Arquivo: Notes

Por Traduzir …

Workplan

• (done) Partir da bulk-superlattice e dopar com Na e Li nos sites T e O
• (done) Criar base da densidade de carga tipo 2 para o Na e Li

A supercelula do bulk está em:
blafis:/home/coutinho/research/f4-tcnq/fcc-si317/bulk/res.fcc-si317h172

• (done) Calculate Na$^{(0/+)}$ and Li$^{(0/+)}$ in a Si-NC in a box to understand the effect of the superlattice
• Produce figures with levels of Li, Na and TCNQ without charge correction;
• Calculate migration barrier of Li$^{(0)}$ and Na$^{(0)}$ in the NC core (9 NEB images);
• Calculate absorption barrier of Li$^{(0)}$ and Na$^{(0)}$ into the NC (9 NEB images);
• Calculate migration barrier of Li$^{(+)}$ and Na$^{(+)}$ in the NC core (9 NEB images);
• Calculate absorption barrier of Li$^{(+)}$ and Na$^{(+)}$ into the NC (9 NEB images);

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Basis

parameter{ecut=200}
species{pot=11-Na-9, wfbas=atom-pppp, cdbas=atom-2-5xs, normalize}
species{pot=3-Li-3,  wfbas=atom-pppp, cdbas=atom-2-5xs, normalize}

Energies and Levels

==============================================================================================================================
str            E(0)            E(+)            E_rel       E(0)-E(+)   E(-)-E(0)   E(0)-E*(+)  E*(-)-E(0)
               [Hr]            [Hr]            [eV]        [eV]        [eV]        [eV]        [eV]
------------------------------------------------------------------------------------------------------------------------------
fcc-bulk      -1353.86075     -1353.92476                  1.74180     3.60905     1.61979     3.73106
------------------------------------------------------------------------------------------------------------------------------
Na - fcc
   Th         -1401.44947     -1401.58313      0.02503     3.63707                 3.51506
   Oh         -1401.42752     -1401.56457      0.62232     3.72932                 3.60731
   [111]      -1401.45039     -1401.58465      0.00000     3.65340                 3.53139
   c-Th       -1401.39612     -1401.52659      1.47676     3.55027                 3.42826
------------------------------------------------------------------------------------------------------------------------------
Li - fcc
   Th         -1361.17023     -1361.29105      0.50178     3.28768                 3.16567
   Oh         -1361.14869     -1361.26373      1.08791     3.13040                 3.00839
   [111]      -1361.16714     -1361.29867      0.58586     3.57911                 3.45710
   c-Th       -1361.18867     -1361.31927      0.00000     3.55381                 3.43180
==============================================================================================================================
fcc-bulk   E(-)=-1353.72812 Hr

Starred values include a Madelung correction (see F4-TCNQ molecules in Si-NC superlattices).

Increasing superlattice parameter

==============================================================================================================================
str            E(0)            E(+)            E_rel       E(0)-E(+)   E(-)-E(0)   E(0)-E*(+)  E*(-)-E(0)
               [Hr]            [Hr]            [eV]        [eV]        [eV]        [eV]        [eV]
------------------------------------------------------------------------------------------------------------------------------
fcc80-bulk    -1353.67707     -1353.60655                 -1.91895     0.14585    -2.09223     0.31914
------------------------------------------------------------------------------------------------------------------------------
Na - fcc80
   Th         -1401.23826     -1401.24548      0.85770     0.19647                 0.02319
   Oh         -1401.23834     -1401.24591      0.85553     0.20599                 0.03271
   [111]      -1401.26978     -1401.27374      0.00000     0.10776                -0.06552
   c-Th       -1401.21868     -1401.21753      1.39050    −0.03129                -0.20457
------------------------------------------------------------------------------------------------------------------------------
Li - fcc80
   Th         -1360.95887     -1360.92842      1.22179    −0.82859                -1.00187
   Oh         -1360.95934     -1360.92641      1.20900    −0.89607                -1.06935
   [111]      -1360.97748     -1360.97515      0.71539    −0.06340                -0.23668
   c-Th       -1361.00377     -1361.00300      0.00000    −0.02095                -0.19423
==============================================================================================================================
fcc80-bulk E(-)=-1353.67171

As F4-TCNQ molecules in Si-NC superlattices, we applied a Madelung correction (starred values), using the following parameters. The super-lattice constant is now $a_0=4.2334~~\text{nm}$ (80.00 a.u.) and the nanocrystal’s volume is maintained, so the new estimation for the macroscopic dielectric constant is $\epsilon_{SL}=4.50$. For a fcc lattice we have $E_M(q=1)=0.17328~\text{eV}$.
No structural relaxation was performed in these cases.

Results from the previous tables are pictured below.

Levels (0/+) of Na (black) and Li (orange) with Madelung corrections for several structures (solid lines) and the effect of increasing superlattice constant (dashed lines).

Ion diffusion

Absorption barriers

A work in progress …

Migration barriers

A work in progress …

Si-NC in Vacuum

An atom-centered Si-NC (Si₂₁₁H₁₄₀) was placed in the corner of a simple cubic box (80 a.u.). Self-consistent electron relaxation cycle was performed to find the minimized total energy and then the process was repeated after adding a point charge in the center of the box.

Dump files of electron density and potencials are no longer available in blafis due to its large size (~40GB per file). They are still available upon request.

After fiddling with the code, I got the following result using these conditions

SiNC.dump
SiNCpc.dump
1.0                           ! Localized charge [...,-2,-1,+1,+2,...]
0.0 0.0 0.0                   ! Averaging stencil diameters along X, Y and Z [Bohr]
1                             ! nspin [1 - Spin average, 2 - Spin polarized]
450 450 450                   ! Grid [nga, ngb, ngc]
 80.000     0.000     0.000   ! a1 lattice vector [Bohr]
  0.000    80.000     0.000   ! a2 lattice vector [Bohr]
  0.000     0.000    80.000   ! a3 lattice vector [Bohr]
0.0 0.0 0.0                   ! Center of symmetry sphere/slab (z-coord) [Bohr]
20.000                        ! Cut-off radius/half-width sphere/slab [Bohr]
200                           ! Number of bins for plot

Note that the stencil diameter is zero, ie, no local averaging is carried out. The source code can be found at blafis:/home/coutinho/aimpro/screening/devel/screen.f90

Stencil diameter dependency

Now we need to make a few tests, namely, increase the stencil diameter and produce (1) a plot with local rho functions vs radius for several stencil diameters, and (2) a plot with cumulative rho functions vs radius for several stencil diameters. Use stencil diameters of D = (0, 1, 5, 10, 20) atomic units. Mind that the stencil diameters are directly set at the input files (and not its radii as in some input file comments suggest).
Results are pictured below.
Calculations with stencil diameters of D= (30, 40) were abandoned due to high computational costs.

Size effects

Tests with smaller and larger SI-NCs are in progress …

Surface modification

Data available in the following files:
blafis:~tiago/queue/sinc/radial.Si377.*.norm.dat.d0 (normalized)
blafis:~tiago/queue/sinc/nc-sc.Si377.H196/scr/radial.dat.d0 (un-normalized charge density for -H)
blafis:~tiago/queue/sinc/nc-sc.Si377.F196/scr/radial.dat.d0 (un-normalized charge density for -F)

The same atom-centered Si-NC passivated with hydrogen was also passivated with Fluorine (Si₂₁₁F₁₄₀) and -OH (Si₂₁₁O₁₀₀H₈₀) and placed in the same simple cubic box (80 a.u.). After structural relaxation, self-consistent electron relaxation cycle was performed to find the minimized total energy and then the process was repeated after adding a point charge in the center of the box. The following plots sumarize the obtained results.

H passivation

F passivation

OH passivation

Summary

Medium (host) effects

Calculations for SiNC embeded in diferent mediums are on hold!

I. Optimization of the Si-NC superlattice

We produced superlattices of atom-centered H-terminated Si-NCs. We performed a self-consistent lattice/atom/electron relaxation cycle to find the Si-NC superlattices that minimized the total energy. Two situations were study, stacked and connected NCs arranjed in two superlattices, simple cubic (sc) and face-centered cubic (fcc) superlattices.

          | stacked             | connected           |
diameter  | fcc      | sc       | fcc      | sc       |
-------------------------------------------------------
 ~ 2 nm   | 63.3929    42.0278    52.1310    40.4852

*all values in a.u.

Soon, I’ll be adding the other NCs sizes.