Tunable Mixed-Valence Doping toward Record Electrical Conductivity in a Three-Dimensional Metal-Organic Framework

Area of Science:

• Materials Science
• Solid-State Chemistry
• Nanotechnology

Background:

• Metal-organic frameworks (MOFs) are porous crystalline materials with diverse applications.
• Achieving high electrical conductivity in MOFs remains a significant challenge.
• Tuning electronic properties of MOFs is crucial for advanced electronic devices.

Purpose of the Study:

• To develop a novel mixed-valence iron-tetrazolate metal-organic framework (MOF).
• To investigate the electrical conductivity and its tunability in the synthesized MOF.
• To understand the electronic mechanisms governing the conductivity.

Main Methods:

• Partial oxidation of an iron-tetrazolate MOF using ambient atmosphere.
• Single-crystal conductivity measurements over a range of temperatures.
• Variable-temperature conductivity analysis to determine activation energy.
• Electronic spectroscopy (UV-Vis-NIR) to probe electronic transitions.
• Electronic band structure calculations.

Main Results:

• A mixed-valence iron-tetrazolate MOF was successfully synthesized.
• Single-crystal conductivities were tunable over 5 orders of magnitude, exceeding 1 S/cm.
• The material exhibited the highest conductivity reported for a 3D-connected MOF.
• A low activation energy of 160 meV was observed.
• Evidence of intervalence charge transfer between Fe²⁺ and Fe³⁺ centers was found, supported by spectroscopy and calculations.

Conclusions:

• Partial oxidation and induction of metal-based mixed valency is an effective strategy for high conductivity in MOFs.
• The synthesized MOF demonstrates significant potential for applications in electronic devices.
• Systematic tuning of conductivity is achievable through controlled mixed valency.