In Situ Observation of Current Generation in ZnO Nanowire Based

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In situ observation of current generation in ZnO nanowire based nanogenerators using a CAFM integrated into an SEM Chao Wen, Xu Jing, Frank F. Hitzel, Chengbin Pan, Günther Benstetter, and Mario Lanza ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00447 • Publication Date (Web): 27 Mar 2019 Downloaded from http://pubs.acs.org on March 28, 2019

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ACS Applied Materials & Interfaces

In situ observation of current generation in ZnO nanowire based nanogenerators using a CAFM integrated into an SEM Chao Wen1, Xu Jing1, Frank F. Hitzel2, Chengbin Pan1, Guenther Benstetter3, Mario Lanza*1

1 Institute

of Functional Nano & Soft Materials, Collaborative Innovation Center of

Suzhou Nanoscience and Technology, Soochow University, Suzhou, 215123, China. 2 Semilab

Germany GmbH, Geysostraße 13, 38106 Braunschweig, Germany

3 Deggendorf

Institute of Technology, Dieter-Görlitz-Platz 1, 94469 Deggendorf, Germany * Corresponding author E-mail: [email protected]

Abstract In this work, we monitor in situ the movement of the nanowires by using a conductive atomic force microscope integrated into a scanning electron microscope. This setup allows seeing the bending of the nanowires and simultaneously measuring the currents generated. We conclude that the currents generated not only come from piezoelectric effect, but also from contact potential and triboelectric effect. These contributions have been ignored in all previous reports in this field, meaning that the power conversion efficiency of these devices may have been overestimated. Our study helps to clarify the working mechanism of piezoelectric nanogenerators based on ZnO nanowires. Keywords: ZnO nanowire arrays, piezoelectric effect, contact potential, triboelectric effect, conductive atomic force microscope 1

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During the last decade nanowires (NWs) have raised as one of the most promising nanostructures for the fabrication of next generation electromechanical devices, including high-mobility transistors1 ， optoelectronic devices2, strain sensors3, and actuators4. Among all NWs, those that exhibit piezoelectric effect (ZnO, CaN, InN NWs) have gained more interest due to their ability to convert mechanical energy into electrical currents, and the performances shown allow using them as nanogenerators5. Currently, piezoelectric NWs can be synthesized using different approaches, including hydrothermal synthesis6, thermal evaporation synthesis7, pulsed laser deposition8, chemical vapor deposition9, magnetron sputtering10, electron-beam template lithography11 and electrochemical deposition (ED)12. Moreover, piezoelectric NWs with controllable sizes13 can be grown on different substrates (including flexible substrates14) at low temperatures, which is essential for their integration in real devices (such as ZnO NW-based energy harvesters15-16). The first characterization of NWs-based piezoelectric nanogenerators, wich was reported in 2006, used a conductive atomic force microscope (CAFM)5. An array of vertically aligned ZnO NWs was scanned by a metal-varnished CAFM tip with two purposes: i) bending NWs, and ii) measuring the currents generated. After this pioneering work, several reports improved the synthesis of the NWs and the visualization of the currents/potentials generated17-19. However, several issues affecting the performance of ZnO NWs based devices still remain unclear. First, although the synthesis of ZnO NW arrays can be achieved with reasonable good vertical alignment and uniformity in NW diameter, NW clustering is still inevitable20. Therefore, it is hard to ensure whether the current peaks detected via CAFM come from one single NW or from a cluster, making hard to determine what is the real power conversion efficiency

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(PCE). Second, highly dense NW arrays are preferred to enhance the PCE, but at the same time that may result in an insufficient bending due to NW-to-NW collision, reducing the overall performance of the device. Third, topographic and current maps only give information about one specific position, and the entire bending process is difficult to be reliably monitored. This may result in false imaging of the current, as it shows a decay with the time21. And fourth, because the CAFM setup doesn’t allow monitoring the whole NW bending process directly, it is very hard to ensure that all the currents appearing in the current map have a piezoelectric origin. In other words, contact potentials and/or triboelectric effects may also be responsible for the current signals collected by the CAFM22-23 —contact potential is defined as the electrostatic potential between two different electrically conductive or semiconducting materials when they are brought into contact24, and triboelectric effect is defined as the electrification of some materials when they reach frictional contact with another material25. For these reasons, finding characterization methods capable to provide dynamic information about the currents generated in NW-based piezoelectric nanogenerators is essential for a complete understanding of this technology. One possible solution is the combination of different techniques to provide in situ combined information about the devices. Agrawal et al.26 measured the elastic modulus for a series of ZnO nanowires in different shapes by using a TEM with a sample holder containing an AFM cantilever. The AFM tip was controlled by a microelectromechanical system (MEMS), and it was used to apply precise mechanical strain to the NW quantitatively, and the corresponding force could be monitored by this combined system. This allowed in situ observation of the movement of the NW when it was bended with the AFM tip. A similar experiment was carried out by Brown et al.27 using a SEM instead the TEM, and he also calculated the elastic modulus for the NWs 3



employed. However, this technique requires a complex TEM sample preparation, plus the AFM tip has never been used to record the currents generated during NW bending. In this work, we use for the first time a CAFM head integrated into an SEM to characterize in situ the currents generated by NW arrays. Our experiments reveal that the currents generated are not only related to piezoelectric effect, but also to contact potentials and triboelectric effects. This finding is crucial for a better understanding of the electrical properties of NW-based devices, and it can only be assessed with this experimental setup. The growth of ZnO nanowire arrays consisted of three main steps; i) ZnO seed layer deposition, ii) nutrient solution preparation, and iii) chemical hydrothermal synthesis. First, the silicon wafers were cleaned in acetone, alcohol and deionized water (10 min for each step, sequentially) with the assistance of an ultrasonic bath. Then, the ZnO seed layer with a thickness of 5 nm was deposited on the surface of the Si wafer using a magnetron plasma sputtering system (Kurt J. Lesker, model PVD75). Then, the nutrient solution was prepared by mixing 0.01 mol Zn(NO3)2·6H2O, 0.01 mol hexamethylenetetramine (HMTA), 10 mL ammonium hydroxide and 100 mL deionized water. Then, the chemical hydrothermal synthesis was conducted in a sealed teflon-lined stainless steel autoclave (from Shanghai Jing Hong laboratory instrument corporation, model KH100) at 95 C for 3 hours. And finally, the yielded nanowires were washed by deionized water and dried with nitrogen gas. The integration of a CAFM head into an SEM was carried out at the laboratories of Semilab GmbH. A self-designed CAFM head was mounted into the vacuum chamber of an SEM from ZEISS (model Merlin FE-SEM). The schematic of this setup is displayed in Figure 1a. The laser diode, the detector and the preamplifier are located

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inside the vacuum chamber in hermetically encapsulated compartments, and the alignment of the laser path and the exchange of the cantilever and the sample can be performed without breaking the vacuum. The operator is able to control both SEM and AFM systems from one platform, and the positioning of the CAFM tip can be realized by a remote control28. The SEM electron beam could be turned off (when necessary) to avoid the influence from electron bombardment during the measurements. The experiments were carried out in vacuum (at 10-6 Pa) at room temperature. Two different kinds of CAFM tips were used during the tests: non-conductive Si tips, and conductive Pt-Ir coated Si tips. Figure 1b shows the SEM image of the CAFM tip after landing on the ZnO NW array. Figures 1c and 1d show the top view SEM image and AFM topographic map of the ZnO NW array. The topographic map was collected in tapping mode using a Si tip and a very low contact force in order to avoid NW bending, which resulted in a clearcut image. The absence of NW bending during the scan can be clearly seen in the Supplementary Video 1. From Figures 1c and 1d it can be concluded that the average diameter of the NWs is around 250 nm. By cross-sectional SEM images we find that the height of the NWs is 2.5 μm (not shown). The high crystallinity of the ZnO NWs is confirmed by the sharp and stepped top end. In the next step, current maps were collected simultaneously to the topographic ones using a Pt-Ir coated CAFM tip in contact mode. In this case, the topographic maps (see Figure 2a) indicate that the diameter of the NWs is ~500 nm (larger than the ~250 nm detected in Figure 1d). The reason is the larger apex radius and less sharp shape of the Pt-Ir coated Si tip (used in Figure 2a), compared to the Si tip (used in Figure 1d) which cannot penetrate within the narrow spaces between the NWs, and that results in a larger NW diameter. Interestingly, despite the tip/sample contact during the 5



scan does not produce NW bending (as confirmed via SEM), the current maps show conductive spots on top of the NWs (see Figure 2b). These currents cannot be associated to piezoelectric effect, as the NWs do not bend during the scan. This is demonstrated by two facts: i) Supplementary Video 1 shows no NWs bending (as already mentioned); and ii) the topographic maps in Figures 2a (and also 1b) is very clear and has no interferences. In our previous work we demonstrated that NWs bending produces the observation of horizontal lines in the image related to defective scanning, due to tip/sample violent collision29. In order to find out if the currents could be related to the SEM electron gun, some additional experiments have been carried out. First, the experiments have been repeated after turning off the SEM gun, and similar current images have been obtained. Second, the same experiments have been performed in a standard CAFM (not integrated into the SEM), and similar current images have been detected. And third, the CAFM tip has been placed static on the ZnO NWs, and no current has been detected; this observation indicates that that tip movement is necessary to generate the currents, which rules out the effect of the SEM electron gun. Therefore, if the currents observed in Figure 2b are not related to piezoelectric effects nor SEM electron gun, the only feasible explanation is that they may be related to the contact potentials or triboelectric effects produced during the tip movement and the contact with the surface of NWs22-23. In order to detect the piezoelectric response of the ZnO nanowires, current vs. lateral distance (I-X) curves were collected using a Pt-Ir coated Si tip (see Figure 3, and its Supplementary Video 2). To do this experiment, the feedback (force sensor) was turned off, so that the tip can be moved at the desired height without self-retraction. The experiment is described using schematics in Figures 3a: SEM images at different steps of the I-X curve are displayed in Figures 3b-3e, and the consequent I-X plot is displayed 6


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in Figure 3f. As it can be observed, when the tip is far from the NW the electrical signal collected is just noise (Figure 3a.I and 3f.I). In this experiment the background current noise is around 50 pA, a bit higher than in other CAFM experiments30-31 due to the electron bombardment from the SEM. At around 75 nm displacement the CAFM tip touched the NW (Figures 3a.II and 3b), and a current peak appeared in the I-X plot (Figure 3f.II). Note that this current cannot be related to piezoelectric effect, as the SEM images confirm that the NW still didn't bend. Therefore, the high current peak observed between 75 nm and 100 nm in Figure 3f.II should be related to the tip/NW contact potential. After this, the NW gradually starts to bend (see Figures 3a.III and 3c) but almost no current was detected in the I-X curve (Figure 3f.III, from 100 nm to 150 nm). The reasons are: i) the static friction force makes the tip stuck to the contact point, leading to no relative sliding along NW/tip interface, and ii) no piezo-current can be output due to the reversed Schottky barrier between the elongated NW side and metal coated CAFM tip31. At around 150 nm, the I-X plot shows a wide current peak (see Figure 3f.IV), indicating that CAFM tip started to slide along the NW surface (see Figures 3a.IV and 3d), until reaching the compressed side of the NW (see Figures 3a.V and 3e) and finally detaching at around 200 nm. When the tip starts to slide on the elongated surface of the NW (at ~150 nm) some current can be detected despite the presence of a reverse Schottky barrier at the NW/tip interface. Therefore, the origin of that current cannot be piezoelectric, and most probably should be related to the presence of triboelectric effects. As the scan proceeds, the CAFM tip arrives to the compressed surface of the NW, which changes the contact into a forward Schottky barrier5, producing the discharge of the piezoelectric potential. It should be highlighted that it is unclear when the piezoelectric currents start to flow (from Figure 3f one may speculate that the change happens at ~160 nm, i.e. when the currents goes back to zero, but it is 7



very hard to confirm it). Moreover, both triboelectric and piezoelectric phenomena may be produced simultaneously, as the tip may also slide at the compressed region of the NW during the scan. For these reasons, the peak 3f.IV has been associated to both phenomena (no distinction has been made). Up to this point, the whole NW deforming procedure was completed. Note that the distance of 125 nm between the contact point (75 nm) to the separation point (200 nm) is of the same order than the NW diameter, which further confirms the validity of this experiment. It should be highlighted that the currents in static regime (Figure 3f.III), i.e. after the contact potential currents vanish and before the CAFM tip starts to slide laterally, are almost negligible (as stated above). But in fact, that is not completely true because some small current peaks can be indeed observed. The low current values and discontinuous nature of these peaks indicates that they may be related to uncontrollable and intermittent tip/sample contact and/or ploughing effects32. Our experiments indicate that the observation of these current peaks is less repetitive than the currents related to contact, triboelectric and piezoelectric effects. Therefore, it is not possible to make any effective use of them, and for this reason in this manuscript they will not be further discussed. Finally, a Pt-Ir coated Si tip was approached on the top of a single nanowire without inducing bending, and at the same time the electron beam was focused on one side of the NW (see Figures 4a and 4b). Thanks to the accurate force sensing capability of the built-in AFM, this setup is able to detect the force signal from the AFM tip caused by the electron beam from the SEM hitting the NW periodically. To do this experiment the beam current was increased from 1 nA to 15 nA to amplify the piezoelectric feedback. Figure 4c is the corresponding amplitude vs. frequency spectrum collected at an electron beam frequency of 18 kHz. As it can be seen, a strong piezo-force signal 8


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with an exact same frequency was detected. In order to make sure that the feedback really comes from the NW, we reduced the SEM scanning frequency to 12 kHz and 7 kHz successively and (as expected) the piezo-force signal followed the changes perfectly (Figure 4d and 4e). Besides, the piezo-force signal decreased significantly when we zoomed out the SEM scanning (Figure 4f). Since the SEM was working at 18 kHz, the thermal effect can be ruled out because the NW expansion will not be able to follow such a high frequency. In conclusion, we have mounted a CAFM into the chamber of an SEM and used it to characterize in situ the generation of currents in ZnO NWs arrays. Our experiemnts indicate that the currents detected with the CAFM when scanning the ZnO NW arrays are not only related to the piezoelectric effect, but also to contact potentials and triboelectric effects at the tip/sample junction. The implications of these findings are huge because they demonstrate that all previous reports in this field ignored these contributions, and therefore they may have overestimated the real power conversion efficiency of the nanogenerators. Furthermore, the setup here developed could be very useful for the study of a wide range of flexible and conductive nanomaterials (e.g. two dimensional materials, NWs), and it may also be useful in the field of micro and nano electromechanical systems.

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Supplementary Material

Two supplementary videos are provided. In the first one, the CAFM tip scans the surface of the nanowires without bending them. In the second one, which actually is considered a multimedia view of Figure 3, the CAFM tip is placed on a single nanowire and the contact force is increased to monitor its bending in real time with the SEM.

Acknowledgements

This work has been supported by the Young 1000 Global Talent Recruitment Program of the Ministry of Education of China, the Ministry of Science and Technology of China (grant no. BRICS2018-211-2DNEURO), the National Natural Science Foundation of China (grants no. 61502326, 41550110223, 11661131002, 61874075), the Jiangsu Government (grant no. BK20150343), the Ministry of Finance of China (grant no. SX21400213) and the Young 973 National Program of the Chinese Ministry of Science and Technology (grant no. 2015CB932700). The Collaborative Innovation Center of Suzhou Nano Science & Technology, the Jiangsu Key Laboratory for CarbonBased Functional Materials & Devices, the Priority Academic Program Development of Jiangsu Higher Education Institutions, and the 111 Project from the State Administration of Foreign Experts Affairs are also acknowledged. Additional support has been from the Engineering and Physical Sciences Research Council, UK under grant number EP/K01739X/1, Leverhulme Trust under grand number RPG-2016-135, and The Worshipful Company of Scientific Instrument Makers.

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In Situ Observation of Current Generation in ZnO Nanowire Based

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