Graphene's behavior depends on where it sits

Aug 13, 2012 by David L. Chandler

When you look at a gift-wrapped present, the basic properties of the wrapping paper — say, its colors and texture — are not generally changed by the nature of the gift inside.

But surprising new experiments conducted at MIT show that a one-atom-thick material called , a form of pure carbon whose atoms are joined in a chicken-wire-like lattice, behaves quite differently depending on the nature of material it’s wrapped around. When sheets of graphene are placed on substrates made of different materials, fundamental properties — such as how the graphene conducts electricity and how it interacts chemically with other materials — can be drastically different, depending on the nature of the underlying material.

“We were quite surprised” to discover this altered behavior, says Michael Strano, the Charles and Hilda Roddey Professor of Chemical Engineering at MIT, who is the senior author of a paper published this week in the journal Nature Chemistry. “We expected it to behave like graphite” — a well-known form of carbon, used to make the lead in pencils, whose structure is essentially multiple layers of graphene piled on top of each other.

But its behavior turned out to be quite different. “Graphene is very strange,” Strano says. Because of its extreme thinness, in practice graphene is almost always placed on top of some other material for support. When that material underneath is silicon dioxide, a standard material used in electronics, the graphene can readily become “functionalized” when exposed to certain chemicals. But when graphene sits on boron nitride, it hardly reacts at all to the same chemicals.

“It’s very counterintuitive,” Strano says. “You can turn off and turn on graphene’s ability to form chemical bonds, based on what’s underneath.”

The reason, it turns out, is that the material is so thin that the way it reacts is strongly affected by the electrical fields of atoms in the material beneath it. This means that it is possible to create devices with a micropatterned substrate — made up of some silicon dioxide regions and some coated with boron nitride — covered with a layer of graphene whose chemical behavior will then vary according to the hidden patterning. This could enable, for example, the production of microarrays of sensors to detect trace biological or chemical materials.

Qing Hua Wang, an MIT postdoc who is the lead author of the paper, says, “You could get different molecules of a delicate biological marker to interact [with these regions on the graphene surface] without disrupting the biomolecules themselves.” Most current fabrication techniques for such patterned surfaces involve heat and reactive solvents that can destroy these sensitive biological molecules.

Ultimately, graphene could even become a protective coating for many materials, Strano says. For example, the one-atom-thick material, when bonded to copper, completely eliminates that metal’s tendency to oxidize (which produces the characteristic blue-green surface of copper roofs). “It can completely turn off the corrosion,” he says, “almost like magic … with just the whisper of a coating.”

To explain why graphene behaves the way it does, “we came up with a new electron-transfer theory” that accounts for the way it is affected by the underlying material, Strano says. “A lot of chemists had missed this,” and as a result had been confused by seemingly unpredictable changes in how graphene reacts in different situations. This new understanding can also be used to predict the material’s behavior on other substrates, he says.

James Tour, a professor of chemistry and of computer science at Rice University who was not involved in this research, says, “This is the first systematic study of the substrate’s effect on graphene’s chemical reactivity. This is a very carefully conducted study with convincing results. I predict that it will become a frequently cited publication.”

Wang adds that “it’s a pretty general result” that can be used to predict the chemical behavior of many different configurations. “We think other groups can take this idea and really develop different things with it,” she says. Tour agrees, saying, “The graphene-sensing community will be inspired by this work to explore many more substrates in an effort to optimize graphene reactivity.”

As for the MIT team, she says, “the next step is, we’re digging into the details of how bilayer graphene reacts. It seems to behave differently” than the single-layer material.

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antialias_physorg
3.7 / 5 (3) Aug 13, 2012
A couple of months ago there wa ans article on physorg on how layers below the surface could affect the properties of surface atoms (as a simulation). This seems to corroborate this simulation.

Fascinating stuff. With the right mix of substrate and surface layer we may eb able to get whole new classes of materials. Especially for scientists working on battery anodes/cathodes, fuel cells and catalysts in gerenral this should open up whole new vistas.

Might also be used in patterning graphene for ultra-thin electronics: Put it on a structured layer (template) which causes the structured parts to react (be etched off) and the unstructured part to remain. Peel. Rinse repeat.
Satene
1 / 5 (3) Aug 13, 2012
..fascinating stuff..
It's not so strange if we realize, the properties of graphene layer are determined just with fact, it was peeled off the bulk graphite. Therefore, if we place graphene sheet back to the conductive graphene, it's properties will return back to this one of bulk graphite. Now we can ask, what will happen with graphene layer, if we place it to some inert flat nonconductive material, like the mica or glass? It's properties will indeed not change. From this we may deduce, that the properties of graphene will depend on the character of the supporting material easily. Such an experiments were indeed carried out many times before already.

It means, we can remain fascinated with achievements of technology and physical research, but we shouldn't become ignorant sheep and forget all previous results and articles published here about it. Or the time, which you'd spent here will be wasted.
phwjdeng
1 / 5 (2) Aug 13, 2012
It is strange that this result is considerd to be surprising. The properties of graphnene depend on the materials underneath.
Satene
1.6 / 5 (5) Aug 13, 2012
It's not so strange, if we realize, how scientific journalism is working. It considers, the readers have zero memory and usually is even correct with it.
The understanding, what happens with electrons within graphene, when it's separated from the rest of graphite is not so difficult to imagine, if we realize that 1) all electrons are repulsing each other 2) they're still attracted to carbon atoms, which held them together. The layer of electrons at the carbon mesh is behaving like water layer covering the chicken wire mesh. Many layers of such mesh can hold a substantial amount of water between layers, when they're stacked together. But when we remove one mesh from the stack, the excessive water will drip off.
At the case of electron fluid the excessive electrons cannot be removed from peeled layer so easily - so that the isolated layer of graphene will exhibit an excess of electrons. The electrons would be more compressed there, than between many layers of graphene.
Satene
1.6 / 5 (5) Aug 13, 2012
The compression of electrons has a great impact to the mobility of electrons, because the electrons interact more strongly mutually under such a situation and they tend to ignore their bounds to carbon atoms. It's like when we introduce a stress to human civilization: the people will start to ignore the moral rules and the society will become more chaotic and unpredictable.

At the extreme case the compression of electrons may lead into superconductivity, but at the case of graphene this effect is only local and temporal: the electrons are moving in ballistic mechanism and they're doing quantum jumps during it. It increases their mobility and their conductivity significantly. But when the another conductive layer is attached to the isolated graphene sheet, then the electrons of graphene will interact with electrons of neighbouring conductive layer and their mutual stress will go down. As the result, the behaviour of graphene sheet will become more similar to the one of bulk graphite
ValeriaT
2.3 / 5 (3) Aug 13, 2012
The researchers acknowledge that it is already known that the substrate affects the graphene properties. What they did was attempt to explain why it affects the properties. They controlled where reactions took place on a graphene sheet by patterning the underlying SiO2 substrate with octadecyltrichlorosilane (OTA) ink. Nitrobenzene groups would then covalently bond to the surface of the graphene over the top of the SiO2 areas, but not over the OTA areas. The physics behind this is that the substrate material will generate local electron-hole puddles in the graphene, thereby changing it's reactivity in those areas by causing local fluctuations in the Fermi level. Places where the Fermi level fluctuates with higher amplitude will have higher reactivity than areas with smaller fluctuations. They suggest applications of their patterning method in biosensing.
Infinion
1 / 5 (3) Aug 14, 2012
could someone kindly explain why Satene and ValeriaT are being downvoted? What exactly was 'wrong' with their statements?
Satene
1 / 5 (2) Aug 14, 2012
You shouldn't expect any matter-of-facts answer, because PhysOrg attracts mostly dumb religious trolls, which is apparent with their plain downvoting of all cold fusion related thread, for example. It's partially the result of the selection of posters with PO moderators. Trolls are brake of every progress, not just in physics.
antialias_physorg
5 / 5 (1) Aug 14, 2012
attracts mostly dumb religious trolls, which is apparent with their plain downvoting of all cold fusion related thread

You know what? When there actually is an article on cold fusion then my guess is that cold fusion related posts will probably not be downvoted.
Bringing cold fusion issues to threads which have nothing to do with cold fusion just engenders the same reaction that creationist babble gets when it's posted on threads that have nothing to do with creationism. Cold fusion trolls are identical to religious trolls in that respect.

It's nice to have a pet peeve, pet theory, belief, conspiracy theory or whatnot. But posting it in threads that have nothing whatsoever to do with it gets on other people's nerves, you agree?

Or how would you feel if someone expounded in length on every thread on physorg about how great sportsteam X is? You'd think it obnoxious, right?

There you are, then.
Satene
1 / 5 (2) Aug 14, 2012
my guess is that cold fusion related posts will probably not be downvoted
Your "guess" is apparently wrong. BTW You're one of most apparent trolls here - which explains, why you're getting upvoted so often here in community of another trolls.
posting it in threads that have nothing whatsoever to do with it gets on other people's nerves
It's irrelevant comment (a pointless verbiage actually), as all my posts here were strictly related to topic (with exception of these last ones) - and they were still downvoted.
antialias_physorg
5 / 5 (1) Aug 14, 2012
which explains, why you're getting upvoted so often here in community of another trolls.

Make up your mind. Which is it. Do trolls get upvoted or downvoted?

But maybe people get upvoted who actually contribute to the topic at hand? Or people who actually respond to questions posed? Ever think about that?
Satene
1 / 5 (3) Aug 15, 2012
Do trolls get upvoted or downvoted?
Of course they do. At first, just the trolls do like creation of sockpuppets dedicated to voting, if not organizing of voting mafias - not the normal users. At second, in the community of trolls the greatest troll is always valued celebrity - just check the Hitler and his Nazi Germany. At third, the trolls like you can always rationalize their trolling for themselves. They do trolling, because they do believe, they're absolutely right with it.
Satene
1 / 5 (3) Aug 15, 2012
It's not difficult to recognize voting troll. The "lite" account is typical sockpuppet, as he doesn't use his account for posting - only for abusing of voting functionality.
antialias_physorg
3 / 5 (2) Aug 15, 2012
just check the Hitler and his Nazi Germany

Godwin. You lose.

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