Hana Yoon

Vertical Epitaxial Co5Ge7 Nanowire and Nanobelt Arrays on a Thin Graphitic Layer for Flexible Field Emission Displays

Vertically aligned single-crystalline Co5 Ge7 nanowire (NW) and nanobelt arrays are grown on a very thin graphite layer as well as a curved graphite layer with a good epitaxial lattice match. Co5 Ge7 NW arrays, thus grown, show very efficient field emission properties comparable to those of carbon nanotubes and may be used for flexible field emission displays in the future.

Composition-Tuned ConSi Nanowires: Location-Selective Simultaneous Growth along Temperature Gradient

We report the simultaneous and selective synthesis of single-crystalline Co(n)Si NWs (n = 1-3) and their corresponding crystal structures--simple cubic (CoSi), orthorhombic (Co(2)Si), and face-centered cubic (Co(3)Si)--following a composition change. Co(n)Si NWs were synthesized by placing the sapphire substrates along a temperature gradient. The synthetic process is a successful demonstration of tuning the chemical composition in Co(n)Si NWs. The synthesis and detailed crystal structure of single-crystalline Co(2)Si and Co(3)Si are reported for the first time including the bulk and the nanostructure phases. The electrical and magnetic properties of Co(2)Si NWs are investigated and compared with those of CoSi NWs.

Itinerant Helimagnetic Single-Crystalline MnSi Nanowires

We report the synthesis of free-standing MnSi nanowires via a vapor transport method with no catalyst and measurements of their electrical and magnetic properties for the first time. The single-crystalline MnSi nanowire ensemble with a simple cubic (B20) crystal structure shows itinerant helimagnetic properties with a T(c) of about 30 K. A single MnSi nanowire device was fabricated by a new method using photolithography and a nanomanipulator that produces good ohmic contacts. The single-nanowire device measurements provide large (20%) negative magnetoresistance and very low electrical resistivity of 544 microOmegacm for the MnSi nanowire.

Diffusion-Driven Crystal Structure Transformation: Synthesis of Heusler Alloy Fe3Si Nanowires

We report fabrication of Heusler alloy Fe(3)Si nanowires by a diffusion-driven crystal structure transformation method from paramagnetic FeSi nanowires. Magnetic measurements of the Fe(3)Si nanowire ensemble show high-temperature ferromagnetic properties with T(c) >> 370 K. This methodology is also successfully applied to Co(2)Si nanowires in order to obtain metal-rich nanowires (Co) as another evidence of the structural transformation process. Our newly developed nanowire crystal transformation method would be valuable as a general method to fabricate metal-rich silicide nanowires that are otherwise difficult to synthesize.

Structure-Induced Ferromagnetic Stabilization in Free-Standing Hexagonal Fe1.3Ge Nanowires

Single-crystalline free-standing hexagonal Fe(1.3)Ge nanowires (NWs) are synthesized for the first time using a chemical vapor transport process without using any catalyst. Interestingly, Fe(1.3)Ge NWs are found to be ferromagnetic at room temperature, while bulk Fe(1.3)Ge has the lower critical temperature of 200 K. We perform first-principles density functional calculations and suggest that the observed strong ferromagnetism is attributed to the reduced distances between Fe atoms, increased number of Fe-Fe bonds, and the enhanced Fe magnetic moments. Both experimental and theoretical studies show that the magnetic moments are enhanced in the NWs, as compared to bulk Fe(1.3)Ge. We also modulate the composition ratio of as-grown iron germanide NWs by adjusting experimental conditions. It is shown that uniaxial strain on the hexagonal plane also enhances the ferromagnetic stability.

Epitaxy-driven vertical growth of single-crystalline cobalt nanowire arrays by chemical vapor deposition

Highly oriented single-crystalline ferromagnetic Co nanowire (NW) arrays were synthesized on sapphire substrates via a single-step chemical vapor deposition (CVD) method. On an m-cut sapphire substrate, Co NWs were vertically grown in epitaxial relationship with the substrate without using any catalysts or templates. On an r-cut sapphire substrate, Co NWs were horizontally grown in two perpendicular directions. Furthermore, we report that the Co NWs were transformed into Co3O4 nanotubes by thermal annealing under dilute O2 conditions. Such formation of hollow structures is ascribed to favored outward diffusion of Co ions. The present vertically aligned arrays of single-crystalline Co NWs could be utilized for advanced magnetic memory applications owing to their uniform orientations.

Mussel-inspired surface functionalization of porous carbon nanosheets using polydopamine and Fe3+/tannic acid layers for high-performance electrochemical capacitors

A facile mussel-inspired surface modification of interconnected porous carbon nanosheet (IPCN) electrodes is demonstrated through the formation of a polydopamine coating and the subsequent layer-by-layer deposition of ferric ions (Fe3+) and tannic acid, with the aim of developing high-performance electrochemical capacitors. After the deposition of the polydopamine coating, the specific capacitance increases by ∼40% as compared to that of an unmodified IPCN electrode. This increase in the capacitance can be explained based on the pseudocapacitance induced by the catechol groups of polydopamine. Furthermore, the electrodes coated with both polydopamine and layers of Fe3+ and tannic acid exhibit an additional increase in the capacitance to ∼244 F g−1 at 5 mV s−1, which is ∼83% higher than that of the unmodified IPCN electrode. This is attributable to the presence of many redox moieties, which are introduced by polydopamine and tannic acid. Furthermore, the strong interactions between the Fe3+ ions and the catechol groups result in improved capacitance retention even after 1000 cycles. The mussel-inspired surface modification of IPCN electrodes demonstrated in this work can potentially be exploited for developing novel pseudocapacitive electrode materials with excellent performances.

Frequency comb transferred by surface plasmon resonance

Frequency combs, millions of narrow-linewidth optical modes referenced to an atomic clock, have shown remarkable potential in time/frequency metrology, atomic/molecular spectroscopy and precision LIDARs. Applications have extended to coherent nonlinear Raman spectroscopy of molecules and quantum metrology for entangled atomic qubits. Frequency combs will create novel possibilities in nano-photonics and plasmonics; however, its interrelation with surface plasmons is unexplored despite the important role that plasmonics plays in nonlinear spectroscopy and quantum optics through the manipulation of light on a subwavelength scale. Here, we demonstrate that a frequency comb can be transformed to a plasmonic comb in plasmonic nanostructures and reverted to the original frequency comb without noticeable degradation of <6.51 × 10−19 in absolute position, 2.92 × 10−19 in stability and 1 Hz in linewidth. The results indicate that the superior performance of a well-defined frequency comb can be applied to nanoplasmonic spectroscopy, quantum metrology and subwavelength photonic circuits.

Three-dimensionally kinked high-conducting CoGe nanowire growth induced by rotational twinning

We have synthesized single-crystalline horizontal and free-standing monoclinic CoGe nanowire (NW) arrays on high-k dielectric Y-stabilized ZrO2 (110) substrates via a chemical vapor transport process without using any catalysts. Horizontal NWs are grown epitaxially on the substrate. Three-dimensionally (3D)-kinked NWs are grown from the tip of the horizontal NWs homoepitaxially initiated by rotational twinning. Electrical measurements show that both horizontal and 3D-kinked CoGe NWs have low resistivity. The 3D-kinked NWs as well as free-standing metallic CoGe NWs integrated on Y-stabilized ZrO2 substrates could find applications as effective on-chip interconnects and nanoelectrodes for highly integrated nanoelectronic devices and as platforms for fuel cells and as efficient catalysts.

Truncated Tetrahedron Seed Crystals Initiating Stereoaligned Growth of FeSi Nanowires

We have synthesized epitaxially grown freestanding FeSi nanowires (NWs) on an m-Al(2)O(3) substrate by using a catalyst-free chemical vapor transport method. FeSi NW growth is initiated from FeSi nanocrystals, formed on a substrate in a characteristic shape with a specific orientation. Cross-section TEM analysis of seed crystals reveals the crystallographic structure and hidden geometry of the seeds. Close correlation of geometrical shapes and orientations of the observed nanocrystals with those of as-grown NWs indicates that directional growth of NWs is initiated from the epitaxially formed seed crystals. The diameter of NWs can be controlled by adjusting the composition of Si in a Si/C mixture. The epitaxial growth method for FeSi NWs via seed crystals could be employed to heteroepitaxial growth of other compound NWs.