Molecular hydrogen (H₂) or deuterium (D₂) condensed in a low-temperature STM results in a new type of imaging resolution - scanning tunneling hydrogen microscopy (STHM). The microscope operated in the STHM regime images the inner structure of large organic flat lying molecules as well as intermolecular interactions in organic monolayer films.

Scanning tunneling microscopy (STM) is among the few techniques that initiated the nanotechnological revolution. The history of the STM started with the determination of the atomic structure of the famous 7x7 reconstruction of the Si(111) surface. Nowadays the STM is increasingly often applied to the studies of chemically complex systems. One of the examples of such systems are organic molecular films [1]. Fundamental understanding of the structure of such films is a key problem in a variety of research fields ranging from organic electronics to catalysis and biophysics. In general, however, this task remains a challenging problem as, due to their complex chemical structure, the molecules experience numerous types of interactions, a delicate interplay of which finally determines the structure of a molecular film. In such circumstances, an effective microscopy tool resolving the atomic-scale architecture and visualizing the interactions between the individual atomic and molecular species, is indispensable.

Unfortunately the conventional STM does not generally reach the desired imaging conditions since it only images electronic states located in a narrow energy interval close to the Fermi level [2]. Because these states are quite loosely bound to the nuclei, STM micrographs often only vaguely resemble the underlying structure defined by the positions of the atomic cores. To overcome this limitation the STM must become sensitive to another property of the sample, one that closer follows the structure. One of the ways to do that is to add an extra element, a sensor perceptive to the chosen property. To allow readout of the sensor, a transducer, converting the sensor state into the junction conductance, must be employed.

It turns out that the "sensor-transducer" scheme described above is realized when H₂ or D₂ condenses in the junction of an ordinary STM operating in ultra-high vacuum (UHV) at temperatures below 10K [3].

The gas deposited at such conditions physisorbs on cold parts of the STM, including the sample and the tip. When the gas coverage reaches a certain level, the junction's dI/dV(V) spectra develop characteristic conductance spikes at voltages of ±V_inel (fig. 2a). Such a spectrum means that the junction has a bias dependent structure and can be reversibly switched between two structurally different states. Scanning with the junction in the low-bias state at |V|<<V_inel, we observe a new type of resolution (fig. 1 and the middle panel of fig. 2b) - the scanning tunneling hydrogen microscopy or simply STHM contrast [4]. Strikingly, the STHM images closely resemble the chemical structure of imaged molecules (bottom panel, fig. 2b). The STHM contrast does follow a property different from the density of states (DOS) typically imaged by the conventional STM. Clearly, the observed change is due to the gas molecules present beneath the apex of the STM tip. Interestingly, these molecules are excited away from the tip apex by inelastically tunneling electrons at |V|>V_inel. As a result the junction switches back reversibly into the conventional DOS imaging mode (upper panel, fig. 2b). In this way the same junction can be used in two different imaging regimes by simply changing the tunneling bias: the STHM at |V|<<V_inel and conventional STM regime at |V|>V_inel, as shown in figure 2.

The reversible switching of the structure can be used to compare the conductance behavior of the junction with and without the adsorbed gas [4]. The main feature of the STHM imaging mechanism derived from such comparison is the following: a gas molecule caught in the junction plays a dual role of the sensor and transducer (see the upper panel of fig. 3). The position of the gas molecule relative to the tip can be seen as readout of the nanoscopic force sensor detecting interactions of the gas molecule with the sample. This readout is transduced into the junction conductance by another mechanism based on short-range repulsion acting between the tip and the gas molecule. The repulsive force stems from the Pauli exclusion principle prohibiting any overlap between a closed electronic shell of the molecule and electronic states of the tip. When the gas molecule approaches the tip, its molecular orbitals push away the tip states, thus decreasing the tip DOS at the Fermi level and reducing the tunneling conductance.