Juan A. Colón Santana

Effects of Gd Doping and Oxygen Vacancies on the Properties of EuO Films Prepared via Pulsed Laser Deposition

We have successfully prepared EuO films on Si(100) wafers via pulsed laser deposition (PLD). It is well recognized that EuO grows with texture growth along (100) but the addition of 4% Gd changes the lattice constant and the texture growth to (111) as well as having a profound influence on the magnetic properties. The differences in the effects between Gd doping and oxygen vacancies, both expected to be n-type (donor state) dopants in EuO, are discussed.

Ce-doped EuO: Magnetic properties and the indirect band gap

We have prepared and investigated thin films of EuO doped with the rare-earth element cerium. X-ray diffraction, scanning electron microscopy, and energy dispersive x-ray spectroscopy were used to determine the quality of these films prepared by pulsed laser deposition. Ce doping leads to an enhanced Curie temperature near 150 K, close to that seen for oxygen-deficient EuO1−x. However, the magnetization of Ce-doped EuO exhibits differences from that observed for Gd-doped and oxygen-deficient samples. The high-resolution angular-resolved photoemission from Ce-doped EuO reveals filling of conduction-band states near the X point. This indicates that the band gap in EuO is indirect, and that at 2% doping Ce-doped EuO1−x is at least semimetallic.

The K-shell Auger electron spectrum of gadolinium obtained using neutron capture in a solid state device

Highly doped or alloyed Gd2O3 in HfO2 films form heterojunction diodes with silicon. Single neutron capture events can be identified with a Hf0.85Gd0.15O1.93 to n-type silicon heterojunction. With long pulse integration times and suppression of the smaller pulses, there is agreement between the key pulse height spectral features and those predicted by Monte Carlo simulations. The latter align very well with the decay channels of the Gd following neutron capture, particularly those involving the Gd K-shell Auger electron resonances.

Reversible Mn segregation at the polar surface of lithium tetraborate

We find Mn surface segregation for single crystals of Mn doped Li2B4O7, nominally Li1.95Mn0.05B4O7(001), but as the temperature increases, evidence of this Mn surface segregation diminishes significantly. At room temperature, the surface photovoltaic charging is significant for this pyroelectric material but is quenched at a temperature well below that seen for the undoped Li2B4O7 samples. The suppression of surface charging in the region of 120 °C that accompanies the temperature of Mn dissolution in the bulk of Li2B4O7, i.e., the reversal of Mn surface segregation (215 °C), suggests that along the (001) direction, ionic transport must be considered as significant.

Magnetoelectric Fe 2TeO 6 thin films

We demonstrate that Fe2TeO6 is a magnetoelectric antiferromagnet with voltage-controllable boundary magnetization. This provides experimental evidence of the theoretical prediction that boundary magnetization is a universal property of magnetoelectric antiferromagnets including boundary magnetization at a surface orthogonal to the polar direction. Highly (110) textured Fe2TeO6 thin films were grown by pulsed laser deposition, terminating in Te-O at the (110) surface due to a surface reconstruction. Magnetic dc susceptibility measurements of both Fe2TeO6 powder and thin film samples confirm antiferromagnetic long-range order. Finally, measurements of x-ray magnetic circular dichroism combined with photoemission electron microscopy (XMCD-PEEM) provide a lower bound to the spin and angular magnetic moment of the surface Fe ions.

Oxidative peeling of carbon black nanoparticles

We demonstrate that layered carbon black nanoparticles can be oxidatively peeled via the reaction with potassium permanganate in sulfuric acid. As a result of this reaction, outer layers of carbon nanoparticles “peel” off due to high levels of oxidation while the less oxidized inner cores, though they exhibit remarkable solubility in water, remain mostly intact.

Changes in molecular film metallicity with minor modifications of the constitutive quinonoid zwitterions

Molecular films of quinonoid zwitterions, of the general formula C6H2(O)2(NHR)2, have been shown to display electronic properties highly dependent on the nature of the N-substituent R when deposited on gold substrates. The different spacing and organization of the molecules can lead to molecular films with semi-metal or dielectric behavior. With the long term goal to establish how packing effects in the solid state correlate with properties as thin films, we first attempted to identify by X-ray diffraction analysis candidate molecules showing suitable packing arrangements in the crystalline state. To this end, we have prepared a series of new functionalized, enantiopure or sterically-hindered quinonoid zwitterions and established the crystal structure of those with R = CH2–CH2–Ph (6), CH2–CH2–CH2–Ph (7), CH2–CH2–CH2–CH2–Ph (8), CH2–CH2–CH(Ph)–Ph (9), CH(CH3)–Ph (12), CH(CH2–CH3)–Ph (13), CH2–((4–CH3)–C6H4) (15), CH2–((4–NH2)–C6H4) (19) and CH2–CH2–((3,4–(OCH3)2)–C6H3) (24). An analysis of the crystal packing of three molecules, 5, 13 and 15, selected as illustrative examples for comparisons, was carried out and it was unexpectedly found that these chemically very similar molecules gave rise to different packing in the bulk, with resulting thin films showing different electronic properties. Various methods have been used for the characterization of the films, such as synchrotron radiation-based FTIR spatial spectra-microscopy, which provided an anchoring map of zwitterion 15 on a patterned substrate (Au/SiO2) showing its selective anchoring on gold. This is one of the best examples of preferential anchoring of a zwitterion and the sole example of spatial localization for a quinonoid zwitterion thin film. We have also used combined photoemission and inverse photoemission spectra and the data were compared to occupied and unoccupied DFT density state calculations.

Magnetoelectric Fe2TeO6thin films

We demonstrate that Fe2TeO6 is a magnetoelectric antiferromagnet with voltage-controllable boundary magnetization. This provides experimental evidence of the theoretical prediction that boundary magnetization is a universal property of magnetoelectric antiferromagnets including boundary magnetization at a surface orthogonal to the polar direction. Highly (110) textured Fe2TeO6 thin films were grown by pulsed laser deposition, terminating in Te-O at the (110) surface due to a surface reconstruction. Magnetic dc susceptibility measurements of both Fe2TeO6 powder and thin film samples confirm antiferromagnetic long-range order. Finally, measurements of x-ray magnetic circular dichroism combined with photoemission electron microscopy (XMCD-PEEM) provide a lower bound to the spin and angular magnetic moment of the surface Fe ions.