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Register a new userTheoretical insight into the interaction of O2 and H2O molecules with the perfect and the defective InSe monolayers
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By using first-principles calculation, the interaction of O2 and H2O molecules with the pristine and the defective InSe monolayers is studied. It is predicted that the single Se and In vacancies exhibit significantly enhanced chemical activity toward the adsorbates compared with the perfect InSe lattice site, and the Se vacancies have a much higher chemical activity than the In vacancies. H2O molecule should be only physisorbed on the various InSe monolayers at ambient conditions, according to the calculated energies. The doping of the various InSe monolayers is discussed by the physisorbed H2O. The vacancies show a much higher chemical activity toward O2 than H2O. Although O2 molecules are still physisorbed on the pristine InSe monolayer, they will be chemisorbed on the defective InSe monolayers. Especially, our calculated energies suggest that the surface oxidation of the 2D InSe semiconductor should be dominated by the defects that expose under-coordinated host atoms, especially In atoms. Our theoretical results can help better understanding the doping and the oxidation of the 2D InSe semiconductor under ambient conditions.
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In layered LiNixMnyCozO2 cathode material for lithium-ion batteries, the spins of transition metal (TM) ions construct a two-dimensional triangular networks, which can be considered as a simple case of geometrical frustration. By performing neutron powder diffraction experiments and magnetization measurements, we find that long-range magnetic order cannot be established in LiNixMnyCozO2 even at low temperature of 3 K. Remarkably, the frustration parameters of these compounds are estimated to be larger than 30, indicating the existence of strongly frustrated magnetic interactions between spins of TM ions. As frustration will inevitably give rise to lattice instability, the formation of Li/Ni exchange in LiNixMnyCozO2 will help to partially relieve the degeneracy of the frustrated magnetic lattice by forming a stable antiferromagnetic state in hexagonal sublattice with nonmagnetic ions located in centers of the hexagons. Moreover, Li/Ni exchange will introduce 180{deg} superexchange interaction, which further relieves the magnetic frustration through bringing in new exchange paths. Thus, the variation of Li/Ni exchange ratio vs. TM mole fraction in LiNixMnyCozO2 with different compositions can be well understood and predicted in terms of magnetic frustration and superexchange interactions. This provides a unique viewpoint to study the Li/Ni ions exchange in layered Li(NixMnyCoz)O2 cathode materials.
Understanding the reactions in M-O2 cells (M = Li or Na) is of great importance for further advancement of this promising technology. Computational modelling can be helpful along this way, but an adequate approach is needed to model such complex systems. We propose a new scheme for modelling processes in M-O2 cells, where reference energies are obtained from high-level theory, CCSD(T), while the interactions of reaction intermediates with catalyst surfaces are extracted from computationally less expensive DFT. The approach is demonstrated for the case of graphene-based surfaces as model catalysts in Li-O2 and Na-O2 cells using the minimum viable mechanism. B-doped graphene was identified as the best catalyst among considered surfaces, while pristine graphene performs poorly. Moreover, we show that the inclusion of dispersion corrections for DFT has a significant impact on calculated discharge and charge potentials and suggests that long-range dispersion interactions should always be considered when graphene-based materials are modelled as electrocatalysts. Finally, we offer general guidelines for designing new ORR catalysts for M-O2 cells in terms of the optimization of the interactions of catalyst surface with reaction intermediates.
Aims. In this paper we investigate the detectability of the molecular oxygen in icy dust grain mantles towards astronomical objects. Methods. We present a systematic set of experiments with O2-H2O ice mixtures designed to disentangle how the molecular ratio affects the O2 signature in the mid- and near-infrared spectral regions. All the experiments were conducted in a closed-cycle helium cryostat coupled to a Fourier transform infrared spectrometer. The ice mixtures comprise varying thicknesses from 8 $times$ 10$^{-3}$ to 3 $mu$m. The absorption spectra of the O2-H2O mixtures are also compared to the one of pure water. In addition, the possibility to detect the O2 in icy bodies and in the interstellar medium is discussed. Results. We are able to see the O2 feature at 1551 cm$^{-1}$ even for the most diluted mixture of H2O : O2 = 9 : 1, comparable to a ratio of O2/H2O = 10 % which has already been detected in situ in the coma of the comet 67P/Churyumov-Gerasimenko. We provide an estimate for the detection of O2 with the future mission of the James Webb Space Telescope (JWST).
Oxygen is widely used to tune the performance of chalcogenide phase-change materials in the usage of phase-Change random access memory (PCRAM) which is considered as the most promising next-generation non-volatile memory. However, the microscopic role of oxygen in the write-erase process, i.e., the reversible phase transition between crystalline and amorphous state of phase-change materials is not clear yet. Using oxygen doped GeTe as an example, this work unravels the role of oxygen at the atomic scale by means of ab initio total energy calculations and ab initio molecular dynamics simulations. Our main finding is that after the amorphization and the subsequent re-crystallization process simulated by ab initio molecular dynamics, oxygen will drag one Ge atom out of its lattice site and both atoms stay in the interstitial region near the Te vacancy that was originally occupied by the oxygen, forming a dumbbell-like defect (O-VTe-Ge), which is in sharp contrast to the results of ab initio total energy calculations at 0 K showing that the oxygen prefers to substitute Te in crystalline GeTe. This specific defect configuration is found to be responsible for the slower crystallization speed and hence the improved data retention of oxygen doped GeTe as reported in recent experimental work. Moreover, we find that the oxygen will increase the effective mass of the carrier and thus increases the resistivity of GeTe. Our results unravel the microscopic mechanism of the oxygen-doping optimization of phase-change material GeTe, and the present reported mechanism can be applied to other oxygen doped ternary chalcogenide phase-change materials.
The internal quantum efficiency (IQE) of an organic photovoltaic device (OPV) is proportional to the number of free charge carriers generated and their conductivity, per absorbed photon. However, both the IQE and the quantities that determine it, for example, electron-hole binding, charge separation, electron-hole recombination, and conductivity, can only be inferred indirectly from experiments. Using density functional theory, we calculate the excited-state formation energy, charge transfer, and zero-bias conductance in the singlet ground state and triplet excited state across polymer/fullerene and polymer/single walled carbon nanotube (SWNT) OPV donor/acceptor bulk heterojunctions. Specifically, we compare polythiophene (PT) and poly(3-methylthiophene-2,5-diyl) (P3MT) as donors and C$_{60}$ chains with (6,4), (6,5), and (10,5) SWNTs as acceptors. We find the conductivity increases substantially for both the excited triplet relative to the singlet ground state and for PT compared with P3MT due to the increased charge transfer and the resulting improvement in donor/acceptor level alignment. Similarly, the (6,4) SWNT, with a larger SWNT band gap and greater conductivity than fullerenes, provides the highest conductivities of 5 and 9% of the theoretical maximum for electron and hole carriers, respectively. This work has important implications for both the optimization of polymer/SWNT bulk heterojunctions and the design of new OPV bulk heterojunctions in silico.
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