Abstract
We theoretically investigate external surface charge effects on piezoelectric potential of ZnO and AlN nanowires (NWs) and nanotubes (NTs) under uniform compression. The free carrier depletion caused by negative surface charges via surface functionalization on vertically compressed ZnO and AlN NWs/NTs is simulated using finite element calculation; this indicates the enhancement of piezoelectric potential is due to the free carriers (electrons) being fully depleted at the critical surface charge density. Numerical simulations reveal that full coverage of surface charges surrounding the NTs increases the piezoelectric output potential exponentially within a relatively smaller range of charge density compared to the case of NWs for a typical donor concentration (∼1017 cm−3). The model can be used to design functional high-power semiconducting piezoelectric nanogenerators.
Highlights
Zinc oxide (ZnO) is an important semiconducting piezoelectric material, which lacks central symmetry in wurtzite structures
More recently, it has been discovered that the screening effect due to free charge carriers in the conduction band by doping is undesirable for piezoelectric nanogenerators made of as-grown n-type ZnO NWs,[8] unlike in the case of insulating piezoelectric materials such as lead zirconate titanate (PZT), barium titanate (BTO), and polyvinylidene fluoride (PVDF).[9,10,11]
The enhanced piezoelectric potential obtained by varying the surface charge density in three different cases—1) surface charges on top of NWs (Fig. 1(a)), 2) charges surrounding the surface of NWs (Fig. 1(b)), and 3) charges surrounding the surface of NTs (Fig. 1(c))—are presented and discussed numerically
Summary
Zinc oxide (ZnO) is an important semiconducting piezoelectric material, which lacks central symmetry in wurtzite structures. More recently, it has been discovered that the screening effect due to free charge carriers in the conduction band by doping is undesirable for piezoelectric nanogenerators made of as-grown n-type ZnO NWs,[8] unlike in the case of insulating piezoelectric materials such as lead zirconate titanate (PZT), barium titanate (BTO), and polyvinylidene fluoride (PVDF).[9,10,11] The interplay of such effects for laterally bent and vertically compressed NWs has been investigated.[8,12] in order to overcome this effect, the idea of controlling the screening effect by imposing external surface charges on the NWs/NTs system has been invalidated theoretically on a continuum level. An accurate model to consider such a charge effect is necessary in order to design functional high-power piezoelectric nanogenerators. We performed theoretical and finite element analysis of the surface charge effect on the piezoelectric potential of vertically compressed ZnO and aluminum nitride (AlN) NWs and nanotubes (NTs) under thermodynamic equilibrium.
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