Layered two‐dimensional (2D) transition metal dichalcogenides (TMDCs) often form 2D sheets and some of these show an indirect to direct band gap transition as the number of layers decreases from that of the bulk structure. Recently, a new one‐dimensional (1D) material of Nb2Se9 is successfully prepared by solid state reaction. This material is semiconducting and composed of periodically stacked single‐chain atomic crystals (SCAC) where the SCACs form inorganic bulk crystals due to strong bonds within the chain but with weak inter‐chain interactions. To determine the potential applications of our newly developed 1D nanowire, theoretical prediction of its material properties is performed. As a first step, the band structures of bundles of Nb2Se9 SCACs, which are composed of 1–7 single chains, are calculated by using density functional theory. Unlike the bulk structure of Nb2Se9, the chain bundles composed of up to 21 single SCACs would have a direct band gap. Accordingly, it is expected that an Nb2Se9 bundle SCAC with a diameter of 3.6 nm can cause the electronic transition without being disturbed by the phononic environment due to the direct band gap, and can therefore be used in photoluminescence applications.

Recently, we synthesized a one-dimensional (1D) structure of V2Se9. The 1D V2Se9 resembles another 1D material, Nb2Se9, which is expected to have a direct band gap. To determine the potential applications of this material, we calculated the band structures of 1D and bulk V2Se9 using density functional theory by varying the number of chains and comparing their band structures and electronic properties with those of Nb2Se9. The results showed that a small number of V2Se9 chains have a direct band gap, whereas bulk V2Se9 possesses an indirect band gap, like Nb2Se9. We expect that V2Se9 nanowires with diameters less than ∼20 Å would have direct band gaps. This indirect-to-direct band gap transition could lead to potential optoelectronic applications for this 1D material because materials with direct band gaps can absorb photons without being disturbed by phonons.

We study excitation energy transfer (EET) in a model three-site system with a mixed-quantum classical dynamics method, by focusing on the effect of an underdamped vibration. We construct two types of models where the underdamped vibration mode is included either in the quantum subsystem or in the classical bath. We show that the two models yield practically equivalent results despite the different depictions of the vibration. In particular, both models consistently demonstrate accelerations of population relaxation induced by quasi-resonant vibration. This indicates that intricate features of EET dynamics that have been frequently ascribed to the quantal nature of vibrations, such as vibronic mixing, can be successfully reproduced by using physically equivalent but classically described bath modes. The mechanism behind the observed quantum-classical correspondence is proposed. We also systematically examine how the structure of the spectating continuum phonon modes affects the vibronic resonance and observe that phonon modes with different time scales influence the resonance in different manners.

The enlargement of the Stern layer distance caused by this ion exchange improves the dispersibility of (Mo3Se3−)∞ chains and also prevents the re-bundling and aggregation of nanowires in aqueous solutions, even at high concentrations (1 mg mL−1).