N,N'-dimethylperylene-3,4,9,10-bis(dicarboximide) on alkali halide(001) surfaces

TL;DR

This study investigates the growth and structure of DiMe-PTCDI molecules on KBr(001) and NaCl(001) surfaces using atomic force microscopy, revealing wire formation, substrate interactions, and superstructures with implications for molecular assembly.

Contribution

It provides detailed experimental and computational insights into the growth mechanisms and superstructures of DiMe-PTCDI on alkali halide surfaces, highlighting differences between KBr and NaCl substrates.

Findings

Molecular wires form with ~10 nm width and up to 600 nm length.

Wires grow along specific crystallographic directions.

Different stability and superstructure arrangements on KBr and NaCl.

Abstract

The growth of N,N'-dimethylperylene-3,4,9,10-bis(dicarboximide) (DiMe-PTCDI) on KBr(001) and NaCl(001) surfaces has been studied. Experimental results have been achieved using frequency modulation atomic force microscopy at room temperature under ultra-high vacuum conditions. On both substrates, DiMe-PTCDI forms molecular wires with a width of 10 nm, typically, and a length of up to 600 nm at low coverages. All wires grow along the [110] direction (or [1$\bar{1}$0] direction, respectively) of the alkali halide (001) substrates. There is no wetting layer of molecules: Atomic resolution of the substrates can be achieved between the wires. The wires are mobile on KBr surface but substantially more stable on NaCl. A p(2 x 2) superstructure in brickwall arrangement on the ionic crystal surfaces is proposed based on electrostatic considerations. Calculations and Monte-Carlo simulations using empirical potentials reveal possible growth mechanisms for molecules within the first layer for both substrates, also showing a significantly higher binding energy for NaCl(001). For KBr, the p(2 x 2) superstructure is confirmed by the simulations, for NaCl, a less dense, incommensurate superstructure is predicted.