  Vanadium dioxide (VO2) is a popular candidate for electronic and optical
switching applications due to its well-known semiconductor-metal transition.
Its study is notoriously challenging due to the interplay of long and short
range elastic distortions, as well as the symmetry change, and the electronic
structure changes. The inherent coupling of lattice and electronic degrees of
freedom opens the avenue towards mechanical actuation of single domains. In
this work, we show that we can manipulate and monitor the reversible
semiconductor-to-metal transition of VO2 while applying a controlled amount of
mechanical pressure by a nanosized metallic probe using an atomic force
microscope. At a critical pressure, we can reversibly actuate the phase
transition with a large modulation of the conductivity. Direct tunneling
through the VO2-metal contact is observed as the main charge carrier injection
mechanism before and after the phase transition of VO2. The tunneling barrier
is formed by a very thin but persistently insulating surfacelayer of the VO2.
The necessary pressure to induce the transition decreases with temperature. In
addition, we measured the phase coexistence line in a hitherto unexplored
regime. Our study provides valuable information on pressure-induced electronic
modifications of the VO2 properties, as well as on nanoscale metal-oxide
contacts, which can help in the future design of oxide electronics.
