Photoluminescence as a probe of molecular organization in PDI8-CN2 ultra-thin films Aldo Brillante a , Tommaso Salzillo a,n , Raffaele G. Della Valle a , Elisabetta Venuti a , Francesco Borgatti b , Eugenio Lunedei b , Fabiola Liscio c , Silvia Milita c , Cristiano Albonetti b a Department of Industrial Chemistry “Toso Montanari” and INSTM-UdR Bologna, University of Bologna, I-40136 Bologna, Italy b Consiglio Nazionale delle Ricerche – Istituto per lo Studio dei Materiali Nanostrutturati (CNR-ISMN), Via P. Gobetti 101, 40129 Bologna, Italy c Consiglio Nazionale delle Ricerche – Istituto per la Microelettronica e i Microsistemi (CNR-IMM), Via P. Gobetti 101, 40129 Bologna, Italy article info Article history: Received 5 February 2017 Received in revised form 13 March 2017 Accepted 24 March 2017 Available online 25 March 2017 Keywords: PDI8-CN2 Thin film Excimer emission GXRD AFM abstract Photoexcited ultra-thin films of the organic semiconductor N,N'-bis(n-octyl)-dicyanoperylene-3,4:9,10- bis dicarboximide (DPI8-CN2), grown on thermal Si/SiO 2 , exhibit an intense room temperature emission, strongly dependent on molecular coverage, even for sub-monolayer thicknesses. The luminescence spectra are characterized by a highly structured, isolated molecule emission in the sub-monolayer regime (coverage o30%) and by a condensed-state singlet exciton fluorescence temporally evolving (within 0.5 ns) toward an unstructured, energetically relaxed, excimer-like emission, for thicker films. Once a complete monolayer is formed, only the unstructured excimer emission can be detected. The experi- mental findings are interpreted in terms of progressive deposition of nearly not interacting molecules, followed by islands formation where a strong dimeric coupling takes place, upon increasing the coverage. A thorough investigation by means of AFM and in-situ X-ray diffraction confirms the proposed picture. & 2017 Elsevier B.V. All rights reserved. 1. Introduction Among organic semiconductors, π-π stacked perylene diimide derivatives have proved to be valid materials for organic electro- nics, due to their high chemical and thermal stability linked to excellent electron mobility and photo-luminescence properties [1]. The emission of this class of compounds is dominated by strong π- π intermolecular interactions and excimeric and charge transfer exciton states can be generated by optical excitation [2]. When using ultra-thin films, down to sub-monolayer (ML) regime [3], the study of the properties and of the dynamics of the excited states can probe the spatial arrangement of the molecules, with possible hints on their organization as assemblies at the interface [4]. The optical and spectroscopic properties of films of N,N'-bis(n- octyl)-dicyanoperylene-3,4:9,10-bis dicarboximide (PDI8-CN2, depicted in the inset of Fig. 1), a typical representative of n-type semiconductors [5,6], have been widely reported in the recent literature, together with their morphological study and their OFET applications [7–12]. More recently, in an accurate study, the film optical absorption spectra have been discussed in terms of intra- and inter-layer electronic coupling of the chromophore, starting from sub-ML thickness up to 40 nm (about 20 ML) [13]. Lastly, structural and electrical characterizations of thin films grown by High Vacuum (HV) sublimation, performed in-situ during the de- position and complemented with ex-situ Atomic Force Microscopy (AFM) measurements have been reported. That work has given indications of a quasi Layer-by-Layer (LbL) growth, and of the presence of defects/molecules not aggregated in a crystal phase, which heal/crystalize during the deposition, affecting the charge transport properties [14]. Subject of this paper is the study of the photoluminescence (PL) of HV grown ultra-thin films (sub-ML and ML) PDI8-CN2, sup- ported by morphological and structural analyses. This work shows how, by inspecting the PL spectra of ultra-thin films, from sub-ML to few ML thick, information on the molecular arrangement on the substrate can be inferred. The studied films exhibit an intense PL, even in case sub-ML coverage, upon steady-state excitation at 450 nm. Two kinds of emission have been identified: the first one originates from the initial, randomly oriented, set of not-inter- acting (or weakly interacting) molecules, the second one from the organization of the molecules in a condensed system, within the first few MLs. This latter molecular organization gives rise to lu- minescence profiles associated to the radiative recombination of the first singlet excited state of molecular aggregates (at λ  630 nm), progressively relaxing to a pair-like excimer emission (for λ Z675 nm), which gains intensity on the low energy side of the spectrum, whilst the monomer-like emission correspondingly decreases. By a further increasing of coverage, the film emission Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jlumin Journal of Luminescence http://dx.doi.org/10.1016/j.jlumin.2017.03.058 0022-2313/& 2017 Elsevier B.V. All rights reserved. n Corresponding author. E-mail address: tommaso.salzillo@unibo.it (T. Salzillo). Journal of Luminescence 187 (2017) 403–409