M. MeyyappanDirector, Center for Nanotechnology

NASA Ames Research CenterMoffett Field, CA 94035

email: meyya@orbit.arc.nasa.govweb: 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

TEM IMAGES

www.udel.edu ; www.nano-lab. com ; www.thermo.com

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