Different forms of ring deuterated polyaniline with different conductivity have been characterized by solid state deuteron nuclear magnetic resonance.;Quadrupole echo (QE) spectra of all forms of polyaniline consist of a superposition of lineshapes for nearly rigid aromatic rings and a small fraction of rings which undergo fast 180?? flips. The intensity of the fast flipping component is temperature dependent and different for conductive emeraldine salt (ES) and non-conductive emeraldine base (EB). This is a manifestation of the different structure and morphology of these polymers.;Simultaneous measurements of QE lineshapes and the relaxation time anisotropies allowed an accurate description of motion in polyanilines. Slow, small-angle libration in an asymmetric cone provided the best description for the "rigid" fractions of EB and ES. The broadening of deuteron QE lineshapes is consistent with the presence of a distribution of cone angles. Relaxation time measurements also reveal a relatively narrow distribution of librational rates for the EB sample. For ES, the magic angle spinning (MAS) spectra show the existence of two resolved signals with different relaxation rates, which are ascribed to microscopic domains with very different electrical properties. The unexpectedly short relaxation time found for nonconductive domains in ES can be explained by the presence of localized, unpaired electrons.;Spin count experiments proved that in highly conductive ES samples, loss of NMR signal intensity occurs not only because of high RF reflectance but also because of irreversible dephasing before signal acquisition due to interactions of nuclear spins with localized unpaired electrons.;Deuteron MAS spectra provided unique information about small frequency shifts. Compared to non-conductive EB, conductive emeraldine salts have an additional manifold of spinning sidebands, which is shifted ∼5.8 ppm towards higher frequencies. These shifted sidebands arise from quasi-metallic regions of the sample, where deuteron spins interact with delocalized electrons (Knight shift). The experimental temperature dependence of the intensity of the shifted peak can be explained using models developed for amorphous semiconductors. The observation of a Knight shift has an important consequence for the theory of electrical conduction in polyaniline: it implies that polarons are the charge carriers.

With the advent of thin film technology and more recently its applications in microelectronics and control of surface properties, the interest in mechanical properties of thin films has grown tremendously. Mechanical defects such as creep, fracture and adhesion loss, play a very important role in physical instabilities of thin film materials. An acoustic microscope has been built to study mechanical properties of thin-films. The microscope operates at a nominal frequency of 50 MHz. Rayleigh surface waves velocities on the surface of film-substrate systems were measured from V(z) curves generated by the acoustic microscope. V(z) curves are produced from interference between the Rayleigh surface wave and the specularly reflected waves. Technologically important materials, non-stoichiometric titanium nitride (TiN{dollar}\sb{lcub}\rm x{rcub}{dollar}) films and diamond films, were fabricated by using magnetron plasma deposition and hot filament chemical vapor deposition (HFCVD) on Si (100) and Si (111) substrates. Spectra from XPS (X-ray Photoelectron Spectroscopy) were used to determine the chemical composition of the films and SEM (Scanning Electron Microscope) micrographs were taken to study the morphology of the films. Rayleigh surface wave velocity measurements on TiN{dollar}\sb{lcub}\rm x{rcub}{dollar} films show a sharp increase in velocity at x = 0.7. A comparison with the phase diagram of TiN {dollar}\sb{lcub}\rm x{rcub}{dollar} suggests that the sharp increase in velocity might be due to a crystal structural transition from tetragonal {dollar}\varepsilon{dollar}-Ti{dollar}\sb2{dollar}N to fcc {dollar}\delta{dollar}-TiN.

Recent progress and a coordinated national research program have brought considerable effort to bear on the synthesis and application of carbon nanostructures for field emission. at the College of William and Mary, we have developed field emission arrays of vertically oriented graphene (carbon nanosheets, CNS) that have demonstrated promising cathode performance, delivering emission current densities up to 2 mA/mm2 and cathode lifetime > 800 hours. The work function ( & phis;) of CNS and other carbonaceous cathode materials has been reported to be &phis;∼4.5-5.1 eV. The application of low work function thin films can achieve several orders of magnitude enhancement of field emission.;Initially, the intrinsic CNS field emission was studied. The mean height of the CNS was observed to decrease as a function of operating time at a rate of ∼0.05 nm/h (I 1 ∼ 40 muA/mm2). The erosion mechanism was studied using a unique UHV diode design which allowed line-of-site assessment from the field emission region in the diode to the ion source of a mass spectrometer. The erosion of CNS was found to occur by impingement of hyperthermal H and O neutrals and ions generated at the surface oxide complex of the Cu anode by electron stimulated desorption. Techniques for minimizing this erosion are presented. ; The Mo2C (&phis;∼3.7 eV) beading on CNS at previously reported carbide formation temperatures of ∼800 ?? C was circumvented by physical vapor deposition of Mo and vacuum annealing at ∼300??C which resulted in a conformal Mo2C coating and stable field emission of 1 ∼ 50 muA/mm2. For a given applied field, the emission current was > 102 greater than uncoated CNS.;ThO2 thin film coatings were presumed to be even more promising because of a reported work function of &phis; ∼2.6 eV. The fundamental behavior of the initial oxidation of polycrystalline Th was studied in UHV (p < 1x10-11 Torr), followed by studies of thin film coatings on Ir and thermionic emission characteristics. Although a work function of 3.3 eV was determined by a RichardDushman plot, activation of the thin film was not achieved at T < 1700??C. Rather, the deposited ThO2 film decomposed, surface diffused and aggregated into stable ThO2(111) crystallites.;Thin film ThO2 coatings deposited on CNS initially demonstrated excellent field emission (up to ∼2 muA/mm2) and apparently activated spontaneously without significant thermal energy. Fowler-Nordheim plots suggested a work function of &phis; ∼2.6 eV. Undesired beading and ThO2 surface diffusion away from active emission sites resulted in rapidly deteriorating performance at higher field emission currents. Techniques that should provide a more stable ThO2/CNS conformal coating are presented.;The impact of thin films of Mo2C and ThO2on the magnitude of field emission from carbon nanosheets (CNS) was substantial. For a given field emission current density, J ∼2 muA/mm 2, the necessary applied field for uncoated CNS was ∼12 V/mum, but only ∼8 V/mum when coated with Mo2C (&phis;∼3.7 eV) and ∼5 V/mum when coated with ThO2 (&phis;∼2,6 eV). The mechanism for enhanced emission and the stability of the coatings are discussed, with special focus on the activation of ThO2 thin films. The major limitation observed in these studies has been the difference in surface energy of the graphene and the coatings which resulted in a tendency for the films to bead and separate from active emission sites at elevated currents. Suggested techniques to prevent this unwanted surface diffusion are presented.

An RFI plasma enhanced chemical vapor deposition (PECVD) system and a large-scale RF plasma source immersion ion implantation (PSII) system were designed and built to study two forms of 3-D surface processing, PECVD and PSII. Using the RFI PECVD system, Ti-6Al-4V substrates were coated with diamond-like carbon films with excellent tribological and optical properties. as an innovation, variable angle spectroscopic ellipsometry (VASE) was successfully applied for non-destructive, 3-D, large-area tribological coatings quality investigation.;Based on the experience with the RFI PECVD system, a large-scale RFICP source was designed and built for the PSIL Langmuir probe and optical emission spectroscopy studies indicated that the RFI source produced stable, uniform, and clean plasma. MAGIC code was for the first time used to model PSII process, addressing different target geometries and boundaries, materials, plasma parameters, illustrated sheath formation and evolution, field distribution, ion and electron trajectories, ion incident angles, and dose distributions, which are critical for PSII design and understanding.;The RF PSII system was developed into a versatile large-area, uniform, 3-D surface processing apparatus, capable of PSII, PVD, PECVD, and in situ surface cleaning and interface properties modification, for multilayer, multi-step, and high performance surface engineering. Using the RFI PSII system, for the first time, PSII was studied as a mask-based surface layer conversion technique, for pattern writing by implantation as an alternative to current deposition-based and ink-based direct write technologies. It operates at low substrate temperature, keeps the original surface finish and dimensions, and avoids adhesion problem. A different operating mode of the RF source was discovered to perform biased sputtering of high purity quartz, which turned the RFI PSII system into a novel integrated RF PSII/PVD system for large-area, uniform, nitrogen-doped, and hydrogen-free SiO2 films deposition at low substrate temperatures. Nitrogen-doped SiO2 films with excellent optical properties were deposited on semiconductor, metal, and polymer substrates with excellent adhesion. Ellipsometry was used again for non-destructive SiO2 coatings investigation. FEL test electrodes processed by PSII/PVD showed suppressed field emission. A group of transition metals and an FEL test electrode were also implanted by nitrogen using the PSII mode and analyzed.

Group III-Nitride semiconductors: indium nitride (InN), gallium nitride (GaN), aluminum nitride (AlN) and their alloys continue to attract significant scientific interest due to their unique properties and diverse applications in photonic and electronic applications. Group-III nitrides have direct bandgaps which cover the entire spectral range from the infrared (InN) to the ultraviolet (GaN) and to the deep ultraviolet (AlN). This makes III-nitride materials suitable for high-efficient and energy-saving optoelectronic devices, such as light-emitting diodes (LEDs) and laser diodes (LDs). The Nobel Prize in Physics 2014 was awarded for the invention of efficient GaN blue LEDs, which further accelerated the research in the field of group III-nitride materials. GaN and related alloys are also suitable for high-temperature, high-power and high-frequency electronic devices with performance that cannot be delivered by other semiconductor technologies such as silicon (Si) and gallium arsenide (GaAs). For example, GaN-based high electron mobility transistors (HEMTs) have been widely adopted for radio frequency (RF) communication and power amplifiers, high-voltage power switches in radars, satellites, and wireless base stations for 5G.  Recently, nitrogen (N)-polar group-III nitrides have drawn much attention due to their advantages over their metal-polar counterparts in e.g. HEMTs. These include feasibility to fabricate ohmic contacts with low resistance, an enhanced carrier confinement with a natural back barrier, and improved device scalability. Despite intensive research, the growth of micrometer-thick high-quality N-polar GaN based materials remains challenging. One of the major problems to develop device-quality N-polar nitrides is the high surface roughness, which results from the formation of hexagonal hillocks or step-bunching. Another significant hurdle is the unintentional polarity inversion, which reduces the crystalline quality and prohibits device fabrication.  This licentiate thesis focuses on the development of N-polar AlN and GaN heterostructures on SiC substrates for HEMT RF applications. The overall aim is to exploit the advantages of the hot-wall MOCVD concept to grow high-quality N-polar HEMT structures for higher operational frequencies and improved device performance. In order to achieve this goal, special effort is dedicated to understanding the effects of growth conditions and substrate orientation on the structural properties and polarity of AlN, GaN and AlGaN grown by hot-wall MOCVD. N-polar AlN nucleation layers (NLs) with layer by layer growth mode and step-flow growth mode can be achieved on on-axis and 4_ offaxis SiC (000¯1), respectively, by carefully controlling V/III ratio and growth temperature. Utilizing scanning transmission electron microscopy (STEM) we have established a comprehensive picture of the atomic arrangements, local polarity and polarity evolution in AlN, GaN/AlN and AlGaN/GaN/AlN in the cases of low-temperature and high-temperature AlN NLs both for on-axis and off-axis substrates. We have shown that typically employed methods for polarity determination using potassium hydroxide wet etching could not provide conclusive results in the case of mixed-polar AlN as Al-polar domains may be easily over-etched and remain undetected. Atomic scale electron microscopy is therefore needed to accurately determine the polarity. We further have developed growth strategy and have optimized the epitaxial process for N-polar GaN, and have demonstrated high quality N-polar AlGaN/GaN/AlN heterostructures. / <p>Additional funding agencies: Chalmers University of technology; ABB; Ericsson; Epiluvac; FMV; Gotmic; Saab; SweGaN; UMS; Swedish Foundation for Strategic Research under Grants No. FL12-0181, No. RIF14-055, and No. EM16-0024; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University, Faculty Grant SFO Mat LiU No.2009- 00971.</p>

This thesis was directed at learning more about the unusual electronic environment near hydrogen within strong hydrogen bonds. "Strong" hydrogen bonds are unique in that the hydrogen atom is symmetrically located, or nearly so, between two electronegative atoms; the bond energies are relatively large. In a "normal" hydrogen bond the hydrogen atom is bonded to, and thus physically closer to, a parent atom, and only weakly attracted to another electronegative atom; bond energies are typically small. To examine these bonds, deuterium was substituted for hydrogen and the electric quadrupole coupling constant (QCC) of deuterium was measured using field cycling nuclear magnetic resonance. The electric quadrupole moment of deuterium is sensitive to changes in the surrounding electric field gradient, and is thus a good probe of the immediate electronic structure. The results show that the temperature dependence of the QCC is opposite to, and much larger than, what one would normally expect to observe for deuterium. The QCC is found to decrease strongly with decreasing temperature. This project was the first to study in detail the temperature dependence of deuterium QCCs in strong hydrogen bonds. The magnitude of the deuterium QCCs for the diacetates was found to be strongly depressed relative to typical values for deuterium. These results parallel large shifts in the infrared vibrational frequencies observed in many molecules which contain strong hydrogen bonds. The asymmetry parameter, which is a measure of the departure from axial symmetry of the electric field gradient (EFG) at deuterium, was found to be unusually large for what are known to be linear, or nearly linear, three-center bonds. Based on ab initio Hartree-Fock calculations aimed at determining the EFG at H in the archetypal bifluoride ion, F-H-F$\sp-$, the electronic charge density is drastically depleted at H. It is believed that the large reduction in the charge density allows the deuterium EFG to be highly sensitive to the shape of the charge distribution on the atoms to which deuterium is bonded. If these atoms are at points of low crystallographic symmetry, the polarization of these adjacent atoms by other nearby atoms may cause the EFG to depart substantially from being axially symmetric. Also obtained from the molecular orbital calculations for bifluoride ion were the total electronic energy and the electric field gradient at H. From these calculations potential function models for the asymmetric stretch and the bend were constructed. An attempt was made to correlate the predictions made by these models for the temperature dependence of the deuteron quadrupole coupling constant in bifluoride ion with the experimentally observed results for the diacetates.

No description available.

Formamidinium lead iodide perovskite (FAPI) holds promise to be used in high–efficiency solar cells. A comprehensive understanding of the immediate surface properties of this material can provide insights into its interaction with other materials and guide future engineering of its interface with other constituents of the solar cell. In this thesis, the orientations of the formamidinium cations on flat FAI and PbI2 terminated (001) surfaces of the of α–FAPI were studied with classical and ab initio molecular dynamics simulations at a temperature of 300 K. The cations on the FAI terminated surface displayed a preference to lie flat on the surface, while the cations near the PbI2 terminated surface displayed a preference to be oriented in such a way that the molecular planes were perpendicular to the surface. Average near edge X–ray fine structure (NEXAFS) spectra at the carbon and nitrogen K–edges were simulated from the trajectories of the FAI terminated structure and compared with existing experimental spectra obtained from a clean FAPI surface under ultrahigh vacuum. By comparing the experimental and calculated NEXAFS spectra it was observed that the distribution of formamidinium orientations which contribute to the experimental NEXAFS spectra is different from the one seen in the outermost FAI layer from the molecular dynamics simulation. This observation can be explained by the finite probing depth of the X–ray or the fact that the surface is not perfectly FAI terminated in the experiment. The more uniform distribution of formamidinium orientations one layer below the surface resulted in simulated NEXAFS spectra which were more similar to the experimental ones.

Two stable molecules, A and B, often form a stable dimer system, AB$\sp+$, when one of the neutral molecules is ionised. In certain regards, these molecules parallel van der Waals molecules except that the ion-dipole or ion-induced dipole forces play a key role. In the first part of this study we attempt to determine the binding energies, equilibrium geometries, and spectroscopic information for certain sub-classes of bimolecular cations at various post Hartree-Fock levels of theory. Firstly, we studied rare gas dimer cations using standard theoretical methods, e.g. various configuration interaction and Moller-Plesset techniques, where extensive comparison with experimental results and with the results of 'benchmark' post Hartree-Fock methods is made. This comparison has the objective of calibrating the methods used before embarking on calculations of other bimolecular cations for which no information is available. This computational methods are used to study a sequence of bimolecular cations consisting of a rare gas atom and a molecular cation. In particular, we considered the following sequence of molecules: NeHF$\sp+$, NeHCl$\sp+$, NeH$\sb2$O$\sp+$, ArHF$\sp+$, ArHCl$\sp+$, and NeH$\sb2$O$\sp+$. On the second part of this study molecular dynamics simulation are used to investigate the trapping of simple spherical species in rare gas matrices at low temperature. The present study examines the nature of the matrix/host material (i.e. crystalline, amorphous, or polycrystalline) and especially explores the site(s) of the trapped species in the matrix. Also of interest is the nature and degree of the matrix distortion near the trapped molecule, and how is the molecular structure affected by the size of the trapped molecule. At the same time, we intend to bring specific structural details of solid state physics to play a major role in the matrix isolation studies, which have primarily devoted little effort to a full characterization of the matrix. The molecular dynamics techniques used seeks to reproduce details of actual experimental deposition methods, i.e. freezing of the sample, slow spray-on deposition, and pulsed deposition.

Quantum spin liquids (QSLs) may roughly be defined as states possessing sufficiently high quantum fluctuations that they impede long range magnetic order. Various electron interactions are currently being studied in order to physically realize such states in condensed matter systems since they can host fractionalized excitations. The purpose of our study is to examine two interactions established as important in the literature while not having been paired together. We consider a bond-dependant $J$-$\Gamma$ ladder, comprised of an alternating symmetric exchange of spin components, mediated by $\Gamma$, along with a Heisenberg interaction controlled by $J$. By parameterizing these couplings by an angle $\phi$, we produce a phase diagram of the system using the Infinite Density Matrix Renormalization Group (iDMRG) numerical technique. In order to classify the phases, we search for discontinuities in the entanglement spectrum for bonds along one of the legs and the rungs of the ladder while also looking at divergences in the susceptibility of the energy. These criteria reveal a possible 10 phases hosted by the system, with 7 of them showing some form of magnetic ordering seen directly from the spin correlations and by applying magnetic fields in appropriate directions. Moreover, known points in the phase diagram can be adiabatically connected to other points within the same phase by tuning $J$ or $\Gamma$. The remaining three phases however show no obvious long-range magnetic order while also having large contributions to the entanglement spectrum. Such phases, showing interesting initial signs, are discussed further in our study. / Thesis / Master of Science (MSc)