Talk:Optical properties of carbon nanotubes/GA3

GA Review

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I am putting this article on hold because the introduction and some other parts of the article require more context for readers unfamiliar with the subject. In particular, the lead section needs to be expanded by at least one more paragraph: see WP:LEAD. Also, a paragraph or two giving context about carbon nanotubes without requiring readers to click that link, either in the lead or right afterwards, will help contextualize the subject.

This is my first good article review. I may seek a mentor to help me. I will also not be in a hurry to finish this task because I have exams this week, and I'm just here to take a break from studying stuff I actually need to know. :) Please be patient.

In my university I am acquainted with a professor who is actively involved in researching carbon nanotubes. I will not disclose his name or my university here, but if you want I can give you the info by email, and maybe we can get an expert review in addition to whatever help I can give.

Aside from the context issue, the article looks within reach of passing GA, and I'll give it consideration after I get a chance to confirm that the images and references are in order. Crystal whacker (talk) 04:10, 15 December 2008 (UTC)

Additional request: I see in the page history that this article has been nominated for Good Article before now. Can you tell me why the article did not pass last time, and what the reviewer recommended to do for this time? Crystal whacker (talk) 04:13, 15 December 2008 (UTC)

User:Nergaal opposed because the lead was too short see Talk:Optical_properties_of_carbon_nanotubes/GA1 and Gary King opposed because of lack of citations. See Talk:Optical_properties_of_carbon_nanotubes/GA2.Headbomb {^ταλκ_{κοντριβς} – WP Physics} 04:38, 15 December 2008 (UTC)

Thank you, Headbomb. I will consult you and Gary King before deciding on the fate of this article if I finish the review; hopefully we will agree on the result. Crystal whacker (talk) 04:25, 22 December 2008 (UTC)

I just did a quick check of images and references. Images are all properly licensed, high quality and relevant to the subject. The second and third images lack a caption. I think all images should have captions even if referred to in the text because some readers (myself included) like to skim the images and skip the text. Also for these images it helps to explain the significance of the data. Captions on the other images are good.

References are all properly formatted. If I'm in the mood I might wikilink Science (journal) etc. but it's not essential. Crystal whacker (talk) 04:32, 22 December 2008 (UTC)

I finished reading the article and did link the journal titles. Some of those should have their own articles. I'm satisfied with the style of the article. The writing is choppy in some places, but I've tried to fix that. I need another run through it to make sure I understand the content well enough to list it as a good article. At this point I'm basically satisfied that it's GA material but I need a little more time to make sure. Crystal whacker (talk) 16:47, 30 December 2008 (UTC)

More thorough review and questions

I checked two of the reference articles in my local university library. Based on that, I gained a better understanding of the (n, m) nomenclature, and rewrote the appropriate section to increase clarity. It is a sufficiently important naming scheme to belong in the main article carbon nanotube.

That leaves me with other questions about content. The writing is of sufficient quality to pass GA, and the references are first-class, but I want to make sure I understand everything. So I'm asking a few questions about things I didn't understand even on the second or third reading of this article.

• What is a Van Hove singularity? The linked-to article doesn't explain it except in mathematical form.

Articles Density of states (DOS) and Van Hove singularity should provide an answer. Please write "to do" and questions in their talk pages and someone (maybe even myself) will hopefully answer it. Those articles are essential for Solid-state physics, which is ranked as top importance (DOS is high importance). It is a cheating to answer this question (and other questions below) in two words, but I'll try. Imagine that electron in a solid has a probability P to have certain energy E, and that P(E) function is called density of states (just for simplicity sake; all this explanation is incorrect, strictly speaking). This function has certain "resonance" energies (say at energies of atomic transitions; though this is even farther from the truth - it is the exact atomic composition of a solid that determines those resonances). At those resonance energies, P(E) shows some kinks called van Hove singularities. The shape of those kinks strongly depends on dimensionality of the solid (1D,2D,3D). Those singularities first appeared only in calculations, and it was quite a surprise to find them in the experiment, e.g., via a shape of optical absorption and luminescence lines from carbon nanotubes.

• How does the existence of a singularity define metallic versus semiconducting nanotubes? Take me back to the definition of those terms. As I recall, a metal has many available electrons and can readily undergo transitions from the conduction band to the valence band; an insulator cannot undergo these transitions because there is too large an energy gap between conduction and valence bands; and a semiconductor has intermediate properties, and can be tuned to allow or diallow transitions. What energy level differences determine metallic versus semiconducting nanotubes? Why does the (n, m) index or chiral angle make a difference?

The shape of the DOS is very sensitive to exact composition of a solid and thus even to a slight change in the angle of the nanotube "rolling"; this shape determines the (energy) position of those resonance features, which in solids are called "bands". There are always lots of them, but essential is the "highest band being filled up by electrons". If this band is filled up then the material is semiconducting or insulating, if not then it is a metal.

• Kataura plot: Figure 3 of Kataura's 1999 Synthetic Metals paper shows "armchair" nanotubes as a separate symbol up to (5, 5) diameter, but drops the distinction at larger diameters. I wonder if plotting specific types of nanotube indexes, such as only the armchairs or only the zigzags, yields a more consistent trend than the overall Kataura plot, which has a general downward trend in bandgap energy as diameter increases, but deviates frequently as stated.

Yes, selecting only armchair or zigzag tubes will smoothen the Kataura plot. However, physicists like those genuine high-frequency oscillations at low (n,m). Perhaps it gives a pleasant feeling of understanding a complex behavior.

• Photoluminescence: In metals the "hole" is "rapidly" filled by another electron, so no exciton forms. How fast is "rapidly"? How come electrons are unavailable to fill the holes in a semiconducting nanotube?

Hard to say how fast, however, if it is faster than the luminescence relaxation time (i.e. "time to emit photon from a nanotube", which is roughly in the range 100 ps - few nanosecond) then no luminescence should be observed. Imagine a semiconductor as a train where all seats are taken, but no standing is allowed. If a vacant seat is created, only a couple of nearby electrons can move into it (and they are not much up to it as they got their own seats). In this picture, a metal is a train where all seats a taken, but standing is allowed, and thus there are plenty of guys running around, looking for a seat. When it becomes available, it gets filled right away.

• Why is PL not detected for armchair or zigzag nanotubes? Are these metallic?

Some are metallic. For others, absence of PL follows from calculations. A "simple" explanation to that is when you measure PL at certain energy, you always need to excite it with a somewhat higher energy (or you can't tell where is excitation light and where is PL). The energy difference between excitation and PL (stokes shift) is taken by a phonon (atomic vibration). Not only energy, but also momentum should conserve, i.e. that vibration should have a certain "momentum" (or symmetry or "geometry"). Such phonons are not available in armchair or zigzag nanotubes.

• "The ovals in the map define (S₂₂, S₁₁) pairs, which unique identify (n, m) index of a tube." How do ovals define transition pairs? What exactly is the PL spectroscopy seeing to correspond to transition pairs rather than a single transition?

The center of the oval has two energy coordinates (S₂₂, S₁₁). Note that this is not a general property of PL, but of the experimental PL routine to find (S₂₂, S₁₁) in the nanotube. In other words, we find a resonant S₂₂ energy which results in most efficient PL at S₁₁. In a similar fashion, we can also find a resonant S₃₃, S₄₄ .. energies which result in most efficient PL from S₁₁, but (S₂₂, S₁₁) pairs are most informative. The broadening of the ovals is because no tubes are exactly the same. The difference between nanotubes and many other solids is that in the latter, the top of S₁₁ and the S₂₂, S₃₃, etc. bands are mixed up (no clear resonances).

• Raman: "Raman scattering in SWCNTs is resonant, i.e., only those tubes are probed which have one of the bandgaps equal to the exciting laser energy." How does that work? What if the exciting laser energy is higher than the bandgap energy - why won't the transition occur anyway?

Ideally, it will not, because the electron has zero probability to have this energy (in that specific nanotube). This is a peculiar property of materials with narrow bands.

• Radial breathing mode (RBM) - what causes it? Does breathing occur constantly at ambient conditions, or is it induced by incoming radiation? What about for other modes such as D and G?

RBM, as any other vibration, can be induced in many ways, e.g. by heat. However, its wavelength (~0.1 mm) is much easier to detect not directly, but via an optical shift in Raman scattering.

• Big picture question: why does the chiral angle of a nanotube affect the electronic properties? Maybe science can't answer why, but I leave wondering if there's any theme or if all we've got is an ensemble of fascinating data.

Slight change in a relative atomic positions (especially for carbon) can turn an insulator (diamond) into a conductor (graphite). Same with nanotubes.

• You can answer any question by starting a new line with ## immediately below the question in the edit window. Saying "look it up in the reference" is fine; any answer is helpful to me. Thanks in advance. Crystal whacker (talk) 20:40, 6 January 2009 (UTC)

"##" would start a subnumbering inside your numbered list. I couldn't quickly find an easy way (please tell me if you know) - most operators break down initial numbering - and simply deleted all numbering. No adequate answers on those questions will fit into this page. One has to read quite a few books to get all details.NIMSoffice (talk) 01:31, 7 January 2009 (UTC)

Thanks for those answers. I especially understood the train analogy. I'm sorry about the numbering scheme: I should have asked you to write "#:" instead of "##", but it's okay as it stands. Crystal whacker (talk) 17:21, 7 January 2009 (UTC)

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The images, which are all on the right, get squished on larger monitors. I suggest staggering them left and right per MOS:IMAGE. Gary King (talk) 21:15, 6 January 2009 (UTC)

Let me work on that. Crystal whacker (talk) 17:22, 7 January 2009 (UTC)

Passed

Okay, this has waited long enough. Any lingering issues with images will not prevent the article from being a GA. I don't know if the images will display well on low-resolution monitors, but if anyone has a better way to do it, please go ahead and try it.

Congratulations to NIMSoffice on a job well done! Crystal whacker (talk) 23:10, 8 January 2009 (UTC)