Bonding properties of silicon (001) wafer

[Skullmonkee]

Hello,
I have been doing some research with self assembled monolayers. I was wondering if a molecule terminating in -OH was able to bond with a silicon wafer in atmospheric conditions?

 

[Reshma]

Hello,
I have been doing some research with self assembled monolayers. I was wondering if a molecule terminating in -OH was able to bond with a silicon wafer in atmospheric conditions?

Si is hydrophobic by nature, so atmospheric water vapour may not have much influence. But if you want to form monolayers using silanol (SiOH) linkages, you can consider oxidising the Si wafer first.

 

[Kmenex]

The bond for Silanol will be pretty weak though, will hold at atmospheric but depending on the nature of your research might be pretty useless.

 

[Skullmonkee]

Thanks for the replies.
Si forms an oxide which is responsible for making the surface natively hydrophobic. It's possible to remove the oxide with things like piranah solution to make the surface hydrophilic, in which case silano linkages can form.

However, specifically what i was referring to was the possibility of a percentage of the native oxide layer being disrupted, in which case silanol linkages might form slowly over time. Say, if the silicon surface is submerged in something like ethanol or methanol for 24-48 hrs.

Thanks for any furthur help

 

[Kmenex]

I just realized that when the SiOH bonds to the wafer that it's going to bond covalently and actually be pretty strong, however your looking to see if -OH can form i don't see how it's going to given that there will be strong repulsion between that head group and the SiO surface.

Also i did not know that you needed to remove the oxygen from the SiO surface in order to get SiOH bonds to form, for the Si bonds to the oxygen in such a case.

Also, why are you interested in the -OH group? and what is the molecular species you wish to assemble on the surface?

 

[Skullmonkee]

Hi,

Yes, generally for functionalisation of the silicon surface (SiO2) the native oxide layer should be removed. This can be done using a strong oxidizer which hydoxylates the surface of the silicon, leaving -OH groups (SiOH). These groups can react.

In my case however, i did not want the silicon to bond. The oxide layer was not purposefully removed by an oxidizer. Instead, the substrate was incubated in an ethanol solution containing thiol groups terminating in a hydroxyl group (HS-(CH¬2)11-(OCH2CH2)3-OH). These groups were used to bond to another part of the substrate made out of gold (via Au-S bonds). When the substrate was removed from the solution it could be washed removing all species from non-bonding surfaces (ie. the silicon). This would theoretically leave gold areas with attached thiols, and silicon areas with nothing attached.

However, i am under the impression that during incubation of the substrate in the solution, the thiol molecules may have also bonded to the silicon via -OH ends. For this to occur, a percentage of the the SiO2 layer on the surface of the silicon substrate would have to have been converted to Si-O-H

 

[MATLABdude]

I also do some work with silane-chemistry self-assembled monolayers. In my experience, Piranha clean or a gentle oxygen (and some suggest water) plasma are great ways to ACTIVATE the surface SiO2 (from say, the native oxide) and make it more accepting of silanes. A small percentage of silanols will form in the presence of moisture, but many more will form when treated as previous. Then again, I purposefully make use of the native oxide, rather than thermally growing a thicker one.

If you can do things in a dry box, heating the substrate up to around 120 C will also dehydrate the surface, eliminating your silanols. If you don't mind completely removing the native oxide layer, have you considered using a Buffered Oxide Etch or hydrofluoric acid? They're meant for stripping SiO2, and I've been able to completely remove (silane) SAMs from silicon substrates this way. WARNING: Despite being relatively weak acids, Buffered Oxide Etch and (high concentrations of) hydrofluoric acid are extremely poisonous, and if you need to use them (and don't know how), find someone who knows what they're doing to give you a hand!
https://www.physicsforums.com/showthread.php?t=312420

I'm not sure what impact HF might have upon your thiols (it shouldn't have an impact on the gold layer), but you can probably determine the impact with a little experiment. Actually, have you used any techniques to look for (or better yet, quantize) the amount of sulphur on the gold versus silicon surfaces? If not, this will help you determine the extent of the problem.

L. T. Zhurvalev did a lot of work on (albeit) silica and hydroxylated surfaces--some of that (or the follow-on work by others) might be of interest to you.