Xiaogan Liang

Traditional robotic vehicle control algorithms, implemented on digital devices with firmware, result in high power consumption and system complexity. Advanced control systems based on different device physics are essential for the advancement of sophisticated robotic vehicles and miniature mobile robots. Here, we present a nanoelectronics-enabled analog control system mimicking conventional controllers' dynamic responses for real-time robotic controls, substantially reducing training cost, power consumption, and footprint. This system uses a reservoir computing network with interconnected memristive channels made from layered semiconductors. The network's nonlinear switching and short-term memory characteristics effectively map input sensory signals to high-dimensional data spaces, enabling the generation of motor control signals with a simply trained readout layer. This approach minimizes software and analog-to-digital conversions, enhancing energy and resource efficiency. We demonstrate this system with two control tasks: rover target tracking and drone lever balancing, achieving similar performance to traditional controllers with ~10-microwatt power consumption. This work paves the way for ultralow-power edge computing in miniature robotic systems.

Despite the unique advantages of the memristive switching devices based on two-dimensional (2D) transition metal dichalcogenides, scalable growth technologies of such 2D materials and wafer-level fabrication remain challenging. In this work, we present the gold-assisted large-area physical vapor deposition (PVD) growth of Bi2Se3 features for the scalable fabrication of 2D-material-based crossbar arrays of memristor devices. This work indicates that gold layers, prepatterned by photolithography processes, can catalyze PVD growth of few-layer Bi2Se3 with 100-folds larger crystal grain size in comparison with that grown on bare Si/SiO2 substrates. We also present a fluid-guided growth strategy to improve growth selectivity of Bi2Se3 on Au layers. Through the experimental and computational analyses, we identify two key processing parameters, i.e., the distance between Bi2Se3 powder and the target substrate and the distance between the leading edges of the substrate and the substrate holder with a hollow interior, which plays a critical role in realizing large-scale growth. By optimizing these growth parameters, we have successfully demonstrated cm-scale highly-selective Bi2Se3 growth on crossbar-arrayed structures with an in-lab yield of 86%. The whole process is etch- and plasma-free, substantially minimizing the damage to the crystal structure and also preventing the formation of rough 2D-material surfaces. Furthermore, we also preliminarily demonstrated memristive devices, which exhibit reproducible resistance switching characteristics (over 50 cycles) and a retention time of up to 106 s. This work provides a useful guideline for the scalable fabrication of vertically arranged crossbar arrays of 2D-material-based memristive devices, which is critical to the implementation of such devices for practical neuromorphic applications.

2D-layered materials (e.g., graphene and transition metal dichalcogenides) have attracted huge attention due to their unique mechanical and electrical properties. Emerging research efforts, which seek to combine device characterization and high-resolution electron micrography analysis for 2D-layered device features, demand nano/microlithographic techniques capable of producing ordered 2D material patterns on ultrathin membranes with nanoscale thicknesses. However, such membranes are so fragile that most conventional lithographic techniques can be hardly performed on them to generate 2D material patterns. Our previous works have demonstrated that the rubbing-induced site-selective (RISS) deposition method can produce arbitrary 2D semiconductor (e.g., MoS2 and Bi2Se3) patterns on regular device substrates. This fabrication route prevents the vulnerable 2D-layered structures from the detrimental damage introduced by plasma etching and resist-based lithography processes. In this work, we explore the applicability of RISS for directly producing 2D material patterns on nanomembranes. Specifically, this work shows that a polymeric interfacing layer on the rubbing template features, which can effectively prevent stress concentration during the rubbing process, is crucial to successful implementation of RISS processes on nanomembranes. Furthermore, we carried out the mechanics simulation of the Von Mises stress and pressure distribution on the RISS-processed membrane to identify the optimal rubbing load, which can generate sufficient triboelectric charge for material deposition but no damage to the membrane. Using this approach, we have successfully demonstrated the deposition of Bi2Se3 patterns on 25 nm SiOx nanomembranes and high-resolution transmission electron micrography characterization of the crystallographic structures.

Device researchers have been actively developing novel diagnostic biosensors using metallic nanoparticle-based plasmonic immunoassays.[1] In these biosensing devices, photodetectors with high photoresponsivity and low internal noise levels are integrated to detect the marginal optical signals change induced by biomarker binding events (i.e., Antigen-antibody binding). Recently, atomically thin layers of molybdenum disulfide (MoS 2 ) have been integrated with plasmonic components to enable fast and sensitive colorimetric monitoring of disease-related biomarkers.[2] However, such biosensors are mostly operated in the ultraviolet/visible range, which needs a purification step to separate out a variety of unwanted biomaterials that absorb visible light and takes several hours of preparation steps. Thus, it would be amenable to develop biosensors for biomolecular detections or immunoassays in the whole blood (WB) analytes without any sample preparations.[3] To address the issues mentioned above, recent studies have demonstrated the engineered gold nanoparticles (AuNPs) that have localized surface plasmonic resonance (LSPR) shift around near-infrared (NIR) wavelength region. However, a high-sensitivity photodetector under NIR region is still required to detect marginal signal changes due to the target biomarker binding events. Meanwhile, the MoS 2 material has been reported to exhibits superior photo-response characteristics.[4] MoS 2 is a transition metal dichalcogenide material of which single- and multi-layer films exhibit an efficient electron-hole pair generation rate under photoexcitation and therefore high photo absorption as compared to silicon. Therefore, a systematical study on MoS 2 photoconductors to optimize the optoelectronic properties under near-infrared (NIR) (λ = 650 nm) operation can enable zero-preparation WB immunoassays combined with specially engineered AuNPs. In this work, we study the photo-response properties (i.e., Photoresponsivity spectrum) of in-plane MoS 2 photodetectors as the function of their geometric dimensions and fabrication conditions. Recent study shows that an annealing temperature after exfoliation can etch the upper layers of MoS2 flakes and clean stains on the device and therefore improve device performances (e.g., mobility). [5] This work enables the NIR operation capabilities of plasmonic colorimetric biosensing by introducing the optimized MoS 2 photodetector fabrication practice. This approach reduces assay preparation times and mitigate background interference even with WB analytes and thereby enable the WB point-of-care immunoassays. References [1] Zhou, W., Gao, X., Liu, D. and Chen, X., Chemical reviews, 115(19), pp.10575-10636. (2015). [2] Park, Y., Ryu, B., Deng, Q., Pan, B., Song, Y., Tian, Y., Alam, H.B., Li, Y., Liang, X. and Kurabayashi, K., Small, 16(1), p.1905611 (2020). [3] Wang, Y., Qian, W., Tan, Y. and Ding, S., 23(7), pp.1166-1170. (2008). [4] Britnell, L., Ribeiro, R. M., Eckmann, A., Jalil, R., Belle, B. D., Mishchenko, A., ... & Novoselov, K. S. Science, 340(6138), 1311-1314. (2013). [5] Islam, A., Lee, J. and Feng, P.X.L., Journal of Applied Physics, 123(2), p.025701. (2018). [6] Lu, X., Utama, M. I. B., Zhang, J., Zhao, Y., & Xiong, Q., Nanoscale, 5(19), 8904-8908. (2013).

Herein we report the assessment of the effects of shockwave (SW) impacts on adult rat hippocampal progenitor cell (AHPC) neurospheres (NSs), which are used as in vitro brain models, for enhancing our understanding of the mechanisms of traumatic brain injury (TBI). The assessment has been achieved by using culture dishes and a new microchip. The microchip allows the chemicals released from the brain models cultured inside the cell culture chamber under SW impacts to diffuse to the nanosensors in adjacent sensor chambers through built-in diffusion barriers, which are used to prevent the cells from entering the sensor chambers, thereby mitigating the biofouling issues of the sensor surface. Experiments showed the negative impact of the SW on the viability, proliferation, and differentiation of the cells within the NSs. A qPCR gene expression analysis was performed and appeared to confirm some of the immunocytochemistry (ICC) results. Finally, we demonstrated that the microchip can be used to monitor lactate dehydrogenase (LDH) released from the AHPC-NSs subjected to SW impacts. As expected, LDH levels changed when AHPC-NSs were injured by SW impacts, verifying this chip can be used for assessing the degrees of injuries to AHPC-NSs by monitoring LDH levels. Taken together, these results suggest the feasibility of using the chip to better understand the interactions between SW impacts and in vitro brain models, paving the way for potentially establishing in vitro TBI models on a chip.

Memristors based on 2D semiconductors such as MoS2 and its derivative materials exhibit analog switching behaviors capable of emulating some synaptic functions, including short-term plasticity, long-term potentiation, and spike-time-dependent-plasticity. Additional investigation is needed to realize reliable control of such synaptic behaviors for practical device implementation. To meet this scientific need, we fabricated MoS2-based memristors and studied their paired-pulse facilitation (PPF) and long-term memory characteristics under different pulse programming settings. This research has provided a guideline for identifying the programming settings for different neuromorphic processes. For example, a specific setting resulting in PPF > 30% and long-term conductance change < 20% has been identified to be suited for processing real-time temporal information. Furthermore, this research also indicates that the MoS2 memristor keeps having an almost constant relative change in conductance but greatly enhanced drive current level under laser illumination. This behavior can enable an easy integration of such memristive devices with state-of-the-art controller circuits for practice neuromorphic control applications.

This chapter presents a review of recent research activities seeking to implement two-dimensional transition metal dichalcogenides for making memory and memristive devices as well as demonstrating preliminary neuromorphic systems. The first section will discuss the special electronic and morphological properties of transition metal dichalcogenides, which could be investigated and further exploited for realizing memory, memristive, and neuromorphic functions. These attractive characteristics related to layered materials have led to several prototype memory and memristive devices, including multi-bit transistor-based memories, memristive transistors, memristors exhibiting analogue switching characteristics, multi-terminal memristors enabling ionic coupling functions, and neuromorphic light-response devices capable of emulating synaptic and neuronal responses to external simulations. The subsequent sections will review these different types of memory and memristive devices based on transition metal dichalcogenides and present the discussion on their technical advantages and disadvantages as compared to the state-of-the-art counterpart devices as well as the potential features that need further research effort.

Bismuth selenide (Bi 2 Se 3 ), a layered semiconductor, has attracted a great deal of attention as a thermoelectric material as well as a potential topological insulator. Here, we present a work showing that Bi 2 Se 3 can also be used for making memristive devices capable of directly processing analog video signals. In this work, Bi 2 Se 3 memristors are produced by multiplexing rubbing-induced site-selective growth, which potentially enables scalable implementation of such memristor arrays for constructing large-scale neuromorphic systems. The fabricated Bi 2 Se 3 memristors exhibit prominent memristive switching characteristics under the application of time-sequential voltage pulses. Especially, such a Bi 2 Se 3 memristor exhibits a reliable dependence of memristive responses on the duty cycle of programming pulses, fast recovery behavior from a dynamically modulated state, and a large drive current. These properties could be employed for extracting spatiotemporal information from analogue signals and realizing practical neuromorphic sensory functions. Our additional tests strongly imply that the memristive output of a Bi 2 Se 3 memristor in response to analogue video scanline signals could be implemented to construct future hardware-based computer vision systems capable of rapidly acquiring graphic information and directly actuating robotic systems with minimal data transmission and energy consumption. Finally, we attribute the observed memristive characteristics to field-mediated drift and diffusion of the selenium vacancies in the Bi 2 Se 3 layers. The simulated memristive response based on this hypothesis model is consistent with the experimental result. This work provides a potentially upscalable device solution to realize memristor-based neuromorphic sensory or edge computing systems.

An embodiment of a method of depositing graphene includes bringing a stamp into contact with a substrate over a contact area. The stamp has at least a few layers of the graphene covering the contact area. An electric field is developed over the contact area. The stamp is removed from the vicinity of the substrate which leaves at least a layer of the graphene substantially covering the contact area.