2021-02, Ebenhoch, Carola, Schmidt-Mende, Lukas

Memristors are deemed to be the electrical twin to biological synapses. They enable emulation of human memory functionalities such as learning, memorizing, and forgetting. The present hydrothermally grown titanium dioxide nanowire array memristive devices have shown to be able to mimic synaptic behaviors. As well as spike‐rate dependent plasticity, excitatory postsynaptic currents, and paired pulse facilitation, high endurance, and on/off ratios for the nanowire arrays are presented. Decay fitting of postsynaptic currents with Kohlrausch's equation shows lifetimes of few milliseconds up to several hundred seconds, offering the possibility of a short‐term to long‐term memory transition. Furthermore, a strong dependence of the lifetime of the signals on the frequency and amplitude of the stimulation pulses is observed.

2020-04-01, Seewald, Tobias, Schütz, Emilia R., Ebenhoch, Carola, Schmidt-Mende, Lukas

Power conversion efficiencies of lead halide perovskite solar cells have rapidly increased in the decade since their emergence, reaching 25% this year. However, reliable film uniformity and device stability remain hard to achieve and often require precise compliance with complicated protocols, which hampers upscaling towards industrial applications. Here, we explore the potential of an alternative route towards high-quality perovskite films: The reaction between a preexisting perovskite film and methylamine (MA) gas has been shown to possess the striking ability to both improve lm morphology and increase grain size drastically, boosting device performance. This promising post-deposition treatment could provide the means to decouple film quality from the initial deposition process, thus promising to facilitate upscaling and lowering production costs. Furthermore, such MA gas treatments show great promise regarding the stability of fabricated devices, as they open up the opportunity to reduce or even eliminate the adverse role of grain boundaries in film degradation.

2021, Ebenhoch, Carola, Gänsler, Thomas, Schupp, Stefan, Hagner, Matthias, Frank, Anna, Scheu, Christina, Schmidt-Mende, Lukas

Single nanowires and networks are considered as promising candidates for miniaturized memristive devices for brain-inspired systems. Moreover, single crystalline nanostructures are useful model systems to gain a deeper understanding in the involved switching mechanism of the investigated material. Here, we report on hydrothermally grown single crystalline Nb₃O₇(OH) nanowires showing a complementary resistive switching (CRS) behavior. The CRS characteristics can be related to an oxygen vacancy migration at the electrode/metal hydroxide interface. Therefore, an oxygen plasma treatment is used to reduce the oxygen vacancy content, resulting in a total reduction of the device conductivity. Furthermore, temporal resolved current–voltage measurements demonstrate the dependence of the destructive readout process of the resistance states on the voltage amplitude and polarity.

2018-08, Nawaz, Asmat, Wong, Ka Kan, Ebenhoch, Carola, Zimmermann, Eugen, Zheng, Zhaoke, Akram, Muhammad Nadeem, Kalb, Julian, Wang, Kaiying, Fakharuddin, Azhar, Schmidt-Mende, Lukas

High performance in perovskite solar cells is often achieved using compact metal oxide layers or their mesoporous analogues. One dimensional scaffold materials such as nanorods or nanotubes are also employed in order to improve charge collection, however, perovskite pore-filling in these nanostructures is rather low. Herein, we introduce a method to more efficiently fill the pores in two most common nanostructure architectures namely, TiO2 nanorods and nanotubes. The method employs recrystallization of perovskite films in a methylamine rich environment– the so called perovskite healing. Whereas the scanning electron microscopy imaging revealed an improved pore-filling and formation of large perovskite grains upon healing, the complementary photoluminescence and electrical characterizations revealed improved charge transfer in the healed films than their pristine rivals. We also report a notable improvement in photoconversion efficiency and a better stability under continuous light soaking in the healed perovskite films.

2020-05-20, Ebenhoch, Carola, Kalb, Julian, Lim, Joohyun, Seewald, Tobias, Scheu, Christina, Schmidt-Mende, Lukas

In the present study, the memristive characteristics of hydrothermally grown TiO₂ nanorod arrays, particularly, the difference in the retention-time of the resistance state are investigated in dependence of the array growth temperature. A volatile behavior is observed and related to a redistribution of oxygen vacancies over time. It is shown, that the retention time increases for increasing array growth temperatures from several seconds up to 20 minutes. The relaxation behavior is also seen in the current-voltage characteristics, which do not show the common unipolar, bipolar, or complementary switching behavior. Instead, the temporal evolution depends on the duration of the applied voltage and on the nanowire growth temperature. Therefore, electronic measurements are combined with scanning electron and scanning transmission electron microscopy, so that the amount of oxygen defect-rich grain boundaries in the upper part of the nanowires can be linked to the differences in the current-voltage behavior and retention time.