[Pw_forum] how to judge movement of HOMO and LUMO

[Rajdeep Banerjee]

Dear Sclauzero  Gabriele, Giuseppe Mattioli and Jos C. Conesa,
thanks a lot for all the beautiful and
valuable tricks and suggestions. I'll take a look and get back if any
problem arises.

Thanks and regards,
Rajdeep Banerjee
JNCASR, Bangalore,
India
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[Pw_forum] how to judge movement of HOMO and LUMO

[Rajdeep Banerjee]

Dear Prof. Marzari,
   thank you very much for all your suggestions and
concerns. I'm really overwhelmed !
   I've looked at work function calculation (thanks
to the links you provided). Now, I have a monolayer of crystalline
non-polar semiconductor material and I can calculate the planar and
macroscopic averages for unstrained and strained cases. But I'm still a
little bit confused about fixing the reference. Should I take macroscopic
average of Hartree potential at vacuum as the reference ?

I've thought about the following steps (in light of your previous
explanation):

1. Do a bulk calculation and find out the macroscopic average
2. Do the strained monolayer and see where the macroscopic average sits
with respect to vacuum level.
   But then I don't understand why do we need a bulk calculation if I can
get the vacuum level (in sigle-layer) and can align it for all strain cases
and refer the HOMO-LUMO directly to that !

  I know I have a lot of doubts and I truly
appreciate your patience and all your helpful suggestions.

Thanks,
Rajdeep Banerjee
JNCASR
Bangalore, India
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[Pw_forum] how to judge movement of HOMO and LUMO

[Nicola Marzari]

If you have a thin film none of the discussion on the forum applies
to you. The disucssion was only for bulk materials in 3d.

So, for your thin film, at each level of strain you can calculate
simultanneously the band structure, and where the vacuum level is,
so you are in good shape.

nicola


On 05/06/2014 08:53, Rajdeep Banerjee wrote:
> Dear Prof. Marzari,
> thank you very much for all your suggestions
> and concerns. I'm really overwhelmed !
> I've looked at work function calculation
> (thanks to the links you provided). Now, I have a monolayer of
> crystalline non-polar semiconductor material and I can calculate the
> planar and macroscopic averages for unstrained and strained cases. But
> I'm still a little bit confused about fixing the reference. Should I
> take macroscopic average of Hartree potential at vacuum as the reference ?
>
> I've thought about the following steps (in light of your previous
> explanation):
>
> 1. Do a bulk calculation and find out the macroscopic average
> 2. Do the strained monolayer and see where the macroscopic average sits
> with respect to vacuum level.
> But then I don't understand why do we need a bulk calculation if I
> can get the vacuum level (in sigle-layer) and can align it for all
> strain cases and refer the HOMO-LUMO directly to that !
>
>I know I have a lot of doubts and I truly
> appreciate your patience and all your helpful suggestions.
>
> Thanks,
> Rajdeep Banerjee
> JNCASR
> Bangalore, India
>
>
> ___
> Pw_forum mailing list
> Pw_forum at pwscf.org
> http://pwscf.org/mailman/listinfo/pw_forum
>

-- 

--
Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL

[Pw_forum] how to judge movement of HOMO and LUMO

["José C. Conesa"]

Dear Rajdeep Banerjee,
In a recent paper of mine (http://dx.doi.org/10.1021/jp306160c) I used 
the van de Walle & Martin's scheme, later used by Pasquarello, to 
compute the band alignment at ZnO/TiO2(anatase) interfaces. The purely 
electrostatic (ionic + Hartree) potential was used as reference. At 
least the results agreed with experimental observations from Gr?tzel's 
group on certain DSSCs. What I would stress here is that one should pay 
attention also (as I did in that work) to the need of obtaining as 
reliable VB and CB positions as possible - which implies at least a good 
reproduction of the bandgap, a well known issue. A way to do so, without 
having to carry out rather expensive GW calculations, may be to use a 
PBE0 hybrid functional with the fraction of HF exchange tuned to the 
optical dielectric constant (a suggestion by Lucia Reining's group); see 
the mentioned paper as well as 
http://dx.doi.org/10.1016/j.cattod.2012.08.039 for details and references.
Good luck,
Jos? Carlos

El 04/06/2014 14:10, Sclauzero Gabriele escribi?:
> Dear Rajdeep Banerjee,
>
>  I think what is used as reference potential is the bare ionic potential 
> + Hartree potential (V_bare + V_H). I kind of justification for that is given 
> in this older paper by van de Walle & Martin 
> (http://journals.aps.org/prb/pdf/10.1103/PhysRevB.35.8154) about band lineups 
> at heterointerfaces.
>
> I agree with Nicola that the problem is subtle, given that a pure bulk 
> calculation for an infinite solid cannot provide absolute energy levels.
> Hopefully someone with clearer ideas than me will be able to suggest 
> alternative solutions.
>
> Best,
>
> GS
>
>
> Dear Sclauzero  Gabriele,
>  I looked at the paper you suggested 
> (http://journals.aps.org/prl/pdf/10.1103/PhysRevLett.101.046405) where it was 
> said that the "common reference level (phi) " is calculated as the "cell 
> average of the local potential originating from the ionic pseudopotentials" 
> which I guess can be calculated from the pseudopotentials used. Can you 
> suggest any functional form for this calculation of phi ? or indicate how to 
> generate it from the pseudopotentials used?
>
> As far as I understand, after defining phi, the CBM and VBM can be described 
> by:
>
>
> CBM = Ec - phi
> VBM = Ev - phi
>
> where Ec and Ev are LUMO and HOMO respectively.
>
> Thank you very much for all the help.
>
> Rajdeep Banerjee
> JNCASR,
> Bangalore, India
>
> ___
> Pw_forum mailing list
> Pw_forum at pwscf.org
> http://pwscf.org/mailman/listinfo/pw_forum
>
>
> Dr. Gabriele Sclauzero
> Materials Theory - ETHZ
> ETH Zurich, HIT G 43.2
> Wolfgang-Pauli-Str. 27
> 8093 Z?rich, Switzerland
>
> Phone +41 44 633 94 10
> Fax +41 44 633 14 59
> gabriele.sclauzero at mat.ethz.ch
> www.theory.mat.ethz.ch
>
>
> ___
> Pw_forum mailing list
> Pw_forum at pwscf.org
> http://pwscf.org/mailman/listinfo/pw_forum

-- 
Jos? C. Conesa
Director
Instituto de Cat?lisis y Petroleoqu?mica, CSIC
Marie Curie 2, Cantoblanco
28049 Madrid - Spain
Tel. +34-915854766

[Pw_forum] how to judge movement of HOMO and LUMO

[Giuseppe Mattioli]

Dear Rajdeep Banerjee,
There is also a "dirty trick" that can be used (with some success, at least in 
my experience, and if you do not want to be very subtle...:-)) to align 
the eigenvalues. If your lattice is not too tight, you can try to insert a He 
atom inside the largest interstitial site. If you put the He always in 
the same site, and provided that it does not mix too much with the lattice 
charge density (you can check it with projwfc.x), then you can use the 1s 
He level to realign the eigenvalues of different systems.
HTH

Giuseppe

On Wednesday 04 June 2014 14:10:55 Sclauzero Gabriele wrote:
> Dear Rajdeep Banerjee,
> 
> I think what is used as reference potential is the bare ionic potential
> + Hartree potential (V_bare + V_H). I kind of justification for that is
> given in this older paper by van de Walle & Martin
> (http://journals.aps.org/prb/pdf/10.1103/PhysRevB.35.8154) about band
> lineups at heterointerfaces.
> 
> I agree with Nicola that the problem is subtle, given that a pure bulk
> calculation for an infinite solid cannot provide absolute energy levels.
> Hopefully someone with clearer ideas than me will be able to suggest
> alternative solutions.
> 
> Best,
> 
> GS
> 
> 
> Dear Sclauzero  Gabriele,
> I looked at the paper you suggested
> (http://journals.aps.org/prl/pdf/10.1103/PhysRevLett.101.046405) where it
> was said that the "common reference level (phi) " is calculated as the
> "cell average of the local potential originating from the ionic
> pseudopotentials" which I guess can be calculated from the
> pseudopotentials used. Can you suggest any functional form for this
> calculation of phi ? or indicate how to generate it from the
> pseudopotentials used?
> 
> As far as I understand, after defining phi, the CBM and VBM can be
> described by:
> 
> 
> CBM = Ec - phi
> VBM = Ev - phi
> 
> where Ec and Ev are LUMO and HOMO respectively.
> 
> Thank you very much for all the help.
> 
> Rajdeep Banerjee
> JNCASR,
> Bangalore, India
> 
> ___
> Pw_forum mailing list
> Pw_forum at pwscf.org
> http://pwscf.org/mailman/listinfo/pw_forum
> 
> 
> Dr. Gabriele Sclauzero
> Materials Theory - ETHZ
> ETH Zurich, HIT G 43.2
> Wolfgang-Pauli-Str. 27
> 8093 Z?rich, Switzerland
> 
> Phone +41 44 633 94 10
> Fax +41 44 633 14 59
> gabriele.sclauzero at mat.ethz.ch
> www.theory.mat.ethz.ch
> 
> 
> ___
> Pw_forum mailing list
> Pw_forum at pwscf.org
> http://pwscf.org/mailman/listinfo/pw_forum



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   ResearcherID: F-6308-2012

[Pw_forum] how to judge movement of HOMO and LUMO

[Sclauzero Gabriele]

Dear Rajdeep Banerjee,

I think what is used as reference potential is the bare ionic potential + 
Hartree potential (V_bare + V_H). I kind of justification for that is given in 
this older paper by van de Walle & Martin 
(http://journals.aps.org/prb/pdf/10.1103/PhysRevB.35.8154) about band lineups 
at heterointerfaces.

I agree with Nicola that the problem is subtle, given that a pure bulk 
calculation for an infinite solid cannot provide absolute energy levels.
Hopefully someone with clearer ideas than me will be able to suggest 
alternative solutions.

Best,

GS


Dear Sclauzero  Gabriele,
I looked at the paper you suggested 
(http://journals.aps.org/prl/pdf/10.1103/PhysRevLett.101.046405) where it was 
said that the "common reference level (phi) " is calculated as the "cell 
average of the local potential originating from the ionic pseudopotentials" 
which I guess can be calculated from the pseudopotentials used. Can you suggest 
any functional form for this calculation of phi ? or indicate how to generate 
it from the pseudopotentials used?

As far as I understand, after defining phi, the CBM and VBM can be described by:


CBM = Ec - phi
VBM = Ev - phi

where Ec and Ev are LUMO and HOMO respectively.

Thank you very much for all the help.

Rajdeep Banerjee
JNCASR,
Bangalore, India

___
Pw_forum mailing list
Pw_forum at pwscf.org
http://pwscf.org/mailman/listinfo/pw_forum


Dr. Gabriele Sclauzero
Materials Theory - ETHZ
ETH Zurich, HIT G 43.2
Wolfgang-Pauli-Str. 27
8093 Z?rich, Switzerland

Phone +41 44 633 94 10
Fax +41 44 633 14 59
gabriele.sclauzero at mat.ethz.ch
www.theory.mat.ethz.ch

[Pw_forum] how to judge movement of HOMO and LUMO

[Rajdeep Banerjee]

Dear Sclauzero  Gabriele,
I looked at the paper you suggested (
http://journals.aps.org/prl/pdf/10.1103/PhysRevLett.101.046405) where it
was said that the "common reference level (phi) " is calculated as the
"cell average of the local potential originating from the ionic
pseudopotentials" which I guess can be calculated from the pseudopotentials
used. Can you suggest any functional form for this calculation of phi ? or
indicate how to generate it from the pseudopotentials used?

As far as I understand, after defining phi, the CBM and VBM can be
described by:


CBM = Ec - phi
VBM = Ev - phi

where Ec and Ev are LUMO and HOMO respectively.

Thank you very much for all the help.

Rajdeep Banerjee
JNCASR,
Bangalore, India
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[Pw_forum] how to judge movement of HOMO and LUMO

[Nicola Marzari]

Dear Rajdeep,

do note that the change in the quantities CBM or VBM as you write them
below as a function of strain would not give the physical change in
CBM or VBM - as you strain the crystal, the change in CBM and VBM below
would include the physical electrostatic term, but I think they
also include an unphysical strain-dependent term that depends
on to the way the the  divergent Ewald sums that are implicitly set to
zero change as a function of strain.

This contribution might be small, but I really don't know - it's a
subtle problem, and you should really become familiar with e.g. the
calculation of work functions is done (that do not need these
subtleties) and with the theory of flexoelectricity.

nicola


On 03/06/2014 18:42, Rajdeep Banerjee wrote:
> Dear Sclauzero  Gabriele,
>  I looked at the paper you suggested
> (http://journals.aps.org/prl/pdf/10.1103/PhysRevLett.101.046405) where
> it was said that the "common reference level (phi) " is calculated as
> the "cell average of the local potential originating from the ionic
> pseudopotentials" which I guess can be calculated from the
> pseudopotentials used. Can you suggest any functional form for this
> calculation of phi ? or indicatehow to generate it from the
> pseudopotentials used?
>
> As far as I understand, after defining phi, the CBM and VBM can be
> described by:
>
>
> CBM = Ec - phi
> VBM = Ev - phi
>
> where Ec and Ev are LUMO and HOMO respectively.
>
> Thank you very much for all the help.
>
> Rajdeep Banerjee
> JNCASR,
> Bangalore, India
>
>
>
> ___
> Pw_forum mailing list
> Pw_forum at pwscf.org
> http://pwscf.org/mailman/listinfo/pw_forum
>

-- 

--
Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL

[Pw_forum] how to judge movement of HOMO and LUMO

[Rajdeep Banerjee]

Dear Sclauzero  Gabriele,
thanks a lot for this helpful
reference. I'll read up and try to find out the way and report back as soon
as I can.

Thanks and regards,
Rajdeep Banerjee
JNCASR
Bangalore, India
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[Pw_forum] how to judge movement of HOMO and LUMO

[Rajdeep Banerjee]

Dear Prof. Nicola Marzari,
 thank you very much for your excellent
suggestions. I'll surely look deep into the matter and report back to you
as soon as I can.

Thanks,
Rajdeep Banerjee
JNCASR
Bangalore , India
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[Pw_forum] how to judge movement of HOMO and LUMO

[Sclauzero Gabriele]

Dear Rajdeep Banerjee,

the following paper

http://journals.aps.org/prl/pdf/10.1103/PhysRevLett.101.046405

and related work in the Pasquarello group (EPFL) could also be of help to solve 
your problem.

Best,


GS


Dear all,
   I want to look at the movement of valence band maxima (VBM / HOMO) 
and conduction band minima (CBM/LUMO) of a semiconductor for different values 
of applied tensile strain. The problem is how to fix a reference point to look 
at such a movement?
1. I can put Fermi energy as reference ... but for a semiconductor it doesn't 
mean anything as long as its in the band gap
2. I can put VBM as Fermi level and take that as reference ... but then I won't 
see VBM movement
3. I can use mid-point between HOMO and LUMO ... but then both the band edges 
will shift by same amount
4. I can fix an arbitrary point in the band gap as reference and check the 
movement without any shift ... but I'm a bit sceptical about this chice though.

   Please advise. Thanks in advance.

warmest regards,
Rajdeep Banerjee
JNCASR
Bangalore, India
___
Pw_forum mailing list
Pw_forum at pwscf.org
http://pwscf.org/mailman/listinfo/pw_forum


Dr. Gabriele Sclauzero
Materials Theory - ETHZ
ETH Zurich, HIT G 43.2
Wolfgang-Pauli-Str. 27
8093 Z?rich, Switzerland

Phone +41 44 633 94 10
Fax +41 44 633 14 59
gabriele.sclauzero at mat.ethz.ch
www.theory.mat.ethz.ch

[Pw_forum] how to judge movement of HOMO and LUMO

[Nicola Marzari]

Dear Rajdeep,


Raffaele Resta pointed out this earlier paper:
http://journals.aps.org/prb/pdf/10.1103/PhysRevB.41.12358 .

As you can see the problem is quite subtle, and for the time
being it looks like you need to deal with a non-polar semiconductor.

Another suggestion was to look at some of the recent work on
flexoelectricity by Stengel, and by Vanderbilt - so in general
you have a fairly complex (and thus exciting) problem at hand.

 nicola



On 01/06/2014 15:59, Nicola Marzari wrote:
>
>
> Dear Rajdeep,
>
>
> good question, and I'd be intrigued to know the answer.
>
> First thing that comes to mind would be to do a reverse process
> to what is used to calculate the work function.
>
> 1) in the bulk calculation, calculate both the VBM and the CBM,
> and refer it to the planar average of the Hartree potential (a lot
> of details here: http://infoscience.epfl.ch/record/32432, see fulltext
> link, or in our 2009 PRB). This way, you know where the bands sit with
> respect to another well defined (but still relative) bulk quantity.
>
> 2) now you need to determine where the average of the Hartree sits with
> respect to the vacuum level - you look at a thick slab (appropriately
> strained, to match uniaxial or biaxial strain above) and see where the
> Hartree sits with respect to the vacuum level. This would still have
> the problem that the different between Hartree and vacuum would be
> affected by the surface (termination, resconstruction). But I guess
> that is the case also in ARPES data - so I wonder if you really need
> to go through all this process (I still think so, because as you
> strain the crystal, the change you see in the VBM or CBM could be
> in part physical, in part computational, due to the lack of an
> absolute reference).
>
> If anyone has more experience in this, they'd be very welcome!
>
>   nicola
>
>
> On 01/06/2014 07:21, Rajdeep Banerjee wrote:
>> Dear all,
>>  I want to look at the movement of valence band maxima (VBM /
>> HOMO) and conduction band minima (CBM/LUMO) of a semiconductor for
>> different values of applied tensile strain. The problem is how to fix a
>> reference point to look at such a movement?
>> 1. I can put Fermi energy as reference ... but for a semiconductor it
>> doesn't mean anything as long as its in the band gap
>> 2. I can put VBM as Fermi level and take that as reference ... but then
>> I won't see VBM movement
>> 3. I can use mid-point between HOMO and LUMO ... but then both the band
>> edges will shift by same amount
>> 4. I can fix an arbitrary point in the band gap as reference and check
>> the movement without any shift ... but I'm a bit sceptical about this
>> chice though.
>>
>>  Please advise. Thanks in advance.
>>
>> warmest regards,
>> Rajdeep Banerjee
>> JNCASR
>> Bangalore, India
>>
>>
>> ___
>> Pw_forum mailing list
>> Pw_forum at pwscf.org
>> http://pwscf.org/mailman/listinfo/pw_forum
>>
>

-- 

--
Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL

[Pw_forum] how to judge movement of HOMO and LUMO

[Nicola Marzari]

Dear Rajdeep,


good question, and I'd be intrigued to know the answer.

First thing that comes to mind would be to do a reverse process
to what is used to calculate the work function.

1) in the bulk calculation, calculate both the VBM and the CBM,
and refer it to the planar average of the Hartree potential (a lot
of details here: http://infoscience.epfl.ch/record/32432, see fulltext
link, or in our 2009 PRB). This way, you know where the bands sit with
respect to another well defined (but still relative) bulk quantity.

2) now you need to determine where the average of the Hartree sits with
respect to the vacuum level - you look at a thick slab (appropriately
strained, to match uniaxial or biaxial strain above) and see where the
Hartree sits with respect to the vacuum level. This would still have
the problem that the different between Hartree and vacuum would be
affected by the surface (termination, resconstruction). But I guess
that is the case also in ARPES data - so I wonder if you really need
to go through all this process (I still think so, because as you
strain the crystal, the change you see in the VBM or CBM could be
in part physical, in part computational, due to the lack of an
absolute reference).

If anyone has more experience in this, they'd be very welcome!

nicola


On 01/06/2014 07:21, Rajdeep Banerjee wrote:
> Dear all,
> I want to look at the movement of valence band maxima (VBM /
> HOMO) and conduction band minima (CBM/LUMO) of a semiconductor for
> different values of applied tensile strain. The problem is how to fix a
> reference point to look at such a movement?
> 1. I can put Fermi energy as reference ... but for a semiconductor it
> doesn't mean anything as long as its in the band gap
> 2. I can put VBM as Fermi level and take that as reference ... but then
> I won't see VBM movement
> 3. I can use mid-point between HOMO and LUMO ... but then both the band
> edges will shift by same amount
> 4. I can fix an arbitrary point in the band gap as reference and check
> the movement without any shift ... but I'm a bit sceptical about this
> chice though.
>
> Please advise. Thanks in advance.
>
> warmest regards,
> Rajdeep Banerjee
> JNCASR
> Bangalore, India
>
>
> ___
> Pw_forum mailing list
> Pw_forum at pwscf.org
> http://pwscf.org/mailman/listinfo/pw_forum
>

-- 

--
Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL

[Pw_forum] how to judge movement of HOMO and LUMO

[Rajdeep Banerjee]

Dear all,
   I want to look at the movement of valence band maxima (VBM /
HOMO) and conduction band minima (CBM/LUMO) of a semiconductor for
different values of applied tensile strain. The problem is how to fix a
reference point to look at such a movement?
1. I can put Fermi energy as reference ... but for a semiconductor it
doesn't mean anything as long as its in the band gap
2. I can put VBM as Fermi level and take that as reference ... but then I
won't see VBM movement
3. I can use mid-point between HOMO and LUMO ... but then both the band
edges will shift by same amount
4. I can fix an arbitrary point in the band gap as reference and check the
movement without any shift ... but I'm a bit sceptical about this chice
though.

   Please advise. Thanks in advance.

warmest regards,
Rajdeep Banerjee
JNCASR
Bangalore, India
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