• Bulletin of the Korean Chemical Society
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• v.17 no.12
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• pp.1112-1117
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• 1996

The electronic structures of twenty-seven isomers of a macrocyclic fulleropyrrolidine are investigated with semi-empirical extended Huckel (EH) molecular orbital method. The geometry of each isomer is determined by the molecular mechanics and dynamics methods based on UFF (universal force field) empirical force field. The calculated geometries, such as the carbon-carbon distances of the fullerene moiety, are in good agreement with those of related fullerene derivatives. The EH calculation shows that the formation of macrocyclic pyrrolidine ring on fullerene moiety results in the reduction of the HOMO-LUMO energy gap. From the graphical analysis of the DOS (density of states), PDOS (projected DOS), and MOOP (molecular orbital overlap population) curves, we can find that this reduction is due to splitting of the HOMO of fullerene moiety, which results from the symmetry-breaking and the distortion of the buckminsterfullerene framework from its ideal icosahedral structure.

• Journal of the Korean Institute of Telematics and Electronics D
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• v.34D no.7
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• pp.37-44
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• 1997

A dadaptive refinement tetrahedron mesh has been presented for using in full band GaAs monte carlo simulation. A uniform tetrahedron mesh is used without regard to energy values and energy variety in case of the past full band simulation. For the uniform tetrahedron mesh, a fine tetrahedron is demanded for keeping up accuracy of calculation in the low energy region such as .GAMMA.-valley, but calculation time is vast due to usin gthe same tetrahedron in the high energy region. The mesh of this study, thererfore, is consisted of the fine mesh in the low energy and large variable energy region and rough mesh n the high energy. The density of states (DOS) calculated with this mesh is compared with the one of the uniform mesh. The DOS of this mesh is improved th efive times or so in root mean square error and the ten times in the correlation coefficient than the one of a uniform mesh. This refinement mesh, therefore, can be used a sthe basic mesh for the full band GaAs monte carlo simulation.

• Advances in nano research
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• v.7 no.3
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• pp.209-221
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• 2019

Graphene, with impressive electronic properties, have high potential in the microelectronic field. However, graphene itself is a zero bandgap material which is not suitable for digital logic gates and its application. Thus, much focus is on graphene nanoribbons (GNRs) that are narrow strips of graphene. During GNRs fabrication process, the occurrence of defects that ultimately change electronic properties of graphene is difficult to avoid. The modelling of GNRs with defects is crucial to study the non-idealities effects. In this work, nearest-neighbor tight-binding (TB) model for GNRs is presented with three main simplifying assumptions. They are utilization of basis function, Hamiltonian operator discretization and plane wave approximation. Two major edges of GNRs, armchair-edged GNRs (AGNRs) and zigzag-edged GNRs (ZGNRs) are explored. With single vacancy (SV) defects, the components within the Hamiltonian operator are transformed due to the disappearance of tight-binding energies around the missing carbon atoms in GNRs. The size of the lattices namely width and length are varied and studied. Non-equilibrium Green's function (NEGF) formalism is employed to obtain the electronics structure namely band structure and density of states (DOS) and all simulation is implemented in MATLAB. The band structure and DOS plot are then compared between pristine and defected GNRs under varying length and width of GNRs. It is revealed that there are clear distinctions between band structure, numerical DOS and Green's function DOS of pristine and defective GNRs.

• Bulletin of the Korean Chemical Society
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• v.13 no.4
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• pp.367-375
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• 1992

The electronic structure change caused by insertion of hydrogen into the interstices of Pd cluster is studied. Several properties such as energy, reduced overlap population (ROP), electon density (ED) and density of states (DOS) are calculated by Extended Huckel Method. Various types of clusters are considered. The same is performed on Ni and Pt and all the results are compared. The results show that the hydrogen atom in Pd is stabilized remarkably but its wave function is almost unperturbed. The fact is compatible with noticeable solubility of hydrogen in Pd but may not be a positive enough evidence to rationalize the claimed cold fusion phenomenon. It is also found that a remarkable charge transfer from Pd atom to hydrogen atom occurs.

• Proceeding of EDISON Challenge
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• 2016.03a
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• pp.280-284
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• 2016

본 연구에서는 Density functional theory(DFT) 계산을 이용하여, $MoS_2$의 Mo와 S를 다른 원자로 치환 했을 때 $2H-MoS_2$ monolayer의 basal plane에서 HER활성을 향상시켰다. 특히 Ge와 Rh를 치환한 경우, ${\Delta}G_H$가 각각 0.03eV, 0,07eV로 최적에 가까운 HER활성이 나타났다. 다른 원자의 치환이 Fermi level 근처의 DOS(density of states)를 높여, ${\Delta}G_H$을 0에 가깝게 낮출 수 있음을 확인하였다. 또한 치환되는 원자의 농도, 그리고 strain을 변화시켜 농도와 strain의 증가에 따른 ${\Delta}G_H$ 감소를 발견했다. 이로써 각치환되는 원자마다, 치환 농도와 strain을 함께 변화시켜 ${\Delta}G_H$을 낮출 수 있었다. ${\Delta}G_H$가 0에 가까운(${\pm}{\pm}0.2eV$ 이내) 원자종류, 치환 농도, strain의 여러 조합을 찾았다.

• Advances in nano research
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• v.14 no.1
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• pp.1-15
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• 2023

Driven by the scaling down of transistor node technology, graphene became of interest to many researchers following the success of its fabrication as graphene nanoribbons (GNRs). However, during the fabrication of GNRs, it is not uncommon to have defects within the GNR structures. Scaling down node technology also changes the modelling approach from the classical Boltzmann transport equation to the quantum transport theory because the quantum confinement effects become significant at sub-10 nanometer dimensions. The aim of this study is to examine the effect of Stone-Wales defects on the electronic properties of GNRs using a tight-binding model, based on Non-Equilibrium Green's Function (NEGF) via numeric computation methods using MATLAB. Armchair and zigzag edge defects are also implemented in the GNR structures to mimic the practical fabrication process. Electronic properties of pristine and defected GNRs of various lengths and widths were computed, including their band structure and density of states (DOS). The results show that Stone-Wales defects cause fluctuation in the band structure and increase the bandgap values for both armchair GNRs (AGNRs) and zigzag GNRs (ZGNRs) at every simulated width. In addition, Stone-Wales defects reduce the numerical computation DOS for both AGNRs and ZGNRs. However, when the lengths of the structures increase with fixed widths, the effect of the Stone-Wales defects become less significant.

• Proceedings of the Korean Vacuum Society Conference
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• 2011.08a
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• pp.108-108
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• 2011

Organometallic complexes containing unpaired spins, such as metalloporphyrin or metallophthalocyanine, have extensively studied with increasing interests of their promising model systems in spintronic applications. Additionally, the use of these complexes as an acceptor molecule in chemical sensors has recently received great attentions. In this presentation, we have investigated adsorption of nitric oxide (NO) molecules at Co-porphyrin molecules on Au(111) surfaces with scanning tunneling microscopy and spectroscopy at low temperature. At the location of Co atom in Co-porphyrin molecules, we could observe a Kondo resonance state near Fermi energy in density of states (DOS) before exposing NO molecules and the Kondo resonance state was disappeared after NO exposing because the electronic spin structure of Co-porphyrin were modified by forming a cobalt-NO bonding. Furthermore, we could locally control the chemical reaction of NO dissociations from NO-CoTPP by electron injections via STM probe. After dissociation of NO molecules, the Kondo resonance state was recovered in density of state. With a help of density functional theory (DFT) calculations, we could understand that the modified electronic structures for NO-Co-porphyrin could be occurred by metal-ligand hybridization and the dissociation mechanisms of NO can be explained in terms of the resonant tunneling process via molecular orbitals.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.17 no.7
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• pp.691-696
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• 2004

The electronic state and chemical bonding of $\beta$-MnO$_2$ with transition metal dopants were theoretically investigated by DV-X$_{\alpha}$ (the discrete variational X$_{\alpha}$) method, which is a sort of the first principles molecular orbital method using the Hartree-Fock-Slater approximation. The calculations were performed with a $_Mn_{14}$ MO$_{56}$ )$^{-52}$ (M = transition metals) cluster model. The electron energy level, the density of states (DOS), the overlap population, the charge density distribution, and the net charges, were calculated. The energy level diagram of MnO$_2$ shows the different band structure and electron occupancy between the up spin states and down spin states. The dopant levels decrease between the conduction band and the valence band with the increase of the atomic number of dopants. The covalency of chemical bonding was shown to increase and ionicity decreased in increasing the atomic number of dopants. Calculated results were discussed on the basis of the interaction between transition metal 3d and oxygen 2p orbital. In conclusion it is expected that when the transition metals are added to MnO$_2$ the band gap decreases and the electronic conductivity increases with the increase of the atomic number of dopants. the atomic number of dopants.

• Journal of Magnetics
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• v.10 no.1
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• pp.1-4
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• 2005

We investigated the electronic structures and the magnetic properties of Ti-based intermetallic system of CoTi/FeTi/CoTi(110) surface and interface by using the all-electron full potential linearized augmented plane wave (FLAPW) method within the generalized gradient approximation (GGA). The calculated magnetic moments of interface Co and Fe atoms are 0.65 and 0.15 μ/sub B/, respectively. Surface and interface magnetism of CoTi/FeTi/CoTi(110) are discussed using the calculated density of states (DOS) and the spin densities.

• Proceedings of the Korean Vacuum Society Conference
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• 2016.02a
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• pp.217-217
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• 2016

The amorphous InGaZnO (a-IGZO) is widely accepted as a promising channel material for thin-film transistor (TFT) applications owing to their outstanding electrical properties [1, 2]. However, a-IGZO TFTs have still suffered from their bias instability with illumination [1-4]. Up to now, many researchers have studied the sub-gap density of states (DOS) as the root cause of instability. It is well known that defect states can influence on the performances and stabilities of a-IGZO TFTs. The defects states should be closely related with the deposition condition, including sputtering power, and pressure. Nevertheless, it has not been reported how these defects are created during conventional RF magnetron sputtering. In general, during conventional RF magnetron sputtering process, negative oxygen ions (NOIs) can be generated by electron attachment in oxygen atom near target surface and then accelerated up to few hundreds eV by a self-bias; at this time, the high energy bombardment of NOIs induce defects in oxide thin films. Recently, we have reported that the properties of IGZO thin films are strongly related with effects of NOIs which are generated during the sputtering process [5]. From our previous results, the electrical characteristics and the chemical bonding states of a-IGZO thin films were depended with the bombardment energy of NOIs. And also, we suggest that the deep sub-gap states in a-IGZO as well as thin film properties would be influenced by the bombardment of high energetic NOIs during the sputtering process.In this study, we will introduce our novel technology named as Magnetic Field Shielded Sputtering (MFSS) process to prevent the NOIs bombardment effects and present how much to be improved the properties of a-IGZO thin film by this new deposition method. We deposited a-IGZO thin films by MFSS on SiO2/p-Si and glass substrate at various process conditions, after which we investigated the morphology, optical and electrical properties of the a-IGZO thin films.