-
[show abstract]
[hide abstract]
ABSTRACT: Scaling-down of phase change materials to a nanowire (NW) geometry is critical to a fast switching speed of non-volatile memory devices. Herein, we report novel composition-phase-tuned GeSbTe NWs, synthesized by a chemical vapor transport method, which guarantees promising applications in the field of nano-scale electric devices. As the Sb content increased, they showed a distinctive rhombohedral-cubic-rhombohedral phase evolution. Remarkable superlattice structures were identified for the Ge(8)Sb(2)Te(11), Ge(3)Sb(2)Te(6), Ge(3)Sb(8)Te(6), and Ge(2)Sb(7)Te(4) NWs. The coexisting cubic-rhombohedral phase Ge(3)Sb(2)Te(6) NWs exhibited an exclusively uniform superlattice structure consisting of 2.2 nm-period slabs. The rhombohedral phase Ge(3)Sb(8)Te(6) and Ge(2)Sb(7)Te(4) NWs adopted an innovative structure; 3Sb(2) layers intercalated the Ge(3)Sb(2)Te(6) and Ge(2)Sb(1)Te(4) domains, respectively, producing 3.4 nm and 2.7 nm period slabs. The current-voltage measurement of the individual NW revealed that the vacancy layers of Ge(8)Sb(2)Te(11) and Ge(3)Sb(2)Te(6) decreased the electrical conductivity.
Nano Letters 01/2013; · 13.20 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: An amorphous alloy, Cu43Zr43Al7Be7, was synthesized. The alloy showed a large supercooled liquid region (115 °C), a significant glass forming ability (Φ12 mm)
and considerable strain to fracture (8–9%), which collectively have not been observed in other Cu-based amorphous alloys.
The alloy has a unique microstructure characterized by atomic-scale phase separation, which most likely resulted from the
large difference in the mixing enthalpy between the binary pairs. This study discusses a possible mechanism underlying the
simultaneous enhancement in the GFA and plasticity by considering the atomic packing state and atomic-scale compositional
separation resulting from Al and Be.
Metals and Materials International 05/2012; 13(1):21-24. · 1.18 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The electrical transport of individual ZnO nanorod devices manufactured by focused ion beam (FIB) was investigated by the direct measurement of electrical resistance at electrode junctions of cross-sectioned devices using two nanoprobes. The cathodoluminescence (CL) measurements were also performed to evaluate the crystallinity at the center and edge of the cross-sectioned ZnO nanorods. The electrical transport of the individual ZnO nanorod device depends strongly on the crystallinity of the ZnO nanorod itself and the carbon contents at Pt junctions. The ZnO-Au junction of the device acted as the fastest path for electrical transport.
Journal of Nanoscience and Nanotechnology 02/2012; 12(2):1466-70. · 1.56 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: AlxTi1-xN/CrN multilayer coatings were fabricated by magnetron sputtering and those hardness variations were studied by observing the crack propagation and measuring the chemical bonding state of nitrides by Ti addition. While AlN/CrN multilayer shown stair-like crack propagation, AlxTi1-xN/CrN multilayer illustrated straight crack propagation. Most interestingly, Ti addition induced more broken nitrogen bonds in the nitride multilayers, leading to the reduction of hardness. However, the hardness of Al0.25Ti0.75N/CrN multilayer, having high Ti contents, increased by the formation of many Ti-N bond again instead of Al-N bond. From these results, we found that linear crack propagation behavior was dominated by broken nitrogen bonds in the AlxTi1-xN/CrN multilayer coatings.
Journal of Nanoscience and Nanotechnology 02/2012; 12(2):1581-4. · 1.56 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: AIN/CrN multilayer hard coatings with various bilayer thicknesses were fabricated by a reactive sputtering process. The microstructural and mechanical characterizations of multilayer coatings were investigated through transmission electron microscope (TEM) observations and the hardness measurements by nano indentation. In particular, the variation of chemical bonding states of the bilayer nitrides was elucidated by near edge X-ray absorption fine structure (NEXAFS) spectroscopy. Many broken nitrogen bonds were formed by decreasing the bilayer thickness of AIN/CrN multilayer coatings. Existence of optimum AIN/CrN multilayer coatings thickness for maximum hardness could be explained by the competition of softening by the formation of broken nitrogen bonds and strengthening induced by decreasing bilayer thickness.
Journal of Nanoscience and Nanotechnology 02/2012; 12(2):1476-9. · 1.56 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The structure of multi-wall carbon nanotubes has been attributed previously
to disordered stacking of the graphene planes.
01/2012;
-
Jong-Hyun Seo,
Youngdong Yoo,
Na-Young Park,
Sang-Won Yoon,
Hyoban Lee,
Sol Han,
Seok-Woo Lee,
Tae-Yeon Seong,
Seung-Cheol Lee,
Kon-Bae Lee,
Pil-Ryung Cha,
Harold S Park,
Bongsoo Kim, Jae-Pyoung Ahn
[show abstract]
[hide abstract]
ABSTRACT: We report that defect-free Au nanowires show superplasticity on tensile deformation. Evidences from high-resolution electron microscopes indicated that the plastic deformation proceeds layer-by-layer in an atomically coherent fashion to a long distance. Furthermore, the stress-strain curve provides full interpretation of the deformation. After initial superelastic deformation, the nanowire shows superplastic deformation induced by coherent twin propagation, completely reorientating the crystal from <110> to <100>. Uniquely well-disciplined and long-propagating atomic movements deduced here are ascribed to the superb crystallinity as well as the radial confinement of the Au nanowires.
Nano Letters 08/2011; 11(8):3499-502. · 13.20 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We report that twin-free single-crystalline Pd nanowire (NW) arrays grow epitaxially in a selected pattern on a substrate. Parallel aligned Pd NWs are synthesized on a SrTiO(3) (110) substrate in a very high density. On a SrTiO(3) (001) substrate, Pd NWs grow horizontally in two perpendicular directions. Vertical Pd NWs are synthesized instead of horizontal NWs when a c-cut sapphire substrate is employed. We reveal that the atomic structure of the substrate surface determines the geometry and orientation of seeds, which in turn direct the growth patterns of the NWs. The interface energy between the NW material and the substrate is also critical in determining the NW growth pattern. Polarization-dependent localized surface plasmon resonance of as-synthesized epitaxial Pd NW arrays is investigated for application as a plasmonic platform.
ACS Nano 05/2010; 4(5):2919-27. · 10.77 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Electron tomography and high-resolution transmission electron microscopy were used to characterize the unique three-dimensional (3D) structures of twinned Zn(3)P(2) (tetragonal) and InAs (zinc blende) nanowires synthesized by the vapor transport method. The Zn(3)P(2) nanowires adopt a unique superlattice structure that consists of twinned octahedral slice segments having alternating orientations along the axial [111] direction of a pseudo cubic unit cell. The apexes of the octahedral slice segment are indexed as six equivalent <112> directions at the [111] zone axis. At each 30 degrees turn, the straight and zigzagged morphologies appear repeatedly at the <112> and <011> zone axes, respectively. The 3D structure of the twinned Zn(3)P(2) nanowires is virtually the same as that of the twinned InAs nanowires. In addition, we analyzed the 3D structure of zigzagged CdO (rock salt) nanowires and found that they include hexahedral segments, whose six apexes are matched to the <011> directions, linked along the [111] axial direction. We also analyzed the unique 3D structure of rutile TiO(2) (tetragonal) nanobelts; at each 90 degrees turn, the straight morphology appears repeatedly, while the in-between twisted form appears at the [011] zone axis. We suggest that the TiO(2) nanobelts consist of twinned octahedral slices whose six apexes are indexed by the <011>/<001> directions with the axial [010] direction.
Nano Letters 04/2010; 10(5):1682-91. · 13.20 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We have synthesized epitaxial Au, Pd, and AuPd nanowire arrays in vertical or horizontal alignment on a c-cut sapphire substrate. We show that the vertical and horizontal nanowire arrays grow from half-octahedral seeds by the correlations of the geometry and orientation of seed crystals with those of as-grown nanowires. The alignment of nanowires can be steered by changing the atom flux. At low atom deposition flux vertical nanowires grow, while at high atom flux horizontal nanowires grow. Similar vertical/horizontal epitaxial growth is also demonstrated on SrTiO(3) substrates. This orientation-steering mechanism is visualized by molecular dynamics simulations.
Nano Letters 02/2010; 10(2):432-8. · 13.20 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Stacked AA graphite has been synthesized using a high-density dc plasma in hydrogen-methane mixtures. Graphene layers have been grown epitaxially with 2-1 registration between the AA graphitic edges and the (111) surface of diamond. In addition, a new graphite crystal structure containing AA(') graphene layers, where alternate planes are translated by half the hexagon width, is formed by 1-1 registry. The resulting interplanar distances of the AA graphite at the interface range from 2.20 A for the 1-1 registration to 4.40 A for the 2-1 registration and have been measured directly by high-resolution transmission electron microscopy (TEM). The appearance of the characteristic d-spacings, 3.55, 2.15, 1.80, 1.75 (not fully resolved), and 1.25 A in the selective area diffraction patterns from the TEM, are consistent with reflections from the (001), (100), (102), (002), and (110) planes of the AA graphite. Simulation of the diffraction patterns, employing the structural factors of graphene, confirms the existence of AA graphite.
The Journal of chemical physics 01/2009; 129(23):234709. · 3.09 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We report on the formation of thermally stable and low-resistance Ti/Au-based ohmic contacts to n-type GaN (4.0×1018cm−3) by using a W barrier layer. It is shown that the electrical characteristic of the sample is considerably improved upon annealing
at 900°C for 1min in a N2 ambient. The contacts produce the specific contact resistance as low as 6.7×10−6Ωcm2 after annealing. The Norde and current–voltage methods are used to determine the effective Schottky barrier heights (SBHs).
It is shown that annealing results in a reduction in the SBHs as compared to that of the as-deposited sample. Auger electron
spectroscopy (AES), scanning transmission electron microscopy (STEM) and X-ray diffraction (XRD) examinations show that nitride
and gallide phases are formed at the contact/GaN interface. Based on the AES, STEM and XRD results, a possible ohmic formation
mechanism is described and discussed.
Journal of Materials Science Materials in Electronics 12/2008; 20(1):9-13. · 1.08 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: In this study, we synthesized Cu–Zr binary alloys reinforced with an ultrafine eutectic microstructure. The alloys consisted of alternating layers of a hard superlattice phase and a ductile Cu phase with a very fine interlamellar spacing of ∼60 nm. The superlattice phase enhanced the strength of the alloys while the laminated composite structure helped improve their plasticity, making their mechanical properties comparable to those of the earlier reported high strength alloys. This paper discusses the fundamental microstructural aspects that influence the mechanical properties of these alloys.
Journal of Materials Research. 06/2008; 23(07):1987 - 1994.
-
[show abstract]
[hide abstract]
ABSTRACT: Electron tomography and high-resolution transmission electron microscopy were used to characterize the unique three-dimensional structures of helical or zigzagged GaN, ZnGa2O4, and Zn2SnO4 nanowires. The GaN nanowires adopt a helical structure that consists of six equivalent <011> growth directions with the axial [0001] direction. We also confirmed that the ZnGa2O4 nanosprings have four equivalent <011> growth directions with the [001] axial direction. The zigzagged Zn2SnO4 nanowires consisted of linked rhombohedrons having the side edges matched to the <110> direction and the [111] axial direction.
Nano Letters 02/2008; 8(2):551-7. · 13.20 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Two unique helical zinc gallate (ZnGa2O4) nanostructures were synthesized by thermal evaporation using the zinc selenide (ZnSe) nanowires; helical ZnGa2O4 nanowire rolls either on a straight ZnSe nanowire support or without any support. They all consist of single-crystalline cubic ZnGa2O4 crystals without any dislocation over the entire helical structure and have four equivalent growth directions of 011 with the axial direction of [001]. We suggest that the lattice matching with the ZnSe nanowires would be an important factor in determining the growth direction of the helical ZnGa2O4 nanowires.
Journal of the American Chemical Society 09/2005; 127(31):10802-3. · 9.91 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We characterized the structure and magnetic properties of Mn-incorporated GaP nanowires synthesized by thermal evaporation of GaP/Mn powders. The nanowires consist of twin-crystalline zinc blende GaP grown with the [111] direction and doped with about 1 at. % Mn. They are often sheathed with the bumpy amorphous outerlayers containing high concentrations of Mn and O. The ferromagnetic hysteresis curves at 5 and 300 K and temperature-dependent magnetization provide evidence for the ferromagnetism with the Curie temperature higher than room temperature. Magnetic properties of individual nanowires have been measured, showing a large negative magnetoresistance equal to about -5% at 5 K. We suggest that the Mn doping of GaP nanowires would form a dilute magnetic semiconductor.
The Journal of Physical Chemistry B 06/2005; 109(19):9311-6. · 3.70 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Nitrogen (N)-doped carbon nanotubes (CNTs) were heated to 1000 degrees C under an ultrahigh vacuum. X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge structure (XANES) reveal three different N structures; graphitelike, pyridine-like, and molecular N(2). The vibrationally resolved XANES peaks of N(2) were first observed, suggesting the existence of molecular N(2) as intercalated and trapped forms. The annealing process can decrease the average N content from 6.3 at. % to 3.3 at. %, mainly by releasing molecular N(2). Electron energy-loss spectroscopy (EELS) confirms that the annealing releases molecular N(2) from the CNTs.
The Journal of Physical Chemistry B 03/2005; 109(5):1683-8. · 3.70 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The devices using individual ZnO nanowire have been manufactured by FIB. Its specific resistance and microstructural characterization has been investigated using nano manipulator and transmission electron microscopy (TEM) observations. The specific resistance was 0.2–0.4 Ω cm. With increasing the RTA temperature, the specific resistance began to be decreased and was abruptly decreased at the RTA temperature above 500 °C. The Pt junction of as-manufactured device consisted of the Pt nanoparticles of 5 nm and the amorphous carbon of 9.1 wt.%. After RTA, the size of Pt nanoparticles grew up to 100 nm, the contents of carbon were decreased within the Pt junction, and the conductivity was enhanced due to Au diffusion into the Pt junction. It was strongly suggested that the contents of carbon is the most important factor for the electrical enhancement.
Thin Solid Films.
