Nanostructured Au(111)/Oxide epitaxial heterostructures with tailoring plasmonic response by a one-step strategy

In this work, we present a strategy for developing epitaxial incommensurate nanostructured Au/oxide heterostructures with tunable plasmonic response. Previously, high-quality single-phase and single-oriented alfa-Fe2O3(0001) thin films were achieved, which have been used as a template for noble-metal epitaxial deposition. The complex systems have been grown by pulsed laser deposition on two different types of oxide substrates: alfa-Al2O3(0001) and SrTiO3(111). A one-step procedure has been achieved tailoring the isolated character and the morphological features of Au nanostructures through the substrate temperature during Au growth, without altering the structural characteristics of the hematite layer that is identified as a single iron oxide phase. The epitaxial character and the lattice coupling of Au/oxide bilayers are mediated through the sort of oxide substrate. Single-oriented Au(111) islands are disposed with a rotation of 30° between their crystallographic axes and those of alfa-Fe2O3(0001). The Au(111) and SrTiO3(111) lattices are collinear, while a rotation of 30° happens with respect to the alfa-Al2O3(0001) lattice. The crystallographic domain size and crystalline order of the hematite structure and the Au nanostructured layer are dependent on the substrate type and the Au growth temperature, respectively. Besides, the functional character of the complex systems has been tested. The localized surface plasmons related to Au nanostructures are excited and controlled through the fabrication parameters, tuning the optical resonance with the degree of Au nanostructuring.This work was supported by the Ministerio Español de Ciencia, Innovación y Universidades (MCIU), and the Consejo Superior de Investigaciones Cientificas (CSIC) through the project PIE-2010 6 OE 013. J.L.-S. and E.E. acknowledge the FPI fellowship and the Torres Quevedo contract (ref: PTQ-14-07289). The ESRF, MCIU, and CSIC are acknowledged for the provision of synchrotron radiation facilities. The authors thank Dr M.A. García for the useful discussion about the plasmonic results and Carlos Beltrán for technical support during the experiments at the BM25 beamline at The ESRF

Selective thermal evolution of native oxide layer in Nb and Nb3Sn-coated SRF grade Nb: An in-situ angular XPS study

This contribution discusses the results of an in-situ angular XPS study on the thermal evolution of the native oxide layer on Nb3Sn and pure Nb. XPS data were recorded with conventional spectrometers using an AlK(alpha) X-ray source for spectra collected up to 600 C, and an MgK(Alpha) X-rays source for temperatures above 600 C. The effect of the thickness, composition, and thermal stability of that oxide layer is relevant to understanding the functional properties of superconducting radiofrequency (SRF) cavities used in particle accelerators. There is a consensus that oxide plays a role in surface resistance (Rs). The focus of this study is Nb3Sn, which is a promising material that is used in the manufacturing of superconducting radiofrequency (SRF) cavities as well as in quantum sensing, and pure Nb, which was included in the study for comparison. The thermal evolution of the oxide layer in these two materials is found to be quite different, which is ascribed to the influence of the Sn atom on the reactivity of the Nb atom in Nb3Sn films. Nb and Sn atoms in this intermetallic solid have different electronegativity, and the Sn atom can reduce electron density around neighbouring Nb atoms in the solid, thus reducing their reactivity for oxygen. This is shown in the thickness, composition, and thermal stability of the oxide layer formed on Nb3Sn. The XPS spectra were complemented by grazing incident XRD patterns collected using the ESRF synchrotron radiation facility. The results discussed herein shed light on oxide evolution in the Nb3Sn compound and guide its processing for potential applications of the Nb3Sn-based SRF cavities in accelerators and other superconducting devices

Improving the co and ch4 gas sensor response at room temperature of alpha-fe2o3(0001) epitaxial thin films grown on srtio3(111) incorporating au(111) islands

In this work, the functional character of complex -Fe2O3(0001)/SrTiO3(111) and Au(111) islands/ -Fe2O3(0001)/SrTiO3(111) heterostructures has been proven as gas sensors at room temperature. Epitaxial Au islands and -Fe2O3 thin film are grown by pulsed laser deposition on SrTiO3(111) substrates. Intrinsic parameters such as the composition, particle size and epitaxial character are investigated for their influence on the gas sensing response. Both Au and -Fe2O3 layer show an island-type growth with an average particle size of 40 and 62 nm, respectively. The epitaxial and incommensurate growth is evidenced, confirming a rotation of 30 between the in-plane crystallographic axes of -Fe2O3(0001) structure and those of SrTiO3(111) substrate and between the in-plane crystallographic axes of Au(111) and those of -Fe2O3(0001) structure. -Fe2O3 is the only phase of iron oxide identified before and after its functionalization with Au nanoparticles. In addition, its structural characteristics are also preserved after Au deposition, with minor changes at short-range order. Conductance measurements of Au(111)/ -Fe2O3(0001)/SrTiO3(111) system show that the incorporation of epitaxial Au islands on top of the -Fe2O3(0001) layer induces an enhancement of the gas-sensing activity of around 25% under CO and 35% under CH4 gas exposure, in comparison to a bare -Fe2O3(0001) layer grown on SrTiO3(111) substrates. In addition, the response of the heterostructures to CO gas exposure is around 5–10% higher than to CH4 gas in each case.This work has been supported by the Ministerio Español de Ciencia e Innovación (MICINN) and the Consejo Superior de Investigaciones Cientificas (CSIC) through the projects PIE-2010-OE-013- 200014, PIE 2021-60-E-030 and RTI2018-095303-A-C52. The ESRF, MICINN and CSIC are acknowledged for the provision of synchrotron radiation facilities. A.S. acknowledges financial support from Comunidad de Madrid for an “Atracción de Talento Investigador” Contract (2017-t2/IND5395)

Characterization of a robust Co-II fluorescent complex deposited intact on HOPG

The new diimine fluorescent ligand ACRI-1 based on a central acridine yellow core is reported along with its coordination complex [Co-2(ACRI-1)(2)] (1), a fluorescent weak ferromagnet. Due to the strong fluorescence of the acridine yellow fluorophore, it is not completely quenched when the ligand is coordinated to Co-II. The magnetic properties of bulk complex 1 and its stability in solution have been studied. Complex 1 has been deposited on highly ordered pyrolitic graphite (HOGP) from solution. The thin films prepared have been characterized by AFM, time-of-flight secondary ion mass spectrometry (TOF-SIMS), grazing incidence X-ray diffraction (GIXRD), X-ray absorption spectroscopy (XAS), X-ray magnetic circular dichroism (XMCD) and theoretical calculations. The data show that the complex is robust and remains intact on the surface of graphite

Formation of titanium monoxide (001) single-crystalline thin film induced by ion bombardment of titanium dioxide (110)

© 2015 Macmillan Publishers Limited. All rights reserved. A plethora of technological applications justify why titanium dioxide is probably the most studied oxide, and an optimal exploitation of its properties quite frequently requires a controlled modification of the surface. Low-energy ion bombardment is one of the most extended techniques for this purpose and has been recently used in titanium oxides, among other applications, to favour resistive switching mechanisms or to form transparent conductive layers. Surfaces modified in this way are frequently described as reduced and defective, with a high density of oxygen vacancies. Here we show, at variance with this view, that high ion doses on rutile titanium dioxide (110) induce its transformation into a nanometric and single-crystalline titanium monoxide (001) thin film with rocksalt structure. The discovery of this ability may pave the way to new technical applications of ion bombardment not previously reported, which can be used to fabricate heterostructures and interfaces.Peer Reviewe

The role of silicon oxide in the stabilization and magnetoresistance switching of Fe3O4/SiO2/Si heterostructures

In this work we analyze the role of the SiO2 layer in the functionality of Fe3O4/SiO2/Si heterostructures, which have been proved to present a strong potential for spin-based applications. Nevertheless, a complete control of the interfaces properties is fundamental for application. In this work, high quality heterostructures are fabricated avoiding chemical exchange and achieving good quality interfaces. The chemical interaction between the Fe3O4 and SiO2 layers during the heterostructures manufacture is deeply analyzed and its role on the transport, magnetic and magneto transport response is revealed. It is proven that during Fe3O4 deposition a competitive interplay happens between the catalytic action of Fe atoms, the transport of dissociated oxygen through SiO2 and the stabilization of Fe3O4. A defective silicon oxide layer is found to grow on top of the native SiO2 enabling the formation of single phase Fe3O4 layer. Such a defective layer and the granular character of the Fe3O4 determine the magnetic and transport response of the heterostructures. The present results prove that the defects in the SiO2 layer induce the switching of the MR sign, so that anomalous positive MR at RT exceeding 17% at 80 kOe is obtained in heterostructures with 19 nm thick magnetite layer, while conventional negative MR response is obtained for thicker films.The authors are grateful to the BM25-SpLine Staff for their valuable help and for the financial support from the Spanish MCIN and Consejo Superior de Investigaciones Científicas [Grant No. 2010-6-OE-013 and 2021-6-OE-030]. Dr. Óscar Rodríguez de la Fuente and Dr. Noemí Carmona Tejero are also acknowledge for SEM and TEM measurements

High- and low-energy x-ray photoelectron techniques for compositional depth profiles: destructive versus non-destructive methods

Hard x-ray photoelectron spectroscopy (HAXPES), angle-resolved x-ray photoelectron spectroscopy (ARXPS) and x-ray photoelectron spectroscopy (XPS) with simultaneous Ar+ bombardment are used to obtain chemical information and concentration depth profiles of thin film oxides on Cr, Al, Si substrata and to explore the capabilities of analyzing buried interfaces at depths above 10 nm in Cr–O–Al thin films mixed oxides deposited on Si substrata. ARXPS and HAXPES are non-destructive techniques and within the photon energy range (7.5–15 keV) and the emission angle range (0◦–70◦) used, both techniques provide equivalent information, ARXPS being more sensitive to the surface morphology. XPS and simultaneous sputtering with Ar+ is a destructive technique and effects such as atomic mixing are unavoidable; however, the comparative study with HAXPES allowed the measurement of key parameters for the understanding of the ion–matter interaction such as the mixing extent and the interface broadening.Ministerio de Ciencia e Innovación CSD2008-00023 (CONSOLIDER-Ingenio 2010)Ministerio de Ciencia e Innovación MAT2008-06618-C0

Formation of titanium monoxide (001) single-crystalline thin film induced by ion bombardment of titanium dioxide (110)

A plethora of technological applications justify why titanium dioxide is probably the most studied oxide, and an optimal exploitation of its properties quite frequently requires a controlled modification of the surface. Low-energy ion bombardment is one of the most extended techniques for this purpose and has been recently used in titanium oxides, among other applications, to favour resistive switching mechanisms or to form transparent conductive layers. Surfaces modified in this way are frequently described as reduced and defective, with a high density of oxygen vacancies. Here we show, at variance with this view, that high ion doses on rutile titanium dioxide (110) induce its transformation into a nanometric and single-crystalline titanium monoxide (001) thin film with rocksalt structure. The discovery of this ability may pave the way to new technical applications of ion bombardment not previously reported, which can be used to fabricate heterostructures and interfaces

Soft magnetism in single phase Fe3Si thin films deposited on SrTiO3(001) by pulsed laser deposition

The development of devices relying on spin phenomena requires of an ideal spin polarized electron source. This can be achieved by taking advantage of half-metallic full Heusler alloy thin films. However, their implementation requires a controlled growth of stoichiometric films with large activation volumes. In this work, we report on the growth of epitaxial Fe3Si ultra-thin films by pulsed laser deposition on SrTiO3(001) substrates, analyzing the effect of deposition temperature in the structural, morphological and magnetic properties of the deposited films. We conclude that optimal compromise between phase purity and interface quality is obtained at 200 ºC, obtaining the best magnetic response under this condition

Neutron reflectometry and hard X-ray photoelectron spectroscopy study of the vertical segregation of PCBM in organic solar cells

The composition of the active layer of a benchmark functional glass/ITO/[P3HT:PCBM][1.1:1]/Al organic solar cell has been studied by neutron reflectometry (NR) and hard X-ray photoelectron spectroscopy (HAXPES). Thermal annealing was performed in several steps and NR and HAXPES were recorded for every temperature. By fitting the NR results to a model composed of several layers, the scattering length density (SLD) distribution through the sample was obtained, and from this SLD profile, the evolution of the composition of the active layer as a function of temperature was established. For the outer layers, HAXPES results confirm the composition evolution. The results show that PCBM tends to segregate reducing the initial concentration of PCBM in the central part of the active layer and increasing its concentration towards both interfaces. The effect of the Al electrode as studied by HAXPES on the nearest zone of the active layer (up to 50 nm) is to stabilize the P3HT depletion in this area, an effect which is not affected by thermal annealing