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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 (&lt;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.

MATERIALS FOR HUMANITY (MH 21)

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

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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.

poster

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.

The effect of moiré superstructures on topological edge states in twisted bismuthene homojunctions

Phosphorene, which is known as black phosphorus (BP), possesses many extraordinary properties, such as high charge mobility and on/off ratio, and thickness-dependent direct bandgap from 0.3 eV to 2 eV. And recently, blue phosphorus (Blue P) with unique effects like fluorination-induced quantum spin Hall insulators synthesized by molecular beam epitaxy (MBE) fulfills the family of two-dimension (2D) phosphorene. Silicene, the 2D silicon allotrope analogue of graphene, was predicted to present interesting structures and properties like an electronic structure hosting Dirac cones. A great deal of work has been done to synthesize phosphorene and silicene and further exploit their properties. And van der Waals heterostructures create a new platform through vertically stacked layer by layer by multiple 2D materials, for exploring innovative structures or functionalities of developing devices. It is very promising for constructing phosphorene-silicene heterojunctions grown by the MBE method.

Construction of silicene/phosphene heterojunction structure by molecular beam epitaxy

The kagome lattice, a two-dimensional network of corner-sharing triangles, is an extensively known platform to investigate the interplay of electron topology and strong correlations. The realization and electronic structure study of kagome bulk crystals, such as Fe3Sn2, Co3Sn2S2, CoSn et al, are reported recently in experiments. However, the complex interlayer interactions in kagome bulk crystals may probably disrupt the characteristic electronic structure like flat band and Dirac bands. Here, we report the growth of hexagonal FeGe bilayer nanoflakes on Ag(111) substrate in MBE method and the electronic structures studied using scanning tunneling microscopy/spectroscopy. Our qPlus atomic force microscopy measurements show that the as-grown FeGe nanoflakes are terminated with germanium hexagonal lattices preferentially. Moreover, the spectroscopic mapping across the nanoflake combined with the DFT calculations indicates that the FeGe nanoflakes possess topological nontrivial edge states. The growth of FeGe bilayer nanoflakes in two-dimensional limit paves the way to examine the pristine electronic structures of kagome lattice without interlayer interactions.

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