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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.

MATERIALS FOR HUMANITY (MH 21)

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

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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.

Growth of kagome metal FeGe bilayer nanoflakes — a scanning probe study

Wound infections following surgical procedures or traumatic injuries is a leading cause of disabilities and deaths. 1,2 In particular, the ubiquitous Pseudomonas aeruginosa pathogen is known to cause infections in chronic wounds. 3 Pseudomonas aeruginosa produces bacteria metabolites such as pyocyanin (PYO), which inhibits wound repair in a concentration dependent manner from 1 – 50 µM.

A Portable SERS Sensor for Pyocyanin Detection in Simulated Wound Fluid and through Swab Sampling

1) Odour released from wounds can be used as an indication of bacterial infections, and it contains volatile bacteria metabolites. 2) Trimethylamine (TMA) is one volatile metabolite produced by pathogens commonly isolated from wounds (P. aeruginosa, S. aureus). In infected wounds, TMA concentration typically ranges from 30-300 ppm. 3) We aim to develop a flexible, colorimetric TMA sensor that can be applied to detect wound infections.

Colorimetric Trimethylamine Sensor for Detection of Wound Infections

Photodynamic therapy (PDT) involves the activation of photosensitizers (PS) with a UV-visible (UV-vis) light source resulting in free radical production for tumor eradication. However, UV-vis light penetrates poorly and undergoes scattering in biological tissues. Hence, the use of upconversion nanoparticles (UCNPs) and near-infrared (NIR) light has been gaining traction in recent years. NIR light can relieve the scattering effect and has better tissue penetration, while UCNPs can convert NIR light to UV-vis light for PS activation. In this study, we report the efficacy of UCNPs clusters, generated by controlled assembly method and loaded with Chlorin e6 (Ce6), in inducing toxicity in vitro. 
An amphiphilic polymer, poly(styrene-co-maleic anhydride)(PSMA), was used to program UCNPs assembly by adjusting the polymer-to-UCNPs weight ratio (4.4 to 26.25). During particle assembly, Ce6 was loaded in the gap zones formed between connecting UCNPs. Adjusting the weight ratio led to the assembly of UCNPs cluster sizes from ~24nm to 140nm. UV absorption spectra revealed a concomitant increase in Ce6 loading with increasing cluster size. Cluster sizes of 24nm (single), 60nm (small), and 120nm (big) were used in subsequent studies. When excited with 980nm laser, energy transfer from UCNP to encapsulated Ce6 results in singlet oxygen formation. Big clusters showed the most superior singlet oxygen production, followed by small and single clusters. However, small clusters delivered the most Ce6 in 3D spheroids (MFI:1519) compared to free Ce6 (542.9), big (1170), and single clusters (837.1). Small clusters emerged top in killing 3D spheroids when excited with a visible light source. 
Although big clusters were superior in forming singlet oxygen, small clusters were the best nanocarrier for penetrating and killing 3D spheroids. The results indicated that the cluster sizes have different strengths, which can be exploited to facilitate their use as nanocarriers for cancer PDT.



Controllable Assembly of Upconversion Nanoparticles Enhanced Tumor Cell Penetration and Killing Efficiency

Upconversion is an optical process that involves the conversion of lower-energy photons into higher-energy photons. In this study, we have rationally developed photoswitching UCNPs with triple and non-overlapping emissive colors (i.e., red, green and blue) in response to three near-infrared (NIR) excitations (i.e., 1550 nm, 808 nm and 980 nm). Unlike their traditional counterparts, these triple emissive UCNPs are designed with a simple core–tri-shell structure and all the red, green, and blue upconversion luminescence reply on the single Yb3+ sensitizer ions. These specially developed UCNPs are believed to hold great potentials in tumor-targeted bioimaging and synergistic therapy.

Next from MATERIALS FOR HUMANITY (MH 21)

Dielectric Engineering for 2D Materials Based Functional Devices

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Spintronics with heavy metals, topological insulators and antiferromagnets

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