Shane C. Street

Growth of epitaxial thin films of the ordered double perovskite La2NiMnO6 on different substrates

Epitaxial thin films of La2NiMnO6, a ferromagnetic semiconductor, have been fabricated on different substrates by pulsed laser deposition. The x-ray diffraction and Raman scattering observations reveal that the films are single crystalline and have an orthorhombic structure. The magnetic properties of the films, including the coercive field, remanent magnetization, and Curie temperature, are strongly dependent on the choice of the substrate. The optimized films exhibit a magnetic moment of 4.63μB∕f.u. at 5K, with a Curie temperature close to 280K. The film characteristics are promising for potential device applications in information storage, spintronics, and sensors.

El Niño impact on mollusk biomineralization-implications for trace element proxy reconstructions and the paleo-archeological record.

Marine macroinvertebrates are ideal sentinel organisms to monitor rapid environmental changes associated with climatic phenomena. These organisms build up protective exoskeletons incrementally by biologically-controlled mineralization, which is deeply rooted in long-term evolutionary processes. Recent studies relating potential rapid environmental fluctuations to climate change, such as ocean acidification, suggest modifications on carbonate biominerals of marine invertebrates. However, the influence of known, and recurrent, climatic events on these biological processes during active mineralization is still insufficiently understood. Analysis of Peruvian cockles from the 1982–83 large magnitude El Niño event shows significant alterations of the chemico-structure of carbonate biominerals. Here, we show that bivalves modify the main biomineralization mechanism during the event to continue shell secretion. As a result, magnesium content increases to stabilize amorphous calcium carbonate (ACC), inducing a rise in Mg/Ca unrelated to the associated increase in sea-surface temperature. Analysis of variations in Sr/Ca also suggests that this proxy should not be used in these bivalves to detect the temperature anomaly, while Ba/Ca peaks are recorded in shells in response to an increase in productivity, or dissolved barium in seawater, after the event. Presented data contribute to a better understanding of the effects of abrupt climate change on shell biomineralization, while also offering an alternative view of bivalve elemental proxy reconstructions. Furthermore, biomineralization changes in mollusk shells can be used as a novel potential proxy to provide a more nuanced historical record of El Niño and similar rapid environmental change events.

Thin Metal Films Deposited on Dendrimer Monolayers

Chemical interactions and the evolution of surface morphology resulting from deposition of Cr and Co onto dendrimer monolayers have been investigated by XPS, RAIRS, and AFM. Evidence is presented for formation of metal nitrides and mediation of film morphology as a function of the metal and generation number of the dendrimer comprising the monolayer.

Nanotribological studies of dendrimer-mediated metallic thin films

Recent studies on ultra-thin Au films on dendrimer-mediated substrates showed that the nanohardness of thin metal films is significantly increased by the presence of a dendrimer interlayer. This increase is attributed to the enhanced adhesion and modified growth mode of the dendrimer-mediated Au films as well as the confined nature of the dendrimer interlayer itself. This paper presents the results of extensive nanotribological studies of 10 nm Ti and thin films deposited onto the PAMAM dendrimer-mediated Si wafers using direct current magnetron sputtering. Ramped load nanoscratch tests were performed using a Nano Indenter® II system. X-ray photoelectron spectroscopy was employed to investigate the chemical interactions between the metal films, the interlayers and the environment. The nanoscratch surfaces and friction coefficient profiles for the thin films indicate that the scratch critical load is increased by the presence of dendrimer interlayer; that is the scratch load bearing capacity is improved. This is related to the formation of a mixed nanocomposite and increased interfacial adhesion. Critical load improvement is also related to the generation (size) of the dendrimer. The effect is more pronounced for the thicker G8 dendrimer than that for the G4 dendrimer interlayer. The scratch failure mechanism studied through atomic force microscopy analyses of the scratches tracks is similar to the observed wear behavior of the dendrimer-mediated Cr and Cu films. These results are important for the development and potential application of polymer-mediated metallic thin film tribo-materials.

Molecular Interlayers and the Mechanism of Abrasive Wear of Ultrathin Metal Films

Striking differences have been observed in the deformation modes associated with nanoscratch events on ultrathin Cu layers deposited on SiOx with and without a dendrimer interlayer. In the absence of the dendrimer monolayer significant lateral deformation and distinct ridge formation along the wear track indicative of plowing and wedge formation are observed. By contrast, dendrimer monolayer-mediated films exhibit restricted lateral deformation yielding ridgeless scratches apparently formed in a nearly pure cutting mode.

Kinetic energy influences on the growth mode of metal overlayers on dendrimer mediated substrates

Evidence is presented here for kinetic energy influences on the growth mode and chemical interactions of Cr, Co, and Cu deposited by sputtering and evaporative methods on self-assembled monolayers of amine-terminated poly(amidoamine) (PAMAM) dendrimers on SiOx. X-ray photoelectron spectroscopy (XPS) measurements show that metal nitrides are formed upon deposition of the more reactive metals and the extent of reaction increases with increasing kinetic energy of the deposited species. A peak observed at 398.5 eV in the N 1s spectrum following Cu deposition apparently indicates bond breaking in the internal amido branches of the dendrimer structure but not formation of the metal nitride. Atomic force microscopy (AFM) reveals obvious changes in the surface topography due to the dendrimer adlayer, particularly for the less reactive Cu.Evidence is presented here for kinetic energy influences on the growth mode and chemical interactions of Cr, Co, and Cu deposited by sputtering and evaporative methods on self-assembled monolayers of amine-terminated poly(amidoamine) (PAMAM) dendrimers on SiOx. X-ray photoelectron spectroscopy (XPS) measurements show that metal nitrides are formed upon deposition of the more reactive metals and the extent of reaction increases with increasing kinetic energy of the deposited species. A peak observed at 398.5 eV in the N 1s spectrum following Cu deposition apparently indicates bond breaking in the internal amido branches of the dendrimer structure but not formation of the metal nitride. Atomic force microscopy (AFM) reveals obvious changes in the surface topography due to the dendrimer adlayer, particularly for the less reactive Cu.

Surface characterization of Co/CNx granular films fabricated by nanolamination

Cobalt–carbon thin films proposed for use as granular magnetic media are generally prepared by co-deposition sputtering. An alternative method is nanolamination of the component layers (media and matrix) followed by annealing. We have produced and characterized thin-film granular structures prepared from nanolaminate layers of Co and CNx. For the as-deposited samples, only metallic cobalt is observed with XPS and a constant nitrogen concentration of x ∼ 0.15 was measured for the carbon nitride layers. The annealed films have oxidized cobalt in the very near surface region. Atomic force microscopy measurements show that the surface of the film roughens significantly upon annealing, with the RMS roughness increasing from 0.2 to 1.0 nm. Thus, it appears that the nanostructural evolution caused by the annealing process, which gives rise to grain formation, also degrades the smooth surface of the CNx capping layer and exposes some of the cobalt to ambient. The potential implications of these observations on tribological performance are explored. Copyright

Spectroscopic studies of the dodecanuclear chromium complex Cr12O9(OH)3(pivalate)15: confirmation of the presence of twelve Cr(III) centers and the crystal structure of Cr12O9(OH)3(pivalate)15.2PrOH.9H2O

Abstract The complex originally reported to be ‘Cr12O12(pivalate)15’ has been examined by a number of spectroscopic techniques in order to elucidate the chromium valence states. By charge balance arguments, the complex could, for example, contain nine Cr(III) and three Cr(IV) centers, contain 11 Cr(III) and one Cr(VI) center or could possess three protons, not detected by X-ray crystallography, and 12 Cr(III) centers. While a recent neutron diffraction study has suggested the existence of three additional and disordered deuterons (and thus 12 Cr(III) centers), the complex has been studied by several complementary spectroscopic techniques which confirm the formula to be Cr12O9(OH)3(pivalate)15.

Surface Chemistry of ZnO Nanowires for Nanosensor Applications

In this work, surface modification of ZnO nanowires (NWs) with oleic acid is reported. ZnO NWs were synthesized in a customized chemical vapor deposition furnace and were characterized with scanning electron microscopy, X-ray diffraction, and energy dispersive spectroscopy. Raman and infrared spectroscopies were used to determine that oleic acid modifies the ZnO NW surface with the carboxylate group attached to the surface. The study of the surface chemistry of ZnO NWs provides information about the class of compounds that modify ZnO NWs. This information will assist in tailoring the interface of NWs with appropriate compounds for fabricating nanosensors with high selectivity.

Spectroscopic analysis of the tribological behavior of a model boundary layer lubricant.

Highly ordered alkanethiol self-assembled monolayers (SAMs) on gold substrates are suitable models of boundary layer lubricants and may be used in actual nanoscale device applications. Here, such monolayers were studied by spectroscopic methods as a function of tribological wear (rubbing) using a pin-on-disk microtribometer. The coefficient of friction (COF) (ratio of the frictional force to the load) was measured with the tribometer, and reflectance infrared spectra and X-ray photoelectron spectra were obtained as the monolayer film failed and the COF changed. The results show that it is possible to correlate disorder in the monolayer film with tribological failure of the film, and that continued rubbing produces a chemical change in the monolayer film. Disorder in the monolayer is distinct from the influence of wear in the underlying gold substrate. Aged SAMs, having sulfonate rather than thiol headgroups and initially less well ordered, behave differently to the well-ordered freshly prepared SAMs. Interestingly, they show a lower COF over many more cycles of exposure to the rubbing pin. The impact of the mechanism of film failure in boundary layer lubrication is discussed.