Hyeon Ki Park

Fermi Level Engineering of Single-Walled Carbon Nanotubes by AuCl3 Doping

We investigated the modulation of optical properties of single-walled carbon nanotubes (SWCNTs) by AuCl 3 doping. The van Hove singularity transitions (E 11 (S), E 22 (S), E 11 (M)) in absorption spectroscopy disappeared gradually with an increasing doping concentration and a new peak appeared at a high doping concentration. The work function was downshifted up to 0.42 eV by a strong charge transfer from the SWCNTs to AuCl 3 by a high level of p-doping. We propose that this large work function shift forces the Fermi level of the SWCNTs to be located deep in the valence band, i.e., highly degenerate, creating empty van Hove singularity states, and hence the work function shift invokes a new asymmetric transition in the absorption spectroscopy from a deeper level to newly generated empty states.

Reduction-Controlled Viologen in Bisolvent as an Environmentally Stable n-Type Dopant for Carbon Nanotubes

Various viologens have been used to control the doping of single-walled carbon nanotubes (SWCNTs) via direct redox reactions. A new method of extracting neutral viologen (V(0)) was introduced using a biphase of toluene and viologen-dissolved water. A reductant of sodium borohydride transferred positively charged viologen (V(2+)) into V(0), where the reduced V(0) was separated into toluene with high separation yield. This separated V(0) solution was dropped on carbon nanotube transistors to investigate the doping effect of CNTs. With a viologen concentration of 3 mM, all the p-type CNT transistors were converted to n-type with improved on/off ratios. This was achieved by donating electrons spontaneously to CNTs from neutral V(0), leaving energetically stable V(2+) on the nanotube surface again. The doped CNTs were stable in water due to the presence of hydrophobic V(0) at the outermost CNT transistors, which may act as a protecting layer to prevent further oxidation from water.

Doping strategy of carbon nanotubes with redox chemistry

The chemical doping of single-walled carbon nanotubes (SWCNTs) has been an important issue in tailoring the electronic structures of SWCNTs. This paper proposes a strategy for controlling the doping types and doping concentrations by choosing the reduction potential of a dopant relative to the redox potential of SWCNTs. For this purpose, the redox potential plot in terms of the chirality and diameter was generated based on theoretical calculations, which were in good agreement with the experimental data obtained from individually separated SWCNTs. The change in the electronic structures of the SWCNTs with the various dopants was clearly observed by absorption and Raman spectroscopy, and was explained well by the redox potential argument. This principle was tested further by fabricating transparent conducting films followed by doping. Doping with Au3+ resulted in a sheet resistance of 100 Ω sq−1 at 90% transmittance. This SWCNT doping strategy for both n-type and p-type materials can be generalized to a wide range of nanostructures, such as nanowires and nanoparticles.

Fast Tandem Ring-Opening/Ring-Closing Metathesis Polymerization from a Monomer Containing Cyclohexene and Terminal Alkyne

We report extremely fast tandem ring-opening/ring-closing metathesis polymerization of a monomer containing two rather unreactive functional groups: cyclohexene and a terminal alkyne. When a third-generation Grubbs catalyst was used at low temperature, this tandem polymerization produced polymers with controlled molecular weights and narrow polydispersity indices. To explain this extremely fast polymerization, its reaction mechanism was studied. This new type of controlled polymerization allowed for the preparation of block copolymers using other conventional living metathesis polymerizations. The diene on the backbone of the polymer was postfunctionalized by sequential Diels-Alder and aza-Diels-Alder reactions, which led to selective functionalization depending on the stereochemistry of the diene.

Tandem Ring-Opening/Ring-Closing Metathesis Polymerization: Relationship between Monomer Structure and Reactivity

Monomers containing either cycloalkenes with low ring strain or 1-alkynes are poor monomers for olefin metathesis polymerization. Ironically, keeping two inactive functional groups in proximity within one molecule can make it an excellent monomer for metathesis polymerization. Recently, we demonstrated that monomer 1 having cyclohexene and propargyl moieties underwent rapid tandem ring-opening/ring-closing metathesis (RO/RCM) polymerization via relay-type mechanism. Furthermore, living polymerization was achieved when a third-generation Grubbs catalyst was used. Here, we present a full account on this tandem polymerization by investigating how various structural modifications of the monomers affected the reactivity of the tandem polymerization. We observed that changing the ring size of the cycloalkene moieties, the length of the alkynes, and linker units influenced not only the polymerization rates but also the reactivities of Diels-Alder reaction, which is a post-modification reaction of the resulting polymers. Also, the mechanism of tandem polymerization was studied by conducting end-group analysis using (1)H NMR analysis, thereby concluding that the polymerization occurred by the alkyne-first pathway. With this mechanistic conclusion, factors responsible for the dramatic structure-reactivity relationship were proposed. Lastly, tandem RO/RCM polymerization of monomers containing sterically challenging trisubstituted cycloalkenes was successfully carried out to give polymer repeat units having tetrasubstituted cycloalkenes.

Cascade Polymerization via Controlled Tandem Olefin Metathesis/Metallotropic 1,3-Shift Reactions for the Synthesis of Fully Conjugated Polyenynes

We demonstrate the first example of cascade polymerization by combining olefin metathesis and metallotropic 1,3-shift reactions to form unique conjugated polyenynes. Rational design of monomers enabled controlled polymerization, and kinetic investigation of the polymerization mechanism was conducted.

Versatile Tandem Ring-Opening/Ring-Closing Metathesis Polymerization: Strategies for Successful Polymerization of Challenging Monomers and Their Mechanistic Studies

Tandem ring-opening/ring-closing metathesis (RO/RCM) results in extremely fast living polymerization; however, according to previous reports, only monomers containing certain combinations of cycloalkenes, terminal alkynes, and nitrogen linkers successfully underwent tandem polymerization. After examining the polymerization pathways, we proposed that the relatively slow intramolecular cyclization might lead to competing side reactions such as intermolecular cross metathesis reactions to form inactive propagating species. Thus, we developed two strategies to enhance tandem polymerization efficiency. First, we modified monomer structures to accelerate tandem RO/RCM cyclization by enhancing the Thorpe-Ingold effect. This strategy increased the polymerization rate and suppressed the chain transfer reaction to achieve controlled polymerization, even for challenging syntheses of dendronized polymers. Alternatively, reducing the reaction concentration facilitated tandem polymerization, suggesting that the slow tandem RO/RCM cyclization step was the main reason for the previous failure. To broaden the monomer scope, we used monomers containing internal alkynes and observed that two different polymer units with different ring sizes were produced as a result of nonselective α-addition and β-addition on the internal alkynes. Thorough experiments with various monomers with internal alkynes suggested that steric and electronic effects of the alkyne substituents influenced alkyne addition selectivity and the polymerization reactivity. Further polymerization kinetics studies revealed that the rate-determining step of monomers containing certain internal alkynes was the six-membered cyclization step via β-addition, whereas that for other monomers was the conventional intermolecular propagation step, as observed in other chain-growth polymerizations. This conclusion agrees well with all those polymerization results and thus validates our strategies.

Highly concentrated diameter selective nanodispersion of single-walled carbon nanotubes in water

Nanodispersion of single-walled carbon nanotubes (SWCNTs) has been systematically investigated with the use of sodium dodecyl sulfate (SDS) and poly(vinylpyrrolidone) (PVP) surfactant in de-ionized water. A high concentration of nanodispersed SWCNTs up to 0.08 mg/mL was achieved with introduction of an additional dispersant of PVP by optimizing surfactant concentration, sonication time, and centrifugation speed, which was crucial to obtaining a high concentration of SWCNTs in the supernatant solution. We also demonstrate that diameters of the nanodispersed nanotubes can be sorted out by controlling the centrifugation speed and furthermore the saturated SWCNT concentration was nearly constant, independent of the initial concentration at high centrifugation speed. Two dispersion states were identified depending on the centrifugation speed: an intermediate dispersion of nanodispersion mixed with macrodispersion (I) and nanodispersion (II). This was verified by Raman spectroscopy, scanning probe microscopy, optical absorption spectroscopy, and photoluminescence measurements. The obtained SWCNT solution was stable up to about ten days. Some aggregated SWCNT solution after a long period of time was fully recovered to initial state of dispersion after re-sonication for a few minutes. Our systematic study on high concentration nanodispersion of SWCNTs with selective diameters provides an opportunity to extend the application areas of high quality SWCNTs in large quantity.