Microscopy. Marc McGuigan North Quincy High School Thursday, May 11, 2006

Transcription

1 Microscopy Marc McGuigan North Quincy High School Thursday, May 11, 006

2 Outline Activity Introduction Electromagnetic Spectrum Visible Light Light Microscope AFM Scanning Electron Microscopy Near-Field Microscopy

3 What is it?

4 What is it?

5 What is it?

6 What is it?

7 What is it?

8 What is it?

9 What is it?

10 What is it?

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12 Visible Light Diagram Source (bottom): Molecular Expressions Optical Microscopy Primer,

13 Light Microscopy Microscopy Issues Magnification Resolution Contrast Brightness Focus Adjusting the contrast of an image Diagram Source (left and center): Molecular Expressions Optical Microscopy Primer,

14 Light Microscopy Brightness Contrast Focus Resolution Diagram Source: How Stuff Works,

15 Resolution A smaller wavelength leads to better resolution! The interference pattern of single-slit diffraction and the relative intensities of such a pattern Light from two sources passes through an aperture If you are viewing something 5 cm away the maximum resolution of your eye is mm Diagram Source (top two): P. M. Fishbane, S. Gasiorowicz, and S.T. Thornton, Physics for Scientists and Engineers, Volume I (Prentice Hall, Upper Saddle River, 1996). Diagram Source (bottom two): Molecular Expressions Optical Microscopy Primer,

16 Lenses Diagram Source (left, top, lower right): Molecular Expressions Optical Microscopy Primer,

17 Light Microscope eyepiece Intermediate image Tube lens objective specimen condenser Diagram Source (all): Molecular Expressions Optical Microscopy Primer,

18 Beating the Diffraction Limit d min = λ NA d min ( visible) μm Alternatives Scanning Tunneling Microscope Why use visible light? Contrast Easier Sample Preparation Atomic Force Microscope Scanning Electron Microscope Transmission Electron Microscope Near-Field Imaging

19 How small is a nanometer? Atom: 1 x m Cell: 1 x 10-5 m ( m) ( m) 1 nanometer is m (1 billionth of a meter) An ipod Nano is about 7 million nanometers thick A human hair is about 100,000 nanometers thick ipod Nano thickness : 7 x 10-3 m (~0.007 m)

20 Atomic Force Microscopy Resolution ~ 1 nm How it Works A cantilever probe is scanned over the surface of a sample. As the probe scans the surface it is deflected by surface features. This deflection is measured by laser light reflected off the probe. Advantages High Resolution 3D Image Disadvantages Small scan size Slow scan speed

21 Scanning Electron Microscope (SEM) SEM image of a snow crystal Resolution~10 s of nm How it Works A beam of electrons is scanned over the sample. These electrons cause the sample to emit electrons. It is these emitted electrons that are detected to produce the image. Advantages Large scan area 3D images Disadvantages Sample must be conductive Sample placed in a vacuum Image Source: Wikipedia,

22 What is it? Ant head Pollen Asbestos fibers Spider Web Velcro

23 What is it? Needle and thread Human Hair Popped Popcorn

24 Near Field Imaging History 198 Synge Idea (6) Strong light source behind thin metal film 100 nm diameter hole to illuminate biological sample Sample less than 100 nm away from source Discusses ideas in letters to Albert Einstein (7) 197 E. A. Ash and G. Nicholls (3) Passed microwaves (3 cm) through 1.5 mm aperture Scanned over grating and were able to resolve 0.5 mm lines and 0.5 mm gaps in grating 1984 Pohl, Denk, Duerig (IBM) (SNOM) Lewis group (Cornell) (NSOM) Subwavelength aperture at apex of sharp transparent probe tip that is coated with metal Diagram Source: Molecular Expressions Optical Microscopy Primer,

25 Evanescent Waves ( ) 1 1 sin sin θ θ n n = = 1 1 sin n n θ c ( ) t z k x k i z x e E ω r ( ) t z k x k i z x e E ω + r ( ) k n k k z x = + ( ) k n k k z x = + ( ) sin θ β k n k k z z = = = ( ) cos θ k k x = n ( ) ( ) sin cos θ θ = n n n n ( ) ( ) t i z x n n k in e E ω θ θ sin sin ( ) t z i x e E ω β γ + ( ) sin = n n k n θ γ Diagram Source: K. Iizuka, Elements of Photonics: In Free Space and Special Media, Volume 1 (John Wiley & Sons, New York, 00). (4) Total Internal Reflection Wave vectors propagating in k space

26 Evanescent Waves Evanescent Waves on a Corrugated Metal Surface Evanescent Waves on an Array of Metal Pins k x + kz = To satisfy boundary conditions: k xn ( nk) This can be re-written as: π π = m λ d λzm m = k zm d π = m d The value of k xn is imaginary for high values of m and the waves are evanescent waves d c = λ Above d c k x is always imaginary and all the waves in x are evanescent waves. Diagram Source: K. Iizuka, Elements of Photonics: In Free Space and Special Media, Volume 1 (John Wiley & Sons, New York, 00).

27 Modes of Near Field Imaging NSOM Configurations Different types of scanning near field optical microscopes (c) collection/illumination (a) collection (b) illumination (a) Aperture NSOM (b) Aperture-less NSOM (c) Scanning tunneling optical microscope (d) oblique collection (e) oblique illumination (f) Dark field Diagram (left) Source: M. A. Paesler and P. J. Moyer, Near Field Optics: Theory, Instrumentation, and Applications (John Wiley & Sons, New York, 1996). Diagram (right) Source: B. Hecht et al., J. Chem. Phys. 11, 7761 (000).

28 NSOM Setup Standard NSOM Setup (a) Illumination Tips (8) Heating and pulling method - Optical fiber is heated with CO laser and pulled on both sides of heated area Chemical etching method - Hydrofluoric acid used to etch glass fiber Fiber coated with metal Nanoparticle (Tip Enhanced) (b) Collection and Redistribution (c) Detection Diagram (left) Source: B. Hecht et al., J. Chem. Phys. 11, 7761 (000). Diagram (right) Source: Molecular Expressions Optical Microscopy Primer,

29 Aperture NSOM Resolution: nm Aluminum-coated aperture probes 300 nm (a), (b) prepared by pulling (c), (d) prepared by etching 300 nm (a), (c) macroscopic shape, SEM and optical image (b), (d) SEM close-up of the aperture region Problems (9) Difficult to create smooth aluminum coating on nanometer scale Flat ends of the probes are not good for high resolution topographic imaging Absorption of light by metal coating causes significant heating Diagram (left) Source: B. Hecht et al., J. Chem. Phys. 11, 7761 (000). Diagram (right) Source: Molecular Expressions Optical Microscopy Primer,

30 Tip-Enhanced NSOM Schematic of experimental setup for tip-enhanced near field Induced surface charge density in metal probe Resolution: 10-0 nm The incident field should be polarized along the tip axis to maximize field enhancement Need large near field enhancement so the signal can be detected in the far field Left: Incident wave polarized perpendicular to tip axis Right: Incident wave polarized along tip axis Causes for Enhanced Electric Field: (10) Electrostatic lightning rod effect (depends on geometry) Surface plasmon resonances (depend on excitation wavelength and geometry) Diagram (left) Source: A. Hartschuh, M. R. Beversluis, A. Bouhelier, and L. Novotny, Phil. Trans. R. Soc. Lond. A. 36, 807 (004). Diagram (right) Source: L. Novotny, R. X. Bian, and X. S. Xie, Phys. Rev. Lett. 79, 645 (1997).

31 Near Field Images a b SRAM: 10 x 10 µm (a) AFM topography image, (b) NSOM image Diagram Source: Nanonics Imaging Ltd.

32 Near Field Images a b SRAM after Chemical Mechanical Polishing: 1 x 1 µm (a) AFM image, (b) NSOM image Diagram Source: Nanonics Imaging Ltd. (8)

33 Conclusion There are many factors that affect images obtained with a conventional light microscope Magnification Brightness Contrast Focus Resolution The diffraction of light waves limits the maximum resolution attainable with conventional light microscopy. Electron microscopy allows researchers to obtain high resolution images of a sample. Near-field microscopy involves illuminating a sample with visible light and scanning the sample with a probe that is positioned close to the surface. Near-field imaging techniques allow researchers produce images with a resolution better than 100 nm.

34 References 1. Molecular Expressions Optical Microscopy Primer, How Stuff Works, 3. P. M. Fishbane, S. Gasiorowicz, and S.T. Thornton, Physics for Scientists and Engineers, Volume I (Prentice Hall, Upper Saddle River, 1996). 4. Wikipedia, 5. K. Iizuka, Elements of Photonics: In Free Space and Special Media, Volume 1 (John Wiley & Sons, New York, 00). 6. B. Hecht et al., J. Chem. Phys. 11, 7761 (000). 7. M. A. Paesler and P. J. Moyer, Near Field Optics: Theory, Instrumentation, and Applications (John Wiley & Sons, New York, 1996). 8. P. N. Prasad, Nanophotonics (John Wiley & Sons, Hoboken, 004). 9. E. J. Sanchez, L. Novotny, and X. S. Xie, Phys. Rev. Lett. 8, 4014 (1999). 10. A. Hartschuh, M. R. Beversluis, A. Bouhelier, and L. Novotny, Phil. Trans. R. Soc. Lond. A. 36, 807 (004). 11. L. Novotny, R. X. Bian, and X. S. Xie, Phys. Rev. Lett. 79, 645 (1997). 1. Nanonics Imaging Ltd.