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CCH Optoelectronics Research Group Main Page

VIEWS: 2 PAGES: 19

									  Characterization of CNT using
  Electrostatic Force Microscopy

                       Vadim Karagodsky

 Probing induced defects in individual carbon nanotubes using electrostatic force
  microscopy, T. S. Jespersen et al., Appl. Phys. A 88, 309–313 (2007).

 Charge Trapping in Carbon Nanotube Loops Demonstrated by Electrostatic Force
  Microscopy, T. S. Jespersen et al., Nano Lett, 5, 1838-41 (2005).

 Characterization of Carbon Nanotubes on Insulating Substrates using
 Electrostatic Force Microscopy, T.S. Jespersen et al. Electronic Properties of Novel
 Nanostructures, 786, 135-138 (2005)
   AFM basic operation – oscillating mode
 The cantilever is oscillated
  at its resonant frequency.
 The oscillation is detected
  by the reflected laser at the
  photodiode.
 The atomic forces between
  the tip and the sample
  modulate the oscillation.
 The cantilever is raised
  until the oscillations get
  back to initial state.
 The raised distance is
  proportional to the surface
  topography.
AFM tip and resolution
AFM tip and resolution
AFM tip and resolution

            Atoms of sodium chloride
            sensed by AFM (2007).
              EFM - basic operation

Similar to AFM, but:
 Voltage Vs is applied.
 The tip is raised to tens nm
   above the surface.
 At this height, the atomic
  forces can be neglected but
  the Coulomb force remains.
 The information is
   obtained from phase shift.
 The Coulomb force has a
   longer range, and therefore
   the CNTs appear hugely
   amplified in diameter.        (Vs=-5V h=60nm)
                 AFM vs. EFM
AFM:
(-) Relatively slow
(-) Small scanning areas
        (tens m across)
(+) high resolution
        (CNT diameter)
(+) can work on
        conducting surfaces.
                 AFM vs. EFM
AFM:                           EFM:
(-) Relatively slow            (+) Rapid
(-) Small scanning areas       (+) Larger surfaces
        (tens m across)              (hundreds m across)
(+) high resolution            (+) Provides electrical info.
        (CNT diameter)         (-) low resolution.
(+) can work on                (-) Does not work on
        conducting surfaces.          conducting surfaces.
EFM – phase shift information
                      EFM – Basic theory
Harmonic oscillator equation:
d 2 z 0 dz
            0 2 z  A0 sin D t 
dt 2 Q dt
                        EFM – Basic theory
Harmonic oscillator equation:
d 2 z 0 dz
            0 2 z  A0 sin D t 
dt 2 Q dt
Solution:
z  Z 0 , D  sin D t   

Z 0 , D   A0     D  0         
                                           2
                          2        2
                                                0 2D 2 Q
              1 0D                              1              
   arctan                          arctan                 2Q
              Q D 2  0 2                    2Q /     2      
                        EFM – Basic theory
Harmonic oscillator equation:
d 2 z 0 dz
            0 2 z  A0 sin D t 
dt 2 Q dt
Solution:
z  Z 0 , D  sin D t   

Z 0 , D   A0     D  0         
                                           2
                          2        2
                                                0 2D 2 Q
              1 0D                              1                  
   arctan                          arctan                     2Q
              Q D 2  0 2                    2Q /         2      
Frequency shift and phase shift:
         F'          F'       Q      Q
    1 1                    F '   C ''VS 2
          k           2k      2 k     2 2k
                                                               
                        EFM – Basic theory
Harmonic oscillator equation:
d 2 z 0 dz
            0 2 z  A0 sin D t 
dt 2 Q dt
Solution:
z  Z 0 , D  sin D t   

Z 0 , D   A0     D  0         
                                           2
                          2        2
                                                0 2D 2 Q
              1 0D                              1                  
   arctan                          arctan                     2Q
              Q D 2  0 2                    2Q /         2      
Frequency shift and phase shift:
                                                                       0      60kHz
         F'          F'       Q      Q                             k       2.8N/m
    1 1                    F '   C ''VS 2
          k           2k      2 k     2 2k                            Q       225
                                                                      ||     <2 deg
                        EFM – Basic theory
Harmonic oscillator equation:
d 2 z 0 dz
            0 2 z  A0 sin D t 
dt 2 Q dt
Solution:
z  Z 0 , D  sin D t   

Z 0 , D   A0     D  0         
                                           2
                          2        2
                                                0 2D 2 Q
              1 0D                              1                  
   arctan                          arctan                     2Q
              Q D 2  0 2                    2Q /         2      
Frequency shift and phase shift:
                                                                       0      60kHz
         F'          F'       Q      Q                             k       2.8N/m
    1 1                    F '   C ''VS 2
          k           2k      2 k     2 2k                            Q       225
                                                                      ||     <2 deg
Experiments relying on EFM
  Quick estimation of CNT density
    using phase-shift histogram.
        Experiments relying on EFM
Proving that CNT loops can trap surface charges.
(a) Two loops initially contain charges
        Experiments relying on EFM
Proving that CNT loops can trap surface charges.
(a) Two loops initially contain charges
(b) The lower loop was discharged by grounded AFM tip.
        Experiments relying on EFM
 Identification of special CNT structures (loops) with
 respect to predefined alignment marks in large
 samples (100m X 100m).


Low resolution EFM scan
resolved the CNTs.




AFM scan with the same
resolution did not
resolve the CNTs.
               Conclusions
 EFM is a powerful technique for rapid
  characterization of CNTs on insulating surfaces.

 EFM can operate under ambient conditions.

 In the experiments reported here, EFM was found to
  be at least 100 times more time efficient than AFM.

 EFM provides electrostatic information that is not
  available through topographic AFM scans.

 EFM cannot be used on conducting surfaces.

 EFM does not provide CNT diameter information.

								
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