Direct observation of molecular structure and dynamics at the interface between a solid wall and an organic solution by scanning tunneling microscopy

Abstract

A molecular monolayer adsorbed at the interface between the basal plane of graphite and an organic solution of didodecylbenzene has been observed in situ by scanning tunneling microscopy at a resolution of 0.2 nm. The monolayer forms a 2D polycrystal with crystallite sizes in the range of a few nanometers.The predominant domain boundary is described by a glide plane. Fast image recording allowed, for the first time, direct observation of the molecular dynamics on the time scale of 100 ms. In particular, the motion of domain boundaries could be associated with diftusing "free volume. " PACS numbers: 68.45.v, 61.16.Di, 68.55.Jk The molecular structure at the interface between a solid wall and a molecular liquid is driven by forces which are important, e.g. , for a better understanding of lubrication and adhesion, the orientation of liquid crystals through surfaces, or molecular recognition phenomena at interfaces. ' The order in a molecular monolayer is also a matter of theoretical interest, since true crystalline order cannot exist at any finite temperature in two dimensions.Experimentally, new 2D phenomena have been found for rare-gas adsorbates by synchrotron x-ray scattering ' or for small molecules like nitrogen, ethane, or hexane by neutron scattering, all adsorbed from the gas phase onto graphite.Also low-energy electron diffraction has been employed, e.g. , to the hexane layer. However, in the latter case radiation damage or organic molecules is a serious limitation. Much less is known for organic adsorbate layers at the solid-Auid interface. Part of the reason is the experimental difhculty in determining directly the structure in situ, since only a few surface-sensitive methods give atomic-scale information about the internal interface between two condensed phases.Scanning tunneling microscopy (STM) is particularly suitable for the structure determination of monolayers, provided the monolayer is su%ciently immobilized and also contributes enough to the contrast in STM. An important advantage of STM over other high-resolution electron-based surface-science tools is the fact that (1) it is a true local probe able to investigate, e.g. , only partly ordered or nanocrystalline structures, and (2) it can be operated both under ultrahigh-vacuum conditions and in various Auid ambients, thereby allowing also the study of the internal interface between a conducting solid and a Auid. One of the erst molecular layers investigated by STM was a coadsorbate of benzene and carbon monoxide on Rh(111), prepared under ultrahigh-vacuum conditions. Previous demonstrations of molecular imaging at the solid-Auid interface were on the liquid crystal noctylcyanobiphenyl ' as well as on a solution of the alkane dotriacontane on highly oriented pyrolytic graphite (HOPG), ' which both exhibit a highly ordered interfacial phase. However, so far neither packing defects nor the dynamics in the monolayers could be observed and analyzed on the molecular scale. In the present study we demonstrate that at the solid-Auid interface STM gi...

Molecular chirality at surfaces

“…In 1991 Rabe and Buchholz [63] reported the first supramolecular chiral motifs ever observed with an STM. Adsorbed from solution onto hopg para-bisdodecylbenzene self-assembles into supramolecular chiral lamellae (Fig.…”

“…This applies to 1,4-bisdodecylbenzene (Fig. 9b) as well [63]. As explanation for the missing achiral forms, a denser packing of the 2D crystal or a better alignment of the alkylchains with the underlying substrate must be considered.…”

Video-STM, LEED and X-ray diffraction investigations of PTCDA on graphite

“…These experiments also proved that in LEED experiments beam damage, though present, does not in principle prohibit investigations of organic monolayers, if a suitable registration technique is used. For molecular imaging by STM the adsorbates must be sufficiently immobile on short time scales, either owing to packing constraints (close packing), or to strong enough binding interactions between the molecule and the substrate, while longer time scale molecular dynamics can be detected by STM [14].…”

Soft oxidation of graphite studied by XPS and STM

“…Then, the question of imaging by STM of created organic groups arises. Because the images exhibit at first sight some similarity with those obtained for organic molecules deposited on graphite [9][10][11][12][13], one can suggest that the oxidation creates some kind of organic layer constitued of alkyl chains terminated by organic groups. However this hypothesis is inconsistent with the separation of 0.2 nm found between rows while the distance between alkyl chains is around 0.45 nm [10].…”