RESEARCH PAPER Mechanisms of nucleation and growth in the formation of charge transfer nanocrystals Tingting Li & Scott Melis & Chaitali Bagade & Aisha Khatib & Roman Kosarzycki & Grace Maglieri & Xinran Zhang & Edward Van Keuren Received: 8 January 2019 /Accepted: 25 June 2019 # Springer Nature B.V. 2019 Abstract Nanocrystals of the charge transfer com- pound perylene:7,7′,8,8′-tetracyanoquinodimethane (TCNQ) were created using the reprecipitation method, a simple technique for inducing crystalli- zation in solution by the rapid addition of a misci- ble non-solvent. The nanocrystal size was followed in situ using dynamic light scattering, showing rapid formation of crystals with sizes on the order of 100 nm followed by much slower growth. The complexation of the electron donor perylene and electron acceptor TCNQ was monitored in situ through the appearance of the charge transfer band in the infrared. Both 1:1 and 3:1 cocrystal stoichi- ometries could be obtained, but the ratio of the components did not always result in the same stoi- chiometry in the nanocrystals. In particular, a sam- ple prepared with equal molar amounts of the con- stituents contained both 1:1 and 3:1 forms, as well as pure perylene and TCNQ nanocrystals. This sample also demonstrated non-classical nucleation, with solid particles forming immediately, but longer range order appearing later. Keywords Charge transfer complex . Cocrystals . Nanoprecipitation . Reprecipitation method Introduction Over the past several decades, there has been growing development of organic materials for optical and electronic applications. Foremost has been the work on organic light emitting diodes (LEDs), which have achieved considerable commercial success. However, there has also been strong interest in organic materials for devices such as photovol- taic cells, optical detectors, and organic field effect transis- tors (OFETs). A range of materials have been studied, both polymeric and small molecular system (Kumar et al. 2014). Crystalline small molecule semiconductors are one class of materials that have been extensively investigated. More than two decades ago, crystals of aromatic mole- cules such as pentacene and oligothiophene were fabri- cated into working OFETs with high on/off current ratios and good switching speeds (Horowitz 1998). More re- cently, there has been considerable interest in charge transfer (CT) cocrystals consisting of strong electron donors and acceptors (Goetz et al. 2014). A main advan- tage of these materials is the additional flexibility offered by the ability to change the donor and/or acceptor mol- ecule. Recent reports suggest that particular combina- tions of donor and acceptor molecules can lead to CT crystals that have high electron and hole mobilities and are therefore ideal for ambipolar OFET applications (Zhang et al. 2018; Zhu et al. 2014) or novel devices such as light-emitting transistors (Muccini 2006). J Nanopart Res (2019) 21:147 https://doi.org/10.1007/s11051-019-4593-3 Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11051-019-4593-3) contains supplementary material, which is available to authorized users. T. Li : S. Melis : C. Bagade : A. Khatib : R. Kosarzycki : G. Maglieri : X. Zhang : E. Van Keuren (*) Department of Physics and Institute for Soft Matter Synthesis and Metrology, Georgetown University, Washington, DC 20057, USA e-mail: erv@georgetown.edu