# Presentation: 880520

## CAREER: Synthesis and Electronic/Electrical Properties of Carbon Nanotube JunctionsWenzhi Li	Florida International University	DMR 0548061

- One of the objectives of this research is to study and understand fundamental synthesis mechanism of new functional materials: Y-shaped carbon nanotube junctions (Y-junctions).
- Figure a shows a scanning electron microscope (SEM) image of a Y-junction. It consists of many branches and forms a multiple junction. Figure b shows a transmission electron microscopy (TEM) image of a Y-junction. The outer and inner diameters of the Y-junction are 70 nm and 15 nm, respectively.
- Energy Dispersive X-ray (EDX) spectroscopy and X-ray diffraction (XRD) are employed to determine the chemical composition of the catalyst, which is catalyzing the formation of the Y-junction nanotubes. EDX shows that the as-synthesized material consists of C, O, Co, Mg, and S. XRD further indicates that C, CoS, MgO, and MgS are presented in the sample. Initial analysis of the EDX and XRD data indicates that CoS nanoparticles are promoting the formation of the branched carbon nanotubes.

**Notes:**
Research objective:
Carbon nanotubes have demonstrated to be extraordinary nanomaterial with remarkable electronic properties, and they can be either metallic or semiconducting, depending on their diameter and chirality (helicity). As self-assembled architecture of straight carbon nanotubes, Y-shaped carbon nanotube junction (Y-junction) has inspired further interest in fundamental research and application due to their unique structure and nonlinear transport properties. A Y-junction carbon nanotube can have metal-metal, metal-semiconductor and semiconductor-semiconductor junctions, and these rich electronic properties will make it promising functional component for nanoelectronics applications. One of the objectives of this research is to identify the factors that determine the formation of the branched nanotubes and to completely understand the growth mechanism of the Y-junction nanotubes.
Introduction:
After the discovery of Y-junctions formed from arc-discharge method, Y-junctions have been synthesized by chemical vapor deposition (CVD) technique. Despite the successful synthesis of Y-junctions by many research groups including us, the growth mechanism still remains unclear, and it is not understood which synthesis parameters have induced the formation of Y-junctions in CVD process. In this proposed research, we intend to understand the growth mechanism of Y-junctions through intensive research activities, including (1) identifying and studying the factors that induce the growth of Y-junctions; (2) studying the growth thermodynamics; and (3) investigating the relation between the type of carbon topological defects (for example, carbon pentagons, heptagons, and octagons) and the chiralities of the three branches. 
Experimental techniques:
Y-junction carbon nanotubes were synthesized by chemical vapor deposition (CVD) technique, in which  thiophene (C4H4S) was used as carbon precursor and cobalt, magnesium, and calcium nitrates were used as catalyst precursors. A systematic experiment was carried out to catch the conditions at which the Y-junction nanotubes could form. The structure of the Y-junction nanotubes was studied by scanning electron microscopy and transmission electron microscopy. Energy dispersive X-ray spectroscopy and X-ray diffraction spectroscopy were employed to investigate the chemical composition of catalyst nanoparticles, which catalyzed the formation of branched nanotubes.    
Results:
It is found that cobalt nitrate and magnesium nitrate (or calcium nitrate) are very effective catalyst precursors for synthesizing Y-junction nanotubes. When C4H4S is used as carbon source, Y-junction carbon nanotubes have been synthesized by using cobalt nitrate/magnesium nitrate or cobalt nitrate/calcium nitrate with ratio of Co/Mg or Co/Ca in the range of 5 – 2000 wt%  at temperature of 1000 C and pressure of 200 Torr.  Catalyst with 20 wt% of Co/Mg has proved most efficient for synthesizing high yield Y-junction nanotubes with straight branches. Multiple branched Y-junction nanotubes  are often formed, and all the branches in a multiple Y-junction have the same diameter, see Fig. a and b. It is strikingly surprising that the distribution of angles between the adjacent branches for all the Y-junctions are similar. The habitual distribution of angles between the branches may imply a strong duplicability of the Y-junctions during their growth.
It is well known that the diameter of CVD grown carbon nanotubes is determined by the size of the catalyst nanoparticles. If a catalyst nanoparticle split to two smaller ones during the growth of the nanotube, then a Y-junction with one thick stem and two thin branches should be formed. On the other hand, if two catalyst nanoparticles, which were growing nanotubes, merged together and formed a bigger one, then a Y-junction nanotube with two thin branches and one thick branch should be formed. Actually, these type of Y-junctions had not been observed in our samples. In our experiment, all the braches in a Y-junction nanotube have the same diameter (Fig. a and b). It is speculated that a Y-junction nanotube is catalyzed by and grown on a single catalyst particle, rather than through the split or mergence of catalyst nanoparticles.
The effect of carbon precursor on the formation of Y-junction nanotubes has been investigated by replacing thiophene (C4H4S) with methane (CH4 ) or acetylene (C2H2) while keep the other synthesis parameters the same. The experimental result shows that Y-junction carbon nanotubes can not form, instead regular (linear) nanotubes or amorphous carbon particles are grown. This result demonstrates that not only the catalyst precursors but also the carbon source are playing important role in the formation of branched nanotubes. By comparing the three type of carbon sources, it is not difficult to point out that sulfur decomposed from C4H4S is critical to the formation of Y-junction nanotubes. In order to understand how the sulfur participates in the formation of Y-junctions, we performed chemical composition examination on the catalyst after nanotube growth by energy dispersive X-ray spectroscopy (EDX) any X-ray diffraction (XRD). EDX shows the exist of sulfur (s) besides the expected C, Co, Mg and O (Fig. c), and XRD (not shown here) confirms further the presence of four phases, C(Y-junction nanotubes), MgO, MgS, and CoS. Since magnesium nitrate along has been used as catalyst precursor and C4H4S as carbon source to synthesize nanotubes, but Y-junction nanotubes are not formed, indicating that MgS can not growth branched nanotubes. Therefore, CoS nanoparticles are critical in the formation of branched nanotubes. Further experiments show that when only cobalt nitrate is used as catalyst precursor and C4H4S as carbon source (note: CoS will be formed), Y-junctions can not be formed either. This result indicates that isolated CoS from MgO or MgS can not grow Y-junctions and CoS needs the assistance of MgO or MgS in growing Y-junction nanotubes. The current research result indicates that CoS is the catalyst responsible for the formation of Y-junction nanotubes but the presence of MgO and MgS are also indispensable. Further research is required to confirm the findings and reveal the detailed functions of each phase. The research result is very important for further study on the growth mechanism and growth thermodynamics of Y-junction carbon nanotubes. 

Significance of the results: 
A simple yet effective procedure has been developed and an appropriate catalyst combination has been found for the synthesis of branched carbon nanotubes, the obtained result of chemical composition of the catalyst and existing phases will allow us to study further the function of the catalyst in the branch formation and the growth thermodynamics of the Y-junctions. The successful synthesis of Y-junction nanotubes will enable us to start to investigate the intrinsic electrical/electronic properties although the graphitization of the Y-junction carbon nanotubes needs to be substantially improved. 

Future plans: 
Future research includes the following aspects:
Detailed study will be carried out to understand how exactly the catalyst induce the formation of branched nanotubes. Furthermore, since the catalyst identification was done after the growth, all structure and chemical properties of the catalyst particles at the growth conditions (for example at 1000 C and 200 Torr) were unable to be revealed. Detailed investigations, especially an in-situ examination of the nanopartilces, are required to understand the function of catalyst and the growth mechanism of the Y-junction nanotubes. 
We will carry out extensive and intensive studies on the effect of all the growth parameters including carbon source, catalyst composition, pressure, and temperature on the formation of Y-junction nanotubes aimed at , establishing a thermodynamic model for the formation of Y-junction carbon nanotubes.
We will investigate the intrinsic electronic properties of the Y-junctions and the impact of the nanostructures of Y-junctions on their electronic properties, and to provide insight into the ultimate correlation between the electronic properties and nanostructures of Y-junctions. 

## CAREER: Synthesis and Electronic/Electrical Properties of Carbon Nanotube JunctionsWenzhi Li	Florida International University	DMR 0548061

**Education and Outreach:**
- Two graduate students (Bimal Pandey and Tao Zhang) are taking part in the proposed nanoscale material research. They are preparing their first scientific papers basing on their research findings. Two undergraduate students (David Puldon and Richard Galvez) did their independent study under the PI’s supervision and they were involved in the research on Y-junction nanotubes and carbon nanotube/ceramic composites.
- Four Miami-Dade high school students took summer research in the PI’s laboratory on nanomaterial synthesis and thin film deposition.  Students have gained hands-on skills in chemical vapor deposition and electron beam deposition techniques, and have had chance to observe their own samples (carbon nanotubes) by using scanning electron microscope. Three of the four students will continue their research in the Fall semester in 2006.
-  Collaborating with the FIU “*Physics Learning Center”* and the “*Center for High Energy Physics Research & Education Outreach”*, the PI has given lab tours and demonstrated his research results to about 150 K-12 students, parents, and teachers. This activity will generate strong student and public interest in and understanding of nanoscience and nanotechnology.