Effect of SAM physisorbed on graphene - some questions

[thund3rcz]

Hello everyone! :)

I'm sorry that I'm posting something that might be obvious, but I'm still struggling to capture all the concepts behind the fundamental physics of this model.

Let's say, I have a 2D sheet of graphene, on which, there's a physisorbed layer of molecular dipoles (vdW bonding, no covalent bonds). Eventhough graphene is semi-metal, I consider it in this model as a metalic surface, sicne there's no band-gap = can I do that? I'm not 100% sure, but I think I can.

I understand that these dipoles can (when aligned in the same direction) modulate the workfunction of a surface when aligned: my vision is that there's a desk capacitor with an electric field between the poles of the dipole, and the electron that want to escape the material also has to get through this extra field, which either increases or decreases the energy needed depending on the direction. Is this (simplified) view right?

Okay, so, this modifies the work function of the surface. The work function is linked to the difference between Fermi level and vacuum level, and since it changes, the only variable in this equation is the Fermi level - does that mean the Ef changes with the adsorbed dipoles? Isnt Fermi level shifting equivalent to doping? But doping = charge transfer, right? From the scientific works I read, they generally agree that there can be a charge transfer during adsorption, but some say it's only during chemisorption. Can there be a charge transfer (=doping?) during physisorption, when the molecules are bound only by noncovalent interaction?
And, if the dipoles in the layer orient randomly (that means if they try to minimize the energy), will they still induce some change to the properties of the surface?

Hope it's not too many questions at once. I tried my best at grasping the concepts, read scientific works, some books, but failed to find a source that would 100% answer all my questions. Many thanks to anyone who can clarify some of these questions!
Have a great day, :-)
Matt

 

[eljose79]

Hello Matt,

Thank you for your post and questions. It's great to see someone so interested in understanding the fundamental physics behind a model.

To answer your first question, yes, you can consider graphene as a metallic surface in this model. While graphene does have semi-metallic properties, it can still be treated as a metal in certain situations.

Your understanding of how aligned dipoles can modulate the work function of a surface is correct. The electric field between the poles of the dipole affects the energy needed for an electron to escape the material, thus changing the work function.

Regarding your question about the Fermi level, yes, it will change with the adsorbed dipoles. This can be considered as a type of doping, but not in the traditional sense of charge transfer. Doping in this case refers to the modification of the electronic properties of the surface due to the presence of the adsorbed dipoles.

As for the charge transfer during physisorption, it is possible but not as common as during chemisorption. Physisorption involves noncovalent interactions between the surface and the adsorbate, so any charge transfer would be minimal.

If the dipoles in the layer are oriented randomly, there will still be some change in the properties of the surface, but not as significant as when they are aligned in the same direction.

I hope this helps clarify some of your questions. If you have any further inquiries, please don't hesitate to ask. Keep up the curiosity and have a great day as well!