Freestanding non-covalent thin films of the propeller-shaped polycyclic aromatic hydrocarbon decacyclene

Abstract

Molecularly thin, nanoporous thin films are of paramount importance in material sciences. Their use in a wide range of applications requires control over their chemical functionalities, which is difficult to achieve using current production methods. Here, the small polycyclic aromatic hydrocarbon decacyclene is used to form molecular thin films, without requiring covalent crosslinking of any kind. The 2.5 nm thin films are mechanically stable, able to be free-standing over micrometer distances, held together solely by supramolecular interactions. Using a combination of computational chemistry and microscopic imaging techniques, thin films are studied on both a molecular and microscopic scale. Their mechanical strength is quantified using AFM nanoindentation, showing their capability of withstanding a point load of 26 ± 9 nN, when freely spanning over a 1 μm aperture, with a corresponding Young's modulus of 6 ± 4 GPa. Our thin films constitute free-standing, non-covalent thin films based on a small PAH.

Fig. 1. Langmuir–Blodgett experiments and thin film formation of decacyclene.

a Schematic representation of the compression–decompression of decacyclene molecules at the air–water interface in a Langmuir–Blodgett through. b Representative experimental Langmuir–Blodgett isotherms for two consecutive compression/decompression cycles on a single sample, showing surface pressure as a function of mean molecular area. Black lines are isotherms computed using MD simulations, for systems containing 30 (●) and 60 (■) molecules, respectively. Reported values are averaged over 3 separate MD simulations with error bars indicating standard deviations. Insert shows the molecular structure of decacyclene. c Optimized geometry of a decacyclene dimer computed at PBE/6-31 G(d,p) (Supplementary Structure 5).

Fig. 2. MD simulation on decacyclene.

a MD simulation box (5.0 × 5.0 × 20.0 nm³) containing water molecules represented by a space filling model, and the random, pre-equilibration position of decacyclene molecules (30) on both sides thereof represented by a licorice model in gray. Protons explicitly drawn on water, but omitted from decacyclene. b Top view of a. c Same system as b, after equilibration at 300 K. d Same system as c, after compression to a surface pressure of 30 mN m⁻¹. e Top view of c after equilibration at 300 K. Differently colored arrows added to highlight different stacking domains. f Same system as in d after compression to a surface pressure of 30 mN m⁻¹. g Highlight of molecules found in f showing the onset of roof-tiling. h Distribution of the tilt angle (Φ) per decacyclene molecule as derived from the MD simulations. The definition of the tilt angle (Φ) as the arc between the plane of a decacyclene molecule (yellow) and the x–y plane of the water surface, is illustrated in the top left. Shown are the distributions of the tilt angles for systems consisting of 60 (blue) and 30 (red) decacyclene molecules, before (light) and after (dark) compression to a surface pressure of 30 mN m⁻¹. Distribution curves were obtained via Gaussian broadening with default standard deviation and normalized per amount of decacyclene molecules, using a Kernel Density Estimation to produce this plot with N_bins = 1897.

Fig. 3. Thin film characterization.

a AFM image of a decacyclene film transfered unto a Si/SiO₂ wafer (For additional images see SI Fig. S5). b Height profile of the area marked in blue in a, showing a film thickness of 2.5 ± 0.7 nm. c SEM image of a decacyclene film free-standing over 0.6 µm diameter apertures. Ruptured thin films are indicated by arrows. Insert shows a zoom on the same grid, illustrating minor film defects as dark regions. d SEM image of a near-defect free decacyclene film free-standing over a 2 μm aperture.

Fig. 4. AFM nanoindentation of decacyclene films spanning over a 1 µm circular aperture.

Representative AFM images before a and after b nanoindentation. White arrow in a indicates the point of the indentation with the AFM tip. c Representative force-indentation curve. Red curve shows the fit in the elastic regime. Red arrow points to a small drop in the slope of the curve. Above this point the fit of the elastic properties is no longer valid. The green arrow shows the breaking event when the thin film fully ruptures. d Histogram of the penetration force needed to rupture the thin film. e Histogram of the force inducing non-elastic changes in the thin films. f Effective Young’s modulus of decacyclene film.