Vanadyl acetylacetonate VO(acac)2 dissolved in methanol solution was investigated as precursor for the growth of VOx layers by AP liquid injection MOCVD on SiO2-precoated
glass at 375–450 °C temperatures; the highest temperature studied (450 °C) gave the best results in terms of layer crystallinity. The influence of O2 flow rate (from 0 to 80 sccm) on the degree of VOx film crystallinity, morphology and microstructure
was studied for the 450°C growth temperature. The deposited layers were characterized by XRD, Raman, SEM and IR reflectance-transmittance. Thermochromic VO2 layers having switching temperature 60°C were obtained; the capability to control by altering
the process parameters the microstructure and properties of VO2 films deposited on SiO2-precoated glass may be important for thermochromic coatings applications, such as ‘smart windows’. Thus, vanadyl acetylacetonate VO(acac)2 proved to be a successful
precursor for producing vanadium dioxide thermochromic coatings by liquid injection MOCVD.[i]
[i] D. Vernardou, M.E. Pemble 1, D.W. Sheel, Surf. Coat. Technology, 2004, Vol.188–189, p. 250-254, « Vanadium oxides prepared by liquid injection MOCVD using vanadyl acetylacetonate » doi.org/10.1016/j.surfcoat.2004.08.037, www.sciencedirect.com/science/article/abs/pii/S0257897204006723
Vanadyl(IV) acetylacetonate (VO(acac)2) combined with tungsten (VI) ethoxide (W(OEt)6) (0.1 M solution in methanol) was applied as precursor for the growth of W-doped VO2(M) layers by atmospheric‐pressure direct liquid injection (AP-DLI) MOCVD on commercial SiO2-precoated glass substrates. A reduction in thermochromic transition temperature (Tc) from 60 °C in VO2(M) to 35 °C in V0.98W0.02O2 was achieved. (thus such layers may potentially find application as smart glazing units). The variation of atomic percentages of W and O2 flow rates on layer properties was investigated. The grown VO2(W) layers were characterized by XRD, RBS, SEM, IR reflectance‐transmittance, Raman spectroscopy and their crystallinity, composition, morphology, optical properties, uniformity, and layer oxidation state was evaluated. The influence of dopant concentration on Tc in the most applicable range for solar window coatings and parameters for obtaining the single-phase films were studied.[i]
[i] D. Vernardou, M. E. Pemble, D. W. Sheel, Chem. Vapor Dep., 2007, Vol.13, Iss.4, p.158-162, « Tungsten‐Doped Vanadium Oxides Prepared by Direct Liquid Injection MOCVD », doi.org/10.1002/cvde.200606527, //onlinelibrary.wiley.com/doi/abs/10.1002/cvde.200606527
Bis-acetylacetonatovanadyl VO(acac)2 was studied as vanadium precursor for the growth of V2O5 layers (a candidate electrochromic material) by the microwave plasma MOCVD
on the indium tin oxide (ITO)-coated fused SiO2 substrates (obtained by similar method using In(acac)3 and Sn(thd)2 precursors). The deposited films were found by XRD and XPS to be slightly O-deficient V2O5 layers, having ~70% transmittance and the 400 nm
absorption edge. The variation of color of the films (due to the reduction of V atoms) was observed by performing the cyclic voltammograms in a 1 M LiClO4/γ-butyrolactone electrolyte; the deposited layers proved to be suitable for electrochromic material
applications.[i]
[i] H. Watanabe, Ken-ichi Itoh, O. Matsumoto, Thin Solid Films, 2001, Vol. 386, Iss. 2, p.281-285, « Properties of V2O5 thin films deposited by means of plasma MOCVD », https://doi.org/10.1016/S0040-6090(00)01674-6, https://www.sciencedirect.com/science/article/pii/S0040609000016746
VO(hfac)2 (SuperOx, 98%) (combined with H2O vapor as oxygen source) was applied for the deposition of VO2 layers on single crystal R-Al2O3 substrates by MOCVD in inert atmosphere in hot-wall tubular reactor at 400°C growth temperature. The temperature of the sublimator of VO(hfac)2 precursor was 100°C. The Ar carrier flow rate was 3.6 l/h, resulting in the 0.01 Torr pressure in the system. H2O vapor was introduced by injection of liquid water (with 30 mcl/min rate at 0.5 Hz frequency) into the hot zone (100°C). After deposition the samples were annealed in the reactor at 600°C/ 1 h under low pressure (0.01 Torr, Ar flow 3.6 l/h.
Vanadium dioxide is a material where the insulator-to-metal phase transition allows to control dielectric properties and thus to use it as a triggering element for photonic applications in the wide spectral range from optical to terahertz (THz) frequencies.
The VO2 thin films deposited by MOCVD from VO(hfac)2 precursor demonstrated emission of THz radiation both in insulating and conductive phase states under femtosecond pulse irradiation; the efficiency of THz emission increased up to 30 times after the insulator-to-metal
phase transition. This process is fundamentally forbidden in the bulk material but was obtained in thin films, the crucial importance of interface contributions was revealed by polarization analysis of the emitted radiation. Displacement photocurrents at the
VO2–air and VO2–substrate interfaces induced by the incident laser light are determining the properties of the THz radiation emitted by VO2. Fundamental symmetry restrictions are not applicable to problems of nonlinear optics of thin films, as
was demonstrated by the conversion of optical into THz radiation in VO2 films.[i]
[i] M. Esaulkov, P. Solyankin, A. Sidorov, L. Parshina, A. Makarevich, Q. Jin, Q. Luo, O. Novodrvorsky, A. Kaul, E. Cherepetskaya, A. Shkurinov, V. Makarov, X.-Ch. Zhang, Optica 2015, Vol. 2, No. 9, p. 790, « Emission of terahertz pulses from vanadium dioxide films undergoing metal–insulator phase transition », https://www.osapublishing.org/optica/abstract.cfm?uri=optica-2-9-790, https://www.osapublishing.org/DirectPDFAccess/288E62B8-B322-1D3F-5E394E51FC069716_324838/optica-2-9-790.pdf?da=1&id=324838&seq=0&mobile=no
Unconventional precursor vanadyl bis(1,1,1,5,5,5-hexafluoro-2,4-pentanedionate)(hydrate) VO(hfac)2(H2O) was applied as vanadium precursor for the growth of VO2 rutile-type
thin films by thermal CVD using N2 as carrier gas. In this case the oxidation state V (4+) in the precursor is maintained in the layer and the water molecule coordinated to the metal in the precursor, was found
to greatly favor the decomposition of the precursor as an internal source of H+ ions [[i]].
[i] Barreca D., Depero L.E., Franzato E., Rizzi G.A., Sangaletti L., Tondello E., Vettori U., J. Electrochem. Soc. 146 (1999) 551.
The decomposition of vanadyl bis(1,1,1,5,5,5-hexafluoro-2,4-pentanedionate)(hydrate) VO(hfac)2(H2O) in O2 atmosphere in thermal MOCVD conditions, targeting to obtain V2O5 layers (i.e. with the V metal oxidized from (4+) to (5+) oxidation state), was not very efficient - only very thin V2O5 films have been obtained.
Therefore the advantages of PECVD were used to deposit V2O5 layers using VO(hfac)2(H2O) as precursor on on borosilicate and soda-lime glass as substrates. The deposition rate and layer microstructure control was increased using high reactivity of oxygen species in a plasma in PECVD condition (RF power (300 W, 13.56 MHz) was delivered to four electrodes capacitively coupled and surrounding a quartz reactor). The VO(hfac)2(H2O) precursor was evaporated at 70°C and introduced into the reactor through an injection nozzle positioned above the heated substrates. The target of the experiments was to obtain single phase crystalline V2O5 films , investigating the correlations existing between the chemico-physical properties of the coatings and growth conditions.
The deposition conditions (substrate temperature 200-370°C, total pressure 50-70mbar, O2 fow 2.5-20 sccm, layer thickness 95-178 nm) are summarised in Table 1
The grown V2O5 samples were characterized by XRD, XPS and SIMS measurements, AFM was used to study
their surface morphology. Impedance Spectroscopy (IS) was applied for the analysis of the electrical properties of grown V2O5 films. (this technique provides information on the material conduction mechanisms).[i]
[i] D. Barreca, G.A. Battiston, F. Caccavalec, V. di Noto, R. Gerbasi, A. Gregori, G.A. Rizzi, A. Tiziani, E. Tondello, J. Phys. IV France 9 (1999), Pr8-529, “A PE-MOCVD route to V2O5 nanostructured thin films”, https://www.researchgate.net/profile/Davide_Barreca/publication/289140312_A_PE-MOCVD_route_to_V2O5_nanostructured_thin_films/links/570ca9e908ae8883a1fff4ed/A-PE-MOCVD-route-to-V2O5-nanostructured-thin-films.pdf