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.

Robust formation of amorphous Sb2S3 on functionalized graphene for high-performance optoelectronic devices in the cyan-gap.

Despite significant progress in the fabrication and application of semiconductor materials for optical emitters and sensors, few materials can cover the cyan-gap between 450 and 500 nm. We here introduce a robust and facile method to deposit amorphous Sb2S3 films with a bandgap of 2.8 eV. By exploiting the tunable functionality of graphene, a two-dimensional material, efficient deposition from a chemical was achieved. Ozone-generated defects in the graphene were shown to be required to enhance the morphology and quality of the material and comprehensive characterization of the seed layer and the Sb2S3 film were applied to design an optimal deposition process. The resulting material exhibits efficient carrier transport and high photodetector performance as evidenced by unprecedented responsivity and detectivity in semiconductor/graphene/glass vertical heterostructures. (112 A/W, 2.01 × 1012 Jones, respectively).

Ink-jet patterning of graphene by cap assisted barrier-guided CVD.

Barrier-guided CVD growth could provide a new route to printed electronics by combining high quality 2D materials synthesis with scalable and cost-effective deposition methods. Unfortunately, we observe the limited stability of the barrier at growth conditions which results in its removal within minutes due to hydrogen etching. This work describes a route towards enhancing the stability of an ink-jet deposited barrier for high resolution patterning of high quality graphene. By modifying the etching kinetics under confinement, the barrier film could be stabilized and high resolution barriers could be retained even after 6 hours of graphene growth. Thus produced microscopic graphene devices exhibited an increase in conductivity by 6 orders of magnitude and a decrease in defectiveness by 48 times yielding performances that are superior to devices produced by traditional lithographical patterning which indicates the potential of our approach for future electronic applications.

Increasing the doping efficiency by surface energy control for ultra-transparent graphene conductors.

Graphene's attractiveness in many applications is limited by its high resistance. Extrinsic doping has shown promise to overcome this challenge but graphene's performance remains below industry requirements. This issue is caused by a limited charge transfer efficiency (CTE) between dopant and graphene. Using AuCl3 as a model system, we measure CTE as low as 5% of the expected values due to the geometrical capacitance of small adsorbate clusters. We here demonstrate a strategy for enhancing the CTE by a two-step optimization of graphene's surface energy prior to AuCl3 doping. First, exposure to UV ozone modified the hydrophilicity of graphene and was found to decrease the cluster's geometric capacitance, which had a direct effect on the CTE. Occurrence of lattice defects at high UV exposure, however, deteriorated graphene's transport characteristics and limited the effectiveness of this pretreatment step. Thus, prior to UV exposure, a functionalized polymer layer was introduced that could further enhance graphene's surface energy while protecting it from damage. Combination of these treatment steps were found to increase the AuCl3 charge transfer efficiency to 70% and lower the sheet resistance to 106 Ω/γ at 97% transmittance which represents the highest reported performance for doped single layer graphene and is on par with commercially available transparent conductors.