C C Chen1, R S Chen1, T Y Tsai1, Y S Huang1,4, D S Tsai2and K K Tiong3
1Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan
2Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan
3Department of Electrical Engineering, National Taiwan Ocean University, Keelung 202, Taiwan
Received 4 September 2004, in final form 27 October 2004 Published 12 November 2004
Online atstacks.iop.org/JPhysCM/16/8475 doi:10.1088/0953-8984/16/47/002
Abstract
Self-assembled and well aligned RuO2 nanorods (NRs) have been grown on sapphire (SA) substrates via metal–organic chemical vapour deposition (MOCVD), using bis(ethylcyclopentadienyl)ruthenium as the source reagent.
The surface morphology, structural, and spectroscopic properties of the as-deposited NRs were characterized using field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), selected-area electron diffractometry (SAD), x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), and micro-Raman spectroscopy. FESEM micrographs reveal that vertically aligned nanorods (NRs) were grown on SA(100), while the NRs on the SA(012) were grown with a tilt angle of∼35◦from the normal to the substrates. TEM and SAD measurements showed that the RuO2 NRs with square cross-section have the long axis directed along the [001] direction.
The XRD results indicate that the RuO2 NRs are (002) oriented on SA(100) and (101) oriented on SA(012) substrates. A strong substrate effect on the alignment of the RuO2 NRs growth has been demonstrated and the probable mechanism for the formation of these NRs has been discussed. XP spectra show the coexistence of higher oxidation state of ruthenium in the as-grown RuO2NRs. Micro-Raman spectra show the red-shift and peak broadening of the RuO2signatures with respect to that of the bulk counterpart which may be indicative of a phonon confinement effect for these NRs.
1. Introduction
Fabrication of one-dimensional (1D) nanoscaled materials, such as nanowires, nanotubes (NTs), and nanorods (NRs), has attracted considerable attention owing to their fundamental
4 Author to whom any correspondence should be addressed.
0953-8984/04/478475+10$30.00 © 2004 IOP Publishing Ltd Printed in the UK 8475
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interest in science and potential in developing nanodevices [1, 2]. The development of nanodevices might benefit from the distinctive morphology, huge surface area, and high aspect ratio of nanotubes and nanorods. Down-scaling a broad range of materials, especially the oxide materials, to 1D nanoscopic structure is currently the focus of a rapidly growing scientific community. Wide band gap semiconductor ZnO nanorods are brilliantly prepared and well characterized [3–5]. The electrically insulating oxides of nanostructured SiO2[6], TiO2[7, 8], SnO2 [9], GeO2[10], In2O3[11], Ga2O3[12], and VOx[13] have also been synthesized and studied. Among the numerous metallic oxides, electrically conducting RuO2and IrO2belong to one family with unique properties, whose nanostructures are not well cultivated and merit extensive investigation [14, 15].
RuO2belongs to the family of transition metal dioxide compounds with a rutile structure.
Because of its high electrical conductivity, chemical stability, and excellent diffusion barrier properties, it has found applications in thick film resistors [16, 17], electrode materials of electrochemical devices [18, 19]. It is also a candidate electrode material for ferroelectric random-access memory [20, 21], and a buffer layer for superconducting thin films [22]. In addition, RuO2NRs with the counterpart IrO2NRs [23] have been demonstrated to be candidate materials for field-emission cathodes of vacuum microelectronic devices and field-emission displays owing to the high chemical stability and high aspect ratio [24].
The synthesis of RuO2 NRs has been previously reported, via a template-based method [25]. For practical applications, we develop a simpler method to fabricate large area and high density RuO2 NRs on sapphire substrates. The 1D growth behaviour of RuO2
is found to be highly correlated with the oxygen-rich ambient, growth temperature, and orientations of the substrate rather than the catalyst. The surface morphology, structural, and spectroscopic properties of the as-deposited NRs were examined by using field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), selected-area electron diffractometry (SAD), x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), and micro-Raman investigation. A strong substrate effect on the alignment of the RuO2
NRs is observed, and the probable mechanism for the formation of NRs structure is discussed.
2. Experimental details
2.1. Growth of RuO2nanorods
A cold-wall and vertical-flow MOCVD system was used to prepare the samples on SA(100) and SA(012) substrates. The organometallic precursor bis(2,2,6,6-tetramethyl-3,5-heptanedionato) (1,5-cylooctadiene) ruthenium supplied by Strem Chemicals was utilized for chemical vapour deposition of RuO2 samples. There were two different flow paths for oxygen carrier gas, connecting to the growth chamber. The first one is a bypass flow path, which is designed for controlling the chamber pressure, while the second is heated to the designated temperature and is used for transporting the source vapour to the growth chamber.
Two independent temperature controllers were also mounted in the system to monitor the temperatures of the gas transfer line(Tt) and the precursor reservoir (Tp). Both Tt and Tp
were kept at a constant value of 90–110◦C to avoid the condensation of the precursor during vapour-phase transport. High purity oxygen was used as both carrier gas and reactive gas with the flow rate adjusted to 100 sccm. During deposition, the substrate temperature and pressure of the CVD chamber were controlled at 450◦C and 10–50 Torr, respectively. The growth rate was controlled by adjusting the total pressure of the growth chamber and the partial pressure of the organometallic reagent.
RuO2nanorods on sapphire 8477
Figure 1. (a) The 30◦perspective view and (b) top view FESEM micrographs of the vertically aligned RuO2square nanorods grown on SA(100) substrate.
2.2. Characterization of RuO2nanorods
The micrographs of RuO2 samples were recorded using a JEOL-JSM6500F field-emission scanning electron microscope. TEM images and SAD patterns were recorded to check the nanostructure and preferential growth direction of the individual RuO2NRs (JEOL-200FXII TEM). X-ray diffraction patterns were recorded on a Rigaku RTP300RC spectrometer to examine the growth orientation over a large area. Surface compositions of RuO2 NRs were analysed with x-ray photoelectron spectroscopy (XPS) using a Thermo VG Scientific Theta Probe system under the base pressure of 10−9 Torr. The Al Kα1486.68 eV line was the x-ray source and the Ag 3d5/2 line at 368.26 eV was the calibration reference. XPS peak positions and integrated intensities were obtained through curve fitting, using Thermo VG Scientific: Avantage v1.68 software [26]. Raman scattering spectroscopy was used to extract microstructural information about the RuO2NRs by using a Jobin-Yvon T64000 micro-Raman system, equipped with an Ar ion laser having an excitation wavelength of 514.5 nm which was focused on the sample using an optical microscope.
3. Results and discussion
As illustrated in figure 1, the FESEM images show vertically well aligned RuO2NRs grown on SA(100) substrate. The estimated edge size and the length of the NRs are around 20–40 nm
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Figure 2. FESEM images of the well aligned RuO2nanorods grown on SA(012) substrate: (a) a 30◦perspective view and (b) a cross-view. The self-assembled nanorods were grown with identical tilt angle (∼35◦) from the normal to the substrate.
and 0.5–2.0µm, respectively. The top view (figure 1(b)) images of the overall rods reveal clear solid squares with the edges parallel to each other. This result indicates that the rods standing on the substrate are perfectly vertical and follow the same in-plane orientation.
As illustrated in figure 2, the FESEM images show high density and well aligned RuO2
NRs grown on SA(012) substrate. The self-assembled NRs were grown with identical tilt angle (∼35◦) from the normal to the substrate. The estimated size, length, and packing density are 15–40 nm, 1–4µm, and 175 ± 25 µm−2, respectively.
Figure 3(a) shows the TEM image focused on a single RuO2 nanorod. Figure 3(b) is the SAD pattern taken from the rod sidewall. The SAD pattern has been identified to be the [110] zone pattern, indicating that the rod walls belong to the{110} facets and the preferential growth direction of the RuO2rod is along the [001] direction (c-axis). A schematic plot of the RuO2rod is shown in figure 3(c).
Typical XRD patterns of the well aligned RuO2 NRs grown on SA(100) and SA(012) substrates are shown in figures 4(a) and (b), respectively. As shown in figure 4(a), the single RuO2(002) diffraction peak at ∼59.6◦reveals the uniquely single directional growth of RuO2
NRs along [001] for the sample grown on SA(100). As depicted in figure 4(b), a peak at around 35◦is indexed to the (101) plane of rutile RuO2, indicating that all the RuO2(101) planes are parallel to the substrate plane. The results from electron diffraction and x-ray diffraction
RuO2nanorods on sapphire 8479
(110) (110)
(a)
(b) (c)
c axis
Figure 3. (a) The TEM image focused on a single RuO2nanorod. (b) The SAD pattern taken from the rod sidewall in (a). (c) A schematic diagram of the RuO2nanorod.
20 30 40 50 60 70 80
Intensity( a. u.)
2θ (degree)
SA( 0 1 2 ) SA( 0 2 4 )
RuO2( 1 0 1 )
(b)
SA(012)
SA( 3 0 0 )
RuO2( 0 0 2 ) SA(100)
(a)
Figure 4. The XRD patterns for the RuO2 nanorods grown on (a) SA(100) and (b) SA(012) substrates.
suggest that the growth mechanism of RuO2nanorods is the same as that for IrO2[14], even though the morphological appearances of these two conducting oxides are somewhat different.
Growth with (001) and (101) orientations of RuO2on SA substrates can be explained on the basis of the lattice relationship. The lattice parameters are a= b = 0.449 nm and c = 0.311 nm for RuO2[27], a= b = 0.476 nm and c = 1.299 nm for sapphire [28]. The lattice misfit at the interface produces strain energy when the RuO2is nucleated. The orientation that minimizes the lattice misfit and produces the smallest strain energy will be preferred. Schematic plots of the epitaxial relationships of RuO2/SA(100) and RuO2/SA(012) are shown in figures 5 and 6, respectively. The growth pattern results in the smallest lattice mismatch between the RuO2
NRs and the substrates. According to the plot shown in figure 5, the in-plane orientation of RuO2[100] along SA[010] shows a mismatch of∼ − 5.7% while RuO2[010] along SA[010]
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Figure 5. The schematic drawing of the epitaxial relationship between RuO2and SA(100): (a) the RuO2(001) plane; (b) the SA(100) plane; (c) epitaxy of RuO2(001)/SA(100). (d) The schematic diagram of the orientation relationship between the RuO2nanorod and the SA(100) substrate.
shows a mismatch of∼38.1%. Therefore the overall orientation relationship between the RuO2
nanorods and SA(100) can be described as RuO2(001) SA(100) and RuO2[100] SA[010].
A schematic drawing of the epitaxial relationship of RuO2 and SA(012) which resulted in a tilted growth of RuO2 NRs is shown in figure 6. The lattice spacing of RuO2[0¯10] along SA[100] shows a mismatch of∼−5.7% while RuO2[¯101] along SA[¯1¯21] shows a mismatch of∼7.8%. Therefore the overall orientation relationship between the RuO2 nanorods and SA(012) can be described as RuO2(101) SA(012) and RuO2[0¯10] SA[100].
Possible interpretation of the substrate effect on the tilted growth of RuO2 NRs can be understood as follows: initially, the deposition of RuO2starts from the epitaxy of{101} planes on the SA(012) surface. Since the long axis of nanorods is along the [001] direction, the growth rate of (00l) planes should be the highest in this case. Then the tilted growth occurs along the [001] direction which is∼35◦ from the normal to the SA(012) substrate or RuO2(101) plane. This process leads to vertical 1D growth of RuO2NRs on the sapphire substrate and can likewise be explained by the initial RuO2(001) nucleation on the SA(100) plane. Since the RuO2(001) plane is normal to the SA(100) plane, under this anisotropic growth condition, the RuO2(001) nuclei should elongate along {001} orientation and form vertically aligned NRs in the same direction. The schematic diagrams of the orientation relationship between RuO2
NRs and SA(100)/SA(012) substrates are shown in figures 5(d) and 6(d), respectively.
The strong substrate effect on the alignment of the RuO2nanorods during growth has been demonstrated, where either the vertical or tilted alignment of the RuO2rods can be understood from the lattice misfits at the interface. However, the upward growth of rods from the interface with morphology of square cross-section is another issue of scientific interest. We shall discuss the possible origin of the morphology in terms of the c-axis directional growth mechanism.
Directional growth of RuO2along the c-axis is always observed regardless of whether vertical
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Figure 6. The schematic drawing of the epitaxial relationship between RuO2and SA(012): (a) the RuO2(101) plane; (b) the SA(012) plane; (c) epitaxy of RuO2(101)/SA(012). (d) The schematic diagram of the orientation relationship between the RuO2nanorod and the SA(012) substrate.
or tilted alignments of the square rods were formed. The [001] growth direction is preferred since the (002) plane is the least stable among (110), (101), and (002) planes [29]. In addition, the{110} planes are the most stable crystal planes for rutile structure materials [29]. Thus the RuO2growth proceeds to eliminate the (002) planes via forming its perpendicular{110}
planes; accordingly the crystal elongates in the [001] direction with a square geometry.
The RuO2NRs were also examined using XPS to determine their composition, particularly the relative content of carbon impurities. Unfortunately the C 1s peak overlaps with Ru 3d3/2 at the binding energy∼284 eV in the XPS survey scan, thus making direct measurement of the carbon content impossible. However, the Ru 3d5/2peak at∼280 eV does not overlap with any carbon signals, and the 3d5/2/3d3/2peak intensity ratio of RuO2NRs was approximately 3:2, which closely corresponds to the theoretical value [30]. If carbon were indeed present in the RuO2NRs, the observed 3d5/2/3d3/2integration ratio would show a clear deviation from the ideal value of 3:2. Thus, slow scans over these regions of interest, followed by a least-squares curve fit and deconvolution process to estimate the 3d5/2/3d3/2ratio, should give information on the relative carbon content. In this work, no carbon contamination was being detected.
The XPS spectra of Ru 3d and O 1s core electrons obtained from RuO2NRs on SA(100) are shown in figures 7(a) and (b), respectively. For comparison, the corresponding XPS spectra of a RuO2single crystal are also displayed in figures 7(c) and (d), respectively. Both Ru and O 1s core-level spectra for the RuO2NRs and single crystal exhibit asymmetric lineshapes; this has been proven to be an intrinsic property in previous study of metallic rutile oxides [31, 32].
The accurate peak positions have been determined by curve fitting using a mixed Gaussian
8482 C C Chen et al 3d line and (d) O 1s line for the RuO2single crystal.
and Lorentzian lineshape after treatment of the background with the Shirley function. The Ru 3d signal of the NRs shows the Ru atoms have two different binding states. The peaks identified as [Ru4+] 3d5/2and 3d3/2at 280.3 and 284.5 eV, respectively, are attributed to the 4+ oxidation state of ruthenium, and are similar to those of the RuO2 single crystal which are at 280 and 285.1 eV, respectively. Another two broader features at 281.9 and 286.6 eV, which are respectively higher than those of [Ru4+] 3d5/2and 3d3/2by 1.6 and 2.1 eV, are also observed for the NRs. A similar result is also observed for the O 1s XPS line of the RuO2
NRs, in which the O 1s signal shows a doublet, similar to the observed for O 1s for the single crystal. The position of the main peak of the doublet at 529.9 eV for NRs is close to that for O 1s for the RuO2single crystal (530.3 eV) and an additional broader feature with a higher binding energy of∼531.6 eV is also observed for the NRs. These extra features located at higher binding energy sites of Ru 3d and O 1s might indicate the existence of an impurity with a higher oxidation state in the RuO2NRs. Similar spectral features have been observed for RuO2films and attributed to the presence of RuO3[33]. Quantitative analysis of the peak areas above the background Shirley function, taking into account the sensitivity factors from the Thermo VG Scientific Avantage v1.68 software [26], indicates that the compositions of the NRs are 32±2% and 68±2% for Ru and O, respectively. The excess of oxygen during growth is a probable reason for the presence of higher oxidation states of ruthenium. It seems that the occurrence of excess oxygen is a compositional feature of the RuO2NRs and the implication of this feature for the growth of RuO2 NRs is not known at this stage and requires further investigation.