Publications
-
Atomic coordinates and polarization map around a pair of 1/2 a[011-] dislocation cores produced by plastic deformation in relaxor ferroelectric PIN-PMN-PT
Liu Y., Niu R.-M., Moss S.D., Finkel P., Liao X.-Z., Cairney J.M. Journal of Applied Physics; 129 (23, 234101) 2021. 10.1063/5.0049036. IF: 2.546
The core structures of dislocations are crucial for understanding the plastic deformation mechanisms and the functional properties of materials. Here, we use the scanning transmission electron microscopy imaging techniques of high-resolution high angle annular dark field and integrated differential phase contrast to investigate the atomic structure of a pair of climb-dissociated 1/2a[011-] dislocations in a bending-deformed relaxor ferroelectric Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb1/3)O3-PbTiO3 single crystal. Cations at one dislocation core are found to arrange in the same way as the climb-dissociated 1/2a[011-] dislocation core in SrTiO3, while the other one is different. Oxygen depletion was observed at both dislocation cores. Geometric phase analysis of the lattice rotation shows opposite signs at both sides of the dislocations, demonstrating the strain gradient, which is known to give rise to flexoelectric polarization. Using the peak finding method, the polarization (a combination of ferroelectric and flexoelectric) around dislocations was mapped at the unit-cell scale. The polarization direction obtained is consistent with that predicted based on the flexoelectric effect in a perovskite oxide with [011] geometry. Head-to-head positively charged and tail-to-tail negatively charged domain walls were revealed based on the polarization map, suggesting a new way to stabilize charged domain walls via dislocations. A distinct dislocation core configuration has been observed, and a unit-cell scale polarization map helps understand the flexoelectric effects (coupling between strain gradient and polarization) around dislocations in a relaxor ferroelectric. © 2021 Author(s).
-
Correlating Surface Crystal Orientation and Gas Kinetics in Perovskite Oxide Electrodes
Gao R., Fernandez A., Chakraborty T., Luo A., Pesquera D., Das S., Velarde G., Thoréton V., Kilner J., Ishihara T., Nemšák S., Crumlin E.J., Ertekin E., Martin L.W. Advanced Materials; 33 (20, 2100977) 2021. 10.1002/adma.202100977. IF: 30.849
Solid–gas interactions at electrode surfaces determine the efficiency of solid-oxide fuel cells and electrolyzers. Here, the correlation between surface–gas kinetics and the crystal orientation of perovskite electrodes is studied in the model system La0.8Sr0.2Co0.2Fe0.8O3. The gas-exchange kinetics are characterized by synthesizing epitaxial half-cell geometries where three single-variant surfaces are produced [i.e., La0.8Sr0.2Co0.2Fe0.8O3/La0.9Sr0.1Ga0.95Mg0.05O3−δ/SrRuO3/SrTiO3 (001), (110), and (111)]. Electrochemical impedance spectroscopy and electrical conductivity relaxation measurements reveal a strong surface-orientation dependency of the gas-exchange kinetics, wherein (111)-oriented surfaces exhibit an activity >3-times higher as compared to (001)-oriented surfaces. Oxygen partial pressure ((Formula presented.))-dependent electrochemical impedance spectroscopy studies reveal that while the three surfaces have different gas-exchange kinetics, the reaction mechanisms and rate-limiting steps are the same (i.e., charge-transfer to the diatomic oxygen species). First-principles calculations suggest that the formation energy of vacancies and adsorption at the various surfaces is different and influenced by the surface polarity. Finally, synchrotron-based, ambient-pressure X-ray spectroscopies reveal distinct electronic changes and surface chemistry among the different surface orientations. Taken together, thin-film epitaxy provides an efficient approach to control and understand the electrode reactivity ultimately demonstrating that the (111)-surface exhibits a high density of active surface sites which leads to higher activity. © 2021 Wiley-VCH GmbH
-
Deconvolution of Phonon Scattering by Ferroelectric Domain Walls and Point Defects in a PbTiO3Thin Film Deposited in a Composition-Spread Geometry
Bugallo D., Langenberg E., Ferreiro-Vila E., Smith E.H., Stefani C., Batlle X., Catalan G., Domingo N., Schlom D.G., Rivadulla F. ACS Applied Materials and Interfaces; 13 (38): 45679 - 45685. 2021. 10.1021/acsami.1c08758. IF: 9.229
We present a detailed analysis of the temperature dependence of the thermal conductivity of a ferroelectric PbTiO3 thin film deposited in a composition-spread geometry enabling a continuous range of compositions from ∼25% titanium deficient to ∼20% titanium rich to be studied. By fitting the experimental results to the Debye model we deconvolute and quantify the two main phonon-scattering sources in the system: ferroelectric domain walls (DWs) and point defects. Our results prove that ferroelectric DWs are the main agent limiting the thermal conductivity in this system, not only in the stoichiometric region of the thin film ([Pb]/[Ti] ≈ 1) but also when the concentration of the cation point defects is significant (up to ∼15%). Hence, DWs in ferroelectric materials are a source of phonon scattering at least as effective as point defects. Our results demonstrate the viability and effectiveness of using reconfigurable DWs to control the thermal conductivity in solid-state devices. © 2021 American Chemical Society.
-
Direct Visualization of Anti-Ferroelectric Switching Dynamics via Electrocaloric Imaging
Vales-Castro P., Vellvehi M., Perpiñà X., Caicedo J.M., Jordà X., Faye R., Roleder K., Kajewski D., Perez-Tomas A., Defay E., Catalan G. Advanced Electronic Materials; 7 (12, 2100380) 2021. 10.1002/aelm.202100380. IF: 7.295
The large electrocaloric coupling in PbZrO3 allows using high-speed infrared imaging for visualizing anti-ferroelectric switching dynamics via the associated temperature change. It is found that in ceramic samples of homogeneous temperature and thickness, switching is fast due to the generation of multiple nucleation sites, with devices responding in the millisecond range. By introducing gradients of thickness, however, it is possible to change the dynamics to propagation limited, whereby a single-phase boundary sweeps across the sample like a cold front, at a speed of ≈20 cm s−1. Additionally, introducing thermostatic temperature differences between two sides of the sample enables the simultaneous generation of a negative electrocaloric effect on one side and a positive one on the other, yielding a Janus-like electrocaloric response. © 2021 Wiley-VCH GmbH
-
Effect of Humidity on the Writing Speed and Domain Wall Dynamics of Ferroelectric Domains
Spasojevic I., Verdaguer A., Catalan G., Domingo N. Advanced Electronic Materials; 2021. 10.1002/aelm.202100650. IF: 7.295
The switching dynamics of ferroelectric polarization under electric fields depends on the availability of screening charges in order to stabilize the switched polarization. In ferroelectrics, thin films with exposed surfaces investigated by piezoresponse force microscopy (PFM), the main source of external screening charges is the atmosphere and the water neck, and therefore relative humidity (RH) plays a major role. Here, it is shown how the dynamic writing of domains in BaTiO3 thin films changes by varying scanning speeds in the range of RH between 2.5% and 60%. The measurements reveal that the critical speed for domain writing, which is defined as the highest speed at which electrical writing of a continuous stripe domain is possible, increases non-monotonically with RH. Additionally, the width of line domains shows a power law dependence on the writing speed, with a growth rate coefficient decreasing with RH. The size of the written domains at a constant speed as well as the creep-factor μ describing the domain wall kinetics follow the behavior of water adsorption represented by the adsorption isotherm, indicating that the screening mechanism dominating the switching dynamics is the thickness and the structure of adsorbed water structure and its associated dielectric constant and ionic mobility. © 2021 The Authors. Advanced Electronic Materials published by Wiley-VCH GmbH
-
Energy exponents of avalanches and Hausdorff dimensions of collapse patterns
Casals B., Salje E.K.H. Physical Review E; 104 (5, 054138) 2021. 10.1103/PhysRevE.104.054138. IF: 2.529
A simple numerical model to simulate athermal avalanches is presented. The model is inspired by the "porous collapse"process where the compression of porous materials generates collapse cascades, leading to power law distributed avalanches. The energy (E), amplitude (Amax), and size (S) exponents are derived by computer simulation in two approximations. Time-dependent "jerk"spectra are calculated in a single avalanche model where each avalanche is simulated separately from other avalanches. The average avalanche profile is parabolic, the scaling between energy and amplitude follows E∼Amax2, and the energy exponent is ϵ = 1.33. Adding a general noise term in a continuous event model generates infinite avalanche sequences which allow the evaluation of waiting time distributions and pattern formation. We find the validity of the Omori law and the same exponents as in the single avalanche model. We then add spatial correlations by stipulating the ratio G/N between growth processes G (linked to a previous event location) and nucleation processes N (with new, randomly chosen nucleation sites). We found, in good approximation, a power law correlation between the energy exponent ϵ and the Hausdorff dimension HD of the resulting collapse pattern HD-1∼ϵ-3. The evolving patterns depend strongly on G/N with the distribution of collapse sites equally power law distributed. Its exponent ϵtopo would be linked to the dynamical exponent ϵ if each collapse carried an energy equivalent to the size of the collapse. A complex correlation between ϵ,ϵtopo, and HD emerges, depending strongly on the relative occupancy of the collapse sites in the simulation box. © 2021 American Physical Society.
-
Local and correlated studies of humidity-mediated ferroelectric thin film surface charge dynamics
Gaponenko I., Musy L., Domingo N., Stucki N., Verdaguer A., Bassiri-Gharb N., Paruch P. npj Computational Materials; 7 (1, 163) 2021. 10.1038/s41524-021-00615-4. IF: 12.241
Electrochemical phenomena in ferroelectrics are of particular interest for catalysis and sensing applications, with recent studies highlighting the combined role of the ferroelectric polarisation, applied surface voltage and overall switching history. Here, we present a systematic Kelvin probe microscopy study of the effect of relative humidity and polarisation switching history on the surface charge dissipation in ferroelectric Pb(Zr0.2Ti0.8)O3 thin films. We analyse the interaction of surface charges with ferroelectric domains through the framework of physically constrained unsupervised machine learning matrix factorisation, Dictionary Learning, and reveal a complex interplay of voltage-mediated physical processes underlying the observed signal decays. Additional insight into the observed behaviours is given by a Fitzhugh–Nagumo reaction–diffusion model, highlighting the lateral spread and charge passivation process contributors within the Dictionary Learning analysis. © 2021, The Author(s).
-
Low-Voltage Magnetoelectric Coupling in Fe0.5Rh0.5/0.68PbMg1/3Nb2/3O3-0.32PbTiO3 Thin-Film Heterostructures
Zhao W., Kim J., Huang X., Zhang L., Pesquera D., Velarde G.A.P., Gosavi T., Lin C.-C., Nikonov D.E., Li H., Young I.A., Ramesh R., Martin L.W. Advanced Functional Materials; 31 (40, 2105068) 2021. 10.1002/adfm.202105068. IF: 18.808
The rapid development of computing applications demands novel low-energy consumption devices for information processing. Among various candidates, magnetoelectric heterostructures hold promise for meeting the required voltage and power goals. Here, a route to low-voltage control of magnetism in 30 nm Fe0.5Rh0.5/100 nm 0.68PbMg1/3Nb2/3O3-0.32PbTiO3 (PMN-PT) heterostructures is demonstrated wherein the magnetoelectric coupling is achieved via strain-induced changes in the Fe0.5Rh0.5 mediated by voltages applied to the PMN-PT. We describe approaches to achieve high-quality, epitaxial growth of Fe0.5Rh0.5 on the PMN-PT films and, a methodology to probe and quantify magnetoelectric coupling in small thin-film devices via studies of the anomalous Hall effect. By comparing the spin-flop field change induced by temperature and external voltage, the magnetoelectric coupling coefficient is estimated to reach ≈7 × 10−8 s m−1 at 325 K while applying a −0.75 V bias. © 2021 Wiley-VCH GmbH
-
Mechanical reading of ferroelectric polarization
Stefani C., Langenberg E., Cordero-Edwards K., Schlom D.G., Catalan G., Domingo N. Journal of Applied Physics; 130 (7, 0059930) 2021. 10.1063/5.0059930. IF: 2.546
Flexoelectricity is a property of dielectric materials whereby they exhibit electric polarization induced by strain gradients; while this effect can be negligible at the macroscale, it can become dominant at the nanoscale, where strain gradients can turn out to be tremendous. Previous works have demonstrated that flexoelectricity coupled with piezoelectricity enables the mechanical writing of ferroelectric polarization. When considering ferroelectric materials with out-of-plane polarization, the coupling of piezoelectricity with flexoelectricity can insert a mechanical asymmetry to the system and enable the distinction of oppositely polarized domains, based on their nanomechanical response. Using atomic force microscopy and, more specifically, contact resonance techniques, the coupling of flexoelectricity to piezoelectricity can be exploited to mechanically read the sign of ferroelectric polarization in a non-destructive way. We have measured a variety of ferroelectric materials, from a single crystal to thin films, and domains that are polarized down always appear to be stiffer than oppositely polarized domains. In this article, we demonstrate experimentally that the phenomenon is size-dependent and strongly enhanced when the dimension of the material is reduced to nanoscale in thin films. Ultimately, we demonstrate how the sensitivity in mechanical reading of ferroelectric polarization can be improved by appropriately tuning the mechanical stiffness of the cantilevers. © 2021 Author(s).
-
Metallic Diluted Dimerization in VO2 Tweeds
Sandiumenge F., Rodríguez L., Pruneda M., Magén C., Santiso J., Catalan G. Advanced Materials; 33 (9, 2004374) 2021. 10.1002/adma.202004374. IF: 30.849
The observation of electronic phase separation textures in vanadium dioxide, a prototypical electron-correlated oxide, has recently added new perspectives on the long standing debate about its metal–insulator transition and its applications. Yet, the lack of atomically resolved information on phases accompanying such complex patterns still hinders a comprehensive understanding of the transition and its implementation in practical devices. In this work, atomic resolution imaging and spectroscopy unveils the existence of ferroelastic tweed structures on ≈5 nm length scales, well below the resolution limit of currently used spectroscopic imaging techniques. Moreover, density functional theory calculations show that this pretransitional fine-scale tweed, which on average looks and behaves like the standard metallic rutile phase, is in fact weaved by semi-dimerized chains of vanadium in a new monoclinic phase that represents a structural bridge to the monoclinic insulating ground state. These observations provide a multiscale perspective for the interpretation of existing data, whereby phase coexistence and structural intermixing can occur all the way down to the atomic scale. © 2021 Wiley-VCH GmbH
-
Non-linear nanoscale piezoresponse of single ZnO nanowires affected by piezotronic effect
Lozano H., Catalán G., Esteve J., Domingo N., Murillo G. Nanotechnology; 32 (2, 025202) 2021. 10.1088/1361-6528/abb972. IF: 3.874
Zinc oxide (ZnO) nanowires (NWs) as semiconductor piezoelectric nanostructures have emerged as material of interest for applications in energy harvesting, photonics, sensing, biomedical science, actuators or spintronics. The expression for the piezoelectric properties in semiconductor materials is concealed by the screening effect of the available carriers and the piezotronic effect, leading to complex nanoscale piezoresponse signals. Here, we have developed a metal-semiconductor-metal model to simulate the piezoresponse of single ZnO NWs, demonstrating that the apparent non-linearity in the piezoelectric coefficient arises from the asymmetry created by the forward and reversed biased Schottky barriers at the semiconductor-metal junctions. By directly measuring the experimental I-V characteristics of ZnO NWs with conductive atomic force microscope together with the piezoelectric vertical coefficient by piezoresponse force microscopy, and comparing them with the numerical calculations for our model, effective piezoelectric coefficients in the range d 33eff ∼ 8.6 pm V-1-12.3 pm V-1 have been extracted for ZnO NWs. We have further demonstrated via simulations the dependence between the effective piezoelectric coefficient d 33eff and the geometry and physical dimensions of the NW (radius to length ratio), revealing that the higher d 33eff is obtained for thin and long NWs due to the tensor nature proportionality between electric fields and deformation in NW geometries. Moreover, the non-linearity of the piezoresponse also leads to multiharmonic electromechanical response observed at the second and higher harmonics that indeed is not restricted to piezoelectric semiconductor materials but can be generalized to any type of asymmetric voltage drops on a piezoelectric structure as well as leaky wide band-gap semiconductor ferroelectrics. © 2020 IOP Publishing Ltd.
-
Origin of large negative electrocaloric effect in antiferroelectric PbZr O3
Vales-Castro P., Faye R., Vellvehi M., Nouchokgwe Y., Perpiñà X., Caicedo J.M., Jordà X., Roleder K., Kajewski D., Perez-Tomas A., Defay E., Catalan G. Physical Review B; 103 (5, 054112) 2021. 10.1103/PhysRevB.103.054112. IF: 4.036
We have studied the electrocaloric response of the archetypal antiferroelectric PbZrO3 as a function of voltage and temperature in the vicinity of its antiferroelectric-paraelectric phase transition. Large electrocaloric effects of opposite signs, ranging from an electrocooling of -3.5 K to an electroheating of +5.5K, were directly measured with an infrared camera. We show by calorimetric and electromechanical measurements that the large negative electrocaloric effect comes from an endothermic antiferroelectric-ferroelectric switching, in contrast to dipole destabilization of the antiparallel lattice, previously proposed as an explanation for the negative electrocaloric effect of antiferroelectrics. © 2021 American Physical Society.
-
Oxidation processes at the surface of BaTiO3 thin films under environmental conditions
Spasojevic I., Sauthier G., Caicedo J.M., Verdaguer A., Domingo N. Applied Surface Science; 565 (150288) 2021. 10.1016/j.apsusc.2021.150288. IF: 6.707
Dissociation and adsorption of water on ferroelectric oxide surfaces play important role in the processes of screening and switching dynamics of ferroelectric polarization, as well as in catalytic processes which can be additionally coupled with light, temperature or vibration stimuli. In this work, we present XPS study of ferroelectric BaTiO3thin films and determine the entanglement between surface chemistry, polarization direction and stability, by observing changes upon time exposure to environmental conditions, heating in O2atmosphere and irradiation in vacuum. We devote special attention to Ba 3d spectral region and identify two different oxidation states of O atoms in the compounds of Ba. While this second specie was generally attributed to Ba in surface compounds where it has different oxygen coordination than in the bulk, based on the XPS results of oxygen annealed thin films, we demonstrate that this so far neglected component, corresponds to barium peroxide (BaO2) and identify it as important active specie for the study of screening mechanisms closely related with catalytic activity present in this ferroelectric material. Finally, we report on chemically assisted polarization switching in thin films induced by heating in vacuum or exposure to X-Ray radiation due to the formation of positive surface electric field created by oxygen or electron vacancies, respectively. © 2021 The Authors
-
PbS-Pb-Cu xS Composites for Thermoelectric Application
Li M., Liu Y., Zhang Y., Han X., Xiao K., Nabahat M., Arbiol J., Llorca J., Ibañez M., Cabot A. ACS Applied Materials and Interfaces; 13 (43): 51373 - 51382. 2021. 10.1021/acsami.1c15609. IF: 9.229
Composite materials offer numerous advantages in a wide range of applications, including thermoelectrics. Here, semiconductor-metal composites are produced by just blending nanoparticles of a sulfide semiconductor obtained in aqueous solution and at room temperature with a metallic Cu powder. The obtained blend is annealed in a reducing atmosphere and afterward consolidated into dense polycrystalline pellets through spark plasma sintering (SPS). We observe that, during the annealing process, the presence of metallic copper activates a partial reduction of the PbS, resulting in the formation of PbS-Pb-CuxS composites. The presence of metallic lead during the SPS process habilitates the liquid-phase sintering of the composite. Besides, by comparing the transport properties of PbS, the PbS-Pb-CuxS composites, and PbS-CuxS composites obtained by blending PbS and CuxS nanoparticles, we demonstrate that the presence of metallic lead decisively contributes to a strong increase of the charge carrier concentration through spillover of charge carriers enabled by the low work function of lead. The increase in charge carrier concentration translates into much higher electrical conductivities and moderately lower Seebeck coefficients. These properties translate into power factors up to 2.1 mW m-1 K-2 at ambient temperature, well above those of PbS and PbS + CuxS. Additionally, the presence of multiple phases in the final composite results in a notable decrease in the lattice thermal conductivity. Overall, the introduction of metallic copper in the initial blend results in a significant improvement of the thermoelectric performance of PbS, reaching a dimensionless thermoelectric figure of merit ZT = 1.1 at 750 K, which represents about a 400% increase over bare PbS. Besides, an average ZTave = 0.72 in the temperature range 320-773 K is demonstrated. © 2021 American Chemical Society.
-
Piezoelectricity in nominally centrosymmetric phases
Aktas O., Kangama M., Linyu G., Catalan G., Ding X., Zunger A., Salje E.K.H. Physical Review Research; 3 (4, 043221) 2021. 10.1103/PhysRevResearch.3.043221. IF: 0.000
Compound phases often display properties that are symmetry forbidden relative to their nominal, average crystallographic symmetry, even if extrinsic reasons (defects, strain, or imperfections) are not apparent. Specifically, breaking the macroscopic inversion symmetry of a centrosymmetric phase can dominate or significantly change its observed properties while the detailed mechanisms and magnitudes of the deviations of symmetry breaking are often obscure. Here, we choose piezoelectricity as a tool to investigate macroscopic inversion-symmetry breaking in nominally centrosymmetric materials as a prominent example and measure resonant piezoelectric spectroscopy (RPS) and Resonant Ultrasound Spectroscopy (RUS) in 15 compounds, 18 samples, and 21 different phases, including unpoled ferroelectrics, paraelectrics, relaxors, ferroelastics, incipient ferroelectrics, and isotropic materials with low defect concentrations, i.e., NaCl, fused silica, and CaF2. We exclude the flexoelectric effect as a source of the observed piezoelectricity yet observe piezoelectricity in all nominally cubic phases of these samples. By scaling the RPS intensities with those of RUS, we calibrate the effective piezoelectric coefficients using single-crystal quartz as standard. Using this scaling we determine the effective piezoelectric modulus in nominally nonpiezoelectric phases, finding that the "symmetry-forbidden"piezoelectric effect ranges from ∼1 to 10-5 pm/V (∼0.5% to ∼2×10-5% of the piezoelectric coefficient of poled ferroelectric lead zirconate titanate). The values for the unpoled ferroelectric phase are only slightly higher than those in the paraelectric phase. The extremely low coefficients are well below the detection limit of conventional piezoelectric measurements and demonstrate RPS as a convenient and ultrahighly sensitive method to measure piezoelectricity. We suggest that symmetry-breaking piezoelectricity in nominally centrosymmetric materials and disordered, unpoled ferroelectrics is a common phenomenon. © 2021 authors. Published by the American Physical Society.
-
Pyroelectric thin films - Past, present, and future
Velarde G., Pandya S., Karthik J., Pesquera D., Martin L.W. APL Materials; 9 (1, 010702) 2021. 10.1063/5.0035735. IF: 5.096
Pyroelectrics are a material class that undergoes a change in polarization as the temperature of the system is varied. This effect can be utilized for applications ranging from thermal imaging and sensing to waste-heat energy conversion to thermally driven electron emission. Here, we review recent advances in the study and utilization of thin-film pyroelectrics. Leveraging advances in modeling, synthesis, and characterization has provided a pathway forward in one of the more poorly developed subfields of ferroelectricity. We introduce the complex physical phenomena of pyroelectricity, briefly explore the history of work in this space, and highlight not only new advances in the direct measurement of such effects but also how our ability to control thin-film materials is changing our understanding of this response. Finally, we discuss recent advances in thin-film pyroelectric devices and introduce a number of potentially new directions the field may follow in the coming years. © 2021 Author(s).
-
Strong strain gradients and phase coexistence at the metal-insulator transition in VO2 epitaxial films
Rodríguez L., Sandiumenge F., Frontera C., Caicedo J.M., Padilla J., Catalán G., Santiso J. Acta Materialia; 220 (117336) 2021. 10.1016/j.actamat.2021.117336. IF: 8.203
The proximity of a thermodynamic triple point and the formation of transient metastable phases may result in complex phase and microstructural trajectories across the metal-insulator transition in strained VO2 films. A detailed analysis using in-situ synchrotron X-ray diffraction unveils subtle fingerprints of this complexity in the structure of epitaxial films. During phase transition the low-temperature monoclinic M1 phase is constrained along the {111}R planes by the coexisting high-temperature R phase domains, which remain epitaxially clamped to the substrate. This geometrical constraint induces counteracting local stresses that result in a combined tilt and uniaxial in-plane compression of M1 domains, and a concomitant anomalous cR-axis elongation. This mechanism progressively transforms the M1 phase into the transitional triclinic phase (T), and ultimately into the monoclinic M2 phase, generating strong strain and tilt gradients that remain frozen after the complete transformation of the R phase upon cooling to RT. The transformation path of VO2 films, the complex competition between stable and metastable VO2 polymorphs and its impact on the structure of the low temperature monoclinic state, provide essential insights for understanding the electronic and mechanical properties of the films at the nanoscale, as well as to control their use in functional devices. © 2021
