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shell type organic–inorganic heterostructure
Superlattices and Microstructures 77 (2015) 91–100
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Improved opto-electrical behavior of ZnO/C60 core/shell type organic–inorganic heterostructure M. Krishnaveni, Suganthi Devadason ⇑ Department of Physics, Karunya University, Coimbatore 641 114, Tamil Nadu, India
a r t i c l e
i n f o
Article history: Received 27 October 2014 Accepted 30 October 2014 Available online 20 November 2014 Keywords: ZnO nanorods C60 Core/shell Heterostructures Optical Electrical
a b s t r a c t We report a successful synthesis of ZnO/C60 core/shell-type organic–inorganic hybrid nanorod arrays (NRAs) using simple chemical route. X-ray diffraction pattern (XRD) reveals hexagonal crystal structure of ZnO along with the face-centered cubic (fcc) structure of C60. Detailed Fourier transform infrared spectroscopy (FTIR) inferred the attachment of C60 on the surface of ZnO NRAs. HRTEM observation confirmed 3 nm thickness of a very thin C60 nanocrystals shell layer formation. HRSEM image shows the existence of C60 on vertically aligned ZnO NRAs. UV–visible absorption spectra have two absorption edges corresponding to the energy band gap values of 3.05 and 1.83 eV which arise due to core ZnO and shell C60 respectively. Photoluminescence spectra reveal a strong enhancement in the NBE emission and suppression in the deep-level emission on the surface modified ZnO NRAs. The measured electrical resistivity under illumination for ZnO/C60 core/ shell is lower than that of bare ZnO NRAs. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Zinc oxide is developing as an auspicious material for applications in printable and flexible electronics due to its wide band gap semiconductor nature [1]. Among the various ZnO nanostructures, one-dimensional ZnO nanorods have attracted great interest for their immeasurable potential applications in fabricating nano-scale devices. Recently, in material science carbon nano-materials are ⇑ Corresponding author. Tel.: +91 9952287436. E-mail address: [email protected] (S. Devadason). http://dx.doi.org/10.1016/j.spmi.2014.10.036 0749-6036/Ó 2014 Elsevier Ltd. All rights reserved.
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extensively studied [2–4] and opening up an innovative research area because of the numerous studies on their incredible electrical properties and diverse ranging potential applications [5]. C60 (fullerene) molecule is considered as the third allotrope of carbon which comprises of 20 hexagonal and 12 pentagonal rings creating a soccer or bucky-ball shape that are held together by weak van der Waals interactions wherein all carbon atoms are placed at the nodes of a series of hexagons and pentagons [6]. To enhance a material’s thermal, mechanical or chemical stability, one can be modify the functionality, charge and reactivity of the material through surface coating with preferred material [7]. In particular, as C60 is an excellent electron acceptor when combined with ZnO, the organic molecules adsorption promises the fine-tuning of electronic properties [8]. Owing to their exclusive structures and interesting properties, research involving C60 has been increased in the fields of physics, chemistry and material science [9]. Furthermore, C60 possess wide range of applications in superconductivity [10], organic thin film transistor [11], organic solar cells [12], etc. Huang et al. reported that infiltration of fullerene as a functional interlayer onto the ZnO nanorod arrays (NRAs) provides significant improvement in the organic/inorganic hybrid solar cell device performance [13]. Specifically, for excellent potential applications in next-generation nonvolatile memory devices, inorganic nanostructures containing hybrid inorganic/organic composites are now developed as excellent candidates [4]. However, surface modification of inorganic semiconductor nanostructure is essential to increase the charge transport at the inorganic/organic interface in hybrid photovoltaic devices [14]. Therefore, it is imperative to facilitate the surface coating of zero-dimensional (0-D) C60 shell layer onto the one-dimensional (1-D) ZnO NRAs in order to obtain ZnO/C60 core/shell composites which provide a path way for tailoring the structural, electrical and optical properties. In the present work, an attempt has been made to report the systematic study of ZnO/C60 core/shell-type hybrid NRAs via simple chemical route by observing the modification in their structural, morphological, optical and electrical properties using various characterization techniques.
2. Experimental method 2.1. Synthesis of core ZnO NRAs Two-step chemical bath deposition method [15] has been employed to grow well-aligned ZnO NRAs. In the first step, 0.1 M concentration of zinc acetate dihydrate [(CH3COO)2 Zn2H2O] was dissolved in ethanol and then diethanolamine was added in drops to act as the stabilizer. This mixture was stirred at 50 °C for 1 h to develop a homogeneous, clear and transparent solution. After ageing for a day, the obtained precursor solution was coated on glass substrates using spin coater spun at the rate of 3000 rpm for 30 s. To remove the solvent and organic residuals, the films were dried at 200 °C for 2 min after each coating. The process from spin coating to drying was repeated several times to obtain a layer of dense and uniformly distributed ZnO seed nanoparticles on glass substrate. Then the ZnO seed layer coated substrates were post annealed at 400 °C for 1 h in a muffle furnace. The prepared ZnO seed layer film is 70 nm in thickness which gives a good support for the growth of NRAs. In the second step, the seeded films were suspended in a closed beaker containing 150 ml of aqueous solution having an equal molar (20 mM) of zinc acetate dihydrate and hexamethylenetetramine [C6H12N4]. The whole set-up was kept inside a hot air oven and the reaction temperature was maintained at 90 °C for 3 h to grow ZnO NRAs. Finally, the substrates were removed from the reaction bath, cleaned using distilled water to eliminate the residual salts and dried in air. Thus, well-aligned ZnO NRAs were grown on the seed layer using the two-step process.
2.2. Synthesis of C60 shell layer The deposition of C60 shell layer on to the ZnO NRAs has been carried out using spin coating technique [14]. 0.045 g of C60 powder purchased from sigma Aldrich with 99.95% purity was dispersed in 3 ml of chlorobenzene [C6H5Cl] and the resultant dispersed solution was spun over the ZnO NRAs at the rate of 3000 rpm for 30 s. The spin coating process was repeated up to 7 times to accomplish a
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uniform C60 shell layer distribution on the entire surface of ZnO NRAs. The transformation of ZnO film surface from white to gray color indicates the formation of C60 shell layer over ZnO NRAs. Microstructural studies have been carried out using X-ray Shimadzu XRD-6000 diffractometer with Cu Ka radiation recorded in the range 4–90°. Fourier transform infrared spectroscopy (FTIR) analysis was carried out using IR prestige-21 Shimadzu Fourier transform infrared spectroscopy. The detailed microstructural properties were analyzed using JEOL JEM 2100 high resolution transmission electron microscope (HRTEM) and surface morphological studies were performed by FEI Quanta FEG 200-high resolution scanning electron microscope (HRSEM) attached with energy dispersive X-ray analysis (EDX). UV–visible–NIR spectra were recorded using JASCO-670 UV/VIS/NIR spectrometer. Room temperature photoluminescence (PL) analysis was performed using a Fluoromax-4 spectrophotometer. The current to voltage (I–V) measurements were carried out using an automated testing device by National Instruments (NI PXI-1044).
3. Result and discussion 3.1. X-ray diffraction analysis of ZnO/C60 core/shell NRAs Fig. 1 gives the X-ray diffraction spectra for the bare ZnO NRAs and ZnO/C60 core/shell thin films. On comparing the XRD spectra of hybrid with the bare ZnO NRAs film, both exhibit a sharp and intense diffraction peak at 34.5° corresponding to the (0 0 2) plane of ZnO demonstrating the high crystallinity with the preferred c-axis growth orientation of ZnO NRAs. There are other weak diffraction peaks centered at 31.7°, 36.4°, 47.6°, 63.0°, 68.0° and 72.7° belonging to (1 0 0), (1 0 1), (1 0 2), (1 0 3), (1 1 2) and (0 0 4) planes respectively of hexagonal wurtzite crystal structure of core ZnO NRAs [JCPDS card no.
Fig. 1. X-ray diffraction patterns of bare ZnO and ZnO/C60 core/shell thin films.
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79-0205]. It is to be noted for the ZnO/C60 core/shell structure, the XRD spectrum reveals the presence of C60 shell layer along with the ZnO NRAs which is confirmed by the appearance of four additional diffraction characteristic peaks positioned at 10.8°, 17.7°, 20.7° and 21.7° corresponding to the well matched (1 1 1), (2 2 0), (3 1 1) and (2 2 2) reflecting planes respectively of face-centered cubic (fcc) structure of C60 [JCPDS card no. PDF #812220].
3.2. FTIR analysis of ZnO/C60 core/shell NRAs The surface modification of ZnO nanorods with C60 has been analyzed by FTIR studies. Fig. 2 shows FTIR spectrum for bare ZnO and ZnO/C60 core/shell NRAs recorded in the wavenumber range from 400 to 4000 cm1. The broad absorption band appearing at 3433 cm1 for bare ZnO is attributed to the OAH stretching vibrations of hydroxyl groups due to the adsorption of water on the surface of ZnO nanorods which is found to slightly shift to lower wavenumber region of 3431 cm1 for ZnO/C60 core/shell NRAs because of the surface modification of ZnO NRAs with C60 molecule. The peaks observed between 2300 and 2900 cm1 are assigned to CAH stretching vibration mode of the alkane groups [16]. The two bands located at 1591 and 1421 cm1 in bare ZnO accounting to the asymmetric and symmetric stretching vibrations of carboxyl groups (ACOOH) has been shifted to 1618 and 1428 cm1 in ZnO/C60 which clearly indicates the chemisorption of C60 on the surface of ZnO nanorods by carboxyl groups. The peaks positioned at 1184 and 1024 cm1 correspond to deformation mode and the stretching vibration of CAO respectively. The appearance of an additional new peak at 525 cm1 in the finger print region (<1000 cm1) belongs to C60 [17] which confirms the successful functionalization of ZnO NRAs by C60. The peak at 422 cm1 due to the ZnAO stretching vibrations of ZnO nanorods is found to shift to a lower wavenumber region of 420 cm1 for the ZnO/C60 core/shell NRAs which may be attributed to the surface changes of ZnO NRAs.
Fig. 2. FTIR-analysis for bare ZnO and ZnO/C60 core/shell NRAs.
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3.3. HRTEM analysis of ZnO/C60 core/shell NRAs Crystallography and microstructure of the as-synthesized sample is further characterized by HRTEM analysis. Fig. 3(a)–(c) shows HRTEM images of ZnO/C60 core/shell nanorod under different magnifications. Obviously, adsorption of 3 nm thickness of C60 shell layer has been established by the appearance of a fine dark band of C60 wrapping around the ZnO nanorod as shown in Fig. 3(a). Evidently, Fig. 3(a)–(c) shows the existence of spherical shaped C60 bucky-balls of diameter varying from 3 nm onwards decorate (marked by arrows) the surface of ZnO nanorod. The corresponding SAED pattern of ZnO/C60 core/shell nanorod shown in Fig. 3(d) represents the monocrystalline ZnO nanorod and polycrystalline C60 of the core/shell heterostructure. 3.4. HRSEM analysis of ZnO/C60 core/shell NRAs Fig. 4(a) and (b) shows the typical top and side-view HRSEM images of C60 coated vertically aligned ZnO NRAs. The measured length and diameter of the ZnO nanorods are found to be in the range of 1.3–1.7 lm and 50–100 nm respectively. The images depict the growth of bigger clusters of agglomerated C60 crystallites which are loaded on the top surface of ZnO NRAs. This is again affirmed in the side-view HRSEM image as shown in Fig. 4(b), wherein the ZnO NRAs are capped by C60 nanocrystals. Compared with HRTEM images, it is difficult to discern the presence of C60 nanocrystals
Fig. 3. (a-c) Low and high magnification HRTEM images and (d) the corresponding selected area electron diffraction pattern (SAED) of ZnO/C60 core/shell nanorod thin film.
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Fig. 4. (a) and (b) HRSEM images of top and side-view of ZnO/C60 core/shell NRAs.
using the low resolution HRSEM images. Fig. 5 shows energy dispersive X-ray analysis of ZnO/C60 hybrid core/shell structure. The compositional analysis confirms the presence of elements such as Zn, O, C, Au and Si found in the sample. The EDX spectrum contains Si peak which comes from the substrate and the presence of Au peak is due to the gold sputtering of the sample for HRSEM investigation. 3.5. UV–visible analysis of ZnO/C60 core/shell NRAs Fig. 6 represents UV–visible absorption spectra of bare ZnO and ZnO/C60 core/shell heterostructure. C60 being a narrow band gap material compared with ZnO, a notable enhancement is observed in the absorption spectrum after the loading of C60 on the surface of ZnO NRAs. There are two absorption edges noticed in ZnO/C60 core/shell structure, one is at 402 nm corresponding to bare ZnO nanorods while the other is related to C60 nanocrystals which makes ZnO to extend its absorption capability
Fig. 5. EDX spectra of ZnO/C60 core/shell thin film.
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Fig. 6. Room temperature absorption spectra of bare ZnO and ZnO/C60 core/shell NRAs.
of light in the visible region [18]. The presence of two distinct absorption edges corresponding to ZnO and C60 reveals the spatial separation of two semiconducting phases [19]. In addition, it is also noted that the absorption edge of ZnO in the C60 loaded film slightly shifts to longer wavelength region, 402 nm (red-shifted) as compared to the absorption edge of bare ZnO (382 nm) which might be favored by the effective adsorption of C60 resulting in electronic transition between the C60 shell and the ZnO core. This can be ascribed to the photo-excitation of electrons between highest occupied molecular orbital and lowest unoccupied molecular orbital (HOMO–LUMO) levels from C60 which are then injected into the conduction band of ZnO [8]. The optical energy band gaps have been derived to find the assignment of the above two absorption edges with the help of following equation:
ðahtÞ ¼ Bðht Eg Þ
m
ð1Þ
where B is a constant, h is the plank’s constant, t is the frequency of the incident light, Eg is the optical energy band gap of the material and m is an exponent which is 1/2 for allowed direct transitions. Fig. 7 shows the plot of (aht)2 versus energy (ht) of ZnO/C60 core/shell NRAs and Eg values are determined by extrapolating the linear portion of the curve to meet the energy axis, (aht)2 = 0. According to the literature reported by many researchers using different kinds of techniques, the narrow band gap semiconductor C60 has optical energy gap between the HOMO–LUMO [20] levels vary from 1.5 to 2.3 eV [21,22]. Moreover, the energy band gap value progressively varies for bare C60 thin film based on the crystallinity of the film. The optical energy band gap value is close to 1.5 eV for highly crystalline C60 films but for highly disordered films it is near 2.3 eV [23]. However, the present work gives two different energy band gap values of 3.05 eV corresponding to bare ZnO and 1.83 eV related to C60 which reveals the crystalline nature of C60 shell layer. 3.6. PL-analysis of ZnO/C60 core/shell NRAs PL measurements were performed to investigate the optical quality of the prepared samples. Fig. 8 depicts PL-spectra of ZnO NRAs before and after deposition of C60 shell layer with the excitation wavelength of 350 nm. The spectra clearly displays a strong near-band-edge (NBE) emission peak centered at 380 nm due to the recombination of excitons while there is a broad deep-level yellow emission centered at 574 nm corresponding to trap emissions for both ZnO and ZnO/C60 core/shell NRAs. It is found that the sharp NBE emission is greatly enhanced while there is suppression observed in the defect emission after incorporating C60 shell layer which indicates the surface modification of ZnO nanorods
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Fig. 7. Plot of (aht)2 versus ht of ZnO/C60 core/shell heterostructure thin film.
Fig. 8. Room temperature PL-spectra of bare ZnO and ZnO/C60 core/shell NRAs.
on improving the optical quality of the film. This might be due to the electronic interactions intensely occurring between ZnO core and C60 shell layers. Chen et al. reported that the visible emission of ZnO is highly dominated by surface defects which are reduced due to the lesser probability of number of transferred electrons to be trapped at the defect states of ZnO nanorod surface upon wrapping with C60 shell layer. Evidently, the existence of C60 layer causes the electron–hole separation and passivates the defect states on ZnO NRAs [14]. 3.7. Electrical studies In order to study the electrical behavior of the bare ZnO and ZnO/C60 core/shell NRAs current to voltage (I–V) measurements have been done under dark and light illumination conditions using silver
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Fig. 9. Plot of I–V characteristics of bare ZnO and ZnO/C60-core/shell NRAs under dark and light illumination.
contact and the results are shown in Fig. 9. It is observed that both films exhibit ohmic behavior, which is a basic requirement for a good conductor. The measured electrical resistivity values for the bare ZnO NRAs are 2.86 102 and 5.43 103 X cm for dark and light illuminations and the corresponding values for ZnO/C60 core/shell NRAs are 9.69 102 and 2.23 103 X cm. There is a notable decrease in resistivity from dark and illuminated conditions for both bare ZnO and surfaced modified ZnO NRAs that reveals the existence of photo-excited charge carriers and enhancement in effective separation of excitons [14]. Noticeably, there is a reduction in resistivity under light illumination after incorporating C60 shell layer to provide short and continuous pathways for electron transport process [13]. This result is well supported with the optical absorption spectra (Fig. 6) showing a significant increase in absorption all over the UV and visible region for the film coated with C60. Further, this can be validated by the enhancement in PL intensity of surface modified ZnO NRAs (Fig. 8) to be attributed to the effective exciton separation coupled with reduced surface defect states. This result reveals that C60 shell layer improves the electrical property of ZnO NRAs towards the application of photosensitive devices. 4. Conclusions ZnO/C60 core/shell-type heterostructure has been successfully synthesized on glass substrate using simple chemical route. XRD results reveal the mixed crystal structures of hexagonal ZnO along with the face-centered cubic C60. FTIR-studies give evidence for the occurrence of surface modification on ZnO NRAs by C60. HRTEM images confirm the formation of ZnO/C60 core/shell structure while HRSEM images depict the presence of C60 loaded on the surface of ZnO NRAs. UV–visible absorption spectrum has two distinct absorption edges corresponding to core ZnO and shell C60. PL-measurements reveal that the coating of C60 as shell layer enhances the optical quality of core ZnO NRAs by increasing the intensity of NBE emission and reducing the surface defect emission. The electrical resistivity of surface modified ZnO NRAs by C60 is significantly reduced under illumination supporting the features of photo-induced charge carrier generation and enhancement in separation of excitons. Hence, ZnO/C60 core/shell structure is an excellent choice for fabricating organic/inorganic hybrid optoelectronic devices. Acknowledgements We gratefully acknowledge the Central Research Facility, Karunya University for the support in taking XRD, UV-visible and PL-spectra measurements. We also thank SAIF, IIT Madras, and PSG Institute of Advanced Studies, Coimbatore for extending HRSEM and HRTEM facilities.
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References [1] M. Voigt, M. Klaumunzer, A. Ebel, F. Werner, G. Yang, R. Marczak, E. Spiecker, D.M. Guldi, A. Hirsch, W. Peukert, J. Phys. Chem. C 115 (2011) 5561–5565. [2] F. Schedin, A.K. Geim, S.V. Morozov, E.W. Hill, P. Blake, M.I. Katsnelson, K.S. Novoselov, Nat. Mater. 6 (2007) 652–655. [3] J.C. Meyer, A.K. Geim, M.I. Katsnelson, K.S. Novoselov, T.J. Booth, S. Roth, Nature 446 (2007) 60–63. [4] D.I. Son, T.W. Kim, J.H. Shim, J.H. Jung, D.U. Lee, J.M. Lee, W.I. Park, W.K. Choi, Nano Lett. 10 (2010) 2441–2447. [5] D.I. Son, B.W. Kwon, J.D. Yang, D.H. Park, B. Angadi, W.K. Choi, J. Mater. Chem. 22 (2012) 816–819. [6] Y.M. Guang, D.M. Run, L.B. Bing, Chin. Phys. B 22 (2013) 098109. [7] X. Wang, P. Hu, Y. Fangli, L. Yu, J. Phys. Chem. C 111 (2007) 6706–6712. [8] D. Bonifazi, A. Salomon, O. Enger, F. Diederich, D. Cahen, Adv. Mater. 14 (2002) 802–805. [9] Y.C. Feng, J.Y. Fan, W.Z. Guang, P.L. Long, S.T. Long, Z.Y. Bin, Chin. Phys. B 20 (2011) 076105. [10] H. Takeya, R. Kato, T. Wakahara, K. Miyazawa, T. Yamaguchi, T. Ozaki, H. Okazaki, Y. Takano, Mater. Res. Bull. 48 (2013) 343–345. [11] R.C. Haddon, A.S. Perel, R.C. Morris, T.T.M. Palstra, A.F. Hebard, R.M. Fleming, Appl. Phys. Lett. 67 (1995) 121–123. [12] S. Yoo, B. Domercq, B. Kippelen, Appl. Phys. Lett. 85 (2004) 5427–5429. [13] J.S. Huang, C.Y. Chou, C.F. Lin, Sol. Energy Mater. Sol. Cells 94 (2010) 182–186. [14] C.T. Chen, F.C. Hsu, S.W. Kuan, Y.F. Chen, Sol. Energy Mater. Sol. Cells 95 (2011) 740–744. [15] L.L. Yang, Q.X. Zhao, M. Willander, J. Alloys Compd. 469 (2009) 623–629. [16] D. Krishnan, T. Pradeep, J. Cryst. Growth 311 (2009) 3889–3897. [17] C. Li, Y. Chen, Y. Wang, Z. Iqbal, M. Chhowalla, S. Mitra, J. Mater. Chem. 17 (2007) 2406–2411. [18] K. Katsumata, N. Matsushita, K. Okada, Int. J. Photoenergy 2012 (2012) 1–9. [19] G. Guerguerian, F. Elhordoy, C.J. Pereyra, R.E. Marotti, F. Martin, D. Leinen, J.R.R. Barrado, E.A. Dalchiele, Nanotechnology 22 (2011) 505401. [20] W. Zhou, S. Xie, S. Qian, G. Wang, L. Qian, J. Phys.: Condens. Matter 8 (1996) 5793–5800. [21] S.H. Yang, C.L. Pettiette, J. Conceicao, O. Cheshnovsky, R.E. Smalley, Chem. Phys. Lett. 139 (1987) 233–238. [22] R.W. Lof, M.A. van Veenendaal, B. Koopmans, H.T. Jonkman, G.A. Sawatzky, Phys. Rev. Lett. 68 (1992) 3924–3927. [23] N. Bajwa, K. Dharamvir, V.K. Jindal, A. Ingale, D.K. Avasthi, J. Appl. Phys. 94 (2003) 326–333.
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Improved opto-electrical behavior of ZnO/C60 core/shell type organic–inorganic heterostructure M. Krishnaveni, Suganthi Devadason ⇑ Department of Physics, Karunya University, Coimbatore 641 114, Tamil Nadu, India
a r t i c l e
i n f o
Article history: Received 27 October 2014 Accepted 30 October 2014 Available online 20 November 2014 Keywords: ZnO nanorods C60 Core/shell Heterostructures Optical Electrical
a b s t r a c t We report a successful synthesis of ZnO/C60 core/shell-type organic–inorganic hybrid nanorod arrays (NRAs) using simple chemical route. X-ray diffraction pattern (XRD) reveals hexagonal crystal structure of ZnO along with the face-centered cubic (fcc) structure of C60. Detailed Fourier transform infrared spectroscopy (FTIR) inferred the attachment of C60 on the surface of ZnO NRAs. HRTEM observation confirmed 3 nm thickness of a very thin C60 nanocrystals shell layer formation. HRSEM image shows the existence of C60 on vertically aligned ZnO NRAs. UV–visible absorption spectra have two absorption edges corresponding to the energy band gap values of 3.05 and 1.83 eV which arise due to core ZnO and shell C60 respectively. Photoluminescence spectra reveal a strong enhancement in the NBE emission and suppression in the deep-level emission on the surface modified ZnO NRAs. The measured electrical resistivity under illumination for ZnO/C60 core/ shell is lower than that of bare ZnO NRAs. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Zinc oxide is developing as an auspicious material for applications in printable and flexible electronics due to its wide band gap semiconductor nature [1]. Among the various ZnO nanostructures, one-dimensional ZnO nanorods have attracted great interest for their immeasurable potential applications in fabricating nano-scale devices. Recently, in material science carbon nano-materials are ⇑ Corresponding author. Tel.: +91 9952287436. E-mail address: [email protected] (S. Devadason). http://dx.doi.org/10.1016/j.spmi.2014.10.036 0749-6036/Ó 2014 Elsevier Ltd. All rights reserved.
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extensively studied [2–4] and opening up an innovative research area because of the numerous studies on their incredible electrical properties and diverse ranging potential applications [5]. C60 (fullerene) molecule is considered as the third allotrope of carbon which comprises of 20 hexagonal and 12 pentagonal rings creating a soccer or bucky-ball shape that are held together by weak van der Waals interactions wherein all carbon atoms are placed at the nodes of a series of hexagons and pentagons [6]. To enhance a material’s thermal, mechanical or chemical stability, one can be modify the functionality, charge and reactivity of the material through surface coating with preferred material [7]. In particular, as C60 is an excellent electron acceptor when combined with ZnO, the organic molecules adsorption promises the fine-tuning of electronic properties [8]. Owing to their exclusive structures and interesting properties, research involving C60 has been increased in the fields of physics, chemistry and material science [9]. Furthermore, C60 possess wide range of applications in superconductivity [10], organic thin film transistor [11], organic solar cells [12], etc. Huang et al. reported that infiltration of fullerene as a functional interlayer onto the ZnO nanorod arrays (NRAs) provides significant improvement in the organic/inorganic hybrid solar cell device performance [13]. Specifically, for excellent potential applications in next-generation nonvolatile memory devices, inorganic nanostructures containing hybrid inorganic/organic composites are now developed as excellent candidates [4]. However, surface modification of inorganic semiconductor nanostructure is essential to increase the charge transport at the inorganic/organic interface in hybrid photovoltaic devices [14]. Therefore, it is imperative to facilitate the surface coating of zero-dimensional (0-D) C60 shell layer onto the one-dimensional (1-D) ZnO NRAs in order to obtain ZnO/C60 core/shell composites which provide a path way for tailoring the structural, electrical and optical properties. In the present work, an attempt has been made to report the systematic study of ZnO/C60 core/shell-type hybrid NRAs via simple chemical route by observing the modification in their structural, morphological, optical and electrical properties using various characterization techniques.
2. Experimental method 2.1. Synthesis of core ZnO NRAs Two-step chemical bath deposition method [15] has been employed to grow well-aligned ZnO NRAs. In the first step, 0.1 M concentration of zinc acetate dihydrate [(CH3COO)2 Zn2H2O] was dissolved in ethanol and then diethanolamine was added in drops to act as the stabilizer. This mixture was stirred at 50 °C for 1 h to develop a homogeneous, clear and transparent solution. After ageing for a day, the obtained precursor solution was coated on glass substrates using spin coater spun at the rate of 3000 rpm for 30 s. To remove the solvent and organic residuals, the films were dried at 200 °C for 2 min after each coating. The process from spin coating to drying was repeated several times to obtain a layer of dense and uniformly distributed ZnO seed nanoparticles on glass substrate. Then the ZnO seed layer coated substrates were post annealed at 400 °C for 1 h in a muffle furnace. The prepared ZnO seed layer film is 70 nm in thickness which gives a good support for the growth of NRAs. In the second step, the seeded films were suspended in a closed beaker containing 150 ml of aqueous solution having an equal molar (20 mM) of zinc acetate dihydrate and hexamethylenetetramine [C6H12N4]. The whole set-up was kept inside a hot air oven and the reaction temperature was maintained at 90 °C for 3 h to grow ZnO NRAs. Finally, the substrates were removed from the reaction bath, cleaned using distilled water to eliminate the residual salts and dried in air. Thus, well-aligned ZnO NRAs were grown on the seed layer using the two-step process.
2.2. Synthesis of C60 shell layer The deposition of C60 shell layer on to the ZnO NRAs has been carried out using spin coating technique [14]. 0.045 g of C60 powder purchased from sigma Aldrich with 99.95% purity was dispersed in 3 ml of chlorobenzene [C6H5Cl] and the resultant dispersed solution was spun over the ZnO NRAs at the rate of 3000 rpm for 30 s. The spin coating process was repeated up to 7 times to accomplish a
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uniform C60 shell layer distribution on the entire surface of ZnO NRAs. The transformation of ZnO film surface from white to gray color indicates the formation of C60 shell layer over ZnO NRAs. Microstructural studies have been carried out using X-ray Shimadzu XRD-6000 diffractometer with Cu Ka radiation recorded in the range 4–90°. Fourier transform infrared spectroscopy (FTIR) analysis was carried out using IR prestige-21 Shimadzu Fourier transform infrared spectroscopy. The detailed microstructural properties were analyzed using JEOL JEM 2100 high resolution transmission electron microscope (HRTEM) and surface morphological studies were performed by FEI Quanta FEG 200-high resolution scanning electron microscope (HRSEM) attached with energy dispersive X-ray analysis (EDX). UV–visible–NIR spectra were recorded using JASCO-670 UV/VIS/NIR spectrometer. Room temperature photoluminescence (PL) analysis was performed using a Fluoromax-4 spectrophotometer. The current to voltage (I–V) measurements were carried out using an automated testing device by National Instruments (NI PXI-1044).
3. Result and discussion 3.1. X-ray diffraction analysis of ZnO/C60 core/shell NRAs Fig. 1 gives the X-ray diffraction spectra for the bare ZnO NRAs and ZnO/C60 core/shell thin films. On comparing the XRD spectra of hybrid with the bare ZnO NRAs film, both exhibit a sharp and intense diffraction peak at 34.5° corresponding to the (0 0 2) plane of ZnO demonstrating the high crystallinity with the preferred c-axis growth orientation of ZnO NRAs. There are other weak diffraction peaks centered at 31.7°, 36.4°, 47.6°, 63.0°, 68.0° and 72.7° belonging to (1 0 0), (1 0 1), (1 0 2), (1 0 3), (1 1 2) and (0 0 4) planes respectively of hexagonal wurtzite crystal structure of core ZnO NRAs [JCPDS card no.
Fig. 1. X-ray diffraction patterns of bare ZnO and ZnO/C60 core/shell thin films.
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79-0205]. It is to be noted for the ZnO/C60 core/shell structure, the XRD spectrum reveals the presence of C60 shell layer along with the ZnO NRAs which is confirmed by the appearance of four additional diffraction characteristic peaks positioned at 10.8°, 17.7°, 20.7° and 21.7° corresponding to the well matched (1 1 1), (2 2 0), (3 1 1) and (2 2 2) reflecting planes respectively of face-centered cubic (fcc) structure of C60 [JCPDS card no. PDF #812220].
3.2. FTIR analysis of ZnO/C60 core/shell NRAs The surface modification of ZnO nanorods with C60 has been analyzed by FTIR studies. Fig. 2 shows FTIR spectrum for bare ZnO and ZnO/C60 core/shell NRAs recorded in the wavenumber range from 400 to 4000 cm1. The broad absorption band appearing at 3433 cm1 for bare ZnO is attributed to the OAH stretching vibrations of hydroxyl groups due to the adsorption of water on the surface of ZnO nanorods which is found to slightly shift to lower wavenumber region of 3431 cm1 for ZnO/C60 core/shell NRAs because of the surface modification of ZnO NRAs with C60 molecule. The peaks observed between 2300 and 2900 cm1 are assigned to CAH stretching vibration mode of the alkane groups [16]. The two bands located at 1591 and 1421 cm1 in bare ZnO accounting to the asymmetric and symmetric stretching vibrations of carboxyl groups (ACOOH) has been shifted to 1618 and 1428 cm1 in ZnO/C60 which clearly indicates the chemisorption of C60 on the surface of ZnO nanorods by carboxyl groups. The peaks positioned at 1184 and 1024 cm1 correspond to deformation mode and the stretching vibration of CAO respectively. The appearance of an additional new peak at 525 cm1 in the finger print region (<1000 cm1) belongs to C60 [17] which confirms the successful functionalization of ZnO NRAs by C60. The peak at 422 cm1 due to the ZnAO stretching vibrations of ZnO nanorods is found to shift to a lower wavenumber region of 420 cm1 for the ZnO/C60 core/shell NRAs which may be attributed to the surface changes of ZnO NRAs.
Fig. 2. FTIR-analysis for bare ZnO and ZnO/C60 core/shell NRAs.
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3.3. HRTEM analysis of ZnO/C60 core/shell NRAs Crystallography and microstructure of the as-synthesized sample is further characterized by HRTEM analysis. Fig. 3(a)–(c) shows HRTEM images of ZnO/C60 core/shell nanorod under different magnifications. Obviously, adsorption of 3 nm thickness of C60 shell layer has been established by the appearance of a fine dark band of C60 wrapping around the ZnO nanorod as shown in Fig. 3(a). Evidently, Fig. 3(a)–(c) shows the existence of spherical shaped C60 bucky-balls of diameter varying from 3 nm onwards decorate (marked by arrows) the surface of ZnO nanorod. The corresponding SAED pattern of ZnO/C60 core/shell nanorod shown in Fig. 3(d) represents the monocrystalline ZnO nanorod and polycrystalline C60 of the core/shell heterostructure. 3.4. HRSEM analysis of ZnO/C60 core/shell NRAs Fig. 4(a) and (b) shows the typical top and side-view HRSEM images of C60 coated vertically aligned ZnO NRAs. The measured length and diameter of the ZnO nanorods are found to be in the range of 1.3–1.7 lm and 50–100 nm respectively. The images depict the growth of bigger clusters of agglomerated C60 crystallites which are loaded on the top surface of ZnO NRAs. This is again affirmed in the side-view HRSEM image as shown in Fig. 4(b), wherein the ZnO NRAs are capped by C60 nanocrystals. Compared with HRTEM images, it is difficult to discern the presence of C60 nanocrystals
Fig. 3. (a-c) Low and high magnification HRTEM images and (d) the corresponding selected area electron diffraction pattern (SAED) of ZnO/C60 core/shell nanorod thin film.
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Fig. 4. (a) and (b) HRSEM images of top and side-view of ZnO/C60 core/shell NRAs.
using the low resolution HRSEM images. Fig. 5 shows energy dispersive X-ray analysis of ZnO/C60 hybrid core/shell structure. The compositional analysis confirms the presence of elements such as Zn, O, C, Au and Si found in the sample. The EDX spectrum contains Si peak which comes from the substrate and the presence of Au peak is due to the gold sputtering of the sample for HRSEM investigation. 3.5. UV–visible analysis of ZnO/C60 core/shell NRAs Fig. 6 represents UV–visible absorption spectra of bare ZnO and ZnO/C60 core/shell heterostructure. C60 being a narrow band gap material compared with ZnO, a notable enhancement is observed in the absorption spectrum after the loading of C60 on the surface of ZnO NRAs. There are two absorption edges noticed in ZnO/C60 core/shell structure, one is at 402 nm corresponding to bare ZnO nanorods while the other is related to C60 nanocrystals which makes ZnO to extend its absorption capability
Fig. 5. EDX spectra of ZnO/C60 core/shell thin film.
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Fig. 6. Room temperature absorption spectra of bare ZnO and ZnO/C60 core/shell NRAs.
of light in the visible region [18]. The presence of two distinct absorption edges corresponding to ZnO and C60 reveals the spatial separation of two semiconducting phases [19]. In addition, it is also noted that the absorption edge of ZnO in the C60 loaded film slightly shifts to longer wavelength region, 402 nm (red-shifted) as compared to the absorption edge of bare ZnO (382 nm) which might be favored by the effective adsorption of C60 resulting in electronic transition between the C60 shell and the ZnO core. This can be ascribed to the photo-excitation of electrons between highest occupied molecular orbital and lowest unoccupied molecular orbital (HOMO–LUMO) levels from C60 which are then injected into the conduction band of ZnO [8]. The optical energy band gaps have been derived to find the assignment of the above two absorption edges with the help of following equation:
ðahtÞ ¼ Bðht Eg Þ
m
ð1Þ
where B is a constant, h is the plank’s constant, t is the frequency of the incident light, Eg is the optical energy band gap of the material and m is an exponent which is 1/2 for allowed direct transitions. Fig. 7 shows the plot of (aht)2 versus energy (ht) of ZnO/C60 core/shell NRAs and Eg values are determined by extrapolating the linear portion of the curve to meet the energy axis, (aht)2 = 0. According to the literature reported by many researchers using different kinds of techniques, the narrow band gap semiconductor C60 has optical energy gap between the HOMO–LUMO [20] levels vary from 1.5 to 2.3 eV [21,22]. Moreover, the energy band gap value progressively varies for bare C60 thin film based on the crystallinity of the film. The optical energy band gap value is close to 1.5 eV for highly crystalline C60 films but for highly disordered films it is near 2.3 eV [23]. However, the present work gives two different energy band gap values of 3.05 eV corresponding to bare ZnO and 1.83 eV related to C60 which reveals the crystalline nature of C60 shell layer. 3.6. PL-analysis of ZnO/C60 core/shell NRAs PL measurements were performed to investigate the optical quality of the prepared samples. Fig. 8 depicts PL-spectra of ZnO NRAs before and after deposition of C60 shell layer with the excitation wavelength of 350 nm. The spectra clearly displays a strong near-band-edge (NBE) emission peak centered at 380 nm due to the recombination of excitons while there is a broad deep-level yellow emission centered at 574 nm corresponding to trap emissions for both ZnO and ZnO/C60 core/shell NRAs. It is found that the sharp NBE emission is greatly enhanced while there is suppression observed in the defect emission after incorporating C60 shell layer which indicates the surface modification of ZnO nanorods
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Fig. 7. Plot of (aht)2 versus ht of ZnO/C60 core/shell heterostructure thin film.
Fig. 8. Room temperature PL-spectra of bare ZnO and ZnO/C60 core/shell NRAs.
on improving the optical quality of the film. This might be due to the electronic interactions intensely occurring between ZnO core and C60 shell layers. Chen et al. reported that the visible emission of ZnO is highly dominated by surface defects which are reduced due to the lesser probability of number of transferred electrons to be trapped at the defect states of ZnO nanorod surface upon wrapping with C60 shell layer. Evidently, the existence of C60 layer causes the electron–hole separation and passivates the defect states on ZnO NRAs [14]. 3.7. Electrical studies In order to study the electrical behavior of the bare ZnO and ZnO/C60 core/shell NRAs current to voltage (I–V) measurements have been done under dark and light illumination conditions using silver
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Fig. 9. Plot of I–V characteristics of bare ZnO and ZnO/C60-core/shell NRAs under dark and light illumination.
contact and the results are shown in Fig. 9. It is observed that both films exhibit ohmic behavior, which is a basic requirement for a good conductor. The measured electrical resistivity values for the bare ZnO NRAs are 2.86 102 and 5.43 103 X cm for dark and light illuminations and the corresponding values for ZnO/C60 core/shell NRAs are 9.69 102 and 2.23 103 X cm. There is a notable decrease in resistivity from dark and illuminated conditions for both bare ZnO and surfaced modified ZnO NRAs that reveals the existence of photo-excited charge carriers and enhancement in effective separation of excitons [14]. Noticeably, there is a reduction in resistivity under light illumination after incorporating C60 shell layer to provide short and continuous pathways for electron transport process [13]. This result is well supported with the optical absorption spectra (Fig. 6) showing a significant increase in absorption all over the UV and visible region for the film coated with C60. Further, this can be validated by the enhancement in PL intensity of surface modified ZnO NRAs (Fig. 8) to be attributed to the effective exciton separation coupled with reduced surface defect states. This result reveals that C60 shell layer improves the electrical property of ZnO NRAs towards the application of photosensitive devices. 4. Conclusions ZnO/C60 core/shell-type heterostructure has been successfully synthesized on glass substrate using simple chemical route. XRD results reveal the mixed crystal structures of hexagonal ZnO along with the face-centered cubic C60. FTIR-studies give evidence for the occurrence of surface modification on ZnO NRAs by C60. HRTEM images confirm the formation of ZnO/C60 core/shell structure while HRSEM images depict the presence of C60 loaded on the surface of ZnO NRAs. UV–visible absorption spectrum has two distinct absorption edges corresponding to core ZnO and shell C60. PL-measurements reveal that the coating of C60 as shell layer enhances the optical quality of core ZnO NRAs by increasing the intensity of NBE emission and reducing the surface defect emission. The electrical resistivity of surface modified ZnO NRAs by C60 is significantly reduced under illumination supporting the features of photo-induced charge carrier generation and enhancement in separation of excitons. Hence, ZnO/C60 core/shell structure is an excellent choice for fabricating organic/inorganic hybrid optoelectronic devices. Acknowledgements We gratefully acknowledge the Central Research Facility, Karunya University for the support in taking XRD, UV-visible and PL-spectra measurements. We also thank SAIF, IIT Madras, and PSG Institute of Advanced Studies, Coimbatore for extending HRSEM and HRTEM facilities.
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