Oxidation processes at the surface of BaTiO3 thin films under environmental conditions

Altres ajuts: the ICN2 is funded by the CERCA programme/Generalitat de Catalunya. I. S. acknowledges support of the Secretaria d'Universitats i Recerca - Departament d'Empresa i Coneixement - Generalitat de Catalunya and the European Social Fund (ESF) (FI grant reference 2020 FI_B2 00157).Dissociation and adsorption of water on ferroelectric oxide surfaces play important role in the processes of screening and switching dynamics of ferroelectric polarization, as well as in catalytic processes which can be additionally coupled with light, temperature or vibration stimuli. In this work, we present XPS study of ferroelectric BaTiOthin films and determine the entanglement between surface chemistry, polarization direction and stability, by observing changes upon time exposure to environmental conditions, heating in Oatmosphere and irradiation in vacuum. We devote special attention to Ba 3d spectral region and identify two different oxidation states of O atoms in the compounds of Ba. While this second specie was generally attributed to Ba in surface compounds where it has different oxygen coordination than in the bulk, based on the XPS results of oxygen annealed thin films, we demonstrate that this so far neglected component, corresponds to barium peroxide (BaO) and identify it as important active specie for the study of screening mechanisms closely related with catalytic activity present in this ferroelectric material. Finally, we report on chemically assisted polarization switching in thin films induced by heating in vacuum or exposure to X-Ray radiation due to the formation of positive surface electric field created by oxygen or electron vacancies, respectively

Control of lateral composition distribution in graded films of soluble solid systems A1-xBx by partitioned dual-beam pulsed laser deposition

Altres ajuts: AGAUR agency (project 2017SGR)Lateral compositionally-graded thin films are powerful media for the observation of phase boundaries aswell as for high-throughputmaterials exploration.We herein propose amethod to prepare epitaxial lateral compositionally-graded films using a dual-beampulsed laser deposition (PLD)method with two targets separated by a partition. Tuning the ambient pressure and the partition-substrate gap makes it possible to control of the gradient length of the deposits at the small sizes (≤ 10 mm) suitable for commercial oxide single crystal substrates. A simple Monte Carlo simulation qualitatively reproduced the characteristic features of the lateral thickness distribution. To demonstrate this method, we prepared (1-x)PbTiO-xPbZrO and (1-x)LaMnOLaSrMnO films with lateral composition gradient widths of 10 and 1 mm, respectively, with the partitioned dual PLD

Low-pressure RF remote plasma cleaning of carbon-contaminated B4C-coated optics

Altres ajuts: the research by HMF is supported by funding from the "Generalitat de Catalunya, Departament d'Empresa i Coneixement" within the "Doctorats Industrials" program (dossier no. 2014 DI 037)Boron carbide (BC)-due to its exceptional mechanical properties-is one of the few existing materials that can withstand the extremely high brilliance of the photon beam from free electron lasers (FELs) and is thus of considerable interest for optical applications in this field. However, as in the case of many other optics operated at modern accelerator-, plasma-, or laser-based light source facilities, BC-coated optics are subject to ubiquitous carbon contaminations. These contaminations-that are presumably produced via cracking of CH and CO molecules by photoelectrons emitted from the optical components-represent a serious issue for the operation of the pertinent high performance beamlines due to a severe reduction of photon flux and beam coherence, not necessarily restricted to the photon energy range of the carbon K-edge. Thus, a variety of BC cleaning technologies have been developed at different laboratories with varying success [1]. Here, we present a study regarding the low-pressure RF plasma cleaning of a series of carbon-contaminated BC test samples via an inductively coupled O/Ar and Ar/H remote RF plasma produced using the IBSS GV10x plasma source following previous studies using the same RF plasma source [2, 3]. Results regarding the chemistry, morphology as well as other aspects of the BC optical coatings and surfaces before and after the plasma cleaning process are reported

Influence of the relative molecular orientation on interfacial charge-transfer Excitons at donor/acceptor Nanoscale heterojunctions

We address the impact of the relative orientation between donor (D) and acceptor (A) molecules at the D/A heterojunction on the exciton dissociation. For this purpose, two-dimensional heterojunctions of diindenoperylene (DIP) and N,N'-dioctyl-3,4,9,10-perylene tetracarboxylicdiimide (PTCDI-C) deposited onto SiO/Si are grown, which exemplify two model interfaces with the π-staking direction either perpendicular or parallel to the interface. Aspects related to the morphology of the heterojunctions and charge photogeneration are studied by scanning probe force methods and photoluminescence (PL) spectroscopy. Results from PL spectroscopy indicate that the exciton dissociation is influenced by the different relative molecular orientations of A and D. For the configuration with stronger orbital overlap between A and D at the interface, the exciton dissociation is dominated by recombination from an interfacial charge-transfer state. © 2014 American Chemical Society

Carbon incorporation in MOCVD of MoS2 thin films grown from an organosulfide precursor

Altres ajuts: CERCA Programme/Generalitat de CatalunyaWith the rise of two-dimensional (2D) transition-metal dichalcogenide (TMD) semiconductors and their prospective use in commercial (opto)electronic applications, it has become key to develop scalable and reliable TMD synthesis methods with well-monitored and controlled levels of impurities. While metal-organic chemical vapor deposition (MOCVD) has emerged as the method of choice for large-scale TMD fabrication, carbon (C) incorporation arising during MOCVD growth of TMDs has been a persistent concern-especially in instances where organic chalcogen precursors are desired as a less hazardous alternative to more toxic chalcogen hydrides. However, the underlying mechanisms of such unintentional C incorporation and the effects on film growth and properties are still elusive. Here, we report on the role of C-containing side products of organosulfur precursor pyrolysis in MoS2 thin films grown from molybdenum hexacarbonyl Mo(CO)6 and diethyl sulfide (CH3CH2)2S (DES). By combining in situ gas-phase monitoring with ex situ microscopy and spectroscopy analyses, we systematically investigate the effect of temperature and Mo(CO)6/DES/H2 gas mixture ratios on film morphology, chemical composition, and stoichiometry. Aiming at high-quality TMD growth that typically requires elevated growth temperatures and high DES/Mo(CO)6 precursor ratios, we observed that temperatures above DES pyrolysis onset (â 600 °C) and excessive DES flow result in the formation of nanographitic carbon, competing with MoS2 growth. We found that by introducing H2 gas to the process, DES pyrolysis is significantly hindered, which reduces carbon incorporation. The C content in the MoS2 films is shown to quench the MoS2 photoluminescence and influence the trion-To-exciton ratio via charge transfer. This finding is fundamental for understanding process-induced C impurity doping in MOCVD-grown 2D semiconductors and might have important implications for the functionality and performance of (opto)electronic devices

Stabilizing edge fluorination in graphene nanoribbons

The on-surface synthesis of edge-functionalized graphene nanoribbons (GNRs) is challenged by the stability of the functional groups throughout the thermal reaction steps of the synthetic pathway. Edge fluorination is a particularly critical case in which the interaction with the catalytic substrate and intermediate products can induce the complete cleavage of the otherwise strong C-F bonds before the formation of the GNR. Here, we demonstrate how a rational design of the precursor can stabilize the functional group, enabling the synthesis of edge-fluorinated GNRs. The survival of the functionalization is demonstrated by tracking the structural and chemical transformations occurring at each reaction step with complementary X-ray photoelectron spectroscopy and scanning tunneling microscopy measurements. In contrast to previous attempts, we find that the C-F bond survives the cyclodehydrogenation of the intermediate polymers, leaving a thermal window where GNRs withhold more than 80% of the fluorine atoms. We attribute this enhanced stability of the C-F bond to the particular structure of our precursor, which prevents the cleavage of the C-F bond by avoiding interaction with the residual hydrogen originated in the cyclodehydrogenation. This structural protection of the linking bond could be implemented in the synthesis of other sp2-functionalized GNRs

p-Type Ultrawide-Band-Gap Spinel ZnGa2O4: New Perspectives for Energy Electronics

The family of spinel compounds is a large and important class of multifunctional materials of general formulation AB2X4 with many advanced applications in energy and optoelectronic areas such as fuel cells, batteries, catalysis, photonics, spintronics, and thermoelectricity. In this work, it is demonstrated that the ternary ultrawide-band-gap (∼5 eV) spinel zinc gallate (ZnGa2O4) arguably is the native p-type ternary oxide semiconductor with the largest Eg value (in comparison with the recently discovered binary p-type monoclinic β-Ga2O3 oxide). For nominally undoped ZnGa2O4 the high-temperature Hall effect hole concentration was determined to be as large as p = 2 × 1015 cm–3, while hole mobilities were found to be μh = 7–10 cm2/(V s) (in the 680–850 K temperature range). An acceptor-like small Fermi level was further corroborated by X-ray spectroscopy and by density functional theory calculations. Our findings, as an important step toward p-type doping, opens up further perspectives for ultrawide-band-gap bipolar spinel electronics and further promotes ultrawide-band-gap ternary oxides such as ZnGa2O4 to the forefront of the quest of the next generation of semiconductor materials for more efficient energy optoelectronics and power electronics

Molecular Approach for Engineering Interfacial Interactions in Magnetic/Topological Insulator Heterostructures

Altres ajuts: Interreg V-A España-Francia-Andorra (EFA 194/16 TNSI)Controlling interfacial interactions in magnetic/topological insulator heterostructures is a major challenge for the emergence of novel spin-dependent electronic phenomena. As for any rational design of heterostructures that rely on proximity effects, one should ideally retain the overall properties of each component while tuning interactions at the interface. However, in most inorganic interfaces, interactions are too strong, consequently perturbing, and even quenching, both the magnetic moment and the topological surface states at each side of the interface. Here, we show that these properties can be preserved using ligand chemistry to tune the interaction of magnetic ions with the surface states. By depositing Co-based porphyrin and phthalocyanine monolayers on the surface of BiTe thin films, robust interfaces are formed that preserve undoped topological surface states as well as the pristine magnetic moment of the divalent Co ions. The selected ligands allow us to tune the interfacial hybridization within this weak interaction regime. These results, which are in stark contrast with the observed suppression of the surface state at the first quintuple layer of BiSe induced by the interaction with Co phthalocyanines, demonstrate the capability of planar metal-organic molecules to span interactions from the strong to the weak limit

Control of spin-orbit torques by interface engineering in topological insulator heterostructures

(Bi$_{1-x}$Sb$_x$)$_2$Te$_3$ topological insulators (TIs) are gathering increasing attention owing to their large charge-to-spin conversion efficiency and the ensuing spin-orbit torques (SOTs) that can be used to manipulate the magnetization of a ferromagnet (FM). The origin of the torques, however, remains elusive, while the implications of hybridized states and the strong material intermixing at the TI/FM interface are essentially unexplored. By combining interface chemical analysis and spin-transfer ferromagnetic resonance (ST-FMR) measurements, we demonstrate that intermixing plays a critical role in the generation of SOTs. By inserting a suitable normal metal spacer, material intermixing is reduced and the TI properties at the interface are largely improved, resulting in strong variations in the nature of the SOTs. A dramatic enhancement of a field-like torque, opposing and surpassing the Oersted-field torque, is observed, which can be attributed to the non-equilibrium spin density in Rashba-split surface bands and to the suppression of spin memory loss.Comment: This document is the unedited Author's version of a Submitted Work that was subsequently accepted for publication in Nanoletters, \copyright American Chemical Society after peer revie