[Pw_forum] workfunction and efield

[Stefano Baroni]

On Sep 16, 2009, at 3:17 PM, 程迎春 wrote:

> 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

which equals plus or minus infinity, in the presence of an applied  
electric field ...

> and its value is equivalent to the mean electrostatic potential  
> energy across the metal surface  minus the Fermi energy.

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 ...

> And thus the question whether electric field can reflect the Fermi  
> level of a certain material surface arises.

the above statement is beyond my understanding capabilities ...

>
> 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?

this question does not have an answer until one understand what is  
meant by "work function in a finite field" (the usual definition does  
not apply)

> 2) Can I use the method calculating work function under zero efield  
> to calculate work function under finite efield?

NO, because of the above: the electrostatic potential does not tend to  
a cosnatant far from the surface

> 3) Can the plot number 11 in pp.x package deal with the potential  
> under finite efield?

no idea whatsoever

SB

---
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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