Atomic-Scale Imaging of Transferred Graphene Nanoribbons for Nanoelectronic Device Integration.

On-surface synthesis enables the fabrication of atomically precise graphene nanoribbons (GNRs) with properties defined by their shape and edge topology. While this bottom-up approach provides unmatched control over electronic and structural characteristics, integrating GNRs into functional electronic devices requires their transfer from noble metal growth surfaces to technologically relevant substrates. However, such transfers often induce structural modifications, potentially degrading or eliminating GNRs' desired functionality - a process that remains poorly understood.

Graphene-driven correlated electronic states in one dimensional defects within WS.

Tomonaga-Luttinger liquid (TLL) behavior in one-dimensional systems has been predicted and shown to occur at semiconductor-to-metal transitions within two-dimensional materials. Reports of one-dimensional defects hosting a Fermi liquid or a TLL have suggested a dependence on the underlying substrate, however, unveiling the physical details of electronic contributions from the substrate require cross-correlative investigation. Here, we study TLL formation within defectively engineered WS atop graphene, where band structure and the atomic environment is visualized with nano angle-resolved photoelectron spectroscopy, scanning tunneling microscopy and spectroscopy, and non-contact atomic force microscopy.

Permanent Dipole Moment in a Quantum-Confined Two-Dimensional Metal Revealed by Electric Double Layer Gating.

The tunable optical properties of metals through size-dependent quantum effects have attracted attention due to synthesis of chemically stable, ultrathin, and two-dimensional metals. Gate tunability, from the reduced screening of low-dimensional metals, adds an additional route for control over optical properties. Here, two-dimensional (2D) Ga is synthesized via confinement heteroepitaxy and patterned into electric-double-layer (EDL) gated transistors.

Layer-Dependent Charge-State Lifetime of Single Se Vacancies in WSe_{2}.

Defect engineering in two-dimensional semiconductors has been exploited to tune the optoelectronic properties and introduce new quantum states in the band gap. Chalcogen vacancies in transition metal dichalcogenides in particular have been found to strongly impact charge carrier concentration and mobility in 2D transistors as well as feature subgap emission and single-photon response. In this Letter, we investigate the layer-dependent charge-state lifetime of Se vacancies in WSe_{2}.

Route to Enhancing Remote Epitaxy of Perovskite Complex Oxide Thin Films.

Remote epitaxy is taking center stage in creating freestanding complex oxide thin films with high crystallinity that could serve as an ideal building block for stacking artificial heterostructures with distinctive functionalities. However, there exist technical challenges, particularly in the remote epitaxy of perovskite oxides associated with their harsh growth environments, making the graphene interlayer difficult to survive. Transferred graphene, typically used for creating a remote epitaxy template, poses limitations in ensuring the yield of perovskite films, especially when pulsed laser deposition (PLD) growth is carried out, since graphene degradation can be easily observed.

Large-Area Intercalated Two-Dimensional Pb/Graphene Heterostructure as a Platform for Generating Spin-Orbit Torque.

A scalable platform to synthesize ultrathin heavy metals may enable high-efficiency charge-to-spin conversion for next-generation spintronics. Here, we report the synthesis of air-stable, epitaxially registered monolayer Pb underneath graphene on SiC (0001) by confinement heteroepitaxy (CHet). Diffraction, spectroscopy, and microscopy reveal that CHet-based Pb intercalation predominantly exhibits a mottled hexagonal superstructure due to an ordered network of Frenkel-Kontorova-like domain walls.

Influence of Substrate-Induced Charge Doping on Defect-Related Excitonic Emission in Monolayer MoS.

Many applications of transition metal dichalcogenides (TMDs) involve transfer to functional substrates that can strongly impact their optical and electronic properties. We investigate the impact that substrate interactions have on free carrier densities and defect-related excitonic (X) emission from MoS monolayers grown by metal-organic chemical vapor deposition. C-plane sapphire substrates mimic common hydroxyl-terminated substrates.

Tailoring amorphous boron nitride for high-performance two-dimensional electronics.

Two-dimensional (2D) materials have garnered significant attention in recent years due to their atomically thin structure and unique electronic and optoelectronic properties. To harness their full potential for applications in next-generation electronics and photonics, precise control over the dielectric environment surrounding the 2D material is critical. The lack of nucleation sites on 2D surfaces to form thin, uniform dielectric layers often leads to interfacial defects that degrade the device performance, posing a major roadblock in the realization of 2D-based devices.

A substitutional quantum defect in WS discovered by high-throughput computational screening and fabricated by site-selective STM manipulation.

Point defects in two-dimensional materials are of key interest for quantum information science. However, the parameter space of possible defects is immense, making the identification of high-performance quantum defects very challenging. Here, we perform high-throughput (HT) first-principles computational screening to search for promising quantum defects within WS, which present localized levels in the band gap that can lead to bright optical transitions in the visible or telecom regime.

Air-Stable, Large-Area 2D Metals and Semiconductors.

Two-dimensional (2D) materials are popular for fundamental physics study and technological applications in next-generation electronics, spintronics, and optoelectronic devices due to a wide range of intriguing physical and chemical properties. Recently, the family of 2D metals and 2D semiconductors has been expanding rapidly because they offer properties once unknown to us. One of the challenges to fully access their properties is poor stability in ambient conditions.

Charge state-dependent symmetry breaking of atomic defects in transition metal dichalcogenides.

The functionality of atomic quantum emitters is intrinsically linked to their host lattice coordination. Structural distortions that spontaneously break the lattice symmetry strongly impact their optical emission properties and spin-photon interface. Here we report on the direct imaging of charge state-dependent symmetry breaking of two prototypical atomic quantum emitters in mono- and bilayer MoS by scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM).

Tuning the Fermi Level of Graphene by Two-Dimensional Metals for Raman Detection of Molecules.

Graphene-enhanced Raman scattering (GERS) offers great opportunities to achieve optical sensing with a high uniformity and superior molecular selectivity. The GERS mechanism relies on charge transfer between molecules and graphene, which is difficult to manipulate by varying the band alignment between graphene and the molecules. In this work, we synthesized a few atomic layers of metal termed two-dimensional (2D) metal to precisely and deterministically modify the graphene Fermi level.

Combined Experimental and Computational Insight into the Role of Substrate in the Synthesis of Two-Dimensional WSe.

Synthesis of large-area transition-metal dichalcogenides (TMDs) with controlled orientation is a significant challenge to their industrial applications. Substrate plays a vital role in determining the final quality of monolayer materials grown via the chemical vapor deposition process by controlling their orientation, crystal structure, and grain boundary. This study determined the binding energy and equilibrium distance for tungsten diselenide (WSe) monolayers on crystalline and amorphous silicon dioxide and aluminum dioxide substrates.

Tuning commensurability in twisted van der Waals bilayers.

Moiré superlattices based on van der Waals bilayers created at small twist angles lead to a long wavelength pattern with approximate translational symmetry. At large twist angles (θ), moiré patterns are, in general, incommensurate except for a few discrete angles. Here we show that large-angle twisted bilayers offer distinctly different platforms.

Spin-Stabilization by Coulomb Blockade in a Vanadium Dimer in WSe.

Charged dopants in 2D transition metal dichalcogenides (TMDs) have been associated with the formation of hydrogenic bound states, defect-bound trions, and gate-controlled magnetism. Charge-transfer at the TMD-substrate interface and the proximity to other charged defects can be used to regulate the occupation of the dopant's energy levels. In this study, we examine vanadium-doped WSe monolayers on quasi-freestanding epitaxial graphene, by high-resolution scanning probe microscopy and calculations.

Watching (De)Intercalation of 2D Metals in Epitaxial Graphene: Insight into the Role of Defects.

Intercalation forms heterostructures, and over 25 elements and compounds are intercalated into graphene, but the mechanism for this process is not well understood. Here, the de-intercalation of 2D Ag and Ga metals sandwiched between bilayer graphene and SiC are followed using photoemission electron microscopy (PEEM) and atomistic-scale reactive molecular dynamics simulations. By PEEM, de-intercalation "windows" (or defects) are observed in both systems, but the processes follow distinctly different dynamics.

Atomic structures and interfacial engineering of ultrathin indium intercalated between graphene and a SiC substrate.

Two-dimensional metals stabilized at the interface between graphene and SiC are attracting considerable interest thanks to their intriguing physical properties, providing promising material platforms for quantum technologies. However, the nanoscale picture of the ultrathin metals within the interface that represents their ultimate two-dimensional limit has not been well captured. In this work, we explore the atomic structures and electronic properties of atomically thin indium intercalated at the epitaxial graphene/SiC interface by means of cryogenic scanning tunneling microscopy.

Toward High-Performance p-Type Two-Dimensional Field Effect Transistors: Contact Engineering, Scaling, and Doping.

n-type field effect transistors (FETs) based on two-dimensional (2D) transition-metal dichalcogenides (TMDs) such as MoS and WS have come close to meeting the requirements set forth in the International Roadmap for Devices and Systems (IRDS). However, p-type 2D FETs are dramatically lagging behind in meeting performance standards. Here, we adopt a three-pronged approach that includes contact engineering, channel length () scaling, and monolayer doping to achieve high performance p-type FETs based on synthetic WSe.

Elucidating the Mechanism of Large Phosphate Molecule Intercalation Through Graphene-Substrate Heterointerfaces.

Intercalation is the process of inserting chemical species into the heterointerfaces of two-dimensional (2D) layered materials. While much research has focused on the intercalation of metals and small gas molecules into graphene, the intercalation of larger molecules through the basal plane of graphene remains challenging. In this work, we present a new mechanism for intercalating large molecules through monolayer graphene to form confined oxide materials at the graphene-substrate heterointerface.

Dilute Rhenium Doping and its Impact on Defects in MoS.

Substitutionally doped 2D transition metal dichalcogenides are primed for next-generation device applications such as field effect transistors (FET), sensors, and optoelectronic circuits. In this work, we demonstrate substitutional rhenium (Re) doping of MoS monolayers with controllable concentrations down to 500 ppm by metal-organic chemical vapor deposition (MOCVD). Surprisingly, we discover that even trace amounts of Re lead to a reduction in sulfur site defect density by 5-10×.

Observation of Rich Defect Dynamics in Monolayer MoS.

Defects play a pivotal role in limiting the performance and reliability of nanoscale devices. Field-effect transistors (FETs) based on atomically thin two-dimensional (2D) semiconductors such as monolayer MoS are no exception. Probing defect dynamics in 2D FETs is therefore of significant interest.

Ultrascaled Contacts to Monolayer MoS Field Effect Transistors.

Two-dimensional (2D) semiconductors possess promise for the development of field-effect transistors (FETs) at the ultimate scaling limit due to their strong gate electrostatics. However, proper FET scaling requires reduction of both channel length () and contact length (), the latter of which has remained a challenge due to increased current crowding at the nanoscale. Here, we investigate Au contacts to monolayer MoS FETs with down to 100 nm and down to 20 nm to evaluate the impact of contact scaling on FET performance.

Impact of Large Gate Voltages and Ultrathin Polymer Electrolytes on Carrier Density in Electric-Double-Layer-Gated Two-Dimensional Crystal Transistors.

Electric-double-layer (EDL) gating can induce large capacitance densities (∼1-10 μF cm) in two-dimensional (2D) semiconductors; however, several properties of the electrolyte limit performance. One property is the electrochemical activity which limits the gate voltage () that can be applied and therefore the maximum extent to which carriers can be modulated. A second property is electrolyte thickness, which sets the response speed of the EDL gate and therefore the time scale over which the channel can be doped.