Self-Assembly of Small Polycyclic Aromatic Hydrocarbons on Graphite:  A Combined Scanning Tunneling Microscopy and Theoretical Approach

Abstract

Self-assembled monolayers of chrysene and indene on graphite have been observed and characterized individually with scanning tunneling microscopy (STM) at 80 K under low-temperature, ultrahigh vacuum conditions. These molecules are small, polycyclic aromatic hydrocarbons (PAHs) containing no alkyl chains or functional groups that are known to promote two-dimensional self-assembly. Energy minimization and molecular dynamics simulations performed for small groups of the molecules physisorbed on graphite provide insight into the monolayer structure and forces that drive the self-assembly. The adsorption energy for a single chrysene molecule on a model graphite substrate is calculated to be 32 kcal/mol, while that for indene is 17 kcal/mol. Two distinct monolayer structures have been observed for chrysene, corresponding to highand low-density assemblies. High-resolution STM images taken of chrysene with different bias polarities reveal distinct nodal structure that is characteristic of the molecular electronic state(s) mediating the tunneling process. Density functional theory calculations are utilized in the assignment of the observed electronic states and possible tunneling mechanism. These results are discussed within the context of PAH and soot particle formation, because both chrysene and indene are known reaction products from the combustion of small hydrocarbons. They are also of fundamental interest in the fields of nanotechnology and molecular electronics.

Polycyclic aromatic hydrocarbons from asphalt binder: extraction and characterization

“…[14][15][16][17][18] The size and shape of the molecular structure found on the Au-HPYT surface, seen in Figure 2, showed similarities to some images of coronene adsorbed on modified HOPG and also to that adsorbed on gold under electrochemical potential. 11,14 The image of coronene published by Yoshimoto et al 14 resembles that obtained in this experimental work.…”

Scanning Tunneling Microscopy Images of Alkane Derivatives on Graphite: Role of Electronic Effects

“…[1][2][3] Because of the accessibility of this system using STM probes, the technique is widely utilized to study model systems that elucidate fundamental driving forces in selfassembly. [3][4][5][6][7][8][9] Very often the structural assignment is based on simply overlaying plausible molecular models onto the STM images. However, the clean graphite surface itself is one of the prototypical examples of the role that electronic effects can play in an STM image, where only every second atom appears under typical scan conditions.…”

Abrikosov-like lattices in organic crystals on graphite surface

“…One of these studies, very similar to ours, is the work of Florio et al [15] which used several experimental and computational tools (including MD simulations) to study the self-assembly of chrysene and indene molecules on a graphite surface. Despite the most complex molecular self-assemblies occurring with molecules presenting both directional and non-directional non-bonding interactions [1,5], here we report results on the dynamics of networks of different PAHs subjected to only van der Waals forces obtained by MD simulations.…”