Environmentally Tight TiO₂–SiO₂ Porous 1D‐Photonic Structures

Abstract

materials with different refractive indexes (e.g., TiO 2 and SiO 2 ), has been proposed as a way of tailoring the optical behavior of broadband antireflection coatings used to maximize the performance of photovoltaic cells. [8] Porosity is also a key characteristic when preparing Bragg stacks acting as optical and optofluidic sensors for the detection of vapors or for liquid monitoring, respectively. [9] According to the principles of the effective medium approximation theory, [10] porosity can be utilized to adjust the overall refraction index of thin films as an average of the contributions of air (n air = 1.0) and film material (i.e., n b ). However, the adjustment of the optical properties of porous films faces the critical problem of their environmental instability due to the condensation of water (n w = 1.31) and other vapors into their open pores. Although this effect has been utilized for the optical detection of vapors using stacks of thin films in the form of Bragg structures, [9a,b,10] the affectation of optical properties in multilayers acting as 1D photonic structures can be a serious problem when dealing with passive optical systems. [12] This is clear in solar cells and other outdoor applications where optical instabilities due to water condensation have to be suppressed by polymer encapsulation during device assembly. In the present work, we propose the use porosity as an additional parameter to control the optical properties of multilayer stacks but, simultaneously, ensuring its environmental tightness and therefore, a stable outdoor functioning. To enhance film porosity we use the technique of magnetron sputtering at oblique angles (MS-OAD). [13] This geometrical configuration, either by e-beam evaporation or magnetron sputtering (MS), induces the growth of nanocolumnar films with porosities that may reach up to 50-60% of the film volume. [14] Highly porous OAD films and multilayers have been utilized in the pass for the fabrication of antireflection layers for passive optical applications, [7,11a,b] photo catalytic layers, [5,10a] humidity and other vapor sensors [10a,b,15] or optofluidic devices. [9c,d,16] Herein, we firstly prove that the optical behavior of MS-OAD multilayers in the form of Bragg reflectors (BR) or Bragg microcavities (BM) can be tailored by adjusting the porosity of the TiO 2 and SiO 2 thin films (e.g., making the TiO 2 compact and the SiO 2 highly porous, as proposed in previous works [7,12c] ). Then, using an optical monitoring procedure previously applied to study vapor Although thin film porosity is the basis of many optical sensors, it can be deleterious for a stable optical behavior of passive optical elements due to the condensation of water and other vapors in their pores. This paper proposes a new strategy for the magnetron sputtering (MS) fabrication of environmentally tight SiO 2 -TiO 2 porous multilayers. Thin films of these two oxides deposited in an oblique angle configuration (MS-OAD) present a nanocolumnar and highly porous nanostructure and, as ...

Form Birefringence in Resonant Transducers for the Selective Monitoring of VOCs under Ambient Conditions

“…In powder materials, as well as in the investigated BBM, this particular shape can be attributed to the progressive filling of pores of different sizes with a condensate: at low P / P 0 values (up to ca. 0.1 in the curve of Figure c) nanopores with a size smaller than 2 nm become filled with condensed water vapor, followed by a progressive adsorption and condensation onto large mesopores ( P / P 0 range between ∼0.1 and ∼0.6) and their eventual filling with liquid in the interval between ∼0.6 and 1.0. , Similar optical isotherm curves have been reported for porous and optically isotropic BMs prepared by a large variety of procedures. − For BM prepared by OAD, − these curves have been used for the development of humidity and other VOC detectors, although water/compound identification was not possible in these experiments.…”

“…According to effective medium theories, a reasonable assumption is that the effective refractive index of each stacked layer varies as its void space is partially or completely filled with condensed vapors. This behavior has been extensively studied in conventional porous BMs. ,,− , Working with the BBM, it might be possible that the difference in polarizability along the fast and slow optical axes also varies upon vapor adsorption. To check this point and to fully characterize the optical response of the BBM to the adsorption/condensation of vapors, we have followed the changes in the filling and birefringence strength parameters as a function of the vapor pressure P / P 0 ratio, where P 0 is the saturation partial pressure of the examined vapor at room temperature.…”

“…Other classical strategies developed some years ago for vapor and liquid detection using nanostructured porous silicon 1D photonic systems also failed to selectively identify a given compound though, for biomolecular applications, selective detection of minority components in a fluid has been achieved via pore surface functionalization with molecules with proved specificity toward a target compound. , A classical methodology for volatile organic compound (VOC) monitoring with 1D photonic structures, particularly BMs, consists of recording a so-called optical adsorption isotherm. This procedure has been used for porosity characterization in nanostructured multilayers − and photonic sensor applications, in the latter case using sculptured photonic structures prepared by oblique angle deposition (OAD). , An optical adsorption isotherm is obtained plotting the redshift of the BM resonant peak with respect to vapor pressure. These optical adsorption isotherms are similar in shape to those obtained with other layered or powder materials by means of volumetric, gravimetric, or ellipsoporosimetry methods. ,− A basic assumption for their interpretation is that, according to effective medium theories, filling the void volume with condensates produces a change in the effective refractive index of the stacked layers and therefore a redshift of the resonant peak.…”

Low temperature nucleation of thermochromic VO2 crystal domains in nanocolumnar porous thin films

“…Sculptured VO 2 thin films have also been analyzed in the literature due to their singular morphology. These type of films are formed by tilted nanocolumnar structures with diameter and separation distances in the order of 100 nm, with a large amount of porosity [27][28][29], which creates an effective medium with lower refractive index than its compact counterpart, improving the film transparency [30]. Most remarkable result was found in [31], where films with composition V 2 O 5 were grown using the so-called glancing angle configuration.…”

High Performance Multilayered Organosilicon/Silicon Oxynitride Water Barrier Structure Consecutively Deposited by Plasma-Enhanced Chemical Vapor Deposition at a Low-Temperature

“…To date, the conventional encapsulation method has been achieved by sealing the OLED device with a getter in an inert atmosphere using a rigid glass lid or metal, which causes the device to become more bulky and is incompatible with flexible application. Accordingly, thin and flexible barrier coatings with the high transmittance around visible wavelengths, such as SiO x , Si 3 N 4 , and AlO x , prepared by physical or chemical vapor deposition are potential candidates for thin film encapsulation (TFE) application [7][8][9][10]. However, the degree of the WVTR for these single barrier films [~10 0 -10 −2 g/m 2 /day as it was coated onto the polyethylene terephthalate (PET) substrate] was orders-of-magnitude higher than the requirement for the TFE on the OLED (hereafter referred to as TFE-OLED) packaging.…”