43902-G10
Novel, Ultra-Low Resistance Materials Prepared by Chemical Separation of Metallic Single Walled Carbon Nanotubes
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 material or their composites 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 and MIT: 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.
Separation efficiency for metallic SWNT using our approach depends upon the selective chemical reaction of a molecular handle onto SWNT according to their electronic type. Therefore, we investigated structure-reactivity relations to understand and correlate the reactivity of a number of electron acceptors (molecular handles). In addition to 4-hydroxy benzene diazonium reagent, which was investigated in Year 1, we screened other diazonium reagents having different oxidation potentials compared to hydroxyl group, and investigated their effect on the reaction selectivity for metallic over semiconducting SWNT. Among screened diaonium reagents, carboxy benzene diazonium reagent showed highest selectivity for metallic SWNT than others, and furthermore, di-carboxyl groups showed higher selectivity than mono carboxyl group. This is due to higher oxidation potential of di-carboxyl group compared to that of mono-carboxyl group, lowering reaction activity of diazonium reagent with SWNT. This result can also be directly utilized in increasing separation efficiency, because the charge density, which determines the electrophoretic mobility of functionalized SWNT, will be doubled when di-carboxyl groups are attached to SWNT compared to mono-hydroxyl group.
We demonstrated in Year 1 that we were able to separate metallic SWNT from semiconducting SWNT utilizing the property of 4-hydroxy phenyl chemical handles, which was exclusively attached to metallic SWNT. In Year 2, we prepared metallic SWNT film using the separated metallic SWNT in section 2.1. We confirmed that all hydroxy phenyl functional groups, used for separation of metallic SWNT, were completely removed upon high temperature annealing (>600oC), and all the electrical property of SWNT were restored to the level of original SWNT. The measured sheet resistance of metallic SWNT film is 46% lower than that of SWNT mixture film, indicating that metallic SWNT has higher conductivity than unseparated SWNT mixture.
And also, based on the results obtained in previous section, we investigated the effect of newly developed chemical handles on the separation efficiency. We measured the electrophoretic mobilities of 4-hydroxy and di-carboxyl phenylated SWNT and found out that the speed of motion is approximately twice higher for di-carboxy phenylated SWNT than 4-hydroxy phenylated SWNT, as expected. The estimated electrophoretic mobility is 1.7x10-8m2/Vs for di-carboxyl and 8.8x10-9m2/Vs for 4-hydroxy phenylated SWNT. Higher electrophoretic mobility obtained with di-carboxy phenyl groups can increase the separation efficiency and also contribute to the scalability of this separation process by shortening the time of separation and increasing the injection speed of SWNT.
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.
We showed that metallic SWNT film, prepared using separated metallic SWNT based on our selective chemical method, exhibit 46% lower sheet resistance than unseparated SWNT mixture film. We also have shown that di-carboxyl functional group showed highest reaction selectivity for metallic SWNT over semiconducting SWNT among various electron acceptor functional groups. We also found out that the speed of motion is approximately twice higher for di-carboxy phenylated SWNT than 4-hydroxy phenylated SWNT in electrophoretic separation. Both of these findings would contribute to increase the separation efficiency and scale up the separation process, enabling our group to investigate other applications in addition to power transmission in the future.
