Dynamic interfaces in an organic thin film

Abstract

Low-dimensional boundaries between phases and domains in organic thin films are important in charge transport and recombination. Here, fluctuations of interfacial boundaries in an organic thin film, acridine-9-carboxylic acid on Ag(111), have been visualized in real time and measured quantitatively using scanning tunneling microscopy. The boundaries fluctuate via molecular exchange with exchange time constants of 10-30 ms at room temperature, with length-mode fluctuations that should yield characteristic f(-1/2) signatures for frequencies less than approximately 100 Hz. Although acridine-9-carboxylic acid has highly anisotropic intermolecular interactions, it forms islands that are compact in shape with crystallographically distinct boundaries that have essentially identical thermodynamic and kinetic properties. The physical basis of the modified symmetry is shown to arise from significantly different substrate interactions induced by alternating orientations of successive molecules in the condensed phase. Incorporating this additional set of interactions in a lattice-gas model leads to effective multicomponent behavior, as in the Blume-Emery-Griffiths model, and can straightforwardly reproduce the experimentally observed isotropic behavior. The general multicomponent description allows the domain shapes and boundary fluctuations to be tuned from isotropic to highly anisotropic in terms of the balance between intermolecular interactions and molecule-substrate interactions.

Fig. 1.

Structural models and image of ACA on Ag(111). Shown are the molecular structure of ACA (A) and the arrangement of ACA molecules in the ordered phase as measured by using STM (B) and corresponding molecular models (C and D) determined previously by using STM, infrared reflection absorption spectroscopy (IRRAS), and x-ray photoelectron spectroscopy (XPS) measurements (15, 16) (top and perspective views). Alternating ACA molecules along the head-to-tail H-bonded chains are tilted with respect to the substrate by 45°. The molecular dimensions are a_S ∼ 2a in the [110̄] direction and a_CE $\sim 2\sqrt{3}$a in the [112̄] direction, where a = 0.289 nm is the near-neighbor distance on Ag(111).

Fig. 2.

STM image of ordered islands and surrounding disordered phase. r_CE and r_S indicate the distance from center to edges, with aspect ratio r_CE/r_S close to 1. The scanning conditions are V_s = −0.65 V and I = 34 pA.

Fig. 3.

Measurements of boundary fluctuations. The white arrows indicate the scan directions. (A) STM image of boundaries of ordered and disordered phases. The boundary is perpendicular to the [110̄] direction and the scan direction is along [110̄] direction (CE boundary). (Inset) Boundary perpendicular to [112̄] direction with the scan direction along the [112̄] direction (S boundary). The scanning conditions are V_s = −0.95 V and I = 47 pA. (B) Pseudoimages of boundary fluctuations, with line scan size 50 nm (horizontal axis), line scan time 51.2 ms, and total measurement time 102.4 s (vertical axis) for 2,000 lines. (Left) Boundary perpendicular to the [110̄] direction with the scan direction along the [110̄] direction (CE boundary). (Right) Boundary perpendicular to the [112̄] direction with the scan direction along the [112̄] direction (S boundary).

Fig. 4.

Typical correlation function G(t) (filled black circles, right axis, CE boundary) and autocorrelation function C(t) (filled black triangles, left axis, S boundary) data. The red curve is the fit for G(t) using Eq. 3 and extracts the best-fit values: G₀ = 3.86 ± 0.28 nm², 1/n = 0.52 ± 0.07. The blue curve is the fit for C(t) using Eq. 3 and extracts the best-fit values: C(0) = 1.81 ± 0.36 nm² and τ_c = 1.77 ± 0.49 s.

Fig. 5.

Schematic illustration of the molecular system with variable substrate interactions as well as intermolecular interactions. U and D correspond to up/down orientations of the molecular tilt as shown in Fig. 1, and G corresponds to an orientation with most favorable interaction with the substrate, possibly perfectly horizontal. The parameters ε_ii correspond to lateral interactions between molecules of orientation i = U, D, or G, and the parameters ε_i correspond to interaction of molecules of orientation i = U, D, or G with the substrate.

Fig. 6.

Images of Monte Carlo simulations. (Upper) A condensed strip with side (S) boundaries in equilibrium with the dense gas phase. (Lower) A condensed strip with end (CE) boundaries. The areas enclosed in the boxes are shown in expanded view to the right. Both simulations use a (139 × 150 nm) (240a_S × 150 a_CE) system with periodic boundary conditions. Simulations were initiated with two perfectly straight boundaries, and the images shown are after 300,000 sweeps. The simulation temperature 371 K was chosen somewhat higher than experiment to permit more rapid equilibration between the phases, and the simulations determined the coexistence chemical potential as −0.675 eV.

Fig. 7.

Simulation of ^∼G(y) and fits for β̃ by using Eq. 11 for the boundaries shown. For the CE boundary, L = 150 nm, and the best-fit value β̃ = 103 meV/nm. For the S boundary, L = 139 nm, and fit value β̃ = 102 meV/nm.