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Title

Multilayer Growth of Porphyrin‑Based Polyurea
Thin Film Using Solution‑Based Molecular Layer
Deposition Technique

Author(s)Uddin, S. M. Nizam; Nagao, Yuki

Citation

Langmuir, 33(44): 12777‑12784

Issue Date

2017‑10‑12

Type

Journal Article

Text version

author

URL

http://hdl.handle.net/10119/15727

RightsS. M. Nizam Uddin and Yuki Nagao, Langmuir, 2017,33(44), pp.12777‑12784. 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.7b03450

Description

Japan Advanced Institute of Science and Technology

Multilayer Growth of Porphyrin-Based Polyurea Thin Film Using
Solution-Based Molecular Layer Deposition TechniqueS. M. Nizam Uddin†, Yuki Nagao†*†

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

Abstract
Controllable synthesis of organic thin film materials on solid surfaces is a challenging issue
in the research field of surface science as it is affected by several physical parameters. In thiswork, we demonstrated a solution-based molecular layer deposition (MLD) approach toprepare porphyrin-based covalent organic molecular networks on a 3-aminopropyl
trimethoxysilane (APTMS) modified substrate surface using the urea coupling reaction
between 1,4-phenylene diisocyanate (1,4-PDI) and 5,10,15,20-tetrakis-(4-aminophenyl)
porphyrin (H2TAPP) at room temperature (22 ± 2 ºC). Multilayer growth was investigatedunder different relative humidity (RH) conditions of the reaction chamber. Sequential
molecular growth at low relative humidity (≤10% RH) was observed using UV-vis absorption
spectroscopy and atomic force microscopy (AFM). The high-RH condition shows limited
film growth. Infrared spectroscopy (IR) and X-ray photoelectron spectroscopy (XPS)
revealed the polyurea bond formation in sequential multilayer thin films, demonstrating thatstepwise multilayer film growth was achieved using the urea coupling reaction.

1

Introduction
Fabrication of functionalized organic thin films has become a burgeoning area in the research
field of surface science over the last few decades.1-4 Scientists from different backgrounds
have introduced versatile new methods for thin film synthesis using top-down and bottom-up
approaches.5-6 Bottom-up approach offers an opportunity to synthesize the nanostructure on
the substrate surface by stacking the atoms and/or molecules on each other with less defects,
homogenous chemical composition and better short- and long-range ordering.6-8 Bottom-up
approach includes the various thin film fabrication techniques such as Layer-by-layer (LbL),9
Langmuir-Blodgett (LB),10-11 spin coating,12 microwave13 and electrochemical deposition.14
Among all of these, molecular level LbL thin film fabrication known as molecular layer
deposition (MLD) has gained an extensive attention in today’s research world because of its
simplicity and its attractive control over film thickness, composition, and conformality
through sequential self-limiting surface reaction at the single-molecule level.15-18 As
covalently bonded films possess much higher chemical and thermal stability (because of theirchemical characteristics)19-21 compared to the conventional MLD film fabrication based on
non-covalent interactions such as coulombic interactions22-23 and hydrogen bonding.24 To date,most MLD processes have been conducted through polymerization of volatile bifunctionalmonomers

undervacuumconditions

topreparepolyamide,25-27

polyimide,28-29

polyurethane,30 and polyurea15,31-32 based nanostructures. The urea coupling reaction offers asignificant benefit to avoid the byproduct entrapment within the polymer film as no
byproduct is produced in this reaction process.15,26 In addition, the interlayer linking
functionality of urea moiety has the ability to form hydrogen-bonded network that helps toform molecular network on the substrate surface.33 The use of urea functionalities to enhance
the stability in aromatic supramolecular structures is also well-known because of theirhydrogen-bonding ability between adjacent urea groups along with the π−π stacking
2interaction between neighboring aromatic building blocks. Several research groups including
Bent34-36 and Blanchard33 have utilized bifunctional monomers to fabricate polyurea thinfilms including interlayer hydrogen-bonding between adjacent urea groups. Park and coworkers reported a multilayer polyurea thin film using same symmetrical tetrafunctionalmonomers.37 Recently, our research group has demonstrated a cross-linked polyurea thin filmusing a different symmetrical bifunctional and tetrafunctional monomers.15
In this work, the solution-based MLD technique is chosen to establish porphyrin-based
covalent organic thin film through the urea coupling reaction between bifunctional 1,4phenylene diisocyanate (1,4-PDI) and tetrafunctional 5,10,15,20-tetrakis-(4-aminophynyl)
porphyrin (H2TAPP) monomers (Scheme 1). The choice of these monomers was based on
their chromophore properties in the multilayer thin film and commercial availability. Two
different wavelength regions of phenylurea and porphyrin in UV-vis absorption spectroscopy
assisted to investigate the multilayer film growth at different RH conditions of the reaction
chamber. The main focus of this paper is on demonstrating the use of conventional
isocyanate-amine chemistry to fabricate porphyrin-based polyurea multilayer assemblies.
Systematic characterization of the sequential molecular growth was accomplished using UVvis absorption spectroscopy and atomic force microscopy (AFM). Moreover, chemical
characteristics of the multilayer organic thin film were characterized using infrared
spectroscopy (IR) and X-ray photoelectron spectroscopy (XPS).

3

Scheme 1. Schematic representation of MLD process to fabricate porphyrin-based
multilayer thin film

4

Experimental
Materials
Silicon wafer (Si (100), p-type) and quartz (SiO2) substrates were purchased from Electronics
and Materials Corp. Ltd. and Sendai Sekiei Glass Seisakusho respectively. 5,10,15,20tetrakis-(4-aminophynyl) porphyrin (H2TAPP, >95%), 1,4-phenylene diisocyanate (1,4-PDI,>98%) and 3-aminopropyltri methoxysilane (APTMS, >96%) were purchased from Tokyo
Chemical Industry Co. Ltd., Japan. Before use, 1,4-PDI was further purified by sublimation.
HPLC (99.7%) grade of ethanol and 2-propanol was used for substrate modification with
APTMS. Super dehydrated (H2O, ≤0.001%) chloroform (CHCl3), tetrahydrofuran (THF),

toluene and N, N-dimethylformamide (DMF) were used for the formation of multilayer thinfilms. All solvents were purchased from Wako Pure Chemical Industries Ltd., Japan, andwere used as-received without further purification.
Modification of Solid Substrate with APTMS
SiO2 (25 × 15 mm2, t = 0.5 mm) and Si (30 × 20 mm2, t = 0.5 mm) substrates were cleaned bysonication for 3 × 15 min using 2-propanol (HPLC grade, 99.7%) to remove organic
contaminants. To obtain the amine-functionalized self-assembled monolayers (SAMs), clean
SiO2 and Si substrates were immersed into 10 mM ethanol solution of APTMS for 1 hr withmild shaking at room temperature (22 ± 2 ºC) under an Ar atmosphere. After taking out fromthe APTMS solution, the substrates were washed with HPLC grade (99.7%) of ethanol(twice) and 2-propanol (twice) consecutively with sonication for 5 min each, and were finallyrinsed with 2-propanol and dried with an Ar stream in a clean bench. Characterizations of the
APTMS-modified substrate are shown in Figures S1–S3.

5

Fabrication of Multilayer Thin Film on Modified Solid Substrate
Amine-functionalized substrates were used for the fabrication of multilayer thin films in a
RH controlled chamber at different relative humidity (RH) condition. The RH of the reaction
chamber was controlled by using Ar gas flow and monitored by RH sensor. Fabrication ofmultilayer thin film was carried out in super dehydrate solvents (H2O, ≤0.001%) at room
temperature with the following steps: (i) the amine-functionalized substrate was immersed

into 4.4 mM 1,4−PDI in the mixture solvent of CHCl3 and THF (8:2, v/v) for 25 min. Thenthe substrate was rinsed successively with two beakers of toluene and two beakers of THF
separately for each 2 min washing. (ii) The resulting isocyanate functionalized substrate was
immersed into 1.5 × 10-1 mM H2TAPP in the mixture solvent of CHCl3 and THF (8:2, v/v)
for 25 min, with subsequent washing successively using two beakers of DMF and two
beakers of THF separately for each 2 min washing to remove the unreacted precursors onto
the deposited film and dried under Ar atmosphere. This bilayer deposition obtained from step(i) and (ii) was regarded as the 1 MLD cycle. The steps described above were repeated tofabricate the multilayer films used for characterization.
Characterization
UV-vis absorption spectra of the multilayer thin film deposited on the SiO2 substrates were
measured using a UV-vis spectrometer (Jasco V-630BIO-IM; Jasco Corp., Japan). The
APTMS-modified SiO2 substrate was used as a reference sample. Transmission Fouriertransform infrared (FTIR) was used to characterize the MLD film formation. IR spectra werecollected with a Fourier transform infrared (FTIR) spectrometer (Nicolet 6700, Thermo
Fisher Scientific Inc.) equipped with a mercury cadmium telluride (MCT) detector. An
APTMS-modified Si substrate was used as a reference sample. The signal noise has been
subtracted by smoothing process. Tapping mode atomic force microscopy (AFM) was used to6analyze the film surface morphology and roughness. AFM images were collected using a
digital AFM system (NanoScope IIIa; Veeco Instruments). Silicon probes were used as acantilever (SI-DF3FM; Nanosensors Corp.) with a resonance frequency of 60-66 kHz and aspring constant of 2.8–4.4 N/m. The measurements were taken under an air atmosphere witha scan rate of 0.4 Hz and scan sizes of 500 nm × 500 nm and 1 μm × 1 μm. Multilayer film
thickness was measured using AFM (VN-8000; Keyence Co.) equipped with a DFM/SS
mode cantilever (OP-75041; Keyence Co.) and white interference microscope (BW-S506,
Nikon Co.). For thickness measurement, samples were prepared by covering a portion of baresubstrate with thin Au coating followed by APTMS and multilayer thin film formation.
Finally the thin Au coated portion was carefully removed from the substrate surface by using
cotton bar before thickness measurements. Measurements were taken at four different
positions, at least, to estimate the average film thickness. The thickness of APTMS-modified
substrate was used as a baseline thickness, which was subtracted from the subsequent film
thickness of the multilayer thin film includes APTMS. To confirm the elemental
characteristics of the thin films, X-ray photoelectron spectroscopy (XPS) study was
performed using a DLD spectrometer (Kratos Axis-Ultra; Kratos Analytical Ltd.) with an Al
Kα radiation source (1486.6 eV). Energy and component separations were conducted using
bundled vision processing software with pure Gaussian profiles with a Shirley background.

7

Results and Discussion
To achieve controlled film growth throughout the deposition process, optimized reaction
conditions such as monomer concentration, immersion time, and physisorption wereascertained carefully as shown in Figures S4–S6. For this work, moisture-sensitive 1,4-PDI(containing isocyanate group) was used as one precursor,33,37 which may have different
degrees of self-polymerization with respect to RH conditions of the reaction chamber.
Considering this issue, the effect of RH on film growth was investigated using UV-vis
absorption spectroscopy. Figure 1 depicts the UV-vis absorption spectra of 1 MLD cycle
films as a function of RH condition of the reaction chamber. The absorbance increased
significantly at 260 nm and decreased at 430 nm from 54 ± 2% to 10 ± 2% RH of the reaction
chamber. But the absorbance remains almost same in the cases of 5 ± 2% and 10 ± 2% RH,
representing a tentative RH condition for the fabrication of porphyrin-based polyurea thinfilm. The higher number of phenylurea formation (at 260 nm)15,33,38 at high–RH reduced thereaction possibilities of porphyrin precursors. At high RH condition of the reaction chamber,hydrolysis of isocyanate group is expected to form the corresponding carbamic acid at the

Figure 1. UV-vis absorption spectra of 1 MLD cycle films on the quartz slides as afunction of RH condition of the reaction chamber (immersion time = 25 min/0.5 MLD
cycle).
8presence of water vapor that might be converted to amine with the evolution of carbon
dioxide.33 Thus, another isocyanate group containing 1,4-PDI molecule can react with thecarbamic acid intermediate or amine to form corresponding urea as the final product of selfpolymerization (Scheme 2).33 As a consequence the top surface of the film could be blocked
by the self-polymerization of 1,4-PDI molecules. These might reduce the availability of
isocyanate groups that is required for the molecular growth of H2TAPP through the urea
coupling reaction (Scheme 3).
Scheme 2. Hydrolysis of isocyanate group to form a carbamic acid (intermediate)
and the subsequent decomposition to produce amine, leading self-polymerization of
1,4-PDI molecules

Scheme 3. Reaction between isocyanate and amine group to form urea linkage

9For clarification, XPS spectra of 1 MLD cycle films were taken at both low (10 ± 2%) andhigh (34 ± 2%) RH conditions of the reaction chamber. The N 1s XPS fine scan spectra
showed that the peak area of urea response prepared at the high–RH condition is 70% larger
than the film prepared at the low–RH condition. The characteristic iminic (=N-) and pyrrolic(N–H) peak areas of porphyrin showed 10% lower response from low to high RH conditions(Figure S7). That can only occur when 1,4-PDI becomes self-polymerized at the high–RH
condition. Figure 2 portrays the continuous increase of absorbance as a function of MLD
cycles at 10 ± 2% RH of the reaction chamber, indicating stepwise film growth.15,37,39
Stepwise film growth at 10 ± 2% RH shows almost similar tends with 5 ± 2% RH condition(Figure S8a). However, limiting film growth was observed in a high–RH condition (FigureS8b). These results signify that, at high RH, a considerable amount of 1,4-PDI selfpolymerization blocks the reactive surface sites that leads to a lower accessibility of theporphyrin molecules for molecular growth.25,34 During multilayer film growth, 1 MLD cycle
film showed higher absorbance at 260 nm compared to the growth sequence of following

(a)

(b)

Figure 2. (a) UV-vis spectra of multilayer thin film as a function of MLD cycles on
APTMS-modified quartz slides and (b) linear plot of absorption intensity vs. thenumber of MLD cycles at 10 ± 2 % RH of the reaction chamber.

10

MLD cycles. It might have occurred as a result of capped region caused by the double
reaction of 1,4-PDI precursors because the molecular population is largest at the initial stepsof film growth.18,40
To demonstrate the growth behavior, thickness of 2, 5, 8, and 10 MLD cycle thin films were
investigated using AFM and white interference microscope (Figure S9). Figure 3 depicts thesequential growth behavior with the number of MLD cycles, confirming stepwise film
formation at10 ± 2 % RH of the reaction chamber.18,23 The film growth rate for 1,4PDI/H2TAPP film was found to be approximately 0.80 ± 0.03 nm/MLD cycle. The thicknessof AFM results are well matched to the white interference microscope results. This growthrate is lower than the combined molecular length of 1,4-PDI and H2TAPP molecules, whichis estimated as approximately 2.08 nm, based on the constituent bond length of 1,4-PDI andH2TAPP molecules. Deviation of the growth rate can be attributed to several factors. First,
the film growth direction is not perpendicular to the substrate surface. In fact, the tilted
configuration of MLD film with respect to the surface might dominate the structure.35 It is
expected that the multilayer films are growing at an average angle of 64° from the substrate

Figure 3. Film thickness as a function of number of MLD cycles prepared at 10 ± 2 %RH of the reaction chamber.
11

surface normal. Second, a fraction of incident precursors may react with the reactive film
surface to form a capped region.18,34
From the discussion presented above, it was not confirmed that the multilayer film growthoccurred by a urea coupling reaction between isocyanate and amine groups. Therefore, IR
spectroscopic measurements were taken to investigate chemical bonding into the molecular
films. Figure 4 presents the IR spectra for 5 and 10 MLD cycle thin films. Several
characteristic peaks were observed in the IR fingerprint region. The band around 1635 and
1670 cm-1 can be assigned as amide I band resulting from the H-bonded and free υ(C=O)
stretching vibration mode of urea group respectively. The amide II band as δ(N–H) bending
vibration was observed around 1510 cm-1. Peaks around 1311 and 1180 cm-1 can be assigned
for the asymmetric υas(N–C–N) and symmetric υs(N–C–N) stretching modes respectively. All
characteristic peaks confirmed the formation of urea linkage.15,33,35-36,41 In addition, the outof-plane γ(N−H) vibration of pyrrole in free-base porphyrin was observed at around 730 and
800 cm-1. A shoulder type υ(N–H) band was also observed at around 965 cm-1 for in-plane N–
H vibration of pyrrole.42-43 Furthermore, the absorbance increased with the number of MLD

Figure 4. IR spectra of 5 and 10 MLD cycle thin films on the APTMS-modified Si
substrates.
12

cycles, reflecting that the polyurea-based growth behavior occurred along with the multilayer
film formation. No peak was observed at around 2270 cm-1 for υas(N=C=O) stretching,36
indicating the absence of unreacted isocyanate groups in the films (Figure S10).
For further characterization, XPS was used to explore the chemical characteristics of themolecular networks. Elemental fine scan spectra revealed the atomic environment andchemical bonding in the multilayer thin film (Figure 5a–5c). The N 1s XPS fine scan spectra
for 10 MLD cycle film showed five different types of components (Figure 5a). The main

(a)

(b)

(c)

(d)

Figure 5. N 1s XPS fine scan spectra for (a) H2TAPP powder and (b) 10 MLD cycle
film. (c) C 1s XPS fine scan spectra for 10 MLD cycle thin film and (d) XPS survey
spectrum for APTMS, 1 MLD, 5 MLD, and 10 MLD cycle thin films.

13

peak at 399.8 eV represents the urea (O=C–N–H) groups.15,35,37 The lowest binding energy
peak at 397.9 eV and higher binding energy peak at 400.4 eV correspond to the iminic (=N-)
and pyrrolic (N–H) groups respectively.47 The intermediate binding energy peak at 399.4 eV
resulted from the non-hydrogen bonded free amine (NH2).15,44-45 Finally, the minor peak at
the higher binding energy around 401.9 eV represents the hydrogen bonded amine andnitrogen shakeup satellite of porphyrin.44-45 The binding energies of N 1s XPS fine scan
spectra for 10 MLD cycle film showed good matching with the N 1s XPS fine scan spectra
for H2TAPP powder, as shown in Table 1.

Table 1. Peak assignments of the N 1s XPS fine scan spectra for H2TAPP powder
and 10 MLD cycle films

SampleH2TAPP
powder10 MLD
cycle film

=N(Iminic)

Free NH2

N–H

H-bonded NH2 /(Pyrrolic) N-shakeup satellite

O=C–N–H

397.9 eV399.4 eV

400.4 eV401.9 eV-397.9 eV399.4 eV

400.4 eV401.9 eV399.8 eV

Consideration of the peak area ratio of the deconvoluted N-components revealed that the urea
(O=C–N–H) and free amine (NH2) responses are 3.2 and 1.3 times higher than the pyrrolic(N–H) group respectively. Indicating that, on average, only 2–3 amine groups of eachporphyrin molecule took part in the reaction during stepwise film growth. Some additionalurea and free amines might be observed because of the capping reaction of incident
precursors34 and unreacted amine and isocyanate groups which were easily converted toamine by humid air. In addition, C 1s XPS spectra for 10 MLD cycle film showed four
14

components (Figure 5c). The lowest binding energy peak at 284.5 eV is attributed to thearomatic carbon (C=C) and C–H bonds.15 The highest binding energy peak at 288.5 eV
corresponds to the carbonyl (C=O) group of urea linkage.15,45 And the intermediate binding
energy peaks at 285.7 eV and 286.5 eV denote the sp2 C–N and C=N bond respectively.45 No
isocyanate peak was observed from N 1s and C 1s XPS fine scan spectra.46 These
assignments of N 1s and C 1s XPS fine scan spectra are consistent with IR results and werepreviously reported polyurea-based works.15,33-37,41,44-45 All results confirmed the urea bondformation consistently into the multilayer thin film. Figure 5d presents survey spectra of
APTMS, 1 MLD, 5 MLD, and 10 MLD cycle thin films. Results show that N 1s and C 1s
peak intensities increase concomitantly with decreasing Si 2p peak intensity from APTMS to
10 MLD cycle thin films, indicating that the multilayer film covers the Si/SiO2 surface.
Additionally, it is noteworthy that N 1s peak intensity for 1 MLD cycle film is higher than thecalculated one MLD cycle of 5 and 10 MLD cycle thin films. That result might have occurred
due to the presence of unreacted amine-functionalized APTMS (H-bonded amine)45 and thecapped region caused by the double reaction of 1,4-PDI or H2TAPP precursors because themolecular population is larger at the initial steps of film growth.18,34 These results are relevantwith the observation of UV-vis absorption response for multilayer film growth (Figure 2).
The topography of SiO2-supported polyurea thin films was also investigated using tapping
mode AFM, as shown in Figure 6. The 2D and 3D AFM height images of 5 and 10 MLD
cycle thin films show homogeneous surface within 500 nm × 500 nm area, which indicates
the well deposition process. It was made up of numerous small grain-like domains along thehorizontal axis with the diameter of ca. 10–20 nm, which increases slightly from 5 to 10
MLD cycle thin films. The root mean square (RMS) roughness was also investigated; whereroughness increased from 0.36 to 0.48 nm for 5 and 10 MLD cycle thin films respectively. In
comparison to the bare SiO2 substrate and APTMS-modified SiO2 substrate, 5 and 10 MLD
15cycle films show greater domain size and RMS roughness (Figure S11), which signifies themolecular layer growth on the APTMS-modified Si substrate.15,23
(a)

(b)

Figure 6. (a) 2D and (b) 3D AFM height images of 5 MLD and 10 MLD cycle thinfilms. Scan size: 500 nm × 500 nm. Data scale 10 nm.
In this covalent network system, results suggest that the humidity at 10 ± 2% RH could be an
acceptable condition for the achievement of porphyrin-based polyurea multilayer thin film
networks on solid surface.

Conclusions
We have demonstrated the solution-based MLD thin film growth of porphyrin-based covalentmolecular networks on a APTMS-modified substrate surface using the urea coupling reaction
between 1,4-PDI and H2TAPP at room temperature. UV-vis absorption spectra showed
stepwise multilayer film growth at ≤10% RH condition of the reaction chamber, whereas the16high RH condition showed film growth-limiting behavior. Presumably, numerous 1,4-PDI
self-polymerization took place at high RH conditions. This might block the reactive surface
area that leads to a lower accessibility of porphyrin molecules for multilayer film growth.
White interference microscope confirms the film thickness measured by AFM and indicatesthat the multilayer films prepared at 10 ± 2% RH of the reaction chamber are growing at an
average angle of 64° relative to the substrate surface normal. FTIR spectra of multilayer thinfilms showed four characteristic IR vibrational bands for urea linkage along with the in-planeand out-of-plane vibrational modes of pyrrole from metal free porphyrin. The IR responses ataround 1635 and 1670 cm-1 confirmed the presences of hydrogen bonded and free urea
respectively into the multilayer thin film. The IR signal also increased significantly with thenumber of MLD cycles, representing the sequential multilayer film growth caused byconsecutive urea coupling reaction. Furthermore, deconvoluted high resolution N 1s and C 1s
XPS fine scale spectra represent an additional evidence of urea bond formation into themultilayer thin films. Peak area ratio of the deconvoluted N-component revealed that, on
average, only 2-3 amine groups of each porphyrin molecule took part in the urea coupling
reaction for multilayer film formation.

ASSOCIATED CONTENT
Supporting Information
UV-vis, IR, XPS, white interference microscope and AFM studies of substrate surfacemodification and MLD cycle thin film formation. This materials 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
17

Notes
The authors declare no competing financial interest.

ACKNOWLEDGMENTS
The authors thank Dr. Md. A. Rashed and Dr. Salinthip Laokroekkiat for their advice andtechnical assistances. This work was financially supported by The Murata Science
Foundation, Japan.

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