Characterization Scale. OM vstem vs SEM. Microscope. electron m. Transmission electron m. 1,000,000x. 200kV. 5-20kV

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1 Characterization Scale OM vstem vs SEM Microscope Bare eyes Optical Microscope Resolution 10µm 0.1-1µm - Magnification x Light source Visible light Visible light Specimen Bulk Material Polished Scanning electron m. 0.01µm ,000x electrons Bulk material Transmission electron m. 0.1nm 1,000-1,000,000x electrons Thin foils OM SEM 5-20kV 200kV

2 How SEM works In theory, an imaging source should be able to resolve an object the size of half the wavelength of the imaging energy. electrons have a much smaller wavelength than visible light. Before SEM there was TEM TEM takes electrons from a source and through condenser and objective electromagnetic lenses, focuses the beam on an area of the sample. If the sample is thin enough for the electrons to travel through, the projector lens will project an interference pattern, (or the image) onto a phosphorus screen below. SEM vs TEM ADV: extremely high resolution. LIMITATIONS: - because the electron beam has to travel through the sample, sample preparation is usually required to make the sample thin enough. - since the beam is traveling though the sample, the sample bulk and not the surface is being imaged. G.Cambaz, G.Yushin, Y.Gogotsi, V.Lutsenko, Anisotropic Etching of SiC Whiskers, Nano Letters, 6, 3, p.548, (cover article)

3 Limitations of using electrons electrons will not freely travel through air - there are enough molecules in air to easily absorb an electron beam. Therefore, the electron source, lenses, and sample must all be under a vacuum since electrons are electrically charged, the sample needs to be conductive enough to dissipate this charge. Source of electrons: Electron Gun Most SEMs have a hot cathode source, usually a tungsten filament similar to that in an light bulb. When such a filament is heated by passing current through it, it not only emits light, but an electron cloud forms around the filament. Left on their own, they remain in the cloud and are reabsorbed into the filament when the current is removed. The speed of the electrons emitted from this gun is controlled by the amount of potential (accelerating voltage) applied to the cathode and anode plates. As in a light microscope, we need lenses to control the flow of electrons, however, the glass lenses of a light microscope will not work. Instead, an electron microscope uses electromagnetic lenses. Plates are attached to a scan generator, the beam can be made to scan lines across the sample similar to the way a television tube scans. How is the image formed? When the incident beam electrons come to the sample 3 things can happen: 1. It can pass through the sample without colliding with any of the sample atoms (matter is mostly space). 2. It can collide with electrons from the sample atoms, creating secondary electrons. 3. It can collide with the nucleus of the sample atom, creating a backscattered electron.

4 The incident beam is composed of highly energized electrons. If one of these electrons collides with a sample atom electron, it will knock it out of its shell. This electron is called a secondary electron and is weak in energy (nearly 100 volts). If these secondary electrons are close enough to the sample surface, they can be collected to form an SEM image. The incident beam electron loses little energy in this collision. In fact, a single electron from the beam will produce a shower of thousands of secondary electrons until it doesn't have the energy to knock these electrons from their shells If the incident beam collides with a nucleus of a sample atom, it bounces back out of the sample as a backscattered electron. These electrons have high energies and because a sample with a higher density will create more of them, they are used to form backscattered electron images, which generally can discern the difference in sample densities An electron detector is placed in the sample chamber. By having a 10 kev positive potential on its face, it attracts the secondary electrons emitted from the sample surface. One advantage of this biased detector is that it can attract secondary electrons emitted from sides of the sample which are physically blocked from the detector face. This greatly reduces shadowing effects in SEM images How is the contrast formed? In secondary imaging mode, as the incident beam scans across the sample's surface topography, secondary electrons are emitted from the sample. If the beam travels into a depression or hole in the sample, the amount of secondary electrons that can escape the sample surface is reduced and the image processing places a corresponding dark spot on the screen. Conversely, if the incident beam scans across a projection or hill on the sample, more secondary electrons can escape the sample surface, and the image processing places a bright spot on the screen. This form of image processing is only in gray scale which is why SEM images are always in black and white. In backscattered imaging mode, as the incident beam scans across the sample's surface topography, backscattered electrons are emitted from the sample. A low atomic weight area of the sample will not emit as many backscattered electrons as a high atomic weight area of the sample. In reality, the image is mapping out the density of the sample surface. Some of the sample topography does affect the amount of backscattered electron emission, so the image formed shows some of the topography mixed with the sample density.

5 Lead-tin solder - a good sample for a secondary/backscattered comparison since the solder separates out into two phases. How EDX works? Energy-dispersive X-Ray analysis 2500X Secondary Electron Image of Solder 2500X Back Scattered Electron Image of Solder Chemical composition analysis X-rays emitted from the sample atoms are characteristic in energy (wavelength) not only the element of the parent atom, but also which shells lost electrons and which shells replaced them. When the incident beam bounces through the sample creating secondary electrons, it leaves thousands of the sample atoms with holes in the electron shells where the secondary electrons used to be. If these "holes" are in inner shells, the atoms are not in a stable state. To stabilize the atoms, electrons from outer shells will drop into the inner shells, however, because the outer shells are at a higher energy state, to do this the atom must lose some energy in the form of X-rays. Example: Iron If innermost shell (the K shell) electron of an iron atom is replaced by an L shell electron, a 6400 ev K alpha X-ray is emitted from the sample Or, if the innermost shell (the K shell) electron of an iron atom is replaced by an M shell electron, a 7057 ev K beta X-ray is emitted from the sample Or, if the L shell electron of an iron atom is replaced by an M shell electron, a 704 ev L alpha X-ray is emitted from the sample EDX Spectrum of Iron would have three peaks; An L alpha at 704 ev, a K alpha at 6400 ev, and a K Beta at 7057 ev.

6 How to Quantify elements by EDX Element Atomic % Weight % Al Si Ti Mapping the elements found in an SEM image by X-ray analysis By placing dots on the screen when an X-ray count of the particular element is received, an image is formed that mimics the SEM image, except the contrast is formed by the elemental X-ray emission. Ex: cross section through a lead/tin solder joint on a nickel plated copper lead wire. Cr Mn Fe Ni Nb Mo combination of the nickel, lead, and tin maps. Project PPT: Application of nanotechnology in electronics Summary (not more than 1 page) Ex: Application of Nanotubes in Chemical sensors Solar Cells How does it work? What is measured/calculated? What is the advantage to use CNT? Show examples, figures (SEM, TEM, AFM pic)