Alexandra Suvorova

Magnesium oxide as a candidate high-κ gate dielectric

Magnesium oxide (MgO) thin films with sharp interfaces were deposited by sputtering of a Mg target on Si. The film stack was characterized using spectroscopic ellipsometry and transmission electron microscopy and the film static dielectric constant (κ) and interface traps were determined. An amorphous SiO2 layer was found at the MgO∕Si interface as a result of subcutaneous Si oxidation. κ for the MgO films was found to be about twice that of SiO2, and the interface trap densities of MgO∕Si were found to be comparable with SiO2∕Si, rendering MgO competitive with all presently considered high-κ dielectrics.

A New Metal-Free Carbon Hybrid for Enhanced Photocatalysis

Carbon nitride (C3N4) is a layered, stable, and polymeric metal-free material that has been discovered as a visible-light-response photocatalyst. Owing to C3N4 having a higher conduction band position, most previous studies have been focused on its reduction capability for solar fuel production, such as hydrogen generation from water splitting or hydrocarbon production from CO2. However, photooxidation ability of g-C3N4 is weak and has been less explored, especially for decomposition of chemically stable phenolics. Carbon spheres prepared by a hydrothermal carbonization of glucose have been widely applied as a support material or template due to their interesting physicochemical properties and the functional groups on the reactive surface. This study demonstrated that growth of carbon nanospheres onto g-C3N4 (CN-CS) can significantly increase the photooxidation ability (to about 4.79 times higher than that of pristine g-C3N4) in phenol degradation under artificial sunlight irradiations. The crystal structure, optical property, morphology, surface groups, recombination rate of electron/hole pairs, and thermal stability of CN-CS were investigated by a variety of characterization techniques. This study contributes to the further promising applications of carbon nitride in metal-free catalysis.

Er2O3 as a high-K dielectric candidate

Erbium oxide (Er2O3) films have been deposited by metal organic chemical vapor deposition on Si(001) using tris(isopropylcyclopentadienyl)erbium. The impact of Si surface passivation by the metal organic prior growth initiation was investigated. The correlation between the Er2O3 films structure, the optical response, the static dielectric constant (K), and density of interface traps is discussed. An Er-silicate interfacial layer with a thickness of 1.5nm, a static dielectric constant of 10–12.4, and a density of interface traps of 4.2×1010cm2eV−1 measured for a film with a physical thickness of 8.2nm (with an equivalent oxide thickness of 2.7nm) render Er2O3 an interesting candidate as a high-K dielectric.

Structural and optical properties of ZnO thin films by rf magnetron sputtering with rapid thermal annealing

Epitaxial ZnO thin films were grown on c-plane sapphire substrates by rf magnetron sputtering at room temperature followed by a rapid thermal annealing process. We found that crystallinity of the films was strongly affected by the partial oxygen pressure during deposition. Both x-ray diffraction and transmission electron microscopy studies revealed that the ZnO films grow epitaxially predominantly with aligned ZnO domains. An unresolved excitonic resonance was observed in the optical absorption spectrum. Nevertheless, refractive index and absorption edge of the ZnO films are similar to that of single crystal ZnO.

Transient enhanced diffusion of aluminum in SiC during high temperature ion implantation

6H–SiC wafers were implanted at room temperature (RT) and at 1700 °C high temperature (HT) with 50 keVAl+ ions to doses from 1.4×1014 to 1.4×1016 cm−2. Compared to samples implanted at RT, the samples implanted at high temperature display considerable aluminum redistribution. The diffusion of Al is shown to be a transient effect with different decay times in the near-surface region and in the bulk. Investigation of the crystalline structure indicated that in the near-surface region dislocation loops grow in size and Al precipitates are formed as the dose of Al implanted at HT is increased. Changes in the structure of the implanted layer may have a strong effect on the redistribution of Al. The observed redistribution can be explained by a dissociative diffusion mechanism during the high-temperature implantation.

Precipitation of iron silicate nanoparticles in early Precambrian oceans marks Earth's first iron age

The early ocean was characterized by anoxic, iron-rich (ferruginous) conditions before the rise of atmospheric oxygen ∼2.45 b.y. ago. A proxy for ferruginous conditions in the ancient ocean is the deposition of banded iron formations (BIFs), which are iron- and silica-rich chemical sediments whose constituents were largely derived from seawater. Although experiments simulating ancient ocean chemistry support the rapid growth of iron-silicate phases, the main iron precipitates are hypothesized to have been ferric oxyhydroxides. The paradox between the prevailing reducing conditions and the deposition of oxidized iron phases is explained by biologically mediated oxidation in the water column. New high-resolution microscopy of BIFs and shales throughout the 2.63–2.45 b.y. old Hamersley Group, Australia, reveals the presence of vast quantities of nanometer-sized iron-silicate particles in laminated chert. The nanoparticles are finely disseminated in early diagenetic chert and locally define sedimentary lamination, indicating that they represent relicts of the original sediments. By inference from experimental studies simulating the composition of the early Precambrian ocean, we suggest that the nanoparticles precipitated from anoxic seawater enriched in silica and dissolved iron, and were silicified upon deposition. The prevalence of iron-silicate nanoparticles implies that they were pervasive background precipitates in ferruginous, silica-enriched oceans, forming the primary sediments of BIFs during periods of enhanced submarine mafic volcanism. Our results imply that silicate precipitation was a major sink of seawater iron and silica before the Great Oxidation Event and, because of the reactivity of nanoparticle surfaces, may also have influenced the transport and geochemical cycling of trace metals and nutrients. Our hypothesis that the basic building blocks of BIFs were predominantly iron-silicate muds rather than iron oxides and/or hydroxides may lead to new insights into seawater chemistry on the early Earth and the role of biology in the deposition of BIFs.

Thermally stable coexistence of liquid and solid phases in gallium nanoparticles

Gallium (Ga), a group III metal, is of fundamental interest due to its polymorphism and unusual phase transition behaviours. New solid phases have been observed when Ga is confined at the nanoscale. Herein, we demonstrate the stable coexistence, from 180 K to 800 K, of the unexpected solid γ-phase core and a liquid shell in substrate-supported Ga nanoparticles. We show that the support plays a fundamental role in determining Ga nanoparticle phases, with the driving forces for the nucleation of the γ-phase being the Laplace pressure in the nanoparticles and the epitaxial relationship of this phase to the substrate. We exploit the change in the amplitude of the evolving surface plasmon resonance of Ga nanoparticle ensembles during synthesis to reveal in real time the solid core formation in the liquid Ga nanoparticle. Finally, we provide a general framework for understanding how nanoscale confinement, interfacial and surface energies, and crystalline relationships to the substrate enable and stabilize the coexistence of unexpected phases.

Demonstrating the capability of the high-performance plasmonic gallium-graphene couple

Metal nanoparticle (NP)-graphene multifunctional platforms are of great interest for exploring strong light-graphene interactions enhanced by plasmons and for improving performance of numerous applications, such as sensing and catalysis. These platforms can also be used to carry out fundamental studies on charge transfer, and the findings can lead to new strategies for doping graphene. There have been a large number of studies on noble metal Au-graphene and Ag-graphene platforms that have shown their potential for a number of applications. These studies have also highlighted some drawbacks that must be overcome to realize high performance. Here we demonstrate the promise of plasmonic gallium (Ga) nanoparticle (NP)-graphene hybrids as a means of modulating the graphene Fermi level, creating tunable localized surface plasmon resonances and, consequently, creating high-performance surface-enhanced Raman scattering (SERS) platforms. Four prominent peculiarities of Ga, differentiating it from the commonly used noble (gold and silver) metals are (1) the ability to create tunable (from the UV to the visible) plasmonic platforms, (2) its chemical stability leading to long-lifetime plasmonic platforms, (3) its ability to n-type dope graphene, and (4) its weak chemical interaction with graphene, which preserves the integrity of the graphene lattice. As a result of these factors, a Ga NP-enhanced graphene Raman intensity effect has been observed. To further elucidate the roles of the electromagnetic enhancement (or plasmonic) mechanism in relation to electron transfer, we compare graphene-on-Ga NP and Ga NP-on-graphene SERS platforms using the cationic dye rhodamine B, a drug model biomolecule, as the analyte.

ZrO2 film interfaces with Si and SiO2

The interface formed by the thermal oxidation of sputter-deposited Zr metal onto Si(100)- and SiO2-coated Si(100) wafers was studied in situ and in real time using spectroscopic ellipsometry (SE) in the 1.5–4.5 photon energy range and mass spectrometry of recoiled ions (MSRI). SE yielded optical properties for the film and interface and MSRI yielded film and interface composition. An optical model was developed and verified using transmission electron microscopy. Interfacial reaction of the ZrO2 was observed for both substrates, with more interaction for Si substrates. Equivalent oxide thicknesses and interface trap levels were determined on capacitors with lower trap levels found on samples with a thicker SiO2 underlayer. In addition to the optical properties for the intermixed interface layer, the optical properties for Zr metal and unreacted ZrO2 are also reported.

GaMg alloy nanoparticles for broadly tunable plasmonics.

Manipulating the properties of well understood materials systems for novel technological applications can be achieved by creating nanoscale structures and mixed compounds. The design of novel nanomaterials that underpin plasmonic applications drives the rapid progress in advancing plasmonic materials over the last few years. [ 1 , 2 ] Control of metal nanoparticle shape, [ 3 , 4 ] density, and spacing [ 5 ] is continually improving with novel synthetic techniques. Nevertheless, to continue advancing plasmonics, new metallic nanostructure systems must be developed that offer unique properties and are superior to Ag, [ 6 ] Au, [ 3 , 7 ] and mixtures thereof—the most widely exploited metals [ 8–10 ] and bimetallic systems. [ 11 ]