The electrical conductivity of aluminium doped zinc oxide (AZO, ZnO:Al) materials depends on doping induced defects and grain structure. This study aims at relating macroscopic electrical conductivity of AZO nanoparticles with their atomic structure, which is non-trivial because the derived materials are heavily disordered and heterogeneous in nature. For this purpose we synthesized AZO nanoparticles with different doping levels and narrow size distribution by a microwave assisted polyol method followed by drying and a reductive treatment with forming gas. From these particles electrically conductive, optically transparent films were obtained by spin-coating. Characterization involved energy-dispersive X-ray analysis, wet chemical analysis, X-ray diffraction, electron microscopy and dynamic light scattering, which provided a basis for a detailed structural solid-state NMR study. A multinuclear ( 27 Al, 13 C, 1 H) spectroscopic investigation required a number of 1D MAS NMR and 2D MAS NMR techniques (T 1 -measurements, 27 Al-MQMAS, 27 Al-1 H 2D-PRESTO-III heteronuclear correlation spectroscopy), which were corroborated by quantum chemical calculations with an embedded cluster method (EEIM) at the DFT level. From the combined data we conclude that only a small part of the provided Al is incorporated into the ZnO structure by substitution of Zn. The related 27 Al NMR signal undergoes a Knight shift when the material is subjected to a reductive treatment with forming gas. At higher (formal) doping levels Al forms insulating (Al, H and C containing) side-phases, which cover the surface of the ZnO:Al particles and increase the sheet resistivity of spin-coated material. Moreover, calculated 27 Al quadrupole coupling constants serve as a spectroscopic fingerprint by which previously suggested point-defects can be identified and in their great majority be ruled out.

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In this contribution the preparation and structural characterization of nanoscale fluorine doped tin-oxide (SnO2:F, FTO) is described. By using a microwave assisted polyol approach, nanoparticles with different doping levels are prepared, which show narrow size distribution as measured by X-ray diffraction, electron microscopy and dynamic light scattering. They were converted into electrically conductive optically transparent films at 500 °C by a specific thermal treatment (500 °C in air followed by 250 °C in forming gas), exhibiting a specific resistivity of (1.9 × 10−1 Ω cm). Solid-state MAS NMR and 119Sn Mössbauer spectroscopy were used to study how F atoms are incorporated into the SnO2:F nanoparticles. Distance constraints were determined by 119Sn{19F} REDOR, fluorine-doping homogeneity by homonuclear dipolar recoupling experiments (SR66 2). Cross-polarization was used to investigate the immediate environment of the dopant. The experiments were supplemented by first-principles quantum-chemical calculations for possible defect site models. The combined data strongly indicate that F doping is not directly related to an increase in charge-carrier concentration, even though F atoms do occupy O vacancy sites in SnO2:F. For this study we have implemented background compensated NMR 2D pulse-sequences which reliably suppress the fluorine background originating from the NMR probe. Moreover we show that cluster calculations on the basis of the extended embedded ion method (EEIM) can be used to study the structure of diluted defects in crystalline host structures and predict NMR properties.

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