ICMCTF2015 Session C1: Recent Advances in Optical Thin Films and Nanomaterials

Carbide and nitride of transition elements are natural candidate materials for wear resistance applications as protective coatings. Extreme hardness, high wear resistance, low friction coefficient, chemically stability of transition metal(s) doped-diamond-like carbon (DLC) films make them popular. From this point of view, transition metals, Ti and Ta doped hydrogeneted DLC films were deposited by closed-field unbalanced magnetron sputtering system onto aluminum, Ti6Al4V and magnesium materials in Ar/N₂/C₂H₂ atmosphere. The friction and wear properties of Ti:Ta-based DLC coating were investigated for tribological applications. The coated specimens were characterized by SEM, XPS and X-ray diffraction techniques. Reciprocation multipass and pin-on-disc wear tests were carried out to compare their friction and wear properties. Hardness measurements was performed by micro indentation. A multi-mode operation was used as sliding fatigue, like multi-pass scratching in the same track at different loads. Failure mechanisms were discussed according to SEM examinations of the scratch tracks. Our results suggest that Ti-Ta-doped DLC film shows very dense microstructure, high hardness with low CoF and high wear resistance.

Noble metal nanoparticles (NPs) have been used for many centuries, providing different colours in Roman glasses or in the windows of medieval cathedrals. Nowadays, the interest in nanocomposite materials containing those metal NPs embedded in dielectric matrices, such as TiO₂ or Al₂O₃, is related with their potential use for a wide range of advanced technological applications, including colour filters, bio- and optical sensors, absorption elements of solar cells, enhancement of electrical/thermal conductivity of coatings, photocatalytic antibacterial and pollutant-degradation materials, gas sensors and Surface Enhanced Raman Spectroscopy (SERS). Most of these applications, particularly, those in the decorative field, rely on the so-called localized surface plasmon resonance (LSPR) absorption, which is governed by the type of the noble metal NPs, their distribution, size and shape and as well as of the dielectric properties of the matrix.

The main focus of the presentation will be placed in the demonstration on how the morphological and structural changes are a function of the annealing temperature, as well as on the particular amounts, shape, size and distribution of the noble metal particles dispersed in the dielectric matrix. Since changes in size, shape and distribution of Au clusters are fundamental parameters for tailoring of the LSPR effect, a set of films with a wide range of Au concentration was prepared. The films were deposited by DC magnetron sputtering, and in order to promote the clustering of the Au nanoparticles, the as-deposited samples were subjected to an in-air annealing protocol.

Many optical instruments are present in the artificial satellites orbiting around the Earth. Such instruments, as spectrometers, imagers, interferometers, contain typically a number of thin-film optical filters and coated optics.

Special thin-film filters are often required and represent the critical elements for the construction of the instrument itself. Two examples will be described: a spatially variable filter for an image spectrometer dedicated to the observation of Earth and a near-infrared narrow-band filter dedicated to the detection of the oxygen emission lines during the lightning phenomena in the atmosphere. In the first case the main difficulty consists in the accurate control of a graded film thickness over a dimension of few millimeters. On the contrary, in the second case the diameter of the filter is larger than 100 mm and the main problem is to maintain high thickness uniformity over the whole surface.

Besides the optical performance, there are additional requirements that should be taken into account when the coatings are manufactured. An important issue in space is the dimension and weight of all components, thus the optical filters should contribute to the realization of miniaturized instruments. Another critical issue is the survival of optical coatings in the space environment that is necessary for the correct operation of space instrumentation. In fact, either the failure or the variation of the performance of a coating could compromise the success of the mission. Therefore is advisable to investigate in advance the behavior of optical coatings when exposed to space radiation and extreme temperatures.

Representative thin-film materials and optical filters have been selected for testing and all coatings have been deposited by energetic techniques to ensure high-density materials with a better resistance to the space environment. The effect of particle irradiation, especially protons and ultraviolet solar photons, on the coating performance will be shown. The influence of other space environmental conditions, as contamination and low-temperature excursions, on the behavior of coatings will be discussed as well.

The synergy among all effects of space conditions remains still an open problem to which the experiments on the International Space Station may give an important contribution.

We develop a technology to obtain Al-doped ZnO (AZO) polycrystalline films with a well-defined single (0001) orientation deposited by a direct-current magnetron sputtering (DC-MS) to achieve low-resistivity AZO films. In this work, our aim is to clarify the characteristics of AZO films deposited by DC-MS and radio-frequency MS (RF-MS) and to develop novel technology to mix the benefits of the two different MS technologies. We deposited 500-nm-thick AZO films on glass substrates (@200 ºC). Al₂O₃ contents in the sputtering target was 2.0 wt.%.

Firstly, we clarify the characteristics of AZO films deposited by DC-MS and RF-MS. We deposited the films with varying the ratio of the DC power (P_DC) and RF power (P_RF). We found the critical ratio, P_RF/(P_DC+P_RF)=0.14, causing substantial changes in structural and electrical properties. The AZO films deposited with a ratio, P_RF/(P_DC+P_RF), of 0.0, i.e., by only DC-MS, showed poor crystallinity: the (002) and (004) peak intensities were very weak compared with those of the other samples and a small peak of (006) diffraction could be observed. For the AZO films deposited with the critical ratio, we found that all the three peaks above had strong intensity and the (101) peak had diminished intensity. For the AZO films deposited with a ratio of 1.0, by only RF-MS method, we found not only a further increase in the intensities of the three peaks but also a complete absence of the (101) reflection. We obtained the following results of Hall effect measurements: (1) AZO deposited by RF-MS showed low carrier concentration (N) of 4.13×10²⁰ cm^-3; (2) AZO deposited by DC-MS showed low Hall mobility (μ_H) of 29.3 cm²/Vs; (3) AZO films deposited with the critical ratio exhibited low resistivity (ρ) of 2.47×10^-4 Ωcm with N=6.88×10²⁰ cm^-3 and μ_H=36.8 cm²/Vs.

Then, we develop smart technology for the achievement of AZO films with well-defined single (0001) orientation. We propose “critical layer (CL)” that extremely affects the growth orientation of AZO films deposited by DC-MS. We deposited 10-nm-thick AZO films as the CLs on glass substrates by RF-MS to successively deposit AZO films by DC-MS. For the AZO films with the CLs, we found not only a further increase in the intensities of the three peaks but also a complete absence of the (101) reflection. We obtained the following results in this study: the AZO films with the CLs exhibited the lowest ρ=2.32×10^-4 Ωcm with the highest N=7.00×10²⁰ cm^-3 and highest μ_H=38.6 cm²/Vs. We will clarify key factors limiting carrier transport of AZO films theoretically.

This work has been supported by JSPS (Kakenhi No.26790050), Grant-in-Aid for Young Scientists (B).

LED lighting has attracted attention for a long time since its potential in high efficiency, effective energy saving and elongated life time. The lighting device and its package are requested for high performance, strengthened thermal and mechanical stability, and improved reliability nowadays. Designing phosphor containing glass thin layer for light conversion is one of the important techniques in the pursuit for high performance lighting. In present study, the borosilicate and aluminophosphate glasses with low characteristic temperatures to replace conventionally adopted epoxy matrix in the phosphor containing system is applied. The developed of B-Si-Bi-Zn glass has low characteristic temperatures 447 and 626°C for T_g and T_c, namely glass transition and crystallization points, respectively. Glass ceramic phosphor (GCP) thin layer is formed by the YAG phosphors driven into the designed B-Si-Bi-Zn glass substrate under heat treatments with various temperature and time parameters. Through SEM observation, the phosphors powders are well distributed in glass substructure surface layer and the deepest GCP layer thickness reaches 15 µm. The light emission could be manipulated from blue to white then to yellow with heat treatment time and temperature control. The microstructure, phase distribution about glass/ceramic interaction, and the optical performance of CIE coordinates of the GCP layer are investigated. In addition, the developed P₂O₅-Al₂O₃-R₂O-Bi₂O₃ (R=Li and Na)quaternary system with even lower characteristic temperature is analyzed with the glass transition point around 320°C.

Between all the optical systems used to collect the incoming solar radiation, the solar receiver that must deliver hot air is one of the key component. To design it, porous ceramic foams appear very interesting in the range of 300-1200°C because of their large specific surface which directly heats the air flowing through them. Generally, porous SiC-based materials are chosen as solar receivers because of their high temperature mechanical resistance [1]. Note that materials used as solar receivers have to efficiently absorb the visible-near infrared waves and they have to simultaneously reflect the mid and far-infrared rays. This rule defines here the spectral selectivity. However, selected SiC compounds absorb all rays in the whole visible-infrared spectral domain making them inappropriate for our purpose [2].

To solve this challenging issue, a first step is to enhance the spectral selectivity of SiC receivers by depositing a suitable coating with appropriate optical properties on the whole solid network. YBa₂Cu₃O_6+x coatings, when their thicknesses are higher than 400 nm and for x~0.8-0.9, are likely to show low reflectivity in the far and mid infrared spectral range and a high absorptivity in the visible spectral range at T = 20°C [3].

However, the spectral selectivity of YBaCuO depends on the oxygen stoichiometry, which is governed by the thermal treatment used to synthesize it. The sol-gel route is used to easily control both the stoichiometry and the nanostructure of the synthesized oxide and to coat SiC substrates. SiC pellets with YBa₂Cu₃O_6+x-typecoatings are studied, in particular their optical properties. The influence of the oxygen stoichiometry and the microstructure on the optical properties at room temperature are then evidenced.

This work is conducted in the framework of OPTISOL, project supported by the French National Agency (A.N.R.)

[1] T. Fend, R.- Pitz-Paal, O. Reutter, J. Bauer, and B. Hoffschmidt, “Two novel high-porosity materials as volumetric receivers for concentrated solar radiation,” Sol. Energy Mater. Sol. Cells, vol. 84, no. 1–4, pp. 291–304, Oct. 2004.

[2] B. Rousseau, S. Guevelou, G. Domingues, J. Vicente, C. Caliot, and G. Flamant, “Prediction of the radiative properties of reconstructed alpha-SiC foams used for concentrated solar applications,” in MRS Online Proceedings Library, 2013, pp. 1545:mrss13–1545–k01–07.

[3] R. Lobo, C. Allançon, F. Gotor, J.-M. Bassat, L. Jean-Pierre, P. Odier, D. Krystoff, F. Gervais, C. Champeaux, P. Marchet, and A. Catherinot, “Analysis of infrared-visible-near-ultraviolet reflectivity of conducting and superconducting oxides,” Phys. C Supercond., pp. 1071–1072, 1994.

Adhesion is a key functionality for any kind of coating/substrate system. For optical applications, metallic and dielectric coatings are of major interest, in particular for crucial materials such as silver on glass or dielectrics on polymers. Within an adhesion study, four different cases have been investigated in more detail: effect of substrate thickness on adhesion for dielectrics on polymers (1) and for metals on glass (2) as well as the effect of process parameters on adhesion for dielectrics on polymers (3) and for metals on glass (4).

Adhesion, i.e. adhesive strength, is defined by definition as force per area as tensile stress with the unit N/mm² = MPa. There are only two technologies for tensile stress application available, first the single-sample pull-off test in a tensile testing machine and second the multiple-sample pull-off test within a centrifuge. Both tests require bonded test stamps on the coating. The multiple-sample test within a centrifuge was chosen as it enables either the statistics or the ranking of samples under identical testing conditions. After testing, a microscopic inspection is required in order to determine the failure pattern according to ISO 10365 and to correlate the failure event to distinct interfaces, the effect of adhesion promoters or process parameters.

Transparent and Conducting oxide films owe their ubiquitous properties to the presence of resident chemical and structural defect states that are intentionally introduced by doping or that are created during post deposition thermal treatment, or irradiation. To achieve low resistivity in these films, both charge carrier density and carrier mobility must necessarily be maximized. However, these two variables are closely coupled and exhibit a somewhat inverse relationship with one another. The challenge is to discover optimum chemistries, compositions, and structures that will maintain high transparency under conditions of low film resistivity. A vast amount of phenomenological data in the literature supplemented by electronic structure simulations allows development of ad hoc guidelines for the rational identification of dopants and matrix systems. This information paired with innovative concepts for improving charge mobility can lead not only to improved film performance, but also to improved film robustness in operating environments. A conceptual picture of TCO films is presented here that is intended to germinate fresh ideas that will underpin subsequent development of these technologically important materials. Both PVD and solution deposited n-type free carrier and p-type polaron conducting films will be used as examples to illustrate these concepts.

Silver nanoparticles were intercalated between layers of transparent dielectric films such as PTFE, titanium dioxide and amorphous carbon. All nanoparticles and films were produced by rf magnetron sputtering in the same vacuum chamber. Samples were analyzed by X- ray photoelectron Spectroscopy and X-ray diffraction. The nanoparticles size was determined by HRTEM. Optical properties, reflectance and transmittance, in the range 300-1100 nm were measured and the plasmon resonance frequency was related to the size of silver nanoparticles according to the Mie theory. Antibacterial effects were studied on Eschericia coli and Staphylococcus aureus.