journal homepage: www.elsevier.com/locate/nanoenergy Available online at www.sciencedirect.com RAPID COMMUNICATION Piezoelectric nanogenerator with a nanoforest structure Myeong-Lok Seol, Ji-Min Choi, Jee-Yeon Kim, Jae-Hyuk Ahn, Dong-Il Moon, Yang-Kyu Choi n Department of Electrical Engineering, KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea Received 30 December 2012; received in revised form 11 March 2013; accepted 23 April 2013 Available online 7 May 2013 KEYWORDS Nanogenerator; Metal-assisted Ćche- mical etching Ć(mac- etch); Energy harvesting; Nanoforest; Piezoelectric; Barium titanate (BTO) Abstract Piezoelectric nanogenerators have been intensively developed in terms of their materials and applications; however, only modest structural progress has been made due to limitations in the growth mechanisms of nano-materials. In this work, a piezoelectric nanogenerator based on a nanoforest structure is introduced. Through a metal-assisted chemical etching (mac-etch) process, densely packed vertical nanowires and nanovoids are fabricated. The nanoforest structure serves as a bottom electrode, which increases the interfacial area with a subsequently deposited piezoelectric material, in this case barium titanate (BaTiO 3 ). In the proposed fabrication method, more various piezoelectric materials can be used for a piezo- electric device compared to previously reported methods because the process relies not on the growth mechanism but on the deposition technique. The proposed nanoforest structured nanogenerator produces a 4.2 times enhanced power density compared to the control generator, which uses the same material but has a flat topology. The strong relationship between the enhancement ratio and the nanoforest height is found. Both the experiment and the simulation data present a consistent trend of a gradual increase with a short height and saturation at a tall height. & 2013 Elsevier Ltd. All rights reserved. Introduction Energy harvesting technology has been widely investigated for future sustainable power sources. In particular, energy harvesting using piezoelectricity attracts special interests due to its abundant energy sources, scalability, and high energy conversion efficiency. By virtue of the rapid devel- opment of nanofabrication techniques, a piezoelectric energy generator using a nanostructure, also known as a nanogenerator, was introduced by Wang and Song [1]. The prototype nanogenerator utilized a vertically standing ZnO nanowire array, while the tip of an atomic force microscope (AFM) was used for the electrical contact and to apply mechanical force. With further developments in recent 2211-2855/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.nanoen.2013.04.006 n Corresponding author. Tel.: +82 42 350 3477. E-mail address: ykchoi@ee.kaist.ac.kr (Y.-K. Choi). Nano Energy (2013) 2, 1142–1148