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Strong temperature and substrate effect on ZnO nanorod flower structures in modified chemical vapor condensation growth
Current Applied Physics 10 (2010) 942–946
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Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Strong temperature and substrate effect on ZnO nanorod flower structures in modified chemical vapor condensation growth S.R. Haldar a, A. Nayak b, T.K. Chini a, S. Bhunia a,* a b
Surface Physics Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700064, India Department of Physics, Presidency College, 86/1, College Street, Kolkata 700073, India
a r t i c l e
i n f o
Article history: Received 12 August 2009 Received in revised form 26 September 2009 Accepted 15 November 2009 Available online 22 November 2009 Keywords: ZnO Nanorods Flower structures
a b s t r a c t We have reported low temperature growth (300 °C) of ZnO nanorod flower structures by depositing zinc acetate vapor on Ge (100) substrate in the form of a jet using chemical vapor condensation technique. The flowers were comprised of hierarchical arrangement of highly crystalline ZnO nanorods oriented isotropically around a common nucleus. The temperature window for stability of these structures was found to be very narrow and the formation of the flowers was highly depended on the type of the substrates used. The flower morphology changed to a different hemispherical shape when the growth temperature was increased by only 50 °C while decreasing the growth temperature of the same degrees resulted in an amorphous deposition of ZnO. The temperature and substrate effect has been explained on the basis of adatom kinetics during growth. X-ray diffraction and TEM study revealed wurtzite ZnO nanorods with lattice constants a and c of 3.2 and 5.19 Å, respectively. The flower structures showed strong room temperature photoluminescence having pure excitonic transition at around 3.298 eV. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Realization of technologically useful nanomaterials will depend not only on the quality of the crystal and their surface chemistry, but also on their special design, orientation and arrangement [1,2]. Therefore, development of morphologically controllable synthesis of nano or microstructures of materials and clever design by means of crystal growth of mesoscopic structures composed of nanostructured material is urgently important. Zinc oxide (ZnO) is an exceptionally important semiconductor with a direct wide band gap (3.37 eV) at room temperature and large exciton binding energy (60 meV) [3]. The non-central symmetric crystallographic structure, spontaneous surface polarization characteristics and ability to grow in an abundant variety of self organized nanostructures make it one of the most exciting oxide nanostructures for investigating nanoscale physical and chemical phenomena [4– 11]. These low dimensional nanostructures exhibit novel properties including near-UV laser emission [1], gas and piezoelectric sensing [12], photovoltaics and find potential applications in nano-devices exploiting these properties [13]. Very recently, growth of exotic self organized 3-D architectures of ZnO such as nanoflowers, comprised of nanowires and other nanostructures have been reported and it was suggested that these ‘designed’
* Corresponding author. Tel.: +91 33 23375348; fax: +91 33 23374637. E-mail address: [email protected] (S. Bhunia). 1567-1739/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2009.11.077
structures may improve the physical properties and chemical activities of ZnO [14–18]. Many of these reported flower-like structures were grown by wet chemical synthesis method. In this paper, we will report a novel low temperature catalyst-free growth of self-organized 3-D assembly of ZnO nanorod flower structures by a modified chemical vapor condensation method and study the effect of growth temperature and type of substrates on the stability of the structures.
2. Experimental The ZnO flower structures were nominally grown on (100) oriented Ge substrates by a modified chemical vapor condensation method using zinc acetate dihydrate (Zn(CH3COO)22H2O) powder as source material at a growth temperature of 300 °C for 60 min. Sapphire (1000) and glass substrates were also placed next to Ge in the reactor to understand the variation in flower morphology under the identical growth condition. To study the effect of temperature on the structures, two additional growth runs were carried out at 250 °C and 350 °C. For each experiment, about 200 mg of zinc acetate powder was taken as source material in a confined cylindrical quartz crucible with a small orifice at the top and placed inside a vertical muffle furnace at its bottom. The substrate was kept vertically above the crucible with the growth surface facing the orifice. Heating rate of the system was maintained at 5 °C/
S.R. Haldar et al. / Current Applied Physics 10 (2010) 942–946
min and the source to substrate distance was kept at 4 cm, both of which are very sensitive to the formation of stable flower structures. Zinc acetate undergoes thermal decomposition at the given temperature producing ZnO vapor which then passes through the orifice in the form of vapor jet and condensed on the substrate. This arrangement was crucial for the formation of stable flower structures, without which we obtained randomly oriented ZnO nanorods throughout the substrate. The morphological and structural properties of the flower structures were investigated by scanning electron microscopy (SEM) and X-ray diffraction techniques. The optical properties were studied by photoluminescence (PL) measurements at room temperature using a He–Cd laser excitation at 325 nm.
3. Results and discussions Fig. 1(a) shows SEM micrograph of a typical ZnO nanorod flower structure grown at 300 °C for 60 min on Ge (100) substrate. Each structure was composed of densely spaced and multi-stacked ZnO nanorods grown radially outward from a common nucleation centre and oriented isotropically, forming architecture with the resemblance to a flower. Such common nucleation found to occur when the sublimation vapors were allowed to pass through the nozzle. High magnification SEM images (Fig. 1(b)) reveal that each nanorod of the flowers was highly crystalline, exhibiting crystal facets with (0001) orientation typical to ZnO. When the number density of the flowers grown on the substrate was low, their shapes were nearly isotropic and regular with only one nucleation point at the centre (as shown in Fig. 1(a)). In the region where number density was relatively large so that the distance between two adjacent nuclei is smaller than the free diameter of each flower, then two growth fronts directed towards opposite nuclei overlap and a sin-
943
gle flower with elliptical shape develop with two nuclei at the centre, as shown in Fig. 1(c). Similarly, when three nucleations take place close to each other, overlapping of two growth fronts could be observed (Fig. 1(d)). No intermixing of the adjacent growth fronts at the overlap region occurs indicating the growth process terminates in this direction when no further substrate space was available. These observations suggest that growth of the structures is strongly limited by surface diffusion process of the adatoms on the substrate. Fig. 2(a) shows the TEM micrograph of individual nanorods. The high resolution lattice image shown in Fig. 2(b) suggests the nanorods were single crystalline and the distance between two lattice planes was about 0.53 nm which correspond to the d-spacing of the (0001) crystal planes of wurtzite ZnO. The corresponding selected area electron diffraction pattern shown in the inset of Fig. 2(b) further substantiates the crystallinity of the ZnO nanorods and the growth direction. When the samples were grown at 350 °C, the shape of the flowers changed to a new kind of hemispherical morphology, as shown in Fig. 3(a). These structures appear to be composed of nano-nail type of structures, which have been reported earlier for ZnO [19–21]. The tips of the nano-nails were oriented towards the centre of the hemisphere and the flat heads could be seen on the surface, as has been explained in reference [20]. In our study, this is evident from the high resolution SEM image of the lower portion of the flowers shown in Fig. 3(b). The main difference in growth mechanism between flower structures grown at 300 °C and 350 °C is the type of mass transport from the central nucleation region of the flowers towards the periphery. Mass transport occurs during crystal growth through surface diffusion process of the adatoms, which is governed by surface kinetics and is strongly dependent on substrate types and temperature. The diffusion coefficient, Ds and the diffusion length, ks of adatoms on a crystalline surface can be given as
Fig. 1. (a) SEM micrograph of a typical ZnO nanorod flower structure grown at 300 °C for 60 min on Ge (100) using chemical vapor condensation technique. (b) High magnification SEM image showing crystal facets of individual ZnO nanorod of the flower structure. (c) and (d) SEM micrographs of two and three overlapping flower structures, respectively, showing the demarcation boundaries.
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S.R. Haldar et al. / Current Applied Physics 10 (2010) 942–946
Fig. 2. (a) TEM micrograph of individual ZnO nanorods of the flower structures grown at 300 °C for 60 min on Ge (100). (b) High resolution lattice image of a ZnO nanorod. Inset shows selected area electron diffraction pattern of ZnO nanorod.
Fig. 3. (a) SEM micrograph of ZnO flower structure grown at 350 °C for 60 min. (b) High resolution SEM image of the marked portion of (a).
Es
Es
Ds ¼ a2 mekT ¼ D0 ekT
ð1Þ
and
ks ¼ ae
Ed Es 2kT
ð2Þ
where a is the distance between adjacent adsorption/lattice sites, m is the vibrational frequency of the single adatom migration and Es, Ed are energy of hopping to adjacent sites and for desorption from the surface, respectively. The diffusion coefficient and the diffusion length could be related through the Einstein’s relation given as
ks ¼
pffiffiffiffiffiffiffiffi Ds s
ð3Þ
where s is a characteristic time describing a duration the adatom undergoes in the diffusion process. This characteristic time could be the mean time the adatom spends on the surface either before being captured in the lattice site or desorbed from the surface. The former definition is more relevant in the low temperature regime, as in our case. Substituting Eqs. (1) and (2) into Eq. (3) gives
ks ¼
pffiffiffiffiffiffiffiffiffi Es D0 se kT
ð4Þ
and
1
Ed
s ¼ ekT m
ð5Þ
In the low temperature regime, where Ed >> kT, the variation in
s between two growth temperatures of 300 °C and 350 °C can be neglected. Under such approximation, both Ds and ks increase with increasing temperature and vice versa. Thus for growth at 300 °C, the diffusive mass transport rate towards the outer region might
be smaller than the incorporation rate itself at the growing tip of the nanorods. For growth at 350 °C, the mass transport rate might exceed the incorporation rate and as a result, adatoms accumulate towards the end of the nanorods leading to the formation of the ‘heads’ of the nano-nails. When grown at a lower temperature of 250 °C, no flower structures could be observed and resulted in an amorphous deposition of ZnO all over the substrate. At low temperature, the lack of sufficient thermal energy hinders the surface diffusion process, which is necessary to produce crystalline materials. The role of surface diffusion process in forming the self organized structures was further elaborated from the study of growth under similar condition on (0001) oriented sapphire which has similar crystal structure like that of ZnO and on amorphous substrate such as glass and the results are shown in Fig. 4(a) and (b), respectively. It is evident from Eqs. (1) and (2) that both the diffusion coefficient and the diffusion length of the adatoms on the surface depend on the spacing of the adjacent adsorption sites under hopping, which can be taken as same as the surface lattice constant and the vibrational frequency. For amorphous glass, the random distribution of these surface sites leads to inefficient materials transport on the surface. Hence, though the nanorods were grown under the given growth condition, they were formed all over the surface but the self organized architectures of the nanorods could not be built. On the other hand when growth was carried out on sapphire, we obtained much thinner nanorods, all oriented at some angle to the substrate. No growth along the plane of the substrate was observed, which was prominent when grown on Ge. The ZnO nanorods, when grown on sapphire, tend to orient along different crystallographic directions of sapphire, including the common (0001) direction, similar to that observed for growth of InAs whis-
S.R. Haldar et al. / Current Applied Physics 10 (2010) 942–946
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Fig. 4. SEM micrographs of ZnO nanorods grown at 300 °C on (a) sapphire (0001) and (b) glass.
kers on GaAs [22]. Because of crystalline nature of the sapphire substrate, the surface diffusion of adatoms was prominent and hence no deposition of ZnO was observed on the surface. In both the substrates of glass and sapphire, the common feature of nucleated growth of nanorods starting from a common origin could still be observed. The crystalline structures of the nanorods of the flower structures grown on Ge substrates were evaluated by X-ray diffraction (XRD) method using Cu Ka radiation with a conventional h–2h goniometer. Fig. 5 shows XRD patterns of two types of flower structures grown at 300 °C and 350 °C along with that of the sample grown at 250 °C. Both the flower structures exhibit diffraction peaks at 31.7°, 34.4°, 36.2°, 47.5°, 56.6°, 62.9°, 67.8°, 68.9° corresponding to (100), (002), (101), (102), (110), (103), (112), (201) of ZnO with the hexagonal wurtzite structure. The calculated lattice constants a and c were 3.2 and 5.19 Å, respectively for both the samples. The samples grown at 250 °C exhibited broad and wide peak around 34o implying the amorphous nature of the film. The photoluminescence spectra (Fig. 6) of the flower structures grown on Ge substrates at 300 °C showed only pure free excitonic transition [23] centred at about 3.298 eV (376 nm) with full width at half maxima (FWHM) of about 132 meV. No defect related mid gap transitions were present in these samples. The samples grown at 350 °C on Ge also showed similar band gap related excitonic transition, but a strong and broad green emission peak centred around 500 nm also appeared. This deep-level emission peak is ~ oz-Herna´ndez et. al. [27] have very common in ZnO [24–26]. Mun proposed that this emission peak could be due to an electronic transition between oxygen vacancies (VO), which act as donors
Fig. 6. Room temperature photoluminescence spectra of ZnO flower structures grown at (a) 300 °C and (b) 350 °C on Ge (100) substrates.
and an electronic level generated by zinc vacancies (VZn) or zinc interstitial (Zni). These point defects are predominant in ZnO [28,29], which are easily incorporated in the crystals during growth due to high vapor pressures of both Zn and O. The vapor pressures of elements depend strongly on temperature and hence, the concentration of these defects increases with increase in growth temperature. In our case, the samples grown at 300 °C might have less defect concentrations and as a result, no defect related transitions could be observed. On the other hand, the samples grown at a relatively higher temperature of 350 °C had larger probability of incorporating the defect complexes causing the green emission observed in the PL spectra. The samples grown at lower temperature of 250 °C did not show any appreciable luminescence intensity because of their amorphous nature. 4. Summary
Fig. 5. X-ray diffraction pattern of ZnO flower structures grown at (a) 350 °C, (b) 300 °C and (c) 250 °C on Ge (100) substrates.
In this paper, we have demonstrated self organized hierarchical arrangement of ZnO nanorods in the form of flowers on Ge (100) substrates by a small modification of vapor condensation technique of zinc acetate dihydrate at low temperature of 300 °C by allowing the vapors to pass in the form of a jet. TEM and X-ray diffraction studies indicate that the nanorods comprising the flowers were crystalline in nature having wurtzite phase with lattice constant a and c of 3.2 and 5.19 Å, respectively. The morphology of the flower structures was found to be very sensitive to growth temperature and the type of substrates used. When the samples were grown at a relatively higher temperature of 350 °C, the constitu-
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S.R. Haldar et al. / Current Applied Physics 10 (2010) 942–946
ents of the flowers changed from nanorods to an apparently hierarchical organization of nano-nail like structures leading to a hemispherical structure. The flower structures were not formed when grown at a lower temperature of 250 °C but only amorphous deposition of material was found on the substrate. When grown on sapphire (0001) and glass substrates, we could only obtain uniform and randomly oriented nanorods, respectively. The growth behaviour has been explained through qualitative surface kinetic models of the adatoms. The nanorod flower structures grown at 300 °C showed pure free excitonic PL emission at 3.249 eV at room temperature and the hemispherical structures grown at 350 °C showed defect related mid gap transition in addition to the near band gap transition. Apart from the obvious interest from crystal growth’s point of view, these highly textured flower structures, especially those grown at 300 °C might show improved photovoltaic response because of the presence of large surface area and good structural and optical properties. Acknowledgements S.R. Haldar thanks CSIR, Govt. of India for financial support. References [1] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897. [2] H.Q. Yan, R.R. He, J. Pham, P.D. Yang, Adv. Mater. 15 (2003) 402. [3] Ü. Özgür, Y.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Dog˘an, V. Avrutin, S.J. Cho, H. Morkoç, J. Appl. Phys. 98 (2005) 41301. [4] Z.L. Wang, X.Y. Kong, Y. Ding, P.X. Gao, W.L. Hughes, R. Yang, Y. Zhang, Adv. Funct. Mater. 14 (2004) 943.
[5] D. Banerjee, J.Y. Lao, D.Z. Wang, J.Y. Huang, Z.F. Ren, D. Steeves, B. Kimball, M. Sennett, Appl. Phys. Lett. 83 (2003) 2061. [6] R. Wahab, S.G. Ansari, Y.S. Kim, M. Song, H.-S. Shin, Appl. Surf. Sci. 255 (2009) 4891. [7] J.J. Wu, S.C. Liu, C.T. Wu, K.H. Chen, L.C. Chen, Appl. Phys. Lett. 81 (2002) 1312. [8] X.Y. Kong, Y. Ding, R. Yang, Z.L. Wang, Science 303 (2004) 1348. [9] W.I. Park, G.C. Yi, M. Kim, S.J. Pennycook, Adv. Mater. 14 (2002) 1841. [10] J.Y. Lao, J.G. Wen, Z.F. Ren, Nano. Lett. 2 (2002) 1287. [11] U. Pal, J.G. Serrano, P. Santiago, G. Xiong, K.B. Ucer, R.T. Williams, Optical Materials 29 (2006) 65. [12] E. Comini, G. Fagila, G. Sberveglieri, Z. Pan, Z.L. Wang, Appl. Phys. Lett. 81 (2002) 1869. [13] H. Kind, H. Yan, B. Messer, L. Mathew, P. Yang, Adv. Mater. 14 (2002) 158. [14] G.H. Du, F. Xu, Z.Y. Yuan, G.V. Tendeloo, Appl. Phys. Lett. 88 (2006) 243101. [15] R. Wahab, S.G. Ansari, Y.S. Kim, H.K. Seo, G.S. Kim, G. Khang, H.-S. Shin, Mater. Res. Bull. 42 (2007) 1640. [16] J. Liu, X. Huang, Y. Li, J. Duan, H. Ai, Mater. Chem. Phys. 98 (2006) 523. [17] X. Xia, Z. Ye, G. Yuan, L. Zhu, B. Zhao, Appl. Surf. Sci. 253 (2006) 909. [18] S.R. Haldar, A. Nayak, T.K. Chini, S.K. Ray, N. Yamamoto, S. Bhunia, Phys. Status Solidi A (in press, doi:10.1002/pssa.200925223). [19] X. Songa, Y. Zhang, J. Zheng, X. Li, J. Phys. Chem. Sol. 68 (2007) 1681. [20] G. Shen, J.H. Cho, C.J. Lee, Chem. Phys. Letts. 401 (2005) 414. [21] Yu-H. Lin, C-C. Lin, J-M. Wu, U-S. Chen, J-R. Chen, H.C. Shih, Thin Sol. Film. 517 (2008) 1225. [22] K. Hiruma, M. Yazawa, T. Katsuyama, K. Ogawa, K. Haraguchi, M. Koguchi, J. Appl. Phys. 77 (1995) 447. [23] N. Ohashi, T. Sekiguchi, K. Aoyama, T. Ohgaki, Y. Terada, I. Sakaguchi, T. Tsurumi, H. Haneda, J. Appl. Phys. 91 (2002) 3658. [24] U. Pal, P. Santiago, J. Phys. Chem. B 109 (2005) 15317. [25] Y. Hu, H.J. Chen, J. Appl. Phys. 101 (2007) 124902. [26] S.A. Studenikin, N. Golego, M. Cocivera, J. Appl. Phys. 84 (1998) 2287–2294. ~ oz-Herna´ndez, A. Escobedo-Morales, U. Pal, Cryst. Growth and Design 9 [27] G. Mun (2009) 297. [28] A.F. Kohan, G. Ceder, D. Morgan, C.G. Van de Walle, Phys. ReV. B 61 (2000) 15019. [29] S.B. Zhang, S.H. Wei, A. Zunger, Phys. ReV. B 63 (2001) 075205.
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Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Strong temperature and substrate effect on ZnO nanorod flower structures in modified chemical vapor condensation growth S.R. Haldar a, A. Nayak b, T.K. Chini a, S. Bhunia a,* a b
Surface Physics Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700064, India Department of Physics, Presidency College, 86/1, College Street, Kolkata 700073, India
a r t i c l e
i n f o
Article history: Received 12 August 2009 Received in revised form 26 September 2009 Accepted 15 November 2009 Available online 22 November 2009 Keywords: ZnO Nanorods Flower structures
a b s t r a c t We have reported low temperature growth (300 °C) of ZnO nanorod flower structures by depositing zinc acetate vapor on Ge (100) substrate in the form of a jet using chemical vapor condensation technique. The flowers were comprised of hierarchical arrangement of highly crystalline ZnO nanorods oriented isotropically around a common nucleus. The temperature window for stability of these structures was found to be very narrow and the formation of the flowers was highly depended on the type of the substrates used. The flower morphology changed to a different hemispherical shape when the growth temperature was increased by only 50 °C while decreasing the growth temperature of the same degrees resulted in an amorphous deposition of ZnO. The temperature and substrate effect has been explained on the basis of adatom kinetics during growth. X-ray diffraction and TEM study revealed wurtzite ZnO nanorods with lattice constants a and c of 3.2 and 5.19 Å, respectively. The flower structures showed strong room temperature photoluminescence having pure excitonic transition at around 3.298 eV. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Realization of technologically useful nanomaterials will depend not only on the quality of the crystal and their surface chemistry, but also on their special design, orientation and arrangement [1,2]. Therefore, development of morphologically controllable synthesis of nano or microstructures of materials and clever design by means of crystal growth of mesoscopic structures composed of nanostructured material is urgently important. Zinc oxide (ZnO) is an exceptionally important semiconductor with a direct wide band gap (3.37 eV) at room temperature and large exciton binding energy (60 meV) [3]. The non-central symmetric crystallographic structure, spontaneous surface polarization characteristics and ability to grow in an abundant variety of self organized nanostructures make it one of the most exciting oxide nanostructures for investigating nanoscale physical and chemical phenomena [4– 11]. These low dimensional nanostructures exhibit novel properties including near-UV laser emission [1], gas and piezoelectric sensing [12], photovoltaics and find potential applications in nano-devices exploiting these properties [13]. Very recently, growth of exotic self organized 3-D architectures of ZnO such as nanoflowers, comprised of nanowires and other nanostructures have been reported and it was suggested that these ‘designed’
* Corresponding author. Tel.: +91 33 23375348; fax: +91 33 23374637. E-mail address: [email protected] (S. Bhunia). 1567-1739/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2009.11.077
structures may improve the physical properties and chemical activities of ZnO [14–18]. Many of these reported flower-like structures were grown by wet chemical synthesis method. In this paper, we will report a novel low temperature catalyst-free growth of self-organized 3-D assembly of ZnO nanorod flower structures by a modified chemical vapor condensation method and study the effect of growth temperature and type of substrates on the stability of the structures.
2. Experimental The ZnO flower structures were nominally grown on (100) oriented Ge substrates by a modified chemical vapor condensation method using zinc acetate dihydrate (Zn(CH3COO)22H2O) powder as source material at a growth temperature of 300 °C for 60 min. Sapphire (1000) and glass substrates were also placed next to Ge in the reactor to understand the variation in flower morphology under the identical growth condition. To study the effect of temperature on the structures, two additional growth runs were carried out at 250 °C and 350 °C. For each experiment, about 200 mg of zinc acetate powder was taken as source material in a confined cylindrical quartz crucible with a small orifice at the top and placed inside a vertical muffle furnace at its bottom. The substrate was kept vertically above the crucible with the growth surface facing the orifice. Heating rate of the system was maintained at 5 °C/
S.R. Haldar et al. / Current Applied Physics 10 (2010) 942–946
min and the source to substrate distance was kept at 4 cm, both of which are very sensitive to the formation of stable flower structures. Zinc acetate undergoes thermal decomposition at the given temperature producing ZnO vapor which then passes through the orifice in the form of vapor jet and condensed on the substrate. This arrangement was crucial for the formation of stable flower structures, without which we obtained randomly oriented ZnO nanorods throughout the substrate. The morphological and structural properties of the flower structures were investigated by scanning electron microscopy (SEM) and X-ray diffraction techniques. The optical properties were studied by photoluminescence (PL) measurements at room temperature using a He–Cd laser excitation at 325 nm.
3. Results and discussions Fig. 1(a) shows SEM micrograph of a typical ZnO nanorod flower structure grown at 300 °C for 60 min on Ge (100) substrate. Each structure was composed of densely spaced and multi-stacked ZnO nanorods grown radially outward from a common nucleation centre and oriented isotropically, forming architecture with the resemblance to a flower. Such common nucleation found to occur when the sublimation vapors were allowed to pass through the nozzle. High magnification SEM images (Fig. 1(b)) reveal that each nanorod of the flowers was highly crystalline, exhibiting crystal facets with (0001) orientation typical to ZnO. When the number density of the flowers grown on the substrate was low, their shapes were nearly isotropic and regular with only one nucleation point at the centre (as shown in Fig. 1(a)). In the region where number density was relatively large so that the distance between two adjacent nuclei is smaller than the free diameter of each flower, then two growth fronts directed towards opposite nuclei overlap and a sin-
943
gle flower with elliptical shape develop with two nuclei at the centre, as shown in Fig. 1(c). Similarly, when three nucleations take place close to each other, overlapping of two growth fronts could be observed (Fig. 1(d)). No intermixing of the adjacent growth fronts at the overlap region occurs indicating the growth process terminates in this direction when no further substrate space was available. These observations suggest that growth of the structures is strongly limited by surface diffusion process of the adatoms on the substrate. Fig. 2(a) shows the TEM micrograph of individual nanorods. The high resolution lattice image shown in Fig. 2(b) suggests the nanorods were single crystalline and the distance between two lattice planes was about 0.53 nm which correspond to the d-spacing of the (0001) crystal planes of wurtzite ZnO. The corresponding selected area electron diffraction pattern shown in the inset of Fig. 2(b) further substantiates the crystallinity of the ZnO nanorods and the growth direction. When the samples were grown at 350 °C, the shape of the flowers changed to a new kind of hemispherical morphology, as shown in Fig. 3(a). These structures appear to be composed of nano-nail type of structures, which have been reported earlier for ZnO [19–21]. The tips of the nano-nails were oriented towards the centre of the hemisphere and the flat heads could be seen on the surface, as has been explained in reference [20]. In our study, this is evident from the high resolution SEM image of the lower portion of the flowers shown in Fig. 3(b). The main difference in growth mechanism between flower structures grown at 300 °C and 350 °C is the type of mass transport from the central nucleation region of the flowers towards the periphery. Mass transport occurs during crystal growth through surface diffusion process of the adatoms, which is governed by surface kinetics and is strongly dependent on substrate types and temperature. The diffusion coefficient, Ds and the diffusion length, ks of adatoms on a crystalline surface can be given as
Fig. 1. (a) SEM micrograph of a typical ZnO nanorod flower structure grown at 300 °C for 60 min on Ge (100) using chemical vapor condensation technique. (b) High magnification SEM image showing crystal facets of individual ZnO nanorod of the flower structure. (c) and (d) SEM micrographs of two and three overlapping flower structures, respectively, showing the demarcation boundaries.
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S.R. Haldar et al. / Current Applied Physics 10 (2010) 942–946
Fig. 2. (a) TEM micrograph of individual ZnO nanorods of the flower structures grown at 300 °C for 60 min on Ge (100). (b) High resolution lattice image of a ZnO nanorod. Inset shows selected area electron diffraction pattern of ZnO nanorod.
Fig. 3. (a) SEM micrograph of ZnO flower structure grown at 350 °C for 60 min. (b) High resolution SEM image of the marked portion of (a).
Es
Es
Ds ¼ a2 mekT ¼ D0 ekT
ð1Þ
and
ks ¼ ae
Ed Es 2kT
ð2Þ
where a is the distance between adjacent adsorption/lattice sites, m is the vibrational frequency of the single adatom migration and Es, Ed are energy of hopping to adjacent sites and for desorption from the surface, respectively. The diffusion coefficient and the diffusion length could be related through the Einstein’s relation given as
ks ¼
pffiffiffiffiffiffiffiffi Ds s
ð3Þ
where s is a characteristic time describing a duration the adatom undergoes in the diffusion process. This characteristic time could be the mean time the adatom spends on the surface either before being captured in the lattice site or desorbed from the surface. The former definition is more relevant in the low temperature regime, as in our case. Substituting Eqs. (1) and (2) into Eq. (3) gives
ks ¼
pffiffiffiffiffiffiffiffiffi Es D0 se kT
ð4Þ
and
1
Ed
s ¼ ekT m
ð5Þ
In the low temperature regime, where Ed >> kT, the variation in
s between two growth temperatures of 300 °C and 350 °C can be neglected. Under such approximation, both Ds and ks increase with increasing temperature and vice versa. Thus for growth at 300 °C, the diffusive mass transport rate towards the outer region might
be smaller than the incorporation rate itself at the growing tip of the nanorods. For growth at 350 °C, the mass transport rate might exceed the incorporation rate and as a result, adatoms accumulate towards the end of the nanorods leading to the formation of the ‘heads’ of the nano-nails. When grown at a lower temperature of 250 °C, no flower structures could be observed and resulted in an amorphous deposition of ZnO all over the substrate. At low temperature, the lack of sufficient thermal energy hinders the surface diffusion process, which is necessary to produce crystalline materials. The role of surface diffusion process in forming the self organized structures was further elaborated from the study of growth under similar condition on (0001) oriented sapphire which has similar crystal structure like that of ZnO and on amorphous substrate such as glass and the results are shown in Fig. 4(a) and (b), respectively. It is evident from Eqs. (1) and (2) that both the diffusion coefficient and the diffusion length of the adatoms on the surface depend on the spacing of the adjacent adsorption sites under hopping, which can be taken as same as the surface lattice constant and the vibrational frequency. For amorphous glass, the random distribution of these surface sites leads to inefficient materials transport on the surface. Hence, though the nanorods were grown under the given growth condition, they were formed all over the surface but the self organized architectures of the nanorods could not be built. On the other hand when growth was carried out on sapphire, we obtained much thinner nanorods, all oriented at some angle to the substrate. No growth along the plane of the substrate was observed, which was prominent when grown on Ge. The ZnO nanorods, when grown on sapphire, tend to orient along different crystallographic directions of sapphire, including the common (0001) direction, similar to that observed for growth of InAs whis-
S.R. Haldar et al. / Current Applied Physics 10 (2010) 942–946
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Fig. 4. SEM micrographs of ZnO nanorods grown at 300 °C on (a) sapphire (0001) and (b) glass.
kers on GaAs [22]. Because of crystalline nature of the sapphire substrate, the surface diffusion of adatoms was prominent and hence no deposition of ZnO was observed on the surface. In both the substrates of glass and sapphire, the common feature of nucleated growth of nanorods starting from a common origin could still be observed. The crystalline structures of the nanorods of the flower structures grown on Ge substrates were evaluated by X-ray diffraction (XRD) method using Cu Ka radiation with a conventional h–2h goniometer. Fig. 5 shows XRD patterns of two types of flower structures grown at 300 °C and 350 °C along with that of the sample grown at 250 °C. Both the flower structures exhibit diffraction peaks at 31.7°, 34.4°, 36.2°, 47.5°, 56.6°, 62.9°, 67.8°, 68.9° corresponding to (100), (002), (101), (102), (110), (103), (112), (201) of ZnO with the hexagonal wurtzite structure. The calculated lattice constants a and c were 3.2 and 5.19 Å, respectively for both the samples. The samples grown at 250 °C exhibited broad and wide peak around 34o implying the amorphous nature of the film. The photoluminescence spectra (Fig. 6) of the flower structures grown on Ge substrates at 300 °C showed only pure free excitonic transition [23] centred at about 3.298 eV (376 nm) with full width at half maxima (FWHM) of about 132 meV. No defect related mid gap transitions were present in these samples. The samples grown at 350 °C on Ge also showed similar band gap related excitonic transition, but a strong and broad green emission peak centred around 500 nm also appeared. This deep-level emission peak is ~ oz-Herna´ndez et. al. [27] have very common in ZnO [24–26]. Mun proposed that this emission peak could be due to an electronic transition between oxygen vacancies (VO), which act as donors
Fig. 6. Room temperature photoluminescence spectra of ZnO flower structures grown at (a) 300 °C and (b) 350 °C on Ge (100) substrates.
and an electronic level generated by zinc vacancies (VZn) or zinc interstitial (Zni). These point defects are predominant in ZnO [28,29], which are easily incorporated in the crystals during growth due to high vapor pressures of both Zn and O. The vapor pressures of elements depend strongly on temperature and hence, the concentration of these defects increases with increase in growth temperature. In our case, the samples grown at 300 °C might have less defect concentrations and as a result, no defect related transitions could be observed. On the other hand, the samples grown at a relatively higher temperature of 350 °C had larger probability of incorporating the defect complexes causing the green emission observed in the PL spectra. The samples grown at lower temperature of 250 °C did not show any appreciable luminescence intensity because of their amorphous nature. 4. Summary
Fig. 5. X-ray diffraction pattern of ZnO flower structures grown at (a) 350 °C, (b) 300 °C and (c) 250 °C on Ge (100) substrates.
In this paper, we have demonstrated self organized hierarchical arrangement of ZnO nanorods in the form of flowers on Ge (100) substrates by a small modification of vapor condensation technique of zinc acetate dihydrate at low temperature of 300 °C by allowing the vapors to pass in the form of a jet. TEM and X-ray diffraction studies indicate that the nanorods comprising the flowers were crystalline in nature having wurtzite phase with lattice constant a and c of 3.2 and 5.19 Å, respectively. The morphology of the flower structures was found to be very sensitive to growth temperature and the type of substrates used. When the samples were grown at a relatively higher temperature of 350 °C, the constitu-
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ents of the flowers changed from nanorods to an apparently hierarchical organization of nano-nail like structures leading to a hemispherical structure. The flower structures were not formed when grown at a lower temperature of 250 °C but only amorphous deposition of material was found on the substrate. When grown on sapphire (0001) and glass substrates, we could only obtain uniform and randomly oriented nanorods, respectively. The growth behaviour has been explained through qualitative surface kinetic models of the adatoms. The nanorod flower structures grown at 300 °C showed pure free excitonic PL emission at 3.249 eV at room temperature and the hemispherical structures grown at 350 °C showed defect related mid gap transition in addition to the near band gap transition. Apart from the obvious interest from crystal growth’s point of view, these highly textured flower structures, especially those grown at 300 °C might show improved photovoltaic response because of the presence of large surface area and good structural and optical properties. Acknowledgements S.R. Haldar thanks CSIR, Govt. of India for financial support. References [1] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897. [2] H.Q. Yan, R.R. He, J. Pham, P.D. Yang, Adv. Mater. 15 (2003) 402. [3] Ü. Özgür, Y.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Dog˘an, V. Avrutin, S.J. Cho, H. Morkoç, J. Appl. Phys. 98 (2005) 41301. [4] Z.L. Wang, X.Y. Kong, Y. Ding, P.X. Gao, W.L. Hughes, R. Yang, Y. Zhang, Adv. Funct. Mater. 14 (2004) 943.
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