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We present electronic properties of a charge transfer material consisting of Manganese(ii)Phthalocyanine (MnPc) and 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), investigated by means of photoemission spectroscopy and electron energy-loss spectroscopy, as well as supporting density functional theory calculations. We report the successful formation of a bulk material characterized by a strong interaction of the molecular compounds which affects the optical properties significantly. Our investigations reveal a significant charge transfer, whereas the MnPc molecule is oxidized and F4TCNQ is reduced. The valence band data indicate a full charge transfer between the two partners. The electronic excitation spectrum reveals a relatively small energy gap of MnPc/F4TCNQ of about 0.7 eV, which is related to a charge transfer excitation.

Niobium oxide devices exhibit threshold switching behavior which enables their use as selectors in memory arrays or as locally active devices for neuromorphic computing. Among the basic dynamical phenomena appearing in non-linear circuits, the oscillations generated in a relaxation oscillator, which is making use of the negative differential resistance (NDR) effect of a threshold switching device, are of special significance for the design of neuromorphic electronic systems. Here, the necessary requirements for the emergence of oscillations of this kind in a simple relaxation oscillator circuit and their influence on the shape of the measured quasi-static I– characteristic of the threshold switch are examined. In the corresponding experiments multiple NDR regions were found to appear in the quasi-static I– characteristic of the threshold switch concurrently with the occurrence of oscillations. The observed ‘multiple NDR phenomenon’ is therefore merely a measurement artefact due to the averaging effect associated to the operating principles of the source measure unit (SMU) utilized to measure the device current and voltage. In this work, we analyzed how the emergence of oscillatory behavior in the relaxation oscillator depends upon the device layer stack composition. The probability of the appearance of oscillations within a large current range can be increased by decreasing the oxygen content in the sub-stoichiometric bottom layer of a niobium oxide bi-layer stack. It is shown that this trend is caused by the resulting decrease in the value of the product between thermal capacitance and thermal resistance of the threshold switching device. Furthermore, the changed stack composition reduces the variability and changes the forming voltage, which goes hand in hand with a change of the threshold voltage.

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In this work the I–V characteristics of a niobium oxide-based threshold switching device were optimized to match the requirements for its application in neuromorphic circuits. Those neuromorphic circuits rely on coupled oscillators utilizing the volatile resistive switching effect of the memristor. A large voltage extension of the negative differential resistance region of the threshold switch enables enhanced signal amplification, and, furthermore, can lead to a better tolerance to device variability. A symmetric switching behavior as well as a high device stability for the operation in both voltage polarities is mandatory to allow the integration in circuits that utilize the connection of several threshold switching devices operated in different polarities. These properties are similarly important for the adoption of the threshold switches as selector devices in bipolar resistive memory arrays. Furthermore, a low forming voltage is desirable because it leads to a better control during the forming step. To meet all those requirements the application of multilayer stacks consisting of niobium and niobium oxide layers is proposed and their optimization is investigated in detail.

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