Substrate-Free, Self-Standing ZnO Thin Films Antonino Gulino,* Fabio Lupo, and Maria E. Fragala ` Dipartimento di Scienze Chimiche, UniVersita ` di Catania and I.N.STM UdR of Catania, V.le A. Doria 6, 95125 Catania, Italy ReceiVed: May 5, 2008; ReVised Manuscript ReceiVed: June 13, 2008 Substrate-free ZnO thin films have been obtained by metal organic chemical vapor deposition. Mild heating (37-51 °C) of the Zn(C 5 F 6 HO 2 ) 2 · 2H 2 O · CH 3 (OCH 2 CH 2 ) n OCH 3 (n ) 2, 3, 4) adduct precursors produced thermally stable liquid compounds that were easily evaporated. Rapid quenching to room temperature of the reactor quartz tube caused self-exfoliation of ZnO films deposited on the walls, thus giving flexible films having a few hundred nanometer thickness. No exfoliation was observed from films grown on flat silica substrates, even in the case of rapid quenching to room temperature. The obtained films were characterized by X-ray diffraction, UV-vis spectra, secondary electron microscopy, and transmission electron microscopy. The film thickness was evaluated by SEM cross-sections. Present ZnO films are semiconducting with an allowed direct transition at 3.3 eV. Introduction Control of nanodimensionality of metal oxide semiconductors remains a crucial goal for many technologies since particular morphologies might have distinguished properties for special applications. 1 In this perspective, self-standing substrate-free films are of relevance since the substrate-film interface affects optical, electrical, and mechanical properties of thin crystalline films. Conversely, properties of films unaffected by the substrate-film interaction may significantly differ from their substrate-affected counterparts. 1b,j Zinc oxide (ZnO) adopts the wurtzite structure (space group P6 3 mc) pictorially described in terms of hexagonal close packing (hcp) of oxygen ions stacked along the [001] direction with cations occupying one-half of the tetrahedral sites. 1b,e,2 It is an intrinsic n-type semiconductor having lattice defects consisting of oxygen vacancies and involving interstitial (Zn 2+ -2e - ) pairs and cation and anion vacancies (0 M , 0 O ). 2 The band gap width of ∼3.3 eV, 3 in polycrystalline ZnO, shows anomalous behavior because of the existence of potential barriers at the grain boundaries 4 while the resistivities depend on the synthetic procedure and/or on the presence of dopants within the ZnO lattice. 5 Both its direct wide band gap and large exciton binding energy (60 meV) 1b,h make ZnO one of the most important functional materials with almost unique properties in terms of near-UV emission, optical transparency, electric conductivity, piezoelectricity, and sensing characteristics. 6 For instance, double-side ZnO films can show chemisorption on both surfaces, thus improving sensing detection limits. Besides ZnO represents an important emerging 1D nanomaterial. 1b,f,h,7 Moreover, the most common varistor is represented by bulk ZnO material sandwiched between two electrodes. 8 Varistors show nonohmic current-voltage behaviors and are usually incorporated in electronic circuits as protection devices. They show high resistances at low voltages, and low resistances at high voltages; hence, they divert a large current, due to excessive transient voltages, from the sensitive components. It turns out that a varistor remains nonconductive under voltages well below the “clamping value” while, upon excessively high transient pulses, they melt, burn, vaporize, or otherwise are damaged or destroyed, thus protecting the whole circuit. In this perspective, substrate-free thin ZnO films could have varistor applications in very sensible electronic circuits operating at low currents. Nanostructured ZnO has been obtained using a large variety of techniques. 1h,6,9,10 We already reported on the metal organic chemical vapor deposition (MOCVD) of ZnO thin films using the low-melting (37-51 °C) zinc hexafluoroacetylacetonate dihydrate polyether, Zn(C 5 F 6 HO 2 ) 2 · 2H 2 O · CH 3 (OCH 2 CH 2 ) n - OCH 3 (n ) 2, 3, 4) complexes (hexafluoroacetylacetonate ) 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate) as precursors. 11 MOCVD from liquids has revealed a choice of election since it allows elevated reproducibility associated with constant precursor evaporation (hence constant mass-transport) rates for given source temperatures. The Zn(C 5 F 6 HO 2 ) 2 · 2H 2 O · polyether complexes are liquid (at MOCVD conditions) and allow MOCVD of ZnO from liquid sources. 11 Therefore, in the present investigation we report on the first ever case of substrate-free ZnO thin films showing a thickness of a few hundred nanometers. Experimental Details The Zn(C 5 F 6 HO 2 ) 2 · 2H 2 O · CH 3 (OCH 2 CH 2 ) 2 OCH 3 (1), Zn- (C 5 F 6 HO 2 ) 2 · 2H 2 O · CH 3 (OCH 2 CH 2 ) 3 OCH 3 (2), and Zn(C 5 F 6 - * Corresponding author. E-mail: agulino@dipchi.unict.it, fax: +39-095- 580138. Figure 1. Arrhenius log-plot of the evaporation rates versus 1/T of present source precursors. (1) R )-0.99832, SD ) 0.06421, P < 0.0001. (2) R )-0.99604, SD ) 0.08348, P < 0.0001. (3) R ) -0.9966, SD ) 0.0842, P < 0.0001. J. Phys. Chem. C 2008, 112, 13869–13872 13869 10.1021/jp8039466 CCC: $40.75  2008 American Chemical Society Published on Web 08/19/2008