High‐Performance Two‐Dimensional Polydiacetylene with a Hybrid Inorganic–Organic Structure

Abstract

Many materials used in organic electronics are conjugated organic polymers with a backbone of alternating double and single bonds along which electrons can flow. Owing to their structural flexibility and tunable electronic properties, conducting polymers show great promise in flexible, inexpensive, large-area applications such as flexible displays, radio frequency identification devices (RFIDs), smart cards, nonvolatile memory, and sensors. The one-dimensional nature of these polymers can be misleading. Thin films of conjugated polymers still have disordered structures, with twists and bends in their individual polymer strands. For most applications, the challenge is to uniformly align the polymer chains to control the efficiency with which light and electrons can be transported by the polymer molecules. One way to do this is to incorporate stable inorganic cross-linkers into conjugated organic polymer chains. Inorganic cross-linkers provide greater control of the alignment, stability, and electronic properties of organic polymer chains to form hybrid organic– inorganic structure materials. Hybrid organic–inorganic conducting polymers are ideal materials for organic electronics because they offer the structural flexibility and tunable optoelectronic properties of their organic components in addition to the stable and elegant electrical properties of inorganic components. In this study, we developed high-performance two-dimensional (2D) polydiacetylene 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 saturated surface reactions and results in the formation of an organic monolayer in each sequence. We combined conjugated organic polymer layers with inorganic cross-linkers, to produce 2D polydiacetylenes. The inorganic layers constitute an extended structure bonded by powerful covalent interactions resulting in a high carrier mobility as well as good thermal and mechanical stabilities. The advantages of the MLD technique include accurate control of film thickness, large-scale uniformity, excellent conformality, good reproducibility, multilayer processing capability, sharp interfaces, and excellent film qualities at low temperatures. The construction of 2D polydiacetylene thin films can be accomplished with monolayer precision by ligand-exchange reactions of diethylzinc (DEZ) and hexadiyne diol (HDD) with UV polymerization, whereby the OH group on both ends of the diol sequentially react with the ethyl group of DEZ to produce bridging alkanes. Zinc oxide cross-linked polydiacetylene (ZnOPDA) thin films may have enhanced carrier mobility or other favorable properties owing to their 2D structures. Figure 1 shows a schematic outline for the present layerby-layer synthesis of two-dimensional (2D) polydiacetylene films with zinc oxide cross-linkers. First, an ethyl zinc oxide monolayer was formed by exposing a substrate to the DEZ molecule in the MLD chamber. The DEZ molecule was chemisorbed on substrate surfaces rich in hydroxy groups through a ligand-exchange reaction to formC2H5ZnO species. Second, the ethyl group of the chemisorbed ethyl zinc molecule on substrates was replaced by an OH group of HDD with the living ethane to form a diacetylene layer. The OH group of the diacetylene layer provides an active site for the exchange reaction of the next DEZ molecule. Third, the diacetylene molecules were polymerized by UV irradiation to form a polydiacetylene layer. The zinc oxide cross-linked polydiacetylene (ZnOPDA) thin films were grown under vacuum by repeated sequential adsorptions of DEZ and HDD with UV polymerization. The expected monolayer thickness for the ideal model structure of ZnOPDA is about 6 . In a true MLD process, the surface reactions must be selfterminating and complementary to yield a uniform, conformal, and high-quality polydiacetylene thin film. To verify that the surface reaction for HDD is indeed self-terminating, the dosing time was varied between 1 and 20 s. The growth rate of the ZnO-PDA films was measured by an ellipsometer at growth temperatures in the range 100–150 8C. The obtained film thickness per cycle for HDD became saturated when the dosing time exceeded 10 s (Figure 2a). This result indicates that HDD undergoes a self-terminating reaction with DEZ that is absorbed on Si substrates. Figure 2b shows that the growth rate as a function of the DEZ dosing time is quickly saturated when the pulse time exceeded 0.5 s. This result suggests that DEZ undergoes a fast self-terminating reaction with HDD adsorbed on the substrates. All the self-terminating growth experiments were performed over 100 cycles. The [*] S. Cho, G. Han, Prof. M. M. Sung Department of Chemistry, Hanyang University Seoul 133-791 (Korea) Fax: (+82)2-2220-2555 E-mail: smm@hanyang.ac.kr