Imaging metal-like monoclinic phase stabilized by surface coordination effect in vanadium dioxide nanobeam

In correlated systems, intermediate states usually appear transiently across phase transitions even at the femtosecond scale. It therefore remains an open question how to determine these intermediate states—a critical issue for understanding the origin of their correlated behaviour. Here we report a surface coordination route to successfully stabilize and directly image an intermediate state in the metal-insulator transition of vanadium dioxide. As a prototype metal-insulator transition material, we capture an unusual metal-like monoclinic phase at room temperature that has long been predicted. Coordinate bonding of L-ascorbic acid molecules with vanadium dioxide nanobeams induces charge-carrier density reorganization and stabilizes metallic monoclinic vanadium dioxide, unravelling orbital-selective Mott correlation for gap opening of the vanadium dioxide metal–insulator transition. Our study contributes to completing phase-evolution pathways in the metal-insulator transition process, and we anticipate that coordination chemistry may be a powerful tool for engineering properties of low-dimensional correlated solids.

, Room-temperature Raman spectra of this metastable region before and after the heating treatment, respectively. The measured region exhibits the characteristic peaks of VO 2 (M1) phase before heating treatment; while the Raman spectra of the same region matches well with the VO 2 (R) after heating treatment 1 . These results indicate a homogenous phase in this metastable region.
Supplementary Figure 4 | Schematic of the scanning near-field infrared microscope. The near-field images of the AA-treated VO 2 nanobeam were performed on NeaSNOM (Neaspec GmbH Co.) with the resolution of about 10 nm. The incident infrared laser beam excites metallic tip and the backscattered light is recorded to achieve the near-field infrared image of the sample. Optical images and Raman spectra of the VO 2 nanobeams treated in oxalic acid, ethylene glycol, glucose and NaBH 4 solution. Note: the reducing ability of oxalic acid, ethylene glycol and glucose is weaker than L-ascorbic acid, while reducing ability of NaBH 4 is stronger than L-ascorbic acid. The peak labeled by "*" in Raman spectra belongs to the silicon substrate.

Supplementary Figure 7 | Schematic diagram shows the energy level alignment
of the AA 2species and rutile phase VO 2 , confirming the electron transfer from the highest occupied molecular orbital (HOMO) of AA 2species to rutile VO 2 nanobeam.

Supplementary Note 1
In order to confirm that the stabilization of rutile metal and metastable metal-like monoclinic phases is due to the AA treatment, the control experiments were performed. VO 2 nanobeams were treated in the pure water without adding AA and other conditions kept unchanged. As shown in Supplementary Figure 2, it can be seen that the VO 2 nanobeam exhibits a homogeneous bright reflection of insulating monoclinic phase and no dark domains emerged at the two ends of the nanobeam. The Raman spectra further demonstrates the VO 2 nanobeam keeps monoclinic phase at room temperature after treatment in pure water 1 . Thus, these results indicate the emergency and stabilization of rutile metal phase at room temperature is attributed to the AA molecules in our treatment.

Supplementary Note 2
As exhibited in Supplementary Figure 5, the quantitative DFM signal at different V g shows that there are three different DFM response regions in all V g , which confirms that the metal-like monoclinic intermediate phase has higher carrier density than mid phase, and lower carrier density than end phase.

Supplementary Note 3
In order to further detect how the AA molecules attached on the surface of VO 2 nanobeams, we have simulated the vibrational modes of enolic C-O connecting with the surface V atoms. After geometry optimization, the vibrational modes are calculated for free AA molecule (gas phase) and adsorbed molecule. By comparing the noticeable differences in frequencies, the structure information of the adsorbed molecule can be determined. According to the simulated vibrational modes, we found that vibration peak of the enolic C-O vibration is at 1386 cm -1 when connected with the V atoms. The calculated value is close to the observed new peak at 1390 cm -1 in our ATR-FTIR spectrum, suggesting the formation of C-O-V complexes on the surface of the AA-treated VO 2 nanobeam.

Supplementary Note 4
To inspect whether other electron-donating molecules can be utilized to trigger the phase engineering along the VO 2 nanobeams, parallel experiments of the oxalic acid, ethylene glycol, glucose, NaBH 4 treatment on the VO 2 nanobeams were also performed. As illustrated in Supplementary Figure 6, it can be clearly seen that the VO 2 nanobeams exhibit no color change and the Raman spectra indicate the VO 2 nanobeams still keep monoclinic phase after these treatments. Thus, we deem that the interaction between L-ascorbic acid and VO 2 nanobeam is a complicated process and effective regulation of VO 2 MIT using chemical molecules needs two critical factors.
First, the treated molecules could coordinately bond on the surface of VO 2 nanobeams to form a stable complex. Second, the suitable match of energy level between the chelated molecules and VO 2 is indispensable to induce the charge transfer.

Supplementary Note 5
In order to confirm that the electrons can transfer from the surface binding AA species to VO 2 nanobeam, we calculate the molecular orbital energy of the surface AA species and the Fermi level of rutile phase VO 2 as shown in Supplementary Figure 7. Of note, in our case, the AA treatment of VO 2 nanobeam is at the temperature of 80 °C, where the VO 2 nanobeam is in the metallic rutile phase, thus we calculate the Fermi level of the rutile VO 2 . In addition, from the reaction process, we can see that the AA molecules bind to the VO 2 surface through the ortho-substituted hydroxyl groups of the enolic C-O-H and remove the two H + ions. In this regard, we calculate the molecular orbital energy of the surface binding AA 2species (AA molecule removing two H + ions) as the electron transfer precursor. From the calculated results, the HOMO energy of binding AA 2species is -2.0 eV and the Fermi level of rutile VO 2 is -4.5 eV compared with the vacuum level, which allows the electrons transfer from the HOMO of AA species to rutile VO 2 nanobeam.