Question

All surface probe microscopy (SPM) techniques involve a probe which interacts with a sample surface. Atomic Force Microscopy (AFM) and Scanning Transmission Microscopy (STM) are two SPM techniques that operate on the basis of different fundamental principles, though they have many similar aspects. Describes the underlying principles of AFM and STM for measuring surfaces

Answer 1

Scanning Tunneling Microscopy (STM)

The STM system is a surface imaging tool with sub-nano meter resolution. It’s working principle is based on quantum tunneling of electrons between the surface and the STM tip. When the conducting tip of the STM is brought very close to a surface to be examined, a bias voltage is applied between the tip and the surface which causes electrons to tunnel through the intervening vacuum from the surface to the tip resulting in a tunneling current. As the tip is moved across the surface, changes in the surface height or density of states cause measurable changes in the tunneling current. The tip is tracked across the surface to cover an area and obtain a nanoscale surface image where the contrast is generated by changes in the tunneling current.

Shown above is an STM image of silicon 7 x 7 structure. The STM system may also be used to manipulate a surface directly using the very fine STM tip. This technique was discovered by IBM scientists and was an amazing breakthrough for the field of nanotechnology.

STM systems may also be modified and used for a number of different techniques such as photon scanning microscopy (PSTM) where photons are tunneled from the surface, scanning tunneling potentiometry (STP), and spin polarized scanning tunneling microscopy (SPSTM). Additionally, due to the ability of this system to move individual atoms, it is possible to isolate a single molecule such as an organic semiconductor material and measure its HOMO-LUMO gap directly.




Atomic Force Microscopy (AFM)

This technique is related to STM and uses some of the same equipment. The AFM technique is also an imaging tool, but has much higher resolution then an STM system. An AFM system consists of a cantilever with a sharp tip at its end that is used to scan the specimen surface. The cantilever is typically very small with a tip of radius of curvature on the order of nanometers for very high resolution.

When the tip is brought into proximity of a sample surface, forces between the tip and the sample lead to a deflection of the cantilever according to Hooke's law. The forces measured are typically van der Waals forces, but may include many types such as mechanical contact force, chemical forces, or other electrostatic forces. The deflection of the cantilever may be measured very accurately using a laser and photodiode system as shown or some other optical method.

The AFM can be used to study a wide variety of samples (i.e. plastic, metals, glasses, semiconductors, and biological samples such as the walls of cells and bacteria). Unlike STM or scanning electron microscopy it does not require a conductive sample. However there are limitations in achieving atomic resolution. The physical probe used in AFM imaging is not ideally sharp. As a consequence, an AFM image does not reflect the true sample topography, but rather represents the interaction of the probe with the sample surface. This is called tip convolution