Gwang Hoon Jun

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.

Enhanced Thermal Conductivity of Epoxy–Graphene Composites by Using Non‐Oxidized Graphene Flakes with Non‐Covalent Functionalization

Homogeneous distribution of graphene flakes in a polymer matrix, still preserving intrinsic material properties, is key to successful composite applications. A novel approach is presented to disperse non-oxidized graphene flakes with non-covalent functionalization of 1-pyrenebutyric acid and to fabricate nanocomposites with outstanding thermal conductivity (∼1.53 W/mK) and mechanical properties (∼1.03 GPa).

Enhanced Electrical Networks of Stretchable Conductors with Small Fraction of Carbon Nanotube/Graphene Hybrid Fillers

Carbon nanotubes (CNTs) and graphene are known to be good conductive fillers due to their favorable electrical properties and high aspect ratios and have been investigated for application as stretchable composite conductors. A stretchable conducting nanocomposite should have a small fraction of conductive filler material to maintain stretchability. Here we demonstrate enhanced electrical networks of nanocomposites via the use of a CNT-graphene hybrid system using a small mass fraction of conductive filler. The CNT-graphene hybrid system exhibits synergistic effects that prevent agglomeration of CNTs and graphene restacking and reduce contact resistance by formation of 1D(CNT)-2D(graphene) interconnection. These effects resulted in nanocomposite materials formed of multiwalled carbon nanotubes (MWCNTs), thermally reduced graphene (TRG), and polydimethylsiloxane (PDMS), which had a higher electrical conductivity compared with MWCNT/PDMS or TRG/PDMS nanocomposites until specific fraction that is sufficient to form electrical network among conductive fillers. These nanocomposite materials maintained their electrical conductivity when 60% strained.

Fast P3HT Exciton Dissociation and Absorption Enhancement of Organic Solar Cells by PEG-Functionalized Graphene Quantum Dots.

PEG-functionalized graphene quantum dots (GQDs) are shown to promote fast exciton dissociation in organic solar cells. Short-chain PEG promotes the most favorable interaction with other organic layers, and the overall efficiency is improved by 36% when compared to the reference devices. The mechanism of enhancement is shown to be increased absorption due to fewer charges remain-ing in the bound state.

High conductivity and stretchability of 3D welded silver nanowire filled graphene aerogel hybrid nanocomposites

Many studies have investigated stretchable conductors, fabricated from silver nanowire (AgNW)-containing nanocomposites, due to their outstanding electrical properties and high-aspect-ratio structures. To retain good conductivity with a high degree of elongation, nanocomposites used as stretchable conductors should maintain a well-connected electrical network with a low mass fraction of AgNWs. A three-dimensional (3D) graphene aerogel (GA) frame provides an excellent scaffold for conductive networks consisting of AgNWs. This research describes, for the first time, the synthesis of a new type of nanostructure, AgNW/GA, formed by coating 3D GA frames with AgNWs. AgNW/GA/polydimethylsiloxane (PDMS) nanocomposites exhibited high electrical conductivity (∼40 S cm−1) and stable conductivity retention at up to 60% elongation. In addition, the electrical properties of the AgNW/GA/PDMS nanocomposites could be further enhanced by structural changes in the GA induced by optical welding.

Enhanced electromagnetic interference shielding behavior of Graphene Nanoplatelet/Ni/Wax nanocomposites

We report on the electromagnetic interference shielding behavior of Graphene Nanoplatelet (GNP)/Ni/Wax nanocomposites fabricated by a molecular-level mixing process. Blended powders of GNP and Ni nanoparticles with an average size of 80 nm were fabricated by this simple process. Strong interfacial bonding and homogeneous attachment between the two components (graphene and nickel) was achieved by applying functional groups on the GNP. Nano-sized powders were also prepared by the ball-milling process to demonstrate the contribution of the process to shielding properties. After mixing two different GNP/Ni powders in paraffin wax, the electromagnetic interference shielding behavior of GNP/Ni was investigated. The GNP/Ni/Wax sample of 0.7 mm thickness prepared by the molecular-level mixing process showed a shielding effectiveness value of 40 dB in the X-band, while GNP/Ni/Wax samples prepared by the ball-milling process showed a value of 20 dB. The enhancement can be achieved by facilitating charge transfer and providing magnetic dipoles which interact with electromagnetic waves. Functional groups on the GNP not only provide nucleation sites for Ni nanoparticles of uniform size but help the GNP/Ni powders to be homogeneously dispersed in wax by enhancing interfacial interactions between GNP/Ni and wax.

Design and application of carbon nanomaterials for photoactive and charge transport layers in organic solar cells

Commercialization of organic solar cell (OSC) has faltered due to their low power conversion efficiency (PCE) compared to inorganic solar cell. Low electrical conductivity, low charge mobility, and short-range light absorption of most organic materials limit the PCE of OSCs. Carbon nanomaterials, especially carbon nanotubes (CNTs) and graphenes, are of great interest for use in OSC applications due to their high electrical conductivity, mobility, and unique optical properties for enhancing the performance of OSCs. In this review, recent progress toward the integration of carbon nanomaterials into OSCs is described. The role of carbon nanomaterials and strategies for their integration into various layers of OSCs, including the photoactive layer and charge transport layer, are discussed. Based on these, we also discuss the prospects of carbon nanomaterials for specific OSC layers to maximize the PCE.