Available online at ciencedirect.com SCIENCE IRECT JOURNALOF CRYST ELSEVIER Journal of Crystal Growth 282(2005)131-136 www.elsevier.com/locate/jcrysgro Catalyst-free large-quantity synthesis of ZnO nanorods by a vapor-solid growth mechanism: Structural and optical properties A. Umar. S.h. Kim.Y-S. Lee. K.S. Nahm. Y B. Hahn School of Chemical Engineering and Technology and Nanomaterials Research Centre, Chonbuk National Unicersity, Chonju 561-756 Republic of Korea Received 28 March 2005: accepted 27 April 2005 Available online 13 june 2005 Communicated by R. Kern Abstract The formation of high-density Zno nanorods has been achieved by a vapor-solid growth mechanism using metallic zinc powder and oxygen gas as source materials for zinc and oxygen, respectively. General morphological studies indicated that the as-grown products are flower-shaped containing several hundreds of nanorods. The obtained nanorods have a diameter of 150-250 nm while their lengths are 5-10 um. The detailed structural analysis revealed that the Zno nanorods exhibit a single crystalline wurtzite hexagonal structure and preferentially oriented in the c-axis direction. Room temperature Raman scattering and photoluminescence studies found that the as-grown ZnO nanorods have good crystal quality with the hexagonal wurtzite phase containing very less structural defects. C 2005 Elsevier B V. All rights reserved PACS:68.65.+g;78.55Et:78.66Hf Keywords: Al. High-resolution transmission electron microscopy: Al. Optical characterization; Al. ZnO nanorods; A2. Vapor-solid 1. Introduction gap (3.37ev) and high exciton binding energy (60 mev) is an excellent candidate for the fabrica A highly applicable and widely used II-VI tion of nanoelectronic and photonic devices [1, 2 emiconducting material, ZnO, with a wide band Because of its wide band gap and high exciton binding energy much larger than ZnSe(22 meV) uthor.Tel:+82632702439 and Gan(25 meV), it has an opportunity to ax:+82632702306 recognize itself as a versatile material and con- E-mail address: ybhahn(@ chonbuk ac kr(Y B. Hahn) siderably acknowledged because of its catalytic, 0022-0248/Ssee front matter C 2005 Elsevier B.V. All rights reserved doi:l0.1016/ jcrysgro.2005.0409
Journal of Crystal Growth 282 (2005) 131–136 Catalyst-free large-quantity synthesis of ZnOnanorods by a vapor–solid growth mechanism: Structural and optical properties A. Umar, S.H. Kim, Y.-S. Lee, K.S. Nahm, Y.B. Hahn School of Chemical Engineering and Technology and Nanomaterials Research Centre, Chonbuk National University, Chonju 561-756, Republic of Korea Received 28 March 2005; accepted 27 April 2005 Available online 13 June 2005 Communicated by R. Kern Abstract The formation of high-density ZnOnanorods has been achieved by a vapor–solid growth mechanism using metallic zinc powder and oxygen gas as source materials for zinc and oxygen, respectively. General morphological studies indicated that the as-grown products are flower-shaped containing several hundreds of nanorods. The obtained nanorods have a diameter of 150–250 nm while their lengths are 5–10 mm. The detailed structural analysis revealed that the ZnOnanorods exhibit a single crystalline wurtzite hexagonal structure and preferentially oriented in the c-axis direction. Room temperature Raman scattering and photoluminescence studies found that the as-grown ZnOnanorods have good crystal quality with the hexagonal wurtzite phase containing very less structural defects. r 2005 Elsevier B.V. All rights reserved. PACS: 68.65.+g; 78.55.Et; 78.66.Hf Keywords: A1. High-resolution transmission electron microscopy; A1. Optical characterization; A1. ZnO nanorods; A2. Vapor–solid mechanism 1. Introduction A highly applicable and widely used II–VI semiconducting material, ZnO, with a wide band gap (3.37 eV) and high exciton binding energy (60 meV) is an excellent candidate for the fabrication of nanoelectronic and photonic devices [1,2]. Because of its wide band gap and high exciton binding energy much larger than ZnSe (22 meV) and GaN (25 meV), it has an opportunity to recognize itself as a versatile material and considerably acknowledged because of its catalytic, ARTICLE IN PRESS www.elsevier.com/locate/jcrysgro 0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.04.095 Corresponding author. Tel.: +82 63 270 2439; fax: +82 63 270 2306. E-mail address: ybhahn@chonbuk.ac.kr (Y.B. Hahn)
A. Umar et al/ Journal of Crystal Growth 282(2005)131-136 electrical, optoelectronic, and photochemical flow of high-purity nitrogen carrier gas with a fle properties [3-6]. It is known that exciton binding rate of 10 sccm(standard cubic centimeter per energy must be much greater than the thermal minute). When the furnace temperature reached energy at room temperature to show the efficient the desired growth temperature, the oxygen gas exciton laser action at room temperature. So the was flowed at 20 sccm during the whole growth low threshold intensity, high chemical stability and period. The typical growth time for the synthesis low growth temperature make the Zno a nice of these Zno nanorods was 1-1.5h. After the candidate for room temperature UV lasing [7-10]. growth process, the white colored products were Due to the aforementioned fields and applications, deposited near the outlet of the quartz tube. These needless to say, the hexagonal wurtzite Zno could products were scratched from the quartz tube and be one of most important materials for future examined in terms of structural and optical research and applications. a wide variety of Zno properties nanostructures have been synthesized by the General morphology of the deposited ZnO various fabrication techniques and are reported nanorods was observed using the scanning elec in the literature till date [11-20 tron microscopy(SEM) while the detailed struc In this paper, we report the catalyst-free larg tural characterization was done by the scale synthesis of ZnO nanorods grown by the transmission electron microscope(TEM)equipped thermal evaporation method using a rapid thermal with the selected area electron diffraction(SAED) reactor, in which the required temperatures are patterns. The crystallinity and crystal phase of the attainable in a short time. High-purity, commer- deposited structures were investigated by X-ray cially available zinc powder and oxygen gas were diffraction (XRD) pattern measured with Cu-Ko used as source materials for the zinc and oxygen, radiation. The room temperature Raman scatter respectively. Furthermore, the structural and ing and photoluminescence(PL)spectroscopy with optical properties of the deposited ZnO nanorods the Ar(513. 4 nm) and He-Cd(325 nm) laser line were studied in detail as the exciton sources, respectively, were used to examine the optical properties of the as-grown Zno nanorods 2. Experimental details The synthesis of high-density ZnO nanorods 3. Results and discussions was carried out by the rapid thermal, chemical vapor deposition process, which contains a hor- 3. 1. Structural characterization and growth izontal quartz tube furnace with the halogen lamp mechanism of the synthesized high-density heating system having the heating rate of 10C/s. catalyst-free ZnO nanorods A high-purity metallic Zn powder(99.99%)and oxygen gas(99.999%) have been used as precur To investigate the morphologies of the sors of Zn and oxygen, respectively. The source ynthesized ZnO nanostructures, SEM was used material, metallic zinc powder, was put into a Fig. l(a and b) shows the low and high magnifica quartz boat and loaded inside the quartz tube tion images of the as-grown materials. These urnace. Nitrogen gas (99.999%)was used as a images clearly indicated high-density, radially carrier gas and to create an inert atmosphere inside grown, flower-type structures containing several the furnace during the whole reaction process hundreds of nanorods in one array. The average Before starting the reaction, the chamber pressure diameter of one Zno nanorod was about was lowered to 3 Torr using a rotary vacuum 150-250 nm while their lengths were 5-10 um. pump, which was slightly increased after the The diameters of most nanorods are almost same introduction of the reactant gases. The source throughout their length and all exhibited smooth material, metallic zinc powder, was rapidly heated and clean surfaces with a slight reduction up to the temperature ranges 500-620C under a diameter at their tips. The full width of one
electrical, optoelectronic, and photochemical properties [3–6]. It is known that exciton binding energy must be much greater than the thermal energy at room temperature to show the efficient exciton laser action at room temperature. So the low threshold intensity, high chemical stability and low growth temperature make the ZnOa nice candidate for room temperature UV lasing [7–10]. Due to the aforementioned fields and applications, needless to say, the hexagonal wurtzite ZnOcould be one of most important materials for future research and applications. A wide variety of ZnO nanostructures have been synthesized by the various fabrication techniques and are reported in the literature till date [11–20]. In this paper, we report the catalyst-free largescale synthesis of ZnOnanorods grown by the thermal evaporation method using a rapid thermal reactor, in which the required temperatures are attainable in a short time. High-purity, commercially available zinc powder and oxygen gas were used as source materials for the zinc and oxygen, respectively. Furthermore, the structural and optical properties of the deposited ZnOnanorods were studied in detail. 2. Experimental details The synthesis of high-density ZnOnanorods was carried out by the rapid thermal, chemical vapor deposition process, which contains a horizontal quartz tube furnace with the halogen lamp heating system having the heating rate of 10 1C/s. A high-purity metallic Zn powder (99.99%) and oxygen gas (99.999%) have been used as precursors of Zn and oxygen, respectively. The source material, metallic zinc powder, was put into a quartz boat and loaded inside the quartz tube furnace. Nitrogen gas (99.999%) was used as a carrier gas and to create an inert atmosphere inside the furnace during the whole reaction process. Before starting the reaction, the chamber pressure was lowered to 3 Torr using a rotary vacuum pump, which was slightly increased after the introduction of the reactant gases. The source material, metallic zinc powder, was rapidly heated up to the temperature ranges 500–620 1C under a flow of high-purity nitrogen carrier gas with a flow rate of 10 sccm (standard cubic centimeter per minute). When the furnace temperature reached the desired growth temperature, the oxygen gas was flowed at 20 sccm during the whole growth period. The typical growth time for the synthesis of these ZnOnanorods was 1–1.5 h. After the growth process, the white colored products were deposited near the outlet of the quartz tube. These products were scratched from the quartz tube and examined in terms of structural and optical properties. General morphology of the deposited ZnO nanorods was observed using the scanning electron microscopy (SEM) while the detailed structural characterization was done by the transmission electron microscope (TEM) equipped with the selected area electron diffraction (SAED) patterns. The crystallinity and crystal phase of the deposited structures were investigated by X-ray diffraction (XRD) pattern measured with Cu-Ka radiation. The room temperature Raman scattering and photoluminescence (PL) spectroscopy with the Ar+ (513.4 nm) and He–Cd (325 nm) laser line as the exciton sources, respectively, were used to examine the optical properties of the as-grown ZnOnanorods. 3. Results and discussions 3.1. Structural characterization and growth mechanism of the synthesized high-density catalyst-free ZnO nanorods To investigate the morphologies of the synthesized ZnOnanostructures, SEM was used. Fig. 1(a and b) shows the low and high magnification images of the as-grown materials. These images clearly indicated high–density, radially grown, flower-type structures containing several hundreds of nanorods in one array. The average diameter of one ZnOnanorod was about 150–250 nm while their lengths were 5–10 mm. The diameters of most nanorods are almost same throughout their length and all exhibited smooth and clean surfaces with a slight reduction in diameter at their tips. The full width of one ARTICLE IN PRESS 132 A. Umar et al. / Journal of Crystal Growth 282 (2005) 131–136
A. Umar et al/ Journal of Crystal Growth 282(2005)131-136 三 Fig. 2. XRD pattern of the grown Zno nanorods: the indexed peaks are corresponding to the typical wurtzite hexagonal structure for the grown products of some unreacted zinc in the as-grown structures The high intensity and narrow spectral width of the obtained Zno peaks from the XRD patterns show that the as-grown nanorods are highly crystalline with very less impurit Additional structural characterizations of as- Fig. I.Low(a)and high(b)magnified SEM images of Zno grown Zno nanorods were carried out using nanorods synthesized using metallic zinc powder and oxygen the TEM equipped with the SAEd patterns gas as source materials for zinc and oxygen, respectively. Fig. 3(a-c) shows the low-magnification TEM presenting the general morphologies of the synthesized individual Zno nanorods and reveal flower-like structure is about 10-12 um. It is shown that the average diameter of the deposited that the distributions of these flowers -like struc- nanorod is between 150 and 250nm with the tures are uniform. In addition to this. the zno length of few micrometers. The diameters of the nanorods are randomly grown and they originated nanorods are almost same throughout their length from the centre of the flower. It seems that the with smooth and clean surfaces and mostly central part of the flower-shaped structures pro- exhibited a slight reduction in diameter at their vides a root for the growth of these Zno nanorods. tips. The corresponding SAed patterns of these To determine the crystallinity and crystal planes nanorods are shown in their insets indicating of the as-grown structures, the synthesized Zno the single crystallinity for the grown structures nanorods were analyzed with the XRD patterns. Fig. 3(d), the high-resolution TEM (HRTEM Fig. 2 shows the obtained peaks in the XRD image of the single crystalline ZnO nanorods, spectrum which is typically indexed to the wurtzite corroborates that the grown nanorods are single hexagonal phase of the bulk Zno, indicating that crystalline and grown along the [000 1] direction. the grown structures are c-axis oriented. Origina- The lattice spacing, corresponds to the d-spacing tion of one Zn peak at 38.9 indicated the existence of [000 1] crystal planes of the wurtzite ZnO, is
flower-like structure is about 10–12 mm. It is shown that the distributions of these flowers-like structures are uniform. In addition to this, the ZnO nanorods are randomly grown and they originated from the centre of the flower. It seems that the central part of the flower-shaped structures provides a root for the growth of these ZnOnanorods. To determine the crystallinity and crystal planes of the as-grown structures, the synthesized ZnO nanorods were analyzed with the XRD patterns. Fig. 2 shows the obtained peaks in the XRD spectrum which is typically indexed to the wurtzite hexagonal phase of the bulk ZnO, indicating that the grown structures are c-axis oriented. Origination of one Zn peak at 38.91 indicated the existence of some unreacted zinc in the as-grown structures. The high intensity and narrow spectral width of the obtained ZnOpeaks from the XRD patterns show that the as-grown nanorods are highly crystalline with very less impurities. Additional structural characterizations of asgrown ZnOnanorods were carried out using the TEM equipped with the SAED patterns. Fig. 3(a–c) shows the low-magnification TEM images presenting the general morphologies of the synthesized individual ZnOnanorods and reveals that the average diameter of the deposited nanorod is between 150 and 250 nm with the length of few micrometers. The diameters of the nanorods are almost same throughout their length with smooth and clean surfaces and mostly exhibited a slight reduction in diameter at their tips. The corresponding SAED patterns of these nanorods are shown in their insets indicating the single crystallinity for the grown structures. Fig. 3(d), the high-resolution TEM (HRTEM) image of the single crystalline ZnOnanorods, corroborates that the grown nanorods are single crystalline and grown along the [0 0 0 1] direction. The lattice spacing, corresponds to the d-spacing of [0 0 0 1] crystal planes of the wurtzite ZnO, is ARTICLE IN PRESS Fig. 1. Low (a) and high (b) magnified SEM images of ZnO nanorods synthesized using metallic zinc powder and oxygen gas as source materials for zinc and oxygen, respectively. Fig. 2. XRD pattern of the grown ZnOnanorods: the indexed peaks are corresponding to the typical wurtzite hexagonal structure for the grown products. A. Umar et al. / Journal of Crystal Growth 282 (2005) 131–136 133
A. Umar et al/ Journal of Crystal Growth 282(2005)131-136 (d) Fig 3(a-c) Low magnification and (d) high-resolution TEM images of Zno nanorods indicated the [0001 growth direction with the istance between two fringes is 0.52 nm, (inset of Fig. 3a, c and d) selected area electron diffraction(SAED) pattern images of the 0.52 nm for the grown nanorods and confirms that supersaturation state, the grown droplets lead to the grown nanostructures are preferentially or- the formation of nanostructures. The typical iented in the c-axis direction The hrtem results characteristic of the vls mechanism is the are consistent with the Saed patterns(Fig 3(d), presence of metal particles capped at the end of nset). The electron diffraction patterns and the grown nanostructures. However, no metal HRTEM images support the XRD results catalyst is used in our synthesis process of ZnO To explain the mechanism for the growth of the nanorods, so vapor-solid mechanism is presumed deposited ZnO nanorods, the vapor-solid mechan- instead of the VLS model. The Zn atoms were ism is proposed instead of conventionally used continuously evaporated from the quartz boat vapor-liquid-solid (VLS) model. In the Vls during the heating process. In the presence of mechanism [21], the source vapor reacted with oxygen, the Zn vapors adsorbed on the surface of the metal particles, which acts as a catalyst, and quartz tube react with oxygen and form the Zno formed the alloy droplets. After reaching at the nuclei. As the reactant concentration increases, the
0.52 nm for the grown nanorods and confirms that the grown nanostructures are preferentially oriented in the c-axis direction. The HRTEM results are consistent with the SAED patterns (Fig. 3(d), inset). The electron diffraction patterns and HRTEM images support the XRD results. To explain the mechanism for the growth of the deposited ZnOnanorods, the vapor–solid mechanism is proposed instead of conventionally used vapor-–liquid–solid (VLS) model. In the VLS mechanism [21], the source vapor reacted with the metal particles, which acts as a catalyst, and formed the alloy droplets. After reaching at the supersaturation state, the grown droplets lead to the formation of nanostructures. The typical characteristic of the VLS mechanism is the presence of metal particles capped at the end of the grown nanostructures. However, no metal catalyst is used in our synthesis process of ZnO nanorods, so vapor–solid mechanism is presumed instead of the VLS model. The Zn atoms were continuously evaporated from the quartz boat during the heating process. In the presence of oxygen, the Zn vapors adsorbed on the surface of quartz tube react with oxygen and form the ZnO nuclei. As the reactant concentration increases, the ARTICLE IN PRESS Fig. 3. (a–c) Low magnification and (d) high-resolution TEM images of ZnOnanorods indicated the [0 0 0 1] growth direction with the distance between two fringes is 0.52 nm, (inset of Fig. 3a, c and d) selected area electron diffraction (SAED) pattern images of the corresponding structures. 134 A. Umar et al. / Journal of Crystal Growth 282 (2005) 131–136
A. Umar et al/ Journal of Crystal Growth 282(2005)131-136 ZnO nuclei individually grow in upward direction wurtzite hexagonal phase of ZnO [23]. In addition in the form of nanorods. The Zno nanorods are to this, two very short and suppressed peaks at 332 grown along the [0001 direction which was and 389 cm-I are assigned to be as e2H-E2L substantiating from the HRTEM image and (multi phonon) and AIT modes respectively. The SAED patterns(Fig 3(d)) origination of a very short peak at 579 cm attributed as EIl was also observed. The appear- 3. 2. Optical properties of synthesized high-density ance of EIL mode is supposed to be because of the catalyst-free ZnO nanorods structural defects (oxygen vacancies, zinc inter stitial and free carriers)and impurities etc. The Raman scattering and photoluminescence stu- higher intensity and narrower spectral width of the dies, performed at room temperature, have been Raman active E, mode indicated that the employed to know the optical properties of the as- grown Zno nanorods have good crystal qu grown ZnO nanorods. With a wurtzite crystal with a hexagonal wurtzite crystal phase structure, Zno belongs to the Ct space group Fig. 5 demonstrates the room temperature PL having the two formula units per primitive cell and spectra of the synthesized ZnO nanorods.The all the atoms occupying the C3y symmetry. Near appearance of two peaks, a strong, dominated and the centre of the Brillouin zone, the group theory high intensity peak at 388 nm in the UV region predicts the existence of the different optic modes while a suppressed and week band at 510 nm in the T=A+2B1+E1+2E,. The Al. El, and 2E, visible region, was observed in the spectrum. The modes are Raman active while additionally the Al Uv emission is also called as near band edge and El are infrared active and split into long- emission and originated by the recombination of itudinal(LO)and transverse(To)optical compo- the free excitons through an exciton-exciton nents [22]. Fig. 4 shows the typical Raman collision process. The green band in the visible scattering of the synthesized products. A domi- region, known as deep level emission, is generally nated, sharp and strong intensity peak at explained by the radial recombination of the 4371cm- was observed in the spectrum which photo-generated hole with the electrons which is assigned as the optical phonon E2. The E2 mode belong to the singly ionized oxygen vacancies [24] corresponds to the band characteristic for the In our case, the UV emission is dominated over the Fig. 4. Typical Raman scattering spectrum of the asgrow Fig. 5. Photoluminescence spectrum of the synthesized Zno Zno nanorods with the Ar(513. nm) laser line as the exciton nanorods at room temperature using a He-Cd laser with ar sources excitation wavelength of 325 nm
ZnOnuclei individually grow in upward direction in the form of nanorods. The ZnOnanorods are grown along the [0 0 0 1] direction which was substantiating from the HRTEM image and SAED patterns (Fig. 3(d)). 3.2. Optical properties of synthesized high- density catalyst-free ZnO nanorods Raman scattering and photoluminescence studies, performed at room temperature, have been employed to know the optical properties of the asgrown ZnOnanorods. With a wurtzite crystal structure, ZnObelongs to the C4 6v space group having the two formula units per primitive cell and all the atoms occupying the C3V symmetry. Near the centre of the Brillouin zone, the group theory predicts the existence of the different optic modes: G ¼ A1 þ 2B1 þ E1 þ 2E2: The A1, E1, and 2E2 modes are Raman active while additionally the A1 and E1 are infrared active and split into longitudinal (LO) and transverse (TO) optical components [22]. Fig. 4 shows the typical Raman scattering of the synthesized products. A dominated, sharp and strong intensity peak at 437.1 cm1 was observed in the spectrum which is assigned as the optical phonon E2. The E2 mode corresponds to the band characteristic for the wurtzite hexagonal phase of ZnO [23]. In addition to this, two very short and suppressed peaks at 332 and 389 cm1 are assigned to be as E2H2E2L (multi phonon) and A1T modes respectively. The origination of a very short peak at 579 cm1 attributed as E1L was also observed. The appearance of E1L mode is supposed to be because of the structural defects (oxygen vacancies, zinc interstitial and free carriers) and impurities etc. The higher intensity and narrower spectral width of the Raman active E2 mode indicated that the asgrown ZnOnanorods have good crystal quality with a hexagonal wurtzite crystal phase. Fig. 5 demonstrates the room temperature PL spectra of the synthesized ZnOnanorods. The appearance of two peaks, a strong, dominated and high intensity peak at 388 nm in the UV region while a suppressed and week band at 510 nm in the visible region, was observed in the spectrum. The UV emission is also called as near band edge emission and originated by the recombination of the free excitons through an exciton–exciton collision process. The green band in the visible region, known as deep level emission, is generally explained by the radial recombination of the photo-generated hole with the electrons which belong to the singly ionized oxygen vacancies [24]. In our case, the UV emission is dominated over the ARTICLE IN PRESS Fig. 4. Typical Raman scattering spectrum of the as-grown ZnOnanorods with the Ar+ (513.4 nm) laser line as the exciton sources. Fig. 5. Photoluminescence spectrum of the synthesized ZnO nanorods at room temperature using a He–Cd laser with an excitation wavelength of 325 nm. A. Umar et al. / Journal of Crystal Growth 282 (2005) 131–136 135
A. Umar et al/ Journal of Crystal Growth 282(2005)131-136 green level emission. It has been reported that the References improvement in the crystal quality such as low structural defects, oxygen vacancies, zinc inter- [1] H. Kind, H. Yan, M. Law, B. Messer, P. Yang, Ad stitial and decrease in the impurities may cause Mater.14(2002)158 the appearance of a sharp and strong UV emission 2M. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. and a suppressed and weak green emission [25]. So Weber, R. Russo, P. Yang, Science 292(2001)1897. 3 D.S. King, R.M. Nix, J. Catal. 160(1996)76 the presence of a strong UV emission and a weak 4 T Minami, Mater. Res. Soc. Bull. 25(2000)38 green emission from the synthesized ZnO nanor- 5 D.M. Bagnall, Y F. Chen, Z. Zhi ao. S. Koy ods indicated that the as-grown structures have M.Y. Shen, T Goto, Appl. Phys (1997)2230 good crystal quality with less structural defects [6J. Zhong, A H. Kitai, P. Ascher, ff. Electrochem This pl result is also consistent with the tem and Soc.140(1993)3644 [7 P. Zu, Z.K. Tang, G.K. L. Wong. M. Kawasaki, A Raman observations Ohtomo. H. Koinuma, Y Segawa. Solid State Commun [8 H Cao, J.Y. Xu, E.W.Seelig, R P.H. Chang, Appl. Phys 4. Conclusions Lett.76(20002997 9 P. Yang, H. Yan, S. Mao, R. Russo, J. Johnson, R R. He. H -J. Choi. Ad Large-quantity with high-density Zno nanorod Funct. Mater. 12(200 have been synthesized by the thermal evaporation []S. Lee, Y.H. Im, Y B Korean j. Chem. Eng method using metallic zinc powder and oxygen gas as source materials for zinc and oxygen, respec- [I A. Sekar, S.H. Kim, A Y B. Hahn, J. Crystal ively. The general morphological studies indicated . S. Nahm. Y B. Hahn, J that the grown structures are flower type contain Growth27702005)479 ing several hundreds of nanorods in one array. The 13]B P. Zhang, N.T. Binh, K. Wakatsuki, Y. Segawa.Y nanorods having the average diameter of Yamada, N. Usami, M. Kawasaki, H. Koinuma, App 150-250nm with a length of 5-10 um while the Phys.Le84(2004)4098 full array of one flower is about 10-12 um. The [14] W L. Hughes, Z.L. Wang, Appl. Phys. Lett. 82(2003) detailed structural analyses reveal that the Zno [15)P X Gao Z.L. Wang, Appl. Phys. Lett. 84(2004)2883 nanorods exhibited a single crystalline wurtzite [16] x.Y. Kong, Z.L. Wang, Nano Lett. 3(2003)1625 hexagonal structure and preferentially grown in 17xY. Kong, Y. Ding, R.s. Yang, Z.L. Wang, Science 303 the c-axis direction. Room temperature Raman (2004)1348 [18]JY Lao, J.Y. Huang D.Z. Wang, Z.F. Ren Nano Lett. 3 scattering indicate that the as-grown nanorods have the hexagonal wurtzite phase with good [19]PX. Gao, Z.L. Wang. J. Phys. Chem. B 106(2002) crystal quality and very less structural defects. The 12653 PL spectra showed a strong UV emission at 20 J. Yu. Lao, J.G. Wen, Z.F. Ren, Nano Lett. 2(2002)1287. 88 nm, but a suppressed and weak green emission 221 M.H. Huang, Y. Wu, H. Feick, N. Tran, E. Weber. P Yang, Adv. Mater. 13 (2001)113 at 510nm 22J. M. Calleja, M. Cardona, Phys. Rev. B 16(1977) 23]YJ. Xing, Z.H. Xi, Z.Q. Xue, X.D. Zhang, JH R.M. Wang. J. Xu. Y. Song S L. Zhang. D.P. Yu Acknowledgement Phys.Lett.83(2003)1689 24 K. Vanheusden. C H Seager, W L. Warren D.R. Tallant. J.A. Voigt, JAppl. Phys. 79(1996)7983 This work was supported by the Brain Korea 21 25] D.M. Bagnall, Y.F. Chen, Z. Zhu, T. Yao, S. Koyama, project M.Y. Shen, T Goto, Appl. Phys. Lett. 73(1998)103
green level emission. It has been reported that the improvement in the crystal quality such as low structural defects, oxygen vacancies, zinc interstitials and decrease in the impurities may cause the appearance of a sharp and strong UV emission and a suppressed and weak green emission [25]. So the presence of a strong UV emission and a weak green emission from the synthesized ZnOnanorods indicated that the as-grown structures have good crystal quality with less structural defects. This PL result is also consistent with the TEM and Raman observations. 4. Conclusions Large-quantity with high-density ZnOnanorods have been synthesized by the thermal evaporation method using metallic zinc powder and oxygen gas as source materials for zinc and oxygen, respectively. The general morphological studies indicated that the grown structures are flower type containing several hundreds of nanorods in one array. The nanorods having the average diameter of 150–250 nm with a length of 5–10 mm while the full array of one flower is about 10–12 mm. The detailed structural analyses reveal that the ZnO nanorods exhibited a single crystalline wurtzite hexagonal structure and preferentially grown in the c-axis direction. Room temperature Raman scattering indicate that the as-grown nanorods have the hexagonal wurtzite phase with good crystal quality and very less structural defects. The PL spectra showed a strong UV emission at 388 nm, but a suppressed and weak green emission at 510 nm. Acknowledgement This work was supported by the Brain Korea 21 project in 2005. References [1] H. Kind, H. Yan, M. Law, B. Messer, P. Yang, Adv. Mater. 14 (2002) 158. [2] M. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897. [3] D.S. King, R.M. Nix, J. Catal. 160 (1996) 76. [4] T. Minami, Mater. Res. Soc. Bull. 25 (2000) 38. [5] D.M. Bagnall, Y.F. Chen, Z. Zhu, T. Yao, S. Koyama, M.Y. Shen, T. Goto, Appl. Phys. Lett. 70 (1997) 2230. [6] J. Zhong, A.H. Kitai, P. Mascher, W.J. Puff, Electrochem. Soc. 140 (1993) 3644. [7] P. Zu, Z.K. Tang, G.K.L. Wong, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Solid State Commun 103 (1997) 459. [8] H. Cao, J.Y. Xu, E.W. Seelig, R.P.H. Chang, Appl. Phys. Lett. 76 (2000) 2997. [9] P. Yang, H. Yan, S. Mao, R. Russo, J. Johnson, R. Saykally, N. Morris, J. Pham, R. He, H.-J. Choi, Adv. Funct. Mater. 12 (2002) 323. [10] S. Lee, Y.H. Im, Y.B. Hahn, Korean J. Chem. Eng. 22 (2005) 334. [11] A. Sekar, S.H. Kim, A. Umar, Y.B. Hahn, J. Crystal Growth 277 (2005) 471. [12] A. Umar, S. Lee, Y.S. Lee, K.S. Nahm, Y.B. Hahn, J. Crystal Growth 277 (2005) 479. [13] B.P. Zhang, N.T. Binh, K. Wakatsuki, Y. Segawa, Y. Yamada, N. Usami, M. Kawasaki, H. Koinuma, Appl. Phys. Lett. 84 (2004) 4098. [14] W.L. Hughes, Z.L. Wang, Appl. Phys. Lett. 82 (2003) 2886. [15] P.X. Gao, Z.L. Wang, Appl. Phys. Lett. 84 (2004) 2883. [16] X.Y. Kong, Z.L. Wang, Nano Lett. 3 (2003) 1625. [17] X.Y. Kong, Y. Ding, R.S. Yang, Z.L. Wang, Science 303 (2004) 1348. [18] J.Y. Lao, J.Y. Huang, D.Z. Wang, Z.F. Ren, Nano Lett. 3 (2003) 235. [19] P.X. Gao, Z.L. Wang, J. Phys. Chem. B 106 (2002) 12653. [20] J. Yu. Lao, J.G. Wen, Z.F. Ren, Nano Lett. 2 (2002) 1287. [21] M.H. Huang, Y. Wu, H. Feick, N. Tran, E. Weber, P. Yang, Adv. Mater. 13 (2001) 113. [22] J.M. Calleja, M. Cardona, Phys. Rev. B 16 (1977) 3753. [23] Y.J. Xing, Z.H. Xi, Z.Q. Xue, X.D. Zhang, J.H. Song, R.M. Wang, J. Xu, Y. Song, S.L. Zhang, D.P. Yu, Appl. Phys. Lett. 83 (2003) 1689. [24] K. Vanheusden, C.H. Seager, W.L. Warren, D.R. Tallant, J.A. Voigt, J.Appl. Phys. 79 (1996) 7983. [25] D.M. Bagnall, Y.F. Chen, Z. Zhu, T. Yao, S. Koyama, M.Y. Shen, T. Goto, Appl. Phys. Lett. 73 (1998) 1038. ARTICLE IN PRESS 136 A. Umar et al. / Journal of Crystal Growth 282 (2005) 131–136