Alber, I.

Burr, L.

Toimil-Molares, M. E.

Trautmann, C.

GSI Repository

Thermal Instability of Porous Gold NanowiresL. Burr#,1, I. Alber1, M.E. Toimil-Molares1 and C. Trautmann1,21GSI, Darmstadt, Germany, 2Technische Universität Darmstadt, GermanyDue to their high surface-to-volume ratio, porous metaland semiconductor nanowires (NWs) are attracting stronginterest as promising devices e.g. in electronics, catalysis and sensorics. Under working conditions, NW devicesshould stand elevated temperatures, it is thus necessary toinvestigate their thermal stability.Theoretical modelling by Nichols and Mullins pre-dicted that, while annealing, solid cylinders with initialradius R0 break into a row of spheres with diameter3.78R0 spaced by 2¥2ʌR0 in a process governed by sur-face diffusion [1]. Recent publications showed that Cuand Au NWs are thermally instable at temperatures muchlower than the melting point of their bulk counterpart, andtransform into a row of spheres [2,3]. Here we study thethermal instability of porous Au NWs by annealing andscanning electron microscopy (SEM) investigations.AuAg alloy solid NWs were electrochemically grown in the nanochannels of etched ion-track polycarbonate templates. The membranes were fabricated by irradiationwith Au ions (~ GeV) at the UNILAC accelerator andsubsequent chemical etching. Nanowires with length ~ 2 Pm and diameter  ~ 110 nmwere synthesized inside the etched channels using a cyanidic electrolyte containing KAu(CN)2 and KAg(CN)2in 1:1 ratio, applying a potential U = -1.1V vs. Ag/AgClreference electrode. Details of the synthesis process aregiven in [4]. After deposition, the polymer membrane isremoved in a dichloromethane solution, and thenanowires were drop-casted on Si wafers. Figure 1: SEM images of a given AuAg NW (a) beforeand (b) after HNO3 treatment.To create porous NWs, the Si wafer with AuAg alloyNWs is dipped for 3h in concentrated nitric acid. Figure 1depicts the same AuAg alloy NW (a) before and (b) afterthe nitric acid treatment. In this case, the diameter of the wire is reduced from 105 to 85 nm. Nitric acid dissolvesAg atoms but does not react with Au. The formation of nanoscale porosity can be explained by diffusion of Auadatoms on the NW surface and formation of Au-richclusters during Ag dissolution [4]. We observe that theporosity and the diameter reduction depend sensitively onthe initial Au/Ag composition and NW diameter.Figure 2 depicts the porous NWs after annealing at 200,300, and 500 °C for 1h on 290-μm thick silicon wafers(heating rate of 9 °C/min).Figure 2: SEM pictures of porous nanowires after 1h an-nealing at temperatures (a) 200 °C, (b) 300 °C, and(c) 500 °C (Scale bar: 100nm).Karim et al. previously reported that several Pm-longsolid Au NWs with diameter ~ 80 nm maintain their cy-lindrical morphology during 1 hour annealing at tempera-tures between 200 and 500 °C, and display morphologicalinstabilities after annealing at 600 °C.In contrast, the morphology of the porous NWs as syn-thesized in this work starts to change already at 200 °C(Fig. 2a). At higher temperatures, they transform intochains of spheres (Fig. 2b). At 500 °C, spheres with sizesvarying between ~ 30 and 120 nm, and inter-sphere sepa-rations ranging between 30 and 200 nm are observed (Fig.2c). Further experiments are currently underway to under-stand the relation between initial wire porosity and di-mensions, and the final morphology of the chain ofspheres.These first results indicate that porous nanostructuresundergo morphological transformation at lower tempera-tures as their solid counterparts due to their higher sur-face-to-volume ratio. At the same time, this behaviourenables the fabrication of chains of closely spaced nanoparticles interesting for, e.g., surface plasmon inves-tigations.References[1] F. A. Nichols and W. W. Mullins, J. Appl. Phys. 36(1965) 1826..[2] M. E. Toimil Molares et al. Appl. Phys. Lett. 85(2004) 5337.[3] S. Karim et al. Nanotechnology 17 (2006) 5954.[4] I. Alber, “Synthesis and Plasmonic Properties ofMetallic Nanowires and Nanowires Dimers,” Ruprecht - Karls - Universität Heidelberg, PhD Thesis 2012.*Work supported by HGS-HIRe.# l.Burr@gsi.deGSITemplate2007GSI SCIENTIFIC REPORT 2012 PNI-INHOUSE-EXP-48417

Thermal Instability of Porous Gold Nanowires

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Thermal Instability of Porous Gold Nanowires

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