[Pw_forum] Fail to predict semiconductor

["José C. Conesa"]

Dear Iwan,
In the paper you quote, the Ti:ZnO case has the Ti levels inside the ZnO 
conduction band (see fig. 2) - and this even though the latter band has 
been pushed up by applying an ad hoc shift to the Zn 4s levels, so that 
the correct gap of ZnO is obtained. Thus it does not seem that the 
calculation reported there predicts this system to be a semiconductor. 
Of course, experimentally it may happen that the material prepared, 
especially if it was made in even mildly oxidizing environment, contains 
cation vacancies, possibly located close to the Ti atoms (assuming these 
to be at tetrahedral sites - which is not a preferred coordination for 
Ti) so that it could behave as semiconductor.
In any case computing correctly band gaps and level positions in these 
systems is tricky even with post-DFT treatments. I try to avoid if 
possible DFT+U (treating differently different electrons is not very 
elegant), and to use hybrids instead. One problem with the latter is 
that the fraction of Fock exchange to be used can be questioned. Indeed 
one same fraction is not valid for both TiO2 and ZnO. You can approach 
the situation better if that fraction is tuned to the dielectric 
constant of the material. If you are interested in this specific issue I 
invite you to look at my recent papers J. Phys. Chem. C116 (2012) 18884 
and Catal. Today in press (doi: 10.1016/j.cattod.2012.08.039) where this 
solution is explained better, although its suitability for possibly 
metallic systems like yours can be questioned as well.
Regards,
José-Carlos

El 23/01/2013 15:56, Iwan Darmadi escribió:
>  Dear Mr.Jose,
>
> Yes, I do. But, as far as I know, transition metal doped zno is 
> semiconductor even theoretically (according for example PRB 79/165202 
> and ). So I assumed that without cation vacancy, Ti doped ZnO might be 
> semiconductor also.
>
> Regards,
> ID
> ****
> *
> *Iwan Darmadi*
>  Undergrad.Student - Department of Physics
>  Universitas Indonesia
>
> ------------------------------------------------------------------------
> *From:* Jose C. Conesa <jcconesa at icp.csic.es>
> *To:* Iwan Darmadi <iwan_darmadi at rocketmail.com>; PWSCF Forum 
> <pw_forum at pwscf.org>
> *Sent:* Wednesday, January 23, 2013 3:09 PM
> *Subject:* Re: [Pw_forum] Fail to predict semiconductor
>
> Dear Iwan,
> Do you know whether the experimentally known Ti doped ZnO contains 
> cation vacancies?
> Good luck,
> José Carlos
>
> El 23/01/2013 6:50, Iwan Darmadi escribió:
>> Dear all,
>>
>> I have calculated electronic structure of Ti doped ZnO in both GGA 
>> and GGA+U scheme. Both scheme predicts Ti doped ZnO is metallic. In 
>> contrary, Ti doped ZnO is well known as semiconductor experimentally. 
>> At first glance, I thought it was local minimum problem of DFT+U 
>> (like FeO problem in Mr. Himmetoglu's tutorial). Then I try to copy 
>> Mr. Himmetoglu's trick to override a "suspected" fully occupied 
>> orbitals of Ti. Sadly, nothing change,  it's still a metallic.
>>
>> Now, I am confused whether this is a really local minimum problem or 
>> intrinsic limitation of DFT it self.
>>
>> Do anyone here have suggestions so I can get semiconductor Ti doped 
>> ZnO in the calculation ?
>>
>> Ps.
>> I have also attached my input and output file.
>> ****
>> *
>> *Iwan Darmadi*
>>  Undergrad.Student - Department of Physics
>>  Universitas Indonesia
>>
>>
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>
>
> -- 
> José C. Conesa
> Instituto de Catálisis y Petroleoquímica, CSIC
> Marie Curie 2, Cantoblanco
> 28049 Madrid - Spain
> Tel. +34-915854766
>
>

-- 
José C. Conesa
Instituto de Catálisis y Petroleoquímica, CSIC
Marie Curie 2, Cantoblanco
28049 Madrid - Spain
Tel. +34-915854766

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