STM beyond vacuum tunnelling, a route to ultra high resolution

Direct imaging is a fast and reliable method for the characterization of surfaces. When it comes to small surface structures in the size of the features e.g. in todays computer processors, classical optical imaging methods fail in resolving these structures. With the invention of the scanning tunnelling microscope (STM) for the first time it became possible to image the structure of surfaces with atomic precision. However, the STM fails in resolving complex chemical structures like e.g. organic molecules. The lack of chemical sensitivity in STM images can be overcome by the condensation of molecular hydrogen or deuterium in the STM junction. Images recorded in the so-called scanning tunnelling hydrogen microscopy (STHM) closely resemble the chemical structure of different organic molecules. However, the mechanism behind the contrast formation has not been addressed so far. Here we show that the origin of the STHM contrast is a single hydrogen (H$_{2}$) or deuterium (D$_{2}$) molecule located directly below the tip apex that acts as a combined sensor and signal transducer. Together with the tip the gas molecule forms a nano-scale force sensor, comparable to sensors in atomic force microscopy (AFM), which probes the total electron density (TED) of the surface trough the Pauli repulsion and converts this signal into variations of the junctions’ conductance again via Pauli repulsion. Other than the sensors in conventional scanning force techniques, due to its size, the sensor of the STHM junction is intrinsically insensitive to long-range forces, usually limiting the image resolution. The insensitivity to long-range forces results in a high image resolution, so that even small changes in the TED leave a mark in obtained STHM images. The resolution hereby reaches an unprecedented level as can be seen by the direct imaging of local intermolecular interactions like e.g. hydrogen bonds appear with remarkable clarity in STHM images of organic layers. Thus, besides the identification of chemical species of different adsorbates, the STHM mode allows the study of interactions between adsorbates which e.g. lead to their self organization on the surface. Therefore, the STHM mode may give important insight in the driving mechanisms behind the formation and composition of matter on the atomic level. However, the STHM mode, in which a single H$_{2}$ (D$_{2}$) molecule probes the TED of the surface, is only one example of a broader class of sensors. It is conceivable, that by an appropriate choice of the molecule in the junction, other surface properties can be imaged which are usually inaccessible by other imaging techniques