This Week’s Science Roundup

This week, there have been some interesting papers dealing with new magnetic materials; using thin-films in new and interesting ways (such as in transistor memory devices and gate dielectrics in carbon nanotube transistors); nanoscale photonics using nanowires and nanotubes; exploring the possibility of creating quantum dots in graphene using electrostatic potential barriers; using scanning tunneling microscopy to look at the Kondo effect in molecules and carrier dynamics in p-n junctions while they’re being operated; figuring out what part of the brain is responsible for our wandering minds; and two interesting applications of quantum mechanics in biology - theoretically considering phonon-assisted tunneling of electrons in elucidating how we smell, and using computational quantum mechanical calculations to study protein splicing. Whew.

Nanoscale/Condensed Matter-Related:
- Hybrid metal-organic materials that are magnetic at room temperature
- Thin-film ferromagnetic devices whose magnetization is modulated via an applied electric field
- A new organic (pentacene) thin-film field-effect transistor (FET) as a possible non-volatile memory device
- Using self-assembled monolayers (SAMs) as the gate dielectric in carbon nanotube FETs
- Nanoscale photonics: nanowire LEDs and nanotube coaxial cables
- Creating quantum dots electrostatically in graphene
- Manipulating the Kondo effect in molecular systems using STM
- Using STM to study carrier dynamics in a p-n junction

Bio-related:
- Why your brain wanders when you’re bored
- Could Humans Recognize Odor by Phonon Assisted Tunneling?
- Studying Protein Cleavage Using Quantum Mechanical Calculations

I’m going to start off with magnetism in two novel systems. Researchers in British Columbia (Canada) have reported recent success in developing hybrid metal-organic materials that are magnetic at room temperature - a goal that has proved to be elusive for a good twenty years now. These materials are desirable because they would open up a route to producing magnets chemically, under conditions that are far less extreme than the extremely high temperatures required for metallurgically producing magnets today. In addition, the organic nature of these new magnets could potentially lead to interesting properties that one could exploit in technological applications - for example, controlling magnetic properties using light! The main point of this paper, however, is more practical: not only have the researchers produced such room temperature organic magnets, but the synthesis process is sufficiently flexible that it should be possible to produce a wide range of similar materials by tweaking the process a bit, and these in turn could have many more interesting properties. (Up)

European scientists, on the other hand, have recently reported a thin-film ferromagnetic device whose magnetization is modulated via an applied electric field. The thin-film is metallic, which means that the electric field changes the electric density at the surface of the device - so it isn’t crazy to expect that the magnetic properties will change, as well. The trick is in getting everything to work out how you want it to, and this is what these researchers have done. Their device geometry relies on the use of an electrolytic cell containing the 2nm thick film (FePt or FePd) in contact with propylene carbonate electrolyte, with two Pt or Pd electrodes across the device (grown on a MgO substrate). Again, numerous technological applications of this exist (such as alternative low-power memory devices), and the key is that this device structure can be applied to other thin-film systems. (Up)

Speaking of thin-films and technological applications, a team of researchers from Korea and Cambridge (UK) have recently reported a new organic (pentacene) thin-film field-effect transistor (FET) they’ve made that functions as a very fast memory device (programming speeds ~ 1 microsecond, or a million times faster than the previous record). Non-volatile flash memory devices that retain their information when switched off are currently made from semiconducting materials, and are ubiquitous in innumerable electronic devices for obvious reasons. These researchers’ goal is similar, but instead uses organic thin-film FETs which are cheaper/easier to make, and are more versatile (since they are ’soft’ and may be applicable to flexible electronic components, for example). There’s a nice review of the principles involved, and what the Korea/Cambridge researchers did, in Nature. (Up)

While I’m on the subject of thin-films and FETs, I might as well briefly mention one more paper more relevant to my line of work (kind of): using self-assembled monolayers (SAMS) as the gate dielectric in carbon nanotube FETs. Gating is very important in these nanoscale field-effect transistor devices - after all, the charge density in the ‘channel’ - be it a carbon nanotube, nanowire, or something else - is modulated via the effect of an electric field produced by a ‘gate’ electrode. The strength of this is clearly very dependent on the distance between the gate electrode and the FET channel, and this can influence the characteristics of the transistor - however, the two can’t be touching, or else the device would be shorted out. So, they’re separated by an insulating dielectric material, the size and properties of which play a role in device performance. Typically, a heavily doped (and hence effectively metallic) Si ‘back-gate’ is used as the gate, with several hundred nm of SiO2 dielectric separating the nanotube from the backgate, and the source and drain electrodes being lithographically patterned on the surface of the sample. In this paper, a group of German scientists have utilized a 2 nm thick silane-based organic SAM on a 4nm thick SiO2 layer as the gate dielectric (see the image at left - the thin red curved line indicated by the arrow is the nanotube, while the four thick yellow-red-green lines are the electrodes). Apparently only one other paper has previously been published utilizing a SAM as the dielectric in such devices, and the previous paper didn’t address the crucial (and obvious) issue of whether the SAM is stable when it’s bombarded by electrons during the lithographic process of patterning the source and drain electrodes on the sample. In this work, the authors study this issue, and are able to successfully produce devices which operate at relatively low voltages. (Up)

I’m going to switch topics slightly now (still very much nano, though) to two papers on nanoscale photonics. I’ll keep this short because both of these papers have been blogged about elsewhere, the first by Doug Natelson and the second by myself - but I feel compelled to include these for completeness, and because the papers are so darn cool. Leo Kouwenhoven and the quantum transport group at Delft (Netherlands) have done some amazing stuff, and the latest of their creations is a single quantum dot InP nanowire light-emitting diode. On the other hand, researchers in Boston (USA) and Germany have made coaxial cables out of carbon nanotubes, showing that their devices are faithful analogs of conventional coax cables to transmit visible light (with propagation via the conventional TEM mode); see my previous post on this for a pretty picture. (Up)

Here’s a theory paper that I found interesting, mostly because I’ve been really into graphene this past year or so (as well as pretty much everyone else interested in nanoscale/mesoscopic electronics, I think), ever since the seminal 2005 Geim and Kim papers. (I plan on writing a reasonably accessible article on why graphene’s getting so much attention these days at some point. Eventually. Perhaps.) Anyway, a lot of work’s been done since then from a lot of different angles, but the main appeal of graphene is for its electronic properties. (I may be biased). For example, quantum dots have been fabricated in a range of nanostructures, from carbon nanotubes to nanowires and nanocrystals; and of course, in two-dimensional electron gases (2DEGs) in semiconductor heterostructures like GaAs/AlGaAs, as in the days of olde. These latter quantum dots were made electrostatically - that is, biased electrodes were used to produce electrostatic potential barriers defining a quantum ‘well’ in which a finite number of electrons could be confined (Kouwenhoven and Marcus have a great review article on this). Electrons in graphene kind of look like electrons in a 2DEG (after all, they are confined to a plane). Can quantum dots be similarly created in graphene using electrostatic barriers? This recent theoretical paper by Silvestrov and Efetov (in Germany and Russia, respectively) suggests a means by which this can be done. But it’s not as simple as transferring the physics over from 2DEGs. For starters, the density of electrons in graphene is huge: ~ 4 x 10¹⁵ cm^-2. In addition, electrons in graphene behave relativistically (the fancy term is that they are ‘emergent Dirac fermion quasiparticles’, or something of that sort). One crucial consequence of this is that unlike electrons in a 2DEG potential well, which are described by the non-relativistic Schrödinger equation, electrons in graphene are described by the Dirac equation and can penetrate through potential barriers, no matter high or wide. Clearly, things aren’t all that simple. What the authors do in this paper is utilize the interesting electronic structure of graphene (e.g. its 6 k-points) and consider the transverse degrees of freedom in the electron motion to propose a means of confining these electrons using an external electrostatic potential. Very cool, at least as I see it - I can’t wait to see what the experimentalists working with graphene do with this. (Up)

Now that I’ve talked about some interesting devices, perhaps I should talk about another crucial part of any experiments dealing with nanoscale structures - microscopy, or ‘how the doors to the nanoworld were opened‘, as some have termed it. (My own research deals with a certain kind of microscopy, so the experimental and philosophical importance of good microscopy and other instrumentation is something dear to my heart.) There are two recent papers that I’ve found quite interesting, both dealing with Scanning Tunneling Microscopy (STM). The first, in line with the idea of studying electronic properties of small systems and correlated-electron effects, deals with manipulating the Kondo effect in molecular systems. The Kondo effect is a very rich effect resulting from the interaction between electrons and magnetic impurities, and Kouwenhoven and Glazman have written a great review on the subject. In a nutshell, while the resistance of ‘normal’ metals decreases up to a constant saturation value as their temperature is lowered, when electrons ’see’ magnetic atoms they scatter from it in such a way that the resistance actually increases as the temperature is lowered. This gives rise to all sorts of effects, and can tell you quite a bit about your system, being observed in a number of systems including carbon nanotubes and quantum dots - and so, it’s quite a big deal nowadays.

In turn, STM has been used for a good deal of time now (ever since the pioneering work of Don Eigler at IBM) for single-atomic manipulation purposes, and has been applied (among other things) to studying these magnetic impurities. For example, in the early part of this decade Eigler and co-workers constructed an ellipsoidal quantum corrall from magnetic Co atoms, placing a single Co atom at one focus. By studying the density of states, they were able to image both the Kondo resonance at the single atom, and another smaller Kondo resonance at the other focus. Anyway, a good deal of work has gone into observing the Kondo effect in magnetic molecules, which are appealing for ‘spintronics‘ applications, and the point is that these authors have applied a modified STM to observing the Kondo effect in a two-dimensional molecular assembly (in the spirit of Eigler), and have furthermore manipulated it in a tunable; that is, they can control the ‘Kondo temperature’ by tweaking the number of nearest-neighbor molecules using the STM tip. This is very exciting, since it provides a means of studying how this phenomenon arises, and how it can be manipulated for technological applications. (Click the image for more details - each individual yellow hexagon in the image is a molecule, while the monochrome images in the rightmost column are calculated electron standing wave patterns, and the red circle indicates the centermost molecule). In addition, the authors have a very cool movie of how their samples were produced on their website. (Up)

In another neat application using STM to study interesting physics, a group of researchers in Japan have mapped charge carriers in a GaAs p-n junction using light-modulated scanning tunneling spectroscopy. While this work doesn’t reveal any new physics, just the fact that the researchers can actually see the carrier dynamics in these junctions is quite exciting. They did this by modifying an STM, coupling it with a laser pulse that is used to ‘free’ carriers in the crystal in a manner that circumvents a typical problem with using straight-up STM for this application; namely, the fact that these charge carriers like to collect around the tip and make interpretation of the data somewhat difficult. I wonder what they’ll be applying this to next? (Up)

Now let’s switch gears a bit to the world of biology. In work that I wouldn’t necessarily term ground-breaking, but still very interesting, a group of researchers have studied what happens to the brain when the mind wanders, and have concluded that this phenomenon is associated with activity in a specific default network of cortical regions. The researchers assembled a group of subjects to perform boring tasks for which they’d been ‘overtrained’, and studied their brains using functional magnetic resonance imaging (fMRI). While this work doesn’t do anything to elucidate why our minds wander, it does point to where in the brain researchers should focus their efforts. (Up)

And I’ll end with two nice examples of using quantum mechanics in biology, one of which I already blogged about when it was on the arxiv (it’s in the most recent issue of PRL) - could humans detect odors by phonon-assisted tunneling of electrons within nose receptors - that is, activating receptors in the nose via a tunneling process mediated by odorant molecules? (Hint: the answer is ‘yes’, and the researchers have the calculations to back it up.) The other comes from work done by a group at Rensselaer Polytechnic Institute using quantum mechanical simulations (e.g. DFT) on supercomputers to study a means by which certain proteins are cleaved and find results that are in line with experiments. (Up)

That’s all for this week!

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• Weekly Roundup from Science of the Invisible, 8th December 2006
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