Poly( p -xylylene)–molybdenum oxide nanocomposite thin films of different thicknesses and inorganic filler content are synthesized by low-temperature vapor deposition polymerization. The structure of the nanocomposites and its evolution during thermal annealing is studied by wide angle X-ray scattering and X-ray absorption spectroscopy. It is found that the molybdenum oxide nanoparticles are amorphous in both the as-deposited and annealed composite films. The short-range order characteristic of orthorhombic molybdenum trioxide is preserved in the nanoparticles; however, a noticeable disordering of the structure together with a decrease in the effective oxidation state of molybdenum are revealed. Both an increase in the filler content and thermal annealing lead to a decrease in the bandgap of the composites, which is related to the increase in the nanoparticle size. It is shown that thermal annealing improves the stability of the resistive switching (RS) characteristics in memristors based on the synthesized nanocomposites, which creates an opportunity for the application of these materials as the active layer of memristive devices.

Методом полимеризации на поверхности из газовой фазы синтезированы образцы тонкопленочных композитов на основе поли-пара-ксилилена и оксида молибдена с различной толщиной и концентрацией неорганического наполнителя. Методами рентгеновского рассеяния и спектроскопии поглощения рентгеновского излучения исследована структура синтезированных композитов и ее изменение при термическом отжиге. Обнаружено, что наночастицы оксида молибдена как в исходных, так и в отожженных образцах остаются аморфными. При этом в наночастицах сохраняются элементы локального порядка, характерные для орторомбического триоксида молибдена, однако происходит заметное разупорядочение структуры с понижением эффективной степени окисления молибдена. Увеличение концентрации наполнителя и отжиг приводят к уменьшению ширины запрещенной зоны в наночастицах оксида молибдена, что, по всей видимости, связано с увеличением размера наночастиц. Показано, что отжиг приводит к улучшению стабильности характеристик резистивного переключения в мемристорах на основе синтезированных композитов, что открывает возможность использования данных материалов в качестве активного слоя мемристивных устройств.

The effect of the dose of sterilizing γ irradiation on the properties of polylactide-sponge materials is studied. It is found that irradiation significantly affects the weight-average molecular weight that decreases from 180 to 83 kDa at an irradiation dose of 60 kGy as well as the mechanical properties of the materials. The elastic modulus decreases at an irradiation dose of 25 kGy and above.

The evolution of the morphology of island poly(chloro-p-xylylene) films formed on silicon substrates by vapor deposition polymerization is investigated by atomic force microscopy. The dependences of the effective thickness of the island coating, the number density of polymer islands, and their average size on the surface coverage are studied. The maximal density of polymer islands and the surface coverage corresponding to the transition to the coalescence regime are estimated. Within the framework of the theory of dynamic scaling, the size distribution of islands and the size distribution of their "capture zones" are analyzed. It is shown that, at low degrees of filling of the substrate, before the coalescence of islands, these distributions are described by scaling functions corresponding to the model of reaction-limited aggregation. The size of the critical nucleus is estimated from the size distributions of the "capture zones" of polymer islands.

Thin-film protective coatings based on poly(p-xylylene) (PPX) and poly(chloro-p-xylylene) (PCPX) have been synthesized by vapor deposition polymerization (VDP) in a vacuum and deposition in a nitrogen flow. Comparison of characteristics of the PPX coatings by atomic force microscopy, infrared spectroscopy, and thermogravimetric analysis has shown that the films deposited in a nitrogen flow, unlike the films prepared by VPD in a vacuum, contain a large amount of low-molecular-weight fractions that worsen the protective characteristics of the films. In addition, the growth rate of these films is too low for practical applications. Analysis of the PCPX-based coatings prepared by these methods has shown a high similarity in composition and structure at comparable growth rates of the coatings. Owing to the high transparency and hydrophobicity of PCPX coatings prepared by deposition in a nitrogen flow, they can be used for the conservation of historical artifacts and written monuments on paper media.

In this study, we employed several experimental techniques to investigate structure and magnetic properties of poly( p -xylylene)–MnSb composites synthesized by low-temperature vapor deposition polymerization technique and MnSb films deposited at various temperatures. The presence of MnSb nanocrystallites in the studied films was verified by the results of X-ray diffraction, electron microscopy and Raman spectroscopy studies. The obtained data revealed the formation of Sb-rich sublayer with well-oriented Sb grains near the susbtrate, which seems to act as a buffer for the consequent poly( p -xylylene)–MnSb or MnSb layer growth. Increasing the polymer content results in qualitative change of surface morphology of studied films. At high polymer content the hybrid nanocomposite with MnSb nanoparticles embedded into poly( p -xylylene) matrix is formed. All investigated samples demonstrated detectable ferromagnetic response at room temperature, while the parameters of this response revealed a complex correlation with nominal composition, presented crystal phases and surface morphology of studied films. Estimated values of the Curie temperature of the samples are close to that of bulk MnSb.

Nowadays there is a growing interest in wearable and biocompatible computing systems that are safe for the human body. Memristive devices are prospective for such tasks owing to a number of their attractive properties, in particular, the multilevel character of resistive switching, or plasticity, which allows them to emulate synapses in hardware neuromorphic networks (NNs). The use of local learning rules for such NNs, for example, bioinspired spike-timing-dependent plasticity (STDP), has firmly established itself in recent years. In biological systems the basic STDP can be modified in the presence of neuromodulators (e.g. dopamine). This effect is believed to be essential for important biological functions such as reinforcement learning (RL), memory and others. The goal of this work was to demonstrate that such dopamine-like modulated STDP can be used in memristors based on a biocompatible polymer, parylene (poly-p-xylylene, or PPX). We have studied memristors both in the form of single Cu/PPX/ITO devices and in the form of crossbar Cu/PPX/Au structures. It was found that, in addition to stable memristive characteristics suitable for NNs, these devices can also change their conductance by means of bioinspired STDP rules, including dopamine-like modulated STDP window realized by introducing the coefficients for neuron spike amplitudes. The amplitude coefficients from −1 (inhibitory mode) to 1 (excitatory mode) of pre- and post-spikes, reflecting the 'dopamine' concentration, in various combinations allow observing the STDP window not only of the usual shape, but also of the anti-STDP, bell and anti-bell shapes. The obtained results demonstrate that the development of memristors based on PPX provides prospects for hardware realization of bio-inspired spiking NNs with RL ability.

The paper presents the results of a study at room temperature of the quantization effect of the conductivity of memristive structures based on the organic material poly-p-xylylene with resistive switching. Measurement methods are shown and a comparative analysis of the manifestation of the effect when switching structures to a high-resistance and low-resistance state is carried out. The possibility of setting stable quantum states of conductivity in memristive structures based on poly-p-xylylene is demonstrated. It is shown that some of these states have short-term, and some long-term stability. The results obtained open up new possibilities for using the quantization effect of conductivity in the implementation of neuromorphic systems.

Исследованы эффекты резистивного переключения 2-го порядка в мемристорах на основе поли-n-ксилилена (PPX). Обнаружено уменьшение времени переключения, вызванное эффектами 2-го порядка. Результаты указывают на возможность использования обнаруженных эффектов в нейроморфных вычислительных системах (НВС).

The results of studying the room-temperature conductance quantization of memristive structures based on poly-p-xylylene organic material with resistive switching are presented. The measurement procedures are shown and comparative analysis of manifestation of the effect upon switching structures to the high and low resistive states is presented. The possibility of specifying the stable quantum states of the conductance in memristive structures based on poly-p-xylylene is demonstrated. It is shown that some of these states possess short-term stability, while others possess long-term stability. These results open up new possibilities for using the conductance quantization effect in the implementation of neuromorphic systems.

In this study, structure and magnetic properties of poly(p-xylylene)-MnSb nanocomposite films with different inorganic filler content were investigated. The thickness of all studied films was about 200-300 nm as determined by scanning electron microscopy. It was found that the filler concentration strongly affects surface morphology of the composite films. AFM revealed that the film with the lowest MnSb content has globular surface mophology, presumably formed by polymeric grains with small MnSb nanoparticles on their surface. Ferromagnetic hysteresis loop was observed for all studied films at room temperature with the saturation magnetization values correlating with the MnSb content.

Нейроморфные вычислительные сети (НВС) с синаптическими связями на основе мемристоров могут обеспечить значительно большую эффективность аппаратной реализации биоподобных спайковых нейронных сетей, чем цифровые синаптические элементы на основе комплементарной технологии. Для реализации энергоэффективных и в перспективе самообучаемых НВС необходимо, чтобы сопротивление мемристора, связывающего пре- и постсинаптический нейроны, могло быть изменено по локальным правилам, например по правилам пластичности, зависящей от времени прихода пре- и постсинаптического импульсов (STDP). На примере мемристивных структур Cu/поли-пара-ксилилен (РРХ)/оксид индия-олова (ITO), у которых верхний электрод (медь) выступал в качестве пресинаптического входа, а нижний (ITO) — в качестве постсинаптического, продемонстрирована возможность обучения мемристоров по правилам STDP. Найдены оптимальные значения амплитуды и длительности импульсов для прямоугольной и треугольной форм обучающих импульсов. Полученные результаты открывают перспективы создания автономных НВС, способных к обучению с учителем и без него для решения сложных когнитивных задач.

In this study, wide-angle X-ray diffraction, X-ray absorption spectroscopy, and transmission electron microscopy were employed to address the crystalline structure and morphology of poly(p-xylylene)-PbS nano composite thin films prepared by vapor deposition polymerization as well as their evolution upon thermal annealing. It was found that as-synthesized samples with different PbS contents demonstrate similar diffraction patterns that cannot be fully ascribed to a decrease in crystallite size, indicating distorted crystal structure of PbS nanoparticles compared to the bulk PbS. X-ray absorption spectroscopy reveals wide distribution of Pb-S bond lengths with a minimum value of 2.67 angstrom, which can be attributed to the presence of molecular (PbS)(n) clusters in the studied films. It was shown that thermal annealing can be used to control the size of PbS nanoparticles and, as a consequence, optical properties of the composite films. The UV-vis absorption spectra demonstrate pronounced red shift of the absorption edge correlated with the growth of PbS nanoparticles upon annealing. Comprehensive analysis of several theoretical models describing the effect of nanoparticles size on optical band gap of the composite material has been performed and compared with the experimental data.

Flexible memristive structures based on poly-para-xylelene layers, which exhibit stable resistive switchings and are resistant to bendings with radii up to 10 mm, have been prepared and studied. A two-step scheme of setting the resistive state of the memristive structure is proposed, which is based on monitoring the compliance current passing through the structure. The results obtained allow memristive structures based on poly-para-xylelene layers to be used for neuromorphic computational systems and biocompatible “wearable” electronics.

—Neuromorphic computer networks (NCNs) with synaptic connections based on memristors can provide much greater efficiency in the hardware implementation of bio-inspired spiking neural networks than digital synaptic elements based on complementary technology. To achieve energy-efficient and, in the long-term, self-learning NCNs, the resistance of a memristor connecting pre- and postsynaptic neurons needs to be changeable according to local rules, e.g., according to the rules of spike-timing-dependent plasticity—STDP. The possibility of memristor training according to STDP rules was demonstrated by the example of Cu/poly- p -xylylene (PPX)/indium tin oxide (ITO) memristive structures, in which the top electrode (copper) acted as the presynaptic input, and the bottom (ITO), as the postsynaptic input. The optimal pulse amplitude and duration values are found for rectangular and triangular training pulses. The results open up prospects for creating autonomous NCNs capable of supervised and unsupervised learning to solve complex cognitive problems.

This work presents the results of the fabrication and investigation of flexible memristive structures based on parylene layers, which demonstrate stable resistive switching and are resistant to bends up to 10 mm radii. It is also proposed a two-step scheme for establishing the resistive state of the memristive structure, based on control of the value of the limiting current flowing through the structure. Obtained results open the possibility of using memristive structures based on parylene layers for neuromorphic computing systems and biocompatible "wearable" electronics.