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EPIR effect of Cu2O films by electrochemical deposition
Current Applied Physics 14 (2014) 1282e1286
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Current Applied Physics journal homepage: www.elsevier.com/locate/cap
EPIR effect of Cu2O films by electrochemical deposition €rner b, V.V. Marchenkov c D.W. Shi a, C.J. Luo a, C.P. Yang a, *, R. Yang a, H.B. Xiao a, K. Ba a Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Faculty of Physics & Electronic Science, Hubei University, Wuhan 430062, China b €ttingen, F.Hund Platz 1, Go €ttingen D-37077, Germany Department of Physics, University of Go c Russian Acad. Sci., Inst. Met. Phys., Ural Branch, Ekaterinburg 620219, Russia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 May 2014 Accepted 2 July 2014 Available online 10 July 2014
Cuprous oxide (Cu2O) films and Cu/Cu2O/Cu/FTO sandwich structures were prepared by electrochemical deposition on conductive FTO substrates with different pH value conditions but constant deposition potential. The phase composition, crystal structure and microstructure of the Cu2O films were characterized by XRD, SEM and EDS as well as by ElectricePulseeInducedeResistance (EPIR) perturbation. In particular, the switching effects of the Cu/Cu2O/Cu/FTO device are examined in this work. The result shows that the EPIR-effect is large for the Cu/Cu2O/Cu/FTO device at room temperature and strongly related to the pH value of the solution. In both acidic and neutral conditions, for example at pH ¼ 5, 6 and 7, the EPIR effect is significant and decreases with increasing pH value. It disappears when the pH value goes further into the alkaline regime, i.e. pH ¼ 8, 9 and 10. Space charge barriers at the interface of electrode and Cu2O are used to explain the IeV characteristic of the layer structure and the EPIR-effect. © 2014 Elsevier B.V. All rights reserved.
Keywords: Electrochemical deposition Interfacial effects EPIR effect Resistive memory
1. Introduction Resistive random access memory (RRAM), based on the Resistive Switching (RS) effect, is a new concept and becomes a hot topic of information storage in recent years. Compared to traditional flash memories, RRAM's are faster in performance, bitalterable and lower voltage which makes them good candidates for application in both embedded and Solid State Disk (SSD) systems. As one of the RS effects, the EPIR effect arouses particular interest as it does occur at room temperature and also well meets the requirements of the new generation storage device of RRAM. In 2000, Liu et al. first found this effect in the Pr0.7Ca0.3MnO3 (PCMO) thin films, in which the value of the resistance can be switched between high and low states by changing the polarity of the applied pulsed electric field [1]. The value of resistance will be set at the low state when loaded with a positive pulse electric field, while it can be switched into the high state by changing the polarity of the pulse. Both the low and high resistive states can be maintained for a long time and can be converted into each other by switching the polarity of the electrical pulse field. Thus, the EPIR effect has attracted extensive attention from research groups all over the world. As a consequence, different models were
* Corresponding author. Tel.: þ86 027 88665447. E-mail address: [email protected] (C.P. Yang). http://dx.doi.org/10.1016/j.cap.2014.07.006 1567-1739/© 2014 Elsevier B.V. All rights reserved.
proposed to understand the physical factors and mechanism. According to literatures, the physics of the EPIR effect include intrinsic body effects [1,2], interfacial effects [3e6], conductive domains tunneling [7], ion redox effects [8] and others. However, to date there is no model that can explain all the experimental phenomena of EPIR in transition metal oxides, and so the physical mechanisms of EPIR are still far from being solved. In addition to perovskite manganites, EPIR effect is also observed in binary oxides, such as in NiO, TiO2, Nb2O5, ZnO, Cu2O et al. [9e14]. Cuprous oxide Cu2O is a typical semiconductor with a band gap of 1.9e2.2 eV, having a stable performance and good compatibility. Nanoscale Cu2O films have excellent optical, electrical and photocatalytic properties, also considerable EPIR effect is observed at room temperature. But the physical mechanism and influence factors of EPIR effect have not been understood well for Cu2O. However, it was assumed in Refs. [12e14] that the EPIR effect appears in Cu2O due to the interfacial effects. To verify this assumption, the Cu/Cu2O/Cu/FTO sandwich structures with different interfaces were prepared by electrochemical deposition in electrolytes with different pH values. It was supposed that the grain size of films will depend on pH values. The results show that the EPIR effect is closely related to the pH value prepared the sample. For samples prepared in acidic and neutral conditions (pH 7), Cu2O has a significant EPIR effect. But the EPIR effect of Cu2O decreases with increasing pH value and even disappears completely for the samples prepared in alkaline electrolytes.
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The phase constituents, crystal structure, microstructure of Cu2O films were analyzed by X-ray diffraction (D/MAX e IIIC, Japan), scanning electron microscopy (JSM6510LV, Japan) and EDS techniques, respectively. We used an ion sputtering instrument (JFC-1600, Japan) to fabricate point shaped copper electrodes on the Cu2O films for measuring. The DC electrical transport was measured by the 2-wire method using a computer controlled Keithley 2400 Sourcemeter. The Cu/Cu2O/Cu/FTO sandwich structures and a schematic view of transport measurements are shown in Fig. 5. 3. Results and discussion 3.1. The electrochemical processes of producing Cu2O films
Fig. 1. XRD patterns of Cu2O thin films electrodeposited at different pH values.
The acidic and neutral mixture solutions consist of 0.1 mol/L CH3COONa and 0.02 mol/L (CH3COO)2Cu. The acetic acid and acetate ions are used as buffer for keeping the pH value at about 5.6 in the electrolyte. Cu2O films can be formed on FTO glass by the following reactions: Hþ þ CH3COO 4 CH3COOH.
(1)
2. Experiments
Cu2þ þ 2e / Cu.
(2)
In this work, we use a standard three e electrode deposition system to prepare the Cu2O samples [15,16], in which a conductive glass (FTO; 10 U/,, fluorine-doped tin oxide made by the Japan Asahi company), a saturated calomel electrode (SCE) and a platinum electrode were used as the substrate, reference electrode and counter electrode, respectively. The FTO glasses were cleaned by acetone, ethanol, followed by an ultrasonic treatment in pure water before electrochemical deposition. Acidic or alkaline electrolytes were prepared in ultrapure water with a resistivity of 18 U cm at room temperature. The acidic and neutral electrolytes are mixed solutions of 0.1 mol/L CH3COONa and 0.02 mol/L (CH3COO)2Cu, in which the pH value of the electrolyte was adjusted by 1 mol/L CH3COOH and 2 mol/L NaOH for ranging the pH value from 5 to 7. The alkaline electrolytes were made from the mixture of 0.4 mol/L CuSO4 and 3 mol/L C3H6O3 (lactic acid), with the pH value ranging from 8 to 10. The depositing process of cuprous oxide was controlled by a CHI660D electrochemical workstation (CH Instruments, China). The electrolyte was put into a water bath slot at a constant temperature of 60 C and kept stirring. Then, the work potential was adjusted to 0.245 V vs SCE in acid/ neutral condition and 0.4 V vs SCE in alkaline condition. Finally, we deposited for 60 min to synthesize Cu2O and then the films were cleaned and dried for investigation.
2Cu2þ þ 2e þ H2O / Cu2O þ 2Hþ.
(3)
Cu2O þ 2Hþ þ 2e / 2Cu þ H2O.
(4)
In the three-electrode system, the working electrode and the counter electrode are used as cathode and anode of the electrolytic cell, respectively. At the anode, the OH ions loose electrons and generate oxygen while the Cu2þ ion receives electrons to generate Cu2O at the cathode (Reaction 3). In the acidic electrolyte, the CH3COO ion works as a buffer and keeps the acidity of the solution constant. If we regard the reaction as a dynamic process, the concentration of Hþ ions is expected to increase gradually which results in decreasing pH value according to reaction 3. Consequently, reaction 3 to produce Cu2O will be slow down while reaction 4, which dissolves Cu2O, will become dominant. Therefore, it is difficult to obtain Cu2O in a strongly acidic solution (pH ＜ 4). This is concordant with our experimental fact that Cu2O can not be formed when pH ¼ 2 and 3. On the other hand, a Cu2O cubic phase may be achieved when reaction 3 is dominant since then the acidity and Hþ concentration of the electrolyte decrease, which is also confirmed by our experiments, i.e., Cu2O forms when pH ¼ 5 and 6. In alkaline solutions consisting of 0.4 mol/L CuSO4 and 3 mol/ L C3H6O3, the hydroxyl ion will be produced by hydrolysis of
Fig. 2. Scanning electron micrographs of a typical electrodeposited Cu2O film. The preparation conditions are as follows, applied potential: 0.245 V vs SCE; bath temperature: 55 C; CH3COONa concentration: 0.1 mol/L and (CH3COO)2Cu concentration: 0.02 mol/L; pH: 5.8; time: 60 min.
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Fig. 3. The cross section micrographs of Cu2O films prepared at different pH value: (a) pH ¼ 5 and (b) pH ¼ 10.
lactate ions (L2) in reaction 5. In order to avoid production of Cu(OH)2 precipitates, lactic acid has been added as complexant to form lactic acid copper ions (CuL2 2 ) [17], which can combine with hydrogen ions (OH) to bond stabilize the pH value of the solution, like in reaction (6). According to the reaction 7 and 8, Cu2O can be formed during the process of Cu2þ reduction combined with OH. However, it is also possible that the copper ions are reduced to metallic copper according to the reactions 2 and 9 [17].
L2 þ H2O / HL þ OH.
(5)
3 CuL2 2 þ OH /½CuL 2 ðOHÞ :
(6)
2 2CuL2 þH2 O: 2 þ 2e þ2OH /Cu2 O þ 4L
(7)
[CuL2(OH)]3 þ 2e / Cu2O þ 4L2 þ H2O.
(8)
Fig. 4. EPIR measurements on Cu/Cu2O/Cu/FTO devices deposited at different pH value: (a) pH ¼ 5; (b) pH ¼ 6; (c) pH ¼ 7; (d) pH ¼ 8; (e) pH ¼ 9 and (f) pH ¼ 10.
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Fig. 5. 2-wire IeV characteristic curves of Cu/Cu2O/Cu/FTO devices prepared at different pH value: (a) pH ¼ 5 and (b) pH ¼ 10. 2 CuL2 2 þ 2e /Cu þ 2L :
(9)
3.2. Microscopic structure The XRD patterns of Cu2O films prepared at different pH values are shown in Fig. 1. They are consistent with that of the standard cubic phase of Cu2O, indicating that neither Cu nor CuO secondary phases are mixed with the target phase Cu2O. The samples show a preferential orientation along the [111] direction since the intensity of the [111] peak is stronger in comparison to other directions [18]. One reason might be that the number of oxygen atoms per unit area ranks as (100) < (110) < (111) for the polycrystalline planes [19]. It is known that the density of oxygen and its activity in hydroxyl, carboxyl and hydroxyl groups influences the growth directions of Cu2O crystals, i.e. the preferred growth is along the [111] direction. In addition, we find that polycrystalline Cu2O films are deposited in better quality under acid and neutral conditions than under alkaline conditions since the Bragg reflections are stronger. Fig. 2 shows the SEM images of the Cu2O films. It can be seen in Fig. 2(a) that the grain size of Cu2O is about 0.4 mm. In Fig. 2(b), we can clearly see the two separate layers with thicknesses 1.110 mm and 0.169 mm for the Cu2O film and the FTO glass respectively. Fig. 2(b) also shows that the uniform, dense, smooth and chemically stable Cu2O films can be prepared by electrochemical deposition. Moreover, the electrochemical deposition method can also be used to prepare Cu2O (photovoltaic) p-n homojunctions [26]. In Fig. 3, the cross section micrographs of Cu2O films show remarkable difference of samples prepared at between acidic and alkaline solution. The pH values of preparative solutions are 5 and 10 respectively as shown in Fig. 3(a) and (b). From Fig. 3(b), the grain size increases significantly bigger than Fig. 3(a). It is shown that alkaline solution could improve Cu2O films crystallization. 3.3. The EPIR effect in cuprous oxide films The EPIR effect of Cu/Cu2O/Cu/FTO devices was measured using the two-wire method, in which two copper points are fabricated on Cu2O films as the electrodes by sputtering. Usually, the resistance measured using 2-wire includes the contributions from grains, grain boundaries and the interface resistance between electrode and the surface of the sample. After testing different pulse parameters, we find that the EPIR of Cu/Cu2O/Cu/FTO devices can be observed best at the optimal conditions: voltage amplitude A ¼ 6 V, pulse duration t ¼ 0.001 s and measuring voltage U0 ¼ 0.1 V. Fig. 4 demonstrates that the Cu/Cu2O/Cu/FTO devices show an EPIR effect at room temperature, which is strongly dependent on the pH values of preparation. The magnitude of the EPIR effect is significant for the samples prepared at the pH ¼ 5, 6 and 7, and it decreases with increasing the pH value. With further increasing pH, i.e. for the alkaline conditions of pH ¼ 8, 9 and 10, the EPIR effect even disappears.
Usually, a nonlinear IeV characteristic is a necessary condition for the EPIR effect, which is confirmed by previous experiments [5,20e25]. Fig. 5 shows a nonlinear but asymmetric characteristic IeV for Cu2O films and an EPIR effect deposited at pH 7 (see Fig. 5(a)), and the EPIR effect disappears when the IeV becomes linear for the samples formed at pH > 7 (see Fig. 5(b)). Nonlinear IeV characteristic does not only appear at the region near grain boundaries, but also between electrode and sample surface due to the behavior of the surface space charge layer. In previous research work, it was found that the electrochemical deposition method can be used to produce n-type Cu2O thin films using weak acidic electrolytes [27], but p-type Cu2O occurs when alkaline electrolytes with lactic acid as complexant are used [28]. In theory, the IeV characteristic of a single contact between a metal and a semiconductor (M/S) is that of a depletion layer for an n-type semiconductor if the work function of metal is greater than that of semiconductor (FM > FS), i.e. a Schottky barrier is formed. In contrast, for the same condition of FM > FS, the IeV characteristic will be linear for the p-type semiconductor. The work function of copper electrode is 4.65 eV and it is larger than that of 4.48 eV for Cu2O. So the contact is expected to be non-ohmic for n-type Cu2O, which is formed in acidic conditions. Then the nonlinear IeV and EPIR are expected to observe for the samples at pH ¼ 5, just as we see in the experimental data in Fig. 5(a). However, the contact changes into ohmic and no EPIR is expected to appear if we increase the pH of solution into an alkaline condition since no Schottky barrier formed for the p-type Cu2O, also consistent with our experiment in Fig. 5(b).
4. Conclusion Under potentiostatic cathodic reduction, cuprous oxide (Cu2O) films and Cu/Cu2O/Cu/FTO devices were prepared at different pH values. An EPIR effect was found in the Cu/Cu2O/Cu/FTO devices. The EPIR effect is strongly dependent on the preparation conditions at room temperature. In both acidic and neutral condition, for example at pH ¼ 5, 6 and 7, the EPIR effect is significant and decreases with increasing pH value till it disappears when the value goes up to pH values of 8, 9 and 10. It was shown that the EPIR effect of Cu2O films is strongly depend on the interfaces which result from the preparation conditions.
Acknowledgment The authors thank the financial support of the National Natural Science Foundation of China (Grant No. 11174073 and 11104065), the Innovative Research Groups Foundation of Education Bureau of Hubei Province (Grant No. T201301) and the AvH Foundation of Germany.
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References [1] S.Q. Liu, N.J. Wu, A. Ignatiev, Appl. Phys. Lett. 76 (2000) 2749. [2] K. Aoyama, K. Waku, A. Asanuma, Y. Uesu, T. Katsufuji, Appl. Phys. Lett. 85 (2004) 1208. [3] A. Sawa, T. Fujii, M. Kawasaki, Y. Tokura, Appl. Phys. Lett. 85 (2004) 4073. [4] A. Baikalov, Y.Q. Wang, B. Shen, B. Lorenz, S. Tsui, Y.Y. Sun, Y.Y. Xue, C.W. Chu, Appl. Phys. Lett. 83 (2003) 957. [5] S.S. Chen, C.P. Yang, L.F. Xu, F.J. Yang, H.B. Wang, H. Wang, L.B. Xiong, Y. Yu, I.V. Medvedevac, K. B€ arner, Sol. State Commun. 150 (2010) 240. [6] S. Tsui, A. Baikalov, J. Cmaidalka, Y.Y. Sun, Y.Q. Wang, Y.Y. Xue, C.W. Chu, L. Chen, A.J. Jacobson, Appl. Phys. Lett. 85 (2004) 317. nchez, Phys. Rev. Lett. 92 (2004) 178302. [7] M.J. Rozenberg, I.H. Inoue, M.J. Sa [8] R. Yang, X.M. Li, W.D. Yu, X.D. Gao, D.S. Shang, X.J. Liu, X. Cao, Q. Wang, L.D. Chen, Appl. Phys. Lett. 95 (2009) 072105. [9] Y. Nishi, J.R. Jameson, in: Device Research Conference, 2008, pp. 271e274. [10] D.C. Kim, S. Seo, S.E. Ahn, D.-S. Suh, M.J. Lee, B.-H. Park, I.K. Yoo, I.G. Baek, H.J. Kim, E.K. Yim, J.E. Lee, S.O. Park, H.S. Kim, U-In Chung, J.T. Moon, B.I. Ryu, Appl. Phys. Lett. 88 (2006) 202102. [11] K. Kinoshita, T. Tamura, M. Aoki, Y. Sugiyama, H. Tanaka, Appl. Phys. Lett. 89 (2006) 103509. [12] A. Chen, S. Haddad, Y.C. Wu, T.N. Fang, S. Kaza, Z. Lan, Appl. Phys. Lett. 92 (2008) 013503. [13] S. Hong, D.X. Long, I. Hwang, J.S. Kim, Y.C. Park, S.O. Kang, B.H. Park, Appl. Phys. Lett. 99 (2011) 052105.
[14] S.O. Kang, S. Hong, J. Choi, J.S. Kim, I. Hwang, I.S. Byun, K.S. Yun, B.H. Park, Appl. Phys. Lett. 95 (2009) 092108. [15] M. Izaki, T. Shinagawa, K.T. Mizuno, Y. Ida, M. Inaba, A. Tasaka, J. Phys. D Appl. Phys. 40 (2007) 3326e3329. [16] M. Yin, C.K. Wu, Y.B. Lou, C. Burda, J.T. Koberstein, Y.M. Zhu, S. O'Brien, J. Am. Chem. Soc. 127 (2005) 9506e9511. [17] S. Leopold, M. Herranen, J.-O. Carlsson, L. Nyholm, J. Electrochem. Chem. 547 (2003) 45e52. [18] T.D. Golden, M.G. Shumsky, Y. Zhou, R.A. VanderWerf, R.A. Van Leeuwen, J.A. Switzer, Chem. Mater. 8 (1996) 2499e2504. [19] L.C. Wang, N.R. de Tacconi, C.R. Chenthamarakshan, K. Rajeshwar, M. Tao, Thin Solid Films 515 (2007) 3090e3095. [20] Z.G. Chen, Y.W. Tang, Z.J. Jia, L.S. Zhang, J. Inorg. Mater. 20 (2005) 367e372. [21] Daniel Hsu, J.G. Lin, W.F. Wu, J. Magn. Magn. Mater. 310 (2007) 978. [22] M. Villafuerte, S.P. Heluani, G. Juarez, D. Comedi, Phys. B 398 (2007) 321e324. [23] A. Beck, J.G. Bednorz, Ch Gerber, C. Rossel, D. Widmer, Appl. Phys. Lett. 77 (2000) 139. [24] A. Sawa, T. Fujii, M. Kawasaki, Y. Tokura, Appl. Phys. Lett. 88 (2006) 232112. [25] D.S. Shang, Q. Wang, L.D. Chen, R. Dong, X.M. Li, W.Q. Zhang, Phys. Rev. B 73 (2006) 245427. [26] C.M. McShane, W.P. Siripala, K.S. Choi, J. Phys. Chem. Lett. 1 (2010) 2666e2670. [27] C.M. McShane, K.S. Choi, J. Am. Chem. Soc. 131 (2009) 2561e2569. [28] C.C. Hu, J.N. Nian, H. Teng, Sol. Energy Mater. Sol. Cells 92 (2008) 1071e1076.
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
EPIR effect of Cu2O films by electrochemical deposition €rner b, V.V. Marchenkov c D.W. Shi a, C.J. Luo a, C.P. Yang a, *, R. Yang a, H.B. Xiao a, K. Ba a Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Faculty of Physics & Electronic Science, Hubei University, Wuhan 430062, China b €ttingen, F.Hund Platz 1, Go €ttingen D-37077, Germany Department of Physics, University of Go c Russian Acad. Sci., Inst. Met. Phys., Ural Branch, Ekaterinburg 620219, Russia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 May 2014 Accepted 2 July 2014 Available online 10 July 2014
Cuprous oxide (Cu2O) films and Cu/Cu2O/Cu/FTO sandwich structures were prepared by electrochemical deposition on conductive FTO substrates with different pH value conditions but constant deposition potential. The phase composition, crystal structure and microstructure of the Cu2O films were characterized by XRD, SEM and EDS as well as by ElectricePulseeInducedeResistance (EPIR) perturbation. In particular, the switching effects of the Cu/Cu2O/Cu/FTO device are examined in this work. The result shows that the EPIR-effect is large for the Cu/Cu2O/Cu/FTO device at room temperature and strongly related to the pH value of the solution. In both acidic and neutral conditions, for example at pH ¼ 5, 6 and 7, the EPIR effect is significant and decreases with increasing pH value. It disappears when the pH value goes further into the alkaline regime, i.e. pH ¼ 8, 9 and 10. Space charge barriers at the interface of electrode and Cu2O are used to explain the IeV characteristic of the layer structure and the EPIR-effect. © 2014 Elsevier B.V. All rights reserved.
Keywords: Electrochemical deposition Interfacial effects EPIR effect Resistive memory
1. Introduction Resistive random access memory (RRAM), based on the Resistive Switching (RS) effect, is a new concept and becomes a hot topic of information storage in recent years. Compared to traditional flash memories, RRAM's are faster in performance, bitalterable and lower voltage which makes them good candidates for application in both embedded and Solid State Disk (SSD) systems. As one of the RS effects, the EPIR effect arouses particular interest as it does occur at room temperature and also well meets the requirements of the new generation storage device of RRAM. In 2000, Liu et al. first found this effect in the Pr0.7Ca0.3MnO3 (PCMO) thin films, in which the value of the resistance can be switched between high and low states by changing the polarity of the applied pulsed electric field [1]. The value of resistance will be set at the low state when loaded with a positive pulse electric field, while it can be switched into the high state by changing the polarity of the pulse. Both the low and high resistive states can be maintained for a long time and can be converted into each other by switching the polarity of the electrical pulse field. Thus, the EPIR effect has attracted extensive attention from research groups all over the world. As a consequence, different models were
* Corresponding author. Tel.: þ86 027 88665447. E-mail address: [email protected] (C.P. Yang). http://dx.doi.org/10.1016/j.cap.2014.07.006 1567-1739/© 2014 Elsevier B.V. All rights reserved.
proposed to understand the physical factors and mechanism. According to literatures, the physics of the EPIR effect include intrinsic body effects [1,2], interfacial effects [3e6], conductive domains tunneling [7], ion redox effects [8] and others. However, to date there is no model that can explain all the experimental phenomena of EPIR in transition metal oxides, and so the physical mechanisms of EPIR are still far from being solved. In addition to perovskite manganites, EPIR effect is also observed in binary oxides, such as in NiO, TiO2, Nb2O5, ZnO, Cu2O et al. [9e14]. Cuprous oxide Cu2O is a typical semiconductor with a band gap of 1.9e2.2 eV, having a stable performance and good compatibility. Nanoscale Cu2O films have excellent optical, electrical and photocatalytic properties, also considerable EPIR effect is observed at room temperature. But the physical mechanism and influence factors of EPIR effect have not been understood well for Cu2O. However, it was assumed in Refs. [12e14] that the EPIR effect appears in Cu2O due to the interfacial effects. To verify this assumption, the Cu/Cu2O/Cu/FTO sandwich structures with different interfaces were prepared by electrochemical deposition in electrolytes with different pH values. It was supposed that the grain size of films will depend on pH values. The results show that the EPIR effect is closely related to the pH value prepared the sample. For samples prepared in acidic and neutral conditions (pH 7), Cu2O has a significant EPIR effect. But the EPIR effect of Cu2O decreases with increasing pH value and even disappears completely for the samples prepared in alkaline electrolytes.
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The phase constituents, crystal structure, microstructure of Cu2O films were analyzed by X-ray diffraction (D/MAX e IIIC, Japan), scanning electron microscopy (JSM6510LV, Japan) and EDS techniques, respectively. We used an ion sputtering instrument (JFC-1600, Japan) to fabricate point shaped copper electrodes on the Cu2O films for measuring. The DC electrical transport was measured by the 2-wire method using a computer controlled Keithley 2400 Sourcemeter. The Cu/Cu2O/Cu/FTO sandwich structures and a schematic view of transport measurements are shown in Fig. 5. 3. Results and discussion 3.1. The electrochemical processes of producing Cu2O films
Fig. 1. XRD patterns of Cu2O thin films electrodeposited at different pH values.
The acidic and neutral mixture solutions consist of 0.1 mol/L CH3COONa and 0.02 mol/L (CH3COO)2Cu. The acetic acid and acetate ions are used as buffer for keeping the pH value at about 5.6 in the electrolyte. Cu2O films can be formed on FTO glass by the following reactions: Hþ þ CH3COO 4 CH3COOH.
(1)
2. Experiments
Cu2þ þ 2e / Cu.
(2)
In this work, we use a standard three e electrode deposition system to prepare the Cu2O samples [15,16], in which a conductive glass (FTO; 10 U/,, fluorine-doped tin oxide made by the Japan Asahi company), a saturated calomel electrode (SCE) and a platinum electrode were used as the substrate, reference electrode and counter electrode, respectively. The FTO glasses were cleaned by acetone, ethanol, followed by an ultrasonic treatment in pure water before electrochemical deposition. Acidic or alkaline electrolytes were prepared in ultrapure water with a resistivity of 18 U cm at room temperature. The acidic and neutral electrolytes are mixed solutions of 0.1 mol/L CH3COONa and 0.02 mol/L (CH3COO)2Cu, in which the pH value of the electrolyte was adjusted by 1 mol/L CH3COOH and 2 mol/L NaOH for ranging the pH value from 5 to 7. The alkaline electrolytes were made from the mixture of 0.4 mol/L CuSO4 and 3 mol/L C3H6O3 (lactic acid), with the pH value ranging from 8 to 10. The depositing process of cuprous oxide was controlled by a CHI660D electrochemical workstation (CH Instruments, China). The electrolyte was put into a water bath slot at a constant temperature of 60 C and kept stirring. Then, the work potential was adjusted to 0.245 V vs SCE in acid/ neutral condition and 0.4 V vs SCE in alkaline condition. Finally, we deposited for 60 min to synthesize Cu2O and then the films were cleaned and dried for investigation.
2Cu2þ þ 2e þ H2O / Cu2O þ 2Hþ.
(3)
Cu2O þ 2Hþ þ 2e / 2Cu þ H2O.
(4)
In the three-electrode system, the working electrode and the counter electrode are used as cathode and anode of the electrolytic cell, respectively. At the anode, the OH ions loose electrons and generate oxygen while the Cu2þ ion receives electrons to generate Cu2O at the cathode (Reaction 3). In the acidic electrolyte, the CH3COO ion works as a buffer and keeps the acidity of the solution constant. If we regard the reaction as a dynamic process, the concentration of Hþ ions is expected to increase gradually which results in decreasing pH value according to reaction 3. Consequently, reaction 3 to produce Cu2O will be slow down while reaction 4, which dissolves Cu2O, will become dominant. Therefore, it is difficult to obtain Cu2O in a strongly acidic solution (pH ＜ 4). This is concordant with our experimental fact that Cu2O can not be formed when pH ¼ 2 and 3. On the other hand, a Cu2O cubic phase may be achieved when reaction 3 is dominant since then the acidity and Hþ concentration of the electrolyte decrease, which is also confirmed by our experiments, i.e., Cu2O forms when pH ¼ 5 and 6. In alkaline solutions consisting of 0.4 mol/L CuSO4 and 3 mol/ L C3H6O3, the hydroxyl ion will be produced by hydrolysis of
Fig. 2. Scanning electron micrographs of a typical electrodeposited Cu2O film. The preparation conditions are as follows, applied potential: 0.245 V vs SCE; bath temperature: 55 C; CH3COONa concentration: 0.1 mol/L and (CH3COO)2Cu concentration: 0.02 mol/L; pH: 5.8; time: 60 min.
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Fig. 3. The cross section micrographs of Cu2O films prepared at different pH value: (a) pH ¼ 5 and (b) pH ¼ 10.
lactate ions (L2) in reaction 5. In order to avoid production of Cu(OH)2 precipitates, lactic acid has been added as complexant to form lactic acid copper ions (CuL2 2 ) [17], which can combine with hydrogen ions (OH) to bond stabilize the pH value of the solution, like in reaction (6). According to the reaction 7 and 8, Cu2O can be formed during the process of Cu2þ reduction combined with OH. However, it is also possible that the copper ions are reduced to metallic copper according to the reactions 2 and 9 [17].
L2 þ H2O / HL þ OH.
(5)
3 CuL2 2 þ OH /½CuL 2 ðOHÞ :
(6)
2 2CuL2 þH2 O: 2 þ 2e þ2OH /Cu2 O þ 4L
(7)
[CuL2(OH)]3 þ 2e / Cu2O þ 4L2 þ H2O.
(8)
Fig. 4. EPIR measurements on Cu/Cu2O/Cu/FTO devices deposited at different pH value: (a) pH ¼ 5; (b) pH ¼ 6; (c) pH ¼ 7; (d) pH ¼ 8; (e) pH ¼ 9 and (f) pH ¼ 10.
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Fig. 5. 2-wire IeV characteristic curves of Cu/Cu2O/Cu/FTO devices prepared at different pH value: (a) pH ¼ 5 and (b) pH ¼ 10. 2 CuL2 2 þ 2e /Cu þ 2L :
(9)
3.2. Microscopic structure The XRD patterns of Cu2O films prepared at different pH values are shown in Fig. 1. They are consistent with that of the standard cubic phase of Cu2O, indicating that neither Cu nor CuO secondary phases are mixed with the target phase Cu2O. The samples show a preferential orientation along the [111] direction since the intensity of the [111] peak is stronger in comparison to other directions [18]. One reason might be that the number of oxygen atoms per unit area ranks as (100) < (110) < (111) for the polycrystalline planes [19]. It is known that the density of oxygen and its activity in hydroxyl, carboxyl and hydroxyl groups influences the growth directions of Cu2O crystals, i.e. the preferred growth is along the [111] direction. In addition, we find that polycrystalline Cu2O films are deposited in better quality under acid and neutral conditions than under alkaline conditions since the Bragg reflections are stronger. Fig. 2 shows the SEM images of the Cu2O films. It can be seen in Fig. 2(a) that the grain size of Cu2O is about 0.4 mm. In Fig. 2(b), we can clearly see the two separate layers with thicknesses 1.110 mm and 0.169 mm for the Cu2O film and the FTO glass respectively. Fig. 2(b) also shows that the uniform, dense, smooth and chemically stable Cu2O films can be prepared by electrochemical deposition. Moreover, the electrochemical deposition method can also be used to prepare Cu2O (photovoltaic) p-n homojunctions [26]. In Fig. 3, the cross section micrographs of Cu2O films show remarkable difference of samples prepared at between acidic and alkaline solution. The pH values of preparative solutions are 5 and 10 respectively as shown in Fig. 3(a) and (b). From Fig. 3(b), the grain size increases significantly bigger than Fig. 3(a). It is shown that alkaline solution could improve Cu2O films crystallization. 3.3. The EPIR effect in cuprous oxide films The EPIR effect of Cu/Cu2O/Cu/FTO devices was measured using the two-wire method, in which two copper points are fabricated on Cu2O films as the electrodes by sputtering. Usually, the resistance measured using 2-wire includes the contributions from grains, grain boundaries and the interface resistance between electrode and the surface of the sample. After testing different pulse parameters, we find that the EPIR of Cu/Cu2O/Cu/FTO devices can be observed best at the optimal conditions: voltage amplitude A ¼ 6 V, pulse duration t ¼ 0.001 s and measuring voltage U0 ¼ 0.1 V. Fig. 4 demonstrates that the Cu/Cu2O/Cu/FTO devices show an EPIR effect at room temperature, which is strongly dependent on the pH values of preparation. The magnitude of the EPIR effect is significant for the samples prepared at the pH ¼ 5, 6 and 7, and it decreases with increasing the pH value. With further increasing pH, i.e. for the alkaline conditions of pH ¼ 8, 9 and 10, the EPIR effect even disappears.
Usually, a nonlinear IeV characteristic is a necessary condition for the EPIR effect, which is confirmed by previous experiments [5,20e25]. Fig. 5 shows a nonlinear but asymmetric characteristic IeV for Cu2O films and an EPIR effect deposited at pH 7 (see Fig. 5(a)), and the EPIR effect disappears when the IeV becomes linear for the samples formed at pH > 7 (see Fig. 5(b)). Nonlinear IeV characteristic does not only appear at the region near grain boundaries, but also between electrode and sample surface due to the behavior of the surface space charge layer. In previous research work, it was found that the electrochemical deposition method can be used to produce n-type Cu2O thin films using weak acidic electrolytes [27], but p-type Cu2O occurs when alkaline electrolytes with lactic acid as complexant are used [28]. In theory, the IeV characteristic of a single contact between a metal and a semiconductor (M/S) is that of a depletion layer for an n-type semiconductor if the work function of metal is greater than that of semiconductor (FM > FS), i.e. a Schottky barrier is formed. In contrast, for the same condition of FM > FS, the IeV characteristic will be linear for the p-type semiconductor. The work function of copper electrode is 4.65 eV and it is larger than that of 4.48 eV for Cu2O. So the contact is expected to be non-ohmic for n-type Cu2O, which is formed in acidic conditions. Then the nonlinear IeV and EPIR are expected to observe for the samples at pH ¼ 5, just as we see in the experimental data in Fig. 5(a). However, the contact changes into ohmic and no EPIR is expected to appear if we increase the pH of solution into an alkaline condition since no Schottky barrier formed for the p-type Cu2O, also consistent with our experiment in Fig. 5(b).
4. Conclusion Under potentiostatic cathodic reduction, cuprous oxide (Cu2O) films and Cu/Cu2O/Cu/FTO devices were prepared at different pH values. An EPIR effect was found in the Cu/Cu2O/Cu/FTO devices. The EPIR effect is strongly dependent on the preparation conditions at room temperature. In both acidic and neutral condition, for example at pH ¼ 5, 6 and 7, the EPIR effect is significant and decreases with increasing pH value till it disappears when the value goes up to pH values of 8, 9 and 10. It was shown that the EPIR effect of Cu2O films is strongly depend on the interfaces which result from the preparation conditions.
Acknowledgment The authors thank the financial support of the National Natural Science Foundation of China (Grant No. 11174073 and 11104065), the Innovative Research Groups Foundation of Education Bureau of Hubei Province (Grant No. T201301) and the AvH Foundation of Germany.
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References [1] S.Q. Liu, N.J. Wu, A. Ignatiev, Appl. Phys. Lett. 76 (2000) 2749. [2] K. Aoyama, K. Waku, A. Asanuma, Y. Uesu, T. Katsufuji, Appl. Phys. Lett. 85 (2004) 1208. [3] A. Sawa, T. Fujii, M. Kawasaki, Y. Tokura, Appl. Phys. Lett. 85 (2004) 4073. [4] A. Baikalov, Y.Q. Wang, B. Shen, B. Lorenz, S. Tsui, Y.Y. Sun, Y.Y. Xue, C.W. Chu, Appl. Phys. Lett. 83 (2003) 957. [5] S.S. Chen, C.P. Yang, L.F. Xu, F.J. Yang, H.B. Wang, H. Wang, L.B. Xiong, Y. Yu, I.V. Medvedevac, K. B€ arner, Sol. State Commun. 150 (2010) 240. [6] S. Tsui, A. Baikalov, J. Cmaidalka, Y.Y. Sun, Y.Q. Wang, Y.Y. Xue, C.W. Chu, L. Chen, A.J. Jacobson, Appl. Phys. Lett. 85 (2004) 317. nchez, Phys. Rev. Lett. 92 (2004) 178302. [7] M.J. Rozenberg, I.H. Inoue, M.J. Sa [8] R. Yang, X.M. Li, W.D. Yu, X.D. Gao, D.S. Shang, X.J. Liu, X. Cao, Q. Wang, L.D. Chen, Appl. Phys. Lett. 95 (2009) 072105. [9] Y. Nishi, J.R. Jameson, in: Device Research Conference, 2008, pp. 271e274. [10] D.C. Kim, S. Seo, S.E. Ahn, D.-S. Suh, M.J. Lee, B.-H. Park, I.K. Yoo, I.G. Baek, H.J. Kim, E.K. Yim, J.E. Lee, S.O. Park, H.S. Kim, U-In Chung, J.T. Moon, B.I. Ryu, Appl. Phys. Lett. 88 (2006) 202102. [11] K. Kinoshita, T. Tamura, M. Aoki, Y. Sugiyama, H. Tanaka, Appl. Phys. Lett. 89 (2006) 103509. [12] A. Chen, S. Haddad, Y.C. Wu, T.N. Fang, S. Kaza, Z. Lan, Appl. Phys. Lett. 92 (2008) 013503. [13] S. Hong, D.X. Long, I. Hwang, J.S. Kim, Y.C. Park, S.O. Kang, B.H. Park, Appl. Phys. Lett. 99 (2011) 052105.
[14] S.O. Kang, S. Hong, J. Choi, J.S. Kim, I. Hwang, I.S. Byun, K.S. Yun, B.H. Park, Appl. Phys. Lett. 95 (2009) 092108. [15] M. Izaki, T. Shinagawa, K.T. Mizuno, Y. Ida, M. Inaba, A. Tasaka, J. Phys. D Appl. Phys. 40 (2007) 3326e3329. [16] M. Yin, C.K. Wu, Y.B. Lou, C. Burda, J.T. Koberstein, Y.M. Zhu, S. O'Brien, J. Am. Chem. Soc. 127 (2005) 9506e9511. [17] S. Leopold, M. Herranen, J.-O. Carlsson, L. Nyholm, J. Electrochem. Chem. 547 (2003) 45e52. [18] T.D. Golden, M.G. Shumsky, Y. Zhou, R.A. VanderWerf, R.A. Van Leeuwen, J.A. Switzer, Chem. Mater. 8 (1996) 2499e2504. [19] L.C. Wang, N.R. de Tacconi, C.R. Chenthamarakshan, K. Rajeshwar, M. Tao, Thin Solid Films 515 (2007) 3090e3095. [20] Z.G. Chen, Y.W. Tang, Z.J. Jia, L.S. Zhang, J. Inorg. Mater. 20 (2005) 367e372. [21] Daniel Hsu, J.G. Lin, W.F. Wu, J. Magn. Magn. Mater. 310 (2007) 978. [22] M. Villafuerte, S.P. Heluani, G. Juarez, D. Comedi, Phys. B 398 (2007) 321e324. [23] A. Beck, J.G. Bednorz, Ch Gerber, C. Rossel, D. Widmer, Appl. Phys. Lett. 77 (2000) 139. [24] A. Sawa, T. Fujii, M. Kawasaki, Y. Tokura, Appl. Phys. Lett. 88 (2006) 232112. [25] D.S. Shang, Q. Wang, L.D. Chen, R. Dong, X.M. Li, W.Q. Zhang, Phys. Rev. B 73 (2006) 245427. [26] C.M. McShane, W.P. Siripala, K.S. Choi, J. Phys. Chem. Lett. 1 (2010) 2666e2670. [27] C.M. McShane, K.S. Choi, J. Am. Chem. Soc. 131 (2009) 2561e2569. [28] C.C. Hu, J.N. Nian, H. Teng, Sol. Energy Mater. Sol. Cells 92 (2008) 1071e1076.