M. Meyyappan Director, Center for Nanotechnology NASA Ames Research Center Moffett Field, CA 94035

Transcription

1 M. Meyyappan Director, Center for Nanotechnology NASA Ames Research Center Moffett Field, CA web: 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.

3 OPTICAL MICROSCOPES Image construction for a simple biconvex lens

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 M OBJ X M EYE

6 Rayleigh criterion for resolution x ~ 0.2µ ; 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 : λ = h / mv λ: wavelength associated with the particle h: Plank s constant ^-34 J.s; mv: momentum of the particle m_e: ^-31 kg; e ^-19 coloumb P.E ev = mv 2 /2 => λ = 12.3/ VÅ V of 60kV, λ= 0.05 Å => x ~ 2.5 Å Microscopes using electrons as illuminating radiation TEM & SEM

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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: kv; Vacuum pumps: Torr

10 Schematic of E Gun & EM lens Magnification: 10, ,000; Resolution: 1 nm-0.2 nm

11 TEM IMAGES ; com ;

12 Schematic of SEM Physics dept, Chalmers university teaching material

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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

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.

17 Scanning tunneling microscope I α e -2κd I: Tunneling current; κ (decay const.) = 2mϕ/ h d: tip-sample distance ; spm.aif.ncsu.edu

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 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

20 STM Images HOPG: ambient Physics dept, IISc, India Si(7X7): UHV Courtesy: RHK Tech.

21 Nasa nano group

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 T c superconductor T: 250mK.

24 Scanning tunneling potentiometry Platinum film Physics dept, IISc, India

25 ATOMIC FORCE MICROSCOPE ;

26 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

27 AFM Images Mica: digital instruments; Grating:

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 (

30 Thermal evaporation Physical Sputtering Chemical (CVD) Plasma deposition Spin coating Dip coating Self-assembling monolayers Molecular beam epitaxy (can be physical or chemical) Laser ablation Sol-gel processing

31 Thermal evaporation - Old technique for thin film dep. - Sublimation of a heated material onto a substrate in a vacuum chamber # ( φ e / kt) - Molecular flux cm 2.s = N 0 exp φ e = 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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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 TiO 2 particles for solar cells - Optical fibers - Anti-reflection coatings (automotive) - Aerogels as filler layer to replace air in double-pane structures

35 Check 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