Self-Assembled Multilayer Structure and Enhanced Thermochromic

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Self-assembled multilayer structure and enhanced thermochromic performance of spinodally decomposed TiO2-VO2 thin film Guangyao Sun, Huaijuan Zhou, Xun Cao, Rong Li, Masato Tazawa, Masahisa Okada, and Ping Jin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12476 • Publication Date (Web): 03 Mar 2016 Downloaded from http://pubs.acs.org on March 6, 2016

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ACS Applied Materials & Interfaces

Self-assembled

Multilayer

Thermochromic

Performance

Structure of

and

Spinodally

Enhanced Decomposed

TiO2-VO2 Thin Film Guangyao Sun,a,† Huaijuan Zhou,a,† Xun Cao,a Rong Li,a Masato Tazawa,b Masahisa Okada,b Ping Jina,b* a.

State Key Laboratory of High Performance Ceramics and Superﬁne Microstructure,

Shanghai institute of Ceramics, Chinese Academy of Sciences, Dingxi 1295, Changning, Shanghai, 200050, China. b

Materials Research Institute for Sustainable Development, National Institute of

Advanced Industrial Science and Technology, Nagoya 463-8560, Japan

*

Corresponding authors:

Prof. Ping Jin State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China E-mail: [email protected] Tel.: +86 21 6990 6208. Fax: +86 21 6990 6221.

† These authors contributed equally to this work

ACS Paragon Plus Environment


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Abstract: Composite films of VO2-TiO2 were deposited on sapphire (11-20) substrate by co-sputtering method. Self-assembled well-ordered multilayer structure with alternative Ti- and V-rich epitaxial thin layer was obtained by thermal annealing via a spinodal decomposition mechanism. The structured thermochromic films demonstrate superior optical modulation upon phase transition, with significantly reduced transition temperature. The results provide a facile and novel approach to fabricate smart structures with excellent performance.

Key words: thin film, self-assembling, multilayer structure, spinodal decomposition, phase transition, smart structure.

1. Introduction Vanadium dioxide (VO2) is a renowned oxide owing to its reversible first-order semiconductor-metal phase transition (SMT) between low-temperature insulating state (monoclinic M phase) and high-temperature metallic state (tetragonal rutile R phase) at a critical temperature of ~68 oC 1. The thermally driven SMT is regarded as one of the most dramatic phenomena observed in condensed matter science since it is always accompanied by a tremendous contrast between the two phases in optical, electrical and magnetic properties

2-6

. In particular, the M phase is almost transparent

to infrared light while the R phase is highly reflective, maintaining visible light transmittance. For this reason, VO2-based materials, especially transparent VO2-based thin films, provide a promising application in energy-saving smart windows 7-12. Although VO2-based thin films have been extensively studied

13

, it is still

thought to be an important research subject to improve its physical stability and optical properties. One of the most efficient way is to design multilayer and multifunctional film structure, where numerous structural designs and fabricated methods have been applied on 11, 14-18. On the other hand, Z. Hiroi et al 19-20 provided a new spinodal decomposition method to obtain alternatively Ti-rich and V-rich TixV1-xO2 multilayer structure by annealing uniform bulk TiO2/VO2 (TVO) solid


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solution. Spinodal decomposition is a mechanism for phase separation which can be exploited to control the microstructure at the nanometer scale. Basically there are two routes to fulfill the phase separation from uniform solid solution. One is the nucleation and growth process, in which tiny aggregates of one phase begin to form in a matrix of the other phase, finally resulting in a random mixture of the two phases. The other is the spinodal decomposition, in which a solid solution becomes thermodynamically unstable against a minimal composition fluctuation, and a nearly sinusoidal composition modulation occurs and develops

21

. Spinodal systems

commonly involve two phases with similar crystal structures and similar materials properties, and the obtained well-ordered superlattice could be characterized by long-range spatial correlation, quasi-periodic, and self-organized. It was firstly reported in SnO2-TiO2 system by Padurow systems, e.g. Al2O3-Cr2O3

23

and AlN-SiC

22

, and then other oxide and non-oxide

24

, were found to decompose through

spinodal mechanism. The spinodal decomposition in TVO system was discovered by Zanma and Ueda in 1998 and investigated in details on bulk materials by Z. Hiroi et al recently

19-20

.

TVO system is attractive not only on account of the dramatic SMT properties of VO2 and the stable wide band gap insulator of TiO2, but also because Ti was proved to be an efficient dopant to modulate the thermochromic properties of VO2. To our best knowledge, there was no report on spinodal decomposing TVO thin film now. In this paper, TVO thin films with spinodal structures were prepared in a room temperature sputtering-annealing method for the first time, and relevant structure and optical properties were characterized and analyzed. Our research provides a novel promising synthesis way of VO2-based smart window material.

2. Experimental Amorphous TVO thin films were firstly deposited by magnetron sputter method on a-plane (11-20) sapphire substrate, and then annealed for spinodal decomposition. The deposition of typical amorphous TVO samples (A1) were carried out by



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co-sputtering of VO2 ceramic target of 70 W DC power and TiO2 ceramic target of 50 W RF power at room temperature with Ar and O2 flow of 39 and 1 sccm, respectively. Spinodal decomposition samples (SD1) were then obtained by annealing the amorphous samples at 550 oC for over 10 h with the vacuum degree of 1 mTorr. For comparison, crystalline solid solution TVO (C1) were prepared with almost the same conditions as A1, expect that the substrate temperature was kept at 450 oC while sputtering. Single component VO2 thin film sample (V1) and TiO2 thin film sample (T1) were prepared by alternatively 70 W DC sputtering of VO2 ceramic target and 50 W RF sputtering of TiO2 ceramic target, respectively. The diagram of the preparation processes for these samples (C1, A1, T1, V1 and SD1) was presented in Fig. 1. Thin film X-ray diffraction (XRD) analysis was conducted on a Rigaku Ultima IV diffractometer with Cu Kα radiation (λ=1.5418 Å) using θ-2θ scanning model. The transmittance of the films in the wavelength range from 350 nm to 2600 nm at 20 oC and 80 oC was measured using a UV-Vis spectrophotometer (HITACHI U-3100) with a temperature controlling unit. Transmission electron microscopy (TEM) observations were carried out in a JEOL-2010F electron microscope equipped with an EDS analyzer. X-ray photoemission spectroscopy (XPS) analysis were conducted on ThermoFisher ESCAlab250.

3. Results and Discussion Distinct from the previous reports on TVO bulk materials 19-20, we found that the spinodal decomposition of the TVO thin film systems was hard to realize by directly annealing the crystalline solid solution samples C1. Herein, we provided the room temperature sputtering-annealing method (RTS-A). As shown in Fig.1, the spinodally decomposed sample SD1 was prepared by annealing amorphous TVO (A1), which was deposited by room temperature co-sputtering of TiO2 and VO2 ceramic targets. Crystalline solid solution TVO (C1) were prepared with almost the same co-sputtering power and substrate temperature of 450 oC. Single component thin films of VO2 (V1) and TiO2 (T1), acted as the control group, were obtained by alternatively sputtering of single target.


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The atomic ratio of Ti to V and the value of x in the TixV1-xO2 thin films should be the same in A1 and C1, and it was determined by XPS analysis conducted on crystalline samples A1. The peak areas corresponding to V 2p and Ti 2p in XPS spectra were employed to estimate the value of x, as shown in Fig.2(b and c). The molar ratio of V to Ti is 3.58, and therefore the composition is written as Ti0.22V0.78O2. The XPS spectra is calibrated by the C 1s peak (284.6 eV) from adventurous hydrocarbon contamination on the sample surface, thus there is one C 1s peak in the survey spectrum in Fig.2a. All the other peaks are assigned to Ti, V, and O, indicating the high purity of A1 sample. The main fitting peaks of Ti 2p3/2 and V 2p3/2 are centered at 458.85 eV and 516.16 eV, respectively and match well with the positions for Ti4+ in TiO2 (458.80 eV) and V4+ in VO2 (516.30 eV), confirming the tetravalency of both Ti and V elements, indicating the high quality of our sputtering thin films. For the XRD results, only one peak appears in sample C1 (2θ=36.8o), thus herein, we focus on the XRD patterns of samples SD1, C1, A1, V1 and T1 in the diffraction angle ranging from 35o to 37.5o, as shown in Fig. 3. The differences among the five samples can be clearly observed. The peak located at 36.8o for sample C1 is indexed to (101) plane of TVO thin film, in agreement with the previous research15. The diffraction angle of 36.8o locates between ideal (101) plane of rutile TiO2 (36.16o, PDF:78-1510) and (101) plane of tetragonal VO2 (37.13o, PDF:79-1655), which confirms the formation and the rutile structure of C1 solid solution. No peak can be observed in the pattern of sample A1, indicating that amorphous structure has obtained by room temperature sputtering. After annealing for enough time, the (101) peak split into two set of peaks, indicating that a complete phase separation has fulfilled in SD1. The peaks at lower angle (36.2o) and higher angle (37.0o) are indexed to Ti-rich and V-rich phases in the decomposed sample, respectively, which can later be proved by TEM and EDS results. The Ti-rich angle is little higher than the diffraction angle of pure rutile TiO2 thin film (TiO2(R), sample T1) and the V-rich angle is little lower than pure monoclinic VO2 thin film (VO2(M), sample V1), indicating the composition of the separated phases are V-doped TiO2 and Ti-doped VO2, respectively.



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Z. Hiroi et al

19

found that XRD peak split in TVO bulk solid solution occurs

only for hkl reflections with l≠0, which means that spinodal modulation in TVO system occurred along the c axis, i.e. [001] direction. According to the lattice parameters of rutile TiO2 (a=4.582, b=2.953, PDF:78-1510) and tetragonal VO2 (a=4.554, b=2.8557, PDF:79-1655), the lattice mismatch in TVO system is 0.61% along a axis and 3.3% along c axis and thus the modulation direction should be chosen to c axis direction to minimize the elastic strain energy at the interface. Similar phenomenon was observed in TiO2/SnO2 system

25-26

. In this work, TVO thin film is

deemed to share the same mechanism, and therefore the multilayer should be parallel to that of the a-axis of a-plane sapphire (11-20) substrates and tilted away from the interface based on the (101) orientation of the TVO thin films. A schematic diagram of the microstructure of the spinodally decomposed TVO thin film is showed in Fig.4. Further structure analysis was carried out by Transmission Electron Microscopy (TEM). TEM proves that sample A1, with the thickness of 220 nm, was nearly amorphous since few crystal sections can be recognized in the thin film layer in Fig.5b. It is exciting to find out that there is a clear epitaxial crystalline layer with the thickness of ~ 5 nm appears in the interface between the sapphire substrate and the amorphous TVO film. The interplanar crystal spacing is measured at 2.45 Å and matches well with the XRD diffraction angles of sample C1 (36.8 o, d=2.44 Å) in Fig.3. It is reasonable to induce that the sapphire substrate is effective in promoting epitaxial growth, and the epitaxial crystalline thin layer can act as seeds in the following annealing process. After decomposition, the morphology of thin film turns out to be porous but the thickness of the film remains ~ 200 nm. A grain boundary can be distinguished in Fig. 5d, which provides a direct evidence of phase separation. The crystal planes parallel to substrate is measured at 2.48 Å and 2.42 Å, matches well with the d value of the diffraction angles of 36.2o (2.48 Å) and 37.0o (2.43 Å) in XRD patterns of sample SD1 (Fig.3). For V-rich phase, we consider that the semiconductor-metal transition occurred during the HR-TEM testing process since the temperature rise up under the bombardment of electron beam, and thus V-rich phase in Fig. 5d exhibited the rutile


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structure of VO2 (VO2 (R)). The orientation relationship between the separated phases and a-plane sapphire substrates can be written as: (101) TiO2(R) // (11-20) Al2O3 (101) VO2(R) // (11-20) Al2O3 The chemical analysis of Ti and V were conducted by EDS on a selected area, as shown in Fig. 6(a~d). In Fig.6(c) and (d), the alternate change in Ti and V contents along the red line reveals the parallel antiphase modulations, which can be clearly observed in the enlarged Fig. 6(f) and (g). Fig. 6(f) and (g) confirm that the direction of the decomposed multilayer structure is, as expected, not parallel but slant to the a-sapphire substrate, which is in agreement with Fig. 4. By the way, according to the XPS analysis (Fig.2), the average composition of thin film is Ti0.22V0.78O2, thus the Ti-rich layer is expected to be thinner than the V-rich layer, which is in fact observed in Fig. 6(c, d, f, g). Combined with the XRD, TEM and EDS results, we conclude that the spinodal phase separation can be successfully realized during annealing process of amorphous TVO thin films and results in an alternative Ti-rich and V-rich parallel arrangement multilayer structure. The UV–vis–NIR transmittance spectra of the composite films were characterized at 20 oC (before phase transition) and 80 oC (after phase transition) for detecting their optical modulation capability, as shown in Fig.7. The application of VO2 for smart windows relies on the enhancement in both luminous transmittance (Tlum) and solar modulating ability (∆Tsol). For all the samples, the luminance transmittance (Tlum, 400-700 nm) and solar transmittance (Tsol, 350-2600 nm) were obtained based on the measured spectra using the following equation: Tρ = ∫ψ ρ (λ )T (λ )d λ / ∫ψ ρ (λ )d λ

(1)

∆Tsol = Tsol −20o C −Tsol−80o C

(2)

where T(λ) means the transmittance at wavelength λ; ρ denotes lum or sol for calculations; ߰௟௨௠ is the standard efficiency function for photopic vision; and ߰௦௢௟ is the solar irradiance spectrum for an air mass of 1.5 (corresponding to the sun standing 37o above the horizon). The optical features of samples were summarized



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and listed in Table 1. Based on Fig. 7 and Table 1, the amorphous samples A1 and TiO2 thin film T1 doesn’t show optical modulation ability while the crystalline solid solution sample C1, VO2 thin film V1 and spinodal decomposition SD1 do. For sample SD1 and C1, the Tlum in both semiconductor phase and metallic phase keeps almost the same. Nevertheless, the ∆Tsol is increased from 6.7% for sample C1 to 13.1% for sample SD1. In terms of the luminous transmittance between the sample SD1 and V1, the Tlum undergoes an increase in both semiconductor phase (from 18.5% to 22%) and metallic phase (from 18.6% to 20.9%), and ∆Tsol increases from 11.5% to 13.1%. The optimized optical switching performance in sample SD1 reveals the enhanced effect of superior optical modulation upon the phase transition and the Ti-rich layer in designed TVO multilayer displayed as an antireflection layer, just like TiO211. Further systematic experiments of Ti/V content and different substrate are required to obtain much more ∆Tsol improvement. In Fig.7(c), the hysteresis loops of C1, V1 and SD1 were measured at 2000 nm. The critical temperature (Tc) of semiconductor-metal phase transition is defined as the central temperature of the hysteresis loop. In Fig.7(c), C1 does not display the SMT transition, as reported in other research 15. The values of Tc are 50.0 oC and 56.3 oC for sample SD1 and V1, respectively. Compared to the bulk VO2 single crystal, whose transition temperature is ~68 oC, the decline of Tc in sample V1 is mainly ascribed to the stress arising from the ion bombardment during the depiction process and from the coefficient of thermal expansion (CTE) difference between the VO2 film and the sapphire substrate. It is worthy to note that the sample SD1 also suffered from a larger decline of Tc value although previous researches pointed out that the Tc value of VO2 would increase by Ti doping

27-29

. Similar to the sample C1, the shrink of Tc is also

connected with the stress from the ion bombardment and TEC difference. Moreover, the contribution from the lattice mismatching (Fig.7(d)) cannot be excluded. Research 30

has pointed out that the anisotropic strain caused by lattice mismatching can cause

the reduction in Tc, and the anisotropic compression pressure along the c axis is the most effective strain. As mentioned above, the spinodal modulation of our spinodally


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decomposed TVO thin film occurred along the c axis, and the lattice of Ti-rich was larger than V-rich since the diffraction angle of Ti-rich phase was lower, which means that there is tensile stress perpendicular to c axis and compressive stress parallel to c axis in V-rich phase. Ladd 30 reported that anisotropic compression along c axis could theoretically reduce Tc by -12 oC /GPa. Compared to the pure VO2 thin film (sample C1), the VTO thin film (sample SD1) exhibits a decrease of 6.3 oC in Tc, which means a huge anisotropic pressure of ≥0.53GPa has been achieved in V-rich layer. It is hard to achieve such a large value of pressure in regular multilayer method. As a consequence, spinodal decomposition provides us a new insight to lower the transition temperature and achieve huge anisotropic stress, which would be significant to further scientific research on VO2 and the application of VO2-based smart windows. Table.1 Optical properties of samples. Tlum(%) Sample

Tsol(%)

∆Tsol

∆T2000nm

Tc

20oC

80oC

20oC

80oC

(%)

(%)

o

A1

67.6

66.6

71.6

70.6

1.0

1.0

--

C1

24.5

22.8

29.1

22.4

6.7

31.2

--

T1

84.6

84.7

85.2

85.2

0

0

--

V1

18.5

18.6

26.0

14.5

11.5

44.2

56.3

SD1

22.0

20.9

33.2

20.1

13.1

56.7

50.0

C

4. Conclusions We prepared spinodally decomposed TVO thin film in room temperature sputtering-annealing

method

(RTS-A)

and

characterized

its

structure

and

thermochromic performance. The self-assembled multilayer thin film has a well-ordered alternative Ti-rich and V-rich parallel arrangement and can increase luminous transmittance, improve solar modulating ability and reduce the critical temperature simultaneously. Furthermore, the spinodally decomposed TVO thin film sample contained huge anisotropic stress, which is hard to achieve by other method, making the decomposed sample more promising in optical switching utilization. The



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self-assembled multilayer structure and relatively outstanding optical regulating properties indicated that spinodal decomposition may provide a novel insight for fabricating VO2-based smart windows materials.

Acknowledgments This study was financially supported by the Science and Technology Commission of Shanghai Municipality (STCSM, No.:14DZ2261203, 5157021410).

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Reducing Thermal Hysteresis Width of VO2 by Ti Doping: A Joint Experimental and Theoretical Study. J. Phys. Chem. C 2014, 118 (33), 18938-18944. (30) Ladd, L. A.; Paul, W., Optical and Transport Properties of High Quality Crystals of V2O4 near Metallic Transition Temperature. Solid State Commun. 1969, 7 (4), 425-428.



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Figure Captions: Fig.1 Schematic diagram of the synthetic route for (a) amorphous TVO (sample A1) and spinodally decomposed TVO (sample SD1), (b) crystalline solid solution TVO (sample C1), (c) pure VO2 (sample V1) thin film and (d) pure TiO2 thin film (sample T1).

Fig.2 (a) XPS spectrum of A1sample and high-resolution scan of (b) V 2p and O 1s, and (c) Ti 2p spectrum

Fig.3 XRD patterns of sample SD1, C1, A1, V1 and T1.

Fig.4 Schematic diagram of the microstructure of spinodally decomposed TVO.

Fig.5 TEM images for (a and b) sample A1 and (c and d) sample SD1.

Fig.6 EDS element analysis for SD1. (c and d) showed the Ti and V content along the red line which was drawn in TEM image (a and b) of SD1. (e, f and g) displayed the elemental mapping results of the selected area (b) with Al (e, in orange color), Ti (f, in yellow color) and V (g, in green color).

Fig.7 UV-vis-near-infrared transmittance spectra of samples in (a) A1, C1 and SD1; (b) T1, V1 and SD1, where L represented the spectra detected at low temperature (20oC) and H represented the high temperature (80oC). (c) Temperature-varied transmittance hysteresis loop plots of C1, V1 and SD1 at 2000 nm during heating (solid) and cooling (hollow) cycles. (d) Schematic drawing of Ti-rich and V-rich layer and stress analysis in spinodally decomposed TVO. Black arrows revealed the direction of lattice mismatching induced stress and strain of V-rich layer.


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Fig.1 Schematic diagram of the synthetic route for (a) amorphous TVO (sample A1) and spinodally decomposed TVO (sample SD1), (b) crystalline solid solution TVO (sample C1), (c) pure VO2 (sample V1) thin film and (d) pure TiO2 thin film (sample T1).



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Fig.2 (a) XPS spectrum of A1sample and high-resolution scan of (b) V 2p and O 1s, and (c) Ti 2p spectrum


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Fig.3 XRD patterns of sample SD1, C1, A1, V1 and T1.



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Fig.4 Schematic diagram of the microstructure of spinodally decomposed TVO.


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Fig.5 TEM images for (a and b) sample A1 and (c and d) sample SD1.



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Fig.6 EDS element analysis for SD1. (c and d) showed the Ti and V content along the red line which was drawn in TEM image (a and b) of SD1. (e, f and g) displayed the elemental mapping results of the selected area (b) with Al (e, in orange color), Ti (f, in yellow color) and V (g, in green color).


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Fig.7 UV-vis-near-infrared transmittance spectra of samples in (a) A1, C1 and SD1; (b) T1, V1 and SD1, where L represented the spectra detected at low temperature (20oC) and H represented the high temperature (80oC). (c) Temperature-varied transmittance hysteresis loop plots of C1, V1 and SD1 at 2000 nm during heating (solid) and cooling (hollow) cycles. (d) Schematic drawing of Ti-rich and V-rich layer and stress analysis in spinodally decomposed TVO. Black arrows revealed the direction of lattice mismatching induced stress and strain of V-rich layer.



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Table of Contents/Abstract Graphic


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Fig.1 Schematic diagram of the synthetic route for (a) amorphous TVO (sample A1) and spinodally decomposed TVO (sample SD1), (b) crystalline solid solution TVO (sample C1), (c) pure VO2 (sample V1) thin film and (d) pure TiO2 thin film (sample T1) 100x71mm (300 x 300 DPI)



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Fig.2 (a) XPS spectrum of A1sample and high-resolution scan of (b) V 2p and O 1s, and (c) Ti 2p spectrum 653x183mm (72 x 72 DPI)


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Fig.3 XRD patterns of sample SD1, C1, A1, V1 and T1 252x337mm (72 x 72 DPI)



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Fig.4 Schematic diagram of the microstructure of spinodally decomposed TVO. 193x124mm (149 x 149 DPI)


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Fig.5 TEM images for (a and b) sample A1 and (c and d) sample SD1 383x378mm (72 x 72 DPI)



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Fig.6 EDS element analysis for SD1. (c and d) showed the Ti and V content along the red line which was drawn in TEM image (a and b) of SD1. (e, f and g) displayed the elemental mapping results of the selected area (b) with Al (e, in orange color), Ti (f, in yellow color) and V (g, in green color) 340x160mm (149 x 149 DPI)


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Fig.7 UV-vis-near-infrared transmittance spectra of samples in (a) A1, C1 and SD1; (b) T1, V1 and SD1, where L represented the spectra detected at low temperature (20oC) and H represented the high temperature (80oC). (c) Temperature-varied transmittance hysteresis loop plots of C1, V1 and SD1 at 2000 nm during heating (solid) and cooling (hollow) cycles. (d) Schematic drawing of Ti-rich and V-rich layer and stress analysis in spinodally decomposed TVO. Black arrows revealed the direction of lattice mismatching induced stress and strain of V-rich layer 466x350mm (72 x 72 DPI)



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Table of Contents/Abstract Graphic 77x36mm (149 x 149 DPI)


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Self-Assembled Multilayer Structure and Enhanced Thermochromic

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