Ultrathin reduced graphene oxide films as transparent top-contacts for light switchable solid-state molecular junctions

Article abstract of DOI:10.1002/adma.201300607

A new type of solid-state molecular junction is introduced, which employs reduced graphene oxide as a transparent top contact that permits a self-assembled molecular monolayer to be photoswitched in situ, while simultaneously enabling charge-transport measurements across the molecules. The electrical switching behavior of a less-studied molecular switch, dihydroazulene/vinylheptafulvene, is described, which is used as a test case. Copyright

Full text of DOI:10.1002/adma.201300607

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Ultrathin Reduced Graphene Oxide Films as Transparent
Top-Contacts for Light Switchable Solid-State Molecular
Junctions
Tao Li, Martyn Jevric, Jonas R. Hauptmann, Rune Hviid, Zhongming Wei, Rui Wang,
Nini E. A. Reeler, Erling Thyrhaug, Søren Petersen, Jakob A. S. Meyer, Nicolas Bovet,
Tom Vosch, Jesper Nygård, Xiaohui Qiu, Wenping Hu, Yunqi Liu, Gemma C. Solomon,
Henrik G. Kjaergaard, Thomas Bjørnholm, Mogens Brøndsted Nielsen, Bo W. Laursen,*
and Kasper Nørgaard*
The use of molecules as active electronic components in com-
putational circuitry is a longstanding goal of molecular elec-
tronics.[1] The development of synthetic molecules endowed
with desirable functionalities, and the incorporation of these
molecules into reliable solid-state electronic devices, have been
two major challenges since the dawn of molecular electronics.
From the perspective of functional molecules, species pos-
sessing at least two interconvertible molecular states are defined
as molecular switches, which have been studied extensively for
a broad range of nanoelectronic and nanomechanical applica-
tions.[2] In particular, the successful transduction of different
charge or conformational states of the molecules into electrical
switching signals show great promise for building memories
and logic devices at the molecular level.[3] Among a number of
external stimuli, light offers a non-invasive, low-cost and easily-
addressable way for triggering molecular switches in electrical
devices.[2b,c] To date, a variety of photochromic molecules have
been devised, and several well-studied families of photochromic
molecules, such as azobenzenes,[3f,4] diarylethenes[3b,5] and
spiropyrans,[6] have presented potential opportunities for opto-
electronic applications. A less explored photochromic system,
the dihydroazulene (DHA)/vinylheptafulvene (VHF) system
was introduced in the 1980s.[7] DHA undergoes a light-induced
ring-opening reaction to the corresponding VHF, which in
turn undergoes a thermally activated ring-closure reaction back
to DHA. The photoreactions of DHA derivatives to the cor-
responding VHFs are often characterized by higher quantum
yields than the trans to cis isomerization of azobenzenes, and
it was shown that the absorption maximum of VHFs can be
strongly tuned by substituent groups.[8] Single-molecule elec-
tronic measurements on the DHA/VHF system have recently
been undertaken,[9a,b] as well as a first study of DHA/VHF
switches immobilized as a monolayer on a gold surface.[9c]
From a device-fabrication perspective, the first step is to
unambiguously make contacts to molecular components while
retaining their functionality.[1c,10] While various test beds for
contacting and probing single molecules and their ensembles
have been reported,[11] molecular junctions based on self-assem-
bled monolayers (SAMs)[12] can be constructed and operated in a
more reproducible manner, more amenable to mass production
and inte-gration.[13] However, direct vapor deposition of metals
onto SAMs has proven to be very invasive and damaging,[10a,14]
normally resulting in a high percentage of short-circuits.[15] To
address this problem, several methods for making soft contacts
to SAMs have been developed.[10a,16] Among these methods
introducing a conductive interlayer between metal and SAMs
worked well to prevent shorts, achieving high device yield and
operational stability.[11g,13,17] Particularly, by using poly(3,4-eth-
ylenedioxythiophene) stabilized with poly(4-styrenesulphonic
acid) (PEDOT:PSS) as the interlayer, a light-controlled solid-
state switching device based on monolayers of diarylethenes
was successfully constructed,[5c] which was a breakthrough for
large-area functional molecular electronic applications. Never-
theless, for specific optoelectronic applications, PEDOT:PSS
(up to 100 nm thick) with an auxiliary semi-transparent top Au
contact[5c] would limit the efficiency of light in addressing the
underlying molecules, due to low transmittance.
Dr. T. Li, Dr. M. Jevric, R. Hviid, Dr. Z. Wei,
N. E. A. Reeler, E. Thyrhaug, S. Petersen, J. A. S. Meyer,
Dr. N. Bovet, Prof. T. Vosch, Prof. G. C. Solomon,
Prof. H. G. Kjaergaard, Prof. T. Bjørnholm,
Prof. M. B. Nielsen, Prof. B. W. Laursen, Prof. K. Nørgaard
Nano-Science Center & Department of Chemistry
University of Copenhagen
Universitetsparken 5, 2100 Copenhagen, Denmark
E-mail: bwl@nano.ku.dk; kn@nano.ku.dk
Dr. J. R. Hauptmann, Prof. J. Nygård
Nano-Science Center
Center for Quantum Devices & Niels Bohr Institute
University of Copenhagen
Universitetsparken 5, 2100 Copenhagen, Denmark
Dr. Z. Wei, N. E. A. Reeler, J. A. S. Meyer
Sino-Danish Centre for Education and Research (SDC)
Dr. R. Wang, Prof. X. Qiu
National Center for Nanoscience and Technology
Beijing 100190, P. R. China
Prof. W. Hu, Prof. Y. Liu
Beijing National Laboratory for Molecular Sciences
Key Laboratory of Organic Solids
Institute of Chemistry
Chinese Academy of Sciences
Beijing 100190, P. R. China
DOI: 10.1002/adma.201300607
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Figure 1. a) Schematic view of molecular test bed with rGO thin film as transparent top contact. b) Schematic cross section of the molecular junction
and molecular structure of DHA 1 and VHF 2 illustrating the thermo-optical switching. Charge transport measurements are made through adjacent
micropore junctions in series.
Very recently, our group[18] as well as Lee et al.[19] indepen-
dently reported the successful use of reduced graphene oxide
(rGO)[20] fi lm as the interfacial layer between molecules and
top metal electrodes. In particular, benefiting from the high in-
plane conductivity of rGO films compared with molecular com-
ponents, we introduced a new test bed in which rGO films can
perform as conductive top contacts and interconnects between
SAMs in series, obviating the need for an additional top metal
contact. Herein, by utilizing the high optical transparency of
ultrathin rGO films in the above-mentioned double junction
layout, we report a new type of light switchable solid state device
based on monolayers of photo switchable molecules (Figure 1).
The transparent rGO film and special double junction layout
permits the SAMs to be photoswitched in situ while simulta-
neously enabling charge transport measurements across the
molecules. Bidirectional conductance switching events with an
average on/off ratio of 5–7 were recorded and ascribed to the
structural changes of DHA/VHF isomers. This study is the first
demonstration of the use of graphene materials as transparent
top-contacts for light switchable solid-state molecular junctions,
and highlights the great potential for graphene materials in
molecular electronics.
The structural difference between interconvertible DHA
1 and VHF 2 in solution, with regard to conjugation, spatial
shape, and dipole moment, was reflected by significantly dif-
ferent UV–vis absorption maxima of the two species (Figure 2a).
DHA 1 exhibited an absorption maximum (λmax) at 358 nm
while VHF 2 exhibited a red-shifted absorption maximum at
476 nm. The reverse reaction occurred thermally and was sig-
nificantly accelerated by increasing the temperature. Revers-
ible conformational change between DHA 1 and VHF 2 can
be induced by alternating light irradiation and heat treatment.
The compounds were stable enough to survive several cycles
without any sign of decomposition. Similar UV–vis absorp-
tion spectra for other DHA/VHF couples have been reported
previously,[8,21] however, very little is known about the pho-
tochromic properties of DHA/VHF molecules in the solid state.
We employed Raman spectroscopy to probe the interconvertible
molecular states of DHA 1 and VHF 2 in the form of solid-state
powders. A part of the Raman spectrum of the DHA 1 pow-
ders is shown in Figure 2b. The Raman band arising from the
–CN groups was observed at 2246 cm−1 and had a FWHM of
≈6 cm−1 . After recording the Raman spectrum, the powders
were exposed to UV light. Upon UV exposure, a distinct color
change was observed going from transparent to red (Figure 2c).
Furthermore, the powder initially exhibited a bright blue fluo-
rescence (see Supporting Information), which rapidly disap-
peared during photoconversion. A new spectrum after UV expo-
sure showed a small peak at 2211 cm−1 from VHF 2 in addition
to the original peak at 2246 cm−1 arising from DHA 1, indi-
cating the DHA 1 had been partially converted into VHF 2. The
ratio between the integrated CN-peaks from DHA 1 and VHF 2
in this spectrum is 2:1. Our theoretical calculations indicate that
the peak from VHF 2 is roughly 10 times more intense than the
peak from DHA 1, leading to the conclusion that the majority
of the powder was still present as DHA 1. Most likely, the pho-
toconversion only took place in the top molecular layers of the
powder and the system would slowly convert back to initial
DHA 1, evidenced by a decreased intensity of the VHF 2 peak
over time (see Supporting Information). In order to fully pho-
toconvert the material, DHA 1 powder was dissolved in toluene
and placed under UV exposure for 15 min. The solution shifted
from colorless to deep red (Figure 2a). The solution was drop-
cast on a metal surface and set to dry in the dark. When dry,
another Raman spectrum was recorded. All the DHA 1 seemed
to have converted into VHF 2 since the peak at 2246 cm−1
had disappeared and only a peak at 2209 cm−1 with a FWHM
of ≈16 cm−1 was present (Figure 2b). The experimental Raman
spectroscopy data match closely the theoretical Raman spec-
troscopy calculation on the DHA 1 and VHF 2 (ab initio calcu-
lations, see Supporting Information).
After characterizing the DHA 1/VHF 2 couple in solution and
as powders, the next crucial step is to assemble them as SAMs
on metal surfaces without losing their photoswitching ability.
The acetyl-protected thiolate anchoring group was synthesized
in a meta configuration on a phenylene unit to decouple the
photochromic core from the electrodes in an effort to reduce
surface quenching (Figure 1). The SAM was formed on clean
and flat Au substrates. Electrochemical characterization by
cyclic voltammetry (CV) indicated that the surface properties
of the Au substrate were significantly changed by the coverage
of VHF 2 SAMs (see Supporting Information). X-ray photoelec-
tron spectroscopy (XPS) was employed to determine the quality
of the SAMs. The measurements were performed with angles
of 0°, 40° and 60° between the sample surface and the detector.
The data for different angles indicated that clean and uniform
monolayers were present on the Au surface. Figure 3a depicts
the S 2p spectrum for VHF 2 SAMs at 0°. No obvious changes
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μs-trans-VHF = 15.7 Debye).[22] Thus, the hydrophobicity of the
SAM surface should be reduced when converting DHA to
VHF.[23] The contact angle of VHF 2 SAM was found to be
63 1°. It reached 82 2° after 30 min thermal treatment at
70 °C, and reverted to its initial state after exposure to UV light
(365 nm) for 2 h. Optical images in Figure 3b depict a visible
change of surface wettability triggered by external stimuli. This
interconvertible change (≈19°) was attributed to two distinct
molecular states (VHF 2 and DHA 1) of the SAM. Reversible
wettability transitions can be reproduced for at least 5 cycles by
alternating heating and UV irradiation (Figure 3b), albeit with
prolonged UV exposure in the last few rounds (likely due to
degradation of the SAM). Collectively, XPS and water contact
angle measurement confirmed that the DHA 1/VHF 2 couple
can be immobilized onto an Au surface as high-quality SAMs
with their thermo-optical activity retained.
For the fabrication of reliable molecular junctions, well-
defined junction areas, minimized parasitic leakage current
and non-destructive contacts to the molecules are impor-
tant considerations. An insulating layer of aluminium oxide
(≈30 nm) was patterned by electron beam lithography (EBL)
to define Au bottom contacts in micropores (2 and 4 μm in
diameter). In this way, the molecules were only assembled
inside the pores (Figure 1). The presence of a SAM inside the
micropore cavity was confirmed by Kelvin-probe force micros-
copy (see Supporting Information). The preparation of rGO
thin films as a soft top contact was described in our previous
work, where they were shown to form reliable (>90% yield) and
highly uniform contacts in the applied micropore devices.[18,24]
rGO thin films have been studied extensively as a transparent
and flexible electronic material.[20a,c] In the present work, we
transferred the rGO films onto quartz (Figure 3c, inset) and
recorded optical transmission spectra of the films with varied
thickness (Figure 3c). These ultrathin films (<10 nm) were
confirmed to have fairly high and uniform light transmit-
tances across the UV–vis region. A decrease of transmittance
with increased film thickness was also observed, as expected.
Conversely, thicker films normally possess higher in-plane
conductivity.[18,20c] Concerning both optical and electrical prop-
erties, we chose ≈5 nm thick rGO films with up to 25 000 S m−1
in-plane conductivity and higher than 80% transmittance for
device fabrication. The optical transmittance of these rGO films
are found to be superior to the previously applied PEDOT:PSS
Au contacts (See Suporting Information for comparison). After
the molecules were self-assembled onto Au bottom electrodes
inside micropores, the rGO thin films were transferred onto the
SAMs as a top contact. The hydrophilic aluminium oxide, low
aspect-ratio (<1:50) holes, and flexibility of rGO films all facili-
tated the transfer process. Figure 3d shows optical and atomic
force microscopy (AFM) images of a working device covered
by a rGO film. The rGO thin film follow the topography of the
substrate very well making up potentially an ideal contact to the
SAM at the bottom of the micro well.
Figure 2. a) UV–vis absorption spectroscopy of DHA 1 and VHF 2 in
MeCN. Irradiation of DHA 1 (1.0 × 10−5 M) with UV light (365 nm)
induces ring-opening reaction to VHF 2. The initial DHA 1 and final VHF
2 absorption spectra are indicated by thick lines. VHF 2 undergoes a
thermally-induced ring-closure reaction back to DHA 1. b) Raman spectra
of solid-state powders composed of DHA 1 only, a mixture of DHA 1 and
VHF 2, and VHF 2 only. c) Upon UV exposure, the color of the powders
changed from transparent to deep red. Initially bright blue fluorescence
was also observed which rapidly disappeared during photoconversion.
The scale bar is 20 μm.
were observed after thermal annealing at 70 °C for 0.5 h (DHA 1
SAMs, not shown). Under both conditions, the S 2p signals
showed only one doublet with S 2p3/2 at 162 eV attributed to an
S atom bound to Au. No signal was observed at 164 eV (from
S–C or S–H bonds) and no peak appeared at 168 eV for oxi-
dized S. At higher detection angles, where XPS is the most sur-
face sensitive, traces of S at 164 eV were detected and traces of
oxidized S from air annealed sample were observed. The data
thus verified that the molecules were bound to the surface via
S–Au for both DHA 1 and VHF 2 SAMs. Data analysis revealed
a negligible change of SAM thickness (d) for the as-prepared
VHF 2 sample and the after-annealing sample (DHA 1), both
yielding thicknesses around d = 1.4–1.5 nm (further XPS infor-
mation is presented in Table S1 in the Supporting Information).
Calculations have revealed that the dipole moment of
VHF is about twice that of DHA (μDHA = 6.0, μs-cis-VHF = 11.6,
The solid-state molecular junctions were measured in a
double-junction configuration, as illustrated in Figure 1b.
rGO thin films not only made a non-destructive contact to the
SAMs, but also acted as the conductive interconnect allowing
measurements on SAMs in series. More significantly, trans-
parent rGO allows incident light to easily access the underlying
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Figure 3. a) S 2p XPS signals for VHF 2 SAMs. b) Optical images of water droplets on surfaces of DHA 1 (left) and VHF 2 (right) SAMs, and revers-
ible wettability transitions as a function of alternating thermal treatment and UV irradiation. c) Optical transmission spectra of rGO films with a series
of film thicknesses. Inset, a ≈7 nm-thick rGO film transferred onto quartz for transmission measurement. d) Optical and AFM images of rGO films
transferred onto bottom electrodes. The ultrathin flexible rGO film follows the topography of the electrode patterns and ensures a good contact to the
SAMs at the bottom of the microholes. The texture of the film is clearly visible.
molecules. The measurement started from junctions based
on VHF 2 SAMs. As the photochromic molecules were sensi-
tive to light while in solution, it was more straightforward to
use fully converted VHF 2 for the SAM formation rather than
pure DHA 1. Both 2 μm and 4 μm diameter junctions were
measured.
independent control experiments were conducted. J–V char-
acteristics of junctions without SAMs (rGO only), with SAMs
of non-switching C12 alkane thiols and conjugated OPE3 are
shown in Figure 4d–f, respectively. For rGO-only junctions,
ohmic behavior was observed and the current densities were
orders of magnitude higher. Neither thermal treatment nor
UV irradiation influenced the I–V characteristics. For junc-
tions with a monolayer of non-switchable molecules (Au/C12/
rGO/C12/Au and Au/OPE3/rGO/OPE3/Au), the influence of
UV irradiation was also negligible, while thermal treatment
resulted in a slight increase of conductivity of less than a factor
of 2 (Figure 4e,f). This small irreversible thermal annealing
effect is the only change that was observed in monolayers of the
non-switchable molecules, and optical switching was observed
only for the DHA/VHF couple. Collectively, it is evident that
the observed conductance switching is a molecular feature, that
is to say due to the interconversion between DHA 1 and VHF 2
isomers induced by optical and thermal stimuli.
The current magnitude of the DHA/VHF devices is in the
range expected for molecular junctions, as observed by the two
reference systems (C12 and OPE). The rGO electrode, on the
other hand, is orders of magnitude more conducting[18] thus
ruling out changes in the rGO conductance as the origin of
the observed device response. In previous measurements on
single molecules coupled only to one electrode with a tunnel-
ling gap to the other, the VHF isomer was shown to be more
The current density versus applied voltage (J–V) characteris-
tics for molecular junctions based on VHF 2 monolayers before
and after thermal annealing (70°C for 0.5 h, DHA 1) is shown
in Figure 4a. The data is averaged from at least 24 junctions
(both 2 μm and 4 μm diameter junctions) of the same batch.
An increase in J was observed after thermal treatment. Statistics
based on more than 200 junctions confirmed that SAMs of DHA
1 showed higher conductivity than that of VHF 2 (Figure 4b).
Conductance values were calculated from linear fits of the low-
bias region (−0.3 V ≤ V ≤ 0.3 V), and the corresponding his-
togram indicates an averaged conductance on/off ratio of 5–7
(Figure 4b). Exposure to UV irradiation in turn resulted in a
decrease of J and these bidirectional conductance switching
characteristics were reproduced by alternating thermal treat-
ment and UV irradiation (Figure 4c). We emphasize the fact
that UV irradiation normally did not fully recover the low-con-
ducting state of VHF 2 SAMs (see Supporting Information),
resulting in a decrease of the on/off ratio (Figure 4c, inset). To
confirm that the conductance switching is intrinsically due to
structural changes of the molecular components in the devices,
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Figure 4. a ) J–V characteristics of VHF 2 (as prepared) and DHA 1 (70 °C for 0.5 h) isomers self-assembled in the molecular junctions. The data are
averaged from at least 24 junctions of the same batch. b) Conductance on/off ratio histogram (purple bars) based on more than 200 junctions. SAMs
of DHA 1 show a higher conductivity than that of VHF 2 by an averaged factor of 5–7. The blue and red bars correspond to non-switchable OPE3 and
dodecanethiol molecular junctions, respectively. c) Representative bidirectional conductance switching characteristics induced by alternatingheating
and UV irradiation. UV irradiation normally does not fully recover the low-conducting state of VHF 2 and thus results in a decrease of the on/off ratio
(inset). d,e,f) J–V characteristics of junctions without SAMs (rGO only), with a SAM of dodecanethiol and with a SAM of OPE3, respectively.
conducting than DHA.[9] In the present case, through-bond tun-
nelling along the molecular backbone is expected to dominate
the charge-transport. Moreover, the junction conductivity is
an average of many molecules in a monolayer. The increased
backbone flexibility of the VHF 2 isomer gives rise to a larger
number of possible molecular conformations in the SAM,
compared with the much more constricted conformations
of DHA 1. This difference in conformational freedom is sup-
ported by ab initio calculations (see Supporting Information).
Hence, the surface morphology could play a significant role
in determining the conductivity of the device, as the different
molecular conformers will have varying coupling strengths to
the physisorbed top electrode. Specifically, the reduced confor-
mational flexibility in DHA 1 means that all the conformations
are of approximately similar height, that is to say, all the mole-
cules are equally close to the top contact and can contribute to
the charge transport. Conversely, for VHF 2 the transmission
through the conformations corresponding to relatively short
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of the Danish Ministry of Science, Technology and Innovation (Grant
09-065274), the European Union seventh Framework Programme
(FP7/2007–2013) under the grant agreement no 270369 (“ELFOS”) and
the Lundbeck Foundation. The authors thank Dr. Tue Hassenkam for
fruitful discussions.
tunnelling length (possibly more highly conducting states, for
example, injection through the cyano group) could be partially
screened by taller conformations (lower conductive states, for
example, injection into the 7-membered ring). Thus, the mono-
layer DHA/VHF device can be considered an electromechanical
switch, differing from individual single-molecule electronic
measurements. It is in this regard interesting to make a com-
parison with the conducting AFM studies of SAMs of cis/trans-
isomeric azobenzenes on a Au substrate.[3f ] These studies
reveal that the cis-isomer is more highly conducting than the
trans-isomer due to a smaller tunnelling barrier length. The
effective length of the cis-isomer is only from the Au–S bond to
the exposed azo group, touching the AFM tip, while it is the full
length of the molecule for the trans-isomer. In contrast, calcula-
tions reveal that trans-azobenzenes are significantly more con-
ducting than cis-azobenzenes in regards to transport through
the entire molecules.[25] We anticipate that the rGO top-contact
test bed can be employed generally for a wide range of light-
switchable monolayer junctions, some of which could exhibit
improved device performances depending on the nature of the
molecules and their monolayers.
Received: February 5, 2013
Revised: April 9, 2013
Published online:
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In summary, we have studied the reversible photoisomeri-
zation of DHA/VHF derivatives 1/2 in solution, in the solid
state and in SAMs, and analyzed its effect on the SAM thick-
ness, surface wettability, and electrical conductivity. More
significantly, by using solution-processed ultrathin rGO films
as a transparent, soft top-contact, we successfully constructed
solid-state conductance switches operated by thermo-optical
stimuli. The flexible, conductive and transparent rGO films
permit molecular monolayers to be photoswitched in situ
while simultaneously enabling charge transport measure-
ments across the molecules. The double-junction geom-
etry with well-defined electrode area and spacing obviates
the need for an additional top metal contact and provides
a simple reproducible architecture. Experimental results
showed that SAMs of DHA 1 displayed a higher conductivity
than that of VHF 2 with an average current on/off ratio of
5–7 (or >3 when accounting for thermal annealing effects).
By alternating thermal annealing and UV irradiation, bidi-
rectional conductance switching could be achieved. Inde-
pendent control experiments confirmed that the conduct-
ance switching properties were due to the intrinsic structural
change of photochromic molecules. These results dem-
onstrate the immense potential of graphene materials for
molecular electronic applications, for example, by exploiting
the high transparency, high conductivity and the mechanical
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