Antoine Barnabé

p-Type conducting transparent characteristics of delafossite Mg-doped CuCrO2thin films prepared by RF-sputtering

The growth of technologically relevant compounds, Mg-doped CuCrO2 delafossite thin films, on a quartz substrate by radio-frequency sputtering is reported in this work. The deposition, performed at room temperature, leads to a nanocrystalline phase with extremely low roughness and high density. Delafossite characteristic diffraction peaks were obtained as a function of the thermal treatment under primary vacuum. The electrical conductivity was optimized until 1.6 S cm−1 with an optical transmittance of 63% in the visible range by a 600 °C annealing treatment under primary vacuum applied for 4 h. The transport properties were analyzed by Seebeck and Hall measurement, integrated spectrophotometry and optical simulation. These measurements highlighted degenerated semiconductor behavior using a hopping mechanism with a high hole concentration (1021 cm−3) and a low mobility (0.2 cm2 V−1 s−1). The direct optical bandgap of 3.3 eV has been measured according to Taucs relationship. A refractive index of 2.3 at a wavelength of 1100 nm has been determined by spectroscopic ellipsometry and confirmed by two independent modellings of the optical transmittance and reflectance spectra. All these p-type TCO optoelectronic characteristics have led to the highest Haackes figure of merit (1.5 × 10−7 Ω−1) reported so far for such delafossite materials

Synthesis and Thermostructural Studies of a CuFe1−xCrxO2 Delafossite Solid Solution with 0 ≤ x ≤ 1

In this work, different CuFe(1-x)Cr(x)O(2) compositions with 0 <or= x <or= 1 were prepared by a standard solid-state reaction. These oxides crystallize with the delafossite structure. The phase stability and thermal behavior of the complete CuFe(1-x)Cr(x)O(2) solid solution was studied by thermogravimetric analysis and high-temperature X-ray diffraction experiments under an air atmosphere up to 1000 degrees C. For x = 0, CuFeO(2) is oxidized into the spinel (CuFe(2)O(4)) and copper monoxide (CuO) phases, whereas for x = 1, CuCrO(2) is thermally stable. For all of the intermediate compositions (0 < x < 1), complex oxidation, reduction, and phase transitions between delafossite and spinel have been observed. chromium tends to stabilize the stoichiometric delafossite phase, while iron favors the delafossite-to-spinel phase transition.

Ac conductivity and dielectric properties of CuFe1−xCrxO2 : Mg delafossite

The electrical and dielectric properties of CuFe1−xCrxO2 (0 ≤ x ≤ 1) powders, doped with 3% of Mg and prepared by solid-state reaction, were studied by broadband dielectric spectroscopy in the temperature range from −100 to 150 °C. The frequency-dependent electrical and dielectric data have been discussed in the framework of a power law conductivity and complex impedance and dielectric modulus. At room temperature, the ac conductivity behaviour is characteristic of the charge transport in CuFe1−xCrxO2 powders. The substitution of Fe3+ by Cr3+ results in an increase in dc conductivity and a decrease in the Cu+–Cu+ distance. Dc conductivity, characteristic onset frequency and Havriliak–Negami characteristics relaxation times are thermally activated above −40 °C for x = 0.835. The associated activation energies obtained from dc and ac conductivity and from impedance and modulus losses are similar and show that CuFe1−xCrxO2 delafossite powders satisfy the BNN relation. Dc and ac conductivities have the same transport mechanism, namely thermally activated nearest neighbour hopping and tunnelling hopping above and below −40 °C, respectively.

Thermoelectric and Transport Properties of Delafossite CuCrO2:Mg Thin Films Prepared by RF Magnetron Sputtering

P-type Mg doped CuCrO2 thin films have been deposited on fused silica substrates by Radio-Frequency (RF) magnetron sputtering. The as-deposited CuCrO2:Mg thin films have been annealed at different temperatures (from 450 to 650 °C) under primary vacuum to obtain the delafossite phase. The annealed samples exhibit 3R delafossite structure. Electrical conductivity σ and Seebeck coefficient S of all annealed films have been measured from 40 to 220 °C. The optimized properties have been obtained for CuCrO2:Mg thin film annealed at 550 °C. At a measurement temperature of 40 °C, this sample exhibited the highest electrical conductivity of 0.60 S·cm−1 with a Seebeck coefficient of +329 µV·K−1. The calculated power factor (PF = σS²) was 6 µW·m−1·K−2 at 40 °C and due to the constant Seebeck coefficient and the increasing electrical conductivity with measurement temperature, it reached 38 µW·m−1·K−2 at 220 °C. Moreover, according to measurement of the Seebeck coefficient and electrical conductivity in temperature, we confirmed that CuCrO2:Mg exhibits hopping conduction and degenerates semiconductor behavior. Carrier concentration, Fermi level, and hole effective mass have been discussed.

Integration of P-CuO Thin Sputtered Layers onto Microsensor Platforms for Gas Sensing

P-type semiconducting copper oxide (CuO) thin films deposited by radio-frequency (RF) sputtering were integrated onto microsensors using classical photolithography technologies. The integration of the 50-nm-thick layer could be successfully carried out using the lift-off process. The microsensors were tested with variable thermal sequences under carbon monoxide (CO), ammonia (NH3), acetaldehyde (C2H4O), and nitrogen dioxide (NO2) which are among the main pollutant gases measured by metal-oxide (MOS) gas sensors for air quality control systems in automotive cabins. Because the microheaters were designed on a membrane, it was then possible to generate very rapid temperature variations (from room temperature to 550 °C in only 50 ms) and a rapid temperature cycling mode could be applied. This measurement mode allowed a significant improvement of the sensor response under 2 and 5 ppm of acetaldehyde.

Highly Sensitive Sputtered ZnO:Ga Thin Films Integrated by a Simple Stencil Mask Process on Microsensor Platforms for Sub-ppm Acetaldehyde Detection

The integration of a 50-nm-thick layer of an innovative sensitive material on microsensors has been developed based on silicon micro-hotplates. In this study, integration of ZnO:Ga via radio-frequency (RF) sputtering has been successfully combined with a low cost and reliable stencil mask technique to obtain repeatable sensing layers on top of interdigitated electrodes. The variation of the resistance of this n-type Ga-doped ZnO has been measured under sub-ppm traces (500 ppb) of acetaldehyde (C2H4O). Thanks to the microheater designed into a thin membrane, the generation of very rapid temperature variations (from room temperature to 550 °C in 25 ms) is possible, and a rapid cycled pulsed-temperature operating mode can be applied to the sensor. This approach reveals a strong improvement of sensing performances with a huge sensitivity between 10 and 1000, depending on the working pulsed-temperature level.

Lithium trapping as a degradation mechanism of the electrochromic properties of all-solid-state WO3//NiO devices

There has been keen interest for years in the research of all-solid-state transmittance-type electrochromic (EC) devices due to their various applications especially in ‘‘smart windows’’. A step forward has been taken in the successful preparation of full multilayered devices with enlarged optical contrast and fast switching response. However, limited durability remains a severe issue. Upon cycling, EC devices suffer from decline of charge capacity as well as optical modulation while the detailed degradation mechanisms remain poorly understood. Here, we demonstrate unambiguous ion-trapping evidence to interpret the charge density decay of the EC device induced using various voltammetric cycling protocols, namely long-term cycling and accelerated cycling. Pronounced comparable ion trapping occurs in cathodically colored WO3 films whatever the cycling procedure is, suggesting the existence of the trapping ‘‘saturation’’ phenomenon. From second-ion-mass-spectroscopy analysis, the 7Li+/184W+ ratio in the degraded WO3 films is more than 100 while it is almost zero in the as-prepared films. In contrast, for anodically colored NiO, a larger number of trapped cations is determined in the long-term cycled films than in the accelerated ones. In combination with X-ray-photoelectron-spectroscopy, variable bonding energies indicate that the ions are trapped at different types of sites, depending on the cycling procedure, and they can reside in the structural channels or break the network chains to form new chemical bondings, thus resulting in a significant color difference. In addition, a clear upward trend in the trapped Li concentration along with depth is observed. All our findings provide a deep insight into the degradation phenomenon taking place in electrochromic films as well as in full devices and offer valuable information for the understanding of micro mechanisms.

Determination of modified figure of merit validity for thermoelectric thin films with heat transfer model: Case of CuCrO2:Mg deposited on fused silica

Thermoelectric performance of a material is determined using a figure of merit (FOM) determined as ZT (ZT = σS2T/κ where σ is the electrical conductivity, S is the Seebeck coefficient, κ is the thermal conductivity, and T is the temperature). In the case of a thin film, it is normal in the first approach to consider calculating the FOM by using the thermal conductivity of the film. However, both the thermal influence of the substrate and the emissivity of the film must also be taken into account. In the present work, the heat transfer model is used in order to study the influence of the thermal conductivity, the thickness, and the emissivity of the film on the thermal gradient of the stack (substrate + thin film). The limits of these three parameters are determined in order to have the temperature variation due to the presence of the film compared to the substrate alone that remains less than 1%. Under these limits, the thermal conductivity of the substrate can be taken into account instead of the thermal conductivity of the thin film, and a modified FOM (Z’T) can be calculated. The present study leads to the determination of the validity of modified ZT. In the case of CuCrO2:Mg thin films, the model shows that the use of Z’T is valid. The calculated value of Z’T with the measured Seebeck coefficient and the electrical conductivity as a function of the temperature for 100 nm thick films and the temperature dependent thermal conductivity taken from the literature reached 0.02 at 210 °C. A thermoelectric module made with this material showed 10.6 nW when 220 °C is applied at the hot side.

FIB plan view lift-out sample preparation for TEM characterization of periodic nanostructures obtained by spinodal decomposition in Co1.7Fe1.3O4 thin films.

There is a miscibility gap in the CoFe2O4–Co3O4 phase diagram. In this miscibility gap, the oxides can be subjected to a spinodal transformation. It has already been observed in oxides consisting of crystals greater than or equal to 100 nm that spinodal decomposition leads to the formation of two alternating iron-rich and cobalt-rich spinel phases. The pseudo-periodic alternation occurs approximately every 5 nm. In the miscibility gap, thin films of pure iron cobaltites, consisting of crystallites of the order of 10 nm in diameter and around 300 nm in thickness, undergo transformation when they are treated at 600 °C for several hours. X-ray diffraction and Raman spectroscopy clearly reveal this transformation, which is accentuated as a function of the treatment time. An electron microscopy study of the cross-sections (view of the films along their thickness), confirms the progressive separation of the former spinel oxide in iron-rich and cobalt-rich spinel phases, without however revealing a pseudo-periodic organization of these phases, whatever the time of treatment. In an attempt to reveal this organization, a specific method of preparation has been implemented to extract the upper part of the films parallel to their basic plane and to observe the crystallites in plan view. The alternation of the iron- and cobalt-rich phases could, however, only be found in the largest crystallites. It seems that the nanometric size of the crystallites prevents the establishment of a pseudo-periodic organization of the phases during the periodic transformation. The observation of compositional anomalies in the grain boundaries seems to support the hypothesis related to a nanometric effect of the crystallization.

Correction: Sinnarasa, Inthuga, et al. Thermoelectric and Transport Properties of Delafossite CuCrO2:Mg Thin Films Prepared by RF Magnetron Sputtering. Nanomaterials 2017, 7, 157

The authors wish to make the following correction to this paper [1]. In Equation (8), logarithm (ln) term is missing[...].