Semiconductor nanowires directly grown on graphene – towards wafer scale transferable nanowire arrays with improved electrical contact John P. Alper, ab Albert Gutes, ab Carlo Carraro a and Roya Maboudian * ab We present for the first time the growth of dense arrays of silicon and silicon carbide nanowires directly on graphene as well as methods of transferring these novel hybrids to arbitrary substrates. Improved electrical contact for SiC nanowire/graphene hybrid is demonstrated in the application of a robust supercapacitor electrode. 1 Introduction The eld of nano-electronics is rapidly evolving as new and useful scale-dependent properties associated with nano-mate- rials are discovered. Arrays of semiconducting nanowires are particularly attractive for a number of applications. In secondary battery technology, silicon nanowires (SiNWs) have been shown to reduce the mechanical stresses associated with lithium insertion and extraction. 1 This may enable rechargeable batteries with over ten times the specic capacity of current graphite anodes. SiNW arrays also exhibit greatly enhanced broadband light absorption due to decreased reectance and transmittance as compared to solid lms, 2 and are promising in the eld of thin lm photovoltaics. In addition to Si, the nanoscale electrical and morphological properties of silicon carbide NWs (SiCNWs) including high aspect ratio and low electron affinity indicate them as excellent candidates for eld emission cathodes. 3 To date, SiCNW emitters have been demonstrated with a lower turn-on eld and higher current densities than Si based devices. 4 SiCNWs also show promise as stable materials in electrochemical environments as encoun- tered in aqueous supercapacitors. 5 Typically these semiconducting nanowire arrays are grown from rigid conductive substrates which provide electrical contact to the array. A exible conductive substrate however provides many advantages in terms of the applications described above. Batteries and supercapacitors both benet from smaller form factors such as the typical rolled cylinder cell. The labor and mounting materials for solar panel installation, 15% of the total cost per Watt, 6 may be signicantly reduced by “roll out” light-weight modules. Flexibility also provides greater opportu- nities for integration of energy storage and harvesting technol- ogies with fabric for use in clothing or lightweight building supplies. In order to realize these benets, methods must be developed for transferring arrays of nanowires to exible substrates while maintaining good electrical contact. Considering the latter issue rst, forming electrical contact to nanomaterials is a non-trivial task which has received much attention. Optical lithography, 7 electron beam lithography, 8 dip- pen nanolithography, 9 and focused ion beam methods, 10 have been used to contact individual nanowires, nanorods, and nanotubes; however, these are not applicable to arrays of nanomaterials at scale. Evaporation of conductors onto the tips of nanowires is a method which addresses a larger array of nanomaterials, but requires a polymer deposition to prevent shorting, and partial removal of the polymer to expose the nanowires. 11 The post-growth transfer of nanomaterials arrays has been as well the subject of much study. Polymers have been utilized to form a supportive matrix around vertically aligned nanowire arrays which are then transferred to a secondary substrate aer mechanical removal from the initial growth substrate. The resulting nanowires may be in a vertical, 12,13 or horizontal orientation. 14 Electrical contact is then made to the array by contact evaporation. However, the application of polymer may require a later polymer removal step unless the polymer is incorporated into the device architecture. Wet approaches such as the Langmuir–Blodgett technique are an alternative method of depositing arrays of nanomaterials on substrates. 15,16 This obviates the need to stabilize the entire array prior to transfer, as the materials need only to be dispersed on the surface of a deposition liquid. Still the resulting material lacks precision control of electrical contact points and thus it is not favourable for scalable, array-based devices. a Department of Chemical and Biomolecular Engineering, University of California at Berkeley, Berkeley, CA, 94720 USA. E-mail: maboudia@berkeley.edu b Center of Integrated Nanomechanical Systems, University of California at Berkeley, Berkeley, CA, 94720, USA Cite this: DOI: 10.1039/c3nr00367a Received 21st January 2013 Accepted 20th March 2013 DOI: 10.1039/c3nr00367a www.rsc.org/nanoscale This journal is ª The Royal Society of Chemistry 2013 Nanoscale Nanoscale COMMUNICATION Downloaded by University of California - Berkeley on 09/04/2013 21:11:45. Published on 21 March 2013 on http://pubs.rsc.org | doi:10.1039/C3NR00367A View Article Online View Journal