The sol-gel method is a novel technique for the preparation of thin films. In this research, gadolinium oxide (Gd 2O 3) and gadolinium oxide europium doped (Gd 2O 3:Eu 3+) films prepared via the sol-gel and dip coating methods were investigated. In addition to the elaboration on the sol-gel preparation routes and additional observations of the films’ surface morphology as characterized by scanning electron microscope (SEM), we determined via viscosity measurements that the sols were stable for 398 days. FTIR analysis of the Gd 2O 3 and Gd 2O 3:Eu 3+ dip coated films was made to monitor the decomposition and oxidation reactions that occurred during processing as well as process stability.
Gadolinium oxide films have received increasing attention due to their novel electronic and optoelectronic properties. It has been determined that these films possess high refractive index, large band gap, high resistivity and relative permittivity, and an innate ability to be readily doped with rare earth ions [
In essence, the sol-gel process usually consists of four steps. In the first step, the desired colloidal particles are dispersed in a liquid, from which the sol will be formed. The next step involves the deposition of the sol solution that produces the coatings on the substrates by spraying, dipping or spinning. Next, the particles in the sol are polymerized through the removal of the stabilizing components and produce a gel in a state of a continuous network. The fourth and final step consists of a final heat treatment that pyrolyzes the remaining organic or inorganic components with the gel thereby forming an amorphous or crystalline coating [
The undoped sol was prepared by making a 3.2 mmol solution of gadolinium pentantdionate hydrate (99.9%, Sigma Aldrich) in 95 mL of methanol (99.9%, Sigma Aldrich) at 40˚C and stirred vigorously for 1 hr. Finally, 1.5 mL of 2,4-pentanedione (99%, Alfa Aesar) was added to the solution and was stirred overnight. No additional water was added to the sol. The sol was filtered through a 0.22 µm filter prior to being used.
The Gd2O3:Eu+3 sol was prepared by making a 3.2 mmol solution of gadolinium pentantdionate hydrate (99.9%, Sigma Aldrich) in 95 mL of methanol (99.9%, Sigma Aldrich) at 40˚C and stirred vigorously for 1 hr. Separately, a 1.2 mmol solution of europium (III) nitrate (99.9%, Sigma Aldrich) in 90 mL of methanol was prepared. To the prepared precursor solution, 1 mL of europium (III) nitrate was added, and the solution was stirred vigorously for 1 hr. Finally, 1.5 mL of 2,4-pentanedione (99%, Alfa Aesar) was added to the solution and was stirred overnight. No additional water was added to the sol. The sol was filtered through a 0.22 µm filter prior to being used.
The thin films were obtained by dipping a precleaned substrate into the filtered sols. The cleaning procedure of the borosilicate glass (BSG) used in this study consisted of a solution bath of detergent, followed by an acetone bath, and finished with a methanol bath. The BSG substrates (Corning® microscope slides) with an area of 75 × 25 mm2 were used in this study. Before coating, the sols were filtered through a 0.22 µm filter to remove dust or contamination from the deterioration of the sols. Extreme care was taken to avoid contamination at all stages of the coating process. Deposition of the film from the solution was executed via a dip coating apparatus. The motorized, lifting sample holder shaft was used to soak the substrates into the sol and to raise them carefully with a smooth movement of 3.9 mm/min. 10 and 50 successive coatings of undoped and europium doped Gd2O3 sols were cast on the substrate in the laboratory and clean room environment. The solution cup and the motor were isolated from vibration in order to ensure that the liquid surface remains completely undisturbed. The coatings were performed using an HWTL-01 Desktop Dip Coater (MTI Corporation, USA) to reduce dust contamination. The films were cast in a Class 1000 clean room to determine if there were any detectable variations in the surface morphology, physical and optical properties of the films.
Complete conversion to oxide film requires a drying stage and a heat treatment in order to obtain a dense and a hard coating. The layers were heat-treated at 100˚C for 5 min between coatings by direct insertion into the CMF-1100 Programmable Compact Muffle Furnace (MTI Corporation, USA). The heating rate was very slow (1.5˚/min) up to the final required temperature (300˚C and 500˚C) in order to reduce strain coming from the slight mismatch between the dilatation coefficient of the layer and the substrate [
The FTIR spectra of the modified and unmodified Gd2O3 and Gd2O3:Eu3+ films deposited on BSG under atmosphere and clean room conditions were collected in transmission mode at room temperature with a spectrophotometer (Nicolet iS10FTIR) equipped with the Smart iTR accessory. Each spectrum, recorded in the frequency region of 4000 - 400 cm−1, was the average of 32 scans at a resolution of 0.4 cm−1.
In order to ascertain the stability of the sol, viscosity measurements were taken. The viscosity of the sol was measured with a HYDRAMOTION Viscolite VL7-100B-d15 rheometer DV digital programmable viscometer at room temperature (RT). The synthesized films were filtered through a 0.22 μm filter prior to each viscosity measurement being made. The films were stored over the course of 398 days at ambient conditions.
The surface morphology of the Gd2O3 and Gd2O3:Eu3+ films annealed for 3 hours at 300˚C and 500˚C was investigated and analyzed by SEM observations with a Jeol SEM Model JSM-6390lv with an acceleration voltage of 10 kV. The samples were directly mounted to the sample holder by placing silver paste on the outer edges of the substrate. The center of the samples was imaged and examined for their morphological features―see SEM images in section 3.3.
prepared in methanol and ethanol solvent mediums with the reaction time varied.
As shown in
The heating rate and solution viscosity are two critical parameters that must be taken into account in order to produce crack-free films. Figures 6(a)-8(b) show SEM photographs of the modified and unmodified Gd2O3 films that were prepared with 10 and 50 layers and annealed at 300˚C and 500˚C. The images show that crack-free
films can be obtained at low viscosity (~0.3 cP for methanol based and ~0.8 cP for ethanol based sol gels) with a heating rate of 1.5˚/min, and no crack appears even with 50 repeated coatings. Samples annealed at 300˚C exhibit crystallite growth that resembles islands and what appears to be the initial formation of crystallites that resembles fiber networks. In contrast, samples annealed at 500˚C exhibit crystallite growth that resembles fiber networks. It has been reported in the literature that these films become crystalline annealed between 500˚C- 2200˚C [
From this work, we have used the sol-gel and dip coating methods to synthesize undoped Gd2O3and Gd2O3:Eu3+ films in methanol and ethanol solvent mediums for planar optical waveguides. We determine from this study that the annealing process and the number of layers directly influence the physical properties and surface morphology of the films. We also determine that the fabricated films possess detectable levels of organic materials, which can be minimized with increased annealing time. Viscosity measurements revealed that the ethanol and methanol based sols were stable over the course of 398 days. More investigation will be done on this material to determine the presence of potential structural defects in the material, verify the occupational sites of Eu3+ ions, and to ascertain the optical and physical character of the films.
We greatly acknowledge the support of the Alabama Space Grant Consortium Fellowship Program, Title III funding, DHS grant 2012-DN-077-ARI065-04, and IDC-HSHDM/Evans Allen Funding for supporting this research. We would like to thank Dr. A.K. Batra for his invaluable feedback and discussion. We would also like to thank Ms. Sheral Roberson for her assistance with the formatting and graphics, and Mr. Garland Sharp for his diligence in ensuring the functionality of the dip coating equipment.