Photoinduced Strain Release and Phase Transition Dynamics of Solid-Supported Ultrathin Vanadium Dioxide

Abstract

The complex phase transitions of vanadium dioxide (VO2) have drawn continual attention for more than five decades. Dynamically, ultrafast electron diffraction (UED) with atomic-scale spatiotemporal resolution has been employed to study the reaction pathway in the photoinduced transition of VO2, using bulk and strain-free specimens. Here, we report the UED results from 10-nm-thick crystalline VO2 supported on Al2O3(0001) and examine the influence of surface stress on the photoinduced structural transformation. An ultrafast release of the compressive strain along the surface-normal direction is observed at early times following the photoexcitation, accompanied by faster motions of vanadium dimers that are more complex than simple dilation or bond tilting. Diffraction simulations indicate that the reaction intermediate involved on picosecond times may not be a single state, which implies non-concerted atomic motions on a multidimensional energy landscape. At longer times, a laser fluence multiple times higher than the thermodynamic enthalpy threshold is required for complete conversion from the initial monoclinic structure to the tetragonal lattice. For certain crystalline domains, the structural transformation is not seen even on nanosecond times following an intense photoexcitation. These results signify a time-dependent energy distribution among various degrees of freedom and reveal the nature of and the impact of strain on the photoinduced transition of VO2.

Large non-thermal contribution to picosecond strain pulse generation using the photo-induced phase transition in VO2

“…Therefore, we conclude that after PIPT the VO 2 film expands along the rutile axis during a short time~a/s and then slowly shrinks. Such non-monotonic behavior has been observed earlier in some electron diffraction experiments [46][47][48][49][50][51] .…”

“…Using transducers grown on differently oriented sapphire or other substrates may enable control over the direction in which the largest nonthermal strain generation occurs thus opening a pathway for a further optimization. This and the complex path which the lattice takes following photoexcitation 46,50 suggests that VO 2 transducers with different orientations grown on various substrates may allow the generation of, not only compressive-tensile strain pulses, but also shear ones. Furthermore, the progress in growth of high-quality VO 2 films 57,58 enables tuning their transition temperature and even stabilizing various phases at ambient conditions, which can be further utilized for PIPT-induced generation of strain pulses with various parameters.…”

Natural and induced growth of VO2 (M) on VO2 (B) ultrathin films

“…Accordingly, the growth of the VO 2 (B) polymorph is prevented, which promotes that of the VO 2 (M) polymorph and leads to the coexistence of both VO 2 (B) and VO 2 (M) phases for films with thickness t  >  t c . These observations therefore indicate that strain significantly influence the competing nature of VO 2 polymorphs growth, so as it does for the ultrafast structural transition dynamics following above-gap photoexcitation 26 . Likewise, they strongly suggest that the surface properties of the VO 2 (B) film, which play a key role in the observed phase growth change from VO 2 (B) to VO 2 (M), should definitely be considered to further control the growth of these polymorphs.…”

Bidirectional micro-actuators based on eccentric coaxial composite oxide nanofiber

“…The FFT of 30 °C has two more rows of diffraction spots than 100 °C, which were marked by white circles. This indicates a phase transition of VO2 from monoclinic to tetragonal [28][29][30], as shown in Fig. S2 in the ESM.…”