17 research outputs found
Graphene functionalised by laser ablated V2O5 as highly sensitive NH3 sensor
Graphene has been recognized as a promising gas sensing material. The
response of graphene-based sensors can be radically improved by introducing
defects in graphene using, e. g., metal or metal oxide nanoparticles. We have
functionalised CVD grown, single layer graphene by applying pulsed laser
deposition (PLD) of V2O5 which resulted in a thin V2O5 layer on graphene with
average thickness of ~0.6 nm. According to Raman analysis, PLD process also
induced defects in graphene. Compared to unmodified graphene, the obtained
chemiresistive sensor showed considerable improvement of sensing ammonia at
room temperature. In addition, also the response time, sensitivity and
reversibility were essentially enhanced due to graphene functionalisation by
laser deposited V2O5. This can be explained by increased surface density of gas
adsorption sites introduced by high energy atoms in laser ablation plasma and
formation of nanophase boundaries between deposited V2O5 and graphene.Comment: 22 pages, 6 figure
Optical and mechanical properties of nanolaminates of zirconium and hafnium oxides grown by atomic layer deposition
Nanolaminates of ZrO2 and HfO2 were grown by atomic layer deposition, using metal halides and water as precursors, on silicon and fused quartz substrates at 300 degrees C. The crystalline phase composition, optical refraction, and mechanical performance of the multilayers were influenced by the relative contents of the constituent metal oxides. The crystal growth in as-deposited HfO2 dominantly led to the monoclinic phase, whereas ZrO2 was partially crystallized as its metastable and hard tetragonal polymorph. The hardness and elasticity of the nanolaminate structures could be modified by varying the amounts of either oxide contributing to the crystallographic order formed in the solid films. The refractive indexes depended on the nanolaminate structure.Peer reviewe
Phase stability and oxygen-sensitive photoluminescence of ZrO2:Eu,Nb nanopowders
This work was supported by institutional research funding ( IUT34-27 and IUT2-14 ) of the Estonian Ministry of Education and Research .We studied structure and oxygen-sensitive photoluminescence (PL) of ZrO2:Eu,Nb nanocrystalline powders synthesized via a sol-gel route and heat-treated up to 1200 °C. The material containing only 2 at% Eu3+ was predominantly monoclinic, whereas 8 at% of Eu3+ stabilized tetragonal phase. Comparable amount of niobium co-doping effectively suppressed the formation of tetragonal phase. PL of Eu3+ ions was observed under direct excitation at 395 nm. PL decay kinetics showed that the luminescence was partially quenched, depending on doping concentrations and ambient atmosphere. At 300 °C, the PL intensity of all samples systematically responded (with up to 70% change) to changing oxygen content in the O2/N2 mixture at atmospheric pressure. At low doping levels, the dominant factor controlling the PL intensity was an energy transfer from excited PL centers to randomly distributed defects in the ZrO2 lattice. We argue that the charge transfer between the defects and adsorbed oxygen molecules alters the ability of the defects to quench Eu3+ luminescence. At high doping levels, another type of sensor response was observed, where some Eu3+ emitters are effectively switched on or off by the change of ambient gas. A remarkable feature of the studied material is a reversing of the sensor response with the variation of the Nb concentration.Estonian Ministry of Education and Research IUT34-27 and IUT2-14; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART
Electric and Magnetic Properties of Atomic Layer Deposited ZrO2-HfO2 Thin Films
Atomic layer deposition method was employed to deposit thin films consisting of ZrO2 and HfO2. Zirconia films were doped with hafnia and vice versa, and also nanolaminates were formed. All depositions were carried out at 300 degrees C. Most films were crystalline in their as-deposited state. Zirconia exhibited the metastable cubic and tetragonal phases by a large majority, whereas hafnia was mostly in its stable monoclinic phase. Magnetic and electrical properties of the films were assessed. Un-doped zirconia was ferromagnetic and this property diminished with increasing the amount of hafnia in a film. All films exhibited ferroelectric-like behavior and the polarization curves also changed with respect to the film composition. (C) The Author(s) 2018. Published by ECS.Peer reviewe
Atomic layer deposited nanolaminates of zirconium oxide and manganese oxide from manganese(III)acetylacetonate and ozone
ProducciĂłn CientĂficaAtomic layer deposition method was used to grow thin films consisting of ZrO2 and MnOx layers. All depositions were carried out at 300 ÂşC. Some deposition characteristics of the manganese(III)acetylacetonate and ozone process were investigated, such as crystallinity and the dependence of growth rate on the deposition temperature. All films were partly crystalline in their as-deposited state. Zirconium oxide contained cubic and tetragonal phases of ZrO2, while the manganese oxide was shown to consist of cubic Mn2O3 and tetragonal Mn3O4 phases. All the films exhibited nonlinear saturative magnetization with hysteresis, as well as resistive switching characteristics.Fondo Europeo de Desarrollo Regional (projects TK134 and TK141)Ministerio de EconomĂa, Industria y Competitividad (project TEC2017-84321-C4-2-R)Estonian Research Agency (projects PRG4 and PRG753
Structure and behavior of ZrO2-graphene-ZrO2 stacks
ProducciĂłn CientĂficaZrO2-graphene-ZrO2 layered structures were built and their crystallinity was characterized before resistive switching measurements. Thin nanocrystalline ZrO2 dielectric films were grown by atomic layer deposition on chemical vapor deposited graphene. Graphene was transferred, prior to the growth of the ZrO2 overlayer, to the ZrO2 film pre-grown on titanium nitride. Nucleation and growth of the top ZrO2 layer was improved after growing an amorphous Al2O3 interface layer on graphene at lowered temperatures. Studies on resistive switching in such structures revealed that the exploitation of graphene interlayers could modify the operational voltage ranges and somewhat increase the ratio between high and low resistance states.Fondo Europeo de Desarrollo Regional (project TK134)Estonian Research Agency (grants PRG753 and PRG4)Ministerio de EconomĂa, Industria y Competitividad (grant TEC2017-84321-C4-2-R
Atomic layer deposition and properties of ZrO2/Fe2O3 thin films
Thin solid films consisting of ZrO2 and Fe2O3 were grown by atomic layer deposition (ALD) at 400 degrees C. Metastable phases of ZrO2 were stabilized by Fe2O3 doping. The number of alternating ZrO2 and Fe2O3 deposition cycles were varied in order to achieve films with different cation ratios. The influence of annealing on the composition and structure of the thin films was investigated. Additionally, the influence of composition and structure on electrical and magnetic properties was studied. Several samples exhibited a measurable saturation magnetization and most of the samples exhibited a charge polarization. Both phenomena were observed in the sample with a Zr/Fe atomic ratio of 2.0.Peer reviewe
Electrical and magnetic properties of atomic layer deposited cobalt oxide and zirconium oxide nanolaminates
ProducciĂłn CientĂficaNanolaminates of ZrO2 and Co3O4 were atomic layer deposited on silicon and titanium nitride at 300 °C. Films were confirmed to be polycrystalline in the as-deposited state, with the cubic phase dominating in both oxides. All films exhibited resistive switching characteristics and charge polarization and ferromagnetic behavior. Also, the relative permittivities of the films were measured and the dispersion functions modelled.Ministerio de EconomĂa, Industria y Competitividad (TEC2017-84321-C4-2-R
Ag Sputter-Deposited on MnO<sub>2</sub>-Carbon Nanotube Nanocomposites as Electrocatalysts for Oxygen Reduction Reaction in Alkaline Media
As energy demand increases, new energy conversion methods are also sought. In this study, two MnO2 and multiwalled carbon nanotube (MWCNT) composites were prepared and decorated with silver using magnetron sputtering, to evaluate their electrocatalytic activity towards the oxygen reduction reaction (ORR). Three nominal thicknesses of Ag layers were used, 5, 10 and 20 nm. The physicochemical characterisation was carried out using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, as well as X-ray photoelectron spectroscopy. The substrate materials (MnO2-MWCNT) were also investigated by X-ray diffraction analysis. The electrochemical studies of the ORR revealed that the activity and stability of the composite catalysts depend on the substrate material and the Ag layer thickness