Tran Viet Cuong

Chemical functionalization of graphene sheets by solvothermal reduction of a graphene oxide suspension in N-methyl-2-pyrrolidone

We report a simple and effective method for reducing and functionalizing graphene oxide into chemically converted graphene by solvothermal reduction of a graphene oxide suspension in N-methyl-2-pyrrolidone (NMP). Graphene oxide sheets were functionalized by free radicals generated during heating of NMP in the presence of air. The degree of functionalization was easily controlled by manipulating the reduction time. High functionalized solvothermally reduced graphene oxide (STRG) shows superior dispersibility in various organic solvents, while slightly functionalized STRG shows excellent electrical conductivity. The superior dispersibility of highly functionalized STRG in organic solvents was attributed to the steric effect of functionalized groups on the surface of STRG sheets. Free-standing STRG paper that was reduced for 1 h exhibited electrical conductivity as high as 21600 S m−1, while the dispersibility of STRG that was reduced for 5 h was as high as 1.4 mg mL−1.

Superior conductive polystyrene – chemically converted graphene nanocomposite

The polystyrene–chemically converted graphene composite (PS-CCG) prepared by solution blending followed by compression molding, exhibited a percolation threshold as low as 0.19 vol.% and an electrical conductivity as high as 72.18 S m−1 at only ∼2.45 vol.%. The superior electrical conductivity of PS-CCG is the result of the combination of high electrical conductivity of CCG and the good dispersion of the nanofiller in PS matrix. The thermal properties of polystyrene were greatly improved upon addition of a small amount of CCG. The onset decomposition temperature of the PS-CCG increased by approximately 60 °C at 0.19 vol% of CCG loading. The mechanical properties of the PS-CCG were also affected by CCG loading. The storage modulus in the glassy region was enhanced by about 28% at 1.94 vol.% of CCG loading.

Novel conductive epoxy composites composed of 2-D chemically reduced graphene and 1-D silver nanowire hybrid fillers

In this study, 1-D Ag nanowires (NWs), 2-D chemically reduced graphene (CRG), and hybrid CRG–Ag NW fillers were investigated for use as conductive epoxy composites. By combining the 2-D CRG with 1-D Ag NWs, the percolation limit of the Ag NWs decreased from 30 wt% to 10 wt% and the electrical conductivity was dramatically enhanced because of the decreased tunneling resistance between the Ag NWs due to the thin 2-D conductive CRGs. Their thermal and mechanical properties were also improved due to the chemical crosslinking effects between CRGs and the hardener in the epoxy matrix as well as the physical crosslinking effects between nano-structures and polymer chains. The break strength of the CRG/Ag-NW/epoxy composite was 50% higher than that of the pure epoxy resin.

Graphene network on indium tin oxide nanodot nodes for transparent and current spreading electrode in InGaN/GaN light emitting diode

We report a device that combines indium tin oxide (ITO) nanodot nodes with two-dimensional chemically converted graphene (CCG) films to yield a GaN-based light emitting diode (LED) with interesting characteristics for transparent and current spreading electrodes for the potential use in the ultraviolet region. The current-voltage characteristics and electroluminescence output power performance showed that CCG network on ITO nanodot nodes operated as a transparent and current spreading electrode in LED devices.

Temperature-dependent photoluminescence from chemically and thermally reduced graphene oxide

Temperature-dependent photoluminescence (PL) of graphene oxide (GO) reduced with hydrazine and heat has been measured to investigate the effect of reduction type on the bandgap of the reduced GO. Nitrogen functionalities formed in the hydrazine-treated GO were responsible for a strong localization of carriers that caused in a fluctuation in PL peak position with temperature. The intensity of C-OH peak was relatively low in the heat-treated GO, indicating that raising temperature facilitated the removal of hydroxyl groups, resulting in larger sp2 domain size and smaller bandgap energy.

Enhanced light output power of near UV light emitting diodes with graphene / indium tin oxide nanodot nodes for transparent and current spreading electrode.

We report GaN-based near ultraviolet (UV) light emitting diode (LED) that combines indium tin oxide (ITO) nanodot nodes with two-dimensional graphene film as a UV-transparent current spreading electrode (TCSE) to give rise to excellent UV emission efficiency. The light output power of 380 nm emitting UV-LEDs with graphene film on ITO nanodot nodes as TCSE was enhanced remarkably compared to conventional TCSE. The increase of the light output power is attributed to high UV transmittance of graphene, effective current spreading and injection, and texturing effect by ITO nanodots.

Synthesis of highly concentrated suspension of chemically converted graphene in organic solvents: Effect of temperature on the extent of reduction and dispersibility

We report the effect of temperature on the extent of graphene oxide reduction by hydrazine and the dispersibility of the resulting chemically converted graphene (CCG) in polar organic solvents. The extent of graphene oxide reduction at high temperatures was only slightly higher than at low temperatures (30–50 °C), while the dispersibility of the resulting CCG in organic solvents decreased markedly with increasing temperature. The low dispersibility of CCGs prepared at high temperatures was greatly affected by reduction and influenced by the formation of an irreversible agglomerate of CCG at high temperatures. The reduction of graphene oxide at low temperatures is necessary to prepare highly dispersible CCG in organic solvents. CCG prepared at 30 °C is dispersible in N-methyl-2-pyrrolidone concentrations as high as 0.71 mg/mL. The free-standing paper made of this CCG possessed an electrical conductivity of more than 22,000 S/m, one of the highest values ever reported.

Low-voltage solution-processed graphene transistors based on chemically and solvothermally reduced graphene oxide

We have developed solution-processed reduced graphene oxide (RGO) transistors with ion gel gate dielectrics. The combination of solution-processed high-capacitance ion gel gate dielectrics and spray-coated RGO films yielded high-performance RGO transistors that operated below 4 V. Two reduction processes were applied to GO: (i) chemical reduction by hydrazine and (ii) solvothermal reduction in N-methyl-2-pyrrolidone. Chemical reduction provided a more efficient route to reduce GO than solvothermal reduction, and the resulting RGO films yielded higher electron and hole mobilities than films based on solvothermal methods. Temperature-dependent transport studies revealed that higher mobilities in RGO films based on chemical reduction result from (i) more effective delocalization of the charge carriers, (ii) more numerous localized states near the Fermi energy, and (iii) a longer optimum hopping distance, compared to those for films based on solvothermal reduction.

Strain-Induced Compositional Fluctuation and V-Defect Formation in Green-InGaN/GaN Multi-Quantum Wells Grown on Sapphire and Freestanding GaN Substrates

Spatial variation of V-defects and emission characteristics of green-InGaN/GaN multiple quantum wells (MQWs) grown on sapphire and freestanding GaN substrates have been studied. Near-field scanning microscopy exhibits uniformly distributed nanoscale bright spots with a single emission wavelength for MQWs grown on freestanding GaN, whereas, the nanoscale spots get accumulated resulting in large bright areas of micron scale with fluctuated emission wavelengths in case of sapphire. Homoepitaxially grown MQWs reveal large area uniformity in cathodoluminescence emission but a large contrast is observed for sapphire case.

Formation and ferromagnetic properties of FeSi thin films

In this work, the growth and ferromagnetic properties of e-FeSi thin film on Si(100) substrate prepared by molecular beam epitaxy are reported. The inter-diffusion of Fe layer on Si(100) substrate at 600 °C results in polycrystalline e-FeSi layer. The determined activation energy was 0.044 eV. The modified magnetism from paramagnetic in bulk to ferromagnetic states in e-FeSi thin films was observed. The saturated magnetization and coercive field of e-FeSi film are 4.6 emu/cm3 and 29 Oe at 300 K, respectively.