Mechanical/Electrical Characterization of Zno Nanomaterial Based on AFM/Nanomanipulator Embedded in SEM

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Article Mechanical/Electrical Characterization of ZnO Nanomaterial Based on AFM/Nanomanipulator Embedded in SEM

Mei Liu, Weilin Su, Xiangzheng Qin, Kai Cheng, Wei Ding, Li Ma, Ze Cui, Jinbo Chen, Jinjun Rao, Hangkong Ouyang * and Tao Sun *

Shanghai Key Laboratory of Intelligent Manufacturing and Robotics, School of Mechatronic Engineering and Automation, Shanghai University, Shanghai 200072, China; [email protected] (M.L.); [email protected] (W.S.); [email protected] (X.Q.); [email protected] (K.C.); [email protected] (W.D.); [email protected] (L.M.); [email protected] (Z.C.); [email protected] (J.C.); [email protected] (J.R.) * Correspondence: [email protected] (H.O.); [email protected] (T.S.); Tel.: +86-21-6613-0621 (H.O.)

Abstract: ZnO nanomaterials have been widely used in micro/nano devices and structure due to special mechanical/electrical properties, and its characterization is still deﬁcient and challenging. In this paper, ZnO nanomaterials, including nanorod and nanowire are characterized by atomic force microscope (AFM) and nanomanipulator embedded in scanning electron microscope (SEM) respectively, which can manipulate and observe simultaneously, and is efﬁcient and cost effective. Surface morphology and mechanical properties were observed by AFM. Results showed that the average Young’s modulus of ZnO nanorods is 1.40 MPa and the average spring rate is 0.08 N/m. Electrical properties were characterized with nanomanipulator, which showed that the ZnO nanomaterial have cut-off characteristics and good schottky contact with the tungsten probes. A two-probe strategy Citation: Liu, M.; Su, W.; Qin, X.; was proposed for piezoelectric property measurement, which is easy to operate and adaptable to Cheng, K.; Ding, W.; Ma, L.; Cui, Z.; Chen, J.; Rao, J.; Ouyang, H.; et al. multiple nanomaterials. Experiments showed maximum voltage of a single ZnO nanowire is around Mechanical/Electrical 0.74 mV. Experiment criteria for ZnO manipulation and characterization were also studied, such as Characterization of ZnO acceleration voltage, operation duration, sample preparation. Our work provides useful references Nanomaterial Based on for nanomaterial characterization and also theoretical basis for nanomaterials application. AFM/Nanomanipulator Embedded in SEM. Micromachines 2021, 12, 248. Keywords: nanomanipulator; atomic force microscope; ZnO; mechanical/electrical characterization; https://doi.org/10.3390/mi12030248 piezoelectric property

Academic Editors: Daniel Ahmed and Mehmet Remzi Dokmeci 1. Introduction Received: 9 January 2021 Nanomaterials have a wide range of applications, whose characterization, including Accepted: 25 February 2021 Published: 28 February 2021 force, electricity, heat and light, etc. is of great signiﬁcance [1–4]. For instance, ceramic nanomaterials are often used as friction protective ﬁlms, so its micro/nano hardness,

Publisher’s Note: MDPI stays neutral Young’s modulus, fracture strength and so on need to be tested [5]. Metal or semiconductor with regard to jurisdictional claims in nanowires are commonly used as connecting wires or functional construction materials published maps and institutional afﬁl- in nano-devices, so that its electrical properties need to be characterized [6]. Performance iations. optimization and reliability improvement of nano-devices rely on nanomaterials. ZnO is one of the most important nanomaterials in nanotechnology [7]. It is a wide bandgap semiconductor, which has piezoelectric and pyroelectric properties [8–10]. With piezoelectric properties, ZnO nanomaterials were used to prepare piezoelectric nanogenerators and nanosensors with different structures to transform mechanical energy into Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. electrical energy [11–13]. However, current research on the mechanical/electrical perfor- This article is an open access article mance of ZnO is still insufﬁcient, and further exploration is still in demand. distributed under the terms and SEM and AFM are widely used in basic micro/nano operation and characterization, conditions of the Creative Commons research of micro/nano science and technology has been continuously innovated and Attribution (CC BY) license (https:// improved [14–16]. Nanostructures and nanomaterials with excellent mechanical/electrical creativecommons.org/licenses/by/ properties have been discovered continuously, which has an important impact on the 4.0/). development of life science, information and communication, new materials, and many

Micromachines 2021, 12, 248. https://doi.org/10.3390/mi12030248 https://www.mdpi.com/journal/micromachines Micromachines 2021, 12, 248 2 of 11

other fields [17]. AFM force measurement technology provides a technical platform for micro/nano structures mechanical properties characterization [18–20]. SEM is able to integrate MEMS equipment and carry out secondary development because of its large vacuum chamber [21]. Most of present mechanical/electrical characterization methods cannot simultaneously measure and observe nanomaterials, limiting work efficiency. The SEM-embedded nanomanipulator and AFM can simultaneously operate and characterize the performance of micro and nano samples. In this paper, ZnO nanomaterials, including nanorod and nanowire are characterized by AFM and nanomanipulator embedded in SEM respectively. Surface morphology and mechanical properties were observed by AFM. Electrical properties were characterized with nanomanipulator, which showed that the ZnO nanomaterial have cut-off characteristics and good schottky contact with the tungsten probes. Piezoelectric property was also measured. A two-tip approach for piezoelectric measurement was proposed, which is flexible and adaptable. Different with previous publications, piezoelectric properties of a single ZnO nanowire was measured instead of ZnO arrays, which is easy to operate and cost-effective. It is easier to control experimental variables. Experiment criteria for ZnO manipulation and characterization were also studied, such as acceleration voltage, operation duration, sample preparation.

2. Theoretical Modeling Analysis 2.1. Mechanical/Electrical Characterization Principles One-dimensional materials such as ZnO nanorods and ZnO nanowires have excellent physical properties due to the size effect, which is an ideal component for constructing nano devices. Research on mechanical properties of materials is helpful to understand internal structure and offer guidance for performance improvement. Nano indentation technology is a widely used method to measure mechanical properties of nanomaterials. Young’s modulus and rigidity are measured with AFM embedded in SEM. Nano indentation technology is based on elastic contact theory. Principle of this technique is to form a force-displacement curve by pressing down sample with a known type of probe and then lifting up the probe. The elastic-plastic properties of nanomaterials were recorded by force-displacement curve. Electrical characterization of ZnO nanorods were carried out by a nano manipulator embedded in SEM. Two tungsten probes serve as electrodes, which contacts with ZnO nanorod, establishing a conductive circuit together with a power supply. The equivalent Micromachines 2021, 12, x FOR PEERelectric REVIEW circuit of metal-semiconductor-metal structure is shown in the Figure1. There are3 of 11 two probe-ZnO metal semiconductor junctions in the loop. When probes get into contact with ZnO, the forbidden band of semiconductor at the interface bends, forming a Schottky barrier, whose presence leads to a high interfacial resistance [22].

(a) (b)

FigureFigure 1. Equivalent 1. Equivalent electric electric circuit circuit of metal-semiconductor-metal of metal-semiconductor-metal structure. structure. (a) Probe (a) Probe is marked is marked in blue. in When blue. semiconduc-When semicon- tor ZnOductor nanorod ZnO nanorod come into come contact into withcontact probe with electrode, probe electrode, a barrier a is barrier formed is at formed the contact at the point contact which point is calledwhich depletion is called depletion layer. (b) Three resistors in the circuit. 𝑅 and 𝑅 are resistance of the depletion layer, 𝑅 is resistance of the layer. (b) Three resistors in the circuit. RD1 and RD2 are resistance of the depletion layer, Rs is resistance of the ZnO nanorod. ZnO nanorod.

Electrical conductivity is an important parameter to measure the ability to transfer current. Electrical conductivity (σ) can be calculated by Equation (1). 1 L R= × (1) σ S where L is the distance between two contact points of probe and ZnO nanowire, S is the cross-sectional area of ZnO nanowires and R is resistance of ZnO nanowires.

2.2. Piezoelectric Properties Characterization Principles ZnO nanomaterials possess piezoelectric properties. Nanogenerator made of piezoelectric nanomaterials has become a hot research topic [23–25]. Principle of piezoelectric properties measurement, i.e., the two-probe strategy, is shown in Figure 2. Two tungsten probes would be connected to sample to form an electrical circuit. One probe holds the nanowire, another one is used to deform the nanowire. As shown in Fig- ure 2, when the probe is away from the sample, it would be electrically neutral. When the probe gradually moves to the right and touches ZnO nanomaterial, it exerts a force on the ZnO nanomaterial, which will deform. As ZnO nanomaterial have piezoelectric properties, deformation can generate piezoelectric electric field inside nanomaterial, thus generating a piezoelectric potential. The potential will be correlated with the deformation extent [26–30].

Figure 2. Principle of piezoelectric properties characterization. Yellow rectangle represents a single ZnO nanowire. ZnO nanowire deforms when pressed by probe which is marked in blue. Two probes contact ZnO nanowire to form an electrical circuit. A digital multimeter measures voltage generated by ZnO nanowire deformation.

The piezoelectric constant is a significant parameter that measures conversion of mechanical energy to electrical energy by ZnO. We defined a shear piezoelectric constant to

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(a) (b) Figure 1. Equivalent electric circuit of metal-semiconductor-metal structure. (a) Probe is marked in blue. When semiconductor ZnO nanorod come into contact with probe electrode, a barrier is formed at the contact point which is called depletion layer. (b) Three resistors in the circuit. 𝑅 and 𝑅 are resistance of the depletion layer, 𝑅 is resistance of the ZnO nanorod.

Electrical conductivity is an important parameter to measure the ability to transfer current. Electrical conductivity (σ) can be calculated by Equation (1). 1 L R= × (1) σ S Micromachines 2021, 12, 248 where L is the distance between two contact points of probe and ZnO nanowire, S is the3 of 11 cross-sectional area of ZnO nanowires and R is resistance of ZnO nanowires.

2.2. Piezoelectric Properties Characterization Principles Electrical conductivity is an important parameter to measure the ability to transfer current.ZnO nanomaterials Electrical conductivity possess piezoelectric (σ) can be pr calculatedoperties. byNanogenerator Equation (1). made of piezoelectric nanomaterials has become a hot research topic [23–25]. Principle of piezoelectric properties measurement, i.e., the two-probe strategy,1 L is shown in Figure 2. R = × (1) Two tungsten probes would be connected toσ sampleS to form an electrical circuit. One probe holds the nanowire, another one is used to deform the nanowire. As shown in Fig- where L is the distance between two contact points of probe and ZnO nanowire, S is the ure 2, when the probe is away from the sample, it would be electrically neutral. When the cross-sectional area of ZnO nanowires and R is resistance of ZnO nanowires. probe gradually moves to the right and touches ZnO nanomaterial, it exerts a force on the ZnO2.2. nanomaterial, Piezoelectric which Properties will Characterization deform. As Zn PrinciplesO nanomaterial have piezoelectric properties, deformation can generate piezoelectric electric field inside nanomaterial, thus gener- ZnO nanomaterials possess piezoelectric properties. Nanogenerator made of piezo- ating a piezoelectric potential. The potential will be correlated with the deformation extent electric nanomaterials has become a hot research topic [23–25]. Principle of piezoelectric [26–30]. properties measurement, i.e., the two-probe strategy, is shown in Figure2.

FigureFigure 2. Principle 2. Principle of piezoelect of piezoelectricric properties properties characterization. characterization. Yellow Yellow rectangle rectangle represents represents a single a single ZnO ZnOnanowire. nanowire. ZnO ZnO nanowire nanowire deforms deforms when when pre pressedssed by by probe probe which isis markedmarked in in blue. blue. Two Two probes probescontact contact ZnO ZnO nanowire nanowire to form to fo anrm electrical an electrical circuit. circuit. A digital A digi multimetertal multimeter measures measures voltage voltage generated generated by ZnO nanowire deformation. by ZnO nanowire deformation.

The piezoelectricTwo tungsten constant probes wouldis a significant be connected parameter to sample that measures to form an conversion electrical circuit. of me- One chanicalprobe energy holds to the electrical nanowire, energy another by ZnO. one We is used defined to deforma shear thepiezoelectric nanowire. constant As shown to in Figure2, when the probe is away from the sample, it would be electrically neutral. When the probe gradually moves to the right and touches ZnO nanomaterial, it exerts a force on the ZnO nanomaterial, which will deform. As ZnO nanomaterial have piezoelectric properties, deformation can generate piezoelectric electric field inside nanomaterial, thus generating a piezoelectric potential. The potential will be correlated with the deformation extent [26–30]. The piezoelectric constant is a significant parameter that measures conversion of mechanical energy to electrical energy by ZnO. We defined a shear piezoelectric constant Qshear to estimate piezoelectric capability of ZnO nanowire: dshear = F , where Qshear is the electric charge generated on two ends of the deformed nanowire, F is the force deform ZnO nanowire which is exerted by probe.

2.3. Contact Strategy between Probe and the Nanorod Object-probe contact strategy plays an important role in the experiment. For characterization, probe gets close to and then touch the sample. Probe pose is key to improve experimental efﬁciency and yield, as probe angle will affect operation efﬁciency, as shown in Figure3. Micromachines 2021, 12, x FOR PEER REVIEW 4 of 11

estimate piezoelectric capability of ZnO nanowire: d , where Qshear is the electric charge generated on two ends of the deformed nanowire, F is the force deform ZnO nanowire which is exerted by probe.

2.3. Contact Strategy between Probe and the Nanorod Object-probe contact strategy plays an important role in the experiment. For charac- Micromachines 2021, 12terization,, 248 probe gets close to and then touch the sample. Probe pose is key to improve 4 of 11 experimental efficiency and yield, as probe angle will affect operation efficiency, as shown in Figure 3.

FigureFigure 3. Force 3. Force interaction interaction between between probe probe and and nanoro nanorodsds with with different different interaction interaction angles. angles.

It is known thatIt is knownat microscale, that at microscale,van der Wa vanals derforce Waals is proportional force is proportional to contact to area, contact area, and and in vacuum,in vacuum, van der vanWaals der force Waals between force between any two anyobjects two is objects generally is generally attractive. attractive. Fig- Figure3 shows three types of contact strategies between the tip and the sample. Probe 1 touches the ure 3 shows three types of contact strategies between the tip and the sample. Probe 1 sample with a point, probe 2 has an acute angle with the sample, while probe 3 is nearly touches the sample with a point, probe 2 has an acute angle with the sample, while probe parallel with the sample. The van der Waals forces of probe 1 and probe 3 are the lowest 3 is nearly parallel with the sample. The van der Waals forces of probe 1 and probe 3 are and highest. In electrical properties characterization, probe touches the sample and both the lowest and highest. In electrical properties characterization, probe touches the sample get into contact, leaves the sample after measurement is over, and contact strategy 1 and and both get into contact, leaves the sample after measurement is over, and contact strat- 2 should be adopted. For sample pickup, probe and sample get into contact, the probe egy 1 and 2 should be adopted. For sample pickup, probe and sample get into contact, the takes the sample away, so contact strategy 3 should be used, to strength adhesion force probe takes the sample away, so contact strategy 3 should be used, to strength adhesion between the two. force between the two. 3. Materials and Equipment 3. Materials and Equipment Experimental setup is shown in Figure4. The nanomanipulator (Canada/Toronto ExperimentalNano Instrumentationsetup is shown Inc,in Figure NB202-SEM) 4. The and nanomanipulator AFM (Canada/Toronto (Canada/Toronto Nano Instrumentation Nano InstrumentationInc, EM-AFM) Inc, NB202-SEM) can be installed and in AFM the vacuum (Canada/Toronto chamber of Nano SEM Instrumenta- (SU3500, Hitachi, Japan). tion Inc, EM-AFM)The manipulator can be installed has a in coarse the vacuum and ﬁne chamber three-degree-of-freedom of SEM (SU3500, (DOF)Hitachi, mode Ja- that can be pan). The manipulatorswitched in has real a timecoarse according and fine to three-degree-of-freedom operational tasks. The probe (DOF) was mode ST-20-0.5 that from GGB can be switched in real time according to operational tasks.µ The probe was ST-20-0.5◦ from Micromachines 2021, 12, x FOR PEER REVIEWIndustries Inc, with a tip diameter of 0.5 m and taper of 10 . Original ZnO nanorod5 of 11 GGB Industries(diameter: Inc, with 100–2000 a tip diameter nm, length: of 0.5 μ 3–10m andµm) taper and of nanowire 10°. Original (diameter: ZnO nanorod 50–120 nm, length: (diameter: 100–20002–20 µm) nm, powder length: was 3–10 purchased μm) and fromnanowire Nanjing (diamete XFNANOr: 50–120 Materials nm, length: Tech Co., 2– Ltd, China. 20 μm) powder was purchased from Nanjing XFNANO Materials Tech Co., Ltd, China. For mechanical/electrical properties characterization of ZnO nanorods, 40 mg ZnO nanorod powder were mixed with 2 ml alcohol. A pipette was used to drop the solution onto a silicon wafer, which is attached to the corresponding sample stage after dried. For piezoelectric test, a tiny amount of nanowire powder was glued on a small piece of carbon tape, which is placed on one arm of the nanomanipulator, and characterized with the other two tips. According to abovementioned working principles, the mechanical/electrical/piezoelectric properties were measured and recorded.

Figure 4. Mechanical/electrical characterization experimentalexperimental apparatus. apparatus. On On the the left left is is the the electrical/piezoelectric electrical/piezoelectric character- charac- terizationization system system (The (The point point that that a probe a probe contacts contacts ZnO ZnO nanorods nanorods is called is called irradiation irradiation point, point, asred as red dotted dotted line line marks), marks), in the in themiddle middle is the is SEM,the SEM, and and on theon rightthe right is the is morphologythe morphology and and mechanical mechanical characterization characterization system. system.

4. ResultsFor mechanical/electrical and Discussions properties characterization of ZnO nanorods, 40 mg ZnO nanorodFigure powder 5 shows were the mixed random with distribution 2 ml alcohol. of observed A pipette nanorod was used and to dropnanowire the solution on the substrate. It can be seen from the figure that samples randomly distributed on the substrate. Some samples gather or pile up in three-dimensional space, resulting in aggregation, as red-highlighted in Figure 5. Correspondingly, samples cannot be directly contacted and interacted, i.e., not all samples are suitable for operation and characterization. Only clear and single nanorod/nanowire can be picked. Criteria of suitable ZnO nanorod/nanowire for mechanical/electrical characterization are defined as follows: it must exist independently without winding, stacking and contacting; nanomaterial and silicon wafer should be kept clean and without attachments.

(a) (b) Figure 5. ZnO nanorods and ZnO nanowires. (a) ZnO nanorod marked by blue dotted line is suitable for operation and observation and marked by red dotted line means stacked ZnO nanorods. (b) ZnO nanowires marked by red dotted line are sample for piezoelectric properties characterization.

SEM image scanning lacks operational depth information feedback. In experiments, probe manipulation relies on experience. In this paper, local scanning method is adopted to ensure high reliability of real-time visual feedback and accuracy of absolute probe positioning. Height information of probe and sample is confirmed by focusing. The sample was selected as reference and the probe was moved to touch sample by constantly adjusting nanomanipulator arm.

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onto a silicon wafer, which is attached to the corresponding sample stage after dried. For piezoelectric test, a tiny amount of nanowire powder was glued on a small piece of carbon Figure 4. Mechanical/electrical characterization experimental apparatus. On the left is the electrical/piezoelectric charac- tape, which is placed on one arm of the nanomanipulator, and characterized with the other terization system (The point that a probe contacts ZnO nanorods is called irradiation point, as red dotted line marks), in the middle is the SEM, and twoon the tips. right is the morphology and mechanical characterization system. According to abovementioned working principles, the mechanical/electrical/piezoelectric properties4. Results and were Discussions measured and recorded. 4. ResultsFigure and 5 shows Discussions the random distribution of observed nanorod and nanowire on the substrate. It can be seen from the figure that samples randomly distributed on the sub- Figure5 shows the random distribution of observed nanorod and nanowire on the strate. Some samples gather or pile up in three-dimensional space, resulting in aggrega- substrate. It can be seen from the ﬁgure that samples randomly distributed on the substrate. tion, as red-highlighted in Figure 5. Correspondingly, samples cannot be directly con- Some samples gather or pile up in three-dimensional space, resulting in aggregation, as tacted and interacted, i.e., not all samples are suitable for operation and characterization. red-highlighted in Figure5. Correspondingly, samples cannot be directly contacted and in- Only clear and single nanorod/nanowire can be picked. Criteria of suitable ZnO nano- teracted, i.e., not all samples are suitable for operation and characterization. Only clear and singlerod/nanowire nanorod/nanowire for mechanical/electrical can be picked. characterization Criteria of suitable are defined ZnO nanorod/nanowire as follows: it must forex- mechanical/electricalist independently without characterization winding, stacking are deﬁned and contacting; as follows: nanomaterial it must exist and independently silicon wa- withoutfer should winding, be kept stacking clean and and without contacting; attachments. nanomaterial and silicon wafer should be kept clean and without attachments.

(a) (b)

FigureFigure 5.5. ZnOZnO nanorodsnanorods andand ZnOZnO nanowires.nanowires. (a)) ZnOZnO nanorodnanorod markedmarked byby blueblue dotteddotted lineline isis suitablesuitable forfor operationoperation andand observationobservation andand markedmarked by by red red dotted dotted line line means means stacked stacked ZnO ZnO nanorods. nanorods. (b )(b ZnO) ZnO nanowires nanowires marked marked by redby red dotted dotted line line are sampleare sample for piezoelectricfor piezoelectric properties properties characterization. characterization.

SEM image scanning lacks operational depth information feedback. In experiments, probe manipulation relies on experience. In this paper, locallocal scanningscanning methodmethod isis adoptedadopted to ensure ensure high high reliability reliability of of real-time real-time visual visual feedback feedback and and accuracy accuracy of absolute of absolute probe probe po- positioning.sitioning. Height Height information information of of probe probe and and sample sample is is confirmed conﬁrmed by by focusing. The sample was selectedselected asas referencereference and and the the probe probe was was moved moved to to touch touch sample sample by by constantly constantly adjusting adjust- nanomanipulatoring nanomanipulator arm. arm.

4.1. Electron Beam Irradiation The point that a single probe contact with nanomaterial is called electron beam irradiation point, as shown in Figure4. Electron beam irradiation is also one of the factors affecting contact between probe and its object [31]. As the probe tip and ZnO nanomaterial were in contact, their adhesion force increases with the electron beam irradiation time, whose increase rate depends on electron beam current, in accordance with EBID (electron beam-induced deposition). Even after suspending the irradiation, the adhesion force caused by electron beam irradiation increases irreversibly. Experiment has proved that possibility of samples attaching to the tip increases with operation time, as shown in Figure6, so acceleration voltage of SEM and contact duration between probe and sample should be deﬁned tactfully. Micromachines 2021, 12, x FOR PEER REVIEW 6 of 11

4.1. Electron Beam Irradiation The point that a single probe contact with nanomaterial is called electron beam irradiation point, as shown in Figure 4. Electron beam irradiation is also one of the factors affecting contact between probe and its object [31]. As the probe tip and ZnO nanomaterial were in contact, their adhesion force increases with the electron beam irradiation time, whose increase rate depends on electron beam current, in accordance with EBID (electron beam-induced deposition). Even after suspending the irradiation, the adhesion force caused by electron beam irradiation increases irreversibly. Micromachines 2021, 12, 248 Experiment has proved that possibility of samples attaching to the tip increases 6with of 11 operation time, as shown in Figure 6, so acceleration voltage of SEM and contact duration between probe and sample should be defined tactfully.

FigureFigure 6. Release/adsorption probability probability vs. vs. irradiation irradiation time time between between probe probe and nanorod. Contact strategystrategy 3 3 was used. SEM SEM acceleration acceleration voltage voltage is is 5 5 kV. kV. Adsorption probability reached over 95% when contact time exceeds 4 minutes.

AsAs seen fromfrom thethe Figure Figure6, 6, when when contact contact strategy strategy 3 in 3 Figure in Figure3 was 3 used,was used, the nanowire- the nanowire-probeprobe contact contact area is considerable,area is considerable, when irradiation when irradiation time exceeds time 240exceeds s, ZnO 240 nanowire s, ZnO nan- was owirealmost was adsorbed almost to adsorbed the tip irreversibly. to the tip irreversibly. For piezoelectricity For piezoelectricity characterization, characterization, probe need probeto pick need up a to single pick ZnOup a single nanowire ZnO from nanowire tape which from tape adsorb which a large adsorb number a large of nanowires.number of nanowires.Accordingly, Accordingly, sample-probe sample-probe contact duration contact should duration last fourshould minutes last four at least. minutes at least. 4.2. Morphology and Mechanical Properties of ZnO Nanorods 4.2. Morphology and Mechanical Properties of ZnO Nanorods Mechanical properties of ZnO nanorods have a very important impact on their applica- Mechanical properties of ZnO nanorods have a very important impact on their ap- tions in micro/nano devices [32–34]. Therefore, it is of great signiﬁcance to characterize the Micromachines 2021, 12, x FOR PEERplications REVIEW in micro/nano devices [32–34]. Therefore, it is of great significance to character-7 of 11 mechanical properties of ZnO nanorods. Surface topography of a ZnO nanorod is shown izein Figurethe mechanical7a,b. Combining properties with of SEM, ZnO itnanoro is easyds. to Surface locate ZnO topography nanorod, of and a ZnO characterize nanorod itis shownwith AFM in Figure efﬁciently. 7a,b. Combining with SEM, it is easy to locate ZnO nanorod, and characterize it with AFM efficiently.

(a) (b)

FigureFigure 7. Morphology 7. Morphology and and mechanical mechanical properties properties of ZnO of ZnO nanorods nanorods in the in rangethe range of 2 ofµm 2 ×μm2 µ×m. 2 μ (am.) AFM(a) AFM is scanning is scanning a a singlesingle ZnO ZnO nanorod. nanorod. (b) Morphology(b) Morphology of an of observed an observed nanorod. nanorod.

ForFor selected selected ZnO ZnO nanorod nanorod in thisin this experiment, experiment, the the average average Young’s Young's modulus modulus and and springspring rate rate of ZnO of ZnO nanorods nanorods are 1.40are 1.40 MPa MPa and and 0.08 0.08 N/m, N/m, respectively. respectively.

4.3.4.3. Electrical Electrical Properties Properties of ZnO of ZnO Nanorods Nanorods AcceleratingAccelerating voltage voltage was was adjusted adjusted to 30 to kV30 whenkV when probes probes contact contact with with ZnO ZnO nanorod, nanorod, wherewhere strategy strategy 2 was 2 was used, used, and and irradiation irradiation time time was was set 15set minutes,15 minutes, which which was was used used to to strengthenstrengthen adhesion adhesion between between probe probe and and nanorod. nanoro Thisd. This operation operation can can effectively effectively reduce reduce contactcontact resistance resistance between between probe probe and and nanorods. nanorods. Contact Contact interaction interaction is shown is shown in Figure in Figure4. 4. Contact between probe tip and the end surface of ZnO nanorods is a typical metal semiconductor contact. Through nanomanipulator embedded in SEM, it is connected with external electrical test module to form a current circuit. The I-V characteristics of the probe tip and the contact barrier are measured and evaluated. Three groups of ZnO nanorods with different lengths were tested, as shown in Figure 8.

Figure 8. I-V curve of three groups of nanorods. Curve 1, curve 2 and curve 3 correspond to ZnO nanorods with length of 10 μm, 20 μm and 30 μm respectively, with same diameters of 2 μm. The three I-V curves are nearly symmetrical, which indicates that ZnO nanorods and probe form almost equal contact barriers between the forward bias junction and reverse bias junction.

When the Schottky barrier is in positive or reverse bias state, the depletion layer in ZnO nanorods increases with the gradual increase of bias voltage. When the critical bias

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(a) (b) Figure 7. Morphology and mechanical properties of ZnO nanorods in the range of 2 μm × 2 μm. (a) AFM is scanning a single ZnO nanorod. (b) Morphology of an observed nanorod.

For selected ZnO nanorod in this experiment, the average Young's modulus and spring rate of ZnO nanorods are 1.40 MPa and 0.08 N/m, respectively.

4.3. Electrical Properties of ZnO Nanorods Accelerating voltage was adjusted to 30 kV when probes contact with ZnO nanorod, Micromachines 2021, 12, 248 where strategy 2 was used, and irradiation time was set 15 minutes, which was used7 of to 11 strengthen adhesion between probe and nanorod. This operation can effectively reduce contact resistance between probe and nanorods. Contact interaction is shown in Figure 4. Contact between probe tip and the end surface of ZnO nanorods is a typical metal semiconductorContact between contact. probeThrough tip nanomanipula and the end surfacetor embedded of ZnO in nanorods SEM, it is is connected a typicalmetal with externalsemiconductor electrical contact. test module Through to form nanomanipulator a current circuit. embedded The I-V incharacteristics SEM, it is connected of the probe with tipexternal and the electrical contact test barrier module are tomeasured form a current and evaluated. circuit. The Three I-V characteristicsgroups of ZnO of nanorods the probe withtip and different the contact lengths barrier were aretested, measured as shown and in evaluated. Figure 8. Three groups of ZnO nanorods with different lengths were tested, as shown in Figure8.

FigureFigure 8. 8. I-VI-V curve curve of of three three groups groups of of nanorods. nanorods. Curv Curvee 1, 1, curve curve 2 2 and and curve curve 3 3 correspond correspond to to ZnO ZnO nanorodsnanorods with with length length of of 10 10 μµm,m, 20 20 μµmm and and 30 30 μµmm respectively, respectively, with with same same diameters diameters of of 2 2 μµm.m. The The threethree I-V I-V curves curves are are nearly nearly symme symmetrical,trical, which which indicates indicates that that ZnO ZnO nanorods nanorods and and probe probe form form almost almost equal contact barriers between the forward bias junction and reverse bias junction. equal contact barriers between the forward bias junction and reverse bias junction.

WhenWhen the the Schottky Schottky barrier barrier is is in in positive positive or or reverse reverse bias bias state, state, the the depletion depletion layer layer in in ZnOZnO nanorods nanorods increases increases with with the the gradual gradual increase increase of of bias bias voltage. voltage. When When the the critical critical bias bias is reached, the conducting channels in the nanorods will be completely exhausted. This depletion behavior is similar to clamping of conductive channels in a ﬁeld effect transistor. And the current will show a cut-off characteristic after clamping. The experiment shows that the current will not change when the forward voltage reaches 4.5 V and the reverse voltage −3.8 V. The results show that ZnO nanorods and tungsten probes form a good back-to-back Schottky contact and show a cut-off characteristic.

4.4. Electrical Conductivity of ZnO Nanowires Electrical conductivity of ZnO nanowires are measured and evaluated by nanomanipulator embedded in SEM. Three groups of ZnO nanowires with different lengths were tested, as shown in Figure9. As can be seen in Figure9d, the blue and orange bars represent the resistance and electrical conductivity of ZnO nanowires, respectively. Resistance increases with measured length, consistent with theoretical predictions. But the electrical conductivity also increased with the increase of measured length, although the change is not signiﬁcant. This may be due to the poor heat dissipation in the vacuum chamber of SEM. The current equipment can not completely exclude the inﬂuence of temperature, which can be improved by setting thermocouple inside SEM or other ways in future research. Finally, the average electrical conductivity of ZnO nanowires is calculated as 480.9 S/m. Micromachines 2021, 12, x FOR PEER REVIEW 8 of 11

is reached, the conducting channels in the nanorods will be completely exhausted. This depletion behavior is similar to clamping of conductive channels in a field effect transistor. And the current will show a cut-off characteristic after clamping. The experiment shows that the current will not change when the forward voltage reaches 4.5 V and the reverse voltage −3.8 V. The results show that ZnO nanorods and tungsten probes form a good back-to-back Schottky contact and show a cut-off characteristic.

4.4. Electrical Conductivity of ZnO Nanowires Electrical conductivity of ZnO nanowires are measured and evaluated by nanoma- Micromachines 2021, 12, 248 nipulator embedded in SEM. Three groups of ZnO nanowires with different lengths were8 of 11 tested, as shown in Figure 9.

(a) (b)

FigureFigure 9. 9.ElectricalElectrical conducti conductivityvity measurement. measurement. (a (a––cc) )Lengths Lengths of of ZnO ZnO nanowires nanowires are 20.9, 30.9,30.9, 40.940.9 µμmm withwith samesame diameters diame- tersof 1.06of 1.06µm. μm. (d )( Blued) Blue bars bars represent represent resistance resistance and and red red bars bars represent represent electrical electrical conductivity conductivity correspond correspond to ZnO to ZnO nanowires nan- owireswith lengthwith length of 20.9 ofµ 20.9m, 30.9 μm,µ 30.9m and μm 40.9 andµ m.40.9 μm.

4.5.As Piezoelectric can be seen Properties in Figure of ZnO9d, the Nanomaterials blue and orange bars represent the resistance and electricalDue conductivity to the small of size ZnO and nanowires, ability to re supplyspectively. energy Resistance to micro/nano increases devices, with meas- metal– uredoxide-based length, consistent piezoelectric with energy theoretical harvesters predictions. have recently But the gainedelectrical an conductivity enormous amount also increasedof attention. with the Here increase piezoelectric of measured properties length, of aalthough singleZnO the change nanowire is not is measuredsignificant. by Thisnanomanipulator may be due to the embedded poor heat in dissipatio SEM withn ain two-tip the vacuum strategy. chamber Experimental of SEM. The apparatus current is equipmentshown in thecan Figurenot completely4, and experimental exclude the process influence are shownof temperature, in Figure 10whicha–d. can be im- provedThe by setting two-tip thermocouple strategy is that, inside a clear, SEM singleor other ZnO ways nanowire in future on research. the carbon Finally, tape the was averageselected, electrical probe IIconductivity got close to of the ZnO nanowire nanowires below is itscalculated tip, as the as nanowire480.9 S/m. moved slightly with the probe, the two got into contact. The probe II position was adjusted to increase 4.5.contact Piezoelectric area until Properties it exceeds of ZnO 2 µ m.Nanomaterials The contact was exposed with e-beam for at least 4 min. AccordingDue to the to Figuresmall size6, probe and IIability and nanowireto supply willenergy be fixedto micro/nano together firmly.devices, As metal–ox- diameter ide-basedof ZnO nanowirepiezoelectric is lower energy than harvesters that of ZnOhave nanorod,recently gained it is easier an enormous to separate amount from tape, of increasing experiment efficiency. The nanowire was then taken away from the tape by probe II. Probe I got into contact with free end the nanowire with the same strategy with probe II. E-beam is turned off to eliminate electromagnetic noise. Probe I went downward a specific distance with the nanowire free end, called deformation, the transient maximum voltage was recorded, as

shown in Figure 10. As deformation increases, the voltage will also increase. Deformation ranges from 0 to 6 µm. Accordingly, the output voltage maximum varies from 0 to 0.74 mV,in accordance with previous literature [12]. The shear piezoelectric constant is calculated to be around 9.91 × 10−8 C/N [35]. It is much higher than previous literature (26 pC/N) [36]. The discrepancy may be results of different crystal direction or deformation range. More accurate and reasonable piezoelectric constant will be our future work, with the benefit of more accurate microforce sensors. More accurate and reasonable piezoelectric constant will be our future work, with the benefit of more accurate microforce sensors. Micromachines 2021, 12, x FOR PEER REVIEW 9 of 11

attention. Here piezoelectric properties of a single ZnO nanowire is measured by nano- Micromachines 2021, 12, 248 manipulator embedded in SEM with a two-tip strategy. Experimental apparatus is shown9 of 11 in the Figure 4, and experimental process are shown in Figure 10a–d.

(a) (b) (c)

(d) (e)

FigureFigure 10. 10. PiezoelectricPiezoelectric characteristic measurement.measurement. ( a(a)) Probe Probe IIⅡ getsgets close close to to ZnO ZnO nanowire. nanowire. (b ()b Probe) Probe IIⅡ touchestouches and and picks picks up upZnO ZnO nanowire. nanowire. (c) Probe(c) Probe II takesⅡtakes the nanowirethe nanowire away away from from the tape. the tape. (d) Probe (d) Probe I deformsⅠdeforms the nanowire. the nanowire. (e) Piezoelectric (e) Piezoelectric voltage voltagevs. deformation. vs. deformation. Deformation Deformation ranges from ranges 0 to from 6 µm. 0 to Accordingly, 6 μm. Accordingly, the output the voltage output varies voltage from varies 0 to 0.74from mV. 0 ton 0.74= 15. mV. n = 15. Our work provides theoretical basis for the application of ZnO nanowires, such as nanogenerators.The two-tip Thestrategy two-tip is that, strategy a clear, is simple single and ZnO efficient, nanowire and on the the probes carbon are tape free fromwas selected,abrasion. probe Improving Ⅱ got close its repeatability, to the nanowire adaptability below its and tip, reliability as the nanowire is our future moved work. slightly with the probe, the two got into contact. The probe Ⅱ position was adjusted to increase contact5. Conclusions area until it exceeds 2 μm. The contact was exposed with e-beam for at least 4 min. AccordingPrecise to operation/characterization Figure 6, probe Ⅱ and nanowire of nanomaterials will be fixed is stilltogether one of firmly. the most As importantdiameter ofchallenges ZnO nanowire in the is construction lower than ofthat micro/nano of ZnO nanorod, scale components. it is easier to Mechanical/electrical separate from tape, increasingproperties experiment characterization efficiency. of ZnO nano-materials relies on nanomanipulator and AFM embeddedThe nanowire in SEM. was Surface then morphology taken away andfrom mechanical the tape by properties probe Ⅱ. Probe wereobserved Ⅰ got intoby contact AFM. withElectrical free end properties the nanowire were characterizedwith the same with strategy nanomanipulator, with probe Ⅱ. E-beam which showed is turned that offthe to eliminateZnO nanomaterial electromagnetic have cut-off noise. characteristics Probe Ⅰ went downward and good schottky a specific contact distance with with the tungstenthe nan- owireprobes. free Piezoelectric end, called property deformation, was also the measured, transient and maximum the two-tip voltage strategy was was recorded, convenient, as shownefficient in andFigure easy 10. to As expand deformation to other incr materials.eases, the Our voltage work will provides also increase. useful references for nanomaterialDeformation characterization ranges from 0 and to 6 also μm. theoretical Accordingly, basis the for output nanomaterials voltage maximum application. varies from 0 to 0.74 mV, in accordance with previous literature [12]. The shear piezoelectric constantAuthor Contributions: is calculated Conceptualization,to be around 9.91 M.L., 10 H.O., C/N T.S. [35]. and It W.S.; is much methodology, higher than M.L.; previous software, literatureK.C.; validation, (26 pC/N) M.L., [36]. W.S. The and discrepancy K.C.; formalanalysis, may be J.C.results and of J.R.; different investigation, crystal X.Q.; direction resources, or deformationM.L.; data curation, range. W.S.;More writing—original accurate and reasonable draft preparation, piezoelectric W.S.; constant writing—review will be andour editing,future M.L.; visualization, W.D.; supervision, L.M.; project administration, Z.C.; funding acquisition, M.L., work, with the benefit of more accurate microforce sensors. More accurate and reasonable H.O. and T.S. All authors have read and agreed to the published version of the manuscript. piezoelectric constant will be our future work, with the benefit of more accurate micro- forceFunding: sensors.This research was funded by the National Natural Science Foundation of China (No. 51205245, 61573236); the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, and Shanghai Science and Technology Committee of China (No. 14JC1491500). Conflicts of Interest: The authors declare no conflict of interest.

Micromachines 2021, 12, 248 10 of 11

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