Page 493 - Carrahers_Polymer_Chemistry,_Eighth_Edition

456 Carraher’s Polymer Chemistry

sometimes the same, to that employed in atomic force microscopy (AFM). Essentially, light is
directed through the probe tip onto the sample from just immediately above the sample surface.
The light emanating from the probe tip is smaller than the light’s wavelength and spreads out over
the surface. This results in the maximum influence occurring at the surface with little contribution

from regions nearby (such as within 30 nm), resulting in little out-of-focus light. Depending upon
the surface and sample thickness, the light is measured as absorption or fluorescence and collected

and recorded electronically. NSOM can be fitted onto a regular optical microscope or coupled with
scanning probe microscopy (SPM).

13.2.3 ELECTRON MICROSCOPY

The upper limit of magnification for optical microscopes is about 2,000 times. Thus, additional
forms of microscopy have been developed that allow near to actual atomic observation of polymer
surfaces. Electron microscopy utilizes an electron beam to act as the sensing radiation in place of
light. High-energy electrons take on wave character as they approach the speed of light. The wave-
length is inversely proportional to the electron speed or velocity. When accelerated over large volt-
ages, electrons can be made to travel at speeds to permit wavelengths on the order of 0.003 nm. The
electron beam is focused and the image is formed using magnetic lenses. The two most common
forms of electron microscopy are transmission electron microscopy (TEM) and scanning electron
microscopy (SEM).
In SEM, the surface of the polymeric surface is scanned using an electron beam with the refl ected
or back scattered beam of electrons collected and displayed on a cathode ray tube screen. The image
represents the surface contour of the scanned material. Because the surface must be conductive,

most polymer surfaces must be overlaid with a conductive coating. Magnifications up to about
50,000 are carried out using SEM.
Transmission electron microscopy utilizes an image formed by an electron beam that passes
through the sample. This allows internal microstructures to be determined. Structural details of
materials can be observed on an atomic level by looking at contrasts in the image caused by various
concentrations of different elements. Very thin films are employed. Under good conditions, magni-

fications up to one million are possible employing TEM.
13.2.4 SPM

Scanning probe microscopy encompasses a group of surface-detection techniques that include
AFM and scanning tunneling microscopy (STM) that allow the topographic profiling of surfaces.

SPM techniques investigate only the outermost few atomic layers of the surface with nanometer
resolutions and at times atomic level resolution.
Scanning tunneling microscopy is generally used with electrically conductive materials applied
to polymeric materials giving overlays consisting of conducting material layered over the surface of
–10
the sample. STM experiments typically require extremely low pressures less than 1 × 10 mbar.
By comparison, AFM can be run under room conditions and does not require the use of electrically
conductive material. In with STM, the metallic tip is held close (about 0.5–1 nm) to the surface.
A voltage, applied between the tip and sample surface, drives a tunneling current. The conductive
surface reconstructs the atomic positions via minimizing the surface free energy. This gives topo-

graphic superstructures with specific electronic states which are recorded as surface contours or
images.
Atomic force microscopy can be run under room conditions. AFM can be performed in either
of two forms—a contact mode and a noncontact mode. It does not require the use of electrically
conductive material since (in the contact mode) the tip actually “touches” the surface rather than
residing immediately above it as is the case in STM. In both the contact and the noncontact mode,
light is used as the sensing source rather than an applied voltage. In contact AFM, a cantilever with

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