Bog G. Kim

Super-tough carbon-nanotube fibres

The energy needed to rupture a fibre (its toughness) is five times higher for spider silk than for the same mass of steel wire, which has inspired efforts to produce spider silk commercially. Here we spin 100-metre-long carbon-nanotube composite fibres that are tougher than any natural or synthetic organic fibre described so far, and use these to make fibre supercapacitors that are suitable for weaving into textiles.

Improving the mechanical properties of single-walled carbon nanotube sheets by intercalation of polymeric adhesives

Organic polymers, such as poly(vinyl alcohol), poly(vinyl pyrrolidone), and poly(styrene), were intercalated into single-walled carbon nanotube sheets by soaking the sheets in polymer solutions. Even for short soak times, significant polymer intercalation into existing free volume was observed. Tensile tests on intercalated sheets showed that the Young’s modulus, strength, and toughness increased by factors of 3, 9, and 28, respectively, indicating that the intercalated polymer enhances load transmission between nanotubes.

Continuous carbon nanotube composite fibers: properties, potential applications, and problemsElectronic supplementary information (ESI) available: frontispiece figure. See http://www.rsc.org/suppdata/jm/b3/b312092a/

Using solution spinning, which involves an intermediate gel-state, we obtained exceptionally strong carbon nanotube fibers that are tougher than either spider silk or any fiber used for mechanical reinforcement. We use these fibers to make 100 micron diameter supercapacitors and electronic textiles. Per weight, the energy needed to break these fibers is about 4× higher than spider dragline silk and 20× higher than steel wire. This article describes this advance, comparisons with the prior art, potential applications, and present barriers for large volume applications.

Graphyne: Hexagonal network of carbon with versatile Dirac cones

FIG. 1. (Color online) Schematic representations of (a) graphene, and (b) α, (c) β, and (d) γ graphyne. Red quadrangles indicate unit cells. (e) The hopping matrix elements along a carbon triple bond in graphyne. The carbon atoms 1 and 4 are at vertices of a hexagon in graphyne. (f) Effective direct hopping matrix element between the two carbon atoms 1 and 4 in (e).

Two-dimensional normal-state quantum oscillations in a superconducting heterostructure

Semiconductor heterostructures provide an ideal platform for studying high-mobility, low-density electrons in reduced dimensions. The realization of superconductivity in heavily doped diamond, silicon, silicon carbide and germanium suggests that Cooper pairs eventually may be directly incorporated in semiconductor heterostructures, but these newly discovered superconductors are currently limited by their extremely large electronic disorder. Similarly, the electron mean free path in low-dimensional superconducting thin films is usually limited by interface scattering, in single-crystal or polycrystalline samples, or atomic-scale disorder, in amorphous materials, confining these examples to the extreme ‘dirty limit’. Here we report the fabrication of a high-quality superconducting layer within a thin-film heterostructure based on SrTiO3 (the first known superconducting semiconductor). By selectively doping a narrow region of SrTiO3 with the electron-donor niobium, we form a superconductor that is two-dimensional, as probed by the anisotropy of the upper critical magnetic field. Unlike in previous examples, however, the electron mobility is high enough that the normal-state resistance exhibits Shubnikov–de Haas oscillations that scale with the perpendicular field, indicating two-dimensional states. These results suggest that delta-doped SrTiO3 provides a model system in which to explore the quantum transport and interplay of both superconducting and normal electrons. They also demonstrate that high-quality complex oxide heterostructures can maintain electron coherence on the macroscopic scales probed by transport, as well as on the microscopic scales demonstrated previously.

Writing polarization bits on the multiferroic BiMnO3 thin film using Kelvin probe force microscope

We report the multiferroic properties of epitaxially (100) oriented BiMnO3 thin film on (100) LaAlO3 substrate and preferentially (111) oriented BiMnO3 thin film on (111) Pt/TiO2/SiO2/Si substrate. Nano-size bits of ferroelectric polarization on the BiMnO3 thin film on (111) Pt/TiO2/SiO2/Si substrate can be easily written and read by Kelvin force microscope (KFM). We found that, for the preferentially (111) oriented BiMnO3 thin film, only ferroelectric polarization has been induced at the low writing biases, which makes the writing and reading process simple. This suggests that the preferentially oriented BiMnO3 thin film is a potential candidate for the high-density data storage device based on KFM.

Structural evolution of a high Tc ferroelectric (x)Bi(Zn1∕2Ti1∕2)O3–(1−x)PbTiO3 solid solution

We have studied the structural phase transition of solid solutions (x)Bi(Zn1∕2Ti1∕2)O3–(1−x)PbTiO3. The temperature evolution of the x-ray diffraction patterns of the θ-2θ scans around {110} has been performed to check the structural phase transitions: The (101)∕(011) peak shifts to a larger value, while the (110) peak shifts to a smaller value with increasing temperature. Finally, the peaks merge to the same value at the Curie temperature, indicating a tetragonal to the cubic phase transition. The structural phase transition temperature Tc coincides with the Curie temperature and increases as the increasing Bi(Zn1∕2Ti1∕2)O3 composition (x). The evolution of the lattice constant, unit cell volume, and tetragonality as functions of temperature and composition is discussed.

Optical evidence of multiphase coexistence in single crystalline(La,Pr,Ca)MnO3

We investigated temperature (T)- and magnetic field-dependent optical conductivity spectra σ(ω) of a La 5 / 8 - y Pr y Ca 3 / 8 MnO 3 (y 0.35) single crystal, showing intriguing phase coexistence at low T. At T C <T <T C O , a dominant charge-ordered phase produces a large optical gap energy of ∼0.4 eV. At T<T C , at least two absorption bands newly emerge below 0.4 eV. Analyses of σ(ω) indicate that the bands should be attributed to a ferromagnetic metallic phase and a charge-disordered phase that coexist with the charge-ordered phase. This optical study clearly shows that La 5 / 8 - y Pr y Ca 3 / 8 MnO 3 (y0.35) is composed of multiphases that might have different lattice strains.

Kinetically controlled thin-film growth of layered β- and γ-NaxCoO2 cobaltate

We report growth characteristics of epitaxial β-Na0.6CoO2 and γ-Na0.7CoO2 thin films on (001) sapphire substrates grown by pulsed-laser deposition. Reduction of the deposition rate could change the structure of the NaxCoO2 thin film from a β phase with an island growth mode to a γ phase with a layer-by-layer growth mode. The γ-Na0.7CoO2 thin film exhibits spiral surface growth with multiterraced islands and highly crystallized texture compared to that of the β-Na0.6CoO2 thin film. This heterogeneous epitaxial film growth can give an example of the strain effect of physical properties and growth dynamics of NaxCoO2 as well as the subtle nature of structural change.

Terahertz Birefringence in Zinc Oxide

The birefringence of zinc oxide (ZnO) in the terahertz (THz) frequency range is measured using a parallel-polarization configuration THz time-domain spectrometer and compared with the result of an ab initio calculation. The measured birefringence of 0.180 at 1 THz shows good agreement with the calculated value of 0.170 from full phonon consideration, both of which are about 20 times larger than the birefringence in the visible range. It is found that the difference of the transverse optical and longitudinal optical (TO–LO) phonon splitting between the optical phonon branches (A1 and E1) predominantly contributes to the huge birefringence of ZnO in the THz frequency region.