Seiichi Sato

Abstract This study examined the potential of silicon dioxide (SiO₂) nanofilms as dynamic resistive elements in secure chaos-based communication. The nanofilms exhibited a pinched hysteresis in their current–voltage (I–V) characteristics, reminiscent of their memristive behavior. To describe this response, we implemented an equivalent circuit model incorporating a resistor–capacitor element and antiparallel diodes in MATLAB/Simulink and LTspice. We observed qualitative agreement with the experimental data. These nanofilms were then integrated into a Lorenz-based chaotic circuit to evaluate their feasibility as symmetric keys for secure communication. The simulation results confirmed that only the receivers equipped with identical SiO₂ nanofilms successfully decoded the transmitted signal, demonstrating the potential of this approach for secure wired communication.

Conductive porous Si films composed of Si nanoparticles were produced by depositing a thiol-modified Si nanoparticle ink and then sublimating the thiol-modification surface, leaving Si–S bonds on the surface. By exchanging atmospheric gas such as N₂ and O₂, these S-terminated porous Si films reversibly changed electrical resistances in a dynamic range of five orders of magnitude, indicating that the resistance change was due to O₂ adsorption/desorption on the S-terminated surface.

Memristors, capable of altering their resistance based on past voltage inputs, have emerged as promising components for various applications, including resistive memory, neuromorphic devices, and chaotic circuits. Over the years, a variety of nanomaterials and nanodevices, such as nanocrystal arrays [1], metal-doped dielectrics [2,3], and ferroelectric switching devices [4], have been reported to exhibit memristive properties. In this study, we demonstrated memristor-like behavior in metal-contacted SiO₂ nanofilms. Unlike conventional methods used for inducing memristive behavior in oxide films, such as adjusting the density distribution of mobile ions or creating/disrupting conductive filaments within the oxide, we leveraged the unique electrical conductivity of the nanofilms to achieve memristor-like current–voltage (I–V) characteristics. This conductivity likely results from the many-body effects arising from the electron-electron and/or electron-hole interactions between metals and SiO₂ nanofilms [5]. A notable deviation from conventional memristors was observed when the resistance shifted from low to high resistance states as the voltage returned to 0 V. Despite this behavior, the SiO₂ nanofilms exhibited a pinched hysteresis loop in their I–V characteristics, and were thus suitable for application in chaotic circuits. The use of SiO₂ nanofilms offers advantages such as facile integration into Si microfabrication processes and realization of memristor-like properties through the intrinsic characteristics of SiO₂, eliminating the need for precise doping processes. Herein, we investigated the alternating current characteristics of SiO₂ nanofilms in contact with Au films and Ag-pastes. The SiO₂ nanofilms were thermally grown on low-resistance Si substrates, followed by the deposition of metal electrodes. The deposited metals served not only as electrodes for injecting carriers into the nanofilms but also as inducers of many-body effects. SiO₂ nanofilms with thicknesses of ~1 nm, which facilitated easy carrier tunneling, exhibited resistance-specific behaviors in their I–V characteristics. Conversely, films with thicknesses exceeding 50 nm, which prevented carrier injection into SiO₂, showed capacitance-specific properties. Memristor-like behavior was achieved by adjusting the SiO₂ film thicknesses to their intermediate values. For example, a 9-nm-thick SiO₂ nanofilm demonstrated a resistance change from over 1 kΩ·cm during voltage increase to below 10 Ω·cm during voltage decrease. This phenomenon is attributed to the SiO₂ nanofilms acting as wide-bandgap semiconductors upon metal contact, as well as in the presence of Schottky barriers and/or Fowler–Nordheim tunneling at the metal/SiO₂ interfaces. Based on our experimental results, we propose utilization of SiO₂ nanofilms in chaotic circuits for secure internet of things (IoT) device communication. Simulations were conducted using LTspice and MATLAB Simulink to encrypt digital signals by modulating chaotic voltage oscillation rates with the digital signals. The modulated voltage wave patterns were transmitted with white noise added to mimic real-world scenarios. At the receiving end, decryption was achieved using a similar SiO₂ nanofilm-integrated chaotic circuit, representing a form of hardware based symmetric key encryption. Simulations showed that the image data encryption/decryption performance degraded only slightly with increasing noise levels, even at signal-to-noise ratios of approximately 10 dB. Thus, the proposed device exhibits secure communication capabilities comparable to those of conventional memristor–incorporated circuits [6]. References [1] F. Wang, M. Yu, X. Chen, J. Li, Z. Zhang, Y. Li, G. Zhang, K. Shi, L. Shi, M. Zhang, T. Lu, J. Zhang, Smart Mater. 4, e1135 [13 pages] (2023). [2] W. Tong, W. Wei, X. Zhang, S. Ding, Z. Lu, L. Liu, W. Li, C. Pan, L. Kong, Y. Wang, M. Zhu, S. Liang, F. Miao, Y. Liu, Nano Lett. 23, 9928–9935 (2023). [3] X. Yan, Y. Shao, Z. Fang, X. Han, Z. Zhang, J. Niu, J. Sun, Y. Zhang, L. Wang, X. Jia, Z. Zhao, Z. Guo, Appl. Phys. Lett. 122, 042101 [7 pages] (2023). [4] J. Qin, B. Sun, G. Zhou, T. Guo, Y. Chen, C. Ke, S. Mao, X. Chen, J. Shao, Y. Zhao, ACS Mater. Lett. 5, 2197–2215 (2023). [5] Y. Murata, Chem. Record 15, 557–594 (2015). [6] R. Vishwakarma, R. Monani, A. Hedayatipour, A. Rezaei, Dis. Inter. Things 3, 2 [17 pages] (2023).

Materials that reversibly modify their electrical resistance in response to external stimuli have attracted the interest of many researchers due to their potential applications in low-energy consumption memories and memristive devices for constructing electrical neural networks. For example, various chalcogenide glasses, such as Ge₂Sb₂Te₅ [1] and Ag- or In-incorporated Sb₂Te [2], can change resistances by 3–6 orders of magnitude, originating from the phase changes between amorphous (high resistance) and crystal (low resistance) states due to rapid and gradual Joule heating. Similar dynamic ranges of resistance changes have been observed in metal-doped glasses [3], such as Ta:Ta₂O₅, Cu:Ta₂O₅, Ag:SiO₂, Ni:NiO, and nanocomposite Sb-SiO₂, using the conductive filament growth/rupture in glasses. Furthermore, devices containing ferroelectric materials, such as Pt/BaTiO₃/Nb:SrTiO₃ tunnel junctions and Sc-doped AlN/MoS₂ ferroelectric field-effect transistors, exhibit similar dynamic ranges by ferroelectric polarization switching. Among these, nanometer-sized Si is a unique material whose resistance can be controlled by near-infrared optical and electrical stimulations [4]. This is the advantage of incorporating resistance change materials into optical communication systems, resulting in the construction of optoelectronic neural networks. However, the following shortcomings of nanometer-sized Si hinder its applications: (1) serious conductivity degradation that proceeds in air and (2) insufficient dynamic ranges of the resistance changes. Recently, we produced sulfur (S)-terminated Si nanoparticles, whose surface is difficult to oxidize even in the air [5]. Because S atoms, which are known to act as donors in Si, are located at the outermost surface of nanoparticles, the electron doping from S atoms can be considerably influenced by the surface-adsorbed molecules. Considering the large surface-to-volume ratio in nanoparticles, the electrical resistances of Si nanoparticle films can potentially be largely changeable through adsorption/desorption of electron-withdrawing molecules, such as O₂ and halogens. In this study, we demonstrated that surface S termination considerably reduces resistances in Si nanoparticles, and that S-terminated nanoparticles can reversibly change resistances similar to hitherto reported resistance change materials. Figure 1 shows current–voltage characteristics of the S-terminated Si nanoparticles measured under N₂ and O₂ atmospheres. As shown in the figure, changing the atmospheric gas significantly altered the current. For example, by changing the atmospheric gas from N₂ to O₂, the resistance at 2 V changed from 5×10⁶ Ω to greater than 10¹² Ω and vice versa. This reversible resistance change is due to O₂ adsorption/desorption on the film surface. The electron-withdrawing nature of O₂ may induce band bending at the nanoparticle surface as depicted in the figure, and the induced energy barrier may prevent the interparticle transport of electrons, which agrees with the small energy barrier found in our conductive atomic force microscopy measurements. The dynamic range of the resistance change was remarkably comparable to the ranges of the conventional phase change materials, indicating that by terminating the surface with S atoms, Si nanomaterials can be used not only in sensors but also photo-stimulated synaptic devices by controlling the amount of the adsorbed O₂, for example, through light irradiation [4]. References [1] Z. Song, R. Wang, Y. Xue, S. Song, Nano Res. 15, 765–772 (2022). [2] K. D. Shukla, N. Saxena, S. Durai, A. Manivannan, Sci. Rep. 6, 37868 [7 pages] (2016). [3] S. Gao, G. Liu, Q. Chen, W. Xue, H. Yang, J. Shang, B. Chen, F. Zeng, C. Song, F. Pan, R.-W. Li, ACS Appl. Mater. Interfaces 10, 6453–6462 (2018). [4] L. Yin. C. Han, Q. Zhang, Z. Ni, S. Zhao, K. Wang, D. Li, M. Xu, H. Wu, X. Pi, D. Yang, Nano Energy 63, 103859 [11 pages] (2019). [5] S. Sato, T. Dobashi, S. Matsuda, Chem. Eng. J. 268, 356–361 (2015). Figure 1 <p></p>