Scanning Electron Microscopy

Transcription

1 Scanning Electron Microscopy Amanpreet Kaur 1

2 Scanning Electron Microscopy What is scanning electron microscopy? Basic features of conventional SEM Limitations of conventional SEM Looking at wet samples in the high vacuum SEM Cryo-SEM and Environmental SEM 2

3 SEM & TEM Electron gun Electron gun Condenser lens Objective lens Specimen Projector lens Fluorescent screen Specimen Digital camera Scanning electron microscope Transmission electron microscope Workshop: 31 st October 3

4 SEM Scanning electron microscope 4

5 Electron sources for SEM Schottky Field Emission Source High brightness Tungsten thermionic source Energy spread < 0.5 ev Low brightness Energy spread ~ 1-2eV 5

6 SEM Scanning electron microscope 6

7 Interaction of high-energy electrons with specimen 7

8 Interaction of electrons with specimen S = secondary B = backscattered (A are auger) 8

9 Secondary and backscattered electrons What are secondary electrons? They are electrons dislodged from the specimen itself. What are backscattered electrons? They are primary electrons that have entered the sample and then escaped back out. 9

10 Detectors for SEM Secondary electrons Backscattered electrons Everhart Thornley or solid state detector (silicon diode). Usually positioned around the final lens or inside. Everhart Thornley detector (ETD): Scintillator photomultiplier system. 10

11 Information given by different signals Secondary electrons Topographic information Backscattered electrons Compositional information X-rays - Spectra - Maps Session on X-ray analysis and Elemental Mapping: 21 st November 11

12 Origin of topographic contrast Everhart Thornley detector 12

13 Detector efficiency contrast pseudo-octahedral crystal of the compound [NC 6 H 8 ][GaGe 3 S 8 ] a mixed Germanium and Gallium Sulphide framework material 50% collected somewhat bright 10% collected - Dark 100% collected - Bright 13

14 Early history of SEM 1935, M. Knoll, Germany proposed concept of a scanning electron microscope. 1942, Zworykin et al., RCA Laboratories, USA - first working SEM. 1950s, C.W. Oatley et al., Engineering Department, Cambridge - major improvements in electron optics and system, and secondary electron detection (Everhart - Thornley detector). First backscattered detector. 1965: first commercial SEM, Cambridge Instrument Company, Stereoscan Mark 1. 14

15 EM Lab s high vacuum SEM 15

16 Conventional SEM images Compact disc Polypropylene spherulites Integrated circuit 16

17 Problems with conventional SEM Can only operate at low pressure (10-5 Torr) No wet samples Surface must be electrically conducting Non-conducting samples have to be coated 17

18 Looking at wet samples in the high vacuum SEM Animal and plant tissues up to 98% water. Options for examining these materials in the high vacuum SEM: Freeze drying Chemical fixation, critical point drying 18

19 Critical point drying Evaporative drying of specimens can cause collapse of structures, mainly due to effects of surface tension. Effect can be reduced by substitution of water with a liquid with a lower surface tension Specimen prepared by CPD: Hyphae and spores in Stilton cheese 19

20 Sample prepared by fixation & critical point drying Euphorbia seed. Amal Al Hasan, SBS. 20

21 Cryo - SEM Workshop: 7 th November 21

22 Cryo SEM of food Potato showing starch grains Scale bar 100 μm Probiotic yoghurt with bacteria. Scale bar 5μm 22

23 Environmental SEM Special pumping system allows water vapour to be introduced into chamber. Cooled specimen to retain moisture. New (backscattered) electron detector. Cooled specimen Workshop: 7 th November 23

24 Introducing water vapour into the chamber enables uncoated samples to be examined Secondary electrons from sample strike water molecules. Positively charged water molecules are attracted to negatively charged sample. Negative charge at sample surface is neutralized.. 24

25 EM Lab s FEI Quanta 600 ESEM Gaseous secondary electron detector fits on objective lens. Pressures up to 20 Torr - samples typically cooled to 5 o C. 25

26 Environmental SEM Raspberry buds : Hiroyuki Imanishi, Plant Sciences 26

27 Becoming an EMLab user Sherrie Foo 27

28 Booking system 28

29 Remaining workshops Date Topic 31 October November November November November 2017 Transmission Electron Microscopy Environmental & Cryo SEM Biological specimen preparation for SEM X-ray Analysis and Elemental Mapping Biological specimen preparation for TEM 29

30 Electron Microscopy Laboratory Questions? Demo 30