Nano Materials and Physics
Authors: Ren Z. F. 
Published: October 2008

Aligning carbon nanotubes in any way desired is very important for many fundamental and applied research projects. In this talk, I will first discuss how to grow them with controlled diameter, length, spacing, and periodicity using catalyst prepared by magnetron sputtering, electron beam (e-beam) lithography, electrochemical deposition, and nanosphere self-assembly. Then I will present our results of field emission property of both the aligned carbon nanotubes grown on flat substrates and random carbon nanotubes grown on carbon cloth. For the aligned carbon nanotubes arrays, I will present the preliminary results of using them as photonic band gap crystals and nanoantennae. As an alternative material of carbon nanotubes, ZnO nanowires have been grown in both aligned fashion on flat substrates and random fashion on carbon cloth. Using these ZnO nanowires, good field emission properties were observed. Furthermore, I will present our recent studies on the electrical breakdown and transport properties of a single suspended nanotube grown on carbon cloth by a scanning electron microscope probe incorporated into a high resolution transmission electron microscope. As part of the potential applications, I will also discuss our recent success on molecules delivery into cells using carbon nanotubes. Finally I will talk about our most recent endeavor on achieving thermoelectric figure-of-merit (ZT) higher than 2 using our unique nanocomposite approach. Plasma-enhanced chemical vapor deposition (PECVD) was discovered by my group in 1998 to be able to grow aligned carbon nanotubes [1]. Catalyst film was first deposited by magnetron sputtering. According to the thickness of the catalytic film, aligned carbon nanotubes were grown with different diameters and spacing, and different length depending the growth time. However, the two major drawbacks are 1) that the location of where the nantoube grows can not be controlled, 2) that the spacing between the nanotubes can not be varied too much. Therefore, we immediately explored to grow aligned carbon nanotubes with the location and spacing controls using e-beam lithography [2]. Unfortunately the cost is so high that the e-beam is not suited for large scale commercialization that requires only an average site density control not the exactly location, for example, electron source. It is the cost issue that made us to develop electrochemical deposition to make catalyst dots that can be separated more than 10 micormeters between dots [3]. With such arrays, we tested many samples for field emission properties and found the optimal site density [4]. However, for applications that require the location controls, for example, photonic band gap crystals, electrochemical deposition can not be satisfactory. It is this kind of application that led us to develop the nanosphere self-assembly technique in large scale [5]. For field emission, we found that ZnO nanowires are good field emitters comparable to carbon nanotubes if they are grown with the right diameter and spacing. Here I will discuss the field emission properties of ZnO nanowires as an alternative material to carbon nanotubes [6]. Us a special kind of carbon nanotubes made by PECVD, we discovered a highly efficient molecular delivery technique, named nanotube spearing, based on the penetration of Ni-particle embedded nanotubes into cell membranes by magnetic field driving. DNA plasmids encoding the enhanced green fluorescent protein (EGFP) sequence were immobilized onto the nanotubes, and subsequently speared into targeted cells. We have achieved the unprecedented high transduction efficiency in Bal17 B-lymphoma, ex vivo B cells, and primary neurons with high viability. This technique may provide a powerful tool for high efficient gene transfer in a variety of cells, especially, the hard-to-transfect cells [7]. Conventional transport studies of multiwall carbon nanotubes (MWNTs) with only the outmost wall contacted to the electrodes via side-contact shows that a MWNT is a ballistic conductor with only the outmost wall carrying current. Here we show, by using end-contact in which every wall is contacted to the electrodes, that every wall is conducting, as evidenced by a significant amount of current drop when an innermost wall is broken at high-bias. Remarkably, the breakdown of each wall was initiated in the middle of the nanotube, not at the contacts, indicating diffusive electron transport. Using end-contact, we were able to probe the conductivity wall-by-wall and found that each wall is indeed either metallic, or semiconducting, or pseudogap-like. These findings not only reveal the intrinsic physical properties of MWNTs but also provide important guidance to MWNT-based electronic devices [8]. At the end of the talk, if time permits, I will talk about our ongoing effort on improving the figure-of-merit (ZT) of thermoelectric materials using a nanocomposite strategy to mimic the structure of the superlattice of PbTe/PbSe and Bi2Te3/Sb2Te3 hoping to reduce the thermal conductivity by a factor of 2–4 while maintaining the electrical conductivity. To make a close to 100% dense nanocomposite, we started with nanoparticles synthesis, then consolidation using both the traditional hot press and the direct current fast-heat, named plasma pressure compact, to preserve the nano size of the component particles. So far, we have seen thermal conductivity decrease by a factor of 2 in the systems of Si/Ge, PbeTe/PbSe, Bi2Te3/Sb2Te3, indicating the potential of improving ZT by a factor of 2.