Nanomaterials

Taught by: Dr. Jain Lin
Taken: SS 2015

Exam 2 Review

Characterization of Nanomaterials

Contrast Enhancement

Brightfield

  • Sensors placed parallel to direct beam
  • Makes background brighter than object

Darkfield

  • Sensor place at an angle/offset from direct beam
  • Makes object brighter than background

Microscope Resolution

  • White light resolution ~ 300nm
  • Numerical Aperture (NA) ~ 1.2-1.5 for and optical microscope
  • Wavelength \downarrow \Rightarrow resolution \uparrow

Electron-Specimen Interaction

  • BSE and SE come off top of specimen
  • Scattered and unscattered electrons transmit through specimen
  • X-rays off top of specimen

Back Scattered Electrons (BSE)

  • Scattering angle > 180^{\circ}
  • Atomic number \uparrow \Rightarrow BSE \uparrow \Rightarrow brighter image

Secondary Electrons (SE)

  • Collision between incoming electrons and loosely bonded electrons

Electron Microscope

Scanning Electron Microscope (SEM)

  • Collects BSE and SE
  • Provides morphology information

Energy Dispersive X-Ray Spectroscopy (EDS)

  • X-rays emitted during SEM
  • Provides element information (element mapping)

Transmission Electron Microscope (TEM)

  • Collects BSE and SE like SEM but also collects transmitted electrons
  • Provides crystal structure in addition to the information SEM provides

X-Ray Photoelectron Spectroscopy (XPS)

  • Kinetic energy of electrons to reveal binding energies
  • Determines elements by binding energies
  • Can differentiate between isotopes

X-Ray Diffractometer (XRE)

  • Detects phase shifts in x-rays through a material
  • Provides information on crystal structures (bond length, bond angle, atom positions)

Scanning Probe Microscopy

Atomic Force Microscopy (AFM)

  • Drags a probe across the tops of a specimen
  • Measures tomography and some mechanical properties

Scanning Tunneling Microscopy (STM)

  • Measures electric current between a probe and the specimen
  • Provides surface information

Nano Composites

Polymer vs. Ceramic vs. Metal

  • Ceramic = high strength, low toughness
  • Polymer = high toughness, low strength
  • Metal = medium strength, medium toughness

What Are Composites

  • Consists of a matrix material base and added reinforcement material
  • Create new and improved properties from the combination of materials

Polymer/Nano Clay

Phase Separated (Microcomposites)

  • Polymer is unable to enter silicate
  • Poor mixing leaves regions properties largely dependant on single materials

Intercalated (Nanocomposite)

  • Single polymer inbetween silicate layers
  • Overall material properties starting to not have localized zones

Exfoliated/Delaminated

  • Silicate layers are dispersed in polymer matrix (complete mixing)
  • Properties are very different even compared to the microcomposite

XRE of Possible Polymer/Nano Clay Samples

d = \frac{\lambda}{2\sin{\theta}}

d - Distance bewteen layers
\lambda - Wave length
\theta - Phase angle

\theta \uparrow \Rightarrow d \downarrow

XRE: Intensity vs. Phase Angle
XRE: Intensity vs. Phase Angle

A = Phase Separated
B = Partially Exfoliated
C = Fully Exfoliated
D = Intercalated

CNT's/Polymer Nanocomposites

  • Carbon nanotubes are favorable due to high aspect ratio and low percolation threshold
  • CNT's need to be well dispersed otherwise they crate high stress concentration sites and lower the mechanical properties
  • Improve mechanical properties by exceeding aspect ratio threshold for CNT/Polymer composites
  • Pass percolation threshold to significantly increase electrical properties

Interface and Load Transfer

  • Changes structure, phase transitions, molecular mobility
  • AFM can measure its shear force/stress/strength
  • Can deflect cracks away from center material

Nanosensors

Why Nanosensors

  • Large surface to volume ratio
  • Debye length comparable to radius
    • Debye length is the part of the material that contrubutes to electron transport
  • Can travel from interior to surface faster

Parts Needed for a Sensor

  • Detectors
  • Receptors
  • Conductors

Preferable Sensor Characteristics

  • Sensitivity
    • How much does the measured value change
  • Response time
    • How quickly does the measured value change when stimulated
  • Recover time
    • How quickly does the measured value return to steady state after the stimulus has been removed
  • Selectivity
    • Does the sensor react to stimulii other than what you're trying to sense
  • Cyclic performance
    • Will the sensor continue to work repeatedly

Example

CNT, Field Effect Transistor

  • Detects a resistance change of the CNT when gasses or other molecules bind to the outside