High‐Performance Piezoelectric Nanogenerators Using Two‐Dimensional Flexible Top Electrodes

Abstract

But to date, the performance of these devices is still low, not because of the low amount of energy generated in the cell-actually that is quite decent-but due to the diffi culty of collecting it: the main problem relies on the electrode/nanowires interfaces, specially the top one, which is mechanically mounted on the NWs. Traditionally, piezoelectric nanogenerators have been fabricated using all kind of materials as top electrode, including electronic polymers, [ 7,8 ] Au-coated polyethersulfone (PES), [ 9 ] and Cr-coated Al 2 O 3 [ 10 ] or Pt-coated Silicon [ 11 ] stacks, but they are unable to physically contact all the nanowires (a rigid top electrode can only contact the tallest NWs), therefore wasting much energy. Xu et al. [ 10 ] optimized the performance of these devices by tuning the morphology of the top electrode and, more specifi cally, he used the paired nanotip-to-nanowire brushes to provide a better nanowires/electrode interface. Wang and his co-workers used Poly-methyl methacrylate (PMMA) to improve the mechanical stability of the entire cell. [ 11 ] But, even if these techniques increase the performance of the cell, the nanowires may lose a high degree of mobility. Recently, Choi et al. [ 4 ] suggested that the use of a graphene/PEN (polyethylene naphthalate) electrode can improve the effi ciency of these devices due to the high carriers' mobility of graphene. In their prototype the nanowires were fi rst directly grown on a graphene/PEN structure, and the top graphene electrode was later integrated: the graphene was fi rst transferred to a different PEN substrate and then the cell was mounted. However, this methodology can bring about a major associated problem: the low density of nanowires (≈6%) that actually contact the top electrode, which results in a large contact resistance. [ 4,9 ] In this work, we present a novel cell that overtakes the performance of all designs previously reported by directly transferring a graphene sheet on top of the nanowires before mounting the top electrode. The conductive and fl exible graphene fl ake can perfectly adapt to the rough topography of the nanowires, contacting up to ≈73% of all the nanowires. Moreover the high fl exibility of the graphene top electrode introduces spring-like effects and, as a result, the novel cells show higher currents during longer times. Figure 1 shows the schematic of the fabrication method we followed to build the nanogenerators. First, the nanowires were vertically grown on a silicon substrate (Figure 1 a) using the hydrothermal method to synthesize zinc oxide nanowire array [ 12 ] (see the Experimental Section). In the next step a sheet of graphene is directly transferred on the nanowires (Figure 1 b), and fi nally a rigid copper top electrode is mounted to form the nanogenerator (Figure 1 c). Figure 2 a,b shows the typical top and cross-sectional scanning electron microscope images of the nanowires (respectively), revealing that their diameters range from 20 to 100 nm and their typical height is ≈3 µm. TheFinding new methods t...

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