Electrical, Optical and Thermal Transport Properties of Oxygen

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C: Energy Conversion and Storage; Energy and Charge Transport

Electrical, Optical and Thermal Transport Properties of Oxygen Deficient Amorphous WO (2.5 < x < 3) Films x

Gowoon Kim, Hai Jun Cho, Yu-Miin Sheu, and Hiromichi Ohta J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b02448 • Publication Date (Web): 31 May 2019 Downloaded from http://pubs.acs.org on May 31, 2019

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The Journal of Physical Chemistry

Electrical, Optical and Thermal Transport Properties of Oxygen Deficient Amorphous WOx (2.5 < x < 3) Films

Gowoon Kim1,2, Hai Jun Cho1,2*, Yu-Miin Sheu3,4, and Hiromichi Ohta1,2*

1Graduate

School of Information Science and Technology, Hokkaido University,

N14W9, Kita, Sapporo 060−0814, Japan 2Research

Institute for Electronic Science, Hokkaido University, N20W10, Kita,

Sapporo 001−0020, Japan 3Department

of Electrophysics, National Chiao Tung University, Hsinchu, 30010,

Taiwan 4Center

for Emergent Functional Matter Science, National Chiao Tung University,

Hsinchu 30010, Taiwan

ABSTRACT: Oxygen deficient tungsten oxide (WOx) is known as active materials 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

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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 increasing the +5 concentrations, which also enhances the thermal conductivity due to the fact that heat can be carried by additional conduction electrons. As the +4 states 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.

1. INTRODUCTION Tungsten oxide (WOx, where x is O/W ratio ranges from 3 to 2) is well-known as an active material of electrochromic displays (ECDs) after the discovery of its electrochromic effect by Granqvist1, 2. Today, WOx-based ECDs are widely applied for the smart windows3. Furthermore, WOx is also attracting attention as an active material for many other useful applications such as anode material of Li-ion batteries4, photocatalyst5, gas sensors6, and organic solar cells7. These unique functional properties of WOx are mainly driven from the change in 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


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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 the following.2 (Colorless transparent insulator) WO3 + xA+ + xe− ↔ AxWO3 (Dark blue conductor) , where x is < 1, A is H, Li, Na, or K and e− is the electron charge. Since tungsten ion prefers several valence states including +6, +5, or +4, a fraction of W6+ in WO3 become W5+ or W4+ when A+ ions are introduced. An occupied O 2p and a vacant W 5d0 compose the valence band maximum and conduction band minimum that is responsible for 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 red light absorption. Thus, insight into correlations between x and the valence state of W ion can reveal more unique properties of WOx, applicable for developing new devices.

The main challenge in isolating the W ion valence state effect is the existence of multiple stable WOx phases. This issue can be resolved by investigating 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 a-WOx10, 11, its influence on the electrical, optical, and thermal properties are still remain unclear due to the lack of systematic studies. Some recent works reported that the proton or lithium insertion can demonstrate a continually tuning of the valence state by electron injections.12, 13 However, the intrinsic nature of a-WOx is still unclear because of the fact that the distribution of H+ or Li+ is difficult to clarify. Therefore, we focused on oxygen deficient a-WOx thin films to clarify the relationship



between the valence state of W ion and the properties of a-WOx continuously against x. We believe that the results would provide useful information to improve recently developed a-WOx-based electrochromic transistors (ECTs)14-16, which utilized reversible electrochromism of the active material to change the valence state of its transition metal ions, enabling the control of both the electrical resistance and visible transmission. Such electric components have great potential for advanced memory devices.

Here we show the relationship between x, the valence state of W ion, the electrical, optical, and thermal properties of a-WOx. We fabricated several a-WOx thin films, with x ranging from 2.982 to 2.511, by pulsed laser deposition (PLD) technique under various oxygen atmospheres at room temperature (RT). Although the +6 dominant films were electrical insulators with transparency in the visible region, both optical transmissivity and electrical resistivity decreased drastically with increasing the +5 concentrations. Meanwhile, the thermal conductivity increased due to the contributions from conduction electrons. However, the +4 dominant films showed slightly higher resistivity compared to the +5 dominated films while maintaining optical opacity. These results indicate that the redox of tungsten between +6 and +5 creates switchable functionalities from a-WOx, paving new paths for the future design of unique devices like ECTs.

2. EXPERIMENTAL SECTION 2.1.

Fabrication and Analyses of the Films.

The a-WOx films were deposited by pulsed laser deposition (PLD) on alkaline-free glass


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substrates (10 × 10 × 0.7 mm, EAGLE XG®, Corning) at room temperature (RT). The KrF excimer laser pulses ( = 248 nm, ~1 J cm−2 pulse−1, 20 ns pulse width, 10 Hz) were irradiated on the WO3 ceramic target for 10 min under the oxygen pressure (PO2) from 10−2 to 2.8 Pa. The deposition rate was 8 − 17 pm pulse−1.

Firstly, the resultant films were analyzed by the glancing angle incidence X-ray diffraction (GIXRD) with the incident angle of 0.5°. The film thickness, bulk density, and the root mean square roughness of the resultant films were measured by X-ray reflectivity (XRR, Cu Kα1,  = 1.54059 Å, ATX-G, Rigaku Co.) measurements. The film thicknesses were 49 − 105 nm.

X-ray photoelectron spectroscopy (XPS) was used to analyze the valence state of W in the resultant a-WOx films. We used the Al Kα to generate the photoelectrons, and a flood gun was used to prevent charging at RT. We measured the W 4f (30 − 42 eV) and O 1s (528 − 535 eV) core level spectra. All the spectra were calibrated with C 1s peak at 284.8 eV.17

2.2.

Optical Properties of the Films.

Optical transmission and reflection spectra of the resultant films were measured using an ultraviolet-visible-NIR spectrometer (UV-Vis-NIR, SolidSpec-3700, Shimadzu Co.) at RT. The absorption coefficient (α) was extracted using the observed transmission (T) and reflection (R) from the relationship of α = d−1·ln [(1−R)·T−1], where d is the film thickness.18



2.3.

Electron Transport Properties of the Films.

Electrical resistivity (ρ) was measured by d.c. four probe method with van der Pauw electrode configuration from RT to 30 K. Thermopower (S) was measured at RT by creating a temperature difference (ΔT) of ~10 K across the film using two Peltier devices. The thermo-electromotive force (ΔV) and ΔT were measured simultaneously, and the S-values were obtained from the slope of the ΔV–ΔT plots. Detail of our S measurement has been described elsewhere.19

2.4.

Thermal Transport Properties of the Films.

Cross-plane thermal conductivity (κ) of the resultant films was measured by the time domain thermo-reflectance (TDTR, PicoTR, PicoTherm Co.). The 100-nm-thick Mo was deposited on the a-WOx films as a transducer by dc sputtering. The results were simulated using the packaged software developed by PicoTherm. For the simulation, we used the specific heat capacity (Cp), the bulk density and the film thickness as free parameter in the films. In order to minimize the number of free parameters, we used constant value for bulk density = 10200 kg m−3, Cp =250 J Kg−1 K−1 and κ =31 W m−1 K−1 for Mo transducer and bulk density = 2350 kg m−3, Cp = 768 J Kg−1 K−1 and κ =1 W m−1 K−1 for the glass substrate, which were measured separately.

3. RESULTS AND DISCUSSION 3.1. Structure and Valence State of Tungsten. From the glancing angle incidence X-ray diffraction (GIXRD) measurements (See Figure S1 in the Supporting Information), only halo pattern from the sample, ranging q/2π ~2.8−4 nm–1, together with that from the substrate (q/2π ~2.6 nm–1) was detected


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[Figure 1(a)], confirming the a-WOx films for all the samples discussed here. The bulk density of the a-WOx films gradually increased from ~5.8 g cm−3 to ~9.3 g cm−3 with the reduced oxygen pressure (PO2) [Figure 1(b)]. Since the density of crystalline WOx increases from 7.1 g cm−3 to 10.8 g cm−3 with decreasing x from 3 to 2, the observed increase of the density suggests a reduced x from the decreased PO2. The a-WOx films deposited under high PO2 (> 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 Pa) [Figure 1(c)], most likely due to a formation of the porous structure at higher PO2.20 Comparing density between a-WOx and c-WOx, we can expect that various a-WOx films, with x ranging from 3 to ~2.5, were successfully prepared.

In order to determine the x values, we analyzed the a-WOx films using the X-ray photoelectron spectroscopy (XPS). Although the W 4f doublet peaks (W 4f7/2: ~36 eV, W 4f5/2: ~38 eV) were clearly observed for films with the grown condition PO2 = 2.8 Pa, they became broader with the decreased PO2. An additional peak appeared around 33 eV when PO2 = 0.01 Pa [Figure 2(a)]. On the other hand, tuning PO2 leads to an insignificant change in the O 1s spectra, which are composed of O2− peak ~530.8 eV and OH− related peak at 532.1 eV [Figure 2(b)]. 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 de-convoluted 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 eV) and W4+ (5d2, 33.4 ± 0.1 eV)21, 22 by Lorentzian (30 %) - Gaussian peak with Shirley background subtraction (Figure S2 in the Supporting Information). The ratio of W6+ (5d0) to the total W ion has inverse proportionality to PO2, from 100 % to ~60 % upon the reduced PO2 from 2.8 Pa to 1.5 Pa, while that of W5+ (5d1) to the total W ion increased almost linearly from 0 % to ~35 % [Figure 2(c)]. The further reduction of PO2 (PO2 < 1.5 Pa) leads to the increment of W4+ (5d2), thus gradually declining the W5+. It should be noted that the concentration of W5+ was maximized for PO2 ~1.2−1.5 Pa. To further clarify, using area of each valence states, we calculated O/W ratio x = W6+ × 3.0 + W5+ × 2.5 + W4+ × 2.0., where Wn+ (n = 3.0, 2.5, or 2.0) denotes the percentage of the W ion states and the multiplied value means oxygen contents of each W valence state. From this analysis, we confirmed that our a-WOx films have O/W ratio from 2.982 (PO2 = 2.8 Pa) to 2.511 (PO2 = 0.01 Pa) and there is a linear relationship between PO2 and x [Figure 2(d)]. Summarizing these results, we can confirm the successful fabrication of a-WOx films with various x.

3.2. Optical Properties of the a-WOx Films. With the decline of x, the color of the a-WOx films changed from colorless transparent to metallic black, as summarized in Figure S3 in the Supporting Information. Meanwhile, the optical transmission decreased dramatically, due to the formation of oxygen deficiency.11, 23, 24 Figure 3(a) and Figure S4 in the Supporting Information display the absorption versus x. At the high O/W ratio (x > 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, agreeing well with that of the reported value (3.2 – 3.5 eV).25-27 The Drude-like absorption due to the


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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 that a small dispersion of the conduction band. On the other hand, the Drude-like absorption displays 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 3(b)]. 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, and 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 a-WOx films. Figure 4(a) shows the resistivity (ρ) of various a-WOx films at RT. With the reduced x from 2.982 to 2.642, ρ dramatically decreased from 2 × 104 Ω cm to 3 × 10−3 Ω cm. Crystalline WOxs (c-WOx, white and grey square: polycrystalline WOx, black square: epitaxial WOx) also show similar behaviors with the amorphous ones, but there exists a two-order-of-magnitude difference, likely due to the higher mobility. Figure 4(b) shows ρ−T curves for the a-WOx films from 300 K 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 4(c)]. 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 Boltzmann constant, electric charge, density of states, carrier electron concentration, and transport constant (typically 0 ≤ A ≤ 2), respectively28, the decreasing tendency of –S indicates that the 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 the x. The −S – σ relation of the a-WOx films at the x ≤ 2.642 is located at slightly higher −S side than that at the x ≥ 2.779. It should be noted that similar behavior of –S was reported in the WOx-ECT when protonation16 and the –S did not return to the original value after oxidation. We extracted the activation energy of σ (Ea) of the a-WOx films around RT by assuming Arrhenius-type thermal activation of the σ [Figure 4(d)]. The Ea decreased from 94 meV 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, the Ea increased to 29 meV with decreasing the 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 c-WOx (white square) for comparison. Although c-WOx shows 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 ranging from ~0.9 W m−1 K−1 (x = 2.982) to ~1.6 (x =


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2.642), which is far lower than that of the c-WOx [Figure 5 (a)]. Note that the porous structure of the a-WOx films (x >2.93) deposited at higher PO2 (>2.5 Pa) would show 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 a-WOx films, we defined the κlat as κint (intrinsic thermal conductivity). We extracted the κint from L·σ·T, where L is the Lorentz number (2.4453 × 10−8 W Ω K−2) and T is 300 K [Figures 5(c) and 5(d)]. Both κobsd and κelectron (grey dashed line) of c-WOx has linear dependency on σ, indicating the growth of electronic contribution to the thermal conductivity upon the reduced x [Figure 5(c)]. On the other hand, there is a scattering of the observed κobsd for lower σ of a-WOx, it shows increasing behavior with increasing σ [Figure 5(d)]. 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 5(b)], we found that κint of a-WOx films is ranging from ~0.85 W m−1 K−1 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 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 due to the fact that there is no periodic lattice structure in a-WOx.

3.5. Discussion. From the results, we observed the 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 a-WOx films high values of x, W5+ valence state increases with decreasing oxygen and, simultaneously, EF gradually increased and the growing W5+ valence state provides free electron as heat and electron carrier. 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 W4+ valence state cannot affect the electrical properties. Therefore, –S and ρ start to increases although x decreases more. This can be supported from the disappearance of Drude-like behavior in the absorption spectra. Consequently, it demonstrates that free electrons from W5+ solely contribute to heat transport.

4. CONCLUSIONS 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 increasing the concentration of +5 states. Meanwhile, heat can be carried by additional free conduction electrons, increasing thermal conductivity. However, the +4 dominant films showed 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.

SUPPOORTING INFORMATION


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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.XXXXXXX.

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; The decay curves of the TDTR phase signal of the a-WOx thin films.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected] ORCID Gowoon Kim: 0000-0002-5803-839X Hai Jun Cho: 0000-0002-8642-4183 Yu-Miin Sheu: 0000-0002-4495-0708 Hiromichi Ohta: 0000-0001-7013-0343 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 interests.



ACKNOWLEDGEMENTS 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.


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Figure 1. Structural features of the resultant WOx thin films under various oxygen pressures. (a) Glancing angle incidence X-ray diffraction patterns at RT. The incident angle of the X-ray was fixed at 0.5°. (b) Bulk densities of the resultant a-WOx films, which were measured by the XRR patterns (See Figure S1 in the Supporting Information for further details). (c) The surface roughness (≡root mean square roughness) of the resultant a-WOx films, which was also evaluated by the XRR measurements. The surface roughness remarkably increases from ~0.4 nm to ~1.7 nm, when the oxygen pressure exceeds 1 Pa.



Figure 2. XPS spectra of the a-WOx thin film surfaces. (a) W 4f spectra for various films grown with different oxygen pressure. (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) De-convoluted valence state plotted as a function of oxygen pressure for +6 to +4. (d) The relationship between O/W ratio, x and oxygen pressure.


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Figure 3. Optical features of the resultant a-WOx thin films with various O/W ratio. (a) Absorption coefficient versus photon energy for various x. (b) The absorption coefficient extracted from 1.4 eV for various x.



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 Arrhenius plot.


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Figure 5. Heat transport features of the a-WOx thin films in the cross-plane 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 = L·σ·T, 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, plotted as a function of 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).



Table of Contents Graphic


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Electrical, Optical and Thermal Transport Properties of Oxygen

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