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In a correlated system, the intermediate states appear in a transient state across phase transitions, even at the femtosecond scale. As such, it has been a constant challenge how Scientists can determine these intermediate states, especially when it is fundamental for understanding their correlated behavior.
A team of Researchers from China have now developed a surface coordination route to successfully stabilize and image the intermediate state (metal-insulator transition) of vanadium dioxide.
Vanadium dioxide is seen as a prototype correlated material, i.e. a material where the electronic (fermionic) structure includes an electronic correlation to be accurate. Vanadium dioxide exhibits a sharp first-order metal-insulator transition (MIT) near room temperature and is accompanied by a lattice change on the picosecond timescale.
Vanadium dioxide has been proffered as a beacon of hope for various applications including in ultrafast switching techniques, Mottronics and memristors.
The Researchers have employed a surface coordination method to stabilize and directly image an intermediate state in the metal-insulator transition of VO2. The Researchers took single crystalline VO2 nanobeams, reacted them and treated them, then transferred them onto substrates for characterization.
For characterization, the Researchers used an optical microscope (Olympus), dark field microscopy (DFM, XE-120 microscope, Park Systems Corp) with atomic force microscopy tips (AFM, NSC18/Cr-Au, Mikromasch) to take images. Selected area electron diffraction (SAED, JEOL JEM-ARF200F), Raman spectroscopy (LABRAM-HR confocal laser micro Raman spectrometer), scattering-type near-field microscopy (NeaSNOM, Neaspec GmbH Co.) and attenuated total reflectance-Fourier transform infra-red spectroscopy (Shimadzu IRPrestige-21 spectrometer, with a Specac Ltd ZnSe ATR cell and a PIKE Technologies Inc. ZnSe grating polarizer).
As a new metal–insulator transition material, the Researchers managed to capture an unusual metal-like monoclinic VO2 intermediate phase. There was an induced coordination effect produced from L-ascorbic acid (AA) ligands onto the VO2 nanobeams. The AA molecules chelated on the surface and caused a reorganization of the one-dimensional (1D) charge carrier density and a non-equilibrium metal state in the monoclinic lattice.
Metal-like monoclinic VO2 was stabilized over several micrometers and exhibited a significantly higher charge-carrier density compared to its insulating counterpart. This stabilization showed that the insulating gap of the VO2 nanobeams possessed an orbital-selective Mott correlation, borne out of a Peierls and Mott transition, to open the gap of the vanadium dioxide metal–insulator transition.
The Mott transition was found to be orbital-selective, due to the charge occupancy in the t2g orbitals influencing the Mott instability in the VO2 metal–insulator transition. The competition between orbitals to occupy the charge is now known to play a significant role in determining the electronic phases and the lattice in a VO2 system.
There are 3 t2g orbitals in VO2, as there are 3 of t2g electron orbitals in the vanadium ions. Depending on the lattice phase, the Researchers found varying orbital properties and orientations. In the rutile phase, the t2g orbitals overlap at the Fermi level, producing an isotropic metal electronic state. However, when VO2 transforms into the monoclinic Bravais lattice, not only does it become insulating, but the dxy orbital becomes more occupied and reduces the occupation of the dxz and dyz orbitals. This mechanism is what causes the redistribution of charge and forms a 1D electronic state along the V atom chains.
During the coordination reaction, electrons were injected from the AA ligands, which caused the rutile phase to be stable at room temperature. However, it was found in the monoclinic phase that there was an insufficient number of injected electrons for lattice stabilization, hence, the lattice undergoes a reorganization mechanism to stabilize itself.
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The Researchers believe that the discovery will open alternative coordination chemistry pathways for the regulation of electronic properties in low-dimensional correlated materials. The research also contributes heavily to completing phase-evolution pathways in the metal-insulator transition process, and could be used in the future to specifically engineer the properties in low-dimensional correlated materials.
Sources and Further Reading
- “Imaging metal-like monoclinic phase stabilized by surface coordination effect in vanadium dioxide nanobeam”- Li Z., Nature Communications, 2017, DOI: 10.1038/ncomms15561