The sol-gel process is based on the hydrolysis and condensation
of molecular precursors. These molecular precursors are usually metal alkoxides.
However, hydrolyzed metal ions in aqueous solutions exhibit also sol-gel
transition . This technique is very convenient for the synthesis
of oxides or multicomponent oxides such as ceramics or glasses. The crystalline
structure and the atomic homogeneity can be tailored by controlling the
process parameters. Usually, complete crystallized structures are obtained
at lower temperatures than those of conventional ceramic processes. The
preparation of thin films from sol-gel solutions is today one of the main
applications of the sol-gel technique for the deposition of oxides with
a wide variety of properties.
Thermochromic materials are characterized by a
semiconductor-to-metal transition occurring from a reversible change in their
crystalline structure as a function of the temperature. This change have been
observed in transition-metal oxides [2,3] such as Ti2O3,
Fe3O4, Mo9O26 and in several
Magneli phases of vanadium oxide, VnO2n-1. Among them, VO2
has been received most attention because of the large reversible change of
electric, magnetic and optical properties at temperatures around 70°C (4).
During the semiconductor-to metal-transition, the optical properties of vanadium
dioxide are characterized by a sharp decrease in optical transmission in the
infrared spectrum. This is coupled with an increasing in its reflectivity.
Because of this anomalous behavior, vanadium dioxide has been presented as an
attractive thin film material for electrical or optical switches, optical
storage, laser protection, and solar energy control for windows space
The transition temperature of vanadium dioxide may be decreased by the
addition of high-valent transition metals such as niobium, molybdenum or
tungsten. Trivalent cations (Cr3+ and Al3+) increase the transition temperature.
The hysteresis profile associated with the transition depends on the
microstructure and crystallinity.
Among the techniques of thin-film deposition, sol-gel technique
have been used to prepare vanadium dioxide thin films [5-13]. VO2
films can be prepared by using tetravalent alkoxide precursor such as vanadium
tetrabutoxide [7,8], In this process, VO2
films are synthesized after a heat treatment at 600°C under nitrogen
atmosphere. Vanadium dioxide has been also obtained from pentavalent vanadium
precursors including vanadium oxo isopropoxide [5, 6, 9, 10, 11], and
vanadium pentoxide [12, 13]. These processes generally require a reducing
atmosphere (vacuum, CO/CO2, H, Noxal 3) for the synthesis of crystalline VO2
Figure 1, shows a view of a sol-gel VO2 thin films
on a silica substrates.
Figure 1 : VO2 thin films of several thickness deposited onto
Dried films are prepared by direct deposition vanadium oxo
isopropoxide /Isopropanol solution. Solvent evaporation, partial hydrolysis and
condensation occur after film deposition by spin coating (3000 rpm, 15 sec) and
infrared heating at 60°C in air at ambient humidity. The green color of the
films suggests that some pentavalent vanadium species are reduced to tetravalent
vanadium. Upon drying, vanadium oxo-polymers, [VO(OR)3-x(OH)x]n
and [V2O5-x(OR)x]n, rather than hydrated
vanadium oxides, are formed. In addition, the as-deposited coatings are
amorphous and do not exhibit the typical layered structure of hydrated vanadium
oxides. Crystalline VO2 is subsequently formed when the films are
submitted to a heat treatment of 500°C for 2 hours under a reducing atmosphere
of Noxal 3 (Ar-H2( 5%)) . In figure 1, the three samples show the
effect of the thickness on the visual aspect of films. Thickness is varied by
depositing successive layers at room temperature, with a drying step in between
depositions. From the left to the right, one, three and five layers were
deposited. From SEM observations, the cross-section thickness were, 70 nm, 400
nm and 500 nm respectively.
The properties of VO2 films are dependent of the
microstructure and crystallinity. These can be controlled by the experimental
process parameters, such as the molecular precursors, heat treatments and
The change in transition temperature has also been investigated
in sol-gel VO2 doped thin films . In these doped films,
switching characteristics, such as transition temperature, hysteresis loop, and
light blocking properties, are modified.
SEM characterization of heat the treated samples shown in figure 1 revealed
crystallite sizes of about 50 nm for the sample having 70 nm thickness, 100 nm
to 300 nm for the sample with 400 nm of thickness, and 100 to 400 for the sample
with 500 nm thickness. Relatively mono-dispersed grain sizes were observed on
films having thickness up to 300 nm.
The optical switching behavior of a 300 nm thick film is shown
in figure 2.
Figure 2 : Optical hysteresis at 2.5 µm of a sol-gel prepared VO2
The optical transmission axis is shown on a log scale to better
see the transmission minimum obtained at the metal phase. A large hysteresis
loop of about 20 °C is observed. This is probably related to the microstructure
and some residual carbon, observed by SIMS . The anomalous hysteresis
shape, characterized in the cooling step, has been interpreted  as
the result of the existence of crystallite texture domains differing in
crystalline orientation and size.
VO2 thin films were deposited also onto ZnS and
Germanium substrates. Coatings having good homogeneity and optical properties
were deposited on the ZnS substrates. However, poor coating adhesion was
observed. Thickness greater than 100 nm could not be deposited. In addition,
good quality coatings were obtained on germanium.
crystalline phase was observed by XRD is shown in figure 3.
: X-ray diffraction of a vanadium dioxide film using germanium
transmission at the low temperature (semiconductor) state and at the high
temperature (metal) state is shown in the 4. Because of the wide infrared window
of germanium, we could study the blocking properties of VO2 sol-gel thin films
in the range of 1 µm to 20 µm. Infrared transmission lower than 0.5% was
measured for VO2 coatings when heated at 100°C.
Figure 4 :
Infrared transmittance at the semiconductor state (22°C)
and metal state (100°C) of a sol-gel VO2 thin film on germanium.
Among the transition metal oxide exhibiting semiconductor to
metal structural phase transition, vanadium dioxide is the most spectacular
example. This is due to the amplitude of physical property changes during this
phase transition. This has led to, vanadium dioxide being studied, not only from
the fundamental point of view, but also for potential applications in
electronics, optics or opto-electronics This interest is especially generated by
large changes of resistivity, optical transmission, and reflection in the
infrared. These changes are associated with a hysteresis loop when the material
is heated and cooled around the temperature of phase transition.
Because of the ability to change the transition temperature by
doping, Lee et al  and more recently P. Jin et al. ,
suggested that tungsten doped vanadium dioxide can be used in energy
efficient-windows. These smart windows or electrochromic displays find special
applications in the architectural, automotive and aerospace sectors .
Roach  pointed that, due to the changes in
reflectivity during the phase transition, VO2 films can be used
as a kind of optical disc medium and demonstrate holographic storage.
Bit recording on VO2 films using a near-infrared
laser was demonstrated , Stability during long-term storage and over
108 time-cycles of write and erase were achieved without
degradation. Switching time of about 30 nsec and writing energy of the order of
a few mJ/cm2 were reported . Bit density has been
estimated to be 350 bits/mm. Such low threshold recording energy and erase-rewritabilty
encourage the use of VO2 films as a recording media .
More recently, the use of VO2 thin films was
suggested in ultrafast optical switching devices. The high-temperature metallic
state was attained in 5ps by using femto second laser excitation at 780 nm was
In summary, vanadium dioxide is an interesting candidate for modern
applications of active thin films in optical or electric  switches.
The sol-gel process is an attractive approach for preparing these films.
The applicability of this kind of coatings needs more work on device feasibility,
reproducibility and scale-up processes.
This work was made in the Laboratoire de Chimie de la
Matière Condensée of the University Pierre et
Marie Curie where Prof. Jacques Livage generously accepted to provide me
with his assistance and collaboration. In addition, this work was partially
financed by the French Office National d'Etudes et de Recherches
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Dr. Guillaume Guzman works for Corning in the Materials, Surface and Interface Group. His
research focus on soft chemistry applications such as coatings, microstructuring surfaces and
surface treatments on glasses, ceramics or composites
and organic/inorganic materials.
He realized research on ferroelectric materials, intercalation compounds for lithium ion batteries,
optical materials, barrier layers, phase separation systems, bioactive surfaces, electrochromic
and infrared thermochromic materials. His Ph.D. (Physics and Chemistry Department, Sao
Paulo University, Sao Carlos, Brazil, 1991) and post-doctoral research (Chemistry of
Condensed Mater Laboratory, Pierre et Curie University, Paris, France, 1993) treated
fundamental aspects on multicomponents bulk and coating ceramics, and coating/substrate
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