Biomedical subjects

Jisoon Ihm

Publications and source records attributed to Jisoon Ihm.

8 recordsLinked to original sources

PubMed2006

Combinatorial search for optimal hydrogen-storage nanomaterials based on polymers.

We perform an extensive combinatorial search for optimal nanostructured hydrogen-storage materials among various metal-decorated polymers using first-principles density-functional calculations. We take into account the zero-point vibration as well as the pressure- and temperature-dependent adsorption-desorption probability of hydrogen molecules. An optimal material we identify is Ti-decorated cis-polyacetylene with reversibly usable gravimetric and volumetric density of 7.6 wt % and 63 kg/m(3), respectively, near ambient conditions. We also propose "thermodynamically usable hydrogen capacity" as a criterion for comparing different storage materials.

Journal Article↗

PubMed2006

Conformational molecular switch of the azobenzene molecule: a scanning tunneling microscopy study.

We propose to utilize azobenzene as a nanomolecular switch which can be triggered by transmitting electrons above threshold biases. The effect is explained by an electron impact trans-cis conformational change of the isolated azobenzene molecules. The molecular electronic states of both isomers have been measured with spatially resolved scanning tunneling microscopy or spectroscopy, leading to suggested transition pathways of the electron-induced isomerization.

Journal Article↗

PubMed2005

Electrical switching in metallic carbon nanotubes.

We present first-principles calculations of quantum transport which show that the resistance of metallic carbon nanotubes can be changed dramatically with homogeneous transverse electric fields if the nanotubes have impurities or defects. The change of the resistance is predicted to range over more than 2 orders of magnitude with experimentally attainable electric fields. This novel property has its origin that backscattering of conduction electrons by impurities or defects in the nanotubes is strongly dependent on the strength and/or direction of the applied electric fields. We expect this property to open a path to new device applications of metallic carbon nanotubes.

Journal Article↗

PubMed2005

Paired gap states in a semiconducting carbon nanotube: deep and shallow levels.

Several paired, localized gap states were observed in semiconducting single-wall carbon nanotubes using spatially resolved scanning tunneling spectroscopy. A pair of gap states is found far from the band edges, forming deep levels, while the other pair is located near the band edges, forming shallow levels. With the help of a first-principles study, the former is explained by a vacancy-adatom complex while the latter is explained by a pentagon-heptagon structure. Our experimental observation indicates that the presence of the gap states provides a means to perform local band-gap engineering as well as doping without impurity substitution.

Journal Article↗

PubMed2004

Zipper mechanism of nanotube fusion: theory and experiment.

We propose a new microscopic mechanism to explain the unusually fast fusion process of carbon nanotubes. We identify the detailed pathway for two adjacent (5,5) nanotubes to gradually merge into a (10,10) tube, and characterize the transition states. The propagation of the fused region is energetically favorable and proceeds in a morphology reminiscent of a Y junction via a zipper mechanism, involving only Stone-Wales bond rearrangements with low activation barriers. The zipper mechanism of fusion is supported by a time series of high-resolution transmission electron microscopy observations.

Journal Article↗

PubMed2003

Magnetism in all-carbon nanostructures with negative Gaussian curvature.

We apply the ab initio spin density functional theory to study magnetism in all-carbon nanostructures. We find that particular systems, which are related to schwarzite and contain no undercoordinated carbon atoms, carry a net magnetic moment in the ground state. We postulate that, in this and other nonalternant aromatic systems with negative Gaussian curvature, unpaired spins can be introduced by sterically protected carbon radicals.

Journal Article↗

PubMed2003

Orbital hybridization and charge transfer in carbon nanopeapods.

We investigate the electronic structure of the fullerenes encapsulated inside carbon nanotubes, the so-called nanopeapods, using the first-principles study. The orbital hybridization of LUMO+1 (the state above the lowest unoccupied molecular orbital) of C60, rather than LUMO as previously proposed, with the nanotube states explains the peak at approximately 1 eV in recent scanning-tunneling-spectroscopy (STS) data. For the endohedral metallofullerenes nested in the strained nanotube, the charge transfer shifts the relative energy levels of the different states and produces a spatial modulation of the energy gap in agreement with another STS experiment.

Journal Article↗

PubMedDate not supplied

Field emission properties of carbon nanotubes coated with boron nitride.

Field emission properties of carbon nanotubes coated with a single layer of boron nitride are calculated using the first-principles pseudopotential method. At lower bias voltage, the emission current of the coated nanotube is comparable to that of the bare carbon nanotube and is dominated by the contribution from localized states at the tip of the tube. At higher voltage, newly generated hybridized states between the carbon nanotube tip and the even-membered boron nitride rings contribute significantly to the emission current because they experience a low tunneling barrier compared with the bare carbon nanotube case. Our results suggest that the insulator coating can, besides protecting the nanotube tip from the attack of gas molecules, substantially enhance the field emission current.

Boron Compounds↗