High Pressure-Driven Magnetic Disorder and Structural Transformation in Fe3 GeTe2 : Emergence of a Magnetic Quantum Critical Point.

Among the recently discovered 2D intrinsic van der Waals (vdW) magnets, Fe3 GeTe2 (FGT) has emerged as a strong candidate for spintronics applications, due to its high Curie temperature (130 - 220 K) and magnetic tunability in response to external stimuli (electrical field, light, strain). Theory predicts that the magnetism of FGT can be significantly modulated by an external strain. However, experimental evidence is needed to validate this prediction and understand the underlying mechanism of strain-mediated vdW magnetism in this system. Here, the effects of pressure (0 - 20 GPa) are elucidated on the magnetic and structural properties of Fe3 GeTe2 by means of synchrotron Mössbauer source spectroscopy, X-ray powder diffraction and Raman spectroscopy over a wide temperature range of 10 - 290 K. A strong suppression of ferromagnetic ordering is observed with increasing pressure, and a paramagnetic ground state emerges when pressure exceeds a critical value, PPM ≈ 15 GPa. The anomalous pressure dependence of structural parameters and vibrational modes is observed at PC ≈ 7 GPa and attributed to an isostructural phase transformation. Density functional theory calculations complement these experimental findings. This study highlights pressure as a driving force for magnetic quantum criticality in layered vdW magnetic systems.

Correction: Structural, magnetic and hyperthermia properties and their correlation in cobalt-doped magnetite nanoparticles.

[This corrects the article DOI: 10.1039/D1RA07407E.].

Structural, magnetic and hyperthermia properties and their correlation in cobalt-doped magnetite nanoparticles.

Cobalt doped magnetite nanoparticles (Co x Fe3-x O4 NPs) are investigated extensively because of their potential hyperthermia application. However, the complex interrelation among chemical compositions and particle size means their correlation with the magnetic and heating properties is not trivial to predict. Here, we prepared Co x Fe3-x O4 NPs (0 ≤ x ≤ 1) to investigate the effects of cobalt content and particle size on their magnetic and heating properties. A detailed analysis of the structural features indicated the similarity between the crystallite and particle sizes as well as their non-monotonic change with the increase of Co content. Magnetic measurements for the Co x Fe3-x O4 NPs (0 ≤ x ≤ 1) showed that the blocking temperature, the saturation magnetization, the coercivity, and the anisotropy constant followed a similar trend with a maximum at x = 0.7. Moreover, 57Fe Mössbauer spectroscopy adequately explained the magnetic behaviour, the anisotropy constant, and saturation magnetization of low Co content samples. Finally, our study shows that the relaxation loss is a primary contributor to the SAR in Co x Fe3-x O4 NPs with low Co contents as well as their potential application in magnetic hyperthermia.

One-step synthesis of NaLu80-xGdxF4:Yb183+/Er23+(Tm3+) upconversion nanoparticles for in vitro cell imaging.

Upconversion nanoparticles (UCNPs) possess a unique type of photoluminescence (PL) in which lower-energy excitation is converted into higher-energy emission via multi-photon absorption processes. In this work, we have used a facile one-step hydrothermal method promoted water solubility to synthesis NaLuGdF4:Yb3+/Er3+(Tm3+) UCNPs coated with malonic acid (MA). Scanning electron microscopy images and X-ray diffraction patterns reveal sphere-shaped UCNPs with an average size of ~80nm crystallized in the cubic NaLuF4 structure. The characteristic vibrations of cubic UCNPs have been taken into account by using Fourier-transform infrared spectroscopy. Based on PL studies, we have determined an optimal concentration of Gd3+ doping. The dependence of upconversion PL intensity on Gd3+ concentration is discussed via the results of magnetization measurements, which is related to the coupling/uncoupling of Gd3+ ions. Particularly, our study reveals that carboxyl-functionalized NaLuGdF4:Yb3+/Er3+(Tm3+) UCNPs have a relatively high cell viability with HeLa cells.

Minor hysteresis patterns with a rounded/sharpened reversing behavior in ferromagnetic multilayer.

Hysteresis of ferromagnetic system exhibits a fundamental stimulus-response behavior, thereby casting all the important macromagnetic system parameters such as coercivity, nucleation field, saturation magnetization, and hysteresis loss. Recently, increasing attention has been paid to exploration of relatively less understood minor loop behavior, since faster operation of magnetic devices is inevitably accompanied by minor hysteresis behavior from cycling among unsaturated ferromagnetic states. Here, we report our microscopic investigation of unusual minor hysteresis loop behavior, represented by rounded or sharpened response of minor hysteresis loop of (CoFeB/Pd)4 multilayer film. It is observed that rounded and sharpened response in the minor hysteresis response could be manifested under proper conditions. The minor loop behavior has been systematically investigated by direct microscopic magnetic domain observation using magneto-optical Kerr microscopy. The rounded response of magnetization at the reversing external field along the minor hysteresis curve, so far neglected or considered as one of 'unusual' behaviors, has been found to be elaborately controllable by tuning the reversing field strength and the field sweep rate for multilayers with low repeat numbers. Variable roundedness of the minor hysteresis loop is understandable based on the analysis of magnetic domain dynamics such as domain nucleation and the domain wall velocity.

Au-Coated Honeycomb Structure as an Efficient TCO-Free Counter-Electrode for Quantum-Dot-Sensitized Solar Cells.

This study reports the fabrication of a Petri dish patterned with cylindrical micro-cavities that are produced using a one-step solvent-immersion phase-separation process. The developed 3D honeycomb Petri dish is coated with a Au film through a sputtering method to be an efficient Au-coated FTO-free electrode for quantum-dot-sensitized solar cells. Due to the high specific active surface area of the electrode with the Au-coated honeycomb structure, the energy conversion efficiency of devices that use this electrode is 5.2 % compared to 4.4 and 4.7 % by devices using an Au-coated flat Petri dish and an Au-coated FTO electrode, respectively. This design strategy offers excellent potential for the fabrication of highly efficient counter electrodes with FTO-free substrates of flexible photovoltaic devices.

Tunable luminescent emission characterization of type-I and type-II systems in CdS-ZnSe core-shell nanoparticles: Raman and photoluminescence study.

We used wet chemical methods to synthesize core-shell nanocrystalline samples CdS(d)/ZnSe N , where d = 3-6 nm and N = 1-5 are the size of CdS cores and the number of monolayers grown on the cores, respectively. By annealing typical CdS(d)/ZnSe N samples (with d = 3 and 6 nm and N = 2) at 300 °C for various times t an = 10-600 min, we created an intermediate layer composed of Zn1-x Cd x Se and Cd1-x Zn x S alloys with various thicknesses. The formation of core-shell structures and intermediate layers was monitored by Raman scattering and UV-vis absorption spectrometers. Careful photoluminescence studies revealed that the as-prepared CdS(d)/ZnSe N samples with d = 5 nm and N = 2-4, and the annealed samples CdS(3 nm)/ZnSe2 with t an ≤ 60 min and CdS(6 nm)/ZnSe2 with t an ≤ 180 min, show the emission characteristics of type-II systems. Meanwhile, the other samples show the emission characteristics of type-I systems. These results prove that the partial separation of photoexcited carriers between the core and shell is dependent strongly on the engineered core-shell nanostructures, meaning the sizes of the core, shell, and intermediate layers. With the tunable luminescence properties, CdS-ZnSe-based core-shell materials are considered as promising candidates for multiple-exciton generation and single-photon sources.

Theoretical model for optical properties of porphyrin.

We propose a simple model to interpret the optical absorption spectra of porphyrin in different solvents. Our model successfully explains the decrease in the intensity of optical absorption at maxima of increased wavelengths. We also prove the dependence of the intensity and peak positions in the absorption spectra on the environment. The nature of the Soret band is supposed to derive from π plasmon. Our theoretical calculations are consistent with previous experimental studies.

Defect-induced ferromagnetism of mechanical-milled BaTi0.98Mn0.02O3 nanoparticles.

Room-temperature ferromagnetism was found in BaTi0.98Mn0.02O3 nanoparticles fabricated by solid-state reaction and mechanical-ball milling. The nature of ferromagnetism was systematically investigated by means of X-ray diffraction (XRD), electron spin resonance (ESR), superconducting quantum interference device (SQUID), and X-ray absorption (XAS). A weak ferromagnetism was found in the defect-rich samples with the milling time (t(m)) longer than 6 hrs (i.e., t(m) > 6 hrs), while defect-poor ones with t(m) = 0-6 hrs exhibited mainly the paramagnetic properties. Detailed analyses of ESR and XAFS data indicated the shift in the oxidation state of Mn from 4+ to 3+ after milling, and both Mn3+ and Mn4+ were incorporated into the Ti sites of the BaTiO3-tetragonal host lattice. With the results obtained, we believe that exchange interactions due to Mn(4+)-Mn4+ and/or Mn(3+)-Mn3+ dipole pairs are responsible for broad ESR signals in the Lorentzian shape and the paramagnetic behaviors of the samples. Meanwhile, the weak ferromagnetic interaction is generated from various point defects such as Ti-site cation vacancies or Ti(3+)-Ti3+ super-exchange interaction pairs.

Repulsive interactions of a lipid membrane with graphene in composite materials.

The van der Waals interaction between a lipid membrane and a substrate covered by a graphene sheet is investigated using the Lifshitz theory. The reflection coefficients are obtained for a layered planar system submerged in water. The dielectric response properties of the involved materials are also specified and discussed. Our calculations show that a graphene covered substrate can repel the biological membrane in water. This is attributed to the significant changes in the response properties of the system due to the monolayer graphene. It is also found that the van der Waals interaction is mostly dominated by the presence of graphene, while the role of the particular substrate is secondary.

Electric potential profile of a spherical soft particle with a charged core.

The electrostatic potential profile of a spherical soft particle is derived by solving the Poisson-Boltzmann equations on a spherical system both numerically and analytically. The soft particle is assumed to consist of an ion-permeable charged outer layer and a non-permeable charged core with constant charged density. The contribution of the core to the potential profile is calculated for different charges and dielectric constants. Our results show that the charged core heavily influences the local potential within the soft particle. By contrast, the potential distribution outside the particle in the salt solution is found to be weakly dependent on the core features. These findings are consistent with previous experiments showing the minor impact of the core of the MS2 virus on its overall electrical properties. Our studies also indicate that while a change in temperature from 290 K to 310 K only slightly varies the potential, the ionic strength in the range of 1-600 mM has a significant effect on the potential profile. Our studies would provide good understanding for experimental research in the field of biophysics and nanomedicine.

Raman scattering in Me-doped ZnO nanorods (Me = Mn, Co, Cu and Ni) prepared by thermal diffusion.

We have investigated normal and resonant Raman scattering in Me-doped ZnO nanorods (Me = Mn, Co, Cu and Ni) prepared by thermal diffusion. Experimental results show that the normal Raman spectra consist of the conventional modes associated with wurtzite ZnO and impurity-related additional modes. Under resonant conditions, only longitudinal optical (LO) phonon scattering and its overtones are observed. The number of LO phonon lines and their relative intensity depend on the doping element and level. For the nanorods doped with Cu and Ni, we have observed LO phonon overtones up to eleventh order. This situation does not happen for the Mn-doped nanorods, which show only five LO phonon modes. By co-doping Mn and Co into the ZnO host lattice, however, the LO phonon overtones up to eleventh order are observed again. The nature of this phenomenon is explained by means of the study of XRD, TEM and photoluminescence.