A simple approach to analyze layer-dependent optical properties of few-layer transition metal dichalcogenide thin films.

Intense interest in transition metal dichalcogenides (TMDs) is driven, among other things, by their intriguing layer-dependent optical properties down to the monolayer and strong excitonic effects. Experimental determination of optical constants such as the complex refractive index and dielectric function of large-area TMDs thin films is prone to large uncertainties at the few-layer level using a single measurement technique such as spectroscopic ellipsometry (SE). In this work, we demonstrate an approach to accurately determine the optical constants by combining transmission spectroscopy with SE. Using MoS2 as an example, the prototypical TMD material, we demonstrate our approach on transparent (Quartz, Al2O3) and absorbing (e.g. Si/SiO2) substrates. We find that pre-characterized optical properties from transmission spectroscopy significantly improve the quantitative accuracy of optical constants obtained using SE on both transparent and absorbing substrates. We show that the extracted optical constants from samples deposited on transparent substrates are highly reliable in obtaining layer-dependent optical properties of TMDs while data on absorbing Si/SiO2 substrate suffer from strong substrate contribution. Overall, we strongly conclude that SE must be combined with transmission spectroscopy to obtain optical constants with high quantitative accuracy. Our approach, though demonstrated on few-layer films, will be applicable to monolayer and bilayer TMDs.

Magnetic, structural and magnetocaloric properties of Ni-Si and Ni-Al thermoseeds for self-controlled hyperthermia.

Self-controlled hyperthermia is a non-invasive technique used to kill or destroy cancer cells while preserving normal surrounding tissues. We have explored bulk magnetic Ni-Si and Ni-Al alloys as a potential thermoseeds. The structural, magnetic and magnetocaloric properties of the samples were investigated, including saturation magnetisation, Curie temperature (TC), and magnetic and thermal hysteresis, using room temperature X-ray diffraction and magnetometry. The annealing time, temperature and the effects of homogenising the thermoseeds were studied to determine the functional hyperthermia applications. The bulk Ni-Si and Ni-Al binary alloys have Curie temperatures in the desired range, 316 K-319 K (43 °C-46 °C), which is suitable for magnetic hyperthermia applications. We have found that TC strictly follows a linear trend with doping concentration over a wide range of temperature. The magnetic ordering temperature and the magnetic properties can be controlled through substitution in these binary alloys.

Spectroscopic determination of phonon lifetimes in rhenium-doped MoS2 nanoparticles.

We investigated the infrared vibrational properties of pristine and Re-substituted MoS2 nanoparticles and analyzed the extracted phonon lifetimes in terms of multiple scattering events. Our measurements reveal both size- and doping-dependent changes that we attribute to grain boundary scattering and charge and mass effects, respectively. By contrast, Born charge is affected only by size. These findings illustrate the utility of reaching beyond traditional bulk semiconductors and quantum dots to explore how doping and confinement impact carrier-phonon interactions in low-dimensional semiconducting nanomaterials.

Colloidal synthesis of magnetic CuCr2S4 nanocrystals and nanoclusters.

Nanocrystals and nanoclusters of the room-temperature magnetic spinel CuCr(2)S(4) have been synthesized using a facile solution-based method. The synthesis involves hot injection of an excess of 1-dodecanethiol (1-DDT) into a boiling coordinating solvent containing CuCl(2) and CrCl(3)·6H(2)O. Using octadecylamine (ODA) as a solvent yields cube-shaped nanocrystals with an average size of 20 ± 2 nm, while with oleylamine (OLA), nanoclusters with an average size of 31 ± 2.5 nm are obtained. In both cases, powder X-ray diffraction patterns confirmed the formation of the pure spinel phase without any impurities. While the synthesized powders are superparamagnetic near room temperature, they exhibit ferromagnetic behavior at lower temperatures, with magnetization (M(S)) values of 30 emu/g (1.63 µ(B)/f.u.) and 33 emu/g (1.79 µ(B)/f.u.) for the ODA- and OLA-capped nanocrystals and nanoclusters, respectively, at 5 K.

Nanoscale switching characteristics of nearly tetragonal BiFeO3 thin films.

We have investigated the nanoscale switching properties of strain-engineered BiFeO(3) thin films deposited on LaAlO(3) substrates using a combination of scanning probe techniques. Polarized Raman spectral analysis indicates that the nearly tetragonal films have monoclinic (Cc) rather than P4mm tetragonal symmetry. Through local switching-spectroscopy measurements and piezoresponse force microscopy, we provide clear evidence of ferroelectric switching of the tetragonal phase, but the polarization direction, and therefore its switching, deviates strongly from the expected (001) tetragonal axis. We also demonstrate a large and reversible, electrically driven structural phase transition from the tetragonal to the rhombohedral polymorph in this material, which is promising for a plethora of applications.

Controlled growth of monodisperse self-supported superparamagnetic nanostructures of spherical and rod-like CoFe2O4 nanocrystals.

Novel shape- and structural-controlled superparamagnetic nanostructures composed of self-supported spherical and rod-like CoFe(2)O(4) colloidal nanocrystals have been prepared by thermolysis of a stoichiometric Co(2+)Fe(2)(3+)-oleate complex in organic solution with periodic injection of hexane for controlling the nucleation, assembly, and growth of the nuclei.

Magnetic field control of charge excitations in CoFe2O4.

We combine magnetic circular dichroism and photoconductivity with prior optical absorption and first principles calculations to unravel spin-charge interactions in the high Curie temperature magnet CoFe2O4. In addition to revising the bandgap hierarchy, we reveal a broad set of charge transfer excitations in the spin down channel which are sensitive to the metamagnetic transition involving the spin state on Co centers. We also show photoconductivity that depends on an applied magnetic field. These findings open the door for the creation and control of spin-polarized electronic excitations from the minority channel charge transfer in spinel ferrites and other earth-abundant materials.

Giant resistive switching in mixed phase BiFeO3via phase population control.

Highly-strained coherent interfaces, between rhombohedral-like (R) and tetragonal-like (T) phases in BiFeO3 thin films, often show enhanced electrical conductivity in comparison to non-interfacial regions. In principle, changing the population and distribution of these interfaces should therefore allow different resistance states to be created. However, doing this controllably has been challenging to date. Here, we show that local thin film phase microstructures (and hence R-T interface densities) can be changed in a thermodynamically predictable way (predictions made using atomistic simulations) by applying different combinations of mechanical stress and electric field. We use both pressure and electric field to reversibly generate metastable changes in microstructure that result in very large changes of resistance of up to 108%, comparable to those seen in Tunnelling Electro-Resistance (TER) devices.

Correction: Giant resistive switching in mixed phase BiFeO3via phase population control.

Correction for 'Giant resistive switching in mixed phase BiFeO3via phase population control' by David Edwards et al., Nanoscale, 2018, 10, 17629-17637.

Syntheses and magnetic properties of Cr2Te3 and CuCr2Te4 nanocrystals.

The Cr-based tellurides are attractive material systems for fundamental studies and potential applications. Colloidal syntheses of ferromagnetic Cr(2)Te(3) and CuCr(2)Te(4) nanocrystals with uniform morphology and narrow size distribution are reported together with their detailed magnetic characterisation.