Jang Ah Kim

Reduction of metal contact resistance of graphene devices via CO2 cluster cleaning

We report on a cleaning technique using CO2 clusters for large-scale mono-layer graphene fabricated via chemical vapor deposition (CVD) and its application to reduce contact resistance of the CVD graphene device. We found that polymeric residues, i.e., polymethyl methacrylate and photoresist which are generated during transfer and patterning of graphene, can be effectively removed via rapid shrinkage, induced by thermal energy transfer to low temperature CO2 clusters. By applying the CO2 clusters to the cleaning of the interface between metal and graphene, the metal contact resistance of the fabricated graphene field effect transistor was lowered to 26.6% of pristine graphene. The contact resistance shows the best result at an optimized CO2 cluster cleaning condition with a flow rate of 20 l/min, and the resistance was further lowered to 270 Ω μm when a gate bias of −40 V was applied. We expect that the proposed CO2 cluster cleaning to be a very promising technique for future device application using 2-di...

A novel nanometric DNA thin film as a sensor for alpha radiation

The unexpected nuclear accidents have provided a challenge for scientists and engineers to develop sensitive detectors, especially for alpha radiation. Due to the high linear energy transfer value, sensors designed to detect such radiation require placement in close proximity to the radiation source. Here we report the morphological changes and optical responses of artificially designed DNA thin films in response to exposure to alpha radiation as observed by an atomic force microscope, a Raman and a reflectance spectroscopes. In addition, we discuss the feasibility of a DNA thin film as a radiation sensing material. The effect of alpha radiation exposure on the DNA thin film was evaluated as a function of distance from an 241Am source and exposure time. Significant reflected intensity changes of the exposed DNA thin film suggest that a thin film made of biomolecules can be one of promising candidates for the development of online radiation sensors.

Construction and characterization of Cu2+, Ni2+, Zn2+, and Co2+ modified-DNA crystals

We studied the physical characteristics of modified-DNA (M-DNA) double crossover crystals fabricated via substrate-assisted growth with various concentrations of four different divalent metallic ions, Cu(2+), Ni(2+), Zn(2+), and Co(2+). Atomic force microscopy (AFM) was used to test the stability of the M-DNA crystals with different metal ion concentrations. The AFM images show that M-DNA crystals formed without deformation at up to the critical concentrations of 6 mM of [Cu(2+)], 1.5 mM of [Ni(2+)], 1 mM of [Zn(2+)], and 1 mM of [Co(2+)]. Above these critical concentrations, the M-DNA crystals exhibited deformed, amorphous structures. Raman spectroscopy was then used to identify the preference of the metal ion coordinate sites. The intensities of the Raman bands gradually decreased as the concentration of the metal ions increased, and when the metal ion concentrations increased beyond the critical values, the Raman band of the amorphous M-DNA was significantly suppressed. The metal ions had a preferential binding order in the DNA molecules with G-C and A-T base pairs followed by the phosphate backbone. A two-probe station was used to measure the electrical current-voltage properties of the crystals which indicated that the maximum currents of the M-DNA complexes could be achieved at around the critical concentration of each ion. We expect that the functionalized ion-doped M-DNA crystals will allow for efficient devices and sensors to be fabricated in the near future.

A 2D DNA lattice as an ultrasensitive detector for beta radiations.

There is growing demand for the development of efficient ultrasensitive radiation detectors to monitor the doses administered to individuals during therapeutic nuclear medicine which is often based on radiopharmaceuticals, especially those involving beta emitters. Recently biological materials are used in sensors in the nanobio disciplines due to their abilities to detect specific target materials or sites. Artificially designed two-dimensional (2D) DNA lattices grown on a substrate were analyzed after exposure to pure beta emitters, (90)Sr-(90)Y. We studied the Raman spectra and reflected intensities of DNA lattices at various distances from the source with different exposure times. Although beta particles have very low linear energy transfer values, the significant physical and chemical changes observed throughout the extremely thin, ∼0.6 nm, DNA lattices suggested the feasibility of using them to develop ultrasensitive detectors of beta radiations.

InP/ZnS-graphene oxide and reduced graphene oxide nanocomposites as fascinating materials for potential optoelectronic applications.

Our recent studies on metal-organic nanohybrids based on alkylated graphene oxide (GO), reduced alkylated graphene oxide (RGO) and InP/ZnS core/shell quantum dots (QDs) are presented. The GO alkylated by octadecylamine (ODA) and the QD bearing a dodecane thiol (DDT) ligand are soluble in toluene. The nanocomposite alkylated-GO-QD (GOQD) is readily formed from the solution mixture. Treatment of the GOQD composite with hydrazine affords a reduced-alkylated-GO-QD (RGOQD) composite. The structure, morphology, photophysical and electrical properties of GOQDs and RGOQDs are studied. The micro-FTIR and Raman studies demonstrate evidence of the QD interaction with GO and RGO through facile intercalation of the alkyl chains. The field emission scanning electron microscopy (FESEM) and high resolution transmission electron microscopy (HRTEM) images of the GOQD composite show heaps of large QD aggregates piled underneath the GO sheet. Upon reduction to RGOQDs, the QDs become evenly distributed on the graphene bed and the size of the clusters significantly decreases. This also facilitates closer proximity of the QDs to the graphene domains by altering the optoelectronic properties of the RGOQDs. The X-ray photoelectron spectroscopy (XPS) results confirm QDs being retained in the composites, though a small elemental composition change takes place. The XPS and the fluorescence spectra show the presence of an In(Zn)P alloy while the X-ray diffraction (XRD) results show characteristics of the tetragonal indium. The photoluminescence (PL) quenching of QDs in GOQD and RGOQD films determined by the time correlated single photon counting (TCSPC) experiment demonstrates almost complete fluorescence quenching in RGOQDs. The conductance studies demonstrate the differences between GOQDs and RGOQDs. Investigation on the metal-oxide-semiconductor field-effect transistor (nMOSFET) characteristics shows the composite to exhibit p-type channel material properties. The RGOQD exhibits much superior electrical conductance as a channel material compared to the GOQD due to the close proximity of the QDs in the RGOQD to the graphene surface. The transfer characteristics, memory properties, and on/off ratios of the devices are determined. A mechanism has been proposed with reference to the Fermi energies of the composites estimated from the ultraviolet photoelectron spectroscopy (UPS) studies.

Combining protein-shelled platinum nanoparticles with graphene to build a bionanohybrid capacitor.

The electronic properties of biomolecules and their hybrids with inorganic materials can be utilized for the fabrication of nanoelectronic devices. Here, we report the charge transport behavior of protein-shelled inorganic nanoparticles combined with graphene and demonstrate their possible application as a bionanohybrid capacitor. The conductivity of PepA, a bacterial aminopeptidase used as a protein shell (PS), and the platinum nanoparticles (PtNPs) encapsulated by PepA was measured using a field effect transistor (FET) and a graphene-based FET (GFET). Furthermore, we confirmed that the electronic properties of PepA-PtNPs were controlled by varying the size of the PtNPs. The use of two poly(methyl methacrylate) (PMMA)-coated graphene layers separated by PepA-PtNPs enabled us to build a bionanohybrid capacitor with tunable properties. The combination of bioinorganic nanohybrids with graphene is regarded as the cornerstone for developing flexible and biocompatible bionanoelectronic devices that can be integrated into bioelectric circuits for biomedical purposes.

Coverage percentage and raman measurement of cross-tile and scaffold cross-tile based DNA nanostructures.

We present two free-solution annealed DNA nanostructures consisting of either cross-tile CT1 or CT2. The proposed nanostructures exhibit two distinct structural morphologies, with one-dimensional (1D) nanotubes for CT1 and 2D nanolattices for CT2. When we perform mica-assisted growth annealing with CT1, a dramatic dimensional change occurs where the 1D nanotubes transform into 2D nanolattices due to the presence of the substrate. We assessed the coverage percentage of the 2D nanolattices grown on the mica substrate with CT1 and CT2 as a function of the concentration of the DNA monomer. Furthermore, we fabricated a scaffold cross-tile (SCT), which is a new design of a modified cross-tile that consists of four four-arm junctions with a square aspect ratio. For SCT, eight oligonucleotides are designed in such a way that adjacent strands with sticky ends can produce continuous arms in both the horizontal and vertical directions. The SCT was fabricated via free-solution annealing, and self-assembled SCT produces 2D nanolattices with periodic square cavities. All structures were observed via atomic force microscopy. Finally, we fabricated divalent nickel ion (Ni(2+))- and trivalent dysprosium ion (Dy(3+))-modified 2D nanolattices constructed with CT2 on a quartz substrate, and the ion coordinations were examined via Raman spectroscopy.

DNA reusability and optoelectronic characteristics of streptavidin-conjugated DNA crystals on a quartz substrate†

We demonstrate self-assembled double-crossover (DX) DNA crystal growth and coverage on quartz by the substrate assisted growth method. Here we introduced the novel concept of a reusability process to fabricate the DX crystals on a given substrate with a 10 nM DNA concentration, which is equal to the saturation concentration and this concentration is enough to fully cover the substrate with DX crystals. Also the DX crystals with biotinylated oligonucleotides (DXB) were constructed based on the structure of immobile crossover branched junctions. Upon streptavidin conjugation with DNA, the Raman band intensities were increased as compared to pristine DX crystals which indicate the strong bonding between the biotinylated DXB crystals and streptavidin. The DX crystals showed a relatively higher current than streptavidin conjugated DXB crystals because of the electrically insulating characteristic of streptavidin. Furthermore, the optical band gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) as well as the second band onset were determined and discussed. The HOMO–LUMO band gaps of DNA crystals, DNA crystals with biotins, and DNA crystals with biotins and streptavidin showed an inverted V-shape and the second band onsets – consistent with the electrical characteristics – revealed the increasing behavior with protein conjugation.

High-Purity Amino-Functionalized Graphene Quantum Dots Derived from Graphene Hydrogel

The unique properties of graphene quantum dots (GQDs) make them interesting candidate materials for innovative applications. Herein, we report a facile method to synthesize amino-functionalized graphene quantum dots (AF-GQDs) by a hydrothermal reaction. Graphene oxide (GO) was synthesized by Hummer’s method where ultra-small GO sheets were obtained by a prolonged oxidation process followed by sonication using an ultrasonic probe. Subsequently, graphene hydrogel (GH) was also obtained by a hydrothermal synthesis method. Proper care was taken during synthesis to avoid contamination from water soluble impurities, which are present in the precursor, GO solution. Following the treatment of GH in ammonia, ultra-small amino-functionalized graphene fragments (AF-GQDs) were formed, which detached from the GH to eventually disperse evenly in the water without agglomerating. This modified synthesis process enables the formation of high-purity AF-GQDs (99.14%) while avoiding time-consuming synthesis procedures. Our finding shows that AF-GQDs with sizes less than 5nm were well dispersed. A strong photoluminescence (PL) emission at ∼410nm with 10% PL quantum yield was also observed. These AF-GQDs can be used in many bio applications in view of their low cytotoxicity and strong fluorescence that can be applied to cell imaging.

Surface Cleaning of Graphene by CO2 Cluster

Graphene has attracted researchers due to its unique physical properties [1]. However, residues on surface can act as contaminants which further have adverse effects on its performance. As synthetic graphene has inherent surface roughness which can also affect the weak adhesion of layers and leakage points. In order to improve the mechanical and electrical properties, the graphene surface should be uncontaminated. In general practice wet cleaning methods, containing hazardous chemical and solvents are used to remove the residues from graphene surface [2, 3]. To avoid chemicals, mechanical cleaning of graphene using contact mode atomic force microscopy (AFM) has been tried. However, the contact mode AFM cleaning is a limited in cleaning area and the cleaning procedure takes a long time. Recently, CO2 cluster cleaning shows benefits that overcomes these problems. Herein we report the use of CO2 cluster to clean the graphene surface without affecting its inherent properties for the first time. The CO2 cluster treated graphene samples were evaluated by AFM for its roughness change and residual contamination.