Figure 1 5μm, 10μm and 20μm long SiNWs SEM images before and after charge/disP5091 charge cycling. SEM images before charge/discharge SCH727965 chemical structure cycling of a) 5 μm SiNWs, b) 10 μm SiNWs, c) 20 μm SiNWs and SEM images after charge/discharge cycling
of d) 5 μm SiNWs, e) 10 μm SiNWs, f) 20 μm SiNWs. Figure 2 Cyclic voltammetry of symmetrical SiNWs/SiNWs micro-ultracapacitors for several SiNWs lengths. Figure 3 Symmetrical SiNWs/SiNWs micro-ultracapacitors Galvanostatic charge/discharge for several SiNWs lengths at ±10μA cm −2 . Results and discussion SiNWs growth by CVD From 50-nm gold colloids, a NWs density of ≈3.108 NWs cm−2, with diameters of ≈50 ± 5 nm, has been obtained for all electrodes. With growth times of 10, 20, and 40 min, electrodes with SiNWs lengths of 5 μm ± 10 nm (a in Figure 4), 10 μm ± 10 nm (b in Figure 4), and 20 μm ± 20 nm (c in Figure 4), respectively, have been obtained. Gold colloids are kept on top of the SiNWs (inserted in a in Figure 4) without any influence Pictilisib manufacturer on the electrochemical behavior. SiNWs length, diameter, and density determination from the SEM images provides an estimation of SiNWs volume and by calculation with silicon density, an estimation of SiNWs mass (respectively, ≈12, 24, and 48 μg for
5, 10 and 20 μm NWs). The developed surface cannot be accurately determined from the SEM images. With the dopant/SiH4 ratio equal to 4.10−3 for all samples, we obtain a doping level of 4.1019 cm−3[21]. Figure 4 Capacitance stability of symmetrical SiNWs/SiNWs micro-ultracapacitors during galvanostatic charge/discharge cycling at ±5 μA cm −2 . Capacitance stability of a) symmetrical bulk Si/Si micro-ultracapacitor and symmetrical SiNWs/SiNWs micro-ultracapacitors with b) 5 μm long Hydroxychloroquine nmr SiNWs, c) 10 μm long SiNWs and d) 20 μm long SiNWs. Electrochemical characterization of SiNWs/SiNWs micro-ultracapacitors All devices show a quasi-ideal capacitive behavior. Cyclic voltammetry curves are rectangular.
Galvanostatic charge/discharge curves are triangular and symmetrical, which indicates that only very few losses occur between the charge and the discharge. An unexpected lower voltage for 1 M NEt4BF4 in PC is used to stay in the system electrochemical stability window (ESW) and avoid side reactions. In fact, the system ESW is smaller than the one obtained on platinum for this electrolyte due to silicon oxidation at a potential below the electrolyte one [15, 16]. Device capacitance increase with the SiNWs length can be seen on cyclic voltammetry curves (Figure 1) and on galvanostatic charge/discharge curves (Figure 2). In fact, for the first one, capacitance is proportional to the current density difference inside the two curves (Formula 1) and for the second one it is inversely proportional to the discharge slope (Formula 2).