Sung Ju Hong

N-type graphene induced by dissociative H2 adsorption at room temperature

Studies of the interaction between hydrogen and graphene have been increasingly required due to the indispensable modulation of the electronic structure of graphene for device applications and the possibility of using graphene as a hydrogen storage material. Here, we report on the behaviour of molecular hydrogen on graphene using the gate voltage-dependent resistance of single-, bi-, and multi-layer graphene sheets as a function of H2 gas pressure up to 24 bar from 300 K to 345 K. Upon H2 exposure, the charge neutrality point shifts toward the negative gate voltage region, indicating n-type doping, and distinct Raman signature changes, increases in the interlayer distance of multi-layer graphene, and a decrease in the d-spacing occur, as determined by TEM. These results demonstrate the occurrence of dissociative H2 adsorption due to the existence of vacancy defects on graphene.

Scalable Production of Highly Sensitive Nanosensors Based on Graphene Functionalized with a Designed G Protein-Coupled Receptor

We have developed a novel, all-electronic biosensor for opioids that consists of an engineered μ-opioid receptor protein, with high binding affinity for opioids, chemically bonded to a graphene field-effect transistor to read out ligand binding. A variant of the receptor protein that provided chemical recognition was computationally redesigned to enhance its solubility and stability in an aqueous environment. A shadow mask process was developed to fabricate arrays of hundreds of graphene transistors with average mobility of ∼1500 cm2 V–1 s–1 and yield exceeding 98%. The biosensor exhibits high sensitivity and selectivity for the target naltrexone, an opioid receptor antagonist, with a detection limit of 10 pg/mL.

Formation of flat, relaxed Si1−xGex alloys on Si(001) without buffer layers

Atomically flat, fully strained Si1−xGex layers with thicknesses ranging from 40 to 240nm were grown on Si(001) at 450°C by ultrahigh-vacuum chemical vapor deposition and subjected to annealing at 1000°C for 20min to induce relaxation. In order to minimize surface diffusion during annealing and thereby inhibit strain-induced roughening in favor of misfit dislocation formation, SiO2 capping layers are deposited prior to annealing. The overall process results in smooth, relaxed alloy layers without the necessity of using several-μm-thick compositionally graded buffer layers.

Verification of electron doping in single-layer graphene due to H2 exposure with thermoelectric power

We report the electron doping of single-layer graphene (SLG) grown by chemical vapor deposition (CVD) by means of dissociative hydrogen adsorption. The transfer characteristic showed n-type doping behavior similar to that of mechanically exfoliated graphene. Furthermore, we studied the thermoelectric power (TEP) of CVD-grown SLG before and after exposure to high-pressure H2 molecules. From the TEP results, which indicate the intrinsic electrical properties, we observed that the CVD-grown SLG is n-type doped without degradation of the quality after hydrogen adsorption. Finally, the electron doping was also verified by Raman spectroscopy.

Probing Spin-charge Relation by Magnetoconductance in One-dimensional Polymer Nanofibers

Polymer nanofibers are one-dimensional organic hydrocarbon systems containing conducting polymers where the non-linear local excitations such as solitons, polarons and bipolarons formed by the electron-phonon interaction were predicted. Magnetoconductance (MC) can simultaneously probe both the spin and charge of these mobile species and identify the effects of electron-electron interactions on these nonlinear excitations. Here we report our observations of a qualitatively different MC in polyacetylene (PA) and in polyaniline (PANI) and polythiophene (PT) nanofibers. In PA the MC is essentially zero, but it is present in PANI and PT. The universal scaling behavior and the zero (finite) MC in PA (PANI and PT) nanofibers provide evidence of Coulomb interactions between spinless charged solitons (interacting polarons which carry both spin and charge).

Apparent Power Law Scaling of Variable Range Hopping Conduction in Carbonized Polymer Nanofibers

We induce dramatic changes in the structure of conducting polymer nanofibers by carbonization at 800 °C and compare charge transport properties between carbonized and pristine nanofibers. Despite the profound structural differences, both types of systems display power law dependence of current with voltage and temperature, and all measurements can be scaled into a single universal curve. We analyze our experimental data in the framework of variable range hopping and argue that this mechanism can explain transport properties of pristine polymer nanofibers as well.

Competition between electron doping and short-range scattering in hydrogenated bilayer graphene on hexagonal boron nitride

We studied the electron doping of bilayer graphene (BLG) on hexagonal boron nitride (h-BN) by dissociative H2 adsorption. Charge transfer phenomena were investigated by the gate voltage (Vg)-dependent electrical conductivity σ(Vg) and Raman spectroscopy with respect to the H2 exposure. The shift of the charge neutrality point toward the negative Vg region was observed and the charge scattering mechanism was found with the variation of σ(Vg). The charge transfer at the interface as well as the lattice distortion of BLG due to hydrogenation were verified by Raman spectroscopy. From these results, we concluded that the electron doping and short-range scattering in the BLG exposed to high H2 pressure (11 bar) are intrinsic features, which were achieved using a van der Waals interface consisting of BLG and h-BN.

Electrical and thermoelectric transport by variable range hopping in reduced graphene oxide

This study investigated the transport properties of single-layer reduced graphene oxides (rGOs). The rGOs were prepared by the bubble deposition method followed by thermal reduction. The crossover of the transport mechanism from Efros-Shklovskii (ES) variable range hopping (VRH) between the localized states to Mott-VRH was observed near 70 K using the temperature-dependent conductance. The ES-VRH conduction below 70 K is apparent in the electric field dependence of the field-driven hopping transport in the high-electric field regime. We also figure out that the thermoelectric power is consistent with the 2D Mott VRH above 70 K. We argue that the VRH conduction results from the topological disorders of rGO as confirmed by Raman spectroscopy. This infers that the average distance between defects is approximately 2.0 nm.

Fabrication and independent control of patterned polymer gate for a few-layer WSe2 field-effect transistor

We report the fabrication of a patterned polymer electrolyte for a two-dimensional (2D) semiconductor, few-layer tungsten diselenide (WSe2) field-effect transistor (FET). We expose an electron-beam in a desirable region to form the patterned structure. The WSe2 FET acts as a p-type semiconductor in both bare and polymer-covered devices. We observe a highly efficient gating effect in the polymer-patterned device with independent gate control. The patterned polymer gate operates successfully in a molybdenum disulfide (MoS2) FET, indicating the potential for general applications to 2D semiconductors. The results of this study can contribute to large-scale integration and better flexibility in transition metal dichalcogenide (TMD)-based electronics.

Probing variable range hopping lengths by magneto conductance in carbonized polymer nanofibers

Using magneto transport, we probe hopping length scales in the variable range hopping conduction of carbonized polyacetylene and polyaniline nanofibers. In contrast to pristine polyacetylene nanofibers that show vanishing magneto conductance at large electric fields, carbonized polymer nanofibers display a negative magneto conductance that decreases in magnitude but remains finite with respect to the electric field. We show that this behavior of magneto conductance is an indicator of the electric field and temperature dependence of hopping length in the gradual transition from the thermally activated to the activation-less electric field driven variable range hopping transport. This reveals magneto transport as a useful tool to probe hopping lengths in the non-linear hopping regime.