Resistive switching memory performance in oxide hetero-nanocrystals with well-controlled interfaces

Enhancing Long-Term Memory in Carbon-Nanotube-Based Optoelectronic Synaptic Devices for Neuromorphic Computing

This study investigates the impact of spin-coating speed on the performance of carbon nanotube (CNT)-based optoelectronic synaptic devices, focusing on their long-term memory properties. CNT films fabricated at lower spin speeds exhibited a greater thickness and density compared to those at higher speeds. These denser films showed enhanced persistent photoconductivity, resulting in higher excitatory postsynaptic currents (EPSCs) and the prolonged retention of memory states after UV stimulation. Devices coated at a lower spin-coating speed of 2000 RPM maintained EPSCs above 70% for 3600 s, outperforming their higher-speed counterparts in long-term memory retention. Additionally, the study demonstrated that the learning efficiency improved with repeated UV stimulation, with fewer pulses needed to achieve the maximum EPSC in successive learning cycles. These findings highlight that optimizing spin-coating speeds can significantly enhance the performance of CNT-based synaptic devices, making them suitable for applications in neuromorphic computing and artificial neural networks requiring robust memory retention and efficient learning.

Crystal orientation of epitaxial film deposited on silicon surface

Direct growth of oxide film on silicon is usually prevented by extensive diffusion or chemical reaction between silicon (Si) and oxide materials. Thermodynamic stability of binary oxides is comprehensively investigated on Si substrates and shows possibility of chemical reaction of oxide materials on Si surface. However, the thermodynamic stability does not include any crystallographic factors, which is required for epitaxial growth. Adsorption energy evaluated by total energy estimated with the density functional theory predicted the orientation of epitaxial film growth on Si surface. For lower computing cost, the adsorption energy was estimated without any structural optimization (simple total of energy method). Although the adsorption energies were different on simple ToE method, the crystal orientation of epitaxial growth showed the same direction with/without the structural optimization. The results were agreed with previous simulations including structural optimization. Magnesium oxide (MgO), as example of epitaxial film, was experimentally deposited on Si substrates and compared with the results from the adsorption evaluation. X-ray diffraction showed cubic on cubic growth [MgO(100)//Si(100) and MgO(001)//Si(001)] which agreed with the results of the adsorption energy.

Boosting Thermoelectric Performance in Epitaxial GeTe Film/Si by Domain Engineering and Point Defect Control

This study demonstrates a simultaneous realization of ultralow thermal conductivity and high thermoelectric power factor in epitaxial GeTe thin films/Si substrates by a combination of the interface introduction by domain engineering and the suppression of Ge vacancy generation by point defect control. We formed epitaxial Te-poor GeTe thin films having low-angle grain boundaries with a misorientation angle close to 0° or twin interfaces with a misorientation angle close to 180°. The control of interfaces and point defects gave rise to ultralow lattice thermal conductivity of ∼0.7 ± 0.2 W m^-1 K^-1. This value was the same in the order of magnitude as the theoretical minimum lattice thermal conductivity of ∼0.5 W m^-1 K^-1 calculated by the Cahill-Pohl model. At the same time, the GeTe thin films exhibited a high thermoelectric power factor because of the suppression of Ge vacancy generation and a small contribution of grain boundary carrier scattering. The outstanding combined technique of domain engineering and point defect control can be a great approach for developing high-performance thermoelectric films.

Tunable Thermal Switch via Order-Order Transition in Liquid Crystalline Block Copolymer

Organic material-based thermal switch is drawing much attention as one of the key thermal management devices in organic electronic devices. This study aims at tuning the switching temperature (T_S) of thermal conductivity by using liquid crystalline block copolymers (BCs) with different order-order transition temperature (T_tr) related to the types of mesogens in the side chain. The BC films with low T_tr of 363 K and high T_tr of 395 K exhibit reversible thermal conductivity switching behaviors at T_S of ∼360 K and ∼390 K, respectively. The BC films also exhibit thermal conductivity variation originating from the anisotropy of the internal structures: poly(ethylene oxide) domains and liquid crystals. These results demonstrate that the switching behavior is attributed to an order-order transition between BC films with vertically arranged cylinder domains and the ones with ordered sphere domains. This highlights that BCs become a promising thermal conductivity switching material with tailored T_S.

Self-Assembly Nanopillar/Superlattice Hierarchical Structure: Boosting AlGaN Crystalline Quality and Achieving High-Performance Ultraviolet Avalanche Photodetector

As a burgeoning wide-band gap semiconductor material, AlxGa1-xN alloy has attracted great attention for versatile applications due to its superior properties. However, its poor crystalline quality has restricted the employment of AlGaN on electronic devices for a long time. Herein, we proposed a nanopillar/superlattice hierarchical structure for AlGaN epitaxy to boost the crystalline quality. The scale-controllable AlN nanopillar template is fabricated from a nickel self-assembly process. AlGaN initiates the epitaxial laterally overgrowth mode based on the nanopatterned template. In addition, the AlxGa1-xN/AlyGa1-yN superlattice structure could effectively block the propagation of threading dislocation segments. The kinetics of the dislocation and epitaxy process in the hierarchical structure is intuitively demonstrated and analyzed. Consequently, the dislocation density of AlGaN grown by this method is significantly reduced by more than 30 times compared to the AlN template. No threading dislocation segments were observed in the 4 μm TEM field of view. Moreover, based on the hierarchical structure, we also fabricated an AlGaN ultraviolet avalanche photodiode (APD). The APD exhibits superior performance, achieving a maximum gain of 1.3 × 105 and high responsivity of 1.46 A/W at 324 nm. The reliability of the nanopillar/superlattice AlGaN epitaxial procedure is anticipated to shed new light on the nitride semiconductor material, further bringing a breakthrough to wide-band gap electronic devices.

Preparation of Multilayered Core-Shell Fe3O4-SnO2-C Nanoparticles via Polymeric/Silane-Amino Functionalization

Multilayered core–shell Fe₃O₄-SnO₂-C nanoparticles were prepared via surface treatment and carbonization at atmospheric pressure. Fe₃O₄-SnO₂ nanoparticles were prepared by the carboxylation of the pivotal particles (Fe₃O₄) with an anionic surfactant to immobilize SnO₂ nanoparticles. A method was proposed to externally surround hydrophilic carbon with amine-forming materials, polyethyleneimine (PEI), and (3-Aminopropyl) triethoxysilane (APTES). The synthesis strategy was based on the electrostatic bonding of the introduced amine group with the hydroxyl group on the carbon precursor and the carbonization of the coating layer by the catalytic reaction of sulfuric acid.

Electron Doping Effect in the Resistive Switching Properties of Al/Gd1-xCaxMnO3/Au Memristor Devices

We report on the resistive switching (RS) properties of Al/Gd1-xCaxMnO3 (GCMO)/Au thin-film memristors. The devices were studied over the whole calcium substitution range x as a function of electrical field and temperature. The RS properties were found to be highly dependent on the Ca substitution. The optimal concentration was determined to be near x = 0.9, which is higher than the values reported for other similar manganite-based devices. We utilize an equivalent circuit model which accounts for the obtained results and allows us to determine that the electrical conduction properties of the devices are dominated by the Poole-Frenkel conduction mechanism for all compositions. The model also shows that lower trap energy values are associated with better RS properties. Our results indicate that the main RS properties of Al/GCMO/Au devices are comparable to those of other similar manganite-based materials, but there are marked differences in the switching behavior, which encourage further exploration of mixed-valence perovskite manganites for RS applications.

Phonon transport in the nano-system of Si and SiGe films with Ge nanodots and approach to ultralow thermal conductivity

Phonon transport in the nano-system has been studied using well-designed nanostructured materials to observe and control the interesting phonon behaviors like ballistic phonon transport. Recently, we observed drastic thermal conductivity reduction in the films containing well-controlled nanodots. Here, we investigate whether this comes from the interference effect in ballistic phonon transport by comparing the thermal properties of the Si or Si0.75Ge0.25 films containing Ge nanodots. The experimentally-obtained thermal resistance of the nanodot layer shows peculiar nanodot size dependence in the Si films and a constant value in the SiGe films. From the phonon simulation results, interestingly, it is clearly found that in the nanostructured Si film, phonons travel in a non-diffusive way (ballistic phonon transport). On the other hand, in the nanostructured SiGe film, although simple diffusive phonon transport occurs, extremely-low thermal conductivity (∼0.81 W m-1 K-1) close to that of amorphous Si0.7Ge0.3 (∼0.7 W m-1 K-1) is achieved due to the combination of the alloy phonon scattering and Ge nanodot scattering.

High-Uniformity Threshold Switching HfO2-Based Selectors with Patterned Ag Nanodots

High-performance selector devices are essential for emerging nonvolatile memories to implement high-density memory storage and large-scale neuromorphic computing. Device uniformity is one of the key challenges which limit the practical applications of threshold switching selectors. Here, high-uniformity threshold switching HfO2-based selectors are fabricated by using e-beam lithography to pattern controllable Ag nanodots (NDs) with high order and uniform size in the cross-point region. The selectors exhibit excellent bidirectional threshold switching performance, including low leakage current (<1 pA), high on/off ratio (>108), high endurance (>108 cycles), and fast switching speed (≈75 ns). The patterned Ag NDs in the selector help control the number of Ag atoms diffusing into HfO2 and confine the positions to form reproducible filaments. According to the statistical analysis, the Ag NDs selectors show much smaller cycle-to-cycle and device-to-device variations (C V < 10%) compared to control samples with nonpatterned Ag thin film. Furthermore, when integrating the Ag NDs selector with resistive switching memory in one-selector-one-resistor (1S1R) structure, the reduced selector variation helps significantly reduce the bit error rate in 1S1R crossbar array. The high-uniformity Ag NDs selectors offer great potential in the fabrication of large-scale 1S1R crossbar arrays for future memory and neuromorphic computing applications.