Structure study of electrodeposited ZnO nanowires - Semantic Scholar
Microelectronics Journal 36 (2005) 625–628 www.elsevier.com/locate/mejo
Structure study of electrodeposited ZnO nanowires Y. Leprince-Wanga,*, A. Yacoubi-Ouslima, G.Y. Wangb a
Laboratoire de Physique des Mate´riaux Divise´s et Interfaces (LPMDI), CNRS-UMR 8108, Universite´ de Marne la Valle´e, 5 Bd. Descartes, 77454 Marne la Valle´e Cedex 2, France b Centre d’E´tudes de Chimie Me´tallurgique (CECM), CNRS-UPR 2801, 15 rue G. Urbain, 94407 Vitry sur Seine Cedex, France Available online 6 June 2005
Abstract In this work, we report on the structure study of electrodeposited ZnO nanowires. The samples were mounted as a working electrode and the deposition was performed in a classical three electrodes electrochemical cell. For obtaining ZnO nanowires, the working electrode was a polycarbonate membrane with a random distribution of nanometric pores, gilded one side to ensure electric contact. The morphology and structure characterizations of the different diameters ZnO nanowires were carried out by transmission electron microscopy (TEM) and highresolution transmission electron microscopy (HRTEM). The electrons pattern diffraction confirmed the same crystal structure of electrodeposited ZnO nanowires indexed by X-ray diffraction (XRD) on electrodeposited ZnO thin films: hexagonal ZnO phase with cell parameters aZ0.32584 nm and cZ0.52289 nm. Both TEM investigations and HRTEM images reveal a monocrystalline structure for electrodeposited ZnO nanowires. A roughness of few nanometers on the wire surface was observed. Meanwhile, no preferential growth direction has been obviously detected. q 2005 Elsevier Ltd. All rights reserved. Keywords: Zinc oxide (ZnO); Electrodeposition; Nanowires; Transmission electron microscopy (TEM)
1. Introduction Zinc oxide (ZnO) has been one of the most promising oxide semiconductor materials because of its good optical, electrical, and acoustic characteristics. It can be used in many areas, such as field-emission displays, transparent conducting windows in solar cells, and gas sensors [1–6]. ZnO is an n type II–VI semiconductor with a wide bandgap of 3.3 eV [7], and a direct structure at room temperature with Wurtzite crystal structure: aZ0.325 nm and cZ 0.512 nm [8]. ZnO nanowires are usually prepared by Physical Vapor Deposition (PVD) [9–11] or Chemical Vapor Deposition (CVD) [12,13]. The nanowires obtained by PVD or CVD have generally a good crystalline quality and an important length (more often they are in nanobelts form). But these techniques require a sophistical and expensive equipment, because they ask to work in vacuum and/or at high * Corresponding author. Tel.: C33 1 60 95 72 76; fax: C33 1 60 95 72 97. E-mail address: [email protected] (Y. Leprince-Wang).
0026-2692/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2005.04.033
temperature. Electrochemical deposition is a simple method and less onerous, widely used in industry. Electrodeposition of metal oxide films has been used by many research teams because of the preparation from aqueous solutions presents several advantages over the above techniques. [6,14–18]. However, there are still few studies concerning the electrodeposition of metal oxide nanowires, specially in the case of ZnO nanowires [19]. Here, we report on the structure study of the ZnO nanowires by electrochemical deposition, a simple and low temperature method. The morphology and structure characterizations of the nanowires ZnO were carried out by TEM and HRTEM.
2. Experimental details ZnO nanowires were synthesized by electrochemical deposition within a porous polycarbonate membrane. The sizes of pores range from 90 nm down to 10 nm in diameter. In the following, the so-called M90, M60, M30 and M10 type of membranes correspond to nominal pore diameters of 90, 60, 30 and 10 nm, respectively. The pore density varies from 6!108 to 2!109 pores/cm2 (Table 1). Inverse optical
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Table 1 Nominal and experimental characteristics of the polycarbonate membranes [22]. Membrane
Thickness (mm)
Pore density (pores/cm2)
Pore nominal diameter (nm)
Wire measured diameter (nm)
M90 M60 M30 M10
16 16 16 6
1!109 2!109 2!109 6!108
90 60 30 10
w150 w100 w60 w30
microscope and scanning electron microscope (SEM) were employed to examine the membrane characteristics provided by manufacture [20]. From Table 1, we can note that the measured wire diameter is systematically larger than the nominal pore size. In order to ensure a good electrical contact, a thin gold layer of about 50 nm thickness was firstly evaporated onto the bottom of the membrane. Track-etched membranes were used as a template in this work. The nanometric pores were fabricated by using high-energy particles that created nuclear track damage first, followed by a suitable chemical etching [21]. For optimizing ZnO electrodeposition conditions, we used firstly some conductor substrates for ZnO thin films fabrication: stainless steel, gilded silver and gold discs. The thin films as obtained serve also for XRD measurements in order to determine crystal structure parameters. Electrodeposition of pure ZnO nanowires was performed in potassium chloride electrolytic solution. The chloride based electrolytic aqueous solutions contained a mixture of 0.1 M reagent grade potassium chloride and 5 mM reagent grade zinc chloride (from ACROS). The hydrogen peroxide concentration was adjusted to 5 mM by injecting an accurate volume of a reagent grade stabilized 30% H2O2 (from Prolabo). The electrochemical bath obtained is quasi-neuter, its pH value being about 6.85. The solution was prepared with ultra pure water (O18 MU) from a Millipore system. ZnO electrodeposition consists in generating hydroxide ions at the electrode surface by cathodically reducing an oxygen precursor. Three oxygen precursors have been described in the literature: molecular oxygen [6,23], nitrate ions [24,25] and hydrogen peroxide [26]. H2O2 presents some interesting advantages as a precursor: its use is easy and convenient since H2O2 is highly soluble in aqueous medium and it does not produce undesirable by-product by reduction. The proposed stepwise mechanism is quite simple. The reduction of hydrogen peroxide leads to the formation of hydroxide ions: H2 O2 C 2eK 0 2OHK
(1)
The overall deposition reaction of ZnO in the presence of H2O2 can be reasonably written as: Zn2C C 2OHK0 ZnO C H2 O
(2)
Electrodeposition of ZnO nanowires was performed using a computer-controlled Radiometer Analytical PGZ 301 potentiostat-galvanostat. The plating system was based on a classical three-electrode device. The reference electrode was a commercial saturated mercury sulfate electrode (SSE). The counter-electrode was a platinum grid. The working electrode was a polycarbonate membrane with a random distribution of nanometric pores. Before electrolysis, the electrochemical cell was immersed into stirred electrolyte for about 15 min to obtain maximum wettability of the membrane pores. Both potentiostatic mode and galvanostatic mode were used for obtaining ZnO nanowires. For potentiostatic mode, a constant cathodic potential of about K1500 mV (VSSE) was used. For galvanostatic mode, the current intensity employed was various according to the used membrane: about K0.1 mA for M90 and M60 type of membranes; K0.05 mA for M30 and M10 type of membranes. In all cases, the potential value (VSSE) was kept at about K1500 mV. We noted that the nanowires obtained by the galvanostatic mode are greater numbers than the potentiostatic mode. The electrolyte temperature was maintained in the stirred solution at 70 8C during electrodeposition by a thermostated bath. As the ZnO wires are nanometer sized, classical TEM specimen thinning such as mechanical polishing or ion milling is not necessary. A very simple preparation procedure was employed: a piece of as-deposited sample (about 2 mm2, gold layer on the surface) with nanowires embedded in the porous membrane was firstly put onto a copper TEM grid covered with a carbon film, installed on a microscope slide. A drop of chloroform then deposited above them, the polycarbonate substrate was progressively dissolved and evaporated. The gold film was removed before solvent evaporation. XRD experiments were performed with a Philips PW 1830 diffractometer using Co Ka radiation (lZ 0.17890 nm). TEM and HRTEM observations were carried out using a Topcon 002B transmission electron microscope operating at 200 kV.
3. Results and discussion The thin films ZnO were firstly made on conductor substrates under same electrochemical conditions in order to determine crystal structure parameters by XRD. Fig. 1 shows the XRD spectra of the electrodeposited ZnO thin film (w2.0 mm in thickness) on a stainless steel substrate, indexed to the hexagonal crystal structure with cell parameters aZ0.32584 nm and cZ0.52289 nm. The electrodeposited ZnO thin films show generally a polycristalline structure and no preferential growth direction. The electrodeposited thin films appear a transparent aspect on polished substrates. There are an excellent adherence between thin film and substrate. We note that the electrodeposited ZnO
Y. Leprince-Wang et al. / Microelectronics Journal 36 (2005) 625–628
Fig. 1. XRD spectra of the electrodeposited ZnO thin film on a stainless steel substrate (VSSEZK1500 mV).
has the cell parameters slightly larger than the literature ones, above all, for c parameter. Fig. 2 presents the TEM observations results showing general quality of the electrodeposited ZnO nanowires: (a) M90 type nanowires are about 150 nm in diameter and about 2.0 mm in length; and (b) M60 type nanowires are about 100 nm in diameter and about 1.6 mm in length. The same crystal structure indexed by XRD, hexagonal crystal structure, was confirmed by electrons pattern diffraction. From TEM investigation, no preferential growth direction has been obviously detected for electrodeposited ZnO nanowires. TEM images show also that the diameter size is homogeneous from one wire to another for a same type of membranes. At the same time, we note that the diameter varies along the wire length. The smallest end is
Fig. 2. TEM images showing a general view of the electrodeposited ZnO nanowires from membranes M90 (a) and M60 (b) (IZK0.1 mA).
627
the beginning of the nanowires growth, as proved by electrodeposited Sb nanowires using same template method [22]. This phenomenon is less manifest for the M30 and M10 type nanowires (see following). Electrodeposited ZnO nanowires have generally a good crystal quality with a monocrystalline structure. Both electron diffraction patterns and HRTEM images indicate the single crystalline characteristic for most of the electrodeposited ZnO nanowires. Fig. 3(a) shows an individual nanowire from M30 membrane with [001] direction growth. The nanowire is about 65 nm in diameter and about 1.5 mm in length. HRTEM image evidences at the atomic scale its single crystalline character (Fig. 3(b)). A roughness of several nanometers has been also evidenced by the HRTEM observations on the wire surface, which is mainly related to the pores quality of the commercial template substrates. The observed contrast on HRTEM image is the thickness contrast due to the nanowire roughness. For the smallest M10 nanowires, their diameter is quite homogeneous along whole wire length, but a more important roughness is observed (Fig. 4(a)). The important roughness of M10 type nanowire is probably related to the membrane production. In fact, M90, M60 and M30 type membranes are produced by Wathman (UK); M10 type membranes are produced by Osmonics (USA). Those membranes are initially destined for filtration function, so their pore wall’s quality is not controlled. Both electrons pattern diffraction and HRTEM image (Fig. 4(b)) indicate the good quality monocrystalline structure of the nanowire (w30 nm in diameter, w1.3 mm in length). Electrons pattern diffraction shows a same diffracting plane (100) of hexagonal ZnO than the nanowire
Fig. 3. Both TEM observation (a) and HRTEM image (b) showing a M30 type individual wire with [100] growth direction (IZK0.05 mA).
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above all, for c parameter. No preferential growth direction has been obviously detected. Both TEM investigations and HRTEM images show the single crystalline character of electrodeposited ZnO nanowires. We observed a homogeneous diameter distribution from one wire to another for a same type of membranes. However, the diameter varies along the wire length for M90 and M60 type of nanowires. This variation is reduced for M30 and M10 type of nanowires. A roughness of few nanometers on the wire surface is observed which is related to the pores quality of the commercial template substrates. This roughness increases when the diameter of the nanowires decreases.
References
Fig. 4. Monocrystalline structure of an individual M10 type nanowire. Both TEM image (a) and HRTEM image (b) showing an important roughness on the wire surface (IZK0.05 mA).
of Fig. 3 with a rotation, indicating the growth direction form an angle of 348 to [001] direction. Viewing TEM images of all type nanowires, we note that the smaller the pore diameter of the membrane is, the bigger the discrepancy between the nominal pore size and the measured wire diameter is. We observed also that the roughness on the wire surface increases when their diameter decreases. The quality of nanowires provided by template method crucially depends on the host pore quality. Parameters such as diameter, density, cylinder form and surface roughness of the pores, play important roles in quality control of the nanowire fabrication. However, few articles have been dedicated to a careful examination of those factors [27]. In the future, we would find the substrates more adapted. Of cause, they would be compatible with electrochemical bath’s conditions and easy to remove after electrodeposition.
4. Conclusion ZnO nanowires with different diameters were successfully synthesized by electrodeposition within nanoporous polycarbonate membrane. The growth morphology and crystalline quality of individual nanowires have been carefully investigated by TEM and HRTEM observations. Electrodeposited ZnO nanowires have a hexagonal crystal structure with aZ0.32584 nm and cZ0.52289 nm. Those cell parameters are slightly larger than the literature ones,
[1] L. Stolt, J. Hedstro¨m, J. Kessler, M. Ruckh, K.O. Velthaus, H.W. Schock, Appl. Phys. Lett. 62 (1993) 597–599. [2] R.H. Mauch, J. Hedstro¨m, D. Lincot, et al., Proceedings of 22nd IEEE Photovoltaic Specialists Conference, Las Vegas, New York, 1991. [3] K.S. Weißenrieder, J. Mu¨ller, Thin Solid Films 300 (1997) 30–41. [4] W.C. Shih, M.S. Wu, J. Cryst. Growth 137 (1994) 319–325. [5] D. Gal, G. Hodes, D. Lincot, H.W. Schock, Thin Solid Films 361–362 (2000) 79–83. [6] Th. Pauporte´, D. Lincot, Electrochimica Acta 45 (2000) 3345–3353. [7] H.L. Hartnagel, A.L. Dawar, A.K. Jain, C. Jagadish, Semeconducting Transparent Thin Films, Institute of Physics Publishing, Bristol/Philadelphia, 1995. [8] S. Peulon, Thesis, Universite´ Paris VI, France, 1995. [9] L. Dai, X.L. Chen, W.J. Wang, T. Zhou, B.Q. Hu, J. Phys.: Condens. Matter. 15 (2003) 2221–2226. [10] Y.C. Kong, D.P. Yu, B. Zhang, W. Fang, S.Q. Feng, Appl. Phys. Lett. 78 (2001) 407–409. [11] Y. Zhang, H.B. Jia, R.M. Wang, C.P. Chen, X.H. Luo, D.P. Yu, Appl. Phys. Lett. 83 (2003) 4631–4633. [12] Y.W. Wang, L.D. Zhang, G.Z. Wang, X.S. Peng, Z.Q. Chu, C.H. Liang, J. Cryst. Growth 234 (2002) 171–175. [13] B.Y. Geng, T. Xie, X.S. Peng, Y. Lin, X.Y. Yuan, G.W. Meng, L.D. Zhang, Appl. Phys. A 77 (2003) 363–366. [14] Th. Pauporte´, T. Yoshida, A. Goux, D. Lincot, J. Electroanal. Chem. 534 (2002) 55–64. [15] Z.R. Yu, X.D. Jia, J.H. Du, J.Y. Zhang, Solar Energy Mater. Solar Cells 64 (2000) 55–63. [16] J. Lee, H. Varela, S. Uhm, Y. Tak, Electrochem. Comm. 2 (2000) 646–652. [17] A. Helfen, S. Merkourakis, Y.S. Wang, M.G. Walls, E. Roy, K. YuZhang, Y. Leprince-Wang, Solid State Ionics, in press, 2004. [18] M. Musiani, Electrochimica Acta 45 (2000) 3397–3402. [19] M.J. Zheng, L.D. Zhang, G.H. Li, W.Z. Shen, Chem. Phys. Lett. 363 (2002) 123–128. [20] E. Roy, P. Fricoteaux, K. Yu-Zhang, J. Nanosci. Nanotech. 1 (2001) 323–329. [21] S.A. Whatman, Avenue Einstein, B-1348 Louvain-la-Neuve, Belgium. [22] E. Roy, Thesis, Universite´ de Marne la Valle´e, France, 2002. [23] S. Peulon, D. Lincot, J. Electrochem. Soc. 145 (1998) 864–874. [24] M. Izaki, T. Omi, Appl. Phys. Lett. 68 (1996) 2439–2440. [25] T. Yoshida, S. Ide, T. Sugiura, H. Minoura, Trans. Mater. Res. Soc. Jpn 25 (2000) 25. [26] Th. Pauporte´, D. Lincot, J. Electrochem. Soc. 148 (2001) C310–C314. [27] L. Piraux, S. Dubois, J.L. Duvail, A. Radulescu, J. Mater. Res. 14 (1999) 3042–3048.
Structure study of electrodeposited ZnO nanowires Y. Leprince-Wanga,*, A. Yacoubi-Ouslima, G.Y. Wangb a
Laboratoire de Physique des Mate´riaux Divise´s et Interfaces (LPMDI), CNRS-UMR 8108, Universite´ de Marne la Valle´e, 5 Bd. Descartes, 77454 Marne la Valle´e Cedex 2, France b Centre d’E´tudes de Chimie Me´tallurgique (CECM), CNRS-UPR 2801, 15 rue G. Urbain, 94407 Vitry sur Seine Cedex, France Available online 6 June 2005
Abstract In this work, we report on the structure study of electrodeposited ZnO nanowires. The samples were mounted as a working electrode and the deposition was performed in a classical three electrodes electrochemical cell. For obtaining ZnO nanowires, the working electrode was a polycarbonate membrane with a random distribution of nanometric pores, gilded one side to ensure electric contact. The morphology and structure characterizations of the different diameters ZnO nanowires were carried out by transmission electron microscopy (TEM) and highresolution transmission electron microscopy (HRTEM). The electrons pattern diffraction confirmed the same crystal structure of electrodeposited ZnO nanowires indexed by X-ray diffraction (XRD) on electrodeposited ZnO thin films: hexagonal ZnO phase with cell parameters aZ0.32584 nm and cZ0.52289 nm. Both TEM investigations and HRTEM images reveal a monocrystalline structure for electrodeposited ZnO nanowires. A roughness of few nanometers on the wire surface was observed. Meanwhile, no preferential growth direction has been obviously detected. q 2005 Elsevier Ltd. All rights reserved. Keywords: Zinc oxide (ZnO); Electrodeposition; Nanowires; Transmission electron microscopy (TEM)
1. Introduction Zinc oxide (ZnO) has been one of the most promising oxide semiconductor materials because of its good optical, electrical, and acoustic characteristics. It can be used in many areas, such as field-emission displays, transparent conducting windows in solar cells, and gas sensors [1–6]. ZnO is an n type II–VI semiconductor with a wide bandgap of 3.3 eV [7], and a direct structure at room temperature with Wurtzite crystal structure: aZ0.325 nm and cZ 0.512 nm [8]. ZnO nanowires are usually prepared by Physical Vapor Deposition (PVD) [9–11] or Chemical Vapor Deposition (CVD) [12,13]. The nanowires obtained by PVD or CVD have generally a good crystalline quality and an important length (more often they are in nanobelts form). But these techniques require a sophistical and expensive equipment, because they ask to work in vacuum and/or at high * Corresponding author. Tel.: C33 1 60 95 72 76; fax: C33 1 60 95 72 97. E-mail address: [email protected] (Y. Leprince-Wang).
0026-2692/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2005.04.033
temperature. Electrochemical deposition is a simple method and less onerous, widely used in industry. Electrodeposition of metal oxide films has been used by many research teams because of the preparation from aqueous solutions presents several advantages over the above techniques. [6,14–18]. However, there are still few studies concerning the electrodeposition of metal oxide nanowires, specially in the case of ZnO nanowires [19]. Here, we report on the structure study of the ZnO nanowires by electrochemical deposition, a simple and low temperature method. The morphology and structure characterizations of the nanowires ZnO were carried out by TEM and HRTEM.
2. Experimental details ZnO nanowires were synthesized by electrochemical deposition within a porous polycarbonate membrane. The sizes of pores range from 90 nm down to 10 nm in diameter. In the following, the so-called M90, M60, M30 and M10 type of membranes correspond to nominal pore diameters of 90, 60, 30 and 10 nm, respectively. The pore density varies from 6!108 to 2!109 pores/cm2 (Table 1). Inverse optical
626
Y. Leprince-Wang et al. / Microelectronics Journal 36 (2005) 625–628
Table 1 Nominal and experimental characteristics of the polycarbonate membranes [22]. Membrane
Thickness (mm)
Pore density (pores/cm2)
Pore nominal diameter (nm)
Wire measured diameter (nm)
M90 M60 M30 M10
16 16 16 6
1!109 2!109 2!109 6!108
90 60 30 10
w150 w100 w60 w30
microscope and scanning electron microscope (SEM) were employed to examine the membrane characteristics provided by manufacture [20]. From Table 1, we can note that the measured wire diameter is systematically larger than the nominal pore size. In order to ensure a good electrical contact, a thin gold layer of about 50 nm thickness was firstly evaporated onto the bottom of the membrane. Track-etched membranes were used as a template in this work. The nanometric pores were fabricated by using high-energy particles that created nuclear track damage first, followed by a suitable chemical etching [21]. For optimizing ZnO electrodeposition conditions, we used firstly some conductor substrates for ZnO thin films fabrication: stainless steel, gilded silver and gold discs. The thin films as obtained serve also for XRD measurements in order to determine crystal structure parameters. Electrodeposition of pure ZnO nanowires was performed in potassium chloride electrolytic solution. The chloride based electrolytic aqueous solutions contained a mixture of 0.1 M reagent grade potassium chloride and 5 mM reagent grade zinc chloride (from ACROS). The hydrogen peroxide concentration was adjusted to 5 mM by injecting an accurate volume of a reagent grade stabilized 30% H2O2 (from Prolabo). The electrochemical bath obtained is quasi-neuter, its pH value being about 6.85. The solution was prepared with ultra pure water (O18 MU) from a Millipore system. ZnO electrodeposition consists in generating hydroxide ions at the electrode surface by cathodically reducing an oxygen precursor. Three oxygen precursors have been described in the literature: molecular oxygen [6,23], nitrate ions [24,25] and hydrogen peroxide [26]. H2O2 presents some interesting advantages as a precursor: its use is easy and convenient since H2O2 is highly soluble in aqueous medium and it does not produce undesirable by-product by reduction. The proposed stepwise mechanism is quite simple. The reduction of hydrogen peroxide leads to the formation of hydroxide ions: H2 O2 C 2eK 0 2OHK
(1)
The overall deposition reaction of ZnO in the presence of H2O2 can be reasonably written as: Zn2C C 2OHK0 ZnO C H2 O
(2)
Electrodeposition of ZnO nanowires was performed using a computer-controlled Radiometer Analytical PGZ 301 potentiostat-galvanostat. The plating system was based on a classical three-electrode device. The reference electrode was a commercial saturated mercury sulfate electrode (SSE). The counter-electrode was a platinum grid. The working electrode was a polycarbonate membrane with a random distribution of nanometric pores. Before electrolysis, the electrochemical cell was immersed into stirred electrolyte for about 15 min to obtain maximum wettability of the membrane pores. Both potentiostatic mode and galvanostatic mode were used for obtaining ZnO nanowires. For potentiostatic mode, a constant cathodic potential of about K1500 mV (VSSE) was used. For galvanostatic mode, the current intensity employed was various according to the used membrane: about K0.1 mA for M90 and M60 type of membranes; K0.05 mA for M30 and M10 type of membranes. In all cases, the potential value (VSSE) was kept at about K1500 mV. We noted that the nanowires obtained by the galvanostatic mode are greater numbers than the potentiostatic mode. The electrolyte temperature was maintained in the stirred solution at 70 8C during electrodeposition by a thermostated bath. As the ZnO wires are nanometer sized, classical TEM specimen thinning such as mechanical polishing or ion milling is not necessary. A very simple preparation procedure was employed: a piece of as-deposited sample (about 2 mm2, gold layer on the surface) with nanowires embedded in the porous membrane was firstly put onto a copper TEM grid covered with a carbon film, installed on a microscope slide. A drop of chloroform then deposited above them, the polycarbonate substrate was progressively dissolved and evaporated. The gold film was removed before solvent evaporation. XRD experiments were performed with a Philips PW 1830 diffractometer using Co Ka radiation (lZ 0.17890 nm). TEM and HRTEM observations were carried out using a Topcon 002B transmission electron microscope operating at 200 kV.
3. Results and discussion The thin films ZnO were firstly made on conductor substrates under same electrochemical conditions in order to determine crystal structure parameters by XRD. Fig. 1 shows the XRD spectra of the electrodeposited ZnO thin film (w2.0 mm in thickness) on a stainless steel substrate, indexed to the hexagonal crystal structure with cell parameters aZ0.32584 nm and cZ0.52289 nm. The electrodeposited ZnO thin films show generally a polycristalline structure and no preferential growth direction. The electrodeposited thin films appear a transparent aspect on polished substrates. There are an excellent adherence between thin film and substrate. We note that the electrodeposited ZnO
Y. Leprince-Wang et al. / Microelectronics Journal 36 (2005) 625–628
Fig. 1. XRD spectra of the electrodeposited ZnO thin film on a stainless steel substrate (VSSEZK1500 mV).
has the cell parameters slightly larger than the literature ones, above all, for c parameter. Fig. 2 presents the TEM observations results showing general quality of the electrodeposited ZnO nanowires: (a) M90 type nanowires are about 150 nm in diameter and about 2.0 mm in length; and (b) M60 type nanowires are about 100 nm in diameter and about 1.6 mm in length. The same crystal structure indexed by XRD, hexagonal crystal structure, was confirmed by electrons pattern diffraction. From TEM investigation, no preferential growth direction has been obviously detected for electrodeposited ZnO nanowires. TEM images show also that the diameter size is homogeneous from one wire to another for a same type of membranes. At the same time, we note that the diameter varies along the wire length. The smallest end is
Fig. 2. TEM images showing a general view of the electrodeposited ZnO nanowires from membranes M90 (a) and M60 (b) (IZK0.1 mA).
627
the beginning of the nanowires growth, as proved by electrodeposited Sb nanowires using same template method [22]. This phenomenon is less manifest for the M30 and M10 type nanowires (see following). Electrodeposited ZnO nanowires have generally a good crystal quality with a monocrystalline structure. Both electron diffraction patterns and HRTEM images indicate the single crystalline characteristic for most of the electrodeposited ZnO nanowires. Fig. 3(a) shows an individual nanowire from M30 membrane with [001] direction growth. The nanowire is about 65 nm in diameter and about 1.5 mm in length. HRTEM image evidences at the atomic scale its single crystalline character (Fig. 3(b)). A roughness of several nanometers has been also evidenced by the HRTEM observations on the wire surface, which is mainly related to the pores quality of the commercial template substrates. The observed contrast on HRTEM image is the thickness contrast due to the nanowire roughness. For the smallest M10 nanowires, their diameter is quite homogeneous along whole wire length, but a more important roughness is observed (Fig. 4(a)). The important roughness of M10 type nanowire is probably related to the membrane production. In fact, M90, M60 and M30 type membranes are produced by Wathman (UK); M10 type membranes are produced by Osmonics (USA). Those membranes are initially destined for filtration function, so their pore wall’s quality is not controlled. Both electrons pattern diffraction and HRTEM image (Fig. 4(b)) indicate the good quality monocrystalline structure of the nanowire (w30 nm in diameter, w1.3 mm in length). Electrons pattern diffraction shows a same diffracting plane (100) of hexagonal ZnO than the nanowire
Fig. 3. Both TEM observation (a) and HRTEM image (b) showing a M30 type individual wire with [100] growth direction (IZK0.05 mA).
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above all, for c parameter. No preferential growth direction has been obviously detected. Both TEM investigations and HRTEM images show the single crystalline character of electrodeposited ZnO nanowires. We observed a homogeneous diameter distribution from one wire to another for a same type of membranes. However, the diameter varies along the wire length for M90 and M60 type of nanowires. This variation is reduced for M30 and M10 type of nanowires. A roughness of few nanometers on the wire surface is observed which is related to the pores quality of the commercial template substrates. This roughness increases when the diameter of the nanowires decreases.
References
Fig. 4. Monocrystalline structure of an individual M10 type nanowire. Both TEM image (a) and HRTEM image (b) showing an important roughness on the wire surface (IZK0.05 mA).
of Fig. 3 with a rotation, indicating the growth direction form an angle of 348 to [001] direction. Viewing TEM images of all type nanowires, we note that the smaller the pore diameter of the membrane is, the bigger the discrepancy between the nominal pore size and the measured wire diameter is. We observed also that the roughness on the wire surface increases when their diameter decreases. The quality of nanowires provided by template method crucially depends on the host pore quality. Parameters such as diameter, density, cylinder form and surface roughness of the pores, play important roles in quality control of the nanowire fabrication. However, few articles have been dedicated to a careful examination of those factors [27]. In the future, we would find the substrates more adapted. Of cause, they would be compatible with electrochemical bath’s conditions and easy to remove after electrodeposition.
4. Conclusion ZnO nanowires with different diameters were successfully synthesized by electrodeposition within nanoporous polycarbonate membrane. The growth morphology and crystalline quality of individual nanowires have been carefully investigated by TEM and HRTEM observations. Electrodeposited ZnO nanowires have a hexagonal crystal structure with aZ0.32584 nm and cZ0.52289 nm. Those cell parameters are slightly larger than the literature ones,
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