Scanning Probe Microscopy SPM

 Scanning Probe Microscopy (SPM): 

The scanning probe microscopy is a general term for a wide variety of microscopic 

techniques, which measure the morphology and properties of surfaces on the atomic scale.  

This includes the following: 

Scanning Tunneling Microscopy (STM) – which studies the surface topography and electronic 

structure, Atomic Force Microscopy (AFM) – which studies the surface topography, surface 

hardness and elastic modulus, Lateral Force Microscopy (LFM) – which studies the relative 

frictional properties,  Scanning Thermal Microscopy (SThM) –  which studies the thermal 

conductivity, Magnetic Force & Electric Force Microscopies (MFM & EFM) – which study 

the magnetic and electric properties. 

The techniques of STM and AFM are discussed below, since these are widely used: 

   

Principle:  The general principle for all the scanning probe microscopes is that a sharper 

probe (or a very fine tip) is used to scan the surface of the sample with much lower force and 

obtain the topography and morphology information. 

Scanning tunneling microscope: When a sharp tip made of a conducting material is brought 

close to a  conducting sample, overlapping of the electron clouds between the two surfaces 

will occur. If a potential is given between them a current of electrons is formed, which is 

often referred as “tunneling”  current,  and the effect is known as “tunneling” effect.  This 

effect is largely depended on the distance between the tip and the sample material. Hence, if 

the scanning tip is controlled by a high precision motion device made of piezo-electric 

material, the distance between the tip and the sample can be measured during a scanning 

through a feedback loop control of the piezo-electric element. By this way the sample can be 

scanned with sub-angstrom precision.   

Atomic force microscope:  This technique operates by measuring the forces between the 

sample and the tip, and the sample need not be a conducting material. Here, the tip is brought 

close enough to the sample surface to detect the repulsive force between the atoms of the tip 

material and the sample. The probe tip is mounted at the end of a cantilever of a low spring 

constant and the tip-to-sample spacing is held fixed by maintaining a constant and very low 

force on the cantilever. Hence, if the tip is brought close to the sample surface, the repulsive 

force will induce a bending of the cantilever. This bending can be detected by a laser beam, 

which is reflected off the back of the cantilever. Thus by monitoring the deflection of the 

cantilever, the surface topography of the sample can be tracked. Since the force maintained 

on the cantilever is in the range of inter-atomic forces (about 10-9 

Newton), this technique 

derived the name “atomic force” microscopy. 

AFM operates at two modes: 

Repulsive or contact mode – which detects the repulsive forces between the tip and sample; 

Attractive or non-contact mode – which detects the van der waals forces that act between the 

tip and sample. 

Instrumentation:  

Scanning tunneling  microscope:  It mainly consists of a scanner, probe motion sensor 

composed of piezo-electric material, micro probe, etc. 

Atomic force microscope:  It mainly consists of a scanner, cantilever, laser source, photo-

diode detector, micro-probe, etc.

Applications: Both STM and AFM find applications widely in material sciences especially 

for surface studies on a nano scale range. While STM finds its applications in the 

characterization of surface structure (including the electronic structure), AFM finds its 

applications in measuring the  hardness of materials. Sometimes, AFM can be used in the 

study of “depth profile” of the deposited oxide layer on to a material.  

Disadvantages: A limitation to STM is that it can study only the conducting samples, since 

the technique is based on the tunneling current between two conducting areas. Hence, it 

doesn’t lend itself to the study of non-conducting materials. In fact, the AFM had been 

developed to encounter this  problem. These methods require  special sample preparation 

techniques, which are tedious, like, thin sectioning, electo-polishing, various mechanical 

cutting and polishing techniques, etc.