Long and narrow, free of defects, and soluble: graphene nanoribbons by bottom-up synthesis

(PhysOrg.com) -- Electronic components based on graphene could render our current silicon-based electronics obsolete. Graphene, a more recently discovered form of carbon, consists of two-dimensional sheets of aromatic six-membered carbon rings in a honeycomb arrangement. In contrast to extended graphene layers, narrow graphene nanoribbons have semiconducting properties and could thus be candidates for electronic applications.

Klaus Müllen and a team from the Max Planck Institute for Polymer Research in Mainz have now introduced a new method for the synthesis of long, narrow graphene ribbons with defined dimensions in the journal Angewandte Chemie.

Previously, graphene ribbons were mainly cut out of larger graphene sheets or were obtained by slitting open carbon nanotubes lengthwise. However, with these methods it is impossible to produce ribbons that have a precisely established ratio of width to length as well as defined edges. These details are important because they determine the electronic properties of the ribbons. The search has thus been on for a method that allows controlled production of very narrow graphene ribbons—an extremely difficult challenge. The German researchers working with Müllen are now well on the way to overcome it. They are not starting with large structures to cut up (top-down); instead they are building their ribbons from smaller components (bottom-up).

The building blocks selected by Müllen and his team are long chains of aromatic six-membered carbon rings called polyphenlyenes. In contrast to previous approaches, they did not produce straight chains; instead they made them with a flexible, zigzagging, bent backbone. Furthermore, they attached hydrocarbon side-chains to the backbone to increase the solubility in organic solvents, which allows the compounds to be synthesized and processed in solution.

The next step is a reaction that splits off hydrogen (dehydrogenation). This causes a ring-closing reaction in each pointy tip of the zigzag, forming a new aromatic six-membered carbon ring that shares a side with three neighboring rings—the chain transforms in to a narrow ribbon.

In this way, the team was able to produce a series of different nanoribbons with lengths reaching over 40 nm. The width of the ribbon was defined by the size of the “points” of the polyphenylene precursor. The resulting ribbons are free of defects and soluble in common organic solvents.

“We have been the first to demonstrate that structural perfection can be achieved by the classical bottom-up synthesis of defined graphene nanoribbons,” says Müllen. “The solubility of the ribbons is an important requirement for the large-scale production of electronic components.”

Explore further: Nanoribbons for graphene transistors

More information: Klaus Müllen, Graphene Nanoribbons by Chemists: Nanometer-Sized, Soluble, and Defect-Free, Angewandte Chemie International Edition, Permalink to the article: dx.doi.org/10.1002/anie.201006593

nuge

not rated yet Feb 08, 2011

Would someone please clarify something for me: is graphene actually a sheet of benzene rings bonded together at each of the corners, or what? This makes me wonder if you could make a similar structure to buckyballs or nanotubes with Benzene rings.

PinkElephant

5 / 5 (3) Feb 08, 2011

@nuge,

You can think of it as a sheet of benzene rings, except the classical definition of a benzene ring is a hexagon of C with H atoms at each corner. In graphene, there is no H, only pure C. And it's just one continuous sheet of C. You can visually parse it into "rings", but there are no distinct rings in the sheet (each C atom belongs equally to 3 adjacent "rings".) The chemistry is a bit different, because of this: the different affinity of valence electrons toward C vs. H in benzene, compared to uniform and symmetric affinity in a pure-C sheet.

Similar to buckyballs and nanotubes, a graphene sheet is just one big holistic macromolecule. You can parse it into a collection of hexagonal rings, just like you can parse a buckyball into a bunch of hexagons and pentagons. But in the end, it's a single entity, and its electronic properties derive from its integrated structure (rather than any artificially delineated substructure.)

Ah, righto. Thanks PinkElephant. So there are delocalised electrons in the middle of the rings?

5 / 5 (2) Feb 08, 2011

So there are delocalised electrons in the middle of the rings?

Well... sort of. Firstly, they aren't actually in the middle of the rings (nor would they be in benzene.) Delocalized electrons are simply free to skip from one atom to another immediately neighboring one.

So in graphene, on average at any given instant 3 of the 4 valence electrons on each C atom are binding it covalently to its 3 neighboring C atoms, while the 4th electron is randomly migrating through the molecule.

Thanks, you've been very helpful. One more question though - is the delocalised electron the reason for the high conductivity of graphene? And how good is the conductivity anyway, compared to metals like Gold?

Yes, the delocalized electron is responsible for the high conductivity. Or to put it more accurately, the pool of loosely bound electrons is responsible.

Though as opposed to metals, in graphene, due to the structure of the molecule, electrons can only migrate along the 2D plane of the graphene sheet (they cannot easily jump from sheet to sheet in bulk graphite.) So conductivity really depends on how well-aligned the electric field is with the graphene sheet's plane. It also depends on orientation of the sheet: current travels easier in some directions than in others along the sheet, due to how the bond axes align with the electric field.

The conductivity of isolated graphene is excellent. At room temperature it can be more conductive than any metal, when it is suspended in vacuum. However, when laid against a substrate, at room temperature the thermal vibration modes induced by the substrate can reduce carrier mobility significantly; by how much depends on the substrate.

yoatmon

not rated yet Feb 09, 2011

On the nano scale, exact measurements have proven that the tensile strength of graphene is 217x that of steel. Its conductivity is 100 times higher than that of copper. Sequential conductivity ratings for:
1) Graphene = 5700
2) Silver = 66.5
3) Copper soft / hard = 57 / 56
4) Aluminum = 35
5) Brass = 15 - 19
Values are derived x = 1 : specific resistance.

Oops I missed Gold. =s 45 and is somewhere between copper and aluminum. An almost unequalled property of gold is its resistance to oxidize.

Though as opposed to metals, in graphene, due to the structure of the molecule, electrons can only migrate along the 2D plane of the graphene sheet (they cannot easily jump from sheet to sheet in bulk graphite).

Researchers at MIT were the first to successfully grow multiwalled CNTs on a graphene sheet. This enables the electrons to move just as easily in the 3rd dimension as in the other two within one plane.

antialias

A surface (albeit a toridal one in the case of a CNT) is still topologically a 2D surface.

That may well be; it doesn't make my statement less viable.

kcameron

yoatmon, Where are you getting the 100x copper conductivity number? In my searches, I've only found values that show marginally better bulk conductivity than copper. The 100x numbers are for current carrying capacity which is not the same thing. High current carrying capacity is a useful property for nano-scale circuit interconnects since it avoids failures due to electro-migration.

100X copper conductivity would revolutionize many fields; especially electric motors and inductors. Unfortunately, it doesn't look like it's feasible.

How about superconductivity? What temperature does it superconduct at, and is that still only in the plane of the sheet? If so, that would be a pretty interesting effect.

And another thing, what is the magnetic behaviour of graphene like?

not rated yet Feb 10, 2011

It should be noted that the astounding properties of graphene have been proven mainly on the nano scale. It is very difficult to maintain these properties when scaling up to the macro level but not impossible.

yoatmon, Where are you getting the 100x copper conductivity number?

Ref. to wikipedia:
//en.wikipedia.org/wiki/Graphene

And another thing, what is the magnetic behaviour of graphene like?

Ref. to Berkeley News Bulletin:
//newscenter.lbl.gov/news-releases/2010/07/29/graphene-under-strain/

StandingBear

not rated yet Feb 13, 2011

Ever seen a honeycomb with its six sided cells? Pass a plane through the comb at planarly parallel to its top surface...thats where the bees come out sports fans, the top.....and what you have is graphene, graphically. Really I am not goophin around!

yoatman, I'd seen the wiki page before but I went back and checked it anyway. I don't see where the current carrying capacity is mentioned. The electrical conductivity numbers aren't clearly stated but the closest reference I could find is "The corresponding resistivity of the graphene sheet would be 10−6 Ω·cm". Copper resistivity is about 1.7x10-6 Ω·cm. So graphene's conductivity is about 1.7X that of copper; a far cry from 100X.

1.7X would still be pretty good. Given the greatly reduced density and ability to withstand much higher temperatures, graphene wires may someday lead to substantially improved electric motors, etc.

yoatman, The relevant entry from the wiki page is "The corresponding resistivity of the graphene sheet would be 10−6 Ω·cm". That corresponds to conductivity of about 1.7X that of copper: a far cry from 100X.

Given graphene's greatly reduced density and ability to withstand high temperatures, it would still make for a substantial improvement to motors, etc.

yoatman, The relevant entry from the wiki page is "The corresponding resistivity of the graphene sheet would be 10-6 ohm-cm". That corresponds to conductivity of about 1.7X that of copper: a far cry from 100X.

Given graphene's greatly reduced density and ability to withstand high temperatures, it would still make for a substantial improvement to motors, etc.

Sorry for the repeats. The web interface kept telling me that the formatting was wrong but apparently accepted the comments anyway.

Apology NOT accepted.

not rated yet Feb 14, 2011

------------------------------------------------------
"Copper resistivity is about 1.7x10-6 Ω·cm. So graphene's conductivity is about 1.7X that of copper; a far cry from 100X."
------------------------------------------------------
Check out e. g. : (http//www)
.electronicsweekly.com/Articles/2009/08/05/46677/georgia-tech-claims-100x-copper-conductivity-for-graphene-interconnect.htm

I had seen that electronics weekly article before. It is about current density; not conductivity. Unfortunately, the article's author also got confused about the difference.

Here's the relevant excerpt from that article:
"This makes them very robust in resisting electromigration and should greatly improve chip reliability. The current carrying capacity is at least two orders of magnitude higher than copper at these size scales"

Again: current carrying capacity != conductivity.