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Title

Fabrication and Characterization of Cross‑linked
Organic Thin Films with Nonlinear Mass Densities

Author(s)Rashed, Md. A; Laokroekkiat, Salinthip; Hara,
Mitsuo; Nagano, Shusaku; Nagao, Yuki

Citation

Langmuir, 32(23): 5917‑5924

Issue Date

2016‑05‑13

Type

Journal Article

Text version

author

URL

http://hdl.handle.net/10119/14268

RightsMd. A. Rashed, Salinthip Laokroekkiat, MitsuoHara, Shusaku Nagano and Yuki Nagao, Langmuir,
2016, 32(23), pp.5917‑5924. This document is theunedited author's version of a Submitted Workthat was subsequently accepted for publication in
Langmuir, copyright (c) American Chemical Society
after peer review. To access the final edited andpublished work, see
http://dx.doi.org/10.1021/acs.langmuir.6b00540

Description

Japan Advanced Institute of Science and Technology

Fabrication and Characterization of Cross-linked
Organic Thin Films with Nonlinear Mass Densities
Md. A. Rashed,† Salinthip Laokroekkiat,† Mitsuo Hara,‡
Shusaku Nagano,§ Yuki Nagao†*†

School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi,
Ishikawa 923-1292, Japan

Department of Molecular Design ＆ Engineering, Graduate School of Engineering, Nagoya University,‡

Furo-cho, Chikusa, Nagoya 464-8603, Japan§

Nagoya University Venture Business Laboratory, Nagoya University, Furo-cho, Chikusa,
Nagoya 464-8603, Japan

ABSTRACT
Urea (bonded) cross-linked multilayer thin film by sequential deposition of bifunctional
and tetrafunctional molecular building blocks was demonstrated. Multilayer growth as afunction of deposition cycles was inspected using UV-vis absorption spectroscopy.
From IR results, three characteristic infrared bands of amide I, amide II, and asymmetric(N-C-N) stretching band confirmed the formation of polyurea networks by alternate
dipping into solutions of amine and isocyanate functionalities monomers. The
deconvoluted component from the C 1s and N 1s spectra from X-ray photoelectron
spectroscopy (XPS) shows clear evidence of stable polyurea networks. Enhancement ofstructural periodicity with film growth was demonstrated by grazing incidence small
1angle X-ray scattering (GI-SAXS) measurements. The thin film near the substratesurface seems to have an amorphous structure. However, molecular ordering improvesin the surface normal direction of the substrate with a certain number of deposited layers.
Constant mass density was not observed with deposition cycles. The mass density
increased upto 16% within deposited layers from proximate layers to those extending
away from the substrate surface. This difference in packing density might derive fromthe different degrees of cross-linking among layers proximate to the substrate surface
and extending away from the surface.
Keywords: molecular layer deposition, covalent bond, multilayer films, polyurea,
layer-by-layer

1. INTRODUCTION
Cross-linked organic thin films have garnered much attention recently for their higher
density,1 better mechanical strength,2 and chemical stability.3,4 Demand for cross-linked
nanoscale organic thin films has increased rapidly for conventional applications such asorganic thin film transistors5,6, purification membranes etc.7 However, the synthesis ofthin films with sub-nanometer scale is still challenging. For the synthesis of organic thinfilms, spin-coating8 and Langmuir–Blodgett (LB)9 are well-known and usefultechniques. Another simple and versatile technique to prepare multilayer organic thin2

films is molecular layer deposition (MLD)10–12, which uses a layer-by-layer (LbL)13,14
fashion to prepare multilayer thin films with sub-nanometer scale. To date, most MLD
processes have been conducted with polymerization reaction of volatile bifunctionalmonomers under vacuum condition. Polymeric thin films have been reported such aspolyamides,15–17 polyimides,18–22 polyurethane,23 polyureas,1, 24–27 polythiourea,28 andpolyester.29 However, this vapor-based MLD technique is inapplicable for monomers
with high molar mass and low vapor pressure.30 Because of the high boiling point ofmonomers, excess adsorbed monomers onto the substrate surface by vapor deposition
cannot be removed completely, which hinders single molecular layer deposition.
Moreover, with vacuum-based reaction system, it is difficult to use a catalyst or pHchange in the reaction media to make a non-spontaneous reaction a spontaneous one.26
Solution-based MLD offers an exclusive route to overcoming these problems. Staffordand co-workers reported multilayer cross-linked polyamide thin film using
solution-based molecular layer-by-layer (mLbL) synthesis. They used bifunctionalm-phenylene diamine and trifunctional trimesoyl chloride monomers to fabricate
organic multilayer thin films.31,32 Recent reports by Qian et al. and Kim et al. show
cross-linked polyamide and polyurea thin films by polymerization of the same
symmetrical tetrafunctional monomers to produce three-dimensional (3D) covalent3

bonded molecular networks via LbL dipping technique.30, 33
Our approach to fabrication of 3D covalent bonded multilayer thin films is based onsequential deposition of different symmetrical monomers, such as bifunctional
1,3-phenylene diisocyanate (PDI) and tetrafunctional tetrakis(4-aminophenyl)methane(TAPM), on a 3-aminopropyltrimethoxysilane (APTMS) modified amine terminatedsurface. The longer urea chains with combinations of different symmetrical monomers
lead to different cross-linking states within multilayer growth. This different degree ofcross-linking might be facilitated by changing the molecule–molecule interaction
proximate to the substrate surface and extending away from the substrate surface. It isreasonable to consider that intermolecular interaction apart from the substrate surface isconsiderably higher than the intermolecular interactions among monomers near thesurface. It is also rational to infer that molecular networks extending away from thesubstrate surface exhibit higher cross-linking than molecular networks located near thesubstrate surface. Presumably, this different degree of cross-linking provides films withvariable densities among layers (with a certain thickness) proximate to the substratesurface and extending away from the substrate surface (Figure 1).

4

Figure 1. Schematic illustration of molecular layer deposition (MLD) process on a solid
surface and the chemical structure of 1,3-PDI and TAPM monomers.
This work shows the synthesis and characterization of a new class of cross-linked
multilayer polyurea thin films using 3D covalent bonded molecular networks withnonlinear mass densities via solution-based MLD technique. Infrared spectroscopy (IR)and X-ray photoelectron spectroscopy (XPS) confirmed the urea bond formation
between the reaction of amine and isocyanate functionalities. UV-vis absorption
spectroscopy demonstrated multilayer growth. The thickness of multilayer polyurea
films on the silicon substrate was measured using atomic force microscopy (AFM) and
X-ray reflective (XRR) techniques. The mass density and degree of molecular orderingwith film growth were examined, respectively, using X-ray reflectivity (XRR) analysis
5and grazing incidence small angle X-ray scattering (GI-SAXS) measurements.
2. EXPERIMENTAL
2.1. MaterialsAsmolecularbuilding

blocks,tetrakis(4-aminophenyl)methane(TAPM)

and

1,3-phenylene diisocyanate (PDI) were purchased, respectively, from Aldrich Chemical
Co. Inc., USA and Tokyo Chemical Industry Co. Ltd., Japan. Using recrystallization
technique,

98%

pure

commercial

PDI

was

purifiedfurther.

3-Aminopropyltrimethoxysilane (APTMS, >96%) was purchased from Tokyo Chemical
Industry Co. Ltd., Japan, and was used as received. All solvents were purchased assuper-dehydrated and AR grade from Wako Pure Chemical Industries Ltd., Japan andwere used without further purification.
2.2. Fabrication of Multilayer Thin Films on Solid Surface
Multilayer thin films were synthesized onto quartz substrates (25 × 15 mm2, t = 0.5 mm)
and silicon wafers (30 × 20 mm2, t = 0.525 mm). Prior to multilayer formation,
substrates were cleaned using 2-propanol with sonication (three times each, 15 minduration), and were then dried and kept in the clean bench. The cleaned substrates were
immersed into 10 mM amine functionalized 3-aminopropyltrimethoxysilane (APTMS)
in ethanol solution at room temperature for 1 hr under Ar atmosphere with constant6

stirring. After taking out from the APTMS solution, substrates were cleaned using
ethanol (twice) and 2-propanol (twice) consecutively with sonication and finally rinsed
with 2-propanol and dried. These modified substrates were used for multilayer thin film
formation using an automatic layer-by-layer (LbL) system. Layer deposition was carriedout in the following procedures: (i) amine functionalized substrate was immersed into
17.6 mM 1,3-PDI in 1,4-dioxane and toluene (3:1, v/v) solution for 5 min. Then, thesubstrate was rinsed in five separate containers: two beakers of 1,4-dioxane and toluene
mixture at the previously described ratios, two beakers of dry THF, and finally one
beaker of chloroform. (ii) After washing, the isocyanate-terminated substrate was
immersed into 3.78 mM TAPM in 1,4-dioxane and toluene (3:1, v/v) solution for 5 min,followed by rinsing successively in two beakers with 3:1 (v/v) ratio of 1, 4-dioxane andtoluene mixture, and two beakers of THF. The deposited substrate was dried under Ar
atmosphere. This bilayer deposition process described above (steps (i) and (ii)) was
designated as a single cycle of molecular layer deposition (MLD). Multilayer thin films
were formed by repeating the procedure described above in a desired number of cycles.
In every cycle, the amine-terminated surface is regarded as a top surface.

72.3. Characterization
Several analytical techniques have been applied to investigate multilayer growth, toexplore the physical and chemical properties of the polyurea thin films, and to elucidate
the film surface morphology. UV-vis absorption spectra of the molecular networks
fabricated on quartz substrates were obtained using a UV-vis spectrometer (Jasco
V-630BIO-IM; Jasco Corp. Japan). The APTMS-modified quartz substrate was used asa background. Fourier-transform infrared (FTIR) spectroscopy was used to investigate
the urea bond formation of the thin film on the substrate. Infrared spectra were collectedusing an FTIR spectrometer (Nicolet 6700; Thermo Fisher Scientific Inc.) equippedwith a mercury cadmium telluride (MCT) detector. An APTMS-modified Si wafer wasused as a background. Spectra were taken at 8° from the surface normal. The signal
noise has been subtracted by smoothing process. To check the surface morphology, an
atomic force microscope (AFM, Nanoscope IIIa; Veeco Instruments) with tapping mode
was used. Silicon cantilevers (SI-DF3FM; Nanosensors Corp.) with a spring constant
between 2.8 Nm-1 and 4.4 Nm-1 and resonance frequency of 79–89 kHz were used. The
measurements were taken, respectively, under an air atmosphere with a scan rate of 0.4
Hz and scan size of 5 × 5 μm2 and 0.5 × 0.5 μm2. Film thickness was measured using an
AFM (VN–8000; Keyence Co.) equipped with a DFM/SS mode cantilever (OP-75041;
8

Keyence Co.). Thickness was measured at least four positions on each sample and got
an average value. Then XPS study was performed using a DLD spectrometer (Kratos
Axis-Ultra; Kratos Analytical Ltd.) with an Al Kα radiation source (1486.6 eV). Energy
calibration and component separation were conducted using the bundled software withpure Gaussian profiles and a Shirley background. X-ray reflectivity (XRR) analysis was
performed using a high-resolution diffractometer (ATX/G; Rigaku Corp.; Japan) with an
R-AXIS IV two-dimensional (2D) detector. The diffractometer was equipped with Cu
Kα radiation (λ= 0.1542 nm) and divergence of 0.01°. The reflective oscillation curves
were fitted using software (GIXRR; Rigaku Corp.; Japan). Curve fitting areas werechosen between 0.3° to 3°. Grazing incidence small angle X-ray scattering (GI-SAXS)
analysis was performed using a diffractometer (FR-E; Rigaku Corp., Japan) with beam
size of approximately 300 × 300 µm2. The camera length was 300 mm. The sample
stage

was

composedofthegoniometerandaverticalstage(ATS-C316-EM/ALV-300-HM; Chuo Precision Industrial Co. Ltd.). The X-ray
incidence angle varied between 0.21° and 0.22°. The X-ray exposure time was fixed for
4 hr.

9

3. RESULTS AND DISCUSSION
To confirm the multilayer growth of MLD process, transparent UV-vis spectra of thinfilms were measured as a function of the MLD cycles (Figure 2). The absorption
maxima were centered at 268 nm. The absorbance intensities continuously increased
within deposition cycles represents the multilayer growth carried out. The inset of
Figure 2 shows layer growth near the substrate up to 10 MLD cycles and extending
away layer growth above 10 MLD cycles followed two distinctive linear trends. This
results suggested non-linear growth for MLD cycles.

Figure 2. UV-vis spectra of multilayer thin film as the number of deposition cycles onquartz substrate. Inset: absorption peak intensity at 268 nm as a function of 0–30
deposition cycles.
10

As the absorbance intensity increases (Figure 2) with the number of deposition cycles,
thickness also changes with the number of deposition cycles. Film thickness wasinvestigated using AFM and XRR techniques. For AFM thickness measurements, films
were partly scratched and the height difference was evaluated for film thickness.
Thicknesses of the 10, 20, and 30 MLD cycle films were respectively, 4.34, 6.82, and9.33 nm thick. These results show growth rates (avg.) of 0.434 nm / cycle, 0.341 nm /cycle and 0.311 nm / cycle, respectively, for 10, 20, and 30 MLD cycle films. The
thickness of AFM results is well-matched to the XRR results, as shown in Figure 3.
Results of XRR data are discussed later. The non-uniform growth rate (avg.) per cyclein 10, 20, and 30 MLD cycle films might be attributed to different packing
conformations (loose/dense) per unit volume (cm3) of different depth into the films.
This density (g/cm3) change phenomenon will be discussed next. Moreover, thisthickness value is lower than the combined molecular length of 1,3-PDI and TAPM,
which is estimated as approximately 1.44–1.61 nm / cycle. Some reports of theliterature have described that growth rates are much less than ideal repeating unit lengthfor MLD thin film deposition.1,11,12 This deviation can be explained with several reasons.
Thinner growth could be observed because of the tilting of organic molecular networks.
In “double” reactions, the bifunctional monomer stops the layer growth by forming a11

“capped” region on the reactive film surface.10,11 Finally, steric hindrance due to bulky
precursor may also affect the accessibility of surrounding reactive sites and cause
submonolayer coverage.1 Though, the growth rate of the polyurea thin film is lower than
expected, the film thickness increased with MLD cycles, indicating multilayer
deposition. This phenomenon is consistent with the UV-vis results discussed above.

Figure 3. Thickness profiles of 10, 20, and 30 MLD cycle films by AFM (red circle) andXRR (blue circle).
This increase of UV-vis absorbance intensity and changes of thickness with MLD
cycles does not confirm the formation of a urea bond (covalent networks) into thin films.
Polyurea networks formation through reaction between amine and isocyanate
functionalities was inspected with infrared (IR) spectroscopy. Figure 4 presents IR
12spectra of 10, 20, and 30 MLD cycle thin films. Several characteristic peaks wereobserved in the region of 1200–1800 cm-1. Spectra of the higher wavenumber region are
shown in Supporting Information as Figure S2. Peaks around 1650 cm-1 – 1690 cm-1 canbe assigned to the amide I band with the(C=O) stretching vibration mode of the urea

group. The band at 1510 cm-1 can be assigned to the amide II band with (N–H) bending
vibration. These two bands and thea (N-C-N) asymmetric stretching band at 1300 cm-1

suggest urea linkage formation. The wavenumbers of these characteristic peaks are
consistent with others reported in the literature of polyurea linkage,26,27,34,35 and assist to
confirm polyurea networks in as-synthesized thin films. In addition, a shoulder-type
band near 1535 cm-1 and a sharp band near 1603 cm-1 are respectively attributable toaromatic ring breathing and aromatic ring stretching.

13

Figure 4. IR spectra of 10, 20, and 30 MLD cycle thin films in the infrared vibrational
region for urea bonds. Spectra were measured under a nitrogen atmosphere.
Along with infrared spectroscopy, X-ray photoelectron spectroscopy (XPS) can also
be applied to investigate the bonding to as-synthesized polyurea thin films. The atomic
environments and chemical bonding to the thin films can be discussed using XPS finescans spectra. Figure 5 depicts C 1s and N 1s XPS fine scan spectra of 10, 20, and 30
MLD cycle films. Survey scan spectra are shown in Supporting Information (Figure
S3).

14

Figure 5. XPS fine scan spectra of C 1s (top (a–c)) and N 1s (bottom (d–f)) at 10, 20,
and 30 MLD cycle films.
The C 1s fine scan spectra showed three different types of carbon species in all
MLD films. The lowest binding energy peak at 284.5 eV resulted from electron richaromatic carbon. The highest binding energy peak near 288.6 eV resulted from thecarbonyl carbon (C=O) from urea linkage. The intermediate peak at 285.9 eV resulted
from the combination of alkyl carbon and substituted aromatic carbon linked with urea.
These assignments of the C 1s fine scan spectra are consistent with other reports of thepolyurea-based and thiourea-based MLD literature.27,28 These assignments confirm the15

urea bond formation into thin films. The IR results also reinforce urea bond formation,
indicating that the reaction between amine and isocyanate functionalities form polyurea
networks via a urea-coupling reaction.
The N 1s fine scan also contained three isolated peaks. The intermediate binding
energy peak at 399.9 eV corresponds to urea groups.27,33 The lower binding energy at
399.3 eV and higher binding energy at 401.3 eV are attributed respectively to
non-hydrogen bonded and hydrogen-bonded free amine groups in polyurea thin films.36
N 1s for isocyanate was not detected because the unreacted isocyanate groups areconverted easily to amine by exposure to humid air.33
From Table 1, results show that the peak intensity percentage of N 1s for urea linkage
increases from 74% to 85% with layer growth. The urea density apparently increased
with layer growth. This revealed that different degrees of cross-linking were formed
among layers proximate to the surface and extending away from the surface. Thisdegree of cross-linking might also influence the mass density (g/cm3) of 10, 20, and 30
MLD cycle films.

16

Table 1. Peak widths and intensity of the deconvoluted N 1s in 10, 20, and 30 MLD
cycle films
N 1s in urea

N 1s in amine

N 1s in H-bonded amine

Binding Energy (eV)

399.9

399.3

401.4

FWHM (eV)

1.49

1.49

1.49

Peak intensity (%)

73.97

15.57

10.44

Binding Energy (eV)

399.9

399.3

401.3

FWHM (eV)

1.44

1.49

1.49

Peak intensity (%)

84.0

6.41

9.59

Binding Energy (eV)

399.9

399.2

401.3

FWHM (eV)

1.46

1.50

1.49

Peak intensity (%)

84.82

4.30

10.87

10 Cycles

20 Cycles

30 Cycles

However, inspection of the peak intensity ratio of three carbon species is notapplicable because of the presence of adventitious carbon (AC) in thin films. This AC
carbon might influence the intensity ratio of carbon species. Finally, UV-vis, IR, and
XPS results revealed that sequential deposition of amine and isocyanate functionalities
produces polyurea networks and demonstrates multilayer growth.

17

To evaluate the film thickness, film mass density, and interface roughness, XRR
measurements were performed. XRR is a powerful tool for accurate measurement ofmultilayer thin film thickness, film mass density, and interface roughness.37–39 Figure 6
shows XRR curves for 10, 20, and 30 MLD cycle films. From the XRR simulated
fringes of Figure 6, film thickness, film mass density, and roughness of 10, 20, and 30
MLD cycle films were estimated. For XRR fitting curves, the simulated fringe
satisfactorily fit with the experimental fringe profile in the case of 10 MLD cycles witha uniform density. In the case of 20 and 30 MLD cycle films, the results of satisfactory
fit with a uniform density could not be obtained. More close fit has been tried with abilayer model as shown in Figure S6. The fit results were found to improve at thesufficiently satisfactory level. The densities around the boundary between the freeinterface layer and interfacial layer might intergrade. These results suggest the
non-uniform density structure in the thicker films. From XRR measurements, therespective thicknesses of 10, 20, and 30 MLD cycle films were found to be 4.55, 6.59,
and 9.44 nm thick. This replies nonlinear thickness growth (in average) per cycle withrespect to MLD cycles. AFM results support this nonlinear thickness growth as shownin Figure 3. Therefore, it is clear to say that the films density should vary with numberof deposition cycles.
18

Figure 6. XRR profiles of different MLD cycle thin films. Solid lines and rhombus dot
represent experimental and fitting data.

XRR density profiles (Figure 7) showed that the film mass density of 10 MLD cycle
film is constant through the film depth. However, for 20 and 30 MLD cycle films, themass density is not uniform throughout the range of film depths. The mass density of
the free interface (f/i) region is ca. 16% higher than that of the interfacial layer (int.)
region.

19

Figure 7. Film mass density (g/cm3) profiles of simulated models for (a) 10 MLD cycle,(b) 20 MLD cycle, and (c) 30 MLD cycle thin films as a function of distance from thefree interface.
These phenomena might occur because of different packing densities proximate to
the surface (up to 3.5–5 nm thickness) and extending away from the surface (thicker
than 3.5–5 nm). The XPS results support this hypothesis. We assume that these densitychanges might be observed according to the different degrees of interpenetration amongorganic molecular networks or different degrees of cross-linking. Since, cross-linking
introduces denser film1 and the XPS results suggest that urea density increases withlayer growth. This fact suggests that different degrees of cross-linking occurred among
the layers near to the surface and extending away from the surface.

The propensity of cross-linking among molecular networks proximate to the substratesurface and extending away from the substrate surface can be considered withcorresponding molecule–molecule interaction. Molecule–molecule interaction in layers
20extending away from the substrate surface might be considerably higher than it is near
the substrate surface. Thereby, it is possible to change the degree of cross-linking
proximate to the surface and extending away from the surface. This different degree ofcross-linking can also be examined with changes of molecular volume (Å3) in single
repeating unit within-layer deposition. Reportedly cross-linking increases the film
density corresponding to the decrease in the molecular volume of each repeating
unit.1 The repeating unit can be defined by the reactivity of four amine functional groups
as one TAPM molecule with four isocyanate groups as two 1,3-PDI molecules. The
molecular volume of each repeating unit can be extracted approximately by the
mass-to-density ratio using the molecular mass of the single repeating unit and the film
density obtained from XRR density profiles. This analysis revealed that the molecularvolume in the free interface (f/i) region of 20 and 30 MLD cycle films decreases
compared to the interfacial (int.) region (Table 2). The molecular volume of the single
repeating unit in the free interface (f/i) regions of 20 and 30 MLD cycle films are,respectively, 722 Å3 and 696 Å3. In contrast, the molecular volumes of the single
repeating unit in interfacial layers (int.) region of 20 and 30 MLD cycle films are,respectively, 830 Å3 and 813 Å3. This result reflects the presence of different degrees ofcross-linking into thin films. This phenomenon is responsible for introducing nonlinear
21

mass densities into thin films.
Table 2. Molecular volume of single repeated unit in 10, 20, and 30 MLD cycle films
MLD
cycles

Total
Thickness (nm)Thickness(nm)

Mass density(g/cm3)

10
20

4.55
6.59

30

9.44

4.55
4.89 (int.)
1.70 (f/i)
3.73 (int.)
5.71 (f/i)

1.33
1.40 (int.)
1.61 (f/i)
1.43 (int.)
1.67 (f/i)

Molecular
volume (Å3)
874
830 (int.)
722 (f/i)
813 (int.)
696 (f/i)

To increase the film mass density with deposition cycles, structural change mightoccur. These structural properties of the thin films were evaluated using GI-SAXS.
Figure 8 portrays 2D and 1D GI-SAXS profiles of the 30 MLD cycle film on a Si wafer.
The GI-SAXS profiles for a 10 MLD cycle film are shown in Supporting Information as
Figure S8.

Figure 8. 2D GI-SAXS imaging plate pattern of the 30 MLD cycle film on a Si wafer.
1D profile in the out-of-plane direction extracted from the 2D GI-SAXS pattern.

22

A clear diffraction was observed in 30 MLD cycle film in 2D imaging plate at theout-of-plane direction at 2θ z = 12.5° (Figure 8) with a corresponding d-spacing value of
0.7 nm. However, no diffraction peak was observed at the out-of-plane direction in the
10 MLD cycle film. This suggests that the threshold thickness should be considered formolecular ordering of the thin film. This result indicates that molecular ordering seems
to be improved to the surface normal direction of the substrate with a certain number ofdeposited layers. In the in-plane direction, no diffraction peak was observed in 10, 20,
or 30 MLD cycle films, which indicates that the MLD thin film exhibits less molecularordering in the in-plane direction.
4. CONCLUSIONS
Using solution-based MLD technique in ambient conditions, we synthesized 3D
covalent bonded organic thin films with variable densities from sequential exposure ofbifunctional 1,3-PDI and tetrafunctional TAPM precursors. Our layer-assembled thinfilms exhibited multilayer growth. Fabricated thin films included three characteristic
infrared vibrational bands for urea linkage, confirming polyurea networks’ existence inthe thin films. Furthermore, in XPS study, peak splitting within N 1s and C 1s
corresponded to the values expected for polyurea films. Results showed that structural
periodicity and mass density (g/cm3) of the thin films increased concomitantly with the23

number of MLD cycles. These unusual changes might result from different degrees ofcross-linking of organic molecular networks proximate to the substrate surface andextending away from the surface. This approach of synthesizing organic molecular
networks with higher degrees of cross-linking can be extended to porous substrates thatcan be used for ion separation membranes.
ASSOCIATED CONTENT
Supporting Information
IR, XPS, AFM, XRR, and GI-SAXS studies of different MLD cycle thin films. This
material is available free of charge via the internet at http://pubs.acs.org .
AUTHOR INFORMATION
Corresponding Author*ynagao@jaist.ac.jp Phone: +81(Japan)-761-51-1541, Fax: +81(Japan)-761-51-1149,
Address: 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan
Notes
The authors declare that they have no competing interest, financial or otherwise, related
to this study.

24

ACKNOWLEDGMENTS
The authors thank Dr. Masashi Akabori of Japan Advanced Institute of Science andTechnology (JAIST) for support in conducting atomic force microscopy. This work was
financially supported by the Japan Society for the Promotion of Science (JSPS) throughthe Funding Program (GR060) for Next Generation World-Leading Researchers (NEXTprogram), initiated by the Council for Science and Technology Policy (CSTP). This
work was partially supported by the Murata Science Foundation, Japan and Ogasawara
Foundation, Japan.
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