Figure 4. Characterization of the artificial haptic perception neurons. a) The circuit diagram of the haptic perception neuron. b)IV curves of the piezoresistive sensor with different applied pressures/weights. c) The output resistance of sensor as a function of weight. The solid line is fitting result by Equation (3). d) Neuronal response of the haptic perception neuron under a constant bias voltage (5 V, 20 μs) and the influence of varying pressure. The output frequencies are 0.45, 0.55, 0.75, and 0.95 MHz at the pressures/weights of 100, 150, 200, and 250 g, respectively e) Recognition of braille characters (“p”, “k”, “u”) using tactile sensing. Without convex patterns (that is, two sensors have no pressure), the output frequency is zero. With convex patterns both on the left and right (that is, two sensors both have pressure by applying 100 g), the output frequency is ~1.1 MHz. With convex patterns only on one of the left and right (that is, the only one of two sensors has pressure), the output frequency is ~0.4 MHz.
2.3. Artificialtemperature perception neuron based on VO2volatile memristor
In human sensory system, temperature sensation is also an indispensable sensory ability, besides tactile sensation, which can help the human body respond to the temperature of the outside world so that the central nervous system can initiate a motor response, for example, avoiding injury of the human body. The VO2 device is inherently sensitive to temperature due to its thermally-driven metal-insulator transition,[41, 42] and hence this feature can be exploited to construct an artificial temperature perception neuron, as illustrated in Figure 5 a. Figure 5b firstly shows theI-V characteristics of VO2 volatile memristor at different temperatures ranging from 284 to 306 K, showing apparent impact on the TS characteristics. Only the VO2 device was heated in the probe station during the temperature-elevated test, while the other components were all kept at room temperature. In order to further investigate the impact of temperature on the characteristics of VO2 memristor, critical paramters including V th andV hold are extracted. Figure 5c further showsV th and V hold values at different temperatures. One can see that both V thand V hold decrease as the temperature increases, which agrees with the switching behavior of VO2 observed in the previous study and variation in switching voltages originates from thermal processes induced by Joule heating and its dissipation.[43-46] In order to exclude the possibility of stochastic fluctuations, we have systematically measured the cycle-to-cycle variation of the VO2 memristor under different temperatures (Figure S11). The experimental results in Figure S11 demonstrate that the temperature dependence of the threshold voltages in the VO2 memristor is much more significant than the parameter fluctuations and hence contribute to the temperature sensing capability of the sensory neuron. We have characterized the operation temperature range of our VO2 device. The results demonstrate that the TS characteristics remain stable at least below 0 °C but gradually disappear above 35 °C (Figure S12, Supporting Information), which could be related to the low phase transition temperature of VO2 (~340 K)[47]. This issue can potentially be addressed by using Mott system with higher phase transition temperature, such as NbO2 (~1080 K)[48].
Lee et al.[44] proposed a Joule heating model to point out that the relationship betweenV th,V hold and temperature (T ). In this model, the mott transition occurs when the voltage-induced Joule heating is sufficient to raise the crystal temperature to the mott transition temperature. Assuming the heat obtained in the VO2crystal is mainly due to the balance between resistive Joule heating and heat loss from the crystal to the environment via heat conduction, this relation can be expressed by the following simple heat equation:
(4)
where C , T , R th, and R are the heat capacitance, temperature, effective thermal resistance, and resistance of the VO2 film. V VO2is the applied voltage to the VO2 film andT e is the environmental temperature. Equation (4) is a first differential equation, where the relationship betweenV th, V hold andT e can be obtained by integrating (see Supporting Information), which shows the T e-dependence ofV th and V hold. BothV th and V hold decreased with increasing T e.
Furthermore, Figure 5d illustrates the oscillation frequency of the structure shown in Figure 5a as a function of R Lat different temperatures from 284 to 306 K, when a constant voltage pulse (5 V, 20 μs) is applied. The oscillation frequency generally increases linearly with decreased resistance, and the oscillation frequency is increased at evelated temperature whenR L is fixed (Figure 5d). Figure 5e further shows the oscillation waveforms at varied temperature from 284 to 306 K, with constant R L of 3.6 kΩ and the same input voltage pulse (5 V, 20 μs). Once again the output oscillation frequency increases (from 0.3 to 0.8 MHz) as the temperature increases (from 284 to 306 K). Since the oscillation is activated betweenV hold and V th, the amplitude of oscillation decreases as the temperature increases, which is consistent with the results in Figure 5b,c. Therefore, the VO2 neuron in Figure 5a can realize temperature perception based on intrinsic thermal sensitivity of VO2, which is able to perceive temperature changes and serves as a temperature sensory neuron.