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Structural and photoluminescence characterization of vertically aligned multiwalled carbon nanotubes coated with ZnO by magnetron sputtering
Thin Solid Films 520 (2012) 4816–4819
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Structural and photoluminescence characterization of vertically aligned multiwalled carbon nanotubes coated with ZnO by magnetron sputtering N. Ouldhamadouche a, b, 1, A. Achour a,⁎, 1, I. Musa a, K. Ait Aissa a, F. Massuyeau a, P.Y. Jouan a, M. Kechouane b, L. Le Brizoual a, E. Faulques a, N. Barreau a, M.A. Djouadi a a b
Institut des Matériaux Jean Rouxel, Université de Nantes, CNRS, 2 rue de la Houssinière BP 32229 44322 Nantes cedex 3, France Laboratoire de Physique des Matériaux, Université des Sciences et de la Technologie Houari Boumediene, BP 32 El Alla. 16111, Bab Ezzouaur, Algeria
a r t i c l e
i n f o
Available online 29 October 2011 Keywords: Zinc oxide nanostructure MWCNTs DC and RF-sputtering Photoluminescence
a b s t r a c t Zinc oxide (ZnO) nanostructures are very attractive in various optoelectronic applications such as light emitting devices. A fabrication process of these ZnO nanostructures which gives a good crystalline quality and being compatible with that of micro-fabrication has significant importance for practical application. In this work ZnO films with different thicknesses were deposited by RF-sputtering on vertically aligned multiwalled carbon nanotube (MWCNTs) template in order to obtain ZnO nanorods. The obtained hybrid structures (ZnO/MWCNTs) were characterized by scanning electron microscopy, X-ray diffraction, transmission electron microscopy, and time resolved photoluminescence spectroscopy (PL). Results show that the ZnO/MWCNTs have a nanorod structure like morphology with a good crystalline quality of the deposited ZnO on the MWCNTs. PL measurements reveal an enhancement of the band edge signal of ZnO/MWCNTs which is three times of magnitude higher compared to the ZnO film deposited on silicon. Moreover, the intensity enhancement varies as function of the ZnO thickness. Such hybrid structures are promising for optoelectronic application, such as blue–violet sources. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Zinc oxide (ZnO) is an important II–VI direct band gap semiconductor. It is commonly used in various devices, such gas sensors, transparent electrode in solar cells and Ultra Violet (UV) light emitting diodes. [1,2]. Due to its wide band gap (3.3 eV at room temperature) and large exciton binding energy of 60 meV [3,4], ZnO is considered as one of the most promising materials for optoelectronic applications. ZnO nanostructures, such as nanorods and nanowalls [5], are receiving more attention due to their promising application in high efficient UV to blue light sources [6]. Some authors have also reported the fabrication of hybrid structures based on ZnO and carbon nanotubes (CNTs) in order to enhance the UV emission of ZnO. Such enhancement is usually attributed to surface plasmon effect coming from CNTs [7]. This cooperative effect between these two materials (ZnO and CNTs) is expected to show interesting optical properties for fabricating future optoelectronic devices [8]. In addition, coating CNTs with ZnO, allows to obtain 1D nanostructured ZnO that can have high efficient UV emission. Some synthesis techniques for coating vertically aligned or random oriented CNTs with ZnO, such as atomic layer deposition, sol–gel process and laser process have been reported [9–11]. Even through these methods can give homogenous
⁎ Corresponding author. E-mail address: [email protected] (A. Achour). 1 These authors contributed equally. 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.10.069
coating, they consist of more than one step process and usually lead to a bad crystalline quality of the deposited ZnO which give rise to emission in the green region (defect emission) [12] and quench of the band edge signal in the UV region [9,12]. In contrast, DC or RF magnetron sputtering is widely used in the semiconductor technology manufacturing and can produce high quality ZnO films with controlled thickness and morphology [13]. Here we present a simple and straightforward method to produce ZnO nanorods structure like by RF sputtering of ZnO target directly on the as prepared vertically aligned multiwalled carbon nanotube (MWCNTs) template. We also report on photoluminescence enhancement of the band edge signal from this hybrid structure (ZnO/MWCNTs) where the defect emission is almost absent. 2. Experimental details MWCNTs were grown on (001) doped silicon by Distributed Electronic Cyclotron Resonance (DECR)–Plasma Enhanced Chemical Vapour Deposition (PECVD). Experimental details are already reported elsewhere [14]. Briefly few nanometers thick of amorphous carbon was deposited as a buffer layer by IPVD system, then nickel nanoparticles catalyst (~ 50 nm in diameter) were electrochemically deposited prior to MWCNTs growth. The MWCNTs were grown by (DECR)–PECVD in acetylene plasma, diluted in ammonia (1:4), at a constant substrate temperature of 600 °C (Pressure: 0.2 Pa; Microwave power: 125 W). After that, zinc oxide was deposited on the as prepared
N. Ouldhamadouche et al. / Thin Solid Films 520 (2012) 4816–4819
4817
Fig. 1. SEM image of as prepared vertically aligned MWCNTs tilted at 45° (a) TEM image of MWCNTs showing nickel nanoparticle at the top (b). The inset is high magnification image of MWCNTs wall showing its high degree of crystallinity.
vertically aligned MWCNTs by RF magnetron sputtering from 4 ″ diameter of ZnO target. Argon (99.999%) as an inert gas was introduced through a mass flow controller. Sputtering deposition was performed under a gas pressure of 2.67∙ 10 − 2 Pa without intentional heating. The input power was kept at 350 W. The ZnO thickness was measured with a surface profilometer on glass and silicon substrate loaded in the same reactor during ZnO deposition on MWCNTs. The measured thicknesses are 500 nm, 800 nm, 1000 nm and 1500 nm. The ZnO/ MWCNTs hybrid structures were characterized by X-ray diffraction (XRD), using X-ray diffractometer D500 MOXTEK with Cu Kα radiation (λ = 0.1540 nm) in the θ–2θ Bragg Brentano configuration. The surface morphology and structural characterization were carried out by Field
Emission Gun Scanning Electron Microscopy (FEG-SEM) using JEOL JSM 7600F1 apparatus and Field Emission Gun Transmission Electron Microscope (FEG-TEM; Hitachi HF 2000) operating at 200 kV. Samples were micro-delaminated with a diamond tip and directly transferred onto the TEM copper grids. Ultrafast Photoluminescence (PL) experiments were carried out with a femtosecond laser system (Spectra Physics Hurricane X) delivering 100 fs pulses at 1 kHz, and 1 W of mean power. Samples of ZnO/MWCNTs and ZnO reference film (1000 nm thick) deposited on Si were excited at 267 nm by triple harmonic generation. Transient signals were spectrally dispersed into an imaging spectrograph. The time-resolved emission spectra were detected with a streak camera of temporal resolution b 20 ps.
Fig. 2. Cross section SEM images of (a) 500 nm, (b) 1000 nm, (c) 800 nm and 1500 nm thick ZnO encapsulating vertically aligned MWCNTs. (Scale bar: 1 μm).
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Intensity (arb.units)
(101)
ZnO/Si: 1000 nm
(100)
Fig. 1 (a) shows SEM image (tilted at 45°) of the as prepared vertically aligned MWCNTs deposited on silicon substrate. The MWCNTs have a length of 1 to 2 μm and an average diameter of ~50 nm with a tip growth mechanism (nickel nanoparticle at the top) as it is shown in Fig. 1 (b). The inset image shows a high degree of crystallinity of the deposited MWCNTs. The MWCNTs did not form a very dense carpet and a gap spacing of 100 to 300 nm is observed between two vertically aligned MWCNTs. This allows a deeper deposition of ZnO between these MWCNTs. The SEM images of Fig. 2a, b, c and d show the hybrid structures of MWCNTs coated respectively with 500, 800, 1000 and 1500 nm thick ZnO. These architectures consist of nanorod structure like with a convex end pointing to vertical direction with sizes that increases with the thickness of the layer deposited. Moreover both crystallite size and top film density increase with the thickness of the deposited ZnO. As the sputtering time increases, the coating thickness increases and ZnO film fully encapsulates the MWCNTs. The ZnO appeared to form tip-shaped single crystalline particles attaching to the top surface of the nanotubes which may result in the higher luminescence intensity of band edge emission of ZnO coated MWCNTs. TEM image of ZnO (800 nm thick) coated MWCNTs is shown in Fig. 3 (a). It can be seen that the ZnO is deposited directly on the carbon surface and closely encapsulates the MWCNTs along all its length. However, only few ZnO in the form of small grains with an average size of ~5 nm is deposited on its bottom as it can be seen from Fig. 3(b). Inter planar spacing of 2.48 A°, 2.78 A°, and 2.58 A° corresponding to (101), (100) and (002) ZnO planes respectively can be measured from the selected area electron diffraction pattern (SAED) of Fig. 3(c). The hexagonal structure of the ZnO coating with a (002) preferential orientation, also can be confirmed by nano-diffraction patterns (Fig. 3d). The same structure is confirmed by XRD pattern shown in Fig. 4 where the peaks can be assigned to (100), (002), (101) and (102) planes of pure ZnO. It should be noted that ZnO films deposited directly on silicon substrate (ZnO/Si) exhibit a single orientation along (002) direction in comparison to ZnO on MWCNTs (Fig. 4) which exhibits a preferred c-axis orientation. The preferential orientation of ZnO films could be attributed to the competition between the factors of the surface and interface influence including surface energy, strain energy and electrostatic energy [15]. From a theoretical point of view, the preferential orientation of the ZnO nanoparticles along (002) orientation can be explained satisfactory by the minimization of the strain energy of the nanoparticles [16]. The observed orientations change and peak positions shift may happen in order to minimize surface and/or strain energy of ZnO film during its deposition. On silicon
(002)
3. Results and discussion
ZnO/MWCNTs: 1500 nm
ZnO/MWCNTs: 800 nm
ZnO/MWCNTs: 500 nm
20
30
40
50
60
2θ (degree) Fig. 4. XRD pattern of nanocomposites with different thicknesses of ZnO deposited on MWCNTs (ZnO/MWCNTs) and ZnO deposited on silicon substrate (ZnO/Si).
substrate, strain energy could prevail over surface energy. Thus the film grows with (002) plans parallel to its surface. These plans would be of lower strain energy. On MWCNTs nanostructure which possesses large specific surface, the surface energy could be of importance as well. In the case of ZnO on MWCNTs, a shift of diffraction peaks position towards higher angles is observed as thickness increased from 800 to 1000 nm followed by a shift towards lower angles as the thickness of ZnO is further increased. Fig. 5 shows the room temperature PL spectra of the ZnO/MWCNTs hybrid structure and the ZnO film deposited on silicon substrate (ZnO/Si). In the case of ZnO/Si, the strong peak at 390 nm is attributed to the band edge emission of ZnO which corresponds to the emission peak of excitons recombination [17–19]. The peaks that are situated at 414, 459 and 512 nm are usually attributed to the oxygen vacancies and/or interstitial zinc ions and oxygen interstitials in the ZnO lattice [17–19] but they can more probably assigned to interference phenomenon [20] since the energy calculated from subtraction of two nearest peaks remain constant with a value of about 0.3 eV. Concerning ZnO/MWCNTs composites PL spectra, one can observe a broader and asymmetric peak at 390 nm which increases and reaches a maximum value for 800 nm thick ZnO. Its intensity is 3 times higher than the signal obtained for ZnO/Si sample. However, this intensity decreases with further increasing the thickness up to 1500 nm of ZnO but still remains more intense than the one obtained for ZnO/Si. Furthermore, the relatively sharp UV emission peak is an
Fig. 3. TEM image of ZnO (800 nm thick) coated MWCNTs (a), high magnification of the selected region (b), SAED pattern of the ZnO /MWCNTs hybrid structure (c) and (d) nanodiffraction patterns.
PL intensity (arb.units)
N. Ouldhamadouche et al. / Thin Solid Films 520 (2012) 4816–4819
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structure with a good crystalline quality of ZnO coating. The photoluminescence intensity reveals that such a nanorod like structure has higher luminescence intensity of ZnO band edge emission without appreciably defects contribution. The enhancement of excitonic emission varies with the variation of ZnO thickness. The intense UV peak indicates ZnO of the hybrid structures is highly crystalline containing low amount of defects. Moreover, the significantly enhanced PL of ZnO by interfacing with the MWCNTs highlights the importance and potential utility of using CNTs as template for nanostructuration purpose in order to develop multifunctional optoelectronic devices.
ZnO/MWCNTs: 1000 nm
ZnO/MWCNTs: 800 nm
Acknowledgments
ZnO/MWCNTs: 1500 nm ZnO/MWCNTs: 500 nm
The authors would like to thank E. Gautron, N. Stephant and S. Grolleau for their help in TEM and SEM analyses.
ZnO/Si: 1000 nm
350
400
450
500
550
600
650
700
Wavelength (nm) Fig. 5. Time resolved PL spectra of nanocomposites with different thicknesses of ZnO deposited on MWCNTs (ZnO/MWCNTs) and ZnO deposited on silicon substrate (ZnO/Si).
indication of good structural quality of ZnO film deposited on MWCNTs. The increase of the band edge emission intensity of ZnO/MWCNTs compared to ZnO/Si film can be explained by the increase of the surface-to-volume ratio of ZnO/MWCNTs hybrid structure which has a nanorod structure like with a convex end (nano cones at the top). In fact, the peak intensity increases with ZnO thickness up to 800–1000 nm and decreases for higher thicknesses due to the decrease of the surface area (Fig. 2). The band edge emission intensity of 500 nm thick ZnO deposited on MWCNTs is lower than those of 800 nm and 1000 nm even though the surface area of 500 nm thick ZnO seems to be higher (Fig. 2). This could be attributed to an improvement of the ZnO crystalline quality when thickness increases from 500 nm up to 800–1000 nm [21]. 4. Conclusion In summary, vertically aligned MWCNTs were grown on silicon substrates by plasma enhanced chemical vapour deposition and coated with ZnO using RF magnetron sputtering. The MWCNTs are well encapsulated by the ZnO coating and form a hybrid ZnO/MWCNTs
References [1] J.H. Lim, C.K. Kang, K.K. Kim, D.K. Huang, S.J. Park, Adv. Mater. 18 (2006) 2720. [2] G. Hu, H. Gong, E.F. Chor, P. Wu, Appl. Phys. Lett. 89 (2006) 251102. [3] U. Ozgur, I. Ya, C. Alivov, A. Liu, M.A. Teke, S. Reshchikov, V. Dogan, S.J. Avrutin, H. Cho, Morkoc, J. Appl. Phys. 98 (2005) 041301. [4] D.C. Look, Mater. Sci. Eng. B. 80 (2001) 383. [5] Y. Wu, H. Yan, M. Huang, B. Messer, J.H. Song, P. Yang, Chem. Eur. J. 8 (2002) 1260. [6] M. Al-Suleiman, A. Che Mofor, A. El-Shaer, A. Bakin, H.H. Wehmann, A. Waag, J. Appl. Phys. Lett. 89 (2006) 231911. [7] S. Kim, D.H. Shin, C.Oh. Kim, S.W. Hwang, S.-H. Choi, Seungmuk Ji, Ja-Yong Koo, J. Appl. Phys. Lett. 94 (2009) 213113. [8] Y. Zhu, H.I. Elim, Y.-L. Foo, T. Yu, Y. Liu, Wei Ji, J.-Y. Lee, Z. Shen, A.T.-S. Wee, J.T.-L. Thong, C.-H. Sow, Adv. Mater. 18 (2006) 587. [9] X.L. Li, C. Li, Y. Zhang, D.P. Chu, W.I. Milne, H.J. Fan, Nanoscale Res. Lett. 5 (2010) 1836. [10] L.-P. Zhu, G.-H. Liao, W.-Y. Huang, Li-Li Ma, Y. Yang, Ying Yu, S.-Y. Fu, Mater. Sci. Eng. B. 163 (2009) 194. [11] B. Aissa, C. Fauteux, My A. El Khakania, J. Mater. Res. 24 (2009) 313. [12] A.B. Djurisi, Y.H. Leung, K.H. Tam, Y.F. Hsu, L. Ding, Y.C. Zhong, K.S. Wong, W.K. Chan, H.L. Tam, K.W. Cheah, D.L. Phillips, Nanotechnology 18 (2007) 095702. [13] S. Rahmane, B. Abdallah, A. Soussou, E. Gautron, P.Y. Jouan, L. Le Brizoual, N. Barreau, A. Soltani, M.A. Djouadi, Phys. Status Solidi 1604 (2010) 1608. [14] T.M. Minea, S. Point, A. Gohier, A. Granier, C. Godon, F. Alvarez, Surf. Coat. Technol. 200 (2005) 1101. [15] H.X. Wei, W.J. Jie, W. Huang, J. Zhu, Y.R. Li, J. Cryst. Growth 311 (2009) 2391. [16] M.A. Tagliente, M. Massaro, Nucl. Instrum. Methods Phys. Res. 266 (2008) 1055. [17] J. Joo, S.G. Kwon, J.H. Yu, T. Hyeon, Adv. Mater. 17 (2005) 1837. [18] S.Y. Bae, H.W. Seo, H.C. Choi, J. Park, J. Phys. Chem. B 108 (2004) 12318. [19] S. Cho, J. Ma, Y. Kim, G.K.L. Wong, J.B. Ketterson, J. Appl. Phys. Lett. 75 (1999) 2761. [20] Y.-G. Wang, N. Ohashia, H. Ryoken, H. Haneda, J. Appl. Phys. Lett. 100 (2006) 114917. [21] E.S. Shim, H.S. Kang, J.S. Kang, J.H. Kim, S.Y. Lee, Appl. Surf. Sci. 186 (2002) 474.
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Structural and photoluminescence characterization of vertically aligned multiwalled carbon nanotubes coated with ZnO by magnetron sputtering N. Ouldhamadouche a, b, 1, A. Achour a,⁎, 1, I. Musa a, K. Ait Aissa a, F. Massuyeau a, P.Y. Jouan a, M. Kechouane b, L. Le Brizoual a, E. Faulques a, N. Barreau a, M.A. Djouadi a a b
Institut des Matériaux Jean Rouxel, Université de Nantes, CNRS, 2 rue de la Houssinière BP 32229 44322 Nantes cedex 3, France Laboratoire de Physique des Matériaux, Université des Sciences et de la Technologie Houari Boumediene, BP 32 El Alla. 16111, Bab Ezzouaur, Algeria
a r t i c l e
i n f o
Available online 29 October 2011 Keywords: Zinc oxide nanostructure MWCNTs DC and RF-sputtering Photoluminescence
a b s t r a c t Zinc oxide (ZnO) nanostructures are very attractive in various optoelectronic applications such as light emitting devices. A fabrication process of these ZnO nanostructures which gives a good crystalline quality and being compatible with that of micro-fabrication has significant importance for practical application. In this work ZnO films with different thicknesses were deposited by RF-sputtering on vertically aligned multiwalled carbon nanotube (MWCNTs) template in order to obtain ZnO nanorods. The obtained hybrid structures (ZnO/MWCNTs) were characterized by scanning electron microscopy, X-ray diffraction, transmission electron microscopy, and time resolved photoluminescence spectroscopy (PL). Results show that the ZnO/MWCNTs have a nanorod structure like morphology with a good crystalline quality of the deposited ZnO on the MWCNTs. PL measurements reveal an enhancement of the band edge signal of ZnO/MWCNTs which is three times of magnitude higher compared to the ZnO film deposited on silicon. Moreover, the intensity enhancement varies as function of the ZnO thickness. Such hybrid structures are promising for optoelectronic application, such as blue–violet sources. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Zinc oxide (ZnO) is an important II–VI direct band gap semiconductor. It is commonly used in various devices, such gas sensors, transparent electrode in solar cells and Ultra Violet (UV) light emitting diodes. [1,2]. Due to its wide band gap (3.3 eV at room temperature) and large exciton binding energy of 60 meV [3,4], ZnO is considered as one of the most promising materials for optoelectronic applications. ZnO nanostructures, such as nanorods and nanowalls [5], are receiving more attention due to their promising application in high efficient UV to blue light sources [6]. Some authors have also reported the fabrication of hybrid structures based on ZnO and carbon nanotubes (CNTs) in order to enhance the UV emission of ZnO. Such enhancement is usually attributed to surface plasmon effect coming from CNTs [7]. This cooperative effect between these two materials (ZnO and CNTs) is expected to show interesting optical properties for fabricating future optoelectronic devices [8]. In addition, coating CNTs with ZnO, allows to obtain 1D nanostructured ZnO that can have high efficient UV emission. Some synthesis techniques for coating vertically aligned or random oriented CNTs with ZnO, such as atomic layer deposition, sol–gel process and laser process have been reported [9–11]. Even through these methods can give homogenous
⁎ Corresponding author. E-mail address: [email protected] (A. Achour). 1 These authors contributed equally. 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.10.069
coating, they consist of more than one step process and usually lead to a bad crystalline quality of the deposited ZnO which give rise to emission in the green region (defect emission) [12] and quench of the band edge signal in the UV region [9,12]. In contrast, DC or RF magnetron sputtering is widely used in the semiconductor technology manufacturing and can produce high quality ZnO films with controlled thickness and morphology [13]. Here we present a simple and straightforward method to produce ZnO nanorods structure like by RF sputtering of ZnO target directly on the as prepared vertically aligned multiwalled carbon nanotube (MWCNTs) template. We also report on photoluminescence enhancement of the band edge signal from this hybrid structure (ZnO/MWCNTs) where the defect emission is almost absent. 2. Experimental details MWCNTs were grown on (001) doped silicon by Distributed Electronic Cyclotron Resonance (DECR)–Plasma Enhanced Chemical Vapour Deposition (PECVD). Experimental details are already reported elsewhere [14]. Briefly few nanometers thick of amorphous carbon was deposited as a buffer layer by IPVD system, then nickel nanoparticles catalyst (~ 50 nm in diameter) were electrochemically deposited prior to MWCNTs growth. The MWCNTs were grown by (DECR)–PECVD in acetylene plasma, diluted in ammonia (1:4), at a constant substrate temperature of 600 °C (Pressure: 0.2 Pa; Microwave power: 125 W). After that, zinc oxide was deposited on the as prepared
N. Ouldhamadouche et al. / Thin Solid Films 520 (2012) 4816–4819
4817
Fig. 1. SEM image of as prepared vertically aligned MWCNTs tilted at 45° (a) TEM image of MWCNTs showing nickel nanoparticle at the top (b). The inset is high magnification image of MWCNTs wall showing its high degree of crystallinity.
vertically aligned MWCNTs by RF magnetron sputtering from 4 ″ diameter of ZnO target. Argon (99.999%) as an inert gas was introduced through a mass flow controller. Sputtering deposition was performed under a gas pressure of 2.67∙ 10 − 2 Pa without intentional heating. The input power was kept at 350 W. The ZnO thickness was measured with a surface profilometer on glass and silicon substrate loaded in the same reactor during ZnO deposition on MWCNTs. The measured thicknesses are 500 nm, 800 nm, 1000 nm and 1500 nm. The ZnO/ MWCNTs hybrid structures were characterized by X-ray diffraction (XRD), using X-ray diffractometer D500 MOXTEK with Cu Kα radiation (λ = 0.1540 nm) in the θ–2θ Bragg Brentano configuration. The surface morphology and structural characterization were carried out by Field
Emission Gun Scanning Electron Microscopy (FEG-SEM) using JEOL JSM 7600F1 apparatus and Field Emission Gun Transmission Electron Microscope (FEG-TEM; Hitachi HF 2000) operating at 200 kV. Samples were micro-delaminated with a diamond tip and directly transferred onto the TEM copper grids. Ultrafast Photoluminescence (PL) experiments were carried out with a femtosecond laser system (Spectra Physics Hurricane X) delivering 100 fs pulses at 1 kHz, and 1 W of mean power. Samples of ZnO/MWCNTs and ZnO reference film (1000 nm thick) deposited on Si were excited at 267 nm by triple harmonic generation. Transient signals were spectrally dispersed into an imaging spectrograph. The time-resolved emission spectra were detected with a streak camera of temporal resolution b 20 ps.
Fig. 2. Cross section SEM images of (a) 500 nm, (b) 1000 nm, (c) 800 nm and 1500 nm thick ZnO encapsulating vertically aligned MWCNTs. (Scale bar: 1 μm).
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N. Ouldhamadouche et al. / Thin Solid Films 520 (2012) 4816–4819
Intensity (arb.units)
(101)
ZnO/Si: 1000 nm
(100)
Fig. 1 (a) shows SEM image (tilted at 45°) of the as prepared vertically aligned MWCNTs deposited on silicon substrate. The MWCNTs have a length of 1 to 2 μm and an average diameter of ~50 nm with a tip growth mechanism (nickel nanoparticle at the top) as it is shown in Fig. 1 (b). The inset image shows a high degree of crystallinity of the deposited MWCNTs. The MWCNTs did not form a very dense carpet and a gap spacing of 100 to 300 nm is observed between two vertically aligned MWCNTs. This allows a deeper deposition of ZnO between these MWCNTs. The SEM images of Fig. 2a, b, c and d show the hybrid structures of MWCNTs coated respectively with 500, 800, 1000 and 1500 nm thick ZnO. These architectures consist of nanorod structure like with a convex end pointing to vertical direction with sizes that increases with the thickness of the layer deposited. Moreover both crystallite size and top film density increase with the thickness of the deposited ZnO. As the sputtering time increases, the coating thickness increases and ZnO film fully encapsulates the MWCNTs. The ZnO appeared to form tip-shaped single crystalline particles attaching to the top surface of the nanotubes which may result in the higher luminescence intensity of band edge emission of ZnO coated MWCNTs. TEM image of ZnO (800 nm thick) coated MWCNTs is shown in Fig. 3 (a). It can be seen that the ZnO is deposited directly on the carbon surface and closely encapsulates the MWCNTs along all its length. However, only few ZnO in the form of small grains with an average size of ~5 nm is deposited on its bottom as it can be seen from Fig. 3(b). Inter planar spacing of 2.48 A°, 2.78 A°, and 2.58 A° corresponding to (101), (100) and (002) ZnO planes respectively can be measured from the selected area electron diffraction pattern (SAED) of Fig. 3(c). The hexagonal structure of the ZnO coating with a (002) preferential orientation, also can be confirmed by nano-diffraction patterns (Fig. 3d). The same structure is confirmed by XRD pattern shown in Fig. 4 where the peaks can be assigned to (100), (002), (101) and (102) planes of pure ZnO. It should be noted that ZnO films deposited directly on silicon substrate (ZnO/Si) exhibit a single orientation along (002) direction in comparison to ZnO on MWCNTs (Fig. 4) which exhibits a preferred c-axis orientation. The preferential orientation of ZnO films could be attributed to the competition between the factors of the surface and interface influence including surface energy, strain energy and electrostatic energy [15]. From a theoretical point of view, the preferential orientation of the ZnO nanoparticles along (002) orientation can be explained satisfactory by the minimization of the strain energy of the nanoparticles [16]. The observed orientations change and peak positions shift may happen in order to minimize surface and/or strain energy of ZnO film during its deposition. On silicon
(002)
3. Results and discussion
ZnO/MWCNTs: 1500 nm
ZnO/MWCNTs: 800 nm
ZnO/MWCNTs: 500 nm
20
30
40
50
60
2θ (degree) Fig. 4. XRD pattern of nanocomposites with different thicknesses of ZnO deposited on MWCNTs (ZnO/MWCNTs) and ZnO deposited on silicon substrate (ZnO/Si).
substrate, strain energy could prevail over surface energy. Thus the film grows with (002) plans parallel to its surface. These plans would be of lower strain energy. On MWCNTs nanostructure which possesses large specific surface, the surface energy could be of importance as well. In the case of ZnO on MWCNTs, a shift of diffraction peaks position towards higher angles is observed as thickness increased from 800 to 1000 nm followed by a shift towards lower angles as the thickness of ZnO is further increased. Fig. 5 shows the room temperature PL spectra of the ZnO/MWCNTs hybrid structure and the ZnO film deposited on silicon substrate (ZnO/Si). In the case of ZnO/Si, the strong peak at 390 nm is attributed to the band edge emission of ZnO which corresponds to the emission peak of excitons recombination [17–19]. The peaks that are situated at 414, 459 and 512 nm are usually attributed to the oxygen vacancies and/or interstitial zinc ions and oxygen interstitials in the ZnO lattice [17–19] but they can more probably assigned to interference phenomenon [20] since the energy calculated from subtraction of two nearest peaks remain constant with a value of about 0.3 eV. Concerning ZnO/MWCNTs composites PL spectra, one can observe a broader and asymmetric peak at 390 nm which increases and reaches a maximum value for 800 nm thick ZnO. Its intensity is 3 times higher than the signal obtained for ZnO/Si sample. However, this intensity decreases with further increasing the thickness up to 1500 nm of ZnO but still remains more intense than the one obtained for ZnO/Si. Furthermore, the relatively sharp UV emission peak is an
Fig. 3. TEM image of ZnO (800 nm thick) coated MWCNTs (a), high magnification of the selected region (b), SAED pattern of the ZnO /MWCNTs hybrid structure (c) and (d) nanodiffraction patterns.
PL intensity (arb.units)
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structure with a good crystalline quality of ZnO coating. The photoluminescence intensity reveals that such a nanorod like structure has higher luminescence intensity of ZnO band edge emission without appreciably defects contribution. The enhancement of excitonic emission varies with the variation of ZnO thickness. The intense UV peak indicates ZnO of the hybrid structures is highly crystalline containing low amount of defects. Moreover, the significantly enhanced PL of ZnO by interfacing with the MWCNTs highlights the importance and potential utility of using CNTs as template for nanostructuration purpose in order to develop multifunctional optoelectronic devices.
ZnO/MWCNTs: 1000 nm
ZnO/MWCNTs: 800 nm
Acknowledgments
ZnO/MWCNTs: 1500 nm ZnO/MWCNTs: 500 nm
The authors would like to thank E. Gautron, N. Stephant and S. Grolleau for their help in TEM and SEM analyses.
ZnO/Si: 1000 nm
350
400
450
500
550
600
650
700
Wavelength (nm) Fig. 5. Time resolved PL spectra of nanocomposites with different thicknesses of ZnO deposited on MWCNTs (ZnO/MWCNTs) and ZnO deposited on silicon substrate (ZnO/Si).
indication of good structural quality of ZnO film deposited on MWCNTs. The increase of the band edge emission intensity of ZnO/MWCNTs compared to ZnO/Si film can be explained by the increase of the surface-to-volume ratio of ZnO/MWCNTs hybrid structure which has a nanorod structure like with a convex end (nano cones at the top). In fact, the peak intensity increases with ZnO thickness up to 800–1000 nm and decreases for higher thicknesses due to the decrease of the surface area (Fig. 2). The band edge emission intensity of 500 nm thick ZnO deposited on MWCNTs is lower than those of 800 nm and 1000 nm even though the surface area of 500 nm thick ZnO seems to be higher (Fig. 2). This could be attributed to an improvement of the ZnO crystalline quality when thickness increases from 500 nm up to 800–1000 nm [21]. 4. Conclusion In summary, vertically aligned MWCNTs were grown on silicon substrates by plasma enhanced chemical vapour deposition and coated with ZnO using RF magnetron sputtering. The MWCNTs are well encapsulated by the ZnO coating and form a hybrid ZnO/MWCNTs
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