M. Meyyappan Director, Center for Nanotechnology NASA Ames Research Center Moffett Field, CA 94035 email: email@example.com web: http://www.ipt.arc.nasa.gov Guest Lecturer: Dr. Geetha Dholakia Nanoscale Imaging T ools
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 hav e 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
Rayleigh criterion for resolution Δx ~ 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 : = h λ / 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/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
Schematic of E Gun & EM lens Magnification: 10,000 – 100,000; Resolution: 1 nm 0.2 nm
Physics dept, Chalmers university teaching material
Electron scattering from specimen www.unl.edu • Resolution depends on spot size • Typically a few nanometers • Topographic scan range: order of mm X mm • X rays: e lemental analysis
Some SEM images CNT in an array Blood platelet Dia: 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.
Scanning tunneling microscope I e-2 d I: Tunneling current; (decay const.) = 2m / h d: tip sample distance
www.mpi halle.mpg.de ; spm.aif.ncsu.edu
Operational modes and requirements • Topography (conducting • Vibration isolation: 0.001nm surfaces and biological • Reliable tip sample samples). positioning • ST Spectroscopy (from IV • Electrical and acoustic noise obtain the DOS). isolation • STP(spatial variation of • Stability against thermal drift potential in a current carrying film). • Good tips • BEEM (Interfacial properties, • STM Mechanical stability Schottky barriers).
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
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.kuleu ven.ac.be ; www.chem.sci.gu.edu.au
AFM modes of operation • Contact mode Force: nano newtons • Non contact mode Force: femto newtons Freq. of oscillation 100kHz • Intermittent contact • Image any type of sample.
Park Scientific handbook
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 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 prin ters (http://web.mit.edu/tdp/www)
Thermal evaporation • Spin coating • Physical Sputtering • Dip coating • Chemical (CVD) • Self assembling • Plasma deposition monolayers • Molecular beam epitaxy (can be physical or chemical) • Laser ablation • Sol gel process ing
• Thermal evaporation Old technique for thin film dep. Sublimation of a heated material onto a substrate in a vacuum ⎛ #
⎞ chamber cm2 ⎝ .s ⎠ (−φ/kT) e Mole φe cular flux = N exp 0 = 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. € € €
• 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 TiO particles for solar cells 2 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 c
ontrol of composition, microstruc ture, surface structure