Scanning Electron Microscope Jamie Goings

Theory• Conventional microscopes use light and glass lenses• SEM uses electrons and magnetic lenses to create magnification • Electron beam ‘traces’ over object, interacting with surface and dislodges surface electrons• Detector collects electrons, and registers different levels of brightness• Scanned onto monitor dot by dot, row by row

Instrumentation – Electron Gun1. Thermionic Guns – most common voltage is applied that heats filament (usually tungsten)

2. Field Emission Guns Creates strong electrical field to pull e away from atoms

Anode attracts e causing them to accelerate down into a beam

II. Lenses Magnetic lenses direct electron beam into wanted path.

III. Scanning Coils-Additional magnetic field use voltage to move beam back and forth across sample. -Can adjust magnification here by adjusting scan area

IV. Sample Chamber-must be sturdy and isolated from vibration-adjustable position-vacuum chamber – keeps e beam clear of air particles and sample free of dust

V. Detectors1. Secondary Electron Detector- has a 300V positive charged metal cage to

attract e- collects e dislodged from surface - number of e collected per ‘dot scan’

determined brightness of spot- creates image2. Energy Dispersive X-Ray Detector (EDX)- elemental analysis- analyzes x-rays emitted from specimen - detects number of x-rays vs their energy-

energy of x-ray is specific to element it was emitted from

EDX Spectrogram

Sample / Sample PrepSample-must be conductive-must be able to withstand vacuum – no liquids-15/15 mm older machines-200/200 mm modern

Sample Prep-clean dust or debris-sputter coating-biological samples are dehydrated and dried

Benefits / Limitations

Benefits-High depth of field-High resolution – high magnification-Can adjust focus, contrast, brightness-Computer controls-3D image-With EDX is both qualitative and quantitative

Limitations-Generates radiation-Needs to be clean!!!

Work Cited:

Oatley, C. W. The Scanning Electron MicroscopeGabriel, Biological Scanning Electron MiscroscopySpringer-Verlag, Methods of Preparation for Electron Microscopyhttp://www.purdue.edu/rem/rs/graphics/sem2.gifhttp://www.herguth.com/technical/sem.pdfhttp://static.howstuffworks.com/gif/scanning-elecron-microscope-illustration.jpg (image)