Surface-Controlled Crystal Alignment of Naphthyl End-Capped Oligothiophene on Graphene: Thin-Film Growth Studied by in Situ X-ray Diffraction

Abstract

We report on the microstructure, morphology, and growth of 5,5´-bis(naphth-2yl)-2,2´-bithiophene (NaT2) thin films deposited on graphene, characterized by grazingincidence X-ray diffraction (GIXRD) and complemented by atomic force microscopy (AFM) measurements. NaT2 is deposited on two types of graphene surfaces: custom-made samples where CVD-grown graphene layers are transferred onto a Si/SiO 2 substrate by us and common commercially transferred CVD graphene on Si/SiO 2 . Pristine Si/SiO 2 substrates are used as a reference. The NaT2 crystal structure and orientation depend strongly on the underlying surface, with the molecules predominantly lying-down on the graphene surface (face-on orientation) and standing nearly out-of-plane (edge-on orientation) on the Si/SiO 2 reference surface. Post growth GIXRD and AFM measurements reveal that the crystalline structure and grain morphology differ depending on whether there is polymer residue left on the graphene surface. In situ GIXRD measurements show that the thickness dependence of the intensity of the (111) reflection from the crystalline edge-on phase does not intersect zero at the beginning of the deposition process, suggesting that an initial wetting layer, corresponding to 1-2 molecular layers, is formed at the surface-film interface. By contrast, the (111) reflection intensity from the crystalline face-on phase grows at a constant rate as a function of film thickness during the entire deposition.

Crystal Orientation Imaging of Organic Monolayer Islands by Polarized Light Microscopy

“…The confirmation of crystal orientation in bulk or thin-films was performed by X-ray diffraction, 3,[7][8][9][10][11][12][13][14][15][16] low-energy electron diffraction (LEED), [17][18][19] photoemission electron microscopy, 20 optical measurements, 2,[14][15][16][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38] and atomic force microscopy (AFM). 2,30,[39][40][41][42][43] For small molecule organic materials that are soluble in organic solvents, X-ray diffraction measurement and absorption spectroscopy 2,15,16,21,22 have been used to investigat...…”

Revealing structural evolution occurring from photo-initiated polymer network formation

“…5c and Supplementary Table 2 29 . Illuminating the sample under conditions where the incident angle is above the critical angle allow the penetration of the full thin film and so yield information over the full thickness [30][31][32][33] . The beamline setup includes the UV-LED, a Vis spectroscopy setup to confirm changes in thickness of the thin film during light exposure as well as a camera to observe any macroscopic change.…”

Work function difference of naphthyl end-capped oligothiophene in different crystal alignments studied by Kelvin probe force microscopy

“…and it has been proposed that PMMA residues influence the molecular orientation, when NaT2 molecules are deposited on top [29]. Finally, a thin film of preheated (230 °C) NaT2 was deposited onto the heated, graphene-covered substrates by physical vapor deposition (PVD).…”

“…Huss-hansen et al used grazing incidence X-ray diffraction (GIXRD) to study an OSC thin film consisting of 5,50-bis(naphth-2-yl)-2,20-bithiophene (NaT2) deposited on substrates of pristine SiO 2 and substrates of SiO 2 covered with high-quality graphene [29]. NaT2 is a rod-shaped, naphthyl end-capped oligothiophene forming upright standing molecular structures when deposited on SiO 2 , and additionally forming face-on structures when deposited on graphene (Fig.…”

“…Two kinds of face-on structures have been observed with the angle between the b-axis and the substrate being either 0 • or 22 • . Both face-on structures have the same lattice constants as the upright standing molecules and only the angle with respect to the substrate is altered [29]. Huss-hansen has also documented the photo-absorption of NaT2 to be approximately 2.5 times larger for face-on orientation compared to upright standing structures, and thus NaT2 deposited on graphene could be an attractive candidate for use in OPTs [29].…”