Junhyeok Bang

Diffusion and thermal stability of hydrogen in ZnO

We perform both first-principles calculations and kinetic Monte Carlo (kMC) simulations to study the diffusion and thermal stability of hydrogen in ZnO. The migration energy of a substitutional hydrogen (HO) is 1.7eV, much higher than the value of 0.4–0.5eV for an interstitial hydrogen (Hi). Using as input the calculated energy barriers for H diffusion, kMC simulations show that while Hi diffuses out at low temperature, the thermal stability of HO is maintained up to 475°C, in good agreement with the annealing data. In addition, our calculations suggest that injected hydrogen from air turns into HO, causing n-type conductivity.

Electron-Rich Driven Electrochemical Solid-State Amorphization in Li-Si Alloys

The physical and chemical behaviors of materials used in energy storage devices, such as lithium-ion batteries (LIBs), are mainly controlled by an electrochemical process, which normally involves insertion/extraction of ions into/from a host lattice with a concurrent flow of electrons to compensate charge balance. The fundamental physics and chemistry governing the behavior of materials in response to the ions insertion/extraction is not known. Herein, a combination of in situ lithiation experiments and large-scale ab initio molecular dynamics simulations are performed to explore the mechanisms of the electrochemically driven solid-state amorphization in Li-Si systems. We find that local electron-rich condition governs the electrochemically driven solid-state amorphization of Li-Si alloys. This discovery provides the fundamental explanation of why lithium insertion in semiconductor and insulators leads to amorphization, whereas in metals, it leads to a crystalline alloy. The present work correlates electrochemically driven reactions with ion insertion, electron transfer, lattice stability, and phase equilibrium.

Understanding the presence of vacancy clusters in ZnO from a kinetic perspective

Vacancy clusters have been observed in ZnO by positron-annihilation spectroscopy (PAS), but detailed mechanisms are unclear. This is because the clustering happens in non-equilibrium conditions, for which theoretical method has not been well established. Combining first-principles calculation and kinetic Monte Carlo simulation, we determine the roles of non-equilibrium kinetics on the vacancies clustering. We find that clustering starts with the formation of Zn and O vacancy pairs (VZn − Vo), which further grow by attracting additional mono-vacancies. At this stage, vacancy diffusivity becomes crucial: due to the larger diffusivity of VZn compared to VO, more VZn-abundant clusters are formed than VO-abundant clusters. The large dissociation energy barriers, e.g., over 2.5 eV for (VZn − Vo), suggest that, once formed, it is difficult for the clusters to dissociate. By promoting mono-vacancy diffusion, thermal annealing will increase the size of the clusters. As the PAS is insensitive to VO donor defects, o...

Structural and electronic properties of crystalline InGaO3 (ZnO)(m)

Based on theoretical calculations, we find that the crystal structure of InGaO3(ZnO)m consists of an alternating stack of a wurtzite (Ga∕Zn)–O block and an In–O octahedral layer. In the (Ga∕Zn)–O block, the Ga atoms favor a modulated boundary structure against a flat boundary structure. The band spectrum shows that hole carriers are spatially confined whereas electrons move more freely through the whole crystal. The characteristics of a superlattice structure appears especially in the flat boundary structure. The band gap decreases with m due to the reduction in the quantum confinement effect.

Localization and one-parameter scaling in hydrogenated graphene

We report a metal-insulator transition in disordered graphene with low coverages of hydrogen atoms. Hydrogen interacting with graphene creates short-range disorder and localizes states near the neutrality point. The energy range of localization grows with increasing of H concentration. Calculations show that the conductances through low-energy propagating channels decay exponentially with sample size and are well fitted by one-parameter scaling function, similar to a disorder-driven metal-insulator transition in two-dimensional disordered systems.

Molecular doping of ZnO by ammonia: a possible shallow acceptor

Stable p-type doping of ZnO has been a major technical barrier for the application of ZnO in optoelectronic devices. While p-type conductivity for nitrogen-doped ZnO has been repeatedly reported, its origin remains mysterious. Here, using first-principles calculation, we predict that an ammonia molecule could counterintuitively assume a Zn site and form a substitutional defect, (NH3)Zn. By comparing with other molecular dopants (N2 and NO) on the Zn site and N on the O site (NO), we found that (NH3)Zn is thermodynamically the most stable defect under O-rich conditions. The stability is attributed to the formation of a strong dative bond of the ammonia molecule with a neighbouring O atom. The (NH3)Zn defect is neutral regardless of the Fermi level of the system, but it can capture a H donor forming (NH4)Zn, which becomes an acceptor. Experimental evidence for the existence of this Zn-site N acceptor is provided based on a comparison of calculated and measured N 1s X-ray photoelectron spectra. Accurately calculating the (0/−) transition level for this and other N-based acceptors has been hindered by the theoretical method used. Experimental studies are called for to clarify its (0/−) transition level.

Electronic structure and transport properties of hydrogenated graphene and graphene nanoribbons

The band gap opening is one of the important issues in applications of graphene and graphene nanoribbons (GNRs) to nanoscale electronic devices. As hydrogen strongly interacts with graphene and creates short-range disorder, the electronic structure is significantly modified by hydrogenation. Based on first-principles and tight-binding calculations, we investigate the electronic and transport properties of hydrogenated graphene and GNRs. In disordered graphene with low doses of H adsorbates, the low-energy states near the neutrality point are localized, and the degree of localization extends to high-energy states with increasing adsorbate density. To characterize the localization of eigenstates, we examine the inverse participation ratio and find that the localization is greatly enhanced for the defect levels, which are accumulated around the neutrality point. Our calculations support the previous result that even with a low dose of H adsorbates, graphene undergoes a metal–insulator transition. In GNRs, relaxations of the edge C atoms play a role in determining the edge structure and the hydrocarbon conformation at low and high H concentrations, respectively. In disordered nanoribbons, we find that the energy states near the neutrality point are localized and conductances through low-energy channels decay exponentially with sample size, similar to disordered graphene. For a given channel energy, the localization length tends to decrease as the adsorbate density increases. Moreover, the energy range of localization exceeds the intrinsic band gap.

Regulating energy transfer of excited carriers and the case for excitation-induced hydrogen dissociation on hydrogenated graphene

Understanding and controlling of excited carrier dynamics is of fundamental and practical importance, particularly in photochemistry and solar energy applications. However, theory of energy relaxation of excited carriers is still in its early stage. Here, using ab initio molecular dynamics (MD) coupled with time-dependent density functional theory, we show a coverage-dependent energy transfer of photoexcited carriers in hydrogenated graphene, giving rise to distinctively different ion dynamics. Graphene with sparsely populated H is difficult to dissociate due to inefficient transfer of the excitation energy into kinetic energy of the H. In contrast, H can easily desorb from fully hydrogenated graphane. The key is to bring down the H antibonding state to the conduction band minimum as the band gap increases. These results can be contrasted to those of standard ground-state MD that predict H in the sparse case should be much less stable than that in fully hydrogenated graphane. Our findings thus signify the importance of carrying out explicit electronic dynamics in excited-state simulations.

The effect of impurities on hydrogen bonding site and local vibrational frequency in ZnO

For isovalent impurities such as Be, Mg, Ca, Sr, and Cd and group-I element such as Na in ZnO, first-principles local-density-functional calculations show that the interstitial position of H depends on the type of impurities, either occupying a bond center (BC) site or an antibonding (AB) site adjacent to the impurity atom. The AB site is more favorable in the vicinity of Na, Ca, Sr, and Cd, while the stable position is the BC site in the case of Be. We find that both electronegativity and atomic size play a role in switching the H interstitial position between the BC and AB sites. Previous studies have suggested that two infrared lines observed at 3611 and 3326 cm−1 result from hydrogen atoms positioned at BC and AB sites, respectively. The results for the H bonding sites and defect concentrations suggest that Ca is the most probable impurity as the origin of the infrared line at 3326 cm−1. However, for impurities around which H is positioned at the AB site, the calculated local vibrational frequencies a...

Carrier-Multiplication-Induced Structural Change during Ultrafast Carrier Relaxation and Nonthermal Phase Transition in Semiconductors

While being extensively studied as an important physical process to alter exciton population in nanostructures at the fs time scale, carrier multiplication has not been considered seriously as a major mechanism for phase transition. Real-time time-dependent density functional theory study of Ge_{2}Sb_{2}Te_{5} reveals that carrier multiplication can induce an ultrafast phase transition in the solid state despite that the lattice remains cold. The results also unify the experimental findings in other semiconductors for which the explanation remains to be the 30-year old phenomenological plasma annealing model.