Won Woo Lee

Graphene as an Interfacial Layer for Improving Cycling Performance of Si Nanowires in Lithium-Ion Batteries

Managing interfacial instability is crucial for enhancing cyclability in lithium-ion batteries (LIBs), yet little attention has been devoted to this issue until recently. Here, we introduce graphene as an interfacial layer between the current collector and the anode composed of Si nanowires (SiNWs) to improve the cycling capability of LIBs. The atomically thin graphene lessened the stress accumulated by volumetric mismatch and inhibited interfacial reactions that would accelerate the fatigue of Si anodes. By simply incorporating graphene at the interface, we demonstrated significantly enhanced cycling stability for SiNW-based LIB anodes, with retentions of more than 2400 mAh/g specific charge capacity over 200 cycles, 2.7 times that of SiNWs on a bare current collector.

Reversible and Irreversible Responses of Defect-Engineered Graphene-Based Electrolyte-Gated pH Sensors

We have studied the role of defects in electrolyte-gated graphene mesh (GM) field-effect transistors (FETs) by introducing engineered edge defects in graphene (Gr) channels. Compared with Gr-FETs, GM-FETs were characterized as having large increments of Dirac point shift (∼30-100 mV/pH) that even sometimes exceeded the Nernst limit (59 mV/pH) by means of electrostatic gating of H(+) ions. This feature was attributed to the defect-mediated chemisorptions of H(+) ions to the graphene edge, as supported by Raman measurements and observed cycling characteristics of the GM FETs. Although the H(+) ion binding to the defects increased the device response to pH change, this binding was found to be irreversible. However, the irreversible component showed relatively fast decay, almost disappearing after 5 cycles of exposure to solutions of decreasing pH value from 8.25 to 6.55. Similar behavior could be found in the Gr-FET, but the irreversible component of the response was much smaller. Finally, after complete passivation of the defects, both Gr-FETs and GM-FETs exhibited only reversible response to pH change, with similar magnitude in the range of 6-8 mV/pH.

Metal-Coated Silicon Nanowire Plasmonic Waveguides

We report that a metal-coated silicon nanowire functions as a plasmonic waveguide. Measurements showed that plasmonic waveguide modes propagated efficiently through a chemically synthesized silicon nanowire with a diameter of ~80 nm coated with silver. The propagation lengths for transverse-magnetic and transverse-electric modes were estimated to be ~8.05 and ~6.61 µm, respectively. Numerical simulations of the propagation length and mode profile of each plasmonic waveguide mode agreed with the experimental results. These plasmonic waveguides with highly smooth surfaces, fabricated through a bottom-up approach, represent a meaningful step toward the demonstration of an ultracompact subwavelength-scale plasmonic integrated circuit.

Graphene meshes decorated with palladium nanoparticles for hydrogen detection

We fabricated flexible and transparent hydrogen gas sensors based on a palladium-decorated graphene mesh. Electron microscopy analysis confirmed that ~2–3 nm diameter Pd nanoparticles were uniformly dispersed on the graphene mesh surfaces. The sensors were highly transparent with an average optical transmission of >90% to visible light and they exhibited good electrical stability with a resistance change of 1.5% under a tensile strain of 1.1% in a cyclic bending-unbending test. Compared with graphene-based sensors, the graphene mesh sensors exhibited a faster response to hydrogen gas with sensitivity as high as ~5% at a low concentration of 10 ppm H2/air, even at room temperature. The enhanced H2 detection characteristic of the graphene mesh sensors is attributed to the existence of edges.

Three-dimensional epitaxy of single crystalline semiconductors by polarity-selective multistage growth

Despite recent advances in three-dimensional (3D) fabrication, the epitaxial growth of finely controlled, single crystalline 3D structures is still constrained. In this study, we demonstrate the step-by-step growth of hierarchical 3D architectures composed of single crystalline semiconductors. To achieve this goal, we first established control of the preferential growth direction of ZnO crystals by polarity-selective crystallization during the low-temperature solution-phase synthesis. The time-dependent analysis showed that the binding of a citrate additive to the positively charged, top (0001) surface of ZnO crystals led to an inversion of the growth anisotropy from predominantly the vertical direction to the lateral direction with more than a 10 times increase in the width-to-height ratio. With further fine optimization of the charge balance, we minimized the interruption of citrate ions for epitaxial and iterative stacking of Zn and O ions, and achieved single-crystalline hexaplates. This feature was further combined with multistage epitaxial growth of the nanocrystal constituents, producing a 3D, single crystalline semiconductor with excellent luminescent characteristics.

Characterization of Performance of a Mobile MIMO Antenna in Free Space

This letter presents an antenna design method for a dual antenna with high isolation for mobile stations (MSs) and includes a verification method for improving the correlation performance of an MS that uses a proposed multiple-input-multiple-output (MIMO) antenna by using MIMO over-the-air (OTA) channel models. The large-scale parameters for the MIMO OTA channel models are important because they determine the fixed geometrical parameters to be used for channel impulse response realization. The correlation performance of an MS has a strong relation to the cross-polarization discrimination (XPD) of the second branch antenna both numerically and experimentally. This study applies modern technology to implement MIMO antenna switch high isolation by selecting Band 5 to have low-frequency fabricated antenna samples that have several XPD values, thereby comparing their characteristics.

Three-dimensionally-architectured GaN light emitting crystals

We demonstrate the epitaxial growth of three-dimensional (3D) GaN single crystal arrays through metal–organic vapor phase epitaxy (MOVPE) on lattice-matched ZnO templates that were achieved via hydrothermal growth in which we controlled the position, size and morphology of the layer. To prevent collapse of the crystals during MOVPE growth, graphene sheets were employed as a protection/mask layer, and initial GaN growth was performed at a relatively low temperature under hydrogen-depleted conditions. Temperature-dependent growth behaviors of GaN crystals on diverse types of ZnO templates can facilitate the control of hexagonal crystal shapes including pencil, tent and plate shapes. Furthermore, a 3D light emitting crystal array was demonstrated through subsequent growth of high-quality n-type and p-type GaN layers.

Synthesis and transfer of Si nanowire arrays embedded in photo-sensitive polymer films for non-planar electronics

In this study, hybrid composite films (HCFs) consisting of aligned Si nanowires (SiNWs) and a photoresist polymer were prepared and their applicability to nanomaterial assembly and pattern transfer onto various substrates, including curved surfaces, was demonstrated. The synthesis and transfer of the HCFs can be scaled up to large areas. Transfer of the HCFs, when performed immediately after a UV-exposure step, enabled direct patterning of fine features on curved surfaces. The unique feature of this approach was further exploited to fabricate a new type of SiNW electronic device that can conform to non-planar surfaces without strain.

Anomalous Photovoltaic Response of Graphene-on-GaN Schottky Photodiodes

Graphene has attracted great attention as an alternative to conventional metallic or transparent conducting electrodes. Despite its similarities with conventional electrodes, recent studies have shown that a single-atom layer of graphene possesses unique characteristics, such as a tunable work function and transparencies for electric potential, reactivity, and wetting. Nevertheless, a systematic analysis of graphene and semiconductor junction characteristics has not yet been carried out. Here, we report the photoresponse characteristics of graphene-on-GaN Schottky junction photodiodes (Gr-GaN SJPDs), showing a typical rectifying behavior and distinct photovoltaic and photoelectric responses. Following the initial abrupt response to UV illumination, the Gr-GaN SJPDs exhibited a distinct difference in photocarrier dynamics depending on the applied bias voltage, which is characterized by either a negative or positive change in photocurrent with time. We propose underlying mechanisms for the anomalous photocarrier dynamics based on the interplay between electrostatic molecular interactions over the one-atom-thick graphene and GaN junction and trapped photocarriers at the defect states in the GaN thin film.

Controlling electric potential to inhibit solid-electrolyte interphase formation on nanowire anodes for ultrafast lithium-ion batteries

With increasing demand for high-capacity and rapidly rechargeable anodes, problems associated with unstable evolution of a solid-electrolyte interphase on the active anode surface become more detrimental. Here, we report the near fatigue-free, ultrafast, and high-power operations of lithium-ion battery anodes employing silicide nanowires anchored selectively to the inner surface of graphene-based micro-tubular conducting electrodes. This design electrically shields the electrolyte inside the electrode from an external potential load, eliminating the driving force that generates the solid-electrolyte interphase on the nanowire surface. Owing to this electric control, a solid-electrolyte interphase develops firmly on the outer surface of the graphene, while solid-electrolyte interphase-free nanowires enable fast electronic and ionic transport, as well as strain relaxation over 2000 cycles, with 84% capacity retention even at ultrafast cycling (>20C). Moreover, these anodes exhibit unprecedentedly high rate capabilities with capacity retention higher than 88% at 80C (vs. the capacity at 1C).Lithium-based rechargeable batteries suffer from unstable evolution of solid-electrolyte interphase on the electrode surface. Here, the authors provide an approach to inhibiting SEI formation by controlling electric potential distribution across electrolyte and electrode.