An NC-AFM and KPFM study of the adsorption of a triphenylene derivative on KBr(001)

Abstract

SummaryThe adsorption on KBr(001) of a specially designed molecule, consisting of a flat aromatic triphenylene core equipped with six flexible propyl chains ending with polar cyano groups, is investigated by using atomic force microscopy in the noncontact mode (NC-AFM) coupled to Kelvin probe force microscopy (KPFM) in ultrahigh vacuum at room temperature. Two types of monolayers are identified, one in which the molecules lie flat on the surface (MLh) and another in which they stand approximately upright (MLv). The Kelvin voltage on these two structures is negatively shifted relative to that of the clean KBr surface, revealing the presence of surface dipoles with a component pointing along the normal to the surface. These findings are interpreted with the help of numerical simulations. It is shown that the surface–molecule interaction is dominated by the electrostatic interaction of the cyano groups with the K+ ions of the substrate. The molecule is strongly adsorbed in the MLh structure with an adsorption energy of 1.8 eV. In the MLv layer, the molecules form π-stacked rows aligned along the polar directions of the KBr surface. In these rows, the molecules are less strongly bound to the substrate, but the structure is stabilized by the strong intermolecular interaction due to π-stacking.

Molecular Design and Control Over the Morphology of Self‐Assembled Films on Ionic Substrates

“…[ 44 ] If only the interdigitation and π-stacking M-M interactions were present, then 2D line structures would probably be formed. The added cyano groups, which are believed to be involved in hydrogen bonding to enhance M-M interactions, [ 45 ] also provide site-specifi c M-S interactions, [ 17 ] allowing other types of fi lm structure to be stabilized.…”

“…Some molecules are able to form wire structures through π-stacking, [ 27,28 ] other examples include stabilization of the molecular layers via hydrogen bonding, [ 19 ] and a few experiments report on the use of polar groups to enhance adsorption onto ionic substrates. [ 17,29,30 ] However, there is no single set of design criteria for the morphology and stability of such fi lms that can satisfy a wide range of possible applications. For example, molecular electronics applications may require that the molecules selfassemble in large, highly ordered, and defect-free domains.…”

“…[ 15 ] However, applications, such as optoelectronic and molecular devices, require the use of bulk insulators in order to avoid the coupling of molecular orbitals with the surface. [ 16 ] The formation of organic layers on insulators has been achieved in the past by either strengthening the molecule-surface (M-S) interaction by means of appropriate anchoring groups, [ 17,18 ] or by reinforcing the molecule-molecule (M-M) interaction. [ 19,20 ] Several attempts have also been made to infl uence the balance between M-S and M-M interactions.…”

“…In order to characterize the produced monolayers (MLs) with molecular resolution we employ noncontact atomic force microscopy (NCAFM) as this provides a nondestructive method to study the delicate structure of molecular layers, which has been successfully applied in the past to image individual molecules, aggregates, and fi lms on insulating surfaces. [ 17,[31][32][33][34][35][36][37][38][39][40][41][42] However, while a number of supramolecular structures have been observed, [35][36][37]39,40 ] the mechanisms of structure formation are not fully understood. Moreover, studies demonstrating high levels of control over the structure formation on insulators are still rare due to the subtle balance of M-M and M-S interactions.…”

Bicomponent Supramolecular Architectures at the Vacuum–Solid Interface

“…This field is currently burgeoning, and there is a need to develop bicomponent supramolecular assembly on insulating surfaces by simultaneously maximizing intermolecular (e.g., by complementary H-bonding) and molecule-substrate interactions (e.g., using electrostatic forces in the form of dipoles 399,400 and local charges 401 ).…”