ARTICLE pubs.acs.org/JPCC
Facile Synthesis of Single Crystalline Metallic RuO2 Nanowires and Electromigration-Induced Transport Properties Yumin Lee,† Byeong-Uk Ye,‡ Hak ki Yu,§ Jong-Lam Lee,§ Myung Hwa Kim,*,† and Jeong Min Baik*,‡ †
Department of Chemistry & Nano Science, Ewha Womans University, Seoul, 120-750, Korea School of Mechanical and Advanced Materials Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan, Korea § Department of Materials Science and Engineering, Division of Advanced Materials Science, Pohang University of Science and Technology (POSTECH), Pohang, Gyeongbuk 790-784, Republic of Korea ‡
bS Supporting Information ABSTRACT: We report a simple strategy to synthesize highly crystalline ruthenium dioxide (RuO2) nanowires by atmospheric pressure chemical vapor deposition in supercooled liquid nanodroplets without the use of catalyst particles. The nanowires are single crystals with no discernible amorphous layers or defects. The pretreatment of the substrate is found to be effective for the position controlled growth of the nanowires as small as 10 nm. The RuO2 nanowires have extremely low electrical resistivity of 62.5 ( 8.8 μΩ 3 cm and uniform electrical materials characteristics, regardless of size. By passing high current (>107 A/cm2) through the nanowires, very small nanogaps of less than 5 nm are produced, which can be explained by a heuristic model, in which electromigrative and surface diffusional effects are included.
’ INTRODUCTION One-dimensional (1-D) nanostructural materials such as nanowires, nanorods, nanotube, and nanobelts have been extensively studied as building blocks in optoelectronic devices for alternative renewable energy applications and as heterogeneous catalysts owing to the modification of their chemical, mechanical, electrical, and optical properties from those of the bulk.1-4 Among them, ruthenium dioxide (RuO2) is of great interest as a promising candidates as electrodes in electrochemical devices and capacitors and optoelectronic devices due to extremely low resistivity, excellent chemical and thermal stability, and a good diffusion barrier property.5-8 In particular, anhydrous single crystal RuO2 in the bulk has a metallic electronic conductivity of ∼2 104 S cm-1, unlike most metal oxides, while the hydrous form of RuO2 also shows high proton conductivity. Owing to the high catalytic activity, there also has been a consistent interest in 1-D RuO2 nanomaterials as excellent electrodes for applications in sensing and catalysis.6,7 Conventional RuO2 nanowires are usually prepared by thermal oxidation or chemical vapor deposition of appropriate Ru-based precursors and reactive sputtering using pure Ru metal targets.6,9-11 In this paper, we report a facile approach for synthesizing single crystalline RuO2 nanowires without catalyst, by atmospheric pressure chemical vapor deposition (APCVD), based on the thermal conversion from supercooled liquid nanodroplets.12 The attractive features of this method are simplicity, low cost, and high throughput as well as the high crystalline properties of the nanowires. The morphologies of the nanowires can be also r 2011 American Chemical Society
controlled by varying only the He/O2 ratio and temperature. Following their synthesis, we focus on the investigation of the physical properties, including the microstructural and electrical properties, of RuO2 nanowires. Failure phenomena at high current density are also investigated. Our results suggest the potential application of the nanowires as interconnects in nanosized semiconductor devices.
’ EXPERIMENTAL SECTION The growth of RuO2 nanowires was carried out on various substrates such as a single crystalline Si(001), a 200-nm silicacovered Si(001) wafer, plasma etched Si, and Al2O3 fiber. Specifically, RuO2 nanowire (NW) was synthesized by the vapor transport without catalyst in a three-zone horizontal quartz tube furnace, 2.5 cm in diameter and 122 cm long under atmospheric pressure. Ten milligrams of fine meshed RuO2 (99.9%, Aldrich) powder was first loaded at the center of a 6 cm long quartz boat without further purification. The substrate on a quartz boat was then introduced into the furnace at a point approximately ∼15 cm downstream of the RuO2 powder source. The quartz boat and its RuO2 charge were cleansed of impurities by first placing it at the center of the quartz tube furnace under He (99.999%) carrier gas flowing for ∼10 min at a gas flow rate of 300 sccm before heating. After that, the furnace temperature was Received: January 15, 2011 Revised: February 9, 2011 Published: February 28, 2011 4611
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Figure 2. High-magnification SEM (a, b) and TEM (c) images of the as-grown RuO2 nanowires. The nanowire is single crystalline, showing a [001] direction. The facets at the tip consist of (001), (111), (111), and {110} side planes. Figure 1. Low-magnification (a) and high-magnification (b) SEM images of the as-grown RuO2 nanowires on a Si substrate synthesized by simple vapor transport of RuO2 powders. X-ray diffraction spectrum (c) and Raman spectrum (d) of the grown nanowires.
rapidly increased to a temperature in the range of 950-1000 °C with flowing He (99.999%) carrier gas of 300 sccm at the rate of 100 °C/min. The nanowire growth proceeded as high-purity He (99.999%, 300 sccm) and O2 (99.9%, 10 sccm) flowed for 2 h. The temperature of the region in which the nanowires grew on the substrate was measured as ∼650 °C. The furnace was then allowed to cool to room temperature under flowing He. The product that was collected on various substrates was characterized by scanning electron microscopy (SEM) in conjunction with energy dispersive X-ray spectroscopy and X-ray diffraction (XRD). The crystal structures of RuO2 nanowires were also imaged by high-resolution transmission electron microscopy (HRTEM, FEI Titan TEM/STEM at 300 kV) at room temperature. Samples for TEM imaging were prepared by touching the nanowire-covered substrate to a TEM grid, thereby transferring some of nanowires to the grid. Raman scattering measurements of a single nanowire were carried out on wellspaced single RuO2 nanowires transferred to a Pyrex glass slide. Raman spectra were recorded in the backscattering configuration using a confocal microscope (Renishaw InVia System), with a 100 (0.9 NA) microscope objective, which both focused the laser beam (∼1 μm) and collected the backscattered light. Raman spectra were excited with 632.8 nm He-Ne laser light. Low powers were used to ensure that the nanowires did not decompose by localized laser heating. Optimal results were obtained with 0.57 mW laser power and 200-s integration times.
’ RESULTS AND DISCUSSION Figure 1a shows SEM images of the as-grown RuO2 nanowires on a Si substrate synthesized by a simple vapor transport of RuO2 powder and shows a high density of RuO2 nanowires more than 10 μm long. The nanowires grown were straight and did not show the catalyst particles at the end of the tip. Some droplets approximately 40 nm in size also appear in the vicinity of the nanowires in the image of Figure 1b, reminiscent of nanowires grown by the nanodroplets, very similar to previous reports on VO2 nanowires.11 It was also observed that the growth of
nanowires was sensitively changed by controlling the content of the oxygen gas at the growth temperature, as shown in Figure S1 (Supporting Information). Without a flow of oxygen gas, the nanowire growth was not observed. On the other hand, morphologies of the nanowires grown under high oxygen flow were rather close to those of microsized crystals. Thus, the growth of RuO2 nanowires with high aspect ratios was obtained within a limited range of oxygen flow rate. Using the same growth condition, Figure S2 (Supporting Information) shows the successful applications of the RuO2 nanowire growth on the O2plasma etched Si substrate and Al2O3 fiber in the manner of hierarchical nanostructures. Interestingly, the dimensions of the nanowires formed on the Si pillar made by the O2-plasma etching process were remarkably small sizes of less than 10 nm, which is generally unusual for the vapor phase growth process. In addition, the nanowires can be grown in a controlled position by using a surface functionalization technique, as shown in the inset of Figure 1a. These results are currently being investigated and will be the subject of a separate publication. The corresponding X-ray diffraction spectrum and Raman spectrum, shown in parts c and d of Figure 1, confirm that the phase of the nanowires are tetragonal in structure, growing in the [001] crystallographic direction along the c axis of the nanowires. The detailed crystal structure of the grown RuO2 nanowires was investigated by high-magnification SEM and TEM images of nanowires. A low-magnification TEM image of a representative RuO2 nanowire and selected area diffraction (SAED) pattern taken from the same sample are shown in Figure S3 (Supporting Information). A SAED pattern taken along the [110] zone axis indicates that a RuO2 nanowire is a single crystalline tetragonal structure with a [001] growth direction. It is clearly seen that nanowires are terminated by a prismatic form with two shape categories characterized by polyhedral or rectangular cross sections and each with well-defined facets, as shown in Figure 2a,b. These growth characteristics and crystal structures of RuO2 nanowire are similar to those of previous results.8,15 Figure 2c shows that a HRTEM image of a tip of RuO2 nanowire clearly reveals lattice fringes, indicating that the RuO2 nanowire is single crystalline. The lattice spacing of adjacent planes is about 0.312 nm, corresponding to that between the (001) planes of tetragonal RuO2, showing that nanowires grow along the [001] direction parallel to a {110} family of planes, in exact agreement with the fast Fourier transform (FFT) of the lattice-resolved 4612
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image (Figure 2c). It is also clearly seen that nanowires terminated by a prismatic form, have a (001) growth front plane, {110} side planes, and (111) and (111) intersection planes between (001) and (111) plane and (001) plane. It is well-known that the grow rate is strongly dependent on the surface energies of the crystal planes. Thus, a prismatic shape of nanowires is believed to be attributed to the relative growth rates of (001) > (111) and (111) > {110} planes. Although the growth mechanism of RuO2 nanowires resembles sublimation followed by recrystallization, a process referred to as vapor-solid (VS) growth due to the absence of the catalytic particle at the tip of a nanowire, we see the nanodroplets on the side well of nanowires as mentioned before. The presence of these nanodroplets might indicate the growth mechanism would be similar to the supercooled nanodroplets growth process for VO2 nanowire growth. Nanometer-sized liquid droplets formed at temperatures below the bulk melting point become supercooled as they grow through Ostwald ripening or coalescence and can be exploited to grow nanowires without any catalyst. In our growth mode, on the other hand, it should be noted that vaporizing active precursors during CVD growth might be gaseous RuO4 species by reaction with oxygen carrier gas, which is a highly volatile and very low melting point intermediate. Because of the highly oxidizing condition at high temperature, the formation of gaseous RuO4 species can be much more favorable and then may have an important role to nucleate RuO2 single crystals on a substrate. However, it is necessary to explore this mechanism in greater detail in order to fully demonstrate the preferential and unidirectional crystal growth of RuO2. The electrical properties of these RuO2 nanowires are characterized by transferring the nanowires onto a thermally grown silicon dioxide layer on a p-type silicon wafer substrate. Source and drain electrodes were fabricated using conventional photolithography with the Ni/Au (20/200 nm) electrodes deposited using an e-beam evaporator without further annealing. The contacts are found to be Ohmic at low applied voltage of -0.1 to 0.1 V because the work function of Ni (5.04 eV) is comparable to the electron affinity of RuO2 (4.87 eV). By examining several I-V curves, the total resistances of the RuO2 nanowires were found to be in the range of 40-125 Ω at room temperature, varying with the cross-sectional area and distance between the electrodes. The total resistance extracted from the I-V measurements for a nanowire with given length (LNW) and width (WNW) (cross-sectional area, ANW = WNW2) is usually given by Rt ¼ 2Rc þ RNW
ð1Þ
Figure S4 (Supporting Information) shows a plot of the measured resistance (normalized by length) versus square of crosssectional area. We see that the observed dependence of R/L decreases approximately as A-0.97. The reciprocal dependence suggests that the resistance is not greatly affected by the existence of a near surface depletion region due to the presence of surface charges on the nanowires, as previously reported in p-doped Si nanowires.13,14 Previous results showed that the charge balance between positive surface charges and negative space charges in the depleted region decreased with the ratio of the surface-charge density to the carrier concentration in conducting area, which implies that the high current density of RuO2 nanowires does not influence surface depletion region greatly. To compute the nanowire resistance (RNW), we first compute the resistivity
Figure 3. The representative I-V curve of RuO2 nanodevice carried out at room temperature. (Inset) SEM image of single nanowire spanning gold contacts separated by about 3 μm. (b) Total resistance as a function of length/area. The resistivity of 14 nanowires was calculated to be 62.5 ( 8.8 μΩ 3 cm.
FNW from the slope (Figure 3b) of the measured total resistance as a function of length/area. This produces a value of 62.5 ( 8.8 μΩ 3 cm, which is lower than the value (∼200 μΩ 3 cm) reported for RuO2 nanowires synthesized by reactive sputtering systems6 and much more comparable to the reported resistivity of 40 μΩ 3 cm for a RuO2 thin film.15 The results of durability and reliability tests of the RuO2 nanowire were obtained by performing I-V measurements under a higher voltage at room temperature, as shown in Figure 4a. At low values of the applied voltage, the current increased linearly with increasing voltage with a slope corresponding to a conductance value of 8.93 10-3 Ω-1. That is, the nanowire behaved as an Ohmic resistor (110 Ω) with conductance corresponding approximately to that of several metals (Ag, Au, etc). The conductance value decreased slowly to 7.51 10-3 Ω-1 as the voltage increased. The small decrease in conductance with increasing voltage observed in this region we believe is due to a small degree of resistive heating of the nanowire. Above approximately 0.61 V (for the particular nanowire illustrated), the conductance drops suddenly and irreversibly to 0, indicating the formation of a nanogap. The inset of Figure 4a shows a SEM image of a nanowire connecting gold contacts separated by ∼3 μm after the gap-forming voltage was reached. The ensuing ∼5 nm (may be less) gap, generally formed in the middle of the nanowire devices, similar what was observed for metallic nanowires,6,16 is shown at higher magnification in the inset to Figure 4a. Local melting might be reasonably attributed to be the cause of failure, because it can be considered that the middle of the nanowires has the highest temperature under resistive selfheating, as reported in other metallic nanowires.6,16-18 However, we have two points to be considered. In those papers, the current dropped suddenly to 0 without any significant change in the slope of the I-V curve just before the gap. It is also worth noting the formation of very small gap (107 A/cm2) through the nanowires, very small nanogaps of less than 5 nm are produced, explained by a heuristic model, in which electromigrative and diffusial effects are included. ’ ASSOCIATED CONTENT
bS
Supporting Information. Additional information as discussed in the text. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (J.M.B.),
[email protected] (M.H.K.).
’ ACKNOWLEDGMENT This research was supported by Basic Science Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (No. 2010-0013062 and 2010-0022028) and by National Nuclear R&D Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education, Science and 4614
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Technology (No. 2010-0018642). We are thankful to Prof. Martin Moskovits (UCSB) for fruitful discussions. Also, M.H.K. gratefully acknowledges Prof. Joon Woo Park for providing experimental equipment.
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