Reconfigurable Low-Power TiO2 Memristor for Integration of Artificial Synapse and Nociceptor.

Bio-mimetic advanced electronic systems are emerging rapidly, engrossing their applications in neuromorphic computing, humanoid robotics, tactile sensors, and so forth. The biological synaptic and nociceptive functions are governed by intricate neurotransmitter dynamics that involve both short-term and long-term plasticity. To emulate the neuronal dynamics in an electronic device, an Ag/TiO2/Pt/SiO2/Si memristor is fabricated, exhibiting compliance current controlled reversible transition of volatile switching (VS) and non-volatile switching (NVS). The origin of the VS and NVS depends on the diameter of the conducting filament, which is explained using a field-induced nucleation theory and validated by temporal current response measurements. The switching delay of the device is used to determine the characteristic nociceptive behaviors such as threshold, relaxation, inadaptation, allodynia, and hyperalgesia. The short-term and long-term retention loss attributed to the VS and NVS, respectively, is used to emulate short-term memory and long-term memory of the biological brain in a single device. More importantly, synergistically modulating the VS-NVS transition, the complex spike rate-dependent (SRDP) and spike time-dependent plasticity (STDP) with a weight change of up to 600% is demonstrated in the same device, which is the highest reported so far for TiO2 memristors. Furthermore, the device exhibits very low power consumption, ∼3.76 pJ/spike, and can imitate synaptic and nociceptive functions. The consolidation of complex nociceptive and synaptic behavior in a single memristor facilitates low-power integration of scalable intelligent sensors and neuromorphic devices.

Bipolar Resistive Switching in TiO2 Artificial Synapse Mimicking Pavlov's Associative Learning.

Memristive devices are among the most emerging electronic elements to realize artificial synapses for neuromorphic computing (NC) applications and have potential to replace the traditional von-Neumann computing architecture in recent times. In this work, pulsed laser deposition-manufactured Ag/TiO2/Pt memristor devices exhibiting digital and analog switching behavior are considered for NC. The TiO2 memristor shows excellent performance of digital resistive switching with a memory window of order ∼103. Furthermore, the analog resistive switching offers multiple conductance levels supporting the development of the bioinspired synapse. A possible mechanism for digital and analog switching behavior in our device is proposed. Remarkably, essential synaptic functions such as pair-pulse facilitation, long-term potentiation (LTP), and long-term depression (LTD) are successfully realized based on the change in conductance through analog memory characteristics. Based on the LTP-LTD, a neural network simulation for the pattern recognition task using the MNIST data set is investigated, which shows a high recognition accuracy of 95.98%. Furthermore, more complex synaptic behavior such as spike-time-dependent plasticity and Pavlovian classical conditioning is successfully emulated for associative learning of the biological brain. This work enriches the TiO2-based resistive random-access memory, which provides information about the simultaneous existence of digital and analog behavior, thereby facilitating the further implementation of memristors in low-power NC.

Strain-mediated ferromagnetism and low-field magnetic reversal in Co doped monolayer [Formula: see text].

Strain-mediated magnetism in 2D materials and dilute magnetic semiconductors hold multi-functional applications for future nano-electronics. Herein, First principles calculations are employed to study the influence of biaxial strain on the magnetic properties of Co-doped monolayer [Formula: see text]. The non-magnetic [Formula: see text] shows ferromagnetic signature upon Co doping due to spin polarization, which is further improved at low compressive (-2 %) and tensile (+2 %) strains. From the PDOS and spin density analysis, the opposite magnetic ordering is found to be favourable under the application of compressive and tensile strains. The double exchange interaction and p-d hybridization mechanisms make Co-doped [Formula: see text] a potential host for magnetism. More importantly, the competition between exchange and crystal field splittings, i.e. ([Formula: see text]), of the Co-atom play pivotal roles in deciding the values of the magnetic moments under applied strain. Micromagnetic simulation reveals, the ferromagnetic behavior calculated from DFT exhibits low-field magnetic reversal (190 Oe). Moreover, the spins of Co-doped [Formula: see text] are slightly tilted from the easy axis orientations showing slanted ferromagnetic hysteresis loop. The ferromagnetic nature of Co-doped [Formula: see text] suppresses beyond [Formula: see text] strain, which is reflected in terms of decrease in the coercivity in the micromagnetic simulation. The understanding of low-field magnetic reversal and spin orientations in Co-doped [Formula: see text] may pave the way for next-generation spintronics and straintronics applications.

Effect of Charge Injection on the Conducting Filament of Valence Change Anatase TiO2 Resistive Random Access Memory Device.

The recent observation of stable quantized conductance in anatase TiO2 resistive random access memory (ReRAM) devices opens up a new pathway toward the realization of brain-inspired neuromorphic computing devices. Herein, for the first time, ab initio calculations are implemented to understand the resistive switching phenomena in anatase TiO2. Oxygen vacancy configurations with different charge states are studied to gain insight into the ON and OFF states of ReRAM devices. Among the trivacancy configurations, the Vo+ state is observed to induce highly dispersed defect states within the bandgap forming a charge density channel where the carriers behave as free electrons leading to the formation of a conducting filament (CF). On the contrary, the breakdown of the CF is noticed by the removal of an oxygen vacancy from the trivacancy configuration. In this OFF state, the defect state carriers are found to be highly localized. In addition, we have also investigated the effect of charge injection on the crystal field symmetry of the CF. The reduction of symmetry due to the trivacancy configuration lowers the eg manifold energy, whereas the divacancy configuration lowers the t2g manifold energy.

Thermal conductivity of free-standing silicon nanowire using Raman spectroscopy.

Low dimensional systems, nanowires (NWs), in particular, have exhibited excellent optical and electronic properties. Understanding the thermal properties in semiconductor NWs is very important for their applications in electronic devices. In the present study, the thermal conductivity of a freestanding silicon NW is estimated by employing Raman spectroscopy. The advantage of this technique is that the excitation source (laser) acts as both the heater and probe. The variations of the first-order Raman peak position of the freestanding silicon NW with respect to temperature and laser power are recorded. From the analysis of effective laser power absorbed by exposed silicon NW and a detailed Raman study along with the concept of longitudinal heat distribution in silicon NW, the thermal conductivity of the freestanding silicon NW of ∼112 nm diameter is estimated to be ∼53 W m-1 K- 1.