High‐Temperature Photochromism of Fe‐Doped SrTiO₃ Caused by UV‐Induced Bulk Stoichiometry Changes

Abstract

The impact of UV irradiation on Fe-doped SrTiO 3 (Fe:STO) single crystals is investigated at elevated temperatures. Illumination leads to incorporation of oxygen into the single crystals and thus to a decreasing oxygen vacancy concentration and oxidation of Fe 3+ to Fe 4+ . The Fe 4+ ions cause a color change from transparent/brownish to black. This photo chromic blackening due to stoichiometry changes at elevated temperatures is irreversible at room temperature, but annealing at high temperatures, for example at 700 °C, can restore the original stoi chiometry and color. Absorbance changes due to UV irradiation are monitored by ex situ and in situ UV-vis spectroscopy experiments and changes in electrical properties are measured by van der Pauw measurements and in-plane electrochemical impedance spectroscopy. After 1140 min of illumination at 440 °C, for example, electrical measurements reveal a conductivity increase by more than a factor of 5 due to the enhanced hole concentration in blackened Fe:STO. In addition, UV illumination increases the oxygen chemical potential up to a calculated p(O 2 ) of more than 10 9 Pa in Fe:STO. Hence, UV light can be used to tune the color, but also electrical properties of Fe:STO by directly impacting the bulk defect concentrations.

Cation non-stoichiometry in Fe:SrTiO3 thin films and its effect on the electrical conductivity

“…The polycrystalline targets were prepared by a mixed oxide route, starting with SrCO 3 , TiO 2 and Fe 2 O 3 (Sigma Aldrich, Germany). Stoichiometric as well as nonstoichiometric (3,5,7, and 11 % Sr excess compared to the sum of Fe and Ti) amounts of the solids were accurately weighed, thoroughly mixed in an agate mortar and calcined at 1000 °C for 2 h under ambient conditions. For a stoichiometric cation composition (i.e.…”

“…SrTiO 3 (STO) single crystals and thin films are currently a cornerstone for the study of multiple phenomena such as resistive switching in memristive devices [1][2][3][4], photoconductivity [5], increased oxygen incorporation under UV light [6,7], photovoltages in bulk SrTiO 3 [8,9], and interfacial conductivity, especially with LaAlO 3 [10,11]. However, SrTiO 3 thin films are not fully understood yet and show great variability in prpertiesdepending on the fabrication conditions.…”

Impact of Oxygen Non‐Stoichiometry on Near‐Ambient Temperature Ionic Mobility in Polaronic Mixed‐Ionic‐Electronic Conducting Thin Films

“…[21,22] While this mechanism was regarded as an aging effect, determining the lifetime of capacitors or gate dielectrics, progress in nanometric processing methods has enabled the design of novel devices that take advantage of the mobile ionic species, with versatile functionalities that cannot be achieved solely by electronic effects. New memory devices based on ionic defect motion have emerged, for example, in the fields of electronics (e.g., memristors), [12,[23][24][25][26][27][28][29] magnetics (e.g., magnetoionics), [30][31][32] optics (e.g., electrochromic devices), [33][34][35][36] and ferroelectrics, [37][38][39] often matching the performance of their electronic counterparts, at lower energy consumption and smaller device footprint. Moreover, these new devices are considered as promising candidates for progressing beyond traditional computational architectures.…”

Direct measurement of oxygen mass transport at the nanoscale

“…However, the deployment of strategies for increasing mass transport at the interface level is often hindered by a lack of understanding of the fundamental underpinnings and controlling mechanisms, which remain largely undisclosed to date. 11,17 Several techniques based on electrical conductivity relaxation, 18 weight changes, 19 electrochemical impedance spectroscopy, 20 and photochromic effects, [21][22][23] are widely employed in order to retrieve global information on ion transport in oxides, but are insufficient for the study of local kinetics at the nanoscale.…”