M. MeyyappanDirector, Center for Nanotechnology
NASA Ames Research CenterMoffett Field, CA 94035
email: [email protected]: http://www.ipt.arc.nasa.gov
Guest Lecturer: Dr. Geetha DholakiaNanoscale Imaging Tools
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.
OPTICAL MICROSCOPES
Image construction for a simple biconvex lens
Important parameters
• Magnification: Image size/Object size
• Resolution: Minimum distance between two objects that can still be distinguished by the microscope.
Schematic of a simple optical microscope
Total visual magnification
MOBJ X MEYE
www.microscopy.fsu.edu
Rayleigh criterion for resolutionΔx ~ 0.2μ
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 of incident
radiation and on the numerical aperture.
THE ELECTRON MICROSCOPES
de Broglie : λ = h / mv λ: wavelength associated with the particle
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 => Δx ~ 2.5 Å
Microscopes using electrons as illuminating radiation
TEM & SEM
Components of the TEM
1. Electron Gun: Filament, Anode/Cathode2. Condenser lens system and its apertures3. Specimen chamber4. Objective lens and apertures5. Projective lens system and apertures6. Correctional facilities (Chromatic, Spherical, Astigmatism)
7. Desk consol with CRTs and camera
Transformers: 20-100 kV; Vacuum pumps: 10-6 – 10-10 Torr
Schematic of E Gun & EM lens
Magnification: 10,000 – 100,000; Resolution: 1 nm-0.2 nm
www.udel.edu
Schematic of SEM
Physics dept, Chalmers university teaching material
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
Some SEM imagesCNT in an array
Blood plateletDia: 7
CNT: NASA nanotech group; Blood cell: www. uq.edu. au
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
Scanning tunneling microscope
I: Tunneling current; (decay const.) = 2m/ h
d: tip-sample distancewww.mpi-halle.mpg.de ; spm.aif.ncsu.edu
I e-2d
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
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
STM Images
HOPG: ambient
Si(7X7): UHVCourtesy: RHK Tech.
Physics dept, IISc, India
Nasa nano group
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
Scanning tunneling spectroscopy
• dI/dV DOS of sample
• J.C. Davis Group, Berkeley.
• Effect of Zn impurity on a
high Tc superconductor
• T: 250mK.
Scanning tunneling potentiometry
Platinum film
Physics dept, IISc, India
ATOMIC FORCE MICROSCOPE
www.fys.kuleuven.ac.be ; www.chem.sci.gu.edu.au
AFM modes of operation
• Contact mode
Force: nano newtons • Non-contact modeForce: femto newtons
Freq. of oscillation 100kHz
• Intermittent contact• Image any type of
sample.
Park Scientific handbook
AFM Images
Mica: digital instruments; Grating: www.eng.yale.edu
Acronyms galore!
• MFM: Magnetic force microscopy
• EFM: Electrostatic force microscopy
• TSM: Thermal scanning microscopy
• NSOM: Near field scanning optical microscope
• Top-down techniques take a bulk material, machine it, modify it into the desired shape and product
- classic example is manufacturing of integrated circuitsusing 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)
• Physical
• Chemical (CVD)
• Plasma deposition
• Molecular beam epitaxy(can be physical or chemical)
• Laser ablation
• Sol-gel processing
Thermal evaporation
Sputtering• Spin coating
• Dip coating
• Self-assembling monolayers
• Thermal evaporation- Old technique for thin film dep.- Sublimation of a heated material onto a substrate in a
vacuumchamber
- 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
where they are deposited.
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• 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
• 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
• 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