[Pw_forum] workfunction and efield

[程迎春]

Thank you for your reply.
Recently I have read a paper published in Nano letters titled "Tuning the
Graphene work function by electric field effect"(
http://pubs.acs.org/doi/pdf/10.1021/nl901572a).  The main conclusion is that
the work function of graphene can be adjusted as the gate voltage tunes the
Fermi level across the charge neutrality point.  The tunable work function
makes graphene an attractive material for low contact barrier electrodes.
Motivated by this attractive application, I tried to calculate the work
function under external electric field.

Before the calculation, I firstly tried to understand the definition of the
work function and how it is calculated in Pwscf package.  The literature (J.
Phys.: condens. Matter 11(1999) 2689-2696) mentioned in the description of
the workFct_example directory shows the macroscopic-average method to the
calculation of work functions.  The work function is the minimum energy
required to extract one electron to an infinite distance form the surface
and its value is equivalent to the mean electrostatic potential energy
across the metal surface  minus the Fermi energy.  And thus the question
whether electric field can reflect the Fermi level of a certain material
surface arises.

The example31 shows how to perform electronic structure calculations using
pwscf package for a system undergoing the presence of a static homogeneous
finite electric field.  As a test, I firstly calculated the work function of
graphene at zero external electric field and the calculated value is
consistent with the previous published paper(PRL_94_236602) and the
experimental result.  Secondly, I calculated the work function at finit
external electric field, and then I use the pp.x and average.x to get the
work function as at the zero external efield.  It seems that the calculated
work function value is different from that under zero external efield.  But
I don't know whether the calculated work function value is reasonable.

My questions are as follows:
1) Whether the work function would be changed under a finite external
efield?
2) Can I use the method calculating work function under zero efield to
calculate work function under finite efield?
3) Can the plot number 11 in pp.x package deal with the potential under
finite efield?

Any suggestion or comment is appreciated. Thank you in advance.


sincerely,




2009/9/16 Stefano Baroni <baroni at sissa.it>

> Hi. before calculating anything, I think it is important to define and
> understand what one is going to calculate. Would you please give us a
> definition of "work function under an external field" and compare/contrast
> it to the accepted definition of "workfunction at zero field"? Thanks - SB
>   On Sep 16, 2009, at 3:53 AM, 程迎春 wrote:
>
>   Dear pwscf users,
>         The PWscf can calculate the workfunction in slab model and the
> electronic structure under external efield.  I am concerning that whether it
> it correct to calculate the workfunction of a certain materials under
> external efield?  Thank you in advance.
>
> --
> 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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>     ---
> Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center - Trieste
> http://stefano.baroni.me [+39] 040 3787 406 (tel) -528 (fax) /
> stefanobaroni (skype)
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-- 
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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