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

Impacts of hot electron diffusion, electron-phonon coupling, and surface atoms on metal surface dynamics revealed by reflection ultrafast electron diffraction

Metals exhibit nonequilibrium electron and lattice subsystems at transient times following femtosecond laser excitation. In the past four decades, various optical spectroscopy and time-resolved diffraction methods have been used to study electron-phonon coupling and the effects of underlying dynamical processes. Here, we take advantage of the surface specificity of reflection ultrafast electron diffraction (UED) to examine the structural dynamics of photoexcited metal surfaces, which are apparently slower in recovery than predicted by thermal diffusion from the profile of absorbed energy. Fast diffusion of hot electrons is found to critically reduce surface excitation and affect the temporal dependence of the increased atomic motions on not only the ultrashort but also sub-nanosecond times. Whereas the two-temperature model with the accepted physical constants of platinum can reproduce the observed surface lattice dynamics, gold is found to exhibit appreciably larger-than-expected dynamic vibrational amplitudes of surface atoms while keeping the commonly used electron-phonon coupling constant. Such surface behavioral difference at transient times can be understood in the context of the different strengths of binding to surface atoms for the two metals. In addition, with the quantitative agreements between diffraction and theoretical results, we provide convincing evidence that surface structural dynamics can be reliably obtained by reflection UED even in the presence of laser-induced transient electric fields.

Decoupling the metal insulator transition and crystal field effects of VO2

VO2 is a highly correlated electron system which has a metal-to-insulator transition (MIT) with a dramatic change of conductivity accompanied by a first-order structural phase transition (SPT) near room temperature. The origin of the MIT is still controversial and there is ongoing debate over whether an SPT induces the MIT and whether the Tc can be engineered using artificial parameters. We examined the electrical and local structural properties of Cr- and Co-ion implanted VO2 (Cr-VO2 and Co-VO2) films using temperature-dependent resistance and X-ray absorption fine structure (XAFS) measurements at the V K edge. The temperature-dependent electrical resistance measurements of both Cr-VO2 and Co-VO2 films showed sharp MIT features. The Tc values of the Cr-VO2 and Co-VO2 films first decreased and then increased relative to that of pristine VO2 as the ion flux was increased. The pre-edge peak of the V K edge from the Cr-VO2 films with a Cr ion flux ≥ 1013 ions/cm2 showed no temperature-dependent behavior, implying no changes in the local density of states of V 3d t2g and eg orbitals during MIT. Extended XAFS (EXAFS) revealed that implanted Cr and Co ions and their tracks caused a substantial amount of structural disorder and distortion at both vanadium and oxygen sites. The resistance and XAFS measurements revealed that VO2 experiences a sharp MIT when the distance of V-V pairs undergoes an SPT without any transitions in either the VO6 octahedrons or the V 3d t2g and eg states. This indicates that the MIT of VO2 occurs with no changes of the crystal fields.

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

Picosecond strain pulses are a versatile tool for investigation of mechanical properties of meso- and nano-scale objects with high temporal and spatial resolutions. Generation of such pulses is traditionally realized via ultrafast laser excitation of a light-to-strain transducer involving thermoelastic, deformation potential, or inverse piezoelectric effects. These approaches unavoidably lead to heat dissipation and a temperature rise, which can modify delicate specimens, like biological tissues, and ultimately destroy the transducer itself limiting the amplitude of generated picosecond strain. Here we propose a non-thermal mechanism for generating picosecond strain pulses via ultrafast photo-induced first-order phase transitions (PIPTs). We perform experiments on vanadium dioxide VO₂ films, which exhibit a first-order PIPT accompanied by a lattice change. We demonstrate that during femtosecond optical excitation of VO₂ the PIPT alone contributes to ultrafast expansion of this material as large as 0.45%, which is not accompanied by heat dissipation, and, for excitation density of 8 mJ cm⁻², exceeds the contribution from thermoelastic effect by a factor of five.

Ultrafast driving of vanadium dioxide can induce a large structural phase transition, which can be used to generate picosecond strain pulses. Here the authors show that the photo-induced phase transition can contribute 0.45% strain without causing undesirable heating.

Cross-Examination of Ultrafast Structural, Interfacial, and Carrier Dynamics of Supported Monolayer MoS2

In this Letter, the ultrafast structural, interfacial, and carrier dynamics of monolayer MoS₂ supported on sapphire are cross-examined by the combination of ultrafast electron diffraction (UED) and transient reflectivity techniques. The out-of-plane motions directly probed by reflection UED suggest a limited anisotropy in the atomic motions of monolayer MoS₂, which is distinct from that of related materials such as graphene and WSe₂. Besides thermal diffusion, the MoS₂-sapphire interface exhibits structural dynamics trailing those of the overlaying MoS₂ and are in stark contrast with the sapphire bulk, which is consistent with the limited thermal boundary conductance. These structural dynamics provide justification for the determination of carriers being trapped by defects in ∼600 fs and releasing energy within a few picoseconds. The rich findings attest to the strength of combining techniques with real-time optical and direct structure probes for a detailed understanding of dynamical processes in functional materials.

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

This work examines the synthesis of single phase VO₂ (B) thin films on LaAlO₃ (100) substrates, and the naturally-occurring and induced subsequent growth of VO₂ (M) phase on VO₂ (B) films. First, the thickness (t) dependence of structural, morphological and electrical properties of VO₂ films is investigated, evidencing that the growth of VO₂ (B) phase is progressively replaced by that of VO₂ (M) when t > ~11 nm. This change originates from the relaxation of the substrate-induced strain in the VO₂ (B) films, as corroborated by the simultaneous increase of surface roughness and decrease of the c-axis lattice parameter towards that of bulk VO₂ (B) for such films, yielding a complex mixed-phase structure composed of VO₂ (B)/VO₂ (M) phases, accompanied by the emergence of the VO₂ (M) insulator-to-metal phase transition. Second, the possibility of inducing this phase conversion, through a proper surface modification of the VO₂ (B) films via plasma treatment, is demonstrated. These natural and induced VO₂ (M) growths not only provide substantial insights into the competing nature of phases in the complex VO₂ polymorphs system, but can also be further exploited to synthesize VO₂ (M)/VO₂ (B) heterostructures at the micro/nanoscale for advanced electronics and energy applications.

Chemical and thermal properties of VO2 mechanochemically derived from V2O5 by co-milling with paraffin wax

A novel mechanochemical reduction process of V₂O₅ to VO₂ was established by milling with paraffin wax (PW, average molecular weight 254–646), serving as a reductant. The reduction progressed with increasing milling time and mass ratio V₂O₅ : PW (MRVP). The mechanochemically derived VO₂ became phase pure after milling for 3 h with an MRVP of 30 : 1 and exhibited a reversible polymorphic transformation between tetragonal and monoclinic phases at around 53–60 °C and 67–79 °C during heating and cooling, respectively. The latent heat was above 20 J g⁻¹ in both processes, being superior to those of commercial VO₂. Doping of starting V₂O₅ with Cr, Mo or W at 1 at% in the form of oxide did not increase the latent heat. This is another difference from the conventionally prepared doped VO₂. These anomalous heat storage properties of mechanochemically derived VO₂ were discussed mainly on the basis of X-ray photoelectron spectroscopy V_2p3/2 peaks combined with ion etching. The observed relatively high heat storage capacity of undoped VO₂ is primarily ascribed to the abundance of V⁴⁺ ionic states introduced during milling with PW, which were stabilized with simultaneously introduced structural degradation throughout the entire particles. The possible role of a remaining small amount of PW was also discussed.

Reduction of V₂O₅via a mechano-chemical route brings about unique electronic states of vanadium. The resulting VO₂ exhibits high latent heat storage during heating (a) and cooling (b).