Myung Jin Park

Balancing Light Absorptivity and Carrier Conductivity of Graphene Quantum Dots for High-Efficiency Bulk Heterojunction Solar Cells

Graphene quantum dots (GQDs) have been considered as a novel material because their electronic and optoelectronic properties can be tuned by controlling the size and the functional groups of GQDs. Here we report the synthesis of reduction-controlled GQDs and their application to bulk heterojunction (BHJ) solar cells with enhanced power conversion efficiency (PCE). Three different types of GQDs--graphene oxide quantum dots (GOQDs), 5 h reduced GQDs, and 10 h reduced GQDs--were tested in BHJ solar cells, and the results indicate that GQDs play an important role in increasing optical absorptivity and charge carrier extraction of the BHJ solar cells. The enhanced optical absorptivity by rich functional groups in GOQDs increases short-circuit current, while the improved conductivity of reduced GQDs leads to the increase of fill factors. Thus, the reduction level of GQDs needs to be intermediate to balance the absorptivity and conductivity. Indeed, the partially reduced GQDs yielded the outstandingly improved PCE of 7.60% in BHJ devices compared to a reference device without GQDs (6.70%).

Origin of White Electroluminescence in Graphene Quantum Dots Embedded Host/Guest Polymer Light Emitting Diodes

Polymer light emitting diodes (PLEDs) using quantum dots (QDs) as emissive materials have received much attention as promising components for next-generation displays. Despite their outstanding properties, toxic and hazardous nature of QDs is a serious impediment to their use in future eco-friendly opto-electronic device applications. Owing to the desires to develop new types of nano-material without health and environmental effects but with strong opto-electrical properties similar to QDs, graphene quantum dots (GQDs) have attracted great interest as promising luminophores. However, the origin of electroluminescence from GQDs incorporated PLEDs is unclear. Herein, we synthesized graphene oxide quantum dots (GOQDs) using a modified hydrothermal deoxidization method and characterized the PLED performance using GOQDs blended poly(N-vinyl carbazole) (PVK) as emissive layer. Simple device structure was used to reveal the origin of EL by excluding the contribution of and contamination from other layers. The energy transfer and interaction between the PVK host and GOQDs guest were investigated using steady-state PL, time-correlated single photon counting (TCSPC) and density functional theory (DFT) calculations. Experiments revealed that white EL emission from the PLED originated from the hybridized GOQD-PVK complex emission with the contributions from the individual GOQDs and PVK emissions.

Surface-Engineered Graphene Quantum Dots Incorporated into Polymer Layers for High Performance Organic Photovoltaics.

Graphene quantum dots (GQDs), a newly emerging 0-dimensional graphene based material, have been widely exploited in optoelectronic devices due to their tunable optical and electronic properties depending on their functional groups. Moreover, the dispersibility of GQDs in common solvents depending on hydrophobicity or hydrophilicity can be controlled by chemical functionalization, which is particularly important for homogeneous incorporation into various polymer layers. Here we report that a surface-engineered GQD-incorporated polymer photovoltaic device shows enhanced power conversion efficiency (PCE), where the oxygen-related functionalization of GQDs enabled good dispersity in a PEDOT:PSS hole extraction layer, leading to significantly improved short circuit current density (Jsc) value. To maximize the PCE of the device, hydrophobic GQDs that are hydrothermally reduced (rGQD) were additionally incorporated in a bulk-heterojunction layer, which is found to promote a synergistic effect with the GQD-incorporated hole extraction layer.

Graphene quantum dots-decorated ZnS nanobelts with highly efficient photocatalytic performances

Hybrid nanostructures combining inorganic materials and graphene have shown great potential for the environmentally friendly treatment of effluents. Herein, graphene quantum dots (GQDs)-decorated ZnS nanobelts have been synthesized via a facile hydrothermal method. The electrostatic attraction of two materials and the thermal reduction of graphene are the main driving forces to fabricate well-defined composite nanostructures. GQDs in GQD/ZnS nanocomposites have been found to exist discretely and uniformly on the surfaces of ZnS nanobelts. The photocatalytic activity of GQD/ZnS nanocomposites has been found to be highest at a GQD/ZnS mass ratio of 8 × 10−4. The photocatalytic rate constant (0.0046 min−1) of GQD/ZnS nanocomposites having the optimized GQD content in the photodegradation reaction of rhodamine B has been found to be 14 times higher than that of commercially available ZnS powder. Decorated GQDs introduce an additional visible-light response and serve as electron collectors and transporters to block electron–hole recombination efficiently, enhancing the photocatalytic performances of ZnS nanobelts immensely.

Continuous Films of Self‐Assembled Graphene Quantum Dots for n‐Type Doping of Graphene by UV‐Triggered Charge Transfer

The demands to examine components serving as one of the active layers in heterostructures of 2D materials have been recently increasing. Nanomaterials synthesized from a solution process and their self-assembly can provide a promising route to build a new type of mixed dimensional heterostructures, and several methodologies have been reported previously to construct 2D assemblies from colloidal nanostructures in solution. Graphene quantum dots (GQDs), receiving much interest due to the tunable optical band gap and the capability of chemical functionalization, are considered as emerging nanomaterials for various optoelectronic and biological applications. This study fabricates a closely packed GQDs film (GQDF) from colloidal solutions using a solvent-assisted Langmuir Blodgett method, and investigates the optical and electrical characteristics of the heterostacked graphene/GQD film (G/GQDF) structures. It is observed that the GQDF plays a role not only as a buffer layer that isolates Chemical Vapor Deposited graphene (CVD graphene) from undesired p-doping but also as a photoactive layer that triggers n-doping of the heterostacked CVD graphene film. The n-doping density of the G/GQDF device is proportional to UV irradiation time, but its carrier mobility remains constant regardless of doping densities, which are unique characteristics that have not been observed in other doping methods.

Graphene quantum dots prevent α-synucleinopathy in Parkinson’s disease

Graphene quantum dots inhibit the formation of alpha-synuclein fibrils and induce their dissociation in vitro, and display neuro-protective properties in in vivo models of Parkinson’s disease, with no appreciable long-term toxicity.AbstractThough emerging evidence indicates that the pathogenesis of Parkinson’s disease is strongly correlated to the accumulation1,2 and transmission3,4 of α-synuclein (α-syn) aggregates in the midbrain, no anti-aggregation agents have been successful at treating the disease in the clinic. Here, we show that graphene quantum dots (GQDs) inhibit fibrillization of α-syn and interact directly with mature fibrils, triggering their disaggregation. Moreover, GQDs can rescue neuronal death and synaptic loss, reduce Lewy body and Lewy neurite formation, ameliorate mitochondrial dysfunctions, and prevent neuron-to-neuron transmission of α-syn pathology provoked by α-syn preformed fibrils5,6. We observe, in vivo, that GQDs penetrate the blood–brain barrier and protect against dopamine neuron loss induced by α-syn preformed fibrils, Lewy body/Lewy neurite pathology and behavioural deficits.