DOI: 10.1021/cg1008297
Quantum Mechanically Guided Design of Transition Metal Alloyed RuO2 Nanorods
2010, Vol. 10 4531–4536
Denis Music,* Felix H.-U. Basse, and Jochen M. Schneider Materials Chemistry, RWTH Aachen University, D-52056 Aachen, Germany Received June 22, 2010; Revised Manuscript Received August 19, 2010
ABSTRACT: Using ab initio calculations, we have probed the effect of all 4d transition metals as potential alloying elements for RuO2 (space group P42/mnm, prototype rutile) on the phase stability and Seebeck coefficient. Nb additions were identified to increase both the phase stability and the Seebeck coefficient. Based on this design proposal, the ternary compound was synthesized by combinatorial reactive sputtering. X-ray diffraction data suggest that Nb is incorporated in the rutile structure with the solubility limit in the range 3-4 atom %. Nanorod formation is observed at Nb contents g2.9 atom %. Rutile coordination is present in all specimen, while with increasing Nb content Nb2O5 coordination appears. This may be understood based on our ab initio molecular dynamics simulations. Surface coarsening on the atomic scale occurs due to O cross-linking of two neighboring NbO6 octahedra. Hence, it is reasonable to assume that NbO6 octahedra contribute toward the experimentally observed formation of nanorods.
1. Introduction RuO2 (space group P42/mnm, prototype rutile) exhibits interesting transport properties, such as low resistivity of 40 μΩ cm as well as large thermal and chemical stability.1 RuO2 is used in microelectrical devices, such as electrodes for high dielectric thin films.1 Nanostructured RuO2 was suggested to be a candidate for interconnects and optoelectronics.2 The Seebeck coefficient of RuO2 was reported to be 4 μV/K at room temperature,3 and when doped with 2 mol % BaO, the value is 90 μV/K at 700 °C.4 Hence, RuO2 is also a promising candidate for thermoelectric devices. Lately, substantial efficiency improvements have been achieved by nanostructuring.2,5 Rutile RuO2 thin films have been reported to form nanorods.3,6,7 The proposed mechanism is the evaporation of Ru hyperoxides, such as RuO3 and RuO4, which act as precursors for nanorod formation.3,6 This was confirmed by our classical molecular dynamics (MD) simulations.3 Furthermore, based on ab initio calculations,3 two effects of the nanorod formation on the Seebeck coefficient were observed: (i) increase due to additional states in the vicinity of the Fermi level and (ii) slight decrease due to oxygen point defects (volatile species). It is clear that doping/alloying enhances the transport properties of RuO2. However, this is not known if doping/alloying can be combined with beneficial effects of nanostructuring. In this work, it is our ambition to identify suitable alloying elements for RuO2, based on quantum mechanically guided design, which improve the transport properties and phase stability. Using ab initio calculations, we probe all 4d transition metals and identify Nb to be the best choice. Based on this design proposal, Nb alloyed RuO2 thin films are grown by combinatorial reactive sputtering. Nb is incorporated in the rutile structure. Nanorods are formed and Nb2O5 coordination appears at Nb contents g2.9 atom %. This may be understood based on our ab initio MD data. Surface coarsening on the atomic scale occurs due to O cross-linking of two
neighboring NbO6 octahedra. Hence, it is reasonable to assume that NbO6 octahedra contribute toward the experimentally observed formation of nanorods. 2. Theoretical Methods 2.1. Ab Initio Calculations. The theoretical study was carried out in two parts. First ab initio calculations were performed and then ab initio MD simulations were made. For the ab initio part of our study, density functional theory was used,8 as implemented in the Vienna ab initio simulation package (VASP),9,10 wherein the projector augmented wave potentials with the generalized gradient approximation are employed.11-13 The following parameters were applied: the convergence criterion for the total energy of 0.01 meV, Bl€ ochl corrections for the total energy,11 a cutoff of 500 eV, and integration in the Brillouin zone according to Monkhorst-Pack14 with 5 5 5 k-points. RuO2 supercells (2 2 2) with 48 atoms (1 Ru atom replaced with 4d elements) were relaxed with respect to atomic positions and cell volumes. The energy of formation was calculated using the following expression: RuO2 þ 0:06M f Ru0:94 M0:06 O2 þ 0:06Ru
ð1Þ
*To whom correspondence should be addressed. E-mail: music@ mch.rwth-aachen.de. Fax: þ49 241 80 22295.
where M stands for 4d elements, which were relaxed here within the following space groups: P63/mmc (Y, Zr, Tc, Ru, Cd), Im3m (Nb, Mo), and Fm3m (Pd, Ag). For all transition metals probed, the equilibrium lattice parameters and bulk moduli are
