On Sep 16, 2009, at 3:17 PM, ??? 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) La morale est une logique de l'action comme la logique est une morale de la pens?e - Jean Piaget Please, if possible, don't send me MS Word or PowerPoint attachments Why? See: http://www.gnu.org/philosophy/no-word-attachments.html -------------- next part -------------- An HTML attachment was scrubbed... URL: http://www.democritos.it/pipermail/pw_forum/attachments/20090916/39255da2/attachment.htm
