174 Search Results

2D materials exhibiting in‐plane anisotropy enable new applications in directional energy transport and polarized optical response. Silver phenylchalcogenides (AgEPh) – including mithrene (AgSePh), tethrene (AgTePh), and thiorene (AgSPh) – represent an exciting new addition to this family, with optical response spanning the visible to near‐UV. Here, excitonic anisotropy is predicted and characterized in this family of materials using a combination of ab initio theory and optical micro‐spectroscopy of single‐crystalline flakes. Using density functional theory and GW with the Bethe–Salpeter equation calculations, it is revealed that all AgEPh compounds exhibit anisotropic electronic band structure and host multiple delocalized excitons with in‐planemore » anisotropy. Room‐temperature polarization‐resolved optical micro‐spectroscopy shows that orthogonally polarized excitons with similar energy lead to nearly isotropic absorption in AgSPh, whereas energy separation between excitonic resonances in AgSePh and AgTePh leads to strong absorption and emission anisotropy. Cryogenic reflectance micro‐spectroscopy further reveals exciton fine structure in AgSePh, reconciling the discrepancies between room‐temperature experiments and theoretical predictions. Finally, it is demonstrated that the optical response of thicker AgEPh crystals is influenced by photonic effects arising from finite crystal size. Overall, this work advances the understanding of the relationship between anisotropic structure, composition, and excitonic properties in AgEPh, providing a foundation for technological integration.« less

Molecular beam epitaxy (MBE) is a promising synthesis technique for both heterostructure growth and epitaxial integration of ferroelectric BaTiO₃. However, direct measurement of the remnant polarization (P_r) has not been previously reported in MBE-grown BaTiO₃ films. We report the in situ growth of an all-epitaxial SrRuO₃/BaTiO₃/SrRuO₃ heterostructure on Nb-doped SrTiO₃ (001) substrates by hybrid MBE using metal–organic precursors. This capacitor structure consisting of 16 nm SrRuO₃/40 nm BaTiO₃/16 nm SrRuO₃ shows hysteretic polarization–electric field (P–E) curves with P_r ∼ 15 μC cm^–2 at frequencies ranging from 500 Hz to 20 kHz, after isolating the intrinsic ferroelectric response from non-ferroelectric contributionsmore » using the Positive-Up-Negative-Down (PUND) method. We hypothesize that the asymmetry in switching behavior and current leakage has origins in structural defects. Furthermore, this work opens the door to defect-engineered ferroelectric BaTiO₃-based heterostructures grown by hybrid MBE for future electronic, photonic and spintronic applications.« less

Achieving deterministic control over the properties of low-dimensional materials with nanoscale precision is a long-sought goal. Mastering this capability has a transformative effect on the design of multifunctional electrical and optical devices. Here, we present an ion-assisted synthetic technique that enables precise control over the material composition and energy landscape of two-dimensional (2D) atomic crystals. Our method transforms binary transition-metal dichalcogenides, like MoSe2, into ternary MoS2αSe2(1-α) alloys with systematically adjustable compositions, α. By piecewise assembly of the lateral, compositionally modulated MoS2αSe2(1-α) segments within 2D atomic layers, we present a synthetic pathway toward the realization of multicompositional designer materials. Our techniquemore » enables the fabrication of advanced 2D structures with arbitrary boundaries, dimensions as small as 30 nm, and fully customizable energy landscapes. Our optical characterizations further showcase the potential for implementing tailored optoelectronics in these engineered 2D crystals.« less

Over the past two decades, graphene has been intensively studied because of its remarkable mechanical, optical, and electronic properties. Initial studies were enabled by manual “Scotch Tape” exfoliation; nearly two decades later, this method is still widely used to obtain chemically pristine flakes of graphene and other 2D van der Waals materials. Unfortunately, the yield of large, pristine flakes with uniform thickness is inconsistent. Thus, significant time and effort are required to exfoliate and locate flakes suitable for fabricating multilayer van der Waals heterostructures. Here, we describe a relatively affordable tabletop device (the “eXfoliator”) that can reproducibly control key parametersmore » and largely automate the exfoliation process. In a typical exfoliation run, the eXfoliator produces 3 or more large (≥ 400 μm²) high-quality graphene monolayer flakes, allowing new users to produce such flakes at a rate comparable to manual exfoliation by an experienced user. Furthermore, we use an automated mapping system and a computer vision algorithm to locate candidate flakes. Our results provide a starting point for future research efforts to identify more precisely which parameters matter for the success of exfoliation and to optimize them.« less

Double-perovskite oxides (DPOs) with two transition metal ions ($$A_{2}BB'O_{6}$$) offer a fascinating platform for exploring exotic physics and practical applications. Studying these DPOs as ultrathin epitaxial films on single crystalline substrates can add another dimension to engineering electronic, magnetic, and topological phenomena. Understanding the consequence of polarity mismatch between the substrate and the DPO would be the first step toward this broad goal. Here, we investigate this by studying the interface between a prototypical insulating DPO $$Nd_{2}NiMnO_{6}$$ and a wide band gap insulator $$SrTiO_{3}$$. The interface is found to be insulating in nature. By combining several experimental techniques and densitymore » functional theory, we establish a site-selective charge compensation process that occurs explicitly at the Mn site of the film, leaving the Ni sites inert. We further demonstrate that such surprising selectivity, which cannot be explained by existing mechanisms of polarity compensation, is directly associated with their electronic correlation energy scales. This study establishes the crucial role of Mott physics in polar compensation process and paves the way for designer doping strategies in complex oxides.« less

Excitation transfer across the interfaces between graphene, perylenetetracarboxylic diimide (PTCDI), and titanyl phthalocyanine (TiOPc) was studied by using transient absorption and photoluminescence spectroscopy. Both photoluminescence quenching and transient absorption measurements confirm the presence of a type-II interface between PTCDI and TiOPc. While the graphene/PTCDI interface is expected to exhibit type-I behavior, transient absorption measurements indicate that only electrons transfer from PTCDI to graphene, with no evidence of hole transfer. Density functional theory calculations reveal significant ground-state electron transfer from graphene to PTCDI, resulting in band bending that prevents excited holes from transferring from PTCDI to graphene. This feature is exploitedmore » in a trilayer heterostructure of graphene/PTCDI/TiOPc, where the spatial separation of photoexcited electrons and holes in graphene and TiOPc, respectively, leads to the formation of long-lived photoexcitations with a lifetime of approximately 500 ps. Furthermore, spatially resolved transient absorption measurements reveal the immobile nature of these excitations, confirming that they are charge-transfer excitons rather than free electrons and holes. Furthermore, these results provide valuable insights into the complex interlayer photoexcitation transfer properties and demonstrate precise control over the layer population and the recombination lifetime of photocarriers in such hybrid heterostructures.« less

We report an experimental investigation of photocarrier dynamics in a heterostructure formed by a WSe₂ monolayer stacked on a multilayer α-RuCl₃ flake. The samples were fabricated using mechanical exfoliation and dry transfer techniques. Photoluminescence measurements showed that the excitonic photoluminescence of WSe₂ is quenched by more than 3 orders of magnitude upon contact with α-RuCl₃. Transient absorption measurements revealed an ultrashort photocarrier lifetime of 0.5 ps in the heterostructure. Spatially resolved transient absorption microscopy measurements were employed to probe the transport properties of these photocarriers, resulting in a room-temperature diffusion coefficient of 370 cm² s^–1, which is higher than themore » exciton diffusion coefficients of most previously studied 2D semiconductors. Furthermore, these results suggest that α-RuCl₃-doped WSe₂ could be utilized in high-speed optoelectronic devices.« less

Sodium-ion batteries (SIBs) have become one of the most promising candidates for large-scale energy storage applications. Metal chalcogenides anode materials based on alloying or conversion reactions are widely studied because of their high theoretical capacities and rich redox reactions. However, their intrinsic limitations such as high voltage hysteresis and large volume expansion hinder their further applications. The construction of heterostructures has become an attractive strategy to alleviate the above issues. Here, the formation of built in electric fields (BIEFs) at the heterointerfaces will accelerate the migration of Na⁺ and electrons. Moreover, heterostructures can also enhance the structural stability, generate moremore » active sites and provide additional capacity. It is worth noting that heterointerfacial properties play a significant role in promoting the overall electrochemical performance of the heterostructures. However, a systematic understanding of their interfacial engineering is currently lacking. This article reviews the research progress of metal chalcogenides-based heterostructure anode materials in the near term. First, the definition, classification and the roles of heterostructures are introduced. Second, the detailed research progress of the metal chalcogenide-based heterostructures anodes in SIBs is discussed. Finally, the future prospects and potential research directions of the heterostructures for batteries are discussed.« less

We demonstrate an approach to creating nanoscale potentials in van der Waals layers integrated with a buried programmable ferroelectric layer. Using ultra-low-voltage electron beam lithography (ULV-EBL), we can program the ferroelectric polarization in Al_1–xB_xN (AlBN) thin films, generating structures with sizes as small as 35 nm. We demonstrate the ferroelectric field effect with a graphene/vdW stack on AlBN by creating a p–n junction. This resist-free, high-resolution, contactless patterning method offers a new pathway to integrate ferroelectric films with a wide range of two-dimensional layers including transition-metal dichalcogenides (TMD), enabling arbitrary programming and top-down creation of multifunctional devices.

In the hexagonal, corundum-like structure, α-Ga₂O₃ has a bandgap of ~5.1 eV, which, combined with its relatively small electron effective mass, high Baliga's figure of merit, and high breakdown field, makes it a promising candidate for power electronics. Ga₂O₃ is easy to dope n-type, but impossible to dope p-type, impeding the realization of some electronic device designs. Developing a lattice-matched p-type material that forms a high-quality heterojunction with n-type Ga₂O₃ would open new opportunities in electronics and perhaps optoelectronic devices. In this work, we studied Ir₂O₃ as a candidate for that purpose. Using hybrid density functional theory calculations we predictmore » the electronic band structure of α-Ir₂O₃ and compare that to α-Ga₂O₃, and study the stability and electronic properties of α-(Ir_xGa_1–x)₂O₃ alloys. We discuss the band offset between the two materials and compare it with recently available experimental data. We find that the Ir d bands that compose the top of the valence band in α-Ir₂O₃ are much higher in energy than O p bands in α-Ga₂O₃, possibly enabling effective p-type doping. Finally, our results provide an insight into using the Ir₂O₃ or Ir₂O₃-Ga₂O₃ alloys as p-type material lattice-matched to α-Ga₂O₃ for the realization of p–n heterojunctions.« less