Fabrication of a new type of organic-inorganic hybrid superlattice films combined with titanium oxide and polydiacetylene
© Yoon et al; licensee Springer. 2012
Received: 10 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
We fabricated a new organic-inorganic hybrid superlattice film using molecular layer deposition [MLD] combined with atomic layer deposition [ALD]. In the molecular layer deposition process, polydiacetylene [PDA] layers were grown by repeated sequential adsorption of titanium tetrachloride and 2,4-hexadiyne-1,6-diol with ultraviolet polymerization under a substrate temperature of 100°C. Titanium oxide [TiO2] inorganic layers were deposited at the same temperatures with alternating surface-saturating reactions of titanium tetrachloride and water. Ellipsometry analysis showed a self-limiting surface reaction process and linear growth of the nanohybrid films. The transmission electron microscopy analysis of the titanium oxide cross-linked polydiacetylene [TiOPDA]-TiO2 thin films confirmed the MLD growth rate and showed that the films are amorphous superlattices. Composition and polymerization of the films were confirmed by infrared spectroscopy. The TiOPDA-TiO2 nanohybrid superlattice films exhibited good thermal and mechanical stabilities.
PACS: 81.07.Pr, organic-inorganic hybrid nanostructures; 82.35.-x, polymerization; 81.15.-z, film deposition; 81.15.Gh, chemical vapor deposition (including plasma enhanced CVD, MOCVD, ALD, etc.).
Organic-inorganic hybrid superlattice films have an attractive potential for the creation of new types of functional materials by combining organic and inorganic properties. The hybrid superlattice films provide both the stable and distinguished optical or electrical properties of inorganic constituents and the structural flexibility of organic constituents. Furthermore, such hybrid superlattice films show unique optical and electrical properties which differ from their constituents [1–3]. They provide the opportunity for developing new materials with synergic effects, leading to improved performance or useful properties. A key factor to utilize organic-inorganic hybrid films is the ability to prepare high quality multilayers in the simplest and most reliable method. The ability to assemble one monolayer of hybrid films at a time provides control over thickness, composition, and physical properties with a single-layer precision. Such monolayer control provides an important path for the creation of new hybrid materials for organic-inorganic electronic devices and molecular electronics.
Recently, we developed two-dimensional polydiacetylene [PDA] with hybrid organic-inorganic structures using molecular layer deposition [MLD] . MLD is a gas-phase layer-by-layer growth process, analogous to atomic layer deposition [ALD] that relies on sequential, self-limiting surface reactions [5–13]. In the MLD method, the high-quality organic PDA thin films can be quickly formed with monolayer precision under ALD conditions (pressure, temperature, etc.). The MLD method can be combined with ALD to take advantages of the possibility of obtaining organic-inorganic hybrid thin films. The advantages of the MLD technique combined with ALD include accurate control of film thickness, good reproducibility, large-scale uniformity, multilayer processing ability, and excellent film qualities. Therefore, the MLD method with ALD [MLD-ALD] is an ideal fabrication technique for various organic-inorganic nanohybrid thin films.
Herein, we report a fabrication of titanium oxide cross-linked polydiacetylene [TiOPDA]-titanium oxide [TiO2] organic-inorganic nanohybrid thin films using the MLD-ALD method. In this MLD process, the PDA organic layers were grown by repeated sequential ligand-exchange reactions of titanium tetrachloride [TiCl4] and 2,4-hexadiyn-1,6-diol [HDD] with UV polymerization. The TiO2 inorganic nanolayers were prepared by ALD using TiCl4 and water. The prepared TiOPDA-TiO2 nanohybrid thin films exhibited good thermal and mechanical stability.
Preparation of Si substrates
The Si (100) substrates used in this research were cut from p-type (100) wafers with a resistivity in the range of 1 to 10 Ω cm. The Si substrates were initially treated by a chemical cleaning process proposed by Ishizaka and Shiraki which involved degreasing, HNO3 boiling, NH4OH boiling (alkali treatment), HCl boiling (acid treatment), rinsing in deionized water, and blow-drying with nitrogen to remove contaminants and grow a thin protective oxide layer on the surface .
Atomic layer deposition of TiO2 thin film
The oxidized Si (100) substrates were introduced into the ALD system Cyclic 4000 (Genitech, Daejon, Korea). The TiO2 thin films were deposited onto the substrates using TiCl4 (99%; Sigma-Aldrich Corporation, St. Louis, MO, USA) and water as ALD precursors . Ar served as both a carrier and a purging gas. The TiCl4 and water were evaporated at 30°C and 20°C, respectively. The cycle consisted of a 1-s exposure to TiCl4, 5-s Ar purge, 1-s exposure to water, and 5-s Ar purge. The vapor pressure of the Ar in the reactor was maintained at 100 mTorr. The TiO2 thin films were grown at 100°C under a pressure of 100 mTorr.
Molecular layer deposition
TiOPDA thin films were deposited onto the Si substrates using TiCl4 and HDD (99%; Sigma-Aldrich Corporation, St. Louis, MO, USA) in the MLD chamber. Ar served as both a carrier and a purging gas. TiCl4 and HDD were evaporated at 30°C and 80°C, respectively. The cycle consisted of a 1-s exposure to TiCl4, 5-s Ar purge, 10-s exposure to HDD, and 50-s Ar purge. The vapor pressure of the Ar in the reactor was maintained at 100 mTorr. The deposited HDD layer was exposed to UV (254 nm, 100 W) for 30 s. The TiOPDA thin films were grown at 100°C under a pressure of 100 mTorr.
The thicknesses of the thin films were evaluated using an ellipsometer (AutoEL-II, Rudolph Research Analytical, Hackettstown, NJ, USA). UV-Visible [Vis] and Fourier transform infrared [FTIR] spectra were obtained using a UV-Vis spectrometer (Agilent 8453 UV-Vis, Agilent Technologies Inc., Santa Clara, CA, USA) and an FTIR spectrometer (FTLA 2000, ABB Bomem, Quebec, Quebec, Canada), respectively. All X-ray photoelectron [XP] spectra were recorded on a Thermo VG Sigma Probe spectrometer (FEI Co., Hillsboro, OR, USA) using Al Kα source run at 15 kV and 10 mA. The binding energy scale was calibrated to 284.5 eV for the main C 1s peak. Each sample was analyzed at a 90° angle relative to the electron analyzer. The samples were analyzed by a JEOL-2100F transmission electron microscope (JEOL Ltd., Akishima, Tokyo, Japan). Specimens for cross-sectional transmission electron microscopy [TEM] studies were prepared by mechanical grinding and polishing (approximately 10-μm thick) followed by Ar-ion milling using a Gatan Precision Ion Polishing System (PIPS™ Model 691, Gatan, Inc., Pleasanton, CA, USA).
We developed TiOPDA-TiO2 organic-inorganic hybrid superlattice films by MLD combined with ALD. In the MLD process, TiOPDA organic layers were grown under vacuum by repeated sequential adsorptions of 2,4-hexadiyne-1,6-diol and titanium tetrachloride with UV polymerization. In the ALD process, TiO2 inorganic nanolayers were deposited at the same chamber using alternating surface-saturating reactions of titanium chloride and water. The TiOPDA-TiO2 nanohybrid thin films that were prepared exhibit good thermal and mechanical stability, large-scale uniformity, and sharp interfaces.
This work was supported by the Seoul R&BD program (ST090839) and by the Korea Science and Engineering Foundation (KOSEF) funded by the Ministry of Education, Science and Technology (MEST) (No. 2009-0092807).
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