Page 172 - Science at the nanoscale

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Nanotools and Nanofabrication
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n is the refractive index of the medium in which the lens operates.
λ is the wavelength of light illuminating or emanating from (in the
case of fluorescence microscopy) the sample. The quantity n sin α
is also known as the numerical aperture. For the best lenses, α is
◦
about 70 (sin α = 0.94), the shortest wavelength of visible light
is blue (λ = 400 nm), and the typical high resolution lenses are oil
immersion lenses (n = 1.5). Substituting these values in Eq. (8.2),
we determine R to be approximately 170 nm, i.e., the resolution
limit of a light microscope using visible light is about 200 nm.
There is another consideration in developing optical micro-
scopes with high magnification, namely, the light ray has to be
focused very tightly onto the samples. This means such an optical
microscope would have limited depth of focus. For example, a typ-
ical optical lens with a magnification of 100× would have a depth
of focus of about 1-2 microns. As a result, if the object we are
interested in seeing is a sizable 3D object (> a few microns thick),
then we would only be able to obtain a sharp image of part of the
object.
8.2 ELECTRON MICROSCOPY
In the early 1930’s, many scientists and engineers realised that
they have reached the theoretical limit of the resolving power
of an optical microscope. In order to be able to “see” the finer
details of objects such as biological cells, scientists started to
develop new type of microscopes that make use of fast-moving
electrons instead of light. Microscopes that make use of fast-
moving electrons are known as electron microscopes and they are ch08
generally classified into two types: the Scanning Electron Micro-
scope (SEM) and the Transmission Electron Microscope (TEM).
The idea is to direct a focused beam of electrons towards a small
part of an object in vacuum and detect various signals generated
due to the interaction of the electrons with the object. Images can
be generated depending on the contrast in the magnitude of the
signals obtained when the beam of focused electrons is scanned
across the object. The first electron microscope was invented by
Ernst Ruska and Max Knoll from Germany. In 1986, the Physics
Nobel Prize was co-awarded to Ernst Ruska for the development
of electron microscopy.