confirmed by XRD) was found to make up about 25% of the mass measured after the growth and this was subtracted from the total mass to obtain the mass of the Si part of the NW structure only The electrochemical properties were evaluated by both cyclic voltammetry and galvanostatic cycling. They were performed in a glass cell with a three-electrode configuration, with the SiNWs on the stainless steel substrate as the working electrode and Li foil as both reference and counter electrodes. The electrolyte was 1.0 M LiPF6 in 1: 1 w/w ethylene carbonate: diethyl carbonate(Ferro Corporation). No binders or conducting carbon were used. All measurements were made in an Argon-filled glove box using an EcoChemie PGStaT100 potentiostat. Scans were typically performed from 2.0 to o01V ys li/li Characterization was done using scanning electron microscopy(FEI XL30 Sirion), transmission electron microscopy(Philips CM20-FEG, acceleration voltage 200 kV), and X-ray diffraction(PANalytical X'Pert, Cu Ka-radiation). For transmission electron microscopy (TEM) characterization, the NWs were deposited via a dry transfer stamping technique onto lacey carbon film on copper grids. A 200 kV Hitachi HD-2300A scanning transmission electron microscope(STEM)was used to do chemical analysis with a high sensitivity energy dispersive X-ray spectrometer(eDs, nORan System). In all cases where electrochemically cycled SiNWs were analyzed, great care was taken to limit exposure to air in order to preserve the compositional state of the NWs and avoid oxidation. Nw samples were transported in Teflon-sealed glass vials filled with Ar to the characterization instrument and exposed to air for 1 min during sample loading For X e 2007 Nature Publishing Group© 2007 Nature Publishing Group confirmed by XRD) was found to make up about 25% of the mass measured after the growth and this was subtracted from the total mass to obtain the mass of the Si part of the NW structure only. The electrochemical properties were evaluated by both cyclic voltammetry and galvanostatic cycling. They were performed in a glass cell with a three-electrode configuration, with the SiNWs on the stainless steel substrate as the working electrode and Li foil as both reference and counter electrodes. The electrolyte was 1.0 M LiPF6 in 1:1 w/w ethylene carbonate: diethyl carbonate (Ferro Corporation). No binders or conducting carbon were used. All measurements were made in an Argon-filled glove box using an EcoChemie PGSTAT100 potentiostat. Scans were typically performed from 2.0 to 0.01 V vs. Li/Li+ . Characterization was done using scanning electron microscopy (FEI XL30 Sirion), transmission electron microscopy (Philips CM20-FEG, acceleration voltage 200 kV), and x-ray diffraction (PANalytical X’Pert, Cu K
-radiation). For transmission electron microscopy (TEM) characterization, the NWs were deposited via a dry transfer stamping technique onto lacey carbon film on copper grids. A 200 kV Hitachi HD-2300A scanning transmission electron microscope (STEM) was used to do chemical analysis with a high sensitivity energy dispersive X-ray spectrometer (EDS, NORAN System). In all cases where electrochemically cycled SiNWs were analyzed, great care was taken to limit exposure to air in order to preserve the compositional state of the NWs and avoid oxidation. NW samples were transported in Teflon-sealed glass vials filled with Ar to the characterization instrument and exposed to air for < 1 min during sample loading. For X-