Growth and Characterization of Well-Aligned RuO2 Nanocrystals on

Science and Technology, Taipei 106, Taiwan, and Department of Electrical ... Taiwan. ReceiVed May 1, 2006; ReVised Manuscript ReceiVed August 31, 2006...
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Growth and Characterization of Well-Aligned RuO2 Nanocrystals on Oxide Substrates via Reactive Sputtering Korotcov,†

Alexandru Kwong-Kau Tiong§

Hung-Pin

Hsu,†

Ying-Sheng

Huang,*,†

Dah-Shyang

Tsai,‡

and

CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 11 2501-2506

Departments of Electronic Engineering and Chemical Engineering, National Taiwan UniVersity of Science and Technology, Taipei 106, Taiwan, and Department of Electrical Engineering, National Taiwan Ocean UniVersity, Keelung 202, Taiwan ReceiVed May 1, 2006; ReVised Manuscript ReceiVed August 31, 2006

ABSTRACT: Well-aligned, densely packed RuO2 nanocrystals (NCs) have been grown on sapphire (SA), LiNbO3 (LNO), and LiTaO3 (LTO) substrates with different orientations via reactive magnetron sputtering using a Ru metal target. The surface morphology and structural and spectroscopic properties of the as-deposited NCs are characterized using field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and micro-Raman spectroscopy (RS). FESEM micrographs reveal that NCs grown on SA(100)/LNO(100) are vertically aligned, whereas the NCs on SA(012)/LTO(012) and SA(110) contain singly and doubly tilted alignments, respectively, with a tilted angle of ∼35° from the normal to the substrates. NCs grown on SA(001) show in-plane alignment with the mosaic structure. The XRD results indicate that the NCs are (001), (101), and (100) oriented on SA(100)/LNO(100), SA(012)/LTO(012)/SA(110), and SA(001) substrates, respectively. A strong substrate effect on the alignment of the RuO2 NCs deposition has been demonstrated. XPS analyses reveal the coexistence of higher oxidation states of Ru in the as-deposited RuO2 NCs. The Raman spectra show the red-shift and asymmetric peak broadening of the RuO2 signatures with respect to that of the bulk counterpart, which are attributed to both the size and residual stress effects, whereas the intensity of certain modes follows the selection rules for the different oriented NCs. 1. Introduction In the past decade, one-dimensional (1D) nanocrystals (NCs) have attracted enormous attention because of their fundamental interests and promises in the fabrication of nanodevices.1,2 Oxides are currently a major topic in nanoscopic research activities, and the search for binary oxides nanocrystals has been rather successful all over the periodic table.3 RuO2 crystallized in a tetragonal rutile structure and exhibited metallic conductivity at room temperature.4 Because of the unique combination of metallic conductivity and oxide nature, RuO2 has been used in thick film resistors5,6 and as electrode material for electrochemical devices,7,8 electrodes in ferroelectric random-access memory,9,10 electrochemical capacitors for energy storage,11-13 and field-emission (FE) cathodes for vacuum microelectronic devices and FE displays.14-16 As a result of these diverse applications, there is a growing need to develop easy and reliable methods for growing different RuO2 phases as micro- or nanophase forms. Various methods such as metalorganic chemical vapor deposition (MOCVD),17,18 reactive sputtering,19,20 pulsed laser deposition,21 molecular beam epitaxy,22 electroplating followed by annealing,23 pyrolysis of metal-organics,24 reduction of RuO4,25,26 and electrochemical oxidation of Ru metal surface27,28 have been employed for this purpose. Among these methods, the reactive radio frequency magnetron sputtering (RFMS) is a simple technique for fabricating large area structures, which has several advantages, including better control of the growth conditions and a single deposition step to obtain the nanostructures. * To whom correspondence should be addressed. Phone: (886)227376385. Fax: (886)2-27376424. E-mail: [email protected]. † Department of Electronic Engineering, National Taiwan University of Science and Technology. ‡ Department of Chemical Engineering, National Taiwan University of Science and Technology. § Department of Electrical Engineering, National Taiwan Ocean University.

In this work, we report the growth of well-aligned, densely packed RuO2 NCs by reactive RFMS using a Ru metal target at 200 °C on different oriented single-crystal oxide substrates. The surface morphology and structural and spectroscopic properties of the as-deposited NCs were examined. The growth behavior of RuO2 NCs has been found to be highly correlated with the pressure, rf power, substrate temperature, and orientations of the substrates. A strong substrate effect on the alignment of the NCs is observed, and the probable mechanisms for the formation of NC structure will be discussed. The Raman spectra show the red-shift and asymmetric peak broadening of the RuO2 signatures, which are attributed to both the size and residual stress effects, whereas the intensity of certain modes follows the selection rules for the different oriented NCs. 2. Experimental Section The study was performed using a homemade high-vacuum RFMS system. A schematic diagram with system basic parameters and substrate treatment procedure can be found elsewhere.29 The sputtering target was a 1 in. Ru (99.95%) metal. The substrates used in this study were sapphire (SA) with different orientations: SA(001), SA(100), SA(012), and SA(110); LiNbO3 (LNO) (100) and LiTaO3 (LTO) (012). All the substrates had the dimensions ∼10 × 10 mm2, and at least one side of each substrate was polished. Each substrate was pretreated through a standard cleansing procedure: consecutive cleansing with acetone and methanol in an ultrasonic bath for 10 min to prevent any contamination on the substrate surface. A manually controlled shutter was located in front of the target. Two separate gas lines, each equipped with a mass flow controller, were used to control the Ar and O2 flow rates with an accuracy of 0.1 standard cubic centimeter per minute (sccm). The sample holder was approximately 50 mm from the target and can be heated to a maximum temperature of 500 °C. To promote uniform transfer of heat to the substrates, we placed a thin coating of melted In between the substrate and sample holder. Care was taken to confine all the In metal to only the back of the substrate. The sputtering chamber was evacuated with a turbo-molecular pump and has a base pressure of ∼1 × 10-5 mbar. The reactive sputtering was carried out in a mixture of argon (11 sccm) and oxygen (1 sccm) gases. O2 was introduced over the substrate into the sputtering chamber with Ar

10.1021/cg060254h CCC: $33.50 © 2006 American Chemical Society Published on Web 10/04/2006

2502 Crystal Growth & Design, Vol. 6, No. 11, 2006 atmosphere. A working pressure of 7.5 mbar, RF generator power of 65 W, substrate temperature Ts of 200 °C, and a 180 min deposition time were used in the experiment. For comparison purposes, RuO2 single crystals (SC) were also grown by the open-flow chemical vapor transport technique with oxygen as the transporting agent.30 The gaseous oxide, RuO3 or RuO4, was formed when oxygen was passed over Ru metal powder at a temperature of around 1350 °C and a flow rate of ∼100 cm3min-1. The volatile RuO3 or RuO4 gas then decomposed and crystallized into highly ordered single crystals of RuO2 at about 1050°C. The morphology of the RuO2 1D NCs was recorded using a JEOLJSM6500F field-emission scanning electron microscope with an accelerating voltage of 15 kV. Crystal structures were analyzed using a Rigaku D/Max-RC X-ray diffractometer (XRD) equipped with Cu KR radiation source and Ni filter. The chemical binding states of RuO2 NCs were analyzed by XPS using a Thermo VG Scientific Theta Probe system under a base pressure of 1.3 × 10-7 Pa. The Al KR 1486.68 eV line was used as the X-ray source. Before the measurement, the system was calibrated using the Ag 3d5/2 line at 368.26 eV. The XPS operating conditions were published elsewhere.31 XPS peak positions and integrated intensities were obtained through the curve fitting, using Thermo VG Scientific: Avantage version 2.13 software. Mixed Lorentzian/Gaussian curve fitting with no asymmetry after the treatment of background by Shirley function was applied. The charge compensations were determined using Ru 3p1/2 and Ru 3p3/2 peak positions as references, from which any charging can be measured. Raman spectroscopy was used to extract nanostructural information of the RuO2 NCs. Raman spectra were recorded on a Renishaw inVia micro-Raman system with a1800 grooves/mm grating and an optical microscope with a 50× objective at room temperature. The same microscope was used to collect the signal in backscattering geometry. The Ar-ion laser beam of the 514.5 nm excitation line with a power of about 1 mW was focused on a spot size ∼5 µm in diameter.

Korotcov et al.

Figure 1. Typical FESEM images (30° perspective view and crosssection view) of the vertically aligned RuO2 NCs grown on (a) SA(100) and (b) LNO(100) substrates and their (c) X-ray diffraction patterns.

3. Results and Discussion 3.1. RuO2 on SA(100) and LNO(100). The FESEM images illustrated in images a and b of Figure 1 show the RuO2 NCs grown on SA(100) and LNO(100) substrates with vertically aligned growth behavior. The rodlike densely populated RuO2 NCs have an edge size and length of about 45 ( 5 nm and 530 ( 10 nm, respectively. The typical XRD patterns of the vertically aligned RuO2 NCs grown on SA(100) and LNO(100) depicted in Figure 1c show the preferable orientation of the nanostructures along RuO2 [001] (2θ ≈ 59.5°). For the RuO2 NCs on the LNO(100) substrate, a weak diffraction signal indexable to the RuO2(301) plane (2θ ≈ 69.4°) can be distinguished. This fact proves the predominantly (001) oriented growth on SA(100) and LNO(100) with a small presence of (301) growth orientation on LNO(100) substrates, which can be predicted according to the lattice relationship of substrate and nanostructure interfaces. A similar growth behavior of IrO2, a counterpart of RuO2, grown on SA(100) by reactive RFMS was observed by our group earlier.29 The preferable oriented growth of RuO2(001) along [001] is explained by examining the RuO2 and SA(100)/LNO(100) planar structures at the atomic level, which can to be correlated to the epitaxial relation between the rutile lattice of RuO2 and the underlying single-crystal substrates. The main assumption is that the SA(100) and LNO(100) surfaces are terminated by dislocated oxygen atoms, as in the single crystals. The schematic diagrams illustrated in Figure 2 show the atomic arrangements on RuO2(001) and SA(100), LNO(100) planes. The lattice parameters for RuO2 are a ) b ) 4.49 Å and c ) 3.11 Å (JCPDS 88-0322); for sapphire, they are a ) b ) 4.76 Å and c ) 12.99 Å (JCPDS 10-0173), and a ) b ) 5.15 Å and c ) 13.86 Å for LiNbO3 (JCPDS 20-631). The incoming Ru atoms have sufficient mobility to minimize the lattice misfit and align themselves in a RuO2(001) arrange-

Figure 2. Schematic plots of the lattice relationships between RuO2 and SA(100) and LNO(100) substrates: (a) RuO2(001) plane; (b) SA(100) plane; (c) LNO(100) plane; (d) RuO2(001) on SA(100); and (e) RuO2(001) on LNO(100).

ment because the oxygen arrangement of the underlying substrates is similar to that of RuO2(001) oxygen atoms. Thus, the growth relationship can be described as RuO2(001)[100] // SA(100)[010] and RuO2(001)[100] // LNO(100)[010]. These alignments produce residual stress due to mismatch values of -5.67% (i.e., ((4.49-4.76) Å /4.76 Å)×100%) along RuO2[100], +3.46% ((4.49-4.34) Å/4.34 Å) along RuO2[010] for the NCs grown on SA(100), and -12.82% (i.e., (4.49-5.15) Å/5.15 Å) along RuO2[100], -3.23% ((4.49-4.64) Å/4.64 Å) along RuO2[010] for the NCs grown on LNO(100). Therefore, we conclude that the c-axis directional growth18 RuO2[001] with

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Figure 4. Schematic plots of the lattice relationships between RuO2 and SA(012), LTO(012), and SA(110) substrates: (a) RuO2(101) plane; (b) SA(012) plane; (c) LTO(012) plane; (d) SA(110) plane; (e) RuO2(101) on SA(012); (f) RuO2(101) on LTO(012); and (g) RuO2(101) on SA(110).

Figure 3. Typical FESEM images (30° perspective view and crosssection view) of the tilted RuO2 NCs grown on (a) SA(012), (b) LTO(012), and (c) SA(110) substrates and their (d) X-ray diffraction patterns.

the lattice mismatch-minimizing mechanism29 explain the vertical growth of RuO2 NCs on SA(100) and LNO(100) substrates, which exhibit the templates for the RuO2(001) plane formation. 3.2. RuO2 on SA(012), LTO(012), and SA(110). The micrographs of self-assembled, densely packed RuO2 NCs on SA(012), LTO(12), and SA(110) are illustrated in Figure 3. These rodlike nanostructures exhibit regularly tilted NCs with identical tilt angles (∼35°) from the normal to substrate. Moreover, the NCs on the SA(110) substrate reveal symmetrical doubly tilted aligned directions. The possible explanation of these unique directional growths will be discussed below. The FESEM images reveal that RuO2 NCs have an average diameter and length of 45 ( 5 nm and 530 ( 10 nm, respectively. Figure 3d shows the typical XRD patterns of the regularly tilted RuO2 NCs deposited on SA(012), LTO(12), and SA(110). Two peaks can be indexed as (101) and (202) diffraction planes at around ∼35.0 and 74.6° 2θ, respectively, indicating parallel in-plane RuO2(101) orientation. Here, we observe anisotropic growth and as a result, film formation is restricted by the inplane mismatch. Thus, the deposited Ru and O atoms are stacked into a 1D nanostructure in the c-direction, with RuO2 plane formation following the substrate orientation. The probable allowed orientation of the NCs to the substrate interfaces are RuO2(101) // SA(012), or LTO(012), or SA(110). To determine the directions of planar deposition, we have to examine the atomic arrangements of the appropriate surfaces. Figure 4 illustrates the schematic plots of the atom arrangements and lattice relationships between RuO2 and the SA(012), LTO-

(012), and SA(110) surfaces. According to the argument of the minimization of the oxide sublattice structural mismatch, the possible NC-substrates alignment can be described as RuO2(101)[010] // SA(012)[100] and RuO2(101)[010] // LTO(012)[100]. The relationship of RuO2(101) and SA(110) oxide sublattices reveals the possibility of forming two equivalent structural domains with 180° rotation symmetry, and, in this way, it leads to doubly tilted nanostructure formation with the following orientations: RuO2(101)[1h01] // SA(110)[11h0] with RuO2[1h1h1] or RuO2[111h] // SA[22h1]. The alignments mentioned above produce directional mismatches on SA(012) of -5.67% ((4.49-4.76) Å/4.76 Å), +6.43% ((5.46-5.13) Å/5.13 Å) along RuO2[010] and RuO2[101h], respectively; mismatches on LTO(012) of -12.82% ((4.49-5.15) Å/5.15 Å), -0.18% ((5.46-5.47) Å/5.47 Å) along RuO2[010] and RuO2[101h], respectively; and mismatches on SA(110) along RuO2 [010] of +3.46% ((4.49-4.34) Å/4.34 Å); -4.88% ((5.46-5.74) Å/5.74 Å) along one side and +1.68% ((5.46-5.37) Å/5.37 Å) along the other side of the unit cell along RuO2[1h01] and +1.43% {(7.08-7.00) Å/7.00 Å} along RuO2 [1h1h1]. Here, we used the lattice parameters a ) b ) 5.15 Å and c ) 13.76 Å for LiTaO3 (JCPDS 29-836). Concluding this part of the study, we have to emphasize that the minimization of the oxide sublattice structural mismatch together with the c-directional growth mechanism are the two main driving forces for forming either the tilted or vertical RuO2 1D NCs. Moreover, we should notice that the conversion of film to nanocrystals is guided by internal and/or external factors. Here, the c-directional growth mechanism is referred to as the internal factor and the other parameters such as growth conditions, the choice of substrates, and substrate orientations are classified as the external factors. It is obvious that the external factors such as the substrate orientation combining with the temperature of

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Figure 5. Typical FESEM images (30° perspective view and crosssection view) of the mosaic RuO2 NCs grown on (a) SA(001) substrate and (b) their X-ray diffraction patterns.

the substrate can also influence the internal factor such as energetically favorable surface for the incoming atoms (cdirectional growth mechanism) and initiate the preferable plane orientation of RuO2 NCs, whereby the incoming atoms will stick onto the lower energy sites. The c-directional growth mechanism comes from the anisotropy of the crystal structure that results in different growth rate for the different directions of NCs. 3.3. RuO2 on SA(001). Figure 5a depicts the FESEM images of in-plane RuO2 nanoplates or nanowalls that are aligned in a 3-fold mosaic structure on the SA(001) substrate. The length of these plates is between 30 and 100 nm, the width is between 10 and 15 nm, and the height is approximately 300 nm. Similar results with columnar microstructure on SA(001) were achieved via MOCVD by G. Wang et al.17 and reactive RFMS by Q. Wang et al.19 using the following growth conditions: Ar:O2 flows ) 1:1, a working pressure of 2.0 Pa, RF generator power of 300 W, substrate temperature Ts of 150 °C. In both cases, the orientation of the structures is dictated by the epitaxial relation between the RuO2 lattice and sapphire single-crystal substrate. XRD patterns illustrated in Figure 5b reveal a preferential crystalline alignment of RuO2 grown on SA(001) along [100] (2θ ≈ 40.3° for (200) and 2θ ≈ 86.4° for (400) diffraction peaks). By examining the atomic arrangement of the relevant epitaxial surfaces illustrated in Figure 6, the mosaic structure formation can be understood. As mentioned in previous sections, the oxygen layer of substrate represents the template onto which the RuO2 is deposited. The initial Ru and O atoms that impinge on the SA(001) surface should prefer the atom sites at the locations as indicated in Figure 6. According to the oxygen atoms arrangement, the best structural match is to position the RuO2(100) nanostructure on SA(001) such that RuO2[010] is along SA[100], [010], and [110], thus forming the three equivalent structural domains intersecting at an angle of 120°. The results are directional mismatches of +7.99% along RuO2[001] (i.e., (3.11-2.88) Å/2.88 Å), and -5.67% along RuO2[010] (i.e., (4.49-4.76) Å/4.76 Å). The maximized mismatch of RuO2 along [001], together with the minimized mismatch along [010] and the c-directional growth mechanism, explains the discontinuous growth and formation of the nanostructured RuO2 on SA(001). 3.4. XPS Analysis. X-ray photoelectron spectroscopy is frequently used as a complementary technique for assigning oxidation states and the stoichiometry of the oxides. A slow scan XPS was performed on a Ru 3d doublet (5/2 and 3/2) and

Korotcov et al.

Figure 6. Schematic plots of the lattice relationships between RuO2 and SA(001) substrate: (a) RuO2(100) plane; (b) SA(001) plane; and (c) RuO2(100) on SA(001).

Figure 7. Typical peak-fitted slow scan XPS spectra of the (a) Ru 3d and (b) O 1s lines of the RuO2(001) NCs.

oxygen O 1s peaks in the binding energy ranges of 278-290 eV and 526-536 eV, respectively. Figure 7 shows the peakfitted slow scan XPS spectra of (a) Ru 3d and (b) O 1s coreelectron peaks obtained from the reactive RuO2(001) NCs deposited on SA(100). The C 1s peak, which appears at ∼284 eV, can overlap with Ru4+ 3d3/2 or it satellite peak. To get rid of the possible carbon contamination, we deployed the surface Ar ion etching for 10 s before the measurements. The peak intensity ratio of 3d5/2/3d3/2 after the cleaning procedure was determined and revealed an approximate value of 3:2, which corresponds closely to the theoretical value due to the spinorbit interactions of d-electrons.32 The Ru 3d and O 1s core level XPS spectra of reactive RFMS nanostructures were compared with those of a single crystal.18 All the samples investigated in the study exhibit similar XPS spectra. The Ru 3d signal of NCs shows two different binding states of ruthenium atoms and exhibit asymmetric line shapes. The peaks are identified as Ru 3d5/2 and 3d3/2 at ∼280.5 and ∼284.7 eV, respectively, which are close to those of single crystal at 281.1 and 285.1 eV, respectively, and are attributed to the 4+ oxidation state of Ru. Quantitative deconvolution of the XPS spectra revealed two additional features with broader character at ∼281.7 and ∼286.6 eV, respectively, for the Ru

Well-Aligned RuO2 Nanocrystals via Reactive Sputtering

Figure 8. Raman spectra of the RuO2: (a) RuO2 single crystal; (b) RuO2(001) on SA(100); (c) RuO2(001) on LNO(100); (d) RuO2(101) on SA(012); (e) RuO2(101) on LTO(012); (f) RuO2(101) on SA(110); and (g) RuO2(100) on SA(001).

3d5/2 and 3d3/2 regions and are higher than the major peaks by ∼1.2 and ∼1.9 eV, respectively. The origin of these satellite peaks, which were also observed on oxidized Ru metal surface, is still controversial in the literature and has been claimed to be related to excitation of the RuO2 plasmon,33 the surface species of Ru high bonding states,34 or the final-state screening effect.35 The XPS results of the O 1s peak show doublet signals with broader satellite at higher binding energy situated at ∼531.0 eV. The major peak position at ∼529.5 eV is similar to that of O 1s of the RuO2 single crystal and corresponds to oxygen in the metal-O-metal bond.36 We have tentatively assigned these broader extra features located at higher-binding-energy sites of Ru 3d and O 1s to the existence of an impurity of higher oxidation states in the RuO2 NCs. Similar spectral features were also observed for RuO2 films37,38 and IrO2 NCs and films29,39 and were attributed to the presence of higher oxidation states. Quantitative analysis of the peaks area allows us to determine the surface composition of RuO2 reactive RFMS nanostructures. The estimated ratio is about 29:71 and indicates the higher surface oxygen content in comparison with single crystals. The excess oxygen have also been observed in nanorods grown by MOCVD,18 columnar microstructural RuO2,19 and in RuO2 thin films.38 3.5. Raman Scattering Analysis. Vibrational properties of nanoscaled materials are interesting from practical and fundamental points of view because of their changes related to the grain size spatially confined effects. The Raman scattering results of nanocrystalline materials are very important in the understanding and clarification of their physical properties. Moreover, as shown in our previous work,40 RS can be used for the structural analysis of NCs. The first-order Raman spectra of the RuO2 SC and NCs are shown in Figure 8. Only three modes are Raman-active in the range of measurements (450-800 cm-1) with symmetries Eg, A1g, and B2g, where the first is a doublet and the latter two are singlets.41 The RS spectrum of RuO2 SC was fitted with mixed Lorentzian/Gaussian curves (Figure 8a, open circle line). Relative Raman intensities of the Eg, A1g, and B2g phonon modes for the various polarization configurations for RuO2 are similar

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for IrO2 and can be found elsewhere.40 The Raman scattering results of RuO2 NCs are consistent with the ascriptions given by selection rules for different planes. The selection rules maintain that Eg, A1g, and B2g modes are allowed for all polarization configurations for the (101) plane, and the B2g and Eg modes, respectively, are forbidden for all configurations from the scattering from (100) and (001) planes. It is worth noting that the intensities of Raman signals of all differently oriented RuO2 NCs used in this study follow the selection rules reasonably well and depend on the orientation of nanostructures. The appearance of a weak Raman signal of the normally forbidden mode might indicate observation of the scattering from the other planes of the NCs, as evidenced from the shape of the nanostructures’ tips and/or from the strain-induced Raman tensor, which may break down the selection rules. Thus, the RS results show the usefulness of Raman scattering in the determination of NCs orientations and are consistent with the XRD patterns of RuO2 samples. Also, the RS results describe the trend of peaks’ red-shifts and the asymmetric line shape broadening when the sizes become nanometric. The open-circle lines in Figure 8b-g are the RS fitted by the modified special correlation (MSC) model,40 which enables the separation of the Raman shift into two components. The first component corresponds to a nanometric size and leads to asymmetric broadening of the red-shifted Raman signals. The values of the red-shift extracted by MSC analysis are similar for all samples and are about 4 cm-1. The second component relates to residual stress effect red shift and exhibited different values for different substrates. For the samples grown on SA(100), SA(012), and SA(110) with minimal mismatch, this component is about 2 cm-1, and on LNO(100), LTO(012), and SA(001) is about 4 cm-1 because of a higher mismatch between the substrates and RuO2 NCs. We have assigned this shift to the residual stress effect induced by the tensile strain. These assignments are consistent with that of Rosenblum et al.,42 who reported the observation of blueshift on the three strongest lines induced by hydrostatic pressure in the RuO2 single crystal. Thus, we have shown that RS analyses can be utilized to extract the structural information of NCs such as the preferable NC orientation, the nanometric size effect, and the stress-induced effects. 4. Conclusion Well-aligned, densely packed RuO2 nanocrystals have been grown on sapphire SA(100), LNO(100), SA(012), LTO(012), SA(110), and SA(001) substrates by reactive RFMS using a Ru metal target. We have demonstrated that RFMS is a simple method that has several advantages, including better control of the growth conditions and a single deposition step to obtain the nanostructures. The results of morphological and structural investigations reveal that NCs grown on SA(100)/LNO(100) are vertically aligned RuO2(001) NCs, whereas the NCs on SA(012)/LTO(012) and SA(110) contained singly and doubly tilted aligned RuO2(101) NCs, respectively. The resulting NCs grown on SA(001) reveal a RuO2(100) nanomosaic structure. The strong substrate effect on the RuO2 NCs’ alignment has been explained as the combined effects of the minimization of the oxide sublattice structural mismatch and the c-directional growth mechanism. XPS analyses reveal a Ru:O ratio of about 29:71, which has been attributed to the higher oxidation state of Ru. RS analyses show the usefulness of Raman scattering in extracting the structural information of RuO2 NCs. The intensities of Raman signals follow the selection rules and depend on the

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orientation of RuO2 NCs. The red-shift and peak broadening of the RuO2 are attributed to both the size and residual stress effects. Acknowledgment. The authors acknowledge the support of the National Science Council of Taiwan under Contracts NSC94-2120-M-011-001 and NSC94-2112-M-019-001. References (1) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353-389. (2) Rao, C. N. R.; Govindaraj, A. Nanotubes and Nanowires; Royal Society of Chemistry: London, 2005. (3) Patzke, G. R.; Krumeich, F.; Nesper, R. Angew. Chem., Int. Ed. 2002, 41, 2446-2461. (4) Chen, R. S.; Huang, Y. S.; Chen, Y. L.; Chi, Y. Thin Solid Films 2002, 413, 85-91. (5) Dziedzic, A.; Golonka, L. J. Mater. Sci. 1988, 23, 3151-3155. (6) Khanna, P. K.; Bhatnagar, S. K.; Sisodia, M. L. J. Phys. D: Appl. Phys. 1988, 21, 1796-1801. (7) Ferro, S.; de Battisti, A. J. Phys. Chem. B 2002, 106, 2249-2254. (8) Kuhn, A. T.; Mortimer, C. J. J. Electrochem. Soc. 1973, 120, 231234. (9) Kim, T. Y.; Kim, D.; Chung, C. W. Jpn. J. Appl. Phys. 1997, 36 Part 1, 6494-6499. (10) Bai, G. R.; Tsu, I. F.; Wang, A.; Foster, C. M.; Murray, C. E.; Dravid, V. P. Appl. Phys. Lett. 1998, 72, 1572-1574. (11) Hu, C. C.; Chen, W. C.; Chang, K. H. J. Electrochem. Soc. 2004, 151, A281-A290. (12) Conway, B. E.; Birss, V.; Wojtowicz, J. J. Power Sources 1997, 66, 1-14. (13) Ke, Y. F.; Tsai, D. S.; Huang, Y. S. J. Mater. Chem. 2005, 15, 21222127. (14) Hsieh, C. S.; Tsai, D. S.; Chen, R. S.; Huang, Y. S. Appl. Phys. Lett. 2004, 85, 3860-3862. (15) Hsieh, C. S.; Wang, G.; Tsai, D. S.; Chen, R. S.; Huang, Y. S. Nanotechnology 2005, 16, 1885-1891. (16) Cheng, C. L.; Chen, Y. F.; Chen, R. S.; Huang, Y. S. Appl. Phys. Lett. 2005, 86, 103104. (17) Wang, G.; Hsieh, C. S.; Tsai, D. S.; Chen, R. S.; Huang, Y. S. J. Mater. Chem. 2004, 14, 3503-3508. (18) Chen, C. C.; Chen, R. S.; Tsai, T. Y.; Huang, Y. S.; Tsai, D. S.; Tiong, K. K. J. Phys.: Condens. Matter 2004, 16, 8475-8484. (19) Wang, Q.; Gilmer, D.; Fan, Y.; Franciosi, A.; F. Evans, D.; Gladfelter, W. L.; Zhang, X. F. J. Mater. Res. 1997, 12, 984-996. (20) Cheng, K. W.; Lin, Y. T.; Chen, C. Y.; Hsiung, C. P.; Gan, J. Y.; Yeh, J. W.; Hsieh, C. H.; Chou, L. J. Appl. Phys. Lett. 2006, 88, 043115.

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