Reui-San Chen,aYing-Sheng Huang,*aYa-Min Liang,bDah-Shyang Tsai,bYun Chic and Ji-Jung Kaid
aDepartment of Electronic Engineering, National Taiwan University of Science and Technology, 43 Keelung Road, Section 4 Taipei 106, Taiwan, Republic of China
bDepartment of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan, Republic of China. E-mail: [email protected]
cDepartment of Chemistry, National Tsing Hua University, Hsinchu 300, Taiwan, Republic of China
dDepartment of Engineering and System Science, National Tsing Hua University, Hsinchu 300, Taiwan, Republic of China
Received 19th May 2003, Accepted 29th July 2003
First published as an Advance Article on the web 13th August 2003
Iridium dioxide (IrO2) nanorods with pointed tips have been grown on Si(100) and transition-metal-coated-Si(100) substrates, via metal–organic chemical vapor deposition (MOCVD), using (MeCp)Ir(COD) as the source reagent. The as-deposited nanorods were characterized using field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). FESEM micrographs revealed that the majority of the nanorods are a wedge shape in cross section and converge at top; occasionally several of them pack into a column of a spiral tip. The vertical alignment and packing density are significantly improved by prior deposition of a thin layer of Ti on Si. TEM and XRD results indicate that the sputtered Ti thin layer erects the nanorods in the c-axis direction. XPS spectra show that iridium in IrO2nanorods also exist in a higher oxidation state.
1 Introduction
Fabrication of one-dimensional (1-D) nano-scaled materials such as rods and wires has gained considerable attention owing to a fundamental interest in science and to the potential use of these materials in nanodevices. To investigate the 1D carrier confinement effect, many studies have focused on the preparation and characterization of nanorods and nanowires of semiconductors and their binary compounds such as Si,1–4 GaAs,5–7GaN8–10and InP.11Recently, a few studies have been extended to oxide systems. For instance, ZnO has been extensively studied due to its wide bandgap (3.37 eV) and large free exciton binding energy (60 meV),12–15while related MgO has been investigated with a view to improving the critical current densities of superconductors.16 This rush for the 1D-materials has led to further studies of electrical insulating oxides such as SiO2,17 GeO218
and Ga2O3,19,20 whereas conducting oxide materials such as RuO2, OsO2and IrO2are relatively unexplored, except for their synthesis using carbon nanotubes as templates.21
Of these conductive oxides, IrO2has been investigated for use in applications such as optical switches for electrochromic devices,22 durable electrodes for chlorine or oxygen evolu-tion,23 and thin film electrodes for nonvolatile ferroelectric random access memory devices (FRAM).24In addition, IrO2is a candidate material for field emission cathodes of vacuum microelectronic devices and field emission displays owing to its low surface work function (4.23 eV) and high chemical stability.25,26Apparently, IrO2with nanometer-sized tips will be particularly appealing in the manufacture of field emitters.
Recently, such a nanosized IrO2tip was prepared using the metal–organic chemical vapor deposition (MOCVD) techni-que.27We have now focused on tuning the growth behavior of IrO2nanorods by modifying the substrate surface by sputtering various transition metal thin layers on a Si wafer. Our effort
shows that the degree of vertical alignment can be considerably improved by prior deposition of a thin titanium (Ti) layer on the Si(100) surface. Results of structural and compositional analyses are also discussed.
2 Experimental
2.1 Growth of IrO2nanorods
The CVD experiments were carried out in a vertical 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 the temperature range of 100–120 uC to avoid precursor con-densation during the vapor-phase transport. High purity oxygen, flow rate 100 sccm, was used as the carrier gas. The substrate temperatures were kept at 350 uC and the system pressure was held within the range of 10–50 Torr. IrO2 nanorods were grown on several transition-metal-sputtered silicon wafers and silicon wafers without metal coating. A dramatic difference in the growth behavior of the nanorods on two substrates, Si(100) and Ti-coated-Si(100) wafer, was observed and compared. The Ti-coated-Si(100) wafer was prepared by direct-current (dc) sputtering (1 minute) to deposit a Ti layer of y100 A˚. Before each experiment, the Si wafers were rinsed with a diluted HF solution (2 min) to remove the SiO2 coating on the surface, followed sequentially by de-ionized water and acetone, and then dried under nitrogen.
2.2 Characterization of IrO2nanorods
The micrographs of IrO2 nanorods were recorded using a JEOL-JSM6500F field-emission scanning electron microscope.
X-Ray diffraction patterns recorded on a Rigaku RTP300RC spectrometer were used to examine the growth orientation over
DOI: 10.1039/b305602n J. Mater. Chem., 2003, 13, 2525–2529 2525
This journal is # The Royal Society of Chemistry 2003
a large area. TEM images and electron diffraction patterns were recorded to check the preferential growth direction of individual IrO2 nanorods (JEOL 2010F FEG TEM). The chemical composition of IrO2 samples was determined by X-ray photoelectron spectroscopy using a Thermo VG Scien-tific Theta Probe system under a base pressure of 1029Torr.
The Al-Ka 1486.68 eV line was used as the X-ray source and the Ag 3d5/2line at 368.26 eV as the calibration reference before measurement. XPS peak positions and integrated intensities were obtained by curve fitting using Thermo VG Scientific:
Avantage v1.68 Software.28
3 Results and discussion
As illustrated in Fig. 1, FESEM images show that rod-like IrO2
crystals grown on the Si(100) substrate have a wedge-shaped geometry and almost all of them have sharp tips. The esti-mated diameters and packing density are 75–150 nm and 18 ¡ 2 mm22, respectively. The neighboring IrO2 rods on the Si substrate appear to stem from one common growth center and extend outwards. The orientations of these neighboring rods are rather erratic. To encourage these nanorods to grow in a more orderly manner, Al, Au, Cu, Ni (5%Ti), In and Ti layers, 100 A˚ thick, were sputtered on the Si(100) substrates prior to the CVD experiments. Of these metal layers, only Ti and In have substantial control on the nanorod orientations. The micrographs of IrO2nanorods grown on the other four metal layers, look similar to that in Fig. 1. The IrO2nanorods grown on the Ti layer, illustrated in Fig. 2, exhibit the most significant improvement in alignment. Figs. 2(a) and 2(b) show that the rod diameters are 50–100 nm with a distribution that is less dispersed than that of the IrO2rods directly grown on the Si substrate. The packing density of the nanorods on the Ti layer is increased to 45 ¡ 5 mm22. The size of the nanorods depends on the growth temperature, i.e. a higher temperature produces a larger nanorod. Sometimes IrO2wedge-shaped nanorods can
pack themselves into columns with an interesting spiral tip grown on top, depicted in Figs. 2(c) and 2(d). The diameters of these columns are larger, in the range of 100–250 nm.
A schematic diagram and a bright-field TEM image that emphasizes the tip of a typical wedge-shaped IrO2nanorod are shown in Figs. 3(a) and 3(b). The corresponding electron diffraction pattern taken along the [010] zone axis is depicted in Fig. 3(c). The electron diffraction pattern reveals the main diffraction dots indexed as the {002}, {101} and {200} planes.
The [001] direction shown by the arrow is parallel to the long axis of the nanorod, indicating that the c-axis is the preferred growth direction. Formation of an IrO2 tetragonal rutile structure in these rod-like crystals is confirmed by electron diffraction analysis.
XRD patterns of the IrO2nanorods on the Si and Ti-coated Si substrates as well as a JCPDS (No.15-870) standard pattern are shown in Fig. 4. Reflections of the two IrO2samples are indexed as (110), (101), (200), (211), (002) and (112) in the rutile structure. The XRD pattern, Fig. 4(a), of nanorods deposited on the Si substrate shows a much more intense (101) reflection and slightly more intense (002) and (112) reflections, compared with the JCPDS reference in Fig. 4(c). In contrast, the diffraction pattern of nanorods on the Ti–Si substrate (see Fig. 4(b)) exhibits a dramatic increase in (002) and (112) reflection intensities, and a large decrease in the (101) reflection intensity. The variations in the relative reflection intensities of the XRD patterns shown in Fig. 4 reveal the differences in crystal orientations of the nanorods grown over a large area.
The (101) signal comes from the side-walls of rod-shaped crystals, which has been confirmed in the electron diffraction analysis mentioned above. Since a large number of nanorods on the Si substrate are tilted, the (101) crystal planes are preferentially exposed. Hence the (101) reflection in Fig. 4(a) is much more intense than the (110) reflection, which is supposed to be the most intense reflection if the crystal orientation is randomly arranged, Fig. 4(c). When the nanorods are aligned vertically, the intensity of the (101) reflection drastically decreases since a portion of the (101) plane is overshadowed by neighboring nanorods, Fig. 4(b). On the other hand, the intensities of the (002) and (112) reflections increase. Quantita-tively, the integrated intensity ratio I(002)/I(101) increases from 0.37 (Fig. 4(a)) to 7.10 (Fig. 4(b)); meanwhile, I(112)/I(101) ratio increases from 0.32 (Fig. 4(a)) to 5.53 (Fig. 4(b)). The (002) and (112) reflections are derived from the facets at the front end of the IrO2nanorods. When many of the nanorods are erected vertically, the tops of the rods are exposed, hence, the (002) and (112) intensities increase. Therefore the intensity ratios, I(002)/I(101) and I(112)/I(101), could be used as a measure of the degree of vertical alignment. The two intensity ratios for IrO2 on the Ti–Si substrate are the highest of the substrates to be tested.
The XPS spectra of Ir 4f and O 1s core-electron obtained from IrO2nanorods on Ti–Si are shown in Figs. 5(a) and 5(b), respectively. For comparison, the corresponding XPS spectra of a IrO2single crystal are also displayed in Figs. 5(c) and 5(d).
Both Ir 4f and O 1s core-level spectra for the IrO2nanorods and single crystal exhibit asymmetric line-shapes, which have been proved to be an intrinsic property in a previous study.28 The accurate peak positions have been determined by curve fitting using Gaussian and Lorentzian line-shape mixing after treatment of the background by the Shirley function. The Ir 4f signal of the nanorods shows the iridium atoms to have two different binding states. The peaks identified as [Ir41] 4f7/2and 4f5/2at 62.0 and 65.0 eV, respectively, are attributed to the 41 oxidation state of iridium, and are similar to those of the IrO2 single crystal which have values of 61.7 and 64.7 eV, respec-tively. Another two broader features at 63.1 and 66.0 eV, respectively, higher than that of [Ir41] 4f7/2and 4f5/2by 1.0 eV, are observed. A similar result is also observed for the O 1s XPS line of the IrO2nanorods, in which the O 1s signal shows a Fig. 1 FESEM images of the IrO2 nanorods grown on a Si(100)
substrate. (a) Top view; (b) 45u cross sectional view.
2526 J. Mater. Chem., 2003, 13, 2525–2529
doublet, instead of the one peak observed for O 1s of the single crystal. The position of the main peak of the doublet at 530.5 eV is the same as that of O 1s of the standard IrO2 single crystal29 and an additional broader feature with a higher binding energy ofy531.6 eV is observed. These extra features located at higher-binding-energy sites of Ir 4f and O 1s might indicate the existence of an impurity with a higher oxidation state in the IrO2nanorods. Similar spectral features have been observed for RuO2and attributed to the presence of RuO3.30–33 Quantitative analysis of the peak areas indicates the composi-tions of the nanorods are 30 ¡ 2% and 70 ¡ 2% for Ir and O,
respectively. The presence of a higher oxidation state of iridium is considered to be the reason for the excess oxygen. The 10%
excess oxygen is also found in the samples of IrO2nanorods grown on the Si substrate. It seems, therefore, that the excess oxygen is a compositional feature of the IrO2 nanorods themselves. However, the implication of this for the growth of c-axis preferred IrO2nanorods is not known at this stage.
Our previous study has shown that if the growth temperature is over 400uC or the chamber pressure is lower than 20 Torr, the growth of IrO2 nanorods is suppressed and a densely packed IrO2thin film forms.27A substantial amount of iridium metal has been detected in these IrO2thin films, in contrast to the excess oxygen observed in the nanorods. We have also checked the roots of the IrO2nanorods and their morphology Fig. 2 FESEM images of the IrO2nanorods grown on the Ti-coated-Si substrate. (a) Top view; (b) 45u cross sectional view; (c) and (d) views of a nanorod with a larger spiral-shaped tip.
Fig. 3 (a) Schematic diagram of a typical IrO2nanorod; (b) a TEM image focussing on the tip of a nanorod and (c) its corresponding electron diffraction pattern taken along the [010] zone axis.
Fig. 4 XRD patterns for the IrO2nanorods grown on (a) the Si(100) substrate and (b) the Ti-coated-Si substrate. (c) XRD patterns of an IrO2powder standard taken from JCPDS (15-870).
J. Mater. Chem., 2003, 13, 2525–2529 2527
at the initial growth stage. The growth mechanism appears to be different to the vapor–liquid–solid (VLS)1,2,34and catalyst-assisted growth processes.11–13 Although a unified conclusion on growth mechanism has not been reached at this point, the importance of an oxygen-rich environment and the high-density nucleation sites provided by titanium on the 1-D vertical growth of IrO2 nanorods is recognized in this investigation.
4 Conclusion
IrO2 nanorods have been grown on Si(100) and transition-metal-coated-Si(100) substrates at a deposition temperature of 350 uC and under an oxygen pressure of 10–50 Torr. The majority of the IrO2nanorods have a wedge-shape geometry and a sharp tip. Their vertical alignment and packing density are significantly improved by a prior deposition of a thin layer of Ti on the Si substrate. TEM measurements indicate that the IrO2 rods have a preferred c-axis growth orientation. The presence of iridium in a higher oxidation state in the IrO2 nanorods is detected by XPS. The growth mechanism of the IrO2nanorod by MOCVD is different from that of ordinary vapor–liquid–solid or catalyst-mediated growth mechanisms.
Acknowledgements
The authors would like to thank the National Science Council of Taiwan for the financial support.
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