SIMS2015 Session SI1-TuM: Surface & Interface Analysis

Self-Assembled Monolayers (SAMs) comprise a broadly applied nanotechnological system, in which molecules are chemically bonded to a substrate in an ordered and oriented fashion. The influence of the strength of the molecule-substrate bond on the structure and stability of SAMs are still poorly understood even for the most simple system of methanethiol on Au(111). While it is a common concept in chemistry that strengthening of one bond results in weakening of the adjacent ones, no results have been published if and how this effect protrudes further into the molecular backbone. By binding molecules to a surface in a form of SAMs, the strength of a primary bond can be selectively altered. Here, we report that by using static SIMS, we are able to observe for the first time positional oscillations in stability of consecutive bonds along the adsorbed molecule, with the amplitudes diminishing with increasing distance from the molecule-metal interface. To explain these observations, we have performed molecular dynamics simulations and density functional theory calculations. Our calculations show that the observed oscillation effects in chemical bond stability have a very general nature in chemistry related to breaking the translational symmetry in molecules. In the talk we will discuss two experiments. In the first experiment [1] as a model SAMs for SIMS analysis we have selected two homologous series of biphenyl substituted alkanethiols, for which either sulphur atoms (BPnS) or selenium atoms (BPnSe) act as a binding groups to the Au(111) substrate. In the second experiment [2] we use this SIMS approach for the analysis of purely aromatic SAMs/Au(111) based on naphthalene.

[1] Ossowski et al. „Oscillations in Stability of Consecutive Chemical Bonds Revealed by Ion-Induced Desorption” Angew. Chem. Int. Ed.2015, 54, 1336-1340.

[2] Ossowski et al. „Thiolate versus Selenolate: Structure, Stability and Charge Transfer Properties” ACS Nano, 2015, 9, 4508-4526.

Vertical heterostructures composed of ultra-thin films are an intense area of research due to their unique properties that result from structural planar confinement. Interfaces play a fundamental role in device performance, therefore requiring characterization tools that are both in-depth chemically selective and surface-morphology sensitive. Here, we propose a characterization methodology combining (micro-)Raman spectroscopy, atomic force microscopy (AFM) and time of flight secondary ion mass spectrometry (TOF-SIMS) to provide structural information, morphology, and planar chemical composition at virtually the atomic level, aimed specifically at studying ultra-thin vertical heterostructures. We give a general overview of the strength of this technique when applied to various devices that are linked to solar cells¹, hydrogen catalysis², copper interconnects³ and two dimensional vertical heterostructures⁴.

References

1. Jeramy D. Zimmerman, Brian E. Lassiter, Xin Xiao, Kai Sun, Andrei Dolocan, Raluca Gearba, David A. Vanden Bout, Keith J. Stevenson, Piyumie Wickramasinghe, Mark E. Thompson, and Stephen R. Forrest, “Control of Interface Order by Inverse Quasi-Epitaxial Growth of Squaraine/Fullerene Thin Film Photovoltaics”, ACS Nano 7, 9268 (2013).

2. Sean P. Berglund, Huichao He, William D. Chemelewski, Hugo Celio, Andrei Dolocan, and C. Buddie Mullins, “p‑Si/W₂C and p‑Si/W₂C/Pt Photocathodes for the Hydrogen Evolution Reaction”, Journal of the American Chemical Society 136, 1535 (2014).

3. Tyler D.-M. Elko-Hansen, Andrei Dolocan, and John G. Ekerdt, “Atomic Interdiffusion and Diffusive Stabilization of Cobalt by Copper During Atomic Layer Deposition from Bis(N-tert-butyl-N′-ethylpropionamidinato) Cobalt(II)”, The Journal of Physical Chemistry Letters 5 (7), 1091 (2014).

Biomedical devices and sensors often predominantly consist of a pure metal component or an alloyed metal. The surfaces of many biomedically relevant metals and alloys (e.g., Ti based) are covered with oxide layers. The latter are terminated with hydroxyl groups and consequently exhibit similar affinities to similar functional groups. Selective modification of different oxide substrates consequently is a challenging task, in contrast to noble metal – metal oxide composites where the difference in affinity to certain functional groups (e.g. thiols and silanes) can be exploited. The successful, selective self-assembled monolayer (SAM) formation on different metal oxides, however, could allow for an easy, substrate induced lateral structuring of the chemistry of designated positions in a device consisting of non-noble metals.

We designed a strategy to produce selectively modified mixed metal oxide surfaces and evaluated each step with XPS and ToF-SIMS. ZrO₂ and TiO₂ were chosen as a model system for the development of a general route to selectively assemble monolayers of functional molecules on metal oxide surfaces. Checkerboard patterns of the two oxides could be successfully modified with two different molecules in a four step procedure, in which initially both metal oxides are coated with a SAM of the first molecule, which is then selectively decomposed on the TiO₂-areas and replaced with a second SAM after reactivation. Micro- to nanoscale structures were investigated.