• 한국진공학회:학술대회논문집
• /
• 한국진공학회 2009년도 제38회 동계학술대회 초록집
• /
• pp.94-94
• /
• 2010

Scanning tunneling microscopy (STM) was used to study the electronic structure of cobalt(II) tetraphenylporphyrin (CoTPP) on the Fe/W(110) substrate. Clover-like conformation of CoTPP was observed and showed bias dependent STM images. The central Co(II) ion of this porphyrin was protruded on the positive biases, but it was depressed on the negative biases. On the positive biases, the phenyl rings of CoTPP appeared to be bright contrary to the invisible pyrrole rings. These results were compared the first-principles calculations using GGA and GGA+U to elucidate the influence of the Fe substrate. GGA+U results agreed well with the experimental results; however, GGA did not. These results show that proper treatment of the on-site Coulomb repulsion of the Fe ions is crucial to describe the electronic structure of this system. By the comparison between the GGA+U calculations on the Fe substrate and the gas phase calculations, it can be noted that chemical potential shift occurred accompanying charge transfer from the Fe ions of the substrate to the pyrrole ligand of the porphyrin.

• 센서학회지
• /
• 제31권5호
• /
• pp.318-323
• /
• 2022

Barium titanate (BaTiO₃) is considered to be a beneficial ceramic material for multilayer ceramic capacitor (MLCC) applications because of its high dielectric constant and low dielectric loss. Numerous attempts have been made to improve the physical properties of BaTiO₃ in response to recent market trends by employing multicomponent alloying strategies. However, owing to its significant number of atomic combinations and unpredictable physical properties, finding a traditional experimental approach to develop multicomponent systems is difficult; the development of such systems is also time-consuming. In this study, 168 new structures were fabricated using special quasi-random structures (SQSs) of Ba_1-xCa_xTi_1-yZryO₃, and 1680 physical properties were extracted from first-principles calculations. In addition, we built an integrated database to manage the computational results, and will provide big data solutions by performing data analysis combined with AI modeling. We believe that our research will enable the global materials market to realize digital transformation through datalization and intelligence of the material development process.

• 한국결정성장학회지
• /
• 제17권5호
• /
• pp.192-195
• /
• 2007

Structural and electronic properties of bulk GaN with nitrogen vacancy($V_N$), oxygen substitution on nitrogen site($O_N$), and complex of nitrogen vacancy and oxygen substitution on nitrogen site($V_N-O_N$) were investigated using the first principle calculations. It was found that stability of defect formation is dependent on the epilayer growth conditions. The complex of $V_N-O_N$ is energetically the most favorable state in a condition of Ga-rich, however, oxygen substitution in nitrogen site is the most favorable state in N-rich condition. The electronic property of complex with negative charge states at $\Gamma$ point was changed from semiconductor to metal. On the contrary, the properties of nitrogen vacancy except for neutral charge state have shown the semiconductor characteristics at $\Gamma$ point. In the oxygen substitution on nitrogen site, the energy differences between conduction band minimum and Fermi level were smaller than that of defect-free GaN.

• Journal of Magnetics
• /
• 제21권1호
• /
• pp.6-11
• /
• 2016

The magnetic anisotropy energy (MAE) and the saturation magnetization $B_s$ of (110) $Fe_nCo_n$ heterostructures with n = 1, 2, and 3 are investigated in first-principles within the density functional theory by using the precise full-potential linearized augmented plane wave (FLAPW) method. We compare the results employing two different exchange correlation potentials, that is, the local density approximation (LDA) and the generalized gradient approximation (GGA), and include the spin-orbit coupling interaction of the valence states in the second variational way. The MAE is found to be enhanced significantly compared to those of bulk Fe and Co and the magnetic easy axis is in-plane in agreement with experiment. Also the MAE exhibits the in-plane angle dependence with a two-fold anisotropy showing that the $[1{\overline{I}}0]$ direction is the most favored spin direction. We found that as the periodicity increases, (i) the saturation magnetization $B_s$ decreases due to the reduced magnetic moment of Fe far from the interface, (ii) the strength of in-plane preference of spin direction increases yielding enhancement of MAE, and (iii) the volume anisotropy coefficient decreases because the volume increase outdo the MAE enhancement.

• 한국재료학회지
• /
• 제30권12호
• /
• pp.666-671
• /
• 2020

The electronic structure and magnetic properties of chalcopyrite (CH) AlGaAs2 with dopant Mn at 3.125 and 6.25 % concentrations are investigated using first-principles calculations. The CH AlGaAs2 alloy is a p-type semiconductor with a small band-gap. The AlGaAs2:Mn shows that the ferromagnetic (FM) state is the most energetically favorable one. The Mn-doped AlGaAs2 exhibits FM and strong half-metallic ground states.The spin polarized Al(Ga,Mn)As2 state (Al-rich system) is more stable than the (Al,Mn)GaAs2 state (Ga-rich system), which has a magnetic moment of 3.82mB/Mn. The interaction between Mn-3d and As-4p states at the Fermi level dominates the other states.The states at the Fermi level are mainlyAs-4p electrons, which mediate strong interaction between the Mn-3d and As-4p states. It is noticeable that the FM ordering of dopant Mn with high magnetic moment originates from the As(4p)-Mn(3d)-As(4p) hybridization, which is attributed to the partially unfilled As-4pbands. The high FM moment of Mn is due to the double-exchange mechanism mediated by valence-band holes.

• 한국표면공학회:학술대회논문집
• /
• 한국표면공학회 2017년도 춘계학술대회 논문집
• /
• pp.126-126
• /
• 2017

Using first principles calculations we unveil fundamental mechanism of hydrolysis reactions of two hazardous chemicals $PCl_3$ and $POCl_3$ with molecular water clusters nearby. It is found that the water molecules play a key role as a catalyst significantly lowing the activation barriers by transferring its protons to the reaction intermediates. Interestingly, torsional angles of molecular complexes at transition states are identified as a vital descriptor on the reaction rate. Analysis of charge distribution over the complexes further reinforces the finding with atomic level correlation between the torsional angle and variation of the orbital hybridization state of P in the complex. Electronic charge separation (or polarization) enhances thermodynamic stability of the activated complex at transition state and reduces the activation energy through hydrogen bonding network with water molecules nearby. Calculated potential energy surfaces (PES) for the hydrolysis reactions of $PCl_3$ and $POCl_3$ depict their two contrastingly different profiles of double- and triple-deep wells, respectively. It is ascribed to the unique double-bonding O=P in the $POCl_3$. Our results on the activation free energy show well agreements with previous experimental data within $7kcalmol^{-1}$ deviation.

• 반도체디스플레이기술학회지
• /
• 제19권3호
• /
• pp.49-54
• /
• 2020

We studied the electronic and magnetic properties for the Mn-doped chalcopyrite (CH) AlAs, GaAs, and AlGaAs₂ semiconductor by using the first-principles calculations. The chalcopyrite AlGaP₂, AlGaAsP, and AlGaAs₂ compounds have a semiconductor characters with a small band-gap. The interaction between Mn-3d and As-4p states at the Fermi level dominate rather than the other states. The ferromagnetic ordering of dopant Mn with high magnetic moment is induced due to the Mn(3d)-As(4p) strong coupling, which is attributed by the partially filled As-4p bands. The holes are mediated with keeping their 3d-electrons, therefore the ferromagnetic state is stabilized by this double-exchange mechanism. We noted that the ferromagnetic state with high magnetic moment is originated from the hybridized As(4p)-Mn(3d)-As(4p) interaction mediated by the holes-carrier.

• 한국표면공학회:학술대회논문집
• /
• 한국표면공학회 2016년도 추계학술대회 논문집
• /
• pp.97-97
• /
• 2016

Development of advanced Li-ion battery cells with high durability is critical for safe operation, especially in applications to electric vehicles and portable electronic devices. Understanding fundamental mechanism on the formation of a solid-electrolyte interphase (SEI) layer, which plays a substantial role in the electrochemical stability of the Li-ion battery, in a cathode was rarely reported unlike in an anode. Using first-principles density functional theory (DFT) calculations and ab-initio molecular dynamic (AIMD) simulations we demonstrate atomic-level process on the generation of the SEI layer at the interface of a carbonate-based electrolyte and a spinel $LiMn_2O_4$ cathode. To accomplish the object we calculate the energy band alignment between the work function of the cathode and frontier orbitals of the electrolyte. We figure out that a proton abstraction from the carbonate-based electrolyte is a critical step for the initiation of an SEI layer formation. Our results can provide a design concept for stable Li-ion batteries by optimizing electrolytes to form proper SEI layers.

• Coupled systems mechanics
• /
• 제6권1호
• /
• pp.29-40
• /
• 2017

This work employed density functional theory to investigate the structural and ferroelectric properties of the Ruddlesden-Popper (RP) phase of lead titanate, $Pb_2TiO_4$, as well as its phase transitions with epitaxial strain. A wealth of novel structural instabilities, which are absent in the host $PbTiO_3$ material, were identified in the RP phase through phonon soft-mode analysis. Our calculations showed that the ground state of $Pb_2TiO_4$ is antiferroelectric, distinct from the dominant ferroelectric phase in the corresponding host material. In addition, applied epitaxial strain was found to play a key role in the interactions among the instabilities. The induction of a sequence of antiferroelectric and antiferrodistortive (AFD) phase transitions by epitaxial strain was demonstrated, in which the ferroic instability and AFD distortion were cooperative rather than competitive, as is the case in the host $PbTiO_3$. The RP phase in conjunction with strain engineering thus represents a new approach to creating ferroic orders and modifying the interplay among structural instabilities in the same constituent materials, enabling us to tailor the functionality of perovskite oxides for novel device applications.

• Journal of Magnetics
• /
• 제13권4호
• /
• pp.115-119
• /
• 2008

The authors predicted that $Co_2HfSi$, a $Co_2$-based full Heusler alloy, is being a half-metallic ferromagnet by first-principles calculations using the all electron full-potential linearized augmented plane wave method which adopts the generalized gradient approximation. The integer value of the calculated total magnetic moment of 2.00 ${\mu}_B$ per formula unit and a spin gap of 0.69 eV in spin down state confirmed the half-metallicity of bulk $Co_2HfSi$. For the $Co_2HfSi$(001) surface, we considered two possible surface terminations, namely, Co terminated and HfSi terminated surfaces. It was found that half-metallicity was retained at the HfSi-terminated surface but not at the Co-terminated surface. The magnetic moment of surface Co atoms in the Co-terminated surface was slightly lower than that of Co atoms in deep inner-layers, whereas the magnetic moments of Hf and Si atoms at the HfSi-terminated surface were almost same as those in deep inner-layers.