Jinkyoung Yoo

In situ atomic-scale imaging of electrochemical lithiation in silicon

In lithium-ion batteries, the electrochemical reaction between the electrodes and lithium is a critical process that controls the capacity, cyclability and reliability of the battery. Despite intensive study, the atomistic mechanism of the electrochemical reactions occurring in these solid-state electrodes remains unclear. Here, we show that in situ transmission electron microscopy can be used to study the dynamic lithiation process of single-crystal silicon with atomic resolution. We observe a sharp interface (~1 nm thick) between the crystalline silicon and an amorphous Li(x)Si alloy. The lithiation kinetics are controlled by the migration of the interface, which occurs through a ledge mechanism involving the lateral movement of ledges on the close-packed {111} atomic planes. Such ledge flow processes produce the amorphous Li(x)Si alloy through layer-by-layer peeling of the {111} atomic facets, resulting in the orientation-dependent mobility of the interfaces.

Enhanced field emission properties from well-aligned zinc oxide nanoneedles grown on the Au∕Ti∕n-Si substrate

The authors investigated the field emission from vertically well-aligned zinc oxide (ZnO) nanoneedles grown on the Au∕Ti∕n-Si (100) substrate using metal organic chemical vapor deposition. The turn-on field of ZnO nanoneedles was about 0.85V∕μm at the current density of 0.1μA∕cm2, and the emission current density of 1mA∕cm2 was achieved at the applied electric field of 5.0V∕μm. The low turn-on field of the ZnO nanoneedles was attributed to very sharp tip morphology, and the high emission current density was mainly caused by the formation of the stable Ohmic contact between the ZnO nanoneedles and Au film.

Improving the functionality of carbon nanodots: doping and surface functionalization

Distinct from conventional carbon nanostructures, such as fullerene, graphene, and carbon nanotubes, carbon nanodots (C-dots) exhibit unique properties such as strong fluorescence, high photostability, chemical inertness, low toxicity, and biocompatibility. Various synthesis routes for C-dots have been developed in the last few years, and now intense research efforts have been focused on improving their functionality. In this aspect, doping and surface functionalization are two major ways to control the chemical, optical, and electrical properties of C-dots. Doping introduces atomic impurities into C-dots to modulate their electronic structure, and surface functionalization modifies the C-dot surface with functional molecules or polymers. In this review, we summarize recent progress in doping and surface functionalization of C-dots for improving their functionality, and offer insight into controlling the properties of C-dots for a variety of applications ranging from biomedicine to optoelectronics to energy.

Nanophotonic switch using ZnO nanorod double-quantum-well structures

The authors report on time-resolved near-field spectroscopy of ZnO∕ZnMgO nanorod double-quantum-well structures (DQWs) for a nanometer-scale photonic device. They observed nutation of the population between the resonantly coupled exciton states of DQWs. Furthermore, they demonstrated switching dynamics by controlling the exciton excitation in the dipole-inactive state via an optical near field. The results of time-resolved near-field spectroscopy of isolated DQWs described here are a promising step toward designing a nanometer-scale photonic switch and related devices.

GaN/In1−xGaxN/GaN/ZnO nanoarchitecture light emitting diode microarrays

We studied the fabrication and electroluminescent (EL) characteristics of GaN/In1−xGaxN/GaN/ZnO nanoarchitecture light emitting diode (LED) microarrays consisting of position-controlled GaN/ZnO coaxial nanotube heterostructures. For the fabrication of nanoarchitecture LED arrays, n-GaN, GaN/In0.24Ga0.76N multiquantum well (MQW) structures and p-GaN layers were deposited coaxially over the entire surface of position-controlled ZnO nanotube arrays grown vertically on c-plane sapphire substrates. The nanoarchitecture LEDs exhibited strong green and blue emission from the GaN/GaN/In0.24Ga0.76N MQWs at room temperature. Furthermore, the origins of dominant EL peaks are also discussed.

Photoluminescent characteristics of Ni-catalyzed GaN nanowires

The authors report on time-integrated and time-resolved photoluminescence (PL) of GaN nanowires grown by the Ni-catalyst-assisted vapor-liquid-solid method. From PL spectra of Ni-catalyzed GaN nanowires at 10K, several PL peaks were observed at 3.472, 3.437, and 3.266eV, respectively. PL peaks at 3.472 and 3.266eV are attributed to neutral-donor-bound excitons and donor-acceptor pair, respectively. Furthermore, according to the results from temperature-dependent and time-resolved PL measurements, the origin of the PL peak at 3.437eV is also discussed.

Quantum confinement effect in ZnO∕Mg0.2Zn0.8O multishell nanorod heterostructures

We report on photoluminescence measurements of Mg0.2Zn0.8O∕ZnO∕Mg0.2Zn0.8O multishell layers on ZnO core nanorods. Dominant excitonic emissions in the photoluminescence spectra show a blueshift depending on the ZnO shell layer thickness attributed to the quantum confinement effect in the nanorod heterostructure radial direction. Furthermore, near-field scanning optical microscopy clearly shows sharp photoluminescence peaks from the individual nanorod quantum structures, corresponding to subband levels.

Tailoring Lithiation Behavior by Interface and Bandgap Engineering at the Nanoscale

Controlling the transport of lithium (Li) ions and their reaction with electrodes is central in the design of Li-ion batteries for achieving high capacity, high rate, and long lifetime. The flexibility in composition and structure enabled by tailoring electrodes at the nanoscale could drastically change the ionic transport and help meet new levels of Li-ion battery performance. Here, we demonstrate that radial heterostructuring can completely suppress the commonly observed surface insertion of Li ions in all reported nanoscale systems to date and to exclusively induce axial lithiation along the [111] direction in a layer-by-layer fashion. The new lithiation behavior is achieved through the deposition of a conformal, epitaxial, and ultrathin silicon (Si) shell on germanium (Ge) nanowires, which creates an effective chemical potential barrier for Li ion diffusion through and reaction at the nanowire surface, allowing only axial lithiation and volume expansion. These results demonstrate for the first time that interface and bandgap engineering of electrochemical reactions can be utilized to control the nanoscale ionic transport/insertion paths and thus may be a new tool to define the electrochemical reactions in Li-ion batteries.

Mapping carrier diffusion in single silicon core-shell nanowires with ultrafast optical microscopy.

Recent success in the fabrication of axial and radial core-shell heterostructures, composed of one or more layers with different properties, on semiconductor nanowires (NWs) has enabled greater control of NW-based device operation for various applications. (1-3) However, further progress toward significant performance enhancements in a given application is hindered by the limited knowledge of carrier dynamics in these structures. In particular, the strong influence of interfaces between different layers in NWs on transport makes it especially important to understand carrier dynamics in these quasi-one-dimensional systems. Here, we use ultrafast optical microscopy (4) to directly examine carrier relaxation and diffusion in single silicon core-only and Si/SiO(2) core-shell NWs with high temporal and spatial resolution in a noncontact manner. This enables us to reveal strong coherent phonon oscillations and experimentally map electron and hole diffusion currents in individual semiconductor NWs for the first time.

Fabrication and photoluminescent characteristics of ZnO∕Mg0.2Zn0.8O coaxial nanorod single quantum well structures

The authors report on fabrication and photoluminescent (PL) properties of ZnO∕Mg0.2Zn0.8O coaxial nanorod quantum structures with various quantum well and barrier layer thicknesses. Employing catalyst-free metal-organic vapor-phase epitaxy, coaxial nanorod single quantum well structures were fabricated by the alternate heteroepitaxial growth of ZnO and Mg0.2Zn0.8O layers over the entire surfaces of the ZnO nanorods with fine thickness controls of the layers. The quantum confinement effect of carriers in coaxial nanorod quantum structures depends on the Mg0.2Zn0.8O quantum barrier layer thickness as well as the thickness of the ZnO quantum well layer. The temperature-dependent PL characteristics of the coaxial nanorod quantum structures are also discussed.