R.S. Chen
a, Y.S. Huang
a,∗, Y.M. Liang
b, D.S. Tsai
b, K.K. Tiong
caDepartment of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan
bDepartment of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan
cDepartment of Electrical Engineering, National Taiwan Ocean University, Keelung 202, Taiwan
Abstract
Conductive iridium dioxide (IrO2) nanorods have been successfully grown on the Si(1 0 0) substrates via metalorganic chemical vapor deposition (MOCVD). A wedge-shaped morphology and naturally formed sharp tips are observed for IrO2nanorods using field-emission scanning electron microscopy (FESEM). High-resolution transmission electron microscopy (TEM) image and electron diffraction pattern show the growth of IrO2 nanorods preferentially along c-axis. Structure and composition of IrO2 nanorods have also been characterized using the techniques of Raman spectroscopy and X-ray photoelectron spectroscopy (XPS), respectively. It is noted that the IrO2nanorods are self-mediated instead of the conventional vapor–liquid–solid (VLS) approach or catalyst-mediated method.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Iridium dioxide nanorod; Field-emission scanning electron microscopy; Transmission electron microscopy; Raman spectroscopy; X-ray photoelectron spectroscopy
1. Introduction
Fabrication of one-dimensional (1D) nano-scaled materi-als such as rods and wires has gained considerable attention owing to their fundamental interests in science and potential in developing nanodevices. Among them, most researches have focused on the semiconductor materials such as Si[1], GaAs[2], GaN[3], and InP[4]. Recently, a few studies have extended to the oxide systems. For instance, ZnO was exten-sively studied due to its wide bandgap and large free exciton binding energy [5,6], while related MgO was investigated for improving the critical current densities of the supercon-ductors [7]. This rush for the 1D materials has further ex-panded into the electrical insulating oxides such as SiO2[8], GeO2[9], and Ga2O3[10], whereas the conducting oxides such as RuO2, OsO2, and IrO2 are relatively unexplored, except their synthesis in carbon nanotubes[11].
Recently, IrO2has been reported to be a candidate mate-rial for field emission cathodes of vacuum microelectronic devices and field emission displays owing to its low sur-face work function (4.23 eV) and high chemical stability [12,13]. Apparently, IrO2 with nanometer-sized tips will be particularly appealing in manufacturing field emitters.
∗Corresponding author.
E-mail address: [email protected] (Y.S. Huang).
Recently, such a nanosized IrO2tip has been realized using metalorganic chemical vapor deposition (MOCVD) tech-nique [14]. In this report, a detailed characterization for the IrO2 nanorods grown on Si(1 0 0) substrate are carried out. The morphology, structure, and composition of IrO2 nanorods are studied using field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Raman spectroscopy and X-ray photoelectron spec-troscopy (XPS). The results are analyzed and discussed.
2. Experimental
2.1. Growth of IrO2nanorods
The CVD experiments were carried out in a verti-cal cold-wall reactor using a low-melting iridium source reagent, (MeCp)Ir(COD), supplied by Strem Chemicals.
Both the precursor reservoir and the transport line were controlled in a temperature range between 100 and 120◦C to avoid precursor condensation during the vapor-phase transport. An oxygen flow rate of 100 sccm was used to convey precursor vapor and to invoke the growth reaction.
A high oxygen pressure in the range of 10–60 Torr was ap-plied, while deposition temperature was fixed at 350◦C. An investigation concerning this specific temperature at 350◦C for IrO2nanorod growth was described elsewhere[14]. IrO2
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2004.04.050
274 R.S. Chen et al. / Journal of Alloys and Compounds 383 (2004) 273–276
nanorods were directly grown on Si(1 0 0) wafers. All wafers before deposition were cleaned by the RCA procedure.
2.2. Characterization of the samples
The as-deposited samples were characterized by means of several techniques. A JEOL-JSM6500F FESEM was used to observe the morphology of nanorods. TEM images and elec-tron diffraction patterns, taken on a JEOL 2010F FEG TEM instrument, were used to determine the crystalline qual-ity and preferential growth direction of the individual IrO2 nanorod. Raman scattering measurements were utilized to examine the microstructure of IrO2nanorods. Raman spectra were recorded at room temperature utilizing the back scatter-ing mode on a Renishaw Raman Microscope System 2000.
The system is also equipped with an optical microscope for focusing of the 514.5 nm excitation line from the Ar-ion laser beam onto the sample. Chemical composition of sam-ples was examined by XPS using a Thermo VG Scientific Theta Probe system under the base pressure of 10−10Torr.
The Al K␣ 1486.68 eV line was the X-ray source and the Ag 3d5/2line at 368.26 eV was the calibration reference before measurement. XPS peak positions and integrated intensities were obtained through the curve fitting, using Thermo VG Scientific: Avantage v1.68 Software[15].
3. Results and discussion
Fig. 1 shows the nanosize rod-shaped IrO2 crystals grown on the Si(1 0 0) substrate. These IrO2 rods reveal wedge-shaped morphology and naturally formed sharp tips.
The estimated diameters and packing density are 75–150 nm and 25 ± 2 m−2, respectively. The energy dispersive spectrometry (EDS) shows a 1:2 atomic composition ratio expected for the IrO2stoichiometry.
A schematic diagram of a single IrO2 nanorod and its bright-field TEM image that emphasized the tip position
Fig. 1. FESEM images of the IrO2nanorods grown on Si(1 0 0) substrate.
Fig. 2. (a) The schematic diagram of a typical IrO2 nanorod; (b) TEM image focused on the tip of a nanorod; and (c) its corresponding electron diffraction pattern taken along [0 1 0] zone axis.
are illustrated inFigs. 2a and b, respectively. The electron diffraction pattern taken along the [0 1 0] zone axis is de-picted in Fig. 2(c). The electron diffraction pattern reveals the main diffraction dots indexed as {0 0 2}, {1 0 1}, and
R.S. Chen et al. / Journal of Alloys and Compounds 383 (2004) 273–276 275
Fig. 3. Raman spectrum of the IrO2nanorods grown on Si(1 0 0) substrate.
{2 0 0} planes. According to the diffraction pattern, [0 0 1]
direction is parallel to the long axis of the rods indicating that it is the preferred growth direction. Formation of ru-tile IrO2with single-crystalline structure is confirmed by the electron diffraction analysis.
The first-order Raman spectrum of the IrO2 nanorods grown on the Si(1 0 0) substrate is displayed inFig. 3. The experimental data of Raman spectra are presented as open circles. The rutile IrO2 phase of the as-deposited samples is further confirmed by the occurrence of three major Ra-man modes; namely, the Eg, B2g, and A1g modes, observed in the vicinity of 561, 728, and 752 cm−1. The observa-tion is consistent with the previous assignment [16]. The peak positions and the full-width at half maximum (FWHM) of these Raman signals can be accurately determined from the curve fitting procedure utilizing a Lorentzian line-shape functional form and plotted as solid line as shown inFig. 3.
The FWHMs of Eg, B2g, and A1gare evaluated to be 17± 2, 27± 2, and 12 ± 2 cm−1, respectively. The observed value of 17 cm−1for the FWHM of the Egband is close to that for IrO2single crystals (12 cm−1)[17], showing the formation of a high quality, nearly stress-free IrO2rod-shape crystals on Si substrate.
The XPS spectra of Ir 4f and O 1s core-electron obtained from the IrO2 nanorods on Si(1 0 0) are shown inFigs. 4a and b, respectively. Both Ir 4f and O 1s core-level spectra for IrO2 rods and single crystal exhibiting asymmetric line-shapes had been proved to be an intrinsic prop-erty from previous study [18]. The accurate peak posi-tions are determined through curve fitting using mixed Gaussian–Lorentzian line-shape after the treatment of back-ground by Shirley function. The Ir 4f signals having central energies at 62.0 and 64.9 eV are attributed to the oxide states of Ir atoms having 4+ valences in IrO2compound and are identified as [Ir4+] 4f7/2 and 4f5/2, respectively. The Ir 4f peak positions of the nanorods are close to that of single crystal (∼61.7 and ∼64.7 eV) [18], giving unambiguous evidence for the formation of IrO2 single crystals. The O
Fig. 4. The XPS spectra of (a) Ir 4f line and (b) O 1s line for the IrO2
nanorods grown on Si(1 0 0) substrate.
1s XPS line of the IrO2nanorods (seeFig. 4b) reveals two binding states of oxygen. The peak identified as [O2−] 1s with central energy at 530.4 eV is consistent with the value of 530.1 eV for IrO2single crystal[18], indicating that this part of oxygen atoms are well bound to Ir atoms within the IrO2compound. Another peak with broader character is de-tected at higher energy site of 531.8 eV. This feature cannot be eliminated after several runs of Ar+ion sputtering while the C 1s signal resulted from the surface contamination has been nearly eliminated. The result is different from the re-port by Kodintsev et al., in which the IrO2films prepared by thermal decomposition revealed an additional O 1s feature at 531.5 eV. They indicated that this unstable feature could be eliminated through ion bombardment and was attributed to the existence of hydroxide[19]. The broader feature at 531.8 eV for the present experiment might be attributed to the existence of oxygen-containing impurities within the samples.
Our previous study showed that if the growth temper-ature is over 400◦C or the chamber pressure lower than 10 Torr, growth of the IrO2 nanorods would be suppressed and instead we would observe the formation of a densely
276 R.S. Chen et al. / Journal of Alloys and Compounds 383 (2004) 273–276
packed IrO2 thin film [14]. Substantial amount of iridium metal have also been detected in these IrO2thin films. We have also checked the roots of IrO2 nanorods and their morphology at the initial growth stage. The growth mecha-nism appeared different from the vapor–liquid–solid (VLS) [1,20] and the catalyst-assisted growth process [4–6]. Al-though a unified conclusion on growth mechanism is not reached at this point, the importance of an oxygen-rich environment on the 1D-vertical growth of IrO2nanorods is recognized in this study.
4. Conclusion
IrO2 nanorods have been successfully grown on the Si(1 0 0) substrates by MOCVD under a relatively high oxygen ambient pressure range around 10–60 Torr and low temperature of 300–350◦C. FESEM micrographs show the IrO2 nanorods with wedge-shaped morphology and natu-rally formed sharp tips. High-resolution TEM image and electron diffraction pattern indicate that the IrO2 nanorods were grown preferentially along c-axis. The measure-ments of Raman spectra show the high quality and nearly stress-free rod-shape crystals of IrO2 on Si substrate. The XPS spectra of IrO2nanorods are quantitatively identical to that of IrO2single crystal. The formations of IrO2nanorods are self-mediated instead of the conventional VLS approach or catalyst-mediated method.
Acknowledgements
The authors R.S.C. and Y.S.H. would like to thank for the financial supports of the National Science Council of Taiwan under Project No. NSC 91-2112-M-011-001.
References
[1] A.M. Morales, C.M. Lieber, Science 279 (1998) 208.
[2] W.S. Shi, Y.F. Zheng, N. Wang, C.S. Lee, S.T. Lee, Appl. Phys.
Lett. 78 (2001) 3304.
[3] W. Han, S. Fan, Q. Li, Y. Hu, Science 277 (1997) 1287.
[4] J. Wang, M.S. Gudiksen, X. Duan, Y. Cui, C.M. Lieber, Science 293 (2001) 1455.
[5] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, Y. Yang, Science 292 (2001) 1897.
[6] M.H. Huang, Y. Wu, H. Feick, N. Tran, E. Weber, P. Yang, Adv.
Mater. 13 (2001) 113.
[7] P. Yang, C.M. Lieber, Science 273 (1996) 1836.
[8] Y.Q. Zhu, W.B. Hu, W.K. Hsu, M. Terrones, N. Grobert, J.P. Hare, H.W. Kroto, D.R.M. Walton, H. Terrones, J. Mater. Chem. 9 (1999) 3173.
[9] Z.G. Bai, D.P. Yu, H.Z. Zhang, Y. Ding, Y.P. Wang, X.Z. Gai, Q.L. Hang, G.C. Xiong, S.Q. Feng, Chem. Phys. Lett. 303 (1999) 311.
[10] Y.C. Choi, W.S. Kim, Y.S. Park, S.M. Lee, D.J. Bae, Y.H. Lee, G.S. Park, W.B. Choi, N.S. Lee, J.M. Kim, Adv. Mater. 12 (2000) 746.
[11] B.C. Satishkumar, A. Govindaraj, M. Nach, C.N.R. Rao, J. Mater.
Chem. 10 (2000) 2115.
[12] B.R. Chalamala, Y. Wei, R.H. Reuss, S. Aggarwal, B.E. Gnade, R.
Ramesh, J.M. Bernhand, E.D. Sosa, D.E. Golden, Appl. Phys. Lett.
74 (1999) 1394.
[13] B.R. Chalamala, Y. Wei, R.H. Reuss, S. Aggarwal, S.R. Pe-russe, B.E. Gnade, R. Ramesh, J. Vac. Sci. Technol. B 18 (2000) 1919.
[14] R.S. Chen, Y.S. Chen, Y.S. Huang, Y.L. Chen, Y. Chi, C.S. Liu, K.K. Tiong, A.J. Carty, Chem. Vap. Deposition 9 (2003) 301.
[15] Thermo VG: Scientific, Avantage Software, West Sussex, UK.
[16] Y.S. Huang, S.S. Lin, C.R. Huang, M.C. Lee, T.E. Dann, F.Z. Chien, Solid State Comm. 70 (1989) 517.
[17] P.C. Liao, C.S. Chen, W.S. Ho, Y.S. Huang, K.K. Tiong, Thin Solid Films 301 (1997) 7.
[18] G.K. Wertheim, H.J. Guggenheim, Phys. Rev. B 22 (1980) 4680.
[19] I.K. Kodintsev, S. Trasatti, M. Rubel, A. Wieckowski, N. Kaufher, Langmuir 8 (1992) 283.
[20] R.S. Wagner, W.C. Ellis, Appl. Phys. Lett. 4 (1964) 89.