Transcript Slide 1
Scanning (and Transmission) Electron Microscopies Andrew van Bommel February 21st, 2006 SEM Electron Source (GUN) • The majority use a thermionic gun: – A voltage is applied to a tungsten filament (cathode): it is heated and e-s are produced – The electrons are accelerated to the anode – Electrons can exit a small (<1mm) hole to move down the EM column (in a vacuum) for imaging Electron Source • The thermionic gun ensures that: – All electrons are emitted from a point source (since they are accumulated in a space charge before they leave the anode) – The electrons have similar energies – Only electrons parallel to the optical axis can exit the gun Field-Emission Gun (FESEM) • Does not heat a filament- also called a cold cathode field emitter • The cathode (W) is placed in a electrical potential gradient • Cathode: a tungsten wire fashioned into a sharp point (radius of less than 100 nm) • Can produce electron beam of 2 nm: theoretically resolution limit (3-6X SEM) Electron Beam • Stream of electrons are ‘focused’ by a series of metal apertures and magnetic lenses • The magnetic lenses are circular electromagnets with the ability to project a precise circular magnetic field in a particular region (they ‘steer’ the electron beam) • Magnetic lenses are similar to optical lenses: – Focal length, angle of divergence – Spherical aberration, Chromatic aberration Sample Preparation • Non-conductive materials must be coated in gold so that they conduct (and therefore interact with) the beam of electrons Specimen Interaction • In SEM: – Backscatter: electrons are reflected by atoms – Secondary electrons: from material Backscattered Electrons • AKA elastic scattering: little change in energy of scattered electrons • Backscatter arises from interaction of electrons with nucleus: atoms with higher mass scatter Cu more • Can get contrasts: Al Secondary Electrons • Electrons emitted from the specimen with < 50eV (only small amt of energy transferred) • Electrons produced by interactions with energetic electron beam and weakly bonded electrons (metals, insulators, semiconductors) • Detection : – Electrons Scintillator photons photomultiplier conversion into electric current detection – Process in order to pass through vacuum env. Scanning (in SEM and STEM) • A set of coils scans or sweeps the electron beam (like a television) • The instrument counts the interactions of the electron beam at a certain location (the more reactions, the brighter the pixel) • The pattern can be scanned 30 times per sec. Specimen Interaction • In TEM: – Elastically and inelastically scattered electrons TEM Electron Scattering • The brightness of the image is proportional to the amount of unscattered electrons • Atoms of small mass will leave more electrons unscattered, so they will appear brighter in the micrograph Display • SEM: the interactions are displayed on a CRT screen • TEM: the transmitted electrons (after interacting with the sample) strike a phosphor image screen with the brightness of the image dependant on the number of electrons hitting the screen Analytical Techniques • EDX: Energy Dispersive X-Ray Analysis- find info on elementary composition from characteristic X-rays given off by sample (good for elements with high z) • EELS: Electron Energy Loss Spectroscopyuses the characteristic energy loss of transmitted electrons to find info on elements (composition, bonding, electronic structure) (good for elements with low z) Nano Applications • Nanomeasurements in TEM (Microsc. and Microanal., 6 (2000) 224-230) • Bending modulus of a single carbon nanotube has been measured Nano Applications • A method for characterizing carbon nanotubes (J. Electron Microsc. 50 (2001) 321-324) • Heated C NTs to 600°C removed contamination and stabilized the structure Nano Applications • Three-dimensional STEM for observing nanostructures (J. Electron Microsc. 50 (2001) 235-241) • Specimen stage rotated, e-s from all directions Nanomaterials • See Microscopy Research and Technique, Nanomaterials Characterization Using Microscopy Vol. 64, Issue 4-5 • Other Jounals: – – – – Journal of Electron Microscopy Microscopy and Microanalysis Electron Microscopy Reviews Journal of Microscopy