O2 sensing dynamics of BiFeO3 nanofibers: effect of minor carrier compensation.

In this paper we investigate O(2) sensing dynamics in BiFeO(3) (BFO) nanofibers at various concentrations and temperatures, by using a combined experiment and computer simulation approach. Samples of pristine BFO, Ni-doped BFO, and Pb-doped BFO nanofibers were prepared. By incorporating Ni and Pb, additional acceptor states are introduced in BFO. Density functional theory calculations show that Ni prefers to substitute Fe site while Pb substitutes Bi site, resulting in a new deep donor originating from Ni interstitial defects, along with oxygen vacancies (V(o)). We find that both the sensing response and recovery time are shorter in samples made of pristine BFO nanofibers than in Ni- and Pb-doped nanofiber samples. We interpret the observed sensing dynamics through charge transport theory of the major (acceptors) and minor (donors) carriers, and found that the minor carrier compensation plays a significant role in determining the response and recovery time of the sensor device. This minor carrier compensation charge transport mechanism will provide new insights into more robust sensor development strategies, and into the research of ion-electron coupling in chemical dynamics of semiconductors.

Transition metal-doped BiFeO3 nanofibers: forecasting the conductivity limit.

We investigate the limiting electrical conductivity of BiFeO3 (BFO) nanofibers via first-principles modelling and experiments. Based on a semi-empirical approach, all transition metals are first screened for their suitability to form an acceptor in BFO. The resultant candidates (e.g., Ni, Cu and Ag) are further studied by more sophisticated electronic structure theory and experiments. Accordingly, a systematic approach in forecasting the electrical conduction in BFO nanofibers is established. The calculated results show that Ag(+) cations prefer substitutions of Bi(3+) while Ni(2+) and Cu(2+) prefer substitution of Fe(3+) sites to form acceptors. All three metals contribute to an increased overall hole concentration which may lead to a conductivity limit in BFO. These predictions were confirmed consistently through the synthesis and electrical testing of Ni-, Cu- and Ag-doped BFO nanofibers. Finally, our results indicate that the conductivity limit is approached by Ni doping in BFO. The methodology presented here may be extended to search for the doping conductivity limits of other semiconductors of interest.

The role of Bi vacancies in the electrical conduction of BiFeO3: a first-principles approach.

We employ first-principles methods to study the mechanism controlling the electrical conduction in BiFeO3 (BFO). We find that under oxygen-rich conditions, Bi vacancies (V(Bi)) have lower defect formation energy than O vacancies (V(O)) (-0.43 eV vs. 3.35 eV), suggesting that V(Bi) are the acceptor defects and control the conductivity of BFO, making it a p-type semiconductor. In order to obtain further insight into the conduction mechanism, we calculate the effect of donor (Sn(4+)) and acceptor (Pb(2+)) impurities in BFO. Results indicate that Sn impurities prefer to substitute Fe sites to form shallow donor defects, which compensate the acceptor levels derived from V(Bi). Meanwhile, Pb atoms favour the substitution of Bi sites to form acceptor defects, reducing the overall concentration of holes (h(+)). Theoretical findings were later surveyed by current-voltage characteristics of Sn- or Pb-doped BFO nanofibers. This study is of general interest in carrier transport in charge compensation semiconductors, and of particular relevance within the context of defect-mediated conductivity in BFO.

Enhanced stability of gold colloids produced by femtosecond laser synthesis in aqueous solution of CTAB.

Femtosecond laser ablation of gold in an aqueous solution of cetyl trimethylammonium bromide (CTAB) is shown to produce nanoparticle suspensions with superior colloidal stability compared to other surfactants, with shelf lives exceeding 2 months even at low concentrations of CTAB, below 1 mM. CTAB also helps control nanoparticle size with mean diameters of 6.3, 5.6, and 4.7 nm obtained in 0.1, 0.5, and 1 mM concentrations of CTAB respectively, compared to 11.9 nm obtained in pure deionized water under same ablation conditions. The size distributions produced with low concentrations of CTAB are comparable to those produced by other surfactants, typically used at high concentrations.