Improvement of Dispersibility of Graphene Oxide by Surface Modification with Rare Earths

Rare earth-modified graphene oxide (RE-M-GO) materials were successfully prepared by infiltration and heating modifier method. The morphology and phase structure of RE-M-GO were characterized by scanning electron microscopy(SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and energy dispersive spectrometer(EDS). The changes of the chemical structure were indicated by Fourier transform infrared (FTIR). X-ray photoelectron spectroscopy(XPS) was used to study the chemical state of the surface elements of graphene oxide which showed that the rare earth elements were added to the graphene oxide functional groups through the coordination reaction. Additionally, the findings concluded that the effect of modification by Ce is more obvious than La elements and the RE-M-GO materials prepared by the heating modifier method had better dispersibility than infiltration. With activating effect, the rare earth elements grafting to graphene oxide will contribute to its combination with other materials.

Phase Diagrams of the Excitonic Insulator State: Analyzing the Excitonic Susceptibility

In this paper, the formation of the excitonic insulator state in the rare-earth chalcogenides hasbeen investigated through the extended Falicov-Kimball model. Adapting the unrestricted Hartree-Fockapproximation, we have derived a set of explicitly self-consistent equations determining expectationvalues and the excitonic susceptibility in the system. Analyzing the excitonic susceptibility, we haveestablished phase diagrams of the excitonic insulator state depending on the model parameters. The phasestructures confirmed the excitonic insulator state is found at low temperature and between two criticalvalues of the Coulomb interaction. The effect of the external pressure on the formation of the excitonicinsulator state is also shown.

Identification of rare-earth minerals associated to K-feldspar: Capacsaya project in Peru

A recently discovered the rare-earth-rich site in Capacsaya, located at 123 km northwest of Cusco, at the south of Peru, contains significant quantities of light and heavy rare-earth elements such as neodymium, lanthanum, cerium, europium, and yttrium. This work reports the identification of rare-earth elements and their associated minerals using scanning electron microscopy, energy-dispersive X-ray spectroscopy and X-ray diffraction analyses. Five (5) samples extracted from different locations at the Capacsaya site were characterized and identified K-feldspar as the mineral associated with the rare-earth elements in a representative sample with a high concentration of lanthanum and cerium. The results showed rare-earth elements contained within the mineral phase monazite, being cerium the dominant element in the phase (La, Ce, Nd)PO$$_4$$ 4 . Finally, through the electrostatic separation process we demonstrate that it was possible to achieve an efficient separation of the K-feldspar phase in the particle size range 75–150 $$\upmu$$ μ m.

Rare earth nitrides and their applications in magnetic tunnel junctions

<p>In this thesis, we investigate the rare earth nitrides, a family of materials containing many intrinsic ferromagnetic semiconductors, with a particular focus on GdN and SmN.We investigate the rare earth nitride formation reaction, explore some properties of GdN and SmN, and finally manufacture and measure magnetic tunnel junctions which incorporate rare earth nitrides. The investigations of the reaction and properties of the materials are used to improve and understand the magnetic tunnel junctions. All samples and devices are grown at room temperature, giving polycrystalline rare earth nitride films. We show that a rare earth surface can catalytically break theN2 molecule at ambient temperature and low pressures. We follow the nitrogen reacting with the rare earth to form a rare earth nitride in real time via conductance measurements. By comparing the N2 cracking, reaction, and diffusion at both a RE and a REN surface we propose a pressure range in which the nitrogen content in SmN can be manipulated and conclude that the nitrogen in the top monolayers in a SmN film is mobile. In the investigation of GdN and SmN, we find that the conductivity of SmN follows the same behaviour as GdN when changing the N2 pressure during deposition. We follow the conductance change in SmN during deposition and propose a minimum thickness for room temperature deposited SmN films for consistent conductivity measurements. We report structural and magnetic changes in GdN which has been exposed to N-ions. We also present data on materials making ohmic contact to both GdN and SmN. Finally, we report the manufacturing and investigation of magnetic tunnel junctions using GdN and SmN electrodes with a GaN tunnel barrier. A new pattern design produces 20 devices, in a single deposition, which show consistent behaviour and expands on previous work on this topic. The main focus of the investigation is the J-V characteristics of the magnetic tunnel junctions which shows clear non-linear behaviour arising from tunnelling through the GaN. A Simmons fit to the J-V characteristics yields a barrier height of 0:8 eV and barrier thicknesses close to experimentally determined thicknesses. The J-V characteristics are investigated with changing temperature and changing applied magnetic field to investigate the effect of the ferromagnetism of the GdN and SmN electrodes. The tunnel magnetoresistance (TMR) of the devices show two contributions, a low-temperature TMR contribution and a 50K TMR contribution, and the maximum TMR for all devices are between 100% to 600%. The devices can withstand current densities up to 4000A/cm² and voltages up to 5V which is promising for a wide range of future applications.</p>

Rare earth nitrides and their applications in magnetic tunnel junctions

<p>In this thesis, we investigate the rare earth nitrides, a family of materials containing many intrinsic ferromagnetic semiconductors, with a particular focus on GdN and SmN.We investigate the rare earth nitride formation reaction, explore some properties of GdN and SmN, and finally manufacture and measure magnetic tunnel junctions which incorporate rare earth nitrides. The investigations of the reaction and properties of the materials are used to improve and understand the magnetic tunnel junctions. All samples and devices are grown at room temperature, giving polycrystalline rare earth nitride films. We show that a rare earth surface can catalytically break theN2 molecule at ambient temperature and low pressures. We follow the nitrogen reacting with the rare earth to form a rare earth nitride in real time via conductance measurements. By comparing the N2 cracking, reaction, and diffusion at both a RE and a REN surface we propose a pressure range in which the nitrogen content in SmN can be manipulated and conclude that the nitrogen in the top monolayers in a SmN film is mobile. In the investigation of GdN and SmN, we find that the conductivity of SmN follows the same behaviour as GdN when changing the N2 pressure during deposition. We follow the conductance change in SmN during deposition and propose a minimum thickness for room temperature deposited SmN films for consistent conductivity measurements. We report structural and magnetic changes in GdN which has been exposed to N-ions. We also present data on materials making ohmic contact to both GdN and SmN. Finally, we report the manufacturing and investigation of magnetic tunnel junctions using GdN and SmN electrodes with a GaN tunnel barrier. A new pattern design produces 20 devices, in a single deposition, which show consistent behaviour and expands on previous work on this topic. The main focus of the investigation is the J-V characteristics of the magnetic tunnel junctions which shows clear non-linear behaviour arising from tunnelling through the GaN. A Simmons fit to the J-V characteristics yields a barrier height of 0:8 eV and barrier thicknesses close to experimentally determined thicknesses. The J-V characteristics are investigated with changing temperature and changing applied magnetic field to investigate the effect of the ferromagnetism of the GdN and SmN electrodes. The tunnel magnetoresistance (TMR) of the devices show two contributions, a low-temperature TMR contribution and a 50K TMR contribution, and the maximum TMR for all devices are between 100% to 600%. The devices can withstand current densities up to 4000A/cm² and voltages up to 5V which is promising for a wide range of future applications.</p>

Effect of Rare-Earth Doping on the Structural and Optical Properties of the Ag3AsS3 Crystals

Non-linear optical (NLO) materials allow the production of the coherent laser beam in the difficult frequency ranges of the electromagnetic spectrum. Aiming to explore new classes of the NLO materials with high optical performance in the infrared (IR) region, in this work, we investigated the effect of the rare earth doping (Pr, Eu, Yb) on the crystal structure and optical properties of the Ag3AsS3 crystals. The performed analysis of the XRD patterns indicates that the rare earth elements are located in the Ag sites of the crystal lattice. As a result, the second harmonic generation (SHG) intensity, which determines the effectiveness of the NLO materials, increases with the increase of rare earth dopant content up to 1.0 %. Using the absorption analysis and Raman spectroscopy, we show that the increase in the SHG intensity can be related to the slight decrease of the bandgap, as well as with the increased electron-phonon interaction in rare-earth-doped Ag3AsS3 crystals. Considering the discovered enhancement of the SHG intensity, accompanied by the low melting temperature, this work offers rare-earth-doped Ag3AsS3 crystals as potential candidates for the non-linear optical applications for the infrared frequency range.