Published: March 04, 2011 r2011 American Chemical Society 6968 dx.doi.org/10.1021/jp200822y | J. Phys. Chem. C 2011, 115, 6968–6974 ARTICLE pubs.acs.org/JPCC Tuning the Conduction Mechanism in Niobium-Doped Titania Nanoparticle Networks Hynek N emec,* Zolt  an Mics, Martin Kempa, and Petr Ku  zel Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 18221 Prague 8, Czech Republic Oliver Hayden Corporate Technology, Siemens AG, CT T DE HW3, Guenther-Scharowsky-Strasse 1, 91050 Erlangen, Germany Yujing Liu, Thomas Bein, and Dina Fattakhova-Rohlfing Department of Chemistry and Biochemistry, University of Munich, Butenandtstrasse 5-13, 81377 Munich, Germany 1. INTRODUCTION Nanoscaling and nanopatterning introduce additional func- tional properties to existing materials, which opens a way to the conception of novel devices and techniques. For example, fabrication of transparent conducting oxides in the form of nanoparticles can significantly enrich the scope of the available materials in addition to dense films, enabling manufacturing of conducting composites, nanostructured transparent electrodes, or low-temperature printing of patterned electrodes. However, the decrease of grain dimensions to the nanoscale increases the role of the surface, which dramatically alters the dielectric properties and electron transport in the nanoparticle-based materials. The measured macroscopic conductivity in a sample composed of assembled nanosized particles is influenced (besides the intrinsic bulk properties of the material) by several factors such as electron confinement effects, energy of surface states, difference in surface and core composition of nanoparti- cles, electron scattering on surface defects and on grain bound- aries, and connectivity of nanoparticles in the sample, just to name a few. The ability to resolve and characterize the individual factors controlling the total macroscopic conductivity is of extreme importance for the optimization of charge carrier transport properties in nanoscaled materials. Many of these factors can be assessed from the electromag- netic response measured in a broad frequency range. A very pertinent spectral domain for the investigation of nanoscaled materials is the terahertz (THz) range. First of all, different conductivity mechanisms lead to qualitatively different conduc- tivity spectra in the terahertz region, and it is straightforward to distinguish between the response of delocalized electrons (described, for example, by the Drude formula) and electrons localized in potential wells. 1,2 The electron confinement strongly affects the conductivity spectra if the particle size is comparable to or smaller than the electron diffusion length l D on the time scale of one period of the probing radiation [l D ≈ (D/f) 1/2 , where D is the diffusion coefficient and f is the probing frequency]. Terahertz frequencies are thus optimal for the investigation of electron transport within and among nanometer-sized particles of common semiconductors. 3 Finally, the measured conductivity spectra reflect the distribution of depolarization fields, which are inherently related to the morphology of the nanomaterial. 4,5 Received: January 26, 2011 Revised: February 15, 2011 ABSTRACT: Networks of niobium-doped TiO 2 anatase nano- particles with variable doping concentrations were investigated by time-domain terahertz spectroscopy and microwave impe- dance spectroscopy. A detailed description of their electromag- netic response is proposed; the model takes into account the depolarization fields of inhomogeneous samples and allows us to understand the conductive and dielectric response of in- dividual nanoparticles. We find that electron hopping is the dominating contribution to the conductivity at terahertz fre- quencies and that the dielectric losses of TiO 2 nanoparticles are enhanced in comparison with bulk anatase. The conductive properties of nanoparticles can be tuned via synthesis conditions and thermal posttreatment. In particular, annealing at elevated temperatures improves the nanoparticle crystallinity, reduces the density of structural defects, and enhances the conductive percolation of the network. The developed model of the conduction processes can be helpful for interpretation of charge transport in other semiconducting nanoscale materials.