Electrical and optical studies on carbon nanotube arrays

Single-walled carbon nanotubes (SWNTs) have been studied as a prominent class of high performance electronic materials for next generation electronics. Their geometry dependent electronic structure, ballistic transport and low power dissipation due to quasi one dimensional transport, and their capability of carrying high current densities are some of the main reasons for the optimistic expectations on SWNTs. However, device applications of individual SWNTs have been hindered by uncontrolled variations in characteristics and lack of scalable methods to integrate SWNTs into electronic devices. One relatively new direction in SWNT electronics, which avoids these issues, is using arrays of SWNTs, where the ensemble average may provide uniformity from device to device, and this new breed of electronic material can be integrated into electronic devices in a scalable fashion. This dissertation describes (1) methods for characterization of SWNT arrays, (2) how the electrical transport in these two-dimensional arrays depend on length scales and spatial anisotropy, (3) the interaction of aligned SWNTs with the underlying substrate, and (4) methods for scalable integration of SWNT arrays into electronic devices. The electrical characterization of SWNT arrays have been realized by polymer electrolyte-gated SWNT thin film transistors (TFTs). Polymer electrolyte-gating addresses many technical difficulties inherent to electrical characterization by gating through oxide-dielectrics. Having shown polymer electrolyte-gating can be successfully applied on SWNT arrays, we have studied the length scaling dependence of electrical transport in SWNT arrays. Ultrathin films formed by sub-monolayer surface coverage of SWNT arrays are very interesting systems in terms of the physics of two-dimensional electronic transport. We have observed that they behave qualitatively different than the classical conducting films, which obey the Ohm’s law. The resistance of an ultrathin film of SWNT arrays is indeed non-linear with the length of the film, across which the transport occurs. More interestingly, a transition between conducting and insulating states is observed at a critical surface coverage, which is called percolation limit. The surface coverage of conducting SWNTs can be manipulated by turning on and off the semiconductors in the SWNT array, leading to the operation principle of SWNT TFTs. The percolation limit depends also on the length and the spatial orientation of SWNTs. We have also observed that the percolation limit increases abruptly for aligned arrays of SWNTs, which are grown on single crystal quartz substrates. In this dissertation, we also compare our experimental results with a two-dimensional stick network model, which gives a good qualitative picture of the electrical transport in SWNT arrays in terms of surface coverage, length scaling, and spatial orientation, and briefly discuss the validity of this model. However, the electronic properties of SWNT arrays are not only determined by geometrical arguments. The contact resistances at the nanotube-nanotube and nanotube-electrode (bulk metal) interfaces, and interactions with the local chemical groups and the underlying substrates are among other issues related to the electronic transport in SWNT arrays. Different aspects of these factors have been studied in detail by many groups. In fact, I have also included a brief discussion about electron injection onto semiconducting SWNTs by polymer dopants. On the other hand, we have compared the substrate-SWNT interactions for isotropic (in two dimensions) arrays of SWNTs grown on Si/SiO2 substrates and horizontally (on substrate) aligned arrays of SWNTs grown on single crystal quartz substrates. The anisotropic interactions associated with the quartz lattice between quartz and SWNTs that allow near perfect horizontal alignment on substrate along a particular crystallographic direction is examined by Raman spectroscopy, and shown to lead to uniaxial compressive strain in as-grown SWNTs on single crystal quartz. This is the first experimental demonstration of the hard-to-achieve uniaxial compression of SWNTs. Temperature dependence of Raman G-band spectra along the length of individual nanotubes reveals that the compressive strain is non-uniform and can be larger than 1% locally at room temperature. Effects of device fabrication steps on the non-uniform strain are also examined and implications on electrical performance are discussed. Based on our findings, there are discussions about device performances and designs included in this dissertation. The channel length dependences of device mobilities and on/off ratios are included for SWNT TFTs. Time response of polymer-electrolyte gated SWNT TFTs has been measured to be ~300 Hz, and a proof-of-concept logic inverter has been fabricated by using polymer electrolyte gated SWNT TFTs for macroelectronic applications. Finally, I dedicated a chapter on scalable device designs based on aligned arrays of SWNTs, including a design for SWNT memory devices.

Quantitative characterization of physico-chemical properties of the active layers of reverse osmosis and nanofiltration membranes, and their relation to membrane performance

The main objectives of this dissertation were: (i) to develop experimental and analytical procedures to quantify different physico-chemical properties of the ultra-thin (~ 100 nm) active layers of reverse osmosis (RO) and nanofiltration (NF) membranes and their interactions with contaminants; (ii) to use such procedures to evaluate the similarities and differences between the active layers of different RO/NF membranes; and (iii) to relate characterization results to membrane performance. Such objectives were motivated by the current limited understanding of the physico-chemical properties of active layers as a result of traditional characterization techniques having limitations associated with the nanometer-scale spatial resolution required to study these ultra-thin films. Functional groups were chosen as the main active layer property of interest. Specific accomplishments of this study include the development of procedures to quantify in active layers as a function of pH: (1) the concentration of both negatively and positively ionized functional groups; (2) the stoichiometry of association between ions (i.e., barium) and ionized functional groups (i.e., carboxylate and sulfonate); and (3) the steric effects experienced by ions (i.e., barium). Conceptual and mathematical models were developed to describe experimental results. The depth heterogeneity of the active layer physico-chemical properties and interactions with contaminants studied in this dissertation was also characterized. Additionally, measured concentrations of ionized functional groups in the polyamide active layers of several commercial RO/NF membranes were used as input in a simplified RO/NF transport model to predict the rejection of a strong electrolyte (i.e., potassium iodide) and a weak acid (i.e., arsenious acid) at different pH values based on rejection results at one pH condition. The good agreement between predicted and experimental results showed that the characterization procedures developed in this study serve as useful tools in the advancement of the understanding of the properties and structure of the active layers of RO/NF membranes, and the mechanisms of contaminant transport through them.

Design of advanced reverse osmosis and nanofiltration membranes for water purification

Most commercially available reverse osmosis (RO) and nanofiltration (NF) membranes are based on the thin film composite (TFC) aromatic polyamide membranes. However, they have several disadvantages including low resistance to fouling, low chemical and thermal stabilities and limited chlorine tolerance. To address these problems, advanced RO/NF membranes are being developed from polyimides for water and wastewater treatments. The following three projects have resulted from my research. (1) Positively charged and solvent resistant NF membranes. The use of solvent resistant membranes to facilitate small molecule separations has been a long standing industry goal of the chemical and pharmaceutical industries. We developed a solvent resistant membrane by chemically cross-linking of polyimide membrane using polyethylenimine. This membrane showed excellent stability in almost all organic solvents. In addition, this membrane was positively charged due to the amine groups remaining on the surface. As a result, high efficiency (> 95%) and selectivity for multivalent heavy metal removal was achieved. (2) Fouling resistant NF membranes. Antifouling membranes are highly desired for “all” applications because fouling will lead to higher energy demand, increase of cleaning and corresponding down time and reduced life-time of the membrane elements. For fouling prevention, we designed a new membrane system using a coating technique to modify membrane surface properties to avoid adsorption of foulants like humic acid. A layer of water-soluble polymer such as polyvinyl alcohol (PVA), polyacrylic acid (PAA), polyvinyl sulfate (PVS) or sulfonated poly(ether ether ketone) (SPEEK), was adsorbed onto the surface of a positively charged membrane. The resultant membranes have a smooth and almost neutrally charged surface which showed better fouling resistance than both the positively charged NF membranes and commercially available negatively charged NTR-7450 membrane. In addition, these membranes showed high efficiency for removal of multivalent ions (> 95% for both cations and anions). Therefore, these antifouling surfaces can be potentially used for water softening, water desalination and wastewater treatment in a membrane bioreactor (MBR) process. (3) Thermally stable RO membranes. Commercial RO membranes cannot be used at temperature higher than 45°C due to the use of polysulfone substrate, which often limits their applications in industries. We successfully developed polyimides as the membrane substrate for thermally stable RO membranes due to their high thermal resistance. The polyimide-based composite polyamide membranes showed desalination performance comparable to the commercial TFC membrane. However, the key advantage of the polyimide-based membrane is its high thermal stability. As the feed temperature increased from 25oC to 95oC, the water flux increased 5 - 6 times while the salt rejection almost kept constant. This membrane appears to provide a unique solution for hot water desalination and also a feasible way to improve the water productivity by increasing the operating temperature without any drop in salt rejection.