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n-ZnO heterojunction
Applied Surface Science 257 (2011) 4919–4922
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
AFM analysis of piezoelectric nanogenerator based on n+ -diamond/n-ZnO heterojunction Zhengzheng Shao a,b,∗ , Liaoyong Wen b , Dongmin Wu b , Xueao Zhang a , Shengli Chang a , Shiqiao Qin a a b
Center of Materials Science, College of Science, National University of Defense Technology, 49, Yanwachi Street, 410073 Changsha, China i-Lab, Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, 398, Ruoshui Road, 215125 Suzhou, China
a r t i c l e
i n f o
Article history: Received 30 July 2010 Received in revised form 7 December 2010 Accepted 28 December 2010 Available online 4 January 2011 Keywords: ZnO nanorod Piezoelectric nanogenerator Nitrogen-doped diamond Heterojunction AFM
a b s t r a c t We have demonstrated a high performance piezoelectric nanogenerator by scanning a diamond-coated conductive tip on ZnO nanorod arrays in an AFM system with contact-mode. About 95% ZnO nanorods generate piezoelectric current due to the excellent mechanical and electrical properties of the tip. The tip’s nitrogen-doped diamond coating is the key factor to maintain effective physical contact and electrical contact to ZnO nanorods, leading to efficient piezoelectric generation. Rectifying n+ –n heterojunction is formed when the nitrogen-doped diamond tip contacted with a ZnO nanorod, which plays an important role in accumulating and releasing piezoelectric charges of the piezoelectric nanogenerator. Our research indicates that conductive diamond film is an ideal electrode for this type of piezoelectric nanogenerator.
1. Introduction One-dimensional zinc oxide (ZnO) nanostructures are very important II–VI group semiconducting materials with unique electrical, optoelectronic and piezoelectric properties. A variety of nanodevices based on one-dimensional ZnO nanostructures have been exploited [1–5]. One of the most attractive properties of ZnO nanorod is the coupling of piezoelectric and semiconductor properties, which enables novel applications as piezoelectric nanogenerators [6], piezoelectric gated diodes [7], piezoelectric field-effect transistors [8,9] and piezoelectric strain sensors [10]. Among them, piezoelectric nanogenerator has the potential of converting mechanical energy into electrical energy in nanoscale by bending a ZnO nanorod [11]. Atomic force microscopy (AFM) is a versatile tool for studying and manipulating nanoscopic systems [12–14]. In order to study the current generating processes in a single ZnO nanorod, AFM has been introduced to bend the ZnO nanorod and to monitor the current signal using a Pt-coated conductive tip [11]. As a piezoelectric material, the bending ZnO nanorod creates a strain field and charge separation across itself. And the rectifying characteristic of the Schottky barrier formed between the Pt tip and the ZnO nanorod
∗ Corresponding author at: Center of Materials Science, College of Science, National University of Defense Technology, 49, Yanwachi Street, 410073 Changsha, China. Tel.: +86 0731 84573294; fax: +86 0731 84573293. E-mail address: [email protected] (Z. Shao). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.12.147
© 2011 Elsevier B.V. All rights reserved.
leads to charge accumulation and release. By replacing the tip with a zigzag electrode coated with Pt film, a direct current piezoelectric nanogenerator was assembled [15]. However, the metal (usually is Pt or Au) electrode of the piezoelectric nanogenerator will be worn out easily, resulting in poor electrical conductivity and piezoelectric current output. As a substitute for the Pt tip, the diamond-coated conductive tip has good electrical conductivity and anti-wear properties [16], which lead to its widely application in conductive AFM technology. In this paper, we present a prototype of piezoelectric nanogenerator based on n+ -diamond/n-ZnO heterojunction, by contact scanning a diamond-coated conductive tip on ZnO nanorod arrays. Around 95% ZnO nanorods generate piezoelectric current, which indicates the conductive diamond film is an ideal electrode for the piezoelectric nanogenerator. The discharge details of the ZnO nanogenerator and the rectify property of n+ -diamond/n-ZnO heterojunction were analyzed. 2. Experimental details The film of wurtzite ZnO nanorod arrays on silicon wafer was synthesized by two-step wet-chemical method [17,18]. Firstly, a silicon wafer was rinsed three times in ethanol solution containing ZnO nanocrystals, which were prepared by mixing 0.03 M NaOH solution with 0.01 M zinc acetate dehydrate solution in ethanol, and stirring for 2 h at 60 ◦ C. After each rinse, the wafer was annealed at 150 ◦ C for 0.5 h to ensure good adhesion of ZnO nanocrystals to the wafer surface. Then, hydrothermal ZnO growth was carried out by
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Fig. 2. Topography (a) and current mapping (b) captured by the AFM system when the nitrogen-doped diamond tip contact-scanning over the ZnO nanorod arrays, with a scanning area of 5 m × 5 m and a scanning frequency of 1 Hz.
electrical signal were recorded by the AFM system simultaneously.
Fig. 1. (a) SEM image of the ZnO nanorod arrays with an average diameter of 70 nm and length of 2 m. (b) Schematic diagram showing the experimental design of piezoelectric nanogenerator in AFM system.
immersion the wafer upside-down in the nutrient solution, which was composed of 0.05 M zinc nitrate hydrate and 0.05 M hexamethylenetetramine at 95 ◦ C for 3 h. The wafer was then removed from the solution, rinsed with deionized water and dried. Scanning electron microscope (SEM) image Fig. 1(a) shows the wafer is coated with a highly uniform and dense array of ZnO nanorods, with an average diameter of 70 nm and length of 2 m. Piezoelectric generating characterization of ZnO nanorod was carried out by Veeco Multimode AFM system with an NT-MDT DSP11 diamond-coated conductive tip. The diamond coating is heavily doped with nitrogen for better electric conduction, and the carrier concentration is about 1020 cm−3 . The tip curvature radius is about 60 nm. Fig. 1(b) shows the scheme of piezoelectric generating test of the ZnO nanorod arrays in the AFM system. To start with, the film of the ZnO nanorod arrays was electric connected to the current sensing module of the AFM system using silver paste, and a complete current loop was formed when the diamond tip scanning on the ZnO nanorod arrays in contact mode. The surface topography of the ZnO nanorods and the
3. Results and discussion Fig. 2 shows the topography (a) and current signals (b) recorded simultaneously when the diamond tip scanning over the aligned ZnO nanorod arrays. The topographic image of ZnO nanorod arrays is distorted because the ZnO nanorod can be easily bent by the scanning tip. As a result of the piezoelectric effect, current signals were generated from the bent ZnO nanorods, and recorded by the current sensing module. From the 2D current image Fig. 2(b), we calculated the density of the current pulse is about 41/m2 and the highest peak is about 35 pA. The density of ZnO nanorods is about 43/m2 from the SEM image, which means about 95% ZnO nanorods generate current signals when scanned by the diamond tip. For comparison only about 40% ZnO nanorods could generate piezoelectric current signals when scanned by the Pt tip [11]. The outstanding performance of the diamond tip is related to the diamond coating’s unique properties. First of all, the diamond coating has high wear resistance, so that the tip will not breakdown in contact-scanning, and good mechanical contact with ZnO nanorods can be achieved. Moreover, the diamond coating is nitrogen doped for good electrical conductivity [19]. Those two outstanding properties make the diamond-coated tip the first choice in the conductive AFM technology.
Z. Shao et al. / Applied Surface Science 257 (2011) 4919–4922
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Fig. 3. (a) Relationships between the topography and the current output in a single scanning line extracting from Fig. 2. The upper line is about ZnO nanorods’ topography and the lower line is about the current signal. (b) An enlargement of the dotted line frame in Fig. 3(a) shows the detailed relationship between the topography and current output. (c) A series of schematic processes about the diamond tip scanning over a single ZnO nanorod according to Fig. 3(b).
In order to analyzing the relationships between the topography of the ZnO nanorod and the detected current signal, two corresponding scanning lines were selected from the topography image and current image (Fig. 2). As shown in Fig. 3(a), the line upside is the topography of ZnO nanorods, and the line downside is the output current signals. As we can see that a corresponding current discharge appears with a topographical peak. Fig. 3(b) is an enlargement of the dotted line frame in Fig. 3(a), which shows detailed relationships between the topography and current output. According to Fig. 3(b), we can divide the processes of a current discharge into three steps (Fig. 3(c)). Firstly, the diamond tip begins to contact with the sidewall of the ZnO nanorod at location (a) and applies a lateral force to bend it. The tip is going upwards at the same time. No current signal is detected in this process. Next, the tip ascents to the highest point (b) corresponding to the top end of the ZnO nanorod and current signal is detected, the current signal increases in intensity at first and then decreases in this process from (b) to (d), the largest current IMax is detected at location (c). The applied force from the tip can be divided into a downward pressure force FP and a frictional force Ff parallel to the nanorod’s end face. Finally, the tip separates from the nanorod and descends. In this process, on current signal is detected. As shown in Fig. 4(a), the current–voltage (I–V) characteristics curve of the diamond tip/ZnO nanorod heterojunction reveals an obvious rectifying property. The I–V tests about diamond tip/Au film contact and Ag paste/ZnO film contact show ohmic electrical conductive characteristics (the inset figure in Fig. 4(a)). So that we confirm that the rectifying feature is caused by the heterojunction of diamond tip/ZnO nanorod, relating to the potential barrier at the junction interface. Diamond is a wide band-gap (Eg = 5.47 eV) semiconductor with an electron affinity ≈ 0.5 eV [20]. When heavily doped with nitrogen, diamond has an electronic characteristics as n+ -type [21]. On the other hand, ZnO (Eg = 3.37 eV) is a semiconductor with an electron affinity x eV [22]. And the native ZnO behaves as an n-type semiconductor because of Zn interstitial [23]. As a result, the contact between nitrogen-doped diamond and ZnO is an n+ –n heterojunction, in which only the majority carriers (electrons) impact the space-charge region. Fig. 4(b) shows the energy
band diagram of n+ -diamond/n-ZnO heterojunction. The potential barrier of the conductive band near the interface is the major factor to effect the electrons’ motion under a bias voltage. Because the dopant concentration in diamond tip is much higher than in ZnO
Fig. 4. (a) I–V characteristics of diamond tip and ZnO nanorod contact, which shows an obvious rectifying property. The inset figure shows the diamond tip-Au film contact and Ag paste-ZnO film contact are both ohmic contacts. (b) Energy band diagram of the n+ -diamond/n-ZnO heterojunction.
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nanorod, most of the potential drop is carried by the ZnO, leading to a rectifying n+ -diamond/n-ZnO heterojunction [24]. The mechanism proposed previously for the Pt tip/ZnO nanorod based piezoelectric nanogenerator applies for the present case after a few modifications [11,25]. As the diamond tip started to deflect the ZnO nanorod, a positive potential is produced at the stretched side of the nanorod, while a negative potential is induced at the compressed side [26,27]. Because of the redistribution of carriers in the nanorod under the piezoelectric field, the positive potential is seriously screened by the carriers, whereas the negative potential is well preserved [28]. The potential of the diamond tip is zero. No current signals were observed due to the presence of the reversebiased heterojunction barrier of diamond tip/ZnO nanorod contact. When the diamond tip goes up to the top end of the nanorod, a current is produced because of the presence of the forward-biased heterojunction barrier. At the same time, the piezoelectric charges are released from the ZnO nanorod as a formation of current discharge. After the diamond tip separates from the ZnO nanorod, the current discharge is finished. 4. Conclusion In summary, we have demonstrated a high performance piezoelectric nanogenerator using diamond tip contact-scanning ZnO nanorod arrays in AFM system. Excellent wear and conductive properties are the key factors to maintain effective physical contact and electrical contact. Rectifying n+ –n heterojunction is established when the nitrogen-doped diamond tip contacts with the native n-typed ZnO nanorod, and plays an important role in accumulating and releasing piezoelectric potential of the nanogenerator. Our works shows that conductive diamond film is an ideal electrode for the piezoelectric nanogenerator. Acknowledgement This work was supported by the National High Technology Research and Development Program of China (Grant No. 2009AA01Z114). References [1] S. Yodyingyong, Q. Zhang, K. Park, C.S. Dandeneau, X. Zhou, D. Triampo, G. Cao, ZnO nanoparticles and nanowire array hybrid photoanodes for dye-sensitized solar cells, Appl. Phys. Lett. 96 (2010) 73115. [2] F.S. Chiena, C. Wanga, Y. Chana, H. Linb, M. Chenb, R. Wu, Fast-response ozone sensor with ZnO nanorods grown by chemical vapor deposition, Sens. Actuators B 144 (2010) 120–125. [3] L.W. Ji, S.M. Peng, Y.K. Su, S.J. Young, C.Z. Wu, W.B. Cheng, Ultraviolet photodetectors based on selectively grown ZnO nanorod arrays, Appl. Phys. Lett. 94 (2009) 203106.
[4] R.N. Gayen, S. Dalui, A. Rajaram, A.K. Pal, Modulation of field emission properties of vertically aligned ZnO nanorods with aspect ratio and number density, Appl. Surf. Sci. 255 (2009) 4902–4906. [5] W. Hong, S. Song, D. Hwang, S. Kwon, G. Jo, S. Park, T. Lee, Effects of surface roughness on the electrical characteristics of ZnO nanowire field effect transistors, Appl. Surf. Sci. 254 (2008) 7559–7564. [6] S.S. Lin, J.H. Song, Y.F. Lu, Z.L. Wang, Identifying individual n- and p-type ZnO nanowires by the output voltage sign of piezoelectric nanogenerator, Nanotechnology 20 (2009) 365703. [7] H.Jr. He, C.L. Hsin, J. Liu, L.J. Chen, Z.L. Wang, Piezoelectric gated diode of a single ZnO nanowire, Adv. Mater. 19 (2007) 781–784. [8] C.S. Lao, Q. Kuang, Z.L. Wang, M. Park, Y.L. Deng, Polymer functionalized piezoelectric-FET as humidity/chemical nanosensors, Appl. Phys. Lett. 90 (2007) 262103–262107. [9] S.S. Kwon, W.K. Hong, G. Jo, J. Maeng, T.W. Kim, S. Song, T. Lee, Piezoelectric effect on the electronic transport characteristics of ZnO nanowire field-effect transistors on bent flexible substrates, Adv. Mater. 20 (2008) 4557–4562. [10] J. Zhou, Y.D. Gu, P. Fei, W.J. Mai, Y.F. Gao, R.S. Yang, G. Bao, Z.L. Wang, Flexible peizotronic strain sensor, NanoLetters 8 (2008) 3035–3040. [11] Z.L. Wang, J.H. Song, Piezoelectric nanogenerators based on zinc oxide nanowire arrays, Science 312 (2006) 242–246. [12] J.A. Kellar, J.M.P. Alaboson, Q.H. Wang, M.C. Hersam, Identifying and characterizing epitaxial graphene domains on partially graphitized SiC, 0001. . . surfaces using scanning probe microscopy, Appl. Phys. Lett. 96 (2010) 143103. [13] H. Kaczmarek, H. Chaberska, AFM and XPS study of UV-irradiated poly(methyl methacrylate) films on glass and aluminum support, Appl. Surf. Sci. 255 (2009) 6729–6735. [14] M. Grodzicki, S. Smolarek, P. Mazur, S. Zuber, A. Ciszewski, Characterization of Cr/6H–SiC(0 0 0 1) nano-contacts by current-sensing AFM, Appl. Surf. Sci. 256 (2009) 1014–1018. [15] X.D. Wang, J.H. Song, J. Liu, Z.L. Wang, Direct-current nanogenerator driven by ultrasonic waves, Science 316 (2007) 102–105. [16] K.H. Chung, D.E. Kim, Wear characteristics of diamond-coated atomic force microscope probe, Ultramicroscopy 108 (2007) 1–10. [17] L.E. Greene, M. Law, J. Goldberger, F.J.J.C. Kim, Y. Zhang, R.J. Saykally, P. Yang, Low-temperature wafer-scale production of ZnO nanowire arrays, Angew. Chem. Int. Ed. 42 (2003) 3031–3034. [18] L. Vayssieres, Growth of arrayed nanorods and nanowires of ZnO from aqueous solution, Adv. Mater. 15 (2003) 464–466. [19] H.S. Huang, H.M. Cheng, L.J. Lin, Coating tips used in electrical scanning probe microscopy with W and AuPd, Appl. Surf. Sci. 252 (2005) 2085–2091. [20] J.B. Cui, R. Graupner, J. Ristein, L. Ley, Electron affinity and band bending of single crystal diamond (1 1 1) surface, Diam. Relat. Mater. 8 (1999) 748–753. [21] R.G. Farrer, On the substitutional nitrogen donor in diamond, Solid State Commun. 7 (1969) 685–688. [22] D.C. Look, G.C. Farlow, P. Reunchan, S. Limpijumnong, S.B. Zhang, K. Nordlund, Evidence for native-defect donors in n-type ZnO, Phys. Rev. Lett. 95 (2005) 225502–225504. [23] P.S. Xu, Y.M. Sun, C.S. Shi, X.F. Qiang, P.H. Bin, Native point defect states in ZnO, Chin. Phys. Lett. 18 (2001) 1252–1253. [24] B.L. Sharma, PurohitF R.K., Semiconductor Heterojunction, Pergamon Press, Oxford, 1974. [25] P.X. Gao, J.H. Song, J. Liu, Z.L. Wang, Nanowire piezoelectric nanogenerators on plastic substrates as flexible power sources for nanodevices, Adv. Mater. 19 (2007) 67–72. [26] Y.F. Gao, Z.L. Wang, Electrostatic potential in a bent piezoelectric nanowire. The fundamental theory of nanogenerator and nanopiezotronics, NanoLetters 7 (2007) 2499–2505. [27] Z.Z. Shao, L.Y. Wen, D.M. Wu, X.F. Wang, X.A. Zhang, S.L. Chang, A continuum model of piezoelectric potential generated in a bent ZnO nanorod, J. Phys. D: Appl. Phys. 43 (2010) 245403. [28] Z. Shao, L. Wen, D. Wu, X. Zhang, S. Chang, Influence of carrier concentration on piezoelectric potential in a bent ZnO nanorod, J. Appl. Phys. 108 (2010) 1.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
AFM analysis of piezoelectric nanogenerator based on n+ -diamond/n-ZnO heterojunction Zhengzheng Shao a,b,∗ , Liaoyong Wen b , Dongmin Wu b , Xueao Zhang a , Shengli Chang a , Shiqiao Qin a a b
Center of Materials Science, College of Science, National University of Defense Technology, 49, Yanwachi Street, 410073 Changsha, China i-Lab, Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, 398, Ruoshui Road, 215125 Suzhou, China
a r t i c l e
i n f o
Article history: Received 30 July 2010 Received in revised form 7 December 2010 Accepted 28 December 2010 Available online 4 January 2011 Keywords: ZnO nanorod Piezoelectric nanogenerator Nitrogen-doped diamond Heterojunction AFM
a b s t r a c t We have demonstrated a high performance piezoelectric nanogenerator by scanning a diamond-coated conductive tip on ZnO nanorod arrays in an AFM system with contact-mode. About 95% ZnO nanorods generate piezoelectric current due to the excellent mechanical and electrical properties of the tip. The tip’s nitrogen-doped diamond coating is the key factor to maintain effective physical contact and electrical contact to ZnO nanorods, leading to efficient piezoelectric generation. Rectifying n+ –n heterojunction is formed when the nitrogen-doped diamond tip contacted with a ZnO nanorod, which plays an important role in accumulating and releasing piezoelectric charges of the piezoelectric nanogenerator. Our research indicates that conductive diamond film is an ideal electrode for this type of piezoelectric nanogenerator.
1. Introduction One-dimensional zinc oxide (ZnO) nanostructures are very important II–VI group semiconducting materials with unique electrical, optoelectronic and piezoelectric properties. A variety of nanodevices based on one-dimensional ZnO nanostructures have been exploited [1–5]. One of the most attractive properties of ZnO nanorod is the coupling of piezoelectric and semiconductor properties, which enables novel applications as piezoelectric nanogenerators [6], piezoelectric gated diodes [7], piezoelectric field-effect transistors [8,9] and piezoelectric strain sensors [10]. Among them, piezoelectric nanogenerator has the potential of converting mechanical energy into electrical energy in nanoscale by bending a ZnO nanorod [11]. Atomic force microscopy (AFM) is a versatile tool for studying and manipulating nanoscopic systems [12–14]. In order to study the current generating processes in a single ZnO nanorod, AFM has been introduced to bend the ZnO nanorod and to monitor the current signal using a Pt-coated conductive tip [11]. As a piezoelectric material, the bending ZnO nanorod creates a strain field and charge separation across itself. And the rectifying characteristic of the Schottky barrier formed between the Pt tip and the ZnO nanorod
∗ Corresponding author at: Center of Materials Science, College of Science, National University of Defense Technology, 49, Yanwachi Street, 410073 Changsha, China. Tel.: +86 0731 84573294; fax: +86 0731 84573293. E-mail address: [email protected] (Z. Shao). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.12.147
© 2011 Elsevier B.V. All rights reserved.
leads to charge accumulation and release. By replacing the tip with a zigzag electrode coated with Pt film, a direct current piezoelectric nanogenerator was assembled [15]. However, the metal (usually is Pt or Au) electrode of the piezoelectric nanogenerator will be worn out easily, resulting in poor electrical conductivity and piezoelectric current output. As a substitute for the Pt tip, the diamond-coated conductive tip has good electrical conductivity and anti-wear properties [16], which lead to its widely application in conductive AFM technology. In this paper, we present a prototype of piezoelectric nanogenerator based on n+ -diamond/n-ZnO heterojunction, by contact scanning a diamond-coated conductive tip on ZnO nanorod arrays. Around 95% ZnO nanorods generate piezoelectric current, which indicates the conductive diamond film is an ideal electrode for the piezoelectric nanogenerator. The discharge details of the ZnO nanogenerator and the rectify property of n+ -diamond/n-ZnO heterojunction were analyzed. 2. Experimental details The film of wurtzite ZnO nanorod arrays on silicon wafer was synthesized by two-step wet-chemical method [17,18]. Firstly, a silicon wafer was rinsed three times in ethanol solution containing ZnO nanocrystals, which were prepared by mixing 0.03 M NaOH solution with 0.01 M zinc acetate dehydrate solution in ethanol, and stirring for 2 h at 60 ◦ C. After each rinse, the wafer was annealed at 150 ◦ C for 0.5 h to ensure good adhesion of ZnO nanocrystals to the wafer surface. Then, hydrothermal ZnO growth was carried out by
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Fig. 2. Topography (a) and current mapping (b) captured by the AFM system when the nitrogen-doped diamond tip contact-scanning over the ZnO nanorod arrays, with a scanning area of 5 m × 5 m and a scanning frequency of 1 Hz.
electrical signal were recorded by the AFM system simultaneously.
Fig. 1. (a) SEM image of the ZnO nanorod arrays with an average diameter of 70 nm and length of 2 m. (b) Schematic diagram showing the experimental design of piezoelectric nanogenerator in AFM system.
immersion the wafer upside-down in the nutrient solution, which was composed of 0.05 M zinc nitrate hydrate and 0.05 M hexamethylenetetramine at 95 ◦ C for 3 h. The wafer was then removed from the solution, rinsed with deionized water and dried. Scanning electron microscope (SEM) image Fig. 1(a) shows the wafer is coated with a highly uniform and dense array of ZnO nanorods, with an average diameter of 70 nm and length of 2 m. Piezoelectric generating characterization of ZnO nanorod was carried out by Veeco Multimode AFM system with an NT-MDT DSP11 diamond-coated conductive tip. The diamond coating is heavily doped with nitrogen for better electric conduction, and the carrier concentration is about 1020 cm−3 . The tip curvature radius is about 60 nm. Fig. 1(b) shows the scheme of piezoelectric generating test of the ZnO nanorod arrays in the AFM system. To start with, the film of the ZnO nanorod arrays was electric connected to the current sensing module of the AFM system using silver paste, and a complete current loop was formed when the diamond tip scanning on the ZnO nanorod arrays in contact mode. The surface topography of the ZnO nanorods and the
3. Results and discussion Fig. 2 shows the topography (a) and current signals (b) recorded simultaneously when the diamond tip scanning over the aligned ZnO nanorod arrays. The topographic image of ZnO nanorod arrays is distorted because the ZnO nanorod can be easily bent by the scanning tip. As a result of the piezoelectric effect, current signals were generated from the bent ZnO nanorods, and recorded by the current sensing module. From the 2D current image Fig. 2(b), we calculated the density of the current pulse is about 41/m2 and the highest peak is about 35 pA. The density of ZnO nanorods is about 43/m2 from the SEM image, which means about 95% ZnO nanorods generate current signals when scanned by the diamond tip. For comparison only about 40% ZnO nanorods could generate piezoelectric current signals when scanned by the Pt tip [11]. The outstanding performance of the diamond tip is related to the diamond coating’s unique properties. First of all, the diamond coating has high wear resistance, so that the tip will not breakdown in contact-scanning, and good mechanical contact with ZnO nanorods can be achieved. Moreover, the diamond coating is nitrogen doped for good electrical conductivity [19]. Those two outstanding properties make the diamond-coated tip the first choice in the conductive AFM technology.
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Fig. 3. (a) Relationships between the topography and the current output in a single scanning line extracting from Fig. 2. The upper line is about ZnO nanorods’ topography and the lower line is about the current signal. (b) An enlargement of the dotted line frame in Fig. 3(a) shows the detailed relationship between the topography and current output. (c) A series of schematic processes about the diamond tip scanning over a single ZnO nanorod according to Fig. 3(b).
In order to analyzing the relationships between the topography of the ZnO nanorod and the detected current signal, two corresponding scanning lines were selected from the topography image and current image (Fig. 2). As shown in Fig. 3(a), the line upside is the topography of ZnO nanorods, and the line downside is the output current signals. As we can see that a corresponding current discharge appears with a topographical peak. Fig. 3(b) is an enlargement of the dotted line frame in Fig. 3(a), which shows detailed relationships between the topography and current output. According to Fig. 3(b), we can divide the processes of a current discharge into three steps (Fig. 3(c)). Firstly, the diamond tip begins to contact with the sidewall of the ZnO nanorod at location (a) and applies a lateral force to bend it. The tip is going upwards at the same time. No current signal is detected in this process. Next, the tip ascents to the highest point (b) corresponding to the top end of the ZnO nanorod and current signal is detected, the current signal increases in intensity at first and then decreases in this process from (b) to (d), the largest current IMax is detected at location (c). The applied force from the tip can be divided into a downward pressure force FP and a frictional force Ff parallel to the nanorod’s end face. Finally, the tip separates from the nanorod and descends. In this process, on current signal is detected. As shown in Fig. 4(a), the current–voltage (I–V) characteristics curve of the diamond tip/ZnO nanorod heterojunction reveals an obvious rectifying property. The I–V tests about diamond tip/Au film contact and Ag paste/ZnO film contact show ohmic electrical conductive characteristics (the inset figure in Fig. 4(a)). So that we confirm that the rectifying feature is caused by the heterojunction of diamond tip/ZnO nanorod, relating to the potential barrier at the junction interface. Diamond is a wide band-gap (Eg = 5.47 eV) semiconductor with an electron affinity ≈ 0.5 eV [20]. When heavily doped with nitrogen, diamond has an electronic characteristics as n+ -type [21]. On the other hand, ZnO (Eg = 3.37 eV) is a semiconductor with an electron affinity x eV [22]. And the native ZnO behaves as an n-type semiconductor because of Zn interstitial [23]. As a result, the contact between nitrogen-doped diamond and ZnO is an n+ –n heterojunction, in which only the majority carriers (electrons) impact the space-charge region. Fig. 4(b) shows the energy
band diagram of n+ -diamond/n-ZnO heterojunction. The potential barrier of the conductive band near the interface is the major factor to effect the electrons’ motion under a bias voltage. Because the dopant concentration in diamond tip is much higher than in ZnO
Fig. 4. (a) I–V characteristics of diamond tip and ZnO nanorod contact, which shows an obvious rectifying property. The inset figure shows the diamond tip-Au film contact and Ag paste-ZnO film contact are both ohmic contacts. (b) Energy band diagram of the n+ -diamond/n-ZnO heterojunction.
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nanorod, most of the potential drop is carried by the ZnO, leading to a rectifying n+ -diamond/n-ZnO heterojunction [24]. The mechanism proposed previously for the Pt tip/ZnO nanorod based piezoelectric nanogenerator applies for the present case after a few modifications [11,25]. As the diamond tip started to deflect the ZnO nanorod, a positive potential is produced at the stretched side of the nanorod, while a negative potential is induced at the compressed side [26,27]. Because of the redistribution of carriers in the nanorod under the piezoelectric field, the positive potential is seriously screened by the carriers, whereas the negative potential is well preserved [28]. The potential of the diamond tip is zero. No current signals were observed due to the presence of the reversebiased heterojunction barrier of diamond tip/ZnO nanorod contact. When the diamond tip goes up to the top end of the nanorod, a current is produced because of the presence of the forward-biased heterojunction barrier. At the same time, the piezoelectric charges are released from the ZnO nanorod as a formation of current discharge. After the diamond tip separates from the ZnO nanorod, the current discharge is finished. 4. Conclusion In summary, we have demonstrated a high performance piezoelectric nanogenerator using diamond tip contact-scanning ZnO nanorod arrays in AFM system. Excellent wear and conductive properties are the key factors to maintain effective physical contact and electrical contact. Rectifying n+ –n heterojunction is established when the nitrogen-doped diamond tip contacts with the native n-typed ZnO nanorod, and plays an important role in accumulating and releasing piezoelectric potential of the nanogenerator. Our works shows that conductive diamond film is an ideal electrode for the piezoelectric nanogenerator. Acknowledgement This work was supported by the National High Technology Research and Development Program of China (Grant No. 2009AA01Z114). References [1] S. Yodyingyong, Q. Zhang, K. Park, C.S. Dandeneau, X. Zhou, D. Triampo, G. Cao, ZnO nanoparticles and nanowire array hybrid photoanodes for dye-sensitized solar cells, Appl. Phys. Lett. 96 (2010) 73115. [2] F.S. Chiena, C. Wanga, Y. Chana, H. Linb, M. Chenb, R. Wu, Fast-response ozone sensor with ZnO nanorods grown by chemical vapor deposition, Sens. Actuators B 144 (2010) 120–125. [3] L.W. Ji, S.M. Peng, Y.K. Su, S.J. Young, C.Z. Wu, W.B. Cheng, Ultraviolet photodetectors based on selectively grown ZnO nanorod arrays, Appl. Phys. Lett. 94 (2009) 203106.
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