NANO LETTERS
Self-Assembly and Conductive Properties of Molecularly Linked Gold Nanowires
2004 Vol. 4, No. 1 19-22
Tue Hassenkam, Kasper Moth-Poulsen, Nicolai Stuhr-Hansen, Kasper Nørgaard, M. S. Kabir, and Thomas Bjørnholm* Nano-Science Center, The UniVersity of Copenhagen, UniVersitetparken 5, 2100 KøbenhaVn Ø, Denmark Received September 8, 2003; Revised Manuscript Received November 6, 2003
ABSTRACT A pseudo one-dimensional molecular electronic network consisting of segments of gold nanowires separated by 1−3 nm wide gaps and interconnected by thiol endcapped oligo(phenenylenevinylene)s (OPVs, 1.3 nm − 1.9 nm long) has been fabricated by lipid templated selfassembly. The electronic properties of the networks have been characterized in three situations in which: (i) only lipid molecules reside in the gap between gold wire segments or (ii) OPV molecules which are too short to bridge the gap or (iii) OPV molecules that are long enough to bridge the gap. The resulting network conductivity increases by 2−3 orders of magnitude with increasing covalent contact between OPV molecules and electrodes. An order of magnitude estimate of the low-bias conductance reveals G ≈ 50 nS for one OPV molecule which is long enough to covalently bind to both electrodes bridging the gap.
Conjugated organic molecules are becoming increasingly important as the active component in electronic devices, both in the form of low-tech high market volume applications (e.g., organic light emitting diodes)1 or as components in nanoscale devices based on a few molecules or a single molecule.2-9 The latter development is closely tied to a parallel development of controlled preparation of materials and objects with a critical length scale in the nanometer range.10 Although most approaches of current commercial importance are based on “top-down” lithographic techniques, alternative strategies such as nano- and atomic scale manipulation by scanning probe techniques as well as selfassembly approaches are becoming increasingly important.11,12 In particular, self-assembly methods13-18 can potentially overcome some of the shortcomings of lithographic techniques. An attractive scenario for future device fabrication would therefore be a combination of lithography and self-assembly, where the microstructured scaffold and the interface to the outside world are manufactured by conventional lithographic techniques, while the nanostructured functional units are self-assembled at designated sites of the device.19 In the present paper we demonstrate how nanosized gaps between gold nanowires (Figure 1B) assembled from thiol protected gold nanoparticles20 in the domain boundaries inherent to surfactant lipid systems19,21 can be cross-linked with thiol end-capped oligo(phenylenevinylene)s (OPVs)22 * Corresponding author. E-mail:
[email protected]. 10.1021/nl034752d CCC: $27.50 Published on Web 12/10/2003
© 2004 American Chemical Society
(Figure 1C), leading to a conductive molecular electronic network. This is illustrated in Figure 1A where an image of the surface potential of a network connected to two macroscopic electrodes at different potentials (∆U ) 1/2 V) reveals a gradual potential drop through the network as expected when the current is confined to the network. This effect is only observed when the network has been cross linked with OPV molecules. The electronic properties of this pseudo onedimensional molecular electronic circuit has been further characterized by using a conductive tip AFM23 as a contact to the network (Figure 2). By measuring the I-V characteristics at different separations between the macroscopic electrode and the conductive AFM tip we have eliminated contact resistance and extracted the intrinsic network conductivity (Figure 3). Investigations of different combinations of gap size D (≈ 1.5-3 nm) and OPV length d (≈ 1.3-1.9 nm) show that the conductance changes by several orders of magnitude depending of the details of the contact zone between molecule and electrode (cf. below). Molecules which can bind covalently to both gold nanowire segments in the gap have the highest conductance which we estimate to be ≈ 50 nS corresponding to a resistance of 20 MΩ. This order of magnitude estimate, corresponding to a low source-drain bias, is in fair agreement with recent theoretical24 and experimental2-8 studies, as discussed further below. The molecular electronic network was formed by spreading a mixture of lipid surfactant dipalmitoylphosphatidylcholine (DPPC) and 0.44 molpercent alkylthiol-protected gold
Figure 2. (Top) Resistance of a typical pseudo one-dimensional gold nanowire network as a function of the distance to a macroscopic electrode. The example shown consists of dodecanethiol stabilized gold nanoparticles reacted with molecule 2. (Bottom) AFM image of a selected gold wire. The resistance of the wire is measured at selected distances (marked by black cross) from the prefabricated gold electrode, which is situated in the left of the image.
Figure 1. (A) Kelvin force microscope image of a network of molecularly linked gold nanowire confined to a domain boundary inherent to a lipid system. The nonconductive phosphorlipid constitutes the white background. A potential of 0.5 V is applied to two prefabricated gold electrodes which appears in the top left and lower right side of the map. (B) High-resolution TEM micrograph of gold wire segments. The spacing of each wire segment was measured by TEM to be 3.0 nm for wires of dodecanethiol stabilized gold nanoparticles and 1.7 nm for wires of pentanethiol stabilized gold nano particles. These distances are in good agreement with X-ray reflectivity measurements on gap size between adjacent gold nanoparticles26 C) The thiol end-capped OPV molecules used in the present study, 1 trans-4,4′-Bis(acetylthio)stilbene and 2 (E,E)-1,4-bis[4-(acetylthio)styryl]benzene, both prepared according to litterature.22,27 The S-S′ distance was estimated using semiempirical PM3 calculations, giving S-S′ distances of 1.3 and 1.9 nm, respectively.
nanoparticles on a water surface as described in ref 19. The Langmuir monolayer was compressed to 30-40 mN/m and subsequently transferred horizontally (Langmuir-Scha¨fer) 20
to a solid support with a preformed network of gold electrodes prepared by UV/e-beam lithography. The electrodes could be connected to external wiring, thus enabling transport measurements through the network (Figure 1A). Alternatively, the AFM-probe was used as one of the electrodes and one of the static electrodes on the substrate as the other in a two-probe transport measurement of given pseudo one-dimensional network (Figure 2). This setup was inspired by a setup developed by Frisbie et al.23 which was able to obtain I-V curves at specific points on a substrate between the AFM tip and a predefined electrode on the substrate surface. The setup involves using the AFM in two modes.25 First a topography map is obtained, then the conducting tip is positioned and put into contact with the surface at a predetermined point. When good contact is established, I-V curves are recorded and the network conductance, G, is calculated from the slope of the I-V curves corresponding to a low bias (Ubias < 1 V). This procedure was repeated for a number of distances between tip and electrode, allowing the length dependence of the network conductance to be established. A typical example is shown in Figure 2. From the slope of such a plot (slope ) R/l, where R is the resistance and l the length of the network, respectively) combined with the measured values of the cross-sectional area of the network, A, (typically 3 nm high and 30 nm wide), we calculated the conductivity of the network as σ ) l/(R‚A). The above procedure was repeated after the sample had been exposed for 20 min to a 10-4 M Nano Lett., Vol. 4, No. 1, 2004
Figure 3. (Top) Graphical representation of average size of the gaps between gold wire segments (D) compared to the length of the molecules (d) corresponding to (i) bare dodecanethiol-stabilized gold nanowires, (ii) dodecanethiol-stabilized gold nanowires reacted with molecule 1, (iii) dodecanethiol-stabilized gold nanowires reacted with molecule 2, and (iv) pentanethiol-stabilized gold nanowires reacted with molecule 2. (Bottom) Measured network conductivity vs d-D corresponding to i-iv. The conductivity of bare pentanethiol and dodecanethiol-stabilized nanowires is close to the detection limit. The σ value of 0.05 S/m should hence be regarded as an upper estimate.
solution of OPV molecules deprotected by ethylenediamine in an argon flushed solution of acetonitrile/water 10:1 (v/v) to expose the free thiol end-group. Subsequently, the sample was washed with acetonitrile and water (MilliQ grade). In this way the conductivity of the four different situations illustrated in Figure 3 was measured. All conductance values are averages over 2-3 measurements. Since the gaps between the segments of gold nanowires are defined by the length of the alkylthiols initially protecting the gold nanoparticles, we were able to tune the gap size between 1.7 and 3.0 nm by changing the length of the alkylthiols. AFM images taken just before and after the OPV treatment revealed that the morphology and dimensions of the network were unchanged by the treatment. This implies that the segments of gold nanowires did not move, and thus Nano Lett., Vol. 4, No. 1, 2004
that the gap sizes between the segments was left unaltered by the treatment. Figure 3 summarizes the results of measurements in four different gap-molecule geometries (denoted i-iv) showing that the conductivity of the network without OPV molecules in the gap (i) is lower than 0.05 S/m, which corresponds to the detection limit of our setup. If the gap is twice as large as the length of the extended molecules, we observe an increase in the network conductivity to about 0.4 S/m (ii). Further extension of the molecular length allows the molecules in the gap to overlap resulting in conductivities around 1.5 S/m (iii). Finally molecules that are long enough to bind covalently to both sides of the gap result in network conductivities around 18 S/m (iv). Assuming that the latter value corresponds to a network of (single molecule)-(single cluster) complexes with a cross-sectional area corresponding to one particle (3 nm × 3 nm) and the length of one molecule connected to one particle (≈ 3 nm) we get an order of magnitude of the single molecule conductance amounting to G ) (18 S/m × 9 × 10-18m2)/ (3 × 10-9m) ≈ 5 × 10-8 S or R ) 1/G ≈ 20 MΩ. These values are in good agreement with a number of previous publications that all report on estimated single molecule conductances in this range.3,4,8 A recent theoretical study reports a reduction in conductance by 2 orders of magnitude if one of the covalent bonds to the electrodes is replaced by a van der Waals gap.24 This situation resembles the change in conductivity we observe when going from situation (iii) to (iv) in Figure 3 which is of the same order of magnitude. In summary we have produced a “molecular electronic circuit board” in which molecules can be plugged in to nanosized gaps between segments of gold nanowires. The entire circuit is produced by a series of purely chemical manipulations. Subsequent transfer to electrode arrays allows us to address the electronic properties of the circuit and estimate the order of magnitude and relative changes of single molecule conductances of molecules in covalent and/or van der Waals contact with the electrodes. Acknowledgment. This research was supported by the Danish Research Councils and the European Union (NANOMOL Project). We thank Jonas Nyvold Pedersen and Peter Toke Ahlgren for their contribution to the experiments. Fruitfull discussions with Dr. Mathias Brust (University of Liverpool) are gratefully acknowledged. References (1) Junji, K. Phys. World 1999, 12, 27-30. (2) Liang, W. J.; Shores, M. P.; Bockrath, M.; Long, J. R.; Park, H. Nature 2002, 417, 725-729. (3) Reichert, J.; Ochs, R.; Beckmann, D.; Weber, H. B.; Mayor, M.; von Lohneysen, H. Phys. ReV. Lett. 2002, 88, 176804. (4) Kergueris, C.; Bourgoin, J. P.; Palacin, S.; Esteve, D.; Urbina, C.; Magoga, M.; Joachim, C. Phys. ReView B 1999, 59, 12505-12513. (5) Park, J.; Pasupathy, A. N.; Goldsmith, J. I.; Chang, C.; Yaish, Y.; Petta, J. R.; Rinkoski, M.; Sethna, J. P.; Abruna, H. D.; McEuen, P. L.; Ralph, D. C. Nature 2002, 417, 722-725. (6) Liang, W. J.; Shores, M. P.; Bockrath, M.; Long, J. R.; Park, H. Nature 2002, 417, 725-729. (7) Park, J.; Pasupathy, A. N.; Goldsmith, J. I.; Chang, C.; Yaish, Y.; Petta, J. R.; Rinkoski, M.; Sethna, J. P.; Abruna, H. D.; McEuen, P. L.; Ralph, D. C. Nature 2002, 417, 722-725. 21
(8) Kubatkin, S.; Danilov, A.; Hjort, M.; Cornil, J.; Bredas, J. L.; StuhrHansen, N.; Hedegård, P.; Bjørnholm, T. Nature 2003, 425, 698701. (9) Nitzan, A.; Ratner, M. A. Science 2003, 300, 1384-1389. (10) Acc. Chem. Res. 1999, 5 (special issue on Nanoscale Materials). (11) Moriarty, P. Rep. Prog. Phys. 2001, 64, 297-381. (12) Andres, R. P.; Bielefeld, J. D.; Henderson, J. I.; Janes, D. B.; Kolagunta, V. R.; Kubiak, C. P.; Mahoney, W. J.; Osifchin, R. G. Science 1996, 273, 1690-1693. (13) Kiely, C. J.; Fink, J.; Brust, M.; Bethell, D.; Schiffrin, D. J. Nature 1998, 396, 444-446. (14) Li, M.; Schnablegger, H.; Mann, S. Nature 1999, 402, 393-395. (15) Pileni, M. P. J. Phys. Chem. B 2001, 105, 3358-3371. (16) Fendler, J. H. Chem. Mater. 2001, 13, 3196-3210. (17) Sanchez, C.; Soler-Illia, G. J. D. A.; Ribot, F.; Lalot, T.; Mayer, C. R.; Cabuil, V. Chem. Mater. 2001, 13, 3061-3083. (18) Bjørnholm, T.; Hassenkam, T.; Reitzel, N. J. Mater. Chem. 1999, 9, 1975-1990. (19) Hassenkam, T.; Norgaard, K.; Iversen, L.; Kiely, C. J.; Brust, M.; Bjørnholm, T. AdV. Mater. 2002, 14, 1126-1130. (20) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801-802. (21) Nielsen, L. K.; Bjørnholm, T.; Mouritsen, O. G. Nature 2000, 404, 352.
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(22) Stuhr-Hansen, N.; Christensen, J. B.; Harrit, N.; Bjørnholm, T. J. Org. Chem. 2002, 68, 1275-1282. (23) Loiacono, M. J.; Granstrom, E. L.; Frisbie, C. D. J. Phys. Chem. B 1998, 102, 1679-1688. (24) Brandbyge, M.; Stokbro, K.; Taylor, J.; Mozos, J. L.; Ordejon, P. Phys. ReV. B 2003, 67, 193104. (25) Experimental Note: Samples were examined by AFM using a Digital Instruments Nanoscope IIIa scanning probe microscope with the extender module and signal access module, working in Tapping mode. The transport measurements setup utilized force calibration mode for the AFM tip using a very low engange/disengage range. Electrical connection to the tip was achieved through the signal access module. The I-V characteristics were recorded using a DC Keithley 2400 voltage source and a Keithley 6514 electrometer. A 1 × 10-4 M solution of OPV in acetonitrile/H2O (10:1) was used for the exchange of dodecanethiol with the OPV molecules. Samples were immersed in 1 mL OPV solution and flushed with Ar for 20 min. Ethylenediamine (10 µL) was added to deprotect the acetyl thiols. The mixture was kept under Ar and allowed to react for 20 min. The samples were washed in acetonitrile and water to remove unreacted material. (26) Nørgaard, K.; Weygand, M. J.; Kjaer, K.; Brust, M.; Bjørnholm, T. Faraday Discuss. 2003, 125, in press. (27) Stuhr-Hansen, N. Synth. Commun. 2003, 33, 641-646.
NL034752D
Nano Lett., Vol. 4, No. 1, 2004
