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Magnetic skyrmions are nanoscale spin structures which can be stabilized at room temperature in multilayer films [1, 2]. In particular, the amorphous multilayer film presents a pinning-free material platform, enabling efficient spin-orbit-torque manipulation of skyrmion dynamics. The device-relevant configuration of dots is of immense interest for realizing skyrmionic tunnel junctions for next-generation memory and computing applications [3]. Here, we first establish an amorphous multilayer composite stack comprising [Pt/CoB(x)/Ir]3/[Pt/CoB(y)/MgO], where x: 1.7-2.2 nm and y: 1-1.4 nm, with demonstrable skyrmions of sizes ~100 nm stabilized at zero field (ZF). By varying both the bottom (x) and top (y) CoB thicknesses, the magnetic anisotropy, Keff, can be tuned by an order from -0.03 to 0.126 MJ/m3. Next, we present ZF stabilization of skyrmions confined in such multilayer nanodots over a range of Keff and geometric sizes (w: 300 – 1000nm). Kerr magnetometry characterization of these nanodot arrays shows a monotonic increase in squareness with decreasing w. Expectedly, at ZF, we observe a gradual evolution in magnetic textures from the labyrinthine stripe to skyrmion phase, and eventually to the uniform magnetisation phase with w reduction (Figure 1). Notably, the stabilization of ZF skyrmions is observed over a wide w range from 400 to 700 nm across samples and skyrmion multiplets are stabilized in dots with low Keff. Finally, we demonstrate that at intermediate OP applied fields, skyrmion multiplets can be stabilized across varying w. Micromagnetic simulations are performed to model the geometric confinement effect of the nanodots on the skyrmion nucleation probability and size. These results provide a platform for harnessing the properties of nanoscale skyrmions in confined geometries for device applications.

MATERIALS FOR HUMANITY (MH 21)

Study of Confinement Effects on Skyrmions in Amorphous Multilayer Nanodots

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technical paper

Emulating advanced synaptic functions of the human brain with electronic devices contributes an important step toward constructing high-efficiency neuromorphic systems. Ferroelectric materials are promising candidates as synaptic weight elements in neural network hardware due to their controllable polarization states. However, the increased depolarization field at the nanoscale and the complex fabrication process of the traditional ferroelectric materials hamper the development of high-density, low power and highly sensitive synaptic devices. Here we report the implementation of 2D ferroelectrics α-In2Se3 as an active channel material to emulate typical synaptic functions. The α-In2Se3-based synaptic device features multi-mode operations, enabled by the coupled ferroelectric polarization under various voltage pulses applied at both drain- and gate-terminals. Moreover, the energy consumption can be reduced to ~ 1 pJ by using high-κ dielectric (Al2O3). The successful control of ferroelectric polarizations in α-In2Se3 and its application in artificial synapses are expected to inspire the implementation of 2D ferroelectric materials for future neuromorphic systems. （Acknowledgment: This work was supported by the Science and Engineering Research Council of A*STAR (Agency for Science, Technology and Research) Singapore, under Grant No. A20G9b0135.

Intrinsic Polarization Coupling in 2D α-In2Se3 toward Artificial Synapse with Multi-mode Operations

Two-dimensional (2D) van der Waals heterostructure (vdWH)-based floating gate devices show great potential for next-generation nonvolatile and multilevel data storage memory. However, high program voltage induced substantial energy consumption, which is one of the primary concerns, hinders their applications in low-en- ergy-consumption artificial synapses for neuromorphic computing. In this study, we demonstrate a three-terminal floating gate device based on the vdWH of tin disul- fide (SnS2), hexagonal boron nitride (h-BN), and few-layer graphene. The large electron affinity of SnS2 facilitates a significant reduction in the program voltage of the device by lowering the hole-injection barrier across h-BN. Our floating gate device, as a nonvolatile multilevel electronic memory, exhibits large on/off current ratio (~10 ), good retention (over 10 s), and robust endurance (over 1000 cycles). Moreover, it can function as an artificial synapse to emulate basic synaptic func- tions. Further, low energy consumption down to ~7 picojoule (pJ) can be achieved owing to the small program voltage. High linearity (<1) and conductance ratio (~80) in long-term potentiation and depression (LTP/LTD) further contribute to the high pattern recognition accuracy (~90%) in artificial neural network simulation. The proposed device with attentive band engineering can promote the future development of energy-efficient memory and neuromorphic devices.

Band-tailored van der Waals heterostructure for multilevel memory and artificial synapse

To sense is the ability to extract crucial information from the environment. However, the environment is dynamic, light levels during the day differ from the night. While current photodetectors show high sensitivity towards light stimuli, their dynamic range is limited by the ambient conditions. Fundamentally, they are unable to adapt chemically as well as our eyes. Here, a MoO3 - based electrochromic photodetector is demonstrated to sense with 2 adaptation modes – photopic and scotopic vision. The adjustment of sensitivity in demand depending on the requirement paves the way for environmentally adaptable devices.

Artificial eye that adapts to the environment - electrochromic photodetector

The association of sensory stimuli in the environment forms the basis of learning. Classical conditioning is a key benchmark of associative learning capabilities. While classical conditioning had been emulated in synaptic devices, the suppression of conditioning is yet to be explored. Here we emulate such inhibitory behaviour with an associatively coupled optoelectronic synapse which demonstrates altered plasticity behaviour. Such adaptable synapses are crucial building blocks in neuromorphic circuits capable of learning.

Emulating latent inhibition effects in the conditioning paradigm with an optoelectronic artificial synapse

We experimentally study the spin–orbit torque (SOT) in PtRh/heterostructures by varying the composition of PtRh alloy. By performing dc-biased spin-torque ferromagnetic resonance and second-harmonic measurements in PtxRh1−x/ferromagnet heterostructures, we find that the effective damping-like spin-torque efficiency and spin Hall conductivity are 0.18 and 3.8 × 105̵ h/2e Ω−1 m−1 for Pt0.9Rh0.1, respectively, with a low resistivity of 46.9 μΩ cm. Furthermore, current induced SOT switching in PtRh/Co is investigated. The critical current density for SOT switching decreases with an increase in the Rh composition of the PtRh alloy, which can be understood by domain wall assisted switching. Due to a large spin Hall conductivity, a relatively low resistivity, and sustainability of the high temperature process, the PtRh alloy could be an attractive spin source for SOT applications. Ref: 1. APL Mater. 9, 041105 (2021).

Composition dependence of spin– orbit torques in PtRh/ferromagnet heterostructures

Two-dimensional magnetic materials have attracted much attention due to their intrinsic ferromagnetism and potential application in spintronic devices. Thickness-dependent magnetic behavior has previously been observed in chemical vapor deposition-grown chromium selenide. However, the low-dimensional structure in non-layered chromium selenide which plays a crucial role in determining the low-dimensional magnetic order, needs further study. Here, we report the structure-dependent magnetic properties in monolayer CrSe2 and Cr2Se3 grown by molecular beam epitaxy. By controlling the growth temperature, monolayer CrSe2 and Cr2Se3 can be fabricated in a controllable manner. In the monolayer CrSe2 , 1T-CrSe2 with a lattice constant of 3.3 Å has a metallic character, co-existing with an antiferromagnetic 1T″ phase with a 2 × 2 surface periodicity and a gap opening of ~50 meV, confirmed by the scanning tunneling microscopy study. Monolayer CrSe2 can be transformed into the air-stable Cr2Se3 with a lattice constant of 3.6 Å by annealing at 300 ºC. X-ray magnetic circular dichroism (XMCD) measurements combined with DFT calculations reveal that whilst the MBE-grown monolayer CrSe2 is antiferromagnetic, monolayer Cr2Se3 is ferromagnetic with a Curie temperature of ~200 K. This work demonstrates the structural diversity in non-layered chromium selenide with thickness down to one monolayer, and the critical effect of different structures on its electronic and magnetic properties.

Diverse structures and magnetic properties in non-layered monolayer chromium selenide

2D devices usually rely on deposited or transferred insulators as the dielectric layer, resulting in various challenges in device compatibility and fabrication complexity. Here, we demonstrate a controllable and reliable oxidation process to turn 2D semiconductor HfS2 into native oxide, HfOx , which shows good insulating property and clean interface with HfS2 . We then incorporate the HfOx /HfS2 heterostructure into a flash memory device, achieving a high on/off current ratio of ~105 , a large memory window over 60 V, good endurance and long retention time over 103 seconds. Especially, the memory device can work as an artificial synapse to emulate basic synaptic functions and feature good linearity and symmetry in conductance change during long-term potentiation/depression processes. Our work provides a simple and effective approach for integrating high-k dielectrics into 2D material-based memory and synaptic devices.

Dielectric Engineering for 2D Materials Based Functional Devices

Using in-situ TEM¹ we follow the nanoscale wet etch chemistry and dynamics of amorphous silicon (a-Si) and crystalline silicon (c-Si) nanopillars in real-time, showing that: 1. Intermediate reaction products generated during alkaline wet-etching first aggregate as nanoclusters on the a-Si surface and subsequently nanoclusters detach, diffuse, and finally dissolve in the etchant solution. 2. Control experiments with Isopropyl alcohol (IPA) and molecular dynamics simulations¹ suggest that hydroxyl groups (OH- ) on the Si surface are primarily responsible for holding the nanoclusters of the intermediates at the nanopillar surface. 3. c-Si etching can controllably be switched between different crystallographic planes. This Si etch anisotropy arises from dynamic interaction between the byproducts and the Si surface. Preliminary data from ergy-filtered TEM, STEM and electron energy-loss spectroscopy (EELS) show promissing results and in future using these techniques, we aim to obtain a deeper understanding of the reaction dynamics.

Scaling-down silicon nanowires to sub-10 nm in alkaline solutions for next-generation 3D device application

Motivation - Atomically thin transition metal dichalcogenide (TMDC) monolayer (ML) have gained significant attention in recent years. For Tungsten Disulfide (WS2 ) ML in particular, an electron mobility higher than 1000 cm2V−1 s −1 was predicted [1] thus leading to a significant motivation to conduct experimental work on the growth of this material using scalable growth techniques such as Chemical Vapour Deposition (CVD) [2,3].

Growth of Aligned WS2 Monolayer on Vicinal Surface of Sapphire

Using the molecular beam epitaxy, bismuthene homostructure consisting of a hexagonal bismuthene monolayer on a rectangular black-phosphorus-like bismuth monolayer was realized. The STM/STS and nc-AFM measurements revealed moiré superlattices with twist angle in the homostructure. Most importantly, for the first time, the topological edge states in the bismuthene were found to be well modulated by the moirésuperstructure. Combined theoretical calculations identified that the topological edge states were strongly controlled by atomic site-specific interface interactions between the two bismuth monolayers, suggesting the moiré-like modulation of topological edge states.

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