Dongjea Seo

Membranes of complex oxides like perovskite SrTiO ₃ extend the multi‐functional promise of oxide electronics into the nanoscale regime of 2D materials. Here, it is demonstrated that freestanding oxide membranes supply a reconfigurable platform for nano‐photonics based on propagating surface phonon polaritons. Infrared near‐field imaging and spectroscopy enabled by a tunable ultrafast laser are applied to study pristine nano‐thick SrTiO ₃ membranes prepared by hybrid molecular beam epitaxy. As predicted by coupled mode theory, it is found that strong coupling of interfacial polaritons realizes symmetric and antisymmetric hybridized modes with simultaneously tunable negative and positive group velocities. By resolving reflection of these propagating modes from membrane edges, defects, and substrate structures, their dispersion is quantified with position‐resolved nano‐spectroscopy. Remarkably, polariton negative dispersion is found to be both robust and tunable through choice of membrane dielectric environment and thickness, and proposes a novel design for in‐plane Veselago lensing harnessing this control. This work lays the foundation for tunable transformation optics at the nanoscale using polaritons in a wide range of freestanding complex oxide membranes.

Doping allows precise tuning of the electronic properties in 2D materials, optimizing their performance for applications such as complementary metal‐oxide‐semiconductor (CMOS) technology. However, developing reliable p‐type 2D semiconductors remains challenging due to intrinsic defects or unintentional n‐type doping. This study presents robust p‐type monolayer WSe₂ field‐effect transistors (FETs) using phase‐engineered WSe₂/WSe_yO_x building blocks created via an atomic oxidation process (AOP). The findings reveal that when bilayer WSe₂ is exposed to AOP, the top layer undergoes self‐limited oxidation to WSe_yO_x with no detectable oxidation of the bottom layer. This result is confirmed by Raman spectroscopy, X‐ray photoelectron spectroscopy, and Kelvin probe force microscopy. This process has further been used to demonstrate a well‐controlled and fully encapsulated WSe_yO_x/WSe₂/WSe_yO_x heterostructure, ensuring symmetrical protection and stability of the WSe₂ channel region. The surface charge transfer doping using WSe_yO_x provides the capability to selectively modulate the carrier concentration in a WSe₂ without altering the intrinsic properties of the channel. This non‐destructive method simplifies the fabrication of p‐type 2D FETs with monolithic, phase‐engineered heterostructures, facilitating seamless integration into next‐generation device architectures.

2D semiconductors have shown great potential for application to electrically tunable optoelectronics. Despite the strong excitonic photoluminescence (PL) of monolayer transition metal dichalcogenides (TMDs), their efficient electroluminescence (EL) has not been achieved due to the low efficiency of charge injection and electron–hole recombination. Here, multioperation‐mode light‐emitting field‐effect transistors (LEFETs) consisting of a monolayer WSe₂ channel and graphene contacts coupled with two top gates for selective and balanced injection of charge carriers are demonstrated. Visibly observable EL is achieved with the high external quantum efficiency of ≈6% at room temperature due to efficient recombination of injected electrons and holes in a confined 2D channel. Further, electrical tunability of both the channel and contacts enables multioperation modes, such as antiambipolar, depletion,and unipolar regions, which can be utilized for polarity‐tunable field‐effect transistors and photodetectors. The work exhibits great potential for use in 2D semiconductor LEFETs for novel optoelectronics capable of high efficiency, multifunctions, and heterointegration.

A finite Schottky barrier and large contact resistance between monolayer MoS₂ and electrodes are the major bottlenecks in developing high-performance field-effect transistors (FETs) that hinder the study of intrinsic quantum behaviors such as valley-spin transport at low temperature. A gate-tunable graphene electrode platform has been developed to improve the performance of MoS₂ FETs. However, intrinsic misalignment between the work function of pristine graphene and the conduction band of MoS₂ results in a large threshold voltage for the FETs, because of which Ohmic contact behaviors are observed only at very high gate voltages and carrier concentrations (∼10¹³ cm⁻²). Here, we present high-performance monolayer MoS₂ FETs with Ohmic contact at a modest gate voltage by using a chemical-vapor-deposited (CVD) nitrogen-doped graphene with a high intrinsic electron carrier density. The CVD nitrogen-doped graphene and monolayer MoS₂ hybrid FETs platform exhibited a large negative shifted threshold voltage of −54.2 V and barrier-free Ohmic contact under zero gate voltage. Transparent contact by nitrogen-doped graphene led to a 214% enhancement in the on-current and a fourfold improvement in the field-effect carrier mobility of monolayer MoS₂ FETs compared with those of a pristine graphene electrode platform. The transport measurements, as well as Raman and X-ray photoelectron spectroscopy analyses before and after thermal annealing, reveal that the atomic C-N bonding in the CVD nitrogen-doped graphene is responsible for the dominant effects of electron doping. Large-scale nitrogen-doped graphene electrodes provide a promising device platform for the development of high-performance devices and the study of unique quantum behaviors.