Characterization of Nanomaterials… And the magnification game!

During today’s notes, there will be a picture every other slide During today’s notes, there will be a picture every other slide. Try to guess what common household item you’re looking at (it has been magnified quite a bit!!!)

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Observations and Measurement: Studying physical properties related to nanometer size Needs: Extreme sensitivity Extreme accuracy Atomic-level resolution http://www.viewsfromscience.com/ documents/webpages/nanocrystals.html

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Characterization Techniques Structural Characterization Scanning electron microscopy (SEM) Transmission electron microscopy (TEM) X-ray diffraction (XRD) Scanning probe microscopy (SPM) (includes AFM) Chemical Characterization Optical spectroscopy Electron spectroscopy Structure of butterfly wing

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Structural Characterization Techniques are already used for crystal structures X-Ray Diffraction Many techniques are already used for studying the surfaces of bulk material (They provide topographical images) Scanning Probe Microscopy (AFM & STM) Electron Microscopes http://www.nanowerk.com/nanotechnology/videos/Nano_The_next_dimension.php (5-10) DEMO: Lattice model & laser/skewer

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Electron Microscopes Are used to count individual atoms What can electron microscopes tell us? Morphology Size and shape Topography Surface features (roughness, texture, hardness) Crystallography Organization of atoms in a lattice

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Crystallography Crystallography affects properties Crystals have atoms in ordered lattices Amorphous: no ordering of atoms Crystallography affects properties Properties: strength; electrical; etc.

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Microscopes: History Light microscopes Electron Microscopes 500 X to 1500 X magnification Resolution of 0.2 µm Limits reached by early 1930s Optical microscopes have a resolution limit of 200 nm, meaning they cannot be used to measure objects smaller than 200 nm. (wavelength of visible light ~400 nm). Electron Microscopes Use focused beam of electrons instead of light * Transmission Electron Microscope (TEM) * Scanning Electron Microscope (SEM) http://en.wikipedia.org/wiki/File:Optical_microscope_nikon_alphaphot_%2B.jpg

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Electron Microscopy Steps to form an image: Stream of electrons formed by an electron source and accelerated toward the specimen Electrons confined and focused into thin beam Electron beam focused onto sample Electron beam affected as interacts with sample Interactions / effects are detected Image is formed from the detected signals

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Electron Microscopes Electron Beam Sample Detection Accelerated and focused using deflection coils Energy: 200 - 1,000,000 eV Sample TEM: conductive, very thin! SEM: conductive http://video.google.com/videoplay?docid=-5489601762301542658&q=transmission+electron+ microscope&total=6&start=0&num=10&so=0&type=search&plindex=1 http://www.vcbio.science.ru.nl/en/image-gallery/electron/ Detector – CRT For SEM, beam rastered over surface Detection TEM: transmitted e- SEM: emitted e- Source: Virtual Classroom Biology

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EM Resolution Wavelength dependent on: Resolution dependent on: wavelength of electrons () NA of lens system Wavelength dependent on: Electron mass (m) Electron charge (q) Potential difference to accelerate electrons (V) d = resolution (spacing so that features don’t blur together) n = index of refraction of medium btw source and lens(1 in perfect vacuum) Alpha: half angle of cone of light from specimen plane accepted by the object (radians) – very small for EMs (spot size??) N sin a = NA (numerical aperture)

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Transmission EM Magnification: ~50X to 1,000,000X E-beam strikes sample and is transmitted through film Scattering occurs Unscattered electrons pass through sample and are detected http://en.wikipedia.org/wiki/File:MicroscopesOverview.svg Elastic Scattering No energy loss Diffraction patterns Inelastic Scattering Occurs at heterogeneties (defects, grain boundaries) http://www.hk-phy.org/atomic_world/tem/tem04_e.html Source: Wikipedia

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Scanning EM Magnification: ~10X to 300,000X E-beam strikes sample and electron penetrate surface Interactions occur between electrons and sample Electrons and photons emitted from sample Emitted e- or photons detected http://en.wikipedia.org/wiki/File:MicroscopesOverview.svg Numerical aperture http://www.youtube.com/watch?v=bfSp8r-YRw0 http://www.csmate.colostate.edu/cltw/cohortpages/viney_off/atomhistory.html http://virtual.itg.uiuc.edu/training/EM_tutorial/#segment 1_6 Source: Wikipedia

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SEM: Electron Beam Interactions Valence electrons Inelastic scattering Can be emitted from sample “secondary electron” Atomic nuclei Elastic scattering Bounce back - “backscattered electrons” Core electrons Core electron ejected from sample; atom excited To return to ground state, x-ray photon or Auger electron emitted + valence e- core e- nucleus Valence e-: E transferred to atom’s electron, if enough E transferred, can be emitted from sample; these secondary electrons have E <50 eV Atomic nuclei: e- bounce off with same energy; samples with high atomic numbers cause more backscattering

Electron Spectroscopy Auger e- Ground state e- emitted; excited state Relaxes to ground state Energy X-ray e- or photon strikes atom; ejects core e- e- from outer shell fills inner shell hole Energy is released as X-ray or Auger electron EDS: Energy Dispersive X-ray Spectroscopy AES: Auger Electron Spectroscopy http://www.youtube.com/watch?v=BsLpwGam_gI (2:30 for electron spectroscopy) Emitted energy is characteristic of a specific type of atom because each atom has its own unique electronic structure and energy levels.

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Electron Spectroscopy Emitted energy is characteristic of a specific type of atom Each atom has its own unique electronic structure and energy levels AES is a surface analytical technique <1.5 nm deep AES can detect almost all elements EDS only detects elements Z > 11 EDS can perform quantitative chemical analysis To study deeper using AES, much etch, repeat, etch, repeat

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SEM and TEM Comparison SEM makes clearer images than TEM SEM has easier sample preparation than TEM TEM has greater magnification than SEM SEM has large depth of field SEM is often paired with detectors for elemental analysis (chemical characterization)

SEM and TEM Data Images Ag thin film deposited on Si substrate (thermal or e-beam evaporation) TCNQ (7,7,8,8-tetracyanoquinodimethane) powder and Ag thin film are enclosed in a vacuum glass tube, then heated in a furnace. http://nami.eng.uci.edu/projects/Agtcnq.htm

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Chemical Characterization Optical Spectroscopy Absorption and Emission Photoluminescence (PL) Infrared Spectroscopy (IR or FTIR) Raman Spectroscopy Electron Spectroscopy Energy-Dispersive X-ray Spectroscopy (EDS) Auger Electron Spectroscopy (AES)

Optical Spectroscopy: Absorbance/Transmittance Absorbance: electron excited from ground to excited state Emission: electron relaxed from excited state to ground state Transmittance: “opposite” of absorbance: A = -log(T) Radiation only penetrates ~50 nm N&N Fig. 8.10

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Scanning Probe Microscopy (SPM) AFM & STM Measure forces Many types of forces (dependent on tip) Electrostatic Force Microscopy Distribution of electric charge on surface Magnetic Force Microscopy Magnetic material (iron) coated tip magnetized along tip axis Scanning Thermal Microscopy Scanning Capacitance Microscopy Capacity changes between tip and sample

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Scanning Tunneling Microscopy (STM) Developed by Binnig and Rohrer in 1982 Tunneling Very dependent on distance between the two metals or semiconductors By making the distance 1 nm smaller, tunneling will increase 10X

Scanning Tunneling Microscopy (STM) Instrument: Scanning Tip Extremely sharp Metal or metal alloys (Tungsten); Conductive Mounted on a stage that controls position of tip in all three dimensions Typically kept 0.2 - 0.6 nm from surface Tunneling Current: ~ 0.1 - 10 nA Resolution: 0.01 nm (in X and Y directions) 0.002 nm in Z direction Source: Univ. of Michigan

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Scanning Tunneling Microscopy (STM) Constant Current Mode: As tip moves across the surface, it constantly adjusts height to keep the tunneling current constant Uses a feedback mechanism Height is measured at each point Constant Height Mode: As tip moves across surface, it keeps height constant Tunneling current is measured at each point Constant current: directly related to electron charge density (not just topography) Constant height: faster (no feedback loop)

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Atomic Force Microscopy (AFM) Can be used for most samples Measures: Small distances: Van der Waals interactions Larger distances: Electrostatic interactions (attraction, repulsion) Magnetic interactions Capillary forces (condensation of water between sample and tip) Source: photonics.com Source: Nanosurf

Atomic Force Microscopy (AFM) http://virtual.itg.uiuc.edu/training/AFM_tutorial/ Scan tip across surface with constant force of contact Measure deflections of cantilever

Scanning Probe Techniques Some instruments combine STM and AFM Other tip-surface force microscopes: Magnetic force microscope Scanning capacitance microscope Scanning acoustic microscope Uses: Imaging of surfaces Measuring chemical/physical properties of surfaces Fabrication/Processing of nanostructures Nanodevices http://virtual.itg.uiuc.edu/training/AFM_tutorial/

Summary: Techniques used to study nanostructures Bulk characterization techniques Information is average for all particles Surface characterization techniques Information about individual nanostructures Bulk: XRD Surface: SPM, TEM

Mosquito Eye 1

Salt & Pepper 2

Deer Tick Head 3

Foot of House Fly 4

Porcupine Quill 5

Tip of Dentist’s Drill 6

Toilet Paper 7

Velcro 8

Staple in Paper 9

Scratch & Sniff Paper 10

Needle & Thread 11

12 Toothbrush bristles

13 Used Q-tip

Dust 14

Used Dental Floss 15

Human Eyelashes 16

Instant Coffee Granule 17

Butterfly Wing 18

19 Red Blood cells & Virus