Materials – Page 6

Transport in carbon nanotubes

The electronic (band) structure of carbon nanotubes (CNT) can be roughly understood from that of graphene by imposing periodic boundary conditions along one axis. We immediately run into a problem – how did the two edges of the graphene join? Is the longitudinal axis oriented directly parallel with para- positions of the hexagons, or is it at a slight angle? Another way to put the same question, does a curve around the nanotube’s periphery and joining unit cell centers describe a circle or a spiral?
It turns out that the band structure critically depends on just this geometry. CNT’s can be metallic, semi-conductive or semi-metallic according to how they zip up. (The nanotube’s diameter plays a lesser role.) Experimentally, there is no good way to synthesize only one type.
A review posted this week explores the electronic structure of CNT’s in some detail. Among the many implications are that electrons in CNT’s display surprising spin-orbit coupling, which affects the lifetime of prepared spin states, among other things.
Futhermore, electrons can display very strong correlation. In many CNT’s, a valence electron has an effective Coulomb radius in the same order of magnitude as the radius of the CNT. Therefore, electrons “line up” along the nanotube axis, unable to get out of each other’s way. (This is, of course, a classical picture. Quantally, the electrons are indistinguishable.)

Reference: Laird et al, “Quantum transport in carbon nanotubes”, arXiv:1403.6113.

Large-scale computation of materials properties

Quantum chemical property calculations in the 10K to 1M atom range are now feasible. Over the last few months, several preprints have been posted to the arXiv. Traditional solid state theory assumes periodic boundary conditions, which is equivalent to assuming a structure of infinite extent. Quantum chemical techniques, by contrast, have always assumed a finite collection of atoms. And until recent times, “finite” meant on the order of 10-100 atoms!

With advances in algorithmics and in massively parallel computing, it became feasible in 2010 to compute at least some properties of material systems containing on the order of one million atoms [1]. Given that nano-scale electronic devices (for example, nano-dots and short transistor gates) can be well modelled with < 1M atoms, this is a timely development. Another approach, not yet to the 1M atom scale, is given in [2].

I include two papers addressing device transport properties [1,3]. The second is not as sophisticated as the first, but has the advantage of being more accessible.

To quote Richard Hamming, “The purpose of computing is insight, not numbers.” So it is in large-scale theoretical calculations. How do we know that all the (millions of) numbers coming out have any resemblance to reality? Papers [4,5] present a methodical looks at various of the theoretical techniques with the objective of answering how reliable we might find these methods to be in practical application.

Paper [6] considers theoretical methods to design energy storage and photoemissive materials.

Finally, paper [7] picks up the theme of inverse design. Given some desired properties, how can one design a material displaying those properties? In an abstract sense, inverse design is a generalization of the designer potentials approach to self-assembly that I reported on some time ago.

References:

[1] Lee et al., “Million Atom Electronic Structure and Device Calculations on Peta-Scale Computers”, arXiv:1003.4570

[2] Mohr et al., “Daubechies Wavelets for Linear Scaling Density Functional Theory”, arXiv:1401.7441

[3] Gross et al, “Kwant: a software package for quantum transport”, arXiv:1309.2926

[4] Keller and Reiher, “Determining Factors for the Accuracy of DMRG in Chemistry”, arXiv:1401.5497

[5] Shullenberger and Mattsson, “Quantum Monte Carlo applied to solids”, arXiv:1310.1047

[6] Nemeth, “Materials Design by Quantum-Chemical and other Theoretical/Computational Means: Applications to Energy Storage and Photoemissive Materials”, arXiv:1312.0699

[7] Weymuth and Reiher, “Inverse Quantum Chemistry: Concepts and Strategies for Rational Compound Design”, arXiv:1401.1512

Graphene oxidation mechanism

The Nouchi group has discovered two methods for inducing oxidation in graphene. In the first, they noted that graphene oxidized while performing Raman spectroscopy. The presence of water is required. They further established that, while exposure to water alone did not change the characteristics of the graphene film, exposure to water simultaneously with UV irradiation did change the electron transport properties of the film.

In the second experiments, the group explored the reaction between graphene and oxygen caused by UV irradiation, and the physisorption of oxygen molecules when a gate voltage is applied to graphene field effect transistors. Irradiation of positively biased graphene gates results in oxygen desorption, while similar irradiation of negatively biased gates results in etching of the graphene. This etching proceeds predominately from the edges.

N Mitoma, R Nouchi, and K Tanagaki, “Photo-oxidation of graphene in the presence of water”, arXiv:1311.3447.
N Mitoma and R Nouchi, “Gate-controlled ultraviolet photo-etching of graphene edges”, arXiv:1311.3452.

Protecting two-dimensional materials during TEM

It is by now routine to achieve sub-atomic resolution (a few 10’s of pm) by aberration-corrected TEM and by STEM. However, reducing the signal to noise ratio sufficient for the highest resolution requires exposing the sample to fairly high electron dose. The electron irradiation causes sample damage while the image is being acquired. Several mechanisms may contribute: electron knock-on (in which an imaging electron transfers enough momentum to an atom to displace the atom from its original position), radiolysis (in which chemical bonds are rearranged, leading the target material to change into something else), heating (including sample melting), and Coulomb explosion (due to sample charging).

In viewing two-dimensional materials, one cannot average the imaging over depth; a two-dimensional material is by definition all surface. So any damage immediately and irreversibly degrades the image.

Two groups, one at Manchester, the other at Ulm, have independently discovered that it is possible to stablize MoS2 during (S)TEM imaging. This is done making a sandwich of graphene/MoS2/graphene. The Ulm group imaged with aberration-corrected TEM, while at Manchester, they used STEM. Additions to the image from the graphene layers, being periodic, can be subtracted out, leaving only the image of the study material, MoS2. It is an interesting coincidence that the two groups compared identical material stacks.

References:
“The pristine atomic structure of MoS2 monolayer protected from electron radiation
damage by graphene”, G Algara-Siller, et al. [arXiv:1310.2411]
“Control of Radiation Damage in MoS2 by Graphene Encapsulation”, R Zan et al. [ACS Nano (DOI: 10.1021/nn4044035), see also arXiv:1310.4012]

Contamination and doping

Not surprisingly, properties of a two-dimensional material, being all surface, may be exquisitely susceptible to contamination. (See “Experimenting with monolayers” and links therein.) This general observation has been recently confirmed in MoS2 as well.

On the other hand, this susceptibility can be exploited for band engineering, doping, etc. Two recent preprints on the arXiv make interesting reading. See arXiv:1308.4924, discussing organometallic complexes of graphene, as well as arXiv:1308.1645, discussing the reversible grafting of naphthylmethyl radicals to graphene.

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