Youngmi Cho

Formation, manipulation, and elasticity measurement of a nanometric column of water molecules

Nanometer-sized columns of condensed water molecules are formed by an atomic-resolution force microscope operated in ambient conditions. An unusual stepwise decrease of the force gradient associated with the ultrathin water bridge in the tip-substrate gap is observed during its stretch, exhibiting regularity in step heights (approximately 0.5 N/m) and plateau lengths (approximately 1 nm). Such quantized elasticity is indicative of an atomic-scale stick slip at the tip-water interface. A thermodynamic-instability-induced rupture of the water meniscus (5 nm long and 2.6 nm wide) is also found. This work opens a high-resolution study of the structure and interface dynamics of a nanometric aqueous column.

Magnetic field sensing scheme using CoFeB∕MgO∕CoFeB tunneling junction with superparamagnetic CoFeB layer

The authors investigated the tunneling magnetoresistance (TMR) of CoFeB∕MgO∕CoFeB tunnel junctions by varying the thickness (tCoFeB) of the top CoFeB layer. Linear and hysteresis-free switching was observed in junctions with tCoFeB⩽10A, while normal tunneling behavior occurred for tCoFeB>10A. The field sensitivity and the sensing field range were found to be controlled by varying the thickness of the sensing layer. This finding means that the magnetic tunneling junction (MTJ) provides a scheme for magnetic field sensing, which has a simple sensor design and low power consumption. The magnetic properties of the sensing layer with tCoFeB⩽10A were found to show the characteristics of superparamagnetism. Although the detailed mechanism of TMR in MTJs with a superparamagnetic layer is not fully understood at present, this phenomenon is observed repeatedly. Therefore, this sensing scheme would be an alternative method for overcoming the problems in magnetic sensors with a crossed magnetization pattern.

Variation in the properties of the interface in a CoFeB∕MgO∕CoFeB tunnel junction during thermal annealing

Variation in the quality of the interface in a CoFeB∕MgO∕CoFeB tunnel junction during thermal annealing was investigated using x-ray photoemission spectroscopy. The formation of B oxide and the reduction of Fe oxide at the bottom interface after thermal annealing near Ta=300°C were found to enhance the tunneling magnetoresistance ratio significantly. At the same time, an asymmetry of the conductance (dV∕dI) in the bias polarity and a local minimum of conductance in a positive bias state were measured which were attributed to the presence of a minority state at the bottom interface. The authors believe that the existence of the Bloch state was also responsible for the failure of the application of the Brinkman-Dynes-Rowell or Simmons models to the CoFeB∕MgO∕CoFeB junction.

Cation disorder as the major electron scattering source in crystalline InGaZnO

We theoretically investigate the electron transport mechanism in crystalline In-Ga-Zn oxides using the semi-classical transport theory. The site disorder of Ga and Zn atoms is treated based on the virtual crystal approximation. The valence difference between Ga3+ and Zn2+, modeled by screened Coulomb potential, plays a critical role in determining the total electron mobility. The temperature and carrier-density dependences of the calculated electron mobility are in excellent agreement with experimental data. The unusual behavior of electron mobility is explained based on the energy-dependent relaxation time and underlying band structure.

First-principles study on secondary electron emission of MgO surface

We theoretically investigate secondary-electron-emission properties of MgO when noble gases are incident on the surface. We consider both potential and kinetic emission mechanisms. For the potential emission through Auger neutralization, densities of states and vacuum level are obtained from the first-principles calculations. It is found that secondary-emission coefficients decrease in the following sequence of surface directions; (111)-OH>(100)>(110), a tendency that is in agreement with experimental observations. For a surface model including F center, the secondary-emission coefficient substantially increases for Kr and Xe. To investigate the kinetic emission mechanism by an energetic ion impinging on MgO surfaces, first-principles molecular dynamics simulations are performed. Dynamic up-shifts of antibonding states between ions and oxygen atoms are found to lead to the secondary-electron emission at kinetic energies as low as 30 eV. Various collision conditions are compared based on the temporal interval during which excited states stay within the conduction band.We theoretically investigate secondary-electron-emission properties of MgO when noble gases are incident on the surface. We consider both potential and kinetic emission mechanisms. For the potential emission through Auger neutralization, densities of states and vacuum level are obtained from the first-principles calculations. It is found that secondary-emission coefficients decrease in the following sequence of surface directions; (111)-OH>(100)>(110), a tendency that is in agreement with experimental observations. For a surface model including F center, the secondary-emission coefficient substantially increases for Kr and Xe. To investigate the kinetic emission mechanism by an energetic ion impinging on MgO surfaces, first-principles molecular dynamics simulations are performed. Dynamic up-shifts of antibonding states between ions and oxygen atoms are found to lead to the secondary-electron emission at kinetic energies as low as 30 eV. Various collision conditions are compared based on the temporal inter...

Intrinsic nature of visible-light absorption in amorphous semiconducting oxides

To enlighten microscopic origin of visible-light absorption in transparent amorphous semiconducting oxides, the intrinsic optical property of amorphous InGaZnO4 is investigated by considering dipole transitions within the quasiparticle band structure. In comparison with the crystalline InGaZnO4 with the optical gap of 3.6 eV, the amorphous InGaZnO4 has two distinct features developed in the band structure that contribute to significant visible-light absorption. First, the conduction bands are down-shifted by 0.55 eV mainly due to the undercoordinated In atoms, reducing the optical gap between extended states to 2.8 eV. Second, tail states formed by localized oxygen p orbitals are distributed over ∼0.5 eV near the valence edge, which give rise to substantial subgap absorption. The fundamental understanding on the optical property of amorphous semiconducting oxides based on underlying electronic structure will pave the way for resolving instability issues in recent display devices incorporating the semiconducting oxides.

Molecular dynamics study on low-energy sputtering properties of MgO surfaces

In an effort to understand microscopic processes occurring between MgO protective layers and impinging plasma ions in a discharge cell of plasma-display panel, sputtering properties of MgO(100) surface by He, Ne, and Xe atoms are studied with molecular dynamics simulations. Interatomic potentials between constituent atoms are fitted to first-principles data sets for representative configurations. Various incident directions of ions are considered with kinetic energies under 100eV. It is found that sputtering yields for the Ne atom are largest among tested noble gases. The angle dependence of sputtering yields indicates that (111)-oriented MgO films are much more vulnerable to ion attacks than (100)-oriented layers. A surface model including the monolayer step is also studied and it is found that the yields increase substantially for grazing-angle incidence.

Field emission of carbon nanotubes and electronic structure of carbon nanopeapods

We calculate the electron emission from nanostructures under an applied electric field using the ab initio pseudopotential method. The transition rates of the electrons are calculated by integrating the time-dependent Schr€ equation for the states initially inside the emitter. A localized basis set is used for obtaining the eigenstates before emission and the potential that drives the field emission. The calculated electron-tunneling graph is quite linear in the short-time region, giving the transition rate within the simulation time. We have applied this new method to the field emission of carbon nanotubes as a test. Then we calculate the electronic structure of the fullerenes encapsulated inside the carbon nanotubes, the so-called carbon ‘‘nanopeapods’’. The fullerenes may or may not contain metallic atoms such as gadolinium or potassium. There is an interesting effect of strain when the diameter of the carbon nanotube is smaller than that of the inserted fullerene plus twice the van der Waals distance. 2002 Published by Elsevier Science B.V.

Field Emission and Lifetime Characteristics of Titanium-coated Carbon Nanotubes

We investigated the effect of titanium (Ti)-coated carbon nanotubes (CNTs) theoretically and experimentally. We found that adsorption of single Ti atom lowers the work function of CNTs by density functional calculations. Also, Ti-coated CNTs showed the largest increase in local density of states around Fermi level under electric fields. Coating of Ti metal on CNTs was carried out by electroless plating method. Ti-coated CNTs were mixed with conductive pastes, and then screen-printed. The measurement of field emission carried out using a diode structure showed that the electron emission of Ti-coated CNT films uniformly had a field of 3.7 V/mum at a current density of 100 mu/cm2 (1/500 duty), as compare to bare CNT films showed a field of 5.5 V/mum. Furthermore, the lifetime of our CNT samples was about a few times as much as that of the pristine ones. Therefore, Ti-coated CNTs improved the characteristics of CNT-based field emission emitters

Origin of asymmetry of tunneling conductance in CoFeB/MgO/CoFeB tunnel junction

We investigated the top and bottom interfaces of a CoFeB∕MgO∕CoFeB tunnel junction using transmission electron microscope (TEM) and x-ray photoemission spectroscopy (XPS) in order to understand the origin of the asymmetry of dI∕dV in terms of bias polarity. It was found, from a TEM image, that there is no clear cut at the top interface, while the bottom interface has relatively clean boundary. Furthermore, XPS data show that more hydroxides were formed at the top interface than at the bottom interface. These indicate that the hydroxides would hinder the epitaxial crystallinity at the interface in CoFeB∕MgO∕CoFeB tunnel junctions. Therefore, it is most likely that the asymmetry of dI∕dV is caused by the disappearance of minority Bloch state, which is closely correlated with the existence of hydroxides at the top interface of a CoFeB∕MgO∕CoFeB tunnel junction.