Research Topic

Scanning Probe Microscopy

We use scanning probes to follow local structure and properties while pressure, gas composition, temperature, light, and electrical bias are controlled.

Conventional surface science begins with a clean, well-defined sample in ultrahigh vacuum. Functional materials, however, operate in gases, liquids, and electrical contacts, where the stable surface can be different from the one prepared in vacuum. The difficulty is not simply reaching a higher pressure. The same location must remain measurable while the environment changes.

Scanning probe microscopy is well suited to this problem because one probe can measure several local responses. STM provides atomic structure and electronic states on conductive surfaces. AFM extends the experiment to semiconducting and insulating materials and can map topography, friction, adhesion, current, and surface potential. Spectroscopy is then used to identify the chemistry behind the contrast.

The operating surface may not be the vacuum surface.

Gas pressure changes adsorbate coverage, surface free energy, and sometimes the atomic structure itself. At room temperature, the chemical-potential difference between ultrahigh vacuum and atmospheric pressure is large enough to stabilize surface states that are absent in a conventional vacuum experiment. This is the pressure gap: a structure measured before or after reaction is not necessarily the structure present during reaction.

Schematic showing pressure-dependent adsorption and restructuring of a surface — Increasing pressure can change both adsorbate coverage and surface structure. Adapted from Choi et al., ACS Nano (2020).

One instrument spans the complete pressure history.

Our ambient-pressure AFM design combines an ultrahigh-vacuum preparation chamber with a separate measurement chamber and gas manifold. A sample can be prepared and characterized in UHV, transferred without air exposure, and then measured from approximately 10⁻¹⁰ mbar to 1 bar in a selected gas. The same platform supports topography, lateral force, local conductance, and force spectroscopy.

Schematic of the ultrahigh-vacuum preparation chamber and ambient-pressure AFM chamber — UHV preparation chamber and variable-pressure AFM chamber. Adapted from Choi et al., Rev. Sci. Instrum. (2018).

Atomic contrast must survive a change in pressure.

A variable-pressure instrument is useful only if pressure cycling does not erase spatial registration or atomic resolution. On HOPG, atomic stick–slip contrast was retained in oxygen from UHV to 120 mbar and after evacuation. The result demonstrates that frictional contrast can be followed over many orders of magnitude in pressure without replacing the sample or probe.

Atomic stick-slip lateral-force image of HOPG measured in 120 mbar oxygen — Atomic stick–slip image of HOPG measured at 120 mbar O₂. Adapted from Choi et al., Rev. Sci. Instrum. (2018).

Force spectroscopy detects changes that topography can miss.

On freshly cleaved HOPG, oxygen produced little change in adhesion, whereas methane exposure increased the pull-off force strongly above about 10⁻³ mbar. Part of that increase remained after evacuation, consistent with hydrocarbon adsorption on the surface or tip. The experiment shows why local mechanics should be measured together with morphology: a surface can become chemically different before a new structure is obvious in topography.

Plot of adhesion force on HOPG during methane dosing and evacuation — Pressure-dependent adhesion during CH₄ dosing and evacuation. Adapted from Choi et al., Rev. Sci. Instrum. (2018).

Current work

Measure the transformation, not only its endpoints.

S² Lab applies variable-environment AFM and STM to hybrid perovskites and metal oxides under water, reactive gases, light, electrical bias, and temperature. Our aim is to track the same area from a defined starting surface into its working state, combine structural and mechanical or electrical contrast, and use complementary spectroscopy to assign the chemical process responsible for the change.

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