The paper describes the effect of doping with hydrogen and tungsten by means of plasma-immersion ion implantation (PIII) on the properties of vanadium dioxide and hydrated vanadium pentoxide films. It is shown that the parameters of the metal-insulator phase transition in VO2 thin films depend on the hydrogen implantation dose. Next, we explore the effect of PIII on composition, optical properties, and the internal electrochromic effect (IECE) in V2O5·
Vanadium being a transition metal forms a large number of oxide phases with different V atom oxidation degrees:
In stoichiometric VO2 single crystals, the MIT occurs at a temperature of 340 K. It is a first-order phase transition: a latent heat of 4.27 kJ/mol is absorbed at heating and released at cooling; there is a temperature hysteresis whose width
In addition, in MOM structures with vanadium dioxide, as well as in structures based on some other transition metal oxides, the phenomenon of electrical switching associated with MIT is observed [
Vanadium pentoxide demonstrates electrochromic behavior under lithium or hydrogen intercalation due to the formation of
The electrochromic effect has potential applications in making optical indicators, displays, optoelectronic switches, sensors, and so on. In the hydrated vanadium pentoxide V2O5·
The IECE in vanadium pentoxide xerogel takes place when a DC voltage is applied to the film across the two metal electrodes. At the flowing current, a bright red spot appears and gradually grows under the negative electrode. This phenomenon is associated with the migration of protons, which are present in the xerogel water phase, toward the cathode and formation of red-colored highest polyvanadic acids, particularly, hexavanadic Н4V6O
Moreover, electrical switching is observed in MOM structures based on vanadium pentoxide gel films after electroforming which results in formation of a VO2 channel inside the film between metal electrodes [
All the foregoing allows us to conclude that the alloying of vanadium oxide films is of undoubted interest, since it provides an opportunity for varying the MIT and switching parameters in a wide range, as well as for the improvement of the IECE manifestation and performance in V2O5 xerogel films.
Current methods of alloying, particularly those based on thermal-diffusion processing, have a disadvantage associated with an inhomogeneous distribution of the dopant. In addition, thermal-diffusion doping requires substantial heating of the sample. Doping at the stage of synthesis (e.g., with magnetron sputtering using a V-W target or the introduction of tungsten oxide into the initial sol in the production of films by the sol-gel method) also results in an inhomogeneous distribution of impurities and even phase separation. The ion-beam implantation method requires high ion energies, while the penetration depth of alloying elements in the material is unacceptably high, and the ion-beam implantation systems themselves require complex beam focusing and sample movement devices, so implantation at low energies and small depths of doping becomes ineffective. In addition, both ion-beam and heat treatment can be accompanied by a change in oxygen stoichiometry, in particular, by oxidation of VO2 to higher oxides of the
Thus, the modification of the properties of vanadium oxide films during their doping by the method of plasma-immersion ion implantation is a topical task. In this paper we report the results of the effect of doping with hydrogen and tungsten by means of plasma-immersion ion implantation on the structure, composition, and physical properties of vanadium dioxide and hydrated vanadium pentoxide films. The metal-insulator transition, electrochromic effect, and electrical switching in these materials are studied, and their modification under doping is examined.
In the case of doping with hydrogen, the samples were hydrogenated using a specially designed PIII setup schematically depicted in Figure
(a) Block diagram of PIII setup and (b) the setup modification for W implantation.
The irradiation dose
The parameters of PIII are presented in Table
Parameters of plasma-immersion ion implantation.
Parameter, unit | Implantation of H into VO2 | Implantation of H into V2O5 gel | Implantation of W into V2O5 gel |
---|---|---|---|
Discharge current, A | 9.5 | 9 | 10 |
Discharge voltage, V | 69.5 | 75 | 55 |
Cathode heating current, A | 65 | 60 | 65 |
Pressure, Pa | 4 | 4 | 1.2 (Ar); 3.8 (W(CO)6) |
Gas flow, m3Pa/s | 0.0018 (H2) | 0.0018 (H2) | 0.0018 (Ar) |
|
2 | 2 | 2 |
|
20 | 50 | 50 |
|
10 | 10 | 5 |
|
1–5 | 5–30 | 1–5 |
|
1-2 | 2 | 1.7 |
When implanting tungsten, the discharge was ignited in argon. A crucible with tungsten hexacarbonyl W(CO)6 was placed into the working chamber and heated up to 200°C (Figure
Molecules W(CO)6 are ionized in the discharge and implanted into the sample.
The gas composition during the experiment was monitored with a quadrupole mass spectrometer Thermo Scientific DSQ II. From the mass spectra (Figure
: Mass spectrum of gas in the experimental chamber when the crucible with W(CO)6 is heated.
It should be noted that the dose of tungsten implantation could not be determined from the current value according to (
Film samples of 200 nm thick polycrystalline vanadium dioxide VO2 on glass-ceramic substrates were fabricated using an AJA ORION 5 setup by reactive magnetron sputtering in a mixture of argon and oxygen (the partial pressures of Ar and O2 were 4.3 and 0.7 Torr, and the DC generator power was 200 W) with subsequent annealing at a temperature of 500°C.
The V2O5·
The samples were characterized by X-ray diffraction (XRD) analysis, optical (VIS-NIR) spectroscopy, dispersive Raman spectrometry, nuclear magnetic resonance (NMR) spectroscopy, and thermal gravimetric analysis (TGA).
The XRD analysis was carried out using a DRON-3 diffractometer with CuK
The conductivity temperature dependence of VO2 films was measured using the four-point probe method. The temperature was measured with a chromel–alumel thermocouple and a Keithley 2000 multimeter. The voltage and current at probes were measured by a Keithley 2400 sourcemeter.
For the manifestation of the IECE in V2O5·
The switching effect was studied in a sandwich-type structure Au-V2O5·
The XRD analysis (Figure
XRD patterns of Si substrate
Figures
Conductivity temperature dependence of VO2 film: initial sample 1
Conductivity temperature dependence of VO2 film: initial sample 2
In the work [
Thus, it is demonstrated that the PIII technique allows one to perform hydrogenation of vanadium dioxide films and vary the concentration of hydrogen in the samples. Hydrogen is retained in the films for a long time, and the hydrogen concentration that is sufficient for metallization (i.e., for the metal tetragonal phase to be preserved at room temperature) is attained.
The XRD analysis shows that the initial vanadium oxide film obtained by the sol-gel method is an amorphous V2O5·
XRD patterns of V2O5 gel film:
Qualitatively, these curves are identical; that is, there are no sharp structural changes during hydrogenation in the films. However, the first maximum of the curve for the hydrogenated film is shifted toward larger angles as compared to that of the original sample, which indicates a decrease in the interlayer spacing (Table
Interlayer spacing (
Sample | Number in Figure |
2 |
|
|
---|---|---|---|---|
Initial film |
|
7.65 | 11.55 | 11625 |
Hydrogenated |
|
8.80 | 10.04 | 6805 |
1 day after hydrogenation |
|
8.68 | 10.17 | 6850 |
1 week after hydrogenation |
|
8.10 | 10.91 | 7744 |
2 weeks after hydrogenation |
|
7.47 | 11.82 | 11082 |
During the hydrogen implantation, the film is heated to about 100°C by radiation from the HCPS hot cathode. According to [
At the electrochromic coloration of an initial (nonhydrogenated) film, analogous changes occur [
The maxima at
In order to track the recovery process, the XRD patterns of the film have also been obtained at 1, 7, and 14 days after hydrogenation (see Figure
For the Raman spectra measurements, the samples of crystalline V2O5 have been obtained by annealing of the xerogel films in a muffle furnace at a temperature of 400°C. According to the XRD data (Figure
XRD patterns of V2O5·
Raman spectra of crystalline V2O5:
For the study of the influence of hydrogenation (with the dose of
Colored spot area as a function of time of voltage application:
The colored spot can persist up to 24 hours, and when applying the reverse voltage, the spot disappears for 1–5 minutes. The films withstand up to 104 coloring-bleaching cycles. From Figure
Next we have studied the optical properties of the hydrogenated and colored samples. The
Reflectance (a) and transmittance (b) spectra of V2O5 xerogel film:
The optical bandgap
From these measurements, the optical bandgap has been found to be
51V MAS NMR spectra of both initial and hydrogenated films (Figure
(a) 51V MAS NMR spectra. Not marked signals are spinning sidebands. (b) 1Н MAS NMR spectra.
The partial V5+ → V4+ reduction seems to be supported by the 900 cm−1 Raman peak intensity increase (Figure
To further affirm this inference, we have conducted an additional experiment, namely, tested another method of hydrogenation, by holding the sample in hot (125°C) glycerin. Heating causes the decomposition of glycerin to form glycerol aldehyde and molecular hydrogen [
Provided that the formed hydrogen gas can not escape through the surface, it turns out to be dissolved in liquid glycerin. If some hydrogen molecule decomposition catalyst (e.g., vanadium pentoxide) is present in the reaction chamber, then atomic hydrogen appears and penetrates into the V2O5 film where it splits into protons and electrons. The vanadium pentoxide film changes its color from brown to blue-green. However, in this case, no changes occur in the concentration of pentavalent vanadium. The internal electrochromic effect in the film hydrogenated by such a way is much brighter than that in the nonhydrogenated film. Unfortunately, this method of the IECE enhancement can hardly be applied in practice because of an uncontrollable film delamination from the substrate during the hot glycerin treatment. Nonetheless, this experiment proves that the IECE manifestation after hydrogenation depends not solely on hydrogen excess, but primarily on the vanadium valence state.
In the 1H MAS NMR spectra (Figure
TGA of V2O5·1.8H2O:
From the results of TGA (Figure
Thus, the TGA results are consistent with the 1H NMR data. The total amount of hydrogen decreases after implantation. However, the implanted hydrogen differs from the hydrogen composed in water. First of all, the implanted hydrogen is not bound in the film; that is, it possesses a higher mobility. Perhaps this explains the enhancement of the electrochromic effect in V2O5·
In summary we have studied the internal electrochromic effect in vanadium pentoxide xerogel film and the influence of hydrogenation on its parameters. The PIII-assisted hydrogen insertion has been found to benefit the coloration rate at a specific mutual arrangement of electrodes. Particularly, if the negative electrode is located on the nonhydrogenated region and the positive electrode on the hydrogenated one, the IECE is more pronounced, while at the reverse polarity (or if both electrodes are on hydrogenated region), the IECE is practically suppressed. This is associated with the fact that the migration of protons from the region enriched with hydrogen is substantially enhanced, while it is impaired at the reverse polarity.
The 51V MAS NMR analysis and Raman spectroscopy have revealed a partial reduction of V5+ to V4+ oxidation state at hydrogenation. This accounts for the fact that electrochromic coloration does not occur in the hydrogenated region, because the formation of highest polyvanadic acids, responsible for the V2O5·
Electrical switching due to MIT has been first revealed for vanadium dioxide [
Switching in VO2 is associated with the current-induced Joule heating of the sample up to
In our experiments, after EF in the film of pure hydrated vanadium pentoxide, a vanadium dioxide channel forms where the MIT can occur. The current-voltage characteristic of the structure becomes S-shaped with
(a) Schematic sandwich structure: diameter of Au top electrodes is 0.9 mm. (b)
Implantation of tungsten greatly influences the
(a)
Note that the doping with metal ions has a profound influence on the phase transition behavior and transition temperature of VO2 [
One can assume that for a high W implantation dose a transformation of vanadium pentoxide xerogel into VO2, brought about by heating and ion bombardment, occurs simultaneously with doping. That is, in this case some amount of VO2 : W might be formed at plasma treatment. That is why no preliminary EF is required, and the resistivity jump at switching decreases: compare
Thus, the implantation of small doses of tungsten into vanadium pentoxide films significantly improves their switching parameters. When implanting relatively large doses, the switching effect is observed without a preliminary electroforming process, but in case of EF, switching vanishes due to either metallization or breakdown.
In this paper, we have studied the effect of doping with hydrogen and tungsten on the properties of vanadium dioxide and hydrated vanadium pentoxide films. For carrying out the efficient doping process, the plasma-immersion ion implantation method has been developed and implemented.
It is shown that the implantation of hydrogen by the PIII method in vanadium dioxide films leads to the suppression of the MIT and, at a hydrogen concentration greater than 10.5 at.%, to the VO2 metallization at
Also in this study the internal electrochromic effect in V2O5·
Thus, we conclude that a method has been found for substantial increase of the rate of electrochromic coloring of hydrated vanadium pentoxide films without the use of an electrolyte. The observed phenomenon can find application in various electrochromic devices [
Next we have studied the switching effect in the metal-V2O5·
Finally, we would like to comment on the doping method used in the present study. As has been shown in the review [
The authors declare that they have no conflicts of interest.
This work was supported by the RF Ministry of Education and Science, Project no. 16.5857.2017/8.9 (state program), and by the Flagship University Development Program of Petrozavodsk State University (2017–2021). The authors thank Yu. Kolyagin and L. Lugovskaya for their help with NMR (Yu. Kolyagin) and XRD (L. Lugovskaya) measurements and useful discussions.