Nitrogen-doped carbon nanotubes for self-powered memristive systems

Abstract

Memristive devices are one of the promising candidates for creating neuromorphic systems due to the possibility of multilevel switching, low operating voltages and high scalability. However, as with any passive element, the memristor requires an external bias voltage to operate, which requires the inclusion of a power source in the circuit. In this regard, of great interest are works on the creation of self-powered memristive systems consisting of connecting in series a memristor and a nanogenerator that converts the energy of the external environment into electrical energy [1, 2]. Such a memristive system has a high potential for applications in aerospace and implantable electronics. At the moment, the first self-powered memristive and sensor systems based on metal oxides and piezoelectric nanogenerators (PENG) have already been developed [2]. The main problems in this area are to reduce the size of the nanogenerator and to match the output parameters of the nanogenerator and the input parameters of the memristor. In the framework of this work, these problems are being resolved by creating a self-powered memristive system based on nitrogen-doped carbon nanotubes (N-CNTs). Previously, we studied the memristive properties of N-CNTs and showed that nanotubes demonstrate reproducible multilevel switching with a resistance ratio in the high- and low-resistance states (HRS/LRS) of about 4⋅105 [3, 4]. It was found that the memristive effect in N-CNTs is due to the incorporation of nitrogen atoms into the nanotube structure and the formation of an internal piezoelectric field [4]. As part of further studies, it was found that an array of vertically aligned N-CNTs is a promising material for creating PENG: the generated output voltage is hundreds of mV and the current generated by single nanotube reaches hundreds of nA [5]. The results obtained allow us to speak about the possibility of developing a self-powered memristive system by connecting in series a memristor and PENG based on N-CNTs. To optimize the output characteristics of the PENG, in particular, the amplitude of the generated voltage, and the input switching voltage of the N-CNT-based memristor, studies were carried out to increase the piezoelectric response and reduce the switching voltage of the N-CNT resistance by changing the concentration of the dopant nitrogen in the nanotube growth process. It was found that it is necessary to grow N-CNTs with a doping nitrogen concentration of up to 12% and a high aspect ratio of length to diameter (more than 60) to create PENG with an output voltage of up to 2 V. These N-CNT parameters are provided at a low growth temperature (500–550 C°) and high ratio of acetylene and ammonia flows (1:5 - 1:6). On the contrary, the N-CNTs with a small aspect ratio (less than 30) and doping nitrogen concentrations of 4–6% are required for the manufacture of memristors with a minimum switching voltage (about 2 V), These N-CNT parameters are provided by increasing the growth temperature to 615 C° and reduction in growth time. The obtained results can be used in the development of self-powered memristive and sensor systems based on nitrogen-doped carbon nanotubes.