For the third, or is it the fourth time now, this is an empirically observable fact which has nothing at all to do with materialism. Pick any monism or dualism you like it won't go away. Are you going to keep ignoring this objection?
Let's be clear. If the causes of physical phenomena were something other than physical, this could not be acommodated by our laws of physics. We would see anomalies when examining the functioning of the brain, effects that had no visible physical cause.
Any scientist who made such an earth-shattering discovery would be instantly famous, and yet none of them appear to be even looking for this. No one who understands anything about the brain thinks that it does anything other than follow the laws of physics.
This doesn't mean that materialism is true. Science can't prove materialism. Conscousness could still exist as an epiphenomenon. But you want it to be more than that.