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Nanotechnology

Using nanophotonics to extract structural information on single-walled carbon nanotubes

The bandgap photoluminescence of single-walled carbon nanotubes can now be translated into a structural description.
8 August 2006, SPIE Newsroom. DOI: 10.1117/2.1200607.0326

Nanomaterials, by virtue of their structure and size, invite new applications in materials science, biology, and medicine. In this context, understanding the semiconducting properties of single-walled carbon nanotubes (SWNTs) is important for several current and developing materials-science nanophotonic applications. These include, inter alia, field-effect transistors, field-emitters for display technology, memory devices, semiconductor devices and interconnects. Other SWNT properties are important in biology, prompting their use as near-IR biosensor probes and in developing microarray technologies, which are particularly important for DNA technology.1

Unlike ordinary materials, nanoparticles present certain difficulties when researchers study their fluorescence properties to obtain structural information. For example, SWNTs emit fluorescence in the near-IR (∼1.55eV to <0.62eV) instead of in the UV or visible regions. This requires appropriate detection technology. Since SWNTs exhibit wide variations in diameter and helicity depending on the manufacturing method, the determination of structure based on fluorescence properties is not a trivial exercise. Hence the need for specialized instrumentation to study nanoparticles and their derivatives.

The main underlying principle of the photoluminescence behavior of semiconducting nanomaterials is the quantum confinement effect, which stipulates that, when the size of a semiconducting nanocrystal or nanotube is less than the size of the Bohr exciton radius of the bulk material, the bandgap energy is inversely proportional to the nanoparticle size.2,3 Quantum-confinement properties predict that smaller-diameter SWNTs generally have higher energy absorbance and emission properties than larger diameter species composed of the same material. These unique SWNT semiconducting energetic properties were first reported by the Weisman group at Rice University, where researchers performed spectrofluorometric measurements on SWNTs to extract information on their diameter and helical architecture.4,5

The SWNT diameter inversely correlates with the absorption and emission energies and bandgaps between their valence and conductance bands. The major breakthrough of the Weisman group was the observation that treating bulk SWNT preparations with surfactants and sonication allowed SWNT aggregates to mono-disperse as individual SWNT-micelles. Only the separated SWNTs exhibited photoluminescence, because aggregates were energetically self-quenched.

There were five key initial observations were as follows. First, the decreasing absorption and emission energetic properties of the individual SWNT species correlated directly with the diameter estimates obtained by analysis of the well-known radial breathing modes from Raman spectroscopy. Second, the semiconducting SWNTs were species with specific parameters for the n,m coordinates that coincided with predicted bandgaps between the valence and conductance bands. Third, metallic and semi-metallic SWNTs with continuous valence and conductance bands exhibited little or no photoluminescence. Fourth, the photon-absorption energies of the respective transitions between valence band 2 (v2) and conductance band 2 (c2) correlated with the photon-emission energies of the transitions from the c1 to v1 bands for individual SWNTs. Finally, the coordinate energies of the v2 – c2 and c1 – v1 transitions mapped to the diameter and helical properties of individual SWNTs with specific n,m coordinates.

To conduct this type of research, commercial instruments—specially configured for collecting excitation vs. emission maps (EEMs) for semiconducting SWNT samples—have now become available from HORIBA Jobin Yvon, manufacturers of the NanoLog® spectrofluorometer. This instrument only requires a few minutes for the EEMs to reveal the bandgap energies for each individual SWNT in a given mixture.

A typical configuration may include the following main components: a double-grating excitation monochromator, an imaging emission spectrograph, a multichannel liquid-N2-cooled InGaAs detector, and a 450W Xenon lamp (see Figure 1). The emission spectrometer has selectable gratings in a turret mount for rapid, easy acquisition of near-IR spectra. One grating (150 grooves/mm, 1200nm blaze) has single-shot coverage >500nm, with a detector that is sensitive from 800–1700nm. EEMs are fully instrument-corrected for the detector's dark-signal and spectral response, as well as the lamp's spectral output. Additionally, compiled EEMs can be globally analyzed using the exclusive Nanosizer® software to determine the SWNT composition. A double-convolution algorithm (U.S. patent pending) is also included in the software package to simultaneously compute excitation and emission wavelength-coordinate lineshape parameters for each SWNT species.


Figure 1. Shown is the NanoLog spectrofluorometer, specifically designed for SWNT fluorescence measurements. Quantitative sample analysis is easily performed with the companion Nanosizer analysis software.
 

Figures 2A and B shows comparative EEM data (solid lines) and simulations results (contour-maps) obtained for two different SWNT suspensions. The first suspension (Figure 2A) was manufactured using a high-pressure carbon-monoxide method (HiPCO) and the second (Figure 2B) with a cobalt-molybdenum catalytic method (CoMoCAT). Figure 2A identifies five main HiPCO species while Figure 2B identifies four main CoMoCAT species in the regions of interest. Figure 2C shows a comparative helical map of the species found in Figure 2A and B. It plots the helical angle versus SWNT diameter (in nanometers) against intensity of emission (symbol size/color). The data clearly shows that the SWNT samples can be distinguished by their different size and helical distributions (Figure 2C):6,7 while the HiPCO suspension forms a broad-size distribution of SWNTs (0.6 to 1.3nm dia.) with many helical angles and more than 50 species, the CoMoCAT suspension has a narrower average size (∼0.8nm dia.) and a smaller helical-angle distribution. Only two species (6,5 and 7,5) account for ∼58% of the intensity.


Figure 2. Quantum excitation-emission (A and B) and helical (C) maps of HiPCO and CoMoCAT SWNT suspensions, obtained using a NanoLog. The solid lines (A and B) are data and the color contours are simulations. Symbol sizes (C) show relative amplitudes for HiPCO (circles) and CoMOCAT (squares), each normalized to 1. The R2 values for the simulations are 0.997 (HiPCO) and 0.999 (CoMoCAT).
 

The field of SWNTs is pursuing a ‘holy grail’, namely the ability to synthesize or isolate SWNTs in bulk (kg quantities) under controlled conditions to yield specific n,m species. These efforts will obviously improve a great number of semiconductor-related applications, currently limited because of the mixed species present in current SWNT preparations. For researchers interested in SWNT structures and properties, our Nanolog/Nanosizer system should be of assistance.


Authors
Adam Gilmore, Stephen Cohen
Molecular Microanalysis Division, Horiba Jobin Yvon
Edison, NJ
Dr. Gilmore is currently an applications scientist at Horiba Jobin Yvon. He has more than 15 years of experience with photoluminescence instrumentation and plays a key role in nanophotonics instrument and software development and support for Horiba Jobin Yvon. In addition, Dr. Gilmore was invited to deliver a keynote address on the subject of Nanophotonics instrumentation at the 2006 SPIE European meeting.

References:
1. D. A. Heller, E. S. Jeng, T.-K. Yeung, B. M. Martinez, A. E. Moll, J. B. Gastala, M. S. Strano, Optical detection of DNA conformational polymorphism on single-walled carbon nanotubes,
Science,
Vol: 311, pp. 508-511, 2006.