REPORTS The observed sharp voltage output for Zno grounded. Note that v. and vs are the volt port process. Because the compressed side of NWs was not observed for metal film(fig. $4), ages produced by the Pz effe are each the semiconductor Zno Nw has negative po- aligned carbon nanotubes(fig. S5), or aligned typically larger than a few tens (22). The tential /. and the stretched side has positive wO, NWs(fig. S6) under identical or similar potential is created by the displace- potential (s ), two distinct transport processes experimental conditions. These data rule out the of the Zn" cations with respect to the o will occur across the Schottky barrier possibility of friction or contact potential as the anions, a result of the Pz effect in the We now consider the case of a ZnO Nw source of the observed response. crystal structure; thus, these ionic charges cannot without an Au particle at the top. In the first step, t the efficiency of the electric power gener- freely move and cannot recombine without the AFM conductive tip that induces the de- by this process can be calculated as follows. releasing the strain( Fig. 3D). The potential dif- formation is in contact with the stretched surface The output electrical energy from one Nw in ference is maintained as long as the deforma- of positive potential V.( Fig 3, D and E). The one piezoelectric harge(PZD) event is tion is in place and no foreign free charges Pt metal tip has a potential of nearly zero, I AWpzp = VoC/h2 (15), where Vo is the peak (such as from the metal contacts)are injected. 0, so the metal tip-Zno interface is negatively voltage of the discharge output. For simplicity, The contacts at the top and the base of the biased for Al=Vm-v.<0. Because the as- we approximately take the nw as a 2D object NW are nonsymmetric; the bottom contact is to synthesized Zno Nws behave as n-type semi- for easy analytical calculation. The elastic the Zno film in contact with Ag paste, so the conductors, the Pt metal-ZnO semiconductor deformation energy (WELD created by the effective contact is between ZnO and Ag. The (M-S)interface in this case is a reverse-biased AFM tip for displacing the Nw is WelD= electron affinity(E )of ZnO is 4.5 ev(23)and Schottky diode(Fig. 3E), and little cur 3Yly2 /22(15), where Y is the elastic modu- the work function(o)of Ag is 4.2 eV; there is flows across the interface. In the second step. the NW, and ym is the maximum deflection of is ohmic. At the tip of the Nw, Pt has pressed side of the Nw(Fig. 3F), the metal the Nw. Dissipation of Wen mainly occurs ev, and the Pt-Zno contact is a Schottky tip-Zno interface is positively biased for Al in three ways: (1) mechanical resonance/ (24, 25)and dominates the entire trans- L=m-5 >0. The M-S interface in this ibration after releasing the Nw(Fig. 2F) (i) Pz discharge(AWpp) for each cycle of the vibration; and (ii) friction/viscosity from the Fig. 3. Transport is governed environmental medium. The mechanical reso- y a metal-semiconductor nance of the Nw continues for many cycles, shottky barrier for the Pz but it is eventually damped by the viscosity of TV>o 5l Zno Nw (see movies S1 and the medium. Each cycle of the vibration gen- S2). (A) Schematic definition erates AWpzp, but the AFm tip in the present of a nw and the coordi- experimental design catches only the energ tion system.(B)Longi- generated in the first cycle of vibration. Taking tudinal strain e distribution into account the energy dissipated by the nw lEco in the first cycle of vibration AWED (15),the deflected by an AFM tip from efficiency of converting mechanical energy to the side, the data were electrical energy is△Wpa/△ W.. Therefore, simulated by FEMLAB for a an efficiency of 17to30% has been received闷l≈0 Zno NW of length 1 Ht for a cycle of the resonance(table SI). The an aspect ratio of 10.(C)The large efficiency is likely due to the extremely PZ-induced electric field E large deformation that can be bome by the RL stribution in the nw nanowire(18) Potential distribution in the The physical principle for creating the Pz Nw as a result of the pz ef- discharge energy arises from how the piezo- fect. (E and F) Contacts be- electric and semiconducting properties of Zno ween the AFM tip and are coupled. For a vertical, straight Zno Nw e semiconductor zno nw (Fig 3A), the deflection of the Nw by an AFM boxed area in(D)] at two re- tip creates a strain field, with the outer surface Ag △v=vV versed local contact potentials being stretched(positive strain e)and the inner (positive and negative), show- electric field E, along the nw e direction)is G ed Schottky rectifying then created inside the nw volume through the havior, respectively (see PZ effect, E. =E_/d, where d is the ot). This oppositely biased coefficient(19)along the nw direction that is shottky barrier across the normally the positive c axis of Zno, with the Zn W preserves the Pz charges atomic layer being the front-terminating layer and later produces the dis- (20, 21). The PZ field direction is closely paral- charge output. The inset shows lel to the z axis(Nw direction) at the outer a typical current-voltag surface and antiparallel to the z axis at the in (-v relation characteristic surface(Fig. 3C). Under the first-order approx- (n-type) Schottky barrier. The process in(E is to separate and maintain the charges as well as build up imation, across the width of the Nw at the top the potential. The process in(F is to discharge the potential and generates electric current. (G and H) d,the electric potential distribution from the Contact of the metal tip with a Zno NW with a small Au particle at the top. The PZ potential is built up in ompressed to the stretched side surface is the displacing process(G), and later the charges are released through the compressed side of the Nw approximately between V. to V.* [with VH (H).(D) Contact of the metal tip with a Zno NW with a large au particle at the top. the charges are u3TlymV4Ld (15), where T is the thickness of gradually leaked" out through the compressed side of the Nw as soon as the deformation occurs the Nw). The electrode at the base of the Nw is thus, no accumulated potential will be created 244 14ApriL2006Vol312ScieNcewww.sciencemag.orgThe observed sharp voltage output for ZnO NWs was not observed for metal film (fig. S4), aligned carbon nanotubes (fig. S5), or aligned WO3 NWs (fig. S6) under identical or similar experimental conditions. These data rule out the possibility of friction or contact potential as the source of the observed VL response. The efficiency of the electric power gener￾ated by this process can be calculated as follows. The output electrical energy from one NW in one piezoelectric discharge (PZD) event is DWPZD 0 V0 2C/2 (15), where V0 is the peak voltage of the discharge output. For simplicity, we approximately take the NW as a 2D object for easy analytical calculation. The elastic deformation energy (WELD) created by the AFM tip for displacing the NW is WELD 0 3YIy2 m/2L3 (15), where Y is the elastic modu￾lus, I is the moment of inertia, L is the length of the NW, and ym is the maximum deflection of the NW. Dissipation of WELD mainly occurs in three ways: (i) mechanical resonance/ vibration after releasing the NW (Fig. 2F); (ii) PZ discharge (DWPZD) for each cycle of the vibration; and (iii) friction/viscosity from the environmental medium. The mechanical reso￾nance of the NW continues for many cycles, but it is eventually damped by the viscosity of the medium. Each cycle of the vibration gen￾erates DWPZD, but the AFM tip in the present experimental design catches only the energy generated in the first cycle of vibration. Taking into account the energy dissipated by the NW in the first cycle of vibration DWELD (15), the efficiency of converting mechanical energy to electrical energy is DWPZD/DWELD. Therefore, an efficiency of 17 to 30% has been received for a cycle of the resonance (table S1). The large efficiency is likely due to the extremely large deformation that can be borne by the nanowire (18). The physical principle for creating the PZ discharge energy arises from how the piezo￾electric and semiconducting properties of ZnO are coupled. For a vertical, straight ZnO NW (Fig. 3A), the deflection of the NW by an AFM tip creates a strain field, with the outer surface being stretched (positive strain e) and the inner surface compressed (negative e) (Fig. 3B). An electric field Ez along the NW (z direction) is then created inside the NW volume through the PZ effect, Ez 0 ez/d, where d is the PZ coefficient (19) along the NW direction that is normally the positive c axis of ZnO, with the Zn atomic layer being the front-terminating layer (20, 21). The PZ field direction is closely paral￾lel to the z axis (NW direction) at the outer surface and antiparallel to the z axis at the inner surface (Fig. 3C). Under the first-order approx￾imation, across the width of the NW at the top end, the electric potential distribution from the compressed to the stretched side surface is approximately between Vs – to Vs þ Ewith Vs m 0 m3Tkymk/4Ld (15), where T is the thickness of the NW). The electrode at the base of the NW is grounded. Note that Vs þ and Vs – are the volt￾ages produced by the PZ effect, which are each typically larger than a few tens of volts (22). The potential is created by the relative displace￾ment of the Zn2þ cations with respect to the O2– anions, a result of the PZ effect in the wurtzite crystal structure; thus, these ionic charges cannot freely move and cannot recombine without releasing the strain (Fig. 3D). The potential dif￾ference is maintained as long as the deforma￾tion is in place and no foreign free charges (such as from the metal contacts) are injected. The contacts at the top and the base of the NW are nonsymmetric; the bottom contact is to the ZnO film in contact with Ag paste, so the effective contact is between ZnO and Ag. The electron affinity (Ea ) of ZnO is 4.5 eV (23) and the work function (f) of Ag is 4.2 eV; there is no barrier at the interface, so the ZnO-Ag contact is ohmic. At the tip of the NW, Pt has f 0 6.1 eV, and the Pt-ZnO contact is a Schottky barrier (24, 25) and dominates the entire trans￾port process. Because the compressed side of the semiconductor ZnO NW has negative po￾tential Vs – and the stretched side has positive potential (Vs þ), two distinct transport processes will occur across the Schottky barrier. We now consider the case of a ZnO NW without an Au particle at the top. In the first step, the AFM conductive tip that induces the de￾formation is in contact with the stretched surface of positive potential Vs þ (Fig. 3, D and E). The Pt metal tip has a potential of nearly zero, Vm 0 0, so the metal tip–ZnO interface is negatively biased for DV 0 Vm – Vs þ G 0. Because the as￾synthesized ZnO NWs behave as n-type semi￾conductors, the Pt metal–ZnO semiconductor (M-S) interface in this case is a reverse-biased Schottky diode (Fig. 3E), and little current flows across the interface. In the second step, when the AFM tip is in contact with the com￾pressed side of the NW (Fig. 3F), the metal tip–ZnO interface is positively biased for DV 0 VL 0 Vm – Vs – 9 0. The M-S interface in this Fig. 3. Transport is governed by a metal-semiconductor Schottky barrier for the PZ ZnO NW (see movies S1 and S2). (A) Schematic definition of a NW and the coordi￾nation system. (B) Longi￾tudinal strain ez distribution in the NW after being deflected by an AFM tip from the side. The data were simulated by FEMLAB for a ZnO NW of length 1 mm and an aspect ratio of 10. (C) The corresponding longitudinal PZ-induced electric field Ez distribution in the NW. (D) Potential distribution in the NW as a result of the PZ ef￾fect. (E and F) Contacts be￾tween the AFM tip and the semiconductor ZnO NW [boxed area in (D)] at two re￾versed local contact potentials (positive and negative), show￾ing reverse- and forward￾biased Schottky rectifying behavior, respectively (see text). This oppositely biased Schottky barrier across the NW preserves the PZ charges and later produces the dis￾charge output. The inset shows a typical current-voltage (I-V) relation characteristic of a metal-semiconductor (n-type) Schottky barrier. The process in (E) is to separate and maintain the charges as well as build up the potential. The process in (F) is to discharge the potential and generates electric current. (G and H) Contact of the metal tip with a ZnO NW with a small Au particle at the top. The PZ potential is built up in the displacing process (G), and later the charges are released through the compressed side of the NW (H). (I) Contact of the metal tip with a ZnO NW with a large Au particle at the top. The charges are gradually ‘‘leaked’’ out through the compressed side of the NW as soon as the deformation occurs; thus, no accumulated potential will be created. REPORTS 244 14 APRIL 2006 VOL 312 SCIENCE www.sciencemag.org