Growth and modelling of spherical crystalline morphologies of molecular materials

Abstract

Crystalline, yet smooth, sphere-like morphologies of small molecular compounds are desirable in a wide range of applications but are very challenging to obtain using common growth techniques, where either amorphous films or faceted crystallites are the norm. Here we show solvent-free, guard flow-assisted organic vapour jet printing of non-faceted, crystalline microspheroids of archetypal small molecular materials used in organic electronic applications. We demonstrate how process parameters control the size distribution of the spheroids and propose an analytical model and a phase diagram predicting the surface morphology evolution of different molecules based on processing conditions, coupled with the thermophysical and mechanical properties of the molecules. This experimental approach opens a path for exciting applications of small molecular organic compounds in optical coatings, textured surfaces with controlled wettability, pharmaceutical and food substance printing and others, where thick organic films and particles with high surface area are needed.

Printing of small molecular medicines from the vapor phase

“…The broadening of diffraction peaks in the printed films is attributed to reductions in the average size of the crystallites 41 , compared to that in the original powder. This reduction in crystallite size upon printing occurs due to supersaturation of the API vapor above the substrate, which reduces the critical nucleus size in the formation of new crystallites throughout the deposition process, discussed elsewhere 21 , 25 .

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“…The process is controlled via several parameters 20 , which regulate the deposition rate, the deposit shape, as well as the resulting film morphology. Previously 21 – 23 , we demonstrated how OVJP can provide morphological control and generate thick (>500 nm) and large area deposits, while also enabling additive patterning with high material utilization efficiency (>70%). When film thickness exceeds 200 nm, the films can evolve into unique three-dimentional nano and microstructures 21 , such as shown in Fig.…”

“…Previously 21 – 23 , we demonstrated how OVJP can provide morphological control and generate thick (>500 nm) and large area deposits, while also enabling additive patterning with high material utilization efficiency (>70%). When film thickness exceeds 200 nm, the films can evolve into unique three-dimentional nano and microstructures 21 , such as shown in Fig. 1c , where a film of fluorescein has evolved nanolobes of 500 nm diameter.…”

“…Although VTE is a widely used technique for inorganic and organic material deposition, its scalability for the pharmaceutical industry is limited due to relatively low, controlled deposition rates (on the order of Å per s) and low material yield. Additionally, due to the deposition mechanism, some morphologies are difficult to access 21 – 23 , while significant cross-contamination risk of the common evaporation chamber and parasitic deposits makes it poorly suited for drug discovery. To overcome the challenges outlined above, we adapt a process originally developed to achieve continuous, solvent-free, large-scale, high-throughput, yet ultra-precise printing of small-molecular organic semiconductors: organic vapor jet printing (OVJP) 22 .…”

“…1c , where a film of fluorescein has evolved nanolobes of 500 nm diameter. The effect of source temperature and deposition duration on size and shape of the structures has been previously demonsrated 20 , 21 . Unlike other vapor-based deposition techniques (VTE, chemical vapor deposition, atomic layer deposition, and others), OVJP can be performed at atmospheric pressure and in some cases in ambient atmosphere 24 , 25 .…”