Structure, Wettability, and Thermal Stability of Organic Thin-Films on Gold Generated from the Molecular Self-Assembly of Unsymmetrical Oligo(ethylene glycol) Spiroalkanedithiols

Abstract

Organic thin-films on gold were prepared from a set of new, custom-designed bidentate alkanethiols possessing a mixture of normal alkane and methoxy-terminated tri(ethylene glycol) chains. The new unsymmetrical spiroalkanedithiol adsorbates were of the form [CHO(CHCHO)(CH)]-[CH(CH)]C[CHSH] where n = 3 and 14; designated EG3C7-C7 and EG3C7-C18, respectively. Their corresponding self-assembled monolayers (SAMs) on gold were characterized and compared with monothiol SAMs derived from an analogous normal alkanethiol (C18SH) and an alkanethiol terminated with an oligo(ethylene glycol) (OEG) moiety (i.e., EG3C7SH). Ellipsometric data revealed reduced film thicknesses for the double-chained dithiolate SAMs, which perhaps arose from the phase-incompatible merger of a hydrocarbon chain with an OEG moiety, contributing to disorder in the films and/or an increase in chain tilt. The comparable wettabilities of the SAMs derived from EG3C7SH and EG3C7-C7, using water as the contacting liquid, are consistent with exposure of the OEG moieties at both interfaces, whereas the lower wettability of the SAM derived from EG3C7-C18 is consistent with exposure of hydrocarbon chains at the interface. The data collected by X-ray photoelectron spectroscopy confirmed the formation of the new OEG-terminated dithiolate SAMs, and also revealed them as less densely packed monolayers due in part to the large molecular cross section of the OEG moieties and to their double-chained structure with dual surface bonds. Mixed SAMs formed from pairs of monothiols having chain compositions analogous to those of the chains of the new dithiols showed that an EG3C7SH/heptanethiol-mixed SAM and the EG3C7-C7 SAM produced almost identical characterization data, revealing the favorable film formation dynamics for adsorbate structures where the alkyl chains can assemble beneath the phase-incompatible OEG termini. For the mixed SAM formed from EG3C7SH/C18SH, the data indicate that the EG3C7SH component failed to incorporate in the film, demonstrating that the blending of phase-incompatible chains is sometimes best accomplished when both chains exist on a single adsorbate structure. Furthermore, the results of solution-phase thermal desorption tests revealed that the OEG-terminated films generated from the bidentate EG3C7-C7 and EG3C7-C18 adsorbates exhibit enhanced thermal stability when compared to the film generated from monodentate EG3C7SH. In a brief study of protein adsorption, the multicomponent SAMs showed a greater ability to resist the adsorption of fibrinogen on their surfaces when compared to the SAM derived from C18SH, but not better than the monolayer derived from EG3C7SH.

Growth of Metal Halide Perovskite, from Nanocrystal to Micron-Scale Crystal: A Review

“…This surface will confine the dispersed solution and guide the growth direction of perovskite crystals. As with the self-assembled organic molecular technology [82], hybrid perovskite molecules are favorable to assembly along the crystal surface of the substrate. This lateral crystal growth is a common method via epitaxial lateral overgrowth (ELO) and the vertical direction is restricted by geometrical confinement.…”

Unsymmetrical Spiroalkanedithiols Having Mixed Fluorinated and Alkyl Tailgroups of Varying Length: Film Structure and Interfacial Properties

“…This concept can be readily applied to films derived from organic molecules covalently bound to surfaces by incorporating multiple attachment points on the adsorbate to create multiple interactions between the adsorbate and the metal surface. Several examples of designs of such bidentate adsorbates, which have been used to enhance film stability on metal substrates, include aromatic-based bidentate adsorbates [ 56 , 57 , 58 , 59 ], spiroalkanedithiolate-based adsorbates [ 60 , 61 , 62 , 63 ], and trithiolate-based adsorbates [ 64 , 65 , 66 , 67 , 68 , 69 ]. Using the spiroalkanedithiol (SADT) structure, well-packed and highly oriented SAMs can be generated by utilizing an adsorbate structure having a pair of relatively long alkyl chains [ 60 ].…”

“…A major attractive feature of this design is the relatively simple synthetic approach, which can open new avenues for the synthesis of a large variety of adsorbates. A recent example includes the development of film compositions where two phase-incompatible groups were incorporated into the same molecule, yielding homogeneously mixed SAMs [ 62 , 63 ]. Important to note are previous efforts toward the development of mixed-chained adsorbates originating from a single point, including the use of dialkyl sulfides by Whitesides [ 70 ], as well as other methods highlighted in the literature [ 51 ].…”

Unsymmetrical Spiroalkanedithiols Having Mixed Fluorinated and Alkyl Tailgroups of Varying Length: Film Structure and Interfacial Properties

“…Several examples of designs of such bidentate adsorbates, which have been used to enhance film stability on metal substrates, include aromatic-based bidentate adsorbates [44][45][46][47], spiroalkanedithiolate-based adsorbates [48][49][50][51], and trithiolate-based adsorbates [52][53][54][55][56][57]. Using the spiroalkanedithiol (SADT) structure, well-packed and highly oriented SAMs can be generated by utilizing an adsorbate structure having a pair of relatively long alkyl chains [48].…”

“…A major attractive feature of this design is the relatively simple synthetic approach, which can open new avenues for the synthesis of a large variety of adsorbates. A recent example includes the development of film compositions where two phase-incompatible groups were incorporated into the same molecule, yielding homogeneously mixed SAMs [50,51].…”