NANO 230 Micro/Nano Characterization Scanning Electron Microscopes

Microscopes Light Microscopes Electron Microscopes Magnification: 500 X to 1000 X Resolution: 0.20 µm Limits reached by early 1930s Color images Sample in air Electron Microscopes Magnification: 1,000,000 X Resolution: <1 nm down to 0.5 A (TEM) Use focused beam of electrons instead of light “Lenses” are coils, not glass Sample in vacuum Scanning Electron Microscope (SEM) Transmission Electron Microscope (TEM)

Scanning Electron Microscopy (SEM) Provides information about: Topography of sample or structure Chemical composition near the surface of sample Magnification: ~30X to 500,000X Resolution Nanometer scale Dependent on: wavelength of electrons () Numerical aperture of lens system (NA) Electron gathering ability of the objective Electron providing ability of the condenser Most widely used techniques for characterization of nanomaterials and nanostructures. Developed in 1942 First commercial instruments in 1965 Scans beam of electrons across sample Easily adjusted magnification over a very large range Resolution: down to a few nanometers Numerical aperture of lens system is made of two parts: objective and condenser. For this reason, the numerical aperture of the lens system is dependent on two measures: - electron gathering ability of the objective - electron providing ability of the condenser Numerical aperture is a number that should be engraved on the system

SEM Instrument Electron beam Spot size ~5 nm Energy ~200 - 50,000 eV (electron volts) Rastered over surface of sample Emitted electrons collected on a cathode ray tube (CRT) to produce SEM images Source of radiation in this technique is electrons. These electrons are focused into a very fine, focused beam that has a spot size ~ 5 nm and energy from a few hundred electron volts up to 50,000 eV (also known as 50 KeV) Rastered? rectangular formation of parallel scanning lines that guide the electron beam Deflection coils? Sample Prep: Attach to Al “stub” with conductive carbon tape or paste Sputter-coat non-conductive samples

SEM: How it works Electron beam strikes surface and electrons penetrate surface Interactions occur between electrons and sample Electrons and photons emitted from sample Emitted electrons captured on CRT SEM image made from detected electrons http://www.youtube.com/watch?v=bfSp8r-YRw0&feature=related http://www.youtube.com/watch?v=fToTFjwUc5M&feature=related

SEM: Electron Beam Interactions Valence electrons Inelastic scattering: Energy transferred to atomic electron If atomic electron has high enough energy can be emitted from sample “Secondary electron” if energy of emitted electron <50 eV Atomic nuclei “Backscattered electrons” Elastic scattering: e- bounce off with same amount of energy Atoms with high atomic numbers cause more backscattering Core electrons Core electron ejected from sample; atom becomes excited To return to ground state, x-ray photon or Auger electron emitted High Energy electrons from beam hit sample and have three different types of interactions, depending on what they hit. If they hit a valence electron that belongs to an atom in the sample, inelastic scattering occurs. Called secondary electron because it is low energy and because the primary electron (one from the electron beam) is very high energy. If the electron beam electrons hit an atomic nucleus, elastic scattering occurs. Backscattering can NOT be used for chemical identification (even though it gives us relative information about atomic number of atoms in your sample). However, if you know your sample is made up of to different tpyes of atoms which differ greatly in atomic number, it is possible to see the contrast between these two types of atoms in the SEM image. This “excited state” simply means that all of its electrons are not in their “ground state” (draw picture?) These two emissions can then be used for chemical characterization (to be discussed later)

SEM instrumentation Courtesy of the Science Education Resource Center, Carlton College, Carlton, PA

Some definitions Stigmation: Collimation: correcting asymmetries in horizontal v. vertical focus seen as “streakiness” Collimation: creation of parallel path particles typically no control over

Improving Images: Spot size electron spot radius (rms) Especially useful to improve focus at high mag Minimize spot size: Decrease working distance Increase current on focusing lens Trade-offs: Smaller area covered Lower beam current (worse contrast)

Improving Images: Depth of field How many planes are in focus at once Related to distance that beam stays narrow Especially useful to see detail on rough surfaces: Maximize DOF: Decrease aperture size Decrease magnification Increase working distance Trade-off: Lower magnification Flower v. background

Improving Images: Signal-to-Noise Signal-to-noise ratio: contrast between interacting and non-interacting surfaces Especially useful to gain more fine detail Maximize S/N ratio: High beam current Slow scan rate Trade-off: Much larger spot size