Molecular electronics - Integration of single molecules in electronic circuits

Article abstract of DOI:10.2533/000942902777680144

Electronic devices made with organic molecules may one day play an important role in integrated circuits. However, the research is still in its infancy. The mechanically controlled break-junction technique allows the measurement of the current through a s

Full text of DOI:10.2533/000942902777680144

NANOSCIENCE AND NANOTECHNOLOGY
494
CHIMIA 2002, 56, No. 10
Chimia 56 (2002) 494–499
© Schweizerische Chemische Gesellschaft
ISSN 0009–4293
Molecular Electronics – Integration of
Single Molecules in Electronic Circuits
Marcel Mayor* and Heiko B. Weber*
Abstract: Electronic devices made with organic molecules may one day play an important role in integrated
circuits. However, the research is still in its infancy. The mechanically controlled break-junction technique
allows the measurement of the current through a single molecule junction. The current–voltage relations in-
dicate a nonlinear conductance. Information about the junction can be provided by comparative studies of
different molecules. It turns out that the conduction properties depend strongly on the molecular structure.
This, in turn, encourages the idea of tailoring the electronic properties by an appropriate design of the
molecule.
Keywords: Break-junction · Electron transport · Molecular electronics · Molecular wires ·
Single-molecule studies
Introduction and Brief Review
potential of this approach is not only the acts with the dipole moments of the mole-
very small size of molecules, giving rise to cules and destroys the ordered film. Re-
For almost four decades, silicon-based inte- the hope for further reductions of the fea- cently, memory devices could be fabricated
grated circuits have continuously reduced ture sizes on the chip. The production cost by deposition of tailor-made supramolecu-
the feature sizes of devices. This miniatur- of a device with its electronic function de- lar systems like catenanes [4] or rotaxanes
ization, described by Moore’s law, has been fined by chemical synthesis may become [5] as LB-films between electrodes. Elec-
mainly driven by the desire to reduce the much cheaper than a device with the corre- trochemically active components on the
cost per functional unit. However, in the sponding function based on multi-step clas- molecular building blocks of the super-
last few years it became obvious that fur- sical semiconductor production technolo- molecule allow individual components to
ther decrease in feature size will become gy. However, currently this remains specu- be addressed by voltage pulses. Thereby,
increasingly difficult due to physical limi- lation as the technology to integrate and to changes in charge result in configurational
tations, accompanied by an enormous in- address molecules or molecular assemblies changes of the supermolecule. The setup al-
crease in expenses. This explains the grow- on a chip has not yet been developed. But lows reversible switching between two dis-
ing interest for alternative concepts to build the research activity has strongly increased crete configurations of the supermolecule,
electronic devices.
A very promising approach is the inte- perimental breakthroughs.
gration of molecular structures to supple- The idea of profiting from molecular low voltage and have different tunneling
in the last few years [1], driven by some ex- which are shown in Scheme 1 for the cate-
nane. As both configurations are stable at
ment specific functionalities on a semi- structures in order to perform electronic characteristics, the actual state of the switch
conductor chip. The physical properties of functions has its roots in the 1970s. In a pi- can therewith be read out at lower voltage.
such molecule-on-chip devices will depend oneering theoretical paper Aviram and Rat-
For controlled use of molecular struc-
strongly on the molecular structure. Con- ner described the behavior of a molecule, tures, a detailed understanding of the inter-
sequently, the electronic function of the consisting of two separated acceptor and play between the structure and the electron-
device may be given by careful design and donor π-systems, between two metallic ic properties is crucial. Until the 1980s,
tailor-made chemical synthesis. The huge electrodes [2]. Due to the energetic differ- such studies on molecular structures were
ences of the frontier orbitals of the two limited to bulk properties [6]. With the de-
π-systems, they predicted the characteris- velopment of scanning probe methods in-
tics of a rectifier for such a device. Nearly vestigations of individual molecules be-
two decades later, the proof of principle came experimentally accessible [7]. For
of this concept could be shown with example, the conductance of individual
donor–acceptor molecules organized in a conjugated molecular rods was qualitative-
*Correspondence: Dr. M. Mayor and Dr. H.B. Weber
Langmuir Blodget (LB) film [3]. Sand- ly investigated by comparison of AC and
wiched between two metallic leads, the LB- DC STM studies on a gold surface covered
film displayed for a few cycles rectifying with an insulating thioalkane self-assem-
characteristics. Unfortunately, the rather bled monolayer (SAM) and longer mole-
weak van der Waals interaction of the mol- cular rods, situated in defect positions of the
ecules with the surface was insufficient to SAM [8]. The common topological and
withstand the applied voltage, which inter- conductance maxima were interpreted as
Forschungszentrum Karlsruhe GmbH
Institute for Nanotechnology
P.O. Box 3640
D–76021 Karlsruhe
Tel.: + 49 724 782 6392
Fax: + 49 724 782 5685
E-Mail: marcel.mayor@int.fzk.de;
heiko.weber@int.fzk.de
www.fzk.de/int/

NANOSCIENCE AND NANOTECHNOLOGY
495
CHIMIA 2002, 56, No. 10
symmetries along the rod axes. First corre-
lation studies between the overall resist-
ance of the electrode–molecule–electrode
setup and the investigated molecular struc-
ture are presented. Theoretical studies ac-
company the experiments and support the
design of future molecular structures with
defined electronic functions.
Mechanically Controlled Break-
Scheme 1. A catenane molecule with two metastable conformations which can be converted Junction (MCB)
by reversible reduction/oxidation, used in [4] for resistive memory devices. The device is formed
by a Langmuir-Blodget film of these molecules, sandwiched between electrodes. The reduc-
A single-molecule contact requires a
tion/oxidation can then be steered by applying an appropriate voltage pulse ( 2 V). Thus
information can be written on the device. As both conformations have different conductivities,
the information can be read out at low voltages.
pair of electrodes separated from each
other by exactly the length of the molecule.
The MCB technique provides a pair of
electrodes with adjustable distance. The
the effects of the protruding conjugated gold contacts and the I/V characteristics method is based on a lithographically fabri-
rods. In another approach, the lateral extent were investigated as a function of the elec- cated gold structure consisting of two large
of a SAM was limited by the use of pores in trode distance. Although these are both areas connected with a thin gold bridge
a silicon nitride membrane with a diameter pioneering experiments, they lack any in- (thickness 20–50 nm). This gold structure is
of about 40 nm. A SAM of 2'-amino-4- formation about the number of molecules patterned on an insulating polyimide film
ethynylphenyl-4'-ethynylphenyl-5'-nitro-1- between both electrodes.
on a flexible substrate. Reactive ion etching
benzenethiol was deposited on a gold sur- Our own experiments, which we pres- removes the polyimide around the gold
face in a pore and covered with a gold elec- ent here, try to address several new topics. structure, underetches the gold bridge and
trode [9]. The device displayed a negative A further development of the MCB tech- leaves it freely suspended. The final struc-
differential resistance (NDR: increase in nique as tweezers for molecular structures ture on the substrate is shown in Fig. 2
voltage causes a decrease in current above is presented. Molecular rods are designed (left). The substrate is fixed in a three point
a certain threshold) at temperatures below and synthesized in order to analyze the setup that allows the substrate to be me-
60K. The device architecture and its cur- situation on a molecular level between the chanically bent (Fig. 2, right). In a vacuum
rent/voltage (I/V) characteristics are sche- two electrodes of the MCB. To investigate chamber the substrate is carefully bent
matically represented in Fig. 1. This I/V the question whether the above-mentioned while the electric resistance between the
characteristic turned out to be stable and re- NDR originates from the molecular struc- two large gold areas is monitored. Thereby,
producible. However, whether this uncon- ture or from the device architecture, the cor- the thin gold bridge is elongated until it
ventional behavior arises from the molecu- responding molecule with two thiol func- finally breaks, which is observed as an im-
lar structure or from the device architecture tions was synthesized and investigated in a mediate increase of the resistance. Release
has not yet been clarified and is the topic of MCB. Additional information about the of the bending tension allows the two bro-
current investigations.
number of investigated molecules in a ken ends of the gold structure to approach.
Attempts to contact a single molecule MCB was gained by synthesizing and in- The extremely flat architecture results in a
with two electrodes have already been vestigating molecular rods with different distance resolution of the two electrodes of
made. A first mechanically controlled
break-junction (MCB) experiment was
made with a fixed thin gold wire with a pre-
determined breaking point on a flexible
substrate [10]. The gold wire was exposed
to a para-dithiobenzene solution such that a
SAM of these molecules was formed on the
surface. By careful bending of the substrate
while observing the current through the
gold wire, the breaking of the wire could be
determined. When the bending tension was
released again, the two broken ends ap-
proach each other. At a given point, the first
molecule is expected to touch the opposite
side, forming thiol-gold bonds to both elec-
trodes. The electric transport through this
setup was investigated and interpreted as
the transport through bridging molecules.
To improve this MCB technique the gold
wire was replaced by lithographically fab-
ricated gold structures [11]. Terthiophenes,
which were thiol functionalized on both
increases strongly and breaks down. The phenomenon that the current decreases when the
ends, were immobilized between the two voltage is increased is called negative differential resistance (NDR).
Fig. 1. a) Sketch of the experiment in which a self-assembled molecular film is sandwiched be-
tween two gold films [9]. This film is placed in a nanopore in a Si3N4 membrane, which restricts
the lateral dimension of the device. b) These devices show an unusual I/V relation at low tem-
peratures: the current is strongly suppressed at low voltages. At V ~ 2 V, the current suddenly

NANOSCIENCE AND NANOTECHNOLOGY
496
CHIMIA 2002, 56, No. 10
better than a tenth of an Ångstrom. The set-
up is therefore ideal to immobilize molecu-
lar rods with lengths in the order of a few
nanometers.
Integration of Single Molecules in
Electronic Circuits by MCBs
So far, no microscopy technique is suit-
able to analyze the situation between the
two electrodes of a MCB. Information can
only be gained by analysis of the conduc-
tance data and comparison between appro-
priately designed different molecules. Aba-
sic requirement for the molecule design is a
rod-like structure with two thiol endgroups
for covalent linkage on the gold surfaces.
The experiment discussed above, display-
ing a NDR for a SAM of a rod-like mole-
cule, inspired our first investigations. As
the molecular rod of the SAM was thiol-
Fig. 2. The mechanically controlled break-junction setup. Left panel: SEM-picture of a litho-
graphically produced gold film. The designated breaking point (in the center of the picture, 50
nm large) is a freely suspended gold bridge which forms the electrode pair once it is broken.
Right panel: Sketch of the bending mechanism. The setscrew is moved upwards to bend the
flexible substrate until the gold bridge on top breaks. By bending back and forth, the electrode
gap can be tuned with sub-Ångstrom resolution.
functionalized on one side only, the con- gins in the molecules’ structure or the de- acetyl-protected thiol functions are separat-
nection of the SAM to the top electrode is vice architecture, the corresponding rod- ed by about 2 nm. The acetyl-protected
not clear and may be the origin of the ob- like structure 1 with acetyl-protected thiol amino function and the nitro function in the
served unexpected I/V behavior. To investi- functions on both ends was synthesized central benzene ring of 1 result in a dipole
gate the current transport through this struc- (Scheme 2). 1 is a stiff and conjugated moment along the rod axis. For a single
ture in further details and to address the π-system, consisting of three benzene cores molecule immobilized between two elec-
question whether the observed NDR ori- connected with acetylene bridges. The two trodes, a dipole moment along the rod axis
Scheme 2. Synthesis
of the molecular rods
1, 2 and 3.

NANOSCIENCE AND NANOTECHNOLOGY
497
CHIMIA 2002, 56, No. 10
may result in a dependence of the I/V char- the opposite side is still acetyl protected. the I/V characteristics. In addition, sym-
acteristics on the direction of the applied The rod-like structures consisting of conju- metric I/V curves were recorded in the
current.
gated π-systems orient themselves in the locked-in state only for the symmetric
Of particular interest is a control exper- strong electric field towards the electrode molecular rod 2', while only asymmetric
iment with an immobilized molecular rod on the opposite side. When the electrode I/Vs were observed with the asymmetric
having a symmetry plane perpendicular to gap is closed further, a sudden rise in the rod 1'. This indicates, among other argu-
the molecules axis through its center. For current is observed. This current is insensi- ments, that most likely a single molecular
such a setup similar I/V characteristics in tive to small changes in distance, displaying rod is bridging the two electrodes in the
both current directions would be expected. a fairly stable lock-in situation, probably lock-in situation. If the junction was formed
The molecular rod 2 was synthesized for due to the first molecule bridging the two by many molecules in parallel with random
this purpose. With two acetyl-protected electrodes. All displayed I/V curves are orientation, one would rather expect a sym-
thiol functions on both ends, 2 has an in- recorded in this locked-in situation. Further metric I/V characteristic.
version center in its center. However, in its approach of the electrodes results in a
A sudden breakdown of the current, as
immobilized form, 2' has a symmetry plane strong increase of the current. Sometimes, a described for the SAM device (Fig. 1; [9])
perpendicular to its rod-axis. Both mole- second plateau with doubled current could at temperatures below 60K with a compa-
cules were synthesized with acetyl-protect- be observed and was interpreted as the con- rable molecular structure, was not observed
ed thiol functions. This protection group is tribution of a second molecule bridging the for the asymmetric rod 1'. As our experi-
known to deprotect spontaneously on gold gap.
surfaces and to slow down the kinetics of
ments were recorded at room temperature,
The I/V curves of 1' and 2' observed at this may be a possible explanation for the
SAM formation [12]. Both molecular rods room temperature in the lock-in state are lack of NDR. However, very recent experi-
were investigated repeatedly in a MCB displayed in Fig. 3. Comparable overall re- ments at temperatures below 30K have also
[13].
sistances through the setup of the order of failed to display an NDR. This suggests that
The broken MCB was opened to about 1 MΩ at 1 V were observed for both mole- the described NDR results from the archi-
10 nm and a 5×10–4 M solution of the mol- cules. This was expected, as both rod-like tecture of the device and does not have its
ecules in THF was applied for 30 s. After structures consist of comparable conjugat- origin in the molecular structure alone.
extensively rinsing with THF under N2 at- ed π-systems, namely in para-position
Quantum chemical studies of the inves-
mosphere, the sample chamber was evacu- ethynyl-connected aromatic systems. While tigated systems reproduce the phenomenol-
ated to ~ 5×10–7 mbar. At an applied volt- the I/V characteristics of the symmetric rod ogy observed in the experiments. To take
age of about 1 V the two electrodes were 2' was not affected by current inversion, the influence of the electrodes approxima-
slowly brought closer together.At this stage 1' displayed substantial differences upon tively into account, a Au29 cluster was
of the experiment we assume that individ- current inversion (Fig. 3). With the rod attached on both sides of the molecule
ual molecules are immobilized on the gold symmetry an intrinsic property of the mo- (Fig. 4). This supermolecule is still
surface of one electrode with the thiol func- lecular structure, for the first time a molec- tractable in DFT calculations. The main in-
tion on one end, while the thiol function on ular property was reflected and identified in terest was dedicated to charge redistribu-
Fig. 3. a) Current-voltage relations (I/V: grey
lines) and differential conductance (dI/dU:
black lines) of the molecular junction 1'. The
conductance is clearly nonlinear and asym-
metric with respect to current inversion (posi-
tive voltages correspond to one current direc-
tion through the molecule, negative voltages
to the opposite). The peak-like structures in
dI/dU probably indicate transport through
molecular orbitals. b) The corresponding
measurement with 2'. The data are (in good
approximation) symmetric with respect to cur-
rent inversion. The comparison of both meas-
urements proves that the I/Vs reflect intrinsic
properties of the molecules: their symmetry.
Fig. 4. To compute the properties of the sin-
gle-molecule junction, a DFT calculation was
performed with the symmetric molecule 2' at-
tached to two Au29 clusters, which simulate
the influence of the electrodes.

NANOSCIENCE AND NANOTECHNOLOGY
498
CHIMIA 2002, 56, No. 10
tion in the presence of finite electric fields, scaffolding [15]. Spectroscopic investiga- plied voltage. The overall resistance of the
which correspond to the applied voltage. It tions of trans-platinum(II)-linked acetylene electrode–3'–electrode device is with 5–50
turns out that the polarizability of the su- structures have shown the pure σ-character GΩ about three orders of magnitude larger
permolecule, which is related to the con- of the acetylene-carbon-platinum(II) bond than observed for the conjugated rods 1'
ductivity, behaves in a qualitatively similar and hence the potential of the platinum(II) and 2', pointing out the potential of the
fashion to the experimental observations ion as conjugation passive linker or inter- trans-platinum(II) connection as mechani-
[14].
rupter of the π-system. Therefore, the cally rigid but electronically interrupting
metalorganic trans-platinum(II) ethynyl linker in molecular systems. The I/V rela-
complex 3 was designed and synthesized tionship of 3' resembles the behavior of a
by the conversion of trans-bis(triphenyl- classical insulator and can be described by
Correlation of Molecular Structure
vs. Electronic Properties
phosphine)platinum(II)
(4-ethynyl)phenyl thioacetat [16], as shown gular tunnel barrier with a tunnel barrier
chloride with a quantum-mechanical model for a rectan-
The above experiment comparing the in Scheme 2. The resulting molecular rod height of 2.5 eV (circles in Fig. 6).
I/V characteristics with the molecule sym- 3 has again acetyl-protected thiol functions
metry already demonstrated the depend- at both ends that allow immobilization be-
ence of electronic properties on the molec- tween two gold electrodes. 3 crystallizes Summary, Conclusion and
ular structure. However, even though the from chloroform in single crystals, suitable Perspective
differences in symmetry were reflected by for X-ray analysis (Fig. 5). The platinum
the observed I/Vs, the overall electronic ion is coordinated in a square-planar
The results point out that individual
properties like threshold voltage of the con- arrangement by two trans-triphenylphos- molecules can be contacted with the MCB
ductivity and resistance of the device with phines and trans-acetylene ligands, result- technique. Together with tailor-made
the integrated molecule were comparable. ing in a sulfur-to-sulfur distance of 1.83 nm. chemical synthesis, the combination be-
Electronic transport is expected through
To investigate the electronic properties comes a beautiful tool to investigate the
one of the frontier orbitals of the π-system. of 3, similar protocols as for 1 and 2 were correlation between molecular structure
As both rods 1 and 2 are composed of sim- used. It turned out that after deposition of 3 and electronic property in detail. The sub-
ilar components (ethynyl-connected aro- on the electrodes, the applied voltage while stantial effect of the molecular structure on
matic systems) with similar connections approaching the two electrodes had to be the electronic properties that has already
(para-position for acetylenes and thiols), set at a higher value of ~ 4 V. Again a stable been demonstrated indicate the potential of
their conjugated π-systems are comparable lock-in situation was observed at room tem- molecules in future hybrid devices as func-
and similar electronic properties are not at perature and reproducible I/V curves were tional units. Currently, we are focusing on
all surprising.
recorded. Further approach of the elec- new molecular structures, designed and
But what happens if the conjugation on trodes occasionally resulted in a doubling synthesized to perform specific electronic
a molecular rod is interrupted? If the elec- of the current, probably due to a second functions. Among them are molecular rec-
tronic transport is through a frontier orbital molecule in the break-junction. The strong tifiers, molecular memory devices and mo-
of the π-system, such a molecular rod con- analogy to the behavior of 1 and 2 in the lecular rods that may display NDR effects.
sisting of two divided π-systems should break-junction leads to the assumption that The studies presented here are still on a ba-
display different electronic properties. most likely a single molecular rod 3 is sic research level and contribute to the un-
Naïvely, one would expect that the I/V bridging the two electrodes in the locked-in derstanding of a detailed structure–proper-
characteristics are dominated by the inter- state. The I/V curves recorded for 3' are ty correlation. The methods may even allow
rupting structure, which is the bottleneck shown in Fig. 6. They are symmetric with the study of critical parameters of integrat-
for electronic transport. Platinum(II) ions respect to voltage inversion and display an ed molecules like stability or fatigue prop-
are well-known rigid linkers in acetylene exponential increase of the current with ap- erties. However, to profit from the accumu-
Fig. 5. Molecular structure of 3 in the crystal,
obtained from X-ray structure analysis.

NANOSCIENCE AND NANOTECHNOLOGY
499
CHIMIA 2002, 56, No. 10
Fig. 6. The current-voltage relation of 3' (lines). The resistance of the device with 3' is more than
three orders of magnitude higher than with 1' and 2'. The shape of the I/V indicates insulating
behavior. The circle symbols are theoretical data, which assume a simple rectangular potential
barrier with a barrier height of 2.5 eV.
[5] Y. Luo, C.P. Collier, J.O. Jeppesen, K.A.
Nielsen, E. Delonno, G. Ho, J. Perkins,
H.-R. Tseng, T. Yamamoto, J.F. Stoddart,
J.R. Heath, ChemPhysChem 2002, 3,
519–525.
[6] For an overview: ‘Electronic Materials:
The Oligomer Approach’, Ed. K. Müllen,
G. Wegner, Wiley-VCH, Weinheim, 1998.
[7] G. Binning, H. Rohrer, C. Gerber, E.
Weibel, Phys. Rev. Lett. 1982, 49, 57–61.
[8] L.A. Bumm, J.J.Arnold, M.T. Cygan, T.D.
Dunbar, T.P. Burgin, L. Jones II, D.L. Al-
lara, J.M. Tour, P.S. Weiss, Science 1996,
271, 1705–1707.
lated knowledge and to integrate molecular
structures in semiconductor technologies is
still a huge challenge. The parallel integra-
tion and addressing of larger numbers of
molecules is with the methods presented
here not achievable [17] and future innova-
tive ideas and concepts are sought to over-
come this drawback.
Acknowledgement
We gratefully acknowledge D. Beckmann,
J. Reichert, R. Ochs, H. v. Löhneysen, C. v.
Hänisch, F. Weigend, R. Ahlrichs, M. Fischer,
M. Di Leo, C. Didschies, M. Elbing, J.M. Lehn,
S.T. Balaban, M. Hettler, O. Wollersheim and
the Strategiefond from the Helmholtz Founda-
tion for financial support.
[9] J. Chen, M.A. Reed, A.M. Rawlett, J.M.
Tour, Science 1999, 286, 1550–1552.
[10] M.A. Reed, C. Zhou, C.J. Mullen, T.P.
Burgin, J.M. Tour, Science 1997, 278,
252–254.
[11] C. Kegueris, J.-P. Bourgoin, S. Palacin,
D. Esteve, C. Urbina, M. Magoga,
C. Joachim, Phys. Rev. B 1999, 59,
12505–12513.
Received: August 28, 2002
[12] J.M. Tour, L.R. Jones II, D.L. Pearson,
J.J.S. Lamba, T.P. Burgin, G.M. White-
sides, D.L. Allara, A.N. Parikh, S.V. Atre,
J. Am. Chem. Soc. 1995, 117, 9529–9534.
[13] J. Reichert, R. Ochs, D. Beckmann, H.B.
Weber, M. Mayor, H. v. Löhneysen, Phys.
Rev. Lett. 2002, 88, 176804-1-176804-4.
[14] H.B. Weber, J. Reichert, F. Weigend,
R. Ochs, D. Beckmann, M. Mayor, R.
Ahlrichs, H. v. Löhneysen, Chem. Phys.
2002, 281, 113–125.
[15] P. Siemsen, U. Gubler, C. Bosshard, P.
Günter, F. Diederich, Chem. Eur. J. 2001,
7, 1333–1341.
[16] M. Mayor, C. v. Hänisch, H.B. Weber, J.
Reichert, D. Beckmann, Angew. Chem.
Int. Ed. 2002, 41, 1183–1186.
[1] For an overview: C. Joachim, J.K.
Gimzewski, A. Aviram, Nature 2000, 408,
541–548; M.A. Reed, J.M. Tour, Scientif-
ic American 2000, 282, 86–93; ‘Molecular
Electronics: Science and Technology’, Ed.
A. Aviram, M. Ratner, Ann. N. Y. Acad.
Sci. New York, 1998, 852; ‘Molecular
Electronics II’ Ed. A. Aviram, M. Ratner,
V. Mujica, Ann. N. Y. Acad. Sci. New York,
2002, 960.
[2] A.Aviram, M.R. Ratner, Chem. Phys. Lett.
1974, 29, 277–283.
[3] G.J. Ashwell, Thin Solid Films 1990, 186,
155–165.
[4] C.P. Collier, G. Mattersteig, E.W. Wong,
Y. Luo, K. Beverly, J. Sampaio, F.M. Ray-
mo, J.F. Stoddart, J.R. Heath, Science
2000, 289, 1172–1175.
[17] The integration of LB films between
crossbar electrodes as discussed in [4] and
[5] are first steps towards parallel integra-
tion.