NANO LETTERS
Nanowire Lithography: Fabricating Controllable Electrode Gaps Using Au−Ag−Au Nanowires
2005 Vol. 5, No. 6 1071-1076
Shuhong Liu,† Jeffrey B.-H. Tok,‡ and Zhenan Bao*,† Department of Chemical Engineering, Stanford UniVersity, Stanford, California 94305, and Chemistry and Materials Science Directorate, Lawrence LiVermore National Laboratory, LiVermore, California 94551 Received March 25, 2005; Revised Manuscript Received May 1, 2005
ABSTRACT A method to fabricate nanowire electrodes possessing controllable gaps is described. The method relies on electrochemical deposition and selective chemical etching or heating to selectively remove the Ag segment of Au−Ag−Au nanowires. Because the thickness of the Ag segment directly dictates the size of the nanogap, the gap width can be easily controlled during the nanowire fabrication process. Herein, we demonstrate gaps with 2 µm, 100 nm and 20 nm widths via the above-mentioned approaches. In addition, we observed that small gaps (∼20 nm) can be formed through annealing Au−Ag−Au nanowires at 200 °C in air. Electrical contact between nanowire electrodes and contact pads is studied. Using nanowire electrodes with a 100 nm gap, we subsequently fabricate organic field effect transistors (FETs) with regioregular poly(3hexylthiophene).
The fabrication of nanoscale electronic devices is an area of intense research.1-4 One important experimental challenge is making electrical contact to nanometer-scale objects. Scanning tunneling microscopy has proven to be a useful approach.5-7 However, it would be more desirable to employ nanofabricated electrodes in a fixed planar geometry because they offer greater mechanical stability and allow the straightforward implementation of electrostatic gate modulation.8 Several methods for fabricating narrow electrode gaps have been demonstrated, which include a mechanical break junction,9 electromigration,10 controlled electrochemical plating,11 selective chemical deposition,12 shallow-angle evaporation,13 electron-beam lithography,14 and dip-pen lithography.15 However, the complexity, high cost, and low yield of these techniques have limited their general applications. In this letter, we report a simple method of fabricating nanowire electrodes with well-defined gaps ranging from tens of nanometers to micrometers. The method relies on electrochemical deposition and selective chemical etching or heating to remove the B segment of an A-B-A-type metallic-striped nanowire. As a demonstration of this concept, we have employed Au-Ag-Au three-component nanowires that are 250 nm in diameter and 6 µm in length, which can be easily seen under an optical microscope. The thickness of the Ag segment dictates the size of the nanogap, which is easily controlled during * Corresponding author. E-mail:
[email protected]. † Stanford University. ‡ Lawrence Livermore National Laboratory. 10.1021/nl050581w CCC: $30.25 Published on Web 05/17/2005
© 2005 American Chemical Society
the nanowire synthesis process. Nanowires were prepared to contain Ag segments of 2 µm, 100 nm and 20 nm. The Ag segment was subsequently removed by chemical etching to result in the corresponding electrode gaps. We believe that nanometer-scale separations can be achieved when we shrink the nanowire diameter and employ singlecrystalline nanowire electrodes. In addition, we observed that Ag nanowires became fragmented under annealing at 200 °C in air. These results suggest that heat treatment is potentially a useful and convenient method for making nanometer-scale gaps. Using the obtained nanowire electrodes with a 100 nm gap, we fabricated organic FETs by depositing regioregular poly(3-hexylthiophene) (P3HT) from the solution phase. A schematic process for fabricating the nanowire electrodes is shown in Figure 1. First, three batches of Au-Ag-Au nanowires (commercially available from Nanoplex Technologies, Inc., Menlo Park, CA)16,17 with different Ag segment lengths (2 µm or 100 or 20 nm) were functionalized with 11-mercaptoundecanoic acid (MUA) and dispersed in ethanol. Second, drops of the nanowire suspension were placed onto a SiO2/Si substrate patterned with gold contact pads with 4 µm separations. After solvent evaporation, the substrate was heated at 200 °C for 20 min in air to improve the electrical contact between nanowires and the contact pads. Finally, the substrate was dipped into a 4 M nitric acid solution for 20 s, followed by rinsing with distilled water. Nanowire electrodes as shown in Figure 1 were subsequently obtained.
Figure 1. Schematic for the nanowire electrode fabrication process. (A) Au-Ag-Au three-component nanowires being drop-cast onto contact pads and (B) selective chemical etching to remove Ag parts.
Figure 2. (A) SEM images of a nanowire lying across contact electrode pads and (B-D) gaps with different designed separations of 2 µm and 100 and 20 nm, respectively. The inset of B shows nanowire electrodes connected with contact electrode pads.
Figure 2A shows a nanowire bridging two contact pads before etching. After etching with nitric acid, the Ag segment was removed, resulting in a 2 µm gap as shown in Figure 2B. It is important to note that the Au segments did not move during the etching process because the heat treatment effectively removes the MUA coating and immobilizes the Au wires. Figure 2B-D show the SEM images of the resulting electrode gaps. For the 2 µm Ag segment, the observed gap width is close to the desired separation. For the 100 nm Ag segment, the average gap distance observed is also very close to 100 nm, with the smallest gap width being ∼30 nm, which may be due to the roughness of the Au-Ag interface. For the designed 20 nm gap, SEM did not clearly show the gap structure across the 250-nmdiameter nanowire, but we can nonetheless see the separation to be around 20 nm. Applying a voltage across the electrodes, we obtained a linear current-voltage (I-V) plot, and the current was about 10-10 A at 1 V bias, indicating that a small portion of the gap was shorted. 1072
One important issue to be considered is the electrical contact between the nanowire electrodes and the contact pads. To determine if the contact is functional, we placed singlecomponent Au nanowires coated with MUA onto contact electrodes, followed by the same heat treatment (200 °C for 20 min) in air. The measured IV properties are summarized in Table 1. The measured resistance across two electrodes decreased by several orders of magnitude after heat treatment, which we attributed to the MUA desorption at high temperature.18 We also measured the IV properties of pure gold nanowires (without MUA coating) after the same heat treatment and obtained the same result, indicating that our heat treatment is effective in removing MUA molecules. The computed resistivity of these gold nanowires is of the same order of magnitude as that for bulk gold, indicating that the electrical contact between nanowire electrodes and the contact pads is reasonable. Besides heat treatment, we found that the electrical contact between the Au nanowire and contact pads could also be Nano Lett., Vol. 5, No. 6, 2005
Table 1. Resistance of Au Nanowires under Different Treatment Conditionsa Au nanowires under varying conditions
computed resistivity (Ω m)
MUA-coated Au, no heat treatment
0.001
MUA-coated Au, heating at 200 °C for 20 min
8 × 10-8
bare Au, heating at 200 °C for 20 min
8 × 10-8
MUA-coated Au, repeated IV scan (nth testing)
(1) 5.0 × 10-2, (2) 2.5 × 10-2 (3) 2.2 × 10-2, (4) 5.7 × 10-3 (5) 1.40 × 10-7, (6) 1.41 × 10-7 (7) 1.42 × 10-7, (8) 1.39 × 10-7
bulk gold19
2.2 × 10-8
a MUA-coated Au refers to gold nanowires functionalized with 11mercaptoundecanoic acid (MUA), and bare Au refers to pure gold nanowires.
improved with repeated IV scanning at 1 V. The conductance across two electrodes was increased by ∼5 orders of magnitude after the fifth scan, with the increase being less pronounced after that. A possible explanation is that the heat generated by the passing current results in the desorption of thiol molecules from the gold surface.18 After the fifth scan, the increase in conductance was minimal because most of the MUA molecules were removed during previous scans. The nanowires were also exposed to UV ozone for 20 min to remove the MUA molecules, but no change in resistance was observed. This is probably the result of inefficient exposure of the MUA molecules between the Au nanowire and contact pads. Interestingly, the Ag nanowires undergo significant changes after heat treatment in air, which is visible under the optical microscope (Figure 3A and B). Using SEM, we found that the 2 µm Ag segment was severely fragmented, resulting in several gaps between two gold components (Figure 3C). To obtain a single gap, we employed nanowires with shorter Ag segments (100 and 20 nm) and heated them under the same conditions. As seen from Figure 3D and E, at least a single partial gap is formed in the Ag segment. It is likely that elevated temperature and prolonged heat treatment resulted in the observed gap. This heating process may potentially be a useful method for making ultrasmall gaps because the change in the nanowires is a gradual process. To better understand why the Ag component in an Au-Ag-Au nanowire changes upon heating, pure Ag nanowires were also heated under the same conditions to observe if Au segments contributed to the morphological changes in Ag. As shown in Figure 3F, pure Ag nanowires also showed similar changes upon heating, indicating that Ag alone is responsible for the morphological changes. The structural change in the Ag segment during heat treatment is hypothesized to be related to its polycrystalline nature. Heating results in larger grains, which then causes the material to shrink. Thus, elevated heat treatment correspondNano Lett., Vol. 5, No. 6, 2005
ingly results in a faster shrinking rate for Ag. To support this hypothesis, we subjected the nanowires to different elevated temperatures for a similar period of time and observed that the morphological changes in Ag became more visible with increasing temperature (Figure 4 A-H). Both the oxidation of Ag and the decomposition of AgO may contribute to the breaking of the Ag segment. Oxygen is known to have high permeability through silver,20 and the oxidation of Ag occurs spontaneously even at room temperature. We subsequently treated the nanowires at 200 °C in N2 and observed that the morphological change in Ag was not as severe as for those treated in air. The gradual decomposition of AgO begins at temperatures above 100 °C and becomes significant with temperatures above 200 °C.21 It was observed that the discontinuity in Ag became more visible upon heating at a temperature above 200 °C. Using the 100-nm-gap nanowire electrodes, organic field effect transistors (FETs) are made using similar approaches to those previously reported.22,23 A highly doped Si wafer is used as the gate electrode, whereas a thermally grown dry oxide (300 nm) is used as the dielectric layer. Au contact pads with 4 µm separations were lithographically patterned. Nanogap electrodes were prepared using the same process described earlier. The device structure is shown in Figure 5A, and its 100 nm gap is bridged by spin coating regioregular P3HT (Mn ≈ 20 kD) from a dilute (0.05 mg/mL) chloroform solution at 2000 rpm. This concentration was carefully chosen so that there was no observable field effect between a pair of contact pads without nanowire electrode gaps. The devices were kept under vacuum (100 mTorr) for 4 h to remove residual solvent before electrical testing. Under different gate voltages, we observed a clear field effect in which the drain-source current (Ids) scales up with increasing gate voltage (Vgs) (Figure 5A). The obtained device characteristics are similar to those with metal electrodes fabricated through electron beam lithography with 30 nm gaps24 and single-walled carbon nanotube electrodes with 5-6 nm gaps.22 The off current for P3HT devices is relatively high because of partial oxidation by O2, which is commonly seen with P3HT FETs. The I-V characteristics did not show saturation regions due to the short channel effect, normally observed for organic transistors with channel lengths (distance between drain and source electrodes) of less than a few micrometers.23 The field effect observed is most likely due to a small patch of film deposited between the nanogap with most polymers randomly oriented and possibly a few polymer chains bridging the nanogap because the average length of polymer used in this case is not sufficiently long. To confirm that the field effect observed is due to the film deposited onto the nanowire electrodes instead of the contribution from the 4 µm gap between the contact pads, we also tested the IV properties of devices without nanowires between metal contacts. As shown in Figure 5B, no field effect was detected, thus indicating that the P3HT deposits were discontinuous at the 4 µm length scale, and the discontinuity resulted in no electrical connections between contact pads. 1073
Figure 3. Optical micrographs of Au-Ag-Au nanowires (length of Ag ) 2 µm) (A) before and (B) after heat treatment at 200 °C for 20 min in air. SEM images of Au-Ag-Au nanowires with different thickness of Ag segments (C-E: 2 µm and 100 and 20 nm, respectively) and pure Ag nanowires (F) after heat treatment at 200 °C for 1 h in air.
In conclusion, we report a simple method for making nanowire electrodes with controllable gaps by selective chemical etching of B segments from A-B-A nanowires. It was also observed that heat treatment can make discontinuous Ag nanowires. This is also a potentially useful method for making very small gaps within nanowires. An attractive advantage of this method is that the entire process involves only electrochemical synthesis, heating, and selective chemical etching of the Au-Ag-Au nanowires, all of which are remarkably simple. Additionally, the gap distance can be predesigned to be in the range of the micrometer to nanometer scale, which is easily controlled during the nanowire synthesis process. We have designed three different 1074
gaps (2 µm, 100 nm and 20 nm) using nanowires with 250 nm diameters and have observed that the obtained gaps have overall sizes very close to the predesigned gaps. The gap sizes do show slight deviations across the nanowires, especially for the 20 nm gaps, which we attribute mainly to the morphology of Au and Ag nanowires grown by electrochemical plating. Using this approach, nanometer-scale gaps are obtainable if either the diameter of the nanowires is reduced or single-crystal nanowires are employed. Anodized alumina membranes containing 20 nm pore sizes used for nanowire growth are commercially available, and the conditions for synthesizing single-crystalline Au and Ag nanowires have been reported.25 Employing the nanowires Nano Lett., Vol. 5, No. 6, 2005
Figure 4. Optical micrographs of Ag nanowires (A) before and (B-H) after heat treatment at different temperatures (B-H: 75, 125, 170, 180, 190, 200, and 250 °C) for 20 min in air. Scale bar ) 6 µm.
Figure 5. Device structures and room-temperature Ids-Vds characteristics for P3HT FETs. (A) Nanowire electrodes with a 100 nm gap are used as source and drain electrodes. (B) Contact pads without nanowires (channel width W ) 250 µm, channel length L ) 4 µm). Doped Si serves as a back gate and SiO2 with a thickness of 300 nm serves as the dielectric layer. S and D correspond to source and drain, respectively.
as electrodes, we have also demonstrated the fabrication of nanoscale organic FETs in the nanowire gaps. The combination of “nanowire lithography” with self-assembly approaches26 will potentially allow the fabrication of large Nano Lett., Vol. 5, No. 6, 2005
arrays of nanogaps. Furthermore, conjugated polymers with thiol or isocyanide substitutions and different molecular lengths can be used in these nanogaps to study their chargetransport properties along polymer backbones. 1075
Acknowledgment. We thank Sharron Penn, Frances Wong, and Gabriela Chakarova from Nanoplex Technologies, Inc., for the nanowire synthesis and Alejandro L. Briseno for helpful discussions. S.L. acknowledges financial support from a Stanford Graduate Fellowship. Work performed in LLNL is carried out under the auspices of the U.S. Department of Energy by the University of California under contract No. W-7405-Eng-48. References (1) Goldhaber-Gordon, D.; Montemerlo, M. S.; Love, J. C.; Opiteck, G. J.; Ellenbogen, J. C. Proc. IEEE 1997, 85, 521-540. (2) Heath, J. R.; Ratner, M. A. Phys. Today 2003, 5, 43-49. (3) Nitzan, A.; Ratner, M. A. Science 2003, 300, 1384-1389. (4) Reed, M. A.; Lee, T. Molecular Nanoelectronics; American Scientific Publishers: Stevenson Ranch, CA, 2003. (5) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones, L.; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 275, 1705-1707. (6) Donhauser, Z. J.; Mantooth, B. A.; Kelly, K. F.; Bumm, L. A.; Monnell, J. D.; Stapleton, J. J.; Price, D. W.; Rawlett, A. M.; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 2001, 292, 2303-2307. (7) Cui, X. D.; Primak, A.; Zarate, X.; Tomfohr, J.; Sankey, O. F.; Moore, A. L.; Moore, T. A.; Gust, D.; Harris, G.; Lindsay, S. M. Science 2001, 294, 571-574. (8) Khondaker, S. I.; Yao, Z. Appl. Phys. Lett. 2002, 81, 4613-4615. (9) Reed, M. A.; Zhou, C.; Muller, J.; Burgin, T. P.; Tour, J. M. Science 1997, 278, 252-254.
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NL050581W
Nano Lett., Vol. 5, No. 6, 2005
