Conductance switching and mechanisms in single-molecule junctions

Article abstract of DOI:10.1002/anie.201304301

A molecular switch: The conductance of a new molecular device based on graphene-molecule junctions was reproducibly switched between open and closed states under irradiation of external light (see picture). Three photochromic diarylethene derivatives with different substituents were used as key elements of the molecular devices. Copyright

Full text of DOI:10.1002/anie.201304301

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DOI: 10.1002/anie.201304301
Optoelectronics
Conductance Switching and Mechanisms in Single-Molecule
Junctions**
Chuancheng Jia, Jinying Wang, Changjiang Yao, Yang Cao, Yuwu Zhong, Zhirong Liu,
Zhongfan Liu,* and Xuefeng Guo*
From its very start, one of the most intriguing motivations of
molecular electronics is to provide unique and low-cost
solutions for electronic functions based on molecules, such as
diodes, transistors, switches, and memristors, since molecules
are probably the smallest units still capable of offering a rich
structural variety.[1,2] However, the ability to control the
conductance of molecules at the molecular level by an
external mode is still a formidable challenge in this field.
Here we report the observation of reproducible conductance
switching triggered by external light on a new platform of
graphene–molecule junctions, where three photochromic
diarylethene derivatives with different substituents are used
as key elements. Analyses of both transition voltage spec-
troscopy and first-principles calculations consistently reveal
tunable molecule–electrode coupling, thus demonstrating the
photogated inflection (Vtrans) transition when the charge-
transport mechanism changes from direct to Fowler–Nord-
heim (F-N) tunneling.
We chose diarylethene derivatives as photosensitive
molecular bridges because they, as a typical family of
photochromic molecules, can undergo reversible transitions
between two distinct isomers with open/closed conformations
when exposed to light irradiation (Figure 1a).[3] The closed
isomer is nearly planar, but the open isomer adopts a bent
conformation with its thiophene rings twisted about 618 out of
the plane from the cyclopentene ring. Correspondingly, these
two isomers display different absorption spectra, that of the
closed form extends towards longer wavelengths up to the
visible region, suggesting the delocalization of p electrons
over the entire structure (see Figure S1 in the Supporting
Information). In the open form, however, delocalization of
the p electrons is restricted to each half of the molecule and
electronic communication through the unsaturated bond of
the middle ring is interrupted. Another remarkable feature of
the diarylethene molecules used in this study is that only
negligible changes in the molecular length (ꢀ 0.2 ꢀ) happen
when they switch back-and-forth between open/closed states
(Figure S2 and Table S1). In conjunction with their superior
thermal stability and fatigue resistance, these significant
electronic and structural properties place diarylethene mol-
ecules as ideal candidates for building light-driven molecular
switches as demonstrated theoretically[4] and experimen-
tally.[5] However, a longstanding challenge is to conserve
these promising properties in solution when the diarylethene
molecules are sandwiched between solid-state molecular-
scale electrodes.[2,6] One major reason is due to the quenching
effect of the photoexcited states of the diarylethene molecules
by the electrodes,[5e,f] which strongly stresses the importance
of the molecule–electrode coupling strength to the device
performance.[2,7]
To tailor the energy level alignments at the molecule–
electrode interface, in this study we intend to modify diary-
lethene backbones with rationally designed side and anchor-
ing groups (1–3 in Figure 1b). This modification has two
specific considerations. The first is to substitute the hydro-
genated cyclopentene in 1 by the fluorinated unit (2). In
comparison with the hydrogenated cyclopentene, the fluori-
nated unit is electron-withdrawing and thereby decreases the
electron density on the central alkene unit and increases the
fatigue resistance of the photochromic properties.[3a] The
second is to further introduce a methylene group (CH2)
between the terminal amine group and the functional center
on each side (3). The incorporation of the saturated CH2
groups can cut off p-electron delocalization, thus largely
decoupling the electronic interaction between molecules and
electrodes. Theoretical calculations were performed to pre-
dict the electronic structures of the molecule–electrode
contacts as shown in Figure 1c (Table S2). Indeed, the
energy levels of 2 are lower than those of 1 because of the
electron-withdrawing effect of the fluorinated unit, which is
consistent with electrochemical measurements of similar
systems.[8] For 3, the energy levels are even lower. More
importantly, the calculated molecular orbital diagrams reveal
a lower orbital density of states (DOS) at the C sites of the
CH2 groups (Figure 1c), which implies that the CH2 groups
decrease the strong electronic coupling between diarylethene
molecules and electrodes. These results demonstrate the
potential of molecular engineering as an efficient tool for
tuning the molecule–electrode coupling strength. This tuna-
[*] C. Jia,[+] J. Wang,[+] Y. Cao, Prof. Z.-R. Liu, Prof. Z.-F. Liu,
Prof. X.-F. Guo
Center for NanoChemistry
Beijing National Laboratory for Molecular Sciences
State Key Laboratory for Structural Chemistry of Unstable and
Stable Species, College of Chemistry and
Molecular Engineering, Peking University
Beijing 100871 (P. R. China)
E-mail: zfliu@pku.edu.cn
guoxf@pku.edu.cn
C. Yao, Prof. Y.-W. Zhong
Institute of Chemistry, Chinese Academy of Sciences
Beijing 100190 (P. R. China)
[+] These authors contributed equally to this work.
[**] We acknowledge primary financial support from MOST (grant
number 2012CB921404) and NSFC (grant numbers 21225311 and
51121091).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201304301.
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Figure 1. a) Switching of graphene–diarylethene junctions. b) Molecular structures of 1–3. c) Calculated molecular energy levels and related
molecular orbital diagrams (LUMO+1, LUMO, HOMO and HOMO-1) of 1–3. ECB is the work function of graphene.
bility is of great importance in molecular junctions because it
allows precise construction of the contact interface as well as
adjustment of the interfacial electronic structure and explo-
ration of the intrinsic transport mechanism, depending on the
molecular structure to be used.
tively (More data for 1 or 3-reconnected molecular junctions
can be found in Figures S3–S5). Remarkably, photoinduced
significant consistent changes from the low-conductance (off)
state to the high-conductance (on) state were observed for all
the working devices. This observation should be ascribed to
the isomerization from the open, unconjugated conformation
to the closed, conjugated conformation under UV illumina-
tion (Figure 1a). To better understand the switching phenom-
enon through diarylethene molecules, the energy dependence
of the transmission spectra at zero bias voltage was calculated
as shown in Figure 2c,d and Figure S6. Clearly, the calculated
transmission spectra of open and closed conformations are
distinctively different near the Fermi level. Since the HOMO
and LUMO of diarylethene molecules with open conforma-
tions are localized frontier orbitals (Figure 1c), which are not
good conductive channels for electrons passing through the
junctions, there is not any significant transmission channel
located at the energy range from À0.05 to 2.00 eV (Fig-
ure 2c). On the contrary, the HOMO and LUMO of diary-
lethene molecules with closed conformations are delocalized
p-conjugated orbitals (Figure 1c), which provide good con-
ductive channels, thus leading to two significant transmission
peaks located at each side of the Fermi level (about À0.05 eV
and 1.2 eV; Figure 2d). For molecular electronic devices, the
nearest transmission peaks at each side of the Fermi level
Syntheses of 1–3 with the requisite amine anchors are
detailed in the Supporting Information. The graphene–diary-
lethene junctions were fabricated by a dash-line lithographic
(DLL) method described in detail elsewhere,[9] using chemical
vapor deposition (CVD) grown high-quality single-layer
graphenes on copper foils.[10] One important feature of this
approach is that the contacts made by covalent amide bond
formation are robust and thus can tolerate broad external
stimuli. In conjunction with the remarkable properties of
graphene electrodes, the ease of device fabrication and the
device stability position graphene–molecule junctions as
a new-generation testbed for molecular electronics. On the
basis of the statistical data, the average connection yields
were found at 25% for 1, 25% for 2, and 31% for 3, which are
reasonable to afford single-molecule junctions in most cases
as detailed in the Supporting Information.[9]
Figure 2a shows the comparison of the current–voltage
(I–V) curves of a 2-reconnected molecular junction before
and after irradiation of UV light from a low-intensity
handheld UV lamp (about 100 mWcmÀ2, 365 nm), respec-
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Both afore-achieved theoretical and experimental results
demonstrate that photoisomerization of diarylethenes
between open/closed states is able to change the molecular
energy levels at the contact interface, thus offering a chance to
study intrinsic charge-transport mechanisms in molecular
junctions. In most instances, charge transport in molecular
junctions can be simplified as electron tunneling through
a barrier (Figure 3a).[11] In the zero-bias limit, the shape of the
Figure 2. Switching properties of a 2-reconnected molecular junction
as a representative. a) I–V characteristics of 2 with open (red line) and
closed (dark line) states at VG =0 V. The inset shows the enlarged I–V
curve for the open state. b) The current-versus-time curve of the same
device at VD =50 mV and VG =0 V. c–d) The zero-bias voltage trans-
mission (T0) spectra of 2-reconnected junctions with c) open and
d) closed configurations. Both open and closed isomers are con-
structed to have similar angles with graphene (Figure S7). The full red
downward-pointing and full blue upward-pointing triangles stand for
the energies of the MPSH and isolated molecules, respectively. The
red and blue rectangles mark out the transmission peaks of p-HOMO
and p-LUMO.
corresponds well to the HOMO and LUMO of the molecules.
Then we further calculated the molecular projected self-
consistent Hamiltonian (MPSH) spectra on the entire photo-
chromic core between graphene electrodes (Figure S8). The
perturbed HOMO (p-HOMO) and LUMO (p-LUMO) retain
most features of the molecular orbitals, especially for p-
HOMO. Because p-HOMOs are much closer to the Fermi
level in all systems, they mainly dominate the carrier transport
behavior at low bias voltages. On the basis of the theoretical
results, consequently, it is no surprise that conformational
changes from open to closed form of individual molecules
when sandwiched between graphene point contacts lead to
the conductance switching of devices from the off state to the
on state, consistent with the experimental observation above.
In addition to the switching effect, we also found that the
on/off ratios are very high, gradually increasing from 60 for
1 and 200 for 2 to 300 for 3. This is reasonable because among
these three molecules, the transmission spectrum of the closed
form for 3 shows the largest difference in comparison with
that for the open form (Figure 2, Figure S6, and Table S3).
This is in excellent agreement with our original design
(Figure 1), which implies that the on/off conductance ratios
highly depend on the side groups outside the switching core
for single-molecule junctions.[4] Thus, the conductance switch-
ing is dominated by the changes in the molecular energy levels
as a result of either conformation transformation or the side
group substitution. These optoelectronic switching phenom-
ena are very reproducible since we observed the consistent
photoswitching effect in many devices (at least 19 working
devices for 1, 7 for 2, and 13 for 3).
Figure 3. a) F-N plots of the I–V characteristics for the same 2-
reconnected molecular junction used in Figure 2 with open (top) and
closed (bottom) states. The insets show the conventional barrier
S
model to qualitatively explain the inflection of the F-N curve. EF and
D
EF are the Fermi energies of source and drain electrodes, and Vtrans is
the voltage at which the inflection takes place. Note that the figures
are drawn for HOMO-mediated electron tunneling. b) The resonant
tunneling model to qualitatively explain the inflection of the F-N
curves.
tunneling barrier is rectangular. When the applied bias
exceeds the barrier height, the shape of the barrier changes
to be triangular, and the Fowler–Nordheim (F-N) tunneling
or field emission occurs (Figure 3a insets). From the plot of
ln(I/V2) against VÀ1 (F-N plot), an inflection point (Vtrans) can
be obtained, which is equal to the effective barrier height and
corresponds to the energy offset between the Fermi level of
the electrode and the closest frontier molecular orbital.
Figure 3a shows the typical F-N plots of the I–V character-
istics with open/closed states for the same 2-bridged molec-
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ular junction used in Figure 2a (More data for 1 and 3 can be
found in Figure S9). We found that Vtrans of the open state of 2
was about 0.41 V, less than that of the closed state (about
0.69 V). Similar data were also obtained in 1 (about 0.73 and
0.96 V for the open/closed states, respectively). Interestingly,
in the case of 3, we achieved Vtrans of about 0.63 V for the open
state, but a smaller Vtrans value of about 0.28 V for the closed
state, which is opposite to the findings in 1 and 2. This
observation should be ascribed to changes in the electronic
structures of the molecule–electrode contacts as shown in
Figure 1c, again proving the capability of modulating the
molecular energy levels at the contact interface by molecular
design (and thus the molecule–electrode coupling). For 3-
reconnected molecular junctions in which the work function
of graphene locates well between HOMO and LUMO in both
conformations, the charge-transport mechanism perfectly fits
to the barrier model shown in Figure 3a. The effective barrier
height for charge transport in the closed state is lower than
that of the open state. This is in excellent consistence with the
theoretical calculations which shows that the HOMO level
dominating charge transport is closer to the work function of
graphene for the closed form in comparison with the case of
the open form (Figure 1c and Table S2). Importantly, this is
direct evidence for experimentally observing the photogated
Vtrans transition when the charge-transport mechanism varies
from a direct to the Fowler–Nordheim (F-N) tunneling.
In addition to the simple tunneling barrier model, another
more real model for electron transport is the resonant
tunneling, which can clearly explain the Vtrans transition
observed above even for 1 and 2 in which the work function of
graphene is lower than the HOMO energy level of the closed
form of molecules (Figure 1c). On the basis of the Landauer
formula[12] and theoretical predictions,[13] the inflection in the
F-N plot takes place when a certain amount of the tail of the
resonant peak enters the bias window (Figure 3b). Therefore,
to better reveal the transition mechanism, we systematically
calculated the transmission spectra under the different biases
from 0 to 1.0 V, in which we have taken into the account the
polarization effects arising from the electric field (Figure 4
and Figures S10–S13).[14] Using a 2-reconnected molecular
junction as a representative, we found that when the bias
reached 0.5 V in the open state, the transmission peak of the
HOMO began to come into the bias window, thus providing
an additional conductive channel (Figure 4a). Furthermore,
from the MPSH spectra (Figure S11a), the slight increase of
the HOMO delocalization was also observed at the bias of
0.5 V. These should contribute to the inflection in the F-N plot
as the resonant tunneling occurs. As for the closed state of 2,
the transmission peaks are much closer to the electrode Fermi
level because of the decreased HOMO–LOMO gap and thus
the transmission peak of the HOMO comes into the bias
window at a smaller bias (0.2 V). However, the strength of the
transmission peak is very low (about 10À4), which has the
negligible contribution to the conductance. But, when the bias
increased to 0.7 V, we observed an obvious increase of the
transmission peak strength probably because of the polar-
ization effect under the enhanced electric field (Figure 4b).
On the other hand, we also detected a significant decrease of
the LUMO delocalization in the MPSH spectra at the same
bias (Figure S11b). This might be due to the quantum
interference or other quantum effects[15] by which the
decreased LUMO state leads to the higher transmission
peak of the HOMO. Therefore, it is because of the enhanced
transmission peak strength that the inflection of the F-N plot
emerges at a bias of 0.7 V for the closed state of 2. The same
analysis for a 1-reconnected junction generates very similar
results: Vtrans = 0.7 and > 1.0 V for open and closed states,
respectively (Figure S12). In the case of 3, however, the
opposite values were achieved, 0.6 V for the open state and
0.1 V for the closed state (Figure S13). These results are
consistent with the experimental observations as well as the
discussions using the barrier model.
In summary, this study demonstrates the ability to
integrate electronic functions based on molecules into
electronic nanocircuits. We achieved optoelectronic switching
from the off state to the on state when individual photo-
chromic diarylethenes sandwiched between graphene point
contacts transform between two states of conjugation. Spec-
troscopic analyses based on both barrier and resonant
tunneling models prove the tunability of the molecule–
electrode coupling strength through molecular engineering
and consistenty support the experimental observation of the
photogated tunneling transition in molecular junctions. This
demonstration also provides novel insight into designing new
types of molecule-based devices for revealing the relationship
between charge-transport mechanisms and the electronic
structures of molecular junctions.
Received: May 19, 2013
Published online: July 1, 2013
Keywords: charge transfer · diarylethene compounds ·
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graphene · molecular devices · optoelectronics
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