Atomic Layer Deposition of V1–xMoxO2 Thin Films, Largely Enhanced

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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 6601−6607

Atomic Layer Deposition of V1−xMoxO2 Thin Films, Largely Enhanced Luminous Transmittance, Solar Modulation Xinrui Lv,†,‡ Yunzhen Cao,*,† Lu Yan,† Ying Li,† Yuzhi Zhang,† and Lixin Song† †

Key Laboratory of Inorganic Coating Materials CAS, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: V1−xMoxO2 thin films were fabricated by nanolamination of VO2/MoO3 alternating layers using atomic layer deposition (ALD) process, in which tetrakis-dimethyl-amino vanadium(IV) [V(NMe2)4] and molybdenum hexacarbonyl(VI) [Mo(CO)6] were used as vanadium and molybdenum precursors, respectively. The dopant content of V1−xMoxO2 films was controlled by adjusting MoO3 cycle percentage (PMo) in ALD pulse sequence, which varied from 2 to 10%. Effects of PMo on V1−xMoxO2 crystal structure, morphology, semiconductor-to-metal transition properties, and optical transmittance were studied. A linear reduction of phase transition temperature (Tc) by approximately −11 °C/cycle % Mo was observed for V1−xMoxO2 films within PMo ≤ 5%. Notably, dramatic enhanced luminous transmittance (Tlum = 63.8%) and solar modulation (ΔTsol = 23.5%) were observed for V1−xMoxO2 film with PMo = 7%. KEYWORDS: atomic layer deposition (ALD), VO2/MoO3 nanolaminates, V1−xMoxO2 films, semiconductor-to-metal transition (SMT), thermochromic performance

1. INTRODUCTION Atomic layer deposition (ALD) is a state-of-the-art technique for thin-film deposition with outstanding characteristics such as uniformity, conformality, and high-density.1 This technique relies on alternate pulsing of precursors onto substrate surface and subsequent chemisorption or surface reaction of the precursors, resulting in its self-limiting surface reaction mechanism.2 Recently, ALD technique has been studied to produce doped films,3,4 which were fabricated by nanolaminated packing of alternative layers of different constituents. Because the nanolaminates at the scale of nanometer thickness are initially in nonequilibrium condition with large interface energy,3 it is thermodynamically favorable to form a structure with lower energy through interlayer diffusion.5 Several doped films were successfully fabricated by ALD nanolamination process, such as Al-doped ZnO,4,6 Ge-doped ZnO,7 and Zndoped TiO2.3 The doped films were considered to have a uniform dopant solubility after postannealing process, which was suggested by secondary ion mass spectrometry from the depth profiles.3 Moreover, films with different dopant contents could be attained by varying the cycle ratio of the binary © 2018 American Chemical Society

constituents in the nanolamination process. Therefore, the ALD nanolamination process makes it possible to attain a continuously adjustable solubility of doped films with good uniformity. Vanadium dioxide (VO2) undergoes a reversible semiconductor-to-metal transition from monoclinic phase (M1, P21/c) to rutile phase (R, P42/mnm) at 341 K.8 Accompanied with the transition, drastic changes in optical and electrical behaviors occur, which is promising for a variety of practical applications such as smart windows,9 infrared stealth material,10 and switching devices.11 However, relatively high-phasetransition temperature (Tc) of pure VO2 film restricts its practical use. It was reported that doping could efficiently reduce Tc (e.g., −23 K/at. % by W12 and −15 K/at. % by Mo13). Besides, VO2 film with Mo as dopant can considerably increase vis−NIR transmittance in semiconductor and metallic states.14 Because the efficient reduction of Tc and considerable Received: October 30, 2017 Accepted: January 30, 2018 Published: January 30, 2018 6601

DOI: 10.1021/acsami.7b16479 ACS Appl. Mater. Interfaces 2018, 10, 6601−6607

Research Article

ACS Applied Materials & Interfaces

vacuum (∼1 Pa) to obtain crystallized V1−xMoxO2 films, and the VO2/ MoO3 alternating layers were expected to penetrate and intermix with each other,3 resulting in a ternary compound of V1−xMoxO2. 2.2. Characterizations. The composition of V1−xMoxO2 films were analyzed by Rutherford backscattering spectrometry (RBS) measurement with a 2 MeV He+ beam. The recorded BRS data were simulated based on the SIMNRA program.19 The crystal phases of films were identified by X-ray powder diffraction (XRD, Bruker D2 Phaser) using Cu Kα (λ = 0.15406 nm) radiation. The oxidation states of V and Mo in V1−xMoxO2 films were measured by X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific ESCAlab250) using Al Kα as the exciting source. The XPS spectra were calibrated by the C 1s peak (285 eV) from hydrocarbon contamination on the sample surface. Surface morphologies of the films were observed by field-emission scanning electron microscopy (FESEM, FEI Magellan 400). The resistance−temperature hysteretic curves of films were measured by physical property measurement system (Quantum Design PPMS-9(dxl)), and phase-transition temperatures Tc were calculated by differentiating the hysteretic curves, that is, d[log(R)]/dT for both heating and cooling branches. Spectral transmittances of the films on fused silica were measured by UV− vis−NIR spectrophotometers (Cary 500, Varian, America). The integral visible transmittance (Tlum, 380−780 nm) and solar transmittance (Tsol, 240−2500 nm) were obtained based on the measured spectra using the following equation

enhanced transmittance in vis−NIR region, Mo-doped VO2 thin film is promising for commercial applications such as smart windows. Several studies have been systematically reported using ALD technique to deposit VO2 thin films.15,16 However, to our knowledge, doped VO2 films by ALD process have not been reported yet. To deposit Mo-doped VO2 film by ALD nanolamination method, one prerequisite is that the reactor temperature should meet the requirement of the deposition temperature for both VO2 and MoO3. It is thereby crucial to find proper ALD precursors for these two constituents. The deposition temperature for ALD VO2 has already been determined as 150−200 °C, using tetrakis-dimethyl-amino vanadium(IV) as vanadium precursor, in our previous study.16 In addition, studies indicate that the temperature for MoO3 deposition is 152−17217 or 165−175 °C18 using Mo(CO)6 as molybdenum precursor. Therefore, the overlapping temperature range of ALD VO2 and MoO3 can be used for the fabrication of Mo-doped VO2 films. In present work, the ALD process to obtain V1−xMoxO2 films with different Mo contents was studied, and the crystal structure, microstructure, and thermochromic performances of the films were systematically investigated. Dramatic enhanced luminous transmittance (Tlum = 63.8%) and solar modulation (ΔTsol = 23.5%) were observed for V1−xMoxO2 film, which almost surpasses all of the thermochromic performances of other VO2-based films reported to date.

Tlum/sol =

∫ φlum/sol(λ)T(λ) d(λ)/∫ φlum/sol(λ) d(λ)

(1)

where T(λ) denotes the transmittance at wavelength λ, φlum is the standard luminous efficiency function for the photonic vision of human eyes,20 and φsol is the solar irradiance spectrum for the air mass 1.5 (corresponding to the sun standing 37° above the horizon).21 The solar modulation ability was calculated as ΔTsol = Tsol,l − Tsol,h.

2. EXPERIMENTAL SECTION 2.1. Deposition of V1−xMoxO2 Thin Films. To deposit V1−xMoxO2 thin films, the ALD process of MoO3 film was first studied, using Mo(CO)6 (98%, Aldrich) as molybdenum precursor and plasma-enhanced O2 as reactant gas (see the Supporting Information for more details). Because the MoO3 ALD window has been determined as 150−170 °C (see Figure S1) and VO2 ALD window as 150−200 °C in our previous study,16 the overlapped temperature range of 150−170 °C was assumed to deposit selected VO2/MoO3 nanolaminates. In the experiment, all of the nanolaminations were deposited at 160 °C. The ALD process of VO2/ MoO3 nanolaminates consists of alternate certain cycles of VO2 and MoO3, which is shown in Table 1. The whole process with a total of

3. RESULTS AND DISCUSSION 3.1. Effect of MoO3 Cycle Percentage (PMo) on Film Composition, Structure, and Morphology of V1−xMoxO2 Film. The RBS spectra of the annealed V1−xMoxO2 films with different PMo are shown in Figure 1. The experimental signals

Table 1. Summary of VO2/MoO3 Nanolaminates with Various Percentage of MoO3 Cycles (PMo) percentage of MoO3 cycles (PMo)

number of MoO3 cycles (NMo)

number of VO2 cycles (NV)

number of supercycles

0 2% 3.2% 5% 7% 10%

0 2 4 5 7 10

100 98 121 95 93 90

5 5 4 5 5 5

Figure 1. RBS of the annealed V1−xMoxO2 films with MoO3 cycle percentage PMo from 2 to 10%. The solid lines present experimental data and the open circles present simulated data. The calculated Mo contents x are shown in the brackets.

500 ALD cycles was composed of four or five supercycles of VO2/ MoO3 bilayers, with each supercycle consisting of NV-cycles of VO2 and NMo-cycles of MoO3. Therefore, MoO3 cycle percentage could be calculated as PMo = NMo/(NV + NMo) × 100%. To attain films with different Mo contents, various MoO3 cycle percentages were controlled using altered cycle numbers of ALD VO2 and MoO3. Each VO2 cycle consists of a timing sequence of V(NMe2)4(3s)− Ar(8s)−H2O(0.6s)−Ar(3s) determined in previous study,16 whereas each MoO3 cycle consists of a timing sequence of Mo(CO)6(1s)− Ar(4s)−O2(1s)−Ar(3s). The VO2/MoO3 nanolaminated films were deposited on Si(100) substrates and fused silica substrates under 150− 170 °C. The as-deposited films were annealed at 500 °C for 60 min in

(solid line) are in a good coincidence with the simulated curves (open circle). Because RBS signal is highly sensitive to heavy atoms with large atomic numbers, the signal of Mo is significant in the spectra despite the low value of MoO3 cycle percentage. The Mo content is determined to be x = 0.024 (PMo = 2%), x = 0.055 (PMo = 3.2%), x = 0.092 (PMo = 5%), x = 0.104 (PMo = 7%), and x = 0.108 (PMo = 10%). 6602

DOI: 10.1021/acsami.7b16479 ACS Appl. Mater. Interfaces 2018, 10, 6601−6607

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Figure 2. XRD patterns of V1−xMoxO2 films with different MoO3 cycle percentage PMo deposited on (a) Si(100) substrates and (b) fused silica substrates.

Figure 3. (a) V 2p−O 1s and (b) Mo 3d XPS spectra of annealed V1−xMoxO2 films with different MoO3 cycle percentage PMo.

suggesting that the crystallinity of VO2(R) enhances as the Mo content increases. Moreover, the peak positions of VO2(R) (110) shift slightly toward smaller angle compared with that of pure VO2(R) at 27.68° (JCPDS card no. 44-0253). These peak shifts indicate a wider plane space as a result of the replacement of V in VO2 lattice by Mo with a larger ionic radius. The similar variation trend is also observed for V1−xMoxO2 films deposited on fused silica, as shown in Figure 2b. The oxidation states of vanadium and molybdenum in V1−xMoxO2 films were studied by XPS. The oxidation state of V was determined by calculating the binding energy difference (Δ) between O 1s and V 2p3/2. It was reported that Δ = 12.8

Figure 2a shows the XRD patterns of annealed V1−xMoxO2 films deposited on Si(100) substrates. The V1−xMoxO2 film with PMo = 2% exhibits the same diffraction peak at 27.9° as the undoped VO2(M1),16 corresponding to (011) plane. However, with PMo increasing to 3.2%, a weaker but wider peak appears with a large full width at half maximum (FWHM) of 0.35° and shifts slightly to a smaller angle, suggesting a rather low crystallinity and a considerable distortion in VO2(M1) phase. When PMo further increases to 5, 7, and 10%, the films change into VO2(R) totally with the peaks at around 27.6°, corresponding to VO2(R) (110) plane. The peak intensity increases with PMo accompanied by the decreased FWHM, 6603

DOI: 10.1021/acsami.7b16479 ACS Appl. Mater. Interfaces 2018, 10, 6601−6607

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ACS Applied Materials & Interfaces eV for V5+, 14.35 eV for V4+, and 14.84 eV for V3+, respectively.22 Figure 3a shows V 2p−O 1s photoelectron spectra of V1−xMoxO2 films after 10 s of Ar ion etching to remove the surface oxide layer of films because VO2 is metastable and is easily oxidized in air. The fitted peaks of O 1s at around 530.5 eV denote the low-binding-energy component ascribed to the O 1s core peak of O2− bound to V ions,23 whereas the peaks centered at 531.5−532 eV denote highbinding-energy components that might relate to hydroxides, organic oxygen, or oxygen vacancies.23 Only one fitted peak is observed in each V 2p spectra for the samples with PMo = 2% and PMo = 5%, indicating that only one vanadium valence state presents in the films. Because Δ = 14.31 eV for film with PMo = 2% and Δ = 14.35 eV for film with PMo = 5%, the valence state of vanadium is determined as V4+ according to the literature. Additionally, the binding energies of V 2p3/2 in samples with smaller Mo contents are slightly higher than that of VO2 film in our previous research,16 that is, 515.96 eV for PMo = 0 and about 516.2 eV for PMo = 2 and 5%, which is in agreement with the other research of Mo-doped VO2 films.24 However, when Mo is heavily doped in VO2 films (PMo = 10%), a new fitted peak of V 2p3/2 appears at a lower binding energy of 515.35 eV with Δ = 15 eV, denoting the oxidation state of V3+. Similar result was found in W-doped VO2 by Tang et al.25 It was proposed that each W6+ in VO2 lattice breaks up a V4+−V4+ bond, and two W 3d electrons are transferred to the nearest neighbor V4+, resulting in a V3+−W6+ and a V3+−V4+ pair. Because Mo and W are congeners in the periodic table, it is reasonable to suggest that the presence of V3+ might be caused by the electron transferred from Mo to V in the heavily doped film. The oxidation states of molybdenum on the surface (without etching) of V1−xMoxO2 films were measured by XPS (Figure 3b) because Mo6+ ions are easily reduced to lower states when etched with Ar ions.14 The spectra of each sample consist of Mo 3d5/2 and Mo 3d3/2 spin−orbit components. Each Mo 3d component consists of two fitted peaks, indicating two molybdenum states that exist in the V1−xMoxO2 films. Peaks centered at around 233.1 eV (Mo 3d5/2) and 236.2 eV (Mo 3d3/2) with Δ = 3.15 eV are corresponding to Mo6+.26 Besides, there are peaks at around 231.3 and 234.4 eV, which are ascribed to lower oxidation state because of the lower binding energies. It has previously been suggested that BE = 231.2 eV (Mo 3d5/2) and BE = 234.3 eV (Mo 3d3/2) correspond to Mo5+.26 Being coincident with the literature, the lower oxidation state of molybdenum is determined as Mo5+; thus, the oxidation states of Mo are the mixture of Mo5+ and Mo6+. The surface morphologies of annealed VO2 and V1−xMoxO2 films are observed by FESEM, as shown in Figure 4. Continuous films with similar morphology are observed in Figure 4a (VO2 film) and 4b (V1−xMoxO2 film with PMo = 2%), whereas the latter shows more crystal boundaries with smaller grain size. Films with PMo = 3.2, 5, and 7% in Figure 4c−e demonstrate similar morphology with porous structures. The film with PMo = 10% (Figure 4f) exhibits relatively lower porosity and smoother surface compared with the films with PMo from 3.2 to 7%. Similar porous microstructure of VO2 has been reported in previous research. Porous VO2 films have also been fabricated by Xu et al.27 upon thermal annealing process. Pauli et al.28 obtained the atomically flat amorphous VO2 layer using pulsed-laser deposition, which is mainly converted into crystalline isolated islands upon annealing. In our study, the formation of noncontinuous porous films may be caused by a

Figure 4. FESEM images of (a) annealed VO2 film and V1−xMoxO2 films with MoO3 cycle percentage of (b) 2, (c) 3.2, (d) 5, (e) 7, and (f) 10%.

dewetting process29 that occurred at the interfacial compound of VO2/MoO3 during annealing. Considering that the continuous films (VO2 and V1−xMoxO2 film with PMo = 2%) show crystal structure of VO2(M1), whereas porous films (V1−xMoxO2 films with PMo from 3.2 to 10%) show crystal structure of VO2(R), it can be inferred that the difference of dewetting properties between VO2(M1) and VO2(R) might cause variation in the microstructure for films with different Mo content. 3.2. Effect of MoO3 Cycle Percentage (PMo) on Electrical and Optical Properties of V1−xMoxO2 Films. Figure 5a describes the resistance−temperature curves (R−T) of V1−xMoxO2 films during heating and cooling processes, and Figure 5b depicts their derivative curves (d[log(R)]/dT−T), in which the solid lines and dot lines represent heating branches and cooling branches, respectively. The phase-transition temperature Tc is determined as the average of two peak positions of d[log(R)]/dT−T curves upon heating and cooling, and the hysteresis loop width ΔH is calculated as the temperature difference of these two peaks. The temperaturedependent resistance change is considered as the ratio of resistance at low temperature to that at high temperature, that is, RL/RH. It is clear that Tc drops dramatically with Mo doping, that is, 66.7 °C for VO2 film and 46.0, 31.1, 13.5, and −11.1 °C for doped films with PMo of 2, 3.2, 5, and 10%, respectively. Additionally, the presence of Mo efficiently narrows ΔH from 10 °C for the undoped film in our previous work16 to almost 0.5 °C for the doped film with PMo = 5%. The shrinkage of ΔH with increasing dopant content agrees with other reports.13,30 However, the temperature range of transition, determined as the temperature between the start and the end of significant resistance change, widens with the increase of PMo. The resistance change across transition (RL/RH) reduces from 2.5 orders of magnitude for the undoped VO2 to just one order of magnitude for doped films, which is mainly because of the lower resistance at semiconductor phase because Mo acts as the 6604

DOI: 10.1021/acsami.7b16479 ACS Appl. Mater. Interfaces 2018, 10, 6601−6607

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Figure 5. (a) V1−xMoxO2 film resistance as a function of temperature. (b) Derivative curves (d[log(R)]/dT−T) of 5a with the inset of relationship between phase-transition temperature Tc and MoO3 cycle percentage PMo.

Figure 6. (a) Transmittance of VO2 film and V1−xMoxO2 films within the spectrum of UV−vis−NIR region at the semiconductor phase (solid lines) and at metallic phase (dash lines). (b) Integral luminous transmittance (Tlum,l for the semiconductor state and Tlum,h for the metallic state) and solar modulation (ΔTsol) with different MoO3 cycle percentage PMo of V1−xMoxO2 films.

electron donor.31 The inset in Figure 5b illustrates a linear reduction of Tc by −11 °C/cycle % Mo for V1−xMoxO2 films with PMo ≤ 5%. However, when the PMo is further increased to 10%, Tc deviates from the trend of the linear fit, presenting a lower reduction efficiency. Figure 6a depicts the transmittance of VO2 film and V1−xMoxO2 films deposited on fused silica. Compared with the transmittance of VO2 film, the transmittance of Mo-doped films is increased almost in the whole UV−vis−NIR range, and the enhancement of transmittance in visible region for films in the semiconductor state (solid line) is more significant than that in the metallic state (dash line). Additionally, it is observed that doping slightly reduces infrared modulation (ΔTIR) across phase transition, that is, ΔTIR at 2.0 μm is 57.1% for VO2 film, and 44.2, 41.3, and 49.9% for Mo-doped film with PMo = 2%, PMo = 5%, and PMo = 7%, respectively. ΔTIR drops to the lowest value for the film with PMo = 5%, which has the largest hole size indicated by the FESEM images. The decrease of ΔTIR may be becuase of the porous structure of doped films.32 Figure 6b shows the corresponding integral luminous transmittance (Tlum,l for the semiconductor state and Tlum,h for the metallic state) and solar modulation (ΔTsol) calculated from the spectra in Figure 6a. As PMo increases from 0 to 7%, Tlum,l increases from 46 to 71.6%, whereas Tlum,h first increases from 52.1 to 60.9% (PMo = 2%) and then drops to 56% (PMo = 7%). The average luminous transmittance (Tlum) for V1−xMoxO2 films with PMo = 5% and PMo = 7% is 64.4 and 63.8%, respectively, which ideally meets the requirement (preferably Tlum > 60%) of smart windows with good indoor illumination.33 Moreover,

accompanied with the dramatically increased Tlum, the solar modulation ΔTsol is increased from 6.4% (Tsol,l = 51.6% and Tsol,h = 45.2%) for VO2 film to 23.5% (Tsol,l = 72.1% and Tsol,h = 48.6%) for V1−xMoxO2 film with PMo = 7%. The drastic enhancement of both luminous transmittance and solar modulation ability may be because of the porous structure of V1−xMoxO2 films, which has been demonstrated in previous reports. Chen et al.34 observed a similar phenomenon (Tlum,l > Tlum,h) when studying the thermochromic property of W-doped VO2 nanorods. Lu et al.35 also found that the luminous transmittance of periodic micropatterned VO2 in the semiconducting state was larger than that in the metallic state. Liu et al.33 obtained similar optical behavior with VO2 nanocomposite. They proposed that VO2 nanocomposite has a lower refractive index in the spectrum range of vis−NIR below its critical transition temperature (Tc), which reduces reflection and enhances luminous transmittance at a lower temperature. Therefore, the relationship between the microstructure of the V1−xMoxO2 films and their complex refractive index will be investigated in the further study. Numerous methods have been attempted to fabricate VO2 base materials with increased Tlum and ΔTsol, such as doping,34,36 multilayer structuring,37 biomimetic nanostructuring,38 nanoporous structuring,39 and nanoparticle-based materials.33 Table 2 summarizes some of the best thermochromic performances with different VO2-based materials from previous researches. It can be seen that both Tlum and ΔTsol of the VO2-based materials have increased to some extent compared with the single-layered VO2 film. However, Tc of 6605

DOI: 10.1021/acsami.7b16479 ACS Appl. Mater. Interfaces 2018, 10, 6601−6607

ACS Applied Materials & Interfaces

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Table 2. Different VO2-Based Materials with the Best Thermochromic Performances materials

Tlum (%)

ΔTsol (%)

Tc (°C)

41 60.4 45 44.5 50 59 60 63.8

6.7 14.1 12.1 7.1 14.7 12 35 23.5

60 64.3 60 unreported unreported unreported 30

single-layered VO2 films Zr-doped VO2 foils36 TiO2/VO2/TiO2/VO2/TiO237 moth-eyed nanostructure38 nanoporous VO2 films39 VO2/Si−Al nanocomposite33 VO2/hydrogel hybrid40 V1−xMoxO2 films in this work 41

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xinrui Lv: 0000-0002-0817-8300 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Rongxu Bai (Beneq Oy) for his assistance during ALD deposition. The current work is supported by the laboratory foundation of Chinese Academy of Sciences (grant no. 16S083).

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many of these materials are too high to be used in the practical application. It is worth noticing that Long et al.40 have fabricated VO2/hydrogel hybrid, which integrated inorganic with organic thermochromic materials, possessing an unprecedented value of ΔTsol = 35% with an average Tlum = 60%. However, the fabrication of these hybrids is troublesome, timeconsuming, and less possible to apply into mass production. The fabrication method of V1−xMoxO2 films in our work is simple and scalable to mass production. Besides, to our knowledge, the thermochromic performances of V1−xMoxO2 films almost surpass all of the performances reported to date. With the advantages of largely decreased Tc, drastic-enhanced Tlum and ΔTsol, and easily fabricated method to mass production, the ALD V1−xMoxO2 films are promising for the application for smart windows.

REFERENCES

(1) George, S. M. Atomic Layer Deposition: An Overview. Chem. Rev. 2010, 110, 111−131. (2) Leskelä, M.; Ritala, M. Atomic Layer Deposition (ALD): From Precursors to Thin Film Structures. Thin Solid Films 2002, 409, 138− 146. (3) Su, C.-Y.; Wang, C.-C.; Hsueh, Y.-C.; Gurylev, V.; Keic, C.-C.; Perng, T.-P. Enabling High Solubility of ZnO in TiO2 by Nanolamination of Atomic Layer Deposition. Nanoscale 2015, 7, 19222−19230. (4) Geng, Y.; Guo, L.; Xu, S.-S.; Sun, Q.-Q.; Ding, S.-J.; Lu, H.-L.; Zhang, D. W. Influence of Al Doping on the Properties of ZnO Thin Films Grown by Atomic Layer Deposition. J. Phys. Chem. C 2011, 115, 12317−12321. (5) Tromp, R. M.; Hannon, J. B. Thermodynamics of Nucleation and Growth. Surf. Rev. Lett. 2002, 9, 1565−1593. (6) Elam, J. W.; George, S. M. Growth of ZnO/Al2O3 Alloy Films Using Atomic Layer Deposition Techniques. Chem. Mater. 2003, 15, 1020−1028. (7) Lee, S.-H.; Lee, J.-H.; Choi, S.-J.; Park, J.-S. Studies of Thermoelectric Transport Properties of Atomic Layer Deposited Gallium-Doped ZnO. Ceram. Int. 2017, 43, 7784−7788. (8) Morin, F. J. Oxides Which Show a Metal-To-Insulator Transition at the Neel Temperature. Phys. Rev. Lett. 1959, 3, 34−36. (9) Kamalisarvestani, M.; Saidur, R.; Mekhilef, S.; Javadi, F. S. Performance, Materials and Coating Technologies of Thermochromic Thin Films on Smart Windows. Renewable Sustainable Energy Rev. 2013, 26, 353−364. (10) Mao, Z.; Wang, W.; Liu, Y.; Zhang, L.; Xu, H.; Zhong, Y. Infrared Stealth Property Based on Semiconductor (M)-to-Metallic (R) Phase Transition Characteristics of W-Doped VO2 thin films Coated on Cotton Fabrics. Thin Solid Films 2014, 558, 208−214. (11) Rini, M.; Hao, Z.; Schoenlein, R. W.; Giannetti, C.; Parmigiani, F.; Fourmaux, S.; Kieffer, J. C.; Fujimori, A.; Onoda, M.; Wall, S.; Cavalleri, A. Optical Switching in VO2 Films by Below-Gap Excitation. Appl. Phys. Lett. 2008, 92, 181904. (12) Tazawa, M.; Jin, P.; Tanemura, S. Optical Constants of V1‑xWxO2 Films. Appl. Opt. 1998, 37, 1858−1861. (13) Hanlon, T. J.; Coath, J. A.; Richardson, M. A. MolybdenumDoped Vanadium Dioxide Coatings on Glass Produced by the Aqueous Sol-Gel Method. Thin Solid Films 2003, 436, 269−272. (14) Liu, S.-J.; Fang, H.-W.; Su, Y.-T.; Hsieh, J.-H. Metal-Insulator Transition Characteristics of Mo- and Mn-Doped VO2 Films Fabricated by Magnetron Cosputtering Technique. Jpn. J. Appl. Phys. 2014, 53, 063201. (15) Park, H.; Kim, B.; Lee, S. H.; Kim, H. Study of a Vanadium Precursor for VO2 Thin-Film Growth in the Atomic Layer Deposition Process by Multiscale Simulations. J. Phys. Chem. C 2016, 120, 28193− 28203. (16) Lv, X.; Cao, Y.; Yan, L.; Li, Y.; Song, L. Atomic Layer Deposition of VO2 Films with Tetrakis-Dimethyl-Amino Vanadium(IV) as Vanadium Precursor. Appl. Surf. Sci. 2017, 396, 214−220.

4. CONCLUSIONS V1−xMoxO2 thin films were deposited by nanolamination using ALD technique with V(NMe2)4/H2O and Mo(CO)6/O2 (plasma-enhanced) as precursors. The ALD window for the deposition of VO2/MoO3 nanolaminates was experimentally determined as 150−170 °C. The dopant content of V1−xMoxO2 films could be controlled by adjusting ALD MoO3 and VO2 cycle numbers. The XRD patterns showed that VO2(R) could be obtained at room temperature at MoO3 cycle percentage PMo ≥ 5%. The oxidation valence of V was V4+ in films with small Mo contents (PMo = 2%, PMo = 5%), whereas it was the mixture of V4+ and V3+ in the heavily doped film (PMo = 10%). The oxidation valence of Mo was the mixture of Mo6+ and Mo5+ for all V1−xMoxO2 films. The FESEM images showed morphologies of doped films that were porous when PMo ≥ 2%. Besides, the relationship between the phase-transition temperature Tc and the Mo content of V1−xMoxO2 was studied through the electrical performances of the films. Tc decreased dramatically with the increase of PMo, and the linear reduction in Tc by approximately −11 °C/cycle % Mo was observed when PMo ≤ 5%. The optical transmittance of V1−xMoxO2 films deposited on fused silica was increased almost in the whole UV−vis−NIR region with the enhanced solar modulation as well, that is, Tlum,l = 71.6%, Tlum,h = 56%, and ΔTsol = 23.5% for film with PMo = 7%, which is beneficial to the applications such as smart window.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b16479. Deposition and characterization of ALD MoO3 thin films (PDF) 6606

DOI: 10.1021/acsami.7b16479 ACS Appl. Mater. Interfaces 2018, 10, 6601−6607

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ACS Applied Materials & Interfaces

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DOI: 10.1021/acsami.7b16479 ACS Appl. Mater. Interfaces 2018, 10, 6601−6607