Spatially Resolved Covalent Functionalization Patterns on Graphene

Article abstract of DOI:10.1002/anie.201810119

Spatially resolved functionalization of 2D materials is highly demanded but very challenging to achieve. The chemical patterning is typically tackled by preventing contact between the reagent and material, which brings various accompanying challenges. Photochemical transformation on the other hand inherently provides remote high spatiotemporal resolution using the cleanest reagent—a photon. Herein, we combine two competing reactions on a graphene substrate to create functionalization patterns on a micrometer scale via the Mitsunobu reaction. The mild reaction conditions allow introduction of covalently dynamic linkages, which can serve as reversible labels for surface- or graphene-enhanced Raman spectroscopy characterization of the patterns prepared. The proposed methodology thus provides a pathway for local introduction of arbitrary functional groups on graphene.

Full text of DOI:10.1002/anie.201810119

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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Accepted Article
Title: Spatially resolved covalent functionalization patterns on
graphene
Authors: Leoš Valenta, Petr Kovaříček, Václav Valeš, Zdeněk Bastl,
Karolina A Drogowska, Timotheus A Verhagen, Radek
Cibulka, and Martin Kalbáč
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201810119
Angew. Chem. 10.1002/ange.201810119
Link to VoR: http://dx.doi.org/10.1002/anie.201810119
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Spatially resolved covalent functionalization patterns on
graphene
Leoš Valenta,[a,b] Petr Kovaříček,*[a] Václav Valeš,[a] Zdeněk Bastl,[a] Karolina A. Drogowska,[a]
Timotheus A. Verhagen,[c] Radek Cibulka,[b] Martin Kalbáč*[a]
Abstract: Spatially resolved functionalization of 2–D materials is
highly demanded but very challenging to achieve. The chemical
patterning is typically tackled by preventing contact between the
reagent and material, with various accompanying challenges.
Photochemical transformation on the other hand inherently provides
remote high spatiotemporal resolution using the cleanest reagent – a
photon. Here, we combine two competing reactions on a graphene
substrate to create functionalization patterns on a micrometre scale
via the Mitsunobu reaction. The mild reaction conditions allow
introduction of covalently dynamic linkages, which can serve as
reversible labels for surface- or graphene-enhanced Raman
spectroscopy characterization of the patterns prepared. The proposed
methodology thus provides a pathway for local introduction of arbitrary
functional groups on graphene.
two approaches[31] because photochemistry responds to
illumination patterns without the need for a resist or spatially
confined reagent deposition.
The critical challenge for the surface functionalization of the
2–D materials is unambiguous characterization of the reaction
products facing the detection and resolution limits of many
methods due to the strict monolayer nature of the materials.[32]
Typical methods such as Raman spectroscopy[33–35] and X-ray
photoelectron spectroscopy[36] (XPS) provide only limited insight
into the actual chemistry taking place at the surface, an issue
which has been tackled recently by the use of surface- and
graphene-enhanced Raman spectroscopy (SERS and GERS,
respectively)[14,37] and also by other methods.[38,39] Application of
SERS/GERS allows direct assignment of characteristic
vibrational modes to particular species and thus provides solid
evidence for species being grafted to the material.
Graphene holds a prominent position among two-dimensional (2–
D) materials due to its unique properties,[1–3] which can be further
tuned for particular application[4–6] by several methods. Among
them, chemical functionalization offers the broadest variety of
possibilities.[7–9] Although several protocols for covalent grafting
have been reported in the literature,[10–15] only recently has the
advancement towards specific applications revealed the need for
spatially resolved methods that would provide patterned surface
functionalization on the 2–D material,[16–18] aiming at sophisticated
sensor arrays or device mass production.[19] This non-trivial task
has been approached from several directions, using for instance
photolithography,[20] dip-pen lithography[21–23] or various writing
techniques.[24–26] Using covalent modification for spatially
resolved functionalization typically relies on preventing contact of
the reagents with graphene, thus implying the use of a resist
mask[27] or controlled deposition[28,29] at the nanoscale, which
either adds more steps (with concomitant contamination[30]) or
severely limits the processing speed, respectively. Photochemical
reactions represent a unique alternative to the abovementioned
Here, we report on the use of a photo-modulated Mitsunobu
reaction on oxygen plasma-treated monolayer graphene to create
patterned functionalization. In this setup, the oxygenated
graphene serves both as the substrate for introduction of
carboxylate species as well as the reagent in the reductive de-
oxygenation by triphenylphosphine (PPh3). By the use of a
photomask
and
UV
light
inducing
azodicarboxylate
decomposition in irradiated areas, spatially resolved
functionalization patterns are created, which can then be detected
by Raman and SERS spectroscopy. The proposed method
provides a way to form spatially resolved arbitrary functional
group 2–D structures on graphene in an easy, clean and rapid
manner.
The Mitsunobu reaction involves PPh3 activated by dialkyl
azodicarboxylate (i-propyl in our case, DIAD) to convert a
hydroxyl into an easily leaving group, which is then replaced by a
nucleophile, such as carboxylate. On the oxygenated graphene,
both hydroxy and epoxy functions can undergo this
transformation. Several reports in the literature also employ
chemical reduction of graphene oxide by various agents,[40]
among others hydrazine[41] or phosphine[42] derivatives. Finally,
azodicarboxylates are sensitive to irradiation in the blue/UV
region, leading to their photochemical decomposition.[43] We have
thus outlined a system in which oxygenated graphene is
subjected to the Mitsunobu reaction under photomasked UV
irradiation, anticipating that a) the non-irradiated areas will
undergo the Mitsunobu substitution reaction and b) at the
irradiated part DIAD will decompose, leaving unreacted PPh3
available for oxygenated graphene reduction (Scheme 1).
[a]
L. Valenta, Dr. P. Kovaříček, Dr. V. Valeš, Dr. Z. Bastl, Dr. M.
Kalbáč, Dr. K. A. Drogowska
J. Heyrovský Institute of Physical Chemistry of the Czech Academy
of Sciences, Dolejškova 2155/3, 182 23 Praha, Czech Republic
E-mail: petr.kovaricek@jh-inst.cas.cz, martin.kalbac@jh-inst.cas.cz
L. Valenta, Prof. R. Cibulka
University of Chemistry and Technology Prague, Technická 5, 166
28 Praha, Czech Republic
[b]
[c]
Dr. T. A. Verhagen
Department of Condensed Matter Physics, Faculty of Mathematics
and Physics, Charles University, Ke Karlovu 5, 121 16 Prague 2,
Czech Republic
Supporting information for this article is given via a link at the end of
the document.
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Scheme 1. a) Scheme of chemical transformation of monolayer CVD-grown
graphene. Oxygen–plasma treatment causes introduction of, among others,
hydroxy- and epoxy-groups which can either be removed by the reverse
reaction with PPh3, or subjected to nucleophilic Mitsunobu substitution in the
presence of DIAD and PPh3. b) By photochemical decomposition of DIAD,
spatially resolved functionalization-reduction patterns can be prepared using a
photomask.
Figure 1. Raman spectra of as-transferred graphene on Si/SiO2 wafer (black),
after oxidation in oxygen plasma (red) and reduced back to pristine form by
PPh3 (blue). Oxidation leads to a considerable D mode visible in the spectra, but
the reduction leads to almost complete removal of oxidative defects.
Graphene was synthesized on copper foil using the chemical
vapor deposition method (CVD) and transferred onto Si/SiO2
wafers (300 nm SiO2 layer) by a polymer-assisted copper etching
method, as described previously.[30,44] Material quality was
confirmed by spectroscopic Raman mapping[33–35] showing the
typical G and 2D modes at approximately 1600 and 2700 cm-1,
respectively, without any detectable D mode. Oxygen plasma
treatment was then performed using the previously published
protocol,[45] which resulted in appearance of clear D mode at 1346
cm-1 (Figure 1). This process produces various oxygen–
containing functional groups on graphene, namely hydroxyls,
epoxy bridges, carbonyl and carboxyl functions,[46,47] it is therefore
necessary first to investigate the feasibility of the oxygenated
graphene reduction by PPh3. The sample with median ID/IG ratio
of 0.51 was immersed into the PPh3 solution (20 mM) in
dichloroethane (DCE) for 12 hours at room temperature, after
which the sample showed a strong decrease of the D band to
median ID/IG = 0.17. Using the published model,[48] the change in
the D mode intensity corresponds to the average ‘defect’ distance
changing from 16 nm to 29 nm. Hence, PPh3 is a feasible reducing
agent for oxygenated graphene, yet some oxidative defects,
presumably carbonyl and carboxyl functions, may remain at the
surface (Figure 1). It is also important to note that the reduction
efficiency decreases rapidly with increasing oxidation degree (i.e.
with an ID/IG ratio above 0.7) as a result of graphene over–
oxidation to carbonyl and carboxyl stages.
In the Mitsunobu reaction, hydroxy and epoxy groups on the
oxygen plasma–treated graphene were subjected to formal
nucleophilic exchange by p-bromobenzoic acid.[49] The bromine
atom is a suitable probe for quantification of the on-surface
reaction yield. Performing the reaction on oxygenated graphene
with PPh3 and DIAD (20 mM each reagent) in DCE for three hours
under ambient conditions did not result in a significant change of
the ID/IG ratio, indicating that PPh3 activation by DIAD is a faster
process than graphene reduction and thus the concentration of
sp3 ‘defects’ in the monolayer remains rather constant during the
reaction. Based on the XPS data, the C:Br atomic concentration
ratio was determined to be 60:1 as a result of substitution of only
part of the oxygen-containing groups created by the plasma
treatment. The product of the Mitsunobu reaction was further
confirmed by SERS after evaporation of 12.5 nm silver film on top
of the sample. The SERS spectra provided homogeneously
distributed characteristic vibrational bands, which have been
attributed to the p-bromobenzoate ester species bound to the
surface. The signal assignment was performed using a bulk-
synthesized benzyl p-bromobenzoate as
a reference (see
Supplementary information, SI) as follows (in cm-1): 1586 C=C str.,
1280 and 1140 for asym and sym C-O-C str., 1073 for Ar-Br, 930
for C-C-O str. and 896 for C-O-C def; other bands overlap with
graphene signals and were not considered in the assignment.
Photochemical decomposition of DIAD provides the
opportunity to switch the reaction pathway on oxygenated
graphene remotely with high spatiotemporal resolution. To verify
the concept, we have drop-casted the reaction mixture onto
oxygen-plasma treated graphene, placed a half-masked cover
slide to create only a thin liquid film and placed the sample under
a laboratory UV lamp for three hours. After removal of the mask
and sample washing, Raman spectra were recorded on both
irradiated and photomasked regions. The non-irradiated area
showed merely an unchanged ID/IG ratio of 0.57, but at the
irradiated parts, the ID/IG ratio decreased to 0.27. These results
clearly demonstrate that decomposition of the DIAD reagent in the
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10.1002/anie.201810119
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irradiated parts leaves PPh3 in the solution, which is then
responsible for the reductive de-oxygenation of graphene, but in
Visualization of the functionalization patterns by SERS is a
powerful technique. However, in some cases it can be impractical
the masked regions the Mitsunobu reaction proceeds undisturbed. because evaporation of the silver film on the sample is a
destructive step; thus, we have designed a different approach
Spatial control of the reaction pathway is in principle limited
only by the diffraction limit of UV light, therefore it should be
using GERS and dynamic covalent bonds.[37,50] In this setup,
Raman signals of
a given molecule are enhanced (and
possible to adopt photolithography to the fine patterning of the
graphene functionalization. As described above, different
transformations were anticipated to occur in irradiated vs. non–
irradiated areas. The reaction was performed in a DCE liquid film
in between the Si/SiO2 substrate with oxygenated graphene and
a metal on a glass photomask. After three hours of reaction under
UV light at 30 mM concentration of all components, the sample
was thoroughly washed and 12.5 nm thick Ag film was evaporated
on top of it. Results of the SERS mapping are shown in Figure 2.
Indeed, the mapped intensity of graphene’s 2D mode and the
ester C-O-C and C-C-O vibrations are fully complementary,
showing the 5-μm wide stripe pattern imprinted by the photomask.
This pattern is in fact even visible in optical microscope because
the Ag film exhibited slightly different morphology as observed
with scanning electron microscopy.
luminescence quenched) by interactions with graphene when
more fundamental conditions are met without the need for further
sample processing. Also, the dynamic linkage between graphene
and the selected molecule enables for subsequent removal of the
labelling species from the surface and its reuse for the final
application. Hence, we have performed the photomasked
Mitsunobu reaction with 4-formylbenzoic acid in acetonitrile in a
similar manner as in the previous case and afterwards reacted
with 7-(diethylamino)coumarin-3-carbohydrazide. The reaction
sequence functionalized the CVD graphene with aldehyde
moieties, which then underwent reversible condensation with the
hydrazide to provide the corresponding acylhydrazone. The
coumarin species features a bandgap which is in resonance with
a 532 nm excitation laser and thus the essential condition for
GERS is fulfilled.[37] Optical images did not show any visible
features on the surface; however, Raman mapping clearly
resolved the photoimprinted stripped patterns periodically
changing between strong coumarin Raman signals and pristine
graphene spectra (Figure 3).
Figure 3. Functionalization patterns visualized by GERS. a) Raman spectra
recorded at irradiated (black) and non-irradiated (green) areas showing strong
GERS enhancement of coumarin moiety. b) Optical picture with area subjected
to Raman mapping shown in green, no macroscopic features can be observed.
c) Raman map with plotted intensity of coumarin signals (red scale bar = 5 μm).
Figure 2. a) Raman spectra of graphene recorded after oxidation (black) and
after photomasked Mitsunobu reaction at non-irradiated (red) and irradiated
(blue) regions. b) Image taken after Ag film evaporation, a line is visible due to
different film morphology. c) SERS spectra taken at irradiated (red) and non-
irradiated (blue) areas, C-O-C def vibration is only present in non-irradiated part
(d) and vice versa 2D mode appears only at irradiated one (e), band at 850 cm-
1 due to the omnipresent C-O def vibration (black scale bar = 5 μm).
We have demonstrated a photochemical spatially resolved
functionalization using a Mitsunobu reaction and/or reduction of
oxygenated CVD graphene. Mild graphene oxidation by oxygen
plasma generates predominantly hydroxy and epoxy groups on
the sp2 carbon monolayer, both of which were efficiently reduced
by the action of PPh3. In presence of DIAD, these groups are
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Keywords: graphene • patterned functionalization • Raman
spectroscopy • Mitsunobu reaction • photochemistry
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Entry for the Table of Contents
Layout 1:
COMMUNICATION
Patterns of chemical functional groups
are grafted to graphene via
Leoš Valenta, Petr Kovaříček,* Václav
Valeš, Zdeněk Bastl, Karolina A.
Drogowska, Timotheus A. Verhagen,
Radek Cibulka, Martin Kalbáč
photomodulated Mitsunobu reaction.
Enhanced Raman spectroscopy
employing covalently dynamic
linkages provide easy and direct
mapping of the structures prepared.
Arbitrary patterns with micrometer
resolution can be prepared.
Page No. – Page No.
Spatially resolved covalent
functionalization patterns on
graphene
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