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1 DOI: /NCHEM.1332 Light triggered self-construction of supramolecular organic nanowires as metallic interconnects Vina Faramarzi 1,2, Frédéric Niess 1,3, Emilie Moulin 3, Mounir Maaloum 1,3, Jean-François Dayen 1,2, Jean-Baptiste Beaufrand 1,2, Silvia Zanettini 1,2, Bernard Doudin 1,2 *, and Nicolas Giuseppone 1,3 * Index Supplementary characterizations of STANWs Supplementary Figure 1 Patterned electrodes.2 Supplementary Figure 2 Current vs voltage measurements...3 Supplementary Figure 3 Size distribution of STANWs in the gap.4 Supplementary Figure 4 Differential conductance measured at 200 K in vacuum using an AC bridge technique...5 Control Experiments A. Measurements on empty gaps...6 Supplementary Figure 5 Intensity/voltage measurements of the empty gap at high voltages.7 B. Blind test experiments on several triarylamine derivatives 8 Supplementary Table 1 Summary of the control experiments 9 C. Imaging nanotrenches...10 Supplementary Figure 6 SEM and AFM imaging of a nanotrench filled with STANWs, after rinsing, of 50 Ω resistance...11 D. Electrical properties of filled nanotrenches..12 NATURE CHEMISTRY 1

2 2 DOI: /NCHEM.1332 Supplementary characterizations of STANWs Supplementary Figure 1 Patterned electrodes. a, Scanning electron microscopy (SEM) image of the nanotrench electrodes. b, SEM zoom indicating the pseudo-four points connections, limiting the series resistance of the interconnects at less than 2 Ω. c, SEM zoom of the nanotrench (marked by the red square in (b)) showing a typical gap of length at less than 100 nm. d, Atomic force microscopy (AFM) topography large scale image of a nanotrench electrode. e, AFM medium scale topography image of the region defined by the red square in (b). f, AFM medium scale phase image of the region defined by the red square in (b). NATURE CHEMISTRY 2

3 3 Supplementary Figure 2 Current vs voltage measurements. a, I/(V) of an empty reference nanotrench (width = 100 µm; length = 0.08 µm) in ambient athmospheric conditions (no solvent). b, I/(V) measurements of the nanotrench (width = 100 µm; length = 0.08 µm) after self-assembly of 1 upon light irradiation (P=10 W.cm -2 during 10 s), after rinsing and drying the device, and showing the effect of the pre-voltage applied during the initial light irradiation promoting the STANWs formation (diamonds: 0.01 V; circles: 0.1 V; squares: 0.3 V). The two-point measurement set-up includes track resistance of typically Ω resulting in a sample potential drop smaller than the indicated voltage scale. NATURE CHEMISTRY 3

4 4 Supplementary Figure 3 Size distribution of STANWs in the gap. a, AFM image of the closed gap with STANWs (surface scale 500x500 nm 2 ). b, Topography extracted from image (a). c, Length distribution of STANWs observed from a typical gap and obtained from a series of AFM phase image as shown in (a and b). The short length average distribution is due to residual non-connecting wires at the surface and which survive the washing of the device. n.b.: The resolution of the AFM images of the STANWs cannot be as good as the one obtained in our initial article 1. Indeed, here the fibers make a high angle with the surface because of the presence of a step between the two electrodes (see Methods and image (b)). The presence of this height considerably affects the resolution. The AFM cantilevers used here are Arrow probe (from NanoAndMore). These probes are very sharp (<10 nm of curvature radius) and made from monolithic silicon, which is highly doped to dissipate static charge. The STANWs diameters were measured taking into account the convolution of the image by the AFM tip during the imaging process. Here, we measured a portion where a bimodal distribution can be found, although several domains only present the longest population. NATURE CHEMISTRY 4

5 5 Supplementary Figure 4 Differential conductance measured at low temperatures, in vacuum, using an AC bridge technique. a, Time evolution of the conductance, after a 0.4V voltage stress, stopped at t = 0. b, Another sample measured at 200K, showing parabolic di/dv without hysteresis. NATURE CHEMISTRY 5

6 6 DOI: /NCHEM.1332 Control Experiments We here present a series of measurements mostly aimed at checking that the observed remarkable conductivity of our samples does not result from metallic shorts originating from the electrodes interconnects. A. Measurements on empty gaps Supplementary Fig. 5 presents typical test measurements of 80 nm long trenches separating the electrodes, without the presence of organic self-assembly. Data were taken at room temperature under ambient conditions, similar to those during the self-assembly. We observed the following: i) As-made nanotrenches can sustain a minimum of 5 V bias, with typical leakage currents not exceeding a few pa. The observed current in Supplementary Fig. 5a is mostly due to the parasitic capacitive part of the circuit. When measured under vacuum conditions, bias up to 40 V with pa leakage current values are observed. ii) When immersed in C 2 H 2 Cl 4, the leakage current increases up to na values, with applied bias exceeding 3 V. A possible origin for the leakage current is the water content of the solvent, which might be non negligible under ambient atmosphere conditions (Supplementary Fig. 5b). This is confirmed by the lack of reproducibility of such measurements, with leakage currents possibly one order of magnitude smaller than those of fig. 5b. iii) We tested that illumination at power of similar magnitude to the one used for selfassembly did not create photoconduction artifacts (Supplementary Fig. 5c). iv) Solutions of triarylamine molecules that do not self-assemble because of a chemical modification (molecule 4 of Supplementary Table 1), under illumination, showed leakage currents in the na range (3 V bias) (Supplementary Fig. 5d) typically seven orders of magnitude smaller than those obtained on successfully self-assembled systems. From this series of test experiments, we can deduce that no electromigration of metal, nor photoconduction due to Ti inclusions or polluted SiO substrate, occur, even under NATURE CHEMISTRY 6

7 7 experimental conditions significantly more stressful than those used during the procedure of self-assembly described in our main paper. Supplementary Figure 5 Intensity/voltage measurements of the empty gap at high voltages. a, I/(V) of an empty reference nanotrench (width = 100 µm; length = 0.08 µm) without any solvent. b, I/(V) of the reference nanotrench (width = 100 µm; length = 0.08 µm) before light irradiation. The negligible residual current is attributed to ionic impurities in solution. c, I/(V) measurements of the nanotrench (width = 100 µm; length = 0.08 µm) immersed in C 2 H 2 Cl 4, and upon white light irradiation (density 10 W.cm -2 ). d, I/(V) curve of a reference nanotrench (width = 100 µm; length = 0.08 µm) covered with a solution of molecule 4 (1 mg.ml -1 in C 2 H 2 Cl 4 ), after white light illumination, characteristic of a chemically and redox related sample, but without self-assembly properties. NATURE CHEMISTRY 7

8 8 DOI: /NCHEM.1332 B. Blind test experiments on several triarylamine derivatives We have previously described that triarylamine analogues 1-6 (see Supplementary Table 1) behave differently upon light stimulation in chlorinated solvent 1. Indeed, we have determined by 1 H NMR experiments that compounds 1-3 self-assemble in CDCl 3 solutions upon light stimulation, while compounds 4-6 do not produce stacks. This structure-property relationship highlights that several features should be simultaneously operating to produce STANWs: a) a triarylamine core; b) an amide group for hydrogen bondings (the chlorine having no role in the self-assembly); and c) alkyl chains or benzylic groups on the phenols for hydrophobic or stacking secondary interactions. In the series 1-6, redox behaviours are however similar and our control experiment can discriminate between parasitic electrochemical processes that would be initiated by the production of triarylamonium species, and the conductivity related to the subsequent formation of self-assembled STANWs. The control experiments were performed using a blind test protocol: the person who prepared the solutions did not perform the conductivity experiments and the label on each vial was kept coded until the measurements were performed and analyzed. Each sample was measured under the same conditions: we applied a voltage of 1 Volt to solutions of triarylamines 1-6 (1 mg.ml -1 ), together with a 100 W irradiation at constant distance ( 10 W.cm -2 ) for a period of 10 seconds; then, I/V dependence was measured for each gap. Results are summarized in Table S1 for each derivative and by comparison to the expected behaviour determined in solution 1. The correlations clearly show that the conductivity is self-assembly dependent. n.b. When the gaps were closed, the resistance values were measured in the same range for the various derivatives. NATURE CHEMISTRY 8

9 9 Supplementary Table 1 Summary of the control experiments performed in blind test conditions to determine the influence of the self-assembly on the conductivity starting from derivatives 1-6. NATURE CHEMISTRY 9

10 10 C. Imaging nanotrenches We extensively checked the morphology and the difference of the electrodes before and after self-assembly. We used scanning electron microscopy (SEM) and atomic force microscopy (AFM) to obtain qualitative and quantitative information on the nanotrenches. Supplementary Fig. 1 and Fig. 1c show the appearance of the nanotrenches prior to selfassembly, and Fig. 1b,d,e, Supplementary Fig. 3, and Supplementary Fig. 6 provide SEM and AFM imaging information after self-assembly. SEM imaging of the filled trenches provide limited contrast from the organic spacer. Supplementary Fig. 6 a,b illustrate a series of systematic SEM micrographs along the whole length of a filled trench of 50 Ω resistance value. Even by exaggerating the contrast better to image the interconnects, it was not possible to visualize them. The information given by SEM is important however because we cannot detect any presence of large metallic bridges that would explain the observed conductivity. Even under the hypothesis that small metallic residues might exist, they could not sustain the current and voltage applied to the sample (see discussion in the next section electrical properties of filled nanotrenches ). In contrast, AFM provides clear indication of fiber-like structures filling the trenches, with diameters expected from self-assembly morphology observed in solution. In addition, by applying a 2 times stronger force on the AFM tip for imaging, the fibers appeared very brittle. Fig. 6c,d reveal that the fibers can be easily broken, thus illustrating the organic nature of the STANWs and their tight attachment at each electrode, as well as their suspension over the trench. NATURE CHEMISTRY 10

11 11 Supplementary Figure 6 SEM and AFM imaging of a nanotrench filled with STANWs, after rinsing, of 50 Ω resistance. a, One typical region of the trench is shown, where no high-contrast bridges can be seen. The dark spots on the electrodes are solvent or solution residues. b, Zoomed image of the nanotrench, without indication of continuous electrical interconnects. c, Corresponding AFM imaging indicates that the trench imaged by SEM is indeed fully filled with STANWs. Some nonoriented remaining material can be seen close to the gap. d, The AFM imaging is very sensitive to the applied force value, which has to be maintained within a very narrow interval to preserve the STANWs; a slight increase in the applied force causes destruction of the organic suspended fibers bridging the two electrodes. NATURE CHEMISTRY 11

12 12 D. Electrical properties of filled nanotrenches Stress voltage experiments were performed on self-assembled and cleaned samples, at lower temperatures (200 K) and vacuum environment. Currents of a few tens of ma and bias of 1 V were reproducibly observed on several samples, with illustrating data given in Supplementary Fig. 4. The observed currents and voltages of the samples provide more arguments against the hypothesis of electrodes metallic debris bridging the nanotrench. On a separate set of experiments using a similar setup by the same researchers 2, the electromigration of Au and Ni nanoscale constrictions was investigated for the purpose of studying electrical properties of ultra-small metallic bridges. In these experiments, metallic constrictions of typically 50 nm width patterned by e-beam were exposed to stress of a few ma and a few hundreds of mv. Such values were large enough to initiate electromigration, ultimately resulting in breaking of the nanocontact. If we recall that the bridging particles should be more electrically fragile than patterned continuous metallic lines, we would need several tens of such small structures to sustain the high current shown in Supplementary Fig. 4. If we suppose that the current involves nanoparticle hot points, the related current density that can be estimated is in the range A.cm -2, and cannot lead to resistance values with long-term stability as observed in Supplementary Fig. 4. Finally, if metallic residues do appear, we expect the electrodes to exhibit a corresponding loss of material. When performing experiments involving successive self-assembly, no morphological difference within the SEM resolution was found, confirming that the metallic electrodes were left essentially undamaged by the self-assembly. 1 2 Moulin, E. et al. The hierarchical self-assembly of charge nanocarriers: a highly cooperative process promoted by visible light. Angew. Chem. Int. Ed. 49, (2010). Beaufrand, J.-B., Dayen, J.-F., Kemp, N. T., Sokolov, A., & Doudin, B. Magnetoresistance signature of resonant states in electromigrated Ni nanocontacts, Appl. Phys. Lett. 98, (2011). NATURE CHEMISTRY 12