Fac-Alq 3 and Mer-Alq 3 Nano/Microcrystals with Different Emission and Charge-Transporting Properties By Hai Bi, Hongyu Zhang, Yu Zhang, Hongze Gao, Zhongmin Su, and Yue Wang* Since the first efficient organic light-emitting devices (OLEDs) based on tris(8-hydroxyquinoline)aluminum (Alq 3 ) reported by Tang and VanSlyke, Alq 3 has been employed as an electro- n-transporting or host-emitting material in OLEDs because of its excellent electron-transporting ability as well as film formation properties. [1–7] In addition to the intensive studies on the optimization of Alq 3 -based devices for high performance, a great deal of effort has been invested in its polymorphs and photophysical properties. [8–15] Generally, Alq 3 molecules adopt mer -Alq 3 (mer ¼ meridional) isomeric form with C 1 symmetry in both solution and solid state, and fac-Alq 3 ( fac ¼ facial) with C 3 symmetry could only been obtained by annealing mer -Alq 3 solid at a very high temperature (around 400 8C). These two isomeric forms exhibit quite different physical properties such as color and fluorescence, that is, green emission for light-yellow mer -Alq 3 and blue emission for whitish fac-Alq 3 . Along with the progress on the studies of Alq 3 bulk materials, increased effort has been directed toward the construction of Alq 3 -based nano or microstructured materials, particularly, 1D nanostructures, to further understand their photophysical properties in the nanoscale and explore novel optoelectronic applications. [16–22] Although some green-emitting mer-Alq 3 nanocrystals in a or b phases have been obtained via different processes, the prepara- tion of blue-emitting fac-Alq 3 nanocrystals in d or g phase is still an important issue. [23] It was demonstrated that the fac-Alq 3 crystalline phase could only be achieved by annealing mer-Alq 3 at around 400 8C for at least several tens of minutes. [12] Therefore, the reported approaches such as vapor deposition and a solution-based route, [16–21] which have been employed to prepare mer-Alq 3 nanocrystals, are not suitable for the fabrication of fac-Alq 3 nanocrystals. From this viewpoint, the development of an efficient approach suitable to prepare fac-Alq 3 nanocrystals is a highly important issue regarding the fundamental physical properties of fac-Alq 3 -based nanostructures. Since different phases of an organic functional material often have obviously different optical and electronic properties, it can be expected that fac-Alq 3 will display distinct transporting properties compared to mer -Alq 3 . The mer-Alq 3 has already been proven to be an excellent n-type semiconducting material; however, the charge-transporting property of the fac-Alq 3 has not yet been studied in detail. To understand the transporting nature of fac-Alq 3 and develop its potential applications for optoelec- tronics, the preparation of a fac-Alq 3 -based film is also urgently required. Following these points, we have mainly concentrated our attention on the development of an efficient approach to prepare fac-Alq 3 nano/microstructures as well as a uniform thin film thereof. In this contribution, we report a facile method for the controllable fabrication of high-quality fac-Alq 3 and mer -Alq 3 1D nano/microstructures as well as their nano/microstructure-based films. This approach is significant because it not only enables the preparation of high-quality fac-Alq 3 nanostructures and thin films but also provides the pathway to evaluate the transporting nature of the produced fac-Alq 3 film. The uniform films comprised fac-Alq 3 nano/microcrystals forming on substrates, which were pre-coated with electrodes, thus, allowing us to directly characterize their electrical properties such as carrier transport. Indeed, a single-carrier device based on fac-Alq 3 film fabricated in situ exhibited an excellent hole-transporting character. To obtain mer-Alq 3 and fac-Alq 3 nano/microcrystals, a double-film annealing process was developed (Fig. 1). Firstly, two Alq 3 films with a thickness of about 500 nm each n were prepared on glass substrates by a common vacuum deposition technique at room temperature; these Alq 3 films were nearly amorphous, as verified by X-ray diffraction measurement (see Supporting Information, Fig. S1). Then, the two films were stacked together with the help of gravity in a face-to-face fashion and horizontally placed into a small chamber, which was filled COMMUNICATION www.MaterialsViews.com www.advmat.de Figure 1. Process for the preparation of mer-Alq 3 and fac-Alq 3 nano/ microcrystals. [*] Prof. Y. Wang, H. Bi, Dr. H. Zhang, Y. Zhang, Dr. H. Gao, Prof. Z. Su State Key Laboratory of Supramolecular Structure and Materials College of Chemistry, Jilin University Changchun 130012 (P.R. China) E-mail: yuewang@jlu.edu.cn DOI: 10.1002/adma.200903094 Adv. Mater. 2010, 22, 1631–1634 ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1631