Design For Accessibility: Getting it right from the start
Nanoscale Science and Technology
1. M. Meyyappan
Director, Center for Nanotechnology
NASA Ames Research Center
Moffett Field, CA 94035
email: meyya@orbit.arc.nasa.gov
web: http://www.ipt.arc.nasa.gov
Guest Lecturer: Dr. Geetha Dholakia
Nanoscale Imaging Tools
2. Overview of microscopy
• Optical Microscope
• Electron Microscopes
Transmission electron microscope
Scanning electron microscope
• Scanning probe microscopes
Scanning tunneling microscope
Atomic force microscope
NOTE: This talk has been put together from material available
in books, various websites, and from data obtained by NASA
nanotech group. I have given acknowledgements where ever
possible.
4. Important parameters
• Magnification: Image size/Object size
• Resolution: Minimum distance between two
objects that can still be distinguished by the
microscope.
5. Schematic of a simple optical microscope
Total visual magnification
MOBJ X MEYE
www.microscopy.fsu.edu
6. Rayleigh criterion for resolution
x ~ 0.2Δ μ
www.microscopy.fsu.edu ; www.imb-jena.de
Please check the first web site to watch a Java Applet on the dependence of Rayleigh criterion on λ of incident
radiation and on the numerical aperture.
7. THE ELECTRON MICROSCOPES
de Broglie : = h / mvλ
: wavelength associated with the particleλ
h: Plank’s constant 6.63 10^-34 J.s;
mv: momentum of the particle
m_e: 9.1 10^-31 kg; e 1.6 10^-19 coloumb
P.E eV = mv2
/2 => λ = 12.3/√VÅ
V of 60kV, λ= 0.05 Å => x ~ 2.5Δ Å
Microscopes using electrons as illuminating radiation
TEM & SEM
8.
9. Components of the TEM
1. Electron Gun: Filament, Anode/Cathode
2. Condenser lens system and its apertures
3. Specimen chamber
4. Objective lens and apertures
5. Projective lens system and apertures
6. Correctional facilities (Chromatic, Spherical, Astigmatism)
7. Desk consol with CRTs and camera
Transformers: 20-100 kV; Vacuum pumps: 10-6
– 10-10
Torr
10. Schematic of E Gun & EM lens
Magnification: 10,000 – 100,000; Resolution: 1 nm-0.2 nm
www.udel.edu
14. Electron scattering from specimen
• Resolution depends on spot size
• Typically a few nanometers
• Topographic scan range: order of mm X mm
• X rays: elemental analysis
www.unl.edu
15. Some SEM images
CNT in an array
Blood
platelet
Dia: 7µ
CNT: NASA nanotech group; Blood
cell: www. uq.edu. au
16. Scanning probe microscopy
• 1982 Binning & Rohrer, IBM
Zurich.
• STM, AFM & Family.
• Resolution:
Height: 0.01nm, XY: 0.1nm
• Local tip-sample interaction:
Tunneling (electronic
structure), Van der Waal’s
force, Electric/Magnetic fields.
• Advantages: atomic resolution,
non destructive imaging, UHV,
ambient/liquids, temperatures.
• Diverse fields: materials
science, biology, chemistry,
tribology.
www.spm.phy.bris.ac.uk
18. Operational modes and requirements
• Topography (conducting
surfaces and biological
samples).
• ST Spectroscopy (from IV
obtain the DOS).
• STP(spatial variation of
potential in a current carrying
film).
• BEEM (Interfacial properties,
Schottky barriers).
• Vibration isolation: 0.001nm
• Reliable tip - sample
positioning
• Electrical and acoustic noise
isolation
• Stability against thermal drift
• Good tips
• STM Mechanical stability
19. Electronics
• Current to voltage converter: Gain 108
-1010
• Bias Circuit
• Feedback Electronics: Error amplifier, PID
controller, few filters.
• Scan Electronics: +X -X +Y -Y ramp signals
(generated by the DA card).
• HV Circuit amplifies the scan voltages and the
feedback signal to ± 100 V from ± 10 V.
• Data acquisition and image display
22. More pictures
• 2.6 nm X 2.6 nm self
assembled organic
film. Molecular
resolution.
NASA nano group
• Quantum corral
Fe on Cu(111)
Courtesy: Eigler, IBM Almaden
23. Scanning tunneling spectroscopy
• dI/dV α DOS of sample
• J.C. Davis Group, Berkeley.
• Effect of Zn impurity on a
high Tc superconductor
• T: 250mK.
28. Acronyms galore!
• MFM: Magnetic force microscopy
• EFM: Electrostatic force microscopy
• TSM: Thermal scanning microscopy
• NSOM: Near field scanning optical
microscope
29. • Top-down techniques take a bulk material, machine it, modify it into the
desired shape and product
- classic example is manufacturing of integrated circuits
using a sequence of steps sush as crystal growth, lithography, deposition,
etching, CMP, ion implantation…
⇑
(Fundamentals of Microfabrication: The Science of
Miniaturization, Marc J. Madou, CRC Press, 2002)
⇓
• Bottom-up techniques build something from basic materials
- assembling from the atoms/molecules up
- not completely proven in manufacturing yet
Examples:
Self-assembly
Sol-gel technology
Deposition (old but is used to obtain nanotubes, nanowires, nanoscale films…)
Manipulators (AFM, STM,….)
3-D printers (http://web.mit.edu/tdp/www)
31. • Thermal evaporation
- Old technique for thin film dep.
- Sublimation of a heated material onto a substrate in a
vacuum
chamber
- Molecular flux = N0 exp
= activation energy
- heat sources for evaporation (resistance, e-beam, rf, laser)
• Sputtering
- The material to be deposited is in the form of a disk (target)
- The target, biased negatively, is bombarded by positive ions
(inert gas ions such as Ar+
) in a high vacuum chamber
- The ejected target atoms are directed toward the substrate
#
cm
2
.s
⎛
⎝
⎞
⎠ −φe /kT( )
φe
32.
33. • Versatile process for making ceramic and glass materials (powders, coatings,
fibers… variety of forms).
• Involves converting from a liquid ‘solution’ to a solid ‘gel’
• Start with inorganic metal salts or metal alkoxides (called precursors); series of
hydrolysis and polymerization reactions to prepare a colloidal suspension (sol).
• Next step involves an effort to get the desirable form
- thin film by spin or dip coating
- casting into a mold
• Further drying/heat treatment, wet gel is converted into desirable final product
• Aerogel: highly porous, low density material obtained by removing the liquid in
a wet gel under supercritical conditions
34. • Ceramic fibers can be drawn from the gel by adjusting the
viscosity
• Powders can be made by precipitation, or spray pyrolysis
• Examples
- Piezoelectric materials such as lead-zircomium-titanate (PZT)
- Thick films consisting of nano TiO2 particles for solar cells
- Optical fibers
- Anti-reflection coatings (automotive)
- Aerogels as filler layer to replace air in double-pane structures
35. • Check http://www.mit.edu/tdp/www
• Solid freeform fabrication, currently working only at sub-mm
level, is amenable for nanoscale prototyping
• Works by building parts in layers. Starts with a CAD model for
the structure
• Each layer begins with a thin distribution of powder spread over
the surface of a powder bed
• Technology similar to ink-jet printing
• A binder material selectively joins particles where the object
formation is desired
• A piston is lowered that leads to spreading the next layer
• Layer-by-layer process is repeated
• Final heat treatment removes unbound powder
• Allows control of composition, microstructure, surface structure
