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<p dir="ltr">Molecular materials exhibiting bistability between two states are intriguing candidates for next generation electronic devices. Two similar classes of materials, known as spin crossover (SCO) and valence tautomers (VT) respectively, are of particular interest due to their multifunctional properties, which are controllable via several external parameters, such as temperature, light irradiation, pressure, magnetic field, and electric field. In recent years, considerable research has been dedicated to better understanding the underlying principles that govern the behavior of these materials, so that their implementation into nano-based devices might be achieved.</p><p dir="ltr">In this report, a systematic study of the valence tautomer molecule [Co(sq)(cat)(3-tpp)₂] is presented. In the first chapter, the phenomenon of valence tautomerism (VT) occurring in coordination compounds is introduced and described from the perspective of Crystal Field Theory (CFT). Further, the molecular structure and physical properties of the [Co(sq)(cat)(3-tpp)₂] molecule are explored. The properties of the ferroelectric material Polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), and the 2-D Mxene Ti₃C₂ are also discussed.</p><p dir="ltr">The next section details equipment development and experimental methods. Thin films of VT molecules were prepared from solution via a drop-casting approach. For thin film analysis, we have developed a custom made, fully automated Vibrating Sample Magnetometer (VSM) with a sensitivity on the order of 1 × 10^-5 emu, as well as a fully automated, variable temperature, under vacuum electron transport stage, and a magneto-optic Kerr effect apparatus (MOKE). Additional experimental methods used to characterize the VT thin films include X-ray Absorption Spectroscopy (XAS), UV-visible Spectrometry (UV-Vis) and Differential Scanning Calorimetry. Experimental results obtained from these techniques are discussed and analyzed in the third section. PVDF-HFP polarization dependent isothermal spin state switching of [Co(sq)(cat)(3-tpp)₂] is also discussed as well as the effects of doping [Co(sq)(cat)(3-tpp)₂] with Ti₃C₂, followed by a conclusion and an outline of future work.</p>

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In this report, the operation of a normally-off vertical gallium nitride (GaN) metal-oxide field effect transistor with a threshold voltage of 5 V is demonstrated. A crucial step during device fabrication is the formation of the highly n-doped source layer. The authors infer that the use of molecular beam epitaxy (MBE) is highly beneficial for suppressing diffusion of the magnesium (Mg) p-type dopants from the body layer grown by metal-organic vapor phase epitaxy into the source cap. Repassivation of the previously activated Mg acceptors by a hydrogen out-diffusion treatment is suppressed in the ultrahigh vacuum growth environment. Structural and electrical data indicate that the defect density of the GaN substrate is currently limiting device performance much more compared to other effects like varying surface morphology resulting from fluctuations in III/N stoichiometry during the MBE growth.

The predicted acceptor impurity nature of carbon in hexagonal GaN grown by molecular-beam epitaxy (MBE) is revisited spectroscopically in the energy range between 1.6 and 3.5 eV. Photoluminescence (PL) spectra from ultra-pure GaN and material doped with carbon at a level of 1·18 cm⁻³ differ significantly in the energy range between 3.0 and 3.3 eV depending on the Ga/N stoichiometry during MBE growth. Acceptor-like features formerly attributed to carbon, beryllium or magnesium incorporation are found for both, undoped and carbon-doped GaN. The intensity of these features depends on the Ga/N stoichiometry during growth. While for Ga-lean surface regions, exhibiting multiple 10 nm deep pits, the observed PL features are found to be less intense compared to Ga-rich surface regions, the situation reverses for carbon-doped material. For all samples, the intensity of the yellow luminescence band around 2.2 eV is weak. The results point at crystal defects and the unintentionally present oxygen as the origin of the spectroscopic features traditionally attributed to carbon in GaN.