Scanning Tunneling Microscopy (STM)

The primary tool we use in our experiments is the scanning tunneling microscope (STM). The heart of the STM is a fine metal wire, called the STM tip, placed in close proximity to a conducting surface as diagrammed in the figure below.

If the STM tip is close enough to the surface then an applied voltage between the tip and sample will cause electrons to tunnel through the junction. The tunneling current is exponentially dependent on the junction width and increases by a decade per Angstrom as the tip is brought closer to the surface. In typical systems a tip-sample separation of 0.5 nm will produce currents of ~1 nA for biases of 1V.

Electrons from occupied states of the tip tunneling into unoccupied states of the sample.

Microscopy

One can exploit the sensitive dependence of the tunneling current to image surfaces with atomic resolution. By scanning the tip along the surface and monitoring the current, one can resolve the surface topography directly underneath the tip. The tip motion is controlled using piezoelectric materials, which can position the tip with sub Angstrom resolution. In typical scanning modes the current is kept constant and the perpendicular tip deflection needed to keep the current fixed is monitored as the tip is moved laterally along the surface. An image of the Ag(001) surface taken in this manner is shown alongside. Each bright spot corresponds to a single Ag atom.

Spectroscopy

While the dependence of current on position reveals the geometric structure of the surface, the dependence of current on voltage give information about its electronic structure. This relationship is demonstrated by Equation 1. This expression shows that the tunneling current is approximately proportional to the integral of all the electronic states between the Fermi Energy and the tunneling bias.

The first derivative of the tunneling current (Eqn. 2) with respect to voltage (dI/dV) is thus proportional to the local density of electronic states below the tip, at the given tunneling voltage. Thus by measuring dI/dV as a function of voltage, one can probe the electronic states at that particular point on the surface.

In general, peaks in the first derivative correspond to electronic resonances, such as atomic states, of the system being probed. One can go a step further and combine STM microscopy and spectroscopy, to image the spatial distribution of the observed electronic states.

Manipulation

Another powerful STM capability is the ability to move atoms and molecules. This is achieved by placing the tip close enough to the surface adsorbate so that the tip-adsorbate attraction is comparable to the surface corrugation barrier. In this regime, the molecule will follow the tip wherever it is moved along the surface. One can then retract the tip, without causing the molecule to desorb from the surface.