Tunneling Phenomena in NanoParticle Composites

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Tunneling Phenomena in Nano-Particle Composites
REU Intern Alex Di Sciullo Jones Mentor Dr.
Adam Huang Graduate Student Feng Pan and Arun
Madyala
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Background

• Study interactions contributing to conductivity
  in a nano-composite
• Conductive nano-particles in an insulative matrix
• For experimentation
• Nano-particles Carbon Black (CB)
• Matrix Silicone elastomer

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Background ctd.

• Percolation Threshold vs. Tunneling
• Percolation threshold particles assumed to be in
  contact with each other, electrons flow through
• Tunneling particles separated by insulator, jump
  from particle-to-particle across composite

Insulator
Conductor
Current

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Purpose

• Problem tunneling is not thought to exist in
  particles beyond around 10nm apart
• Under 35 CB, spacing larger and yet material is
  still conductive
• If not tunneling, what is responsible

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Method

• Take images using electrical microscopy
• Surface Potential
• Electric Field Microscopy (EFM)
• Observe changes in interaction between CB
  particles at different s

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Atomic Force Microscope (AFM)

• Function
• Cantilever passes over a sample (contact,
  tapping, lift)
• Laser reflects off cantilever and signal received
  by photodetector
• Signal processed
• Height, amplitude, phase
• Uses
• Topography
• Electric Field
• Surface Potential

Picture from http//www3.physik.uni-greifswald.de/
method/afm/eafm.htm
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Atomic Force Microscope Ctd.
Camera
Head
Cantilever Holder
Microscope
Stage
Probe
Front View of AFM
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Topographical Scan

• Contact
• Probe dragged along surface to get data
• Microfilms with Feng Pan
• Tapping
• Probe oscillates near resonant frequency
• Data taken when probe comes in contact with sample

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Samples

• Plastic cartridges
• Hold composite sample
• Two leads to control applied voltage
• Composite
• Percentages range from 6 to 20 CB
• Surface flat pressed against glass slide

Composite
Plastic
Leads
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Sample Topography

• Tapping mode
• Soft sample difficult to scan
• Amplitude images of CB composite
• Signal perceived by photodetector

8 CB
15 CB
20 CB
All scanned images have scan width of 5µm
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Scan Surface Potential

• Surface-Mount Resistor
• Hard but rough sample
• Ideally uniform resistance calibration
• Should no see topographical image in surface
  potential
• See flat image with constant slope of
  increasing/decreasing voltage

Potential cross-section
Same Scan
10V across sample
Height
Surface Potential
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Scan for Electric Field

• Scan 7 CB sample for electric field
• Lift mode
• Only some areas visible through EFM

Same Scan
9 V bias applied through probe (sample grounded)
No bias applied
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Future Work

• Take time to become proficient at scanning for
  surface potential EFM
• Scan surface potential of CB composite samples
• Make observations how CB concentration changes
  the images

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To Close

• Thank you for your time and Im glad to answer
  any questions you might have.