Solution-processed reduced graphene oxide films as electronic contacts for molecular monolayer junctions

Article abstract of DOI:10.1002/anie.201105895

Reduction to the essential: A highly conductive and soft carbon interlayer of reduced graphene oxide (rGO) prevents the formation of a filamentary current path, achieving both high yield and true molecular effects in monolayer-based molecular devices. Jun

Full text of DOI:10.1002/anie.201105895

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Communications
DOI: 10.1002/anie.201105895
Molecular Electronics
Solution-Processed Reduced Graphene Oxide Films as Electronic
Contacts for Molecular Monolayer Junctions**
Sohyeon Seo, Misook Min, Junghyun Lee, Takhee Lee, Sung-Yool Choi, and Hyoyoung Lee*
In monolayer-based molecular electronics, functionalized
molecules are utilized as active components; in electronic
devices, these functionalities are accessed through stable
electrical contacts.[1] In such molecular junctions, a metallic
contact can be used to complete a molecular electronic
circuit. For practical applications, however, the formation of a
soft contact is highly desired to avoid the possibility of a metal
filamentary short circuit or molecular damage caused by the
penetration of metal atoms into molecular monolayers during
direct metal deposition.[2,3] Therefore, contact fabrication
techniques such as indirect metal evaporation,[4] surface-
diffusion-mediated deposition,[5] and soft organic interlayer
coating[6–9] have been developed for molecular electronic
devices. One successful technique used for the formation of
stable molecular junctions involves the use of a conducting
layer of an organic material such as single-walled nanotubes
(SWNTs),[9] a conducting polymer (e.g., poly(3,4-ethylene-
molecular functionality is required for the further develop-
ment of these molecular devices. To develop a highly sensitive
interlayer with a high device yield, it is crucial to create an
interlayer that is stable, both chemically and electrically, with
a soft contact that is able to communicate between the
functional molecular monolayer and a metal electrode. To
optimize the sensitivity of the interlayer, its thickness should
be easily controllable by varying the number of layers. For a
stable junction, the interlayer should protect the molecular
monolayers from the penetration of hot metal nanoparticles
during the vapor deposition of a metallic top electrode
because most functional molecular monolayers, which are
only a few nanometres thick (< 2–3 nm), are vulnerable to
this contamination.
Chemically exfoliated graphene oxide (GO) consists of
atomically thin sheets of oxidized graphite that are dispersible
in various solvents[12,13] and can be used to produce dispersible
reduced graphene oxide (rGO) by chemical reduction.[12] The
conductivity of rGO is comparable to that of SWNT[14,15] and
is increased by thermal treatment, similar to graphite.[16,17]
Graphene and rGO are composed of sp2 carbon networks and
can act as electrodes for electronic devices.[18] One notable
advantage of the use of rGO for organic electronics is its
solution processability compared with that of graphene. For
example, an rGO film electrode can be easily prepared by
spin-coating an rGO solution[19] onto a substrate or by spin-
coating with GO followed by vapor reduction.[20] Addition-
ally, the strong p–p interactions between rGO nanosheets
result in a graphite interlayer distance of approximately
0.34 nm, leading to a high conductivity that is comparable to
that of highly oriented pyrolytic graphite (HOPG).[20] Elec-
trical conduction in rGO thin films is treated as that through a
semimetal such as graphite, and the contact resistance in rGO
devices is negligible.[21] Because rGO has high chemical
stability, mechanical strength, and a work function compara-
ble to that of gold,[22] rGO has been considered to be a
promising candidate for interfacial electronic contacts in
monolayer molecular electronics.
dioxythiophene):poly(4-styrenesulphonic
acid),
PEDOT:PSS),[6–8] or multilayered graphene[10] as a top
electrode or a bridging interfacial layer between the active
molecules and a top electrode. These layers can provide
proper contact resistance to facilitate true molecular effects in
monolayer-based devices composed of alkanethiols/alkane-
dithiols,[6,10] photoisomers,[8] p-conjugated organic mole-
cules,[9] or metal complexes.[7,11]
Nonetheless, although molecule-dependent electronic
transport has been thoroughly investigated, previous inter-
layer junctions have not allowed for a clear elucidation of the
intrinsic properties of functionalized molecules such as
memory components in molecular monolayer devices. A
reliable device system with a highly sensitive interlayer with
[*] Dr. S. Seo, M. Min, Dr. J. Lee, Dr. H. Lee
National Creative Research Initiative, Center for Smart Molecular
Memory, Department of Chemistry, Samsung-SKKU Graphene
Center, Sungkyunkwan University
300 Cheoncheon-dong, Jangan-gu, Suwon, Gyeonggi-do 440-746
(Korea)
E-mail: hyoyoung@skku.edu
Herein, we report the development of a solution-pro-
cessed electronic contact between rGO thin films and
molecular components in monolayer-based devices. The
rGO contact allows for stable monolayer junctions of
alkanethiol monolayers (e.g., molecular resistors) and
redox-active metal complex monolayers[23] (e.g., molecular
memories) that prevent the formation of metallic short
circuits and exhibit excellent junction preservation. The
effects of monolayer thickness and molecular functionality
were examined in novel molecular junctions using rGO
interlayers. Through the semimetallic rGO interlayer contact,
the current hysteresis loops and threshold conductance
Dr. T. Lee
Department of Materials Science and Engineering, Gwangju
Institute of Science and Technology
Gwangju (Korea)
Dr. S.-Y. Choi
Electronics and Telecommunications Research Institute (ETRI)
Daejeon (Korea)
[**] This work was supported by the Creative Research Initiatives
(project title: Smart Molecular Memory) of MEST/NRF.
Supporting information for this article including material synthesis
and characterization is available on the WWW under http://dx.doi.
org/10.1002/anie.201105895.
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voltages of a redox-active metal complex monolayer were
measured in a new molecular contact system, showing true
molecular effects that depend on molecular redox properties.
Solvent-dispersed rGO sheets were produced by the
reduction of GO sheets dispersed in water using a reaction
mixture of hydrazine and aqueous ammonia (Figure S1; see
Supporting Information for synthesis and characterization).
The conductivity of the synthesized rGO film (with thickness
of 6.5 ꢀ 0.5 mm) was approximately 7600 ꢀ 200 Smꢁ1,[14] thus
indicating the production of rGO sheets with much higher
conductivities than that of highly doped PEDOT:PSS.[6]
Schematic diagrams of the fabrication processes for
monolayer-based devices using rGO interlayers are presented
in Figure 1. Well-dispersed rGO sheets in solution were spin-
coated onto an Au bottom electrode. An Au top electrode was
vapor-deposited on the rGO film (to approximately 10 nm
thickness) at a low deposition rate of 0.1 ꢀsꢁ1. No hysteresis
or metallic filamentary conductance was observed in the
current–voltage (I/V) curves, hence indicating that the rGO
interlayer successfully prevented the penetration of top-metal
particles (Figure 1a). The low-bias resistance was determined
to be 160 ꢀ 5 W (at 1.0 V with 95% confidence), as measured
in 20 devices (> 99% yield).
tions. Self-assembled monolayers (SAMs) of alkanethiols
(e.g., 1-octanethiol (C8), 1-decanethiol (C10), and 1-dodec-
anethiol (C12)) behaved as molecular resistors in the
molecular junctions (Figure 1b). The resistance of the C10
SAM device was 4500 ꢀ 50 W (at 1.0 V with 95% confidence),
as measured in 50 devices (> 99% yield). Furthermore, the
SAMs of bis-thiophenylterpyridine-transition metal(II) com-
plexes ([MTII(tpyphs)2], where MTII = FeII, RuII, or CoII)
behaved as molecular memories in the molecular junctions
(Figure 1c). The I/V curves of the Au/rGO film/[CoII-
(tpyphs)2] monolayer/Au device showed sharp conductance
switching and large hysteresis loops. A cross-section of the
functionalized monolayer-based devices showed that the rGO
interlayer protected the SAM well (Figure S2).
Figure 2 shows the evaluation of the electron transport
behaviors of the molecular junctions of three alkanethiol
SAMs. In Figure 2a, current density is plotted on a logarith-
mic scale as a function of applied voltage for the alkanethiol
monolayers; the error bars represent the standard deviation
and corresponding average values (with 95% confidence) for
50 devices. Current density was observed to decrease as the
molecular length of the alkanethiols was increased. The
current density of the alkanethiol monolayer junctions was
exponentially dependent on the molecular length, thus
indicating that the dominant conductance mechanism was
tunnelling.[24,25] The I/V charac-
The contact effects of the rGO interlayer in molecular
electronic devices were examined in monolayer-based junc-
teristics shown in Figure 2b
indicate that the three alkane-
thiol devices exhibited no
noticeable degradation after
storage under vacuum condi-
tions for 30 days. Figure 2c
presents current density plots
at several biases as a function of
molecular length. The current
density (J) plot as a function of
the barrier width (d) can be
described by the expression J /
exp(ꢁbd), where b is the tun-
neling decay coefficient. Here,
the average value of b was
0.82 ꢀ 0.12 ꢀꢁ1, which is in
agreement with previously
reported values for alkanethiol
junctions.[10,26,27] This result con-
firms that the new rGO inter-
layer device system operates
reliably to give molecular trans-
port properties through mono-
layer junctions.
The molecular conductance
states that mediate electron
transport in the molecular elec-
tronic devices were determined
from their I/V characteristics
Figure 1. Fabrication of a soft electronic contact for monolayer-based molecular devices. a) An Au/rGO
film/Au junction and its current–voltage (I/V) characteristics, exhibiting low resistance. b), c) Au/rGO
film/molecular monolayer/Au junctions and I/V characteristics. The alkanethiol self-assembled monolayer
(SAM) in (b) shows high resistance, and the metal complex SAM in (c) shows hysteretic conductance
switching.
such as the threshold voltage
for conductance switching. The
redox potentials of electro-
chemically active molecules
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redox couples were measured against a saturated calomel
electrode (SCE) as reference and converted to vacuum
levels; Vabs(eV) = 4.7 eV + E0(SCE),[28] where E0 is the
formal redox potential and 4.7 eV was approximated based
on the vacuum level of the SCE reference electrode.
Furthermore, the electronic transition between molecular
redox states that appears at 450–650 nm caused by metal-
to-ligand charge transfer (MLCT) is related to the energy
gap between the highest occupied molecular orbital
(HOMO) and the lowest unoccupied orbital (LUMO;
Figure 3b). The HOMO–LUMO gap energies were calcu-
lated using the absorption energies calculated by the
extrapolation of the MLCT peaks. Consequently, the
redox molecular states (e.g., HOMO and LUMO) of the
[MTII(tpyphs)2] monolayer were available from the electro-
chemical measurements and the UV/Vis absorption meas-
urements shown in Figure 3.
On the other hand, redox-active molecular monolayers
showed unique I/V characteristics when incorporated into a
novel electronic contact system. Figure 4a–c shows repre-
sentative I/V curves on a linear scale for each Au/rGO
film/[MTII(tpyphs)2] monolayer/Au junction exhibiting con-
ductance switching between low-conducting and high-
conducting states. The threshold conductance voltages in
Figure 2. Effects of monolayer thickness on new molecular junctions with
an rGO interlayer. a), b) Current density–voltage profiles plotted on a log
scale for alkanethiol monolayers in rGO interlayer junctions after process-
ing (a) and after 30 days of storage (b). Alkanethiol SAMs are 1-
octanethiol (C8, black), 1-decanethiol (C10, red), and 1-dodecanethiol
(C12, blue). c) Plots of current densities at different voltages versus the
molecular lengths of the alkanethiol SAMs from the plots in (a).
are intrinsic characteristics, and electron transport in these
molecules occurs across their molecular redox states when the
molecules are placed between two electrodes.[28] However,
the interlayers establishing electronic contact in monolayer-
based electronic devices may or may not be sensitive to
conductance switching at the molecular level due to the redox
reaction potential. Thus, the precise mechanism for the
current hysteresis observed as a function of conductance
switching in redox-active monolayer junctions has not yet
been elucidated. As a characteristic aspect of the conductance
switching of redox-active monolayers, the molecular charging
energy involves the electrochemical potential of a molecular
conducting state.[28] A redox-active transition metal(II) com-
plex has discrete redox states that are induced by the
coordination of a conjugated ligand to a metal center, which
produces metal-center-dependent redox reaction potentials in
the associated cyclic voltammograms (CVs; Figure 3a). The
formal redox potentials measured with respect to MIII/MII
Figure 4. Effects of molecular functionality in new molecular junctions
with an rGO interlayer. I/V characteristics on a linear scale for Au/rGO
film/[MTII(tpyphs)2] monolayer/Au junctions show conductance switch-
ing at approximately 1.75 V for [FeII(tpyphs)2] (a), at approximately
2.05 V for [RuII(tpyphs)2] (b), and at approximately 2.20 V for [CoII-
(tpyphs)2] (c). The threshold conductance voltages observed in each
molecular junction from 50 devices are indicated as Vth in each I/V
curve.
Figure 3. Characterization of redox-active molecular monolayers.
a) Solid-state cyclic voltammograms (CVs) of [MII(tpyphs)2] SAMs on
ITO substrates in HClO4 (0.1m) at a scan rate of 0.1 Vsꢁ1; b) solid-
state UV/Vis spectra of [MII(tpyphs)2] films on ITO substrates.
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the [MTII(tpyphs)2] monolayer devices were dependent on
the central metal atoms of [MTII(tpyphs)2] (e.g., 1.5–1.9 V
for [FeII(tpyphs)2], 1.7–2.1 V for [RuII(tpyphs)2], and 1.9–
2.3 V for [CoII(tpyphs)2], which were measured in 50
devices for each complex). The conductance switching
behavior of these molecules is a consequence of molecular
resonant tunneling due to the energetic alignment of the
molecular state with the Fermi level of the metal at a
specific voltage. The current hysteresis loop also resulted
from the switching between two states, with electron charge
involving the electrochemical reaction of the redox-active
molecules. Thus, the molecular charging energy at the high-
conducting state corresponds to the energy difference
between the electrochemical potential of the redox-active
molecules and the Fermi level of the metal. The threshold
conductance voltages of each [MTII(tpyphs)2] are in good
agreement with the energy gaps between the Au Fermi
level and the electron affinity level of each metal complex.
The electron affinity level (i.e., LUMO) of the metal
complexes was obtained from the ionization energy (i.e.,
HOMO) and the MLCT electronic transition energy (i.e.,
the HOMO–LUMO gap). From the CV data shown in
Figure 3a, the formal redox potentials (i.e., the ionization
potentials corresponding to the HOMO) of these com-
plexes were determined to be 5.69 eV, 5.85 eV, and 4.87 eV
below vacuum for [FeII(tpyphs)2], [RuII(tpyphs)2], and
[CoII(tpyphs)2] respectively. As determined from the UV/
Vis absorption spectra (Figure 3b), the HOMO–LUMO
energy gaps for the complexes were 1.95 eV, 2.34 eV, and
2.18 eV for [FeII(tpyphs)2], [RuII(tpyphs)2], and [CoII-
(tpyphs)2] respectively. The corresponding LUMO levels of
the complexes were then calculated to be 3.74 eV, 3.51 eV,
and 2.93 eV below vacuum for [FeII(tpyphs)2], [RuII-
(tpyphs)2], and [CoII(tpyphs)2], respectively. Therefore, the
theoretical threshold conductance voltages were determined
by the energy gap between the metal Fermi level (5.1–5.3 eV
for Au)[23,28,29] and each LUMO level; the average values were
approximately 1.7 ꢀ 0.2 eV, 1.8 ꢀ 0.2 eV, and 2.2 ꢀ 0.2 eV for
[FeII(tpyphs)2], [RuII(tpyphs)2], and [CoII(tpyphs)2]. These
calculated values agree well with the experimental values (see
Figure 4a–c).
Figure 5. Memory effect of Au/rGO film/[MTII(tpyphs)2] monolayer/Au
devices. Write/multiple read/erase/multiple read operations are shown in
the left panel and the corresponding ON and OFF retentions are shown in
the right panel for molecular monolayers of [RuII(tpyphs)2] (a), [FeII-
(tpyphs)2] (b), and [CoII(tpyphs)2] (c).
layers. The write/erase operations were also achieved by
continuous reading at + 1.5 Vafter pulsing voltages for 1.5 s at
+ 3.0 V for the ON state and 0.0 V for the OFF state. WRER
cycles for a [RuII(tpyphs)2] device were conducted over 40
times (Figure 5a) for one test set, and good reproducibility
was observed in the following five test sets. The memory
effects of [FeII(tpyphs)2]- and [CoII(tpyphs)2]-monolayer-
based rGO junctions were also successfully programmed
(Figure 5b for [FeII(tpyphs)2] and Figure 5c for [CoII-
(tpyphs)2]). The retention time of each ON and OFF state
in all of the memory devices was sustained for a minimum of
10 min, which is characteristic of nonvolatile memory.
To demonstrate the performance of a molecular memory
component in the rGO interlayer junctions, memory reversi-
bility tests for the Au/rGO film/[MTII(tpyphs)2] monolayer/Au
junction were examined by write/multiple read/erase/multiple
read (WRER) operations[7,9] (Figure 5). At 1.5 V, represent-
ing an intermediate reading voltage, the ON/OFF current
ratio for the high-conducting state (ON) and the low-
conducting state (OFF) of [MTII(tpyphs)2] monolayers was
shown to be approximately five. Such small ON/OFF ratios
have often been observed in monolayer-based memory
devices containing redox-active molecules,[7,9] whereas large
ON/OFF ratios have been reported in spin-coated thick
films.[30,31] As film thickness is increased, the OFF current
sharply decreases, leading to an increased ON/OFF ratio.[31]
Thus, molecular monolayers that are a few nanometers thick
(< 2–3 nm) are expected to lead to small ON/OFF ratios.
Although large ratios are advantageous, they are not always
required for potential applications,[32] especially in very thin
In conclusion, solution-processed rGO films were dem-
onstrated as universal electronic contacts that can be used to
elucidate the electronic functionality of molecular resistors or
molecular memories in molecular electronic devices. Due to
the semimetallic property of rGO thin films, they exhibit high
conductivity; the rGO-interlayer contact showed interesting
electronic characteristics comparable to those of a metallic
contact system such as gold nanoparticle and nanowire
contacts. The rGO electronic contact represents a new test
bed for solution-processable and universal applications of
monolayer-based molecular devices with high sensitivity
because of its molecular functionality and high device yield.
Experimental Section
Fabrication of rGO-interlayer devices with SAMs: The synthesized
rGO was dissolved in DMF (0.5 mgmLꢁ1). The supernatant of the
rGO solution was used for spin-coating to produce an rGO film on an
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Au electrode for monolayer molecular devices. Au bottom electrodes
(80 nm thick) were deposited on Ti (5 nm)/SiO2 (300 nm)/Si by
electron-beam evaporation at a low deposition rate (0.1 ꢀsꢁ1).
[MTII(tpyphs)2] (3 mm; see Supporting Information for synthesis) in
DMF was used to produce a SAM on the Au bottom electrodes by a
36 h immersion after deprotection of the thioacetate groups by adding
approximately 15 mL of aqueous ammonia (28 wt%, Aldrich). SAMs
of alkanethiols (e.g., C8, C10, and C12, all purchased from Aldrich)
were prepared by immersing the Au substrate in 2 mm alkanethiol in
ethanol for 24 h. All substrates were prepared by chemical cleaning
using piranha solution (H2SO4:H2O2 at 3:1, a very strong oxidant
requiring caution in handling), washing with deionized water and
ethanol, and drying with N2 gas. The well-dispersed rGO sheets in
solution were spin-coated onto the SAMs, and Au top electrodes
(100 ꢁ 100 mm2) 60 nm thick were then vapor-deposited (0.1 ꢀsꢁ1) on
the rGO film/SAM/Au bottom electrodes. The electrical character-
istics of the devices were measured by a Keithley 4200-SCS semi-
conductor characterization system in vacuum.
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Keywords: molecular electronics ·
molecular monolayer junctions · nanotechnology ·
reduced graphene oxide · self-assembly
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