Je Min Yoo

Vapor-phase molecular doping of graphene for high-performance transparent electrodes.

Doping is an essential process to engineer the conductivity and work-function of graphene for higher performance optoelectronic devices, which includes substitutional atomic doping by reactive gases, electrical/electrochemical doping by gate bias, and chemical doping by acids or reducing/oxidizing agents. Among these, the chemical doping has been widely used due to its simple process and high doping strength. However, it also has an instability problem in that the molecular dopants tend to gradually evaporate from the surface of graphene, leading to substantial decrease in doping effect with time. In particular, the instability problem is more serious for n-doped graphene because of undesirable reaction between dopants and oxygen or water in air. Here we report a simple method to tune the electrical properties of CVD graphene through n-doping by vaporized molecules at 70 °C, where the dopants in vapor phase are mildly adsorbed on graphene surface without direct contact with solution. To investigate the dependence on functional groups and molecular weights, we selected a series of ethylene amines as a model system, including ethylene diamine (EDA), diethylene triamine (DETA), and triethylene tetramine (TETA) with increasing number of amine groups showing different vapor pressures. We confirmed that the vapor-phase doping provides not only very high carrier concentration but also good long-term stability in air, which is particularly important for practical applications.

Ultraclean Patterned Transfer of Single-Layer Graphene by Recyclable Pressure Sensitive Adhesive Films

We report an ultraclean, cost-effective, and easily scalable method of transferring and patterning large-area graphene using pressure sensitive adhesive films (PSAFs) at room temperature. This simple transfer is enabled by the difference in wettability and adhesion energy of graphene with respect to PSAF and a target substrate. The PSAF-transferred graphene is found to be free from residues and shows excellent charge carrier mobility as high as ∼17,700 cm(2)/V·s with less doping compared to the graphene transferred by thermal release tape (TRT) or poly(methyl methacrylate) (PMMA) as well as good uniformity over large areas. In addition, the sheet resistance of graphene transferred by recycled PSAF does not change considerably up to 4 times, which would be advantageous for more cost-effective and environmentally friendly production of large-area graphene films for practical applications.

Non-destructive electron microscopy imaging and analysis of biological samples with graphene coating

In electron microscopy, charging of non-conductive biological samples by focused electron beams hinders their high-resolution imaging. Gold or platinum coatings have been commonly used to prevent such sample charging, but it disables further quantitative and qualitative chemical analyses by energy dispersive spectroscopy (EDS). Here we report that graphene-coating on biological samples enables non-destructive high-resolution imaging by scanning electron microscopy (SEM) as well as chemical analysis by EDS, utilizing graphenes transparency to electron beams, high conductivity, outstanding mechanical strength, and flexibility. We believe that the graphene-coated imaging and analysis would provide us a new opportunity to explore various biological phenomena unseen before due to the limitation in sample preparation and image resolution, which will broaden our understanding on the life mechanism of various living organisms.

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.

Graphene-Based Nanomaterials

Graphenes are unique nanomaterials which was recently made compatible for physiological milieu, and thus find a way to be used as nanocarriers. The surface engineering abilities of nano-graphene oxides (nano-GOs) or graphene quantum dots (GQDs) in their own outstanding photoluminescence property have made it possible to be used for imaging or drug delivery. Though many successes were not reported in the application of GQDs in vivo, toxicity and safety of GOs in vitro and in vivo are being investigated. Interestingly, GOs are found to be degraded in vivo. In addition to much effort to use GOs as drug carriers, there have been recent trials to apply GOs for magnetic resonance imaging (MRI) and positron emission tomography (PET) or even multiplexed imaging with MRI/fluorescence or MRI/PET. The product has the composition of chelator-conjugated GOs loaded with iron oxide nanoparticles, and a variety of radionuclides such as 68Ga, 64Cu or 125I and 131I and 177Lu were used for PET or SPECT imaging and for radionuclide therapy, respectively. Multiplexed imaging or theranostics will enable targeted delivery, imaging in vivo with PET/SPECT and/or MRI and ex vivo confirmation of the tissues in preclinical studies.