[Pw_forum] workfunction and efield

[程迎春]

> Efermi - average potential far from the surgace
> the average potential far from the surface does not approach a constant in
> the presence of an electric field, so you have to understand exactly what
> the experimentalists measure,
> when you are done, I would be pleased to learn that ...
>
In the experiment, they apply scanning kelvin probe microscope(SKPM)
techniques to demonstrate that the work function can be controlled over a
wide range by electric field effect (EFE).   SKPM is an atomic force
microscope (AFM) based experimental technique can map the surface potential
variation of a sample surface relative to that of a metallic tip.  The
relative surface contact potential difference measured can be related to the
difference in work function between the metallic tip and graphene sample
surface.  The work function of the metallic tip is insensitive to the gate
voltage due to the large density of states near the Fermi level.  So the
variation of relative surface contact potential difference reflects the EFE
modulation of work function of graphene sample.  The authors conclude that
the behavior of EFE tuned work function of graphene sample can be explained
by the change of Efermi in graphene sample.


Now I understand that in the experiment the local surface potential is
measured.  The local surface potential corresponds to the work function
difference between the metal tip and the graphene surface .  The general
definition of work function is : average potential far from the surface -
Efermi.  Though the average potential far from the surface does not approach
a constant in the presence of an electric field, the difference of the
average potential between the metal tip and graphene equal zero.  So local
surface potential is : Efermi(metal tip) - Efermi(graphene).  So I think
that the title of the paper published in Nano letters "Tuning the graphene
work function by EFE" means "Tuning the Fermi energy level of graphene by
EFE".

Thank Prof. Stefano Baroni very much.
 <http://www.gnu.org/philosophy/no-word-attachments.html>




-- 
Y. C. Cheng
Department of Physics
Nanjing University
Nanjing 210093
P. R. China
Tel: 86-25-83592907
Email: yccheng.nju at gmail.com
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