Effect of a CdSe Layer on the Thermo- and Photochromic

Article Cite This: J. Phys. Chem. C 2019, 123, 17083−17091

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Effect of a CdSe Layer on the Thermo- and Photochromic Properties of MoO3 Thin Films Deposited by Physical Vapor Deposition M. Morales-Luna,*,† M. A. Arvizu,‡ M. Peŕ ez-Gonzaĺ ez,§,∥ and S. A. Tomaś ⊥

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†

Escuela de ingeniería y ciencias, Centro del agua, Tecnológico de Monterrey, Av. Eugenio Garza Sada 2501 Sur, Nuevo León, A.P. 64849 Monterrey, México ‡ Centro de Investigación y Desarrollo Tecnológico en Energías Renovables, Universidad Politécnica de Chiapas, A.P. 29150 Suchiapa, Chiapas, México § Unidad Profesional Interdisciplinaria en Ingeniería y Tecnologías Avanzadas del IPN, Ciudad de México 07340, México ∥ ́ Area Académica de Matemáticas y Física, Instituto de Ciencias Básicas e Ingeniería, Universidad Autónoma del Estado de Hidalgo, Carretera Pachuca-Tulancingo Km. 4.5, Col. Carboneras, C.P. 42184, Mineral de la Reforma, Hidalgo, México ⊥ Departamento de Física, Centro de Investigación y de Estudios Avanzados del IPN, A.P. 14-740, Ciudad de México 07360, México S Supporting Information *

ABSTRACT: The influence of a cadmium selenide (CdSe) layer on the thermochromic and photochromic properties of molybdenum trioxide (MoO3) thin films was studied. The films were deposited on glass substrates by thermal evaporation in vacuum using two different bilayer configurations, namely, substrate/MoO3/CdSe (SMC) and substrate/CdSe/MoO3 (SCM). The film thicknesses for the MoO3 and CdSe layers were ca. 250 and 20 nm, respectively. The thermochromic effect was evaluated in the annealing temperature range from 25 to 225 °C, in the presence of air. The characteristic optical absorption band attributed to the color center formation, centered at 820 nm, indicated enhanced thermo- and photochromic effects for both bilayer systems relative to monolayer MoO3 thin films. For the thermochromic effect, this improvement was more pronounced when CdSe was the upper layer, i.e., for the SMC system. Regarding the photochromic effect, the films were irradiated with UV light for several exposure times within the lapse of 30−180 min. While both bilayer systems presented better photochromic response than pure MoO3 thin films, the SCM system exhibited better photochromic response. These results are explained in terms of the optical, structural, and surface chemistry properties of the films.

1. INTRODUCTION Since the last few decades, the excessive production of pollutant gases such as carbon dioxide (CO2) has been blamed as the main factor behind climate change.1,2 Lots of efforts have been made to mitigate this effect; an important goal is the production of “clean” energy, but also, a lot of attention has been paid to technologies aimed at optimizing the energy use. In the latter case, chromogenic materials are used to reduce the need for artificial lighting, as well as heating and cooling systems to regulate indoor temperature. These compounds have the ability to form color centers when exposed to certain external agents. For example, photochromic materials change their coloration when excited by irradiation,3 while the thermochromic composites change their optical properties when subjected to different temperatures.4 On the other hand, electrochromic materials form color centers when they are under the influence of an applied external electric field.5 Among chromogenic samples, certain transition-metal oxides, for instance, vanadium dioxide (VO2), tungsten trioxide (WO3), and molybdenum trioxide (MoO3), are known to possess photo-, thermo-, or electrochromic properties. In some © 2019 American Chemical Society

cases, more than one of these properties are presented simultaneously6−9 with great potential for use in several technological applications due to their outstanding physical and chemical properties.4−6,10−13 A variety of heterostructures have been studied to improve their thermochromic, photochromic, and electrochromic properties, including VO2/ZnO, VO2/SnO2, VO2/WO3, Aunanoparticles/MoO 3 , Pt/MoO 3 , WO 3 /MoO 3 , ZrO 2 / V1−xWxO2/ZrO2, and ZnSe/MoO3. All of these configurations have been synthesized with the objective of improving a given chromogenic property, typically having in mind specific applications.12,14−17 The increased photochromic and thermochromic responses in these heterostructures have been explained in terms of the formation of a Schottky barrier created between the metal and the oxide that inhibits the fast recombination of the excited electrons. Another method to improve the chromogenic properties of MoO3 thin films Received: March 27, 2019 Revised: June 19, 2019 Published: June 20, 2019 17083

DOI: 10.1021/acs.jpcc.9b02895 J. Phys. Chem. C 2019, 123, 17083−17091

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The Journal of Physical Chemistry C

Figure 1. X-ray diffraction patterns for (a) MoO3 and (b) SMC thin films, either kept at room temperature or annealed at 225 °C. For comparison, JCPDS data for the wurtzite structure of CdSe is included. (c) XRD patterns for MoO3 and SCM thin films irradiated for 180 min and, in the bottom, for the SCM system annealed at 225 °C.

2.2. Film Characterization. On the one side, thermochromism was induced in the samples by means of thermal treatments in air in a conventional furnace (Barnstead/ Thermolyne 1300) for 2 h. The annealing temperature was varied in the range of 25−225 °C, with steps of 25 °C. On the other side, photochromism was achieved by irradiating the samples with a high-power xenon lamp (Oriel, 6271) held at 700 W. The samples were placed at 10 cm from the lamp. Irradiation in air was performed for several periods of time between 0 and 180 min. Different spectroscopic and nonspectroscopic techniques were used to perform the optical, structural, and surface chemistry characterizations. UV−vis spectrophotometry in the range from 300 to 1000 nm was accomplished with a Perkin-Elmer Lambda instrument. Raman spectroscopy was conducted on a Horiba Jobin Yvon microRaman spectrometer, using a He−Ne laser with an excitation line of around 632.8 nm and an output power of 20 mW. X-ray diffraction (XRD) patterns were obtained with a Siemens D5000 system using the Cu Kα line. X-ray photoelectron spectroscopy (XPS) was achieved with a Thermo Scientific KAlpha spectrometer using the Al Kα excitation line at 1487 eV along with an energy step size of 0.10 eV, a pass energy of 50 eV, and a spot size of 400 μm. An XPS rotary depth profile analysis using an Ar+ ion gun was carried out to analyze the elemental distribution of the SMC system. At each sputtering level, the Ar+ gun was turned on for 30 s with an intensity of 1 keV. The spot size used was 400 μm, with a pass energy of 150 eV and a speed rotation of 2 rpm.

consisted in the deposition of an intermediate layer of a II−VI semiconductor, specifically cadmium sulfide (CdS).18 This proposal is based on the injection of electrons from the CdS layer into the MoO3 layer. The reason for using CdS is because it has a lower band-gap energy (∼2.4 eV) than MoO3 (∼3.0 eV). This idea has been further developed to explore the effect of zinc selenide (ZnSe) doping on a MoO3 thin film.19 In this case, the inclusion of ZnSe particles in the bulk of molybdenum oxide films enhanced both the thermochromism and photochromism. In addition, the effect of a ZnSe layer on the thermochromic properties of MoO3 thin films has been proven successful by our group.17 In the present work, thermal evaporation in vacuum was used to deposit two types of bilayer systems, i.e., substrate/ MoO3/CdSe (SMC) and substrate/CdSe/MoO3 (SCM). Cadmium selenide (CdSe) was chosen due to its importance in optoelectronic applications, light-emitting devices, and nanosensors, among other uses.20 In addition, the small band gap (∼1.7 eV) of CdSe is important to improve the photochromic and thermochromic properties of MoO3. As discussed below, the position of this layer turned out to play a relevant role in the specific chromogenic phenomena studied.

2. EXPERIMENTAL DETAILS 2.1. Thin-Film Preparation. The substrate/MoO3, substrate/MoO 3 /CdSe (SMC), and substrate/CdSe/MoO 3 (SCM) systems were deposited by thermal evaporation in vacuum. High-purity MoO3 (99.9%, Sigma-Aldrich) and CdSe (99.9%, Electronic Space Products International) powders were used as precursors. Prior to film deposition, either 0.5 g of MoO3 or 0.4 g of CdSe powders were successively placed in crucibles mounted in a vacuum chamber. The chamber was then evacuated to a base pressure of ca. 2.5 × 10−5 Torr. During the evaporation process, a current of ∼1.8 A allowed sublimation of the powders until an approximate thickness of 250 nm for MoO3 films and 20 nm for the CdSe layer was reached, as measured with a quartz crystal oscillator. The difference between the SCM and SMC bilayer systems is only the order in which the MoO3 and CdSe thin films were deposited.

3. RESULTS AND DISCUSSION 3.1. Structural Characterization by XRD. Figure 1a shows the X-ray diffractograms of MoO3 thin films deposited by thermal evaporation in vacuum at room temperature (RT), as well as the corresponding MoO3 films annealed in air at 225 °C. These patterns indicate an amorphous structure for both types of samples, evidencing that the thermal treatments, within this temperature range, do not induce crystallinity. On the other hand, exhibited in Figure 1b are the diffraction patterns for as-deposited and annealed SMC arrays displaying weak peaks at 2θ = 24.06, 25.50, 42.12, and 49.96° associated 17084


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annealed samples. A detailed set of Raman spectra for MoO3, SCM, and SMC systems at different irradiation times are shown in Figure S1 (Supporting Information) and will be discussed in more detail in Section 4. 3.3. Chromogenic Performance. In Figure 3a, the absorption spectra for the annealed MoO3 thin films show

with the (100), (002), (110), and (112) crystallographic planes of the wurtzite structure of CdSe (JCPDS−ICDD card number 04-001-7283). These peaks are barely noticeable due to the fact that the CdSe layer is very thin. Pal et al. also found the wurtzite phase for CdSe thin films deposited by a similar evaporation technique.21 As a complement, Figure 1c shows XRD patterns for MoO3 and SCM samples irradiated for 180 min, as well as SCM films annealed at 225 °C. Again, neither irradiation nor thermal annealing induces any crystallization in these films. 3.2. Raman Spectroscopy Analysis. The Raman spectra for the MoO3, SCM, and SMC arrays at RT are seen in Figure 2a. For MoO3, the spectrum shows important features in two

Figure 3. Optical absorption spectra for thin films annealed at different temperatures: (a) MoO3, (b) SCM, and (c) SMC systems. The thermal treatments were performed in the range of 25−225 °C, with steps of 25 °C, for 2 h.

the fundamental absorption edge and the formation of a band, centered at 820 nm, attributed to the color center formation due to thermal annealing, i.e., it manifests the thermochromic response of the material. Similar behaviors were presented by the SMC and SCM systems, as, respectively, seen in Figure 3b,c, with the coloration band being more pronounced for the bilayer systems. Among these systems, the SMC configuration exhibits the highest thermochromic response. To quantify these differences, it is important to analyze the evolution of the absorbance at the maximum of the coloration band as a function of the annealing temperature. Plotted in Figure 4 is the evolution of the absorption modulation, defined as ΔA(T) = A(T) − A0, where A(T) is the absorbance at 820 nm for a sample annealed at temperature T and A0 is the absorbance at 820 nm for the corresponding as-deposited sample. In general, this plot indicates a monotonic increase of the thermochromic response with the temperature for all of the arrangements, with the SMC system achieving the highest absorbance at the highest annealing temperature analyzed. Moreover, it has been reported that the thermochromic coloration of MoO3 exhibits a saturation point at around 200 °C, in agreement with the results obtained in this work.19,25 The photochromic response for all types of films is shown in Figure 5. In this case, the coloration band is formed as a result of UV irradiation and is more evident as the exposure time increases. The photochromic response is higher for the bilayer samples, but unlike the thermochromic effect, the highest photochromic response at 820 nm is obtained for the SCM system (see Figure 6).

Figure 2. Raman spectra for the MoO3, SCM, and SMC systems (a) maintained at RT and (b) annealed at 200 °C. The dotted line indicates features associated with the substrate. The dashed lines indicate CdSe signals and the diamonds are associated with MoO3 bands.

regions. In the low-frequency region from 150 to 500 cm−1, usually known as the region of deformation modes,22 the weak band centered at 223 cm−1 is associated with the Mo−O−Mo bond,22−24 while the dotted line indicates features associated with the substrate.25 The second region at high frequencies lies between 600 and 1000 cm−1; here, the band centered at around 992 cm−1 is related to the Mo−O bond.23−26 With regard to the SCM system, its spectrum shows similar peaks to those displayed by the MoO3 sample (at 992 and 223 cm−1). In this case, it is important to note the widening of the peak located at 223 cm−1, which could be due to the superposition of two Raman signals, one of them (223 cm−1) associated with MoO3 and another one (211 cm−1) related to the longitudinal optical (LO) dispersion mode of CdSe.27 The Raman spectrum for the SMC system displays peaks at 211 and 414.6 cm−1, which are distinctive of the CdSe compound.28,29 In Figure 2b, the Raman spectra for the three systems annealed at 200 °C confirm that the thermal treatments at this temperature do not modify their structure and, in particular, MoO3 maintains its amorphous phase. It should be mentioned that the Raman spectra corresponding to the samples irradiated for 180 min exhibit similar features as those of the 17085


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Figure 6. Absorbance modulation at 820 nm as a function of the irradiation time for the MoO3, SCM, and SMC systems.

Figure 4. Absorbance modulation at 820 nm as a function of the annealing temperature for the MoO3, SCM, and SMC thin films. In all cases, the thermal treatments were performed for 2 h. The photographs illustrate the state of the films.

Figure 7. Band-gap energy for the MoO3, SCM, and SMC systems as a function of (a) the annealing temperature and (b) irradiation time. Figure 5. Optical absorption spectra for thin films irradiated at different times: (a) MoO3, (b) SCM, and (c) SMC systems.

from 2.94 to 2.75 eV as the annealing temperature increases in the range of 25−225 °C. In the case of the SCM and SMC designs, Egap drops in the same range from 2.74 to 2.60 eV and from 2.82 to 2.70 eV, respectively (see Figure 7a). For MoO3 thin films, the slight increase in Egap with the annealing temperature has also been observed by other groups, which have deposited this oxide by different synthesis techniques.30−32 This effect has been associated with a structural rearrangement of the MoO3 lattice, specifically a reduction of point defects.30 In addition, our group recently found that the crystallization process in MoO3 takes place at higher temperatures, with respect to annealing in air, when the thermal treatments are performed in inert atmospheres.25,33 This information implies that, for the SMC system, the CdSe layer causes a slow rearrangement process in the MoO3 lattice as compared to the case where MoO3 is directly exposed to oxygen. For all of the irradiated samples, the band-gap energy remains essentially constant (see Figure 7b).

A complementary analysis of the optical properties allowed the calculation of the band-gap energy (Egap) for the MoO3, SCM, and SMC samples. We used Tauc’s method, described by the equation (αhν)n = A(hν − Egap), where the value of n depends on whether the material has a direct or indirect transition, α is the optical absorption coefficient, hν is the incident photon energy, and A is a proportionality constant. MoO3 is an indirect-transition semiconductor; therefore, a value of n = 1/2 was chosen. Egap was thus obtained by an extrapolation of the linear region in a (αhν)1/2 vs (hν − Egap) plot (see Figure S2 in the Supporting Information). Figure 7a,b depicts the evolution of the band-gap energy as a function of the annealing temperature and irradiation time for thermochromic and photochromic films, respectively. Thermal treatments have a stronger effect on the band gap than irradiation treatments. For the MoO3 thin film, Egap decreases 17086


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outcome supports the idea that electrons can be captured in lattice anion vacancies, i.e., they can be trapped in a midband state between the valence (VB) and conduction (CB) bands upon thermal excitation. The experimental evidence in support of this hypothesis was provided by Rabalais et al.36 These authors investigated the coloration mechanism in substoichiometric MoO3 thin films and claimed that electrons trapped in positively charged anion vacancies are responsible for the formation of a small band in the XPS valence band spectrum, displayed in the binding energy (BE) range from 0.5 up to 1.8 eV below the Fermi level (BE = 0 eV). This point will be discussed in more detail below. As seen in Figure 9, the XPS VB spectra reveal a main band ranging from ca. 3 to 10 eV attributed to the O 2p orbital. In

Illustrated in Figure S3a are the optical absorption spectra for CdSe thin films annealed at different temperatures. It is observed that the thermal treatments do not modify these spectra, indicating that the changes in optical absorption for the multilayer systems arise only from the thermochromic response of MoO3. Similarly, Figure S3b reveals that the optical absorption spectra for CdSe thin films irradiated for different times do not undergo any noticeable change, confirming that the photochromic response comes just from the MoO3 layer. 3.4. XPS Analysis. The surface chemistry of the MoO3 and SMC samples was characterized by X-ray photoelectron spectroscopy. Illustrated in Figure 8a is the high-resolution

Figure 8. HR-XPS spectra of the Mo 3d state for MoO3 thin films at (a) RT and (b) 225 °C. Shown are the Mo 3d3/2 and Mo 3d5/2 components. For the fitting analysis, two doublets associated with the Mo6+ (in cyan) and Mo5+ (in dark blue) oxidation states were considered. The arrows indicate the obtained energy separation for each doublet.

Figure 9. XPS valence band spectra for MoO3 thin films at RT and 225 °C. The valence band maximum was determined from the intercept of the solid lines with the binding energy axis. The weak bands magnified in the inset correspond to localized states.

XPS (HR-XPS) spectrum of the Mo 3d state for the asdeposited MoO3 film, where the spin−orbit splitting characteristic of this level is observed. This spectrum was deconvolved into two doublets, each of them related to a specific oxidation state. The binding energies for the Mo 3d5/2 and Mo 3d3/2 peaks of the most intense doublet, identified with the Mo6+ oxidation state, are 232.60 and 235.70 eV, respectively. Two constraints were considered for the deconvolution process, the peak-to-peak separation (ΔMo 3d = Mo 3d3/2 − Mo 3d5/2) and the peak area ratio; the latter is related to the (2j + 1) subshell degeneracies (area(Mo 3d5/2)/area(Mo 3d3/2) = 3/2), where j is the total angular momentum quantum number.25 Unlike other works, where constraints are strictly fixed, here the constraints were allowed to vary slightly around the expected, reported values. For the Mo6+ oxidation state of the as-deposited MoO3 samples (Figure 8a), the fitting analysis yielded the values ΔMo6+ 3d = 3.10 eV and area(Mo6+ 3d5/2)/ area(Mo6+ 3d3/2) = 1.5, both of which are in accordance with the previous studies.25,34,35 The other doublet placed at 231.50 and 234.80 eV, related to the Mo5+ state, presents ΔMo5+ 3d and area(Mo5+ 3d5/2)/area(Mo5+ 3d3/2) values of 3.30 eV and 1.7, respectively; both values are close to those reported in the literature and are also within the resolution of the spectrometer (±0.2 eV). It is important to point out that, as the annealing temperature increases from room temperature up to 225 °C, the peak area of the Mo5+ oxidation state raises to 27% with respect to the as-deposited sample (see Figure 8b). This

particular, the shoulder at ∼5 eV has been associated with O 2p−metal d bonding interactions, which are related to the O 2p-σ and 2p-π orbitals.37 Recently, our group deposited amorphous MoO3 thin films by radio-frequency reactive magnetron sputtering and found that, depending on the deposition parameters, several oxidation states are produced, including Mo6+, Mo5+, and Mo4+.34 Specifically, the samples with more populated Mo4+ and Mo5+ oxidation states showed a feature in the region between 0 and 2 eV that was attributed to localized states (F-centers). In the present work, both the asdeposited and the thermally treated films display almost the same signal in this region (see the inset of Figure 9). Furthermore, the position of the valence band maximum (VBM) was determined from the intercept of the lower edge of the VB spectra with the binding energy axis. The VBM values varied from 2.70 eV at RT to 2.90 eV at 225 °C, in close agreement with those obtained for sputtered MoO3 thin films.25,34 Additionally, the stoichiometry of the MoO3−x samples was analyzed from the HR-XPS spectra. For the asdeposited samples and those annealed at 225 °C, the values obtained were MoO2.79 and MoO2.87, respectively. This implies that oxygen chemisorption processes take place on the MoO3−x surface as the temperature increases. Figure 10a depicts the Se 3d core-level spectrum for the SMC film annealed at 225 °C, where the spin−orbit splitting is not clearly resolved. The fitting analysis specified that the components Se 3d5/2 and 3d3/2 are centered at 54.67 and 55.50 17087


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Figure 11. HR-XPS spectra of the O 1s state for MoO3 thin films at (a) RT and (b) 225 °C.

Moreover, it is seen that for annealed samples, the intensity of the lattice oxygen signal increased with respect to the other two peaks, indicating an enhancement in Mo−O bonds. To further investigate the interaction between O and Mo, the Auger parameter, α, was obtained by summing the kinetic energy (KE) of the Auger peak and the binding energy of each element, i.e., α = KE + BE. For oxygen, the O KLL and O 1s lines were used to calculate αO. For molybdenum (αMo), the Mo M45N23V and Mo 3d5/2 contributions were considered. Additionally, the chemical-bonding interaction between molybdenum and oxygen was calculated as ΔMo = O 1s−Mo 3d5/2. The obtained data, presented in Table 1, show that both parameters are higher for the annealed sample, indicating a stronger interaction with molybdenum as the annealing temperature increases. Finally, the XPS depth profile for the SMC bilayer system is presented in Figure 12. Both the CdSe−MoO3 and the MoO3−substrate interfaces are clearly visualized. As indicated by the first levels of ion bombardment, the elements with the highest contribution to the first layer are Cd and Se. This layer is removed after three etching levels, which coincides with an expected increase in the Mo and O signals. The MoO3 layer is observed in the etching cycle range of 3−43. For higher etching cycles, a drop in the molybdenum signal is observed and, at the same time, an increase in the O and Si signal is perceived, indicating that the glass substrate has been reached.

Figure 10. HR-XPS spectra for the SMC thin film annealed at 225 °C showing the (a) Se 3d and (b) Cd 3d states. The characteristic spin− orbit splitting of these orbitals is noticed. One doublet was deconvolved in each case. The arrows indicate the calculated energy separations.

eV, respectively, with a peak-to-peak separation of ΔSe 3d = 0.83 eV, a value close to that found by Zhu et al.;38 additionally, the area(Se 3d5/2)/area(Se 3d3/2) ratio was 1.5. Figure 10b shows the typical doublet of the Cd 3d5/2 and 3d3/2 lines placed at 405.60 and 412.52 eV, respectively. The peak separation of approximately ΔCd 3d = 6.92 eV indicates a pure Cd−Se bonding, in accordance with the value obtained by Sharma et al.;39 in this case, the area(Cd 3d5/2)/area(Cd 3d3/2) ratio yielded a value of 1.2. The elemental chemical compositions were calculated as 42.1 and 57.9 atom % for Cd and Se, respectively. To get a better insight of the interaction between oxygen and molybdenum upon thermal annealing, a detailed analysis of the high-resolution XPS spectrum of the O 1s state was made for both the as-deposited and annealed MoO3 thin films. As shown in Figure 11, this spectrum was deconvolved into three specific components: one peak centered at 530.53 eV, associated with oxygen linked to Mo in the bulk (Olattice, i.e., Mo−O bonds), and two other signals centered at 532.07 and 533.18 eV, related to superficial oxygen (Osurface, related to Mo−OH interactions) and adsorbed oxygen (O adsorb , attributed to C−OH/C−O−C bonds), respectively. These signals are in agreement with studies carried out elsewhere.34,40 Here, it is particularly important to note the evolution of the peak associated with oxygen adsorption as the annealing temperature increases. To quantify this evolution, the change in the Osurface/Oadsorb area ratio was determined for samples maintained at RT and samples annealed at 225 °C; the obtained values were 6.1 and 5.5, respectively. The decrease in the Osurface/Oadsorb area ratio as the annealing temperature increased from RT up to 225 °C, estimated at ca. 10%, is an evidence of oxygen adsorption on the sample surface.

4. DISCUSSION OF THE COLORATION MECHANISM In this section, the possible mechanism of enhanced coloration is discussed. On the one hand, it has been found that the coloration of all of the three types of samples is improved by increasing the annealing temperature. In particular, the MoO3 and SCM thin films present a maximum coloration when the temperature reaches 200 °C, after which point the films start bleaching. This saturation point was described in an earlier work by Tomás et al.19 On the other hand, for the SMC system, a coloration increase is observed even at 225 °C. This result is related to the amorphous nature of the samples, as it has been proved that the color change is more efficient when MoO3 is amorphous; here, this characteristic is preserved even at 225 °C.3,25 It has also been demonstrated that the thermochromic coloration of MoO3 is enhanced when treated at high temperatures in inert atmospheres, specifically, in 17088


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The Journal of Physical Chemistry C Table 1. Auger Parameter (α = KE + BE) and Chemical-Bonding Interaction (ΔMo = O 1s−Mo 3d5/2) element or compound Mo β-RbSm(MoO4)2 CsMnMoO3F3 Oxyfluoride MoO3 (RT) MoO3 (225 °C)

αO = O KLL + O 1s source monochromated Al Kα nonmonochromatic Al Kα nonmonochromatic Al Kα monochromated Al Kα monochromated Al Kα

αMo = Mo M45N23V + Mo 3d5/2 ΔMo = O 1s−Mo 3d5/2

[eV]

[eV]

[eV]

αMo error respect to Mo [%]

415 1041.9

reference 41

413.9

297.9

0.27

42

414.3

297.6

0.17

43

1043.1

415.6

297.96

0.14

1043.2

416.8

298.16

0.43

this work this work

the II−VI semiconductor. Alternatively, when CdSe is used as a back layer, it can profit from the radiation not absorbed by MoO3 and then provide photogenerated electrons to the oxide. Nevertheless, when MoO3 becomes colored, less appropriate radiation reaches the semiconductor layer. In this sense, for photochromic purposes, the idea of using the semiconductor as a dopant seems to be more convenient.19 In addition to the evolution of the absorption spectra, for different irradiation times, a more detailed analysis of the Raman spectra reveals the evolution of the characteristic band at around 400 cm−1. Cruz-San Martiń et al.8 have claimed that the increase in the intensity of this band is strongly associated with the formation of bonds between Mo5+ states and oxygen. This effect is caused by a reduction of Mo6+ to Mo5+ states,44 which takes place after excited electrons are trapped in oxygen vacancies; in this case, the electrons are excited by the action of UV irradiation. The evolution of the Raman peak intensity is shown in Figure S1. This behavior corroborates that the SMC system displays more color centers than the SCM array and the MoO3 thin film.

Figure 12. XPS depth profile for a representative SMC thin film showing the elemental composition of the different layers.

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CONCLUSIONS The thermal evaporation in vacuum has proven to be an effective, simple technique for the deposition of MoO3 thin films with good chromogenic properties. The use of a CdSe thin film as either an upper or back layer increased the chromogenic response of MoO3. However, regarding thermochromism and photochromism, the SMC and SCM systems performed quite differently. While the thermochromic response for the SMC configuration was clearly superior to that for the SCM and MoO3 systems, the highest photochromic response was displayed by the SCM array. In the case of photochromism, these results were explained in terms of the electron injection provided by the II−VI semiconductor layer to the oxide, whereas in the case of thermochromism, CdSe plays the role of a barrier layer limiting the replenishment of oxygen vacancies in the MoO3 thin film.

argon.25 Considering this information, it can be said that the deposition of a CdSe layer over MoO3 (SMC) works as a barrier, preventing the interaction of oxygen with MoO3; accordingly, it simulates a reductive atmosphere, and, as a result, an increase of the thermochromic response is observed. Furthermore, while the thermochromic response for the SCM array is worse than that for the SMC array, it resulted better with respect to MoO3. This can be explained by considering that the coloration in MoO3 arises from electrons generated by thermal excitation, which are subsequently trapped by oxygen vacancies to form color centers. Because of the smaller band gap of CdSe, with respect to MoO3, an extra amount of electrons is generated in the chalcogenide, which could lead to an additional formation of color centers if they reach the oxide matrix.18 However, the exposition of MoO3 to air during thermal treatments leads to the replenishment of oxygen vacancies, limiting the formation of new color centers. For photochromic samples, the presence of the semiconductor layer is also valuable. It must be mentioned that the film temperature never exceeded 40 °C during irradiations. At these relatively low temperatures, both the thermochromic effect and the replenishment of oxygen vacancies can be neglected. Accordingly, photons are responsible for the generation of electron−hole pairs in the CdSe layer and, consequently, for the injection of these electrons into the MoO3 layer. The fact that the SMC configuration resulted worse than the SCM one could be due to low electron injection into the oxide, despite an intense light absorption by

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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/acs.jpcc.9b02895. Raman spectra for MoO3, SCM, and SMC systems at different irradiation times (Figure S1); Tauc model linear fitting of representative MoO3 thin films (Figure S2); and absorption spectra for CdSe thin films at different (a) irradiation times and (b) annealing temperatures (Figure S3) (PDF) 17089


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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

M. Morales-Luna: 0000-0002-0351-8118 M. Pérez-González: 0000-0002-8511-0465 S. A. Tomás: 0000-0003-2663-1233 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was partially supported by the Mexican Council for Science and Technology (CONACyT) (Project Nos. 168605 and 205733). M.A.A. thanks CONACyT for financial support to work at Universidad Politécnica de Chiapas for one year within the “Retención” program. The technical assistance of E. Ayala, M. Guerrero, and A. Garciá Sotelo is acknowledged.

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Effect of a CdSe Layer on the Thermo- and Photochromic

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