Significant Changes in Electrical Resistance of Conductive Porous Silicon Films Produced from Thiol-Modified Silicon Nanoparticle Inks
Authors: Sato Seiichi , Tsubota Shuhei , Ichinohe Takahisa 
Published: December 2024

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].
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Figure 1