Polymer self-assembly and thin film deposition in supercritical fluids

Patterning of flexible electronic devices using large-area printing techniques is the focus of intense research due to their promise of producing low-cost, light-weight, and flexible devices. The successful integration of advanced materials like semiconductor nanocrystals, carbon nanotubes and polymer semiconductors into microscale electronic devices requires deposition techniques that are robust, scalable, and enable fine patterning. To this end, we have established a deposition technique that leverages the unique solubility properties of supercritical fluids. The technique is the solution-phase analog of physical vapour deposition and allows thin films of a semiconducting polymer to be grown without the need for in-situ chemical reactions. To demonstrate the flexibility of the technique, we demonstrated precise control over the location of material deposition using a combination of photolithography and resistive heating. The versatility of the technique is demonstrated by creating a patterned film on the concave interior of a silicone hemisphere, a substrate that cannot be patterned via any other technique. More generally, the ability to control the deposition of solution processed materials with lithographic accuracy provides the long sought-after bridge between top-down and bottom-up self-assembly. In addition, we investigated the self-assembly of polymers in supercritical fluids by depositing thin films and studying their morphology using polarized optical microscopy and grazing incidence wide angle x-ray scattering. We summarized our observations with a two-step model for film formation. The first step is pre-aggregation in solution whereby the local crystalline order is established, and the solution turbulence can easily disrupt the solution-phase self-assembly. The second step to film formation is the longer length scale organization that is influenced by the chain mobility on the surface. We identified pressure and solvent additive as two powerful tools to facilitate the local crystalline order and longer length scale organization. The work demonstrated key insights necessary to optimizing thin-film morphologies and principles for understanding self-assembly in supercritical fluids that could be applied to self-assembly of materials in other contexts. Finally, we developed a simple empirical model based on classical thermodynamics that highlights the interplay of intermolecular interactions and solvent entropy and describes both the temperature and pressure dependence of polymer solubility in supercritical fluids

RADIO FREQUENCY SIGNAL PROCESSING WITH MICROELECTROMECHANICAL RESONATING SYSTEMS

This thesis presents a study of the dynamics and applications of a high frequency micromechanical (MEMS) resonator. Mechanical systems, which have been scaled in dimension to the micron scale, show promise for replacing electrical resonant systems, which have larger physical size and lower performance. MEMS resonators can also be integrated into a chip containing conventional field effect transistors. A process incorporating both frequency dependent resonant systems as well as analog and digital electronics will enable all hardware in a communication architecture to be placed on a single silicon chip. In this study, a micron-sized circular membrane, suspended in the middle and clamped on the periphery, forms the basis of the resonant mechanical system. A small degree of curvature is fabricated into the resonator, which serves to stiffen the device and hence increase the frequency range. A microheater, defined in proximity to the resonator, is used to induce motion in the membrane. The frequency dependent response of the membrane is then detected through either interferometric or piezoresistive techniques. Resistive actuation and detection allow the membrane and actuators to be fabricated into a single plane of silicon, facilitating integration of the complete MEMS system. It is demonstrated how both the resonators and transducers can be implemented into two CMOS processes. Both designs incorporate the mechanical system as well as the solid-state electronics for output signal detection into a single fabrication process. Finally, the dynamics of the MEMS resonator, both in the linear and non-linear regime, are explored. The micron-sized mechanical system is demonstrated to perform several types of signal processing that are critical for wireless communication architectures. These studies shed new light on how the nonlinear dynamics of these systems may be characterized and harnessed for new applications

FAST-PYROLYSIS OF BIOMASS RELATED MODEL COMPOUNDS: A NOVEL APPROACH TO EXPERIMENTAL STUDY AND MODELING

Fast pyrolysis is a potentially attractive method for converting biomass to a low energy-density liquid (bio-oil) that can be further upgraded for use as fuel. Currently there is no agreement concerning the reaction pathways and mechanisms for pyrolysis of any individual component of biomass. This information is important for optimization of the fast-pyrolysis process. The work was divided into four areas, 1--development and validation of analytical methods and reactors, 2--the utilization of these methods to study pyrolysis of biomass and related models, 3--use of available biomass conversion pathways to propose potential integration with the existing fuel and chemicals markets, and 4--a proposed kinetic and multiphase reactor model for the physical and chemical processes that occur during pyrolysis

Measurements of Doping-Dependent Microwave Nonlinearities in High-Temperature Superconductors

I first present the design and use of a near-field permeability imaging microwave microscope to measure local permeability and ferromagnetic resonant fields. This microscope is then modified as a near-field nonlinear microwave microscope to quantitatively measure the local nonlinearities in high-Tc superconductor thin films of YBa2Cu3O7-d (YBCO). The system consists of a coaxial loop probe magnetically coupling to the sample, a microwave source, some low- and high-pass filters for selecting signals at desired frequencies, two microwave amplifiers for amplification of desired signals, and a spectrum analyzer for detection of the signals. When microwave signals are locally applied to the superconducting thin film through the loop probe, nonlinear electromagnetic response appearing as higher harmonic generation is created due to the presence of nonlinear mechanisms in the sample. It is expected that the time-reversal symmetric (TRS) nonlinearities contribute only to even order harmonics, while the time-reversal symmetry breaking (TRSB) nonlinearities contribute to all harmonics. The response is sensed by the loop probe, and measured by the spectrum analyzer. No resonant technique is used in this system so that we can measure the second and third harmonic generation simultaneously. The spatial resolution of the microscope is limited by the size of the loop probe, which is about 500 mm diameter. The probe size can be reduced to ~ 15 mm diameter, to improve the spatial resolution. To quantitatively address the nonlinearities, I introduce scaling current densities JNL(T) and JNL'(T), which measure the suppression of the super-fluid density as , where J is the applied current density. I extract JNL(T) and JNL'(T) from my measurements of harmonic generation on YBCO bi-crystal grain boundaries, and a set of variously under-doped YBCO thin films. The former is a well-known nonlinear source which is expected to produce both second and third harmonics. Work on this sample demonstrates the ability of the microscope to measure local nonlinearities. The latter is proposed to present doping dependent TRS and TRSB nonlinearities, and I use my nonlinear microwave microscope to measure the doping dependence of these nonlinearities

Bimetal Temperature Compensation for Waveguide Microwave Filters

Microwave communication devices have become ubiquitous in the past decade. As an increasing number of systems compete for spectrum, guard bands have shrunk to increase bandwidth efficiency. The frequency behaviour of microwave devices is affected by thermal expansion. In order to avoid interference with adjacent bands, microwave components must exhibit high temperature-stability in most communications applications. Thermally stable materials can be used to construct temperature-stable components. However, this approach requires an expensive mass and cost trade-off. Temperature compensated aluminum resonators and filters provide major advantages in cost and mass. This work proposes that a compensating tuning screw with a temperature-dependent effective length be constructed by mounting a bimetallic compensator at the end of a mounting screw. This so-called bimetal tuning-screw can be used to produce temperature-compensated resonators and filters. There are several advantages to this approach. Compensation can be tuned by adjusting the depth of the bimetal, simply by adjusting the mounting screw. Since there are no moving parts inside the cavity or filter, and the bimetal can be plated, there are no additional sources of passive intermodulation. Also, this design is simple to implement for waveguide designs in general. In order to compensate for temperature drift, it is useful to quantify uncompensated drift. Temperature drift for a lossless linearly expanding RF component is derived from Maxwell's equations. For the lossy case, it is demonstrated that the resulting formula is approximately true, and that the quality of this approximation is excellent for practical levels of temperature range and thermal expansion. Experimental results are provided that demonstrate bimetal compensation under uniform-temperature conditions for a single aluminum resonator. Measured drift of the compensated resonator is -0.38 ppm/°C, compared to -23 ppm/°C for an uncompensated resonator. Measured drift for a bimetal-compensated 4-pole filter prototype is 2.35 ppm/°C. A method for adjusting compensation for a filter is also provided. Multiphysics simulations are used to examine power handling for bimetal-compensated filters. It is demonstrated that power-handling can be improved by reducing the effective length of the compensator to improve heat conduction to the cavity or filter

Ultra-sensitive YBCO nanoSQUIDs for the investigation of magnetic nanoparticles

Superconducting quantum interference devices (SQUIDs) are used in an impressively large variety of applications requiring sensitive detection of magnetic flux. In recent years, there has been a growing scientific and technological interest in the development of nanoSQUIDs, i.e. strongly miniaturized SQUIDs with lateral size on the sub-micrometer scale, that can be used to detect the magnetization of small spin systems like individual magnetic nanoparticles. The development of nanoSQUIDs is a major research topic at the Physics Institute - Experimental Physics II. In this thesis, we first review the major achievements obtained so far on the development of sensitive nanoSQUIDs in Tübingen, based on Nb and YBa2Cu3O7 (YBCO) as superconductors. This part emphasizes the advantages offered by YBCO nanoSQUIDs, fabricated on bicrystal SrTiO3 (STO) substrates, regarding enhanced ranges of temperature and magnetic field, over which those nanoSQUIDs can be operated. Regarding the application of YBCO nanoSQUIDs fabricated on STO bicrystal substrates, we have studied the occurrence of magnetic-field-driven nucleation and annihilation of magnetic vortices in individual ultrasmall ferromagnetic Co particles by YBCO nanoSQUID magnetometry. We demonstrate that the Co particles reveal bi-stable magnetization states at zero applied field, with the vortex state being the ground state. This topic is important in order to understand the thermal and temporal stability of noncollinear and other nontrivial spin textures, e.g., vortices or skyrmions, confined in ultrasmall ferromagnets. Improving the sensitivity and long-time stability of YBCO nanoSQUIDs are in the focus of the research activities presented within this thesis. A process for the fabrication of YBCO nanoSQUIDs on MgO bicrystal substrates has been developed. The lower dielectric permittivity of MgO, as compared to STO, offers the possibility to realize YBCO nanoSQUIDs without the need of a resistively shunting Au layer on top of the YBCO film. This in turn offers a significant increase of the characteristic voltage of the grain boundary Josephson junctions intersecting the SQUID loop, which should significantly improve the sensitivity of the nanoSQUIDs. We demonstrate that YBCO nanoSQUIDs patterned by focused Ga ion beam (Ga FIB) milling on MgO bicrystals can have non-hysteretic current voltage characteristics (IVCs) at 4.2K even without Au as shunting layer, which shows the high potential to further improve the flux sensitivity. We further clarify the evolution of the electric transport and noise properties at 4.2K of YBCO nanoSQUIDs on bicrystal MgO substrates, upon decreasing the thickness of the Au film used as a resistively shunting layer. Moreover, we compare the performance of YBCO nanoSQUIDs fabricated on STO and MgO bicrystals at 77K and 4.2 K. A new approach based on heteroepitaxially grown superlattices was implemented in order to improve the flux sensitivity of nanoSQUIDs. We report on the fabrication and characterization of nanopatterned dc SQUIDs with grain boundary Josephson junctions based on heteroepitaxially grown YBCO/STO superlattices on STO bicrystal substrates. Nanopatterning is performed by Ga FIB milling. The electric transport properties and thermal white flux noise of superlattice nanoSQUIDs are comparable to single layer YBCO devices on STO bicrystals. However, we find that the superlattice nanoSQUIDs have more than an order of magnitude smaller low-frequency excess flux nois. We attribute this improvement to an improved microstructure at the grain boundaries forming the Josephson junctions in our YBCO nanoSQUDs. Last but not least, we developed a novel weak link in YBCO thin films based on an artificial bottom-up technology, i.e., by using Ga FIB milling to prepare nanogrooves in single crystal STO substrates, prior to YBCO thin film growth. This technique combined with cutting edge equipment like extreme ultraviolet lithography could provide a cost-effective and reliable pathway for scaling up superconducting circuits operating at liquid-nitrogen temperature

Initiated chemical vapor deposition of polymeric thin films : mechanism and applications

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2005.Includes bibliographical references.Initiated chemical vapor deposition (iCVD) is a novel technique for depositing polymeric thin films. It is able to deposit thin films of application-specific polymers in one step without using any solvents. Its uniqueness of in situ surface polymer synthesis distinguishes iCVD from conventional processes such as spin-on deposition and plasma-enhanced chemical vapor deposition. It allows engineering polymers to be made with specific microscale properties translating to well-defined macroscale behaviors. In this thesis work, two application-specific polymers based on poly(2-hydroxyethyl methacrylate) (PHEMA) and poly(cyclohexyl methacrylate) (PCHMA) were synthesized using iCVD. PHEMA thin films with specific degrees of cross-linking leading to well-defined structural, thermal, wetting, and swelling properties were made in a single vacuum step by simply adjusting chamber conditions. Cross-linked PCHMA thin films were synthesized for use as sacrificial layers for microfabrication. Such films of engineering polymers cannot be made using conventional methods. A study of the polymerization mechanism was included to serve as a groundwork for increased understanding of iCVD as a thin- film deposition method.(cont.) Growth rates and molecular weights, crucial parameters for polymeric thin films, were found to be highly dependent on the surface concentrations of monomers, leading to the conclusion that polymer formation occurs predominantly on the surface of the substrate. This conclusion also infers that controlling the surface concentrations of monomers can lead to copolymers/terpolymers with well-defined compositions, which was demonstrated in the iCVD of PHEMA-based thin films. iCVD therefore can be extended to complex polymer systems with multiple monomeric building blocks. Photo- initiatied chemical vapor deposition (piCVD) using a volatile photoinitiator is introduced for the first time in this thesis. piCVD possesses all the benefits of iCVD over conventional processes but uses a photochemical initiation mechanism that simplifies chamber design and potentially allows self-patterning during deposition.by Kelvin Chan.Ph.D