Use of the Carbothermal Route to Prepare Anisotropic Single-Crystal

Use of the Carbothermal Route to Prepare Anisotropic Single-Crystal Platinum Nanostructures with Low Resistivity Dai-Liang Ma and Hsuen-Li Chen* Department of Materials Science and Engineering, National Taiwan UniVersity, 1, SEC. 4, RooseVelt Road, Taipei, Taiwan

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 5 2030–2035

ReceiVed August 30, 2008; ReVised Manuscript ReceiVed February 21, 2009

ABSTRACT: We have used a carbothermal process in the absence of a catalyst or template to prepare two-dimensional (2D) platinum (Pt) nanoplatelets and one-dimensional (1D) Pt nanobelts on sapphire surfaces. This paper describes the first examples of the growth of Pt nanobelts and mesobelts. It appears that the presence of adequate quantities of diamond and Y2O3 powder at a molar ratio was a critical factor controlling the Pt gas species to form nanobelts at a relative low level of supersaturation. When the growth time was 15 h, we obtained ultralong (0.5 mm) mesobelts and microsheets. Electrical measurements indicated that the single-crystal Pt nanobelt had an extremely low resistivity of 16.8 µΩ cm and a failure current density of greater than 1 × 107 A cm-2. These unique Pt nanobelts are potentially attractive nanoscale building blocks for use as interconnects in nanoelectronic devices. Research into noble-metal nanostructures is stimulated by the fascinating size- and shape-dependent properties of these nanomaterials. Because of their unique and tunable properties, they hold promise for various applications in optics, electronics, information storage, biological labeling, and imaging.1 Platinum (Pt) is one of the most important noble metals,2-4 especially for its application in the fields of molecular scale electronic devices,5 biosensors,6 and catalysts.7,8 For example, because platinum is inert, highly thermally stable in air, and does not cause contamination of the integrated circuit, it is the most common choice for use in integrated circuit repair and modification and bottom electrode.9 Also, because of its nice performance toward the detection of hydrogen peroxide, a typical enzymatic product, platinum electrodes, and platinum nanostructure modified electrodes have been widely used for fabrication of biosensors.10 Because Pt has a face-centered cubic (fcc) structure, there is no crystallographic driving force for anisotropic growth. As a result, Pt atoms generally assemble to form faceted spheres (e.g., singlecrystal cuboctahedrons)11 and, in particular, few reports exist describing the growth of two-dimensional (2D) and one-dimensional (1D) Pt nanostructures. In spite of that, it is common for other fcc metals (like Ag, Au) to form 2D nanostructures, such as triangular and hexagonal platelets, when transition-metal colloids are prepared in solution through the reduction of metal salts.12-14 Most reports on the preparation of Pt nanostructures have been limited to the synthesis of nanoparticles (NPs) or polycrystalline nanorods using either solution-phase techniques15 or template-directed synthesis16 for examples of synthesized 2D and 1D Pt nanostructures, respectively. Tan et al. prepared 2D Pt NPs by the reduction of their salts with a weak reductant - potassium bitartrate. In their study, Pt NPs have two dominant shape distributions, triangular and square.17 For an instance of synthesized 1D Pt nanostructures, Zhao et al. synthesized polycrystalline Pt nanowire array electrode by dc electrodeposition of Pt into the pores of an anodic aluminum oxide (AAO) template on Ti/Si substrate.18 Until now, singlecrystalline Pt nanowires have been synthesized successfully only when using so-called polyol process methods, which have been exploited by many research groups to produce NPs from various materials.19,20 The key to the preparation of a wirelike morphology is the use of a polymeric capping reagent (PVP) and the introduction of seeds; for example, Chen et al. reported the solution-phase synthesis of single-crystal Pt nanowires (ca. 7 nm in diameter, 200 nm long) on polymeric or ceramic sphere substrates.21,22 The disadvantage of the polyol process is that it is often difficult to * Corresponding author. E-mail: [email protected].

completely remove the agglomerates via sonication to obtain a phase of pure Pt nanowires; thus, such nanowires may not be suitable for use in developing devices. In this study, we have been exploring simple ways to prepare 2D Pt platelets (triangular and hexagonal platelets) and 1D Pt nanostructures (nanobelts, mesobelts) on sapphire by employing carbonthermal procedures wherein diamond powder is one of the reactants. For the growth of 1D nanostructures, in particular, we found that the presence of adequate quantities of diamond and Y2O3 powder at a molar ratio was a critical factor controlling the Pt gas species to form nanobelts at a relatively low level of supersaturation. Because we used no metal catalysts and no metal droplets appeared at the tip of each nanobelt after its growth, it is likely that the growth of Pt nanobelts occurs by a vapor-solid (VS) mechanism; nevertheless, the growth mechanism remains unclear. We found that the Pt nanobelts having high crystalline quality exhibited excellent electronic transport. Experimental Section. Growth of the Pt nanostructures was conducted in a high-temperature furnace (1250-1500 °C) in air at 1 atm. The source materials, Pt sheets (0.5 g), diamond powder (4.8 g), and yttrium oxide (Y2O3) powder (2.7 g), were placed in a high-purity alumina crucible and positioned at the center of the furnace. The substrate, c-plane (0001) sapphire, was placed facedown on the alumina crucible. Typically, the source temperature was ramped at 10 °C min-1 to 1500 °C and then maintained at that temperature for a period of time ranging from 30 min to 15 h. The morphologies of the Pt nanostructures were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray energy-dispersive spectrometry (EDX). The electron transport properties of the Pt nanostructures were determined using a multiprobe nanoelectronic measurement (MPNEM) system. The various morphologies obtained under the growth conditions are summarized in Figure 1.

Results and Discussion. Morphology Characterization of Pt Nanostructures. In our present experiments, the morphology of Pt nanostructures was examined by SEM and TEM. The SEM images (images A and B in Figure 2) demonstrate that 2D Pt platelets and 1D Pt nanostructures could be easily produced on a large scale using the present method. As displayed in Figure 2A, we formed two dominant shape distributions, hexagonal and triangular 2D Pt platelets (40% in hexagonal platelets, 20% in triangular structures, 10% in 1D nanostructures, and 30% in irregular structures) after a source material consisting of Pt sheets and diamond powder was heated in the absence of Y2O3 under air (1 atm) at 1500 °C for 30 min. Moreover, the hexagonal platelets are the most dominant shape distribution. In our study, the triangular

10.1021/cg800964s CCC: $40.75  2009 American Chemical Society Published on Web 03/31/2009

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Crystal Growth & Design, Vol. 9, No. 5, 2009 2031 with the results reported by Lofton et al. where the diffraction patterns of the triangular and hexagonal plates of other fcc metals (gold and silver) indicated that the main faces were {111}.24 Furthermore, there is ample evidence that depositing platinum at a sufficiently high temperature in an ultra high vacuum system to form hexagonal and triangular shapes.25-27 Among these previous reports, such as Michael Bott’s group, have been proposed that triangular and hexagonal 2D Pt plates could be formed by layerby-layer growth (2D growth).

Figure 1. Schematic illustration of the different morphologies at different growth conditions. 2D platelets of Pt grown on sapphire are formed after heating the source material (Pt sheets and diamond powder). A few Pt platelets and nanobelts grown either vertically aligned or lying parallel on sapphire are formed after heating the source material (Pt sheets, diamond powder, Y2O3 powder) under air at 1500 °C for 30 min. With a growth time of up to 15 h, ultralong Pt mesobelts and elongated Pt platelets grow either vertically aligned or lying parallel on sapphire.

Figure 2. (A) SEM image of Pt platelets, hexagonal and triangular in shape, formed after heating the source materials (Pt sheets, diamond powder) under air at 1500 °C for 30 min. (B) SEM image showing anisotropic growth of 1D Pt nanobelt either vertically aligned or lying parallel on sapphire after heating the source material (Pt sheets, diamond powder, Y2O3 powder) under air at 1500 °C for 30 min. (C) SEM image shown as 70° tilted view taken from (B). (D) SEM image of an individual Pt nanobelt with very smooth surface and good crystalline quality. The width to thickness ratio is about 8.5. (E, F) SEM images of mesobelts lying parallel on sapphire and 2D particles growing at an angle out of sapphire and after heating the source material (Pt sheets, diamond powder, Y2O3 powder) under air at 1500 °C for 15 h, respectively.

and hexagonal platelets that formed on sapphire had widths of a few micrometers, thicknesses that ranged from tens to hundreds of nanometers, and width-to-thickness ratios of ca. 6-10. Because Pt has an fcc crystal structure, the surface energy of its various crystallographic facets usually increases in the order γ{111} < γ{100} < γ{110}. The shape of a single-crystalline platelet is defined by its Wulff construction.23 For our triangular and hexagonal 2D platelets, we believe that the {111} surfaces were maximized because of their weakest surface energies, consistent

Next, we used the present method approach to prepare singlecrystal 1D Pt nanobelts and mesobelts for the first time. The source materialssPt sheets, diamond, and Y2O3 powdersswere heated under air at 1500 °C for 30 min in the presence of adequate quantities of diamond and Y2O3 powders at a molar ratio of 33:1. Apart from a few 2D particles, we obtained Pt nanobelts exhibiting very smooth surfaces and good crystallinity. Figure 2B displays an SEM image of the as-grown Pt nanobelts that were aligned either vertical or parallel to the sapphire surface. Figure 2C presents an image of the vertically aligned (70° titled) Pt nanobelts from Figure 2B. The Pt nanobelts had widths ranging from 20 to 300 nm, lengths extending up to tens of micrometers, and width-to-thickness ratios of 3-9. Figure 2D displays an individual 170-nm-wide, 20-nm-thick Pt nanobelt having an almost rectangular cross-section. Upon the growth time being extending from 30 min to 15 h, while the temperature was maintained at 1500 °C, a remarkable change occurred: the Pt nanobelts lying on the sapphire grew into ultralong mesobelts (lengths: ca. 0.2-0.5 mm; Figure 2E) and elongated 2D Pt plates having widths and thicknesses on the scale of micrometers (width-to-thickness ratio ≈ 6) grew at an angle from the sapphire surface (Figure 2F). Figure 3A presents a low-magnitude TEM image of several straight Pt nanobelts exhibiting good crystallinity. Each Pt nanobelt had a uniform width of ca. 50 nm along its entire length. EDX spectroscopy of one such nanobelt (Figure 3B) revealed that it consisted of Pt only, i.e., without any traces of yttrium oxide or carbon contamination; the peak for copper arose from the TEM grid. We obtained TEM images to characterize the direction of growth and the crystal structure of these Pt nanobelts. The direction of growth of the Pt nanobelt was in agreement with two electron diffraction (ED) patterns (Figure 3C) taken from the root and stem portions of an individual Pt nanobelt. Both ED patterns were indexed to the {111}, {220}, and {311} facets and exhibited rectangular symmetry corresponding to the 〈112〉 zone axis. All of the diffraction spots could also be indexed to single-crystalline Pt possessing fcc phase. The ED studies indicated the same direction of growth along the [111] axis as that previously reported for single-crystalline Pt wires prepared via solutionphase synthesis.22 The inset at the bottom of Figure 3C displays Pt nanobelts (confirmed by EDX spectroscopy) nucleating from the corners of a rectangular Pt particle; this phenomenon is similar to that occurring during the growth of Pb nanobelts when using the solution-liquid-solid (SLS) method.28,29 Figure 3D presents a TEM image of the tip of the 40-nm-wide Pt nanobelt marked by the circle in Figure 3C. The tip of this Pt nanobelt was relatively rough when compared with the enclosed smooth side surfaces (as indicated by the arrow). This morphology implies that the growth of the Pt nanobelts occurred through adhesive growth30 in the longitudinal direction; it also suggests that the growth rate in the longitudinal direction may have been higher than that in the lateral directions. Nevertheless, our experimental observations revealed two other growth directions: along the [100] and [110] facets. Most of Pt nanobelts grew in the [111] and [110] directions without any Pt droplets attached on either end. In contrast, Pt nanowires

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Figure 3. (A) TEM image of several straight Pt nanobelts with good crystalline quality. Each Pt nanobelt has uniform width (below 50 nm) along its entire length. (B) EDX spectrum of Pt nanobelt. EDX spectrum reveals that the nanobelt consists of only Pt without any trace of yttrium oxides or carbon contamination. The copper composition was contributed from the TEM grid. (C) Electron diffraction (ED) patterns taken from the root and stem portions of an individual Pt nanobelt (indicated by arrow), respectively. The inset at the bottom shows that a nanobelt nucleating from the corner of a rectangular Pt particle (indicated by arrow) confirmed by EDX. (D) TEM image of extremely rough tip of a Pt nanobelt indicated by circle in (C) and the arrow indicated the rough tip portion of Pt nanobelt.

synthesized using solution-phase methods grow only in the [111] direction. Figure 4A displays the TEM image of a Pt nanobelt possessing a good crystalline structure and a very smooth surface in the [110] growth direction. The diffraction pattern taken perpendicular to the top face of the Pt nanobelt (Figure 4B) exhibited 6-fold symmetry along with a [111] zone axis. Thus, this Pt nanobelt is single-crystalline in nature and possesses a {111} top surface. Wang and Sun et al. observed the appearance of 1/3{422} forbidden spots within the typical selected area electron diffraction (SAED) patterns of fcc metallic nanowires (e.g., silver and lead) synthesized in solution.31,32 Such forbidden spots can appear in a single-crystalline fcc metal when two twin planes are aligned parallel to one another. Note, however, that no such 1/3{422} forbidden spots appear in the SAED pattern of the Pt nanobelt. Thus, the Pt nanobelts grown through thermal evaporation were of single-crystal quality and twin-free. Figure 4C displays the TEM image of a 2D Pt platelet obtained after a long growing time (15 h). The diffraction pattern of this 2D Pt platelet (Figure 4D) indicates that the main face is {111}. Interestingly, we observed faints spots within the {220} spots, marked by the circle in Figure 4D, which were indexed as 1/3{422} spots. Lofton et al. suggested that these spots provided evidence that twin planes were present and directed the growth of gold and silver platelets.24 Germain et al. reported that (111) stacking faults lying parallel to the (111) surface and extending across the entire silver nanodisk were responsible for creating 1/3{422} reflections in the [111] pattern.33 Thus, we believe that such twins may also exist within the 2D Pt platelets.

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Figure 4. (A) TEM image of the Pt nanobelt with good crystalline structure and smooth surface in [110] growth direction. (B) The diffraction pattern exhibiting 6-fold symmetry recorded along the [111] zone axis without 1/3{422} forbidden spots within {220}. (C) TEM image of long growing times, up to 15 h, 2D Pt platelet. (D) The diffraction pattern of 2D Pt platelet indicated that main face is {111} face 1/3{422} forbidden spots within {220} as indicated by a circle.

Growth Mechanism of the Generalized 1D and 2D Pt Nanostructures. Among several strategies developed for the vapor-phase synthesis of 2D and 1D nanostructures, in principle, it is possible to process any solid material into 2D and 1D nanostructures by controlling the supersaturation at relatively low levels, respectively.34-36 For example, Sears et al. in a series of whiskers grown from vapor phase studies on various solid materials such as Ag and Zn found that the experimentally estimated supersaturation for the growth of 1D whisker is less than or approximately equal to the calculated supersaturation for the 2D nucleation. Above these experimental supersaturations the growth of massive crystals, which presumably involves surface nucleation, occurs experimentally and 1D whiskers do not grow.36 The carbothermal route is noteworthy because it provides a general method for preparing crystalline 1D nanowires of many of these materials, which include oxides, nitrides, carbides, and metals such as Zn, W.37 The method itself is quite simple and involves heating a mixture of an oxide with carbon in an appropriate atmosphere.

The well-known carbothermal process reported by Rao et al.,37 the first step of which is the generation of reactive species in the vapor phase. This step can be achieved either by heating to high temperature or by laser ablation. Carbon plays the role of facilitating the formation of the reactive species and the exothermic nature of the oxidation of carbon also helps in attaining high local temperatures. Carbon (activated carbon or carbon nanotubes) in mixture with an oxide produces suboxidic vapor species which react with C, O2, N2, or NH3 to produce the desired nanostructures. The mechanism of the formation of nanowires under the carbothermal route involves vapor-solid (VS) or vapor-liquid-solid (VLS) growth processes. In the VS process, evaporation, chemical reduction or reaction generates a vapor species, which is transported and condensed onto

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Figure 5. (A) SEM image of an individual Pt nanobelt with about 78 nm width lying at a angle on a sapphire substrate. (B) SEM image with high magnification of the tip of Pt nanobelt indicated by arrow in A. The width-to-thickness ratio is about 3. (C) SEM image of a manipulator making a two-terminal connection to a single Pt nanobelt to measure its electrical transport properties. (D) The intrinsic nanobelt resistvity, Fnw, can be extracted from a linear fit of R - Rc vs A/L. The slop of the fit (solid line) yields Fnw) 16.8 µΩ cm (Rc ) 552.8 Ω can be extracted from the values of R against values of nanobelt length).

a substrate. In the VLS process, the growth is promoted by a liquid-solid interface, generally evidenced by the presence of droplets at the tips of the nanowires. Our process is basically based on the typical carbothermal process; because of the limitations of the experimental furnace, which is a semiopen system and high-temperature reacted environment, we could control only very few parameters such as temperature and the molar ratio of source materials. Keeping other conditions constant (i.e., 1500 °C, 30 min), it was found to be favorable for the formation of 1D Pt nanobelts if the presence of yttrium oxide (Y2O3) powder along with diamond powder and Pt sheets (Figure 2B). In contrast, 2D Pt platelets were produced only when using diamond powders and Pt sheets to be source materials (Figure 2A). Thus, based on the evidence of previous literature reviewed, we believed that 1D and 2D Pt nanostructures could be prepared by controlling the supersaturation of the Pt gas species at relatively low levels, respectively. We have examined the role played Y2O3 powder by EDX, Auger spectroscopic (AES), and X-ray photoelectron spectroscopy (XPS) analyses (data not shown here). On the basis of these analyzed results, Y2O3 does not seem to enhance the yield of Pt gas species, nor did we find any traces of Y within our Pt nanostructures or on the surface of the sapphire substrates after the growth process. Powell et al. found that mixtures of yttrium oxide (Y2O3; melting point: 2690 °C) and Pt powders undergo chemical interactions that inhibit the sintering of the mixture, thereby stabilizing the open pore network of Pt at temperatures up to 1510 °C.38 In addition, it is interesting to note that a few 1D Pt nanostructures could also be produced in the absence of Y2O3 as shown in Figure 2A. Therefore, we suspect that Y2O3 did not participate in any vapor phase reactions during the growth of our Pt nanostructures on sapphire; it served its main function as a textural promoter stabilizing the source material and inhibiting the high evaporation rate of Pt caused by the high local temperature arising from the exothermal reaction forming CO2. Moreover, for the growth of 1D Pt nanobelts, the presence of Y2O3 may be one of possible parameters to control the

relatively low level of supersaturation of the Pt gas species. As regarding to the role of diamond, except the above-mentioned in carbonthermal process, we conducted experiments using three different molar ratios of diamond (0, 0.2, and 0.4 moles) along with a fixed amount of Y2O3 and Pt sheets under air at 1500 °C for 30 min. The surface of the sapphire was clean of source materials in the absence of diamond powder. Increasing the molar ratio of diamond powders to 0.2 moles, 1D and 2D Pt nanostructures coexisted grown on sapphire. Dominated 1D Pt nanostructures were obtained when diamond powder was 0.4 moles. This observation implies that diamond could also be a factor for controlling the concentration of the Pt gas species. Thus, the growth mechanism of 1D Pt nanostructures could be understood on the basis of a vapor-solid mechanism because extensive microscopic investigations reveal no catalyst/liquid droplets at the ends of the nanobelts. Also, based on the experiments that were conducted in steady air environments, the relevant reactions can be written as follows

Pt(s) + O2(g) f PtO2(v)

(1)

CO2(g) + C(s) f CO(g)

(2)

PtO2(v) f Pt(s) + O2(g)

(3)

PtO2(v) + 2CO(g) f Pt(s) + 2CO2(g)

(4)

It has to be noted that our experiments were carried out at temperatures ranging from 1250-1500 °C under steady air atmosphere. From a survey of the literature regarding the phenomena of Pt heated in air,39-41 it appears that molecular PtO2 plays the main role in the chemical-vapor-transport (CVT) reaction.42 Thus, the first step involves the formation of PtO2 in vapor phase. We infer that diamond and air are reacted to form CO and CO2. The second step involves that CO2 further reacts with carbon to form CO. Therefore, we believe that the exothermic nature of the oxidation of carbon not only helps to increase the temperature in the bottom of crucible higher than

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the sapphire placed onto the lid of the crucible but also increases the PtO2 vapor pressure over the equilibrium vapor pressure. The third step is that Pt gets precipitates from the supersaturated PtO2 vapor phase followed by the formation of Pt nuclei on sapphire. And, the fourth step is that PtO2 and CO are reacted to form Pt, which also may possibly act like the other Pt source for the formation of Pt nuclei. Furthermore, the oxidation of CO on Pt metal surfaces is one of the most widely studied subjects in surface chemistry; it is a model system for describing heterogeneous catalysis,43,44 which suggest that CO oxidation results in the production of highly excited CO2. Therefore, we rationally infer that once the Pt nuclei are formed, which could not be oxidized to form volatized PtO2 again in the containing CO atmosphere and further obtain newly Pt atoms for crystal growth. In this work, because of the limitations of the experimental furnace and reaction at such high temperatures (1500 °C), it is not easy to determine exactly which reactions occur. Further studies in this direction would be of interest. Nevertheless, the mode of growth of the Pt nanobelts and 2D platelets either vertically aligned or epitaxially on sapphire remains unclear. Transport Properties of Pt Nanobelts. With applicability of Pt nanobelts in mind, we investigated the room-temperature electron transport properties associated with a set of six Pt nanobelts having widths ranging from 72 to 78 nm. In situ I-V analysis and manipulation of Pt nanobelts were performed in a multiprobe nanoelectronics measurement (MPNEM) system within a scanning electron microscope. We applied the two-terminal method for electrical transport measurements at high vacuum to minimize influence from the environment.

Figure 5A displays the SEM image of an individual Pt nanobelt having a width of ca. 78 nm and lying at an angle on the sapphire substrate; Figure 5B presents a high-magnitude image of the tip. Although it was very difficult to measure the thickness of every Pt nanobelt using SEM, especially those lying parallel to the sapphire substrate, we estimated that, in general, the width-to-thickness ratio was ca. 3. Thus, we considered the rectangular cross-sectional area of a Pt nanobelt to be ca. 1/3 D2, where D is the measured width of the nanobelt. Figure 5C displays the SEM image of a manipulator forming a twoterminal connection to a single Pt nanobelt, thereby allowing us to measure its electrical transport properties. The two nanotips were used to apply the voltage and measure the current through the nanobelt. Assuming for simplicity that the contact resistance (Rc) was the same for every measurement, we obtain the equation R - Rc ) FnwL/A, where R is measured by the MPNEM system, Fnw is the intrinsic nanobelt resistivity, and L is the effective Pt nanobelt length (distance between the two nanotips). To determine the contact resistance, we plotted the resistance (R) as a function of the nanobelt length (L); we extracted a value of Rc of 552.8 Ω from the best-fit intercept value. We determined the intrinsic nanobelt resistivity, Fnw, to be 16.8 µΩ cm from the slope of a linear fit of the plot of R Rc versus A/L (Figure 5D). The resistance of this Pt nanobelt is slightly higher than that of bulk Pt (10.58 µΩ cm), but it is well below the values (33 ( 5 µΩ cm) reported by other groups probing the intrinsic transport properties of 70-nm-diameter polycrystalline Pt nanowires.44,45 Therefore, from the value of the intrinsic nanobelt resistivity, we infer that electron transport within the single-crystal Pt nanobelts was less affected by either surface or defect scattering mechanisms. We analyzed a total of three Pt nanobelts for breakdown tests using two-terminal measurements. Typically, the Pt nanobelts broke down at current levels of 0.2-0.7 mA when the voltage was greater than 0.5 V; the maximum current densities were greater than 1 × 107 A

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cm-2. This value is much higher than the value of 65 kA cm-2 reported by De Mazi et al., and is comparable to that reported for silver nanobeams (1.8 × 108 A cm-2).46 Conclusion. In terms of thermodynamics, it is a grand challenge to grow triangular and hexagonal 2D platelets and 1D nanostructures from fcc metals via thermal processing. In this study, we used carbon-assisted thermal evaporation in the absence of a catalyst to control the assembly of Pt platelets and nanobelts on sapphire surfaces. An ED study revealed that twin planes existed within the Pt microsheets obtained after long reaction times (up to 15 h). Also, we used the present approach in the absence of a template to prepare, for the first time, 1D Pt nanobelts and mesobelts. It appears that the presence of adequate quantities of diamond and Y2O3 powders at a molar ratio was a critical factor controlling the Pt gas species to form nanobelts at a relatively low level of supersaturation. We obtained ultralong (0.5 mm) mesobelts after increasing the growth time to 15 h. Electrical measurements indicated that the highly crystalline Pt nanobelts had a low resistivity of 16.8 µΩ cm and a failure current density of greater than 1 × 107 A cm-2. These unique Pt nanobelts are attractive nanoscale building blocks for potential use as interconnects in nanoelectronic devices.

Acknowledgment. We thank the Center of Nano-Science and Technology (CNST) in the University System of Taiwan (UST) for technical assistance with multiprobe nanoelectronics measurement (MPNEM) system.

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