We calculated the longitudinal acoustic phonon limited electron mobility of 14 two dimensional semiconductors with composition of MX 2 , where M (= Mo, W, Sn, Hf, Zr and Pt) is the transition metal, and X is S, Se and Te. We treated the scattering matrix by deformation potential approximation. We found that out of the 14 compounds, MoTe 2 , HfSe 2 and HfTe 2 , are promising regarding to the possible high mobility and finite band gap. The phonon limited mobility can be above 2500 cm 2 V −1 s −1 at room temperature.

Monolayer transition-metal dichalcogenides (TMDCs) MX2 (M = Mo, W, Zr, Hf, etc; X = S, Se, Te) have become well-known in recent times for their promising applications in thermoelectrics and field effect transistors. In this work, we perform a systematic study on the thermoelectric properties of monolayer ZrSe2 and HfSe2 using first-principles calculations combined with Boltzmann transport equations. Our results point to a competitive thermoelectric figure of merit (close to 1 at optimal doping) in both monolayer ZrSe2 and HfSe2, which is markedly higher than previous explored monolayer TMDCs such as MoS2 and MoSe2. We also reveal that the higher figure of merits arise mainly from their low lattice thermal conductivity, and this is partly due to the strong coupling of acoustic modes with low frequency optical modes. It is found that the figure of merits can be better optimized in n-type than in p-type. In particular, the performance of HfSe2 is superior to ZrSe2 at a higher temperature. Our results suggest that monolayer ZrSe2 and HfSe2 with lower lattice thermal conductivity than usual monolayer TMDCs are promising candidates for thermoelectric applications.

Neither of the two typical two-dimensional materials, graphene and single layer MoS2, are good enough for developing semiconductor logical devices. We calculated the electron mobility of 14 two-dimensional semiconductors with composition of MX2, where M (=Mo, W, Sn, Hf, Zr and Pt) are transition metals, and Xs are S, Se and Te. We approximated the electron phonon scattering matrix by deformation potentials, within which long wave longitudinal acoustical and optical phonon scatterings were included. Piezoelectric scattering in the compounds without inversion symmetry is also taken into account. We found that out of the 14 compounds, WS2, PtS2 and PtSe2 are promising for logical devices regarding the possible high electron mobility and finite band gap. Especially, the phonon limited electron mobility in PtSe2 reaches about 4000 cm2·V-1·s-1 at room temperature, which is the highest among the compounds with an indirect bandgap of about 1.25 eV under the local density approximation. Our results can be the first guide for experiments to synthesize better two-dimensional materials for future semiconductor devices.

For the first time, large-area, vertically oriented few-layered hafnium disulfide (V-HfS 2 ) nanosheets have been grown by chemical vapor deposition. The individual HfS 2 nanosheets are well [001] oriented, with highly crystalline quality. Far different from conventional van der Waals epitaxial growth mechanism for two-dimensional transition metal dichalcogenides, a novel dangling-bondassisted self-seeding growth mechanism is proposed to describe the growth of V-HfS 2 nanosheets: difficult migration of HfS 2 adatoms on substrate surface results in HfS 2 seeds growing perpendicularly to the substrate; V-HfS 2 nanosheets inherit the growth direction of HfS 2 seeds; V-HfS 2 nanosheets further expand in the in-plane direction with time evolution. Moreover, the V-HfS 2 nanosheets show strong and broadened photons absorption from near infrared to ultraviolet; the V-HfS 2 -based photodetector exhibits an ultrafast photoresponse time of 24 ms, and a high photosensitivity ca. 10 3 for 405 nm laser.

Besides its promising high electron mobilities at room temperature, PtSe2 has a finite bandgap sensitively dependent on the number of monolayers combined by the van der Waals interaction according to our calculations based on the density functional theory. It was found that the frontier orbitals of the valence band maximum and the conduction band minimum are mainly contributed by pz and px+y orbitals of Se, which are sensitive to the out-of-plane and the in-plane lattice constants, respectively. The van der Waals force enhances the bonding out-of-plane, which in turn influences the bonding in-plane. We explain that the layer number dependent bandgap has the same electronic reason as the strain dependent bandgap based on the scenario above. This work shows the flexibilities of tuning the electronic and optical properties of PtSe2 in a wide range, which provides an advantage for applications of PtSe2 in sensors.

Jahn‐Teller effect of C60 monoanion in the first electronically excited states was theoretically investigated. The orbital vibronic coupling parameters for t1g next lowest unoccupied molecular orbitals were derived from the Kohn‐Sham orbital levels calculated using a frozen phonon approach with both hybrid B3LYP and CAM‐B3LYP functionals, which take long‐range interaction correction into consideration. With these coupling parameters, the vibronic states of first excited C60− were derived by exactly diagonalizing dynamical Jahn‐Teller Hamiltonian. The results showed that dynamical Jahn‐Teller effects are more significant in the first excited C60− than those in the ground electronic states. This work also clarified that CAM‐B3LYP gives results closer to experimental data than B3LYP.

The superexchange theory predicts dominant antiferromagnetic kinetic interaction when the orbitals accommodating magnetic electrons are covalently bonded through diamagnetic bridging atoms or groups. Here we show that explicit consideration of magnetic and (leading) bridging orbitals, together with the electron transfer between the former, reveals a strong ferromagnetic kinetic exchange contribution. First-principles calculations show that it is comparable in strength with antiferromagnetic superexchange in a number of magnetic materials with diamagnetic metal bridges. In particular, it is responsible for a very large ferromagnetic coupling (−10 meV) between the iron ions in a Fe 3+-Co 3+-Fe 3+ complex. Furthermore, we find that the ferromagnetic exchange interaction turns into antiferromagnetic by substituting the diamagnetic bridge with magnetic one. The phenomenology is observed in two series of materials, supporting the significance of the ferromagnetic kinetic exchange mechanism.

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