Electron Microscope

Light Resolution  The resolution of a microscope is limited by the diffraction of light. Single diffractionSingle diffraction  The minimum size observable with visible light is about 200 nm. Airy diskAiry disk Visible light down to 400 nmVisible light down to 400 nm Small wavelength – no diffraction Large wavelength – diffraction

Electron Diffraction  The diffraction limit of an electron is based on its wavelength. Related to momentum  Energy of 110 V creates a wavelength of nm. Nonrelativistic energy Rest energy MeV hc = 1240 eV nm

Microscope Revisited  An optical microscope uses light to form an image.  The objective and eyepiece are the primary lenses.  A condenser lens is sometimes used to focus light onto the object. objective intermediate image eyepiece

Electron Gun  A hot filament produces electrons attracted to the positive anode.  The Whenelt cap uses a negative potential to focus electrons onto the axis. Potential about 500 V Space charge electrons from 1 mm gap towards anode  Electrons accelerate through a gap in the anode plate. Point source beam for optics Monochromatic beam Images: University of Nebraska, Lincoln

Electron Lens  Electrons are charges particles. Bent by magnetic fields  Circular magnets act as lenses. Spherical lens equations  Apertures are thin metal disks to block scattered electrons. Typical hole size  m objective intermediate image projector object image magnified image aperture

Transmission  A transmission electron microscope passes electrons through a sample.  The source is an electron gun.  Condenser lenses and create a narrow coherent beam. Spot size lensSpot size lens Intensity lensIntensity lens Adjustable apertureAdjustable aperture  Intermediate and projector lenses increase magnification.  Image recorded on screen. Light areas - more electronsLight areas - more electrons Images: University of Nebraska, Lincoln

Scanning  A scanning electron microscope reflects electrons off of a sample.  The electron gun and condenser create the electron beam. Fine and course current control  Electric coils sweep the beam over the sample. Microsecond time scale  The objective lens targets a spot on the sample. Reflection measured Images: University of Nebraska, Lincoln

Images  Electron microscopes are able to make images with many times the detail of optical microscopes.  At right, a mosquito head with an SEM. Top at x200 magnification Bottom at x1000 magnification next Museum of Science, Boston