arXiv subjects

Young Hee Lee

Publications and source records attributed to Young Hee Lee.

41 records · Page 3Linked to original sources

arXiv2015

Selective amplification of primary exciton in monolayer MoS2

Optoelectronics applications for transition-metal dichalcogenides are still limited by weak light absorption and their complex exciton modes are easily perturbed by varying excitation conditions, because they are inherent in atomically thin layers. Here, we propose a method of selectively amplifying the primary exciton (A0) among the exciton complexes in monolayer MoS2 via cyclic re-excitation of cavity-free exciton-coupled plasmon propagation. This was implemented by partially overlapping a Ag nanowire (NW) on a MoS2 monolayer separated by a thin SiO2 spacer. Exciton-coupled plasmons in the NW enhance the A0 radiation in MoS2. The cumulative amplification of emission enhancement by cyclic plasmon travelling reaches ~20-fold selectively for the A0, while excluding other B exciton and multiexciton by significantly reduced band-filling, without oscillatory spectra implying plasmonic cavity effects.

cond-mat.mtrl-sci↗

arXiv2015

Charge Transport in Polycrystalline Graphene: Challenges and Opportunities

Graphene has attracted significant interest both for exploring fundamental science and for a wide range of technological applications. Chemical vapor deposition (CVD) is currently the only working approach to grow graphene at wafer scale, which is required for industrial applications. Unfortunately, CVD graphene is intrinsically polycrystalline, with pristine graphene grains stitched together by disordered grain boundaries, which can be either a blessing or a curse. On the one hand, grain boundaries are expected to degrade the electrical and mechanical properties of polycrystalline graphene, rendering the material undesirable for many applications. On the other hand, they exhibit an increased chemical reactivity, suggesting their potential application to sensing or as templates for synthesis of one-dimensional materials. Therefore, it is important to gain a deeper understanding of the structure and properties of graphene grain boundaries. Here, we review experimental progress on identification and electrical and chemical characterization of graphene grain boundaries. We use numerical simulations and transport measurements to demonstrate that electrical properties and chemical modification of graphene grain boundaries are strongly correlated. This not only provides guidelines for the improvement of graphene devices, but also opens a new research area of engineering graphene grain boundaries for highly sensitive electrobiochemical devices.

cond-mat.mtrl-sci↗

arXiv2014

The characterization of Co-nanoparticles supported on graphene

The results of density functional theory (DFT) calculations and measurements of X-ray photoelectron (XPS) and X-ray emission (XES) spectra of Co-nanoparticles dispersed on graphene/Cu composites are presented. It is found that for 0.02nm and 0.06nm Co coverage the Co atoms form islands which are strongly oxidized under exposure at the air. For Co (2nm) coverage the upper Co-layers is oxidized whereas the lower layers contacting with graphene is in metallic state. Therefore Co (2 nm) coverage induces the formation of protective oxide layer providing the ferromagnetic properties of Co nanoparticles which can be used as spin filters in spintronics devices.

cond-mat.mtrl-sci↗

arXiv2013

Ultrafast zero balance of the oscillator-strength sum rule in graphene

Oscillator-strength sum rule in light-induced transitions is one general form of quantum-mechanical identities. Although this sum rule is well established in equilibrium photo-physics, an experimental corroboration for the validation of the sum rule in a nonequilibrium regime has been a long-standing unexplored question. The simple band structure of graphene is an ideal system for investigating this question due to the linear Dirac-like energy dispersion. Here, we employed both ultrafast terahertz and optical spectroscopy to directly monitor the transient oscillator-strength balancing between quasi-free low-energy oscillators and high-energy Fermi-edge ones. Upon photo-excitation of hot Dirac fermions, we observed that the ultrafast depletion of high-energy oscillators precisely complements the increased terahertz absorption oscillators. Our results may provide an experimental priori to understand, for example, the intrinsic free-carrier dynamics to the high-energy photo-excitation, responsible for optoelectronic operation such as graphene-based phototransistor or solar-energy harvesting devices.

cond-mat.mes-hall↗

arXiv2011

Controlled Growth of ZnO Nanowire, Nanowall, and Hybrid Nanostructures on Graphene for Piezoelectric Nanogenerators

Precise control of morphologies of one-dimensional (1D) or 2D nanostructures during growth has not been easily accessible, usually degrading the device performance and therefore limiting applications to various advanced nanoscale electronics and optoelectronics. Graphene could be a platform to serve as a substrate for both morphology control and direct use of electrodes due to its ideal monolayer flatness with π electrons. Here, we report that by using graphene directly as a substrate, vertically well-aligned ZnO nanowires and nanowalls were obtained systematically by controlling Au catalyst thickness and growth time, without invoking significant thermal damage on the graphene layer during thermal chemical vapor deposition of ZnO at high temperature of about 900 oC. We further demonstrate a piezoelectric nanogenerator that was fabricated from the vertically aligned nanowire-nanowall ZnO hybrid/graphene structure generates a new type of direct current.

cond-mat.mtrl-sci↗