Operation Modes of STM

When a bias voltage is applied between tip and substrate to induce electron tunneling, STM images can be obtained in several operation modes  

Constant Height Mode Alternatively, on (atomically) flat surfaces, the tip-sample distance can be kept constant by disabling the feedback (Fig. 10.5b). The variations of the tunneling current are then recorded. Constant height mode is often used to atomically resolve surfaces and does not give any direct topographic information. However, it is particularly useful, when high scan speeds are desired (e.g. for video STM). 

STM at the liquid-solid interface bears many advantages compared to operation under ultrahigh vacuum conditions First of all, it combines ease in operation and low infrastructure costs, but also it is better suited for investigations of large and 

When an STM is operated in air, in general a water film is present on all surfaces. STM at the liquid-solid interface on the other hand offers control and tuning of the solvent properties as a function of the particular substrate and/or molecules. When measuring at the liquid-solid interface, STM is performed in a sealed fluid cell, otherwise the operation time is limited by the evaporation of the solvent. 

Due to its many advantages, STM at the liquid-solid interface [27] has provided detailed insight into the molecule-substrate (epitaxy [60]) and molecule-molecule interactions (e.g. hydrogen bonding, metal complexation) responsible for the ordering of molecules on the atomically flat surface. It is possible to induce chemical reactions at the liquid-solid interface, via external stimuli (e.g. light), or by manipulation with the STM tip, where the location and orientation of functional groups could be controlled [27]. 

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The liquid-solid interface (Fig. 10.24c) can directly be investigated operating the tip submerged in liquid [59]. This important method was already introduced in the above section operation modes of STM . Additionally, an electric potential can be applied to the surface bio-molecules successfully 2-D crystallized and imaged with AFM on mica in a pH-controlled surrounding buffer [31]. 

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