Single zinc oxide nanowires (ZnO NWs) are promising for nanogenerators because of their excellent semiconducting and piezoelectric properties, but characterizing the latter efficiently is challenging. As reported here, an electrical breakdown strategy was used to construct single ZnO NWs with a specific length. With the high operability of a nanomanipulator in a scanning electron microscope, ZnO-NW-based two-probe and three-probe structures were constructed for fabricating AC/DC nanogenerators, respectively. For a ZnO NW, an AC output of between −15.31 mV and 5.82 mV was achieved, while for a DC nanogenerator, an output of ∼24.3 mV was realized. Also, the three-probe structure’s output method was changed to verify the distribution of piezoelectric charges when a single ZnO NW is bent by a probe, and DC outputs of different amplitudes were achieved. This study provides a low-cost, highly convenient, and operational method for studying the AC/DC output characteristics of single NWs, which is beneficial for the further development of nanogenerators.

HIGHLIGHTS

  • A method for reworking NWs to a specific length was achieved using an electrical breakdown strategy based on the constructed electrical loop, and its success rate is high.

  • With the maneuverability provided by multiple operating units of a nanomanipulator, AC output was investigated for a single ZnO NW with a two-probe structure, as well as DC output with a three-probe structure.

  • In the three-probe structure, different DC outputs were achieved by changing the position of the NW–probe contact point and the wiring state of the probe, while the distribution of surface piezoelectric potential along the NW’s axis was studied.

Continuous breakthroughs in the integration and miniaturization of electronic circuits have reduced the energy consumption of electronic devices to milliwatts or even microwatts.1 Being small and consuming little energy, microelectronic devices are used widely as wearable electronic devices, microsensors, and Internet of Things nodes.2–5 However, despite the low power consumption of these devices, challenges remain in meeting their proliferation with relying solely on traditional power supply methods, which require frequent replacement and thus not only hamper long-term stable operation but also increase operating costs.

An effective approach to the above problem is to use nanogenerators, which convert the abundant mechanical energy available in daily life into electrical energy. Two common types of nanogenerator are triboelectric6–9 and piezoelectric,10–15 of which piezoelectric nanogenerators are particularly promising for self-powering microelectronic devices because of being lightweight and having high power density and long lifespan.10–15 

The piezoelectric effect is the generation of electric charge on the surface of an anisotropic material in response to an applied external force, and commonly used piezoelectric materials include aluminum nitride, gallium nitride, and zinc oxide (ZnO). Of these materials, ZnO is highly regarded because of its desirable semiconductor, piezoelectric, and environmentally friendly properties.16–19 

However, most studies of piezoelectric nanogenerators to date have been focused on nanowire (NW) arrays, which can achieve output voltages of tens of volts. Nevertheless, these NW-based nanogenerators require lengthy and complex preparation and must be coordinated with specific substrates and electrodes, often on centimeter scale. Instead, investigating the power generation characteristics of single ZnO NWs is important because they are the fundamental structures for array-based nanogenerators, and doing so offers to address the requirements of further miniaturization. ZnO NWs have several distinctive features, including quasi-lattice perfection (lack of dislocations), nanoscale dimensions, and a large surface-to-volume ratio. Their hexagonal structure exhibits considerable intrinsic polarization, which facilitates the accumulation of charges at the heterojunction interface. Also, the absence of symmetric centers in ZnO NWs gives them strong piezoelectric properties.20 

Wang and Song21 demonstrated the power generation capabilities of a single vertically grown ZnO NW on a substrate using a silicon probe coated with platinum; grounding the substrate enabled the detection of a positive DC output when the probe bent the NW. The same group developed an AC nanogenerator22 by placing a fine piezoelectric wire on a flexible substrate and securing it with electrodes; when the substrate deformed, the NW was bent, leading to the generation of AC output via piezoelectric charges and the Schottky contact between the electrodes and the NW.

While AC/DC nanogenerators based on ZnO NWs have been studied previously, these generator structures often necessitate specific processing steps, such as manipulating the NW–substrate and NW–electrode interfaces. As a result, the exploration of various piezoelectric properties of ZnO NWs has been limited. Zhou et al.23 addressed this limitation by constructing a mechanical–electrical triggering system in which a gas gun was used to bend the NW, enabling output detection via a probe placed at the side. This structure allowed for multi-point contact with the stretched and compressed surfaces of the NW by adjusting the probe’s position. However, the deformation induced by the gas gun could not be controlled precisely, and it was suggested that there is a nonuniform piezoelectric potential along the axis of an ultra-long NW during deformation. Therefore, constructing ZnO NWs with specific lengths is also crucial for accurate piezoelectric characterization.

To address the aforementioned issues, we propose using a nanomanipulator in a scanning electron microscope (SEM) to enable the implementation of multi-probe-based AC/DC nanogenerators (note that DC means that the signal is unidirectional). By constructing an electrical loop using a two-probe structure, we were able to adjust the NW’s length via an electrical breakdown strategy. Leveraging the maneuverability provided by multiple operating units of the nanomanipulator, we investigated both AC output using a two-probe structure and DC output using a three-probe structure for a single ZnO NW. Furthermore, we achieved different DC outputs by varying the contact point between the NW and the probe and the wiring configuration of the probe in the three-probe structure. This study offers an effective operational approach for studying the AC/DC output of a single NW, thereby realizing low-cost, highly convenient, and operationally versatile characterization of its piezoelectric characteristics.

The ZnO NWs used in this study were purchased from Nanjing XF-NANO Materials Tech Co., Ltd. in China. Figure 1(a) shows a SEM image of the ZnO NWs used in this study, which had lengths of 15–50 μm and diameters of 50–550 nm. The process for picking up a single ZnO NW was the same one that we used previously.24 A nanomanipulation system (LifeForce TNI, Canada) in a SEM (Hitachi SU 3500) was used to pick up and move NWs for nanogenerator fabrication. Furthermore, a digital multimeter (Keithley DMM 6500) and a source measure unit (Keithley 2280S semiconductor characterization system) were connected electrically to the nanomanipulator for piezoelectric characterization and energy supply, respectively.

FIG. 1.

Morphology of ZnO nanowires (NWs) and experimental principles: (a) SEM image of ZnO NWs; (b) schematic of two-probe structure; (c) schematic of three-probe structure.

FIG. 1.

Morphology of ZnO nanowires (NWs) and experimental principles: (a) SEM image of ZnO NWs; (b) schematic of two-probe structure; (c) schematic of three-probe structure.

Close modal

The nanomanipulator in our study had four units, each of which could move independently in the X, Y, and Z directions. Separate nanorobotic arms were installed on the four units of the manipulator, and each robotic arm was equipped with a probe that could be connected electrically to the peripheral measuring instruments. Therefore, this device could be used to manipulate NWs and study their piezoelectric output characteristics. In summary, two-probe and three-probe structures were constructed using the nanomanipulator in order to construct AC and DC nanogenerators, respectively, as shown in Figs. 1(b) and 1(c).

To obtain ZnO NWs with specific lengths, we used an electrical breakdown strategy to reprocess the picked-up single ZnO NW based on a two-probe structure [see Fig. 1(b)]. Probe 1 picks up the NW and remains in a fixed position. First, position information about the NW end and probe-2 tip in the Z direction is obtained by adjusting the focal plane, then probe 2 is moved so that they are in the same plane. Second, to form a current loop, probe 2 is adjusted to make contact with the NW end. Next, an external source measure unit is used to supply a safe voltage (5 V) to the current loop, while the SEM electron beam is focused on the contact point, resulting in a stable electrical connection by maintaining this state for 5 min, as shown in Fig. 2(a). Because of the influence of the geometric parameters of the NW on the electrical breakdown voltage, we applied a scanning voltage (in steps of 0.5 V) until the NW breakdown.

FIG. 2.

Fabrication of NWs with a specific length using electrical breakdown strategy: (a) SEM image of two-probe structure; (b) SEM image of NW after breakdown; (c) SEM image of NW’s bead-like structure after breakdown; (d) ratio (i.e., proportion) vs. ε (30 measurements).

FIG. 2.

Fabrication of NWs with a specific length using electrical breakdown strategy: (a) SEM image of two-probe structure; (b) SEM image of NW after breakdown; (c) SEM image of NW’s bead-like structure after breakdown; (d) ratio (i.e., proportion) vs. ε (30 measurements).

Close modal

Figure 2(b) shows a SEM image of a NW after breakdown, defining the effective length of the picked-up NW as the part between the two contact points of the NW and the tungsten probes (i.e., the length of the NW through which current flows after being energized). The results show that the fracture point of the NW appears in the middle of effective length, indicating that this method is effective for reprocessing the length of the picked-up NW. Therefore, the expected NW length can be obtained by adjusting the contact position between probe 2 and the NW.

To ensure experimental repeatability, 30 electrical breakdown tests were conducted in our study. However, occasional morphological changes after NW breakdown were found in our experiments [see Fig. 2(c)], while the NW produced a distinct bead-like structure, probably due to the unpredictable recrystallisation of the NW from single crystal to polycrystalline during the breakdown process to restore the energy balance.25 For statistical convenience, we define the longer segment from the probe–NW contact point to the NW fracture point after breakdown as L1 and the other segment as L2, then we define ε = L1/L2 as the expected deviation of a NW of specific length, and the closer ε is to 1, the better the result. Figure 2(d) shows the results of 30 electrical breakdown tests, which show that 86.7% of the NWs had ε = 1–1.4. In addition, it might be possible in the future to control ε more accurately by decreasing the diameter of the probe tip or by narrowing the step value of the applied voltage (for NWs of different diameters and lengths, the breakdown voltages all fluctuate within a small range, i.e., greater than 20 V,26 and therefore the step value can be further narrowed when the applied voltage is close to 20 V), but this puts higher demands on the experimental setup, and thus further research is needed after upgrading the equipment. Note that the appearance of the bead-like structure is always accompanied by significant deviations, which may be due to the uncertainty of recrystallisation making the NW’s fracture location unpredictable. This strategy is effective for reprocessing the picked-up NWs to obtain expected specific NWs.

The two-probe structure was used to construct an AC nanogenerator. Both ends of the NW were connected electrically to an external digital multimeter via probes, moving probe 2 to the NW’s free end. With probe 2 touching the NW as the sampling starting point, the nanomanipulator was controlled to bend the NW in a stepwise manner perpendicular to the NW axis until the two were separated. Note that each step moved the same distance, and the moving distance was determined according to the SEM magnification (this distance was unknown because of the insufficiency of the nanomanipulator’s feedback). Also, the time interval between each step was approximately the same (the nanomanipulator was controlled manually).

To verify that the output was generated by the NW (and not from electron beam irradiation), we conducted a switching polarity test of a two-probe AC nanogenerator. The test was performed by switching the polarity of probes 1 and 2 only with the wiring of the external digital multimeter without changing the NW’s deformation conditions [see Fig. 3(a)]. Figure 3(b) shows the output of the two-probe nanogenerator with positive connection, indicating an AC output during the NW’s bending process. Positive voltage represents the piezoelectric charges generated at the moment of NW deformation, while negative voltage corresponds to the NW’s tendency to restore charge equilibrium when the NW–probe contact is relatively stationary (i.e., the stationary moment after probe 2 stepping). Note that the positive and negative voltages are not of the same amplitude, which may be due to different times of NW piezoelectric charge generation and recovery. Generation depends mainly on NW’s deformation, while recovery is the tendency of the NW itself to return to its initial state. When the digital multimeter was connected in reverse, the output was also reversed, and it formed an asymmetric output before switching polarity, which may have been caused by a bias current in the measurement system that was superimposed on the output when connected in the forward direction and canceled part of the output when reversed, in agreement with previous work.22 Therefore, the true signal generated by this AC nanogenerator is the average of the corresponding output amplitudes for forward and reverse connections, which are −15.31 mV and 5.82 mV for a ZnO NW with a diameter of 255 nm and a length of 31.7 μm.

FIG. 3.

Piezoelectric testing of AC/DC nanogenerator based on a single ZnO NW: (a) schematic of switching polarity test [left: forward connection (FC), right: reverse connection (RC)]; (b) output of AC nanogenerator during FC (left) and RC (right); (c) SEM image of three-probe DC nanogenerator; (d) output of DC nanogenerator during FC (left) and RC (right).

FIG. 3.

Piezoelectric testing of AC/DC nanogenerator based on a single ZnO NW: (a) schematic of switching polarity test [left: forward connection (FC), right: reverse connection (RC)]; (b) output of AC nanogenerator during FC (left) and RC (right); (c) SEM image of three-probe DC nanogenerator; (d) output of DC nanogenerator during FC (left) and RC (right).

Close modal

For the DC nanogenerator, probe 1 (grounded) picked up the NW, followed by moving probe 3 near the NW’s free end (upper surface in SEM field of view) and probe 2 to the middle of the NW while touching its lower surface [see Fig. 3(c)]. A digital multimeter was connected to probes 1 and 2, and its output was recorded while moving probe 3 to bend the NW along its vertical direction. Again, the step value of probe 3 was determined based on the SEM magnification, and the value was kept constant during the experiment (i.e., the NW’s free end produced the same displacement each time). As shown in Fig. 3(d), the nanogenerator produced DC output when both forward and reverse connections were made, and the corresponding output amplitudes were essentially the same (minor differences may have been caused by the irradiated electron beam), in agreement with the results of previous work.23 Unlike the AC output, because probe 1 was always grounded, when the digital multimeter was connected to probes 1 and 2, DC output was always detected. When the NW bends, it generates positive piezoelectric charges on its stretched side and negative charges on its compressed side. Also, because probe 2 always touches the compressed side and remains motionless, which makes contact between it and the NW relatively stable, the output at probe 2 is essentially the same for each further step of probe 3. For the fabricated DC nanogenerator (the ZnO NW was 323 nm in diameter and 40.7 μm in length), a DC output of ∼24.3 mV was achieved.

To realize a DC nanogenerator with multiple outputs, we changed the position of probe 2 and the connection between the three-probe structure and the digital multimeter [see Fig. 4(a)]. As shown in Figs. 4(b)4(d), the input conditions of probe 3 were kept constant (i.e., step length, starting position, and moving direction), and only probe 2 was moved along the NW’s axis. Note that the deformation between probes 1 and 2 is negligible because probe 2 is always close to probe 3 and probe 3 moves a minimal distance relative to the NW length. The distance between the tips of probes 2 and 3 was the effective length (L). The results show that the maximum output (Vmax) is −21.7 mV when L is 3.17 μm (there is no risk of collision between probes 2 and 3 at this value of L), which decreases as L increases, and Vmax reduces to −12.7 mV when L rises to 7.43 μm. Figure 4(e) shows the relationship between L and Vmax, which tends to decrease as L increases because the NW’s deformation only occurs at the free end by probe 3, and the polarized piezoelectric charges tend to concentrate where the deformation is large. We also obtained the output at the stress concentration by connecting probes 1 and 3 to a digital multimeter, and the output was converted to positive and Vmax was as high as 28.8 mV because of the contact of probe 3 with the NW’s stretched surface, which again verified that the polarized charge tends to be distributed near the stress point. Note that the output of probe 3 is more unstable compared to the negative output, which is caused by the tendency of probe 3 to slide relative to the NW during the bending process. Furthermore, the output grows exponentially as the diameter of the NW decreases,27 and the NW’s deformation affects the output as well,28,29 so in further work the output of the NW-based nanogenerator can be increased by selecting a NW with a smaller diameter or increasing its deformation.

FIG. 4.

Output of three-probe DC nanogenerator (ZnO NW diameter: 421 nm, length: 38.9 μm): (a) schematic of output variation of DC nanogenerator; (b)–(d) output with probes 1 and 2 connected to digital multimeter [L = (b) 3.17 μm, (c) 5.65 μm, and (d) 7.43 μm]; (e) L vs. maximum output (Vmax); (f) output with probes 1 and 3 connected to digital multimeter.

FIG. 4.

Output of three-probe DC nanogenerator (ZnO NW diameter: 421 nm, length: 38.9 μm): (a) schematic of output variation of DC nanogenerator; (b)–(d) output with probes 1 and 2 connected to digital multimeter [L = (b) 3.17 μm, (c) 5.65 μm, and (d) 7.43 μm]; (e) L vs. maximum output (Vmax); (f) output with probes 1 and 3 connected to digital multimeter.

Close modal

We have successfully constructed AC/DC nanogenerators using single ZnO NWs, leveraging the high operability of a nanomanipulator in a SEM. Recognizing that the NW length significantly impacts the fabrication of piezoelectric devices, we proposed a NW reprocessing method based on an electrical breakdown strategy. This approach yielded a success rate of 86.7% for a deviation (ϵ) between 1 and 1.4. In contrast to the complex fabrication processes involved in conventional ZnO NW piezoelectric devices, our study used a nanomanipulator with multiple moving units to construct two-probe and three-probe structures for fabricating AC/DC nanogenerators. We achieved an AC output ranging from −14.27 mV to 3.45 mV (for a ZnO NW with a diameter of 255 nm and a length of 31.7 μm) and a DC output of ∼24.3 mV (for a ZnO NW with a diameter of 323 nm and a length of 40.7 μm). The distribution of piezoelectric charges when a single ZnO NW was bent by a probe was successfully verified using a three-probe DC nanogenerator. The results show that charges tended to concentrate at the NW–probe contact point, with a faster decay of piezoelectric charges observed as the distance from the contact point increased. This work provides a valuable reference method for constructing AC/DC nanogenerators based on single ZnO NWs, emphasizing low cost, high convenience, and high operability. It is an important contribution for further exploration of the piezoelectric properties of ZnO NWs and the ongoing development of nanogenerators.

This work was supported by the Research Fund Program of the Guangdong Provincial Key Laboratory of Fuel Cell Technology (Grant No. FC202204).

The authors have no conflicts to disclose.

The data that support the findings of this study are available within the article.

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Mei Liu was born in Hebei. She holds a doctoral degree and is an associate professor in the School of Mechatronic Engineering and Automation at Shanghai University. Her research interests are in micro/nano-manipulation and micro/nano-sensing.

Mengfan He was born in Henan and is a graduate student in the School of Mechatronic Engineering and Automation at Shanghai University. His research interests are in micro/nano-operation and piezoelectric nanogenerators.

Zhiming Wang was born in Shanghai. He has a Ph.D. degree and is a professor in the School of Mechatronic Engineering and Automation at Shanghai University. His research interests are in intelligent CNC technology and micro-operated machinery.