1 © 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com Recently significant focus has been given to nanoscale mate- rials due to their very interesting size-dependent proper- ties, [1,2] especially electrical conductivity [3] and magnetic coercivity. [4] Further, nanomaterials with high aspect ratios have been fabricated via a broad array of methods, [5–7] but one method that presents great potential due to its inex- pensive nature and ability to produce diverse materials with various morphologies is electrospinning. [8,9] Previously purely metallic nanofibers have been fabricated via water-based polymeric and metallic precursor electrospinning, but the use of low metal precursor concentrations only surfaced the measurement of the magnetic [10–13] and electrical [14,15] prop- erties, and lacked the analysis detailing the effect of crystal size and density on the coercivity or electrical conductivity due to low loading of metal precursors. In the current work, metal/ceramic acetate precursors and polyvinyl alcohol have been used due to their novel ability to generate homogenous distributions with very high loadings, [16] thereby maintaining proper solution properties for electrospinning. Nanofibers containing a metal/ceramic-acetate-to-polymer mass ratio of 4:1 were electrospun under ambient conditions, removed from the collector plate, and subsequently thermally treated. Particular attention is paid to the crystal morphology within the nanofiber diameter following thermal treatment as this dictates both the magnetic and electrical properties of these nanofibers. Figure 1 presents a bank of transmission electron micro- scopy (TEM) images of nanofibers comprised solely of four different metals (columns) where the thermal treatment con- ditions were varied (rows). Different nanofiber samples are treated at various thermal treatments to control the crystal size, density, and morphology within the nanofiber matrix in an effort to optimize electrical and magnetic properties. A low temperature (400 °C) under flowing inert atmosphere was used to generate nanofibers with small, discrete crystal- line domains supported within an amorphous metal nanofiber matrix (Scheme 1). Energy dispersive X-ray spectroscopy Metal Nanofibers with Highly Tunable Electrical and Magnetic Properties via Highly Loaded Water-Based Electrospinning Nathaniel S. Hansen, Daehwan Cho, and Yong Lak Joo* (EDX) confirmed that the amorphous regions had extremely low carbon content ( <0.25%) and were primarily metallic with slightly elevated oxygen concentrations. These samples were subsequently microtomed and viewed under TEM, as presented in Figure 1b, and it was found that in all cases the crystal domains are evenly distributed throughout the fiber in all cases. Homogeneous radial crystal distribution was con- firmed by averaging the relative radial distance of each crystal found in 20 TEM images of microtomed samples displaying uniformity of crystal concentration. The second thermal treatment consisted of a low temperature (400 °C) thermal treatment under air followed by a second low temperature (400 °C) treatment under flowing inert atmosphere (Scheme 2). A similar thermal treatment was used previously [10] to pro- duce oxidized crystal after the air treatment and then reduce the crystal back during the inert treatment. As can be seen by Figure 1c, this produces isotropic crystals connected at narrow regions generating nanofibers of connected crystals with the amorphous region nearly removed. These results differ from those previously reported [10] due to the new ability to load a large amount of metal precursors homogeneously distrib- uted throughout the nanofiber allowing for well connected domains of crystals with extremely low concentrations of amorphous metal. Finally, the third thermal treatment tested was a high temperature (800 °C) under inert atmosphere as presented in Figure 1d (Scheme 3). This thermal treatment produced purely crystalline nanofibers across the entire fiber diameter in the copper and nickel case, but produced large crystals of iron and cobalt supported in an amorphous nanofiber matrix in the pure iron and cobalt case. The electrical conductivities were then tested for these materials and presented on a log scale in Figure 2 along with the known bulk electrical conductivity. As can be seen in Figure 2a, the second thermal treatment scheme which gen- erated nanofibers void of amorphous regions with isotropic crystals connected to each other produced the highest elec- trical conductivity near 10 6 to 10 7 S m -1 , consistently about an order of magnitude below the electrical conductivity of the bulk material. The high-temperature thermal treatment (Scheme 3) produced very high electrical conductivities in the copper and nickel case where amorphous region was reduced to the extent that pure metal diameters were seen. However, in the iron and cobalt case large crystals connected by amor- phous electrically resistive regions, which as expected dem- onstrated significantly lower electrical conductivities. The first DOI: 10.1002/smll.201102087 Metal Nanofibers N. S. Hansen, Dr. D. Cho, Prof. Y. L. Joo 340 Olin Hall, Cornell University Ithaca, NY, 14853, USA E-mail: ylj2@cornell.edu small 2012, DOI: 10.1002/smll.201102087