## Photonics

#### Modeling MoS2 on Graphene using FDTD’s 2D structures

• Chris Kopetski
Ansys Employee

Simulating MoS2 (0.615nm thick) on a layer of Graphene requires an extremely small mesh when using standard 3D material models. I think the 2D object and material model (i.e. https://support.lumerical.com/hc/en-us/articles/360042244874-Graphene-surface-conductivity-material-model) are better for this type of simulation. I get sensible results for MoS2 and Graphene on their own, but combining them in a single simulation doesn't work.

Note: I took the values for dielectric constant of MoS2 from http://journals.aps.org/prb/pdf/10.1103/PhysRevB.90.205422

Question migrated from the Lumerical KX

• Chris Kopetski
Ansys Employee
The main advantage of using 2D objects to model thin layers like Graphene is that it allows you to use a mesh size that's much larger than the layer thickness. For example, you might use a 10nm mesh, even though the physical thickness of the graphene is 1nm. However, this doesn't work if you have two very thin layers. The algorithm can only handle one '2D material' for each mesh point. The meshing algorithm basically sees the two objects (graphene & MoS2) as being directly on top of each other. The end result is that one is ignored.
Unfortunately, I don't think you'll be able to use two 2D materials directly on top of each other. I can think of two possible solutions, although I must admit that I'm not sure which one is better.
Use a 2D material model for one of the materials (i.e. Graphene) and a bulk 3D model for the other (i.e. MoS2). This will require a small mesh to resolve the bulk 3D material, but it should allow you to correctly model the system.
Create a single 'effective' 2D material model that represents the behavior of the combined graphene-MoS2 structure. The advantage of this approach is that it would allow you to use a larger mesh size. However, it would require some thought and careful testing to determine if it's possible to create such a model that accurately represents the combined materials.