www.afm-journal.de FULL PAPER www.MaterialsViews.com 577 wileyonlinelibrary.com © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2012, 22, 577–584 Eric R. Meshot, Keval D. Patel, Sameh Tawfick, K. Anne Juggernauth, Mostafa Bedewy, Eric A. Verploegen, Michaël F. L. De Volder, and A. John Hart* 1. Introduction Fullerenes (C 60 ) are a uniquely monodisperse nanoscale building block and can be assembled into nano- and microscale crystals with diverse and controllable sizes and shapes using evaporation of dispersions. [1–6] Crystalline C 60 is a direct-band gap semiconductor, [7] an excellent electron acceptor, [7,8] and calculations show that contacts between C 60 molecules in a closely packed crystal have very low thermal conductance. [9] These features make C 60 attractive for applications including diodes, [8,10] field-effect transistors, [11] pho- toelectrics, [5,12] and thermoelectrics. [9] C 60 crystals are generally made in bulk or deposited randomly on substrates; how- ever, directed placement, alignment, and interconnection of C 60 crystals is typically necessary for their utilization in devices, and this remains a challenge. We demonstrate that aligned carbon nanotube (CNT) films can spatially direct the crystallization of C 60 from solution, therefore enabling integration of hybrid thin-film devices by combining standard photolithography and two-step capillary self-assembly process. Uniquely, we find that C 60 rods crystallize preferentially par- allel to films of horizontally aligned CNTs, thus creating a self-organized directional assembly, wherein C 60 crystals are pierced and interconnected by CNT conduits. We find that the kinetics of C 60 crystallization are 8-fold faster on CNTs than on bare Si, but the dimensions of the rods are the same. The resultant hybrid film is advantageous for energy devices, especially considering that C 60 crystals alone are poor electrical conductors. To this end, we demonstrate that the C 60 –CNT hybrid films have high photoconductive gain under UV irradiation with a responsivity as high as 10 5 A W −1 at low biases (± 0.5 V). Overall, the rational design and fabrication of templates that induce ordered crystallization, and the under- standing of how nanostructured surfaces affect crystallization kinetics, are important topics for applications including organic electronics, [13] composite materials, and protein science. [14,15] 2. Synthesis of C 60 -CNTs Hybrid Films Directed crystallization of C 60 on CNTs is achieved, as shown in Figure 1. First, vertically aligned CNT “forests” are grown from lithographically patterned catalyst on silicon wafers using a standard catalytic chemical vapor deposition (CVD) process. [16,17] The forests comprise multi-walled CNTs with a mean diameter of 11 ± 2.8 nm. [18] Next, the vertical CNTs are Photoconductive Hybrid Films via Directional Self- Assembly of C 60 on Aligned Carbon Nanotubes Hybrid nanostructured materials can exhibit different properties than their constituent components, and can enable decoupled engineering of energy conversion and transport functions. Novel means of building hybrid assem- blies of crystalline C 60 and carbon nanotubes (CNTs) are presented, wherein aligned CNT films direct the crystallization and orientation of C 60 rods from solution. In these hybrid films, the C 60 rods are oriented parallel to the direction of the CNTs throughout the thickness of the film. High-resolution imaging shows that the crystals incorporate CNTs during growth, yet grazing- incidence X-ray diffraction (GIXD) shows that the crystal structure of the C 60 rods is not perturbed by the CNTs. Growth kinetics of the C 60 rods are enhanced 8-fold on CNTs compared to bare Si, emphasizing the importance of the aligned, porous morphology of the CNT films as well as the selective surface interactions between C 60 and CNTs. Finally, it is shown how hybrid C 60 –CNT films can be integrated electrically and employed as UV detectors with a high photoconductive gain and a responsivity of 10 5 A W −1 at low biases (± 0.5 V). The finding that CNTs can induce rapid, directional crystal- lization of molecules from solution may have broader implications to the science and applications of crystal growth, such as for inorganic nanocrystals, proteins, and synthetic polymers. DOI: 10.1002/adfm.201102393 E. R. Meshot, K. D. Patel, S. Tawfick, K. A. Juggernauth, M. Bedewy, Prof. A. J. Hart Department of Mechanical Engineering University of Michigan 2350 Hayward Street, Ann Arbor, MI 48104, USA E-mail: ajohnh@umich.edu Dr. E. A. Verploegen Stanford Synchrotron Radiation Lightsource SLAC National Accelerator Laboratory 2575 Sand Hill Road, Menlo Park, CA 94025, USA Dr. M. F. L. De Volder IMEC, Kapeldreef 75 3001 Heverlee, Belgium Department of Mechanical Engineering KULeuven, Celestijnenlaan 300B, 3001 Leuven, Belgium