Intramolecular and Intermolecular Interactions in ... - ACS Publications

Jun 17, 2016 - Department of Chemistry, Sookmyung Women's University, Seoul ... Department of Chemistry, Hanyang University, Seoul, 133-791, Korea...
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Intramolecular and Intermolecular Interactions in Hybrid Organic− Inorganic Alucone Films Grown by Molecular Layer Deposition Yi-Seul Park,† Hyein Kim,† Boram Cho,‡ Chaeyun Lee,† Sung-Eun Choi,† Myung Mo Sung,‡ and Jin Seok Lee*,† †

Department of Chemistry, Sookmyung Women’s University, Seoul 140-742, Korea Department of Chemistry, Hanyang University, Seoul, 133-791, Korea



S Supporting Information *

ABSTRACT: Investigation of molecular interactions in polymeric films is crucial for understanding and engineering multiscale physical phenomena correlated to device function and performance, but this often involves a compromise between theoretical and experimental data, because of poor film uniformity. Here, we report the intramolecular and intermolecular interactions inside the ultrathin and conformal hybrid organic−inorganic alucone films grown by molecular layer deposition, based on sequential and self-limiting surface reactions. Varying the carbon chain length of organic precursors, which affects their molecular flexibility, caused intramolecular interactions such as double reactions by bending of the molecular backbone, resulting in formation of hole vacancies in the films. Furthermore, intermolecular interactions in alucone polymeric films are dependent on the thermal kinetics of molecules, leading to binding failures and cross-linking at low and high growth temperatures, respectively. We illustrate these key interactions and identify molecular geometries and potential energies by density functional theory calculations. KEYWORDS: polymeric films, molecular interactions, alucone, molecular layer deposition, surface reaction produce organic thin films; however, there are several factors to consider when using these methods to produce thin film, including degradation, polarity, swelling, and environmental concerns, any of which can result in random or horizontal orientation19 of the molecules in the films. Molecular layer deposition (MLD) is a promising method for obtaining conformal ultrathin organic films using vapor-phase organic precursors, while their composition and thickness can be controlled at the molecular level.20 This process is based on self-saturating reactions between the organic precursors and the substrate surface,21 and it is similar to the atomic layer deposition (ALD) process used to produce inorganic films.22 The use of vapor-phase approaches to fabricate organic films ensures that pure materials are used as precursors,5 and these approaches allow films to be deposited on nanostructured substrates with arbitrary and complex surfaces.23 With MLD,

1. INTRODUCTION Organic thin films consisting of polymeric materials are used in a wide range of applications, such as organic field-effect transistors (OFETs),1 organic photovoltaic devices (OPVCs),2 and organic light-emitting diodes (OLEDs),3 all of which can be fabricated into desired shapes on complex nanodevice surfaces. Precise control of the composition,4 conformality,5 thickness,6 density,7 and molecular orientation8 of organic thin films is necessary, because all of these properties affect aspects of a device’s performance, such as carrier mobility9 and light outcoupling efficiency.10 In particular, the electrical,11 optical,12 and mechanical13 properties of polymeric organic thin films are dependent on their intramolecular and intermolecular interactions between the organic molecules in the films, which are affected by molecular stacking14 and the physical conformation15 of the polymer chains. However, the effects of these interactions have not been thoroughly investigated, because of the difficulty of fabricating organic thin films with vertical dimensionality. Generally, solution-based processes, such as spin coating,16 sol−gel process,17 and self-assembly,18 can easily © XXXX American Chemical Society

Received: February 12, 2016 Accepted: June 17, 2016

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DOI: 10.1021/acsami.6b01856 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 1. Molecular layer deposition of alucone films. (a) Schematic diagram of homemade MLD chamber equipped with in situ FTIR spectroscopy. Schematic for growth mechanism of hybrid organic−inorganic alucone MLD films using TMA and three types of diol with different carbon chain lengths, (b) EDO, (c) BDO, and (d) HDO, as inorganic and organic precursors, respectively.

and density of the produced alucone films were investigated by ex situ analysis. All molecular geometries and energies were predicted by performing density functional theory (DFT) calculations.

new reaction pathways can be developed to produce various polymeric films by designing molecular sequences24 that lead to the formation of polyamide,25 polyimide,26 polyurethane,8 polyurea,6 and other organic−inorganic hybrid thin films combining organic and inorganic precursors.27 Hybrid organic−inorganic thin films based on aluminum can be produced from metal alkyls and organic diols as inorganic and organic precursors, respectively. These aluminum alkoxide polymers have a composition approximated by (−Al−O−R− O−)n and are known as “alucones”.28 The flexibility of the molecular backbone in organic diol precursors is dependent on the length of the carbon chain, which, in turn, affects the coupling reaction by bending the molecular geometry.7 Indeed, the molecule’s flexibility can lead to different intramolecular interactions. To date, many researchers have studied various aspects of the MLD growth process, such as self-limiting surface reactions,29 factors that control film thickness composition,30 and molecular orientation31 within films, with regard to precursor types,32,33 growth temperature,34 and other parameters, and focused on the MLD film properties. However, to thoroughly understand the unique properties of thin films, it is necessary to investigate the intramolecular and intermolecular interactions in organic thin films consisting of polymer chains. In this work, we present for the first report of the intramolecular and intermolecular interactions between molecular layers in hybrid organic−inorganic alucone films produced by MLD, which are affected by molecular flexibility and thermal kinetics. These films were fabricated through coupling reactions between trimethyl aluminum (TMA) and one of three diols with different carbon chain lengths (1,2-ethanediol, EDO; 1,4butanediol, BDO; or 1,6-hexanediol, HDO) as inorganic and organic precursors, respectively. The surface that occurred with varying doses of TMA and organic diol was monitored using in situ Fourier transform infrared (FTIR) spectroscopy. In addition, the composition, growth rate, surface morphology,

2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. The inorganic precursor, trimethylaluminum (TMA; Al(CH3)3, 97%), was purchased from Sigma−Aldrich. The organic precursors (three diols with different carbon chain lengths), 1,2-ethanediol (EDO; HO(CH2)2OH, 99.8%), 1,4-butanediol (BDO; HO(CH2)4OH, ≥99%), and 1,6-hexanediol (HDO; HO(CH2)6OH, 99%), were also purchased from Sigma− Aldrich. Isopropyl alcohol ((CH3)2CHOH, 99.5%), sulfuric acid (H2SO4, 95.0%), and hydrogen peroxide (H2O2, 34.5%) were purchased from Samchun. Si(100) wafers were purchased from LG Siltron. 2.2. Preparation of Hydroxyl-Terminated Substrate. Si(100) wafers were used as a substrate, which were cut into 1 cm (width) × 1 cm (length) pieces. The substrates were rinsed using isopropyl alcohol and dried by nitrogen gas. After that, they were treated with Piranha solution (3:1 ratio of sulfuric acid:hydrogen peroxide) at least 30 min to create a hydroxyl-terminated surface and rinsed with deionized water several times. They then were dried under nitrogen gas and loaded into the chamber for molecular layer deposition (MLD). It was heated to growth temperature of 120 °C and maintained for 30 min before MLD reaction. 2.3. Fabrication of Alucone MLD Films. All hybrid organic− inorganic alucone MLD films were deposited in our homemade MLD chamber equipped with in situ Fourier-transform infrared spectroscopy (FTIR) apparatus (Figure 1a), as described previously.35 The MLD chamber was heated using halogen lamp and heating jacket to keep uniform heat at growth temperature. Also, it is important to optimize the organic precursor temperature, dose time, and purge time, because the MLD reaction is based on the organic chemistry in the gas phase. Precursors were filled into a bubbler, which can be introduced into the MLD chamber. TMA, the inorganic precursor, was held at room temperature, because of its sufficient vapor pressure. In order to induce organic reactions using sufficient vapor pressure of organic precursors, we investigated the precursor temperature until its pressure reaches at B

DOI: 10.1021/acsami.6b01856 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 2. Surface reactions and elemental composition of alucone MLD films. (a) In situ FTIR spectra after the TMA and EDO surface reactions during alucone MLD film growth at 120 °C. TMA is the spectrum after TMA exposure onto a hydroxyl-terminated SiO2 particle substrate, and EDO is the spectrum after EDO exposure following saturation of TMA exposure. (b) In situ FTIR spectra of initial hydroxylated SiO2 particle substrate and after 5, 10, and 30 cycles during deposition of alucone MLD film using TMA and EDO at 120 °C. (c) Wide XPS survey scan of the (TMA/ EDO)300 alucone MLD film on Si(100) wafer at 120 °C. (d) Thickness of (TMA/diol)n alucone MLD films on Si(100) wafers versus number of MLD cycles with three types of diols, EDO, BDO, and HDO, as measured by ellipsometry, where n is the number of cycles. The dotted line represents the ideal growth rate based on