Covalently patterned graphene surfaces by a force-accelerated Diels-Alder reaction

Article abstract of DOI:10.1021/ja4042077

Cyclopentadienes (CPs) with Raman and electrochemically active tags were patterned covalently onto graphene surfaces using force-accelerated Diels-Alder (DA) reactions that were induced by an array of elastomeric tips mounted onto the piezoelectric actuators of an atomic force microscope. These force-accelerated cycloadditions are a feasible route to locally alter the chemical composition of graphene defects and edge sites under ambient atmosphere and temperature over large areas (～1 cm²).

Full text of DOI:10.1021/ja4042077

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Communication
Covalently Patterned Graphene Surfaces by
a Force Accelerated Diels-Alder Reaction
Shudan Bian, Amy M Scott, Yang Cao, Yong Liang, Sílvia Osuna, K. N. Houk, and Adam B. Braunschweig
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja4042077 • Publication Date (Web): 11 Jun 2013
Downloaded from http://pubs.acs.org on June 14, 2013
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Covalently Patterned Graphene Surfaces by a Force
Accelerated Diels-Alder Reaction
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Shudan Bian, Amy M. Scott, Yang Cao, Yong Liang, Sílvia Osuna, K. N. Houk, * and
1
Adam B. Braunschweig *
1
Department of Chemistry, University of Miami, Coral Gables, FL 33146
2
Department of Chemistry and Biochemistry, University of California, Los Angeles, CA 90095
Supporting Information Placeholder
ABSTRACT: Cyclopentadienes (CPs) with Raman and
electrochemically active tags were patterned covalently onto
graphene surfaces using forceꢀaccelerated DielsꢀAlder (DA)
reactions that were induced by an array of elastomeric tips
mounted onto the piezoelectric actuators of an atomic force
microscope. These forceꢀaccelerated cycloadditions are a feasible
route to locally alter the chemical composition of graphene
defects and edge sites under ambient atmosphere and temperature
molecules into patterns onto the basal plane of graphene through
the formation of two new C–C bonds (Fig. 1a).
2
over large areas (~1 cm ).
Graphene has become the focus of considerable research attention
because of its high conductivity, 2D structure, and superior
1
mechanical properties. Patterning onto the basal plane of
graphene may increase the bandgap of graphene, a boon for
integrated optics and electronics, or for the fabrication of
2
grapheneꢀbased sensors. However,
a consequence of the
stabilizing πꢀconjugation of graphene is that the basal plane is
resistant towards chemical functionalization, so carrying out
organic chemistry on graphene siteꢀspecifically is particularly
challenging. Scalable methods to covalently pattern organic
molecules onto graphene have not, to the best of our knowledge,
been demonstrated. Functional molecules have been anchored to
Figure 1. a) DA oligomerization reaction between functionalized CP and
a single layer graphene (SLG) surface. b) Cy3ꢀcontaining Raman active 1
and ferroceneꢀcontaining electrochemically active 2 ink molecules used to
confirm forceꢀaccelerated patterning. c) An elastomeric tipꢀarray. d) The
tipꢀarray coated with an ink mixture (red) consisting of a CP and
poly(ethylene glycol) (PEG). e) The inked tip array is pushed into the
SLG surface. f) Following rinsing of the surface to remove the PEG and
excess CP, only the covalently immobilized molecules remain on the
surface.
3
graphene using noncovalent interactions or bonding to oxidized
2
d,4
5
defect sites and edges.
Alternatively, photochemical, dipolarꢀ
6
2b,7
cycloadditions, and diazonium salt
reactions couple organics
directly with the basal plane of graphene, albeit not in patterns
and with an input of energy that would denature or destroy soft
matter. The recent report by Haddon et al. proposing that single
layer graphene (SLG) participates in DA reactions as a dienophile
To demonstrate that force accelerated cycloadditions
could covalently pattern large areas (~1 cm ) of SLG sheets, we
2
used an elastomeric tip array mounted onto the piezoelectric
actuators of an atomic force microscope (AFM) to exert a
localized force between functionalized CPs and SLG sheets.
These tip arrays are commonly used for Polymer Pen
o
8
at temperatures as low as 50 C over 3 h suggested to us
conditions that could be used to scalably pattern graphene at
ambient temperatures and atmosphere. Because of their negative
11
Lithography, where patterns are formed by ink transfer from the
tips to the surface through an aqueous meniscus. Moreover, their
9
activation volumes, cycloaddition reactions are significantly
2
accelerated in pressurized reaction vessels, and the Reinhoudt and
Ravoo groups have covalently micropatterned various surfaces by
large areas (>1 cm ) and the computer controlled movement of
the piezoactuators that hold the array provide high throughput and
I
I
inducing Cu ꢀfree Huisgen and DA reactions through applied
flexible pattern design. We have recently induced both the Cu ꢀ
1
0
12
force between inked elastomeric stamps and surfaces. Thus, we
reasoned that a localized force exerted between SLG and a diene
would accelerate the DA reactions and thereby immobilize
catalyzed azideꢀalkyne click reaction and the Staudinger
13
ligation on Au and SiO surfaces with these tip arrays under
2
ambient temperatures and pressures, confirming the suitability of
these tipꢀarrays for covalently immobilizing soft matter
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nondestructively through selective organic transformations.
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Because the elastomeric tips also compress upon contact with
surfaces, they can apply a predictable force between molecular
1
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inks and a surface, so in this experiment, the position, force, and
reaction time can all be controlled precisely to pattern surfaces
2
over cm areas with micrometerꢀscale features.
Ramanꢀactive cyanine 3 (Cy3) containing CP 1 and
electrochemicallyꢀactive ferrocene CP 2 (Fig. 1b) were designed
to characterize the bonding upon cycloaddition between the SLG
surface and the CPs. CPs react quickly in DA reactions because of
their geometric preorganization, and as a result, they have been
8a,15
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utilized already in the context of surface patterning.
The
Haddon group used Raman spectroscopy to follow a DA reaction
ꢀ
1
on graphene and found that the D band at 1324 cm that
2
corresponds to the A1g breathing vibration of sp carbon rings,
Figure 2. a) Light microscopy image (10x magnification) of 2 x 3 dot
arrays of a mixture containing 1 and PEG, with varying dwell times (30,
7, 24, 21, 18, 15 min), that were patterned by each pen in the tip array.
which is suppressed in pure graphene, increases significantly
because the introduction of defects or covalently adsorbed
molecules reduces the symmetry of the graphene lattice. The ratio
of the Dꢀ and Gꢀband integrations (I /I ) is a measure of the
2
Scale bar is 200 ꢁm. Inset is a magnified image of one array. Scale bar is
–1
D
G
20 ꢁm. b) Raman mapping image (1324 cm , D band) of 2 x 3 dot arrays
1
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degree of functionalization of graphene.
Alternatively,
of 1 covalently immobilized onto the SLG. Scale bar is 20 ꢁm. c) AFM
image of a single feature printed onto the SLG. d) Height profile of a
single feature of 1 patterned onto SLG.
electrochemistry can confirm the immobilization of the ink onto
the surface and quantify the density of surfaceꢀbound
12,17
molecules.
Molecules 1 and 2 were synthesized and characterized
The spectra associated with different points on the
Raman map further confirmed that the changes in bonding were
produced because of localized DA reactions (Fig. 3). A Raman
spectrum taken at a point where the inked tips were pressed into
1
13
by H NMR, C NMR, and highꢀresolution mass spectrometry,
and all analytical data were consistent with the proposed
18
structures. The 8500 tip PDMS arrays with a tipꢀtoꢀtip spacing
the surface had peaks of 1 as well as an increased I /I value of
of 80 or 160 ꢁm were prepared following previously published
D G
11
0
.56, compared to 0.14 for the unaltered surface, 0.17 where the
literature protocols. To induce the DA reaction between 1 and
the SLG surface, 1 (0.8 mg, 1.2 mmol) and poly(ethylene glycol)
tips had been pressed into the surface in the absence of 1, and
19
ꢀ
1
ꢀ1
0
.16 for the original SLG surface. The changes in the ID/I
G
(
PEG) (2000 g mol , 10 mg ml ) in 0.8 mL 60 : 20 THF:H O,
2
indicate that the forceꢀinduced reaction of 1 with SLG converts
which was sonicated to ensure solution homogeneity, were spin
coated (2000 rpm, 2 min) onto a tip array. The PEG matrix that
encapsulates the CPs ensures even distribution of ink across the
tip array and that ink transport from the tips to a surface is
2
3
some C–C bonds in the SLG from sp to sp consistent with
,
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previous observations for DA reactions on SLG surfaces.
1
2
predictable and reproducible. The tips were mounted onto the zꢀ
piezo of an AFM that was specially equipped with an apparatus to
hold the tip arrays, an environmental chamber to regulate the
humidity, and customized lithography software to control the
1
8
position, force, and dwellꢀtime of the tips (Fig. 1c). A 2x3 dot
pattern of 1 with featureꢀtoꢀfeature spacing of 20 ꢁm was
produced by each tip in the array by pushing the tips into the SLG
surface (SLG on 285 nm SiO ) at times ranging from 15ꢀ30 min
2
14
and a force of ~100 mN at each spot. The transfer of the 1/PEG
mixture into 2x3 patterns and an approach dot to level the arrays
on the surface was confirmed by light microscopy (Fig. 2a).
After washing the surfaces with EtOH and H O to
2
remove unreacted 1 and PEG, the surface bonding was analyzed
by Raman microscopy (WITec, 633 nm excitation). A Raman
map of the surface that was obtained following force accelerated
printing of 1 revealed a 2 x 3 pattern of features where I was
D
elevated significantly compared to surrounding areas (Fig. 2b).
The 20 ꢁm spacing between features in the 2 x 3 patterns
observed in the Raman map matched with the pattern of features
1
8
printed by the pen array. The elevated ID was observed at all
points where the tips were pressed into the surface for all dwell
times above 15 min, but poor signal was obtained for dwell times
below 12 min. Importantly, control experiments where 1 was not
present in the ink mixture or where 1 was present, but force was
not applied to the surface upon ink transfer, did not produce
similar patterns or significantly elevated I /I , confirming that the
Figure 3. a) Raman spectrum of 1 on a SiO
from a map with force accelerated printing of 1 (Fig. 2b) taken at a
position with increased Dꢀband intensity (I ) compared to unreacted SLG
also has peaks corresponding to 1. c) Raman spectrum from the map of
the same printed surface taken at a position without increased I . d)
2
surface. b) Raman spectrum
D
G
D
diene and force are both necessary for changes in bonding to
occur. AFM height images show the presence of elevated surfaces
where the dienes had been printed (Fig. 2c and 2d), with a height
of ~3.5 nm, which is too high for a monolayer of 1.
D
Raman spectrum of control experiment where PEG and force were applied
to the SLG surface without 1, taken at a point where the tips were pushed
into the SLG surface. e) Raman spectrum of pure SLG. Red lines mark
–
1
peaks of 1 (11124, 1157, 1178, 1272, 1387, 1594 cm ). Blue lines mark
–1
SLG peaks (1324, 1584 cm ).
2
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Electrochemically active CP 2 was patterned onto SLG
similar protocol described above, and the
immobilization density of 2 on the SLG surface was analyzed by
bonds cannot be functionalized through DA reactions with CPs,
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following
a
and only some special edges, comparable to defect sites, will be
reactive. However, once the CP has been attached to the edge
positions, it might either activate nearby bonds or itself react. The
DA reaction of a second CP on the grapheneꢀCP cycloadduct
(functionalized on bond “a”) was also calculated (Fig. 5b). Five
additional bonds were evaluated (“a”, “b”, “c”, “d”, and “e”, Fig.
5b). The new reaction enthalpies for the CP addition on bonds
“a”, “b”, and “c” of the grapheneꢀCP cycloadduct (Table S2) are
practically unchanged. This clearly indicates that the
functionalization at the edge bond “a” does not favor the
subsequent CP addition on the graphene lattice. The reaction
enthalpies on neighboring bonds “d” and “e” are 1.6 and 37.4
kcal/mol, respectively. Cycloaddition on bond “e” is impossible
because of high endothermicity. Bond “d” can be viewed as the
edge bond on a 4x4 graphene model and thus possesses a
reactivity comparable to bond “a”. The enthalpy of 1.6 kcal/mol
on bond “d” indicates that the CP group has in fact deactivated its
nearby bonds by steric hindrance.
cyclic voltammetry (CV). Each tip in the pen array produced a 2 x
2
3
dot pattern over the 1 cm area with an average feature edge
2
length of 7.1 ꢁm and area of 50.4 ꢁm . Following washing of the
surface with EtOH and H O to remove the PEG and unreacted 2,
2
CV was carried using a Ag/AgCl reference electrode, a Pt counter
electrode, and the patterned SLG as the working electrode. A
o
strong redox peak at E = 590 mV (vs. Ag/AgCl) confirmed the
+
presence of the ferrocene (fc)/ferrocenium (fc ) reversible redox
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couple from 2 (Fig. 4), which is shifted anodically compared to fc
because of the ester linking the fc to the CP. Control experiments
where 2 was deposited without force did not result in any
+
observable current corresponding to the fc/fc redox couple after
washing, confirming that force is necessary to induce the DA
reaction under these conditions. The linear relationship between
peak current and scan rate confirmed that 2 is immobilized on the
1
7,19
SLG surface,
but that the localized changes in bonding from
2
3
sp to sp do not prevent conduction through the SLG.
ϚΊ
Interestingly, the increase in ΔE with scan rate ( Ϛ/ ) as well as
ϚΊ
the slope of 0.7 in the ln(Ϛ/) vs. ln(I), indicates complexity in the
charge transfer from the fc to the SLG. From integration of the
1
4
ꢀ2
CV, a charge density ( fc) of (5.34 ± 0.76) x 10 cm was
Г
1
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obtained, which is significantly higher than would be expected
from the increase in the Dꢀband intensity of the Raman spectrum.
Figure 5. a) The 5x5 graphene model with the representative addition
sites in red. b) DA reaction sites of the second CP on grapheneꢀCP
cycloadduct. c) Structures of graphene, grapheneꢀCP cycloadduct, and the
following CP dimerization product, reaction enthalpies in kcal/mol.
From CV data, we determine that approximately 1 fc is
present on the surface for every 5 graphene double bonds, while
calculations indicate that such a high coverage is not attainable
because most of the graphene double bonds are unreactive with
CP. How can the differences between experiment and
computation be explained? Inspired by recent report of the
Figure 4. Cyclic voltammetry (CV) of 2 on SLG using a Pt counter
electrode and Ag/AgCl reference electrode in 0.1 M HClO electrolyte.
4
The inset shows the relationship between Ln(scan rate) and Ln(current).
20
functionalization of graphene by polymerization, we postulate
that CP also oligomerizes through DA reactions. Fig. 5c shows
structures of the grapheneꢀCP cycloadduct and its CP
dimerization product. The double bond of the grapheneꢀCP
cycloadduct resembles that of norbornene. The reaction enthalpy
for the cycloaddition of a second CP is –25.8 kcal/mol. This is
significantly more exothermic than any of the reactions of
graphene calculated above (the most reactive site on graphene
model is bond “a” with ∆H of –11.3 kcal/mol). Once one CP
reacts with a reactive edge or defect on graphene, the second CP
can react with the norbornene double bond. This can be repeated
because CP is well known to dimerize and polymerize through
To further study the reaction between CP and SLG as
well as explain the CV and AFM results, DFT calculations were
conducted for DA reactions of CP on three representative bonds
in a 5x5 graphene model (Fig. 5a). Hydrogenꢀsubstituted edges
2d,4
were used, although the nature of the edge is likely complex.
The corner bond “a” can be viewed as the joint part of zigzag and
armchair edges of graphene. Another periphery bond “b”
represents the edges, and the center bond “c” most resembles the
pristine graphene interior. All structures were optimized at
(
U)M06ꢀ2X/6ꢀ31G(d) level, and single point energy calculations
were carried on the optimized structures at the (U)M06ꢀ2X/6ꢀ
21
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DA reactions. Oligomerization of CP induced by initial Dielsꢀ
Alder reaction at a graphene defect is preferred over multiꢀsite
functionalization. On the basis of the CV characterization and
feature height of 3.5 nm, which would correspond to a degree of
3
11G(d,p) level. We report here only the situation of graphene
acting as dienophile, and CP as diene. Two more reaction
18
pathways were also calculated but found to be unfavorable.
Computational results (Table S1) show that the Dielsꢀ
Alder cycloadditions at bonds “a” and “b” have reaction
enthalpies of –11.3 and –1.4 kcal/mol, respectively. However,
bond “c” involves unfavorable, endothermic enthalpy (36.6
kcal/mol) under standard conditions. The bond “c” most
resembles the interior of pristine graphene. The thermochemical
calculations of single CP on graphene demonstrate that center
18
CP oligomerization of 15, the functionalization degree of
graphene is about 1.3%. Finally, the dependence of ꢂE with scan
rate of the CV and the slope of 0.7 in the inset of Fig. 4 can be
attributed to hopping of electrons through the fc chains appended
to the CP oligomers.
3
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In conclusion, SLG sheets on SiO substrates have been
patterned covalently with oligomers of organic small molecules
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through a forceꢀaccelerated DA reaction induced on graphene
defect and edge sites by an array of pyramidal elastomeric tips.
The changes in bonding were characterized by Raman
microscopy, cyclic voltammetry, and electronic structure
calculations, and the results are consistent with micrometer scale
features composed of covalently immobilized molecules patterned
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2
over large (cm ) areas. Importantly, these reactions occur at
ambient temperature and atmosphere, while accessing one of the
most versatile reactions in organic chemistry. Future studies will
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and elucidate the role of force on reaction rate, reversibility, and
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could find utility in sensors, electronics, and optical devices.
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ASSOCIATED CONTENT
Supporting Information
Organic synthesis, microfabrication and lithography, Raman and
electrochemical characterization, control experiments, and computational
details. This material is available free of charge via the Internet at
http://pubs.acs.org.
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B. J.; Reinhoudt, D. N. Angew. Chem. Int. Ed. 2006, 45, 5292; b)
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AUTHOR INFORMATION
(
Corresponding Author
a.braunschweig@miami.edu, houk@chem.ucla.edu
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ACKNOWLEDGMENT. A.B.B. is grateful to the Air Force Office of
Scientific Research (Young Investigator Award #FA9550ꢀ11ꢀ1ꢀ0032) and
the National Science Foundation (DBIꢀ115269) for generous support, to
Prof. Chad A. Mirkin for use of the Raman microscope, and Prof. A. E.
Kaifer for helpful discussions. K.N.H. is grateful to the National Science
Foundation (CHEꢀ1059084) for financial support. S.O. acknowledges the
European Community for the postdoctoral fellowship (PIOFꢀGAꢀ2009ꢀ
52856). Calculations were performed on the Hoffman2 Cluster at UCLA
and the Extreme Science and Engineering Discovery Environment
XSEDE), which is supported by the NSF (OCIꢀ1053575).
(
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2
(18) See Supporting Information for details.
(19) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce,
A. M. J. Am. Chem. Soc. 1990, 112, 4301.
(
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(20) Ma, X.; Li, F.; Wang, Y.; Hu, A. Chem. Asian J. 2012, 7,
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B.; Chiu, H. Y.; Grill, A.; Avouris, P. Science 2010, 327, 662; b)
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Shih, C. J.; Ham, M. H.; SanchezꢀYamagishi, J. D.; Watanabe,
K.; Taniguchi, T.; Kong, J.; JarilloꢀHerrero, P.; Strano, M. S. Nat.
Chem. 2012, 4, 724; c) RodriguezꢀLopez, J.; Ritzert, N. L.; Mann,
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Wang, F.; Zhang, X. Nature 2011, 474, 64.
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5) Liu, H. T.; Ryu, S. M.; Chen, Z. Y.; Steigerwald, M. L.;
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Figure 1. a) DA oligomerization reaction between functionalized CP and a single layer graphene (SLG)
surface. b) Cy3-containing Raman active 1 and ferrocene-containing electrochemically active 2 ink molecules
used to confirm force-accelerated patterning. c) An elastomeric tip-array. d) The tip-array coated with an ink
mixture (red) consisting of a CP and poly(ethylene glycol) (PEG). e) The inked tip array is pushed into the
SLG surface. f) Following rinsing of the surface to remove the PEG and excess CP, only the covalently
immobilized molecules remain on the surface.
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Figure 2 a) Light microscopy image (10x magnification) of 2 x 3 dot arrays of a mixture containing 1 and
PEG, with varying dwell times (30, 27, 24, 21, 18, 15 min), that were patterned by each pen in the tip
array. Scale bar is 200 ꢀm. Inset is a magnified image of one array. Scale bar is 20 ꢀm. b) Raman mapping
image (1324 cm–1, D band) of 2 x 3 dot arrays of 1 covalently immobilized onto the SLG. Scale bar is 20
ꢀ
m. c) AFM image of a single feature printed onto the SLG. d) Height profile of a single feature of 1
patterned onto SLG.
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Figure 3 a) Raman spectrum of 1 on a SiO2 surface. b) Raman spectrum from a map with force accelerated
printing of 1 (Fig. 2b) taken at a position with increased D-band intensity (ID) compared to unreacted SLG
also has peaks corresponding to 1. c) Raman spectrum from the map of the same printed surface taken at a
position without increased ID. d) Raman spectrum of control experiment where PEG and force were applied
to the SLG surface without 1, taken at a point where the tips were pushed into the SLG surface. e) Raman
spectrum of pure SLG. Red lines mark peaks of 1 (11124, 1157, 1178, 1272, 1387, 1594 cm–1). Blue lines
mark SLG peaks (1324, 1584 cm–1).
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Figure 4 Cyclic voltammetry (CV) of 2 on SLG using a Pt counter electrode and Ag/AgCl reference electrode
in 0.1 M HClO4 electrolyte. The inset shows the relationship between Ln(scan rate) and Ln(current).
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Figure 5. a) The 5x5 graphene model with the representative addition sites in red. b) DA reaction sites of
the second CP on graphene-CP cycloadduct. c) Structures of graphene, graphene-CP cycloadduct, and the
following CP dimerization product, reaction enthalpies in kcal/mol.
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