Experimental and ab Initio Studies of Deep-Bulk Traps in Doped Rare-Earth Oxide Thick Films

Lanthanum-doped CeO2 is a promising semiconductor for gas sensing. A combined study applying impedance spectroscopy and first-principles calculations was performed for pure and lanthanum-doped samples. The results showed a strong influence of the localized Ce 4f states on the electrical conduction processes and an electrical resistance increase as a function of the exposure to vacuum and air atmospheres. After its modification with a rare-earth element along with exposure to reducing and oxidizing atmospheres, the observed behavior suggested the presence of multitraps, which depended on the described equilibrium between the oxygen vacancies (Vo x ↔ VO· ↔ VO· ) in a disordered deep-bulk trap location. According to the DFT results, the multitraps were formed with the creation of an oxygen vacancy far from the doping atom. They were considered to be responsible for the phenomena modifying the Debye-like response. The transfer of electrons from Ce(III) to the adsorbed oxygen species, decreasing the number of electrons in the 4f state, reduced the electrical conductivity by the hopping frequency dependence of the total resistance and capacitances. This was probably due to the interactions between defective oxygen and metallic species.Fil: Silva Rosa Rocha, Leandro. Universidade Federal do São Carlos; BrasilFil: Schipani, Federico. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Aldao, Celso Manuel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Cabral, Luís A.. Universidade Federal do São Carlos; BrasilFil: Simoes, Alexandre Z.. Faculdade de Engenharia de Guaratinguetá, Unesp; BrasilFil: Macchi, Carlos Eugenio. Universidad Nacional del Centro de la Provincia de Buenos Aires.. Facultad de Ciencias Exactas. Departamento de Física; Argentina. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Tandil. Sede Tandil del Centro de Investigaciones En Fisica E Ingenieria del Centro de la Provincia de Buenos Aires.; ArgentinaFil: Marques, Gilmar Eugenio. Universidade Federal do São Carlos; BrasilFil: Ponce, Miguel Adolfo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Longo, Elson. Universidade Federal do São Carlos; Brasi

Metal-insulator transition and electrically-driven memristive characteristics of SmNiO3 thin films

The correlated oxide SmNiO3 (SNO) exhibits an insulator to metal transition (MIT) at 130 {\deg}C in bulk form. We report on synthesis and electron transport in SNO films deposited on LaAlO3 (LAO) and Si single crystals. X-ray diffraction studies show that compressively strained single-phase SNO grows epitaxially on LAO while on Si, mixed oxide phases are observed. MIT is observed in resistance-temperature measurements in films grown on both substrates, with charge transport in-plane for LAO/SNO films and out-of-plane for Si/SNO films. Electrically-driven memristive behavior is realized in LAO/SNO films, suggesting that SNO may be relevant for neuromorphic devices

Hole selective contacts based on transition metal oxides for c-Ge thermophotovoltaic devices

© 2022 Elsevier. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/Thermophotovoltaics has become a very attractive solution for heat-to-electricity conversion due to its excellent conversion efficiencies. However, further research is needed to reduce the device cost which is typically based on III-V semiconductors. To tackle this limitation, crystalline germanium (c-Ge) has been proposed as an excellent substrate for low-cost devices. One of the key advances behind high system efficiencies is the excellent reflectance of the out-of-band photons at the rear surface of the photovoltaic device. These photons with lower energy than the absorber bandgap are reflected back to the thermal emitter reducing its thermal losses. In this work, we explore the performance of hole selective contacts based on evaporated transition metal oxides (MoOx, VOx, WOx) to be introduced at the rear surface of c-Ge devices. Regarding electrical properties, we characterize the selectivity of the contact by measuring effective surface recombination velocity (Seff) and contact resistivity (¿C). Best results are obtained with MoOx contacted by Ag/ITO with Seff = 588 cm/s and ¿C = 55.6 mO cm2 which can be improved by using gold as a metal contact leading to Seff = 156 cm/s and ¿C = 60.9 mO cm2. Regarding out-of-band reflectance, it is better for the case of Ag/ITO/MoOx contact with 87.5% compared to 78.9% for Au/MoOx when a 1473 K black body spectrum is used. Device simulations show potential system efficiencies in the range of 18–19% which are comparable to the best reported efficiencies using c-Ge thermophotovoltaic devices.This work has been supported by the Spanish government under projects PID2019-109215RB-C41 (SCALED), PID2020-116719RB-C41 (MATER ONE) and PID2020-115719RB-C21 (GETPV) funded by MCIN/ AEI/10.13039/501100011033. The authors would like to thank the master student Oscar Llados ´ and Guillem Ayats for their help in processing the samples, Dr. Alejandro Datas from Instituto de Energía Solar (IES) in Madrid for providing the c-Ge wafers and fruitful discussions.Peer ReviewedPostprint (published version

Electronic, thermoelectric and optical properties of vanadium oxides: VO2, V2O3 and V2O5

Correlated electrons in vanadium oxides are responsible for their extreme sensitivity to external stimuli such as pressure, temperature or doping. As a result, several vanadium oxides undergo insulator-to-metal phase transition (IMT) accompanied by structural change. Unlike vanadium pentoxide (V3O3), vanadium dioxide (VO3) and vanadium sesquioxide (V3O3) show I MT in their bulk phases. In this study, we have performed one electron Kohn-Sham electronic band-structure calculations of VO3, V3O3 and V2O5 in both metallic and insulating phases, implementing a full ab-initio simulation package based on Density Functional Theory (DFT), Plane Waves and Pseudopotentials (PPs). Electronic band structures are found to be influenced by crystal structure, crystal field splitting and strong hybridization between O2p and V3d bands. “Intermediate bands”, with narrow band widths, lying just below the higher conduction bands, are observed in V2O5 which play a critical role in optical and thermoelectric processes. Similar calculations are performed for both metallic and insulating phases of bulk VO2 and V2O3. Unlike in the metallic phase, bands corresponding to “valence electrons” considered in the PPs are found to be fully occupied in the insulating phases. Transport parameters such as Seebeck coefficient, electrical conductivity and thermal (electronic) conductivity are studied as a function of temperature at a fixed value of chemical potential close to the Fermi energy using Kohn-Sham band structure approach coupled with Boltzmann transport equations. Because of the layered structure and stability, only V2O5 shows significant thermoelectric properties. All the transport parameters have correctly depicted the highly anisotropic electrical conduction in V2O5. Maxima and crossovers are also seen in the temperature dependent variation of Seebeck coefficient in V2O5, which can be consequences of “specific details” of the band structure and anisotropic electron-phonon interactions. For understanding the influence of phase transition on transport properties, we have also studied transport parameters of VO2 for both metallic and insulating phases. The Seebeck coefficient, at experimental critical temperature of 340K, is found to change by 18.9 µV/K during IMT, which lies within 10% of the observed discontinuity of 17.3 µV/K. Numerical methods have been used to analyze the optical properties of bulk and thin films of VO2, V2O3, and V2O5, deposited on Al2O3 substrates, from infrared to vacuum ultraviolet range (up to 12 eV). The energies corresponding to the peaks in the reflectivity-energy (R-E) spectra are explained in terms of the Penn gap and the degree of anisotropy is found to be in the order of V2O3 \u3c VO2 \u3c V2O5. The effective number of electrons participating in the optical transitions is described using the “sum rule”. The optical absorption is found to occur followed by the transitions of d electrons as well as the transitions from O2p to V3d states. In the Honeywell microbolometer structure, the bolometer sensing element has been chosen to be VOx, with x equal to 1.8, along with other layers of Si3N4, air, Al and Si. The room temperature spectral emissivity of such a layered structure is analyzed using Multi-Rad, a simulation package that utilizes thin film optics in the form of matrix method of multilayers. Calculations show that the Si3N4 layer provides the much desired linear performance of the VOx based bolometer

PEO Coatings with Active Protection Based on In-Situ Formed LDH-Nanocontainers

In the present work, for the first time Zn-Al layered double hydroxide (LDH) nanocontainers were grown in-situ on the surface and in the pores of plasma electrolytic oxidation (PEO) layer and then loaded with a corrosion inhibitor to provide an active protection. The developed LDH-based conversion process ensures partial sealing of the pores and provides an effective corrosion inhibition on demand leading to increased fault-tolerance and self-healing properties. The structure, morphology and composition of the LDH-sealed PEO coatings on 2024 aluminum alloy were investigated using SEM, TEM/FIB, XRD and GDOES. Electrochemical impedance spectroscopy and scanning vibrating electrode techniques show a remarkable increase in the corrosion resistance and fault tolerance when PEO coating is sealed with a LDH-inhibitor treatment

Lithiation and Characterisation by Ion Beam Techniques of V2O5 Thin Films

In this work, studies on the mechanism of lithium ion intercalation (Li+) in vanadium pentoxide thin films (V2O5) were performed. The intercalation was induced by electrochemical experiments and the conditions and parameters used were optimised to allow structural, optical and electrical characterisation. Rutherford Backscattering Spectrometry (RBS) and Nuclear Reaction Analysis (NRA) ion beam techniques, performed at the Centre for Nuclear Technologies (CTN/IST), were used to locate and quantify the presence of Li+ ions in the crystalline structure of the material. In addition, the effect of pre-lithiation by ion implantation on V2O5 thin films was studied. V2O5 thin films were deposited by electron beam assisted evaporation (EBPVD) on glass substrates with an ITO layer. The samples were subjected to cyclic voltammetry (CV) and chronoamperometry (CA) as a procedure for intercalating Li+ ions in the V2O5 structure. Optical characterization by visible spectroscopy revealed a recovery of the film’s initial transmittance state after the sample was subjected to five cycles of CV. The corresponding I(V) curves displayed a comparable symmetry of peak anodic and cathodic currents indicating reversibility of Li+ intercalation. The X-Ray Diffractograms (XRD) of these samples showed the presence of an orthorhombic structured V2O5 with a preferential orientation in the (0 0 1) plane. The intercalation resulted in an increase in interplanar spacing of 3% along the lattices’ c-axis, which varied proportionally with the applied voltage. RBS and NRA spectra revealed distinct peaks, characteristic of 7Li+ ions and it was possible to quantify Li as an atomic percentage of the samples’ composition along its depth. It was also observed that the depth profile for Li extended beyond the ITO layer. The electro-optical characterisation of Li+ ion implantation found a reduction in the samples’ intercalation reversibility, with depth profiles suggesting Li entrapment. It was possible to quantify and detect the presence of Li+ ions and correlate these results with the structural expansion induced by the intercalation

Electrochemical and photoelectrochemical properties of nickel oxide (NiO) with nanostructured morphology for photoconversion applications

The cost-effective production of chemicals in electrolytic cells and the conversion of the radiation energy into electrical energy in photoelectrochemical cells (PECs) require the use of electrodes with large surface area, which possess either electrocatalytic or photoelectrocatalytic properties. In this context nanostructured semiconductors are electrodic materials of great relevance because of the possibility of varying their photoelectrocatalytic properties in a controlled fashion via doping, dye-sensitization or modification of the conditions of deposition. Among semiconductors for electrolysers and PECs the class of the transition metal oxides (TMOs) with a particular focus on NiO interests for the chemical-physical inertness in ambient conditions and the intrinsic electroactivity in the solid state. The latter aspect implies the existence of capacitive properties in TMO and NiO electrodes which thus act as charge storage systems. After a comparative analysis of the (photo)electrochemical properties of nanostructured TMO electrodes in the configuration of thin film the use of NiO and analogs for the specific applications of water photoelectrolysis and, secondly, photoelectrochemical conversion of carbon dioxide will be discussed. © 2018 Bonomo, Dini and Decker

Studies on electrochromatic materials and devices

This thesis investigates electrochromic thin films needed to construct a variable transmission electro chromic device. Such a device is made of 5 layers sandwiched between 2 pieces of glass: two electronic transparent conducting layers, an optically active electro chromic layer (W03), a ion-conducting polymer electrolyte and an ionstorage layer (NiOx, TiOx, VOx, VzTiyOx) . Electrochromic NiOx thin films were produced by R.F. magnetron sputtering and electrodeposition techniques and studied under proton intercalation. A visible transmittance modulation of 0.70 and 0.80 and a visible coloration efficiency of 35 and 100 cm2.C-1 for a thickness of 300 and 200 nm were obtained for sputtered and chemically-deposited NiOx films respectively. Anodic films are extremely porous and soft. Under the mechanical stresses of ionic insertion/extraction they degrade more quickly than the compact nanostructure of physically deposited films. When studied under lithium intercalation, sputtered NiOx films exhibit a nucleation loop observed in cyclic voltammetry indicating the growth of a new phase and are seen to degrade quickly. NiOx films were not seen to be potential candidates for EC applications using Lt intercalation. W03, TiOx and VOx thin films were deposited by R.F. magnetron sputtering and studied under Lt intercalation/deintercalation. Optimised W03 films exhibited good electro chromic properties: a visible transmittance modulation of 0.82 and a visible coloration efficiency of 49 cm2 . C-l for a thickness of 450 nm. Electrochromic properties of TiOx films were seen to not strongly depend on the sputtering process parameters whereas VOx films showed a stronger dependence. TiOx films are able to store a limited quantity of charge Q = 13 mC.cm-2 for thicknesses greater than 13 nm. They are transparent in both charged and uncharged states T V,u and Tv,ch> 0.80, and are stable upon charge insertion/extraction. VOx films can store a much larger quantity of charge Q = 35 mC.cm-2 for a thickness of 70 nm. They are yellow in the uncharged state and bluish in the charged state: Tv,u and Tv,ch > 0.70, and the charge insertion/extraction process is seen to evolve during the initial cycles. Both TiOx and VOx films did not show all the required electrochromic properties for EC applications. The main achievement of this work was the development of highly durable vanadium/titanium mixed oxide thin films. Work was carried out on different VITi ratios using specific deposition techniques developed for that purpose. Films with a vanadium to titanium ratio of about 50 % showed optimum performance characteristics for passive ion storage layer applications. Such layers deposited on ITO exhibited high visible transmittance: Tv,ch > 0.62, and a relatively low visible modulation (0.20), with high storage capacity Q > 40 mC.cm-2 for a thickness of80 nm. The laminated W03IPAAUAlVzTiyOx EC device was assembled and exhibited under specific switching conditions encouraging properties: a visible transmittance modulation > 0.50 over more than 105 cycles

Metal-Insulator-Semiconductor Photodetectors

The major radiation of the Sun can be roughly divided into three regions: ultraviolet, visible, and infrared light. Detection in these three regions is important to human beings. The metal-insulator-semiconductor photodetector, with a simpler process than the pn-junction photodetector and a lower dark current than the MSM photodetector, has been developed for light detection in these three regions. Ideal UV photodetectors with high UV-to-visible rejection ratio could be demonstrated with III–V metal-insulator-semiconductor UV photodetectors. The visible-light detection and near-infrared optical communications have been implemented with Si and Ge metal-insulator-semiconductor photodetectors. For mid- and long-wavelength infrared detection, metal-insulator-semiconductor SiGe/Si quantum dot infrared photodetectors have been developed, and the detection spectrum covers atmospheric transmission windows