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Electrostatic properties of carbon nanotubesand CNT-based devices
using scanning force techniques
Thierry MELIN
Institut d’Electronique, de Microélectroniqueet de Nanotechnologie
IEMN CNRS-UMR8520 Villeneuve d’Ascq FRANCE
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Thierry MELIN, Heinrich DIESINGER CNRS research scientists
M. Zdrojek (2006), D. Brunel (2008) PhD studentsS. Barbet (2008), L. Borowik
D. Deresmes Technical support
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Our aim :
Probing the electrostatics of nanostructures and nanodevicesusing electrical techniques derived from atomic force microscopy
Electric Force Microscopy (EFM) detecting chargesKelvin Force Microscopy (KFM) probing surface potentials
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I - Electrostatics of individual nanotubes (SWCNTs, MWCNTs)
II - Coupled transport and electrostatic measurements (SWCNTs)[ carbon nanotubes as memory devices ]
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I - Electrostatics of individual nanotubes (SWCNTs, MWCNTs)
II - Coupled transport and electrostatic measurements (SWCNTs)
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Charge injection - Charge detection
Charge injection Charge detection (EFM)
Q Q
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Principle of charge detection
Force gradient on the tip
Fz = Fz(zo)+ gradz Fz .(z - zo)
“ - Δk ”
Q
cantilever resonance frequency shift
fexc
effective change in cantilever stiffness
2π f0= km
sensitivity down to 1e in specific cases
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Illustration : silicon nanoparticles 1/2
Topographyimage
EFM image(VEFM=-8V)
50 nm scale 40 Hz scale
Uncharged silicon nanoparticles (capacitive interaction)
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Illustration : silicon nanoparticle 2/2
+
+ ++ +
- -- -
EFM (Δf0) 300x300nm
- 2V
+2V
Injection @+6V(~ +170 e)
+ ++ +
- -- -
- 2V
+2V
Injection @ -6V(~-170 e)
-
-
+
Silicon nanoparticles after charge injection
T. Mélin et al., Appl. Phys. Lett., 78 5054 (2002)
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Charge injection in carbon nanotubes
Charge injection Vinj=-5V Delocalized charge patternDifferent FWHMs between chargeand capacitive EFM patterns
~30 nm diameter MWCNT VEFM=-3V, 10Hz color scale
M. Zdrojek et al., J.Appl Phys (2006)
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Abrupt discharge phenomena
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• Abrupt discharges when scanning along the nanotubes
[MWCNT with 18 nm diameter, VEFM=-2V, 20Hz color scale]
AFM EFM EFM
[MWCNT with 18 nm diameter, Vinj=-7V (3 min) detection VEFM=-3V]
Abrupt discharges in MWCNTs
MWCNT with 21 nm diameter, Vinj=-5V (2 min) VEFM=-3VAFM : 50 nm color scale ; EFM : 20 Hz color scale
Preferential charge emission sites with density : < a few µm-1
MWCNTs can carry an out-of-equilibrium charge on SiO2 !!
M. Zdrojek et al., J. Appl. Phys. 100 114326 (2006)
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MWCNTs versus SWCNTs
SWCNT 3.0 nm diameter
Charge injection : Vinj=-6V, 30s, charge detection : VEFM=-3V
distance [µm]0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
distance [µm]0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
MWCNT 15.0 nm diameter
nanotube charge signal
M. Zdrojek et al., Phys. Rev. Lett. 96 039703 (2006)
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Charging mechanisms
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•
•
Charging mechanisms ?
Capacitive responseto the tip bias
??
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SiO2e=200nm
dCNT
Intrashellconduction
Intershellconduction
Vinj
Transverse dielectric properties of nanotubes
⇒ Response to the electric fieldgenerated by the EFM tip
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substrate
oxide
MWCNT
EFM tip-eVinj
0
Energy
substrate
oxide
MWCNT
EFM tipΔV
Response to the electric field at the EFM tip : λ = 2 πε0 ε / ln(4e/dCNT) . ΔV
Energy diagrams during charge injection
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Capacitive prediction λ = 2πε0 ε / ln(4e/dCNT)
(a) analytical ε=(εox+1)/2 (b) numerical calculations
Experimental data points
and (c) :
λ = πε0 ε / ln(4e/dCNT) . Etip. dCNT
Comparison with experiments
M. Zdrojek et al., Phys. Rev. B 77 033404 (2008)
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Inner-shell charging of MWCNTs
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• Charge storage in the first inner metallic shells
(coll. A. Mayer, Univ Namur, Belgium)
Inner-shell charging of MWCNTs
M. Zdrojek et al., Phys. Rev. B 77 033404 (2008)
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Electron interactions in SWCNTs
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Charge distribution in SWCNTs 1/3
in vacuum on SiO2
-e-e
Charge accumulationat the nanotube ends ?
φ= charge enhancement factorRatio of the charge density
at the end of the CNT (10%L) with respect to its middle
[ coll. Z. Wang, M. Devel,Univ. Franche Comté France ]
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Charge distribution in SWCNTs 2/3
Experimental evidencefor charge enhancement at the CNT cap
Nanotube radius (nm)E
nhan
cem
entf
acto
r
Enhancement as a function of the CNT radius
Z. Wang et al., Phys. Rev. B 78 085425 (2008)
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I - Electrostatics of individual nanotubes (SWCNTs, MWCNTs)
II - Coupled transport and electrostatic measurements (SWCNTs)- description of experiments- analysis of coupled transport and KFM measurements- model
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I - Electrostatics of individual nanotubes (SWCNTs, MWCNTs)
II - Coupled transport and electrostatic measurements (SWCNTs)- description of experiments- analysis of coupled transport and KFM measurements- model
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Electrostatic excitation : ac+dc voltage to the AFM tip
Principle of surface potential measurementsusing Kelvin force microscopy
~Vdc
Vac
Electrostatic force
1ω force component
Nullification of the 1ωforce component
Vdc=Vs
Measurementof the surface potential
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AFM image
Vds=-3V
Coupled tranport and KFM experiments
CNT-FET subjected to a local charge perturbation
Transportmeasurements
electrostaticmapping
EFM,KFM
charge injection in the SiO2 layer
I(Vg)
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Vinj=-6V
AFMKFM (before injection)
0V
1,5V
1 µm
KFM (after injection)
Experimental procedure
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I - Electrostatics of individual nanotubes (SWCNTs, MWCNTs)
II - Coupled transport and electrostatic measurements (SWCNTs)- description of experiments- analysis of coupled transport and KFM measurements- model
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AFM
AFM
Vinj = -6V
Negative charge injection close to the CNT-FET
~-200e at ~200nm from the CNTPositive effective gate voltage shift (!)
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AFM
AFM
Vinj = +6V
Positive charge injection close to the CNT-FET
~+200e at ~200nm from the CNT Negative effective gate voltage shift (!)
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KFM images (Vinj>0)
AFM KFMKFMBefore injection After injectionBefore injection After injectionAFM KFMKFMBefore injection After injectionBefore injection After injection
Vinj = 6V
50 mV
200 mV
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Vs
(mV
)
Nanotube response probed by KFM (Vinj>0)
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Vinj = +6V
Vs
(mV
)
Nanotube response probed by KFM (Vinj>0)
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AFM KFMKFMBefore injection After injectionBefore injection After injection
Injection at Vinj = -6V
Vinj = -6V
50 mV
200 mV
KFM images (Vinj<0)
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Nanotube response probed by KFM (Vinj<0)
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Vinj = -6V
Nanotube response probed by KFM (Vinj<0)
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I - Electrostatics of individual nanotubes (SWCNTs, MWCNTs)
II - Coupled transport and electrostatic measurements (SWCNTs)- description of experiments- analysis of coupled transport and KFM measurements- model
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Qualitative model
gate
Local charge
S D
RC RC’Ctube-lc
Clc-gate
Q+ Ctube-gate
Q- Q-Q-
- Nanotube coupled to electrodes by tunnel contacts- screening of the local (Q+) charge by (Q-) charges from the electrodes- delocalized (Q-) charge emission pattern along the nanotube
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Sensitivity of CNTs as charge detectors
Charge over distance
Gat
evo
lata
gesh
ift In our case :100e@100nm
Additional exp. point :Single charge
in a nanocrystal closeto the CNT channel
(1µm-thick oxide device)[Gruneis et al. Nanolett. (2007)]
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- Electrostatics of individual nanotubes (SWCNTs, MWCNTs)
-Coupled transport and electrostatic measurements[ carbon nanotubes as memory devices ]
Conclusion - Prospects
Electrostatic properties of CNTs and CNT-devicesusing scanning force techniques
Prospects
UHV – AFM (EFM, KFM etc …) low-temperature measurements
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AFM
KFM
AFM
KFM
-0,2V
-0,1V
Vd = 0V Vs = 0V Vd = 0,15 V Vs = 0VVg = 0V Vg = 0V
300 nm
Prospects