Jongseob Kim

The Role of Intrinsic Defects in Methylammonium Lead Iodide Perovskite

One of the major merits of CH3NH3PbI3 perovskite as an efficient absorber material for the photovoltaic cell is its long carrier lifetime. We investigate the role of the intrinsic defects of CH3NH3PbI3 on its outstanding photovoltaic properties using density-functional studies. Two types of defects are of interest, i.e., Schottky defects and Frenkel defects. Schottky defects, such as PbI2 and CH3NH3I vacancy, do not make a trap state, which can reduce carrier lifetime. Elemental defects like Pb, I, and CH3NH3 vacancies derived from Frenkel defects act as dopants, which explains the unintentional doping of methylammonium lead halides (MALHs). The absence of gap states from intrinsic defects of MALHs can be ascribed to the ionic bonding from organic-inorganic hybridization. These results explain why the perovskite MALHs can be an efficient semiconductor, even when grown using simple solution processes. It also suggests that the n-/p-type can be efficiently manipulated by controlling growth processes.

Structures, binding energies, and spectra of isoenergetic water hexamer clusters: Extensive ab initio studies

We investigated five lowest energy structures of the water hexamer (ring, book, bag, cage, and prism) using extensive ab initio calculations. High levels of theory using various basis sets were employed. On the basis of Mo/ller–Plesset second order perturbation (MP2) calculations using a large basis set [9s6p4d2 f1g/6s4p2d]+diffuse(2sp/s), the lowest energy structure with zero point energy (ZPE) correction is the cage conformer, followed by the book (within 0.1 kcal/mol) and the prism (within 0.2 kcal/mol). The spectra of the five conformers have been investigated. The predicted rotational constants and dipole moments of the cage conformer are in good agreement with the experiment [Liu et al., Nature 381, 501 (1996)] as compared to other structures. This proves that the experiment surely found the cage structure, which was first reported by one of the authors [Kim et al., Chem. Phys. Lett. 131, 451 (1986)]. However, the five structures would still be nearly isoenergetic within 0.7 kcal/mol at 0 K. Above ∼...

Comparative ab initio study of the structures, energetics and spectra of X−⋅(H2O)n=1–4 [X=F, Cl, Br, I] clusters

X−⋅(H2O)n=1–4 [X=F, Cl, Br, I] have been studied using high level ab initio calculations. This extensive work compares the structures of the different halide water clusters and has found that the predicted minimum energy geometries for different cluster are accompanied by several other structures close to these global minima. Hence the most highly populated structures can change depending on temperature due to the entropy effect. As the potential surfaces are flat, the wide-ranging zero point vibrational effects are important at 0 K, and not only a number of low-lying energy conformers but also large amplitude motions can be important in determining structures, energies, and spectra at finite temperatures. The binding energies, ionization potentials, charge-transfer-to-solvent (CTTS) energies, and the O–H stretching frequencies are reported, and compared with the experimental data available. A distinctive difference between F−⋅(H2O)n and X−⋅(H2O)n (X=Cl, Br, I) is noted, as the former tends to favor inter...

STRUCTURES AND ENERGETICS OF THE WATER HEPTAMER : COMPARISON WITH THE WATER HEXAMER AND OCTAMER

In spite of a spate of studies of various water clusters, a few theoretical studies on the water heptamer are available. State-of-the-art ab initio calculations are thus carried out on twelve possible water heptamer structures to explore the conformation as well as spectroscopic properties of this water cluster. Two three-dimensional cagelike structures comprised of seven-membered cyclic rings with three additional hydrogen bondings were found to be the lowest-lying energy heptamer conformers. The global minimum energy structure was found to be 0.5 kcal/mol lower than the other. The zero-point energy uncorrected and corrected binding energies of the global minimum energy structure are 55.2 and 37.9 kcal/mol, respectively. An almost two-dimensional ring conformer, which is only 1 kcal/mol above the global minimum at 0 K, could be more stable above 150 K. The vibrational spectra of different heptamer conformers are discussed and compared with the spectra of the hexamer and octamer water clusters.

What is the global minimum energy structure of the water hexamer? The importance of nonadditive interactions

The global minimum energy structures of the water hexamer predicted by widely used analytic water potentials are very different from each other, while the cyclic hexamer does not appear to be a low‐lying energy structure. However, high levels of ab initio calculation predict that a number of low‐lying energy conformers including the cyclic conformer are almost all isoenergetic due to the balance of two‐body and nonadditive interactions. For modeling of water potentials, we suggest that the binding energy of the dimer be between −5.0 and −4.7 kcal (mol dimer)−1, while the three‐body corrections be taken into account to a large extent.

Structures, energetics, and spectra of aqua‐sodium(I): Thermodynamic effects and nonadditive interactions

Using extensive ab initio calculations including electron correlation, we have studied structures, thermodynamic quantities, and spectra of hydrated sodium ions [Na(H2O)+n (n=1–6)]. Various configurations were investigated to find the stable structures of the clusters. The vibrational frequency shifts depending on the number of water molecules were investigated along with the frequency characteristics depending on the presence/absence of outer‐shell water molecules. The thermodynamic quantities of the stable structures were compared with experimental data available. Entropy‐driven structures for n=5 and particularly for n=6 are noted in the calculations, which can explain the peculiar experimental thermal energies. On the other hand, the enthalpy effect to maximize the number of hydrogen bonds of the clusters with the surrounding water molecules seems to be the dominant factor to determine the primary hydration number of Na+ in aqueous solution. The nonadditive interactions in the clusters are found to be...

Strain-Driven Electronic Band Structure Modulation of Si Nanowires

One of the major challenges toward Si nanowire (SiNW) based photonic devices is controlling the electronic band structure of the Si nanowire to obtain a direct band gap. Here, we present a new strategy for controlling the electronic band structure of Si nanowires. Our method is attributed to the band structure modulation driven by uniaxial strain. We show that the band structure modulation with lattice strain is strongly dependent on the crystal orientation and diameter of SiNWs. In the case of [100] and [111] SiNWs, tensile strain enhances the direct band gap characteristic, whereas compressive strain attenuates it. [110] SiNWs have a different strain dependence in that both compressive and tensile strain make SiNWs exhibit an indirect band gap. We discuss the origin of this strain dependence based on the band features of bulk silicon and the wave functions of SiNWs. These results could be helpful for band structure engineering and analysis of SiNWs in nanoscale devices.

Charge transfer to solvent (CTTS) energies of small X−(H2O)n=1–4 (X=F, Cl, Br, I) clusters: Ab initio study

Charge transfer to solvent (CTTS) energies of small halide–water clusters, X−(H2O)n=1–4 (X=F, Cl, Br, I) have been studied using first-order configuration interaction as well as time dependent discrete Fourier transform density functional methods. The only available experimental data are the recently reported CTTS energies for I−(H2O)n=1–4 clusters by Johnson and co-workers [D. Serxner, C. E. H. Dessent, and M. A. Johnson, J. Chem. Phys. 105, 7231 (1996)]. These results are in good agreement with our predicted values. The calculated CTTS energies indicate that there is regularity in the change of CTTS energies with respect to the change of halide anion as well as the cluster size. Our investigations have shown that this observed trend of CTTS energies of X−(H2O)n clusters could be quantitatively explained by the ionization potential of the halide anions and the binding energies of the respective clusters.

Characteristics of high-k Al2O3 dielectric using ozone-based atomic layer deposition for dual-gated graphene devices

We present characteristics of dual-gated graphene devices with an Al2O3 gate dielectric formed by an O3-based atomic-layer-deposition process. Raman spectra reveal that a O3 process at 25°C on single-layered graphene introduces the least amount defects, while a substantial number of defects appear at 200 °C. This graphene device with O3-based Al2O3 dielectric demonstrates a heterojunction within the graphene sheet when applying VTG and VBG and possesses good dielectric properties with minimal chemical doping, including a high dielectric constant ∼8, low hysteresis width ∼0.2 V, and low leakage current and a carrier mobility of 5000 cm2/V s 25°C in ambient.

Structures, energetics, and spectra of fluoride–water clusters F−(H2O)n, n=1–6: Ab initio study

F−(H2O)n (n=1–6) clusters have been studied using ab initio calculations. This is an extensive work to search for various low-lying energy conformers, for example, including 13 conformers for n=6. Our predicted enthalpies and free energies are in good agreement with experimental values. For n=4 and 6, both internal and surface structures are almost isoenergetic at 0 K, while internal structures are favored with increasing temperature due to the entropic effect. For n=5, the internal structure is favored at both 0 and 298 K under 1 atm. These are contrasted to the favored surface structures in other small aqua–halide complexes. The ionization potential, charge-transferto-solvent (CTTS) energy, and O–H stretching vibrational spectra are reported to facilitate future experimental work. Many-body interaction potential analyses are presented to help improve the potential functions used in molecular simulations. The higher order many-body interaction energies are found to be important to compare the energetics ...