Anna Szwajca

Zinc oxide as a defect-dominated material in thin films for photovoltaic applications – experimental determination of defect levels, quantification of composition, and construction of band diagram

In the present work, thin ZnO layers were synthesized by the sol-gel method with subsequent spin-coating on Si(100). We show that the detailed analysis of lab-recorded photoemission spectra in combination with Kelvin probe data yielded the work function, ionization energy, and valence band - Fermi level separation - and hence enabled the construction of band diagrams of the examined layers. With small modifications in preparation, very different films can be obtained. One set shows a homogeneous depth-dependent n carrier distribution, and another a significant carrier concentration gradient from n-type conductivity to almost metal-like n(+) character. Likewise, the surface morphology can be tuned from a uniform, compact surface with spherical single-nm sized grain-like features to a structured surface with 5-10 nm tall crystallites with (002) dominating crystal orientation. Based on the band-bending and the energy levels observed, defects of contradictory nature, i.e. acceptor-donor-trap (ADT) properties, were identified. These defects may be groups of point defects, with opposite character. The ADT states affect the energy levels of the oxide layers and due to their nature cannot be considered in the photoemission experiment as mutually independent. The versatile nature of the synthesis provides us with the opportunity to tune the properties with a high degree of freedom, at low processing costs, yielding layers with an exotic electronic structure. Such layers are interesting candidates for applications in photovoltaic and nanoelectronic devices.

Deprotonation of N-phenacyl- and N-acetonyl-4-cyanopyridinium halides with 1,4-diazabicyclo[2,2,2]octane

Abstract In N-phenacyl- and N-acetonyl-4-cyanopyridinium halides (1a·Br and 1b·Cl) the N+CH2 hydrogens are replaced by deuterium in D2O or CH3OD solutions, similarly as in β-diketones, proving a prototropic equilibrium 1⇌2. The rate constants have been measured for deprotonation of 1a·Br and 1b·Cl with 1,4-diazabicyclo[2,2,2]octane (DABCO) in solutions. UV, FTIR, 1H and 13C NMR spectra are consistent with the ylide structure of the deprotonated species. N-phenacyl-4-cyanopyridinium ylide (3a) is relatively stable in the solid state and unstable in solutions. The observed parallel changes in ΔS≠ and the time of disappearance of the ylide absorption band suggest that solvation is responsible for the ylide stability. The UV–Vis spectra of 1a·Br and 1b·Cl with DABCO show solvent dependent isosbestic points, which suggest that the decomposition reactions of ylides are very probably uniform. The PM3 and B3LYP calculations were carried out to investigate the structures of the cations, ylides and their complexes with ammonia and DABCO. Ylides are formed by abstraction of hydroxy proton from enol form (2) via complex 8.

Experimental and quantum chemical evidences for C-H· · ·N hydrogen bonds involving quaternary pyridinium salts and pyridinium ylides

Abstract The rates of equilibrium of eight quaternary pyridinium salts containing activated methylene groups with the corresponding ylides have been measured in DMSO solution containing strong bases (DBU, MTBD and P1-tBu). The effect of the ring substituents on the deprotonation rate is greater than that of the methylene substituents. This observation is consistent with the energy of C–H⋯N hydrogen bonds and the elongation of the C–H bond participating in hydrogen bonds estimated by the PM3 and BLYP/6-31G(d,p) calculations. The UV spectra and 1 H NMR spectra of selected ylides have been measured. The solvent effect on the proton chemical shifts of quaternary pyridinium halides and ylides is discussed. Semiempirical and DFT calculations were carried out to investigate the structure of ylides. In ylides, the α-pyridine protons are more acidic than the methine proton.

Fluorinated SAMs on Si(001) surface: Surface electronic properties and structural aspects

GRAPHICAL ABSTRACT Abstract Due to their insulating properties, small organic molecules deposited on the silicon surfaces, can be used as organic transistors. Since the molecular resistance is known to increase with increasing content of electron withdrawing groups, the self-assembled monolayers (SAMs) with the fluorocarbon moieties have been used to study the electronic properties of underlying solid substrate. Moreover, fluorinated SAMs may undergo charge transfer with leading materials designed for organic electronic devices. This minireview focuses on the structure of underlying silicon surface after wet-chemical routes towards the formation of silicon–organic monolayers and describes ways of potential changes in the electronic properties of covered silicon surface.

Synthesis, characterization, and thermal and surface properties of co- and terpolymers based on fluorinated α-methylstyrenes and styrene

Conventional bulk radical co- and terpolymerizations of α-fluoromethylstyrene (FMST) or/and α-trifluoromethylstyrene (TFMST) with styrene (ST) initiated by α,α′-azobis(isobutyronitrile) (AIBN) are presented. The resulting poly(F-ST-co-ST) copolymers and poly(TFMST-ter-ST-ter-FMST) terpolymers were characterized by 1H, 19F and/or 13C NMR spectroscopy that evidenced the incorporation of fluorinated α-methylstyrenes and enabled the assessment of the molar percentages of base units (in copolymers, 10.2–49.7 mol% of FMST and 10.6–48.3 mol% of TFMST and in terpolymers, F-ST mol% ranging between 5.2–38.4 and 3.7– 14.5 for FMST and TFMST, respectively). The molecular weights were in the range of 1500–23 700 g mol−1, 1500–14 600 g mol−1 and 6900–10 900 g mol−1 for poly(FMST-co-ST) and poly(TFMST-co-ST) copolymers and poly(TFMST-ter-ST-ter-FMST) terpolymers, respectively. The bulkier CF3 group induced a slightly lower reactivity of the TFMST comonomer. From the extended Kelen–Tudős (EK–T) linear method, the kinetics of the copolymerizations led to the determination of the reactivity ratios, ri, of both comonomers for each copolymerization system (rFMST = 0.08 ± 0.02 and rST = 0.72 ± 0.04, rTFMST = 0.00 and rST = 0.64 ± 0.01 at 70 °C), showing that F-ST monomers were less reactive than ST and retarded the rate of polymerization, and thus reduced the molecular weights. However, in the case of terpolymerizations where all three monomers were incorporated into polymeric chains, the retarding effects of F-STs were less noticeable, indicating that a termonomer induced copolymerization occurred. Finally, the thermal properties of these copolymers showed that the presence of fluorinated monomer units incorporated into the polystyrenic structure promoted an increase of the glass transition temperatures of the resulting copolymers up to 114 °C [poly(TFMST-co-ST) copolymer] and a slightly better thermal stability than that of polystyrene. Furthermore, the relationships between the surface structure of varied fluorinated copolymers and their wetting and oleophobic properties showed that below 80 mol% of styrene repeating units, the incorporation of 20–40 mol% of FMST or 10–20 mol% of TFMST in the copolymer structure caused a major change in the contact angle of the copolymers, reaching an unexpectedly high water contact angle of up to 147°.