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• Bonkerud, Julie; Zimmermann, Christian; Weiser, Philip Michael; Vines, Lasse & Monakhov, Eduard (2021). On the permittivity of titanium dioxide. Scientific Reports. 11. doi: 10.1038/s41598-021-92021-5. Full text in Research Archive Show summary

  Abstract Conductive rutile TiO 2 has received considerable attention recently due to multiple applications. However, the permittivity in conductive, reduced or doped TiO 2 appears to cause controversy with reported values in the range 100–10,000. In this work, we propose a method for measurements of the permittivity in conductive, n-type TiO 2 that involves: (i) hydrogen ion-implantation to form a donor concentration peak at a known depth, and (ii) capacitance–voltage measurements for donor profiling. We cannot confirm the claims stating an extremely high permittivity of single crystalline TiO 2 . On the contrary, the permittivity of conductive, reduced single crystalline TiO 2 is similar to that of insulating TiO 2 established previously, with a Curie–Weiss type temperature dependence and the values in the range 160–240 along with the c-axis.

• Bonkerud, Julie; Zimmermann, Christian; Weiser, Philip Michael; Aarholt, Thomas; Verhoeven, Espen Førdestrøm & Vines, Lasse [Show all 8 contributors for this article] (2020). Fabrication and characterization of Schottky barrier diodes on rutile TiO2. Materials Research Express. 7(6). doi: 10.1088/2053-1591/ab9777. Full text in Research Archive
• Bonkerud, Julie; Zimmermann, Christian; Herklotz, Frank; Weiser, Philip Michael; Seiffert, Christoph & Verhoeven, Espen Førdestrøm [Show all 8 contributors for this article] (2020). Electrically-active defects in reduced and hydrogenated rutile TiO2. Semiconductor Science and Technology. ISSN 0268-1242. 36(1). doi: 10.1088/1361-6641/abc854. Full text in Research Archive
• Weiser, Philip Michael; Zimmermann, Christian; Bonkerud, Julie; Vines, Lasse & Monakhov, Eduard (2020). Donors and polaronic absorption in rutile TiO2 single crystals. Journal of Applied Physics. ISSN 0021-8979. 128(14). doi: 10.1063/5.0027434. Full text in Research Archive Show summary

  We have used a combination of optical absorption and electrical conductivity measurements to study the effect of the main donor on small polarons in rutile TiO2 single crystals rendered n-type conductive by hydrogenation or doping with Nb. The electrical conductivity measured at 295 K for hydrogenated samples shows a clear correlation with the interstitial hydrogen (Hi) concentration, which is consistent with reports that Hi is the main shallow donor in rutile TiO2. Conductive samples exhibit two distinct optical absorption bands in the IR spectral region, at ω1 = 6500 cm−1 (∼0.8 eV) and ω2 = 3100 cm−1 (∼0.4 eV), which are present in both hydrogen-rich and Nb-doped samples. The intensities of the absorption bands are proportional to the electrical conductivity, and they exhibit an Arrhenius-like temperature dependence for temperatures between 25–50 K and 50–100 K for H-doped and Nb-doped samples, respectively. The thermal activation energies (EAs) for the absorption bands depend strongly on the main donor: ω2 exhibits EA(H) and EA(Nb) of ∼4 and ∼10 meV, respectively, whereas ω1 shows EA(H) and EA(Nb) of ∼1 and ∼2 meV, respectively. The combination of temperature-dependent data for the optical absorption bands and interstitial deuterium (Di)-small polaron vibrational lines support a model where the thermal activation is associated with the reconfiguration of small polarons involving Ti sites far away from the donor. The thermal activation of the optical absorption bands gives us insight into the dynamics of donor-dependent small polaron reconfiguration in rutile TiO2.

• Zimmermann, Christian; Bonkerud, Julie; Herklotz, Frank; Sky, Thomas Neset; Hupfer, Alexander & Monakhov, Eduard [Show all 8 contributors for this article] (2018). Influence of annealing atmosphere on formation of electrically-active defects in rutile TiO2. Journal of Applied Physics. ISSN 0021-8979. 123(16), p. 161572-1–161572-8. doi: 10.1063/1.5011136. Full text in Research Archive Show summary

  Electronic states in the upper part of the bandgap of reduced and/or hydrogenated n-type rutile TiO2 single crystals have been studied by means of thermal admittance and deep-level transient spectroscopy measurements. The studies were performed at sample temperatures between 28 and 300 K. The results reveal limited charge carrier freeze-out even at 28 K and evidence the existence of dominant shallow donors with ionization energies below 25 meV. Interstitial atomic hydrogen is considered to be a major contributor to these shallow donors, substantiated by infrared absorption measurements. Three defect energy levels with positions of about 70 meV, 95 meV, and 120 meV below the conduction band edge occur in all the studied samples, irrespective of the sample production batch and the post-growth heat treatment used. The origin of these levels is discussed in terms of electron polarons, intrinsic point defects, and/or common residual impurities, where especially interstitial titanium atoms, oxygen vacancies, and complexes involving Al atoms appear as likely candidates. In contrast, no common deep-level defect, exhibiting a charge state transition in the 200–700 meV range below the conduction band edge, is found in different samples. This may possibly indicate a strong influence on deep-level defects by the post-growth heat treatments employed.

• Arcadipane, E.; Sanz, R; Amiard, G.; Boninelli, S.; Impellizzeri, G & Privitera, V. [Show all 11 contributors for this article] (2016). Single-crystal TiO2 nanowires by seed assisted thermal oxidation of Ti foil: Synthesis and photocatalytic properties. RSC Advances. 6(60), p. 55490–55498. doi: 10.1039/c6ra09088e. Full text in Research Archive Show summary

  TiO2 nanowires (NWs) can improve the advantageous photocatalytic properties of TiO2 by increasing the active surface area. Here we investigate the synthesis of TiO2 NWs by thermal oxidation, studying the role of temperature, annealing time, and gas flow rates. The optimal thermal growth conditions were found to be 800 °C for 4 h in a mixed gas flow of Ar and O2. Morphological and structural characterizations, carried out by scanning and transmission electron microscopy, and X-ray diffraction, indicated a TiO2 rutile monocrystalline structure of the nanowires. An in situ thermal growth analysis was performed by means of an environmental electron microscope, providing additional insights into the TiO2 NWs growth dynamics. The photocatalytic properties were studied by using the degradation rate of methylene blue under UV light. TiO2 NWs revealed a 70% improvement of the degradation rate compared to a reference TiO2 bulk sample. Moreover, NWs were additionally annealed in forming gas (5% H2 in N2) in order to promote the formation of oxygen vacancies and an increase of the carrier density. Indeed, the photocatalytic activity of the NWs treated in forming gas was 3 times higher than that of the reference TiO2 bulk sample. The photonic efficiency and the quantum efficiency, also showed an increase versus bulk TiO2 of about 20% in the presence of NWs, and of about 100% with the forming gas annealing. Moreover, this increase in the photocatalytic activity after the forming gas annealing also correlates with the disappearance of a deep level recombination center as observed by deep level transient spectroscopy.

• Marcon, Valentina; Breiby, Dag Werner; Wojciech, Pisula; Dahl, Julie; Kirkpatrick, James & Patwardhan, Sameer [Show all 8 contributors for this article] (2009). Understanding Structure-Mobility Relations for Perylene Tetracarboxydiimide Derivatives. Journal of the American Chemical Society. ISSN 0002-7863. 131(32), p. 11426–11432. doi: 10.1021/ja900963v. Full text in Research Archive

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• Arcadipane, E; Sanz, R; Amiard, G; Boninelli, S; Impellizzeri, G & Privitera, V [Show all 11 contributors for this article] (2017). Correction: Single-crystal TiO2 nanowires by seed assisted thermal oxidation of Ti foil: synthesis and photocatalytic properties (RSC Adv. (2016) 6 (55490-55498) DOI: 10.1039/C6RA09088E. RSC Advances. 7(19), p. 11222–11222. doi: 10.1039/c7ra90019h. Full text in Research Archive

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