In addition to the formation of surfaces layers with composition differing from the bulk, the presence of particles at the surface of old wires was observed in the TEM on some alloy nanowires.
Figure II-25: HRTEM (A and C) and HAADF (D) images and a corresponding EDX linescan (B) measured on Au40Ag60 nanowires (diameter 85 nm) with surface nanoparticles before dealloying. Length and position of the linescan with respect to the nanowire is represented by the arrows
Some wires were studied in the TEM several months after transfer on the substrate in contrast to the previous results where TEM and EDX studies were performed 3 days after membrane dissolution. Figure II-25 shows HRTEM and HAADF images of such Au40Ag60 nanowires with diameter 85 nm exhibiting the presence
of nanoparticles at their surface and a corresponding EDX linescan. The nanoparticles are at the surface of the nanowires but do not look tightly bound to the wire. HRTEM imaging (Figure II-25C) shows that the nanoparticles are crystalline and the lattice parameter of the particles coincide with that of gold or silver. EDX analysis of one particle (Figure II-25B) reveals that the particle is about 100% composed of Ag. The EDX linescan also evidences the net separation between the Ag particle and the wire (about 5 nm).
Figure II-26: STEM images of nanowires with different compositions and diameters presenting surface nanoparticles after several weeks in air. (A, B, and C) Au40Ag60 nanowires with respective diameter of 85, 45, and 35 nm after several weeks in air. (D) 35 nm large Ag nanowires with pronounced coverage with particles
Consistently with the results introduced previously, a silver shell is also present on the Au40Ag60nanowire, even after several months of storage in air. The Ag nature of the nanoparticles, confirmed by EDX as well as HRTEM, contrast with
other studies where the formation of nanoparticles at the surface of pure Ag nanowires was attributed to the formation of Ag2S.196–198
The presence of particles is also demonstrated by STEM investigations on NWs stored on the wafer in air for several weeks as shown in Figure II-26. The investigation reveals that nanoparticles form on all wires that contain Ag, independently on their diameter. Figure II-26A, B, and C show Au40Ag60 nanowires of diameter 85, 45 and 35 nm respectively. Numerous particles appear on small nanowires whereas almost no particles are observed on 85 nm wires at this magnification. The connection between the particles and the wires is not steady, thus quantification of the particle density is not reasonable. Nevertheless, SEM observations show the tendency of an increasing particle number with increasing Ag content in the wires. In the case of pure Ag nanowires with diameter of 35 nm, the particles have dimensions comparable to the wire diameter, which leads to discontinuous wires as shown circled in red in Figure II-26D. This behavior is clearly different from Rayleigh instability, which forms regularly spaced spherical droplets with diameter larger than the wire diameter. It has to be emphasized that, no particles could be found on nanowires directly after membrane dissolution and transfer to the substrate.
Figure II-27: SEM images of (A) Au40Ag60and (B) Au60Ag40 nanowires with 85 nm diameter stored on a silicon substrate at 60 °C in air for 72h
The influence of different storage conditions has also been considered. Nanowires stored as dichloromethane suspended solution for long periods (before
transfer) do not exhibit nanoparticles directly after the transfer (not shown). Similarly, nanowires kept on the substrate in dichloromethane are not covered by particles. However, it cannot be excluded that the nanoparticles are washed away when taking the sample out of the organic solvent. Nanowires stored at higher temperature (60 °C) in air feature very fast the presence of particles as shown in Figure II-27D and E. Thus, the formation of particles directly depends on the surrounding media and is enhanced by temperature. These considerations are consistent with the surface diffusion and segregation already mentioned. Surprisingly, particles can also be observed on Au60Ag40 nanowires Figure II-27E that exhibit Au surface enrichment (see section 3.3.5). However, such wires also exhibit inhomogeneity of the gold layer that may explain the particle evolution. Our observations raise the question of the stability of the nanowires containing Ag. Further systematic investigations are necessary to better understand the influence of the temperature and other external parameters on the diffusion of Au and Ag as well as to comprehend the surface segregation occurring on the alloy nanowires.
4. Summary
The synthesis as well as the characterization of porous gold nanowires has been presented in this chapter. First, the analysis of the electrodeposition process evidences that cyanide based electrolytes enable high current efficiencies and depend on the metal ion type and concentration. Cyclic voltammetry confirms that the reduction of Au salt can follow a 2-step path at intermediate voltage or, like Ag, a one-step reduction at very negative voltage. In addition, Au and Ag can be co-deposited from a single bath electrolyte. Operating in a recessed geometry (i.e. in nanochannels) exhibits similar reduction processes and the strong current density increase is mainly attributed to non-faradaic currents such as capacitance and uncompensated resistance of the electrolyte in the nanochannels. Because the potentiostatic reduction of Au and Ag at - 1.1 V occurs close to diffusion-limited regime, the growth of the nanowires can be precisely controlled, with the
differentiation of the various deposition steps: initiation, wire growth, and cap growth. Moreover, the composition is expected to be homogeneous along the wire deposition and can be tailored by monitoring the constitution of the electrolyte. The wires fabricated are cylindrical, independently on their composition and for all diameters equal or below 100 nm, whereas larger wires have bullet like endings. XRD and TEM demonstrate that all nanowires studied are polycrystalline, independently on the diameter and composition. The grains are several micron long and often separated by double grain boundaries consisting in two grain boundaries spaced by only few tenth of nanometers. Compositional analysis by EDX in the TEM shows surface segregation in alloy nanowires. Ag surface enrichment of 1 to 4 nm is observed on all measured Au40Ag60 nanowires whereas a discontinuous Au layer is detected on Au60Ag40 nanowires. Additionally, the formation of Ag nanoparticles has been observed on Ag containing nanowires with various compositions and seems to be enhanced by temperature. The dealloying of nanowires with composition of Au40Ag60 gives rise to porous nanowires with ligaments between 5 and 30 nm. Consequently, thin nanowires with diameter below 50 nm before dealloying are not always continuous after HNO3 treatment. TEM analysis confirms that the crystalline
defects seem neither to be affected by dealloying nor to influence the process. After dealloying, the residual Ag is of only few percent and is mainly situated in small Ag rich domains. Surface roughness is obtained by dealloying of Au60Ag40 nanowires and no change in composition is observed.