This thesis reports on synthesis, microstructure and properties of binary and ternary carbide thin films deposited by dc magnetron sputtering. These materials are interesting since they exhibit a wide range of useful properties, such as high hardness, resistance to wear and oxidation, and high electrical conductivity. Here, an early transition metal (M) and carbon (C) have been used as the basis, often with the addition of a second M-element or an A-group element (A). In these systems nanocomposites, metastable solid solutions, multilayers, or Mn+1AXn-phases have been deposited. The Mn+1AXn-phases are a group of nanolaminated compounds with a unique mixture of metallic and ceramic properties. In general X is carbon or nitrogen, although here only carbon has been used. Epitaxial MAX-phase thin films of Ti2AlC, Ti3AlC2 and V2GeC have been deposited for the first time. They have been studied with emphasis on phase stability, phase composition and nucleation characteristics to gain deeper insights into their growth. The microstructure of the films was characterized by electron microscopy and X-ray diffraction. In addition, bond strength characteristics have been studied by soft X-ray spectroscopy and complementary calculations within DFT. Their mechanical and electrical properties have been studied, and the results are discussed on the basis of their electronic structure. Furthermore, by interleaving the Ti3SiC2 MAX-phase with TiC0.67 a multilayer structure has been formed, for which a new intrusion-type deformation behaviour has been described. A new concept in the design of nanocomposite films has been developed, whereby a solid solution of a weak carbide-forming element in the carbide structure creates a driving force for surface segregation of C. This concept has been verified both theoretically and experimentally for the Ti-Al-C and Ti-Fe-C systems. It has been shown by pin-on-disc measurements that this surface segregation leads to graphitization and consequently a very low friction coefficient for these films. Finally, it has been demonstrated that low-friction films with tunable magnetic properties can be achieved in the Ti-Fe-C system.

Presented here is a study of the electronic properties and molecular stacking structure of four novel X-shaped anthracene based organic semiconductors utilizing near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and density functional theory (DFT) calculations. These materials have been found to exhibit high charge carrier mobility when used in organic thin film transistors without an annealing step. Angle resolved NEXAFS show local molecular order through polarization dependence in C 1s → π* transitions, and that the plane of the anthracene core is oriented nearly normal to the plane of the substrate. DFT calculations were used examine electronic structure and the effects of molecular geometry, showing that the highest occupied molecular orbital (HOMO) conjugation extends to the thiophene end groups. The attachment of the thiophene end group is determined to modify intermolecular interaction, resulting in either a cofacial or herringbone structure. With the understanding of how these materials form an ordered crystal structure, future fabrication of new materials may be directed towards a preference for crystallization without annealing. A study with applications for organic photovoltaic devices was also undertaken to examine the thin film stacking structure of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). NEXAFS measurements show that the side chain lifts the energy degeneracy of the C60 molecular orbitals around the chain attachment. This breaks the spatial π -orbital symmetry of the lowest unoccupied molecular orbital (LUMO) of the C60 backbone which is observed through polarization dependence of π* transitions. The intensity dependence is further analyzed to determine the bulk crystal structure of PCBM.

Presented here is a study of the electronic properties and molecular stacking structure of four novel X-shaped anthracene based organic semiconductors utilizing near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and density functional theory (DFT) calculations. These materials have been found to exhibit high charge carrier mobility when used in organic thin film transistors without an annealing step. Angle resolved NEXAFS show local molecular order through polarization dependence in C 1s → π* transitions, and that the plane of the anthracene core is oriented nearly normal to the plane of the substrate. DFT calculations were used examine electronic structure and the effects of molecular geometry, showing that the highest occupied molecular orbital (HOMO) conjugation extends to the thiophene end groups. The attachment of the thiophene end group is determined to modify intermolecular interaction, resulting in either a cofacial or herringbone structure. With the understanding of how these materials form an ordered crystal structure, future fabrication of new materials may be directed towards a preference for crystallization without annealing. A study with applications for organic photovoltaic devices was also undertaken to examine the thin film stacking structure of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). NEXAFS measurements show that the side chain lifts the energy degeneracy of the C60 molecular orbitals around the chain attachment. This breaks the spatial π -orbital symmetry of the lowest unoccupied molecular orbital (LUMO) of the C60 backbone which is observed through polarization dependence of π* transitions. The intensity dependence is further analyzed to determine the bulk crystal structure of PCBM.

The nonlinear electrodynamic characteristics and presence of vortex dynamics in pseudo 2-dimensional microbridges make them attractive to design novel passive and active microwave circuits. Before such applications could be feasibly accomplished, a greater understanding of the the these devices are necessary, by a complete DC, microwave and optoelectronic characterization. A cryostat design and construction is discussed including the creation of test beds for DC characterization. Coplanar waveguide (CPW) design methodology is presented and used for the creation of CPWs for microwave characterization. Microbridges and meander lines are also embedded into the CPWs for determining the microwave performance of said devices and for optoelectronic characterizations. Results are compared against accepted results from theory and simulations, introducing vortices as explanations for device behaviour. Feasibility of these devices as single photon detectors is discussed.

With the use of grand canonical molecular dynamics, we studied the slow ompression(0.01m/s) of very thin liquid films made of equimolar mixtures of short and long alkane chains (hexane and hexadecane), and branched and unbranched alkanes (phytane and hexadecane). Besides comparing how these mixtures behave under constant speed compression, we will compare their properties with the behavior and structure of the pure systems undergoing the same type of slow compression. To understand the arrangement of the molecules inside the confinement, we present segmental and molecular density profiles, average length and orientation of the molecules inside well layered gaps. To observe the effects of the compression on the fluids, we present the number of confined molecules, the inlayer orientation, the solvation force and the inlayer diffusion coefficient, versus the thickness of the gap. We observe that pure hexadecane, although liquid at this temperature, starts presenting strong solid-like behavior when it is compressed to thicknesses under 3nm, while pure hexane and pure phytane continue to behave liquid-like except at 1.3nm when they show some weak solid-like features. When hexadecane is mixed with the short straight hexane, it remains liquid down to 2.8nm at which point this mixture behaves solid-like with an enhanced alignment of the long molecules not seen in its pure form; but when hexadecane is mixed with the branched phytane the system does not present the solid-like features seen when hexadecane is compressed pure.

No description available.

In this Thesis the magnetic and structural properties of thin epitaxial Fe, Co, and Ni films are discussed. Some of the in-situ prepared samples were used to characterize the degree of circular polarization of the newly installed beamline D1011 at MAX-lab. By means of x-ray magnetic circular dichroism (XMCD) and utilizing the associated magneto optic sum rules, the orbital (ml) and spin (ms) moments are determined directly in mB/atom with elemental specificity. The extended x-ray absorption fine structure (EXAFS) measurements yield site specific information on the local crystallographic structure. These measurements were performed using the circular x-rays of several beamlines. The influence of the degree of spatial source coherence lspat of the x-rays was characterized by means of Fresnel diffractometry. A correlation between enhanced XAS white line intensities and higher values of lspat was established for 20 ML Fe, Co, and Ni films on Cu(100). The degree of circularly polarized x-rays (Pc) at beamline D1011 at MAX-lab was characterized by studying Fe films on Cu(100) by means of XMCD. The maximum value of Pc is experimentally determined to Pc =0.85. The Au/Co/Au trilayer system was studied as a function of Co thickness, temperature, and Au cap thickness. A 10 mono-layer (ML) Co film, with an Au cap of 20 Å, shows a spin reorientation transition (SRT) from an in-plane to an out-of-plane easy direction as the temperature is lowered from 300 K to 200 K. The magnetic properities of these Co films are very different to what is found for bulk samples due to, in particular, the broken symmetry at the interfaces. The thickness dependent spin reorientation transition in the Fe/Ag(100) system was characterized by means of XMCD and EXAFS measurements. 3 ML Fe films show an out-of-plane easy direction with an 125% enhanced orbital moment as compared to the 25 ML Fe in-plane film. Simulations of the Fe L-edge EXAFS indicate the bulk Fe bcc structure for film thicknesses of 6-25 ML Fe. For 3 ML Fe strong deviations from this bcc phase is observed. Ultrathin Co films deposited on flat and vicinal Cu(111) in the thickness region 1-25 ML were studied by means of XMCD and scanning tunneling microscopy (STM). The vicinal Cu(111) Co deposition leads to the formation of elongated islands preferentially oriented along the step edges. In connection to this particular Co growth mode we observe an increase of both the orbital and the spin moment on the vicinal Cu(111) of about 25% relative to what was observed for Co on flat Cu(111).

The hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) are widely used as switching elements in LCD displays and large area matrix addressed senor arrays. In recent years, a-Si:H TFTs have been used as analog active components in OLED displays. However, a-Si:H TFTs exhibit a bias induced metastability. This problem causes both threshold voltage and subthreshold slope to shift with time when a gate bias is applied. These instabilities jeopardize the long-term performance of a-Si:H TFT circuits. Nevertheless a-Si:H TFTs show an exponential transfer characteristic in the subthreshold region. Moreover, the typical power consumptions for TFTs in the subthreshold region are in the order of nano-watts, thus making them suitable for low power design. For these reasons, a-Si:H TFT I-V characteristics in the forward subthreshold operation are investigated. First, we have derived the static and dynamic models of a-Si:H TFT in the forward subthreshold region. Second, we have verified our theoretical models with experimental results. Third, we have proven that a-Si:H TFT experiences no subthreshold slope degradation or threshold voltage shift in the forward subthreshold operation. Finally, we have studied a-Si:H TFT current mirror circuit applications. Measurements regarding the fidelity of current matching in the forward subthreshold region have been performed, and results are shown.

The recent developments of high resolution flat panel imagers have prompted interests in fabricating smaller on-pixel transistors to obtain higher fill factor and faster speed. This thesis presents fabrication and modeling of short channel amorphous silicon (a-Si:H) vertical thin film transistors (VTFT). <br /><br /> A variety of a-Si:H VTFTs with different channel lengths, from 100 nm to 1 &mu;m, are successfully fabricated using the discussed processing steps. Different structural and electrical characteristics of the fabricated device are measured. The results of I-V and C-V characteristics are comprehensively discussed. The 100 nm channel length transistor performance is diverged from regular long channel TFT characteristics, as the short channel effects become dominant in the device, giving rise to necessity of having a physical model to explain such effects. <br /><br /> An above threshold model for a-Si:H VTFT current characteristics is extracted. The transport mechanisms are explained and simulated for amorphous silicon material to be used in the device model. The final model shows good agreement with experimental results. However, we used numerical simulation, run in Medici, to further verify the model validity. Simulation allows us to vary different device and material parameters in order to optimize fabrication process for VTFT. The capacitance behavior of the device is extensively studied alongside with a TFT breakdown discussion.

Microfluidics is gaining in importance due to its wide ranging benefits and applicability in chemical and biological analysis. Although traditional microfluidic devices are created with glass or silicon based fabrication technologies, polymer based devices are gaining in popularity. Soft lithography and replica molding are techniques for the rapid prototyping of such devices, utilizing Polydimethylsiloxane (PDMS) as the dominant material. Other benefits include its low costs and ease of fabrication. Even though soft lithography is a well researched and developed fabrication process, new applications have been discovered in which the technology can be applied. Often, changes in the fabrication process are necessary for their application in other areas of research. This thesis will address several microfluidic applications using soft lithography. These areas of research include microfluidic thermometry, isoelectric focusing (IEF), and micromixers. In microfluidic thermometry, a novel thin film PDMS/Rhodamine B has been developed allowing whole-chip temperature measurements. In addition, compatibility problems between Rhodamine B and PDMS microfluidic devices were resolved. The thin film fabrication process, experimental results, and issues with its use are discussed. Future work and attempts at improving the thin film performance are also provided. IEF involves applications in which samples are separated according to its electrostatic charge. Two types of IEF applications are shown in which soft lithography has been shown to be beneficial to its development and performance. In isoelectric focusing with the use of thermally generated pH gradients, soft lithography allows for the rapid design, production and testing of different channel layouts. In general, due to PDMS insulation and overall low heat transfer rates, the temperatures detected are more gradual than those previously reported in literature. IEF using carrier ampholytes are also discussed, with preliminary results in which devices fabricated using soft lithography are compared to commercially available IEF cartridges. Its fabrication issues are discussed in detail. In micromixers, soft lithography fabrication issues and overall integration with flow mechanisms is discussed. In general it is difficult to perform mixing in the microscale due to the predominantly laminar flow and flow rate restrictions. Channel geometry is insignificant, as can be seen through numerical simulations.