PHYS-E0541:Special Course in Physics Gas phase synthesis of carbon nanotubes for thin film application. Electron Microscopy. for

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1 PHYS-E0541:Special Course in Physics Gas phase synthesis of carbon nanotubes for thin film application Electron Microscopy for Introduction to Electron Microscopy Carbon Nanomaterials (nanotubes) Dr. Hua Jiang Nano-Technology Group, VTT Processes, Hua P.O. Jiang Box 1000, FIN VTT, Espoo Nanomicroscopy Center, Aalto University Carbon Nanomaterials Carbon nanobuds combine nanotubes with fullerenes, which appear to bud off the tubes. (Source: Canatu.com) (source: AZoNano) 1

2 Carbon nanotubes were discovered in TEM Aalto University Honorary Doctor, Oct. 10, 2014 (source: wikipedia) Electron Microscopy images This is a CNT sample 200 nm Are you able to tell what types of microscopy images they are? 2

3 Electron Microscopy images This is a CNT sample 50 nm 50 nm 50 nm TEM BF STEM DF SEM This lecture is designed for you to recognize and analyze those images, and extract as much structural info as possible from the sample. Scope of Electron Microscopy Scanning Electron Microscopy (SEM) Transmission Electron Microscopy (TEM) Scanning Transmission Electron Microscopy (STEM) 3

4 Electron Microscopy CRT CRT Specimen Specimen Viewing screen HAADF detector BF/DF detector SEM TEM STEM Electron object interaction high energy electrons e - Backscattering electrons Auger electrons SEM Secondary electrons Visible light X-ray EDS / WDS Interaction between electrons and specimen sample If the specimen is thin enough for electron transparencey Elastically scattered electrons TEM / ED Direct Beam (background) Inelastically scattered electrons EELS / EFTEM TEM: transmssion electron microscopy ED: electron diffraction EDS: energy dispersive spectrometry EELS: electron energy loss spectrometry EFTEM: energy filtering TEM SEM: scanning electron microscopy WDS: wavelength dispersive spectrometry 4

5 SEM: Scanning Electron Microscope --- Basic priciples --- Image contrast mechanism --- SEM performance SEM basics 5

6 Electron-matter interactions SE: secondary electrons BSE: backscattered electrons X-ray: characteristic of elements Electron-matter interactions E0 Incident high energy electron a few nm nm X-ray K L M E0- DE in the µm range SE: secondary electrons BSE: backscattered electrons X-ray: characteristic of elements 6

7 Secondary electron imaging Topographical observation The number of secondary electrons depends greatly on the surface topography from a very thin layer of the speciment. Thus, the SE are considered to be the most suitable signal for observing a specimen s surface morphology SE image (Courtesy: JEOL Company) Backscattered electron imaging Compositional observation The number of backscattered electrons are mainly determined by the average atomic number of the substances composing the specimen surface. SE image BSE image (Courtesy: JEOL Company) 7

8 X-ray imaging Elemental analysis (mapping) Characteristic X-rays are emitted from a specimen when an elelctron beam irradiates it. By detecting and analyzing the characteristic X-rays, we can identify the elements contained in the sample either qualitatively or quantitatively. BSE image Aluminum map Calcium map (Courtesy: JEOL Company) SEM Performance Important imaging parameters (conditions): high voltage beam diameter (spot size) beam current HV For the highest imaging resolution: beam diameter, d p, must be as small as possible; For better image quality (S/N ratio): emission current, i p, must be as large as possible For good image contrast for a fine surface structure: use lower accelarating voltage a few nm nm in the µm range One needs to optimize the operating condition for best imaging. 8

9 Effects of acceletating voltage 30kV 5 kv (Courtesy: JEOL Company) Low voltage secondary electron images of SWCNTs supported on the holey SiO 2 thin film ( ~ 2 nm diameter ) 2 kv 2 kv 100nm 5 nm 300 V 100 V 50nm 5 nm 5 nm (Courtesy: JEOL Company) 9

10 SEM: a few highlights In SEM, electrons are focused into a small probe and being scanned over the surface of the specimen. A variety of signals might be detected to form images: Secondary electron: topography Backscattered electrons: compositional X-ray: elemental distribution SEM performance is mainly determined by several parameters: Accelerating voltage Beam diameter (spot size) Beam current Best images are always achieved by optimizing the imaging conditions. TEM: Transmission Electron Microscope Imaging techniques Diffraction analysis Spectroscopy STEM Applications 10

11 Signals picked up for TEM Auger electrons Visible light e - Backscattering electrons SEM Secondary electrons X-ray EDS Elastically scattered electrons (S)TEM / ED Direct Beam (background) Thin specimen for electron transparencey? Inelastically scattered electrons EELS / EFTEM TEM Specimen General requirements: Small: Thin: 3mm in diameter nm* in thickness Remember: Your microscope (TEM) is only as good as the sample that you put into it!!! * 1 nm = 10-9 meter 1 nm = 10 Å 11

12 TEM: Diffraction & Imaging 200 kv e - YBa 2 Cu 3 O 7-d sample Obj. Lens diffraction Y Ba O Cu/O image An image represents the structure in real space at a certain resolution; The diffractionis an reproduction of the structure in reciprocal space. Electron Diffraction From Carbon Nanotubes (22, 8): d = 2.11nm 12

13 Chirality: Description of CNT structure graphene sheet SWNT The best way to visualize the property of helicity is to imagine rolling up a piece of graphene sheet into a tube. (Source: Wikipedia) 0,13 0,14 0,1 0,2 0,3 1,1 0,4 0,5 0,6 2,2 0,7 0,8 0,9 3,3 0,10 0,11 0,12 ( a, D ) Û ( n, m ) 0 4,4 L 5,5 a 6,6 7,7 8,8 10,5 9,9 10,10 11,11 0,0 1,0 2,0 3,0 4,0 5,0 6,0 7,0 8,0 9,0 10,0 11,0 12,0 13,0 14,0 15,0 16,0 13

14 Electron Microscopy for SWCNTs Arm-chair Zig-zag Chiral From ( a, d ) to ( n, m ) --- conventional method Tube axis a d 2 d 1 d 3 The chiral index : ( m, n) = (21, 9) 2a The chiral angle: tana = 1 3 æ d ö ç2-1 è d3 ø 2 = The equatorial oscillation is described as a 0-order Bessel function: o 17 f I 0( p RD0) µ J 0 ( prd0) 2 The tube diameter: 1 d µ = nm f 14

15 Forming an EDP of a SWCNT in TEM Normal incidence --- H Jiang et al, PRB 74 (2006), Incident electrons a carbon nanotube (real space) O.L. Intensities? line spacings? (geometry) The CNT reciprocal countpart tube axis Planar Ewald sphere Layer-lines: intensity analysis --- H Jiang et al, PRB 74 (2006), L 2 L 1 L 0 ( D R) 2 J 0 p 0 ( D R) J 2 n p 0 ( D R) J 2 p m 0 d L 0 Profile L1 Profile L2 Profile 15

16 Layer-lines: geometry analysis --- H Jiang et al, CARBON 45 (2007), 662 x i = d i d Totally calibration-free An Example NIST VAMAS TWA-34 SWCNT sample Hua Jiang 2010 (6, 5) d 2 d 3 16

17 1,0 2,0 3,0 4,0 5,0 6,0 7,0 8,0 9,0 10,0 11,0 12,0 13,0 14,0 15,0 16,0 17,0 1,1 2,1 3,1 4,1 5,1 6,1 7,1 8,1 9,1 10,1 11,1 12,1 13,1 14,1 15,1 16,1 2,2 3,2 4,2 3,3 5,2 6,2 7,2 8,2 9,2 10,2 11,2 12,2 13,2 14,2 15,2 16,2 4,3 5,3 6,3 7,3 8,3 9,3 10,3 11,3 12,3 13,3 14,3 15,3 4,4 The Chirality map was determined from the EDPs of 49 individual nanotubes: Metallic tubes: 14% Semiconducting tubes: 86% 5,4 6,4 7,4 8,4 9,4 10,4 11,4 12,4 13,4 14,4 15,4 5,5 6,5 7,5 8,5 9,5 10,5 11,5 12,5 13,5 14,5 6,6 7,6 8,6 9,6 10,6 11,6 12,6 13,6 14, nm 7,7 8,7 9,7 10,7 11,7 12,7 13,7 1 nm 8,8 9,8 10,8 11,8 12,8 13,8 9,9 10,9 11,9 12,9 10,10 11,10 12,10 11,11 NIST VAMAS TWA-34 SWCNT sample Hua Jiang 2010 A diffraction pattern can be misleading Combining imaging and diffraction: We see what we diffract!! 17

18 An image may also mislead us The resolution is important in interpreting an image correctly. 2 nm Cs-corrected HREM analysis of a DWCNT taken at 80kV nm 2 nm (32, (17, 10) 18

19 How do CNTs grow? CO as carrier gas and carbon source with ferrocene and CO 2 Fe nanoparticles are formed by thermal decomposition of ferrocene SWCNTs are grown in the gas flow via Bouduard reaction: Fe CO + CO = C(s) + CO 2 Controlled concentration ~ particles/cm 3 High individual SWCNT fraction SWCNT networks formed by filtration at the reactor outlet The TEM sample was collected by placing a grid on filter during collection (collection time: 30 sec.) CO 2 TEM grid 880 C How do CNTs grow from nano-catalyst particles? 19

20 Seeing is believing A TEM shoots not only still images but also movies --- an in-situ study TEM: a few highlights In a conventional TEM, coherent electrons are spread onto a thin (or small) object. They pass though the sample, and carry its structural information. Assisted by the objective lens in the microscope, they form: Electron diffraction pattern (EDP) Electronic micrograph (image) Both image and EDP analysis can mislead us, unless we understand things and do things correctly. TEM is not just an imaging tool, but can also be utilized as a chemical reactor, where one can introduce chemical reaction, and observe the reaction process in real time. As an example, we can grow CNTs in an electron microscope and observe the growth process. 20