Aron
Pekker
ab,
Mingguang
Chen
ac,
Elena
Bekyarova
ab and
Robert C.
Haddon
*abcd
aCenter for Nanoscale Science and Engineering, University of California, Riverside, California 92521, USA. E-mail: haddon@ucr.edu
bDepartment of Chemistry, University of California, Riverside, California 92521, USA
cDepartment of Chemical and Environmental Engineering, University of California, Riverside, California 92521, USA
dDepartment of Physics, King Abdulaziz University, Jeddah 21589, Saudi Arabia
First published on 17th November 2014
The electrical conductivity of single-walled carbon nanotube (SWNT) networks is strongly enhanced by the high vacuum e-beam deposition of transition metals. In the present communication we demonstrate that it is possible to accomplish the same chemical functionalization reactions at room temperature beginning with simple organometallic precursors. We show that the photochemically induced reactions of solutions of Cr(CO)6, Cr(η6-benzene)(CO)3, and Cr(η6-benzene)2 with thin films of semiconducting, metallic and non-separated SWNT films all lead to strongly enhanced conductivities which produce consistent results for each SWNT type among the three organometallic reagents. We conclude that all three of these reactions lead to the generation of covalent (η6-SWNT)Cr(η6-SWNT) interconnects which provide conducting pathways in the SWNT films and our results broaden the applicability of the transition metal bis-hexahapto-bond as an electronically conjugating linkage between graphene surfaces.
Conceptual insightsThe building block of carbon nanotubes and graphene is the graphitic surface of sp2 hybridized carbon atoms which are incorporated in continuous, periodic poly-benzenoid structures. Due to their high degree of perfection there is no simple way to covalently interconnect such structures without disrupting the electronic band structure and introducing sp3 carbon atoms into the lattice. The introduction of rehybridized carbon atoms into these graphitic structures has been shown to decrease the electrical conductivity and the mobility of the carriers and to lead to localized structures. In this report we extend our use of the covalent organometallic hexahapto-bond in the conjugation of these delocalized graphene surfaces by exploring photochemical routes for the preparation of (η6-SWNT)Cr(η6-SWNT) junctions (SWNT = single-walled carbon nanotubes), in random networks of SWNT thin films. We show that standard organometallic chromium precursors may be utilized to improve the conductivity of metallic, semiconducting, and non-separated SWNT thin films by the use of simple photochemical reactions utilizing the appropriate wavelength of light. The work supports the idea that the minimal rehybrization which is implicit in the formation of organometallic hexahapto-linkages to conjugated graphene surfaces is the only form of covalent bonding which is capable increasing the conductivity of these networks. |
The preparation of hexahapto-bis(arene) metal complexes may be accomplished by a number of different routes, but only two have found general application:10 the metal vapor synthesis (MVS) technique of Timms11 and the solution-based Fischer–Hafner process.12 The preparation of hexahapto-arene-chromium-tricarbonyls can be accomplished by the solution reaction of Cr(CO)6 with the appropriate arene under thermal or photochemical conditions,13,14 although neither process proceeds to the fully substituted Cr(η6-arene)2 complexes.15 We have found that variants of some of these gas phase MVS and solution-based thermal reactions are capable of realizing the organometallic functionalization of extended, periodic π-electrons systems such as single-walled carbon nanotubes (SWNT) and graphene.8,9,16–18 The MVS reactions are accomplished within the high vacuum chamber of an e-beam evaporation system and this has allowed us to show that the formation of these hexahapto-bis(η6-SWNT) metal complexes dramatically increases the conductivity of thin films of SWNTs.
In the present communication we introduce facile solution-based routes for the ambient temperature introduction of organometallic atomic interconnects between the graphene surfaces of SWNTs which obviate the need for high vacuum reaction vessels. We show that photochemical reactions of reagents such as Cr(CO)6 and Cr(η6-benzene)(CO)3 are very effective in forming Cr(η6-SWNT)2 complexes which act as interconnects between the graphitic surfaces, as evidenced by an abrupt increase in conductivity on irradiation of thin films of these materials in the presence of the appropriate reagents. Surprisingly we find that room temperature treatment of the SWNT films with solutions of Cr(η6-benzene)2 spontaneously produces the same effect apparently as a result of incidental exposure to room light.
Typically we first measured the pristine film conductivities on overnight annealing under argon in order to remove residual solvent and atmospheric contaminants and to provide a baseline for the organometallic reactions. The protocol for the experiments is shown in Fig. 2 and consists of the exposure of a suitably wired SWNT thin film (thickness, t = 8 nm), to a solution of the organometallic precursor by dipping the film in an acetonitrile solution of the reagent (concentration ≈ 1 × 10−3 M) in a glove box followed by the measurement of the conductivity of the SWNT thin film at ambient temperature. For these experiments we included three types of SWNT thin films, as described in the experimental section: semiconducting (SC-), metallic (MT-), and non-separated (NS-) SWNTs; the transport properties of these films have been previously characterized in connection with our MVS experiments.9,22 The conductivities of the pristine films are sensitive to atmospheric doping (particularly SC-SWNTs), but representative room temperature conductivities of the annealed films are as follows: σ ≈ 3 S cm−1 (SC-SWNT), 350 (MT-SWNT), 300 (NS-SWNT).
The irradiation of matrices containing Cr(CO)6 with UV light is known to lead to the successive dissociation of the CO ligands which are replaced with molecules from the surrounding matrix;23,24 the high reactivity of these intermediates leads to complexation with inert ligands such as acetonitrile,25 alkanes,26 dihydrogen,27 and inert gases (Ne, Ar, Kr, Xe).23,24
We found that SWNT films in the presence of Cr(CO)6, in which the solvent had been removed by evaporation, did not experience a discernible change in conductivity on irradiation and thus we conclude that acetonitrile complexes25 such as Cr(CO)3(CH3CN)3 must be implicated in these reactions and we have previously shown that this complex reacts readily with exfoliated graphene samples to give (η6-graphene)Cr(CO)3.7 The subsequent reactions with Cr(CO)6 were conducted without removal of the acetonitrile and we postulate the following reaction pathway (Scheme 1).
As may be seen in Fig. 3a, the response of the conductivity is essentially complete in the first 25 s of UVC irradiation (λ = 254 nm) and we were unable to detect differences in the reactivity of the three SWNT films toward the species generated from the photolysis of Cr(CO)6; the use of UVA light (λ = 365 nm) produced no discernible change in the film conductivities. The changes in conductivity are discussed in detail later, but they show the same qualitative behavior as was found previously in the MVS experiments,9 with the SC-SWNT films experiencing the strongest enhancement of the conductivity (by a factor of more than two orders of magnitude in the present experiments).
We found that SWNT films previously exposed to an acetonitrile solution of Cr(η6-benzene)(CO)3 when irradiated with UVA light (λ = 365 nm) led to a virtually identical set of conductivity responses (Fig. 3b), although on a slightly shorter time scale and in this case the use of UVC light (λ = 254 nm) produced no discernible effect on the film conductivities. The photochemistry of Cr(η6-benzene)(CO)3 is known to be complex,26,28 but based on the wavelength of the light found to initiate reaction, we postulate the following reaction mechanism (Scheme 2), as Cr(η6-benzene)(CO)3 is known to undergo both arene exchange and CO loss under irradiation with light.29
The exposure of the SWNT films to a solution of Cr(η6-benzene)2 in acetonitrile in the presence of room light (filtered through the plexiglass of the glove box), lead to immediate reaction as evidenced by the instantaneous response of the conductivity (Fig. 3c).
The benzene ring is usually regarded as kinetically inert toward arene exchange reactions in Cr(η6-benzene)2 and benzene is noted for its ability to replace other arenes under appropriate conditions.10,15,30,31 Furthermore Cr(η6-benzene)2 is known to be quite stable to irradiation and it does not normally undergo light-induced arene exchange.29 However, based on the redox potentials of Cr(η6-C6H6)2 (reduction potential of Cr(η6-C6H6)2+, E1/2 = −0.69 V against SCE in DME)15 and SC-SWNTs (reduction potential of electric arc SWNTs, E1/2 ≈ −0.5 V against SCE in DMSO),32 it seems likely that the reaction may proceed by a series of electron transfer processes as the metallocene cations generally undergo facile arene exchange reactions.10,31
In Scheme 3 we show the reaction as a photochemical process which requires visible light for its initiation, but as may be seen in Fig. 4 it seems the reaction is able to proceed in the absence of light, but at a much slower rate in accord with the redox potentials discussed above which are thermodynamically sufficient for spontaneous electron transfer (Scheme 3).
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Fig. 4 Evolution of the conductivity of semiconducting (SC-) SWNT films after dipping in Cr(η6-benzene)2 solution, in the presence of room light and in the dark. |
It is apparent from Fig. 3 that the photochemical reactions of all three organometallic species lead to the same modification of the electronic structure of the SWNT films and the final conductivities show good agreement after chromium functionalization: σ ≈ 300 S cm−1 (SC-SWNT), 500 S cm−1 (MT-SWNT), 1250 S cm−1 (NS-SWNT), which gives the following conductivity enhancements: (σ − σ0)/σ0 ≈ 100 (SC-SWNT), 1 (MT-SWNT), 4 (NS-SWNT), where σ is SWNT film conductivity after functionalization and σ0 is conductivity of the pristine SWNT film. These values exceed the previously determined MVS results which gave values of: (σ − σ0)/σ0 ≈ 70 (SC-SWNT) and 0.6 (MT-SWNT), for films of thickness, t = 8 nm.9 We assign the slightly improved conductivities of the solution processed films to the ability of the solvent to penetrate and uniformly disperse the reagents throughout the films thereby allowing contact with SWNT junctions within the film that are not accessible to the line-of-sight flux of metal atoms that is characteristic of high vacuum evaporation processes. For films of thickness, t = 2 nm the MVS experiments gave values of (σ − σ0)/σ0 ≈ 130 (SC-SWNT) and 0.8 (MT-SWNT).9 Previous experiments show that the stoichiometry of Cr atoms necessary to bring about these conductivity enhancements is very low.19,21
As noted above, the reaction rates estimated from the conductivities, suggest the following order of reagent reactivities under the specified conditions: Cr(η6-benzene)2 > Cr(η6-benzene)(CO)3 > Cr(CO)6, whereas under most circumstances the reverse order might be expected in reactions of these reagents with free arenes.10,15 Nevertheless the high reactivities shown in Fig. 3 are in accord with our frontier molecular orbital theory (FMO) analysis of the electronic structure of graphene, in which the high lying HOMOs and low lying LUMOs together with the presence of appropriate FMOs suggests that graphene and related small band gap nanomaterials such as SWNTs might be very reactive in certain circumstances, including organometallic functionalization chemistry.8,33
The conductivity enhancements of the MT-SWNTs are less than in the case of the SC-SWNTs which may be related to the different symmetries of the FMOs in the case of arm chair and semiconducting SWNTs which will affect the interaction with the transition metal d-orbitals;34 furthermore two-terminal conductances measured across MT-MT junctions of pristine SWNTs are comparable to the two-terminal conductances of the individual SWNTs.35
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