[Paper Review] Two-dimensional bimetal-embedded expanded phthalocyanine monolayers: a class of multifunctional materials with fascinating properties

Debing Long, Nikolay V. Tkachenko+5 more|arXiv (Cornell University)|Jul 11, 2023

2D Materials and ApplicationsMaterials Science3 citations

TL;DR

This study proposes two-dimensional bimetal-embedded expanded phthalocyanine monolayers (TM2EPc, TM = Sc–Zn) as a new class of multifunctional 2D materials with exceptional structural stability, tunable electronic properties, and strong light absorption. First-principles calculations reveal that Cr2EPc exhibits ferromagnetic Dirac half-metallicity with a high Curie temperature (325 K), while Fe2EPc becomes half-metallic under compressive strain, and heterojunctions like Ni2EPc/2H-WSe2 achieve a power conversion efficiency of 25.19%, highlighting strong potential for spintronic and photovoltaic applications.

ABSTRACT

The expanded phthalocyanine (EPc) single-layer sheets with double transition metals (labeled as TM2EPc, TM = Sc-Zn) are predicted to be a new class of two-dimensional (2D) metal-organic materials with a series of favorable functional properties by means of systematic first-principle calculations and molecular dynamics simulations. The strong coordination between metal and EPc substrate accounts for the excellent structural stability. Chemical bonding analysis has demonstrated the absence of TM-TM bonding. Each metal center is isolated, but connected to the organic framework by four 2c-2e TM-N σ-bonds to form an extended 2D network. Unexpectedly, it is found that the V2EPc is an antiferromagnetic metal with Dirac cone, while Cr2EPc exhibits ferromagnetic Dirac half-metallicity, which is not common in 2D materials. Excitingly, the ferromagnetic Cr2EPc and antiferromagnetic Mn2- and Fe2-EPc have high magnetic transition temperatures of 223, 217, and 325 K, respectively, which are crucial for the practical applications of spintronics. Cr2EPc can maintain the Dirac half-metallicity under -6 % ~ 2 % biaxial strains, and Fe2EPc can transform from semiconductor to half-metal by applying -6 % ~ -10 % compressive strains. Additionally, the TM2EPc monolayers exhibit a full response to visible light and some materials have strong absorption in the ultraviolet and infrared regions in addition to visible light, showing extraordinary solar light-harvesting ability. Notably, the designed type-II heterojunctions Fe2EPc/SnC, Co2EPc/GeS, and Ni2EPc/2H-WSe2 have high power conversion efficiency (PCE > 15%), especially the PCE of Ni2EPc/2H-WSe2 reaches 25.19%, which has great potential in solar cell applications. All these desired properties render 2D TM2EPc monolayers promising candidates for future applications in nanoelectronics, spintronics,optoelectronics, and photovoltaic devices.

Motivation & Objective

• To explore the structural and electronic stability of two-dimensional bimetal-embedded expanded phthalocyanine monolayers (TM2EPc) with transition metals (TM = Sc–Zn).
• To investigate the magnetic, electronic, and optical properties of TM2EPc monolayers for potential applications in spintronics and optoelectronics.
• To evaluate the performance of TM2EPc-based heterojunctions in photovoltaic devices, particularly their power conversion efficiency (PCE).

Proposed method

• Systematic first-principle calculations based on density functional theory (DFT) were employed to analyze the electronic structure, bonding, and stability of TM2EPc monolayers.
• Molecular dynamics simulations were used to assess the thermal and structural stability of the 2D TM2EPc systems.
• Chemical bonding analysis was performed to characterize the nature of metal–ligand interactions, focusing on 2c-2e σ-bonds between TM and nitrogen atoms.
• Strain engineering was applied to probe the tunability of electronic properties, particularly the transition from semiconductor to half-metallic behavior.
• Type-II heterojunctions were designed between TM2EPc and 2D semiconductors (e.g., SnC, GeS, 2H-WSe2) to evaluate photovoltaic performance.
• Power conversion efficiency (PCE) was calculated for heterojunctions to assess solar cell potential.

Experimental results

Research questions

• RQ1 Can bimetal-embedded expanded phthalocyanine monolayers (TM2EPc) maintain structural stability with isolated transition metal centers connected via 2c-2e σ-bonds?
• RQ2 What are the magnetic and electronic properties of TM2EPc monolayers, particularly under strain and at finite temperatures?
• RQ3 Can TM2EPc-based heterojunctions achieve high power conversion efficiency suitable for practical solar cell applications?
• RQ4 How does strain influence the transition from semiconductor to half-metallic behavior in Fe2EPc?
• RQ5 What is the extent of solar light harvesting in TM2EPc monolayers, including absorption in UV, visible, and IR regions?

Key findings

• Cr2EPc exhibits ferromagnetic Dirac half-metallicity with a high magnetic transition temperature of 325 K, indicating strong potential for room-temperature spintronic applications.
• V2EPc is predicted to be an antiferromagnetic metal with a Dirac cone, a rare and promising feature in 2D materials.
• Mn2EPc and Fe2EPc show antiferromagnetic and ferromagnetic ordering with high magnetic transition temperatures of 217 K and 325 K, respectively.
• Fe2EPc transforms from a semiconductor to a half-metal under compressive strain of -6% to -10%, demonstrating tunable electronic behavior.
• The Ni2EPc/2H-WSe2 heterojunction achieves a power conversion efficiency of 25.19%, exceeding 15% and indicating strong photovoltaic potential.
• All TM2EPc monolayers exhibit full response to visible light, with some showing strong absorption in UV and IR regions, enabling exceptional solar light-harvesting capability.