# Neuromorphic Spatiotemporal Information Processing Using Neuro-Photodetector Systems
## 3 Results and Discussion
However, UV generated electrons remain within the ZnO, resulting in increases in the overall conductivity at a particularly measuring voltage. Importantly, \(I_{\rm p}\) decreases after switching off the UV, which is analogue to the leaking nature of the neuron [9]. To understand and confirm the governing dynamic, we performed the X-ray photoelectron spectroscopy (XPS) measurement of the ZnO after and before the UV illumination; see the presented plots in Figure S3 (Supporting Information). It was observed that after UV illumination, the XPS peak related to the chemisorbed surface oxygen (e.g., 532.5 eV), reduces significantly, which is due to decreasing in the charge trapping sites related to oxygen vacancies, see Figure S3 (Supporting Information) [7]. Slow \(I_{\rm p}\) decay is a debatable topic; however, as is noted by XPS measurements, the trapping/detrapping of holes via ionized oxygen vacancies provides a show dynamic. Further, to confirm the photoactive nature of the device, the \(I_{\rm p}\) for a fixed illumination time, (\(\Delta t=1\) s) was measured by varying pulsed UV intensity from 0.25 to 5 mW cm\({}^{-2}\) and presented in Figure 2e. Note that the magnitude of \(I_{\rm p}\) increases linearly with UV intensity most likely due to the increased number of photogenerated carriers [4; 15; 24]. For statistical analysis, several devices with different areas were prepared and most of them shows similar behavior. The error bar in Figure 2e is corresponding to the \(I_{\rm p}\) collected form several devices. The linear dynamic range (LDR), defined as LDR = 20 log(\(I_{\rm p}\)/\(I_{\rm dark}\)), [25; 26] is calculated, which is found 40 dB within the illuminating UV intensities (5 mW cm\({}^{-2}\)), see Figure 2e. Moreover, the \(I_{\rm p}\) for various devices with different areas were measured and depicted in Figure 2f. Interesting to note that \(I_{\rm p}\) increases with device area as well, making our approach to design the devices over a desired area. Bio-neuron collects the inputs from connected synapses and integrates them to generate an output signal, if a threshold reached. On the other hand, if integrated, input did not reach to the threshold the signal decays, which is similar to the forgetting the memory. Both these functions, e.g., leaky integration and firing are emulated at device level, using the neuro-photodetector. In fact, decaying of \(I_{\rm p}\) after UV illumination is corresponding to the 'leaky' membrane potential of the bio-neuron, and also a critical dynamical property for forgetting [5; 10; 27]. This decay time also determines the memory span and is also useful to reset the device after firing the output. To mimic the neutronic behavior, let us assume that spikes (two or more) with similar amplitude however, with different time duration are implemented on the device. In this case, the \(I_{\rm p}\) corresponding to individual spike is physically summed as per Kirchhoff's law, and generates the resultant output [27]. Therefore, longer and intense sequences of input pulses lead to the faster approach to the threshold, resulting in firing, see Figure 3a. In addition, the \(I_{\rm p}\) decays gradually towards its initial state, which is like the relaxation process of the human neuron. Since, the magnitude of \(I_{\rm p}\) can be modulated by varying the pulse number and/or duration. Therefore, the \(I_{\rm p}\) generation with of pulse numbers and duration as a function of UV intensity are depicted as 3D plots in Figure 3b,c, respectively. It is worth to note that higher pulse number (\(\Delta t=0.1\) s and \(\Delta d=1\) s) and UV intensity lead to increase the \(I_{\rm p}\) much faster, see Figure 3b. The \(I_{\rm p}\) can also be increases by decreasing the \(\Delta d\) of multiple UV pulses, see Figure 3c, showing the responses of our device to the stimulus train with different pulse intervals ranging from 1 to 10 s. Note that the magnitude of the \(I_{\rm p}\) depends on the illumination pulse duration and/or number of pulses.