Among the layered transition metal dichalcogenides (TMDs) that can form stable two-dimensional crystal structures, molybdenum disulfide (MoS) has been intensively investigated because of its unique properties in various electronic and optoelectronic applications with different band gap energies from 1.29 to 1.9 eV as the number of layers decreases. To control the MoS layers, atomic layer etching (ALE) (which is a cyclic etching consisting of a radical-adsorption step such as Cl adsorption and a reacted-compound-desorption step via a low-energy Ar-ion exposure) can be a highly effective technique to avoid inducing damage and contamination that occur during the reactive steps. Whereas graphene is composed of one-atom-thick layers, MoS is composed of three-atom-thick S-Mo-S layers; therefore, the ALE mechanisms of the two structures are significantly different. In this study, for MoS ALE, the Cl radical is used as the adsorption species and a low-energy Ar ion is used as the desorption species. A MoS ALE mechanism (by which the S, Mo, and S atoms are sequentially removed from the MoS crystal structure due to the trapped Cl atoms between the S layer and the Mo layer) is reported according to the results of an experiment and a simulation. In addition, the ALE technique shows that a monolayer MoS field effect transistor (FET) fabricated after one cycle of ALE is undamaged and exhibits electrical characteristics similar to those of a pristine monolayer MoS FET. This technique is also applicable to all layered TMD materials, such as tungsten disulfide (WS), molybdenum diselenide (MoSe), and tungsten diselenide (WSe).

Recent theoretical studies on geometric and chemical modification strategies, band engineering, and charge carrier dynamics of TiO2 nanoparticles are discussed.

Investigation of charge carrier recombination dynamics is central to understanding and further enhancing the photocatalytic activity of water splitting and other photochemical reactions catalyzed by TiO 2 . In this study, we carried out nonadiabatic molecular dynamics calculations combined with real-time time-dependent density functional theory to investigate the effects of size and shape on charge recombination in TiO 2 nanoparticles (NPs). Using the Wulff construction method, we considered both octahedral (10, 35, and 84 TiO 2 units) and cuboctahedral (29, 78, and 97 TiO 2 units) nanoclusters with size varying from 1 to 3 nm. Generally, the recombination rates decreased with increasing NP size. We rationalized the trend in terms of average transition energy, exciton binding energy (ΔE ex ), nonadiabatic coupling (NAC), and pure-dephasing time. The relaxation times increased with increasing NP size, as the NAC and ΔE ex decreased. The cuboctahedral clusters showed smaller ΔE ex compared to the octahedral clusters. For the octahedral clusters, the smaller NAC and shorter dephasing time contributed to longer relaxation, despite smaller transition energy, as the size increased. However, the influence of the NAC, transition energy, and dephasing time were intertwined for the cuboctahedral clusters. The smaller NAC of the 97-unit cluster rationalized its longer relaxation time compared to the 29-unit cluster, but the presence of a singly coordinated oxygen atom greatly reduced the transition energy, thus leading to a shorter relaxation time compared to the 78-unit cluster. Our results provide a detailed understanding on the effects of size and shape on the charge carrier dynamics in TiO 2 nanoclusters, separating these effects from other factors, such as presence of defects, dopants, and adsorbates that are hard to control precisely in experiments.

Oxygen vacancies in TiO 2 nanoparticles are important for charge carrier dynamics, with recent studies reporting contradictory results on TiO 2 nanoparticle photocatalytic activity. We demonstrate that ground state multiplicity, defect levels, and formation energies depend strongly on vacancy location. Quantum dynamics simulations show that charges are trapped within several picoseconds and recombine over a broad range of time scales from tens of picoseconds to nanoseconds. Specifically, nanoparticles with missing partially coordinated surface oxygens showed fast recombination, while nanoparticles with missing highly coordinated subsurface oxygens or singly coordinated oxygens at tips showed slow recombination, even slower than in the pristine system. The results are rationalized by energy gaps and electron−hole localization, the latter determining nonadiabatic coupling and quantum coherence time. The diverse charge recombination scenarios revealed by the nonadiabatic dynamics simulations rationalize the contradictory experimental results for photocatalytic activity and provide guidelines for rational design of nanoscale metal oxides for solar energy harvesting and utilization.