A standard SEM is typically used for low-to-medium magnification (10-50,000×) imaging of conductive samples. For non-conductive samples, a conductive coating of carbon, gold, chromium, etc. must be applied to avoid charging effects. A variable-pressure SEM (low vacuum or environmental) is used for non-conductive specimens like glass, polymers/paint and biological materials without applying a conductive coating to the specimen.

In the scanning electron microscope (SEM), samples are irradiated with a focused beam of electrons in a vacuum chamber. These electrons interact with the sample in numerous ways, creating various radiation types including the production of secondary electrons, backscattered electrons and characteristic x-rays. These signals are used for imaging and compositional analysis at high spatial resolution.

Compared to light microscopy, SEM offers higher resolution (as good as 0.6 nm), greater depth of field, longer working distances and better surface sensitivity. Images of backscattered electrons (BE) provide compositional information, since the BE signal intensity of a material is proportional to its average atomic number. X-rays generated by the electron beam are detected and analyzed with an energy dispersive x-ray spectrometer (EDS). EDS spectra can be used to obtain a qualitative and quantitative elemental analysis at micrometer spatial resolution.

Instrumentation:

McCrone Associates uses several scanning electron microscopes:

JEOL JSM-6480LV low vacuum SEM. Large sample chamber and super conical lens allow examination of samples up to 8” in diameter and at high tilt angles. The system is integrated with INCA and ISIS microanalysis systems from Oxford Instruments. Operation in low vacuum (LV) mode permits examination and x-ray microanalysis of non-conductive samples without coating. The computer-controlled stage and digital imaging system allow automated particle and feature analysis and unattended x-ray mapping.
Resolution: 3.5 nm
Accelerating Voltage: 0.3 to 30 kV
Magnification: 18× to 300,000×

JEOL JSM-6460LV low vacuum SEM. Similar capabilities to the JSM-6480. Equipped with an Oxford Instruments INCA microanalysis system for EDS microanalysis, and Renishaw SEM-SCA (structural and chemical analyzer) system with inVia Raman microscope for Raman, cathodoluminescence and photoluminescence spectroscopies.
Resolution: 3.5 nm
Accelerating Voltage: 0.3 to 30 kV
Magnification: 18× to 300,000×

JEOL JSM-7500F field emission SEM (FESEM). Ultrahigh resolution with magnifications up to 1,000,000×. A wide range of operating modes and detectors are optimized for surface imaging of materials at the nanometer scale.
Resolution: 0.6 nm
Accelerating Voltage: 0.1 to 30 kV
Magnification: 25× to 1,000,000×