Film growth, adsorption and desorption kinetics of indigo on SiO2

Abstract

Organic dyes have recently been discovered as promising semiconducting materials, attributable to the formation of hydrogen bonds. In this work, the adsorption and desorption behavior, as well as thin film growth was studied in detail for indigo molecules on silicon dioxide with different substrate treatments. The material was evaporated onto the substrate by means of physical vapor deposition under ultra-high vacuum conditions and was subsequently studied by Thermal Desorption Spectroscopy (TDS), Auger Electron Spectroscopy, X-Ray Diffraction, and Atomic Force Microscopy. TDS revealed initially adsorbed molecules to be strongly bonded on a sputter cleaned surface. After further deposition a formation of dimers is suggested, which de-stabilizes the bonding mechanism to the substrate and leads to a weakly bonded adsorbate. The dimers are highly mobile on the surface until they get incorporated into energetically favourable three-dimensional islands in a dewetting process. The stronger bonding of molecules within those islands could be shown by a higher desorption temperature. On a carbon contaminated surface no strongly bonded molecules appeared initially, weakly bonded monomers rather rearrange into islands at a surface coverage that is equivalent to one third of a monolayer of flat-lying molecules. The sticking coefficient was found to be unity on both substrates. The desorption energies from carbon covered silicon dioxide calculated to 1.67 ± 0.05 eV for multilayer desorption from the islands and 0.84 ± 0.05 eV for monolayer desorption. Corresponding values for desorption from a sputter cleaned surface are 1.53 ± 0.05 eV for multilayer and 0.83 ± 0.05 eV for monolayer desorption.

Macroscale Biomolecular Electronics and Ionics

“…Inducing this “standing” orientation of indigo molecules can be achieved by using low surface‐energy aliphatic gate dielectric modification layers . The crystal structures of indigo and tyrian purple are thoroughly characterized, including thin‐film polymorphism . Growing large single crystals of indigo and its derivatives has proven to be straightforward, most commonly via sublimation.…”

Adsorption, desorption, and film formation of quinacridone and its thermal cracking product indigo on clean and carbon-covered silicon dioxide surfaces

“…Thus, the desorption behavior of QA suggests that no desorbable wetting layer exists above the strongly bonded first layer, which would exhibit first-order desorption reaction. This is in contradiction to most organic molecules, which typically form such desorbable wetting layers in which the molecules are either more weakly (as, e.g., for indigo 10 or rubicene 11 on SiO 2 ) or more strongly bonded (as, e.g., for hexaphenyl on mica 8 or pentacene on gold 12 ) than in the 3D islands. Instead, in the present case, even for small coverages (above the strongly bonded first layer), dewetting has to take place and 3D islands are formed.…”

“…Again, indigo first desorbs from a weakly bonded α-state, which for higher coverage transforms into the β-state, due to dewetting, as already observed for pure indigo adsorption/desorption. 10 For QA, which desorbs as γ-peak, again a well pronounced zero-order desorption is observed, down to lowest coverage. Thus, also on this case, no wetting layer exits and islands are already formed for the lowest coverage.…”

“…We have attributed the α-peak to a weakly bonded layer, in which the molecules probably form dimers, due to intermolecular hydrogen bonds, thus weakening the bonding to the substrate. 10 Upon further exposure, dewetting of the weakly bonded indigo molecules sets in and indigo islands are formed. This leads to a decrease of the α-peak and to the evolution of a secondary desorption peak at around 400 K, labelled β.…”

“…7 In fact, the β-peak is very similar to desorption spectra obtained after deposition of pure indigo on SiO 2 . 10 At very low exposure (<3 Å), no desorption takes place at all. Apparently, the flat lying molecules in the first monolayer are so strongly bonded, in particular due to the additional hydrogen bonds, that they rather decompose during sample heating than desorb.…”