Harnessing Quantum Capacitance in 2D Material/Molecular Layer Junctions for Novel Electronic Device Functionality.

Two-dimensional (2D) materials promise advances in electronic devices beyond Moore's scaling law through extended functionality, such as non-monotonic dependence of device parameters on input parameters. However, the robustness and performance of effects like negative differential resistance (NDR) and anti-ambipolar behavior have been limited in scale and robustness by relying on atomic defects and complex heterojunctions. In this paper, we introduce a novel device concept that utilizes the quantum capacitance of junctions between 2D materials and molecular layers. We realized a variable capacitance 2D molecular junction (vc2Dmj) diode through the scalable integration of graphene and single layers of stearic acid. The vc2Dmj exhibits NDR with a substantial peak-to-valley ratio even at room temperature and an active negative resistance region. The origin of this unique behavior was identified through thermoelectric measurements and ab initio calculations to be a hybridization effect between graphene and the molecular layer. The enhancement of device parameters through morphology optimization highlights the potential of our approach toward new functionalities that advance the landscape of future electronics.

Selective activation of MoS2 grain boundaries for enhanced electrochemical activity.

Molybdenum disulfide (MoS2) has emerged as a promising material for catalysis and sustainable energy conversion. However, the inertness of its basal plane to electrochemical reactions poses challenges to the utilization of wafer-scale MoS2 in electrocatalysis. To overcome this limitation, we present a technique that enhances the catalytic activity of continuous MoS2 by preferentially activating its buried grain boundaries (GBs). Through mild UV irradiation, a significant enhancement in GB activity was observed that approaches the values for MoS2 edges, as confirmed by a site-selective photo-deposition technique and micro-electrochemical hydrogen evolution reaction (HER) measurements. Combined spectroscopic characterization and ab-initio simulation demonstrates substitutional oxygen functionalization at the grain boundaries to be the origin of this selective catalytic enhancement by an order of magnitude. Our approach not only improves the density of active sites in MoS2 catalytic processes but yields a new photocatalytic conversion process. By exploiting the difference in electronic structure between activated GBs and the basal plane, homo-compositional junctions were realized that improve the photocatalytic synthesis of hydrogen by 47% and achieve performances beyond the capabilities of other catalytic sites.

Self-Expansion Based Multi-Patterning for 2D Materials Fabrication beyond the Lithographical Limit.

Two-dimensional (2D) materials are promising successors for silicon transistor channels in ultimately scaled devices, necessitating significant research efforts to study their behavior at nanoscopic length scales. Unfortunately, current research has limited itself to direct patterning approaches, which limit the achievable resolution to the diffraction limit and introduce unwanted defects into the 2D material. The potential of multi-patterning to fabricate 2D materials features with unprecedented precision and low complexity at large scale is demonstrated here. By combining lithographic patterning of a mandrel and bottom-up self-expansion, this approach enables pattern resolution one order of magnitude below the lithographical resolution. In-depth characterization of the self-expansion double patterning (SEDP) process reveals the ability to manipulate the critical dimension with nanometer precision through a self-limiting and temperature-controlled oxidation process. These results indicate that the SEDP process can regain the quality and morphology of the 2D material, as shown by high-resolution microscopy and optical spectroscopy. This approach is shown to open up new avenues for research into high-performance, ultra-scaled 2D materials devices for future electronics.

Nitrogen Pretreatment of Growth Substrates for Vacancy-Saturated MoS2.

Two-dimensional transition-metal dichalcogenides (2D TMDCs) are considered promising materials for optoelectronics due to their unique optical and electric properties. However, their potential has been limited by the occurrence of atomic vacancies during synthesis. While post-treatment processes have demonstrated the passivation of such vacancies, they increase process complexity and affect the TMDC's quality. We here introduce the concept of pretreatment as a facile and powerful route to solve the problem of vacancies in MoS2. Low-temperature nitridation of the sapphire substrate prior to growth provides a nondestructive method to MoS2 modification without introducing new processing steps or increasing the thermal budget. Spectroscopic characterization and atomic-resolution microscopy reveal the incorporation of nitrogen from the sapphire surface layer into chalcogen vacancies. The resulting MoS2 with nitrogen-saturated defects shows a decrease in midgap states and more intrinsic doping as confirmed by ab initio calculations and optoelectronic measurements. The demonstrated pretreatment method opens up new routes toward future, high-performance 2D electronics, as evidenced by a 3-fold reduction in contact resistance and a 10-fold improved performance of 2D photodetectors.

Vacancy-plane-mediated exfoliation of sub-monolayer 2D pyrrhotite.

Conventional exfoliation exploits the anisotropy in bonding or compositional character to delaminate 2D materials with large lateral size and atomic thickness. This approach, however, limits the choice to layered host crystals with a specific composition. Here, we demonstrate the exfoliation of a crystal along planes of ordered vacancies as a novel route toward previously unattainable 2D crystal structures. Pyrrhotite, a non-stoichiometric iron sulfide, was utilized as a prototype system due to its complex vacancy superstructure. Bulk pyrrhotite crystals were synthesized by gas-assisted bulk conversion, and their diffraction pattern revealed a 4C superstructure with 3 vacancy interfaces within the unit cell. Electrochemical intercalation and subsequent delamination yield ultrathin 2D flakes with a large lateral extent. Atomic force microscopy confirms that exfoliation occurs at all three supercell interfaces, resulting in the isolation of 2D structures with sub-unit cell thicknesses of 1/2 and 1/4 monolayers. The impact of controlling the morphology of 2D materials below the monolayer limit on 2D magnetic properties was investigated. Bulk pyrrhotite was shown to exhibit ferrimagnetic ordering that agrees with theoretical predictions and that is retained after exfoliation. A complex magnetic domain structure and an enhanced impact of vacancy planes on magnetization emphasize the potential of our synthesis approach as a powerful platform for modulating magnetic properties in future electronics and spintronics.

2D Material-Enabled Optical Rectennas with Ultrastrong Light-Electron Coupling.

Optical rectennas extend the electromagnetic wave rectification process into the visible regime and provide a route toward high-performance photodetection and energy harvesting. Here, the promise of 2D materials toward on-chip optical rectennas is demonstrated. A self-aligned patterning process yields lateral MIM structures where a nanometer-sized air gap separates a 2D material contact from a metal electrode. This device can be scalably produced in large arrays using established microfabrication techniques. Different from previous approaches, the performance of the 2D rectenna can be adjusted through electrostatic gating. Optimization of the band alignment leads to strong rectification at wavelengths around 500 nm and clear polarization control. Comparison of wavelength-dependent rectenna performance with a photon-assisted tunneling model reveals a tenfold increase in photon-electron coupling over nanotube-based rectennas. The results highlight the potential of 2D material-based rectennas for future quantum computing applications.

Chemical vapor deposition merges MoS2 grains into high-quality and centimeter-scale films on Si/SiO2.

Two-dimensional molybdenum disulfide (MoS2) has attracted increasing attention due to its promise for next-generation electronics. To realize MoS2-based electronics, however, a synthesis method is required that produces a uniform single-layer material and that is compatible with existing semiconductor fabrication techniques. Here, we demonstrate that uniform films of single-layer MoS2 can be directly produced on Si/SiO2 at wafer-scale without the use of catalysts or promoters. Control of the precursor transport through oxygen dosing yielded complete coverage and increased connectivity between crystalline MoS2 domains. Spectroscopic characterization and carrier transport measurements furthermore revealed a reduced density of defects compared to conventional chemical vapor deposition growth that increased the quantum yield over ten-fold. To demonstrate the impact of enhanced scale and optoelectronic performance, centimeter-scale arrays of MoS2 photosensors were produced that demonstrate unprecedentedly high and uniform responsivity. Our approach improves the prospect of MoS2 for future applications.

QD/2D Hybrid Nanoscrolls: A New Class of Materials for High-Performance Polarized Photodetection and Ultralow Threshold Laser Action.

Nanoscrolls are a class of nanostructures where atomic layers of 2D materials are stacked consecutively in a coaxial manner to form a 1D spiral topography. Self-assembly of chemical vapor deposition grown 2D WS2 monolayer into quasi-1D van der Waals scroll structure instigates a plethora of unique physiochemical properties significantly different from its 2D counterparts. The physical properties of such nanoscrolls can be greatly manipulated upon hybridizing them with high-quantum-yield colloidal quantum dots, forming 0D/2D structures. The efficient dissociation of excitons at the heterojunctions of QD/2D hybridized nanoscrolls exhibits a 3000-fold increased photosensitivity compared to the pristine 2D-material-based nanoscroll. The synergistic effects of confined geometry and efficient QD scatterers produce a nanocavity with multiple feedback loops, resulting in coherent lasing action with an unprecedentedly low lasing threshold. Predominant localization of the excitons along the circumference of this helical scroll results in a 12-fold brighter emission for the parallel-polarized transition compared to the perpendicular one, as confirmed by finite-difference time-domain simulation. The versatility of hybridized nanoscrolls and their unique properties opens up a powerful route for not-yet-realized devices toward practical applications.