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It is well known that holes in oxides such as Ga2O3 localize and distort the lattice, forming small polarons. Here we explore the formation of polarons in (100) Ga2O3 nanolayers, which have been experimentally demonstrated. Using density functional theory at the hybrid functional level, we show that polarons can localize on oxygen atoms, with two of the symmetrically inequivalent (3-fold coordinated) O sites leading to stable polarons and the 4-fold coordinated position being metastable. This metastable polaron requires the breaking of a Ga–O bond and leads to a large displacement of a Ga atom. The combination of such a large displacement and a 2D layer should allow for experimental observation by using high-resolution transmission electron microscopy. 

(AlxGa1−x)2O3 alloys are frequently used in heterostructures with monoclinic Ga2O3, resulting in a large conduction-band offset, which leads to charge-carrier confinement, a property that is desirable for device applications. However, when (AlxGa1−x)2O3 alloys are n-type doped with Si, the most efficient shallow donor, there is a significant reduction in the number of charge carriers when the Al content of the alloys is greater than 26%, rendering intentional doping ineffective. Here, we show that this compensation is due to cation vacancies forming in response to donor doping. We use density functional theory with the HSE06 hybrid functional to study cation vacancies in monoclinic AlGaO3 and monoclinic Al2O3. We find that vacancies prefer to occupy split-vacancy configurations, similar to vacancies in Ga2O3. Furthermore, by comparing the formation energy of the vacancy with the formation energy of Si donors, we show that vacancies are lower in energy than Si donors, independent of the Fermi level, as soon as the alloys contain more than 16% Al under O-poor conditions. Therefore, cation vacancies will compensate the donor doping, explaining experimental observations. 

V₂O₅is a promising battery electrode material that has several polymorphs with very different structural properties, however, our hybrid density functional theory calculations show that all polymorphs have similar electronic properties. 

The moiré potential in rotationally misfit two-dimensional (2D) heterostructures has been used to build artificial exciton and electron lattices, which have become platforms for realizing exotic electronic phases. Here, we demonstrate a different approach to create a superlattice potential in 2D crystals by using the near field of an array of polar molecules. A bilayer of titanyl phthalocyanine (TiOPc), consisting of alternating out-of-plane dipoles, is deposited on monolayer MoS2. Time-resolved two-photon photoemission spectroscopy reveals a pair of interlayer exciton states with an energy difference of ∼0.1 eV, which is consistent with the electrostatic potential modulation induced by the TiOPc bilayer as determined by density functional theory calculations. Because the symmetry and the period of this potential superlattice can be changed readily by using molecules of different shapes and sizes, molecule/2D heterostructures can be promising platforms for designing artificial exciton and electron lattices. 

The effect of the energy valley on interlayer charge transfer in transition metal dichalcogenide (TMD) heterostructures is studied by transient absorption spectroscopy and density functional theory. First-principles calculations confirm that the Λmin valley in the conduction band of few-layer WSe2 evolves from above its K valley in the monolayer (1L) to below it in 4L. Heterostructure samples of 𝑛⁢L−WSe2/1⁢L−MoS2, where 𝑛=1,2,3, and 4, are obtained by mechanical exfoliation and dry transfer. Photoluminescence spectroscopy reveals a thickness-dependent WSe2 band structure and efficient interlayer charge transfer. Transient absorption measurements show that the electron transfer time from the Λmin valley of 4L WSe2 to the K valley of MoS2 is on the order of 30 ps. This process is much slower than the K-K charge transfer in 1L/1L TMD heterostructures. The momentum-indirect interlayer excitons formed after charge transfer have lifetimes >1 ns. 

Abstract Continuous device downsizing and circuit complexity have motivated atomic-scale tuning of memristors. Herein, we report atomically tunable Pd/M1/M2/Al ultrathin (<2.5 nm M1/M2 bilayer oxide thickness) memristors using in vacuo atomic layer deposition by controlled insertion of MgO atomic layers into pristine Al 2 O 3 atomic layer stacks guided by theory predicted Fermi energy lowering leading to a higher high state resistance (HRS) and a reduction of oxygen vacancy formation energy. Excitingly, memristors with HRS and on/off ratio increasing exponentially with M1/M2 thickness in the range 1.2–2.4 nm have been obtained, illustrating tunneling mechanism and tunable on/off ratio in the range of 10–104. Further dynamic tunability of on/off ratio by electric field is possible by designing of the atomic M2 layer and M1/M2 interface. This result probes ways in the design of memristors with atomically tunable performance parameters.