Soon Hyung Hong

Tuning the Photoluminescence of Graphene Quantum Dots through the Charge Transfer Effect of Functional Groups

The band gap properties of graphene quantum dots (GQDs) arise from quantum confinement effects and differ from those in semimetallic graphene sheets. Tailoring the size of the band gap and understanding the band gap tuning mechanism are essential for the applications of GQDs in opto-electronics. In this study, we observe that the photoluminescence (PL) of the GQDs shifts due to charge transfers between functional groups and GQDs. GQDs that are functionalized with amine groups and are 1-3 layers thick and less than 5 nm in diameter were successfully fabricated using a two-step cutting process from graphene oxides (GOs). The functionalized GQDs exhibit a redshift of PL emission (ca. 30 nm) compared to the unfunctionalized GQDs. Furthermore, the PL emissions of the GQDs and the amine-functionalized GQDs were also shifted by changes in the pH due to the protonation or deprotonation of the functional groups. The PL shifts resulted from charge transfers between the functional groups and GQDs, which can tune the band gap of the GQDs. Calculations from density functional theory (DFT) are in good agreement with our proposed mechanism for band gap tuning in the GQDs through the use of functionalization.

Versatile Carbon Hybrid Films Composed of Vertical Carbon Nanotubes Grown on Mechanically Compliant Graphene Films

[*] Prof. S. O. Kim, D. H. Lee, J. E. Kim, T. H. Han, J. W. Hwang, Prof. S. W. Jeon, Prof. S. H. Hong, Prof. W. J. Lee Department of Materials Science and Engineering, KAIST Daejeon 305-701 (Republic of Korea) E-mail: sangouk.kim@kaist.ac.kr Dr. S. Y. Choi Convergence Components and Materials Laboratory Electronics and Telecommunication Research Institute (ETRI) Daejoen 305-700 (Republic of Korea)

Enhanced mechanical properties of graphene/copper nanocomposites using a molecular-level mixing process.

RGO flakes are homogeneously dispersed in a Cu matrix through a molecular-level mixing process. This novel fabrication process prevents the agglomeration of the RGO and enhances adhesion between the RGO and the Cu. The yield strength of the 2.5 vol% RGO/Cu nanocomposite is 1.8 times higher than that of pure Cu. The strengthening mechanism of the RGO is investigated by a double cantilever beam test using the graphene/Cu model structure.

Enhanced Electrical Conductivity of Nanocomposites Containing Hybrid Fillers of Carbon Nanotubes and Carbon Black

Nanocomposites reinforced with hybrid fillers of carbon nanotubes (CNTs) and carbon black (CB) are developed, aiming at enhancing the electrical conductivity of composites with balanced mechanical properties while lowering the cost of the final product. Epoxy-based nanocomposites were prepared with varying combinations of CNTs and CB as conducting fillers, and their electrical and mechanical properties were evaluated. It was shown that the addition of CNTs in CB composites enhanced the electrical conductivity of composites: a low percolation threshold was achieved with 0.2 wt % CNTs and 0.2 wt % CB particles. The CB particles also enhanced the ductility and fracture toughness of nanocomposites, confirming the synergistic effect of CB as a multifunctional filler. The novelty of this work lies in the synergy arising from the combination of two conducting fillers with unique geometric shapes and aspect ratios as well as different dispersion characteristics, which have not been specifically considered previously.

High‐Strength Carbon Nanotube Fibers Fabricated by Infiltration and Curing of Mussel‐Inspired Catecholamine Polymer

Carbon nanotubes (CNTs) have received extensive attention due to their extraordinary properties in electronic conduction, [ 1 ] heat transfer, [ 2 ] and mechanical strength. [ 3 ] Materials with unparallel performance, such as super-strong, lightweight e-textiles, can be fabricated from CNTs, suggesting a future revolution in materials science. Thus, the emerging CNT technology will largely depend on the development of effective spinning and post-spinning processes to realize such unprecedented materials. Two widely implemented strategies for fabricating CNT fi bers are in-solution [ 4–7 ] and solid-state spinning techniques. The in-solution spinning of CNTs can produce continuous CNT fi bers; however, homogeneous dispersion of CNTs in the solvent is necessary for proper spinning. Moreover, the properties of the CNT fi bers strongly depend on the methods of CNT dispersion. An alternative strategy is solid-state spinning, [ 8–17 ] which allows avoidance of CNT dispersion in solvents and for various post-spinning processes to be applied with ease. Twisting, [ 10–16 ] densifi cation, [ 9 , 18 ] and infi ltration [ 8 , 19,20 ] are examples of post-spinning processes, and the main purpose of the spinning and post-spinning processes is to enhance the mechanical properties of CNT fi bers. Despite the effort that has been made, however, fabrication of strong CNT fi bers remains a great challenge.

Scalable exfoliation process for highly soluble boron nitride nanoplatelets by hydroxide-assisted ball milling.

The scalable preparation of two-dimensional hexagonal boron nitride (h-BN) is essential for practical applications. Despite intense research in this area, high-yield production of two-dimensional h-BN with large-size and high solubility remains a key challenge. In the present work, we propose a scalable exfoliation process for hydroxyl-functionalized BN nanoplatelets (OH-BNNPs) by a simple ball milling of BN powders in the presence of sodium hydroxide via the synergetic effect of chemical peeling and mechanical shear forces. The hydroxide-assisted ball milling process results in relatively large flakes with an average size of 1.5 μm with little damage to the in-plane structure of the OH-BNNP and high yields of 18%. The resultant OH-BNNP samples can be redispersed in various solvents and form stable dispersions that can be used for multiple purposes. The incorporation of the BNNPs into the polyethylene matrix effectively enhanced the barrier properties of the polyethylene due to increased tortuosity of the diffusion path of the gas molecules. Hydroxide-assisted ball milling process can thus provide simple and efficient approaches to scalable preparation of large-size and highly soluble BNNPs. Moreover, this exfoliation process is not only easily scalable but also applicable to other layered materials.

Enhanced Mechanical Properties of Epoxy Nanocomposites by Mixing Noncovalently Functionalized Boron Nitride Nanoflakes

The influence of surface modifications on the mechanical properties of epoxy-hexagonal boron nitride nanoflake (BNNF) nanocomposites is investigated. Homogeneous distributions of boron nitride nanoflakes in a polymer matrix, preserving intrinsic material properties of boron nitride nanoflakes, is the key to successful composite applications. Here, a method is suggested to obtain noncovalently functionalized BNNFs with 1-pyrenebutyric acid (PBA) molecules and to synthesize epoxy-BNNF nanocomposites with enhanced mechanical properties. The incorporation of noncovalently functionalized BNNFs into epoxy resin yields an elastic modulus of 3.34 GPa, and 71.9 MPa ultimate tensile strength at 0.3 wt%. The toughening enhancement is as high as 107% compared to the value of neat epoxy. The creep strain and the creep compliance of the noncovalently functionalized BNNF nanocomposite is significantly less than the neat epoxy and the nonfunctionalized BNNF nanocomposite. Noncovalent functionalization of BNNFs is effective to increase mechanical properties by strong affinity between the fillers and the matrix.

Grain refinement assisted strengthening of carbon nanotube reinforced copper matrix nanocomposites

Carbon nanotube reinforced copper matrix (CNT/Cu) nanocomposites were fabricated by the modified molecular-level mixing process, which produces a homogeneous dispersion of CNTs in a fine grained metal matrix. These nanocomposites, consisting of 1.5μm Cu matrix grains and 5vol% of multiwall CNTs, show a high strengthening capability; enhancing the yield strength of unreinforced Cu by 2.3 times. The enhanced yield strength stems from the reinforcing effect of CNTs and additional hardening of the Cu matrix by grain refinement. The results reveal that the metallurgical treatment of the matrix is important for the development of high-strength CNT/metal nanocomposites.

Highly entangled carbon nanotube scaffolds by self-organized aqueous droplets

We present a facile and versatile directed assembly process for highly entangled carbon nanotube (CNT) scaffolds. Macroporous polymer/CNT nanocomposites were prepared by a ‘breath figure’ method. After pyrolysis of the nanocomposites, highly stable CNT scaffolds with diverse morphologies such as monolayered or multilayered cellular films or individual CNT rings were prepared. The cellular CNT scaffolds demonstrated high electrical conductivity and field-emission properties, which is potentially useful for various applications in electronics and energy storage devices.

Direct Insulation‐to‐Conduction Transformation of Adhesive Catecholamine for Simultaneous Increases of Electrical Conductivity and Mechanical Strength of CNT Fibers

Increase in conductivity and mechanical properties of a carbon nanotube (CNT) fiber inspired by mussel-adhesion chemistry is described. Infiltration of polydopamine into an as-drawn CNT fiber followed by pyrolysis results in a direct insulation-to-conduction transformation of poly(dopamine) into pyrolyzed-poly(dopamine) (py-PDA), retaining the intrinsic adhesive function of catecholamine. The py-PDA enhances both the electrical conductivity and the mechanical strength of the CNT fibers.