xperimentally, both the topography(fee monitored as the tip scanned over the nano- back signal from the scanner)(Fig 2A)and the scanned over the vertically aligned Nws, the wires(note the defined polarity of the voltage corresponding output e images across NWs were bent consecutively. The bending dis- signal). No external voltage was applied in the load( Fig. 2B)were recorded simultaneous- tance was directly recorded in the topography any stage of the experiment ly when the AFM tip was scanned over the image, from which the maximum bending deflec- tion distance and the elastic modulus of the nw as well as the density of Nws that have been scanned by the tip were directly derived(13) In the v image (like discharge peaks) were observed. These peaks, typically about 4 to 50 times the noise level, are rather sharp and narrow, and some- times one or two pixels represent one voltage peak because of the limited scanning speed of the AFM tip, so that the color distribution in the plot is not easy to display (fig. S2 ). By reducing the scan range and increasing the scan frequen- cy, more complete profiles of the discharge peaks y (um) 1.5 were captured (fig. S3). Most of the voltage peaks are -6 to 9 mV in height. The density of NWs contacted by the tip is counted 2A to be -20/um, and the average NWs whose voltage output events 40% of the Nws were contacted. the voltage output line profiles across one Nw acquired at a time interval of l min is shown in Fig 2C. Because the dwell time for each data int (or pixel) is 2 ms, which is longer than the age lifetime of the voltage peak of -06m Fig 2D), the peak at which V, reached the max imum was possibly missed by the"slow"scan D ning tip, so E in Fig. 2C). A sharp peak can be identified con- > tinuously at the location of the Nw and in the Woutput voltage in each scan of the tip. When R the tip started to deflect the NW, no voltage output was observed (Fig 2D): V was detected when the deflection of the Nw approached its maximum. When the Nw was released by the AFM tip, V dropped to zero, indicating that the the end of the afm scan over the nw Fig. 2. Electromechanically coupled discharging process of aligned piezoelectric Zno NWs The shape of the discharge peak can be observed in contact mode. (A and B)Topography image (A) and corresponding output voltage further improved and analyzed by maximizing characterized by only a couple of data points, which made it difficult to display the data by rainbow scanning range. Shown in Fig. 2E is a line color (see fig. $2).(C) A series of line profiles of the voltage output signal when the AFM tip profile of I, when the tip was scanned over a scanned across a vertical NW at a time interval of 1 min The data show the registration of the gle Nw at a scanning velocity of 12.394 um/s. electric signal with the location of the Nw and the reproducibility of the event over an extended The full width at half maximum (FWHM)of the period of time. Colors represent the outputs required for a series of scans. In (A) to(), the scanning speed of the tip was 12.081 um/s, and the time spent to acquire and output one scan voltage peak was estimated to be t-06ms. n be qualitatively described an of the NW, indicating that the discharge occurs when the tip is in contact with the compressed side alent circuit (l4)(Fig. 2E). The Nw is approx- the NW. When the tip touches the Nw, PZ charges start to accumulate but no discharge occurs. imated as a resistor R, and a capacitor C Discharge occurs when the deflection reaches nearly the maximum ym Note that the lateral (including the contribution from the syste deflection y includes the shape and contact geometry of the tip, which must be subtracted to derive The lifetime of the output voltage Across the true deflection of the NW.(E) Line profile of the voltage output signal when the AFM tip scans the load R, (note the polarity of the voltage) across a vertical NW at 12.394 um/s. the time spent to acquire and output one scan point was 0.05 is t=(RL Ru)C(5). For the experiment we ms, which was achieved at the maximum scan frequency of the AFM. The inset is an equivalent have designed, the resistance of the NW r, circuit of the measurement to be used for simulating the discharging process. ( F)The resonance is negligible relative to RL(16, 17). Thus, the vibration of a NW after being released by the AFM tip, showing that the stored elastic energy is equivalent capacitance of the Nw and the transferred mainly into vibrational energy after creating the pz discharge event. system is C≈tR1≈1.2pF www.sciencemag.orgscIencEVol31214ApriL2006 243resistance RL 0 500 megohms was continuously monitored as the tip scanned over the nano￾wires (note the defined polarity of the voltage signal). No external voltage was applied in any stage of the experiment. Experimentally, both the topography (feed￾back signal from the scanner) (Fig. 2A) and the corresponding output voltage (VL) images across the load (Fig. 2B) were recorded simultaneous￾ly when the AFM tip was scanned over the aligned NW arrays. In contact mode, as the tip scanned over the vertically aligned NWs, the NWs were bent consecutively. The bending dis￾tance was directly recorded in the topography image, from which the maximum bending deflec￾tion distance and the elastic modulus of the NW as well as the density of NWs that have been scanned by the tip were directly derived (13). In the VL image, many sharp output peaks (like discharge peaks) were observed. These peaks, typically about 4 to 50 times the noise level, are rather sharp and narrow, and some￾times one or two pixels represent one voltage peak because of the limited scanning speed of the AFM tip, so that the color distribution in the plot is not easy to display (fig. S2). By reducing the scan range and increasing the scan frequen￾cy, more complete profiles of the discharge peaks were captured (fig. S3). Most of the voltage peaks are È6 to 9 mV in height. The density of NWs contacted by the tip is counted from Fig. 2A to be È20/mm2 , and the average density of NWs whose voltage output events had been captured by the tip in Fig. 2B is È8/mm2 , thus È40% of the NWs were contacted. The location of the voltage peak is directly registered at the site of the NW. A time series of the voltage output line profiles across one NW acquired at a time interval of 1 min is shown in Fig. 2C. Because the dwell time for each data point (or pixel) is 2 ms, which is longer than the average lifetime of the voltage peak of È0.6 ms (Fig. 2D), the peak at which VL reached the max￾imum was possibly missed by the Bslow[ scan￾ning tip, so that VL shows a chopped top (arrows in Fig. 2C). A sharp peak can be identified con￾tinuously at the location of the NW and in the NW output voltage in each scan of the tip. When the tip started to deflect the NW, no voltage output was observed (Fig. 2D); VL was detected when the deflection of the NW approached its maximum. When the NW was released by the AFM tip, VL dropped to zero, indicating that the output of piezoelectricity was detected toward the end of the AFM scan over the NW. The shape of the discharge peak can be further improved and analyzed by maximizing the tip scanning frequency and reducing the scanning range. Shown in Fig. 2E is a line profile of VL when the tip was scanned over a single NW at a scanning velocity of 12.394 mm/s. The full width at half maximum (FWHM) of the voltage peak was estimated to be t È 0.6 ms. The damping behavior of the voltage peak can be qualitatively described with an equiv￾alent circuit (14) (Fig. 2E). The NW is approx￾imated as a resistor RI and a capacitor C (including the contribution from the system). The lifetime of the output voltage VL across the load RL (note the polarity of the voltage) is t 0 (RL þ RI )C (15). For the experiment we have designed, the resistance of the NW RI is negligible relative to RL (16, 17). Thus, the equivalent capacitance of the NW and the system is C , t/RL , 1.2 pF. Fig. 2. Electromechanically coupled discharging process of aligned piezoelectric ZnO NWs observed in contact mode. (A and B) Topography image (A) and corresponding output voltage image (B) of the NW arrays. The discharging process was so rapid that each discharge event was characterized by only a couple of data points, which made it difficult to display the data by rainbow color (see fig. S2). (C) A series of line profiles of the voltage output signal when the AFM tip scanned across a vertical NW at a time interval of 1 min. The data show the registration of the electric signal with the location of the NW and the reproducibility of the event over an extended period of time. Colors represent the outputs required for a series of scans. In (A) to (C), the scanning speed of the tip was 12.081 mm/s, and the time spent to acquire and output one scan point was 2 ms. (D) Line profiles from the topography (red) and output voltage (blue) images across a NW. The peak of the voltage output corresponds approximately to the maximum deflection of the NW, indicating that the discharge occurs when the tip is in contact with the compressed side of the NW. When the tip touches the NW, PZ charges start to accumulate but no discharge occurs. Discharge occurs when the deflection reaches nearly the maximum ym. Note that the lateral deflection y includes the shape and contact geometry of the tip, which must be subtracted to derive the true deflection of the NW. (E) Line profile of the voltage output signal when the AFM tip scans across a vertical NW at 12.394 mm/s. The time spent to acquire and output one scan point was 0.05 ms, which was achieved at the maximum scan frequency of the AFM. The inset is an equivalent circuit of the measurement to be used for simulating the discharging process. (F) The resonance vibration of a NW after being released by the AFM tip, showing that the stored elastic energy is transferred mainly into vibrational energy after creating the PZ discharge event. REPORTS www.sciencemag.org SCIENCE VOL 312 14 APRIL 2006 243