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Nucleation of Organic Crystals onto 2D materials

Periodic Reporting for period 3 - NOC2D (Nucleation of Organic Crystals onto 2D materials)

Reporting period: 2018-09-01 to 2020-02-29

The formation of crystalline solids from liquid-phase precursor is a central idea in materials chemistry. Organic crystal structures can be found in a large number of products, including food, explosives, pigments and pharmaceuticals. Control of molecular assembly is therefore a fundamental problem for both research and industry and it involves substantial scientific and economic challenges. For example, polymorphism is crucial for drug manufacturers because the crystal structure, morphology and size, can all affect the stability, efficacy and production cost of the drug. Therefore, it is essential to achieve a deeper understanding on the molecular processes happening at the early stage of crystallization. Although important results have been obtained, our understanding on how a crystal of organic molecules nucleates on a surface is still poor. To go beyond state-of-the art we need techniques able to probe rare nucleation events with nanoscale resolution and very high sensitivity, providing direct insights on the structure of the nuclei and their interaction with the environment.

​The aim of NOC2D is to use 2D crystals to open new horizons in the understanding of nucleation of organic crystals by using a multi-disciplinary approach, which combines chemical engineering, material chemistry, graphene physics and sensors technology. Graphene, a single layer of graphite, will allow preparing advanced surface templates and to perform nucleation experiments that would be impossible or too difficult to achieve with other templates. In particular, graphene will be used both as surface template and as sensor to probe nucleation events in real time. We will combine electrical and optical readouts to investigate molecular interactions during nucleation with chemical recognition and nanoscale resolution. This will strongly improve our understanding of the basic phenomena which control heterogeneous nucleation from liquid-phase precursors.
The crystallisation protocol was optimised (sample set-up, control environmental conditions, concentration, volume , etc) using silicon as a reference substrate and glycine, as reference molecule. Glycine was selected because it has been widely used and its polymorphs can be easily distinguished by Raman spectroscopy. Both homogeneous and heterogenous nucleations were investigated. The substrates used are silicon and graphene, produced by chemical vapour deposition (CVD). We first optimised the transfer of graphene from copper to silicon in order to minimise contaminations, then we started to look at how graphene affects the polymorphs outcomes of the crystals. In addition to CVD graphene, other types of graphene were used, produced with different methods and containing variable oxygen content to tune the surface properties of graphene.
In situ Raman analysis during evaporation of the solvent was carried out. The results however are rather difficult to understand and more work is required to explain the changes observed in the Raman spectrum.
Far field Raman spectroscopy was performed on glycine crystals, graphene nanoribbons and electro-chemically exfoliated graphene. Tip-Enhanced Raman Spectroscopy on electro-chemically exfoliated graphene was attempted, but failed.
There are no previous works looking at crystallisation on graphene, so all results obtained are currently beyond state of art.