Chemical infiltration during atomic layer deposition: metalation of porphyrins as model substrates

Article abstract of DOI:10.1002/anie.200900426

New uses for ALD: By applying standard metal oxide atomic layer deposition (ALD) to two types of porphyrins, site-specific chemical infiltration of substrate molecules is achieved: Diethylzinc can diffuse into the interior of porphyrin supramolecular stru

Full text of DOI:10.1002/anie.200900426

Communications
DOI: 10.1002/anie.200900426
Hybrid Nanostructures
Chemical Infiltration during Atomic Layer Deposition: Metalation of
Porphyrins as Model Substrates**
Lianbing Zhang,* Avinash J. Patil, Le Li, Angelika Schierhorn, Stephen Mann, Ulrich Gꢀsele,
and Mato Knez*
[
7]
Atomic layer deposition (ALD) is a gas-phase deposition
process based on successive self-terminating gas–solid reac-
tions. During the process, the template substrate is exposed to
precursor molecules from the gas phase, which ideally
promotes adsorption of a monolayer on the surface. After
purging the excess precursor and subsequent exposure to a
second gaseous precursor, reaction on the surface of the
substrate leads to formation of a layer of the desired material.
The layer thickness is controlled by the number of the
reaction cycles. Owing to the precise thickness control and
broad range of operating temperatures, ALD has recently
been used for coating various structures, including thermally
and chemically sensitive organic and biological macromole-
for hybrid material fabrication, or templates for the
[
8]
fabrication of nanostructures. Such materials normally
contain various functional groups that are potentially reactive
with ALD precursors. Precursors can diffuse through polymer
layers during the ALD process, but the chemical interactions
between the precursors and the substrate during the diffusion
[
3,9]
process has only been proposed.
Herein, we use J-aggregate nanostructures composed of
meso-tetra(p-phenylsulfonato)porphyrin (TSPP) or meso-tet-
raphenylporphyrin (H TPP) as model substrates for standard
metal oxide ALD, and show for the first time that the
2
[
1–3]
cules.
Much work has also been performed to develop new
precursors and new processes to increase the versatility of
ALD. For example, a number of studies have been devoted to
the investigation of the interface chemistry during the
deposition process to achieve better control over the depo-
sition rate and the area selectivity of the deposition. Several
[
4–6]
reviews about ALD have been recently published.
To date, only little attention has been paid to the chemical
interactions between the precursors and the substrate under-
neath the macroscopic interface formed during the ALD
process. It is essential to understand such interactions,
particularly as an increasing number of organic soft materials
are being used in ALD processes to fulfill various tasks: for
[
1]
example, as masks for area-selective deposition, scaffolds
precursor can infiltrate the substrate at the molecular level
and induce site-specific metalation. Self-assembly of proton-
ated TSPP molecules into J-aggregate nanotapes occurs
through electrostatic interactions between the negatively
charged sulfonato groups and positively charged pyrrole
[
*] L. Zhang, Prof. U. Gꢀsele, Dr. M. Knez
Max-Planck-Institut fꢁr Mikrostrukturphysik
Weinberg 2, 06120 Halle/Saale (Germany)
Fax: (+49)345-551-1223
E-mail: lianbing@mpi-halle.de
[10]
amine groups,
which are also potential active sites for
mknez@mpi-halle.de
interactions with ALD precursors. Moreover, the protonated
and deprotonated monomer and the J-aggregate of TSPP
Dr. A. J. Patil, Prof. S. Mann
Centre for Organized Matter Chemistry, School of Chemistry
University of Bristol (UK)
[11]
have distinct light absorption profiles,
which provides a
L. Li
convenient method for analysis. Specifically, we show that
processing the J-aggregates with standard ZnO-ALD results
in infiltration of the metal precursor diethylzinc into the TSPP
J-aggregates and induces metalation of TSPP molecules with
zinc(II) ions. Formation of the metalloporphyrins was con-
firmed by UV/Vis absorption and mass spectrometry. Sim-
Institut fꢁr Planzenphysiologie
Martin-Luther-Universitꢂt, Halle (Germany)
Dr. A. Schierhorn
Institut fꢁr Biochemie und Biotechnologie
Martin-Luther-Universitꢂt, Halle (Germany)
[
**] This work is supported by the International Max Planck Research
School for Science and Technology of Nanostructures (Nano-
IMPRS) at Halle. M.K. gratefully acknowledges the financial support
by the German Federal Ministry of Education and Research (BMBF)
with the Contract No. FKZ: 03X5507.
ilarly, the reaction between diethylzinc and H TPP as
2
substrate with a half cycle of the ALD process also resulted
in zinc(II) intercalation into the porphyrin cavity. Such a half
cycle is analogous to a simple vapor-phase exposure of a
substrate to the precursor in a vacuum chamber. This process
can therefore be considered to be vapor-phase metalation.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200900426.
4
982
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2009, 48, 4982 –4985

Angewandte
Chemie
The use of ALD in the presence of a porphyrin-based
substrate is shown in Figure 1a. SEM images of TSPP J-
aggregates after ALD of TiO (500 cycles of Ti(OiPr) /H O)
2
4
2
Figure 2. UV/Vis absorption spectra of a) TSPP J-aggregates and J-
aggregates with different metal oxides deposited by ALD and sus-
pended in water at pH 1, and b) native and coated TSPP J-aggregates
with various metal oxide ALDs suspended in water at pH7. c) MALDI-
Figure 1. a) The metal oxide ALD process on porphyrins. b,c) SEM
images of TSPP J-aggregates after TiO deposition with 500 cycles of
2
Ti(OiPr) /H O. Inset: the hollow structure of the TiO coating after
TOF mass spectrum of TSPP J-aggregates after ZnO deposition (200
4
2
2
+
disassembly of J-aggregates following treatment with water (scale bar:
mm). d) TEM image of TSPP J-aggregates coated with 20 nm Al O .
cycles Et
2
Zn/H
2
O). ZnTSPP: m/z 995.922 ([M] ), TSPP: m/z 935.035
+
1
([M+H] ).
2
3
oxide coating, or localized mechanical disruption in the films
incurred during the re-suspension process.
showed a network of long bundles of coated nanofilaments on
the surface of the silicon wafer (Figure 1b). Viewed in cross-
section, the bundles of filament-like structures were observed
below the metal oxide layer (Figure 1c). Incubating the
scratched sample in water caused disassembly and dissolution
of the TSPP J-aggregates at their open ends, thus producing
the hollow metal oxide structures (inset, Figure 1c). TEM
studies of the hybrid nanostructures after Al O coating
Interestingly, the UV/Vis absorption spectrum of J-
aggregates subjected to ZnO deposition and suspended in
water at pH 7 was different to that observed after coating with
Al O3 or TiO . The ZnO/J-aggregate composites showed
2
2
absorption maxima at 422 and 556 nm instead of the
2
À
4À
characteristic peaks for H TSPP , H TSPP , or J-aggregates
4
2
(434 nm; 413 nm and 515 nm; 491 nm and 706 nm, respec-
tively; Figure 2b and Supporting Information). The presence
of the Soret band at 422 nm for the ZnO/J-aggregates
indicates significant modification of the TSPP molecules
with regard to changes to the electron density of the
porphyrin ring. The results are consistent with the formation
of the metalloporphyrin ZnTSPP, which has absorption
2
3
(
Figure 1d) showed that the J-aggregate filaments were
entrapped in a well-defined metal oxide shell when the
coating layer remained intact during the dispersion process.
Owing to its higher mass density, the metal oxide layer
showed an enhanced contrast in the TEM compared to the J-
aggregates. These results demonstrate that deposition of
metal oxides by ALD at low temperatures takes place without
significant disruption of the J-aggregate superstructure.
UV/Vis spectroscopy studies of the J-aggregate hybrid
nanostructures re-suspended in water at pH 1 showed the
same absorption profile as untreated J-aggregates (Fig-
ure 2a). A band at 434 nm, which is characteristic for the
[12,13]
maxima in aqueous solution at 422 and 556 nm.
Metal-
ation of the porphyrin molecules by reaction of diethylzinc
with TSPP during the ZnO-ALD process was confirmed by
mass spectrometry (Figure 2c), in which the main signal
+
corresponds to ZnTSPP (calculated m/z 996 [M] ). The signal
+
for the free-base TSPP (m/z 935 [M+H] ) had only very low
2
À
H TSPP monomer, was observed, along with a red-shifted
intensity, and there were no signals corresponding to TSPP
conjugated with more than one zinc(II), indicating that salt
formation with the sulfonato groups did not occur.
The above results were also confirmed by undertaking the
ZnO-ALD process in the presence of the water-insoluble
4
Soret band at 491 nm and broad peak at 706 nm correspond-
ing to the J bands of the aggregates. Thus, in acidic solvent,
the absorption spectra indicated that the supramolecular
organization of the TSPP J-aggregate remained intact during
the ALD process. Moreover, suspensions of the J-aggregate
hybrid material in water at pH 7 showed absorption peaks at
porphyrin, H TPP, which contains phenyl groups instead of
2
the sulfonatophenyl side chains. Moreover, to further inves-
tigate the mechanism of diethylzinc infiltration and zinc(II)
complexation with the porphyrin molecules, we performed a
2
À
4
13 and 515 nm, which indicate deprotonation of H TSPP to
4
4
À
H TSPP and associated disassembly of the J-aggregates
2
(
Figure 2b and Supporting Information). These results indi-
pseudo-deposition of ZnO on H TPP involving a half-cycle
2
cate that the J-aggregates were still accessible to hydroxide
ions, possibly because of the formation of a porous metal
ALD process composed of 100 pulses of diethylzinc without
water pulses. Note that such a process is identical to the
Angew. Chem. Int. Ed. 2009, 48, 4982 –4985
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 4983

Communications
exposure of H TPP to diethylzinc vapor in a vacuum
2
chamber, suggesting that this procedure could also be
performed in suitable setups that are different from ALD.
No changes were observed in the microscopic structure of the
dried H TPP structures before and after the half-cycle ALD
2
(
Figure 3a,b). After 100 pulses of diethylzinc in the ALD
chamber, an absorption maximum at 420 nm in the Soret
Scheme 1. Chemical reaction between diethylzinc and porphyrins TSPP
and H TPP during ALD.
2
isopropoxide and trimethylaluminum, which exists predom-
[14,15]
inantly as a dimer in the vapor phase,
infiltration and successive metalation.
may impede
Metalation of porphyrin has mostly been performed in
aqueous or organic media, and to overcome the slowness of
the complexation reaction, either high temperature or
[
16,17]
catalysts have to be applied.
Thus, metalation of por-
phyrins from vapor phase could serve as a novel method to
synthesize metalloporphyrins with several advantages com-
pared to metalation in solution. In particular, the solvent-free
environment of the vapor-phase reaction enables the supra-
molecular structures of free-base porphyrins to be metalated
in situ in the absence of catalysts and at lower temperatures
and short reaction times. Moreover, zinc porphyrins have
higher light-to-energy efficiencies than free-base porphyrins
Figure 3. SEM images of H TPP dissolved in acetone and dried on a
2
silicon wafer a) before, and b) after 100 cycles of Et Zn pulses by a
2
half-cycle ALD. c) UV/Vis absorption spectra in acetone of H TPP,
2
ZnTPP (99+ %, Sigma–Aldrich), and H TPP treated with Et Zn.
2
2
[18–20]
d) MALDI-TOF mass spectrum of H TPP after treatment with a half-
as sensitizers for solar cells,
and several reports inves-
2
+
cycle ALD (100 pulses of Et Zn). H TPP: m/z 614.980 ([M+H] ),
tigating porphyrin-dye-sensitized ZnO and TiO solar cells
2
2
2
+
ZnTPP: m/z 676.087 ([M] ).
[21–23]
have been published,
suggesting that similar configura-
tions might be achieved in a one-step process using ALD. For
example, the dye molecules could be conveniently metalated
to enhance the light-electrical energy conversion efficiency,
and metal oxide films of desired thickness can be deposited
within an ALD chamber simply by varying the pulse
sequences. In addition, infiltration of ALD precursors and
induction of chemical reactions has general significance for
the application of ALD in various technological fields. For
example, it has been suspected that CÀC double bonds were
band region was observed, which was identical to the
spectrum recorded for a standard sample of ZnTPP (Fig-
ure 3c). Mass spectrometry analysis of H TPP after the half-
2
+
cycle ALD confirmed the formation of ZnTPP (m/z 676 [M] ;
Figure 3d). Identical results were also achieved when a
standard ALD deposition of ZnO (with pulses of both
diethylzinc and water) was applied to H TPP. TSPP could also
2
[
24]
be converted into ZnTSPP with diethylzinc pulses only (see
the Supporting Information).
accessible to trimethylaluminum, a standard ALD metal
precursor, and physical infiltration in the absence of chemical
[
25,26]
Our results indicate that metalation of both TSPP and
interactions has been also proposed by George et al.
In
H TPP can be achieved with a ZnO-ALD process, despite the
another example, infiltration by ALD induced significant
mechanical charges to spider silks, of which the chemical
2
different water-solubility and self-assembly properties of
those different porphyrins. The conversion of the major part
of TSPP molecules into their metalloporphyrin counterparts
demonstrates that diethylzinc can reach the interior of
compact nanostructures of the TSPP J-aggregates. The
diethylzinc pulse itself was sufficient to convert the porphyr-
ins into the corresponding metalloporphyrins, which confirms
that chemical infiltration occurs during the exposure time of a
[27]
nature is not yet known in detail. Certainly, the occurrence
of chemical infiltration will depend on both the parameters of
the ALD process and chemical properties of the template and
the precursors; for example, the length of exposure and
purging time, the molecule size, functional groups on the
template, and the reactivity of the involved precursors and
groups.
half cycle, and metalation of TSPP and H TPP originates from
In summary, by processing standard metal oxide ALD on
2
direct decomposition of diethylzinc by the pyrrolic N-bound
hydrogen atoms of the free-base porphyrins, followed by the
complexation of zinc(II) (Scheme 1). Metalation was not
observed after treatment with titanium and aluminum
precursors. Although the reason for this is unclear, steric
hindrance owing to the larger molecular size of titanium
TSPP J-aggregates and H TPP, we have shown that the ALD
2
precursor diethylzinc can infiltrate the bulk substrate and
interact with the amine groups at the molecular level, thereby
inducing metalation of the porphyrin molecules. We expect
similar reactions to take place during various vapor-phase
treatments using proteins, peptides, or amine-containing
4
984 www.angewandte.org
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2009, 48, 4982 –4985

Angewandte
Chemie
polymers as substrates. For ALD applications, controlled site-
specific chemical infiltration can be applied as a new method
for in-situ modification of substrates, and can also provide a
new aspect for precursor design. Future work will investigate
the reaction kinetics, parameters affecting the infiltration
depth, and the activity of standard ALD precursors to
frequently used organic materials. The effect of chemical
infiltration should be taken into consideration for future
applications of ALD.
[
[
[
1] M. Knez, A. Kadri, C. Wege, U. Gꢀsele, H. Jeske, K. Nielsch,
Nano Lett. 2006, 6, 1172.
2] L. Niinistꢀ, J. Pꢁivꢁsaari, J. Niinistꢀ, M. Putkonen, M. Nieminen,
Phys. Status Solidi A 2004, 201, 1443.
3] A. Sinha, D. W. Hess, C. L. Henderson, J. Vac. Sci. Technol. B
2006, 24, 2523.
[4] R. L. Puurunen, J. Appl. Phys. 2005, 97, 121301.
[
[
5] M. Knez, K. Nielsch, L. Niinistꢀ, Adv. Mater. 2007, 19, 3425.
6] M. Leskelꢁ, M. Ritala, Angew. Chem. 2003, 115, 5706; Angew.
Chem. Int. Ed. 2003, 42, 5548.
[
7] X-H. Liang, S. M. Geoge, A. W. Weimer, N-H. Li, J. H. Black-
son, J. D. Harris, P. Li, Chem. Mater. 2007, 19, 5388.
Experimental Section
[8] G. M. Kim, S. M. Lee, G. H. Michler, H. Roggendorf, U. Gꢀsele,
M. Knez, Chem. Mater. 2008, 20, 3085.
[9] A. Sinha, D. W. Hess, C. L. Henderson, J. Vac. Sci. Technol. B
2007, 25, 1721.
[10] R. Rotomskis, R. Augulis, V. Snitka, R. Valiokas, B. Liedberg, J.
Phys. Chem. B 2004, 108, 2833.
[11] P. J. Meadows, E. Dujardin, S. R. Hall, S. Mann, Chem.
Commun. 2005, 3688.
[
11]
The preparation of TSPP J-aggregates is described elsewhere. For
the deposition of metal oxides by ALD, a suspension of TSPP J-
aggregates was dried in air on a laboratory film (Parafilm) or on a
silicon wafer. The sample was subsequently transferred into the ALD
chamber (Savannah 100, Cambridge Nanotechnology Inc.) and
evacuated at 508C (in the case of Parafilm) or 808C (silicon wafer)
at 0.1 Torr for 1 h to remove remaining water from the J-aggregates.
After drying, the deposition of metal oxides was performed by
alternating exposure to the precursors with 100, 200, or 500 cycles at
[12] R. Fujiyoshi, T. Arai, M. Katayama, J. Radioanal. Nucl. Chem.
1994, 185, 133.
5
0 or 808C at 0.2 Torr. Trimethylaluminum, titanium isopropoxide,
[13] A. Farajtabar, F. Gharib, P. Jamaat, N. Safari, J. Chem. Eng. Data
2008, 53, 350.
and diethylzinc were used for the deposition of Al O , TiO , and ZnO
2
3
2
as the first precursor, and water as the second precursor. The pulsing
[14] N. Muller, D. E. Pritchard, J. Am. Chem. Soc. 1960, 82, 248.
[15] W. R. Salaneck, R. Bergman, J. E. Sundgren, A. Rockett, T.
Motooka, J. E. Greene, Surf. Sci. 1988, 198, 461.
[16] R. J. Abraham, G. R. Bedford, D. McNeille, B. Wright, Org.
Magn. Reson. 1980, 14, 418.
[17] M. Inamo, A. Tomita, Y. Inagaki, N. Asano, K. Suenaga, M.
Tabata, S. Funahashi, Inorg. Chim. Acta 1997, 256, 77.
[18] J. Rochford, D. Chu, A. Hagfeldt, E. Galoppini, J. Am. Chem.
Soc. 2007, 129, 4655.
time was 0.2 s for Al Me , 1.0 s for Ti(OiPr) , 0.3 s for Et Zn, and 1.3 s
2
6
4
2
for water. For all pulses, 60 s of exposure time and 60 s of purging time
were used. Purging was carried out with argon with a flow rate of 20
standard cubic centimeters per minute. After the ALD process, the
sample was examined in a scanning electron microscope (JEOL
6
701F). The hybrid nanostructures were dispersed in distilled water at
various pH values and studied by UV/Vis absorption spectroscopy
Lambda 25 Spectrophotometer, Perkin-Elmer Inc.; UV-1602 UV/
Vis Spectrophotometer, Shimadzu) using quartz cuvettes with a
0 mm path length. For transmission electron microscopy (JEOL
(
[19] M. Tanaka, S. Hayashi, S. Eu. , T. Umeyama, Y. Matano, H.
Imahori, Chem. Commun. 2007, 2069.
1
JEM-1010), a 10 mL aliquot of the dispersion was dropped onto
carbon-coated copper grids (Plano) and allowed to stand for 2 min.
The sample was air-dried after removing the excess liquid with filter
paper.
The MALDI-TOF mass spectra were acquired with an Ultra-
flex II TOF-TOF mass spectrometer (Bruker Daltonics). For the
sample preparation, 0.5 mL of a saturated solution of a-cyano-4-
hydroxycinnamic acid in acetone was deposited onto the sample
target. A 1 mL aliquot of the sample was injected into a small drop of
water previously deposited onto the matrix surface.
[20] J. Jasieniak, M. Johnston, E. R. Waclawik, J. Phys. Chem. B 2004,
108, 12962.
[21] J. Rochford, E. Galoppini, Langmuir 2008, 24, 5366.
[22] Q. Zhao, M. Yu, T. Xie, L. Peng, P. Wang, D. Wang, Nano-
technology 2008, 19, 245706.
[23] A. Forneli, M. Planells, M. A. Sarmentero, E. Martinez-Ferrero,
B. C. OꢂRegan, P. Ballester, E. Palomares, J. Mater. Chem. 2008,
18, 1652.
[24] C.-Y. Chang, F.-Y. Tsai, S.-J. Jhou, M.-J. Chen, Org. Electron.
2008, 9, 667.
[
25] C. A. Wilson, P. K. Grubbs, S. M. George, Chem. Mater. 2005, 17,
Received: January 22, 2009
Revised: April 16, 2009
5625.
[26] C. A. Wilson, J. A. McCormick, A. S. Cavanagh, D. N. Gold-
Published online: June 2, 2009
stein, A. W. Weimer, S. M. George, Thin Solid Films 2008, 516,
6175.
[
27] S. M. Lee, E. Pippel, U. Gꢀsele, C. Dresbach, Y. Qin, C. V.
Chandran, T. Brꢁuniger, G. Hause, M. Knez, Science 2009, 324,
488.
Keywords: atomic layer deposition · metalation · porphyrinoids ·
vapor-phase infiltration · zinc
.
Angew. Chem. Int. Ed. 2009, 48, 4982 –4985
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 4985