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We report the synthesis of SrMnO 3- F  perovskite oxyfluoride thin films using a vapor transport method to fluorinate as-grown SrMnO 2.5 epitaxial thin films. The influence of the fluoropolymer, which acts as a fluorine vapor source, was investigated by utilizing polyvinyl fluoride (PVF), polyvinylidene difluoride (PVDF) and polytetrafluoroethylene (PTFE) in the reaction. The same process was carried out with polyethylene (PE) to isolate the role of carbon in the vapor transport process. The F distribution was probed by X-ray photoemission spectroscopy, which confirmed the incorporation of F into the films and revealed higher F concentrations in films exposed to PVF and PVDF compared to PTFE. The c-axis parameter expands after fluorination, a result consistent with density functional theory calculations that attribute the volume expansion to elongated Mn-F bonds compared to shorter Mn-O bonds. Using X-ray absorption spectroscopy, we show that the fluorination process reduces the nominal Mn oxidation state suggesting that F accommodate [11]. Early synthesis efforts of transition metal oxyfluorides were carried out by solid-state reactions at around 1000°C [12]. In order to reduce the synthesis temperature, topochemical fluorination of metal oxide precursors with fluorine sources including F 2 , NH 4 F, MF 2 (M = Ba, Cu, Ni, Zn), and XeF 2 was utilized, significantly reducing the reaction temperature [5]. This fluorination method has been reported in producing powder samples of copper, titanium, iron, manganese and other metal oxyfluorides [3][4][5][6]13].An alternative approach to fluorination at low temperatures was introduced by Slater [14], who demonstrated that polyvinylidene difluoride (PVDF) can be used as a fluorine vapor source when decomposed in close proximity to metal oxide powders. Following the work of Slater, PVDF has been applied in fluorination of other metal oxide samples in powder form, especially in producing the perovskite-related oxyfluoride materials [9,[15][16][17]. The use of fluoropolymers can further reduce the fluorination temperature to 180°C for PVDF and 330°C for polytetrafluoroethylene (PTFE) [11], and mitigate the formation of secondary phases.Additionally, polymer-based fluorination has been applied to the synthesis of oxyfluoride thin films, carried out as post-growth reaction on as-grown films [18][19][20][21]. To date, these fluorination studies of thin films have all utilized PVDF as the fluorine source, while PTFE has been only investigated in producing bulk oxyfluorides [22]. To the best of our knowledge, polyvinyl fluoride (PVF) has not been reported as a fluorinating agent yet. Thus, there has been no systematic report of how the choice of fluoropolymer influences the fluorination reaction and resultant oxyfluoride films.We have synthesized epitaxial SrMnO 3- F  (SMOF) oxyfluoride thin films by fluorinating the as-grown SrMnO 2.5 (SMO) films with PVF, PVDF, and PTFE. We find that the use of PVDF and PVF results in more F incorporation than PTFE; however, the crystalline...

Measuring time-resolved photoexcited properties in semiconductors is critical to the design and improvement of light-harvesting devices. Although ultrafast pump–probe spectroscopy offers a promising route to understand carrier recombination mechanisms and quantify lifetimes, thermal contributions to the transient optical response can be significant and need to be properly accounted for to isolate carrier-induced contributions. We demonstrate the use of broadband ultrafast optical spectroscopy on type I heterostructures as a means to isolate transient effects that are solely thermal in nature. Specifically, we use transient absorption and reflectance spectroscopy to measure the time-resolved optoelectronic changes in photoexcited epitaxial bilayers of LaFeO3/LaMnO3 and monolithic thin films of these materials. Experiments and complementary numerical modeling reveal that thermal effects dominate the transient absorption and reflectance spectra above the band gap. Fitting the dynamics with a thermal diffusion model yields thermal conductivities of 6.4 W m–1 K–1 for LaFeO3 and 2.2 W m–1 K–1 for LaMnO3. In LaFeO3, an additional photoinduced absorption feature below the band gap at ∼1.9 eV is assigned primarily to photoexcited carriers and persists for over 3 ns. This work provides a direct demonstration of how thermal and electronic contributions can be separated in transient optical spectroscopies, enabling new insights into dynamical optical properties of semiconductors.

Recently, topotactic fluorination has become an alternative way of doping epitaxial perovskite oxides through anion substitution to engineer their electronic properties instead of the more commonly used cation substitution. In this work, epitaxial oxyfluoride SrMnO2.5-F films were synthesized via topotactic fluorination of SrMnO2.5 films using polytetrafluoroethylene (PTFE) as the fluorine source. Oxidized SrMnO3 films were also prepared for comparison with the fluorinated samples. The F content, probed by X-ray photoemission spectroscopy (XPS), was systematically controlled by adjusting fluorination conditions. Electronic transport measurements reveal that increased F content (up to  = 0.14) systematically increases the electrical resistivity, despite the nominal electron-doping induced by F substitution for O in these films. In contrast, oxidized SrMnO3 exhibits a decreased resistivity and conduction activation energy.A blue-shift of optical absorption features occurs with increasing F content. Density functional theory calculations indicate that F acts as a scattering center for electronic transport, controls the observed weak ferromagnetic behavior of the films, and reduces the inter-band optical transitions in the manganite films.These results stand in contrast to bulk electron-doped La1-xCexMnO3, illustrating how aliovalent anionic substitutions can yield physical behavior distinct from A-site substituted perovskites with the same nominal B-site oxidation states.

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