[QE-users] Dangling bonds with CdS

[Giuseppe Mattioli]

Dear Tamas and Kumar
only to add a few words to the (very pertinent) Tamas' reply:
1) Semiconductor surfaces can undergoes very complex reconstruction  
patterns (the 7x7 Si(111) reconstruction being likely the most famous  
case). You will not find complex reconstructions by simply cutting and  
relaxing the slabs, and it is best to search in experimental  
literature, in order not to waste time simulating unphysical systems  
which are never going to converge.

2) Dangling bonds can be also created inside crystals (e.g, by atomic  
vacancies). It is not clear what kind of dangling bond you are  
referring to.

3) There is an additional problem when you want to simulate some  
process happening on the surface (e.g., adsorptions of molecules).  
Various strategies can be used, including the saturation of dangling  
bonds on one side of the slab by "pseudohydrogen atoms" having  
fractional charge mimicking that of the pristine bond broken by  
cleavage.

This said, you should "make an educated guess" [cit. John Malkovich  
:-)] and ask less general questions which may (or may not) generate  
more useful answers.

HTH
Giuseppe

Quoting Tamas Karpati <tkarpati at gmail.com>:

> Dear Dr. Kumar,
>
> I guess dangling bonds are written about in DFT textbooks of the
> physicists' style
> (in quantum chemistry, ie. clusters rather than crystals, it is less
> of a problem).
>
> The typical problem is that you cut chemical bonds when cleave the
> crystal (to get a slab).
> In case of bonds originally dominated by ionic (Coulomb) forces you
> may still have a closed
> shell system (ie. just paired electrons) after the cleavage. As for
> the more covalent
> bonds, radicals are generated and such high multiplicity electronic
> states (large
> magnetizations in terms of QE/PW.x inputs) reorganize to the more  
> stable closed
> shell systems by changing geometry (you need to reoptimize their  
> geometry) and
> simultaneously forming new bonds. This way every few surface atom
> pairs get closer
> and such bonds form, ie. bonds that were dangling after you made the  
> cut are now
> in covalent bonds again (no dangling anymore).
> This you can model by reoptimizing with low (not sure but probably
> zero) magnetization.
>
> CdS has strong bonds carrying both ionic (maybe less) and covalent nature
> (more of the latter). Depending on the cleaving plane you applied to
> the crystal,
> you have a high chance to see new Cd-S bonds form. Less probable is  
> that you'll
> have Cd-Cd bonds or -S-S- bridges but these are also possible if your surface
> atoms are situated so.
>
> Another way of treating such systems is to keep the structure as you have cut
> from the crystal and apply a high starting_magnetization in a PW/relax job.
>
> Be careful, though, as the chemistries you describe by the above two methods
> (reorganization vs. high magnetization) are living in two distant Universes.
>
> I hope this helps,
>   Tamas
>
> On Thu, Nov 12, 2020 at 8:13 AM Dr. SUNIL KUMAR  
> <suniliitd14 at gmail.com> wrote:
>>
>> Dear QE developers and Users.
>> I am struggling to carry out a DFT simulation of CdS with dangling  
>> bonds using Quantum Espresso DFT packages. I am unable to  
>> understand the phenomena of Dangling bond and its implementation in  
>> QE DFT simulation. I would like to request to you all, kindly  
>> suggest me some tutorial and sample QE scripts for CdS with  
>> Dangling bonds. I will be grateful to you.
>>
>> Thanks
>> With regards
>> SUNIL
>> Dr. Sunil Kumar
>> Ph.D (Chemical Engg. IIT Delhi)
>> M.Tech (Chemical Engg. IIT Delhi)
>> B.Tech (Chemical Engg. IET-CSJMU Kanpur)
>> Scientist-C and Assistant Professor
>> CSIR-National Metallurgical Laboratory Jamshedpur-831007
>> http://www.nmlindia.org/
>> https://scholar.google.co.in/citations?user=OchYqugAAAAJ&hl=en&oi=sra
>>
>> _______________________________________________
>> Quantum ESPRESSO is supported by MaX (www.max-centre.eu)
>> users mailing list users at lists.quantum-espresso.org
>> https://lists.quantum-espresso.org/mailman/listinfo/users
> _______________________________________________
> Quantum ESPRESSO is supported by MaX (www.max-centre.eu)
> users mailing list users at lists.quantum-espresso.org
> https://lists.quantum-espresso.org/mailman/listinfo/users



GIUSEPPE MATTIOLI
CNR - ISTITUTO DI STRUTTURA DELLA MATERIA
Via Salaria Km 29,300 - C.P. 10
I-00015 - Monterotondo Scalo (RM)
Mob (*preferred*) +39 373 7305625
Tel + 39 06 90672342 - Fax +39 06 90672316
E-mail: <giuseppe.mattioli at ism.cnr.it>