Scanning Electron Microscopy: An Introduction. Dr. Bill Miller Sacramento City College

Transcription

1 Scanning Electron Microscopy: An Introduction Dr. Bill Miller Sacramento City College

2 Scanning Electron Microscopy (SEM) Images Ant s Head Alan Hicklin, Spectral Imaging Facility, UCD

3 SEM Images Ant s Stinger Alan Hicklin, Spectral Imaging Facility, UCD

4 SEM Images Murine mast cells on a Self-Assembled Monolayer on Gold Jie-Ren Li, Spectral Imaging Facility, UCD

5 SEM Images Scanning Electron Micrograph of the Surface of a Kidney Stone Kempf, E. K.

6 SEM Images Single Gold Nanoparticle on Glass Tip Nanonics Co.

7 Hitachi S-4100 T SEM

8 Hitachi S-4100 T SEM

9 Optical Microscopy eyepiece objective lens sample condenser lens light collector light source

10 Optical Microscopy light source light collector condenser lens sample objective lens eyepiece

11 Optical Microscopy vs. SEM Iowa State SEM page

12 Optical Microscopy vs. SEM Light Source is replaced by an electron source: Why electrons? 1. Visible light has a wavelength of nm = m. 2. Electrons have a wavelength that depends on their. h mv h = Planck s constant = x J s m = mass of an electron = 9.1 x g = 9.1x kg v = velocity of an electron = 20% of the speed of light = 6.0 x 10 7 m/s 3. = 1.2 x m = nm!!

13 Optical Microscopy vs. SEM How is the electron beam created? Method 1: Heating a tungsten filament A. Tungsten has a high melting point and a low vapor pressure. T m = 3695 K B. Apply a voltage/current to the Tungsten to heat it up (similar to an incandescent light bulb). C. At >2500 K, tungsten will emit electrons (and light and heat). D. Typical operating T=2800 K.

14 Optical Microscopy vs. SEM How is the electron beam created? Method 2: Cold Field Emission Gun (Cold FEG) A. An extraction voltage is applied to a sharp Tungsten tip. B. Operating temperature is 300 K. C. Electrons are preferentially extracted from the very tip of a metal towards another positively charged metal. D. Requires flashing of tip.

15 Optical Microscopy vs. SEM How is the electron beam created? Method 2: Cold Field Emission Gun (Cold FEG) A. An extraction voltage is applied to a sharp Tungsten tip. B. Operating temperature is 300 K. C. Electrons are preferentially extracted from the very tip of a metal towards another positively charged metal. D. Requires flashing of tip.

16 Comparison of Electron Guns

17 Optical Microscopy vs. SEM How are the electrons accelerated? With an accelerating voltage! The anode is a positively charged plate that attracts the electrons from the tip. Those that miss the anode continue past it at a higher speed. Accelerating voltages range from 500 v to 30 kv. The higher the voltage, the faster the electrons!

18 Optical Microscopy vs. SEM How are the electrons focused? Using a magnetic lens : By passing a current through a wire, a magnetic field is created. This magnetic field bends the path of an electron.

19 How is the image created? Optical Microscopy vs. SEM

20 Optical Microscopy vs. SEM How is the image created? Optical Microscopy: Lenses expand and focus image Detector records entire image at once

21 Optical Microscopy vs. SEM How is the image created? Magnetic lenses focus the electron beam to a point. Scanning coils raster this point across the surface. As the electrons smash into the surface, there are three types of interactions 1. Secondary electrons are ejected and X-rays are emitted. 3. Backscattered electrons are ejected. Each type of radiation tells us something about the sample.

22 SEM Sample Preparation SEM is conducted under high vacuum of 1 x 10 5 mbar or less. For Cold FEG, use < 1 x mbar bar = 1 atm = 760 torr This is so that the electrons don t hit gas particles. At these pressures all water immediately evaporates, so samples must be dry. Biological samples must be freeze dried to keep their shape and structures.

23 SEM Sample Preparation The emission current in a tungsten filament SEM is typically 200 µa, which is roughly electrons per second. SEM samples are typically coated in gold to conduct these electrons away from the surface of the sample so that the sample does not become charged. For cold FEG, the amount of current going into the sample is 20 times less (and, of course, it s more complicated than this) but: little to no charging of surface = no gold coating necessary!

24 Secondary Electrons y axis secondary electron electron from gun nucleus x axis n=1 n=2 n=3 Type of scattering: Typical Kinetic Energy (KE) of a secondary electron: What secondary electrons tells us about the sample:

25 Secondary Electron Image Ant s Head Alan Hicklin, Spectral Imaging Facility, UCD

26 X-ray Emission y axis secondary electron electron from gun nucleus x axis n=1 n=2 n=3 Typical Wavelength of X ray: What X rays tell us about the sample:

27 SE and X-ray Emission Analysis 3.7 kev/photon = 6.0 x J/photon hc E ( J s)( m/s) J Kidney Stone Digital Microscopy Facility, Mount Allison University m

28 SE and X-ray Emission Analysis Kidney Stone Digital Microscopy Facility, Mount Allison University

29 Back Scattered Electrons electron from gun y axis nucleus x axis n=1 n=2 n=3 Type of scattering: Typical Kinetic Energy (KE) of electron: What backscattered electrons tell us about the sample:

30 SE and Back Scattered Electron (BSE) Image SE Image BSE Image Iron Particle on Carbon ETH-Zurich

31 Our Project In collaboration with Professor Gang-yu Liu (Department of Chemistry) and The Spectral Imaging Facility At UC Davis Research Experiment Accomplish 2 Main Goals: 1. Scientifically: Study Human Hair Samples using Field Emission SEM A. Field Emission SEM allows imaging of hair samples in their natural state B. Previous studies have established a general relationship between ethnicity and diameter of hair for gold-coated samples. We hope to quantify this relationship using uncoated hair samples and image analysis software. C. Further studies potentially looking at pollen samples, clay samples, bat hair samples and many other samples.

32 Our Project In collaboration with Professor Gang-yu Liu (Department of Chemistry) and The Spectral Imaging Facility At UC Davis Alan Hicklin, Staff Scientist Research Experiment Accomplish 2 Main Goals: 2. Educationally: Involve community college students in all aspects of this project A. Learning about SEM B. Participating in this study by donating a hair sample and receiving back an SEM image of your hair including diameter analysis. C. Participate in this study by analyzing hair samples for their diameter on your computer. D. Participate in this study by completing a second seminar on the specifics of using the Hitachi S-4100T Field Emission SEM. Then, accompany me to UC Davis to prepare and analyze your own hair sample. E. Participate in this study as a $500 paid intern: includes all of the above many times over and helping to coordinate the project.

33 Preliminary Results

34 Preliminary Results 75 µm Miller

35 Preliminary Results hair not cleaned with soap Miller damaged hair Miller

36 Preliminary Results 58 µm Trotter 99 µm Sanchez

37 Preliminary Results 303 µm cat whisker different scale 55 µm dog fur

38 Preliminary Results cat whisker close-up

39 Preliminary Results pollen grain

40 Conclusions/Summary Scanning Electron Microscopy 1. The SEM uses a beam of electrons scanned across the surface of a sample. 2. Collecting secondary electrons produces a 3-D image of the surface. 3. Collecting x-rays allows an elemental analysis of the surface. 4. Collecting back scattered electrons allow an elemental mapping of the surface. Research Project 1. Hair samples have been imaged using a Field Emission SEM without any conductive coating on the sample. 2. These hair images have a variety of thicknesses. 3. There are opportunities for student involvement in this project.

41 Acknowledgements Thank you to Gang-yu Liu and Sacramento City College for support. Thank you to Alan Hicklin and Jie-Ren Li for scientific assistance. Thank you to Scott Trotter, Eric Sanchez, Ceanne Brunton and Tam Le for assistance in acquiring images of their hairs.