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| Within [[materials science]], the '''optical properties of carbon nanotubes''' refer specifically to the [[absorption spectroscopy|absorption]], [[photoluminescence]] ([[fluorescence]]), and [[Raman spectroscopy]] of [[carbon nanotube]]s. Spectroscopic methods offer the possibility of quick and non-destructive characterization of relatively large amounts of [[carbon nanotube]]s. There is a strong demand for such characterization from the industrial point of view: numerous parameters of the [[Carbon nanotube#Synthesis|nanotube synthesis]] can be changed, intentionally or unintentionally, to alter the nanotube quality. As shown below, optical absorption, photoluminescence and Raman spectroscopies allow quick and reliable characterization of this "nanotube quality" in terms of non-tubular carbon content, structure (chirality) of the produced nanotubes, and structural defects. Those features determine nearly any other properties such as optical, mechanical, and electrical properties.
| |
| | |
| [[Carbon nanotubes]] are unique "one dimensional systems" which can be envisioned as rolled single sheets of [[graphite]] (or more precisely [[graphene]]). This rolling can be done at different angles and curvatures resulting in different nanotube properties. The diameter typically varies in the range 0.4–40 nm (i.e. "only" ~100 times), but the length can vary ~10,000 times, reaching 18.5 cm. Thus the nanotube [[aspect ratio]], or the length-to-diameter ratio, can be as high as 132,000,000:1,<ref name="Longest">{{cite journal|doi=10.1021/nl901260b|pmid=19650638|title=Fabrication of Ultralong and Electrically Uniform Single-Walled Carbon Nanotubes on Clean Substrates|year=2009|author=Xueshen Wang ''et al.''|journal=Nano Letters|volume=9|issue=9|pages=3137–41|bibcode = 2009NanoL...9.3137W }}</ref> which is unequalled by any other material. Consequently, all the properties of the carbon nanotubes relative to those of typical semiconductors are extremely [[anisotropic]] (directionally dependent) and tunable.
| |
| | |
| Whereas mechanical, electrical and electrochemical ([[supercapacitor]]) properties of the carbon nanotubes are well established and have immediate [[Carbon nanotube#Potential and current applications|applications]], the practical use of optical properties is yet unclear. The aforementioned tunability of properties is potentially useful in [[optics]] and [[photonics]]. In particular, light-emitting diodes ([[LED]]s)<ref name="led1">
| |
| {{cite journal
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| |author=J. A. Misewich ''et al.''
| |
| |title=Electrically Induced Optical Emission from a Carbon Nanotube FET
| |
| |journal=[[Science (journal)|Science]]
| |
| |volume=300 |issue=5620 |pages=783–786
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| |year=2003
| |
| |doi=10.1126/science.1081294
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| |pmid=12730598
| |
| |bibcode = 2003Sci...300..783M }}</ref><ref name="led2">
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| {{cite journal
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| |author=J. Chen ''et al.''
| |
| |title=Bright Infrared Emission from Electrically Induced Excitons in Carbon Nanotubes
| |
| |journal=[[Science (journal)|Science]]
| |
| |volume=310 |issue=5751 |pages=1171–1174
| |
| |year=2005
| |
| |doi=10.1126/science.1119177
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| |pmid=16293757
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| |bibcode = 2005Sci...310.1171C }}</ref> and [[Photodiode|photo-detectors]]<ref>
| |
| {{cite journal
| |
| |author=M. Freitag ''et al.''
| |
| |title=Photoconductivity of Single Carbon Nanotubes
| |
| |journal=[[Nano Letters]]
| |
| |volume=3 |issue=8 |pages=1067–1071
| |
| |year=2003
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| |doi=10.1021/nl034313e
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| |bibcode = 2003NanoL...3.1067F }}</ref> based on a single nanotube have been produced in the lab. Their unique feature is not the efficiency, which is yet relatively low, but the narrow selectivity in the [[wavelength]] of emission and detection of light and the possibility of its fine tuning through the nanotube structure. In addition, [[bolometer]]<ref>
| |
| {{cite journal
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| |author=M. E. Itkis ''et al.''
| |
| |title=Bolometric Infrared Photoresponse of Suspended Single-Walled Carbon Nanotube Films
| |
| |journal=[[Science (journal)|Science]]
| |
| |volume=312 |issue=5772 |pages=413–416
| |
| |year=2006
| |
| |doi=10.1126/science.1125695
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| |pmid=16627739
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| |bibcode = 2006Sci...312..413I }}</ref> and optoelectronic memory<ref>
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| {{cite journal
| |
| |author=A. Star ''et al.''
| |
| |title=Nanotube Optoelectronic Memory Devices
| |
| |journal=[[Nano Letters]]
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| |volume=4 |issue=9 |pages=1587–1591
| |
| |year=2004
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| |doi=10.1021/nl049337f
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| |bibcode = 2004NanoL...4.1587S }}</ref> devices have been realised on ensembles of single-walled carbon nanotubes.
| |
| | |
| ==Terminology==
| |
| This article uses the following abbreviations:
| |
| | |
| * Carbon nanotube (CNT)
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| * Single wall carbon nanotube (SWCNT)
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| * Multiwall carbon nanotube (MWCNT)
| |
| | |
| However, C is often omitted in scientific literature,<ref name="sinnott"/> so NT, SWNT and MWNT are more commonly used. Also, "wall" is often exchanged with "walled".
| |
| | |
| ==Electronic structure of carbon nanotube==
| |
| [[Image:CNTnames.png|thumb|The (''n'', ''m'') nanotube naming scheme<ref name="sinnott">
| |
| {{cite journal
| |
| |author=S. B. Sinnott and R. Andreys
| |
| |title=Carbon Nanotubes: Synthesis, Properties, and Applications
| |
| |journal=Critical Reviews in Solid State and Materials Sciences
| |
| |volume=26 |issue=3 |pages=145–249
| |
| |year=2001
| |
| |doi=10.1080/20014091104189
| |
| |bibcode = 2001CRSSM..26..145S }}</ref><ref name="dress">
| |
| {{cite journal
| |
| |author=M. S. Dresselhaus ''et al.''
| |
| |title=Physics of Carbon Nanotubes
| |
| |journal=[[Carbon (journal)|Carbon]]
| |
| |volume=33 |issue=7 |pages=883–891
| |
| |year=1995
| |
| |doi=10.1016/0008-6223(95)00017-8
| |
| }}</ref> can be thought of as a vector ('''C'''<sub>h</sub>) in an infinite [[graphene]] sheet that describes how to "roll up" the graphene sheet to make the nanotube. '''T''' denotes the tube axis, and '''a'''<sub>1</sub> and '''a'''<sub>2</sub> are the unit vectors of graphene in real space.]]
| |
| | |
| [[Image:ArmchairCNT.png|thumb|left|Armchair nanotube]]
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| [[Image:ZigzagCNT.png|thumb|left|Zigzag nanotube]]
| |
| A single-wall carbon nanotube can be imagined as graphene sheet rolled at a certain "chiral" angle with respect to a plane perpendicular to the tube's long axis. Consequently, SWCNT can be defined by its diameter and chiral angle. The chiral angle can range from 0 to 30 degrees. | |
| | |
| However, more conveniently, a pair of indices (''n'', ''m'') is used instead. The indices refer to equally long [[unit vector]]s at 60° angles to each other across a single 6-member [[carbon ring]]. Taking the origin as carbon number 1, the '''a'''<sub>1</sub> unit vector may be considered the line drawn from carbon 1 to carbon 3, and the '''a'''<sub>2</sub> unit vector is then the line drawn from carbon 1 to carbon 5. (See the upper right corner of the diagram at right.) To visualize a CNT with indices (''n'', ''m''), draw ''n'' '''a'''<sub>1</sub> unit vectors across the graphene sheet, then draw ''m'' '''a'''<sub>2</sub> unit vectors at a 60° angle to the '''a'''<sub>1</sub> vectors, then add the vectors together. The line representing the sum of the vectors will define the circumference of the CNT along the plane perpendicular to its long axis, connecting one end to the other.<ref name="sinnott"/><ref name="dress"/><ref name="eklund">
| |
| {{cite journal
| |
| |author=P. C. Eklund ''et al.''
| |
| |title=Vibrational Modes of Carbon Nanotubes; Spectroscopy and Theory
| |
| |journal=[[Carbon (journal)|Carbon]]
| |
| |volume=33 |issue=7 |pages=959–972
| |
| |year=1995
| |
| |doi=10.1016/0008-6223(95)00035-C
| |
| }}</ref> In the diagram at right, '''C'''<sub>h</sub> is a (4, 2) vector: the sum of 4 unit vectors from the origin directly to the right, then 2 unit vectors at a 60° angle down and to the right.
| |
| | |
| Tubes having ''n'' = ''m'' (chiral angle = 30°) are called "armchair" and those with ''m'' = 0 (chiral angle = 0°) "zigzag". Those indices uniquely determine whether CNT is a [[metal]], [[semimetal]] or [[semiconductor]], as well as its [[band gap]]: when |''m'' – ''n''| = 3''k'' (''k'' is [[integer]]), the tube is metallic; but if |''m'' – ''n''| = 3''k'' ± 1, the tube is semiconducting. The nanotube diameter ''d'' is related to ''m'' and ''n'' as
| |
| | |
| : <math> d = \frac{a}{\pi} \sqrt{(n^2 + nm + m^2)}.</math>
| |
| | |
| In this equation, ''a'' = 0.246 nm is the magnitude of either unit vector '''a'''<sub>1</sub> or '''a'''<sub>2</sub>.
| |
| | |
| The situation in multi-wall CNTs is complicated as their properties are determined by contribution of all individual shells; those shells have different structures, and, because of the synthesis, are usually more defective than SWCNTs. Therefore, optical properties of MWCNTs will not be considered here.
| |
| | |
| ==Van Hove singularities==
| |
| [[Image:SSPN41.PNG|right|400px|<!--Caption is inside the figure-->]]
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| [[File:DOS multdim.jpg|thumb|left|180px|A bulk 3D material (blue) has continuous DOS, but a 1D wire (green) has Van Hove singularities.]]
| |
| | |
| Optical properties of carbon nanotubes derive from electronic transitions within one-dimensional [[density of states]] (DOS). A typical feature of one-dimensional crystals is that their DOS is not a continuous function of energy, but it descends gradually and then increases in a discontinuous spike. In contrast, three-dimensional materials have continuous DOS. The sharp peaks found in one-dimensional materials are called [[Van Hove singularity|Van Hove singularities]].
| |
| | |
| Van Hove singularities result in the following remarkable optical properties of carbon nanotubes:
| |
| | |
| *Optical transitions occur between the ''v''<sub>1</sub> − ''c''<sub>1</sub>, ''v''<sub>2</sub> − ''c''<sub>2</sub>, etc., states of semiconducting or metallic nanotubes and are traditionally labeled as ''S''<sub>11</sub>, ''S''<sub>22</sub>, ''M''<sub>11</sub>, etc., or, if the "conductivity" of the tube is unknown or unimportant, as ''E''<sub>11</sub>, ''E''<sub>22</sub>, etc. Crossover transitions ''c''<sub>1</sub> − ''v''<sub>2</sub>, ''c''<sub>2</sub> − ''v''<sub>1</sub>, etc., are [[Forbidden mechanism|dipole-forbidden]] and thus are extremely weak, but they were possibly observed using cross-polarized optical geometry.<ref>
| |
| {{cite journal
| |
| |author=Y. Miyauchi ''et al.''
| |
| |title=Cross-Polarized Optical Absorption of Single-Walled Nanotubes Probed by Polarized Photoluminescence Excitation Spectroscopy
| |
| |journal=[[Physical Review B]]
| |
| |volume=74
| |
| |issue=20 |pages=205440
| |
| |year=2006
| |
| |doi=10.1103/PhysRevB.74.205440
| |
| |arxiv = cond-mat/0608073 |bibcode = 2006PhRvB..74t5440M }}</ref>
| |
| | |
| *The energies between the Van Hove singularities depend on the nanotube structure. Thus by varying this structure, one can tune the optoelectronic properties of carbon nanotube. Such fine tuning has been experimentally demonstrated using UV illumination of polymer-dispersed CNTs.<ref name="iak1">
| |
| {{cite journal
| |
| |author=K. Iakoubovskii ''et al.''
| |
| |title=Midgap Luminescence Centers in Single-Wall Carbon Nanotubes Created by Ultraviolet Illumination
| |
| |journal=[[Applied Physics Letters]]
| |
| |volume=89
| |
| |issue=17 |pages=173108
| |
| |year=2006|url=http://pubman.nims.go.jp/pubman/item/escidoc:1587355:1/component/escidoc:1587354/Apl173108.pdf
| |
| |doi=10.1063/1.2364157
| |
| |bibcode = 2006ApPhL..89q3108I }}</ref>
| |
| | |
| *Optical transitions are rather sharp (~10 meV) and strong. Consequently, it is relatively easy to selectively excite nanotubes having certain (''n'', ''m'') indices, as well as to detect optical signals from individual nanotubes.
| |
| | |
| ==Kataura plot==
| |
| [[Image:KatauraPlot.jpg|thumb|150px|In this Kataura plot, the energy of an electronic transition decreases as the diameter of the nanotube increases.]]
| |
| | |
| The band structure of carbon nanotubes having certain (''n'', ''m'') indexes can be easily calculated.<ref>
| |
| {{cite web
| |
| |author=S. Maruyama
| |
| |title=Shigeo Maruyama's Fullerene and Carbon Nanotube Site
| |
| |url=http://reizei.t.u-tokyo.ac.jp/~maruyama/nanotube.html
| |
| |accessdate=2008-12-08
| |
| }}</ref> A theoretical graph based on this calculations was designed in 1999 by [[Hiromichi Kataura]] to rationalize experimental findings. A Kataura plot relates the nanotube diameter and its bandgap energies for all nanotubes in a diameter range.<ref name="kataura">
| |
| {{cite journal
| |
| |author=H. Kataura ''et al.''
| |
| |title=Optical Properties of Single-Wall Carbon Nanotubes
| |
| |url=http://staff.aist.go.jp/h-kataura/Kataura-Synth-Met-103-2555.pdf
| |
| |journal=[[Synthetic Metals]]
| |
| |volume=103
| |
| |issue=1–3 |pages=2555–2558
| |
| |year=1999
| |
| |doi=10.1016/S0379-6779(98)00278-1
| |
| }}</ref> The oscillating shape of every branch of the Kataura plot reflects the intrinsic strong dependence of the SWCNT properties on the (''n'', ''m'') index rather than on its diameter. For example, (10, 1) and (8, 3) tubes have almost the same diameter, but very different properties: the former is a metal, but the latter is semiconductor.
| |
| {{clr}}
| |
| | |
| ==Optical absorption==
| |
| [[Image:CNTabsorption.jpg|thumb|left|300px|Optical absorption spectrum from dispersed single-wall carbon nanotubes]]
| |
| | |
| [[Ultraviolet-visible spectroscopy|Optical absorption]] in carbon nanotubes differs from absorption in conventional 3D materials by presence of sharp peaks (1D nanotubes) instead of an absorption threshold followed by an absorption increase (most 3D solids). Absorption in nanotubes originates from electronic transitions from the ''v''<sub>2</sub> to ''c''<sub>2</sub> (energy ''E''<sub>22</sub>) or ''v''<sub>1</sub> to ''c''<sub>1</sub> (''E''<sub>11</sub>) levels, etc.<ref name="sinnott"/><ref name="kataura"/> The transitions are relatively sharp and can be used to identify nanotube types. Note that the sharpness deteriorates with increasing energy, and that many nanotubes have very similar ''E''<sub>22</sub> or ''E''<sub>11</sub> energies, and thus significant overlap occurs in absorption spectra. This overlap is avoided in photoluminescence mapping measurements (see below), which instead of a combination of overlapped transitions identifies individual (''E''<sub>22</sub>, ''E''<sub>11</sub>) pairs.<ref name="dwnt1"/><ref name="dwnt2"/>
| |
| | |
| Interactions between nanotubes, such as bundling, broaden optical lines. While bundling strongly affects photoluminescence, it has much weaker effect on optical absorption and Raman scattering. Consequently, sample preparation for the latter two techniques is relatively simple.
| |
| | |
| Optical absorption is routinely used to quantify quality of the carbon nanotube powders.<ref>
| |
| {{cite journal
| |
| |author=M. E. Itkis ''et al.''
| |
| |title=Comparison of Analytical Techniques for Purity Evaluation of Single-Walled Carbon Nanotubes
| |
| |journal=[[Journal of the American Chemical Society]]
| |
| |volume=127 |issue=10 |pages=3439–48
| |
| |year=2005
| |
| |doi=10.1021/ja043061w
| |
| |pmid=15755163
| |
| }}</ref>
| |
| | |
| The spectrum is analyzed in terms of intensities of nanotube-related peaks, background and pi-carbon peak; the latter two mostly originate from non-nanotube carbon in contaminated samples. However, it has been recently shown that by aggregating nearly single chirality semiconducting nanotubes into closely packed Van der Waals bundles the absorption background can be attributed to free carrier transition originating from intertube charge transfer.<ref name="agg">
| |
| {{cite journal
| |
| |author=Jared J. Crochet ''et al.''
| |
| |title=Electrodynamic and Excitonic Intertube Interactions in Semiconducting Carbon Nanotube Aggregates
| |
| |journal=[[ACS Nano]]
| |
| |volume=5 |issue=4 |pages=2611–2618
| |
| |year=2011
| |
| |doi=10.1021/nn200427r
| |
| }}</ref>
| |
| {{clr}}
| |
| | |
| ===Carbon nanotubes as a black body===
| |
| An ideal [[black body]] should have [[emissivity]] or [[absorbance]] of 1.0, which is difficult to attain in practice, especially in a wide [[spectrum|spectral range]]. Vertically aligned "forests" of single-wall carbon nanotubes can have absorbances of 0.98–0.99 from the [[far-ultraviolet]] (200 nm) to [[far-infrared]] (200 μm) wavelengths. [[Super black]], a coating based on chemically etched [[nickel]]-[[phosphorus]] [[alloy]], is another material approaching the absorption of 1.0.
| |
| | |
| These SWNT forests ([[buckypaper]]) were grown by the super-growth CVD method to about 10 μm height. Two factors could contribute to strong light absorption by these structures: (i) a distribution of CNT chiralities resulted in various bandgaps for individual CNTs. Thus a compound material was formed with broadband absorption. (ii) Light might be trapped in those forests due to multiple reflections.<ref>
| |
| {{cite journal|author=Zu-Po Yang et al.|title=Experimental Observation of an Extremely Dark Material Made By a Low-Density Nanotube Array|journal=[[Nano letters]]|volume=8 |pages=446–451|year=2008|doi=10.1021/nl072369t|pmid=18181658|issue=2
| |
| |bibcode = 2008NanoL...8..446Y }}</ref><ref>{{cite journal|author=K. Mizuno ''et al.''|title=A black body absorber from vertically aligned single-walled carbon nanotubes|journal=[[Proceedings of the National Academy of Sciences]]|volume=106 |pages=6044–6077|year=2009|doi=10.1073/pnas.0900155106|pmid=19339498|issue=15|pmc=2669394
| |
| |bibcode = 2009PNAS..106.6044M }}</ref><ref>
| |
| {{cite journal|author=K. Hata ''et al.''|title=Water-Assisted Highly Efficient Synthesis of Impurity-Free Single-Walled Carbon Nanotubes|url=http://keithcu.com/wiki/images/b/b3/Water.pdf|journal=[[Science (journal)|Science]]|volume=306 |pages=1362–1364|year=2004|doi=10.1126/science.1104962|pmid=15550668|issue=5700|bibcode = 2004Sci...306.1362H }}</ref>
| |
| | |
| {| style="font-size: 95%; text-align:center;" class="wikitable" border="0"
| |
| |+ Reflectance measurements<ref>{{cite journal |author=L. Mizuno''et al.'' |year=2009 |title=Supporting Information |journal= Proceedings of the National Academy of Sciences|volume= 106|issue= 15|page= |doi=10.1073/pnas.0900155106|pages= 6044–7|bibcode = 2009PNAS..106.6044M |pmid=19339498 |pmc=2669394}}</ref>
| |
| |-
| |
| !
| |
| ! UV-to-near IR
| |
| ! Near-to-mid IR
| |
| ! Mid-to-far IR
| |
| |-
| |
| | Wavelength, μm
| |
| | 0.2-2
| |
| | 2–20
| |
| | 25–200
| |
| |-
| |
| | Incident angle, °
| |
| | 8
| |
| | 5
| |
| | 10
| |
| |-
| |
| | Reflection
| |
| | Hemispherical-directional
| |
| | Hemispherical-directional
| |
| | Specular
| |
| |-
| |
| | Reference
| |
| | White reflectance standard
| |
| | Gold mirror
| |
| | Aluminum mirror
| |
| |-
| |
| | Average reflectance
| |
| | 0.0160
| |
| | 0.0097
| |
| | 0.0017
| |
| |-
| |
| | Standard deviation
| |
| | 0.0048
| |
| | 0.0041
| |
| | 0.0027
| |
| |}
| |
|
| |
| ==Luminescence==
| |
| [[Image:PLmap1.jpg|thumb|420px|Photoluminescence map from single-wall carbon nanotubes. (''n'', ''m'') indexes identify certain semiconducting nanotubes. Note that PL measurements do not detect nanotubes with ''n'' = ''m'' or ''m'' = 0.]]
| |
| | |
| ===Photoluminescence (Fluorescence)===
| |
| Semiconducting single-walled carbon nanotubes emit near-infrared light upon photoexcitation, described interchangeably as [[fluorescence]] or [[photoluminescence]] (PL). The excitation of PL usually occurs as follows: an electron in a nanotube absorbs excitation light via ''S''<sub>22</sub> transition, creating an electron-hole pair ([[exciton]]). Both electron and hole rapidly relax (via [[phonon]]-assisted processes) from ''c''<sub>2</sub> to ''c''<sub>1</sub> and from ''v''<sub>2</sub> to ''v''<sub>1</sub> states, respectively. Then they recombine through a ''c''<sub>1</sub> − ''v''<sub>1</sub> transition resulting in light emission.
| |
| | |
| No excitonic luminescence can be produced in metallic tubes — electron can be excited, thus resulting in optical absorption, but the hole is immediately filled by another electron out of many available in metal. Therefore no exciton is produced.
| |
| | |
| ===Salient properties===
| |
| | |
| *Photoluminescence from SWCNT, as well as optical absorption and Raman scattering, is linearly polarized along the tube axis. This allows monitoring of the SWCNTs orientation without direct microscopic observation.
| |
| *PL is quick: relaxation typically occurs within 100 [[picosecond]]s.<ref name="relax">
| |
| {{cite journal
| |
| |author=F. Wang ''et al.''
| |
| |title=Time-Resolved Fluorescence of Carbon Nanotubes and Its Implication for Radiative Lifetimes
| |
| |journal=[[Physical Review Letters]]
| |
| |volume=92
| |
| |issue=17 |pages=177401
| |
| |year=2004
| |
| |doi=10.1103/PhysRevLett.92.177401
| |
| |pmid=15169189
| |
| |bibcode=2004PhRvL..92q7401W
| |
| }}</ref>
| |
| *PL efficiency was first found to be low (~0.01%),<ref name="relax"/> but later studies measured much higher quantum yields. By improving the structural quality and isolation of nanotubes, emission efficiency increased. A quantum yield of 1% was reported in nanotubes sorted by diameter and length through gradient centrifugation,<ref name="PLqy">
| |
| {{cite journal
| |
| |author=Jared Crochet ''et al.''
| |
| |title=Quantum Yield Heterogeneities of Aqueous Single-Wall Carbon Nanotube Suspensions
| |
| |journal=[[Journal of the American Chemical Society]]
| |
| |volume=129|pages=8058–805
| |
| |year=2007
| |
| |doi=10.1021/ja071553d
| |
| |pmid=17552526
| |
| |issue=26
| |
| }}</ref> and it was further increased to 20% by optimizing the procedure of isolating individual nanotubes in solution.<ref>
| |
| {{cite journal
| |
| |author=S-Y Ju ''et al.''
| |
| |title=Brightly Fluorescent Single-Walled Carbon Nanotubes via an Oxygen-Excluding Surfactant Organization
| |
| |journal=[[Science (journal)|Science]]
| |
| |volume=323|pages=1319–1323
| |
| |year=2009
| |
| |doi=10.1126/science.1166265
| |
| |pmid=19265015
| |
| |issue=5919
| |
| |bibcode = 2009Sci...323.1319J }}</ref>
| |
| *The spectral range of PL is rather wide. Emission wavelength can vary between 0.8 and 2.1 micrometers depending on the nanotube structure.<ref name="dwnt1">
| |
| {{cite journal
| |
| |author=K. Iakoubovskii ''et al.''
| |
| |title=IR-Extended Photoluminescence Mapping of Single-Wall and Double-Wall Carbon Nanotubes
| |
| |journal=[[Journal of Physical Chemistry B]]
| |
| |volume=110 |issue=35 |pages=17420–17424
| |
| |year=2006 |url=http://pubman.nims.go.jp/pubman/item/escidoc:1587357:1/component/escidoc:1587356/jpcb17420.pdf
| |
| |doi=10.1021/jp062653t
| |
| |pmid=16942079
| |
| }}</ref><ref name="dwnt2">
| |
| {{cite journal
| |
| |author=K. Iakoubovskii ''et al.''
| |
| |title=Optical Characterization of Double-wall Carbon Nanotubes: Evidence for Inner Tube Shielding
| |
| |journal=[[Journal of Physical Chemistry C]]
| |
| |volume=112 |issue=30 |pages=11194–11198
| |
| |year=2008|url=http://pubman.nims.go.jp/pubman/item/escidoc:1587353:1/component/escidoc:1587352/jpcc11194.pdf
| |
| |doi=10.1021/jp8018414
| |
| }}</ref>
| |
| *Excitons are apparently delocalized over several nanotubes in single chirality bundles as the photoluminescence spectrum displays a splitting consistent with intertube exciton tunneling.<ref name="agg">
| |
| {{cite journal
| |
| |author=Jared J. Crochet ''et al.''
| |
| |title=Electrodynamic and Excitonic Intertube Interactions in Semiconducting Carbon Nanotube Aggregates
| |
| |journal=[[ACS Nano]]
| |
| |volume=5 |issue=4 |pages=2611–2618
| |
| |year=2011
| |
| |doi=10.1021/nn200427r
| |
| }}</ref>
| |
| *Interaction between nanotubes or between a nanotube and another material may quench or increase PL.<ref>
| |
| {{cite journal
| |
| |author=B. C. Satishkumar ''et al.''
| |
| |title=Reversible fluorescence quenching in carbon nanotubes for biomolecular sensing
| |
| |journal=[[Nature Nanotechnology]]
| |
| |volume=2|pages=560–564
| |
| |year=2007
| |
| |doi=10.1038/nnano.2007.261
| |
| |pmid=18654368
| |
| |issue=9
| |
| }}</ref> No PL is observed in multi-walled carbon nanotubes. PL from double-wall carbon nanotubes strongly depends on the preparation method: [[Chemical vapor deposition|CVD]] grown DWCNTs show emission both from inner and outer shells.<ref name="dwnt1"/><ref name="dwnt2"/> However, DWCNTs produced by encapsulating [[fullerenes]] into SWCNTs and annealing show PL only from the outer shells.<ref>
| |
| {{cite journal
| |
| |author=T. Okazaki ''et al.''
| |
| |title=Photoluminescence Quenching in Peapod-Derived Double-Walled Carbon Nanotubes
| |
| |journal=[[Physical Review B]]
| |
| |volume=74|url=http://pubman.nims.go.jp/pubman/item/escidoc:1587378:1/component/escidoc:1587377/Prb153404.pdf
| |
| |issue=15 |pages=153404
| |
| |year=2006
| |
| |doi=10.1103/PhysRevB.74.153404
| |
| |bibcode = 2006PhRvB..74o3404O }}</ref> Isolated SWCNTs lying on the substrate show extremely weak PL which has been detected in few studies only.<ref>
| |
| {{cite journal
| |
| |author=N. Ishigami ''et al.''
| |
| |title=Crystal Plane Dependent Growth of Aligned Single-Walled Carbon Nanotubes on Sapphire
| |
| |journal=[[Journal of the American Chemical Society]]
| |
| |volume=130 |issue=30 |pages=9918–9924
| |
| |year=2008
| |
| |doi=10.1021/ja8024752
| |
| |pmid=18597459
| |
| }}</ref> Detachment of the tubes from the substrate drastically increases PL.
| |
| | |
| *Position of the (''S''<sub>22</sub>, ''S''<sub>11</sub>) PL peaks depends slightly (within 2%) on the nanotube environment (air, dispersant, etc.). However, the shift depends on the (''n'', ''m'') index, and thus the whole PL map not only shifts, but also warps upon changing the CNT medium.
| |
| | |
| ===Applications===
| |
| *Photoluminescence is used for characterization purposes to measure the quantities of semiconducting nanotube species in a sample. Nanotubes are isolated (dispersed) using an appropriate chemical agent ("dispersant") to reduce the intertube quenching. Then PL is measured, scanning both the excitation and emission energies and thereby producing a PL map. The ovals in the map define (''S''<sub>22</sub>, ''S''<sub>11</sub>) pairs, which unique identify (''n'', ''m'') index of a tube. The data of Weisman and Bachilo are conventionally used for the identification.<ref name="bachilo">
| |
| {{cite journal
| |
| |author=R. B. Weisman and S. M. Bachilo
| |
| |title=Dependence of Optical Transition Energies on Structure for Single-Walled Carbon Nanotubes in Aqueous Suspension: An Empirical Kataura Plot
| |
| |journal=[[Nano Letters]]
| |
| |volume=3 |issue=9 |pages=1235–1238
| |
| |year=2003
| |
| |doi=10.1021/nl034428i
| |
| |bibcode = 2003NanoL...3.1235W }}</ref>
| |
| | |
| *Nanotube fluorescence has been investigated for the purposes of imaging and sensing in biomedical applications.<ref name="cherukuri">
| |
| {{cite journal
| |
| |author=Paul Cherukuri, Sergei M. Bachilo, Silvio H. Litovsky, and R. Bruce Weisman |title=Near-Infrared Fluorescence Microscopy of Single-Walled Carbon Nanotubes
| |
| in Phagocytic Cells
| |
| |journal=[[Journal of the American Chemical Society]]
| |
| |volume=126 |pages=15638-15639
| |
| |year=2004
| |
| |doi=10.1021/ja0466311}}</ref> <ref name="welsher">
| |
| {{cite journal
| |
| |author=Kevin Welsher, Sarah P. Sherlock, and Hongjie Dai
| |
| |title=Deep-tissue anatomical imaging of mice using carbon nanotube fluorophores in the second near-infrared window
| |
| |journal=[[Proceedings of the National Academy of Sciences]]
| |
| |volume=108 |issue=22 |pages=8943–8948
| |
| |year=2011
| |
| |doi=10.1073/pnas.1014501108 }}</ref> <ref name="barone">
| |
| {{cite journal
| |
| |author=Paul W. Barone, Seunghyun Baik, Daniel A. Heller, and Michael S. Strano
| |
| |title= Near-infrared optical sensors based on single-walled carbon nanotubes
| |
| |journal=[[Nature Materials]]
| |
| |volume=4 |pages=86-92
| |
| |year=2005
| |
| |doi=10.1038/nmat1276 }}</ref>
| |
| | |
| ===Sensitization===
| |
| Optical properties, including the PL efficiency, can be modified by encapsulating organic dyes ([[carotene]], [[lycopene]], etc.) inside the tubes.<ref>
| |
| {{cite journal
| |
| |author=K. Yanagi ''et al.''
| |
| |title=Light-Harvesting Function of β-Carotene Inside Carbon Nanotubes
| |
| |journal=[[Physical Review B]]
| |
| |volume=74
| |
| |issue=15 |pages=155420
| |
| |year=2006|url=http://pubman.nims.go.jp/pubman/item/escidoc:1587358:1/component/escidoc:1587358/Prb155420.pdf
| |
| |doi=10.1103/PhysRevB.74.155420
| |
| |bibcode = 2006PhRvB..74o5420Y }}</ref><ref>
| |
| {{cite journal
| |
| |author=K. Yanagi ''et al.''
| |
| |title=Photosensitive Function of Encapsulated Dye in Carbon Nanotubes
| |
| |journal=[[Journal of the American Chemical Society]]
| |
| |volume=129 |issue=16 |pages=4992–4997
| |
| |year=2007|url=http://pubman.nims.go.jp/pubman/item/escidoc:1587376:1/component/escidoc:1587375/jacs4992.pdf
| |
| |doi=10.1021/ja067351j
| |
| |pmid=17402730
| |
| }}</ref> Efficient energy transfer occurs between the encapsulated dye and nanotube — light is efficiently absorbed by the dye and without significant loss is transferred to the SWCNT. Thus potentially, optical properties of a carbon nanotube can be controlled by encapsulating certain molecule inside it. Besides, encapsulation allows isolation and characterization of organic molecules which are unstable under ambient conditions. For example, Raman spectra are extremely difficult to measure from dyes because of their strong PL (efficiency close to 100%). However, encapsulation of dye molecules inside SWCNTs completely quenches dye PL, thus allowing measurement and analysis of their Raman spectra.<ref>
| |
| {{cite journal
| |
| |author=Y. Saito ''et al.''
| |
| |title=Vibrational Analysis of Organic Molecules Encapsulated in Carbon Nanotubes by Tip-Enhanced Raman Spectroscopy
| |
| |journal=[[Japanese Journal of Applied Physics]]
| |
| |volume=45
| |
| |issue= 12 |pages=9286–9289
| |
| |year=2006
| |
| |doi=10.1143/JJAP.45.9286
| |
| |bibcode = 2006JaJAP..45.9286S }}</ref>
| |
| | |
| ===Cathodoluminescence===
| |
| [[Cathodoluminescence]] (CL) — light emission excited by electron beam — is a process commonly observed in TV screens. An electron beam can be finely focused and scanned across the studied material. This technique is widely used to study defects in semiconductors and nanostructures with nanometer-scale spatial resolution.<ref>
| |
| {{cite journal
| |
| |author=S. J. Pennycook ''et al.''
| |
| |title=Observation of Cathodoluminescence at Single Dislocations by STEM
| |
| |journal=[[Philosophical Magazine A]]
| |
| |volume=41 |issue=4 |pages=589–600
| |
| |year=1980
| |
| |doi=10.1080/01418618008239335
| |
| |bibcode = 1980PMagA..41..589P }}</ref> It would be beneficial to apply this technique to carbon nanotubes. However, no reliable CL, i.e. sharp peaks assignable to certain (''n'', ''m'') indices, has been detected from carbon nanotubes yet.
| |
| | |
| ===Electroluminescence===
| |
| If appropriate electrical contacts are attached to a nanotube, electron-hole pairs (excitons) can be generated by injecting electrons and holes from the contacts. Subsequent exciton recombination results in [[electroluminescence]] (EL). Electroluminescent devices have been produced from single nanotubes.<ref name="led1"/><ref name="led2"/><ref>
| |
| {{cite journal
| |
| |author=M. Freitag ''et al.''
| |
| |title=Hot Carrier Electroluminescence from a Single Carbon Nanotube
| |
| |journal=[[Nano Letters]]
| |
| |volume=4 |issue=6 |pages=1063–1066
| |
| |year=2004
| |
| |doi=10.1021/nl049607u
| |
| |bibcode = 2004NanoL...4.1063F }}</ref>
| |
| | |
| ==Raman scattering==
| |
| {{main|Raman spectroscopy}}
| |
| [[Image:CNTRaman.jpg|thumb|300px|Raman spectrum of single-wall carbon nanotubes]]
| |
| | |
| Raman spectroscopy has good spatial resolution (~0.5 micrometers) and sensitivity (single nanotubes); it requires only minimal sample preparation and is rather informative. Consequently, Raman spectroscopy is probably the most popular technique of carbon nanotube characterization. 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.<ref name="fantini">
| |
| {{cite journal
| |
| |author=C. Fantini ''et al.''
| |
| |title=Optical Transition Energies for Carbon Nanotubes from Resonant Raman Spectroscopy: Environment and Temperature Effects
| |
| |journal=[[Physical Review Letters]]
| |
| |volume=93 |issue=14 |pages=147406
| |
| |year=2004
| |
| |doi=10.1103/PhysRevLett.93.147406
| |
| |pmid=15524844
| |
| |bibcode=2004PhRvL..93n7406F
| |
| }}</ref><ref name="filho"/> Several scattering modes dominate the SWCNT spectrum, as discussed below.
| |
| | |
| Similar to photoluminescence mapping, the energy of the excitation light can be scanned in Raman measurements, thus producing Raman maps.<ref name="fantini"/> Those maps also contain oval-shaped features uniquely identifying (''n'', ''m'') indices. Contrary to PL, Raman mapping detects not only semiconducting but also metallic tubes, and it is less sensitive to nanotube bundling than PL. However, requirement of a tunable laser and a dedicated spectrometer is a strong technical impediment.
| |
| | |
| ===Radial breathing mode===
| |
| Radial breathing mode (RBM) corresponds to radial expansion-contraction of the nanotube. Therefore, its frequency ''ν''<sub>RBM</sub> (in cm<sup>−1</sup>) depends on the nanotube diameter ''d'' as, ''ν''<sub>RBM</sub>{{=}} A/''d'' + B (where A and B are constants dependent on the environment in which the nanotube is present. For example B=0 for individual nanotubes.) (in nanometers) and can be estimated<ref name="fantini"/><ref name="filho"/> as {{nowrap|''ν''<sub>RBM</sub> {{=}} 234/''d'' + 10}} for SWNT or {{nowrap|''ν''<sub>RBM</sub> {{=}} 248/''d'' }} for DWNT, which is very useful in deducing the CNT diameter from the RBM position. Typical RBM range is 100–350 cm<sup>−1</sup>. If RBM intensity is particularly strong, its weak second [[overtone]] can be observed at double frequency.
| |
| | |
| ===Bundling mode===
| |
| The bundling mode is a special form of RBM supposedly originating from collective vibration in a bundle of SWCNTs.<ref>
| |
| {{cite conference
| |
| |author=H. Kataura ''et al.''
| |
| |title=Bundle Effects of Single-Wall Carbon Nanotubes
| |
| |url=http://staff.aist.go.jp/h-kataura/IWEPNM00-Kataura.pdf
| |
| |booktitle=AIP Conference Proceedings | |
| |volume=544 |pages=262 | |
| |year=2000
| |
| }}</ref>
| |
| | |
| ===G mode===
| |
| Another very important mode is the G mode (G from graphite). This mode corresponds to planar vibrations of carbon atoms and is present in most graphite-like materials.<ref name="eklund"/> G band in SWCNT is shifted to lower frequencies relative to graphite (1580 cm<sup>−1</sup>) and is split into several peaks. The splitting pattern and intensity depend on the tube structure and excitation energy; they can be used, though with much lower accuracy compared to RBM mode, to estimate the tube diameter and whether the tube is metallic or semiconducting. A wider pattern indicates semiconducting nature of the nanotube.
| |
| | |
| ===D mode===
| |
| ''D'' mode is present in all graphite-like carbons and originates from structural defects.<ref name="eklund"/> Therefore, the ratio of the ''G''/''D'' modes is conventionally used to quantify the structural quality of carbon nanotubes. High-quality nanotubes have this ratio significantly higher than 100. At a lower functionalisation of the nanotube, the ''G''/''D'' ratio remains almost unchanged. This ratio gives an idea of the functionalisation of a nanotube.
| |
| | |
| ===G' mode===
| |
| The name of this mode is misleading: it is given because in graphite, this mode is usually the second strongest after the G mode. However, it is actually the second overtone of the defect-induced D mode (and thus should logically be named D'). Its intensity is stronger than that of the D mode due to different [[selection rules]].<ref name="eklund"/> In particular, D mode is forbidden in the ideal nanotube and requires a structural defect, providing a phonon of certain angular momentum, to be induced. In contrast, G' mode involves a "self-annihilating" pair of phonons and thus does not require defects. The spectral position of G' mode depends on diameter, so it can be used roughly to estimate the SWCNT diameter.<ref name="dwnt2"/> In particular, G' mode is a doublet in double-wall carbon nanotubes, but the doublet is often unresolved due to line broadening.
| |
| | |
| Other overtones, such as a combination of RBM+G mode at ~1750 cm<sup>−1</sup>, are frequently seen in CNT Raman spectra. However, they are less important and are not considered here.
| |
| | |
| ===Anti-Stokes scattering===
| |
| All the above Raman modes can be observed both as [[Raman scattering#Raman_scattering:_Stokes_and_anti-Stokes|Stokes and anti-Stokes]] scattering. As mentioned above, Raman scattering from CNTs is resonant in nature, i.e. only tubes whose band gap energy is similar to the laser energy are excited. The difference between those two energies, and thus the band gap of individual tubes, can be estimated from the intensity ratio of the Stokes/anti-Stokes lines.<ref name="fantini"/><ref name="filho">
| |
| {{cite journal
| |
| |author=A. G. Souza Filho ''et al.''
| |
| |title=Stokes and Anti-Stokes Raman Spectra of Small-Diameter Isolated Carbon Nanotubes
| |
| |journal=[[Physical Review B]]
| |
| |volume=69 |issue=11 |pages=115428
| |
| |year=2004
| |
| |doi=10.1103/PhysRevB.69.115428
| |
| |bibcode = 2004PhRvB..69k5428S }}</ref> This estimate however relies on the temperature factor ([[Boltzmann factor]]), which is often miscalculated – focused laser beam is used in the measurement, which can locally heat the nanotubes without changing the overall temperature of the studied sample.
| |
| | |
| ==Rayleigh scattering==
| |
| Carbon nanotubes have very large [[aspect ratio]], i.e., their length is much larger than their diameter. Consequently, as expected from the [[Maxwell's equations|classical electromagnetic theory]], elastic light scattering (or [[Rayleigh scattering]]) by straight CNTs has anisotropic angular dependence, and from its spectrum, the band gaps of individual nanotubes can be deduced.<ref>
| |
| {{cite journal
| |
| |author=M. Y. Sfeir ''et al.''
| |
| |title=Probing Electronic Transitions in Individual Carbon Nanotubes by Rayleigh Scattering
| |
| |journal=[[Science (journal)|Science]]
| |
| |volume=306 |issue=5701 |pages=1540–1543
| |
| |year=2004
| |
| |doi=10.1126/science.1103294
| |
| |pmid=15514117|bibcode = 2004Sci...306.1540S }}</ref><ref>
| |
| {{cite journal
| |
| |author=Y. Wu ''et al.''
| |
| |title=Variable Electron-Phonon Coupling in Isolated Metallic Carbon Nanotubes Observed by Raman Scattering
| |
| |journal=[[Physical Review Letters]]
| |
| |volume=99
| |
| |issue=2 |pages=027402
| |
| |year=2007
| |
| |doi=10.1103/PhysRevLett.99.027402
| |
| |pmid=17678258
| |
| |bibcode=2007PhRvL..99b7402W
| |
| |arxiv = 0705.3986 }}</ref>
| |
| | |
| Another manifestation of Rayleigh scattering is the "antenna effect", an array of nanotubes standing on a substrate has specific angular and spectral distributions of reflected light, and both those distributions depend on the nanotube length.<ref>
| |
| {{cite journal
| |
| |author=Y. Wang ''et al.''
| |
| |title=Receiving and Transmitting Light-Like Radio Waves: Antenna Effect in Arrays of Aligned Carbon Nanotubes
| |
| |journal=[[Applied Physics Letters]]
| |
| |volume=85
| |
| |issue=13 |pages=2607
| |
| |year=2004
| |
| |doi=10.1063/1.1797559
| |
| |bibcode = 2004ApPhL..85.2607W }}</ref>
| |
| | |
| ==See also==
| |
| {{colbegin|3}}
| |
| *[[Carbon nanotubes in photovoltaics]]
| |
| *[[Carbon nanotube]]
| |
| *[[Selective chemistry of single-walled nanotubes]]
| |
| *[[Potential applications of carbon nanotubes]]
| |
| *[[Resonance Raman spectroscopy]]
| |
| *[[Allotropes of carbon]]
| |
| *[[Graphene]]
| |
| *[[Nanoflower]]
| |
| *[[Buckypaper]]
| |
| *[[Hiromichi Kataura]]
| |
| *[[Mechanical properties of carbon nanotubes]]
| |
| {{colend}}
| |
| | |
| ==References==
| |
| {{reflist|2}}
| |
| | |
| ==External links==
| |
| *[http://stacks.iop.org/1367-2630/5/i=1/a=E04 Selection of free-download articles on carbon nanotubes (New Journal of Physics)]
| |
| *[http://staff.aist.go.jp/h-kataura/kataura-pub.html Publications of H. Kataura] — many of older ones are downloadable
| |
| *[http://www.aist.go.jp/pr/nanotech2009/pdf/a5_e.pdf Carbon Nanotube Black Body (AIST nano tech 2009)]
| |
| | |
| {{good article}}
| |
| | |
| [[Category:Carbon nanotubes]]
| |