. The oscillation period,which is 1 nm(5 Ag layers) ,is in good agreement with the calculated values when the Ag thickness is greater than 1.5 nm. While the thickness of the Ag spacer layer decreases to 1 nm,the oscillation coupling varies from calculations,which can be attributed to the intermixing of the interlayers according to the annealing results.
Yu Gu Fei Zeng Fang Lv Yu-li Gu Pei-yong Yang Feng Pan
Research interest in ZnO nanostructures derives from their excellent luminescent properties and availability of low cost fabricating and processing,which hold promise for the development of electronic and optoelectronic nanodevices.In this review,we focus on the progress in synthesis,properties and nanodevices of ZnO nanorod(NR)arrays and nanotetrapods(NTPs).Recent work done by the authors are also presented.After a brief introduction to the controlled fabrication methods for the highly-ordered ZnO NR arrays and NTPs,we present some aspects of the fundamental properties,especially optical performance,of ZnO NRs/NTPs.Then,we provide an overview of the applications to functional nanodevices based on individual NR and NTP of ZnO.It is demonstrated that different morphologies of ZnO nanostructures have salient effects on their properties and applications.Although much progress has been achieved in the fundamental and applied investigations of ZnO NRs/NTPs over the past decade,many obstacles still remain,hampering further development in this field.Finally,some longstanding problems that warrant further investigation are addressed.
Ag/Fe multilayers with well compositional modulation periodicity of 4-60 nm were prepared at room temperature by evaporation deposition using an ultra high vacuum (UHV) chamber. Their microstructure and hardness were investigated using XRD, TEM and nanoindentation. The fcc/bcc type multilayers show a textured polycrystalline growth with Ag (111) and Fe (110) in Ag layers and Fe layers, respectively. The hardness increases with decreasing periodicity and approaches the maximum of 6.36 GPa at the periodicity of 4 nm. The peak hardness is 1.51 times mixture value. The experimental results are well explained by the dislocation-image force-based model developed by Lehoczky.
