Transient dynamics of the phase transition in VO2 revealed by mega-electron-volt ultrafast electron diffraction

Vanadium dioxide (VO2) exhibits an insulator-to-metal transition accompanied by a structural transition near room temperature. This transition can be triggered by an ultrafast laser pulse. Exotic transient states, such as a metallic state without structural transition, were also proposed. These unique characteristics let VO2 have great potential in thermal switchable devices and photonic applications. Although great efforts have been made, the atomic pathway during the photoinduced phase transition is still not clear. Here, we synthesize freestanding quasi-single-crystal VO2 films and examine their photoinduced structural phase transition with mega-electron-volt ultrafast electron diffraction. Leveraging the high signal-to-noise ratio and high temporal resolution, we observe that the disappearance of vanadium dimers and zigzag chains does not coincide with the transformation of crystal symmetry. After photoexcitation, the initial structure is strongly modified within 200 femtoseconds, resulting in a transient monoclinic structure without vanadium dimers and zigzag chains. Then, it continues to evolve to the final tetragonal structure in approximately 5 picoseconds. In addition, only one laser fluence threshold instead of two thresholds suggested in polycrystalline samples is observed in our quasi-single-crystal samples. Our findings provide essential information for a comprehensive understanding of the photoinduced ultrafast phase transition in VO2.

In the monoclinic insulating phase (called the M1 phase), V atoms dimerize and form zigzag chains (called the superstructure for convenience in this work).In the rutile (tetragonal) metallic phase (called the R phase), V−V dimers and related zigzag chains disappear.The unit cell in the M1 phase is nearly twice larger than that in the R phase, resulting in more X-ray or electron diffraction peaks (called superstructure peaks for convenience in this work) in the M1 phase.To understand the phase transition, VO2 was studied by various experimental methods, such as terahertz (THz) spectroscopy [26][27][28]30,31 , optical spectroscopy 24,25 , X-ray absorption spectroscopy (XAS) 36 , ultrafast electron diffraction (UED) 17-19,35,37,38, , and ultrafast X-ray diffraction (UXRD) 20 .
However, the true nature of the phase transition is still not yet fully understood.For instance, while the SPT involves both the loss of the V−V dimerization and the crystal symmetry transformation from monocline to tetragon 39,40 , the related atomic pathway has not been fully revealed 17,36 .
The very first UED (in a reflected electron diffraction mode) experiment with femtosecond resolution 17 on single crystalline VO2 proposed that V − V dimers first dilate to form an intermediate phase without dimers but with V zigzag chains.Then V−V bonds begin tilting to eliminate the zigzag chain, leading to the R-phase 17 .UED experiment 19 on polycrystalline VO2 proposed that V atoms might first reach an intermediate structure, which is akin to the M2 phase (a phase with a special configuration of V zigzag chains 40 ).UXRD experiment have found that the disordered movement of V atoms played an important role in the light-induced ultrafast phase transition 20 .Recent UED experiment on single-crystal VO2 indicated that V−V dimers dilate and tilt simultaneously in a coherent way in the very early stage of the photoinduced SPT (PSPT) 37 .However, so far, no information about crystal symmetry transformation has been obtained in diffraction experiments.SPT from the M1 to R phase will not only change the fractional coordinates of V atoms within a unit cell, but also cause a change in the lattice constants of  and  and the angle between them 39 .An ultrafast diffraction conoscopy experiment showed that VO2 films grown on an Al2O3 substrate could sustain an intermediate biaxial structure that is different from the initial M1 phase (the M1 phase is also a biaxial phase) for several hundreds of femtoseconds, which provides some indirect information that the disappearance of V−V dimers and zigzag chains may not coincide with the change of the crystal symmetry in PSPT 34 .
Furthermore, previous UED experiments on polycrystalline VO2 samples suggested a photoexcited long-lived monoclinic metallic phase (called the mM phase) lasting for hundreds of picoseconds 18 .This phase is isostructural to the M1 phase 18,35,36,38 .The key evidence in this UED experiment was that the laser fluence threshold for non-superstructure peaks is smaller than that for superstructure peaks 18 .However, the experimental uncertainty is considerable large due to the low signal-to-noise ratio in polycrystalline samples.Recently, only one threshold was observed in time-resolved XAS 36 .UED on a single crystalline VO2 sample thinned by focused ion beam from a bulk single crystal also gave some indication of one threshold 37 .
In this work, we succeed in fabricating high quality quasi-single crystalline VO2 freestanding films for the first time and revisited the PSPT using mega-electron-volt (MeV) UED with 50 fs resolution 41 .Due to the high signal-to-noise ratio and the special domain structures in quasi-single crystalline samples, we revealed the comprehensive atomic pathway, including the melting of V−V dimers and the transformation from monoclinic symmetry to tetragonal symmetry.We present direct evidence that the disappearance of superstructure does not coincide with the change of the crystal symmetry.We found that V−V dimers melt and V atoms reach the same fractional coordinates as that in the R phase within 200 fs, resulting in a transient monoclinic structure without V−V dimers and zigzag chains.Then, it continues to evolve from this transient monoclinic structure to the final tetragonal structure within 5 picoseconds.In addition, the laser fluence thresholds are accurately measured benefited from the well separation of the superstructure peaks from other diffraction peaks.A single threshold was obtained within the experimental uncertainty.

Results
Preparation of quasi-single crystalline freestanding VO2 films.The state-of-the-art UED technique uses transmission mode, so freestanding films are required.Recently, Sr3Al2O6 (SAO) has been used as a water-soluble sacrificial layer to obtain freestanding single crystalline films by pulsed laser deposition (PLD) 42 .Directly growing ultrathin freestanding single-crystal films on SAO has been demonstrated for materials with perovskite structures such as STO 42 , La0.7Sr0.3MnO3 42, BaTiO3 43 , BiFeO3 44 , and LaMnO3 45 that have lattice parameters close to SAO.However, this method does not apply to VO2 due to the large lattice mismatch.Our solution for obtaining freestanding VO2 films by PLD is to add an ultrathin STO(111) buffer layer between the VO2 and SAO.By limiting the thickness of STO buffer layer to 2 nm, we can minimize the effect from the buffer layer on our diffraction measurements.We grew 20 nm SAO on a bulk SrTiO3(111) (STO) substrates followed by a 2 nm single-crystal STO(111) film as the buffer layer for the VO2 growth.Then we grew ~ 40 nm VO2 on the STO(111) buffer layer.The layout of the sample is shown in Fig. 1(a).
The epitaxial growth of VO2 films on the STO(111) surface was systematically studied in the previous report 46  Diffraction spots from the 2-nm STO buffer layer were also observed, and are indicated by "STO".and S2).Red/green/black colors in Fig. 1(d) represent the contributions from three 120 o domains, respectively.The simulation is in good agreement with the measurements.The elongated shape along the tangential direction in Fig. 1(c) is related to two effects.First, three sets of diffraction spots from 120 o in-plane domains distribute in the tangential direction, seen from Fig. 1(d).They partially overlapped in experiments, which makes the spots broaden along the tangential direction.Second, the shape of crystal grains is anisotropic (see details in Fig. S1 in SM), which also makes the spots broader in the tangential direction than in the radial direction.UED is way bigger than the grain size in the film.The momentum resolution in Fig. 2(a) is not as good as that in Fig. 1(c).There are two reasons.First, unlike the TEM, the electron beam is not focused in UED to achieve a high temporal resolution.Second, the higher kinetic energy in the MeV UED also slightly lowers the momentum resolution.As a result, the diffraction spots close to each other in Fig. 1(c) merge together in Fig. 2(a).For instance, 102 and 3 02 become one spot.For simplicity, we use 102 to index this big spot.102 and 3 02 spots are only existed in the M1 phase related to the V−V dimers.They are superstructure spots.The structure fact of 102 and 3 02 are zero in the R phase, so they both disappear during the SPT from the M1 phase to R phase.
Similarly, 002 , 200, and 2 02 spots are also overlapped.We use 200 as the simple index.In the previous diffraction experiments in a single crystalline sample 37 , the intensities of 002 , 200, and 2 02 spots all enhance during the SPT from the M1 phase to R phase.Therefore, it is reasonable to trace the intensities of the big 102 and 200 spots to study the dynamics of the SPT.
As expected, the superstructure spots disappear above the transition temperature (Fig. 2(a)right).Fig. 2(b) presents the line profiles of a superstructure peak along the direction indicated by the white line in Fig. 2(a).At 300 K, the diffraction peak is well-defined.At 345 K, the diffraction peak disappears and the background is slightly enhanced due to the thermal effect.To simulate the polycrystalline sample to some extent, we plot the azimuthally averaged diffraction intensity in Fig. 2(c).The superstructure peaks (100, 1 02 and 102) are influenced by the neighboring nonsuperstructure peak (200) with very high intensity, which renders accurate determination of the threshold difficult in polycrystalline samples 18 .Following the temperature dependence of the intensity of 102 peak, a clear SPT with a temperature hysteresis loop was observed (see details in SM Fig. S3).
Ultrafast evolution of the diffraction spots in VO2.The VO2 sample shows an ultrafast response to the laser pump.Figure 2(d) present the normalized intensity differences between the diffraction pattern at t = 1.5 ps after photoexcitation and that at t = 0 (without photoexcitaion) under the low ( ~ 5 mJ/cm 2 at 240 K) and high ( ~ 13 mJ/cm 2 at 300 K) laser fluence, respectively., are clearly observed.There is no SPT under low fluence (see data with 1.5 ps in SM Fig. S4).A similar coherent phonon was detected in the UXRD experiment 20 , but had not observed in previous UED experiments primarily due to the insufficient temporal resolution.In contrast, under a high fluence, the intensity drops by more than 70% within 200 fs, and no oscillation was detected.The absence of oscillation implies a fast structural transition, in agreement with the UXRD experiments 20 .Qualitatively consistent with previous UED experiments 18 , the intensity of the non-superstructure 200 peak increases after photoexcitation, as shown in Fig. 2 (f).However, the timescale dichotomy in the intensity evolution of the superstructure and non-superstructure diffraction peaks observed in polycrystalline samples 18,35,38 is not observed in our measurements.Rather, the intensities of all the peaks changed with nearly the same femtosecond timescale, also consistent with UXRD on singlecrystal samples 20 .
Interestingly, we can trace the ultrafast structure evolution by following not only the peak intensity but also the peak width due to the high signal-to-noise ratio and the domain structure in our quasi-single crystalline samples.Presented in Fig. 2(g), the peak width of 200 (W200) along the x-direction (indicated by a green line in Fig. 2(a)) decreased with a picosecond timescale under a high fluence, while it remained constant under a low fluence.To do the analysis on the peak width along x-direction with high signal-to-noise ratio, we superposed the intensities of 200 spot along the y-direction.Since no SPT occurs under a low fluence, the decrease of W200 under a high fluence is very likely related to the SPT.In fact, the decrease of W200 can be understood based on the difference in the lattice constants  and  , as well as the angle (β) between  and  , in the monoclinic structure.As we show in Fig. 1, there are three 120 o domains plus twin domains in our samples, which has two implications for the peak width.First, the separation between the diffraction spots from different 120 o domains is correlated to β .Second, the twin domains originated from the difference in  and  cause the additional peak splitting.With the gradual suppression of twin domains and increase in β, six diffraction spots (Fig. 1(d)) inside a big 200 spot get closer and closer (see more details in SM Fig. S5).As a result, the peak width becomes narrower.The inset of Fig. 2(g) indicates that the peak width decreases in a picosecond timescale, implying that a monoclinic structure persists for several picoseconds even after the complete disappearance of the superstructure in VO2.3(d)-(f).The inserts present six diffraction spots inside the big 200 spot observed in Fig. 2(a).In the initial M1 phase (Figs.3(a) and (d)), the sample consists of the superstructure and twin domains.In the first process, the V − V dimers dilate and tilt simultaneously, which is characterized by the very fast disappearance of the superstructure peaks (Fig. 2(e)).This fast process has been observed in previous scattering studies and was thought to end up in the R phase in a femtosecond timescale 18,20 .It should be noted that in diffraction experiments, the intensity of diffraction peaks is mainly affected by the fractional coordinates of atoms in the unit cell.
Therefore, the disappearance of the superstructure peaks does not necessarily mean that VO2 is in the R phase, but rather it means that V atoms are in the same fractional coordinates as in the R phase.
The evolution of the width of 200 peak in Fig. 2(d) indicates that the fast process does not lead to the R phase, but to an intermediate monoclinic structure without V−V dimers and/or zigzag chains.Figs.3(b) and (e) sketch the final stage of this fast process at t = 200 fs.The V−V dimers and/or zigzag chains no longer exist, and V atoms are located at the same fractional coordinates as in the R phase.Correspondingly, there is no superstructure spot, but aM and cM remains different and β does not reach that in the R phase.Hence, the twin domains still exist after the first process.
The second process is the transition from the monoclinic symmetry to the tetragonal symmetry through the lattice expansion, mainly along the aM direction.This process is characterized by the continual decrease of W200 (Fig. 2 (d)).It is a slow process and nearly finishes within 5 ps, ending up in the R phase sketched in Figs. 3 (c) and (f) with aM = cM and no twinning.The second process is not distinguishable from the first process within the first 200 fs because the movement of V atoms in the first process expands aM simultaneously.The second process suggests that a transient monoclinic structure without V−V dimers and/or zigzag chains exists after the first process and lasts for several picoseconds.
Fluence thresholds measurements.To determine the fluence thresholds for the PSPT, we measured the diffraction intensity at a time delay of 10 ps when the electron and lattice largely reaches a quasi-equilibrium condition.Fig. 4(a) shows the fluence dependence of 102 peak measured at 300 K.A distinct kink is observed at ~ 3.1 mJ/cm 2 .The intensity decreases slowly below this value, while a fast decrease is observed above this value.Because the 102 peak is directly associated with the V−V dimerization, the presence of a kink indicates a fluence threshold for SPT, e.g. a critical energy is needed to melt the V−V dimers for the transition to the R phase.Therefore, at 300 K, we have the threshold (Fc1) of ~ 3.1 mJ/cm 2 .For the 200 peak, a kink is also observed at ~ 3.1 mJ/cm 2 , as shown in Fig. 4(b).Thus, we obtained the threshold (Fc2) of ~ 3.1 mJ/cm 2 .This is in stark contrast to previous work in polycrystalline samples where Fc1 is much larger than Fc2 18 .We repeated the measurements at several temperatures (280 K, 260 K, 240 K, and 220 K) and the threshold for the 102 and 200 peaks are consistently very similar (see details in SM Fig. S6).We also measured the fluence threshold for the change of W200 as shown in Fig. 4(c).Again, it has a similar threshold (Fc3) of ~ 3.1 mJ/cm 2 .
It should be noted that the diffraction measurements do not yield the direct measurements of free-electron density that confirm metallicity.Ref. 18

Discussion
The pathway with two processes we proposed during the PSPT in VO2 differs from the proposed two steps model 17 .In the previous model, the first step is the dilation of the V-V dimers, followed by the tilting of the V-V bonds.Therefore, two timescales were expected for the decay of the superstructure peaks.However, only one timescale following the superstructure peak was observed by the UXRD 20 and UED 37 experiments in the single crystalline samples.While two timescales were observed by UED in the polycrystalline VO2 samples, they were extracted from superstructure and non-superstructure peaks 18 .Some optical measurements observed two processes with two timescales of < 10 ps and > 45 ps 49 which is not directly related to the ultrafast phase transition.In our experiments, only one timescale was obtained based on the superstructure peak, which is in agreement with UXRD and UED in the single-crystal samples.A single timescale in the superstructure peak indicates a direct movement of V atoms; in other word, the V-V dimers dilate and tilt at the same time to the fractional coordinates the same as that of the R phase, which is the first process in our proposal.The second process revealed in our experiments is the transformation from a transient monoclinic structure to the tetragonal R phase.Recent ultrafast X-Ray hyperspectral imaging experiments 50 observed two time constants of 132 fs and 4.75 ps, and they are not related to dilation and tilting 17 .Considering the fact that these time constants are close to the timescales we observed, it is possible that they might be related to the two processes revealed in our experiments, which would be interesting to explore in the future.
The second process can hardly be explored in polycrystalline samples by UED.Although we found this new process in quasi-single crystalline samples, we still cannot tell the exact route how V atoms move as a function of time because of the in-plane domains.High quality freestanding single crystals are need to completely solve this problem.Preliminary results have been obtained in single-crystal samples 37 .However, much more efforts are needed to have well controlled freestanding single crystalline samples.In principle, by tracing the intensity of various diffraction spots in single crystals, we can determine the position of V atoms through the structure factor.
In summary, we studied the PSPT in the quasi-single crystalline VO2 samples with the MeV UED.The highly improved signal-to-noise ratio and temporal resolution not only allowed us to observe the coherent phonon related to the zigzag motion of V atoms connecting the M1 and R phases under a low laser fluence, but also revealed two processes with different timescales when

Transmission electron microscopy
The TEM experiments were conducted using a Talos F200X STEM.The accelerating voltage was 200 kV.The selected area aperture is 200 m in diameter and the diffracting region is 4 m in diameter.The electron beam was incident on the sample along the [010] direction of VO2.
The electron beam in this MeV UED system has a kinetic energy of about 3 MeV and is compressed with a double-bend achromatic magnet system.An electron-multiplying CCD camera and a phosphor screen were used to measure the diffraction pattern.A 800-nm Ti:Sapphire laser with a pulse width of approximately 30 fs was used as the pump laser.The radius spot size of the pump laser was 1.5 mm.The pump laser was near-normal incidence.More details of the MeV UED setup can be found in the literature 41 .A repetition rate of 100 Hz or 50 Hz was used to mitigate the heat accumulation.The temporal resolution was about 50 fs (FWHM) for the measurements of the ultrafast dynamic.The temporal resolution was 100 fs to increase the signal-to-noise ratio for the measurements of the thresholds.
. The normal direction of the film is [010], which nicely matches the UED experiments because V−V dimers exist in the (010) plane.By the strict definition, VO2/STO(111) film is a (010) textured film.Unlike usual textured films with lots of in-plane orientations, VO2/STO(111) film only has three in-plane orientations (called three 120 o domains, for convenience) in the R phase.In the M1 phase, each domain will further form twins. Details about the crystal structure are presented in Fig. S1 in the Supplemental Material (SM).To be distinct from usual textured films, we call VO2/STO(111) films quasi-single crystalline films.After removing SAO by etching in water, we obtained relatively large flakes of freestanding 40-nm VO2/2-nm STO films.The lateral size of the flakes varies around 0.5 × 0.5 mm.See more details about the samples in SM Fig. S1.The freestanding films are transferred to a transmission electron microscope (TEM) grid with ultrathin amorphous carbon film for MeV UED measurements.The amorphous carbon film effectively reduces heat accumulation from the optical pump with a repetition rate of 100 Hz.Static electron diffractions of quasi-single-crystal VO2.The static electron diffraction pattern at 300 K (M1 phase) measured by 200 KeV TEM is shown in Fig. 1(c) with Miller indexes shown for several representative diffraction spots.All Miller indexes use the M1-phase's representation.

Fig. 1 (
Fig. 1(d) shows the simulated ideal TEM diffraction pattern in the M1 phase considering three 120 o domains and the twinning in each domain.The twinning in the M1 phase results from the difference in the lattice constants ( = 5.356 Å and  = 5.383 Å 39,47 ) (see more details in SM Figs.S1 and S2).Red/green/black colors in Fig. 1(d) represent the contributions from three 120 o

Figure 1 .
Figure 1.Static electron diffractions of VO2 by TEM.(a) Schematic layout of the VO2 films for UED experiments.(b) Crystal structures of VO2 in the R phase and M1 phase.Only V atoms are presented.Red solid lines in the M1 phase represent V−V dimers.Hexahedrons with black lines are the unit cells.(c) Static electron diffraction pattern measured at 300 K by TEM.Incident electron beam is along [010] direction of VO2.(d) Simulated TEM diffraction pattern in the M1 phase with three domains and twinning.Red/green/black colors represent the contributions from three 120 o domains, respectively.

Figure 2 .
Figure 2. Ultrafast evolution of the diffraction spots in VO2.(a) Static electron diffraction patterns measured at 300 K (left) and 345 K (right) with the 3-MeV electrons in UED. Green line labels the direction where we trace the peak width.(b) Line profiles along the while line in (a) at 300 K and 345 K, respectively.(c) Azimuthally averaged intensity of the diffraction pattern at 300 K.
Figure 2(a) presents the static diffraction pattern measured by the 3-MeV electrons in the MeV UED instrument below and above the phase transition temperature.It should be noted that three 120 o domains equally contribute to the diffraction intensities because the electron beam size (∼ 150 µm in diameter) in

Figure 2 (
Figure 2(e) shows the transient intensity change of the 102 peak.Under a low fluence, the intensity only drops by about 13% and oscillations of 6 THz related to a coherent phonon of the zigzag motion of V atoms20,25,29,48 , are clearly observed.There is no SPT under low fluence (see

Figure 3 .
Figure 3. Transient monoclinic structure without V−V dimers in VO2.Schematic of (a) Monoclinic M1 phase with V−V dimers and zigzag chains at t = 0, (b) Transient monoclinic structure without V−V dimers and zigzag chains at t = 200 fs and (c) R phase at t = 5 ps.Orange dots represent the projected positions of V atoms on the (010) plane.Black lines present the V−V dimers.Orange lines are a guide for the eyes.Blue quadrilateral indicates the unit cell using M1phase's representation.Bottom panels are the sketch of twin domains.Red and green dots indicate the diffraction spots from two domains of different orientations and the twin domains respectively.And the separation between the two spots is exaggerated for clarity.
suggested an interesting photoinduced mM phase with a very long lifetime (sever hundreds of picoseconds).Besides the optical measurements, one of the key evidences is the existence of two thresholds in UED.Since only one threshold is detected in our experiments, the proposed long lifetime mM phase in Ref. 18 very likely does not exist in our quasi-single crystalline samples.

Figure 4 .
Figure 4. Measurements of the laser fluence.Fluence dependence of the changes measured at t = 10 ps after photoexcitation for (a) 102 peak intensity, (b) 200 peak intensity, and (c) 200 peak width.
SPT occurs: the fast destruction of V−V dimers and zigzag chains within ~ 200 fs and the relatively slow transformation from the monoclinic symmetry to the tetragonal symmetry within ~ 5 ps.Only a single fluence threshold was observed for different diffraction peaks.Our work promotes the comprehensive understanding of the ultrafast PSPT in VO2 and will stimulate more theoretical and experimental efforts in the future.MethodsSample preparation.Quasi-single-crystal VO2 films were epitaxially grown on SrTiO3 (111) (STO) substrates by pulsed laser deposition with a KrF excimer laser (λ = 248 nm, Coherent) using a V2O5 target.The base pressure of our PLD system is 8×10−10  Torr.The STO(111) substrate was first annealed at 1000 o C for 1 hour in oxygen at 1×10−6  Torr to get obtain a clean and flat surface.Then, a 20-nm SAO (111) sacrificial layer followed by a 2-nm STO (111) buffer layer was deposited at 880 o C under 5×10 −6 Torr oxygen pressure.The thickness of SAO and STO was monitored by the reflection high energy electron diffraction (RHEED) oscillations.The laser repetition rate is 5 Hz, and the fluence is 1.5 J/cm 2 .After that, 40 nm quasi-single-crystal VO2(010) films was deposited on the STO (111) buffer layer at 450 o C in 3 mTorr oxygen pressure.The laser repetition rate is 10 Hz, and the fluence was 1 J/cm 2 .We used deionized water to remove the SAO layer and transferred the 40 nm VO2(010)/2 nm STO (111) freestanding film to a TEM grid with ultrathin (~ 5 nm) amorphous carbon film for 200 KeV TEM and 3 MeV UED measurements.Thickness of VO2 film were checked by AFM.