Prof. Abdennaceur Karoui

Goals: Develop a process and a tool for handling and processing carbon nanotubes (CNTs) to effectively use them in advanced solar cell concept. Design the tool using finite element analysis of electrokenitic of microfluidic solution containing low concentration of CNTs. Fabricate the MEMS and test the process of disantanglement, aligning, and sorting CNTs.

Carbon nanotubes (CNTs) are known to twist and entangle due to van der Waals forces, and length minimization subsequent energy minimization needs. These make processing CNTs individually or as a group of organized elements impossible, which lead one to consider them almost useless, albeit they draw a lot of attention of scientists and hopes of technologists. Aside from structural nanomaterials (e.g., nanocomposite), in electronics and optoelectronics, it is practically useless to modify the host material with CNTs (or to use them directly) if these tubes don't have at least some directionality. For nanocomposites the sought enhancement of mechanical properties could still be achieved through incorporating a small quantity of twisted CNT, or even a "spool of CNT threads". Contrary to this, in electronics, optoelectronics, and drug delivery,... it is necessary that the employed CNTs lead, even in small quantity, to a strong material property through regimentation of these CNTs. Usually, researchers focus on obtaining with CNT some extreme properties, such as, exceptional directional electric conduction, extraordinary thermal conduction, very strong optical anisotropy, and some nanoscale pieozoelectricity,... We focused on solving the CNT twisting problem and the entanglement in relation with our research focus. The method is also used to sort ot nanorods (for instance TiO₂ , ZnO , CuO nanorods) and align them.

        For our research on new concepts for advanced solar cells, we have been focusing on novel electric contacts for nano-structured photovoltaic cells. For such cells the electric contacts are of primary importance. They must collect charge carriers from very confined spaces (from few nanometers to deca-nanometers) and as soon as the carriers are photo-generated, i.e., within nanoseconds. Any delayed or remotely collected photo-generated charge carrier causes the carrier to be lost by recombination, a common phenomenon that severely degrades solar cell efficiency. 

         To solve this critical problem, we have devised a new method that consists in using NTs to collect charges where and as soon as they are generated. However, the implementation of this idea has been extremely challenging, since NTs severely entangle with each others, hence, they do not have preferential conduction direction. We approached this crucial material issue using a micro-electro-mechanical system, that we refer to as “Nanotube Handler MEMS^©”. It is a micro-fluidic tool that allows organization of nanotubes in useful manner and permits assembling them in very long nano-contacts. A side from the photovoltaic application, aligned NTs can be designed to make NT based biosensors and high performance nano-electronic devices. Figure 1 shows SEM images of nano-channels and electrodes in the fabricated MEMS tool.

       The design and the sizing required modeling of a NT-containing microfluidic solution by Finite Element Analysis (FEA), where electro-osmosis theory was used. The MEMS design itself was very complex and the understanding of the behavior of CNTs in the microfluidic solution and their flow were scientifically challenging with totally new issues, practically in every step. It has a large number of elements and nano-features that work in synch to produce sufficient quantity of aligned NTs. We aimed at using frontier processing, where techniques for making the nano-channels and vertical nano-electrodes were invented. This undertaking was a pioneering type of work and resulted challenges were tackled nano-fabrication and the characterization of the fabricated layers and features.

Fig. 1: Channels and electrodes of the MEMS, where the NT microfluidic solution flows. The tagged NTs are visualized by nanoscale fluorescence spectroscopy imaging.

         The testing of the Nanotube Handler is in progress. Microfluidic flow of boron nitride nanotubes (NTs) under various conditions are being tested. After marking the NTs with fluorescein they are dissolved in the microfluidic solution and injected in the MEMS. The NTs are visualized, in-operando, using a high-resolution fluorescence microscope. They are traced during their motion inside the mixing chambers of the Nanotube Handler as well as during their separation and their flow inside nanochannels.

The MEMS nanofabrication work was done at Cornell University under the sponsorship of the National Nanotechnology Infrastructure Network, the NSF nanofabrication network.