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Ultraviolet-emitting javelin-like ZnO nanorods by thermal evaporation: Growth mechanism, structural and optical properties
Chemical Physics Letters 440 (2007) 110–115 www.elsevier.com/locate/cplett
Ultraviolet-emitting javelin-like ZnO nanorods by thermal evaporation: Growth mechanism, structural and optical properties A. Umar a, S.H. Kim a, E.-K. Suh b, Y.B. Hahn
a,b,*
a
School of Chemical Engineering and Technology and Nanomaterials Processing Research Centre, Chonbuk National University, Jeonju 561-756, South Korea b Department of Semiconductor Science and Technology, Semiconductor Physics Research Centre, Chonbuk National University, Jeonju 561-756, South Korea Received 20 February 2007; in final form 29 March 2007 Available online 7 April 2007
Abstract Ultraviolet-emitting, single-crystalline aligned javelin-like zinc oxide (ZnO) nanorods were synthesized on copper foil by thermal evaporation at low temperature of 500 C without catalysts or additives. Detailed structural characterizations confirmed that the formed products are single-crystalline, they possess a wurtzite hexagonal phase and are grown along the c-axis direction. Raman-active opticalphonon E2 mode at 437 cm1 with sharp and strong UV emission at 381 nm in room-temperature photoluminescence spectrum showed that the obtained ZnO nanorods have a good crystal quality with excellent optical properties. Moreover, a detailed growth mechanism was proposed for the formation of the javelin-like ZnO nanorods. 2007 Elsevier B.V. All rights reserved.
1. Introduction With a wide band gap (3.37 eV) and large excitonic binding energy (60 meV) at room-temperature, the II–VI semiconductor ZnO possesses a great potential application in the fabrication of a variety of electronic and optoelectronic devices, particularly in the ultraviolet (UV) lightemitting and laser-diodes with high-efficiency and low threshold [1]. Hitherto, various studies have been made on both synthesis and applications of ZnO nanostructures grown by a variety of fabrication techniques, but still it is desirable to have defect-free, high-crystalline and UV-emitting ZnO nanostructures for their potential applications in nanodevices, especially in the exploitation of practical blueUV laser devices [2–16]. Previously, Kong et al. reported * Corresponding author. Address: School of Chemical Engineering and Technology and Nanomaterials Processing Research Centre, Chonbuk National University, Jeonju 561-756, South Korea. Fax: +82 63 270 2306. E-mail addresses: [email protected] (A. Umar), ybhahn@ chonbuk.ac.kr (Y.B. Hahn).
0009-2614/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2007.04.006
the ultraviolet-emitting ZnO nanowires using zinc and selenium powder at 1100 C [9]. Recently, ZnO hexagonal nanoprisms with ultraviolet-emitting properties were grown onto the Au-coated Si(1 0 0) substrate at 910 C [10]. Kim et al. also demonstrated the synthesis of ultraviolet-emitting ZnO nanowires onto pre-deposited hexagonal Zn nanoplates on CaF2(1 1 1) substrate at about 950 C [16]. Additionally, according to the results regarding the growth of good quality ZnO nanostructures reported in the literature, high temperature and/or catalyst or additives are required. Besides, Wang et al. reported the synthesis of ZnO hexagonal arrays of nanowires grown on nanorods at low temperature of about 550 C, but these nanostructures exhibit a strong green emission which is related to the structural defects and oxygen vacancies in the deposited structures [7]. Here, we report the synthesis of highly crystalline ultraviolet-emitting, vertically aligned javelinlike ZnO nanorods on copper foil at 500 C without the use of metal catalysts or additives. Due to the good crystal quality and excellent optical properties, the grown structures may be highly applicable for the fabrication of efficient optoelectronic devices operating at
A. Umar et al. / Chemical Physics Letters 440 (2007) 110–115
room-temperature. Furthermore, the detailed growth mechanism and structural properties of the formed nanostructures are also discussed based on the crystallographic habits of wurtzite hexagonal ZnO and structural characterization techniques, respectively. 2. Experimental details The synthesis of javelin-like ZnO nanorods was carried out by the thermal evaporation technique. Commercially available copper foil (200 mm, 99%) was used as a substrate. A 2 · 2 cm piece of copper foil was ultrasonically cleaned by the isopropyl alcohol and acetone in sequence. High-purity metallic zinc powder (99.999%) and oxygen were used as source materials for the zinc and oxygen, respectively. A quartz boat containing the metallic zinc powder was placed at the centre of the furnace while the substrate was put adjacent to the quartz boat. Before being heated, the tube was evacuated to 101–103 Torr and kept under vacuum throughout the synthesizing process. After evacuation of the quartz tube, the furnace was rapidly heated using a halogen lamp heating system and oxygen and nitrogen were introduced continuously into the reaction chamber with the flow rates of 500 sccm and 250 sccm, respectively. The reaction lasted for 30–90 min. After termination of the reaction, the furnace was very slowly cooled down to room temperature and the deposition of a white gray colored product was observed onto the whole surface of the substrate, which was examined with various structural and optical characterizations.
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3. Results and discussion 3.1. Structural and optical properties of ultraviolet-emitting javelin-like ZnO nanorods The general morphologies of the as-grown javelin-like ZnO nanorods were observed by the field emission scanning electron microscopy (FESEM). In these images, it is clearly seen that two pieces of ZnO nanorods, i.e. hexagonal-shaped nanorods and inverted nanocones, are connected to each other in such a manner that the shaft of the nanocones is attached to the upper portion of the hexagonal nanorod to form a javelin-like ZnO nanorod. Fig. 1a and b show the low and high-magnification images of javelin-like ZnO nanorods, which reveal that the formed nanorods are vertically aligned and are grown onto the whole substrate surface in a high density. The typical diameters of the basal hexagonal nanorods are 650 ± 100 nm, while the diameters of the inverted cone-shaped nanorods are 250 ± 50 nm at their shaft to 600 ± 50 nm at their top, which shows a perfectly hexagonal flat surface (cap). The lengths of the javelin-like ZnO nanorods are several micrometers and they all exhibited a perfectly hexagonal, clean and smooth surface throughout their lengths. To determine the crystallinity and crystal phases of the asgrown ZnO nanorods, X-ray diffraction (XRD) patterns were measured with Cu Ka radiation, and are shown in Fig. 1c. The origination of two sharp and strong peaks at 34.4 and 72.6 was indexed as ZnO(0 0 0 2) and ZnO(0 0 0 4), respectively, of the wurtzite structure of
Fig. 1. (a) Low and (b) high magnification FESEM images; (c) XRD pattern; and (d) EDX spectrum of vertically aligned javelin-like ZnO nanorods.
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ZnO, indicating that the nanorods are single-crystalline with the wurtzite hexagonal phase and are preferentially grown in the c-axis direction. No other peak related to impurity and source material was observed, which confirmed that grown products are pure ZnO. To investigate the chemical composition of the as-grown aligned javelinlike ZnO nanorods, an energy dispersive X-ray (EDX) spectrometry has been performed and is shown in Fig. 1d. It is clearly observed from the spectrum that the obtained nanorods are formed in a proper stoichiometry of zinc and oxygen and are composed of zinc and oxygen only. Additionally, two other peaks for copper are also seen from the spectrum which is related to the copper substrate. The detailed structural characterization of the asgrown javelin-like ZnO nanorods was studied by transmission electron microscopy (TEM). Fig. 2a shows the basal portion of the nanorods and is fully consistent with the FESEM observations. The typical diameters of the basal hexagonal nanorods are in the range of 650–700 nm and show a taper form at their tip for the further growth of inverted nanocones. The corresponding selected area electron diffraction pattern (SAED) confirms the [0 0 0 1] growth direction (inset (a)). Fig. 2b shows the high-resolution transmission electron microscopy (HRTEM) image (circled portion shown in (a)) which represents that the lattice fringes are separated by 0.52 nm, corresponding to the (0 0 0 1) plane of the ZnO. The corresponding SAED pattern is consistent with the HRTEM observation (inset (b)). Fig. 2c demonstrates the upper portion of nanorods and reveals that the diameters of nanorods gradually decrease from the top to the root which
Fig. 2. (a) Low magnification TEM image of the basal portion of the javelin-like ZnO nanorods and its corresponding SAED pattern (inset); (b) high resolution TEM image observed from the circled portion in (a) and its SAED pattern (inset); (c) low magnification TEM image of the upper portion (inverted cone-shaped nanorod) of the javelin-like ZnO and its corresponding SAED pattern (inset); (d) high resolution TEM image observed from the circled portion in (c) and its SAED pattern (inset).
forms the inverted cone-like morphology. The inverted nanocones have smooth and clean surfaces and the top portion exhibits a flat surface with the typical diameter of about 600 ± 50 nm. The corresponding SAED pattern confirms that the inverted nanocones are c-axis oriented (inset (c)). The HRTEM image of the tip portion of the nanocone exhibits that the measured plane spacing is about 0.52 nm, corresponding to the (0 0 0 1) fringes perpendicular to the growth direction, which further confirms single-crystallinity and [0 0 0 1] growth direction for the deposited nanostructures (Fig. 2d). The corresponding SAED pattern has good agreement with the HRTEM observation (inset (d)). Fig. 3a shows the Raman-scattering spectrum of the javelin-like ZnO nanorods, measured with 514.5 nm laser-line as the excitation source. With a wurtzite hexagonal phase, ZnO belongs to the space group C46V , in which one primitive cell includes two formula units where all the atoms occupy the C3V symmetry. Group theory predicts the existence of the following optic modes: A1 + 2B1 + E1 + 2E2, near the centre of Brillouin zone. The A1, E1 and E2 modes are Raman active, while A1 and E1 are both Raman and infra-red active. Moreover, A1 and E1 are split into two optical components: longitudinal (LO) and transverse (TO) [17]. The Raman spectrum shows a sharp, strong and dominant peak at 437 cm1 attributed to ZnO non-
Fig. 3. (a) Typical Raman scattering and (b) room-temperature PL spectrum of the as-grown javelin-like ZnO nanorods.
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polar optical phonon E2 mode and a very suppressed and small peak, assigned as E1L mode, at 582 cm1 due to structural defects and oxygen vacancies [4]. Furthermore, a peak at 331 cm1 assigned to be E2H – E2L (multi-phonon process) can be found only when the ZnO is single crystal [3]. Hence, owing to a higher intensity and a narrower spectral width of E2-mode affirmed that the as-grown javelin-like ZnO nanorods have good crystal quality with a wurtzite hexagonal phase. The photoluminescence (PL) spectrum of the as-synthesized javelin-like ZnO nanorods was studied using a He–Cd (325 nm) laser-line as the exciton source at room temperature and is shown in Fig. 3b. The spectrum exhibits only a sharp intensive UV emission peaked at 381 nm, called near band edge emission which originated due to a recombination of free-excitons through an exciton–exciton collisionprocess [4]. No peak in the visible region, known as deep level emission, was observed. The origination of the green emission in the PL spectrum is associated with the intrinsic centers such as oxygen vacancies (VO), oxygen interstitials (Oi) or antisite oxygen (OZn), zinc vacancies (VZn), and zinc interstitials (Zni). Vanheusden et al. reported that green emission appears due to the radial recombination of photogenerated hole with a singly ionized charged state of the oxygen vacancy [18]. Dijken et al. have proposed that the origination of green emission is due to the transition from the conduction band to the deeply trapped holes [19]. Additionally, it is also demonstrated that the improvement in crystal quality (decrease of impurities, and structural defects such as zinc interstitials and oxygen defects etc.)
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may cause a high UV emission to the deep level emission ratio which results in the appearance of a high near band edge emission at room temperature [15]. Hence, the presence of only an intensive UV emission in the observed PL spectrum indicated that the obtained javelin-like ZnO nanorods have good crystallinity and exhibit an excellent optical property. 3.2. Growth mechanism of ultra-violet emitting javelin-like ZnO nanorods A metal catalyst was neither used nor detected hence we attribute that the growth process for the formation of javelin-like ZnO nanorods in our experiments is vapor–solid (VS) process [4] rather than conventionally and commonly used vapor–liquid–solid (VLS) process [20]. On the basis of observed results, here we propose a plausible growth mechanism for the formation of javelin-like ZnO nanorods. Fig. 4 demonstrates a schematic sketch for the formation of aligned javelin-like ZnO nanorods. Basically, the formation of these nanorods can be divided into two parts: nucleation and growth. As the temperature of the furnace was ramped up to 500 C under the continuous flow of O2 and N2, it provides sufficient temperature for the evaporation of metallic zinc powder (m.p. = 419 C). The generated Zn vapors reacted with oxygen in the gaseous phase via a simple chemical reaction: Zn(g) + O2(g) ! ZnO(g) (a). These formed ZnO vapors then condensed and nucleated in the form of ZnO nanocrystals with the characteristic ZnO hexagonal morphologies onto the whole substrate
Fig. 4. Schematic illustration of the growth mechanism for the formation of ultraviolet-emitting javelin-like ZnO nanorods grown by thermal evaporation process.
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surface, which provide the energetically favored sites for the further adsorption of incoming species as the reaction proceeds (b). The formed hexagonal ZnO nanocrystals are the building blocks for the formation of the final products. With a wurtzite hexagonal phase, the ZnO has polar and non-polar faces. In the polar ZnO crystals, the zinc and oxygen atoms are arranged alternatively along the caxis and the top surfaces are Zn-terminated (0 0 0 1) which are catalytically active, while the bottom surfaces are oxygen-terminated ð0 0 0 1Þ which are chemically inert [4,20]. Therefore, the obtained hexagonal nanocrystals exhibited top Zn-terminated (0 0 0 1) and bottom oxygen-terminated ð0 0 0 1Þ surfaces (b). This is nucleation process for the formation of javelin-like ZnO nanorods. Therefore, due to the continuous feeding of reactants (oxygen and zinc), the newly incoming atoms arrange in a proper cation–anion ratio onto the previously formed hexagonal ZnO nanocrystals, which lead to the formation of final javelin-like ZnO nanorods (c and d). Actually, the growth of final javelinlike nanorods can be further divided into two steps in which the first step involves the growth of hexagonal nanorods (c) while the second step is the formation of inverted nanocones grown over the hexagonal nanorods (d). It is known that the morphology of a particular crystal is determined by the slowest growing faces and the growth depends upon the relative growth velocities of different planes in the ZnO crystals [11,12]. Moreover, the polar faces with surface dipoles are thermodynamically less stable than non-polar faces and often undergo rearrangement to minimize their surface energy and also tend to grow more rapidly [20]. Here, the growing pattern of basal hexagonal nanorods is fully consistent with that of ideal hexagonal ZnO crystal model in which the crystal growth velocities in the various crystal facets are ½0 0 0 1 > ½0 1 1 1 > ½0 1 1 0 > ½0 0 0 1 under hydrothermal conditions [11,12]. Thus, the maximum growth rate in the ZnO crystals is along the [0 0 0 1] direction. Its growth rate is about twice as compared to the ½0 1 1 0 crystal facets, while growth along the ½0 1 1 1 is in between the two. Hence, the basal hexagonal nanorods were grown along the [0 0 0 1] direction, while the top and side surfaces were formed by the ±(0 0 0 1) and six crystallographic equivalent f0 1 1 0g planes, respectively. As far as the growing pattern of ZnO is concerned, it is strongly hemimorphic; therefore, both the ends of ZnO crystals are bounded with the ±(0 0 0 1) faces. Generally, it is accepted that a fast growing plane generally tends to disappear, leaving behind the slower growing forms with lower surface energy. Therefore, the (0 0 0 1) face is not an equilibrium surface; hence it is bounded by the growth facets of f0 1 1 1g which have higher Miller indices but lower specific surface energy compared to the (0 0 0 1) facets. Since the specific surface free energy (r) and appropriate area of the (0 0 0 1) faces are higher than the other growth facets, therefore the higher index faces predominate [12,21]. It is reported that after the growth of f0 1 1 1g facets, the (0 0 0 1) planes are the most likely remaining facets which appear on the ZnO crys-
tals [12]. ZnO belongs to the P63mc space group with polar surfaces; positively charged, catalytically active Zn-(0 0 0 1) surfaces and negatively charged, chemically inert O-(0 0 0 1) surfaces [5]. Therefore, upper surfaces of the obtained first step growth of hexagonal nanorods exhibited a catalytically active Zn-terminated (0 0 0 1) top facet which provides a preferred site for the further growth of the second step of inverted nanocones in the [0 0 0 1] direction via self-catalytic growth process [3,10,22]. The javelin-like nanorods exhibited a two layer growth, i.e. the first layer is basal hexagonal nanorods and the next layer is inverted nanocones. It is reported that if the ZnO crystal grows in a layer by layer manner, then after the end of the first layer growth, the next layer can grow along the [0 0 0 1] direction [23]. Moreover, in the ZnO crystal growth, the highest growth rate is along the c-axis direction and large facets are usually f0 1 1 0g and f2 1 1 0g non-polar surfaces rather than the polar {0 0 0 1} surfaces [12,13]. It is also reported that necking in the ZnO nanorods can occur if there is a change in the temperature [14]. Therefore, we expect that the second step growth occurs during the very slow cooling down process. It is worthwhile to note that the cooling rate is important for the formation of javelin-like ZnO nanorods. The fast cooling rate under the same experimental conditions leads to the formation of flat hexagonal nanorods. As the top surfaces of the obtained nanorods are catalytically active (0 0 0 1) facets, hence the top area can absorb the incoming vapor species (Zn and oxygen) efficiently, as it is the energetically favored area. At high temperature, the mobility of ZnO molecular species becomes high, therefore the ZnO species deposited to the base and surface area moved to the top area. In this manner, the top area grows not only in the longitudinal direction but also in the transverse direction, which results in the enlargement of the cap area, and finally an inverted cone-shaped morphology can be obtained. It is interesting to note that the base diameters of the inverted nanocones are equal to the platform sizes of the Zn-terminated (0 0 0 1) surface of basal hexagonal nanorods as evident from structural observations (Figs. 1 and 2), which is consistent with the previously reported results [2,7]. Moreover, the caps of the inverted nanocones exhibit a perfect hexagonal shape because of the hexagon crystal structures of ZnO. 4. Conclusion In summary, ultraviolet-emitting, single-crystalline aligned javelin-like ZnO nanorods were synthesized, in a large quantity by thermal evaporation at low temperature without catalysts or additives. Extensive structural characterizations confirmed the single-crystallinity and [0 0 0 1] growth direction for the as-grown nanorods. A sharp and strong Raman-active optical-phonon E2 mode at 437 cm1 in the Raman spectrum reveals the good crystallinity with the wurtzite hexagonal phase for the deposited nanorods. An intensive UV emission in the PL spectrum confirms an excellent optical property which suggests that
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it may be highly applicable for the fabrication of efficient nano-optoelectronic devices, operating at room temperature, in the near future. Furthermore, a plausible growth mechanism is proposed based on the structural characterization and crystallographic habits of wurtzite hexagonal ZnO. Acknowledgement This work was supported by the Korea Science and Engineering Foundation Grant funded by the Korean Government (MOST) (R01-2006-000-11306-0). References [1] X. Duan, Y. Huang, Y. Cui, J. Wang, C.M. Lieber, Nature 409 (2001) 66. [2] A. Umar, Y.B. Hahn, Appl. Phys. Lett. 88 (2006) 173120. [3] Y.H. Yang et al., Appl. Phys. Lett. 87 (2005) 183109. [4] A. Umar, Y.B. Hahn, Nanotechnology 17 (2006) 2174. [5] Z.L. Wang, X.Y. Kong, J.M. Zuo, Phys. Rev. Lett. 91 (2003) 185502. [6] Q. Ahsanulhaq, A. Umar, Y.B. Hahn, Nanotechnology 18 (2006) 115603. [7] R.C. Wang, C.P. Liu, J.L. Huang, S.-J. Chen, Appl. Phys. Lett. 86 (2005) 251104.
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[8] A. Umar, J.P. Jeong, E. –K. Suh, Y.B. Hahn, Korean J. Chem. Eng. 23 (2006) 860. [9] Y.C. Kong, D.P. Yu, B. Zhang, S.Q. Feng, Appl. Phys. Lett. 78 (2001) 407. [10] D.F. Liu et al., Nanotechnology 16 (2005) 2665. [11] R.A. Laudise, A.A. Ballman, J. Phys. Chem. 64 (1960) 688. [12] R.A. Laudise, E.D. Kolb, A.J. Caporaso, J. Am. Ceram. Soc. 47 (1964) 9. [13] S. Kar, A. Dev, S. Chaudhuri, J. Phys. Chem. B 110 (2006) 17848. [14] S. Mahmud, M. Abdullah, J. Chong, A. Mohammad, M. Zakaria, J. Cryst. Growth 287 (2006) 118. [15] D.M. Bagnall, Y.F. Chen, Z. Zhu, T. Yao, S. Koyama, M.Y. Shen, T. Goto, Appl. Phys. Lett. 73 (1998) 1038. [16] T.W. Kim, T. Kawazoe, S. Yamazaki, M. Ohtsu, Appl. Phys. Lett. 84 (2004) 3358. [17] J.M. Calleja, M. Cardona, Phys. Rev. B 16 (1977) 3753. [18] K. Vanheusden, C.H. Seager, W.L. Warren, D.R. Tallant, J.A. Voigt, J. Appl. Phys. 79 (1996) 7983. [19] A. van Dijken, E.A. Meulenkamp, D. Vanmaekelbergh, A. Meijerink, J. Phys. Chem. B 104 (2000) 1715. [20] P.X. Gao, Z.L. Wang, J. Phys. Chem. B 108 (2004) 7534. [21] A.N. Mariano, R.E. Hanneman, J. Appl. Phys. 34 (1963) 384. [22] A. Umar, B. Karunagaran, E.-K. Suh, Y.B. Hahn, Nanotechnology 17 (2006) 4072. [23] L.W. Yang, X.L. Wu, Y. Xiong, Y.M. Yang, G.S. Huang, P.K. Chu, G.G. Siu, J. Cryst. Growth 283 (2005) 332.
Ultraviolet-emitting javelin-like ZnO nanorods by thermal evaporation: Growth mechanism, structural and optical properties A. Umar a, S.H. Kim a, E.-K. Suh b, Y.B. Hahn
a,b,*
a
School of Chemical Engineering and Technology and Nanomaterials Processing Research Centre, Chonbuk National University, Jeonju 561-756, South Korea b Department of Semiconductor Science and Technology, Semiconductor Physics Research Centre, Chonbuk National University, Jeonju 561-756, South Korea Received 20 February 2007; in final form 29 March 2007 Available online 7 April 2007
Abstract Ultraviolet-emitting, single-crystalline aligned javelin-like zinc oxide (ZnO) nanorods were synthesized on copper foil by thermal evaporation at low temperature of 500 C without catalysts or additives. Detailed structural characterizations confirmed that the formed products are single-crystalline, they possess a wurtzite hexagonal phase and are grown along the c-axis direction. Raman-active opticalphonon E2 mode at 437 cm1 with sharp and strong UV emission at 381 nm in room-temperature photoluminescence spectrum showed that the obtained ZnO nanorods have a good crystal quality with excellent optical properties. Moreover, a detailed growth mechanism was proposed for the formation of the javelin-like ZnO nanorods. 2007 Elsevier B.V. All rights reserved.
1. Introduction With a wide band gap (3.37 eV) and large excitonic binding energy (60 meV) at room-temperature, the II–VI semiconductor ZnO possesses a great potential application in the fabrication of a variety of electronic and optoelectronic devices, particularly in the ultraviolet (UV) lightemitting and laser-diodes with high-efficiency and low threshold [1]. Hitherto, various studies have been made on both synthesis and applications of ZnO nanostructures grown by a variety of fabrication techniques, but still it is desirable to have defect-free, high-crystalline and UV-emitting ZnO nanostructures for their potential applications in nanodevices, especially in the exploitation of practical blueUV laser devices [2–16]. Previously, Kong et al. reported * Corresponding author. Address: School of Chemical Engineering and Technology and Nanomaterials Processing Research Centre, Chonbuk National University, Jeonju 561-756, South Korea. Fax: +82 63 270 2306. E-mail addresses: [email protected] (A. Umar), ybhahn@ chonbuk.ac.kr (Y.B. Hahn).
0009-2614/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2007.04.006
the ultraviolet-emitting ZnO nanowires using zinc and selenium powder at 1100 C [9]. Recently, ZnO hexagonal nanoprisms with ultraviolet-emitting properties were grown onto the Au-coated Si(1 0 0) substrate at 910 C [10]. Kim et al. also demonstrated the synthesis of ultraviolet-emitting ZnO nanowires onto pre-deposited hexagonal Zn nanoplates on CaF2(1 1 1) substrate at about 950 C [16]. Additionally, according to the results regarding the growth of good quality ZnO nanostructures reported in the literature, high temperature and/or catalyst or additives are required. Besides, Wang et al. reported the synthesis of ZnO hexagonal arrays of nanowires grown on nanorods at low temperature of about 550 C, but these nanostructures exhibit a strong green emission which is related to the structural defects and oxygen vacancies in the deposited structures [7]. Here, we report the synthesis of highly crystalline ultraviolet-emitting, vertically aligned javelinlike ZnO nanorods on copper foil at 500 C without the use of metal catalysts or additives. Due to the good crystal quality and excellent optical properties, the grown structures may be highly applicable for the fabrication of efficient optoelectronic devices operating at
A. Umar et al. / Chemical Physics Letters 440 (2007) 110–115
room-temperature. Furthermore, the detailed growth mechanism and structural properties of the formed nanostructures are also discussed based on the crystallographic habits of wurtzite hexagonal ZnO and structural characterization techniques, respectively. 2. Experimental details The synthesis of javelin-like ZnO nanorods was carried out by the thermal evaporation technique. Commercially available copper foil (200 mm, 99%) was used as a substrate. A 2 · 2 cm piece of copper foil was ultrasonically cleaned by the isopropyl alcohol and acetone in sequence. High-purity metallic zinc powder (99.999%) and oxygen were used as source materials for the zinc and oxygen, respectively. A quartz boat containing the metallic zinc powder was placed at the centre of the furnace while the substrate was put adjacent to the quartz boat. Before being heated, the tube was evacuated to 101–103 Torr and kept under vacuum throughout the synthesizing process. After evacuation of the quartz tube, the furnace was rapidly heated using a halogen lamp heating system and oxygen and nitrogen were introduced continuously into the reaction chamber with the flow rates of 500 sccm and 250 sccm, respectively. The reaction lasted for 30–90 min. After termination of the reaction, the furnace was very slowly cooled down to room temperature and the deposition of a white gray colored product was observed onto the whole surface of the substrate, which was examined with various structural and optical characterizations.
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3. Results and discussion 3.1. Structural and optical properties of ultraviolet-emitting javelin-like ZnO nanorods The general morphologies of the as-grown javelin-like ZnO nanorods were observed by the field emission scanning electron microscopy (FESEM). In these images, it is clearly seen that two pieces of ZnO nanorods, i.e. hexagonal-shaped nanorods and inverted nanocones, are connected to each other in such a manner that the shaft of the nanocones is attached to the upper portion of the hexagonal nanorod to form a javelin-like ZnO nanorod. Fig. 1a and b show the low and high-magnification images of javelin-like ZnO nanorods, which reveal that the formed nanorods are vertically aligned and are grown onto the whole substrate surface in a high density. The typical diameters of the basal hexagonal nanorods are 650 ± 100 nm, while the diameters of the inverted cone-shaped nanorods are 250 ± 50 nm at their shaft to 600 ± 50 nm at their top, which shows a perfectly hexagonal flat surface (cap). The lengths of the javelin-like ZnO nanorods are several micrometers and they all exhibited a perfectly hexagonal, clean and smooth surface throughout their lengths. To determine the crystallinity and crystal phases of the asgrown ZnO nanorods, X-ray diffraction (XRD) patterns were measured with Cu Ka radiation, and are shown in Fig. 1c. The origination of two sharp and strong peaks at 34.4 and 72.6 was indexed as ZnO(0 0 0 2) and ZnO(0 0 0 4), respectively, of the wurtzite structure of
Fig. 1. (a) Low and (b) high magnification FESEM images; (c) XRD pattern; and (d) EDX spectrum of vertically aligned javelin-like ZnO nanorods.
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ZnO, indicating that the nanorods are single-crystalline with the wurtzite hexagonal phase and are preferentially grown in the c-axis direction. No other peak related to impurity and source material was observed, which confirmed that grown products are pure ZnO. To investigate the chemical composition of the as-grown aligned javelinlike ZnO nanorods, an energy dispersive X-ray (EDX) spectrometry has been performed and is shown in Fig. 1d. It is clearly observed from the spectrum that the obtained nanorods are formed in a proper stoichiometry of zinc and oxygen and are composed of zinc and oxygen only. Additionally, two other peaks for copper are also seen from the spectrum which is related to the copper substrate. The detailed structural characterization of the asgrown javelin-like ZnO nanorods was studied by transmission electron microscopy (TEM). Fig. 2a shows the basal portion of the nanorods and is fully consistent with the FESEM observations. The typical diameters of the basal hexagonal nanorods are in the range of 650–700 nm and show a taper form at their tip for the further growth of inverted nanocones. The corresponding selected area electron diffraction pattern (SAED) confirms the [0 0 0 1] growth direction (inset (a)). Fig. 2b shows the high-resolution transmission electron microscopy (HRTEM) image (circled portion shown in (a)) which represents that the lattice fringes are separated by 0.52 nm, corresponding to the (0 0 0 1) plane of the ZnO. The corresponding SAED pattern is consistent with the HRTEM observation (inset (b)). Fig. 2c demonstrates the upper portion of nanorods and reveals that the diameters of nanorods gradually decrease from the top to the root which
Fig. 2. (a) Low magnification TEM image of the basal portion of the javelin-like ZnO nanorods and its corresponding SAED pattern (inset); (b) high resolution TEM image observed from the circled portion in (a) and its SAED pattern (inset); (c) low magnification TEM image of the upper portion (inverted cone-shaped nanorod) of the javelin-like ZnO and its corresponding SAED pattern (inset); (d) high resolution TEM image observed from the circled portion in (c) and its SAED pattern (inset).
forms the inverted cone-like morphology. The inverted nanocones have smooth and clean surfaces and the top portion exhibits a flat surface with the typical diameter of about 600 ± 50 nm. The corresponding SAED pattern confirms that the inverted nanocones are c-axis oriented (inset (c)). The HRTEM image of the tip portion of the nanocone exhibits that the measured plane spacing is about 0.52 nm, corresponding to the (0 0 0 1) fringes perpendicular to the growth direction, which further confirms single-crystallinity and [0 0 0 1] growth direction for the deposited nanostructures (Fig. 2d). The corresponding SAED pattern has good agreement with the HRTEM observation (inset (d)). Fig. 3a shows the Raman-scattering spectrum of the javelin-like ZnO nanorods, measured with 514.5 nm laser-line as the excitation source. With a wurtzite hexagonal phase, ZnO belongs to the space group C46V , in which one primitive cell includes two formula units where all the atoms occupy the C3V symmetry. Group theory predicts the existence of the following optic modes: A1 + 2B1 + E1 + 2E2, near the centre of Brillouin zone. The A1, E1 and E2 modes are Raman active, while A1 and E1 are both Raman and infra-red active. Moreover, A1 and E1 are split into two optical components: longitudinal (LO) and transverse (TO) [17]. The Raman spectrum shows a sharp, strong and dominant peak at 437 cm1 attributed to ZnO non-
Fig. 3. (a) Typical Raman scattering and (b) room-temperature PL spectrum of the as-grown javelin-like ZnO nanorods.
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polar optical phonon E2 mode and a very suppressed and small peak, assigned as E1L mode, at 582 cm1 due to structural defects and oxygen vacancies [4]. Furthermore, a peak at 331 cm1 assigned to be E2H – E2L (multi-phonon process) can be found only when the ZnO is single crystal [3]. Hence, owing to a higher intensity and a narrower spectral width of E2-mode affirmed that the as-grown javelin-like ZnO nanorods have good crystal quality with a wurtzite hexagonal phase. The photoluminescence (PL) spectrum of the as-synthesized javelin-like ZnO nanorods was studied using a He–Cd (325 nm) laser-line as the exciton source at room temperature and is shown in Fig. 3b. The spectrum exhibits only a sharp intensive UV emission peaked at 381 nm, called near band edge emission which originated due to a recombination of free-excitons through an exciton–exciton collisionprocess [4]. No peak in the visible region, known as deep level emission, was observed. The origination of the green emission in the PL spectrum is associated with the intrinsic centers such as oxygen vacancies (VO), oxygen interstitials (Oi) or antisite oxygen (OZn), zinc vacancies (VZn), and zinc interstitials (Zni). Vanheusden et al. reported that green emission appears due to the radial recombination of photogenerated hole with a singly ionized charged state of the oxygen vacancy [18]. Dijken et al. have proposed that the origination of green emission is due to the transition from the conduction band to the deeply trapped holes [19]. Additionally, it is also demonstrated that the improvement in crystal quality (decrease of impurities, and structural defects such as zinc interstitials and oxygen defects etc.)
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may cause a high UV emission to the deep level emission ratio which results in the appearance of a high near band edge emission at room temperature [15]. Hence, the presence of only an intensive UV emission in the observed PL spectrum indicated that the obtained javelin-like ZnO nanorods have good crystallinity and exhibit an excellent optical property. 3.2. Growth mechanism of ultra-violet emitting javelin-like ZnO nanorods A metal catalyst was neither used nor detected hence we attribute that the growth process for the formation of javelin-like ZnO nanorods in our experiments is vapor–solid (VS) process [4] rather than conventionally and commonly used vapor–liquid–solid (VLS) process [20]. On the basis of observed results, here we propose a plausible growth mechanism for the formation of javelin-like ZnO nanorods. Fig. 4 demonstrates a schematic sketch for the formation of aligned javelin-like ZnO nanorods. Basically, the formation of these nanorods can be divided into two parts: nucleation and growth. As the temperature of the furnace was ramped up to 500 C under the continuous flow of O2 and N2, it provides sufficient temperature for the evaporation of metallic zinc powder (m.p. = 419 C). The generated Zn vapors reacted with oxygen in the gaseous phase via a simple chemical reaction: Zn(g) + O2(g) ! ZnO(g) (a). These formed ZnO vapors then condensed and nucleated in the form of ZnO nanocrystals with the characteristic ZnO hexagonal morphologies onto the whole substrate
Fig. 4. Schematic illustration of the growth mechanism for the formation of ultraviolet-emitting javelin-like ZnO nanorods grown by thermal evaporation process.
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surface, which provide the energetically favored sites for the further adsorption of incoming species as the reaction proceeds (b). The formed hexagonal ZnO nanocrystals are the building blocks for the formation of the final products. With a wurtzite hexagonal phase, the ZnO has polar and non-polar faces. In the polar ZnO crystals, the zinc and oxygen atoms are arranged alternatively along the caxis and the top surfaces are Zn-terminated (0 0 0 1) which are catalytically active, while the bottom surfaces are oxygen-terminated ð0 0 0 1Þ which are chemically inert [4,20]. Therefore, the obtained hexagonal nanocrystals exhibited top Zn-terminated (0 0 0 1) and bottom oxygen-terminated ð0 0 0 1Þ surfaces (b). This is nucleation process for the formation of javelin-like ZnO nanorods. Therefore, due to the continuous feeding of reactants (oxygen and zinc), the newly incoming atoms arrange in a proper cation–anion ratio onto the previously formed hexagonal ZnO nanocrystals, which lead to the formation of final javelin-like ZnO nanorods (c and d). Actually, the growth of final javelinlike nanorods can be further divided into two steps in which the first step involves the growth of hexagonal nanorods (c) while the second step is the formation of inverted nanocones grown over the hexagonal nanorods (d). It is known that the morphology of a particular crystal is determined by the slowest growing faces and the growth depends upon the relative growth velocities of different planes in the ZnO crystals [11,12]. Moreover, the polar faces with surface dipoles are thermodynamically less stable than non-polar faces and often undergo rearrangement to minimize their surface energy and also tend to grow more rapidly [20]. Here, the growing pattern of basal hexagonal nanorods is fully consistent with that of ideal hexagonal ZnO crystal model in which the crystal growth velocities in the various crystal facets are ½0 0 0 1 > ½0 1 1 1 > ½0 1 1 0 > ½0 0 0 1 under hydrothermal conditions [11,12]. Thus, the maximum growth rate in the ZnO crystals is along the [0 0 0 1] direction. Its growth rate is about twice as compared to the ½0 1 1 0 crystal facets, while growth along the ½0 1 1 1 is in between the two. Hence, the basal hexagonal nanorods were grown along the [0 0 0 1] direction, while the top and side surfaces were formed by the ±(0 0 0 1) and six crystallographic equivalent f0 1 1 0g planes, respectively. As far as the growing pattern of ZnO is concerned, it is strongly hemimorphic; therefore, both the ends of ZnO crystals are bounded with the ±(0 0 0 1) faces. Generally, it is accepted that a fast growing plane generally tends to disappear, leaving behind the slower growing forms with lower surface energy. Therefore, the (0 0 0 1) face is not an equilibrium surface; hence it is bounded by the growth facets of f0 1 1 1g which have higher Miller indices but lower specific surface energy compared to the (0 0 0 1) facets. Since the specific surface free energy (r) and appropriate area of the (0 0 0 1) faces are higher than the other growth facets, therefore the higher index faces predominate [12,21]. It is reported that after the growth of f0 1 1 1g facets, the (0 0 0 1) planes are the most likely remaining facets which appear on the ZnO crys-
tals [12]. ZnO belongs to the P63mc space group with polar surfaces; positively charged, catalytically active Zn-(0 0 0 1) surfaces and negatively charged, chemically inert O-(0 0 0 1) surfaces [5]. Therefore, upper surfaces of the obtained first step growth of hexagonal nanorods exhibited a catalytically active Zn-terminated (0 0 0 1) top facet which provides a preferred site for the further growth of the second step of inverted nanocones in the [0 0 0 1] direction via self-catalytic growth process [3,10,22]. The javelin-like nanorods exhibited a two layer growth, i.e. the first layer is basal hexagonal nanorods and the next layer is inverted nanocones. It is reported that if the ZnO crystal grows in a layer by layer manner, then after the end of the first layer growth, the next layer can grow along the [0 0 0 1] direction [23]. Moreover, in the ZnO crystal growth, the highest growth rate is along the c-axis direction and large facets are usually f0 1 1 0g and f2 1 1 0g non-polar surfaces rather than the polar {0 0 0 1} surfaces [12,13]. It is also reported that necking in the ZnO nanorods can occur if there is a change in the temperature [14]. Therefore, we expect that the second step growth occurs during the very slow cooling down process. It is worthwhile to note that the cooling rate is important for the formation of javelin-like ZnO nanorods. The fast cooling rate under the same experimental conditions leads to the formation of flat hexagonal nanorods. As the top surfaces of the obtained nanorods are catalytically active (0 0 0 1) facets, hence the top area can absorb the incoming vapor species (Zn and oxygen) efficiently, as it is the energetically favored area. At high temperature, the mobility of ZnO molecular species becomes high, therefore the ZnO species deposited to the base and surface area moved to the top area. In this manner, the top area grows not only in the longitudinal direction but also in the transverse direction, which results in the enlargement of the cap area, and finally an inverted cone-shaped morphology can be obtained. It is interesting to note that the base diameters of the inverted nanocones are equal to the platform sizes of the Zn-terminated (0 0 0 1) surface of basal hexagonal nanorods as evident from structural observations (Figs. 1 and 2), which is consistent with the previously reported results [2,7]. Moreover, the caps of the inverted nanocones exhibit a perfect hexagonal shape because of the hexagon crystal structures of ZnO. 4. Conclusion In summary, ultraviolet-emitting, single-crystalline aligned javelin-like ZnO nanorods were synthesized, in a large quantity by thermal evaporation at low temperature without catalysts or additives. Extensive structural characterizations confirmed the single-crystallinity and [0 0 0 1] growth direction for the as-grown nanorods. A sharp and strong Raman-active optical-phonon E2 mode at 437 cm1 in the Raman spectrum reveals the good crystallinity with the wurtzite hexagonal phase for the deposited nanorods. An intensive UV emission in the PL spectrum confirms an excellent optical property which suggests that
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it may be highly applicable for the fabrication of efficient nano-optoelectronic devices, operating at room temperature, in the near future. Furthermore, a plausible growth mechanism is proposed based on the structural characterization and crystallographic habits of wurtzite hexagonal ZnO. Acknowledgement This work was supported by the Korea Science and Engineering Foundation Grant funded by the Korean Government (MOST) (R01-2006-000-11306-0). References [1] X. Duan, Y. Huang, Y. Cui, J. Wang, C.M. Lieber, Nature 409 (2001) 66. [2] A. Umar, Y.B. Hahn, Appl. Phys. Lett. 88 (2006) 173120. [3] Y.H. Yang et al., Appl. Phys. Lett. 87 (2005) 183109. [4] A. Umar, Y.B. Hahn, Nanotechnology 17 (2006) 2174. [5] Z.L. Wang, X.Y. Kong, J.M. Zuo, Phys. Rev. Lett. 91 (2003) 185502. [6] Q. Ahsanulhaq, A. Umar, Y.B. Hahn, Nanotechnology 18 (2006) 115603. [7] R.C. Wang, C.P. Liu, J.L. Huang, S.-J. Chen, Appl. Phys. Lett. 86 (2005) 251104.
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