Large work function shift of gold induced by a novel perfluorinated azobenzene-based self-assembled monolayer

Article abstract of DOI:10.1002/adma.201201737

Tune it with light! Self-assembled monolayers on gold based on a chemisorbed novel azobenzene derivative with a perfluorinated terminal phenyl ring are prepared. The modified substrate shows a significant work function increase compared to the bare metal. The photo-conversion between trans and cis isomers chemisorbed on the surface shows great perspectives for being an accessible route to tune the gold properties by means of light. Copyright

Full text of DOI:10.1002/adma.201201737

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Large Work Function Shift of Gold Induced by a Novel
Perfluorinated Azobenzene-Based Self-Assembled
Monolayer
Núria Crivillers, Silvio Osella, Colin Van Dyck, Giovanni M. Lazzerini, David Cornil,
Andrea Liscio, Francesco Di Stasio, Shabbir Mian, Oliver Fenwick, Federica Reinders,
Markus Neuburger, Emanuele Treossi, Marcel Mayor,* Vincenzo Palermo,*
Franco Cacialli,* Jérôme Cornil,* and Paolo Samorì*
Dedicated to the memory of Dr. Gianluca Latini
Chemisorption of self-assembled monolayers (SAMs) is a uni-
versal method to modify numerous surface properties, which
allows interfaces to be optimized for the application of choice.[1]
Because of this reason SAMs are currently widely used as
key components in the fabrication of various optoelectronic
devices, such as organic thin-film transistors (OTFTs),[2] organic
light-emitting diodes (OLEDs), organic photovoltaic (OPV)
and organic memory (OMEM) cells.[3] When chemisorbed on
metallic surfaces, SAMs can be used for the tuning of both the
metal work function and surface wettability,[4] thus enabling the
optimization of the charge injection/extraction at metal–semi-
conductor interfaces[5] and the control over the molecular order
(including the degree of crystallinity) within the semiconducting
layer deposited on top of the SAMs.[6] The latter is crucial for
the optimization of the transfer integral, representing the elec-
tronic coupling of neighboring molecules, and the polaronic
relaxation energy, to promote the charge transport within the
active material and ultimately enhance the device performance.
It has been demonstrated that subtle changes in the chemical
structure of the molecules forming the SAMs can dramati-
cally change the resulting properties of the device.[4,7] It is now
known and widely exploited for device fabrication that chemi-
sorption of n-alkanethiols and partially perfluorinated thiols on
Au electrodes decreases and increases the metal work function,
respectively.[8] The use of one versus the other depends on the
nature of the active molecular material, i.e., n-type, p-type, or
ambipolar, that is employed to fabricate the specific device.
Azobenzenes are extensively employed photochromic sys-
tems because of their accessible and reversible isomerization
process that occurs upon illumination with UV and visible
light between the trans and cis states and vice versa, respec-
tively.[9] Among the vast variety of applications of azobenzene,
the photoinduced conformational change of azobenzene-based
SAMs has been used for the development of responsive mole-
cular wires,[10] which were successfully integrated in an organic
field-effect transistor to photochemically modulate the charge
injection at the metal-semiconductor interface.[11] One physical
property that was found to be photochemically modified is the
work function of Au.[12] Unfortunately, this result was obtained
using bi-component SAMs, which thus integrated, in a poorly
controlled way, two molecular building blocks featuring diverse
electrical and electronic properties. We have recently reported a
single-component responsive SAM, based on an azo-biphenyl
molecule, exhibiting a photo-modulable work-function when
chemisorbed on Au electrodes.[13] Here, we have designed and
synthesized a new thiolated azobenzene molecule exposing a
perfluorinated benzene head group. These SAMs obtained by
chemisorption on Au exhibit a markedly large shift of the metal
work function when compared to the non-fluorinated analog.
Dr. N. Crivillers, Prof. P. Samorì
ISIS & icFRC, Université de Strasbourg & CNRS
8 allée Gaspard Monge, 67000 Strasbourg, France
E-mail: samori@unistra.fr
S. Osella, C. Van Dyck, Dr. D. Cornil, Dr. J. Cornil
Laboratory for Chemistry of Novel Materials
University of Mons
Place du Parc 20, B-7000 Mons, Belgium
E-mail: Jerome.Cornil@umons.ac.be
Dr. G. M. Lazzerini, Dr. F. Di Stasio, Prof. S. Mian,
Dr. O. Fenwick, Prof. F. Cacialli
Department of Physics and Astronomy and London
Centre for Nanotechnology
University College London
Gower Street, London WC1E 6BT, England
E-mail: f.cacialli@ucl.ac.uk
Dr. A. Liscio, Dr. E. Treossi, Dr. V. Palermo
ISOF-CNR, via Gobetti 101, 40129 Bologna, Italy
E-mail: palermo@isof.cnr.it
F. Reinders, M. Neuburger, Prof. M. Mayor
Department of Chemistry
University of Basel
St. Johannsring 19, 4056 Basel, Switzerland
E-mail: marcel.mayor@unibas.ch
E. Treossi
Laboratorio MISTER, via Gobetti 101, 40129 Bologna, Italy
Prof. M. Mayor
Karlsruhe Institute of Technology (KIT)
Institute for Nanotechnology
P.O. Box 3640, 76021 Karlsruhe, Germany
Prof. S. Mian
Department of Physics
McDaniel College
2 College Hill, Westminster, MD 21157, USA
DOI: 10.1002/adma.201201737
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Figure 1. a) Schematic representation of the FAZO-SAM in its trans (left) and cis (right) state. b) UV-vis spectra of 1 in solution (2.89 × 10-5 M in
chloroform) and (c) in SAM chemisorbed on gold.
This shift is comparable to other fluorinated SAMs on Au such
as those based on pentafluorobenzene thiol (PFBT),[14] but here
due to the presence of an azobenzene moiety, the work function
can be later altered by irradiating the SAM at a specific wave-
length, thus widening the functionality of the monolayer.
The synthesis of the fluorinated azobenzene compound 1
(FAZO) was accomplished in six steps, as detailed in the Sup-
porting Information. All compounds were characterized by
NMR-spectroscopy, mass spectrometry and elemental analysis.
Furthermore, single crystals suitable for X-ray analysis were
obtained for 1 by slow evaporation of a saturated CDCl3 solu-
tion. The solid-state structure corroborated the identity of 1 in
its trans form.
The preparation of the SAM of 1 (FAZO-SAM) was accom-
plished by immersion of a freshly gold evaporated substrate in
a 0.1 mM solution in CHCl3 for 48 hours. The SAM formation
was carried out in the dark to obtain the SAM in its trans state
(Figure 1a). On the contrary, for inducing the adsorption of the
cis FAZO molecules on the substrate, a 0.1 mM solution of 1
was initially irradiated with UV light for 1h and then the sub-
strate was immersed in the solution for 23 hours keeping the
irradiation ON during the formation of the SAM.[15] After the
corresponding immersion time, the functionalized substrates
were removed from the solution, vigorously rinsed with CHCl3
and dried under a N2 stream.
The reversible isomerisation process between trans and cis
forms of 1 in solution was followed by UV-vis absorption spec-
troscopy. Figure 1b shows the evolution of the absorption spec-
trum upon irradiation with UV light (366 nm), which clearly
demonstrates the photo-induced isomerisation of 1 after ca.
45 minutes. The peak at 340 nm corresponds to the trans form,
which decreases upon irradiation. Conversely, 15 minutes of
white light irradiation were sufficient to induce the complete
back isomerization. The same technique was employed to char-
acterize the SAM prepared on a transparent 25 nm thick gold
film evaporated on quartz. In Figure 1c the UV-vis absorption
spectrum of the non-functionalized gold on quartz substrate
(red) and that of the AZO modified one (black) are reported.
The bare gold substrate shows a non-symmetric broad absorp-
tion band centered at approximately 510 nm that can be attrib-
uted to the gold surface plasmon resonance (SPR).[16] Upon
chemisorption of the AZO molecules the intensity of this band
decreases as a result of the formation of the Au-S bond, and a
new band with a maximum centered at 345 nm appears, which
is attributed to the π–π∗ transition in the trans state, indicating
the formation of the FAZO-SAM.
The water contact angle (CA) measurement of a freshly
prepared FAZO-SAM revealed a strongly hydrophobic nature
of the trans FAZO-SAM (CA = 94.0° 2.3°). This value is in
agreement with other reported fluorinated SAMs,[17] hence
supporting the successful SAM formation observed by UV-vis
absorption measurements. To investigate the trans to cis isom-
erisation of the chemisorbed molecules, the same sample was
rinsed with water and ethanol, dried under N2 stream and
exposed for one hour to UV (365 nm) irradiation. The obtained
CA value of the irradiated sample featured a larger error bar
(91.6° 7.3°), and most importantly did not reveal a signifi-
cant difference when compared to the initial trans SAM. To
avoid any possible damage of the surface during the first CA
measurement which could affect the yield of isomerization, a
freshly prepared trans SAM was illuminated with UV for two
hours: in this case, the obtained value was 88.0° 0.8°. The
small difference observed between CA values measured before
and after irradiation does not unambiguously prove the occur-
rence of the trans to cis isomerisation in chemisorbed mole-
cules forming the SAM on Au but on the contrary it does not
exclude the possibility that this change is observable. Taking
into account that the molecule has a terminal perfluorinated
benzene ring, one hypothesis to explain the last measurement
is that although the molecules are in their cis state, the fluorine
atoms are not completely hidden and hence do not promote
a significant change in the final wettability. This observation
is confirmed by calculated optimized geometries (see the Sup-
porting Information), providing evidence for the exposure
of some of the fluorine atoms in the cis SAM. Atomic force
microscopy (intermittent contact mode) images were acquired
on both trans and cis based SAMs. Although relevant structural
information of the monolayer could not be extracted the meas-
ured root mean square roughness (Rrms) showed an increase
from 2 1 Å to 4 1 Å when comparing the uncoated sur-
face of Au(111) with the same surface once coated with the
FAZO-SAM.
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The electronic properties of the gold coated with a FAZO-
SAM in its trans and cis isomer, and in particular its work func-
tion (Φ), were experimentally studied by macroscopic Kelvin
Probe measurements (KP), Kelvin Probe Force Microscopy
(KPFM), Photoelectron Spectroscopy (PS) under atmospheric
conditions, and by Electroabsorption (EA) spectroscopy.[18]
Theoretical calculations were also performed to achieve a better
understanding of the influence of the SAM in the two isomeric
forms on the Au work function.
Upon sample irradiation with UV light (365 nm) for ∼1 hour
only a small decrease in Φ (30 meV) was monitored (see meas-
urements in Supporting Information). The relatively modest
difference observed prompted us to study two independent
systems, i.e. the separate functionalization of the gold with
FAZO molecules in their trans and cis state. Ambient PS
measurements, done on “ex situ” freshly prepared trans and
cis-FAZO-SAMs on Au(111)/mica, revealed Φ trans-FAZO/Au(111)
=
5.36 0.02 eV and Φ = 5.25 0.02 eV, providing
cis-FAZO/Au(111)
The change in the Φ of Au upon formation of the SAM was
studied by EA[19] measurements, performed on an LED incorpo-
rating the trans FAZO-SAM, with the following structure: ITO/
Au(5 nm)/FAZO-SAM/F8BT(100 nm)/Al(50 nm). A detailed
experimental description can be found in reference.[13,18] All
measurements were carried out at room temperature by irra-
diating with a monochromated beam (from a Xe-lamp) at
λ = 505 nm, while applying a sinusoidal voltage VAC = 0.5 V
at a frequency f = 2 kHz, and with the optical probe entering
the LED through the semitransparent ITO/Au electrode. For
calculating the work function of the modified gold, we consider
the possibility of pinning unlikely since the LUMO of F8BT is
accepted to lie in the range 3.2–3.5 eV and the Al work func-
tion only decreases below 3.5 eV upon exposure to significant
amount of oxygen during or after evaporation (the work func-
tion of Al in UHV is ∼4.1–4.2 eV).[20] In addition, previous KP
measurements of our evaporated Al electrodes gave an average
Al work function in our diodes of 3.70–3.80 eV. Assuming a
work function of the cathode of 3.75 0.05 eV, we calculated
evidence for a large Φ variation between the two isomers,
with a ΔΦtrans-cis of ∼110 meV (see measurements in Sup-
porting Information). As expected the Φ values obtained by
PS are few hundreds of meV lower than those measured by
KP; this is due to the fact that the KP method measures an
average of all Φ values found in potentially different (and/or
non-homogenous) domains existing under the probe head.
Differently, PS is most sensitive to the lowest work function
in the probed area, as the secondary electrons with the lowest
kinetic energy are used for the determination.[21] Interestingly,
KPFM revealed a similar work function shift when a trans
FAZO-SAM was irradiated with UV light (i.e., ΔΦtrans-cis); this
shift amounts to 90 meV with Φ = 5.50 0.01 eV and 5.41
0.01 eV for the trans- and cis- states, respectively. KPFM meas-
urements were used to monitor the Φ variation of the sample
during in situ UV light/dark cycles (Figure 2b). Differently
from the previously studied non-fluorinated AZO-SAM,[13]
the two switching kinetics are similar showing a simple expo-
nential behaviour with a characteristic time of ∼100 min. The
simple exponential decay suggests that the switching can be
described in terms of a single mechanism and that all FAZO
molecules chemisorbed within an area of about 100 μm2 are
activated at the same time, i.e. no serial activation as observed
in the case of AZO-SAM.
the work function of the functionalized gold electrode, Φanode
,
by using the equation Vnull = (Φanode–Φcathode)/e, where Vnull is
the zero-crossing voltage of the EA signal and e is the electronic
charge. The obtained Vnull was 2.02 0.06 V, which corresponds
to a Φanode of 5.77 0.1 eV for the FAZO functionalized gold
electrode. Figure 2a shows the EA signal as a function of the
applied DC bias, which is compared with the recently reported
non-fluorinated AZO SAM[13] to show the large Φ shift caused
by the fluorinated terminal benzene ring.
All techniques consistently displayed higher Φ values for the
trans isomer based SAM compared to the corresponding cis.
This trend, which is opposite compared to our previous find-
ings on a non-fluorinated azobenzene derivative, is due to the
presence of a perfluorinated terminal phenyl ring featuring
strong electron-withdrawing characteristics. Interestingly, our
result on the FAZO-SAM is in line with previously reported
works on other azobenzene based SAMs having an ending CN
group exposed to the surface.[12]
Macroscopic KP measurements revealed a large Au work
function shift compared with bare gold (Φ = 5.12 eV[13]):
freshly prepared trans FAZO-SAMs on Au/glass were found
to display a Φ shift up to 5.78 0.02 eV, in perfect agreement
with the EA results (with 0.02 being the standard deviation).
Figure 2. (a) DC voltage scan of the EA signal towards positive bias. The zero crossing voltages are 1.39 V (5.15 0.10 eV) for AZO (black circles)
and 2.02 V (5.77 0.1 eV) for FAZO (white circles). (b) KPFM on the FAZO-SAM on Au(111). The work function variation is monitored in situ while
irradiating the initial trans SAM and then following the thermal back conversion (in dark) from the cis to the trans SAM.
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contains two independent molecules chemisorbed within a her-
ringbone packing on a slab made five layers of gold; the chosen
number of gold layers ensures a good convergence of the results
when varying the thickness of the slab (details about the theo-
retical methodology and geometry optimizations are reported in
the supporting information). The work function of gold modified
with the cis and trans isomers are 5.06 eV and 5.28 eV, respec-
tively, which represents a decrease by −0.20 eV and an increase
of +0.02 eV with respect to the clean metal surface. The higher Φ
observed for the trans form is consistent with the experimental
data although the calculated ΔΦ (220 meV) is higher than the
experimental one (90 meV) measured with the KPFM. The com-
parison of the absolute Φ values is most likely hampered by the
actual nature of the gold sample and by the presence of contam-
inants, which can shield the intrinsic Φ of the sample surface
both in the KP and the KPFM measurements.
In order to investigate the bond dipole versus molecular con-
tributions to this shift, we have also computed the potential pro-
file of the free SAM in Figure 3a. For the cis form, the molecular
contribution is −0.85 eV, leading to a bond dipole of +0.65 eV.
A close value of −0.72 eV is obtained for the trans form, which
translates into a bond dipole about +0.74 eV. According to our
calculations, the shift of the work function is thus driven by the
two counter-acting contributions.
In Figure 3b we have plotted the evolution of the generated
bond dipole when moving from the left to the right side of the
interface for both isomers as well as the difference between
cis and trans isomers. This difference is zero on the left side
and amounts to a total value of 96 meV on the right side. This
graph illustrates that the bond dipole does not build up exclu-
sively in the vicinity of the Au-S bond since the dipole is sig-
nificantly growing from the sulfur atom to the middle part of
the molecule. The bond dipole difference between cis and trans
forms is about 30 meV near the sulfur atom to be compared
with the final difference of about 96 meV. The bond dipole
stops increasing in the cis isomer when the second phenyl ring
is reached, while it grows until the fluorinated ring is reached
in the trans configuration.
The contribution of the molecular backbone to ΔΦ arises
from its dipole moment. For the cis isomer, the dipole moment
of the radical evolves from 3.20 D in the gas-phase optimized
geometry (as calculated in a large unit cell to avoid intermo-
lecular interactions) to 3.24 D and 3.12 D for isolated radical
molecules adopting the structure of the non-equivalent units
in the SAM; the dipole changes for the trans isomer from
2.86 D to 2.58 D and 2.46 D, respectively. Depolarization effects
next come into play to modify these individual dipoles when
accounting for the intermolecular interactions between the two
inequivalent molecules within the SAM.[22] The total dipole of
the unit cell based on the two independent radical molecules is
4.66 D for cis and 3.75 D for trans, thus showing a depolariza-
tion of about 30% compared to the isolated molecules (3.24 +
3.12 = 6.36 D for cis and 2.58 + 2.46 = 5.04 D for trans). The
use of periodic conditions further reduce the dipole of the cell
by up to 70%, leading finally to a similar dipole value of 1.41 D
for cis and 1.21 D for trans per cell, thus rationalizing the larger
ΔVSAM contribution for the cis form.
Figure 3. (a) Plane averaged potentials of the cis (top) and trans (middle)
SAMs on the gold (111) surface. The zero energy is set at the Fermi level
so that the work function is directly readable. We display curves for the bare
Au(111) gold surface in its relaxed state (black), the gold surface covered by
the SAM (red), and the free SAM layer (green). In the left part, we can identify
the five gold layers and the SAM contribution moving away from the surface
(see Supporting Information for more details). The two arrows on the right
side points to the work function shift of the full system (light grey) and of the
contribution arising from the free SAM (red). The difference between these
shifts corresponds to the bond dipole. (b) Evolution of the bond dipole at
the metal/SAM interface from the left to the right side. The blue and red
lines correspond to the trans and cis isomers, respectively. Reference ver-
tical lines correspond to the gold surface and different positions along the
molecular backbone in the superimposed trans geometry. The bond dipole
first decreases in the vicinity of the Au-S bond due to the repulsion of the
gold electronic cloud at the surface by the atoms of the deposited molecule
(i.e., pillow effect), then grows to reach its final value. The green solid line
corresponds to the difference between the bond dipoles of the two isomers.
This shows that the bond dipole is not confined in the Au-S bond region.
Figure 3a displays the calculated plane averaged potential pro-
files associated to the optimized structures of the unit cell for
the cis and trans isomers. For both isomeric forms, the unit cell
In summary, we have demonstrated that the modulation
of the gold work function can be easily achieved following an
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it is the case for the novel fluorinated azobenzene reported here.
In addition to the significant modification of the energetics
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Supporting Information is available from Wiley Online Library or from the
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Acknowledgements
This work was supported by the EC Marie-Curie IEF-OPTSUFET
(PIEF-GA-2009-235967), ITN-SUPERIOR (PITN-GA-2009-238177) and
RTN-THREADMILL (MRTN-CT-2006-036040), the NanoSci-E+ project
SENSORS, the ERC project SUPRAFUNCTION (GA-257305), and
the International Center for Frontier Research in Chemistry (icFRC).
The work in Mons is further supported by the Belgian National Fund
for Scientific Research (FNRS). C.V.D. and J.C. are FNRS research
fellows. Financial supports by the Swiss Nationals Science Foundation
(SNF) and the Swiss Nanoscience Institute (SNI) are also gratefully
acknowledged.
Received: April 30, 2012
Revised: June 20, 2012
Published online:
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Adv. Mater. 2012,
DOI: 10.1002/adma.201201737
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