Nanotechnology Lect.10

Characterization Methods
This lecture summarizes some of the methods used for imaging and characterization of nanomaterials, meaning materials with at least one dimension at the nanoscale level (1–100 nm). These include nanostructured surfaces, nanoparticles, nanoporous materials, etc. The aim of this lecture is to answer the question: How are nanomaterials imaged and characterized?
There are many methods available to image nanostructured materials (e.g. a nanostructured surface) and to characterize their physical and chemical properties. Here, only a short review and description of these methods is provided.
In general, two fundamental types of characterization methods exist: imaging by microscopy and analysis by spectroscopy. The methods employed have been developed specifically to meet the characterization needs of nanomaterials.
Microscopy
An optical microscope uses visible light (i.e. electromagnetic radiation) and a system of lenses to magnify images of small samples. For this reason, it is also called a light microscope. Optical microscopes are the oldest and simplest of the microscopes. The resolution limit of an optical microscope is governed by the wavelength of visible light (As a general rule of thumb, the resolution is about half the wavelength used in the measurement). Visible light is the part of the electromagnetic spectrum with wavelengths between 400 and 700 nm and the resolving power of an optical microscope is around 0.2 ?m or 200 nm: thus, for two objects to be distinguishable, they need to be separated by at least 200 nm. Single objects smaller than this limit are not distinguishable: they are seen as ?fuzzy objects‘. This is known as the ?diffraction limit‘ of visible light.
In order to overcome the limitations set by the diffraction limit of visible light, other microscopes have been designed which use other beams: rather than light, they use electron beams to illuminate the sample. Electron microscopes have much greater resolving power than light microscopes that use electromagnetic radiation and can obtain much higher magnifications of up to two million times, while the best light microscopes are limited to magnifications of 2000 times. Both electron and light microscopes have resolution limitations, imposed by the wavelength of the radiation used. The greater resolution and magnification of the electron microscope is because the wavelength of an electron (its de Broglie wavelength) is much smaller than that of a photon of visible light.