Solution-Based Property Tuning of Black Phosphorus

Recent trends in covalent functionalization of 2D materials

Covalent functionalization of the surface is more crucial in 2D materials than in conventional bulk materials because of their atomic thinness, large surface-to-volume ratio, and uniform surface chemical potential. Because...

In Situ Cleaning and Fluorination of Black Phosphorus for Enhanced Performance of Transistors with High Stability

Most two-dimensional (2D) semiconductors suffer from intrinsic instability under ambient conditions, especially 2D black phosphorus (BP). Although much effort has been made to study the passivation of 2D materials against corrosion by oxygen and water molecules, facile and effective passivation with long-term stability is still challenging; in particular, selective passivation, which is critical for integration into nanoelectronics, is still lacking. Here, we develop a novel passivation route for BP using a fluorinated self-assembled thin film of PFSA (perfluorosulfonic acid, PFSA), where the surface modifier with high hydrophobicity on BP presents extremely stable characteristics over five months under ambient conditions. Moreover, we report for the first time in situ cleaning and selective fluorination of only BP flakes on a SiO₂/Si substrate by a spin-coating process followed by ultrasonication, which was attributed to the formation of P-F covalent bonds on the BP surface. Selectively fluorinated BP shows not only enhanced stability in air but also electrical properties of the BP field-effect transistor (FET), with the on-current of the BP FET increasing and presenting enhanced carrier mobility (125 cm² V^-1 s^-1) and on/off ratio (10⁴). This significant finding sheds light on fabricating vertical 2D heterostructures to realize high performance and reliability with versatile 2D materials. This work demonstrates an emerging passivation approach for long-term stability together with superior electrical properties, which paves the way for integrating 2D semiconductors into critical channel materials in FETs that are favorable for next-generation digital logic circuits.

Synchronous Enhancement for Responsivity and Response Speed in In2Se3 Photodetector Modulated by Piezoresistive Effect

Although single ultra-high-performance indicators have been achieved based on two-dimensional (2D) semiconductors, the comprehensive performances of the photodetectors of them are not so desirable. The response speed and responsivity are two key figures of merit for photodetectors, while these two parameters are always mutually suppressive and can not be synchronously satisfied. Here, we proposed a feasible strategy that can simultaneously improve the responsivity and response speed of In₂Se₃-based photodetectors by applying the mechanical strain and producing the piezoresistive effect, which can synergistically modulate the band structure and boost the overall photodetecting performances. Through studying the optoelectronic properties of In₂Se₃ photodetector under strain modulations, we found that the responsivity under 0.65% tensile strain is improved by almost 68.6% on average, while responsivity under 0.65% compressive strain is lowered by about 57.3% in the wavelength range of 200-1000 nm. More importantly, the response speed of the In₂Se₃-based photodetector under two different mechanical strains rises distinctly (from 244 to 214 and 180 μs, accordingly). The strain-engineering can accommodate the band structure and enhance the electric and optical properties of the semiconducting crystals, ultimately realizing high-performance photodetectors. The strategy proposed in this work for improving the performance of photodetectors provides a promising route to practical applications in next-generation optoelectronic devices.

Black Phosphorus-IGZO van der Waals Diode with Low-Resistivity Metal Contacts

There have been a few studies of heterojunctions composed of two-dimensional transition-metal dichalcogenides (TMDs) and an oxide layer, but such studies of high-performance electric and optoelectronic devices are essential. Such heterojunctions with low-resistivity metal contacts are needed by the electronics industry to fabricate efficient diodes and photovoltaic devices. Here, a van der Waals heterojunction composed of p-type black phosphorus (p-BP) and n-type indium-gallium-zinc oxide (n-IGZO) films with low-resistivity metal contacts is reported, and it demonstrates high rectification. The low off-state leakage current in the thick IGZO film accounts for the high rectification ratio in a sharp interface of p-BP/n-IGZO devices. For electrostatic gate control, an ionic liquid is introduced to achieve a high rectification ratio of 9.1 × 10⁴. The photovoltaic measurements of p-BP/n-IGZO show fast rise and decay times, significant open-circuit voltage and short-circuit current, and a high photoresponsivity of 418 mA/W with a substantial external quantum efficiency of 12.1%. The electric and optoelectronic characteristics of TMDs/oxide layer van der Waals heterojunctions are attractive for industrial applications in the near future.