Forschung im HLRN-Verbund 2011

31Figure 1: Model for a subnanometer-sized anatase wire.by complexes which cause much smaller latticestrain than isolated acceptors. The investigation ofsuch phenomena will be carried out in the presentproject year. We intend to conclude our studies onbulk defects by investigating the very controversialissue of the intrinsic defects (oxygen vacancy andtitanium self-interstitial). These defects are of crucialimportance in explaining the room temperatureferromagnetism observed in thin anatase films.In the last phase of our investigations on TiO 2 ,we will concentrate on low-dimensional systems,besides thin films mainly on nanowires. In structureswith lower dimensionality, the large surfaceto volume ratio will change the energetic relationsbetween dopants and intrinsic defects, andnanostructures can be expected to be more flexiblein accommodating volume changes. Recentexperimental work has shown the existence of extremelythin anatase nanowires, and their capabilityto incorporate an unusually high amount of nitrogen.Our simulation of the HRTEM (high resolutiontransmission electron microscopy) picturesshow that the wire structure which was proposedby the experimentalists to explain their data, is notcorrect. Based on some preliminary calculations,we suggest another model, which is more in accordancewith the observed HRTEM picture. Usingour new models, we will investigate the nitrogenuptake and the effect of surface functionalizationon the electronic structure of anatase nanowires.Finally, we intend to use our results to establish aresearch direction into phase changes induced bydefect aggregation in TiO 2 . Such phenomena areat the heart of TiO 2 based memristors, to be usedfor high density storage with resistance switching(RRAM).More Information1. P. Deák, B. Aradi, T. Frauenheim, E. Janzén,and A. Gali, Phys. Rev. B 81, 153203 (2010).2. P. Deák, A. Gali, B. Aradi, and T. Frauenheim,in Advanced calculations for defects inmaterials, eds. A. Al-kauskas, P. Deák, J.Neugebauer, A. Pasquarello, and C. G. Van deWalle [Wiley-VCH, Berlin 2011] online at DOI:10.1002/pssb.2010462103. A. Gali, E. Janzén, P. Deák, G. Kresse, and E.Kaxiras, Phys. Rev. Lett. 103, 186404 (2009)FundingUniversity BremenChemie