ICMCTF2019 Session C3+C2+C1-ThM: Thin Films for Energy-related Applications II/Novel Oxide Films for Active Devices/Optical Metrology in Design, Optimization, and Production of Multifunctional Materials

Capacitive deionization (CDI) is considered to be one of the most promising technologies for many water treatment and purification applications. To meet the demand for pursuing high efficiency of desalination, the key component, CDI electrode, requires high chemical stability, high specific surface area, high water wetting ability, and suitable porous structure for ion electrosorption. In this study, a facile approach involving alkali treatment followed by post-annealing was utilized to develop nanoflaky titanium dioxide (TiO₂) structure on titanium (Ti) foil. Furthermore, the feasibility of using nanoflaky TiO₂ grown Ti as electrode for CDI application was evaluated.

The results showed that the grown TiO₂ possessed a porous nanoflaky structure with a mixed-phase of anatase and rutile; in addition, the porous size is in the range of 30 ~ 100 nm. Such microstructure characteristics implies a high specific surface area. Superhydrophilic surface property was also obtained for this nanoflaky TiO₂ structure. According to the cyclic voltammetry results, the nanoflaky TiO₂ grown Ti electrode exhibited electrosorption/electrodesorption ability in 1 M sodium chloride solution, indicating the stability and regeneration. Finally, the deionization performance of flow-through CDI device using nanoflaky TiO₂ grown Ti electrode was demonstrated.

Supercapacitors have become a popular energy storage device in the past ten years, owing to its excellent properties in many aspects, like electrical conductivity, cycle stability, and higher power density than lithium battery. However, the biggest challenge for supercapacitors to compete with the Li battery is energy density, i.e., its discharging time is still insufficient to satisfy for many desired applications. Therefore, we have studied a novel supercapacitor electrode made out of a mixed transition metal oxide grown on Ni foam to combat this problem. The mixed transition metal oxide is MnCo2O4 was synthesized using a one-pot synthesis involving hydrothermal treatment. We show that simply varying the precursor concentration would lead to the formation of MnCo2O4 nanostructures having different morphologies. The resulting MnCo2O4 provides more oxidation states that can participate in the electron transfer than monoxide. Several material characterizations and electrochemical tests were performed. Moreover, we demonstrate that the novel electrode gives very excellent specific capacitance of 1,740 F/g and stability after 5,000 cycles. Effect of the material characteristics on the electrochemical performance is discussed.

One of the main limitations of microalgae culture in a photobioreactor is the low efficiency of sunlight conversion. To improve the overall photoconversion efficiency and provide energy support, a photobioreactor (PBR) and photovoltaic technology (PV) coupled system can be developed.

Semi-transparent thin film solar cells based on hydrogenated amorphous silicon technology could be directly placed on the PBR’s surface. PV cells absorb a part of the incident light to produce electricity while being transparent in the specific photosynthesis wavelength range. To reach this objective, very thin solar cells must be produced. Decreasing the active layer thickness (intrinsic a-Si:H layer) of solar cells induces a better transparency but also a lower efficiency. As a consequence, we worked on the compensation of efficiency losses of thin solar cells by improving optical properties of substrate surfaces. Texturing process using reactive ion etching was applied on glass substrate to obtain a light scattering property on its back side and an antireflective property on the upper side. We studied the influence of those both textured surfaces on a-Si:H solar cells properties.

First, by texturing the back side of the substrate, the influence of light scattering was investigated. That should improve the light absorption in PV solar cells. The best result shows a raise of the short circuit current (Jsc) of the thin solar cells from 8.5 to 10 mA/cm², while the other parameters (the open circuit voltage (Voc) and the fill factor (FF)) remained the same. Thus the efficiency increased from 3.7 to 4.5 %.

In a second step, the influence of an antireflective texture of the substrate upper side was studied. The antireflection texture with light scattering effect gives the best results on PV cells. Short circuit current of this thin solar cell goes to 11.2 mA/cm² and its efficiency increased to 5.2 %.

The influence of these solar cells, used as optical filter, on the microalgae production rate was investigated. The photosynthesis peaks of microalgae correspond to wavelengths [400 - 500 nm] and [550 – 700 nm]. The first peak of photosynthesis is absorbed by the solar cell. Furthermore, an important diffusion of light in the solar cells also induces losses of transmitted light to microalgae for the second peak of photosynthesis. Finally growing test of Phaeodactylum Tricornutum shows that PV optical filtering does not have influence on growth rate. Optical modifications of substrate applied to very thin solar cells allowed to keep a high PV efficiency while maintaining the growing rate of microalgae.

The new generation of engineered materials is required to perform under extreme conditions of stress, temperature and irradiation, among others. Nano multilayer (NM) materials have been shown to be promising candidates to overcome current structural and functional limitations. To date, most of the studied NM systems are comprised of metal/metal layers, where the optical properties do not represent an area of interest. Therefore, in this work we extended the structural potential of metal/metal NM configurations to ceramic/ceramic NMs which display a broad range of properties but in particular, they offer the possibility to modulate the optical properties depending on their morphology, grain size, layer thickness, and composition. Three different systems composed of alternated layers of AlN and SiO_xwere studied. For each sample, the number of layers and the individual thicknesses were varied until a total thickness of 1 µm was reached. The optical properties of the multilayers were determined using UV-Vis-NIR spectroscopy, revealing the formation of uniform and smooth interfaces with an average optical transparency above 80% that extends through the entire region. Furthermore, the microstructure and plastic behavior were also studied and correlated with the multilayer configuration of the synthesized systems.

The subwavelength scale periodic arrangement of nanostructures, so-called artificially engineered nano-structures exhibit distinct optical, mechanical, and magnetic properties when they are compared with their bulk counterparts which has recently gained a growing interest due to its potential applications in various optical and optoelectronic systems such as lenses, solar cells, photodetectors, and sensors [1-3]. Tailoring aforementioned inherent properties cannot merely possible with the material choices (ie. elemental composition), but also the size and shape of these artificial structures play a significant role. Unraveling the mechanisms that influence and control the optical properties of highly-porous, periodic, and three dimensional arrangements of nanoplasmonic structures can offer new approaches for the development of next generation sensors. Particularly, both glancing angle deposition and atomic layer deposition can be used to create periodic nanostructures with multiple constituent materials, so-called heterostructured metamaterials.[4]

In this study, we employ a two-source (ie. Au and Si) electron-beam-evaporated, ultra-high-vacuum glancing angle deposition which allows for the fabrication of highly-ordered and spatially-coherent super-lattice type Au-Si slanted columnar heterostructured thin films. We perform a combinatorial spectroscopic generalized ellipsometry and finite-element method calculation analysis to determine anisotropic optical properties. We observe the occurrence of a strong locally enhanced dark quadrupole plasmonic resonance mode (bow-tie mode) in the vicinity of the gold junctions, with a tunable and geometry dependent frequency in the near-infrared spectral range. In addition, inter-band transition-like modes are observed in the visible to ultra-violet spectral regions. Using finite element method, we demonstrate that changes in the index of refraction due to the concentration variation of a chemical substance environment (gaseous or liquid) within a porous nanoplasmonic structure can be detected by transmitted intensity alterations down to 1 ppm sensitivity.

References

[1] Kabashin, A. V., et al. Nature materials 8.11 (2009): 867.

[2] Schmidt, Daniel, and Mathias Schubert. Journal of Applied Physics 114.8 (2013): 083510.

[3] Frölich, Andreas, and Martin Wegener. Optical Materials Express 1.5 (2011): 883-889.

[4] Sekora, Derek, et al. Applied Surface Science 421 (2017): 783-787.

Rapidly growing applications of p-type Copper nitride (Cu₃N) films in optical storage media, photovoltaics etc. has motivated us to study Cu₃N thin films which were deposited on glass and silicon substrates by reactive magnetron sputtering at 150˚C from a metallic copper target. Until now, few researchers have studied the p-type conductivity of Cu₃N films which is low compared to the result obtained in our experiment. In this work, we discuss the effects of working pressure on the microstructures, electrical, and optical properties of the Cu₃N films. The working pressures were varied from 5 mtorr to 23 mtorr while gas flow rate was kept constant at N₂/(Ar+N₂)%=40%. When the working pressure increases, the Cu₃N (111) peak intensity decreases as evident from XRD studies. Meanwhile, conduction type changes from n-type to p-type. When working pressure is increased to 15 mtorr, the resistivity is 1.575 Ω•cm and the sample shows p-type conduction. This is possibly due to the formation of many copper vacancies (i.e. vacancies at Cu cation sites) in the films. When the working pressure is 5 mtorr, a Cu (111) pattern was observed from selected area electron diffraction (SAED) by TEM analysis. It disappears upon increasing the working pressure to 15 mtorr. It was also found that the ratio of Cu²⁺/Cu⁺ increases from 0.39 to 0.93 when the working pressure is raised from 5 mtorr to 20 mtorr. More substitution of Cu²⁺ for Cu⁺ results in the formation of more Cu vacancies, which leads to the transition in conduction from n-type to p-type. Finally, n-type Cu₃N/p-type Cu₃N homojunctions and n-type ZnO/p-type Cu₃N heterojunctions diodes were fabricated. It was found that homojunction devices Al/n-type Cu₃N/p-type Cu₃N do not show significant rectification effects. As we observed, at ±3Volts, the I_on/I_off was only 0.24. Whereas, in heterojunction devices Al/n-type ZnO/p-type Cu₃N, a higher I_on/I_off of 3118 can be achieved. Heterojunction devices outperform the homojunction devices instead of interfacial issues indicating the superior electrical properties which are explained considering the mismatch in the built-in potentials of the p-n junctions.

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