Optical properties of carbon nanotubes

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Optical properties of carbon nanotubes (CNTs) are important both for science and technology: Optical (and other) measurements on CNTs - an ideal one-dimensional solid - allowed thorough experimental testing of many solid-state theories. On the practical side, remarkable optical properties have been discovered, which might result in commercial single-molecule LEDs or photodetectors.

Terminology

Grammatically, carbon nanotube, double-wall carbon nanotube and multi-wall carbon nanotube should be abbreviated as CNT, DWCNT and MWCNT, respectively, however, C is very often omitted in scientific literature, and NT, DWNT and MWNT are actually more common than their C-ed counterparts. Also, "wall" is often exchanged with "walled".

Electronic structure of carbon nanotube

Main article: carbon nanotube
The (n,m) nanotube naming scheme can be thought of as a vector (Ch) in an infinite graphene sheet that describes how to "roll up" the graphene sheet to make the nanotube. T denotes the tube axis, and a1 and a2 are the unit vectors of graphene in real space.

Single-wall carbon nanotube can be imagined as graphene sheet rolled at a certain "chiral" angle. Consequently, SWCNT can be defined by its diameter and chiral angle. However, more conveniently, a pair of indexes (m,n) is used instead. Tubes having n = m are called "armchair" and those with m = 0 "zigzag". Those indexes uniquely determine whether CNT is a metal, semimetal or semiconductor, as well as its band gap: when |m-n| = 3k (k is integer) the tube is metallic and it is semiconducting if |m-n| = 3k + 1 or 3k - 1. The nanotube diameter d is related to m and n as

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

Optical properties of carbon nanotubes derive from electronic transitions within one-dimensional density of states. The major difference of one-dimensional nanotube from 3D materials is presence of sharp van Hove singularities, which result in the following remarkable optical properties of carbon nanotubes:

  • Optical transitions occur between the v1-c1, v2-c2, etc., states of semiconducting or metallic nanotubes and are traditionally labeled as S11, S22, M11, etc., or, if the "conductivity" of the tube is unknown or unimportant, as E11, S22, etc. Crossover transitions c1-v2, c2-v1, etc., are dipole-forbidden and thus are extremely weak, but they were possibly observed using cross-polarized optical geometry.[1]
  • 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.[2]
  • Optical transitions are rather sharp (~10 meV) and strong. Consequently, it is relatively easy to selective excite nanotubes of certain type, as well as to detect optical signals from individual nanotubes.

Kataura plot

The band structure of carbon nanotubes having certain (m,n) indexes can be calculated.[3] However, the calculation is complex and not accurate enough. Fortunately, an experimental graph named "Kataura plot", relating the nanotube diameter and its bandgap energies, has been designed in 1999. [4] The oscillating shape of every branch of the Kataura plot reflects the intrinsic strong dependence of the SWCNT properties on the (m,n) index rather than on its diameter. For example, (10,0) and (8,3) tubes have almost the same diameter, but very different properties: the former is a metal, but the latter is semiconductor.

Optical absorption

Optical absorption spectrum from dispersed single-wall carbon nanotubes

Optical absorption originates from electronic transitions from the c2 to v2 (energy E22) or c1 to v1 (E11) levels, etc. 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 E22 or E11 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 (E22,E11) pairs.

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.[5] 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.

Luminescence

File:DWNTpl.jpg
Photoluminescence map from double-wall carbon nanotubes, see[6]

Excitation mechanism

Photoluminescence (PL) is one of the important tools for nanotube characterization. The excitation of PL usually occurs as follows: electron in nanotube absorbs excitation light via S22 transition creating an electron-hole pair (exciton). Both electron and hole rapidly relax (via phonon-assisted processes) from c2 to c1 and from v2 to v1 states, respectively. Then they recombine through a c1-v1 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 efficiency is yet rather low - typically 0.01%.[7]
  • The spectral range of PL is rather wide - emission wavelength can vary between 0.8 and 2.1 micrometers depending on the nanotube structure.[8][6]
  • Interaction between nanotubes or between nanotube and another material (e.g., substrate) quenches PL. For this reason, no PL is observed in multi-wall carbon nanotubes. PL from double-wall carbon nanotubes strongly depends on how they were prepared: CVD grown DWCNTs show emission both from inner and outer shells.[8][6] However, DWCNTs produced by encapsulating fullerenes into SWCNTs and annealing show PL only from the outer shells.[9] Isolated SWCNTs lying on the substrate show extremely weak PL which has been detected in few studies only.[10] Detachment of the tubes from the substrate drastically increases PL.
  • Position of the (S22,S11) PL peaks depends slightly (within 2%) on the nanotube environment (air, dispersant, etc.). However, the shift depends on the (m,n) index, and thus the whole PL map not only shifts, but also warps upon changing the CNT medium.

Applications

Because of low efficiency, no commercial application of PL from pure carbon nanotubes is viable yet. However, PL is widely used to deduce (m,n) indexes: first 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 (S22,S11) pairs, which unique identify (m,n) index of a tube. The data of Weisman and Bachillo are conventionally used for the identification.[11]

Sensitization

Optical properties, including the PL efficiency, can be modified by encapsulating organic dyes (carotene, lycopene, etc.) inside the tubes. [12][13] 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. [14]

Cathodoluminescence

Cathodoluminescence (CL) - light emission excited by electron beam - is a process commonly observed in TV screens. 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. It would be beneficial to apply this technique to carbon nanotubes. However, no reliable CL, i.e. sharp peaks assignable to certain (m,n) indexes, has been detected from carbon nanotubes yet.

Electroluminescence

If appropriate electrical contacts are attached to a nanotube then electron-hole pairs (excitons) can be generated by injecting electrons and holes from the contacts. Subsequent exciton recombination results in electroluminescence (EL). Electroluminescent devices - single molecule LEDs - have been produced from single nanotubes.[15][16][17]

Raman scattering

Main article: Raman spectroscopy
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. 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. Those maps also contain oval-shaped features uniquely identifying (m,n) indexes. 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 sub>RBM (in cm-1) depends on the nanotube diameter (in nanometers) and can be estimated as RBM = 238/d, which is very useful in deducing the CNT diameter from the RBM position. Typical RBM range is 100-350 cm-1. 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.[18]

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. G band in SWCNT is shifted to lower frequencies relatively to graphite (1580 cm-1) 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.

D mode

D mode is also present in all graphite-like carbons and originates from structural defects. Therefore the ratio of the G/D modes is conventionally used to quantify the quality of carbon nanotubes. High-quality nanotubes have this ratio significantly higher than 100.

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 specific selection rules. Its position is diameter dependent, and thus can be used to roughly estimate the SWCNT diameter.[6] In particular, G' mode is a doublet in double-wall carbon nanotubes, often unresolved due to line broadening.

Other overtones are frequently seen in CNT Raman spectra, such as a combination of RBM+G mode at ~1750 cm-1. However, they are less important and are not considered here.

References

  1. ^ "Cross-polarized optical absorption of single-walled nanotubes probed by polarized photoluminescence excitation spectroscopy" Phys. Rev. B 74 (2006) 205440
  2. ^ "Midgap luminescence centers in single-wall carbon nanotubes created by ultraviolet illumination" Appl. Phys Lett. 89 (2006) 173108
  3. ^ Prof. Shigeo Maruyama
  4. ^ "Optical Properties of Single-Wall Carbon Nanotubes" Synthetic Metals 103 (1999) 2555
  5. ^ "Comparison of Analytical Techniques for Purity Evaluation of Single-Walled Carbon Nanotubes" J. Am. Chem. Soc. 127 (2005) 3439
  6. ^ a b c d "Optical Characterization of Double-wall Carbon Nanotubes: Evidence for Inner Tube Shielding" J. Phys. Chem. C 112 (2008) 11194
  7. ^ a b "Time-Resolved Fluorescence of Carbon Nanotubes and Its Implication for Radiative Lifetimes" Phys. Rev. Lett. 92 (2004) 177401
  8. ^ a b "IR-extended photoluminescence mapping of single-wall and double-wall carbon nanotubes" J. Phys. Chem. B 110 (2006) 17420
  9. ^ "Photoluminescence quenching in peapod-derived double-walled carbon nanotubes" Phys. Rev. B 74 (2006) 153404
  10. ^ "Crystal Plane Dependent Growth of Aligned Single-Walled Carbon Nanotubes on Sapphire" J. Am. Chem. Soc. 130 (2008) 9918
  11. ^ Nano Letters 3 (2003) 1235
  12. ^ "Light-harvesting function of b-carotene inside carbon nanotubes" Phys. Rev. B 74 (2006) 155420
  13. ^ "Photosensitive function of encapsulated dye in carbon nanotubes" J. Am. Chem. Soc. 129 (2007) 4992
  14. ^ "Vibrational Analysis of Organic Molecules Encapsulated in Carbon Nanotubes by Tip-Enhanced Raman Spectroscopy" Jap. J. Appl. Phys. 45 (2006) 9286
  15. ^ "Bright infrared emission from electrically induced excitons in carbon nanotubes" Science 310 (2005) 1171
  16. ^ "Electrically Induced Optical Emission from a Carbon Nanotube FET" Science 300 (2003)783
  17. ^ "Hot Carrier Electroluminescence from a Single Carbon Nanotube" Nano letters 4 (2004) 1063
  18. ^ AIP Conference Proceedings 544 (2000) pp. 262

See also

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