Messages

31

General Questions and Answers / Re: Silicene nanoribbon configuration

« on: January 16, 2012, 06:44 »

Ok, one more question to the list 

6) In some papers i have seen people working on tunable band gaps i.e, by increasing the perpendicular GATE electric field (by varying gate voltage) the band gap of the material in question starts to open up or show some variation.When i create a graphene junction  and apply the required bias i didn't see any available 'Bandstructure' calculation module in the analysis tab of script generator.Is there a way (direct or indirect)in ATK-VNL by which i can calculate the bandgap at various gate voltages.

Thanks

32

General Questions and Answers / Re: Silicene nanoribbon configuration

« on: January 16, 2012, 06:26 »

Hi, i have some basic questions to ask (Please forgive if questions look repetitive  )

Considering decent accuracy of results

1)What value of MonkhorstPackGrid(?,?,?)  points should be used when i want to calculate 'density of states' of a graphene or silicene nanoribbon.

2)What value of MonkhorstPackGrid(?,?,?)  points should be used when i want to calculate 'Device density of states' of a graphene or silicene FET junction.

3)What value of MonkhorstPackGrid(?,?,?)  points should be used when i want to calculate 'transmission spectrum' of a graphene or silicene nanoribbon.

4)What value of MonkhorstPackGrid(?,?,?)  points should be used when i want to calculate 'transmission spectrum' of a graphene/silicene junction.

5)How a gate region must be designed?? , (Considering Z-A-Z...image attached) by default in graphene junction enabling the gate spreads the gate region only in middle of armchair region. i.e, Gate covers only the '-A-' portion of Z-A-Z junction.When i design the gate for silicene or graphene FET should i use this default spread of gate?? or the gate should ideally span all the three (Z-A-Z) regions???

33

General Questions and Answers / Re: Silicene nanoribbon configuration

« on: January 13, 2012, 13:10 »

thanks

34

General Questions and Answers / Re: Silicene nanoribbon configuration

« on: January 13, 2012, 13:01 »

yes thats why the custom bandgap finder finds a very low 0.4meV  but iam still confused b/w the different values of normal band gap and direct band gap at the same K point!!

35

General Questions and Answers / Re: Silicene nanoribbon configuration

« on: January 13, 2012, 12:47 »

Thanks for the code...i have run this script ... if the  bandgap is at 'K' point  can i use it by changing the route to [K,K]?

36

General Questions and Answers / Re: Silicene nanoribbon configuration

« on: January 13, 2012, 12:31 »

actually the bandstructure is of buckled silicene crystal and not of a nanoribbon.The bandgap is obtained at K point.I have attached the script.
the concerned nc file is here: http://lts.cr/i/02f8f8

37

General Questions and Answers / Re: Silicene nanoribbon configuration

« on: January 13, 2012, 10:43 »

I used this band gap finder analyzer script for 'buckled silicene' using (GGA-PBE) for bandstructure calculations

This is the log:

Analyzing bandstructure number 0 Spin.Up
Valence band maximum    -0.0009 eV at [0.3332, 0.3334,0.0000]   
Conduction band minimum -0.0004 eV at [0.3332, 0.3334,0.0000]   
Band gap                 0.0004 eV

Direct Band gap          0.0027 eV at [0.3332, 0.3334,0.0000]  

A band gap in general is the difference between (C.B min and V.B max) and a direct band gap is the one where the 'k' momentum space points remain the same during transition but here i am getting confused as the log file says 'direct' and 'normal' band gaps have different values at the same 'k' points(0.3,0.3,0)..

i.e, at the same k points the band gap is 0.4meV and again at the same k point the direct band gap is 2.7meV

how is this possible???.Can you please explain the difference here. 

38

General Questions and Answers / Re: Silicene nanoribbon configuration

« on: January 13, 2012, 06:29 »

i must say..that was extremely helpful 

39

General Questions and Answers / Re: Silicene nanoribbon configuration

« on: January 12, 2012, 12:00 »

Hi,

i am trying to calculate the bandgap of nanoribbons accurately.The Bandstructure plot can be zoomed (see image attached )but it doesn't contain vertical minor grid lines so i am unable to read out the exact values.Can you please suggest some remedy for this.I saw the tutorial for (Si) band gap calculation and tried to use the code  by calculating minima and maxima of conduction and valence band respectively but i am getting absurd results as i increase the width of ribbons.I am getting  negative bandgap values?? for eg, -1.03292958 -1.19040904 -1.03755011 in (all in eV)

here are the lines from tutorial:

energies = bandstructure.evaluate()
e_valence_max = energies[0][3]
e_conduction_min = energies[0][4]
i_valence_max=0
i_conduction_min=0

# Locate extrema
for i in range(energies.shape[0]):
    #find maximum of valence band
    if (energies[3] > e_valence_max):
        e_valence_max=energies[3]
        i_valence_max=i
    #find minimum of conduction band
    if (energies[4] < e_conduction_min):
        e_conduction_min=energies[4]
        i_conduction_min=i

# Print out results   
print 'Valence band maximum (eV) ',e_valence_max, 'at ',
print bandstructure.kpoints()[i_valence_max]
print 'Conduction band minimum (eV)',e_conduction_min, 'at ',
print bandstructure.kpoints()[i_conduction_min]
print 'Band gap = %7.4f eV ' % (e_conduction_min-e_valence_max)




Please help me in calculating exact values of band gap.Considering accuracy of bandgaps what number of points per segment i should use??? 400 ..800 or something else

40

General Questions and Answers / Re: Silicene nanoribbon configuration

« on: January 9, 2012, 07:33 »

This means that for the stable minimum energy cell if i try to use a loop i have to deal with 2 variables??? 'y' and 'z'.(Since x is kept fixed at 10A).OR just variation in 'y' will do.Please comment.
I am only working with Cartesian coordinates.By selecting all the atoms in fractional coordinates  i just stretch the atoms and again revert back to Cartesian mode in order to check if bond lengths are approximately matched.

41

General Questions and Answers / Silicene nanoribbon configuration

« on: January 7, 2012, 11:33 »

Hi,

I have used the out of plane geometry of silicon atoms and created a silicene nanoribbon.(see image attached).I want to know what appropriate k-points and mesh cut off i need to use for this arrangement??...generally i have seen something like (1X1X100).As this  arrangement is periodic in Z direction and i want to simulate isolated ribbons what value should i select  for 'x' in primitive vectors of unit cell??.Do i have to keep it large say(10A to 20A) so that there is minimum interaction between them.

Now in order to get a minima point in energy-vs-vol curve i want to set 'x'  primitive vector to a large value and select all atoms in fractional coordinates mode and stretch them to a proper value of 'y' so that Si-Si bond lengths are approximately matched with my previous 'bulk silicene crystal' calculation .Once this is done i will vary 'z' primitive vector and calculate energy at each of those z-points(or volume) to get correct structure of unit cell lattice having minimum energy.Then i will go ahead and perform force optimization (to a low value 0.005eV/A or something..) keeping x,y,z all "ticked". This should give me the stable structure of ribbon.Is this method ok?? OR should i do force optimization first and then play with lattice?

Please comment and advice on k-points,mesh cut off and above methodology of obtaining energetically favorable configuration of ribbons.

42

General Questions and Answers / Re: Silicene bandstructure calculation

« on: December 30, 2011, 11:50 »

correction: the second attached image is of rotation at 20degrees not 10. 

43

General Questions and Answers / Re: Silicene bandstructure calculation

« on: December 30, 2011, 11:44 »

yes generally GGA calculated lattice values tend be higher than LDA  ones and there might be differences in the values because of different simulation packages that people use.... thanks for the code snippet.

But i still don't understand how the bond lengths will change on rotation as i only change the angle and even after rotating about y axis (say, 1degree,2 degree and so on..) i have checked in z-matrix the bond lengths remains the same as selected earlier.

see the attached image( 0 degrees and after rotation by 10 degrees )as the second atom gains some 'z ' coordinate as well
pls note i just calculated energy at each such 'new points' after rotation steps without including any kind of optimization.

Thanks for all the help given by forum members and moderators..and yesss wishing a very happy new year

44

General Questions and Answers / Re: Silicene bandstructure calculation

« on: December 30, 2011, 10:55 »

thanks a lot for this info..

45

General Questions and Answers / Re: Silicene bandstructure calculation

« on: December 30, 2011, 09:19 »

mesh cut off 500eV, k points 20X20X3

I just used the 'already optimized' planar silicene geometry and rotated one atom about y axis (keeping other fixed at origin) just to get various Cartesian coordinates and the bond length was obviously not changed by this(fixed at 2.21)..no extra optimization was done as the tiring rotation exercise was just performed to get various points and calculating energy at each of those points.Rotation was done b/w (5 to 19 degrees in steps of 0.2 degree each  ).
Is  rotation needed to be done in xz,or xy axis

LDA-PZ energy minima at (15.6) degrees corresponds to buckling in z direction by 0.52A
GGA-PBE energy minima at (around 12 degrees)  corresponds to buckling in z direction by 0.4A
I get 2 lowered minimas in LDAplot but only one in gga.

Literature values of buckling is somewhere around 0.44A

Although i am not very good at coding but yes pls do share the rotational coding method if i find it comfortable to use i may use it  .