The discovery of a two-dimensional electron gas (2DEG) at the LaAlO/SrTiO interface has resulted in the observation of many properties not present in conventional semiconductor heterostructures, and so become a focal point for device applications. Its counterpart, the two-dimensional hole gas (2DHG), is expected to complement the 2DEG. However, although the 2DEG has been widely observed , the 2DHG has proved elusive. Herein we demonstrate a highly mobile 2DHG in epitaxially grown SrTiO/LaAlO/SrTiO heterostructures. Using electrical transport measurements and in-line electron holography, we provide direct evidence of a 2DHG that coexists with a 2DEG at complementary heterointerfaces in the same structure. First-principles calculations, coherent Bragg rod analysis and depth-resolved cathodoluminescence spectroscopy consistently support our finding that to eliminate ionic point defects is key to realizing a 2DHG. The coexistence of a 2DEG and a 2DHG in a single oxide heterostructure provides a platform for the exciting physics of confined electron-hole systems and for developing applications.

Epitaxial growth suffers from the mismatches in lattice and dangling bonds arising from different crystal structures or unit cell parameters. Here, we demonstrate the epitaxial growth of 2D MoS ribbon on 1D CdS nanowires (NWs) via surface and subsurface defects. The interstitial Cd in the (12̅10) crystal plane of the [0001]-oriented CdS NWs are found to serve as nucleation sites for interatomically bonded [001]-oriented MoS, where the perfect lattice match (∼99.7%) between the (101̅1) plane of CdS and the (002)-faceted in-plane MoS result in coaxial MoS ribbon/CdS NWs heterojunction. The coaxial but heterotropic epitaxial MoS ribbon on the surface of CdS NWs induces delocalized interface states that facilitate charge transport and the reduced surface state. A less than 5-fold ribbon width of MoS as hydrogen evolution cocatalyst exhibits a ∼10-fold H evolution enhancement than state of the art Pt in an acidic electrolyte, and apparent quantum yields of 79.7% at 420 nm, 53.1% at 450 nm, and 9.67% at 520 nm, respectively.

Alloying two-dimensional transition metal dichalcogenides (2D TMDs) is a promising avenue for band gap engineering. In addition, developing a scalable synthesis process is essential for the practical application of these alloys with tunable band gaps in optoelectronic devices. Here, we report the synthesis of optically uniform and scalable single-layer MoW S alloys by a two-step chemical vapor deposition (CVD) method followed by a laser thinning process. The amount of W content ( x) in the MoW S alloy is systemically controlled by the co-sputtering technique. The post-laser process allows layer-by-layer thinning of the MoW S alloys down to a single-layer; such a layer exhibits tunable properties with the optical band gap ranging from 1.871 to 1.971 eV with variation in the W content, x = 0 to 1. Moreover, the predominant exciton complexes, trions, are transitioned to neutral excitons with increasing W concentration; this is attributed to the decrease in excessive charge carriers with an increase in the W content of the alloy. Photoluminescence (PL) and Raman mapping analyses suggest that the laser-thinning of the MoW S alloys is a self-limiting process caused by heat dissipation to the substrate, resulting in spatially uniform single-layer MoW S alloy films. Our findings present a promising path for the fabrication of large-scale single-layer 2D TMD alloys and the design of versatile optoelectronic devices.

Despite a longstanding controversy surrounding TiO materials, TiO polymorphs with heterojunctions composed of anatase and rutile outperform individual polymorphs because of the type-II energetic band alignment at the heterojunction interface. Improvement in photocatalysis has also been achieved via black TiO with a thin disorder layer surrounding ordered TiO. However, localization of this disorder layer in a conventional single TiO nanoparticle with the heterojunction composed of anatase and rutile has remained a big challenge. Here, we report the selective positioning of a disorder layer of controlled thicknesses between the anatase and rutile phases by a conceptually different synthetic route to access highly efficient novel metal-free photocatalysis for H production. The presence of a localized disorder layer within a single TiO nanoparticle was confirmed for the first time by high-resolution transmission electron microscopy with electron energy-loss spectroscopy and inline electron holography. Multiple heterojunctions in single TiO nanoparticles composed of crystalline anatase/disordered rutile/ordered rutile layers give the nanoparticles superior electron/hole separation efficiency and novel metal-free surface reactivity, which concomitantly yields an H production rate that is ∼11-times higher than that of Pt-decorated conventional anatase and rutile single heterojunction TiO systems.

The breaking of symmetry across an oxide heterostructure causes the electronic orbitals to be reconstructed at the interface into energy states that are different from their bulk counterparts . The detailed nature of the orbital reconstruction critically affects the spatial confinement and the physical properties of the electrons occupying the interfacial orbitals. Using an example of two-dimensional electron liquids forming at LaAlO/SrTiO interfaces with different crystal symmetry, we show that the selective orbital occupation and spatial quantum confinement of electrons can be resolved with subnanometre resolution using inline electron holography. For the standard (001) interface, the charge density map obtained by inline electron holography shows that the two-dimensional electron liquid is confined to the interface with narrow spatial extension (~1.0 ± 0.3 nm in the half width). On the other hand, the two-dimensional electron liquid formed at the (111) interface shows a much broader spatial extension (~3.3 ± 0.3 nm) with the maximum density located ~2.4 nm away from the interface, in excellent agreement with density functional theory calculations.

Mixed phase 2D Mo0.5W0.5S2 catalyst exhibits multifunctional catalytic performance in cathode resulting in high performance Li–S batteries.

We report wafer-scale growth of atomically thin, three-dimensional (3D) van der Waals (vdW) semiconductor membranes. By controlling the growth kinetics in the near-equilibrium limit during metal-organic chemical vapor depositions of MoS2 and WS2 monolayer (ML) crystals, we have achieved conformal ML coverage on diverse 3D texture substrates, such as periodic arrays of nanoscale needles and trenches on quartz and SiO2/Si substrates. The ML semiconductor properties, such as channel resistivity and photoluminescence, are verified to be seamlessly uniform over the 3D textures and are scalable to wafer scale. In addition, we demonstrated that these 3D films can be easily delaminated from the growth substrates to form suspended 3D semiconductor membranes. Our work suggests that vdW ML semiconductor films can be useful platforms for patchable membrane electronics with atomic precision, yet large areas, on arbitrary substrates.

In heterogeneous catalysts, metal−oxide interactions occur spontaneously but often in an undesired way leading to the oxidation of metal nanoparticles. Manipulating such interactions to produce highly active surface of metal nanoparticles can warrant the optimal catalytic activity but has not been established to date. Here we report that a prior reduced TiO 2 support can reverse the interaction with Pt nanoparticles and augment the metallic state of Pt, exhibiting a 3-fold increase in hydrogen production rate compared to that of conventional Pt/TiO 2 . Spatially resolved electron energy loss spectroscopy of the Ti valence state and the electron density distribution within Pt nanoparticles provide direct evidence supporting that the Pt/TiO 2 /H 2 O triple junctions are the most active catalytic sites for water reduction. Our reverse metal−oxide interaction scheme provides a breakthrough in the stagnated hydrogen production efficiency and can be applied to other heterogeneous catalyst systems composed of metal nanoparticles with reducible oxide supports.