MoS2 nanowire

[sitangshu]

How can I make an MoS2 Nanowire of 1 micro meter length??

[zh]

It may be too long for the simulation or the structural visualization.

[sitangshu]

can you let me know for 10nm long MoS2 nanowire?

Thanks

[Nordland]

I think it is important to understand why you want a 1 micro meter MoS2, since as long as the wire is homogeneous in structure you will just have to simulate a single piece/sample of the wire, that properly is no longer than 5 Angstrom.

All the properties you could derive from the simulation will be equally good for this length, and you will be able to perform the simulation in minutes.

[sitangshu]

Hi,

This 1 micro meter length is needed to put it between 2 electrodes and to measure the electrical properties like resistance. Since, the resistance depends whether the system is diffusive or ballistic, I think the results for MoS2 nanowire will be entirely different from that of 5 Angstrom and 10 nm.
Isn't it?
However, your suggestion may help me to get the band structure and the band gap of the wire.
Hence, can you please let me know how to build such MoS2 nanowire asap ?

Thanks,

[zh]

For a perfect MoS2 nanowire (i.e., defect-free or no disorder), its resistance or conductivity may be ballistic, like the case of CNT. If you wants to study of the effect of defects and disorder on the electronic transport properties of such system, you may need a longer nanowire in the simulations.  Otherwise, the use of too long MoS2 wire will be a waste of computing resource.

The generation of the atomic coordinates of  a MoS2 nanowire is not unique, and also cannot be obtained automatically using VNL. A possible way for a MoS2 (001) nanowire is outlined as follows:
1) Create the structure of bulk MoS2, which may be selected from the crystal DataBase of VNL;
2) Drag (transfer) it into the "Builder" tool of VNL;
3) Use the "Repetition" in the "Builder" to extend the x and y directions of MoS2, according to the wanted radius of nanowire;
4) According to the wanted shape of nanowire, selectively delete some of atoms;
5) Adjust the lattice vectors of A1 and A2  to make the unit cell  orthogonal;
6) According to the wanted length of nanowire, extend the z direction of MoS2 using the "Repetition" in the "Builder".

[sitangshu]

Hi,

Thanks for your quick reply and valuable suggestions.
Can you let me know how to passivate MoS2 nanowire dangling bonds with Hydrogen. Is there any script available??

[zh]

No such script file at current stage.

Since the atomic bondings in MoS2 are quite different from the case of silicon, the passivation of dangling bonds in MoS2 by hydrogen may not make sense for some cases, e.g., the dangling bonds around Mo atoms.

[ramkrishna]

How can I make MoS2 monolayer, bilayer, quadrilayer ??

[zh]

It is quite easy.
First, create a bulk system of MoS2 using the "Builder" or find out the crystal structure of bulk MoS2 from the "Crystal DataBase" of VNL.
Then, use the "Reptition" " in the "Builder" to extend the z direction of MoS2 to 4 atomic layers and modify the lattice constant of c (c should be set large enough to get  thick vacuum layer). This will give the quardilayer. For the monolayer and bilayer, they can be obtained by deleting the atoms in the 2 or 3 atomic layers.

[ramkrishna]

Dear Sir,
        I have done the the band structure calculation of the layers according to your previous suggestion but I can't get proper band structure and also get very low band gap value. Attached please find the builder file for the quardilayer. I have checked with both DFT and Huckel method with various k-points. Please let me know if there is any mistake in my model.

I also want to know that if I just repeat in z direction by  "Reptition"  in the "Builder" to extend the z direction of MoS2 to 4 atomic layers then, will it take care of the Van der Waals interactions between the layers?

Thanks
Ramkrishna

[zh]

The setup for the bulk configuration of 4 atomic layers of MoS2 may be wrong. In particular, the value of "vector_b" for the crystal is not correct. Since MoS2 is a hexagonal crystal, the a and b lattice vectors for the primitive unit cell can be described as follows:
vector_a = [0.5 *a, -0.5*sqrt(3.0)* a, 0]
vector_b = [0.5*a,  0.5*sqrt(3.0)*a, 0]
,where a is one of the lattice constants of bulk MoS2.

The manipulation mentioned by you does not take the Van der Waals interaction between the atomic layers into account. The Van der Waals interaction can be properly described the DFT-D2 method of Grimme or other Vdw-DFT functionals (see http://www.fhi-berlin.mpg.de/th/Meetings/DFT-workshop-Berlin2011/presentations/2011-07-15_Tkatchenko_Alexandre.pdf). However these are implemented in the current version of ATK. 

[ramkrishna]

I have checked with that lattice vectors but still can't get reasonable band gap. Earlier you had mentioned that I have to modify the lattice constant of b (b should be set large enough to get  thick vacuum layer). So if I use the lattice vectors as you say in your previous post then how can I fit a thick vacuum layer? I can understand, something is wrong in my bulk configuration but can't find out properly. If it is possible then, can you please send me builder file?

I am using the current version of atk (11.2.3) but still I can't see no variation if I will increase the layers.

[zh]

Sorry, there is a typo in my previous reply, in which I mentioned the increasing of lattice vector b for a vacuum layer. It should be lattice vector c.

For the coordinates of atoms in the primitive unit cell of bulk 2H-MoS2, they can be referred to the paper: 
Ricardo Grau-Crespo and Rafael Lo´pez-Cordero, Phys. Chem. Chem. Phys.,  2002,  4,  4078–4079.

The attached scripts for the bulk configuration of 2H-MoS2 and n-layers of MoS2 (n=1, 2, 3, 4) are constructed  according to the atomic coordinates listed on the Table 1 of the above paper. The experimental lattice constants of 2H-MoS2(a = 3.16 angstrom, c = 12.296 angstrom) are used.

[ramkrishna]

I have done the band structure calculation with that builder scripts using Huckel as well as DFT with k points (11X11X11), but I can't get any significant change between bulk, 1-layer, 2-layer band structure. According to the literatures( like http://pubs.acs.org/doi/abs/10.1021/nl903868w ) there should be a drastic change between these and the 3, 4-layers do not show any band structure at all. Please help me.