Applications of Carbon Nanostructures

Although research on solid C60 and related materials is still at an early stage, these materials are already beginning to show many exceptional properties, some of which may lead to practical applications. Small-scale applications will come first; then targeted applications utilizing the special properties of fullerenes are expected to follow [20.1,2].

Many of the fullerene applications that have been identified thus far have been related to the unusual properties of fullerenes discussed earlier in this book, such as the nonlinear processes associated with the excited molecular triplet states (§13.7), the charge transfer between certain polymers and C60 (§13.5), phototransformation properties (§13.3.4), and the strong bonding between C60 and metal (or Si) surfaces (§17.14). This chapter first covers applications associated with optical excitations (§20.1) because they are so pervasive, followed by applications to electronics (§20.2), materials and electrochemical applications (§20.3 and §20.4), and finally miscellaneous other applications including nanotechnology, coatings, free-standing membranes, tribology, separations, and sensors (§20.5). The concluding section (§20.6) gives a summary of patents and efforts to commercialize the applications previously discussed in the chapter.

20.1. Optical Applications

Several of the unusual optical properties exhibited by fullerenes show promise for applications. The nonlinear properties of optical absorption in the excited triplet state give rise to optical limiting devices (§20.1.1)

and photorefractive devices (§20.1.3), while photoinduced charge transfer between polymer and C60 constituents in C60-polymer composites gives rise to photoconductivity applications (§20.1.2), photodiodes (§20.2.2), and photovoltaic devices (§20.2.4).

20.1.1. Optical Limiter

One promising application for fullerenes is as an optical limiter because of the large magnitude of the nonlinear third-order electric susceptibility and nonlinear optical response, which in turn arise from basic optical properties associated with the interplay between allowed and forbidden transitions and between singlet and triplet state transitions [20.3] (see §13.7). Optical limiters are used to protect materials from damage by intense incident light pulses via a saturation of the transmitted light intensity with increasing exposure time and/or incident light intensity.

Outstanding performance for C60 relative to presently used optical limiting materials has been observed at 5320 Â for 8 ns pulses (see Fig. 20.1) using solutions of C60 in toluene and in chloroform (CH3C1) [20.5]. Although C70 in similar solutions also shows optical limiting action, the performance of C60 was found to be superior. It was found, in fact, that more than 100 photons per C60 molecule could be absorbed repetitively in a single nanosecond optical pulse (at a wavelength of 532 nm) [20.6],

The proposed mechanism for the optical limiting is that C60 and C70 are more absorptive in the infrared (IR) and visible regions of the spectrum

Fig. 20.1. Optical limiting response of a variety of reverse saturation absorption active materials to light pulses at a wavelength of 532 nm [20.4],

when these molecules are excited from the triplet excited state than when excited from the singlet ground state (see §13.2.3) [20.6], The higher absorption associated with the triplet state (relative to the ground state) can be simply understood in terms of an increased transition probability through dipole-allowed transitions and the larger number of available final states. The larger availability of final triplet states arises because the irreducible representations associated with the triplet states Fr are found by taking the direct product Flg ® Fs, where Flg denotes the transformation properties of the S = 1 spin state and Ts denotes the irreducible representations of the orbital singlet states (see Table 12.4 in §12.5).

Fullerene molecules reach the metastable triplet state through the following process, as shown in Fig. 13.7. The absorption of a photon takes an electron from the singlet S0 ground state to an excited singlet state 5,, which decays rapidly to the lowest excited singlet state S\. This is followed by an intersystem crossing (5! -*■ 7,) to the lowest triplet state, Tx, although other Sj —> Tt, intersystem crossings also may occur from higher-lying St states before the 5, 5, transitions takes place. Because electrons in the triplet state T, can be excited via dipole-allowed processes, while electrons in the ground state can be excited only by dipole-forbidden processes near the optical absorption edge, the nonlinear absorption coefficient of C60 near 2 eV can be increased by populating the metastable triplet state. Even though the decay time in the triplet state is usually in the fis range in a solid film, the population of the triplet state increases rapidly as the light intensity for optical excitation increases, and consequently the absorption coefficient also increases. These physical properties allow use of C60 as an optical limiter [20.5], For the optical limiting of fast pulses (on a nanosecond time scale) a single C60 molecule was found to be able to transfer 200-300 eV (i.e., ~100 photons of energy in the visible range) of energy per pulse from the C60 molecules to vibrational excitations of the fullerene and/or solvent molecules without dissociation of either the fullerene or the solvent molecules [20.6],

Figure 20.2 shows that the nonlinear absorption of the triplet state gives rise to a very large increase in absorption at infrared frequencies, while Fig. 20.1 shows excellent optical limiting properties of C60 in comparison with other materials which have either been considered or used for optical limiting applications [20.4]. It is suggested by Fig. 20.2 that Qq would have even better performance as an optical limiter near ~0.8 eV (pumped by two photon absorption followed by intersystem crossing) than at 532 nm (2.33 eV). Because of the long lifetime of the triplet exciton in frozen solution (50 ms) [20.8,9], it has been suggested that C60 would have superior performance characteristics as an optical limiter if used in a solid matrix solution [20.10]. Although C60 and C70 offer attractive characteristics for use

Fig. 20.2. Absorption spectrum of a toluene solution of Cm. The inset shows the induced absorption of C^ associated with optical transitions from excited triplet states. Note that the wavelength scale of the inset lines up with that for the main figure [20.7] .

Wavelength (nm)

Fig. 20.2. Absorption spectrum of a toluene solution of Cm. The inset shows the induced absorption of C^ associated with optical transitions from excited triplet states. Note that the wavelength scale of the inset lines up with that for the main figure [20.7] .

as an optical limiter material, damage effects such as are induced by high-intensity (~2xlOu W/cm2) light pulses of very short duration (e.g., 300 fs duration at a wavelength of 612 nm) [20.11] require further investigation.

20.1.2. Photoexcited Cm~Polymer Composites

As discussed §13.5, experiments on C60-polymer composites [20.12-16] have shown that a very fast (subpicosecond) photoinduced electron transfer occurs from the polymer into the nearby C60 molecules, thereby forming a metastable Cg0 anion and a mobile hole on the polymer backbone. The mobile hole is responsible for the high photoconductivity of the composite [20.12,16,17]. A polymer-C60 heterojunction, therefore, would be expected to exhibit a photovoltaic response with a charge separation photoinduced across the interface. These C60-polymer composites and heterojunctions have been investigated for their potential as photoconductors (§20.1.2), rectifying diodes (§20.2.2), for their photorefractive effects (§20.1.3), and for photovoltaic applications (§20.2.4) [20.18-23].

Studies on the conducting polymer PHT [poly(3-hexylthiophene)] [20.24] showed that the addition of 10-30 mol % of C60 to form a polymer C60-composite suppresses the interband optical absorption edge of the pristine PHT at ~2 eV and also quenches the PHT photoluminescence, thus indi-

eating that C60 acts as an acceptor dopant (i.e., a C60 anion is formed). Consistent with this view is the appearance at high C60 doping levels (~30%) of a broad optical absorption band at 1.1 eV (see §13.4.1), shown previously to be associated with the presence of the C60 anion [20.25,26]. However, also present is the characteristic strong absorption band of C60 (neutral molecules), which suggests that it is the C60 molecules with the shortest distances to the polymer chain that are predominantly involved in the photo-induced steady-state charge transfer, and the more distant C60 molecules remain neutral. Several groups showed that even the neutral C6U molecules might be involved in the suppression of the PHT photoluminescence via a two-step process involving photoexcitation of the polymer and subsequent electron transfer to the C60 to form the C60 anion with some carrier hopping from a C60 anion to a neutral C60 molecule [20.12,16,24].

Photoconducting devices are another interesting application of Copolymer composites. Polyvinylcarbazole (PVK) doped with a mixture of C60 and C70 was the first fullerene-polymer system reported to exhibit exceptionally good photoconductive properties and a high potential for xerographic applications [20.18,27]. The striking enhancement of the pho-toinduced discharge curve of PVK by doping with a few percent of C60 is shown in Fig. 13.34 [20.18], The charge generation efficiency 4> is increased by more than a factor of 50 at a wavelength of 500 nm and by a factor of ~4 at 340 nm by doping PVK with as little as -2.7% CM (by weight). The performance of the fullerene-PVK composite in these experiments was found to compare favorably with one of the best commercial polymer photoconductors (thiapyrylium-doped polycarbonate).

Materials useful for commercial xerography applications should exhibit low dark conductivity, large charge generation efficiency (f>, and a fast, complete discharge of the surface charge. For light with a wavelength A > 350 nm, efficient photoinduced discharge occurs in C60-polymer composites for both positive and negative charging, but for the strongly absorbing region (A < 350 nm), the discharge occurs only for positive charging [20.18,28]. The charge generation efficiency <j> is enhanced by an electric field, and the observed electric field dependence of <f> is explained by a model due to Onsager [20.29], which assumes that upon absorption the incident photon creates a bound electron-hole pair, where the electron and hole are separated by a distance r0. The thermalized electron-hole pairs either recombine or are separated into free electrons and holes which are collected in the photocurrent. The fraction of absorbed photons giving rise to free or unbound electron-hole pairs (denoted by 4>Q) determines the efficiency of the material as a photoconductor. The fitting of the data for the PVK-C60 composite to the Onsager model yields values for r0 — 19 A and <t>0 = 0.90.

The use of fullerenes to enhance the photoconductivity and charge generation efficiency of polymers has been extended to other polymer systems, seeking both enhanced performance and a better understanding of the science behind the device performance. In C60-conjugated-polymer composites, at long wavelengths the fullerenes serve both as light absorbers and as sensitizers for the photoconductivity. But at shorter wavelengths (higher photon energies) light is primarily absorbed by the polymer close to the surface, and in this case an electron in the polymer is excited to a state lying above the lowest excited state for C60, thereby inducing charge transfer of the electron to C60 as shown in Figs. 13.35 and 13.37. This charge transfer separates the electron and the hole, thus enhancing hole conduction along the polymer chain. Although high charge generation efficiency was first found for the C60-PVK composite (see Fig. 13.34), later work showed that the charge generation efficiency is even higher for C60-PMPS (phenylmethylpolysilane) than for C60-PVK at low electric fields, as shown in Figs. 13.34 and 20.3.

While photoinduced electron transfer with high quantum efficiency was observed for conjugated polymers (see §13.5), such as poly(para-phenylene-vinylene) and its derivatives (e.g., MEH-PPV) [20.16], poor

Fig. 20.3. Comparison of photoinduced discharge curves of a 3.2-/nm-thick film of un-doped PMPS (phenylmethylpolysilane) and a similar fullerene-doped film under illumination by a 50 mW/cm2 tungsten lamp light source [20.28]. See Fig. 13.34 for similar results for Qo-PVK [20.18], time (sec)

Fig. 20.3. Comparison of photoinduced discharge curves of a 3.2-/nm-thick film of un-doped PMPS (phenylmethylpolysilane) and a similar fullerene-doped film under illumination by a 50 mW/cm2 tungsten lamp light source [20.28]. See Fig. 13.34 for similar results for Qo-PVK [20.18], performance was observed for several other polydiacetylene (PDA) derivatives [20.16], This was attributed to the strong, excitonic binding energy between electrons and holes in those polymers, in contrast to an exci-ton interaction that is apparently well screened in PPV [20.16]. Further evidence for this proposed explanation is needed. Enhanced photoconductivity by C60 doping has also been reported in another conducting polymer poly(3-octyldithiophene) or PODT [20.30] near 1.9 eV and 3.5 eV for C60 doping in the range 1-10 mol %. A related polymer P30T [poly(3-octyldithiophene)] also shows enhanced photoconductivity upon addition of C60 [20.16], Although research progress in this field is strongly dependent on the close collaboration between physicists and chemists, it is the chemical synthesis and polymer processing that become especially important in the ultimate commercialization of this class of materials in practical devices.

20.1.3. Photorefractivity in C60-Polymer Composites

The enhancement of the photoconductivity of polymeric films by the addition of a few weight percent of C60 has led to the development of highly efficient photorefractive polymeric films [20.19-21,31]. We first briefly review the principles underlying photorefractive devices in polymer composites [20.21].

A photorefractive device acts like a transmission grating, by diffracting, or bending, a transmitted light beam away from the forward direction. One coherent light source is used to create (or write) a grating in the photorefractive medium, and a second source, incident from the other side of the device, is used to read the result of the writing source. The effect of the grating is to diffract the reading beam, and this photorefracted beam is monitored with a detector.

In the photorefractive medium, the "grating" is created by the superposition of two interfering, coherent beams derived from the same laser. A sinusoidal, spatial modulation of light intensity from these beams bathes the sample, and photogenerates carriers in proportion to the local light intensity. These carriers migrate by diffusion (in an inorganic, photorefractive medium) or by the action of an applied electric field (in polymeric, photorefractive materials) away from the location where the carriers were generated to occupy deep trap sites, thereby setting up a steady-state, spatially modulated space charge field. This "field grating" is converted by the linear electro-optic effect into an "index grating," or a spatial modulation of the refractive index coefficient, which can be used to diffract a third, "read" beam incident from the other side of the device into a detector.

The experimental apparatus used to study the photorefractive effect is shown schematically in Fig. 20.4. The presence of the C60 additive in the photorefractive film stimulates the photogeneration of mobile holes in the polymer. These mobile holes are soon trapped and then set up a modulated space charge field (i.e., the grating). A beam from an InGa-AlP semiconductor diode laser (687 nm wavelength) is incident from one side of the sample and is used to read the index grating written by the interference grating induced by the He-Ne laser from the other side of the film (see Fig. 20.4). The response due to these three beams is a fourth, photo-diffracted signal beam monitored by the detector Du and D2 monitors the incident intensity of the "reading" beam. The elements Pj, Mj, and BSj (j = 1,2,...) in Fig. 20.4 represent optical polarizers, mirrors, and beam splitters, respectively, and the optical wavelength is 687 nm.

The initial studies of the photorefractive effect were carried out in the Car-polymer composite PVK:C60:DEANST [20.22], where the DEANST, or diethyl aminonitrostyrene, was added as a second-order, nonlinear optical molecular constituent to the polyvinylcarbazole (PVK)-C60 composite. The photorefractive performance of this PVK-polymer composite was found to be very promising. However, substantial improvements in per-

Fig. 20.4. Schematic diagram of the apparatus used for writing a pattern on a 100-/u.m-thick film of the PVK-TCP:Cm:DEANST polymeric composite for photorefractive studies. The He-Ne laser writes the grating, which is then read by the InGa-AlP semiconductor diode laser [20.21],

formance were made by lowering the glass transition temperature of the polymer through the addition of a plasticizer TCP (tricresyl phosphate), which improves the applied field-induced, noncentrosymmetric alignment of the DEANST molecules. The actual photorefractive device is a three-layer structure with the 100-/j,m-thick photorefractive PVK-TCP:C60:DEANST film sandwiched between two transparent indium-tin oxide (ITO) electrodes. These electrodes are used to pole the device (align the DEANST molecules by an applied electric field), as well as to induce a drift of the photogenerated holes [20.20]. In Fig. 20.5 we display time-resolved diffraction intensity data for PVK:C60:DEANST (the plasticizer was not present

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