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. 

Transmission Electron Microscopy TEM

Transmission Electron Microscopy (TEM): 

Principle:    In this technique, a beam of high-energy electrons (typically 100 - 400keV) is 

collimated by magnetic lenses and allowed to pass through a specimen under high vacuum. 

The transmitted beam and a number of diffracted beams can form a resultant diffraction 

pattern, which is imaged on a fluorescent screen kept below the specimen. The diffraction 

pattern gives the information regarding lattice  spacing and symmetry of the structure under 

consideration. Alternatively, either the transmitted beam or one of the diffracted beams can 

be made to form a magnified image of the sample on the viewing screen as bright-and dark-

field imaging modes respectively, which give  information about the size and shape of the 

micro-structural constituents of the material. High-resolution image, that contains 

information about the atomic structure of the material, can be obtained by recombining the 

transmitted beam and diffracted beams together.    

Instrumentation:    It comprises of a tungsten filament or LaB6 or a field emission gun as 

source of electron beam, objective lens, imaging lens, CCD camera, monitor, etc.   

Applications:  Transmission electron microscopy is used to study the local structures, 

morphology, and dispersion of multi-component polymers, cross sections & crystallization of 

metallic alloys and semiconductors, microstructure of composite  materials, etc. The 

instrument can be extended to include other detectors like Energy Dispersive Spectrometer 

(EDS) or Energy Loss Spectrometer (ELS) to study about the local chemistry of the material 

similar to SEM technique.   

Disadvantages:  The instrumentation is complicated and needs high vacuum. Sample 

preparation is very time consuming. Some materials, especially polymers, are sensitive to 

electron beam irradiation which results in the loss of crystallinity and/or mass. 

Scanning Electron Microscopy SEM

Scanning Electron Microscopy (SEM): 

Principle:  In this technique, an electron beam is focused onto the sample surface kept in a 

vacuum by electro-magnetic lenses (since electron possesses dual nature with properties of 

both particle and wave an electron beam can be focused or condensed like an ordinary light) 

The beam is then rastered or scanned over the surface of the sample. The scattered electron 

from the sample is then fed to the detector and then to a cathode ray tube through an 

amplifier, where the images are formed, which gives the information on the surface of the 

sample. 

Instrumentation:  It comprises of a heated filament as source of electron beam, condenser 

lenses, aperture, evacuated chamber for placing the sample, electron detector, amplifier, CRT 

with image forming electronics, etc. 

Applications: Scanning electron microscopy has been applied to the surface studies of metals, 

ceramics, polymers, composites and biological materials for both topography as well as 

compositional analysis. An extension (or sometimes conjunction to SEM) of this technique is 

Electron Probe Micro Analysis (EPMA), where the emission of X-rays, from the sample 

surface, is studied upon exposure to a beam of high energy electrons. Depending on the type 

of detectors used this method  is classified in to two as:  Energy Dispersive Spectrometry 

(EDS) and Wavelength Dispersive Spectrometry (WDS). This technique is used extensively 

in the analysis of metallic and ceramic inclusions, inclusions in  polymeric materials, 

diffusion profiles in electronic components.   

Disadvantages:  The instrumentation is complicated and needs high vacuum for the optimum 

performance.

Analysis Through Microscopy

ANALYSIS THROUGH MICROSCOPY 

The techniques described here are not for the simple, ordinary optical microscopes which use 

light for the magnification. These employ electron beam and mechanical probes to magnify 

the surfaces under study. 

X-Ray Diffractometry XRD

 X-Ray Diffractometry (XRD):

Principle:  In this technique the primary X-rays are made to fall on the sample substance

under study. Because of its wave nature, like light waves, it gets diffracted to a certain angle.

This angle of diffraction, which differs from that of the incident beam, will give the

information regarding the crystal nature of the substance. The wavelength of the X-rays can

be varied for the application by using a grating plate.

Instrumentation :  It consists of X-ray tube for the  source, monochromator and a rotating

detector.

Applications : The diffraction of X-rays is a good tool to study the nature of the crystalline

substances. In crystals the ions or molecules are arranged in well-defined positions in planes

in three dimensions. The impinging X-rays are reflected by each crystal plane. Since the

spacing between the atoms and hence the planes can’t be same or identical for any two

chemical substances, this technique provides vital information regarding the arrangement of

atoms and the spacing in between them and also to find out the chemical compositions of

crystalline substances. The sample under study can be of either a thin layer of crystal or in a

powder form. Since, the power of a diffracted beam is dependent on the quantity of the

corresponding crystalline substance, it is  also possible to carry out quantitative

determinations.

X-Ray Photo-emission Spectrometry XPS

 X-Ray Photo-emission Spectrometry (XPS): 

Principle: When a primary X-ray beam of precisely known energy impinges on sample 

atoms, inner shell electrons are ejected and the energy of the ejected electrons  is measured. 

The difference in the energy of the impinging  X-ray and the ejected  electrons gives the 

binding energy (Eb) of the electron to the atom. Since,  this binding energy of the emitted 

electron depends on the energy of the electronic orbit and the element it can be used to 

identify the element involved. Further, the chemical form or environment of the atom affects 

the binding energy to a considerable extent to give rise to some chemical shift, which can be 

used to identify the valence state of the atom and its exact chemical form. This technique is 

mostly referred as Electron Spectroscopy for Chemical Analysis (ESCA).  

 

An associated process with this method is that when the electron is ejected from the inner 

orbital a vacancy is left with. Hence, another electron from the outer orbits may fall to fill the 

vacancy and by doing so emits X-ray fluorescence. The energy of this X-ray fluorescence is 

sometimes transferred to a second electron to make it to be ejected. This second electron thus 

emitted is termed as  Auger electron  and the method  Auger Spectroscopy  (after French 

Physicist  Pierre Auger).  Its applications are more or less similar to ESCA and both the 

methods are used in conjunction since in both cases the energies involved are similar.   

 

Instrumentation:  It consists of a radiation source for primary X-rays, monochromator, the 

energy analyzer (to resolve the electrons generated from the samples by energy) and detector 

to measure the intensity of the resolved electrons. The analysis is done in high vacuum. 

Applications : It is mainly used for surface analysis, especially in the qualitative identification 

of the elements in a sample. Based on the chemical shifts, the chemical environment around 

the atoms can also be estimated. This measurement is useful in determining the valence states 

of the atoms present in various moieties in a sample. Quantitative measurements can be made 

by determining the intensity of the ESCA lines of each element. 

 

Disadvantages : High vacuum is necessary for the system to avoid the low energy electrons to 

be collided with other impurities, which may result in low sensitivity. It is not possible to 

detect the impurities at the ppm or ppb levels. The whole instrumentation is highly 

complicated.

X-Ray Fluorescence Spectrometry XFS

X-Ray Fluorescence Spectrometry (XFS) : 

Principle: When a sample is placed in a beam of primary X-rays, part of it will be absorbed 

and the atoms get excited, by the ejection of electrons present in K and L shells. While 

relaxing they re-emit X-rays of characteristic wavelength. This re-emitted X-rays are called 

secondary or fluorescent X-rays  and hence the name for this technique. Since, the 

wavelength of the fluorescence is characteristic of the element being excited, measurement of 

the wavelength and intensity enables to carry out the qualitative and quantitative analyses. 

Instrumentation:  It comprises of a source for primary X-rays, collimators, analyzing crystal 

and detector.  

Applications:    It is one of the non-destructive methods in the elemental analysis of solid or 

liquid samples for major and minor constituents. Most of the elements in the periodic table, 

both metals and nonmetals, respond to this technique. Detection limit is between 10 to 100 

ppm. One of the significant uses of this method is in the medical field in identifying and 

determining the sulfur in protein.  

Disadvantages:  The sensitivity gets affected for elements with lower atomic numbers, 

particularly elements with atomic number  lower than 15 are difficult to analyze. The 

sensitivity is also limited by matrix absorption, secondary fluorescence and scattering of the 

particles. Instruments are often large, complicated and costly. 

Generation of X-Rays

Generation of X-Rays :  When metals, like copper, molybdenum, tungsten, etc., are 

bombarded directly with a stream of high energy electrons or radioactive particles, X-rays  

(wavelengths of order 0.1-100Å) are emitted because of the transitions involving K-Shell and 

L-Shell electrons. This can be simply expressed as follows: 

A cathode in the form of a metal wire when  electrically heated gives off electrons. If a 

positive voltage, in the form of an anode (target comprised of the metals mentioned above), is 

placed near these electrons, the electrons are accelerated toward the anode. Upon striking the 

anode, the electrons transfer their energy to the metallic surface, which then gives off X-ray 

radiation. This is referred as primary X-rays.  

The following is the schematic diagram for the process:

Note:  The wavelength of the emitted X-ray is characteristic of the element being 

bombarded. Hence with some modifications this process can be used as a tool for 

qualitative and quantitative elemental analysis by measuring the wavelength and emission 

intensity of the X-rays respectively. This forms the basis for Electron Probe Microanalysis! 

Differential Scanning Caloriemeter DSC

Differential Scanning Caloriemeter (DSC): 

This technique is more or less similar to DTA except that it measures the amount of heat 

absorbed or released by a sample as it is heated or cooled or kept at constant temperature  

(isothermal). Here the sample and reference material are simultaneously heated or cooled at a 

constant rate. The difference in temperature between them is proportional to the difference in 

heat flow (from the heating source i.e. furnace), between the two materials. It is the well-

suited technique in the detection and further studies of liquid crystals. This technique is 

applied to most of the polymers in evaluating the curing process of the thermoset materials as 

well as in determining the heat of melting and melting point of thermoplastic polymers, glass 

transition temperature (Tg.), endothermic & exothermic behaviour. Through the adjunct 

process of isothermal crystallization it provides information regarding  the molecular weight 

and structural differences between very similar materials. The instrumentation is exactly 

similar to that of DTA except for the difference in obtaining the results.

Differential Thermal Analysis DTA

 Differential Thermal Analysis (DTA):

This technique measures the temperature difference between a sample and a reference

material as a function of temperature as they  are heated or cooled or kept at a constant

temperature (isothermal). Here the sample and reference material are simultaneously heated

or cooled at a constant rate. Reaction or transition temperatures are then measured as a

function of the temperature difference between  the sample and reference.  It provides vital

information of the materials regarding their  endothermic and exothermic behaviour at high

temperatures. It finds most of its applications in analysing and characterising clay materials,

ceramic, ores, etc.

Thermomechanical Analysis TMA

Thermomechanical Analysis (TMA): 

Thermomechanical analysis is  the measurement of a material’s behaviour, ie. expansion or 

contraction, when temperature and a load is applied. A scan of dimensional changes related 

to temperature (at constant load) or load  (at constant temperature) provides valuable 

information about the sample’s mechanical properties. One of the most important 

applications of TMA is the characterization of composite and laminate materials, through the 

study of glass transition temperature and the expansion coefficient. Another application is in 

the quantitative measurement of extension and  contraction observed in textile fibres, thin 

films and similar materials.

Thermogravimetric Analysis TGA

Thermogravimetric Analysis (TGA):

In this technique the change in sample weight is measured while the sample is heated at a

constant rate (or at constant temperature), under air (oxidative) or nitrogen (inert)

atmosphere. This technique is effective for quantitative analysis of thermal reactions that are

accompanied by mass changes, such as evaporation, decomposition, gas absorption,

desorption and dehydration. The following is the simplified diagram for the instrumentation:

The micro-balance plays a significant role, during measurement the change in sample mass

affects the equilibrium of the balance. This  imbalance is fed back to a force coil, which

generates additional electromagnetic force to recover equilibrium. The amount of additional

electromagnetic force is proportional to the mass change. During the heating process the

temperature may go as high as 15000

 C inside the furnace.

THERMAL ANALYSIS

THERMAL ANALYSIS 

The technique of thermal analysis actually comprises of a series of methods, which detect the 

changes in the physical and mechanical properties of the given substance by the application 

of heat or thermal energy. The physical properties include mass, temperature, enthalpy, 

dimension, dynamic characteristics, etc. It finds its application in finding the purity, integrity, 

crystallinity and thermal stability of the chemical substances under study. Sometimes it is 

used in the determination of  the composition of complex mixtures. This technique has been 

adopted as testing standard in quality control in the production field, process control and 

material inspection. It is applied in wide fields,  including, polymer, glass, ceramics, metals, 

explosives, semiconductors, medicines and foods.

Gas Chromatography GC

 GAS CHROMATOGRAPHY (GC)  

Principle: Here an inert carrier gas (Helium or Nitrogen) acts as the mobile phase. This will 

carry the components of analyte mixture and elutes through the column. The column usually 

contains an immobilized stationary phase. The technique can be categorised depending on the 

type of stationary phase as follow: 

Gas Solid Chromatography (GSC) -  here the stationary phase is a solid which has a large 

surface area at which adsorption of components of the analyte takes place. The separation is 

possible based on the differences in the adsorption power and diffusion of gaseous analyte 

molecules. The application of this method is limited and is mostly used in the separation of 

the low-molecular-weight gaseous species like carbon monoxide, oxygen, nitrogen and lower 

hydrocarbons.

Gas Liquid Chromatography (GLC) - this is the most important and widely used method for

separating and determining the chemical components of volatile organic mixtures. Here the

stationary phase is a liquid that is immobilized on the surface of a solid support by adsorption

or by chemical bonding. The separation of the mixture into individual components is by

distribution ratio (partition) of these anayte components between the gaseous phase and the

immobilized liquid phase. Because of its wide applications most of  the GCs are configured

for the GLC technique.  

Instrumentation :  The instrumentation for GC is different from that of HPLC in that the

injection port, column and detector are to be heated to a pre-specified temperature. Since the

mobile phase here is a gas (carrier gas) the components present in the analyte mixture should

be vaporised, so that it can be effectively carried through the column. The basic

instrumentation for GC includes a carrier gas cylinder with regulator, a flow controller for the

gas, an injection port for introducing the sample, the column, the detector and the recorder.

An outlay is as follow:

In the above illustration the  injection port, column oven and detector are hot zones. The

success of this technique requires the appropriate selection of the column and the temperature

conditions at which the column to be maintained throughout the analysis. Basically the

columns for GC are classified as analytical columns and preparative columns. The analytical

columns are of two types: packed column and open-tubular or capillary column. Both differ

in the way the stationary phases are stacked inside.

In the instrumentation of GC detectors play  unique role. There are a number of detectors,

which vary in design, sensitivity and selectivity. Detectors in GC are designed to generate an

electronic signal when a gas other than the carrier gas elutes from the column. Few examples

and applications of the detectors are:

Thermal Conductivity Detector (TCD) - this operates on the principle that gases eluting from

the column have thermal conductivity different from that of the carrier gas. It is the universal

detector (detects most of the analytes) and is non-destructive and hence used with preparative

GC, but less sensitive than other detectors.

Flame Ionization Detector (FID) -  it is one of the important detectors where the column

effluent is passed into a hydrogen flame and the flammable components are burned. In this

process a fraction of the molecules gets fragmented into charged species as positive and negative.

While positively charged ions are drawn to a collector, negatively charged ions are

attracted to positively charged burner head, this creates an electric circuit and the signal is

amplified. The FID detector is very sensitive, but destroys the sample by burning. It only

detects organic substances that burn and fragment in a hydrogen flame (e.g. hydrocarbons).

Hence its usage is restricted for preparative GC and for inorganic substances which do not

burn.  

Electron Capture Detector (ECD)  -  this is another type of ionization detector which utilises

the beta emissions of a radioactive source, often nickel-63, to cause the ionization of the

carrier gas molecules, thus generating electrons which constitute an electrical current. This

detector is used for environmental and bio-medical applications. It is especially useful for

large halogenated hydrocarbons and hence in the analysis of halogenated pesticide residues

found in environmental and bio-medical samples. It is extremely sensitive. It does not destroy

the sample and thus may be used for the preparative work.

Nitrogen/Phosphorus Detector (NPD) -  the design of the detector is same to that of the FID

detector except that a bead of alkali metal salt is positioned just above the flame. It is also

known as ‘Thermionic Detector’. It is useful for the phosphorus and nitrogen containing

pesticides, the organophosphates and carbamates. The sensitivity for these compounds are

very high since the fragmentation of the other organic compounds are minimized.

Flame Photometric Detector (FPD) -  here a flame photometer is incorporated into the

instrument. The principle is that the sulfur or phosphorus compounds burn in the hydrogen

flame and produce light emitting  species. This detector is  specific for organic compounds

containing sulphur or phosphorus. It is very selective and very sensitive.

Electrolytic Conductivity Detector (ECD Hall) -  this otherwise known as ‘Hall detector’,

converts the eluting gaseous components into ions in liquid solution and then measures the

electrolytic conductivity of the solution in a conductivity cell. The conversion to ions is done

by chemically oxidizing or reducing the components with a “reaction gas” in a small reaction

chamber. This detector is used in the analysis of organic halides and has excellent sensitivity

& selectivity, but is a destructive detector.

The recent developments allow the GC to be  coupled with other analytical techniques like

Infra Red Spectrometry (Gas Chromatography-Infrared Spectrometry, GC-IR)   and    Mass

Spectrometry  (Gas Chromatography- Mass Spectrometry, GC-MS).   These are termed as

‘hyphenated techniques’, and are very efficient for qualitative analysis as very accurate and

precise information like mass or IR spectrum of the individual sample components are readily

obtained as they elute from the GC column. It saves time and reduces the steps involved for a

component to be separated and analysed.

Disadvantages: Samples must be volatile and thermally stable below about 4000

 C. No single

universal detector is available and most commonly used detectors  are non-selective. One

should take much care in the analytical steps  starting from the selection of the column, the

detector and must define the temperatures of all the three ports viz., injection port, column

oven and detector. An improper programming on these will lead to erratic results.

High Performance Size Exclusion Chromatography

High Performance Size Exclusion Chromatography:  This technique is for separating 

dissolved species on the basis of their size and particularly applicable to high-molecular-  

weight species like oligomers and polymers to determine their  relative sizes and molecular 

weight distributions. Here, the stationary phase is polymer resin, which contains small pores. 

If the components to be separated are passed through the column the small sized particles can 

easily enter into these pores and their mobility is retarded. Whereas the large sized particles, 

which can’t enter into these pores can come out of the column fast and elude first. Thus the 

separation of various sized particles is possible  through variations in the elution time. It is 

classified into two categories based on the nature of the columns and their packing as:

Gel Filtration Chromatography - which uses hydrophilic packing  to separate polar species 

and uses mostly aqueous mobile phases. This  technique is mostly used to identify the 

molecular weights of large sized proteins & bio-molecules.  

Gel Permeation Chromatography -  which uses hydrophobic packing to separate nonpolar 

species and uses nonpolar organic solvents. This technique is used to identify the molecular 

weights of polymers.

Instrumentation :  The basic HPLC system consists of  a solvent (mobile phase) reservoir, 

pump, degasser, injection device, column and  detector. The pump draws the mobile phase 

from the reservoir and pumps it to the column through the injector. At the end of the column 

(effluent end), a detector is positioned. Mostly UV absorption detector is used. In the case of 

analytical studies, after the detection the eluents are collected in waste bottles. In the case of 

preparative studies the eluents are fractionally collected for further studies. Most of the 

HPLC design will be the same as described for all the four main groups previously described. 

However, there can be differences in selecting the specific detectors for particular type of 

analysis, say for example, with ion-exchange chromatography, detectors commonly used are 

conductivity detectors for obvious reasons. Other important detectors for HPLC separations 

include refractive index detector, fluorescence  detector and mass selective detector. The 

following is the most generalised outlay of the HPLC system:   

Disadvantages :  Column performance is very sensitive, which depends on the method of 

packing. Further, no universal and sensitive detection system is available.

High Performance Ion-Exchange Chromatography

High-Performance Ion-Exchange Chromatography:  This method is used to separate 

mixtures of ions (organic or inorganic),  and finds its application mostly in protein 

separations. The stationary phase consists of very small polymer resin “beads” which have 

many ionic bonding sites on their surface, termed as Ion Exchange Resins. This resin can be 

either an anion exchange resin, which possesses positively charged sites to attract negative 

ions, or a cation exchange resin, which possesses negatively charge sites to attract positive 

ions. If the analyte mixture which contains mixture of ions is introduced into the column 

packed with suitable ion-exchange resin, selected ions will be attached or bonded on to the 

resin, thus being separated from other species that do not bond. Later, these attached ions can 

be dislodged from the column by repeated elution with a solution that contains an ion that 

competes for the charged groups on the resin surface, in other words, which has high affinity 

for the charged sites on the resin than the analyte ions. Thus the analyte ions get exchanged 

and separated from the column. 

High Performance Partition Chromatography

High-Performance Partition Chromatography:  It is the most widely used liquid 

chromatographic procedures to separate most kinds of organic molecules. Here the 

components present in the analyte mixture distribute (or partition) themselves between the 

mobile phase and stationary phase as the  mobile phase moves through the column. The 

stationary phase actually consists of a thin liquid film either adsorbed or chemically bonded 

to the surface of finely divided solid particles. Of these the latter is considered more 

important and has a distinct stability advantage. It is not removed from the solid phase either 

by reaction or by heat and hence it is more popular. It finds wide applications in various 

fields, viz., pharmaceuticals, bio-chemicals, food products, industrial chemicals, pollutants, 

forensic chemistry, clinical medicine, etc.