Covalent Functionalization and Passivation of Exfoliated Black Phosphorus via Aryl Diazonium Chemistry

Abstract: Functionalization of atomically thin nanomaterials enables the tailoring of their chemical, optical, and electronic properties. Exfoliated black phosphorus, a layered two-dimensional semiconductor exhibiting favorable charge carrier mobility, tunable bandgap, and highly anisotropic properties, is chemically reactive and degrades rapidly in ambient conditions. In contrast, here we show that covalent aryl diazonium functionalization suppresses the chemical degradation of exfoliated black phosphorus even following weeks of ambient exposure. This chemical modification scheme spontaneously forms phosphorus-carbon bonds, has a reaction rate sensitive to the aryl diazonium substituent, and alters the electronic properties of exfoliated black phosphorus, ultimately yielding a strong, tunable p-type doping that simultaneously improves field-effect transistor mobility and on/off current ratio. This chemical functionalization pathway controllably modifies the properties of exfoliated black phosphorus, thus improving its prospects for nanoelectronic applications.

• The paper shows that covalent attachment of aryl groups significantly passivates exfoliated BP, enhancing its stability under ambient conditions.
• The paper demonstrates that the functionalization enables controllable p-type doping, improving electronic properties such as on/off current ratios and carrier mobility.
• The paper employs XPS, Raman spectroscopy, and DFT calculations to confirm strong P-C bond formation and to quantify the extent of chemical modification.

Covalent Functionalization and Passivation of Exfoliated Black Phosphorus via Aryl Diazonium Chemistry

The paper "Covalent Functionalization and Passivation of Exfoliated Black Phosphorus via Aryl Diazonium Chemistry" offers a comprehensive investigation into the modification of exfoliated black phosphorus (BP) through aryl diazonium chemistry. This technique shows potential in enhancing the stability and electronic properties of BP, presenting a significant advancement for the practical use of this nanomaterial in electronic applications.

Introduction to the Research

Exfoliated black phosphorus, a promising two-dimensional semiconductor, exhibits desirable characteristics such as high carrier mobility, a tunable bandgap, and anisotropic transport properties. However, its rapid degradation under ambient conditions has restricted its application scope. Through covalent functionalization employing aryl diazonium chemistry, this study aims to address these limitations. The functionalization primarily focuses on forming strong, covalent phosphorus-carbon bonds, thereby improving BP's chemical stability and enabling controlled p-type doping.

Methodology

The study utilized mechanical exfoliation to produce few-layer BP, followed by immersion in solutions of aryl diazonium salts—specifically 4-nitrobenzene-diazonium (4-NBD) and 4-methoxybenzene-diazonium (4-MBD). The chemical modifications were verified using techniques such as X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The research leveraged density functional theory (DFT) calculations to ascertain the thermodynamic favorability of the reaction, confirming covalent bond formation and consequent lattice distortion.

Key Findings

1. Chemical Passivation: The successful covalent binding of aryl groups onto BP results in notable passivation. Atomic force microscopy (AFM) confirmed the preservation of BP's morphology even after weeks of ambient exposure, contrary to unmodified BP that degraded significantly.
2. Controllable Doping: The covalent functionalization facilitated tunable p-type doping, enhancing the on/off current ratios and field-effect transistor mobility. The electron transfer rate in aryl diazonium reactions, dictated by the electronic properties of the diazonium ions, afforded controlled modification levels.
3. Spectroscopic Evidence: XPS and Raman spectroscopy provided robust evidence for the formation of P-C bonds, correlating the extent of functionalization with the modification duration and concentration. Notably, 4-NBD induced more rapid changes compared to 4-MBD, attributed to differing electron affinities and reduction potentials.

Implications

The implications of this research are multifaceted. Practically, the stabilization and enhancement of BP make it a viable candidate for future use in nanoelectronics and optoelectronic devices. The p-type doping mechanism also allows for refined control over electronic properties, potentially expanding the role of BP in semiconductor technologies. Theoretically, the research opens avenues for exploring further chemical modifications of phosphorene and other two-dimensional materials, extending the tunable property range accessible through strategic molecular chemistry.

Future Prospects

This study sets a foundation for further investigations into the diversification of chemical strategies applied to phosphorene and potentially other layered materials. Future research could explore different functional groups and their effects on BP, possibly leading to new avenues in the development of phosphorus-based nanomaterials with tailored properties for specific applications. Additionally, integrating these chemically stabilized materials in device architectures could bridge the gap between laboratory findings and industrial-scale applications.

In conclusion, the covalent functionalization and subsequent passivation of black phosphorus via aryl diazonium chemistry represent a notable advancement in the application and stability of this material. The findings demonstrate a promising strategy for stabilizing and functionally enhancing BP, thereby augmenting its utility in the advancing field of nanotechnology.