Eunhee Hwang

One-pot reduction of graphene oxide at subzero temperatures

We report a new reducing agent system: hydriodic acid with trifluoroacetic acid, which can chemically convert graphene oxide into reduced graphene oxide at temperatures below 0 °C in solution. This is the first achievement to produce reduced graphene oxide at subzero temperature with a mass production.

Dual Functions of Highly Potent Graphene Derivative–Poly-l-Lysine Composites To Inhibit Bacteria and Support Human Cells

Dual-function poly(L-lysine) (PLL) composites that function as antibacterial agents and promote the growth of human cell culture have been sought by researchers for a long period. In this paper, we report the preparation of new graphene derivative-PLL composites via electrostatic interactions and covalent bonding between graphene derivatives and PLL. The resulting composites were characterized by infrared spectroscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy. The novel dual function of PLL composites, specifically antibacterial activity and biocompatibility with human cells [human adipose-derived stem cells and non-small-cell lung carcinoma cells (A549)], was carefully investigated. Graphene-DS-PLL composites composed of 4-carboxylic acid benzene diazonium salt (DS) generated more anionic carboxylic acid groups to bind to cationic PLLs, forming the most potent antibacterial agent among PLL and PLL composites with high biocompatibility with human cell culture. This dual functionality can be used to inhibit bacterial growth while enhancing human cell growth.

An electrolyte-free flexible electrochromic device using electrostatically strong graphene quantum dot-viologen nanocomposites.

A strong electrostatic MV(2+) -GQD nanocomposite provides an electrolyte-free flexible electrochromic device wih high durability. The positively charged MV(2+) and negatively charged GQD are strongly stabilized by non-covalent intermolecular forces (e.g., electrostatic interactions, π-π stacking interactions, and cation-π electron interactions), eliminating the need for an electrolyte. An electrolyte-free flexible electrochromic device fabricated from the GQD-supported MV(2+) exhibits stable performance under mechanical and thermal stresses.

Cancer Therapy Using Ultrahigh Hydrophobic Drug-Loaded Graphene Derivatives

This study aimed to demonstrate that curcumin (Cur)-containing graphene composites have high anticancer activity. Specifically, graphene-derivatives were used as nanovectors for the delivery of the hydrophobic anticancer drug Cur based on pH dependence. Different Cur-graphene composites were prepared based on polar interactions between Cur and the number of oxygen-containing functional groups of respective starting materials. The degree of drug-loading was found to be increased by increasing the number of oxygen-containing functional groups in graphene-derivatives. We demonstrated a synergistic effect of Cur-graphene composites on cancer cell death (HCT 116) both in vitro and in vivo. As-prepared graphene quantum dot (GQD)-Cur composites contained the highest amount of Cur nano-particles and exhibited the best anticancer activity compared to the other composites including Cur alone at the same dose. This is the first example of synergistic chemotherapy using GQD-Cur composites simultaneous with superficial bioprobes for tumor imaging.

Can Commonly Used Hydrazine Produce n‐Type Graphene?

A simple chemical method to obtain bulk quantities of N-doped, reduced graphene oxide (rGO) sheets (see figure) as an n-type semiconductor through the treatment of as-prepared GO sheets with the commonly used reducing reagent hydrazine, followed by rapid thermal annealing (RTA) is described.

Synthesis of Highly n‐Type Graphene by Using an Ionic Liquid

The development of an n-type, graphene-based semiconductor is currently a significant research interest. Graphene is easily p-doped by adsorbates such as oxygen and moisture and thus a p-type semiconductor can be easily prepared. However, the development of an n-type semiconductor is required to fabricate a complementary circuit, and nitrogendoped (N-doped) materials would be useful in real device applications. Chemical doping is an important method used to modulate the electrical properties of graphene. Both theoretical calculations and detailed experiments have proved that chemical doping with foreign atoms, such as nitrogen, is an effective approach to achieve n-type semiconductors. [1] nType semiconductors can be obtained by replacing carbon atoms with nitrogen atoms in the graphene framework. The lone electron pairs of nitrogen atoms play an important role in producing a delocalized conjugated system with sp 2 hybridized carbon frameworks [2–4] that can enhance the reactivity and electrocatalytic properties of graphene. Substitutional N-doped multilayer graphene sheets were synthesized by adding NH3 gas during the chemical vapor deposition (CVD) growth of graphene, [1] and the monolayer growth of N-doped graphene sheets by using poly(methyl methacrylate) (PMMA) and pyridine was recently reported. [5, 6] The nitrogen-doping behavior of graphene and reduced graphene oxide (rGO) through electrical joule heating and thermal annealing in NH3, respectively, has been reported to produce an n-type semiconductor. [7, 8] However, the systematic investigation of graphene doping is required to achieve a large N-doping effect in real device applications. A high atomic percentage of dopant nitrogen is important in the fabrication of n-type rGO with a large shift of the Dirac point (DP). The catalyst-free synthesis of N-doped graphene by thermal annealing of graphene oxide (GO) with melamine through a bulk reaction was reported to provide a high atomic percentage of nitrogen on the rGO surface. [9] How

Tuning of n‐ and p‐Type Reduced Graphene Oxide Transistors with the Same Molecular Backbone

As silicon integrated circuits approach their theoretical limits, many researchers have turned to non-silicon-based materials in the hope of discovering new materials upon which the next generation of electronic devices will be based. Carbon nanomaterials are promising alternative candidates to replace silicon in device technologies. Recently, graphene has been widely investigated for its fieldeffect properties, and transistors composed of the material have been used in some applications. Graphene is a monolayer of graphite in which a carbon backbone forms a twodimensional (2D) hexagonal structure as a zero-gap semiconductor. Field-effect transistors (FETs) using graphene as a channel between the source and drain suffer from the absence of a bandgap. To open and tune an energy gap in graphene, various approaches have been developed to improve the semiconducting properties, such as those involving quantum dots, nanoribbons, nanomeshes, and graphene binding to particular substrates. Chemical doping is one such subject that draws attention. The type and concentration of electron or hole carriers in graphene can be controlled by the introduction of metals or molecules onto the graphene surface. Several chemical species are known to produce doping effects in mechanically exfoliated graphene. n-Type graphene FETs doped with electron-donating materials have been observed when graphene FETs have been exposed to titanium and potassium with charged impurity scattering, TiO2 [9b] , NH3 [10] and CO vapors, polymer (polyethyleneimine), or small molecules (1,5-naphthalenediamine and 9,10-dimethylanthracene). Alternatively, p-type graphene FETs doped with electron-withdrawing materials have been observed when graphene FETs have been exposed to H2O and NO2 vapors or small molecules (4-bromobenzenediazonium tetrafluoroborate, tetrasodium 1,3,6,8-pyrenetetrasylfonic acid, and 9,10-dibromoanthracene). Thus, to realize the pand n-doping effects of graphene FETs, the introduction of functionalized molecules that have electron-withdrawing or -donating properties is necessary. Until now, however, there is still no clear understanding of the molecular-doping effect, even though various functionalized molecules have been tested for pand n-type graphene FETs. The main reason for the unclear doping effect of the molecules is that the previous molecules selected for por ntype had different backbones, leading to limited understanding of the origin of the molecular-doping effect. For a clear demonstration of this effect, it is necessary for the doped molecules to have the same molecular backbone and to possess a series of functional groups. In addition, semiconducting channels in graphene layers have been prepared by means of several different methods. A simple and easy method is based on the creation of colloidal suspensions through the use of a graphene oxide (GO) precursor to achieve individual layers. GO is easily exfoliated and well dispersed in water, allowing simple spin-coating onto any substrate. Field-effect devices fabricated through the spin-casting of GO layers, followed by reduction into reduced graphene oxide (rGO) typically exhibit semiconducting behavior with low conductivity. We choose the pyrene molecule backbone for several reasons. First, pyrene derivatives could stably bind to the rGO channel through strong p–p interactions between their aromatic rings and the graphene. These p-electron-rich pyrene derivatives were reported as a stable biosensor and also as intercalated molecules for the dispersion of graphene. Second, we could attach withdrawing or donating groups to the pyrene moiety with a simple reaction and easy purification. In the present work, rGO FETs with few layers of rGOs were successfully prepared as a semiconducting channel through the reduction of GO and modification with pyrene derivatives including 1-aminopyrene (Py-NH2), 1-nitropyrene (Py-NO2), 1-pyrenecarboxylic acid (Py-CO2H), and 1-pyrenesulfonic acid (Py-SO3H) (Figure 1). The p-electron-rich pyrene backbone was designed for immobilization on the p-electron-rich rGO channel through

Superconductivity at 7.4 K in Few Layer Graphene by Li-intercalation

Superconductivity in graphene has been highly sought after for its promise in various device applications and for general scientific interest. Ironically, the simple electronic structure of graphene, which is responsible for novel quantum phenomena, hinders the emergence of superconductivity. Theory predicts that doping the surface of the graphene effectively alters the electronic structure, thus promoting propensity towards Cooper pair instability (Profeta et al (2012) Nat. Phys. 8 131-4; Nandkishore et al (2012) Nat. Phys. 8 158-63) [1, 2]. Here we report the emergence of superconductivity at 7.4 K in Li-intercalated few-layer-graphene (FLG). The absence of superconductivity in 3D Li-doped graphite underlines that superconductivity in Li-FLG arises from the novel electronic properties of the 2D graphene layer. These results are expected to guide future research on graphene-based superconductivity, both in theory and experiments. In addition, easy control of the Li-doping process holds promise for various device applications.

Changes in major charge transport by molecular spatial orientation in graphene channel field effect transistors

Changes in major charge transport of graphene channel transistors in terms of the spatial orientation of adsorbed functional molecules were demonstrated. In contrast to the horizontally (physically) bound molecules, the vertically (chemically) bound molecules did not change major charge carriers of graphene channels, revealing the molecular orientation-dependent doping effects.

Chemically modulated graphene quantum dot for tuning the photoluminescence as novel sensory probe

A band gap tuning of environmental-friendly graphene quantum dot (GQD) becomes a keen interest for novel applications such as photoluminescence (PL) sensor. Here, for tuning the band gap of GQD, a hexafluorohydroxypropanyl benzene (HFHPB) group acted as a receptor of a chemical warfare agent was chemically attached on the GQD via the diazonium coupling reaction of HFHPB diazonium salt, providing new HFHPB-GQD material. With a help of the electron withdrawing HFHPB group, the energy band gap of the HFHPB-GQD was widened and its PL decay life time decreased. As designed, after addition of dimethyl methyl phosphonate (DMMP), the PL intensity of HFHPB-GQD sensor sharply increased up to approximately 200% through a hydrogen bond with DMMP. The fast response and short recovery time was proven by quartz crystal microbalance (QCM) analysis. This HFHPB-GQD sensor shows highly sensitive to DMMP in comparison with GQD sensor without HFHPB and graphene. In addition, the HFHPB-GQD sensor showed high selectivity only to the phosphonate functional group among many other analytes and also stable enough for real device applications. Thus, the tuning of the band gap of the photoluminescent GQDs may open up new promising strategies for the molecular detection of target substrates.