We investigate the optical control possibilities of spin-valley qubit carried by single electrons localized in nanostructures of monolayer TMDs, including small quantum dots formed by lateral heterojunction and charged impurities. The quantum controls are discussed when the confinement induces valley hybridization and when the valley hybridization is absent. We show that the bulk valley and spin optical selection rules can be inherited in different forms in the two scenarios, both of which allow the definition of spin-valley qubit with desired optical controllability. We also investigate nuclear spin induced decoherence and quantum control of electron-nuclear spin entanglement via intervalley terms of the hyperfine interaction. Optically controlled two-qubit operations in a single quantum dot are discussed.
Controlled flow of spin and valley pseudospin is key to future electronics exploiting these internal degrees of freedom of carriers. Here we discover a universal possibility for generating spin and valley currents by electric bias or temperature gradient only, which arises from the anisotropy of Fermi pockets in crystalline solids. We find spin and valley currents to the second order in the electric field, as well as their thermoelectric counterparts, i.e. the nonlinear spin and valley Seebeck effects. These second-order nonlinear responses allow two unprecedented possibilities to generate pure spin and valley flows without net charge current: (i) by an AC bias; or (ii) by an arbitrary inhomogeneous temperature distribution. As examples, we predict appreciable nonlinear spin and valley currents in two-dimensional (2D) crystals including graphene, monolayer and trilayer transition metal dichalcogenides, and monolayer gallium selenide. Our finding points to a new route towards electrical and thermal generations of spin and valley currents for spintronic and valleytronic applications based on 2D quantum materials.
Ionic covalent organic frameworks
(COFs) consisting of an anionic
or cationic skeleton and corresponding counterions have demonstrated
great potential in many application fields such as ion conduction,
molecular separation, and catalysis. However, arranging anionic and
cationic groups into the same COF to form zwitterionic materials is
still unexplored. Herein we design the synthesis of three zwitterionic
COFs as attractive porous hosts for SO2/CO2 separation
and anhydrous proton conduction. The separated cationic and anionic
groups in zwitterionic COFs’ channels can act as two different
polar sites for SO2 adsorption, allowing zwitterionic COFs
to achieve a high SO2 adsorption capacity (216 mL/g, 298
K) and impressive SO2/CO2 selectivity (118,
298 K). Furthermore, after loading with triazole/imidazole, the zwitterionic
groups in COFs’ channels can induce complete proton carrier
deprotonation, producing more freely migrating protons. The free protons
migrate along a continuous hydrogen-bonding network in zwitterionic
COFs’ channels, leading to outstanding anhydrous proton conductivity
up to 4.38 × 10–2 S/cm, which is much higher
than other N-heterocyclic-doped porous materials under anhydrous conditions.
Proton dissociation energy calculations combined with frequency-dependent
dielectric analysis give insight into the role of zwitterionic COFs
for proton conductivity. Our work provides the possibility to design
well-defined zwitterionic frameworks for gas separation and ion conduction.
The proton dissociation degrees and dielectric properties of proton carriers doped COFs with neutral, polar, Lewis base and positively charged sites are investigated to get better understanding of structure–conductivity relationship.
The hydrogen evolution reaction (HER) from water through electrocatalysis using highly active noble metal-free catalysts as an alternative to precious Pt-based catalysts holds great promise for clean and renewable energy systems. Here, MoC nanosheets well-regulated in N-doped carbon (MCNS/NC)are facilely achieved by in situ confining carburization of a Mo-based inorganic-organic lamellar mesostructure at 900 °C, and demonstrated for the first time as an ultra-efficient and durable HER catalyst. The MCNS/NC displays a very low onset overpotential of ∼0 mV and an overpotential of 19 mV at 10 mA cm with a small Tafel slope of 28.9 mV dec in acid media, which is remarkably superior to most other transition metal-based catalysts and comparable to commercial Pt/C. The HER kinetics are further studied by EIS and Tafel slope analysis, suggesting the dominant Volmer-Tafel mechanism. An "outside-in" carburization mechanism for the evolution of MCNS/NC is proposed through detailed investigation of the samples annealed at different temperatures. This study highlights the importance of designing well-regulated functional nanostructures combined with a conductive carbonaceous matrix for versatile applications.
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