How hard and costly is it to manufacture a few nanometer-wide substance containing layers?

How hard and costy is it to manufacture
a layer of substance a few nanometers wide and how useful such structures would be?

Can you say more about your question? What kind of material? Silicon?

berkeman said:

Can you say more about your question? What kind of material? Silicon?

Such materials as metals and metals' oxides

You mean like nanoparticles?

https://en.wikipedia.org/wiki/Nanoparticle

Well, semiconductors currently have nanometer-size features, and are pretty important (obviously).

Nanoparticles have some interesting applications...

What applications do you have in mind?

DyerMaker said:

How hard and costy is it to manufacture a layer of substance a few nanometers wide and how useful such structures would be?

A few nanometres would be something like a dozen atoms thick.
It would be difficult to accurately control the thickness.

Colour-selective mirrors are typically made from thicker coatings.
https://en.wikipedia.org/wiki/Anti-reflective_coating

Does Graphene count? It's not a metal, but it conducts electricity and heat as well as metals. There are lots of YouTube videos about making it, like this one:

DaveE said:

Does Graphene count? It's not a metal, but it conducts electricity and heat as well as metals. There are lots of YouTube videos about making it, like this one:

This Youtube video reminds me of when I was doing vacuum deposition a few years ago.

berkeman said:

You mean like nanoparticles?

https://en.wikipedia.org/wiki/Nanoparticle

Well, semiconductors currently have nanometer-size features, and are pretty important (obviously).

Nanoparticles have some interesting applications...

What applications do you have in mind?

I have in mind, first of all, such application as MIM diodes.

DyerMaker said:

I have in mind, first of all, such application as MIM diodes.

What reading have you been doing about MIM diodes? Has there been recent research and development into them since the initial work a few decades ago?

https://en.wikipedia.org/wiki/Metal–insulator–metal

If you are talking about thin films that are a few nm thick then they are very, very common, there are lots of cleanrooms that can make such films. Tools such as atomic layer deposition (ALD) systems are very good at making thin insulting films and are widely used in commercial production and often available even in "regular" university cleanrooms.
These films can then be patterned to make very narrow strips; a few tens of nm wide is "easy" using electron beam lithography; structures more narrow than that can be made but the yield does suffer.