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Fine-Tunable Absorption of Uniformly Aligned Polyurea Thin Films for Optical Filters using Sequentially Self-Limited Molecular Layer Deposition Yi-Seul Park, Sung-Eun Choi, Hyein Kim, and Jin Seok Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02142 • Publication Date (Web): 19 Apr 2016 Downloaded from http://pubs.acs.org on April 25, 2016

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ACS Applied Materials & Interfaces

Fine-Tunable Absorption of Uniformly Aligned Polyurea Thin Films for Optical Filters using Sequentially Self-Limited Molecular Layer Deposition Yi-Seul Park, Sung-Eun Choi, Hyein Kim, Jin Seok Lee* Department of Chemistry, Sookmyung Women’s University, Seoul 140-742, Korea KEYWORDS: Molecular layer deposition, self-limiting surface reaction, alignment, polyurea, fine resonance

ABSTRACT: Development of methods enabling the preparation of uniformly aligned polymer thin films at the molecular level is a prerequisite for realizing their optoelectronic characteristics as innovative materials; however, these methods often involve a compromise between scalability and accuracy. In this study, we have grown uniformly aligned polyurea thin films on a SiO2 substrate using molecular layer deposition (MLD) based on sequential and self-limiting surface reactions. By integrating plane-polarized Fourier-transform infrared, Raman spectroscopic tools and density functional theory (DFT) calculations, we demonstrated the uniform alignment of polyurea MLD films. Furthermore, the selective-wavelength absorption characteristics of thickness-controlled MLD films were investigated by integrating optical measurements and

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finite-difference time-domain (FDTD) simulations of reflection spectra, resulting from their thickness-dependent fine resonance with photons, which could be used as color filters in optoelectronics.

1. INTRODUCTION Organic thin films have attracted significant interest due to new approaches for the preparation of organic electronics,1 chemical sensors,2 protective coatings,3 catalysis,4 and diffusion barrier layers.5 The performances of organic thin films rely on the precise control of thickness, composition, and ordering of the films.6,7 To date, various organic films have been examined for organic field-effect transistor (OFET) applications, but they have much lower charge mobility than single crystals and polycrystals due to their randomly oriented molecules.8 In particular, the optimal performances of organic thin films would be expected when the molecules in the film are uniformly oriented to the substrate. The molecular orientation has recently been considered important for good device performance because the intermolecular interactions have specific characteristics in accordance with their geometry and electronic structures.9 However, the molecular orientation has not been sufficiently discussed because of the difficulties in fabricating molecularly organic films.10 Recently, surface antireflection coating have been attracted many researchers to improve optical device performance, which exhibits broadband low reflectance and high transmittance of incident light.11,12 In conventional method for antireflection, it can be achieved by single- or multi-layer coating of transparent quarter wavelength layers such as SiOx, TiOx, or SixNy, which have intermediate and gradient refractive indies, using chemical vapor deposition, sputter


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deposition, and wet deposition on the surface of glass or silicon.13 However, it is difficult to select materials which have proper refractive indies, and effectively perform only in limited spectral range due to limited thickness control. One possible approach is vapor-phase molecular layer deposition (MLD), which is a powerful technique for fabricating conformal ultra-thin organic films with controlled composition and thickness at the molecular level.14 Although various organic films have been developed over the past two decades using MLD such as polyamide,15 polyimide,16 polyurethane,17 and polyurea,18 many researchers have focused on MLD growth processes including self-limiting surface reactions19 and factors that control the film thickness and composition.20 In order to completely understand the unique properties of organic films, it is necessary to investigate the molecular interaction and orientation in these films in detail. To demonstrate the unique characteristics of polymer MLD films, we have previously explored polyurethane MLD films, which are useful for facilitating the production of uniformly aligned microcrystal arrays and for serving as a matrix.17 In this study, we fabricated uniformly aligned polyurea MLD films by repeating alternative vapor exposures of p-phenylenediisocyanate (PDI) and p-phenylenediamine (PDA) organic precursors for sequential and self-limiting surface reactions on SiO2 substrates. By integrating plane-polarized Fourier-transform infrared (FTIR) and Raman spectroscopic tools, we demonstrated the uniform alignment of polyurea MLD films, which corresponded well with the results showing π-π stacking between adjacent aromatic rings in polyurea films from density functional theory (DFT) calculations. Furthermore, the selective-wavelength absorption characteristics of these films were investigated by integrating optical measurements and finitedifference time-domain (FDTD) simulations of reflection spectra, resulting from their thicknessdependent fine resonance with photons.


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2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. The organic precursors, p-penylenediisocyanete (PDI; C6H4(NCO)2), and p-penylenediamine (PDA; C6H4(NH2)2, 99.0 %), were 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. All of the reagents were used without further purification. 300-nm-thick SiO2 covered Si(100) wafers were purchased from LG Siltron. 2.2. Preparation of hydroxyl-terminated substrate. 300-nm-thick SiO2 wafers were diced into small pieces with dimensions of 1 cm (width) × 1 cm (length) for use as the substrate. Prior to film deposition, the substrate was cleaned with isopropyl alcohol to remove any particles and then dried using nitrogen gas. The wafer pieces were subsequently dipped into a fresh Piranha solution (3:1 ratio of sulfuric acid:hydrogen peroxide) for more than 30 min to obtain a hydroxyl-terminated surface and were then rinsed with deionized water. The wafer pieces were dried with nitrogen gas, and loaded into the molecular layer deposition (MLD) chamber; the temperature was ramped to 110 °C and held for 30 min. 2.3. Fabrication of Polyurea MLD Films. Organic polyurea MLD films were fabricated in our homemade hot wall viscous flow vacuum MLD chamber equipped with in situ Fouriertransform infrared spectroscopy (FTIR) apparatus (Figure S1, Supporting Information). The halogen lamp and heating jacket surrounding the system produced uniform heat at desired temperature, which was controlled gradually along the gas flow path to prevent precursor condensation. Each precursor was transferred into a bubbler to be introduced into the vacuum MLD chamber. The organic precursors, PDI and PDA, were heated to 90 °C, and 105 °C,


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respectively, to achieve reasonable vapor pressure. The sequence of the polyurea MLD film deposition process is shown in Table S1. After 60 s PDI dosing with a 30 sccm flow of Ar carrier gas, the MLD chamber was maintained for 30 s to provide a sufficient opportunity for surface reaction; this period was called “exposure”. During dosing and exposure, a gate valve in the pump line was closed. The chamber was purged with Ar for 120 s at a flow rate of 400 sccm to remove residual PDI vapor, and then the MLD chamber was evacuated for 30 s to ensure that no precursors remained in the chamber. These sequences of dose, exposure, purge, and evacuation were repeated with PDA. A single cycle comprised the binary reactions between PDI and PDA described in steps 1-8 (Table S1). Polyurea MLD films of a desired thickness can be fabricated by repeating this cycle. As shown in the reactor schematic in Figure S1, during the PDI and PDA cycles, residual gas was directed through a water-cooled trap to eliminate further reactions in the pump line. 2.4. In situ Characterization. For the in situ FTIR measurement, a pressed SiO2 particle substrate, which has a high surface area, was used to enhance the signal-to-noise ratio. The schematic illustration in Figure S2 shows the preparation process for the pressed SiO2 particle substrate. A blank steel use stainless (SUS) grid is placed on a piece of weighing paper on the bottom die. The SiO2 powder is then evenly distributed over the grid, and a second piece of weighing paper is placed on top. After the second die is placed on top, the die assembly is placed into the hydraulic press, and 15,000 lb of pressure is applied, as shown in Figure S2. Upon removal of the grid from the die, there are mats of SiO2 powder both on top of the grid and inside each of the holes of the grid. The mats of SiO2 powder on the top layer of the grid are easily removed by gentle scraping with a metal spatula. However, the powder that is pressed into the grid holes is rigidly held and withstands gentle scraping. Figure S3 shows scanning electron


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microscopy (SEM) images of (a) the blank SUS grid, which has 100 mesh/inch, (b) the SiO2 particle substrate after SiO2 powder was pressed in the grid, and (c) the pressed SiO2 powder surface in a single grid hole. The IR beam is directed from the FTIR spectrometer (Nicolet is10, Thermo-Fisher) and transmitted through the ZnSe IR window and the sample. After passing through the sample, the IR beam is transmitted through ZnSe IR window again, reflected off two focusing mirrors, and focused onto a mercury cadmium telluride type B (MCT-B) IR detector (Figure S1). The ZnSe IR windows are resistant to heat and humidity, so they are appropriate for our MLD system. Moreover, two gate valves in front of the ZnSe IR windows protected the windows from reactants during MLD reactions. After sufficient purging, the two gate valves are opened, and the IR beam is transmitted. To prevent reaction of the precursors in the pump line, a water-cooled trap was added into the pump line. The spectra were collected with 200 scans at 4 cm-1 resolution. 2.5. Plane-Polarized Grazing Angle FTIR Spectroscopy. The plane-polarized grazing angle FTIR measurements were equipped in a FTIR spectrometer using a mercury cadmium telluride type A (MCT-A) detector. The incident IR beam was reflected with angle of 80° to the surface normal. The incoming IR beam was polarized at the desired angle using a manual polarizer (Pike Technologies). The spectra were collected with 200 scans at 4 cm-1 resolution. A cleaned bare 300-nm-thick SiO2 covered Si(100) wafer was used as a background reference. Both H2O and CO2 peaks were subtracted as necessary using baseline correction. 2.6. Ex situ Characterization. The refraction index, n, and film thickness were measured by ex situ ellipsometry (Gaertner Scientific Corp., L2W15S830) at a wavelength of 632.8 nm He-Ne


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laser light. Thickness was measured in at least 4 different spots on each sample to confirm the uniformity of the polyurea MLD film. Surface morphology was measured by a NX-10 (Park Systems) atomic force microscopy (AFM) using cantilever coated with 30 nm-thick aluminum in a noncontact mode with a scan size of 3 µm × 3 µm. X-ray photoelectron spectroscopy (XPS) analysis utilized an Axis-HIS tool (Kratos Inc.). An Al monochromator anode was used to analyze the polymer samples. Co-axial charge neutralization was used for charge compensation. All scans were calibrated by 284.5 eV C 1s peak as reference. Survey scans were collected to measure the elemental composition of each sample using an energy step of 0.1 eV. The X-ray beam diameter was 600 × 400 µm with 216 W power. Atomic compositions were calculated by determining peak area integrals, and peaks were fit using casa XPS program and combination of Gaussian and Lorentzian_GL(30) method. 2.7. DFT calculations. All calculations and modeling simulations were conducted with DMol3 program of the Material Studio 8.0 suite as software. The basis set a level of theory were chosen based on the results of a validation study for optimized molecular geometry of (PDI/PDA)1 polyurea MLD film using 1 cycle of PDI and PDA on the 300-nm-thick SiO2 substrate. 2.8. Raman Spectroscopy. We obtained the Raman spectrum of polyurea MLD film sample with a homemade micro-Raman spectroscopy system. In micro-Raman spectroscopy, the 514.5 nm line of an Ar ion laser was used as the excitation source with a power of ~1 mW. The heating effect can be neglected at this power range. The laser beam was focused onto the polyurea MLD film sample by a 50× microscope objective lens (0.8 Numerical Aperture). The collected scattered light was dispersed by a Shamrock SR 303i spectrometer (1200 grooves/mm) and was detected with a CCD detector.


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2.9. Optical simulations. The optical properties of the polyurea MLD films were simulated by finite difference time domain (FDTD) solvers supplied by Lumerical Solutions, Inc. Twodimensional (yz plane) models were designed by inputting the thickness of the polyurea MLD film. Perfectly matched layer boundary conditions were imposed in the xy plane and along z axis. A plane wave pulse source was launched along the z axis, and monitors were set to compute the electric field distributions in the films.

3. RESULTS AND DISCUSSION 3.1. Sequentially self-limited surface reactions of polyurea MLD films. All MLD reactions were performed in our homemade MLD chamber specialized for in situ FTIR measurements (Figure S1 and Table S1). The MLD growth of the polyurea film is based on the sequential reaction of two bifunctional organic precursors, PDI and PDA (Figure 1a). The polyurea MLD reactions are performed on hydroxylated silicon oxide surface.15 As depicted in the Figure 1a, the hydroxyl-terminated surface reacted with PDI to obtain an isocyanate-terminated surface, which is then reacted with PDA to form the urea linkage. These two sequences compose a single binary cycle. The polyurea film can be fabricated by repeating this cycle to obtain the desired thickness.21 Generally, MLD is based on self-limiting sequential surface chemistry. To fabricate highquality polymer films with good performance, the optimized condition for the polymer film must be determined by identifying the surface chemistry and self-limiting properties using in situ FTIR spectroscopy.15 The porous SiO2 particle substrate, which has a high surface area, was used for in situ FTIR measurements to obtain a high signal-to-noise ratio (Figure S2 and S3). Figures


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1b–c show the in situ FTIR spectra for the surface chemistry during (PDI/PDA)N polyurea MLD on a SiO2 particle substrate, in which “N” means number of cycles. All peak assignments in the FTIR spectra are shown in Table S2.18 The FTIR spectrum of the initial SiO2 particle substrate shows a broad O-H stretching vibration peak at 3100–3750 cm-1 and a sharp peak at 3743 cm-1 (black line in Figure 1b). The sharp peak is attributed to the isolated free O-H stretching vibration of the hydroxylated SiO2 particle substrate.22 After the first PDI exposure saturation, the O-H stretching vibration peaks are nearly removed, and a new N=C=O stretching vibration peak arises at 2270 cm-1 (red line in Figure 1b). Moreover, the N-H stretching vibration, C=O stretching vibration (amide I), CO-N-H bending vibration, and C-N stretching vibration (amide II) appear in the FTIR spectrum.15 These results indicate that the PDI precursors reacted with the hydroxyl group on the surface and formed the urethane linkage at 110 °C, as expected. The subsequent PDA exposure further increased the absorbance of the N-H, amide I, and amide II vibrational features, and removed the N=C=O stretching vibration peak (blue line in Figure 1b). This result provides convincing evidence that the amine group reacted with the surfaceterminated diisocyanate group and that the urea linkage was formed simultaneously. However, the aromatic C-H stretching vibrations are not shown because of detection limitations. Moreover, the FTIR spectra of the initial hydroxylated SiO2 particle substrate and (PDI/PDA)N polyurea MLD growth after 1, 5, and 10 cycles at 110 °C are shown in Figure 1c. All of the peaks are enhanced with an increasing number of cycles during (PDI/PDA)N polyurea MLD film growth.23 These results indicate that the urea bond is formed continuously during the MLD growth cycle at 110 °C. The self-limiting surface reaction property of (PDI/PDA)N polyurea MLD film was confirmed by monitoring the normalized absorbance of the N-H and N=C=O stretching vibrations with


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respect to the PDI and PDA dose time.20 Figure 1d shows the normalized absorbance of the N-H and N=C=O stretching vibrations during the PDA dose on the isocyanate-terminated surface. The absorbance of the N-H stretching vibration increases, while the absorbance of the N=C=O stretching vibration decreases. When the surface is exposed to PDA for about 30 s, the absorbance begins to be saturated. On the other hand, the normalized absorbance of N=C=O stretching vibrations during PDI exposure on the amine-terminated surface increases, as shown in Figure S4. The intensity saturations of the absorbance peaks imply that no additional reactions proceed on the surface, and the MLD reaction is self-limited on the SiO2 particle surface to both exposure of PDI and PDA during the polyurea MLD growth reaction. Figure 1e shows the thickness of the (PDI/PDA)N polyurea MLD film measured by ellipsometry as a function of the number of cycles. The thickness of polyurea MLD film increases linearly as the number of cycle increases, indicating that the complete MLD reactions continuously proceeded with accurate chemical bonding even with a high number of MLD cycles. Previous reports show a decrease in the growth rate with an increased number of cycles.24 However, in our system, the growth rate of the polyurea MLD film was measured at about 0.4 nm per cycle until 150 MLD cycles without decrease. This result implies that it is difficult to occur double reactions25 during growth of polyurea MLD film because the molecular geometries for double reaction have a higher value in potential energy, as shown in Figure S5. 3.2. Surface morphology and composition of polyurea MLD films. The surface properties of the polyurea MLD film, which was fabricated under the optimized condition, are investigated. Figures 2a–c show the surface roughness of the polyurea MLD film deposited on a SiO2 substrate at 110 °C with different numbers of cycles. These images show a considerably smooth surface texture regardless of the number of cycles. The root-mean square (RMS) values of


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(PDI/PDA)50, (PDI/PDA)150, and (PDI/PDA)300 are 0.30, 0.54, and 0.34 nm, respectively. Although these were polymer films, their RMS values were comparable or quite low compared to those of organic-inorganic hybrid films.26 The line profiles show that the smoothness was maintained at a higher number of cycles (Figure 2d). To obtain the detailed atomic composition of the (PDI/PDA)100 MLD film, X-ray photoelectron spectroscopy (XPS) studies were performed. When the chemical bonds were formed ideally consistent with Figure 1a, the atomic ratios of C:N:O are 7:2:1. In the case of the (PDI/PDA)100 MLD film, the atomic ratios of C:N:O are 7.25:2.08:1 in which the oxygen composition from the SiO2 substrate is considered. Compared to the ideal atomic ratio of polyurea MLD film, as shown in Figure 2e and Figure S6, the slight difference in the atomic ratio possibly exists due to oxygen in the SiO2 substrate, and by adventitious carbon contamination in air. 3.3. Molecular orientation of polyurea MLD films. To investigate the molecular orientations comprising the (PDI/PDA)N polyurea MLD film, plane-polarized grazing angle FTIR measurements were performed for (PDI/PDA)300 (Figures 3a–b). A stretching vibration can be excited only if the transition moment of the vibration has a component parallel to the polarization plane of the electric field vector of the IR beam. Therefore, we can assume the orientation of the molecules from the position and intensity of the stretching vibrational peaks.17,27 If the polymer has an ordered structure, the symmetric and asymmetric stretching vibrations were excited separately depending upon the polarization plane. Interestingly, the plane-polarized grazing FTIR spectra show that the N-H asymmetric stretching vibrational peak at 3330 cm-1 is shifted to the symmetric stretching vibrational peak at


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3270 cm-1 as the dihedral angle between the plane of polarization and the substrate increased from 0o (s polarization, incident beam is parallel to the substrate) to 90o (p polarization, incident beam is perpendicular to the substrate) (Figure 3a).28 The N-H stretching vibration peak is shifted depending on the polarization dihedral angle when the urea linkages are aligned with a specific orientation to the substrate. Moreover, the decrease in the intensity of the amide I (C=O stretching) vibrational peaks with the increase in the polarization angle from 0o to 90o verifies that the C=O bond in the polyurea backbone is oriented parallel to the substrate (Figure 3b). The intensities of the amide I vibrational peaks are nearly removed when the dihedral angle is above 30o. Furthermore, we also prepared randomly oriented (PDI/PDA) polyurea film by spin coating with the slurry polyurea in ethyl acetate on a SiO2 substrate for comparison with the molecularoriented polyurea MLD film. In contrast to (PDI/PDA)N polyurea MLD film, randomly oriented (PDI/PDA) polyurea film has no shift in the N-H stretching vibration peak (Figure S7).17 Moreover, in the case of the C=O vibrational peak, the intensity remained nearly invariant regardless of polarization dihedral angles between the plane of polarization and the substrate. In previous studies, it was reported that the angle between the aligned molecules and substrate could be changed by hydrogen bonding, intermolecular stacking, or molecular flexibility, resulted from inter- and intra-molecular interactions in MLD films.17 In addition, this angle can be affected by the density of functional groups on the substrate. To clear up any confusion regarding substrate issue in this study, we used the SiO2 wafer as a substrate with same density of functional groups on the starting surface of the substrate, and fabricated polyurea MLD films on the SiO2 substrates. And, to confirm the inter- and intra-molecular interaction and orientation of the polyurea backbone after the first cycle on SiO2 surface, we designed the molecular model as (PDI/PDA)1 couples which were chemisorbed on hydroxylated SiO2 substrate, and conducted


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the density functional theory (DFT) calculations.18 In this geometry, (PDI/PDA)1 is tilted about 35.2o from the SiO2 substrate, so the growth rate per cycle was calculated to be 0.6 nm (Figure 3c). The calculation result shows that (PDI/PDA)1 on the SiO2 substrate prefers a nearly planar structure because of resonance stabilization by π-π stacking with a distance of 0.49 nm between two adjacent aromatic rings (blue dotted line in inset of Figure 3d).28 In order to investigate interchain organization inside of polyurea MLD film, the (PDI/PDA)150 polyurea MLD film were measured by grazing-incidence wide-angle X-ray scattering (GIWAXS), which revealed that polyurea MLD film was comprised of out-of-plane with face-on π-π stacking with a distance of 0.46 nm (Figure S8), which corresponds well with DFT calculation (Figure 3d). The measured growth rate 0.4 nm per cycle (Figure 1e), which is slightly low, falls within the range expected for the growth rate from the DFT calculation, in which the relatively low value indicates that the polyurea backbones grew at an angle tilted less than 35.2o to the substrate because the hydrogen bonding between the oxygen and hydrogen in neighboring polyurea backbone induces stacking as shown by the red dotted line in inset of Figure 3d.18,29 The C=O bond is parallel to substrate (Figure 3d), which corresponds with the result of Figure 3b. Inelastic light scattering has been utilized as a potential technique for identifying the molecular orientation using Raman spectroscopy,30 which shows the polarization dependence between different angles of the incident laser. The Raman spectrum of the (PDI/PDA)250 polyurea MLD film is shown in Figure 3e. This spectrum shows a Raman shift at 3075 cm-1 and 3227 cm-1, whose peaks are C-H stretching vibrations in the aromatic rings of the (PDI/PDA)250 polyurea MLD film (Figure S9), showing the polarization dependence from 0° to 360° of the incident 514.5-nm Ar laser (Figure 3f). If the molecular orientation is aligned as mentioned in the DFT calculation, where the aromatic rings face up (Figures 3c–d), the maximum and minimum


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intensities of the polarization-dependent Raman shift are observed for polarizations parallel and perpendicular to the molecular orientation, respectively.31 The polar plots of the peak at 3227 cm1

suggest that the molecules comprising the (PDI/PDA)250 polyurea MLD film are well aligned,

which corresponds to the results from plane-polarized FTIR measurements (Figures 3a–b) and DFT calculations (Figures 3c–d). We also conducted an additional measurement at different locations on the (PDI/PDA)250 polyurea MLD films to investigate the in-plane orientation of molecules. Considering polar plots shown in Figure S10, the oriented direction of molecules was different in different locations, indicating that there are some grains in the in-plane of this films. However, the polarization dependence of the Raman scattering intensities shows a similar tendency in respective grains. Therefore, we believe that there are some grain boundaries in the (PDI/PDA)250 polyurea MLD films, and the molecules are aligned in respective grains. 3.4. Optical properties of polyurea MLD films with different thicknesses. Furthermore, the optical properties of (PDI/PDA)N polyurea MLD films, whose thickness is controlled at molecular level with different numbers of cycles, were investigated by measuring the wavelength-dependent reflections. In general, reflections in the thin film are attributed to optical interference when the film thickness belongs to the coherent length of the incident light source.32 The homemade experimental setup for reflection measurements is illustrated in Figure 4a, which used a white light-emitting diode (LED) as the light source (Figure S11). A Si wafer was used for reference in measuring the reflection. In this measurement, when the photon from the light source irradiates the (PDI/PDA)N polyurea MLD film in the normal direction, the photons are reflected, except for the absorption resonance into the thin film, whose peak is dependent on thickness.33 To demonstrate optical properties of wavelength-dependent reflections, the reflection spectra of (PDI/PDA)N/SiO2 polyurea MLD films were measured (Figure 4b). Here, N


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= 37, 65, and 122 cycles, and the thicknesses of the films were determined to be 15, 26, and 49 nm, respectively, by ellipsometry. The 300-nm-thick SiO2 substrate was used as a control, which is represented by a black line in Figure 4b. The absorption peak shifted to longer wavelengths as the thickness of (PDI/PDA)N polyurea MLD film increased, which showed wavelength-selective coupling to the thin film. These results were verified by simulating the reflection spectra using the finite-difference time-domain (FDTD) method, which is useful to calculate the optical properties of nanostructured materials over the entire wavelength range (Figure 4c).34,35 These results corresponded to the experimental reflection spectra with respect to the absorption peak position as a function of the number of cycles. The absorption peaks were plotted as a function of the number of cycles for both the experiments and simulations, showing linear dependence with the film thickness (Figure 4d). Thus, it is possible to achieve specific-wavelength absorption resonance in the visible range using one of the advantages of the MLD technique, which allows fine-tuning of the film thickness at the molecular level.

4. CONCLUSIONS In conclusion, the data presented here demonstrate that, by integrating plane-polarized FTIR and Raman spectroscopic tools to allow for an in situ and ex situ analysis approach, our MLD technology makes it possible to demonstrate molecular orientations in polymeric films. We fabricated uniformly aligned polyurea thin films by MLD based on the sequential reaction of two bifunctional organic precursors, PDI and PDA. The surface chemistry and self-limiting reactions, which are unique characteristics of MLD, were confirmed by our homemade MLD chamber equipped with in situ FTIR measurements. In addition, the results of the polarization angle-


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dependent FTIR spectra and Raman shift support that the molecules composing the polyurea MLD film are highly oriented, which is caused by π-π stacking among adjacent aromatic rings. Furthermore, the optical properties of the polyurea MLD films were investigated by measuring the reflection spectra, resulting in thickness-dependent absorption resonance, whose results were attributed to precise control of conformal film thickness. Indeed, because of the strong correlation between the molecular orientations of the organic components and the physical properties of the polymeric film devices, we believe that this fine-tunable MLD technique can contribute to advances in polymer-based engineering by controlling the film thickness at a molecular level on large area with conformity and obtaining coating materials with proper indices by designing molecule. In addition, it can provide deeper insights into conceptional growth processes of polymeric films for use in a wide range of applications such as selectivewavelength absorption-based solar cells, photodetectors, and optical filters.


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FIGURES

Figure 1. Sequentially self-limited molecular layer deposition of (PDI/PDA)N polyurea film. (a) Schematic polyurea MLD film using p-phenylenediisocyanate (PDI) and p-phenylenediamine (PDA). (b) In situ FTIR spectra of the hydroxylated SiO2 particle substrate, after the first and second exposure of PDI and PDA for saturation. (c) In situ FTIR spectra of the initial hydroxylated SiO2 particle substrate for 1, 5, and 10 cycles of deposition of the polyurea MLD film. (d) Normalized absorbance of the N=C=O stretching vibration and N-H stretching vibration versus PDA dose time when the PDA was exposed on the isocyanate-terminated surface. (e) Thickness profiles of the (PDI/PDA)N polyurea MLD film as a function of the number of cycles, measured by ellipsometry.


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Figure 2. Surface roughness and elemental composition of (PDI/PDA)N polyurea MLD film grown on SiO2 substrate. Three-dimensional atomic force microscopy (AFM) images and RMS roughness values of (a) (PDI/PDA)50, (b) (PDI/PDA)150, and (c) (PDI/PDA)300 polyurea MLD film, respectively. (d) The line profiles of the polyurea MLD film surface at the line shown in (a– c) represented by blue, green, and red, which are 50, 150, and 300 cycles, respectively. (e) The wide XPS survey scans of (PDI/PDA)100 polyurea MLD film.


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Figure 3. Molecular orientation of (PDI/PDA)N polyurea MLD film. Plane-polarized grazing angle (80° with respect to the surface normal) FTIR spectra of (a) amine (N-H stretching) and (b) amide I (C=O stretching) in (PDI/PDA)250 polyurea MLD film with different polarization dihedral angles between the plane of polarization and the substrate. (c) Calculated molecular geometries and intermolecular interactions of the (PDA/PDA)1 polyurea MLD film using 1 cycle of PDI and PDA on the SiO2 substrate, which are obtained by energy minimization from DFT calculation. Polyurea molecules tethered on the SiO2 substrate are tilted about 35.2° toward the substrate. (d) Parallel orientation of the C=O bond linkage in the polyurea backbone to the substrate, which are indicated by the black dotted line, and hydrogen bonding between H atom of N-H bond and the O atom of C=O bond, which are represented by red dotted line in the area


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outlined by the black square. In addition, two adjacent aromatic rings of polyurea molecules formed π-π stacking with a distance of 0.49 nm, as shown in blue dotted line in the inset of d. (e) Raman spectrum of the molecular vibration in (PDI/PDA)250 polyurea MLD film using a 514.5 nm Ar ion laser as the excitation source. (f) Polar plot of the peak with an asterisk in e, showing the polarization dependence of the Raman scattering intensities of (PDI/PDA)250 polyurea MLD film using the incident laser beam from 0° to 360°. The red curve indicates fitting for the Waveform sine square function.


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Figure 4. Reflection spectra of the (PDI/PDA)N polyurea MLD film with different thicknesses. (a) Schematic of the homemade experimental setup for reflection measurements. (b) Experimental reflection spectra of (PDI/PDA)N/SiO2 polyurea MLD film with different cycles (N = 37, 65, and 122), which show the maximum absorption resonance depending on the film thickness. (c) The FDTD simulations of the reflection spectra of the (PDI/PDA)N/SiO2 polyurea MLD films using 1.6 of refractive index. (d) Experimental and simulated peak position at the maximum absorption resonance as a function of the number of cycles.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI:10.1021/acs.chemmater.5b03XXX. MLD setup, MLD sequence, substrate preparation of in situ FTIR spectroscopy, SEM images of SUS grid, peak assignments of in situ FTIR spectroscopy, self-limiting reactions of PDI exposure, high-resolution XPS, plane-polarized grazing angle FTIR spectra of randomly deposited polyurea film, binding sites, measurement of Raman spectroscopy, and measurement of reflection spectra are shown in Supporting Information. (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions Y.-S.P., S.-E.C. and J.S.L conceived and designed the experiments with the additional support from H.K. for controlled experiment design. Y.-S.P. and S.-E.C. carried out most of the experiments, and H.K. conducted DFT calculations. Y.-S.P., S.-E.C. and J.S.L wrote the paper. All authors analyzed the data, discussed the results, and commented on the manuscript. Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This

work

was

supported

by

Nano·Material

Technology

Development

Program

(2012M3A7B4034986) funded by the National Research Foundation and the Pioneer Research Center Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (2012-0009562). Additionally, it was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2015R1A2A2A01005556).

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Table of Contents and Synopsis

The uniformly aligned polyurea thin films were grown on the SiO2 substrate by molecular layer deposition based on sequential and self-limiting surface reactions. These MLD films were fabricated by repeated alternative exposure of p-phenylenediisocyanate (PDI) and pphenylenediamine (PDA) organic precursors on SiO2 wafers, which was confirmed by in situ Fourier-transform infrared (FTIR) spectroscopy. And, we demonstrated the uniform alignment of polyurea MLD films by integrating plane-polarized FTIR and Raman spectroscopic tools and density functional theory (DFT) calculations.


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Fine-Tunable Absorption of Uniformly Aligned ... - ACS Publications

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