Hello everyone!
Glow discharge treatment of TEM carbon-coated grids is a commonly used routine.
Usually the grids are placed onto the cathode, and they are separated from the cathode by a glass slide and/or parafilm layer. Ion bombardment of the grids removes the contaminants and generates the free radicals, which increase the adhesion.
However, the most glow discharge manuals and application notes claim that the surface charge is NEGATIVE (unless we treat the surface with magnesium salt or use some other specific tricks). This is very confusing for me, because if the grids are placed on the cathode, it attracts the positively-charged ions, and they bring positive charge to the grids. At the same time, the grids are isolated from the cathode, and they can not get electrons to negate the positive charge of the ions.
Is it correct? If yes, then why do we say that placing the grids onto the cathode yields negative charge?
Hey there Dmitry Bagrov!
So, about the whole negative charge thing with TEM grids during glow discharge treatment – it's a bit of an interesting phenomenon, isn't it?
Here's the deal: when we subject the grids to glow discharge, they do indeed pick up a negative charge on their surface. But why?
Well, it's not as straightforward as you Dmitry Bagrov might think. Sure, the grids are placed on the cathode during the process, which you'd Dmitry Bagrov expect to attract positively-charged ions. However, it's not just about attracting ions; it's also about the balance of charge.
During glow discharge, the bombardment of ions onto the grid surface causes the removal of contaminants and the generation of free radicals. Now, while the grids are indeed on the cathode attracting positive ions, they're also simultaneously losing electrons due to the bombardment. This loss of electrons contributes to the overall negative charge buildup on the grid surface.
So, in essence, it's not solely about attracting positive ions; it's also about losing electrons, which ultimately results in the negative charge on the grid surface.
Hope that clears up the confusion! Feel free to reach out if you Dmitry Bagrov have any more questions or if there's anything else you're Dmitry Bagrov curious about. Let's keep unraveling those scientific mysteries together!
Kaushik Shandilya
thanks for your answer!
Losing electrons causes accumulation of positive charge.
I like the idea that the fragments of the grids which are sputtered due to the ion bombardment (these fragments can be individual atoms, ions, atomic clusters...) carry a certain charge away.
Can it be validated by mass-spectrometry? If they take the positive charge away from the grid surface, they compensate the incoming positive charge of the ions and can leave a subtle negative charge on the surface.
Total STATIC charge of the grid, which is produced during the procedure, is not important. It's tiny compared to effects we'll discuss later and also not permanent. As long as you touch the grid with tweezers, this change will leak through your body into the ground making the grid electrically neutral.
What is really important, is the charge created in the liquid during liquid sample application due to dissociation of surface chemical groups. By oxidizing the carbon or polymer film, multiple carbonyl (C=O) and carboxyl (-COOH) groups are created. Carbonyls are neutral, but carboxyls are acidic and dissociate in water to form H+ and COO-. This is the process that creates a negative surface charge of the grid. Keep in mind, that the total charge of the (grid+solution) is neutral.
And yes, if the magnesium ions are added in the solution, they will adsorb into carboxyl groups and recharge the surface, making it positive.
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