is there a way to simulate a cluster of quantum dots?

[zhySarah]

Hi,

I am thinking of using a molecule (Cd₂S₂) to simulate a cluster of CdS Quantum dots, but not sure how to build one. Any comments or suggestions? 

PS: Is there a tutorial teaching how to adsorb a molecule to a nanotube (or any other configuration?)

[Umberto Martinez]

You can start from this tutorial: http://docs.quantumwise.com/tutorials/benzene_au111.html
and also go through these one to familiarize with the Builder:
http://docs.quantumwise.com/tutorials/vnl_builder.html

[sumit34]

After building quantum dots, is it possible to use them in Synopsys tools for TCAD simulations?

[Anders Blom]

No, that is not possible. TCAD has no atomic resolution for things like this.

[sumit34]

Sir,
I want to analyze the variation in bandgap with the change in number of atoms of Quantum dots. For this, I am using silicon Quantum dots. I have noticed the change in bandstructure but not in bandgap when considering the bandgap as minimum energy difference between conduction and valence band.
Attached are the bandstructures that I have obtained by varying the number of atoms in 3D.

I have used the repeat option to repeat the atoms in the three directions. It does not allow the control on number of atoms. Could you any other way by which the atoms number can be increased in the structure.

[Anders Blom]

Seems like you are just computing a larger and larger periodic supercell of Si, so that will not change anything (just take longer time). A cluster will have vacuum around it, and you can build one using the  Wulff construction tool in the Builder.

[sumit34]

Thank you sir!
So I should build a cluster using Wulff construction tool for analyzing the variation in bandgap with the change in number of atoms of Quantum dots?

[Anders Blom]

You can try that, yes, although real quantum dots are rarely as regular in shape as the Wulff construction gives; more typically they are pyramidicial or similar.

[sumit34]

Sir,
I tried the Wulff construction tool for analyzing the variation in bandgap. Unfortunately, after having Wulff cluster the bandstructure option in analysis does not show up. So, how can I find bandgap after having Wulff cluster.

[Anders Blom]

Only periodic structure have a band structure... You can use the HOMO/LUMO gap instead for a cluster.

[sumit34]

Sir,
The Wulff construction tool does not provide an option to increase the atoms in  a discrete manner. I noticed that number of atoms in a cluster is dependent on the radius of the cluster. Is there any way I can set up the number of atoms as my wish in a cluster.

[Ulrik G. Vej-Hansen]

Just to clarify, do you want to add single atoms to an existing Wulff construction, or would you like to add whole layers?

In both cases, the resulting structures are not really Wulff constructions anymore, though they can of course be perfectly valid nanoparticle structures.

[sumit34]

I want to vary the number of atoms by certain number (e.g. increase 2 atoms). In Wulff construction, number increases with radius. There is no such control on number of atoms.

Let say I want to create a structure with 15 atoms. How will I do it?

[Anders Blom]

That's more a question for you than for us.  What does a nanocluster with 15 atoms look like? I certainly don't know and wouldn't know without doing the simulation. I guess there are many answers, but you could just put 15 atoms in a cube relatively close to each other (so they bond reasonably but also don't repel dramatically) and optimize the geometry with DFT. Or make a Wulff construction that is close to 15, then remove some atoms (somewhat symmetrically) to make it 15, and optimize. The other natural question I would have is, why 15? What's magical about that number? Have you or someone else synthesized clusters of that exact atom count in the lab? If so, or otherwise, is there experimental evidence of the shape?

[sumit34]

I had put 15 as a random number. 
Actually, I want to obtain the bandgap (HOMO-LUMO gap) variation with respect to the number of atoms. So, I need to vary the number of atoms in a definite proportion so that appropriate graph is obtained.