The behavior of thiacarbocyanine dyes on the surface of few-layered hexagonal boron nitride

Nellissen, A.-C. and Steeno, R. and Vandenwijngaerden, J.B.F. and De Feyter, S. and Mertens, S.F.L. and Van der Auweraer, M. (2022) The behavior of thiacarbocyanine dyes on the surface of few-layered hexagonal boron nitride. Dyes and Pigments, 208: 110790.

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Abstract

The adsorption and self-assembly of several thiacarbocyanine dyes on hexagonal boron nitride (hBN) was investigated by combining steady-state spectroscopy and atomic force microscopy. The adsorption isotherms indicate that at saturation the density of the cationic TDC (5,5-dichloro-3-3′-diethyl-9-ethyl-thiacarbocyanine) molecules on hBN is similar to that of TD2 (3-3′-diethyl-9-ethyl-thiacarbocyanine) molecules, while the densities of TD0 (3-3′-diethyl-thiacarbocyanine) molecules and the zwitterionic THIATS (5,5-dichloro-3-3′-disulfopropyl-9-ethyl-thiacarbocyanine) molecules are significantly higher. The intermolecular distances between neighboring adsorbed dyes, calculated from these saturation densities indicate a flat-on adsorption for TDC, TD2 and TD0 and a partial edge-on adsorption for THIATS. AFM micrographs of the adsorbed TDC, TD0 and THIATS molecules indicate that already at low dye concentrations in the solution, where only a small fraction of the hBN surface is covered, the dye molecules already form upon adsorption to hBN aggregates of at least 10 nm, separated by areas where no adsorbed dye molecules can be detected. The resolution of the micrographs was however insufficient to show details of the packing of the adsorbed molecules. For THIATS, the thickness of the adsorbed layer is compatible with an edge-on adsorption, while for TDC and TD0, the thickness of the adsorbed layer is twice the thickness expected for flat-on adsorption. The exciton interaction extracted from the steady-state spectroscopy of the adsorbed dyes is much smaller than observed for H- or J-aggregates of the same dyes in solution or adsorbed to other surfaces such as Langmuir films or silver halides. For TDC, TD2 and TD0, the values of the exciton interaction are compatible with a close packed flat-on adsorption. Hence, optimizing the interaction between the adsorbed dye and hBN rather than between the adsorbed dye molecules governs the packing of the adsorbed dye molecules. The observation of the spectral shifts attributed to the exciton interaction at low average coverage of the hBN surface indicates that the exciton interaction already occurs at low coverage of the hBN surface. This observation is in agreement with the AFM micrographs, which show clustering of the adsorbed dye molecules already at low coverages.