Highly Luminescent Monolayers Prepared by Molecular Layer Deposition

Molecular layer deposition (MLD) is a solvent-free technique to prepare organic molecular layers with a similar level of control and perfection as that known from inorganic films grown by atomic layer deposition (ALD). In both, MLD and ALD, the deposition process relies on the self-limiting chemical reaction of two precursors on the surface of the substrate. A plethora of MLD processes has been demonstrated.[1] Among them, hybrid materials made from typical metal-organic precursors and alcohols, so called metalcones, have received considerable attention.[2] However, there is only very limited work on MLD of molecular systems which provide functionalities like charge transport or luminescence.[3; 4]

In our work, a general avenue for the controlled preparation of conformal and highly luminescent monolayers of metal chelate complexes is presented, among them tris(8-hydroxyquinolinato)aluminium (Alq₃) and bis(8-hydroxyquinoline)zinc (Znq₂). Alq₃ is an extremely important model compound for organic electronic applications - it has been the electron transporter and luminescent material in the first efficient hetero-structure organic light emitting diode (OLED). The controlled formation of Alq₃ or Znq₂ monolayers is achieved by functionalization of the substrate with amino groups which serve as initial docking sites for trimethyl aluminum (TMA) or diethyl zinc (DEZ) molecules which datively bind to the amine (see figure).[5] Subsequent exposure to 8-Hydroxyquinoline (8-HQ) results in the self-limiting formation of highly luminescent monolayers on arbitrary surfaces, e.g. curved 3D objects or highly porous silica aerogels. The growth mechanism is studied by in-situ quartz crystal microbalance and ex-situ by highly sensitive (time-resolved) optical absorption/emission spectroscopy and photo-electron spectroscopy. Applications of these thin conformal luminescent layers will be discussed.

[1]       S.M. George, B. Yoon, R.A. Hall, A.I. Abdulagatov, Z.M. Gibbs, Y. Lee, D. Seghete, B.H. Lee, in, Atomic Layer Deposition of Nanostructured Materials, Wiley-VCH Verlag GmbH & Co. KGaA, 2011, pp. 83-107.
[2]       B.H. Lee, B. Yoon, A.I. Abdulagatov, R.A. Hall, S.M. George, Advanced Functional Materials 23 (2013) 532-546.
[3]       O. Nilsen, K.R. Haug, T. Finstad, H. Fjellvåg, Chemical Vapor Deposition 19 (2013) 174-179.
[4]       O. Nilsen, K. Klepper, H. Nielsen, H. Fjellvaåg, ECS Transactions 16 (2008) 3-14.
[5]       A. Räupke, et al., ACS Applied Materials & Interfaces 6 (2014) 1193-1199.