Photoinduced C-C reactions on insulators toward photolithography of graphene nanoarchitectures

Article abstract of DOI:10.1021/ja412868w

On-surface chemistry for atomically precise sp² macromolecules requires top-down lithographic methods on insulating surfaces in order to pattern the long-range complex architectures needed by the semiconductor industry. Here, we fabricate sp²-carbon nanometer-thin films on insulators and under ultrahigh vacuum (UHV) conditions from photocoupled brominated precursors. We reveal that covalent coupling is initiated by C-Br bond cleavage through photon energies exceeding 4.4 eV, as monitored by laser desorption ionization (LDI) mass spectrometry (MS) and X-ray photoelectron spectroscopy (XPS). Density functional theory (DFT) gives insight into the mechanisms of C-Br scission and C-C coupling processes. Further, unreacted material can be sublimed and the coupled sp²-carbon precursors can be graphitized by e-beam treatment at 500 °C, demonstrating promising applications in photolithography of graphene nanoarchitectures. Our results present UV-induced reactions on insulators for the formation of all sp ²-carbon architectures, thereby converging top-down lithography and bottom-up on-surface chemistry into technology.

Full text of DOI:10.1021/ja412868w

Article
pubs.acs.org/JACS
Photoinduced C−C Reactions on Insulators toward Photolithography
of Graphene Nanoarchitectures
Carlos-Andres Palma,*,† Katharina Diller,† Reinhard Berger,‡ Alexander Welle,§ Jonas Bjork,∥
̈
Jose Luis Cabellos,⊥ Duncan J. Mowbray,⊥ Anthoula C. Papageorgiou,† Natalia P. Ivleva,¶ Sonja Matich,□
Emanuela Margapoti,□ Reinhard Niessner,¶ Bernhard Menges,‡ Joachim Reichert,† Xinliang Feng,‡
Hans Joachim Rader,‡ Florian Klappenberger,*,† Angel Rubio,*,⊥,# Klaus Mullen,*,‡
̈
̈
and Johannes V. Barth†
†Physik-Department, Technische Universitat Munchen, James-Franck-Strasse 1, 85748 Garching, Germany
̈
̈
‡Max-Planck-Institut fur Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany
̈
§Institut fur Funktionelle Grenzflachen, Karlsruher Institut fur Technologie, Hermann-von-Helmholz-Platz 1, 76344
̈
̈
̈
Eggenstein-Leopoldshafen, Germany
∥Department of Physics, Chemistry and Biology, IFM, Linkoping University, 58183 Linkoping , Sweden
̈
̈
⊥Nano-Bio Spectroscopy Group and ETSF Scientific Development Centre, Departamento de Física de Materiales, Centro de Física
de Materiales CSIC-UPV/EHU-MPC DIPC, Universidad del País Vasco UPV/EHU, Av. Tolosa 72, E-20018 San Sebastian, Spain
¶Chair for Analytical Chemistry, Institute of Hydrochemistry, Technische Universitat Munchen, Marchioninistr. 17, 81377 Munchen,
́
̈
̈
̈
Germany
□Walter Schottky Institut and Physik-Department, Technische Universitat Munchen, Am Coulombwall 4a, 85748 Garching,
̈
̈
Germany
#Fritz-Haber-Institut Max-Planck-Gesellschaft, Berlin, Germany
S
* Supporting Information
ABSTRACT: On-surface chemistry for atomically precise sp2
macromolecules requires top-down lithographic methods on
insulating surfaces in order to pattern the long-range complex
architectures needed by the semiconductor industry. Here, we
fabricate sp2-carbon nanometer-thin films on insulators and
under ultrahigh vacuum (UHV) conditions from photocoupled
brominated precursors. We reveal that covalent coupling is
initiated by C−Br bond cleavage through photon energies
exceeding 4.4 eV, as monitored by laser desorption ionization
(LDI) mass spectrometry (MS) and X-ray photoelectron spectroscopy (XPS). Density functional theory (DFT) gives insight
into the mechanisms of C−Br scission and C−C coupling processes. Further, unreacted material can be sublimed and the
coupled sp2-carbon precursors can be graphitized by e-beam treatment at 500 °C, demonstrating promising applications in
photolithography of graphene nanoarchitectures. Our results present UV-induced reactions on insulators for the formation of all
sp2-carbon architectures, thereby converging top-down lithography and bottom-up on-surface chemistry into technology.
index metallic surfaces5,7 as a catalytic requirement for C−C
INTRODUCTION
■
On-surface engineering of covalent sp2 architectures1,2 is a
growing discipline that strongly relies on scanning probe
characterization on well-defined surfaces. Today a hand-full of
molecular and atomically precise materials3−5 have been
fabricated by exploiting thermomolecular reactions occurring
mostly at metallic interfaces. Just recently, molecular materials
of technological importance belonging to the family of
graphene nanoribbons have been synthesized by the C−Br
dissociation and aryl−aryl coupling of a brominated precursor
on Au(111) followed by dehydrogenation at 400 °C4 for
graphitization.6 Using well-defined single-crystal playgrounds,
most on-surface synthesis work has been carried out on low-
coupling. The alternate approach of using nonmetallic
substrates has been rarely investigated.8,9 More importantly,
the aforementioned on-surface reactions occur in 2D confine-
ment with reduced dimensionality. Thus, the thermomolecular
reaction conditions are highly dependent on diffusion of the
precursors on the surface.10 The colossal task simultaneously
predicting the structure chemistry and the final absolute spatial
orientation of a molecular structure, represents a major obstacle
for the transition of on-surface chemistry into technology. Such
Received: December 18, 2013
Published: February 13, 2014
© 2014 American Chemical Society
4651
dx.doi.org/10.1021/ja412868w | J. Am. Chem. Soc. 2014, 136, 4651−4658

Journal of the American Chemical Society
Article
a
Scheme 1. Coupling Reaction Intermediates and Graphitization
a
(a) Upon UV irradiation in nanometer-thin films under UHV conditions, 10-bromo-9,9′-bianthryl (bba, 432 g·mol−1) can react producing 9,10-
tetraanthrylene, (ta, 706 g·mol−1), Br-ta (786 g·mol−1) or the ta isomer (ta′), among others. (b) 10,10′-dibromo-9,9′-bianthryl (dbba, 510 g·mol−1)
undergoing analogous UV-induced reactions. (c−f) General procedure: (c) molecular beam epitaxy (MBE) of precursor multilayers (bba or dbba)
on a mica or silicon oxide/silicon wafer, (d) monitoring UV reactions via laser desorption ionization (LDI) mass spectrometry. (e) UV irradiation of
multilayers and (f) e-beam annealing treatment of multilayers. Yellow and orange drawings refer to the molecular structures in (a) and (b).
Figure 1. Photoinduced C−C coupling reaction intermediates via direct laser desorption ionization. (a) LDI MS-ToF spectra for bba with (black)
and dbba (red) at 20 mJ cm−2 irradiation power using a 3.7 eV (337 nm) nanosecond-pulsed laser. (b) Detail of the bba [C56H34Br]+ region
showing the Br-ta structure in Scheme 1. (c) Detail of the bba [C84H51Br]+ region. (d) Detail of the dbba n = 2 oligomer [C56H32Br2]+ region and
(e) the dbba n = 3 oligomer region showing a tribrominated adduct [C84H48Br3]+. Simulated isotopic distributions are shown in blue dotted lines in
(b−e). (f) TD-DFT calculated C−Br dissociation adiabatic pathways for dbba, where 1.92 Å is the equilibrium distance. Dissociation channels (1)
and (2) can be (indirectly) excited 3.7 and 4.4 eV, respectively, at a C−Br bond distance of 1.96 eV.
control is essential for the fabrication of large-area carbon sp2
circuits.2 To simplify this challenge, the semiconductor industry
requires a top-down approach for directly patterning on-surface
chemistry products. One alternative is to photochemically
process tightly packed mono- or multilayers directly2,11 and to
subsequently remove unreacted material. Here we show that
photoreacting nanometer-thin films of dibromo-bianthryls on
insulators produce coupled sp2-carbon multilayers from C−C
coupling reaction intermediates (Scheme 1a,b). After UV
irradiation of the evaporated material on mica substrates, the
coupled sp2-carbon multilayers can be dehydrogenated, i.e.
graphitized, at as low as 500 °C using e-beam irradiation, thus
opening up technological applications (Scheme 1c,f). The
studies present the formation of the coupled multilayers both as
a concerted and a two-step process, undergoing C−Br
photodissociation and C−C coupling. The use of filtered UV
light in conjunction with time-dependent density functional
theory (TD-DFT) calculations suggests that the dissociation
mechanism is occurring through resonant excitation and
subsequent intersystem crossing to a repulsive triplet state.
Unlike UV-induced reactions of multilayers, X-ray photo-
electron spectroscopy reveals how thermal treatment of the
multilayers alone results in almost complete loss (desorption)
of the material, presenting coupled sp2-carbon multilayers as
4652
dx.doi.org/10.1021/ja412868w | J. Am. Chem. Soc. 2014, 136, 4651−4658

Journal of the American Chemical Society
Article
Figure 2. Monitoring C−Br photodissociation and sp2-carbon signal through X-ray photoelectron spectroscopy. (a,b) The XPS measurements of
UV-irradiated bba nanometer-thin films using two different filters indicate that wavelengths between 280 and 225 nm are responsible for C−Br
dissociation. (d,e) UV-irradiated dbba (no filter). (c,f) Atomic force microscopy imaging of the respective samples to the l.h.s. showing the last
preparation step: for (c) bba and (f) dbba multilayers after UV irradiation and flash annealing. The inset in (f) shows a 500 × 500 nm2 survey.
technologically important photoresists. Subsequent annealing
of UV-irradiated multilayers results in detection of long,
nanometer-wide sp2-carbon nanoarchitectures in the film
morphology and graphitization, revealed by atomic force
microscopy (AFM) and Raman spectroscopy, respectively.
Finally, we give perspectives toward atomically precise
fabrication in nanometer-thin films on insulating substrates.
species (dimers, trimers, etc.) yields 70% C−Br dissociation
efficiency at laser powers of 20 mJ cm−2. For the dibromo
precursor, dbba, the peaks of the LDI spectra at 862 m/z are a
clear indication of the formation of an n = 2 oligomer
[C56H32Br2]+ (Figure 1a,d red line). Here also the n = 3
oligomer [C84H48Br3]+ is detected. Again for dbba nonselective
oligomerization reactions occur, as demonstrated by the
formation of side products, mainly triple brominated n = 2
adduct [C56H32Br3]+ at exactly 941 m/z (Figure 1a,e in green).
Such a triple brominated product is attributed to a structure as
depicted in the right panel of Scheme 1b. Also for dbba, the
C−Br scission product [C28H16Br]+ is observed at 431 m/z.
Note that oligomers are not observed when using a 7,7,8,8-
tetracyanoquinodimethane (TCNQ) matrix in conjuction with
dbba or bba, in the analogous matrix-assisted laser desorption
ionization (MALDI) technique. Moreover, we emphasize that
the LDI of the pristine 9′9-bianthracene compound (C28H18)
does not show the presence of oligomers. This excludes
dimerization through alternative well-known anthracenic [4 +
4] photocyclization14 mechanisms with such bulky precursors
and under our conditions (Figure S1a in SI). It is worth noting
that LDI also provokes oligomerization in solid-state powders
of brominated pyrene compounds and other species (see
Figures S1b and S2 in SI).
To study the C−Br scission process from first principles, we
performed TD-DFT calculations on the dissociation of the C−
Br bond in dbba. We made use of an excitation matrix overlap
approach (see SI) to find adiabatic reaction pathways along the
singlet and triplet excitations. Although relatively inexpensive
computationally, our level of theory shows excellent agreement
with reported15 quantum chemical calculations on the
dissociation of bromobenzene (first singlet and triplet non-
dissociative adiabatic channels appear at 4.8 and 4.0 eV,
respectively, in our calculations and 4.7 and 4.0, respectively, in
the diabatics of Liu et al.,15 see Figure S3 in SI for the full
bromobenzene adiabatic pathway). Figure 1f shows the
adiabatic reaction pathways for dbba along the C−Br reaction
coordinate, starting at the ground state C−Br bond length of
RESULTS AND DISCUSSION
■
The photodissociation of aryl halides is a well-known process,12
much less is known about the mechanism and reactivity of
photogenerated aryl radicals toward highly selective polymer-
ization reactions.9,13 Time-of-flight (ToF) laser desorption
ionization (LDI, using a 3.7 eV nitrogen laser) mass
spectrometry (MS) provides a state-of-the-art nanochemistry
method for identifying both photodissociation and C−C
coupling intermediate structural isomers. Figure 1a−e shows
the MS at positive ion detection for the LDI powder samples of
bba and dbba. The peaks at higher masses are indications of
newly formed C−C bonds following the suggested reaction
products in Scheme 1a,b. For bba (Figure 1a black line) the
debrominated precursor, [C28H17]+ observed at 354 m/z, is a
clear indication of C−Br dissociation. Then, the two carbon
isotropic distributions starting at 705 and 707 m/z (Figure 1a,
blue [C56H34]+) can be attributed to the structures ta, ta′ and
their isomers. The main product of the bba LDI appears
between 785 and 786 m/z (Figure 1a,b black line) and is
assigned to Br-ta [C56H34Br]+ (Scheme 1, in magenta). This is
the first indication that nonselective photoinduced reactions
can occur, that is, reactions not leading to a well-defined desired
ta isomer. The unexpected observation of bba trimers
[C84H51Br]+ which are detected at 1138 m/z (Figure 1a,c in
yellow), confirms such nonselective reactions. The intensity of
the debrominated bba peak suggests that the radical yield in
LDI experiments is close to 40%. However, considering
[C28H17]+ as a radical intermediate for all reactions and an
homogeneous ionization process, summing all the reacted
4653
dx.doi.org/10.1021/ja412868w | J. Am. Chem. Soc. 2014, 136, 4651−4658

Journal of the American Chemical Society
Article
Figure 3. DFT insight into C−Br dissociation C−C coupling mechanisms. (a) Heterolytic C−Br dissociation in vacuum produces a radical pair with
3.8 eV in energy. (b) The spontaneous reaction of an aryl-radical with anthracene and subsequent dehydrogenation has the same activation barrier.
(c) A face-on concerted dissociation−coupling bimolecular mechanism showing C−C coupling in an intermediate state (IntS1) and subsequent C−
Br dissociation. The path features a lower activation energy than the heterolytic dissociation in (a) by 0.9 eV. C, Br, and H atoms are depicted by
gray, red, and white, respectively.
1.92 Å. The first (1) pair of triplet and singlet (Figure 1f red
and black lines, respectively) dissociation channels cross the
vertically excited ground state (Figure 1f, dotted lines) close to
4.0 eV (310 nm). These channels involve transitions from the
anthracene π orbitals to the opposite (lower energy) or
adjacent (higher energy) antibonding C−Br σ* orbital. For a
C−Br bond stretching of 0.05 Å, which is consistent with the
temperatures studied, the vertical excitation energy is
approximately 3.7 eV. This suggests the (1) singlet channels
are responsible for the observed C−Br dissociation in the LDI
experiments. However, since the probability of an excitation to
(1) is rather low when dbba is exposed to a filtered UV lamp,
we observe C−Br dissociation only for energies above 4.4 eV
(vide infra). This is consistent with excitation to one of the
singlet attractive channels between 4.4 and 5.5 eV, followed by
an intersystem crossing to a second (2) pair of repulsive triplet
channels 5.5 eV (225 nm). These (2) triplet channels involve
transitions from the 2py or 2pz states of the Br to the
antibonding C−Br σ* orbital. This second dissociation channel
is also in agreement with bromoanthracene photophysics
experiments, which identify such high-lying triplets as the
main C−Br dissociation channels.16 Both orbitals are present in
the dibrominated single anthracene (dba, Figure S2d in SI),
and thus the dissociation channels in dba and dbba are similar.
To provide evidence for the main photodissociation channel
behind C−Br photodissociation, we use XPS to examine the
C−Br scission under two different light-irradiation conditions
and sources. The UHV-prepared multilayer films of dbba (as
used in the LDI experiments) and the model system bba were
investigated (see Experimental Section). Figure 2a,b shows the
Br3d (a) and C1s (b) spectra of the bba multilayer. In the Br3d
region of the pristine bba (Figure 2a continuous black line) two
peaks are discernible: the Al2p line of the mica substrate which
was used as a reference for the calibration of the energy scale
(see Experimental Section and SI) and the Br3d line of the bba
molecules. Both peaks consist of a doublet, whose small energy
splitting does not allow them to be resolved separately due to
the limited resolution of the experimental setup. For the chosen
energy reference the fitting of the Br3d peaks results in binding
energies of Br3d5/2 = 70.6 eV and Br3d3/2 = 71.7 eV. These
values are typical for Br in a covalent bond with carbon17 and
similar to those for bromobenzene18 and exclude their
assignment to neutral bromine (Br03d5/2 = 69 eV19). In
combination with the cross-section-corrected20 C1s/Br3d area
ratio, this indicates that intact molecular layers are obtained
with our preparation routine. The C1s region (Figure 2b
continuous black line) exhibits one single peak at 284.8 eV. As
opposed to the LDI experiments, no change in the XP spectra
could be observed for the samples irradiated by a 3.7 eV pulser
laser (4 1 mW cm−2 irradiation). Note that the same (but
unfocused) laser source as in the LDI experiments was used.
This observation points toward the previous LDI photo-
dissociation experiments being either thermally assisted
(because of high temperatures in the matrix-assisted LDI
plume21) or due to high fields during the ionization process, in
agreement with the previous TD-DFT discussion. With this in
mind, low-pass filters were installed in conjunction with a 120-
W high-pressure mercury lamp (nominal irradiation 58 10
mW cm−2 at 15.1 cm sample distance) in order to identify
other possible C−Br dissociation channels. Irradiating the bba
samples for 180 min using glass filters with 0.01 transmittance
above 345, 335, 320, 295, and 280 nm leads only to small
changes in the Br3d and C1s signals (Figure 2a,b dark-blue
lines and Figure S9a in SI). If, however, a filter blocking
wavelengths exclusively below 225 nm (transmitting up to 5.5
eV energies) is used, the Br3d peak decreases considerably with
only negligible effects on the C1s signal (Figure 2a,b light-blue
lines). This indicates that a scission of the C−Br bond and a
subsequent desorption of Br occurs. Such a decrease is equal to
that found using unfiltered UV light (see Figure S4 in SI). The
previous filter studies situate the dissociation channel between
4.4 and 5.5 eV, indicating that an intersystem crossing to the
second (2) triplet repulsive adiabatic (red line, Figure 1f) is
behind the main dissociation channel for both dbba and bba in
UV experiments (orange line, Figure 1f).
For dbba, Figure 2d,e illustrates the changes of the XP
spectra upon UV irradiation (without filter), followed by
thermal treatment. As was the case for bba, the amount of Br is
reduced after UV irradiation. Interestingly, the C−Br scission
reaches a stationary point. A constant Br3d peak height is
reached for both bba and dbba, after which the UV irradiation
is no longer effective. After this point the Br3d peak can be
4654
dx.doi.org/10.1021/ja412868w | J. Am. Chem. Soc. 2014, 136, 4651−4658

Journal of the American Chemical Society
Article
Figure 4. Detection of coupled intermediates and Raman spectroscopy of graphitized multilayers. ToF-SIMS of dbba and bba multilayers. (a,b)
Pristine bba drop-casted on gold. (c,d) MBE dbba multilayers on muscovite mica. On insulating muscovite mica, a 21 eV electron flood-gun must be
used to compensate for surface charging during SIMS, producing electron-induced brominated dimers. (e,f) UV-irradiated bba multilayers showing
the main dimer [C56H32]+ signal. (g,h) UV-irradiated dbba multilayers also showing the main dimer [C56H32]+ signal. Multilayers in (e−h) were
flash annealed at 300 °C to remove any remaining bromine and unequivocally avoid false positive brominated dimers signals. (i) Raman spectra (λ0 =
532 nm) of the UV-irradiated multilayer on SiOx/Si wafer after annealing at 500 °C with and without direct e-beam heating. A broad D mode is
shown centered at 1330 cm−1, while the G mode is centered at 1595 cm −1. The A mode corresponds to the Raman active signal of the UV-irradiated
multilayer at 1445 cm−1.
further reduced by flash annealing to 300 °C (red and orange
curves in Figure 2d,e). Here the C1s signal remains constant or
even increases, i.e., the decrease in the Br3d signal is not related
to desorption of the molecular species. In fact, when annealing
dbba multilayers (i.e., without UV irradiation), almost
complete disappearance of both C1s and Br3d peaks is
observed (See Figure S6 in SI). In other words, the precursor
material is desorbed as expected for unreacted monomers. In
UV-irradiated samples, the C1s signal remains constant even
after 2 h of annealing above 400 °C, a temperature much higher
than the sublimation temperature of 160 °C. This is the first
evidence of the formation of higher-molecular weight
compounds by UV irradiation. Notice that the C1s peaks
after irradiation feature, if any, a shift toward the carbon sp2
region (Figure S5 in SI) suggesting that the multilayers remain
sp2-hybridized within the limits of our experimental resolution
(see SI). Figure 2c,f depicts the atomic force microscopy
(AFM) images of the multilayer samples of bba and dbba after
irradiation and annealing to 300 °C. A striking difference is
obvious when comparing the samples in Figure 2c,f. For the
UV-irradiated dbba, highly oriented elongated rods with mean
thickness of 3 nm and longer than 50 nm are easily
distinguished (Figure 2f). These are assigned to ribbon-like
architectures of reacted monomers. It is also interesting that
such UV-irradiated-annealed samples show long-range order, in
contrast to the untreated samples (Figure S10 in SI).
To provide insight into the C−C coupling after or during
C−Br photodissociation, we performed DFT-based transition-
state calculations to find reaction and oligomerization
mechanisms. Figure 3a,b depicts the coupled sp2 multilayer
4655
dx.doi.org/10.1021/ja412868w | J. Am. Chem. Soc. 2014, 136, 4651−4658

Journal of the American Chemical Society
Article
fabrication as a two-step reaction. First, the bromine is
dissociated with an overall barrier of 3.83 eV (Figure 3a).
Second, the reactivity of an aryl radical with an anthracene
(Figure 3b) proceeds in a barrier-less manner, breaking
aromaticity through an sp3 carbon in the intermediate product
(IntS). In a third step, dehydrogenation could occur, with a
barrier for regaining aromaticity of 1.45 eV. This last step
demonstrates that, even in the absence of a reducing agent,
thermally removing monatomic hydrogen is possible. Figure 3c
depicts an alternate concerted face-on bimolecular reaction
pathway, showing that the halogenated carbon, in the vicinity of
another one, will react to an intermediate state (IntS1) with a
barrier of 2.31 eV with respect to the ground state (IS). During
this IntS1 step the C−Br distance changes from 1.92 Å to 2.06
Å, and a new C−C bond (1.63 Å) is formed. In a subsequent
step, the anthracene rehybridizes to two C−C sp2 carbons (1.46
Å) where the bromines leave (the Br−Br bond changes from
3.28 Å in the intermediate state to 2.32 Å). The final state (FS)
is then recovered with a 30% reduced overall barrier than the
nonconcerted mechanism. An extensive study of a bimolecular
photoreaction pathway with TD-DFT is beyond the scope of
the current study. However, the concerted mechanism put
forward in Figure 3c is of arresting importance in solid-state
photochemistry, as it suggests that only a C−Br 0.14 Å distance
change suffices to react two neighboring anthracenes. A
bimolecular pathway might also help explain why photo-
dissociation proceeds faster in doubly brominated dbba than in
single-brominated bba (Figure 2). Further investigation of such
pathways might help to boost reaction selectivity in the solid-
state.
dissociation of the C−Br bond. This unexpected result suggests
that low-energy electrons might also be employed to “click”
ordered π-stacked assemblies of brominated aromatic pre-
cursors via the face-on concerted mechanism described in
Figure 3c.
With the possibility of electron-induced reactions in mind,
the UV-irradiated and annealed multilayers shown in Figure
2c,f were characterized (Figure 4e−h). In such 300 °C annealed
multilayers, electron-induced reactions are unequivocally
avoided because thermal treatment removes any bromine that
remains after UV irradiation (cf. Figure 2d). In fact, no traces of
brominated adducts are observed in Figure 4e−h. This is
consistent with covalently bound bromine having left the
multilayer under UHV conditions. Figure 4e,f depict the SIMS
data for the bba samples. As expected, they show as a major
product above 400 m/z, a mixture of the ta species [C56H34]+ −
2H and [C56H34]+ − 1H (704 and 705 m/z, respectively)
following the photoreaction. Because trianthrylene fragments
are not observed, we assign the previous signals to ionization of
intact rearomatized (−2H) ta′ or deprotonated ta molecules.
For the UV irradiation of dbba, the n = 2 [C56H33]+ oligomers
are also detected, albeit in smaller quantities (Figure 4g,h). It is
noteworthy that SIMS-ToF analysis of dbba graphitized
samples (vide infra) did not reveal the presence of monomers
or dimers. Both observations are consistent with SIMS-ToF
detecting only low-molecular weight polyaromatic compounds,
which in turn indicate a higher degree of C−C coupling in
dbba precursors as compared with bba.
Finally, the dehydrogenation step transforming the coupled
sp2-carbon multilayers into graphitic ones is of particular
technological importance. Thus, Raman microspectroscopy
(RM) measurements were performed for samples prepared on
top of silicon oxide on silicon wafers. Unlike muscovite mica,
silicon wafers can be used to record a RM signal. By using e-
beam thermal-annealing (180 min, positive biasing the sample
and using 10 mA cm−2 electron fluxes) to treat the C−C
coupled multilayers, graphitic modes (D at 1330 cm−1 and G at
1595 cm −1) are clearly observed (Figure 4i black line). While
e-beam cross-linking of aromatic monolayers is a well-known
process,23 the overall strategy presented here is to use e-beam
thermal treatment to provoke the final graphitization step of the
(nano)architectures. This represents an increased level of
molecular control provided by chemical design, photo-
chemistry, and in situ mass spectrometry (cf. Scheme 1c−f).
In fact, the Raman signatures obtained in Figure 4 are typical
for polycrystalline graphene pyrolyzed on gold at 1000 °C24
and pyrolyzed on copper at 700 °C.25 The lower temperature
for which we achieved similar results suggests that there is a
reduced barrier for dehydrogenation of our coupled sp2-carbon
multilayers, tentatively due to cooperative dehydrogenation
between anthracene units, as suggested previously.26 Samples
annealed at ∼500 °C without e-beam treatment also featured
graphitization to some extent: In Figure 4i (red line) the sharp
mode at 1445 cm−1 indicates remaining carbon sp2 material.
Note that higher temperatures were not attempted to avoid
decomposition of SiOx and mica. Unlike the aforementioned
examples of (polycrystalline) graphene, all samples annealed at
500 °C (with and without e-beam treatment) showed
reproducible Raman 2D peaks at ∼2920 cm−1, 2745 cm−1,
and 2545 cm−1 (see Figure S12 in SI). Such articulated features
are not present in disordered and damaged graphitic materials27
and are indications of nanostructured polyaromatic com-
pounds.28
Our LDI, XPS studies, and theoretical analysis aimed at an
atomistic understanding of the C−Br dissociation, C−C
reaction intermediates, and their possible coupling mechanisms
during UV irradiation of nanometer-thin multilayers. In order
to characterize the final photoreacted multilayers, we performed
ToF-SIMS and Raman investigations. Unlike LDI, the SIMS
ionization mechanism involves desorption and ionization
through Bi3 cluster impact.22 This implies that SIMS alone
+
cannot provoke photoinduced C−Br dissociation and coupling.
Thus, ToF-SIMS allows us to confirm the formation of final
C−C coupled products under UV irradiation of large-area
samples. Figure 4a depicts the SIMS-ToF spectrum of the
pristine bba on a gold substrate, showing the [C28H17Br]+
isotopic pattern at 432 m/z. As expected, no tetraanthrylene
(ta) oligomer signal [C56H34]+ at 706 m/z is observed (Figure
4b). Notice that this last spectrum was acquired on a
conductive substrate (Au/mica) in order to avoid the use of
an electron flood gun to compensate for surface charging
during measurement. In fact, when measuring the as-evaporated
dbba on muscovite mica using a 21 eV flood gun, brominated n
= 2 and n = 3 oligomers are observed (Figure 4c,d and Figure
S11 in SI). These reactions are electron-induced, since neither
brominated adducts nor tethraanthrylenes (Figure 4b) are
detected in dbba SIMS studies on gold without the use of a 21
eV electron flood gun. Further inspection of the mass spectra in
Figure 4c reveals a peak at 941 m/z, higher in intensity than the
n = 2 dibrominated oligomer. As for the LDI in Figure 1a, this
peak is identified as the triple brominated oligomer
[C56H32Br3]+ (cf. Figure S11 in SI). As previously discussed,
triple brominated adducts are possible if radicals further react
with unreacted dibrominated precursors (via addition to
anthracene moieties as indicated in Figure 3b). Consequently,
the electron-induced process also provokes heterolytic
4656
dx.doi.org/10.1021/ja412868w | J. Am. Chem. Soc. 2014, 136, 4651−4658

Journal of the American Chemical Society
Article
to sample the first Brillouin zone. Energy barriers were obtained by
transition state calculations following the approach described in ref.26
using a combination of the climbing image nudged elastic band34 (CI-
NEB) and dimer35 methods. In short, CI-NEB was used to find an
initial estimate of the transition state. The estimate was then used to
generate the input (central image and dimer) in the dimer method.
Structural optimizations of both local minima and transition states
were performed until the residual forces were smaller than 0.02 eV/Å.
X-ray Photoelectron Spectroscopy. The XPS experiments were
performed in an UHV chamber with a base pressure in the 10−9
−10−10 mbar regime. A nonmonochromatized Mg Kα X-ray source
(hν = 1253.6 eV) was used for the XPS measurements which were
conducted at RT. The data were obtained using a hemispherical
electron energy analyzer (100 mm radius) operated at a pass energy of
20 eV. The analyzer was equipped with a position sensitive resistive
anode detector for increased sensitivity. Due to the sensitivity of the
molecules to electrons, the measurements were performed without the
use of a flood gun, resulting in a shift of approximately 3.5 eV to higher
binding energies due to charging. This shift was corrected by
referencing all spectra to the mica Al2p peak at 74.4 eV36 (for further
information see SI). Afterward a linear (Br3d), constant (C1s, dbba)
or a Shirley (C1s, bba) background was subtracted. Note that the
magnesium X-ray source and/or the occurrence of secondary electrons
during XPS measurements also induce (high-energy) C−Br photo-
dissociation, as evidenced by reduction of the Br 3d peak (see Figure
S8 in SI). This effect seems to be much more prominent for bba than
for dbba. Our experiments indicate that for bba it takes 320 min of
continuous X-ray exposure to produce the same effect as 180 min of
UV irradiation. Therefore the experiments above were conducted such
that the exposure to X-ray irradiation was minimal and the spectra of
two subsequent measurements did not show any substantial difference
(see Figure S8 in SI).
CONCLUSIONS
■
In summary, UV irradiation of brominated aromatic precursors
readily produces covalently coupled sp2-carbon multilayers in
which oligoanthracenes can be detected. Such coupled sp2-
carbon multilayers undergo graphitization already at 500 °C.
We have provided insight into the photochemical and C−C
coupling mechanisms via (TD-) DFT, consistent with XPS
experiments and MS measurements. In doing so, we have
introduced the field of photoinduced thin-film solid-state
reactions as a convergence between top-down and selective
bottom-up nanofabrication. We foresee that in-depth phys-
icochemical control of nanometer-thin layered (self-)assembly
along with the respective photochemical reaction mechanisms
will provide a prosperous approach for directly patterning
atomically precise graphene macromolecules and ribbons on
insulating substrates. This is regarded as a useful and versatile
strategy toward complex and extended carbon sp2 macro-
molecules on insulators, thereby consolidating electronic/
spintronic macromolecular blueprinting and engineering as a
technological standard.
EXPERIMENTAL SECTION
■
Sample Preparation. For all experiments except when noted
otherwise, samples were prepared under a base pressure in the 10−9
−10−10 mbar regime. Prior to molecule deposition by MBE, ex situ
cleaved muscovite mica substrates were annealed to 300 °C to remove
light contaminants, followed by a deposition of the dbba and bba
molecules using organic MBE at a molecular temperature of 150−160
°C for 90 and 20 min, respectively, onto the muscovite mica which was
kept at room temperature (RT). The films were measured to have a
thickness close to 2 nm by AFM. Synthesis of dbba and bba molecules
was carried out as described elsewhere29 and products were triply
sublimated before experiments to ensure high purity.
Time-of-Flight Laser Desorption Ionization. ToF-LDI and
ToF-MALDI experiments were carried out in a Bruker FLEX2
spectrometers at a baser pressure of 10−6 mbar and a 7-ns pulsed
Nitrogen Laser (337 nm). Dbba multilayers were fixed to a stainless
steel target and the ions accelerated under a 15 kV field. The spectra
collected belongs to single laser pulsing signal without averaging.
MALDI experiments differed from ToF experiments in that a TCNQ
matrix was sublimated on top of the dbba multilayers.
Time-Dependent Density Functional Theory. TD-DFT
calculations, were performed using the Grid-based realspace Projector
Augmented Wave function (GPAW) code, employing the Perdew,
Burke, Ernzerhof (PBE) generalized gradient approximation for the
exchange-correlation (xc)-functional.30 Calculations for the isolated
C14H8Br and C6H5Br molecules were performed with more than 6 Å
of vacuum surrounding the molecules and employing nonperiodic
boundary conditions, forcing both the density and Kohn−Sham wave
functions to go to zero at the boundaries of the unit cell. A
commensurate grid spacing of h = 0.2 Å was employed throughout. In
order to calculate the excitation energies of singlets and triplets, we
first optimized the ground state geometries until all atomic Hellman-
Feynman forces were below 0.05 eV/Å. We subsequently stretched the
Br−C bond in steps of 0.1 Å, performing a further minimization of the
geometry, while keeping the Br−C bond length fixed. In this way we
determined the reaction coordinate for the Br dissociation reaction.
For each relaxed geometry along the ground state dissociation reaction
pathway we performed a separate TD-DFT calculation. From this we
obtained both the singlet and triplet transitions, and their respective
energies and oscillator strengths. The exact description of this
procedure is given in the SI.
Time-of-Flight Secondary Ion Mass Spectrometry. ToF-SIMS
was performed on a TOF.SIMS5 instrument (ION-TOF GmbH,
Munster, Germany), equipped with a Bi cluster primary ion source and
̈
a reflectron type time-of-flight analyzer. UHV base pressure was <5 ×
10−9 mbar. For high mass resolution the Bi source was operated in the
“high current bunched” mode providing short Bi1+ or Bi3+ primary ion
pulses at 25 keV energy and a lateral resolution of approximately 4 μm.
The short pulse length of 1.1 to 1.3 ns allowed for high mass
resolution. The primary ion beam was rastered across a 500 × 500 μm
field of view on the sample, and 128 × 128 data points were recorded.
Primary ion doses were kept below 1011 ions/cm2 (static SIMS limit).
Charge compensation was necessary because of the insulating samples
so that an electron flood gun providing electrons of 21 eV was applied
and the secondary ion reflectron tuned accordingly.
ASSOCIATED CONTENT
* Supporting Information
■
S
Additional ToF-LDI, TD-DFT, XPS, AFM, Raman, ToF-SIMS
experiments, and synthetic methods. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
*c.a.palma@tum.de
■
*florian.klappenberger@tum.de
*angel.rubio@ehu.es
*muellen@mpg.mpip-mainz.de
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Density Functional Theory. DFT calculations for the reaction
pathways were carried out with the VASP code.31 A variant of the van
der Waals density functional32 was used, with the exchange replaced by
an optimized form of the Becke 86 exchange functional.33 The plane
waves were expanded to a cutoff of 400 eV and the Γ-point was used
The authors are especially grateful to Karl Eberle for XPS
measurements and Thomas Schuster for additional LDI
measurements. We gratefully acknowledge silicon wafers from
Jose Antonio Garrido and preliminary Raman measurements
4657
dx.doi.org/10.1021/ja412868w | J. Am. Chem. Soc. 2014, 136, 4651−4658

Journal of the American Chemical Society
Article
Lawrence Berkeley National Laboratory; University of California:
Berkeley, CA, 2009.
(20) Yeh, J. J.; Lindau, I. At. Data Nucl. Data Tables 1985, 32, 1.
(21) Knochenmuss, R.; Zenobi, R. Chem. Rev. 2003, 103, 441.
(22) Kollmer, F. Appl. Surf. Sci. 2004, 231, 153.
from Max Seifert, Stefan Funk, Stefan Niedermayer, and Hai Bi.
C.-A.P. thanks Hristo Iglev for continuous help, Christian
Lutke-Eversloh for additional UV data and Achim Hartschuh
̈
for support. We acknowledge financial support from the
European Research Council Advanced Grant DYNamo (ERC-
2010-AdG-267374) Spanish Grants (FIS2010-21282-C02-01
and PIB2010US-00652), Grupos Consolidados UPV/EHU del
Gobierno Vasco (IT578-13), Ikerbasque and the European
Commission projects CRONOS (Grant 280879-2 CRONOS
CP-FP7). C.-A.P. and K.D. were funded through the ERC
Advanced Grant MolArt (no. 247299). D.J.M. acknowledges
funding through the Spanish “Juan de la Cireva” program (JCI-
2010-08156). J.L.C acknowledges funding through the Mexican
CONACyT program. J.B. acknowledges the National Super-
computer Centre, Sweden, for computational resources.
(23) Geyer, W.; Stadler, V.; Eck, W.; Zharnikov, M.; Golzhauser, A.;
̈
̈
Grunze, M. Appl. Phys. Lett. 1999, 75, 2401.
(24) Turchanin, A.; Weber, D.; Buenfeld, M.; Kisielowski, C.; Fistul,
M. V.; Efetov, K. B.; Weimann, T.; Stosch, R.; Mayer, J.; Golzhauser,
̈
̈
A. ACS Nano 2011, 5, 3896.
(25) Matei, D. G.; Weber, N.-E.; Kurasch, S.; Wundrack, S.;
Woszczyna, M.; Grothe, M.; Weimann, T.; Ahlers, F.; Stosch, R.;
Kaiser, U.; Turchanin, A. Adv. Mater. 2013, 25, 4146.
(26) Bjork, J.; Stafstrom, S.; Hanke, F. J. Am. Chem. Soc. 2011, 133,
̈
̈
14884.
(27) Pimenta, M. A.; Dresselhaus, G.; Dresselhaus, M. S.; Cancado,
L. G.; Jorio, A.; Saito, R. Phys. Chem. Chem. Phys. 2007, 9, 1276.
(28) (a) Huang, H.; Wei, D. C.; Sun, J. T.; Wong, S. L.; Feng, Y. P.;
Neto, A. H. C.; Wee, A. T. S. Sci. Rep. 2012, 2, 983. (b) Dossel, L.;
̈
REFERENCES
■
Gherghel, L.; Feng, X. L.; Mullen, K. Angew. Chem., Int. Ed. 2011, 50,
̈
(1) Grill, L.; Dyer, M.; Lafferentz, L.; Persson, M.; Peters, M. V.;
Hecht, S. Nat. Nanotechnol. 2007, 2, 687.
(2) Palma, C. A.; Samorì, P. Nat. Chem. 2011, 3, 431.
(3) (a) Abel, M.; Clair, S.; Ourdjini, O.; Mossoyan, M.; Porte, L. J.
Am. Chem. Soc. 2011, 133, 1203. (b) Bieri, M.; Treier, M.; Cai, J. M.;
Ait-Mansour, K.; Ruffieux, P.; Groning, O.; Groning, P.; Kastler, M.;
Rieger, R.; Feng, X. L.; Mullen, K.; Fasel, R. Chem. Commun. 2009,
6919.
(4) Cai, J. M.; Ruffieux, P.; Jaafar, R.; Bieri, M.; Braun, T.;
Blankenburg, S.; Muoth, M.; Seitsonen, A. P.; Saleh, M.; Feng, X. L.;
Mullen, K.; Fasel, R. Nature 2010, 466, 470.
(5) Zhang, Y. Q.; Kepci
2540.
(29) Muller, U.; Baumgarten, M. J. Am. Chem. Soc. 1995, 117, 5840.
̈
(30) Perdew, J. P.; Burke, K.; Wang, Y. Phys. Rev. B 1996, 54, 16533.
(31) Kresse, G.; Furthmuller, J. Phys. Rev. B 1996, 54, 11169.
̈
(32) Dion, M.; Rydberg, H.; Schroder, E.; Langreth, D. C.;
̈
̈
̈
Lundqvist, B. I. Phys. Rev. Lett. 2004, 92.
̈
̌
(33) Klimes, J.; Bowler, D. R.; Michaelides, A. J. Phys. Condens. Matter
2010, 22, 022201.
(34) Henkelman, G.; Uberuaga, B. P.; Jonsson, H. J. Chem. Phys.
2000, 113, 9901.
(35) (a) Henkelman, G.; Jonsson, H. J. Chem. Phys. 1999, 111, 7010.
(b) Kaestner, J.; Sherwood, P. J. Chem. Phys. 2008, 128.
(36) Bhattacharyya, K. G. J. Electron Spectrosc. Relat. Phenom. 1993,
63, 289.
̈
̌
ja, N.; Kleinschrodt, M.; Diller, K.; Fischer, S.;
Papageorgiou, A. C.; Allegretti, F.; Bjork, J.; Klyatskaya, S.;
̈
Klappenberger, F.; Ruben, M.; Barth, J. V. Nat. Commun. 2012, 3, 549.
(6) McNaught, A. D.; Wilkinson, A. IUPAC Compendium of Chemical
Terminology, 2nd ed.; International Union of Pure and Applied
Chemistry (IUPAC): London, 1997.
(7) (a) Bjork, J.; Hanke, F.; Stafstrom, S. J. Am. Chem. Soc. 2013, 135,
̈
̈
5768. (b) Ertl, G. Reactions at Solid Surfaces; John Wiley & Sons, Inc.:
Hoboken, NJ, 2009. (c) Zhong, D. Y.; Franke, J. H.; Podiyanachari, S.
K.; Blomker, T.; Zhang, H. M.; Kehr, G.; Erker, G.; Fuchs, H.; Chi, L.
̈
F. Science 2011, 334, 213.
(8) (a) Kittelmann, M.; Nimmrich, M.; Lindner, R.; Gourdon, A.;
Kuhnle, A. ACS Nano 2013, 7, 5614. (b) Bombis, C.; Ample, F.;
̈
Lafferentz, L.; Yu, H.; Hecht, S.; Joachim, C.; Grill, L. Angew. Chem.,
Int. Ed. 2009, 48, 9966. (c) Gutzler, R.; Walch, H.; Eder, G.; Kloft, S.;
Heckl, W. M.; Lackinger, M. Chem. Commun. 2009, 4456.
(9) Sloan, D.; Sun, Y. M.; Ihm, H.; White, J. M. J. Phys. Chem. C.
1998, 102, 6825.
(10) Bieri, M.; Nguyen, M. T.; Groning, O.; Cai, J. M.; Treier, M.;
̈
Ait-Mansour, K.; Ruffieux, P.; Pignedoli, C. A.; Passerone, D.; Kastler,
M.; Mullen, K.; Fasel, R. J. Am. Chem. Soc. 2010, 132, 16669.
̈
(11) Colson, J. W.; Dichtel, W. R. Nat. Chem. 2013, 5, 453.
(12) (a) Han, K. L.; He, G. Z. J. Photochem. Photobiol., C 2007, 8, 55.
(b) Davidson, R. S.; Goodin, J. W.; Kemp, G. Adv. Phys. Org. Chem.
1984, 20, 191. (c) Kadi, M.; Davidsson, J.; Tarnovsky, A. N.;
Rasmusson, M.; Akesson, E. Chem. Phys. Lett. 2001, 350, 93.
(13) Szulczewski, G. J.; White, J. M. Surf. Sci. 1998, 399, 305.
(14) Bouas-Laurent, H.; Castellan, A.; Desvergne, J. P.; Lapouyade,
R. Chem. Soc. Rev. 2000, 29, 43.
(15) Chen, S. F.; Liu, F. Y.; Liu, Y. J. J. Chem. Phys. 2009, 131.
(16) Mcgimpsey, W. G.; Scaiano, J. C. J. Am. Chem. Soc. 1989, 111,
335.
(17) Pijpers, A. P.; Meier, R. J. J. Electron Spectrosc. Relat. Phenom.
2001, 121, 299.
(18) Hunger, R.; Jaegermann, W.; Merson, A.; Shapira, Y.;
Pettenkofer, C.; Rappich, J. J. Phys. Chem. B. 2006, 110, 15432.
(19) Thompson, A.; Attwood, D.; Gullikson, E.; Howells, M.; Kim,
K.-J.; Kirz, J.; Kortright, J.; Lindau, I.; Liu, Y.; Pianetta, P.; Robinson,
A.; Scofield, J.; Underwood, J.; Wiliams, G. X-ray Data Booklet
4658
dx.doi.org/10.1021/ja412868w | J. Am. Chem. Soc. 2014, 136, 4651−4658