Electrical, Optical, and Thermal Transport Properties of Oxygen

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Cite This: J. Phys. Chem. C 2019, 123, 15419−15424

Electrical, Optical, and Thermal Transport Properties of OxygenDeficient Amorphous WOx (2.5 < x < 3) Films Gowoon Kim,†,‡ Hai Jun Cho,*,†,‡ Yu-Miin Sheu,§,∥ and Hiromichi Ohta*,†,‡ †

Graduate School of Information Science and Technology, Hokkaido University, N14W9, Kita, Sapporo 060-0814, Japan Research Institute for Electronic Science, Hokkaido University, N20W10, Kita, Sapporo 001-0020, Japan § Department of Electrophysics, National Chiao Tung University, Hsinchu 30010, Taiwan ∥ Center for Emergent Functional Matter Science, National Chiao Tung University, Hsinchu 30010, Taiwan Downloaded via BOSTON UNIV on August 19, 2019 at 12:40:36 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: Oxygen-deficient tungsten oxide (WOx) is known as an active material for various future applications such as smart displays, photocatalysts, Li-ion battery, and so on. WOx exhibits versatile properties depending on the valence state of W, which can vary from +6 to +4. Therefore, clarifying the relationship between x, the valence state of W ion, and the material properties of WOx is crucial for discovering more unique device applications. In case of crystalline WOx, since WOx has many different phases, the valence state of W cannot be modulated continuously from +6 to +4. On the other hand, there is no phase boundary in amorphous (a-) WOx, the valence state of W ion can be continuously modulated against x, and the effect of valence state of W on the material properties of WOx can be thoroughly examined. Here, we report the electrical, optical, and thermal properties of a-WOx films with several valence states of +6 (d0), +5 (d1), and +4 (d2) for x ranging from 2.511 to 2.982. Although the +6 dominant films were electrical insulators with optical transparency in the visible region, we found that both optical transmissivity and electrical resistivity decreased drastically with increase in the +5 concentrations, which also enhances the thermal conductivity because heat can be carried by additional conduction electrons. As the +4 state became dominant in the film, the resistivity slightly increased, whereas the low visible transmission was maintained. These results suggest that the redox of tungsten between +6 and +5 is attributed to all versatile properties of a-WOx, which would be of great use for developing unique devices in the future. W5+ or W4+ when A+ ions are introduced. An occupied O 2p and a vacant W 5d0 compose the valence band maximum and the conduction band minimum that are responsible for the optical transition in WO3. Hence, WO3 is a colorless transparent insulator. When free carriers are introduced in the conduction band to reduce the W6+ valence, i.e., W5+ (5d1) or W4+ (5d2), the material becomes a dark blue conductor due to the absorption of red light. Thus, insight into correlations between x and the valence state of W ion can reveal more unique properties of WOx that are applicable for developing new devices. The main challenge in isolating the W ion valence state effect is the existence of multiple stable phases of WOx. This issue can be resolved by investigating the oxygen-deficient amorphous (a-) WOx, which does not have well-defined phase boundaries like crystalline WOx. Although several studies have shown the feasibility of the valence state modulation in aWOx,10,11 its influence on the electrical, optical, and thermal properties still remain unclear due to lack of systematic studies. Some recent works have reported that the proton or lithium insertion can demonstrate continuous tuning of the valence

1. INTRODUCTION Tungsten oxide (WOx, where x is the O/W ratio in the range from 3 to 2) is well-known as an active material for electrochromic displays (ECDs) after the discovery of its electrochromic effect by Granqvist.1,2 Today, WOx-based ECDs are widely applied in smart windows.3 Furthermore, WOx is also attracting attention as an active material for many other useful applications such as anode material of Li-ion batteries,4 photocatalyst,5 gas sensors,6 and organic solar cells.7 These unique functional properties of WOx are mainly driven by the change in the valence states of W ion. There are many crystalline phases in WOx, including WO3, WO2.9, WO2.82, WO2.72, and WO2. Thus, the valence state modulation of W from +6 to +4 can be realized in a step-by-step manner.8,9 The valence state of W in WO3 can also be modulated by electrochemically introducing proton (H+) or small alkali ions with fast mobility such as Li+, Na+, and K+. The overall chemical reaction can be expressed as follows.2 (colorless transparent insulator) WO3 + x A+ + xe− ↔ A xWO3 (dark blue conductor)

where x is 2 Pa) showed a lower density than that of c-WO3 (7.1 g cm−3). The surface roughness of these films was larger than that of the a-WO3 films deposited under lower PO2 ( 2.935), the absorption coefficient, α, steeply increased above 3 eV due to the direct transition from O 2p to W 5d (Tauc gap). The Tauc gap of a-WOx films (x > 2.888) was ∼3.25 eV, well in agreement with that of the reported value (3.2−3.5 eV).25−27 The Drude-like absorption due to the free carrier electrons (∼1.4 eV) increased from ∼0 to 105 cm−1 when x decreased from 2.928 to 2.779. The shoulder absorption peak around 3.0 eV is most likely due to the oxygen vacancies.24 It should be noted that the Burstein−Moss shift was not observed, which suggests a small dispersion of the conduction band. On the other hand, the Drude-like absorption displays an inverse proportionality to x, i.e., from 1522 cm−1 (x = 2.982) to 93278 cm−1 (x = 2.779), and it was not observable below x = 2.642 (Figure 3b). The disappearance of Drude-like absorption for x = 2.642 is probably due to the formation of W4+ (5d2) valence state.24 The spectra were continuously changed with decreasing x;

Figure 2. XPS spectra of the a-WOx thin film surfaces. (a) W 4f spectra for various films grown with different oxygen pressures. (b) O 1s spectra. To analyze the XPS spectra of W 4f, we measured the peak area ratio of W6+ (5d0), W5+ (5d1), and W4+ (5d2) valence states (see Figure S2 in the Supporting Information for further details). (c) Deconvoluted valence state plotted as a function of oxygen pressure for +6 to +4. (d) The relationship between O/W ratio, x, and oxygen pressure.

insignificant change in the O 1s spectra, which are composed of the O2− peak at ∼530.8 eV and the OH−-related peak at 532.1 eV (Figure 2b). The OH− peak is likely from ambient contaminations, which cannot be controlled and therefore can be attributed to the lack of tendency in O 1s spectra. To gain insight into the XPS spectra, we deconvoluted the W 4f spectra using the relationship of three spin−orbit doublets of W6+ (5d0, 35.8 ± 0.1 eV), W5+ (5d1, 34.8 ± 0.1 15421

DOI: 10.1021/acs.jpcc.9b02448 J. Phys. Chem. C 2019, 123, 15419−15424

Article

The Journal of Physical Chemistry C

that at the x ≥ 2.779. It should be noted that similar behavior of −S was reported in the WOx-ECT when the protonation16 and −S did not return to the original value after oxidation. We extracted the activation energy σ (Ea) of the a-WOx films around RT by assuming an Arrhenius-type thermal activation of σ (Figure 4d). The Ea decreased from 94 to 6.9 meV with decreasing x until it reached 2.642, suggesting that the EF gradually approached the mobility edge with increasing W5+ concentration. When x was less than 2.642, Ea increased to 29 meV with decreasing x. These results suggest that the reversible modulation of x occurs between the range of 3.00 and 2.64 so that the electron transport properties vary in a linear manner with respect to x, creating a tunable functionality. 3.4. Thermal Transport Properties of the a-WOx Films. Finally, we investigated the relationship between x and the thermal conductivity (κ) of the a-WOx films at RT (see Figure S5 in the Supporting Information), which is summarized in Figure 5. We plotted the reported κ of cWOx (white square) for comparison. Although c-WOx shows an inverse linear dependency of the observed κobsd on x, i.e., 1.3 W m−1 K−1 (x = 2.98)9 to 12 W m−1 K−1 (x = 2.72),9 a-WOx films did not show such tendency; κobsd is in the range from ∼0.9 W m−1 K−1 (x = 2.982) to ∼1.6 W m−1 K−1 (x = 2.642),

finally, the Tauc gap was closed at x = 2.511. These results suggest that the resulting free carriers are predominantly due to the formation of W5+ (5d1). 3.3. Electron Transport Properties of the a-WOx Films. Next, we investigated the electron transport properties of the a-WOx films. Figure 4a shows the resistivity (ρ) of

Figure 4. Electron transport properties of the a-WOx films. (a) Comparison of resistivity versus different x between crystalline WOx ceramics and amorphous WOx films. (b) Temperature-dependent resistivity of the a-WOx films. (c) The relationship between the thermopower and O/W ratio. (d) The activation energy of the electrical conductivity (Ea), which was calculated using the Arrhenius plot.

various a-WOx films at RT. With the reduced x from 2.982 to 2.642, ρ dramatically decreased from 2 × 104 to 3 × 10−3 Ω cm. Crystalline WOxs (c-WOx, white and gray square: polycrystalline WOx, black square: epitaxial WOx) also show behaviors similar to those of the amorphous ones, but there exists a difference of two orders of magnitude, likely due to the higher mobility. Figure 4b shows the ρ−T curves for the aWOx films from 300 to 30 K. The ρ for the entire a-WOx films increased with decreasing temperature, indicating that conduction electrons are thermally activated. The slope of the ρ−T curve decreased continuously, and it was minimized at x = 2.642, below which the slope increased again. Similarly, thermopower (−S) decreased with decreasing x until it reached x = 2.642 (Figure 4c). Since the −S of an n-type semiconductor can be expressed as −S = −(kB/e)[ln(N/n) + A], where kB, e, N, n, and A are the Boltzmann constant, electric charge, density of states, carrier electron concentration, and transport constant (typically 0 ≤ A ≤ 2), respectively,28 the decreasing tendency of −S indicates that EF increased with decreasing x. Thus, the carrier concentration increased with decreasing x until it reached x = 2.642. However, when the x was further reduced (x < 2.642), the ρ and −S slightly increased with decreasing x. The −S−σ relation of the a-WOx films at x ≤ 2.642 is located at a slightly higher −S side than

Figure 5. Heat transport features of the a-WOx thin films in the crossplane direction at RT. (a) Extracted thermal conductivity (κobsd) of amorphous (red circle) films compared with the crystalline (white square19) WOx films for various x. (b) Intrinsic thermal conductivity κint (or lattice thermal conductivity κlat for crystalline) with various x. The κint was obtained from κint = κobsd − κelectron. Electron thermal conductivity κelectron. was obtained as κelectron = LσT, where L is the Lorentz number (2.4453 × 10−8 W Ω K−2) and T is 300 K. (c) The observed total thermal conductivity, κobsd, is plotted as a function of the electrical conductivity (σ) for c-WOx. It is compared with the thermal conductivity contributed solely from the electrons (the dashed line). (d) κobsd plotted as a function of σ for c-WOx. It is also compared with the thermal conductivity contributed solely from the electrons (the dashed line). 15422

DOI: 10.1021/acs.jpcc.9b02448 J. Phys. Chem. C 2019, 123, 15419−15424

The Journal of Physical Chemistry C

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which is far lower than that of c-WOx (Figure 5a). Note that the porous structure of the a-WOx films (x > 2.93) deposited at higher PO2 (>2.5 Pa) would show a slightly lower κobsd. The κobsd is the sum of the lattice thermal conductivity, κlat, and the electronic thermal conductivity, κelectron. Since there is no lattice in the a-WOx films, we defined κlat as κint (intrinsic thermal conductivity). We extracted κint from LσT, where L is the Lorentz number (2.4453 × 10−8 W Ω K−2) and T is 300 K (Figure 5c,d). Both κobsd and κelectron (gray dashed line) of cWOx has a linear dependency on σ, indicating the growth of the electronic contribution to the thermal conductivity upon reduced x (Figure 5c). On the other hand, there is a scattering of the observed κobsd for lower σ of a-WOx, it shows an increasing behavior with increasing σ (Figure 5d). The electronic κelectron (red dashed line) showed a maximum at around x = 2.642 (∼0.25 W m−1 K−1), demonstrating that free carrier electrons of W5+ solely contribute κelectron. The contribution of κelectron in a-WOx is low compared to that in c-WOx due to the low carrier mobility of a-WOx. From the extracted κelectron (Figure 5b), we found that κint of the a-WOx films ranges from ∼0.85 to ∼1.2 W m−1 K−1, which is 1/4−1/3 compared to that of c-WOx (κint = 3 − 4.5 W m−1 K−1 except x > 2.98). It should be noted that the κobsd of x = 2.982 is close to the minimum κmin of WOx ∼0.4 W m−1 K−1, which was calculated using the Cahill model.29 From these results, we concluded that both carrier electron mobility and thermal conductivity of a-WOx are far smaller than those of c-WOx because there is no periodic lattice structure in a-WOx. 3.5. Discussion. From the results, we observed an increase of −S, ρ, and Ea when the x was less than 2.642. At around x = 2.642, W5+ is dominant and maximized in the film. We now discuss the chemical reaction in a-WOx when x is between ∼2.5 and 3. For the a-WOx films with high values of x, the W5+ valence state increases with decreasing oxygen and, simultaneously, EF gradually increases, and the growing W5+ valence state provides free electrons as heat and electron carriers. As x decreases further, up to x ∼2.6, the W4+ valence state forms to reduce the concentration of W5+ valence state. Here, we suggest that the localization of the d2 electrons in the W4+ valence state cannot affect the electrical properties. Therefore, −S and ρ start to increase, although x decreases more. This can be supported by the disappearance of the Drude-like behavior in the absorption spectra. Consequently, it demonstrates that free electrons from W5+ solely contribute to heat transport.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b02448. X-ray reflectivity (XRR) of the a-WOx thin films; XPS spectra around the W 4f peaks of the a-WOx thin films; optical transmission and reflection spectra of the a-WOx thin films; optical absorption coefficient of the a-WOx thin films; decay curves of the TDTR phase signal of the a-WOx thin films (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.J.C.). *E-mail: [email protected] (H.O.). ORCID

Gowoon Kim: 0000-0002-5803-839X Hai Jun Cho: 0000-0002-8642-4183 Hiromichi Ohta: 0000-0001-7013-0343 Author Contributions

G.K. and H.O. performed the sample preparation and electrical, optical, and thermal properties measurements. H.J.C. performed the XPS analyses. Y.-M.S. helped the thermal conductivity analyses. H.O. planned and supervised the project. All authors discussed the results and commented on the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by Grants-in-Aid for Scientific Research A (17H01314) from the JSPS, the Asahi Glass Foundation, and the Mitsubishi Foundation. A part of this work was supported by Dynamic Alliance for Open Innovation Bridging Human, Environment, and Materials, and by the Network Joint Research Center for Materials and Devices. H.J.C. acknowledges the support from Nippon Sheet Glass Foundation for Materials Science and Engineering. Y.M. Sheu acknowledges the Center for Emergent Functional Matter Science of National Chiao Tung University from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan.

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4. CONCLUSIONS

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We have shown systematic studies of the electrical, optical, and thermal properties of a-WOx with several valence states, +6 (d0), +5 (d1), and +4 (d2), and varying values of x from 2.982 to 2.511. Although the +6 dominant films were electrically insulating with transparency in the visible region, both optical transmissivity and electrical resistivity decreased drastically with the increasing concentration of the +5 states. Meanwhile, heat can be carried by additional free conduction electrons, thus increasing the thermal conductivity. However, the +4 dominant films showed a slightly higher resistivity while maintaining optical opacity. These results suggest that the redox of tungsten between +6 and +5 creates switchable properties of a-WOx, providing unique functionalities for device design. 15423

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DOI: 10.1021/acs.jpcc.9b02448 J. Phys. Chem. C 2019, 123, 15419−15424