In 1955 while addressing the National Academy of Sciences Richard Feynman stated "Scientific knowledge is a body of statements of varying degrees of certainty." As usual, Feynman's statement was spot on, and holds true decades later. In his famous "Plenty of Room at the Bottom" lecture Feynman talked about what we now call nanotechnology, and all the different applications. Here I am, half a century later, working "at the bottom" and living in a world of uncertainty. I hope to share some of the exciting discoveries at the nanoscale and explain how they apply to my passion of biotechnology- as well as the everyday world. Learn more about Nicholas Fahrenkopf
My posts are presented as opinion and commentary and do not represent the views of LabSpaces Productions, LLC, my employer, or my educational institution.
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I’m a molecular biologist trapped in the body of someone with a physics degree. I’m a member of a bacteriology lab trapped in a college of “Nanoscale Science and Engineering”. As such, while I try to do cool nanoscale things with biological materials, I’m surrounded by physicists and electrical engineers along with their research projects and problems.
Don’t get me wrong, it is often very interesting and downright “cool stuff”. For now I’ll skip hot electrons and ballistic transport, or density functional theory calculations and focus on some buzz words you might have heard:
In a word, they’re called fullerenes. These materials are made of one thing: carbon. Just carbon, and nothing but carbon. Why are different formulations of carbon so exciting and worth spending millions if not billions of dollars on? As with just about anything in nanotechnology, matter behaves differently at the nanoscale. Graphite (in pencils) is pretty boring. Diamonds, while pretty (and apparently friendly to women) are pretty inert and solid. The carbon allotropes have little to do with their nanoscale cousins, although that’s not to say we can’t turn on into the other.
The 2010 Nobel Prize in Physics was awarded for ground breaking research on graphene- a molecular, hexagonal lattice layer of carbon. How do most people make graphene in the lab? A process called mechanical exfoliation. That is, take a hunk of macroscale graphite, stick Scotch Tape to it, peel off a bit, and then repeatedly stick and unstick that bit of graphite until you get down to a molecular layer, and then stick it on a wafer. You can then tell optically (or through Raman spectroscopy- looking at molecular vibrations) if you got down to a monolayer, or if you only went to a bi- or tri-layer. In conference presentations it is called “mechanical exfoliation” but it is really the “Scotch Tape Method”.
So how does a hunk of graphite and Scotch Tape become a Nobel Prize? Again, it’s all in the nanoscale. In graphene electrons (charge carriers) are effectively massless, and as such their mobility is orders of magnitude faster than in silicon or other semiconductors. In nanoelectronics when you’re talking about orders of magnitude improvement, people start drooling and writing checks. Of course, you can’t make computer chips with Scotch Tape, so there is a ton of research- more recently fruitful research- on wafer scale graphene using, for example chemical vapor deposition (CVD).
With CVD organic precursors (like methane) are allowed to adsorb on your substrate and anneal (or reorganize) into the hexagonal lattice of graphene. Most of the time the deposition is done on a copper film because the solubility of carbon (the precursor) is low in copper. (You don’t want the precursor getting embedded in the substrate- you want it on top.) So then you’re left with a film of extremely highly conductive carbon on a conductive substrate of copper, which isn’t the most useful set up. In fact, in traditional nanoelectronics manufacturing, copper is a “poison” and everything is done to keep it away from your devices. In a cleanroom the tools, and even wafer carriers are “clean” or “dirty” depending on if they’re free of copper or not.
So how do you get the graphene off the copper, and keep your final devices copper free? How do you deal with imperfect films? All current research topics my friends. A better question is- what are you going to do with this? The immediate option is high frequency applications since the electrons in graphene have such a high mobility and can respond so quickly. Others are potential carbon based electronics- building novel devices out of graphene. Also, since it is so thin and therefore flexible you could use it for flex circuits. And as will just about any electrical device these days, you could use it as a sensor.
Personally, I’m not convinced it is worth the effort. In some aspects it is a solution looking for a problem. But from the nanoelectronics industry point of view they’re running into significant road blocks on their quest for smaller and faster circuits. They’ve placed substantial bets on a lot of long shot solutions and if the odds are so low that one of this bets will pay off you need to diversify yourself. From the basic science point of view however, it is a really interesting problem. How does carbon form this structure? What can we learn about electrons, quantum mechanics, and more with this new system? So while researchers can do some pretty cool things with this unique material, don’t expect it to save the world… just yet.
Want to read more about graphene? Check out these recent papers:
In “Harvesting Energy from Water Flow over Graphene” Prashant Dhiman, et. al. describe the harvesting of power from a graphene film- up to 175 W/m2. I find this interesting because it seems like a practical use of graphene that doesn’t require high quality, large films. These harvesters could be incorporated into other MEMS devices to provide power to sensors.
Ivan Vlassiouk and colleagues showed in “Role of Hydrogen in Chemical Vapor Deposition Growth of Large Single-Crystal Graphene” how flowing a partial pressure of hydrogen influences the formation of quality films. It is impressive, as usual, how such a seemingly small change (adding hydrogen gas with CH4 gas) can greatly impact the results of film growth.
I didn’t talk about the other fullerenes besides graphene, but if you were to roll up a sheet of graphene you would get a carbon nanotube. They’re just as interesting as graphene, with their own problems. D. P. Hunley and coworkers published “Crystallographically Aligned Carbon Nanotubes Grown on Few-Layer Graphene Films” which showed that CNTs that are grown on a few layers of graphene tend to orient themselves at 60 degree intervals- the same as the hexagonal lattice of graphene.
Right after I hit publish on this post, I went back to my Google Reader and came across three new papers from ACS Nano with some cool applications for graphene. I haven't read them yet, but they look interesting! (There's even one that looks at stem cells on graphene!)
"Rational Design of Hybrid Graphene Films for High-Performance Transparent Electrodes" by Y. Zhu, et. al.
"Origin of Enhanced Stem Cell Growth and Differentiation on Graphene and Graphene Oxide" by W. C. Lee, et al.
Preview image from Wikimedia Commons. Images from ACS Nano, and Nano Letters are copyright ACS Publications.
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Nice post Nick.
Just one thing. Remember not all of us dominate the lingo, so put some links (or little explanations) in some of the terms.
For example you mention Raman. I kind of remembered that it was a type of spectroscopy, but somebody else may not know what it is.
Also CVD. For me, those 3 letters are CardioVascular Disease, not chemical vapor deposition, and the 3 letters are almost ungoogleable unless you know about the topic.
That last article about the stem cells is very cool, seeing how they can direct the cell lines towards one way or the other just by how the base compound (graphene/graphene oxide) interacts with biological marker. Really cool. And graphene should be perfectly compatible with implantation.
Argh! You're completly right, of course, and thanks for calling me out on it. (Edits have been made.) I really should know better. When I was preparing for my thesis proposal defense I talked about annealing films of hafnium oxide before putting DNA on it. It is a common term in semiconductor manufacturing, and in molecular biology. But "annealing" means very different things to those two groups of people!
Lesson (hopefully) learned!
Great post Nick. Nanotech is coming along way since even a few years ago. Do you have a basic process flow of making the graphene chips? Is is CVD for all layers are you sputter coating some of them? Also what are you using to do the etch? Just curious haven't done anytyhing with graphene.
So there are two ways to get graphene- one is CVD (which would be to flow methane and maybe hydrogen at elevated temperature on copper foil). You then can transfer to a wafer, etch away the copper (either ferric chloride, or I like HCl and H2O2), and then pattern the graphene with photoresist. I think etching can be done in many ways with different plasmas, but I'm not sure. Alternatively if you use mechanical exfoliation you transfer your small bit of graphene onto the wafer that already has some electrodes. Since you won't align perfectly you'd then "draw" small wires from the electrodes to the graphene using focused ion beam deposition.
From there you can make basic logic or at least test circuits, but anything more complex (that might require more layers) is beyond my knowledge. I have no idea what would happen if you tried to put down some oxide on top. You wouldn't need multiple layers of graphene though. The graphene would be your FEOL (front end of the line, or logic layer) and then the rest of a chip stack (BEOL) could still be copper lines and vias.
You can also bury electrodes in the oxide under where you stamp the graphene so you can electrostatically "dope" the graphene to be p or n type which you would need to make a pn junction and other electronic circuits.
I've actually never worked with graphene- although I helped a colleague try a new stamping method to transfer CVD copper (it didn't work). I'm not sure if anyone is making "circuits" or full "chips" yet. Mostly just proof of concept and basic research.