Room-temperature application of VO 2 microstructures on rigid and flexible substrates based on synthesis of crystalline VO 2 solution †

The insulator-to-metal transition (IMT) in vanadium dioxide (VO 2 ) oﬀers temperature-dependent infrared wavelength attenuation, rendering it a great contender for bolometers, optoelectronics, memory devices, smart-windows adaptive thermal camouflage applications. However, the required high temperature processing (up to 600 1 C) of VO 2 inhibits its widespread utilization, particularly in applications with low fabrication temperature limits – such as devices on flexible polymer substrates. This study provides a new method that relies on van der Waals substrates with inert basal planes and hydrophilic surfaces to overcome the hurdle of high processing temperatures. Using this method, highly-oriented, optically active crystalline VO 2 was deposited on both glass and flexible polyethylene terephthalate (PET) substrates. We observe VO 2 crystals during IMT using transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and report 4 60% change in transmission on both glass and flexible PET, as well as demonstrating IR modulation.


Introduction
With ever-increasing global energy demands and continuously worsening climate conditions, it is paramount to develop more sophisticated energy solutions, and to not only rely on renewable energy but to also mitigate excessive energy losses and minimize redundancy. Windows are considered to be a significant source of energy loss in buildings and other structures. Heating and cooling of buildings accounts for B14% of the total energy use in the US. 1 Therefore, reducing unnecessary energy loss from buildings is a vital part of the roadmap towards a sustainable future economy. Minimizing energy loss through windows may be achieved through thermally responsive glazing and coatings, which are an emerging and vital parts of glass technology. 2 Dynamic windows, or smart-windows, change their optical properties by utilizing materials that feature photochromism, electrochromism or thermochromism to overcome the traditionally wasteful nature of windows. 3 Monoclinic vanadium dioxide (VO 2 ) is both electrochromic and thermochromic, and is thus suitable to be applied in smart dynamic coatings, 4-6 since it simultaneously blocks ultraviolet (UV) radiation and modulates infrared (IR) light. [7][8][9][10][11] Crystalline VO 2 experiences thermochromism at B68 1C, when the material switches from a semiconducting monoclinic to a conductive tetragonal configuration. As the material changes crystal configuration, it experiences an increase in free charge carriers, leading to a metallic state. Metallic VO 2 exhibits a dramatically reduced transmission in the IR, microwave and terahertz wavelengths range when compared to the semiconducting phase. [12][13][14][15][16][17][18] The insulator-to-metal transition (IMT) of vanadium oxides depends on the microstructure, morphology and stoichiometry of the oxide. The transition generally occurs within pico/ femtoseconds, 19 making vanadium oxide a promising candidate for electronic switch components in integrated circuits, 20 smart window coatings, [21][22][23] tunable antennas 24,25 and metamaterials. 26 However, the development of commercially viable synthesis methods for VO 2 is hindered, due to inherent difficulties associated with the existence of vanadium oxides of different stoichiometry and pronounced polymorphism. With 15-20 other stable vanadium oxide phases, the formation of VO 2 occurs only over specific ranges of oxygen partial pressures and temperatures. 27,28 Therefore, vapor phase VO 2 synthesis methods such as sputtering [29][30][31][32][33][34] and chemical-vapor deposition (CVD) [35][36][37][38] are widely employed, since they offer a high degree of control over the partial oxygen pressure and can be operated at high temperatures.
Solution-based methods such as sol-gel 10,39,40 and polymer assisted deposition (PAD) 40,41 have also been reported. These methods utilize chemical precursors dissolved in a liquid medium. Such synthesis techniques result in colloidal suspensions, which are excellent for coating large areas using solution processing; however, they still pose limitations on the application of VO 2 solution to temperature-sensitive surfaces and fabrics, since solutions do not undergo IMT without a subsequent annealing step. As a result, they are not suitable for application in a large number of thermally unstable existing window technologies (i.e. acrylic windows) or the retrofitting of existing windows or integration with flexible substrates with low melting points, or fabrics. Importantly, even though hydrothermal processes are widely used in VO 2 synthesis, they do not benefit from the level of morphological and crystallographic control available when using physical or chemical vapor deposition techniques.
Herein, we report a two-step fabrication method that allows the synthesis of fully-functional VO 2 using physical vapor deposition (PVD) techniques. The VO 2 can then be suspended to create a functional colloidal ink that can be applied to any surface, without the need for further post-application annealing. We use highly hydrophilic mica growth substrates to develop this technology, enabling the highly-oriented growth of crystalline VO 2 films through pulsed-DC magnetron sputtering, [42][43][44] while allowing the lift-off into solution postsynthesis. We characterize the VO 2 suspension and observe IMT using in situ transmission electron microscopy (TEM), selected area electron diffraction (SAED) and X-ray photoelectron spectroscopy (XPS), before introducing VO 2 to standard glass and a flexible substrate with low thermal stability (PET) for optical characterization.

VO 2 solution synthesis
The VO 2 thin films are deposited onto mica using pulsed DC magnetron sputtering technique. A vanadium target (99.98% purity) was set up at B120 mm from the substrates with base pressure of B4 Â 10 À7 and working pressure of B3 Â 10 À3 . The Ar : O 2 flow rate of the gas mixture in the sputtering chamber was 13.25 : 6.25 sccm. Sputtering was conducted at 200 W with pulse frequency and reverse time at 25 kHz and 5 ms respectively. VO 2 films were then annealed in a furnace, and evacuated to low vacuum to achieve a pressure of B50-150 mTorr, at 550 1C for 90 min. Mica coated with crystalline VO 2 was immersed in water to allow for delamination (sonication was not essential, but it expediates the delamination of VO 2 crystals). The resultant solution was placed in an oven to undergo slow evaporation overnight at 90 1C, before being allowed to cool down to room temperature. IPA was added to the crystals, forming a VO 2 solution that can be evaporated at room temperature.
X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) XPS analysis was conducted using a Thermo Scientific K-Alpha instrument under ultrahigh vacuum (base pressure o 1 Â 10 À7 Pa). An aluminum Ka X-ray radiation source with energy of 1486.6 eV was used. All core-level spectra of the elements were collected at pass energy of 50 eV and analyzed with Advantage software. The binding energies of all principal elements were referenced to the adventitious carbon (C 1s) of binding energy 284.6 eV. XRD diffractograms are produced using X-ray diffraction powder analyser (D2 Phaser, Bruker).

UV-Vis and bandgap measurements
The UV-Vis absorbance measurements of the aqueous suspension of VO 2 were carried out using an Agilent Cary 7000 UV-Vis-NIR spectrophotometer equipped with an integrating sphere.

Raman spectroscopy
Raman spectra were acquired on a Horiba LabRAM HR Evolution Raman spectrometer using a 532 nm laser with 0.5 mW power and Magnification during the experiment was set to Â100. In this experiment, pure Si was used as substrates.

Scanning electron microscopy (SEM)
The FEI Verios 460L was used to collect scanning electron micrographs. Samples were coated with 5 nm of Pt before being placed in the chamber to prevent surface discharge. FEI Helios Nanolab 600 Dual Beam FIB-SEM was used to collect micrographs of cross sections of VO 2 microstructures on glass.
In situ transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS) TEM and EELS measurements were collected by drop-casting VO 2 crystals onto a Protochips heating chip and mounting it in a Fusion heating/biasing sample holder. TEM in situ analysis for diffraction and imaging were then performed on a JEOL 2100F TEM operating at 200 kV. The following parameters were used (Spot size = 1, alpha = 3, condenser aperture number 3 (CA3), selected area aperture number 2 (SA2)) and the flake was fully covered during the experiment. EELS analysis was performed using the same sample and holder combination in a JEOL F200 TEM operating at 200 kV in STEM mode. A Gatan Continuum S spectrometer was used to collect core loss V L 2,3 edge spectra.
Data was collected at initially conditions (B22 1C) and again at elevated temperature (120 1C) to ensure the transition from the M1 monoclinic phase to the tetragonal phase had completed. As the transition is fully reversible, several temperature sweeps were collected for each data type and for video of the transition process.
In situ X-ray photoelectron spectroscopy (XPS) XPS with in situ heating was performed using a Kratos Axis Supra with monochromated Al Ka X-ray source (1486.6 eV)

Results and discussion
Synthesis of crystalline VO 2 solution VO 2 are deposited directly on mica substrates using pulsed-DC magnetron sputtering at room-temperature for 45 minutes in (O 2 : Ar) ratio of 30%, leading to films with a thickness of B150 nm. The amorphous films undergo crystallization as the films are annealed in partial air at 550 1C for 90 minutes in a vacuum furnace with a pressure between 50-150 mTorr. Subsequently, the grown films are immersed and sonicated in water to separate the functional VO 2 crystals from the mica substrates. Mica is a naturally stratified van der Waals material that features chemically inert basal planes, leading to the grown VO 2 being primarily attached to the surface via weak van der Waals forces. During the delamination process, water intercalates the VO 2 -mica interface, while the mica basal plane experiences H-bonding with H 2 O, donating H + to the interface, leading to the delamination of PVD-grown VO 2 crystals. 45 As an increasing number of crystals are released from the surface, more H-bonding occurs, which in turn leads to more rapid separation and ultimately the effective delamination of the grown VO 2 from the mica surface. Delaminated VO 2 is introduced into isopropyl alcohol (IPA) to enable rapid room-temperature drying before being drop-cast on glass and polyethylene terephthalate (PET) substrates for characterization. Fig. 1a-f visualizes the synthesis process leading to highly oriented crystalline VO 2 , and the mechanism of separation from the mica substrate. An image of the VO 2 suspension is shown in the ESI, † in Fig. S1.

Composition, optical bandgap, Raman spectra and surface morphology of VO 2 crystals
The two-step process reported here includes suspending VO 2 microstructures first in water, then in isopropyl alcohol (IPA). Consequently, it is of great importance to verify that the synthesized VO 2 crystals maintain their stoichiometry and functionality, and do not undergo chemical reactions while being suspended. X-ray photoelectron spectroscopy (XPS) was used to detect any possible changes to the stoichiometry and bonding of the oxygen and vanadium atoms. Fig. S2 (ESI †) shows the XPS survey scan of the VO 2 microstructures synthesised for elemental analysis. The presence V 2p, O 1s peaks and high energy core level peaks OKLL confirms the presence of all the composing elements. Fig. 2a and b show the vanadium and oxygen core-level XPS before and after the separation from the mica substrate. The samples exhibit a V 2p 3/2 peak at B516.4 eV, which is associated with vanadium in the V 4+ oxidation state. [46][47][48][49][50][51][52][53] The O 1s peaks at B530 eV corresponds to V-O bonding, as indicated in the literature, 46,50,54 while 532 eV corresponds to oxygen atoms in the Si-O in the substrate. 55 These findings confirm that the grown crystals as well as the delaminated samples that have been drop-cast on glass substrates for analysis are indeed VO 2 . Overall, aside from the Si-O peak in the drop-cast VO 2 , binding energies and oxidation states correlate well, indicating that the composition of VO 2 crystals withstands the delamination process. The optical properties of the synthesized VO 2 solution were investigated utilizing solution-based UV-Vis spectroscopy. Fig. 3a inset shows the optical absorption spectrum of dropcast VO 2 . Tauc plot analysis was performed to determine the optical bandgap of the synthesized material. Considering composition, oxidation state of vanadium and bulk crystallinity of VO 2 in Fig. 3 a Tauc plot is a beneficial correlation in bandgap between VO 2 synthesized here and reported pristine VO 2 structures. The spectrum indicates that the crystals in solution are VO 2 , since the determined bandgap between V 3d 8 and 3p* of B0.72 eV is characteristic for pristine VO 2 , 56-62 while other possible vanadium oxides such as V 2 O 5 feature wider gaps of B2.35 eV. 62,63 These measurements confirm that there are no other vanadium oxides present since the bandgap measures is characteristic to VO 2 . In Fig. 3a, a B1.95 eV bandgap is present between O 2p p and V 3d 8 , similar to literature reports. 62 This bandgap is born to the admixture of vanadium and oxygen bands in VO 2 , however, this second bandgap is only slightly affected by IMT in VO 2 , since it remains intact while the V 3d 8 and 3p* bandgap collapses. The Raman spectrum of the synthesized VO 2 crystals is shown in Fig. 3b. The observed vibrational modes at 285 and 407 cm À1 are associated with the V-O bending mode. The peak at B143 cm À1 corresponds to the B g symmetry, [64][65][66] while the peak at B97 cm À1 is indicative of a two dimensional (2D) layered architecture, since this mode arises from the translated neighboring layers. [67][68][69][70][71][72][73][74][75] The peak at B520 cm À1 can be attributed to the silicon substrate that was used during the measurement. 76 Other phonon modes at 195 (Ag), 225 (Ag), 304 (Ag), 439 (Bg), 615 (Ag) and 700 cm À1 wavenumbers are reported VO 2 phonon vibrations. [70][71][72][73][74][75] Fig . S3a and b (ESI †) show the X-ray diffraction (XRD) diffractograms for the microstructures before and after the delamination process. Fig. S3a (ESI †) is dominated by the peaks of the mica crystal 77 and due to the potential overlap with the diffraction features of the substrate mica crystals and VO 2 78-80 XRD could not be used to unambiguously analyse the crystal microstructure of the VO 2 crystals structure. Without further steps to remove all mica, some is unavoidably transferred into solution alongside the VO 2 ; however, it is unlikely to affect the optical properties of the solution processed VO 2 due to the wide bandgap and the absence of IMT in mica. Henceforth, in depth analysis to in situ transmission electron microscopy (TEM), selected area electron diffraction (SAED), electron energy loss  View Article Online spectroscopy (EELS) and X-ray photoelectron microscopy (XPS) are relied upon to both examine and verify the monoclinic to tetragonal transition characteristic to VO 2 . The scanning electron microscopy (SEM) image in Fig. 3c shows the VO 2 thin film before the delamination process. The films show some uniformity, while the VO 2 microstructures shown in Fig. 3d are non-uniform and have a different and varying morphology. As parts of the films break during delamination, comparisons in surface roughness using atomic force microscopy (AFM) are not effective, especially in the context of the development of crystalline VO 2 solution. Fig. 3d confirms the layered nature of the VO 2 crystals, revealing sheet sizes in the range of 0.5 to 10 mm. Particle morphology is observed to be not uniform, with substantial size and shape variation, while pinholes are prevalent. Cracking of the VO 2 potentially originates during the manual handling of the mica substrate and diffusion-based delamination process that occurs when samples are immersed in water. Further SEM images displaying similar morphology are shown in the ESI † in Fig. S4a-f. The observed overall consistency in morphology between samples attests to the reproducible nature of the process.

In situ characterization of VO 2 crystals
In situ transmission electron microscopy (TEM), selected area electron diffraction (SAED) and electron energy loss spectroscopy (EELS) The microcrystalline and electronic structure of VO 2 crystals during the temperature-induced transition were studied using high resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED) and electron energy loss spectroscopy (EELS). HRTEM and SAED patterns were analyzed and interpreted utilizing Java Electron Microscopy Software (JEMS) simulations and SingleCrystal 4 software. The data used for analysis is shown in Fig. S5 (ESI †). Fig. 4a-c show HRTEM images of VO 2 drop-cast on a heating chip during different stages of the IMT. The calculated lattice spacing of the VO 2 crystal at room temperature (Fig. 4a) is B3.3 Å along the h110i axis, which is consistent with the monoclinic phase of VO 2 . 81 The VO 2 lattice was found to be highly oriented, and the SAED pattern shows a single crystal over the selected area, which attests to the high crystal quality of the synthesized VO 2 . As the temperature of the sample is increased (Fig. 4a-c), the morphological change of the VO 2 crystals can be observed, leading to images (a) before, (c) during, and (b) after IMT. The dashed red circles in Fig. 4a-c highlight the edge of the crystal, where the change of the crystal phase from monoclinic to tetragonal coordination is particularly apparent through reversible change in contrast, which can be ascribed to change in phase (see ESI, † S5 Video Image 1).
It is observed that both the surfaces and edges of the crystal It is observed that both the surfaces and edges of the crystal transition move through three distinct steps. The monoclinic state is observed at room temperature. During transition, the crystal is in a transient state, which is indicated by the dramatic difference in contrast between Fig. 4c and the structures in Fig. 4a and b. The low contrast observed in the transient state can arise from factors such as thermal fluctuation and boundary movement. Furthermore, it can indicate higher electron transmission through VO 2 microstructures. With the contrast being the result of a higher degree of scattering caused by microstructures in motion in the less stable transient state during IMT. It is noteworthy that the crystal transition is possible to observe, even though it is reported that IMT in VO 2 occurs in the order of pico/femtoseconds 82 due to microstructures undergoing phase transition independently. 83 The microstructural change in Fig. 4a-c is reversible, and this can be seen in Video Image 1 of the VO 2 crystal transformation in the ESI. † It is also noted that the TEM heating chip is heated to 120 1C, even though VO 2 IMT temperature is 68 1C. This ensured the entire heated area of the chip was at above the IMT transition temperature and would remain so for the duration of the required measurements. In the monoclinic states, diffraction patters at VO 2 (À1, À1, 0), (1, 1, 0), (À3, 0, 2), (3, 0, À2), (À2, 1, 2), (2, À1, À2), (À1, 2, 2) and (1, À2, À2) are observed in addition to high-order Lau zone (HOLZ) reflection characteristic to monoclinic VO 2 . IMT as observed by electron diffraction is characterized by the disappearance of the reflection along the h011i axis in the tetragonal state in Fig. 4b and the emergence of diffraction patterns at VO 2 (À1, 1, À1), (1, À1, 1), (À3, 2, 0), (3, À2, 0), (2, À1, À1), and (À2, 1, 1). This observation results from the rearrangement of VO 2 microstructures into linear chains as the microstructures moves between the monoclinic and the tetragonal states. The insets in Fig. 4a and b show selected area electron diffraction (SAED) pattern before and after the transition. As expected, the patterns correspond to the monoclinic and tetragonal phases of VO 2 . 53,84-86 The high switching speed of VO 2 is more apparent in SAED video imaging as well as the in situ collection of SAED patterns, which are provided in the ESI, † S5 Video Image 2. Since microstructures undergo IMT independently, SAED patterns reflect a single microstructure during this process, and SAED patterns for the transient state are not possible to observe. The high switching speed of VO 2 is distinguished in Video Image 2 (ESI †) through the unchanged timestamp (2 : 29 : 214) in the video image before and after IMT, which emphasizes that IMT switching is faster than the resolution the clock provides. The electronic structure of the VO 2 crystal was investigated through in situ EELS measurements before and after IMT. Fig. 4d shows a typical VO 2 EELS spectrum featuring the V-L3, V-L2 and O-K edge onset at 513, 521, 532 eV respectively, and atomic ratio of vanadium to oxygen of B2.0. 87,88 The overall V-L3, V-L2 and O-K edge onset and atomic ratios in the insulating and the metallic states in Fig. 4d remains unchanged, since IMT in VO 2 is a reversible change in electronic, structural and inter/intra band interactions. A subtle change in the electronic structure is observed, indicated by the broadening of the O-K onset in the insulating state. IMT impact on band interactions can be recognized since the O-K edge measures dipole-allowed core-hole interactions between the O 1s to unoccupied O 2p and the V 3d 8 , due to the organization of O 2p in symmetries that allow the mixture of O 2p and V 3d states. 89,90 Fig. 4d highlights the differences in the O-K edge between insulating and metallic VO 2 .
In the insulating state, V 3d 8 * is unoccupied, displaying a lower electron energy loss that leads to a higher full width half maximum (FWHM) of 2.56. In contrast, V 3d 8 * is merged with V 3d 8 and is occupied in the metallic state of VO 2 , causing it to generate a greater electron energy loss and a lower FWHM of 1.72. This change in the O-K edge indicates changes in the V 3d 8 * band during IMT, and is congruent with similar literature reports. 91 In situ EELS measurements indicates the changes in the electronic structure to the V 3d 8 * band in VO 2 during IMT and in situ TEM and SAED observations conclusively verify that VO 2 crystals undergo IMT with changes in morphology. Therefore, we can determine that VO 2 crystals change in both electronic structure and morphology during phase change. However, more accurate in-depth analysis of band transitions and interactions requires the utilization of in situ X-ray photoelectron spectroscopy (XPS).
In situ X-ray photoelectron microscopy (XPS) In a monoclinic VO 2 crystal system, the V 3d 8 splits into two energy bands, V 3d 8 and V 3d 8 *, and the insulating forbidden band of B0.7 eV between V 3dp* and V 3d 8 is formed. When VO 2 undergoes IMT, the forbidden band collapses and the V 3d 8 band extends into the conduction band, forming the metallic tetragonal VO 2 crystal system. 60,92 Consequently, valence band XPS can be used to observe the shift in the V 3d band to the Fermi level when the IMT occurs and the band gap is collapsed. Fig. 5a shows XPS spectra of core-levels at 20 1C (room temperature) and after heating in situ to 120 1C. Symmetric broadening of the V 2p peaks by B1.5 eV is observed above the IMT temperature, which is in agreement with previous reports. 47,93 The broadening is explained by the Coloumb attractive force between the V 2p core-hole and the localized electrons in the V 3d band at the Fermi level, which has been estimated to be 1.5 eV for VO 2 . 94 The instantaneous interactions between the core-hole level and the valence band electrons lead to this artifact in the V 2p 3/2 peak in the core-level spectrum. Fig. 5b shows the valence band spectra of the VO 2 sample at 20 1C and 120 1C. A B0.7 eV prominent shift of the V 3d band towards the Fermi level can be observed (see inset Fig. 5b) after the collapse of the forbidden band.

IMT in VO 2 microstructures on glass, flexible substrates and thermal attenuation
After verifying IMT behavior using in situ characterizations of VO 2 crystals, the functionality of the synthesized VO 2 crystals for thermal camouflage and thermal management was examined. The colloidal VO 2 is drop-cast on glass to collect in situ UV-Vis-IR spectra, in order to ensure that the drop-cast microstructures are optically active. The temperature dependent transmittance data in Fig. 6a and b reveal optical switching of the drop-cast crystals on the glass surface. In Fig. 6b, narrow hysteresis can be observed, which is an artefact of the hysteresis loop for each VO 2 crystal in the drop-cast system. VO 2 microstructures undergo IMT independently, and therefore have an independent hysteresis loop. The difference in the hysteresis loops in bulk VO 2 crystals arises from the presence of a large number of crystals of various size in VO 2 system. 95 Overall, a change in transmission was observed at 2000 nm of over 60%, with complete switching occurring at roughly 70 1C, which is expected for VO 2 .
The colloidal VO 2 system synthesized here can be deposited onto temperature-sensitive substrates such as PET and soda lime glass, which are otherwise incompatible with VO 2 processing, due to their low melting temperatures. Fig. 6c shows a VO 2 film on a flexible PET substrate, featuring the characteristic thermochromic behavior of VO 2 . The PET samples led to an infrared transmission drop of 460%, which is comparable to the results obtained for glass substrates, shown in Fig. 6a. The successful demonstration on PET substrates coated with functional VO 2 , offering temperature dependent transparency The IMT behavior and optical functionality of drop-cast VO 2 microstructures in Fig. 6a and c correlate to the simulation result obtained in Fig. 6d with simulated 3D crystals (inset). Simulated crystals suggest that VO 2 crystals can still undergo IMT, even though they form discontinuous plates. These simulated crystals only approximate SEM images and do not exactly represent the morphology of the VO 2 microstructures measured, so a degree of mismatch between simulated crystals and measurements is to be expected. Moreover, variation in the refractive index between VO 2 microstructures synthesized and values used for simulation leads to different magnitude of change between the insulator and the metal states. 96 Fig. S2d-f in the ESI † examines a cross section of VO 2 crystals drop-cast on glass. The cross section shows an overlap of multiple crystals with different thicknesses. However, this variation does not impact the IMT behavior observed or the optical performance measured for the bulk of the VO 2 microstructures on both glass and PET.
Glass and PET samples were coated with a 50 nm gold layer, in order to conduct IR camera tests to demonstrate the characteristic infrared attenuation of VO 2 microstructures. The metal layer is necessary to increase the accuracy of the thermal camera, since it measures opaque surfaces more accurately than transparent materials and microstructures. Fig. 7a-e show both the optical and infrared images of the VO 2 coated glass and PET, and the control as marked. The control consists of  gold-coated glass and PET, to confirm that the reduction in radiation is representative of VO 2 behavior. Fig. 7b and c show the temperature measured on the surface of the glass and PET control at B30 1C and B100 1C. Fig. 7d and e establish that thermal camouflage is compatible and transferrable to PET. Infrared imaging indicates total reduction in radiation perception of approximately 10 1C when the set temperature is B30 1C and exceeding 50 1C at B100 1C. The infrared imaging in Fig. 7e and f confirms the efficient attenuation of infrared radiation across the entire surface of both glass and PET, and the suitability of VO 2 for IR radiation attenuation even at temperatures as low as 30 1C. Fig. 7f contrasts both glass and PET substrate coated with VO 2 and their application for thermal attenuation. Glass substrates show a greater thermal attenuation than its PET counterpart due to the differences in thermal properties and substrate thickness between the two substrates. Nevertheless, those differences do not contradict the general trends of the substrates thermally responsive properties at different temperatures recorded in Fig. 7f.

Conclusions
Herein a synthesis method is demonstrated that overcomes the high processing temperatures required for producing thermochromic VO 2 films, and the associated limitations in the choice of substrates. Using a van der Waals substrate with inert basal planes and hydrophilic surfaces such as mica allows for the facile delamination of the synthesized oxide into aqueous colloidal suspension. The synthesis strategy allows to effectively separate the high temperature processing steps, enabling the deposition of high quality VO 2 onto arbitrary substrates. TEM, EELS and XPS studies assert the high quality of the transferred VO 2 crystals. The insulator-to-metal transition can be directly observed in operando during temperature-dependent high resolution TEM, leading to the observation of changes in the microcrystalline and electronic microstructures. In situ XPS findings highlighted the impact of forbidden bandgap collapse on both deep core-level spectra and valence band measurements. The optical properties of the deposited films featured excellent optical switching performance and infrared radiation attenuation. Similar optical performance was observed for flexible polyethylene terephthalate (PET) and glass substrates, featuring a change in transmission exceeding 60% at 2000 nm and adaptive thermal attenuation. This work offers the capability to readily synthesize VO 2 and subsequently deposit it from solution at room temperature onto any desired substrates, including substrates that are incompatible with high temperature processing. This extends IMT applications to low-cost, flexible, but temperature sensitive platforms, expanding the scope of future VO 2 -based devices and applications. Furthermore, this method can likely be utilized for the synthesis of other materials that can be grown on mica to introduce colloidal heterostructures.

Conflicts of interest
There are no conflicts to declare.