Fabrication of ZnO nanoflowers on gold coated pillars E. Miele, G.C. Messina, M. Dipalo, V. Shalabaeva, A. Jacassi, S. Panaro, M. Malerba, P. Zilio, F. De Angelis ⇑ Istituto Italiano di Tecnologia, via Morego 30, 16163 Genova Italy article info Article history: Received 29 October 2014 Received in revised form 3 December 2014 Accepted 26 January 2015 Available online 2 February 2015 Keywords: Nanofabrication Nanostructures Nanocrystals Nanopalms Hybrid materials Raman spectroscopy abstract We fabricated flower-like shaped ZnO nanorods on the apexes of hollow gold nanopillars by Focused Ion Beam lithography and hydrothermal growth. The hollow nanopillar geometry was defined on a photoresist by ion-beam induced inversion and subsequently metalized. Then the bottom section of the pillars was protected by a second resist coating, exposing only the nanopillar tips for hydrothermal growth of ZnO. The wurtzite crystalline structure of the ZnO nanorods that composes the flowers was confirmed by Raman spectroscopy. Such novel 3D hybrid metal semiconductor nanostructures are promising materials for catalysis and energy harvesting. Ó 2015 Elsevier B.V. All rights reserved. 1. Introduction In recent years the development and the improvement of tech- niques and methods for realization of three-dimensional (3D) func- tional nanostructures have gained increasing interest among the nanofabrication community [1]. The simple and effective nanofab- rication of 3D structures can pave the way for the implementation of novel efficient photonic/plasmonic devices in optofluidics and optoelectronics [2,3]. In these fields strong research activity is focused on the fabrication of hybrid metal–semiconductor nano- structures [4] that are promising to merge the unique properties of novel semiconductor materials with the functionalities of plasmonic metal materials. Indeed, the possibility of enhancing the electromagnetic field with plasmons, which finds applications in sensing [5,6], magnetic field generation [7] or optical switching [8], can also be suitable for increasing the efficiency of solar cells; such hybrid metal–semiconductor nanostructures are interesting especially in catalysis [9], energy harvesting [10] and solar cells applications [11,12]. In each of the above mentioned research fields, zinc oxide (ZnO) holds a promising potential. Zinc oxide (ZnO) is a large band-gap (Eg = 3.35 eV at 300 K) semiconductor with several advantageous properties: piezoelectricity, high exciton binding energy (60 meV) and field emission capability [13,14], just to name a few. Moreover, ZnO is very attractive because it can be synthesized in nanocrystals with tunable shape via a simple and cost-effective hydrothermal synthesis route [13], which is compatible with the present micro- and nanofabrication technology. This compatibility and attractive properties of ZnO nanocrystals, coupled to metal/ plasmonic structures, can find effective employment in different systems such as sunlight-based fuel production, photobioreactors, and photocatalytic systems [2,15,16]. In this respect, it is crucial to be able to control the geometry and spatial distribution in the three dimensions of both the considered nanocrystals and the associated metal nanostructures, since the performance boost in respect to planar geometries strongly depends on such features [17,18]. For structuring in three dimensions at the nanometer scale, polymer resists can be patterned by a variety of techniques, from plasma etching [19] to various lithography techniques such as direct laser writing [20], gray scale technology, two-photons lithography [21], and 3D interference lithography [22]. In stacked electron beam lithography, subsequent e-beam lithography steps are performed in order to obtain multilayered structures. Although the robustness of the approach (it can be considered as an exten- sion of conventional e-beam lithography), it is not time effective and it is poor in efficacy in terms of aspect ratio of the achieved structures. Direct laser writing (DLW), gray scale technology and two- photon lithography are more time and cost effective approaches to the fabrication of 3D structures with high aspect ratio. However, the major drawback of these techniques is the lack of resolution. A substantial improvement in terms of resolution can be obtained by 3D interference lithography, which on the other hand limits possible patterns to periodic structures [22]. http://dx.doi.org/10.1016/j.mee.2015.01.033 0167-9317/Ó 2015 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: francesco.deangelis@iit.it (F. De Angelis). Microelectronic Engineering 141 (2015) 51–55 Contents lists available at ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee