Oriented Growth of ZnO Crystals on Self-Assembled Monolayers of Functionalized Alkyl Silanes R. Turgeman, † O. Gershevitz, † O. Palchik, † M. Deutsch, ‡ B. M. Ocko, # A. Gedanken,* ,† and C. N. Sukenik* ,† Departments of Chemistry and Physics, Bar-Ilan University, Ramat-Gan - 52900, Israel, and Physics Department, Brookhaven National Laboratory, Upton, New York 11973 USA Received June 10, 2003; Revised Manuscript Received August 27, 2003 ABSTRACT: Highly ordered ZnO crystals of 0.15 μm width and 0.5 μm length were grown on silicon wafers coated with a monolayer of SiCl 3 (CH 2 ) 11 -O-C 6 H 5 molecules. Various techniques (contact angle measurements, ellipsometry, ATR-FTIR) were employed for determining the quality of the monolayer coating. In addition, the bare and silane- coated Si wafers were studied by X-ray reflectivity (XR) and grazing-incidence diffraction (GID) using synchrotron radiation. The results obtained point to a possible relationship between the organization of the self-assembled monolayer (SAM) coating, the dipole moment of the headgroup, and the orientation of the ZnO crystals. 1. Introduction Wide band gap materials have come to the forefront of scientific research because of an increasing need for the short wavelength photonic devices that are the future of optoelectronic technology. One of the important wide band gap materials is ZnO (3.3 eV at 300 K). It is a low cost II-VI semiconductor, which is environmen- tally friendly and has superior electronic and optical properties. It is more resistant to radiation damage than other common semiconductor materials such as Si, GaAs, CdS, and GaN, and thus should be useful for space applications. 1 ZnO is currently in use or being considered for use in electrooptical devices, 2-8 as a piezoelectric transducer, 1,9,10 variastor, 1,11,12 phosphor, 1 UV and microwave absorber, 1,10 and gas sensor, 1,13-15 and to make transparent conducting films. 1,16 The physical properties of crystalline materials de- pend on their crystal morphology and habit, crystallite size, the content of impurities or dopants, and the presence of structural defects. For instance, measure- ments of cathode-luminescence and scanning tunneling spectroscopy of ZnO single crystals show that both surface electrical properties and luminescent charac- teristics depend on the face studied. 17 Polar O-termi- nated surfaces show an intrinsic conduction behavior with a surface band gap ranging from 0.4 to 0.8 eV. Zn terminated surfaces show mainly n-type conduction. The nonpolar faces present either intrinsic or p-type behav- ior. 17 ZnO can be grown using a variety of techniques including vapor-phase transport, 18,19 sol-gel meth- ods, 19,20 hydrothermal growth, 21,22 melt growth, 23 chemi- cal vapor deposition (MOCVD), 24 electrochemical deposition, 25-28 laser ablation, 29 sputtering, 30 molecular beam epitaxy, 31,32 and spraying arc-discharge. 33 All these methods yield high quality ZnO and afford control over the orientation of the crystal grown and its exposed face, and, therefore, over its physical properties. However, large-scale use will require the development of simple, low-cost approaches. One such method is to grow ZnO from aqueous solution at temperatures below 100 °C. The feasibility of ZnO crystal engineering under mild conditions was demonstrated by using sur- factants 34-36 or polymers. 37,38 One of the interesting techniques is growing ZnO on diblock copolymers. 37,38 These polymers are designed as molecular tools, such that one block (“anchor block”) interacts strongly with the mineral surface, whereas the other block (“solution block”) simply keeps the construc- tion site in solution. The block, which interacts with the mineral, is hydrophilic, and the other block is hydro- phobic. It was also found that diblock copolymers could play an important role in determining the morphology of CaCO 3 39,40 and BaSO 4 41 crystals. The idea behind this approach is a mimicking of natural processes. 38,42 In natural processes surfactant- like peptides and glycopeptides interact with nuclei and growing crystals. 43 This interaction is achieved by a combination of two factors. One factor is related to the chemical functional group, and another is related to the structure, shape, orientation, and organization of the surfactants. In accordance with this idea, crystal engineering has been done on SAM and Langmuir-Blodgett (LB) mono- layers. 44 The literature has reported on CaCO 3 crystals grown on functionalized alkanthiols, which were coated as self-assembled monolayers on a surface. 45 Growth of CaCO 3 film under a Langmuir monolayer was also reported. 46 Self-assembly of alkyl silane mono- and multilayers on silicon has been studied recently in detail. 47 There is one report of the crystallization of ZnO thin films on a SAM surface. 48 This work, however, did not address the issue of crystal structure and orientation that are the subject of our work. Furthermore, there is one report about growing ZnO on various uncoated surfaces such as glass, Si wafers, ITO, etc. by Vayssieres and co-workers. 49 They reported that the type and crystallinity of the substrates had almost no influence on the crystal growth and orientation of ZnO micro-rod array. Other investigations of crystal engineering by SAM and LB monolayers have shown that the factors * Corresponding author. Aharon Gedanken Tel: 972-5318315. Fax: 972-5311250. E-mail: gedanken@mail.biu.ac.il. † Department of Chemistry, Bar-Ilan University. ‡ Department of Physics, Bar-Ilan University. # Brookhaven National Laboratory. CRYSTAL GROWTH & DESIGN 2004 VOL. 4, NO. 1 169 - 175 10.1021/cg0340953 CCC: $27.50 © 2004 American Chemical Society Published on Web 09/26/2003