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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.

MATERIALS FOR HUMANITY (MH 21)

Construction of silicene/phosphene heterojunction structure by molecular beam epitaxy

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MH 21 is arranged around 6 themes that focus attention on pressing needs that should be addressed to enhance the well-being of communities and provide a forum for presenting breakthrough discoveries.

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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.

Single lanthanide activator regulated triple excitations-emissions upconversion luminescence

The ferromagnetism in Cr-based monolayers is of current interest (2019 Nat. Nanotechnol. 14 408), however, the Curie temperature is low. How can we enhance the thermal stability of ferromagnetism? Motivated by the recent synthesis of the layered conductive magnet CrCl2(pyrazine)2 (2018 Nat. Chem. 10 1056), we perform first-principles calculations and Monte Carlo simulations to demonstrate that the exfoliated 2D CrCl2(pyrazine)2 monolayer is stable dynamically and thermally, and it is a ferromagnetic half-metal with a sizeable band gap of 2.8 eV in the semiconducting channel, and the strong in-plane Cr–Cr interaction results in a large magnetic anisotropy energy. Moreover, the sheet exhibits a high Curie temperature of 350 K due to the enhanced magnetic exchange interaction resulting from the aromatic property of pyrazine. All of these intriguing features endow 2D CrCl2(pyrazine)2 sheet with good potentials for applications in nanoscale spintronics devices.

2D CrCl2(pyrazine)2 monolayer: high-temperature ferromagnetism and half-metallicity

1) Ni-Mn-Ga system exhibits magnetic shape memory effect as a giant magnetic field-induced strain (MFIS) [1,2]. 2) MFIS is possible due to the high mobility of twin boundaries coupled with large magneto-crystalline anisotropy [1,3]. 3) MFIS was observed only in modulated five-layered (10M) or seven-layered (14M) martensites in alloys with near-stoichiometric compositions [4]. 4) Non-modulated (NM) martensite with tetragonaly disorted L21 is stable in compositions far from stoichiometry [4]. It does not exhibit significiant MFIS [5]. 5) We performed ab initio calculations to reveal, how increasing concentration of excess Mn affects formation and propagation of twin boundaries. An effect of local arrangement of excess Mn in Ga sublattice has been considered as well.

First-Principles study of twin formation in Mn-excess Ni-Mn-Ga alloys

It has been a grand challenge to develop efficient thermoelectric materials for room temperature (300- 500K) applications. Recently 2D Sn2Bi sheet has been synthesized on silicon wafer and exhibits a high band degeneracy, which is favorable for high TE performance. In this work, for the first time we demonstrate that fluorination not only can stabilize Sn2Bi sheet but also can significantly improve the TE performance. Because fluorine has a heavier atomic mass and a larger electronegativity, which can lead to a lower phonon group velocity and larger anharmonicity. Inspired by these advances, we show that fluorinating Sn2Bi sheet can significantly improve the thermoelectric performance at room temperature.

The ultralow thermal conductivity and ultrahigh thermoelectric performance of fluorinated Sn2Bi sheet in room temperature

This work is focused on a theoretical study of the phase stability of solid solutions in Al-Ge and Ag-Pb alloys, in which there are no intermediate phases across the whole composition range. The solubilities of the solid solutions were determined using temperature-dependent free energies of pure elements and respective Al- and Ag-rich solid solutions obtained from ab initio calculations based on density functional theory. All total energy calculations were performed by Vienna Ab initio Simulation Package (VASP) [1] with the help of Projector Augmented-wave potentials [2, 3]. Contributions of vibrational free energy and electron free energy were obtained from Phonopy package [4]. The temperatures, at which the solid solutions become stable, were estimated from in the free energy difference between solid solution and mixture of pure elements according to [5]. For Ag-Pb alloy, cluster expansion method and subsequent Monte Carlo simulations with temperature dependent expansion coefficients [6] were employed in order to calculate the phase boundary as well. Also in both cases, quasi-harmonic approximation was employed to include the effect of volume expansion. Including this effect provides results closer to experimental observations in both cases. The resulting solubilities were compared with experimental ones in phase diagrams obtained from the CALPHAD method. The results correspond very well to the experimental measurements qualitatively, however the calculated temperature values are underestimated. References [1] G. Kresse and J. Hafner, Phys. Rev. B 47, 558 (1993); ibid. 49, 14 251 (1994). [2] P. E. Blöchl, Phys. Rev. B 50, 17953 (1994). [3] G. Kresse and D. Joubert, Phys. Rev. 59, 1758 (1999). [4] A. Togo and I. Tanaka, Scr. Mater., 108, 1-5 (2015) [5] V. Ozolinš and M. Asta, Phys. Rev. Lett., 86, 448 (2001) [6] A. van de Walle and G. Ceder, J. of Phase Equilibria, 23, 348 (2002)

Computational prediction of solubility in Al-Ge and Ag-Pb alloys

Motivation - FeCo alloys, characterized by the highest saturation magnetization, good permeability in magnetic fields, and high Curie temperatures have become indispensable materials for inductive applications such as electrical machines and motors. However, the magnetomechanical properties are ruled by complex temperature-ordering relationships. These effects have been studied experimentally and by ab initio computations, but the microstructural origin is still unconclusive. Alternatively, semi-empirical molecular dynamics (MD) can accurately describe relatively large atomic systems. Interatomic potentials for FeCo alloys are already available but focused on the study of high entropy alloys (HEA), thus there is a lack on the proper description of the thermo-mechanical characteristics. Consequently, a FeCo focused potential is required which can also become an important piece to the study of other FeCo based alloys.

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Construction of silicene/phosphene heterojunction structure by molecular beam epitaxy

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