Péter Ágoston

Size-Dependent Lattice Expansion in Nanoparticles: Reality or Anomaly?

Size-dependent lattice expansion of nanoparticles is observed for many ionic compounds, including metal oxides, while lattice contraction prevails for pure metals. However, the physical origin of this effect, which is of importance for the thermodynamic, chemical and electronic properties of nanoparticles, is discussed controversially. After a survey of the experimental literature, revealing a wide variety of materials with size-dependent lattice expansion, we show that the negative surface stress is the key reason for lattice expansion, while the excess of lattice sums or point defects of various charge states can be excluded as general explanations. Ab initio calculations of surface stresses for various surface structures of metal oxides confirm the model of a surface-induced lattice expansion.

Geometry, electronic structure and thermodynamic stability of intrinsic point defects in indium oxide.

Intrinsic point defects in indium oxide, including vacancies, interstitials as well as antisites, are studied by means of first-principles calculations within density functional theory using the generalized gradient approximation together with on-site corrections. Finite-size effects are corrected by an extrapolation procedure in order to obtain defect formation energies at infinite dilution. The results show that all intrinsic donor defects have shallow states and are capable of producing free electrons in the conduction band. This applies in particular to the oxygen vacancy. Since it has also a low formation energy, we find that the oxygen vacancy should be the major donor in this material explaining the n-type conductivity as well as the non-stoichiometry of indium oxide. In addition, we show that there are a wealth of oxygen dumbbell-like defects which are thermodynamically relevant under oxidizing conditions. Finally, we discuss defect induced changes of the electronic structure.

Limits for n-type doping in In2O3 and SnO2: A theoretical approach by first-principles calculations using hybrid-functional methodology

The intrinsic n-type doping limits of tin oxide (SnO2) and indium oxide (In2O3) are predicted on the basis of formation energies calculated by the density-functional theory using the hybrid-functional methodology. The results show that SnO2 allows for a higher n-type doping level than In2O3. While n-type doping is intrinsically limited by compensating acceptor defects in In2O3, the experimentally measured lower conductivities in SnO2-related materials are not a result of intrinsic limits. Our results suggest that by using appropriate dopants in SnO2 higher conductivities similar to In2O3 should be attainable.

Orientation dependent ionization potential of In2O3: A natural source for inhomogeneous barrier formation at electrode interfaces in organic electronics

The ionization potentials of In(2)O(3) films grown epitaxially by magnetron sputtering on Y-stabilized ZrO(2) substrates with (100) and (111) surface orientation are determined using photoelectron spectroscopy. Epitaxial growth is verified using x-ray diffraction. The observed ionization potentials, which directly affect the work functions, are in good agreement with ab initio calculations using density functional theory. While the (111) surface exhibits a stable surface termination with an ionization potential of ∼ 7.0 eV, the surface termination and the ionization potential of the (100) surface depend strongly on the oxygen chemical potential. With the given deposition conditions an ionization potential of ∼ 7.7 eV is obtained, which is attributed to a surface termination stabilized by oxygen dimers. This orientation dependence also explains the lower ionization potentials observed for In(2)O(3) compared to Sn-doped In(2)O(3) (ITO) (Klein et al 2009 Thin Solid Films 518 1197-203). Due to the orientation dependent ionization potential, a polycrystalline ITO film will exhibit a laterally varying work function, which results in an inhomogeneous charge injection into organic semiconductors when used as electrode material. The variation of work function will become even more pronounced when oxygen plasma or UV-ozone treatments are performed, as an oxidation of the surface is only possible for the (100) surface. The influence of the deposition technique on the formation of stable surface terminations is also discussed.