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43902-G10
Novel, Ultra-Low Resistance Materials Prepared by Chemical Separation of Metallic Single Walled Carbon Nanotubes

Michael S. Strano, University of Illinois (Urbana-Champaign)

1. Objectives

The goal of this project is to isolate metallic single-walled carbon nanotubes (SWNT) for use as bulk materials with electrical resistance many times lower than Cu. Moreover, metallic SWNT have been shown to be near ballistic conductors at room temperature, having a ballistic limit up to 10µm. Considering that the ballistic limit of Cu is about 40nm, SWNT materials can be used in ultra-high efficiency power transmission. For this application, the bulk separation of metallic SWNT from SWNT mixtures is necessary, and is therefore a central focus of this proposal.

We outlined three critical objectives that capitalized on new chemical methods developed primarily in our laboratory at UIUC: Develop and demonstrate methods for the selective reaction of metallic SWNT using a chemical handle for separation (objective 1), use 4-hydroxy phenyl functional group to separate metallic SWNT using electrophoresis (objective 2), and deposition and alignment of isolated metallic SWNT across an electrode gap for electrical transport measurements. (objective 3). In the followings, we summarized the results obtained, based on the objectives we claimed.

2. Results

2.1. Develop and demonstrate methods for the selective reaction of metallic SWNT using 4-hydroxy phenyl functional group.

Highly selective reaction of metallic over semiconducting SWNT with functional group, which will be used as a chemical handle for separation, is the key for the high resolution separation of metallic SWNT. Therefore, we investigated the structure-reactivity relationship for electron-transfer reactions of SWNT with functional groups using 4-hydroxybenzene diazonium as a model electron acceptor. Electron transfer theories are used to explain the difference in reactivities between metallic and semiconducting SWNT. The influence of reagent concentration and external illumination (0.764 mW/cm2) on the reaction selectivity is described by the rate model and confirmed by experiments. Illumination was shown to decrease the selectivity of the reagent to metallic over semiconducting SWNT, due to the greater activity of the reagent in solution when exposed to light. Additionally, we found that optimum reagent concentration exists at which the selectivity for metallic SWNT reaches maximum (0.34 mM).

2.2. Use 4-hydroxy phenyl functional group as a chemical handle to separate metallic SWNT.

Hydroxy phenyl group was selectively attached to metallic SWNT with high selectivity, using the reaction conditions developed in the previous section. Deprotonation in alkaline solution induces a negative charge on the functional group, enabling electrophoretic separation of functionalized SWNT. We applied this concept to enrich metallic and semiconducting fractions separately using the induced differences in electrophoretic mobilities. Free solution electrophoresis was utilized to separate selectively reacted samples into non-mobile and negative electrophoretic mobility (8.8×10-9 m2/Vs) fractions. Raman spectroscopy and UV-vis-nIR absorption spectroscopy confirm both the separation of reacted and unreacted SWNT, and after annealing, the enrichment of metallic and semiconducting SWNT respectively in two distinct fractions.

We also found out that functional groups on SWNT surface can alter the densities of individual SWNT. A volume additivity model is able to predict the density differences between 4-hydroxy phenyl functionalized and non-functionalized SWNT as 91.8 kg/m3. Conversely, the density distribution of between SWNT diameters is 34.1 kg/m3, indicating that chemical functionalization can provide an effective handle to separate out functionalized SWNT utilizing density-induced centrifugation. As a demonstration, we applied this concept to separate SWNT where metallic SWNT are selectively functionalized. The results are verified by Raman spectroscopy where the higher density fraction contains an increased disorder mode, indicating high resolution separation of functionalized-metallic SWNT.

2.3. Deposition and alignment of SWNT across an electrode gap for electrical transport measurements.

We developed a novel scheme to deposit and align individual SWNT between gold electrodes from SWNT solution. This method utilizes the phenomenon that droplets of liquid drying on a surface develop an internal hydrodynamic flow that carries entrained SWNT to the air-liquid-substrate interface. More than 84% of SWNT are aligned in parallel within ±5o relative to the target axis of alignment with this method. Future works will be focused on the measurements of electrical properties of separated metallic SWNT deposited using this method.

3. Conclusion

We have shown that 4-hydroxy phenyl group, selectively attached to metallic over semiconducting SWNT, could be used as chemical handle for separation of metallic SWNT from SWNT mixtures with high efficiency. This functional group increases the electrophoretic mobilities of reacted SWNT by deprotonation at high pH during electrophoresis process, enabling the separation of reacted SWNT from SWNT mixtures. Also, this increases the density of reacted SWNT, enabling the separation of reacted SWNT by density-induced centrifugation. These metallic SWNT separated by using chemical handle can be used for wide applications, such as ultra low resistance material for power transmission and interconnect material of nanoelectronics. Furthermore, these methods, which can separate out different metallic SWNT as well as pure semiconducting SWNT, will enable our group to investigate other applications in addition to power transmission in the future.

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