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In VO$_2$, the explicit origin of the insulator-to-metal transition is stilldisputable between Peierls and Mott insulators. Along with the controversy, itssecond monoclinic (M2) phase has received considerable attention due to thepresence of electron correlation in undimerized vanadium ions. However, theorigin of the M2 phase is still obscure. Here, we study a granular VO$_2$ filmusing conductive atomic force microscopy and Raman scattering. Upon thestructural transition from monoclinic to rutile, we observe directly anintermediate state showing the coexistence of monoclinic M1 and M2 phases. Theconductivity near the grain boundary in this regime is six times larger thanthat of the grain core, producing a donut-like landscape. Our results reveal anintra-grain percolation process, indicating that VO$_2$ with the M2 phase is aMott insulator.

Direct observation of the M2 phase with its Mott transition in a VO$_2$ film