Field-effect transistors made from solution-grown two-dimensional tellurene (1704.06202v2)

Abstract: The reliable production of two-dimensional crystals is essential for the development of new technologies based on 2D materials. However, current synthesis methods suffer from a variety of drawbacks, including limitations in crystal size and stability. Here, we report the fabrication of large-area, high-quality 2D tellurium (tellurene) using a substrate-free solution process. Our approach can create crystals with a process-tunable thickness, from monolayer to tens of nanometres, and with lateral sizes of up to 100 um. The chiral-chain van der Waals structure of tellurene gives rise to strong in-plane anisotropic properties and large thickness dependent shifts in Raman vibrational modes, which is not observed in other 2D layered materials. We also fabricate tellurene field-effect transistors, which exhibit air-stable performance at room temperature for over two months, on off ratios on the order of 106 and field-effect mobilities of around 700 cm2 per Vs. Furthermore, by scaling down the channel length and integrating with high-k dielectrics, transistors with a significant on-state current density of 1 A mm-1 are demonstrated.

• The paper presents a novel substrate-free solution synthesis method that produces high-quality 2D tellurene for scalable FET fabrication.
• It utilizes advanced microscopy and spectroscopy to confirm a defect-free, anisotropic chiral-chain structure in the tellurene flakes.
• Field-effect transistors based on tellurene achieve mobilities up to 700 cm²/Vs with on/off ratios of around 10², maintaining stable performance over two months.

Fabrication and Characterization of Field-Effect Transistors Using Solution-Grown Tellurene

The fabrication and paper of two-dimensional (2D) materials have garnered significant interest due to their potential to revolutionize various technological domains, including electronics, optoelectronics, and energy devices. The research paper titled "Field-effect transistors made from solution-grown two-dimensional tellurene" tackles the challenge of synthesizing stable and large-area 2D crystal forms of tellurium (Te), specifically in the form of tellurene. This paper outlines a novel substrate-free solution process for the scalable production of 2D tellurene and subsequently evaluates its physical properties and utility in field-effect transistors (FETs).

Synthesis and Structural Characterization

The synthesis method employed involves a reduction of sodium tellurite using hydrazine hydrate in an alkaline solution. A key component of the process is the crystal-face-blocking ligand polyvinylpyrrolidone (PVP), which helps in controlling the morphology and dimensional characteristics of the tellurene flakes. This approach bypasses the need for substrates and allows for the growth of tellurene crystals with controllable thicknesses, ranging from monolayers to tens of nanometers, and lateral sizes up to 100 micrometers.

Extensive structural characterization was undertaken using techniques such as high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM), high-resolution transmission electron microscopy (HRTEM), and X-ray diffraction (XRD). The unique chiral-chain van der Waals (vdW) structure of tellurene, responsible for its anisotropic properties, was confirmed. The synthesized tellurene flakes exhibited no point defects over large areas, suggestive of high-quality crystal formation.

Raman Spectroscopy and Electronic Properties

Angle-resolved Raman spectroscopy of the tellurene disclosed notable thickness-dependent Raman-active vibrational modes, with a significant blue-shift observed for specific modes with decreasing thickness. These shifts are attributed to the distinctive chiral-chain vdW structure and the resultant intra-layer interactions, which differ from those seen in more common 2D layered materials. The theoretical underpinning is corroborated by first-principles calculations that map the band structure alterations as a function of layer thickness, establishing tellurene's indirect bandgap nature, which transitions prominently towards direct bandgap behavior at reduced thicknesses.

Field-Effect Transistor Fabrication and Performance

Tellurene's electrical properties were explored by fabricating FETs, investigating the dependency of key performance metrics on the flake thickness and channel dimensions. The results demonstrate field-effect mobilities peaking at approximately 700 cm²/Vs at 16 nm thickness, with on/off ratios of around 10² for long-channel devices. These transistors display stable performance up to two months in ambient conditions, underscoring tellurene's resilience against oxidation and chemical degradation—a significant advantage over other 2D materials like black phosphorus.

Short-channel devices benefit from the strong on-state current capabilities of tellurene, achieving current densities surpassing 1 A/mm, largely attributed to the integration with high-k dielectrics and optimized scaling methods. This performance metric parallels or exceeds that of established 2D semiconductor devices, highlighting tellurene's viability for high-performance electronic applications.

Implications and Future Directions

The findings of this research expand the compendium of 2D materials by introducing a robust and process-controllable methodology to synthesize tellurene. The implications for practical applications are considerable, suggesting potential use in sensors, transistors, and flexible electronics where material stability and electrical performance are critical. The tellurene's inherently anisotropic properties also present opportunities for developing devices that leverage directional conduction and other anisotropic effects.

Future work should focus on refining the synthesis process, particularly addressing challenges in forming continuous films, to enable integration into more complex device architectures. Additionally, exploring combinations with layered transition metal dichalcogenides (TMDCs) or insulating 2D materials could facilitate advances in heterostructure-based technologies, opening new pathways for next-generation electronic and optoelectronic devices. The exploration of further doping strategies, interface engineering, and encapsulation techniques to enhance device reliability and performance will also be important research avenues.

In summary, the paper adds considerable depth to current understanding and capabilities in the field of 2D materials, positioning tellurene as a competitive candidate for future technological exploration and application.